Synthesis and Characterization of CdS Nanoparticles: Its Application to Optical Limiting
A Thesis submitted towards partial fulfillment for the degree of
Master of Philosophy
by
VENKATRAM NALLA
School of Physics University of Hyderabad Hyderabad - 500 046 July 2004
To My Parents, Teachers& Friends
DECLARATION
I here by declare that the matter embodied in the thesis entitled “Synthesis and Characterization of CdS Nanoparticles: Its Applications to Optical Limiting” is the result of investigation carried out by me in the School of Physics, University of Hyderabad, India, under direct supervision of Prof. D. Narayana Rao.
Place: Hyderabad Date: Nalla
VenkatRam
CERTIFICATE This is to certify that Mr. VENKATRAM NALLA has carried out the work described in this dissertation under my direct supervision and this has not been submitted for any degree or diploma at this or any other University.
Place: Date:
(Prof. D. NARAYANA RAO)
Dean
School of Physics
Acknowledgements It is indeed a great pleasure to thank all those who have, directly or indirectly, helped me in successfully completing this thesis. At the outset I wish to thank my guide Prof. D. Narayana Rao who was by my side always, patiently and constantly inspiring, encouraging and guiding me throughout the course.
I have learnt a lot from his meticulous planning and
implementation, dedication and hard work. My association with him for over six months was a rewarding experience. I would like to thank the Dean, School of Physics, for making available all the facilities required for the experiments. I also thank all the non-teaching staff for their co-operation. I would like to specially thank Dr. M.A. Akundi (Xavier University of Louisiana, New Orleans, USA), who helped me in the experiments and also all of my teachers from my childhood to Post Graduation. I wish to thank all my senior lab-mates Prem Kiran, Srinivas, Manoj, Joseph, Shivakiran, Chaitanya, Sree Harsha, Sai Santosh, Vijaya Laxmi, Philip, Shathabdi, Prathap, Prakash, Abhijith for their constant support. I would like to thank my friends and classmates Uday Basker, Sunder, Shivaji Reddy, Ravi, Praveen, Devendra, Satheesh, Pradeep, Ramesh, Sree Kanth, Rajeeb Bramha for their constant encouragement and support and for being with me through the ups and downs during this period. Thanks are also due to all those whose names are missing in this list and have helped me in various stages of my work. Last, and most important of all, I wish to thank my parents and my brother without whose co-operation this task would have been highly impossible.
Table of Contents
Acknowledgment…………………………………………………………………………………..i Synopsis ……………………………………………………………………………………………1 Chapter 1: INTRODUCTION 1.1
Nanoparticles……………………………………………………………………………...3
1.2
Nanoclusters………………………………………………………………………………6 1.2.1 Semiconductor Nanoclusters………………………………………………………..7 1.2.2 Metal Nanoclusters………………………………………………………………….9 1.2.3 Metal Nano Fullerenes…………………...………………………………………….9
1.3
Optical Limiting…………………………………………………………………….…….10 1.3.1 Importance………………………………………………………………………....10 1.3.2 Processes leading to optical limiting…………………………………….………...11 1.3.3 Energy-spreading type optical limiters………………………………………….…12 1.3.4 Two Photon Absorption (TPA)……………………………………….…………...15 1.3.5 Excited State Absorption (ESA)…………………………………………………...16 1.3.6 Free – Carrier Absorption………………………………………………………….17 1.3.7 Optical Nonlinearity of Semiconductors…………………………………………..18 1.3.8 Two-Photon assisted excited state absorption…………………………...………. 18 1.3.9 Rate equations for two level semiconductors…………………………………...…20
Chapter 2: EXPERIMENTAL TECHNIQUES AND FUNDAMENTAL DETAILS SETUP………………………………………..……………….………...……23
2.1
SYNTHESIS
2.2 2.3
Synthesis of CdS…………………………………………………………….………..… 23 Size-Selective Precipitation……………………………………………………………. 24
2.4
Particle Size measurements…………………………………………….………………..24
Chapter 3: CHARACTERIZATION 3.1
Properties of Bulk Cadmium Sulphide (CdS)……………………………………………27
3.2 3.3
XRD……………………………………………………………………………………...28 Optical spectroscopic analysis………………………………………………………..….34
3.4
SEM and EDAX…………………………………………………………………….…...36
3.5
Optical limiting……………………………………………………………………….….37
3.6
Z-scan…………………………………………………………………………………...38 3.6.1 Open-aperture Z-scan for optical limiting…………………………………………39 3.6.2 Experimental Set-Up………………………………………………………………40
3.7
Nonlinear Scattering………………………………………………………………….….45
3.8
Theoretical Fits……………………………………………………………………….….46
Chapter 4: CONCLUSIONS, FUTURE SCOPE & REFERENCES 4.1 4.2
Conclusions……………………………………………………………………………....50 Future scope……………………………………………………………………………...50
4.3
References…………………………………………………………………………….…51
DDDDDD
Abstract
CdS nanoparticles were synthesized using Cadmium Acetate and Thiourea and using Glycerol as a capping agent. The study of the linear absorption spectra of the sample showed a blue shift in the absorption peak as the particle size decreased. Open aperture Z-scan studies were performed on the CdS nanoparticles to investigate the non-linear optical properties. The optical limiting property of CdS nanoclusters was clearly established as we observed only 5% transmission at 1.65GW/cm2 of input intensity of 532nm beam from the Nd: YAG laser. Z-scan measurements showed large non-linear scattering in addition to non-linear absorption at high intensities. The effective scattering coefficient and the two-photon absorption coefficient were estimated. The results also indicate that glycerol is not a good capping agent and a better quality of nanoparticles can be obtained if we use thioglycerol as a capping agent.
CHAPTER – 1 INTRODUCTION
1
1.1 Nanoparticles Nanoparticles are generally categorized as the class of materials that fall between the molecular and bulk solid limits, with an average size between 1 – 50 nm. Nanoparticles exhibit physical and chemical properties different from either the individual molecules or the extended solid, hence attracting an enormous attention during the past two decades. [1-8] The changes in the properties of nanoparticles are driven mainly by two factors, namely the increase in the surface to volume ratio and drastic changes in the electronic structure of the material due to quantum mechanical effects with decreasing particle size. Very often it is an interplay of these two effects that is responsible for the changes in the properties [9] because for sizes as small as a few nanometers, the surface atoms, which can be neglected for a bulk solid material, play a major role in determining the electronic properties. For example, while the melting point of bulk CdS is around 1600 °C, a typical 2.5 nm CdS crystallite melts at a temperature of about 400 °C. [10] Such a depression in the melting point is due to a higher surface energy of the nanoparticles compared to the bulk. Apart from the effect of a large surface area, the material properties undergo drastic changes in their optical and electronic properties as a function of the size below a certain size regime. Fig. (1.1) schematically shows the density of states for bulk solids compared to those of one, two and three dimensionally confined solids such as in a thin film, in a nanowire and in a quantum dot [4].
