Research status and performance optimization of medium-temperature thermoelectric material SnTe

Thermoelectric materials have the ability to directly convert heat into electricity, which have been extensively studied for decades to solve global energy shortages and environmental problems. As a medium temperature (400-800 K) thermoelectric material, SnTe has attracted extensive attention as a promising substitute for PbTe due to its non-toxic characteristics. In this paper, the research status of SnTe thermoelectric materials is reviewed, and the strategies to improve its performance are summarized and discussed in terms of electrical and thermal transport properties. This comprehensive discussion will provides guidance and inspiration for the research on SnTe.     

利用晶体结构工程提升GeSe化合物热电性能的研究

在热电研究领域, Ge Se是一种二维层状结构具有较大带隙的半导体,本征载流子浓度低,热电性能差.在本工作中,采用熔融淬火结合放电等离子活化烧结工艺制备了一系列的Ge Se_1–x Te_x (x=0, 0.05, 0.15, 0.25,0.35, 0.45)多晶样品,研究了Te含量对Ge Se化合物物相结构和热电输运性能的影响规律.结果表明:随着Te含量的增加, Ge Se的晶体结构逐渐由正交相向菱方相转变,使得材料的带隙降低,载流子浓度和迁移率同步增加;同时,晶体对称性的提高增加了化合物的能带简并度,有效提高了载流子有效质量.在这些因素的共同作用下,菱方相Ge Se的功率因子比正交相Ge Se提高约2—3个数量级.此外,菱方相Ge Se具有丰富的阳离子空位缺陷以及铁电特性所导致的声子软化现象,这导致其晶格热导率比正交相Ge Se降低近60%.当Te含量为0.45时,样品在573 K取得最大热电优值ZT为0.75,是本征Ge Se样品的19倍.晶体结构工程是提升Ge Se化合物热电性能的有效途径.     

Resonant enhancement in nanostructured thermoelectric performance via electronic thermal conductivity engineering

The use of an asymmetric broadening in the transport distribution, a characteristic of resonant structures, is proposed as a route to engineer a decrease in electronic thermal conductivity thereby enhancing the electronic figure of merit in nanostructured thermoelectrics. Using toy models, we first demonstrate that a decrease in thermal conductivity resulting from such an asymmetric broadening may indeed lead to an electronic figure of merit well in excess of 1000 in an idealized situation and in excess of 10 in a realistic situation. We then substantiate with realistic resonant structures designed using graphene nano-ribbons by employing a tight binding framework with edge correction that match density functional theory calculations under the local density approximation. The calculated figure of merit exceeding 10 in such realistic structures further reinforces the concept and sets a promising direction to use nano-ribbon structures to engineer a favorable decrease in the electronic thermal conductivity.     

Energy band and charge-carrier engineering in skutterudite thermoelectric materials

The binary CoSb₃ skutterudite thermoelectric material has high thermal conductivity due to the covalent bond between Co and Sb, and the thermoelectric figure of merit, ZT, is very low. The thermal conductivity of CoSb₃ materials can be significantly reduced through phonon engineering, such as low-dimensional structure, the introduction of nano second phases, nanointerfaces or nanopores, which greatly improves their ZT values. The phonon engineering can optimize significantly the thermal transport properties of CoSb₃-based materials. However, the improvement of the electronic transport properties is not obvious, or even worse. Energy band and charge-carrier engineering can significantly improve the electronic transport properties of CoSb₃-based materials while optimizing the thermal transport properties. Therefore, the decoupling of thermal and electronic transport properties of CoSb₃-based materials can be realized by energy band and charge-carrier engineering. This review summarizes some methods of optimizing synergistically the electronic and thermal transport properties of CoSb₃ materials through the energy band and charge-carrier engineering strategies. Energy band engineering strategies include band convergence or resonant energy levels caused by doping/filling. The charge-carrier engineering strategy includes the optimization of carrier concentration and mobility caused by doping/filling, forming modulation doped structures or introducing nano second phase. These strategies are effective means to improve performance of thermoelectric materials and provide new research ideas of development of high-efficiency thermoelectric materials.     

