Vibrational Band Structure Research Articles

First-Principles Calculations on Electronic, Optical, and Phonon Properties of γ-Bi2MoO6.

The wide band gap γ-Bi2MoO6 (BMO) has tremendous potential in emergent solar harvesting applications. Here we present a combined experimental-first-principles density functional theory (DFT) approach to probe physical properties relevant to the light sensitivity of BMO like dynamic and structural stability, Raman and infrared absorption modes, value and nature of band gap (i.e., direct or indirect), dielectric constant, and optical absorption, etc. We solvothermally synthesized wide band gap Pca21 phase pure BMO (≳3 eV) for two different pH values of 7 and 9. The structural parameters were correlated with the stability of BMO derived from elastic tensor simulations. The desired dynamical stability at T = 0 K was established from the phonon vibrational band structure using a finite difference-based supercell approach. The DFT-based Raman modes and phonon density of states (DOS) reliably reproduced the experimental Raman and infrared absorption. The electronic DOS calculated from Heyd-Scuseria-Ernzerhof HSE06 with van der Waals (vdW) and relativistic spin-orbit coupling (SOC) corrections produced a good agreement with the band gap obtained from diffuse reflectance spectroscopy (DRS). The optical absorption obtained from the complex dielectric constant for the HSE06+SOC+vdW potential closely resembled the DRS-derived absorption of BMO. The BMO shows ∼43% photocatalytic efficiency in degrading methylene blue dye under 75 min optical illumination. This combined DFT-experimental approach may provide a better understanding of the properties of BMO relevant to solar harvesting applications.

Moiré Pattern Controlled Phonon Polarizer Based on Twisted Graphene.

Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies. Here, thefirst experimental study is conducted on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements,structural characterizations, and atomistic modeling, reveal a modulation up to 631% in thermal conductance with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode-dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high-frequency thermal modes while having negligible effects on low-frequency acoustic modes near Brillouin zone center. This study offers crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra.

Synthesis and Broadband Photodetection of a P-Type 1D Van der Waals Semiconductor HfSnS3.

1Dvan der Waals (vdW) materials have attracted significant interest in recent years due to their giant anisotropic and weak interlayer-coupled characters. More 1D vdW materials are urgently to be exploited for satisfying the practice requirement. Herein, the study of 1D vdW ternary HfSnS3 high-quality single crystals grown via the chemical vapor transport technique is reported. The Raman vibration modes and band structure of HfSnS3 are analyzed via DFT calculations. Its strong in-plane anisotropic is verified by the polarized Raman spectroscopy. The field-effect transistors (FETs) based on the HfSnS3 nanowires demonstrate p-type semiconducting behavior as well as outstanding photoresponse in a broadband range from UV to near-infrared (NIR) with short response times of ≈0.355ms, high responsivity of ≈11.5AW-1 , detectivity of ≈8.2×1011 , external quantum efficiency of 2739%, excellent environmental stability, and repeatability. Furthermore, a typical photoconductivity effect of the photodetector is illustrated. These comprehensive characteristics can promote the application of the p-type 1D vdW material HfSnS3 in optoelectronics.

Sb2S3 thin films: From first principles to in situ crystalline thin film growth by ultrasonic spray pyrolysis

In this paper, we report deposition of in situ crystallized photoconductive Sb2S3 thin films by direct ultrasonic spray pyrolysis using a non-aqueous precursor solution containing antimony chloride and thioacetamide, free from any complexing agent. Further, the First principles calculations of both vibrational and electronic band structures were done in the framework of Density Functional Theory. Experimentally, a systematic study on the thin film growth by varying substrate temperature (170–290 °C) and relative concentration of Sb/S (0.1–0.4) were evaluated. X-ray diffraction and Raman spectral analysis showed the formation of polycrystalline Sb2S3 films at 200 °C. These thin films were subjected to post deposition treatments of vacuum annealing and rapid thermal processing (RTP) to evaluate changes in their optoelectronic properties. Both as prepared and post thermal treated samples were with uniform surface coverage and compact morphologies. Also, the films were photoconductive with a direct band gap of 1.6 eV. The experimental results were in correlation with our theoretical calculations on the complete Raman spectra. This work offers a direct one step ultrasonic spray pyrolysis process to obtain polycrystalline photoconductive Sb2S3 thin films for various optoelectronic applications.

