Indian Institute of Technology Madras

About: Indian Institute of Technology Madras is a facility organization based out in Chennai, Tamil Nadu, India. It is known for research contribution in the topics: Catalysis & Heat transfer. The organization has 20118 authors who have published 36499 publications receiving 590447 citations.

Low-threshold optical power limiting of cw laser illumination based on nonlinear refraction in zinc tetraphenyl porphyrin

Abstract: The increasing use of low power cw lasers in various applications calls for the design of optical limiters with low thresholds. To this end, the optical nonlinearity exhibited by zinc tetraphenyl porphyrin at low laser powers is utilized to design an optical limiter for low threshold operation. The basic parameter responsible for limiting action, the nonlinear refractive index of the medium, is measured using the Z-scan technique and found to have a value of - 1.4 × 10 - 7 cm 2 / W at the helium–neon laser wavelength of 632.8 nm. The origin of nonlinearity is explained on the basis of the thermal lens model. It is shown that effective optical limiting at desired threshold values can be achieved by the optimal choice of aperture size and experimental geometry.

Indian Summer Monsoon Rainfall: Implications of Contrasting Trends in the Spatial Variability of Means and Extremes

Abstract: India’s agricultural output, economy, and societal well-being are strappingly dependent on the stability of summer monsoon rainfall, its variability and extremes. Spatial aggregate of intensity and frequency of extreme rainfall events over Central India are significantly increasing, while at local scale they are spatially non-uniform with increasing spatial variability. The reasons behind such increase in spatial variability of extremes are poorly understood and the trends in mean monsoon rainfall have been greatly overlooked. Here, by using multi-decadal gridded daily rainfall data over entire India, we show that the trend in spatial variability of mean monsoon rainfall is decreasing as exactly opposite to that of extremes. The spatial variability of extremes is attributed to the spatial variability of the convective rainfall component. Contrarily, the decrease in spatial variability of the mean rainfall over India poses a pertinent research question on the applicability of large scale inter-basin water transfer by river inter-linking to address the spatial variability of available water in India. We found a significant decrease in the monsoon rainfall over major water surplus river basins in India. Hydrological simulations using a Variable Infiltration Capacity (VIC) model also revealed that the water yield in surplus river basins is decreasing but it is increasing in deficit basins. These findings contradict the traditional notion of dry areas becoming drier and wet areas becoming wetter in response to climate change in India. This result also calls for a re-evaluation of planning for river inter-linking to supply water from surplus to deficit river basins.