2
Fig.(1.1): The evolution of the density of states with dimensionality. The plots show the energy (E) versus the density of states (gi ) for a bulk solid, thin film, nanowire and quantum dot.
Nanoparticles belong to the quantum dot regime, where the charge carriers are confined to a narrow region along all three directions in space. The physical properties of semi conducting nanoparticles exhibit distinctive changes compared to those of the bulk for sizes below the exciton Bohr radius. An exciton is composed of an electron and a hole. The distance between the electron and the hole within an exciton is called Bohr radius of the exciton. Typical exciton Bohr radius of semiconductors is of a few nanometers. In large size semiconductors, the exciton can move freely in all directions. When the length of a semiconductor is
3
reduced to the same order as the exciton radius, i.e., to a few nanometers, quantum confinement effect occurs and the exciton properties are modified. Depending on the dimension of the confinement, three kinds of confined structures are defined: quantum well (QW)(thin film), quantum wire (QWR) and quantum dot (QD) shown in Fig (1.1). In a QW, the material size is reduced only in one direction and the exciton can move freely in other two directions. In a QWR, the material size is reduced in two directions and the exciton can move freely in one direction only. In a QD, the material size is reduced in all directions and the exciton cannot move freely in any direction .The normal size of an exciton in a large (bulk) crystal, expressed as an exciton Bohr radius, provides an approximate dimension for the onset of quantum-confinement effects. When an electron–hole pair is squeezed into a nanocrystal with one or more dimensions approaching the bulk exciton Bohr radius, the effective bandgap of the semiconductor increases.
Smaller the
nanocrystal, larger the effective bandgap, and greater the energy of optical emission resulting from electron–hole recombination),
aB, of the bulk
semiconductor. At sizes comparable to and less than aB, the exciton binding energy and the oscillator strength increase due to the enhanced overlap between the electron and hole wave functions. [7] An interesting consequence of changes in the electronic structure is seen in the variation of the band gap with size in semiconductor nanoparticles. For example, the band gap of bulk CdS (2.4 eV) increases to about 4.5 eV for ~ 20 Å nanoparticles. [11,12] Nanoparticles exhibit a pronounced variation in the optical properties upon the variation of size. Emission from the nanoparticles of CdSe, upon excitation with ultra-violet (UV) radiation, can be tuned from the red end of the visible spectrum to almost the UV range by changing the size of the nanoparticles. [13] The emission properties can further be tuned by incorporating impurity atoms into the pure semiconductor lattice. Also known as doping, this process provides new states in the band gap region of the semiconductor. These doped nanoparticles provide an alternate pathway for recombination of the electron-hole pair, thus giving rise to an emission at energy 4
different from that of the host semiconductor. Incorporation of magnetic impurities into the host semiconductor lattice also opens up the interesting possibility of exploiting both the charge and spin degrees of freedom in the same material and therefore such materials can be employed for processing and storing data on the same device, now known as spintronic devices. [14-19]. Different theoretical models describe the electronic and optical properties of the semiconductor nanoparticles. The earliest model to describe the band gap variation with size was the effective mass approximation (EMA)
[20-23]. Since then
many other methods such as the empirical pseudo-potential method (EPM) [2426], the effective bond orbital model (EBOM) [27,28] and the tight-binding (TB) approximation [29-33] have been developed. Apart from the variation of the band gap, a few of these methods also allow the study of the density of states as a function of size. This thesis includes the study of optical properties of CdS nanoparticles. Experimental techniques used for the optical studies include UVVisible absorption spectroscopy.
1.2 Nanoclusters: Nanocluster refers to an aggregate of several tens to thousands of atoms or molecules and its grain diameter is about a nanometer. It does not exist in nature and can be obtained only by artificial techniques. Nanoclusters have unique optical, electrical and magnetic properties that are related to the quantum size effect, and the physical properties strongly depend on the number of atoms or molecules.
5
-Capping agent Fig (1.2): Cd+ ,S- surrounded by capping agent to control reaction and particle size in the formation of nanoclusters. If this material could be created and deposited onto a substrate with sufficient stability, it is expected to be a breakthrough for the information and telecommunication industry because of its unique properties. To accomplish this objective, the following are necessary: the technique to generate clusters with controllable size and stability, investigation of the interaction between the nanocluster and the substrate to which the nanocluster is deposited, a method to maintain the stability of the nanocluster, and the technique to precisely measure the physical properties in order to understand the different types of materials and sizes. The research work on nanomaterials divided into three parts: semiconductor nanoclusters, metal nanoclusters and metal fullerenes. Capping agent plays a crucial role in the chemical synthesis of nanoparticles. It controls particle size and reduces clustering of particles and aggregation by controlling the reaction as shown in Fig (1.2). Here glycerol is used as capping agent. But according to literature thioglycerol is a better capping agent then glycerol. Here glycerol has been tried in order to obtain large size particles as with thioglycerol one can obtain a minimum size of about 800 nm 6
clusters. This was done with the intension to introduce nonlinear scattering along with the nonlinear absorption.
1.2a Semiconductor Nanoclusters Semiconductor nanoclusters are of great interest because of their applications as photocatalysts for solar fuel production and solar detoxification. In such applications the semiconductor particles would absorb light-creating electron-hole pairs and thus catalyzes specific chemical reactions. To be effective as a solar photocatalyst, the semiconductor must have a bandgap that is matched to the solar spectrum, and the energies of the valence and conduction band edges must be compatible with the oxidation and reduction potentials for the reactions involved. Additionally, the material must be resistant to photochemical degradation, and the carrier diffusion time to the surface must be shorter than the electron-hole recombination time. Nanometer-size semiconductor clusters (or nanoclusters) are in principle ideally suited to meet these requirements. Their small size allows fast carrier diffusion to the surface, and they possess the ability to continuously tune the bandgap (by up to 1 eV or more) by changing cluster size as a consequence of quantum confinement of the charge carriers. In recent years the photophysical and photocatalytic properties of quantized semiconductor colloids have been studied extensively. The term colloid refers to a nanocluster with a diameter less than 100 nm with the ability to suspend in an aqueous or nonaqueous medium. Semiconductor particles, which exhibit size-dependent optical and electronic properties, are termed quantized particles or quantum dots. These crystallites are molecular clusters (approximately 15-50 Å in diameter) in which complete electron delocalization has not yet occurred. Under bandgap excitation, these semiconductor colloids act as short-circuited electrodes 7
and promote oxidation and reduction processes on the semiconductor particle surface. The bench-top chemical approach employed for the synthesis of semiconductor colloids results in a high density of defect sites, usually at the semiconductor surface. The nature of these defect sites depends strongly on the experimental conditions of chemical synthesis and controls the charge trapping and recombination of photogenerated charge carriers. Further details on the interfacial electron transfer and photocatalytic properties of semiconductor nanoclusters are well discussed in literature. (34,35).
