First-principles Calculations Research Articles

First-principles study of the electronic and optical properties of two-dimensional PtS2/GaS van der Waals heterostructure

Heterostructures based on two-dimensional materials have received increasing attention due to their extraordinary properties and application potential. In this paper, the electronic and optical properties of the PtS2/GaS van der Waals (vdW) heterostructure as well as the effects of biaxial strain and external electric field are systematically investigated based on first-principles calculations. The PtS2/GaS vdW heterostructure has an interlayer distance of 3.01 Å and is a type-Ⅱ semiconductor of band gap 1.54 eV. Large optical absorption coefficients are observed in both the ultraviolet and the visible regions. Furthermore, its band structure can be effectively tuned by applying biaxial strain and external electric field. The transition between the type-Ⅱ and type-I band alignments can be realized. The absorption spectra and their peaks can be then manipulated effectively by applying biaxial strain with good stability under external electric field. The predicted tunable electronic properties and unique optical absorption properties suggests promising potential for the application of the PtS2/GaS vdW heterostructure in future optoelectronic nanodevices.

Electrostatic doping tunable magnetic transition and half-metallicity in the monolayer CrCTe3.

The electrical manipulation of the magnetic transition and spinpolarized states has attracted extensive attention in the field of spintronics. In this work, we perform a detailed study on the electronic and magnetic properties of the carrier-doped monolayer CrCTe3by using first-principles calculation. It is found that, the magnetic transition from Néel-antiferomagnetic (nAFM) to ferromagnetic (FM) is observed in the case of the electron doping, while for hole doping a magnetic transition sequence of nAFM → zigzag-AFM → FM is observed in the monolayer CrCTe3. Interestingly, the carrier doping induced FM ground state always exhibits half-metallicity with full spin polarization. Moreover, the spin polarity of the half-metallic electronic states is opposite for electron and hole doping, meaning that the spin polarization direction can be tuned by manipulating a gate voltage. The Monte Carlo calculations show that the magnetic transition temperature of the doped FM CrCTe3is rapidly increased with the increasing doping concentration and is extremely expected to achieve room temperature at a suitable doping concentration. These findings demonstrate that the monolayer AFM system possesses a potential application in spintronic devices with electrically tunable spin polarization.

"Fraternal-twin" ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles.

In ferroelectric switching, an applied electric field switches the system between two polar symmetry-equivalent states. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped samarium nickelate (SNO) at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-equivalent states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO6 octahedron, a state unrelated by symmetry but equal in energy under a square epitaxial strain (a = b), resulting in a large change in polarization. We term this unconventional ferroelectric a "fraternal-twin" ferroelectric.&#xD.

Anisotropic optical property of ferroelectric Bi2O2X(X = S,Se,Te) monolayer and strain engineering

Based on the first-principles calculations, ferroelectric Bi2O2X(X = S,Se,Te) monolayers with unequivalent in-plane lattice constants are confirmed to be the ground state, which is consistent with the experiment result (Ghosh et al 2019 Nano Lett. 19 5703–09), and the anisotropic optical property is firstly investigated. We find that the polarizations of Bi2O2X monolayers points along the direction of a-axis, and Bi2O2Te monolayer process the largest polarization. Furthermore, both the biaxial and uniaxial strains are favor for the enhancement of polarization of Bi2O2X monolayers. It should be mentioned that the type of band gap will convert from indirect to direct for Bi2O2Te monolayer when the a-axial tensile strain is larger than 2%. At last, the optical absorption coefficient for Bi2O2X monolayers are calculated, and we obtain that Bi2O2Te monolayer has the strongest optical absorption within the range of visible light, the anisotropy and possible strain engineering to improve the optical absorption are discussed in detail. Our findings are significant in fields of optoelectronics and photovoltaics.

Momentum-Resolved Electronic Structures and Strong Electronic Correlations in Graphene-like Nitride Superconductors.

