Thermal Expansion,Anharmonicity and Temperature‐Dependent Raman Spectra of Single‐ and Few‐Layer MoSe2 and WSe2

We report the temperature‐dependent Raman spectra of single‐ and few‐layer MoSe₂ and WSe₂ in the range 77–700 K. We observed linear variation in the peak positions and widths of the bands arising from contributions of anharmonicity and thermal expansion. After characterization using atomic force microscopy and high‐resolution transmission electron microscopy, the temperature coefficients of the Raman modes were determined. Interestingly, the temperature coefficient of the A²_2u mode is larger than that of the A_1g mode, the latter being much smaller than the corresponding temperature coefficients of the same mode in single‐layer MoS₂ and of the G band of graphene. The temperature coefficients of the two modes in single‐layer MoSe₂ are larger than those of the same modes in single‐layer WSe₂. We have estimated thermal expansion coefficients and temperature dependence of the vibrational frequencies of MoS₂ and MoSe₂ within a quasi‐harmonic approximation, with inputs from first‐principles calculations based on density functional theory. We show that the contrasting temperature dependence of the Raman‐active mode A_1g in MoS₂ and MoSe₂ arises essentially from the difference in their strain–phonon coupling.     

Nanostructures boost the thermoelectric performance of PbS

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.     

Anomalous electronic transport in dual-nanostructured lead telluride

The Pb- and Sb- dual nanostructured PbTe system exhibits anomalous electronic transport behavior wherein the carrier mobility first increases and then decreases with increase in temperature. By combining in situ transmission electron microscopy observations and theoretical calculations based on energy filtering of charge carriers, we propose a plausible mechanism of charge transport based on interphase potential that is mediated by interdiffusion between coexisting Pb and Sb precipitates. These findings promise new strategies to enhance thermoelectric figure of merit via dual and multinanostructuring of miscible precipitates.     

High performance Na-doped PbTe-PbS thermoelectric materials: electronic density of states modification and shape-controlled nanostructures

Thermoelectric heat-to-power generation is an attractive option for robust and environmentally friendly renewable energy production. Historically, the performance of thermoelectric materials has been limited by low efficiencies, related to the thermoelectric figure-of-merit ZT. Nanostructuring thermoelectric materials have shown to enhance ZT primarily via increasing phonon scattering, beneficially reducing lattice thermal conductivity. Conversely, density-of-states (DOS) engineering has also enhanced electronic transport properties. However, successfully joining the two approaches has proved elusive. Herein, we report a thermoelectric materials system whereby we can control both nanostructure formations to effectively reduce thermal conductivity, while concurrently modifying the electronic structure to significantly enhance thermoelectric power factor. We report that the thermoelectric system PbTe-PbS 12% doped with 2% Na produces shape-controlled cubic PbS nanostructures, which help reduce lattice thermal conductivity, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS that allow for enhancements in thermoelectric power factor. These concomitant and synergistic effects result in a maximum ZT for 2% Na-doped PbTe-PbS 12% of 1.8 at 800 K.     

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO₃ bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport.The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible dip at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded.The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.     

Graphene oxide as an enzyme inhibitor: modulation of activity of α-chymotrypsin

We have investigated the efficacy of graphene oxide (GO) in modulating enzymatic activity. Specifically, we have shown that GO can act as an artificial receptor and inhibit the activity of α-chymotrypsin (ChT), a serine protease. Most significantly, our data demonstrate that GO exhibits the highest inhibition dose response (by weight) for ChT inhibition compared with all other reported artificial inhibitors. Through fluorescence spectroscopy and circular dichroism studies, we have shown that this protein-receptor interaction is highly biocompatible and conserves the protein's secondary structure over extended periods (>24 h). We have also explored GO-enzyme interactions by controlling the ionic strength of the medium, which attenuates the host-guest electrostatic interactions. These findings suggest a new generation of enzymatic inhibitors that can be applied to other complex proteins by systematic modification of the GO functionality.     

Dopant Distributions in PbTe-Based Thermoelectric Materials

Atom-probe tomography (APT) is utilized to characterize the dopant distribution in two thermoelectric materials systems: (1) PbTe-2?mol.%SrTe-1?mol.%Na₂Te, and (2) codoped PbTe-1.25?mol.%K-1.4?mol.%Na. We observe the presence of Na-rich precipitates of a few nanometers in diameter in both systems. Both concentration frequency distribution analyses and partial radial distribution functions are employed to analyze the tendency for dopant clustering detected by APT. In the codoped sample, K accumulates significantly in Na-rich precipitates, while in the Sr-containing sample, Sr is homogeneously distributed. High-resolution transmission electron microscopy also reveals the presence of precipitates having platelet morphology, which are oriented parallel to the {001} planes.     

Systematic Study of Oxygen Vacancy Tunable Transport Properties of Few‐Layer MoO3−x Enabled by Vapor‐Based Synthesis

Bulk and nanoscale molybdenum trioxide (MoO₃) has shown impressive technologically relevant properties, but deeper investigation into 2D MoO₃ has been prevented by the lack of reliable vapor‐based synthesis and doping techniques. Herein, the successful synthesis of high‐quality, few‐layer MoO₃ down to bilayer thickness via physical vapor deposition is reported. The electronic structure of MoO₃ can be strongly modified by introducing oxygen substoichiometry (MoO_3?x ), which introduces gap states and increases conductivity. A dose‐controlled electron irradiation technique to introduce oxygen vacancies into the few‐layer MoO₃ structure is presented, thereby adding n‐type doping. By combining in situ transport with core‐loss and monochromated low‐loss scanning transmission electron microscopy–electron energy‐loss spectroscopy studies, a detailed structure–property relationship is developed between Mo‐oxidation state and resistance. Transport properties are reported for MoO_3?x down to three layers thick, the most 2D‐like MoO_3?x transport hitherto reported. Combining these results with density functional theory calculations, a radiolysis‐based mechanism for the irradiation‐induced oxygen vacancy introduction is developed, including insights into favorable configurations of oxygen defects. These systematic studies represent an important step forward in bringing few‐layer MoO₃ and MoO_3?x into the 2D family, as well as highlight the promise of MoO_3?x as a functional, tunable electronic material.     

Ternary Metal Phosphide with Triple‐Layered Structure as a Low‐Cost and Efficient Electrocatalyst for Bifunctional Water Splitting

Development of low‐cost, high‐performance, and bifunctional electrocatalysts for water splitting is essential for renewable and clean energy technologies. Although binary phosphides are inexpensive, their performance is not as good as noble metals. Adding a third metal element to binary phosphides (Ni‐P, Co‐P) provides the opportunity to tune their crystalline and electronic structures and thus their electrocatalytic properties. Here, ternary phosphide (NiCoP) films with different nickel to cobalt ratios via an electrodeposition technique are synthesized. The films have a triple‐layered and hierarchical morphology, consisting of nanosheets in the bottom layer, ≈90–120 nm nanospheres in the middle layer, and larger spherical particles on the top layer. The ternary phosphides exhibit versatile activities that are strongly dependent on the Ni/Co ratios and Ni_0.51Co_0.49P film is found to have the best electrocatalytic activities for both hydrogen evolution reactions and oxygen evolution reactions. The high performance of the ternary phosphide film is attributed to enhanced electric conductivity so that reaction kinetics is accelerated, enlarged surface area due to the hierarchical and three‐layered morphology, and increased local electric dipole so that the energy barrier for the water splitting reaction is lowered.