BITS Faculty Publications
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Item ZnO Nanorod Based Ultra Sensitive and Selective Explosive Sensor(IEEE, 2013-02) Rao, V. RamgopalA small scale (20 μm), ultra sensitive (50 ppb) and highly selective sensor based on ZnO nanostructures using Micro-electro-mechanical system (MEMS) platform has been reported here for the detection of explosive and Volatile Organic Compound (VOC) vapors. Flower and rod like architectures of nanorods were used as a sensing layer. The nanorods prepared via chemical synthesis were uniform with diameters of 50-80 nm and lengths about 3-4 μm. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) reveal that the nanostructures are well oriented with the c-axis, perpendicular to the substrate. A relatively higher selectivity for 2, 4, 6-Trinitrotoluene (TNT) vapors compared to other VOCs at room temperature were observed. The intensity of deep level green emission peak associated with point defects decreases after exposure as revealed from Photoluminescence (PL) spectra.Item Al-doped ZnO thin-film transistor embedded microcantilever as a piezoresistive sensor(AIP, 2013) Rao, V. RamgopalIn this work, an aluminium-doped zinc oxide (AZO) thin film transistor, embedded in a polymer micro-cantilever, is demonstrated for nano-mechanical sensing applications. This device senses the surface stress due to a change in the carrier mobility of the semi-conducting layer. Due to the low Young's modulus and high strain sensitivity of the AZO layer, this micro-cantilever shows a deflection sensitivity of 116 ppm per nanometer of deflection. Also, mechanical characterization of these devices shows that the resonance frequency is in the range of a few tens of kilohertz which is suitable for sensor applications.Item Porphyrin induced changes in charge transport of graphene FET(IEEE, 2016) Rao, V. RamgopalThe transport properties of back-gated graphene field effect transistors (GFETs) can be tuned via chemical doping. In this study, we report alteration of charge transport properties of GFET via 5-(4-hydroxyphenyl)-10,15,20-tri-(p-tolyl) zinc(II) porphyrin (Zn(II)-TTPOH) and its free base counterpart. We propose that, the porphyrin induces p-type doping in graphene.Item Application of Mono Layered Graphene Field Effect Transistors for Gamma Radiation Detection(IEEE, 2018-10) Rao, V. RamgopalIn this work, we report the application of graphene field effect transistors (GFETs) as a gamma radiation sensor. The GFETs were irradiated at room temperature by 60 Co gamma radiation source for 10 kGy and 20 kGy gamma dose. The Electrical measurements and Raman spectroscopy showed that gamma radiation induced p-doping in graphene. Large positive shifts in Dirac point and significant degradation in electron mobility were observed post-gamma irradiation. Thus modulation in transport properties of GFETs was utilized here to measure the absorbed gamma radiations. We propose, a GFET based radiation detector with high sensitivity of + 113 V for 20 kGy gamma dose operating in ambient condition.Item MoS2 based nanomechanical bolometer for combined radiation sensing and the estimation of material properties(IEEE, 2023) Rao, V. RamgopalTwo-dimensional material-based bolometer sensors have unique optoelectronic properties. Here, a transparent and flexible bolometer and photodetector, based on MoS 2 is presented which measures radiation intensity via mechanical transduction and band gap transition. Three different cavity dimensions are fabricated to understand its effect on the bolometer performance. The fabricated devices are used to estimate the gauge factor (GF) and absorption coefficient of the material from the valley and peak data of current-time curves. The GF of 100 and absorption coefficient of 2×106 cm−1 is obtained for the MoS 2 on PDMS device. The work presented here is highly encouraging for the utilization of MoS 2 & PDMS in bolometers, thermal sensors and photodetectors.Item Determining ionizing radiation using sensors based on organic semiconducting material(AIP, 2009-03) Rao, V. RamgopalThe use of organic semiconducting material sensors as total dose radiation detectors is proposed, wherein the change in conductivity of an organic material is measured as a function of ionizing radiation dose. The simplest sensor is a resistor made using organic semiconductor. Furthermore, for achieving higher sensitivity, organic field effect transistor (OFET) is used as a sensor. A solution processed organic semiconductor resistor and an OFET were fabricated using poly 3-hexylthiophene (P3HT), a p-type organic semiconductor material. The devices are exposed to Cobalt-60 radiation for different total dose values. The changes in electrical characteristics indicate the potential of these devices as radiation sensors.Item Polymer microcantilever biochemical sensors with integrated polymer composites for electrical detection(Elsevier, 2009-09) Rao, V. RamgopalMicro fabricated sensors based on nanomechanical motion with piezoresistive electrical readout have become a promising biochemical sensing tool. The conventional microcantilever materials are mostly silicon-based. The sensitivity of the sensor depends on Young's modulus of the structural material, thickness of the cantilever as well as on the gauge