Vinayak P. Dravid

Northwest University, Evanston, Illinois, United States

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Publications (451)2210.4 Total impact

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    ABSTRACT: We report several synergistic effects in Hg alloying of SnTe to enhance the power factor and overall figure of merit ZT. Hg alloying decreases the energy separation between the two valence bands, leading to pronounced band convergence that improves the Seebeck coefficient. Hg alloying of SnTe also significantly enlarges the band gap thereby effectively suppressing the bipolar diffusion. Collectively, this results in high ZT of 1.35 at 910 K for 2% Bi-doped SnTe with 3%HgTe. The solubility limit of Hg in SnTe is less than 3 mol%, and above this level we observe HgTe precipitates in the SnTe matrix, typically trapped at grain boundary triple junctions. The strong point defect scattering of phonons caused by Hg alloying coupled with mesoscale scattering via grain boundaries contributes to a great reduction of lattice thermal conductivity. The multiple synergistic roles that Hg plays in regulating the electron and phonon transport in SnTe provide important new insights into continued optimization of SnTe-based and related materials.
    Energy & Environmental Science 10/2015; 8(1):267-277. DOI:10.1039/C4EE01463D · 15.49 Impact Factor
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    ABSTRACT: Alloy nanoparticles are important in many fields, including catalysis, plasmonics, and electronics, due to the chemical and physical properties that arise from the interactions between their components. Typically, alloy nanoparticles are made by solution-based synthesis; however, scanning-probe-based methods offer the ability to make and position such structures on surfaces with nanometer-scale resolution. In particular, scanning probe block copolymer lithography (SPBCL), which combines elements of block copolymer lithography with scanning probe techniques, allows one to synthesize nanoparticles with control over particle diameter in the 2−50 nm range. Thus far, single-element structures have been studied in detail, but, in principle, one could make a wide variety of multicomponent systems by controlling the composition of the polymer ink, polymer feature size, and metal precursor concentrations. Indeed, it is possible to use this approach to synthesize alloy nanoparticles comprised of combinations of Au, Ag, Pd, Ni, Co, and Pt. Here, such structures have been made with diameters deliberately tailored in the 10− 20 nm range and characterized by STEM and EDS for structural and elemental composition. The catalytic activity of one class of AuPd alloy nanoparticles made via this method was evaluated with respect to the reduction of 4-nitrophenol with NaBH 4. In addition to being the first catalytic studies of particles made by SPBCL, these proof-of-concept experiments demonstrate the potential for SPBCL as a new method for studying the fundamental science and potential applications of alloy nanoparticles in areas such as heterogeneous catalysis.
    Journal of the American Chemical Society 07/2015; DOI:10.1021/jacs.5b05139 · 11.44 Impact Factor
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    ABSTRACT: Three-dimensional (3D) mesostructured semiconductors show promising properties and applications; however, to date, few methods exist to synthesize or fabricate such materials. Metal can diffuse along semiconductor surfaces, and even trace amounts can change the surface behavior. We exploited the phenomena for 3D mesoscale lithography, by showing one example where iterated deposition-diffusion-incorporation of gold over silicon nanowires forms etchant-resistant patterns. This process is facet-selective, producing mesostructured silicon spicules with skeletonlike morphology, 3D tectonic motifs, and reduced symmetries. Atom-probe tomography, coupled with other quantitative measurements, indicates the existence and the role of individual gold atoms in forming 3D lithographic resists. Compared to other more uniform silicon structures, the anisotropic spicule requires greater force for detachment from collagen hydrogels, suggesting enhanced interfacial interactions at the mesoscale. Copyright © 2015, American Association for the Advancement of Science.
