Strain-induced indirect to direct bandgap transition in multilayer WSe2.

Transition metal dichalcogenides, such as MoS2 and WSe2, have recently gained tremendous interest for electronic and optoelectronic applications. MoS2 and WSe2 monolayers are direct bandgap and show bright photoluminescence (PL), whereas multilayers exhibit much weaker PL due to their indirect optical bandgap. This presents an obstacle for a number of device applications involving light harvesting or detection where thicker films with direct optical bandgap are desired. Here, we experimentally demonstrate a drastic enhancement in PL intensity for multilayer WSe2 (2-4 layers) under uniaxial tensile strain of up to 2%. Specifically, the PL intensity of bilayer WSe2 is amplified by ∼ 35× , making it comparable to that of an unstrained WSe2 monolayer. This drastic PL enhancement is attributed to an indirect to direct bandgap transition for strained bilayer WSe2, as confirmed by density functional theory (DFT) calculations. Notably, in contrast to MoS2 multilayers, the energy difference between the direct and indirect bandgaps of WSe2 multilayers is small, thus allowing for bandgap crossover at experimentally feasible strain values. Our results present an important advance toward controlling the band structure and optoelectronic properties of few-layer WSe2 via strain engineering, with important implications for practical device applications.

Degenerate n-doping of few-layer transition metal dichalcogenides by potassium.

We report here the first degenerate n-doping of few-layer MoS2 and WSe2 semiconductors by surface charge transfer using potassium. High-electron sheet densities of ~1.0 × 10(13) cm(-2) and 2.5 × 10(12) cm(-2) for MoS2 and WSe2 are obtained, respectively. In addition, top-gated WSe2 and MoS2 n-FETs with selective K doping at the metal source/drain contacts are fabricated and shown to exhibit low contact resistances. Uniquely, WSe2 n-FETs are reported for the first time, exhibiting an electron mobility of ~110 cm(2)/V·s, which is comparable to the hole mobility of previously reported p-FETs using the same material. Ab initio simulations were performed to understand K doping of MoS2 and WSe2 in comparison with graphene. The results here demonstrate the need of degenerate doping of few-layer chalcogenides to improve the contact resistances and further realize high performance and complementary channel electronics.

Electrolyte-induced inversion layer Schottky junction solar cells.

A new type of crystalline silicon solar cell is described. Superficially similar to a photoelectrochemical cell a liquid electrolyte creates a depletion (inversion) layer in an n-type silicon wafer, however no regenerative redox couple is present to ferry charge between the silicon and a counter electrode. Instead holes trapped in the electrolyte-induced inversion layer diffuse along the layer until they come to widely spaced grid lines, where they are extracted. The grid lines consist of a single-walled carbon nanotube film etched to cover only a fraction of the n-Si surface. Modeling and simulation shows the inversion layer to be a natural consequence of the device electrostatics. With electronic gating, recently demonstrated to boost the efficiency in related devices, the cell achieves a power conversion efficiency of 12%, exceeding the efficiency of dye sensitized solar cells.