CHAPTER 3. SURFACE PASSIVATED CdS NANOPARTICLES AND THEIR PROPERTIES

Transcription

1 37 CHAPTER 3 SURFACE PASSIVATED CdS NANOPARTICLES AND THEIR PROPERTIES 3.1 INTRODUCTION Nanostructured materials [97, 98] form a new category of materials which bridge the gap between the bulk and the molecular levels. The boundaries, as to when a material has the properties of bulk, nanoparticle, are dependent upon the composition and crystal structure of the solid. The dependence of size arises from (1) change of surface - to - volume ratio with size and (2) quantum confinement effects. Quantum confinement [99, 100] modifies the density of states (DOS) near the band - edges. This leads to the blue shift of band - gap energy, when the particle size falls below a particular value (depends on the semiconductor). Due to the high surface - to - volume ratio of the nanoparticles, the electronic quantum states associated with the surface have significant effect on the optical properties. The surface states arise from the unsatisfied bonds at the reconstructed surfaces and may be affected by the non - stoichiometry and voids. The energies of these surface states lie in the band gap of the quantum dot [101]. These surface states can act as temporary traps for electrons, holes or excitons, quenching radiative recombination and reducing the quantum yields. These surface states can be eliminated by surface passivation. Advantages of surface passivation are, in this process, the capping molecules form a layer on the nanoparticle and restrict the growth of the nanoparticles yielding size confinement and narrow size distribution. In addition, eliminate the surface traps and therefore, some of the non - radiative relaxation pathways [102]. These molecules via steric hindrance or by charge stabilization prevent the aggregation of nanoparticles providing good dispersity [103]. They are tailored to meet specific application based parameters like, chemical reactivity and water - solubility. Using appropriate capping

2 38 agent, smaller nanoparticles with excellent optical properties can be possible, so that these molecules would facilitate integration of CdS nanoparticles into biological or chemical systems, polymeric matrices, QLEDs, photovoltaics and other applications [ ]. For this reason, binding of organic capping agents, polymers, inorganic substances or biomaterials to nanoparticles [108] is essential. Hence, this area of nanoparticle research is opening up new challenges and is progressively under research. This process is carried out by depositing an organic or inorganic capping layer over the nanoparticle surface. CdS is an excellent II - VI semiconductor material with a wide band - gap. It used for a host of applications in optoelectronics. In the recent past, surface passivation of CdS nanoparticles was adopted and deeply investigated by many researchers for variety of applications. Ding et. al., [109] and many others [110, 111] investigated the photoconductivity of surface passivated CdS nanoparticles dispersed in polymer matrix. It was observed that the CdS nanoparticles serve as a charge generation sensitizer that enhances the photoconductivity of the polymer significantly because of the properties of high quantum efficiency of photosensitization and high charge transport to conducting polymer. CdS was treated with p - thiocresol to provide good chemical stability and ensure good solubility. Cai et. al., [112] used L - cysteine capped CdS nanoparticles as fluorescence probe to trace Hg (II) in aqueous solution based on fluorescence quenching effect. The fluorescence intensity of the L - cysteine capped CdS nanoparticles significantly quench with the increase of Hg (II) concentration. It was speculated that small HgS particles were formed on the surface of the L - cysteine capped CdS nanoparticles, which facilitated the nonradiative recombination of electron - hole pairs resulting in luminescence quenching. Similarly, Gattas - Asfura and Leblac [113] synthesized the peptide coated CdS QDs for optical detection of Cu (II) and Ag (I). Herron et. al., [114] and another group of researchers [115] have reported a synthetic approach for the thiophenol capped CdS nanoclusters. Size control in any synthesis method is achieved based on the processes that take place between nucleation and seed growth. In any synthetic approach, ideally all the crystal seeds form simultaneously and grow identically until the precursors in the solution are expended. In most of the cases, the initial stage is followed by a

