Pressure-induced insulator-to-metal transitions for enhancing thermoelectric power factor in bismuth telluride-based alloys.

First-principles calculations revealing insulator-to-metal transitions in Bi2Te3 and Bi2Te2Se, at 9 GPa and 12.5 GPa, respectively, match with prior experiments. Our electronic band structure calculations and accompanying Boltzmann transport calculations of thermoelectric properties for Bi2-xSbxTe2-ySey alloys explain and predict large power factor changes induced by pressure. Complex band degeneracy changes preceding insulator-to-metal transitions significantly alter the density of states near the Fermi level, and foster the disentangling of the unfavorable coupling between Seebeck coefficient and electrical conductivity. Our findings on pressure-induced changes in thermoelectric power factor provide insights for designing V2VI3-based high-performance thermoelectric materials through strategies such as alloying, high-pressure processing, and strain engineering.

Microwave synthesis of branched silver nanowires and their use as fillers for high thermal conductivity polymer composites.

We report a rapid synthesis approach to obtain branched Ag nanowires by microwave-stimulated polyvinylpyrrolidone-directed polyol-reduction of silver nitrate. Microwave exposure results in micrometer-long nanowires passivated with polyvinylpyrrolidone. Cooling the reaction mixture by interrupting microwave exposure promotes nanocrystal nucleation at low-surfactant coverage sites. The nascent nuclei grow into nanowire branches upon further microwave exposure. Dispersions of low fractions of the branched nanowires in polydimethylsiloxane yield up to 60% higher thermal conductivity than that obtained using unbranched nanowire fillers. Our findings should be useful for realizing nanocomposites with tailored thermal transport properties for applications.

Bonding-induced thermal conductance enhancement at inorganic heterointerfaces using nanomolecular monolayers.

Manipulating interfacial thermal transport is important for many technologies including nanoelectronics, solid-state lighting, energy generation and nanocomposites. Here, we demonstrate the use of a strongly bonding organic nanomolecular monolayer (NML) at model metal/dielectric interfaces to obtain up to a fourfold increase in the interfacial thermal conductance, to values as high as 430 MW m(-2) K(-1) in the copper-silica system. We also show that the approach of using an NML can be implemented to tune the interfacial thermal conductance in other materials systems. Molecular dynamics simulations indicate that the remarkable enhancement we observe is due to strong NML-dielectric and NML-metal bonds that facilitate efficient heat transfer through the NML. Our results underscore the importance of interfacial bond strength as a means to describe and control interfacial thermal transport in a variety of materials systems.

Molecularly-induced roughness and oxidation in cobalt/organodithiol/cobalt nanolayers synthesized by sputter-deposition and molecular sublimation.

Integrating interfacial molecular nanolayers (MNL) with inorganic nanolayers is of interest for understanding processing-structure/chemistry correlations in hybrid nanolaminates. Here, we report the synthesis of Co/biphenyldithiol (BPDT)/Co nanolayer sandwiches by metal sputter-deposition and molecular sublimation. The density and surface roughness of the Co layers deposited on the native oxide are invariant with the Ar pressure pAr during deposition. In contrast, the Co layer roughness rCo deposited on top of the BPDT MNL increases with pAr, and correlates with a higher degree of Co oxidation. Increased roughening is attributed to MNL-accentuated self-shadowing of low mobility Co atoms at high pAr, which consequently increases Co oxidation. These results indicating MNL-induced effects on the morphology and chemistry of the inorganic layers should be of importance for tailoring nanolayered hybrid interfaces and laminates.

Nanomolecularly-induced Effects at Titania/Organo-Diphosphonate Interfaces for Stable Hybrid Multilayers with Emergent Properties.

