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The possibility to tune the functional properties of nanomaterials is key to their technological applications. Superlattices, i.e., periodic repetitions of two or more materials in one or more dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer systems at the nanoscale, as well as to combine materials that cannot be put together in conventional heterostructures due to the lattice mismatch. In this work, we investigate GaAs/GaP superlattices embedded in GaP nanowires and demonstrate the tunability of their phononic and optoelectronic properties by inelastic light scattering experiments corroborated by ab initio calculations. We observe clear modifications in the dispersion relation for both acoustic and optical phonons in the superlattices nanowires. We find that by controlling the superlattice periodicity, we can achieve tunability of the phonon frequencies. We also performed wavelength-dependent Raman microscopy on GaAs/GaP superlattice nanowires, and our results indicate a reduction in the electronic bandgap in the superlattice compared to the bulk counterpart. All of our experimental results are rationalized with the help of ab initio density functional perturbation theory (DFPT) calculations. This work sheds fresh insights into how material engineering at the nanoscale can tailor phonon dispersion and open pathways for thermal engineering. [ABSTRACT FROM AUTHOR]

We report a first-principles theory-based study of the stability, electronic structure, and optical properties of cluster-assembled materials in various one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanostructures using a cagelike Cd9Te9 cluster as the superatom. The bulk 3D self-assemblies form in 2D stacked structures for different cubic lattices. The face-centered stacking is the most stable compared to the simple cubic, body-centered, and zinc blende-type stackings. The 2D stacks are formed as cluster-assembled monolayers, and the monolayer derived from the face-centered structure is the most stable. Further, the cluster chains (or wires) with more number of intercluster bonds are also seen to be dynamically stable. Due to quantum confinement, the energy gap of CdTe self-assemblies increases from the 3D to 2D to 1D nanostructures. The electronic structure, band gap, dielectric constant, and absorption spectra along with the phonon dispersions are discussed for these self-assembled nanostructures. The dielectric properties of these self-assembled nanostructures point to their potential applications in fabricating solar cells. [ABSTRACT FROM AUTHOR]

Two-dimensional metal halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, and phonon response from single two-dimensional lead iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron-phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.

The weak van der Waals interactions in the out-of-plane direction result in markedly low thermal conductivity in one-dimensional (1D) and two-dimensional (2D) materials, which substantially restricts their applications. Developing three-dimensional (3D) columnar hybrid structures, featuring high thermal conductivity both within and beyond the plane, effectively addresses this challenge. This study investigated a 3D hybrid structure composed of graphene and boron nitride nanotubes (GR-BNNTs) using non-equilibrium molecular dynamics simulations. This approach allowed the examination of the formation mechanisms and key factors influencing thermal rectification (TR) in these materials. Our findings reveal a novel mechanism for independently regulating forward and backward heat fluxes in GR-BNNTs. By manipulating the thermal properties of the BNNTs and the graphene layer, the TR ratio can be controlled flexibly. Additionally, we identify specific strategies for independently adjusting the heat flux, such as altering the intercolumn distance of BNNTs, which impacts the backward flux merely, while applying strain to affect the forward flux merely. This research introduces a novel concept of independent regulation of forward and backward heat fluxes, providing significant insights into phonon thermal transport in 3D hybrid structures.

Phonons are envisioned as coherent intermediaries between different types of quantum systems. Engineered nanoscale devices, such as optomechanical crystals (OMCs), provide a platform to utilize phonons as quantum information carriers. Here we demonstrate OMCs in diamond designed for strong for interactions between phonons and a silicon vacancy (SiV) spin. Using optical measurements at millikelvin temperatures, we measure a line width of 13 kHz ( Q -factor of ∼4.4 × 10 5 ) for a 6 GHz acoustic mode, a record for diamond in the GHz frequency range and within an order of magnitude of state-of-the-art line widths for OMCs in silicon. We investigate SiV optical and spin properties in these devices and outline a path toward a coherent spin-phonon interface.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment. [ABSTRACT FROM AUTHOR]

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO3 nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides. [ABSTRACT FROM AUTHOR]