Analysis of flexural and torsional vibration band gaps in phononic crystal beam

Phononic crystals (PCs) play an important role in the reduction of vibration within a targeted frequency range. Most research of the one-dimensional PCs focuses on the properties or applications of the flexural band gaps; however, practically, the broadband environmental excitations also induce the torsional vibration. Hence, this paper studies a two-degrees-of-freedom local resonance (2DOF-LR) phononic crystal, which can both suppress the propagation of flexural and torsion waves in the same frequency range. To improve the computational efficiency, the Timoshenko theory with a modified transfer matrix (MTM) approach is used for flexural vibration band structure analysis. Through the finite element method (FEM), the vibration attenuation characteristics of the structure within the band gaps are verified. A comprehensive study is conducted to investigate the effects of geometric and material parameters on the attenuation characteristics of the phononic crystal. Finally, a prototype of the proposed phononic crystal is fabricated for the vibration experiment. It is worth noting that a novel experimental setup using the lever principle is designed in this paper to verify the torsional band gap. The frequency ranges of the experimental vibration attenuation match well with the numerical results.

Vacuum-ultraviolet absorption cross-sections of functionalized four-carbon species

Absorption cross-sections were measured in the vacuum ultraviolet from 5.17 – 9.92 eV using differential absorption spectroscopy for 33 four-carbon species: n-butane, trans-2-butene, cis-2-butene, butanal, butyric acid, ethyloxirane, trans-2,3-dimethyloxirane, cis-2,3-dimethyloxirane, 2-methyloxetane, 2,2′-bioxirane, vinyl oxirane, 3,4-epoxybutan-2-one, diacetyl, diethyl ether, ethyl vinyl ether, vinyl acetate, acetic anhydride, 1-butanol, 3-buten-1-ol, 1‑hydroxy-butan-2-one, 1‑hydroxy-butan-3-one, 2,3-epoxybutan-1-ol, 3,4-epoxybutan-1-ol, 2-oxetanemethanol, butanone, methyl vinyl ketone, 4H-1,3-dioxine, allyl formate, 2-oxobutanal, 4-hydroxybutanal, 2-methyloxetan-3-one, cis‑but-2-en-1-ol, and trans-2‑but-enal. Uncertainties were quantified in all cases by accounting for errors in gas-phase concentration, experimental repeatability, and signal-to-noise ratio as a function of photon energy. With the exception of 2-oxobutanal, which is reported with an uncertainty of 10%, convolving the sources of error using the root-sum-square method led to an upper limit of 5% uncertainty above the detection limit, which is largely attributable to chemical purity.The primary objective of the present work is to provide absolute cross-sections along with quantified uncertainty limits. The majority of the absorption spectra, which reflect electronic transitions such as σ → σ* and n → σ*, are reported for the first time and provide insight into fundamental chemical physics, such as vibrational band structure and Rydberg transitions. The quantitative spectra in the present work facilitates the discovery of chemical intermediates that support improvement in the accuracy of computational models for low-temperature combustion and atmospheric chemistry.

