Phonon Coupling Effects Research Articles

Impact of strain and electron–phonon coupling on thermoelectric performance of Germanene

This manuscript describes the thermoelectric properties of monolayer germanene under the influence of biaxial strain using the combined approach of ab initio and semi-classical Boltzmann transport theory. To achieve excellent precision in the estimation of the thermoelectric behavior of strained germanene, the research delves into the temperature-dependent scattering time, particularly emphasizing the electron–phonon coupling effect. Incorporating both optical and acoustic phonons is always crucial and key for precisely estimating the scattering time, surpassing the limitations of the deformation potential approximation method. By examining the impact of strain on monolayer germanene and accounting for its scattering time, this approach provides a more practical means of gauging the thermoelectric performance of germanene under the presence of bi-axial strain. Moreover, the study extends its analysis to doped germanene with bi-axial strain, employing the rigid band approximation to investigate its thermoelectric performance. The research extensively estimates the transport properties for both intrinsic and extrinsic germanene, utilizing the hybrid functional HSE06. Additionally, the lattice thermal conductivity of germanene is estimated and compared for the strained and unstrained conditions. The analysis of thermal conductivity involves considering the effects of group velocity and phonon scattering time, providing insights into the nature of heat transport in strained germanene systems. Overall, this comprehensive study contributes to a deeper understanding of the thermoelectric properties of germanene under strain and lays the foundations for potential applications in electronic and thermal devices.

Achieving Luminous Efficiency Enhancement and Near‐Infrared Emission in Moisture‐Stable 0D Zinc‐Based Halides

AbstractZero‐dimensional (0D) metal halides are attractive due to their structure‐dependent and tunable photoluminescence properties. Herein, a new 0D organic–inorganic hybrid Zn‐based halide, (C4H12N2)ZnBr4, featuring a long‐term stable crystal structure and moisture‐stable PL emission under various extreme conditions is reported. A strong electron–phonon coupling effect enables the Zn‐based halide to display highly efficient blue light at 472 nm with a large Stokes shift of 7385 cm−1. Intriguingly, heterovalent substitution of Cu+‐Zn2+ further enhances the photoluminescence quantum efficiency to 60% as the introduction of Cu+ effectively suppresses the nonradiative recombination process. Besides, the formation of twisted [SbBr4]− tetrahedra via Sb3+‐Zn2+ substitution help to achieve a broadband near‐infrared (NIR) emission (760 nm) with full width at half maxima (FWHM) of 203 nm, enabling the potential applications in night‐vision and nondestructive fruit damage inspection. Detailed structural and optical analyses are used to investigate the photophysical processes of different self‐trapped exciton (STE) emission for pristine and Cu+/Sb3+‐doped 0D metal halides. These findings advance the understanding of spectral regulation mechanism via heterovalent substitution and initiate more exploitation of luminescent metal halides for emerging applications.

Maximizing Upconversion Luminescence of Co-Doped CaF₂:Yb, Er Nanoparticles at Low Laser Power for Efficient Cellular Imaging.

