Theory and Ab Initio Calculation of Optically Excited States—Recent Advances in 2D Materials

Abstract

Over the past decade, the research on these materials systems from both theoretical and experimental exploration has produced many fascinating results, such as the discovery of strongly bound two-and three-particle excited states [12][13][14][15][16][17][18][19][20][21] as well as their time-and spindependent dynamics. [22,23] This review will have three parts. In the first part after this introduction, we will discuss several theoretical and computational methods for the excited-state problems. These methods will be introduced pedagogically and will be accessible to both theoretical and experimental readers. In the second part, we will focus on selected 2D materials systems and defect systems that demonstrate interesting properties related to the optically excited states. We present the understanding of these optically excited states by using the methods introduced in the first part. The last part will conclude this review paper and will include an outlook section that highlights several future opportunities in the development of the theory on the optically excited states.The aim of this review is to provide a survey and pedagogical introduction of various theoretical tools and their applications in the optically excited states of low-dimensional materials systems, rather than to scrutinize a specific theoretical approach or a specific materials system in depth. We will also discuss the key differences in the excited-state properties of materials at different dimensions. The physical origin of this difference will be elucidated from the aspects of many-particle interactions, dimensionality effects, spatial symmetry, and tunability. Interested readers may also refer to the review papers about the electronic valley degree of freedom and the valley-spin locking, [22,24] about optical properties in various 2D materials, [5,23,25,26] about excited states in heterostructures, [27][28][29] about photonics, and optoelectronics using 2D materials. [10,30,31] Apart from the optically excited states which are essentially electronic quasiparticle excitation, many other forms of excited states, such as the phonon, plasmon, and magnon excitations, have demonstrated unique and interesting features in 2D materials. [32][33][34] For example, recent theoretical studies of the phonon transport in graphene and other atomic layered materials have discovered hydrodynamic behavior which is distinct from the kinetic phonon-transport behavior in conventional 3D materials at room temperature. [35,36] However, instead of being comprehensive and covering all aspects of the excited-state research, this review will focus on the electronic excited states that Recent studies of the optical properties of 2D materials have reported unique phenomena and features that are absent in conventional bulk semiconductors. Many of these interesting properties, such as enhanced light-matter coupling, gate-tunable photoluminescence, and unusual excitonic optical selection rules arise from the nature of the two-and multi-particle excited states such as stron...

Nanocavity Clock Spectroscopy: Resolving Competing Exciton Dynamics in WSe2/MoSe2 Heterobilayers

“…Nanocavity clock spectroscopy (NCS) thus provides a new approach to quantify the complex and competing dynamics of multiexcitonic systems complementing conventional ultrafast time-domain spectroscopy. Nanocavity control of exciton dynamics in van der Waals materials over several orders of magnitude additionally opens the door to establishing applications like high-temperature exciton condensation, tunable device fabrication, harnessing coherent emission, and control of highly localized trapping potentials such as those found in Moiré lattices. − …”

Signatures of Dimensionality and Symmetry in Exciton Band Structure: Consequences for Exciton Dynamics and Transport

“…From a theoretical point of view, rate-equation approaches are typically used to model exciton diffusion, but these demand extensive parametrization, , which hinders a predictive understanding of exciton structure–property relations. For near-equilibrium excitons in the low-field limit, ab initio Green’s function-based many-body perturbation theory, within the GW plus Bethe Salpeter equation (GW-BSE) approach, − has been highly successful in predicting optical spectra across a wide variety of materials of different dimensionalities. , GW-BSE methods can also give insight into exciton dynamics, including radiative recombination processes, − multiexciton generation, − and exciton–phonon interactions. − …”

“…For near-equilibrium excitons in the low-field limit, ab initio Green's function-based many-body perturbation theory, within the GW plus Bethe Salpeter equation (GW-BSE) approach, 25−33 has been highly successful in predicting optical spectra across a wide variety of materials of different dimensionalities. 34,35 GW-BSE methods can also give insight into exciton dynamics, including radiative recombination processes, 36−40 multiexciton generation, 41−43 and exciton− phonon interactions. 44−46 Exciton dynamics require knowledge of the exciton band structure to accurately describe the phase space of momentumconserving scattering processes.…”

Theory of the Coherent Response of Magneto-Excitons and Magneto-Biexcitons in Monolayer Transition Metal Dichalcogenides

“…The spin-splitting together with the valley selective circular dichroism of monolayer TMDCs allows to separately access the valley and spin degree of freedom. The complex band structure introduces a variety of distinct exciton configurations [13][14][15] as well as related trion [16][17][18][19][20][21] and biexciton configurations [22][23][24]. Gates, barriers, or common accidental impurities, resulting in doped TMDC samples with pronounced trions besides neutral excitons, motivated numerous experimental investigations of the exciton and trion dynamics [25][26][27][28][29][30][31][32][33][34][35][36][37].…”