Martin N. Wybourne

Acoustic phonon modes of rectangular quantum wires

Acoustic phonon modes of a free-standing rectangular quantum wire composed of cubic crystals are theoretically investigated using an algorithm developed to analyse data from resonant ultrasound spectroscopy. The normal phonon modes are classified according to their spatial symmetries into a compressional mode termed the dilatational mode and non-compressional modes referred to as the flexural, torsional, and shear modes. The formalism that we present is quite general and can be applied to wires of any cubic material. As an example, the dispersion relations are obtained for square and rectangular wires of GaAs, taking into account the anisotropic elasticity of the material. The dispersion curves for a rectangular wire are compared with those of the approximate hybrid modes referred to as the thickness and width modes, and the validity of the modes is discussed. The existence of edge modes is confirmed by examining the spatial distribution of displacement vectors.

INTERFERENCE PHENOMENA DUE TO A DOUBLE BEND IN A QUANTUM WIRE

Narrow channel devices were fabricated using a split‐gate high electron mobility transistor structure in which electrons are forced through a double‐bend discontinuity. The low‐temperature conductance shows a number of peaks in the lowest quantized conductance plateau which correspond qualitatively to resonance effects that are predicted for the geometrical discontinuities of the double bend.

Defect-Tolerant Single-Electron Charging at Room Temperature in Metal Nanoparticle Decorated Biopolymers**

The last three decades have seen a dramatic decrease in the size of microelectronic devices, with the number of devices on a microchip doubling about every eighteen months. As device feature sizes shrink towards quantum scales, this evolution is facing serious technical, fundamental, and economic challenges. To address these issues a number of revolutionary departures from the conventional semiconductor device paradigm are currently being investigated. In particular, nanostructures, in which single electron tunneling and charging effects can be exploited for device applications, (e.g., in quantum cellular automata and organic thin-film transistors) are receiving much attention. The effects of single-electron charging on electron transport in lithographically-defined nanostructures is well-documented. In general, nanostructures at the limits of electron-beam lithography (ca. 15 nm) are too large to achieve clear singleelectron effects at room temperature. Alternative approaches to achieve these effects at room temperature include silicon nanocrystal floating gate devices and focused ion beam deposited structures. Another involves assemblies of metal or semiconductor nanoparticles that have well-defined dimensions down to the molecular scale. At these sizes the inherent capacitance of the system is small enough that single-electron effects are manifest at room temperature. An added advantage is that the chemical nature of these building blocks makes possible the parallel chemical assembly of particle arrays from individual particles with precisely tuned physical properties. In order to utilize nanoparticle building blocks, or even to explore their electrical properties, it is important to be able to assemble and make electrical contact to them. Considerable progress has been made toward the rational assembly of extended nanoparticle arrays, and transport measurements perpendicular to the plane of the arrays have been performed using scanning probe microscopy and thin film electrode sandwich arrangements. Measurements that probe lateral transport (in the plane of the array) in two-dimensional films and patterned arrays such as lines are important for designing planar device applications. Given the finite yield of the chemical assembly process, defects will exist in these arrays and might be expected to adversely influence the transport properties. Although the importance of defect tolerance in chemically-assembled nanoscale electronic circuits has been discussed in the context of other systems, the potential of near room temperature experiments on patterned nanoparticle arrays to probe the defectand disorder-sensitivity remains largely unexplored. Here we present the room-temperature electrical behavior of gold nanoparticles assembled on a biopolymer template deposited between metal electrodes on an insulating substrate. The assemblies were prepared using a straightforward procedure that involves only wet chemical methods. Unambiguous single-electron charging effects are observed that can be understood in terms of the nanoparticle properties and the geometrical constraints imposed by the biopolymer. These results support the idea of using nanoparticles in conjunction with biomolecular organization to achieve nanoscale systems with novel, defect-tolerant current±voltage behavior. Networks of gold nanoparticles were fabricated between the fingers of gold interdigitated array (IDA) electrodes (15 or 2 lm gap) by electrostatic assembly of carboxylic acid modified gold nanoparticles onto the amino side chains of the biopolymer poly-L-lysine (PLL). A thin film of PLL (MW = 54 000 amu) is initially deposited from aqueous methanol containing the alpha-helical form of its hydrobromide salt. The exposed side chains of the dried film were subsequently deprotonated by soaking in dilute base. The 11-mercaptoundecanoic acid-stabilized gold nanoparticles were assembled onto the biopolymer from an organic solvent. The metal-core radius was determined to be 0.7 ± 0.2 nm (±30 %) by transmission electron microscopy (TEM), and the diameter of the core and ligand shell together is estimated to be 4.2 nm. The average length of an extended PLL chain is ca. 30 nm. Current±voltage (I±V) measurements were performed at room temperature with the samples in an electrically shielded vacuum chamber. Control measurements were made on the bare electrodes and again after the PLL had been deposited and deprotonated. The I±V characteristics of the deprotonated PLL and the bare surface were linear (ohmic) without any structure, as shown by curve I in Figure 1a. Importantly, these two sets of control data were indistinguishable, which shows that to within experimental uncertainty the surface conductance of the glass substrate was unaffected by the deprotonated PLL. In contrast, when decorated with nanoparticles, the samples exhibited pronounced nonlinear I±V characteristics, as shown by curve II in Figure 1a. After subtraction of the linear I±V behavior measured before PLL decoration, to within the measurement accuracy the electrical characteristics showed a region of zero conductance at low voltages. The onset of current is characterized by a threshold voltage, VT,