3 39 process called Ostwald ripening, which causes the larger particles to grow at the expenses of little ones, owing to difference in surface energy [116]. The capping molecules are typically organic molecules made of functional heads like nitrogen, phosphorous, oxygen or sulphur atom and a long hydrocarbon chain. They may be long chain or a short chain molecule, with more than one functional groups / coordinating atoms. The ease with which the capping molecule coordinates with the surface of the nanoparticle depends on the coordinating ability i.e. the electro negativity of the coordinating atoms. In particular, molecules having nitrogen atom with a lone pair or a thiol group can form very strong bonds with the nanocrystals surface [117]. The functional heads attach to the nanoparticle surface through covalent, dative or ionic bonds and are termed as capping groups. Most of the capping organic molecules are distorted in shape and larger than the surface site. The distorted shape of the capping molecule may not provide proper coverage of the surface of the atoms due to steric hindrances between the functional groups. Hence, to get small defect free CdS nanoparticles with narrow size distribution selection of capping agents must be done taking into account of the above factors. A few commonly used organic capping agents include phosphates [118], phosphine oxides [119], carboxylic acids and various thiols [120]. In the present work, four different capping agents, polyvinylpyrrolidone (PVP), a polymeric capping agent, ethylenediamine tetra acetic acid (EDTA), a hexadendate ligand, cysteine, with two coordinating sites and triethylamine (TEA), with a single coordinating site, were employed. Each of these capping molecules possesses one to many coordinating sites, through which they bond to the nanoparticle. Hence, the extent of organic - inorganic interface differs in each case. The investigation of the uncapped and capped CdS nanoparticles revealed that capping agents reduce the particle size as well as significantly tune the optical properties. The size of the synthesized CdS nanocrystals differed with each capping agents. Certain capping structures produce smaller sized CdS nanoparticles more easily than others. And it helps to check if the material acts as a nucleation inhibitor rather than a capping agent. The role of each one of them in determining the size and narrow size distribution, preventing agglomeration, inducing phase transition, eliminating the surface defects was investigated.

4 STRUCTURES OF CAPPING AGENTS The structure of the organic capping agents employed is given below: Cysteine Cysteine is an amino acid with the chemical formula HO 2 CCH (NH 2 ) CH 2 SH. It is a low molecular weight metal - binding ligand. The molecular structure of cysteine is shown in the Figure 3.1. This amino acid has three deprotonation sites, i.e., carboxylic acid, amine and thiol sites, which are potential metal - ion - binding sites. It possesses a good stabilizing capability and prevents the particle aggregation [121]. It is a powerful antioxidant and detoxifier. Cysteine was selected for the possible additional benefit of containing double bonded oxygen (carboxylic group) and an amine groups for further conjugation, while thiol group caps the nanoparticle. Figure 3.1 Molecular structure of Cysteine Polyvinylpyrrolidone PVP is water - soluble polymer made from the monomer N - vinylpyrrolidone with the chemical formula (C 6 H 9 NO) n. The molecular structure of PVP is shown in the Figure 3.2. It offers a unique combination of desired properties including good initial tack, transparency, chemical and biological inertness and very low toxicity. For the synthesis of nanomaterials, PVP serves as a polymeric capping agent. It is multidendate. PVP has acted as a key factor in the shape

5 41 controlled synthesis of variety nanocrystals. It caps the nanoparticle surface either through the double - bonded oxygen or via the lone pair of electrons on the nitrogen atom. PVP is considered to control the nanoparticle growth by forming a boundary over it and prevents agglomeration by steric effect due to the repulsive force acting among the polyvinyl groups [122]. PVP is also reported to play a key role in producing nanoparticles / nanostructures with good stability and size / shape uniformity in comparison with many other polymers [123]. Figure 3.2 Molecular structure of PVP Ethylenediamine tetra acetic acid EDTA is a polyamino carboxylic acid, a novel molecule for complexing metal ions with the chemical formula C 10 H 16 N 2 O 8. The molecular structure of EDTA is shown in the Figure 3.3. It is a hexadentate ligand with six donor atoms, two nitrogen lone pair of atoms and four oxygen atoms of carboxylic acid. A characteristic arrangement of these compounds is a nitrogen atom connected with two carboxymethyl groups: - N (CH 2 COOH) 2. This molecule can be treated as a tertiary amine [124]. EDTA can bind with CdS nanoparticle surface as well as chelate with other transition metal ion simultaneously.

6 42 Figure 3.3 Molecular structure of ethylenediamine tetraacetic acid Triethylamine Triethylamine is the chemical compound with the formula N (CH 2 CH 3 ) 3. It is the simplest symmetrically trisubstituted amine, i.e. a tertiary amine. The molecular structure of TEA is shown in the Figure 3.4. At room temperature, it is in the form of liquid. It has only single binding site (monodendate). It caps the nanoparticle surface through the lone pair of electrons on the nitrogen. Figure 3.4 Molecular structure of Triethylamine 3.3 SYNTHESIS OF CdS NANOPARTICLES USING VARIOUS CAPPING AGENTS In the below adopted synthesis procedures, all the chemicals used were of analytical purity and were used without further purification. The final synthesized CdS products were centrifugalized, washed thoroughly and dried in vacuum. Further, the synthesized products were characterized.