Nanoscale hybrid inorganic-organic multilayers are attractive for accessing emergent phenomena and properties through superposition of nanomolecularly-induced interface effects for diverse applications. Here, we demonstrate the effects of interfacial molecular nanolayers (MNLs) of organo-diphosphonates on the growth and stability of titania nanolayers during the synthesis of titania/MNL multilayers by sequential atomic layer deposition and single-cycle molecular layer deposition. Interfacial organo-diphosphonate MNLs result in ∼20-40% slower growth of amorphous titania nanolayers and inhibit anatase nanocrystal formation from them when compared to amorphous titania grown without MNLs. Both these effects are more pronounced in multilayers with aliphatic backbone-MNLs and likely related to impurity incorporation and incomplete reduction of the titania precursor indicated by our spectroscopic analyses. In contrast, both MNLs result in two-fold higher titania nanolayer roughness, suggesting that roughening is primarily due to MNL bonding chemistry. Such MNL-induced effects on inorganic nanolayer growth rate, roughening, and stability are germane to realizing high-interface-fraction hybrid nanolaminate multilayers.

A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly.

Obtaining thermoelectric materials with high figure of merit ZT is an exacting challenge because it requires the independent control of electrical conductivity, thermal conductivity and Seebeck coefficient, which are often unfavourably coupled. Recent works have devised strategies based on nanostructuring and alloying to address this challenge in thin films, and to obtain bulk p-type alloys with ZT>1. Here, we demonstrate a new class of both p- and n-type bulk nanomaterials with room-temperature ZT as high as 1.1 using a combination of sub-atomic-per-cent doping and nanostructuring. Our nanomaterials were fabricated by bottom-up assembly of sulphur-doped pnictogen chalcogenide nanoplates sculpted by a scalable microwave-stimulated wet-chemical method. Bulk nanomaterials from single-component assemblies or nanoplate mixtures of different materials exhibit 25-250% higher ZT than their non-nanostructured bulk counterparts and state-of-the-art alloys. Adapting our synthesis and assembly approach should enable nanobulk thermoelectrics with further increases in ZT for transforming thermoelectric refrigeration and power harvesting technologies.

Seebeck and figure of merit enhancement in nanostructured antimony telluride by antisite defect suppression through sulfur doping.

Antimony telluride has a low thermoelectric figure of merit (ZT < ∼0.3) because of a low Seebeck coefficient α arising from high degenerate hole concentrations generated by antimony antisite defects. Here, we mitigate this key problem by suppressing antisite defect formation using subatomic percent sulfur doping. The resultant 10-25% higher α in bulk nanocrystalline antimony telluride leads to ZT ∼ 0.95 at 423 K, which is superior to the best non-nanostructured antimony telluride alloys. Density functional theory calculations indicate that sulfur increases the antisite formation activation energy and presage further improvements leading to ZT ∼ 2 through optimized doping. Our findings are promising for designing novel thermoelectric materials for refrigeration, waste heat recovery, and solar thermal applications.

Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties.

ZnO is a promising high figure-of-merit (ZT) thermoelectric material for power harvesting from heat due to its high melting point, high electrical conductivity σ, and Seebeck coefficient α, but its practical use is limited by a high lattice thermal conductivity κ(L). Here, we report Al-containing ZnO nanocomposites with up to a factor of 20 lower κ(L) than non-nanostructured ZnO, while retaining bulklike α and σ. We show that enhanced phonon scattering promoted by Al-induced grain refinement and ZnAl(2)O(4) nanoprecipitates presages ultralow κ ∼ 2 Wm( -1) K(-1) at 1000 K. The high α∼ -300 µV K(-1) and high σ ∼ 1-10(4) Ω(-1 )m(-1) result from an offsetting of the nanostructuring-induced mobility decrease by high, and nondegenerate, carrier concentrations obtained via excitation from shallow Al donor states. The resultant ZT ∼ 0.44 at 1000 K is 50% higher than that for the best non-nanostructured counterpart material at the same temperature and holds promise for engineering advanced oxide-based high-ZT thermoelectrics for applications.

Viscoelastic bandgap in multilayers of inorganic-organic nanolayer interfaces.

