低温光电流谱研究单层MoS2中激子、三电子和里德伯态
Excitons, trions and Rydberg states in monolayer MoS2 revealed by low temperature photocurrent spectroscopy
摘要：
本文利用光电流谱研究了低温（T = 5 K）下h-BN封装单层$\textrm{MoS}_2$光电晶体管中的激子跃迁。光谱呈现激子峰的线宽低至8毫电子伏，一个数量级低于早期的光电流光谱测量。我们观察到四个光谱特征对应的中性激子（$\textrm{X}_{\textrm{1s}}^\textrm{A}$和$\textrm{X}_{\textrm{1s}}^\textrm{B}$）和带电的trions（$\textrm{T}^\textrm{A}$和$\textrm{T}^\textrm{B}$）的基态，以及高达8个额外的谱线在能量以上的$\textrm{X}_{\textrm{1s}}^\textrm{B}$过渡，我们属性的里德堡系列的激发态的$\textrm{X}^\textrm{A}$和$\textrm{X}^\textrm{B}$。不同光谱特征的相对强度可以通过施加的栅极和漏极-源极电压来调谐，其中trions和Rydberg激发态在大的栅极电压下变得更加突出。利用二维过渡金属二硫属化物中激子的有效质量理论，我们能够准确地拟合测量的谱线，并明确地将它们与相应的里德堡态联系起来。拟合还允许我们确定单层$\textrm{MoS}_2$的准粒子带隙和自旋轨道分裂，以及$\textrm{X}^\textrm{A}$和$\textrm{X}^\textrm{B}$的激子结合能。
英文摘要：
We investigate excitonic transitions in a h-BN encapsulated monolayer $\textrm{MoS}_2$ phototransistor by photocurrent spectroscopy at cryogenic temperature (T = 5 K). The spectra presents excitonic peaks with linewidths as low as 8 meV, one order of magnitude lower than in earlier photocurrent spectroscopy measurements. We observe four spectral features corresponding to the ground states of neutral excitons ($\textrm{X}_{\textrm{1s}}^\textrm{A}$ and $\textrm{X}_{\textrm{1s}}^\textrm{B}$) and charged trions ($\textrm{T}^\textrm{A}$ and $\textrm{T}^\textrm{B}$) as well as up to eight additional spectral lines at energies above the $\textrm{X}_{\textrm{1s}}^\textrm{B}$ transition, which we attribute to the Rydberg series of excited states of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$. The relative intensities of the different spectral features can be tuned by the applied gate and drain-source voltages, with trions and Rydberg excited states becoming more prominent at large gate voltages. Using an effective-mass theory for excitons in two-dimensional transition-metal dichalcogenides we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states. The fit also allows us to determine the quasiparticle bandgap and spin-orbit splitting of monolayer $\textrm{MoS}_2$, as well as the exciton binding energies of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$.
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