Carrier-phonon Coupling Research Articles

Hole Polaron-Mediated Suppression of Electron-Hole Recombination Triggers Efficient Photocatalytic Nitrogen Fixation.

In the pursuit of successful photocatalytic transformations, challenges persist due to limitations in charge carrier utilization and transfer efficiency, which stemming from rapid recombination. Overcoming these limitations necessitates the exploration of novel mechanisms that enhance the effective separation of photogenerated electron-hole pairs. Herein, deviating from the conventional approach of enhancing carrier migration to separate photogenerated charges and extend their lifetime, the proposal is to directly prevent the recombination of photogenerated electrons and holes by forming hole polarons. Specifically, disordered pores are introduced on the surface of KTaO3 ultrathin sheets, and the clear-cut evidences in electron paramagnetic resonance, photoluminescence, and ultrafast spectroscopy unambiguously confirm the enhanced carrier-phonon coupling, which results in the formation of hole polarons to impede the recombination of photogenerated electron-hole pairs. Taking the challenging nitrogen oxidation reaction as an example, it is found that the hole polarons in atomic-disordered pore KTaO3 ultrathin nanosheets trigger outstanding photo-oxidation performance of nitrogen (N2)to nitrate, with a nitrate-producing rate of 2.1 mg g-1 h-1. This scenario is undoubtedly applicable to a wide variety of photocatalytic reactions due to the common challenge of charge carrier recombination in all photocatalytic processes, manifesting broad implications for promoting photocatalysis performance.

Ultrafast nanometric resolution spatiotemporal measurement of ambipolar diffusivity and carrier-phonon coupling in doped and undoped silicon

Understanding and determination of the ambipolar diffusivity of a semiconductor is fundamental for predicting device behavior and optimizing its performance. Ultrafast pump-probe measurements allow for the determination of energy carrier dynamics with sub-picosecond resolution. Due to the inherent diffusive nature of carriers, measurements with a high spatial resolution are needed, in addition to the traditional pump-probe system, to determine not only the carrier dynamics but also the spatial extent caused by carrier diffusion as well. In this work, a spatiotemporal measurement system with ultrafast temporal and nanometric spatial resolution, together with a comprehensive transport model, is used to determine the ambipolar diffusivity and carrier-phonon energy coupling time in both undoped and doped silicon. The results show that as the carrier density increases, the measured ambipolar diffusivity decreases with minimal variation in the carrier-phonon energy coupling time. In general, this work demonstrates an optical-based method for determining ambipolar diffusivity in a semiconductor material.

Recent advances of computational simulation on polarons in organic-inorganic hybrid perovskites

Organic-inorganic hybrid perovskites are prized for their exceptional properties and ease of use, finding applications in high-efficiency, cost-effective solar cells. Hybrid perovskite materials, belonging to the perovskite family, possess unique crystallographic and electronic structures that benefit from versatile phase transitions and component modifications. The crystal structure and electronic characteristics of perovskites facilitate the generation of polarons, which are quasiparticles that readily form in organic-inorganic hybrid perovskites due to the coupling of excess electrons or holes to ionic vibrations and play a pivotal role in shaping the electronic properties. This review delves into the polarons within organic-inorganic hybrid perovskites, in which the formation and interaction of polarons within the crystal lattice and their profound impact on material properties. Computational simulations offer insights into polaron characteristics, such as phonon density of states, carrier-phonon coupling, and polaron dynamics. Therefore, it is of vital importance to make clear the fundamental properties focusing on polarons in perovskite, since a comprehensive understanding of them would significantly facilitate the enhancement of the electronic and optoelectronic characteristics, and eventually contribute to the release of their full potential in real-world applications

Intrinsic carrier mobility limits in the transparent bipolar semiconductor CuInO2

