Showing papers by "Indian Institute of Science published in 2008"

Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

Abstract: The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on $SiO_2$ back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to $5\times 10^{13} cm^{-2}$, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used $SiO_2$ back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.

Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties

Abstract: MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which d...

Control of jasmonate biosynthesis and senescence by miR319 targets.

Abstract: Considerable progress has been made in identifying the targets of plant microRNAs, many of which regulate the stability or translation of mRNAs that encode transcription factors involved in development In most cases, it is unknown, however, which immediate transcriptional targets mediate downstream effects of the microRNA-regulated transcription factors We identified a new process controlled by the miR319-regulated clade of TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) transcription factor genes In contrast to other miRNA targets, several of which modulate hormone responses, TCPs control biosynthesis of the hormone jasmonic acid Furthermore, we demonstrate a previously unrecognized effect of TCPs on leaf senescence, a process in which jasmonic acid has been proposed to be a critical regulator We propose that miR319-controlled TCP transcription factors coordinate two sequential processes in leaf development: leaf growth, which they negatively regulate, and leaf senescence, which they positively regulate

Rise of the Andes.

Abstract: The surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening, leading to their gradual rise. Recent studies of the Andes indicate that their elevation remained relatively stable for long periods (tens of millions of years), separated by rapid (1 to 4 million years) changes of 1.5 kilometers or more. Periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable, dense lower lithosphere after long-term thickening of the crust and lithospheric mantle.

Raman spectroscopy of graphene on different substrates and influence of defects

Abstract: We show the evolution of Raman spectra with a number of graphene layers on different substrates, $SiO_2/Si$ and conducting indium tin oxide (ITO) plate. The G mode peak position and the intensity ratio of G and 2D bands depend on the preparation of sample for the same number of graphene layers. The 2D Raman band has characteristic line shapes in single and bilayer graphene, capturing the differences in their electronic structure. The defects have a significant influence on the G band peak position for the single layer graphene: the frequency shows a blue shift up to $12 cm^{-1}$ depending on the intensity of the D Raman band, which is a marker of the defect density. Most surprisingly, Raman spectra of graphene on the conducting ITO plates show a lowering of the G mode frequency by $\sim 6cm^{-1}$ and the 2D band frequency by $\sim 20cm^{-1}$. This red-shift of the G and 2D bands is observed for the first time in single layer graphene.

Energy-Efficient Cooperative Relaying over Fading Channels with Simple Relay Selection

Abstract: We consider a cooperative wireless network where a set of nodes cooperate to relay in parallel the information from a source to a destination using a decode-and-forward approach. The source broadcasts the data to the relays, some or all of which cooperatively beamform to forward the data to the destination. We generalize the standard approaches for cooperative communications in two key respects: (i) we explicitly model and factor in the cost of acquiring channel state information (CSI), and (ii) we consider more general selection rules for the relays and compute the optimal one among them. In particular, we consider simple relay selection and outage criteria that exploit the inherent diversity of relay networks and satisfy a mandated outage constraint. These criteria include as special cases several relay selection criteria proposed in the literature. We obtain expressions for the total energy consumption for general relay selection and outage criteria for the non-homogeneous case, in which different relay links have different mean channel power gains, and the homogeneous case, in which the relay links statistics are identical. We characterize the structure of the optimal transmission scheme. Numerical results show that the cost of training and feedback of CSI is significant. The optimal strategy is to use a varying subset (and number) of relay nodes to cooperatively beamform at any given time. Depending on the relative location of the relays, the source, and the destination, numerical computations show energy savings of about 16% when an optimal relay selection rule is used. We also study the impact of shadowing correlation on the energy consumption for a cooperative relay network.

The role of friction stir welding tool on material flow and weld formation

Abstract: In this investigation an attempt has been made to understand the mechanism of friction stir weld formation and the role of friction stir welding tool in it. This has been done by understanding the material flow pattern in the weld produced in a special experiment, where the interaction of the friction stir welding tool with the base material is continuously increased. The results show that there are two different modes of material flow regimes involved in the friction stir weld formation; namely “pin-driven flow” and “shoulder-driven flow”. These material flow regimes merge together to form a defect-free weld. The etching contrast in these regimes gives rise to onion ring pattern in friction stir welds. In addition to that based on the material flow characteristics a mechanism of weld formation is proposed.

