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

Effect of temperature and density on electron attachment and ionization processes

The total electron attachment rate constant ka(〈&epsilon〉 ,T) for n-C4F10 and for n-C6F14 has been measured in a buffer gas of Ar using an electron swarm technique in the mean electron energy range, 〈&epsilon〉, 0.41 to 4.81 eV, over the temperature range, T, 300 to 750 K and over the total gas number density, Nt, 2.25 to 12.88 × 1019 molecules/cm3. The measured ka(〈&epsilon〉 ,T) and the total electron attachment cross section σa(ε, T) obtained from them are reported and discussed. The ka(〈&epsilon〉 ,T) for n—C4F10 first decrease slowly with T between 300 to 400 K then decrease precipitously between 400 and ≈ 500 K and subsequently increase for T ≥ 500 K. The ka{{e) ,T) for n—C6F14 initially increase slowly with T between 300 and 400 K, then decrease precipitously between 400 and ≈ 550 K and subsequently decrease for T \u3e 550 K. For both molecules the overall variation of ka(〈&epsilon〉 ,T) with T depends on 〈ε〉 and was independent of Nt except for n-C4F10 which exhibits an increase with Nt for T ≤ 500 K. From the measured dependence of ka(〈&epsilon〉 ,T) for n-C4F10 on Nt and T the contribution of the nondissociative and dissociative electron attachment to ka(〈&epsilon〉 ,T) was determined as a function of 〈ε〉. From the analysis of these results the effect of temperature on nondissociative electron attachment was quantified. The total electron attachment rate constant ka(〈&epsilon〉 ,T) for SO2F2 in N2 has been measured as a function of 〈&epsilon〉 (0.046 to 0.911 eV), temperature (300 to 700 K) and total gas number density (2.25 to 6.44 × 1019 molecules/cm3). From the measured ka(〈&epsilon〉 ,T) the σa(ε, T) were determined. At T = 300 K the σa(ε, T) exhibits a maximum at ≈ 0.22 eV which is due to dissociative electron attachment and an increase below ≈ 0.1 eV which is due to the formation of parent anions at near zero energies. At T = 400 K the σa(ε, T) posseses only one main peak at ~ 0.13 eV which shifts to lower ε with increasing T so that at 700 K the peak is located at ≈ 0.03 eV. The value σa(εmax, T) of σa(ε, T) at the peak increases by a factor of 32 as T increases from 300 to 700 K. The analysis of the present results and earlier similar work leads to the conclusion that the increase of the dissociative electron attachment with temperature is due to the increase of the internal energy (principally vibrational) of the molecule. The ionization onset, If, for TMPD (N,N,N\u27,N\u27-tetramethyl-p-phenylenediamene) molecules in ethane has been measured for the first time in the number density, N, range 0.09 to 8.01 × 1021 molecules/cm3 and over the temperature T range 293 to 413 K, using a multiphoton ionization conductivity technique. The If was found to depend on both N and T in the ranges studied. At a fixed T{=373K), If was found first to decrease with increasing N and then to level off at number densities of ≈ 5.96 × 1021 molecules/cm3. For N ≥ 6.84 × 1021 molecules/cm3 and T = 293 K the If was found to increase with increasing number density. At a constant number density (N = 3.55 × 1021 molecules/cm3) If decreased with increasing T between 323 and 413 K

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

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

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)

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

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

Infrared imaging using arrays of SiO2 micromechanical detectors

Optics Letters, Volume 37, No. 19, pp. 3966-3968 (October 2012)In this Letter, we describe the fabrication of an array of bimaterial detectors for infrared (IR) imaging that utilize SiO2 as a structural material. All the substrate material underneath the active area of each detector element was removed. Each detector element incorporates an optical resonant cavity layer in the IR-absorbing region of the sensing element. The simplified microfabrication process requires only four photolithographic steps with no wet etching or sacrificial layers..

Enhanced Electron Attachment to Highly-Excited States of Molecules: Implications for Plasma Processing Discharges

Recent studies show that large negative ion densities exist in plasma processing discharges, including those of weakly electronegative gases such as SiH{sub 4} and CF{sub 4}. Also, there is strong evidence that the negative ions could be the precursors for particulate formation in processing discharges. Even though it is now well established that large concentrations of negative ions exist in processing discharges, and that they play a crucial role in such discharges, the source of such high negative ion densities has not been clarified. In particular, gases like SiH{sub 4} and CH{sub 4}, which are commonly used in processing discharges, attach electrons only weakly in their ground electronic states (see the references). Due to the lack of an alternative mechanism, the origin of large negative ion densities in such weakly electronegative gases has been frequently attributed to electron attachment to radicals (molecular fragments) or other byproducts produced in the discharge. This hypothesis had not been tested in direct electron attachment measurements