Theodorian Borca-Tasciuc

Data reduction in 3ω method for thin-film thermal conductivity determination

The 3ω method has been proven to be very useful for determining the thermal conductivity of thin films and their substrates. Several simplifications are often used in determining the thermal conductivity of the films based on the experimentally measured 3ω signal. These simplifications, however, have limited range of applicability. In this work, we present a detailed analysis and mathematical modeling of the 3ω method applied for different experimental conditions. Effects considered include the finite substrate thickness, anisotropic nature of the film and substrate thermal conductivity, the film-substrate thermal property contrasts, the effect of heat capacitance of the heater, and the effect of thermal boundary resistance. Several experimental results are analyzed using the models presented. This work shows that the 3ω method can be extended to a wide range of sample conditions, with anisotropic conductivities in both the substrate and the film, and with small film-substrate conductivity contrast.

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 Graphene by the Controlled Adsorption of Water Molecules

Graphene, a single-atom-thick layer of sp 2 -hybridized carbon atoms, has generated considerable excitement in the scientifi c community due to its peculiar electronic band structure, which leads to unusual phenomena such as the anomalous quantum Hall effect, [ 1,2 ] spin-resolved quantum interference, [ 3 ] ballistic electron transport, [ 4 ] and bipolar supercurrent. [ 5 ] However, pristine graphene is a semimetal with zero bandgap; the local density of states at the Fermi level is zero and conduction can only occur by the thermal excitation of electrons. [ 2 ] This lack of an electronic bandgap is the major obstacle limiting the utilization of graphene in nano-electronic and -photonic devices, [ 6,7 ] such as p–n junctions, transistors, photodiodes, and lasers. The graphene band structure is sensitive to lattice symmetry and several methods have been developed to break this symmetry and open an energy gap. These methods are based on a variety of techniques, such as defect generation, [ 8 ] doping (e.g., with potassium [ 9 ] ), applied bias, [ 10–12 ] and interaction with gases [ 13 ] (e.g., nitrogen dioxide). For instance, in reference [ 12 ] a tunable bandgap of up to 0.25 eV was achieved for electrically gated bilayer graphene by a variable external electric fi eld. Similarly, an internal electric fi eld produced by an imbalance of doped charge between two graphene layers has been shown to open a bandgap. [ 9 ] It has been demonstrated that a gap of ≈ 0.26 eV can be produced by growing graphene epitaxially on silicon carbide substrates. [ 14 ] This gap originated from the breaking of sublattice symmetry due to the graphene–substrate interaction. Patterned adsorption of atomic hydrogen onto the Moire superlattice positions of graphene [ 15 ] has resulted in a bandgap of ≈ 0.73 eV opening, while half-hydrogenated graphene [ 16 ] resulted in a bandgap of ≈ 0.43 eV. A graphene nanomesh structure [ 17 ] has also been shown to exhibit a bandgap. In this graphene structure, lateral quantum confi nement and localization effects due to

Large‐Area Freestanding Graphene Paper for Superior Thermal Management

Large-area freestanding graphene papers (GPs) are fabricated by electrospray deposition integrated with a continuous roll-to-roll process. Upon mechanical compaction and thermal annealing, GPs can achieve a thermal conductivity of as high as 1238.3-1434 W m(-1) K(-1) . The super-thermally conductive GPs display an outstanding heat-spread ability and are more efficient in removing hot spots than Cu and Al foils.

The effect of nanoparticles on the liquid-gas surface tension of Bi2Te3 nanofluids

This work investigates the effect of size and concentration of nanoparticles on the effective gas-liquid surface tension of aqueous solutions of bismuth telluride nanoparticles functionalized with thioglycolic acid. The gas-liquid surface tension is obtained by solving the Laplace-Young equation under experimentally measured boundary conditions and droplet parameters. The results demonstrate that the gas-liquid surface tension depends on concentration as well as nanoparticle size. Solutions containing 2.5 and 10.4 nm nanoparticle diameters have been tested. For both, a minimum surface tension exists within the range of tested mass concentrations. The largest reduction in the surface tension (>50% versus bulk liquid) occurred for the 2.5 nm nanoparticle nanofluid. Accumulation and assembly of the charged nanoparticles at the liquid-gas interface was assumed to be responsible for the surface tension of the nanofluids investigated in this work.

