Synthesis and characterization of group 11, 12 and 13 metal selenocarboxylates potential single molecular precursors for metal selenide nanocrystals 2

Chapter Chapter 5. 5.1 Introduction to NPs Introduction to Nanoparticles General Introduction About half a century ago, Richard Feyman gave his prophetic lecture “Plenty of Room at Bottom.” He outlined in this talk the foundations of nanoscience, and the promise that totally synthetic constructions could eventually be built with molecular scale precision. Nanoscience research has been rapidly increasing across the globe during the past decade. It is now widely accepted by the scientific, industrial, government and business communities, that nanoscience will be integral importance in the development of future technologies. Nanoscience is being touted as the engine that will drive the next industrial revolution. NPs, are referred to particles with size ranges from to 100 nm in diameter. Meanwhile, NC is referred to single crystal with size of to 100 nm. In this thesis, we are using the term “nanosynthesis” to describe the synthesis that product NP. Usually the properties of crystalline solids are ordinarily catalogued without reference to their size. It is only in the regime below 10 nm, called Quantum Dots (QDs) where this variable comes into play. Independent of large number of surface atoms, NCs with the same interior bonding geometry as a known bulk phase often exhibit strong variations in their optical and electrical properties with size.1, These changes arise through systematic transformations in the density of electronic energy levels as a function of size of the interior, known as quantum size effect, which pointed out that NCs are lie in between the atomic and molecular limit of discrete density of electronic state and the extended crystalline limit of continuous bands. Clearly shown in Figure 5.1, NPs contain discrete and large “molecular-like” electronic states and their HOMO – 78 Chapter Introduction to NPs LUMO gap widen with decreasing the particle size. Therefore band gap of NPs can be tuned by changing the particle size.3 Figure 5.1. Schematic illustration of the density of states, along with the changes in the band gap, in semiconductor clusters.3 In the past decade, enormous range of physical properties afforded by sizetuning of semiconductor NCs, a class of materials with so many established applications in electronics, optics, and sensors, has drawn the attention of scientists from diverse disciplines, from synthetic and physical chemists to materials scientists, condensed matter physicists, and electrical engineers. Recent years, the ability to control the surfaces of semiconductors with near atomic precision has led to a further idealization of semiconductor structures: quantum wells, wire, and dots. Such structures should exhibit the idealized variations in density of electronic states predicted by simple particle in a box type model of elementary quantum mechanics, with the continuous levels of the 3d case evolving into the discrete states of the 0dimensional case as shown in Figure 5.2.4 79 Chapter Introduction to NPs Figure 5.2. Idealized density of states for one band of a semiconductor structure of – dimensions.4 5.2 Applications of NPs Colloidal NCs are sometimes referred to as ‘artificial atoms’ because the density of their electronic states – which controls many physical properties – can be widely and easily tuned by adjusting the crystal’s size and shape. The combination of size- and shape-dependent physical properties and ease of fabrication and processing makes NCs promising building blocks for materials with designed functions.5, In the following section, we will present several applications associated with semiconductor NCs and nanostructure engineering. 5.2.1 Luminescence Narrow band (15–20 nm), size-tunable luminescence, with efficiencies at least of order 10%, is observed at room temperature form semiconductor NCs. The origin of this luminescence remains the topic of some controversy. 