Carrier Pockets Research Articles

Boosting the thermoelectric properties of layered SnSb2Te4 compound by microstructure regulation combined with heterovalent halogen substitution

The layered IVV2Te4-based compounds have recently received significant attention due to their intrinsically low lattice thermal conductivities. However, the influence of microstructure regulation and halogen doping on their thermoelectric properties has been scarcely studied. In this work, we systematically investigated the anisotropic thermoelectric transport properties of pristine SnSb2Te4 compounds with varied microstructures manipulated by ball milling. With the help of thermoelectric transport models, the optimal ball milling time is estimated based on the calculation of quality factor. Furthermore, the remarkably improved Seebeck coefficient is achieved by the introduction of halogen elements into Te sites, which is attributed to reduced carrier concentration and converged carrier pockets. As a result, a maximal zT ∼0.4 at 725 K is achieved in SnSb2(Te0·94I0.06)4 sample aligned with the SPS pressure direction with a ball-milling time of 1 h, which demonstrates a 35 % enhancement over the pristine sample. This work provides an insightful guide to elevate the thermoelectric properties of layered IVVI2Te4-based compounds.

Synergy of grain size and texture effect for high-performance Mg3Sb2-based thermoelectric materials

High-performance n-type Mg3Sb2-based thermoelectric materials have attracted much attention due to the six-fold degenerate carrier pockets in the conduction band. This study uncovers the electrical and thermal transport behaviors in different directions with respect to the spark plasma sintering (SPS) pressure that is rarely investigated. Results indicate that the grain size in the ⊥P direction (perpendicular to the SPS pressure direction) is slightly larger than that in the ∥P direction. In addition, a moderate texture is observed, reflecting a preferential orientation of the (001)-plane in the ⊥P direction. The synergy of both grain size and the texture leads to an improved electrical conductivity for Mg3.24Sb1.5Bi0.49Te0.01 compounds with lamellar crystal structures. Consequently, the highest thermoelectric figure of merit ZTpeak ∼1.71 and ZTavg ∼1.12 (almost the best ZT values ever reported for Te-doped Mg3Sb2-based materials) are achieved in the ⊥P direction compared to the ∥P direction.

Significant reduction in semiconductor interface resistance via interfacial atomic mixing

The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the momentum conservation requirement, but also because of the incompatible symmetries of the Bloch wave functions of the two sides. In contrast, atomic mixing at the interface increases the carrier transmittance as the interface roughness opens many nonspecular transmission channels, which greatly reduces the contact resistance compared with the perfect interface. Specifically, we show that disordered interfaces with certain symmetries create more nonspecular transmission. The insights from our study will benefit the future design of high-performance heterostructures with low contact resistance.

Synthesis and thermoelectric properties of Bi-doped SnSe thin films* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0405702) and the National Natural Science Foundation of China (Grant No. 51672179).

Bi doped n-type SnSe thin films were prepared by chemical vapor deposition (CVD) and their structure and thermoelectric properties were studied. The x-ray diffraction patterns, x-ray photoelectron spectroscopy, and microscopic images show that the prepared SnSe thin films were composed of pure SnSe crystals. The Seebeck coefficients of the Bi-doped SnSe were greatly improved compared to that of undoped SnSe thin films. Specifically, Sn0.99Bi0.01Se thin film exhibited a Seebeck coefficient of –905.8 μV⋅K−1 at 600 K, much higher than 285.5 μV⋅K−1 of undoped SnSe thin film. Further first-principles calculations reveal that the enhancement of the thermoelectric properties can be explained mainly by the Fermi level lifting and the carrier pockets increasing near the Fermi level due to Bi doping in the SnSe samples. Our results suggest the potentials of the Bi-doped SnSe thin films in thermoelectric applications.

