Phonon Coupling Strength Research Articles

Exciton-phonon interaction in quasi-two dimensional layered (PEA)2(CsPbBr3)n-1PbBr4 perovskite.

Two-dimensional (2D) Ruddlesden-Popper perovskites with bulky organic cations have attracted extensive attention in light-emitting devices and photovoltaics due to their robust environment stability, tunable luminescent color, strong exciton binding and promising efficiency. A quantum well (QW) structure is spontaneously formed by sandwiching PbBr4 layers into bulky organic cations. However, some intrinsic excitonic mechanisms in these materials still need to be elucidated. In this study, the exciton-phonon interaction of quasi-2D (PEA)2(CsPbBr3)n-1PbBr4 with different PbBr4 layer numbers (n) was analyzed by temperature-varied photoluminescence (PL), scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD). The mechanism of bandgap shifting with temperature was found to be dominated by the thermal expansion effect in the large-n 2D and bulk perovskite, and gradually switched to exciton-phonon interaction in the n = 1 (PEA)2PbBr4 phase, indicating enhanced exciton-phonon interaction in the thinner quantum well structure. Further analysis showed that the enhanced exciton-phonon interaction originated from the longitudinal optical phonon-exciton Fröhlich interaction rather than acoustic phonon-exciton coupling. We believe that our results will benefit the further optimization of light-emitting devices based on 2D perovskites.

Temperature Effects on the First Excited State of the Polaron in an Asymmetric Quantum Pseudodot Under Magnetic Field

Using the variational method of the Pekar type, we investigate the first excited state energy of the polaron in an asymmetric quantum pseudodot under the magnetic field. Temperature effects on the polaron are calculated by employing the quantum statistical theory, and the influences of the chemical potential, the zero point of pseudo-harmonic potential (PHP), the cyclotron frequency, the electron–phonon coupling strength and the transverse and the longitudinal effective confinement lengths are taken into account. The results show that the first excited state energy decreases (increases) when the temperature is increased at lower (higher) temperature region. And it is an increasing function of the chemical potential, the zero point of PHP, the cyclotron frequency and the electron–phonon coupling strength. Simultaneously, it is a decreasing one of the transverse and the longitudinal effective confinement lengths.

Effect of B2O3 content on Eu3+−Surrounding in the system SrO−x.B2O3 containing Eu3+ ions prepared by combustion method

The change of Eu3+− surrounding in the system SrO−x.B2O3 with various x values (x = 0.33, 0.5, 1) synthesized by combustion method was investigated via photoluminescence (PL) spectra and phonon sideband (PSB) spectra. From the analysis of PSB spectra, it is reasonable to suggest that when increasing B2O3 content the electron−phonon coupling strength increases due to the increase in the number of BO− non−bridging bond. By using Judd−Ofelt (JO) theory for PL spectra of Eu3+ ion, the Ω2 intensity parameter was calculated and it also shows the asymmetry of Eu3+− surrounding increase corresponding to the B2O3 content.

Electron–phonon coupling in single-layer MoS2

The electron–phonon coupling strength in the spin–split valence band maximum of single-layer MoS2 is studied using angle-resolved photoemission spectroscopy and density functional theory-based calculations. Values of the electron–phonon coupling parameter λ are obtained by measuring the linewidth of the spin–split bands as a function of temperature and fitting the data points using a Debye model. The experimental values of λ for the upper and lower spin–split bands at K are found to be 0.05 and 0.32, respectively, in excellent agreement with the calculated values for a free-standing single-layer MoS2. The results are discussed in the context of spin and phase-space restricted scattering channels, as reported earlier for single-layer WS2 on Au(111). The fact that the absolute valence band maximum in single-layer MoS2 at K is almost degenerate with the local valence band maximum at Γ can potentially be used to tune the strength of the electron–phonon interaction in this material.

Remote Phononic Effects in Epitaxial Ruddlesden-Popper Halide Perovskites.

Despite their weak nature, van der Waals (vdW) interactions have been shown to effectively control the optoelectronic and vibrational properties of layered materials. However, how vdW effects exist in Ruddlesden-Popper layered halide perovskites remains unclear. Here we reveal the role of interlayer vdW force in Ruddlesden-Popper perovskite in regulating phase-transition kinetics and carrier dynamics based on high-quality epitaxial single-crystalline (C4H9NH3)2PbI4 flakes with controlled dimensions. Both substrate-perovskite epitaxial interaction and interlayer vdW interaction play significant roles in suppressing the structural phase transition. With reducing flake thickness from ∼100 to ∼20 nm, electron-phonon coupling strength decreases by ∼30%, suggesting the ineffectiveness of phonon confinement of the natural quantum wells. Therefore, the conventional understanding that vdW perovskite is equivalent to a multiple quantum well has to be substantially amended due to significant nonlocal phononic effects in the layered crystal, where intralayer interaction is not drastically different from the interlayer force.

