(20d) Towards Direct Bandgap Silicon Via 2D Silicane: A Joint Experimental and Computational Study

Silicon currently dominates the semiconductor industry, and for good reason; it is non-toxic, earth abundant, and we can produce silicon with exquisite purity. Recently, however, the fundamental principle by which we transmit information is changing—we are gradually shifting away from classical electronics devices, in favor of modern quantum-based systems and optoelectronic devices that utilize light, instead of electricity, to transmit information. But this poses a threat for silicon, as silicon is a poor emitter of light, inhibiting its implementation in many optoelectronic devices. To circumvent this, there are many trying to develop new materials for such applications in which traditional silicon devices are unsuitable, but these new materials often fall short where silicon shines most: again, silicon is biocompatible, it is inexpensive, and billions of dollars are invested in established infrastructure dedicated to manufacturing silicon on an industrial scale. A core aspect of our research is fundamentally improving and understanding the ability for silicon to emit light. It is well-known that confining silicon to the nanoscale enables silicon to emit light, and with materials such as graphene on the forefront of research, one promising route is to explore 2D silicon nanosheets.

Monolayer silicon nanosheets were first synthesized in the 1800s by selectively removing calcium from calcium disilicide (CaSi2). The product of this reaction was determined to be silicon that emits light. However, despite over 150 years of research, the literature makes it abundantly clear that there is little consensus regarding the structure of these nanosheets. In this presentation, we combine experimental data and density functional theory (DFT) calculations of XRD, FTIR, Raman, NMR, and optical properties to elucidate the structure and the nature of the electronic transitions in 2D silicon nanostructures.¹ We determine that the silicon nanosheets consist of a corrugated framework that is primarily terminated with hydrogen, making these nanosheets the silicon analog of hydrogenated graphene (graphane). Further, this presentation will explore the optical properties of these nanosheets, illustrate their direct bandgap-like behavior, and highlight the fundamental insight provided by DFT calculations to understand the origin of light emission in quantum confined silicon. These results provide pivotal information towards understanding the structural and optical properties of Si-nanosheets for their integration into next-generation electronic and optoelectronic devices.

(1) Ryan et al., Silicene, Siloxene, or Silicane? Revealing the Structure and Optical Properties of Silicon Nanosheets Derived from Calcium Disilicide, Chem. Mater. 2020, 32, 2, 795-804