The delafossite semiconductor CuInO2 has shown great potential in transparent electronics for its bipolar dopability. However, little is known about the limiting factors about its carrier mobility, which impedes its further development. Applying abinitio Boltzmann transport formalism, here we calculate the intrinsic, phonon-limited carrier mobility of CuInO2 and study its carrier–phonon coupling mechanisms. The calculated room-temperature electron and hole mobilities along the in-plane direction are μe=97.6 cm2V−1s−1 and μh=1.4 cm2V−1s−1, respectively. We find that the electron mobility is limited by the combination of acoustic phonons and polar longitudinal optical (LO) phonons, while the hole mobility is mainly limited by carrier–acoustic phonon scattering. We further show that the electron effective mass and bandgap of CuInO2 can be tuned through strain engineering for improved carrier transport properties. Our work uncovers the underlying factors that govern the intrinsic carrier mobility of the transparent bipolar semiconductor CuInO2 and sheds light on the design and exploration of bipolar conducting transparent conductive oxides (TCOs) based on delafossite semiconductors.

Optical properties of inverted type-I InP quantum dots with near-infrared emission

Unlike conventional type-I InP-based quantum dots (QDs), inverted type-I QDs can push the emission wavelength to the near-infrared (NIR) range. However, the fundamental optical properties of such kinds of QDs have yet to be studied in depth. In this work, we report the optical properties of inverted type-I ZnSe/InP/ZnS QDs with photoluminescence (PL) emission in the first biological window. Using femtosecond ultrafast transient absorption, the electron-injection time, Auger lifetime, and hot-carrier cooling time of the inverted type-I QDs are determined. Their temperature-dependent PL spectrum reveals that these QDs possess a higher exciton-binding energy and weaker carrier-phonon coupling effect than conventional type-I InP-based QDs. Interestingly, the QDs have strong two-photon absorption cross sections in the second biological window, up to 5120 GM at 1200 nm. These results suggest that ZnSe/InP/ZnS QDs have potential applications for light emitting devices and deep-tissue bioimaging.

Influence of neutron irradiation on X-ray diffraction, Raman spectrum and photoluminescence from pyrolytic and hot-pressed hexagonal boron nitride

Hexagonal boron nitride (hBN) is considered as an ideal semiconductor material for solid-state neutron detector, owing to its large neutron scattering section because of the low atomic number of B and excellent physical properties. Here we study the influence of neutron irradiation on crystal structure and on intermediate energy state (IMES) levels induced by the presence of impurities and defects in hBN. Large-size and thick pyrolytic and hot-pressed hBN (PBN and HBN) samples, which can be directly applied for neutron detector devices, are prepared and bombarded by neutrons with different irradiation fluences. The SEM and TEM are used to observe the sample difference of PBN and HBN. X-ray diffraction and Raman spectroscopy are applied to examine the influence of neutron irradiation on lattice structures along different crystal directions of PBN and HBN samples. Photoluminescence (PL) is employed to study the effect of neutron irradiation on IMESs in these samples. We find that the neutron irradiation does not alter the in-plane lattice structures of both PBN and HBN samples, but it can release the inter-layer tensions induced by sample growth of the PBN samples. Interestingly and surprisingly, the neutron irradiation does not affect the IMES levels responsible for PL generation, where PL is attributed mainly from phonon-assisted radiative electron-hole coupling for both PBN and HBN samples. Furthermore, the results indicate that the neutron irradiation can weaken the effective carrier-phonon coupling and exciton transitions in PBN and HBN samples. Overall, both PBN and HBN samples show some degree of the resistance to neutron irradiation in terms of these basic physical properties. The interesting and important findings from this work can help us to gain an in-depth understanding of the influence of neutron irradiation on basic physical properties of hBN materials. These effects can be taken into account when designing and applying the hBN materials for neutron detectors.

Ultrafast phonon diffuse scattering as a tool for observing chiral phonons in monolayer hexagonal lattices

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers has important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here, we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to lead to a strongly valley-polarized chiral phonon distribution when this relaxation mechanism is dominant. This momentum valley phonon polarization is directly measurable using ultrafast phonon diffuse scattering techniques. Using ab initio calculations, we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley depolarization in monolayer TMDs.

