Ammonia Gas Sensor Response of a Vertical Zinc Oxide Nanorod-Gold Junction Diode at Room Temperature

Conventional metal oxide semiconductor (MOS) gas sensors have been investigated for decades to protect our life and property. However, the traditional devices can hardly fulfill the requirements of our fast developing mobile society, because the high operating temperatures greatly limit their applications in battery-loaded portable systems that can only drive devices with low power consumption. As ammonia is gaining importance in the production and storage of hydrogen, there is an increasing demand for energy-efficient ammonia detectors. Hence, in this work, a Schottky diode resulting from the contact between zinc oxide nanorods and gold is designed to detect gaseous ammonia at room temperature with a power consumption of 625 μW. The Schottky diode gas sensors benefit from the change of barrier height in different gases as well as the catalytic effect of gold nanoparticles. This diode structure, fabricated without expensive interdigitated electrodes and displaying excellent performance at room temperature, provides a novel method to equip mobile devices with MOS gas sensors

Review on carbon-derived, solid-state, micro and nano sensors for electrochemical sensing applications

The aim of this review is to summarize the most relevant contributions in the development of electrochemical sensors based on carbon materials in the recent years. There have been increasing numbers of reports on the first application of carbon derived materials for the preparation of an electrochemical sensor. These include carbon nanotubes, diamond like carbon films and diamond film-based sensors demonstrating that the particular structure of these carbon material and their unique properties make them a very attractive material for the design of electrochemical biosensors and gas sensors. Carbon nanotubes (CNT) have become one of the most extensively studied nanostructures because of their unique properties. CNT can enhance the electrochemical reactivity of important biomolecules and can promote the electron-transfer reactions of proteins (including those where the redox center is embedded deep within the glycoprotein shell). In addition to enhanced electrochemical reactivity, CNT-modified electrodes have been shown useful to be coated with biomolecules (e.g., nucleic acids) and to alleviate surface fouling effects (such as those involved in the NADH oxidation process). The remarkable sensitivity of CNT conductivity with the surface adsorbates permits the use of CNT as highly sensitive nanoscale sensors. These properties make CNT extremely attractive for a wide range of electrochemical sensors ranging from amperometric enzyme electrodes to DNA hybridization biosensors. Recently, a CNT sensor based fast diagnosis method using non-treated blood assay has been developed for specific detection of hepatitis B virus (HBV) (human liver diseases, such as chronic hepatitis, cirrhosis, and hepatocellular carcinoma caused by hepatitis B virus). The linear detection limits for HBV plasma is in the range 0.5–3.0 μL−1 and for anti- HBVs 0.035–0.242 mg/mL in a 0.1 M NH4H2PO4 electrolyte solution. These detection limits enables early detection of HBV infection in suspected serum samples. Therefore, non-treated blood serum can be directly applied for real-time sensitive detection in medical diagnosis as well as in direct in vivo monitoring. Synthetic diamond has been recognized as an extremely attractive material for both (bio-) chemical sensing and as an interface to biological systems. Synthetic diamond have outstanding electrochemical properties, superior chemical inertness and biocompatibility. Recent advances in the synthesis of highly conducting nanocrystalline-diamond thin films and nano wires have lead to an entirely new class of electrochemical biosensors and bio-inorganic interfaces. In addition, it also combines with development of new chemical approaches to covalently attach biomolecules on the diamond surface also contributed to the advancement of diamond-based biosensors. The feasibility of a capacitive field-effect EDIS (electrolyte-diamond-insulatorsemiconductor) platform for multi-parameter sensing is demonstrated with an O-terminated nanocrystalline-diamond (NCD) film as transducer material for the detection of pH and penicillin concentration. This has also been extended for the label-free electrical monitoring of adsorption and binding of charged macromolecules. One more recent study demonstrated a novel bio-sensing platform, which is introduced by combination of a) geometrically controlled DNA bonding using vertically aligned diamond nano-wires and b) the superior electrochemical sensing properties of diamond as transducer material. Diamond nanowires can be a new approach towards next generation electrochemical gene sensor platforms. This review highlights the advantages of these carbon materials to promote different electron transfer reactions specially those related to biomolecules. Different strategies have been applied for constructing carbon material-based electrochemical sensors, their analytical performance and future prospects are discussed

