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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. Current research projects include:

Excited state molecular dynamics

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. We have developed a novel method to model photochemical time-dependent processes in extended molecular systems based on the quantum-chemical numerical computations. This technique combines semiempirical Hartree-Fock computations with a time-dependent density matrix calculation of vertical optical excitations, using the ground-state single-electron density matrix as input. Developed parallelized computational code allows us to follow the excited state motion of molecules of greater than 1000 atoms in size for time scales as long as tenths of picoseconds. Modeled excited state dynamics in organic materials often results in the energy localization into “hot spots” forming self-trapped and dynamic nonlinear excitations (“breathers”) competing with incoherent energy and charge transport (for example, see movie describing photoexcited breather formation in section Movies). Such nonlinear and nonadiabatic phenomena are typical for many materials including biomolecules, have characteristic spectroscopic signatures, and can be utilized in many optoelectronic applications. We are currently extending this approach to the first principle methods and semiempirical methods, as well as other organic and inorganic molecular systems. This includes non-adiabatic molecular dynamics and ultrafast spectroscopic probes of such 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.

Time-dependent density functional theory: beyond adiabatic approximation

The time-dependent density functional theory (TDDFT) has rapidly emerged as an extremely useful method for computing molecular excited states and studying the optical response of molecules. Just as was the case for ground-state density functional theory, the number of applications is growing exponentially. However, many applications TDDFT lead to discovery of failures such as inaccurate charge transfer excitations and lack of double excitations. Modern density functional models frequently employ so called adiabatic approximation, which neglects the memory effects in the density and subsequently is a source of many TDDFT drawbacks. We are possibilities for improvement of TDDFT by incorporating nonadiabatic effect into functional models. This includes analytic and numerical studies of failures of nonadiabatic models and developing of appropriate extensions to nonadiabatic dynamical density functionals. Recently we have been developing numerical algorithms in the TDDFT framework allowing computation of excited state properties of large molecular systems at linear scaling cost, i.e. O(N) in both time and memory scaling with the system size N.

Electronic interactions 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.

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.