Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
Nano Lett. 2017 Mar 8;17(3):1610-1615. doi: 10.1021/acs.nanolett.6b04775. Epub 2017 Feb 2.
van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap 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 band gap 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 band gap 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 band gap. A key clue to understanding the mechanism of this band-gap 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 band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.
范德华二维（2D）半导体由于其本征带隙而成为一类具有有前途的器件特性的材料。对于实际应用，理想的情况是通过一种可以普遍应用于这类材料的机制来以受控的方式修改带隙。在这里，我们报告了通过 Rb 原子的原位表面掺杂观察到体相 2H 过渡金属二卤代物（TMD）家族中普遍可调的带隙。一系列角分辨光发射光谱无一例外地表明，TMD 在区角处的带隙在 0.8-2.0 eV 的范围内被调制，其覆盖了从可见光到近红外的广泛光谱范围，从间接带隙到直接带隙的趋势。这种带隙工程机制的一个关键线索是对称破缺和由此产生的自旋劈裂的光谱特征，这可以通过在 TMD 的表面双层内形成二维电偶极层来解释。我们的结果基于范德华半导体的强 2D 性质，确立了表面斯塔克效应作为带隙工程的普遍机制。
