Electronic and optical properties of the ZrS2/HfSe2 van der Waals heterobilayer with native type-II band alignment

Strain-induced modification in thermal properties of monolayer 1 T-ZrS2 and ZrS2/ZrSe2 heterojunction

CONTEXT

This paper systematically analyzes the phonon dispersion curves of single-layer ZrS2, ZrSe2, and ZrS2/ZrSe2 heterostructures under different strains. The phonon spectra and thermal parameters of the three structures were obtained based on the density functional perturbation theory method. The upper limits of strain that different monolayers and heterojunctions can withstand were studied. The monolayers ZrSe2 and ZrS2 can withstand up to 8% biaxial tensile strain, and the ZrS2/ZrSe2 heterojunction can withstand up to 6% biaxial tensile strain. In addition, the van der Waals force of the heterojunction may cause phonon tearing in the vertical direction. The application of biaxial tensile strain can adjust the thermal properties of the system to a large extent, which is similar to the strain effect in the pristine case. When the temperature rises, the entropy enthalpy of the three structures also gradually increases, the free energy gradually decreases, and the heat capacity of the system gradually increases until it tends to be stable. Taking single-layer ZrS2 as an example, we analyzed the change curve of thermal properties of single-layer ZrS2 under tensile strain. The results show that with the gradual increase of strain, the crystal's entropy, enthalpy, and free energy change differently. In addition, the heat capacity increases slowly under high temperatures. When all systems reach the limit strain, the phonon spectrum appears to have an imaginary frequency, and the thermal properties decrease significantly.

METHODS

This paper uses the first-principle calculation method based on density functional theory, and the PBE exchange-correlation function based on generalized gradient approximation (GGA) is selected for a specific calculation. The density functional perturbation theory (DFPT) calculates the full kinetic matrix. Because the lattice constants of ZrS2 and ZrSe2 are similar and have similar periodicity, the corresponding unit cells are used for structural optimization and property calculation. The Brillouin zone is integrated using the K points generated by the Monkhorst-pack method. For single-layers ZrS2 and ZrSe2, 8 × 8 × 1 K-point grid is selected, and for ZrS2/ZrSe2 heterojunction, 8 × 8 × 2 K-point grid is selected. A vacuum layer of 30 Å was added in the vertical direction to avoid interlayer interaction. The non-conservative pseudopotential method is used to optimize the structure, and the optimization convergence is set as follows: the cutoff energy is set to 700 eV, the convergence threshold of the maximum force between atoms is 0.01 eV/Å, the convergence threshold of the maximum energy change is set to 1 × 10-9 eV, and the convergence threshold of the maximum displacement is 0.001 Å.

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.

First principles study on structural, electronic and optical properties of HfS2(1-x)Se2x and ZrS2(1-x)Se2x ternary alloys

Alloying 2D transition metal dichalcogenides (TMDs) with dopants to achieve ternary alloys is as an efficient and scalable solution for tuning the electronic and optical properties of two-dimensional materials. This study provides a comprehensive study on the electronic and optical properties of ternary HfS_2(1−x)Se_2(x) and ZrS_2(1−x)Se_2(x) [0 ≤ x ≤ 1] alloys, by employing density functional theory calculations along with random phase approximation. Phonon dispersions were also obtained by using density functional perturbation theory. The results indicate that both of the studied ternary families are stable and the increase of Selenium concentration in HfS_2(1−x)Se_2(x) and ZrS_2(1−x)Se_2(x) alloys results in a linear decrease of the electronic bandgap from 2.15 (ev) to 1.40 (ev) for HfS_2(1−x)Se_2(x) and 1.94 (ev) to 1.23 (ev) for ZrS_2(1−x)Se_2(x) based on the HSE06 functional. Increasing the Se concentration in the ternary alloys results in a red shift of the optical absorption spectra such that the main absorption peaks of HfS_2(1−x)Se_2(x) and ZrS_2(1−x)Se_2(x) cover a broad visible range from 3.153 to 2.607 eV and 2.405 to 1.908 eV, respectively. The studied materials appear to be excellent base materials for tunable electronic and optoelectronic devices in the visible range.

Adding Selenium to HfS₂ and ZrS₂ two-dimensional materials allows tuning the optical properties in a wide visible spectrum that can be used in various electronic and optical applications, including solar cells.

Two-dimensional layered type-II MS2/BiOCl (M = Zr, Hf) van der Waals heterostructures: promising photocatalysts for hydrogen generation

Our theoretical findings reveal that in-plane biaxial strain tunes the bandgap and induces a transition from indirect to direct semiconductor.