Invention: The Naval Research Laboratory (NRL) has developed a gas sensor composed of an array of vertical nanowires topped by a porous electrode. The sensor responds when a substance of interest absorbs on the nanowires, changing their electrical conductivity. The combination of a vertical orientation and a porous top electrode allows for simultaneous exposure and response from huge numbers of individual nanowires. Because of these attributes, the sensor provides a very high signal/noise ratio and short response time. The sensor responds quickly and reliably to trace levels of NO2, NH3, and DNT, even in high humidity. Individual sensor arrays can be differently functionalized and used together in ensembles to achieve chemical selectivity for different substances in complex mixtures. Sub-centimeter scale is easily achieved, enabling portability even when using multiple sensor arrays. This innovative gas sensor architecture reduces noise and improves batch-to-batch uniformity with a simple, easy fabrication procedure. Experiments on sensing in liquids are pending, but expected to yield similar improvements in performance.
Background: Many types of nanowires, and other nanometer-scale structures of similar dimensions, have been at the heart of a large research effort aimed at studying their unique properties and integrating them into novel devices. For example, many different types of sensors have been fabricated from either single or an array of silicon nanowires to take advantage of the favorable physical, chemical, electrical, and optical properties of nanowires. For many device applications, such as gas sensors, a vertical nanowire orientation is ideal since it maximizes the surface area of nanowires that comes in contact with the environment, while also minimizing the deleterious effects of substrate oxides and other surface chemistry. These deleterious effects include trapping/detrapping of charge carriers, nonselective adsorption of other molecules on the substrate, and steric denial of part of the nanowire’s surface to interact with the target molecule. Furthermore, a large number of such nanowires in an array improves device performance by reducing 1/f noise and other noise types sensitive to the number of carriers. Finally, the vertical nanowire configuration simplifies the fabrication of core-shell nanowire structures, which separate the current generation and collection functions in a photochemical, photogalvanic or radioactivity detector. The main challenge in creating a sensor type device based on vertical nanowire arrays lies in making individual electrical connections to all the nanowires. The few existing approaches have involved embedding the entire nanowire array in some type of a sacrificial material, exposing the tips of the nanowires, and depositing the desired top contact electrode layer. In these cases, the nanowire sensing region is exposed upon removal of the sacrificial material, and the substrate itself serves as the bottom electrode. Methods based on the deposition of a porous gold nanoparticle film on top of the nanowire array and the random gap-bridging of nanowires during growth have also been investigated. In all these approaches, a non-ordered array of vertical nanowires was used as the main sensing element. More importantly, none of these methods are able to create a porous top contact electrode layer with holes of controllable size and distribution.
Advantages: Scalable production technique, capable of parallel batch production, produced with widely available tools, short sampling and regeneration times, reliable responses (even in humidity), CMOS compatible, tailorable to analyte.
Applications: Chemical diagnostics, Bio-diagnostics, Biomedical, Radiation detection, Energy harvesting.
Technology Status: Office of Naval Research Funded Program TRL level of 4
Licensing or CRADA Opportunity: US published patent is available for license to companies with commercial interest. Collaborative research and development is available under a Cooperative Research and Development Agreement (CRADA).
Lead Inventor: Christopher Field and Pehr Pehrsson
Patents: US Published Patent Application US 20120119760
- “Decoupling Diameter and Pitch in Silicon Nanowire Arrays Made by Nanosphere Lithography and Metal-Assisted Chemical Etching.” Adv. Funct. Mater. 24 (2014): 106-116.
- “Periodically Porous Top Electrodes on Vertical Nanowire Arrays for Highly Sensitive Gas Detection.” Nanotechnology 22 (2011): 355501-355506.
- “Vapor Detection Performance of Vertically Aligned, Ordered Arrays of Silicon nanowires with a Porous Electrode.” Analytical Chemistry. 83 (2011): 4724-4728.
Navy Case Number: 100,845; related case - 101,595
Read more about this technology here.
- "Vapor Detection Performance of Veritcally Aligned, Ordered Arrays of Silicon Nanowies with a Porous Electrode." Analytical Chemistry 83 (2011) 4724-4728.
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