7-9 As the size is reduced, the shift between the absorbing and emitting state is observed to increase. Therefore different emission wavelengths could be obtained in principle by changing the particle size, a well known example will be the CdSe NPs as shown in Figure 5.3.10 80 Chapter Introduction to NPs Independent of the exact origin of the luminescence, it does appear to be one property which can be manipulated in useful ways. For example, Korgel and co-workers found that the absorption edge of Si nanowires was significantly blue-shifted as compared with the indirect band gap (~ 1.1 eV) of bulk silicon.11-13 They also observed sharp, discrete features in the absorption spectra and relatively strong “band-edge” photoluminescence. They suggested these optical features most likely originated from quantum confinement effects, although surface state might also make additional contributions.14 Figure 5.3. Room-temperature emission (left) and absorption (right) spectra taken from difference sizes CdSe NPs.10 5.2.2 Biological Labeling Now days, the QDs are widely employed as targeted fluorescent labels in biomedical research applications due to the quantum size effect of the NPs as discussed in earlier.15-17 A chart as shown in Scheme 5.1 illustrated the applications of QDs as multimodal contrast agents in bioimaging. Compared with the organic 81 Chapter Introduction to NPs fluorophores that were previously used as biological labels, QDs not photobleach. Studies have shown that only very small amount of QDs is required to produce a strong signal. Indeed, several studies have reported flickering of some specimens, a phenomenon due to the blinking of a small number of QDs.18, 19 This demonstrates that single QDs can still be observed in immunocytological conditions, with an ultimate sensitivity limit of one QD per target molecule. In addition, NPs also provide a readily accessible range of colors. Recently, different-sized QDs have been embedded into polymeric microbeads at precisely controlled ratio to achieve multicolor optical coding for biological assays.20 Scheme 5.1. Applications of QDs as multimodal contrast agents in bioimaging.17 82 Chapter Introduction to NPs 5.2.3 Light Emitting Diodes Alivisatos et al. and Bawendi et al. are the first research groups demonstrated that light-emitting diodes can be made with polymers and CdSe NCs.21, 22 As previously discussed in section 5.2.1, the NCs emission shift with size. Thus, the output color of NC LEDs can be tuned by varying the particle size as shown in Figure 5.4. In certain electronic devices, the semiconducting polymers can replace the inorganic semiconductors due to the low processing cost, for instance, organic lightemitting diodes (OLED). The extension of OLEDs into the technologically important near-infrared (NIR) spectral range used in telecommunications is more difficult because organic molecules usually display optical activity only at wavelengths shorter than µm. However this shortfall is overcomed by Banin’s group where they created a near-infrared plastic light-emitting diodes using a conjugated polymers and InAsZnSe core shell type of NP.23 Figure 5.4. True color image of CdSe NPs illuminated with UV light. (Image was taken from http://ehf.uni-oldenburg.de/pv/nano/index.html). 5.2.4 Laser Unlike the spherical dots, quantum rods have linearly polarized emission as demonstrated recently by fluorescence measurements on single quantum rods and by 83 Chapter Introduction to NPs theoretical calculations.24 This property of linearly polarized emission, along with the prospect of broad spectral coverage and the chemical accessibility to quantum rods, renders them highly attractive as potential laser materials. Recently, an amplified spontaneous emission was observed for spherical colloidal CdSe QDs in close-packed films where pumping with an amplified femtosecond laser source was used to compete with fast non-radiative Auger decay processes.25 Banin’s group have also observed lasing from CdSe/ZnS quantum rods.26 In their study they observed a linearly polarized lasing signal from the quantum rods and a non polarized lasing from QDs (as shown in Figure 5.5) which prove the advantageous for the utility of the rods as laser chromophores. Further, the Auger rates in rods may be smaller because of their larger size while still allowing color tunability through control of the rod diameter. Figure 5.5. Polarized emission measurements for lasing in (a) NCs and (b) NRs.26 5.2.5 Solar Cell Charge transfer rate is reported to improve the efficiency of polymer photovoltaic devices.27 A faster charge transfer rate can be achieved by chemically bind the organic molecules to the nanocrystalline and bulk inorganic semiconductors, which have a high density of electronic states.28 Alivisatos and his co-workers have 84 Chapter Introduction to NPs demonstrated this concept by showing that CdSe NRs can be used to fabricate readily processed and efficient hybrid solar cells together with conjugated polymer poly(3hexylthiophene).29 The intrinsic features and thus performance of such a device could be tuned by controlling the aspect ratios of the NRs. They also found that NRs were superior to QDs in photovoltaic applications, because they can provide a direct path for electrical transport at much lower loadings. The fabricated photovoltaic device eventually achieved an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. 