Accessibility of quantum effects in mesomechanical systems

We explore the quantum aspects of an elastic bar supported at both ends and subject to compression. If strain rather than stress is held fixed, the system remains stable beyond the buckling instability, supporting two potential minima. The classical equilibrium transverse displacement is analogous to a Ginsburg-Landau order parameter, with strain playing the role of temperature. We calculate the quantum fluctuations about the classical value as a function of strain. Excitation energies and quantum fluctuation amplitudes are compared for silicon beams and carbon nanotubes.

Quantum squeezing of mechanical motion for micron-sized cantilevers

Abstract We show that substantial quantum squeezing of mechanical motion can be achieved for micron-sized cantilever devices using available techniques.

TRANSPORT IN GOLD CLUSTER STRUCTURES DEFINED BY ELECTRON-BEAM LITHOGRAPHY

The near-room temperature current-voltage (I-V) characteristics of small structures made from the metal-cluster material Au55[P(C6H5)3]12Cl6 were studied. It is shown that these electron-beam defined structures have highly nonlinear characteristics with features, including a threshold voltage and scaling behavior, which are consistent with Coulomb charging of individual Au55 cores in a disordered array. Applied radio frequency signals introduce plateaus in the I-V characteristics, which demonstrates the presence of coherent tunneling in these cluster systems.

Elastic instability of nanomechanical beams

We report on elastic instability of nanomechanical SiO2 beams with widths 20 nm<d<110 nm and lengths 5 μm<L<10 μm. The beams are fabricated from a silicon substrate with a 500 nm thermal oxide layer. After release from the silicon substrate by reactive ion etching the beams buckle due to the residual Si/SiO2 strain. The measured buckling displacements of the beams are compared with the predictions of nonlinear continuum elasticity theory. We observe a continuous buckling transition, qualitatively different than the critical transition predicted by Euler buckling theory, which we attribute to system asymmetry. Finally, we determine the effective potential energy of the fundamental buckling mode.

Room-temperature Coulomb-blockade-dominated transport in gold nanocluster structures

In this paper we discuss the near-room-temperature electrical transport characteristics of structures made from ligand-stabilized metal clusters. The structures show threshold behaviour, nonlinear current-voltage characteristics and radio-frequency-induced plateaux consistent with Coulomb-blockade-dominated transport in disordered arrays of metal dots. Samples having triphenylphosphine and octadecanethiol ligand shells are found to have a 3 orders of magnitude difference in current above threshold. We discuss a possible explanation for this observation.

Coulomb-blockade dominated transport in patterned gold-cluster structures

In this paper we present the fabrication and near-room temperature electrical transport properties of structures made from the gold-cluster material Au55[P(C6H5)3]12Cl6. We discuss the use of electron-beam lithography to define the structures laterally and compare the direct current-voltage characteristics of non-patterned and patterned structures. In both cases non-linear behavior is observed with features that are consistent with Coulomb blockade dominated transport in disordered arrays of clusters. Radio frequency induced plateaus in the current-voltage characteristics demonstrate coherent tunneling. Finally, we show that other ligand stabilized gold-cluster materials can be used to form ordered gold-cluster arrays.

Fabrication and near-room temperature transport of patterned gold cluster structures

Ligand stabilized metal clusters are becoming of considerable interest for possible nanoscale electronics applications. In this article, we report the fabrication and near-room temperature electrical transport properties of structures made from the gold-cluster material Au55[P(C6H5)3]12Cl6. While other strategies to produce cluster arrays have been reported, this work is the first to use electron-beam lithography to laterally define the structures. We compare the current–voltage characteristics of nonpatterned and patterned structures, and show that in both cases the nonlinear behavior observed is consistent with Coulomb blockade dominated transport. We argue that charging of individual Au55 cores is responsible for the effects observed.