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Electronic structure and spectroscopy of organic and inorganic nano-materials

Our research is focused on theoretical studies of excited states and optical responses of photoactive organic and inorganic nano-materials such as large organic molecules, biological complexes and semiconductor nano-particles. All electronic processes in these systems are governed by strong electronic correlations and coupling of electrons to molecular structure. We develop quantum chemical approaches for efficient and accurate modeling of electronic structure, dynamics, charge and energy transfer in large molecular systems and apply these methods to different materials such as conjugated polymers, dendrimers, carbon nanotubes, biological light harvesting systems, donor-acceptor complexes, and semiconductor quantum dots. The results of the calculations are then used for modeling of linear (absorption and emission) and nonlinear (including ultrafast frequency and time-resolved spectroscopic probes) optical responses to model experimental data and to understand underlying dynamical photo-physical, photo-chemical and relaxation processes (see our reviews on electronic processes, excited state dynamics and organic photovoltaics). Our current research projects include:

Non-adiabatic excited state molecular dynamics (NA-ESMD)

Almost every photochemical, photophysical, charge and energy transport, or spectroscopic process in organic nano-materials involves dynamics of excited vibronic states and ultimately defines molecular electronic properties and functionalities. When excited by photons of light, functional electronic materials show dynamics that are often characterized by large nonadiabatic couplings between multiple excited states through a breakdown of the Born−Oppenheimer approximation. Following photoexcitation, various nonradiative intraband relaxation pathways can lead to a number of complex processes. Therefore, computational simulation of nonadiabatic molecular dynamics is an indispensable tool for understanding complex light-driven processes such as internal conversion, energy transfer, charge separation, and spatial localization of excitons. Over the years, we have developed a nonadiabatic excited-state molecular dynamics (NA-ESMD) framework that efficiently and accurately describes photoinduced phenomena in extended conjugated molecular systems. We use the fewest-switches surface hopping (FSSH) algorithm to treat quantum transitions among multiple adiabatic excited state potential energy surfaces. Extended molecular systems often contain hundreds of atoms and involve large densities of excited states that participate in the photoinduced dynamics. The NA-ESMD methodology offers a computationally tractable route for simulating hundreds of atoms on 10 ps time scales where multiple coupled excited states are involved. This technique was successfully applied to study ultrafast excited state dynamics and spectroscopies in conjugated polyfluorenes, phenylene-ethynylenes, dendrimers, cycloparaphenylenes, chlorophylls, explosives and many other molecular materials. Section Movies exemplifies examples of complex evolution of photoexcited wavefunctions in several systems. Currently we are developing more accurate numerical algorithms for the NA-ESMD allowing treatment of decoherences and interferences beyond the FSSH method, such as use of semiclassical Monte-Carlo integrators. In addition we are exploring dielectric environment effects on excited state dynamics relying on state-specific solvation within polarizable continuum and quantum/mechanics/molecular mechanics (QM/MM) models.

Design of hybrid perovskite materials for photovoltaics and optoelectronics

The recent discovery of organic-inorganic perovskites offers promising routes for the development of low-cost, solar-based clean global energy solutions for the future Solution-processed organic-inorganic hybrid perovskite planar solar cells, such as CH3NH3PbX3 (X = Cl, Br, I), have achieved high average power conversion efficiency (PCE) values approaching 20%. In 2015 our experimental-theory team at LANL has demonstrated a universally applicable fabrication strategy for the growth of millimeters scale crystals of hybrid perovskites. This allows making hysteresis-free solar cells approaching record 18% light-to-energy conversion efficiency with extraordinary reproducibility and negligible degradation. Our device modeling and DFT calculations characterize charge dynamics at the interface of perovskites in order to aide in materials design and device engineering. Our follow up studies address photodegradation processes in perovskite solar cells. Our theory and experiments elucidate the atomistic origin of light-induced macroscopic degradation tracing this phenomenon to appearance to meta-stable charge trap states (polarons) and then utilize this knowledge to demonstrate photovoltaic devices stable with constant illumination.  Finally, our recent results are on 2D layered hybrid perovskites, which being incorporated into a photovoltaic device exhibited world-record efficiency (13% in the category of 2D materials) and stability for over 3 month under constant light illumination and 65% humidity. The theoretical follow up review aims to provide an interpretive and predictive framework for 3D and 2D layered hybrid perovskite optoelectronic properties

Energy and charge transport in macroscopic molecular assemblies

Following absorption (or leading to emission) of light quanta are fundamentals processes of energy (e.g., in a form of excitons) and charge (e.g., in a form of polarons) transport.  Theoretical modeling of these phenomena in large molecular assemblies have evolved from a consideration of simple models to a more or less comprehensive description utilizing first principles calculations of underlining molecular using toward accessing device scale. We have been developing multiscale modeling toolbox for simulation of energy (using Foester theory) and charge carrier mobility (using Marcus theory) in soft electronic materials, to establish a set of structure-property relationships for rational design of such materials and to suggest new materials based on theoretical prediction. Applications were done for small molecule-based organic photovoltaic materials and natural biological systems containing aromatic amino acid residues.