5.2.6 Sensing Applications The extremely high surface-to-volume ratios associated with the nanostructures make their electrical properties extremely sensitive to species adsorbed on surface. Tao and co-workers have demonstrated this concept using arrays of Cu nanowires that contained nanoscale gaps generated through an automated electrochemical process.30 Upon adsorption of organic molecules onto these nanowires, the quantized conductance was reduced to a fractional value as a result of scattering of conduction electrons by the absorbates. Another demonstration, Penner and co-workers fabricated hydrogen sensors with Pd nanowires supported on the surface of a polymeric thin film as shown in Figure 5.6.31, 32 The resistance of these nanowires varied upon the adsorption of hydrogen molecules. In addition to metal nanowires, Lieber and co-workers have implemented the surface modified semiconductor nanowires as highly sensitive, real time sensors for pH and biological species.33 85 Chapter Introduction to NPs Figure 5.6. (A) Schematic diagram of a platinum mesowire array-based hydrogen sensor or switch. (B) SEM image [400 mm(h) by 600 mm (w)] of the active area of a platinum mesowire array-based hydrogen sensor.31, 32 5.3 Synthesis of Nanomaterials Vast applications of nanomaterials as discussed in the previous section have prompted intensive study of the synthesis of these materials to optimize colloidal semiconductor NCs fabrication. As a result, many new concepts for controlling the size, shape, aspect ratio ( Length Diameter ) and connectivity or coupling of colloidal NCs have been developed first for metal chalcogenide materials, but a unified set of synthesis control concept is now also being applied to other classes of materials, such as metals and metal oxides. After more than two decades, impressive progress has been made towards the tailored synthesis of colloidal NCs that have well-defined structures. A wide variety can now be successfully produced using a number of methods, such as coprecipitation, microemulsion, hydrothermal/solvothermal synthesis and surfactant-controlled growth in a hot organic solvent using either single or multi-source precursors.34, 35 86 Chapter Silver Selenide NCs Figure 6.6. DSC curve of the prepared Ag2Se NCs. 6.2.3 Growth Mechanism of Ag2Se NPs It is well known that amine disrupts the growth of particular crystal planes which leads to the formation of various shapes of NP.90-92, 96 However, in our case, varying the concentration of HDA doesn’t change the morphology of Ag2Se. This suggests the Ag2Se particles have a strong tendency to form the preferred cube shaped NPs and a similar phenomena have been reported for MnS NPs by Cheon and coworkers in 2002.91 This intrinsic property of Ag2Se is believed to be responsible for the formation of various NP shapes (see below). In our study, we noticed that the degradation of the precursor is affected mainly by the reaction temperature and the amount of amine added. A rapid degradation occurred readily at either high temperature or high amine concentration, which would result in a high concentration of nuclei. In general, for a spherical singlephase crystal whose size is smaller than 10 – 20 nm, its surface must be a polyhedron containing high-index crystallographic planes.99 Thus, we found that NPs obtained with high amine concentration at high temperature are mainly small faceted crystals, since more nuclei were generated under these conditions as shown in Figure 6.7. 