Vibrational properties and Raman spectra of pristine and fluorinated blue phosphorene

Unveiling excitons in two-dimensional β -pnictogens

In this paper, we investigate the optical, electronic, vibrational, and excitonic properties of four two-dimensional β -pnictogen materials-nitrogenene, phosphorene, arsenene, and antimonene-via density functional theory calculations and the Bethe-Salpeter equation. These materials possess indirect gaps with significant exciton binding energies, demonstrating isotropic behavior under circular light polarization and anisotropic behavior under linear polarization by absorbing light within the visible solar spectrum (except for nitrogenene). Furthermore, we observed that Raman frequencies red-shift in heavier pnictogen atoms aligning with experimental observations; simultaneously, quasi-particle effects notably influence the linear optical response intensively. These monolayers' excitonic effects lead to optical band gaps optimized for solar energy harvesting, positioning them as promising candidates for advanced optoelectronic device applications.

Angle-Dependent Raman Spectra of Crystal Polymorphs of GaO: A Computational Study

Two crystal polymorphs of GaO consisting of GaO-H and GaO-T monolayers are proposed in this study. Based on the density functional theory calculations, the phonon dispersion demonstrates that both GaO-H and GaO-T monolayers could be stable. The band gaps of GaO-H and GaO-T monolayers are 1.51 and 1.43 eV, respectively. When an external electric field is applied, the band gaps of GaO monolayers are reduced dramatically, down to 0.13 eV with the field of 0.7 V/Å. Because of the decreased symmetry of C3v under an external electric field, more peaks of Raman spectra can be obtained. The angle-dependent Raman spectra ofA ' 1 1 ${{\rm{A}}{{^\prime}}_1^1 }$ andA ' 1 2 ${{\rm{A}}{{^\prime}}_1^2 }$ of GaO-H monolayer, andA 1 g 1 ${{\rm{A}}_{1{\rm{g}}}^1 }$ andA 1 g 2 ${{\rm{A}}_{1{\rm{g}}}^2 }$ of GaO-T monolayer are discussed seperately, with the incident lasers of 488 and 532 nm. Additionally, the Raman intensity distribution shows that the incident light should be parallel to the plane of the GaO monolayer to obtain more comparable Raman spectra. These investigations of the crystal polymorphs of GaO monolayers may guide the experimental investigations of GaO monolayers and potential optoelectronic applications.

Electronic structures, transport properties, and optical absorption of bilayer blue phosphorene nanoribbons

Based on first-principles density functional theory and nonequilibrium Green's function, we study the electronic band structures, the electronic transport properties, and the optical absorption of bilayer blue phosphorene nanoribbons (BPNRs). Both bilayer armchair BPNRs (a-BPNRs) and zigzag BPNRs (z-BPNRs) behave as semiconductors in the narrow nanoribbon case and metals in the wide nanoribbon case, sharply different from their monolayer counterparts where the monolayer a-BPNRs (z-BPNRs) are always semiconducting (metallic). This indicates that interlayer couplings or the increasing layer number may induce the switching of the conductivity of the monolayer BPNRs, which is absent in graphene and phosphorene nanoribbons. Furthermore, we explore the edge states of the energy bands near Fermi energy, and find that there are almost no pure edge-state band branches in the bilayer BPNRs, which can be attributed to the interlayer couplings between the edge-states in one layer and the bulk-states in the other. Consequently, the resulting complex band structures cannot be directly analyzed any more in the framework of the two-body coupling picture just according to the simple band structures of the monolayer BPNRs. Finally, we present the current-voltage characteristics and the optical absorption of the bilayer a-BPNRs and z-BPNRs. The influences of the nanoribbon width and the interlayer couplings on the current and the anisotropic optical absorption can be understood based on the complex energy band structures. This research should be an important reference of extending the field of BPNRs from the monolayer to the bilayer case, and deepen the understanding of the difference between the monolayer and bilayer nanoribbons in different materials.

Energy Storage Mechanism of C12-3-3 with High-Capacity and High-Rate Performance for Li/Mg Batteries

The low specific capacity and Mg non-affinity of graphite limit the energy density of ion rechargeable batteries. Here, we first identify that the monolayer C12-3-3 in sp2-sp3 carbon hybridization with high Li/Mg affinity is an appropriate anode material for Li-ion batteries and Mg-ion batteries via the first-principles simulations. The monolayer C12-3-3 can achieve high specific capacities of 1181 mAh/g for Li and 739 mAh/g for Mg, higher than those of most previous anodes. The Li storage reaction is an "adsorption-conversion-intercalation mechanism", while the Mg storage reaction is an "adsorption mechanism". The 2D carbon material of C12-3-3 displays fast diffusion kinetics with low diffusion barriers of 0.41 eV for Li and 0.21 eV for Mg. As a new carbon-based anode material, the monolayer C12-3-3 will promote the practical application of batteries with high-capacity and high-rate performance.

