High Thermoelectric Performance Originating from the Grooved Bands in the ZrSe3 Monolayer

Enhancing phonon thermal transport in 2H-CrX2 (X = S and Se) monolayers through robust bonding interactions

Inspired by the groundbreaking discovery of the 2H-MoS2 monolayer with outstanding physical properties, the electronic structure, structural stability, and thermal transport of 2H-CrX2 (X = S and Se) monolayers are theoretically evaluated using density functional theory (DFT) calculations and semiempirical Boltzmann transport theory. The 2H-CrX2 (X = S and Se) monolayers are direct semiconductors with the bandgaps of 0.91 and 0.69 eV. The elastic modulus and phonon dispersion curve analysis show that the 2H-CrX2 (X = S and Se) monolayers possess excellent mechanical and dynamic stabilities on account of elastic constants satisfying the Born-Huang criterion and the absence of negative frequencies. The thermal stabilities of the 2H-CrX2 (X = S and Se) monolayers at 300 K are proved by ab initio molecular dynamics (AIMD) simulations, as evidenced by the slight changes in the structural evolution and small fluctuation in total energy. High thermal conductivities of 131.7 and 88.6 W m-1 K-1 are discovered for 2H-CrS2 and 2H-CrSe2 monolayers at 300 K. Further analysis of the phonon group velocity, phonon relaxation time, and Grüneisen parameter shows that the high lattice thermal conductivities of 2H-CrX2 (X = S and Se) monolayers could be attributed to the great bond strength, large Young's modulus, relatively small atomic mass, high phonon group velocity, and long phonon relaxation time. In addition, the various scattering mechanisms are further considered in the calculations of phonon thermal transport to evaluate the effect of the scattering rates of the 2H-CrS2 and 2H-CrSe2 monolayers on the lattice thermal conductivity, and the determinative role is found for the phonon boundary scattering. Our present study would not only offer a fundamental understanding of the thermal transport properties of the 2H-CrX2 (X = S and Se) monolayers, but also provide theoretical guidelines for the experimental investigation of thermal management materials with 2H-phase.

Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu₄TiS₄ and Cu₄TiSe₄ were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu₄TiSe₄ compared to that in Cu₄TiS₄, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu₄TiSe₄. In addition, the strong lattice anharmonicity in Cu₄TiSe₄ enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ_L) of 0.11 W m^-1 K^-1 at room temperature in Cu₄TiSe₄ compared with that of 0.58 W m^-1 K^-1 in Cu₄TiS₄. Owing to the suitable band gaps of Cu₄TiS₄ and Cu₄TiSe₄, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu₄TiSe₄ are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu₄TiS₄, due to its low κ_L, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu₄TiSe₄ demonstrates its great potential for applications in thermoelectric conversion.

Lattice Transformation from 2D to Quasi 1D and Phonon Properties of Exfoliated ZrS2 and ZrSe2

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS₂ ), zirconium diselenide (ZrSe₂ ), zirconium trisulfide (ZrS₃ ), and zirconium triselenide (ZrSe₃ ). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX₂ (where X = S or Se) are observed. For 2D-ZrX₂ , Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX₂ , Raman modes are identical to exfoliated ZrX₃ nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX₂ are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS₂ , a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX₂ are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX₂ compared to 2D-ZrX₂ , which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.

Quasi-One-Dimensional van der Waals Transition Metal Trichalcogenides

The transition metal trichalcogenides (TMTCs) are quasi-one-dimensional (1D) MX3-type van der Waals layered semiconductors, where M is a transition metal element of groups IV and V, and X indicates chalcogen element. Due to the unique quasi-1D crystalline structures, they possess several novel electrical properties such as variable bandgaps, charge density waves, and superconductivity, and highly anisotropic optical, thermoelectric, and magnetic properties. The study of TMTCs plays an essential role in the 1D quantum materials field, enabling new opportunities in the material research dimension. Currently, tremendous progress in both materials and solid-state devices has been made, demonstrating promising applications in the realization of nanoelectronic devices. This review provides a comprehensive overview to survey the state of the art in materials, devices, and applications based on TMTCs. Firstly, the symbolic structure, current primary synthesis methods, and physical properties of TMTCs have been discussed. Secondly, examples of TMTC applications in various fields are presented, such as photodetectors, energy storage devices, catalysts, and sensors. Finally, we give an overview of the opportunities and future perspectives for the research of TMTCs, as well as the challenges in both basic research and practical applications.

