During the grant period of 09/01/2007-08/31/2008, our research supported by the ACS-PRF-G award has been fruitful in scientific merits and promoting multi-disciplinary graduate education. Two graduate students are supported by this grant, combined with departmental teaching assistantships and graduate fellowships. We herein report two research subject that already resulted two publications. These research underlie our future work on understanding laser control of molecular reactions and electronic behaviors in quantum particles.
Christine M. Isborn and Xiaosong Li, “Modeling the doubly-excited state with time-dependent Hartree-Fock and density functional theories”, J. Chem. Phys. (accepted)
Multi-electron excited states have become a hot topic in many cutting-edge research fields, such as the photophysics of polyenes and in the possibility of multi-exciton generation in quantum dots for the purpose of increasing solar cell efficiency. However, obtaining multi-electron excited states has been a major obstacle as it is often done with multi-configurational methods, such as CASSCF or SAC-CI, which involve formidable computational cost for large systems. Although they are computationally much cheaper than multi-configurational wave function based methods, linear response adiabatic TDHF/TDDFT are generally considered incapable of obtaining multi-electron excited states. We have developed a real-time TDHF and adiabatic TDDFT approach that is beyond the perturbative regime. We show that TDHF/TDDFT is able to simultaneously excite two electrons from the ground state to the doubly-excited state, and that the real-time TDHF/TDDFT implicitly includes double-excitation within a superposition state. We also present a multi-reference linear response theory to show that the real-time electron density response corresponds to a superposition of perturbative linear responses of the S0 and S2 states. As a result, the energy of the doubly-excited state can be obtained with several different approaches. We report results on simple two-electron systems, including the energies and dipole moments for the two-electron excited states of H2 and HeH+. These results are compared to those obtained with the full configuration interaction (CI) method.
Ekaterina Badaeva, Christine M. Isborn, Yong Feng, Daniel R. Gamelin, Xiaosong Li, “Theoretical Characterization of Electron Transitions in Transition Metal Doped ZnO Quantum Dots”, to be submitted in Oct. 2008
The interaction between transition metal dopants and semiconductor quantum dots has shown great potential in for nanospintronics applications. Experimental analysis and design of DMS QDs are often based on spectroscopic probes of dopant specific electronic structure, such as charge transfer states and the extent of dopant-semiconductor hybridization. In this article, we present a theoretical characterization of the low-energy (ultraviolet/visible/near-infrared) electronic transitions in Co2+ and Mn2+ doped ZnO quantum dots (QDs) with sizes up to 300 atoms. Standard orbital energy differences are incapable of describing electronic transitions in doped quantum dots because of strong electron-hole interactions. Instead, linear response time-dependent hybrid density functional theory (TDDFT) is used in this investigation. The electron excitations associated with several important absorption peaks are identified, including ligand field transitions, two types of charge transfer (CT) transitions from metal to ZnO conduction band (CB), and ZnO valence band (VB) to CB transitions. As a result of quantum confinement effects, positions of these charge transfer bands strongly depend on the size of quantum dot.
