68 research outputs found
Sorption-induced Static Bending of Microcantilevers Coated with Viscoelastic Material
Absorption of a chemical analyte into a polymercoating results in an expansion governed by the concentration and type of analyte that has diffused into the bulk of the coating. When the coating is attached to a microcantilever, this expansion results in bending of the device. Assuming that absorption (i.e., diffusion across the surface barrier into the bulk of the coating) is Fickian, with a rate of absorption that is proportional to the difference between the absorbed concentration and the equilibrium concentration, and the coating is elastic, the bending response of the coated device should exhibit a first-order behavior. However, for polymercoatings, complex behaviors exhibiting an overshoot that slowly decays to the steady-state value have been observed. A theoretical model of absorption-induced static bending of a microcantilever coated with a viscoelastic material is presented, starting from the general stress/strain relationship for a viscoelastic material. The model accounts for viscoelasticstress relaxation and possible coating plasticization. Calculated responses show that the model is capable of reproducing the same transient behavior exhibited in the experimental data. The theory presented can also be used for extracting viscoelasticproperties of the coating from the measured bending data
Rapid Detection of Analytes with Improved Selectivity Using Coated Microcantilever Chemical Sensors and Estimation Theory
Rapid detection of analytes with improved selectivity is achieved though the use of estimation theory to analyze the response of polymer-coated microcantilever chemical sensors. In general, chemical sensors exhibit partial selectivity and can have relatively long response times. Using estimation theory, it is possible to make short-term response predictions from past data. This makes it possible to use the transient information (response time), often unique to an analyte/coating pair, to achieve an improvement in analyte species recognition while simultaneously allowing for a reduction in the time required for identification and quantification. An extended Kalman filter is used as a recursive online approach to refine the estimate of the sensor\u27s future response. Both identification and quantification are thus possible as soon as the filter estimate achieves a high confidence level. Also, with improved selectivity, identification is possible using fewer sensors in an array
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Novel photon detection based on electronically-induced stress in silicon
The feasibility of microcantilever-based optical detection is demonstrated. Specifically, the authors report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers. In this work, optical transduction techniques were used to measure microcantilever response to photons and study the electronic stress in silicon microcantilevers, and their temporal and photometric response. The photo-generation of free charge carriers (electrons, holes) in a semiconductor gives rise to photo-induced (electronic) mechanical strain. The excess charge carriers responsible for the photo-induced stress, were produced via photon irradiation from a diode laser with wavelength {lambda} = 780 nm. The authors found that for silicon, the photo-induced stress results in a contraction of the crystal lattice due to the presence of excess electron-hole-pairs. In addition, the photo-induced stress is of opposite direction and about four times larger than the stress resulting from direct thermal excitation. When charge carriers are generated in a short time, a very rapid deflection of the microcantilever is observed (response time {approximately} {micro}s)
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Piezoresistive microcantilever optimization for uncooled infrared detection technology
Uncooled infrared sensors are significant in a number of scientific and technological applications. A new approach to uncooled infrared detectors has been developed using piezoresistive microcantilevers coated with thermal energy absorbing materials. Infrared radiation absorbed by the microcantilever detector can be sensitively detected as changes in electrical resistance as function of microcantilever bending. The dynamic range of these devices is extremely large due to measurable resistance change obtained with only nanometer level cantilever displacement. Optimization of geometrical properties for selected commercially available cantilevers is presented. We also present results obtained from a modeling analysis of the thermal properties of several different microcantilever detector architectures
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Optical and infrared detection using microcantilevers