Improvement of the thermoelectric efficiency of pyrene-based molecular junction with doping engineering

In this study, the thermoelectric properties of pyrene molecule doped with boron and nitrogen atom at different sites of molecule are investigated using density functional theory and none-equilibrium Green's function formalism in the linear response regime. Our calculations show that when the impurities are added to the edge of the molecule, the anti-resonant peaks will appear in the transmission diagram in the vicinity of the Fermi energy level. So it increases the thermoelectric figure of merit of the system in comparison with the one that the impurity is located in the center of molecule. Additionally,the seebeck coefficient signs are not the same among the B, N, and N B doped devices, indicating that the types of the carriers can be changed with different types of doping.     

High thermoelectric performance of one graphene nanoribbon with line defects

We investigate the quantum transport through zigzag graphene nanoribbons with embedded “5-7-5”-edge line defects, by means of the non-equilibrium Green's function technique. It is found that when two semi-infinite line defects exist in the nanoribbon, notable Fano antiresonance takes place in the quantum transport process, which enables to drive the apparent thermoelectric effect. We propose this structure to be a promising candidate for improving the thermoelectric efficiency based on graphene nanoribbons.     

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides(TMDs) with different chalcogens on each side(X-TM-Y) have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes(DWNTs) from metal to semiconductor. Using density-function theory(DFT) with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs(X = O and S, inside a tube; Y = S and Se, outside a tube) depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms(X) of outer tube and outer atoms(Y) of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.     

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional(2D) layered materials have potential applications in nanoelectronics, nanophotonics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes(LEDs), and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.     

Effects of copper on the thermoelectric performance parameter

Results are presented on conductivity, Hall effect, and thermo-emf for cadmium antimonide crystals containing 0.001–1% Cu in the range 90 ° to 380 ° K. The parameter z is calculated for the range 140 °–380 ° K for four concentrations.     

Excellent thermoelectric performance in weak-coupling molecular junctions with electrode doping and electrochemical gating

Excellent thermoelectric performance in molecular junctions requires a high power factor, a low thermal conductance, and a maximum figure of merit(ZT) near the Fermi level. In the present work, we used density functional theory in combination with a nonequilibrium Green's function to investigate the thermoelectric performance of carbon chain-graphene junctions with both strong-coupling and weak-coupling contact between the electrodes and the molecules. The results revealed that a room temperature ZT of 4 could be obtained for the weak-coupling molecular junction, approximately one order of magnitude higher than that reached by the strong-coupling junction. The reason for this is that strong interfacial scattering suppresses most of the phonon modes in weak-coupling systems, resulting in ultralow phonon thermal conductance. The influence of electrode width,electrode doping, and electrochemical gating on the thermoelectric performance of the weak-coupling system was also investigated, and the results revealed that an excellent thermoelectric performance can be obtained near the Fermi level.     

Spectral engineering and tunable thermoelectric behavior in a quasiperiodic ladder network

Double-stranded quasiperiodic copper mean arrangement has been studied in respect of its electronic property and thermoelectric signature. The two-arm network is demonstrated by a tight-binding Hamiltonian. The eigenspectrum of such aperiodic mesh that does not convey translational invariance, is significantly dependent on the parameters of the Hamiltonian. It is observed that specific correlation between the parameters obtained from the commutation relation between the on-site energy and overlap integral matrices can eventually modify the spectral nature and generate absolutely continuous energy spectrum. This part is populated by atypical extended states that has a large localization length substantiated by the flow of the hopping integral under successive real space renormalization group method steps. This sounds delocalization of single particle energy states in such non-translationally invariant networks. Further this can be engineered at will by selective choice of the relative strengths of the parameters. This precise correlation has a crucial impact on the thermoelectric behavior. Anomalous nature of thermoelectric coefficient may inspire the experimentalists to frame tunable thermo-devices. Specific correlations can help us to tune the continuous band and determine the band position at will.     

