1. Page | 0
Synthesis and study of Au deposited CdSe
nanocrystallites & T_NT-Bi-CdSe/CdS nano-
heterostructures
SUBMITTED IN PARTIAL FULFILMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
METALLURGICAL ENGINEERING
By
Kartik Venkatraman (11107EN012)
Sidhant Ray (11107EN039)
DEPARTMENT OF METALLURGICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
BANARAS HINDU UNIVERSITY
VARANASI-221005
INDIA
2014-15

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CENTRE OF ADVANCED STUDY
DEPARTMENT OF METALLURGICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
(BANARAS HINDU UNIVERSITY)
VARANASI-221005
CERTIFICATE
This is to certify that project work entitled “Synthesis and study of Au deposited CdSe
nanocrystallites & T_NT-Bi-CdSe/CdS nano-heterostructures” embodies the work carried out
by Mr. Kartik Venkatraman and Mr. Sidhant Ray under our supervision. This work has
been done for the partial fulfillment of the requirement for the award of the degree of
Bachelor of Technology in Metallurgical Engineering, Indian Institute of Technology, BHU,
Varanasi and no part of this thereof has been presented earlier for any other degree.
Approved By
Dr. N.K. Mukhopadhyay Dr. Bratindranath Mukherjee
Professor DST-INSPIRE Faculty
Centre of Advanced Study Centre of Advanced Study
Dept. of Metallurgical Engineering Dept. of Metallurgical Engineering
Indian Institute of Technology (BHU) Indian Institute of Technology (BHU)
Varanasi- 221005 Varanasi- 221005
Forwarded
Prof. R.K. Mandal
Head of the Department
Centre of Advanced study
Dept. of Metallurgical Engineering
Indian Institute of Technology (BHU)
Varanasi-221005

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ACKNOWLEDGEMENT
We express our sincere gratitude and heartiest thanks to Dr. Bratindranath Mukherjee and Prof. N.K.
Mukhopadhyay, Department of Metallurgical Engineering, Indian Institute of Technology (BHU), for
their guidance and constant encouragement and support during the course of our project work.
We would like to acknowledge the help of Prof. R.K. Mandal, Head, Department of Metallurgical
Engineering, Indian Institute of Technology (BHU), for facilitating TEM and SEM characterisation.
We are also thankful to Prof. N. Ravishankar, Materials Research Centre, Indian Institute of Science,
for giving Kartik the opportunity to work under his guidance during the summers of 2014 and
providing various facilities for experimentation and characterization in IISc during the ongoing
semester.
We also thank our friends who directly or indirectly helped us in our project work and completion of
the report in time.
Last but not the least we would like to thank our families for their selfless support.
Kartik Venkatraman Sidhant Ray
11107EN012 11107EN039

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CONTENTS
Topic Page
No.
Certificate 1
Acknowledgement 2
Contents 3
1. Introduction 4-5
2. Literature Survey 6-10
3. Objectives 11
4. Experimental 12
4.1. Synthesis and characterisation of CdSe and Au deposited CdSe QDs 12-13
4.2. Synthesis of T_NT-Bi-CdSe nano-heterostructures 13-16
5.Results and Discussion 17
5.1. Au-CdSe quantum dots 17-29
5.2. T_NT-Bi-CdS/CdSe nano-heterostructures 29-33
6. Conclusions 34
7. Scope for future work 35-36
8. References 37-38

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1. INTRODUCTION
The study of nanometer sized crystallites helps us understand the evolution of collective
characteristics of bulk materials from the discrete behaviour of molecular properties, which
in turn may help us control this evolution to our benefits, given the different rates at which
each bulk characteristic evolves. For example, non-linear optical effects from highly
polarisable excited states and novel photochemical behaviour. Confinement of electronic
and vibrational excitations due to spatial constraints, Quantum Confinement, wherein there
is an increase in the HOMO-LUMO gap due to decrease in size which affects the electronic
structure and photophysics of the nanoparticles is the dictator of physical properties of
nanocrystallites [1]. Quantum Confinement can be achieved specifically or in combination of
any of the three dimensions. Confinement in 1D gives Quantum Wells, in 2D gives Quantum
Wires and in 3D gives Quantum Dots (QDs). As such, Quantum Confinement has numerous
applications from Quantum Dot Sensitized Solar Cells (QDSSCs) to Quantum Light Emitting
Devices (QLEDs).
QDs show notable characteristics that include tunability of band gap energy, narrow
emission spectrum, good photostability, broad excitation spectra, high extinction coefficient
and multiple exciton generation. The main motivation of using QDs as sensitizers is their
size-tuneable energy band gap, which can control their absorption range. Vogel et al.
demonstrated that efficient charge separation can be optimized by tuning the size of the
QDs utilizing the quantization effect [2]. CdX (X=S, Se, Te) based QDs have become
prominent due to their low cost, easy fabrication and high performance. It has been
reported that lightly transition metal doped CdX QDs have enhanced photoluminescence
(PL) properties of the QDs [3]. Also, it was seen that surface plasmon - exciton coupling in
CdS QDs – Au thin film and CdSe QDs – Ag thin film resulted in enhanced photovoltaic
efficiency [4, 5].
Surface plasmons are electromagnetic waves that are trapped by collective oscillations of
free electrons in a metal and that propagate along the metal surface. This trapping leads to
an exponential decay in the electromagnetic field with respect to distance from the surface.

