Silicon Nanoclusters: Simulations


Aaron Puzder Lawrence Livermore National Laboratory

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The field of research surrounding the optical properties of semiconductor nanoclusters has seen enormous growth over the past decade. As the dimensions of a semiconductor are reduced below its exciton Bohr radius, quantum confinement causes its bandgap to widen, the electronic states to become discrete, and the oscillator strength of the smallest electronic transitions to increase. In direct gap II/VI semiconductors such as CdSe, several experiments have reproduced the one-to-one correspondence between the size and the wavelength of absorbed and emitted visible light. The reproducibility and generally straightforward synthesis techniques have made CdSe the cornerstone of emerging technologies such as biological markers and nanostructure lasers. Other semiconductor nanoclusters, specifically those in group IV, are also known to absorb and emit visible light when their size is reduced to the nanometer scale. The efficient photoluminescence (PL) observed in porous silicon and in freestanding silicon nanoclusters suggests that these structures may serve as an alternative for CdSe in certain applications. One benefit of exploiting the optical properties of silicon nanoclusters is its potential to be integrated within existing silicon technologies to create nanoscale optoelectronic devices. Additionally, the biocompatibility of silicon makes it an ideal candidate for replacing fluorescent dyes as biotags.

Despite the efforts being placed on synthesizing and characterizing silicon nanoclusters, the size dependence of the optical gap has, so far, been difficult to reproduce experimentally. As the physical properties of a material are strongly governed by the surface at the nanometer scale (where surface-to-volume ratios are greatly increased), the interplay of quantum confinement and surface properties is still unclear. Few surface-sensitive probes are available, and distinguishing bulk effects from surface effects is difficult. Theoretical modeling is challenging within this size regime as a full quantum mechanical description of both the core and the surface atoms are required to provide accurate and predictive data. Developing a thorough understanding of the electronic and optical properties of silicon nanoclusters, in particular the effect of the surface on their optical properties, is a crucial step toward the utilization of these particles for new technologies. Therefore, theoretical predictions are required to improve our understanding of the influence of the surface on the properties of silicon nanostructures.

The school of thought advocating quantum confinement as the only mechanism responsible for PL in silicon would suggest that, regardless of the specific surface chemistry, the same results should be observed as long as all the surface dangling bonds are saturated. However, recent studies have shown that the surface can affect the optical properties of silicon. The combination of different passivating surfaces, contaminant atoms, and surface reconstructions are often responsible for disagreement among different experiments and among theories in characterizing the gap dependence on size.