Band engineering of B_2H_2 nanoribbons

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B_2H_2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B_2H_2 nanoribbons. Based on first-principles calculations, we have found that all narrow armchair nanoribbons with and without mirror symmetry(ANR-s and ANR-as, respectively) are semiconducting. The energy gap has a relation with the width of the ribbon. When the ribbon is getting wider, the gap disappears. The zigzag ribbons without mirror symmetry(ZNR-as) have the same trend. But the zigzag ribbons with mirror symmetry(ZNR-s) are always metallic. We have also found that the metallic ANR-as and ZNR-s can be switched to semiconducting by applying a tensile strain along the nanoribbon. A gap of 1.10 eV is opened under 16% strain for the 11.0-■ ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B_2H_2 nanoribbon increases its possibility of potential applications in nanodevices.     

Facile synthesis and characterization of SnTe films

In this paper, we reported a novel, simple, and cost-effective route to SnTe films. The films were prepared by a chemical bath method, at room temperature and ambient pressure, using conventional chemicals as starting materials with or without surfactant. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy, respectively. The SnTe film deposited without surfactant consists of nanoparticles (∼100 nm). The film deposited using polyethyleneglycol (PEG) as the surfactant consists of nanoparticles with size of ∼25 nm, whereas the film deposited using polyvinylpyrrolidone (PVP) as the surfactant consists of rough rod-like nanostructures (∼50 in diameter and ∼500 nm in length), besides nanoparticles (∼40-180 nm). The SnTe film deposited with PEG is smoother and denser. The formation mechanism of the SnTe films was proposed.     

Excellent thermoelectric performance predicted in Sb2Te with natural superlattice structure

Using first-principles calculations combined with the Boltzmann transport theory, we explore the thermoelectric properties of natural superlattice (SL) structure Sb₂Te. The results show that n-type Sb₂Te possesses larger Seebeck coefficient of 249.59 (318.87) μV/K than p-type Sb₂Te of 219.85 (210.38) μV/K and low lattice thermal conductivity of 1.25 (0.21) W/mK along the in-plane (out-of-plane) direction at 300 K. The excellent electron transport performance is mainly attributed to steeper density of state around the bottom of conduction band. The ultralow lattice thermal conductivity of Sb₂Te is mainly caused by low phonon group velocity and strong anharmonicity. Further analysis shows that the decrease of group velocity comes from flatter dispersion curves which are contributed by the Brillouin-zone folding. The strong anharmonicity is mainly due to the presence of lone-pair electrons in Sb₂Te. Combining such a high Seebeck coefficient with the low lattice thermal conductivity, maximum n-type thermoelectric figure of merit (ZT) of 1.46 and 1.38 could be achieved along the in-plane and out-of-plane directions at room temperature, which is higher than the reported values of Sb₂Te₃. The findings presented here provide insight into the transport property of Sb₂Te and highlight potential applications of thermoelectric materials at room temperature.     

Band engineering of honeycomb monolayer CuSe via atomic modification

Two-dimensional monolayer copper selenide(Cu Se) has been epitaxially grown and predicted to host the Dirac nodal line fermion(DNLF). However, the metallic state of monolayer Cu Se inhibits the potential application of nanoelectronic devices in which a band gap is needed to realize on/off properties. Here, we engineer the band structure of monolayer Cu Se which is an analogue of a p-doped system via external atomic modification in an effort to realize the semiconducting state.We find that the H and Li modified monolayer Cu Se shifts the energy band and opens an energy gap around the Fermi level.Interestingly, both the atomic and electronic structures of monolayer Cu HSe and Cu Li Se are very different. The H atoms bind on top of Se atoms of monolayer Cu Se with Se–H polar covalent bonds, annihilating the DNLF band of monolayer Cu Se dominated by Se orbitals. In contrast, Li atoms prefer to adsorb at the hexagonal center of Cu Se, preserving the DNLF band of monolayer Cu Se dominated by Se orbitals, but opening band gaps due to a slight buckling of the Cu Se layer. The realization of metal-to-semiconductor transition from monolayer Cu Se to Cu X Se(X = H, Li) as revealed by first-principles calculations makes it possible to use Cu Se in future electronic devices.     

