Abstract 10 April 2015

Controlled Synthesis and Novel Properties of 2-Dimensional Materials: from Doped Graphene to In-plane Heterojunctions and van der Waals Solids

Mauricio Terrones, Department of Physics, Department of Chemistry, Department of Materials Science and Engineering and Center for 2-Dimensional & Layered Materials. The Pennsylvania State University, University Park, Pennsylvania 16802, USA & Institute of Carbon Science and Technology, Shinshu University, Japan

This talk will discuss the synthesis of large-area, high-quality monolayers of nitrogen-, boron- and silicon-doped graphene sheets on Cu foils using ambient-pressure chemical vapor deposition (AP-CVD). Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the defects in the doped graphene samples arrange in different geometrical configurations exhibiting different electronic and magnetic properties. Interestingly, these doped layers could be used as efficient molecular sensors and electronic devices. In addition, the synthesis of hybrid carbon materials consisting of sandwich layers of graphene layers and carbon nanotubes by a self-assembly route will be discussed. These films are energetically stable and could well find important applications as field emission sources, catalytic supports, gas adsorption materials and super capacitors.

Beyond graphene, the synthesis of other 2-Dimensional materials will be described. In particular, we will discuss the synthesis of WS2 and MoS2 triangular monolayers, as well as large area films using a high temperature sulfurization of WOx clusters deposited on insulating substrates. We will show that depending on the substrate and the sizes of the oxide clusters, various morphologies of layered dichalcogenides could be obtained. In addition, photocurrent measurements on these materials will be presented. Our results indicate that the electrical response strongly depends on the laser photon energy. The excellent response observed to detect different photon wavelengths in MoS2, WS2 and WSe2 materials, suggest these materials could be used in the fabrication of novel ultrafast photo sensors. We will also show that these techniques are able to produce in-plane heterojunctions with sharp interfaces of MoS2 and WS2. The material exhibits novel excitonic effects that will be discussed. 

From the theoretical stand point, we have found using first principles calculations, that by alternating individual layers of different metal chalcogenides (e.g. MoS2, WS2, WSe2 and MoSe2) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We demonstrate that it is possible to alternate different chalcogenide layers with graphene and hBN with unprecedented optical and physico-chemical properties.

This research is supported by the Army Research Office through MURI Grant #W911NF-11-1-0362.

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