M.Sc. Project Report – Semester II
Preparation and characterization of silicon
nanoparticle by laser ablation in solution
Submitted By
Dibyendu Dey
Y0121
Under the guidance of
Prof. Ramesh Chandra Budhani
Department of Physics
Indian Institute of Technology, Kanpur
Certificate
This is to certify that Dibyendu Dey (Y0121) has worked on the project
titled “Preparation and characterization of silicon nanoparticle by laser
ablation in solution”. The entire work has been carried out the Condensed
Matter Low Dimensional Material Laboratory.
Dr. R. C. Budhani
Department of Physics
IIT Kanpur
2
Contents
1 Abstract
4
2 Introduction
4
2.1 Nanoparticle and Nanotechnology ………………………………………...4
2.2 Quantum Confinement Effect in Silicon Nanoparticle…………………….4
3 Experiment
6
3.1 Previous Set-up……………………………………………………………..6
3.2 Modification in Previous Set-up…………………………………………....6
3.3 New Set-up………………………………………………………………….6
4 Experimental Apparatus
8
4.1 Nd-YAG Laser ……………………………………………………………..9
4.2 Surfactant …………………………………………………………………..9
4.3 PL apparatus ………………………………………………………………..9
4.4 Spectrometer ………………………………………………………………10
5 Results
11
5.1 Absorption Spectrum ……………………………………………………..11
5.2 PL measurement…………………………………………………………..12
5.2.1 PL measurement of nanoparticle sample in solution…………....12
5.2.2 PL of the dried sample…………………………………………..13
5.3 TEM measurement………………………………………………………...14
5.3.1 TEM of sample in SDS solution………………………………...14
5.3.2 TEM of sample in water………………………………………...15
6 Discussions
16
7 Conclusions
17
8 References
17
3
1.0 Abstract
Silicon nanoparticles are prepared by laser ablation of boron-doped p-type
crystalline silicon wafer in both water and SDS (sodium dodecyl chloride) solution. I also
present the Photo-Luminescence, absorption and particle size property of silicon
nanoparticle. The PL measurement shows a wide spectrum and the tunneling electron
microscopy indicates a particle size of nano-scale order.
2.0 Introduction
Nanotechnology - a broad term, covering the use of techniques to manipulate
matter at an atomic scale. The drastic change in properties in particle having nanosize is
Quantum Mechanical in nature and Quantum Mechanics is a magic! Thus Nanotechnology
is a real revolution because it is changing in a fundamental way how we build things. For
example, if we take a hard material, clay or a ceramic, and powder it down to the
nanoscale, and mix it with a polymer, we may land up getting something which has a
combination of hardness and toughness never seen in the world before. Thus this
technology is growing interests in leaps and bounds. Other features that contribute to
nanotechnology's promise are the expectation of cheap, low polluting mass manufacturing
and the possibility of making things, on the scale of biological building blocks, which
could imitate living systems. So far, most applications involve enhancements of preexisting
materials, but new developments are in the works. Thus nanoscale material fabrication
receives intensive scientific interests as a dynamically growing field of material science.
2.1 Nanoparticle and Nanotechnology
The term “Nanoparticle” is generally used in the area of material science
to represent particles of size less than 100 nanometers, lying intermediate between the
molecular state and bulk state.
Nanoscale material fabrication receives intensive scientific interests in the
growing field of material sciences. This is due to the wide applicability of nano-materials
that exhibit unique properties, usually differing from the corresponding bulk counterpart.
Along with the wide range of applicability, nanoparticle can also be synthesized by a gamut
of different processes like laser-assisted chemical liquid-phase decomposition [1], thermal
decomposition [2], chemical reaction [3], microemulsion method [4], laser ablation [5-8]
etc. Each method has its own importance in terms of control of particle size, cost efficiency
and experimental complicacy. Laser ablation is one of the most cost efficient and easiest
technique for the preparation nanoparticle.