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The electromagnetic fields near metallic surfaces strongly affect the optical properties of
nanocrystalline semiconductor materials. Thus, modifying the density of surface–plasmon
states may enhance the absorptivity and the photoluminescence emission rate and increase
quantum efficiency of semiconductor nanocrystals [4-6].
Large bandgap semiconductor oxide nanowires with tubular architecture are considered the
most optimal geometry for facilitating better photogenerated charge separation and
transport. Fabrication of a “Rainbow” solar cell consisting of QDs stacked inside a T_NT to
absorb visible light was proposed recently, indicating that stacking QDs of different sizes
shall allow maximum visible light absorbance thereby placing us in the vicinity of
theoretically reachable quantum efficiencies. However, it is important to realize that this
proposed increase in efficiency is not only due to broad absorption of sunlight by differential
absorption from a mixture of quantum dots of varied sizes and bandgap energies, but also
relies on the fact that when QDs with varied bandgaps are very closely packed in order of
higher to lower bandgap energy, a QD of higher bandgap absorbs energy corresponding only
to its bandgap and transmits the remaining lower energy to be absorbed by the next QD.
Thereby, energy loss due to thermalization is avoided and harvesting of the broad visible
spectrum of light can be realized. However, nanotubes of wide bandgap host oxides have
dimension typically 103
times higher compared to the size of the QDs. Thus, the proposed
architecture in this form (vertical stacking) is not practical due to the disparity in the scale of
the deposited QDs and the host oxide nanowire. Radial stacking is a theoretically feasible
idea, but not easy to achieve experimentally. Multi segmented semiconductor
nanorods/nanowires with coupled electronic properties are reported and can be prepared
by sequential VLS/SLS growth or ion exchange methods. Radial growth of these 1D
nanostructures on oxide nanotubes can alleviate the problem of QD stacking and make a
large stride towards “Rainbow” solar cell.
In the present work, we synthesize CdSe QDs, study their excitonic and photoluminescence
properties along with microscopic characterization, and then dope it with Au nanoparticles
to study its effect on the optical properties of the QDs. We also synthesize TiO2 nanotubes
and use Bi nanoparticles as nucleation sites for CdSe nanowires, as well as segmented
CdS/CdSe nanowires with photoelectrochemical properties better than CdS or CdSe alone.

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2. LITERATURE SURVEY
Quantum Confinement in semiconductors
Quantum confinement describes how the electronic properties - the organization of energy
levels into which electrons can climb or fall - and optical properties change when the
material sampled is in sufficiently small sizes - typically 12nm or less. Specifically, the
phenomenon results from electrons and holes being squeezed into a dimension that
approaches a critical quantum measurement, called the exciton Bohr radius. Holes are the
positively-charge species left over when an electron vacates its position in a crystal. This
quantum confinement is observed when the electron is bound by a set of energy or
potential barriers.
When the size of the quantum dots is decreased, the gap between the valence band and the
conduction band increases. It was found that by J.W Lee, D.Y Son et al. [7] that as the size of
CdSe quantum dots (QDs) decrease, there is a blue shift in the absorption or the emission
spectrum. Blue shift corresponds to decreasing wavelength or increasing energy whereas
the Red shift corresponds to increasing wavelength.
When the semiconductors are energised by a photon or energy source, the electrons of
valence band are excited to the conduction band. These electrons require energy equivalent
to the band gap energy or higher to get excited to conduction band. This is the energy
required to free the electrons from its bond and enable it to move and conduct. There exist
discrete energy levels in an isolated atom whereas energy bands exist in solid. The free
electrons in the conduction band and the holes present in the valence band are the charge
carriers. On application of potential difference/electric field, the electron and hole move in
different directions to generate a current.
Excitons
It may so happen that the electrons remain bound to holes (like it is bound to protons in the
nucleus of an atom) and create composite objects called Excitons. This electron and hole
pair orbit each other and the radius of this orbit is Bohr exciton radius. It is the most
fundamental excitation. These excitons are unable to move freely in a quantum confined SC.

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Quantum dots exhibit quantum confinement when size of SC nanocrystals is close or smaller
than the natural size of excitons. For CdSe crystals, as the size of QD decreases, the excitons
get squeezed and wavelength of emission decreases. The excitons can be squeezed either
by building very small size QDs or by engineering layers of SCs such that one layer is coated
with another SC layer of different (usually smaller band gap).
The squeezing of excitons in 1D is achieved in quantum wells, whereas in 2D it is achieved in
nanowires and squeezing of excitons in 3D is achieved in quantum dots.
Interaction of metal nanoparticles with light
It was observed from many experiments that when the size of the holes of a metal plate
through which light is allowed to pass became smaller or comparable to the wavelength of
incident light, then the fraction of light transmitted is greater than the area of the holes.
Sometimes, this fraction reached 100% at certain frequency.
This anomalous behaviour led to the concept of Surface Plasmons. The incoming light waves
excite the surface waves on film. The energy of excitation comes from incident light energy.
These surface waves propagate along the metal surface and when they reach the hole, they
simply follow through. These surface waves are nothing but surface plasmons. So, we get
light going through the holes that actually didn’t land on them.
A plasmon is a collective oscillation of a large no. of electrons. In presence of electric field,
the electrons and holes get separated in metal nanoparticles (NP). The negatively charged
layer attracts the positively charged layer and the whole metal film oscillates together. A
plasmon forms a wave on metal surface where the electron density oscillates periodically.
Associated with this e-m wave, there is an increase in electron density at one spot and
decrease in electron density in another which constitutes another wave - Electron Density
Wave. So, a plasmon is a coupled excitation, not just sloshing of fluid of electrons but also
an oscillating e-m field of same frequency.