There is a huge demand for magnetoresistance (MR) sensors with high sensitivity, low energy consumption, and room temperature operation. It is well-known that spatial charge inhomogeneity due to impurities or defects introduces mobility fluctuations in monolayer graphene and gives rise to MR in the presence of an externally applied magnetic field. However, to realize a MR sensor based on this effect is hampered by the difficulty in controlling the spatial distribution of impurities and the weak magnetoresistance effect at the monolayer regime. Here, we fabricate a highly stable monolayer graphene-on-black phosphorus (G/BP) heterostructure device that exhibits a giant MR of 775% at 9 T magnetic field and 300 K, exceeding by far the MR effects from devices made from either monolayer graphene or few-layer BP alone. The positive MR of the G/BP device decreases when the temperature is lowered, indicating a phonon-mediated process in addition to scattering by charge impurities. Moreover, a nonlocal MR of >10 000% is achieved for the G/BP device at room temperature due to an enhanced flavor Hall effect induced by the BP channel. Our results show that electron-phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals. [ABSTRACT FROM AUTHOR]

n this work, we discover anomalously low lattice thermal conductivity (<0.25 W/mK at 300 °C) in the Hg-containing quaternary diamond-like semiconductors within the Cu2IIBIVTe4 (IIB: Zn, Cd, Hg) (IV: Si, Ge, Sn) set of compositions. Using high-temperature X-ray diffraction, resonant ultrasound spectroscopy, and transport properties, we uncover the critical role of the antisite defects HgCu and CuHg on phonon transport within the Hg-containing systems. Despite the differences in chemistry between Hg and Cu, the high concentration of these antisite defects emerges from the energetic proximity of the kesterite and stannite cation motifs. Our phonon calculations reveal that heavier group IIB elements not only introduce low-lying optical modes, but the subsequent antisite defects also possess unusually strong point defect phonon scattering power. The scattering strength stems from the fundamentally different vibrational modes supported by the constituent elements (e.g., Hg and Cu). Despite the significant impact on the thermal properties, antisite defects do not negatively impact the mobility (>50 cm2/(Vs) at 300 °C) in Hg-containing systems, leading to predicted zT > 1.5 in Cu2HgGeTe4 and Cu2HgSnTe4 under optimized doping. In addition to introducing a potentially new p-type thermoelectric material, this work provides (1) a strategy to use the proximity of phase transitions to increase point defect phonon scattering, and (2) a means to quantify the power of a given point defect through inexpensive phonon calculations. [ABSTRACT FROM AUTHOR]

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump–probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron–phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material. [ABSTRACT FROM AUTHOR]

Interactions between elementary excitations, such as carriers, phonons, and plasmons, are critical for understanding the optical and electronic properties of materials. The significance of these interactions is more prominent in low-dimensional materials and can dominate their physical properties due to the enhanced interactions between these excitations. One-dimensional single-walled carbon nanotubes provide an ideal system for studying such interactions due to their perfect physical structures and rich electronic properties. Here we investigated G-mode phonon dynamics in individual suspended chirality-resolved single-walled carbon nanotubes by time-resolved anti-Stokes Raman spectroscopy. The improved technique allowed us to probe the intrinsic phonon information on a single-tube level and exclude the influences of tube–tube and tube–substrate interactions. We found that the G-mode phonon lifetime ranges from 0.75–2.25 ps and critically depends on whether the tube is metallic or semiconducting. In comparison with the phonon lifetimes in graphene and graphite, we revealed structure-dependent carrier–phonon and phonon–phonon interactions in nanotubes. Our results provide new information for optimizing the design of nanotube electronic/optoelectronic devices by better understanding and utilizing their phonon decay channels. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX2 (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe2 bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1–3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E2g, A1g, and A21g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects. [ABSTRACT FROM AUTHOR]

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics. [ABSTRACT FROM AUTHOR]

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton–phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton–phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton–phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale. [ABSTRACT FROM AUTHOR]

Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron–hole (e–h) recombination centers, where the determining element for e–h recombination is still unclear. Taking doped TiO2 as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e–h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e–h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e–h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e–h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control. [ABSTRACT FROM AUTHOR]

