States Effective Mass Research Articles

Chemical Pressure-Induced Unconventional Band Convergence Leads to High Thermoelectric Performance in SnTe.

Band convergence is considered a net benefit to thermoelectric performance as it decouples the density of states effective mass ( ) and carrier mobility (µ) by increasing valley degeneracy. Unlike conventional methods that typically prioritize at the expense of µ, this study theoretically demonstrates an unconventional band convergence strategy to enhance both and µ in SnTe under pressure. Density functional theory calculations reveal that increasing pressure from 0 to 5 GPa moves the Σ-band of SnTe upward, reducing the energy offset between L- and Σ-band from 0.35 to 0.2 eV while preserving the light band feature of the L-band. Consequently, a high power factor (PF) of 119.2 µW cm-1 K-2 at 300 K is achieved for p-type SnTe under 5 GPa. Chemical pressure also induces conduction band convergence, significantly enhancing the PF of n-type SnTe. Additionally, the interplay between pressure-induced phonon modes leads to a moderate increase in lattice thermal conductivity of SnTe below 3 GPa, which combined with the significantly enhanced PF, contributes to a large enhancement in ZT. Consequently, predicted ZT values of 2.12 at 650 K and 2.55 at 850 K are obtained for p- and n-type SnTe, respectively, showcasing substantial performance enhancements.

Full-shell d-orbitals of interstitial Ni and anomalous electrical transport in Ni-based half-Heusler thermoelectric semiconductors

We systematically investigate the anomalous electronic properties and electrical transport induced by full-shell d10-orbitals of the extra interstitial Nii in Ni-based half-Heusler XNi1+xZ semiconductors. The orbitals from the interstitial Nii have the unique d10 configuration, split into high-energy eg4 orbitals and low-energy t2g6 orbitals under the octahedral crystal field. In XIVNi1+xZIV (XIV=Ti, Zr, Hf; ZIV=Sn, Pb), the localized Nii-eg4 states fall within the intrinsic bandgap leading to a reduced bandgap. In XIIINi1+xZV (XIII=Sc, Y; ZV=Sb, Bi), the Nii-eg4 states overlap with the intrinsic valence bands. Additionally, the interstitial Nii perturb the nearest neighbor X atomic coordination, leading to the splitting of degenerate conduction band minimum, which is stronger in XIIINi1+xZV than XIVNi1+xZIV. Trace amounts of interstitial Nii significantly impact the electrical transport properties. The introduction of the extra interstitial Nii reduces the density of states effective mass, the electron group velocity, and the relaxation time, leading to a decrease of the Seebeck coefficient and electrical conductivity at low and medium temperatures. Nevertheless, the introduction of localized Nii-eg4 states within the bandgap as new valence band maximum attenuate the high-temperature bipolar effect at low carrier concentration intervals, thus maintaining a high thermopower at elevated temperatures. Furthermore, the tuning of the Nii-d10 orbitals by solid solution of both XIVNi1+xZIV and XIIINi1+xZV is expected to further optimize the electrical transport.

Lattice Softening and Band Convergence in GeTe-Based Alloys for High Thermoelectric Performance.

GeTe-based alloys have been studied as promising TE materials in the midtemperature range as a lead-free alternate to PbTe due to their nontoxicity. Our previous study on GeTe1-xIx revealed that I-doping increases lattice anharmonicity and decreases the structural phase transition temperature, consequently enhancing the thermoelectric performance. Our current work elucidates the synergistic interplay between band convergence and lattice softening, resulting in an enhanced thermoelectric performance for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16). Sb doping in GeTe0.9I0.1 serves a double role: first, it leads to lattice softening, thereby reducing lattice thermal conductivity; second, it promotes a band convergence, thus a higher valley degeneracy. The presence of lattice softening is corroborated by an increase in the internal strain ratio observed in X-ray diffraction patterns. Doping also introduces phonon scattering centers, further diminishing lattice thermal conductivity. Additionally, variations in the electronic band structure are indicated by an increase in density of state effective mass and a decrease in carrier mobility with Sb concentration. Besides, Sb doping optimizes the carrier concentration efficiently. Through a two-band modeling and electronic band structure calculations, the valence band convergence due to Sb doping can be confirmed. Specifically, the energy difference between valence bands progressively narrows upon Sb doping in Ge1-ySbyTe0.9I0.1 (y = 0, 0.02, 0.05, 0.10, 0.12, 0.14, and 0.16). As a culmination of these effects, we have achieved a significant enhancement in zT for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16) across the entire range of measured temperatures. Notably, the sample with y = 0.12 exhibits the highest zT value of 1.70 at 723 K.

