Sumita Santra

Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices

In this study we report the enhancement of UV photodetection and wavelength tunable light induced NO gas sensing at room temperature using Au-ZnO nanocomposites synthesized by a simple photochemical process. Plasmonic Au-ZnO nanostructures with a size less than the incident wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that leads to a strong absorption, scattering and local field enhancement. The photoresponse of Au-ZnO nanocomposite can be effectively enhanced by 80 times at 335 nm over control ZnO. We also demonstrated Au-ZnO nanocomposites application to wavelength tunable gas sensor operating at room temperature. The sensing response of Au-ZnO nancomposite is enhanced both in UV and visible region, as compared to control ZnO. The sensitivity is observed to be higher in the visible region due to the LSPR effect of Au NPs. The selectivity is found to be higher for NO gas over CO and some other volatile organic compounds (VOCs), with a minimum detection limit of 0.1 ppb for Au-ZnO sensor at 335 nm.

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.

Enhanced sensitivity and selectivity of brush-like SnO2 nanowire/ZnO nanorod heterostructure based sensors for volatile organic compounds

Brush-like SnO2 nanowires have been grown by pulsed laser deposition on ZnO nanorods synthesized by the hydrothermal method. SnO2 nanowire/ZnO nanorod heterostructures have been used for sensing several volatile organic compounds (VOCs). The heterostructure sensor exhibits higher response compared to that of control ZnO nanorods. The potential barriers formed at the SnO2–ZnO and SnO2–SnO2 interface are proposed to be responsible for an improved sensing performance over the pure ZnO nanorods. The effect of the length of SnO2 nanowires on the performance of triethylamine, toluene, ethanol, acetic acid, acetone, and methanol sensing has been studied. It is found that the response to the VOCs greatly depends on the length of the brush-like SnO2 nanowires. The SnO2/ZnO heterostructures can be successfully used to discriminate acetone from other VOCs.

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.

CMOS temperature sensors - concepts, state-of-the-art and prospects

The paper reviews the state-of-the-art in IC temperature sensors. It starts by revisiting the semiconductor theory of thermodiodes and thermotransistors, continues with the introduction of IC temperature sensors, the concepts of VPTAT - voltage proportional to absolute temperature and IPTAT (current proportional to absolute temperature) and discusses the possibility of use of parasitic bipolar transistors as temperature sensors in pure CMOS technology. The next section demonstrates the very high operating temperature of a dasiaspecialpsila thermodiode, well beyond the typical IC silicon junction temperature. This is achieved with a diode embedded in an SOI CMOS micro-hotplate. A discussion on the temperature limits of integrated temperature sensors is also given. The final section outlines the prospects of IC temperature sensors.

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.

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.

Ultra-high temperature (≫ 300°C) suspended thermodiode in SOI CMOS technology

This paper reports for the first time on the performance and long term continuous operation of a suspended silicon on insulator (SOI) thermodiode with tungsten metallisation at temperatures beyond 300degC. The thermodiode has been designed and fabricated with minute saturation currents (due to both small size and the use of SOI technology) to allow an ultra-high temperature range and minimal nonlinearity. It was found that the thermodiode forward voltage drop vs temperature plot remains linear upto 500degC, with a non-linearity error of less than 7%. Extensive experimental results on performance of the thermodiode, fabricated using a CMOS (complimentary metal oxide semiconductor) SOI process have been presented. These results are backed up by infra red measurements and a range of 2D and 3D simulations using ANSYS and ISE software. The on-chip electronics for thermodiode and micro-heater drive, as well as the transducing circuit for the sensor were placed adjacent to the membrane. Moreover, we demonstrate that the thermodiode is considerably more reliable in long-term direct current operation at high temperatures when compared to the more classical resistive temperature detectors (RTDs) using CMOS metallisation layers (Tungsten or Aluminum). Finally, we believe that the thermodiode suffers less of piezojunction/piezo-resistive effects when compared to silicon based RTDs. For this we compare a membrane thermodiode with a reference thermodiode placed on the silicon substrate and assess their relative performance at elevated temperatures.