DMC Study of the Optoelectronic Properties of Diamondoids
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DMC Study of the Optoelectronic Properties of Diamondoids Neil Drummond1, Andrew Williamson2, Richard Needs1, and Giulia Galli2 1 University of Cambridge, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom 2 Lawrence Livermore National Laboratory, Livermore, CA, 94550
ABSTRACT We have performed density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations of the optoelectronic properties of hydrogen-terminated carbon nanoparticles (diamondoids). Both the QMC and DFT results show that quantum confinement effects disappear in diamondoids larger than 1 nm in diameter, which have optical gaps below that of bulk diamond. Furthermore, our QMC calculations predict a negative electron affinity for diamondoids up to 1 nm in diameter. Both these properties result from the delocalized nature of the lowest unoccupied molecular orbital. These calculations were originally described in Reference [1]. INTRODUCTION Diamondoids In recent years there has been considerable interest in the optoelectronic properties of nanometer-sized particles of group IV elements, because it is hoped that such particles may be of use in sensor and display devices. Silicon [2] and germanium [3] nanoparticles have been studied extensively, as they are relatively easy to synthesize and integrate with existing device fabrication technology. Carbon nanoparticles have proved to be more difficult to synthesize and characterize. However, Dahl et al. [4] showed that it is possible to isolate monodisperse samples of carbon nanoparticles from petroleum. These nanoparticles, known as diamondoids, are expected to have some useful optoelectronic properties: (i) the band gap of diamond is in the ultraviolet range, and quantum confinement effects might increase diamondoid optical gaps to even higher energies, enabling a unique set of sensing applications; (ii) certain hydrogenterminated diamond surfaces have negative electron affinities [5], suggesting that diamondoids may also have negative electron affinities. By coating a surface with diamondoids it should be possible to produce new low-voltage electron-emission devices, e.g. for use in field-emission displays. Unfortunately, both theoretical and experimental studies of the optoelectronic properties of diamondoids have so far produced controversial results. Previous experimental studies of diamondoids An x-ray absorption near-edge structure (XANES) study of films of carbon-vapordeposition (CVD) diamond carried out in 1999 [6] indicated that substantial quantum confinement effects are present in diamondoids. Nanoparticles of up to 27 nm in diameter were found to have higher optical gaps than bulk diamond. On the other hand, more recent near-edge x-ray absorption fine structure (NEXAFS) studies of diamondoids produced by hot-filament CVD and extracted from explosive residues [7] found no evidence of elevated optical gaps, even
in diamondoids as small as 4 nm in diameter. The discrepancy was attributed to possible contamination of the samples used in the earlier
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