Phonon Vibration Research Articles

Surface quality and microstructure evolution in fused silica under SF6/Ar reactive ion beam etching

The SF6/Ar reactive ion beam etching (RIBE) was proposed to explore the evolution of surface quality, subsurface defects, surface molecular structure, and chemical composition of fused silica. Also, the mechanism of reactive ion beam etching of fused silica surfaces was discussed. The results show that RIBE can maintain the original surface roughness by choosing suitable process parameters. this etching process can suppress the replication and expansion of subsurface defects and the deposition of reactants without bringing obvious contamination. The concentration of structural defects reaches a minimum at an etching depth of about 2 μm. However, further etching may lead to an increase in the chemical structure defects on the surface. The evolution of the intrinsic ring structure demonstrates that most intrinsic defects tend to reorganize into short (Si-O) ring structures. The etching mechanisms of Ar ions were discussed by simulating the stopping power and energy deposition. Phonon dissipation through atomic displacements and atomic vibrations is an important way of energy loss. Phonon vibrations and atomic dislocations induced by nuclear collisions are not only important factors causing the enhancement of electron-phonon coupling but also important causes of surface structural defects. The researches provide a theoretical basis for in-depth understanding of fused silica surface defect repair.

Lattice Dynamics of Quasi-2D Perovskites from First Principles.

We present the vibrational properties and phonon dispersion for quasi-2D hybrid organic-inorganic perovskites (BA)2CsPb2I7, (HA)2CsPb2I7, (BA)2(MA)Pb2I7, and (HA)2(MA)Pb2I7 calculated from first principles. Given the highly complex nature of these compounds, we first perform careful benchmarking and convergence testing to identify suitable parameters to describe their structural features and vibrational properties. We find that the inclusion of van der Waals corrections on top of generalized gradient approximation (GGA) exchange-correlation functionals provides the best agreement for the equilibrium structure relative to experimental data. We also investigate the impact of the molecular orientation on the equilibrium structure of these layered perovskite systems. Our results suggest ground state ferroelectric alignment of molecular dipoles in the out-of-plane direction is unlikely and support the assignment of the centrosymmetric space group for the low-temperature phase of (HA)2(MA)Pb2I7. Finally, we compute vibrational properties under the harmonic approximation. We find that stringent energy cut-offs are required to obtain well-converged phonon properties, and once converged, the harmonic approximation can capture key physics for such a large, hybrid inorganic-organic system with vastly different atom types, masses, and interatomic interactions. We discuss the obtained phonon modes and dispersion behavior in the context of known properties for bulk 3D perovskites and ligand molecular crystals. While many vibrational properties are inherited from the parent systems, we also observe unique coupled vibrations that cannot be associated with vibrations of the pure constituent perovskite and ligand subphases. Energy dispersion of the low energy phonon branches primarily occurs in the in-plane direction and within the perovskite subphase and arises from bending and breathing modes of the equatorial Pb-I network within the perovskite octahedral plane. The analysis herein provides the foundation for future investigations on this class of materials, such as exciton-phonon coupling, phase transitions, and general temperature-dependent properties.

Killer Phonon Caught: Femtosecond Stimulated Raman Spectroscopy Identifies Phonon-Induced Control of Photophysics in Rubrene Derivatives.

Molecular reaction coordinates are defined by the interplay of a number of orthogonal nuclear coordinates and are inherently multidimensional for large molecules. Identifying how specific nuclear motions along these reaction coordinates can be used to drive and control chemical processes is a promising approach for the optimization of chemical outcomes and targeted synthetic design. Here, we used femtosecond stimulated Raman spectroscopy (FSRS) to quantify the effects of individual phonon nuclear motions on singlet fission in rubrene derivatives. Rubrene readily undergoes singlet fission and is amendable to chemical derivatization, yet the factors that impact the singlet fission yield are not fully understood. Crystal packing is known to play a significant role in both fission and carrier transport, and thus, we focused on the impact of phonon nuclear motions on the photophysics. We used four halogen-substituted rubrene crystals and successfully identified one specific phonon mode that suppresses singlet fission in these crystals. We used FSRS with single-pulse excitation and double-pulse excitation to coherently amplify each phonon mode and quantify its effects on the excited-state process. We found that coherent amplification of the specific phonon vibration involving twisting of the peripheral phenyl rings and tetracene core motions resulted in less ground-state depletion and fewer triplet state absorption. Our study demonstrated that it is possible to use coherent phonon excitation to influence the photophysical outcome, while also showing that FSRS with double-pulse excitation can be a successful tool for quantifying mode-selective contributions to photophysics.

