Nanomanufacturing of Functional Nanostructured Surfaces
for Efficient Light Transport
-SKTfTI
TE
ARCHVES
by
1
C)F
Jeong-Gil Kim
-EHOLULGY
JUL 3 0 2015
B.S., Mechanical Engineering
Seoul National University, 2002
LIBRARIES
M.S., Mechanical Engineering
Seoul National University, 2004
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2015
Massachusetts Institute of Technology 2015. All Rights Reserved.
Signature redacted
-
Signature of Author ........ ........
Dep0ofmen-f1echanical Engineering
April 20, 2015
Signature
redacted
... 0 . . . .
.
Certified by.
George Barbastathis
Professor of Mechanical Engineering
Thesis Supervisor
Accepted by .......
Signature redacted'...............
David E. Hardt
Chairman, Department Committee on Graduate Students
I
Nanomanufacturing of Functional Nanostructured Surfaces
for Efficient Light Transport
by
Jeong-Gil Kim
Submitted to the Department of Mechanical Engineering
on April 20, 2015, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
ABSTRACT
Nanostructured surfaces have given rise to many unique optical properties, such
as broadband anti-reflectivity, structural coloring effects, and enhanced light extraction
from high refractive index materials due to their potential to modulate optical behavior on
their surfaces. This thesis focuses on design, analysis, and fabrication of functional
nanostructured surfaces for efficient light transport, seeking optimized optical
performance, high mechanical robustness, and manufacturability, with the aim of
increasing the practicality of the photonic nanostructures.
First, for the case when light propagates from a low-index material to a highindex material, I designed and fabricated an array of inverted nanocones that realizes
anti-reflectivity with robust mechanical strength. The surface exhibits broadband,
omnidirectional anti-reflectivity due to the axially varying effective refractive index of
the inverted nanocone arrays. The surface also maintains its optical performance after
being externally loaded, thanks to low stress concentration and small deflection of the
inverted nanocone structure. In addition, for multi-optical interfacial surfaces, doublegradient-index nanostructures are proposed and demonstrated in order to achieve ultimate
anti-reflectivity. The top surface, textured with inverted nanocones, maintains high
mechanical robustness.
Second, for the case where light has to be extracted from high-index materials, a
conical photonic crystal is proposed and demonstrated. The tapered conical geometry
suppresses Fresnel reflections at the optical interfaces due to adiabatic impedance
matching. Periodicity of the arrays of cones diffracts light into higher-order modes with
different propagating angles, enabling certain photons to overcome total internal
reflection (TIR). After optimizing the structural geometries to balance Fresnel reflection
and TIR, light yield efficiency is characterized experimentally on scintillator surfaces.
3
In order to enhance the adaptability to industrial manufacturing, the fabrication
methods are based on replicating the photonic nanostructures into a UV-curable polymer,
with the help of laser interference lithography as a method of fabricating a master mold.
Advanced techniques such as vacuum assisted-filling and a selective delaminating
method are also developed to produce nanostructures more effectively.
The novel nanostructured surfaces designed in the thesis, and the ability to
imprint these topographies through several generations, are promising for large-scale
commercial applications where efficient light transport is important.
Thesis Supervisor: George Barbastathis
Title: Professor of Mechanical Engineering
4
Acknowledgements
I would first like to thank my advisor, Professor George Barbastathis, for his guidance
and support throughout all these years, and for serving as a life mentor and a friend.
Working with George has been my great pleasure. Thanks to his patience, trust and all his
advice, I could finish this dissertation, starting from no clue about optics.
I also want to thank Professors Jung-Hoon Chun and Nicholas Fang for serving on my
thesis committee. Professor Chun has given me invaluable input and advice about
research as well as about career and life since my first day at MIT. Professor Fang has
always been willing to spare his precious time for any discussion and provided valuable
insights for my work. I feel privileged to be mentored by such great professors in MIT.
I also would like to thank the members of 3D Optical Systems group. Special thanks to
Hyungryul Johnny Choi, a true friend met in MIT. I cannot imagine going through this
tough process without his friendship and all his help. I have learned a lot from him about
research as well as being inspired by his passionate attitude. I also thank Max Hsieh for
all the discussion we had and for his comments about nanofabrication. All other group
members also deserve my thanks for their support: Yi Liu & Lei Tian couple, Hanhong
Gao, Justin Lee, Adam Pan, Kelli Xu, Shuai Li, Nilu Zhao, Dr. Yunhui Zhu. I also thank
former group members Dr. SeBaek Oh, Dr. Se Young Yang, Dr. Yen-Sheng Lu, Dr.
Satoshi Takahashi and Prof. Chih-Hao Chang, who helped me a lot when I first started a
life at MIT.
This thesis would not have been possible without the help of collaborators. I wish to
express my appreciation to Professor Gareth McKinley and Dr. Kyoo-Chul (Kenneth)
Park for adding enhanced wetting properties to the multifunctional nanotextured surfaces
and all the critical input to this work. I thank Dr. Bipin Singh, Dr. Julie Gardener and Dr.
Rajan Gurjar in RMD and Dr. Arno Knapitsch in CERN for working together as a team
on extracting light from scintillators. I thank my Spanish friends, Sagrario Dominguez
and Ignacio Cornago for their contribution for making master molds.
I would like to thank the members of Korean Graduate Society Association in
Mechanical Engineering (KGSAME) for their support and friendship. I acknowledge the
Samsung Electronics for the financial support throughout my graduate study.
5
I would like to thank my father, mother (in heaven), two sisters and my brother for their
love and all the prayers. I deeply thank my wife, my soul mate, Rami Lee, for her
constant love and trust. My two sons, Jaden and Joel, both born since beginning of this
work, have been sources of my positive energy.
Lastly, I thank God for the gift He has given me, for being with me and for inspiring me
to be a better researcher and a better human.
6
Table of Contents
L ist of F igures.................................................................................................
. . 10
L ist of Tab les ...................................................................................................
. . 18
19
C hapter 1. Introduction. .....................................................................................
1.1. Photonic N anostructures ...................................................................
19
1.2. Properties of Light at Optical Interfaces..........................................
20
1.2.1. Properties of light..................................................................
20
1.2.2. Interfacial optical phenomena...............................................
22
1.2.3. Nanostructures for control of interfacial optical phenomena .... 25
1.3. Nanomanufacturing Methods.............................................................28
1.3.1. Conventional nanofabrication methods .................................
28
1.3.2. Nanoimprint and laser interference lithography .................... 29
1.4. Thesis Overview ...............................................................................
References...............................................................................................
32
. . 34
Chapter 2. Multifunctional Inverted Nanocone Arrays for Anti-reflective Surface
with High Mechanical Robustness and Enhanced Wetting Properties.........38
2.1. Introduction ......................................................................................
. . 38
2.2. Numerical Models for Inverted Nanocones......................................
41
2.2.1. Optical behaviors: Anti-reflection ........................................
41
2.2.2. Wetting Properties: Self-cleaning and Anti-fogging ............. 43
7
2.2.3. Mechanical Robustness.........................................................
46
2.3. Fabrication using Replication Method...............................................49
2.4. Results and Discussion ......................................................................
53
2.5. C onclusion .........................................................................................
. 59
R eferences................................................................................................
. 61
Chapter 3. Double Gradient-Index Nanostructures for Broadband Anti-reflectivity
of Multi-optical Interfaces
65
3.1. Introduction ........................................................................................
65
3.2. Design of Nanostructures for Multi-optical Interfaces ...................... 68
3.3. Fabrication of Double Gradient-Index Nanostructures...................... 71
3.4. Results and Discussion ......................................................................
3.5. C onclusion .........................................................................................
R eferences...............................................................................................
74
. 76
. . 77
Chapter 4. Conical Photonic Crystals for Enhancing Light Extraction Efficiency
from High-Index Materials ...........................................................................
4 .1. Introduction ......................................................................................
79
. . 79
4.2. Analysis of Light Extraction in Near-wavelength Periodic Cone Arrays82
4.2.1. Light extraction using conical photonic crystals ................... 82
4.2.2. Diffraction efficiency analysis...............................................
84
4.3. Optimization using RCWA ................................................................
92
4.4. Fabrication Process ...........................................................................
94
4.4.1. Master fabrication ..................................................................
8
94
4.4.2. Replication process optim ization...........................................
96
4.5. Characterization and Discussion........................................................
99
4.5.1. Fabrication results..................................................................
99
4.5.2 Qualitative characterization ......................................................
102
4.5.3. Quantitative measurement of light yield using PMT...............103
4.6. Conclusion ...........................................................................................
108
References...................................................................................................110
Chapter 5. Conclusion..........................................................................................112
Appendix A. Gibbs Free Energy Density for Prediction of Wetting States on an
Inverted N anocone Surface.........................................................................115
9
List of Figures
Figure 1-1. Schematics of light propagation (a) from a low refractive index material
to a high index material, and (b) the other way around. ..................... 21
Figure 1-2. Examples of light transport from low to high index material and the other
w ay around........................................................................................
Figure 1-3. A schematic of different optical regimes. ...........................................
. . 24
25
Figure 1-4. Various nanostructures including anti-reflective nanostructures and
diffractive nanostructures....................................................................
26
Figure 1-5. C oncept of nanoim print.........................................................................
30
Figure 1-6. A schematic of Lloyd's mirror interference lithography system .......... 32
Figure 2-1. Refractive index profile of (a) a flat surface, (b) nanocone surface and (c)
nanohole (or inverted nanocone) surface. (b) and (c) show gradually
varying refractive indices....................................................................
40
Figure 2-2. FDTD (Finite-difference time-domain) simulations of the optical
performance of the nanostructured surfaces in the wavelength range of
350 nm < 2 < 1800 nm at normal incidence (Oi = 0'): Contour plot of the
fraction of light reflected R(A, HIP) from surfaces textured with either
(A) nanocone arrays or (B) nanohole arrays. Light transmission T(,
HIP) from surfaces textured with either (C) nanocone arrays or (D)
nanohole arrays ...................................................................................
10
42
Figure 2-3. Colored contour maps of the change in the Gibbs free energy density
AG(6,*) as a function of the putative apparent contact angle (Q,,*) with a
water droplet and different normalized vertical position (z/H) of the
water meniscus in the inverted nanocone array (A) with intrinsically
hydrophilic PUA (OE = 800) and (B) with hydrophobic fluorosilane
surface coating (OE
-120').
See Appendix for the Gibbs free energy
density function. The effective apparent contact angle (6*) is related to
the equilibrium contact angle (OL) by the Cassie-Baxter relation that
accounts for the solid-liquid fraction and air-liquid fraction for liquidsolid-air composite state of droplet (in the limit of fully-wetted state (z/H
->
1), the Wenzel relation is used). ro and
#,
are functions of z/H. The
insets show goniometric images of water droplets (V= 10 tl) sitting on
each surface. In both (A) and (B), the blue color represents the locus of
the global minimum in the Gibbs free energy density variation
landscape, the corresponding apparent contact angles (6*= 6,* when AG
= min(AG)) are in good agreement with the experimental results
measured by goniometry shown in the insets. ..................................
45
Figure 2-4. SEM images of high aspect ratio nanocones showing examples of
m echanical instability. .......................................................................
11
47
Figure 2-5. Mechanical robustness calculated by finite element method (FEM): (A)
Stress distribution in a nanocone and a nanohole structure (fabricated
from PUA, Young's modulus E ~ 400 MPa) resulting from a typical
shearing or normal finger force (4 N) applied at the top over a circular
area with a radius of 5mm. The numerical values represent the
maximum Von Mises stresses for each case; (B) Maximum tip
deflections of a nanocone and a nanohole structure (P = 200 nm) under a
lateral shearing force for different aspect ratio of nanocones and
nanoholes. The gray shaded region indicates a tip displacement that is
greater than the pitch of the nanocones...............................................
48
Figure 2-6. Schematic representation of the fabrication process showing (A) the
inverted nanocone arrays replicated from the original nanocone master,
and (B) second generation of replicated nanocone arrays formed and
released using an anti-adhesion layer in UV-curable poly urethane
acrylate (PU A ).1
...............................................................................
50
Figure 2-7. A comparison between two filling conditions: (A) Filling process at
atmospheric pressure and the resultant surfaces after keeping the filling
process for 5 and 10 minutes, (B) Vacuum-assisted filling process and
the corresponding surface images after 5 and 10 minutes of contact time.
The aspect ratio of the replicated structure gets larger with longer time
duration. After 10 minutes, the surface exhibits same aspect ratio with
th e m o ld ............................................................................................
. . 52
Figure 2-8. SEM images of (A) side view of a silica mold fabricated using laser
interference lithography. (B) Replicated inverted nanocone arrays
imprinted in PUA and (C) second generation replicated PUA nanocone
arrays. The aspect ratio of the replicated nanostructure is H/P
tip radius rti ~ 20 nm ..........................................................................
12
4 with
54
Figure 2-9. Enhanced optical transmission of the nanotextured surfaces. (A)
Measured broadband transmission over a wide range of wavelength (350
nm < k < 1400 nm) and (B) optical transmission for transverse electric
(TE) polarized light through the nanotextured and flat fused silica
surfaces is measured by changing the incident angle of a laser source
whose wavelength is 633 nm. At an angle of Oi = 80', the transmission
of the nanohole array is T= 40.3%, but has dropped to T= 23.0% for the
flat silica glass. Each angular point was averaged automatically by the
power meter (Newport, 2832-C) over 100 repeated measurements with
standard deviation of less than 0.0 1% ...............................................
54
Figure 2-10. Optical transmission measurements showing (A) anti-fogging behavior
of superhydrophilic inverted nanocone surfaces and (B) self-cleaning
behavior of water-repelling inverted nanocone surfaces with three
different types of powders coated with thickness of more than 0.5 mm
(SiC particles, Lycopodium spores and white sand grains with average
diameters of 10 prm, 30 [tm and 100 pm, respectively). The error bars
were determined by repeating the measurements three times each; the
large deviations are because of the dynamic nature of the measurement;
the precise location of the droplet impacts, the initial uniformity of the
powder on the surface, and the evolution of the droplets and
contaminants after droplet deposition could only be controlled with
limited precision............................................56
13
Figure 2-11. Mechanical robustness test of PUA surfaces textured with inverted
nanocone arrays. (A) Optical transmissivity of the nanotextured surface
after applying a contact force of 4 N in the normal and shearing
directions of the nanotextured surface through a latex rubber pad
(dimension 8.9 mm x 8.9 mm) repeatedly; (B) after applying normal
force through a Neoprene rubber ball (Young's modulus En, a 5.5 MPa
and radius Rnp =4.8 mm) up to 60 N (Corresponding contact pressure 2
3 MPa, calculated using Hertz contact pressure).12 01 The insets show the
SEM images of the egg-crate nanotexture before and after applying the
force
................................................................................................
. . 58
Figure 2-12. Demonstration of multi functional inverted nanocone surfaces including
anti-reflectivity, antifogging effect and self-cleaning effect. Left side of
each figure shows a result of a flat fused silica glass, and right side of
each figure shows inverted nanocone surfaces..................................60
Figure 3-1. (a) A schematic of double gradient-index (D-GRIN) nanostructures for
multi-optical interfaces and the simplified fabrication process; (b) the
gradient-index profile of D-GRIN nanotextured surface....................67
Figure 3-2. Reflectance calculated using FDTD method as implemented in FDTD
solutions 8.0 for different surfaces: flat silicon surfaces, thin film antireflective coating (ARC) cases and nanostructured surfaces consisting of
either double-cones or single-cones. In all calculations, the periodicity of
the nanocones was 200nm, the height was 800nm (aspect ratio of 4), and
the Palik dispersion model was used [17]. The inset shows the enlarged
reflection spectra for single-cone and double-cone surface cases..........70
Figure 3-3. (a) A schematic of Lloyd's mirror interference lithography system used
for fabricating silicon nanocones and (b) UV replication process used for
encapsulating-polymeric surface textured with inverted nanocones......73
14
Figure 3-4. SEM images of the fabricated samples. (a) Silicon nanocone structures
used as a substrate and a master mold in the replication process; (b) a
side view and (c) a top view of replicated inverted nanocone structures
on PU A surfaces. ...............................................................................
74
Figure 3-5. Anti-reflectivity of the nanotextured surfaces. Broadband reflectivity was
measured over a wide range of wavelength (300 nm < X < 1500 nm) and
com pared with calculated values. .......................................................
75
Figure 4-1. (a) A schematic of light extracting environment for a scintillator; (b) The
concept of conical photonic crystals on a scintillator surface for
enhancing light extraction efficiency.................................................
80
Figure 4-2. Light transmission for (a) GRIN structures with a pitch of 200 nm and a
height of 800 nm, (b) PhC structures with a thickness of 450 nm, and (c)
conical PhC structures with a height of 800 nm coated on an inorganic
scintillator calculated using FDTD. In the simulation, light illumination
is assumed to be a transverse electric (TE) polarized irradiation at a
wavelength of A = 540 nm. The refractive indices of the light extracting
layer and the scintillator are assumed to be 1.82; (d) the comparison of
light transmission among flat surface and nanotextured surfaces
calculated using FDTD and RCW A. ..................................................
83
Figure 4-3. Diffraction efficiency calculated using a numerical method (RCWA) and
analytic equation using the Fourier series...........................................
85
Figure 4-4. A schematic of the light incident on (a) a flat surface and (b) a photonic
crystal surface; (c) a schematic of the photonic crystal surface of the
Bragg diffraction phase matching diagrams between a scintillator and air
in k-space. The large waveguide mode circle has a radius kg = 2nen/
and the small air circle has a radius ko = 2 n ir/l....................................86
15
Figure 4-5. Transmission distribution separated into different diffraction modes for
(a) TE (E-field parallel to the plane of incidence) and (b) TM
polarization calculated using RCWA. A ID triangular grating is
simulated at a wavelength ofL= 420 nm with a refractive index of 1.82
both for the scintillator and the light extracting material. White dotted
lines representing the
O-P
relationship following the analytic equation.
Each line indicates the boundaries confining the area where each
diffraction mode exists, following conservation of the in-plain k-vector.88
Figure 4-6. Transmission versus emission angle and pitch, calculated using RCWA. A
ID triangular grating is simulated at a wavelength of
.
= 420 nm using
(a) TE polarized light and (b) TM polarized light. The refractive index of
the scintillator and the light extracting material was set to 1.82......89
Figure 4-7. Effect of refractive indices on light transmission through conical photonic
crystals (H = 0.8 pm) calculated using RCWA. All the emission and
azimuthal angles (00 < 0, < 900, 00 <
OazimUtlal
< 900) and polarization
components (TE and TM) are incorporated into the simulation......91
Figure 4-8. Optimization results for the pitch and height of the conical photonic
crystal as a light extracting layer on an LSO scintillator surface coupled
with (a) air and (b) index matching liquid (n1 = 1.5). ....................... 92
Figure 4-9. Schematic representation of the master mold fabrication process
consisting of laser interference lithography and subsequent shrinking
m ask etching. ....................................................................................
. . 94
Figure 4-10. Schematic representation of the imprint process the conical photonic
crystals replicated from the original silicon master. ...........................
