Local Phonon Research Articles

Size and environment: The effect of phonon localization on micro-Brillouin imaging

Specifically designed samples have been analyzed to test the ability of Brillouin spectroscopy to provide reliable mechanical characterization of micro and nano-objects. The selected samples are polymeric films, whose transversal sizes from hundreds of nano- to some micro-meters cover the entire range of length-scales relevant in Brillouin scattering process. The experimental data highlight how, the size of the extended collective oscillation (acoustic phonons, in brief) is the lowest spatial resolution reachable in Brillouin mechanical characterization. Conversely, in the limit condition of phonon confinement, the technique provides the mechanical properties of nano-objects whose characteristic size is comparable with the phonon wavelength (⁓300 nm). Investigating acoustically heterogeneous materials, both size of heterogeneity and acoustic mismatch between adjacent regions are shown to be relevant in shaping the Brillouin response. In particular, a transition from a confined to a non-confined condition is obtained modulating the acoustic mismatch between the micro-objects and their local environment. The provided results and the derived analytic models for the data analysis will guide the interpretation of Brillouin spectra acquired in complex nano-structured samples such as cells, tissues or biomimetic materials. Our analysis can therefore generate new insights to tackle fundamental problems in mechanobiology or to characterize new bioengineered materials.

Impeded thermal transport in aperiodic BN/C nanotube superlattices due to phonon Anderson localization

Anderson localization of phonons is a kind of phonon wave effect, which has been proved to occur in many structures with disorders. In this work, we introduced aperiodicity to boron nitride/carbon nanotube superlattices (BN/C NT SLs), and used molecular dynamics to calculate the thermal conductivity and the phonon transmission spectrum of the models. The existence of phonon Anderson localization was proved in this quasi one-dimensional structure by analyzing the phonon transmission spectra. Moreover, we introduced interfacial mixing to the aperiodic BN/C NT SLs and found that the coexistence of the two disorder entities (aperiodicity and interfacial mixing) can further decrease the thermal conductivity. In addition, we also showed that anharmonicity can destroy phonon localization at high temperatures. This work provides a reference for designing thermoelectric materials with low thermal conductivity by taking advantage of phonon localization.

Intrinsic thermoelectric properties in biphenylene nanoribbons and effect of lattice defects

Intrinsic thermoelectric properties in biphenylene nanoribbons (BNRs) and effect of lattice defects are investigated by employing the nonequilibrium Green’s function. A comparative analysis on intrinsic thermoelectric properties between graphene nanoribbons (GNRs) and BNRs is performed. Results show that although the Seebeck coefficient of BNRs is lower than that of the corresponding GNRs, BNRs exhibit better thermoelectric properties than the corresponding GNRs due to the lower phonon thermal conductance of BNRs. It is also shown that the lattice defects can enhance the maximum ZT (ZTmax) in armchair BNRs (ABNRs) and zigzag BNRs (ZGNRs), where the ZTmax can be up to 1.1 and 0.4 at T = 300 K, respectively. Moreover, such the enhancement in defective ZBNRs is more remarkable than that in defective ABNRs. Theoretical analysis reveals that this enhancement of the ZTmax originates from the substantial reduction of phonon thermal conductance due to the strong backscattering by the highly localized phonon states around the lattice defects.

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.

Tuning interfacial thermal conductance of GaN/AlN heterostructure nanowires by constructing core/shell structure

The ability to tune the interfacial thermal conductance of GaN/AlN heterojunction nanowires (NWs) with a core/shell structure is shown using molecular dynamics and non-equilibrium Green’s functions method. In particular, an increase in the shell thickness leads to a significant improvement of interfacial thermal conductance of GaN/AlN core/shell NWs. At room temperature (300 K), the interfacial thermal conductance of NWs with specific core/shell ratio can reach 0.608 nW K−1, which is about twice that of GaN/AlN heterojunction NWs due to the weak phonon scattering and phonon localization. Moreover, changing the core/shell type enables one to vary interfacial thermal conductance relative to that of GaN/AlN heterojunction NWs. The results of the study provide an important guidance for solving the thermal management problems of GaN-based devices.