7 3.3.1 Synthesis of CdS nanoparticles using PVP and Cysteine as capping agents 43 A typical reaction consisted of producing stock solution of cadmium by dissolving 0.5 M of cadmium chloride (CdCl 2 ) in 100 ml of de - ionized water for a few minutes under magnetic stirring. To this solution, one gram of PVP was added and the mixture was thoroughly stirred for one hour of duration followed by the addition of 0.5 M sodium sulphide (Na 2 S). The solution turned yellow shortly after the addition of sulphide solution due to the formation of cadmium sulphide nanoparticle (PVP modified CdS). The resulting precipitates were washed multiple times with acetone and the final precipitates were isolated by centrifugation and dried. To synthesize cysteine capped CdS nanoparticles, the same ratio of cadmium and sulphur sources as in above procedure was maintained. Instead of PVP, 0.5 M of cysteine was added. Uncapped CdS nanocrystals were also synthesized by taking 0.5 M cadmium chloride in 100 ml of water followed by the addition of 0.5 M of sodium sulphide under vigorous stirring, without the addition of PVP or cysteine. In all the synthesis process, same molarity of cadmium and sulphur was maintained. The final products were dried in vacuum Synthesis of CdS nanoparticles using EDTA as capping agents The stock solution of cadmium was obtained by dissolving 0.02 M of cadmium chloride (CdCl 2 ) in 50 ml of ethanol (solution A). Solution B was prepared by adding 0.05 M of thiourea (CH 4 N 2 S) and g of EDTA in another beaker containing 50 ml of ethanol. Under vigorous stirring, the solution B was injected into solution A. The mixture of solution A and solution B was heated at about 50 C for one hour. Gradual color change to yellow indicated the formation of CdS nanoparticles. Further, the synthesized yellow color precipitate was washed and centrifuged with water for several times. Finally, the product was dried in vacuum and was stored for characterizations.

8 Synthesis of CdS nanoparticles using TEA as capping agent CdS nanocrystallites with triethylamine as capping were synthesized as follows: 50 ml of 0.5 M cadmium salt solution was made by dissolving cadmium chloride in ethanol and 50 ml of 0.5 M thiourea solution was prepared by dissolving thiourea in ethanol. Triethylamine solution (0.1 M) was prepared by dissolving triethylamine in ethanol. Thiourea solution and triethylamine solution were mixed and stirred at a constant rate, to which cadmium chloride solution was added. The mixture was heated at about 50 C. This resulted in a yellow solution, indicating the formation of CdS. The solution was stirred continuously for 36 hours. Subsequently, the resulting yellow solid products were centrifugalized, washed using ethanol and finally dried in vacuum. 3.4 RESULTS AND DISCUSSIONS Structural Analysis X - ray powder diffraction probes a large number of crystallites that are statistically oriented. The composition and phase purity of the CdS are examined by XRD. Figure 3.5 (a) - (e) shows the powder XRD patterns of the uncapped, cysteine capped, PVP capped, EDTA capped and TEA capped CdS samples scanned in 2 range from 20 to 80. A number of strong Bragg reflections are observed originating from the synthesized CdS samples. The peak reflections positioned with 2 between 24 to 30 correspond to (1 0 0), (0 0 2), (1 0 1) planes. The reflections from the crystal planes (1 0 2) and (1 0 3) are peaked at 2 = 37 and 48 respectively. These two diffracted reflection peaks from planes (1 0 2) and (1 0 3) are characteristics of hcp phase of CdS [125]. The XRD reflections of all the samples were indexed to hexagonal phase of CdS with standard JCPDS and are listed in Table.3.1. The lattice constants observed are slightly different. Similar, observations in the lattice constants of bare CdS and surface modified CdS were reported by Goldstein et. al., [126]. It is observed that for uncapped and cysteine capped CdS peaks from the planes (1 0 0),