97
Figure 4-11. SEM images of fabricated silicon master molds consisting of tapered
nanostructures. The pitch of the structures is 700 nm, and heights are
varied depending on the fabrication conditions. The heights of the
nanostructures are (a) 170 nm, (b) 260 nm, (c) 320 nm, (d) 770 nm, (e)
1000 nm and (f) 830 nm ........................................................................
16
100
Figure 4-12. SEM images of (a) top view and (b), (c) side view of a replicated PUA
polymer from a silicon mold fabricated using laser interference
lithography. The pitch of the nanostructure is 700 nm with the height h
1 m ......................................................................................................
10 0
Figure 4-13 Images of (a) a flat scintillator surface and (b) nanotextured film on the
scintillator. Reflectance on the top surface is drastically reduced in the
case of nanotextured surface (b). ..........................................................
101
Figure 4-14 (a) A schematic of diffraction effect test for nanotextured scintillator
surface and (b) the picture of experimental set-up................................102
Figure 4-15 (a) An image of the projection screen placed in front of a scintillator
under the room-light condition; (b) Projection screen when the coupling
surface is coated with nanostructure and (c) without nanostructure.....103
Figure 4-16. A schematic representation of scintillator - coupler - PMT stack for
measuring light yield through the conical photonic crystals and the
condition for m easurem ent. ..................................................................
104
Figure 4-17. Light yield enhancement quantified for the scintillators coated with
conical photonic crystals when coupled with air and an optical matching
fluid. (a) symmetrical conical shape case and (b) asymmetrical conical
sh ap e . ....................................................................................................
10 6
Figure 4-18. Demonstration of scintillating mode of different nanostructured
scintillators. UV light (A
365nm) is illuminated on LSO scintillators
coated with and without different types of nanostructures such as GRIN,
PhC and conical PhC structures............................................................109
Figure A-I. A schematic diagram of an inverted nanocone structure. Solid-liquid and
solid-air interfaces are represented by cyan and red colors, respectively.] 16
17
List of Tables
Table 4-1. Experimental conditions for fabricating silicon master mold ............... 95
Table 4-2. Experimental conditions for imprinting conical photonic crystals......98
Table 4-3. Summary of the samples used in qualitative characterization..................105
18
Chapter 1.
Introduction
1.1. PHOTONIC NANOSTRUCTURES
It is a wonder of nature that biological systems have been using photonic
nanostructures to produce various optical effects to manipulate the flow of light for
millions of years. There are a variety of natural photonic structures: A species of
nocturnal insects use nipple arrays (or moth-eye structures) on the cornea to minimize the
reflectivity on their compound eyes [1-7]. Morpho butterfly wings use multiple layers of
cuticle and air to produce the iridescent blue, visible from a great distance [1, 6]. Colors
on beetle shells are engendered by photonic nanostructures [6]. A firefly lantern has
cuticular nanostructures for efficient light extraction when emitting light from its body
[8].
Inspired by such natural photonic structures, in recent decades, scientists have
tried to control interfacial optical phenomena, for example optical reflection and
diffraction, by creating synthetic nanostructures. Nanostructures have given rise to many
unique physical properties, such as broadband, omnidirectional anti-reflectivity [7],
structural coloring effects [6] and light extracting from high refractive index materials [9]
just as we observe in nature. Multidimensional micro/nano structures are key components
in efficient light transport in emerging optical fields due to their potential to modulate
optical behavior on their surface[lO], which can be applied to various real world
applications such as displays, light emitting diodes, energy devices, opto-electric devices,
imaging devices, functional glasses or optical films, to name a few.
However, the nanostructures are limited in mechanical robustness, optimized
performance and manufacturing throughput, resulting in non-practicality in the real
19
world. The nanotextured surface is often vulnerable to the outer environment and
incompatible with severe user behavior. Moreover its performance is not fully optimized
because its design is limited by a lack of understanding of nanoengineering. Also even
with great strides that have been accomplished in nanofabrication over the last decade,
nanomanufacturing is still challenging for many types of nanostructures especially on a
large area. Therefore, it is important to analyze and manipulate the nano-scale patterns,
identify the key parameters that improve performance and enhance the adaptability to
industrial manufacturing. More advanced 3D patterning methods are still necessary for
better performances of these devices as modern optical applications tend to rely largely
on nanofabrication.
In this thesis, I propose, analyze and optimize novel functional nanostructured
surfaces for efficient light transport, considering optimized performance, high mechanical
robustness, compatibility with the outer environment and manufacturability. I study the
physics behind optical nanostructures such as anti-reflection, light extraction, nanostructural mechanics and their wetting properties. I design the novel nanostructures to be
manufacturable and optimize the manufacturing processes given the demands of real
world applications with the aim of fulfilling the growing demand for maximizing light
collection efficiency for consumer electronics, energy devices or space exploration.
1.2. PROPERTIES OF LIGHT AT OPTICAL INTERFACES
1.2.1. Properties of light
When light is transported through an interface consisting of two different media,
there are common phenomena, such as reflection, refraction, absorption and diffraction.
The optical interface is defined by a boundary shared by two media of different refractive
indices. Optical properties and light behavior at the interface depend on the refractive
indices of the two media.
20
The refractive index of a medium is a measure of the propagation properties of
light in the medium. It is geherally a complex number varying with different
wavelengths:
(1-1)
5(,) = n(X) + iK(k)
The real part n(X) or real refractive index characterizes the light propagation
speed. The imaginary part K(X) or extinction index is related to absorption by the
medium. Due to a difference in refractive indices at an optical interface, light may be
reflected back from the interface, and transmitted through the interface after refraction.
Light also can be absorbed by the material if the extinction index is not zero at the
wavelength of the incident light. In addition, when the interface is textured with periodic
structure with a periodicity larger than the wavelength of light, light interference on the
periodic structures induces diffraction, generating multiple orders of light propagating
into different directions.
Reflected Light
Incident Light
Extracted Light
(e < e)
n,
# en1
(a)
(b)
Figure 1-1. Schematics of light propagation (a) from a low refractive index material to a
high index material, and (b) the other way around.
21
1.2.2. Interfacial optical phenomena
Control of interfacial optical phenomena, for example optical reflection and
diffraction, is increasingly important both from the point of view of fundamental
understanding of interfacial properties and of course for engineering applications. For
example, eliminating reflection at an air-glass interface increases the efficiency of lightharvesting devices and display devices. These interfacial phenomena need to be
controlled for enhancing the light transport through optical interfaces.
The transportation of light is categorized into two cases: (a) light travels from a
low index material to a high index material and (b) the other way around.
1.2.2.1. From a low refractive index to high refractive index
First, when light travels from a low index material to a high index material, the
light transmission
is usually affected by the Fresnel reflection, which is an
electromagnetic phenomenon that occurs at optical interfaces due to difference in
refractive indices of media. To quantify the light reflection according to different
refractive indices, Fresnel equation is often used.
For the transverse-magnetic (TM) polarization, the reflectance is:
(tan( 1 ( tan(0 1 +
02) 2
02))
where 0, is the incidence angle and 02
=
ncos61- cos0 2
:_ncos0
1 + cos0 2
sin-'(nisin
01/n2)
2
is the angle of refraction into
medium 2 defined by Snell's law and n1 and n2 are the refractive indices at either side of
the boundary. For the transverse-electric (TE) polarization, the reflectance is:
(sin(0
(
1 -
02)
sin(0 1 + 02)
2
(cos0 1 - ncos0 2
^ cos9 1 + ncos0 2
22
2
If there are multiple layers with a series of refractive indices, the total reflection is
calculated by incorporating all the interference at each layer as well as Fresnel
reflections.
The Fresnel reflection often unwanted phenomenon for most optical applications.
For examples, it creates glare on the surface of display devices and losses in efficiency of
optical and opto-electric devices such as solar cells or photodetectors [11].
Various methods have been developed over time to minimize reflection from a
surface in order to improve the efficiency of the optics by increasing transmission or
absorption for maximizing light collection. One of the most conventional ways to
suppress reflection is the thin film interference method. In order to minimize the optical
reflection at an optical interface, a single- or multi-layered stack of two or morc
alternating optical materials is coated on a substrate.
By coating a quarter-wave layer of
transparent material with refractive index of the square root of the substrate's refractive
index, the light destructively interferes in the film and eventually all the reflected light
can be canceled out. In order to induce complete destructive interference on the surface,
the film thickness should be t=X / 4 ncoating where
)
is the wavelength of light and ncoating
is the refractive index of the coating. Also the complete destructive condition will occur
when the amplitudes of both reflecting waves are identical, which requires the refractive
index of the coating to be ncoating = Vni-2.
Further, by putting multiple layers with alternating refractive indices, multiple
beam interferences induced by partial light reflection and transmission at each optical
interface will generate better anti-reflectivity compared to the single layer case.
While thin film coating method relies on the interference of optical waves and the
performance of this method is limited to a certain range of wavelengths and incidence
angles of incoming light, texturing the surface with nanostructure or high-frequency
surface-relief structure offers better opportunities for anti-reflectivity, which will be
discussed in section 1. 1.3.
23
1.2.2.2. From a high refractive index to low refractive index
There are also cases when light travels from a high index material to a low index
material, where efficient light extraction is crucial such as in many photonic devices
where light is generated from the inside of high refractive index materials such as light
emitting diodes (LEDs) and scintillators [9, 12-17].
When the generated light is coupled with an interface between a high index
material and an outer environment such as air, light transmission is still limited by the
Fresnel reflection induced by a difference in refractive indices between two materials.
Further, total internal reflection (TIR) governed by a critical angle from Snell's law limits
the light extraction efficiency, especially when a refractive index is high due to the low
critical angle. Since light generation in a high index material can be assumed to be
isotropic, all light whose incidence angle is larger than the critical angle should be
reflected back and trapped in the material [18]. The thin film interference method can be
applied here for reducing the Fresnel reflection, but the enhancement of light extraction
efficiency is limited since a large portion of light generated inside of high index materials
still cannot be extracted due to TIR.
A. Transparent (Super-transmissivity)
Enhanced Light Extraction Efficiency
Anti-reflection
& Overcoming TIR
High Transmission
B. Opaque (Anti-reflectivity)
Display
sp
-.---
n2
Sitlao
r--
Scintillator
Anti-reflection
LED
Solar Cell
Figure 1-2. Examples of light transport from low to high index material and the other way
around.
24
1.2.3. Nanostructures for control of interfacial optical phenomena
Various nanostructures have been developed in recent decades to manipulate the
light flow through optical interfaces in order to improve the efficiency of the light
transportation by increasing transmission or absorption (or light collection).
The nanostructures can be roughly classified according to their periodicity (P)
compared to the wavelength (A) of light as shown in Fig. 1-3. Subwavelength optics (P <
i)
represents the regime, in which the effective medium theory dominates, and a
diffractive optics regime (P > {), in which scalar diffraction is applied [19]. There is also
intermediate regime (P ~ A) in between those two regions, mixed properties or Bragg
resonance can be applied. Depending on optical environment and applications, different
geometries of nanostructures can be considered. This paper mainly deals with
nanostructures in subwavelength regime to intermediate regime to maximize the light
transport efficiency.
For the light transport from low to high refractive index, the subwavelength
nanostructures have been developed by the new possibilities of numerical simulation and
-
of micro- and nanofabrication. Moth-eye structures or subwavelength nanocones (Fig. I
4(a)) have been proven to reduce reflection with broadband and omnidirectional
performance [2-5, 20, 21]. These structures provide adiabatic impedance matching
between the air and the glass due to a gradually increasing refractive index towards the
substrate surface, thus significantly reducing Fresnel reflection losses. If the aspect ratio
of nanocones is high, then the nanotextured surface can exhibit enhanced anti-reflectivity
[7].
Subwavelength Optics
Intermediate regime
(P<A)
( P-A)
Diffractive Optics
)
( P> A
Figure 1-3. A schematic of different optical regimes.
25
However, the surface might be too vulnerable to outer mechanical forces such as
fracture and fouling especially when exposed to harsh environments [22]. Further, if there
are multiple optical interfaces such as solar cells covered with protective glass, we have
to consider how to eliminate multiple Fresnel reflections altogether [23, 24].
In this thesis, I propose a new design to consider these issues. An inverted
nanocone shown in Fig. 1-4(b), is suggested for better mechanical robustness with
excellent anti-reflectivity, and double-cone nanostructures, shown in Fig. 1-4(c), can
offer an option for eliminating multiple Fresnel reflections [21].
For the light transport from high to low refractive index, there has been growing
interest in functional micro- and nano-structures that can enhance the light extraction
efficiency of materials such as scintillators and LEDs for the last decade [8, 9, 12, 13, 15,
16, 25, 26]. Even though the subwavelength anti-reflective moth-eye nanostructures (Fig.
1-4(a)) can increase the light transmission by suppressing the Fresnel reflection, their
sub-wavelength scale limits the performance since only the zeroth-order light is allowed
to propagate through the nanostructured interface, and any light propagating beyond the
critical angle cannot be extracted due to TIR [18].
Nanocone
Inverted Nanocone
Double-Cone
(a)
(b)
(c)
(d)
Diffractive
Anti-reflective (P < A)
Figure 1-4. Various nanostructures
diffractive nanostructures.
PhC
including anti-reflective
26
Conical PhC
(e)
( P - A or P > A)
nanostructures
and
Instead, photonic crystals as diffraction gratings shown in Fig. 1-4(d) that utilize
intermediate to diffractive optics regime in Fig. 1-3, have often been used for enhancing
the extraction efficiency to overcome TIR [13]. A photonic crystal is a periodically
repeating structure comprising more than two materials with different refractive indices,
and some light can be diffracted through the structure, and hence can propagate beyond
the critical angle. However, the transmission under the critical angle must be sacrificed,
not only because the Fresnel reflection exists, but also because the zeroth-order should be
divided into several diffracted orders.
Although extensive research has been carried out on enhancing light extraction
efficiency, no nanostructured surface exists which sufficiently exhibits advantages both
for under the critical angle region and beyond the critical anglc region. In order to take
advantage of both under the critical angle region and above the critical angle region, we
propose a conical photonic crystal as shown in Fig. 1-4(e) in this thesis.
When designing and optimizing the nanostructure, we need to consider several
key characteristics of the incident light, which are critical in determining how the incident
light interacts with the optical interfaces or any other structures. The important
characteristics of light are:
e
The wavelength range of the incident light
e
The angle at which the incident light strikes a interface
-
Refractive indices configuration of the interface
*
The type of nanotextured surface to interact with incoming light
27
1.3.
NANOMANUFACTURING METHODS
1.3.1. Conventional nanofabrication methods
Advanced
nanofabrication
methods
are
a
key
technology
in
optical
nanoengineering due to their potential to modulate optical behavior on its surface.
Various nanofabrication methods have been developed in recent decades, and make it
possible to control light-matter interactions in diverse and powerful ways [10].
Electron-beam lithography (EBL) has often been used for the fabrication of
functional nanostructures due to its superior resolution and flexibility to write an arbitrary
layout [20]. However, this method is not suitable for large-area manufacturing for most
of the industrial applications, such as consumer electronics or energy devices, due to its
extremely low throughput and high cost. Other serial writing-based tools such as focused
ion-beam lithography (FIB) [27] and atomic force microscopy (AFM) [28] have similar
limitations to EBL. For example, it would take more than a month to write lTb of
features on a large area surface using these kinds of serial writing tools.
Extreme ultraviolet lithography (EUV or EUVL), as a most advanced optical
lithographic method, has been developed by leading companies in the semiconductor
industry for years as a next-generation lithography. This technology uses an extreme
ultraviolet (EUV) wavelength ({ ~ 13.5 nm), and has the potential to print sub-10nm
features, covering the needs of the semiconductor industry in the next decade [29].
However, the lithography tools are too expensive (- $1 OOM) for most applications other
than the semiconductor industry. Its infrastructure, such as the cleanroom, the optical
mask, and environmental conditioning, is also quite expensive and hard to maintain,
which
makes
this
method difficult to adapt
as a high-throughput,
low-cost
nanomanufacturing tool.
Nanosphere lithography or colloidal self-assembly is a cheaper option than
previous methods with higher throughput [30]. It uses the space between mono-dispersed
colloidal spheres assembled on a substrate surface, which can be transferred to another
28
layer. However, it has long-range order irregularity due to nucleating uncorrelated
domains at multiple locations on the wafer, which results in grain boundaries with a
typical dimensional scale of 1 pm to 100 pm. Colloidal self-assembly also results in a
periodic structure containing a high density of defects inherent to the assembly process,
which can dramatically reduce the optical strength [10].
Nanoimprint lithography, or "nano-replication method" is a powerful alternative
for the fabrication of optical nanostructures [21, 31-33]. It has shown promising results
for the fabrication of various optical devices with nanostructures including wire grid
polarizers, anti-reflective moth-eye structures and micro lens arrays, to name a few.
However, a 3D master mold should be prepared prior to the replication process.
Therefore, a reliable, cost-effective and large-area-compatible master fabrication method
should accompany it.
Laser interference lithography (LIL) [7], a patterning method effective for
periodic nanostructures, can be a good option for preparing a master mold for nanoreplication. It has the potential to create large-area and nearly defect-free nanostructures
[10]. Even though LIL itself has a potential to be a nanomanufacturing tool, it still needs
complicated optical set-up and subsequent etching steps.
Given that most current nanofabrication tools have their pros and cons, combining
two or more methods can possibly give us a better option for nanomanufacturing. If LIL
is first used as a large-area mastering tool, and then the master is then replicated into
multiple substrates using the replication method, this combination may offer the optimal
choice for nanomanufacturing to be large-area compatible, low cost and high throughput.
1.3.2. Nanoimprint and laser interference lithography
1.3.2.1. Nanoimprint lithography
Nanoimprint method is a replication technique shown in Fig. 1-5. Nanoimprint is
a simple, fast and unique cost-effective solution for fabricating nanostructures due to its
29
low initial cost and low material consumption. It creates a resist relief pattern by
deforming the resist physical shape [21, 31, 33]. The simple principles makes
nanoimprint lithography capable of producing sub-10 nm features over a large area,
which can be effectively applied to sub-wavelength optical devices, such as photonic
crystals, wire grid polarizers, and other metamaterials.
Subsequent dry etching is also compatible with removing of the residual layer
produced after the nanoimprinting process, which makes it possible to transfer the
imprinted pattern to materials. But in this paper I will only work with the case where the
imprinted polymer itself is the final target pattern, and thus does not require any
additional etching process.
There are issues to overcome in nanoimprint lithography. First, because it is a
replication method, the process needs a pre-fabricated master mold via some other
nanofabrication technique. In addition, for certain designs of nanostructures, for example,
high aspect ratio nanocones, filling the mold with liquid pre-polymer is not easy. Finally,
the demolding process is challenging, especially when the pattern density is high and
imprinted shape is slender due to increased surface area. These challenges will be
discussed in more detail in experimental sections in each chapter.
Resist
Pattern Transfer
Mold
Pol+mer-Micro/Nano Patterns
Figure 1-5. Concept of nanoimprint
30
1.3.2.2. Laser interference lithography (LIL)
Laser interference lithography is the maskless exposure of a photoresist layer to
more than two coherent laser beams. If its large-area capability can be utilized for
fabricating a master mold for the nanoimprint method, such a combination of LIL and
nanoimprint can provide an easy-to-use, inexpensive, high-throughput nanostructuring
tool package.
