Gravimetric Gas Sensors Research Articles

Design of Zeolitic Imidazolate Framework-8-Functionalized Capacitive Micromachined Ultrasound Transducer Gravimetric Sensors for Gas and Hydrocarbon Vapor Detection

A capacitive micromachined ultrasound transducer (CMUT) was engineered and functionalized with zeolitic imidazolate framework-8 (ZIF-8) dispersed in a photoresist AZ1512HS (AZ) matrix to function as a gravimetric gas sensor. The sensor response was recorded in the presence of nitrogen, argon, carbon dioxide, and methane gases as well as water, acetylene, a propane/butane mixture, n-hexane, gasoline, and diesel vapors. The photoresist matrix alone was found to have a negligible response to all the gases and vapors, except for water vapor. No visible difference in sensor response was detected when switching from nitrogen to methane gas. However, a strong shift in the sensor resonance frequency was observed when exposed to higher hydrocarbons, ranging from 1 kHz for acetylene to 7.5 kHz for gasoline. Even longer-chain hydrocarbons, specifically kerosene and more so diesel, had a significantly reduced sensor frequency shift compared with gasoline. Sensors functionalized with a thin film of AZ+ZIF-8 demonstrated higher sensitivity in their response to a hydrocarbon molecular mass than without functionalization.

Vapor-Based Synthesis of ZIF-8 Thin Films and Gas Adsorption Properties

On-site and real-time measurement of gas concentrations are crucial for both the understanding and the monitoring of industrial and environmental processes. In recent years, there is an increasing need to develop portable multi-gas analysis tools allowing in situ detection of complex gas mixtures mainly due to safety, process and environmental considerations. A promising approach is based on the integration of the different parts of the analytical system (i.e. pre-concentrator, gas chromatography column, gravimetric sensors) in a silicon die by using standard microelectronic technologies. Each of these devices need to be coated by an appropriate functional layer.Metal Organic Frameworks (MOF), hybrid microporous crystalline materials with tuneable properties, are attractive for this type of application regarding their high specific surface area and chemical stability. However, these materials are usually synthetized via solvothermal techniques, which complicates the growth of continuous thin films and their integration in micro-devices. Recently, vapor phase-based routes were reported for the synthesis of MOF thin films [1-2]. These solvent-free synthesis methods hold great economic and environmental perspectives since they can not only avoid the use of solvents and metallic salts, but also reduce contamination and impurity incorporated into the crystals during the synthesis step. Moreover, vapour-phase process are usually preferred to obtain conformal coatings in the high aspect ratio features of devices and they present a good compatibility with the Si technology. These breakthroughs have paved the way for the use of MOF in micro- and nanotechnologies but much still remains to be done to control the growth and understand the potential and the limits of these growth methods.In this work, pinhole-free and crystalline zeolitic imidazolate framework-8 (ZIF-8) layers were grown on different types of substrates and devices that can be used for gas analysis. Si wafers, Quartz Crystal Microbalance, Si micro-pillars arrays and 3D foam were envisaged. First, ZnO films were deposited by Atomic Layer Deposition (ALD). Then, the formation of ZIF-8 was realized through a vapor-solid reaction with a vaporized linker (2-methylimidazole). Continuous ZIF-8 films were obtained and the impact of the process parameters on the MOF growth and on the material properties was studied. Additionally, the composition, roughness and structure of the films were studied by FTIR, EDX, XPS, XRD and AFM experiments. Finally, the as-synthesized films were thermally activated and the porosity was assessed using ellipsometric-porosimetry. The adsorption properties of the films were also investigated using gravimetric gas sensors. Several gases were tested (methanol, toluene, formaldehyde) in order to determine the benefit of ZIF-8 thin films in sensors. It is demonstrated that the synthesis approach based on an ALD deposition followed by a vapor-solid treatment in appropriate conditions is compatible with different kinds of substrates, which allows the use of ZIF-8 as sensitive layer in devices for gas sensing.[1] I. Stassen et al., Nat. Mat., 15, 304–310 (2016)[2] E. Ahvenniemi et al., Chem. Com., 52, 1139-1142 (2016)

