Doping--the intentional introduction of impurities into a material--is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7-9), but not into CdSe (ref. 12)--despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to 'self-purification', an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption--and therefore doping efficiency--is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals--for applications from solar cells to spintronics--can be anticipated.

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First principle, tight binding, and semi-empirical embedded atom calculations are used to investigate a tetragonal phase transformation in gold nanowires. As wire diameter is decreased, tight binding and modified embedded atom simulations predict a surface-stress-induced phase transformation from a face-centered-cubic (fcc) ⟨100⟩ nanowire into a body-centered-tetragonal (bct) nanowire. In bulk gold, all theoretical approaches predict a local energy minimum at the bct phase, but tight binding and first principle calculations predict elastic instability of the bulk bct phase. The predicted existence of the stable bct phase in the nanowires is thus attributed to constraint from surface stresses. The results demonstrate that surface stresses are theoretically capable of inducing phase transformation and subsequent phase stability in nanometer scale metallic wires under appropriate conditions.

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We simulate the optical fields and optical transmission through nanoarrays of silica rings embedded in thin gold films using the finite-difference-time-domain method. By examining the optical transmission spectra for varying ring geometries we uncover large enhancements in the transmission at wavelengths much longer than the usual cutoffs for cylindrical apertures or where the usual planar surface plasmons or other periodic effects from the array could play a role. We attribute these enhancements to closely coupled cylindrical surface plasmons on the inner and outer surfaces of the rings, and this coupling is more efficient as the inner and outer ring radii approach each other. We confirm this hypothesis by comparing the transmission peaks of the simulation with cylindrical surface plasmon ͑CSP͒ dispersion curves calculated for the geometries of interest. One important result is that a transmission peak appears in the simulations close to the frequency where the longitudinal wave number k z in the ring satisfies k z = m / L, where m is an integer and L the length of the aperture, for a normal CSP TE 1 or TM 1 mode. The behavior of the CSP dispersion is such that propagating modes can be sent through the rings for ever longer wavelengths as the ring radii approach, whereas the transmission decreases only in proportion to the ring area.

We use density functional theory ͑DFT͒ and the tight-binding ͑TB͒ method to study the relaxation of narrow Cu, Ni, Au, Pt, and Ag nanowires originally oriented in the ͗001͘ direction with a fcc structure. For a small enough diameter ͑d Ͻ 2 nm͒ each nanowire, under the compressive influence of its own surface stress, spontaneously relaxes to either a ͗110͘ orientation ͑Cu, Ni, Ag͒ or to a bct ͗001͘ orientation ͑Au, Pt͒, both of which are characterized by a compression of the wire axis of at least 30%. To analyze the stability of bct structures, we calculate the elastic constants for the bct phases of these metals under bulk, slab, and nanowire conditions. DFT predicts that only the bct phase in Pt is stable with respect to shear distortions in both the bulk and in nanowires. We find that the surface contribution to the elastic constant for shear, C 66 , helps stabilize the bct phase in Au which would otherwise be unstable under bulk conditions. A large stabilization contribution from the surface also occurs in Ni and Cu, but not enough to overcome the shear instability in the bulk, and these nanowires do not transform to bct, although Cu is nearly stable for very narrow nanowires of width ϳ1 nm. We discuss the interplay of surface and edge effects in the phase change or reorientation of these nanowires and implications of these results on pseudoelasticity or shape memory in fcc metals.