Selective Visual Detection of TNT at the Sub-Zeptomole Level

Abstract: Realizing the limits of sensitivity, while maintaining selectivity, is an ongoing quest. Among the multitude of requirements, national security, early detection of diseases, safety of public utilities, and radiation prevention are some of the areas in need of ultralow detection. Structural, functional, and electronic features of nanomaterials are used to develop reliable analytical methods. Several kinds of surfaceenhanced spectroscopy, surface-enhanced Raman in particular, can be used for such applications; the technique may be further enhanced by spatially separating the analyte and the active plasmonic nanostructure with an insulator, a method known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Creating uniform anisotropic structures with nanoscale attributes by simple solution chemistry and combining analyte-selective chemistry on such surfaces enables ultrasensitive and selective detection methods. Noble metal quantum clusters (QCs), a new family of atomically precise nanomolecules with intense luminescence, along with their protein protected analogues, are highly sensitive and selective for specific analytes. Anchoring such QCs on mesoscale (100 nm to a few mm) particles leads to surface-enhancement of their luminescence and can create a new platform for ultrasensitive detection, especially when combined with the use of optical microscopy. Gold mesoflowers (MFs) are anisotropic materials with unique five-fold symmetric stems containing surface-enhancing nanoscale features. An entire MF is only a few micrometers in size, and its distinct shape allows for unique identification by optical microscopy; thus, changes in the properties of an MF can be used for the immediate and efficient detection of analytes. Herein, we demonstrate the selective detection of 2,4,6trinitrotoluene (TNT) at the sub-zeptomole level (10 21 moles) through a combination of these strategies on a mesostructure. Our method involves anchoring silver clusters, which are comprised of fifteen atoms and embedded in bovine serum albumin (BSA), on silica-coated Au MFs, termed Au@SiO2@Ag15 MFs, and using this system for analyte detection. Syntheses of the various components are described in the experimental section. The Au@SiO2 MFs have a tip-totip length of ca. 4 mm (Supporting Information, Figure S1a). The BSA-protected silver cluster (Ag15), is a red luminescent water-soluble QC prepared by a previously reported procedure (see Figure S2 for essential characterization data). Apart from a high quantum yield (10.7%) in water, it is stable over a wide pH range and exhibits emission in the solid state. We exposed varying concentrations of TNT to Au@SiO2@Ag15 MFs and found that even a concentration of less than one zeptomole of TNT per mesoflower quenches the luminescence of the composite mesoflowers within 1 min. The simultaneous disappearance of the luminescence of Ag15 on theMFand the appearance of the luminescence of another embedded fluorophore allows for easy identification of the analyte. Characterization data for the various composite MFs used in this study are presented in the Supporting Information. The hybrid structures, Au@SiO2@Ag15 MFs, with unique structural attributes are observable under an optical microscope (see Figure S3 for a schematic of the setup used). Dark field microscopic images of theseMFs show their well-defined features; they are star-shaped in a two dimensional projection (Figure 1A). The fluorescence image of the same MF (ca. 490 nm excitation, emitted light was passed through a triplepass filter and imaged) shows a characteristic red emission owing to the QCs anchored on its surface (Figure 1A). Unlike with other spherical single particle sensors, which are difficult to locate and distinguish by light-based microscopy, the welldefined shapes of the MFs ensure that the desired particles alone are analyzed. Furthermore, the analyte adsorption capacity of theMFs is enhanced by the thin inert layer of silica employed as a base. Au core/silica shell structures of this type can provide enhanced fluorescence and Raman scattering. The better stability of the QCs on the silica layer, along with a reduction in the luminescence quenching of the QCs on the MF surface and ease of functionalization are among the added advantages of this material (see the Supporting Information). Exposure of the Au@SiO2@Ag15 MFs to TNT (2.5 mL) at a concentration of one part per trillion (ppt) decreases the luminescence intensity slightly without affecting the optical image (Figure 1B), whereas at one part per billion (ppb) of TNT the luminescence feature disappears completely (Figure 1C; note that the MFs shown in Figure 1A–C are different in each case). For spectral intensity data collected from the surface of these MFs, see the Supporting Information, Figure S4. The quenching of cluster luminescence is due to the formation of a Meisenheimer complex by the [*] A. Mathew, Dr. P. R. Sajanlal, Prof. T. Pradeep DST Unit of Nanoscience (DST UNS), Department of Chemistry, Indian Institute of Technology Madras Chennai 600036 (India) E-mail: pradeep@iitm.ac.in [] Current address: Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology Atlanta, GA 30332-0400 (USA)

Photocatalytic reduction of nitrite and nitrate ions to ammonia on M/TiO2 catalysts

Abstract: Noble metal loaded TiO2 catalysts have been employed as catalysts for the photocatalytic reduction of nitrite and nitrate ions to ammonia. The yield of ammonia was found to depend on the nature, amount of metal and the method of metallization. An optimum metal content is beneficial for the activity. Beyond the optimum content the activity decreases.

Predicting the effective thermal conductivity of carbon nanotube based nanofluids.

Abstract: Adding a small volume fraction of carbon nanotubes (CNTs) to a liquid enhances the thermal conductivity significantly. Recent experimental findings report an anomalously wide range of enhancement values that continue to perplex the research community and remain unexplained. In this paper we present a theoretical model based on three-dimensional CNT chain formation (percolation) in the base liquid and the corresponding thermal resistance network. The model considers random CNT orientation and CNT–CNT interaction forming the percolating chain. Predictions are in good agreement with almost all available experimental data. Results show that the enhancement critically depends on the CNT geometry (length), volume fraction, thermal conductivity of the base liquid and the nanofluid (CNT–liquid suspension) preparation technique. Based on the physical mechanism of heat conduction in the nanofluid, we introduce a new dimensionless parameter that alone characterizes the nanofluid thermal conductivity with reasonable accuracy (~ ± 5%).