The feasibility of micromechanical optical and infrared (IR) detection using microcantilevers is demonstrated. Microcantilevers provide a simple means for developing single- and multi-element sensors for visible and infrared radiation that are smaller, more sensitive and lower in cost than quantum or thermal detectors. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress originating from the bimetallic effect. Bending is proportional to the amount of heat absorbed and can be detected using optical or electrical methods such as resistance changes in piezoresistive cantilevers. The microcantilever sensors exhibit two distinct thermal responses: a fast one ({theta}{sub 1}{sup thermal} < ms) and a slower one ({tau}{sub 2}{sup thermal} {approximately} 10 ms). A noise equivalent temperature difference, NEDT = 90 mK was measured. When uncoated microcantilevers were irradiated by a low-power diode laser ({lambda} = 786 nm) the noise equivalent power, NEP, was found to be 3.5nW/{radical}Hz which corresponds to a specific detectivity, D*, of 3.6 {times} 10{sup 7} cm {center_dot} {radical}Hz/W at a modulation frequency of 20 Hz
Charge amplification concepts for direction-sensitive dark matter detectors
Direction measurement of weakly interacting massive particles in
time-projection chambers can provide definite evidence of their existence and
help to determine their properties. This article demonstrates several concepts
for charge amplification in time-projection chambers that can be used in
direction-sensitive dark matter search experiments. We demonstrate
reconstruction of the 'head-tail' effect for nuclear recoils above 100keV, and
discuss the detector performance in the context of dark matter detection and
scaling to large detector volumes.Comment: 15 pages, 9 figure
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Micromechanical Uncooled Photon Detectors
Recent advances in micro-electro-mechanical systems (MEMS) have led to the development of uncooled infrared detectors operate as micromechanical thermal detectors or micromechanical quantum detectors. The authors report on a new method for photon detection using electronic (photo-induced) stresses in semiconductor microstructures. Photo-induced stress in semiconductor microstructures, is caused by changes in the charge carrier density in the conduction band and photon detection results from the measurement of the photo-induced bending of semiconductor microstructures. Small changes in position (displacement) of microstructures are routinely measured in atomic force microscopy (AFM) where atomic imaging of surfaces relies on the measurement of small changes (< 10{sup -9} m) in the bending of microcantilevers. Changes in the conduction band charge carrier density can result either from direct photo-generation of free charge carriers (electrons, holes) or from photoelectrons emitted from thin metal film surfaces in contact with a semiconductor microstructure which forms a Schottky barrier. In their studies, they investigated three systems: (1) Si microstructures, (2)InSb microstructures and (3) Si microstructures coated with a thin film of Pt. They found that for Si the photo-induced stress results in a contraction of the crystal lattice due to the presence of excess electron-hole-pairs while for InSb photo-induced stress causes the crystal lattice to expand. They present their results and discuss their findings
Micromechanical uncooled photon detectors
Recent advances in micro-electro-mechanical systems (MEMS) have led to the development of uncooled infrared detectors operate as micromechanical thermal detectors or micromechanical quantum detectors. The authors report on a new method for photon detection using electronic (photo-induced) stresses in semiconductor microstructures. Photo-induced stress in semiconductor microstructures, is caused by changes in the charge carrier density in the conduction band and photon detection results from the measurement of the photo-induced bending of semiconductor microstructures. Small changes in position (displacement) of microstructures are routinely measured in atomic force microscopy (AFM) where atomic imaging of surfaces relies on the measurement of small changes (< 10{sup -9} m) in the bending of microcantilevers. Changes in the conduction band charge carrier density can result either from direct photo-generation of free charge carriers (electrons, holes) or from photoelectrons emitted from thin metal film surfaces in contact with a semiconductor microstructure which forms a Schottky barrier. In their studies, they investigated three systems: (1) Si microstructures, (2)InSb microstructures and (3) Si microstructures coated with a thin film of Pt. They found that for Si the photo-induced stress results in a contraction of the crystal lattice due to the presence of excess electron-hole-pairs while for InSb photo-induced stress causes the crystal lattice to expand. They present their results and discuss their findings
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Chemical Detection Based on Adsorption-Induced and Photo-Induced Stresses in MEMS Devices