Although transition-metal nitrides have been widely applied for several decades, experimental investigations of their high-resolution electronic band structures are rare due to the lack of high-quality single-crystalline samples. Here, we report on the first momentum-resolved electronic band structures of titanium nitride (TiN) films, which are remarkable nitride superconductors. The measurements of the crystal structures and electrical transport properties confirmed the high quality of these films. More importantly, from a combination of high-resolution angle-resolved photoelectron spectroscopy and first-principles calculations, the extracted Coulomb interaction strength of TiN films can be as large as 8.5 eV, whereas resonant photoemission spectroscopy yields a value of 6.26 eV. These large values of Coulomb interaction strength indicate that superconducting TiN is a strongly correlated system. Our results uncover the unexpected electronic correlations in transition-metal nitrides, potentially providing a perspective not only to understand their emergent quantum states but also to develop their applications in quantum devices.

Clamping enables enhanced electromechanical responses in antiferroelectric thin films.

Thin-film materials with large electromechanical responses are fundamental enablers of next-generation micro-/nano-electromechanical applications. Conventional electromechanical materials (for example, ferroelectrics and relaxors), however, exhibit severely degraded responses when scaled down to submicrometre-thick films due to substrate constraints (clamping). This limitation is overcome, and substantial electromechanical responses in antiferroelectric thin films are achieved through an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition using operando electron microscopy, such that the in-plane clamping further enhances the out-of-plane expansion, as rationalized using first-principles calculations. In turn, a non-traditional thickness scaling is realized wherein an electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO3 film that is just 100 nm thick. The high performance and understanding of the mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems.

Molecular Mechanisms behind Circular Dichroism Spectral Variations between Channelrhodopsin and Heliorhodopsin Dimers.

Channelrhodopsin (ChR) and heliorhodopsin (HeR) are microbial rhodopsins with similar structures but different circular dichroism (CD) spectra: ChR shows biphasic negative and positive bands, whereas HeR shows a single positive band. We explored the physicochemical factors underlying these differences through computational methods. Using the exciton model based on first-principles computations, we obtained the CD spectra of ChR and HeR. The obtained spectra indicate that the protein dimer structures and the quantum mechanical treatment of the retinal chromophore and its interacting amino acids are crucial for accurately reproducing the experimental spectra. Further calculations revealed that the sign of the excitonic coupling was opposite between the ChR and HeR dimers, which was attributed to the contrasting second term of the orientation factor between the two retinal chromophores. These findings demonstrate that slight variations in the intermolecular orientation of the two chromophores can result in significant differences in the CD spectral shape.

Ultrastrong Coupling between Polar Distortion and Optical Properties in Ferroelectric MoBr2O2.

Tuning the properties of materials by using external stimuli is crucial for developing versatile smart materials. Strong coupling among the order parameters within a single-phase material constitutes a potent foundation for achieving precise property control. However, cross-coupling is fairly weak in most single materials. Leveraging first-principles calculations, we demonstrate a layered mixed anion compound MoBr2O2 that exhibits electric-field switchable spontaneous polarization and ultrastrong coupling between polar distortion and electronic structures as well as optical properties. It offers feasible avenues of achieving tunable Rashba spin-splitting, electrochromism, thermochromism, photochromism, and nonlinear optics by applying an external electric field to a single domain sample and heating, as well as intense light illumination. Additionally, it exhibits an exceptionally large photostrictive effect. These findings not only showcase the feasibility of achieving multiple order parameter coupling within a single material but also pave the way for comprehensive applications based on property control, such as energy harvesting, information processing, and ultrafast control.

Study on the competitive reduction mechanism of C and CO on Fe2O3 in blast furnace ironmaking

The competitive reduction behaviour of C and CO on the surface of Fe2O3 in blast furnaces was studied based on experiments and the first-principles calculations. The material morphology and element distribution of Fe2O3 under the conditions of C, CO and C + CO reduction were compared. In order to further investigate the competitive reduction behaviour of C and CO on the Fe2O3 surface, based on the first-principles calculation, the adsorption mechanism of C and CO on the α-Fe2O3(001) surface was studied by calculating the parameters of bond length, adsorption energy, charge density and state density. The results showed that under the condition of C + CO co-reduction, the reduction degree of Fe2O3 is better. When CO was adsorbed on the α-Fe2O3 (001) surface alone, the adsorption energy of the adsorption structure was −0.549 eV. The state density peak of the CO molecule on the left side of the Fermi level was sharper, and the electron delocalization around the CO molecule was weaker. Compared with the C atom, the CO molecule was less easily adsorbed on the α-Fe2O3 (001) surface, and the structure stability of the adsorbed products generated by the CO molecule was also lower. Compared with the adsorption of C and CO on the α-Fe2O3 (001) surface alone, the co-adsorption energy of C atom and CO molecule was relatively low, which was −7.755 eV, and the co-adsorption structure was more stable, and the presence of CO was more conducive to the interactions between C and α-Fe2O3 (001) surface.

Formation of an extended defect cluster in cuprous oxide

Intrinsic defects and defect clusters play an important role in the room-temperature transport of cuprous oxide. Neutralization of these defects by doping and/or modifying the synthesis process is essential to improve the room-temperature hole mobility in cuprous oxide. Toward this end, we annealed polycrystalline cuprous oxide under Cu-rich conditions, which led to the neutralization of the intrinsic acceptor defect. The concentration of both the acceptor defects ( VCu and VCusplit ) that are already present, reduces by four to five orders of magnitude. This is in accordance with the amount of possible Cu incorporation under different annealing conditions, indicating the backfilling of a large fraction of the Cu vacancies. Unforeseeably, the experimental conditions lead to the creation of yet another higher-order extended defect (3 VCu + 2Cu i ) with a defect level at ≈0.5 eV above the valence band. The formation of such a defect is also indirectly suggested by the analysis of carrier concentration vs. temperature data and first-principles calculations. Such singly ionized higher-order defects with a possibly higher capture cross-section act as more effective traps resulting in reduced hole mobility.

Atomistic Insights into Nucleation of Ferroelastic Domains in PbTiO3.

Understanding the nucleation mechanism of domains is essential for domain engineering of perovskite ferroelectric materials. We proposed and examined atomistic details for nucleating ferroelastic (FS) domains by integrating topological analysis and first-principles calculations. FS domains are crystallographically treated as deformation twins. The conventional shear-shuffle nucleation mechanism under simple shear deformation is ruled out because the 1-layer elementary twinning disconnection (TD) cannot nucleate and glide in a perfect matrix. Thus, the pure-shuffle nucleation mechanism under pure shear deformation is proposed due to kinetically favored atomic shuffling. The coherency stress associated with the coherent nucleus is relaxed via forming misfit dislocations, accompanied by formation and sharpening of diffused (110)m∥(110)d domain walls (DWs). The sharp DWs enable growth of the FS nucleus through successive nucleation and gliding of TDs. These findings enrich the knowledge of domain behavior in perovskite ferroelectric materials.

Anisotropic optoelectronic properties of lead-free Cs3Bi2I9 single-crystal photodetectors

Lead-free halide perovskites have attracted widespread research interest due to their excellent optoelectronic properties and environmental friendliness. As one of the bismuth iodide compounds, Cs3Bi2I9 perovskite has been extensively explored in the field of photovoltaic devices and radiation detectors due to their non-toxic lead-free components and excellent stability. In this work, we successfully grow large-sized Cs3Bi2I9 single crystals (SC) with (l00) and (00l) crystal exposure facets by inverse temperature crystallization method. Under 525 nm light illumination with the intensity 15 mW cm−2 and 7 V bias, the (l00) Cs3Bi2I9 SC shows 2 times higher photocurrent, 3.6 times higher responsivity, and 2.8 times higher detectivity than the (00l) Cs3Bi2I9 SC, respectively. Superior response time in the scale of millisecond is obtained in both (l00) and (00l) Cs3Bi2I9 SCs. Based on the first-principle calculation, the (l00) SC possesses a higher charge distribution density and a wider dispersion distribution than (00l) SC, suggesting that more electrons in the (l00) SC can be excited in a wider range. The tighter arrangement of Cs+ and [BiI6]− octahedra in the (l00) SC than (00l) SC leads to the anisotropic photoelectric performance in (l00) and (00l) Cs3Bi2I9 SCs. Our results provide a strategy for the oriental growth of Cs3Bi2I9 SCs and the design of anisotropic optoelectronic devices with excellent performance.