Strong Coupling of Self‐Trapped Excitons to Acoustic Phonons in Bismuth Perovskite Cs3Bi2I9

AbstractTo assess the potential optoelectronic applications of metal‐halide perovskites, it is critical to have a detailed understanding of the nature and dynamics of interactions between carriers and the polar lattices. Here, the electronic and structural dynamics of bismuth‐based perovskite Cs3Bi2I9 are revealed by transient reflectivity and ultrafast electron diffraction. A cross‐examination of these results combined with theoretical analyses allows the identification of the major carrier–phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into Cs3Bi2I9 form self‐trapped excitons on an ultrafast time scale. However, they retain most of their energy, and their coupling to Fröhlich‐type optical phonons is limited at early times. Instead, the long‐lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub‐ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye–Waller random atomic motions on longer times. The current results suggest the potential use of bismuth‐based perovskites for applications other than photovoltaics to take advantage of the carriers’ stronger self‐trapping and long lifetimes.

Phonons in the 1/f noise of topological insulators

In topological insulators, such as (Bi,Sb)2Te3 and BiSbTeSe1.6, the 1/f noise intensity features intriguing peaks, which develop at some specific temperatures. In search for their microscopic origin, we compared this noise structure with either phonon density of states or Raman spectrum of each topological insulator (TI), respectively. In (Bi,Sb)2Te3, the comparison revealed that the noise peaks track the van Hove singularities in the phonon density of states. The most intense noise peak observed in (Bi,Sb)2Te3 at 50 K is attributed to the thermal motion of the Bi atoms. Other less intense noise peaks are assigned to either a single phonon mode or multi-phonon combinations. We found that thermal vibrations of Bi and Te2 atoms in different symmetry directions are involved in most of the phonon combinations, which stand for the signature of the lattice anharmonicity in noise. The noise increase observed in (Bi,Sb)2Te3 and BiSbTeSe1.6 above a specific temperature threshold is attributed to the strengthening of the carrier–phonon coupling induced by anharmonicity. In the case of BiSbTeSe1.6, we show that all noise singularities are mirrored in the Raman spectrum of a structurally close TI (BiSbTeSe2) in the whole temperature range. This indicates that although transport can be at the surface or in the bulk or both of them, the carrier–phonon interaction is the only source of 1/f fluctuations in TIs. Inherently, these results imply that the microscopic origin of 1/f noise in solid is in the perpetual thermal motion of the atoms.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier-carrier, carrier-phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier-phonon and carrier-carrier interactions in LHP optoelectronic materials.

Estimation of Temperature-Dependent Band Parameters for Bi-Doped SnSe with High Thermoelectric Performance

Recent studies have revealed the outstanding thermoelectric performance of Bi-doped n-type SnSe. In this regard, we analyzed the band parameters for Sn1−xBixSe (x = 0.00, 0.02, 0.04, and 0.06) using simple equations and the Single Parabolic Band model. Bi doping suppresses the carrier-phonon coupling while increasing the density-of-states effective mass. The n-type SnSe is known to have two conduction bands converge near 600 K. Bi doping changes the temperature at which the band convergence occurs. When x = 0.04, its weighted mobility maximized near 500 K, which indicated the possible band convergence. The highest zT of the x = 0.04 sample at mid-temperatures (473–573 K) can be attributed to the engineered band convergence via Bi doping.

Evidence of Polaron Formation in Halide Perovskites via Carrier Effective Mass Measurements

A combination of transient spectroscopy techniques provides an estimate of the time-resolved effective carrier mass and carrier-phonon coupling constant in a photovoltaic absorber, here applied to lead-halide perovskites.

In-depth understanding the temperature-dependent reversible phase transition in CsPbI3−xBrx perovskites and its associated photophysical properties

Temperature-dependent phase transition of CsPbI3−x Brx perovskite is synchronously observed in a reciprocal space, and its photophysical properties are systematically studied by exploring the carrier–phonon coupling and excitons transport.

Spacer Switched Two-Dimensional Tin Bromide Perovskites Leading to Ambient-Stable Near-Unity Photoluminescence Quantum Yield.

