Silicon nanowire trace chemical detector

Trace chemical detection with parts per trillion sensitivity in this nanotechnology-based sensor offers the possibility for real-time chemical agent identification and tracking

Military Technology Sensors

Perforated top electrode and nanowire array underneath

Accurate, reliable standoff sensing of trace vapors in complex environments, such as explosives associated with improvised explosives devices (IEDs), is both a critical need and a significant challenge. The problem is akin to that of locating the proverbial needle in a haystack: trace explosives vapors, present at concentrations several orders of magnitude below their saturated vapor concentration, must be sensed in environments containing other non-target vapors present at many orders of magnitude higher concentration.

A host of technologies have been proposed and deployed for chemical agent detection, but each suffers from deficiencies. Laser measurements are difficult in the gas phase due to low vapor pressures associated with analytes of interest. Chemiresistors and micro-cantilevers have been used as trace explosives sensors, but selectivity remains a problem. More complex approaches such as commercial portable ion mobility spectrometers (IMS) can detect trace explosives solid particulates on swabs, but for such analytes such as TNT and RDX, preconcentration is required before delivery to the IMS.

To address the shortcomings of current trace chemical sensing devices, Navy researchers have developed a microfabricated sensor based on nanowires providing a field deployable, gas or liquid phase chemical detection instrument. Known as Silicon Nanowires in a Vertical Array with a Porous Electrode, or SiN-VAPOR, the technology is comprised of arrays of vertical nanowires with a top electrode with an array of holes. The holes in the top contact layer allow molecules to flow rapidly through it and come in contact with the sensing nanowire region underneath. The resistance of a p-type silicon nanowires increase (decrease) upon exposure to an electron donating (withdrawing) molecule, such as ammonia (nitrogen dioxide) thus generating a conductivity response.

The nanowires may include a chemically selective surface, such that not all compounds, or possibly only one compound, will adsorb onto the nanowires. The support is a silicon wafer or another electrode.

The sensor can be used as a preconcentrator for detection and partial separation of trace vapors. The nanowire arrays 1) serve as high surface area adsorptive substrates for trace vapor adsorption in a noncontact/standoff mode of operation, and 2) enable rapid and controlled Joule heating profiles that provide unique thermal desorption spectra for component analysis by portable multichannel detectors (i.e., mass spectrometer or ion mobility spectrometer).

The nanowire sensor is selective and sensitive, miniature, low power, fast, economical, simple-to-use, and capable of detecting a wide range of analytes in complex environments such as a battlefield or an airport. The initial application of this technology is in explosives detection, but extensions may include applications in sensing food spoilage or in the detection of biological agents.

SiN-VAPOR is protected by US patent 9,422,158 and US patent application 20180237294. The ‘158 patent describes the general structure including the nanowires, support, and electrodes.

The ‘294 application describes the method of heating the electrodes to desorb the analyte and thus using the device as a preconcentrator and front end for mass spectroscopy, ion mobility, and other instrumentation. It further covers the method of chemically activating the nanowires to create a selective surface.

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