105 Chapter Silver Selenide NCs Figure 6.7. Shapes and sizes of Ag2Se NCs produced under different conditions after 15 of heating. In order to understand the mechanism of the shape transformation of Ag2Se NPs, we further examined a series of Ag2Se NCs isolated at 95, 125 and 145 °C as shown in Figure 6.8 and found that the shape of the Ag2Se NCs evolved from small faceted crystals to nanocubes. This shape transformation process further supports our suggestion that the Ag2Se NCs intrinsically grown into cube shaped crystals as discussed before. In addition, SEM image of the bulk Ag2Se (shown in Figure 6.9) obtained from pyrolysis of the precursor at low pressure indicate the presence of cubic shaped Ag2Se crystals, reflecting the strong tendency of [(Ph3P)2Ag2(SeC{O}Ph)2] to form cubic shaped Ag2Se in the absence of any capping agents. Overall, when there is insufficient monomer (high nuclei concentration) in the solution, the nuclei have no choice but to grow into small faceted crystals. However, when the monomer concentration is high enough (low nuclei concentration), the nuclei grew into cubes. To demonstrate the effect of nuclei concentration on the morphology of Ag2Se, we 106 Chapter Silver Selenide NCs isolated the NCs at higher temperatures (165 & 180 °C) and Figures 6.3 & 6.4 illustrate that faceted crystals are formed under these conditions. Figure 6.8. TEM images of silver selenide NPs synthesized using an amine-toprecursor ratio of 50 after 15 of heating at (a) 95 °C, (b) 125 °C and (c) 145 °C. Figure 6.9. (a) SEM image of Ag2Se obtained from the pyrolysis of [(Ph3P)3Ag2(SeC{O}Ph)2]. (b) An enlarged portion of the cube-shaped Ag2Se crystal is shown at the right hand lower corner. 107 Chapter 6.3 Silver Selenide NCs Summary We have demonstrated that nearly monodispersed silver selenide nanocubes and faceted crystals were prepared under mild conditions by chemical activation of a single precursor with amine. A possible growth mechanism for the observed Ag2Se NC shapes has been elucidated based on the available experimental results. It is shown that these Ag2Se NCs have a tendency to grow into cube shaped crystals. Varying the nuclei concentration by changing the reaction temperature enables us to fine tune the shapes of these Ag2Se NCs. Meanwhile, the size of these Ag2Se NCs can be tuned by changing the temperature and the HDA concentration. Our TEM study of Ag2Se particles has revealed that the phase transformation has no effect on the shape or the crystallinity of the NCs, which further proves that the shape and size of the Ag2Se NCs are determined by well-controlled experimental conditions. The morphology of the Ag2Se particles prepared causes them to self-assemble on TEM grids which may implies these NCs are good candidates for synthesizing photonic crystals. 6.4 Synthesis and Methodology All the reactions were performed under the argon atmosphere using standard Shlenk techniques. The synthesis of [(Ph3P)3Ag2(SeC{O}Ph)2] (11) has been discussed in chapter 4. A degassed solution of [(PPh4)3Ag2(SeC{O}Ph)2] (50 mg) in TOP (0.5 mL) was injected into a hot solution (95ºC/125ºC/145ºC/165ºC/180ºC) of HDA. After these components had been mixed, the pale yellow solution rapidly changed to brown. After 15 min, the reaction solution was cooled to room temperature, ca. mL of toluene was added and the product was precipitated by further adding 20 mL of ethanol. The precipitate was centrifuged, washed thoroughly 108 Chapter Silver Selenide NCs with 20 mL of ethanol. The precipitate can either be redispersed by adding small amount of toluene (ca 0.5 mL) and used for TEM study or dried under vacuum and used for XRPD measurements. It is worth to note that the dispersion solution is unstable and will precipitate within 0.5 hour. 109 Chapter Chapter 7. Copper Selenide NPs Synthesis of Cu2-xSe NPs and Microflakes from [(Ph3P)3Cu2(SeC{O}Ph)2] 7.1 Introduction Cu2-xSe is an extrinsic p-type semiconductor with a direct band gap of 2.2 eV and an indirect energy gap of 1.4 eV at x = 0.2.124-127 This indirect energy gap of Cu2-xSe is well within the ideal band gap range of 1.1 – 1.7 eV which is suitable for solar energy materials.128 Besides, it can also be used as the superionic energy materials.129 Thus, it is conceivable that the available of Cu2-xSe in the form of NP may bring in new applications. In the past few years, many synthetic methods for copper selenide have been reported. For example, Qian and co-workers have obtained nanocrystalline Cu2-xSe at room temperature by γ-irradiation of copper acetate and sodium selenosulfate.130 Same group also reported that Cu2-xSe NCs, can be obtained from the redox reaction between CuO and Se in a mixture of ethylenediamine and hydrazine solvent.131 Recently they have shown that rare Cu2-xSe nanotubes can be synthesized from the same reaction conditions by extending the reaction duration from 24 hours to 48 hours.132 Further, few groups have reported well crystalline Cu2-xSe NPs can be produced from sonochemical synthesis,133, single source precursor synthesis.82, 114 134 microwave assisted synthesis,135 and However, all these syntheses suffer from a drawback, which the synthesized Cu2-xSe NPs are