Theoretical study of the nonlinear magnon-phonon coupling in CoF2.

The coupling and interplay between magnon and phonon are important topics for spintronics and magnonics. In this work we studied the nonlinear magnon-phonon coupling in CoF2. First-principles calculations demonstrate that the antiferromagnetic resonance magnon drives a phonon with B1g character; the oscillating driving force has a frequency twice of that of the magnon. Comparing with similar materials indicates a strong correlation between the strength of nonlinear magnon-phonon coupling and the orbital magnetic moment of the magnetic ion. This work pave the way for theoretical study of nonlinear magnon-phonon coupling. .

Interlayer coupling modulated for thermoelectric transport characterization of black arsenic nanoscale devices.

Modulating interlayer coupling modes can effectively enhance the thermoelectric properties of nanomaterials or nanoscale devices. By using density functional theory combined with non-equilibrium Green's function method, we investigate the thermoelectric properties of zigzag-type black arsenic nanoscale devices with varying interlayer coupling modes. Our results show that altering the interlayer coupling mode significantly modulates the thermoelectric properties of the system. Specifically, we consider four coupling modes with different strengths, by modulating different interlayer overlap patterns. Notably, in the weaker interlayer coupling mode, the system exhibits enhanced thermoelectric properties due to increased interface phonon scattering, for example, the M4reaching a peak value of 2.23 atµ= -0.73 eV. Furthermore, we explore the temperature-dependent behavior of each coupling model. The results suggest that the thermoelectric characteristics are more sensitive to temperature variations in the weaker coupling modes. These insights provide valuable guidance for enhancing the thermoelectric performance of nanoscale devices through precise interlayer coupling modulation.

Iodide Vacancy Defects Clustering in Pairs Rather Than in Isolation in a Lead Iodide Perovskite: Identification, Origin, and Implications.

Iodide (I-) vacancy defects are strongly related to the stability of perovskite optoelectronic devices. The I- vacancy in lead iodide perovskites is normally considered to exist in the form of a single isolated defect. However, we determined that the I- vacancies cluster in pairs in specific ways in the typical perovskite of tetragonal CsPbI3. This I- vacancy-vacancy dimer is energetically more favorable than two isolated I- monovacancies. It breaks the symmetry of the Pb-I octahedron, resulting in lattice distortion. Its origin lies in the special lattice distortion effect caused by the electron orbital interaction of the perovskite material. Furthermore, the I- vacancy-vacancy dimer and the associated lattice distortion increase the carrier lifetime by 1.3 times compared to that of the system with two isolated I- monovacancies, but they also compromise its structural stability. This new insight into the I- vacancy defect will enhance our understanding of perovskite optoelectronic devices.

Effect of vacancy defects on anisotropic electronic transport behaviors of CoN4C2based 2D devices: a first-principles study.

The strong anisotropic electronic transport properties of the single-atom-thick material CoN4C2monolayer hold immense importance for the advancement of the electronics industry. Using density functional theory combined with non-equilibrium Green's function systematically studied the electronic structural properties and anisotropic electronic transport properties of the CoN4C2monolayer. The results show that Co, N, and C single-atom vacancy defects do not change the electronic properties of the CoN4C2monolayer, which remains metallic. The pristine device and the devices composed of Co, N single-atom vacancy defects exhibit stronger electronic transport along the armchair direction than the zigzag direction, which exhibit strong anisotropy, and a negative differential resistance (NDR) effect can be observed. In contrast to the results mentioned above, the device with C single-atom vacancy defects only exhibits the NDR effect. Among them, the device with the N single-atom vacancy defect regime exhibits the strongest anisotropy, with anIZ/IAof up to 7.95. Moreover, based on the strongest anisotropy exhibited by N single-atom vacancy defects, we further studied the influence of different sites of the N-atom vacancy on the electronic transport properties of the devices. The results indicate that N-1, N-2, N-3, N-12, N-23, N-123, N-1234, and N-12345 model devices did not change the high anisotropy and NDR effect of the device, and among them the N-1234 exhibits the strongest anisotropy, theIZ/IAreaches 6.12. A significant NDR effect is also observed for the electronic transport along the armchair direction in these devices. However, the current gradually decreases as an increase of the number of N defects. These findings showcase the considerable potential for integration of the CoN4C2monolayer in switching devices and NDR-based multifunctional nanodevices.

Theoretical Study on the Raman Effect Due to Magnons in Two-Dimensional Magnets.

