Synthesis and characterization of group 11, 12 and 13 metal selenocarboxylates potential single molecular precursors for metal selenide nanocrystals 3

Chapter Chapter 8. ZnSe & CdSe NPs Synthesis of ZnSe and CdSe NPs from Neutral Zn(II) and Cd(II) Selenocarboxylates 8.1 Introduction ZnSe is a wide band gap (2.7 eV) semiconductor.142 Due to the quantum confinement effect, their NCs are interesting emitting materials in the blue to the ultra violet range.116 To date, many syntheses on ZnSe NP have been reported. Briefly, Peng and his co-workers have synthesized highly monodispersed ZnSe QDs from ZnO precursor in noncoordinating containing fatty acid or amine.116 Recently, an Italian research group has managed to control the morphology and phase of the ZnSe NCs by conducting the reaction in long-chain alkylamines and alkylphosphines.143 In addition, aqueous soluble ZnSe NP has been prepared by Alexander et al. and they have found that post-preparative irradiation on the synthesized ZnSe NPs greatly improve their luminescence quantum yield.144 A simple one-pot synthesis of ZnSe NP by thermally decomposed the diselenocarbamate precursors in TOP/TOPO has been reported by O’Brien group few years ago.78, 145 CdSe semiconductor has a much narrow band gap (1.74 eV) as compared to ZnSe. Till today, CdSe QD remains an interesting material due to quantum size effects,146 the band gap of CdSe NCs increases as their size decreases, and thus the emission color of the band-edge PL of the NCs shifts continuously from red (centered at 650 nm) to blue (centered at 450 nm) as the size of the NCs decreases. Many applications have been emerged from this remarkable property of CdSe NPs.66 CdSe NCs with close to monodispersed size distribution and high crystallinity became available in the early 1990s by use of dimethylcadmium as the cadmium precursor.67, 120 Chapter 147 ZnSe & CdSe NPs This organometallic approach has been well developed during the past 10 years in terms of the control over the size,10 shape,72, 101, 120, 148 and size/shape distribution of the resulting CdSe NCs. Besides, O’Brien group also demonstrated that high quality CdSe NCs can be obtained through single source precursor method.78, 79, 149 The use of single-source molecular precursor, as initially reported by O’Brien et al., had proven to be an efficient route to high quality, crystalline monodispersed NPs of semiconducting materials.78, 79, 145, 149 Further, Hampden-Smith and our laboratory have demonstrated that high quality group 12 metal sulfides can be synthesized from metal thiocarboxylates.84, 85 Therefore, in this chapter, we will explore the synthesis of ZnSe and CdSe NPs using two neutral metal selenocarboxylate precursors we have prepared, namely, [Cd(SeC{O}Ph)2] and [Zn(SeC{O}Tol)2]·(H2O). The optical property of the synthesized NPs has also been presented. The purpose of using the neutral metal selenocarboxylate over [M(TMEDA)(SeC{O}R)2] or [M(2, 2’-bipy)(SeC{O}R)2] is to avoid TMEDA and 2, 2’-bipy which can also compete with other capping agents. These neutral metal selenocarboxylates eliminate the influential contribution of these additional capping agents on the shape, size and properties of the NCs synthesized. This will help us to further advancement of knowledge in this bottom-up approach to metal selenide NCs. 8.2 Results and Discussion Fast injection of precursor into hot surfactant at elevated temperature can lead to a sudden burst of monomers followed by an instantaneous and short nucleation. This is one of the most important criteria in order to produce a monodispersed NPs which many research groups have demonstrated this in the past.10, 66, 67, 72, 101 TOP is 121 Chapter ZnSe & CdSe NPs commonly used in the nanosynthesis due to its high boiling nature. Besides, it can also stabilize a wide range of complexes in solution, unlike the long chain amine which decomposed the precursor at room temperature.86, 87 We found that [Zn(SeC{O}Ph)2] precursor is not soluble in TOP, thus we didn’t use this precursor in our studies. 