Beyond Graphene: Making New Two-Dimensional Materials for Future Electronics

As a result of the widespread integration of semiconductor technology into all facets of life, the Group IV semiconductors, silicon and germanium, are the most important and ubiquitous materials of the current era. Not only are they the workhorse materials of transistor technology, silicon and germanium are the most prevalent materials in photovoltaics and photodetectors, and have attracted considerable attention as thermoelectric energy generators. Still, the neverending push towards device miniaturization calls for the need to understand the nature of these materials when reduced below the nanoscale.

Here, we will describe our recent development of a new family of two-dimensional (2D) materials based on honeycomb, sp³-hybridized Group IV elements. These ligand-terminated Si, Ge, and Sn “graphane analogues” are an intriguing class of 2D materials that offer the potential to tailor the structure, stability, and properties via covalent chemistry.  The electronic structure of these materials can be systematically controlled by attaching different surface terminating ligands, and even bestowing properties that don’t exist in the normal three-dimensional structure.  For example, with the appropriate surface functionalizing ligand, these 2D materials feature direct band gaps, enhancing silicon and germanium’s performance in photovoltaics, photodetectors, light-emitting diodes, and lasers.   These materials can be synthesized in gram-scale quantities, exfoliated as single-layers, and prepared as thin layers directly on Si and Ge wafers. 

This new class of two-dimensional materials are not only promising building blocks for a variety of conventional semiconductor applications, but also provide a pioneering platform to systematically and rationally control material properties using covalent chemistry. The ability to vary band offsets and band gaps in a single 2D sheet solely by selecting different surface functional groups gives the opportunity to define and study 1D interfaces within a single 2D layer by spatially patterning the surface termination, allowing unparalleled control and the understanding of new physical phenomena.   Overall, this new class of 2D materials is poised to have a great impact not only in the traditional sectors of nanoscience, but also in opening up a new research paradigm in covalently-controlled properties by design.