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

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 in

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Chapter 8 ZnSe & CdSe NPs

120

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,

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147 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

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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

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5 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 5 min, 15 min and 30 min 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 min 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 do 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

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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 do not affect the size of the ZnSe NPs based on the approximate size calculated from UV absorption spectra

as shown in Figure 8.3.

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Figure 8.3a UV-absorption spectra of

ZnSe NPs isolated at various temperatures

after 5 min of heating

Figure 8.3b. UV-absorption spectra of ZnSe NPs isolated at various HDA concentration after 5 min 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 min 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) min 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

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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 min 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

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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 do 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

CdSe NPs isolated at different

Figure 8.6b UV-absorption spectra of CdSe NPs isolated at different TOPO

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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 = 5 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 = 5 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 = 5 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)

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Figure 8.8 (a) Optical absorption (blue) and (b) photoluminescence (red) spectra of CdSe NPs (t = 5 min)

8.2.3 Structural Characterization of ZnSe and CdSe NPs

The XRPD spectra of the synthesized ZnSe and CdSe NPs are shown in Figure 8.9 All the peaks can be indexed according which correspond to cubic phase ZnSe (JCPDS No 037-1463) and hexagonal phase CdSe (JCPDS No 08-0459) The broadening of the XRPD peaks as compared with the bulk is due to the smaller 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

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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 the ZnSe and CdSe NPs measured from the TEM image based on 300 NPs are 7.3 ± 1.1 nm and 4.4 ± 0.5 nm respectively The synthesized ZnSe and CdSe NPs are crystalline as clear lattice fringes of these NPs are visible in the HRTEM images The EDAX analysis on these NCs confirmed the presence of the respective metal and selenium with ratio close to 1:2 Further,

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phosphorus peak is observed in the EDAX spectra as shown in Figure 8.13, which indicated the NC could be capped by TOP and/or TOPO The infrared spectrum of the ZnSe NPs (Figure 8.14) show a broad band at 1078 cm-1 (υsym Zn–O–P), a shift of 68

cm-1 from the characteristic stretch for TOPO (υsym O–P, 1146 cm-1),145 which indicated that the NPs are capped by TOPO Similarly, the O=P stretching in the IR spectrum of CdSe NPs also shifted to 1105 cm-1 (υasym Cd–O–P) and 1032 cm-1 (υsymCd–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

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Figure 8.14 IR spectra of TOPO, ZnSe and CdSe NPs

Monodispersed ZnSe and CdSe NPs have been synthesized from [Zn(SeC{O}Tol)2]·H2O and [Cd(SeC{O}Ph)2] in TOP/TOPO/HDA and TOP/TOPO/HPA surfactants respectively In this study, it is found that trace amounts

of HDA and HPA are crucial for the formation of high quality ZnSe and CdSe NPs In the case of ZnSe, a broad emission peak was observed which could be due to the deep trap emission or the recombination of surface trap In contrast, the synthesized CdSe NPs show a promising luminescence property EDX and IR studies has confirmed the surfaces of these NPs are capped by TOPO Our study shows ZnSe and CdSe NPs that obtained through injection method from neutral metal selenocarboxylates were having better size distribution than those that was previously synthesized from [(2, 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

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8.4 Synthesis and Methodology

8.4.1 Synthesis of [Cd(SeC{O}Ph) 2 ] and [Zn(SeC{O}Tol) 2 ]·(H 2 O)

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 ]·H 2 O, 12 The MeCN in [Na+TolC{O}Se–] (4.00 mmol) solution was removed to dryness under vacuum 30 mL of degassed H2O was then added to the crude sodium monoselenocarboxylate and the insoluble precipitate was filtered off ZnCl2 (0.27 g, 2.00 mmol) dissolved in degassed H2O (10 mL) was added dropwise to the yellow filtrate and a pale yellow precipitate formed immediately The solution was stirred for 1.5 hours at room temperature and the pale yellow precipitate was filtered off and washed with 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 (d 6-DMSO) δH: 2.35 (6H, S, CH3–C6H4C{O}Se), 7.23 (4H, d, J = 6 Hz, meta-proton), 7.95 (4H, d, J =

