Supplementary Material
Plasmonic nanohelix metamaterials with tailorable giant
circular dichroism
J. G. Gibbsa) A. G. Mark, S. Eslami, P. Fischer
Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany
Table of Contents
Notes:
(S1) Block copolymer micellar lithography ....................................................................................2
(S2) Helix fabrication ......................................................................................................................2
(S3) SEM images .............................................................................................................................3
(S4) Circular dichroism and optical rotary dispersion measurements ............................................3
(S5) Relationship between helix pitch and helix major radius .......................................................4
(S6) DDSCAT calculations..............................................................................................................4
(S7) SEM images of separations .....................................................................................................4
(S8) Geometrical seeding .................................................................................................................6
(S9) Individual polarization extinction coefficients ........................................................................6
(S10) ORD for spacing experiment .................................................................................................6
(S11) Author contributions ..............................................................................................................7
(S12) References ..............................................................................................................................8
Figures:
(S1) ..................................................................................................................................................2
(S2) ..................................................................................................................................................2
(S3) ..................................................................................................................................................3
(S4) ..................................................................................................................................................3
(S5) ..................................................................................................................................................4
(S6) ..................................................................................................................................................4
(S7) ..................................................................................................................................................5
(S8) ..................................................................................................................................................6
(S9) ..................................................................................................................................................6
a)
gibbs@is.mpg.de
1
Note S1: Block copolymer micellar lithography
Hexagonally arranged and separated Au nanodot
patterns on Si(100) and glass substrates were produced
by block copolymer micellar nanolithography (BCML),
details of which are described in detail elsewhere.1,2 The
patterns consist of well-spaced (~50-100 nm separation),
roughly hexagonally arranged Au nanodots of ~15 nm in
diameter and ~ 6 nm in height. Ideally the nanodots
should be uniformly sized and spaced in a hexagonal
manner, but naturally small imperfections are inevitable.
A top-view SEM of the patterned substrate is shown in
Figure S1.
Fig. S1: Patterned Si(100)
substrate with the BCML
lithography process. Scale bar
100 nm.
Note S2: Helix fabrication
We first start with micellar lithography-patterned Si(100)
wafers or glass cover slips as shown in Figure S1. These
(a)
nanodots serve as the seeding points for growth. The initial
seed pattern dictates helix spacing and the seed size affects
the final helix wire radius, or minor radius. Next, the
substrate is placed in a vacuum chamber (~10-7 mbar) and
positioned in a manner so that the substrate surface normal
makes a very oblique angle (~85°) with the material
incidence direction shown by the lower arrow in Figure S2
(a). The large angle ensures that the growth of material will
be restricted to the nanodots on the substrate surface due to
the shadowing effect.3,4 As is schematically indicated in
Figure S2 (a), the substrate rotates during deposition as is
(b)
necessary for helix formation. The rate at which the motor
spins is controlled by a computer which relates the amount
of material being deposited to the angular frequency of the
spinning substrate; this is the key to forming the helices
Fig. S2: (a) Schematic of
presented in this Letter. The ratio of material thickness to
the
motor-controlled
rotation rate dictates the morphology of the helices, i.e. helix
substrate holder; (b)
pitch, P, and major radius, a. This deposition technique is
tilted view SEM of
often called glancing angle deposition (GLAD).3-5 It should
example helices. Scale
be noted that the substrate must be patterned before
bar 100 nm.
deposition, and for materials with high adatom surface
mobilities such as Au, it is necessary that the substrate temperature to be substantially lowered;
in our system the substrate is contacted to a dewar of ~77 K to reduce this migration of adatoms.
An example cross-sectional SEM image ~100 nm tall Cu/Ag nanohelices on the wafer is shown
in Figure 2 (b).
2
Note S3: SEM images
(a)
Figure S3 shows cross(b)
section SEM images of the
five
samples
of
Cu
(c)
nanohelix
arrays
with
various pitch; the length is
(d)
held constant at ~100 nm. It
is clear from Figure S3 that
the
morphologies
are
(e)
different for each pitch, P =
20, 40, 60, 80, and 100 nm
Fig. S3 Cross-section SEM images of 5 different pitch sizes
in Figures S3 (a)-(e),
for Cu nanohelices.
respectively. The length of
the helix arrays is held
constant in order to reduce effects of broadening of the wire
radius, r, which is typically seen with GLAD films.
Note S4: Circular dichroism and optical rotary dispersion
measurements
α
Circular dichroism (CD) is measured both by a Jasco 800
spectrometer which is typically used for molecular spectroscopy.
It operates with a photoelastic modulator which switches from
LCP to RCP (~50 kHz). We also measure CD statically by
directly measuring transmission of both LCP and RCP, then
taking the difference between the two and dividing by the total
transmission
given
by
the
following
equation
  TRCP  TLCP / TRCP  TLCP . Measurements are made with an
Fig. S4: Helix major
Ocean Optics 2000 spectrometer; polarization states are
diameter
as
a
generated with a linear polarizer oriented at ±45° with respect to
function of helix
the fast axis of an achromatic (400-800 nm) quarter-wave plate.
pitch for Cu helices.
ORD was measured by passing linearly polarized light through
the sample, then rotating another polarizer through a series of
angles, and fitting the photocurrent of a photomultiplier tube (PMT) corresponding to the light
intensity at a constant voltage. These data were fitted to a I  cos 2  curve using Matlab to find
the minima, i.e. when the polarizers are crossed, where θ is the respective angle between the two
polarizers. A diagram showing how ellipticity is defined for the CD measurements with the Jasco
CD spectrometer is shown in Fig. S4, where the tangent of ellipticity is the ratio of the ellipse's
semi-minor axis to its semi-minor axis.