The virtual lack of information on electronic spectra of divalent lanthanide elements (Ln2+) other than Sm2+, Eu2+, Tm2+, and Yb2+ has prompted us to set for synthesis and characterization of novel Ln2+ systems. First successful attempt concerned SrCl2/Nd2+ single crystals. Here, we report stabilization of divalent dysprosium in a chloride host. Importantly this has been accomplished with Dy ions introduced in a divalent state during synthesis, unlike by -irradiation of Dy3+ systems employed previously. This synthesis method yields good quality SrCl2/Dy2+ single crystals. The electronic absorption spectra of Dy2+ doped in SrCl2 have been recorded with high resolution at liquid helium temperature (4.2 K). Identification of the absorption bands occurring in the spectral range of 5000-45000 cm-1 is achieved. On the basis of theoretical calculations using semiempirical Hamiltonian model, assignment of bands and determination of the Hamiltonian parameters for Dy2+(4f95d¹) configuration is carried out. The experimental and theoretical studies reveal fine structure and zero-phonon transitions and thus enable high-resolution assignment of spectral lines. It is shown that spin-forbidden transitions gain relatively high intensity due to significant admixing of low-spin character to nominally high-spin states. [ABSTRACT FROM AUTHOR]

We demonstrate that coherent acoustic phonons derived from plasmonic nanoparticles can modulate electronic interactions with proximal excitonic molecular species. A series of gold bipyramids with systematically varied aspect ratios and corresponding localized surface plasmon resonance energies, functionalized with a J-aggregated thiacarbocyanine dye molecule, produces two hybridized states that exhibit clear anticrossing behavior with a Rabi splitting energy of 120 meV. In metal nanoparticles, photoexcitation generates coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photogenerated oscillations alter the metal nanoparticle's energetic contribution to the hybridized system and, as a result, change the coupling between the plasmon and exciton. We demonstrate that such modulations in the hybridization are consistent across a wide range of bipyramid ensembles. We also use finite-difference time domain calculations to develop a simple model describing this behavior. Such oscillatory plasmonic-excitonic nanomaterials offer a route to manipulate and dynamically tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications. [ABSTRACT FROM AUTHOR]

The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long time scales (hundreds of nanoseconds). However, time-resolved measurements report valley lifetimes of only a few picoseconds. This has been attributed to mechanisms such as phonon-mediated intervalley scattering and a precession of the valley pseudospin through electron-hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe2. We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton-exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs. [ABSTRACT FROM AUTHOR]

High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ~ 1.5 at 723 K and a record high average ZT > 1.0 at 300-873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu2Te inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ~37 ßW cm-1 K-2 at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ~0.38 Wm-1 K-1 in PbTe-5.5%Cu2Te, approaching the theoretical minimum value of ~0.36 Wm-1 K-1. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties. [ABSTRACT FROM AUTHOR]

Heat dissipation is an increasingly critical technological challenge in modern electronics and photonics as devices continue to shrink to the nanoscale. To address this challenge, high thermal conductivity materials that can efficiently dissipate heat from hot spots and improve device performance are urgently needed. Boron phosphide is a unique high thermal conductivity and refractory material with exceptional chemical inertness, hardness, and high thermal stability, which holds high promises for many practical applications. So far, however, challenges with boron phosphide synthesis and characterization have hampered the understanding of its fundamental properties and potential applications. Here, we describe a systematic thermal transport study based on a synergistic synthesis-experimental-modeling approach: we have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature. Through nanoscale ballistic transport, we have, for the first time, mapped the phonon spectra of boron phosphide and experimentally measured its phonon mean free-path spectra with consideration of both natural and isotope-pure abundances. We have also measured the temperature- and size-dependent thermal conductivity and performed corresponding calculations by solving the three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process. Our finding underscores the promise of boron phosphide as a high thermal conductivity material for a wide range of applications, including thermal management and energy regulation, and provides a detailed, microscopic-level understanding of the phonon spectra and thermal transport mechanisms of boron phosphide. The present study paves the way toward the establishment of a new framework, based on the phonon spectra–material structure relationship, for the rational design of high thermal conductivity materials and nano- to multiscale devices. [ABSTRACT FROM AUTHOR]

