Electrodeposited Thin-Film Micro-Thermoelectric Coolers with Extreme Heat Flux Handling and Microsecond Time Response.

Thin-film thermoelectric coolers are emerging as a viable option for the on-chip temperature management of electronic and photonic integrated circuits. In this work, we demonstrate the record heat flux handling capability of electrodeposited Bi2Te3 films of 720(±60) W cm-2 at room temperature, achieved by careful control of the contact interfaces to reduce contact resistance. The characteristic parameters of a single leg thin-film devices were measured in situ, giving a Seebeck coefficient of S = -121(±6) µV K-1, thermal conductivity of κ = 0.85(±0.08) W m-1 K-1, electrical conductivity of σ = 5.2(±0.32) × 104 S m-1, and electrical contact resistivity of ∼10-11 Ω m2. These thermoelectric parameters lead to a material ZT = 0.26(±0.04), which, for our device structure, allowed a net cooling of ΔTmax = 4.4(±0.12) K. A response time of τ = 20 µs was measured experimentally. This work shows that with the correct treatment of contact interfaces, electrodeposited thin-film thermoelectrics can compete with more complicated and expensive technologies such as metal organic chemical vapor deposition (MOCVD) multilayers.

Low-temperature synthesis and electrocatalytic application of large-area PtTe2 thin films.

The synthesis of transition metal dichalcogenides (TMDs) has been a primary focus for 2D nanomaterial research over the last 10 years, however, only a small fraction of this research has been concentrated on transition metal ditellurides. In particular, nanoscale platinum ditelluride (PtTe2) has rarely been investigated, despite its potential applications in catalysis, photonics and spintronics. Of the reports published, the majority examine mechanically-exfoliated flakes from chemical vapor transport (CVT) grown crystals. This method produces high quality-crystals, ideal for fundamental studies. However, it is very resource intensive and difficult to scale up meaning there are significant obstacles to implementation in large-scale applications. In this report, the synthesis of thin films of PtTe2 through the reaction of solid-phase precursor films is described. This offers a production method for large-area, thickness-controlled PtTe2, potentially suitable for a number of applications. These polycrystalline PtTe2 films were grown at temperatures as low as 450 °C, significantly below the typical temperatures used in the CVT synthesis methods. Adjusting the growth parameters allowed the surface coverage and morphology of the films to be controlled. Analysis with scanning electron- and scanning tunneling microscopy indicated grain sizes of above 1 µm could be achieved, comparing favorably with typical values of ∼50 nm for polycrystalline films. To investigate their potential applicability, these films were examined as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The films showed promising catalytic behavior, however, the PtTe2 was found to undergo chemical transformation to a substoichiometric chalcogenide compound under ORR conditions. This study shows while PtTe2 is stable and highly useful for in HER, this property does not apply to ORR, which undergoes a fundamentally different mechanism. This study broadens our knowledge on the electrocatalysis of TMDs.

Quantifying the factors limiting rate performance in battery electrodes.

One weakness of batteries is the rapid falloff in charge-storage capacity with increasing charge/discharge rate. Rate performance is related to the timescales associated with charge/ionic motion in both electrode and electrolyte. However, no general fittable model exists to link capacity-rate data to electrode/electrolyte properties. Here we demonstrate an equation which can fit capacity versus rate data, outputting three parameters which fully describe rate performance. Most important is the characteristic time associated with charge/discharge which can be linked by a second equation to physical electrode/electrolyte parameters via various rate-limiting processes. We fit these equations to ~200 data sets, deriving parameters such as diffusion coefficients or electrolyte conductivities. It is possible to show which rate-limiting processes are dominant in a given situation, facilitating rational design and cell optimisation. In addition, this model predicts the upper speed limit for lithium/sodium ion batteries, yielding a value that is consistent with the fastest electrodes in the literature.

Large variations in both dark- and photoconductivity in nanosheet networks as nanomaterial is varied from MoS2 to WTe2.