Chemical Bonding Origin of the Thermoelectric Power Factor in Half-Heusler Semiconductors

Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, making the Zintl distinction clearly invalid. This covalence is shown to strongly affect the band structure, thermoelectric properties and response properties in the materials, with improved thermoelectric properties observed for those materials that least follow the Zintl concept. This expands our knowledge of the chemical bonding motifs governing physical properties, and gives a critical view on the simplistic chemical concepts too often applied for design of complex materials.

Low-temperature thermal conductivity of thermoelectric Co1−xMxSi (M = Fe, Ni) alloys

We study the low-temperature electrical and thermal conductivity of CoSi and Co1−xMxSi alloys (M = Fe, Ni; x ≤ 0.06). Measurements show that the low-temperature electrical conductivity of Co1−xFexSi alloys decreases at x > 0.01 by an order of magnitude compared with that of pure CoSi. It was expected that both the lattice and electronic contributions to thermal conductivity would decrease in the alloys. However, our experimental results revealed that at temperatures below 20 K, the thermal conductivity of Fe- and Ni-containing alloys is several times larger than that of pure CoSi. We discuss possible mechanisms of the thermal conductivity enhancement. The most probable one is the dominant scattering of phonons by charge carriers. We propose a simple theoretical model that takes into account the complex semimetallic electronic structure of CoSi with nonequivalent charge carrier pockets. This model explains well the increase of the lattice thermal conductivity with increasing disorder and the linear temperature dependence of thermal conductivity in the Co1−xFexSi alloys below 20 K.

Pressure effect on the topologically nontrivial electronic state and transport of lutecium monobismuthide

Rare-earth monopnictides are predicted to be nontrivial semimetal candidates and show pressure-induced superconductivity. Here, we grow LuBi single crystal and study the magnetization, transport behaviors and electronic band structures to reveal its topological semimetal feature and superconductivity under pressure. At 0 GPa, the quantum oscillations indicate that there are several topologically nontrivial carrier pockets around the Fermi level, among which the hole ones are isotropic in shape, while the electron ones are anisotropic and responsible for the angular magnetoresistance. Upon compression, the superconductivity emerges in the titled compound, showing a similar pressure dependence as that observed in LaBi. Our calculation suggests that the electronic band structures are robust at low- and high-pressure respectively and thus the topological features are always preserved. Besides, the nearly pressure-independent density of state in LuBi indicates that the conventional electron-phonon coupling appears to play a minor role in the superconductivity.

Prediction of enhanced thermoelectric performance in two-dimensional black phosphorus nanosheets

Because of the unique layered structure, black phosphorus (BP) provides a possibility of relatively independent control in electrical and thermal conductivity for thermoelectrical applications. It is therefore of great interest to realize high-performance BP-based thermoelectrics as flexible non-toxic and ultralight devices in spite of the low energy conversion efficiency and structural instability of the bulk BP. In this work, we systematically study the thermoelectric properties for the two-dimensional BP from mono-layer up to quad-layer by first-principles calculations and Boltzmann transport theory. It can be concluded from the calculations that the thermoelectric performance of BP nanosheets can be effectively optimized by tuning the layer thickness. We reveal that the maximum ZT values at 300 and 500 K can reach up to 0.45 and 0.90 in p-type bi-layer BP along armchair direction, respectively, around 5 times higher than that of the bulk at room temperature. The high performance in bi-layer BP is mainly attributed to its highly anisotropic and degenerate carrier pockets. Accordingly, we further propose that the formation of BP/h-BN heterostructure can enhance ZT up to 1.2 at 500 K, which facilitates the real application of thin BP for flexible and eco-friendly thermoelectrics.

High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds

Thermoelectrics are used in energy harvesting technology for power generation and refrigeration, but await breakthroughs with a higher figure of merit ($Z\phantom{\rule{0}{0ex}}T$), especially at room-to-cryogenic temperatures. Via explicit treatment of electron-phonon scattering, this study shows that multipocketed full Heusler compounds Sr${}_{2}$BiAu and Sr${}_{2}$SbAu could feature notably high theoretical $Z\phantom{\rule{0}{0ex}}T$. Stability and defects analysis also suggest that these compounds may be synthesizable and favorably $n$-type. A successful experimental realization of these compounds could pave new grounds in bulk thermoelectrics.