1.2b Metal Nanoclusters Now a day’s, creation of metal nanoclusters and studying properties of the material are necessary in order to develop magnetic recording materials. With using metal nanoclusters recording units of new magnetic memory materials are having ten thousand to a hundred thousand times the capacity of conventional ones.
1.2c Metal Nano Fullerenes Physical appearance of metal fullerenes look like empty fullerenes, which do not have any substance in its inside cage. The unique structure of the metal fullerenes drives the following possibilities. ¾ To study metal habits in pure vacuous space. ¾ To handle the minimal size magnet that human being has ever known. ¾ To study behavior of a single/ a few metals. It had been expected that metal including fullerenes show various characteristics and phenomena such as high temperature super conductivity, molecular marking which replaces radio active materials, molecular making for super molecules and for pharmaceutical use, reaction guiding functional units,
8
molecular size switches and any applications in semiconductor field. [36] Ex. Metal nanotubes, nanotubes are most scientifically useful ones.
1.3a) Optical Limiting Lasers have caused revolutionary changes in many fields of science and technology. Since 1960, six orders of magnitude, from 10-9 to 10-15 seconds, have been added to time-resolved observation of fast phenomena. New subfields of science, including femtochemistry and femtobiology, have been created.
In
addition, the domain of power flux densities has been extended from 1012 to 1019 Wcm-2 by the use of short focused pulses. This has given experimental access to new phenomena, including ultrafast phase transitions in electronic structure, above-threshold ionization of atoms, and high-order harmonic generation and acceleration of relativistic electrons by light pulses. Some representative examples of transient Raman scattering and of impulsive and displacive excitations in molecules and crystals illustrate the usefulness of picosecond and femtosecond pulse techniques [37]. Even though lasers revolutionized major fields of science, the dramatic advances in semiconductor and solid-state lasers that are far more portable, compact and efficient are finding application in the realm of laser weaponry [38]. High-power diode-pumped solid-state lasers promise the military the same advantages they offer industry. With such an advent of high power laser sources over wide range of wavelengths and pulse durations, the necessity for protection of sensors and eyes has enormously increased over the last few years. In this context, optical limiters have received significant attention.
An ideal
limiter exhibits a linear transmission below a threshold and clamps the output to a constant above it, thus providing safety to sensors or eye. The minimum criteria identified for a material to act as an effective optical limiter are (1) Low limiting threshold and large dynamic range (2) High optical damage threshold and stability (3) Sensitive broadband response to long and short pulses (4) Fast response time (5) High linear transmittance through out the sensor band width, optical clarity,
9
and robustness [39]. Wide Variety of organic and inorganic materials is being studied to achieve efficient optical limiting [40]. Various approaches have been developed towards better optical limiting based on, e.g., electro-optical [41], magneto-optical [42], and all-optical [43] mechanisms.
1.3b) Processes leading to optical limiting The first experimental demonstration of optical power limiter reported by Leite et al. [44] is based on the laser induced thermal lens effect using 488 nm cw Ar+ laser beam as incident light and nitrobenzene as the linearly absorbing medium with an aperture in front of the detector. Though the change of power through the aperture was only 3% of the total input power change at high input levels, the original idea and setup is still the basis of most popular optical limiting designs using organic dye solutions [45], semiconductors [46] and other materials [47] as linearly absorbing media. The all-optical limiters rely on materials that exhibit one or more of the nonlinear optical mechanisms: Two-photon absorption (TPA), Reverse Saturable Absorption (RSA), Free carrier absorption, Thermal defocusing and scattering, photorefraction, nonlinear refraction, induced scattering [48]. Enhancement in optical limiting has also been achieved by coupling two or more of these mechanisms, like Self-defocusing in conjunction with TPA [49], TPA of one molecule with excited state absorption (ESA) in another molecule [50]. Different experimental geometries like cascaded limiters [51] are also studied to achieve large figure of merit and dynamic range. Optical limiting can be achieved by means of various nonlinear optical mechanisms, including self-focusing, self-defocusing, induced scattering, inducedrefraction, induced aberration, exited state absorption, two-photon absorption, photorefraction and free-carrier absorption in nonlinear optical media [52]. Although there is a great variety of optical limiting devices most of them can be
10
divided into two categories. One is the energy-spreading type of devices and the other is the energy-absorbing type of devices.
1.3c) Energy-spreading type optical limiters For energy-spreading type of devices, the key requirement is to place an aperture or pinhole in front of a detector. The limiting of the detected laser beam is based on the fact that, after passing through a nonlinear medium, the spatial energy distribution of the transmitted laser beam has changed. When the input laser intensity (or fluence) increases, there will be more portions of the incident laser energy spreading to a wider solid-angle range; as a result the portion passing through the aperture will decrease accordingly. Detector (a) Self-focusing
(b) Self-defocusing
(c) Induced scattering
(d) Induced refraction
(e) Induced aberration
Fig (1.3): Schematic illustrations of energy-spreading type of optical limiters 11
In these cases the observed limiting behavior depends not only on the input laser intensity (or fluence) and the nonlinear medium, but also on the pinhole size and the geometric configuration of the optical system for a given device. For most of this type of devices, thermally induced refractive index change plays a major role. Typical designs for the energy-spreading type of optical limiting devices are schematically shown in Fig (1.3). In all these cases the opening size of the aperture is chosen such that for a very low input fluence (or intensity) level, the transmitted laser beam after passing through the medium can just totally pass through the aperture without blocking. In Fig (1.3) (a) and (b) the design based on self-focusing and selfdefocusing are shown. In both the cases at high input levels the detected energy portion can be significantly reduced due to the energy spreading in the aperture plane. Fig (1.3) (c) shows the optical limiter based on laser-induced and intensity dependent scattering. In this case, the limiting medium is a system of linearly absorbing particles randomly distributed in a transparent host material. For a weak input light beam, the temperature and refractive index changed due to the particles’ absorption in the system are negligible, whereas for strong laser beam, the absorption-induced temperature change of the particles is no longer negligible and each particle forms an individual heating center. As a result of this local heating effect, the medium becomes highly inhomogeneous and the considerable portions of the energy will spread out into a wider spatial range and the portion of the light passed through the aperture will be limited [53]. The device shown in fig (1.3) (d) uses a similar idea, where the medium is a mixed system composed of two microscopic components that have the same static refractive index but are in a different phase states, e.g. one in liquid and other is solid. If one component is transparent and the other is linearly absorptive to the incident laser beam, as a result
of
selective
opto-heating
process,
system
becomes
inhomogeneous in the boundary between the two components [54].
