Session H29: First-principles Modeling of Excited-State Phenomena in Materials VI: Solids and Layered Materials

New Insights into Single- and Multi-Exciton Phenomena in Complex Materials from Ab Initio Many-Body Perturbation Theory

Theoretical predictions of excited-state phenomena in complex materials can lead to better understanding of nanoscale energy conversion mechanisms, for instance in emerging photovoltaic and photocatalytic systems. In this talk, I will discuss recent studies using new ab initio many-body perturbation theory methods within the GW approximation and the Bethe Salpeter equation approach (GW-BSE) to understand and uncover such mechanisms. In one example, I will present a new approach to calculate multi-exciton generation processes in solids from first principles, without empirical input, used to study singlet fission in organic crystals. Applying this approach to crystalline pentacene, we discovered a new exciton—bi-exciton coupling channel, one that is purely Coulombic, with a predicted decay rate comparable to experiments; our results led to new understanding of the role of symmetry and structure in the singlet fission mechanism in the solid state. Additionally, I will discuss our recent progress in calculating excited state properties in complex systems of reduced dimensionality. Selected results will be presented for two materials: monolayer transition metal dichalcogenides with point defects, where the calculated GW-BSE absorbance is strongly affected by the presence of localized defect states; and a new class of multilayered hybrid chalcogenides with 2D-like excitonic behavior that is strongly coupled to their unique structure and chemistry.
  

GW and GW-BSE Methods with Broken Time Reversal Symmetry and Their Applications in Magnetic Systems

The ab initio GW and GW-BSE methods based on many-body perturbation theory (MBPT) play an important role in understanding and predicting the electronic and optical properties of materials. However, broken time-reversal symmetry in magnetic systems poses difficulties in both the formalism and its implementation. Moreover, a Hubbard onsite Coulomb interaction at the DFT level will introduce ambiguous starting point for MBPT calculations. In this work, we extend the GW and GW-BSE methods to systems with broken time-reversal symmetry and strong spin-orbit coupling, and avoid the double-counting issue from the onsite Coulomb potential. This method can be applied to complex magnetic systems or reduced-dimensional systems such as 2D magnets.   

Ordering of the Γ6, Γ7, and Γ8 States, the Band Gap, and the Electron Effective Mass in β-HgS via GW Calculations

Materials with strong spin-orbit coupling may have qualitatively different quasiparticle bandstructures than those calculated with density functional theory. One such solid is β-HgS, the zincblende phase of HgS. Unlike HgSe and HgTe, β-HgS has a band gap. However, the atomic character of the low-energy states has been calculated within the GW approach with two different orderings of the Γ₆, Γ₇, and Γ₈ states. Previous calculations incorporating spin-orbit coupling perturbatively find an ordering of 8, 7, then 6 (from lowest energy to highest), while previous calculations using the fully-relativistic GW approach find an ordering of 6, 8, then 7. We present a bandstructure calculated within the fully-relativistic GW approach that agrees with the ordering of 8, 7, then 6. We also calculate, with high agreement to experiment, the band gap and electron effective mass.   

Layer-resolved optical absorption in single-inorganic layer π-conjugated 2D hybrid perovskites from first principles

Layered 2D hybrid organic-inorganic perovskite materials have provided a more chemically stable and optically tunable alternative to the traditional 3D hybrid perovskites. More recently, attention has shifted towards optical engineering of the organic layer by introducing π-conjugated molecules and chromophores to achieve excited-carrier transfer from the perovskite to the organic groups. In the limit of alternating single inorganic and organic layers, joint optical response could be maximized. In this study, we employ density functional theory and the Bethe-Salpeter equation to predict the ion-resolved optical response in ammonium-propyl-imidazole (API)-PbX₄ (X=I, Br, Cl) materials. We determine in which layers electrons and holes are excited as a function of photon energy. This analysis reveals a strong preference for electron and hole confinement within the perovskite layer at visible photon energies and organic-perovskite layer sharing of electrons and holes under near-UV photon energies. These results provide new insight into the limitations of simple π-conjugated organic layer participation for visible light absorption in layered hybrid perovskite systems.   