Upconversion nanoparticles (UCNPs) are well-reported for bioimaging. However, their applications are limited by low luminescence intensity. To enhance the intensity, often the UCNPs are coated with macromolecules or excited with high laser power, which is detrimental to their long-term biological applications. Herein, we report a novel approach to prepare co-doped CaF2:Yb3+ (20%), Er3+ with varying concentrations of Er (2%, 2.5%, 3%, and 5%) at ambient temperature with minimal surfactant and high-pressure homogenization. Strong luminescence and effective red emission of the UCNPs were seen even at low power and without functionalization. X-ray diffraction (XRD) of UCNPs revealed the formation of highly crystalline, single-phase cubic fluorite-type nanostructures, and transmission electron microscopy (TEM) showed co-doped UCNPs are of ~12 nm. The successful doping of Yb and Er was evident from TEM-energy dispersive X-ray analysis (TEM-EDAX) and X-ray photoelectron spectroscopy (XPS) studies. Photoluminescence studies of UCNPs revealed the effect of phonon coupling between host lattice (CaF2), sensitizer (Yb3+), and activator (Er3+). They exhibited tunable upconversion luminescence (UCL) under irradiation of near-infrared (NIR) light (980 nm) at low laser powers (0.28-0.7 W). The UCL properties increased until 3% doping of Er3+ ions, after which quenching of UCL was observed with higher Er3+ ion concentration, probably due to non-radiative energy transfer and cross-relaxation between Yb3+-Er3+ and Er3+-Er3+ ions. The decay studies aligned with the above observation and showed the dependence of UCL on Er3+ concentration. Further, the UCNPs exhibited strong red emission under irradiation of 980 nm light and retained their red luminescence upon internalization into cancer cell lines, as evident from confocal microscopic imaging. The present study demonstrated an effective approach to designing UCNPs with tunable luminescence properties and their capability for cellular imaging under low laser power.

Electron–phonon coupling effects on optical absorption of armchair graphene nanoribbon in the presence of magnetic field

Electron–phonon coupling effects on optical absorption of armchair graphene nanoribbon in the presence of magnetic field

Electron–phonon coupling and non-equilibrium thermal conduction in ultra-fast heating systems

A discrete unified gas kinetic scheme is developed to solve the Boltzmann transport equation (BTE) with coupled electron and phonon thermal transport, whose time step is not limited by the relaxation time. The key of the present scheme is that the electron/phonon advection, scattering and electron–phonon interactions are coupled together within one time step by solving the BTE again at the cell interface. Numerical results show that the present scheme can correctly predict the electron–phonon coupling effects, and is in agreement with typical two-temperature model and experimental results in existing literatures and our performed time-domain thermoreflectance technique. It can also capture the ballistic effects when the characteristic length is comparable to or smaller than the mean free path where the two-temperature model fails. In addition, two anomalous heat conduction phenomena are predicted by the present scheme, namely, heat flows from phonon to electron in transient thermal grating geometry and re-rise reflected signal appears in Au/Pt bilayer metal structure. The present work can provide theoretical guidance for the study of electron–phonon coupling and offer a useful tool for multi-scale thermal management.

Temperature-dependent excitonic emission characteristics of highly crystallized carbon nitride nanosheets

Highly-crystallized carbon nitride (HCCN) nanosheets exhibit significant potential for advancements in the field of photoelectric conversion. However, to fully exploit their potential, a thorough understanding of the fundamental excitonic photophysical processes is crucial. Here, the temperature-dependent excitonic photoluminescence (PL) of HCCN nanosheets and amorphous polymeric carbon nitride (PCN) is investigated using steady-state and time-resolved PL spectroscopy. The exciton binding energy of HCCN is determined to be 109.26 meV, lower than that of PCN (207.39 meV), which is attributed to the ordered stacking structure of HCCN with a weaker Coulomb interaction between electrons and holes. As the temperature increases, a noticeable reduction in PL lifetime is observed on both the HCCN and PCN, which is ascribed to the thermal activation of carrier trapping by the enhanced electron–phonon coupling effect. The thermal activation energy of HCCN is determined to be 102.9 meV, close to the value of PCN, due to their same band structures. Through wavelength-dependent PL dynamics analysis, we have identified the PL emission of HCCN as deriving from the transitions: σ*–LP, π*–π, and π*–LP, where the π*–LP transition dominants the emission because of the high excited state density of the LP state. These results demonstrate the impact of high-crystallinity on the excitonic emission of HCCN materials, thereby expanding their potential applications in the field of photoelectric conversion.