The thermal transport, mechanical, and optical properties of T-Cu6S2: The influence of Cu6 clusters

Cluster substitution can effectively regulate the physicochemical properties of two-dimensional (2D) materials and has been applied in fields such as catalysis and magnetism. However, the intrinsic mechanism of its regulation of thermal transport properties and the impact of clusters on optical and mechanical properties are not yet clear. In this work, 2D T-Cu6S2 is obtained by replacing the transition metal element M of 2D T-MS2 with the Cu6 clusters. Then, based on the first-principles method, we systematically studied the thermal transport, mechanical, and optical properties of T-Cu6S2. The results show that introducing Cu6 clusters reduces the phonon vibration frequency, and enhances phonon scattering, that is, the coupling effect of harmonic weakening and anharmonic enhancement. This makes the lattice thermal conductivity of T-Cu6S2 at the four-photon level reduced to 0.18 W/mK (at 300 K), which is much lower than that of traditional 2D T-MS2. In addition, we found that the introduction of clusters changes the crystal and electronic structures of T-Cu6S2, enhances the flexibility and ductility of T-Cu6S2, and increases its transmission ability for visible light and ultraviolet light. This study provides insights into the regulation of multiple properties of 2D materials, which benefits the development of 2D functional materials.

An emerging quaternary semiconductor nanoribbon with gate-tunable anisotropic conductance

Two-dimensional noble transition metal chalcogenide (NTMC) semiconductors represent compelling building blocks for fabricating flexible electronic and optoelectronic devices. While binary and ternary compounds have been reported, the existence of quaternary NTMCs with a greater elemental degree of freedom remains largely unexplored. This study presents the pioneering experimental realization of a novel semiconducting quaternary NTMC material, AuPdNaS2, synthesized directly on Au foils through chemical vapor deposition. The ribbon-shaped morphology of the AuPdNaS2 crystal can be finely tuned to a thickness as low as 9.2 nm. Scanning transmission electron microscopy reveals the atomic arrangement, showcasing robust anisotropic features; thus, AuPdNaS2 exhibits distinct anisotropic phonon vibrations and electrical properties. The field-effect transistor constructed from AuPdNaS2 crystal demonstrates a pronounced anisotropic conductance (σmax/σmin = 3.20) under gate voltage control. This investigation significantly expands the repertoire of NTMC materials and underscores the potential applications of AuPdNaS2 in nano-electronic devices.

Anharmonic Phonon Vibrations of Ag and Cu for Poor Lattice Thermal Conductivity in Superionic AgCrSe2 and CuCrSe2

Anharmonic Phonon Vibrations of Ag and Cu for Poor Lattice Thermal Conductivity in Superionic AgCrSe₂ and CuCrSe₂

Decoupling of Rattling Mode and Superconductivity in Filled-Skutterudite BaxIr4Sb12

Abstract The rattling mode, an anharmonic vibrational phonon, is widely recognized as a critical factor in the emergence of superconductivity in caged materials. Here, we present a counterexample in a filled-skutterudite superconductor Ba x Ir4Sb12 (x = 0.8, 0.9, 1.0), synthesized via a high-pressure route. Transport measurements down to liquid 3He temperatures reveal a transition temperature (T c) of 1.2 K and an upper critical field (H c2) of 1.3 T. Unlike other superconductors with caged structures, the Ba x Ir4 X 12 (X = P, As, Sb) family exhibits a monotonic decreasing T c with the enhancement of the rattling mode, as indicated by fitting the Bloch–Grüneisen formula. Theoretical analysis suggests that electron doping from Ba transforms the direct bandgap IrSb3 into a metal, with the Fermi surface dominated by the hybridization of Ir 5d and Sb 5p orbitals. Our findings of decoupled rattling modes and superconductivity distinguish the Ba x Ir4Sb12 family from other caged superconductors, warranting further exploration into the underlying mechanism.

