The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 241-249 the infl uence of nano-particle content, density and size, as well as the infl uence from alloy-scattering and electronic doping effects. In this article, we describe the controlled synthesis and thermoelectric properties of fi ne and uniformly dispersed Ag 2 Te precipitates embedded in PbTe. In contrast to many previously studied PbTe-based systems, [ 5 , 10 , 11 , 13-16 ] the Ag 2 Te precipitates are much larger (50-200 nm), are not isostructural to PbTe and do not introduce considerable electronic doping effect to PbTe. We show that these precipitates scatter the phonons effectively, leading to a low lattice thermal conductivity which approaches the minimum expected of PbTe above 650 K. Moreover, doping with La independently optimizes the carrier concentration and results in a thermoelectric fi gure of merit of 1.6 in La-doped PbTe-Ag 2 Te composites at 775 K. This value is about twice that of the state-of-the-art n-type PbTe [ 1 , 26 ] and arises from the low lattice thermal conductivity at this temperature. (PbTe)1 MicrostructureThe pseudo-binary phase diagram of PbTe-Ag 2 Te (see Figure 1 ) [27][28][29] shows signifi cant and strongly temperature dependent solubility of Ag 2 Te in PbTe. Similar behavior in the PbTe-Sb 2 Te 3 phase diagram has been harnessed to yield Widmanstätten precipitates of Sb 2 Te 3 in a matrix of PbTe. [ 25 ] Here, we utilize the variance in maximum solubility of Ag 2 Te in PbTe, which is about 7-11 mol.% [ 27 , 29 ] at the eutectic temperature of ∼ 970 K and quickly drops to about 1 mol.% at ∼ 770 K. [ 27 ] From these features, one can expect that after melting (step 1 in Figure 1 ) and homogenizing the solid solution at ∼ 970 K (step 2 in Figure 1 ), Ag 2 Te precipitates will be obtained during a lower temperature anneal at ∼ 770 K (step 3 in Figure 1 ).Four compositions of (PbTe) 1 − x (Ag 2 Te) x are considered here ( x = 1.3, 2.7, 4.1, 5.5), all of which have compositions ( Table 1 ) greater than the solubility limit for Ag 2 Te at the annealing temperature (770 K). Following this thermal treatment, Ag-rich precipitates are observed to be homogeneously distributed in the PbTe matrix, as shown by fi eld emission scanning electron microscopy images ( Figure 2 a ). As the Ag 2 Te content increases, the Ag 2 Te is incorporated as a solid solution in the PbTe matrix [27][28][29] ). The open circle at 773 K shows the experimental Ag solubility in PbTe, [ 27 ] consistent with the current study. Starting with a homogeneous melt at a composition of Ag5.5 (point 1), the sample is quenched and then annealed within the single phase region (point 2) for homogenization. Phase separation is then achieved by annealing at 773 K (point 3). up to the solubility limit ( ∼ 1%) at the annealing temperature. [ 27 ] Beyond the solubility limit, the volume fraction of Ag 2 Te particles increases with increasing Ag 2 Te content in the mixture (Figure 2 ), but the Ag content in the PbTe matrix remains constant. The solubility limit is dir...

Semiconductor nanowires show promise for many device applications, but controlled doping with electronic and magnetic impurities remains an important challenge. Limitations on dopant incorporation have been identified in nanocrystals, raising concerns about the prospects for doping nanostructures. Progress has been hindered by the lack of a method to quantify the dopant distribution in single nanostructures. Recently, we showed that atom probe tomography can be used to determine the composition of isolated nanowires. Here, we report the first direct measurements of dopant concentrations in arbitrary regions of individual nanowires. We find that differences in precursor decomposition rates between the liquid catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core. We also present a thermodynamic model that relates liquid and solid compositions to dopant fluxes.

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We quantitatively examine the relative influence of bulk impurities and surface states on the electrical properties of Ge nanowires with and without phosphorus (P) doping. The unintentional impurity concentration in nominally undoped Ge nanowires is less than 2 x 10(17) cm(-3) as determined by atom probe tomography. Surprisingly, P doping of approximately 10(18) cm(-3) reduces the nanowire conductivity by 2 orders of magnitude. By modeling the contributions of dopants, impurities, and surface states, we confirm that the conductivity of nominally undoped Ge nanowires is mainly due to surface state induced hole accumulation rather than impurities introduced by catalyst. In P-doped nanowires, the surface states accept the electrons generated by the P dopants, reducing the conductivity and leading to ambipolar behavior. In contrast, intentional surface-doping results in a high conductivity and recovery of n-type characteristics.