QuantumDots: APrimerINTRO DUCTION C r y stallin e in organic so lid s can be divided electronically into three well-known classes: m etals, semiconductors, and insulators. In these extended solids, atomic orbitals overlap to give nearly continuous electronic energy levels known as bands.1 M etals are electronically characterized by having a partially lled band; semiconductors have a lled band (the valence band) separated from the (mostly) empty conduction band by a bandgap E g , corresponding to the familiar HOMO-LUM O energy gap for small m olecules. Insulators are conceptually the sam e as sem ico nd uctors in their electronic structure, except that the bandgap is larger in insulators (Fig. 1). In terms of E g s, m etals have E g less than ;0.1 eV; semiconductors have E g s from ;0.5 to ;3.5 eV; and insulators have E g . ;4 eV. (1 eV 5 1.602 3 10 2 19 J 5 8065.5 cm 2 1 ). There are some key differences, how ever, b etw een the electronic structure of m olecules and solid-state materials such as semiconductors.

We have developed a method of semiconductor nanostructure fabrication relying on the size and shape of a polynucleotide to dictate the overall structure of an assembly of individual nanoparticles. This is exemplified by our use of the 3455-basepair circular plasmid DNA molecule pUCLeu4 which, when anchored to a suitably derivatized substrate, yields an array of semiconductor nanoparticles matching the shape of the biopolymer stabilizer. The viability of the methodology was confirmed using data from high resolution transmission electron microscopy, selected area electron diffraction, and linear optical absorption spectroscopy. This is a unique demonstration of the self-assembly of mesoscale semiconductor nanostructures using biological macromolecules as templates.

This work describes the formation of porous composite materials based on a combination of bioactive mesoporous silicon and bioerodible polymers such as poly-caprolactone (PCL). The fabrication of a range of composites prepared by both salt leaching and microemulsion techniques are discussed. Particular attention to the influence of Si content in the composite on in vitro calcification assays are assessed. For each system, cytotoxicity and cellular proliferation are explicitly evaluated through fibroblast cell culture assays.

Silicon nanowires (Si NWs), one-dimensional single crystalline, have recently drawn extensive attention, thanks to their robust applications in electrical and optical devices as well as in the strengthening of diamond/SiC superhard composites. Here, we conducted high-pressure synchrotron diffraction experiments in a diamond anvil cell to study phase transitions and compressibility of Si NWs. Our results revealed that the onset pressure for the Si I-II transformation in Si NWs is approximately 2.0 GPa lower than previously determined values for bulk Si, a trend that is consistent with the analysis of misfit in strain energy. The bulk modulus of Si-I NWs derived from the pressure-volume measurements is 123 GPa, which is comparable to that of Si-V NWs but 25% larger than the reported values for bulk silicon. The reduced compressibility in Si NWs indicates that the unique wire-like structure in nanoscale plays vital roles in the elastic behavior of condensed matter.