New Insights into the Role of Weak Electron⁻Phonon Coupling in Nanostructured ZnO Thin Films.

High-quality crystalline nanostructured ZnO thin films were grown on sapphire substrates by reactive sputtering. As-grown and post-annealed films (in air) with various grain sizes (2 to 29 nm) were investigated by scanning electron microscopy, X-ray diffraction, and Raman scattering. The electron–phonon coupling (EPC) strength, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminished by reducing the ZnO grain size, which mainly relates to the Fröhlich interactions. Our finding suggests that in the spatially quantum-confined system the low polar nature leads to weak EPC. The outcome of this study is important for the development of nanoscale high-performance optoelectronic devices.

Phonon Coupling with Excitons and Free Carriers in Formamidinium Lead Bromide Perovskite Nanocrystals.

Organometal halide perovskites in the form of nanocrystals (NCs) have attracted enormous attention due to their unique optoelectronic and photoluminescence (PL) properties. Here, we examine the phase composition and the temperature dependence of emission line width broadening in formamidinium lead bromide (FAPbBr3) perovskite nanocrystals (NCs) for light-emitting applications and identify different charge-carrier scattering mechanisms. Our results show most of the emission is from the orthorhombic phase. The PL line width broadening at high temperature is dominated by the Fröhlich interaction between the free charge carriers and the optical phonons. At low temperatures, the peak of the PL spectrum exhibits a continuous red shift indicating an increase of excitons contribution at lower temperatures, and concurrently the line width also narrows down due to the inhibition of the optical phonons. From the temperature-dependent measurements, the coupling strength of both the charge phonon interaction and the exciton phonon interaction have been determined. The obtained results indicate that the charge phonon coupling strengths are higher compared to the exciton phonon coupling.

Optical absorption of Fröhlich polaron in monolayer transition metal dichalcogenides

We theoretically study the optical absorption of a Fröhlich polaron in monolayer transition metal dichalcogenides (TMDS) on different polar substrates using the Devreese-Huybrechts-Lemmens model, in which both the surface optical phonon modes induced by the polar substrate and the intrinsic longitudinal optical phonon modes have been taken into account. We find that the optical absorption occurs only as the energy of incident photon exceeds the energy of optical phonon. The behaviors of absorption are determined by the interplays between the strength of electron-optical phonon coupling and the energy of optical phonon. The amplitude of absorption can be tuned directly by the internal distance between TMDS and polar substrates. These theoretical results provide significant insight into the infrared absorption in two-dimensional materials.

Reduced carrier trapping in CdSe/ZnS/CdSe heterostructure quantum dots inferred from temperature dependent spectral studies

We report temperature dependent photoluminescence (TDPL) characteristics of CdSe/ZnS core-shell and coupled 0D-2D CdSe/ZnS/CdSe heterostructure quantum dots (HQDs). Anomalous, behavior of temperature dependent PL linewidth seen in core-shell HQDs is ascribed to the relaxation of carriers into interfacial strain related local energy minimum. TDPL characteristics show largest exciton-acoustic phonon and exciton-LO phonon coupling in core-shell HQDs compared to other QDs. These are caused by enhanced deformation potential interaction and enhanced Fröhlich interaction due to the interfacial strain field. Further, relatively lower strength of exciton-acoustic phonon coupling in 0D-2D HQDs is due to relaxation of interfacial strain. Temperature dependence of spectrally integrated PL intensity shows nonradiative relaxation below 150 K due to thermal activation of carriers in surface tarp states while at temperatures above, it is dominated by carrier thermal escape mediated by exciton-LO phonon interaction involving lesser number of phonons in 0D-2D HQDs.