LIL uses quite simple physics. Two beams, obtained by splitting a coherent laser
source, are incident on a photoresist coated on a substrate at a certain angle. Then twobeam interference will generate the periodic pattern on the photoresist, which will be
developed and transferred to other layers. The periodicity of the pattern is:
P
=sn
0
2sin (6)
(1-4)
Lloyd's mirror set-up [34] shown in Figure 1.6 is employed for the fabrication used in
this paper. In the set-up, the top half of the incident beam reflects downward at the top
mirror, and another half of the beam is directly incident on the photoresist coated wafer,
as shown in Figure 1-6. The pitch of the nanostructure can be controlled by tuning the
angle (0) as in eq. (4). It is critical to match the laser intensities on a substrate to retain
the maximum contrast in the interference pattern. A single mod TEMoo laser source is
used here. Other than controlling the angle, exposure dose is another important variable
that needs careful control due to its high sensitivity to intensity contrast and photoresist
patterns. For the same reason, the reflection at the bottom of the resist should be
minimized in order to retain the maximum contrast of the interference light pattern. An
anti-reflection coating is usually incorporated and its thickness and refractive index
should be carefully calculated and determined.
31
Mirror
HeCd Laser ( A =325 nm)
Rotation
stageR
td
p
PR coated sample
j
Figure 1-6. A schematic of Lloyd's mirror interference lithography system.
By combining laser interference lithography for the large-area mastering method,
and nano-replication for the final nanomanufacturing tool, we can maximize the
throughput for fabricating various types of nanostructures, including the novel designs
proposed in this thesis. Moreover, advanced techniques would be applied for fabricating
higher aspect-ratio and denser nanostructures, whose conditions will be discussed later in
the thesis.
1.4. THESIS OVERVIEW
In this thesis, I propose, analyze, optimize and fabricate functional nanostructured
surfaces for efficient light transport, seeking enhanced performance, mechanical
robustness, compatibility with the outer environment and manufacturability.
Chapter 2 studies light transport from a low refractive index material to a high
refractive material for transparent materials. I propose a new design, inverted nanocone
structure,
for
ultimate anti-reflectivity
(or super-transmissivity)
with
enhanced
mechanical robustness of the nanotextured surface. In addition, controllable enhanced
wetting properties of the inverted nanocone surface are discussed as additional
functionalities. The proposed nanostructured surface is fabricated using UV nano32
replication with vacuum-assisted filling method, which satisfies industrial manufacturing
requirements.
Chapter 3 also describes light transport from a low refractive index material to a
high refractive material, but on an opaque substrate, silicon. I propose double gradientindex nanostructures for extremely low reflection for multi-optical interfacial surfaces
such as solar cells covered with an encapsulating layer. Multi-optical interfaces are
textured with tapered nanostructure in order to suppress all the Fresnel reflections. The
top surface is textured with inverted nanocones to maintain high mechanical robustness
and the silicon surface is textured with upright nanocones for ultimate anti-reflective
performance. UV nano-replication with selective delamination technique is also used
here with the help of laser interference lithography for fabricating a master mold.
Chapter 4 presents an efficient light extraction from high refractive material using
a novel type of nanostructure. I design a conical diffraction grating/photonic crystal as a
highly efficient light extraction layer, attempting to balance Fresnel reflection and total
internal reflection. I simulate the structure and optimize it using FDTD and RCWA, and
then fabricate it using same the LIL and nanoimprint technology. The results are
characterized on a scintillator application.
Finally, Chapter 5 states conclusions and offers suggestions for future research.
REFERENCES
1.
P. Vukusic and J. R. Sambles, "Photonic structures in biology," Nature 424, 852855 (2003).
2.
Y. Kanamori, M. Sasaki, and K. Hane, "Broadband antireflection gratings
fabricated upon silicon substrates," Opt. Lett. 24, 1422 (1999).
3.
G. Xie, G. Zhang, F. Lin, J. Zhang, Z. Liu, and S. Mu, "The fabrication of
subwavelength anti-reflective nanostructures using a bio-template," Nanotechnol.
19, 095605 (2008).
4.
Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L.
Pan, H. C. Lo, C. H. Hsu, and Y. H. Chang, "Improved broadband and quasiomnidirectional anti-reflection properties with biomimetic silicon nanostructures,"
Nat. Nanotechnol. 2, 770 (2007).
5.
K. Choi, S. H. Park, Y. M. Song, Y. T. Lee, C. K. Hwangbo, H. Yang, and H. S.
Lee, "Nano-tailoring the surface structure for the monolithic high-performance
antireflection polymer film," Adv. Mater. 22, 3713 (2010).
6.
A. R. Parker and H. E. Townley, "Biomimetics of photonic nanostructures," Nat.
Nanotechnol. 2, 347-353 (2007).
7.
K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Nanotextured silica surfaces with robust superhydrophobicity and
omnidirectional broadband supertransmissivity," ACS nano 6, 3789-3799 (2012).
8.
J. J. Kim, Y. Lee, H. G. Kim, K. J. Choi, H. S. Kweon, S. Park, and K. H. Jeong,
"Biologically inspired LED lens from cuticular nanostructures of firefly lantern,"
Proc. Nat]. Acad. Sci. USA 109, 18674-18678 (2012).
9.
J. J. Wierer, A. David, and M. M. Megens, "III-nitride photonic-crystal lightemitting diodes with high extraction efficiency," Nat. Photonics. 3, 163-169
(2009).
10.
K. A. Arpin, A. Mihi, H. T. Johnson, A. J. Baca, J. A. Rogers, J. A. Lewis, and P.
V. Braun, "Multidimensional architectures for functional optical devices," Adv.
Mater. 22, 1084-1101 (2010).
34
11.
K. X. Wang, Z. Yu, V. Liu, Y. Cui, and S. Fan, "Absorption enhancement in
ultrathin crystalline silicon solar cells with antireflection and light-trapping
nanocone gratings," Nano Lett. 12, 1616-1619 (2012).
12.
H. Ichikawa and T. Baba,' "Efficiency enhancement in a light-emitting diode with
a two-dimensional surface grating photonic crystal," Appl. Phys. Lett. 84, 457459 (2004).
13.
D. H. Kim, C. 0. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone,
Y. Park, W. J. Choi, and Q. H. Park, "Enhanced light extraction from GaN-based
light-emitting diodes with holographically generated two-dimensional photonic
crystal patterns," Appl. Phys. Lett. 87(2005).
14.
J. J. Kim, Y. Lee, H. G. Kim, K. J. Choi, H. S. Kweon, S. Park, and K. H. Jeong,
"Biologically inspired LED lens from cuticular nanostructures of firefly lantern,"
P. Natl. Acad. Sci. USA 109, 18674-18678 (2012).
15.
A. Knapitsch, E. Auffray, C. W. Fabjan, J. L. Leclercq, X. Letartre, R.
Mazurczyk, and P. Lecoq, "Results of photonic crystal enhanced light extraction
on heavy inorganic scintillators," IEEE T. Nucl. Sci. 59, 2334-2339 (2012).
16.
M. Kronberger, E. Auffray, and P. Lecoq, "Improving light extraction from heavy
inorganic scintillators by photonic crystals," IEEE T. Nucl. Sci. 57, 2475-2482
(2010).
17.
P. Pignalosa, B. Liu, H. Chen, H. Smith, and Y. Yi, "Giant light extraction
enhancement of medical imaging scintillation materials using biologically
inspired integrated nanostructures," Opt. Lett. 37, 2808-2810 (2012).
18.
M. Kronberger, E. Auffray, and P. R. Lecoq, "Probing the concepts of photonic
crystals on scintillating materials," IEEE T. Nucl. Sci. 55, 1102-1106 (2008).
19.
M. A. Golub and A. A. Friesem, "Analytic design and solutions for resonance
domain diffractive optical elements," J. Opt. Soc. Am. A Opt. Image. Sci. Vis. 24,
687-695 (2007).
20.
S. A. Boden and D. M. Bagnall, "Tunable reflection minima of nanostructured
antireflective surfaces," Appl. Phys. Lett. 93(2008).
21.
J. G. Kim, H. J. Choi, K. C. Park, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Multifunctional inverted nanocone arrays for non-wetting, selfcleaning transparent surface with high mechanical robustness," Small 10, 24872494 (2014).
35
22.
D. Chandra and S. Yang, "Stability of high-aspect-ratio micropillar arrays against
adhesive and capillary forces," Accounts Chem. Res. 43, 1080-1091 (2010).
23.
B. Sopori, "Silicon nitride processing for control of optical and electronic
properties of silicon solar cells," J. Electron. Mater. 32, 1034-1042 (2003).
24.
H. K. Raut, V. A. Ganesh, A. S. Nair, and S. Ramakrishna, "Anti-reflective
coatings: A critical, in-depth review," Energy Environ. Sci. 4, 3779-3804 (2011).
25.
H. Kasugai, K. Nagamatsu, Y. Miyake, A. Honshio, T. Kawashima, K. lida, M.
Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Kinoshita, and H. Shiomi, "Light
extraction process in moth-eye structure," Phys. Status Solidi. C 3, 2165-2168
(2006).
26.
A. Knapitsch, E. Auffray, C. W. Fabjan, J. L. Leclercq, X. Letartre, R.
Mazurczyk, and P. Lecoq, "Effects of photonic crystals on the light output of
heavy inorganic scintillators," IEEE T. Nucl. Sci. 60, 2322-2329 (2013).
27.
M. L. von Bibra, A. Roberts, and J. Canning, "Fabrication of long-period fiber
gratings by use of focused ion-beam irradiation," Opt. Lett. 26, 765-767 (2001).
28.
M. Tello, A. San Paulo, T. R. Rodriguez, M. C. Blanco, and R. Garcia, "Imaging
cobalt nanoparticles by amplitude modulation atomic force microscopy:
comparison between low and high amplitude solutions," Ultramicroscopy 97,
171-175 (2003).
29.
Y. Tao, S. S. Harilal, M. S. Tillack, K. L. Sequoia, B. O'Shay, and F. Najmabadi,
"Effect of focal spot size on in-band 13.5 nm extreme ultraviolet emission from
laser-produced Sn plasma," Opt. Lett. 31, 2492-2494 (2006).
30.
C. H. Chang, J. A. Dominguez-Gaballero, H. J. Choi, and G. Barbastathis,
"Nanostructured gradient-index antireflection diffractive optics," Opt. Lett. 36,
2354-2356 (2011).
31.
P. R. K. S.Y. Chou, P.J. Renstrom, "Nanoimprint Lithography," J. Vac. Sci.
Technol. B 14, 4129 (1996).
32.
J. G. Kim, H. J. Choi, H. H. Gao, I. Cornago, C. H. Chang, and G. Barbastathis,
"Mass replication of multifunctional surface by nanoimprint of high aspect ratio
tapered nanostructures," 2012 International Conference on Optical Mems and
Nanophotonics (OMN), 71-72 (2012).
36
33.
J. G. Kim, Y. Sim, Y. Cho, J. W. Seo, S. Kwon, J. W. Park, H. Choi, H. Kim, and
S. Lee, "Large area pattern replication by nanoimprint lithography for LCD-TFT
application," Microelectron. Eng. 86, 2427 (2009).
34.
A. Bagal and C. H. Chang, "Fabrication of subwavelength periodic nanostructures
using liquid immersion Lloyd's mirror interference lithography," Opt. Lett. 38,
2531-2534 (2013).
37
Chapter 2.
Multifunctional Inverted Nanocone
Arrays for Anti-reflective Surface with
High Mechanical Robustness and
Enhanced Wetting Properties
2.1. INTRODUCTION
Nanocone structures inspired by natural moth-eye have been widely investigated
as nanostructured anti-reflective surfaces due to the axial gradient in refractive index.
From the point of view of optical behavior, the nanocones act in a similar way to an
anechoic chamber, providing adiabatic impedance matching between the air and the glass
and thus significantly reducing Fresnel reflection losses at optical interfaces.
Control of interfacial physical phenomena across multiple modalities, for example
optical reflection together with surface wettability, is increasingly important both from
the point of view of fundamental understanding of interfacial properties and of course for
engineering applications. For example, eliminating reflection at an air-glass interface
increases the efficiency of light-harvesting devices and display devices.[ 1-3] These same
surfaces must also meet stringent requirements of anti-fogging due to condensation or, in
other cases, must be self-cleaning to avoid accumulation of dust and other particulate
contaminants with low risk of mechanical damage.[4-9] Engineered surfaces that
combine these modal responses are commonly referred to as "multifunctional," and
considerable research effort has been invested in their design, fabrication, replication, and
38
characterization.[10-12] Such surfaces are commonly encountered in nature as observed
for example in the anti-reflectivity of moth eyes[13-19] and the super-hydrophobicity of
lotus leaves that in turn enables self-cleaning.[4-6, 20, 21]
In the past, we have developed a biomimetic multi-functional silica surface by
utilizing a square array of slender tapered nanocone structures with pitch of P ; 200 nm
and an aspect ratio (defined as the ratio of nanocone height to pitch) of H/P Z 5. We
demonstrated broadband (400 nm < , < 1200 nm) omnidirectional (0' < Oincidencc < 750)
anti-reflectivity and robust superhydrophobicity. We also quantified the anti-fogging
behavior resulting from the superhydrophilicity of the same nanotextured surface, by
employing the intrinsic hydrophilicity of glass.[12]
With regard to quantifying performance, we demonstrated that a single figure of
merit: i.e., the nanocone slenderness, or height-to-pitch aspect ratio (H/P), suffices to
characterize both behaviors. The taller the cones and the smaller their diameter, the more
gradual the adiabatic refractive index transition is and the more removed the reflection
becomes from the diffractive regime; both qualities contribute to reducing reflection and
scatter from the surface. Slender nanocone structure (H/P >>) combined with a small
hemispherical tip radius (Rtp ~ 17 nm) leads to high Wenzel roughness (rw ~ 9.7).[6]
When combined with a suitable fluorinated chemical coating, the high aspect ratio of the
features and the small area fraction of the conical tips promote development of a very
stable superhydrophobic Cassie-Baxter state with very low contact angle hysteresis.
However, the slender mechanical structures of needles and nanocones are subject
to high risk of mechanical damage. As one would intuitively expect, and as we verify
with detailed numerical calculations below, the slender nanocones are very sensitive to
damage from stresses arising from external loading through direct mechanical contact or
capillary action.[22-24] Lack of mechanical robustness makes transparent nanocone
surfaces inappropriate for several applications of importance, for example surfaces of
personal digital assistants (PDAs) or cell phones that are subject to repeated shearing and
normal loads due to finger-swiping motions by the user.
39
Here, I present an alternative approach that achieves enhanced anti-reflection and
anti-fogging or self-cleaning property by using the complementary topographic structure:
i.e. a square array of inverted nanocones, i.e. nanohole egg-crate structures on a
substrate.[25-27] The nanohole structures are amenable to a high throughput nanoreplication method that uses a nanocone array sample as a 'negative' master. I show
quantitatively that, in addition to the expected radical improvement in mechanical
robustness, the nanohole arrays exhibit broadband optical transmissivity that is almost
identical with the original nanocone structures, whilst still retaining anti-fogging or selfcleaning properties. These results are supported by both numerical simulations of the
mechanical and wetting properties and by experimental results. This new approach using
nanohole arrays, morcover, yields a significant advantage in terms of replication. This is
because a mold made of nanocones can easily be imprinted to form its own inverse on a
polymeric surface (e.g. poly urethane acrylate) before curing. In following sections I
present numerical simulations of the optical, mechanical and wetting properties, as well
as experimental measurements of the corresponding properties on nanoreplicated
samples.
>n
n
(B)
(C)
n
n,
n
n(z)
n(z)
- --- ------
n2
- -- ---
--n2
Flat surface
z
Nanocone
n2
Nanohole
z
-
(A)
z
Figure 2-1. Refractive index profile of (a) a flat surface, (b) nanocone surface and (c)
nanohole (or inverted nanocone) surface. (b) and (c) show gradually varying refractive
indices.
40
2.2. NUMERICAL MODELS FOR INVERTED NANOCONES
2.2.1. Optical behaviors: Anti-reflection
In this section, we first discuss quantitatively the anti-reflective device aspects of
the inverted nanocone array, in the context of mechanical robustness. As briefly
mentioned in Section 2-1, the low optical reflectivity of our design arises mainly due to
an axially-varying effective refractive index of the tapered structures on a length scale
that is below the wavelength of incident irradiation as shown in Fig. 2-1. The effective
gradient introduced by the tapered nanostructures effectively eliminates the abrupt
discontinuity in the refractive index at the interface between the substrate and air, thus
suppressing Fresnel reflection. This effective gradient in the refractive index is equally
applicable to both of the periodic nanostructures we have described (i.e. either cones or
holes), and in this section we show numerically that indeed they both reduce reflective
losses by a similar extent.
We calculated the reflection and transmission of plane waves at different
wavelengths (350 nm <
)
< 1800 nm) at normal incidence through the nanostructured
surfaces using FDTD (Finite-difference time-domain) based software (FDTD Solutions
8.0). Results are shown in Fig. 2-2 for nanocone arrays, as well as for the inverse
geometry consisting of an array of tapered cavities. The structures were simulated as
square arrays with pitch of P = 200 nm and varying aspect ratio (HIP). For structures
with higher aspect ratio, anti-reflectivity on the surface is enhanced because the gradient
of the effective refractive index becomes progressively smaller and more consistent with
the adiabatic assumption. Especially when the aspect ratio exceeds H/P;> 3, the reflective
loss at a zero incidence angle can be suppressed to less than I% over our entire range of
wavelengths used in the simulation.
41
(A)
0.05
7
6
P0
(B)
Light
7
0.05
6
0.0
0 .0 4
.0
.04
0
0.02
2
0.02
0
0
0.02
30
0.01
0.01
400
1 00
1200
Wavelength (nm)
400
800
1200
1600
0
Wavelength (nm)
(D)
(C)
71
*1
7
619
110999
6
_
5
-
0.98
3
400
10.9
800
0.99
i0.99
o
3.97
800
0.98
04P
0.97
0
0.96
0.96
_09
1200
400
1600
800
1200
1600
095
Wavelength (nm)
Wavelength (nm)
Figure 2-2. FDTD (Finite-difference time-domain) simulations of the optical performance
of the nanostructured surfaces in the wavelength range of 350 nm < A < 1800 nm at
normal incidence (Qi = 0'): Contour plot of the fraction of light reflected R(, HIP) from
surfaces textured with either (A) nanocone arrays or (B) nanohole arrays. Light
transmission T(, HIP) from surfaces textured with either (C) nanocone arrays or (D)
nanohole arrays.