Raman spectroscopy is one of the most useful experimental tools for studying elementary excitations in two-dimensional (2D) materials. The Raman scattering due to phonons was widely employed for detecting structural evolutions, especially those caused by magnetic phase transitions in 2D magnets. A first-principles theory of the Raman scattering effect caused by magnons is still lacking. We theoretically study the magnon Raman effect in 2D magnet CrI3. We propose a first-principles method and have calculated the intensity of circularly polarized Raman signals due to different magnon modes in the CrI3 monolayer and bilayers. The calculated Raman intensities due to magnons in the CrI3 monolayer and the rhombohedral bilayer are consistent with the selection rule deduced from the magnon pseudoangular moment and the parity of magnon modes. We also find that the selection rule is violated in the symmetry-broken monoclinic bilayer due to interlayer coupling.

First-principles predictions of enhanced thermoelectric properties for Cs2SnI2Cl2and Cs2PbI2Cl2monolayers with spin-orbit coupling.

Inspired by the exceptional charge transport properties and ultra-low thermal conductivity of halide perovskite, we investigate the electronic nature, thermal transport, and thermoelectric properties for Ruddlesden-Popper all-inorganic perovskite, Cs2SnI2Cl2and Cs2PbI2Cl2monolayers, using first-principles calculations. During the calculations, spin-orbit coupling has been considered for electronic transport as well as thermoelectric properties. The results show that the Cs2SnI2Cl2and Cs2PbI2Cl2monolayers exhibit high carrier mobility and low thermal conductivity. Stronger phonon-phonon interaction is responsible for the fact that thermal conductivity of Cs2SnI2Cl2monolayer is much lower than that of Cs2PbI2Cl2monolayer. At 700 K, the values of the figure of merit (ZT) for the n-type doped Cs2SnI2Cl2and Cs2PbI2Cl2monolayers are about 1.05 and 0.32 at the optimized carrier concentrations 5.42 × 1012cm-2and 9.84 × 1012cm-2. Moreover, when spin-orbit coupling is considered, the correspondingZTvalues are enhanced to 2.73 and 1.98 at 5.27 × 1011cm-2and 6.16 × 1011cm-2. These results signify that Cs2SnI2Cl2and Cs2PbI2Cl2monolayers are promising thermoelectric candidates.

Spin-resolved transport of multifunctional C18molecule-based nanodevices: a first-principles study.

As is well known, Kasieret alfirst synthesized a cyclic molecule C18, as characterized by high-resolution atomic force microscopy, is a polyalkylene structure in which the 18 carbon atoms are linked by alternating single and triple bonds Kaiseret al(2019Science3651299-301). Early studies have found that the C18molecule has semiconducting properties, suggesting that a similar straight-chain carbon structure could become a molecular device. Inspired by this, an analysis of spin-resolved electronic transport of nanodevices made by C18 sandwiched between zigzag graphyne nanoribbon leads or zigzag graphene nanoribbon leads presents here. The computational results demonstrate that a good spin-filtering effect, spin rectifying effect and an obvious negative differential resistance behavior in designed model devices can be obtained. Moreover, a stable dual-spin filtering effect or diode effect can be occurred in considered model devices with leads in an antiparallel state. The intrinsic mechanisms of molecular nanodevices are explained in detail by analyzing the transmission spectrum under different bias voltage, local density of states, molecular projection Hamiltonian, Current-Voltage (I-V) characteristics, transmission pathways,et al. These results are particularly significant for the development of multifunctional spintronic nanodevices.

Mechanisms of adsorption and diffusion of Na on a VSe2 monolayer with engineering-induced vacancies.

Although research on vacancy engineering of anode materials has sufficiently advanced to obtain heightened battery capacity, the effect on the diffusion barrier underlying the mechanism remains to be elucidated. Herein, we investigated the effect of vacancy engineering on Na adsorption and diffusion on a vanadium diselenide (VSe2) monolayer using first-principles calculations to reveal the underlying physics behind the performance optimization of anode materials in a sodium-ion battery. The results demonstrate that the structure of the substrate is responsible for the difference between the adsorption energy and diffusion barrier that resulted from cation and anion vacancies. As there is an absent Se atom (VSe) on the surface layer of the substrate, diffusion of Na on the surface could become pressurized with a high diffusion barrier up to 0.33 eV and a high adsorption energy (-1.92 eV) to capture additional Na atoms. However, because the V layer is sandwiched between two Se layers, there is less interaction with Na, and the adsorption energy and diffusion barrier are -1.58 and 0.13 eV, respectively, when a V atom is nonexistent (VV). Moreover, the defective VSe2 increased the battery capacity, with little impact on open-circuit voltage. In this work, we analyzed the effect of vacancy engineering on VSe2 monolayer material, which provides theoretical clues for the design of efficient sodium-ion batteries with heightened capacity.