8.2.1 Optical Properties of ZnSe NPs In many occasions, high quality CdSe NPs were synthesized from the combination of TOP and TOPO surfactants.10, 67, 72, 101 However, we were unable to synthesize good quality ZnSe NPs from [Zn(SeC{O}Tol)2]·H2O under similar conditions. In most cases, a colorless solution was obtained and based on the reported syntheses, a yellow solution, indicates the formation of ZnSe NCs, should have formed after the injection of precursor.143, 144 In addition, no precipitate was obtained by adding large amount of MeOH to this colorless solution. Thus it is suspected that the precursor has decomposed in the TOP/TOPO solution, reacted with the surfactants and formed a stable inorganic complex which has a very good solubility. Varying the amount of TOP, TOPO and reaction temperature did not change the result. In fact, similar phenomenon was observed by Philippe and Margaret when they tried to synthesize ZnSe from diethyl zinc in TOP/TOPO solution.150According to them, the dispersed NCs are too small that they cannot be isolated by standard solvent/nonsolvent precipitation. HDA is another high boiling surfactant that has been used extensively for the preparation of various semiconductor NPs.87, 91, 150 When the TOP solution of [Zn(SeC{O}Tol)2]·H2O was injected into hot HDA solution, immediately clear yellow solution was observed indicated the formation of ZnSe NP. However, the NPs were unstable in the HDA solution and precipitated out in less than 122 Chapter ZnSe & CdSe NPs min. An additional of TOPO surfactants was found to improve the stability of the NPs in the reaction solution. The ZnSe NPs were first synthesized at 220 ºC with the molar ratio of precursor to HDA to TOPO fixed at 1:7.4:15.5. The growth of the ZnSe NPs was monitored by recording the optical spectra after min, 15 and 30 and the evolution of the absorption spectra over time for ZnSe is shown in Figure 8.1. Direct band gap method142 was used to calculate the approximate band edge of different ZnSe samples. The detailed steps in deriving the band edge from UV-vis spectrum is shown in the appendix section. Figure 8.1. UV-absorption spectra of ZnSe NPs taken at different time intervals (The absorption spectra at 15 and 30 are identical). From the absorption spectra, it clearly showed that all the samples have similar band edge (2.83 eV) and size (4.8 nm). Thus this indicated the ZnSe NPs not undergo Osward ripening when the heating is prolonged.151 Macrocrystalline ZnSe has an optical band gap of 2.58 eV (480 nm) at room temperature.142 Hence, the band edges for these samples are blue-shifted in relation to the bulk. Further, the blue shift is associated with the ZnSe NPs being smaller than the bulk ZnSe. The 123 Chapter ZnSe & CdSe NPs photoluminescence spectra of these samples show a broad band edge emission maximum at 436 nm (λexc. = 381 nm) as shown in Figure 8.2. Figure 8.2. Photoluminescence spectrum of ZnSe NPs at different time internals. The emission peaks are slightly red shifted in relation to the band edge and this could be attributed to recombination from surface traps.152, 153 A shoulder at 463 nm was observed in the emission spectra of ZnSe NPs and could be due to the deep trap emission. Unfortunately we were unable to identify the emission peak at 413 nm. In fact, similar emission peak has been observed for ZnSe NP by O’Brien’s group.78, 145 According to them, this peak could be attributed to the concentration of NPs in solution was too high which cause a re-emission or the recombination of surface traps. Varying the concentration of HDA and growth temperature not affect the size of the ZnSe NPs based on the approximate size calculated from UV absorption spectra as shown in Figure 8.3. 124 Chapter ZnSe & CdSe NPs Figure 8.3a. UV-absorption spectra of Figure 8.3b. UV-absorption spectra of ZnSe NPs isolated at various temperatures ZnSe NPs isolated at various HDA after of heating. concentration after of heating. 