Longitudinal Optical Phonon Research Articles

Revealing the Polaron State at the MoS2/TiO2 Interface

Interfacial polarons determine the distribution of free charges at the interface and thus play important roles in manipulating the physicochemical properties of hybridized polaronic materials. In this work, we investigated the electronic structures at the atomically flat interface of the single-layer MoS2 (SL-MoS2) on the rutile TiO2 surface using high-resolution angle-resolved photoemission spectroscopy. Our experiments directly visualized both the valence band maximum and the conduction band minimum (CBM) of SL-MoS2 at the K point, which clearly defines a direct bandgap of ∼2.0 eV. Detailed analyses corroborated by density functional theory calculations demonstrated that the CBM of MoS2 is formed by the trapped electrons at the MoS2/TiO2 interface that couple with the longitudinal optical phonons in the TiO2 substrate through an interfacial Fröhlich polaron state. Such an interfacial coupling effect may register a new route for tuning the free charges in the hybridized systems of two-dimensional materials and functional metal oxides.

Discrete Donor–Acceptor Pair Transitions in CH3NH3PbI3 Perovskite Single Crystals

Achieving a better understanding of the physics of defects in halide perovskites (HPs) is a key challenge for improving the efficiency of the devices. Herein, a comprehensive study of the defect emission of CH3NH3PbI3 perovskite single crystals is presented. The emission of the pristine surface of cleaved crystals is systematically investigated based on steady‐state and time‐resolved micro‐photoluminescence (micro‐PL) spectroscopy. Donor–acceptor pair (DAP) recombination is observed due to the presence of native shallow defects. The DAP spectra present an important variability depending on the location on the surface of the crystals due to inhomogeneous defect distribution. A strong blueshift of the emission is measured as a function of excitation power and is explained by fluctuating potential caused by compensated defects. With increasing photocarrier density, a transition from a structureless to a structured DAP emission with several longitudinal optical phonon replicas is observed. The DAP transition is characterized by the redshift of the emission with time and a slow, non‐exponential PL decay. Sharp discrete lines with sub‐meV widths, the most recognizable spectral signature of DAP transition, are revealed. Herein, the apparently contradictory previous observations on defect emission of CH3NH3PbI3 are reconciled by the results and new insights are provided into the properties of defects in HPs.

Influence of four confinement phonon models on the electron self-energy in a finite-square potential quantum-well

In this article, we have used the variational method and four different confinement longitudinal optical (LO) phonon models to calculate the self-energy of the electron–confined LO-phonon interaction in a finite square well potential as a function of the width of the well, and the main results are compared with each other. This potential of the quantum well is well suited for the above purpose as it is more realistic than the infinite square potential quantum well (QW). Based on the calculated results, we show good agreement between the confined optical phonon models in two-dimensional structures. The results obtained show that the Huang-Zhu, the Fuchs-Kliewer, and the Knipp-Reinecke models are suitable for studies of the electron–LO-phonon (e–LO-p) interaction in QWs in which the use of the Fuchs-Kliewer model is the easiest because its theoretical model is very simple but effective. The result also shows that the Ridley model is not suitable for theoretical studies of the e–LO-p interaction in QWs because the electromagnetic boundary condition is not satisfied by this model.

Improved Optical Properties of Lead‐Free Double Perovskite Cs2AgBiBr6 Nanocrystals via Na Ions Doping

AbstractLead‐free double perovskite nanocrystals (NCs) are a promising alternative to lead‐based perovskite NCs for optoelectronic materials due to their nontoxic and good moist‐thermal stability. However, most of the lead‐free double perovskite NCs exhibit unsatisfying photoluminescence quantum yield (PLQY); the metal ions doping is an effective method to improve the optical properties. Here, monodisperse, homogeneous cubic morphology of Na+‐doped Cs2AgBiBr6 NCs is prepared via a colloidal synthesis strategy. When Na+ is doped into Cs2AgBiBr6 NCs, the inversion‐symmetry‐induced parity‐forbidden transition is broken. So, the wave function parity of the self‐trapped excitons at Ag atoms is changed, which allows the radiative transition to occur. This leads to a higher PLQY for Na+‐doped Cs2AgBiBr6 NCs (9.08%) than that for Cs2AgBiBr6 NCs (0.40%), and the exciton lifetime is extended from 1.87 ns for Cs2AgBiBr6 NCs to 17.15 ns for Na+‐doped Cs2AgBiBr6 NCs. The Na+ doping elevates the longitudinal optical phonon energy and exciton binding energy, which results in weaker nonradiative transitions during exciton recombination and improved NC thermal stability. So, the Na+‐doped Cs2AgBiBr6 NCs maybe possess potentially valuable applications in optoelectronic devices.