9 45 (0 0 2), (1 0 1) are well distinguished, with higher crystallinity. Higher crystallinity of the samples is indicated by stronger and narrower XRD peaks. The XRD pattern of the TEA capped CdS nanoparticles shows additional peaks at 2 = 31.8 and These peaks are identified and assigned to cadmium oxide formation (JCPDS ). Formation of CdO is a common process under ambient conditions [127]. PVP, TEA and EDTA capped CdS samples showed broader XRD peaks (overlapped peaks) and relatively poor crystallinity. The peak broadening related to the formation of nanoparticles is more prominent in PVP capped, EDTA capped and TEA capped CdS samples. The broadening of diffraction peaks compared with bulk CdS is due to reduction in particle size and the extensive surface defects [128]. Irrespective of the change in sulphur source and the capping agents, no phase transition was observed from the powder XRD spectra of the synthesized CdS nanoparticles. Many researchers have discussed the effect of cadmium concentration and cadmium source on the crystalline phase of CdS. Lee et. al., [129] reported a transformation from cubic to hexagonal crystalline phase when higher mass of CdCl 2 was used. To study the effect of Cd source on the crystalline phase of CdS, cadmium acetate and cadmium chloride were used as Cd source with thiourea as sulphur source. With cadmium acetate and thiourea, CdS crystallized in cubic phase. On replacing cadmium acetate with cadmium chloride, the crystalline phase transformation from cubic to hexagonal occurred. This was due to the inherent ability of CdCl 2 to modify the stacking sequence of fcc cubic structure to a more stable hexagonal structure of CdS [130]. Thongtem et. al., [131] also reported that the CdS products synthesized using CdCl 2 in combination with other sulphur sources yielded hexagonal. In the present synthesis, cadmium chloride was employed as the cadmium source for the production of CdS nanocrystallites. All the CdS synthesized nanoparticles belong to the hexagonal phase.

10 46 Figure 3.5 XRD pattern of (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d) EDTA capped and (e) TEA capped CdS samples.

11 Table 3.1 Lattice parameters and crystalline phase for uncapped and capped CdS samples 47 CdS samples Phase JCPDS card no. Lattice parameters (Å) Uncapped Hexagonal a = 4.140, c = Cysteine Capped Hexagonal a = 4.140, c = PVP capped Hexagonal a = 3.974, c = EDTA capped Hexagonal a = 4.15, c = TEA capped Hexagonal a = 4.142, c = TEM analyses TEM allows the imaging of individual crystallites and the development of statistical description of the size and shape of the particles in a sample. High magnification imaging with lattice contrast allows the determination of individual crystallite morphology [132]. From the TEM images, one can determine the level of agglomeration, morphology and the size of the nanoparticles. Figures 3.6 and 3.7 show the TEM and HRTEM images of the synthesized CdS nanoparticles. The size distribution histograms are also presented in Figure 3.8. The detailed observations made from TEM images are given below. (i) The level of aggregation is higher for the CdS nanoparticles in the uncapped sample than in the capped (cysteine, EDTA and TEA) samples. Agglomeration in the capped samples is prevented due to the steric effect of the functional groups present on the capped CdS nanoparticles, via physical and chemical bonding [133]. The EDTA capped CdS nanoparticles are absolutely free from agglomeration. The EDTA capped CdS nanoparticles are uniformly dispersed. TEA capped CdS nanoparticles also show less agglomeration. But, in the case of PVP capped CdS nanoparticles, the agglomeration level is high due to the intra - and the intermolecular cross linking of the

12 48 polymer chain PVP. This can be termed as polymer caused aggregation [134]. Aggregation can also occur due to the smaller size of the nanoparticles. (ii) The TEM image shows that the CdS nanoparticles i.e uncapped [Figure 3.6 (a)], capped with cysteine [Figure 3.6 (b)], PVP [Figure 3.6 (c)] and TEA [Figure 3.6 (f)] possess spherical morphology. A striking observation of the representative TEM patterns of the EDTA capped CdS sample [Figure 3.6 (e)] is that most of the nanoparticles exhibit spherical shape, with a slight deviation [Figure 3.6 (d)] in a few CdS nanoparticles. The non - spherical CdS nanoparticles in this sample are in the size range 10 nm to 30 nm. Mixed morphologies have been reported earlier by Manna et. al., [135] in CdSe nanocrystals. Such variations can occur due to the variation in the concentration of the capping agent as the reaction proceeds. It is worth mentioning that EDTA is a multidendate ligand with an efficient coordinating capability with metal ions. Deng et. al., [136] employed EDTA to synthesize sharp tipped nanorods, ultra thin nanowires and many other 3D nanostructures. (iii) Figures. 3.7 (a) - (f) shows the HRTEM images of the synthesized CdS nanoparticles. The average particle size of the uncapped CdS nanoparticle is around 20 nm, cysteine capped CdS nanoparticle is about 18 nm, PVP capped CdS nanoparticle is about 3 nm, and TEA capped CdS nanoparticle is 5 nm. Figure 3.7 (d) and (e) indicates the HRTEM image of EDTA capped CdS nanoparticles of different sizes. In Figure 3.7 (d) the average size of the EDTA capped CdS nanoparticle is about 10 nm and in the Figure 3.7 (e) the average size of CdS nanoparticle is 2 nm.

