Ab initio description of the self-organization, shape and properties of IV-VI and III-V nanocrystals and nanorods

 

G. Kresse

F. Bechstedt

 

 

We aim to apply and develop multiscale methods for the prediction of the electronic properties of nanostructures that seamlessly merge parameter free ab initio calculations with non-selfconsistent screened effective pseudopotential methods. The presently implemented methods already allow one to predict the groundstate electronic properties of small nanostructures containing up to 2000 atoms using density functional theory, and between 10.000 and 100.000 atoms using effective pseudopotentials. Applications with up to 1 million atoms will be possible in the third funding period.

One focus of the third funding period, however, will be on the prediction of excitonic effects, covering optical absorption spectra and photoluminescence. This requires the description of the coupled movement of particle-hole pairs in an external time-dependent field as provided by the Bethe-Salpeter equation. For medium sized systems (200-1000 atoms) we plan to develop a new solver for the Bethe-Salpeter equation that scales favourably with the system size (low complexity). Therewith an efficient treatment of nanostructure will become possible. Furthermore, for very large systems (10.000-100.000 atoms), we will constrain the excitation space to few holes and particles treating the remaining electrons as an effective dielectric medium.

Besides that, conventional DFT methods will be applied to smaller “motives” found in nanostructures. We expect that, with further code improvements, DFT calculations for 1000-2000 atoms will be routinely possible to determine atomic geometries, surface and interface properties and the electronic properties of semiconductor nanostructures.

The considered systems will range from nanodots embedded in a matrix, interfaces between different polytypes and compounds, to free standing nanowires. Ideally the simulations for small to medium sized systems will be performed both, using full ab initio DFT calculations, as well as, screened effective pseudopotentials. As far as nanodots are concerned, we will continue the work on the group IV-VI semiconductors (lead and tin salt), where many fundamental issues, for instance stoichiometry, embedding, interface states, screening, and exciton formation are not yet well understood. For free standing nanowires we will focus on narrow-gap III-V as well as doped narrow-gap group IV materials. Central goals are the understanding of the influence of stacking, surface passivation/orientation, and capping with other semiconducting materials. Systems of special interest are heterocrystalline and core-shell nanowires, as well as C and Sn doped Ge. To complement our methodological developments, we will concentrate on the interplay of structural properties and electronic excitations, in particular, on how electronic and optical properties are influenced by the atomic arrangement, the confinement, the interplay with the matrix and interface effects.


Report 2005-2009 (.pdf)

 

Report 2009-2012 (.pdf)