Quantum transport in driven systems with vibrations: Floquet nonequilibrium Green's functions and the self-consistent Born approximation

We investigate the effects of alternating voltage on nonequilibrium quantum systems with localized phonon modes. Nonequilibrium Green's functions are utilized, with electron-phonon coupling being considered with the $GD$ approximation (self-consistent Born approximation). Using a Floquet approach, we assume periodicity of the dynamics. This approach allows us to investigate the influence of the driven electronic component on the nonequilibrium occupation of the vibrations. It was found that signatures of inelastic transport gained photon-assisted peaks. A simplistic model was proposed and found to be in good agreement with the full model in certain parameter ranges. Moreover, it was found that driving the alternating current at resonance with vibrational frequencies caused an increase in phonon occupation.

Strong Antibonding I (p)-Cu (d) States Lead to Intrinsically Low Thermal Conductivity in CuBiI4.

Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κlat) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κlat of ∼0.34-0.28 W m-1 K-1 in the temperature range 4-423 K with an Umklapp crystalline peak of 1.82 W m-1 K-1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (Ef), which arise from robust copper 3d and iodine 5p interactions, cause copper-iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κlat in CuBiI4. The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper-iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p-d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κlat in CuBiI4.

Local phonon imaging of AlN nanostructures with nanoscale spatial resolution.

We demonstrate local phonon analysis of single AlN nanocrystals by two complementary imaging spectroscopic techniques: tip-enhanced Raman scattering (TERS) and nano-Fourier transform infrared (nano-FTIR) spectroscopy. Strong surface optical (SO) phonon modes appear in the TERS spectra with their intensities revealing a weak polarization dependence. The local electric field enhancement stemming from the plasmon mode of the TERS tip modifies the phonon response of the sample, making the SO mode dominate over other phonon modes. The TERS imaging allows the spatial localization of the SO mode to be visualized. We were able to probe the angle anisotropy on the SO phonon modes in AlN nanocrystals with nanoscale spatial resolution. The excitation geometry and the local nanostructure surface profile determine the frequency position of SO modes in nano-FTIR spectra. An analytical calculation explains the behaviour of SO mode frequencies vs. tip position with respect to the sample.

Role of anharmonicity in the phonon density of states of high temperature superconductors

Owing to the quantum dynamical approach of double time temperature dependent phonon-Green’s functions, the expression for phonon density of states (PDOS) has been developed via an almost complete Hamiltonian which consists of the contributions due to the unperturbed phonons and electrons, anharmonic phonon fields, localized phonon fields and that of electron–phonon interactions. It has been observed that the anhrmonicity play an vital role in the PDOS through energy line width. Enhancement in the PDOS due to electron–phonon interactions has been mainly discussed in this work, which, in turns, will be capable to explain a large number of dynamical properties of crystals even that of superconductors. The temperature dependence of PDOS has been found as a unique feature of the theory, which certainly becomes the outcome of the anharmonic interactions. The presence of electron–phonon interaction parameters in each term is an additional and new feature of the theory.

Effect of surface morphology and interfaces on longitudinal phonon thermal conductivity in Ge(001) and Si/Ge(001) thin-film structures

In this work the method of non-equilibrium molecular dynamics was used to study the longitudinal phonon thermal transport at 300 K in nanoscale homogenous Ge(001) and layered Si/Ge(001) thin films with p(2x1) surface reconstruction along different directions. The appearance of thermal transport anysotropy in the films under consideration, which is due to both the surface morphology and sharp Si/Ge interfaces, has been established. For the direction when the dimers and Si-Ge bonds at the interface lie in a plane parallel to the direction of the heat flow, the lowest thermal conductivity is observed (~5-18 W/(m·K) in the range from ~1 to 27 nm). It is shown that for films with thicknesses of more than 13 nm for all directions, layered films have a lower thermal conductivity compared to homogenous ones. In this case, the role of the surface morphology and interfaces is reduced to different degrees of phonon localization and compensation for more heat-conducting Si layers, respectively. Keywords: thermal conductivity, molecular dynamics, Si/Ge thin-film, surface, interfaces.

Heat flux concentrators based on nanoscale phononic metastructures.