SiOCH Hits the Gas

There is an increasing need for on-site and real-time analyses of complex gas mixtures in many application fields such as industry, air quality, security and medicine. One challenge consists in developing complete and portable multi-gas analysis systems allowing in situ and real-time quantitative analysis of complex gas mixtures. A promising approach is based on the integration of resonating devices on silicon chips by using “standard” microelectronic technologies. The fabrication of these gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer to ensure a better collection and concentration of the target molecules. This enhances the sensitivity in conjunction with the limit of detection. The integration of a chemical sensitive layer on to NEMS fabricated on Si wafers leads to specific material requirements. The use of solvent is prohibited in order to avoid the degradation of the Si nanocantilevers due to capillarity forces,Very thin film thickness (less than few hundred of nanometers) should be deposited on the Si beam in order to avoid filling the gap between the resonant cantilever and the electrodes,The deposition technique should provide good uniformity over a large surface (typically on 200 mm Si wafers) for a collective sensor fabrication.And the material should be optimized to absorb the highest level of targeted gas for a given thickness, reversibly and quickly. In this work, the objective was to develop materials deposited using microelectronic compatible techniques in order to detect light alkanes and aromatic volatile organic compounds such as BTEX (for Benzene, Toluene Ethylbenzene and Xylene). Organosilicate materials are potentially good candidates for this application especially because they are nonpolar or weakly polar materials. In order to better understand the impact of the material chemistry on the detection of hydrocarbon gas and optimize the sensitivity of the chemical layer, a large panel of organosilicate thin films were deposited by different chemical vapor deposition (CVD) techniques. First, SiOCH were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), this technique being the most used in the microelectronic industry for dielectric thin-film deposition. Several precursors (diethoxymethylsilane, trimethylsilane or octamethyltrisiloxane) and deposition conditions were used in order to vary their chemical composition and their physical properties. In these SiOCH, the carbon content is mainly under the form of methyl groups bonded to silicon. At the same time, to allow a better flexibility in term of chemical functions, filament assisted chemical vapor deposition (FACVD) was used for the deposition of organosilicate thin films. Indeed, FACVD allow a better control over the precursor fragmentation pathway in comparison to plasma-based techniques. Then, it is possible to add polar groups (such as ethoxy to in case of FACVD of methyltriethoxysilane) or to introduce siloxane rings in the materials (in case of initiated CVD of cyclosiloxane). Finally, the introduction of additional porosity was investigated as another way to potentially improve the sensitivity of the chemical sensitive layers. By using a porogen approach, it is possible to introduce up to 40 % porosity in a PECVD SiOCH. Using a foaming approach, we can go even further (> 50 %). Film properties after deposition and after potential annealing were investigated using multiple characterization techniques (ellipsometry, FTIR and Tof-SIMS). Porosity was characterized by ellipsometric porosimetry and grazing incidence small angle X-ray Scattering. In these material, pores (if any) are nanometrics and porosity is mainly open and interconnected. Thin film response under toluene or pentane exposures was studied using Quartz Crystal Microbalance functionalized with the different SiOCH.Through the synthesis and characterization of these various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding can be highlighted. It is shown that the hydrophobic nature of SiOCH materials composed of a Si-O-Si backbone with methyl groups bonded to silicon, combined with the presence nanoporosity, lead to very high sensibility. The presence of isolated polar bonds such as ethoxy groups seems beneficial especially for BTEX detection. High partition coefficients toward toluene and pentane can be obtained, at least ten time higher than those measured on more classical polymers such as PDMS. Both deposition techniques (PECVD or FACVD) are able to produce good chemical sensitive layers for the detection of light alkanes and aromatics VOCs and these organosilicates constitute promising solution for the functionalization of NEMS-based gas sensors.Acknowledgment: Developments on FACVD were performed in the frame of a collaboration with TEL, US-Technology Development Center, America.

