Article

A Highly Sensitive Humidity Sensor Based on Ultrahigh-Frequency Microelectromechanical Resonator Coated with Nano-Assembled Polyelectrolyte Thin Films

Authors:
  • Sinopec
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Abstract

We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).

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... These sensors are successfully used for air control in ambient, industry, cars, houses, closed apartments, museums, atomic power stations, etc. [27,28]. For development of the humidity sensors various types of acoustic waves (surface (SAW) [1][2][3][4][5][6][7][8][9][10][11], Love [12][13][14][15][16][17], bulk [18][19][20][21][22][23][24], and Lamb [25,26]) have been used. These waves propagated either in homogeneous crystal substrates or in multi-layered structures containing different sorbent films sensitive towards water vapor adsorption. ...
... Usually, the sensitive films are made of ceramic, semiconducting and polymer materials [28,29]. In recent years they are fabricated from graphen-based layers [5][6][7]17,[20][21][22][23]25,26,[30][31][32] or special materials doped by nano-particles, fullerens, and carbon nano-tubes [8][9][10][11]14,17,24]. For any sorbent material adsorption of water molecules produces, in general, the changes in film density, elasticity, electric conductivity, dielectric permittivity, and temperature, which are detected at the output of the sensor as the changes in acoustic wave velocity, frequency, phase and/ or amplitude. ...
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... In the published paper [1], there is an error in Figure 3. The red curve in Figure 3b was deleted by mistake during the revisions. ...
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... 18,19) Features such as potential high sensitivity and real-time detection capability 20,21) have rendered a microwave resonant platform a favorable candidate for various sensing applications including humidity sensors. 22,23) Many attempts [24][25][26][27] have been made aimed at enhancing the sensitivity and speeding response time of microwave humidity sensors. However, a key problem with much of the literature regarding microwave humidity sensors is that they lack a wide humidity range and suffer from poor reproducibility, which could be attributed to the microwave characteristics of the employed humidity responsive material. ...
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Field effect transistors using ultrathin molybdenum disulfide (MoS(2)) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS(2) devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS(2) field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS(2). Uniform encapsulation of MoS(2) transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.
Article
Al-doped ZnO films were deposited onto SiO2/Si substrates by rf magnetron sputtering system as a CO gas sensor. The dependence of the thin film thickness on CO gas sensing properties was investigated, where the film thickness was varied by controlling the deposition time. The structure of the deposited ZnO:Al films was determined by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The CO gas sensing properties were determined by in situ measurement for surface resistance of the thin film as a function of film thickness, different atmosphere, and operation temperature. It was shown that the films were flat and smooth with (0 0 0 1) preferred orientation. The grain size was increased as the film thickness was increased during deposition. Here, the CO gas sensing properties were relative to the structural characteristics where the maximum sensitivity of 61.6% was obtained at 65 nm film thickness for the operation temperature of 400 °C.
Article
A highly sensitive Surface Acoustic Wave (SAW) sensor system for relative humidity (RH) detection using chemically interactive polyvinyl-alcohol (PVA) film with implemented electronic module is described. The system configured as a dual resonator SAW oscillator consists of two SAW resonators, two RF amplifiers, and a mixer. The mixed frequency has been considered as output of the SAW sensor. The SAW response toward 60% RH has been measured as a frequency change of about −11.5 MHz, at room temperature. A cross-sensitivity of the PVA film toward organic vapours with bonded OH groups has been also detected. The RH sensing characteristics, at room temperature, of the PVA-based dual SAW sensor have been analyzed in terms of sensitivity, calibration curve, detection limit, noise, water-resistance, short–medium term repeatability, aging, and sensing performances comparison.
Article
Mica multilayer thin films were assembled layer-by-layer (LBL) from mica that was modified with carboxylic groups (COOH) and cationic polyelectrolyte (poly(allylamine hydrochloride)) (PAH) on a quartz-crystal microbalance (QCM) for low-humidity sensing applications. The surface characteristics of the thin films were studied for various assembly conditions of pH and ionic strength of modified mica solutions. The thin films were characterized using QCM and by atomic force microscopy (AFM). The thickness of mica-COOH/PAH multilayer thin films increased with assembly pH of modified mica solution. When salt was added, the multilayer films became thicker, smoother and grew more regularly. The mica-COOH/PAH multilayer films that were deposited from the mica-COOH solution with salt at pH 3 was coated on the gold electrode of the QCM exhibited excellent sensitivity (0.524 −ΔHz/Δppmv at 36.1 ppmv) and linearity and a short response time (13 s at 60.4 ppmv).
Article
This research develops a simple template assisted sol-gel process for preparing porous TiO2 for a high performance humidity sensor. Tetraethyl orthosilicate (TEOS) as a template was directly introduced into TiO2 sol formed by the hydrolysis and condensation of titanium alkoxide; the following calcination led to the formation of TiO2-SiO2 composite, and the selective removal of SiO2 by dilute HF solution led to the formation of porous structure in TiO2. The resulting porous TiO2-based sensor exhibits high sensitivity and linear response in the wide relative humidity (RH) range of 11%-95%, with an impedance variation of four orders of magnitude to humidity change. Moreover, it exhibits a rapid and highly reversible response characterized by a very small hysteresis of <1% RH and a short response-recovery time (5 s for adsorption and 8 s for desorption), and a 30-day stability test also confirms its long-term stability. Compared with pure TiO2 prepared by the conventional sol-gel method, our product shows remarkably improved performance and good prospect for a high performance humidity sensor. The complex impedance spectra were used to elucidate its humidity sensing mechanism in detail.