42
2.2.2. Wetting Properties: Self-cleaning and Anti-fogging
Turning to the wetting behavior, the tapered hole geometry resulting from the
inverted nanocone array structure also amplifies the interfacial thermodynamic driving
force that governs the surface wettability. This structural effect can increase either the
intrinsic hydrophilicity of the untreated poly urethane acrylate (PUA) or enhance the
Cassie-Baxter hydrophobicity of the textured nanohole surface after applying a
fluorosilane coating. Figs. 2-3A and 2-3B show the resulting energy landscapes for
different apparent contact angles (O,*) and vertical (z) locations of the water meniscus in
the nanohole, calculated from the three-dimensional topography (shown in Fig. 2-3B) and
for two different equilibrium contact angles, corresponding to OE= 800 for untreated
PUA, and OF= 120' for fluorosilane coated PUA.[28, 29]. The blue color represents the
locus of the global minimum for the change in the Gibbs free energy density in each plot
(See Appendix A for details of the relevant Gibbs free energy density function), which
predicts
that
the
system
corresponding
to
the
apparent
contact
angle
is
thermodynamically stable. The fully-wetted state and the resulting extremely low
apparent contact angle of the untreated PUA nanostructure (0* < 50) can be described by
the canonical Wenzel relation cosO* = rwcosE,[30] where 0* and 0E
are the effective
apparent and equilibrium contact angles for water drops on textured and smooth PUA
surfaces, respectively, and rw is the roughness ratio between the total surface area and the
corresponding projected area. Water spontaneously wicks into the nanohole geometry
because the intrinsic hydrophilicity of the untreated PUA surface is amplified by the
roughness and leads to an inwardly-directed net capillary force that acts on the curved
meniscus in each nanohole. Thermodynamically, the resulting water-solid interface that
replaces the initial dry air-solid interface and the liquid meniscus has a lower total change
in the Gibbs free energy density.[28] The energetically favorable formation of a thin
conformal film of water resulting from rapid wicking into the nanostructure provides the
strong anti-fogging property[31, 32] that is observed experimentally in section 2.4. On
the other hand, the global minimum of the Gibbs free energy density variation for the
43
hydrophobically-modified inverted nanocone structure predicts a composite or CassieBaxter state in which the water droplet sits partially on the peaks of the wetted solid
texture and partially on a raft of air pockets trapped in the nanoholes. The apparent
contact angle of the water droplet can be modeled by the Cassie-Baxter relation, cosO*
r1scosOE - (1-o,), [28, 33-35] where r, =(1 -/4)+7/2x
is the roughness of the actual wetted area and
=1-
I-(1-z/H)]
(H/P)2 +1/4
(t/4)(I - z/H) 2 is the area fraction
of the water-air interface occluded by the texture.[29, 35] It should be noted that ro and $s
are functions of z/H. The apparent contact angle of 0* = 1560 resulting from the Gibbs
free energy density variation calculation in Fig. 2-3B matches well the experimental
measurement results shown in the inset. The high feature density (corresponding to the
number of asperities in 1 mm 2) and the closed nature of the inverted nanocone structure
lead to a highly robust Cassie-Baxter state[36].
44
(A) 1:,
(B) I
og (AG)
0.8
0.8
0
0.1
0.2
0.2
0
20
40
60
80
100
120
120
OP *(*0)
IS
.IG
140
160
ep' (*o)
180
Figure 2-3. Colored contour maps of the change in the Gibbs free energy density AG(61 ,*)
as a function of the putative apparent contact angle (Q*) with a water droplet and
different normalized vertical position (z/H) of the water meniscus in the inverted
nanocone array (A) with intrinsically hydrophilic PUA (OE = 80') and (B) with
1200). See Appendix for the Gibbs free
hydrophobic fluorosilane surface coating (OE
energy density function. The effective apparent contact angle (6*) is related to the
equilibrium contact angle (OE) by the Cassie-Baxter relation that accounts for the solidliquid fraction and air-liquid fraction for liquid-solid-air composite state of droplet (in the
limit of fully-wetted state (z/H -> 1), the Wenzel relation is used). rp and Os are functions
of z/H. The insets show goniometric images of water droplets (V= 10 1 d) sitting on each
surface. In both (A) and (B), the blue color represents the locus of the global minimum in
the Gibbs free energy density variation landscape, the corresponding apparent contact
angles (Q* = Q,* when AG = min(AG)) are in good agreement with the experimental
results measured by goniometry shown in the insets.
45
2.2.3. Mechanical Robustness
The enhancement in the optical transmissivity, attributed to the tapered nature of
the high feature density q of the nanocones, improves further as the aspect ratio (HIP)
increases, just as in the case of the wetting properties of the nanocones.[12] Nevertheless,
there is an evident trade-off between the perfonnance characteristics of these high aspect
ratio and high feature density conical structures and the resulting mechanical robustness
of the texture.
Assuming the same basal area (i.e. the same feature density), the more
slender the cone, the smaller the maximum applied force the cone can resist before a
critical stress for mechanical failure is attained under external loading as in examples
shown in Fig. 2-4. On the other hand, a square array of nanohole structures forms an eggcrate like structure, which is intuitively expected to withstand larger external loads, since
the walls of the nanoholes abut each other in a two-dimensional network. We simulated
and quantitatively compared both the compressive stress, the shear stress and the strain
field resulting from comparable external forces applied to both the nanocone and
nanohole cases using finite element method based software (ANSYS).
The results are shown in Fig. 2-5. In both the nanocone and nanohole geometries,
the simulated geometry consisted of a square array with a pitch of P = 200 nm and aspect
ratio of H/P = 4. The material (PUA) was assumed to be isotropic and perfectly elastic
with modulus E ~ 400 MPa. The external force applied to the nanostructured surface is
set to be 4 N over a circular area with a radius of 5 mm corresponding to an applied load
of 2 pN per a single cone or hole, comparable to what might result from a single finger
pressing uniformly on the nanotextured surface.[37] When a normal force is applied to
the nanocone structures, the stress is clearly concentrated at the conical tips of the
features, whereas for nanoholes the external load is much more evenly distributed over
the entire top surface area as shown in Fig. 2-5A; the stress is therefore lower throughout
the structure. The maximum Von Mises stresses are (3max,
44.8 MPa for the nanocones and
GT max, shear=
shear=
50.4 MPa and
1.34 MPa and (3max,
normal
rmax, normal
=
1.23 MPa for the
nanoholes, respectively. In particular for the nanocones, the maximum stress exceeds the
46
reported bulk yield stress of the PUA material (a-y
17 MPa).[38] Although systematic
scale-dependent changes in the magnitude of the yield stress in nanoscale-modulated
structures is a subject of open investigation,[39] reaching values of peak stress in the
proximity of the nominal or bulk yielding value creates concern about plastic
deformation,
fracture and subsequent degradation in both optical and wetting
performance. Alternatively, one may also look at the lateral displacement of the textured
elements that comprise the multifunctional surface: For a nanocone structure with aspect
ratio of H/P = 4, application of a 4 N shear force over the array results in a tip deflection
of &, = 148 nm, which is larger than the half pitch (P/2 = 100 nm). The large shear strain
is certain to lead to irreversible collapse of the periodic nanostructure. On the other hand,
the walls of the nanohole egg-crate array exhibit a very small deflection (5 < 0.1 nm)
when subjected to the same magnitude and direction of force. In addition, unlike the
slender nanocones, the nanohole egg-crate structures are expected to be free from
buckling and collapse problems.
Breakage
Bending
Irregularization
Figure 2-4. SEM images of high aspect ratio nanocones showing examples of mechanical
instability.
47
Nanocone
Nanohole
50.4 MPa
1.34 MPa
44.8 MPa
1.23 MPa
Shear
Force
Normal
Force
(A)
280240E
E
200160-
4)
120-
0
80-
+
40-
Nanocone
Nanohole
01
2
4
3
5
Aspect Ratio, HIP
(B)
Figure 2-5. Mechanical robustness calculated by finite element method (FEM): (A) Stress
distribution in a nanocone and a nanohole structure (fabricated from PUA, Young's
modulus E ~ 400 MPa) resulting from a typical shearing or normal finger force (4 N)
applied at the top over a circular area with a radius of 5mm. The numerical values
represent the maximum Von Mises stresses for each case; (B) Maximum tip deflections
of a nanocone and a nanohole structure (P = 200 nm) under a lateral shearing force for
different aspect ratio of nanocones and nanoholes. The gray shaded region indicates a tip
displacement that is greater than the pitch of the nanocones.
48
2.3. FABRICATION USING REPLICATION METHOD
As a first step of the replication process, a master mold (40 mm by 40 mm)
comprising of a periodic array of nanoconical features was prepared using laser
interference lithography and subsequent dry etching steps with multiple shrinking masks
as described previously.[12] Using the master nanocone array as a negative mold for the
nanohole array, the UV replication process was performed using the sequence of
operations shown in Fig. 2-6A. First, the master mold was placed in contact and pushed
onto a poly urethane acrylate (PUA) prepolyiner (311 RM, Minuta Tech.) dispensed via
syringe on a glass substrate. After curing the PUA with ultra-violet (UV) light (Tamarack
UV exposure system; with peak wavelength and intensity of 365 nm and 4.5 mW/cm 2
respectively), the mold was carefully detached from the PUA surface. In order to enhance
the adhesion between the imprinted PUA and the fused silica substrate, a silane-type
adhesion promoter layer[40] was applied. Thus the periodic nanocone arrays on the mold
were inversely replicated into the PUA surface, texturing it with a periodic array of
nanoholes. This replicated egg-crate structure was subjected to an additional imprint step
resulting in a second generation textured PUA surface composed of nanocone arrays as
shown in Fig. 2-6B. This second replication was carried out in order to compare the
topography and perfonnance of the inverted nanocone arrays with that of the original
nanocone arrays. In the second imprint, a PDMS anti-adhesion layer[41] was first coated
on the mold surface to prevent the two PUA layers from irreversibly fusing into each
other. To lower the surface energy of the nanohole arrays and make the egg-crate
structure
strongly
hydrophobic,
chemical
vapor
deposition
of
IH,IH,2H,2H-
perfluorodecyltrichlorosilane (Alfa Aesar, 96%) has carried out in an oven at 110 'C for
10 hours.
49
Mold
'-4
Anti-Adhesion
layer
Adhesion
layer
LIV
V,,,yiiyMold
V
*AAA
*
AkAAAAAAAAAAkA
*
*
---
I
Nanocone Arrays
Inverted Nanocone Arrays
(B)
(A)
Figure 2-6. Schematic representation of the fabrication process showing (A) the inverted
nanocone arrays replicated from the original nanocone master, and (B) second generation
of replicated nanocone arrays formed and released using an anti-adhesion layer in UVcurable poly urethane acrylate (PUA). 2 51
50
In addition, a vacuum-assisted filling process was developed and employed in
order to fully fill the nanoholes with PUA prepolymer during the imprint step when the
nanohole arrays were pressed into the liquid polymer[42].
The filling process at normal atmospheric pressure is not straightforward because
tiny air bubble might be trapped in the bottom of the nanoholes as shown in Fig. 2-7A.
Moreover, when the mold surface is coated with an anti-adhesive coating with low
surface energy, the filling process is even harder since the nanotextured surface may
exhibit enhanced hydrophobicity that tends to repel liquid from the surface. At
atmospheric pressure, nanocones are not replicated well, only showing some wrinkles
induced from shrinkage of polymer during UV-curing process when keeping the contact
between liquid prepolymer and the mold for 5 minutes. Even after 10 minutes of contact
time, the aspect ratio of the replicated cone is lower than original mold as shown in Fig.
2-7A.
On the other hand, after applying the vacuum-assisted filling process, the
replication process can be done more effectively as shown in Fig. 2-7B. When dispensing
the liquid polymer on the mold, trapped air bubbles in nanoholes are in the equilibrium
state between surface tension and ambient pressure force. After applying vacuum
condition, air bubbles trapped in the bottom of nanoholes can expand due to Boyle's law
(P oc 1/V). When expanded bubbles merge with adjacent bubbles and get bigger, the air
bubbles move upward and eventually escape the liquid surface. Time required to
eliminate the air is inversely proportional to the vacuum pressure, but the pressure should
be larger than the vapor pressure of the prepolymer.
Given that the vapor pressure of the prepolymer is 74mTorr, I used 100 mTorr
and after 10 minutes of contact time, high aspect ratio nanocone is replicated into the
polymeric surface as shown in Fig. 2-7B.
51
Schematics
10 minutes
5 minutes
Resist
(A) at atmospheric
pressure
Mold
2pm
200nm
Resist
(B) in vacuum
Mold
200nm500nm
Figure 2-7. A comparison between two filling conditions: (A) Filling process at
atmospheric pressure and the resultant surfaces after keeping the filling process for 5 and
10 minutes, (B) Vacuum-assisted filling process and the corresponding surface images
after 5 and 10 minutes of contact time. The aspect ratio of the replicated structure gets
larger with longer time duration. After 10 minutes, the surface exhibits same aspect ratio
with the mold.
52
2.4. RESULTS AND DISCUSSION
In Fig. 2-8 we show scanning electron microscope (SEM) images of a mold of
nanocone arrays fabricated using laser interference lithography, replicated nanohole
arrays on a PUA surface and also second generation replicated nanocone structures on a
PUA surface. The fabricated nanocone and nanohole arrays each have a pitch of 200 nm
and aspect ratio 4:1.
Measured transmission spectra under normal incidence for the inverted nanocone
and the nanocone arrays are shown in Fig. 2-9A. Measurements of the transmission
spectra were carried out using a spectrophotometer (Varian Cary-500i) in the visible to
near infrared range (350 nm < ' < 1400 nm). Both the tapered nanocone and nanohole
arrays exhibit enhanced optical transmittance when compared to a flat fused silica surface
over the range from 450 nm to 1400 nm due to the combined effects of the tapered
geometry and the high aspect ratio of the features. This is consistent with the numerical
calculations shown in Figs. 2-2A and 2-2B. In Figure 2-9B we show the enhanced optical
transmission (T) of the replicated egg-crate patterns at different incident angles from 0' to
80' with transverse electric (TE) polarized irradiation at a wavelength of X = 633 nm. At
angles of 64 = 0', 4 0 'and 80', the transmissivity of the nanohole egg-crate array is T =
95.9%, 90.6% and 40.3% respectively, but these values drop to T = 93.2%, 86.8% and
23.0% respectively for the flat silica glass. While lower optical transmission at larger
incidence angles is expected for both for the flat surface and the egg-crate patterned
surface due to the physics of reflection, the egg-crate patterned surface persistently
exhibits higher transmissivities than the corresponding flat glass surface, even at very
high incidence angles.
53
(A)
(B)
(C)
Figure 2-8. SEM images of (A) side view of a silica mold fabricated using laser
interference lithography. (B) Replicated inverted nanocone arrays imprinted in PUA and
(C) second generation replicated PUA nanocone arrays. The aspect ratio of the replicated
nanostructure is H/P 4 with tip radius rtp~ 20 nm.
(B)
(A)
95
c
9&
801
.2
Incident Light
(TE @ 633nm)
.9601
E 85
a
Nanohole
-Nanocone
- Silica Glass
-
80
400
600
800
1000
1200
Nanohole
40
20L
0
140 0
4o. Nanocone
- Silica Glass
20
40
Incident Angle
Wavelength (nm)
60
80
e, (*)
Figure 2-9. Enhanced optical transmission of the nanotextured surfaces. (A) Measured
broadband transmission over a wide range of wavelength (350 nm < k < 1400 nm) and
(B) optical transmission for transverse electric (TE) polarized light through the
nanotextured and flat fused silica surfaces is measured by changing the incident angle of
a laser source whose wavelength is 633 nm. At an angle of 0; = 80', the transmission of
the nanohole array is T = 40.3%, but has dropped to T = 23.0% for the flat silica glass.
Each angular point was averaged automatically by the power meter (Newport, 2832-C)
over 100 repeated measurements with standard deviation of less than 0.01%.
54
The superhydrophilicity of the nanotextured egg-crate surface shown in the inset
of Fig. 2-3A results in anti-fogging behavior via the formation of a thin wetting film of
water that fills the nanotexture and prevents the formation of micrometric water droplets
on the surface that subsequently scatter light. The anti-fogging behavior is quantified by
measuring the Ot order optical transmission of laser (k = 633 nm) at normal incidence
through both a nanotextured surface and a control surface consisting of a cleaned glass
slide, each of which is exposed to a stream of saturated steam (flow velocity V = 3 m/s
and temperature of 87.6 C). As shown in Fig. 2-10A, the nanotextured film exhibits a
reduction in transmission of less than AT = 4%, whereas the cleaned glass slide shows a
sudden large drop of transmission AT = 94% when placed in the stream of steam because
light is scattered from the microscopic fog droplets that are deposited on the glass
surface. The slow evaporation of these pinned droplets also results in a longer time to
recover complete transmission compared to the nanotextured surface.
The strong water-repellency of fluorosilane-treated tapered nanohole structure
also confers self-cleaning properties to these surfaces. Water droplets (V~ 10 PL) were
dispensed every 5 seconds onto two kinds of samples that were inclined with a tilting
angle of a = 300 (Fig. 2-40B) and covered with three different kinds of common
micrometric contaminants (silicon carbide, lycopodium spores and white sand, with
average diameters of 10 pim, 30 gm and 100 pm, respectively). The change in the optical
transmission with the number of droplets dispensed on the samples is shown in Fig. 210B to illustrate how easily the water droplets can remove different particulate
contaminants from the nanotextured surfaces. When compared to the cleaned glass slide,
the water-repellent nanotextured surface exhibits nearly no residual contaminants, and we
observe full recovery of optical transmission after 3 to 5 drops of water impact the
surface.
55
Duration of exposure to fog
1.00
Cn
Cn
0.8-
E
Co 0.60.4
0)
N
Nanotextured Film
Flat Glass
0.2-
Nanotextured Film
-
--- Flat Glass
0
z
0.0
0
2
4
6
8
10
12
Time (sec)
(A)
Test Setup
1.0C
0
(D
Co
Water droplet
0.8-
E
CA
C
0.6-
cc
Sample
0.4-
-
-
--+-*
sic
/
N
--..
-
- Lycopodium
-A- Sand
on nanotextured film
/
0.2-
-4_
/
0
z
0.0-
00
11
- Lycopodium
on flat glass
Sand
2
3
5
4
Number of Droplets
(B)
Figure 2-10. Optical transmission measurements showing (A) anti-fogging behavior of
superhydrophilic inverted nanocone surfaces and (B) self-cleaning behavior of waterrepelling inverted nanocone surfaces with three different types of powders coated with
thickness of more than 0.5 mm (SiC particles, Lycopodium spores and white sand grains
with average diameters of 10 pim, 30 ptm and 100 pm, respectively). The error bars were
determined by repeating the measurements three times each; the large deviations are
because of the dynamic nature of the measurement; the precise location of the droplet
impacts, the initial uniformity of the powder on the surface, and the evolution of the
droplets and contaminants after droplet deposition could only be controlled with limited
precision.
56
The mechanical robustness of a PUA surface textured with the nanoholes was also
tested experimentally. First, a tapping force of 4 N was applied to the sample along the
normal and shear directions of the nanotextured surface through a latex rubber pad
(McMaster-Carr, 85995K28) with dimensions 8.9 mm by 8.9 mm repeatedly. Each
contact pressing consisted of 5 seconds of force application corresponding to an applied
normal stress of 50 kPa. Fig. 2-1 IA shows that there is nearly no degradation of optical
transmission after repeated loading conditions up to 50 times. Alternatively pressing the
sample with higher force up to 60 N through a Neoprene rubber ball (McMaster-Carr,
1241T4, Young's modulus E, = 5.5 MPa, radius R, = 4.8 mm) resulted in small
degradation of the sample in terms of optical transmission. Small distortions of structures
at the top surface are observed as shown in the inset of the graph. The optical
performance started to degrade at 60 N of loading, (corresponding to pressure of 3 MPa,
calculated using Hertz contact pressure[43]), due to distortion of the periodicity in the
hole array at the top surface, as shown in the inset of Fig. 2-11B.