Thermal transport in amorphous silicon dioxide (a-SiO2) is traditionally treated as random walks of vibrations owing to its greatly disordered structure, which results in a mean free path (MFP) approximately the same as the interatomic distance. However, this picture has been debated constantly and in view of the ubiquitous existence of thin a-SiO2 layers in nanoelectronic devices, it is imperative to better understand this issue for precise thermal management of electronic devices. Different from the commonly used cross-plane measurement approaches, here we report on a study that explores the in-plane thermal conductivity of double silicon nanoribbons with a layer of a-SiO2 sandwiched in-between. Through comparing the thermal conductivity of the double ribbon samples with that of corresponding single ribbons, we show that thermal phonons can ballistically penetrate through a-SiO2 of up to 5 nm thick even at room temperature. Comprehensive examination of double ribbon samples with various oxide layer thicknesses and van der Waals bonding strengths allows for extraction of the average ballistic phonon penetration depth in a-SiO2. With solid experimental data demonstrating ballistic phonon transport through a-SiO2, this work should provide important insight into thermal management of electronic devices. [ABSTRACT FROM AUTHOR]

We report on the discovery of a quasi two-dimensional copper–bismuth nano sheet from ab initio calculations, which we call cubine. According to our predictions, single layers of cubine can be isolated from the recently reported high-pressure CuBi bulk material at an energetic cost of merely ≈20 meV/Ų, comparable to values to separate single layers of graphene from graphite. Our calculations suggest that cubine has remarkable electronic and electrochemical properties: It is a superconductor with a moderate electron–phonon coupling λ = 0.5, leading to Tc ≈ 1 K, and can be readily intercalated with lithium with a high diffusibility, rendering it a promising candidate material to boost the rate capacity of current electrodes in lithium-ion batteries. [ABSTRACT FROM AUTHOR]

Using semiclassical Boltzmann transport theory and density functional formalism, we have systematically studied the thermoelectric performance of layered GeAs2. The figure of merit, ZT value, of this layered structure is found to be 2.78 along the out-of-plane direction, with optimal carrier concentration at 800 K. Analysis of the charge density difference and phonon transport properties allows us to attribute such exceptional thermoelectric properties to strong interlayer interaction between the adjacent layers where quasicovalent bonding is responsible for the enhanced electrical conductivity, while the layered structure accounts for the suppressed lattice thermal conductivity. This study highlights the potential of layered crystals for highly efficient thermoelectric materials. [ABSTRACT FROM AUTHOR]

The van der Waals (vdW) interfaces of two-dimensional (2D) semiconductor are central to new device concepts and emerging technologies in light-electricity transduction where the efficient charge separation is a key factor. Contrary to general expectation, efficient electron-hole separation can occur in vertically stacked transition-metal dichalcogenide heterostructure bilayers through ultrafast charge transfer between the neighboring layers despite their weak vdW bonding. In this report, we show by ab initio nonadiabatic molecular dynamics calculations, that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly promoted by an adiabatic mechanism through phonon excitation occurring on 20 fs, which is in good agreement with the experiment. The atomic level picture of the phonon-assisted ultrafast mechanism revealed in our study is valuable both for the fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as for the design of novel quasi-2D devices for optoelectronic and photovoltaic applications. [ABSTRACT FROM AUTHOR]

We present a new paradigm for understanding optical absorption and hot electron dynamics experiments in graphene. Our analysis pivots on assigning proper importance to phonon-assisted indirect processes and bleaching of direct processes. We show indirect processes figure in the excess absorption in the UV region. Experiments which were thought to indicate ultrafast relaxation of electrons and holes, reaching a thermal distribution from an extremely nonthermal one in under 5-10 fs, instead are explained by the nascent electron and hole distributions produced by indirect transitions. These need no relaxation or ad-hoc energy removal to agree with the observed emission spectra and fast pulsed absorption spectra. The fast emission following pulsed absorption is dominated by phonon-assisted processes, which vastly outnumber direct ones and are always available, connecting any electron with any hole any time. Calculations are given, including explicitly calculating the magnitude of indirect processes, supporting these views. [ABSTRACT FROM AUTHOR]

