Growth and Characterization of Silicon at the 2D Limit

Because bulk silicon has dominated the development of microelectronics over the past 50 years, the recent interest in two-dimensional (2D) materials (e.g., graphene, MoS$_{\mathrm{2}}$, phosphorene, etc.) naturally raises questions regarding the growth and properties of silicon at the 2D limit. Utilizing atomic-scale, ultra-high vacuum (UHV) scanning tunneling microscopy (STM), we have investigated the 2D limits of silicon growth on Ag(111). In agreement with previous reports of \textit{sp}$^{2}$-bonded silicene phases, we observe the temperature-dependent evolution of ordered 2D phases. However, we attribute these to apparent Ag-Si surface alloys. At sufficiently high silicon coverage, we observe the precipitation of crystalline, \textit{sp}$^{3}$-bonded Si(111) domains. These domains are capped with a $\surd $3 honeycomb phase that is indistinguishable from the silver-induced $\surd $3 honeycomb-chained-trimer reconstruction on bulk Si(111). Further \textit{ex-situ }characterization with Raman spectroscopy, atomic force microscopy, cross-sectional transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy reveals that these sheets are ultrathin sheets of bulk-like, (111) oriented, \textit{sp}$^{3}$ silicon. Even at the 2D limit, scanning tunneling spectroscopy shows that these silicon nanosheets exhibit semiconducting electronic characteristics.