Investigating the Electron–Phonon Coupling of Molecular Beam Epitaxy-Grown Hg1−xCdxSe Semiconductor Alloys

Using spectroscopic ellipsometry, the temperature-dependence of the dielectric functions of a series of Hg1−xCdxSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates were investigated. Initially, for each sample, room-temperature ellipsometric spectra were obtained from 35 meV to 6 eV using two different ellipsometers. Subsequently, ellipsometry spectra were obtained from 10 K to 300 K by incorporating a cryostat to the ellipsometer. Using a standard inversion technique, the spectroscopic ellipsometric data were modeled in order to obtain the temperature-dependent dielectric functions of each of the Hg1−xCdxSe thin films. The results indicate that the E1 critical point blue-shifts as a function of Cd-alloy concentration. The temperature-dependence of E1 was fitted to a Bose–Einstein occupation distribution function, which consequently allowed us to determine the electron–phonon coupling of Hg1−xCdxSe alloys. From the fitting results, we obtain a value of 17 ± 2 meV for the strength of the electron–phonon coupling for Hg1−xCdxSe alloy system, which compares nominally with the binary systems, such as CdSe and CdTe, which have values around 38 meV and 16 meV, respectively. This implies that the addition of Hg into the CdSe binary system does not significantly alter its electron–phonon coupling strength. Raman spectroscopy measurements performed on all the samples show the HgSe-like transverse optic (TO) and longitudinal optic (LO) phonons (∼ 130 cm−1 and ∼ 160 cm−1, respectively) for all the samples. While there is a slight red-shift of the HgSe-like TO peak as a function of the Cd-concentration, HgSe-like LO peak does not significantly change with the alloy concentration.

Optical Absorption and EPR Studies on Gamma-Ray Irradiated RE3+-Doped Fluorophosphate Glasses

We have studied the effect of γ-ray irradiation on optical absorption, emission and decay characteristics of RE3+ (RE = Sm, Eu and Dy)-doped fluorophosphate glasses. Electron paramagnetic resonance (EPR) study confirms the POHC and PO3 EC defects induced in glasses by the γ-irradiation. The presence of induced defect centers significantly affects the optical and emission properties. The optical band gap values of the studied systems increased after the γ-ray irradiation. The phonon energy and electron–phonon coupling strength of Eu3+-doped fluorophosphate glass were determined from the phonon sideband analysis. The emission intensity of the RE3+ ions increased significantly after the γ-ray irradiation. The intensity parameter, R is the ratio of the intensities of the 5D0 → 7F2/5D0 → 7F1 transitions of Eu3+ ion and Y/B intensity parameter is the ratio of intensities of the 4F9/2 → 6H13∕2/4F9/2 → 6H15/2 transitions of Dy3+ ion reveal that the local environment around the RE3+ ion changed after the γ-ray irradiation in the present system. The lifetime of excited states of RE3+ ions decreased after the γ-ray irradiation due to the formation of defects induced by the γ-ray irradiation. The CIE color coordinates were determined before and after the γ-ray irradiation for the Dy3+-doped glass system.

Transient Carrier Cooling Enhanced by Grain Boundaries in Graphene Monolayer.

Using a high terahertz (THz) electric field (ETHz), the carrier scattering in graphene was studied with an electric field of up to 282 kV/cm. When the grain size of graphene monolayers varies from small (5 μm) and medium (70 μm) to large grains (500 μm), the dominant carrier scattering source in large- and small-grained graphene differs at high THz field, i.e., there is optical phonon scattering for large grains and defect scattering for small grains. Although the electron-optical phonon coupling strength is the same for all grain sizes in our study, the enhanced optical phonon scattering in the high THz field from the large-grained graphene is caused by a higher optical phonon temperature, originating from the slow relaxation of accelerated electrons. Unlike the large-grained graphene, lower electron and optical phonon temperatures are found in the small-grained graphene monolayer, resulting from the effective carrier cooling through the defects, called supercollisions. Our results indicate that the carrier mobility in the high-crystalline graphene is easily vulnerable to scattering by the optical phonons. Thus, controlling the population of defect sites, as a means for carrier cooling, can enhance the carrier mobility at high electric fields in graphene electronics by suppressing the heating of optical phonons.

Raman Studies on Silicene and Germanene

Two-dimensional (2D) group IV elemental materials are expected to have similar electronic features to their lightest analogue, graphene. The favorable hybridization state of heavier group IV elements is the sp3 state, leading to buckled structures instead of a planar one. These buckled structures could modulate the vibrational properties of these 2D materials by introducing a strain effect. In this review, the authors focus on the recent research on the Raman spectra of silicene and germanene from both the theoretical and experimental sides. The abundant superstructures of silicene and germanene are formed due to the different interaction strengths between silicene/germanene and underlying substrate, providing a way to modulate their phonon vibrational properties. Several factors affecting the vibrational modes are discussed, including the strain, doping, coverage and defect effects. Furthermore, the relationship between electron–phonon coupling strength and these factors is established based on the variation of Raman peak position and linewidth. Finally, the authors provide an overview of the general outlook and challenges for this field.