8.2.2 Optical Properties of CdSe NPs An immediate color change from yellow orange to red was observed when the TOP solution of [Cd(SeC{O}Ph)2] was injected into TOPO (0.8 g) at 220 ºC, indicating the formation of CdSe NPs. The estimated size for the CdSe NPs using the direct band gap method142 for the sample isolated at 10 and 20 are 8.8 nm (1.84 eV) and 8.9 nm (1.83 eV) respectively. When the experiments were repeated with the addition of trace amount of HPA (44.68 mg), we noticed a drastic improvement on the quality of the CdSe NP was noticed as shown in Figure 8.4. As mentioned earlier, Alivisatos et al. have noted that the presence of HPA has improved the growth of CdSe NPs.7 Overall, smaller size CdSe particles were obtained based on the blue shift of UV-absorption spectra of the samples isolated at 10 (2.06 eV, 5.6 nm) and 20 (2.05 eV, 5.7 nm) intervals. Further, the obtained CdSe NPs are monodispersed as the patterns of obtained UV-absorption spectra are resembled to those that reported for monodispersed CdSe NPs.72, 101 A sharp peak at ≈ 433 nm could be assigned to the higher spin-orbit component of the 1s – 1s transition.154 125 Chapter ZnSe & CdSe NPs Figure 8.4. Optical absorption spectra of CdSe NPs synthesized with/without HPA. To examine the effect of HPA on the growth of CdSe NPs, the samples were synthesized in various HPA concentrations and absorption spectra of the 10 samples where recorded. The UV-absorption spectra of these samples were compiled in Figure 8.5. Clearly shown in Figure 8.5, that minimum amount of HPA (e. g., 44.68 mg) is sufficient to produce high quality CdSe NPs. Further, increased the HPA concentration doesn’t lead to smaller CdSe NP. This finding is consistent with those reported by Alivisatos et al.72, 101 126 Chapter ZnSe & CdSe NPs Figure 8.5. Uv-absorption spectra of CdSe NPs synthesized at different HPA concentration. Based on the UV-absorption spectra, varying the TOPO amount (the molar ratio between HPA and TOPO is maintained at 1.5) and growth temperature no affect the size of the CdSe NPs as shown in Figure 8.6. However, we noticed that samples that prepared at higher temperature and high TOPO concentration are more stable after they were redispersed in toluene. Figure 8.6a. UV-absorption spectra of Figure 8.6b. UV-absorption spectra of CdSe NPs isolated at different CdSe NPs isolated at different TOPO temperature after of heating. concentration after of heating. 127 Chapter ZnSe & CdSe NPs The optical absorption spectra of the CdSe measured over time period are shown in Figure 8.7. The optical absorption edge 2.14 eV (t = min, 580 nm) shows a blue shift in relation to bulk CdSe, 1.73 eV (716 nm). The band edges of the subsequent samples are as follows: 2.04 eV (t = 10 min, 607 nm); 2.01 eV (t = 15 min, 617 nm). While the calculated diameter for all the samples are as follows: 5.1 nm (t = min); 5.8 nm (t = 10 min) and 6.0 nm (t = 15 min). Unlike the ZnSe, the size of CdSe grows larger as the time of heating is increased. The photoluminescence spectrum of CdSe (t = min) shows a red shift in relation to the corresponding optical spectra as show in Figure 8.8. The photoluminescence peak is narrow with an emission maximum at 565 nm (λexc. = 450 nm). In the case of CdSe, no deep-trap emission is observed in the photoluminescence spectra. Figure 8.7. Optical spectra of CdSe NPs taken at different time intervals (250 ºC; 2.6 g of TOPO; 44.7 mg of HPA). 128 Chapter 11 Chapter 11. Significant & Future Work Summary, Highlight and Possible Extension for Future Work 11.1 Significant of Current Study In this project, number of Group 11, 12 and 13 metal selenocarboxylates compounds have been synthesized for the first time. Their structures vary from monomeric to polymeric as revealed by the X-ray crystallographic. Majority of the synthesized metal selenocarboxylates are isostructural to the corresponding metal thiocarboxylates. Usually steric hindrance is expected in RC{O}Se– as compared to RC{O}S– due to the large size of selenium atom. Interestingly, our studies demonstrated that the large size of selenium atom which is also more polarizable doesn’t inflict great change on the bonding motif of the RC{O}Se– anion. Monoselenocarboxylate (RC{O}Se–) of Group 11, 12 & 13 metal ions are stable toward air and moisture. However, a stringent experiment conditions e. g., argon environment, purified starting materials and solvents, etc. are required to synthesize these metal selenocarboxylates. In addition, it has been noticed that the physical properties, e. g., color and solubility of these metal selenocarboxylates resembled closely to the corresponding metal thiocarboxylates. It has been observed that the chemistry between metal thio- and selenocarboxylates need not be the same or completely different as what our group has reported earlier. For example, in chapter 2, it is discovered that the ME3 (E = S, Se) trigonal planar