Another
12
the
whole
mechanism shown in fig (1.3) (e) is based on induced aberration. It is well known that the induced refractive index change is a function of the local intensity distribution of the laser beam inside the medium. An irregular spatial distribution of local light intensity may lead to a random refractive index change at higher intensity levels, which may cause severe aberration influences on the wavefront of the transmitted laser beam. By keeping a small pinhole in the focal plane of a focusing lens, the portion of the laser energy passed through the pinhole will decrease as the induced aberration becomes greater [55]. Although, all optical limiting devices of energy spreading type are based on the laser induced refractive index change and featured by using an aperture, in some experimental devices no aperture is used, the limited sensitive area of the detector still plays the role of aperture. Comparing with all other nonlinear optical effects related to refractive index changes, the opto-thermal effect induced refractive index change is considered to be important, even it has a slow temporal response. The specific origins that cause thermally induced refractive index changes can be given as following: (1) The presence of small amount of impurities or external particles giving rise to a nonzero residual linear absorption in transparent and nonresonantly absorptive medium. At higher input laser intensity (or fluence) the small residual linear absorption might be strong enough to create a remarkable thermally induced refractive index change.
(2) For a resonant and linearly
absorbing media such as dye solution or semiconductor crystal, thermally induced refractive index change will be significant even for a weaker cw laser beam or low fluence pulsed laser beam. (3) An aperture involved in front of a detector, while working with a nonlinear absorbing material working with RSA or TPA mechanism, the contribution from the thermal effect may be more responsible for the observed optical limiting behavior than the contribution from pure nonlinear absorption.
13
I in
Iout
N o n lin e a r a b s o r p tio n
B is ta b le d e v ic e
Fig (1.4): (a) Schematic of optical limiting device based on nonlinear absorption and (b) optical bistability.
Materials showing RSA become more strongly absorbing as the input optical intensity (or fluence) is increased. This nonlinear optical response can be exhibited when chromophores have a weak ground-state absorption over some spectral range and strong excited-state absorption in the same wavelength range. Although a variety of materials and mechanisms have been and are being explored for use in optical limiting, interest in reverse Saturable absorber chromophores is increasing for many reasons, few of which are listed here. First, for chromophores having large ratio of excited-state to ground-state absorption cross sections (σex/σg >>1) there is a potential for achieving large nonlinear attenuation and maintaining high linear transmittance. Second, since optical energy is absorbed and converted to heat as opposed to being spread, as in nonlinear refractive or scattering media, the limiting may be more reliable and may be applied in fast (highly convergent) optical systems. Third, chromophores with prompt singlet excited-state absorption and long-lived triplet-state absorption may be effectively used to optical limiting of a wider range of pulse widths (sub-picosecond to microsecond duration). Finally, the ability to modify systematically the photophysical properties of chromophores through rational changes in molecular structure enables molecular engineering approaches to the development of chromophores with enhanced limiting responses [56].
14
1.4d) Two-Photon Absorption (TPA) 2ω
ω ω
g
Fig(1.5)Two photon absorption
Two-photon absorption is an instantaneous non-linearity that involves the absorption of a photon from the field to promote an electron from its initial state to a virtual intermediate state, followed by a second photon, which takes the electron to its final state. Since the intermediate state is a virtual state, no energy is conserved and the system can be represented as a two or three level system as shown in fig (1.5). The attenuation and excited state populations can be given by
∂IT = −α I − β I 2 ∂Z and
∂N 0 βI 2 N = − + 1 ∂t τ1 2hω ∂N 1 βI 2 N = − 1 ∂t 2hω τ1 (1.1) Where α is the linear absorption coefficient, N the number of molecules per unit area in the ground state, β is two-photon absorption coefficient and σ is the absorption state cross section. The TPA coefficient β is a macroscopic parameter characterizing the material.
15
1.3e) Excited State Absorption (ESA) S2 ω S1 ω g
FIG(7): Excited state absorption Excited state absorption involves a sequential process in which a photon is initially absorbed and the molecule remains in an excited state for a finite length of time so that a second photon that arrives during that time is also absorbed to put the molecule into an even higher excited state. A schematic of the excited state absorption process is shown in fig (1.6). The basic difference between the two photon and excited state transitions is that the former involve intermediate extremely short lived virtual states, where as the later involve intermediate real states whose lifetime is not determined by Heisenberg uncertainty relations, but, instead, their life times are determined by the electronic structure of the molecules in the materials. Two photon absorption processes are dependent on the intensity of the incident light where as excited state absorption processes are dependent on the fluence (energy per unit area) of the incident light. The same rate equations that are used in TPA will also apply for ESA.
16
1.3f) Free – Carrier Absorption Free-Carrier absorption is more prevalent in semi conductor materials, where the absorption of a photon with energy greater than the band gap energy, will promote an electron (hole) to the conduction band (valence) by absorbing additional photons. This process is often phonon assisted, although depending on the band structure and the frequency of the optical excitation, it may also be direct. The phonon-assisted phenomenon is referred to as free carrier absorption and it is analogous to ESA in molecular systems. The free carrier absorbance can readily be incorporated into the intensity propagation equation in the following form:
∂I T = −(α o + σ c N c ) I ∂Z
(1.2)
Where αo is the linear absorption coefficient, Nc is the intensity dependent carrier density, and σc is the free carrier absorption cross section
e2 σc = n ocεo m*ω 2τ
(1.3)
Where m* is the effective carrier mass, ω is the optical frequency and τ
is the
free carrier decay time. It has the 1/ω2 dependence of high frequency conductivity and thus most important for infrared radiation in semiconductors. The free carrier density is governed by the rate equation given by
∂N c α o I N c = − ∂t hω τ 17
(1.4)
1.3g) Semiconducting Materials
Semiconducting materials are found to play important role in optical limiting because of their strong TPA and manipulation of the bandgap. The study of non-linear optical processes has become the focus of great attention over the last few decades. Since the celebrated observation of second harmonic generation in a quartz crystal by Franken et al (1961), generally regarded as the birth of nonlinear optics, the field has undergone a rapid expansion. Developments in highpower laser technology have fuelled this expansion, and resulted in the observation of a vast range of effects, in many different media of particular interest recently have been the non-linear optical properties of semiconductor heterostructures. Devices based on optical non-linearities in heterostructures offer great potential for applications, most notably in the fields of optical communications and information technology, themselves rapidly expanding through the availability of cheap high quality lasers.
Quantum mechanical
description of non-linear optical processes in semiconductor superlattices, relating features in the optical response to the superlattice band structure.
