Quasiparticle dynamics and optical properties of alkali metal doped few-layered transition metal dichalcogenides

本项目的目标是揭示和设计新型二维过渡金属二卤化物(TMD)(如MoSe2)的电子和光学性质、电子自旋和相关的多体效应。这项工作包括原位样品合成和功能化以及多光谱方法：一方面，将使用与自旋和角度相关的光电子能谱来研究电子的能带结构和重整化；另一方面，将使用拉曼光谱来研究振动性质和电子-声子耦合。我们可以用来设计物理性质的控制参数是：(1)层数，(2)堆积，(3)合金化，(4)碱金属掺杂和(5)纳米带的横向量子限制。人们认为，这些参数允许对物理性质进行广泛的调节，包括完全自旋极化的电子能带结构、光学带隙、电荷密度波和超导制度。特别是，TMD的层数有望控制自旋极化，因为奇数层没有反转中心，因此具有完全的自旋极化带。由于像MoSe_2这样的TMD的电子结构由两个自旋相反的山谷组成，这些材料的载流子通常被认为是电子谷电子学的基石。不同TMD的堆叠提供了一种具有优异光电性能的新型异质结构。合金化(例如，用W代替Mo或用S代替Se)有望完全控制带隙。碱金属掺杂将半导体TMD转变为金属，而电荷载流子浓度(可由碱(Ne)金属类型控制)决定了电荷密度波或超导制度的主导地位。如果有可能在台阶表面生长TMDs带，就可以设计出非常丰富的激发谱，包括谷相关的选择规则和带折叠的带。由于态密度中van-Hove奇点的存在，最大吸收的能量可以由带宽来控制。该提议的新颖方面涉及费米表面的自旋极化性质将如何影响重整化和散射率，探索全面的掺杂剂(碱和碱土)以及官能化和量子限制方面。这项拟议工作的可行性牢固地基于提出者对掺杂的大块过渡金属化合物和石墨烯的角度分辨光电子能谱工作，以及最近分子束外延在高质量样品合成方面的迅猛发展。这项建议的工作不仅将提供对TMD物理的深刻理解，而且将提供对一般二维材料的深入理解。

The goal of the present project is to unravel and engineer the electronic and optical properties, the electronic spin and related many-body effects in novel two-dimensional transition metal dichalcogenides (TMDs), such as MoSe2. The suggested work comprises of in-situ sample synthesis and functionalization and a multispectroscopic approach: on the one hand, spin- and angle dependent photoemission spectroscopy will be employed in order to investigate the electron energy band structure and renormalizations; on the other hand Raman spectroscopy will be employed to investigate the vibrational properties and electron-phonon coupling. The control parameters by which we can engineer the physical properties are: (1) layer number, (2) stacking, (3) alloying (4) alkali metal doping and (5) lateral quantum confinement into nanoribbons. It is believed that these parameters allow for a wide tuning of the physical properties including a fully spin polarized electron energy band structure, optical bandgap, charge-density waves and superconducting regimes. In particular, the layer number of TMDs is expected to control the spin polarization since odd-numbered layers do not have an inversion center and hence have fully spin polarized bands. Since the electronic structure of TMDs such as MoSe2 consists of two valleys with opposite spin, the carriers these materials are often envisioned as building blocks for valleytronics. Stacking of the different TMDs provides a new type of heterostructure with extraordinary optoelectronic properties. Alloying (for example replacing Mo by W or replacing Se by S) is expected to yield full control over the bandgap. Alkali-metal doping turns semiconducting TMDs into metals and the charge carrier concentration (which can be controlled by alkali(ne) metal type) dictates whether charge density wave or superconducting regimes dominate. If it is possible to grow ribbons of TMDs on stepped surfaces, a very rich excitation spectrum consisting of valley dependent selection rules and zonefolded bands could be engineered. Due to the presence of van-Hove singularities in the density of states, the energy of the maximum absorption can be engineered by the ribbon width. The novel aspects of the suggested proposal are related to how the spin-polarized nature of the Fermi surface will affect the renormalization and scattering rates, exploring the full breadth of dopants (alkali and alkaline earth) and on the functionalization and quantum confinement aspects. The proposed work's feasibility is firmly grounded on the proposers' angle-resolved photoemission work on doped bulk transition metal compounds and graphene and the recent stormy developments in high-quality sample synthesis by molecular beam epitaxy. The suggested work will provide a deep understanding, not only of the physics of TMDs but of two-dimensional materials in general.