Incorporating molecular nanolayers (MNLs) at inorganic interfaces offers promise for reaping unusual enhancements in fracture energy, thermal and electrical transport. Here, we reveal that multilayering MNL-bonded inorganic interfaces can result in viscoelastic damping bandgaps. Molecular dynamics simulations of Au/octanedithiol MNL/Au multilayers reveal high-damping-loss frequency bands at 33 ≤ ν ≤ 77 GHz and 278 ≤ ν ≤ 833 GHz separated by a low-loss bandgap 77 ≤ ν ≤ 278 GHz region. The viscoelastic bandgap scales with the Au/MNL interface bonding strength and density, and MNL coverage. These results and the analyses of interfacial vibrations indicate that the viscoelastic bandgap is an interface effect that cannot be explained by weighted averages of bulk responses. These findings prognosticate a variety of possibilities for accessing and tuning novel dynamic mechanical responses in materials systems and devices with significant inorganic-organic interface fractions for many applications, e.g., smart composites and sensors with self-healing/-destructing mechanical responses.

Engineering thermoelectric and mechanical properties by nanoporosity in calcium cobaltate films from reactions of Ca(OH)2/Co3O4 multilayers.

Controlling nanoporosity to favorably alter multiple properties in layered crystalline inorganic thin films is a challenge. Here, we demonstrate that the thermoelectric and mechanical properties of Ca3Co4O9 films can be engineered through nanoporosity control by annealing multiple Ca(OH)2/Co3O4 reactant bilayers with characteristic bilayer thicknesses (b t ). Our results show that doubling b t , e.g., from 12 to 26 nm, more than triples the average pore size from ∼120 nm to ∼400 nm and increases the pore fraction from 3% to 17.1%. The higher porosity film exhibits not only a 50% higher electrical conductivity of σ ∼ 90 S cm-1 and a high Seebeck coefficient of α ∼ 135 µV K-1, but also a thermal conductivity as low as κ ∼ 0.87 W m-1 K-1. The nanoporous Ca3Co4O9 films exhibit greater mechanical compliance and resilience to bending than the bulk. These results indicate that annealing reactant multilayers with controlled thicknesses is an attractive way to engineer nanoporosity and realize mechanically flexible oxide-based thermoelectric materials.

High electrical conductivity antimony selenide nanocrystals and assemblies.

Antimony selenide is a promising thermoelectric material with a high Seebeck coefficient, but its figure of merit is limited by its low electrical conductivity. Here, we report a rapid and scalable (gram-a-minute) microwave synthesis of one-dimensional nanocrystals of sulfurized antimony selenide that exhibit 10(4)-10(10) times higher electrical conductivity than non-nanostructured bulk or thin film forms of this material. As the nanocrystal diameter increases, the nanowires transform into nanotubes through void formation and coalescence driven by axial rejection of sulfur incorporated into the nanowires from the surfactant used in our synthesis. Individual nanowires and nanotubes exhibit a charge carrier transport activation-energy of <60 meV arising from surface sulfur donor states. Nanocrystal assemblies also show high electrical conductivity, making the nanocrystals attractive building blocks to realize nanostructured thin film and bulk forms of this material for thermoelectric device applications.

Nanoscale heterostructures with molecular-scale single-crystal metal wires.

Creating nanoscale heterostructures with molecular-scale (<2 nm) metal wires is critical for many applications and remains a challenge. Here, we report the first time synthesis of nanoscale heterostructures with single-crystal molecular-scale Au nanowires attached to different nanostructure substrates. Our method involves the formation of Au nanoparticle seeds by the reduction of rocksalt AuCl nanocubes heterogeneously nucleated on the substrates and subsequent nanowire growth by oriented attachment of Au nanoparticles from the solution phase. Nanoscale heterostructures fabricated by such site-specific nucleation and growth are attractive for many applications including nanoelectronic device wiring, catalysis, and sensing.

Injectable, Magnetically Orienting Electrospun Fiber Conduits for Neuron Guidance.

