Vivek Ranjan - Academia.edu (original) (raw)
Address: Jaipur, India
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Papers by Vivek Ranjan
Recent advances in theoretical methods combined with the advent of massively-parallel supercomput... more Recent advances in theoretical methods combined with the advent of massively-parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from first principles. We describe applications in two general areas: (i) novel polymer composites for ultrahigh density capacitors, necessary for pulsed power applications, such as electric rail guns, power conditioning, and dense electronic circuitry, and (ii) ballistic electron transport in novel molecule-on-semiconductor structures exhibiting negative differential resistance. The phase diagram of P(VDF-CTFE), which has an usually high energy density, is determined as a function of the electric field. The calculations explain the origin of the observed ultra-high capacitance and suggest a systematic route, not limited to polymers, for obtaining nanostructured materials with high energy density. Turning to molecular electronics, we investigated porphyrins between Si leads, which are candidates for molecular memories and logic. We show that they exhibit tunable negative differential resistance (NDR). In some cases, huge peak-to-valley ratios are obtained, which should result in excellent switching behavior.
Physical Review Letters, 2007
The phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with chlorotrifluoroethyl... more The phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with chlorotrifluoroethylene (CTFE) are investigated by first-principles calculations. Both PVDF and dilute P(VDF-CTFE) prefer nonpolar structures at zero field, but transform to a polar phase below the breakdown field. The critical field decreases with increasing CTFE content, facilitating the transformation. In disordered P(VDF-CTFE), a distribution of concentrations leads to a range of polar transitions, resulting in ultrahigh energy density. These results explain well experimental observations of very high-energy density in P(VDF-CTFE).
Advances in theoretical methods and parallel super computing allow for reliable ab initio simulat... more Advances in theoretical methods and parallel super computing allow for reliable ab initio simulations of the properties of complex materials. We describe two applications: (i) negative differential resistance (NDR) in self assembled monolayers of ferrocenyl-alkanethiolate on a gold surface, and (ii) interface bonding in polymer/ceramic nanocomposites. Our non-equilibrium Green's function calculations show that electron transport through ferrocenyl-alkanethiolate exhibits strong NDR features at both positive and negative biases, in good agreement with the experimental data. The results suggest that the ferrocenyl group acts like a quantum dot and that the NDR features are due to resonant coupling between the HOMO and the density of states of gold leads. Tuning of the "strength" of the NDR and its implication for the design of molecular devices are also discussed. For polymer/ceramic nanocomposites, we show that direct attachment of alkane chains to ceramic surfaces is not energetically favorable, while silanated chains attach through a bridging OH group with an energy gain
Recent advances in theoretical methods combined with the advent of massively-parallel supercomput... more Recent advances in theoretical methods combined with the advent of massively-parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from first principles. We describe applications in two general areas: (i) novel polymer composites for ultrahigh density capacitors, necessary for pulsed power applications, such as electric rail guns, power conditioning, and dense electronic circuitry, and (ii) ballistic electron transport in novel molecule-on-semiconductor structures exhibiting negative differential resistance. The phase diagram of P(VDF-CTFE), which has an usually high energy density, is determined as a function of the electric field. The calculations explain the origin of the observed ultra-high capacitance and suggest a systematic route, not limited to polymers, for obtaining nanostructured materials with high energy density. Turning to molecular electronics, we investigated porphyrins between Si leads, which are candidates for molecular memories and logic. We show that they exhibit tunable negative differential resistance (NDR). In some cases, huge peak-to-valley ratios are obtained, which should result in excellent switching behavior.
Physical Review Letters, 2007
The phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with chlorotrifluoroethyl... more The phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with chlorotrifluoroethylene (CTFE) are investigated by first-principles calculations. Both PVDF and dilute P(VDF-CTFE) prefer nonpolar structures at zero field, but transform to a polar phase below the breakdown field. The critical field decreases with increasing CTFE content, facilitating the transformation. In disordered P(VDF-CTFE), a distribution of concentrations leads to a range of polar transitions, resulting in ultrahigh energy density. These results explain well experimental observations of very high-energy density in P(VDF-CTFE).
Advances in theoretical methods and parallel super computing allow for reliable ab initio simulat... more Advances in theoretical methods and parallel super computing allow for reliable ab initio simulations of the properties of complex materials. We describe two applications: (i) negative differential resistance (NDR) in self assembled monolayers of ferrocenyl-alkanethiolate on a gold surface, and (ii) interface bonding in polymer/ceramic nanocomposites. Our non-equilibrium Green's function calculations show that electron transport through ferrocenyl-alkanethiolate exhibits strong NDR features at both positive and negative biases, in good agreement with the experimental data. The results suggest that the ferrocenyl group acts like a quantum dot and that the NDR features are due to resonant coupling between the HOMO and the density of states of gold leads. Tuning of the "strength" of the NDR and its implication for the design of molecular devices are also discussed. For polymer/ceramic nanocomposites, we show that direct attachment of alkane chains to ceramic surfaces is not energetically favorable, while silanated chains attach through a bridging OH group with an energy gain