    Science 06/2015; 348(6242):1451-5. DOI:10.1126/science.1257278 · 31.48 Impact Factor
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    ABSTRACT: The requirement for large bulk resistivity in topological insulators has led to the design of complex ternary and quaternary phases with balanced donor and acceptor levels. A common feature of the optimized phases is that they lie close to the p-to n-transition. The tetradymite Bi 2 Te 3-x Se x system exhibits minimum bulk conductance at the ordered composition Bi 2 Te 2 Se. By combining local and integral measurements of the density of states, we find that the point of minimum electrical conductivity at x=1.0 where carriers change from hole-like to electron-like is characterized by conductivity of the mixed type. Our experimental findings, which are interpreted within the framework of a two–band model for the different carrier types, indicate that the mixed state originates from different type of native defects that strongly compensate at the crossover point. 2
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    ABSTRACT: Control of both photonic and plasmonic coupling in a single optical device represents a challenge due to the distinct length scales that must be manipulated. Here, we show that optical metasurfaces with such control can be constructed using an approach that combines top-down and bottom-up processes, wherein gold nanocubes are assembled into ordered arrays via DNA hybridization events onto a gold film decorated with DNA-binding regions defined using electron beam lithography. This approach enables one to systematically tune three critical architectural parameters: (1) anisotropic metal nanoparticle shape and size, (2) the distance between nanoparticles and a metal surface, and (3) the symmetry and spacing of particles. Importantly, these parameters allow for the independent control of two distinct optical modes, a gap mode between the particle and the surface and a lattice mode that originates from cooperative scattering of many particles in an array. Through reflectivity spectroscopy and finite-difference time-domain simulation, we find that these modes can be brought into resonance and coupled strongly. The high degree of synthetic control enables the systematic study of this coupling with respect to geometry, lattice symmetry, and particle shape, which together serve as a compelling example of how nanoparticle-based optics can be useful to realize advanced nanophotonic structures that hold implications for sensing, quantum plasmonics, and tunable absorbers.
    Nano Letters 06/2015; DOI:10.1021/acs.nanolett.5b01548 · 13.59 Impact Factor
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    ABSTRACT: Single crystals of [Pb2BiS3][AuTe2], known as the naturally occurring mineral buckhornite, are grown by a self-flux method using the elements in stoichiometric amounts (evacuated quartz tubes, 850 °C, 2 h; cooling to 650 °C within 3 d).
    ChemInform 06/2015; 46(24). DOI:10.1002/chin.201524012
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    ABSTRACT: We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.
    Journal of the American Chemical Society 04/2015; 137(15). DOI:10.1021/jacs.5b00837 · 11.44 Impact Factor
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    ABSTRACT: Ultra-flexible and transparent metal oxide transistors are developed by doping In2 O3 films with poly(vinylphenole) (PVP). By adjusting the In2 O3 :PVP weight ratio, crystallization is frustrated, and conducting pathways for efficient charge transport are maintained. In2 O3 :5%PVP-based transistors exhibit mobilities approaching 11 cm(2) V(-1) s(-1) before, and retain up to ca. 90% performance after 100 bending/relaxing cycles at a radius of 10 mm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 02/2015; 27(14):n/a-n/a. DOI:10.1002/adma.201405400 · 15.41 Impact Factor
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    ABSTRACT: Maintaining the intrinsic features of mesophases is critically important when employing phospholipid self-assemblies to mimic biomembranes. Inorganic solid surfaces provide platforms to support, guide, and analyze organic self-assemblies, but impose upon them a tendency to form well-ordered phases not often found in biomembranes. To address this, we measured mesophase formation in a thiolate self-assembled monolayer (SAM) of diacyl phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) on Au(111), and provide thermodynamic analysis on the mixing behavior of inequivalent DPPTE acyl chains. Our work has uncovered three fundamental issues that enable mesophase formation: (1) Elimination of templating effects of the solid surface, (2) Weakening intermolecular and molecule-substrate interactions in adsorbates, and (3) Equilibrium through entropy-driven self-assembly. Thus, our work provides a more holistic understanding of phase behavior, from liquid phases to mesophases to highly crystalline phases, in organic self-assemblies on solid surfaces, which may extend their applications in nanodevices and to the wider fields of biology and medicine.
