Search Results (21)

Four-phonon (4 ph) scattering is critically important for describing thermal transport properties in two-dimensional (2D) materials. Incorporating the 4 ph process is crucial for obtaining reliable lattice thermal conductivity (κ_l) and understanding phonon thermal transport. Among emerging 2D materials, monolayer BeN₄ has attracted increasing attention because of its unique structural properties. Here, the influence of 4 ph scattering on the thermal transport behavior of monolayer BeN₄ is comprehensively explored through first-principles calculations. The calculated results demonstrate that, after considering the 4 ph scattering, the κ_l of monolayer BeN₄ at 300 K are reduced by 37.7% and 50.6% along the zigzag and armchair directions, respectively. These findings indicate that monolayer BeN₄ exhibits anisotropy in thermal transport and that 4 ph scattering has a significant impact on thermal transport. The thermal transport is dominated by acoustic phonon branches. Furthermore, the larger κ_l at low temperatures originates from longer phonon lifetimes, larger phonon mean free paths, lower phonon scattering rates, and smaller weighted phase space. In addition, the different channels of 4 ph scattering are systematically analyzed, revealing that the redistribution channel provides the dominant contribution to 4 ph scattering. This investigation provides deeper insight into the thermal transport behavior of monolayer BeN₄ and facilitates its potential applications in nanoelectronic and thermal management devices. Full article

Due to their unique structures, intriguing electronic properties, and potential applications across various fields, Janus materials have attracted extensive attention from the science community. However, the thermal transport properties of Janus systems are less known so far, especially regarding lattice thermal conductivity (LTC). In this work, we establish an accurate machine learning potential by which the phonon Boltzmann transport equation can be iteratively solved to readily predict the LTC of Janus WXY (X, Y = S, Se, Te) monolayers. It is found that the LTC for all three systems decreases monotonically with increasing temperature. Among them, the WTeSe monolayer exhibits the lowest LTC, which can be traced back to the competition between the contributions of phonon group velocity and relaxation time. Interestingly, we demonstrate that the effect of four phonon scattering plays an important role in accurately determining the LTC of these Janus monolayers. Our work also provides an alternative way of effectively predicting the LTC of systems with low symmetry and/or large size. Full article

Accurately understanding and modulating thermal and thermoelectric transport in penta-XP₂ (X = Pd, Pt) monolayers is crucial for their applications in nanoelectronics and energy conversion. We systematically investigate the thermal conductivity and thermoelectric properties of penta-XP₂ monolayers through first-principles calculations, incorporating four-phonon (4ph) scattering and electron–phonon interaction (EPI) effects. The 4ph scattering, particularly Umklapp and redistribution processes, markedly suppresses lattice thermal conductivity by generating substantial thermal resistance and disrupting phonon population distributions. At 300 K, the lattice thermal conductivity is reduced to 0.87 W/mK (80% reduction) for penta-PdP₂ and 1.64 W/mK (79% reduction) for penta-PtP₂ compared to three-phonon-only scattering. Combining this with EPI-optimized electronic transport yields enhanced thermoelectric figures of merit (ZT), increasing from 0.21 to 0.86 for penta-PdP₂ and from 0.11 to 0.34 for penta-PtP₂, alongside a broadened optimal carrier concentration range. These findings highlight momentum-conserving 4ph scattering as a key mechanism for phonon transport modulation and thermoelectric efficiency improvement in penta-XP₂ materials, providing theoretical guidance for designing high-performance nanoscale thermal management and energy conversion devices. Full article

Ca₃(BO₃)₂ microwave dielectric ceramics with space group R-3c (#167) were prepared by cold sintering, and their properties were systematically investigated. Phonon density of state diagrams for the Ca₃(BO₃)₂ lattice were obtained based on first-principles calculations to provide a more comprehensive understanding of the lattice vibrational properties of the material. Raman scattering and infrared reflectance spectroscopy were employed to investigate the lattice vibrational characteristics, identifying two types of vibrational modes: internal modes associated with the planar bending and symmetric stretching vibrations of the [BO₃] group, and external modes linked to the vibrations of the [CaO₆] octahedron. The intrinsic dielectric properties were determined by fitting the experimental data using a four-parameter semi-quantum model. The results demonstrate that the dielectric properties of Ca₃(BO₃)₂ ceramics are primarily influenced by the external vibrational modes. The sample under 800 MPa exhibits optimal dielectric performance, with a dielectric constant (ε_r) of 5.95, a quality factor (Q × f) of 11,836 GHz, and a temperature coefficient of resonant frequency (τ_f) of −39.89 ppm/°C. A simulation of this Ca₃(BO₃)₂ sample as a dielectric substrate was conducted using HFSS to fabricate a microstrip patch antenna operating at 14.97 GHz, which exhibits a return loss (S₁₁) of −25.5 dB and a gain of 7.15 dBi. Full article

