High-resolution scanning tunneling microscope and its adaptation for local thermopower measurements in 2D materials

We present the design, fabrication and discuss the performance of a new combined high-resolution Scanning Tunneling and Thermopower Microscope (STM/SThEM). We also describe the development of the electronic control, the user interface, the vacuum system, and arrangements to reduce acoustical noise and vibrations. We demonstrate the microscope's performance with atomic-resolution topographic images of highly oriented pyrolytic graphite (HOPG) and local thermopower measurements in the semimetal Bi2Te3. Our system offers a tool to investigate the relationship between electronic structure and thermoelectric properties at the nanoscale.

Investigation into the physical characteristics of the compounds XBiSe2 (X = Li, Na or K)

CONTEXT

As new materials, the ternary chalcogenides have recently brought scientists' attention. These materials are a novel class of semiconducting chemical compounds. They allow the increase of the photo-conversion efficiency, the performance, and the cheap energy cost. Such materials also provide a wide range of physical and chemical applications.

METHODS

The used investigation employs Density Functional Theory (DFT) implemented in the Wien2k package to systematically characterize the physical properties of ternary chalcogenide compounds XBiSe2 (X = Li, Na and K). Such method emphasizes their applicability to energy conversion technologies. Scrutinizing their electronic, optical, and thermoelectric properties elucidates the effect of alkali metal substitution on performance metrics. The results not only advance knowledge of these materials' physicochemical behaviors but also reveal their potential for tailored functionalization in next-generation energy and optoelectronic systems, marking a significant stride in material science and application-oriented research.

Grain boundary, electrical transport and thermoelectric properties of the ultra-high rGO amount of C12A7-rGO composites

The Ca12Al14O33 ceramic (C12A7) and reduced graphene oxide (rGO) composite which an ultra-high amount (i.e., 40, 50, 60, and 70 wt%) of rGO (ultra-high amount C12A7/rGO composite) were synthesized by a solid-state reaction process. After the hydraulic press, the heat treatment in the temperature range of 773 K under the argon environment had been performed with the composite pellets for 30 min. XRD results of the C12A7 and all the ultra-high amount C12A7/rGO composites indicated a pure phase of C12A7 ceramic. Raman spectra confirmed the existence of rGO content in all the ultra-high amount C12A7/rGO composites. Raman peaks also suggested reduction of the free O 2 2 - and O 2 - ions from the framework of the ultra-high amount C12A7/rGO composites. SEM image presented the homogeneous grain boundary interface after the heat treatment at 773 K of the C12A7 wrapped by the rGO sheet, the agglomerated rGO sheet, and the rough interface stack of rGO sheets. UV-VIS spectroscopy presented the absorption behavior, direct energy gap, and indirect energy gap modifications of the ultra-high amount C12A7/rGO composites. Electrical conductivity of the ultra-high amount C12A7/rGO composites illustrated larger than 108 times improvement with temperature independence. Range of -5 to -17 μ V / K , temperature dependence, and increased with rGO content increasing Seebeck coefficient were reported. Thermal conductivity of the ultra-high amount C12A7/rGO composites was increased with the rGO content increasing. Both the Power factor (PF) and the figure of merit (ZT) of the ultra-high amount C12A7/rGO composites were temperature dependent and were increased with the rGO content increasing, within the range of 0.4 μ W / m . K 2 of PF and the range of 3 x 10 - 4 of ZT, respectively. These experimental results verified grain boundary, modified energy band, electrical transport properties and thermoelectric properties of C12A7/rGO composites loading with ultra-high content rGO.