6 Hz, ortho-proton) 13C NMR (d 6-DMSO) δC:207.2 (TolC{O}Se), 141.5 (C4), 140.7 (C1), 128.5 (C2/6or C3/5), 127.9 (C3/5or C2/6), 21.3 (CH3–C6H4C{O}Se) ESI-MS (DMSO): m/z 661.0 ([Zn(SeC{O}Tol)3 ] -, 100%), 1120.1 ([Zn(SeC{O}Tol)2]2 + TolC{O}Se-, 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 Yield: 89% Elemental Anal: Calcd for CdSe2C14H10O2 (mol wt 480.56): C, 34.99; H, 2.10 % Found C, 34.65; H, 1.78 %

1H NMR (d 6-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) 13C NMR δc (d 6-DMSO): For

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selenobenzoate ligand: 127.12 (C2/6 or C3/5), 128.61 (C2/6 or C3/5), 131.05 (C4), 142.94 (C1), 201.50 (COSe) ESI-MS (DMSO): m/z 665.0 ([Cd(SeC{O}Ph)3 ] -, 30%), 1144.2 ([Cd(SeC{O}Tol)2]2 + TolC{O}Se-, 100%), 185.2 (PhC{O}Se-, 6%)

8.4.2 Synthesis of ZnSe and CdSe NPs

In a typical experiment, [Zn(SeC{O}Tol)2]·H2O (40 mg) was dissolved in warm TOP (0.5 mL) This solution was then injected into hot TOPO/HDA solution (220/250/280 ˚C) and a decrease in temperature of 20 – 30ºC was observed The color

of the precursor solution immediately changed from pale yellow to intense yellow after the injection The reaction solution was heated for 30 minutes and the heating mantle was removed to stop the growth of the NPs CdSe QD was synthesized using similar strategy, except that 50 mg of [Cd(SeC{O}Ph)2] was dispersed in 8 mL of TOP and injected into hot TOPO/HPA solution (220 & 250 ˚C) The color of the precursor solution changed from orange yellow to bright red after the injection The solution was heated for 15 minutes and the heat source was removed to stop the growth of CdSe NPs For kinetic study, small aliquots were taken from the refluxing solution at different time intervals, diluted appropriately and used to record the UV-vis and PL spectra immediately The solution was cooled to ≈ 70 ºC, an excess of acetone/methanol (1:1) was added, and a flocculant precipitate formed The solid was separated by centrifugation and was three times with methanol The powder can be redispersed in toluene for TEM studies The powders were vacuum dried and used for XRPD measurement

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Chapter 9 Silver Indium Selenide NRs

Phase AgInSe2 NRs

The potential applications of I-III-VI chalcopyrites for nonlinear optical devices and photovoltaic solar cells have long been recognized and studied.156-158AgInSe2, a semiconductor with a band gap of 1.19 eV, is a ternary analogue of CdSe which has been used for a number of electronic devices.159, 160 Extensive studies on the electrical and optical properties of AgInSe2 have been carried out.161 So far, most

of the studies are confined to bulk and thin film AgInSe2 due to the limitation of the available synthetic methods.162 Thus to develop a conventional synthesis route to AgInSe2 NP is vital in the discovery of the novel properties of AgInSe2 nanomaterials

In the previous chapters, we have shown that the metal selenocarboxylates are capable for producing binary metal selenide NPs Thus, in this chapter, we will extend the synthesis to a ternary metal selenide Here, we report a novel one-pot synthesis of close to monodispersed, one-dimensional (1D) AgInSe2 in which the single-source precursor is thermally decomposed in mixed amine and thiol solution In addition, the morphology of the NCs has been tuned from a 1D rodlike structure to close to sphere-shaped NCs Usually tetragonal or cubic AgInSe2 are the thermodynamically stable products in most of the syntheses.163 This synthetic method described here yields a previously unknown orthorhombic phase, AgInSe2, which is isostructural with the well-known AgInS2 (JCPDS No 00-025-1328) phase

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9.2 Structural Characterization of AgInSe 2 NPs

Both OA and DT have been widely used as surfactants for synthesizing various NPs, and they are often reported as reagents that promote the anisotropic growth of NPs.136, 164-167 Hence, we choose OA and DT as our reaction medium for synthesizing AgInSe2 NPs As demonstrated in Figure 9.1a, the synthesized AgInSe2 NPs form a 1D rodlike structure The synthesized NRs are close to monodispersed with dimensions of (50.3 ± 5.0) nm × (14.5 ± 1.8) nm (measured from the TEM images based on 350 NRs) The average aspect ratio of the synthesized NRs is 3.5 ± 0.6 Figure 9.1c shows a HRTEM micrograph of an individual NR where the lattice fringes show that the NPs are well crystallized It is observed that the NRs do not grow on a preferred axis