GAS SENSING PROPERTIES AND TRANSPORT PROPERTIES OF MULTI WALLED CARBON NANOTUBES

Multi walled carbon nanotubes (MWCNT) grown in highly ordered porous alumina templates were incorporated into a resistive gas sensor design and were evaluated for their sensitivities. The material characteristics and electrical properties of the nanotubes were analyzed. A study was undertaken to elucidate the effect of UV light on desorption characteristics and the dependence of sensitivity on (i) thickness of amorphous carbon layers and (ii) flow rates of analyte gases. These sensors were highly responsive to both oxidizing and reducing gases with steady state sensitivities of 5% and 10% for 100ppm of NH3 and NO2 respectively, at room temperature. As part of a comparative study, thick films of MWCNTs grown on Si/SiO2 substrates were integrated into various nano-composite based sensors and were evaluated for their response. Steady state sensitivities as high as 10% and 11% were achieved for 100ppm of NH3 and NO2 respectively, at room temperature. MWCNTs were characterized for their electrical properties by I–V measurements at room temperatures. A typical I-V curve with an ohmic behavior was observed for a device with high work function metals (example: Au, Pt); Schottky behavior was observed for devices with metal contacts having low work functions (example: Al, Cu)

Silicon nanowire based sensor for highly sensitive and selective detection of ammonia

The precise determination of the type and concentration of gases is of increasing importance in numerous applications. Despite the diverse operating principles of today´s gas sensors, technological trends can be summarized with the keyword miniaturization, because of the resulting benefits such as integrability and energy efficiency. This work deals with the development and fabrication of novel nanowire based gas sensors, which in comparison to conventional devices have an advantageous combination of high sensitivity and selectivity with low power consumption and small size. On the basis of grown silicon nanowires, sensors based on the functional principle of classical Schottky barrier field effect transistors with abrupt metal-semiconductor contacts are fabricated. The sensing performance of the devices is investigated with respect to the detection of ammonia. Ammonia concentrations down to 170 ppb are measured with a sensor response of more than 160 % and a theoretical limit of detection of 20 ppb is determined. Selectivity investigations show that no cross sensitivity to most common solvents occurring in living spaces exists. Moisture influences on the device are studied and reveal that the sensor responds within seconds, making it potentially suitable as humidity sensor. Moreover, it is shown that a higher relative humidity and higher temperatures decrease the sensor sensitivity. In terms of possible applications, it is a great advantage that the maximum sensitivity is achieved at 25 °C. With respect to sensitivity and selectivity an enhancement is demonstrated compared to most nanosensors known from the literature. Hence, the technology offers the potential to complement conventional measurement systems in future sensor technology especially in portable applications.Die präzise Bestimmung der Art und Konzentration von Gasen erlangt in zahlreichen Anwendungsgebieten zunehmend an Bedeutung. Trotz der vielfältigen Wirkprinzipien heutiger Gassensoren lassen sich die technologischen Trends mit dem Schlagwort Miniaturisierung zusammenfassen, da sich daraus entscheidende Vorteile wie Integrierbarkeit und Energieeffizienz ergeben. Diese Arbeit beschäftigt sich mit der Entwicklung und Herstellung neuartiger nanodrahtbasierter Gassensoren, welche im Vergleich zu klassischen Sensoren eine vorteilhafte Kombination von hoher Sensitivität und Selektivität bei geringem Stromverbrauch und geringer Größe aufweisen. Auf der Grundlage gewachsener Silizium-Nanodrähte werden Sensoren mit abrupten Metall-Halbleiter-Kontakten hergestellt, welche auf dem Funktionsprinzip klassischer Schottkybarrieren-Feldeffekttransistoren beruhen. Die Eignung der Sensoren wird in Bezug auf die Detektion von Ammoniak untersucht. Dabei kann eine minimale Ammoniakkonzentration von 170 ppb mit einer Signaländerung von mehr als 160 % gemessen werden, wobei die theoretische Nachweisgrenze mit 20 ppb ermittelt wird. Selektivitätsuntersuchungen zeigen, dass keine Querempfindlichkeit gegenüber den am häufigsten in Wohnräumen vorkommenden Lösungsmitteln besteht. Feuchtigkeitseinflüsse auf den Sensor werden untersucht und es wird nachgewiesen, dass der Sensor Ansprechzeiten im Sekundenbereich besitzt, was ihn zu einem potenziell geeigneten Feuchtigkeitssensor macht. Darüber hinaus wird gezeigt, dass eine höhere relative Luftfeuchtigkeit und höhere Umgebungstemperaturen die Sensorsensitivität verringern. In Bezug auf mögliche Einsatzgebiete stellt die maximale Empfindlichkeit bei 25 °C einen großen Vorteil da. Bezogen auf Sensitivität und Selektivität wird somit eine Verbesserung im Vergleich zu den meisten aus der Literatur bekannten Nanosensoren demonstriert. Damit bietet die Technologie das Potential, konventionelle Messsysteme in zukünftiger Sensorik vor allem in portablen Anwendungen zu ergänzen