Article
A fluorescence-based sensing scheme exploiting an environment-sensitive fluorophore embedded in a hydrogel has been developed for measurement of relative humidity (RH). The fluorophore, dapoxyl sulfonic acid (DSA), is incorporated into two different hydrogel films, agarose and a copolymer of acrylamide and 2-(dimethylamino)ethyl methacrylate (DMAEM) cross-linked with N,N'-methylenebisacrylamide. The swelling and contracting of the hydrogels in response to relative humidity alters the polarity of the environment of DSA, stimulating a shift in the emission wavelength. From 0 to 100% RH, acrylamide-DMAEM sensors exhibited a 40 and 15 nm wavelength shift in still air and flowing gas, respectively. Agarose sensors showed a 40 nm wavelength shift from 0 to 100% RH in still air and a 30 nm shift from 0 to 70% RH in flowing gas. Response times for both sensors were 15 min in still air and less than 5 min in flowing gas. The sensing approach is straightforward and cost-effective, yields sensors with characteristics suitable for commercial measurement of RH (i.e., sensitivity, response times, reproducibility), and allows ease of adaptability to specific RH measurement requirements. The results support the potential extension of the method to a wide variety of analytes in the vapor phase and aqueous solution by incorporation of functionalized "smart" hydrogels.
Article
A novel humidity sensor was fabricated by electrospinning deposition of nanofibrous polyelectrolyte membranes as sensitive coatings on a quartz crystal microbalance (QCM). The results of sensing experiments indicated that the response of the sensors increased by more than two orders of magnitude with increasing relative humidity (RH) from 6 to 95% at room temperature, exhibiting high sensitivity, and that, in the range of 20-95% RH, the Log(Deltaf) showed good linearity. The sensitivity of fibrous composite polyacrylic acid (PAA)/poly(vinyl alcohol) (PVA) membranes was two times higher than that of the corresponding flat films at 95% RH. Compared with fibrous PAA/PVA membranes, the nanofibrous PAA membranes exhibited remarkably enhanced humidity sensitivity due to their high PAA content and large specific surface area caused by the formation of ultrathin nanowebs among electrospun fibers. Additionally, the resultant sensors exhibited a good reversible behavior and good long term stability.
Article
A polyelectrolyte multilayer was assembled on top of a patterned PDMS stamp employing the layer-by-layer (LbL) assembly technique. By post-treatment with a base and further cross-linking, a porous multilayer-coated PDMS composite stamp was obtained. With the pore structures acting as an ink reservoir, the multiple printing of proteins was successfully achieved without the need to re-ink the stamp.
Article
The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.
Article
We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
Article
The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.
Article
A humidity sensor based on a single SnO2 nanowire was fabricated. This sensor has fast and sensitive response to relative humidity (RH) in air from a wide range of atmospheres at 30 °C. The response sensitivity of the sensor to RH is linear to 90%.
Article
The introduction of silicon-containing polymers with hydrogen-bond acidic groups have paved the way for increased use of fluorinated hydrogen-bond acidic polymers for chemical sensing for the chemical sensing of organic polymers for acoustic wave sensors. different classes of hydrogen-bond acidic polymers and architectures have now been developed, from linear organic polymers containing fluoroalcohols to silicon-containing polymers with phenolic or fluoroalcohol functionalities in linear and branched architectures. There also exist hyperbranched and polyhedral architectures with the use of hyperbranched materials containing both siloxane and carbosilane linkages. Such classes are easier to make than early polymers and fluorinated reagents and monomers are now more readily available and some of the polymers can be obtained commercially. As such, these polymers have been applied to many other sensing approaches. Applications include sensors for explosives, chemical agents or simulants and volatile organic compounds on various acoustic wave devices, chemiresistors, chemicapacitors and microcantilevers and in fluorescence sensing methods.
Conference Paper
In this paper the feasibility of thin film bulk acoustic resonators (FBAR), for applications in bio- and gas-detection, is shown for the first time. Solidly mounted, ZnO FBARs with frequencies around 2 GHz have been fabricated on silicon substrates. The dependence of the FBAR mass sensitivity on the design of the layer stack has been investigated exhibiting an optimized sensitivity of 2.5 Hz cm<sup>2</sup>/pg. Using a common protein assay the capability of detecting bio-molecules has successfully been proved. Gas sensing has been demonstrated by coating the FBAR with a humidity absorbing polymer. A strong non-linear dependence of the humidity sensitivity on the thickness of the polymer coating has been found. When the polymer thickness is far less than the acoustic wavelength, a pure mass dependent response occurs, leading to a negative shift in resonance frequency. Moreover, as the polymer thickness becomes significant, acoustic influences affect the response and the shift becomes large and positive. A sensitivity to humidity of up to two orders of magnitude higher than that of comparably coated quartz crystal micro-balances has been observed.
Acoustic Wave Filters for Communications
  • K Hashimoto
  • Bulk
Hashimoto, K. RF Bulk Acoustic Wave Filters for Communications; Artech House: St Norwood, MA, USA, 2009; p. 92.