Magnetic electrospun fibers are of interest for minimally invasive biomaterial applications that also strive to provide cell guidance. Magnetic electrospun fibers can be injected and then magnetically positioned in situ, and the aligned fiber scaffolds provide consistent topographical guidance to cells. In this study, magnetically responsive aligned poly-l-lactic acid electrospun fiber scaffolds were developed and tested for neural applications. Incorporating oleic acid-coated iron oxide nanoparticles significantly increased neurite outgrowth, reduced the fiber alignment, and increased the surface nanotopography of the electrospun fibers. After verifying neuron viability on two-dimensional scaffolds, the system was tested as an injectable three-dimensional scaffold. Small conduits of aligned magnetic fibers were easily injected in a collagen or fibrinogen hydrogel solution and repositioned using an external magnetic field. The aligned magnetic fibers provided internal directional guidance to neurites within a three-dimensional collagen or fibrin model hydrogel, supplemented with Matrigel. Neurites growing from dorsal root ganglion explants extended 1.4-3× farther on the aligned fibers compared with neurites extending in the hydrogel alone. Overall, these results show that magnetic electrospun fiber scaffolds can be injected and manipulated with a magnetic field in situ to provide directional guidance to neurons inside an injectable hydrogel. Most importantly, this injectable guidance system increased both neurite alignment and neurite length within the hydrogel scaffold.

Frequency-tunable toughening in a polymer-metal-ceramic stack using an interfacial molecular nanolayer.

Interfacial toughening in composite materials is reasonably well understood for static loading, but little is known for cyclic loading. Here, we demonstrate that introducing an interfacial molecular nanolayer at the metal-ceramic interface of a layered polymer-metal-ceramic stack triples the fracture energy for ~75-300 Hz loading, yielding 40% higher values than the static-loading fracture energy. We show that this unexpected frequency-dependent toughening is underpinned by nanolayer-induced interface strengthening, which facilitates load transfer to, and plasticity in, the polymer layer. Above a threshold interfacial bond strength, the toughening magnitude and frequency range are primarily controlled by the frequency- and temperature-dependent rheological properties of the polymer. These results indicate the tunability of the toughening behavior through suitable choice of interfacial molecular layers and polymers. Our findings open up possibilities for realizing novel composites with inorganic-organic interfaces, e.g., arresting crack growth or stimulating controlled fracture triggered by loads with specific frequency characteristics.

Multifold Electrical Conductance Enhancements at Metal-Bismuth Telluride Interfaces Modified Using an Organosilane Monolayer.

Controlling electrical transport across metal-thermoelectric interfaces is key to realizing high efficiency devices for solid state refrigeration and waste-heat harvesting. We obtain up to 17-fold increases in electrical contact conductivity Σc by inserting a mercaptan-terminated organosilane monolayer at Cu-Bi2Te3 and Ni-Bi2Te3 interfaces, yielding similar Σc for both metals by offsetting an otherwise 7-fold difference. The Σc improvements are underpinned by silane-moiety-induced inhibition of Cu diffusion, promotion of high-conductivity interfacial nickel telluride formation, and mercaptan-induced reduction of Bi2Te3 surface oxides. Our findings should enable incorporating nanomolecular layers with appropriately chosen terminal moieties in thermoelectric device metallization schemes without metal diffusion barriers.

Civil Society-Driven Drug Policy Reform for Health and Human Welfare-India.

The lack of adequate access to opioids in India as analgesics and for agonist therapies, forces millions to live with severe unalleviated pain, or languish with suffering associated with drug dependence. Although India is a major opium exporter, the excessively prohibitive 1985 narcotics law formulated to control harmful use of drugs, impeded the availability and access to opioids for medical and scientific purposes. Amendment of this law in 2014 established a new national regulatory framework for improved access to essential opioid analgesics. This article reflects on key elements and processes that led to this landmark achievement. Unlike quick timelines associated with effecting policy reforms for law enforcement, realizing the 2014 drug policy change primarily to mitigate human suffering, was a 22-year-long process. The most exacting challenges included recognizing the multilayered complexities of the prior policy framework and understanding their adverse impact on field practices to chart an appropriate and viable path for reform. The evolution of an informal civil society movement involving health care professionals, lawyers, media, policy analysts, government officials, and the public was pivotal in addressing these challenges and garnering momentum for reform. The success of the effort for improving access to opioid medications was underpinned by a three-pronged strategy of 1) persuading the executive arm of the government to take interim enabling measures; 2) leveraging judicial intervention through public interest litigation; and 3) crafting a viable policy document for legislative approval and implementation. We hope our findings are useful for realizing drug policy reforms, given the current transformed global policy mandates emphasizing humanitarian, healthcare, and quality-of-life considerations.