Figure 9.1 Low-magnification TEM images of (a) AgInSe2 NRs, (b) AgInSe2 NCs (c) HRTEM micrograph showing the crystal lattice of an individual NR (d) SAED pattern of AgInSe2 NR

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As the reaction temperature is increased to 250 ˚C, only irregularly shaped AgInSe2 NPs are obtained after 2 h of heating as shown in Figure 9.1b It is likely that the precursor is less stable in the amine solution at high temperature and that less monomer is available to promote the anisotropic growth of NCs.101 We found that both DT and OA are equally important in the synthesis of AgInSe2 NRs By heating the precursor in OA alone, irregularly shaped AgInSe2 particles (resembling melted particles) are obtained with unidentified impurities as indicated from their XRPD pattern as shown in Figure 9.2 As in the case of heating with DT alone, only bulk AgInSe2 is obtained

Figure 9.2 (A) TEM images of AgInSe2 obtained from pure oleylamine and (B) XRPD pattern of the corresponding AgInSe2 Peaks labeled in * correspond to the tetragonal phase AgInSe2

The FTIR and EDX spectra of the AgInSe2 NRs are shown in Figure 9.3 and 9.4 Based on the IR spectral studies, it is hard to pin point whether the surface of the NRs are passivated by which capping agent Similar IR spectrum of NP was obtained

by Venugopal et al for the CdSe NP coated by DT.168 In addition, many research labs have reported that the metal sulfide and selenide have good binding affinity toward

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sulfur ligand.92, 169 Thus, we believe that the DT is capped to the surface of the NPs If the AgInSe2 NRs are capped mainly by DT, we should see a sharp sulfur peak in the EDX spectrum However, the sulfur signal in the EDX analysis is very weak as shown

in Figure 9.4 hence we suggested that the surface of the NRs is mainly capped by OA and this is no surprise as from the fact that it has been reported that OA is able to cap the surface of the NPs.91, 116 Therefore, based on the FTIR and EDX analysis, we proposed that that the majority of the surface of the NRs is capped by OA with a slight amount of DT.In the synthesis of AgInSe2 NR, we proposed that the OA acts as both activating agent87 and 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

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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 composition analysis of the synthesized AgInSe2 NRs The copper and carbon signals are from the TEM copper grid The Si signal is from the detector The oxygen signal is from the oxidized AgInSe2

When OA is replaced with an equal amount of HDA, identical AgInSe2 NRs are obtained as shown in Figure 9.5 This shows that the morphology of the AgInSe2NCs is governed by the DT When the reaction time is shortened from 17 to 1.5 h, small NRs mixed with polyhedron-shaped AgInSe2 NPs are obtained as shown in the TEM image (Figure 9.5a) To our surprise, the diameter of the NRs remains unchanged on prolonged heating

Figure 9.5 TEM images of AgInSe2 NRs obtained from (A) mixture of OA and DT

at 185 °C after 1.5 hours of heating (B) Mixture of HDA and DT (The mole ratio of

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141

9.3 Crystal Structure Characterization of AgInSe 2 NRs

An XRPD pattern of the synthesized AgInSe2 NRs and the bulk AgInSe2(synthesized by pyrolyzing the precursor in a vacuum) is shown in Figure 9.6 The broadening of the diffraction peaks is characteristic of NPs As mentioned earlier, 12 crystalline phases of AgInSe2 have been reported.163 However, the XRPD patterns of the synthesized AgInSe2 NRs do not match any of the patterns of the reported AgInSe2 structures, not even that of the most common tetragonal AgInSe2 (JCPDS

No 00-035-01099) Careful examination of the XRPD pattern reveals that it resembles the XRPD pattern of orthorhombic AgInS2 with a consistent shift of the peaks to low angles as shown in Figure 9.6

Figure 9.6 (A) XRPD pattern of AgInSe2 NRs obtained from OA and DT at 185 ˚C (B) Bulk AgInSe2 obtained from pyrolysis of the precursor

By substituting the d-spacing values of the AgInSe2 NRs obtained from XRPD

measurements with the corresponding hkl values from the known orthorhombic

AgInS2 (200), (002) and (040) into equation (1), a new set of cell parameters for the new orthorhombic AgInSe2 has been derived They are: a = 7.223 Å, b = 8.489 Å and

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c = 6.920 Å, V = 424.3 Å3 The new set of d-spacings for the orthorhombic AgInSe2

generated based on the calculated cell parameters and the hkl values from the known

orthorhombic AgInS2 (as shown in Table 9.1) match well with the experimental spacing values as shown in Table 9.1

d-2

2 2

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 is isostructural with the known orthorhombic AgInS2

This could be rationalized if the surfactants adjust the chemical environment

in such a way that the relative stability of one phase over another can be reversed.