A CsPbBr3/CdS-based hybrid bidirectional optoelectronic device with light-emitting, modulation, and detection functions

Optoelectronic integrated circuits, with a broad photonic transportation bandwidth, have emerged as a promising solution to fulfill the escalating demands for high-volume information transportation and processing. However, challenges persist in developing optoelectronic integrated circuits based on low-dimensional nanostructures, including limited integration density and high energy consumption. Here, we demonstrate a bidirectional optoelectronic device by integrating a light-emitting/harvesting CsPbBr3 nanoplate with a waveguiding/modulating/detecting CdS nanobelt. By configuring the CsPbBr3 nanoplate in a Schottky-type device structure with a metal electrode, bright electroluminescence was attained at a bias voltage of 18 V. Thanks to the electric field-tuned phonon-coupling effect, the waveguided light in the CdS nanobelt exhibited a high modulation depth of up to 94%, rendering it an excellent building block as optical modulators and optical switches. Moreover, the integrated nanostructure device showcased functionality in the photodetection mode. The proposed device architecture holds promise for broader applications, potentially extending to other perovskite-coupled II–VI semiconductor optoelectronic integrated circuits for expanding integration capacity and enhancing optoelectronic performance.

Intracavity frequency-doubled yellow laser in an electron-phonon-coupled Nd:YVO4 crystal.

An approach to obtain a yellow laser is demonstrated for the first time to our knowledge by the employment of an Nd3+-doped YVO4 crystal and a LBO frequency-doubling crystal. Differing from the previous stimulated self-Raman radiation of Nd:YVO4, a direct 1176 nm lasing, without a high-intensity intracavity 1064 nm laser, was realized by utilizing an electron-phonon coupling effect and amplifying the thermally activated vibronic transitions. Combining with intracavity frequency-doubling, a yellow laser at 588 nm was obtained. At the pump power of 14.3 W, the output power of the yellow laser was 1.17 W, corresponding to a diode-to-visible efficiency of 8.2%. Moreover, for the first time, the yellow laser at 584 nm with output power of 164 mW was realized by tuning the filter, indicating the great potential of such an electron-phonon coupling laser for a wavelength extension in the yellow regime.

Enhanced ultrafast dynamics and saturable absorption response of Nb2SiTe4/graphene heterostructure for femtosecond mode-locked bulk lasers

Two-dimensional (2D) heterostructure provides a promising opportunity to create new hybrid materials with unique tailored properties for novel electronic and photonic devices. Here, 2D Nb2SiTe4/Graphene (NST/G) heterostructure samples with different thicknesses of Nb2SiTe4 are fabricated by mechanical exfoliation and dry transfer methods. Their photocarrier dynamics and nonlinear optical responses are systematically investigated by pump–probe and Z-scan measurements. The results illustrate that Graphene acts as an electron transfer channel effectively shortening the photocarrier lifetime in NST. Moreover, the thinner of NST, the larger of the photocarrier recombination rate in NST/G heterostructure because of the high surface state density and strong electron–phonon coupling effect. Significantly, the 2D NST/G heterostructure with thinner NST also exhibits a larger nonlinear absorption coefficient and moderate saturable absorption parameters, which endow the high-performance passively mode-locked femtosecond laser. With 2 nm NST/G heterostructure as saturable absorber (SA), passively mode-locked Yb:KYW bulk laser is realized with the pulse width of 415 fs and output power of 498 mW. Our results pave a versatile way for designing desirable photonic devices and applications.

Enhanced interfacial thermal conductance in functionalized boron nitride/polylactic acid nanocomposites: A molecular dynamics study