1.3h) Two-Photon assisted excited state absorption TPA is particularly strong in materials since it can lead to significant population of a two-photon allowed state. Often there are allowed transition from this state to higher states of the system, i.e., exited state absorption (ESA) can ensure from the two-photon pumped state. This occurs in both polyatomic molecules and in semiconductors shown in fig(1.7,1.8), especially when excited with ultrashort pulses. The attenuation and excited state population can be given by
18
∂I T = −αI − βI 2 ∂Z ∂N 1 β I 2 N 1 = − 2hω τ 1 ∂t
(1.5)
Fig (1.7): two level (bands) system of semiconductor.
Fig(1.8): Absorption of two photons in semiconductor nanoparticles, D- donor level, A- acceptor level, VB – valence band, CD- conduction band.
19
1.3i) Rate equations for two level semiconductors The contribution of different nonlinear optical processes leading reverse Saturable absorption (RSA) and optical limiting (OL) are found by rate equations describing various processes and solving them numerically.
∂I T = −αI − βI 2 ∂Z dN dt dN dt
0
1
σ IN = − 0 hω σ IN = 0 hω
σ
0
0
N1 βI 2 − + 2hω τ1
N1 βI 2 + − 2hω τ1
0
α
=
N
0
I 00 = I in .e −αl 1
α = ln ⎛⎜ I in I ⎞⎟ 00 ⎠ l ⎝
ω0 . I 00 .e ω z2 2
I (Z ) =
⎛
ω z = ω 0 ⎜⎜ 1 + ⎝
2
z z 02
⎞ ⎟⎟ ⎠
−τ 1 2 tp
2
.
−2 r 2
.e ω z
2
1 2
(1.6)
20
Where, N0 –number of atoms in ground state N1- number of atoms in exited state α -Linear absorption coefficient β -Two-photon cross section ι1 –life times of state N1 hω
-Energy of the Laser light
I-incident intensity of the Laser light IT-transmitted intensity through sample I00-peak intensity at the focus of the gaussian beam r-radius of curvature of wave front tp- Laser pulse width Z0-Rayleigh range Z- position of the sample ω0-beam waist at focus ωz-diameter of the laser beam at position z l-thickness of the sample(or cuvette) . N-number of molecules per unit area in the ground state σ- absorption state cross section.
21
CHAPTER 2 SYNTHESIS
2.1 Synthesis Setup The figure below describes the experimental set-up used for the synthesis of the CdS nanoclusters. The reaction is carried in the round-bottom flask at 110oC on an oil bath in the nitrogen atmosphere. Water circulator is used to condense the vapors produced during the course of the reaction.
Fig (2.1): experimental setup to chemical synthesis of CdS nanoparticles.
2.2 Glycerol as the capping agent We have synthesized CdS Nanoclusters of eight different sizes using glycerol as the capping agent. The method followed was similar to that reported by Vossmeyer et al. [57], but we used glycerol instead of thioglycerol. In this case 2.35 g of Cadmium Acetate (10.19 mmol), 0.475 g thiourea (6.24 mmol) and 0.64
ml of glycerol (10.95 mmol) in 5ml DMF were taken in a round bottomed flask and heated using oil bath at 100°c / 110°c for about an hour under nitrogen atmosphere as shown fig (2.1). This was followed by refluxing the reaction mixture for about 0-16 hours at 150°c . We have followed the method known in the literature as size selected precipitation technique [57] as discussed in the next section.
2.3 Size-Selective Precipitation. The method of size-selective precipitation has recently been described in detail [13,14] and was used to isolate the clusters. Acetone was added to our solution until the large particles started to precipitate. Were purified by reprecipitation and washing three times with acetone and three times with diethyl ether. We obtained yellow and greenish yellow nanoparticles size is around 3-5nm (according to x-rd). But particles are aggregated each other formed nanoclusters size about 500-800nm (according to SEM) cluster powders. AFM also proved that nanoclusters size about 500nm and particle size is 4-10 nm inside the clusters. We did size selective also for sample 0h reflection, powder was isolated from the precipitate whereas, after successive precipitations with acetone, sample remained in the supernatant was isolated.
2.4 Particle Size measurements Nanoparticles size depends on the reflection time, capping agent and also 0 .9 λ synthesized temperature. Using Debye-Scherrer formula d = β Cos θ , the
particle sizes were calculated. Here d-Diameter of the particle,
λ-Wavelength of X-ray, (Cobalt target Kα1=1.788970A, Kα2=1.792850A, Kβ=1.620790A) β-Full width at half maximum (at 2θ) in X-RD graph. θ -Half of the Bragg angle at maximum peak. XRD Spectra of the samples are taken on INEL X-Ray diffraction spectrometer operates at I=25mA, V=35KV, cobalt target, wavelength λ = 1.78897 o A . This gives an approximate size of the particle.
CHAPTER 3 CHARACTERIZATION
3.1 Properties of Bulk Cadmium Sulphide (CdS)
♦ Crystallographic properties Crystallographic structure: Hexagonal a= 0.4135nm, c= 0.6749nm Color: Red ♦ Physical properties Density: 4.82 g/cm3 Melting point: 1748 °C Hardness: 4 Mohs Thermal conductivity: 15.9 W m-1 K-1 Dielectric constant: 8.28
⊥
C, 8.64
C
Band gap (@ 300 °K): 2.53 eV Specific resistivity: ~ 108 (Ohms cm) Emmission wavelength: 600 nm @ 300 °K ♦ Optical properties Transmission range: 0.5µm -15 µm (2mm thick) Refraction index: no = 2.517, ne = 2.548 ♦ Crystalline planes Orientation: (100), (110), (111),
Fig (3.1): Hexagonal crystal structure of the CdS (Bulk) (a=b) a= 0.4135nm, c= 0.6749nm.
