Growth Mechanisms and Electronic Properties of Vertically Aligned MoS2.

Thin films of layered semiconductors emerge as highly promising materials for energy harvesting and storage, optoelectronics and catalysis. Their natural propensity to grow as oriented crystals and films is one of their distinct properties under recent focal interest. Specifically, the reaction of transition metal films with chalcogen vapor can result in films of vertically aligned (VA) layers, while metal-oxides react with chalcogens in vapor phase to produce horizontally aligned crystals and films. The growth mechanisms of vertically oriented films are not yet fully understood, as well as their dependence on the initial metal film thickness and growth conditions. Moreover, the resulting electronic properties and the role of defects and disorder had not yet been studied, despite their critical influence on catalytic and device performance. In this work, we study the details of oriented growth of MoS2 with complementary theoretical and experimental approaches. We present a general theoretical model of diffusion-reaction growth that can be applied to a large variety of layered materials synthesized by solid-vapor reaction. Moreover, we inspect the relation of electronic properties to the structure of vertically aligned MoS2 and shed light on the density and character of defects in this material. Our measurements on Si-MoS2 p-n hetero-junction devices point to the existence of polarizable defects that impact applications of vertical transition-metal dichalcogenide materials.

Lithographically Patterned Functional Polymer-Graphene Hybrids for Nanoscale Electronics.

Two-dimensional (2D) materials are believed to hold significant promise in nanoscale optoelectronics. While significant progress has been made in this field over the past decade, the ability to control charge carrier density with high spatial precision remains an outstanding challenge in 2D devices. We present an approach that simultaneously addresses the dual issues of charge-carrier doping and spatial precision based on a functional lithographic resist that employs methacrylate polymers containing zwitterionic sulfobetaine pendent groups for noncovalent surface doping of 2D materials. We demonstrate scalable approaches for patterning these polymer films via electron-beam lithography, achieving precise spatial control over carrier doping for fabrication of high-quality, all-2D, lateral p-n junctions in graphene. Our approach preserves all of the desirable structural and electronic properties of graphene while exclusively modifying its surface potential. The functional polymer resist platform and concept offers a facile route toward lithographic doping of graphene- and other 2D material-based optoelectronic devices.