    Langmuir 02/2015; 31(10). DOI:10.1021/la504822q · 4.38 Impact Factor
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    ABSTRACT: Fertilization of a mammalian egg initiates a series of 'zinc sparks' that are necessary to induce the egg-to-embryo transition. Despite the importance of these zinc-efflux events little is known about their origin. To understand the molecular mechanism of the zinc spark we combined four physical approaches that resolve zinc distributions in single cells: a chemical probe for dynamic live-cell fluorescence imaging and a combination of scanning transmission electron microscopy with energy-dispersive spectroscopy, X-ray fluorescence microscopy and three-dimensional elemental tomography for high-resolution elemental mapping. We show that the zinc spark arises from a system of thousands of zinc-loaded vesicles, each of which contains, on average, 10(6) zinc atoms. These vesicles undergo dynamic movement during oocyte maturation and exocytosis at the time of fertilization. The discovery of these vesicles and the demonstration that zinc sparks originate from them provides a quantitative framework for understanding how zinc fluxes regulate cellular processes.
    Nature Chemistry 02/2015; 7(2):130-139. DOI:10.1038/nchem.2133 · 23.30 Impact Factor
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    ABSTRACT: Lithiation-exfoliation produces single to few-layered MoS2 and WS2 sheets dispersed in water. However, the process transforms them from the pristine semiconducting 2H phase to a distorted metallic phase. Recovery of the semiconducting properties typically involves heating of the chemically exfoliated sheets at elevated temperatures. It therefore, has been largely limited to sheets deposited on solid substrates. Here, we report dispersion of chemically exfoliated MoS2 sheets in high boiling point organic solvents enabled by surface functionalization, and the controllable recovery of their semiconducting properties directly in solution. This process connects the scalability of chemical exfoliation with simplicity of solution processing, ultimately enabling a facile method for tuning the metal to semiconductor transitions of MoS2 and WS2 within a liquid medium.
    Journal of the American Chemical Society 01/2015; 137(5). DOI:10.1021/ja5107145 · 11.44 Impact Factor
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    ABSTRACT: Two-dimensional (2D) electronic systems are of wide interest due to their richness in chemical and physical phenomena and potential for technological applications. Here we report that [Pb2BiS3][AuTe2], known as a naturally occurring mineral buckhornite, hosts 2D carriers in single-atom-thick layers. The structure is composed of stacking layers of weakly coupled [Pb2BiS3] and [AuTe2] sheets. The insulating [Pb2BiS3] sheet inhibits interlayer charge hopping and confines the carriers in the basal plane of the single-atom-thick [AuTe2] layer. Magneto-transport measurements on synthesized samples and theoretical calculations show that [Pb2BiS3][AuTe2] is a multiband semimetal with compensated density of electrons and holes, which exhibit high hole carrier mobility of ~1360 cm2/Vs. This material possesses an extremely large anisotropy Г=ρc/ρab≈104, comparable to benchmark 2D materials graphite and Bi2Sr2CaCu2O6+δ. The electronic structure features linear band dispersion at the Fermi level and ultrahigh Fermi velocities of 106 m/s which are virtually identical to that of graphene. The weak interlayer coupling gives rise to the highly cleavable property of the single crystal specimens. Our results provide a novel candidate for a monolayer platform to investigate emerging electronic properties.
    Journal of the American Chemical Society 01/2015; 137(6). DOI:10.1021/ja5111688 · 11.44 Impact Factor
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    ABSTRACT: Graphene-based nanocomposites have been synthesized and tested as electrode materials for high power lithium-ion batteries. In the synthesis of such nanocomposites, graphene is generally introduced by either thermally or chemically reduced graphite oxide (GO), which has poorer electric conductivity and crystallinity than mechanically exfoliated graphene. Here, we prepare few-layer graphene sheet (FLGS) with high electric conductivity, by sonicating expanded graphite in DMF solvent, and develop a simple one-pot hydrothermal method to fabricate monodispersed and ultrasmall Co3O4 nanocubes (about 4 nm in size) on the FLGS. This composite, consisting of homogeneously assembled and high crystalline Co3O4 nanocubes on the FLGS, has shown higher capacity and much better cycling stability than counterparts synthesized using GO as a precursor. The products in different synthesis stages have been characterized by TEM, FTIR and XPS to investigate the nanocube growth mechanism. We find that Co(OH)2 initially grew homogeneously on the graphene surface, then gradually oxidized to form Co3O4 nanoparticle seeds, and finally converted to Co3O4 nanocubes with caboxylated anion as surfactant. This work explores the mechanism of nanocrystal growth and its impact on electrochemical properties to provide further insights into the development of nanostructured electrode materials for high power energy storage.
