Local Phonon Research Articles

Near-field radiative heat transfer between irregularly shaped dielectric particles modeled with the discrete system Green's function method

Near-field radiative heat transfer (NFRHT) between irregularly shaped dielectric particles made of SiO2 and morphology characterized by Gaussian random spheres is studied. Particles are modeled using the discrete system Green's function (DSGF) approach, which is a volume integral numerical method based on fluctuational electrodynamics. This method is applicable to finite, three-dimensional objects, and all system interactions are defined independent of thermal excitation by a generalized system Green's function. The DSGF method is deemed suitable to model NFRHT between irregularly shaped particles after verification against the analytical solution for chains of two and three SiO2 spheres. The NFRHT results reveal that geometric irregularity in particles leads to a reduction of the total conductance from that of comparable perfect spheres at vacuum separation distances smaller than the particle size, a regime in which NFRHT is a surface phenomenon. At vacuum separation distances larger than the particle size, NFRHT becomes a volumetric process, and the total conductance between irregularly shaped particles converges to that of comparable perfect spheres. Spectral analysis reveals, however, that particle irregularity leads to damping and broadening of resonances at all separation distances, thereby highlighting the importance of the DSGF method for spectral engineering in the near field. The reduced spectral coherence when particle size is larger than the vacuum separation distance is attributed to coupling of surface phonon-polaritons within the randomly generated, distorted particle features. For particle size smaller than the vacuum separation distance, resonance broadening and damping is linked with the multiple localized surface phonon modes supported by the composite spherical harmonic morphologies of the Gaussian random spheres.

Interfacial strain-dependent localization of phonons in (CdS)m/(CdTe)n superlattices

Comprehensive results of the acoustic and optical phonon traits are reported for the strained (CdS)m/(CdTe)n superlattices (SLs) using microscopic and macroscopic methods. Simulations by realistic rigid-ion-model provided atypical vibrational characteristics in the growth [001] as well as in-plane ([100], [110]) directions of SLs exhibiting anisotropy of zone-center optical-, acoustic-anti-crossing phonons, mini-gap formation, confinement, and gap modes. In (CdTe)n/(CdS)m and (CdS)n/(ZnS)m, the use of elastic continuum techniques offered accurate values of folded acoustic modes with proper shifts of the longitudinal-optical (ΔωLO) and transverse-optical (ΔωTO) phonons. Changes in ΔωLO have instigated strong motives for calculating the Raman intensity profiles in the graded (CdTe)10-Δ/(CdTe0·5S0.5)Δ/(CdS)10-Δ/(CdTe0·5S0.5)Δ SLs by meticulously including the interfacial layer thickness Δ (≡ 1 to 3 monolayers (MLs)). With Δ (≡ 3 ML), the Raman intensity features have exhibited downwards (upwards) shifts of CdS (CdTe)-like ωLO modes by ∼ - 20 cm−1 (∼+18 cm−1) in good agreement with the strain-induced results. Appraisals of Raman intensity shifts in the confined optical mode regions (see: Supplementary material) are ascribed to the localization/delocalization of atomic amplitudes at the CdS–CdTe interface. We strongly believe that the reported phonon aspects in CdTe/CdS SLs will encourage Raman spectroscopists to validate our theoretical conjectures and engineers to evaluate potentials of the CdTe-based SLs to improve/design electronic devices of many contemporary applications.

Effects of hot phonons and thermal stress in micro-Raman spectra of molybdenum disulfide

Micro-Raman spectroscopy has become an important tool in probing thermophysical properties in functional materials. Localized heating by the focused Raman excitation laser beam can produce both stress and local nonequilibrium phonons in the material. Here, we investigate the effects of hot optical phonons in the Raman spectra of molybdenum disulfide and distinguish them from those caused by thermally induced compressive stress, which causes a Raman frequency blue shift. We use a thermomechanical analysis to correct for this stress effect in the equivalent lattice temperature extracted from the measured Raman peak shift. When the heating Gaussian laser beam is reduced to 0.71 μm, the corrected peak shift temperature rise is 17% and 8%, respectively, higher than those determined from the measured peak shift and linewidth without the stress correction, and 32% smaller than the optical phonon temperature rise obtained from the anti-Stokes to Stokes intensity ratio. This nonequilibrium between the hot optical phonons and the lattice vanishes as the beam width increases to 1.53 μm. Much less pronounced than those reported in prior micro-Raman measurements of suspended graphene, this observed hot phonon behavior agrees with a first-principles based multitemperature model of overpopulated zone-center optical phonons compared to other optical phonons in the Brillouin zone and acoustic phonons of this prototypical transition metal dichalcogenide. The findings provide detailed insight into the energy relaxation processes in this emerging electronic and optoelectronic material and clarify an important question in micro-Raman measurements of thermal transport in this and other two-dimensional materials.