usually polydispersed with nonuniform shape. In previous chapter, we have demonstrated that good quality and shape tunable Ag2Se NCs have been prepared from [(Ph3P)3Ag2(SeC{O}Ph)2] single molecular precursor. Hence, we would like to extend this strategy to synthesize Cu2-xSe NP by 110 Chapter using Copper Selenide NPs a structurally similar metal selenocarboxylate precursor, [(Ph3P)3Cu2(SeC{O}Ph)2] and hopefully, a uniform shape and size Cu2-xSe NP can be obtained. Previously Lu and Vittal have published preliminary account on Cu2-xSe NPs obtained from [(Ph3P)3Cu2(SeC{O}Ph)2] precursor in TOPO/TOP surfactants.114 Here, we describe the synthesis using other surfactants to investigate the effect of solvent on the formation of Cu2-xSe NPs. 7.2 Formation and Characterization of Cu2-xSe NPs and Microflakes 7.2.1 HDA and TOP as Surfactants The TEM images of Cu2-xSe NCs obtained from HDA solution at 190 ˚C are shown in Figure 7.1. The TEM images clearly shown that The Cu2-xSe NCs obtained under this condition are twinned NCs. Further, each Cu2-xSe exhibits different degree of twinning and thus an expected ring type SAED pattern (Figure 7.1d) is obtained from these twinned crystals. In the HRTEM image of the NC (Figure 7.1b), we can clearly seen that two or more NCs are sharing a common crystallographic plane and each of these NCs is highly crystalline as judge from the clearly visible crystal lattices under the electron beam (Figure 7.1c). 111 Chapter Copper Selenide NPs Figure 7.1. (a) Low resolution TEM image of Cu2-xSe NCs. (b) Zoom in TEM image of twinned Cu2-xSe NCs. (c) HRTEM image of a twinned crystal. (d) SAED spectrum of Cu2-xSe NPs. Varying the amount of HDA, temperature and prolong the heating not improve the quality of the Cu2-xSe NCs. According to Wang,99 twinning is one of the most popular planar defects in NCs, and it is frequently observed for face-centered cubic (fcc) structure metallic NCs. This could be the reason that twinning process only observed in the case of cubic Cu2-xSe and not in the orthorhombic Ag2Se NCs. In addition, small particle usually prefer the twinning process due to the low surface and volume energy advantage.99 Thus HDA and TOP might not capped strong to the Cu2-xSe and prevent the twinning. By using a strong coordinating ligand, for example, 112 Chapter Copper Selenide NPs dodecanethiol – a well known capping agent for metal chalcogenides (e. g., Cu2S,136 PbS92) NPs, it may be possible to prevent the twinning process. 7.2.2 DT and TOP as Surfactants Figure 7.2 showing the TEM images of Cu2-xSe NPs obtained at various DT concentrations. As illustrated in the TEM images, the degree of twinning in these Cu2-xSe NPs is lesser than those obtained from TOP and HDA surfactants, which has been discussed in previous section. We noticed more twinning crystals are obtained at lower DT concentration (Figure 7.2a). This could be due to insufficient of DT to cap each Cu2-xSe NPs and prevent them from forming twinned crystals. As shown in the TEM images, the size of the Cu2-xSe NPs are decreasing when we increased the DT amount. This phenomenon is commonly observed when the amount of capping agent was increased.87 The synthesized copper selenide NPs are polydispersed and their measured size are 54.1 ± 11.1 nm (surfactant-to-precursor ratio = 45), 40.3 ± 9.5 nm (surfactant-to-precursor ratio = 90) and 35.4 ± 14.4 nm (surfactant-to-precursor ratio = 180). 113 Chapter Copper Selenide NPs Figure 7.2. TEM images of Cu2-xSe NPs isolated after 10 of heating at 220 ˚C with different DT to precursor molar ratio, (a) 45, (b) 90 and (c) 180. The IR spectrum of the Cu2-xSe NP is shown in Figure 7.3. The signal of the IR peaks for the Cu2-xSe NP is very weak. We have repeated the measurement with different samples and the similar type of spectrum were obtained. However, some characteristic peaks (peaks between 600 – 750 nm; peaks at 1500 nm and peaks in the 114 Chapter Copper Selenide NPs range of 2700 – 3000 nm) of DT still visible in the IR spectrum of Cu2-xSe NP. Thus, it may be concluded that the surfaces of the NPs are capped with DT. Figure 7.3. IR spectrum of Cu2-xSe NP. Low resolution TEM images of Cu2-xSe NPs obtained at various temperatures are shown in Figure 7.4. The Cu2-xSe NPs formed at higher temperature are slightly smaller than those obtained at low temperature (40.3 ± 9.5 nm for 220 ˚C; 28.3 ± 10.1 nm for 250 ˚C and 30.5 ± 6.3 nm for 280 ˚C). This suggested that more nucleic are being formed at high temperatures because in the single injection synthesis, an