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Some features of plasmons are (i) It is a coupled excitation, (ii) they aren’t limited by
diffraction limit of focussed light and can be smaller than wavelength, (iii) plasmons live very
close to the metal surface and so the e-m field strength is highest near surface.
Mie Theory
For nanoparticles much smaller than the wavelength of light, the area of only the dipole
oscillation is the equivalent area that interacts with the radiation [8-12]. For larger
nanoparticles, where the dipole approximation is no longer valid, the plasmon resonance
depends explicitly on the particle size. The larger the particles become, the more
inhomogeneous is their polarization by the incident radiation. The plasmon band, therefore,
red shifts with increasing particle size. At the same time, the plasmon bandwidth increases
with increasing particle size. The plasmon bandwidth can be associated with the dephasing
of the coherent electron oscillation. Large bandwidth corresponds to rapid loss of the
coherent electron motion.
The dielectric constant of the material is size dependent below an average particle diameter
of about 20 nm [9, 13]. Electron - surface scattering is enhanced in such small particles since
the mean free path of the conduction electrons is limited by the physical dimension of the
nanoparticles [14, 15]. For nanoparticles smaller than 20nm (in case of gold), the smaller the
particles, the faster the electrons reach the surface of the particles. The electrons can then
scatter at the surface and lose the coherence more quickly than in larger nanoparticles. As a
consequence the plasmon band width increases with decreasing the radius of the
nanoparticles.
Gans Theory
Shape effects seem to be very pronounced in the optical absorption spectrum of gold
nanoparticles [16-18]. The plasmon resonance absorption band splits into two bands as the
particles become more elongated along one axis [10-12]. As the aspect ratio increases, the
energy separation between the resonance frequencies of the two plasmon bands increases
[10-12].
The high-energy band corresponds to the oscillation of the electrons perpendicular to the
major (long) rod axis and is referred to as the transverse plasmon absorption. This

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absorption band is relatively insensitive to the nanorod aspect ratio and coincides spectrally
with the surface plasmon oscillation of the nanodots [17-19]. The other absorption band at
lower energies is caused by the oscillation of the free electrons along the major (long) rod
axis and is known as the longitudinal surface plasmon absorption.
T_NT-CdS/CdSe nano-heterostructures
TiO2 nanotubes (T_NTs) are well documented as a wide bandgap porous host for directional
transport of photogenerated carriers from dye or quantum dot (QD) based light harvesters
[20, 21]. Charge carrier separation is facilitated by the alignment of the conduction band of
the established heterojunction between T_NT and QDs. However, this has shown much
lesser efficiency compared to the p-n junction based architecture, where the built-in
potential is responsible for charge separation. One of the ways to overcome this issue
effectively is to use p-n junction based light harvesters along with T_NT as transport
medium for electrons. Recent trend shows several reports on heterostructure chalcogenide
NWs as p-n junction based light harvesters demonstrating excellent photochemical
properties [22-25]. Interests lie beneath the idea that a broad spectrum can be absorbed
through different components of the p-n junction and the photogenerated carriers can be
efficiently separated by the built-in potential and transported through the single crystalline
backbone of the heterojunction.
Difficulty associated with the attachment of the visible light absorbing NWs is that, like QDs,
neither they can be attached using a bifunctional linker, nor they can be directly grown
using SILAR technique. Typically, growth of such NWs is based on the S-L-S mechanism and it
requires a low melting seed e.g. Bi nanoparticles (NPs). Therefore, to directly grow these
NWs on TiO2, a seed layer of Bi NPs has to be deposited. Most inorganic salts of Bi are less
soluble and hydrolyze in water as insoluble oxysalts; as a result, it becomes extremely
difficult to prepare uniform dispersion of Bi NPs that can be strongly adhered as islands to
the TiO2 surface. Electrodeposition from the acidic salt solution of Bi generally results in
large crystallites, which are usually deposited only on the top surface of T_NT.
A reductive SILAR technique is developed using Bi-citrate complex to uniformly disperse and
deposit Bi seed NPs. The S-L-S approach facilitates the synthesis of a directly grown
nanowire-nanotube hybrid, a first of its kind. The SLS mechanism functions at elevated

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temperatures and promotes whisker growth of many crystalline elements and compounds
including silicon at 900°C to 1100°C and III-V semiconductors at 40°C to 1000°C[26]. A liquid
metal or alloy droplet affixed to the whisker forms an interface with the growth surface; a
second interface exists between the droplet and the solution phase. Precursors in the
solution phase decompose preferentially at the solution-liquid interface, depositing the
constituent element (or elements) of the crystal phase into solution in the liquid-flux
droplets. Super saturation then supports whisker growth at the liquid-solid interface. The
system selectively places the crystal face giving the lowest liquid-solid interfacial energy at
the liquid-solid interface; rapid growth on this crystal facet at the interface (and the lack of
growth on other facets) produces the whisker morphology.

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3. OBJECTIVES
We aim to synthesize CdSe quantum dots (QDs) and Au deposited CdSe QDs , study their
excitonic and photoluminescence (PL) properties and investigate the science behind the
coupling between excitons and surface plasmons in Au deposited CdSe QDs and the reason
for enhanced lifetime of electrons due to surface plasmon resonance which leads to better
optical properties.
In the present work, we also aim to synthesize TiO2 nanotube (T_NT)-Bi-CdSe nano-
heterostructures, the first of its kind, by solution-liquid-solid mechanism. We plan to
synthesize Bi catalyzed CdSe-CdS segmented nanowires on T_NT, and CdSe, CdS, CdS-CdSe
NWs on only Bi nanoparticles for simplicity of imaging.