13 Figure 3.6 TEM images of (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d), (e) EDTA capped (f) TEA capped CdS samples 49

14 50 Figure 3.7 HRTEM images of (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d) and (e) EDTA capped, (f) TEA capped CdS samples

15 51 (iv) The size distribution histogram [Figure 3.8 (a) - (f)] of the CdS nanoparticles indicated that the average size of the CdS nanoparticles are significantly reduced in the capped CdS samples. Uncapped CdS samples [Figure 3.8 (a)] contain more number of nanoparticles in size 20 nm. In cysteine capped CdS samples [Figure 3.8 (b)], more number of nanoparticles falls in the range of 15 nm. The average size of the CdS nanoparticles is greatly reduced on PVP capping, with a large number of nanoparticles of size 3 nm. The PVP efficiently modifies the Ostwald ripening kinetics to regulate the growth rate of CdS nanoparticles, to give a narrow size distribution. Guo et. al., [137] and Varghese et. al., [138] obtained narrow size distribution of nanoparticles with PVP capping in ZnO and CdSe. The size distribution histogram of EDTA capped CdS sample shows that there more number of nanoparticles in the range 1-6 nm with maximum numbers of particles around 2 nm [Figure 3.8 (d) and (e)]. Figure 3.8 (f) shows that the TEA capped CdS samples contain more number of nanoparticles in the range 5 nm. The TEM results shows that the use of cysteine, PVP, EDTA and TEA as capping agents has significantly reduced the CdS nanoparticle size with respect to the uncapped CdS nanoparticle.

16 52 Figure 3.8 Size distribution Histogram of (a) Uncapped, (b) Cysteine capped, (c) PVP capped (d), (e) EDTA capped and (f) TEA capped CdS samples

17 FTIR Analyses The surface modification of the CdS nanoparticles with cysteine, PVP, EDTA and TEA was confirmed by FTIR spectra as shown in Figure 3.9. Figure 3.9 (a) corresponds to FTIR spectrum of cysteine capped CdS samples. Capping of the CdS nanoparticles with cysteine is accomplished by a thiolate bond between the amino acid and the CdS nanoparticle surface. The vibrations due to S - H stretch mode for cysteine are found in the range cm - 1 [139, 140]. The thiol group has an affinity toward Cd ions. The absence of the S - H mode from the spectra can be attributed to the cleavage of S - H bond and the formation of - S - Cd bond. Quite similar results were earlier reported for cysteine derived silver nanoparticles and gold nanoparticles [141, 142]. In both these cases, the disappearance of the band corresponding to thiol group was the key point. The presence of the bands at 1385 cm -1 and 1580 cm -1 is due to COO symmetric stretch and - NH + 3 asymmetric bending mode [143, 144] respectively. All these observation lead to the conclusion that nanoparticle surface is capped by cysteine via a thiol group. PVP serves as a polymeric capping agent; it was used to stabilize metal nanoparticles like Rh, Ir, Pt, Pd from agglomeration [145]. In all these cases, the carbonyl group of PVP interacts with the nanoparticle. Figure 3.9 (b) shows the FTIR spectrum of PVP capped CdS nanoparticles. Wang et. al., [146] in their work, assigned the peak at 1667 cm -1 to C = O group of pure PVP. In the spectrum of PVP capped CdS, [Figure 3.9 (b)] the band at 1623 cm -1 is attributed to C = O. The peak shift toward lower wave number reveals an interaction between the CdS and the amide group of PVP. This leads to the transfer of the electron cloud of C = O, resulting in the decrease of cloud density around it. The bands at 2928 cm -1 and 2860 cm - 1 correspond to CH 2 stretch modes of pyrrolidone ring and the tertiary C - H stretch of PVP [147], respectively. Figure 3.9 (c) shows the FTIR spectrum of EDTA capped CdS nanoparticles. Spectral characterization of EDTA compounds in the cm - 1 region gives information about the structural changes caused due the metal ion binding to EDTA. For unionized EDTA, strong absorption bands due to the carboxylic group ( COOH) can be identified in the region cm 1 [148],