Crystal Growth and Thermal Properties of Quasi-One-Dimensional van der Waals Material ZrSe3

ZrSe₃ with a quasi-one-dimensional (quasi-1D) crystal structure belongs to the transition metal trichalcogenides (TMTCs) family. Owing to its unique optical, electrical, and optoelectrical properties, ZrSe₃ is promising for applications in field effect transistors, photodetectors, and thermoelectrics. Compared with extensive studies of the above-mentioned physical properties, the thermal properties of ZrSe₃ have not been experimentally investigated. Here, we report the crystal growth and thermal and optical properties of ZrSe₃. Millimeter-sized single crystalline ZrSe₃ flakes were prepared using a chemical vapor transport method. These flakes could be exfoliated into microribbons by liquid-phase exfoliation. The transmission electron microscope studies suggested that the obtained microribbons were single crystals along the chain axis. ZrSe₃ exhibited a specific heat of 0.311 J g^-1 K^-1 at 300 K, close to the calculated value of the Dulong-Petit limit. The fitting of low-temperature specific heat led to a Debye temperature of 110 K and an average sound velocity of 2122 m s^-1. The thermal conductivity of a polycrystalline ZrSe₃ sample exhibited a maximum value of 10.4 ± 1.9 W m^-1 K^-1 at 40 K. The thermal conductivity decreased above 40 K and reached a room-temperature value of 5.4 ± 1.3 W m^-1 K^-1. The Debye model fitting of the solid thermal conductivity agreed well with the experimental data below 200 K but showed a deviation at high temperatures, indicating that optical phonons could substantially contribute to thermal transport at high temperatures. The calculated phonon mean free path decreased with temperatures between 2 and 21 K. The mean free path at 2 K approached 3 μm, which was similar to the grain size of the polycrystalline sample. This work provides useful insights into the preparation and thermal properties of quasi-1D ZrSe₃.

Anomalous Thermoelectric Performance in Asymmetric Dirac Semimetal BaAgBi

Multiple-band degeneracy has been widely recognized to be beneficial for high thermoelectric performance. Here, we discover that the p-type Dirac bands with lower degeneracy synergistically produce a higher Seebeck coefficient and electrical conductivity in topological semimetal BaAgBi. The anomalous transport phenomenon intrinsically originated from the asymmetric electronic structures: (i) complete p-type Dirac bands near the Fermi level facilitate high and strong energy-dependent hole relaxation time; (ii) the presence of additional parabolic conduction valleys allows for a large density of states to accept scattered electrons, leading to an enlarged hole-electron relaxation time ratio and, thus, weakened bipolar effect. In combination with the strong lattice anharmonicity, an exceptional p-type average ZT of 0.42 is achieved from 300 to 600 K, which can be dramatically enhanced to 1.38 via breaking the C_3v symmetry. This work uncovers the underlying mechanisms governing the abnormal transport behavior in Dirac semimetal BaAgBi and highlights the asymmetric electronic structures as target features to discover/design high-performance thermoelectric materials.

Thermoelectric performance of novel single-layer ZrTeSe4

Single-layer ZrTeSe4 is a novel 2D semiconductor as well as a promising candidate for 2D thermoelectric materials.

Strongly Anisotropic Strain-Tunability of Excitons in Exfoliated ZrSe3

The effect of uniaxial strain on the band structure of ZrSe₃ , a semiconducting material with a marked in-plane structural anisotropy, is studied. By using a modified three-point bending test apparatus, thin ZrSe₃ flakes are subjected to uniaxial strain along different crystalline orientations monitoring the effect of strain on their optical properties through micro-reflectance spectroscopy. The obtained spectra show excitonic features that blueshift upon uniaxial tension. This shift is strongly dependent on the direction along which the strain is being applied. When the flakes are strained along the b-axis, the exciton peak shifts at ≈60-95 meV %^-1 , while along the a-axis, the shift only reaches ≈0-15 meV %^-1 . Ab initio calculations are conducted to study the influence of uniaxial strain, applied along different crystal directions, on the band structure and reflectance spectra of ZrSe₃ , exhibiting a remarkable agreement with the experimental results.

Promising thermoelectric candidate based on a CaAs3 monolayer: A first principles study

The CaAs₃ monolayer is a newly predicted two-dimensional material with attractive properties, such as a moderate direct bandgap, high carrier mobility, prominent visible-light absorption, etc. To evaluate its potential applications in thermoelectric (TE) fields, herein, the thermoelectric properties of CaAs₃ monolayers were comprehensively investigated by a first-principles method in combination with Boltzmann transport theory. Our calculated results indicate that the CaAs₃ monolayer has an exceptionally low lattice thermal conductivity of 0.44 W m^-1 K^-1 at 300 K, mainly because of the small group velocity and strong phonon-phonon scattering. The CaAs₃ monolayer also exhibits a high power factor due to the large Seebeck coefficient and electrical conductivity. Therefore, large ZT values of 1.72/1.58 were achieved for the n-type/p-type CaAs₃ monolayer at 800 K. Compared with conventional 2D TE materials, the CaAs₃ monolayer does not contain expensive heavy elements, which is beneficial for its practical applications as a TE material. Our results qualify the CaAs₃ monolayer as a promising candidate for building excellent 2D TE devices.

Anisotropic quasi-one-dimensional layered transition-metal trichalcogenides: synthesis, properties and applications

The strong in-plane anisotropy and quasi-1D electronic structures of transition-metal trichalcogenides (MX3; M = group IV or V transition metal; X = S, Se, or Te) have pronounced influence on moulding the properties of MX3 materials. In particular, the infinite trigonal MX6 prismatic chains running parallel to the b-axis are responsible for the manifestation of anisotropy in these materials. Several marvellous properties, such as inherent electronic, optical, electrical, magnetic, superconductivity, and charge density wave (CDW) transport properties, make transition-metal trichalcogenides (TMTCs) stand out from other 2D materials in the fields of nanoscience and materials science. In addition, with the assistance of pressure, temperature, and tensile strain, these materials and their exceptional properties can be tuned to a superior extent. The robust anisotropy and incommensurable properties make the MX3 family fit for accomplishing quite a lot of compelling applications in the areas of field effect transistors (FETs), solar and fuel cells, lithium-ion batteries, thermoelectricity, etc. In this review article, a precise audit of the distinctive crystal structures, static and dynamic properties, efficacious synthesis schemes, and enthralling applications of quasi-1D MX3 materials is made.

Titanium nitride halides monolayers: promising 2D anisotropic thermoelectric materials

2D graphene-like thermoelectric materials have been extensively explored, however, the studies have mostly focused on the binary compounds and most of them exhibit isotropic electron and phonon transport properties. Here, we investigate the thermoelectric transport properties of ternary TiNX (X  =  F, Cl, Br) monolayers by using first-principles combined with the Boltzmann transport theory. Both electron and phonon anisotropic transport properties are found. The large p -type power factor and the low lattice thermal conductivity along the y  direction give rise to better thermoelectric performance along the y  direction than the x direction, and the highest ZT values at 500 K reach 1.00, 0.89 and 1.17 along the y  direction in p -type doping for TiNF, TiNCl, and TiNBr monolayer, respectively. The anisotropy and the difference of lattice thermal conductivities among TiNX monolayers are discussed in terms of the group velocities, the phonon relaxation time and the three-phonon scattering phase space. These results indicate that TiNX monolayers are promising candidates for 2D anisotropic thermoelectric materials.

Thermoelectric Performance of Two-Dimensional AlX (X = S, Se, Te): A First-Principles-Based Transport Study

By using the first-principles calculations in combination with the Boltzmann transport theory, we systematically study the thermoelectric properties of AlX (X = S, Se, Te) monolayers as indirect gap semiconductors. The unique electronic density of states, which consists of a rather sharp peak at the valence band maxima and an almost constant band at the conduction band minima, makes AlX (X = S, Se, Te) monolayers excellent thermoelectric materials. The optimized power factors at room temperature are 22.59, 62.59, and 6.79 mW m-1 K-2 under reasonable electronic concentration for AlS, AlSe, and AlTe monolayers, respectively. The figure of merit (zT) increases with temperature and the optimized zT values of 0.52, 0.59, and 0.26 at room temperature are achieved under moderate electronic concentration for AlS, AlSe, and AlTe monolayers, respectively, indicating that two-dimensional layered AlX (X = S, Se, Te) semiconductors, especially AlSe, can be potential candidate matrices for high-performance thermoelectric nanocomposites.

Realizing high thermoelectric performance with comparable p- and n-type figure-of-merits in a graphene/h-BN superlattice monolayer

We demonstrate that the superlattice monolayer consisting of light, earth-abundant, and environmentally friendly elements can be designed as perfect TE modules with comparable p- and n-type energy conversion efficiency.