Strong lattice anharmonicity securing intrinsically low lattice thermal conductivity and high performance thermoelectric SnSb2Te4 via Se alloying

Abstract Seeking a material with intrinsically low lattice thermal conductivity is crucial for screening high-performance thermoelectric (TE) materials. Here, the TE properties of SnSb2(Te1−xSex)4 (0 ≤ x ≤ 0.25) samples are systematically investigated for the first time. An intrinsically ultralow lattice thermal conductivity (~0.56 W m−1 K−1 at 320 K and ~0.46 W m−1 K−1 at 720 K) has been observed in SnSb2Te4, which can be ascribed to the weak chemical bonding as well as the bond anharmonicity verified by first-principles calculations. Furthermore, alloying with Se enables the remarkable increase in the Seebeck coefficients, resulting from the optimized carrier concentrations due to the enlarged formation energy of intrinsic SnSb-type antisite defects along with the simultaneous enhancement of density-of-states effective mass from the convergence of multiple carrier pockets. As a result, a peak zT value of 0.5 at 720 K and a significant improvement in average zT (~200%) in SnSb2(Te0.75Se0.25)4 are achieved. This work not only demonstrates the potential of SnSb2Te4-based compounds for practical TE applications, but also provides an insightful guidance to improve TE performance by defect and electronic band engineering.

Electronic, elastic and thermoelectric performance in n-type Sr-filled brittle skutterudite

Recently a noteworthy development in the thermoelectric performance of skutterudites has been achieved by implementing void fillers in the caged structures. In present work, we employ a systematic study on the effect of Sr-filled SrRu4As12 on its thermoelectric properties based on first principles calculations. SrRu4As12 crystallizes in ferromagnetic phase having the brittle nature. On application of mBJ potential the energy levels in the conduction band shift towards the Fermi level, promoting two carrier pockets which is an effective scheme to attain the robust thermopower. SrRu4As12 attribute high Grüneisen parameter of 2.13, which imitates the anharmonic effect that ascribes low lattice thermal conductivity. SrRu4As12 skutterudite demonstrates pleasing transport parameters viz. high Seebeck coefficient along with robust power-factor, which makes SrRu4As12 an attractive material for high-performance thermoelectrics.

Anomalous phonon-mode dependence in polarized Raman spectroscopy of the topological Weyl semimetal TaP

Topological Weyl semimetals (WSMs) have attracted widespread interests due to the chiral Weyl fermions and surface Fermi arcs that enable unique optical and transport phenomena. In this work, we present angle-resolved Raman spectroscopy of TaP, a prototypical noncentrosymmetric WSM, for five excitation wavelengths ranging from 364 to 785 nm. The Raman active modes, $A_1$, $B_1^1$, and $B_1^2$ modes, exhibit two main unique features beyond the conventional Raman theory. First, the relative intensities of Raman active modes change as a function of the excitation wavelength. Second, angle-resolved polarized Raman spectra show systematic deviation from the Raman tensor theory. In particular, the $B_1^1$ mode is absent for 633 nm excitation, whereas the $B_1^2$ mode shows an unusual two-fold symmetry instead of a four-fold symmetry for 488, 532, and 633 nm excitations. These unconventional phenomena are attributed to the interference effect in the Raman process owing to the existence of multiple carrier pockets with almost the same energy but different symmetries.

