Phonon Dispersion Analysis Research Articles

First principle investigation on 2D beryllium chalcogenides for thermoelectric and optical applications

This article reports a detailed investigation of structural, electronic, thermoelectric and optical properties of square lattice beryllium chalcogenides (s-BeX; X = O, S, Se and Te) for the first time. Phonon dispersion analysis is carried out to determine the dynamical stability of the s-BeX compounds, whereas all the s-BeX compounds are found to be energetically and dynamically stable. The electronic properties are studied in terms of electronic band structures and partial density of states of s-BeX compounds. The BeO compound shows an indirect bandgap, while other chalcogenides, viz. BeS, BeSe and BeTe appear to be direct semiconductor materials. The effective mass, mobility and relaxation time for the electron and hole from the band structure of s-BeX are also studied to gain more insight into these considered materials. The thermoelectric properties of the entire s-BeX series are investigated for an extensive temperature range as 50–800 K in terms of thermal conductivity (κ), electrical conductivity (σ), Seebeck coefficient (S) and figure of merit (ZT). The thermoelectric merit of s-BeX compounds is also compared to the zinc-blende and hexagonal-monolayer phases of beryllium chalcogenides, which indicates that the thermoelectric potential of BeX compounds depends on their structural phases. The optical properties such as dielectric function ϵ(ω), refraction index n(ω), reflectivity R(ω), absorption spectra I(ω) and extinction index k(ω) of s-BeX are also investigated in the presence of parallel and perpendicular field polarizations. The present work reveals s-BeO, zb-BeSe and h-BeTe as the best thermoelectric materials in their respective series. Also, these compounds are mainly UV-active and therefore can possibly be applicable as UV-photodetectors and UV protectant materials.

Thermoelectric, spin-dependent optical and quantum transport properties of 2D half-metallic Co2Se3.

In this paper, the thermoelectric, spin-dependent optical and quantum transport properties of a two-dimensional (2D) Co2Se3 monolayer are investigated using first principles calculations. The stability of the Co2Se3 monolayer is confirmed by energy-cohesive calculations and phonon dispersion analysis. According to electronic and magnetic calculations, Co2Se3 is a Dirac half-metal (DHM) with several Dirac cones around the Fermi level in the spin-down channel and exhibits intrinsic ferromagnetism with a high Curie temperature. Investigation of the thermoelectric properties shows that the Co2Se3 monolayer with ZT ∼ 1 at low temperature (T < 300 K) to medium temperature (500 < T < 700 K) can be introduced as a suitable candidate for thermoelectric applications. Spin-dependent optical calculations indicate that the maximum reflection and absorption for 2D Co2Se3 are in the energy ranges of 0-1 and 12-8 eV, respectively, so this structure is a suitable wave reflector in the infrared range (IR) and a good absorbent in the far-ultraviolet (FUV) range. On the other hand, the unique feature of Co2Se3 encouraged us to study the spin-dependent transport properties. A spin-down current as large as 27(30) μA passing through in the x(y)-direction, while the spin-up current passing through the scattering region up to 1.2 (1) V is approximately zero, induces 100% spin filtering properties in the Co2Se3 monolayer, so this structure has great potential for application in spintronic nano-devices.

Structural, Electronic, and Optical Properties of Hexagonal XC6 (X=N, P, As, and Sb) Monolayers.

Based on first-principles calculations, a novel family of two-dimensional (2D) IV-V compounds, XC6 (X=N, P, As and Sb), is proposed. These compounds exhibit excellent stability, as determined from the cohesive energies, phonon dispersion analysis, ab initio molecular dynamics (AIMD) simulations, and mechanical properties. In this type of structure, the carbon atom is sp2 hybridized, whereas the X (N, P, As and Sb) atom is nonplanar sp3 hybridized with one 2pz orbital filled with lone pair electrons. NC6 , PC6 , AsC6 and SbC6 monolayers are intrinsic indirect semiconductors with wide bandgaps of 2.02, 2.36, 2.77, and 2.85 eV (based on HSE06 calculations), respectively. After applying mechanical strain, PC6 , AsC6 and SbC6 monolayers can be transformed from indirect to direct semiconductors. The appropriate bandgaps and well-located band edge positions make XC6 monolayers potential materials for photocatalytic water splitting. XC6 family members also have high absorption coefficients (∼105 cm-1 ) in the ultraviolet region and higher electron mobilities (∼103 cm2 V-1 s-1 ) than many known 2D semiconductors.