Investigating the physical and optoelectronic characteristics of Co2ZrZ compounds: findings from computational analysis and thermoelectric evaluation

CONTEXT AND RESULTS

The study examines the physical characteristics of Co2ZrZ compounds using the Wien2k code and the Anisimov and Gunnarsson approach. Results show metallic attributes in Co2ZrBi and Co2ZrAs, while Co2ZrPb exhibits semi-metallic tendencies. Energy gap evaluations reveal significant infrared transitions, indicating altered electron mobility compensated by increased ultraviolet absorption. These compounds have potential in space solar energy applications due to UV light absorption capabilities, especially in Co2ZrPb. The study also identifies optical phenomena like "super-luminescence" and plasmatic oscillations.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The study uses computational techniques like Wien2k calculation code and Hubbard parameter calculations to investigate Co2ZrPb, a compound with potential for space energy applications. Energy gap assessments are conducted using GGA and mBJ-GGA methods. The study also analyzes the optical behavior of the compounds, including infrared and ultraviolet absorption. The BoltzTraP code is used for thermoelectric investigations, revealing a P-type charge carrier predominance in Co2ZrPb. This comprehensive approach provides valuable insights into electrical conductivity and thermoelectric properties.

Structural, electronic and thermoelectric properties of monolayer TiSe2

CONTEXT AND RESULTS

In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44 eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficients i.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data. The temperature dependence of these coefficients is also reported.

COMPUTATIONAL METHOD

The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1 k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.

Investigation of thermoelectric properties of cadmium selenide CdnSen (n= 7, 11, 13) molecular junctions: a DFT study

CONTEXT

The thermoelectric properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated before and after adding hydrogen atoms. The effects of hydrogen passivation on the transmission and thermopower curves were analyzed. CdSe-diamantane (Cd7Se7) and CdSe-tetramantane (Cd11Se11) junctions exhibited the best thermoelectric performance due to their low surface reconstruction energy, which is attributed to the number of dangling and unsaturated bonds. This study guides the design of new molecular junctions with desired thermoelectric properties.

METHOD

The electrical and thermal properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated using a ballistic quantum transport method based on the non-equilibrium Green's function (NEGF) approach. Thermoelectric properties were calculated for the molecular junctions with different structures before and after hydrogen passivation. Density functional theory (DFT) calculations were performed at the B3LYP level with the 3-21G basis set for the Cd atoms and the 6-31G** basis set for the Se atoms. The SIESTA and GOLLUM codes were used to study the effect of changing the shape and size of each structure on its electrical and thermal characteristics.

Multiscale architectures boosting thermoelectric performance of copper sulfide compound

Owing to their high performance and earth abundance, copper sulfides (Cu_2-x S) have attracted wide attention as a promising medium-temperature thermoelectric material. Nanostructure and grain-boundary engineering are explored to tune the electrical transport and phonon scattering of Cu_2-x S based on the liquid-like copper ion. Here multiscale architecture-engineered Cu_2-x S are fabricated by a room-temperature wet chemical synthesis combining mechanical mixing and spark plasma sintering. The observed electrical conductivity in the multiscale architecture-engineered Cu_2-x S is four times as much as that of the Cu_2-x S sample at 800 K, which is attributed to the potential energy filtering effect at the new grain boundaries. Moreover, the multiscale architecture in the sintered Cu_2-x S increases phonon scattering and results in a reduced lattice thermal conductivity of 0.2 W·m^-1·K^-1 and figure of merit (zT) of 1.0 at 800 K. Such a zT value is one of the record values in copper sulfide produced by chemical synthesis. These results suggest that the introduction of nanostructure and formation of new interface are effective strategies for the enhancement of thermoelectric material properties.

SUPPLEMENTARY INFORMATION

The online version of this article (10.1007/s12598-020-01698-6) contains supplementary material, which is available to authorized users.

