A solvothermal route to wurtzite ZnSe nanoparticles
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Zinc powder reacts with equivalent elemental selenium in solvent ethylenediamine at 120 °C for 6 h to form a complex, which is converted to ZnSe nanoparticles by pyrolysis or protonization. X-ray diffraction results suggest that the as-formed products have wurtzite structure. Transmission electron microscopy observation show that particles with spherical and laminar morphology were produced by pyrolysis and protonization, respectively. The formation of ZnSe nanoparticles is also investigated by infrared and thermal analysis. I. INTRODUCTION
The wide band gap II–VI nanoscale semiconductors are of current interest for optoelectronic applications such as blue lasers, light emitting diodes and optical devices.1–3 An important direct band gap material, ZnSe has been called a “promising” material for the fabrication of visible light-emitting devices.4 Recently, ZnSe-based laser diodes have been successfully demonstrated to operate in a continuous-wave (cw) mode at room temperature.5 II–VI compounds can be prepared in a variety of ways. Molecular precursor routes have been developed by a number of groups for bulk materials or coatings.6–8 The straightforward method is a combination of the elements at elevated temperatures, but it is difficult to get nanoscale materials by this traditional solid state reaction. A low-energy approach is the precipitation of metal chalcogenides from aqueous solutions of the metal cations by use of H2E (E ⳱ S, Se or Te).9 The elemental reactions between zinc or copper with sulfur at reflux in a strongly coordinating solvent such as pyridine and Nmethyimidazole, has been reported to form complexes, which can be thermally decomposed at 500 °C to form binary chalcogenides.10–12 Parkin et al. reported that the metal chalcogenides can be obtained through the reaction of selenium or tellurium with elemental metals in liquid ammonia at room temperature in a pressure vessel.13,14 As-formed zinc selenide is amorphous powder, which can be crystallized to stilleite by annealing (300 °C for 2 h). Li reported another elemental reaction route to produce nanocrystalline ZnSe with zincblende structure in pyridine at 180 °C in an autoclave.15 Recently, an important organometallic synthetic route has been implied
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Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 3, Mar 2000
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to the synthesis of colloid zincblende ZnSe nanocrystals with UV-blue luminescence up to 360 nm,16 similar to that route reported by Murray et al.17 Despite these developments in the preparation of zinc selenide nanocrystallites, stilleite ZnSe with cubic structure was generally reported to form by these methods and few reports touched the synthesis of wurtzite ZnSe nanoparticles. As a member of II–VI compounds, ZnSe crystallize in either zincblende or wurtzite structure. Frequently, ZnSe exists in cubic structure as stilleite in natural mines.18 On the other hand, hexagonal structure for ZnSe is a metastable phase, which has been observed by Pashinskin.19 In
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