As a two-dimensional semimetal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability, and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy EF, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl3-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations corresponding to EF > 1 eV. Near-field imaging of this highly doped form of graphene allows us to characterize plasmons, including their corresponding lifetimes, over a broad frequency range. For bilayer graphene, in contrast to the monolayer system, a phonon-induced dipole moment results in increased plasmon damping around the intrinsic phonon frequency. Strong coupling between intrinsic graphene phonons and plasmons is found, supported by ab initio calculations of the coupling strength, which are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The design of graphene-based composite with high thermal conductivity requires a comprehensive understanding of phonon coupling in nanosized graphene. We extended the two-temperature model to coupled groups of phonons. The study give new physical quantities, the phonon-phonon coupling factor and length, to characterize the couplings quantitatively. Besides, our proposed coupling length has an obvious dependence on system size. Our studies can not only observe the nonequilibrium between different groups of phonons but explain theoretically the thermal resistance inside nanosized graphene. [ABSTRACT FROM AUTHOR]

Thermal management plays a critical role in the design of solid state materials for energy conversion. Lead halide perovskites have emerged as promising candidates for photovoltaic, thermoelectric, and optoelectronic applications, but their thermal properties are still poorly understood. Here, we report on the thermal conductivity, elastic modulus, and sound speed of a series of lead halide perovskites MAPbX3 (X = Cl, Br, I), CsPbBr3, and FAPbBr3 (MA = methylammonium, FA = formamidinium). Using frequency domain thermoreflectance, we find that the room temperature thermal conductivities of single crystal lead halide perovskites range from 0.34 to 0.73 W/m·K and scale with sound speed. These results indicate that regardless of composition, thermal transport arises from acoustic phonons having similar mean free path distributions. A modified Callaway model with Born von Karmen-based acoustic phonon dispersion predicts that at least -70% of thermal conductivity results from phonons having mean free paths shorter than 100 nm, regardless of whether resonant scattering is invoked. Hence, nanostructures or crystal grains with dimensions smaller than 100 nm will appreciably reduce thermal transport. These results are important design considerations to optimize future lead halide perovskite-based photovoltaic, optoelectronic, and thermoelectric devices. [ABSTRACT FROM AUTHOR]

The ability to control the energy flow of light at the nanoscale is fundamental to modern communication and big-data technologies, as well as quantum information processing schemes. However, since photons are diffraction-limited, efforts of confining them to dimensions of integrated electronics have so far proven elusive. A promising way to facilitate nanoscale manipulation of light is through plasmon polaritons-coupled excitations of photons and charge carriers. These tightly confined hybrid waves can facilitate compression of optical functionalities to the nanoscale but suffer from huge propagation losses that limit their use to mostly subwavelength scale applications. With only weak evidence of macroscale plasmon polaritons, propagation has recently been reported theoretically and indirectly, no experiments so far have directly resolved long-range propagating optical plasmons in real space. Here, we launch and detect nanoscale optical signals, for record distances in a wireless link based on novel plasmonic nanotransceivers. We use a combination of scanning probe microscopies to provide high resolution real space images of the optical near fields and investigate their long-range propagation principles. We design our nanotransceivers based on a high-performance nanoantenna, Plantenna, hybridized with channel plasmon waveguides with a cross-section of 20 nm × 20 nm, and observe propagation for distances up to 1000 times greater than the plasmon wavelength. We experimentally show that our approach hugely outperforms both waveguide and wireless nanophotonic links. This successful alliance between Plantenna and plasmon waveguides paves the way for new generations of optical interconnects and expedites long-range interaction between quantum emitters and photomolecular devices. [ABSTRACT FROM AUTHOR]