Polarization-resolved and helicity-resolved Raman spectra of monolayer XP3 (X = Ge and In)

Monolayer XP₃ (X = Ge, In) is a theoretically predicted two-dimensional (2D) material with fascinating adsorption efficiency, foreshadowing its potential applications in the photovoltaic and optoelectronic communities. To achieve a comprehensive understanding of its optical properties and to further boost quickly identifying its specific applications, in this paper we systematically investigated the polarization-resolved and helicity-resolved Raman spectra excited by two commonly used laser lines (532 nm and 633 nm) through density functional theory. The dynamical stability of monolayer XP₃ is demonstrated by phonon dispersion. Monolayer GeP₃ and InP₃ are found to exhibit significantly different point group symmetries and thereby Raman properties due to the big difference in atomic size and electronic configurations between the Ge atom and In atom. Raman anisotropy of monolayer XP₃ has been found when the wave vector of linear polarized incident light is parallel to the monolayer, and all the anisotropic Raman active phonons are categorized in terms of the locations of two (four) maxima in polarization angle dependent Raman intensities of the parallel (perpendicular) configuration. The polarization direction averaged Raman spectra have been further discussed according to the characteristics of light absorbance. The calculations of helicity-resolved Raman spectra indicate a stronger helicity selection rule under helical excitation with the wave vector normal to the monolayer. The present work paves the way for the suitable design, characterization and exploitation of the proposed 2D material with controllable surface properties for applications in electronics and optoelectronics.

Predicting the Raman spectra of ferroelectric phases in two-dimensional Ga2O3 monolayer

We investigate the vibrational properties and Raman spectra of the two-dimensional Ga₂O₃ monolayer, using density functional theory. Two ferroelectric (FE) phases of the Ga₂O₃ monolayer with wurtzite (WZ) and zinc blende (ZB) structures (FE-WZ and FE-ZB, respectively) are considered. The Raman tensor and angle-dependent Raman intensities of two major Raman peaks (A11 and A21) in both FE-WZ (497, and 779 cm^-1) and FE-ZB (481, and 772 cm^-1) Ga₂O₃ monolayers, are calculated for the polarization of scattered light, parallel and perpendicular to that of the incident light. The characteristics of angle-dependent Raman intensities are analyzed. The average non-resonant Raman spectra of the minor peaks in FE-WZ (E¹) and FE-BZ (E¹ and E²) are compared with those of major peaks A11 and A21. These predictions of the Raman spectra of the Ga₂O₃ monolayer may guide the rational design of two-dimensional optical devices.

Theoretical assessment of Raman spectra on MXene Ti2C: from monolayer to bilayer

The structural, vibrational and Raman spectra properties of monolayer and bilayer Ti₂C excited by two commonly used laser lines (532 nm and 633 nm) are investigated by first principles calculations to establish a correlation among layer stacks and optical features for two-dimensional MXenes. The stability of the monolayer and the energetically preferable stacking configuration are demonstrated by phonon dispersion. The monolayer and bilayer Ti₂C systems are found to exhibit different point group symmetries and thereby the Raman properties due to the symmetry breaking of the bilayer structure caused by interlayer van der Waals interactions. We listed all Raman-active modes for monolayer (bilayer) Ti₂C, i.e., one (five) out-of-plane A_1g (A₁) and one (five) pair (pairs) of degenerate in-plane E_g (E) vibration modes. Polarization angle dependent Raman intensity has been discussed in terms of the locations of two (four) maxima in the parallel (perpendicular) configuration, which might be applied in experimentally identifying monolayer and bilayer Ti₂C. The difference in the polarization direction averaged Raman spectra between monolayer and bilayer Ti₂C can be explained by the characteristics of light absorbance.

Structural Features of Y2O2SO4 via DFT Calculations of Electronic and Vibrational Properties

The traditional way for determination of molecular groups structure in crystals is the X-Ray diffraction analysis and it is based on an estimation of the interatomic distances. Here, we report the analysis of structural units in Y₂O₂SO₄ using density functional theory calculations of electronic properties, lattice dynamics and experimental vibrational spectroscopy. The Y₂O₂SO₄ powder was successfully synthesized by decomposition of Y₂(SO₄)₃ at high temperature. According to the electronic band structure calculations, yttrium oxysulfate is a dielectric material. The difference between the oxygen–sulfur and oxygen–yttrium bond nature in Y₂O₂OS₄ was shown based on partial density of states calculations. Vibrational modes of sulfur ions and [Y₂O₂²⁺] chains were obtained theoretically and corresponding spectral lines observed in experimental Infrared and Raman spectra.

Simulated Raman spectra of bulk and low-dimensional phosphorus allotropes

We present a comprehensive theoretical and experimental Raman spectroscopic comparative study of bulk Phosphorus allotropes (white, black, Hittorf's, fibrous) and their monolayer equivalents, demonstrating that the application of the Placzek approximation to density functional theory calculated frequencies allows reliable and accurate reproduction of the bulk spectra at a relatively low computational cost. As well as accurate frequencies, peak intensities are also reproduced with reasonable accuracy. Having established the viability of the method we apply it to other less well characterised phosphorus forms such as isolated P4 cages and the planar blue-phosphorus phase. There are several speculative structural models in the literature for amorphous red phosphorus, and we predict Raman spectra for several of these. Via comparison with experiment this allows us to eliminate many of them such as the P2P2-zigzag chain and connected P4 models. The combination of Density functional theory (DFT) modelling, Placzek approximation for intensities with experimental Raman spectroscopy is demonstrated as a powerful combination for accurate characterisation of phosphorus species.