170-172 For example, Xiao et al have demonstrated that a high-temperature phase ZnS NC

can be obtained at low temperature through colloidal synthesis.171 A close inspection

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143

of randomly selected areas of the AgInSe2 NRs by HRTEM revealed that all the samples are uniform and have similar SAED patterns (Figure 9.1d) consistent with their XRPD patterns XPS (Figure 9.7) and EDX (Figure 9.4) analyses of the synthesized NRs show the 1:1 stoichiometry between Ag and In but slightly less Se than expected, which is consistent with the EDX results of some reported I-III-VI calcopyrites.173 The EDX results of the AgInSe2 NRs are shown in Figure 9.4 and generally, a less amount of selenium is observed This could be due to the increase of the surface-to-volume ratio when particles become smaller, which corresponds to a larger number of stabilizing thiol- or amine-capping groups on the particle surface relative to the number of Se atoms in the particle core Such deficient of chalcogenide phenomenon have been observed in many cases, e g., thiol stabilized CdSe,169 Au – CdSe composite material,174 CuInS2 nanomaterial,175 CdSe.176

Figure 9.7 XPS spectra of the as-prepared AgInSe2 NRs

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On the other hand, a sulfur-containing surfactant could serve as a sulfur source

in the colloidal synthesis.177 If sulfur is gradually incorporated into AgInSe2, the peaks

in the powder XRD pattern should be shifting towards the peaks in AgInS2 Such phenomenon has been commonly observed in the preparation of alloy materials, e g., ZnxCd1-xSe and ZnxCd1-xS.75, 76 As shown in Figure 9.8, same powder XRD patterns were obtained at various DT concentrations This clearly shows that the sulfur from the DT has not diffused into the core of the AgInSe2 NRs If the diffusion of S is possible, an XRPD pattern that matched with AgInS2 should be observed at very high

DT concentration (surfactant-to-precursor ratio = 100)

Figure 9.8 (A) XRPD peak positions for orthorhombic AgInS2 X-ray spectra from AgInSe2 NRs prepared at the following DT concentrations surfactant-to-precursor ratio = 100 (B); 80 (C); 60 (D); 40 (E); 20 (F)

When DT is replaced by TOPO or TOP in the synthesis, a mixture of

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145

former case, and impurities are formed together with the orthorhombic phase product

in the latter case as shown in Figure 9.9 This new phase AgInSe2 NPs also can be obtained as we lowered the heating temperature as shown in Figure 9.10

Figure 9.9 XRPD patterns of AgInSe2 NCs obtained using various surfactants (T = tetragonal; O = Orthorhombic; * = impurities)

Figure 9.10 XRPD patterns of AgInSe2 NPs prepared at the various temperatures

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9.4 NLO Properties of AgInSe 2 NRs

The synthesized AgInSe2 NRs have been investigated for its nonlinear optical responses by using Z-scan and transient absorption techniques with femtosecond laser pulses of photon energy greater than the band gap At excitation irradiance of 20 GW/cm2, AgInSe2 NRs reveal saturation in the nonlinear absorption and optical Kerr nonlinearity as shown in Figure 9.11, with a recovery time determined to be a few ten picoseconds Such large saturable absorption and Kerr nonlinearity give rise to a third-order susceptibility of 1.2 x 10-8 esu and a figure of merit of 98 esu.cm.s-1, making AgInSe2 NRs a promising candidate for saturable absorption devices

Figure 9.11 (a) Open- and (b) closed-aperture Z-scans of 1-mm-thick solution of the

AgInSe2 NRs measured with 200-fs laser pulses of 780-nm wavelength The laser irradiance used is in the range from 5 GW/cm2 to 47 GW/cm2 The solid lines are the best-fit curves calculated by using the Z-scan theory The closed-aperture Z-scan

curves in (b) are shifted vertically for clear presentation (c) Irradiance dependence of

the nonlinear absorption coefficient (α2NRs) and nonlinear refractive index (n 2NRs) for the AgInSe2 NR solution

