Phonon Propagation Research Articles

Development of five-dimensional scanning transmission electron microscopy.

By combining the scanning transmission electron microscopy with the ultrafast optical pump-probe technique, we improved the time resolution by a factor of ∼1012 for the differential phase contrast and convergent-beam electron diffraction imaging. These methods provide ultrafast nanoscale movies of physical quantities in nano-materials, such as crystal lattice deformation, magnetization vector, and electric field. We demonstrate the observations of the photo-induced acoustic phonon propagation with an accuracy of 4ps and 8nm and the ultrafast demagnetization under zero magnetic field with 10ns and 400nm resolution, by utilizing these methods.

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Enhanced phonon resonance by non-uniform surface nanopillars in Si nanowires

Based on the wave nature of phonons, the lattice thermal conductivity of nanowires can be regulated by phonon resonators such as surface nanopillars. In this study, the impact of the height and distance of the resonant nanopillars on the thermal conductivity of Si nanowires were investigated. The results calculated from the molecular dynamics simulations show that with increasing height of the nanopillars, the corresponding thermal conductivity decreases. We found that the nanopillars with non-uniform heights induce stronger phonon resonance effect and can further suppress the thermal conductivity, as compared to the models with nanopillars of uniform heights. On the other hand, changing the distance between two adjacent nanopillars does not obviously affect the phonon propagation. Phonon dispersion relations show the formation of flat bands, which confirms the occurrence of phonon resonances due to the nanopillars. When the heights of the nanopillars become non-uniform, more flat bands were generated, and the phonon group velocity further decreased. Analyses of phonon participation ratio indicate stronger phonon localization induced by the disorder stemming from the non-uniform nanopillar height. Our investigation yields new insights into the physical mechanisms governing heat transport in nanowires with phonon resonators, and thus opens potential new routes to the design of thermoelectric devices.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 from ab initio Lattice Dynamics.

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κL ) and a non-standard κL ∝T-0.491 dependence. At high T, the departure from the standard κL ∝T-1 law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κL =2.98 W mK-1 arises mainly from the conventional particle-like propagation of phonons, there is also asignificant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κL with a weak T-dependence points to a new utility for LaWN3 in energy technology applications, and motivates synthesis and exploration of nitrideperovskites.

Structural, electronic and optical properties of fluorinated bilayer silicene

Silicon (Si) is undoubtedly an excellent material for electronic applications. However, Si is not a suitable material for optoelectronic applications due to its indirect bandgap. Based on the first principles approach, we have studied different stacking (AA and AB) patterns in bilayer silicene. The phonon band structure of AA and AB-stacked “bilayer silicene” reveals that only AB-stacked silicene is stable. Different fluorinated configurations (1F, 2F, 3F and 4F) of AB-stacked bilayer silicene are analyzed in terms of structural parameters and the phonon band structure. Of the four fluorinated configurations, 2F-AB-stacked bilayer silicene has a direct bandgap of 1.23 eV. The direct bandgap allows the carriers to transit from the “valence band” to the “conduction band” with a low probability of phonon generation. Meanwhile, the other configurations either lack stability or have a zero bandgap. The proposed fluorinated bilayer silicene, in addition to having an expected integration with silicon technology, could also be suitable for visible and near-infrared light detection applications.

Thermal Conduction and Phonon Transport in Folded Polyethylene Chains

Using theory and simulations, we have investigated the phonons and their role in thermal energy transport in semicrystalline polyethylenes. Considering alternating stacks of lamellae and amorphous regions, and labeling one polyethylene chain interwoven among two amorphous regions and one lamella, we have explored the underlying mechanism of thermal conductivity of polyethylene in its semicrystalline state. We report that hairpin-like folds at the crystalline–amorphous interface significantly scatter phonons, allowing only less than half of the phonons to transmit through polyethylene backbone. Monitoring the phonon propagation and scattering at the interfaces, we have computed thermal conductivity of semicrystalline polyethylene. We have derived a design principle to control thermal conductivity of semicrystalline polyethylene in terms of lamellar thickness and the number of folds per chain at the crystalline–amorphous interface.

