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Electronic and Optical Properties of Confined Systems
Cooperative Research Team on
Excited State Electronic Structure and Response Functions
Objective: Apply static and time dependent pseudopotential-density functional methods to confined systems such as quantum dots, clusters and molecules to understand the electronic, structural and optical properties of such systems.
FIGURE: Simulated photoemission spectrum for negatively charged six atom silicon cluster compared to experiment (See Ref. 4).
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Optical and dielectric properties of quantum dots. Systems at small length scales often have very different dielectric and optical properties than the corresponding bulk material. For example, silicon has a band gap of about 1.1 eV in the crystalline form, but exhibits a gap twice this size for quantum dots whose diameters are on the order of a nm. In order to understand these systems, we have applied the time dependent local density approximation (TDLDA) to these systems [1]. We found that we could accurately replicated the observed experimental data for silicon quantum dots and related systems [1,2]. In collaboration with the Illinois group, we explained an outstanding problem for silicon quantum dots [3]. Silicon quantum dots were initially believed to exhibit a large Stokes shift; however, this has been demonstrated to be an extrinsic effect [3]. We used TDLDA to examine the effect of oxygen adsorbed on the surface of a quantum dot. We found that oxygen could introduce low lying electronic states. The introduction of these states provides a simple explanation for the large Stokes shift, i.e., the apparent arose because of the presence of oxygen. Currently, we are examining II-VI quantum dots and the evolution of the full optical spectra of silicon quantum dots.
Photoemission and metastability of clusters. Anion clusters of a few dozen silicon atoms, bare and hydrogenated have recently been explored experimentally using photoemission [4]. We have simulated the photoemission spectra of these clusters by using ab initio molecular dynamics. We found that good agreement could be observed between the simulated spectra and the measured results (See the Figure). However, quite frequently the best agreement was not observed for the lowest energy structure. We found that the best agreement was often for isomers with notably higher structural energies, i.e., the measured clusters were not likely in thermal equilibrium. We explained the experimental results by postulating a new rule for predicting the observed structure. Namely, the structures most likely observed were those with the highest electron affinity. We believe these structures capture electrons quickly and do not have time to relax to the equilibrium or lowest energy structure.
Excited states of diatomic molecular systems. In collaboration with the Berkeley group, we are currently examining TDLDA methods compared to GW-Bethe Salpeter equation (GW-BSE) methods. We have initiated this study by examining diatomic molecules such as C2, BN, BeO, SiO. Our initial findings using TDLDA are quite encouraging. We have obtained good agreement between our theoretical calculations and the observed low-lying excitations. We are now implementing GW-BSE methods for the same molecular systems.
References
1. I. Vasiliev, S. Ogut, and J. R. Chelikowsky, "First Principles Density Functional Calculations for Optical Spectra of Clusters and Nanocrystals," Phys. Rev. B 65, 115416 (2002).
2. M.C. Troparevsky, L. Kronik and J.R. Chelikowsky, "Ab initio Absorption Spectra of CdSe Clusters," Phys. Rev. B 65, 033311 (2002).
3. I. Vasiliev, R. M. Martin and J.R. Chelikowsky, "Effect of Surface Oxidation on the Optical Properties of Silicon Nanocrystals," Phys. Rev. B 65, 121302 (2002).
Contact: Jim Chelikowsky, Phone:(512) 232-9083, Fax:(512) 471-8694
Email:jrc@ices.utexas.edu,
Web Page: http://tesla.ices.utexas.edu
Codes available online.