Exploring Thermoelectric Transport Properties and Band Parameters of n-Type Bi2-xSbxTe3 Compounds Using the Single Parabolic Band Model

The n-type Bi2-xSbxTe3 compounds have been of great interest due to its potential to achieve a high thermoelectric performance, comparable to that of p-type Bi2-xSbxTe3. However, a comprehensive understanding on the thermoelectric properties remains lacking. Here, we investigate the thermoelectric transport properties and band characteristics of n-type Bi2-xSbxTe3 (x = 0.1 – 1.1) based on experimental and theoretical considerations. We find that the higher power factor at lower Sb content results from the optimized balance between the density of state effective mass and nondegenerate mobility. Additionally, a higher carrier concentration at lower x suppresses bipolar conduction, thereby reducing thermal conductivity at elevated temperatures. Consequently, the highest zT of ~ 0.5 is observed at 450 K for x = 0.1 and, according to the single parabolic band model, it could be further improved by ~70 % through carrier concentration tuning.

Enhanced power factor and figure of merit through magnesium doping in Sb2Si2Te6

We report here a high thermoelectric performance in magnesium (Mg) doped Sb2Si2Te6. The substitution of Mg2+ for Sb3+ in Sb2Si2Te6 induces a synergetic effect on electrical transport performance, exciting multi-band carrier transport behavior, enhancing carrier concentration, and increasing the density of states effective mass. Such effects lead to the highest power factor of 10.95 μW cm−1 K−2 at 824 K for the Sb1.99Mg0.01Si2Te6, representing a ∼22 % increase compared to pristine Sb2Si2Te6. In addition, Mg doping induces MgSb− point defects phonon scattering, resulting in a low thermal conductivity of ∼0.52 W m−1 K−1 at 824 K. The simultaneous optimization of the power factor and thermal conductivity in Sb1.99Mg0.01Si2Te6 leads to a peak ZT of ∼1.15 at 824 K and an average ZT of ∼0.68 (400−824 K), showing its potential for power generator at medium temperature.

Weakening the spin–orbital coupling for band convergence

Band convergence is a favorable strategy that is utilized to improve the thermoelectric performance in experiment, whereas a theoretical guidance for this purposive design is rare. With high-level first-principles treatment of electronic and transport properties in the emerging ABX (A = Ca, Sr, Ba; B = Cu, Ag; X = P, As, Sb) Zintl systems, a principled scheme via weakening the spin–orbital coupling is proposed based on symmetry and orbital analysis to promote the band convergence. This given rule is numerically confirmed in BaAgX alloys. Despite the intensified intervalley electron–phonon scattering, the alloying of BaAgP in BaAgSb prominently triggers the band convergence and contributes to much larger density of state effective mass, which eventually results in a ∼40% improvement in zT values. This work offers an applicable principle for designing band convergence, which also broads the compositions of thermoelectrics to light and earth-abundant elements.

Decreasing the Carrier Concentration of ZrNiSn: An Opposite Way to the Best N-Type Half-Heusler Thermoelectrics.