Band gap enhancement and vibration reduction of functionally graded sandwich metastructure beam

A novel functionally graded (FG) sandwich metastructure beam is proposed for broadband vibration reduction based on the band gaps of Bragg scattering and local resonance. The coupling band gap performance of FG sandwich metastructure beam is used to improve vibration reduction in the low and medium frequency ranges. An analytical model for vibration transmission and band structure of the FG sandwich metastructure beam is developed. The calculation accuracy of present analytical model is verified by using finite element method (FEM). Theoretical results demonstrate that the hybrid band gap in the low and medium frequency ranges will be generated by coupling the Bragg scattering and locally resonant band gaps, and the bandwidth and location of the hybrid band gap can be enhanced and tuned by adjusting the MFC piezoelectric shunting circuit parameters. It also shows that the starting and stopping frequencies of the hybrid band gap can be flexibly designed by the height ratio and constituent gradient of FG sandwich beam. The results in the study provide a new approach to the design of broadband vibration reduction for the composite sandwich structures.

Phase space crystal vibrations: Chiral edge states with preserved time-reversal symmetry

Phase-space crystal is a solidlike crystalline state in phase space formed by atoms, subject to a time-periodic drive. The authors show that interactions between these atoms can give rise to vibrational band structures with nontrivial Chern numbers and chiral edge states in phase space. Remarkably, these arise even in the presence of time-reversal symmetry in contrast to all topological transport scenarios in real space. Phase-space crystal vibrations offer a new platform for phononic topological transport whereby chiral properties arise naturally.

Molecular docking, experimental FT-IR spectra, UV–Vis spectra, vibrational analysis, electronic properties, Fukui function analysis of a potential bioactive agent – Proflavine

Proflavine, having the molecular formula C13H11N3, is a well-known urinary antiseptic and anticancer medication (3,6-diaminoacridine). In this communication, Quantum chemical computations of Proflavine's geometry have been performed and examined in the ground state. The optimized structure and wavenumbers of the molecule's vibrational bands were investigated using the DFT/B3LYP method and 6–311G (d, p) as the basis set. The calculated vibrational frequencies are compared to experimental IR spectra. The link between thermodynamic characteristics and temperature has been studied. The computed IR frequencies correlate well with the experiments, as indicated by the correlation factor (R2 = 0.99). The UV spectra of the title molecule are calculated by using Time Dependent Density Functional Theory (TD-DFT). The molecule's interactions with other species were described using an analysis of a HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital). Natural bond orbitals (NBO) analysis was used to investigate intramolecular and intermolecular hydrogen bonding and their second-order stabilization energies and conjugative and hyperconjugative interactions. By computing the first hyperpolarizability, nonlinear optical (NLO) analysis was utilized to explore the molecule's nonlinear optical properties.

From vibrational spectroscopy and quantum tunnelling to periodic band structures - a self-supervised, all-purpose neural network approach to general quantum problems.

In this work, a feed-forward artificial neural network (FF-ANN) design capable of locating eigensolutions to Schrödinger's equation via self-supervised learning is outlined. Based on the input potential determining the nature of the quantum problem, the presented FF-ANN strategy identifies valid solutions solely by minimizing Schrödinger's equation encoded in a suitably designed global loss function. In addition to benchmark calculations of prototype systems with known analytical solutions, the outlined methodology was also applied to experimentally accessible quantum systems, such as the vibrational states of molecular hydrogen H2 and its isotopologues HD and D2 as well as the torsional tunnel splitting in the phenol molecule. It is shown that in conjunction with the use of SIREN activation functions a high accuracy in the energy eigenvalues and wavefunctions is achieved without the requirement to adjust the implementation to the vastly different range of input potentials, thereby even considering problems under periodic boundary conditions.

Synthesis and Enhanced Light Photocatalytic Activity of Modulating Band BiOBrXI1-X Nanosheets.