3.2 XRD Using X-Ray Diffraction meter (INEL X-Ray diffraction spectrometer at I=25mA, V=35KV, cobalt target, wavelength λ = 1.78897 o A ) crystal structure of CdS nanoparticles was studied. CdS is a hexagonal structured crystal. The relation between the interplanar spacing and the lattice parameters in case of hexagonal packed structure is given by
1 2 d hkl
=
h 2 + hk + k 2 a2
+
l2 c2
Where a= 0.4135nm, c= 0.6749nm and d=
λ 2 sin θ
, d = interplanar spacing
Figures 3.2 to 3.6 show the XRD spectra of the CdS nanoparticles synthesized at the reflux times of 0,3,9,12,16 hours respectively. The reflection time particle size at 0 hrs is smaller as shown in fig (3.2) .The subsequent sizes are determined for 3, 9, 12, and 16 hrs reflection times
which are as shown in fig (3.3), fig (3.4), fig (3.5), fig (3.6) respectively. Full width at half maximum of all the graphs is shown in fig (3.7). The distribution of particles depends on the reflection time. At high reflection time particles were found to be monodispersed. The reason for this aggregation at high reflection time can be because of the use of glycerol as a capping agent. Size selective precipitation also gives different particle sizes as shown fig (3.8). Precipitated particles are slightly larger than supernated ones. Change in particle size also depends on reaction temperature. We need minimum
temperature to synthesize. The size difference with respect to
temperature is shown fig (3.10). This table below summarizes the particle sizes at different reflection times, at different synthesis temperatures. Reflection
FWHM Synthesis
time
Color of the sample
Size (nm)
Temperature (0C)
0h
5.7
110
Greenish
2.92
3h
4.19
110
Greenish yellow
3.98
9h
3.97
110
Light yellow
4.2
12h
3.88
110
Dark yellow
4.3
110
Greenish
2.87
110
Greenish
2.92
100
Greenish
2.83
0h(supernate 5.8 d) 0h(precipitat 5.7 ed) 0h
5.9
C d S ( W IT H G L Y C E R O L ,0 h r e f le c tio n tim e ) ( 1 ,0 ,0 )
140 120 100
counts
80
(1 ,1 ,0 ) (1 ,1 ,2 )
60 40 20 0 20
40
60
80
100
120
2θ
Fig (3.2): this is at 0hour’s reflection time and synthesized at 1100 C
160
C dS (W ITH G LYC ER O L,3h reflection tim e) (1,0,0)
140 120
counts
100
(1,1,0)
80
(1,1,2)
60 40 20 0 20
40
60
80
100
120
2θ Fig (3.3): this is at 3hour’s reflection time and synthesized at 1100 C
C d S (W IT H G L Y C E R O L ,9 h re fle c tio n tim e ) (1 ,0 ,0 )
150
100
counts
(1 ,1 ,0 ) (1 ,1 ,2 ) 50
0 20
40
60
80
100
120
2θ
Fig (3.4): this is at 9hour’s reflection time and synthesized at 1100 C
c d s (W IT H G L Y C E R O L ,1 2 h re fle c tio n tim e ) 250
(1 ,0 ,0 )
counts
200
150
(1 ,1 ,0 ) (1 ,1 ,2 ) 100
50
0 20
40
60
80
100
120
2θ
Fig (3.5): this is at 12 hour’s reflection time and synthesized at 1100 C
(1 ,0 ,0 )
500
16h
counts
400
300
(1 ,1 ,0 ) 200
(1 ,1 ,2 )
100
0 20
40
60
80
100
120
2θ
Fig (3.6): this is at 16 hour’s reflection time and synthesized at 1100 C added 20 v/v of water in synthesis.
C dS (w ith glycerol at diffrent reflection tim es) 2000
0h 3h 9h 12h 16h
1800 1600 1400
counts
1200
(1,0,0) (1,1,0) (1,1,2)
1000
16h 800
12 hours
600
9 hours 400
3 hours
200
0 hours
0 20
40
60
80
100
120
2θ Fig (3.7): This is at 0,3,9,12, 16 hour’s reflection time and synthesized at 110oC
C d S ( w ith g ly c e r o l,0 h r e fle c tio n ,s iz e s e le c tiv e )
150
(1 ,0 ,0 )
c d s (p re s p e ta te d ) c d s ( s u p e r n a te d )
counts
100
(1 ,1 ,0 )
( 1 ,1 ,2 )
50
0 40
60
80
100
120
2θ
Fig (3.8): x-rd of the CdS (precipitated)&CdS (supernated) at 0 reflection time and synthesized at 1100C.
120 0
100
C d S ( W IT H G L Y C E R O L ,0 h r e f le c tio n tim e , a t1 0 0 C ) ( 1 , 0 ,0 )
counts
80
( 1 ,1 ,0 )
60
( 1 ,1 , 2 )
40
20
0 40
60
80
100
120
2θ
Fig (3.9): this is at 0 hour’s reflection time and synthesized at 1000 C
160
0
C d S ( W IT H G L Y C E R O L ,0 h r e f le c tio n tim e ,a t1 0 0 C ) 0 C d S ( W IT H G L Y C E R O L ,0 h r e f le c tio n tim e ,a t1 1 0 C )
140
( 1 ,0 ,0 )
120
counts
100 80
( 1 ,1 ,0 ) ( 1 ,1 ,2 )
60 40 20 0 20
40
60
80
100
120
140
2θ Fig (3.10): this is at 0 hour’s reflection time and synthesized at 1100 C and 1000C
3.3 Optical spectroscopic analysis The UV/Visible absorption spectrum was recorded at room temperature with Spectrophotometer (SHIMADGU 3101PC). The spectrum shows that absorption depends on particle size. It is found that for small particle sizes the absorption peak shifts towards blue as compared to the larger one as shown in fig (3.11) and also shows the exciton peaks. Fig (3.12) shows the absorption peak of the powder refluxed at 0 hour’s. When glycerol is used as capping agent the change in absorption is not that drastic because of clustering of the particles. Clustering of the particles affects energy levels and increases the bandgap energies. Thus we can conclude that glycerol is not a good capping agent since the clustering of particles is unavoidable, which is not expected of a good capping agent.
4.0
0h 3h 9h 12h 16h
3.5
0h
3.0
absorbance
2.5
3h
2.0
9h
1.5 1.0 0.5
12h 16h
0.0 200
300
400
500
600
700
w ave length λ
Fig (3.11): UV absorption of CdS at 0,3,9,12,16 hour’s reflection time and synthesized at 1100 C.
5
4
ads
3
2
1
CdS powder 0 250
300
350
400
450
500
w a v e le n g th λ
Fig (3.12): UV absorption of CdS powder at 0 hour’s reflection time and synthesized at 1100 C.
3.4 Optical limiting The Optical limiting (OL) properties of the CdS nanoclusters are studied by keeping the sample at the focus in f/5 geometry. This geometry is used as a standard, because human eye is equivalent to f/5 optical geometry [58]. The input energy is varied using calibrated neutral density filters, while the output is collected using a calibrated fast photodiode (FND 100). A typical OL setup is shown in Fig (3.13). The input energy where the transmitted output becomes half of the linear transmittance is called the limiting threshold (I1/2) of the material. This parameter is an important factor for choosing a material as an optical limiter. Each experimental point shown in the OL curves is an average of 128 laser pulses to get a better signal to noise ratio. The experiments are repeated to ascertain the reproducibility and to determine the error. An experimental error of 5-10% is observed due to the modification while preparing the solutions for the required linear transmission of 70 –75 %.
3.5 Z-scan The Z-scan technique is a single beam technique, which allows the determination of the real and imaginary parts of the susceptibility [59]. Fig (3.13) shows a typical Z-scan experimental setup.