A variety of new stable and metastable Li-Sn and Li-Sb intermetallics are presented using the ab initio random structure searching (AIRSS) and species swapping methods. These include LiSn2-P4/mmm, Li2Sn3-P1¯, Li7Sn9-P42/n, Li3Sn2-P21/m, Li5Sn3-Im3¯m, Li2Sn-Cmcm, Li8Sn3-R3¯m, Li3Sn-P32, Li7Sn2-P1¯, Li4Sn-P21, Li5Sn-P6/mmm, Li7Sn-Fmmm, LiSb-P4/mmm, Li8Sb5-Fd3¯m, Li8Sb3-P2/c, Li4Sb-C2/m, Li9Sb2-P3¯m1, Li5Sb-P6/mmm, Li6Sb-R3¯m, Li8Sb-Pc, and Li9Sb-Cmcm. The Li-Sn theoretical voltage curve was calculated to high accuracy mainly from experimentally known structures and shows excellent agreement with experimental electrochemical cycling measurements previously reported. Li2Sn was found on the convex hull to within density-functional theory accuracy, and its mechanical stability was investigated by calculating the density of states of the phonon spectrum. The new structures obtained by AIRSS show a consistent structural evolution of Li-Sn phases as Li concentration is increased. First-principles NMR calculations on the hexagonal and cubic Li3Sb phases are performed. Our NMR results are compared to findings of Johnston et al. (Chem. Mater. 2016, 28, 4032) and proposed as a diagnostic tool to interpret experimental data. [ABSTRACT FROM AUTHOR]

Single crystalline SnSe has been reported to exhibit the high thermoelectric zT value of 2.6 at 923 K along the b-axis direction, due to its low thermal conductivity [Zhao, L. D.; et al. Nature 2014, 508, 373]. However, the strongly anisotropic properties of the orthorhombic structure degrade the thermoelectric performance of polycrystalline SnSe, resulting in a low zT of 0.6 and 0.8 for Ag- and Na-doped SnSe, respectively. Here, we prepared Ag0.01Sn0.99Se1-xSx (x = 0, 0.10, 0.15, 0.20, and 0.35) compounds by melting and hot press sintering. The compounds showed extremely low thermal conductivity (0.11 W m-1 K-1 at 825 K for x = 0.15). Using transmission electron microscopy images, we found that SnS alloying induced numerous nanoscale point defects. A Debye-Callaway model analysis supported the conclusion that the extremely low lattice thermal conductivity could be attributed to the point defect scattering of phonons. This resulted in a high zT of 1.67 at 823 K for the x = 0.15 sample, which is the state-of-the-art zT value for polycrystalline SnSe. Because the compounds are based on the environmentally friendly and cheap materials Sn, Se, and S, they make promising candidates for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Nanowires of complex morphologies, such as kinked wires, have been recently synthesized and demonstrated for novel devices and applications. However, the effects of these morphologies on thermal transport have not been well studied. Through systematic experimental measurements, we show that single-crystalline, defect-free kinks in boron carbide nanowires can pose a thermal resistance up to ~30 times larger than that of a straight wire segment of equivalent length. Analysis suggests that this pronounced resistance can be attributed to the combined effects of backscattering of highly focused phonons and required mode conversion at the kink. Interestingly, it is also found that instead of posing resistance, structural defects in the kink can actually assist phonon transport through the kink and reduce its resistance. Given the common kink-like wire morphology in nanoelectronic devices and required low thermal conductivity for thermoelectric devices, these findings have important implications in precise thermal management of electronic devices and thermoelectrics. [ABSTRACT FROM AUTHOR]

In this work we probe the third-order nonlinear optical property of graphene and hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman spectroscopy. When the energy difference of the two input fields matches the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected, while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure nonlinear optical response. [ABSTRACT FROM AUTHOR]

The photodimerization of anthracene derivatives constitutes a model system for intermolecular [4 + 4] cycloadditions. In this paper we deal with the elusive 9-anthracene-carboxylic case and study the crystal state of the head-to-tail dimer, obtained by the reaction of the monomer in various solvents both in its unary and solvated forms, by X-ray diffraction and confocal Raman microscopy in the lattice phonon region. A number of solvates have been identified, and their structures have been solved and here presented. The 9-anthracene-carboxylic acid dimer appears to be an exemplary case of a molecular crystal easily prone to host solvent molecules in the interstices of the framework generated by homomolecular hydrogen bonds. Alternatively, the hydrogen bonds between solvent molecules and the carboxylic group may establish supramolecular structures of closely packed architectures. Raman microscopy has also allowed us to investigate the short-lived dimer, which is produced in the crystal-to-crystal photoreaction triggered by the irradiation of the monomer single crystal. [ABSTRACT FROM AUTHOR]