Giant hyperferroelectricity in LiZnSb and its origin

Hyperferroelectricity (HyFE) is an intriguing phenomenon in which spontaneous polarization persists under an open-circuit boundary condition (OCBC) despite the existence of a depolarization field. However, small polarization and shallow well depth have been found so far in most hyperferroelectric materials, which severely limits their applications. Using first-principles density functional theory, we report the discovery of a giant hyperferroelectricity in LiZbSb. The HyFE spontaneous polarization is shown to be remarkably large with $\mathrm{P}=0.282\phantom{\rule{0.16em}{0ex}}\mathrm{C}/{\mathrm{m}}^{2}$ under OCBC, which is one order of magnitude greater than the HyFE polarization in ${\mathrm{LiNbO}}_{3}$. Furthermore, HyFE in LiZnSb is found to be exceptionally stable with a well depth of electric free energy $\mathrm{\ensuremath{\Delta}}F=\ensuremath{-}332\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which makes LiZnSb a possible hyperferroelectric solid at room temperature. The origin of the giant hyperferroelectricity in LiZnSb is identified and is attributed to the large mode effective charge of the soft longitudinal-optic phonon and the large high-frequency dielectric constant.

Evidence for Topological Magnon-Phonon Hybridization in a 2D Antiferromagnet down to the Monolayer Limit.

Topological phonons and magnons potentially enable low-loss, quantum coherent, and chiral transport of information and energy at the atomic scale. Van der Waals magnetic materials are promising to realize such states due to their recently discovered strong interactions among the electronic, spin, and lattice degrees of freedom. Here, we report the first observation of coherent hybridization of magnons and phonons in monolayer antiferromagnet FePSe3 by cavity-enhanced magneto-Raman spectroscopy. The robust magnon-phonon cooperativity in the 2D limit occurs even in zero magnetic field, which enables nontrivial band inversion between longitudinal and transverse optical phonons caused by the strong coupling with magnons. The spin and lattice symmetry theoretically guarantee magnetic-field-controlled topological phase transition, verified by nonzero Chern numbers calculated from the coupled spin-lattice model. The 2D topological magnon-phonon hybridization potentially offers a new route toward quantum phononics and magnonics with an ultrasmall footprint.

Enhanced InAs phase formation in the In+- and As+-implanted SiO2 films covered with Si3N4 layers

The Raman spectroscopy and room-temperature photoluminescence were used to study the properties of the In+ and As+ ion-implanted SiO2 films as a function of annealing temperature Ta at 700–1100 °C. No features of the InAs phase were obtained in the Raman spectra of the native SiO2 films irrespective of Ta. In the ion-implanted SiO2 films encapsulated with Si3N4, three Raman peaks at 202 cm−1, 256 cm−1 and 232 cm−1 corresponding to transverse and longitudinal optical phonons in black As and to longitudinal optical phonons in InAs, respectively, were detected. The SiO2 encapsulation effect on the enhanced InAs phase formation is discussed. A photoluminescence of around 556 nm was observed from the encapsulated ion-implanted SiO2 films and reached its intensity maximum after the annealing at 1000 °C.

