Session F22: Theory and Simulation of Excited-State Phenomena in Semiconductors and Nanostructures II

Hot Electron Dynamics at Semiconductor-Molecule Interfaces: Insights from First-Principles Dynamics Simulation

Quantum dynamics of excited electrons is fundamental to functionalities of semiconductor-molecule interfaces that are an integral part of various solar energy conversion and opto-electronic technologies. Thus, developing a predictive and quantitative understanding of the electron dynamics at the atomistic level for such complex interfaces is of great interest. In this talk, I will discuss our recent effort on addressing several challenges in understanding how atomistic features such as surface defects influence these electron dynamical processes. We tackle this problem theoretically by employing a first-principles simulation approach that synergistically combine fewest switches surface hopping, many-body perturbation theory, and first-principles molecular dynamics. New findings from the first-principles simulation will be discussed in the context of a larger effort within Solar Fuels EFRC at UNC Chapel Hill. I will also discuss how the results from atomistic theories pose a conceptual challenge when characterizing these interfacial electron processes for these complex interfaces using a simple kinetic model.   

Nonadiabatic dynamics with spin-orbit couplings

In this talk I will present some recent advances in TDDFT-based nonadiabatic dynamics for molecular systems using Tully’s surface hopping. In particular, I will describe a method for the efficient simulation of intersystem crossing events, which requires the on-the-fly calculation of spin-orbit coupling matrix elements along the trajectories. This approach will be applied to the study of the photophysics of metal-organic complexes in solution and of different carbon nanostructures including graphene nanoflakes and nanotubes with different ‘wrapping’ topologies.   

Real-time TDDFT simulations of time-resolved core-level spectroscopies in solid state systems.

The advent of sub-femtosecond time-resolved core-level spectroscopies based on high harmonic generated XUV pulses has enabled the study of electron dyanamics on characteristic femtosecond time-scales. Unambiguous interpretation of these powerful yet complex spectroscopies however requires the development of theoretical algorithms capable of modeling light-matter interaction across a wide energy range spanning both valence and core orbitals. In this context we present a recent implementation of the velocity-gauge formalism of real-time TDDFT [1] within a linear combination of atomic orbital (LCAO) framework, which facilitates efficient numerical treatment of localized semi-core orbitals. Dynamics and spectra obtained from LCAO based simulations are compared to those from a real-space grid implementation [1]. Potential applications are also illustrated by applying the method towards interpreting recent atto-second time-resolved IR-pump XUV-probe spectroscopies investigating sub-cycle excitation dynamics in bulk silicon [2]. \newline \newline [1] Yabana et al., Phys. Rev. B 85, 045134 (2012) \newline \newline [2] Schultze et al., Science, 346, 1348 (2014)   

Long-range dispersion forces between molecules subject to attosecond pulses from ab initio calculations

The London-van der Waals dispersion forces arising from instantaneously induced dipoles in molecules are a key ingredient in a wide range of phenomena in physics, chemistry, and biology. Therefore, the ability to control and manipulate dispersion forces between atoms and molecules is of great importance. Because those dispersion interactions depend crucially on the electronic properties of the molecular systems, a simple route to achieve this would consist in manipulating their electronic states. The recent development of ultra-short optical pulses has given researchers unprecedented control over the electronic degrees of freedom. These pulses, tailored in their frequency and envelope, allow the generation of a strongly out of equilibrium population of electronic states. In this talk we show how the Hamacker constants characterizing the London-van der Waals interaction between two molecules subject to an optical pulse can be calculated using time-dependent density functional theory (TD-DFT) or standard quantum chemistry methods and present several test cases of molecules subjected to IR and UV attosecond pulses.   