2.2 Quantum Confinement effect in Silicon Nanoparticle
Crystalline silicon being ubiquitous in microelectronic industry is an indirect
bandgap material and therefore incapable of light emission. But for nanostructure porous
silicon, amorphous silicon shows photoluminescence properties. It has created significant
interests in forming nanoparticle of semiconductor materials. It also promises a revolution
in miniaturization of microelectronic industry.
4
Small colloidal semiconductor particle have properties that deviate substantially
from those bulk materials. For semiconductor materials when electrons get excited from the
valence band and moves to the conduction band forming a hole, an electron-hole pair is
formed which is termed as excitons. The electron and hole of the excitons are generally
delocalized over a length (order of Bohr radius) much larger that lattice constant, for
example the exciton Bohr radius for silicon is 5nm. As the size of the semiconductor
cluster approaches a length comparable to this exciton diameter, its electronic properties
start to change substantially and it is known as quantum size effect [9-11]. This can be seen
from the blue shift (increase) of optical band gap as well as enhancement in absorption
cross section. For a small cluster of nanoaprticles, having a few thousands of silicon atoms,
we can model it to be a 3-d potential well where the energy gap between the discrete levels
increase with decreasing diameter of the cluster. Thus the energy gap decreases as the
diameter of nanoparticle gets increased and eventually it becomes continuous as in bulk
materials. This explains the blue shift of band gap as the particle size decreases.
Photoluminescence occurs when electron in conduction band combines with the
holes in the valence band emitting a photon and it happens due to relaxation of electron
with emission of photon. The formation of electron-hole pair is caused by the excitation
from external source of light. Crystalline bulk silicon has an inefficient photo-luminescence
because of the indirect band. For indirect band gap materials, it is required to have a
phonon density with certain minimum momentum for the transition to take care of the
conservation of crystal momentum. As the particle size decreases we can expect to have an
enhancement in photoluminescence and it has been explained by many models eg quasidirect transition [12], Quantum confinement effect [13] etc.
As we know, the band gap increases with the decrease of size, and it may put a
question over the prediction of enhancement of photo-luminescence with decreasing
particle. This can be explained from the Quantum confinement effect in nanocluster of the
silicon nanoparticle. In case of nanoclusters the electrons are confined into a smaller size,
so the energy levels become discrete and the minimum energy for the system becomes
higher than any bigger system. This particular situation can be termed as Quantum
Confinement (QC). For metals the effects of QC isn’t visible because there the electronhole interaction (excitons) is screened off and the electrons behave as merely free particle.
According to QC-effect luminescence is attributed to radiative recombination of
band to band electron-hole pair, confined within such nanoparticle [14]. For nanoclusters
the quantum confinement factors plays a major role in increasing the radiative
recombination rate of the excitons (electron-hole pair). From the expression of the
relaxation time [14] we can see that as the energy between electron-hole pair increases the
relaxation time for the radiative recombination decreases. In other words it enhances the
probability of radiative recombination. For lower band gap energy the “radiative
recombination” become quite low and thus for crystalline bulk silicon radiative
recombination is absent. Thus we can say that the nanocluster acts as materials between a
direct band gap semiconductor and an indirect band gap semiconductor.
Theoretically, thus it be predicted that the photo-luminescence of silicon
nanoclusters is much brighter than a bulk materials with the same power of excitation. But
as the band gap shifts towards higher energy, the excitation frequency increases towards
more UV-range.
5
3.0 Experiment
3.1 Previous Set-up
We have formed the experimental set-up for the laser ablation. It is quite different from our
previous set-up [fig: 1], where the laser is mounted horizontally.
Solution
Target
Nd-YAG
Laser
Fig 1: Schematic Diagram of the Previous Experimental Set-up
3.2 Modification in Experimental Set-up
We have brought quite significant changes from the previous set-up. This makes the laser
ablation process more efficient and controlled.

As in the new set-up [fig:2], the laser is mounted vertically, so the laser light
doesn’t have to pass through the glass surface of the beaker as previously and it
reduces the loss due to absorption .