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4. EXPERIMENTAL
4.1 Synthesis and characterisation of CdSe QDs and Au deposited CDs
The Cadmium precursor used for the synthesis was Cadmium Stearate. The stock precursor
was prepared by using 20mmol of Cadmium Acetate and 40mmol of Stearic Acid. The above
mixture was put in a beaker and was heated at 180⁰C for 30min. Acetic Acid evolved, as
expected, with a unique odour and the solution became more viscous. The solution was
allowed to cool to room temperature and it was then left to solidify for 6hrs. It was scraped
off the beaker and stored in a glass container.
The Selenium precursor used was TBPSe. The stock solution was prepared by ultrasonicating
a solution of 5mmol Se powder and 5ml TBP for 3min till a clear solution was obtained.
Synthesis of CdSe QDs was first attempted with the Hot Injection method. The experimental
set up consisted of a three neck round bottom flask, one neck of which was connected with
a condenser which was further connected with Calcium Chloride in an appropriate glass
tube, which acted as a moisture absorber. One other neck was connected with a Nitrogen
cylinder and the third neck was covered with a rubber septum in which a calibrated
thermocouple was inserted. The injections were made via this third neck.
0.1mmol Cadmium Stearate was added with 25ml Octadecene in the flask and was heated
to 280⁰C till an optically clear solution was obtained, after which 5mmol TOPO was added to
act as a capping ligand. 100μl TBPSe was immediately injected using a plastic syringe. The
heating was done till a brownish red colour was attained. Aliquots were taken from the
reaction vessel at three random time intervals and were quenched in Liquid N2. Due to
experimental problems, the synthesis was not successful.
The synthesis was then attempted via the solvothermal route. 0.1mmol Cadmium Stearate,
100μl TBPSe, 5mmol TOPO and 25ml Octadecene were added in a solvothermal bomb. This
bomb was placed in a steel autoclave which was kept in a box furnace at 200⁰C for 2hrs
after which it was removed and air cooled. The reaction mixture turned wine red in colour
after the synthesis, as expected.

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To test the repeatability of the experiment, synthesis was done again via the solvothermal
route. This time, 0.2mmol Cadmium Stearate, 200μl TBPSe, 10mmol TOPO and 30ml
Octadecene were added in the bomb. The experiment was repeated and again, wine red
colour was attained, demonstrating good repeatability.
5ml of this CdSe solution was used for one experiment of Au deposition. The molar
concentration of Au used was varied from 0.1% to 0.4% to 0.7% to 1.0% of the molar
Cadmium concentration in the CdSe solution. A stock Au precursor was prepared by
ultrasonicating 3.3μmol HAuCl4 in 10ml Oleylamine. Therefore, 0.1% corresponded to 100μl
of the stock solution, 0.4% to 400μl, 0.7% to 700μl and 1.0% to 1ml. For 0.1 mol% Au
deposition, 100μl Au precursor was added to 5ml CdSe solution and the reaction mixture
was subjected to microwave synthesis in a CEM Discover Microwave synthesizer at 150⁰C
for 30min. The same procedure was repeated for different Au concentrations.
UV-Visible Absorption Spectroscopy was done for all synthesized samples in a Lambda
Perkin-Elmer750 UV-Visible Absorption Spectrometer. Transmission Electron Microscopy
was done using Tecnai F30 and Tecnai T20 microscope operating at 300 keV and 200 keV
respectively. TEM sample preparation was done by washing the CdSe solution with Acetone,
drying it and then dispersing it in Hexane, after which it was drop-casted on a C-coated,
copper grid and allowed to dry. Photoluminescence (PL) studies were done using Oriiba Lab
HR Raman 800 spectrometer (laser of wavelength 325nm and 532nm was used for the
micro-PL studies).
4.2 Synthesis of T_NT-Bi-CdSe nanowire heterostructure
The synthesis of anodized T_NT is detailed in this section. Bismuth precursor consists of a
mixture of citric acid (20mM) and bismuth citrate (20mM). NaBH4 (0.1M) was used as a
reducing agent. A standard size T_NT strip was immersed in the bismuth precursor for 30s.
After immersing this strip in DI water for another 30s, reduction of bismuth was performed
with NaBH4. Finally, it was gently placed again in DI water to complete one cycle of bismuth
deposition. Deposition was repeated for 5 cycles. Bismuth decorated T_NT (T_NT-Bi) was
dried in air prior to CdSe nanowire growth. However, annealing of Bi NP decorated T_NT
was avoided to prevent nanoparticle agglomeration. Synthesis of CdSe nanowire was
performed at 280o
C in a flat bottomed three neck flask placed in a Si oil bath which was