    Journal of Power Sources 01/2015; 274:816-822. DOI:10.1016/j.jpowsour.2014.10.106 · 6.22 Impact Factor
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    ABSTRACT: The endothelial cell (EC) lining of the pulmonary vascular system forms a semipermeable barrier between blood and the interstitium and regulates various critical biochemical functions. Collectively, it represents a prototypical biomechanical system, where the complex hierarchical architecture, from the molecular scale to the cellular and tissue level, has an intimate and intricate relationship with its biological functions. We investigated the mechanical properties of human pulmonary artery endothelial cells (ECs) using atomic force microscopy (AFM). Concurrently, the wider distribution and finer details of the cytoskeletal nano-structure were examined using fluorescence microscopy (FM) and scanning transmission electron microscopy (STEM), respectively. These correlative measurements were conducted in response to the EC barrier-disrupting agent, thrombin, and barrier-enhancing agent, sphingosine 1-phosphate (S1P). Our new findings and analysis directly link the spatio-temporal complexities of cell re-modeling and cytoskeletal mechanical properties alteration. This work provides novel insights into the biomechanical function of the endothelial barrier and suggests similar opportunities for understanding the form-function relationship in other biomechanical subsystems.
    Scientific Reports 01/2015; 5:11097. DOI:10.1038/srep11097 · 5.58 Impact Factor
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    ABSTRACT: Despite the complexities of cancer, remarkable diagnostic and therapeutic advances have been made during the past decade, which include improved genetic, molecular, and nanoscale understanding of the disease. Physical science and engineering, and nanotechnology in particular, have contributed to these developments through out-of-the-box ideas and initiatives from perspectives that are far removed from classical biological and medicinal aspects of cancer. Nanostructures, in particular, are being effectively utilized in sensing/diagnostics of cancer while nanoscale carriers are able to deliver therapeutic cargo for timed and controlled release at localized tumor sites. Magnetic nanostructures (MNS) have especially attracted considerable attention of researchers to address cancer diagnostics and therapy. A significant part of the promise of MNS lies in their potential for "theranostic" applications, wherein diagnostics makes use of the enhanced localized contrast in magnetic resonance imaging (MRI) while therapy leverages the ability of MNS to heat under external radio frequency (RF) field for thermal therapy or use of thermal activation for release of therapy cargo. In this chapter, we report some of the key developments in recent years in regard to MNS as potential theranostic carriers. We describe that the r 2 relaxivity of MNS can be maximized by allowing water (proton) diffusion in the vicinity of MNS by polyethylene glycol (PEG) anchoring, which also facilitates excellent fluidic stability in various media and extended in vivo circulation while maintaining high r 2 values needed for T 2-weighted MRI contrast. Further, the specific absorption rate (SAR) required for thermal activation of MNS can be tailored by controlling composition and size of MNS. Together, emerging MNS show considerable promise to realize theranostic potential. We discuss that properly functionalized MNS can be designed to provide remarkable in vivo stability and accompanying pharmacokinetics exhibit organ localization that can be tailored for specific applications. In this context, even iron-based MNS show extended circulation as well as diverse organ accumulation beyond liver, which otherwise renders MNS potentially toxic to liver function. We believe that MNS, including those based on iron oxides, have entered a renaissance era where intelligent synthesis, functionalization, stabilization, and targeting provide ample evidence for applications in localized cancer theranostics.
    Cancer treatment and research 01/2015; 166:51-83. DOI:10.1007/978-3-319-16555-4_3
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    ABSTRACT: One way to image the molecular pathology in Alzheimer's disease is by positron emission tomography using probes that target amyloid fibrils. However, these fibrils are not closely linked to the development of the disease. It is now thought that early-stage biomarkers that instigate memory loss are composed of Aβ oligomers. Here, we report a sensitive molecular magnetic resonance imaging contrast probe that is specific for Aβ oligomers. We attach oligomer-specific antibodies onto magnetic nanostructures and show that the complex is stable and binds to Aβ oligomers on cells and brain tissues to give a magnetic resonance imaging signal. When intranasally administered to an Alzheimer's disease mouse model, the probe readily reached hippocampal Aβ oligomers. In isolated samples of human brain tissue, we observed a magnetic resonance imaging signal that distinguished Alzheimer's disease from controls. Such nanostructures that target neurotoxic Aβ oligomers are potentially useful for evaluating the efficacy of new drugs and ultimately for early-stage Alzheimer's disease diagnosis and disease management.
