Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots

Electron transport through two interacting channels in Azurin-based solid-state junctions

The fundamental question of "what is the transport path of electrons through proteins?" initially introduced while studying long-range electron transfer between localized redox centers in proteins in vivo is also highly relevant to the transport properties of solid-state, dry metal-protein-metal junctions. Here, we report conductance measurements of such junctions, Au-(Azurin monolayer ensemble)-Bismuth (Bi) ones, with well-defined nanopore geometry and ~103 proteins/pore. Our results can be understood as follows. (1) Transport is via two interacting conducting channels, characterized by different spatial and time scales. The slow and spatially localized channel is associated with the Cu center of Azurin and the fast delocalized one with the protein's polypeptide matrix. Transport via the slow channel is by a sequential (noncoherent) process and in the second one by direct, off-resonant tunneling. (2) The two channels are capacitively coupled. Thus, with a change in charge occupation of the weakly coupled (metal center) channel, the broad energy level manifold, responsible for off-resonance tunneling, shifts, relative to the electrodes' Fermi levels. In this process, the off-resonance (fast) channel dominates transport, and the slow (redox) channel, while contributing only negligibly directly, significantly affects transport by intramolecular gating.

Modulation of Carbon Nanotube Electronic Structure by Grain Boundary Defects in RbI on Au(111)

The electronic properties of single-walled carbon nanotubes (SWCNTs) are known to be highly sensitive to environmental effects. Here, we use scanning tunneling microscopy and spectroscopy to investigate the electronic properties of SWCNTs deposited on RbI monolayer films grown on Au(111). We find that grain boundary defects in RbI monolayers cause the appearance of spatially confined localized states in the SWCNTs. Our density functional theory calculations show that grain boundary defects in RbI/Au(111) produce a stabilizing electrostatic potential caused by reduced coordination of iodine atoms at the RbI grain boundary. The presented results may offer insights into the performance of devices involving transport through SWCNTs subjected to external electrostatic disorder.

Influence of Encapsulated Water on Luminescence Energy, Line Width, and Lifetime of Carbon Nanotubes: Time Domain Ab Initio Analysis

In a broad range of applications, carbon nanotubes (CNTs) are in direct contact with a condensed-phase environment that perturbs CNT properties. Experiments show that water molecules encapsulated inside of semiconducting CNTs reduce the electronic energy gap, enhance elastic and inelastic electron-phonon scattering, and shorten the excited-state lifetime. We rationalize the observed effects at the atomistic level using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics. Encapsulated water makes the nanotube more rigid, suppressing radial breathing modes while enhancing and slightly shifting the optical G-mode. Water screens Coulomb interactions and shifts charge carrier energies and wave functions. The screening, together with distortion of the CNT geometry and lifting of orbital degeneracy, produces a luminescence red shift. Enhanced elastic and inelastic electron-phonon scattering explains line width broadening and shortening of the excited-state lifetime. The influence of water on the CNT properties is similar to that of defects; however, in contrast to defects, water creates no new phonon modes or electronic states in the CNTs. The atomistic understanding of the influence of the condensed-phase environment on CNT optical, electronic, and vibrational properties, and electron-vibrational dynamics guides design of novel CNT-based materials.

Self-Trapping of Charge Carriers in Semiconducting Carbon Nanotubes: Structural Analysis

The spatial extent of charged electronic states in semiconducting carbon nanotubes with indices (6,5) and (7,6) was evaluated using density functional theory. It was observed that electrons and holes self-trap along the nanotube axis on length scales of about 4 and 8 nm, respectively, which localize cations and anions on comparable length scales. Self-trapping is accompanied by local structural distortions showing periodic bond-length alternation. The average lengthening (shortening) of the bonds for anions (cations) is expected to shift the G-mode frequency to lower (higher) values. The smaller-diameter nanotube has reduced structural relaxation due to higher carbon-carbon bond strain. The reorganization energy due to charge-induced deformations in both nanotubes is found to be in the 30-60 meV range. Our results represent the first theoretical simulation of self-trapping of charge carriers in semiconducting nanotubes, and agree with available experimental data.

Electron confinement induced by diluted hydrogen-like ad-atoms in graphene ribbons

We report the electronic properties of two-dimensional systems, which are patterned with ad-atoms in two separated regions. By applying band-folding procedures we are able to predict the energies and the spatial distribution of those impurity-induced states.