Robert J Simonson

Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticle arrays

Optical lithography based on microfabrication techniques was employed to fabricate one-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensional nanogaps served as a platform on which organic/nanoparticle films were assembled. Characterization of the gaps was performed with high-resolution TEM, SEM, and electrical measurements. Novel self-assembling attachment chemistry, based on the interaction of silicon with a diazonium salt, was used to iteratively build a multi-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with an easily displaced ligand, a variable conductive path was created across the 1D nanogap. Electrical measurements of the gap showed a dramatic change in the I(V) characteristics after assembly of the nanoparticle film.

Novel one-dimensional nanogap created with standard optical lithography and evaporation procedures

This article details a simple four-step procedure to create a one-dimensional nanogap on a buried oxide substrate that relies on conventional photolithography performed on a stack of silicon/silicon oxide/silicon, metal evaporation, and hydrofluoric acid oxide removal. Once the nanogap was fabricated it was bridged with an assembly of 1,8-octanedithiol and 5 nm Au nanoparticles capped with a sacrificial dodecylamine coating. Before assembly, characterization of the nanogaps was performed through electrical measurements and SEM imaging. Post assembly, the resistance of the nanogaps was evaluated. The current increased from 70 fA to 200 microA at +1 V bias, clearly indicating a modification due to nanoparticle molecule assembly. Control experiments without nanoparticles or octanedithiol did not show an increase in current.

High-speed two-dimensional gas chromatography using microfabricated GC columns combined with nanoelectromechanical mass sensors

We report here for the first time the combination of microfabricated gas chromatography (GC) columns with pneumatic modulation to achieve high-speed comprehensive two-dimensional gas chromatography (GCxGC) using microfabricated components. The GCxGC system is in turn combined with nanoelectromechanical (NEMS) resonator mass sensors that have been coated with a chemically-selective polymer to enhance detection of phosphonate compounds that are useful surrogates for chemical warfare agents (CWA). GC elution peak widths on the order of 20 msec have been achieved. Retention times on the order of 2–4 seconds have been demonstrated for polar compounds, indicating that this microfabricated GCxGC system can be applied for rapid analyses.

Two-Dimensional Modeling and Simulation of Mass Transport in Microfabricated Preconcentrators

The adsorption and desorption behavior of a planar microfabricated preconcentrator (PC) has been modeled and simulated using the computational fluid dynamics (CFD) package CFDRC-ACE+trade. By comparison with the results of a designed experiment, model parameters were determined. Assuming a first-order reaction for the adsorption of a light hydrocarbon chemical analyte onto the PC adsorbent and a unity-value sticking coefficient, a rate constant of 36 500 s-1 was obtained. This compares favorably with the value of 25 300 s-1 obtained by application of the Modified-Wheeler equation. The modeled rate constant depends on the concentration of adsorbent sites, estimated to be 6.94 ldr 10-8 kmol/m2 for the Carboxen 1000 adsorbent used. Using the integral method, desorption was found to be first order with an Arrhenius temperature dependence and an activation energy of 30.1 kj/mol. Validation of this model is reported herein, including the use of Aris-Taylor dispersion to predict the influence of fluidics surrounding the PC. A maximum in desorption peak area with flow rate, predicted from a quadratic fit to the results of the designed experiment, was not observed in the 2-D simulation. Either approximations in the simulated model or the nonphysical nature of the quadratic fit are responsible. Despite the apparent simplicity of the model, the simulation is internally self consistent and capable of predicting performance of new device designs. To apply the method to other analytes and other adsorbent materials, only a limited number of comparisons to experiment are required to obtain the necessary rate constants.

Active MEMS Valves for Flow Control in a High-Pressure Micro-Gas-Analyzer

We present active electrostatic MEMS gas valves for Micro-Gas-Analyzer (MGA) flow control. These unique valves enable extremely low dead volume, highly integrated flow control chips for the MGA application, and potentially others (e.g., propulsion, pneumatic, and thermodynamic microsystems). We have demonstrated low leak rates ( <; 0.025 sccm, <; 0.0025 sccm on a similar passive valve design), high operating pressures 6.9×105 N/m2 (100 psig), a high-pressure record for valves of this size and type, and high flow rates (>; 25 sccm) using control voltages on the order of 100 V. The valve designs presented eliminate charge build-up issues associated with insulating materials and are closely tied to a base-lined microfabrication process (SUMMiT), allowing mass production. Using this process, which incorporates only CMOS compatible materials, eliminates outgassing and absorption problems inherent to microvalve designs that incorporate elastomers or organic bonding layers, and reduces contamination when the valve is part of the chemical analysis flowpath. The results obtained indicate that even higher performance level valves (>; 1.4 × 106 N/m2 or 200 psig operating pressure, at similar control voltage, flow rates, and leak rates) are possible.

