CNT-based Gas Sensors Research Articles

Comparison of printing techniques for the fabrication of flexible carbon nanotube-based ammonia chemiresistive gas sensors

Even though a plethora of printing technologies are currently available and their potential for the fabrication of low-cost and flexible sensors has been widely investigated, systematically based, and statistically sustained comparative studies are missing in the literature. In this work, we compare screen, inkjet, and dispense printing for the fabrication of carbon nanotube (CNT)-based ammonia (NH3) chemiresistive flexible gas sensors for the first time. Moreover, we report the first CNT-based gas sensor fabricated via Voltera printer. The devices were made of a thin layer of spray-coated CNTs and printed silver-based interdigitated electrodes. To draw a thoughtful comparison the same sensor layout, materials, and fabrication flow were used. The device morphological features were acquired through microscopic, atomic force microscope, and 3D images; additionally, the response to NH3 as well as the printing process characteristics for each technique was analyzed. From 300 µm nominal spacing between lines, we obtained a decrease of 25%, 13%, and 5% on the printed spacings with dispense, screen, and inkjet printing, respectively. At 100 ppm of NH3, a maximum response of 33%, 31%, and 27% with the dispense-, inkjet-, and screen-printed sensors were found, respectively. Statistical differences were observed between the mean values on the NH3 response of dispense- compared to the inkjet- and screen-printed sensors, which in effect showed the highest response in the Tukey test. This demonstrated that the fabrication technique employed can induce a different response mainly driven by the printed outcomes. Following a holistic approach that includes the sensor response, the application, the market perspective, and the process versatility, we suggest screen printing as the most suitable method for CNT-based NH3 gas sensor fabrication.

First-Principles Investigation of CH₄ Adsorption on a Singly and Doubly Decorated (8,0) Single Wall Carbon Nanotube With Palladium/Oxygen Species

We employ dispersion-corrected density functional theory to study the adsorption of CH4 molecule on a singly and doubly decorated (8,0) single wall carbon nanotube (CNT80) with palladium atom and oxygen molecule. The obtained optimized energies indicate that double decoration with palladium atom and oxygen molecules significantly enhances the adsorption of methane on the CNT80, while single decoration with oxygen molecules gives rise to a weak physical bonding for methane molecules. Moreover, we observe that methane adsorption decreases the bandgap and the distance of the Fermi level to the top of the valance band of the doubly decorated CNT80. The calculated charge density plots and energy band diagrams demonstrate a charge transfer from doubly decorated CNT80 to CH4. Regarding the p-type nature of the structure, our results support the increment of the conductance of the doubly decorated CNT80 after methane adsorption, in agreement with a real measurement in the ambient conditions. This observation underlines the importance of ambient oxygen in the real performance of CNT-based gas sensors.

Effect of Humidity on Gas Sensing Performance of Carbon Nanotube Gas Sensors Operated at Room Temperature

Carbon nanotubes (CNTs) have shown promising results for gas sensing due to their high surface area. Since humidity has a great impact on the electrical conductivity of resistive CNT gas sensors, we have investigated the change of humidity on their sensing properties. In this study, we fabricated vertically aligned CNT-based gas sensors for the detection of volatile organic compounds. The morphologies and phase structures of the fabricated samples were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR), confirming the presence of CNT with some surface impurities. It was found that a relative humidity increase from 10% to 80% can reduce the electrical conductivity of the sensor by around 4%. On the other hand, for a humidity above 80% the conductivity increased slightly. The fabricated device has been used as a gas sensor for volatile organic compounds, and the cross-sensitivity to humidity was investigated. It was found that in the fabricated sensors, a change in humidity up to 80% results in a 40% decrease in the response for the studied organic compounds.

Simulations of Benzene and Hydrogen-Sulfide Gas Detector Based on Single-Walled Carbon Nanotube over Intrinsic 4H-SiC Substrate.

Carbon nanotubes (CNTs)-based sensors have gained significant importance due to their tremendous electrical and physical attributes. CNT-based gas sensors have high sensitivity, stability, and fast response time compared to that of solid-state sensors. On exposure to a large variety of organic and inorganic compounds, the conductivity of CNT changes. This change in electrical conductivity is being used as a detection signal to detect different target molecules. Hydrogen-sulfide and benzene are hazardous gases that can cause serious health issues in humans. Therefore, it is mandatory to detect their presence in industrial and household environments. In this article, we simulated CNT-based benzene and hydrogen-sulfide sensor with a nanoscale semiconductor device simulator—Quantumwise Atomistix Toolkit (ATK). The change in the device density of states, electric current, and photocurrent in the presence of target molecules have been calculated. The change in photocurrent in the presence of target molecules has been proposed as a novel detection mechanism to improve the sensor selectivity and accuracy. This change in photocurrent as well as electric current in the presence of target molecules can be used simultaneously as detection signals. Our intension in the future is to physically fabricate this simulated device and use photocurrent as well as electric current as detection mechanisms.

