Cadmium selenide: Surface and nanoparticle energetics
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Cadmium selenide (CdSe) belongs to a class of important II–VI semiconductors widely used in optical, sensor, and laser materials and quantum-dot light-emitting diodes. Here we present the first direct calorimetric measurement of the surface energy of wurtzite CdSe. CdSe nanoparticles with particle size between 20 and 60 nm were prepared by a hydrothermal method without additives to control morphology, and the surface energy was derived from the drop solution enthalpies in molten sodium molybdate and from water adsorption calorimetry. The surface energy of the hydrated surface is 1.31 6 0.26 J/m2, whereas that of the anhydrous surface is 1.65 6 0.27 J/m2. These values are significantly lower than those for ZnO and many other oxides.
I. INTRODUCTION
Cadmium selenide (CdSe) belongs to a class of important II–VI semiconductors that includes CdS, CdSe, and ZnS. It can exist as a hexagonal wurtzite phase at atmospheric pressure, a cubic zincblende phase generally formed in thin films, and a rocksalt phase at high pressure.1,2 Recently, there has been strong interest in CdSe nanocrystals and their widespread applications as optical filters,3 sensor and laser materials,4 and quantum-dot lightemitting diodes.5 Since the study of nanosized CdSe provides an opportunity to observe the evolution of material properties with crystal structure and size,6–9 many methods have been used to synthesize CdSe nanocrystals, including solid-state metathesis,10 chemical solution deposition and ionic reaction,11,12 gas–liquid precipitation,13 sonochemical methods,14 molecular precursor methods,15,16 c-irradiation,17 and solvothermal methods.9 The rapidly growing field of CdSe nanocrystals lacks experimental benchmarks for thermodynamic properties. A fundamental issue that remains to be addressed is how size relates to the surface activity and chemical and physical properties. However, no direct experimental data on the surface energy of CdSe appear to be available. High-temperature oxide melt solution calorimetry has been shown to be a powerful and convenient method for studying the surface energies of numerous nanophase oxides, including Al2O3,18 TiO2,19 a-Fe2O3,20 ZnO,21 and ZrO2.22 On the basis of our previous calorimetric study of bulk selenides,23 we extend our research to nanophase selenides. In this work, to derive the surface energies of hydrated and anhydrous wurtzite CdSe surfaces, we performed high-temperature oxide melt solution calorimetry a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.20 720
J. Mater. Res., Vol. 26, No. 5, Mar 14, 2011
http://journals.cambridge.org
Downloaded: 17 Mar 2015
and water adsorption calorimetry on a set of wurtzite CdSe nanoparticles with different surface areas. They were synthesized by a hydrothermal method without additives to control morphology. Structure and size were characterized by x-ray diffraction (XRD) and N2 adsorption. The surface energies for hydrated and anhydrous surfaces of wurtzite CdSe nanocrystals are reported for the first time
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