kernels were presented in both [M(SC{O}Ph)3]– and [M(SeC{O}Tol)3]– (M = Zn2+, Cd2+, Hg2+) anions. It has been proven, in both cases, that these trigonal planar kernels are sustained by the geometrical restraints imposed 170 Chapter 11 Significant & Future Work by the presence of carbonyl groups in the thiobenzoate and selenobenzoate ligands. Furthermore, metal NMR and ESI-MS data have been used to probe the ligand exchange species [M(SeC{O}Tol)n(SC{O}Ph)3–n]– (M = Zn – Hg, n = – 0) in solution. The chloride substitution reaction as observed in chlorinated solvent served as a supporting evidence for proof the lability of these metal thio- and selenocarboxyalate complexes. However these [M(SeC{O}Tol)3]– anions cannot act as metalloligands to bind to alkali metal ions under the experimental conditions employed. This is different from the reactivity of [M(SC{O}Ph)3]– anions. It may be noted that Group 13 metal selenocarboxylate anions are found to have similar chemical properties (e. g., crystal structure, solubility) to the corresponding metal thiocarboxylates. Further, [M(SeC{O}Ph)4]– anion has been used as metalloligand to construct hetero metal aggregates similar to the corresponding thio analogues. In addition, unprecedented bonding modes such as µ3-Se2,O, µ2-Se,O and µ2-Se, have been observed in these hetero metallic aggregates. Unlike the Group 13 metal thiocarboxylates, metathesis reaction between the metal salts and appropriate amounts of the corresponding alkali-metal selenocarboxylates in either H2O or MeCN solvent are found to be a suitable synthetic route to obtain pure Group 13 metal selenocarboxylates.Hence, it highlightes the unique characteristic of these Group 13 metal selenocarboxylates. In this work, an asymmetric dimer and tetrameric silver selenocarboxylates have been synthesized. Interestingly the chemistry of silver selenocarboxylates is found to be different from the thio- analogues and it doesn’t resemble the chemistry of copper selenocarboxylates as well. Another interesting feature that has been discovered in this study is that the chemistry of Group 11 (Cu(I) & Ag(I)) metal 171 Chapter 11 Significant & Future Work selenocaboxylates are often found to behave differently from the corresponding thiocarboxylates. Metal thiocarboxyaltes have been found to be suitable single-source precursors for metal sulfides. In this study, several Group 11 – 13 metal selenocarboxylates have also been proved to be suitable single-source precursors for metal selenide powders (chapter & 4) and a conventional synthetic strategy for the metal selenide bulk materials at relatively low temperature have been developed. Similarly, some of these metal selenocarboxylate precursors allowed us to have access to new ternary metal selenide materials, e. g., AMSe2 (A = Na, K; M = Ga, In), as illustrated in chapter 3. The properties of these ternary selenides are waiting to be explored. As highlighted in chapter 5, NPs offered plenty of interesting properties as compared to the bulk materials. Hence, during the past decade, many research groups have devoted their attentions to develop a conventional synthesis to various NPs. In the second part of this project, some of the synthesized metal selenocarboxylates have been employed in the nanosynthesis. A wide range of metal selenide NPs, e. g., Ag2Se, Cu2-xSe, ZnSe, CdSe, AgInSe2 and CuInSe2 have been synthesized from the corresponding metal selenocarxylate precursors in solution at low temperature. The synthesized NPs, e. g., Ag2Se, ZnSe, CdSe and AgInSe2 are close to monodispersed hence no size selection is required. Further, the synthesized NPs, e. g., ZnSe and CdSe are freely soluble in non-polar solvents, such as toluene. Thus these NPs can be further process to use in other applications. It has been reported that morphology of NPs can influence its physical property. In this project, the growth mechanism of each metal selenide NPs has been well elucidated and crucial parameters that are believed to affect the growth of NPs have been identified in the nanosyntheses. It has been 172 Chapter 11 Significant & Future Work concluded that the shape and size of the metal selenide NPs can be tuned by varying the experiment conditions (heating duration, temperature, surfactant & concentration). In chapter 6, an unprecedented