Band engineering of ZnS by codoping for visible-light photocatalysis

Codoping is demonstrated as an efficient approach to narrow the band gap of ZnS and enhance its photocatalytic activity. Herein, we perform the density-function theory calculations of ZnS by codoping of X (N, F) with transition metals (TM = V, Cu). The band gap is reduced in four different types of codoped ZnS. In particular, Cu_ZnF_S codoping, a charge-compensated donor–acceptor pair, leads to an about 32 % reduction of the energy gap, thus extending the absorption edge to visible-light region. The band gap reduction is due to the upshift of the top valence band comprised with the delocalized hybridizing levels of Cu 3d and S 3p states, and the downshift of the bottom conduction band consisting of F 2s states. Moreover, the larger value of m _e*/m _h* in Cu_ZnF_S–ZnS would result in a lower recombination rate of the electron–hole pairs. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in codoped ZnS. These findings raise the prospect of using codoped ZnS with specifically engineered electronic properties in a variety of photocatalytic applications.     

Analysis of performance and optimum configuration of two-stage semiconductor thermoelectric module

In this paper, the theoretical analysis and simulating calculation were conducted for a basic two-stage semiconductor thermoelectric module, which contains one thermocouple in the second stage and several thermocouples in the first stage. The study focused on the configuration of the two-stage semiconductor thermoelectric cooler, especially investigating the influences of some parameters, such as the current I1 of the first stage, the area A1 of every thermocouple and the number n of thermocouples in the first stage, on the cooling performance of the module. The obtained results of analysis indicate that changing the current I1 of the first stage, the area A1 of thermocouples and the number n of thermocouples in the first stage can improve the cooling performance of the module. These results can be used to optimize the configuration of the two-stage semiconductor thermoelectric module and provide guides for the design and application of thermoelectric cooler.     

Band gap engineering in simultaneous phononic and photonic crystal slabs

We discuss the simultaneous existence of phononic and photonic band gaps in two types of phononic crystals slabs, namely periodic arrays of nanoholes in a Si membrane and of Si nanodots on a SiO₂ membrane. In the former geometry, we investigate in detail both the boron nitride lattice and the square lattice with two atoms per unit cell (these include the square, triangular and honeycomb lattices as particular cases). In the latter geometry, some preliminary results are reported for a square lattice.     

Band structure engineering and defect control of oxides for energy applications

Metal oxides play an essential role in modern optoelectronic devices because they have many unique physical properties such as structure diversity, superb stability in solution, good catalytic activity, and simultaneous high electron conductivity and optical transmission. Therefore, they are widely used in energy-related optoelectronic applications such as photovoltaics and photoelectrochemical(PEC) fuel generation. In this review, we mainly discuss the structure engineering and defect control of oxides for energy applications, especially for transparent conducting oxides(TCOs) and oxide catalysts used for water splitting. We will review our current understanding with an emphasis on the contributions of our previous theoretical modeling, primarily based on density functional theory. In particular, we highlight our previous work:(i) the fundamental principles governing the crystal structures and the electrical and optical behaviors of TCOs;(ii) band structures and defect properties for n-type TCOs;(iii) why p-type TCOs are difficult to achieve;(iv) how to modify the band structure to achieve p-type TCOs or even bipolarly dopable TCOs;(v) the origin of the high-performance of amorphous TCOs; and(vi) band structure engineering of bulk and nano oxides for PEC water splitting. Based on the understanding above, we hope to clarify the key issues and the challenges facing the rational design of novel oxides and propose new and feasible strategies or models to improve the performance of existing oxides or design new oxides that are critical for the development of next-generation energy-related applications.     

High performance thermoelectric Tl9BiTe6 with an extremely low thermal conductivity

Tl 9BiTe (6) exhibits a thermoelectric figure of merit of ZT approximately 1.2 around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of Tl 9BiTe (6) [ 0.39 W/(m times K) at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.