Optical properties of layered non-equilibrium ZnO from first principles

The boron-nitride (BN) phase has been reported for zinc oxide (ZnO) nano-structures and thin films. With the layered graphitic-like structure it exhibits, BN-ZnO has very different optical properties than the equilibrium wurtzite (WZ) phase of bulk ZnO. We use density functional theory to obtain the equilibrium structure of both polymorphs and we solve the Bethe-Salpeter equation to obtain accurate optical spectra. We found a larger band gap and larger optical anisotropy for BN-ZnO, compared to WZ-ZnO. By performing Maxwell's equations simulations, we show that the anisotropy in BN-ZnO leads to clear differences at the absorption onset for thin films. Our results indicate that possible existence of BN-ZnO in nano-structures leads to increased transmission in ultraviolet region, that can be used to optically distinguish both phases. By comparing to experiment, we show that for ultra-thin films, better agreement can be obtained when accounting for BN-ZnO, rather than pure WZ-ZnO.   

Ab Initio Radiative Lifetimes in Gallium Nitride

Wurtzite GaN is the primary semiconductor for efficient solid state lighting. Light emission in materials is regulated by the dynamics of excited carriers, which is not completely understood in GaN. In particular, due to the ultrafast (fs – ps) timescales at play and to the presence of defects and interfaces in devices, the intrinsic radiative recombination rate is challenging to measure in GaN. Here, we present ab initio calculations of the radiative lifetime as a function of temperature in bulk GaN. We compute the exciton energies and wavefunctions using a combination density functional theory and the GW-Bethe Salpeter equation method. An equation for the ab initio temperature dependent radiative lifetime in a bulk crystal is derived using Fermi’s Golden rule and applied to GaN. Combined with previous first principles calculations of excited carrier relaxation in GaN [1], we can obtain from first principles key device parameters such as the hot carrier cooling time and the carrier diffusion lengths, with important technological implications.

[1] V.A. Jhalani, J.J. Zhou, and M. Bernardi. "Ultrafast Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop." Nano Letters 17, 5012 (2017).   

First Principles Analysis on the Excitonic Properties of MoS2/WS2 and MoSe2/WSe2 Heterobilayers

By using the method of many body perturbation theory (MBPT), we investigate the optical absorption spectra and the excitonic properties of MoS2/WS2 and MoSe2/WSe2 heterobilayers. We first correct the independent particle band gap by performing GW calculation on top of ground state density functional theory (DFT) calculations and the optical spectra of the bilayers are calculated by solving Bethe-Salpeter equation (BSE). Two different types of excitons present in this type of heterobilayers; the inter-layer excitons (electron and hole locate in different layers) and the intra-layer ones (electron and hole locate in the same layer). Relative spectral position of inter- and intra-layer excitons w.r.t. each other is determined by the band alignment of the constituent layers and binding energy of the excitons. The spectral position of the inter-layer exciton w.r.t. intra-layer one is crucial for the low energy optical response and the charge carrier dynamics of the heterobilayers. In this work, we show a detail analysis on inter- and intra-layer excitons in MoS₂/ WS₂ and MoSe₂/WSe₂ heterobilayers. We investigate their binding energy and origin as well as possible scenarios which might lead to alter the character of the lowest excitonic peak in the optical spectra of the bilayers.   

Excited-state Dynamics and the Role of Electron-Phonon Interactions in Quantum Materials

Integrated architectures at the atomic-scale can be achieved via 2D materials and their corresponding van der Waals heterostructures with deterministic defect engineering. In this talk I will show several examples of ab initio designed quantum materials via an understanding of the electron-phonon interactions in these. I will demonstrate our new carrier lifetime-driven approach to materials with an ab initio description of electron-phonon and electron-optical interactions in a Feynman diagram many-body framework integrated with a nonequilibrium carrier transport theory method. Finally, I will give an outlook on theory-directed control of excited state and non-equilibrium phenomena to deliver integrated quantum-engineered materials with diverse applications in quantum information science as well as quantum sensing and metrology.   

Picosecond Electronic and Structural Dynamics in Photoexcited Monolayer MoSe2

Monolayers of semiconducting transition metal dichalcogenides (TMDC) are emerging as strong candidate materials for next generation electronic and optoelectronic devices. Prior studies have demonstrated strong light-matter interactions in these materials which suggests optical control of material properties is a promising route for their functionalization. However, the electronic and structural dynamics in response to electronic excitation have not yet been fully elucidated. In this work, we use non-adiabatic quantum molecular dynamics based on time-dependent density functional theory to study lattice dynamics of a model TMDC monolayer of MoSe₂ after electronic excitation and explore the dependence of dynamics on photo-generated electron-hole density. We observe phonon mode softening induced by Fermi-surface nesting, as well as increasing lattice disorder, as measured by the Debye-Waller factor (DWF), with increasing excitation. Furthermore, we find a transition from single-exponential to bi-exponential decay of the DWF at higher electron-hole densities.   