Micro-analytical systems for national security applications

Sandia National Laboratories has a long tradition of technology development for national security applications. In recent years, significant effort has been focused on micro-analytical systems - handheld, miniature, or portable instruments built around microfabricated components. Many of these systems include microsensor concepts and target detection and analysis of chemical and biological agents. The ultimate development goal for these instruments is to produce fully integrated sensored microsystems. Described here are a few new components and systems being explored: (1) A new microcalibrator chip, consisting of a thermally labile solid matrix on an array of suspended-membrane microhotplates, that when actuated delivers controlled quantities of chemical vapors. (2) New chemical vapor detectors, based on a suspended-membrane micro-hotplate design, which are amenable to array configurations. (3) Micron-scale cylindrical ion traps, fabricated using a molded tungsten process, which form the critical elements for a micro-mass analyzer. (4) Monolithically integrated micro-chemical analysis systems fabricated in silicon that incorporate chemical preconcentrators, gas chromatography columns, detector arrays, and MEMS valves.

Passive MEMS Valves With Preset Operating Pressures for Microgas Analyzer

In this paper, we present integrated disk-in-cage poppet valves with tuned spring stiffness for gas flow control of a microgas analyzer. The valves require zero power and close at preset offset pressures (0-35 psig) to switch from gas sample loading onto a preconcentrator to concentrated constituent sample injection into a microgas chromatograph. Air flow rates of 4.5 mL/min at pressures of - 2.5--5 psig (vacuum sample loading) were measured. Hydrogen leak rates of 0.1 muL/s (0.006 mL/min) were measured with valves closed at 15 psig. Analytical and numerical modeling was used to guide design of valve spring constants (ranging from 10 to 1500 N/m) that control the valve open position, flow rate, and closing pressure. The parameter design space is limited to a range of seat overlap, valve size, and spring stiffness that will allow adequate flow rate, sealing, and closing at predictable pressures. A linear curve defining closing pressure as a function of spring constant, valve gap, valve size, and seat overlap fit measured closing pressure data and can be used to predict closing pressure for future designs.

Low leak rate mems valves for micro-gas-analyzer flow control

We present MEMS polysilicon microvalves for flow control of a rapid analytical microsystem (Micro-Gas-Analyzer, MGA). All valve components (boss, seat, springs, electrodes, and stops) are surface micromachined in the SUMMiTTM microfabrication process. The valves have been characterized at high flow rate when open (60 ml/min air), low leak rate when closed (≪0.0025 ml/min Hydrogen, H2), and tunable closing pressures (1 to 35 psig). Active electrostatic valves have been shown to hold closed (voltage on) against a high pressure (≫40 psig) for sample loading, open for gas chromatograph (GC) loading (voltage off), and reclose against low pressure 2–5 psig.

Microfabricated Nitrogen-Phosphorus Detector: Chemically Mediated Thermionic Emission

Many chemical warfare agents and toxic industrial chemicals contain nitrogen and phosphorus atoms. Commercially available benchtop Nitrogen-Phosphorus Detectors (NPDs) for gas chromatographs are highly selective for nitrogen and phosphorus compared to carbon. However, the detection mechanism for these thermionic detectors is poorly understood despite 60 years of use. In addition these detectors require the use of flammable gas and operate at high power. We developed a microfabricated NPD (μNPD) with similar selectivity that does not require the use of flammable gas and uses relatively low power. Our μNPD consists of an alkali metal silicate thin film spray coated onto a microhotplate. The silicate thin film is responsible for providing the thermionic emission necessary for analyte detection. We conducted a series of experiments designed to better elucidate the detection mechanism. Our results indicate that surface catalyzed ionization of nitrogen and phosphorus containing analytes is the most likely mechanism.

Viscoelastic coupling of nanoelectromechanical resonators.

This report summarizes work to date on a new collaboration between Sandia National Laboratories and the California Institute of Technology (Caltech) to utilize nanoelectromechanical resonators designed at Caltech as platforms to measure the mechanical properties of polymeric materials at length scales on the order of 10-50 nm. Caltech has succeeded in reproducibly building cantilever resonators having major dimensions on the order of 2-5 microns. These devices are fabricated in pairs, with free ends separated by reproducible gaps having dimensions on the order of 10-50 nm. By controlled placement of materials that bridge the very small gap between resonators, the mechanical devices become coupled through the test material, and the transmission of energy between the devices can be monitored. This should allow for measurements of viscoelastic properties of polymeric materials at high frequency over short distances. Our work to date has been directed toward establishing this measurement capability at Sandia.