factor of the piezoresistor. UV patternable polymers such as SU-8 have a very low Young's modulus compared to the silicon-based materials. Polymer cantilevers with a piezoresistive material having a large gauge factor and a lower Young's modulus are therefore highly suited for sensing applications. In this work, a spin coatable and photopatternable mixture of carbon black (CB) and SU-8, with proper dispersion characteristics, has been demonstrated as a piezoresistive thin film for polymer microcantilevers. Results on percolation experiments of SU-8/CB composite and fabrication of piezoresistive SU-8 microcantilevers using this composite are presented. With our controlled dispersion experiments, we could get a uniform piezoresistive thin film of thickness less than 1.2 μm and resistivity of 2.7 Ω cm using 10 wt% of CB in SU-8. The overall thickness of the SU-8/composite/SU-8 is approximately 3 μm. We further present results on the electromechanical characterization and biofunctionalization of the cantilever structures for biochemical sensing applications. These cantilevers show a deflection sensitivity of 0.55 ppm/nm. Since the surface stress sensitivity is 4.1 × 10−3 [N/m]−1, these cantilevers can well be used for detection of protein markers for pathological applications.Item Polymer composite-based OFET sensor with improved sensitivity towards nitro based explosive vapors(Elsevier, 2010-06) Rao, V. RamgopalSensors based on the organic field effect transistors (OFETs) have been used for detection of vapors of explosives, e.g. 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 2,4,6-trinitrotoluene (TNT), and dinitrobenzene (DNB). The organic composite used was a combination of P3HT (poly 3-hexylthiophene), CuTPP (CuII tetraphenylporphyrin) and ADB (copolymer of diethynyl-pentiptycene and dibenzyl-ProDOT). The incorporation of ADB into P3HT/CuTPP composite makes the film porous and thereby increases the sensitivity of the sensors by increasing the permeability of the analyte into the sensing polymeric matrix. Secondly, it increases the surface roughness which eventually increases the surface area resulting in an increased sensitivity. Along with an improved sensitivity (enhances the response by around 30% to nitro based explosive compounds), the sensors exhibit an excellent selectivity for nitro based explosive analytes like TNT, RDX, DNB as compared to the vapors of non-explosive oxidizing agents such as nitrobenzene (NB), benzoquinone (BQ) and benzophenone (BP). Porosity was analysed using gas (N2) sorption techniques (BET analysis). Surface roughness was characterized by AFM (atomic force microscopy). EFM (electrostatic force microscopy) was also done to see the effect of ADB polymer in reducing the initial conductivity of the film.Item Nanotechnology Polymer nanocomposite nanomechanical cantilever sensors: material characterization, device development and application in explosive vapour detection(IOP, 2011-06) Rao, V. RamgopalThis paper reports an optimized and highly sensitive piezoresistive SU-8 nanocomposite microcantilever sensor and its application for detection of explosives in vapour phase. The optimization has been in improving its electrical, mechanical and transduction characteristics. We have achieved a better dispersion of carbon black (CB) in the SU-8/CB nanocomposite piezoresistor and arrived at an optimal range of 8–9 vol% CB concentration by performing a systematic mechanical and electrical characterization of polymer nanocomposites. Mechanical characterization of SU-8/CB nanocomposite thin films was performed using the nanoindentation technique with an appropriate substrate effect analysis. Piezoresistive microcantilevers having an optimum carbon black concentration were fabricated using a design aimed at surface stress measurements with reduced fabrication process complexity. The optimal range of 8–9 vol% CB concentration has resulted in an improved sensitivity, low device variability and low noise level. The resonant frequency and spring constant of the microcantilever were found to be 22 kHz and 0.4 N m − 1 respectively. The devices exhibited a surface stress sensitivity of 7.6 ppm (mN m − 1) − 1 and the noise characterization results support their suitability for biochemical sensing applications. This paper also reports the ability of the sensor in detecting TNT vapour concentration down to less than six parts per billion with a sensitivity of 1 mV/ppb.Item Piezoresistive SU-8 Cantilever With Fe(III)Porphyrin Coating for CO Sensing(IEEE, 2012-07) Rao, V. RamgopalCarbon monoxide detection is required for various healthcare, environmental, and engineering applications. In this paper, 5,10,15,20-tetra (4,5-dimethoxyphenyl)-21H,23H-porphyrin iron(III) chloride (Fe(III)porphyrin) coated on a piezoresistive SU-8 microcantilever has been used as a CO sensor. Rapid detection of CO down to 2 ppm has been observed with aforementioned sensors. Cantilevers without Fe(III)porphyrin have not responded to CO exposure. Fe(III)porphyrin-coated cantilever selectivity toward CO has been analyzed by measuring the sensor response to various gases such as N 2 , CO 2 , O 2 , ethanolamine, N 2 O, and moisture. The sensor has exhibited a fast response and recovery times and is fully recoverable after repeated exposures.