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Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe2are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe2as active channel materials. Recently, a phase engineering concept of 2D MoS2materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe2and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe2. We have also shown that metallic phase WSe2can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe2flakes with phase-engineered metallic 1T-WSe2as contact regions and intact semiconducting 2H-WSe2as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 107and mobilities up to 66 cm2/V·s for monolayer WSe2. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials.

I2–II–IV–VI4and I–III–VI2semiconductor nanocrystals have found applications in photovoltaics and other optoelectronic technologies because of their low toxicity and efficient light absorption into the near-infrared. Herein, we report the discovery of a metastable wurtzite-like polymorph of Cu2FeSnSe4, a member of the I2–II–IV–VI4family of semiconductors containing only earth-abundant metals. Density functional theory calculations on this metastable polymorph of Cu2FeSnSe4indicate that it may be a superior semiconductor for solar energy and optoelectronics applications compared to the thermodynamically preferred stannite polymorph, since the former displays a sharper dispersion of energy levels near the conduction band minimum that can enhance electron mobility and suppress hot electron cooling. The experimental optical band gap was measured by the inverse logarithmic derivative method to be direct, in agreement with theory, and in the range of 1.48–1.59 eV. Mechanistic studies reveal that this metastable phase derives from intermediate Cu3Se2nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu2FeSnSe4to the related CuFeSe2material system. Our experimental and computational comparisons between Cu2FeSnSe4and CuFeSe2help explain both the crystal chemistry of CuFeSe2that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu2FeSnSe4. This work provides insight into the importance of elemental composition when designing syntheses for metastable materials.