2D Octagon-Structure Carbon and Its Polarization Resolved Raman Spectra

We predict a new phase of two-dimensional carbon with density functional theory (DFT). It was found to be semimetal with two Dirac points. The vibrational properties and the polarization resolved Raman spectra of the carbon monolayer are predicted. There are five Raman active modes: 574 cm⁻¹ (E_g), 1112 cm⁻¹ (B_1g), 1186 cm⁻¹ (B_2g), 1605 cm⁻¹ (B_2g) and 1734 cm⁻¹ (A_1g). We consider the incident light wave vector to be perpendicular and parallel to the plane of the carbon monolayer. By calculating Raman tensor of each Raman active mode, we obtained polarization angle dependent Raman intensities. Our results will help materials scientists to identify the existence and orientation of octagon-structure carbon monolayer when they are growing it.

Raman spectra of MXenes Zr2X(X=C and N)

We investigate a polarized Raman characterization of Zr₂X (X = C, N), excited by two commonly used laser lines with wavelenghts of 532 nm and 633 nm, based on first principle calculations. The Raman spectra of Zr₂X has two Raman shift peaks which correspond to the degenerate in-plane vibration mode (E_g) and out-of-plane vibration mode (A_1g). Furthermore, we study the polarization angle dependent Raman intensity for both E_g and A_1g modes in parallel and perpendicular configurations for these two materials. We found that the polarization angle dependent Raman intensity is isotropic when the laser line is perpendicular to the Zr₂X plane. There are either only two maxima, or two maxima larger than the other maxima, in the parallel configuration when the laser line is parallel to the Zr₂X plane, which might be useful in identifying the orientation of Zr₂X in experiment. The results show that the locations of the maxima of the polarization angle dependent Raman intensity rarely depend on the exciting laser line, except that of the E_g mode of Zr₂N.

Nanoscale nonreciprocity via photon-spin-polarized stimulated Raman scattering

Time reversal symmetry stands as a fundamental restriction on the vast majority of optical systems and devices. The reciprocal nature of Maxwell's equations in linear, time-invariant media adds complexity and scale to photonic diodes, isolators, circulators and also sets fundamental efficiency limits on optical energy conversion. Though many theoretical proposals and low frequency demonstrations of nonreciprocity exist, Faraday rotation remains the only known nonreciprocal mechanism that persists down to the atomic scale. Here, we present photon-spin-polarized stimulated Raman scattering as a new nonreciprocal optical phenomenon which has, in principle, no lower size limit. Exploiting this process, we numerically demonstrate nanoscale nonreciprocal transmission of free-space beams at near-infrared frequencies with a 250 nm thick silicon metasurface as well as a fully-subwavelength plasmonic gap nanoantenna. In revealing all-optical spin-splitting, our results provide a foundation for compact nonreciprocal communication and computing technologies, from nanoscale optical isolators and full-duplex nanoantennas to topologically-protected networks.

β-As Monolayer: Vibrational Properties and Raman Spectra

Recently, semiconducting and other extraordinary properties of the monolayer of the V-group element have attracted a broad interest and attention. The success of experimentally growing antimonene and black phosphorus makes the arsenic monolayer a reasonable candidate for two-dimensional semiconductors. By using DFT calculation, we investigate the vibrational properties and Raman spectra of the buckled honeycomb monolayer of arsenic (β-As) for four commonly used laser lines. By calculating Raman tensor of each active modes of the β-As monolayer, we obtained polarization angle-dependent Raman intensities when the wave vector of incident light is parallel and perpendicular with the plane of the β-As monolayer. We found that the nonresonant Raman spectra have two peaks at 235 and 305 cm^-1 that correspond to the in-plane vibrating mode E_g and out-of-plane vibrating mode A_1g, which is similar to germanene, blue phosphorene, and β-Sb monolayer Raman spectra. There are two (four) minima and two (four) maxima when the polarization direction of scattered light is parallel (perpendicular) to that of the incident light and the wave vector of the incident light is parallel to the β-As monolayer. The Raman intensities of neither parallel polarization configuration nor perpendicular polarization configuration depend on the polarization direction when the wave vector of incident light is perpendicular to the β-As monolayer. The relation between shapes of the polar plots and relative values of Raman tensor elements is found. The Raman intensities decrease with increasing wavelength of incident laser lines in most cases. Our results will help experimentalists to identify the existence and the orientation of the β-As monolayer while they are growing the β-As monolayer.

Using van der Waals heterostructures based on two-dimensional blue phosphorus and XC (X = Ge, Si) for water-splitting photocatalysis: a first-principles study

BlueP/SiC and BlueP/GeC vdW heterostructures are high-efficiency photocatalysts for water-splitting at pH 0 and 7, respectively.