Interfacial thermodynamics-inspired electrolyte strategy to regulate output voltage and energy density of battery chemistry.

The electrochemical behaviors of battery chemistry, especially the operating voltage, are greatly affected by the complex electrode/electrolyte interface, but the corresponding basis understanding is still largely unclear. Herein, the concept of regulating electrode potential by interface thermodynamics is proposed, which guides the improvement of the energy density of Zn-MnO2 battery. A cationic electrolyte strategy is adopted to adjust the charge density of electrical double layer, as well as entropy change caused by desolvation, thus, achieving an output voltage of 1.6 V (vs. Zn2+/Zn) and a capacity of 400 mAh g-1. The detailed energy storage behaviors are also analyzed in terms of crystal field and energy level splitting. Furthermore, the electrolyte optimization benefits the efficient operation of Zn-MnO2 battery by enabling a high energy density of 532 Wh kg-1 based on the mass of cathode and a long cyclic life of more than 500 cycles. This work provides a path for designing high-energy-density aqueous battery via electrolyte strategy, which is expected to be extended to other battery systems.

Multifunctional 2D g-C4N3/MoS2 vdW Heterostructure-Based Nanodevices: Spin Filtering and Gas Sensing Properties.

Two-dimensional (2D) magnetic materials are the key to the development of the new generation in spintronics technology and engineering multifunctional devices. Herein, the electronic, spin-resolved transmission, and gas sensing properties of the 2D g-C4N3/MoS2 van der Waals (vdW) heterostructure have been investigated by using density functional theory with non-equilibrium Green's function method. First, the g-C4N3/MoS2 vdW heterostructure demonstrates ferromagnetic half-metallicity and superior adsorption capacity for gas molecules. The spin-dependent electronic transport of the g-C4N3/MoS2-based nanodevice is obviously regulated by parallel or anti-parallel spin configuration in electrodes, leading to perfect single-spin conduction behavior with a nearly 100% spin filtering efficiency, a negative differential resistance effect, and other interesting electrical transport phenomena. Moreover, g-C4N3/MoS2 exhibits directional dependency and strong transport anisotropic behavior under bias windows, indicating that the electric current propagates more easily through the vertical direction than the horizontal direction. The physical mechanisms are revealed and analyzed by presenting the bias-dependent transmission spectra in combination with the projected local device density of states. Finally, the g-C4N3/MoS2-based gas sensor is more sensitive to CO, NO, NO2, and NH3 molecules with the chemisorption type. The strong chemical adsorption leads to the formation of electrons on the local scattering center and ultimately affects the transport properties, resulting in the maximum gas sensitivity reaching 6.45 for NO at the bias of 0.8 V. This work not only reveals that the g-C4N3/MoS2 vdW heterostructure with high anisotropy, perfect spin filtering, and outstanding gas sensitivity is a promising 2D material but also provides an insight into the further application in futuristic electronic nanodevices.

Restricting the Formation of Pb-Pb Dimer via Surface Pb Site Passivation for Enhancing the Light Stability of Perovskite.

Poor light stability hinders the potential applications of perovskite optoelectronic devices. Recent experiments have demonstrated that the passivation surface via forming strong chemical bonds (SO4 -Pb, PO4 -Pb, Cl-Pb, O-Pb, and S-Pb) could effectively improve the light stability of perovskite solar cells. However, the underlying reasons are not clear. Herein, the elusive underlying mechanisms of light stability enhancement are explained in detail using first principles calculations. The small polaron model and self-trapped exciton model demonstrate that an iodine vacancy defect on the surface of perovskite could trap a free electron under light illumination, which leads to a significant rearrangement of the Pb-I lattice and creats a new chemical species, i.e., a Pb-Pb dimer bound in the typical perovskite of CH3 NH3 PbI3 . The Pb-Pb dimer distorts the Pb-I octahedral lattice and reduces the defect formation energy of the I atoms. The surface Pb site passivation can prevent the formation of the Pb-Pb dimer, thereby improving the light stability. In addition, the strong ionic bond could better stabilize the Pb site. The in-depth understanding of the light stability and the passivation mechanism in this study can promote the application of perovskite optoelectronic devices.

