Structure and Characteristics of Few-layer Molybdenum Disulfide
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Structure and Characteristics of Few-layer Molybdenum Disulfide E. S. Reifler, N. T. Nuhfer, and E. Towe Carnegie Mellon University 5000 Forbes Ave Pittsburgh, PA 15213, U.S.A. ABSTRACT Layered transition-metal dichalcogenides such as molybdenum disulfide are indirect band gap materials in their bulk form but become direct semiconductors when pared down to a single layer. This paper discusses the structural characteristics and properties of single and few-layer molybdenum disulfide. Specifically, we present aberration-corrected high-resolution transmission electron microscopy (HRTEM) investigations of the structural properties of this material. This information is augmented with data from Raman and photoluminescence spectroscopies on single and few-layer samples. High-resolution TEM images of few-layer and bulk molybdenum disulfide confirm a hexagonal structure for the material. Direct images, along with corresponding fast Fourier transforms, provide valuable information about the crystal structure and reciprocal space lattice of few-layer molybdenum disulfide. One can, for example, determine the in-plane lattice constants experimentally from analysis of the TEM images. Atomic force microscope topographic maps can yield the thickness of a monolayer of molybdenum disulfide; these maps can also be used to determine the thicknesses of multilayered samples. Analysis of combined Raman and photoluminescence spectroscopy data are valuable in confirming the number of layers in molybdenum disulfide samples. Furthermore, the photoluminescence data can provide unique information on the nature of emission from monolayer molybdenum disulfide; it is characteristically different from that of few-layer samples. The spectral location of the monolayer peak emission agrees with what was obtained from theoretical calculations. INTRODUCTION In the past several years, there has been a growing interest in the general study of twodimensional materials. Molybdenum disulfide (MoS2) is one such material among many. The interest in these materials stems from their unique properties and the possibility of using them in devices such as transistors [1] and electron-spin based devices [2] for applications in conventional and flexible electronics [3-5]. To better understand two-dimensional molybdenum disulfide, we investigate the structural and electronic characteristics of monolayer and few-layer MoS2 samples. Layered molybdenum disulfide is a transition-metal dichalcogenide that has the hexagonal crystalline structure shown in Figure 1. The in-plane and out-of-plane lattice constants for 2H-MoS2 are, respectively, a = 3.16Å and c = 12.29Å [6, 7]. Atoms within a layer of MoS2 are bonded covalently. Interlayer bonding on the other hand is dominated by weak van der Waals interactions, making it easy for vertically-stacked layers of MoS2 to be separated. Exfoliation of layers from the bulk allows single and few-layer films of MoS2 to be isolated. Monolayer and few-layer films of MoS2 exhibit properties that are distinctly different from those in the
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