We have used solution processing techniques to fabricate thin-film networks of nanosheets of six different transition metal dichalcogenides; MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2. We have measured both the dark conductivity and the photoconductivity under broad band illumination in the intensity range from 0-1500 W m(-2). The dark conductivity varied from ∼ 10(-6) S m(-1) for MoS2 to ∼ 1 S m(-1) for WTe2, with an apparent exponential dependence on bandgap. All materials studied show photocurrents which rise slowly with time and depend sub-linearly on light intensity, both hallmarks of trap limited processes. Because the photoresponse depends relatively weakly on bandgap, the ratio of photo- to dark conductivity is largest for the sulphides because of their lower dark conductivities. As such, MoS2 and WS2 may be best suited to photo-detection applications. However, due to their lower bandgap and superior conductivity, WSe2 and WTe2 might prove more effective in other applications, for example in photovoltaic cells.

Percolation scaling in composites of exfoliated MoS2 filled with nanotubes and graphene.

Applications of films of exfoliated layered compounds in many areas will be limited by their relatively low electrical conductivity. To address this, we have prepared and characterised composites of a nano-conductor (nanotubes or graphene) embedded in a matrix of exfoliated MoS(2) nanosheets. Solvent exfoliation of MoS(2) nanosheets, followed by blending with dispersions of graphene or nanotubes allowed the formation of such composite films by vacuum filtration. This gave spatially uniform mixtures with fully tuneable nano-conductor content. By addition of the nano-conducting phase, it was possible to vary the electrical conductivity of the composite over nine orders of magnitude. For both filler types the conductivity followed percolation scaling laws both above and below the percolation threshold. In the case of SWNT-filled composites, conductivities as high as ~40 S m(-1) were achieved at volume fractions as low as ~4%.

Solvent exfoliation of transition metal dichalcogenides: dispersibility of exfoliated nanosheets varies only weakly between compounds.

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS(2), MoS(2), MoSe(2), and MoTe(2), in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m(2), implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS(2) and MoSe(2) to have surface energies of ∼75 mJ/m(2), in good agreement with dispersibility measurements. However, this method suggested MoTe(2) to have a considerably larger surface energy (∼120 mJ/m(2)). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen's dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of δ(D) ∼ 18 MPa(1/2), δ(P) ∼ 8.5 MPa(1/2), and δ(H) ∼ 7 MPa(1/2) with all four types of layered material. Knowledge of these properties allows the estimation of the Flory-Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h.

Naltrexone in the treatment of alcohol dependence: a Canadian trial.

OBJECTIVES: Alcohol dependence is a prevalent psychiatric disorder affecting approximately 12% of the adult population at some point in their lifetime. Psychosocial treatments are associated with only modest success rates. The first Canadian clinical trial with naltrexone, an opiate antagonist, was conducted to evaluate its safety and usefulness as an adjunctive treatment in the management of alcohol dependence. METHODS: One hundred twenty alcohol-dependent individuals were assessed to receive treatment with 50 mg of naltrexone orally for 12 weeks in an open-label trial. Patients were seen biweekly and received a concurrent psychosocial intervention. Treatment was conducted at multiple sites in Canada. RESULTS: Fifty-four per cent of subjects completed the entire 12 weeks of treatment. During the study, 39% of patients abstained, while of the individuals reporting drinking at baseline, 86% were consuming less alcohol by their final visit. These reductions were accompanied by a significant decrease in craving for alcohol at week 12, as measured by the Obsessive Compulsive Drinking Scale (P<0.01). Naltrexone was well tolerated and no serious adverse events were experienced. CONCLUSIONS: The data lend support to the hypothesis that endogenous opioid activity is involved in the regulation of alcohol intake, and that antagonists of endogenous opioids decrease craving and drinking. Opiate antagonists such as naltrexone are a new strategy in the treatment of alcohol dependence. Naltrexone can be safely given to female and male alcoholics, is acceptable to patients, and plays a role in reducing alcohol consumption and preventing relapse to heavy drinking.