Effects of conjugated structure on electronic and transport properties in organic-inorganic hybrid superlattices Cd2Se2(C2H4N2)1/2.

By inducingπ-conjugated organic molecule C2H4N2in group II-VI based CdSe network structure materials, the band structures and carrier transport of organic-inorganic hybrid superlattices Cd2Se2(C2H4N2)1/2were investigated via first-principles calculations based on the density functional theory. With different stacking patterns, it is found that the carrier mobility can be modulated by 5-6 orders of magnitude. The physical mechanism of the high carrier mobility in the hybrid structures has been revealed, which means dipole organic layers realize electron delocalization via electrostatic potential difference and build-in electric field. Our calculations shown that the dipole organic layers originate from asymmetricπ-conjugated organic molecules and the charges movement between molecules, while symmetric organic molecules tend to electrostatic balance. And although the electronic transport properties were highly restrained by the flat bands of organic layers around Fermi energy in most structures, we found that the collective electrostatic effect can lead to very high electron mobility in AA1 and AA2 stacking systems, which might be attributed to the superposition of molecule electrostatic potential along with electrons transfer between molecules. Furthermore, it is also found that the anisotropy of electron mobility can be modulated via the difference directions of dipole layers.

Giant two-photon absorption in MXene quantum dots.

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb2C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb2C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb2C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb2C changes to saturable absorption when the incident wavelength is between 400-800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb2C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb2C via morphology or surface termination.

Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes.

The conventional P2-type cathode material Na0.67Ni0.33Mn0.67O2 suffers from an irreversible P2-O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na0.67Ni0.33Mn0.67O2. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg2+ and Ca2+ make high-voltage reactions (oxygen redox and Ni4+/Ni3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg2+ and Ca2+ ions, the adverse effects on high-voltage reactions involving Ni-O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2 exhibits a satisfactory initial energy density of 468.2 W h kg-1 and good capacity retention of 83% after 100 cycles at 50 mA g-1 within the voltage range of 2.2-4.35 V. This work deepens our understanding of the specific effects of Mg2+ and Ca2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

Strain modulation on the spin transport properties of PTB junctions with MoC2 electrodes.

Based on MoC2 nanoribbons and poly-(terphenylene-butadiynylene) (PTB) molecules, we designed MoC2-PTB molecular spintronic devices and investigated their spin-dependent electron transport properties by using spin-polarized density functional theory and the non-equilibrium Green's function method. As a typical MXene material, it is found that the magnetic contribution of MoC2 nanoribbons mainly comes from the delocalized 3d electron of edge Mo atoms. Owing to the obvious spin-splitting near the Fermi level of the MoC2 nanoribbon electrode, the spin states can be effectively injected into the central scattering region under an external bias voltage. In addition, we also studied the effects of z-axis strain on the spin transport properties of the PTB molecular device, where the strain was controlled within the range of -9% to 9%. Under a compressed strain, spin current increases obviously, and the spin-filtering efficiency (SFE) decreases slightly. Nevertheless, under a tensile strain, we found that the SFE increases but spin current decreases. Moreover, z-axis strain can induce a negative differential resistance (NDR) effect at a high bias point. This work would expand the potential applications of new two-dimensional (2D) materials in the field of molecular spintronic devices.

The magnetic proximity effect at the MoS2/CrI3interface.

The vicinity to a two-dimensional magnetic material provides a simple and effective way to break the valley degeneracy of transition-metal dichalcogenides because of the magnetic proximity effect. Based on first-principles calculations, we study the band structure of a MoS2/CrI3van der Waals heterostructure and its manipulation by vertical electric fields. A huge valley splitting of about 19.60 meV, equivalent to an external magnetic fields of about 89.0 T can be generated by an electric field of 0.115 V Å-1. The electric field causes discontinuous changes in the valley splitting. The electric field drives the bands of MoS2across those of CrI3. At the critical electric fields, the interlayer orbital hybridization leads to the energy level repulsion and an abrupt exchange of the band index. We also study the effect of interlayer distance on the valley splitting and observe a more significant electric field modulation. This work deepens our understanding on the interfacial magnetic proximity effect as a result of the orbital hybridization across the van der Waals gap.

Effects of pressure and strain on physical properties of VI3.

