The threat of exposure to toxic chemicals is of great concern. In order to provide a chemical situational awareness, we are developing a new type of chemical sensor based on a novel fabric spectrometer-based colorimetric chemical sensor that is low size, weight, and power (SWaP). We are exploring the key design principles for photonic transducers to enable a new approach to chemical threat sensing. The fabric spectrometer is based on a functional fiber platform in which the semiconductor-containing fiber is miniature in two dimensions and extendable in the third dimension (along the fiber length). By exploring fibers, and films that can be scaled to a fiber geometry, we will enable a new fiber-based chemical threat detector that can serve in textiles as well as other interesting form factors.
Big Data processing tools have become increasingly powerful and have been applied to the area of personal chemical monitoring by companies such as Plume Labs and Rubix, requiring low-cost, capable sensors. The ubiquity of cell phone imagers has allowed for a revisiting of colorimetry as a viable chemical detection method. There has been a great deal of effort put into making colorimetric sensor arrays that can discriminate between a variety of analytes, but mainly in a qualitative sense with limited discussion regarding improving the performance of the sensor as a whole. However, these imaging devices have inherent limitations on their ultimate sensitivity. Other sensor configurations are being evaluated that can greatly enhance the sensitivity to a color change. Dye development continues in an effort to increase the specificity of the sensing event using currently available readout mechanisms, but what has been lacking has been a critical analysis of the readout mechanism for these molecular transducers. This work takes a quantitative approach to encourage a more rational design of colorimetric sensors with specific targets.
We are currently developing a novel fabric spectrometer-based colorimetric chemical sensor that is lightweight, sensitive, and person-borne. This research will enable a new class of chemical sensors with a much flexible form factor to open up a variety of other person-borne and distributed sensing applications.
Chemical warfare agents (CWAs) such as nerve and blister agents are expected to pose continuing and growing dangers for the Warfighter in the future. We investigate a novel chemical detection modality, based on a new platform for colorimetric detection of chemical threats incorporated in hollow fibers, which are miniature in two dimensions and extendable (“extrudable”) in the third dimension (along the fiber length). By exploring fibers, and films that can be scaled to a fiber geometry, we will enable a new fiber-based chemical threat detector that can serve in textiles worn by the Warfighter (e.g., uniform), as well as in non-worn textiles and an outlying fence or perimeter for early detection of a threat cloud near an expeditionary shelter, outpost, encampment, or base.
Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to
hikers, the military, first responders, and third-world countries lacking infrastructure for transporting
heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to
satisfying specifications for commercial and military use, but they still have limited absorption due to
insufficient material efficiency, and therefore are large and too heavy in many cases.
Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in
a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of
significant commercial and military value), in order to enhance absorption and efficiency of solar
blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic
active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al
nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a
predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized
arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the
near infrared region. This impressive absorption enhancement and predicted increase in short-circuit
current density may significantly increase the efficiency and reduce the weight of solar blankets,
enabling their use for commercial and military applications.
Nanoparticles and nanostructures with plasmonic resonances are currently being employed to enhance the efficiency of solar cells. 1-3 Ag stripe arrays have been shown theoretically to enhance the short-circuit current of thin silicon layers. 4 Monolayers of Ag nanoparticles with diameter d < 300 nm have shown strong plasmonic resonances when coated in thin polymer layers with thicknesses < d.5 We study experimentally the diffuse vs. specular scattering from monolayer arrays of Ag nanoparticles (spheres and prisms with diameters in the range 50 – 300 nm) coated onto the front side of thin (100 nm < t < 500 nm) silicon films deposited on glass and flexible polymer substrates, the latter originating in a roll-to-roll manufacturing process. Ag nanoparticles are held in place and aggregation is prevented with a polymer overcoat. We observe interesting wavelength shifts between maxima in specular and diffuse scattering that depend on particle size and shape, indicating that the nanoparticles substantially modify the scattering into the thin silicon film.
We have developed a processing method that employs direct surface imaging of a surface-modified silicon wafer to
define a chemical nanopattern that directs material assembly, eliminating most of the traditional processing steps.
Defining areas of high and low surface energy by selective alkylsiloxane removal that match the polymer period length
leads to defect-free grating structures of poly(styrene-block-methyl methacrylate) (PS-b-PMMA). We have performed
initial studies to extend this concept to other wavelengths beyond 157 nm. In this present paper, we will show that electron beam lithography can also be used to define chemical nanopatterns to direct the assembly of PS-b-PMMA films. Half-pitch patterns resulted in the directed assembly of PS-b-PMMA films. Electron beam lithography can also be used to prepare surfaces for pitch division. Instead of the deposition of an HSQ pinning structure as is currently done, we will show that by writing an asymmetric pattern, we can fill in the space with smaller lamellar period block copolymers to shrink the overall pitch and allow for 15-nm features.
We have developed a processing method that significantly reduces the number of steps necessary to yield a surface that directs block copolymer assembly. This methodology employs a single resistless lithography step that directly changes the surface energy without requiring subsequent material deposition or plasma etching steps. The lithographically defined difference in surface energies acts as a template to direct diblock polymer self-assembly into low-defect periodic structures. Our newly developed lithographically directed self-assembly technique can produce sub-45 nm half pitch lines employing poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and interference lithography. Once assembled into periodic lines of alternating materials, the PMMA block can be removed and the resulting polystyrene features can be used as an etch mask to transfer periodic lines-and-spaces into a silicon substrate.