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The fabrication of in‐plane 2H‐1T′ MoTe₂homojunctions by the flux‐controlled, phase‐engineering of few‐layer MoTe₂from Mo nanoislands is reported. The phase of few‐layer MoTe₂is controlled by simply changing Te atomic flux controlled by the temperature of the reaction vessel. Few‐layer 2H MoTe₂is formed with high Te flux, while few‐layer 1T′ MoTe₂is obtained with low Te flux. With medium flux, few‐layer in‐plane 2H‐1T′ MoTe₂homojunctions are synthesized. As‐synthesized MoTe₂is characterized by Raman spectroscopy and X‐ray photoelectron spectroscopy. Kelvin probe force microscopy and Raman mapping confirm that in‐plane 2H‐1T′ MoTe₂homojunctions have abrupt interfaces between 2H and 1T′ MoTe₂domains, possessing a potential difference of about 100 mV. It is further shown that this method can be extended to create patterned metal–semiconductor junctions in MoTe₂in a two‐step lithographic synthesis. The flux‐controlled phase engineering method could be utilized for the large‐scale controlled fabrication of 2D metal–semiconductor junctions for next‐generation electronic and optoelectronic devices.

Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field‐effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene,¹³C‐rubrene (¹³C₄₂H₂₈), are synthesized and characterized. Vapor‐grown¹³C‐rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm²V⁻¹s⁻¹, intrinsic band‐like transport, and clear Hall behavior in the accumulation layer of air‐gap transistors, are also observed for¹³C‐rubrene crystals. The field‐effect mobility distributions based on 74 rubrene and¹³C‐rubrene devices, respectively, reveal that¹³C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after¹³C‐substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors.