Enhancement of monolayer HfSe2 thermoelectric performance by strain engineering: A DFT calculation

Strain-induced effects on the optoelectronic properties of ZrSe2/HfSe2 heterostructures

CONTEXT

Two-dimensional semiconductor materials have received much attention in recent years due to their wide variety of applications in the field of nano-optoelectronic devices. In this project, we applied stresses ranging from -6 to +6% to the ZrSe2/HfSe2 heterostructure and systematically investigated its electrical and optical properties. It is discovered that stress can effectively modulate the forbidden bandwidth of the ZrSe2/HfSe2 heterojunction; whereas, under compressive stress, the forbidden bandwidth of the material decreases further until the bandgap is zero, leading to the material's transformation from semiconductor to metal. The forbidden band gap of the ZrSe2/HfSe2 heterojunction increases with increasing horizontal biaxial tensile strain. We discovered that the light absorption performance of this heterostructure is significantly better than that of its similar monomolecular layer and that its light absorption intensity can reach an order of magnitude of 104. Under compressive and tensile stresses, the ZrSe2/HfSe2 heterojunctions exhibit different degrees of red or blue shift. The results indicate that constructing ZrSe2/HfSe2 heterojunctions and applying horizontal biaxial stresses to them can significantly modulate the optoelectronic properties of the materials. ZrSe2/HfSe2 heterojunction is a new type of high-performance photogenerated carrier transport device with a wide range of applications.

METHODS

The calculations in this study are carried out the first principles approach of density functional theory, as implemented in the CASTEP module of Materials-Studio2019. The researchers used an ultrasoft reaction potential to calculate the interactions between the ion core and the electrons and applied the Perdew-Burke-Ernzerhof (PBE) and the generalized gradient approximation (GGA) to perform the calculations. The Monkhorst-Pack technique was employed to create the k-point samples utilized for integration on the Brillouin zone, and the k-point grid was uniformly 6 × 6 × 1. In addition, in order to avoid interactions between the atomic layers affecting the properties and stability of the material, such interactions were prevented by adding a 30 Å vacuum layer. Using a plane-wave energy cutoff of 500 eV and the convergence accuracy of the iterative process was set to 1 × 10-5 eV to ensure the accuracy of the computational results, and in addition. The maximum stress in the lattice was limited to less than 0.05 GPa or the interaction force between neighboring atoms was lower than 0.03 eV/Å. For the calculation of the properties of the optical properties, a k-point grid of 18 × 18 × 1 is used for optimization, and the polarization direction of the material is not taken into account, considering that the material is isotropic. This study proposes to apply the Tkatchenko-Scheffler (TS) dispersion correction method in DFT-D to appropriately represent the interlayer van der Waals interaction forces to solve inaccuracies in the computation of van der Waals interactions via density functional theory.

HfSe2: Unraveling the microscopic reason for experimental low mobility

Monolayer HfSe2, in the family of transition metal dichalcogenides (TMDCs), is a potential thermoelectric candidate due to its low thermal conductivity. While its mobility remains low as in other 2D TMDCs is inconceivable for electronic and thermoelectric applications. Earlier theoretical attempts have failed to give justification for the orders of low experimental mobility obtained for monolayer HfSe2. We calculate the carrier mobility in the framework of the density functional perturbation theory in conjunction with the Boltzmann transport equation and correctly ascertain the experimental value. We also calculate the carrier mobility with the previously employed method, the deformation potential (DP) model, to figure out the reason for its failure. We found that it is the strong electron-optical phonon interaction that is causing the low mobility. As the DP model does not account for the optical phonons, it overestimates the relaxation time by an order of two and also underestimates the temperature dependence of mobility. A strong polar type interaction is evidenced as a manifestation of a discontinuity in the first derivative of the optical-phonons at the K and Γ points as well as a dispersive optical phonon at the K point. We also included the spin-orbit coupling which leads to an energy splitting of ∼330 meV and significantly affects mobility and scattering rates.

Tensile strain and finite size modulation of low lattice thermal conductivity in monolayer TMDCs (HfSe2 and ZrS2) from first-principles: a comparative study

With excellent physical and chemical properties, 2D TMDC materials have been widely used in engineering applications, but they inevitably suffer from the dual effects of strain and device size. As typical 2D TMDCs, HfSe₂ and ZrS₂ are reported to have excellent thermoelectric properties. Thermal transport properties have great significance for exerting the performance of materials, ensuring device lifetime and stable operation, but current research is not detailed enough. Here, first-principles combined with the phonon Boltzmann transport equation are used to study the phonon transport inside monolayer HfSe₂ and ZrS₂ under tensile strain and finite size, and explore the band structure properties. Our research shows that they have similar phonon dispersion curve structures, and the band gap of HfSe₂ increases monotonically with the increase of tensile strain, while the bandgap of ZrS₂ increases and then decreases with the increase of tensile strain. Thermal conductivity has obvious strain dependence: with the increase of tensile strain, the thermal conductivity of HfSe₂ gradually decreases, while that of ZrS₂ increases slightly, and then gradually decreases. Reducing the system size can limit the contribution of phonons with a long mean free path, significantly decreasing thermal conductivity through the controlling effect of tensile strain. The mode contribution of thermal conductivity is systematically investigated, and anharmonic properties including mode and frequency-level scattering rates, group velocity and Grüneisen parameters are used to explain the associated mechanism. Phonon scattering processes and channels in various cases are discussed in detail. Our research provides a detailed understanding of the phonon transport and electronic structural properties of low thermal conductivity monolayers of HfSe₂ and ZrS₂, and further completes the study of thermal transport of the two materials under strain and size tuning, which will provide a foundation for further popularization and application.

Phase Formation Behavior and Thermoelectric Transport Properties of S-Doped FeSe2-xSx Polycrystalline Alloys

Some transition-metal dichalcogenides have been actively studied recently owing to their potential for use as thermoelectric materials due to their superior electronic transport properties. Iron-based chalcogenides, FeTe₂, FeSe₂ and FeS₂, are narrow bandgap (~1 eV) semiconductors that could be considered as cost-effective thermoelectric materials. Herein, the thermoelectric and electrical transport properties FeSe₂-FeS₂ system are investigated. A series of polycrystalline samples of the nominal composition of FeSe_2-xS_x (x = 0, 0.2, 0.4, 0.6, and 0.8) samples are synthesized by a conventional solid-state reaction. A single orthorhombic phase of FeSe₂ is successfully synthesized for x = 0, 0.2, and 0.4, while secondary phases (Fe₇S₈ or FeS₂) are identified as well for x = 0.6 and 0.8. The lattice parameters gradually decrease gradually with S content increase to x = 0.6, suggesting that S atoms are successfully substituted at the Se sites in the FeSe₂ orthorhombic crystal structure. The electrical conductivity increases gradually with the S content, whereas the positive Seebeck coefficient decreases gradually with the S content at 300 K. The maximum power factor of 0.55 mW/mK² at 600 K was seen for x = 0.2, which is a 10% increase compared to the pristine FeSe₂ sample. Interestingly, the total thermal conductivity at 300 K of 7.96 W/mK (x = 0) decreases gradually and significantly to 2.58 W/mK for x = 0.6 owing to the point-defect phonon scattering by the partial substitution of S atoms at the Se site. As a result, a maximum thermoelectric figure of merit of 0.079 is obtained for the FeSe_1.8S_0.2 (x = 0.2) sample at 600 K, which is 18% higher than that of the pristine FeSe₂ sample.

Strain-Enhanced Thermoelectric Performance in GeS2 Monolayer

Strain engineering has attracted extensive attention as a valid method to tune the physical and chemical properties of two-dimensional (2D) materials. Here, based on first-principles calculations and by solving the semi-classical Boltzmann transport equation, we reveal that the tensile strain can efficiently enhance the thermoelectric properties of the GeS₂ monolayer. It is highlighted that the GeS₂ monolayer has a suitable band gap of 1.50 eV to overcome the bipolar conduction effects in materials and can even maintain high stability under a 6% tensile strain. Interestingly, the band degeneracy in the GeS₂ monolayer can be effectually regulated through strain, thus improving the power factor. Moreover, the lattice thermal conductivity can be reduced from 3.89 to 0.48 W/mK at room temperature under 6% strain. More importantly, the optimal ZT value for the GeS₂ monolayer under 6% strain can reach 0.74 at room temperature and 0.92 at 700 K, which is twice its strain-free form. Our findings provide an exciting insight into regulating the thermoelectric performance of the GeS₂ monolayer by strain engineering.