Nonequilibrium Green's Function approach to time-resolved photoabsorption

We propose a nonequilibrium Green's function (NEGF) approach to calculate the time-resolved absorption spectrum of nanoscale systems [1]. We can deal with arbitrary shape, intensity, duration and relative delay of the pump and probe fields and include ionization processes as well as hybridization effects due to surfaces. We present numerical simulations of atomic systems using different approximate self-energies and show that electron correlations are pivotal to reproduce important qualitative features. [1] E. Perfetto. A.-M. Uimonen, R. van Leeuwen and G. Stefanucci, Phys. Rev. A 92, 033419 (2015) [2] E. Perfetto, D. Sangalli, A. Marini and G. Stefanucci, Phys. Rev. B, accepted   

AB INITIO DYNAMICS OF AN ELECTRON INTERACTING WITH A LATTICE DEFECT

We study the scattering process of a charge carrier with a defect in a range of bulk and 2D materials. The scattering potential is obtained using density functional theory, the carrier is represented by a gaussian wavepacket, and the dynamics is carried out with a split-operator technique. Our parallel code can model the electron-defect scattering processes in real space and time, with an electron wavepacket of realistic size (100 - 1000 unit cells) and an accuracy typical of ab initio calculations. We apply our approach to model a carrier scattering with a vacancy in silicon and an impurity in monolayer MoS2, obtaining angular dependent scattering cross sections and resonant states.   

Real-Time Time-Dependent DFT Study of Electronic Stopping in Semiconductors under Proton Irradiation

Understanding the detailed mechanisms of how highly energetic charged particles transfer their kinetic energy to electronic excitations in materials has become an important topic in various technologies ranging from nuclear energy applications to integrated circuits for space missions. In this work, we use our new large-scale real-time time-dependent density functional theory simulation [1] to investigate details of the ion-velocity-dependent dynamics of electronic excitations in the electronic stopping process. In particular, we will discuss how point defects in semiconductor materials influence the electronic stopping process under proton irradiation, using silicon carbide (3C-SiC) as a representative material due to its great technological importance. Additionally, we will provide atomistic insights into existing analytical models that are based on the plane-wave Born approximation by examining velocity-dependence of the projectile charge from first-principles simulations. [1] ``Quantum Dynamics Simulation of Electrons in Materials on High-Performance Computers'' A. Schleife, E. W. Draeger, V. Anisimov, A. A. Correa, Y. Kanai, \underline {Computing in Science and Engineering, 16 (5), 54 (2014)}.   

Semiconductors Under Ion Radiation: Ultrafast Electron-Ion Dynamics in Perfect Crystals and the Effect of Defects

Stability and safety issues have been challenging difficulties for materials and devices under radiation such as solar panels in outer space. On the other hand, radiation can be utilized to modify materials and increase their performance via focused-ion beam patterning at nano-scale. In order to grasp the underlying processes, further understanding of the radiation-material and radiation-defect interactions is required and inevitably involves the electron-ion dynamics that was traditionally hard to capture. By applying Ehrenfest dynamics based on time-dependent density functional theory, we have been able to perform real-time simulation of electron-ion dynamics in MgO and InP/GaP. By simulating a high-energy proton penetrating the material, the energy gain of electronic system can be interpreted as electronic stopping power and the result is compared to existing data. We also study electronic stopping in the vicinity of defects: for both oxygen vacancy in MgO and interface of InP/GaP superlattice, electronic stopping shows strong dependence on the velocity of the proton. To study the energy transfer from electronic system to lattice, simulations of about 100 femto-seconds are performed and we analyze the difference between Ehrenfest and Born-Oppenheimer molecular dynamics.   

Role of non-adiabatic carrier dynamics in non-thermal phase transition of Ge-Sb-Te alloy

Non-thermal phase transition driven by femtosecond laser irradiation has been explained by the simple static plasma annealing effect: excitation of a large fraction of valence electrons to conduction bands weakens lattices and leads to the structural phase transition in low temperature. Here, by time-dependent density functional theory and molecular dynamics study of Ge-Sb-Te alloys, we find that the energy-dependent dynamics of excited carriers is critical in determining the phase transition mechanism. For low energy carriers electron-phonon scattering becomes a dominant relaxation process, and for high energy carriers electron-electron scattering remains a dominant relaxation process. As a result, we observe significant ionic temperature increase for low energy excitation, which aids phase transition by thermal effect, and non-thermal phase transition for high energy excitation. This provides a new conceptual framework in understanding fundamental phenomenon of the phase transition.   

