C. Karthik

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.

Tunable bandgap in BiFeO3 nanoparticles: The role of microstrain and oxygen defects

We demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. Defect states due to oxygen and microstrain show a strong dependence on BiFeO3 particle size and have a significant effect on the shape of absorbance curves. Oxygen-defect induced microstrain and undercoordinated oxygen on the surface of BiFeO3 nanoparticles are demonstrated via HRTEM and XPS studies. Microstrain in the lattice leads to the reduction in rhombohedral distortion of BiFeO3 for particle sizes below 30 nm. The decrease in band gap with decreasing particle size is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.

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.

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.

Microstructural Characterization of Next Generation Nuclear Graphites

This article reports the microstructural characteristics of various petroleum and pitch based nuclear graphites (IG-110, NBG-18, and PCEA) that are of interest to the next generation nuclear plant program. Bright-field transmission electron microscopy imaging was used to identify and understand the different features constituting the microstructure of nuclear graphite such as the filler particles, microcracks, binder phase, rosette-shaped quinoline insoluble (QI) particles, chaotic structures, and turbostratic graphite phase. The dimensions of microcracks were found to vary from a few nanometers to tens of microns. Furthermore, the microcracks were found to be filled with amorphous carbon of unknown origin. The pitch coke based graphite (NBG-18) was found to contain higher concentration of binder phase constituting QI particles as well as chaotic structures. The turbostratic graphite, present in all of the grades, was identified through their elliptical diffraction patterns. The difference in the microstructure has been analyzed in view of their processing conditions.

A microprobe technique for simultaneously measuring thermal conductivity and Seebeck coefficient of thin films

We demonstrate a microprobe technique that can simultaneously measure thermal conductivity κ and Seebeck coefficient α of thin films. In this technique, an alternative current joule-heated V-shaped microwire that serves as heater, thermometer and voltage electrode, locally heats the thin film when contacted with the surface. The κ is extracted from the thermal resistance of the microprobe and α from the Seebeck voltage measured between the probe and unheated regions of the film by modeling heat transfer in the probe, sample and their contact area, and by calibrations with standard reference samples. Application of the technique on sulfur-doped porous Bi2Te3 and Bi2Se3 films reveals α=−105.4 and 1.96 μV/K, respectively, which are within 2% of the values obtained by independent measurements carried out using microfabricated test structures. The respective κ values are 0.36 and 0.52 W/mK, which are significantly lower than the bulk values due to film porosity, and are consistent with effective media theory. ...

Effect of Microstrain on the Magnetic Properties of BiFeO3 Nanoparticles

We report on size induced microstrain-dependent magnetic properties of BiFeO3 nanoparticles. The microstrain is found to be high (e > 0.3%) for smaller crystallite sizes (d < 30 nm), and shows a sharp decrease as the particle size increases. The presence of pseudo-cubic symmetry is evidenced for these nanoparticles. Raman spectral studies suggest straightening of the Fe-O-Fe bond angle accompanied by a decrease in FeO6 octahedral rotation for d < 65 nm. The magnetization shows a dip around 30 nm, half the size of spin cycloid length for BiFeO3, due to a decrease in rhombohedral distortion with crystallite size. We also observe a similar trend in the TN with respect to size indicating that the microstrain plays a significant role in controlling the magnetic property of BiFeO3.

Threshold conductivity switching in sulfurized antimony selenide nanowires

We report reversible switching between Ohmic and negative differential resistance states at a threshold voltage in sub-100-nm diameter sulfurized antimony selenide nanowires. We show that threshold switching in our nanowires arises due to high non-equilibrium free carrier concentrations resulting from impact ionization of carriers from defect states traceable to sulfurization and surface dangling bonds. Threshold switching is suppressed because of inhibited carrier generation at air-passivated defect states or at high temperatures due to thermally induced carrier depletion from deep states which preempts impact ionization. Such non-linear phenomena would be important for designing phase-change memories, thermoelectric devices, and sensors using pnictogen chalcogenide nanowires.

Metal-dielectric interface toughening by molecular nanolayer decomposition

Recent work has shown that copper–silica interfaces can be toughened several fold by combining interface functionalization with an organosilane molecular nanolayer (MNL) and thermal annealing. In order to understand the role of annealing-induced MNL instabilities on interface toughness, we studied the effects of interface chemical changes on the fracture toughness of copper–silica interfaces tailored with organosilane or organogermane MNLs. Our results indicate that MNL decomposition into its inorganic constituents and consequent intermixing can provide an interface toughening mechanism. Organogermane–tailored interfaces exhibit higher toughness values due to Ge-diffusion induced copper silicate formation, not observed at organosilane tailored interfaces. These findings show that organic nanolayer decomposition at a buried interface could be exploited to tailor interfacial properties through appropriate choice of MNL chemistry and processing treatments.

Magnetic and electrocatalytic properties of transition metal doped MoS2 nanocrystals

In this paper, the magnetic and electrocatalytic properties of hydrothermally grown transition metal doped (10% of Co, Ni, Fe, and Mn) 2H-MoS2 nanocrystals (NCs) with a particle size 25–30 nm are reported. The pristine 2H-MoS2 NCs showed a mixture of canted anti-ferromagnetic and ferromagnetic behavior. While Co, Ni, and Fe doped MoS2 NCs revealed room temperature ferromagnetism, Mn doped MoS2 NCs showed room temperature paramagnetism, predominantly. The ground state of all the materials is found to be canted-antiferromagnetic phase. To study electrocatalytic performance for hydrogen evolution reaction, polarization curves were measured for undoped and the doped MoS2 NCs. At the overpotential of η = −300 mV, the current densities, listed from greatest to least, are FeMoS2, CoMoS2, MoS2, NiMoS2, and MnMoS2, and the order of catalytic activity found from Tafel slopes is CoMoS2 > MoS2 > NiMoS2 > FeMoS2 > MnMoS2. The increasing number of catalytically active sites in Co doped MoS2 NCs might be responsible for their superior electrocatalytic activity. The present results show that the magnetic order-disorder behavior and catalytic activity can be modulated by choosing the suitable dopants in NCs of 2D materials.In this paper, the magnetic and electrocatalytic properties of hydrothermally grown transition metal doped (10% of Co, Ni, Fe, and Mn) 2H-MoS2 nanocrystals (NCs) with a particle size 25–30 nm are reported. The pristine 2H-MoS2 NCs showed a mixture of canted anti-ferromagnetic and ferromagnetic behavior. While Co, Ni, and Fe doped MoS2 NCs revealed room temperature ferromagnetism, Mn doped MoS2 NCs showed room temperature paramagnetism, predominantly. The ground state of all the materials is found to be canted-antiferromagnetic phase. To study electrocatalytic performance for hydrogen evolution reaction, polarization curves were measured for undoped and the doped MoS2 NCs. At the overpotential of η = −300 mV, the current densities, listed from greatest to least, are FeMoS2, CoMoS2, MoS2, NiMoS2, and MnMoS2, and the order of catalytic activity found from Tafel slopes is CoMoS2 > MoS2 > NiMoS2 > FeMoS2 > MnMoS2. The increasing number of catalytically active sites in Co doped MoS2 NCs might be responsible for...