Strain and size effects on heat transport in nanostructures

The relative role of the residual strain and dimensional scaling on heat transport in nanostructures is investigated by molecular dynamics simulations of a model Lennard-Jones solid. It is observed that tensile (compressive) strains lead to a reduction (enhancement) of the lattice thermal conductivity. A nonhydrostatic strain induces thermal conductivity anisotropy in the material. This effect is due to the variation with strain of the stiffness tensor and lattice anharmonicity, and therefore of the phonon group velocity and phonon mean free path. The effect due to the lattice anharmonicity variation appears to be dominant. The size effect was studied separately in unstrained thin films. Phonon scattering on surfaces leads to a drastic reduction of the thermal conductivity effect which is much more important than that due to strain in the bulk. It is suggested that strain may be used to tailor the phonon mean free path which offers an indirect method to control the size effect.

Surfactant-Directed Synthesis of Branched Bismuth Telluride/Sulfide Core/Shell Nanorods†

Branched core/shell bismuth telluride/bismuth sulfide nanorod heterostructures are prepared by using a biomimetic surfactant, L-glutathionic acid. Trigonal nanocrystals of bismuth telluride are encapsulated by nanoscopic shells of orthorhombic bismuth sulfide. Crystallographic twinning causes shell branching. Such heteronanostructures are attractive for thermoelectric power generation and cooling applications.

THERMAL CONDUCTIVITY OF InAs/ AlSb SUPERLATTICES

In this work, we present experimental studies on the cross-plane thermal conductivity of InAs/AlSb superlattices. The thermal conductivities of MBE-grown InAs/AlSb superlattices are measured using the 3 y method from 80 to 300 K. The influence of the growth temperature and postannealing is investigated. Significant reductions in thermal conductivity are observed in these superlattices compared to the predictions of the Fourier heat conduction theory based on the bulk material properties. These results suggest that the interface conditions strongly influence the thermal conductivity.In this work, we present experimental studies on the cross-plane thermal conductivity of InAs/AlSb superlattices. The thermal conductivities of MBE-grown InAs/AlSb superlattices are measured using the 3 y method from 80 to 300 K. The influence of the growth temperature and postannealing is investigated. Significant reductions in thermal conductivity are observed in these superlattices compared to the predictions of the Fourier heat conduction theory based on the bulk material properties. These results suggest that the interface conditions strongly influence the thermal conductivity.

Phonon engineering in nanostructures for solid-state energy conversion

Solid-state energy conversion technologies such as thermoelectric and thermionic refrigeration and power generation require materials with low thermal conductivity but good electrical conductivity, which are difficult to realize in bulk semiconductors. Nanostructures such as quantum wires and quantum wells provide alternative approaches to improve the solid-state energy conversion efficiency through size effects on the electron and phonon transport. In this paper, we discuss the possibility of engineering the phonon transport in nanostructures, with emphases on the thermal conductivity of superlattices. Following a general discussion on the directions for reducing the lattice thermal conductivity in nanostructures, specific modeling results on the phonon transport in superlattices will be presented and compared with recent experimental studies to illustrate the potential approaches and remaining questions.

Nanotube-assisted protein deactivation

Conjugating proteins onto carbon nanotubes has numerous applications in biosensing, imaging and cellular delivery. However, remotely controlling the activity of proteins in these conjugates has never been demonstrated. Here we show that upon near-infrared irradiation, carbon nanotubes mediate the selective deactivation of proteins in situ by photochemical effects. We designed nanotube-peptide conjugates to selectively destroy the anthrax toxin, and also optically transparent coatings that can self-clean following either visible or near-infrared irradiation. Nanotube-assisted protein deactivation may be broadly applicable to the selective destruction of pathogens and cells, and will have applications ranging from antifouling coatings to functional proteomics.