The relatively low thermal conductivity of biodegradable polylactic acid (PLA) has limited its applications in various fields. To address this issue, the incorporation of nanofillers, such as boron nitride nanosheets (BNNSs), has emerged as an effective method to enhance PLA's thermal properties. However, the thermal conduction of polymer-based nanocomposites is strongly influenced by interfacial thermal resistance. In this study, we investigate the impact of pristine and surface-treated BNNSs on the thermal behavior of PLA using molecular dynamics simulations. To enhance interfacial interactions and reduce chain mobility during heat transfer, we chemically modify the surface of BNNSs by introducing three different functional groups (NH2, OH, and COOH) with varying polarities. Our findings suggest that oxygen-containing groups, namely –OH and –COOH, exhibit stronger interfacial interactions compared to the other cases. We also systematically apply different percentages of these functional groups (i.e., 2.5, 5, and 7.5 %) and observe that a higher number of functional groups leads to a greater improvement in interfacial thermal transport, attributed to the enhanced phonon coupling effect. To complete the discussion, we thoroughly study the influence of random and agglomerated patterns of functional groups distribution.

A‐site coordinating cation engineering in zero‐dimensional antimony halide perovskites for strong self‐trapped exciton emission

AbstractLow‐dimensional hybrid halide perovskites represent a promising class of materials in optoelectronic applications because of strong broad self‐trapped exciton (STE) emissions. However, there exists a limitation in designing the ideal A‐site cation that makes the material satisfy the structure tolerance and exhibit STE emission raised by the appropriate electron–phonon coupling effect. To overcome this dilemma, we developed an inorganic metal‐organic dimethyl sulfoxide (DMSO) coordinating strategy to synthesize a series of zero‐dimensional (0D) Sb‐based halide perovskites including Na3SbBr6·DMSO6 (1), AlSbBr6·DMSO6 (2), AlSbCl6·DMSO6 (3), GaSbCl6·DMSO6 (4), Mn2Sb2Br10·DMSO13 (5) and MgSbBr5·DMSO7 (6), in which the distinctive coordinating A‐site cation [Am‐DMSO6]n+ efficiently separate the [SbXz] polyhedrons. Advantageously, these materials all exhibit broadband‐emissions with full widths at half maxima (FWHM) of 95–184 nm, and the highest photoluminescent quantum yield (PLQY) of 3 reaches 92%. Notably, compounds 2–4 are able to remain stable after storage of more than 120 d. First‐principles calculations indicate that the origin of the efficient STE emission can be attributed to the localized distortion in [SbXz] polyhedron upon optical excitation. Experimental and calculational results demonstrate that the proposed coordinating strategy provides a way to efficiently expand the variety of novel high‐performance STE emitters and continuously regulate their emission behaviors.

Strong plasmon − phonon coupling for graphene/ hBN thermal emitter atomic system

The surface plasmon polaritons of graphene coupled with hyperbolic phonon polaritons of hexagonal boron nitride (hBN) have been considered as the ideal platform to enhance thermal emitter, which provides a powerful tool to implement energy conversion and thermal management. The conventional approaches for the strong plasmon − phonon coupling rely on the bulk hBN materials, which usually have a large footprint and display little tunability. Recently, hBN atomic emitter has been demonstrated that its peak emissivity is hopefully as high as the bulk hBN emitters. Here, we propose a graphene/hBN thermal emitter atomic system to flexibly modify the spectral emissivity by the strong plasmon − phonon coupling effect, which can be separately manipulated by the graphene width, chemical potential of graphene, number of hBN atomic layers and hBN width. Our scheme is hopefully realizing a comparable function with the bulk graphene/hBN system. Furthermore, we employ coupled oscillator model involving three oscillators to investigate the energy exchange and mode evolution between three plasmon − phonon hybridized modes, providing an excellent agreement with simulated and theoretical results. Consequently, optically manipulating plasmon − phonon interaction opens a broad design space for the spectral emissivity control, which explores the mid-infrared applications of advanced graphene thermo-optoelectronics and electrically driven thermal emitters.

