Longitudinal Optical Phonon Research Articles

Phonon Characteristics of Gas-Source Molecular Beam Epitaxy-Grown InAs1−xNx/InP (001) with Identification of Si, Mg and C Impurities in InAs and InN

The lattice dynamical properties of dilute InAs1−xNx/InP (001) epilayers (0 ≤ x ≤ 0.03) grown by gas-source molecular beam epitaxy were carefully studied experimentally and theoretically. A high-resolution Brüker IFS 120 v/S spectrometer was employed to measure the room-temperature infrared reflectivity (IRR) spectra at near-normal incidence (θi = 0). The results in the frequency range of 180–500 cm−1 revealed accurate values of the characteristic In-As-like and In-N-like vibrational modes. For InAs1−xNx alloys, a classical “Drude–Lorentz” model was constructed to obtain the dielectric functions ε~ω in the far IR regions by incorporating InAs-like and InN-like transverse optical ωTO modes. Longitudinal optical ωLO phonons were achieved from the imaginary parts of the simulated dielectric loss functions. The theoretical results of IRR spectra for InAs1−xNx/InP (001) epilayers using a multi-layer optics methodology provided a very good agreement with the experimental data. At oblique incidence (θi ≠ 0), our study of s- and p-polarized reflectance (Rs,p(ω)) and transmission (Ts,p(ω)) spectra allowed the simultaneous perception of the ωTO and ωLO phonons of the InAs, InN and InAs0.97N0.03 layers. Based on the average t-matrix Green’s function theory, the results of local vibrational modes for light SiIn+ donors and SiAs−, CAs− acceptors in InAs were found in good agreement with the existing Raman scattering and infrared spectroscopy data. InInN, however, the method predicted an in-band mode for the MgIn− acceptor while projecting an impurity mode of the SiIn+ donor to appear just above the maximum ωmaxInN[≡595 cm−1] phonon frequency region. In InAs1−xNx/InP (001) epifilms, the comparison of reflectivity/transmission spectra with experiments and the predictions of impurity modes for isoelectronic donor and acceptor impurities in InAs and InN can be valuable for appraising the role of defects in other technologically important semiconductors.

Temperature-dependent photoluminescence of lead-free cesium tin halide perovskite microplates

Tin halide perovskites recently have attracted extensive research attention due to their similar electronic and band structures but non-toxicity compared with their lead analogues. In this work, we prepare high-quality CsSnX 3 (X = Br, I) microplates with lateral sizes of around 1–4 μm by chemical vapor deposition and investigate their low-temperature photoluminescence (PL) properties. A remarkable splitting of PL peaks of the CsSnBr3 microplate is observed at low temperatures. Besides the possible structural phase transition at below 70 K, the multi-peak fittings using Gauss functions and the power-dependent saturation phenomenon suggest that the PL could also be influenced by the conversion from the emission of bound excitons into free excitons. With the increase of temperature, the peak position shows a blueshift tendency for CsSnI3, which is governed by thermal expansion. However, the peak position of the CsSnBr3 microplate exhibits a transition from redshift to blueshift at ∼160 K. The full width at half maximum of CsSnX 3 broadens with increasing temperature, and the fitting results imply that longitudinal optical phonons dominate the electron–phonon coupling and the coupling strength is much more robust in CsSnBr3 than in CsSnI3. The PL intensity of CsSnX 3 microplates is suppressed due to the enhanced non-radiative relaxation and exciton dissociation competing with radiative recombination. According to the Arrhenius law, the exciton binding energy of CsSnBr3 is ∼38.4 meV, slightly smaller than that of CsSnI3.

Rate equations description of the asymmetric double barrier electronic cooler

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments.

Longitudinal-optical-phonon resonant thermal emission efficiency and spectrum control of metal-GaAs surface micro-stripe structures