The photocatalysis technique has been proven to be a promising method to solve environmental pollution in situations of energy shortage, and has been intensively investigated in the field of pollutant degradation. In this work, a band structure-controlled solid solution of BiOBrXI1−X (x = 0.00, 0.05, 0.10, 0.15, 0.20, 1.00) with highly efficient light-driven photocatalytic activities was successfully synthesized via simple solvothermal methods. The phase composition, crystal structure, morphology, internal molecular vibration, optical properties, and energy band structure were characterized and analyzed by XRD, SEM, HRTEM, XPS, Raman, and UV Vis DRS. To evaluate the photocatalytic activity of BiOBrXI1−X, rhodamine B was selected as an organic pollutant. In particular, BiOBr0.15I0.85 displayed significantly enhanced photocatalytic activity by virtue of modulating the energy band position, optimizing redox potentials, and accelerating carrier separation. Moreover, the enhancement mechanism was elucidated on the basis of band structure engineering, which provides ideas for the design of highly active photocatalysts for practical application in the fields of environmental issues and energy conservation.

Vibrational band-structures caused by internal rotations of the boron Wankel rotor B11.

Nuclear quantum effects are often neglected for systems without hydrogen atoms. However some planar boron rotors turn out to exhibit remarkable nuclear quantum effects. Recent experiment on infrared spectroscopy of B13+ shows unexpected spectral broadening which still awaits physical explanation. Here we present quantitative investigations of the vibrational energy levels of B11− up to full dimension. A harmonic-bath averaged Hamiltonian suitable for planar boron rotors is constructed and used to predict typical types of vibrational states of B11−. Band structures caused by internal rotations are found for all the investigated vibrational states. The experimental phenomenon of spectral broadening is thus due to the band structures of the corresponding vibrational levels. The detailed information of the relevant vibrational states reported in the present work may provide valuable references for future investigations of high resolution spectroscopy of B11−.

Vacuum-ultraviolet absorption cross-sections of functionalized cyclic hydrocarbons: Six-membered rings

Absorption cross-sections were measured in the vacuum ultraviolet (9.92–5.17 eV) using differential absorption spectroscopy for a series of six-membered hydrocarbon rings: cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, tetrahydropyran, 3,4-dihydro-2H-pyran, 1,4-dioxane, 1,4-dioxene, cyclohexanol, cyclohexen-4-ol, cyclohexen-3-ol, cyclohexanone, cyclohexen-4-one, cyclohexen-3-one, 3,4-epoxycyclohexene, and 4,5-epoxycyclohexene. In addition, cross-sections for 1-butene and acetic acid were measured for comparison with the literature. Uncertainties were quantified in all cases by accounting for errors in gas-phase concentration, experimental repeatability, and signal-to-noise ratio as a function of photon energy. Convolving the sources of error using the root-sum-square method led to an upper limit of 5% uncertainty above the detection limit, which is largely attributable to chemical purity.The primary objective of the present work is to provide absolute cross-sections along with quantified uncertainty limits. The majority of the absorption spectra, which reflect electronic transitions such as σ → σ* and n → σ*, are reported for the first time and provide insight into fundamental chemical physics, such as vibrational band structure and Rydberg transitions. In addition, the absorption cross-sections enable quantitative isomer-resolved speciation measurements in combustion chemistry, photolysis calculations for laser spectroscopy and atmospheric chemistry, and spectral assignments in astrochemistry.

A Computational and Experimental Investigation of the Phonon and Optical Properties of Au₂P₃.

In a combined experimental and theoretical study of gold phosphide (Au2P3), we investigate its vibrational properties, band structure, and dielectric properties, providing new insight into the properties of this underexplored material. Using a simple synthesis route, Au2P3 thin films were produced, enabling the first reported Raman analysis of this material. Coupled with first-principles calculations of these Raman modes, this analysis reveals that low-frequency vibrations are due to Au or mixed Au to P, and at higher frequencies, they are due to P vibrations. Further band structure and dielectric calculations reveal Au2P3 to be a narrow band (0.16 eV) indirect semiconductor. This work helps to fill major gaps in our understanding of key properties in this material that will benefit future research in this field.

