Events

Dynamics of Excitons and Phonons in Disordered Nanoscale Materials

December 15, 2009 at 3pm/36-428

Sergei Tretiak
Los Alamos National Laboratory

abstract:
Prediction of the optical response and photoinduced processes of molecular and nanomaterials is fundamental to a myriad of technological applications, ranging from sensing, imaging, solar energy harvesting, to future optoelectronic devices. In this talk I will overview several applications of emerging quantum-chemical approaches to model photoinduced dynamics in a number of molecular materials over multiple length and time scales. Using concept of strongly bound excitons, we develop exciton scattering (ES) model, allowing multiscale calculation of electronic spectra in large molecules. Consequently, computational design of molecular structures with desirable electronic and optical properties can be performed using graph-like representation of molecules. To simulate non-adiabatic excited state molecular dynamics and concurrent energy transfer processes in dendrimetrs, we use surface hopping techniques combined with the semiempirical approaches. The results show intricate details of photoinduced vibronic relaxation leading to ultrafast energy transfer. In quantum dots we study the role of surface ligands on the electronic structure and observe strong surface-ligand interactions leading to formation of hybridized states and polarization effects. This opens new relaxation channels for high energy photoexcitations simulated using non-adiabatic time-domain DFT molecular dynamics technique. In the amorphous clusters of conjugated polymers, we find that electron trap states are induced primarily by intra-molecular configuration disorder, while the hole trap states are generated primarily from inter-molecular electronic interactions. All these phenomena define important properties of materials suitable for various technologies such as solar energy conversion.

Representative publications:
C. Wu, S. Malinin, S. Tretiak, and V. Chernyak, “Multiscale Modeling of Electronic Excitations in Branched Conjugated Molecules Using Exciton Scattering Approach,” Phys. Rev. Lett. 100, 057405 (2008).
S. Kilina, E.R. Batista, P. Yang, S. Tretiak, A. Saxena, R.L. Martin, and D.L. Smith ”Electronic structure of amorphous polyfluorene aggregates”, ACS Nano 2 1381-1388 (2008).
D.A. Yarotski, S.V. Kilina, A. Talin, S. Tretiak, O.V. Prezhdo, A.V. Balatsky and A.J. Taylor ”Scanning tunneling microscopy of DNA-wrapped carbon nanotubes”, Nano Lett. 9 12-17 (2009).
S. Kilina, S. Ivanov, V.I. Klimov, and S. Tretiak ”Effect of Surface Ligands on Optical and Electronic Spectra of Semiconductor Nanoclusters”, J. Am. Chem. Soc., 131, 7717-7726 (2009).
S. F. Alberti, V. Kleiman, S. Tretiak, A. Roitberg, ”Nonadiabatic molecular dynamics simulations of the energy transfer between building blocks in a phenylene ethynylene dendrimer”, J. Phys. Chem. A, 113, 7535-7542 (2009).
J. Tao, S. Tretiak, ”Optical absorptions of new blue-light emitting oligoquinolines bearing pyrenyl and triphenyl endgroups investigated with time-dependent density functional theory”, J. Chem. Theory and Comput., 5, 866-872 (2009).

bio:
Sergei Tretiak received his M.Sc. degree (highest honors, 1994) from Moscow Institute of Physics and Technology (Russia) and his Ph.D. degree in 1998 from the University of Rochester where he worked with Prof. Shaul Mukamel. Afterwards he was Director-funded Postdoctoral Fellow (1999-2001) and subsequently became a Technical Staff Member (2001-present) at LANL. Since 2006 he is a member of the DOE funded Center for Integrated Nanotechnologies (CINT). Dr. Tretiak’s research interests center on the relation between optical and chemical properties of organic and semiconductor materials; development of modern computational methods for molecular optical properties; time-dependent density functional theory and semiempirical methods; nonlinear optical response of organic chromophores; adiabatic and non-adiabatic molecular dynamics of the excited states; collective electronic excitations and optical response of confined excitons in conjugated polymers, carbon nanotubes, semiconductor nanoparticles, and molecular aggregates; charge and energy transfer in biological and artificial antenna complexes. Dr. Tretiak has published over 110 scientific publications and has presented more than 90 invited talks.