Raman study of Cd1−xZnxTe phonons and phonon–polaritons—Experiment andab initiocalculations

Backward/near-forward Raman scattering and ab initio Raman/phonon calculations are combined, together with x-ray diffraction and ellipsometry measurements to further inform the debate on the compact phonon behavior of the II–VI Cd1−xZnxTe alloy. The compacity favors the coupling of polar optic modes in both the transverse and longitudinal symmetries via the related (EL,T) long-wave electric fields. The EL-coupling achieves maximum in the Zn-dilute limit, which enhances the (upper) ZnTe-like (impurity) mode at the expense of the (lower) CdTe-like (matrix-like) one, leaving the impression of a unique {Cd-Te,Zn−Te}-mixed longitudinal optic (LO) phonon across most of the composition domain. However, the purely mechanical (non-polar) transverse optic (PM-TO) phonons, that hardly couple, reveal an underlying three-mode {1 × (Cd-Te),2 × (Zn-Te)} fine structure that distinguishes between Zn–Te vibrations in Zn- and Cd-like environments up to second neighbors. Further refinement arises by exploring the phonon–polariton (i.e., polar-TO) regime at large Zn content. On reducing the scattering angle, the ET-coupling develops into a sequential softening of phonon–polaritons from ZnTe- down to CdTe-like ones, which transiently unveils a bimodal pattern behind the Cd–Te signal. Altogether, this results in a (rare) canonical four-mode {2 × (Cd-Te),2 × (Zn-Te)} percolation pattern for Cd1−xZnxTe, i.e., a close II–VI replica of the twin III−V In1−xGaxAs one—yet differing by two apparent LO modes and a sensitivity of bond vibrations limited to first-neighbors. Retrospectively, the difference in sensitivity of bond vibrations to the local environment between In1−xGaxAs (limited to first neighbors) and Cd1−xZnxTe (extending up to second neighbors) emerges as a rule throughout common (covalent) III–V and (ionic) II–VI semiconductor alloys.

Passivation of oxygen vacancy defects in conductive ZnO nanoparticles via low-temperature annealing in NF3

Oxygen vacancies (VO) often exist in wide-bandgap metal oxide semiconductors (MOSs) as deep-level defects and undermine the reliability of various optoelectronic devices based on MOSs. Conventional methods to passivate VO defects, such as high-temperature annealing or plasma treatment, can compromise device performance. This work demonstrates that passivation of VO defects in aluminum-doped ZnO (AZO) nanoparticles can be realized through low-temperature annealing (350 °C) in an NF3 atmosphere. After the NF3 annealing, the longitudinal optical phonon scattering mode in Raman spectroscopy, the visible light emission intensity in photoluminescence spectroscopy, and the oxygen deficiency-related peak in x-ray photoelectron spectroscopy decrease simultaneously, indicating the passivation of VO defects in AZO nanoparticles. As a result, AZO nanoparticles show higher visible light reflectance and better stability of electrical conductivity owing to the suppression of deep-level light absorption and gas molecule adsorption. This work also offers insights into the passivation mechanism of VO defects in MOSs.

Polar Optical-Phonon Dominated Electrical Transport in Ti2CO2 MXene

Two-dimensional Ti2CO2 MXene is promising for applications in electronics and optoelectronics, where high intrinsic mobility is essential to achieving high performance. Therefore, accurate prediction of carrier mobility is important for these types of materials. Here, we show that full electron–phonon coupling (EPC) calculations can accurately predict the carrier mobility for polar materials like Ti2CO2 MXene. Based on full EPC calculations and mode-by-mode analyses of the phonon-limited carrier transport in Ti2CO2 MXene, we demonstrate that the EPC matrix of optical phonons is significantly higher than that of the acoustic modes, and the carrier scattering process is dominated by the longitudinal optical phonon (Fröhlich interaction). Consequently, the calculated carrier mobility of Ti2CO2 at 300 K is 319.64 cm2/Vs for the hole and 16.69 cm2/Vs for the electron at a carrier concentration of nc = 1 × 1012 cm–2, which are over one order of magnitude lower than that predicted by the deformation potential theory method. The present work demonstrates the importance and necessity of considering the full EPC to accurately predict the carrier mobility of MXenes and other polar materials.

