Ultrafast optical wide field microscopy.

We have developed a new imaging method, ultrafast optical wide field microscopy, capable of rapidly acquiring wide field images of nearly any sample in a non-contact manner with high spatial and temporal resolution. Time-resolved images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon nanowire after optical excitation are captured using a two-dimensional smart pixel array detector. These images represent the time-dependent carrier dynamics with high sensitivity, femtosecond time resolution and sub-micrometer spatial resolution.

Electrical spin injection and detection in silicon nanowires through oxide tunnel barriers.

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 µm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.

Adaptable silicon-carbon nanocables sandwiched between reduced graphene oxide sheets as lithium ion battery anodes.

Silicon has been touted as one of the most promising anode materials for next generation lithium ion batteries. Yet, how to build energetic silicon-based electrode architectures by addressing the structural and interfacial stability issues facing silicon anodes still remains a big challenge. Here, we develop a novel kind of self-supporting binder-free silicon-based anodes via the encapsulation of silicon nanowires (SiNWs) with dual adaptable apparels (overlapped graphene (G) sheaths and reduced graphene oxide (RGO) overcoats). In the resulted architecture (namely, SiNW@G@RGO), the overlapped graphene sheets, as adaptable but sealed sheaths, prevent the direct exposure of encapsulated silicon to the electrolyte and enable the structural and interfacial stabilization of silicon nanowires. Meanwhile, the flexible and conductive RGO overcoats accommodate the volume change of embedded SiNW@G nanocables and thus maintain the structural and electrical integrity of the SiNW@G@RGO. As a result, the SiNW@G@RGO electrodes exhibit high reversible specific capacity of 1600 mAh g⁻¹ at 2.1 A g⁻¹, 80% capacity retention after 100 cycles, and superior rate capability (500 mAh g⁻¹ at 8.4 A g⁻¹) on the basis of the total electrode weight.

Axial SiGe heteronanowire tunneling field-effect transistors.

We present silicon-compatible trigated p-Ge/i-Si/n-Si axial heteronanowire tunneling field-effect transistors (TFETs), where on-state tunneling occurs in the Ge drain section, while off-state leakage is dominated by the Si junction in the source. Our TFETs have high I(ON) ~ 2 µA/µm, fully suppressed ambipolarity, and a subthreshold slope SS ~ 140 mV/decade over 4 decades of current with lowest SS ~ 50 mV/decade. Device operation in the tunneling mode is confirmed by three-dimensional TCAD simulation. Interestingly, in addition to the TFET mode, our devices work as standard nanowire FETs with a good I(ON)/I(OFF) ratio when the source-drain junction is forward-biased. The improved transport in both biasing modes confirms the benefits of utilizing bandgap engineered axial nanowires for enhancing device performance.

Mapping carrier diffusion in single silicon core-shell nanowires with ultrafast optical microscopy.

Recent success in the fabrication of axial and radial core-shell heterostructures, composed of one or more layers with different properties, on semiconductor nanowires (NWs) has enabled greater control of NW-based device operation for various applications. (1-3) However, further progress toward significant performance enhancements in a given application is hindered by the limited knowledge of carrier dynamics in these structures. In particular, the strong influence of interfaces between different layers in NWs on transport makes it especially important to understand carrier dynamics in these quasi-one-dimensional systems. Here, we use ultrafast optical microscopy (4) to directly examine carrier relaxation and diffusion in single silicon core-only and Si/SiO(2) core-shell NWs with high temporal and spatial resolution in a noncontact manner. This enables us to reveal strong coherent phonon oscillations and experimentally map electron and hole diffusion currents in individual semiconductor NWs for the first time.

Highly efficient charge separation and collection across in situ doped axial VLS-grown Si nanowire p-n junctions.

VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p-n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of ~50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of ~1.0 µm for electrons and 0.66 µm for holes for as-grown Si nanowires (d(NW) ≈ 65-80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passivation achieved in our as-grown nanowires is comparable to or better than that achieved for nanowires in prior studies at significantly larger diameters. We suggest that the dramatically improved surface recombination velocities may result from the reduced sidewall reactions and deposition in our cold wall CVD reactor.

Are nanoporous materials radiation resistant?

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKA's energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.

Controlling heterojunction abruptness in VLS-grown semiconductor nanowires via in situ catalyst alloying.