Novel electrical properties of moiré graphene systems

In this review, we discuss the electronic structures, topological properties, correlated states, nonlinear optical responses, as well as phonon and electron-phonon coupling effects of moiré graphene superlattices. First, we illustrate that topologically non-trivial flat bands and moiré orbital magnetism are ubiquitous in various twisted graphene systems. In particular, the topological flat bands of magic-angle twisted bilayer graphene can be explained from a zeroth pseudo-Landau-level picture, which can naturally explain the experimentally observed quantum anomalous Hall effect and some of the other correlated states. These topologically nontrivial flat bands may lead to nearly quantized piezoelectric response, which can be used to directly probe the valley Chern numbers in these moiré graphene systems. A simple and general chiral decomposition rule is reviewed and discussed, which can be used to predict the low-energy band dispersions of generic twisted multilayer graphene system and alternating twisted multilayer graphene system. This review further discusses nontrivial interaction effects of magic-angle TBG such as the correlated insulator states, density wave states, cascade transitions, and nematic states, and proposes nonlinear optical measurement as an experimental probe to distinguish the different “featureless” correlated states. The phonon properties and electron-phonon coupling effects are also briefly reviewed. The novel physics emerging from band-aligned graphene-insulator heterostructres is also discussed in this review. In the end, we make a summary and an outlook about the novel physical properties of moiré superlattices based on two-dimensional materials.

Factors influencing self-trapped exciton emission of low-dimensional metal halides

In this review, we mainly summarized the structure distortion, molecular engineering, electron–phonon coupling effect, external temperature and pressure, and metal ion doping that influence the self-trapped exciton emission of low-dimensional metal halides (LDMHs).

Interplay of spin, phonon, and lattice degrees in a hole-doped double perovskite: Observation of spin–phonon coupling and magnetostriction effect

Unlike a typical spin–phonon coupling, an exhibition of unconventional spin–phonon coupling, which is mediated via magnetostriction effect, is reported in a hole-doped double perovskite Pr1.5Sr0.5CoMnO6. Various investigations including electronic and crystal structures, spin structure, transport property, lattice dynamics, and theoretical density of states analysis by density-functional theory (DFT) have been performed. A substantial increase in the mean oxidation states of Co ions and a concurrent abrupt decrease in the resistivity upon Sr doping is observed, thus altering its underlying transport mechanism. An insulating and ferromagnetic (FM) ground state is predicted by DFT calculations. The neutron diffraction data analysis reveals a complex crystal structure of Pr1.5Sr0.5CoMnO6, which consists of B-site disordered monoclinic (P21/n) and orthorhombic (Pnma) structures, highlighting the presence of an anti-site disorder in the system. The analysis also suggests an overall FM ordering of Co/Mn spins below 150 K for the monoclinic phase, whereas no such magnetic ordering is found for the orthorhombic phase. More interestingly, the neutron powder diffraction study perceives the presence of a strong magnetostriction effect in the system. Raman spectroscopy unravels the presence of a spin–phonon coupling, which is essentially mediated by the magnetostriction effect.

Pressure Evolution of Ultrafast Photocarrier Dynamics and Electron-Phonon Coupling in FeTe0.5Se0.5.

Understanding the coupling between electrons and phonons in iron chalcogenides FeTexSe1-x has remained a critical but arduous project in recent decades. The direct observation of the electron-phonon coupling effect through electron dynamics and vibrational properties has been lacking. Here, we report the first pressure-dependent ultrafast photocarrier dynamics and Raman scattering studies on an iron chalcogenide FeTe0.5Se0.5 to explore the interaction between electrons and phonons in this unconventional superconductor. The lifetime of the excited electrons evidently decreases as the pressure increases from 0 to 2.2 GPa, and then increases with further compression. The vibrational properties of the A1g phonon mode exhibit similar behavior, with a pronounced frequency reduction appearing at approximately 2.3 GPa. The dual evidence reveals the enhanced electron-phonon coupling strength with pressure in FeTe0.5Se0.5. Our results give an insight into the role of the electron-phonon coupling effect in iron-based superconductors.

