P. A. Bennett

Ledge-flow-controlled catalyst interface dynamics during Si nanowire growth

Self-assembled nanowires offer the prospect of accurate and scalable device engineering at an atomistic scale for applications in electronics, photonics and biology. However, deterministic nanowire growth and the control of dopant profiles and heterostructures are limited by an incomplete understanding of the role of commonly used catalysts and specifically of their interface dynamics. Although catalytic chemical vapour deposition of nanowires below the eutectic temperature has been demonstrated in many semiconductor-catalyst systems, growth from solid catalysts is still disputed and the overall mechanism is largely unresolved. Here, we present a video-rate environmental transmission electron microscopy study of Si nanowire formation from Pd silicide crystals under disilane exposure. A Si crystal nucleus forms by phase separation, as observed for the liquid Au-Si system, which we use as a comparative benchmark. The dominant coherent Pd silicide/Si growth interface subsequently advances by lateral propagation of ledges, driven by catalytic dissociation of disilane and coupled Pd and Si diffusion. Our results establish an atomistic framework for nanowire assembly from solid catalysts, relevant also to their contact formation.

Local ionic and electron heating in single-molecule junctions

A basic aim in molecular electronics is to understand transport through a single molecule connected to two electrodes. Substantial progress towards this goal has been made over the past decade as a result of advances in both experimental techniques and theoretical methods. Nonetheless, a fundamental and technologically important issue, current-induced local heating of molecules, has received much less attention. Here, we report on a combined experimental and theoretical study of local heating in single molecules (6-, 8- and 10-alkanedithiol) covalently attached to two gold electrodes as a function of applied bias and molecular length. We find that the effective local temperature of the molecular junction first increases with applied bias, and then decreases after reaching a maximum. At fixed bias, the effective temperature decreases with increasing molecular length. These experimental findings are in agreement with hydrodynamic predictions, which include both electron-phonon and electron-electron interactions.

Smart bridges, smart tunnels: Transforming wireless sensor networks from research prototypes into robust engineering infrastructure

We instrumented large civil engineering infrastructure items, such as bridges and tunnels, with sensors that monitor their operational performance and deterioration. In so doing we discovered that commercial offerings of wireless sensor networks (WSNs) are still geared towards research prototypes and are currently not yet mature for deployment in practical scenarios. We distill the experience gained during this 3-year interdisciplinary project into specific advice for researchers and developers. We discuss problems and solutions in a variety of areas including sensor hardware, radio propagation, node deployment, system security and data visualization. We also point out the problems that are still open and that the community needs to address to enable widespread adoption of WSNs outside the research lab.

Classical and quantum transport in focused-ion-beam-deposited Pt nanointerconnects

We study the electrical properties of Pt nanointerconnects, formed on SiO2 substrates by focused-ion-beam deposition. Studies of their temperature-dependent resistivity reveal a small residual-resistivity ratio, and a Debye temperature that differs significantly from that of pure Pt, indicative of the disordered nature of the nanowires. Their magnetoresistance shows evidence for weak antilocalization at temperatures below 10 K, with a phase-breaking length of ∼100 nm, and a temperature dependence suggestive of quasi-one-dimensional interference.

Magnetic iron silicide nanowires on Si(110)

Self-assembled iron silicide nanowires were formed by depositing 1ML of Fe onto Si(110) at 700°C in ultrahigh vacuum. The nanowires have average dimensions of 5nm high ×10nm wide ×μm long, as measured with ex situ atomic force microscopy. High-resolution electron microscopy identifies the crystal structure as cubic FeSi2 with orientation FeSi2(1¯11)∕∕Si(11¯1), FeSi2⟨110⟩∕∕Si⟨110⟩. Magnetometer measurements show a magnetic moment of 0.3Bohr magneton per iron atom at 2K. This magnetic property in metastable cubic FeSi2 nanowires opens up the possibility for high-density data storage and logic applications.