Phonon-based tera-Hertz or mid-infrared emission has been investigated because it can be applied to high-efficiency light sources for optical communication and material diagnostics, where thermal energy can be utilized as an energy source of emission. Particularly, nano- and micro-surface structures have been investigated for versatile emission controllability. We present the polarized longitudinal optical (LO) phonon resonant thermal emission of radiation (LORE) peaked at approximately 8.5 THz in a range of 450–650 K from simple surface Au–GaAs–Au micro-stripe structures on undoped (u-) GaAs wafers, where the metal component, which has been avoided for surface phonon polariton (SPhP) emission, is introduced. This LORE is based on electric dipoles formed by coherently oscillating polarization charges on pairs of interfaces and has a feature of structure size or emission direction-independent frequency, which is quite different from the properties of SPhPs. The dependence of emission properties on structural factors and temperature reveals the approximately evenly matched radiative and nonradiative LO-phonon annihilation rates in the local surface field and shows the controllability of emission and absorption balance based on the surface structures. Higher emission efficiency in the higher temperature region is found for structures with narrow emission windows of less than 2 μm. The decrease in the volume occupied by electric dipoles reduces the reabsorption probability of LORE, which shortens the effective radiative lifetime and, resultantly, increases the fraction of the radiative rate in the total annihilation rate. By comparing the emission from a pure graphite wafer, a short LORE mean time interval of approximately 2 ps is found. The background emission subject to Planck’s law is significantly reduced particularly in the region of 250–300 cm−1 using emission window widths narrower than 2 μm because of the sub-diffraction phenomenon.

Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr3 Perovskite Quantum Dots.

Exciton-phonon interactions elucidate structure-function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton-phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton-phonon interactions cause performance-degrading PL line width broadening and slower electron-hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron-hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton-phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters.

Electronic, optical, phonon, polaron, carrier transport and scintillation properties of Cesium Hafnium Halides Cs2HfX6, (X = CI, Br and I)

In this work, we investigated the electronic, optical, phonon, polaron, and transport properties of cesium hafnium halides Cs2HfX6, X=Cl, Br and I and estimated their upper light yield. Using First-principles calculations combined the exchange-correlation energy of the Generalized Gradient Approximation GGA with the modified Becke–Johnson exchange potential (mBJ) and the spin-orbit coupling (SOC), these materials demonstrated discrete valence and conduction bands with low band dispersion and direct band gap decreasing as Cs2HfCl6 (5.48 eV)→ Cs2HfBr6(4.18eV)→ Cs2HfI6 (2.72 eV). These obtained values are very close to the available experimental ones. Likewise, the band edge curvature of both valence band maximum (VBM) and conduction band minimum (CBM) reflects the heavier effective mass of electron at CBM Cs2HfCl6(7.58m0),Cs2HfBr6(5.94m0),Cs2HfI6(3.92m0) compared to that of the hole at VBM Cs2HfCl6(6.12m0),Cs2HfBr6(3.72m0),Cs2HfI6(2.34m0). Moreover, these materials exhibited a high transmittance rate, in their emission range, about: 93.57 % for Cs2HfCl6 91.25 % for Cs2HfBr6 and 88.1 % for Cs2HfI6, which is adequate to minimize the scintillated photons loss. The phonon dispersion and the normal modes for Γ point were calculated and discussed. Only three longitudinal optical phonon modes were identified active and their contribution to the ionic dielectric constant was calculated. Having First-principles results, polaron properties, the temperature dependence of polaron mobility, and relaxation time were calculated. These materials showed moderate electron (hole)-phonon couplings αe=6.72(αh=6.04) for Cs2HfCl6,αe=8.26(αh=6.54) for Cs2HfBr6 and αe=6.16(αh=4.76) for Cs2HfI6 compared to other polar materials leading to large polaron mass, high scattering rate, small relaxation time, and low electron (hole) room-temperature mobility 0.14cm2V−1s−1 (0.23 cm2V−1s−1) for Cs2HfCl6 0.14cm2V−1s−1 (0.45 cm2V−1s−1) for Cs2HfBr6 and 0.62cm2V−1s−1 (1.81 cm2V−1s−1) for Cs2HfI6which has only a positive role via the increase of the recombination yield. While such low mobility definitely limited their use in solar cell applications. Moreover, the upper light yield of these materials was estimated for the polaron and simple phenomenological models by taking the needed material inputs to these models from First-principles calculations. The results of latter one were 58,290 photon/MeV for Cs2HfCl6 76,685photon/MeV for Cs2HfBr6 and 118,000photon/MeV for Cs2HfI6 compared to the maximum experimental values 54,000 photon/MeV for Cs2HfCl6 61,500photon/MeV for Cs2HfBr6 and 70,000photon/MeV for Cs2HfI6. These computational findings may add strong evidence for suggesting experimental improvement of the crystal growth conditions and explain the remarkable scintillation properties of these materials as new and efficient gamma- and X-rays scintillators.