57
1.00-
0.98
-r
(A)
-
-
0.98-
C
0.96
~
No
0.96
0.94
al
0.94-
C
- _40Shear
0.92
0.90
'F
-
C
0
(B)
Normal Force
+ Shear Force
-..-
N
b
0.92[
0.90
o P sn 3b 4' 5
Normal Force (N)
Number of Pressings
Figure 2-11. Mechanical robustness test of PUA surfaces textured with inverted nanocone
arrays. (A) Optical transmissivity of the nanotextured surface after applying a contact
force of 4 N in the normal and shearing directions of the nanotextured surface through a
latex rubber pad (dimension 8.9 mm x 8.9 mm) repeatedly; (B) after applying normal
force through a Neoprene rubber ball (Young's modulus E,, = 5.5 MPa and radius Rn, =
4.8 mm) up to 60 N (Corresponding contact pressure ~ 3 MPa, calculated using Hertz
contact pressure). 201 The insets show the SEM images of the egg-crate nanotexture
before and after applying the force.
58
2.5. CONCLUSION
We have demonstrated a replication-based approach using a UV photocurable
polymer capable of mass-producing multifunctional nanostructured films consisting of
periodic arrays of inverted nanoholes in an egg-crate structure. These textured structures
have superior anti-reflective and wetting properties compared to flat fused silica glass
surfaces as shown in Fig. 2-12, and enjoy greater mechanical robustness than our earlier
approach.[12]
While retaining the high feature density and high aspect ratio
characteristics of tapered nanostructures that provide multifunctional enhancement of
both the optical and wetting performance, the nanohole arrays also provide high
mechanical robustness regardless of their aspect ratio via stress redistribution across a
broad network of interconnected features. The UV replication method is compatible with
large area imprint[44] or roll-to-roll processes,[45] offering potential advantages such as
low cost and high throughput.
With such processes, it can be anticipated that
multifunctional surfaces can be fabricated in the form of flexible plastic films and thus
applied conformally as an adhesive tape to a broad range of materials such as glass,
silicon and other optical plastics. The process is also compatible with curved substrates.
These nanohole egg-crate structures and the ability to continuously manufacture
structures using roll-to-roll process technology may offer potential for industrial
applications that require combined control of reflectivity and wetting behavior over large
surface areas, such as photovoltaic cells, car windshields and future touch-screen display
devices.
59
Anti-reflectivity
Antifogging
Self-cleaning
Figure 2-12. Demonstration of multi functional inverted nanocone surfaces including
anti-reflectivity, antifogging effect and self-cleaning effect. Left side of each figure
shows a result of a flat fused silica glass, and right side of each figure shows inverted
nanocone surfaces.
60
REFERENCES
1.
C. H. Sun, P. Jiang, and B. Jiang, "Broadband moth-eye antireflection coatings on
silicon," Appl. Phys. Lett. 92, 061112 (2008).
2.
J. W. Leem, Y. M. Song, and J. S. Yu, "Broadband wide-angle antireflection
enhancement in AZO/Si shell/core subwavelength grating structures with
hydrophobic surface for Si-based solar cells," Opt. Express 19, Al155-Al164
(2011).
3.
Y. F. Li, J. H. Zhang, and B. Yang, "Antireflective surfaces based on biomimetic
nanopillared arrays," Nano Today 5, 117-127 (2010).
4.
B. Bhushan, Y. C. Jung, and K. Koch, "Micro-, nano- and hierarchical structures
for superhydrophobicity, self-cleaning and low adhesion," Philos. Trans. R. Soc.,
A 367, 1631 (2009).
5.
M. Nosonovsky and B. Bhushan, "Roughness optimization for biomimetic
superhydrophobic surfaces," Microsyst. Technol. 11, 535-549 (2005).
6.
D. Querd, "Wetting and roughness," Annu. Rev. Mater. Res. 38, 71 (2008).
7.
Y. C. Jung and B. Bhushan, "Mechanically durable carbon nanotube-composite
hierarchical structures with superhydrophobicity, self-cleaning, and low-drag,"
ACS nano 3, 4155-4163 (2009).
8.
B. Bhushan, K. Koch, and Y. C. Jung, "Biomimetic hierarchical structure for selfcleaning," Appl. Phys. Lett. 93(2008).
9.
T. L. Sun, L. Feng, X. F. Gao, and L. Jiang, "Bioinspired surfaces with special
wettability," Accounts Chem. Res. 38, 644-652 (2005).
10.
W. L. Min, B. Jiang, and P. Jiang, "Bioinspired self-cleaning antireflection
coatings," Adv. Mater. 20, 3914 (2008).
11.
J. Zhu, C. M. Hsu, Z. F. Yu, S. H. Fan, and Y. Cui, "Nanodorne solar cells with
efficient light management and self-cleaning," Nano Lett. 10, 1979 (2010).
12.
K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Nanotextured silica surfaces with robust superhydrophobicity and
omnidirectional broadband supertransmissivity," ACS nano 6, 3789-3799 (2012).
61
13.
P. Vukusic and J. R. Sambles, "Photonic structures in biology," Nature 424, 852855 (2003).
14.
Y. Kanamori, M. Sasaki, and K. Hane, "Broadband antireflection gratings
fabricated upon silicon substrates," Opt. Lett. 24, 1422 (1999).
15.
G. Xie, G. Zhang, F. Lin, J. Zhang, Z. Liu, and S. Mu, "The fabrication of
subwavelength anti-reflective nanostructures using a bio-template," Nanotechnol.
19, 095605 (2008).
16.
Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L.
Pan, H. C. Lo, C. H. Hsu, and Y. H. Chang, "Improved broadband and quasiomnidirectional anti-reflection properties with biomimetic silicon nanostructures,"
Nat. Nanotechnol. 2, 770 (2007).
17.
K. Choi, S. H. Park, Y. M. Song, Y. T. Lee, C. K. Hwangbo, H. Yang, and H. S.
Lee, "Nano-tailoring the surface structure for the monolithic high-performance
antireflection polymer film," Adv. Mater. 22, 3713 (2010).
18.
A. Deinega, I. Valuev, B. Potapkin, and Y. Lozovik, "Minimizing light reflection
from dielectric textured surfaces," J. Opt. Soc. Am. A, Opt. Ima. Vis. 28, 770
(2011).
19.
A. R. Parker and H. E. Townley, "Biomimetics of photonic nanostructures," Nat
Nanotechnol. 2, 347-353 (2007).
20.
V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. H.
Anastasiadis, and C. Fotakis, "Biomimetic artificial surfaces quantitatively
reproduce the water repellency of a lotus leaf," Adv. Mater. 20, 4049-4054
(2008).
21.
A. Lafuma and D. Quere, "Superhydrophobic states," Nat. Mater. 2, 457 (2003).
22.
K. T. Lau, S. Q. Shi, and H. M. Cheng, "Micro-mechanical properties and
morphological observation on fracture surfaces of carbon nanotube composites
pre-treated at different temperatures," Compos. Sci. Technol. 63, 1161-1164
(2003).
23.
E. J. Garcia, A. J. Hart, B. L. Wardle, and A. H. Slocum, "Fabrication of
composite microstructures by capillarity-driven wetting of aligned carbon
nanotubes with polymers," Nanotechnol. 18(2007).
62
24.
D. Chandra and S. Yang, "Stability of high-aspect-ratio micropillar arrays against
adhesive and capillary forces," Accounts Chem. Res. 43, 1080-1091 (2010).
25.
A. Nakajima, K. Abe, K. Hashimoto, and T. Watanabe, "Preparation of hard
super-hydrophobic films with visible light transmission.," Thin Solid Films 376,
140-143 (2000).
26.
A. Nakajima, K. Hashimoto, and T. Watanabe, "Recent studies on superhydrophobic films," Monatsh Chem. 132, 31-41 (2001).
27.
K. Choi, S. H. Park, Y. M. Song, C. Cho, and H. S. Lee, "Robustly nano-tailored
honeycomb structure for high-throughput antireflection polymer films," J. Mater.
Chem. 22, 17037-17043 (2012).
28.
A. Marmur, "Wetting on hydrophobic rough surfaces: To be heterogeneous or not
to be?," Langmuir 19, 8343-8348 (2003).
29.
A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley, and R. E. Cohen, "Robust
omniphobic surfaces," P. NatI. Acad. Sci. USA 105, 18200-18205 (2008).
30.
R. N. Wenzel, "Resistance of solid surfaces to wetting by water," Ind. Eng. Chem.
28, 988-994 (1936).
31.
M. Reyssat, L. Courbin, E. Reyssat, and H. A. Stone, "Imbibition in geometries
with axial variations," J. Fluid Mech. 615, 335-344 (2008).
32.
D. Lee, M. F. Rubner, and R. E. Cohen, "All-nanoparticle thin-film coatings,"
Nano Lett. 6, 2305-2312 (2006).
33.
B. Krasovitski and A. Marmur, "Drops down the hill: Theoretical study of
limiting contact angles and the hysteresis range on a tilted plate," Langmuir 21,
3881-3885 (2005).
34.
A. B. D. Cassie and S. Baxter, "Large contact angles of plant and animal
surfaces," Nature 155, 21-22 (1945).
35.
A. J. B. Milne and A. Amirfazli, "The Cassie equation: How it is meant to be
used," Adv. Colloid Interfac. 170, 48-55 (2012).
36.
C. Y. Jeong and C. H. Choi, "Single-step direct fabrication of pillar-on-pore
hybrid nanostructures in anodizing aluminum for superior superhydrophobic
efficiency," ACS Appl. Mater. Inter. 4, 842-848 (2012).
63
37.
S. A. Mascaro and H. H. Asada, "Measurement of finger posture and three-axis
fingertip touch force using fingernail sensors," IEEE T. Robotic Autom. 20, 26-35
(2004).
38.
M. Koshiba, K. K. S. Hwang, S. K. Foley, D. J. Yarusso, and S. L. Cooper,
"Properties of ultra-violet curable polyurethane acrylates," J. Mater. Sci. 17,
1447-1458 (1982).
39.
H. E. H. Meijer and L. E. Govaert, "Mechanical performance of polymer systems:
The relation between structure and properties," Prog. Polym. Sci. 30, 915-938
(2005).
40.
D. E. Packham, Handbook of adhesion, 2nd ed. (John Wiley, Hoboken, N.J.,
2005).
41.
M. J. Lee, N. Y. Lee, J. R. Lim, J. B. Kim, M. Kim, H. K. Baik, and Y. S. Kim,
"Antiadhesion surface treatments of molds for high-resolution unconventional
lithography," Adv. Mater. 18, 3115. (2006).
42.
J. G. Kim, H. J. Choi, H. H. Gao, I. Cornago, C. H. Chang, and G. Barbastathis,
"Mass replication of multifunctional surface by nanoimprint of high aspect ratio
tapered nanostructures," 2012 International Conference on Optical MEMS and
Nanophotonics (OMN), 71-72 (2012).
43.
K. K. L. J. K. L. Johnson, Contact Mechanics (Cambridge University Press.,
1985).
44.
J. G. Kim, Y. Sim, Y. Cho, J. W. Seo, S. Kwon, J. W. Park, H. Choi, H. Kim, and
S. Lee, "Large area pattern replication by nanoimprint lithography for LCD-TFT
application," Microelectron. Eng. 86, 2427 (2009).
45.
S. H. Ahn and L. J. Guo, "Large-area roll-to-roll and roll-to-plate nanoimprint
lithography: a' step toward high-throughput application of continuous
nanoimprinting," ACS nano 3, 2304-2310 (2009).
64
Chapter 3.
Double Gradient-Index Nanostructures
for Broadband Anti-reflectivity
of Multi-optical Interfaces
3.1. INTRODUCTION
Reflection of light is a natural, but often unwanted phenomenon for most optical
applications. It creates glare on the surface of display devices and losses in efficiency of
opto-electric devices such as solar cells or photo-detectors [1, 2].
Various methods have been developed over time to minimize reflection from a
surface in order to improve the efficiency of such devices by increasing transmission or
absorption as previously described in Chapter 1 [3]. Nanotextured surfaces have also
proven to reflection with broadband and omnidirectional performance [1, 3-11]. We have
previously shown that ultra-high aspect ratio nanocones on silicon substrates can provide
enhanced anti-reflectivity [12], but the surface might be too vulnerable to outer
mechanical forces such as fracture and fouling especially when it is exposed to harsh
environments. A cover glass or a plastic-encapsulating layer would protect the
nanostructures, but it would also induce additional Fresnel reflection [13].
In Chapter 2, we also have shown that inverted nanocone structures replicated
into polyurethane acrylate (PUA) effectively suppress the Fresnel reflection by adiabatic
coupling due to a gradually increasing refractive index towards the substrate surface.
Mechanical robustness of the surface textured with the inverted nanocones is also
65
enhanced due to the geometry, which has potential to be applied to the texture of a single
optical interface, the encapsulating layer. However, this case does not consider multiple
optical interfaces, hence there are additional Fresnel reflections to suppress.
Here we propose double gradient-index (D-GRIN) nanostructures for extremely
low reflection at multi-layered optical interfacial surfaces such as encapsulated solar cells
[14]. As shown in Fig. 3-1(a), each surface is textured with tapered nanostructures in
order to suppress Fresnel reflection. The top surface is textured with inverted nanocones,
and the interface between the encapsulating layer and silicon surface is textured with
nanocone structures. Both nanotextured surfaces exhibit broadband omnidirectional antireflectivity due to the gradient-index adiabatic impedance matching effect. In addition,
the top surface textured with inverted nanocones maintains high mechanical robustness
because the nanostructures are connected with each other as a network [15].
The height of the silicon nanocones can be increased as much as the fabrication
technique allows for enhanced anti-reflectivity, without worrying about low mechanical
robustness. This is because the nanotextured silicon surface is protected by the
encapsulating layer. The fabrication processes used in this work are compatible with
high-throughput and large-area production. We employ laser interference lithography
(LIL) first to fabricate an array of silicon nanocones, and then replicate the nanocones
resulting in inverted nanocones on a polymeric encapsulating layer, as shown in Fig. 31(a).
66
Inverted
Nanocone
(a)
Nanocone
(b)
n
n
[
n
Inverted
Nanocone
nno
-
n2
z
Nanocone
n2
z
Figure 3-1. (a) A schematic of double gradient-index (D-GRLN) nanostructures for multioptical interfaces and the simplified fabrication process; (b) the gradient-index profile of
D-GRIN nanotextured surface.
67
3.2. DESIGN OF NANOSTRUCTURES FOR MULTI-OPTICAL
INTERFACES
To quantify the light reflection according to different refractive indices, Fresnel
equation is often used.
For the transverse-magnetic (TM) polarization, the reflectance
is:
(fnicosi
RTM
- n2cos2
(nicosBl + n2COS02)
2
where 01 is the incidence angle and 02 = sin-'(nisin 01/n2) is the angle of refraction into
the second medium defined by Snell's law and ni and n 2 are the refractive indices at
either side of the boundary. For the transverse-electric (TE) polarization, the reflectance
is:
RTE
=
n
0(C0s2
-
n2cos01 2
(3-2)
niCOS02 + n 2 cos61
If there are multiple layers with a series of refractive indices, the total reflection is
calculated by incorporating all the interference at each layer as well as Fresnel
reflections.
Both the nanocones on silicon surface and the inverted nanocones on the top
polymeric surface are two-dimensional square arrays with sub-wavelength periodicity,
designed to serve as an effective medium of refractive index that is gradually increasing
along the direction normal to the surface. This is shown in Fig. 3-1(b).
Nanocone structures mimicking natural moth-eye's structures have been widely
used as anti-reflective structures due to their graded refractive index, namely their filling
factor gradually increasing from the tip to the bottom of nanocones. This minimizes the
optical impedance mismatch between two media and, hence, reduces Fresnel reflection,
68
which is proportional to the square of the index difference between two media as in Eqs.
(3-1) and (3-2).
Similarly, the surface textured with inverted nanocone structures has a refractive
index profile graded along the direction normal to the surface. The gradient in the
refractive index is also from the gradually increasing fill factor of the material from the
top surface to the tip of the inverted cone. This adiabatic index matching eliminates the
reflection. By having double gradient index nanostructures at each optical interface, all
the Fresnel reflections can be suppressed at all the optical interfaces.
The optical performance of the proposed design is calculated using the FDTD
(finite-difference time-domain) method, and compared with flat surface cases and
conventional thin film coating methods, as shown in Fig. 3-2. For nanotextured surface
cases, the periodicity of both nanocones and inverted nanocones is 200 nm, which is
smaller than the wavelengths in the broadband range from ultraviolet to near infrared
light, which are of concern for most opto-electronic devices. Generally, for both
nanocones and inverted nanocones, the higher the aspect ratio (defined as the ratio of
nanocone height to pitch), the less reflection the surface produces [15, 16]. The aspect
ratio of the nanostructures in the calculation is assumed to be 4, which is high enough to
generate a low reflectance of less than 1% in the wavelength range of visible light [12,
15].
69
50
4
-
-
40.-:
-0
300
700
1100
1500
cc 30
Flat Si
Flat Si (w/ glass)
-ARC on Si
-ARC on Si (w/ glass)
- Single cone
-Double cone
20
Single-cone
10
0
300
500
900 1100
700
Wavelength, A (nm)
1300
1500
Double-cone
Figure 3-2. Reflectance calculated using FDTD method as implemented in FDTD
solutions 8.0 for different surfaces: flat silicon surfaces, thin film anti-reflective coating
(ARC) cases and nanostructured surfaces consisting of either double-cones or singlecones. In all calculations, the periodicity of the nanocones was 200nm, the height was
800nm (aspect ratio of 4), and the Palik dispersion model was used [17]. The inset shows
the enlarged reflection spectra for single-cone and double-cone surface cases.
As calculated using Eqs. (3-1) and (3-2) when 01 = 0', flat silicon surfaces with or
without cover glass show high reflection due to the high refractive index of silicon
compared to the refractive index of glass or air. Conventional film-type anti-reflective
coatings exhibit relatively low reflectance, but their anti-reflectivity is limited to a small
band of wavelengths. The optical performance of the thin film method is also limited to a
small range of incidence angles, where the destructive interference condition occurs. On
the other hand, the nanotextured surfaces exhibit extremely low reflectance compared to
all other cases. Both single-cone and double-cone cases show the reflectance of less than
3% over the entire broadband wavelength range. The double-cone nanostructure case
exhibits a broadband reflectance of less than 1 % even after incorporating all the
reflectance from both the interfaces on the top surface and the boundary between silicon
70
and the encapsulating polymeric material. The graph demonstrates that the design of
double gradient-index structure achieves extremely low reflectance for the multi-layered
optical interface.
The anti-reflectivity improvement is marginal compared to the uncoated single
gradient-index structure; more importantly the double-layer provides protection and
mechanical stability. Compared to nanocone structures, which usually have been adapted
as anti-reflective structures, the inverted nanocones that texture the top surface are
mechanically much more robust since they are supported with surrounding structures as a
network [15]. This would offer a clear advantage for silicon solar cells, which often have
to be exposed to harsh environmental conditions.
