基于磁圆二向色谱的单层MoS_2激子能量和线宽温度依赖特性

以二硫化钼(MoS_2)为代表的过渡金属硫属化物属于二维层状材料,样品可以薄至单层.单层MoS_2是一种直接带隙半导体,在纳米逻辑器件、高速光电探测、纳米激光等领域具有广阔的应用前景.在实际应用中,温度是影响半导体材料能带结构和性质的主要因素之一.因此研究单层二维材料能带的温度依赖特性对理解其物理机理以及开展器件应用具有重要的意义.目前,在广泛采用的测量单层MoS_2反射谱的研究中,激子峰往往叠加在一个很强的光谱背底上,难以准确分辨激子的峰位和线宽.基于自行搭建的显微磁圆二向色谱系统,研究了单层MoS_2在65—300 K温度范围内的反射谱和磁圆二向色谱,结果表明磁圆二向色谱在研究单层材料激子能量和线宽方面具有明显的优势.通过分析变温的磁圆二向色谱,得到了不同温度下的A,B激子的跃迁能量和线宽.通过对激子能量和线宽的温度依赖关系进行拟合,进一步讨论了声子散射对激子线宽的影响.

Temperature dependent excitonic transition energies and linewidths of monolayer MoS2 probed by magnetic circular dichroism spectroscopy

Layered transition metal dichalcogenides (TMDs), as a new class of two-dimensional material, have received wide attention of scientific community due to their peculiar electronic and optical properties. Monolayer TMDs such as MoS₂, MoSe₂, WS₂ and WSe₂ are semiconductors with band gap energies in the visible and near-infrared region, which promises the applications in logic nano-devices, ultra-high speed photoelectric detectors and nano-lasers. Temperature has strong influences on the electronic and optical properties of semiconductors, and their applications in photonic and optoelectronic devices. Thus, the research on the temperature dependence of the energy band of monolayer TMDs is important and meaningful. Monolayer MoS₂, as a prototype of TMDs, displays a weak absorption line with a strong background in original reflection or absorption spectra. The strong background has a tremendous influence on the determination of excitonic transition energy and linewidth. In this work, we adopt the reflection magnetic circular dichroism (MCD) spectroscopy in which reflection spectra and MCD spectra can be simultaneously obtained. We demonstrate that the background disturbance is eliminated in the MCD spectra, in contrast to the reflectivity spectra. And we discuss the optimization of our home-built experimental setup in detail. Through the elaborate analysis of the MCD theory, we demonstrate that the excitonic transition energy and linewidth can be directly and accurately extracted from the MCD spectrum. We perform the reflection MCD measurements on monolayer MoS₂ in a temperature range of 65–300 K. The transition energies and linewidths of A and B excitons of monolayer MoS₂ are extracted, respectively. Those functional parameters that describe the temperature dependence of the energy and linewidth of both excitonic transitions are evaluated and analyzed. We find that the broadening of the linewidth is related to the LO phonon scattering, and the linewidth of A exciton is clearly narrower than that of B exciton. The linewidth difference between A and B excitons might result from the stronger inter-valley coupling of B exciton. Our results indicate that MCD spectroscopy, as a modulated spectroscopy by magnetic fields, provides an easy tool to determine the features of monolayer excitons.