The laser light hits the target and the created nanoparticles sputters out of the
surface of the metal plate. If the target is kept horizontal, as in our set-up [Fig: 1], it
is always possible that the particle formed from laser ablation can get deposited on
the target again. So we have put a magnetic stirrer which continuously stirs the
solution so that the deposition can be avoided.

We have used a Teflon holder to hold the sample. The sample holder is formed in
such a shape that the sample doesn’t get toppled while ablation.

We also have used a bi-convex lens of focal length 15cm to focus the laser light on
the target of metal peace this is hold on a Teflon holder. In the previous set-up also
we used this lens for focusing.
3.3 New Set-Up
With all those modification mentioned in sec [3.2], the new set-up has been shown
schematically in fig [2].
6
Fig 2: Schematic Diagram of the Experimental Set-up
Nd-YAG Laser
Lens assembly
Target drive
Teflon target
holder
Target
Stirring capsule
Magnetic Stirrer
7
4.0
Experimental Apparatus
4.1 Nd-YAG Laser:
We have used an Nd-YAG pulse laser of maximum power of 10mJ/pulse. The lasing
medium is one of the most important criterions to characterize lasers. Nd-YAG laser is a
type of solid-state laser where neodymium is doped to the yttrium-aluminum-garnet (YAG)
host crystal to make the lasing medium. This type of laser is important because it can
produce high power. The Nd-YAG laser is Q-switched (gives brief high energy pulses
instead of continuous pulse) because of saturable absorber, which only allows the laser
light to move between the mirrors for a very small period of time. There are different dyes
used as a saturable absorber, which becomes transparent for a considerable exposure to
light and becomes opaque again.
1=1064nm;2 = 532nm; 3 =256nm
1/2
Pumping
KTM
3
Fig 3: Schematic diagrams for the Nd-YAG laser’s working principle
One of the most important properties of this Nd-YAG laser is its second harmonic
generation (SHG). The basic principle of second harmonic generation is based on a
non-linear crystal property. When a monochromatic light impinges on a surface with
lack of symmetry, light of frequency twice that of the incident light can get generated.
Fig 4: The energy level diagram of Nd: YAG [17]
8
4.2 Surfactant
We have used a sodium dodecyl sulfate (SDS) as the synthetic surfactant. Surfactant is
needed to control the size of the nanoparticle and for its chemical and magnetic stability.
When the laser hits the metal surface an embryonic metal particle is formed and it is
followed by a slow particle growth. The SDS coating involves in termination of this growth
and thus with increasing SDS concentration we can expect to have a smaller particle size
Si-nanoparticle
Co
SDS
Fig 5: Schematic diagram for the Polymer coating of the nanoparticle
4.3 PL apparatus
I have taken the PL measurement with an apparatus of model number Flurolog3.22. It works mainly on three steps.

The excitation spectrometer filters a particular frequency out of the light source.
It uses a double grating for this purpose, which give high sensitivity.

The particular frequency of light excites the sample inside the sample
compartment and the output is again passed through an emission spectrometer.
It uses the similar principle as the excitation spectrometer to filter light.

The signal is detected by a detector called PMT (Photo multiplier tube), which
is controlled by a computer interface.

By stepping either or both the spectrometer through a wavelength region and
recording the variation of intensity a spectrum is found.
9
Source
Excitation
Spectrometer
Emission
Spectrometer
Sample
P
M
T
Controller
PC
Fig 6: Schematic diagram for Flurolog-3.22
4.4 Spectrometer (Lambda 40 UV/Visible Spectrometer)
This instrument is used to measure the absorption spectrum of the nanoparticle. A
grating (1052 lines/mm) is used to split the source light (UV/Visible) into a particular
wavelength and falls on the sample of nanoparticle solution to measure the absorption
coefficient. Using the absorption coefficient found from a SDS solution only eliminates
the background caused by the SDS solution.
Reference
Detector
Light
Source
Detector
Grating
Beam-Splitter
Sample
Fig 7: Schematic diagram of the spectrometer’s working principle
10
5 Results
I have taken PL measurement of the nanoparticle solution along with TEM
measurement and absorption spectrum.
