739 publications from this institution
Current research into semiconductor clusters is focused on the properties of quantum dots—fragments of semiconductor consisting of hundreds to many thousands of atoms—with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.
Research capabilities in nanoscience, molecular biology and computation have advanced to the point where it is possible to define research activities in which the development of nano-bio systems will support major DOE science goals. Specifically, we identify two major long term research goals which can motivate research at the intersection of nanoscience and biology: 1) Development of biological-systems-control for bioremediation, carbon dioxide sequestration and tailored biomaterials fabrication. 2) Development of artificial nanosystems with biomimetic functionality but without biological fragility. Basic research in support of these goals can be focused by identifying immediate research challenges involving the integration of physical nanostructures and biological nanostructures (i.e. proteins, with a strong emphasis on membrane-bound proteins) in a program of closely correlated theoretical and experimental research.
In recent years, techniques have been developed to prepare highly monodisperse and crystalline particles of many common semiconductors in the nanometer size regime. These particles, Consisting of a hundred to tens of thousands of atoms, exhibit electronic spectra that evolve in interesting ways with size. Recent experiments on electric field modulation of absorption and emission, resonance Raman scattering and photon echoes will be presented, with a view towards presenting a unified picture of the electronic spectra in these materials.
Nanostructures constructed from metal and semiconductor nanocrystals conjugated to and organized by DNA are an emerging class of materials with collective optical properties. We created discrete pyramids of DNA with gold nanocrystals at the tips. By taking small-angle X-ray scattering measurements from solutions of these pyramids, we confirmed that this pyramidal geometry creates structures which are more rigid in solution than linear DNA. We then took advantage of the tetrahedral symmetry to demonstrate construction of chiral nanostructures.
Transient electric birefringence measurements were performed on dilute solutions of CdSe nanorods. The results confirm the existence of a permanent dipole along the c-crystallographic axis. Measurements on nanorods with different widths and lengths show that the longitudinal permanent dipole moment scales linearly with volume, suggesting it arise from the noncentrosymmetric crystallographic lattice.
We have used synchrotron radiation photoemission to probe the valence and core level electronic structure of compound-semiconductor monodisperse clusters (nanocrystals). These clusters exhibited a 10% or less variation relative to the mean diameter and were attached to the metal substrates via alkane chains. Direct evidence of gap broadening due to size variation in CdS clusters was observed. The novel utilization of alkane chain attachment is the key to eliminating the otherwise debilitating problem of sample charging, as occurs with powders. The quality of sample preparation was confirmed by other methods such as transmission electron microscopy, Raman scattering, and x-ray diffraction. This work provides a direct link between photoemission studies of expitaxial ultrathin films of compound semiconductors, the photon-spectroscopy measurements of cluster powders and the existing theories of quantum confinement in reduced dimensionality structures.
We present two examples of the use of liquid cells to study colloidal inorganic nanocrystals using <italic>in situ</italic> transmission electron microscopy. The first uses a liquid cell to quantify the interaction potential between pairs of colloidal nanocrystals, and the second demonstrates direct imaging of nanocrystal growth and structure in the liquid cell.
A simple approach to obtain end-to-end assemblies of nanorods over macroscopic distances in thin films is described. Nanorods with aspect ratio of 8-12 can be aligned parallel to the surface in an end-to-end fashion by imposing geometric confinement via block copolymer-based supramolecular assemblies. Successful control over the orientation and location of nanorods requires a balance of particle-particle interactions and entropy associated with geometric confinement from the supramolecular framework, as well as consideration of the kinetics of assembly.