N-type ZrNiSn-based alloys reach a record thermoelectric figure of merit zT ≈1.2 by increasing the carrier concentration to 4-5 × 1020 cm-3 . In this work, It is reported that a comparable zT can also be realized in trace Ru-doped ZrNiSn-based alloy at even lower temperature by decreasing the carrier concentration. Compared to the previously reported Co doping, the doping of Ru results in a more effective reduction in carrier concentration, and thus higher Seebeck coefficient, lower electronic thermal conductivity, and enhanced thermoelectric performance. The electronic specific heat coefficient of the ZrNi1- x Rux Sn sample remains constant with increasing Ru content, indicating no obvious change in the density of states effective mass. Theoretical calculations show that the doping of Ru has negligible effect on the bottom of conduction band. The lattice thermal conductivity is further reduced by alloying Ti and Hf at the Zr site, and the bipolar diffusion is suppressed by doping of 0.5 at.% Sb. As a result, Ti0.25 Zr0.5 Hf0.25 Ni0.99 Ru0.01 Sn0.995 Sb0.005 reaches not only a zT value of 1.1 at 773 K but also a record average zT value of 0.8 in 300 to 873 K, demonstrating the effectiveness of trace Ru doping on boosting the thermoelectric performance of ZrNiSn-based alloys.

Polarons in binary Bose–Einstein condensates

Bose polarons are quasiparticles formed through the interaction between impurities and Bose–Einstein condensates. In this paper, we derive an effective Fröhlich Hamiltonian using the generalized Bogoliubov transformation. The effective Fröhlich Hamiltonian encompasses two types of effective interactions: impurity-density (ID) coupling and impurity-spin (IS) coupling. Furthermore, we employ the Lee–Low–Pines variational approach to investigate the relevant properties of Bose polarons induced by the ID and IS coupling. These properties include the ground state energy, effective mass, and average number of virtual phonons. Our findings reveal that the contribution resulting from IS couplings to the ground energy decreases to zero near the miscible–immiscible boundary. Additionally, the increase of the IS coupling induces a greater number of virtual phonons, impeding the movement of impurities and leading to a significant increase in the effective mass of Bose polarons.

Research on the electronic structure of the interface between the LiNbO3 and the magic angle graphene electrode: first principle calculation

In this paper, the interfacial electronic structures of the monolayer, bilayer, and magic-angle graphene/LiNbO3 heterostructures are calculated. The density, energy band, differential charge density, effective mass are calculated and analyzed for the model of structure. The results show that the interface material can adjust the state density, band width, electron concentration and effective mass of the electrons around the Fermi energy level. Therefore, we predict that this structure is feasible in principle for experimental preparation, and can improve the transfer efficiency of carrier, which can provide a high-speed carrier basis for the solar cells.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Towards a new chalcopyrite high-performance thermoelectric semiconductor Cu3InSnSe5 by entropy engineering

A new semiconductor Cu3InSnSe5 is introduced by entropy driven alloying equimolar ratio CuInSe2 and Cu2SnSe3. Cu3InSnSe5 is a p-type semiconductor (Eg ∼0.41 eV) and crystallizes in the chalcopyrite structure with I4_2 m space group. Our density functional theory calculations show that Cu3InSnSe5 exhibits an electronic band structure with multiple valance band peaks, rendering a large density of state effective mass m* of 1.8 me. Therefore, Cu3InSnSe5 has a high power factor of 7.59 μW cm−1 K−2 at 773 K. In addition, Cu3InSnSe5 also has a low lattice thermal conductivity (0.34 W m−1 K−1 at 773 K) owing to its severe lattice distortion arose by entropy enhancement. As a result, the figure of merit reaches as high as 1.08 at 773 K for Cu3InSnSe5. Furthermore, electronic band engineering is adopted to enhance the carrier concentration and m* of Cu3InSnSe5 via the substitution of In for Sn, resulting in ∼28% enhancement in power factor, ∼21% increment in ZT (1.31 at 773 K), and ∼10% enhancement in average ZT (∼ 0.67 at 473 – 773 K) for Cu3In1.01Sn0.99Se5 compared with Cu3InSnSe5 itself. Our work shows Cu3InSnSe5 should be a promising candidate for thermoelectric power generation at medium temperature.

Simultaneous Enhancement of the Power Factor and Phonon Blocking in Nb-Doped WSe2

Transition-metal dichalcogenide WSe2 is a potentially good thermoelectric (TE) material due to its high thermopower (S). However, the low electrical conductivity (σ), power factor (PF), and relatively large lattice thermal conductivity (κL) of pristine WSe2 degenerate its TE performance. Here, we show that through proper substitution of Nb for W in WSe2, its PF can be increased by ∼10 times, reaching 5.44 μW cm-1 K-2 (at 850 K); simultaneously, κL lowers from 1.70 to 0.80 W m-1 K-1. Experiments reveal that the increase of PF originates from both increased hole concentration due to the replacement of W4+ by Nb3+ and elevated thermopower (S) caused by the enhanced density of states effective mass, while the reduced κL comes mainly from phonon scattering at point defects NbW. As a result, a record high figure of merit ZTmax ∼0.42 is achieved at 850 K for the doped sample W0.95Nb0.05Se2, which is ∼13 times larger than that of pristine WSe2, demonstrating that Nb doping at the W site is an effective approach to improve the TE performance of WSe2.

Raising the thermoelectric performance in Pb/In-codoped BiCuSeO by alleviating the contradiction between carrier mobility and lattice thermal conductivity

Owing to the unavoidable electron scattering, introducing phonon-scattering defects to suppress the phonon diffusion in thermoelectric materials will always deteriorate the carrier transport. Therefore, exploring new methods to counterpoise the contradiction between carrier transport and phonon scattering is urgently needed. Herein, present work highlights that Pb/In codoping on BiCuSeO can suppress the phonon propagation while preserving the carrier transport, which alleviates the contradiction between carrier mobility (μ) and lattice thermal conductivity (κl), and improves the thermoelectric performance in Pb/In-codoped BiCuSeO. Multi-scale phonon-scattering centers, such as point defects, dislocations, In2O3/Cu2Se nano-precipitates and phase/grain boundaries, were introduced to promote full-wavelength phonon scattering, which brought about much reduced κl and substantially low total thermal conductivity, and a minimum κl of 0.30 W m−1 K−1 was achieved in Bi0·91Pb0·06In0·03CuSeO at 873 K. More importantly, Pb/In codoping can compensate the increased density of state effective masses caused by Pb single-doping, which resulted in improved μ and the obvious increased μ/κl. In addition, the improved μ also led to considerably increased electrical conductivity and power factor. Finally, the maximum ZT of 1.23 at 873 K and a high ZTavg of 0.72 between 300 and 873 K are achieved in Bi0·93Pb0·06In0·01CuSeO, which is superior to both the pristine and Pb-single-doped counterparts. Our findings elucidate the importance of balancing the carrier transport and phonon scattering during defect modulation of the thermoelectric performance.

High-Power Factor Enabled by Efficient Manipulation Interaxial Angle for Enhancing Thermoelectrics of GeTe-Cu2Te Alloys.

The emerged strategy of manipulating the rhombohedral crystal structure provides another new degree of freedom for optimizing the thermoelectric properties of GeTe-based compounds. However, the concept is difficult to be effectively measured and often depends on heavy doping that scatters carriers severely. Herein, we synergistically manipulate lattice distortion and vacancy concentration to promote the excellent electrical transport of GeTe-Cu2Te alloys and quantify the interaxial angle-dependent density of state effective mass. Distinct from the conventional electronic coupling effect, about 2% substitution of Zr4+ significantly increases the interaxial angle, thereby enhancing the band convergence effect and improving the Seebeck coefficient. In addition, Ge-compensation attenuates the mobility deterioration, leading to improved power factor over the whole temperature range, especially exceeding ∼22 μW cm-1 K-2 at 300 K. Furthermore, the Debye-Callaway model elucidates low lattice thermal conductivity due to strong phonon scattering from Zr/Ge substitutional defects. As a result, the highest figure of merit zT of ∼1.6 (at 650 K) and average zTave of ∼0.9 (300-750 K) are obtained in (Ge1.01Zr0.02Te)0.985(Cu2Te)0.015. This work demonstrates the effective band modulation of Zr on GeTe-based materials, indicating that the modification of the interaxial angle is a deep pathway to improve thermoelectrics.

Unique Microstructures and High Thermoelectric Performance in n–type Bi2Te2.7Se0.3 by the Dual Incorporation of Cu and Y

n-type thermoelectric systems operating near ambient temperature have been significantly understudied. n-type Bi2Te3-based materials have suffered from lack of efficient performance-enhancing strategies and compositional diversity mainly due to the difficulty in efficient doping and alloying to their structure. Herein, we report new n-type thermoelectric system Bi2-xYxTe2.7Se0.3 (x = 0 – 0.020) and optimized Cu0.01Bi1.985Y0.015Te2.7Se0.3 involving unique yttrium selenide-based microstructures. The incorporated Y and Cu atoms serve multiple favorable roles in improving thermoelectric performance of the title samples. A majority of the introduced Y forms either semiconducting Y2Se3 or metallic Y5-δSe7 depending on its chemical pressure, significantly affecting both thermal and charge transport properties. Changing the Y concentration in the nominal composition dynamically changes the composition of the surrounding matrix, microstructures, and their interfaces. The generation of microstructures and compositional variance driven by the Y and Cu incorporation can be understood and controllable in light of hard-soft acid-base principle and structural chemistry of the constituent elements. Cu atoms are more abundant in the interface than the other areas, and richer inside the microscale precipitate than the surrounding matrix, thereby creating large mass and compositional fluctuation. The Raman spectra verify that the incorporated Y and Cu atoms contribute to scattering and softening phonon modes effectively. All these jointly depress the lattice thermal conductivity of the optimized phase Cu0.01Bi1.985Y0.015Te2.7Se0.3 to ∼0.51 W m-1 K-1 at 300 K. The dual incorporation of Cu and Y atoms induces heavier density of states effective mass, thereby improving a magnitude of Seebeck coefficients. Accordingly, power factor increases especially near room temperature, giving ∼39.1 μW cm-1 K-2 at 300 K. Consequently, the Cu0.01Bi1.985Y0.015Te2.7Se0.3 sample achieves a high thermoelectric figure of merit, ZT, of ∼1.20 at 347 K and an average ZT of ∼1.16 from 300 to 423 K. Its ZT of ∼1.09 at 300 K is comparable to the state-of-the-art n-type polycrystalline Bi2Te3 systems.

Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

AbstractThe good co‐existence of midgap state and valence band degeneracy is realized in Bi‐alloyed GeTe through the In‐Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid‐high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m0, and the corresponding carrier mobility still reaches 34.3 cm2 V−1 s−1 at room temperature. Moreover, the Bi‐In‐Cd co‐alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m−1 K−1 at 600 K. As a result, the Ge0.89Bi0.06In0.01Cd0.04Te sample obtains excellent thermoelectric properties of zTmax ≈2.12 at 650 K and zTavg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.

Vacancy Manipulation Induced Optimal Carrier Concentration, Band Convergence and Low Lattice Thermal Conductivity in Nano‐Crystalline SnTe Yielding Superior Thermoelectric Performance

AbstractSynergetic optimization of electrical and thermal transport properties is achieved for SnTe‐based nano‐crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m−1 K−2 for Sn0.96Gd0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd‐doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano‐pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe‐based nanoscale polycrystalline materials.

Enhanced thermoelectric performance in the zintl antimonides (Ca,RE)9Cd4Sb9 (RE = rare-earth metal). Synergy between increased structural complexity and drive towards optimized chemical bonding

The interplay of structural complexity, high carrier mobility, and high density of states effective mass can play a pivotal role in achieving enhanced thermoelectric (TE) performance in candidate materials. In this regard, the Zintl phases represent a class of compounds that are susceptible to harboring these key ingredients. This, in addition to their amenability to various forms of chemical substitution mechanisms makes them a good choice of systems to explore systematically. Here we demonstrate the role-play of these ingredients in achieving excellent TE properties on single-crystals of Ca9–xREyCd4+δSb9 (RE = Ce, Pr, Nd, Sm, Gd, Tb; x ≈ 0.5–0.8, y ≈ 0.5–0.7, δ ≈ 0.25). These phases represent a new addition to the “9–4–9” family with intricate chemical bonding arising from both a purposely introduced disorder on Ca sites and the inherent presence of interstitial Cd positions. Many of the newly synthesized and characterized phases show moderate values of the Seebeck coefficient, lying in the range of 71–116 μV/K at 600 K and evolving as degenerate semiconductors. Simultaneously, the electrical resistivity ρ(T) of the measured samples can be as low as 0.18 mΩ cm at this temperature. As a result, the observed TE power factors in the Ce-, Nd-, and Sm-samples are in the range 6–46 μW/cm.K2. For Ca9–xCexCd4+δSb9, the estimated thermoelectric quality factor B > 0.4 at 300 K, which corresponds to a figure of merit zT ≥ 1. Calculations based on the single parabolic band (SPB) model show that the optimum region for thermoelectric performance requires carrier concentration n = 2–6 × 1019 cm−3 thus providing for an open window to further tune the TE properties.

Crystal Structure and Thermoelectric Properties of Layered Van der Waals Semimetal ZrTiSe4

Semiconductors have been studied as the most promising thermoelectric materials for the past decades due to their high Seebeck coefficient. Semimetals usually have a small Seebeck coefficient due to a small difference in the density of states between electrons and holes, compensating each other in the longitudinal direction. However, recent studies suggested that semimetals with large asymmetry between electrons and holes could be good thermoelectric candidates. Here, we report the crystal structure and thermoelectric properties of layered van der Waals semimetal ZrTiSe4. Micrometer-sized single crystals of ZrTiSe4 were obtained by the solid-state reaction. The single-crystal X-ray diffraction study suggests that this material exhibits a trigonal structure with space group P3̅m1 (no. 164), instead of a P2/m (no. 10) monoclinic phase reported in the previous work. ZrTiSe4 has the same crystal structure as ZrSe2 and TiSe2 with Zr and Ti atoms occupying the same site with equal occupancy. Polycrystalline ZrTiSe4 samples consolidated by cold pressing show a semi-metallic type of resistivity in the temperature range of 2–400 K and an n-type conducting behavior. The samples exhibit an unusually large Seebeck coefficient of −202 ± 11 μV K–1 at 300 K, notably higher than the parent phases of ZrSe2 and TiSe2 as well as other semimetals. Based on the first-principles calculations of the electronic band structure, the large Seebeck coefficient of ZrTiSe4 could arise from the conduction band convergence at the M point, resulting in a large density of states (DOS) effective mass for electrons and a highly asymmetric DOS about the chemical potential. Furthermore, owing to a low sound velocity and strong phonon scattering by boundaries and defects, the thermal conductivity exhibits weak temperature dependence and a low value of 2.2 ± 0.4 W m–1 K–1 at 300 K. This work provides useful insights into the crystal structure and thermoelectric properties of semimetal ZrTiSe4.

Enhanced Thermoelectric Performance of ZnO-Based Thin Films via Interface Engineering

Zinc oxide (ZnO) is a potential thermoelectric material with good chemical and thermal stability as well as an excellent Seebeck coefficient. However, the extremely low carrier concentration brings poor electrical transport properties. Although Gallium (Ga) doping could increase the carrier concentration of ZnO film, its thermoelectric performance is still limited due to the deteriorated Seebeck coefficient and enhanced thermal conductivity. Interface engineering is an effective strategy to decouple electron-phonon interaction for thermoelectric materials. Thus, in this work, GZO (Ga-doped ZnO)/NAZO (Ni, Al co-doped ZnO) multilayer films were designed to further improve the thermoelectric properties of GZO films. It was found that GZO/NAZO multilayer films possessed better electrical conductivity, which was attributed to the increased carrier concentration and Hall mobility. Meanwhile, benefiting from the energy filtering that occurred at GZO/NAZO interfaces, the density of states effective mass increased, resulting in comparable Seebeck coefficient values. Ultimately, an enhanced power factor value of 313 μW m−1 K−2 was achieved in the GZO/NAZO multilayer film, which is almost 46% larger than that of GZO film. This work provides a paradigm to optimize the thermoelectric performance of oxide films and other thermoelectric systems by multilayer structure design with coherent interfaces.