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra. [ABSTRACT FROM AUTHOR]

Hybrid two-dimensional (2D) lead halide perovskites have been employed in optoelectronic applications, including white-light emission for light-emitting diodes (LEDs). However, until now, there have been limited reports about white-light-emitting lead halide perovskites with experimental insights into the mechanism of the broadband emission. Here, we present white-light emission from a 2D hybrid lead chloride perovskite, using the widely known phenethylammonium cation. The single-crystal X-ray structural data, time-resolved photophysical measurements, and density functional theory calculations are consistent with broadband emission arising from strong exciton-phonon coupling with the organic lattice, which is independent of surface defects. The phenethylammonium lead chloride material exhibits a remarkably high color rendering index of 84, a CIE coordinate of (0.37,0.42), a CCT of 4426, and photostability, making it ideal for natural white LED applications. [ABSTRACT FROM AUTHOR]

Phononic coupling can have a significant role in friction between nanoscale surfaces. We find frictional dissipation per atom in carbon nanotube (CNT) oscillators to depend significantly on interface features such as contact area, commensurability, and by end-capping of the inner core. We perform large-scale phonon wavepacket MD simulations to study phonon coupling between a 250 nm long (10,10) outer tube and inner cores of four different geometries. Five different phonon polarizations known to have dominant roles in thermal transport are selected, and transmission coefficient plots for a range of phonon energies along with phonon scattering dynamics at specific energies are obtained. We find that the length of interface affects friction only through LA phonon scattering and has a significant nonlinear effect on total frictional force. Incommensurate contact does not always give rise to superlubricity: the net effect of two competing interaction mechanisms shown by longitudinal and transverse phonons decides the role of commensurability. Capping of the core has no effect on acoustic phonons but destroys the coherence of transverse optical phonons and creates diffusive scattering. In contrast, the twisting and radial breathing phonon modes have perfect transmission at all energies and can be deemed as the enablers of ultralow friction in CNT oscillators. Our work suggests that tuning of interface geometries can give rise to desirable friction properties in nanoscale devices. [ABSTRACT FROM AUTHOR]

The measured frequencies and intensities of different first- and second-order Raman peaks of suspended graphene are used to show that optical phonons and different acoustic phonon polarizations are driven out of local equilibrium inside a submicron laser spot. The experimental results are correlated with a first-principles-based multiple temperature model to suggest a considerably lower equivalent local temperature of the flexural phonons than those of other phonon polarizations. The finding reveals weak coupling between the flexural modes with hot electrons and optical phonons. Since the ultrahigh intrinsic thermal conductivity of graphene has been largely attributed to contributions from the flexural phonons, the observed local nonequilibrium phenomena have important implications for understanding energy dissipation processes in graphene-based electronic and optoelectronic devices, as well as in Raman measurements of thermal transport in graphene and other two-dimensional materials. [ABSTRACT FROM AUTHOR]

By carefully and systematically performing Green-Kubo equilibrium molecular dynamics simulations, we report that the thermal conductivity (κ) of Si nanowires (NWs) does not diverge but converges and increases steeply when NW diameter (D) becomes extremely small (dκ/dD < 0), a long debate of one-dimensional heat conduction in history. The κ of the thinnest possible Si NWs reaches a superhigh level that is as large as more than 1 order of magnitude higher than its bulk counterpart. The abnormality is explained in terms of the dominant normal (N) process (energy and momentum conservation) of low frequency acoustic phonons that induces hydrodynamic phonon flow in the Si NWs without being scattered. With D increasing, the downward shift of optical phonons triggers strong Umklapp (U) scattering with acoustic phonons and attenuates the N process, leading to the regime of phonon boundary scattering (dκ/dD < 0). The two competing mechanisms result in nonmonotonic diameter dependence of κ with minima at critical diameter of 2-3 nm. Our results unambiguously demonstrate the converged κ and the clear trend of κ ∼ D for extremely thin Si NWs by fully elucidating the competition between the hydrodynamic phonon flow and phonon boundary scattering. [ABSTRACT FROM AUTHOR]