Ag2Se nanocube and faceted NCs have been synthesized from a low temperature amine-mediated nanosynthesis (as shown in Scheme 11.1). These unique shape NPs are found to aggregate periodically on the TEM grid, which formed a cubic and hexagonal close pack lattice. The morphology of these NPs is found to inflict such uniform aggregation. Hence, this may imply that these NPs are good candidates for synthesizing photonic crystals. Scheme 11.1 A previously unknown metastable orthorhombic phase AgInSe2 nanorod has been synthesized (as shown in Scheme 11.2). This new phase material is found to be isostructural to AgInS2, hence it may inherit some interesting electronic properties from AgInS2 which is not seen in the usual tetragonal phase AgInSe2. The good solubility of the synthesized AgInSe2 nanorods may open a venue for studying the novel properties of this material, e.g., interesting third-order nonlinear optical properties have been discovered for the first time as discussed in chapter 9. The good 173 Chapter 11 Significant & Future Work solubility of the AgInSe2 nanorods also allows one to utilize the NPs in many applications, e.g. fabrication of polymer thin films containing discrete NCs. Scheme 11.2 11.2 Suggestion for Future Work In this project, a number of new metal selenocarboxylates have been synthesized. In some cases the chemistry of metal selenocarboxylate is found to be independent of the thiocarboxylates. Thus, in future, an extension of current work to telluride analogues is worth pursuing. Besides, the metal tellurocarboxylates might be prominent single-source precursors for metal telluride bulk, thin film and NPs. Transition metals and lanthanides have attracted plenty of attentions recently due to their unpredictable coordination geometry at the metal center, which lead to the formation of interesting structural motifs. In this project, it has been shown that the monoselenocarboxylate ligand is capable of binding both hard and solf metal center using carbonyl oxygen and selenium respectively. Hence, it is worth extending the current study to transition and lanthanides metals. In this project, close to monodispersed ZnSe and CdSe NPs have been synthesized. As discussed in chapter 8, the ZnSe NP shows poor luminescence. Thus, the experimental conditions may be tuned in the future to optimize its optical 174 Chapter 11 Significant & Future Work property. In contrast, CdSe NP shows promising luminescence property. Hence, the next challenge is to modify the surface of this NPs to make it water soluble so that the CdSe NPs can be used in bio-imaging applications. Besides, one may also consider preparing the aqueous soluble QDs directly from water medium by using the zinc or cadmium selenocarboxylate precursors. Alloy and core-shell NPs have attracted attentions since last decade due to their interesting optical properties. This thesis only covered the syntheses of conventional binary or ternary metal selenide NPs. In the future, one can explore the possibility of using multi-source metal selenocarboxylate precursors in a single nanosynthesis to synthesize alloy NPs, for example, ZnxCd1-xSe, CdSexS1-x, CuxAg1xInSe2, AgInSxSe1-x or core-shell NPs. In chapter 6, monodispersed Ag2Se nanocubes have been prepared and fully characterized. The NCs are found to pack uniformly due to the morphology it has possessed. Thus, the next challenge of this work is to arrange these nanocubes on a substrate to achieve a long range order NC supper lattice which allowed the photonic study to be conducted on these Ag2Se NCs. Recently, Alivisatos et al. have reported the cation-exchange reaction, where they gradually replaced the Ag+ in Ag2Se with Cd2+. So far, no one has reported the synthesis for CdSe nanocube. Hence, the synthesized Ag2Se nanocubes may serve as a template to prepare CdSe nanocube. Further, the optical properties of the CdSe nanocubes can be investigated and compared with the rod and spherical shaped CdSe. In this project, the shape of the Ag2Se NPs is found to be governed by the nature of precursor. Thus, in future, one can synthesize different forms of silver selenocarboxylate, e. g., (Ph3P)4Ag4(SeC{O}Ph)4, (Ph3P)Ag(SeC{O}Ph), etc. to 175 Chapter 11 Significant & Future Work prepare the Ag2Se NPs and study the influence of the precursor on the morphology of the NPs. Although it has not been studied here, many of these metal selenocarboxylates could be used as precursors to deposit the corresponding metal selenide thin films using MOCVD or AACVD techniques. Last but not least, the great challenge of the next step is to employ the synthesized NPs for various applications, e. g., sensors, LED, photovoltaic application. In order to succeed in this direction, one may need to modify the surface of these NPs and design a method to study the properties of the NPs. 176 Chapter 12 Chapter 12. 