Ab-Initio Calculations of the Vibrational Properties of Nanostructures
Colloidal semiconductor nanocluster research is a rapidly growing field driven by the attractive idea to tailor material properties by acting on the morphology of the structures. The modification of the optical properties by merely changing the diameter o
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1 Introduction Colloidal semiconductor nanocluster research is a rapidly growing field driven by the attractive idea to tailor material properties by acting on the morphology of the structures. The modification of the optical properties by merely changing the diameter of colloidal quantum dots is one of the figureheads of nanostructure science [1–3]. It is the intense research effort towards the fabrication of nanostructures with favorable properties that has helped to establish most of the knowledge base we rely on today. Till now, the modification of the electronic and optical properties by changing the size of the nanoclusters are well understood theoretically and well controlled experimentally. One open problem of nanostructure science is the effects of temperature on the electronic and optical properties of nanoclusters and hence their vibrational properties. A theory at T D 0 K yields very valuable results to unveil certain aspects of the underlying physics, but to make predictions valid in the real world, where the physical properties such as temperature broadening, quantum coherence dephasing, spin-flip transitions and relaxation of charge carriers are key components [4–6], the effects of vibration and temperature on the dynamical processes must be addressed. The vibrational properties such as the phonon density of states (DOS) and dispersion of bulk semiconductors have been calculated with great accuracy using ab initio density functional perturbation theory (DFPT) since the end of the last century [7]. After the successful applications of DFPT on bulk phonon eigenmodes, ab initio studies on the vibrational properties of semiconductor nanostructures such as fullerenes, nanowires, nanotubes, and nanoclusters with small sizes have been performed [8–10]. However, an accurate density functional theory (DFT) study on the vibrational properties of nanoclusters with the experimentally relevant size of G.Bester () P. Han Max-Planck-Institut f¨ur Festk¨orperforschung, Stuttgart, Germany e-mail: [email protected]; [email protected] W.E. Nagel et al. (eds.), High Performance Computing in Science and Engineering ’12, DOI 10.1007/978-3-642-33374-3 13, © Springer-Verlag Berlin Heidelberg 2013
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few nm diameter has not been reported until now due to the high computational demand. With the computational facility available at the H¨ochst Leistungs Rechenzentrum Stuttgart (HLRS), we have recently calculated confinement and surface effects on the vibrational properties of colloidal semiconductor nanoclusters based on first-principles DFT. We describe how the molecular-type vibrations, such as surface-optical, surface-acoustic, and coherent acoustic modes, coexist and interact with bulk-type vibrations, such as longitudinal and transverse acoustic (LA, TA) and optical (LO, TO) modes. We could link the vibrational properties to structural changes induced by the surface and highlight the qualitative difference between III–Vs and II–VIs semiconductor nanoclusters [11]. We describe the specific
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