In recent years, nanoscale heat flux regulation has been at the forefront of research. Nanoscale heat flux concentration is of potential importance in various applications, but no research has been conducted on local heat flux concentration. In this paper, we designed two heat flux concentrators using patterned amorphous and nanomesh structures, respectively. Using molecular dynamics simulation, we find that the heat flux in the central regions is much higher than that in the adjacent regions, with the concentration ratio arriving at 9-fold. Thus a heat flux concentrator is realized using these nanophononic metastructures. The phonon localization theory was used to explain the underlying mechanism. This work provides a direct design strategy for thermal concentrators using practical nanofabrication technologies.

Thermal Transport Properties of Sulfur-Loaded Carbon-Based Nanotubes and Composite Sulfur Cathodes in Lithium-Sulfur Batteries.

The thermal management of lithium-sulfur batteries with high specific energy has become one of the critical issues for their applications. Carbon-based nanotubes are widely used to construct composite sulfur cathodes. This paper focuses on the thermal transport properties of sulfur-coated and sulfur-embedded boron carbide nanotubes (BCNTs) and carbon nanotubes (CNTs) and their composites using molecular dynamics. It is found that phonon softening and localization play a role in making BCNT exhibit a lower thermal conductivity (TC) than CNT. Furthermore, it is discovered that the sulfur embedded inside the carbon-based nanotube has a greater negative impact on carbon-based nanotube phonon transport. Moreover, the effective medium theory model is not suitable for predicting the effective thermal conductivity of coated sulfur composites, in contrast to its good applicability to embedded sulfur composites. These findings provide an in-depth understanding of the thermal transport properties of composite sulfur cathodes in lithium-sulfur batteries.

High performance magnetostriction of Fe-Cu-Pd ferromagnetic strain glass:Local chemical ordering induced martensitic nanodomains morphology with low modulus

Large magnetostriction with low saturation magnetic field was recently reported in Fe-Pd ferromagnetic strain glass alloys. Such behaviour is highly desirable for miniaturized sensor and actuator applications. The origin of high performance magnetostriction in Fe-Pd ferromagnetic strain glass is suggested to stem from its martensitic nanodomains. However, the atomic formation mechanism underlying these nanodomains and their associated local crystal symmetry breaking have not been well revealed. Moreover, accumulating experimental results indicate that the nano-structure can lead to a small saturation field but cannot guarantee large magnetostriction. Consequently, the necessary conditions for low field triggered large magnetostriction of Fe-Pd ferromagnetic strain glass alloys still remain unclear. In this paper, the chemistry, structure and transformation properties of the Fe98-xCu2Pdx (x = 28.5 ∼ 31) system are systematically explored to discover the origin of the high performance magnetostriction of ferromagnetic strain glass alloys. We found that the structure of ferromagnetic strain glass consists of local chemical order. This leads to tetragonal martensitic nanodomains with the local crystal symmetry breaking. The local chemical order also induces strong local phonon softening and a low modulus during the strain glass transition. Further investigation shows that the combination of martensitic nanodomains and low modulus are the necessary conditions to achieve both large magnetostriction and a low saturation field, because they can result in small energy barriers for the rotation of nanodomains. These findings provide an effective guideline to design high performance magnetostrictive materials and devices.

Phonon-induced breakdown of Thouless pumping in the Rice-Mele-Holstein model

Adiabatic and periodic variation of the lattice parameters can make it possible to transport charge through a system even without net external electric or magnetic fields, known as Thouless charge pumping. The amount of charge pumped in a cycle is quantized and entirely determined by the system's topology, which is robust against perturbations such as disorder and interactions. However, coupling to the environment may play a vital role in topological transport in many-body systems. We study the topological Thouless pumping, where the charge carriers interact with local optical phonons. The semi-classical multi-trajectory Ehrenfest method is employed to treat the phonon trajectories classically and charge carriers quantum mechanically. We find a breakdown of the quantized charge transport in the presence of phonons. It happens for any finite electron-phonon coupling strength at the resonance condition when the pumping frequency matches the phonon frequency, and it takes finite phonon coupling strength away from the resonance. Moreover, there exist parameter regimes with non-quantized negative and positive charge transport. The modified effective pumping path due to electron-phonon coupling accurately explains the underlying physics. In the large coupling regime where the pumping disappears, the phonons are found to eliminate the staggering of the onsite potentials, which is necessary for the pumping protocol. Finally, we present a stability diagram of quantized pumping as a function of time period of pumping and phonon coupling strength.