Spin-Orbit Torque in Single-Molecule Junctions from ab Initio.

The use of electric fields applied across magnetic heterojunctions that lack spatial inversion symmetry has been previously proposed as a nonmagnetic means of controlling localized magnetic moments through spin-orbit torques (SOT). The implementation of this concept at the single-molecule level has remained a challenge, however. Here, we present first-principles calculations of SOT in a single-molecule junction under bias and beyond linear response. Employing a self-consistency scheme invoking density functional theory and nonequilibrium Green's function theory including spin-orbit interaction, we compute the change of the magnetization with the bias voltage and the associated current-induced SOT. Within the linear regime our quantitative estimates for the SOT in single-molecule junctions yield values similar to those known for magnetic interfaces. Our findings contribute to an improved microscopic understanding of SOT in single molecules.

Spontaneous and tunable valley polarization in two-dimensional single-layer LaCl2

Abstract Applying the valley contrasting properties of valleytronic materials to logical operations is the foundation of valleytronic device manufacturing. It is predicted that single-layer LaCl2 is an ferrovalley material with intrinsic and tunable valley polarization through first-principles calculations. It is a ferromagnetic semiconductor (bandgap 0.767 eV) with roughly 1.0 μB per unit cell as well as out of plane magnetization, and the Curie temperature is about 149 K. The tight-binding model considering five orbitals as well as next nearest neighboring hopping get a consistent band structure with the first-principles calculation. The valley polarization changes from 40.49 to 98.51 meV under the biaxial strain of 5% ~ -5%. Therefore, the biaxial strain can be a means to tune the valley polarization. In addition, the valley polarization of the double-layer structure (~ 80 meV) is much greater than that of the single-layer structure (~ 59 meV) due to the increased magnetic moment of the double-layer structure, indicating the potential tunable character by stacking few layers. We believe that single-layer LaCl2 has great potential for device manufacturing and application in the field of valley electronics.

Insight on the Electronic, Elastic and Thermal Properties of Au-Al Intermetallic Compounds Based on First-Principles Calculations

Insight on the Electronic, Elastic and Thermal Properties of Au-Al Intermetallic Compounds Based on First-Principles Calculations

Unexpected thermal transport properties of MgSiO3 monolayer at extreme conditions

The thermal transport properties of mantle minerals are of paramount importance to understand the thermal evolution processes of the Earth. Here, we perform extensively structural searches of two-dimensional MgSiO3 monolayer by CALYPSO method and first-principles calculations. A stable MgSiO3 monolayer with Pmm2 symmetry is uncovered, which possesses a wide indirect band gap of 4.39 eV. The calculations indicate the lattice thermal conductivities of MgSiO3 monolayer are 49.86 W (mK)−1 and 9.09 W (mK)−1 in x and y directions at room temperature. Our findings suggest that MgSiO3 monolayer is an excellent low-dimensional thermoelectric material with high ZT value of 4.58 from n-type doping in the y direction at 2000 K. The unexpected anisotropic thermal transport of MgSiO3 monolayer is due to the puckered crystal structure and the asymmetric phonon dispersion as well as the distinct electron states around the Fermi level. These results offer a detailed description of structural and thermal transport properties of MgSiO3 monolayer at extreme conditions.

