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The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials.

A ferroelectric FET using MoS2 and PVDF-TrFE was fabricated, and the effects of varying the gate voltage scan rate from 200 mV/s to 4 mV/s investigated. Charge mobility, sub-threshold voltage swing and the memory window width depended on the scan rate. Prior to switching on, a negative trans-conductance was observed for all scan rates, followed by a rapid increase in the channel current to the on state. A model based on nucleation of ferroelectric domains and unrestricted domain growth was used to explain these results. By lowering the scan rate, the performance of polymer based FE-FET's can be improved. [ABSTRACT FROM AUTHOR]

Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T′-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T′-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin-orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T′-MoTe2 and implementation in next-generation nanoelectronic devices. [ABSTRACT FROM AUTHOR]

Monolayer materials are sensitive to their environment because all of the atoms are at their surface. We investigate how exposure to the environment affects the electrical properties of CVD-grown monolayer MoS 2 by monitoring electrical parameters of MoS 2 field-effect transistors as their environment is changed from atmosphere to high vacuum. The mobility increases and contact resistance decreases simultaneously as either the pressure is reduced or the sample is annealed in vacuum. We see a previously unobserved, non-monotonic change in threshold voltage with decreasing pressure. This result could be explained by charge transfer on the MoS 2 channel and Schottky contact formation due to adsorbates at the interface between the gold contacts and MoS 2 . Additionally, from our electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 10 12  cm -2 eV -1 , degrading the electrical properties of monolayer MoS 2 .

Two-dimensional materials are promising for a range of applications, as well as testbeds for probing the physics of low-dimensional systems. Tungsten disulfide (WS 2 ) monolayers exhibit a direct band gap and strong photoluminescence (PL) in the visible range, opening possibilities for advanced optoelectronic applications. Here, we report the realization of two-dimensional nanometer-size pores in suspended monolayer WS 2 membranes, allowing for electrical and optical response in ionic current measurements. A focused electron beam was used to fabricate nanopores in WS 2 membranes suspended on silicon-based chips and characterized using PL spectroscopy and aberration-corrected high-resolution scanning transmission electron microscopy. It was observed that the PL intensity of suspended WS 2 monolayers is ∼10-15 times stronger when compared to that of substrate-supported monolayers, and low-dose scanning transmission electron microscope viewing and drilling preserves the PL signal of WS 2 around the pore. We establish that such nanopores allow ionic conductance and DNA translocations. We also demonstrate that under low-power laser illumination in solution, WS 2 nanopores grow slowly in size at an effective rate of ∼0.2-0.4 nm/s, thus allowing for atomically controlled nanopore size using short light pulses.