Morphology-dependent optical properties of a 3D CH3NH3PbBr3 based metal − organic framework

More recently, Metal–Organic Frameworks (MOFs) have displayed extensive promise in the fields of energy storage, solar cells, catalysis, super capacitors, and optoelectronic devices. This work investigates the nonlinear thermo-optical responses of CH3NH3PbBr3 (MAPbBr3) and mix MAPbBr3 with UiO-66-NH2 (MAPbBr3 + MOF) thin films using the Z-scan technique with different incident power of laser ranging from 10 mW to 50 mW. The results show that the third-order nonlinear optical coefficients of MAPbBr3 + MOF thin films were improved obviously than MAPbBr3 thin films. Furthermore, nonlinear optical properties of MAPbBr3 + MOF thin film is evaluated to be improved from 12.5 × 10-8 to 38.2 × 10-8 by decreasing the input power of laser from 50 mW to 10 mW. This treatment repeated for MAPbBr3 thin film, as well. Considering Kubelka-Munk equation, energy gap) Eg for the MAPbBr3 and MAPbBr3 + MOF thin films deposited on glass film is calculated to be 2.565 and 2.530 eV. Due to analyze the optical characterization of the synthesized samples, optical reflection, transmission and absorption were studied. Kramers-Kronig relations and the reflectance data were used for calculation of complex refractive index and dielectric constant. The transverse and the longitudinal optical phonon modes of MAPbBr3 and MAPbBr3 + MOF thin films locate at about 548 and 598 nm, and 590 and 640 nm, respectively. This research opens a new window for wide scale optoelectronic devices in Q-switching for the generation of pulsed lasers including MAPbBr3 with UiO-66-NH2 by detecting the remarkable nonlinear optical characteristics in lower incident power.

Temperature/laser dependence of N3 optical centers in natural type Ia diamonds through photoluminescence spectroscopy

In this work, low-temperature photoluminescence spectroscopy was employed to investigate the N3 optical color center in natural type Ia diamond. The optical properties of the N3 center with regular changes in testing temperature and laser power were studied, and the energy level transition was also discussed. The results showed that the PL intensity of the N3 center enhanced sub-linearly with laser power increased, indicating that the electron transition of the N3 center was mainly radiative recombination with weak Auger recombination participation. In addition, the physical model was employed to analyze the change in the zero phonon line with the increase in the test temperature; it obtained thermal quenching activation energy (25.2 meV), bond softening, and strong interaction with acoustical phonons of the N3 center. The theoretical analysis of the broadening parameters revealed that the longitudinal optical phonon energy was 55.1 meV, and the electron–optic phonon coupling strength was 2.3 meV.

Coherent Phonon Manipulation via Electron-Phonon Interaction for Facilitated Relaxation of Metastable Centers in ZnO.

Methods that allow versatile manipulation of metastable centers in semiconductors are highly important owing to their potential for quantum information processing and computations. In this study, we demonstrate that the electron-phonon interaction enables phonon participation to promote relaxation of metastable centers in ZnO, which is known for its persistent photoconductivity (PPC) effect. Experimentally, we show that continuous infrared (IR) radiation (1064 nm, ∼30 mW/cm2) promotes longitudinal optical phonons via the Fröhlich interaction and increases the PPC relaxation rate by ∼4 folds. More importantly, we discover that coherent phonons activated by an ultrashort pulse IR laser of the same power increased the relaxation rate by ∼1200-fold, as confirmed by ultrafast transient spectroscopy to be correlated to the excitation of coherent acoustic phonons via the inverse piezoelectric effect. We expect this study to provide valuable guidance for the development of novel quantum and photoactive devices.

Adequate UV photoemission from Ga2O3/ZnO/Ga2O3 thin film epistructures

Epistructures of Ga2O3/ZnO/Ga2O3 were grown on c-Al2O3 substrate using pulsed laser deposition route. Thickness of ZnO layer varied to form these epistructures were investigated structurally, micro-structurally and optically using respective techniques. The modification made with the epistructures are observed to change the peak position of longitudinal optical phonon plasmon (LPP) i.e. A1(LO) mode of the wurtzite ZnO. The specific mode position observed suggests the presence of optical phonon confinement, defects and impurities present at the interface of the layer with others. However the photoluminescence spectra recorded on these epistructures showed quenching of unwanted visible emissions generally observed in device quality of ZnO and gallium oxide films/crystals is interesting. Further the temperature dependent PL investigations performed on selected specimens confirmed that the donor and acceptor excitonic emissions observed are purely governed by either the electron phonon interactions or due to the lattice dilation and the electron phonon interactions both, depending on the thickness of a sandwiched layer i.e. ZnO. The results observed herewith are encouraging considering use epistructures in thin film form to fabricate UV detectors where cost, repeatability and ease of manufacturing are one of the prerequisites.