Resonance energy transfer from a dye molecule to graphene

Abstract: We study the distance dependence of the rate of resonance energy transfer from the excited state of a dye to the \pi system of graphene. Using the tight-binding model for the \pi system and the Diraccone approximation, we obtain the analytic expression for the rate of energy transfer from an electronically excited dye to graphene. While in traditional fluorescence resonance energy transfer, the rate has a $(distance)^{-6}$ dependence, we find that the distance dependence in this case is quite different. Our calculation of rate in the case of the two dyes, pyrene and nile blue, shows that the distance dependence is Yukawa type. We have also studied the effect of doping on energy transfer to graphene. Doping does not modify the rate for electronic excitation energy transfer significantly. However, in the case of vibrational transfer, the rate is found to be increased by an order of magnitude due to doping. This can be attributed to the nonzero density of states at the Fermi level that results from doping.

Flavor physics of leptons and dipole moments

Abstract: This chapter of the report of the “Flavor in the era of the LHC” Workshop discusses the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavor structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the standard model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.

Hypoxia and hypoxia-inducible factor-1 alpha modulate lipopolysaccharide-induced dendritic cell activation and function.

Abstract: Dendritic cells (DC) play a key role in linking innate and adaptive immunity. In inflamed tissues, where DC become activated, oxygen tensions are usually low. Although hypoxia is increasingly recognized as an important determinant of cellular functions, the consequences of hypoxia and the role of one of the key players in hypoxic gene regulation, the transcription factor hypoxia inducible factor 1\alpha (HIF-1\alpha), are largely unknown. Thus, we investigated the effects of hypoxia and HIF-1alpha on murine DC activation and function in the presence or absence of an exogenous inflammatory stimulus. Hypoxia alone did not activate murine DC, but hypoxia combined with LPS led to marked increases in expression of costimulatory molecules, proinflammatory cytokine synthesis, and induction of allogeneic lymphocyte proliferation compared with LPS alone. This DC activation was accompanied by accumulation of HIF-1\alpha protein levels, induction of glycolytic HIF target genes, and enhanced glycolytic activity. Using RNA interference techniques, knockdown of HIF-1alpha significantly reduced glucose use in DC, inhibited maturation, and led to an impaired capability to stimulate allogeneic T cells. Alltogether, our data indicate that HIF-1\alpha and hypoxia play a crucial role for DC activation in inflammatory states, which is highly dependent on glycolysis even in the presence of oxygen.

Open-framework structures of transition-metal compounds.

Abstract: Inorganic framework solids are no longer limited to just the silicates and phosphates. Recent research has revealed that carboxylates, arsenates, sulfates, selenates, selenites, germanates, phosphites can also form such structures. One of the emerging areas combines the rich coordination chemistry of the central metal ions of many of these structures with the flexibility and functionality of the organic linkers to give rise to organic-inorganic hybrid compounds. The compounds of the transition-metals appear to provide many variations arising from their coordination preferences, ligand geometry, and the valence state. In addition, the combination of the magnetic nature of the transition metal center with the channel structure of open frameworks suggests interesting potential applications. In this Review the synthesis, structures and properties of the various transition-metal open-framework compounds are discussed.

Changes in the electronic structure and properties of graphene induced by molecular charge-transfer

Abstract: Interaction with electron donor and acceptor molecules such as aniline and nitrobenzene brings about marked changes in the Raman spectrum and the electronic structure of graphene, prepared by the exfoliation of graphitic oxide.

Uptake of H2 and CO2 by Graphene

Abstract: Graphene samples prepared by the exfoliation of graphitic oxide and conversion of nanodiamond exhibit good hydrogen uptake at 1 atm, 77 K, the uptake going up to 1.7 wt %. The hydrogen uptake varies linearly with the surface area, and the extrapolated value of hydrogen uptake by single-layer graphene works out to be just above 3 wt %. The H2 uptake at 100 atm and 298 K is found to be 3 wt % or more, suggesting thereby the single-layer graphene would exhibit much higher uptakes. Equally interestingly, the graphene samples prepared by us show high uptake of CO2, the value reaching up to 35 wt % at 1 atm and 195 K. The first-principles calculations show that hydrogen molecules sit alternately in parallel and perpendicular orientation on the six-membered rings of the graphene. Up to 7.7 wt % of hydrogen can be accommodated on single-layered graphene. CO2 molecules sit alternatively in a parallel fashion on the rings, giving use to a maximum uptake of 37.93 wt % in single-layer graphene. The presence of more t...