Defect‐Driven Light Perception and Memristor Storage with Phase Transition in Vanadium Dioxide

AbstractTunable optical information storage is crucial in artificial retinal systems for mimicking neurobiological visual characteristics. The perception and storage of light signals rely heavily on the regulation of the conductivity states of memristor materials (e.g., transition metal oxides). Controlling light memristor behavior via defects and polymorphic phases remains underexplored and differs from traditional plasticity training via repeated testing. In this study, defect‐driven ultraviolet light perception and memristor storage with phase transitions in vanadium dioxide (VO2) thin films are presented. The effects of oxygen defects and the corresponding polymorphic phases on ultraviolet light memristors are investigated. The dependence of phonon vibrations and insulator–metal transition behavior on defect levels are revealed. Self‐doping and polymorphs enable VO2 to exhibit distinct ultraviolet memristor performance. It is anticipated that defect‐driven light memristors significantly contribute to the realization of artificial synaptic devices and the implementation of advanced electronic neuron systems.

Large and Small Polarons in Highly Efficient and Stable Organic‐Inorganic Lead Halide Perovskite Solar Cells: A Review

Polarons, which arise from the intricate interplay between excess electrons and/or holes and lattice vibrations (phonons), represent quasiparticles pivotal to the electronic behavior of materials. This review reaffirms the established classification of small and large polarons, emphasizing its relevance in the context of recent advances in understanding lead halide perovskites' behavior. The distinct characteristics of large and small polarons stem from the electron–phonon interaction range, which exerts a profound influence on materials’ characteristics and functionalities. Concurrently, lead halides have emerged with exceptional opto‐electronic properties, featuring prolonged carrier lifetimes, low recombination rates, high defect tolerance, and moderate charge carrier mobilities; these characteristics make them a compelling contender for integration of optoelectronic devices. In this review, the formation of both small and large polarons within the lattice of lead halide perovskites, elucidating their role in protecting photogenerated charge carriers from recombination processes, is discussed. As optoelectronic devices continue to advance, this review underscores the importance of unraveling polaron dynamics to pave the way for innovative strategies for enhancing the performance of next‐generation photovoltaic technologies. Future research should explore novel polaronic effects using advanced computational and experimental techniques, enhancing our understanding and unlocking new applications in materials science and device engineering.

Photoluminescence Quenching Upon Growth of Metal Nanoparticles: Quantum-Mechanical Views.

In dictating the optical processes in metal nanoparticles, for instance, quantum nature of free electrons is significantly dominant and plays very crucial roles at the level of nanoscale dimensions of materials. As consequences of the quantum-confinement effects on the conduction electrons, surface-plasmon resonance induced optical absorption and light emission properties of metal nanoparticles are found to be strongly dependent on physical dimensions of the nanomaterials. In addition, surface-confined acoustic vibration (phonon) modes have been experimentally observed to depend on the sizes of the metal nanoparticles. Also, interestingly, tuning of the surface-plasmon resonance condition is found to enhance the intensity of the acoustic Raman modes in metal nanoparticles. The study highlights the role of plasmon-phonon coupling in Co metal nanoparticles embedded in a silica-glass. In the research field of nanosciences and nanotechnologies, extraordinary behaviour and properties of nanoscale matters are investigated. In this context, interesting studies have been discussed in this review article to elaborate optical, chemical and photoluminescence properties of nanoscale Ag metal particles. Subtle detection of optical phenomena associated with the excited many-body electronic processes in the metal nanoparticles, for example, are very interesting but definitely challenging. Here we make an attempt to find out how the thermal growth of Ag metal nanoparticles in a glass matrix snuffs out the light emission from the samples? Quantum mechanical interpretations of the underlying processes about the quenching of photoluminescence phenomena with the growth of the metal nanoparticles will help to fine tune the optical properties of plasmonic systems as well as to harness potential applications of the nanomaterials.