Electric field-induced band modulation of predicted ternary 2D MXC3 [M:X = As:Ge, Sb:Sn and Bi:Pb] with strong stability and optical properties

The present work reports a new series of ternary two-dimensional MXC3 [M:X = As: Ge, Sb: Sn and Bi: Pb] compounds whose stability and existence are confirmed through the phonon dispersion analysis, elastic properties and ab initio molecular dynamics simulation. The developed compounds are further investigated with their electronic and optical properties using density functional theory. We have observed zero bandgap nature in MXC3 compounds, where AsGeC3 shows the graphene-like band structure with Dirac cone at K-point. We have systematically applied the external electric field up to ±0.40 eV/Å to modulate the electronic and optical properties of these compounds. The increasing magnitude of the electric field has shown the increasing nature of bandgap in the AsGeC3 compound, while no effect was observed in case of SbSnC3 and BiPbC3 compounds. Consequently, the bandgap of AsGeC3 compound is tuned up to 0.98 eV which is maximum at an optimum value of the applied electric field of ±0.40 eV/Å. All the three 2D MXC3 compounds have shown the broken symmetry of the structure after the optimum value of the electrical field that can be understood by the maximum tolerance limit of an applied electric field. Our results strongly suggest the excellent mechanical stability of the AsGeC3 compound under the applied electric field. We have also calculated the variation in the optical properties upon the external field for the AsGeC3 compound, which shows a good transparency nature and can be used in the regulation of the smart window applications. We sincerely believe that our outcomes reflect a significant achievement for potential applications in future nanoelectronics and optoelectronic devices along with tunable electronic properties.

Hybrid CaS/CaSe bilayer as a wide temperature range thermoelectric material

Abstract The present work reports a detail density functional investigation on the structure, vibrational, electronic and thermoelectric properties of bilayered CaS and CaSe and their hybrid CaS/CaSe bilayer, based on our recently developed hexagonal monolayers CaS and CaSe as promising thermoelectric materials from a series of calcium chalcogenides CaX (X = O, S, Se and Te). The minimized structures of the considered bilayers are assessed for the dynamical stability with phonon dispersion analysis. In order to look into the electronic properties of stable compounds, we have generated electronic band structure and projected density of states (PDOS) through first principle methods. To study the thermoelectric properties, we have utilized the semi-classical Boltzmann transport equations (BTE) for computing electrical and thermal conductivities, Seebeck coefficient and figure of merit (ZT). Present study reveals hybrid CaS/CaSe bilayer as the best promising thermoelectric material for a wide range of temperature toward future applications in thermopower generation devices.

Structure, stability, electronic and thermoelectric properties of strontium chalcogenides

Abstract The present investigation reports a critical insight on the temperature dependent transport properties of strontium chalcogenides (SrX; X = O, S, Se and Te) in their rock salt (rs-SrX) and hexagonal monolayer (h-SrX) phases, for the first time. For an initial check on the stability of these phases (rs-SrX and h-SrX), we have carried out phonon dispersion analysis under PAW-GGA level of calculations. Although, some of these structures are reported to be unstable under lower level (PAW-LDA) of calculations in past, our PAW-GGA computations reveals all the rs-SrX and h-SrX phases to be dynamically stable. The stability of all these structures along with wide band (indirect) gap certainly motivates us to look for their thermoelectric properties, for the first time. The electronic band structures and projected density of states (PDOS) of these materials are computed to understand their electronic properties. Finally, in order to obtain temperature dependent transport properties for the considered series, we have utilized the semi-classical Boltzmann transport equations (BTE). The Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ) and figure of merit (ZT) are calculated using BTE. Couple of monolayers (h-SrS and h-SrTe) and bulk (rock salt) phases (rs-SrO and rs-SrS) are come up with excellent thermoelectric properties through the present investigation.