Polaron states of the full-configuration defects in metal halide perovskites

The systematical analysis for varieties of defects with different depths and lattice relaxation strengths in metal halide perovskites (MHPs) is a challenging task. Here, we study the energy shifts of the full-configuration defects due to the polaron effect based on the all-coupling variational method in MHPs, where these polaron states are formed stemming from different defect species coupling with the longitudinal optical phonon modes via Frhlich mechanism. We find that the polaron effect results in defect levels varying from tens to several hundreds of meV, which are very close to the correction of defect levels due to the defect-polaron effect, especially for these defects migration proved in the recent experiments in MHPs. These results provide the significant enlightenment not only for analyzing the radiation and non-radiation processes of carriers mediated by defects, but also for optimizing defect effect in the photovoltaic and photoelectric devices based on MHPs materials.

First-principles calculations of improving carrier mobility for [formula omitted]CsPbI3

In the past few years, inorganic perovskite CsPbI3 has attracted great interest due to high conversion efficiency in solar cells. Carrier mobility, as an important physical parameter in optoelectronic devices, directly affected power consumption. In this study, the phonon-limited mobilities of β-CsPbI3 were investigated using Boltzmann transport equation based on first-principles calculations. Using the fully relaxed lattice constants of a = 8.829/c = 6.505 Å, we obtained a mobility of μe,x=151/μe,z = 241 cm2V−1s−1 and μh,x=94/μh,z = 163 cm2V−1s−1. Longitudinal optical phonon associated with the relative vibration between Pb and I atoms was revealed to be the main scattering source, and the scattering mechanism was independent of temperature and carrier type. Biaxial compressive strain of in-plane (ab-plane) decreased mobility, which should be avoided in film growth. Since the exchange correlation functional with GGA-PBE form overestimated the lattice constants, the experimental values were used to correct the results. The band gap was mainly determined by the lattice constant of in-plane, and the mobility was extremely sensitive to the lattice constant in out-of-plane (c-axis). Using experimental lattice constants of a = 8.776/c = 6.214 Å, the mobility was improved to μe,x=503/μe,z = 1084 cm2V−1s−1 and μh,x=373/μh,z = 871 cm2V−1s−1.

Two-photon absorption of FAPbBr3 perovskite nanocrystals

Perovskite nanocrystals (NCs) with high two-photon absorption (TPA) cross-section are of great interest due to their potential applications in three-dimensional optical data storage and multiphoton fluorescence microscopy. Among various perovskite materials, FAPbBr3 NCs show a better development prospect due to their excellent stability. However, there are few reports on their nonlinear optical properties. In this work, the nonlinear optical behavior of FAPbBr3 NCs is studied. The methods of multiphoton absorption photoluminescence saturation and open aperture Z-scan technique were applied to determine the TPA cross-section of FAPbBr3 NCs, which was around 2.76 × 10−45 cm4⋅s⋅photon−1 at 800 nm. In addition, temperature-dependent photoluminescence induced by TPA was investigated, and the small longitudinal optical phonon energy and electron–phonon coupling strength was obtained, which confirm the weak Pb–Br interaction. Meanwhile, it is found that the exciton binding energy in FAPbBr3 NCs was 69.668 meV, which may be ascribed to the strong hydrogen bond interaction. It is expected that our findings will promote the application of FAPbBr3 NCs in optoelectronic devices.

Strain evolution and confinement effect in InAs/AlAs short-period superlattices studied by Raman spectroscopy

Raman spectra of two series of InAs/AlAs short-period superlattices were measured at room temperature to investigate the impact of strain on the phonon modes taking into consideration the confinement effect and interface mode. The evolution of strain in the InAs layer and the AlAs layer was studied in (InAs)2/(AlAs)2 superlattices grown at various temperatures (400–550 °C). While the strain existed in the AlAs layer remained almost constant, the strain in the InAs layer varied significantly as the growth temperature increased from 500 to 550 °C. The confinement effect on the optical phonons was analyzed based on results from (InAs)n/(AlAs)n grown at 450 °C (n = 2, 3, 4, and 5). Additionally, the confinement effect was found to be stronger in shorter periods with higher interface quality. The interface phonon modes were resolved between the longitudinal optical and transverse optical phonon modes, which assist in the rough estimation of the thickness of the layers. The disorder-activated acoustic phonon modes at the low-frequency side were also addressed.