3.3. FABRICATION OF DOUBLE GRADIENT-INDEX
NANOSTRUCTURES
The fabrication processes used in this work are large-area compatible and highthroughput methods, which make the proposed design more practical to real world
applications. The fabrication processes are categorized into two parts: texturing the
silicon surface with a square array of nanocones, and replicating the nanocones into the
inverted nanocones in a polymeric encapsulating layer.
First, the silicon surface is textured with high aspect ratio nanocones using laser
interference lithography and subsequent multiple etching steps, as described in our prior
work [12]. A square array of nanoholes is patterned on photoresist (PR) using a Lloyd's
mirror interferometer
lithographic setup
shown in Fig.
3-3(a), and hydrogen
silsesquioxane (HSQ 14, Dow Corning) is used as a mask for etching the silicon substrate.
Then the nanotextured silicon surface is covered with a UV-curable polyurethane
acrylate layer, whose top surface is textured with inverted nanocones using UV
replication methods as shown in Fig. 3-3(b) to produce an anti-reflective encapsulating
layer. In this process, the pre-fabricated silicon surface consisting of nanocones is used
71
both as a substrate of double gradient-index nanostructures and as a mold for the UV
replication to create inverted nanocones on the top encapsulating surface.
Since neither
the silicon mold nor the silicon substrate, which are made via the same process, are
transparent to UV light, a transparent substrate made of fused silica is first used as a
tentative carrier on which the UV replication process is performed temporarily using UVcurable poly urethane acrylate (PUA). The liquid pre-polymer is first dispensed on the
fused silica substrate, and the silicon mold is placed in contact with and pressurized into
the pre-polymer. After curing the polymer by exposing it to UV light from the transparent
substrate side, the inverted nanocones are formed by detaching the silicon mold from the
cured polymeric surface. The nanotextured polymeric film is then delaminated from the
transparent carrier with the help of precisely controlled surface adhesion between the
fused silica and the film. Finally the delaminated nanotextured film is attached to the
nanotextured silicon substrate using the same UV-curable PUA as an adhesive material
by exposing UV light, which finalizes the fabrication process of the double-index
nanostructures consisting of silicon nanocones and polymeric inverted nanocones on the
encapsulating layer as shown in Fig. 3-3(b).
72
Mirror
HeCd Laser
( A= 325 nm)
Rotation
stage
R coated sample
Pitch =
2sin6
(a)
Mold
an&&
i &AM
I
mmm
AAAAJ
Delaminated Film
0d
IMj
T "TT"TT"T*
-----
I
(b)
Figure 3-3. (a) A schematic of Lloyd's mirror interference lithography system used for
fabricating silicon nanocones and (b) UV replication process used for encapsulatingpolymeric surface textured with inverted nanocones.
73
3.4. RESULTS
AND DISCUSSION
(a)
(b)
(c)
structures used
nanocone
Silicon
(a)
Figure 3-4. SEM images of the fabricated samples.
as a substrate and a master mold in the replication process; (b) a side view and (c) a top
view of replicated inverted nanocone structures on PUA surfaces.
Fig. 3-4 shows scanning electron microscope (SEM) images of silicon nanocones
and replicated inverted nanocones. The silicon nanocones have periodicity of 200 nm and
aspect ratio of 4. The replicated inverted nanocones have the inverted geometry of the
silicon nanocone, but with the same periodicity; this verifies the stability of the
.
replication process. The size of the sample is 1 x 1.5 cm
The optical reflectance spectra are measured using a spectrophotometer (Varian
Cary 500i) in the ultraviolet to near infrared range (300 nm < A < 1500 nm) on the
nanostructured samples, as shown in Fig. 3-5. The measured spectra are also compared
with the numerical values calculated using FDTD. The nanotextured surfaces exhibit
small reflectance over the entire spectral range in the measurement, which is also well
matched with the calculated data. The reflectance of both single-cone and double-cone
surfaces is lower than the case where Si nanocone surface is covered with a flat
encapsulating layer, which is indicated as a dotted gray line in Fig. 3-5. The double-cone
nanostructures show even lower reflectivity (< 1 %) than the single-cone case (< 3 %),
due to more gradual effective refractive index profile induced from an additional antireflective layer. Diffused reflectance was also measured using a diffuse reflectance
74
accessory (Agilent, Praying Mentis) [18] and was found to be negligible (less than the
instrument sensitivity of 0.3%).
The top surface textured with the inverted nanocones also can exhibit selfcleaning effect [14] to avoid accumulation of dust and other particulate contaminants [15]
with low risk of mechanical damage, offering an additional benefit to the solar cell
application.
4
-------------------------------' 3
--- Single-cone under flat polymer (Cal.)
--- Single-cone (Cal.)
-u-Single-cone (Exp.)
- - - Double-cone (Cal.)
--+-Double-cone (Exp.)
&
c 2
CU
0
300
500
700
900
1100
1300
1500
Wavelength, A (nm)
Figure 3-5. Anti-reflectivity of the nanotextured surfaces. Broadband reflectivity was
measured over a wide range of wavelength (300 nm < X < 1500 nm) and compared with
calculated values.
75
3.5. CONCLUSION
In conclusion, the proposed double gradient nanostructures for ultimate antireflectivity for multi-interfacial surfaces demonstrate that the gradient refractive index at
each optical interface effectively reduces the Fresnel reflection. The proposed design
provides a potential to increase the efficiency of the silicon-based devices, such as
encapsulated solar cells or silicon based photodectectors, with long-term duration of
enhanced performance of the devices due to better mechanical robustness on the top
surface and an additional wetting property. Also the fabrication method, which is largearea and manufacturing compatible, makes the proposed design an attractive choice for
surface structures useful to industrial fields related to optics and opto-electronics.
76
REFERENCES
1.
K. X. Wang, Z. Yu, V. Liu, Y. Cui, and S. Fan, "Absorption enhancement in
ultrathin crystalline silicon solar cells with antireflection and light-trapping
nanocone gratings," Nano. Lett. 12, 1616-1619 (2012).
2.
M. E. Motamedi, W. H. Southwell, and W. J. Gunning, "Antireflection surfaces in
silicon using binary optics technology," Appl. Opt. 31, 4371-4376 (1992).
3.
H. K. Raut, V. A. Ganesh, A. S. Nair, and S. Ramakrishna, "Anti-reflective
coatings: A critical, in-depth review," Energy Environ. Sci. 4, 3779-3804 (2011).
4.
Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L.
Pan, H. C. Lo, C. H. Hsu, and Y. H. Chang, "Improved broadband and quasiomnidirectional anti-reflection properties with biomimetic silicon nanostructures,"
Nat. Nanotechnol. 2, 770 (2007).
5.
C. H. Chang, J. A. Dominguez-Gaballero, H. J. Choi, and G. Barbastathis,
"Nanostructured gradient-index antireflection diffractive optics," Opt. Lett. 36,
2354-2356 (2011).
6.
Y. Kanamori, M. Sasaki, and K. Hane, "Broadband antireflection gratings
fabricated upon silicon substrates," Opt. Lett. 24, 1422 (1999).
7.
Y. F. Li, J. H. Zhang, and B. Yang, "Antireflective surfaces based on biomimetic
nanopillared arrays," Nano Today 5, 117-127 (2010).
8.
C. H. Sun, P. Jiang, and B. Jiang, "Broadband moth-eye antireflection coatings on
silicon," Appl. Phys. Lett. 92, 061112 (2008).
9.
J. Zhu, C. M. Hsu, Z. F. Yu, S. H. Fan, and Y. Cui, "Nanodome solar cells with
efficient light management and self-cleaning," Nano Lett. 10, 1979 (2010).
10.
D. Shir, J. Yoon, D. Chanda, J. H. Ryu, and J. A. Rogers, "Performance of
ultrathin silicon solar microcells with nanostructures of relief formed by soft
imprint lithography for broad band absorption enhancement," Nano Lett. 10,
3041-3046 (2010).
11.
P. Spinelli, M. A. Verschuuren, and A. Polman, "Broadband omnidirectional
antireflection coating based on subwavelength surface Mie resonators," Nature
Commun. 3, 692 (2012).
77
12.
S. Dominguez, I. C. Bravo, J. Pirez-Conde, H. J. Choi, J.-G. Kim, and G.
Barbastathis, "Simple fabrication of ultrahigh aspect ratio nanostructures for
enhanced antireflectivity," J. Vac. Sci. Technol. B 32, 030602 (2014).
13.
A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Blasi, A. Heinzel, D. Sporn,
W. Doll, and V. Wittwer, "Antireflective transparent covers for solar devices,"
Solar Energy 68, 357-360 (2000).
14.
J. G. Kim, S. Dominguez, H. J. Choi, I. Cornago, and G. Barbastathis, "Double
cone nanostructures for ultimate anti-reflectivity of encapsulated silicon solar
cells," in International Confirence on Optical Mems and Nanophotonics, (IEEE,
2014), 211 - 212.
15.
.. G. Kim, H. J. Choi, K. C. Park, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Multifunctional inverted nanocone arrays for non-wetting, selfcleaning transparent surface with high mechanical robustness," Small 10, 24872494 (2014).
16.
K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Nanotextured silica surfaces with robust superhydrophobicity and
omnidirectional broadband supertransmissivity," ACS nano 6, 3789-3799 (2012).
17.
E. D. Palik, Handbook qf Optical Constants of Solids (Academic, 1998).
18.
R. Hogue, "'Praying Mantis' diffuse reflectance accessory for UV-Vis-NIR
spectroscopy," Fres. J. Anal. Chem. 339, 68-69 (1991).
78
Chapter 4.
Conical Photonic Crystals for Enhancing
Light Extraction Efficiency from HighIndex Materials
4.1.
INTRODUCTION
Efficient light extraction is crucial for many photonic devices where light is
generated from the inside of high refractive index materials, such as scintillators. Often
used for medical imaging or cosmic radiation detection, scintillators generate visible light
after absorbing high-energy electromagnetic waves such as X-ray or gamma radiation[ 1,
2]. Scintillator materials are generally isotropic, so the visible light they generate is
emitted at all possible angles, whereas the optoelectronic transducer occupies only one
facet as shown in Fig. 4-1(a). When the generated visible light is coupled with an
interface between a high index material and an outer environment such as air, light
transmission is often limited by Fresnel reflection due to the difference in refractive
indices between two materials. Light extraction efficiency is further limited by total
internal reflection (TIR), a limitation that becomes more severe for high refractive index
materials, because of the low critical angle. Since light generation in a scintillator
material can be assumed to be isotropic, all light whose incidence angle is larger than the
critical angle should be reflected back and trapped in the material [3, 4].
Recently, there has been growing interest in functional micro- and nano-structures
that can enhance the light extraction efficiency of materials such as scintillators and
LEDs. Anti-reflective moth-eye nanostructures of subwavelength period can increase the
79
light transmission by suppressing the Fresnel reflection [5-8]. However, their subwavelength scale is not appropriate for the scintillator application since only the zerothorder light is allowed to propagate through the nanostructured interface, and thus TIR still
occurs. Instead, diffraction gratings, sometimes referred to as "photonic crystals", have
also been used for enhancing the extraction efficiency. To overcome TIR, these patterns
are redirecting some of the light to diffraction orders, propagating beyond the critical
angle [1, 3, 4, 9-12]. Typically, these structures consist of square lattices of cylindrical
holes with spacing larger than the wavelength. However, in these cases the transmission
under the critical angle must be partly sacrificed, not only because of Fresnel reflection,
but also because the zeroth-order must now be divided into several diffracted orders
(some of which still do not successfully overcome the TIR problem.) Although extensive
research has been carried out on enhancing light extraction efficiency, no nanostructured
surface exists which sufficiently exhibits advantages both for under the critical angle
region and beyond the critical angle region.
air or
Light
grease
>
extracting
Light
layer
extracting
Scintillator
Scintillator
Reflector
(for visible light)
X- or gamma-rays
(b)
(a)
Figure 4-1. (a) A schematic of light extracting environment for a scintillator; (b) The
concept of conical photonic crystals on a scintillator surface for enhancing light
extraction efficiency
80
Here we propose a conical diffraction grating/photonic crystal as a highly
efficient light extraction layer shown in Fig. 4-1(b), as an attempt to balance more
successfully Fresnel reflection and TIR. Essentially, we combine two concepts: the
conical shape is meant to reduce Fresnel reflection for the zeroth-order of light at a broad
range of angles of incidence, whereas the diffractive structure is meant to redirect at least
a portion of the light that is incident beyond the critical angle. However, the two goals are
in conflict in the sense that the first function (reducing Fresnel reflection) requires a
subwavelength pitch, whereas the second function (diffracting light that would otherwise
be TIR'ed) requires a diffractive structure with period bigger than wavelength.
Fortunately, if the period is near the wavelength, a mix of both effects can be observed
[13-15] and we can hope to balance them effectively to maximize light extraction.
The methodologies for using FDTD and RCWA for this problem, as well as a
preliminary analysis of the effects of pitch on the ability of the structure to retain light
emitted at different angles are presented in Section 4.2. With these insights, we proceed
to fully optimize the structures with respect to all geometrical parameters in Section 4.3.
Fabrication processes and experimental results are in Section 4.4 and 4.5 respectively.
Finally, section 4.6 states conclusions.
81
4.2.
ANALYSIS OF LIGHT EXTRACTION IN NEAR-
WAVELENGTH PERIODIC CONE ARRAYS
4.2.1. Light extraction using conical photonic crystals
We
first calculated
light
transmission
by layering
different
types
of
nanostructured surfaces on an inorganic scintillator surfaces using the FDTD method.
The common conditions for these simulations are transverse electric (TE) polarized
irradiation at a wavelength of 2 = 540 nm (i.e., the peak emission wavelength of GYGAG
crystals), illuminated at different incident angles from 0' to 90'.
Fig. 4-2(a) shows that conventional anti-reflective moth-eye or nanocone
structures with a pitch of Pnanocojle= 2 00 nm exhibit excellent transmissivity when the
incidence angle is below the critical angle (Ocr
33'). The gradient in the index profile of
nanocones minimizes the Fresnel reflection because it satisfies adiabatic impedance
matching between air and the scintillator substrate. However, no light propagating
beyond the critical angle can be extracted, indicating that the sub-wavelength
nanostructure cannot overcome TIR.
When a conventional photonic crystal slab consisting of a square array of holes is
coated on a scintillator surface, some light can be transmitted beyond the critical angle, as
shown in Fig. 4-2(b). Light overcomes TIR beyond the critical angle because it is
diffracted on a periodic surface structure with periodicity larger than the wavelength of
light. However, some light loss is observed when an incidence angle is smaller than the
critical angle, because the Fresnel reflection on both surface types of the array (i.e., the
flat tops and the indented bottoms of the photonic crystals) still exists.
In addition,
some portion of the light is separated into higher diffracted orders. When the surface is
textured with a pillar array, a similar behavior of light transmission is also observed as
depicted in Fig. 4-2(b).
82
1.0
-0200
201.0
0.9
0.9
No
300
0.8
0
0.8
400
0.7
0.7
2
0.6
500
0.6
0.5
600
0.5
700
0.4
800
03
0.4
5
0.3
4
0
0.2
33
0.2
900
0.1
0.1
5
1000
10 20
0
30 40 50 60
Angle (*)
70 80 90
00
0 10
20
30 40 50 60 70
Angle (*)
80
90
(b)
(a)
1.0
180
-Flat
2.
0.7
220
.6
0.5
240
-P = 200 nm (FDTD)
-P = 260 nm(FDTD)
-Flat surface (RCWA)
0.8
2
0
0.
0.
.~0.6
P = 200 nm (RCWA)
E
C
0.4
1
26a_
260
surface (FDTD)
-P = 260 nm (RCWA)
0.4
0.3
280
0.2
0.1
300
0
0
10
20
30
40
50
60
70
0.2
0
0
80
20
30
50
40
Angle (
60
70
80
)
Angle ( )
10
(d)
(c)
a
Figure 4-2. Light transmission for (a) GRIN structures with a pitch of 200 nm and
PhC
height of 800 nm, (b) PhC structures with a thickness of 450 nm, and (c) conical
structures with a height of 800 nm coated on an inorganic scintillator calculated using
FDTD. In the simulation, light illumination is assumed to be a transverse electric (TE)
light
polarized irradiation at a wavelength of A = 540 nm. The refractive indices of the
extracting layer and the scintillator are assumed to be 1.82; (d) the comparison of light
transmission among flat surface and nanotextured surfaces calculated using FDTD and
RCWA.
83
Fig. 4-2(c) shows the light transmission via the proposed conical photonic crystals
on a scintillator surface. Light gains are observed beyond and below the critical angle.
The light extraction beyond the critical angle is due to the lateral periodicity of the
conical photonic crystal generating a diffraction effect. The light transmission under the
critical angle is also higher than that shown in Fig. 4-2(b) or on the flat surface shown in
Fig. 4-2(a), because the reflection is suppressed due to the gradient index effect from the
tapered geometry [5, 6] of the proposed design.
A comparison among conventional subwavelength nanocones, conical photonic
crystals and a flat scintillator surface is shown in Fig. 4-2(d). The conical photonic crystal
shows higher transmission compared to a flat scintillator surface due to the gradient index
effect from the tapered shape of the design, while there also is some light extraction
beyond the critical angle generated by diffraction from the periodicity of the conical
photonic crystals. In Fig. 4-2(d) the results calculated using FDTD are compared with the
values calculated using RCWA, which gives reasonably reliable match with each other
for the range of geometrical parameters considered.
4.2.2. Diffraction efficiency analysis
4.2.1.1 Analytic approach based on Fourier optics
Given that the conical photonic crystal is capable of extracting light from a highindex material, we need to understand the effect of key parameters such as a pitch, height
and refractive index on diffraction efficiency to optimize the structure geometry for an
efficient light extracting layer. We first analytically calculated the diffraction efficiency
using Fourier series, which are useful as a way to break up a periodic function into a set
of different diffraction orders, whose efficiencies are denoted as a coefficient of
expanded terms.
For reasons of simplicity, the conical photonic crystal is assumed to be
a ID cross-section of the proposed structures, i.e. triangular as depicted in Fig. 4-3(a).
The shape of the triangular phase grating g, is expressed as Eqs. (4-1) and (4-2) below:
84
1
Tri-angular Phase Grating (1D)
0.8
0 tt
diffracted Order
-AJI
a
0
th
ch 0.6
V
)2nd
()3th
CD
C
p~ 0.4
29.1%
23.2%
0.2
x
)8.7%
)0.1%
1
0
4
3
2
5
6
7
Pitch (urn)
(b)
(a)
Figure 4-3. Diffraction efficiency calculated using a numerical method (RCWA) and
analytic equation using the Fourier series.
exp (i
go(x)
-m
2
=
-x
0:!
,
P
2
(4-1)
P
2
2m
P
otherwise
.0,
+00
+00C
gt(x)=6
x <
-
2m
-m
X)
cqxexp(i27q-+i27T)
S(x-nP)=
0 (x)x
q=-oo
(4-2)
q=-oo
where P is the pitch of grating, A is the wavelength of incident light, and Cq is the Fourier
coefficient. Here the diffraction efficiency can be estimated as 9q
=
Cq
12, which is a
function of the height of the cones, but not related to the periodicity of the structures.