In situ resistance measurements of epitaxial cobalt silicide nanowires on Si(110)

We have performed in situ resistance measurements for individual epitaxial CoSi2 nanowires (NWs) (approximately 60 nm wide and 5μm long) formed on a Si(110) surface. Two- and four-point probe measurements were done with a multitip scanning tunneling microscope at room temperature. The NWs were well isolated from the substrate by a Schottky barrier with zero-bias resistance of 107Ω. The resistivity of the NWs was 30μΩcm, which is similar to that for high-quality epitaxial films. The NW resistance was essentially unchanged after exposure to air.

Dysprosium silicide nanowires on Si(110)

Dysprosium deposited on Si(110) at 720 °C is observed to form self-assembled silicide nanowire (NW) structures with a single orientation and average dimensions of 15 nm wide and microns long. The NW sides grow into the substrate along inclined Si{111} planes, forming a V-shaped cross section with an interface that is coherent on one side, described by DySi2(0001)//Si(111_) and DySi2[011_0]//Si[1_10], and incoherent on the other. This type of growth represents a physical mechanism for self-assembled NW formation that does not require anisotropic lattice mismatch.

Epitaxial titanium silicide islands and nanowires

The growth of titanium silicide islands formed by reactive deposition of Ti on Si(1 1 1) at T � 850 C has been studied using atomic force microscopy and transmission electron microscopy. The predominant shape is very long and narrow, and can be considered to be a nanowire (NW). Other flat-topped structures coexist with the NWs, including small equilateral triangles and large rectangular plates. Most NWs are oriented along Sih 220 i directions, with typical dimensions 20 nm wide, 10 nm high and several microns long. A minority of NWs are oriented along Sih 224 i. These latter tend to break up into chains of small segments with regular size and spacing. Growth at lower temperature or higher deposition rate results in smaller and more numerous NWs. Length appears to be limited by intersection with other NWs oriented 120 apart. The junction between NWs appears to be incoherent in most cases. The triangular islands are positively identified as fully relaxed C54 TiSi2, while the chains are relaxed C49 TiSi2. The dominant NW structure is incommensurate and is tentatively identified as C49 TiSi2. 2002 Elsevier Science B.V. All rights reserved.

Scanning tunneling microscopy studies of nucleation and growth in a reactive, epitaxial system: Co/Si(111)

We present scanning tunneling microscopy observations of the reaction of cobalt with Si(111)‐(7×7). For deposition at 320 °C (reactive epitaxy), flat‐topped monolayer islands of triangular shape with vertices along 〈112〉Si nucleate on the faulted side of the 7×7 structure then grow in size attaining edge lengths that are quantized to integer multiples of the 7×7 unit cell. A 2×2 reconstruction with large corrugation occurs on some of the islands, and is believed to be an ordered array of silicon adatoms. The shape and orientation of the islands appears to be determined by energetics, not growth kinetics. They coexist with the 7×7 structure, implying that metal atoms readily diffuse through the silicon matrix before attaching to an existing island. From the areal density of islands we estimate an activation energy for ‘‘surface diffusion’’ of 0.8 eV, and argue that the process involves metal atom transport through near‐surface silicon interstitials. At lower temperatures, no ordered silicide forms, while...

Structure and orientation of epitaxial titanium silicide nanowires determined by electron microdiffraction

The crystal structure and epitaxial orientation of self-assembled titanium silicide nanowires (NWs) on Si (111) is determined using transmission electron microdiffraction. The NWs are formed by deposition of ∼1 monolayer Ti on Si(111) at ∼850 °C. Type 1 NWs are oriented with long axis along Si〈2-20〉 and are identified as C49 TiSi2. The most common orientation is C49 [01-3] || Si [112] and C49 (200) || Si (2-20), but several other orientations are also found. Type 2 NWs are oriented with long axis along Si〈224〉 and are identified as B27 TiSi, with orientation B27 [02-1] || Si [111] and B27 (-312) || Si (22-4) + 4°. Most of the NWs are incommensurate and fully strain relaxed. They generally extend below the surface with inclined incoherent interfaces.