Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge0.5Te0.5)3 via Band Engineering.

We report the phase evolution and thermoelectric properties of a series of Co(Ge0.5Te0.5)3-xSbx (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge0.5Te0.5)3 skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb4 rings with Ge2Te2 ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.

Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe.

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb2 Te3 significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.

Electronic structures of 24-valence-electron full Heusler compounds investigated by density functional and GW calculations.

The electronic structures of Fe-based and Ru-based full Heusler compounds have been investigated systematically by density functional theory (DFT) with PBE, PBE + U, and HSE06 exchange-correlation (XC) functionals. In order to have a better systematic and quantitative comparison between the results of different approximations, the average deviation of eigenvalues (ADE) between any two electronic band structures were calculated. From quantitative analysis of the ADEs, we have shown that different XC functionals used in the DFT calculations will result in very different and inconsistent electronic band structures. However, the discrepancies are dramatically reduced and get more consistent band structures after the GW calculations. Furthermore, comparing the experimental and calculated Seebeck coefficients and band-gap values of Fe2VAl, it implies that the GW methods including dynamically screened Coulomb interactions are more reliable than DFT with PBE or HSE06 functionals. Conclusively, contrast to the fact that DFT methods give inconsistent band structures when using different XC functionals, the GW methods have better predictive power for the band structures of Fe-based and Ru-based full Heusler compounds.

Theoretical prediction of superconductivity in monolayer CoO2.

Motivated by the synthesis of the layered structure CoO2via Li atom deintercalation from LixCoO2, herein, we investigated the electronic structure, lattice dynamics, electron-phonon interaction, and superconductivity of monolayer CoO2 using first-principles calculations. This 2D material was predicted to have a ferromagnetic ground state with a metallic band structure and the total magnetization of 0.83µB. Remarkably, the non-spin polarized calculations show that the monolayer CoO2 possesses phonon-mediated superconductivity at 25-28 K owing to its intermediate to strong electron-phonon coupling (EPC). The rather strong EPC in this compound is mainly driven by the acoustic phonons, making this compound one of the highest-temperature superconductors among the existing 2D materials. Moreover, the CoO2 sheets could be synthesized via exfoliation from bulk CoO2 owing to the relatively small interlayer binding energy while maintaining its stability under normal experimental conditions. Compared to its bulk and bilayer counterparts, monolayer CoO2 was found to have highest EPC.

Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe.

Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b, c, and a directions, respectively.

Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1-xBix)Te crystals.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1-xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6-10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm-3 to 3-5 × 1020 cm-3 and the Seebeck coefficient is boosted three times to 75 µVK-1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m-1K-1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1-xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites.

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron-hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials' properties.

Expedite random structure searching using objects from Wyckoff positions.

Random structure searching has been proved to be a powerful approach to search and find the global minimum and the metastable structures. A true random sampling is in principle needed yet it would be highly time-consuming and/or practically impossible to find the global minimum for the complicated systems in their high-dimensional configuration space. Thus the implementations of reasonable constraints, such as adopting system symmetries to reduce the independent dimension in structural space and/or imposing chemical information to reach and relax into low-energy regions, are the most essential issues in the approach. In this paper, we propose the concept of "object" which is either an atom or composed of a set of atoms (such as molecules or carbonates) carrying a symmetry defined by one of the Wyckoff positions of space group and through this process it allows the searching of global minimum for a complicated system to be confined in a greatly reduced structural space and becomes accessible in practice. We examined several representative materials, including Cd3As2 crystal, solid methanol, high-pressure carbonates (FeCO3), and Si(111)-7 × 7 reconstructed surface, to demonstrate the power and the advantages of using "object" concept in random structure searching.

Dependence of Adenine Raman Spectrum on Excitation Laser Wavelength: Comparison between Experiment and Theoretical Simulations.

We acquired the Raman spectra of adenine in powder and aqueous phase using excitation lasers with 532, 633, and 785 nm wavelengths for the region between 300 and 1500 cm-1. In comparison to the most distinct peak at 722 cm-1, the peaks between 1200 and 1500 cm-1 exhibited a characteristic increase in cross-section with decreasing excitation wavelength in both phases. This trend can be reproduced by different density functional theory (DFT) calculations for the adenine molecule in the gas phase as well as in the aqueous phase. Furthermore, from the calculation on the π-stacked dimer, hydrogen-bonded dimer, and trimer, we find that this trend toward excitation laser wavelength is not sensitive to the packing. When comparing the Raman spectra given by different excitation wavelength, one should take care in analyzing the cross-section, and present day DFT calculations are able to capture general trends in the excitation laser wavelength dependence of the Raman activity.

Quasiparticle Scattering in the Rashba Semiconductor BiTeBr: The Roles of Spin and Defect Lattice Site.

Observations of quasiparticle interference have been used in recent years to examine exotic carrier behavior at the surfaces of emergent materials, connecting carrier dispersion and scattering dynamics to real-space features with atomic resolution. We observe quasiparticle interference in the strongly Rashba split 2DEG-like surface band found at the tellurium termination of BiTeBr and examine two mechanisms governing quasiparticle scattering: We confirm the suppression of spin-flip scattering by comparing measured quasiparticle interference with a spin-dependent elastic scattering model applied to the calculated spectral function. We also use atomically resolved STM maps to identify point defect lattice sites and spectro-microscopy imaging to discern their varying scattering strengths, which we understand in terms of the calculated orbital characteristics of the surface band. Defects on the Bi sublattice cause the strongest scattering of the predominantly Bi 6p derived surface band, with other defects causing nearly no scattering near the conduction band minimum.

