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.

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.

Correlated Raman microscopy and transmission electron microscopy were used to study the ordering of {111} planar defects in individual silicon nanowires. Detailed electron diffraction and polarization-dependent Raman analysis of individual nanowires enabled assessments of the stacking fault distribution, which varied from random to periodic, with the latter giving rise to local domains of 2H and 9R polytypes rather than the 3C diamond cubic structure. Some controversies and inconsistencies concerning earlier reports of polytypes in Si nanowires were resolved.

Scanning photocurrent microscopy (SPCM) is used in semiconductor nanowire devices to establish quantitative potential profiles correlated with nonuniformities in electrical resistivity. Surface doping leads to a nonuniform axial photocurrent (a). Surface etching improves the uniformity of the local photocurrent (b) and reduces the radial and axial carrier concentration gradients (c, blue curve after etching).

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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.

The authors report the use of a radio frequency silicon nanowire mechanical oscillator as a lowtemperature nuclear magnetic resonance force sensor to detect the statistical polarization of 1 H spins in polystyrene. In order to couple the 1 H spins to the nanowire oscillator, a magnetic resonance force detection protocol was developed which utilizes a nanoscale current-carrying wire to produce large timedependent magnetic field gradients as well as the rf magnetic field. Under operating conditions, the nanowire experienced negligible surface-induced dissipation and exhibited an ultralow force noise near the thermal limit of the oscillator.

We describe the displacement detection of freestanding silicon [111] nanowires by fiber-optic interferometry. We observe approximately a 50-fold enhancement in the scattered intensity for nanowires 40–60nm in diameter for incident light polarized parallel to the nanowire axis, as compared to perpendicular polarization. This enhancement enables us to achieve a displacement sensitivity of 0.5pm∕Hz for 15μW of light incident on the nanowire. The nanowires exhibit ultralow mechanical dissipation in the range of (2×10−15)–(2×10−14)kg∕s and could be used as mechanical sensors for ultrasensitive scanning probe force measurements.