Tailoring Electrical Transport Across Metal-Thermoelectric Interfaces Using a Nanomolecular Monolayer.

We report a 13-fold increase in electrical contact conductivity Σc upon introducing a 1,8-octanedithiol (ODT) monolayer at Cu-Bi2Te3 interfaces. In contrast introducing ODT at Ni-Bi2Te3 interfaces results in a 20% decrease in Σc. Rutherford backscattering spectrometry, X-ray diffraction and electron spectroscopy analyses indicate that metal-sulfur and sulfur-Bi2Te3 bonds at metal-Bi2Te3 interfaces inhibit chemical mixing, curtail metal-telluride formation, and suppress oxidation. Suppressing p-type Cu2Te favors electrical transport across Cu-metallized n-type Bi2Te3, whereas inhibiting the formation of Ohmic-contact-promoting NixTey compromises the electrical conductance at Ni-Bi2Te3 interfaces. Our findings illustrate that molecular nanolayers could be attractive for manipulating interface chemistry and phase formation for tailoring electrical transport across metal-thermoelectric interfaces for solid-state refrigeration applications.

Harnessing Topological Band Effects in Bismuth Telluride Selenide for Large Enhancements in Thermoelectric Properties through Isovalent Doping.

Dilute isovalent sulfur doping simultaneously increases electrical conductivity and Seebeck coefficient in Bi2 Te2 Se nanoplates, and bulk pellets made from them. This unusual trend at high electron concentrations is underpinned by multifold increases in electron effective mass attributable to sulfur-induced band topology effects, providing a new way for accessing a high thermoelectric figure-of-merit in topological-insulator-based nanomaterials through doping.

A noncontact thermal microprobe for local thermal conductivity measurement.

We demonstrate a noncontact thermal microprobe technique for measuring the thermal conductivity κ with ∼3 µm lateral spatial resolution by exploiting quasiballistic air conduction across a 10-100 nm air gap between a joule-heated microprobe and the sample. The thermal conductivity is extracted from the measured effective thermal resistance of the microprobe and the tip-sample thermal contact conductance and radius in the quasiballistic regime determined by calibration on reference samples using a heat transfer model. Our κ values are within 5%-10% of that measured by standard steady-state methods and theoretical predictions for nanostructured bulk and thin film assemblies of pnictogen chalcogenides. Noncontact thermal microprobing demonstrated here mitigates the strong dependence of tip-sample heat transfer on sample surface chemistry and topography inherent in contact methods, and allows the thermal characterization of a wide range of nanomaterials.

Seebeck tuning in chalcogenide nanoplate assemblies by nanoscale heterostructuring.

Chalcogenide nanostructures offer promise for obtaining nanomaterials with high electrical conductivity, low thermal conductivity, and high Seebeck coefficient. Here, we demonstrate a new approach of tuning the Seebeck coefficient of nanoplate assemblies of single-crystal pnictogen chalcogenides by heterostructuring the nanoplates with tellurium nanocrystals. We synthesized bismuth telluride and antimony telluride nanoplates decorated with tellurium nanorods and nanofins using a rapid, scalable, microwave-stimulated organic surfactant-directed technique. Heterostructuring permits two- to three-fold factorial tuning of the Seebeck coefficient, and yields a 40% higher value than the highest reported for bulk antimony telluride. Microscopy and spectroscopy analyses of the nanostructures suggest that Seebeck tunability arises from carrier-energy filtration effects at the Te-chalcogenide heterointerfaces. Our approach of heterostructuring nanoscale building blocks is attractive for realizing high figure-of-merit thermoelectric nanomaterials.