A comparison of MISiC Schottky-diode hydrogen sensors made by NO, N 2O, or NH 3 nitridations

MISiC Schottky-diode hydrogen sensors with gate insulator grown in three different nitridation gases (nitric oxide (NO), N 2O, and NH 3) are fabricated. Steady-state and transien-t-response measurements are carried out at different temperatures and hydrogen concentrations using a computer-controlled measurement system. Experimental results show that these nitrided sensors have high sensitivity and can give a rapid and stable response over a wide range of temperature. This paper also finds that N 2O provides the fastest insulator growth with good insulator quality and hence the highest sensitivity among the three nitrided samples. The N 2O- nitrided sensor can give a significant response even at a low H 2 concentration of 48-ppm H 2 in N 2, indicating a potential application for detecting hydrogen leakage at high temperature. Moreover, the three nitrided samples respond faster than the control sample. At 300°C, the response times of the N 2O, NO, and NH 3-nitrided sample to the 48-ppm H 2 in N 2 are 11, 11, and 37 s, respectively, as compared to 65 s for the control sample without the gate insulator. © 2006 IEEE.published_or_final_versio

One-Dimensional Oxide Nanostructures as Gas-Sensing Materials: Review and Issues

In this article, we review gas sensor application of one-dimensional (1D) metal-oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. One of the most important steps in fabricating 1D-nanostructure devices is manipulation and making electrical contacts of the nanostructures. Gas sensors based on individual 1D nanostructure, which were usually fabricated using electron-beam lithography, have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools. In the second part of the paper, some critical issues are pointed out including long-term stability, gas selectivity, and room-temperature operation of 1D-nanostructure-based metal-oxide gas sensors

Study of Mos2 and Graphene-Based Heterojunctions for Electronic and Sensing Applications