Oxysulfide Li2BeSO: A potential new material for solid electrolyte predicted from first principles

Solid electrolytes with fast Li+ conductivity and high chemical stability are of particular interest for application in all-solid-state lithium ion batteries. Particle swarm optimization (PSO) here predicts a new material, Li2BeSO, with both high Li+ conductivity and a wide electrochemical window. Two polymorphs are found in different pressure ranges; a tP20 structure is stable from ambient pressure to 12.6 GPa, above which an oI10 structure is stable. Analysis of phonon dispersion and possible decomposition paths reveals that both Li2BeSO polymorphs are dynamically and chemically stable at ambient pressure. This new material is therefore quench recoverable if synthesized under different P–T conditions. Migration energy barriers for Li+ transport in the two polymorphs at 0 GPa are analyzed for different paths and hopping modes, and the optimal Li+ transition path is derived for each structure. Interestingly, oI10-Li2BeSO is found to have a low barrier of ∼70 meV, suggesting it could be a potentially useful solid-state electrolyte. In addition to discovering a new and efficient solid electrolyte, this study advances our understanding of superionic conductors.

The Ultrahigh Quantum Thermal Conductance of Hydrogenated Boron Nanotubes

Using first‐principles calculations and phonon dispersion analysis, the authors demonstrate theoretically that a hydrogenated monolayer borophene sheet can form a stable nanotube structure, denoted as H‐BNT. Combined with non‐equilibrium Green's function method, it is found that although thermal conductance of H‐BNT is lower than its carbon counterpart in the high‐temperature limit, at room temperature the phonon thermal conductance of H‐BNT is only slightly lower (2%) than carbon nanotube with the similar diameter. The high room temperature thermal conductance of boron nanotube is attributed to the light atomic mass of boron, strong interatomic force between boron atoms and high atomic density. Our studies shed light on the nature of quantum thermal transport in low‐dimensional materials, and can help in developing boron‐based nanoscale devices.

First-principles study of structural phase transition, electronic, elastic and thermodynamic properties of C15-type Laves phase TiCr2 under pressure

Phase transition of TiCr2 in C15 (MgCu2), C36 (MgNi2), C14 (MgZn2) structures have been studied by using the projector augmented wave method. It is found that C15-type is the most stable structure, which agrees with the results of Chen et al. At 0 K, the phase boundary of C15 to C36 is 207.79 GPa, and the phase transition from C36 to C14 is 265.61 GPa. Both the transition pressures decrease with increasing temperature. Phonon dispersion and elastic constants are calculated and found that C15-type TiCr2 is mechanically stable according to the elastic stability criteria and phonon dispersion analysis. Moreover, the pressure and temperature dependence of the specific heat, Debye temperature and thermal expansion coefficient are discussed, among them our calculated Debye temperature is consistent with the report of A. sari et al., however, it is far from the results of B. Mayer et al. and Chen et al.

Phonon states of B12P2 crystals: Ab initio calculation and experiment

Using methods of density functional theory (DFT) and Raman spectroscopy (RS) we investigate the phonon structure of the B12P2 crystal. The theory is consistent with the experiment for all twelve Raman-active modes. We have calculated Born charges, oscillator strengths, longitudinal-transverse splitting, and dielectric functions of dipole modes. The results are used to interpret the known IR spectra of B12P2. Analysis of phonon dispersion shows that the crystal is dynamically stable. The energy gap separating acoustic and optical phonons is absent due to the overlap of the phonon states at the Brillouin zone boundary. Vibrations of phosphorus and boron atoms contribute to the low- and high-frequency regions of the phonon density of states, respectively.

Stability, electronic and thermodynamic properties of aluminene from first-principles calculations

Using first-principles calculations based on density functional theory (DFT), we have investigated the structure stability and electronic properties of both buckled and 8-Pmmn phase aluminene. Phonon dispersion analysis reveals that the buckled and 8-Pmmn aluminene are dynamically stable. The band structure shows that both the buckled and 8-Pmmn aluminene exhibit metallic behavior. Finally, the thermodynamic properties are investigated based on phonon properties.

