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In my presentation, I will describe results from two of our recent projects:
1) Carbon Nanotubes: Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogenous environment are effectively subjected to a disorder potential, which can lead to localized electronic states. An important element of the physical picture of such states localized on the nanometer-scale is the existence of a local vibronic manifold resulting from the localization-enhanced electron-vibrational coupling. To investigate these effects, we used Scanning Tunneling Spectroscopy (STS) to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. STS spectra show the vibrational overtones identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. The presence of these vibrational modes in the STS spectra suggests rippling distortion and dimerization of carbon atoms on the SWCNT surface. This study thus experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states.
2) Lead-Sulfide Nanocrystals: The properties of photovoltaic devices based on colloidal nanocrystals are strongly affected by localized sub-bandgap states associated with surface imperfections. To understand the nature of such surface states, a correlation between their properties and the atomic-scale structure of chemical imperfections responsible for their appearance must be established. We used Scanning Tunneling Spectroscopy to visualize the manifold of electronic states in annealed ligand-free lead sulfide nanocrystals supported on the Au(111) surface. Delocalized quantum-confined states and localized sub-bandgap states are identified via spatial mapping. Maps of the sub-bandgap states show localization on non-stoichiometric adatoms self-assembled on the nanocrystal surfaces. Our model study sheds light onto the mechanisms of surface state formation that, in a modified form, may be relevant to the more general case of ligand-passivated nanocrystals, where under-coordinated surface atoms exist due to the steric hindrance between passivating ligands attached to the nanocrystal surface.