Electric field control of electromagnon frequency in multiferroics

Electromagnons, which are coupled polar and magnetic excitations in magnetoelectric materials, are of large interest for electronic and computing technological devices. Using molecular dynamics simulations based on an ab initio effective Hamiltonian, we predict that the frequency of several electromagnons can be tuned by the application of electric fields in the model multiferroic ${\mathrm{BiFeO}}_{3}$, with this frequency either increasing or decreasing depending on the selected electromagnon. In particular, we show that the frequency of electromagnons localized at ferroelectric domain walls can be tuned over a 200 GHz range by realistic dc electric fields. We interpret the realized frequency increase (decrease) by local hardening (softening) of the associated polar phonons which couples to the applied electric field. The increase versus decrease in the electromagnon frequency is further found to be correlated with the real-space localization of such phonons.

Identifying the effect of Ni solubility on the thermoelectric properties of HfNiSn-based half-Heuslers

Phase diagrams provide important guidance for the optimal design of thermoelectric materials. Previous studies demonstrated a specific solubility of Ni atoms in TiNiSn and ZrNiSn, which can impede phonon transport and regulate the electronic band structure. But the thermodynamic investigation and the effect of Ni content on thermoelectric properties for the HfNiSn system are absent. In this work, we constructed an isothermal section phase diagram of the Hf-Ni-Sn ternary system at 1173 K and determined the solubility of Ni in the vacant position to be 0.04 < x < 0.16. At this temperature profile, the stoichiometric ratio HfNiSn is located in the two-phase region of HfNi1+xSn + Sn when it is in equilibrium, and the homogeneity region may be broken by the process of ball milling and sintering. Calculation results indicate that the excess-Ni in HfNiSn will generate impurity levels in the forbidden band and localized phonon modes above the acoustic branch. The intermediate-Heusler phase formed by extra Ni atoms can work as a phonon scattering center and reduce the lattice thermal conductivity near room temperature, but the Sn impurities generated by the disrupted thermal equilibrium will neutralize this effect at high temperatures. Overall, the impact of the Ni-rich environment on HfNiSn-based half-Heusler is very different from that of alloys TiNiSn and ZrNiSn, which is of great significance for the comprehensive understanding of (Ti, Zr, Hf)NiSn system.

High-Throughput Screening of Aperiodic Superlattices Based on Atomistic Simulation-Informed Effective Medium Theory and Genetic Algorithm

Superlattices (SLs) have received great attention as thermoelectric materials because phonon transport in them can be tailored independently without changing the electronic properties to enhance the thermoelectric figure of merit and conversion efficiency. Heterogeneous aperiodic SLs (ap-SLs) have lower thermal conductivity than their periodic counterparts (p-SL) due to Anderson localization of coherent phonons. Due to enormous amounts of layered structure combinations in ap-SLs, it is challenging to efficiently screen through all the combinations for the structure with the minimum thermal conductivity. In this work, a modified effective medium theory (m-EMT) is established to predict the thermal conductivity of Si/Ge p-SLs and ap-SLs. The effects of Anderson localization and layer thickness distribution are considered using a correction function for the degree of randomization (DOR) in layer thickness. The combination of the m-EMT and the genetic algorithm enables high-throughput screening of enormous amounts (∼ 2n, where n is the number of atomic layers) of SL structures with micron-scale total SL thickness. The thermal conductivities of ap-SLs can be constantly reduced to 1.5 W/(m·K) at average periodic layer thicknesses of 2.0 nm and a DOR of 0.8, regardless of the total SL thicknesses. Phonon spatial localization energy distribution further reflects the intense Anderson localization at interfaces, especially for those sandwiched between the highly asymmetric thick and thin layers. This work provides a high-throughput screening tool to theoretically design micron-scale and heterogeneous SLs, with detailed structural guidance for experimentally growing ap-SL with minimum thermal conductivities.