Recently there has been an increasing demand to perform real-time in-situ chemical detection of hazardous materials, contraband chemicals, and explosive chemicals. Currently, real-time chemical detection requires rather large analytical instrumentation that are expensive and complicated to use. The advent of inexpensive mass produced MEMS (micro-electromechanical systems) devices opened-up new possibilities for chemical detection. For example, microcantilevers were found to respond to chemical stimuli by undergoing changes in their bending and resonance frequency even when a small number of molecules adsorb on their surface. In our present studies, we extended this concept by studying changes in both the adsorption-induced stress and photo-induced stress as target chemicals adsorb on the surface of microcantilevers. For example, microcantilevers that have adsorbed molecules will undergo photo-induced bending that depends on the number of absorbed molecules on the surface. However, microcantilevers that have undergone photo-induced bending will adsorb molecules on their surfaces in a distinctly different way. Depending on the photon wavelength and microcantilever material, the microcantilever can be made to bend by expanding or contracting the irradiated surface. This is important in cases where the photo-induced stresses can be used to counter any adsorption-induced stresses and increase the dynamic range. Coating the surface of the microstructure with a different material can provide chemical specificity for the target chemicals. However, by selecting appropriate photon wavelengths we can change the chemical selectivity due to the introduction of new surface states in the MEMS device. We will present and discuss our results on the use of adsorption-induced and photo-induced bending of microcantilevers for chemical detection
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Detection and Characterization of Chemicals Present in Tank Waste
The goal of this three-year project is to develop and demonstrate novel multi-parameter micro-electro-mechanical system (MEMS) sensors that are robust and can be used to simultaneously detect the presence of target chemicals in a mixture, radiation emitted from radioactive materials and the heat generated by the absorption of photons of specific wavelength by the target molecules.The goal of this program is to study and develop effective methods of immobilizing chemical selective phases for improved microsensor performance 1. Investigations of Photo-Induced and Adsorption-Induced Stress in Micromechanical Structures and Photothermal Spectroscopy Microcantilevers respond to chemical stimuli by undergoing changes in their bending and resonance frequency even when a small number of molecules adsorb on their surface. In our present studies, we extended this concept by studying changes in both the adsorption-induced stress and photo-induced stress as target chemicals adsorb on th e surface of microcantilevers. For example, microcantilevers that have adsorbed molecules will undergo photo-induced bending that depends on the number of absorbed molecules on the surface. However, microcantilevers that have undergone photo-induced bending will adsorb molecules on their surfaces in a distinctly different way. Coating the surface of a microstructure with a different material can provide chemical specificity for the target chemicals. Therefore combining measurements of photo-induced and adsorption-induced stress in MEMS devices caused by target molecules with microcalorimetric spectroscopy both the presence and identity of target molecules can be determined. In addition, radioactive chemicals can also be identified by measuring the temperature changes of micromechanical sensors as the absorb emitted radiation. (i) Studies of Adsorption-Induced Stress in MEMS We investigated the effect of absorption of trace amounts of target molecules, 2- mercaptoethanol and diisopropyl methylphosphonate (DIMP), on micromechanical structures. Although gold coated surfaces adsorb DIMP effectively, the selectivity can be substantially improved by first coating the surface with self-assembled monolayers. The chemical selectivity of the layer is based on the interaction of Cu+2 bound to the MEMS surface by a carboxylate-terminated n-alkanethiol monolayer. Microcantilever MEMS devices with such surface coatings were exposed DIMP molecules by flowing a mixture of N2 and DIMP vapor in a chamber containing the microcantilever. The composite self-assembled monolayer coating transiently adsorbs molecules of DIMP vapor, which causes the microcantilever to bend. We found that our derivitized MEMS respond proportionally and reversibly to the presence of DIMP molecules in a way that is distinguishable from any response to common organic solvents such as ethanol, methanol, or acetone. During the exposure time, we observed no measurable change in the resonance frequency of the MEMS
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