BITS Faculty Publications
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Item A Simple and Direct Method for Interface Characterization of OFETs(IEEE, 2007) Rao, V. RamgopalMulti-frequency transconductance technique is successfully applied in this work for the first time for interface characterization of OFETs. Standard charge pumping measurements are used on silicon MOSFETs for the validation of MFT technique. The method is implemented on pentacene as well as the P3HT based OFETs with SiO 2 as the gate dielectric. Our results show interface state densities in the range of 10 12 /cm 2 /eV for both the samples. The P3HT films are also shown to have additional trap centres which respond to frequencies above 100 kHz. Our results therefore clearly indicate that the MFT technique is indeed a highly useful technique for interface characterization of OFETs.Item Organic field effect transistors for explosive and radiation dosimetry applications(IEEE, 2016-11) Rao, V. RamgopalThis paper presents a brief overview of an Organic field effect transistor (OFET) based sensor applications. It highlights recent progress in the field of gas sensing and ionizing radiation sensing using OFETs. Besides step-by-step improvement of conventional devices, novel materials and novel approaches for OFET based gas and radiation sensing are discussed.Item Investigations of enhanced device characteristics in pentacene-based field effect transistors with sol-gel interfacial layer(AIP, 2007-03) Rao, V. RamgopalPentacene films grown on sol-gel silica dielectrics showed a significant enhancement in field effect mobility, threshold voltages, and subthreshold swings. This letter investigates the contributing factors for the enhanced device characteristics. The smoother and more hydrophobic film surfaces of sol-gel silica (rms roughness of ∼1.9Å and water contact angle of ∼75°) induced larger pentacene grains, yielding mobilities in excess of ∼1cm2/Vs at an operating voltage of −20V. Different sol-gel silica film thicknesses showed similar trends in improved performances, indicating that this phenomenon is clearly a semiconductor-dielectric interface phenomenon rather than a bulk dielectric effect.Item Explosive vapor sensor using poly (3-hexylthiophene) and CuII tetraphenylporphyrin composite based organic field effect transistors(AIP, 2008-12) Rao, V. RamgopalOrganic field effect transistors based on poly(3-hexylthiophene) and CuII tetraphenylporphyrin composite were investigated as sensors for detection of vapors of nitrobased explosive compounds, viz., 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 2,4,6-trinitrotoluene (TNT), and dinitrobenzene, which are also strong oxidizing agents. Significant changes, suitable for sensor response, were observed in transistor “on” current (Ion) and conductance (S) after exposure. A similar device response was, however, not observed for oxidizing agents such as benzoquinone and benzophenone. The Fourier transform infrared spectrometry experiments supported the results, where exposure to RDX and TNT vapors resulted in a significant shift in IR peaks.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 Poly(3-hexylthiophene) and hexafluoro-2-propanol-substituted polysiloxane based OFETs as a sensor for explosive vapor detection(Elsevier, 2011-11) Rao, V. RamgopalThe organic field effect transistors (OFETs) with regioregular poly 3-hexylthiophene (rr-P3HT) and hexafluoro-2-propanol-substituted polysiloxane (SXFA) as an organic layer, have been used for detection of explosive vapors with excellent sensitivity of less than 70 ppt for 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and less than 100 ppt for 2,4,6-trinitrotoluene (TNT). The sensor response (% change in saturation current) was found to be 125 ± 10% for TNT and 90 ± 10% for RDX. It was also observed that the incorporation of CuII tetraphenylporphyrin (CuTPP) into rr-P3HT/SXFA matrix resulted in an improved selectivity for the vapors of nitro based analytes (TNT, RDX and DNB) as compared to the vapors of non explosive oxidizing agents such as nitrobenzene (NB), benzoquinone (BQ) and benzophenone (BP). This is attributed to the increased binding of the vapors containing nitro compound to the thin films due to the presence of CuTTP. Spin coated thin films were further characterized by Atomic Force Microscopy (AFM) and Electrostatic Force Microscopy (EFM).Item Organic Sensor Platforms for Environmental and Security Applications(IOP, 2011) Rao, V. RamgopalOrganic sensors based on polymer microcantilevers and organic field effect transistors (OFETs) bring orthogonality to the sensing mechanism for different environmental and security applications. Orthogonolity is an important requirement from the point of reducing the false positives, which is of utmost importance for many applications. Development of polymer nanocomposite microcantilever based sensors for explosive vapour detection was reported. These polymer microcantilevers offer a deflection sensitivity of 1.1ppm for 1nm of deflection, which is the highest sensitivity reported till date. The sensor response to trinitrotoluene (TNT) vapours at a few parts per billion concentration levels was demonstrated. OFETs using poly 3-hexylthiophene (P3HT) and CuII tetraphenylporphyrin (CuTPP) composite as their active material were studied as sensors for detection of various nitro-based explosives. Significant changes were observed in the ON current (Ion) and transconductance (gm) of the OFET sensor after exposure to vapours of various explosive compounds. Sensor selectivity to explosive vapours over strong oxidizing agents was also demonstrated. Also, the change in conductivity for organic semiconducting material as a function of ionizing radiation was studied with an organic semiconducting material in resistor and OFET configurations. 