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placed in a digitally controlled customized heating mantle using 30ml Octadecene (ODE)
(solvent, reducing agent, and surfactant(TOPO,3.88gms)). 0.2mmol Se was dissolved in
0.2ml tri-butylphosphine designated as TBP to prepare the Se precursor. After the addition
of cadmium stearate (0.2mmol) to ODE at 280o
C, 0.2mmol of TBP-Se was also introduced.
Immediately, a T_NT-Bi strip was immersed in the reaction medium and CdSe NW was
allowed to grow. We studied growth for a time period of 4min. For CdS-CdSe segmented
nanowire synthesis, in addition to the above reagents 0.2mmol TBP-S was added to the
solvent. After this, the Ti strip was cleaned with toluene.
Synthesis of Anodized TiO2 Nanotubes
Titanium strips, 4cmx1cm, cut from Ti foil were used for anodization. Each strip was
polished well and, later, ultrasonicated in acetone. Ultrasonication was performed to
remove dirt materials and organic residues. Electrolyte used for anodization consisted of a
fluorinated solution: ammonium fluoride at 0.5% w/w of ethylene glycol & DI water (10%
w/w) of the electrolyte. The cleaned strip was anodized at 40 V (DC) for 2hrs in a two-
electrode cell set-up; platinum was used as the cathode. The distance between the cathode
and the anode was ~1.5cm. Later, annealing of anodized samples was performed at 450o
C in
presence of O2 gas flow for 2hrs to achieve the desired structure.
Bismuth nanoparticle deposition on T_NTs and overall process scheme
As the TiO2 nanotube strip is immersed in the bismuth precursor, precursor is adsorbed to
the TiO2 nanotube surface. Reduction of the precursor was performed with NaBH4 to
deposit bismuth nanoparticles on the oxide nanostructure.
Scheme 1: Scheme of bismuth deposition on anodized T_NT.

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Overall process for synthesizing T_NT-Bi-CdSe nano-hybrid
Scheme 2: Overall process scheme for the synthesis of T_NT-Bi-CdSe nano-hybrid.
For the synthesis of CdS-CdSe segmented nanowires, the above procedure for CdSe-T_NT
nano hybrid synthesis was followed with the addition of 0.2mmol TBP-S which acts as
sulphide precursor and the entire solution was now heated at 3200
C to obtain T_NT-Bi-CdS-
CdSe nano heterostructures.
Synthesis of Bi-CdX (X = S, Se, Te) nanorods
Another set of experiments were conducted wherein no T_NTs were used. TOPO (3.88g)
was melted in a three neck flat bottom flask kept in a Silicon oil bath placed on a digitally
controlled heating mantle. 136mg of the Cd precursor was added to this melt. Also, 25µl of
a Bi precursor was added to this solution. This precursor was prepared by making a 10mM
solution of Ammonium Bismuth Citrate and Oleylamine for which 188mg of Ammonium
Bismuth Citrate was added to 10ml of Oleylamine. The Bi-Cd precursor solution was then
heated to 2800
C reaching which 200µl of TBP=Se was immediately injected into the solution.
This solution was then cooled to room temperature wherein it solidified. Toluene was added
and the solution was kept overnight. Then ethanol was added to the solution to precipitate
the nanostructures. The supernatant solution was then thrown away to collect the
precipitate. More ethanol was added and the solution was centrifuged at 3000rpm for

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5mins. The supernatant solution was thrown again and the concentrated solution was drop
casted over a copper grid for TEM characterization.
Similar experiments were conducted for synthesis of Bi-CdS nanorods and Bi-CdSe-CdS
nano-hetero-structures. TBP-S was used instead of TBP-Se for Bi-CdS synthesis. For hetero-
structural synthesis, after TBP-Se was added to the solution at 2800
C, 136mg of Cd precursor
was again added to the solution followed by raising the temperature to 3200
C, at which
200µl of TBP-S was injected into the solution. Samples were prepared for TEM
characterizations of Bi-CdS nanorods as well as Bi-CdSe-CdS hetero-structures.
Transmission Electron Microscopy was done using Tecnai T20 microscope operating at
200keV. TEM sample preparation was done by washing the Bi-CdSe, Bi-CdS and Bi-CdSe-CdS
solution with ethanol, centrifuging at 3000rpm for 5min, drying it and then dispersing it in
Hexane, after which it was drop-casted on a C - coated, copper grid and allowed to dry.

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5. RESULTS AND DISCUSSION
5.1 Au-CdSe QDs
The UV-Visible absorbance spectrum obtained from the synthesis via Hot Injection method
is shown in Fig. 1.
Fig. 1: UV-Visible absorbance spectrum obtained from samples of the Hot Injection
Synthesis Route at random time intervals.
As we can see from the above graph, there is no visible peak in the 500-600nm wavelength
range which is where the absorbance peak corresponding to CdSe QDs is expected [27].
Even after magnifying the spectrum in the visible region (400-700nm), no peak was
observed, which leads us to the inference that CdSe QDs were not formed during the
synthesis.

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The UV-Visible absorbance spectrum for samples synthesized via solvothermal method is
shown in Fig. 2.
Fig. 2: UV-Visible absorbance spectrum obtained from a sample of the solvothermal
Synthesis Route.
From the above graph, there is an absorbance corresponding to a wavelength range from
500-650nm which is reported to correspond to absorbance by CdSe QDs. The peak is
observed at 540nm.PL spectra of solvothermally synthesised sample is shown in Fig. 3.
Fig. 3: PL spectrum obtained from samples of the solvothermal synthesis route.

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As we can see from the above graph, there are two peaks, one corresponding to a
wavelength of ~560nm and the other, to ~600nm. The ~600nm peak, we believe is due to
dangling bonds and other defects that lead to a low energy emission from the QDs. The
band gap corresponding to the ~560nm peak comes out to be ~2.22eV. The bulk band gap
observed for CdSe is ~1.75eV. This demonstrates the Quantum confinement effect due to
decrease in crystallite size. The Effective mass model gives the following formula to calculate
the size of the crystallites,
where, ε = 10.6, me* = 0.13, mh* = 0.45 for CdSe [28].
Substituting known values in the above equation, the diameter of the crystallites was
estimated to be ~5.2nm, which was confirmed by the TEM images of the sample (Fig. 4), as
shown below. The analysis of TEM results were done using Digital Micrograph .dm3
software and the SAED pattern was indexed to hexagonal CdSe crystal structure
corresponding to the JCPDS #772307.
Fig. 4: TEM images of CdSe QDs synthesized via the solvothermal route.
The UV-Visible Absorbance spectrum for the repeated solvothermal synthesis is shown in
Fig. 5.
As seen from the graph above, again, the absorbance wavelength corresponds to a range of
500-650nm, the expected range of absorbance for CdSe QDs. The absorbance peak is