The low-frequency bandgap characteristics of phononic crystal isolators with multi-hole

A vibration isolator for a floating slab track, based on a locally resonant phononic crystal, has been designed to suppress vibrations caused by the wheel-rail interaction. In this isolator, a circular vibrator is connected to a connector through a rubber layer uniformly embedded in holes. The finite element method is utilized to compute the energy band structure and transmission loss of the vibration isolator, while the mechanism of bandgap is elucidated through the analysis of modes. The static stiffness and dynamic stiffness of the vibration isolator are calculated, which proves that the vibration isolator meets the requirements of engineering applications. The influence of the number of circular holes and the geometric parameters of the vibration isolator on the bandgap is examined. It is discovered that augmenting the number of holes and the hole diameter in the rubber layer leads to a decreased bandgap. A low-frequency locally resonant bandgap of 38.2–97.1 Hz and a relative bandgap bandwidth of 86.9 % are obtained by optimizing the vibration isolator. The vibration isolation performance of a floating slab track equipped with vibration isolators is analyzed and more robust vibration attenuation performance is obtained in the bandgap. The multi-hole phononic crystal vibration isolator is employed in floating slab tracks for rail transportation, effectively mitigating vibrations transmitted to tunnels, thus further minimizing the effects on the surrounding environment.

Dynamic mode decomposition of nonequilibrium electron-phonon dynamics: accelerating the first-principles real-time Boltzmann equation

Nonequilibrium dynamics governed by electron–phonon (e-ph) interactions plays a key role in electronic devices and spectroscopies and is central to understanding electronic excitations in materials. The real-time Boltzmann transport equation (rt-BTE) with collision processes computed from first principles can describe the coupled dynamics of electrons and atomic vibrations (phonons). Yet, a bottleneck of these simulations is the calculation of e–ph scattering integrals on dense momentum grids at each time step. Here we show a data-driven approach based on dynamic mode decomposition (DMD) that can accelerate the time propagation of the rt-BTE and identify dominant electronic processes. We apply this approach to two case studies, high-field charge transport and ultrafast excited electron relaxation. In both cases, simulating only a short time window of ~10% of the dynamics suffices to predict the dynamics from initial excitation to steady state using DMD extrapolation. Analysis of the momentum-space modes extracted from DMD sheds light on the microscopic mechanisms governing electron relaxation to a steady state or equilibrium. The combination of accuracy and efficiency makes our DMD-based method a valuable tool for investigating ultrafast dynamics in a wide range of materials.

Tuning of thermoelectric performance by modulating vibrational properties in Ni-doped Sb2Te3

Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3 thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, including B iso and strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2−x Ni x Te3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 μWm−1K−2 for pristine Sb2Te3 to 2415 μWm−1K−2 for Ni-doped Sb2Te3 sample. Finally, the thermoelectric figure of merit, ZT is found to increase by four times in Sb1.98Ni0.02Te3 than that of its pristine counterpart.

Large geometric polarization and magnetic behavior in the multiferroic quasi-2D SrNiF4 fluoride

In the last decades, multifunctional single-crystals that show multiferroic and magnetoelectric responses have attracted considerable attention due to the potential applications and physical phenomena involved in the entanglement of the ferroic orders. This paper explores the structural, ferroelectric, and magnetic properties of the unexplored layered SrNiF4 fluoride compound. We show that, in terms of the vibrational phonon modes, this fluoride compound shows a tangible ferroelectric behavior with an Ps = 14.8 μC cm−2 standing as the largest among its family. Besides, based on our findings such ferroelectric polarization presents a geometric origin that is strongly entangled with the octahedral rotations within the Γ2− phonon mode and enhanced by the structure’s Γ1+ mode. We also observed that the spontaneous polarization is coupled to the noncollinear antiferromagnetic ordering in the structure allowing a switching of the weak antiferromagnetic component when the polarization is reversed.