increase in the nucleic concentration always caused the reduction in the size of the NP.74 No drastic change on the morphology of the Cu2-xSe NPs as we increased the temperature. Interestingly, no spherical shaped Cu2-xSe NPs (mainly hexagonal and some triangular shaped (see Figure 7.4b)) were obtained high temperature (280 ˚C), even though spherical shaped NPs are usually considered as thermodynamic product.99 Overall we have seen that DT did promote the anisotropic growth in Cu2-xSe NPs (illustrated in Figures 7.2 & 7.3) as compared with the spherical shaped Cu2-xSe obtained by Lu et al. from TOPO/TOP surfactants.114 115 Chapter Copper Selenide NPs Figure 7.4. TEM images of Cu2-xSe NPs isolated after 10 of heating at (a) 220 ˚C; (b) 250 ˚C and (c) 280 ˚C with surfactant-to-precursor ratio at 90. 7.2.3 Ethylenediamine as Surfactants Ethylenediamine is a widely used solvent for the synthesis of various metal chalcogenide NPs.137-141 In some case, interesting morphology like nanotube132 can be obtained using this solvent. We were able to obtain Cu2-xSe as microflakes when the precursor was heated at 105 ˚C in mL of ethylenediamine for an hour. The SEM 116 Chapter Copper Selenide NPs images the Cu2-xSe macro flasks are shown in Figure 7.5. Unfortunately, the particles agglomerate. Figure 7.5. SEM images of Cu2-xSe macro flasks obtained from ethylene diamine. Typical XRPD spectra of the synthesized Cu2-xSe particles are shown in Figure 7.6. All the peaks can be indexed to the cubic phase Cu2-xSe (JCPDS No.: 0006-0680) Figure 7.6. XRPD patterns of Cu2-xSe obtained from (a) ethylenediamine; (b) HDA and (c) DT. 117 Chapter 7.3 Copper Selenide NPs Summary In 2002, Vittal et al. have reported that close to monodispersed spherical Cu2-xSe NPs can be produced by thermally decomposing [(Ph3P)3Cu2(SeC{O}Ph)2] in TOPO/TOP surfactants.114 Here, we employed other surfactants e. g., HDA, DT and ethylenediamine and we have shown that these surfactants induced an anisotropic growth on either nano or macrosize Cu2-xSe particles. However they are not highly monodispersed and uniform shape. High quality Ag2Se NCs were obtained from silver selenocarboxylate precursor. However, similar results were not obtained for Cu2-xSe although an identical metal selenocarboxylate precursor was used. This could be due to a few reasons. First of all, the copper selenocarboxylate appear to be more stable than the silver analogue in solution. E. g., a higher temperature (≤180 ˚C) is required to decompose the copper selenocarboxylate in HDA. Hence, when these two precursors were injected into the hot HDA solution at same temperature; one should expect differences in their nucleation and growth durations due to the different stabilities of these two precursors. This eventually leads to the formation of different sizes and shapes of NPs. Another reason for the dissimilarity in the Ag2Se and Cu2-xSe nanosynthesis is that both metal selenides have different crystal structures, orthorhombic and cubic respectively. Hence it is not surprising that Cu2-xSe nanocube and faceted NCs were not obtained by using the same experimental conditions. In our study, we have demonstrated that the twinning process in the Cu2-xSe NCs can be hindered by using a strongly coordinating capping agent likes DT. 7.4 Synthesis and Methodology All the reactions were performed under the argon atmosphere using standard Shlenk techniques. The synthesis of Cu2-xSe NPs is similar to the Ag2Se NPs that 118 Chapter have been Copper Selenide NPs described in previous chapter. Briefly, 30 mg of [(Ph3P)3Cu2(SeC{O}Ph)2]114 was dissolved in degassed TOP (0.3 mL). This yelloworange solution was later injected through syringe into a pre-heated (180 – 230 ˚C) HDA or DT solution. The solution immediately turns into black indicates the formation of Cu2-xSe NP. This solution was allowed to heat for 10 and the growth of the NP was halted by removing the heat source. ~3 mL of toluene and 20 mL of EtOH were added to the HDA/TOP solution to precipitate out the NCs. The dark color Cu2-xSe NPs were centrifuged out and washed two times with 10 mL of EtOH to ensure all the side products were removed. The Cu2-xSe powder was dried under vacuum and later used for XRPD measurements. Cu2-xSe microflakes were obtained by heating the precursor (30 mg) in pure ethylenediamine at 105 ˚C for an hour. 119 [...]... is unstable and will precipitate within 0.5 hour 109 Chapter 7 Chapter 7 Copper Selenide NPs Synthesis of Cu2-xSe NPs and Microflakes from [(Ph3P)3Cu2(SeC{O}Ph )2] 7.1 Introduction Cu2-xSe is an extrinsic p-type semiconductor with a direct band gap of 2. 