Setup for photolithography on microscopic flakes of 2D materials by combining simple-geometry mask projection with writing.

An optical arrangement and procedure for photolithography on microscopic flakes of two-dimensional materials with an arbitrary shape/size is described. The technique combines projection of demagnified images of simple geometry macroscopic masks with writing. Only a few masks, such as vertical/horizontal slit and square hole, are sufficient to generate most of the required patterns. The setup allows for initially locating the photoresist coated flake on a substrate by imaging it. Thereafter, the automated precise sample stage motion followed by projection of the demagnified mask image is repeated several times to expose the photoresist in the shape of the required pattern. Appropriate light wavelength regimes for imaging and for exposure are chosen through automated optical filter switching. Programming steps for the process are described. The setup allows for direct lithography in one round on microscopic samples without requiring sample shape/size specific masks or predefined position markers. Making of electrode lines of width down to 3 µm, at desired locations on tiny flakes of MoS2, is demonstrated.

Near-Direct Bandgap WSe2/ReS2 Type-II pn Heterojunction for Enhanced Ultrafast Photodetection and High-Performance Photovoltaics.

Pn heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current-rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct heterointerface bandgap for enhanced photogeneration in high light-absorbing few-layer vdW materials remains unexplored. In this work, for the first time, density functional theory calculations show that the heterointerface of few-layer group-6 transition metal dichalcogenide (TMD) WSe2 with group-7 ReS2 results in a sizable (0.7 eV) near-direct type-II bandgap. The interlayer IR bandgap is confirmed through IR photodetection, and microphotoluminescence measurements demonstrate type-II alignment. Few-layer flakes exhibit ultrafast response time (5 µs), high responsivity (3 A/W), and large photocurrent-generation and responsivity-enhancement at the hetero-overlap region (10-100×). Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 µA enable high output electrical power. Finally, long-term air-stability and facile single contact metal fabrication process make the multifunctional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronics.