Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides.

Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.

Electrical Control of Circular Photogalvanic Spin-Valley Photocurrent in a Monolayer Semiconductor.

In a monolayer transition metal dichalcogenide (TMDC) that lacks structural inversion symmetry, spin degeneracy is lifted by strong spin-orbit coupling, and a distinctive spin-valley locking allows for the creation of valley-locked spin-polarized carriers with a circularly polarized optical excitation. When excited carriers also have net in-plane momentum, spin-polarized photocurrents can be generated at ambient temperature without magnetic fields or materials. The behavior of these spin-polarized photocurrents in monolayer TMDC remains largely unexplored. In this work, we demonstrate the tuning of spin-valley photocurrent generated from the circularly polarized photogalvanic effect in monolayer MoS2, including magnitude and polarization degree, by purely electric means at room temperature. The magnitude of spin-polarized photocurrent can be modulated up to 45 times larger, and the polarization degree of the total photocurrent can be tuned significantly (here from 0.5 to 16.6%) by gate control. Combined with the atomic thickness and wafer-scale growth capabilities of monolayer TMDC, the efficient electrical tuning of spin-valley photocurrent suggests a pathway to achieve spin-logic processing by local gate architectures in monolayer opto-spintronic devices.

Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts.

Understanding the electronic transport of monolayer transition metal dichalcogenides (TMDs) and their heterostructures is complicated by the difficulty in achieving electrical contacts that do not perturb the material. Typically, metal deposition on monolayer TMDs leads to hybridization between the TMD and the metal, which produces Schottky barriers at the metal/semiconductor interface. In this work, we apply the recently reported hexagonal boron nitride (h-BN) tunnel contact scheme to probe the junction characteristics of a lateral TMD heterostructure grown via chemical vapor deposition. We first measure the electronic properties across the junction before elucidating optoelectronic generation mechanisms via scanning photocurrent microscopy. We find that the rectification ratio measured using the encapsulated, tunnel contact scheme is almost 2 orders of magnitude smaller than that observed via conventional metal contact geometry, which implies that the metal/semiconductor Schottky barriers play large roles in this aspect. Furthermore, we find that both the photovoltaic as well as hot carrier generation effects are dominant mechanisms driving photoresponse, depending on the external biasing conditions. This work is the first time that this encapsulation scheme has been applied to lateral heterostructures and serves as a reference for future electronic measurements on this material. It also simultaneously serves as a framework to more accurately assess the electronic transport characteristics of 2D heterostructures and better inform future device architectures.