Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo 1Àx W x Se 2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo 1Àx W x Se 2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors.
A giant bandgap reduction in layered GaTe is demonstrated. Chemisorption of oxygen to the Te-terminated surfaces produces significant restructuring of the conduction band resulting in a bandgap below 0.8 eV, compared to 1.65 eV for pristine GaTe. Localized partial recovery of the pristine gap is achieved by thermal annealing, demonstrating that reversible band engineering in layered semiconductors is accessible through their surfaces.
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