Chris Berven

Principles and mechanisms of gas sensing by GaN nanowires functionalized with gold nanoparticles

Electrical properties of a chemical sensor constructed from mats of GaN nanowires decorated with gold nanoparticles as a function of exposure to Ar, N2, and methane are presented. The Au nanoparticle decorated nanowires exhibited chemically selective electrical responses. The sensor exhibits a nominal response to Ar and slightly greater response for N2. Upon exposure to methane the conductivity is suppressed by 50% relative to vacuum. The effect is fully reversible and is independent of exposure history. We offer a model by which the change in the current is caused by a change in the depletion depth of the nanowires, the change in the depletion depth being due to an adsorbate induced change in the potential on the gold nanoparticles on the surface of the nanowires.

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,

Gas Sensing With Mats of Gold-Nanoparticle Decorated GaN Nanowires

We report on the use of mats of gold nanoparticle decorated GaN nanowires as gas sensors. The sensing was by the repeated and reversible measurement of changes in the current-voltage characteristics of the mat of nanowires. The nanowires had diameters of about 200 nm and were many microns long. The mat was grown on a 1-cm diameter sapphire disk and was about 10 thick. The selectivity mechanism is attributed to the details of the surface morphology of the gold nanoparticles decorating the surface of the nanowires. The changes in the currents are attributed to a depletion mechanism in the nanowires due to the formation of a Schottky barrier due to the presence of the gold on the inherently n-type GaN. We were able to detect CO, CH4, CO2, H2, and observed possible evidence of creation of the by-products of the water-gas shift reaction.

Interaction of hybrid nanowire–nanoparticle structures with carbon monoxide

A gas-phase sensor based on a GaN nanowire mat decorated with Au nanoparticles was studied both experimentally and theoretically. The sensor is responsive to CO and H(2) and could be used to study the water-gas-shift reaction, which involves combining CO and H(2)O to produce H(2). It was shown that for catalyzing this reaction using support Au nanoparticles, the sequence in which the reactants are exposed to the catalyst surface is critical. To quantitatively evaluate the sensor response to gas exposure a depletion model was developed that considered the Au nanoparticle-semiconductor interface as a nano-Schottky barrier where variation in the depletion region caused changes in the electrical conductivity of the nanowires.

Background charge fluctuations and the transport properties of biopolymer-gold nanoparticle complexes

The room temperature electrical characteristics of biopolymer-gold nanoparticle complexes show threshold behavior, periodic conductance features, and current–voltage scaling that together indicate the nonlinear transport is associated with single electron charging. Repeated measurements over a period of up to 80 h showed the characteristics change with time. The current–voltage scaling behavior is found to be time independent, while the position of the conductance features shifted randomly over periods of many hours. We show that the time dependence is consistent with a fluctuating background charge distribution and can be understood within the framework of the orthodox model of single electron transport that is modified to account for the relatively large self-capacitance of the nanoparticles.

Towards practicable sensors using one-dimensional nanostructures

Nanomaterials, including nanoparticles, nanowires, nanotubes etc., are the object of much well deserving attention by researchers and the public alike. Because of their novel properties associated with their typically large surface to volume ratios and finite- or quantum-size effects, they offer an avenue of exploration for new and interesting physics, chemistry, biology and materials science. Before it is possible to take advantage of these materials, an understanding of their fundamental properties is needed. Even with an understanding of these properties, in order to create practicable devices, the details of how changes in these fundamental properties (e.g., band-structures) manifest themselves as changes in practically measurable properties (e.g., the current-voltage characteristics) is needed. This review article will examine some recent work that focused on these issues. The first topic is the use of mats of gold-nanoparticle-decorated GaN nanowires as a gas sensor. The second and third developed the theory of using carbon nanotubes as elements of real-world sensors for ions and magnetic fields.

Effect of self-capacitance on the tunneling thresholds in linear arrays of nanoparticles

We consider the electron transport through gated one-dimensional chains of ligand stabilized metal nanoparticles. In such systems the self-capacitance of the core can be larger than the interparticle capacitance. In this regime we show that the self-capacitance cannot be neglected and must be included in the calculation of the free energy differences that govern the tunneling thresholds. We demonstrate the consequence of the self-capacitance on the tunneling thresholds of a single nanoparticle device and a chain of six nanoparticles.

Negative differential conductance observed in a lateral double constriction device

Lateral double point contact devices were fabricated using a split‐gate high electron mobility transistor. The low‐temperature source‐drain characteristics show pronounced S‐shaped negative differential conductance that can be independently controlled by an applied gate bias. The mechanism for the observed switching behavior is believed to be similar to that proposed for heterostructure hot electron diodes.

Nanoparticle based boolean logic

Abstract Arrays of passivated gold nanoparticles have been shown to exhibit single electron transport behavior at room temperature. The ratio of the self-capacitance to the interparticle capacitance is very different to that reported for single electron devices fabricated using traditional methods. This ratio suggests that novel device behavior may be possible in organized arrays of nanoparticles. We report on simulations of single electron logic circuits designed using parameters expected for circuits made using passivated gold nanoparticles. We will show AND- and NAND-gate behavior from an electron-pump gated one-dimensional array of nanoparticles.

Nonequilibrium random telegraph switching in quantum point contacts

Abstract We have investigated nonequilibrium transport through quantum point contact structures in high mobility GaAs/AlGaAs heterostructure. For low source-drain bias the current-voltage characteristics show the expected conductance quantization. At biases above approximately 6 mV, conductance instabilities in the DC current-voltage characteristics are observed which depend on the thermal and light exposure history of the sample. Time-dependent measurements in the regions of instability reveal that random telegraph switching (RTS) between well-defined differential conductance states is occurring. The RTS has been studied as a function of source-drain and gate bias, as well as temperature. The average time in the low and high states is found to depend exponentially on the source-drain and gate bias around some critical bias point. This critical point appears to correspond to a transition when an extra quasi one-dimensional subband crosses the Fermi level. The origin of the switching is believed to be associated with the charging and discharging of shallow donor defects due to DX centers in the AlGaAs.