Strong Carrier-Phonon Coupling in Lead Halide Perovskite Nanocrystals.

We highlight the importance of carrier–phonon coupling in inorganic lead halide perovskite nanocrystals. The low-temperature photoluminescence (PL) spectrum of CsPbBr3 has been investigated under a nonresonant and a nonstandard, quasi-resonant excitation scheme, and phonon replicas of the main PL band have been identified as due to the Fröhlich interaction. The energy of longitudinal optical (LO) phonons has been determined from the separation of the zero phonon band and phonon replicas. We reason that the observed LO phonon coupling can only be related to an orthorhombically distorted crystal structure of the perovskite nanocrystals. Additionally, the strength of carrier–phonon coupling has been characterized using the ratio between the intensities of the first phonon replica and the zero-phonon band. PL emission from localized versus delocalized carriers has been identified as the source of the observed discrepancies between the LO phonon energy and phonon coupling strength under quasi-resonant and nonresonant excitation conditions, respectively.

Development of sodium-zinc phosphate glasses doped with Dy3+, Eu3+ and Dy3+/Eu3+ for yellow laser medium, reddish-orange and white phosphor applications

A spectroscopy analysis of Eu2O3 (2.0mol%) or Dy2O3 (2.0mol%) singly-doped and Dy2O3 (2.0mol%)/Eu2O3 (2.0mol%) codoped sodium-zinc phosphate glasses is performed through their Raman, absorbance and photoluminescence spectra, and decay times. The phonon energy (1124cm−1) and electron–phonon coupling strength (0.028) of the glass host were determined using phonon side band spectroscopy. Laser parameter values (stimulated emission cross section, effective bandwidth, gain bandwidth and optical gain), obtained for the 4F9/2 → 6H13/2 yellow emission of the Dy3+ singly-doped glass, are higher than those reported for others 2mol% Dy2O3-doped oxide glasses, so that the magnitudes of such parameters could be suitable for a low threshold and a high gain of yellow laser emission. Additionally a value of the quantum efficiency close to the unity (0.96) was found for the dysprosium 4F9/2 level luminescence. The intense yellow emission relative to the 4F9/2 → 6H15/2 blue one leads to the Dy3+ singly-doped glass to emit neutral white light of 4647K upon 349nm excitation. The Eu3+ singly-doped glass can emit reddish-orange light of 2166K and red color purity of 97.7% upon 394nm excitation, whose (0.642, 0.351) CIE1931chromaticity coordinates are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. The Dy3+/Eu3+codoped glass emission color can be tuned from warm white light of 3628K upon 349nm excitation to reddish-orange light of 1891 K after 364nm excitation. The warm white light emission is generated mainly by the transitions 4F9/2 → 6H13/2, 6H15/2 of Dy3+ and 5D0 → 7F2 of Eu3+, where Eu3+ is sensitized by Dy3+ through non-radiative energy transfer with an efficiency of 0.22. An electric dipole-dipole interaction into Dy3+ - Eu3+ clusters might be the dominant mechanism in the Dy3+ to Eu3+ energy transfer. Excitations at 349, 364 and 394nm can be obtained with AlGaN, GaN and InGaN based LEDs, respectively, and therefore Dy3+ and/or Eu3+ doped sodium-zinc phosphate glasses pumped by UV LEDs could be appropriate for solid state lighting technology as neutral/warm white and reddish-orange light sources.

Exciton and lattice dynamics in low-temperature processable CsPbBr3 thin-films

Lead-halide based perovskite semiconductors have recently attracted significant attention owing to their rapidly increasing efficiency in solar cells and light-emitting devices. Organic–inorganic lead halide perovskites have gained popularity due to their low-temperature solution processability, low electronic defect density and low structural and thermal disorder. Nonetheless, their single crystals suffer from lower carrier mobility (∼10–100 cm2 V−1 s−1) than those in inorganic semiconductors and slow radiative recombination rates, with average lifetimes exceeding one μs. Recently, CsPbBr3, an all-inorganic lead-bromide perovskite also attracted great attention due to its high thermal stability and fast radiative recombination. Therefore, it is required to explore the exciton and lattice dynamics of solution processed CsPbBr3 thin films. Here, we show that CsPbBr3 exhibits an electron–phonon coupling strength (∼70 meV) comparable with its organic–inorganic hybrid perovskite counterpart CH3NH3PbBr3 (∼60 meV). The main source of electron–phonon coupling was identified to be the longitudinal-optical phonon mode of the PbBr lattice vibration at ∼20 meV. Carrier recombination studies by temperature-dependent steady-state and time-resolved photoluminescence revealed that recombination occurred from lattice-bound carriers at room-temperature and in inert atmosphere. Carrier lifetimes are short and show a distinct temperature-dependence. All findings are supported by first-principles calculations in the framework of density-functional theory.