Electronic heat generation in semiconductors: Non-equilibrium excitation and evolution of zone-edge phonons via electron–phonon scattering in photo-excited germanium

We investigate experimentally and using first-principles theory the generation of phonons and the relaxation of carriers on picosecond timescales across the Brillouin zone of photo-excited Ge by inter-valley electron–phonon scattering. The phonons generated are typical of those generated in semiconductor devices, contributing to the accumulation of heat within the material. We simulate the time-evolution of phonon populations, based on first-principles band structure and electron–phonon and phonon–phonon matrix elements, and compare them to data from time-resolved x-ray diffuse scattering experiments, performed at the Linac Coherent Light Source x-ray free-electron laser facility, following photo-excitation by a 50 fs near-infrared optical pulse. We show that the intensity of the non-thermal x-ray diffuse scattering signal, which is observed to grow substantially near the L-point of the Brillouin zone over 3–5 ps, is due to phonons generated by scattering of carriers between the Δ and L valleys. These phonons have low group velocities, resulting in a heat bottleneck. With the inclusion of phonon decay through 3-phonon processes, the simulations also account for other non-thermal features observed in the x-ray diffuse scattering intensity, which are due to anharmonic phonon–phonon scattering of the phonons initially generated by electron–phonon scattering.

Experimental evidence for the significance of optical phonons in thermal transport of tin monosulfide

The understanding of the lattice dynamics is essential for engineering the thermal transport properties in quantum materials. Based on the canonical point of view, acoustic phonons are believed to be the principal thermal carriers in heat flow. Here, in this work, optical phonons are elucidated to play a pivotal role in determining the lattice thermal conductivity in thermoelectric material SnS by using the state-of-the-art inelastic neutron scattering technique combined with first-principles calculations. Additionally, in contrast to acoustic phonons, optical phonons are observed to exhibit pronounced softening and broadening with temperature. Our observations not only shed light on the significance of the optical phonons in thermal transport but also provide a vital clue to suppress the propagation of optical phonons to optimize the thermoelectric performance of SnS.

Phonon Spectroscopy of Solid Dielectrics

A method for studying solid dielectrics under conditions of nonstationary propagation of subterahertz phonons is presented. The method analyzes the transport characteristics of thermal phonons in the helium (He) temperature range (2–4 K) in diffusion mode and makes it possible to investigate the following: the kinetic characteristics of thermal phonons in single crystals of solid solutions; the contribution to phonon scattering from the concentration, type, and positions of a substitutional impurity, defects comparable with the wavelength (clusters, phase inhomogeneities, dislocations), low-energy excitations of various nature, including those associated with the formation of an equilibrium configuration of vacancies in the anionic sublattice with respect to substitutional impurity cations; equilibrium criteria in the system of nonequilibrium phonons–low-energy excitations; peculiarities of the phonon spectrum of nanostructured materials; the relationship between the diffusion coefficient of subterahertz phonons and the system of grain boundaries (GB), grain size, conditions for the formation of a “gap” in the phonon spectrum of nanostructured ceramics; spectral properties of GB, their relationship with the technological conditions of synthesis, to evaluate the average thickness and acoustic impedance of GB for a sample; the features of thermal phonon transport in amorphous dielectrics (glasses and glasslike materials) near the thermal conductivity “plateau,” the possibility of “gap” formation in the spectrum of phonon states; subterahertz phonon transport in ceramics based on ferroelectrics, electrolyte solid solutions, and cermets.

Localized coherent phonon generation in monolayer MoSe2 from ultrafast exciton trapping at shallow traps.

We report spectroscopic evidence for the ultrafast trapping of band edge excitons at defects and the subsequent generation of defect-localized coherent phonons (CPs) in monolayer MoSe2. While the photoluminescence measurement provides signals of exciton recombination at both shallow and deep traps, our time-resolved pump-probe spectroscopy on the sub-picosecond time scale detects localized CPs only from the ultrafast exciton trapping at shallow traps. Based on occupation-constrained density functional calculations, we identify the Se vacancy and the oxygen molecule adsorbed on a Se vacancy as the atomistic origins of deep and shallow traps, respectively. Establishing the correlations between the defect-induced ultrafast exciton trapping and the generation of defect-localized CPs, our work could open up new avenues to engineer photoexcited carriers through lattice defects in two-dimensional materials.

Twisted grain boundary leads to high thermoelectric performance in tellurium crystals

A twisted grain boundary is introduced in the tellurium crystal to effectively block phonon propagation while maintaining high electron mobility for superior thermoelectric properties.

Resonance-enhanced excitation and relaxation dynamics of coherent phonons in Fe1.14Te.

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te-a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te.

The influence of initial phonon states on the generation of coherent optical phonons

The generation of coherent phonons is theoretically studied using a thermal equilibrium phonon distribution as the initial condition. The time evolution of the coherent phonon amplitude and phonon population is calculated for 4.5-THz phonons with initial phonon distributions at 10 and 300 K. It is determined that the coherent-phonon amplitudes of state-selected phonons are dependent on the initial phonon distribution. The coherent-phonon amplitudes, when considering all phonon states instead of selecting certain phonon levels, are almost the same for both initial distributions. The variance of the coherent-phonon amplitudes is found to oscillate with twice the phonon frequency, and its oscillation amplitude depends on the initial phonon distribution.

