Molybdenum ditelluride (MoTe2) has attracted considerable interest for nanoelectronic, optoelectronic, spintronic, and valleytronic applications because of its modest band gap, high fieldeffect mobility, large spin-orbit-coupling splitting, and tunable 1T′/2H phases. However, synthesizing large-area, high-quality MoTe2 remains challenging. The complicated design of gasphase reactant transport and reaction for chemical vapor deposition or tellurization is nontrivial because of the weak bonding energy between Mo and Te. Here, we report a new method for depositing MoTe2 that entails using physical vapor deposition followed by a post-annealing process in a Te-free atmosphere. Both Mo and Te were physically deposited onto the substrate by sputtering a MoTe2 target. A composite SiO2 capping layer was designed to prevent Te sublimation during the post-annealing process. The post-annealing process facilitated 1T′-to-2H phase transition and solid-phase crystallization, leading to the formation of high-crystallinity few-layer 2H-MoTe2 with a field-effect mobility of ~10 cm 2 /(V•s), the highest among all nonexfoliated 2H-MoTe2 currently reported. Furthermore, 2H-MoS2 and Td-WTe2 can be deposited using similar methods. Requiring no transfer or chemical reaction of metal and chalcogen reactants in the gas phase, the proposed method is potentially a general yet simple approach for depositing a wide variety of large-area, high-quality, two-dimensional layered structures.

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Two-dimensional (2D) molybdenum ditelluride (MoTe 2 ) exhibits an intriguing polymorphic nature, showing stable semiconducting 2H and metallic 1T′ phases at room temperature. Polymorphism in MoTe 2 presents new opportunities in developing phase-change memory, high- performance transistors, and spintronic devices. However, it also poses challenges in synthesizing homogeneous MoTe 2 with a precisely controlled phase. Recently, a new yet simple method using sputtering and 2D solid-phase crystallization (SPC) is proposed for synthesizing high-quality and large-area MoTe 2 . This study investigates the polymorphism control of MoTe 2 synthesis using 2D SPC. The Te/Mo ratio and oxygen content in the as-sputtered films correlate strongly with the final phase and electrical properties of SPC MoTe 2 . Furthermore, the SPC thermal budget may be exploited for stabilizing a deterministic phase. The comprehensive experiments presented in this work demonstrate the versatile and precise controllability on the MoTe 2 phase by using the simple 2D SPC technique.

Effective doping techniques that precisely and locally control the conductivity and carrier polarity, i.e., electron (n-type) or hole (p-type), play a vital role in the remarkable success of Si-based technology and thus are critical for developing useful devices based on two-dimensional layered transition-metal dichalcogenides (TMDs). In contrast to the previous approaches based on either chemical doping or phase transition that requires complex chemicals or a high thermal budget and shows limited tunability and reliability, we propose a simple yet effective electron-beam irradiation (EBI) technique as an alternative for facilitating polarity transformation and transport modulation in selected regions. The EBI process may generate a precise amount of native chalcogen defects in both MoS2 and MoTe2 by controlling the EBI dosage. First-principles simulations support that the presence of native chalcogen vacancies may substantially reduce the band gaps of TMDs. In MoTe2, the progressive evolution of p-type conduction, n-type conduction, to metallic-like conduction can be observed with increasing EBI dosage. The high conductivity of metallic-like MoTe2 induced by EBI is comparable to that in a metallic 1T′-MoTe2, demonstrating the ability to selectively form extremely conductive regions in semiconducting TMDs. The proposed EBI technique could be potentially applied to a wide range of layered TMDs and facilitate the development of high-performance TMD-based devices in the future.

A nontoxic hydrogen-assisted solid Se vapor selenization process (HASVS) technique to achieve a large-area (40 × 30 cm(2)) Cu(In,Ga)Se2 (CIGS) solar panel with enhanced efficiencies from 7.1 to 10.8% (12.0% for active area) was demonstrated. The remarkable improvement of efficiency and fill factor comes from improved open circuit voltage (Voc) and reduced dark current due to (1) decreased interface recombination raised from the formation of a widened buried homojunction with n-type Cd(Cu) participation and (2) enhanced separation of electron and hole carriers resulting from the accumulation of Na atoms on the surface of the CIGS film. The effects of microstructural, compositional, and electrical characteristics with hydrogen-assisted Se vapor selenization, including interdiffusion of atoms and formation of buried homojunction, were examined in detail. This methodology can be also applied to CIS (CuInSe2) thin film solar cells with enhanced efficiencies from 5.3% to 8.5% (9.4% for active area) and provides a facile approach to improve quality of CIGS and stimulate the nontoxic progress in the large scale CIGS PV industry.