Van Der Waals Hybrid Perovskite of High Optical Quality by Chemical Vapor Deposition

Abstract2D hybrid perovksite (RNH3)2PbX4 materials not only serve as ideal platforms to study fundamental physics such as polariton dynamics but also show promise for optoelectronic and electro‐optic applications. However, for the preparation of high optical quality crystals, mechanical exfoliation has to be applied in the past. In this work, the vapor phase growth of single crystalline (C4H9NH3)2PbI4 flakes with high optical quality is reported. Individual single crystalline domains show lateral size about 5–10 µm with defined rectangular shape. Spectroscopic studies show room temperature photoluminescence full width at half maximum of 70 meV and decay lifetime of several nanoseconds, indicating comparably high quality with mechanically exfoliated counterparts. Exciton binding energy 279 ± 46 meV and electron–phonon coupling strength around 20 meV are revealed. This vacuum‐based method may provide a solution for integrating layered perovskites into optoelectronic devices and systems.

Effect of oxygen partial pressure on the density of antiphase boundaries in Fe3O4 thin films on Si(100)

Polycrystalline Fe3O4 thin films were grown on Si(100) substrate by reactive DC sputtering at different oxygen partial pressures PO2 for controlling the growth associated density of antiphase boundaries (APBs). The micro-Raman analyses were performed to study the structural and electronic properties in these films. The growth linked changes in the APBs density are probed by electron–phonon coupling strength (λ) and isothermal magnetization measurements. The estimated values of λ are found to vary from 0.39 to 0.56 with the increase in PO2 from 2.2×10−5 to 3.0×10−5Torr, respectively. The saturation magnetization (saturation field) values are found to increase (decrease) from 394 (5.9) to 439 (3.0)emu/cm3 (kOe) with the increase in PO2. The sharp Verwey transition (∼120K), low saturation field, high saturation magnetization and low value of λ (comparable to the bulk value ∼0.51) clearly affirm the negligible amount of APBs in the high oxygen partial pressure deposited thin films.

Quantum transition and decoherence of levitating polaron on helium film thickness under an electromagnetic field

We have studied the transition probability and decoherence time of levitating polaron in helium film thickness. By using a variational method of Pekar type, the ground and the first excited states of polaron are calculated above the liquid-helium film placed on the polar substrate. It is shown that the polaron transits from the ground to the excited state in the presence of an external electromagnetic field in the plane. We have seen that, in the helium film, the effects of the magnetic and electric fields on the polaron are opposite. It is also shown that the energy, transition probability and decoherence time of the polaron depend sensitively on the helium film thickness. We found that decoherence time decreases as a function of increasing electron–phonon coupling strength and the helium film thickness. It is seen that the film thickness can be considered as a new confinement in our system and can be adjusted in order to reduce decoherence.

Electrothermal Control of Graphene Plasmon-Phonon Polaritons.

Graphene plasmons are known to offer an unprecedented level of confinement and enhancement of electromagnetic field. They are hence amenable to interacting strongly with various other excitations (for example, phonons) in their surroundings and are an ideal platform to study the properties of hybrid optical modes. Conversely, the thermally induced motion of particles and quasiparticles can in turn interact with electronic degrees of freedom in graphene, including the collective plasmon modes via the Coulomb interaction, which opens up new pathways to manipulate and control the behavior of these modes. This study demonstrates tunable electrothermal control of coupling between graphene mid-infrared (mid-IR) plasmons and IR active optical phonons in silicon nitride. This study utilizes graphene nanoribbons functioning as both localized plasmonic resonators and local Joule heaters upon application of an external bias. In the latter role, they achieve up to ≈100 K of temperature variation within the device area. This study observes increased modal splitting of two plasmon-phonon polariton hybrid modes with temperature, which is a manifestation of increased plasmon-phonon coupling strength. Additionally, this study also reports on the existence of a thermally excited hybrid plasmon-phonon mode. This work can open the door for future optoelectronic devices such as electrically switchable graphene mid-infrared plasmon sources.