Thermal Transport Properties of Two-Dimensional Janus MoXSiN2 (X = S, Se, and Te).

The design of Janus materials offers an effective means of regulating both their physical and chemical properties, leading to their application in various fields. However, the underlying mechanism governing the modulation of the thermal transport characteristics through the construction of Janus materials remains unclear. In this work, we introduce VI-group elements into the MoSi2N4 structure, yielding two-dimensional Janus MoXSiN2 (X = S, Se, and Te) and systematically investigate their thermal transport properties based on first-principles calculation methods. Our findings reveal that the lattice thermal conductivities (κl) of MoSSiN2, MoSeSiN2, and MoTeSiN2 are 47.2, 24.3, and 40.6 W/mK at 300 K, respectively, significantly lower than that of MoSi2N4 (224 W/mK). Such low κl values mainly come from the introduction of X atoms, which enhances phonon scattering and reduces phonon vibration frequencies. In addition, MoTeSiN2 exhibits a higher κl compared to MoSeSiN2, contrary to the trend observed in most materials containing VI-group elements, where κl decreases gradually from S to Te. This anomalous behavior can be attributed to the competitive result between its lower phonon vibrational frequency and weaker phonon anharmonicity of MoTeSiN2. This work elucidates the inherent mechanism governing the modulation of thermal transport properties in Janus materials, thereby enhancing the potential application of Janus MoXSiN2 in engineering thermal management.

FE/PDE: a novel approach applied to PC plate structure with multi-scale and multi-physics field coupling

For the more straightforward and more efficient solution of phononic crystal (PC) plate frequency band structure, transmission curve, and vibration mode, in this paper, related theories based on spatial Fourier series expansions, combined with Bloch’s theorem, a novel approach to solve the structural governing equations of PC plate is proposed by using the partial differential equations (PDE) module in the finite element software COMSOL. It is named the FE/PDE (Finite element and partial differential equations) method. The method’s accuracy is verified by comparing the results with those obtained from the traditional method. Systematic elucidation of the application of the method to probe the properties of multi-scale, multi-physics field coupled PC plate. In order to demonstrate the flexibility and scientific validity of the method, a novel nano-piezoelectric PC plate structure is proposed and solved. The method is simple, computationally efficient, and applicable, and provides a new method for investigating the properties of PC plates.

Tunable Photocarrier Dynamics in CuS Nanoflakes under Pressure Modulation.

Two-dimensional materials with a unique layered structure have attracted intense attention all around the world due to their extraordinary physical properties. Most importantly, the internal Coulomb coupling can be regulated, and thus electronic transition can be realized by manipulating the interlayer interaction effectively through adding external fields. At present, the properties of two-dimensional materials can be tuned through a variety of methods, such as adding pressure, strain, and electromagnetic fields. For optoelectronic applications, the lifetime of the photogenerated carriers is one of the most crucial parameters for the materials. Here, we demonstrate effective modulation of the optical band gap structure and photocarrier dynamics in CuS nanoflakes by applying hydrostatic pressure via a diamond anvil cell. The peak differential reflection signal shows a linear blueshift with the pressure, suggesting effective tuning of interlayer interaction inside CuS by pressure engineering. The results of transient absorption show that the photocarrier lifetime decreases significantly with pressure, suggesting that the dissociation process of the photogenerated carriers accelerates. It could be contributed to the phase transition or the decrease of the phonon vibration frequency caused by the pressure. Further, Raman spectra reveal the change of Cu-S and S-S bonds after adding pressure, indicating the possible occurrence of structural phase transition. Interestingly, all of the variation modes are reversible after releasing pressure. This work has provided an excellent sight to show the regulation of pressure on the photoelectric properties of CuS, exploring CuS to wider applications that can lead toward the realization of future excitonic and photoelectric devices modulated by high pressure.

Localized Phonon Transport Study of GaN/SiO2 Core/shell Nanowires under Thermal-Stress Coupling.