This technique is a
simple, sensitive, single beam method that uses the principle of spatial beam distortion to measure both the sign and the magnitude of refractive nonlinearities of optical materials. The experiment uses a Gaussian beam from a laser in tight focus geometry to measure the transmittance of a nonlinear medium as a function of the sample position Z, from the focal
plane. The transmittance characteristics of the sample with a finite aperture depend on the nonlinear refractive index, as elucidated below. This technique has several advantages, some of which are: ¾ Simplicity: No complicated alignment except for keeping the beam centered on aperture. ¾ Data analysis is quick and simple except for some particular conditions. ¾ High sensitivity, capable of resolving a phase distortion of λ/300 provided the sample is of high optical quality. Some disadvantages include: ¾ Stringent requirement of high quality Gaussian TEM00 beam for absolute measurements. ¾ For non-Gaussian beams the analysis is completely different. Relative measurements against a standard samples allows relaxation on requirements of beam shape ¾ Beam walk-off due to sample imperfections, tilt or distortions. ¾ Not suitable for measurement of off-diagonal elements of the susceptibility tensor except when a second non-degenerate frequency beam is employed. The Z-scan technique has been used extensively to study different materials like semiconductors, nanocrystals, semiconductor-doped glasses, liquid crystals, organic materials, biomaterials etc. To enhance it’s sensitivity and applicability new extensions have been added. It is a much more sensitive technique. A comprehensive review of different techniques of Z-scan could be found in the references listed in [60].
3.5a Open-aperture Z-scan for optical limiting Experimental details In the open-aperture Z-scan the sample is scanned across the focus using by a PC controlled stepper motor. A part of the input beam split using a glass plate is monitored using a PD to know the fluctuations in the input laser beam. This beam is used to trigger the boxcar averager used for data collection. The transmitted light is then collected using another large area lens of f ~ 100 mm and a fast photodiode. To ensure that the photodiode does not get saturated neutral density filters are used for attenuation. The photodiode output is fed to a boxcar averager/gated integrator and is finally recorded. In the boxcar averager the gate width is fixed at 30 ns and the signal coming from the PD’s is synchronized to fall within the gate, which reduces the noise level. The number of averages from in boxcar is varied accordingly to obtain a good signal to noise ratio. The averaged signal is then sent to an interfaced ADC card and then to a computer. In general nonlinear absorption arises not only due to two-photon absorption but also from processes like ESA, Multi-photon absorption and TPA generated ESA in organic molecules, free carrier absorption in semi conductors, surface plasmon absorption and inter/intraband transitions in case of nanoparticles. In such a case a generalized rate equations that are appropriate to the material under study have to be solved in order to estimate the nonlinear absorption.
3.5b Experimental Set-Up
Fig (3.13): Schematic diagram of the z-scan open aperture setup. Nd:YAG – laser (Spectra-Physics (Quanta-Ray), 6ns pulse width, fundamental wavelength is1064nm, output wavelength is 532nm, energy is 600mj/pulse, 10 pulses/sec) Ir – iris, BS – beam splitter, L-lens, A – attenuator (Filter), PD – Photo diode, B- Boxcar averager, PC-computer, Sstepper motor, S- sample.
The graph between Z-position and normalized transmittance is shown in fig (3.14 – 3.18) at different reflection times of synthesis. From these graphs it was observed that the variation of absorption with respect to the particle size is not that drastic because of clustering. It was also observed that the sample used for the Z-scan started precipitating in the DMF solution. In order to avoid this problem the sample was constantly rotated through out the scan. The Z-scan plot using this procedure is as shown in fig (3.19) at different input intensities. The Z-scan of pure CdS powder was performed with out using any solution. This was done by coating CdS powder on glass plate that gives saturational absorption as shown in fig (3.20) at different input intensities. By this we mean that the energy level was saturated and started to emit. So these graphs show peak at focal point of the lens.
Normalised Transmittance
1.0
0.9
0.8
0.7
0.6
0 h reflection 0.5 -12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
z(mm)
Fig (3.14): Z-scan of CdS at 0 hour’s reflection time and synthesized at 1100 C, in
DMF solution, at I00=83.15x107 W/cm2.
Normalised Transmittance
1.0
0.9
0.8
0.7
3h reflection
0.6
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
Z(mm)
Fig (3.15): Z-scan of CdS at 3 hour’s reflection time and synthesized at 1100 C, in DMF solution, at I00=83.15x107 W/cm2.
1.2
Normalised Transmittance
1.1 1.0 0.9 0.8 0.7 0.6 0.5
9 h reflection
0.4 -12
-10
-8
-6
-4
-2
0
2
4
6
8
10
Z(mm)
Fig (3.16): Z-scan of CdS at 9 hour’s reflection time and synthesized at 1100 C, in DMF solution, at I00=83.15x107 W/cm2.
1.1 1.0
Normalised Transmittance
0.9 0.8 0.7 0.6 0.5 0.4
12 h reflection
0.3 -12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
Z(mm)
Fig (3.17): Z-scan of CdS at 12 hour’s reflection time and synthesized at 1100 C, in DMF solution, at I00=83.15x107 W/cm2.
Normalised Transmittance
1.0
0.9
0.8
0.7
0.6
16 h reflection
0.5 -12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
Z(mm)
Fig (3.18): Z-scan of CdS 16hour’s of reflection time, in DMF solution, at I00=83.15x107 W/cm2.
Z-SCAN CdS WITH ROTATION 1.3 1.2
Normalised Transmittance
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 -20
7
I00=10.46x10 W/cm 7
I00=41.66x10 W/cm 7
2 2
I00=165.87x10 W/cm -15
-10
-5
2
0
5
10
15
Z(mm)
Fig (3.19): Z-scan of CdS at 0 hour’s reflection time and synthesized at 1100 C, in DMF solution, at different input intensities.
1.9 1.9 1.8
7
7
1.7
2
I00=41.66X10 W/cm
1.7
1.6
1.6
1.5
1.5
T
T
1.8
2
I00=20.88X10 W/cm
1.4
1.4
1.3
1.3
1.2 1.2 -30
-20
-10
0
10
z(mm)
1.1 -30
20
-20
-10
z(mm) 0
10
20
2.0 2.0 1.9
7
I00=83.13X10 W/cm
2
1.9
1.8 1.7
1.8
1.6
1.7
T
T
7
I00=165.87X10 W/cm
2
1.5
1.6
1.4
1.5
1.3 1.4 1.2 -20
-10
0
z(mm)
10
20
30
-20
-10
0
10
20
30
z(mm)
Fig (3.20): Z-scan of CdS at 0 hour’s reflection time and synthesized at 1100 C,
powder on glass plate, at I00=20.88x107 W/cm2, 41.66x107 W/cm2, 43.13x107 W/cm2, 165x107 W/cm2 we can see change in saturalabsorption with respect to intensity.