Synthesis of flower like microstructured CdZnS films for optoelectronic applications by powder vapor transport technique

Cadmium zinc sulfide (CdZnS) is a remarkable semiconducting material used as window layer in solar cells. Herein, the flower’s like microstructured CdZnS film is synthesized by powder vapor transport (PVT) technique. The structural parameters like lattice constant, various diffraction planes, crystallinity, preferential orientation factor and standard deviation of CdZnS film of good quality growing preferentially along (110) orientations. The growth of longitudinal optical phonons modes at 303 and 599 cm−1 confirms the synthesis of CdZnS film. The values of transmittance and refractive index are ranged from 89.9%–95.2% and 1.53–2.67 whereas the values of energy band gap, oscillator energy and dispersion energy are found to be 2.79, 0.73 and 1.73 eV respectively. The values of dielectric constant are increased with increasing photon energy. The smaller values of dissipation factor and volume/surface energy loss functions of CdZnS film is suitable to make it a good quality capacitor. The electrical conductivity (1.68×103 S cm −1 ) of CdZnS film is increased nonlinearly with increasing temperature and the activation energy is found to be 0.31 eV. It is determined that the synthesized CdZnS film may be used as window layer in solar cells and to make good quality capacitor, hence it may be used in optoelectronic applications.

Relaxor Ferroelectric PbNi1/3Ta2/3O3: Synthesis, Structure, Raman Spectra, and Dielectric Susceptibility

The structure of the relaxor ferroelectrics PbNi1/3Ta2/3O3 (PNT) was studied by powder X-ray diffraction. The measurements were performed at 313.5 ± 1 K using a powder, which was prepared by grinding PNT single crystals grown by the spontaneous crystallization. The structure refinement and the fitting of the simulated diffraction pattern to the experimental data were performed by the Rietveld method. It was demonstrated that the grown crystals of PNT have a perovskite structure (sp. gr. Pm3m (221), a = 4.02679(2) Å). Polarized Raman spectra of PNT were recorded at room temperature. The main modes of the light scattering spectra were assigned to the Е1 and А1 components of the transverse optical phonon (ТО1) and the А1 component of the longitudinal optical phonon (LO3). The temperature dependence of the dielectric susceptibility shows a broad frequency-dependent anomaly with a maximum at 89 К at a frequency of 1 kHz.

Description of electron mobilities in epitaxial lanthanum-doped barium stannate films: Influences of LO phonons, threading dislocation, and ionized donor defects

Lanthanum-doped barium stannate (La-doped BaSnO3 or LBSO) has attracted the attention of researchers and engineers because of its wide range of potential applications in electronic and optoelectronic devices. This is due to a combination of its exceptional room temperature (RT) mobility of 320 cm2 V−1 s−1 and high visible range transparency. However, epitaxial LBSO films made using strategic deposition techniques such as molecular beam epitaxy, pulsed laser deposition, and magnetron sputtering show comparatively low RT mobilities, between 24 and 183 cm2 V−1 s−1, and an accurate description of these RT mobilities is still sought. Herein, we provide the underlying scattering mechanisms related to longitudinal optical (LO) phonons, threading dislocation, and ionized donor defects to elucidate the RT mobilities in LBSO epitaxial films. It was found that the total mobility estimated using Matthiessen's rule provided strong quantitative agreement with experimental results. The large polaron mobility based on LO phonon scattering dominated the whole spectrum of electron concentrations in this system. It was an upper bound mobility, i.e., the mobility limit attained at 320 cm2 V−1 s−1. The calculated mobility associated with LO phonon and threading dislocation scatterings adequately verified the experimental results between 150 and 183 cm2 V−1 s−1. The predicted results for all three scattering types were predominant in experimental data at less than 150 cm2 V−1 s−1. These investigations deepen our understanding of mechanisms governing the charge transport scattering in epitaxial LBSO films and pave the way for the development of novel semiconductor thin films for use in electronic and optoelectronic devices.