Strong electron–phonon coupling of Cr3+ ion provides an opportunity for superior sensitivity cryogenic sensing

Nowadays, optical temperature measurement is becoming a reliable and advanced technology. Most of optical temperature sensing materials have been reported in the first biological temperature region or even higher. However, cryogenic optical sensing materials seem to be ignored. Here, Cr3+-doped SrGa12O19 is reported, exhibiting a obvious sharp peak at 696 nm (2E→4A2) and near-infrared emission at 750 nm (4T2→4A2) at 77 K. As the temperature increases, the transition of 2E→4A2 is suppressed and the emission of 4T2 is broadened due to the increasing electron–phonon coupling effect. This feature provides an opportunity to obtain the obvious discrepancy of fluorescence intensity, and further realize the superior cryogenic sensing performance based fluorescence intensity ratio (FIR) method with the maximum absolute sensitivity (Sa) of 0.0085 K−1 and relative sensitivity (Sr) of 2.62 %·K−1 at 77 K. The value of Sr is 138.6 % higher than that of CaHfO3:Cr3+ system and the temperature uncertainty (δT) is one order of magnitude lower than the most popular ruby Al2O3:Cr3+, showing superior cryogenic sensing ability. This work not only exhibits a novel cryogenic sensing material but also provides a train of thought for designing cryogenic thermometer.

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure.

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of Tc up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

Thermally Robust Broadband Near-Infrared Luminescence in the NaGaP2O7:Cr3+ Phosphor

The near-infrared (NIR) phosphor-converted light-emitting diode (pc-LED) is an emerging portable light source and has multifunctional applications based on spectroscopic technology. Nevertheless, it is still difficult to develop broadband NIR materials with strong resistance toward thermal quenching. In this work, a thermally robust broadband NIR phosphor NaGaP2O7:Cr3+ was reported, which has an emission peak located at 793 nm, and the emission spectrum covers 650–1100 nm with a full width at half-maximum (fwhm) of 115 nm. The emission intensity at 150 °C was surprisingly maintained as 85.45% of that at 25 °C, indicating an outstanding ability to resist thermal quenching. A systematic investigation of aspects such as structural rigidity, the band gap of the host, and the electron phonon coupling effect was carried out to understand the mechanism of thermally robust luminescence in this material. Finally, the NaGa0.94Cr0.06P2O7 NIR phosphor and 450 nm InGaN LED chip were combined to produce a prototype NIR light source. This device generates a NIR output power of 44.43 mW and a photoelectric conversion efficiency of 10.4% when driven at 150 mA, demonstrating a good performance of this material for practical applications.

Dominant Energy Carrier Transitions and Thermal Anisotropy in Epitaxial Iridium Thin Films

AbstractHigh aspect ratio metal nanostructures are commonly found in a broad range of applications such as electronic compute structures and sensing. The self‐heating and elevated temperatures in these structures, however, pose a significant bottleneck to both the reliability and clock frequencies of modern electronic devices. Any notable progress in energy efficiency and speed requires fundamental and tunable thermal transport mechanisms in nanostructured metals. In this work, time‐domain thermoreflectance is used to expose cross‐plane quasi‐ballistic transport in epitaxially grown metallic Ir(001) interposed between Al and MgO(001). Thermal conductivities ranges from roughly 65 (96 in‐plane) to 119 (122 in‐plane) W m−1 K−1 for 25.5–133.0 nm films, respectively. Further, low defects afforded by epitaxial growth are suspected to allow the observation of electron–phonon coupling effects in sub‐20 nm metals with traditionally electron‐mediated thermal transport. Via combined electro‐thermal measurements and phenomenological modeling, the transition is revealed between three modes of cross‐plane heat conduction across different thicknesses and an interplay among them: electron dominant, phonon dominant, and electron–phonon energy conversion dominant. The results substantiate unexplored modes of heat transport in nanostructured metals, the insights of which can be used to develop electro‐thermal solutions for a host of modern microelectronic devices and sensing structures.