The past 20 years have witnessed simultaneous multidisciplinary explosions in experimental techniques for synthesizing new materials, measuring and manipulating nanoscale structures, understanding biological processes at the nanoscale, and carrying out large-scale computations of many-atom and complex macromolecular systems. These advances have led to the new disciplines of nanoscience and nanoengineering. For reasons that are discussed here, most nanoparticles do not 'self-assemble' into their thermodynamically lowest energy state, and require an input of energy or external forces to 'direct' them into particular structures or assemblies. We discuss why and how a combination of self- and directed-assembly processes, involving interparticle and externally applied forces, can be applied to produce desired nanostructured materials.

One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.Carrier multiplication is the process in which the absorption of a single, high-energy photon results in the generation of two or more electron-hole pairs. The excess energy of the initially excited electron is used to excite a second electron over the bandgap, rather than being converted into heat through sequential phonon emission. Carrier multiplication is important for the operation for high-speed electronic devices 4 , but is especially relevant for solar cells 1 , because relaxation of hot carriers through phonon emission is a common loss mechanism in bulk semiconductor solar cells. In this context, semiconductor quantum dots are promising building blocks for future solar cells 1 . In addition to the size-tunability of the quantum-dot optical properties, the carrier-multiplication efficiency in quantum dots was reported to be much higher than in bulk materials, where the process is generally referred to as impact ionization. It has been argued that carrier multiplication is more efficient in nanostructured semiconductors owing to quantum-confinement effects causing (1) a slowing of the phonon-mediated relaxation channel 1 and (2) enhanced Coulomb interactions 2 , resulting from forced overlap between wavefunctions and reduced dielectric screening at the quantum-dot surface 3 . In recent years, several femtosecond spectroscopy studies have revealed highly efficient carrier multiplication in PbSe and PbS (refs 2, 5-9) (refs 18, 19) quantum dots. In initial studies, carrier-multiplication efficiencies may have been overestimated owing to several experimental complications, including too high excitation fluences (generating multiple carriers by sequential absorption of multiple photons), lack of stirring of quantum-dot suspensions (causing photo-induced charging) and sample-to-sample variability 19 . Furthermore, recent tight-binding calculations 20 suggest carrier multiplication in quantum dots is not only not enhanced relative to bulk, but is actually lower. Answering the...

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We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

Polydiacetylene (PDA) Langmuir films (LFs) were investigated directly at the air/water interface using in situ synchrotron grazing incidence X-ray diffraction, and ex situ transmissison electron microscopy and diffraction. The films were compressed and polymerized on pure water. A crystallographic model describes the structures and phase transitions of the unpolymerized (monomer) film, via the metastable (blue phase), to the fully stable PDA red phase as a function of irradiation dose. The monomer-to-blue-to-red chromatic phase transitions are accompanied by changes in the in-plane crystal structure and pendant chains packing arrangement from arced alkyl chains (in the monomer and blue phases) to near-vertical closely packed chains in the red phase. Notably, the characteristic linear strand morphology of PDA films can be explained as a direct result of the marked decrease in spacing between adjacent polymer chains upon transition from the blue to the red phase.

We report on the synthesis of the newly discovered cubic phase of tin monosulfide π-SnS and compare its properties to the well-known phase of tin monosulfide, α-SnS.

Size-controlled ZnSe nanoparticles with high extents of atomic positional order are shown to exhibit large size-dependent variations in their local electronic environments. Solid-state ;{77}Se and ;{67}Zn NMR spectra reveal increasingly broad distributions of ;{77}Se and ;{67}Zn environments with decreasing nanoparticle sizes, in contrast with high degrees of atomic positional order established by transmission electron microscopy and x-ray diffraction. First-principles calculations of NMR parameters distinguish between atomic positional and electronic disorder that propagate from the nanoparticle surfaces and yield insights on the order and disorder present.

Ultranarrow (1.8 nm) PbS nanowires are synthesized in a single step, under benchtop conditions at relatively low temperature (90 degrees C). The nanowires exhibit a nearly perfect crystal lattice, high width uniformity, and tight side-by-side registry. Two-dimensional (2D) assembly over large areas (>15 microm2) is achieved using the Langmuir Blodgett method. The wire width can be readily controlled in the range 1.8-10 nm by a surface pressure-induced coalescence reaction, as monitored by transmission electron microscopy and Raman spectroscopy. The fluorescence of the 2D assembly shows strong polarization dependence along the long axis of the wires, making the system potentially suitable for orientation-sensitive devices.