Electrostatic modes of atomic force microscopy have shown to be non-destructive and relatively simple methods for imaging conductors embedded in insulating polymers. Here we use electrostatic force microscopy to image the dispersion of carbon nanotubes in a latex-based conductive composite, which brings forth features not observed in previously studied systems employing linear polymer films. A fixed-potential model of the probe-nanotube electrostatics is presented which in principle gives access to the conductive nanoparticle's depth and radius, and the polymer film dielectric constant. Comparing this model to the data results in nanotube depths that appear to be slightly above the film-air interface. This result suggests that water-mediated charge build-up at the film-air interface may be the source of electrostatic phase contrast in ambient conditions.
This paper describes measurements being made on a series of material systems for the purpose of developing a radiative-transfer model that describes the reflectance of light by granular solids. It is well recognized that the reflectance spectra of granular materials depend on their intrinsic (n(λ) and k(λ)) and extrinsic (morphological) properties. There is, however, a lack of robust and proven models to relate spectra to these parameters. The described work is being conducted in parallel with a modeling effort1 to address this need. Each follows a common developmental spiral in which material properties are varied and the ability of the model to calculate the effects of the changes are tested. The parameters being varied include particle size/shape, packing density, material birefringence, optical thickness, and spectral contribution of a substrate. It is expected that the outcome of this work will be useful in interpreting reflectance data for hyperspectral imaging (HSI), and for a variety of other areas that rely on it.
The goal of this project was to expand upon previously demonstrated single carbon nanotube devices by preparing a more practical, multi-single-walled carbon nanotube (SWNT) device. As a late-start, proof-of-concept project, the work focused on the fabrication and testing of chromophore-functionalized aligned SWNT field effect transistors (SWNT-FET). Such devices have not yet been demonstrated. The advantages of fabricating aligned SWNT devices include increased device cross-section to improve sensitivity to light, elimination of increased electrical resistance at nanotube junctions in random mat devices, and the ability to model device responses. The project did not achieve the goal of fabricating and testing chromophore-modified SWNT arrays, but a new SWNT growth capability was established that will benefit future projects. Although the ultimate goal of fabricating and testing chromophore-modified SWNT arrays was not achieved, the work did lead to a new carbon nanotube growth capability at Sandia/CA. The synthesis of dense arrays of horizontally aligned SWNTs is a developing area of research with significant potential for new discoveries. In particular, the ability to prepare arrays of carbon nanotubes of specific electronic types (metallic or semiconducting) could yield new classes of nanoscale devices.
With the goal of studying the conversion of optical energy to electrical energy at the nanoscale, we developed and tested devices based on single-walled carbon nanotubes functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrated the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggested that upon photoabsorption, the chromophores isomerize to give a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations were used to study the chromophore-nanotube hybrids, and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments supported the notion of dipole changes as the optical detection mechanism.
Rapid identification of aerosolized biological agents following an alarm by particle triggering systems is needed to enable response actions that save lives and protect assets. Rapid identifiers must achieve species level specificity, as this is required to distinguish disease-causing organisms (e.g., Bacillus anthracis) from benign neighbors (e.g., Bacillus subtilis). We have developed a rapid (1-5 minute), novel identification methodology that sorts intact organisms from each other and particulates using capillary electrophoresis (CE), and detects using near-infrared (NIR) absorbance and scattering. We have successfully demonstrated CE resolution of Bacillus spores and vegetative bacteria at the species level. To achieve sufficient sensitivity for detection needs ({approx}10{sup 4} cfu/mL for bacteria), we have developed fiber-coupled cavity-enhanced absorbance techniques. Using this method, we have demonstrated {approx}two orders of magnitude greater sensitivity than published results for absorbing dyes, and single particle (spore) detection through primarily scattering effects. Results of the integrated CE-NIR system for spore detection are presented.
We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism.