First principles investigation on elastic, optoelectronic and thermoelectric properties of KYX (X = Ge, Sn and Pb) half-heusler compounds

Theoretical calculations based on the density functional theory and the Boltzmann semi-classical transport theory have been carried out to examine the structural, elastic, electronic, optical and thermoelectric properties of Potassium- and Yttrium-based half-Heusler (HH) compounds KYX (X = Ge, Sn and Pb). Based on our calculations, KYGe, KYSn, and KYPb HH compounds are mechanically stable, and show semiconductor nature with direct band gaps of 0.852, 0.921, and 0.927 eV, respectively, which are obtained from mBJ level of theory. Moreover, the KYSn is brittle, while the KYGe and KYPb are dutile. The optical results show that these HH compounds have wide absorption band from high energy region of infrarred to ultraviolet region. At high photon energies (beyond of 13 eV), they shows very small reflectivity. Because of their favorable electronic structure, these materials have very good thermoelectric performance with high thermopower and figure of merit. The effect of temperature on thermoelectric properties also is discussed in details.

Realization of non-equilibrium process for high thermoelectric performance Sb-doped GeTe

Pristine GeTe shows inferior thermoelectric performance around unit due to the large carrier concentration induced by the presence of intrinsic high concentration of Ge vacancy. In this study, we report a thermoelectric figure of merit ZT of 1.56 at 700 K, realized in Sb-doped GeTe based thermoelectric (TE) materials via combined effect of suppression of intrinsic Ge vacancy and Sb doping. The non-equilibrium nature during melt spinning process plays very important role. For one thing, it promotes the homogeneity in Ge_1-xSb_xTe samples and refines the grain size of the product. Moreover the persistent Ge precipitated as impurity phase in the traditional synthesis process is found to be dissolved back into the GeTe sublattice, accompanying with a drastic suppression of Ge vacancies concentration which in combination with Sb electron doping significantly reduced the inherent carrier concentration in GeTe. Low carrier concentration, approaching the optimum carrier concentration ∼3.74 × 10^-20 cm^-3 and a high power factor of 4.01 × 10^-3 W m^-1 K^-2 at 750 K are achieved for Ge_0.98Sb_0.02Te sample. In addition, the enhanced grain boundary phonon scattering by refining the grain size through melt spinning (MS) process, coupled with the intensified alloying phonon scattering via Sb doping leads to low thermal conductivity of 1.53 W m^-1 K^-1 at 700 K for Ge_0.94Sb_0.06Te sample. All those contribute to a high ZT value, representing over 50% improvement in the ZT value compared to the Sb free samples, which provides an alternative way for ultrafast synthesis of high performance GeTe based thermoelectric material.

Synthesis and thermoelectric properties of Rashba semiconductor BiTeBr with intensive texture

Bismuth tellurohalides with Rashba-type spin splitting exhibit unique Fermi surface topology and are developed as promising thermoelectric materials. However, BiTeBr, which belongs to this class of materials, is rarely investigated in terms of the thermoelectric transport properties. In the study, polycrystalline bulk BiTeBr with intensive texture was synthesized via spark plasma sintering (SPS). Additionally, its thermoelectric properties above room temperature were investigated along both the in-plane and out-plane directions, and they exhibit strong anisotropy. Low sound velocity along two directions is found and contributes to its low lattice thermal conductivity. Polycrystalline BiTeBr exhibits relatively good thermoelectric performance along the in-plane direction, with a maximum dimensionless figure of merit (ZT) of 0.35 at 560 K. Further enhancements of ZT are expected by utilizing systematic optimization strategies.

Anisotropic thermoelectric properties of layered compound SnSe2

Similar to high performance SnSe thermoelectrics, SnSe₂ is also a layered structured semiconductor. However, its anisotropic thermoelectric properties are less experimentally investigated. In this work, Cl-doped SnSe₂ bulk materials are successfully prepared, and their thermal stability and anisotropic transport properties are systematically studied. Unexpectedly, different from the theoretical prediction and other typical layered thermoelectric compounds like Bi₂Te₃, the out-of-plane zT_c value is higher than in-plane zT_a for the same composition. The zT value is significantly enhanced by Cl doping. A maximum zT_c of ∼0.4 at 673 K is achieved in SnSe_1.88Cl_0.12, twice higher than previously reported Cl-doped SnSe₂ synthesized by the solvothermal method.