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sharper than of that which was synthesized earlier. The work was reproducible and the
synthesis yield high quality QDs.
Fig. 5: UV-Visible absorbance spectrum obtained from a sample of the solvothermal
Synthesis Route.
As seen from the graph above, again, the absorbance wavelength corresponds to a range of
500-650nm, the expected range of absorbance for CdSe QDs. The absorbance peak is
sharper than of that which was synthesized earlier. The work was reproducible and the
synthesis yield high quality QDs.
The TEM images for these CdSe QDs are shown in Fig. 6.

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a) b)
c) d)
Fig. 6: TEM images of CdSe QDs synthesized via the solvothermal route. (a, b) LM image; (c)
corresponding DP; (d) HRTEM image.
As seen from the images, uniform-sized monodispersed nanocrystallites have been formed.
This ensures high quality synthesis. The diffraction pattern coincides with the expected
diffraction pattern of CdSe, inferred from PCPDF. Also, the diffused rings that are seen in the
diffraction pattern speak up for the small size of the nano crystallites.
The UV-Visible Absorbance spectrum and the PL spectrum for Au deposited CdSe are shown
in Fig. 7 and Fig. 8 respectively.

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Fig. 7: UV-Visible Absorbance spectrum of Au deposited CdSe QDs with respect to varying
Au concentration.
Fig. 8: PL spectrum of Au deposited CdSe QDs with respect to varying Au concentration.

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From the above absorbance graph, and comparing it with the previous UV-Visible spectra
for CdSe QDs only, we find that the deposition of Au on CdSe brings about an overall blue
shift in the absorbance peak, which signifies more energy absorption than that obtained for
CdSe QDs alone. This was expected due to the amplification of the exciton energy because
of surface plasmon resonance shown by Au. Moreover, we can also see a red shift due to
increasing Au concentration till 0.7%, after which there is a blue shift on further increasing
the concentration to 1.0%. The reason for this is not understood yet.
The PL spectra clearly show an overall blue shift in the peak corresponding to CdSe. The
CdSe peaks, irrespective of the varying Au concentrations, lie in the range of 525-540nm,
unlike ~560nm for CdSe QDs alone. Also, the Au emission peaks are clearly visible in the
graph in the range of 410-425nm, and there is little shift due to variation in Au
concentration. The collective blue shift due to Au deposition ascertains the fact that higher
energy emission takes place due to the presence of Au in the vicinity of CdSe. This again
invokes the theory of surface plasmon resonance shown by Au to justify the additional
emission energy that is observed experimentally. An important facet that needs to be
looked at is that the PL peaks do not shift with increase in Au concentrations. This is in
contradiction with the shift observed in the case of absorbance spectra. The reason for this
is not yet understood.
The TEM images for Au deposited CdSe are shown in Fig. 9, 10, 11 and 12.
a) (b)
2 1/nm2 1/nm
102
110
201

25. Page | 24
c)
Fig. 9: TEM images of 0.1mol% Au deposited CdSe QDs. (a) LM image; (b) corresponding DP -
the spots correspond to CdSe crystallites, whereas the rings correspond to Au crystallites as
in Fig. 10; (c) HRTEM image.
a)
b)
5 1/nm5 1/nm
111
200
220
311
100 (CdSe)
203 (CdSe)

26. Page | 25
c)
Fig. 10: TEM images of 0.4mol% Au deposited CdSe QDs. (a) LM image; (b) corresponding DP
- the spots correspond to CdSe crystallites, whereas the rings correspond to Au crystallites;
(c) HRTEM image.
a) b)
5 1/nm5 1/nm
312 CdSe
110
203
300
312

27. Page | 26
c)
Fig. 11: TEM images of 0.7mol% Au deposited CdSe QDs. (a) LM image; (b) corresponding DP
- the spots correspond to CdSe crystallites, whereas the rings correspond to Au crystallites;
(c) HRTEM image.
a)
b)
5 1/nm5 1/nm
102
110
204

28. Page | 27
c)
Fig. 12: TEM images of 1.0mol% Au deposited CdSe QDs. (a) LM image; (b) corresponding DP
- the spots correspond to CdSe crystallites, whereas the rings correspond to Au crystallites;
(c) HRTEM image.
From the TEM images and diffraction patterns above, we can say that the monodispersity
observed in CdSe QDs alone was lost due to the deposition of Au. Also, branching is seen to
take place. All these are as shown in Fig. 13. Also, for 1.0mol% Au deposition, separate
clusters of Au nanocrystallites are prominently observed, as shown in Fig. 14. Moreover, the
diffraction patterns, when indexed, show that the rings obtained are of Au crystallites while
the spots observed are of CdSe crystallites. The diffused nature of the rings shows that Au
crystallites are very small-sized with respect to CdSe crystallites. Also, the HR images show
that the Au nano crystallites have very well deposited on the CdSe nano crystallites. This is
supported by higher energy absorption and emission due to Au deposition.
a) b)

29. Page | 28
c)
Fig. 13: Branching and longitudinal deposition of Au nanoparticles in 0.4, 0.7 and 1.0mol%
Au deposited CdSe QDs respectively.
a) b)
c)
Fig. 14: Prominent Au nanocrystallite clusters in 1.0mol% Au deposited CdSe QDs.