A Low-Complexity Detector for Large MIMO Systems and Multicarrier CDMA Systems

Abstract: We consider large MIMO systems, where by 'large' we mean number of transmit and receive antennas of the order of tens to hundreds. Such large MIMO systems will be of immense interest because of the very high spectral efficiencies possible in such systems. We present a low-complexity detector which achieves uncoded near-exponential diversity performance for hundreds of antennas (i.e., achieves near SISO AWGN performance in a large MIMO fading environment) with an average per-bit complexity of just O(NtNr), where Nt and Nr denote the number of transmit and receive antennas, respectively. With an outer turbo code, the proposed detector achieves good coded bit error performance as well. For example, in a 600 transmit and 600 receive antennas V-BLAST system with a high spectral efficiency of 450 bps/Hz (using BPSK and rate-3/4 turbo code), our simulation results show that the proposed detector performs to within about 7 dB from capacity. This practical feasibility of the proposed high-performance, low-complexity detector could potentially trigger wide interest in the theory and implementation of large MIMO systems. We also illustrate the applicability of the proposed detector in the low-complexity detection of high-rate, non-orthogonal space-time block codes and large multicarrier CDMA (MC-CDMA) systems. In large MC-CDMA systems with hundreds of users, the proposed detector is shown to achieve near single-user performance at an average per-bit complexity linear in number of users, which is quite appealing for its use in practical CDMA systems.

Estimating Granger causality from fourier and wavelet transforms of time series data.

Abstract: Experiments in many fields of science and engineering yield data in the form of time series. The Fourier and wavelet transform-based nonparametric methods are used widely to study the spectral characteristics of these time series data. Here, we extend the framework of nonparametric spectral methods to include the estimation of Granger causality spectra for assessing directional influences. We illustrate the utility of the proposed methods using synthetic data from network models consisting of interacting dynamical systems.

Impact Angle Constrained Interception of Stationary Targets

Abstract: In Many advanced guidance applications [1–5], it is required to intercept the target from a particular direction, that is, achieve a certain impact angle.

Adsorption of Sulfonated Dyes by Polyaniline Emeraldine Salt and Its Kinetics

Abstract: A method for the removal of anionic (sulfonated) dyes from aqueous dye solutions using the chemical interaction of dye molecules with polyaniline is reported. Polyaniline (PANI) emeraldine salt was synthesized by chemical oxidation. Sulfonated dyes undergo chemical interactions with the charged backbone of PANI, leading to significant adsorption of the dyes. This phenomenon of selective adsorption of the dyes by PANI is reported for the first time and promises a green method for removal of sulfonated organics from wastewater. The experimental observations from UV-vis spectroscopy, X-ray diffraction, and conductivity measurements rule out the possibility of secondary doping of polyaniline salt by sulfonated dye molecules. A possible mechanism for the chemical interaction between the polymer and the sulfonated dye molecules is proposed. The kinetic parameters for the adsorption of sulfonated dyes on PANI are also reported.

targetTB: A target identification pipeline for Mycobacterium tuberculosis through an interactome, reactome and genome-scale structural analysis

