Roman Jimenez-Diaz

Ultralow power consumption gas sensors based on self-heated individual nanowires

Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

On the photoconduction properties of low resistivity TiO2 nanotubes

TiO(2) nanotubes were synthesized by anodic oxidation of titanium foils using dimethyl sulfoxide and hydrofluoric acid as the electrolyte. The electrical properties of individual nanotube-based devices were evaluated and modeled after exposing some of them to different gas and illumination conditions. Resistivity values fully comparable to those of TiO(2) single crystal anatase (ρ(SA) = 1.09 ± 0.01Ω cm) were found, and their photoconductive characteristics, explained in terms of the Shockley-Read-Hall model for non-radiative recombination in semiconductors, were found to be strongly influenced by the applied experimental conditions such as the surrounding atmosphere. These devices may have potential applications in photocatalytic processes, such as CO(2) reduction or H(2)O splitting, avoiding the interfering effects typical of nanotube arrays.

Localized growth and in situ integration of nanowires for device applications

Simultaneous localized growth and device integration of inorganic nanostructures on heated micromembranes is demonstrated for single crystalline germanium and tin oxide nanowires. Fully operating CO gas sensors prove the potential of the presented approach. With this simple CMOS compatible technique, issues of assembly, transfer and contact formation are addressed.

Direct observation of the gas-surface interaction kinetics in nanowires through pulsed self-heating assisted conductometric measurements

Dynamics of gas-surface interactions determine the limits of the fastest response times of sensors based on metal oxides. Here, the kinetics of adsorption and desorption of gaseous molecules onto the surface of metal oxide nanowires was analyzed through pulsed self-heating assisted conductometric measurements. This approach overcomes gas diffusion, which is typical of conventional porous film based devices, and provides thermal response times fast enough to evaluate the fundamental gas-surface reactions kinetics. Experimental response and recovery times of individual SnO2 nanowires toward oxidizing and reducing gases obtained with the here-proposed methodology were related to the reaction barriers predicted by theoretical models and other experimental techniques.

An experimental method to estimate the temperature of individual nanowires

In this paper, the authors present an effective experimental method to estimate the temperature of individual metal oxide nanowires that can be used to quantify the heating produced in conductometric or other operating conditions. The here-proposed method is based on the analysis of the recovery time of the nanowires resistance after exposure to a gas pulse (0.5 ppm of NO2 in dry air). It is reproducible with different devices always with uncertainties below ±20°C in the temperature range (70-300°C) studied herein. The exploration of alternative gases and nanolithography techniques may help to extend its operating range and its applicability to other materials. In any case, the opportunity to probe temperatures at the nanoscale opens the door to a number of fundamental and applied advancements in the field of nanotechnology.

Individual nanowire chemical sensor system self-powered with energy scavenging technologies

A fully autonomous chemical gas sensor system is presented. This system is based on the exploitation of dissipated power at individual nanowires by Joule effect due to the bias current applied in conductometric measurements (self-heating), which enables heating the tiny mass of these wires up to the optimum temperatures for gas sensing applications. This novel approach only requires few miliwatts to bias, heat and measure the sensors. We also demonstrate that the low-power requirements of these devices can be supplied by state-of-the-art energy scavenging technologies, like thermoelectric microgenerators. For all this, the here-presented system is an important step forward toward fully autonomous and distributed gas sensor networks without the need of battery replacement.

Photoexcited Individual Nanowires: Key Elements in Room Temperature Detection of Oxidizing Gases

Illuminating metal oxide semiconductors with ultra‐violet light is a feasible alternative to activate chemical reactions at their surface and thus, using them as gas sensors without the necessity of heating them. Here, the response at room temperature of individual single‐crystalline SnO2 nanowires towards NO2 is studied in detail. The results reveal that similar responses to those obtained with thermally activated sensors can be achieved by choosing the optimal illumination conditions. This finding paves the way to the development of conductometric gas sensors operated at room temperature. The power consumption in these devices is in range with conventional micromachined sensors.

P2.7.10 Individual Metal Oxide Nanowire contacted onto MEMS substrates for Monitoring of Toxic Species

Metal oxide nanowires are used for their integration in gas nanosensors. These semiconducting nanowires are deposited onto suspended sensing platforms and are electrically contacted (a single nanowire in each substrate) by means of electron beam induced deposition inside a FIB dual beam system. The platforms are equipped with a buried integrated resistor that permits their heating, allowing to tune the reaction of the gas species with the surface of the nanowire, thus controlling the response. These low power consumption nanosensor prototypes exhibit good responses to toxic gases, like CO, NOx and O3, making them suitable for environmental monitoring.

UV photosensors based on individual semiconductor nanowires

We present a functional photo sensing device based on individual metal oxide nanowires with a cost-effective interfacing electronics for computer controlled operation. The photoconductive gain and the dynamic response of the device were improved by optimizing the layout and depositing surface passivation coatings. Photoconductive gains of 6.10 4 and response times below 5s were obtained with the optimized prototypes.