Direct Integration of Carbon Nanotubes in CMOS, Towards an Industrially Feasible Process: A Review

This article reviews our work on direct integration of carbon nanotubes (CNTs) in MEMS vehicles and shows initial results in realizing direct CNT integration in CMOS. Extraordinary properties of CNTs are fully exploited if they are closely integrated with signal processing electronics. An example device is a CNT-based gas sensor, where CNTs with their huge surface-to-volume ratio act as a very sensitive sensing element, and the CMOS circuits provide the necessary signal processing for a device to give a standardized, calibrated, low-noise output signal. Direct synthesis of CNTs in CMOS will enable wafer-level, low-cost fabrication of CNT-based devices. A major challenge is the 800–1000 °C CNT synthesis temperature requirement, whereas CMOS-compatible temperature is <300 °C. We synthesized CNTs on purpose-designed MEMS microheaters by local resistive heating to avoid over-heating the bulk of the chip. An applied electric field guides the CNTs to grow towards a second electrode to close electric circuits by making a Si-CNT-Si system. The process is fully controlled through electrical parameters, indicating that the process can be automated and scaled up to wafer-level for a low-cost, high-volume industrial manufacturing process. Characterization of the synthesized CNTs reveals that the hottest region (∼900 °C) ensures low-diameter, less dense and high-quality CNTs. Also, our statistical analysis shows that the electric field guiding is diameter-dependent, ensuring that only well-ordered low-diameter CNTs become part of an electric circuit. Through electrical analysis of the CNT-Si contacts, we found that the Si-CNT-Si systems show either near-ohmic or non-linear (Schottky-like) behaviours depending on the Si doping level. We also demonstrated functionalization of the locally grown CNTs. For direct CNT synthesis on CMOS chips, a high thermal gradient around the CNT growth structure is required to maintain CMOS-compatible temperature. Several promising designs of CNT growth structures and their thermomechanical simulations are presented. Partially suspended microheaters provide thermal isolation to avoid heating the active CMOS region, while keeping reliable mechanical stability. In our attempt to direct CNT-CMOS integration, a purpose-designed CMOS chip is manufactured using a standard AMS 350 nm CMOS process. Layout designs and optical characterization of several fabricated CMOS microstructures are presented.

Development of ultra-sensitive CNT based gas sensor

The development of nanotechnology has created huge potential to build highly sensitive, low cost, portable sensors with low power consumption. The extremely high surface-to-volume ratio and hollow structure of nanomaterials is ideal for the adsorption of gas molecules. In this study, we developed multi-walled carbon nanotube (MWCNT) paper-based gas sensor which is a low-cost solution and investigate the response of MWCNT gas sensors at room temperature. The multi-walled carbon nanotubes paper was prepared by filtration method. We measured resistance at 5 points for each CNT papers by four probes method to check uniformity. We suppose response fit to the Langmuir isotherm. The results show us: (1) The MWCNT paper gotten by this method has good homogeneity. (2) Sensor response is fitting to the Langmuir isotherm and we certified stable repeatability of this sensor by Langmuir isotherm.

Low-Cost Gas Sensing: Dynamic Self-Compensation of Humidity in CNT-Based Devices.

Solid-state gas sensors are a cost-effective and well-spread alternative to conventional gas sensing, employable in most environments, ranging from homes and offices to harsh industrial scenarios. The emergence of carbon nanotubes (CNTs) as sensing material in solid-state gas sensors paved the way to a new class of devices, which are semitransparent, flexible, and with a remarkably low environmental impact. These devices, however, lack selectivity to other gases and concurring physical phenomena, such as temperature and pressure changes. In this contribution, we show how by measuring the impedance of CNT-based gas sensors at different frequencies, it is possible to evaluate sensitivity coefficient for the immediate compensation of moisture content in the air, while still preserving in the considered ranges of average sensitivities as high as 0.045, 0.112, 7.842 × 10-5, and 0.041 % Z/ppm for ammonia, carbon dioxide, carbon monoxide and ethanol gas, respectively. With this simple approach, it will be possible to develop simple sensor read-out systems, with reduced external electronic, simplifying the route to low-cost and low-power sensor nodes for the internet of things.