Effect of Phosphorus Substitution on Stability, Electronic, and Magnetic Properties of SiC Hybrid

The effect of different concentrations of P-atom-substituted SiC sheet were studied using ab initio calculations within the density functional theory. For dosage concentrations ranging from 1/32 to 8/32, structural, electronic, and magnetic properties are reported and discussed. Thermal stability performed using phonon dispersion analysis and molecular dynamic calculations shows that P-substituting SiC hybrid is an exothermic process. All the substituted systems are indirect band gap semiconductors with strongly bonded C-, Si-, and P-atoms accompanied by an appreciable electron transfer from Si- and P- towards C-atoms. Doping with P decreases the gap of pure SiC and varies it in the interval (1.37–1.81 eV) for different dosages. Moreover, spin-polarized calculations show that the configurations with 1/32 to 4/32 concentrations exhibit a magnetic behavior and Curie temperature is determined for ferromagnetic structures. For dosage concentrations larger than 5/32, the structures are nonmagnetic.

A structural mechanics approach for the phonon dispersion analysis of graphene

A molecular structural mechanics model for the numerical simulation of phonon dispersion relations of graphene is developed by relating the C-C bond molecular potential energy to the strain energy of the equivalent beam-truss space frame. With the stiffness matrix known and further based on the periodic structure characteristics, the Bloch theorem is introduced to develop the dispersion relation of graphene sheet. Being different from the existing structural mechanics model, interactions between the fourth-nearest neighbor atoms are further simulated with beam elements to compensate the reduced stretching stiffness, where as a result not only the dispersion relations in the low frequency field are accurately achieved, but results in the high frequency field are also reasonably obtained. This work is expected to provide new opportunities for the dynamic properties analysis of graphene and future application in the engineering sector.

Vibrational spectra and phonon dispersion analysis of a single-walled zigzag carbon nanotube: A first principles study

This paper reports a vibrational spectroscopic study on a zigzag single-walled carbon nanotube (SWCNT) using the first-principles method based on density functional theory (DFT). The most suitable exchange correlation functional for DFT analysis was determined by comparing the predicted value of band gap of the SWCNT under study with the experimental value reported in the literature. General gradient approximation functional in combination with revised Perdew–Burke–Ernzerh sub-functional was found to give the best results. Using this optimum combination, phonon density of states and phonon dispersion curves have been determined. The analysis of results obtained focuses on symmetry considerations, group theory analysis, segregation of Raman active and infrared (IR) active vibrational modes and interpretation of the Raman and IR spectra obtained. The earlier approaches to the problem rely upon the zone folding technique and force constant models in which structural relaxation factor is not taken care of. An ab initio approach has been adopted by the authors in this work, which is advantageous as it neither depends on some predefined parameter nor does it ignore the structural relaxation factor. Analysis of the Raman spectrum reveals some additional peaks other than the commonly known radial breathing mode, D, G, and G′ bands in the SWCNT spectra, which have been recently reported to be observed experimentally also. Similarly, the theoretically developed IR spectrum for the simulated SWCNT is also in agreement with experimental observations. The methodology presented thus provides a very useful and novel simulation route to predict the vibrational modes, Raman spectra, and IR spectra of SWCNTs theoretically.

Phonon transport in single-layer boron nanoribbons

Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green’s function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene’s. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.

The Effects of the Organic-Inorganic Interactions on the Thermal Transport Properties of CH3NH3PbI3.

Methylammonium lead iodide perovskite (CH3NH3PbI3), the most investigated hybrid organic-inorganic halide perovskite, is characterized by a quite low thermal conductivity. The rotational motion of methylammonium cations is considered responsible for phonon transport suppression; however, to date, the specific mechanism of the process has not been clarified. In this study, we elucidate the role of rotations in thermal properties based on molecular dynamics simulations. To do it, we developed an empirical potential for CH3NH3PbI3 by fitting to ab initio calculations and evaluated its thermal conductivity by means of nonequilibrium molecular dynamics. Results are compared with model systems that include different embedded cations, and this comparison shows a dominant suppression effect provided by rotational motions. We also checked the temperature dependence of the vibrational density of states and specified the energy range in which anharmonic couplings occur. By means of phonon dispersion analysis, we were able to fully elucidate the suppression mechanism: the rotations are coupled with translational motions of cations, via which inorganic lattice vibrations are coupled and scatter each other.