30MΩ/Gy sensitivity for the organic resistor sensor and 28fA/Gy/1µm-width sensitivity for OFF current for the OFET sensor were observed. Moreover, changes in various other electrical parameters for an OFET sensor were also found proportional to the ionizing radiation dose.Item Solution processed photopatternable high-k nanocomposite gate dielectric for low voltage organic field effect transistors(Elsevier, 2012-08) Rao, V. RamgopalWe report an organic field effect transistors (OFETs) with photo-patternable, solution processed nanoparticle composite high-k gate dielectric layer. The dielectric layer consists of Barium Titanate (BT) nanoparticles dispersed in SU-8, which makes it possible to use solution-processable methods to prepare the dielectric layer. The dielectric constant k of the nanoparticle composite films can be tuned over a wide range by varying the concentration of BT particles, which enables lower voltage operation possible with these composite gate dielectric films. OFETs with P3HT as the semiconducting layer have been demonstrated; it was found that the OFETs with the nanocomposite dielectric layer show a significant improvement in the drive current yet retaining the photopatternability, which is an advantage for circuit fabrication. The composite being a high-k enables low voltage operation (∼4 V) compared to pristine SU-8 as a gate dielectric operating at high voltages (∼40 V). Working organic transistors and inverters with a high-k nanocomposite dielectric layer (k > 13) with considerably lower leakage current have been demonstrated. This method allows low cost preparation of OFETs without the complicated gate dielectric deposition methods.Item An organic field effect transistors-based sensing platform for environmental/security applications(World Scientific, 2011) Rao, V. RamgopalOrganic semiconducting material based sensors have been used for various environmental applications. Organic field effect transistors (OFETs) also find their applications in explosive vapor detection and total ionizing radiation dose determination. OFETs using poly 3-hexylthiophene (P3HT), a p-type organic semiconductor material and CuII tetraphenylporphyrin (CuTPP) composite as their active material were investigated as sensors for detection of various nitro-based explosive vapors with greater than parts per billion sensitivity range. Significant changes, suitable for sensor response, were observed in ON current (Ion) and transconductance (gm) extracted from electrical characteristics of the OFET after exposure to vapors of various explosive compounds. However, a similar device response was not observed to strong oxidizing agents such as benzoquinone (BQ) and benzophenone (BP). Also, the use of organic semiconducting material sensors for determining total ionizing radiation dose was studied, wherein the conductivity of the material was measured as a function of total ionizing radiation dose. An organic semiconducting material resistor was exposed to γ-radiation and it was observed that the change in resistance was proportional to the ionizing radiation dose. Changes in various parameters extracted from electrical characteristics of the OFET after γ-radiation exposure resulted in an improved sensitivity. To protect the organic semiconductor layer from the degradation in the ambient the sensors were passivated with a thin layer of silicon nitride.Item Comparison among different algorithms in classifying explosives using OFETs(Elsevier, 2013-01) Rao, V. RamgopalVapour phase detection of explosives using pattern recognition approaches is a very important area of research worldwide. This paper elaborates on the comparison between different algorithms in classifying empirical multiparametric data that are obtained from the explosive vapor sensors based on organic field effect transistors (OFETs). We address the problem of classification by means of statistical comparison among algorithms such as NaiveBayes (NBS), locally weighted learning (LWL), sequential minimal optimization (SMO) and J48 decision tree on data acquired from OFETs. This analysis helps in understanding the nature of data obtained from experiments and in making efficient estimators for the detection of explosives. The correctly classified instances for predicting tested samples using LWL, NBS, SMO and J48 decision tree are 72%, 73%, 80% and 90%, respectively. The future development of standoff explosive detectors will be benefited greatly by a proper choice of these classification approaches.