Since the discovery of graphene, there has been an increase in two-dimensional (2D) materials research for their scalability down to atomic dimensions. Among the analogs of graphene, transition metal dichalcogenides (TMDs) are attractive due to their exceptional electronic and optoelectronic properties. MoS2, a TMD, has several advantages over graphene and the industry workhorse Si, and has been reported to demonstrate excellent transistor performances. The key obstacle in the commercialization of MoS2 technology is low carrier mobility over large areas for top-down devices. Although there were several early reports on synthesis of atomically thin MoS2 with moderate mobility, transferring large area grown films to a substrate of choice leads to interface charges that degrade mobility. In our work, a top-down growth technique for synthesizing large area, 3-5 monolayers (ML) thick MoS2 film have been presented by pre-oxidation of metallic Mo instead of direct sulfidation. The growth temperature was significantly reduced in this method, eliminating free sulfur-induced degradation of the SiO2 gate dielectric. As a result, the leakage current was suppressed by a factor of \u3e108, when compared to a single step direct sulfidation method. Using these thin films, back-gated field effect transistors have been demonstrated with accumulation electron mobility \u3e80 cm2/Vs, on/off \u3e105, and subthreshold swing of 84 mV/dec; which are among the best results for MoS2 based transistors on SiO2 substrate. A hypothesis on current saturation has also been presented, attributing it to charge control rather than velocity saturation. The second part of our work aims at utilizing the best properties both graphene and MoS2 simultaneously by forming a heterojunction of these two atomically thin materials. Interestingly, these two materials have certain contrasting properties, for example, graphene based FETs have poor switching performance while MoS2 based FETs can outperform many state-of-the-art ultra-low power transistors. Fabricating a Schottky diode made of graphene and MoS2 allows the unique properties of these two materials to be combined and has been shown to be useful. A key property of these 2D heterojunctions is that each constituent of the heterojunction is so thin that it may not be able to completely screen an electric field from the second constituent, i.e. the Debye screening length can be greater than the layer thicknesses, so that voltage-induced interfacial tuning is achievable. This capability is unique to thin layers, most practically achieved in 2D heterojunctions, and has been exploited in recent “barristors”, which are 3-terminal devices with Schottky diodes where the barrier height can be tuned by an insulated gate. Such a tunable Schottky diode, similar to a triode vacuum tube is attractive for applications in RF circuits, photodetection and chemical sensing, analog and digital electronics, etc, with all the advantages of solid state devices e.g. high speed, low-cost and compactness. In this work, a graphene/MoS2 heterojunction on SiO2 dielectric has been fabricated to demonstrate a functional barristor device. By varying the gate bias between -20 V and +10 V, the barrier height could be modulated by \u3e0.65 eV, potentially enabling current control over 10 orders of magnitude at room temperature. Using the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of this device, we have also extracted the Richardson’s coefficient and electronic effective mass in MoS2 using a thermionic emission model, which are very important parameters required for proper engineering of these devices. After that, various applications of the barristor device have also been explored. The high optical response of the barristor has demonstrated the presence of photoconductive gain, and has been consistent with the changes in Schottky barrier height caused by the back-gate. The barristor has also been successful as gate-tunable toxic gas sensors, with lowest level detection lying around 100 ppb (parts per billion) for NO2 and 1 ppm (parts per million) for NH3. These observations highlight the potential applications of the graphene/MoS2 barristor for various electronic, optoelectronic and sensing applications. Finally, a mixed dimensional barristor made of graphene/InN nanowire heterojunction with a backgate has been demonstrated. The surface passivation of InN and the tunnel barrier formation at the graphene/NW interface have been achieved through controlled O2 plasma exposure, which has allowed an otherwise ohmic contact to turn into a gate tunable Schottky junction with \u3e1 eV barrier height. This device has been demostrated to perform sub-ppb level trace gas detection, photo-detection with very high sensitivity and a novel gate-controllable memristive action through longer O2 plasma exposure

Tuning the Performance of Nanocarbon-Based Gas Sensors Through Nanoparticle Decoration

Tin dioxide (SnO2) is a well–known gas sensing material, but it becomes sensitive only at elevated temperatures (e.g., above 200 &degC). Nanoparticles (NPs) combined with nanocarbons, such as carbon nanotubes (CNTs) and graphene, form a new class of hybrid nanomaterials that can exhibit fascinating gas sensing performance due to tunable electron transfer between NPs and nanocarbons induced by gas adsorption. Indeed, sensors made of SnO2 NPs&ndascoated CNTs have shown outstanding room–temperature sensing performance to various gases, including those that are undetectable by either SnO2 or CNTs alone. The objectives of this dissertation study are to synthesize various NP–nanocarbon hybrid materials and to fabricate and characterize sensing platforms based on the resulting hybrid nanomaterials. Two simple and efficient methods have been used for the hybrid synthesis. One is a simple NP synthesis and assembly system for NP–nanocarbon hybrid nanomaterials production through combining a mini–arc plasma reactor with electrostatic force–directed assembly. The other is a simple wet–chemical method for direct fabrication of doped SnO2 NP–decorated reduced graphene oxide (RGO) sheets. In particular, CNT/Ag NP and RGO/Ag NP hybrids have been produced for fast, sensitive, and selective detection of NH3. Furthermore, a ternary hybrid of Ag NPs and SnO2 NPs–decorated CNTs has been demonstrated and showed better sensing performance than CNT/SnO2 NP hybrids likely due to the enhanced gas adsorption and electron transfer. Additionally, hybrid sensors of In–doped SnO2 NPs on RGO are shown to exhibit high selectivity to NO2 sensing. Finally, the sensing mechanism for the NP–nanocarbon system has been extensively discussed. Based on this study, we conclude that the sensing performance (including sensitivity, selectivity, and response time) can be fine–tuned by coating nanocarbons with carefully–selected NPs (pure or doped). An attempt has been made to compare the sensing performance of hybrids based on various types of nanocarbons (e.g., multiwalled CNTs, semiconducting single–walled CNTs, RGO). Nanocarbons with superior semiconducting properties as building blocks of hybrid nanomaterials are shown to exhibit better gas sensing performance. This study provides a scientific foundation to engineer practical room–temperature gas sensors with enhanced performance