A Nano Sensor Array for Methyl Salicylate Detection in Gaseous Form

Pathogenic plant diseases can be recognized through volatile organic compounds (VOC) chemical markers that are emitted from the plant surface upon distress. The ability to detect biomarkers from distressed plants sensitively, selectively, accurately, and rapidly could be of enormous benefit to the agricultural industry and crop growers. Among the various VOC, Methyl Salicylate (MeSa) has been identified to be a fingerprint compound that can be used as chemical marker for early detection of diseased crops1,2. Carbon nanotube (CNT) based sensors have been long used in detection of molecular species, due to their extremely high surface to volume ratio, and their p-type semiconducting behavior when connected to metallic electrode3. In this work the development of a CNT-based amperometric gas sensor for MeSa has been undertaken. The CNT gas adsorption capabilities result from their hollow structure and nanometric dimensions. These properties are important for sensitivity as it allows for several molecules to adsorb on to the surface of the sensor. The detection of MeSa is done by monitoring the current flowing through the CNT under specific gate-drain and source-drain potentials. These potentials are determined by careful examination of the change in current under different gate potentials (Trans-conductance) and the change in current under different source-drain potentials. When the MeSa molecule is adsorbed to the CNT surface it behaves as an additional gate potential source and therefore changes the current in the CNT. Selectivity can be achieved with different surface modifications of the CNT. The surface modifications will change the potential the MeSa induces on the CNT, and thus gives a different current profile for the adsorption and desorption of the MeSa molecules to and from the CNT surface.

Carbon Nanotube Compositie Based Sensor for Detecting Hydrogen Molecules

Accuracy and precise measurement of gas values is crucial for recognizing many critical biological and physical signs, as gas values present significant signals for monitoring chemistry, biology, environments and medical industries. Many sensing materials such as metal compounds, composite, nanowires, and polymers, for detecting gas have been investigated, and the merits and demerits of each have been reported [1,2]. Recently, carbon-based nanomaterials, including zero-dimensional fullerenes, one-dimensional carbon nanotubes (CNT), and two-dimensional graphene, have shown probable in gas sensor applications. Many prior works have reported their superior properties, including higher electrical conductivity, mechanical durability and chemical stability, and a larger surface area.In particularly, CNTs are well known material for sensor applications due to their stable graphite configuration, structural endurance under chemical/physical treatment processes, and mechanical characteristics. Moreover, lots of efforts to realize CNTs as chemical sensor material were performed by many groups previously [3]. They reported that the concentrations of electrons and holes in CNTs play important role in sensitivity of the CNT-based gas sensor. However, although many techniques were introduced to fabricate the CNT-based gas sensors, such device manufacturing processes were intricate and have limited repeatability and reliability, which makes hard them to be commercialized.The CNT yarn was introduced, reported and characterized previously by group using simple dry spinning method [4]. The CNT yarn has good electrical property with a high conductivity, and flexible structural property, and chemical stability. In this work, we develop the ability of CNT yarn for detecting hydrogen and carbon monoxide concentrations. Furthermore, we demonstrate that metal nanoparticles on the surface of CNT yarn highly affect the sensitivity of the CNT yarn-based gas sensor. Therefore, this study focused on the analysis and characteristics of the gas sensor using binary composite as gas sensor.

CNT-Based Inkjet-Printed RF Gas Sensor: Modification of Substrate Properties during the Fabrication Process.

This paper presents the feasibility of a fully inkjet-printed, microwave flexible gas sensor based on a resonant electromagnetic transducer in microstrip technology and the impact of the printing process that affects the characteristics of the gas sensor. The sensor is fabricated using silver ink and multi-wall carbon nanotubes (MWCNTs) embedded in poly (3,4-ethylenedioxythiophene) polystyrene (PEDOT: PSS-MWCNTs) as sensitive material for Volatile Organic Compounds (VOCs) detection. Particular attention is paid to the characterization of the printed materials and the paper substrate. The manufacturing process results in a change in relative permittivity of the paper substrate by nearly 20%. Electrical characterization, made in the presence of gas, validates our theoretical approach and the radiofrequency (RF) gas sensor proof of concept.