Microsystems pose unparalleled opportunity in the realm of real-time sample analysis for multiple applications, including Homeland Security monitoring devices, environmental monitoring, and biomedical diagnostics. The need for a universal means of processing, separating, and delivering a sample within these devices is a critical need if these systems are to receive widespread implementation in the industry and government sectors. Efficient particle separation and enrichment techniques are critical for a range of analytical functions including pathogen detection, sample preparation, high-throughput particle sorting, and biomedical diagnostics. Previously, using insulator-based dielectrophoresis (iDEP) in microfluidic glass devices, we demonstrated simultaneous particle separation and concentration. As an alternative to glass, we evaluate the performance of similar iDEP structures produced in polymer-based microdevices and their enhancement through dynamic surface coatings. There are numerous processing and operational advantages that motivate our transition to polymers such as the availability of numerous innate chemical compositions for tailoring performance, mechanical robustness, economy of scale, and ease of thermoforming and mass manufacturing. The polymer chips we have evaluated are fabricated through an injection molding process of the commercially available cyclic olefin copolymer Zeonor{reg_sign}. We demonstrate that the polymer devices achieve the same performance metrics as glass devices. Additionally, we show that the nonionic block copolymer surfactant Pluronic F127 has a strong interaction with the cyclic olefin copolymer at very low concentrations, positively impacting performance by decreasing the magnitude of the applied electric field necessary to achieve particle trapping. The presence of these dynamic surface coatings, therefore, lowers the power required to operate such devices and minimizes Joule heating. The results of this study demonstrate that polymeric microfluidic devices with surfactant coatings for insulator-based dielectrophoresis provide an affordable engineering strategy for selective particle enrichment and sorting.
Power sources capable of supplying tens of watts are needed for a wide variety of applications including portable electronics, sensors, micro aerial vehicles, and mini-robotics systems. The utility of these devices is often limited by the energy and power density capabilities of batteries. A small combustion engine using liquid hydrocarbon fuel could potentially increase both power and energy density by an order of magnitude or more. This report describes initial development work on a meso-scale external combustion engine based on the Stirling cycle. Although other engine designs perform better at macro-scales, we believe the Stirling engine cycle is better suited to small-scale applications. The ideal Stirling cycle requires efficient heat transfer. Consequently, unlike other thermodynamic cycles, the high heat transfer rates that are inherent with miniature devices are an advantage for the Stirling cycle. Furthermore, since the Stirling engine uses external combustion, the combustor and engine can be scaled and optimized semi-independently. Continuous combustion minimizes issues with flame initiation and propagation. It also allows consideration of a variety of techniques to promote combustion that would be difficult in a miniature internal combustion engine. The project included design and fabrication of both the engine and the combustor. Two engine designs were developed. The first used a cylindrical piston design fabricated with conventional machining processes. The second design, based on the Wankel rotor geometry, was fabricated by through-mold electroforming of nickel in SU8 and LIGA micromolds. These technologies provided the requisite precision and tight tolerances needed for efficient micro-engine operation. Electroformed nickel is ideal for micro-engine applications because of its high strength and ductility. A rotary geometry was chosen because its planar geometry was more compatible with the fabrication process. SU8 lithography provided rapid prototypes to verify the design. A final high precision engine was created via LIGA. The micro-combustor was based on an excess enthalpy concept. Development of a micro-combustor included both modeling and experiments. We developed a suite of simulation tools both in support of the design of the prototype combustors, and to investigate more fundamental aspects of combustion at small scales. Issues of heat management and integration with the micro-scale Stirling engine were pursued using CFD simulations. We found that by choice of the operating conditions and channel dimensions energy conversion occurs by catalysis-dominated or catalysis-then-homogeneous phase combustion. The purpose of the experimental effort in micro-combustion was to study the feasibility and explore the design parameters of excess enthalpy combustors. The efforts were guided by the necessity for a practical device that could be implemented in a miniature power generator, or as a stand-alone device used for heat generation. Several devices were fabricated and successfully tested using methane as the fuel.