Excited state properties of materials and nanostrcutures

Ab-initio calculations of excited state properties of condensed systems and nanostructures involve evaluation of dielectric matrices

For example, to obtain photoemission spectra , absorption spectra , electron energy loss spectra , in principle one needs to compute the inverse dielectric matrix. Likewise, calculations of long range correlation effects (e.g. within the Random Phase Approximation (RPA)) involve knowledge of dielectric response functions. We have devised techniques, based on Density Functional Perturbation Theory (DFPT) to obtain approximate yet accurate dielectric matrices from eigenvalue decompositions [1,2]. An example of the use of approximate dielectric matrices are GW calculations for water and ice [3]. We have also applied similar techniques to simplify:

  • Solution of the Bethe-Salpeter Equations (BSE) for molecules and clusters
  • Calculations of RPA correlation energies of molecular crystals [4].

In addition we are carrying out calculations of excited state properties of Silicon and II-VI nanostructures [5,7], at different level of theory (including DFT and TDDFT), to help interpret and understand experiments, e.g. within our CCI collaboration


Si Nanowires





Ball and stick representations of CnHm functionalized Si dots (below) and H-terminated crystalline Si-wires (left).
Si NanCluster

We are studying absorption spectra of hydrogen terminated and functionalized silicon nanowires and nanoclusters [6]. Work on nanoclusters is in collaboration with Prof. G.Zimanyi at UCD and Prof. Adam Gali at the Budapest University of Technology. Work on nanowires is part of the CCI collaboration.


MoS2 nanoparticles
MoS2 nanoparticles: stoichiometric and Mo-doped rods (upper panel) and platelets with different surface terminations.

We have carried out a first principle study of MoS2 nanoparticles [5] which provides a unified explanation of measured photoluminescence spectra and recent STM measurements as a function of size. In addition our calculations suggest ways to engineer the electronic properties of these systems so as to obtain direct band gap 3D layered nanoparticles, or Mo doped metallic nanowires. We are collaborating with the group of Prof. Feng Wang at UCB to further investigate the design of direct band gap chalcogenides [7].


References

  1. "Efficient iterative methods for the calculation of dielectric matrices", H.Wilson, F.Gygi and G.Galli, Phys. Rev. B, 78,113303 (2008).
  2. "Iterative calculations of dielectric eigenvalue spectra", H. F. Wilson, D. Lu, F. Gygi and Giulia Galli,, Phys. Rev. B. , 79, 245106 (2009).
  3. "Dielectric properties of ice and liquid water from first principle calculations", D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett., 100, 147601(2008).
  4. "Ab initio calculation of van der Waals bonded molecular crystals", D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett., 102, 206411 (2009).
  5. "Electronic properties of MoS2 nanoparticles", T. Li, and G.Galli, J. Phys. Chem. C, 111, 16192 (2007).
  6. "High energy excitations in silicon nanoparticles" , A. Gali, Adam, M.Vörös, D.Rocca, G. Zimanyi, and G.Galli, Nanolett. 2009 (in press).
  7. "Emerging Photoluminescence in Monolayer MoS2" A. Splendiani, Y. Zhang, T. Li, J. Kim, C-Y. Chim, G. Galli, and F. Wang (submitted for publication).