For advanced device applications, increasing the compositional abruptness of axial heterostructured and modulation doped nanowires is critical for optimizing performance. For nanowires grown from metal catalysts, the transition region width is dictated by the solute solubility within the catalyst. For example, as a result of the relatively high solubility of Si and Ge in liquid Au for vapor-liquid-solid (VLS) grown nanowires, the transition region width between an axial Si-Ge heterojunction is typically on the order of the nanowire diameter. When the solute solubility in the catalyst is lowered, the heterojunction width can be made sharper. Here we show for the first time the systematic increase in interface sharpness between axial Ge-Si heterojunction nanowires grown by the VLS growth method using a Au-Ga alloy catalyst. Through in situ tailoring of the catalyst composition using trimethylgallium, the Ge-Si heterojunction width is systematically controlled by tuning the semiconductor solubility within a metal Au-Ga alloy catalyst. The present approach of alloying to control solute solubilities in the liquid catalyst may be extended to increasing the sharpness of axial dopant profiles, for example, in Si-Ge pn-heterojunction nanowires which is important for such applications as nanowire tunnel field effect transistors or in Si pn-junction nanowires.

Direct observation of nanoscale size effects in Ge semiconductor nanowire growth.

Progress in the synthesis of semiconductor nanowires (NWs) has prompted intensive inquiry into understanding the science of their growth mechanisms and ultimately the technological applications they promise. We present new results for the size-dependent growth kinetics of Ge NWs and correlate the results with a direct experimental measurement of the Gibbs-Thomson effect, a measured increase in the Ge solute concentration in liquid Au-Ge droplets with decreasing diameter. This nanoscale-dependent effect emerges in vapor-liquid-solid Ge NW growth and leads to a decrease in the NW growth rate for smaller diameter NWs under a wide range of growth conditions with a cutoff in growth at sufficiently small sizes. These effects are described quantitatively by an analytical model based on the Gibbs-Thomson effect. A comprehensive treatment is provided and shown to be consistent with experiment for the effect of NW growth time, temperature, pressure, and doping on the supersaturation of Ge in Au, which determines the growth rate and critical cutoff diameter for NW growth. These results support the universal applicability of the Gibbs-Thomson effect to sub-100 nm diameter semiconductor NW growth.

Integration of nanowire devices in out-of-plane geometry.

We report the fabrication of arrays of single and multiple out-of-plane nanowire devices on a single substrate, an important step for the fabrication of novel three-dimensional devices and the integration of individually addressable nanowires onto current Si planar technology platforms. Vertical nanowire device fabrication can greatly increase device densities; however integrating such devices into arrays with registry to the substrate requires precise control over the number and position of the nanowires. Here we report the directed assembly of gold nanoparticle seeds into patterned arrays for the growth of nanowires using chemical recognition and electrophoretic methods. Chemical recognition provides highly reproducible control of the position and number of nanoparticles per pattern element and is shown to be in good agreement with a simple electrostatic model. Individually addressed out-of-plane, vapor-liquid-solid grown Ge nanowires with single and multiple nanowires per element are fabricated and electrically characterized.

Epitaxy of Ge nanowires grown from biotemplated Au nanoparticle catalysts.

Semiconductor nanowires (NWs) are being actively investigated due to their unique functional properties which result from their quasi-one-dimensional structure. However, control over the crystallographic growth direction, diameter, location, and morphology of high-density NWs is essential to achieve the desirable properties and to integrate these NWs into miniaturized devices. This article presents evidence for the suitability of a biological templated catalyst approach to achieve high-density, epitaxial growth of NWs via the vapor-liquid-solid (VLS) mechanism. Bacterial surface-layer protein lattices from Deinococcus radiodurans were adsorbed onto germanium substrates of (111), (110), and (100) crystallographic orientations and used to template gold nanoparticles (AuNPs) of different diameters. Orientation-controlled growth of GeNWs was achieved from very small size (5-20 nm) biotemplated AuNP catalysts on all of the substrates studied. Biotemplated GeNWs exhibited improved morphologies, higher densities (NW/microm(2)), and more uniform length as compared to GeNWs grown from nontemplated AuNPs on the substrate surfaces. The results offer an integrated overview of the interplay of parameters such as catalyst size, catalyst density, substrate crystallographic orientation, and the presence of the protein template in determining the morphology and growth direction of GeNWs. A comparison between templated and nontemplated growth provides additional insight into the mechanism of VLS growth of biotemplated NWs.

Diameter-dependent electronic transport properties of Au-catalyst/Ge-nanowire Schottky diodes.

We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

Vertical growth of Ge nanowires from biotemplated Au nanoparticle catalysts.