Phonon-cavity electromechanics allows the manipulation of mechanical oscillations similar to photon-cavity systems. Many advances on this subject have been achieved in various materials. In addition, the coherent phonon transfer (phonon Rabi oscillations) between the phonon cavity mode and another oscillation mode has attracted many interest in nanoscience. Here, we demonstrate coherent phonon transfer in a carbon nanotube phonon-cavity system with two mechanical modes exhibiting strong dynamical coupling. The gate-tunable phonon oscillation modes are manipulated and detected by extending the red-detuned pump idea of photonic cavity electromechanics. The first- and second-order coherent phonon transfers are observed with Rabi frequencies 591 and 125 kHz, respectively. The frequency quality factor product fQm ∼ 2 × 1012 Hz achieved here is larger than kBTbase/h, which may enable the future realization of Rabi oscillations in the quantum regime. [ABSTRACT FROM AUTHOR]

Transient changes of the optical response of WS2 monolayers are studied by femtosecond broadband pump-probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate. [ABSTRACT FROM AUTHOR]

We study the early time carrier drift dynamics in CsPbI 3 nanocrystal thin films with a sub 25 ps time resolution. Prior to trapping, carriers exhibit band-like transport characteristics, which is similar to those of traditional semiconductor solar absorbers including Si and GaAs due to optical phonon and carrier scattering at high temperatures. In contrast to the popular polaron scattering mechanism, the CsPbI 3 nanocrystal thin film demonstrates the strongest optical phonon scattering mechanism among other inorganic-organic hybrid perovskites, Si, and GaAs. This ultrafast dynamics study establishes a foundation for understanding the fundamental carrier drift properties that drive perovskite nanocrystal optoelectronics.

Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.

The emergence of wide and ultrawide bandgap semiconductors has revolutionized the advancement of next-generation power, radio frequency, and opto- electronics, paving the way for chargers, renewable energy inverters, 5G base stations, satellite communications, radars, and light-emitting diodes. However, the thermal boundary resistance at semiconductor interfaces accounts for a large portion of the near-junction thermal resistance, impeding heat dissipation and becoming a bottleneck in the devices' development. Over the past two decades, many new ultrahigh thermal conductivity materials have emerged as potential substrates, and numerous novel growth, integration, and characterization techniques have emerged to improve the TBC, holding great promise for efficient cooling. At the same time, numerous simulation methods have been developed to advance the understanding and prediction of TBC. Despite these advancements, the existing literature reports are widely dispersed, presenting varying TBC results even on the same heterostructure, and there is a large gap between experiments and simulations. Herein, we comprehensively review the various experimental and simulation works that reported TBCs of wide and ultrawide bandgap semiconductor heterostructures, aiming to build a structure-property relationship between TBCs and interfacial nanostructures and to further boost the TBCs. The advantages and disadvantages of various experimental and theoretical methods are summarized. Future directions for experimental and theoretical research are proposed.

The thermal conductivity of graphene is limited in the transverse direction due to the exfoliation of the interplanar distance. In this study, the multiwalled carbon nanotubes (MWCNTs) with high thermal conductivities were immobilized into the exfoliated few layers graphene with an objective to enhance the transverse thermal conductivity of MWCNT embedded graphene three-dimensional architectures. Reconstructing the topographically controlled synthetic hybrid nanostructure assemblies in a polymer matrix provides the opportunity to gain insight into the electrical and thermal conduction mechanism. The thickness of the resin layer between the nanohybrid fillers was found to be critical for the phonon transport through the coating whereas this epoxy insulating layer prevents tunneling of the electron. By controlled functionalization and site specific orientations of hybrid nanoscale morphologies in the fiber reinforced composite, the higher mechanical strength could be achieved with increased electrical but reduced thermal boundary resistance of the matrix. [ABSTRACT FROM AUTHOR]