Semiconductor nanostructures with near-unity photoluminescence quantum yields (PLQYs) are imperative for light-emitting diodes and display devices. A PLQY of 99.7 ± 0.3% has been obtained by stabilizing 91% Sn2+ in the Dion-Jacobson (8N8)SnBr4 (8N8-DJ) perovskite with 1,8-diaminooctane (8N8) spacer. The PLQY is favored by a longer spacer molecule and out-of-plane octahedral tilting. The PLQY shows one-month ambient stability under high relative humidity (RH) and temperature. With n-octylamine (8N) spacer, Ruddlesden-Popper (8N)2SnBr4 (8N-RP) also shows PLQY of 91.7 ± 0.6%, but it has poor ambient stability. The 5-300 K PL experiments decipher the self-trapped excitons (STEs) where the self-trapping depth is 25.6 ± 0.4 meV below the conduction band because of strong carrier-phonon coupling. The microsecond long-lived STE dominates over the band edge (BE) peaks at lower excitation wavelengths and higher temperatures. The higher PLQY and stability of 8N8-DJ are due to the stronger interaction between SnBr64- octahedra and 8N8 spacer, leading to a rigid structure.

Boosting the Self-Trapped Exciton Emission in Cs2NaYCl6 Double Perovskite Single Crystals and Nanocrystals.

Halide double perovskites have aroused substantial research interest because of their unique optical properties and intriguing flexibility for various compositional adjustments. Herein, we report the synthesis and photophysics of rare-earth element yttrium (Y)-based double perovskite single crystals (SCs) and nanocrystals (NCs). The pristine Cs2NaYCl6 bulk SCs exhibit a weak sky-blue emission with a low photoluminescence quantum yield (PLQY) of 7.68% based on the self-trapped exciton (STE), while no PL emission was observed for NCs. Excitingly, the STE emission of SCs and NCs is greatly enhanced via Sb3+ doping. The optimized Cs2NaYCl6:Sb3+ SCs and NCs exhibit high PLQYs up to 82.5% and 51.8%, respectively. Theoretical calculations and charge-carrier dynamic studies demonstrate that the giant emission enhancement after Sb3+ doping is related with the enhancement of the sensitization of the emissive STE states, the passivating of the nonradiative carrier trapping processes, and the regulation of carrier-phonon coupling.

Comparison of Coulomb Impurity, Longitudinal Acoustic Phonons, and Surface Optical Phonons Affecting the n = 0 Landau Level in Monolayer Graphene

The influences of a charged Coulombic impurity with screened effect and carrier-phonon interaction on the n = 0 Landau level in monolayer graphene with a polar substrate under a high static magnetic field are discussed to compare the competition among the impurities, the longitudinal acoustic phonons in the graphene plane and the surface optical phonons on the substrate. A method of linear combination operators is used to deal with the position and momentum of a carrier in a magnetic field. The method of Lee-Low-Pines variation with an arbitrary carrier-phonon coupling is adopted to derive the effects of phonons. It is found that the energy gap of n = 0 Landau level opened by carrier-longitudinal acoustic phonons cannot be the main mechanism, whereas both the carrier-surface optical phonon interaction and the carrier-impurity interaction play the main roles in determining the energy splitting.

Time-resolved terahertz spectroscopy for probing the effects of low-temperature annealing on CsPbBr3 evaporated thin-films

The fine-tuning of the growth conditions and post-deposition treatments is of fundamental importance to improve the efficiency of photovoltaic devices based on all-inorganic metal halide perovskites like CsPbBr3. In this work, we used time-resolved terahertz spectroscopy (TRTS) in combination with optical characterization techniques, x-ray diffraction (XRD), and scanning electron microscopy, to probe the different properties induced by a low-temperature (180 °C) annealing treatment on evaporated CsPbBr3 thin-films. We observed a faster build-up and relaxation dynamics in the annealed sample, accompanied by a remarkable decrease of the photoluminescence intensity and minor changes in the photoconductivity and XRD measurements as compared to the as-deposited sample. We estimated for both the samples a mobility of cm2 V−1 s−1. Our results suggest that the lattice reorganization induced by low-temperature annealing of evaporated CsPbBr3 could lead to a different charge carrier-phonon coupling and to an increased contribution of non-radiative recombination channels. We found that TRTS can be effectively used to follow the changes induced by post-deposition thermal annealing of CsPbBr3.