3.6 Nonlinear Scattering In addition to the nonlinear absorption, we have also observed nonlinear scattering at high energies from CdS nanoparticles on DMF solution, which is clearly observed in the far field. Different mechanisms are reported to lead to nonlinear scattering. In a medium consisting of two components at low energy, the medium is rendered homogenous by a good refractive index matching between the two components, whereas at high energy, the intense laser light propagating through the medium makes it a heterogeneous scattering medium because of the Photoinduced refractive index mismatch between the two components. In case of nanoparticles, nonlinear scattering phenomenon is proposed to be due to the induced pseudo-absorbance to the vaporization or the fragmentation of the metal nanoparticles inducing a large light-scattering center around the initial particles. Such vaporization or fragmentation induced by a thermal effect has been reported earlier.[61] The energy concentrated in each particle and available to form the scattering centers is more in the nanoclusters of larger size, and the size of the induced scattering centers increases with time to reach a maximum, leading to more nonlinear scattering.
In case of
nanoclusters suspended in solutions at high energies, the scattering centers produces more numerous fragments confined in the same scattering center. This increases the probability of recombination with the solvent and a more efficient cooling of the scattering centers. Such effects can be efficient in case of nanoclusters in solution state, where as in thin films such a process can lead to an irreversible damage, which was observed at higher energies. The nonlinear scattering (collected at an angle of 5o from the beam axis, sample refluxed as 0 hours) is shown in Fig. 3.21.
0.20 0.18 0.16
scattring
0.14
scat
0.12 0.10 0.08 0.06 0.04 0.02 0.00 1E-3
0.01
0.1 -2
Input Fluence (Jcm )
Fig (3.21): Scattering curve at θ~5o of CdS synthesized at 0 hours reflection time.
3.7 Theoretical Fits Theoretical fits were done for samples refluxed at 0 hrs with rotating samples at different intensities. Using the data the two-photon absorption coefficient, linear scattering coefficient, and nonlinear scattering coefficient were determined. The transmitted intensity with effects of both nonlinear scattering and nonlinear absorption was found by solving the following equation and rate equations (in chapter 1, equation (1.6)) numerically. dI T = −α nl I − βI 2 dz where α nl = α 0 + α s
α s = g s (Δn) 2 Δn = Δn L + Δn Nl
(3.1)
Here ΔnL = (n0DMF-n0CdS), difference of the linear refractive indexes.
ΔnNL= (nnlDMF-n nlCdS),difference of the nonlinear refractive indexes. To solve differential equation we used RK-4 (integration over Z) method. These theoretically fitted graphs are shown in fig (3.22), fig (3.23) and fig (3.24) at different intensities. From these fits we can calculate the two photon absorption coefficient, nonlinear scattering coefficient. For low intensities there was no nonlinear scattering. Nonlinear scattering was observed only at high intensities.
1.1
Normalised Transimittance
1.0
0.9
0.8
0.7
0.6
Exper Data Theoretical fit
7
2
I00=10.46X10 W/cm
0.5 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Z(cm)
Fig(3.22): theoretical fit for the Z-scan curve with rotating sample at 10.46X107 W/cm2 input intensity.
1.1 1.0
Normalised Transimittance
0.9 0.8 0.7 0.6 0.5 0.4 0.3
7
Exper Data Theoretical fit
2
I00=41.66X10 W/cm
0.2 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Z (cm)
Fig(3.23): theoretical fit for the Z-scan curve with rotating sample at 41.66X107 W/cm2 input intensity.
Normalised Transimittance
1.0
0.8
0.6
0.4
0.2
0.0 -2.0
Exper Data Theoretical fit
-1.5
-1.0
-0.5
7
2
I00=165.87X10 W/cm
0.0
0.5
1.0
1.5
Z(cm)
Fig(3.24): theoretical fit for the Z-scan curve with rotating sample at 165.87X107 W/cm2 input intensity.
This table summarizes the results of the theoretical fitted values of the Zscan curves. I00 (W/cm2)
β (cm/W)
g s (Δn0 ) cm-1
g s (Δnnl ) cm-1
α0 cm-1
10.46x107
20X10-8
0.2
0.0
1.27
41.66x107
15X10-8
0.2
0.0
1.27
165.87x107
15X10-8
0.2
9X10-9
1.27
3.8 SEM and EDAX SEM gives the CdS cluster size that is around 700nm as shown in fig (3.25). It clearly shows that particles had clustered. EDAX gives the chemical composition of the sample. It shows that atomic percentage of S=48.97%, Cd=51.03% in CdS nanoclusters shown in fig (3.26).
Fig(3.25): SEM picture of clustered CdS nanoparticles. Fig (3.26): compositions of Cd, S in CdS nanoparticles. This table summarizes the weight, atomic percentages and K-ratios of Cd, S in CdS sample. Cd(L) Weight percentage Atomic percentage K-Ratio
78.51 51.03 0.6459
S(K) 21.49 48.97 0.1529
Chapter4 Conclusions, Future scope & References
4.1Conclusions In this dissertation work the XRD was employed to determine the particle size. In chemical synthesis capping agent plays an important role. The particle size of CdS is found to be 3-6 nm. The result of SEM shows that the CdS clusters size is around 700nm. Thus glycerol that is employed in this work can be concluded as a moderate capping agent because it allows the cluster formation. Clustering of CdS affected the absorption peak that is around 270nm [8] instead of 370nm as expected without clusters. We successfully synthesized CdS nanoparticles and characterized these CdS particles using UV/Visible spectro meter, XRD and SEM. We investigated the optical nonlinear absorption behavior of the CdS particles using Z-scan technique. The results show that optical nonlinear behavior depends on particle size and capping agent. The optical limiting property of CdS nanoclusters was observed only 5% transmition (because of both nonlinear absorption, nonlinear scattering) at 1.65GW/cm2 of input intensity when the probe beam of 532nm from the Nd:YAG laser was used. This was experimentally determined using the Z-scan technique as discussed in chapter-3.
4.2 Future scope Synthesis of nanoparticles, and characterization of their optical behavior is one of the most important research topics. The work presented in the thesis is just an introduction of the optical characterization of nanoparticles. The synthesis CdS nanoparticles with thioglycerol as a capping agent can give interesting results, as thioglycerol is a better capping agent as compared to glycerol. A chemical
synthesis of nanoparticles is easier than any other method and also gives monodispersive fine particles.
More work on nanomaterials synthesis and
characterization is proposed to be done in future. And also TEM picture will give the actual size of nanoparticles, it is proposed to done in future. Semiconducting nanoparticles are more useful in life as quantum dots, qbits (quantum computation), solar cell etc. Making semiconducting nanoparticles and its characterization (optical, electrical and magnetic….) can be a good extension of this project. This dissertation was done using CdS as nanoparticles the same can be done using CdSe, CdTe, ZnS, ZnSe, ZnTe. Optical characterization is just a small area of research in the case of nanoparticles. Future work can be done in electrical and magnetic characterization where these particles in the nano regime can be used in the quantum computing, Qbits….
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