Coherent control of a local phonon in trapped ions using dynamical decoupling

In this paper, we present a dynamical decoupling (DD) technique to coherently control the dynamics of a single local phonon in trapped ions. A 2$\pi$ rotation at a motional sideband transition flips the sign of the relevant local phonon state, resulting in cancellation of the phonon dynamics. In this work, we implement DD using single and multiple blue-sideband pulses to control a single local phonon in two $^{40}{\rm Ca}^{+}$ ions in a linear Paul trap. Our proposed DD technique can be used to engineer coupling between local phonon modes.

Temperature-sensitive hybridization of propagating and localized surface phonon polaritons in polar 4H-SiC nano-resonators

Polar 4H-SiC nano-resonators can host localized surface phonon polaritons (SPhPs) with low optical loss and fully exploit light–matter interactions for promising nanophotonic applications such as surface-enhanced spectroscopies and thermal imaging. To expand the mid-infrared and infrared application spaces, a sound knowledge of temperature effect on infrared dielectric functions and SPhPs of 4H-SiC is required, yet it remains largely unexplored. Herein, we focus on exploiting the temperature influence on dielectric functions, hybridization of propagating and localized SPhPs, and tailed spectral radiation properties of 4H-SiC nanopillar arrays through spectroscopic ellipsometry (SE) measurements as well as multiscale simulations. The 4H-SiC crystal is grown using the physical vapor transport method, and SE experiments measure infrared dielectric functions at temperatures between 300 and 800 K. Finite-element electromagnetic simulations confirm the emerged Monopole and transverse dipoles (TD1 and TD2) resonance modes in 4H-SiC nano-resonators, which agrees with the literature experiment. At high temperatures with strong lattice vibration, the amplitudes of resonant optical absorption peaks gradually decrease and the linewidths broaden, accompanied by the weakened electric resonances. First-principles calculations show that the anharmonic phonon scattering strengthens and less optical phonons are coupled to incident photons as temperature increases. Moreover, the propagating and localized SPhPs’ hybridization and spectral radiation properties of 4H-SiC nanopillar arrays can be largely tailed by modifying the morphology and incident angle of light. This work provides physical insights into the temperature-induced spectral tuning of 4H-SiC nano-resonators and helps exploit their applications in the high-temperature working conditions.

Phonon localization and resonance in thermal transport of pillar-based GaAs nanowires

Exploring the possibility of nanostructures to modulate thermal conductivity (TC) contributes to promote a deeper comprehension of phonon diffusion and transport processes with the design of thermally insulated devices with high ZT values, and the GaAs nanowires (NWs) widely used in optoelectronic and microelectronic devices exhibit nondiffusive phonon thermal transport phenomena attributed to size effects, while ignoring the wave effects of phonons. Here, we simulate the TC of pillar-based GaAs NWs using non-equilibrium molecular dynamics and Monte Carlo simulations. The spatial distribution of density of states, temperature and heat flow distribution clouds, phonon participation rate, dispersion curves and phonon transmittance of atoms were calculated to investigate the phonon thermal transport processes in pillar-based NWs. The calculation results show that the pillar-based surface reduce the TC by 16%, the TC of pristine NW increases with axial and equivalent diameter, and the TC of pillar-based NW increases nonlinearly with axial length and increases with radial length. The phonon-surface scattering intensity is enhanced by the perturbation introduced by the pillared surface with a substantial decrease in phonon transmission capacity and a break in long-wavelength phonon transport even annihilated, which leads to surface phonon localization. Nanopillars not only enhance the phonon-surface scattering intensity at low frequencies, but also reconfigure the dispersion curve to reduce the group velocity. A series of flat resonance phonon modes are generated throughout the whole spectrum due to the hybridization between the local resonance phonon modes of the nanopillar and the phonon modes of the substrate NWs, resulting in the phonon modes shifting to lower frequencies. The pillar-based surface induced surface phonon localization and local resonance phenomenon contributes to the modulation of phonon thermal transport in GaAs-based field-effect transistors.

