Emerging device applications for two-dimensional nanomaterial heterostructures

Two-dimensional materials have emerged as promising candidates for next-generation electronic and optoelectronic applications [1]. As is common for new materials, much of the early work has focused on measuring and optimizing intrinsic properties on small samples (e.g., micromechanically exfoliated flakes) under idealized conditions (e.g., vacuum and/or cryogenic temperature environments). However, real-world devices and systems inevitably require large-area samples that are integrated with dielectrics, contacts, and other semiconductors at standard temperature and pressure conditions. These requirements are particularly challenging to realize for two-dimensional materials since their properties are highly sensitive to surface chemistry, defects, and the surrounding environment. This talk will thus explore methods for improving the uniformity of solution-processed two-dimensional materials with an eye toward realizing scalable processing of large-area thin-films. For example, density gradient ultracentrifugation allows the solution-based isolation of transition metal dichalcogenides (e.g., MoS₂, WS₂, MoSe₂, and WSe₂) and boron nitride with homogeneous thickness down to the single-layer level [2]. Similarly, two-dimensional black phosphorus is isolated in solution with the resulting flakes showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes [3]. In addition to solution processing, this talk will also report on the integration of two-dimensional materials with dielectrics and other semiconductors. In particular, atomic layer deposition of dielectrics on two-dimensional black phosphorus suppresses ambient degradation, thereby preserving electronic properties in field-effect transistors at atmospheric pressure conditions [4]. Finally, p-type semiconducting carbon nanotube thin films are combined with n-type single-layer MoS₂ to form p-n heterojunct- on diodes [5]. The atomically thin nature of single-layer MoS₂ implies that an applied gate bias can electrostatically modulate the doping on both sides of the p-n heterojunction concurrently, thereby providing five orders of magnitude gate-tunability over the diode rectification ratio in addition to unprecedented anti-ambipolar behavior when operated as a three-terminal device. This anti-ambipolar behavior can be generalized to other solution-processed p-n heterojunctions including p-type semiconducting carbon nanotubes and n-type indium gallium zinc oxide [6].