Heteroepitaxial chemistry of zinc chalcogenides on InP nanocrystals for defect-free interfaces with atomic uniformity

Heteroepitaxy on colloidal semiconductor nanocrystals is an essential strategy for manipulating their optoelectronic functionalities. However, their practical synthesis typically leads to scattered and unexpected outcomes due to the intervention of multiple reaction pathways associated with complicated side products of reactants. Here, the heteroepitaxy mechanism of zinc chalcogenide initiated on indium phosphide (InP) colloidal nanocrystals is elucidated using the precursors, zinc carboxylate and trialkylphosphine selenide. The high magnetic receptivity of 77Se and the characteristic longitudinal optical phonon mode of ZnSe allowed for monitoring the sequence of epilayer formation at the molecular level. The investigation revealed the sterically hindered acyloxytrialkylphosphonium and diacyloxytrialkylphosphorane to be main intermediates in the surface reaction, which retards the metal ion adsorption by a large steric hindrance. The transformation of adsorbates to the crystalline epilayer was disturbed by surface oxides. Raman scattering disclosed the pathway of secondary surface oxidation triggered by carboxylate ligands migrated from zinc carboxylate. The surface-initiated heteroepitaxy protocol is proposed to fabricate core/shell heterostructured nanocrystals with atomic-scale uniformity of epilayers. Despite the large lattice mismatch of ZnS to InP, we realised a uniform and interface defect-free ZnS epilayer (~0.3 nm thickness) on InP nanocrystals, as evidenced by a high photoluminescence quantum yield of 97.3%.

Electron energy relaxation in SiGe quantum wells

In this article, we theoretically calculated the hot electron energy relaxation induced by longitudinal acoustic (LA) phonons, longitudinal optical (LO) phonons and importantly two-transverse acoustic (2TA) phonons in two-dimensional SiGe quantum wells, by incorporating dynamic screening effect in LA phonon scattering and hot-phonon effect in LO phonon scattering mechanisms. At the low-temperature regime, the ELR is found to be dominated by LA phonons and at higher temperature ELR is dominated by LO phonons. In the intermediate temperature range, 2TA phonon scattering mechanism is very important and we incorporated in our calculations. The unscreened longitudinal acoustic (LA) phonon due to deformation potential (DP) coupling is dominant over the screened acoustic piezoelectric phonon contribution. At higher temperatures, there is a crossover from ELR due to LA phonons to ELR due to longitudinal optical (LO) phonons with the cross-over temperature being about Te∼40K. The LA phonon screening effect and hot phonon effect is demonstrated to reduce ELR significantly. In the intermediate temperature range (30 K

Controlled synthesis of monodispersed ZnSe microspheres for enhanced photo-catalytic application and its corroboration using density functional theory.

The present work reports enhanced photocatalytic performance of highly crystalline, monodisperse ZnSe microspheres, synthesized by the size-selective, ETDA-assisted hydrothermal method. Systematic studies on time-dependent reaction kinetics and growth parameters indicate dependency of morphology and crystal structure on the volume % of EDTA and dependency of size on the volume % of hydrazine hydrate for ZnSe microspheres. X-ray diffraction studies confirm highly crystalline cubic zinc blende crystal structure with crystallite size in the range of 10-15 nm. Diffuse reflectance spectra show blue shift having a broad absorption peak between 415 and 425 nm, with a band gap of ∼2.6 eV from the K-M plot. Photoluminescence spectra show higher ratio of near band edge emission to deep level emission, confirming the decrease in defect related emission and depicting the higher crystallinity of ZnSe. Raman spectroscopy also confirms the crystalline and pure nature of ZnSe microspheres, from the observation of a high intensity dominant peak at 248 cm-1, attributed to longitudinal optical phonon scattering. Morphological analysis using FE-SEM and HRTEM shows monodispersed microspheres having size ∼2.5 μm, made up of small ZnSe nanocrystals with ∼10 nm size and with an interplanar spacing of ∼0.32 nm, corresponding to zinc blende ZnSe(111) planes. Brunauer-Emmett-Teller analysis indicates type-IV adsorption-isotherms and hysteresis loops, confirming the presence of mesopores on the surface of ZnSe microspheres controlling the diffusion rate of the catalyst. The degradation rate constant for methylene-blue using first-order reaction kinetics confirms improvement in photocatalytic activity by a factor of 7 to 13 times higher than that of bulk ZnSe, which is attributed to the controlled, well-defined morphology of spherical microstructures made up of small-sized ZnSe nanocrystals. Density functional theory based calculations support the preferential adsorption of EDTA at the Se site of the ZnSe(111) surface with an energy of -1.90 eV. The electronic-structure plot demonstrates semiconducting behaviour with a direct band gap of ∼1.51 eV. First-principles calculations confirm enhancement in the photocatalytic water splitting activity of the ZnSe(111) surface via adsorption of intermediates. The improvement in dye degradation can be attributed to the enhanced oxidation process through the formation of intermediates such as O* (-3.13 eV) and HO* (-2.57 eV) at the Se site of the ZnSe surface.

