Prasanta Kumar Guha

Chemically Reduced Graphene Oxide for Ammonia Detection at Room Temperature

Chemically reduced graphene oxide (RGO) has recently attracted growing interest in the area of chemical sensors because of its high electrical conductivity and chemically active defect sites. This paper reports the synthesis of chemically reduced GO using NaBH4 and its performance for ammonia detection at room temperature. The sensing layer was synthesized on a ceramic substrate containing platinum electrodes. The effect of the reduction time of graphene oxide (GO) was explored to optimize the response, recovery, and response time. The RGO film was characterized electrically and also with atomic force microscopy and X-ray photoelectron spectroscopy. The sensor response was found to lie between 5.5% at 200 ppm (parts per million) and 23% at 2800 ppm of ammonia, and also resistance recovered quickly without any application of heat (for lower concentrations of ammonia). The sensor was exposed to different vapors and found to be selective toward ammonia. We believe such chemically reduced GO could potentially be used to manufacture a new generation of low-power portable ammonia sensors.

CMOS integration of inkjet-printed graphene for humidity sensing

We report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10–80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.

Post-CMOS wafer level growth of carbon nanotubes for low-cost microsensors—a proof of concept

Here we demonstrate a novel technique to grow carbon nanotubes (CNTs) on addressable localized areas, at wafer level, on a fully processed CMOS substrate. The CNTs were grown using tungsten micro-heaters (local growth technique) at elevated temperature on wafer scale by connecting adjacent micro-heaters through metal tracks in the scribe lane. The electrical and optical characterization show that the CNTs are identical and reproducible. We believe this wafer level integration of CNTs with CMOS circuitry enables the low-cost mass production of CNT sensors, such as chemical sensors.

Enhanced ammonia sensing at room temperature with reduced graphene oxide/tin oxide hybrid films

Sensitive and selective detection of ammonia at room temperature is required for proper environmental monitoring and also to avoid any health hazards in the industrial areas. The excellent electrical properties of reduced graphene oxide (RGO) and sensing capabilities of SnO2 were combined to achieve enhanced ammonia sensitivity. RGO–SnO2 films were synthesized hydrothermally as well as prepared by mixing different amounts of hydrothermally synthesized SnO2 nanoparticles with graphene oxide (GO). It was observed that the response of the hybrid sensing layer was considerably better than intrinsic RGO or SnO2. However, the best performance was observed in the 10 : 8 (RGO–SnO2) sample. The sample was exposed to nine different concentrations of ammonia in the presence of 20% RH at room temperature. The response of the sensor varied from 1.4 times (25 ppm) to 22 times (2800 ppm) with quick recovery after purging with air. The composite formation was verified by characterizing the samples using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). The results and their significance have been discussed in detail.

Highly proton conducting MoS2/graphene oxide nanocomposite based chemoresistive humidity sensor

This paper reports the development of MoS2/GO nanocomposite based sensing layers for resistive humidity sensors. The MoS2 nanoflakes were synthesized through liquid exfoliation and GO was synthesized using modified Hummers method. The nanocomposite was drop-cast on a Si/SiO2 substrate containing aluminium electrodes to fabricate the sensor device. The best performance was shown by the 1 : 4 (MoS2/GO) composite. Various characterization techniques like Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Fourier Transform Infrared Spectroscopy (FTIR) were used to verify the composite formation. The sensing response was found to lie between 55 times at 35% RH and 1600 times at 85% RH. Such a high response is believed to be because of proton conductivity in the water layer for both MoS2 and GO. The sensor performance was found to be repeatable even after three months of the first measurement with quick response and recovery. Thus the authors believe that the excellent sensitivity coupled with low cost synthesis and resistive sensing will make their work useful to develop new generation humidity sensors.