Semiconductor nanowires are being actively investigated because of their unique physical properties and potential applications in nanoelectronics and optoelectronic devices. However, current hurdles for device integration include the lack of control over the orientation, location, and packing density of nanowires. This communication presents for the first time the use of a unique, bottom-up approach for the catalyzed growth of semiconductor nanowires via biological templating. High-density, vertically oriented growth of Ge nanowires with monodispersed diameters and spacings was achieved through patterning of very small sized (5-20 nm) Au nanoparticles using bacterial surface-layer proteins as a template. We envision the applicability of this biotemplating approach to a variety of nanowires and substrate materials.

Ultrafast electron and hole dynamics in germanium nanowires.

We present the first ultrafast time-resolved optical measurements, to the best of our knowledge, on ensembles of germanium nanowires. Vertically aligned germanium nanowires with mean diameters of 18 and 30 nm are grown on (111) silicon substrates through chemical vapor deposition. We optically inject electron-hole pairs into the nanowires and exploit the indirect band structure of germanium to separately probe electron and hole dynamics with femtosecond time resolution. We find that the lifetime of both electrons and holes decreases with decreasing nanowire diameter, demonstrating that surface effects dominate carrier relaxation in semiconductor nanowires.

Photon control of liquid motion on reversibly photoresponsive surfaces.

The movement of a liquid droplet on a flat surface functionalized with a photochromic azobenzene may be driven by the irradiation of spatially distinct areas of the drop with different UV and visible light fluxes to create a gradient in the surface tension. In order to better understand and control this phenomenon, we have measured the wetting characteristics of these surfaces for a variety of liquids after UV and visible light irradiation. The results are used to approximate the components of the azobenzene surface energy under UV and visible light using the van Oss-Chaudhury-Good equation. These components, in combination with liquid parameters, allow one to estimate the strength of the surface interaction as given by the advancing contact angle for various liquids. The azobenzene monolayers were formed on smooth air-oxidized Si surfaces through 3-aminopropylmethyldiethoxysilane linkages. The experimental advancing and receding contact angles were determined following azobenzene photoisomerization under visible and ultraviolet (UV) light. Reversible light-induced advancing contact-angle changes ranging from 8 to 16 degrees were observed. A large reversible change in contact angle by photoswitching of 12.4 degrees was achieved for water. The millimeter-scale transport of 5 microL droplets of certain liquids was achieved by creating a spatial gradient in visible/UV light across the droplets. A criterion for light-induced motion of droplets is shown to be consistent with the response of a variety of liquids. The type of light-driven fluid movement observed could have applications in microfluidic devices.

Evaporative properties and pinning strength of laser-ablated, hydrophilic sites on lotus-leaf-like, nanostructured surfaces.

Wetting, evaporative, and pinning strength properties of hydrophilic sites on superhydrophobic, nanostructured surfaces were examined. Understanding these properties is important for surface characterization and designing features in self-cleaning, lotus-leaf-like surfaces. Laser-ablated, hydrophilic spots between 250 mum and 2 mm in diameter were prepared on silicon nanowire (NW) superhydrophobic surfaces. For larger circumference pinning sites, initial contact angle measurements resemble the contact angle of the surface within the pinning site: 65-69 degrees . As the drop volume is increased, the contact angles approach the contact angle of the NW surface without pinning sites: 171-176 degrees . The behavior of water droplets on the pinning sites is governed by how much of the water droplet is being influenced by the superhydrophobic NW surfaces versus the hydrophilic areas. During the evaporation of sinapic acid solution, drops are pinned by the spots except for the smaller circumference sites. Pinning strengths of the hydrophilic sites are a linear function of the pinning spot circumference. Protein samples prepared and deposited on the pinning sites for analysis by matrix-assisted laser desorption ionization indicate an improvement in sensitivity from that of a standard plate analysis by a factor of 5.

Strain mapping in nanowires.

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

Tailored surface modification by ion implantation and laser treatment.

An important trend in materials science is the use of increasingly sophisticated methods to control composition and microstructure during processing. Near-surface modification by ion implantation and laser treatment is one of these new methods for tailoring material properties. Novel materials have been formed which are far from thermodynamic equilibrium and which exhibit unexpected and useful properties. The most extensively studied property changes include modified electrical properties of semiconductors and improved wear, hardness, and corrosion resistance of metals. The high degree of control available with energetic beams allows relations between microstructure and properties to be systematically investigated at the atomic level. This article illustrates how ion and laser beam modification is being applied to advance both the technology and the exploratory science of materials.