12.1 Experimental Experimental General All of the following chemicals were used as received without further purification 1) Zn(NO3)2.6H2O (Aldrich) 2) ZnCl2 (Aldrich) 3) CdCl2 (Aldrich) 4) Cd(NO3)2.4H2O (Aldrich) 5) HgCl2 (Aldrich) 6) PPh4Cl (Aldrich) 7) InCl3 (Aldrich) 8) Ga(NO3)3.xH2O (Aldrich) 9) 2, 2’-bipyridyl (Aldrich) 10) Tri-n-octylphosphine Oxide – 90% (Aldrich) 11) Tri-n-octylphosphine – 90% (Aldrich) 12) Hexadecylamine – 98% (Aldrich) 13) Dodecanethiol – 98% (Aldrich) 14) Oleylamine – Technical grade, 70% (Fluka) 15) Ethylenediamine – 99.5% (Aldrich) THF was dried by refluxing with sodium wire using benzophenone as indicator. Acetonitrile was dried using calcium hydride and distilling before use. The rest of the solvents were dried by allowing them to stand over Å molecular sieves overnight. Benzoyl Chloride and p-Toluoyl Chloride were purified by distillation 177 Chapter 12 Experimental under vacuum. Copper grids (200 mesh) coated with amorphous carbon film were purchased from Solid Vision for TEM measurements. 12.1.1 Preparation of Na2Se and K2Se Sodium and potassium selenide were prepared based on the reported syntheses.1 Briefly, in a 2-necked flask, 7.70 g of sodium (335 mmol) and 4.30 g of naphthalene (33.5 mmol) were added to an iced cooled dry (80 mL) THF. The color of the THF solution changed to dark green upon stirring. 12.30 g of grey selenium powder (155.7 mmol) was added slowly to this solution. Large amount of heat was generated when adding the selenium powder into the sodium in THF solution containing the naphthalene catalyst. This mixture was then refluxed under argon atmosphere for overnight. Eventually the color of the solution was changed from dark green to purple and white, which indicated the formation of Na2Se. A slight greenish color was observed if there is an excess of Na. The solution was cooled and then filter under inert atmosphere. The Na and naphthalene were washed off using dry THF. This white powder was kept in dry box for further use. K2Se was prepared using similar procedure as described above, except K metal was used in the synthesis. 12.2 Elemental Analysis All element analyses experiments were performed by the microanalytical laboratory in the Department of Chemistry, National University of Singapore. 178 Chapter 12 12.3 Experimental Infrared Spectroscopy The IR spectra (KBr pellet) of all compounds were recorded on a Bio-Rad FTIR spectrophotometer. 12.4 NMR Spectroscopy H, 31P{1H} and 13C{1H} NMR spectra were recorded with a Bruker ACF300 NMR spectrometer, with chemical shifts referenced to residual non-deuterated solvent and external H3PO4, respectively. 12.5 ESI-MS Mass spectra were obtained with a Finnigan MAT LCQ (ESI) spectrometer. 12.6 TGA Thermogravimetric analysis was recorded on a SDT 2960 simultaneous DTATGA. Approximately 10 mg of the sample was used under inert N2 flow (90 mL/min) and a heating rate of 10 deg.min-1. 12.7 UV-vis Spectroscopy UV-vis spectra were recorded using a Shimadzu UV-2501PC Uv-vis Recording Spectrophotometer. Samples were pre-disssolved or pre-dispersed in approatiate solvents before recording. 12.8 Photoluminescence Spectroscopy Photoluminescence properties were recorded on a Perkin Elmer Luminescence Spectrometer LS50B – 50Hz instrument. 179 Chapter 12 12.9 Experimental X-ray Powder Diffraction X-ray powder diffraction experiments of the various samples were recorded using a D5005 Bruker AXS X-ray diffractometer at 25 °C. 12.10 Single Crystal X-ray Diffraction The diffraction experiments were carried out on a Bruker SMART CCD diffractormeter with a Mo Kα sealed tube. The program SMART2 was used for collecting frames of data, indexing reflection and determination of lattice parameters, SAINT2 for integration of the intensity of reflections and scaling. SADABS3 was used for absorption correction and SHELXTL4 for space group and structure determination and least refinement on F2. All the X-ray crystallographic experiments were performed by Dr. J. J. Vittal. Additional crystallographic data in the form of CIF files are provided as a soft copy in the CD-ROM attached with this thesis. 