Synthesis and Compositional Control of Size Monodisperse Si x Ge 1-x Nanocrystals for Optoelectronic Applications
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Synthesis and Compositional Control of Size Monodisperse SixGe1-x Nanocrystals for Optoelectronic Applications Keith Linehan, Darragh Carolan, Daithi Ó Sé and Hugh Doyle Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland ABSTRACT Alkyl-terminated SixG1-x nanocrystals are prepared at room temperature by co-reduction of Si and Ge precursors by hydride reducing agents within inverse micelles. Compositional control of the alloy silicon-germanium NCs (ca. 3.6 nm) is achieved by varying the relative amounts of each precursor used in the synthesis. Transmission electron microscopy imaging confirmed that the NCs are highly crystalline with a narrow size distribution; optical spectroscopy shows strong quantum confinement effects, with moderate absorption in the UV spectral range, and a strong blue emission with a marked dependency on excitation wavelength. INTRODUCTION Over the last few decades, size-dependent optical and electronic properties of semiconductor nanocrystals (NCs) or quantum dots have made them attractive materials for applications ranging from optoelectronics, photovoltaics, light emission and bioimaging.[1-3] More recently, the increasing number of reported routes for preparation of size monodisperse nanostructures has led to greater interest in modifying the composition in order to achieve control over the band gap of semiconductor nanocrystals (NCs).[4,5] Compositional tailoring of semiconductor NCs is particularly attractive because it allows for band gap modification to be carried out without some of the challenges associated with changing nanocrystal size.[6] For Group IV materials, it is known that Si and Ge are miscible in the solid state and this effect has been used in order to tune material band gaps in nanostructures and quantum wells.[7-9] More recently, theoretical studies have suggested that by suitable control of alloying, stress, band offsets and folding, truly direct band gap semiconducting heterostructures based on silicon and germanium can be realized.[10] This would in turn enhance extinction co-efficients, radiative recombination rates and photoluminesnce quantum yields of SixGe1-x NCs, significantly improving their applicability for optoelectronic devices. Nanostructured SixGe1-x has been prepared using several methods, including physical and chemical vapor deposition,[7] molecular beam epitaxy,[11,12] radio frequency cosputtering,[13,14] non-thermal plasma pyrolysis,[15] thermal disproportionation[16] and laserinduced pyrolysis.[17] Each fabrication method has associated benefits and challenges including process scalability, yield, energy input, crystallinity, polydispersity, and surface chemistry that must be considered.[6] Despite the number of reports describing the preparation of high quality SixGe1-x NCs, no studies have been reported on the chemical synthesis of NCs with controlled composition. Here we report the solution-phase synthesis and characterization of alloy SixGe1-x NCs dispersed in non-polar solvents with well-defined core diameters of ca.
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