30. Page | 29
5.2 T_NT-Bi-CdS/CdSe nano-heterostructures
The SEM micrographs for TiO2 nanotubes and Bi deposited T_NT are shown in Fig.15. They
show clean and clear TiO2 nanotubes with a spherical cross-section. Only the large size Bi
particles are seen in these micrographs but very small Bi particles are also formed indicated
by the growth of CdSe NWs from all over the T_NT surface, this may be due to the use of
very low conc. of Bi.
a) b)
Fig.15: SEM micrographs showing (a)TiO2 nanotubes; (b)Bi coated TiO2 nanotubes.
The SEM micrographs of T_NT-Bi-CdSe nanowires are shown in Fig.16:
Fig.16: SEM micrographs of CdSe nanowires grown on Bi deposited T_NT.
Bi

31. Page | 30
It is clear from the SEM micrographs of Fig.16 that a forest of CdSe NWs has grown from the
Bi NPs deposited over the T_NT. The diameter of these NWs is around 20nm and so they are
expected to show excellent photoelectrical properties due to enhanced quantum
confinement.
The TEM images of Bi-CdS, Bi-CdSe and Bi-CdSe-CdS are shown in Fig.17, 18 and 19
respectively.
a) b)
c) d)
Fig.17: TEM images of Bi-CdS nanowires. a), b), c) LM images; d) DP corresponding to c). The
indexed rings match with the cubic structure of CdS.

32. Page | 31
a) b)
c) d)
Fig.18: TEM images of Bi-CdSe nanowires. a), b), c) LM images; d) DP corresponding to c).
The indexed rings match with the cubic structure of CdSe.

33. Page | 32
a) b)
c) d)
Fig.19 : TEM images of Bi-CdSe-CdS nano-heterostructures. a), b), c) LM images; d) DP
corresponding to c). The indexed rings match with the cubic structure of CdS, while the
streaks match with the cubic structure of CdSe.
As we can see from the images, Bi has acted as a nucleation site and both CdS as well as
CdSe have nucleated from it respectively. We also see nanodots of CdS formed in the
vicinity of the Bi-CdS nanowires as well as those of CdSe formed in the vicinity of the Bi-CdSe
nanowires. The ring diffraction patterns correspond to CdS and CdSe nanodots, while
streaks correspond to CdS and CdSe nanowires respectively. In the case of Bi-CdSe-CdS
nano-heterostructures, the rings have been found to match with the structure of CdS,
showing that the nanodots are predominantly of CdS. Also, the streaks have been found to

34. Page | 33
match with the structure of CdSe, showing that the nanowires are of CdSe. The contrast in
the nanowires images is mainly due to stacking faults. In Bi-CdSe-CdS, however, it may also
be because of an envelope of CdS nanodots over CdSe nanowires. The diameter of the
nanowires is found to be ~20nm. Improvement is required so that nanowires are preferably
formed over nanodots. An important factor could be the absence of Octadecene. We need
to carry out experiments to verify this.

35. Page | 34
6. CONCLUSIONS
From the above results, we can conclude that:
1) Solvothermal synthesis is a good method for preparing CdSe QDs. It is reproducible and
also renders high quality QDs.
2) Au deposition results in increased absorption and emission energy than that of CdSe QDs
alone. This is attributed to surface plasmon resonance shown by Au in vicinity of CdSe
nanocrystallites.
3) Also, the Au deposition is always observed to take place in the longitudinal direction of
the CdSe nanocrystallites. Therefore, the amplification of electromagnetic fields by Au will
be lesser in the transverse direction than in the longitudinal direction.
4) SILAR method is an effective way to deposit Bi nano-islands on the intricate surface of
TiO2 nanotube.
5) The S-L-S approach assisted growth of bismuth catalyzed visible light absorber (CdSe
nanowire) from the T_NT surface, which results in a direct contact establishment between
the semiconductors.
6) The deposition of CdS nanodots in the vicinity of CdSe nanowires where only CdS
nanowires were expected is an interesting observation and the reason for it has to be
thoroughly studied in the future.

36. Page | 35
7. SCOPE FOR FUTURE WORK
The present work has to be continued by getting more involved into determining the
factors contributing to the results which have not yet been understood. Also, the effects of
light doping of transition metal into CdSe QDs have to be studied.
The repeatability of the T_NT-Bi-CdS/CdSe experiments has to be checked. Experiments
have to be performed with the use of Octadecene for cleaner growth of Bi-CdS/CdSe nano-
heterostructures for better TEM characterization and absorbance and photoluminescence
studies. There are many modifications which can be made to the nanowires to improve their
absorption energy range, schematically elucidated in Fig.20.
Fig.20: Modifications proposed to improve absorption energy range of nanowires grown on
TiO2 host nanotubes [28].
The first modification that comes to mind is the development of a p/n junction in the
nanowires assembly. This can be done by depositing PbS nanoparticles on the CdSe-CdS
nanowires. The deposition of Au nanoparticles on CdSe QDs in the first chapter amplified
the energy absorbed and emitted. This is another modification that can be applied to our
assembly. New nano-heterostructures consisting of lanthanide (Ln)-doped NaYF4
nanocrystals dendritically decorated with CdSe quantum dots (QDs) have been synthesized

37. Page | 36
[28]. These materials combine up-converting and semiconducting properties, resulting in
the appearance of sub-band-gap photoconductivity. This can also be applied to our
assembly. Light transitional metal cation doping has been found to increase the time
response of the excited electron by an order of 109
. This modification can improve our
optical properties greatly. Perovskite coating on the CdSe/CdS nanowires helps decrease the
depletion layer and thereby, increase the efficiency of charge transport. The synthesis of
perovskite structure can be done by spin coating of PbI2 and methyl ammonium
iodide(CH3NH2I) on the T_NT-Bi-CdSe-CdS substrate and then heating it to 200o
C.The
perovskite obtained is Pb(CH3NH2)I3.

