Exciton antennas and concentrators from core-shell and corrugated carbon nanotube filaments of homogeneous composition.

There has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core-shell structures with radial gradients in the optical bandgap. We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-order Auger process. Core-shell structures consisting of annular shells of mostly (6,5) SWNTs (E(g)=1.21 eV) and cores with bandgaps smaller than those of the shell (E(g)=1.17 eV (7,5)-0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet-near-infrared wavelength regime provides quasi-singular photoemission at the (8,7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Preferential destruction of metallic single-walled carbon nanotubes by laser irradiation.

Upon laser irradiation in air, metallic single-walled carbon nanotubes (SWNTs) in carbon nanotube thin film can be destroyed in preference to their semiconducting counterparts when the wavelength and power intensity of the irradiation are appropriate and the carbon nanotubes are not heavily bundled. Our method takes advantage of these two species' different rates of photolysis-assisted oxidation, creating the possibility of defining the semiconducting portions of carbon nanotube (CNT) networks using optical lithography, particularly when constructing all-CNT FETs (without metal electrodes) in the future.

Relative optical absorption of metallic and semiconducting single-walled carbon nanotubes.

While it is well-known that tube-tube interaction causes changes (peak red-shift and suppression) in the optical absorption of single-walled carbon nanotubes (SWNTs), we found in this work that, upon bundling, the optical absorption of metallic SWNTs (M11) is less affected compared to their semiconducting counterparts (S11 or S22), resulting in enhanced absorbance ratio of metallic and semiconducting SWNTs (A(M)/A(S)). Annealing of the SWNTs increases this ratio due to the intensified tube-tube interaction. We have also found that the interaction between SWNTs and the surfactant Triton X-405 has a similar effect. The evaluation of SWNT separation by types (metallic or semiconducting) based on the optical absorption should take these effects into account.

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. Here we use Scanning Tunneling Spectroscopy (STS) to investigate, in real space, sub-bandgap states in individual ligand-free PbS nanocrystals. In the majority of studied PbS nanocrystals, spatial mapping of electronic density of states with STS shows atomic-scale variations attributable to the presence of surface reconstructions. STS spectra show that the presence of surface reconstructions results in formation of surface-bound sub-bandgap electronic states. The nature of the surface reconstruction varies depending on the surface stoichiometry, with lead-rich surfaces producing unoccupied sub-bandgap states, and sulfur-rich areas producing occupied sub-bandgap states. Highly off-stoichiometric areas produce both occupied and unoccupied states showing dramatically reduced bandgaps. Different reconstruction patterns associated with specific crystallographic directions are also found for different nanocrystals. This study provides insight into the mechanisms of sub-bandgap state formation that, in a modified form, are likely to be applicable to ligand-passivated nanocrystal surfaces, where steric hindrance between ligands can result in under-coordination of surface atoms.

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. Delocalized quantum-confined states and localized sub-bandgap states are identified, for the first time, via spatial mapping. Maps of the sub-bandgap states show localization on nonstoichiometric adatoms self-assembled on the nanocrystal surfaces. The present 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.