Field-Effect Transistors Based on Few-Layered alpha-MoTe_2

Here we report the properties of field-effect transistors based on few layers of chemical vapor transport grown alpha- MoTe_2 crystals mechanically exfoliated onto SiO_2. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS_2 and MoSe_2, our MoTe_2 field-effect transistors (FETs) are observed to be hole-doped, displaying on/off ratios surpassing 106 and typical sub-threshold swings of ~ 140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/Vs which are comparable to figures previously reported for single or bi-layered MoS_2 and/or for MoSe_2 exfoliated onto SiO_2 at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that both MoS_2 is electron doped, the stacking of MoTe_2 onto MoS_2 could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe_2 is comparable to those of MoS_2 and MoSe_2, the heavier element Te should lead to a stronger spin orbit-coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes.Comment: 29 pages, 7 figure, ACS Nano (in press

Atomically sharp non-classical ripples in graphene

A fundamental property of a material is the measure of its deformation under applied stress. After studying the mechanical properties of bulk materials for the past several centuries, with the discovery of graphene and other two-dimensional materials, we are now poised to study the mechanical properties of single atom thick materials at the nanoscale. Despite a large number of theoretical investigations of the mechanical properties and rippling of single layer graphene, direct controlled experimental measurements of the same have been limited, due in part to the difficulty of engineering reproducible ripples such that relevant physical parameters like wavelength, amplitude, sheet length and curvature can be systematically varied. Here we report extreme (>10%) strain engineering of monolayer graphene by a novel technique of draping it over large Cu step edges. Nanoscale periodic ripples are formed as graphene is pinned and pulled by substrate contact forces. We use a scanning tunneling microscope to study these ripples to find that classical scaling laws fail to explain their shape. Unlike a classical fabric that forms sinusoidal ripples in the transverse direction when stressed in the longitudinal direction, graphene forms triangular ripples, where bending is limited to a narrow region on the order of unit cell dimensions at the apex of each ripple. This non-classical bending profile results in graphene behaving like a bizarre fabric, which regardless of how it is pulled, always buckles at the same angle. Using a phenomenological model, we argue that our observations can be accounted for by assuming that unlike a thin classical fabric, graphene undergoes significant stretching when bent. Our results provide insights into the atomic-scale bending mechanisms of 2D materials under traditionally inaccessible strain magnitudes and demonstrate a path forward for their strain engineering.Comment: 22 pages, 4 figure

Defects and impurities in graphene-like materials

Graphene-like materials could be used in the fabrication of electronic and optoelectronic devices, gas sensors, biosensors, and batteries for energy storage. Since it is almost impossible to work with defect-free or impurity-free materials, it is essential to understand how defects and impurities alter the electronic and vibrational properties of these systems. Technologically speaking it is more important to distinguish between different types of defects (impurities) and determine if their presence is desirable or not. This review discusses these issues and provides an updated overview of the current characterization tools able to identify and detect defects in different forms of graphene.United States. Office of Naval Research. Multidisciplinary University Research Initiative (ONR-MURI-N00014-09-1-1063

Towards the understanding of the graphene oxide structure: How to control the formation of humic- and fulvic-like oxidized debris

Former structural models of graphene oxide (GO) indicated that it consists of graphene-like sheets with oxygen groups, and no attention was paid to the resulting sheet size. We now provide evidence of the complex GO structure consisting of large and small GO sheets (or oxidized debris). Different oxidation reactions were studied. KMnO4 derived GO consists of large sheets (20–30 wt.%), and oxidized debris deposits, which are formed by humic- and fulvic-like fragments. Large GO sheets contain oxygen groups, especially at the edges, such as carbonyl, lactone and carboxylic groups. Humic-like debris consists of an amorphous gel containing more oxygenated groups and trapped water molecules. The main desorbable fraction upon heating is the fulvic-like material, which contains oxygen groups and fragments with high edge/surface ratio. KClO3 in HNO3 or the Brodie method produces a highly oxidized material but at the flake level surface only; little oxidized debris and water contents are found. It is noteworthy that an efficient basal cutting of the graphitic planes in addition to an effective intercalation is caused by KMnO4, and the aid of NaNO3 makes this process even more effective, thus yielding large monolayers of GO and a large amount of humic- and fulvic-like substances.The authors thank the Government of Spain, Ministry for Economy and Competiveness, for financial support of project CTQ2013-44213-R, and Generalitat Valenciana for projects PROMETEOII/2014/007 and ISIC/2012/008. IRP thanks the Government of Spain, Ministry of Science and Education, for PhD Scholarship in the FPU program

Nonlinear optical absorption and reflection of single wall carbon nanotube thin films by ZZ-scan technique

"Both the nonlinear optical transmission and reflection characteristics of HiPco-based single wall carbon nanotube (SWNT) thin films are studied by using the ZZ-scan method with femtosecond laser pulses at a wavelength of 1.46μm1.46μm. The nonlinear absorption coefficient and nonlinear refractive index are obtained as (5.4±2.0)×10−7cm/W(5.4±2.0)×10−7cm∕W and (1.1±0.5)×10−11cm2/W(1.1±0.5)×10−11cm2∕W, respectively, which are considerably greater than those of other optical materials. This large optical nonlinearity is ascribed to (a) homogeneously deposited thin nanotube film on optically transparent barium fluoride, (b) just-resonant excitation condition, and (c) intrinsic saturable absorption feature of SWNTs.