On the vibrational properties of transition metal doped ZnO: Surface, defect, and bandgap engineering

We present a comprehensive study on the structure and optical properties of Mn-and Co-doped ZnO samples prepared via solid-state reaction method with different dopant concentrations and atmospheres. The samples were structural and chemically characterized via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray excited photoelectron spectroscopy. The optical characterization was performed via Raman, photoluminescence, and diffuse photoreflectance spectroscopies. Emphasis was done on the studies of their vibrational properties. The structural data confirm the incorporation of Mn and Co ions into the wurtzite ZnO lattice. It is demonstrated that the usual observed additional bands in the Raman spectrum of transitional metal (TM) doped ZnO are related to structural damage, deriving from the doping process, and surface effects. The promoted surface optical phonons (SOP) are of Fröhlich character and, together with the longitudinal optical (LO) polar phonons, are directly dependent on the ZnO electronic structure. The enhancement of SOP and LO modes with TM-doping is explained in terms of nonhomogeneous doping, with the dopants concentrating mainly on the surface of grains, and a resonance effect due to the decrease of the ZnO bandgap promoted by the introduction of the 3d TM levels within the ZnO bandgap. We also discuss the origin of the controversial vibrational mode commonly observed in the Mn-doped ZnO system. It is stated that the observation of the analyzed vibrational properties is a signature of substitutional doping of the ZnO structure with tuning of ZnO optical absorption into the visible range of the electromagnetic spectrum.

Effect of Mn concentration on the polar optical phonons frequencies in Cd1− xMnxTe thin films

Cd1−xMnxTe (CMT) is a wide bandgap semiconductor that stands out among the ternary compounds in several industrial applications, such as optoelectronic and solar cell devices. Therefore, it is worth understanding the mechanisms of light interaction with CMT produced by growth techniques. In this work, we investigate the room temperature Raman scattering by longitudinal optical phonon modes CdTe-like (LO1) and MnTe-like (LO2) in CMT thin films grown on Si(111) by molecular-beam epitaxy. The well-known linear dependence of LO1 and LO2 frequencies on x is observed in Raman spectra when the excitation photon energy is above the bandgap energy. As expected, for the excitation energy of the incident laser line near the fundamental gap of CMT, the resonance becomes evident in the Raman results. However, for a specific resonance condition due to tuning of the bandgap energy dependence on Mn concentration, the frequencies of the LO1 and LO2 phonon modes remain constant as the average manganese concentration increases to values x≳ 0.30. From micro-photoluminescence spectroscopy investigations, we concluded that for Mn concentration above 0.3, a broad range of optical transitions provides the required conditions for a resonant Raman scattering selected by the incident photon energy. It introduces a resonant selectivity of regions where the incident excitation energy coincides with a bandgap energy associated with a specific value of x, even for samples with nominally different compositions. The results show the sensitivity of resonant Raman to express diagnostics of ternary compound growth.

Fundamental limitations on gain of terahertz quantum cascade lasers

We analyze the main physical processes in quantum cascade lasers with a spatial separation between the region of photon radiation and longitudinal optical (LO) phonon emission, which facilitates the depopulation of the lower level of the optical transition. Our objective is to identify the reasons for the reduction of population inversion at low photon energy and explore methods to enhance it. The expression for population inversion is derived from an equation for a simplified density matrix. This approach allows us to consider the coherence of tunneling between different levels and comprehend its influence on transition probabilities in a straightforward manner. We have found out that the energy uncertainty principle is the fundamental factor limiting population inversion in terahertz lasers. By optimizing the tunneling matrix element between the two regions and the LO phonon emission time, it is possible to significantly increase the population inversion. The optimal value for the matrix element is smaller than its maximum possible value, while the optimal LO phonon emission time exceeds the time achieved during LO phonon resonant emission.

Raman scattering and photoluminescence in In+ and As+ ion-implanted SiO2 layers encapsulated with Si3N4

Raman and photoluminescence spectra were investigated in the In+ and As+ ion-implanted SiO2 films encapsulated with Si3N4 layers as a function of annealing temperature. The optical phonon frequency, as a function of the InAs nanocrystal size, was also calculated within the confined phonon model. The Raman scattering band of around 231 cm−1, close to the low-frequency shifted longitudinal optical phonon mode in the InAs matrix, was observed as the annealing temperature increased to 900 °C. The InAs nanocrystal size of 3 nm was estimated. The strong room-temperature photoluminescence peaking at 550 nm (2.25 eV) was also obtained under the 473 nm wavelength excitation. Its intensity reached a maximum value as the annealing temperature increased to 1000 °C. Its peak position was blue-shifted as the excitation wavelength decreased. The direct irradiative electron and hole recombination in the InAs nanocrystals was proposed as a possible mechanism of this photoluminescence.