Mobility of two-dimensional materials from first principles in an accurate and automated framework

We present a first-principles approach to compute the transport properties of 2D materials in an accurate and automated framework. We use density-functional perturbation theory in the appropriate bidimensional setup with open-boundary conditions in the third direction. The materials are charged by field effect via planar counter-charges. In this approach, we obtain electron-phonon matrix elements in which dimensionality and doping effects are inherently accounted for, without the need for post-processing corrections. This treatment highlights some unexpected consequences, such as an increase of electron-phonon coupling with doping in transition-metal dichalcogenides.We use symmetries extensively and identify pockets of relevant electronic states to minimize the number of electron-phonon interactions to compute; the integrodifferential Boltzmann transport equation is then linearized and solved beyond the relaxation-time approximation. We apply the entire protocol to a set of much studied materials with diverse electronic and vibrational band structures: electron-doped MoS2, WS2, WSe2, phosphorene, arsenene, and hole-doped phosphorene. Among these, hole-doped phosphorene is found to have the highest mobility, with a room temperature value around 600 cm^2/(Vs). Last, we identify the factors that affect most phonon-limited mobilities, such as the number and the anisotropy of electron and hole pockets, to provide a broader understanding of the driving forces behind high mobilities in two-dimensional materials.

Experimental and theoretical investigation of the vibrational band structure of the 1 Πu5-1 Πg5 high-spin system of C2.

Vibrational levels of the recently observed high-spin transition (1 Πu5-1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performing perturbation-assisted double-resonant excitation to the 1 Πu5, υ = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings.

Vibrational band structure of nanoscale phononic crystals

AbstractThe vibrational properties of two‐dimensional phononic crystals are studied with large‐scale molecular dynamics simulations and finite element method calculation. The vibrational band structure derived from the molecular dynamics simulations shows the existence of partial acoustic band gaps along the –M direction. The band structure is in excellent agreement with the results from the finite element model, proving that molecular dynamics simulations can be used to study the vibrational properties of such complex systems. An analysis of the structure of the vibrational modes reveals how the acoustic modes deviate from the homogeneous bulk behavior for shorter wavelengths and hints toward a decoupling of vibrations in the phononic crystal.

Nanotubes based on monolayer blue phosphorus

We demonstrate structural stability of monolayer zigzag and armchair blue phosphorus nanotubes by means of molecular dynamics simulations. The vibrational spectrum and electronic band structure are determined and analyzed as functions of the tube diameter and axial strain. The nanotubes are found to be semiconductors with a sensitive indirect band gap that allows flexible tuning.

A study of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic acid, as aided by DFT dimer analysis

Infrared and Raman spectral measurements in the region 4000–400cm−1 have been carried out for (R)-(+)-Methylsuccinic acid. The vibrational band structures near 3100–3040cm−1 in the IR and near 1650cm−1 in the Raman spectra have indicated the presence of an inter-molecular hydrogen bonding. A DFT dimer model has been proposed that involves O–H⋯OC type of hydrogen bonding. The proposed dimer model has been derived from the three stable monomers computed at RHF/3-21G and B3LYP/6-311+G(d,p) levels of theory. A total of six dimer structures have been considered with a Boltzmann population of 38% for the most stable dimer and 62% for the remaining five dimer populations. A Boltzmann population weighted vibration spectrum has predicted bands, among others, for O–H⋯OC group that are in very good agreement with experiment. All the dimers have the same structure in that the two pairs of –O–H and –OC form a closed cyclic structure with a local center of inversion. This dimer geometry has given rise to one asymmetric mode at 1683 and one symmetric –CO mode at 1637cm−1 corresponding to mutually exclusive an experimental IR band at 1700 and a Raman band at 1651cm−1. Further, the bond length, H⋯O, for the most stable dimer is 1.686Å, being shorter than the sums of van der Waals radii, 2.72Å and the angle between O–H and H⋯O is almost linear (179°) suggesting that the hydrogen bonding is fairly strong.

Vibrational dynamics and band structure of methyl-terminated Ge(111).

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH3-Ge(111) and CH3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.