Department of Physics, University of Washington, Seattle, WA, USA.
Department of Physics, University of Warwick, Coventry, UK.
Nature. 2019 Aug;572(7768):220-223. doi: 10.1038/s41586-019-1402-1. Epub 2019 Jul 17.
The ability to directly monitor the states of electrons in modern field-effect devices-for example, imaging local changes in the electrical potential, Fermi level and band structure as a gate voltage is applied-could transform our understanding of the physics and function of a device. Here we show that micrometre-scale, angle-resolved photoemission spectroscopy (microARPES) applied to two-dimensional van der Waals heterostructures affords this ability. In two-terminal graphene devices, we observe a shift of the Fermi level across the Dirac point, with no detectable change in the dispersion, as a gate voltage is applied. In two-dimensional semiconductor devices, we see the conduction-band edge appear as electrons accumulate, thereby firmly establishing the energy and momentum of the edge. In the case of monolayer tungsten diselenide, we observe that the bandgap is renormalized downwards by several hundreds of millielectronvolts-approaching the exciton energy-as the electrostatic doping increases. Both optical spectroscopy and microARPES can be carried out on a single device, allowing definitive studies of the relationship between gate-controlled electronic and optical properties. The technique provides a powerful way to study not only fundamental semiconductor physics, but also intriguing phenomena such as topological transitions and many-body spectral reconstructions under electrical control.
直接监测现代场效应器件中电子状态的能力 - 例如，在施加栅极电压时成像局部电势能、费米能级和能带结构的变化 - 可以改变我们对器件物理和功能的理解。在这里，我们展示了应用于二维范德华异质结构的微角分辨光发射光谱学（microARPES）具有这种能力。在双端石墨烯器件中，我们观察到费米能级在狄拉克点处的偏移，而在施加栅极电压时，没有检测到色散的变化。在二维半导体器件中，我们看到导带边缘在电子积累时出现，从而确定了边缘的能量和动量。对于单层二硒化钨，我们观察到随着静电掺杂的增加，带隙向下修正了几百毫电子伏特 - 接近激子能量。光学光谱学和 microARPES 都可以在单个器件上进行，从而可以对栅极控制的电子和光学性质之间的关系进行明确的研究。该技术为研究不仅基本半导体物理，而且还为研究拓扑转变和许多体光谱重建等有趣现象提供了一种强大的方法，这些现象可以在电控制下发生。