Polarization-Resolved Electron Energy Gain Nanospectroscopy With Phase-Structured Electron Beams

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Spectral analysis of localized surface phonon polaritons in resonant silicon carbide hollow cylinder array

Manipulation of the amplitude and frequency of resonant optical surface waves in mid-infrared is of great interest for improvement of photonic devices and vibrational molecule sensing applications. Antennas supporting localized surface phonon polaritons (LSPhPs) fold the optical phonons into periodic pillar array to control the scattering process. Energy exchange, mode evolution and near-field coupling mechanism are investigated thoroughly, and it is demonstrated that the transverse dipole mode in the 6H-silicon carbide hollow cylinder array shows excellent absorption efficiency and tunable capability across a wide spectral range. Dependence of local field on structural parameters in the polarized sub-mode is explored to elucidate the optical properties. Near-field coupling is further evaluated by combining the values of current distribution with multipole decomposition. This study also provides a practical guide to establish a general framework for exploring the spectral tuning and coupling mechanisms of LSPhP modes.

Thermal transport in porous graphene with coupling effect of nanopore shape and defect concentration

Thermal conductivity of porous graphene can be affected by defect concentration, nanopore shape and distribution, and it is hard to clarify the effects due to the correlation of those factors. In this work, molecular dynamics simulation is used to compare the thermal conductivity of graphene with three shapes of regularly arranged nanopores. The results prove the dominant role of defect concentration under certain circumstances in reducing thermal conductivity, while the coupling effect of nanopore shape should be noticed. When the atoms at the local phonon scattering area around each nanopore are properly removed, the abnormal increment of thermal conductivity can be detected with the increase of defect concentration. Heat flux vector angles can effectively characterize the local phonon scattering area, which can be used to describe the effect of nanopore shape. The coupling effect of defect concentration and pore shape with similar heat flux path is clarified according to this process. By adjusting vertex angle of triangle defect, there is a balanced state of the effect factors between the variation of defect concentration and the same phonon scattering area. It provides a possible way to describe the weighing factors of the coupling effect. The results suggest a feasible approach to optimize and regulate thermal properties of porous graphene in nanodevice.

Launching Coherent Acoustic Phonon Wave Packets with Local Femtosecond Coulomb Forces.

Generation and manipulation of coherent acoustic phonons enables ultrafast control of solids and has been exploited for applications in various acoustic devices. We show that localized coherent acoustic phonon wave packets can be launched by ultrafast Coulomb forces in a scanning tunneling microscope (STM) using tip-enhanced terahertz electric fields. The wave packets propagate at the speed of the longitudinal acoustic phonon, creating standing waves up to 0.26THz for a 6.4nm thin Au film on mica. The ultrafast lattice displacement can be as large as 5pm and is precisely controlled by varying the tip-sample distance. This nonthermal femtosecond Coulomb-force-based excitation mechanism is applicable in nano-optomechanics for advanced terahertz engineering and opens new perspectives in exploiting coherent phonons at the atomic scale.

Localization of the Optical Phonon Modes in Boron Nitride Nanotubes: Mixing Effect of 10B Isotopes and Vacancies.

We explored the mixing effect of 10B isotopes and boron (B) or nitrogen (N) vacancies on the atomic vibrational properties of (10,0) single-wall boron nitride nanotubes (BNNTs). The forced oscillation technique was employed to evaluate the phonon modes for the entire range (0–100%) of 10B isotopes and atomic vacancy densities ranging from 0 to 30%. With increasing isotope densities, we noticed a blue shift of the Raman-active A1 phonon peak, whereas an increased density of mixed or independent B and N vacancies resulted in the emergence of a new low-frequency peak and the annihilation of the A1 peak in the phonon density of states. High-energy optical phonons were localized as a result of both 10B isotopes and the presence of mixing defects. We found an asymmetrical nature of the localization length with increasing 10B isotope content, which corresponds well to the isotope-inherited localization length of carbon nanotubes and monolayer graphene. The localization length falls abruptly with the increase in concentration of both atomic vacancies (B or N) and mixing defects (10B isotope and vacancies). These findings are critical for understanding heat conduction and nanoscopic vibrational investigations such as tip-enhanced Raman spectra in BNNTs, which can map local phonon energies.

Ultralow and anisotropic thermal conductivity in graphene phononic metamaterials

Phononic metamaterials based on the idea of phonon coherent resonance attract increasing research attention because of their unique properties. In this paper, we have studied the heat transport in graphene phononic metamaterials (GPMs, pillared graphene nanoribbons) using molecular dynamics simulations. The results show that, GPMs have lower thermal conductivities in the different in-plane directions and stronger anisotropy in thermal conductance, compared with graphene nanoribbons (GNRs). As the temperature decreases or the height and width of pillar increases, the GPM thermal conductivity in both directions decreases and the anisotropic ratio increases. The pillar-driven suppression of thermal conductivity along the periodic direction is attributed to the phonon localization and the local resonance leading to the flat bands and band gaps, while the reduction in thermal conductivity along the aperiodic direction is due to the phonon boundary scattering. The thermal anisotropy results from the anisotropic phonon relaxation times and the anisotropic contributions of low-frequency-phonons. Our findings reveal the phonon transport mechanisms of GPMs in different directions, and shed some light on the GPM potential for thermoelectric and directional heat management applications.