18 54 whereas the absorption bands are shifted to the region cm 1 for coordinated EDTA [149]. Alanah and Simona [150] reported that on coordination with metal ion the band at 1697 cm - 1 due to C = O, shifted to lower wave number 1633 cm 1. Kasapo glu et. al., [151] also observed a similar shift of the strong band at 1694 cm -1 associated with C = O bonds of the carboxyl groups of EDTA to 1629 cm -1 which is due to bonding with metal ions. From the observations made, it is evidenced that the absorption band structure at 1650 cm 1 corresponds to the C = O of coordinated to EDTA. Hence, the EDTA coordinates through the carboxyl group with the surface of the CdS nanoparticle. All these data indicate the presence of EDTA on the surface of the CdS nanoparticles. Figure 3.9 (d) corresponds to FTIR spectrum of TEA capped CdS nanoparticles. TEA is a tertiary amine. This does not show any absorption band corresponding to N - H stretching in the region cm -1, as TEA neither have an N - H stretch nor an N - H wag. Generally, the band corresponding C - N stretch at occurs at 1214 cm -1 (non - aromatic) for TEA. From the spectra of TEA capped CdS, it is observed that the absorption band due to C - N stretching occurs at 1180 cm -1. This shift in C - N band is due to the interaction of CdS through the lone pair of electrons on the nitrogen of TEA. Further, the band at 2934 cm -1 corresponds to C - H stretching, the band at 1460 cm -1 corresponds to CH 3 stretching and the band at 1380 cm -1 corressponds to CH 2 bending. All these observations prove the TEA capping to the CdS nanoparticles.

19 55 Figure 3.9 FTIR spectra of (a) Cysteine capped, (b) PVP capped, (c) EDTA capped and (d) TEA capped CdS samples UV - Vis Absorption Analyses The study of optical absorption is important to understand the behavior of the semiconductor nanocrystals. Figures 3.10 (a) - (e) shows the UV - Vis absorption spectra of the uncapped, PVP capped, cysteine capped, TEA capped and EDTA capped CdS, respectively. Optical excitation of electrons across the band gap produces an abrupt absorption at that wavelength corresponding to the band gap energy. This is known as absorption edge. The absorption edge observed for the uncapped CdS is 434 nm. The absorption edges for the capped samples are 433 nm, 365 nm, 340 nm and 388 nm with cysteine, PVP, EDTA and TEA respectively. The observed blue shift in the absorption edge is the reflection of the band gap increase owing to quantum confinement effect [152]. The more is the blue - shift in absorption edge with respect to the uncapped CdS, the smaller are the nanoparticles synthesized. EDTA capped CdS nanoparticles show large blue - shift with respect to the uncapped samples due to large number of nanoparticles are in 2 nm range. Blue - shift of the

20 56 absorption edge observed in PVP capped CdS nanoparticles is relatively lesser with respect to EDTA capped CdS nanoparticles. From the absorption onset observed from the UV - Vis spectra of capped samples, it is clear that the cysteine capped CdS nanoparticles are larger in size. One nanometre blue shift is observed for cysteine capped CdS nanoparticles with respect to the uncapped CdS nanoparticles. This could be due to the presence of similar sized nanoparticles in both the uncapped CdS sample and cysteine capped sample [Figure 3.8 (a) and (b)]. The important observation is that cysteine capping has effectively reduced the size of the larger CdS nanoparticles. All the profiles show excitonic peak (except for TEA capped CdS), which are well shifted from the absorption peak expected at 515 nm for bulk CdS. Absorption spectrum without any peaks have been reported by Marteinz et. al., [153] and by Mahuani et. al.,[154] for CdS and ZnS nanoparticles, respectively. The band gap energies have been determined from ) 2 Vs (h ) graphs shown in Figure The absorption edges and the band gap values are listed in the Table From the table, it observed that the band gap increases as the absorption edges blue - shifts for the cysteine capped, PVP capped CdS, EDTA capped CdS and TEA capped CdS nanoparticles, with respect to the uncapped CdS nanoparticles. The band gap values are approximately same for the uncapped and cysteine capped CdS nanoparticles, as there is only one nanometre shift in the absorption edge.