Investigation for the hydrogen storage properties of XCrH3 (X=Na, K) and KYH3(Y[dbnd]Mo, W) perovskite type hydrides based on first-principles

Perovskite type hydrides have emerged as a focal point in hydrogen storage applications as potential candidates in recent years. However, the currently synthesized perovskite hydrogen storage materials suffer from high hydrogen desorption temperatures and unstable reactions. To identify promising perovskite hydrogen storage materials, this study explores the hydrogen storage properties of XCrH3 (X = Na, K) and KYH3 (YMo, W) based on first-principles (FPs) calculations. The results indicate that NaCrH3 and KCrH3 exhibit half-metallic properties, while KMoH3 and KWH3 are metallic materials with high hydrogen storage safety. Mechanical property analysis reveals that NaCrH3 exhibits the highest hardness, indicating better stability for hydrogen storage. Thermodynamic analysis shows that KCrH3 has a lower hydrogen desorption temperature than the other three compounds, with NaCrH3 having the second-lowest. The gravimetric hydrogen storage capacities of NaCrH3, KCrH3, KMoH3, and KWH3 are 3.85 wt%, 3.21 wt%, 2.19 wt%, and 1.34 wt%, respectively. Overall, NaCrH3 demonstrates superior gravimetric hydrogen storage capacity, stability, and safety, making it a promising candidate for new perovskite type hydrogen storage materials.

High-Performance Sub-10 nm Two-Dimensional SbSeBr Transistors through Transport Orientation.

Exploring two-dimensional (2D) materials with a small carrier effective mass and suitable band gap is crucial for the design of metal oxide semiconductor field effect transistors (MOSFETs). Here, the quantum transport properties of stable 2D SbSeBr are simulated on the basis of first-principles calculations. Monolayer SbSeBr proves to be a competitive channel material, offering a suitable band gap of 1.18 eV and a small electron effective mass (me*) of 0.22m0. The 2D SbSeBr field effect transistor (FET) with 8 nm channel length exhibits a high on-state current of 1869 μA/μm, low power consumption of 0.080 fJ/μm, and small delay time of 0.062 ps, which can satisfy the requirements of the International Technology Roadmap for Semiconductors for high-performance devices. Moreover, despite the monolayer SbSeBr having an isotropic me*, the asymmetrical band trends enable SbSeBr FETs to display transport orientation, which emphasizes the importance of band trends and provides valuable insights for selecting channel materials.

First-principles calculation of orbital Hall effect by Wannier interpolation: Role of orbital dependence of the anomalous position

First-principles calculation of orbital Hall effect by Wannier interpolation: Role of orbital dependence of the anomalous position

LaCl3-based sodium halide solid electrolytes with high ionic conductivity for all-solid-state batteries

To enable high performance of all solid-state batteries, a catholyte should demonstrate high ionic conductivity, good compressibility and oxidative stability. Here, a LaCl3-based Na+ superionic conductor (Na1−xZrxLa1−xCl4) with high ionic conductivity of 2.9 × 10−4 S cm−1 (30 °C), good compressibility and high oxidative potential (3.80 V vs. Na2Sn) is prepared via solid state reaction combining mechanochemical method. X-ray diffraction reveals a hexagonal structure (P63/m) of Na1−xZrxLa1−xCl4, with Na+ ions forming a one-dimensional diffusion channel along the c-axis. First-principle calculations combining with X-ray absorption fine structure characterization etc. reveal that the ionic conductivity of Na1−xZrxLa1−xCl4 is mainly determined by the size of Na+-channels and the Na+/La3+ mixing in the one-dimensional diffusion channels. When applied as a catholyte, the NaCrO2||Na0.7Zr0.3La0.7Cl4||Na3PS4||Na2Sn all-solid-state batteries demonstrate an initial capacity of 114 mA h g−1 and 88% retention after 70 cycles at 0.3 C. In addition, a high capacity of 94 mA h g−1 can be maintained at 1 C current density.