Combined colorimetric and gravimetric CMUT sensor for detection of benzyl methyl ketone

The detection of benzyl methyl ketone (BMK) is of interest as it is a common precursor for the synthesis of (meth)amphetamine. Resonant gravimetric sensors can be used to detect the mass and hereby the concentration of a gas while colorimetric arrays typically have exceptional selectivities to a target analyte. We present a sensor system consisting of a capacitive micromachined ultrasonic transducer (CMUT) and a colorimetric array for detection of BMK. The CMUT is used as a resonant gravimetric gas sensor where the resonance frequency shift due to mass loading of the plate. A single local oxidation of silicon (LOCOS) step was used to define the cavities which were sealed with a Si3N4 plate with a thickness of 100 nm, resulting in a resonance frequency of 38.8 MHz and a mass sensitivity of 28.3zgHz×μm2 or 8.5 × 103 cm2/g This sensitivity is, to our knowledge, the best reported as compared with the state of the art of CMUTs. The CMUTs were functionalized with the same dyes used to fabricate the colorimetric arrays. While both the CMUT and the colorimetric array showed selectivity between BMK and acetone and water, the highest selectivity was achieved with the colorimetric array. Here the analyte BMK resulted in the only signal (color change), while acetone and water did not give any signal. Furthermore, the mass of the BMK was found as a function of time. Thus, the combination of a colorimetric array and a CMUT resulted in a sensor system with a selectivity between BMK, acetone and water and in addition a quantitative value for the mass.

Filament Assisted Chemical Vapor Deposited organosilicate as chemical layer for nanometric hydrocarbon gas sensors

The fabrication of gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer that is uniformly deposited by a solvent-free deposition technique. In this work, we show than an organosilicate layer deposited by Filament-Assisted Chemical Vapor Deposition using methyltriethoxysilane presents very high affinity toward hydrocarbon gas. High partition coefficients toward toluene and pentane were obtained on Quartz Crystal Microbalance sensors after being annealed at 400 °C. It is shown that the hydrophobic nature of this material composed of a SiOSi backbone with methyl groups bonded to silicon, combined with the presence of isolated ethoxy groups and microporosity, allow a very high sensibility in comparison to others SiOCH. Furthermore, the functionalization of NEMS-based gas sensors was demonstrated. This material, deposited using a CMOS compatible technique, constitutes a promising solution for the development of a complete and portable multi-gas analysis system.

Quantitative evaluation of the responses of a gravimetric gas sensor based on mesoporous functionalized silica: Application to 2,4-DNT and TNT detection

The literature reports several attempts to predict the responses of gas sensors using as only input the molecular structures of the analyte and sensing material. However, these are restricted to relatively simple and homogeneous materials. The present paper reports for the first time the semiquantitative estimation of the sensing responses of quartz crystal microbalance sensors based on a complex material, taking relative humidity into account. The sensing material is methylated mesoporous silica, and the analytes are vapors of the nitroaromatic compounds 2,4-dinitrotoluene (2,4-DNT) and trinitrotoluene (TNT). Our approach is based on molecular dynamics simulations. The amount of vapor (water, 2,4-DNT or TNT) adsorbed on the sensing surfaces is obtained as a function of their partial pressure by simulating adsorption isotherms at high temperature and extrapolating the results to ambient conditions. Furthermore, a correction factor is introduced to account for the role of residual water within the mesoporous silica. The order of magnitude of the simulated sensing responses to 2,4-DNT and TNT is found to be consistent with experimental data.

Diketopiperazine receptors: highly selective layers for gravimetric sensors

Diketopiperazine with two peptidic side chains have been evaluated as selective and sensitive coatings for gravimetric gas sensors. Individually coated arrays of multiple quartz crystal microbalances yield characteristic response patterns, which allow identification and monitoring of the individual components of complex aromas or mixtures of different analytes. The diketopiperazides tested here differentiate simple alcohols. Thanks to their facile synthesis, the composition and stereochemistry of their receptor arms can easily be modified and optimized for the detection of a variety special analytes.