5.1 Absorption Spectrum
I took absorption spectrum of the silicon nanoparticle in miliq water solution in
FEAT laboratory. They tasted the silicon nanoparticle solution with eliminating the
background using another beaker of only water. The working principle has been shown in
detail in section [4.4]. The error in the wavelength is 0.3nm, which gives the x-axis error
bar, and the error in absorbance (i.e. the y-axis error bar) is 0.015 in maximum.
1.257
1.20
1.15
1.10
1.05
1.00
0.95
0.90
A
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.455
200.0
300
400
500
600
nm
700
800
900
1000.0
Fig 8: absorption spectrum of Si-nanoparticle solution in water
It clearly shows that there is no absorption peak in the visible region, but there
seems to be an absorption in the ultraviolet region. I fitted a Gaussian peak function [fig: 9]
and the fitting correlation is found to be of the order of 98.6%. This clearly indicates a peak
to be in the shorter wavelength region. Due to the effect of Quantum Confinement the band
gap for this silicon nanoparticle solution increases substantially and it can be clearly seen
from the absorption in the UV region. This QC effect becomes more and sharper as the
particle size decreases. But of course for very small particle size (size much lesser than
exciton diameter), this QC-effect again fades away, which hasn’t till been properly
understood [14].
11
Analysis of the absorption spectrum
1.4
Absorbance [a.u]
1.2
Data: TMP_B
Model: Gauss
Chi^2/DoF
= 0.00024
R^2
= 0.98622
1.0
y0
xc
w
A
0.8
0.47159
-78.17596
264.74013
2166.90474
±0.00065
±44.06816
±18.01273
±1024.78823
0.6
0.4
200
400
600
800
1000
Wavelength [nm]
Fig 9: Plot fitted with a Gaussian distribution
5.2 PL Measurement
I first attempted to take PL measurement on the nanoparticles, both in water and
SDS solution. The solution was divided into five parts where each part is oxidized for few
seconds using an Ozoniser and then the PL measurement was taken.
5.2.1 PL measurement of Si nanoparticle solution in water
The nanoparticle solution in water was first scanned over a continuous
wavelength region to see the absorption spectrum and it was found that the excitation
wavelength is 440nm. So the sample was excited with 440nm wavelength and a PL signal
was detected. Initially the PL signal from the solution seemed to be from the Sinanoparticles. But when a PL measurement was taken on sample of miliq water without
any nanoparticle, the same signal was found.
PL measurement of Si nanoparticle in milliq water
10000
PL measurement of Si nanoparticle in milliq water with small slit exposure
600
6000
Peak=511.6nm
Excitation wavelength=440nm
Output
Output
Excitation wavelength:440nm
8000
700
500
400
4000
2000
300
0
200
450
500
550
600
650
700
750
450
500
wavelength(nm)
550
600
650
wavelength(nm)
Fig 10: PL measurement of nanoparticle miliq water solution
12
700
750
Water has a photoluminescence with same excitation wavelength of 440nm and as
the concentration of the nanoparticle solution isn’t very high, the signal form Silicon
couldn’t supersede the water signal to get a PL of the nanoparticle solution. Thus it
naturally evoked the necessity to bring some process of drying the sample out of water. We
also thought of using some other solution which may be implemented in the future work of
this project.
5.2.1 PL of dried Si nanoparticles sample
The low concentration of the sample hindered from getting a PL signal from
silicon nanoparticles as the signal had to supersede the PL signal from the water. So I had
to dry out the silicon nanoparticles out of the miliq water.
I put the solution in a Centrifuge to separate out bigger particle from the solution.
The Centrifuge was operated at speed of 12000rpm for duration of 35minute. It helps
separating major part of the nanoparticles from the solution. The extra water was poured
off and the container is then ultrasonically dispersed to mix the nanoparticles in rest of the
water. Thus the solution becomes quite dense nanoparticle solution. This small amount of
solution is now kept over an aluminum foil and dried using a hot-blower. We have used
aluminum foil because it doesn’t give any photoluminescence. The dried sample was first
excited with a continuous wavelength region and it showed photoluminescence with an
excitation in the UV region (310nm).
