Pseudo-direct bandgap transitions in silicon nanocrystals: effect on optoelectronics and thermoelectrics

by Vivek Singh, Yixuan Yu, Qi C. -Sun, Brian A. Korgel, and Prashant Nagpal

Nanoscale 2014 10.1039/C4NR04688A

While silicon nanostructures are extensively used in electronics, the indirect bandgap of silicon poses challenges for optoelectronic applications like photovoltaics and light emitting diodes (LEDs). Here, we show that size-dependent pseudo-direct bandgap transitions in silicon nanocrystals dominate the interaction between (photoexcited) charge carriers and phonons, and hence the optoelectronic properties of silicon nanocrystals. Direct measurements of the electronic density of states (DOS) for different sized silicon nanocrystals reveal that these pseudo-direct transitions, likely arising from nanocrystal surface, can couple with quantum-confined silicon states. Moreover, we demonstrate that since these transitions determine the interactions of charge carriers with phonons, they change the light emission, absorption, charge carrier diffusion and phonon drag (Seebeck coefficient) in nanoscaled silicon semiconductor. Therefore, these results can have important implications for design of optoelectronics and thermoelectric devices based on nanostructured silicon.

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