Enhanced Thermoelectric Performance in Li Doped SnS via Carrier Concentration Optimization

Tin sulfide (SnS) has attracted increasing attentions due to its similar band structure and crystal structure to tin selenide (SnSe). The layered structure and strong anharmonicity of these materials result in an ultralow lattice thermal conductivity, which is advantageous for the thermoelectric performance. Further enhancement in thermoelectric properties is expected to improve the carrier concentration and power factor by band structure optimization. In this work, lithium (Li) is proved to be an efficient dopant for SnS and the carrier concentration is increased to 1.3 × 1018 cm-3 at room temperature for Sn0.98Li0.02S. Calculations show that Li doping flattens the edge of the valence band and increases the number of carrier pockets in SnS, which collectively enhance the electronic transport properties. Moreover, the phonon scattering was enhanced by the nano-precipitates and increased boundaries in Sn1-xLixS, resulting in 40% reduction in thermal conductivity at room temperature. Combined with the enhanced power factor (∼3 μW cm-1 K-2 at 848 K) and low thermal conductivity (0.36 W m-1 K-1 at 848 K), the maximum ZT of 0.66 is achieved in Sn1-xLixS bulk samples at 848 K.

Fermi surface complexity, effective mass, and conduction band alignment in n-type thermoelectric Mg3Sb2 – xBix from first principles calculations

Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2 – xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with an increase in the Bi content, the K and M band minima move away from the conduction band minimum CB1 while the singly-degenerate Г band minimum shifts rapidly downward and approaches the conduction band minimum. However, the favorable sixfold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the Г and CB1 band minima. In addition, we show that the connection of the CB1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with an increase in the Bi content, which is unfavorable to the electrical transport. In contrast, reducing the conductivity effective mass with an increase in the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.

Electronic band structure engineering in InAs/InSbAs and InSb/InSbAs superlattice heterostructures

We report a detailed ab initio study of two superlattice heterostructures, one component of which is a unit cell of CuPt ordered InSb0.5As0.5. This alloy part of the heterostructures is a topological semimetal. The other component of each system is a semiconductor, zincblende-InSb, and wurtzite-InAs. Both heterostructures are semiconductors. Our theoretical analysis predicts that the variation in the thickness of the InSb layer in InSb/InSb0.5As0.5 heterostructure renders altered bandgaps with different characteristics (i.e., direct or indirect). The study holds promise for fabricating heterostructures, in which the modulation of the thickness of the layers changes the number of carrier pockets in these systems.

Enhancement of electrical performance via carrier modulation in single crystalline PbTe prepared by Pb-flux method

Single crystalline PbTe samples are prepared by the Pb-flux method according to the stoichiometric ratio of Pb1+xTe (x = 1.5, 2.0, 2.5, and 3.0). Experimental results show that all the samples exhibit n-type conductivity. The order of magnitude for carrier concentration is 1019, which is one order of magnitude higher than that in pure n-type PbTe sample (2.56 × 1018 cm−3). The highest power factor reaches 2.07 × 10−3 W m−1 K−2 at 420 K for the sample with x = 1.5, which is an approximately 30% enhancement compared with the polycrystalline sample (1.59 × 10−3 W m−1 K−2). In a theoretical study on the PbTe system, Te vacancy (VTe) is preferentially formed under the Pb-rich condition. To further understand the effect of VTe on the electrical performance of PbTe thermoelectric material, the electronic structures of PbTe with VTe are determined by first principle calculations. Theoretical calculations reveal that VTe can modify the electronic density of states by introducing resonant level, which can increase the number of carrier pockets. Consequently, carrier concentration has increased remarkably. In this study, a Pb-flux method for synthetizing single crystalline PbTe thermoelectric material is convenient and low cost, and it has potential for mass production in commercial applications.