12.11 Scanning Electron Microscope SEM images of all the compounds were recorded on a JOEL JSMT220A/JSM-5200 Scanning Microscop, with the accelerating voltage of 20 KV. The samples were smeared over a double-sided adhensive tape placed over an aluminum stub. All samples were gold coated before the SEM images were taken. 12.12 TEM TEM and selected area electron diffraction (SAED) patterns were obtained on a 100 kV JEM-100CXII TEM and 300KV JEOL 3010 microscope. The samples were 180 Chapter 12 Experimental prepared by placing a drop of the NP solution dispersed in certain solution, e. g., toluene onto a copper grid, and was allowed to dry in desiccators. The HRTEM measurements were done by Dr. Chris Boothroyd from Institite of Materials Research and Engineering (IMRE) and Mr. Liu Bin Hai from Department of Biological Sciences, National University of Singapore. 12.13 EDX EDX of the sample were measured from the Oxford EDX machine, which coupled to the JEOL 3010 TEM. 12.14 NLO Measurement The NLO measurements for AgInSe2 NRs were done by Assoc. Prof. Ji Wei et al. from Department of Physic, National University of Singapore. References: 1. Thompson, D. P.; Boudjouk, P. J., Org. Chem. 1988, 53, 2109. 2. SMART & SAINT Software Reference Manuals, Version 5.0, Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2000. 3. Sheldrick, G. M., SADABS a software for empirical absorption correction, Version 2.03, University of Göttingen: Göttingen, Germany 2000. 4. SHELTX Reference Manual, Version 6.13, Bruker Analytical X-ray Systems, Inc.: Madison, WI, 200017 181 Appendix Appendix Element Na K Cu K Ga K Se K Peak Area 31835 69715 76578 143085 Area Sigma 699 483 510 660 k factor 1.247 1.312 1.491 1.794 Abs Corrn. 1.000 1.000 1.000 1.000 Weight% Weight% Sigma 7.91 0.16 18.22 0.12 22.74 0.14 51.13 0.18 Atomic% 21.44 17.87 20.33 40.36 Figure A1.EDX spectrum and elemental composition analysis of compound 5. Element KK Cu K Ga K Se K Peak Area 13463 28446 15872 28385 Area Sigma 230 301 239 297 k factor 0.939 1.312 1.491 1.794 Abs Corrn. 1.000 1.000 1.000 1.000 Weight% Weight% Sigma 10.15 0.17 29.97 0.28 18.99 0.26 40.89 0.32 Atomic% 17.06 31.00 17.91 34.03 Figure A2.EDX spectrum and elemental composition analysis of compound 6. 182 Appendix Element KK Cu K Se K In L Peak Area 66080 149536 162412 124956 Area Sigma 1031 691 719 1777 k factor 0.939 1.312 1.794 1.757 Abs Corrn. 1.000 1.000 1.000 1.000 Weight% Weight% Sigma 8.07 0.12 25.51 0.15 37.88 0.20 28.54 0.30 Atomic% 15.44 30.05 35.91 18.60 Figure A3.EDX spectrum and elemental composition analysis of compound 8. 183 Appendix Band edge calculation from UV spectrum For a semiconductor with band gap absorption in UV-vis region, the band edge can be determined from the UV-vis spectrum. In this thesis, the band edge can be obtained from the onset point of the absorption curve in the UV-vis spectrum as shown in the diagram below. The relationship between band edge energy (Eg, eV) and band gap absorption (λg, nm) is given by the equation: Eg·e = hc/ λg, where e is the charge of an electron (1.602 x 10-19 C), h is Planck constant (6.626 x 10-34 Js), c is the speed of light (2.998 x 1017 nmS-1). Therefore Eg = 1240/ λg. Brus equation, as shown below Eg = E + h  1  3.6e + − * * 2d ⋅ e  me m h  4π ⋅ ε ⋅ d can estimate direct band gap semiconductor nanoparticle materials diameter d when Eg was determined from UV-vis spectrum or vice versa. Where E is the band edge 184 Appendix energy of bulk material, me* and mh* are the effective masses of electron and hole, respectively. ε is the dielectric constant. 185 [...]... (TolC{O}Se), 141.5 (C ), 140.7 C 4 (C ), 128 .5 (C or C ), 127 .9 (C or C ), 21 .3 (CH –C H C{O}Se) ESI-MS (DMSO): 1 2/6 3/ 5 3/ 5 2/6 3 6 4 m/z 661.0 ([Zn(SeC{O}Tol) ] , 100%), 1120 .1 ([Zn(SeC{O}Tol) ]2 + TolC{O}Se-, 3 - 2 100%), 199 .3 (TolC{O}Se-, 30 %) TGA for one H2O: 3. 75 % (Calc.); 3. 