An optimized resistive CNT-based gas sensor with a novel configuration by top electrical contact

The paper proposes a new fabrication method of a resistive gas sensor made of vertically aligned carbon nanotubes (CNT) by the perforated electrode. This new configuration can be also utilized with vertically aligned nanomaterial such as ZnO wires. This method contains two aspects, which are getting top electrical contact from MWCNTs and devising hexagonal pattern holes on the sensor surface to ease the gas flow through the sensor. Reaching ambient molecules to these dense CNT forests is always of the essence; also, the top electrical contact of the CNTs is a fabrication challenge. The high sensitivity of proposed CNT-based sensor is attributed to their collective large porous surface along with etched circular holes located at the apexes and center of a hexagonal pattern on sensor top contact. For optimal sensor performance, the simulation approach by ANSYS-FLUENT software has optimized circular holes diameters and distances. The fabrication process proposes an oxygenated plasma treated CNTs with top electrical contact devised by a certain simple method for the very first time. Top electrical contact has been fabricated possible by the procedure of immersing the CNTs in viscosity liquid and providing electrical contact using conductive paste. Well-aligned multi-walled carbon nanotubes (MWCNT) have been grown on glass substrate placed into DC-PECVD unit bearing approximately temperature of 350 °C. In this paper, the resistivity of carbon nanotubes at different temperatures for the ambient gas and linear sensitivity to oxygen gas has been investigated. The sensors with larger diameter nanotubes are more sensitive. Etching patterned holes with 200 μm diameter enhances the sensitivity up to 50%.

Time stability of carbon nanotube gas sensors

In this communication, we report on the long-term stability of a carbon nanotube (CNT) thin-film based NH3 gas sensor. These sensors were fabricated with a reproducible, large-scale, low-cost spray deposition process and are examined in terms of NH3 sensitivity and long-term stability. The devices are measured directly after fabrication; 1-year and 4-years post fabrication while being stored in ambient conditions without further encapsulation. The retainment of up to 96% of response demonstrates the stability of such CNT-based devices. Hence, it solves a critical issue for stepping beyond the proof of concept stage. Not only can CNT-based gas sensors be utilized for years, but such sensors also do not require any special recalibration over the years.

Enhanced response and improved selectivity for toxic gases with functionalized CNT thin film resistors

ABSTRACTHigh surface area of CNTs makes them an extremely sensitive detection element for a wide variety of analytes present in minute quantities of ppb and ppm levels. CNTs does not inherit selectivity for a particular gas/analyte, it can be introduced in CNT based gas sensor by means of functionalization, along with improvement in gas sensor's response. Effects of Carboxylated group and gold nano-particles were studied on the response of CNT sensors towards toxic gases such as NO2 and NH3. Gold nanoparticles have found to enhance the response for NO2 and NH3, while oxygenated functional have brought selectivity towards NH3.

Enhanced sensing of gas molecules by a 99.9% semiconducting carbon nanotube-based field-effect transistor sensor

Carbon nanotubes (CNTs) have been regarded as a promising material for highly sensitive gas sensors due to their excellent material properties combined with their one-dimensional structural advantages, i.e., a high surface-to-volume ratio. Here we demonstrate a CNT-based gas sensor based on assembling highly purified, solution-processed 99.9% semiconducting CNT networks bridged by palladium source/drain electrodes in a field-effect transistor (FET) configuration with a local back-gate electrode. We investigated the gas responses of the CNT-FETs under different controlled operating regimes for the enhanced detection of H2 and NO2 gases using sensors with various physical dimensions. With the aid of the CNTs with high semiconducting purity (99.9%), we achieved excellent electrical properties and gas responses in the sensors and clearly determined that the operating regimes and physical dimensions of the sensors should be appropriately adjusted for enhanced sensing performance, depending on the gas molecules to be detected.

A review on recent advances of CNTs as gas sensors

PurposeThis review paper aims to focus on recent advances of carbon nanotubes (CNTs) to produce gas sensors. Gas sensors are widely used for monitoring hazardous gas leakages and emissions in the industry, households and other areas. For better safety and a healthy environment, it is highly desirable to have gas sensors with higher accuracy and enhanced sensing features.Design/methodology/approachIn this review, the authors focus on recent contributions of CNTs to the technology for developing different types of gas sensors. The design, fabrication process and sensing mechanism of each gas sensor are summarized, together with their advantages and disadvantages.FindingsNowadays, CNTs are well-known materials which have attracted a significant amount of attention owing to their excellent electrical, electronic and mechanical properties. On exposure to various gases, their properties allow the detection of gases using different methods. Therefore, over recent years, researchers have developed several different types of gas sensors along with other types of sensors for temperature, strain, pressure, etc.Originality/valueThe main purpose of this review is to introduce CNTs as candidates for future research in the field of gas sensing applications and to focus on current technical challenges associated with CNT-based gas sensors.