High figure-of-merit and power generation in high-entropy GeTe-based thermoelectrics

The high-entropy concept provides extended, optimized space of a composition, resulting in unusual transport phenomena and excellent thermoelectric performance. By tuning electron and phonon localization, we enhanced the figure-of-merit value to 2.7 at 750 kelvin in germanium telluride-based high-entropy materials and realized a high experimental conversion efficiency of 13.3% at a temperature difference of 506 kelvin with the fabricated segmented module. By increasing the entropy, the increased crystal symmetry delocalized the distribution of electrons in the distorted rhombohedral structure, resulting in band convergence and improved electrical properties. By contrast, the localized phonons from the entropy-induced disorder dampened the propagation of transverse phonons, which was the origin of the increased anharmonicity and largely depressed lattice thermal conductivity. We provide a paradigm for tuning electron and phonon localization by entropy manipulation, but we have also demonstrated a route for improving the performance of high-entropy thermoelectric materials.

Chiral emission induced by the interaction between chiral phonons and localized plasmon

We demonstrate chiral photoluminescence and scattering induced by the interaction between chiral phonons and localized plasmon. In the experiment, we constructed a hybrid structure of single gold nanorods and bilayer graphene. The optical chirality was investigated with a helicity-resolved single-particle spectroscopy technique, including the dark-field scattering and photoluminescence spectra. The single-particle spectra can effectively indicate the chiral phonon in bilayer graphene. That is due to the interaction between achiral local surface plasmon resonance and chiral phonons, which influences the plasmon damping at the interface. We propose a plasmon–phonon coupled spectroscopy method for phonon chirality detection. This method provides an advantage for developing high spatial resolution detection of chiral phonon in low-dimensional materials due to the localization of plasmonic near-field.

Eliashberg theory of the Jahn-Teller-Hubbard model

We study a multiorbital Hubbard model coupled to local Jahn-Teller phonons to investigate the superconducting state realized in fullerides. A weak-coupling approach is employed in combination with a local self-energy approximation. In addition to the normal and anomalous self-energies of the electrons, we consider the phonon self-energy, which allows a self-consistent treatment of the energetics. The frequency dependence of the self-energies and their characteristic coefficients, such as renormalization factors and dampings, are investigated in detail using numerical calculations. It is clarified that the anisotropic phonons play an important role in the stabilization of the superconducting state. By comparing the full results to those without phonon self-energies, we show that the superconductivity is stabilized by the softening of the phonon frequency. The effects of electronic fluctuations are also considered, which leads to the coupling to orbitons, an analog of plasmons in the electron gas. This additional contribution further stabilizes the superconducting state.

Local Phonon Environment as a Design Element for Long-Lived Excitonic Coherence: Dithia-anthracenophane Revisited

The purpose of this study is to investigate the role of a structured immediate phonon environment in determining the exciton dynamics and the possibility of using it as an optimal design element. Through the case study of dithia-anthracenophane, a bichromophore using the Hierarchical Equations Of Motion formalism, we show that the experimentally observed coherent exciton dynamics can be reproduced only by considering the actual structure of the phonon environment. While the slow dephasing of quantum coherence in dithia-anthracenophane can be attributed to strong vibronic coupling to high-frequency modes, vibronic quenching is the source of long oscillation periods in population transfer. This study sheds light on the crucial role of the structure of the immediate phonon environment in determining the exciton dynamics. We conclude by proposing some design principles for sustaining long-lived coherence in molecular systems.

Thermal transport of monolayer amorphous carbon and boron nitride

Amorphous materials feature localization of electrons and phonons that alter the electronic, mechanical, thermal, and magnetic properties. Here, we report calculations of the in-plane thermal conductivities of monolayer amorphous carbon and monolayer amorphous boron nitride, by reverse nonequilibrium molecular dynamics simulations. We find that the thermal conductivities of both monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (ma-BN) are about two orders of magnitude smaller than their crystalline counterparts. Moreover, the ultralow thermal conductivities are independent of the temperature and strain due to their extremely short heat carrier mean free paths. The relation between the structure disorder and the reduction of the thermal conductivity is analyzed in terms of the vibrational density of states and the participation ratio. The ma-BN shows strong vibrational localization across the frequency range, while the MAC exhibits a unique extended G* diffuson mode due to its sp2 hybridization and the broken E2g symmetry. The irregular vibrational patterns are also analyzed. The present results may enable potential applications of MAC and ma-BN in thermal management.