    Nature Nanotechnology 12/2014; 10(1). DOI:10.1038/nnano.2014.254 · 33.27 Impact Factor
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    ABSTRACT: SnTe is an intriguing alternative to its sister compound PbTe in thermoelectric energy conversion because of their electronic and structural similarity; however, it is challenging to optimize its thermoelectric performance to the level of PbTe because of the difficulties in decreasing its intrinsically large hole population and high thermal conductivity arising from the tin vacancies. We demonstrate here that by alloying some AgBiTe2 in SnTe, thus forming AgSnxBiTex+2 compositions the hole concentration can be duly decreased because of the high efficiency of Bi as an electron donor. The lattice thermal conductivity is also decreased due to the strong scattering of phonons (by point defect scattering as well as Ag-rich nanostructures) to achieve a value of 0.7 W m−1 K−1 at 750 K. As a result, a high thermoelectric figure ZT of merit of 1.1 at 775 K is achieved by chemical composition optimization (×15), making lead free SnTe–AgBiTe2 a promising thermoelectric material.
    11/2014; DOI:10.1039/C4TA05530F
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    ABSTRACT: Precipitates in bulk p-type thermoelectric materials, PbTe-SrTe and PbTe-PbS, are studied using three-dimensional (3-D) atom-probe tomography (APT). APT is capable of characterizing chemically materials in 3-D with subnano-scale spatial resolution on an atom-by-atom basis, which enables us to characterize secondary phases in the PbTe matrix as well as the dopant distributions at different imperfections. We demonstrate that APT provides accurate information about the compositions and morphologies of nanoprecipitates. In the PbTe-SrTe system, different morphology of precipitates is observed and the SrTe composition is confirmed. Also, segregation of Na dopants at mesoscale imperfections, dislocations and grain boundaries, and at matrix/precipitate interfaces is observed. In the PbTe-PbS system, PbS precipitates are observed. The PbS precipitates exhibit faceting, and have a morphology that depends on the bulk Na concentration. A predominance of {100} faceted precipitates is observed for 2 mol.% Na. Using 3-D APT, we demonstrate that Na segregation at matrix/precipitate interfaces is most likely responsible for the change in their morphologies, which occurs by reducing the interfacial free energy of {100} facets.
    JOM: the journal of the Minerals, Metals & Materials Society 11/2014; 66(11):2288-2297. DOI:10.1007/s11837-014-1155-0 · 1.76 Impact Factor
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    ABSTRACT: The influence of hierarchical TiO2 inclusions on electrical and thermal transport properties has been investigated in the Ba-filled skutterudite compound Ba0.3Co4Sb12 synthesized by ball milling followed by hot pressing. The hierarchical TiO2 with morphology of nanocrystallite aggregates were prepared using a carbon sphere-templated method. It was found that TiO2 inclusions strongly enhance the Seebeck coefficient while they weakly degrade the electric conductivity. In addition, TiO2 inclusions significantly reduce the lattice thermal conductivity through all-scale length phonon scattering due to these hierarchically-structured TiO2 particle inclusions. ZT values up to 1.2 were obtained for Ba0.3Co4Sb12 with the optimal amount and dispersion of TiO2 inclusions. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites via hierarchical dispersion.
    10/2014; 2(48). DOI:10.1039/C4TA05285D

Publication Stats

9k Citations
2,210.40 Total Impact Points

Institutions

  • 2005–2015
    • Northwest University
      Evanston, Illinois, United States
  • 1970–2015
    • Northwestern University
      • • Department of Materials Science and Engineering
      • • Department of Chemistry
      • • Center for AIDS Research
      Evanston, Illinois, United States
  • 1987–2005
    • Lehigh University
      • Department of Materials Science and Engineering
      Bethlehem, Pennsylvania, United States
  • 1998
    • Brown University
      • Department of Physics
      Providence, Rhode Island, United States