Enhanced electrical transport properties via Pb vacancies in single crystalline PbTe prepared by Te-flux method

In PbTe system, Pb vacancies (VPb) are preferentially formed compared with other intrinsic point defects (anti-sites TePb and interstitials Tei) under the Te-rich condition. In order to explore the effect of VPb on electrical transport properties of PbTe thermoelectric material, the electronic structures of PbTe with VPb are determined by the first principle calculations. Theoretical calculations reveal that the VPb can modify the band structure near the Fermi level, which can increase the number of carrier pockets thus increase the carrier concentration. Based on the calculated results, single crystalline PbTe samples with VPb have been prepared by Te-flux method according to the stoichiometric ratio of PbTe1+x (x = 2.0, 2.5, 3.0 and 3.5). The experimental results show that carrier concentration is optimized significantly by tuning Te-flux content, and it varies from 2.94 × 1018 to 8.01 × 1018 cm−3. The highest electrical conductivity reaches 3.1 × 104 S m−1 at 300 K for x = 3.5. A large Seebeck coefficient of 562 μV/K is obtained at 480 K for x = 2.0. Due to the synergistic effect of the Seebeck coefficient and the electrical conductivity, the highest power factor of 3.17 × 10−3 W m−1 K−2 is achieved at 450 K for x = 3.0.

Thermoelectric properties of p-type cubic and rhombohedral GeTe

We investigate the electronic and thermoelectric properties of GeTe in both cubic and rhombohedral phases. We find that cubic GeTe has an electronic structure with a narrow band gap that is unfavorable at high temperature, where the cubic phase is normally stable. However, cubic GeTe has electronic features that may lead to p-type performance superior to the normal rhombohedral phase at lower temperature. This is explained in part by the combination of light and heavy band character that is very effective in obtaining high thermopower and conductivity. In addition, the valence band edge carrier pockets in cubic GeTe possess the largest anisotropy among cubic IV-VI analogs. These effects are stronger than the effect of band convergence in the rhombohedral structure. The results suggest further study of stabilized cubic GeTe as a thermoelectric.

High electrical transport properties performance enhanced by anti-site defects in single crystalline SnSe

Single crystalline tin selenide (SnSe) has attracted considerable attention due to its record high figure of merit value in medium temperature range. However, their large band gap restricts the electron transport. Herein, we tailor the carrier concentration by offering a variability of the Sn/Se ratio to fine-tune the electrical transport properties. Density functional theory demonstrated that the collaborative optimization of the Seebeck coefficient and the electrical conductivity through optimizing carrier concentration can improve significantly the power factor, and deviation from stoichiometry is an effective strategy to modulate the carrier concentration because anti-site defects SnSe lower the Fermi level and increases the number of carrier pockets in SnSe. Meanwhile, experimental observations confirm that the carrier concentration and electrical conductivity are changed with varied Sn/Se ratio. Through this work, a viable design principle for optimized carrier concentration for achieving high thermoelectric performance is provided.

Ultra-high average figure of merit in synergistic band engineered Sn Na1−Se0.9S0.1 single crystals

Thermal-electricity conversion is one of the most promising routes to harvest heat and convert it as easily storable and deliverable electric energy. Significant progress has been made since the discovery of Seebeck effect in 1821, particularly, the figure of merit zT approached a record high value of 2.6 in 2014. However, for thermoelectric devices, high average zT values (zTave) over the operating temperature range is more important as it is directly related to the conversion efficiency (η). Approaching highly stable and repeatable ultra-high zTave for Te-free materials has been historically challenging over the past century though exciting progress with zTave well above 1.10 was made recently. Here, through synergistic band engineering strategy for single crystalline SnSe, we report a series of record high zTave over a wide temperature range, approaching ∼1.60 in the range from 300 K to 923 K in Na-doped SnSe0.9S0.1 solid solution single crystals, with the maximum zT of 2.3 at 773 K. These ultra-high thermoelectric performance derive from the new multiple valence band extrema near the band edges in SnSe0.9S0.1 and the shift of Fermi level towards the multi-valley bands through Na doping which introduce additional carrier pockets to attend electrical transport. These effects result in an optimized ultrahigh power factor exceeding 4.0 mW m−1 K−2 in Sn0.97Na0.03Se0.9S0.1 single crystals. Combined with the extremely lowered thermal conductivity attributed from the intrinsic anharmonicity and point defect phonon scattering, the series of ultra-high zTave and a record high calculated conversion efficiency of 21% over a wide temperature range are approached.