Activity and Synergy Effects on a Cu/ZnO(0001) Surface Studied Using First-Principle Thermodynamics

Using first-principle thermodynamics, we have studied surface phase diagrams of Cu substitutional ZnO­(000<u>1</u>) surfaces under industrial conditions. On the one hand, the Cu substituted on Zn sites can promote efficient formation of oxygen vacancies on the ZnO­(000<u>1</u>) surface. It can improve the activity on the Cu/ZnO­(000<u>1</u>) surface. On the other hand, metallic monolayers containing certain Cu and Zn atoms can be also formed, accompanied by the oxygen vacancies formation. We have further investigated CO₂ adsorption and reduction on these metallic monolayers. These metallic monolayers prefer to have an intermediate binding strength with the CO₂ molecule. The intermediate binding strength was expected to be optimized for subsequent CO₂ reduction. We have performed further studies and demonstrated successfully the improved catalysis for the subsequent CO₂ reduction on these metallic monolayers. The relevant mechanism can be interpreted with the second synergy effect. The d-band states of these metallic monolayers, supported on the ZnO­(000<u>1</u>) surface, are tuned to shift upward, that is, more close to Fermi level. Therefore, these metallic monolayers indeed exhibit promoted catalysis, in comparison with reported metallic surfaces in the literature

Theoretical Insights into CO₂ Activation and Reduction on the Ag(111) Monolayer Supported on a ZnO(0001) Substrate

First principles calculations are performed to investigate CO₂ adsorption and reduction on Ag(111)/ZnO­(000<u>1</u>) surfaces and interfaces. First, the pristine Ag(111) surface turns out to be quite noble for CO₂ adsorption, as its d-band states are located well below the Fermi level. The d-band states of the Ag(111) surface are subject to a tensile strain slightly shifting toward the Fermi level. However, the d-band center is still far away from the Fermi level. A critical change of the d-band states is obtained when the stretched Ag(111) monolayer is supported on the ZnO­(000<u>1</u>) substrate. The binding ability between the supported Ag(111) monolayer and CO₂ molecule is an intermediate strength. Thus, the CO₂ reduction in the subsequent hydrogenation process is optimized as well. Furthermore, we demonstrate that the stretched Ag(111) monolayer supported on the ZnO­(000<u>1</u>) substrate is indeed stable under H₂-rich conditions. This surface can even maintain the improved ability for CO₂ adsorption and reduction in the presence of Zn impurities

Gate-Controlled Donor Activation in Silicon Nanowires

Due to the proximity to an embedding medium with low dielectric constant (e.g., oxides), semiconductor nanowires have higher impurity ionization energy than their bulk counterparts, resulting lower free carrier density. Using ab initio calculations within density functional theory, we propose a way to reduce the ionization energy in nanowires by fabricating a special cross section with appropriate engineering of doping and an applied gate voltage. We demonstrate on a phosphorus-doped silicon nanowire that the ionization energy can be effectively tuned and the impurity backscattering can also be reduced. For instance, even without special engineering of doping, the free carrier density may increase by 40% in a silicon nanowire with 15 nm diameter and special cross section. Our proposal has profound implications to fabricate nanowire devices with high carrier density

Role of External Stimuli in Engineering Magnetic Phases and Real-Time Spin Dynamics of Co/Mn Oxides

Magnetism in atomically thin two-dimensional (2D) materials can be easily manipulated by alloying, functionalization, external ultrafast laser pulse, strain, electric field, etc. In this work, we have performed a series of spin-resolved density functional theory calculations on 2D magnetic hexagonal transition-metal oxide alloys, CoMnO4. We have explored different alloy patterns and found the most stable magnetic phases in each pattern, resulting in a stable ferromagnetic (FM) ground state depending upon the pattern. We have used Janus functionalization in these materials to tune the magnetic nature of the system from FM to antiferromagnetic (AFM) states. To further control the spin dynamics, we have applied an ultrafast laser pulse to the Janus systems to explore an AFM-to-FM transition process. Finally, applying strain and electric field to the Janus alloys allows us to tune the structure–property relationship in the 2D layers to obtain desirable spin arrangements

Ultrafast Chiral Precession of Spin and Orbital Angular Momentum Induced by Circularly Polarized Laser Pulse in Elementary Ferromagnets

Despite spin (SAM) and orbital (OAM) angular momentum dynamics being well-studied in demagnetization processes, their components receive less focus. Here, we utilize real-time time-dependent density functional theory (rt-TDDFT) to unveil significant x and y components of SAM and OAM induced by circularly left (σ+) and right (σ–) polarized laser pulses in ferromagnetic Fe, Co, and Ni. Our results show that the magnitude of the OAM is an order of magnitude larger than that of the SAM, highlighting a stronger optical response from the orbital degrees of freedom of electrons. Intriguingly, σ+ and σ– pulses induce chirality in the precession of SAM and OAM, respectively, with clear associations with laser frequency and duration. Finally, we demonstrate the time scale of the OAM and SAM precession occurs even earlier than that of the demagnetization process and the OISTR effect. Our results provide detailed insight into the dynamics of SAM and OAM during and shortly after a polarized laser pulse