21 57 Figure 3.10 UV - visible spectra of (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d) EDTA capped and (e) TEA capped CdS samples

22 58 Figure 3.11 Plot of ( ) 2 Vs (h ) (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d) EDTA capped and (e) TEA capped CdS samples

23 59 Table 3.2 Absorption edge and the band gap for the uncapped and capped CdS samples CdS Samples Absorption edge (nm) Band gap (ev) Uncapped Cysteine capped PVP capped EDTA capped TEA capped Photoluminescence Analyses The surface plays a major role in determining physical and chemical properties of nanocrystals. In particular, the PL is extremely sensitive to the surface passivation and special care is required to remove potential trap sites and to achieve high fluorescence quantum yields. Generally, for CdS, two types of emission are possible, the band edge (BE) emission and the surface defect emission that lie in the wavelength range below and above 500 nm [155, 156], respectively. The trap states arise due to the unsaturated dangling bonds on the nanoparticle surface. Recently, Peng et. al., [157] have reported the synthesis of high quality CdS nanocrystals; however the nanocrystals tend to give deep - trap luminescence. Tang et. al., [158] reported that the ethylenediamine capped CdS nanoparticles show the band edge PL peak at 450 nm. Quite often, different synthesis strategies are adopted to passivate the dangling bonds, so as to enhance the BE emission. Zhu et. al., [159] in their work to produce disodium ethylenediamine tetra acetic acid (EDTA) capped CdS nanoparticles, observed two peaks: one at 480 nm was assigned to band edge luminescence and the broad band around 700 nm was ascribed to the surface state emission. These synthesized CdS nanoparticles when doped with europium exhibited a different optical behavior. It was observed that these Eu - doped CdS nanoparticles quenched the surface state emission and enhanced the band edge emission with longer refluxing time. Han et. al., [160, 161] also adopted reflux treatment to eliminate the surface states. Reflux treatment ensures direct contact of the capping agents with

24 nanocrystals surface by removing water. In the present research work, the synthesis of CdS nanoparticles is carried out using water and ethanol as solvent. 60 Figure 3.12 presents the PL spectra of uncapped CdS and capped CdS samples. The PL spectra of uncapped CdS nanoparticles exhibit two emission peaks: a weak blue emission around 440 nm corresponding to BE emission and the broad emission at 708 nm which is due to defects. The emission peak located in the range nm corresponds to red emission, which is associated with sulphur vacancies in CdS nanoparticles [162, 163]. Zhang Jun et. al., [164] reported a weak BE emission around 456 nm and a strong emission at 605 nm, giving an orange emission. The blue emission is attributed to the radiative recombination of elelctron - hole pair. The emission peaks of the uncapped and capped CdS samples are listed in Table 3.3. The PVP capped and cysteine capped CdS nanoparticles also exhibit two similar emission peaks but there is no significant peak shift in band edge emission. There is a considerable peak shift in the defect state emission. The strong interaction between the capping molecule and the nanoparticle surface may result in relatively weak band edge emission and strong broad trap state emission which are due to the self - quenching [89] process. The surface state emission is blue shifted on PVP capping and it is slightly red shifted on cysteine capping, with respect to the uncapped CdS samples. This is because in the process of the ligands passivating the nanoparticle surface, surface reconstruction occurs. Kilina et. al., [165] reported that during surface reconstruction (bond saturation), the removal of surface states lying in the band gap is possible. However, the surface reconstruction is effective only in smaller nanoparticles. The inset in Figure 3.12 shows the expanded view of band edge emission. It is observed that the intensity of BE emission is slightly decreased on capping (PVP and cysteine), while there is considerable enhancement in the intensity of the defect state emission. On capping with EDTA and TEA, the defect state emission is completely eliminated and the band edge emission is enhanced. The band edge emission of EDTA capped CdS nanoparticle peak is at 347 nm. Multiple PL peaks occur at 388 nm and at 404 nm for TEA capped CdS nanoparticles. This could be due to the formation of CdS nanoparticles and CdO nanoparticles (as evidenced by XRD).

25 61 Yang et. al., [166] also observed multiple peaks in the PL spectrum of a sample, containing two kinds of anions (S and St ) and a common cation (Cd 2+). The existence of the multiple peaks was explained due to the formation of CdS nanoparticles and the CdSt + functionalized CdS nanoparticles. The band edge emission is blue shifted with respect to the uncapped CdS nanoparticles on capping with EDTA and TEA. The blue shift in PL emission peak could be attributed to size reduction of CdS nanoparticle [167] and is in good agreement with TEM results. EDTA and TEA molecules efficiently passivate the surface defects and control the nanoparticle growth at their initial stages before Ostwald maturation process. Thus, small sized good quality CdS nanoparticles are synthesized. Table 3.3 Band edge emission and surface state emission peaks for the uncapped and the capped CdS samples CdS samples PL emission peak (nm) Band edge Surface defect Uncapped Cysteine capped PVP capped TEA capped 388, 404 No emission EDTA capped 347 No emission The PL results proved that one can modulate the emission of the CdS nanoparticles by using different capping agents. It is possible to regulate the relative ratio of surface state emission to band edge emission. CdS nanoparticles synthesized using ethanol as solvent exhibit minimum / almost no defect states. By EDTA and TEA capping, the surface state emission was eliminated and the band edge emission was enhanced.