Моделирование поверхностно-объемного заряжения диэлектрика электронами с энергией от 6 до 30 keV

The physico-mathematical model applied to ground tests for geomagnetic plasma exposure based on the combined consideration of surface and bulk processes of transport and charge accumulation for calculating the kinetics of charging high-resistance dielectrics irradiated by medium-energy monoenergetic electrons (from 6 to 30 keV) is proposed. The model takes into account the contribution to the charging of the dielectric by the trains of longitudinal optical phonons generated by each thermalizing primary electron with energy below the band gap of the dielectric, which supplements the induced conduction current due to the generation of electron-hole pairs. As a result, the current induced by trains of longitudinal optical phonons of the tunnel conductivity through free electron traps is introdused, as well as the current induced in the conduction band due to multiphonon ionization the electron traps by trains of longutudinal optical phonons in the region of existence of electric field. Using the example of the α-Al2O3 (sapphire) dielectric, the results of computer simulation of the internal currents distributions, charges, and electric field in the dielectric with the open surface irradiated by monoenergetic electrons with energies from 6 to 30 keV are presented according with the achievement the quasi-equilibrium in the irradiated part of the dielectric and with switching of the energy of primary electrons during the irradiation process.

First-principles study of the bandgap renormalization and optical property of β-LiGaO2

The β-LiGaO2 with an orthorhombic wurtzite-derived structure is a candidate ultrawide direct-bandgap semiconductor. In this work, using the non-adiabatic Allen–Heine–Cardona approach, we investigate the bandgap renormalization arising from electron–phonon coupling. We find a sizable zero-point motion correction of –0.362 eV to the gap at Γ, which is dominated by the contributions of long-wavelength longitudinal optical phonons. The bandgap of β-LiGaO2 decreases monotonically with increasing temperature. We investigate the optical spectra by comparing the model Bethe–Salpether equation method with the independent-particle approximation. The calculated optical spectra including electron–hole interactions exhibit strong excitonic effects, in qualitative agreement with the experiment. The contributing interband transitions and the binding energy for the excitonic states are analyzed.

A remarkable third-order nonlinear optical behavior of single-crystal bromide organic-inorganic lead halide perovskite

The optical and structural aspects of methylamine lead bromide (MAPbBr3) single crystal (SC) are investigated. Based on Kramers–Kronig (KK) equations, the optical and dielectric parameters are investigated. The optical phonons Longitudinal optical (LO = 678 cm−1) and Transversal optical (TO = 578 cm−1) phonon modes are associated with the optical interaction with lattices in the visible region for MAPbBr3 SC. The Z-scan method is used to investigate the third-order nonlinear coefficients of SC hybrid organic–inorganic lead halide perovskites, MAPbBr3 with the 2.19 eV optical band gap energy. According to the nonlinear optical (NLO) properties, MAPbBr3 has a high third-order NLO response. The two-photon absorption (2PA) coefficient is found to be 3 × 10-4 cm W−1 using the transmission open aperture Z-scan technique. The nonlinear refraction (NLR) is obtained in the order of 10-9 cm2 W−1 utilizing a close aperture Z-scan, which revealed self-defocusing phenomena. This research shows that MAPbBr3 is a practical and low-priced nonlinear material for ultrafast photonics applications.