2 eV and an indirect energy gap of 1.4 eV at x = 0 .2 . 124 - 127 This indirect energy gap of Cu2-xSe is well within the ideal band gap range of 1.1 – 1.7... images of Ag2Se NCs prepared at 165 °C for 15 min with surfactant-to-precursor ratio of, a) 25 and b) 50 Figure 6.4 TEM images of faceted Ag2Se NCs prepared at 180 °C for 5 min with surfactant-to-precursor ratio of, a) 25 and b) 50 Ag2Se is known to exist in two phases, a cubic phase above 133 ºC and an orthorhombic phase below 133 ºC . 122 , 123 However, we found no difference in the 103 Chapter 6 Silver Selenide. .. Silver Selenide NCs Figure 6.7 Shapes and sizes of Ag2Se NCs produced under different conditions after 15 min of heating In order to understand the mechanism of the shape transformation of Ag2Se NPs, we further examined a series of Ag2Se NCs isolated at 95, 125 and 145 °C as shown in Figure 6.8 and found that the shape of the Ag2Se NCs evolved from small faceted crystals to nanocubes This shape transformation... growth of Ag2Se nanocubes and examine the roles of parameters critical to the formation of Ag2Se NCs 6 .2 Results and Discussion 6 .2. 1 Formation of Ag2Se NCs from [(Ph3P)3Ag2(SeC{O}Ph )2] Previously, our laboratory have demostrated that Cu2-xSe NPs can be synthesized by thermolysis of copper selenocarboxylate in TOPO/TOP.114 Here we further extend our work by using silver(I) selenocarboxylate [(Ph3P)3Ag2(SeC{O}Ph )2] ... Figure 5 . 12 TEM images of various sizes CdSe NPs.73 5.3.4.1 Single Molecular Precursor Approach Instead of using hazardous ((CH3)2Cd) or inert (e g., CdO) precursors, one can choose to use metal complexes as the single molecular precursors The concept of thermally decomposed single molecular precursor in surfactant to produce NP was first introduced by O’Brien The advantages of using single molecular. .. [Pb(SC{O}Ph )2] .86 However, until today, relatively few have been developed for the syntheses of metal selenide NPs through molecular precursors As shown in previous chapter, most of the metal selenocarboxylates that have been synthesized are suitable precursors for metal selenide powder based on the TGA and XRPD analysis Hence in the second part of this thesis, we devoted our efforts to employ them as molecular. .. preparation of 98 Chapter 6 Silver Selenide NCs Ag2Se NPs,108-110 nanowires111 and nanoscale dendrites. 1 12, 113 In this chapter, we demonstrate one pot synthesis of structurally well defined Ag2Se nanocubes and faceted NCs under mild conditions via the amine mediated synthetic approach from single precursor, [(Ph3P)3Ag2(SeC{O}Ph )2] Further, in this study, we elucidate the formation of Ag2Se faceted NCs and. .. c FC: Faceted crystals; CU: Cubes d The faceted crystals will transform into cube shaped NCs after 2 hours of heating e The size measured is correspond to the faceted crystals b 100 Chapter 6 Silver Selenide NCs 6 .2. 2 Morphology and Characterization of the Ag2Se NCs TEM images of the Ag2Se nanocubes formed at 125 °C after 60 minutes of heating reveal a self-assembled closed-packed monolayer array (Figure... from Ag2Se NCs synthesized at different temperatures 104 Chapter 6 Silver Selenide NCs Figure 6.6 DSC curve of the prepared Ag2Se NCs 6 .2. 3 Growth Mechanism of Ag2Se NPs It is well known that amine disrupts the growth of particular crystal planes which leads to the formation of various shapes of NP.90- 92, 96 However, in our case, varying the concentration of HDA doesn’t change the morphology of Ag2Se... Same group have also reported the syntheses of various anisotropic shaped semiconductor, e g., CdS (Figure 5.13a & b),90 MnS (Figure 5.13c & d),91 and PbS (Figure 5.13e & f) 92 NPs, which were synthesized from single molecular precursors in hexadecylamine surfactant Figure 5 .13 TEM images of various shapes CdS (a – b), MnS (c – d) and PbS (e – f) NPs.90- 92 96 Chapter 5 5.4 Introduction to NPs Aim and . atomic and molecular limit of discrete density of electronic state and the extended crystalline limit of continuous bands. Clearly shown in Figure 5.1, NPs contain discrete and large molecular- like”. adding a micellar solution of Na 2 S to a micellar solution of MnCl 2 and ZnCl 2 . Subsequent hydrothermal treatment of the products increased the average diameter to ~ 12 nm but substantially. 5.13a & b), 90 MnS (Figure 5.13c & d), 91 and PbS (Figure 5.13e & f) 92 NPs, which were synthesized from single molecular precursors in hexadecylamine surfactant. Figure 5 .13.