Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides (1711.11236v1)

Abstract: Two-dimensional (2D) van-der-Waals semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic bandgap. For realistic applications, the ideal is to modify the bandgap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable bandgap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the bandgap of TMDs at the zone corners is modulated in the range of 0.8 ~ 2.0 eV, which covers a wide spectral range from visible to near infrared, with a tendency from indirect to direct bandgap. A key clue to understand the mechanism of this bandgap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of bandgap engineering based on the strong 2D nature of van-der-Waals semiconductors.

Universal Mechanism of Bandgap Engineering in Transition-Metal Dichalcogenides

This paper explores a universal method for bandgap engineering in transition-metal dichalcogenides (TMDs) through surface doping with Rb atoms, demonstrating significant modulation in bandgap and implications for optoelectronic applications. The paper indicates that such a technique can be a broadly applicable mechanism for manipulating the electronic properties of 2H-TMDs, which are critical in the advancing field of 2D materials.

Transition-metal dichalcogenides present a unique opportunity in semiconductor research owing to their intrinsic bandgap properties, particularly in applications involving nanoelectronics and optoelectronics. One of the paper’s core contributions is demonstrating the capability of precise bandgap control through the surface Stark effect, which results from the vertical electric field generated by charged Rb atoms. The authors successfully employed angle-resolved photoemission spectroscopy (ARPES) to observe changes in band structure, revealing a modulation range of 0.8 to 2.0 eV in the bandgap. This range spans across visible and near-infrared spectra, making these findings particularly relevant for optoelectronic device development.

The mechanism relies on the strong 2D van-der-Waals nature of TMDs, with the introduction of electric dipole layers within the surface bilayer contributing to the understanding of the bandgap modulation. The paper promotes Stark effect as a definitive tool in achieving electron doping and bandgap reductions, applicable across a suite of 2H-TMDs, including WS₂, MoS₂, WSe₂, MoSe₂, and MoTe₂. Each material displayed uniform behavior under the surface doping process, showcasing the versatility and consistency of this approach.

The spin splitting result from external electric fields adds layers of complexity to the mechanics of bandgap engineering in TMDs. Spin-layer locking, when disrupted by dopants, leads to doublet spin degeneracy lifting, which influences the indirect-to-direct bandgap transition—a key factor in enhancing the optical properties of these materials.

The implications of these findings are significant for designing field-effect optoelectronic devices using 2D semiconductors. The paper outlines the necessity for evaluating the surface Stark effect, which might considerably influence the efficiency and design of semiconductor devices. The stark mechanism is particularly highlighted for providing tunability, achievable in gated applications through controllable external electric fields or ionic liquid gating.

Future research will likely explore further refinements of the surface doping concentrations and their practical implementations in device fabrication. Considering TMDs' potential, further investigations into their underlying physical phenomena and interfacial potential landscapes might reveal additional scope for device engineering perfection.