Elucidating the Unique Hot Carrier Cooling in Two-Dimensional Inorganic Halide Perovskites: The Role of Out-of-Plane Carrier–Phonon Coupling

Two-dimensional (2D) halide perovskites represent the natural semiconductor quantum wells (QWs), which hold great promise for optoelectronics. However, due to the hybrid structure of Ruddlesden-Popper 2D perovskites, the intrinsic nature of hot-carrier kinetics remains shielded within. Herein, we adopt CsPbBr3 nanoplates as a model system to reveal the intrinsic carrier dynamics in inorganic perovskite QWs. Interestingly, we revealed an ultrafast and hot-phonon-bottleneck (HPB)-free carrier cooling in monodisperse CsPbBr3 QWs, which is in sharp contrast to the bulk and nanocrystalline perovskites. The absence of HPB was attributed to the efficient out-of-plane triplet-exciton-LO-phonon coupling in 2D perovskites because of the structural anisotropy. Accordingly, the HPB can be activated by shutting down the out-of-plane energy loss route through forming the layer-stacked perovskite superlattice. The controllable on and off of HPB may provide new possibilities in optoelectronic devices and these findings deepen the understanding of a hot-carrier cooling mechanism in 2D perovskites.

Revealing the Interaction of Charge Carrier-Phonon Coupling by Quantification of Electronic Properties at the SrTiO3/TiO2 Heterointerface.

Oxide heterointerfaces with high carrier density can interact strongly with the lattice phonons, generating considerable plasmon-phonon coupling and thereby perturbing the fascinating optical and electronic properties, such as two-dimensional electron gas, ferromagnetism, and superconductivity. Here we use infrared-spectroscopic nanoimaging based on scattering-type scanning near-field optical microscopy (s-SNOM) to quantify the interaction of electron-phonon coupling and the spatial distribution of local charge carriers at the SrTiO3/TiO2 interface. We found an increased high-frequency dielectric constant (ε∞ = 7.1-9.0) and charge carrier density (n = 6.5 × 1019 to 1.5 × 1020 cm-3) near the heterointerface. Moreover, quantitative information between the charge carrier density and extension thickness across the heterointerface has been extracted by monochromatic near-field imaging. A direct evaluation of the relationship between the thickness and the interaction of charge carrier-phonon coupling of the heterointerface would provide valuable information for the development of oxide-based electronic devices.

Highly efficient emission in lead-free inorganic vacancy-ordered Sb-Bi-alloyed halide quadruple perovskites

Lead-free vacancy-ordered quadruple perovskites have received extensive attention in optoelectronic applications. We study the optical properties and charge-carrier dynamics of Cs4Mn(Bi1-xSbx)2Cl12 (0 ≤ x ≤ 1) quadruple perovskite single crystals (SCs), which exhibit excellent stability and Mn2+-based photoluminescence (PL) emission. The ratio of Bi:Sb has a great influence on PL intensity. As the Bi3+ content increases, the PL quantum yield (PLQY) shows a monotonically increasing trend and enhances by more than 40 times, attributed to larger energy difference between conduction band minimum and 4T1 state of Mn2+ ions, weaker Mn-Mn coupling interaction due to longer Mn···Mn distance and smaller Jahn-Teller distortion, and lower carrier-phonon coupling. Moreover, Cs4Mn(Bi1-xSbx)2Cl12 nanocrystals were synthesized through top-down ultrasonic crushing method. The PL quenching of Cs4MnBi2Cl12 nanocrystals compared to corresponding SCs arises from surface defect trapping. Our work provides an in-depth understanding of Mn2+-based PL mechanism.