Optimal ultrafast laser pulse-shaping to direct photo-induced phase transitions

Photo-induced phase transitions (PIPT) in quantum and/or complex materials are the epitome of challenging non-equilibrium many-body phenomena, that also have a wide range of potential applications. We present a computational approach to finding optimal ultrafast laser pulse shapes to control the outcome of pump-probe PIPT experiments. The Krotov approach for optimal control is combined with a Keldysh Green's function calculation to describe experimental outcomes such as photoemission, transient single particle density of states and optical responses. Results for a simple model charge density wave system will be presented.   

Multiphysics modeling of non-linear laser-matter interactions for optically active semiconductors

Development of photonic devices for sensors and communications devices has been significantly enhanced by computational modeling. We present a new computational method for modelling laser propagation in optically-active semiconductors within the paraxial wave approximation (PWA). Light propagation is modeled using the Streamline-upwind/Petrov-Galerkin finite element method (FEM). Material response enters through the non-linear polarization, which serves as the right-hand side of the FEM calculation. Maxwell's equations for classical light propagation within the PWA can be written solely in terms of the electric field, producing a wave equation that is a form of the advection-diffusion-reaction equations (ADREs). This allows adaptation of the computational machinery developed for solving ADREs in fluid dynamics to light-propagation modeling. The non-linear polarization is incorporated using a flexible framework to enable the use of multiple methods for carrier-carrier interactions (e.g.\ relaxation-time-based or Monte Carlo) to enter through the non-linear polarization, as appropriate to the material type. We demonstrate using a simple carrier-carrier model approximating the response of GaN.   

Kinetic Density Functional Theory for Plasmonic Nanostructures

We present a quantum kinetic theory of the dynamic response of typical noble metals [1]. The kinetic DFT is derived starting from the master equation of motion for the density matrix, which involves both momentum and energy relaxation processes. Therefore, the quantum system is described by two relaxation parameters, unlike the conventional time-dependent DFT incorporating only one relaxation parameter. This allows us to describe both the absorption of light and the generation of hot plasmonic electrons. The proposed theory can be employed to model and predict a variety of metal and hybrid nanostructures for applications in photocatalysis, sensors, photodetectors, metamaterials, etc. To support this, we show how the formalism can provide insights on several recent experimental results [2-4]. [1] A.O. Govorov, H. Zhang, J. Phys. Chem. C 119, 6181 (2015). [2] L. Weng, H. Zhang, A. O. Govorov, and M. Ouyang, Nature Commun. 5, 4792 (2014). [3] H. Harutyunyan, A.B.F Martinson, D. Rosenmann, L.K. Khorashad, L.V. Besteiro, A.O. Govorov, G.P. Wiederrecht, Nature Nanotech. 10, 770 (2015). [4] W. Li, Z.J. Coppens, L.V. Besteiro, W. Wang, A.O. Govorov, J. Valentine, Nat. Commun. 6, 8379 (2015).   

Electrically induced spontaneous emission in open electronic system

A quantum mechanical approach is formulated for simulation of electroluminescence process in open electronic system. Based on nonequilibrium Green’s function quantum transport equations and combining with photon-electron interaction, this method is used to describe electrically induced spontaneous emission caused by electron-hole recombination. The accuracy and reliability of simulation depends critically on correct description of the electronic band structure and the electron occupancy in the system. In this work, instead of considering electron-hole recombination in discrete states in the previous work, we take continuous states into account to simulate the spontaneous emission in open electronic system, and discover that the polarization of emitted photon is closely related to its propagation direction. Numerical studies have been performed to silicon nanowire-based P-N junction with different bias voltage.