Enhanced Thermoelectric Performance of Yb-Filled Skutterudite with Bottom-Up Formed CoSi2 Nanoparticles.

It is known that Yb-filled skutterudite with excellent thermoelectric performance is promising for a power generation device in the intermediate temperature region. Here we created a new approach to obtain nanostructured materials by adding Si to Co-overstoichiometric Yb-filled skutterudite through high-energy ball milling, which embedded bottom-up formed CoSi2 nanoparticles into grain-refining Yb0.25Co4Sb12, synergistically resulting in the enhanced thermoelectric properties and room-temperature hardness. On one hand, the abundant grain boundaries and phase interfaces effectively blocked the propagation of medium-low frequency phonons, resulting in a lower lattice thermal conductivity. On the other hand, phase interfaces barrier nicely screened a portion of low-energy electrons, leading to an improved power factor. As a result, an enhanced peak ZT value of ∼1.43 at 823 K and a promising average ZT of ∼1.00 between 300 and 823 K were achieved in the Yb0.25Co4Sb12/0.05CoSi2 sample. Meanwhile, such nanostructures also enhanced the hardness through the collective contributions of second phase and fine grain strengthening, which made skutterudite more competitive in practical application.

Phonon Thermal Transport in Bi2Te3 from a Deep-Neural-Network Interatomic Potential

${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is a widely used thermoelectric material with strong anharmonicity. Determination of its thermal conductivity requires consideration of the high-order phonon scattering process, which makes it extremely time consuming and challenging to accurately calculate its thermal conductivity by obtaining high-order force constants based on density-functional theory. In this work, a deep-neural-network potential is developed to reproduce phonon dispersion and predict the lattice thermal conductivity of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The equilibrium molecular dynamics simulations combined with this potential are performed to calculate the lattice thermal conductivity and the results nicely match the experimental values. Meanwhile, we find the generalized gradient approximation with the DFT-D3 functional can accurately reproduce the experimental lattice constants of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and provide a description of the phonon dispersion in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ as well as the local density approximation. Furthermore, we explore the influence of the native point defects on thermal conductivity, and find that $\mathrm{Te}$ vacancies have the most significant effect on the reduction of thermal conductivity, owing to the appreciable inhibition of phonon propagation speed by $\mathrm{Te}$ vacancies, and the additional scattering among original low-frequency optical phonons and the fresh low-frequency optical phonons moving downward from high frequency region, which provides some theoretical guidance for reducing thermal conductivity in experimental research.

Highly thermal conductive graphene/carbon nanotubes films with controllable thickness as thermal management materials

As electronic devices continue to be integrated and miniaturized, the increased system power density leads to a continued increase in operating temperature, ultimately leading to degradation of stability and performance. Therefore, the development of thermal management materials with superior thermal conductivity is urgently needed. Herein, full-carbon graphitized graphene/carbon nanotubes (CNTs) (gGC) films with controlled thickness were fabricated through compositing followed by compaction. The appropriate amount of CNTs doping enlarges the crystallinity and improves the stacking order of the composite film during the structural evolution process. Ultimately, the thermal synergy between CNTs and graphene sheets accelerates the propagation of phonons and endows the free-standing gGC film with good performance. The flexible gGC film shows an in-plane thermal conductivity of 1280.3 W/mK, an electrical conductivity of 6559 S/cm, and the foldability of 10,000 times at a thickness of 30 μm. Moreover, gGC films with controllable thicknesses were successfully prepared through a convenient “multilayer compaction” strategy, which allows thick gGC films to maintain a high level of thermal dissipation, while effectively mitigating the rapid decline in thermal conductivity of the films with increasing thickness. This work provides a general method for the realization of large-scale and convenient production of high-thermally conductive thick films.

Fluorine-terminated functionalized liquid metal/silicon carbide binary nanoparticles for polyvinyl alcohol composite films with high in-plane thermal conductivity and ultra-low dielectric constant

Highly thermally conductive and low dielectric constant are among the most sought-after properties of flexible polymer-based thermal-management materials (TMMs). However, it remains a formidable challenge to develop efficient thermal dissipation materials with fascinating comprehensive performance. In this work, we present an interfacial engineering strategy using trifluoropropyltriethoxysilane (TFPTES) to act as a thermal linker at granular silicon carbide-liquid metal (SiC-LM) binary interfaces (SiC-LM@F) in the polyvinyl alcohol (PVA) matrix, in which the LMs merely partially anchored to the surface of binary filler by a controllable ball milling technology. The strong hydrogen bonding and/or compact “shoulder-to-shoulder” contact between the binary components and matrix interfaces confer an expressway for the propagation of interfacial phonons. For this, the developed PVA/SiC-LM@F composite film demonstrates a high in-plane TC of 10.2 W m−1 K−1 and ultra-low dielectric constant of 1.49 at 106 Hz at 40 wt% SiC-LM@F loading content, which is more superior to those of the other counterparts in this experiment. Additionally, the obtained PVA/SiC-LM@F composite film also reveals favorable tensile strength, excellent electric insulation, and efficient cooling capacity. This interfacial engineering strategy is expected to offer a versatile solution to the design and construction of advanced thermal-management materials (TMMs).