Abstract: Background: Tuberculosis still remains one of the largest killer infectious diseases, warranting the identification of newer targets and drugs. Identification and validation of appropriate targets for designing drugs are critical steps in drug discovery, which are at present major bottle-necks. A majority of drugs in current clinical use for many diseases have been designed without the knowledge of the targets, perhaps because standard methodologies to identify such targets in a high-throughput fashion do not really exist. With different kinds of 'omics' data that are now available, computational approaches can be powerful means of obtaining short-lists of possible targets for further experimental validation. Results: We report a comprehensive in silico target identification pipeline, targetTB, for Mycobacterium tuberculosis. The pipeline incorporates a network analysis of the protein-protein interactome, a flux balance analysis of the reactome, experimentally derived phenotype essentiality data, sequence analyses and a structural assessment of targetability, using novel algorithms recently developed by us. Using flux balance analysis and network analysis, proteins critical for survival of M. tuberculosis are first identified, followed by comparative genomics with the host, finally incorporating a novel structural analysis of the binding sites to assess the feasibility of a protein as a target. Further analyses include correlation with expression data and non-similarity to gut flora proteins as well as 'anti-targets' in the host, leading to the identification of 451 high-confidence targets. Through phylogenetic profiling against 228 pathogen genomes, shortlisted targets have been further explored to identify broad-spectrum antibiotic targets, while also identifying those specific to tuberculosis. Targets that address mycobacterial persistence and drug resistance mechanisms are also analysed. Conclusion: The pipeline developed provides rational schema for drug target identification that are likely to have high rates of success, which is expected to save enormous amounts of money, resources and time in the drug discovery process. A thorough comparison with previously suggested targets in the literature demonstrates the usefulness of the integrated approach used in our study, highlighting the importance of systems-level analyses in particular. The method has the potential to be used as a general strategy for target identification and validation and hence significantly impact most drug discovery programmes.

Space Vector Based Hybrid PWM Techniques for Reduced Current Ripple

Abstract: This paper investigates certain novel switching sequences involving division of active vector time for space vector based pulsewidth modulation (PWM) generation for a voltage source inverter. This paper proposes two new sequences, and identifies all possible sequences, which result in the same average switching frequency as conventional space vector PWM (CSVPWM) at a given sampling frequency. This paper brings out a method for designing hybrid PWM techniques involving multiple sequences to reduce line current ripple. The three proposed hybrid PWM techniques (three-zone PWM, five-zone PWM and seven zone PWM) employ three, five and seven different sequences, respectively, in every sector. Each sequence is employed in a spatial region within the sector where it results in the lowest rms current ripple over the given sampling period. The proposed techniques lead to a significant reduction in THD over CSVPWM at high line voltages. The five-zone technique results in the lowest THD among real-time techniques with uniform sampling, while the seven-zone technique is the best among real-time techniques with twin sampling rates. The superior harmonic performance of the proposed techniques over CSVPWM and existing bus-clamping PWM techniques is established theoretically as well as experimentally.

Nanostructured Cu and Cu@Cu2O core shell catalysts for hydrogen generation from ammonia–borane

Abstract: Copper nanoparticles have been prepared by the solvated metal atom dispersion (SMAD) method. Oxidation of the SMAD prepared copper colloids resulted in Cu@Cu2O core shell structures (7.7 ± 1.8 nm) or Cu2O nanoparticles depending on the reaction conditions. The nano Cu, Cu@Cu2O core shell, and Cu2O particles were found to be catalytically active for the generation of hydrogen from ammonia–borane either viahydrolysis or methanolysis reaction.

Low-Temperature Selective Catalytic Reduction of NO with NH3 over Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, Cu)

Abstract: The catalysts, Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, Cu), were synthesized in anatase phase by solution combustion. Selective catalytic reduction (SCR) of NO with NH3 was investigated over these catalysts. The reaction occurred at the lowest temperature over Ti0.9Mn0.1O2-δ, but the selectivity for N2 was highest over Ti0.9Fe0.1O2-δ. Therefore, both Mn and Fe were substituted in TiO2 (Ti0.9Mn0.05Fe0.05O2-δ). The reaction occurred at low temperature with a high selectivity over this catalyst. In order to understand the reaction mechanism and the nature of the active sites, temperature programmed desorption (TPD) of NH3 and hydrogen uptake studies were carried out. The relation between the Lewis acid sites and SCR window and the relation between Bronsted acid sites and low temperature was established. The order of the SCR reaction with respect to NO, NH3, and O2 was also investigated. It was also shown that the N2 selectivity of the SCR reaction has a strong inverse correlation with the oxidation of ammonia.