PL of dried-Si nanoparticle Sample
PL of dried-Si nanoparticle sample
Slit width:0.1mm
Integration time:0.1ms
Excitation Wavelength:310nm
5500
5000
200000
180000
160000
4000
Output
Output
4500
3500
3000
140000
120000
100000
2500
80000
Slitwidth:0.3nm
Excitation Wavelength:310nm
2000
60000
1500
350
400
450
500
550
350
400
450
500
Wavelength[nm]
Wavelength [nm]
Fig 11: PL of the dried sample of silicon nanoparticle
I have also taken a PL of a simple aluminum foil for the shake of
completeness of the experiment. It doesn’t show any photoluminescence
expectedly.
13
550
300000
:Al Foil
:Sample
Slit Width:0.3nm
Excitation Wavelength:310nm
250000
Output
200000
150000
100000
50000
0
350
400
450
500
550
Wavelength [nm]
Fig 12: PL of the dried sample with an aluminum background
5.3 TEM measurement
I have taken Transmission Electron Microscopy (TEM) of the nanoparticle sample
at ACES. TEM works in the principle that it shines a beam of electron through the Cu-grid.
The beam is collimated to a monochromatic wave, using lenses. When it hits the grid, some
parts get transmitted and some parts get hindered from the particles. Whatever part is
transmitted is projected on a Phosphor screen and thus giving an enlarged image of the
object.
5.3.1 TEM image of Si-nanoparticle in SDS solution
I have already mentioned that SDS (sodium dodeycl sulfate) has been used to form
the polymer coating over nanoparticle [Sec: 4.2]. It helps in separating one nanoparticle
from other. But in the image that isn’t clearly visible. This is due to fault in making the film
over copper grid. Several layers have been formed while our intension was to make single
layer of nanoparticle film. The image was taken in a resolution of 50K; the particle size
can’t be detected due to fault in making the film.
Fig 13: TEM image of Si- nanoparticle in SDS
14
5.3.2 TEM image of Si-nanoparticle in water
As I encountered problem in making film using SDS solution, I used nanoparticle in
water for making film in second attempt. This time the film quality has improved
substantially as can be seen from [Fig:14]. The particle size has been detected to be
between 40-50nm. It was observed that spherical nanoparticles have been struck on the
microgrid.
Fig 14: TEM image of Si-nanoparticle in water
Fig 15: TEM image of Si-nanoparticle in water
15
6 Discussions
Silicon is an indirect band gap semiconductor. Bulk crystalline silicon is an
inherently poor light-emitter. This low-quantum-efficiency of photoluminescence in bulk
crystalline silicon is attributed to the unavailability of phonons needed for an indirect band
gap semiconductor for a transition. However recently, efficient photoluminescence from
bulk crystalline silicon has been reported [15, 16]. Nanocrystalline silicon can be a very
good alternative to get photoluminescence in visible region for such indirect band gap
silicon also. For nanoaparticle of size in the nanometer range, we can consider a 3-d
potential well where the particle has been confined, termed as quantum confinement effect
as mentioned in [Sec: 2.2]. Thus the band gap increases with the size reduction. Thus the
energy of light required for excitation becomes higher and thus get the absorption peak in
UV region as been seen in [Sec: 5.1].
Photoluminescence occurs when electron in the conduction band relaxes to the
valence band emitting a photon. But for indirect band gap semiconductor the radiative
recombination is negligible for the indirect band gap nature. As the particle size decreases
the band structure changes substantially. Thus we can expect to see some different nature
of photoluminescence in this nano-structure. Due to the enhancement of the band gap the
photoluminescence shifts to the visible region and we see PL in room visibly at room
temperature. These phenomena can be explained by quantum confinement effect. The PL is
caused by radiative recombination whose probability is indirectly proportional to the
relaxation time of the electrons in the conduction band. The relaxation time for radiative
recombination rate decreases substantially as the band gap increases. It has been
mathematically shown in ref [14]. Thus we can see the photoluminescence in the visible
region of 400-500nm in Sec [5.2.1]. The small volume of the particle also helps in
confining the electron to a small volume thus the conduction electrons can’t diffuse much.
