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硫族化合物（TMDs）的电子和光学性质，电子自旋和相关的多体效应，如MoSe2。建议的工作包括原位样品合成和功能化和多光谱方法：一方面，将使用依赖自旋和角度的光发射光谱来研究电子能带结构和重整化；另一方面，拉曼光谱将用于研究其振动特性和电子-声子耦合。我们可以设计物理性质的控制参数是：(1)层数，(2)堆叠，(3)合金化，(4)碱金属掺杂和(5)侧向量子约束成纳米带。据信，这些参数允许广泛调整物理性质，包括全自旋极化电子能带结构、光学带隙、电荷密度波和超导体制。特别是，由于奇数层没有反转中心，因此具有完全的自旋极化带，预计TMDs的层数将控制自旋极化。由于tmd（如MoSe2）的电子结构由两个自旋相反的谷组成，因此这些材料的载流子通常被设想为谷电子学的基石。不同tmd的叠加提供了一种具有非凡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转化为金属，而载流子浓度（可由碱金属类型控制）决定了是电荷密度波还是超导体制占主导地位。如果有可能在台阶表面上生长tmd带状，则可以设计出由谷依赖选择规则和带折叠带组成的非常丰富的激发谱。由于态密度中存在范霍夫奇点，最大吸收的能量可以通过带的宽度来设计。该建议的新颖方面涉及费米表面的自旋极化性质如何影响重整化和散射速率，探索掺杂剂（碱和碱土）的全宽度以及功能化和量子限制方面。这项工作的可行性是基于作者在掺杂体过渡金属化合物和石墨烯上的角分辨光发射工作，以及最近通过分子束外延合成高质量样品的迅猛发展。建议的工作将提供一个深刻的理解，不仅是tmd的物理，而且是二维材料的一般。

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.