Strength and Stiffness Response of Coir Fiber-Reinforced Tropical Soil

Abstract: Use of natural fibers in civil engineering construction practice is often advantageous as they are cheap, locally available, biodegradable, and ecofriendly. Among the available natural fibers, coir is produced in large quantities in South Asian countries, such as India, Ceylon, Indonesia, Philippines, etc. and has better mechanical properties, such as tensile strength. In this paper, results on the strength and stiffness behavior of soil reinforced with coir fibers are presented. Soil samples reinforced with coir fibers of different sizes, and made into cylindrical soil specimens were tested in triaxial shear apparatus to determine the strength and stiffness of soil response due to fiber inclusion and the results were compared with that of unreinforced soils. The results show that addition of coir (1-2%) as random reinforcing material increases both strength and stiffness of clay soil considered in the study. In addition, available theoretical models for prediction of strength of fiber-reinforced soil are examined in relation to the results of the present investigation. Analysis shows that the available models are not adequate to capture the strength and stiffness response of coir fiber-reinforced soil. A nonlinear regression model for strength and stiffness response is proposed in the present study.

Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities

Abstract: In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16–52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha−1 y−1, 95% confidence intervals [0.07, 0.39] MgC ha−1 y−1), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y−1) compared with the tree community as a whole (+0.15 % y−1); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y−1), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.

Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques

Abstract: The effects of the interaction of few-layer graphene with electron donor and acceptor molecules have been investigated by employing Raman spectroscopy, and the results compared with those from electrochemical doping. The G-band softens progressively with increasing concentration of tetrathiafulvalene (TTF) which is an electron donor, while the band stiffens with increasing concentration of tetracyanoethylene (TCNE) which is an electron acceptor. Interaction with both TTF and TCNE broadens the G-band. Hole and electron doping by electrochemical means, however, stiffen and sharpen the G-band. The 2D-band position is also affected by interaction with TTF and TCNE. More importantly, the intensity of the 2D-band decreases markedly with the concentration of either. The ratio of intensities of the 2D-band and G-band decreases with an increase in TTF or TCNE concentration, and provides a means for carrier titration in the charge transfer system. Unlike the intensity of the 2D-band, that of the D-band increases on interaction with TTF or TCNE. All of these effects occur due to molecular charge transfer, also evidenced by the occurrence of charge transfer bands in the electronic absorption spectra. The electrical resistivity of graphene varies in opposite directions on interaction with TTF and TCNE, the resistivity depending on the concentration of either compound.

Downscaling precipitation to river basin in India for IPCC SRES scenarios using support vector machine

Abstract: This paper presents a methodology to downscale monthly precipitation to river basin scale in Indian context for special report of emission scenarios (SRES) using Support Vector Machine (SVM). In the methodology presented, probable predictor variables are extracted from (1) the National Center for Environmental Prediction (NCEP) reanalysis data set for the period 1971–2000 and (2) the simulations from the third generation Canadian general circulation model (CGCM3) for SRES emission scenarios A1B, A2, B1 and COMMIT for the period 1971–2100. These variables include both the thermodynamic and dynamic parameters and those which have a physically meaningful relationship with the precipitation. The NCEP variables which are realistically simulated by CGCM3 are chosen as potential predictors for seasonal stratification. The seasonal stratification involves identification of (1) the past wet and dry seasons through classification of the NCEP data on potential predictors into two clusters by the use of K-means clustering algorithm and (2) the future wet and dry seasons through classification of the CGCM3 data on potential predictors into two clusters by the use of nearest neighbour rule. Subsequently, a separate downscaling model is developed for each season to capture the relationship between the predictor variables and the predictand. For downscaling precipitation, the predictand is chosen as monthly Thiessen weighted precipitation for the river basin, whereas potential predictors are chosen as the NCEP variables which are correlated to the precipitation and are also realistically simulated by CGCM3. Implementation of the methodology presented is demonstrated by application to Malaprabha reservoir catchment in India which is considered to be a climatically sensitive region. The CGCM3 simulations are run through the calibrated and validated SVM downscaling model to obtain future projections of predictand for each of the four emission scenarios considered. The results show that the precipitation is projected to increase in future for almost all the scenarios considered. The projected increase in precipitation is high for A2 scenario, whereas it is least for COMMIT scenario. Copyright  2007 Royal Meteorological Society.