Electronic structure calculations for nanomolecular systems

Introducing Molecular Electronics, Lecture Notes in Physics, Ed. G. Cunibert and G. Fagas and K. Richter, Springer, Berlin / Heidelberg Vol. 680, p. 77 - 116 (2005)

Electronic structure calculations for nanomolecular systems

R. Di Felice, A. Calzolari, D. Varsano, A. Rubio

The electronic structure constitutes the fundamentals on which a reliable quantitative knowledge of the electrical properties of materials should be based. Here, we first present an overview of the methods employed to elucidate the ground-state electronic properties, with an emphasis on the results of Density Functional Theory (DFT) calculations on selected cases of (bio)molecular nanostructures that are currently exploited as potential candidates for devices. In particular, we show applications to carbon nanotubes and assemblies of DNA-based homoguanine stacks. Then, to move ahead from the electronic properties to the computation of measurable features in the operation of nanodevices (e.g., transport characteristics, optical yield), we proceed along two different lines to address two non-negligible issues: the role of excitations and the role of contacts. On one hand, for an accurate simulation of charge transport, as well as of optoelectronic features, the ground state is not sufficient and one needs to take into account the excited states of the system: to this aim, we introduce Time-Dependent DFT (TDDFT), we describe the TDDFT frameworks and their relation to the optical properties of materials. We present the application of TDDFT to compute the optical absorption spectra of fluorescent proteins and of DNA bases. On the other hand, the details of the conductor-leads interfaces are of crucial importance to determine the current under applied voltage, and one should compute the transport properties for a device geometry that mimics the experimental setup: to this aim, we introduce a novel development based on Wannier functions. The method, which is a framework for both an in-depth analysis of the electronic states and the plug-in of tight-binding parameters into the Green’s function, is described with the aid of examples on nanostructures potentially relevant for device applications.

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