Likewise, at a given height of cones, the diffraction efficiencies calculated from the
Fourier series should be the same, even with different periodicity of the structures.
However, when the diffraction efficiency obtained from the Fourier series is compared
with the numerical results calculated using RCWA, as shown in Fig. 4-3(b), there is
discrepancy between the two values, especially for the small period regime. Two
diffraction efficiencies obtained from the analytic (numbers on the right side) and
85
numerical calculations (lines), respectively, matched only with each other when the
periodicity of the structure is larger than 3 [tm, which is beyond the range of our interest
because it is larger than the range where gradient the index effect is still effective for
optical photons.
4.2.2.2. Analysis using RCWA method
For the flat surface as shown in Fig. 4-4(a), if the emission angle (0e) is larger
than the critical angle determined by Snell's law 0, = sin-'(n2/nl), all light is internally
reflected inside the material. The relationship from Snell's law can also be explained by
the conservation of the tangential momentum (kg) parallel to the coupling interface, i.e.
phase-matching condition. In the phase-matching diagram in the wave number space
shown in Fig. 4-4(c), only light with k1 = kon.,sinOe smaller than nairko = konsesinO can
radiate into the ambient medium for a flat surface, where nse and nair are the refractive
indices of the scintillator and air, respectively. Therefore light with an angle larger than
the critical angle (Oc) cannot escape the scintillator through the interface.
nl<n 2
n,
in.,
Air
(a)
-2
P
Extracted light
0
n6k
Diffraction factor
AAr
-n
light
(b)
(c)
Figure 4-4. A schematic of the light incident on (a) a flat surface and (b) a photonic
crystal surface; (c) a schematic of the photonic crystal surface of the Bragg diffraction
phase matching diagrams between a scintillator and air in k-space. The large waveguide
mode circle has a radius k,= 2nnA/, and the small air circle has a radius ko = 2nain/L
86
If the in-plain component of the wave vector of emitted light is coupled with a
reciprocal lattice vector or grating vector G, and satisfies the phase matching relation
lkgsin(OL) mGI < ko, where m denotes the order of the diffraction, light can be extracted
into the air as shown in Fig. 4-4(b). The reciprocal lattice vector G of the photonic crystal
can be represented as a function of the periodicity of cones (G = 27/P). The light
extraction through diffraction by the photonic crystal is thus explained by Bragg's
diffraction law [16], which is described as:
7air sin(Q,,) =
On,,,
- sin(,) +MP
P
i
=
1, 2,3.
(4 -3)
where neoating indicates a refractive index of light extracting layer. Equivalently, the
criterion for the extraction of guided light into air can be rewritten as:
0, = sin-'
M -; -
( P
I -
) ncoatn.] ,'
i
n=
,2 3
12,3.
(4 - 4)
White dotted lines depicted in Fig. 4-5 define the boundaries plotted from Eq. (44) in 0,.-P domain for the first three orders of diffracted light. Consequently, the dotted
lines in Fig. 4-5 represent the boundaries confining the area where each diffraction order
is effectively generated.
The diffraction efficiency is also calculated using RCWA as depicted in Figs. 45(a) for TE polarization and 4-5(b) for TM polarization, respectively. The color contours
confirm the relationship between the transmission of diffracted light and the main
parameters such as the periodicity and the emission angle. The transmission of each
diffraction mode was calculated for various periodicities (0 tm < P < 3 [im) and emission
angles (0' < 0,
90') with fixed height H = 0.42 tm.
87
(a)
6 3.
3.0
2.0
t
1.0
02040
60
Angle (*)
3.0
j20
42.0
2 1.C
20
40
60
80
0
303.0-
20
40
2.0
1.0
60
80
2. a
2.
1.0
1.0e
1.
20
40
60
0
80
20
0
20
40
60
80
0
20
40
60
80
0
2040
60
80
0
20
40 60 80
3.
-
2.0
0
(b)
!
1.0
0
80
3.0
40 60
80
3.0
3
3.0
3.0
2.0
2
2.0
2.0
0
20
60
40
Angle (*)
3
80
20
40
60
80
0
20
40
60 80
3 0 -3.0
3.
2.
2.0
2.0
1.0
1.01
0
20
40
60
80
0
2040
60
80
Figure 4-5. Transmission distribution separated into different diffraction modes for (a)
TE (E-field parallel to the plane of incidence) and (b) TM polarization calculated using
RCWA. A 1 D triangular grating is simulated at a wavelength of A = 420 nm with a
refractive index of 1.82 both for the scintillator and the light extracting material. White
dotted lines representing the O-P relationship following the analytic equation. Each line
indicates the boundaries confining the area where each diffraction mode exists, following
conservation of the in-plain k-vector.
88
The relationship between each diffraction order calculated from Eq. (4-4) and the
light transmission becomes obvious in each diffraction mode's transmission calculated
numerically as shown in Fig. 4-5. The areas of the positive
negative
1 s,
2 nd,
Jst, 2 nd,
and
3 rd
modes and
and 3 rd modes, as shown in Figs. 4-5(a) and 4-5(b), are defined by
following the lines of the diffraction modes, respectively. For example, the area above the
white dotted line in Fig. 4-5 is the area where the positive diffraction orders can be
transmitted, and the area confined by dotted lines in Fig. 4-5 indicates the area where the
negative diffraction orders can be transmitted. The
0 th
order is dominated by the critical
angle (O. = 33.3') between the scintillator (nse = 1.82) and air (nair = 1).
Further, the total light transmission was also calculated using RCWA for different
periodicities (0 ptm < P < 3 tm) and emission angles (0' < 0, < 90') with the same height
H = 0.42 pm. The results are shown in Fig. 4-6. The total light transmission is exactly the
same as the summation of all the orders from
0 th
to higher orders shown in Figs. 4-5(a)
and 4-5(b), and hence we can analyze the diffraction efficiency and its contribution to the
total transmission shown in Fig. 4-6.
3.0
1.0
3.0
1.0
2.5
0.8
2.5
0.8
E .0
0.6
2.
1.5
0.4
E 2.0
0.6
2.
1.5
0.4
L
1.0
1.0
0.2
0.2
0.5
0
0.5
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
Angle
Angle (0)
(0)
(b)
(a)
Figure 4-6. Transmission versus emission angle and pitch, calculated using RCWA. A ID
triangular grating is simulated at a wavelength of A= 420 nm using (a) TE polarized light
and (b) TM polarized light. The refractive index of the scintillator and the light extracting
material was set to 1.82.
89
The vertical border at an angle of 33.3' from the critical angle separates the high
transmission area on the left side and relatively low transmission area on the right. Less
light is transmitted beyond the critical angle when the pitch is smaller since little light can
be extracted due to TIR. In particular, when the pitch is smaller than half of the
wavelength, there is no light extraction after the critical angle, as also shown in Fig. 42(a).
More light transmission is observed upon increasing pitch, even beyond the
critical angle due to the diffraction effect. The optical transmission distribution is divided
by additional colored boundaries located across different diagonal directions all along the
graph. It is noted that the lines drawn by Eq. (4-4) are perfectly matched with the
boundary lines, shown in Figs. 4-6(a) and 4-6(b), depicted from numerical calculation,
which clearly proves that the complicated distribution of light transmission through
conical photonic crystals are mainly governed by different diffraction orders. The total
number of diffraction modes and their kinds generated with certain pitch and angle
determine the amount of light extracted through the conical photonic crystal surface.
Even though the complex transmission distribution makes it difficult to select an optimal
pitch for the best possible light extraction efficiency over all the emission angles, we
know where the boundaries originate and what the relationship is between key parameters
such as the pitch and the angle. We can also analyze which diffraction orders contribute
to the total transmission and what parameter we have to choose to increase diffraction
efficiency, which will eventually contribute to light extraction efficiency.
4.2.2.3. Effects of refractive index of light extracting layer
In addition, the refractive index of the light-extracting-layer is an important factor
determining the light extraction efficiency. Generally speaking, the higher the index, the
better transmission as the light transmission is largely reliant on the diffraction effect,
which can be enhanced by a higher index contrast between the nanostructure material and
environment. While some gain is expected even with a low refractive index material, for
example n = 1.5, the gain becomes even higher when we increase the refractive index to
90
2.3, as shown in Fig. 4-7. However, the practical refractive index of the light-extractinglayer is limited by the actual material we can utilize. For example, if the structure is
coated in a form of an imprinted film made of polymer, the practical limit of the layer's
refractive index is at most nmax = 1.67 at the wavelength of 420 nm, assuming the
absorption coefficient of the polymer is almost zero [17]. Nevertheless, if a higher index
polymer with minimal absorption can be developed, such as a TiO 2 mixed polymer [18],
or if an inorganic material such as Si 3N 4 can be patterned into conical photonic crystal
0.4
-
easily, a higher gain is expected.
Refractive Indices
-
0.35
1.5
"1.6
C
0
.0,
-1.7
0.3
-1l.8
Eo
I
0.25
1.9
--- 2
"2.1
-2.2
0.2
0.15
-- --
0.1
0.3
2.3
-
Flat scintillator
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
Pitch (um)
Figure 4-7. Effect of refractive indices on light transmission through conical photonic
crystals (H = 0.8 [tm) calculated using RCWA. All the emission and azimuthal angles (0'
< Oe < 90, 00 <-
Ouaimuthal <- 900) and polarization components (TE and TM) are
incorporated into the simulation.
91
4.3. OPTIMIZATION USING RCWA
While the pitch of the conical photonic crystal is the most critical parameter to
determine the diffraction behavior of the conical photonic crystal, the height can also
affect the diffraction efficiency and hence affect the total transmission of light. If the
height is too low, the surface is close to a flat surface and hence the anti-reflectivity or
diffraction effect is small. There should also be an upper limit for the height, as it
negatively affects the diffraction efficiency of the surface if it is too high. Further, there
are additional potential problems related to the height of cones such as a loss depending
on the material absorption and a difficulty in fabricating the structures. In fact, a height of
approximately 2A is close to the optimal value as shown in the optimization results in Fig.
4-8.
Assuming that we utilize the material with a refractive index of 1.67, which is the
practical limit of the polymer that we can utilize [17], we calculated the optimal pitch and
height
of the
conical
photonic
LSO
on an
crystal
oxyorthosilicate, Lu2 SiO 5 : Ce ) scintillator ( nlLSO
=
1.82 and
(cerium-doped
Xenission
lutetium
420 nm ) when
coupled with air and index matching liquid ( nj = 1.5 ).
0.27
2200
0.64
2200
1800
0.25
1400
1000
1800
0.62
E 1400
0.23
-~
0.6
0.6100
c
6000
600.21
200
600
0.58
200
0.5
1.0
1.5
2.0
Pitch (pm)
2.5
0.5
3.0
1.0
2.0
1.5
Pitch (pm)
2.5
3.0
(b)
(a)
Figure 4-8. Optimization results for the pitch and height of the conical photonic crystal as
a light extracting layer on an LSO scintillator surface coupled with (a) air and (b) index
matching liquid (njl = 1.5).
92
In order to optimize the light extraction efficiency, we have to consider isotropic
light generation in the scintillator [3], which means that the generated light incident on
the coupling surface has all incident and azimuthal angles from 0' to 90'. All the
polarization components (TE and TM) should also be included in the simulation. We first
calculated and averaged out the optical transmission for all the azimuthal angles and
polarization components, and then plotted the averaged value of transmission along
different emission angles (0'
0,
90') for a specific pitch and height of the conical
photonic crystal. In addition, we need to compute the probability for the number of
photons incident on the coupling surfaces as a function of incidence angles, which can be
affected by the geometry of the scintillator, refractive indices of materials and wrapping
conditions [10]. The probability factor can be calculated using Monte-Carlo simulation
[10], and then used for weighting each component of incidence angles when calculating
the total number of photons extracted from each geometry.
The results of the optimization processes are shown in Fig. 4-8. When an LSO
scintillator is coupled with air, the geometry with a periodicity I tm and a height 0.7 pm
shows the best possible extraction efficiency, with a gain of approximately 46%
compared to that of a flat scintillator surface. If the scintillator is coupled with an index
matching liquid with the index of ni/= 1.5, the possible gain is estimated as 12% when
the pitch and the height of the structure is approximately 2.5 [tm and 0.7 pm,
respectively. The air coupled case exhibits higher gain than the index matching liquid
case due to a higher index contrast between the light extracting material and ambient.
The optimal geometry ranges over a certain window rather than a single point, which is
beneficial when fabricating the structure because the window will allow a certain amount
of fabrication tolerance.
We can use the same optimization process for various types of light generating
materials with different refractive indices, wavelengths, and coupling environments. For
examples, related applications such as LEDs or OLEDs can utilize the same concept for
enhancing light extraction through the proposed design and optimization process.
931
4.4. FABRICATION PROCESS
The fabrication process for the conical photonic crystal is divided into two main
parts: (a) fabrication of a silicon master mold using laser interference lithography and
subsequent dry etching processes, and (b) replication of the silicon master mold in UV
curable polymers. By combining those two methods, which are large-area compatible and
high-throughput methods, the proposed design can be more practical to real-world
applications. More details about the fabrication process including optimized process
recipes and several experimental issues are discussed in this section.
4.4.1. Master fabrication
First, a silicon master mold, comprising a periodic array of conical photonic
crystals, was fabricated using laser interference lithography and subsequent dry etching
steps with a shrinking mask [19] as shown in Fig. 4-9. Depending on the optimized
geometry in Fig. 4-8, process parameters were carefully chosen and optimized. The pitch
of a square array of cones was controlled by the angle in the laser interference
lithographic system. The height of the structure was tuned by controlling the time of the
subsequent etching processes. Table 4-1 shows the condition used for fabricating the
silicon master mold.
Loft
Coating TrIayera
0
f~
Ow-
Laser Intefence
Lithographiy
Developing Photoresist
Etching ARC & S102
Etching S
(via shrinking mask)
Figure 4-9. Schematic representation of the master mold fabrication process consisting of
laser interference lithography and subsequent shrinking mask etching.
94
Table 4-1. Experimental conditions for fabricating silicon master mold
Step
Tri-layer coating
Laser Interference
Sub-Step
Si0 2 coating
ARC coating
i-CON-7
Spin coating: 3000 rmp for 60s
Thickness :70 nm
Hard baking: 180'C for 60s
Photoresist coating
PFI-88 A2
Spin coating: 3000 rpm for 60s
Thickness: 220 nm
Soft baking: 90'C for 60s
Double Exposure
0= sin-1 (A / 2P)
lithography
Etching I
(ARC and SiO2 )
Etching II (Si)
Conditions
Thermal evaporated
Thickness: 120 nm
0= 13.42'
Exposure dose = 26 ptJx 2
Developing
Develop: Immerse in CD26 for 60s
Hard baking: 1 10 C for 60s
ARC etching
02 plasma etching
for 30s
SiO 2 etching
CF 4 etching
for 5 minutes
Si Etching
Cl 2 (+Ar), 40sccm, 20mtorr, 100W
for 25 minutes
95
4.4.2. Replication process optimization
The fabricated master was then imprinted into two UV curable polymers with
different refractive indices as shown in Fig. 4-10. The fabricated master mold was first
replicated into the low index polymer (PUA, 311 RM, Minuta Tech.), which worked as a
replica mold. After placing the master mold in contact and pushing it onto the prepolymer, dispensed via syringe on a fused silica substrate, UV light (Tamarack UV
exposure system; with peak wavelength and intensity of 365 nm and 4.5 mW/cm 2
respectively) radiates on it. Then the demolding process was performed to complete the
first replication process. A silane-type adhesion promoter layer was applied on the fused
silica substrate to enhance the adhesion between the imprinted PUA and the substrate.
Using the inversely imprinted PUA surface as a replica mold, an additional
replication process was performed to create the upright conical photonic crystal in the
high refractive index polymer (L2061 B, ACW). This second replication was carried out
in order to coat a scintillator surface with the designed conical photonic crystal structure.
A vacuum-assisted filling process was employed when the PUA replica mold was pressed
in the second pre-polymer in order to fully fill the nanotextured surface with viscous high
refractive index polymer (v ~ 2000cps at 25C) during the second imprint step.
The refractive indices of polymers used here are 1.52 for 311 RM and 1.64 for
L2016B, respectively, at the wavelength of 420 nm, the peak emission wavelength of a
LSO scintillator. More details of the fabrication conditions are shown are Table 4-2.
96
Mold
Adhesion
layer
Conical PhC
Replica Mold
(B)
(A)
Figure 4-10. Schematic representation of the imprint process the conical photonic crystals
replicated from the original silicon master.
97
Table 4-2. Experimental conditions for imprinting conical photonic crystals
Step
Conditions
Ist imprint
Resist: 311 RM
Substrate: Glass slide
Adhesion : Minuta Primer, 11 5'C Hot plate for 15 minutes
UV curing:
365nm, 4.5mW/cm 2 for 1 minutes
Vacuum assisted filling: 10 minutes in 50'C
Pressing Force: No
2
"d imprint
Resist: L2061-B
Substrate: Scintillator (LSO)
Adhesion : Minuta Primer, 11 5'C Hot plate for 15 minutes
UV curing:
365nm, 4.5mW/cm 2 for 3 minutes
Vacuum assisted filling: 10 minutes in 50'C
Pressing Force: ~ 5N on 7.5x7.5 mm 2 for
Demolding: Optional
98
IOs
4.5. CHARACTERIZATION AND DISCUSSION
4.5.1. Fabrication results
In Fig. 4-11 we show scanning electron microscope (SEM) images of silicon
molds of conical photonic crystals fabricated using laser interference lithography with
varying height. Replicated structures on a PUA surface, which are coated on a scintillator
surface, are shown in Fig. 4-12. As shown in Fig. 4-13, the nanotextured surfaces exhibit
reduced reflection on the top surface compared to a flat scintillator surface. The vacuumassisted nanoimprinting used here shows excellent replication quality. The fabricated
nanostructures have a pitch of 0.7 um and a height of lum, which are close to the
optimized geometry calculated from numerical simulation.
More characterizations for light extraction properties are performed qualitatively
and quantitatively in Section 4.5.2 and 4.5.3.
99
(a)
(b)
(c)
(d)
(e)
(f)
Figure 4-11. SEM images of fabricated silicon master molds consisting of tapered
nanostructures. The pitch of the structures is 700 nm, and heights are varied depending on
the fabrication conditions. The heights of the nanostructures are (a) 170 nm, (b) 260 nm,
(c) 320 nm, (d) 770 nrm, (e) 1000 nm and (f) 830 nm.
(a)
(b)
(c)
Figure 4-12. SEM images of (a) top view and (b), (c) side view of a replicated PUA
polymer from a silicon mold fabricated using laser interference lithography. The pitch of
the nanostructure is 700 nm with the height h I !pm.
100
(a)
(b)
Figure 4-13 Images of (a) a flat scintillator surface and (b) nanotextured film on the
scintillator. Reflectance on the top surface is drastically reduced in the case of
nanotextured surface (b).
101
4.5.2 Qualitative characterization
Using the fabricated photonic crystal sample, enhanced light extraction is
qualitatively tested using a white light source and a projection screen. Fig. 4-14 shows the
experimental setup used in this qualitative test.