In order to comprehensively explore the intricate mechanisms of thermo-mechanical interactions, this study employs the molecular dynamics method to investigate the influence of heat flux density, shell thickness and length, as well as stress on the radial interface phonon transport in GaN/SiO2 core/shell nanowire. Additionally, the surface eigenmode decomposition method is employed to analyze the interface phonon dispersion curves. The investigation reveals that with increasing heat flux density, internal thermal stresses intensify, leading to a complex distribution of thermal stresses within the system. Under the influence of thermal stress, the nonlinear acoustic properties interact with phonon scattering, resulting in the pronounced localization of interface phonons. Compressive stress causes an upshift in low-frequency phonons, while tensile stress induces a downward shift in the high-frequency optical branches at the interface. The localized phonon vibrations at the SiO2/GaN interface under nonuniform stress are identified as the primary cause for the abundant presence of nondispersive phonon modes at the radial interface. By elucidating the subtle interplay between lattice vibrations and stress fields, this study offers a novel and profound understanding of thermo-mechanical coupling effects, thereby providing innovative theoretical foundations for the design and performance management of thermoelectric devices.

Symmetry Reduction of Molecular Shape of Cationic Chromophores for High‐Performance Terahertz Generators

AbstractThe symmetry reduction of molecular shape of cationic chromophores as a design strategy to develop organic terahertz (THz) generators is used. Electron donating group (EDG) with an asymmetric shape is introduced into two types of cationic styryl‐quinolinium chromophores, leading to the development of several novel non‐centrosymmetric crystals with high macroscopic optical nonlinearity. In contrast, when an EDG with a symmetric shape is introduced, centrosymmetric crystal structure is obtained in all investigated crystals. For THz wave generation, crystals based on asymmetrically shaped EDG exhibit several optimal crystal characteristics, including a plate‐shape morphology, a large size, and an in‐plane polar axis. Furthermore, over the entire range of molecular phonon vibrations, 0.5–4 THz, the steric hindrance group in the asymmetrically shaped EDG has a strong influence on the suppression of THz phonon vibrations, leading to a low THz absorption coefficient. The crystals based on asymmetrically shaped EDG generate an ≈40 times stronger THz electric field than the inorganic ZnTe crystal and very broad THz spectra, extending up to 16 THz. Thus, the symmetry reduction of the molecular shape of cationic chromophores through the introduction of an asymmetrically shaped EDG is an effective design approach for the development of novel organic THz crystals.

Prediction of a new 2D Janus monolayer MX2Si2Y2 (M = Mo, W and V; X/Y = N, P; X ≠ Y) from density functional theory

We devised the Janus monolayer MX2A2Y2 (M = Mo, W, V, Nb, Ta, Ti, Zr, Hf and Cr; X/Y = N, P and As; A = Si and Ge) and systematically investigated the structural parameters, phonon vibrations, thermal stability, the electronic and mechanical properties. Among the 26 systems, there are six stable Janus monolayers at room temperature: MoN2Si2P2, WN2Si2P2, VN2Si2P2, MoP2Si2N2, WP2Si2N2, CrN2Si2P2. Intriguingly, the MoP2Si2N2 and WP2Si2N2 show non-magnetic semiconductor properties. The MoN2Si2P2 and WN2Si2P2 demonstrate non-magnetic metallic ground states. The VN2Si2P2 and CrN2Si2P2 are magnetic metal with magnetic moments of 1.24 μB and 0.53 μB. Furthermore, the elastic constants of the MSi2N4 and five Janus monolayer MX2Si2Y2 (M = Mo, W and V; X/Y = N, P) meet the stability criteria, indicating they are mechanically stable. The in-plane stiffness of the MX2Si2Y2 increases with the increase of the number of metal atoms and are greatly reduced, compared with the corresponding MSi2N4. In addition, according to Poisson's ratio, the MN2Si2P2 with metallic characteristics are ductile, whereas the MP2Si2N2 with semiconductor characteristic are brittle. In short, the Janus monolayer MX2Si2Y2 enriches the family members of 2D materials. The rich electronic and mechanical properties make the MX2Si2Y2 have high application prospects for future novel mechanical electronic device applications.