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147

To summarize, we have synthesized close to monodispersed AgInSe2 NRs at

185 ˚C in a thiol and amine medium For the first time, orthorhombic AgInSe2 has been synthesized, and this new material may inherit some interesting electronic properties from its isostructural analogue, AgInS2 Recently, our laboratory has reported the synthesis of orthorhombic AgInS2 bulk and NPs.178 With the results we have obtained here, one has the opportunity to do the comparative study on the properties of these two materials in future The good solubility of the synthesized AgInSe2 NRs has opened a venue for studying the novel properties of this material in solution For instance, interesting third-order nonlinear optical properties have been discovered for the first time as discussed here In addition, the good solubility of the AgInSe2 NRs also allows one to utilize the NP in many applications, e.g fabrication

of polymer thin films containing discrete NCs Finally, the work reported here demonstrates that perfect matching of two or more surfactants might be the key to obtaining high-quality NCs for many novel materials This method may be applicable

to other important I-III-VI semiconductor NCs such as 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, in an argon atmosphere At the end of the reaction, a black precipitate was found at the bottom of the flask A small amount of toluene and a large excess of EtOH were added to the reaction solution,

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and AgInSe2 NCs were separated from the reaction solution by centrifugation The synthesized NCs can be dispersed freely in organic solvents such as toluene, CHCl3,

or hexane The NCs were washed with EtOH, dried in a desiccator, and used for structural characterization by XRPD and TEM

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Chapter 10 Copper Indium Selenide NPs

Chapter 10 Synthesis of CuInSe2 NPs

Thin-film photovoltaic devices fabricated with chalcopyrite semiconductors, e g., CuInS2 and CuInSe2 are expected to be highly efficient.179, 180 Although the band gap energy of CuInSe2 (Eg = 1.1 eV) is not well-matched with the solar spectrum for photovoltaic performance, other attributes such as high absorption coefficient and low-cost methods for deposition of thin film make CuInSe2 a promising material for photovoltaic devices.173 Now days, researchers are looking for new approaches, such

as the use of nanotechnology to improve device performance The inclusion of NCline materials in photovoltaic devices181 has been proposed to improve the efficiency of photon conversion (intermediate band solar cell)182, 183 by providing sites for exciton dissociation and pathways for electron transport.29, 184 Few research groups have demonstrated that charge transport efficiency can be improved by using composites of QDs and conductive organic polymers in photovoltaic devices.29, 184-187 Further, QDs are also more resistant to degradation from electron, proton, and alpha particle radiations than the corresponding bulk materials, a preferred characteristic for use in space solar cells.188, 189

In contrast to the overall quantity of QD research, relatively few reports have been published about the synthesis of nanosized CuInSe2.82, 173, 190, 191Briefly, O’Brien and his co-workers are the first group to report the synthesis of CuInSe2 NCs from multi precursors (InCl3, CuCl and TOPSe) in TOP/TOPO solution.82 Later, Hepp et

al. shown that CuInSe2 colloids can be obtained from the thermolysis of [(Ph3P)2CuIn(SePh)4] single precursor in non-coordinating solvent, e g., dioctyl phthalate.173 According to Qian et al., CuInSe2 nanowhiskers can be prepared from

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Chapter 10 Copper Indium Selenide NPs

10.2 Results and Discussion

10.2.1 CuInSe 2 NPs Synthesized from OA and DT Solvents

The XRPD patterns of the black precipitate obtained from thermally decomposed precursor in OA/DT (the surfactants-to-precursor ratio is 100) solution at

185 ˚C is shown in Figure 10.1 However, we were unable to index the XRPD peaks

to any known CuInSe2 Interestingly, we found this XRPD patterns identical to that of AgInSe2 synthesized in same solvents as discussed earlier Hence, this could be due to

a new phase CuInSe2 similar to the AgInSe2 NR Before looking into the possibility of getting a new phase material, we would like to confirm that the peaks obtained in the XRPD analysis is from single component, e g., not from the mixture of two or more species In the TEM images (Figure 10.2), we have observed the different size and shape NPs (large triangular shaped and small polyhedral shaped) were obtained from the one pot synthesis Based on the SAED patterns (Figure 10.2b & d) of these NPs,

we concluded that these two different shape NPs are different materials Careful examine the SAED patterns, we found that the SAED pattern of big triangular particle can be indexed to tetragonal CuInSe2 Therefore the XRPD peaks of the black

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