Tuning of Structural and Magnetic Properties of SrSnO3 Nanorods in Fabrication of Blocking Layers for Enhanced Performance of Dye-Sensitized Solar Cells

Perovskite-based SrSnO3 nanostructures doped with indium are prepared via a facile chemical precipitation method. Prepared nanostructures are used to assemble the dye-sensitized solar cells (DSSCs), and their photovoltaic response and electrochemical impedance spectra are measured. The synthesized samples are subjected to structural, morphological, optical, and magnetic properties. The X-ray diffraction pattern confirms the single-phase orthorhombic (Pbnm) perovskite structure. Local structural and phonon mode variations are examined by Raman spectra. Electron micrographs disclose the nanorods. The elements (Sr, Sn, O, and In) and the existence of oxygen vacancies are identified by X-ray photoelectron spectroscopy analysis. Surface area analysis demonstrates the higher surface area (11.8 m2/g) for SrSnO3 nanostructures. Optical absorption spectra confirm the good optical behavior in the ultraviolet region. The multicolor emission affirms the presence of defects/vacancies present in the synthesized samples. The appearance of interesting ferromagnetic behavior in the prepared samples is due to the presence of F-center exchange interactions. Under the irradiation (1000 W/m2) of simulated sunlight, the DSSC fabricated by 3% In-doped SrSnO3 exhibits the highest η of 5.68%. Hence, the blocking layers prepared with pure and indium-doped samples could be the potential candidates for DSSC applications.

A Qualitative Study of the Disorder Effect on the Phonon Transport in a Two-Dimensional Graphene/h-BN Heterostructure

Recently, massive efforts have been made to control phonon transport via introducing disorder. Meanwhile, materials informatics, an advanced material-discovery technology that combines data-driven search algorithms and material property simulations, has made significant progress and shown accurate prediction ability in studying the target properties of new materials. However, with the introduction of disorder, the design space of random structures is greatly expanded. Global optimization for the entire domain is nearly impossible with the current computer resource even when materials informatics reduces the design space to a few percent. Toward the goal of reducing design space, we investigate the effect of different types of disorders on phonon transport in two-dimensional graphene/hexagonal boron nitride heterostructure using non-equilibrium molecular dynamics simulation. The simulation results show that when the hexagonal boron nitride is distributed disorderly in the coherent phonon-dominated structure, that is, the structure with a period length of 1.23 nm, the thermal conductivity is significantly reduced due to the appearance of coherent phonon localization. By qualitatively analyzing different types of disorder, we found that the introduction of disordered structure in the cross direction with a larger shift distance can further reduce the thermal conductivity. Further physical mechanism analysis revealed that the structures with lower thermal conductivity were caused by weak propagation and strong localization of phonon. Our findings have implications for accelerating machine learning in the search for structures with the lowest thermal conductivity, and provide some guidance for the future synthesis of 2D heterostructures with unique thermal properties.

Strong reduction of thermal conductivity of WSe2 with introduction of atomic defects

The thermal conductivities of pristine and defective single-layer tungsten diselenide (WSe2) are investigated by using equilibrium molecular dynamics method. The thermal conductivity of WSe2 increases dramatically with size below a characteristic of ~5 nm and levels off for broader samples and reaches a constant value of ~2 W/mK. By introducing atomic vacancies, we discovered that the thermal conductivity of WSe2 is significantly reduced. In particular, the W vacancy has a greater impact on thermal conductivity reduction than Se vacancies: the thermal conductivity of pristine WSe2 is reduced by ~60% and ~70% with the adding of ~1% of Se and W vacancies, respectively. The reduction of thermal conductivity is found to be related to the decrease of mean free path (MFP) of phonons in the defective WSe2. The MFP of WSe2 decreases from ~4.2 nm for perfect WSe2 to ~2.2 nm with the addition of 0.9% Se vacancies. More sophisticated types of point defects, such as vacancy clusters and anti-site defects, are explored in addition to single vacancies and are found to dramatically renormalize the phonons. The reconstruction of the bonds leads to localized phonons in the forbidden gap in the phonon density of states which leads to a drop in thermal conduction. This work demonstrates the influence of different defects on the thermal conductivity of single-layer WSe2, providing insight into the process of defect-induced phonon transport as well as ways to improve heat dissipation in WSe2-based electronic devices.