V. Ramgopal Rao

Polymer nanocomposite nanomechanical cantilever sensors: material characterization, device development and application in explosive vapour detection

This 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.

Insights Into the Design and Optimization of Tunnel-FET Devices and Circuits

Improving the on-current has been the focus of enhancing the performance of tunnel field-effect transistors (TFETs). In this paper, we show that the increase in I_ON is not sufficient to improve the circuit performance with TFETs. As TFETs show a drain-barrier voltage in their output characteristics below which the drain current drastically reduces, the rise/fall time significantly increases. This reduces the dynamic noise margin and limits the performance achievable from TFETs. We show that, in TFETs, the delay of the circuit is determined by the rise/fall time rather than by the propagation delay. The saturation voltage is much higher compared with that of complementary metal-oxide-semiconductor (CMOS) devices, leading to a lower gain and a lower static noise margin in digital circuits, as well as impeding the performance of latch/regenerative circuits. We present a design space comprising of I_ON, a drain saturation voltage, and a drain threshold voltage for minimizing the propagation delay of circuits using TFETs. Finally, for the same off-current and speed of operation, TFET devices tend to suffer from a higher gate capacitance compared with CMOS devices. If this behavior is not taken into account during the circuit design, these devices (although designed for low-power applications) can dissipate more power at the same speed of operation than CMOS counterparts.

Modeling of parasitic capacitances in deep submicrometer conventional and high-K dielectric MOS transistors

In deep submicrometer MOSFETs the device performance is limited by the parasitic capacitance and resistance. Hence a circuit model is needed to treat these effects correctly. In this work, we have developed circuit models for the parasitic capacitances in conventional and high-K gate dielectric MOS transistors by taking into account the presence of source/drain contact plugs. The accuracy of the model is tested by comparing the modeled results with the results obtained from three-dimensional (3-D) Monte-Carlo simulations and two-dimensional (2-D) device simulations over a wide range of channel length and oxide thickness. The model is also used to study the dependence of parasitic capacitance on gate length, gate electrode thickness, gate oxide thickness, gate dielectric constant, and spacer width.

Solution-Processed n-Type Organic Field-Effect Transistors With High on / off Current Ratios Based on Fullerene Derivatives

Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP.

Optimization and realization of sub-100-nm channel length single halo p-MOSFETs

Single halo p-MOSFETs with channel lengths down to 100 nm are optimized, fabricated, and characterized as part of this study. We show extensive device characterization results to study the effect of large angle V/sub T/ adjust implant parameters on device performance and hot carrier reliability. Results on both conventionally doped and single halo p-MOSFETs have been presented for comparison purposes.

Fluorescence and piezoresistive cantilever sensing of trinitrotoluene by an upper-rim tetrabenzimidazole conjugate of calix[4]arene and delineation of the features of the complex by molecular dynamics.

A new benzimidazole-functionalized calix[4]arene receptor (R) was synthesized and characterized. The receptor R shows better selectivity toward trinitrotoluene (TNT) compared to the other nitro explosives in solution, which also retains its effectiveness for solid-phase detection. The chemical interactions of the molecule with different nitro explosive analytes were studied by fluorescence spectroscopy and by a molecular dynamics approach. The molecular dynamics studies show a 1:3 complex between R and TNT, and hence high sensitivity was imparted by fluorescence studies. The detection of explosive vapors in ambient conditions was tested by using a sensitive coating layer of R on an SU-8/CB-based piezoresistive cantilever surface. The developed device showed large sensitivity toward TNT compared to cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN) in the solid state at their respective vapor pressures at room temperature. The detection sensitivity of the device was estimated to be 35 mV for TNT at ambient conditions. Moreover, the sensor does not show a response when exposed to humidity. These results demonstrate that R can be used as one of the coating materials for a cantilever for the detection of TNT using piezoresistivity measurement. R can also detect the explosives in solution with high sensitivity and selectivity by fluorescence spectroscopy.

Explosive vapor sensor using poly (3-hexylthiophene) and CuII tetraphenylporphyrin composite based organic field effect transistors

Organic 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.

Strain induced anisotropic effect on electron mobility in C60 based organic field effect transistors

The electron mobility was found to increase (decrease) upon applied compressive (tensile) strain, respectively, when a high-performance flexible C60-based organic field-effect transistor (OFET) was subjected to different bending radii. The observed almost twofold relative change in the electron mobility is considerably larger than that reported before for pentacene-based OFETs. Moreover, the strain dependency of electron mobility in C60 films is strongly anisotropic with respect to the strain direction measured relative to the current flow. Analysis within a hopping-transport model for OFET mobility suggests that the observed strain dependency on electron transport is dominated mostly by the change of inter-grain coupling in polycrystalline C60 films.

Determining ionizing radiation using sensors based on organic semiconducting material

The 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.

A Novel Photoplastic Piezoelectric Nanocomposite for MEMS Applications

This letter reports a photoplastic (SU-8) piezoelectric (ZnO) nanocomposite route for realization of simple and low-cost piezoelectric microelectromechanical systems (MEMS). Integrating the ZnO nanoparticles into a photosensitive SU-8 polymer matrix not only retains the highly desired piezoelectric properties of ZnO but also combines the photopatternability and the optical transparency of the SU-8 polymer. These two aspects, therefore, lead to exciting MEMS applications with simple photolithography-based microfabrication. This approach opens up many new applications in the field of both sensor and energy harvesting.