The van der Waals ferromagnetic material VI3is a magnetic Mott insulator. In this work, we investigate the effects of isotropic and anisotropic pressure on the atomic structure and the electronic structure of VI3using the first-principles method. The in-plane strain induces structural distortion and breaks the three-fold rotational symmetry of the lattice. Both the in-plane and out-of-plane strain widen the conduction and the valence bands, reduce the energy band gap and drive VI3from a semiconductor to a three-dimensional metal. The structural distortion is not the cause of insulator-to-metal transition. Calculations of the magnetocrystalline anisotropy energy indicate an easy-axis to easy-plane transition when the pressure is higher than 2 GPa. The ferromagnetic Curie temperature falls from 63 K at 0 GPa to 25 K at 6 GPa.

The nonlinear optical transition bleaching in tellurene.

To date, outstanding linear and nonlinear optical properties of tellurene, caused by multiple two-dimensional (2D) phases and optical anisotropy, have attracted considerable interest for potential nanophotonics applications. In this work, the ultrafast nonlinear optical (NLO) properties of α-tellurene have been studied via Z-scan and pump-probe techniques at a broadband spectral region. Typical saturable absorption and band filling effects are observed in tellurene due to the Pauli exclusion principle. Analysis using density functional theory (DFT) computation shows the enhancements in NLO response within the ultraviolet-visible absorption spectral region are owing to the increased optical intraband transition in tellurene. Moreover, the effects of varying the photon energy of the probe pulse were explored. Our results indicated that probe pulses with higher photon energies can make smaller differential transmission signal, this effect is found to be negatively correlated with calculated joint density of states (JDOS). These results offer insights into the intrinsic photophysics of 2D tellurene, driving its applications in photonic and optoelectronic fields.

In Situ Defect Induction in Close-Packed Lattice Plane for the Efficient Zinc Ion Storage.

In situ electrochemical activation brings unexpected electrochemical performance improvements to electrode materials, but the mechanism behind it still needs further study. Herein, an electrochemically in situ defect induction in close-packed lattice plane of vanadium nitride oxide (VNx Oy ) in aqueous zinc-ion battery is reported. It is verified by theoretical calculation and experiment that the original compact structure is not suitable for the insert of Zn2+ ion, while a highly active one after the initial electrochemical activization accompanied by the in situ defect induction in close-packed lattice plane of VNx Oy exhibits efficient zinc ion storage. As expected, activated VNx Oy can achieve very high reversible capacity of 231.4 mA h g-1 at 1 A g-1 and cycle stability upto 6000 cycles at 10 A g-1 with a capacity retention of 94.3%. This work proposes a new insight for understanding the electrochemically in situ transformation to obtain highly active cathode materials for the aqueous zinc-ion batteries.

First-principles investigations on a two-dimensional S3N2/black phosphorene van der Waals heterostructure: mechanical, carrier transport and thermoelectric anisotropy.

Isolated monolayer two-dimensional (2D) materials have attracted great attentions due to their unique optical, electrical, mechanical, thermoelectric properties and potential applications in nanoelectronic, optoelectronic and thermoelectric devices. However, it more and more difficult to find high performance and multifunctional monolayer 2D materials. The 2D van der Waals (vdW) heterostructure, which holds two different 2D materials together by vdW interactions, has opened up a new horizon in modulation of the energy band structure, the anisotropy of electrons and phonons, and the improvement of their thermoelectric properties for monolayer 2D materials. In this work, we theoretically investigated the anisotropy in the physical properties of 2D vdW heterostructure comprising of monolayer S3N2and black phosphorene (BP) using first-principles method. It is demonstrated that the AB1stacking is the most stable dynamic and thermodynamics in the S3N2/BP heterostructure with vdW interaction between layers. The Young's modulus and Poisson's ratio of AB1stacking along thexdirection are 3 times of those along theydirection. Based on the Boltzmann transport theory within the relaxation time approximation, we demonstrated that AB1stacking of S3N2/BP vdW heterostructure has significant anisotropy in the electron and phonon transport. Due to larger anharmonicity results in larger three-phonon scattering rates, the thermal conductivity of AB1stacking of this heterostructure is half that of the pristine monolayer BP. We find the one withn-type (p-type) doping exhibits a peak figure of merit (ZT) value of 1.78 (0.52) at 300 K alongxdirection, while those peak ZT value of 2.04 (0.69) alongydirection, exceeding the highest value of the monolayer BP doped withn-type orp-type doping. Our results would pave a way for applications to flexible and thermoelectric 2D materials.