Humidity Sensor Based on High Proton Conductivity of Graphene Oxide

This paper explores the performance of graphene oxide (GO) as humidity sensor. GO was synthesized using modified Hummers and Offeman method, and the sensing layer was characterized using optical microscope, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The sensor devices were fabricated by drop-casting of GO on patterned gold electrodes on Si/SiO2 substrate. GO-based sensor was exposed to six different relative humidity (RH%), and the response of our sensor was found to be excellent due to large proton conduction. The sensor response varied from ~180 times (40% RH) to ~1200 times (88% RH). Our GO-based humidity sensor also showed ultrafast response and recovery times with extremely good repeatability. Also, the role of functional groups in humidity sensing was explored by fabricating the sensor devices by thermally reducing GO for different time durations. We believe GO could potentially be used to develop new-generation ultrasensitive humidity sensor.

Design and simulation of resistive SOI CMOS micro-heaters for high temperature gas sensors

This paper describes the design of doped single crystal silicon (SCS) microhotplates for gas sensors. Resistive heaters are formed by an n+/p+ implantation into a Silicon-On-Insulator (SOI) wafer with a post-CMOS deep reactive ion etch to remove the silicon substrate. Hence they are fully compatible with CMOS technologies and allows for the integration of associated drive/detection circuitry. 2D electro-thermal models have been constructed and the results of numerical simulations using FEMLAB® are given. Simulations show these micro-hotplates can operate at temperatures of 500°C with a drive voltage of only 5 V and a power consumption of less than 100 mW.

Three technologies for a smart miniaturized gas-sensor: SOI CMOS, micromachining, and CNTs - challenges and performance

In this paper we propose a new type of solid-state gas sensor by combining three recent advances, namely silicon-on-insulator CMOS technology, through wafer etching and growth of gas-sensitive carbon nanotubes. We have developed novel tungsten-based CMOS micro-hotplates that offer ultra low power consumption (less than 10 mW at 250degC), on-chip CNT deposition at temperatures up to 700degC, and full integration of CMOS circuitry. Moreover, the tungsten micro-hotplates possess better stability than other CMOS materials such as polysilicon. The multi-walled CNT resistive gas sensors showed a good response to PPB levels of NO2 in air but required additional heating to provide reasonable baseline recovery times. We believe that our approach is attractive for the mass production of low-cost, low-power gas sensors in silicon foundries.

Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing

This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using a dip pen nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) and their characterisation as a low-cost, low-power ethanol sensor. The ZnO nanorods were synthesized hydrothermally and deposited on the MHP that comprise a tungsten micro-heater embedded in a dielectric membrane with gold interdigitated electrodes (IDEs) on top of an oxide passivation layer. The micro-heater and IDEs were used to heat up the sensing layer and measure its resistance, respectively. The sensor device is extremely power efficient because of the thin SOI membrane. The electro-thermal efficiency of the MHP was found to be 8.2 °C mW−1, which results in only 42.7 mW power at an operating temperature of 350 °C. The CMOS MHP devices with ZnO nanorods were exposed to PPM levels of ethanol in humid air. The sensitivity achieved from the sensor was found to be 5.8% ppm−1 to 0.39% ppm−1 for the ethanol concentration range 25–1000 ppm. The ZnO nanorods showed an optimum response at 350 °C. The CMOS sensor was found to have a humidity dependence that needs consideration in real-world application. The sensors were also found to be selective towards ethanol when tested in the presence of toluene and acetone. We believe that the integration of ZnO nanorods using DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol sensor that could be exploited in consumer electronics.

SOI diode temperature sensor operated at ultra high temperatures - a critical analysis

This paper investigates the performance of diode temperature sensors when operated at ultra high temperatures (above 250degC). A low leakage silicon on insulator (SOI) diode was designed and fabricated in a 1 mum CMOS process and suspended within a dielectric membrane for efficient thermal insulation. The diode can be used for accurate temperature monitoring in a variety of sensors such as microcalorimeters, IR detectors, or thermal flow sensors. A CMOS compatible micro-heater was integrated with the diode for local heating. It was found that the diode forward voltage exhibited a linear dependence on temperature as long as the reverse saturation current remained below the forward driving current. We have proven experimentally that the maximum temperature can be as high as 550degC. Long term continuous operation at high temperatures (400degC) showed good stability of the voltage drop. Furthermore, we carried out a detailed theoretical analysis to determine the maximum operating temperature and explain the presence of nonlinearity factors at ultra high temperatures.