Schottky barriers at metal-finite semiconducting carbon nanotube interfaces

Electronic properties of metal-finite semiconducting carbon nanotube interfaces are studied as a function of the nanotube length using a self-consistent tight-binding theory. We find that the shape of the potential barrier depends on the long-range tail of the charge transfer, leading to an injection barrier thickness comparable to half of the nanotube length until the nanotube reaches the bulk limit. The conductance of the nanotube junction shows a transition from tunneling to thermally-activated transport with increasing nanotube length

A non-equilibrium equation-of-motion approach to quantum transport utilizing projection operators

We consider a projection operator approach to the non-equilbrium Green function equation-of-motion (PO-NEGF EOM) method. The technique resolves problems of arbitrariness in truncation of an infinite chain of EOMs, and prevents violation of symmetry relations resulting from the truncation. The approach, originally developed by Tserkovnikov [Theor. Math. Phys. 118, 85 (1999)] for equilibrium systems, is reformulated to be applicable to time-dependent non-equilibrium situations. We derive a canonical form of EOMs, thus explicitly demonstrating a proper result for the non-equilibrium atomic limit in junction problems. A simple practical scheme applicable to quantum transport simulations is formulated. We perform numerical simulations within simple models, and compare results of the approach to other techniques, and (where available) also to exact results.Comment: 16 pages, 5 figure

Raman scattering in current carrying molecular junctions. A preliminary account

This is a preliminary acount of a theory for Raman scattering by current-carrying molecular junctions. The approach combines a non-equilibrium Green's function (NEGF) description of the non-equilibrium junction with a generalized scattering theory formulation for evaluating the light scattering signal. This generalizes our previous study (Phys. Rev. Lett. 95, 206802 (2005); J. Chem. Phys. 124, 234709 (2006)) of junction spectroscopy by including molecular vibrations and developing machinery for calculation of state-to-state (Raman scattering) fluxes within the NEGF formalism. For large enough voltage bias we find that the light scattering signal contains, in addition to the normal signal associated with the molecular ground electronic state, also a contribution from the inverse process originated from the excited molecular state as well as an interference component. The effect of coupling to the electrodes and of the imposed bias on the total Raman scattering as well as its components are discussed. Our result reduces to the standard expression for Raman scattering in the isolated molecule case, i.e. in the absence of coupling to the electrodes. The theory is used to discuss the charge transfer contribution to surface enhanced Raman scattering for molecules adsorbed on metal surfaces and its manifestation in the biased junction.Comment: 46 pages, 7 figure

On optical spectroscopy of molecular junctions

We compare theoretical techniques utilized for description of optical response in molecular junctions, and their application to simulate Raman spectroscopy in such systems. Strong and weak sides of the Hilbert vs. Liouville space, as well as quasiparticles vs. many-body states, formulations are discussed. Common origins of the methodologies and different approximations utilized in different formulations are identified.Comment: 17 pages, 4 figure

Born Oppenheimer Dynamics Near Metal Surfaces

We discuss the usefulness of Born-Oppenheimer potential surfaces for nuclear dynamics for molecules strongly coupled to metal surfaces. A simple model demonstrating the construction of such surface for a molecular junction is discussed.Comment: 5 pages, 2 figure