3
80
Major Diameter (nm)
Note S5: Relationship between helix
pitch and helix major radius
As stated in the main text, for the GLAD
process, there is a linear relationship
between the pitch, P, and the major radius,
a. The relationship is derived by taking
multiple measurements from cross-section
SEM images, like the ones in Figure S3,
with ImageJ. Figure S5 shows the
relationship of a  0.3P0 .
70
60
50
40
30
20
20
40
60
80
100
Pitch (nm)
Fig. S5: Helix major diameter as a
function of helix pitch for Cu helices.
Note S6: DDSCAT calculations
Since Maxwell’s equations for arbitrarily-shaped small particles have no general solution,
numerical simulations are required to perform analysis on the absorption and scattering of light
in the VIS-NIR. We implement a FORTRAN package available from B.T. Draine and P.J. Flatau
called DDSCAT6 to calculate scattering and absorption of our nanohelices in the discrete-dipole
approximation (DDA). It is necessary to provide appropriate locations and polarizabilites for
DDA calculations. DDSCAT returns the extinction coefficient, Qext, which is appropriate to
compare to our data as the total extinction is actually measured. The scattering cross section is
2
given by   Qaeff
in which aeff is the effective radius of the target. An example geometry used
in the calculations is shown in Fig. S6.
Note S7: SEM images of separations
Figure S7 shows SEM top-view images of the five
nanodot separations presented in the text. The series
shows SEM, each image's corresponding FFT, and the
measured separations for each spacing, shown on the
right side of the figure. The purpose of the figure is to
show that the spacing of the nanodot seed patterns
correspond to an increase in helix spacing, as expected,
until reaching around φ ~ 80 nm when the seeding
begins to fail and helices begin forming between seeding
points. The corresponding spectra and descriptions
presented in the main text describes how the optical
density increases for φ>80 and the reason can be seen
here in this figure. The seeding is only effective for φ ~
50-80 nm, although in this range, varying the spacing
does work well, giving another way of tuning the optical
activity of our nanohelix-arrays.
4
Fig. S6: Helix shape used
discrete dipole approximation.
nm
nm
nm
nm
nm
Fig. S7 SEM top view of various separations shown with FFT of the
images. The measured separations are shown on the right.
5
Note S8: Geometrical seeding
0.7
Trans. (%)
The following presents geometry
53 nm
considerations of the deposition
0.6
technique,
which
relies
on
geometrical shadowing. Minimum
0.5
effective separations have been
calculated as   h tan   d , in
0.4
which h is the seed height, γ in this
case is the deposition angle (usually
RCP
α, here not the ORD angle), and d is
0.3
LCP
the diameter of the seed.7 For
400
500
600
700
800
spherical nanodots in the main text
8
with h ~6 nm, d ~14 nm, and a
 / nm
Fig. S8: total transmission for RCP and
deposition angle of 85°, the above
LCP for the φ = 53nm spacing substrate
  83 nm.
analysis
suggests
showing extinction of RCP to LCP is
Although helix formation according
greater for left-handed helices;
to these numbers should still be
restricted to the seeds even for φ =
81 nm, we see the surface number density of helices is actually higher than the nanodot seed
density, which is consistent with top-view SEM images in Note S7. This discrepancy is
explained by variations in h, d, φ, and γ, and the material that inevitably deposits between seeds
at high φ. We therefore conclude that for shadowing growth on h ~6 nm seeds, φ ~ 80 nm is an
upper limit. However, h can be adjusted in BCML by varying the polymer molecular weight.
Note S9: Individual polarization
extinction coefficients
Note S10:
experiment
ORD
for
spacing
ORD measurements for each spacing
presented in the main text are shown in
Figure S9.
ORD(deg.)
Figure S8 shows the individual
extinction coefficients for RCP and
LCP showing the overall magnitudes
of the two.
4
2
0
 (nm)
53
62
69
76
81
-2
-4
300
400
500
600
 / nm
700
800
Figure S9: ORD of the five separation for Cu/Ag nanohelix
arrays.
6
Note S11: Author Contributions
John Gibbs wrote the main text and supplementary material, and performed the majority of the
experiments including helix fabrication and spectroscopy. Andrew Mark helped build the
deposition system and contributed greatly to the fabrication set-up, implemented the use of
DDSCAT in the lab, and assisted in writing. Sahand Eslami performed the majority of the
simulation work with the DDSCAT software. Peer Fischer assisted with the writing of the main
text and is the PI of the laboratory.
7
Note S12: References
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Spatz, J. P.; Mössmer, S.; Hartmann, C.; Möller, M.; Herzog, T.; Krieger, M.;
Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 1999, 16, 407.
Glass, R.; Möller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153.
Robbie, K.; Brett, M. J. Journal of Vacuum Science & Technology a-Vacuum
Surfaces and Films 1997, 15, 1460.
Zhao, Y. P.; Ye, D. X.; Wang, G. C.; Lu, T. M. Nano Letters 2002, 2, 351.
Hawkeye, M. M.; Brett, M. J. Journal of Vacuum Science & Technology A:
Vacuum, Surfaces, and Films 2007, 25, 1317.
Draine, B. T.; Flatau, P. J. Journal of the Optical Society of America a-Optics
Image Science and Vision 1994, 11, 1491.
Jensen, M. O.; Brett, M. J. Ieee Transactions on Nanotechnology 2005, 4, 269.
Glass, R.; Moller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153.
8