First-Principles Study of Honeycomb Borophene on the Mo₂C Substrate

Honeycomb borophene (HB) is an important building block for diverse quantum phase observation and applications. However, freestanding HB is energetically unstable, resulting from electron deficiency. Based on a comprehensive first-principles study, we herein predict that the Mo2C monolayer can serve as an effective two-dimensional substrate to prepare planar HB. It is found that the planar HB layer is energetically favorable on the Mo2C substrate with desirable thermal and dynamical stabilities, benefiting from suitable interfacial interactions and electron transfer from Mo2C to HB. In addition, HB is found to be an effective buffer layer to decouple the electronic interactions and modify metal–semiconductor contact. These insightful results not only indicate that the Mo2C substrate is a promising alternative to synthesizing a stable borophene monolayer with pure honeycomb lattice but also provide hints for applications of HB-based materials in high-performance miniaturized electronic devices

Intrinsically Low Thermal Conductivity in the Most Lithium-Rich Binary Stannide Crystalline Li₅Sn

Using ab initio lattice dynamics and a unified heat transport theory, we compute the lattice thermal conductivity (κL) of Li5Sn, a newly synthesized crystalline material for Li-ion batteries. The weak bonding in the Li-rich environment leads to significant softening of the optical phonon modes, temperature-induced hardening, and strong anharmonicity. This complexity is captured in the particle-like and glass-like components of κL by accounting for the temperature-dependent interatomic force constants acting on the renormalized phonon frequencies and three- and four-phonon scatterings contributing to the phonon lifetime. We predict very low room-temperature κL values of 0.857, 0.599, and 0.961 W/mK for the experimental Cmcm phase and 0.996, 0.908, and 1.385 W/mK for the theoretically predicted Immm phase along the main crystallographic directions. Both phases display complex crystal behavior with glass-like transport exceeding 20% above room-temperature and an unusual κL temperature dependence. Our results can be used to inform system-level thermal models of Li-ion batteries

Nanoscale Multilayer Transition-Metal Dichalcogenide Heterostructures: Band Gap Modulation by Interfacial Strain and Spontaneous Polarization

Using density functional theory calculations, we unveil intriguing electronic properties of nanoscale multilayer transition-metal dichalcogenide (TMDC) heterostructures, (MoX₂)_<i>n</i>(MoY₂)_<i>m</i> (X, Y = S, Se or Te). Our results show that the structural stability and electronic band structure of the TMDC heterostructures depend sensitively on the choice of constituent components and their relative thickness. In particular, the electronic band gap can be tuned over a wide range by the intrinsic mismatch strain and spontaneous electrical polarization at the interface of the heterostructures, which suggests desirable design strategies for TMDC-based devices with an easily adjustable band gap. These interfacial effects also make the electronic properties more susceptible to the influence of a bias electric field, which can induce sensitive and considerable changes in the band gap and even produce a semiconductor–metal transition at relatively low electric fields. Such effective electronic band gap engineering via a combination of internal (i.e., the composition and layer thickness) and external (i.e., a bias field) control makes the TMDC-based heterostructures promising candidates for applications in a variety of nanodevices

Light-Controlled Ultrafast Magnetic State Transition in Antiferromagnetic–Ferromagnetic van der Waals Heterostructures

Manipulating spin in antiferromagnetic (AFM) materials has great potential in AFM opto-spintronics. Laser pulses can induce a transient ferromagnetic (FM) state in AFM metallic systems but have never been proven in two-dimensional (2D) AFM semiconductors and related van der Waals (vdW) heterostructures. Herein, using 2D vdW heterostructures of FM MnS2 and AFM MXenes as prototypes, we investigated optically induced interlayer spin transfer dynamics based on real-time time-dependent density functional theory. We observed that laser pulses induce significant spin injection and interfacial atom-mediated spin transfer from MnS2 to Cr2CCl2. In particular, we first demonstrated the transient FM state in semiconducting AFM–FM heterostructures during photoexcited processes. The proximity magnetism breaks the magnetic symmetry of Cr2CCl2 in heterostructures. Our results provide a microscopic understanding of optically controlled interlayer spin dynamics in 2D magnetic heterostructures and open a new way to manipulate magnetic order in 2D materials for ultrafast opto-spintronics

Tuning Magnetic Anisotropy in Two-Dimensional Metal–Semiconductor Janus van der Waals Heterostructures

In the family of 2D materials, atomically thin magnetic systems are relatively new and highly exploitable. Understanding the spin symmetry in such materials has opened a new path toward controlling the magnetic texture. In this study, we have shown that the plethora of different interface formations in the Janus or pure metal–semiconductor-based van der Waals heterostructures 1T-VXY (X, Y = S, Se, Te)–Cr2A3B3 (A, B = I, Cl, Br) allows us to explore and modify the spin–orbit and ligand–metal interactions to fine-tune magnetic anisotropy and different spin symmetries in these systems. We have utilized the interlayer interactions to modulate spin–orbit coupling (SOC) in heterolayers to regulate the magnetic anisotropy in such systems. We have compared systems with the same compositions and different interfaces, for example, Janus VSTe–Janus Cr2I3Br3 and Janus VTeS–Janus Cr2I3Br3, to show that the first one is an Ising ferromagnet, whereas the second one is an XY ferromagnet because of the SOC effect of the heavy ligand atoms