Spectroscopy and electronic processes in carbon nanotube/graphene materials

Single-wall carbon nanotubes (SWNTs) are one-atom-thick layer of graphene rolled into long rigid cylinders a few nanometers in diameters. The diameter and direction of tube’s rolling (a chiral vector (n,m)) define the main features of SWNTs, such as a semiconductor or metal-like electronic structure. Carbon nanotubes constitute a class of the most promising technological materials with multifarious applications ranging from electronic chips and optoelectronics to medical technologies and sensing. Over the past two years we pioneered applications of methodologies previously developed for molecular materials to SWNTs (in contrast to solid-state approaches being standard in the field). These studies made possible to address many aspects of photophysics of an isolated tube. This includes anharmonic coherent phonon dynamics, quantification of exciton-phonon coupling and Huang-Rhys factors,  characterization of delocalized non-excitonic transitions, effects of Peierls distortion and exciton self-trapping, energetics of dark states, prediction of triplet states, study of transverse polarized absorption in nanotubes, and understanding structure of DNA/SWNT complexes. Many of these results confirm the latest experimental data. Currently we investigate charge transport and energy transfer in the presence of defects, functional groups, intercalating dyes, and intertube interactions. It turns out, control over the reorganization energies for polaronic and excitonic states suggest significant potential for tailoring the electronic properties of carbon nanostructures through chemical engineering.

 

Electronic interactions in semiconductor quantum dots

Colloidal semiconductor quantum dots (QD) can be viewed as nanometer size objects in which quantum confinement occurs in all three dimensions. In QDs, the bulk band structure collapses into discrete atomic-like levels which makes it possible to view QDs as artificial “tunable” atoms with electronic energies that are controlled by QD dimensions. Following the analogy with atoms, QDs can conceivably be manipulated into complex interacting structures that mimic molecules and atomic or molecular solids. Such complex structures possess huge technological potential associated with the versatile tunability of their properties provided by the combined control of energy levels in the individual nanoparticles and collective interactions of nanoparticles in an assembly. First principle calculations of QD (supermolecular approach) presents a significant scientific challenge since the number of atoms grows rapidly with the system size. We are applying a complementary suit of quantum chemical methods ranging from first principles (TDDFT) to semiempirical and minimal models for quantitative evaluation of electronic structure in QD nanosystems working closely with our experimental colleagues. In particular, we have shown that structurally optimized QD assemblies can feature very efficient transfer rate of excitons from smaller to larger dots via electrostatic coupling approaching picosecond time scales. This leads to interesting energy harvesting applications mimicking natural biological complexes. Using density functional theory (DFT) and time-dependent DFT quantum-chemical methodologies, we recently explored the role of surface ligands on the electronic structure and observe strong surface-ligand interactions in quantum dots leading to formation of hybridized states and polarization effects. This opens new relaxation channels for high energy photoexcitations affecting both population (inelastic scattering) and dephasing (elastic scattering) processes. 

Multiscale modeling of electronic excitations using Exciton Scattering (ES) approach

Modern electronic structure calculations involve solving the Schrodinger equation. Substantial numerical cost [O(N3) - O(N5), N being the number of electrons (orbitals) in the system] makes repetitive computations for large molecules prohibitively expensive. This challenge calls for development of multiscale approaches. Notably, any reduction of computational complexity is highly non-trivial, since one need to take an advantage of quantum-mechanical phenomena, such as delocalized wavefunctions, quantum confinement, and electronic coherences. Using concept of strongly bound excitons, we recently developed multiscale exciton scattering (ES) model, which attributes excited states to standing waves in quasi-one-dimensional structures. This method allows electronic spectra for any structure of arbitrary size within the considered molecular family to be obtained with negligible numerical effort. Complex and non-trivial delocalization patterns of photoexcitations throughout the entire molecular structure can then be universally characterized and understood using the proposed ES method, completely bypassing 'supramolecular' calculations. An agreement is within 1-5 meV for most test cases, when the ES results are compared with the reference TD-DFT calculations has been achieved. Consequently, computational design of molecular structures with desirable electronic and optical properties can be performed in real-time using graph-like representation of molecules. Currently we are extending the ES model to calculate various spectroscopic observables and are applying this approach to study the structure of electronic excitations in macromolecules where it constitutes the only feasible option.

 

Nonlinear optical response of conjugated organics

Optical materials with enhanced nonlinear optical (NLO) response have important technological implications such as optical switching and wave-guiding, compact 3D data storage and micro fabrication, chemical and biological sensing, optical power limiting, up-conversion lasing, bio-imaging, etc. Computational design of nonlinear optical materials constitutes a complex fundamental problem and theoretical modeling of optical polarizabilities of molecular systems requires extensive numerical effort since NLO responses often involve strongly correlated excited electronic states. Recently we developed computational approach for modeling nonlinear optical polarizabilities in the framework of adiabatic time-dependent density functional theory (TDDFT) by deriving the respective nonlinear response functions from the equation of motion for the driven single-electron density matrix. This formalism has been coded and applied to calculate one- and two-photon absorption spectra (related to linear and third-order optical responses, respectively) in a series of large donor-acceptor substituted conjugated molecules. Calculated excitation energies corresponding to one and two-photon-absorption maxima are found to be in excellent agreement with experiment with better than 4% accuracy in average which shows a potential of TDDFT for NLO calculations. Using developed TDDFT extensions, we are studying other nonlinear optical responses and molecules including dipolar, quadrupolar, octupolar, branching and dendrimeric systems. Recently, we have investigated two-photon absorption in conjugated energetic molecules constituting a new class of explosives having potential for controlled direct optical initiation through photochemical pathways.