Four quaternary Zintl phase thermoelectric (TE) materials belonging to the Ba_1-xEu_xZn₂Sb₂ (x = 0.02(1), 0.04(1), 0.08(1), 0.15(1)) system were successfully synthesized using the molten Pb-flux or the conventional high-temperature reaction methods. Their crystal structures were characterized by both powder and single-crystal X-ray diffraction analyses, and all four isotypic title compounds adopted the orthorhombic BaCu₂S₂-type (Pnma, Z = 4, Pearson code oP20) structure. The radius ratio criterion, based on the cationic and anionic elements (i.e., r₊/r₋), was successfully verified in the title system, as in our previous reports, where r₊/r₋ > 1 for the BaCu₂S₂-type structure. A series of density functional theory calculations were performed using a hypothetical model with the idealized compositions of Ba_0.75Eu_0.25Zn₂Sb₂, and the results were compared with the ternary parental compound BaZn₂Sb₂ to understand the influence of Eu substituents in the Ba_1-xEu_xZn₂Sb₂ system. A similar overall shape of the density of states (DOS) curves and the near-constant DOS values at E_F before and after the cationic substitution suggest only marginal changes in the carrier concentration. Therefore, carrier mobility has a dominant role in rationalizing the observed variations in the electrical transport properties of the title system. Temperature-dependent TE property measurements proved that an increase in the Seebeck coefficient S and a decrease in electrical conductivity σ were observed as the Eu substituents gradually increased in the Ba_1-xEu_xZn₂Sb₂ system, although the overall S and σ values were lower than those in the parental compound BaZn₂Sb₂. The thermal conductivities of these title compounds were successfully lowered by phonon scattering, but due to the overall smaller electrical transport properties, the observed maximum ZT was 0.49 at 773 K for Ba_0.98(1)Eu_0.02Zn₂Sb₂. Full article

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of Al_xGa_1−xN films with high Al fractions (60–87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal–organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E₂(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) Al_xGa_1−xN films possess large energy gaps E_g in the range of 5.0–5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20–300 K of Al_0.87Ga_0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1–2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field. Full article

The finite element method (FEM) was applied to study the low frequency band gap characteristics of a designed phonon crystal plate formed by embedding a hollow lead cylinder coated with silicone rubber into four epoxy resin short connecting plates. The energy band structure, transmission loss and displacement field were analyzed. Compared to the band gap characteristics of three traditional phonon crystal plates, namely, the square connecting plate adhesive structure, embedded structure and fine short connecting plate adhesive structure, the phonon crystal plate of the short connecting plate structure with a wrapping layer was more likely to generate low frequency broadband. The vibration mode of the displacement vector field was observed, and the mechanism of band gap formation was explained based on the spring mass model. By discussing the effects of the width of the connecting plate, the inner and outer radii and height of the scatterer on the first complete band gap, it indicated that the narrower the width of the connecting plate, the smaller the thickness; the smaller the inner radius of the scatterer, the larger the outer radius; and the higher the height, the more conducive it is to the expansion of the band gap. Full article

The two-dimensional post-transition-metal chalcogenides, particularly indium selenide (InSe), exhibit salient carrier transport properties and evince extensive interest for broad applications. A comprehensive understanding of thermal transport is indispensable for thermal management. However, theoretical predictions on thermal transport in the InSe system are found in disagreement with experimental measurements. In this work, we utilize both the Green–Kubo approach with deep potential (GK-DP), together with the phonon Boltzmann transport equation with density functional theory (BTE-DFT) to investigate the thermal conductivity ($\kappa$) of InSe monolayer. The $\kappa$ calculated by GK-DP is 9.52 W/mK at 300 K, which is in good agreement with the experimental value, while the $\kappa$ predicted by BTE-DFT is 13.08 W/mK. After analyzing the scattering phase space and cumulative $\kappa$ by mode-decomposed method, we found that, due to the large energy gap between lower and upper optical branches, the exclusion of four-phonon scattering in BTE-DFT underestimates the scattering phase space of lower optical branches due to large group velocities, and thus would overestimate their contribution to $\kappa$. The temperature dependence of $\kappa$ calculated by GK-DP also demonstrates the effect of higher-order phonon scattering, especially at high temperatures. Our results emphasize the significant role of four-phonon scattering in InSe monolayer, suggesting that combining molecular dynamics with machine learning potential is an accurate and efficient approach to predict thermal transport. Full article

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N⁰_s, N⁺_s, N⁻_s and N_s-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) N_s⁰, N_s⁺ and N_s⁻ all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) N_s-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that N_s⁺ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that N_s⁺ contributes to, and in the absence of N_s⁰ is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of N_s⁰ indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants. Full article

2D copper-based semiconductors generally possess low lattice thermal conductivity due to their strong anharmonic scattering and quantum confinement effect, making them promising candidate materials in the field of high-performance thermoelectric devices. In this work, we proposed four 2D copper-based materials, namely CuSbS₂, CuSbSe₂, CuBiS₂, and CuBiSe₂. Based on the framework of density functional theory and Boltzmann transport equation, we revealed that the monolayers possess high stability and narrow band gaps of 0.57~1.10 eV. Moreover, the high carrier mobilities (10²~10³ cm²·V⁻¹·s⁻¹) of these monolayers lead to high conductivities (10⁶~10⁷ Ω⁻¹·m⁻¹) and high-power factors (18.04~47.34 mW/mK²). Besides, as the strong phonon-phonon anharmonic scattering, the monolayers also show ultra-low lattice thermal conductivities of 0.23~3.30 W/mK at 300 K. As results show, all the monolayers for both p-type and n-type simultaneously show high thermoelectric figure of merit (ZT) of about 0.91~1.53 at room temperature. Full article