Simulation of ultrafast spin and valley dynamics in two-dimensional materials

We present simulations of how optical experiments probe the spin and valley dynamics in two-dimensional materials - such as single-layer transition metal dichalcogenides (MoS₂, WS₂ and WSe₂). For the simulations, we have developed an ab initio implementation of time-dependent many-body perturbation theory. Simulations include the photo-generation of electrons and holes and the subsequent carrier dynamics, including the scattering mechanisms induced by electron-phonon and electron-electron interactions. We show simulations of pump-probe ultrafast spectroscopy experiments such as time-dependent Kerr rotation [1] and two-colour helicity-resolved pump-probe spectroscopy [2]. We can simulate the selective optical excitation of electron and holes in different valleys of the Brillouin zone. Our simulations shed light on the understanding of the mechanisms driving the intravalley and intervalley spin relaxation dynamics and the valley polarization dynamics in single-layer transition metal dichalcogenides. We demonstrate that the dynamics is largely driven by electron-phonon coupling and obtain good quantitative agreement with the experiments.   

Ab Initio Carrier Dynamics in Twisted 2D Heterostructures

Stacked 2D materials known as van der Waals (vdW) heterostructures are of significant prominence in quantum materials research. Despite a recent surge in exploration of these heterostructures, their fundamental dynamics, particularly the impact of twist angle, are still poorly understood. The promise of these materials is partially rooted in the vast landscape of parameter space available in materials choice for the heterostructure stack. Transition metal dichalcogenides (TMDs), such as MoSe₂ and MoS₂, have received significant attention in particular for their wide array of optoelectronic properties. In this talk, we will show an ab initio Feynman diagram many-body description of dynamics and optoelectronic response of twisted TMD heterostructures. This diagram-based theoretical approach enables understanding of 2D heterostructures and their electron-electron and electron-phonon lifetimes. We will show comparison of our predictions with pump-probe experiments of 2D heterostructures with an emphasis on electron-phonon dynamics.   

Ab Initio Radiative Lifetimes and Angular Dependence of the Photoluminescence in Two-Dimensional Transition Metal Dichalcogenides

We compute from first-principles the radiative lifetimes and the photoluminescence (PL) in two-dimensional (2D) transition metal dichalcogenides (TMDCs) with chemical formula MX2 (M=Mo, W and X=S, Se, Te). We first compute excitons in TMDC monolayers with the GW-Bethe Salpeter equation method. We then derive a new equation for the radiative lifetimes that extends a previous treatment [1] to correctly include the angular dependence of the PL. Besides refining the values of the temperature-dependent radiative lifetimes, our formalism can explain the angular dependence of the PL observed experimentally under linearly polarized light excitation, thus enabling ab initio computations of polar plots of the PL in excellent agreement with experiment. We further discuss how decoherence due to electron-phonon and electron-electron scattering modifies the angular dependence of the PL, and how non-parabolic exciton dispersions are expected to change the radiative lifetimes. Our work presents a direct connection between microscopic exciton states in TMDCs and PL measurements, thus providing computational tools to advance understanding of carrier and exciton dynamics in 2D-TMDCs.

[1] M. Palummo, M. Bernardi, and J. C. Grossman, Nano Letters 15, 2794 (2015)   

Atomic-like high-harmonic generation from two-dimensional materials

The generation of high-order harmonics from atomic and molecular gases enables the production of high-energy photons and ultra-short isolated pulses.
Here, we demonstrate from ab initio simulations that it is indeed possible to generate high-order harmonics from free-standing monolayer materials, with a similar energy cutoff than in atomic and molecular gases.
Electrons driven perpendicularly to the monolayer plane behave qualitatively the same as the electrons responsible for high-harmonic generation (HHG) in atoms, their trajectories being described by the widely used semi-classical model.
Spite of the similarities, the first step and the last step of the well-established three-step model for atomic HHG are remarkable different in the two-dimensional materials from gases.
We show that the electron-electron interaction plays an important role in harmonic generation from monolayer materials, due to strong local-field effects and that the recombination of the accelerated electron wavepacket is modified, due to the infinite extension of the monolayer.
Our results establish a novel and efficient way of generating high-order harmonics based on a solid-state device, with similar energy cutoff and more favorable wavelength scaling of the harmonic yield than in atomic and molecular gases.