Analytical modelling of sensing performance of carbon nanotubes for gas sensing

Carbon nanotubes (CNTs) have recently captured attention as novel materials for nanoelectronic sensors due to their ability to achieve better sensitivity and accuracy in these devices. Up until now, the majority of investigations have focused on experimental studies for sensors. Therefore, there is lack of analytical models comparative to experimental investigations. The proposed model employs a CNT-based field effect transistor (CNT-FET) scheme to model the transport parameters of the CNT-based gas sensor. The CNT conductance is affected on exposure to the target analyte – NH3 gas molecules. The NH3 gas concentration is absorbed on the CNT surface after a chemical reaction between the CNT and NH3. This modulates the current-voltage (I-V) characteristics and conductance of the CNT-based gas sensor. The I-V characteristics of the CNT-based sensor have been proposed as a criterion to detect the effect of gas absorption. Finally, the proposed model was compared with experimental investigation for validation.

Analytical modelling of sensing performance of carbon nanotubes for gas sensing

Carbon nanotubes (CNTs) have recently captured attention as novel materials for nanoelectronic sensors due to their ability to achieve better sensitivity and accuracy in these devices. Up until now, the majority of investigations have focused on experimental studies for sensors. Therefore, there is lack of analytical models comparative to experimental investigations. The proposed model employs a CNT-based field effect transistor (CNT-FET) scheme to model the transport parameters of the CNT-based gas sensor. The CNT conductance is affected on exposure to the target analyte – NH3 gas molecules. The NH3 gas concentration is absorbed on the CNT surface after a chemical reaction between the CNT and NH3. This modulates the current-voltage (I-V) characteristics and conductance of the CNT-based gas sensor. The I-V characteristics of the CNT-based sensor have been proposed as a criterion to detect the effect of gas absorption. Finally, the proposed model was compared with experimental investigation for validation.

Carbon Nanotube Based Gas Sensors toward Breath Analysis.

Breath analysis is a promising method for rapid, inexpensive, noninvasive disease diagnosis and health monitoring owing to the correlative relationship between breath biomarker concentrations and abnormal health conditions. However, current methods to identify and quantify breath components rely on large, bench-top analytical instruments. Carbon nanotube (CNT)-based gas sensors are desirable candidates to replace benchtop instruments because of their sensitive chemical-to-electrical transducer capability, high degree of chemical functionality options, and potential for miniaturization. This review seeks to give an overview of the synthetic methods used to functionalize CNT-based gas sensors, specifically those sensors that target biologically relevant breath markers. Specific examples are provided to highlight the sensing mechanisms behind different classes of CNT hybrid composites. Finally, the current challenges and prospective solutions of applying CNT-based sensors to breath analysis are discussed.

Highly sensitive and selective carbon nanotube-based gas sensor arrays functionalized with different metallic nanoparticles

We present the fabrication and characterization of selective CNT-based gas sensor, which is capable of discriminating between four test gases (NH3, ethanol, CO and CO2). Different metals are utilized in order to alter the response of each sensing element towards the different test gases. We discuss as well the different possibilities for metal combinations in the sensor arrays. The optimum combination between the metallic nanoparticles is obtained through an efficient and quick framework. We study the effect of the thickness of the CNT thin-film on the reproducibility and the response of the sensor array. Furthermore, we investigate the response of the fabricated CNT sensor array towards a mixture of two gases. Such kind of characterization was performed by exposing the CNT sensor array to a mixture of (NH3, ethanol) and (NH3, CO) at different ratios for each mixture. We show that the response towards a mixture of two gases lies in the plane between the data points of each gas separately.

Carbon nanotube gas sensor array for multiplex analyte discrimination

The lack of selectivity toward a particular analyte has always been the primary concern regarding CNT-based gas sensors. For that reason, in here we present a gas discrimination strategy that focuses on the electrode–CNT junction. The junction is shown to play a key role in the sensing mechanism. Resistive gas sensors based on horizontal CNT arrays have been fabricated using various designs and different top-contacting metals: Pt, Pd and Au. Arrays of devices have been exposed to a series of gases to monitor their resistive response. It was found for our system that the sensor response does not significantly change as a function of the device design or the available CNT sensing area in between the anchoring electrodes. On the contrary, responses to gases are observed to be specific to each sensor electrode metal. Exposure of locally passivated devices (for which distinct areas have been covered) to NO2, H2 and NH3 highlights different sensing mechanisms for each gas. Multiplex gas discrimination for room temperature can be achieved by strategically choosing the right metal/CNT combination in a complete sensor system.