56 % (Obs.) [Cd(SeC{O}Ph)2], 13 The synthesis of 13 is similar to 12 except CdCl2 and [Na+PhC{O}Se–] were used in the preparation... d-spacinga 1 2 0 3. 659 3. 665 2 0 0 3. 611 3. 611 0 0 2 3. 460 3. 460 1 2 1 3. 235 3. 239 0 4 0 2 .122 2 .122 3 2 0 2.094 2.086 1 2 3 1.951 1.942 0 4 2 1.809 1.802 3 2 2 1.792 1.781 a d-spacing of the distinct visible peaks from the XRPD measurement With the new set of parameters, all the diffraction peaks can be indexed accordingly, as shown in Figure 9.6 Hence, the synthesized NRs are a new phase of AgInSe2 that... AgGaSe2 and AgGaS2 9.6 Synthesis and Methodology In a typical experiment, 0. 03 mmol (50 mg) of [(Ph3P)2AgIn(SeC{O}Ph)4] (See chapter 3 page 59) was added to a flask containing both OA (1.10 mL, 3. 37 mmol) and DT (0.81 mL, 3. 37 mmol) The precursor dissolved immediately and formed a black solution This solution was then degassed for 15 min in a vacuum and heated to 185 ˚C in an oil bath for 17 h, for example,... 2’bipy)M(SeC{O}R)2] single- source precursors by our group. 155 Thus, the choice of precursor has great influence on the growth of the NPs 133 Chapter 8 8.4 ZnSe & CdSe NPs Synthesis and Methodology 8.4.1 Synthesis of [Cd(SeC{O}Ph)2] and [Zn(SeC{O}Tol)2]·(H2O) The syntheses of Na+TolC{O}Se– and Na+PhC{O}Se– are described in chapter 2 and 3 respectively [Zn(SeC{O}(Tol)2]·H2O, 12 The MeCN in [Na+TolC{O}Se–]... Yield: 89% Elemental Anal: Calcd for CdSe2C14H10O2 (mol wt 480.56): C, 34 .99; H, 2.10 % Found C, 34 .65; H, 1.78 % 1 H NMR (d6-DMSO) δH: 7.96 (4H, d, J = 9 Hz, ortho-proton), 7.58 (4H, t, J = 9 Hz, para-proton), 7.45 (4H, t, J = 6 Hz, meta-proton) 13 C NMR δc (d6-DMSO): For 134 Chapter 8 ZnSe & CdSe NPs selenobenzoate ligand: 127 .12 (C2/6 or C3/5), 128 .61 (C2/6 or C3/5), 131 .05 (C4), 142.94 (C1), 201.50... toluene for TEM studies The powders were vacuum dried and used for XRPD measurement 135 Chapter 9 Chapter 9 Silver Indium Selenide NRs One-Pot Synthesis of New Orthorhombic Phase AgInSe2 NRs 9.1 Introduction The potential applications of I-III-VI chalcopyrites for nonlinear optical devices and photovoltaic solar cells have long been recognized and studied.156-158 AgInSe2, a semiconductor with a band gap of. .. Cd–O–P) and 1 032 cm-1 (υsym Cd–O–P) indicating that the NPs are capped by TOPO Figure 8.11 (a) Low resolution and (b) high resolution TEM images of ZnSe NPs (c) The size distribution of the ZnSe NPs 131 Chapter 8 ZnSe & CdSe NPs Figure 8 .12 (a) Low resolution and (b) high resolution TEM images of CdSe NPs (c) The size distribution of the CdSe NPs Figure 8 . 13 EDX of (a) ZnSe, (b) CdSe NPs 132 Chapter 8... plenty of H2O to removed the unreacted ZnCl2 and [Na+TolC{O}Se–], then it was dried under vacuum and stored at 5 °C for further use Yield: 0.850 g (92 %) Elemental Anal: Calcd for ZnSe2C16H14O2.H2O (mol wt 479.61): C, 40.07; H, 3. 36 % Found C, 40 .31 ; H, 3. 30 % 1H NMR (d6-DMSO) δH: 2 .35 (6H, S, CH –C H C{O}Se), 7. 23 (4H, d, J = 6 Hz, meta-proton), 7.95 (4H, d, J = 3 6 4 6 Hz, ortho-proton) 13 C NMR... capping agent As for DT, it may guide the particles to grow in a 1D manner The growing mechanism of the NR is yet to be investigated Figure 9 .3 IR spectra of AgInSe2 NRs, pure DT, OA and the precursor 139 Chapter 9 Silver Indium Selenide NRs Ag In Se S Ag:In:Se Spectrum 1 24.82 % 31 .34 % 41.51 % 2 .33 % 1:1.26:1.67 Spectrum 2 26.55 % 28.66 % 42.49 % 2 .30 % 1:1.08:1.60 Figure 9.4 EDX and the elemental... size of the NPs In addition, the broad diffuse rings observed in the SAED patterns (as shown in Figure 8.10) also support the fact that the small size of ZnSe and CdSe NPs 129 Chapter 8 ZnSe & CdSe NPs Figure 8.9 XRPD patterns of ZnSe and CdSe NPs Figure 8.10 SAED of (a) ZnSe and (b) CdSe The TEM images of ZnSe and CdSe NPs isolated after 5 min of heating are shown in Figure 8.11 and 8 .12 The size of . = 6 Hz, meta-proton). 13 C NMR δc (d 6 -DMSO): For Chapter 8 ZnSe & CdSe NPs 135 selenobenzoate ligand: 127 .12 (C 2/6 or C 3/ 5 ), 128 .61 (C 2/6 or C 3/ 5 ), 131 .05 (C 4 ), 142.94 (C 1 ),. 100%), 199 .3 (TolC{O}Se - , 30 %). TGA for one H 2 O: 3. 75 % (Calc.); 3. 56 % (Obs.). [Cd(SeC{O}Ph) 2 ], 13 The synthesis of 13 is similar to 12 except CdCl 2 and [Na + PhC{O}Se – ] were. size of ZnSe and CdSe NPs. Chapter 8 ZnSe & CdSe NPs 130 Figure 8.9. XRPD patterns of ZnSe and CdSe NPs. Figure 8.10. SAED of (a) ZnSe and (b) CdSe. The TEM images of ZnSe and