It hinders non-radiative recombination. Thus the quantum confinement model quite
efficiently explains the recombination mechanism which causes photoluminescence. The
PL is also reasoned by pseudo-direct band transition in some papers [14].
The transmission electron micrograph shows a non-uniform particle distribution
and the spherical particle size is much higher than expected. It is due to the inability to
control the particle size during laser ablation. The higher particle size hinders to see the
larger PL and absorption peak. As the exciton diameter of silicon is of the order of 5nm so
the quantum confinement effect becomes prominent for the particle size distribution of that
order. For higher particle size the QC isn’t that prominent. We must use some gas flow to
form a coating and by controlling the flow, we may be able to control the particle size. This
has already been implemented by Li et al. [18]. In fact the surface of the nanoparticle is
very active and thus many of the silicon nanoparticles become oxidized after ablation. It
decreases the PL peak further.
16
7 Conclusions
In this project, I have successfully synthesized nanoparticle by laser ablation in
solution. In first semester, I synthesized cobalt nanoparticle (magnetic) and this semester, I
have synthesized silicon nanoparticle (semiconductor). I have also been involved in putting
the set-up for forming nanoparticle by laser ablation. I characterized the silicon
nanoparticle and gave possible explanations for the characters of the sample. This project
has exposed me to a wide horizon of research
8 References
[1] PhD thesis by Krisztian Kordas at department of Electrical Engineering at the
University of Oulu.
[2] J Connolly, T G St Pierre, M Rutnakornpituk and J S Riffle, J.Phys.D: Appl.
Phys. 37 2475-2482 (2004)
[3] S Misra and H C Padhi, J.Appl.Phys. 75, 4576-4580 (1994)
[4] K Nagai, K Okamoto, H Kondoh, T Yokoyama, T Ohta, arxiv: 9A/2000G263
[5] M S Sibbald, G Chumanov and T M Cotton, J.Phys.Chem 100, 4672-4678
(1996)
[6] Z Pasti, Z E Horvath, G Peto, A Karacs and L Guczi, Applied Surface Science
109/110, 67-73(1997)
[7] J Zheng, C Zhang and R M Dickson Phys.Rev.Lett 93,077402 (2004)
[8] H Yin, T Yamamoto, Y Wada, S Yanagida Materials Chemistry and Physics 83
66-70 (2004)
[9] Vladimir P. Drachev, E. N. Khaliullin, W.Kim, F Alzoubi, S. G. Rautian, V. P.
Safonov, R. L. Armstrong, and V. M. Shalaev, Phys.Rev. B. 69,035318 (2004)
[10] G. Ledoux , O. Guillois, D. Porterat and C. Reynaud, Phys.Rev.B.62,15942
(2000)
[11] H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, T. Nakagiri,
Appl.Phys.Lett.56, 2379 (1990)
[12] M. S. Hybesten , Phys.Rev.lett.72, 1514 (1994)
[13] G. Ledoux, J. Gong, F. Huisken and C. Reynaud, Appl.Phys.Lett.80,4834
(2002)
[14] C. Delerue, G. Allan, and M. lannoo, Phys.Rev.B.48, 11024 (1993)
[15] T. Trupke et al. App. Phys. Letts. 82, 2996 (2003)
[16] Martin A. Green et al. Nature. 412, 805 (2001)
[17] http://www.lasalle.edu/academ/chem/laser_web/ndyag_laser.html
[18] Li et al. Phase Transition 77, 1-2, PP. 131-137, Jan-Feb (2004)
Acknowledgement
I would like to may my sincere thanks to Prof. R. C. Budhani for offering me such an exciting
Project. I would also like to pay my special gratitude to Mr.Rajeev.K.rakshit, Mr. K. Senapati and to all the
members of our lab. I would also like to thanks Prasant who has helped me to take Pl measurement at Dr. A.
Pradhan’s laboratory and also to FEAT laboratory for their help.
17