Structure prediction of the solid forms of methanol: an ab initio random structure searching approach.

Liquid methanol and methanol clusters have been comprehensively studied to reveal their local structure and hydrogen bond networks. However, our understanding of the crystal forms of methanol is rather limited. The known crystal structures of solid methanol, α, ß, and γ, are composed of infinite hydrogen bond chains in their unit cell. The structural diversity of solid methanol is much less than that of liquid methanol, in which both chain and ring structures exist and have been confirmed by experiments. In this study, we employed ab initio random structure searching (AIRSS) to study possible solid methanol structures. AIRSS predicted known solid methanol phases as well as various ring structures that have not been considered. A new possible candidate structure for the δ phase was also discovered. The relative stability of known solid methanol phases and our newly discovered structures were also investigated through dispersion corrected density functional theory. The density functional calculation provides reliable phase transition pressures between the known phases and the searched structures compared with experimental suggestions. In addition, the simulation result indicated that CHO hydrogen bonds play a major role in stabilizing the methanol crystals under high pressures.

Synthesis of two-dimensional Tl(x)Bi(1-x) compounds and Archimedean encoding of their atomic structure.

Crystalline atomic layers on solid surfaces are composed of a single building block, unit cell, that is copied and stacked together to form the entire two-dimensional crystal structure. However, it appears that this is not an unique possibility. We report here on synthesis and characterization of the one-atomic-layer-thick Tl(x)Bi(1-x) compounds which display quite a different arrangement. It represents a quasi-periodic tiling structures that are built by a set of tiling elements as building blocks. Though the layer is lacking strict periodicity, it shows up as an ideally-packed tiling of basic elements without any skips or halting. The two-dimensional Tl(x)Bi(1-x) compounds were formed by depositing Bi onto the Tl-covered Si(111) surface where Bi atoms substitute appropriate amount of Tl atoms. Atomic structure of each tiling element as well as arrangement of Tl(x)Bi(1-x) compounds were established in a detail. Electronic properties and spin texture of the selected compounds having periodic structures were characterized. The shown example demonstrates possibility for the formation of the exotic low-dimensional materials via unusual growth mechanisms.

Li adsorption, hydrogen storage and dissociation using monolayer MoS2: an ab initio random structure searching approach.

Utilizing ab initio random structure searching, we investigated Li adsorption on MoS2 and hydrogen molecules on Li-decorated MoS2. In contrast to graphene, Li can be adsorbed on both sides of MoS2, with even stronger binding than on the single side. We found that high coverages of Li can be attained without Li clustering, which is essential for hydrogen storage and Li ion batteries. Moreover, regarding battery applications, Li diffusion was also found to be easy. The fully-lithiated MoS2 can then adsorb H2 with 4.4 wt%. Interestingly, our calculations revealed that hydrogen molecules can be dissociated at high Li coverage with a minimal energy barrier. We further showed that the dissociated hydrogen atom can readily diffuse on the surface, thus keeping the reaction site active. We therefore propose that Li-MoS2 could be an inexpensive alternative catalyst to noble metals in hydrogen dissociation reactions.

Rapid detection of copper chlorophyll in vegetable oils based on surface-enhanced Raman spectroscopy.

The addition of copper chlorophyll and its derivatives (Cu-Chl) to vegetable oils to disguise them as more expensive oils, such as virgin olive oils, would not only create public confusion, but also disturb the olive oil market. Given that existing detection methods of Ch-Chl in oils, such as LC-MS are costly and time consuming, it is imperative to develop economical and fast analytical techniques to provide information quickly. This paper demonstrates a rapid analytical method based on surface-enhanced Raman spectroscopy (SERS) to detect Cu-Chl in vegetable oils; the spectroscopic markers of Cu-Chl are presented and a detection limit of 5 mg kg(-1) is demonstrated. The analysis of a series of commercial vegetable oils is undertaken with this method and the results verified by a government agency. This study shows that a SERS-based assessment method holds high potential for quickly pinpointing the addition of minute amounts of Cu-Chl in vegetable oils.

A strategy to create spin-split metallic bands on silicon using a dense alloy layer.

To exploit Rashba effect in a 2D electron gas on silicon surface for spin transport, it is necessary to have surface reconstruction with spin-split metallic surface-state bands. However, metals with strong spin-orbit coupling (e.g., Bi, Tl, Sb, Pt) induce reconstructions on silicon with almost exclusively spin-split insulating bands. We propose a strategy to create spin-split metallic bands using a dense 2D alloy layer containing a metal with strong spin-orbit coupling and another metal to modify the surface reconstruction. Here we report two examples, i.e., alloying reconstruction with Na and Tl/Si(111)1 × 1 reconstruction with Pb. The strategy provides a new paradigm for creating metallic surface state bands with various spin textures on silicon and therefore enhances the possibility to integrate fascinating and promising capabilities of spintronics with current semiconductor technology.

Magic numbers of atoms in surface-supported planar clusters.

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.