Peculiarities of pulsed laser deposition of thin InGaAsN films in an active background gas atmosphere

III-V-N compounds are a promising class of solid solutions that have the prospect of being used in optoelectronic devices operating in a wide spectral range up to 3 μm, as well as for increasing the efficiency of photodetectors, lasers in fiber-optic communication lines and telecommunication systems. In this work, the features of various methods for obtaining new III-V-N semiconductor materials are investigated. Thin InGaAsN films were obtained by pulsed laser deposition on GaAs (100) and Si (100) substrates in an active background gas atmosphere. Pulsed laser deposition of thin InGaAsN films was carried out using an In0.02Ga0.98As target in an atmosphere of a highly pure argon-nitrogen mixture at a pressure of 2, 5, and 10 Pa. The source of laser radiation was an AYG:Nd3+ laser with a wavelength of 532 nm (second harmonic), laser radiation energy density of 2.3 J/cm2, pulse repetition rate of 15 Hz, and pulse duration of 10 ns. The substrate temperature was 350 °C, the deposition time was 60 minutes. It is shown that the surface of thin films is textured with microdroplets. It has been established that microdroplets on the surface of a thin film are formed by metallic indium. It has been established that the distribution of indium microdroplets over the film surface is mainly ordered in the form of lines. This phenomenon can be explained by the presence of misfit dislocations. The average size of microdroplets on the surface of the InGaAsN film on GaAs (100) was about 30 nm, and their density did not exceed 0.076 μm–2. For comparison, in films obtained on Si substrates at a pressure of 2 Pa, the highest microdroplet density was 0.26 μm–2. The lowest density of microdroplets on the surface (0.17 μm–2) was noted in samples of a thin film of InGaAsN on Si (100) obtained at a pressure of 10 Pa. It is noted that the intensity of the local phonon vibrational mode LVM InN at a frequency of 430 cm–1 increases with increasing pressure of the argon-nitrogen mixture during pulsed laser deposition in the Raman scattering spectra of InGaAsN films on Si. In the Raman spectra of InGaAsN films on Si, second-order phonon modes LVM InN and LVM GaN were detected at frequencies of 450 cm–1 about 470 cm–1, respectively. This confirms the presence of nitrogen in a thin InGaAsN film obtained by pulsed laser deposition. It is shown that an increase in the pressure of the argon-nitrogen gas mixture during pulsed laser deposition contributes to an increase in the nitrogen concentration in thin InGaAsN films on Si. It has been established that the nitrogen concentration in InGaAsN films obtained at a pressure of 10 Pa on GaAs (100) and Si (100) substrates differs insignificantly and amounts to 1.9 % and 1.8 %, respectively. The presented results will make it possible to create highly efficient photoelectric converters and photodetectors for the near and mid-infrared range up to 3 μm based on the obtained InGaAsN thin films.

Excellent thermoelectric properties of monolayer MoS2-MoSe2 aperiodic superlattices

Effective regulation of phonon thermal conductivity is crucial for the understanding of phonon transmission and improving thermoelectric (TE) performance. Using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method, we investigated the effect of phonon localization induced by aperiodic superlattices on the TE transport properties of superlattices based on monolayer MoS2-MoSe2. The results show that the structural defects obtained by changing the superlattice arrangement can effectively modulate the phonon thermal conductivity independently. In addition, the TE properties of the superlattices increase with temperature and central region length. The maximum value of the TE figure of merit of the aperiodic superlattice can reach 1.38 at 500 K. This implies a new potential strategy for tuning the TE performance of devices by exploiting the arrangements of the aperiodic superlattice.

Effects of localized phonons on interfacial thermal conductance

Interfacial thermal conduction is traditionally described by the transmission of traveling phonons in their bulk counterparts. However, recent experiments suggest that localized phonons at an interface may exert a substantial influence on interfacial thermal conductance. By considering quantum phonon scattering processes at a weakly interacting interface, we analyze the role of localized phonons at the interface involved in the interfacial thermal conduction and discover that there is a coupling between traveling phonons and the even orders of the localized phonons at the interface. Using fourth-order localized phonons as an example, we show that the traveling phonons can collide with the localized phonons at the interface. Such collisions can significantly change the transmission probability of the traveling phonons and thus affect the interfacial thermal conductance, and importantly, such effects become increasingly significant with increasing temperature.