First, white light is illuminated to the side of the scintillator, so that the incidence
angle of the refracted light in the scintillator on the coupling surface is larger than the
critical angle. Then, depending on types of coupling surface, light can be either projected
on the screen or not.
Fig. 4-15 illustrates the diffraction effect of the fabricated sample on a scintillator
surface. If the coupling surface is textured with periodic nanostructures, a rainbow color
pattern is seen on the projection screen (Fig. 4-15(b)) as a result of diffraction from the
nanostructures despite the incidence angle being larger than the critical angle on the
coupling surface. On the other hand, if a flat surface is used as the coupling surface, the
coupling surface and the screen looks black in the same situation (Fig. 4-15(c)), which
implies that there is no light output through the surface due to TIR.
Scintillator
Diffracted
light
ures
TR
Incident
beam
White
Projection
light
Projection Screen
Screen
(b)
(a)
Figure 4-14 (a) A schematic of diffraction effect test for nanotextured scintillator surface
and (b) the picture of experimental set-up.
102
(a)
(b)
(c)
Figure 4-15 (a) An image of the projection screen placed in front of a scintillator under
the room-light condition; (b) Projection screen when the coupling surface is coated with
nanostructure and (c) without nanostructure.
4.5.3. Quantitative measurement of light yield using PMT
The performance of the fabricated samples is also quantitatively characterized
using gamma-ray source (Co-57, 511 keV) and photomultiplier tubes (PMTs) as shown
in Fig. 4-16.
First, the gamma-ray source is placed on the flat top of the scintillator sample,
while the nanotextured surface faces down to the PMT window. The light yield was
measured when the nanostructure is coated on a coupling surface, and the results are
compared to the light yield of a flat scintillator surface. Two types of conical
nanostructures shown in Table 4-3 were tested in the light yield measurement. Sample #1
and #2 have similar geometries with a pitch of 0.7 ptm and a height of approximately 1
tm. Main difference between two samples is whether the structure is symmetrical (i.e.
the same side view of nanostructure from different orthogonal direction). While sample
#1
has a symmetrical square array of nanostructures, sample #2 has asymmetrical
nanostructure. The nanostructures of sample #2 are not fully isolated along one direction
due to the different exposure dose used for that direction in the LIL process. The light
yield was measured under the condition where the surface was the coupled with either air
or optical matching fluid.
103
gamma-rays (Co-57)
Light
Extracting
Layer
-
Source : gamma-rays, Co-57 (511 keV)
-
Scintillator: LSO, peak wavelength of 420nm
-
Photodetector: Photomultiplier tubes (PMT)
-
High index polymer: L2061B (n = 1.64 @ A=420nm)
-
Low index polymer: 311RM (n = 1.52 @ A=420nm)
-
Glass: fused silica
-
Light extracting layer: Conical photonic crystals
Figure 4-16. A schematic representation of scintillator - coupler - PMT stack for
measuring light yield through the conical photonic crystals and the condition for
measurement.
104
Table 4-3. Summary of the samples used in qualitative characterization. Sample #1 has a
symmetrical square array of nanostructures, and sample #2 has an asymmetrical arrays.
Pitch
Sample #
Height
HI Polymer
LI Polymer
SEM
(x & v view)
Sample #1
(symmetric)
0.7 ptm
0.9 gm
I
Sample #2
(asymmetric)
0.7 ptm
L206 1-B
31 iRM
(n = 1.64
(n = 1.52
@ 420 nm)
@ 420 nm)
0.9 ptm
(x view)
i AI
105
It 1; .1111 A
(a)
93.0%
:)
1
1.2
17.3%
9.0%
E
E
o 0 .8
-6
-
t
Cone
Cone
Flat Flat -
PhC - Air
PhC - Grease
Air
Grease
0 0 .4
z
0
50
250
150
450
350
Chanel No.
650
5E 0
750
(b)
74.2%
1.2
11.1%
E
(U
6.4%
---- Cone PhC - Air
-- Cone PhC - Grease
Air
- -.-.
-Flat
Grease
.
0.8
E
-Flat
-
-
U)
0 0.4
z
I
.
--
0
50
150
250
450
350
Chanel No.
550
650
750
Figure 4-17. Light yield enhancement quantified for the scintillators coated with conical
photonic crystals when coupled with air and an optical matching fluid. (a) symmetrical
conical shape case and (b) asymmetrical conical shape.
106
Measured results are plotted in Fig. 4-17 along different channel numbers and
normalized counts. In the graph, since the position on the x-axis represents the relative
light gain, I fit the curve with Gaussian distribution whose variables are shown in Table.
4.4.
Both samples #1 and #2 show enhanced light yield compared to the flat surface,
while sample #1 exhibits more gain. For sample #1, the relative gain compared to a flat
surface is 17.3%s in the air-coupled case, and 9% in the optical-matching-fluid-greasecoupled case. For sample #2, the relative gains are 11.1% and 9% for air-coupled case
and grease-coupled case respectively. The difference may be from the different
geometries of the two samples. Due to the lack of periodicity along one direction in
sample #2, the diffraction effect which is the main mechanism of the light extraction
beyond the critical angle becomes week, which in turn results in lower enhancement. The
discrepancy between the measured data and calculation values is possibly from difference
between optimized design and fabricated geometry as well as polymer's absorption, but
the positive gain from the conical photonic crystals confirms enhancement of the light
extraction by overcoming TIR through the conical photonic crystals.
Table 4-4. Gaussian fitting of the light yield curves and corresponding relative gain in
light yield.
w
A
R2
Flat surface - Air
231.3
42.1
247891
0.998
Conical PhC - Air
257.0
47.3
501253
0.996
Flat surface - Grease
374.6
60.8
408066
0.996
Conical PhC - Grease
398.7
67.0
239396
0.995
107
Gain
-
xC
+11.1%
-
Coupling
+6.4%
4.6. CONCLUSION
I demonstrate that the proposed conical photonic crystal can serve as an efficient
light extracting layer by keeping both anti-reflection and diffraction effects. The tapered
conical geometry suppresses Fresnel reflections at the interfaces due to adiabatic
impedance matching from a gradient index effect. Periodic arrays of nanocone structures
with pitches larger than the wavelength of light diffract light into higher-order modes
with different propagating angles, enabling certain photons to overcome total internal
reflection. The main concept of the proposed structure was verified using the finitedifference time-domain (FDTD) method, in which our results show simultaneous light
yield gains relative to a flat surface both below and above the critical angle. Further
analysis shows how key parameters affect the light extraction efficiency calculated using
rigorous coupled wave analysis (RCWA).
The enhancement of light extraction efficiency of the conical photonic crystals is
further verified by fabrication and characterization. The optimized geometry was
fabricated using a combination of laser interference lithography and nanoimprint method.
The fabricated sample exhibits 17% gain over air-coupled scintillator and 9% gain over
grease (93% gain compared to a flat scintillator coupled with air) experimentally. Further
improvement is expected with higher refractive index polymer such as TiO 2 mixed
materials [18]. The gain that is experimentally obtained here supports the concept of the
conical PhC and the physics behind it. Additional demonstration of enhancing light
extraction using conical photonic crystals compared with other cases is also demonstrated
in Fig. 4-18.
The design described here is potentially applicable to a wide range of light
emitting materials. By using the same concept of design and the optimization process, the
conical photonic crystals can play a part in increasing the light extraction efficiency not
only for scintillators, but also for LEDs and OLEDs.
108
Figure 4-18. Demonstration of scintillating mode of different nanostructured scintillators.
UV light (A = 365nm) is illuminated on LSO scintillators coated with and without
different types of nanostructures such as GRIN, PhC and conical PhC structures.
109
REFERENCES
1.
M. Kronberger, E. Auffray, and P. Lecoq, "Improving light extraction from heavy
inorganic scintillators by photonic crystals," IEEE. T. Nucl. Sci. 57, 2475-2482
(2010).
2.
P. Pignalosa, B. Liu, H. Chen, H. Smith, and Y. Yi, "Giant light extraction
enhancement of medical imaging scintillation materials using biologically
inspired integrated nanostructures," Opt. Lett. 37, 2808-2810 (2012).
3.
M. Kronberger, E. Auffray, and P. R. Lecoq, "Probing the concepts of photonic
crystals on scintillating materials," IEEE T. Nucl. Sci. 55, 1102-1106 (2008).
4.
D. H. Kim, C. 0. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone,
Y. Park, W. J. Choi, and Q. H. Park, "Enhanced light extraction from GaN-based
light-emitting diodes with holographically generated two-dimensional photonic
crystal patterns," Appl. Phys. Lett. 87(2005).
5.
K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Nanotextured silica surfaces with robust superhydrophobicity and
omnidirectional broadband supertransmissivity," ACS nano 6, 3789-3799 (2012).
6.
J. G. Kim, H. J. Choi, K. C. Park, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Multifunctional inverted nanocone arrays for non-wetting, selfcleaning transparent surface with high mechanical robustness," Small 10, 24872494 (2014).
7.
H. Kasugai, K. Nagamatsu, Y. Miyake, A. Honshio, T. Kawashima, K. lida, M.
Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Kinoshita, and H. Shiomi, "Light
extraction process in moth-eye structure," Phys. Status Solidi. C 3, 2165-2168
(2006).
8.
J. Rao, R. Winfield, and L. Keeney, "Moth-eye-structured light-emitting diodes,"
Opt. Commun. 283, 2446-2450 (2010).
9.
A. Knapitsch, E. Auffray, C. W. Fabjan, J. L. Leclercq, X. Letartre, R.
Mazurczyk, and P. Lecoq, "Results of photonic crystal enhanced light extraction
on heavy inorganic scintillators," IEEE T. Nucl. Sci. 59, 2334-2339 (2012).
110
10.
A. Knapitsch, E. Auffray, C. W. Fabjan, J. L. Leclercq, X. Letartre, R.
Mazurczyk, and P. Lecoq, "Effects of photonic crystals on the light output of
heavy inorganic scintillators," IEEE T. Nucl. Sci. 60, 2322-2329 (2013).
11.
J. J. Wierer, A. David, and M. M. Megens, "III-nitride photonic-crystal lightemitting diodes with high extraction efficiency," Nat. Photonics 3, 163-169
(2009).
12.
J. J. Kim, Y. Lee, H. G. Kim, K. J. Choi, H. S. Kweon, S. Park, and K. H. Jeong,
"Biologically inspired LED lens from cuticular nanostructures of firefly lantern,"
Proc. Nati. Acad. Sci. USA 109, 18674-18678 (2012).
13.
R. Magnusson and T. K. Gaylord, "Diffraction regimes of transmission gratings,"
J. Opt. Soc. Am. 68, 809-814 (1978).
14.
M. G. Moharam and T. K. Gaylord, "Diffraction analysis of dielectric surfacerelief gratings," J. Opt. Soc. Am. 72, 1385-1392 (1982).
15.
M. G. Moharam and L. Young, "Criterion for Bragg and Raman-Nath diffraction
regimes," Appl. Opt. 17, 1757-1759 (1978).
16.
H. Ichikawa and T. Baba, "Efficiency enhancement in a light-emitting diode with
a two-dimensional surface grating photonic crystal," Appl. Phys. Lett. 84, 457459 (2004).
17.
R. Morford, W. S. Shih, and J. Dachsteiner, "Press-patterned UV-curable high
refractive index coatings - art. no. 612301," P. Soc. Photo-Opt. Ins. 6123, 1230112301 (2006).
18.
A. Pradana, C. Kluge, and M. Gerken, "Tailoring the refractive index of
nanoimprint resist by blending with TiO2 nanoparticles," Opt. Mater. Express 4,
329-337 (2014).
19.
S. Dominguez, 1. C. Bravo, J. Pdrez-Conde, H. J. Choi, J.-G. Kim, and G.
Barbastathis, "Simple fabrication of ultrahigh aspect ratio nanostructures for
enhanced antireflectivity," J. Vac. Sci. Technol. B 32, 030602 (2014).
111
Chapter 5.
Conclusion
I have analyzed and manipulated the photonic nanostructures, identified the key
parameters that improve light transport efficiency, and enhanced the adaptability to
industrial manufacturing. Seeking to exploit the potential of nanostructures to modulate
optical behavior on their surfaces, I have achieved broadband, omnidirectional antireflectivity and light extracting from high refractive index materials using novel
nanostructured surfaces. High mechanical robustness and additional functionalities were
added to the photonic nanostructures to make them more practical.
I first demonstrated a replication-based approach using a UV-photocurable
polymer capable of mass-producing anti-reflective nanostructured films consisting of
periodic arrays of inverted nanoholes in an egg-crate structure. These textured structures
have superior anti-reflectivity compared to flat fused silica glass surfaces and enjoy
greater mechanical robustness than our earlier approach[l1]. While retaining the high
feature density and high-aspect-ratio characteristics of tapered nanostructures that
provide enhancement of the optical performance, the nanohole arrays also provide high
mechanical robustness regardless of their aspect ratio via stress redistribution across a
broad network of interconnected features. In addition, controllable super-wetting
properties of the inverted nanocone surface are discussed as additional functionalities.
In the multi-optical interfacial surface case, such as in solar cells, the proposed
double-gradient nanostructures for ultimate anti-reflectivity for multi-interfacial surfaces
demonstrate that the gradient refractive index at each optical interface effectively reduces
Fresnel reflection. The proposed design provides a potential to increase the efficiency of
silicon-based devices, such as encapsulated solar cells or photo-detectors, with long-term
duration of enhanced performance of the devices, due to better mechanical robustness
112
and a self-cleaning property on the top surface, which is textured with inverted
nanocones.
Lastly, a conical diffraction grating/photonic crystal as a highly efficient light
extraction layer was developed, attempting to balance Fresnel reflection and total internal
reflection when light is generated inside of high-index materials. The tapered conical
geometry suppresses Fresnel reflections at the interfaces due to adiabatic impedance
matching from a gradient index effect. Periodic arrays of nanocone structures with
pitches larger than the wavelength of light diffract light into higher-order modes with
different propagating angles, enabling certain photons to overcome total internal
reflection.
I theoretically analyzed the light extraction efficiency of the designed
structure, and optimized it using numerical method. The results were characterized on a
scintillator application. The main concept of the proposed structure was verified using
numerical methods, in which our results show simultaneous light yield gains relative to a
flat surface both below and above the critical angle. The gain was experimentally
verified, which supports the concept of the conical photonic crystal and the physics
behind it.
In addition, the fabrication method, which is large-area and manufacturingcompatible, makes the proposed design an attractive choice for surface structures useful
to industrial fields related to optics and opto-electronics. By combining laser interference
lithography for a large-area mastering method, and UV-replication for the final
nanomanufacturing tool, we can maximize the throughput for fabricating various types of
novel nanostructures, including inverted nanocones, double-cone structures, and conical
photonic crystals. Advanced nano-replication techniques such as vacuum-assisted filling
and selective delamination methods were also developed to fabricate the nanostructures.
Beyond this work, there are many more unexplored issues and applications. First,
applying the developed nanostructured surfaces together with their manufacturing
method may offer good future applications where efficient light transport is important.
Enhanced transmissivity of inverted nanocones with high mechanical robustness can be
useful for various applications including surfaces of display devices, car windshields,
113
building glasses, stacks of lenses for cameras, optical films and all functional glasses. The
same nanotextured topography can be used for opaque materials to increase light
collection efficiency of solar cells or photonic sensors. By using the design of the conical
photonic crystals, the light extraction efficiency can be enhanced for applications of lightgenerating materials such as scintillators, LEDs, OLEDs, and transparent display devices.
Second, as an additional issue to be explored, nanomaterial such as highperformance quantum dots and high refractive index polymers should be investigated.
For example, high refractive index polymers such as TiO2 mixed materials[2] can
enhance the light yield of light-generating materials. Also, the underlying physics behind
the nano world, as well as simulation tools, have to be more developed in order to fully
utilize interaction mechanisms between light and nanostructures.
Lastly, for extremely low-cost and fast manufacturing, UV-replication method
can be combined with roll-to-roll processes. With such processes, it can be anticipated
that multifunctional surfaces can be fabricated in the form of flexible plastic films and
thus applied conformally as an adhesive tape to a broad range of materials such as glass,
silicon, and other optical plastics. The process is also compatible with curved substrates.
These nanostructures and the ability to continuously manufacture structures using roll-toroll process technology may offer potential for industrial applications that require
combined control of optical properties and manufacturability over large surface areas.
REFERENCES
I.
K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G.
Barbastathis, "Nanotextured silica surfaces with robust superhydrophobicity and
omnidirectional broadband supertransmissivity," ACS nano 6, 3789-3799 (2012).
2.
A. Pradana, C. Kluge, and M. Gerken, "Tailoring the refractive index of
nanoimprint resist by blending with TiO2 nanoparticles," Opt. Mater. Express 4,
329-337 (2014).
114
Appendix A.
Gibbs Free Energy Density for Prediction
of Wetting States on an Inverted
Nanocone Surface
The imbibition of liquid (water) into the inverted nanocone geometry used in this
work leads to a change in the thermodynamic free energy of the liquid droplet-airtextured solid surface system. The gain (or loss) in the overall free energy associated with
the liquid penetration.11 '
21
can be represented in terms of a contour map (Figure 2-3)
showing the relative value of the Gibbs free energy density (G*) with respect to the
normalized penetration depth (z/H) and putative apparent contact angle (,*).
It
should be noted that the location (in z/H) of the lowest Gibbs free energy density point
represents the globally stable wetting state and the corresponding value of 0,* predicts the
apparent contact angle (0*) of a liquid droplet in the system. We employ the formulation
used in previous studies11' 2 1 and numerically compute the change in the Gibbs free energy
density G* with respect to a reference state of Go* at z/H = 0 in consideration of an
inverted nanocone geometry shown in Figs. 2-2B and A-1. (see the Supporting
Information in the work of Tuteja et al.[2 ] for computation steps in a MATLAB@ (The
Mathworks Inc.) code)
G* = y,7R 2 (-2--2cos0* - sin 2 0*(rO cos0,0 +4
R= R(4 / (2- 3cos* + cos' O ))"/
-- 1))
/47R2
(SI)
(S2)
where yil = liquid-vapor interfacial tension, R = radius of the drop in contact with the
surface at an angle ,*, Ro = original radius of drop (at z/H = 0), ro is the roughness of the
115
wetted area and 0, is the area fraction of the liquid-air interface occluded by the solid
texture.
To numerically calculate 0, and ro as a function of z/H, each actual slender
nanohole is approximated as an inverted nanocone geometry. It should be noted that roo,
value becomes Wenzel roughness (r,,) when z/H = 1.
0, = I
-
(j/4) (I- z/H )2
(S3)
2 +1/4
II-_(I-Z/H) ](H/P)
r.=(I-/4)+/2x
(S4)
P
14
101
z
Area / P2
H
T,
Figure A-1. A schematic diagram of an inverted nanocone structure. Solid-liquid and
solid-air interfaces are represented by cyan and red colors, respectively.
Rerefences
1.
A. Marmur, Langmuir 2003, 19, 8343.
A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley, R. E. Cohen, P. Natl. Acad.
2.
Sci. USA 2008, 105, 18200.
116