Origin of low lattice thermal conductivity and mobility of lead-free halide double perovskites

Lead-free halide double perovskites of Cs2BiAgBr6 and Cs2BiAgCl6 have attracted extensive attention due to potential application in optoelectronic field. The transport behavior plays an important role in electronic devices based on semiconductors, which includes lattice thermal conductivity and carier mobility. Since impurities and defects may exist in the samples, it is difficult to investigate the intrinsic transport behavior by experimental measurements. The theoretical prediction of their transport behaviors can provide a guidance for the application of materials. In this work, we first develop machine learning interatomic potential (MLIP) to explore their lattice thermal conductivities. They exhibit extremely low lattice thermal conductivity at 300 K with the values of κx = 0.122 ± 0.009∕κz = 0.103 ± 0.004 Wm−1K−1 for Cs2BiAgBr6, and κx = 0.352 ± 0.014∕κz = 0.337 ± 0.012 Wm−1K−1 for Cs2BiAgCl6, respectively. It is revealed that the thermal conductivity is mainly controlled by low-frequency phonons. Subsequently, we apply semi-empirical models to predict their mobilities. The mobility is mainly determined by the scattering from longitudinal optical phonon with the values of μe,x= 22.0/μh,x= 18.8cm2V−1s−1 for Cs2BiAgBr6, and μe,x= 19.2/μh,x= 19.5cm2V−1s−1 for Cs2BiAgCl6, respectively. The stretching between Bi and Br/Cl atoms limits the mobility. Finally, the origin of low thermal conductivity and mobility on double perovskites is revealed.

Insights to Carrier-Phonon Interactions in Lead Halide Perovskites via Multi-Pulse Manipulation.

A fundamental understanding of the hot-carrier dynamics in halide perovskites is crucial for unlocking their prospects for next generation photovoltaics. Presently, a coherent picture of the hot carrier cooling process remains patchy due to temporally overlapping contributions from many-body interactions, multi-bands, band gap renormalization, Burstein-Moss shift etc. Pump-push-probe (PPP) spectroscopy recently emerges as a powerful tool complementing the ubiquitous pump-probe (PP) spectroscopy in the study of hot-carrier dynamics. However, limited information from PPP on the initial excitation density and carrier temperature curtails its full potential. Herein, this work bridges this gap in PPP with a unified model that retrieves these essential hot carrier metrics like initial carrier density and carrier temperature under the push conditions, thus permitting direct comparison with traditional PP spectroscopy. These results are well-fitted by the phonon bottleneck model, from which the longitudinal opticalphonon scattering time τLO , for MAPbBr3 and MAPbI3 halide perovskite thin film samples are determined to be 240 ± 10 and 370 ± 10 fs, respectively.

Improved optical properties of lead-free double perovskite Cs2NaBiCl6 nanocrystal via K ions doping

Double perovskite has shown great potential to solve the toxicity and instability problems of lead halide perovskite in practical applications. However, most lead-free double perovskite nanocrystals have low photoluminescence quantum yield (PLQY). Metal ion doping is usually an effective method to improve the optical properties of lead-free double perovskite nanocrystals (NCs). Here, we have prepared monodisperse Cs2NaBiCl6 NCs with uniform morphology and good crystallinity, and a series of Cs2NaBiCl6 NCs with different K+ doping amounts were prepared by colloid synthesis strategy. As the amount of potassium ion doping increased, the absorption spectra of the perovskites shifted towards the red, and the bandgap decreased from 3.56 eV to 3.50 eV. At the same time, the introduction of K+ improved the optical properties of Cs2NaBiCl6 NCs. The PLQY of Cs2Na0.62K0.38BiCl6 NCs reached 7.06 %, which was 4.38 times higher than that of Cs2NaBiCl6 NCs. The lifetime of exciton was increased from 3.42 ns to 12.02 ns by 3.5 times. Thermogravimetric analysis showed that Cs2Na0.62K0.38BiCl6 NCs had better thermal stability than Cs2NaBiCl6 NCs. Temperature-dependent PL spectra showed that Cs2Na0.62K0.38BiCl6 had higher exciton binding energy and longitudinal optical phonon energy than Cs2NaBiCl6 NCs.