This paper proposes a method based on stimulated Brillouin scattering (SBS) for reshaping a temporal Gaussian waveform into a temporal high-intensity step waveform. The theoretical analysis showed that the reshaped temporal waveform depended on the phonon lifetime, the Brillouin gain coefficient, the interaction length between the Stokes and the pump pulse, and the pump energy. It further showed the dynamic evolution of the reshaped temporal waveform with these four parameters. By optimizing these parameters, a temporal step waveform with an intensity of 36.92 MW/cm² was obtained in the experiment. Full article

Nickel films with nanovoids filled with fullerene molecules have been fabricated. The thermoelectric properties of the nanocomposites have been measured from room temperature down to about 30 K. The main idea is that the phonon scattering can be enhanced at the C₆₀/matrix heterointerface. The distribution of atoms within the Ni and Ni-C₆₀ layers has been characterized by Auger depth profiling. The morphology of the grown samples has been checked using cross-sectional scanning electron microscopy (SEM). The Seebeck coefficient and electrical conductivity have been addressed employing an automatic home-built measuring system. It has been found that nanostructuring using Ar⁺ ion treatment increases the thermopower magnitude over the entire temperature range. Incorporating C₆₀ into the resulting voids further increased the thermopower magnitude below ≈200 K. A maximum increase in the Seebeck coefficient has been measured up to four times in different fabricated samples. This effect is attributed to enhanced scattering of charge carriers and phonons at the Ni/C₆₀ boundary. Full article

The optical response of bulk germanium sulfide (GeS) is investigated systematically using different polarization-resolved experimental techniques, such as photoluminescence (PL), reflectance contrast (RC), and Raman scattering (RS). It is shown that while the low-temperature (T = 5 K) optical band-gap absorption is governed by a single resonance related to the neutral exciton, the corresponding emission is dominated by the disorder/impurity- and/or phonon-assisted recombination processes. Both the RC and PL spectra are found to be linearly polarized along the armchair direction. The measured RS spectra over a broad range from 5 to 300 K consist of six Raman peaks identified with the help of Density Functional Theory (DFT) calculations: A${}_{\mathrm{g}}^1$, A${}_{\mathrm{g}}^2$, A${}_{\mathrm{g}}^3$, A${}_{\mathrm{g}}^4$, B${}_{1\mathrm{g}}^1$, and B${}_{1\mathrm{g}}^2$, which polarization properties are studied under four different excitation energies. We found that the polarization orientations of the A${}_{\mathrm{g}}^2$ and A${}_{\mathrm{g}}^4$ modes under specific excitation energy can be useful tools to determine the GeS crystallographic directions: armchair and zigzag. Full article

Brillouin scattering is a dominant inelastic scattering observed in optical fibers, where the energy and momentum transfer between photons and acoustic phonons takes place. Narrowband reflection (or gain and loss) spectra appear in the spontaneous (or stimulated) Brillouin scattering, and their linear dependence of the spectral shift on ambient temperature and strain variations is the operation principle of distributed Brillouin sensors, which have been developed for several decades. In few-mode optical fibers (FMF’s) where higher-order spatial modes are guided in addition to the fundamental mode, two different optical modes can be coupled by the process of stimulated Brillouin scattering (SBS), as observed in the phenomena called intermodal SBS (two photons + one acoustic phonon) and intermodal Brillouin dynamic grating (four photons + one acoustic phonon; BDG). These intermodal scattering processes show unique reflection (or gain and loss) spectra depending on the spatial mode structure of FMF, which are useful not only for the direct measurement of polarization and modal birefringence in the fiber, but also for the measurement of environmental variables like strain, temperature, and pressure affecting the birefringence. In this paper, we present a technical review on recent development of distributed Brillouin sensors on the platform of FMF’s. Full article

In this paper, a novel wrap-around multi-scattering phononic crystal (PC) structure is proposed. Band gaps (BGs) and transmission characteristics of the present structure are calculated using finite element method (FEM). Through the calculations of single-scattering prototype, three complete BGs which are exhibited at low frequency and the fourth wide BG at high frequency are discovered. The transmission features and resonant spectra represented by frequency response function (FRF) shows that apparent resonance directly cause the four specific BGs. By keeping the total area of scatterers unchanged, 2 × 2, 3 × 3 and 4 × 4 scatterers are designed to obtain the change rule of BGs. Furthermore, the size ratio of 2 × 2 scatterers, the number of connection beams are investigated to obtain the regular pattern of acoustic energy transmission and attenuation. The present investigation of multiple-scatter PC structure will provide a solid support on the future design of acoustical functional materials. Full article