First-principles study on structural, electronic, elastic and thermodynamic properties of the full-Heusler alloys Co2YZ (Y=Sc, Cr and Z=Al, Ga)

First-principles calculations based on density functional theory (DFT) and quasi-harmonic Debye model are performed to study the structural, elastic, electronic and thermodynamic properties of the Co 2 YZ (Y=Sc, Cr and Z=Al, Ga) Heusler alloys. It is found that the spin polarization of Co 2 CrZ at Fermi level raises considerably, and Co 2 ScZ keeps the non-magnetic feature when the external pressure increases. Although outstanding performance of half-metallicity appears to Co 2 CrZ, the dynamical stability order exhibits that Co 2 ScZ > Co 2 CrZ as the result of phonon dispersion analysis. The influence of external pressure and temperature on the volume compression ratio, bulk modulus, heat capacity and thermal expansion coefficient are took fully into account and the outcomes from calculation obey the corresponding thermomechanics laws in the quasi-harmonic Debye model.

Stable planar single-layer hexagonal silicene under tensile strain and its anomalous Poisson's ratio

Here, we report the structural and mechanical properties of several two-dimensional (2-D) materials by using first-principles density functional theory calculations. We find that the buckled single-layer silicene could transit to planar hexagonal silicene at a critical tensile strain of 0.20. Phonon dispersion analysis suggests that the planar hexagonal silicene under tension is stable. The Poisson's ratio of silicene and MoS2 shows strong anisotropy: it increases while stretched in the zigzag direction, but decreases when strained in the armchair direction. When stretched in the zigzag direction, the Poisson's ratio of silicene could reach 0.62.

Structural stabilities and electronic properties of fully hydrogenated SiC sheet

The intriguing structural and electronic properties of fully hydrogenated SiC honeycomb sheet are studied by means of ab initio calculations. Based on structure optimization and phonon dispersion analysis, we find that both chair-like and boat-like configurations are dynamically stable, and the chair-like conformer is energetically more favored with an energy gain of 0.03 eV per C atom relative to the boat-like one. The chair-like and boat-like conformers are revealed to be nonmagnetic semiconductors with direct band gaps of 3.84 and 4.29 eV, respectively, both larger than 2.55 eV of pristine SiC sheet. The charge density distributions show that the bondings are characterized with covalency for both chair-like and boat-like conformers.

Phonon Dispersion Analysis as an Indispensable Tool for Predictions of Solid State Polymorphism and Dynamic Metastability: Case of Compressed Silane

Diamond anvil cell experiments suggest that upon compression above 26.5 GPa silane (SiH4) forms a polymeric phase VI, whose crystal structure has not yet been solved. Here we present DFT calculations showing how phonon-guided optimization leads to a polymeric Fdd2 structure which is the lowest-enthalpy polymorph of SiH4 above 26.8 GPa, and which most probably can be identified as the experimentally observed polymeric phase. The new algorithm of predicting the lowest-energy structures enables simultaneous inspection of the potential energy surface of a given system, calculation of its vibrational properties, and assessment of chances for obtaining a metastable ambient-pressure structure via decompression. Our calculations indicate that at room temperature the differences in the vibrational and entropy terms contributing to the Gibbs free energy of different polymorphs of silane are negligible in comparison with corresponding differences in the zero-point energy corrections, in contrast to earlier suggestions. We also show that the Fdd2 polymorph should be metastable upon decompression up to 5 GPa, which suggests the possibility of obtaining a polymeric ambient-pressure form of SiH4. Polymeric silane should exhibit facile thermal decomposition with evolution of molecular hydrogen and thus constitute an efficient (12.5 wt%) material for hydrogen storage.