26 62 Figure 3.12 PL spectra of (a) Uncapped, (b) PVP capped, (c) Cysteine capped, (d) EDTA capped and (e) TEA capped CdS samples. Inset shows the expanded view of the band - edge emission Raman Analyses Raman is a non - destructive technique for structural study of the material to assess sample qualities such as microcrystallinity, homogeneity and surface

27 63 conditions. It is an inelastic process in which incoming photons exchange energy with the crystal vibrational mode. In an ideal bulk crystal, the phonons are represented by plane - wave - like functions having a spatial extension over the entire crystal. However in nanocrystals, the confinement of phonons results in the relaxation of certain selection rules, ultimately leads to asymmetric broadening and low frequency shift of 1 LO ( longitudinal optical phonon modes) Raman peaks [168, 169]. The Figure 3.13 shows the Raman spectra of uncapped and capped CdS nanocrystals respectively. The 1 LO and 2 LO peaks in the Raman spectra of the synthesized CdS samples are listed in Table 3.4. The peak at cm -1 Raman spectrum of uncapped CdS nanoparticle corresponds to the 1 LO and its overtone 2 LO appears at cm -1. It is observed that the extent of Raman shift varies on capping and the 1 LO peaks are broadened for the capped CdS with respect to the uncapped CdS. The 1 LO Raman peaks of the capped CdS are red shifted with respect the uncapped CdS. Spanier et. al., [170] explained that the shift in Raman peaks are due to various factors like phonon confinement, stress, size distribution and defects. This shift in wavelength towards longer wavelength can attributed to phonon confinement and defects. This is also evident from optical absorption studies and TEM analysis. Table LO and 2 LO Raman modes for uncapped and capped CdS samples CdS Samples 1 LO (cm - 1 ) 2 LO (cm - 1 ) Uncapped Cysteine capped PVP capped TEA capped EDTA capped Tanaka et. al., [171] have reported that a red shift and a blue shift in the Raman peaks of the nanocrystallites are the outcome of the tensile and compressive

28 64 stresses affect in the samples. Mohanta et. al., [172] have reported that 1 LO modes of CdS nanocrystals appear at 306 cm -1. The first optical phonon mode was observed at 302 cm -1 for CdS nanocrystals by Nandakumar et. al., [173]. Another important observation made by the same authors, was that the synthesized samples did not exhibit size dependence shift, as the red shift due to phonon confinement is compensated by the blue shift due to the surface pressure effects [174, 175]. Figure 3.13 Raman Spectra of (a) Uncapped, (b) Cysteine capped, (c) PVP capped, (d) EDTA capped and (e) TEA capped CdS samples 3.5 CONCLUSION In summary, uncapped and capped CdS nanoparticles have been synthesized by wet chemical method. The capping agents used were PVP, cysteine, TEA, and EDTA. The average size of the synthesized CdS nanoparticles was determined from the TEM analysis. The average size of all the capped CdS

29 65 nanoparticles was smaller than that of the uncapped CdS nanoparticles. The capping agents cysteine, PVP, EDTA and TEA played a vital role in reducing the nanoparticle size. Capping the CdS with PVP yielded nanoparticles with average size 3 nm of uniform distribution. Capping the CdS with EDTA also resulted in the formation of nanoparticles of 2 nm, but with a broad size distribution. These observations highlights the fact that, (1) PVP acted as a growth inhibitor as well as a size stabilizer and (2) EDTA serves as an inhibitor to agglomeration as well as shape directing agent.the optical properties were investigated using UV - Visible and photoluminescence techniques. Size reduction is evident from the blue - shift in the absorption edge with respect to the bulk CdS. The structural properties were analyzed from the X - ray diffraction techniques. The synthesized CdS nanoparticles belonged to hexagonal phase. The interaction of the capping molecules was confirmed by FTIR analyses..