Diversity of sub-bandgap states in lead-sulfide nanocrystals: real-space spectroscopy and mapping at the atomic-scale.

Colloidal semiconductor nanocrystals have emerged as a promising class of technological materials with optoelectronic properties controllable through quantum-confinement effects. Despite recent successes in this field, an important factor that remains difficult to control is the impact of the nanocrystal surface structure on the photophysics and electron transport in nanocrystal-based materials. In particular, the presence of surface defects and irregularities can result in the formation of localized sub-bandgap states that can dramatically affect the dynamics of charge carriers and electronic excitations.

Spatial Mapping of Sub-Bandgap States Induced by Local Nonstoichiometry in Individual Lead Sulfide Nanocrystals.

The properties of photovoltaic devices based on colloidal nanocrystals are strongly affected by localized sub-bandgap states associated with surface imperfections. A correlation between their properties and the atomic-scale structure of chemical imperfections responsible for their appearance must be established to understand the nature of such surface states. Scanning tunneling spectroscopy is used to visualize the manifold of electronic states in annealed ligand-free lead sulfide nanocrystals supported on the Au(111) surface.

Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots.

Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogeneous 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 mainfold resulting from the localization-enhanced electron-vibrational coupling.

High-stability cryogenic scanning tunneling microscope based on a closed-cycle cryostat.

We report on the design and operation of a cryogenic ultra-high vacuum (UHV) scanning tunneling microscope (STM) coupled to a closed-cycle cryostat (CCC). The STM is thermally linked to the CCC through helium exchange gas confined inside a volume enclosed by highly flexible rubber bellows. The STM is thus mechanically decoupled from the CCC, which results in a significant reduction of the mechanical noise transferred from the CCC to the STM.