TRANSDUCCIÓN OPTOMECÁNICA Y BIESTABILIDADES ÓPTICAS ENMICRORRESONADORES BASADOS EN REFLECTORES DE BRAGG

In cavity optomechanics experiments with ultrashort laser pulses, the temporal dynamics of the optical modes of the system play a fundamental role in the generation and detection of coherent phonons. In this work, we analyze the feasibility of using a second continuous emission laser to optimize the detection efficiency, by setting the fundamentalótical mode to a specific spectral position. Under certain conditions, and through this continuum excitation, it is possible to modify the recovery dynamics of the optical modes after an ultrafast pulsed excitation, to the point of stopping it on a quasiequilibrium. This phenomenon is directly related to the optical bistability of the resonator.

Chemistry in Advancing Thermoelectric GeTe Materials.

ConspectusThe ever-growing energy crisis and the deteriorated environment caused by carbon energy consumption motivate the exploitation of alternative green and sustainable energy supplies. Because of the unique advantages of zero-emission, no moving parts, accurate temperature control, a long steady-state operation period, and the ability to operate in extreme situations, thermoelectrics, enabling the direct conversion between heat and electricity, is a promising and sustainable option for power generation and refrigeration. However, with increasing application potentials, thermoelectrics is now facing a major challenge: developing high-performance, Pb-free, and low-toxic thermoelectric materials and devices.As one group of promising candidates, GeTe derivatives have the potential to replace the widely used thermoelectric materials containing highly toxic elements. In this Account, we summarize our recent progress in developing high-performance GeTe-based thermoelectric materials via exploring innovative strategies to enhance electron transports and dampen phonon propagations. First, we fundamentally illustrate the underlying chemistry and physical reason for an intrinsically high carrier concentration in GeTe, which enormously restrains the thermoelectric performance of GeTe. From our theoretical calculations, the formation energy of Ge vacancy is the lowest among the defects in GeTe, energetically favoring Ge vacancies in the lattice and leading to intrinsically high carrier concentrations. Accordingly, aliovalent doping/alloying is proposed to increase the formation energy of Ge vacancies and decrease the carrier concentration to the optimal level. We then outline the newly developed method to refine the band structures of GeTe with tuned electronic transport. On the basis of the molecular orbital theory, the energy offset between two valence band edges at the L and Σ points in GeTe should be ascribed to the slightly different Ge_4s orbital characters at these two points, which guides the screening of dopants for band convergence. Besides, the Rashba spin splitting is explored to increase the band degeneracy of GeTe. Afterward, we analyze the dampened phonon propagation in GeTe to minimize its lattice thermal conductivity. Alloying with the heavy Sb atoms can shift the optical phonon modes toward low frequency and reinforce the interaction of optical and acoustic phonon modes so that the inherent phonon scattering is enhanced. In addition, planar vacancies and superlattice precipitates can significantly strengthen phonon scattering to result in ultralow lattice thermal conductivity. After that, we overview the finite elemental analysis simulations to optimize the device geometry for maximizing the device performance and introduce the as-developed prototype GeTe-based thermoelectric device. In the end, we point out future directions in the development of GeTe for device applications. The strategies summarized in this Account can serve as references for developing wide materials with enhanced thermoelectric performance.

Structural Optimization and MEMS Implementation of the NV Center Phonon Piezoelectric Device.

The nitrogen-vacancy (NV) center of the diamond has attracted widespread attention because of its high sensitivity in quantum precision measurement. The phonon piezoelectric device of the NV center is designed on the basis of the phonon-coupled regulation mechanism. The propagation characteristics and acoustic wave excitation modes of the phonon piezoelectric device are analyzed. In order to improve the performance of phonon-coupled manipulation, the influence of the structural parameters of the diamond substrate and the ZnO piezoelectric layer on the phonon propagation characteristics are analyzed. The structure of the phonon piezoelectric device of the NV center is optimized, and its Micro-Electro-Mechanical System (MEMS) implementation and characterization are carried out. Research results show that the phonon resonance manipulation method can effectively increase the NV center’s spin transition probability using the MEMS phonon piezoelectric device prepared in this paper, improving the quantum spin manipulation efficiency.