38. Page | 37
8. REFERENCES
1) Bawendi, M.G. et. al., Synthesis and characterisation of monodisperse CdE (E=S,Se,Te)
semiconductor nanocrystallites, J. Am. Chem. Soc. 1993, 115, 8706-8715.
2) Vogel, R. et. al., Study of mesoporous CdS-quantum-dots, J. Phys. Chem. 1994, 98, 3183-3188.
3) Venkatesu, P. et. al., Manganese doped cadmium sulphide (CdS: Mn) quantum particles:
Topological, photoluminescence and magnetic studies, Adv. Mat. Lett. 2013, 4, 202-206.
4) Park, B. et. al., Surface–plasmon-coupled photoluminescence from CdS nanoparticles with Au
films, Solid State Commun. 2012, 152, 1767-1770.
5) Gomez, D.E. et. al., Cooperative coupling of ultracold atoms and surface plasmons, Nano Lett.
2010, 10, 274-278.
6) Choi, H. et. al., Nanostructure sensitization of transition metal oxides for visible-light
photocatalysis, Am. Chem. Soc. Nano 2012, 6, 4418-4427.
7)J.W. Lee, D.Y Son et al., Quantum dot sensitised solar cells with high photocurrent, Scientific
Reports, 2013.
8) Mulvaney , P., 1996, Langmuir, 12, Surface plasmon spectroscopy of nanosized metal particles,
Am. Chem. Soc.,1996, 788.
9) Kreibig , U., and ollmer , M., Optical Properties of Metal Clusters, 2nd
Edition, 1995,Springer,Berlin.
10) Papavassiliou, Alloy Formation of Gold-Silver Nanoparticles and the Dependence of the Plasmon
Absorption on Their Composition G. C., 1980, Prog. Solid st. Chem., 12, 185.
11) Kerker , M., 1969, The Scattering of Light and Other Electromagnetic Radiation ,New York
Academic Press, 645.
12)Bohren , C. F., and Huffman , D. R., Absorption and Scattering of Light by Small Particles, Wiley,
1988, p 520-547.
13) Kreibig , U., and Genzel , U., 1985, Systems of small metal particles: Optical properties and their
structure dependences, Surf. Sci., 1985, 678.
14) Kreibig , U., and von Fragstein , C., The limitation of electron mean free path in small Ag particles
in 1969, Z. Phys., 224, 307.

39. Page | 38
15) Kreibig , U., 1970,Kramers-Kronig analysis of optical constants of small Ag particles, Z. Phys., 234,
307.
16) Yu, Y., Chang , S., Lee , C., and Wang , C. R. C., Gold nanorods: synthesis and optical properties, J.
phys. Chem. B,1997, 101, 34, 6661.
17) vander Zande , B. M. I., Bohmer , M. R., Fokkink , L. G. J., and Schoneberger , C., Optical
Properties of Aligned Rod-Shaped Gold Particles Dispersed in Poly(vinyl alcohol), Films J. phys. Chem.
B,1997, 101, 852.
18) Mohamed, M. B., Ismael, K. Z., Link ,S., and El -Sayed , M. A., Shape control synthesis of metal
nanoparticles, J. phys. Chem. B,1998, 102, 9370.
19) Gomez, D.E., et. al.,Exciton-Plasmon Interactions in metal-semiconductor nanocrystals, Am.
Chem. Soc.,2010, 274,278
20)G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Use of highly-ordered TiO(2)
nanotube arrays in dye-sensitized solar cells.,Nano Lett., 2005, 6, 215.
21)W.-T. Sun, Y. Yu, H.-Y. Pan, X.-F. Gao, Q. Chen and L.M. Peng, CdS quantum dots sensitized TiO2
nanotube-array photoelectrodes, J. Am. Chem. Soc., 2008, 130, 1124.
22)M.-Y. Chen and Y.-J. Hsu, Controllable conversion of plasmonic Cu2−xS nanoparticles to Au2S by
cation exchange and electron beam induced transformation of Cu2−xS–Au2S core/shell
nanostructures, Nanoscale, 2013, 5, 363.
23)S. V. Kershaw, A. S. Susha and A. L. Rogach, Narrow bandgap colloidal metal chalcogenide
quantum dots, Chem. Soc. Rev., 2013, 42, 3033.
24) F. Qian, S. Gradečak, Y. Li, C.-Y. Wen and C. M. Lieber, Core multishell nanowire heterostructures,
Nano Lett. 2005, 5, 2287.
25)S. Yan, Y. Shi, L. Sun, Z. Xiao, B. Sun and X. Xu, Controlled synthesis of NiS nanoparticle/CdS
nanowire heterostructures via solution route, Mater. Sci. Engr. B, 2013, 178, 109.
26)Fudong Wang, Angang Dong, Jianwei Sun, Rui Tang, Heng Yu, and William E. Buhro,Solution-
Liquid-Solid growth of ceystalline III-V nanoparticles, Science, Vol. 270,1995, pp. 1791-1794.
27)Peng, X. et. al.,Alternative route towards CdSe nanocrystallites, Nano Lett. 2001, 1, 333-337.
28) http://education.mrsec.wisc.edu/nanolab/CdSe/