Longitudinal Optical Phonon Research Articles

Solid Solution Behavior of High Performance BiOBrxI1-X Photoanodes Probed Using Solid-State NMR, XRD and Optical Spectroscopy

Bismuth oxyhalides (BIOX) are a family of solution-synthesizable 2D layered semiconductors constituted of earth-abundant elements namely bismuth, oxygen and the halogens. BIOX based thin films and heterojunctions are the subject of immense scientific interest due to their impressive performance in photocatalytic and photoelectrochemical devices. Solid solutions of Br/I bismuth oxyhalides exhibit continuously tunable absorption and emission spectra similar to that observed in mixed halide MAPbClxBryI3-x-y perovskites and mixed anion III-V GaAsxP1-x semiconductors. The excited state decay showed two components for all the mixed halide BiOX semiconductors with a dominant short-lived component in the nanosecond range and a weaker longer-lived component of ~100 ns. The average photoluminescence (PL) lifetime varied from 5.4 ns in bare BiOI to 1.49 ns in bare BiOBr with intermediate compositions exhibiting monotonically increasing average PL lifetimes with increasing iodine content. Using a combination of characterization techniques such as solid-state nuclear magnetic resonance (ssNMR), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and optical spectroscopy, we demonstrate the formation of atomic level solid solutions in Br/I bismuth oxyhalides. Through refinement of the lattice parameters using powder XRD and complementary atomic-level 209Bi SSNMR spectroscopy, we found that Vegard's law was obeyed by the BIOBrxI1-x solid solutions. By varying the Br:I ratio, it is also possible to tune the electronic band gap as well as the energy levels corresponding to the conduction band minimum (ECB, min) and the valence band maximum (EVB, max), thus allowing control over the thermodynamic driving force available to drive chemical reactions using photogenerated or electrically injected charge carriers. Suppression of the longitudinal optical phonon in solid solutions of Br/I bismuth oxyhalides is suggestive of lower phonon scattering of charge carriers and improved electronic transport. A synergistic enhancement of more than 1 order of magnitude in the photoelectrochemical water-splitting performance was obtained for the optimized solid solution (BiOBr0.67I0.33) under AM 1.5G 1 sun illumination. Our results pave the way for a one pot synthesis protocol to form BiOX thin films, powders and colloidal suspensions with deterministic control over the stoichiometry.

Dual role of longitudinal optical phonons for generation of coherent oscillations in gallium arsenide under optical pumping

Dual role of longitudinal optical phonons for generation of coherent oscillations in gallium arsenide under optical pumping

Laser-induced spectral tuning of single quantum dots embedded into microposts cladded with HfO2

Our work investigates the precise tuning of InGaAs quantum dots (QDs) embedded into microposts by leveraging HfO2 crystallization-induced micro-strain via laser annealing. We investigate the efficacy of laser annealing power as a parameter for spectral control, achieving a notable blue shift of QD emissions of up to 5 meV. Through comprehensive Raman thermometry, we reveal consistent dependencies in laser-induced heating relative to micropost diameter, with larger microposts exhibiting superior heat dissipation capabilities and smaller tuning range. For instance, a 5.0 μm micropost demonstrates a maximum local temperature increase of 260 K at 1.82 mW of annealing power, compared to 435 K for a 1.1 μm diameter micropost under the same conditions. By correlating local temperatures derived from the longitudinal optical phonon linewidth of the Raman spectra, with QD emission line blue shift at specific laser powers, the tunability across differing post diameters is studied. Our findings underscore the potential of strain-tuning QDs through laser-induced HfO2 crystallization, offering avenues for scalable resonant single-photon sources applicable in superradiance and multi-photon interference scenarios.

Investigating the optical and magnetic properties of engineered multifunctional graphene oxide and potential application in medical physics and nonlinear optics‏

Herein, the synthesis of multifunctional engineered GOs with the structure of GO@Fe3O4-ligands-CdTe quantum dots (QDs) by an environment-friendly aqueous method was reported. l-cystein (Cys), 3-trimethoxysilyl-propanethiol (TMSP), and glutathione (GSH) were selected as ligands. The as-prepared GO@Fe3O4-ligands-CdTe QDs hybrid nanostructures were characterized by different methods. The findings indicate that the nanocomposites of GO@Fe3O4–ligands–CdTe QDs demonstrate superparamagnetic properties at room temperature. The saturation magnetization values for GO@Fe3O4, GO@Fe3O4-Cys-CdTe, GO@Fe3O4-GSH-CdTe, and GO@Fe3O4-TMSP-CdTe, are 25, 14, 16, and 2 emu g−1, respectively. These nanocomposites show stable and strong luminescence at 580-600 nm. The magnetic nanostructures also reveal the absorption wavelength at 300-490 nm. Using TMSP ligand as a bridge between QDs and magnetic GOs, the luminescence of the nanostructure has not decreased much compared to CdTe QDs. The use of Cys ligand preserves the saturation magnetism of the nanocomposite more than magnetic GO, and the dispersed nanostructure is stable in water. Furthermore, the transverse and longitudinal optical (TO and LO) phonon modes of prepared GO@Fe3O4–ligands–CdTe QDs were obtained by Kramers-Kronig (KK) relations based on fourier-transform infrared (FTIR) spectroscopy. The Z-scan results show that the type of used ligand has a great effect on the nonlinear optical (NLO) properties of the final product. The largest NLO parameters are related to GO@Fe3O4- TMSP -CdTe QDs, and the values of nonlinear absorption coefficient and nonlinear refractive index were obtained as 2.17×10−3(cm/W) and10.27×10−8(cm2/W), respectively. As a result, GO@Fe3O4–ligands–CdTe QDs nanostructures can be considered as promising multifunctional nanostructures for various biomedical applications and NLO devices.

Photon–carrier–spin coupling in a one-dimensional Ni(II)-doped ZnTe nanostructure

Transition metal (TM) ion doping in II–VI semiconductors can produce exciton magnetic polarons (EMPs) and localized EMPs containing longitudinal optical (LO) phonon coupling, which will be discussed in this paper. TM ion doping in II–VI semiconductors for a dilute magnetic semiconductor show emission via magnetic polarons (MPs) together with hot carrier effects that need to be understood via its optical properties. The high excitation power that is responsible for hot carrier effects suppresses the charge trapping effect in low exciton binding energy (8.12 meV) semiconductors, even at room temperature (RT). The large polaron radius exhibits strong interaction between the carrier and MP, resulting in anharmonicity effects, in which the side-band energy overtone to LO phonons. The photon-like polaritons exhibit polarized spin interactions with LO phonons that show strong spin–phonon polaritons at RT. The temperature-dependent photoluminescence spectra of Ni-doped ZnTe show free excitons (FX) and FXs interacting with 2LO phonon–spin interactions, corresponding to 3T1(3F) → 1T1(1G) and EMP peaks with ferromagnetically coupled Ni ions at 3T1(3F) → 1E(1G). In addition, other d–d transitions of single Ni ions (600–900 nm) appear at the low-energy side. RT energy shifts of 14–38 meV are observed due to localized states with density-of-states tails extending far into the bandgap-related spin-induced localization at the valence band. These results show spin–spin magnetic coupling and spin–phonon interactions at RT that open up a more realistic new horizon of optically controlled dilute magnetic semiconductor applications.

Quantum features of low-energy photoluminescence of aluminum nitride films

Photoluminescence of aluminum nitride films at the below bandgap excitation has been studied. It has been found that low-energy (up to 2.02 eV) photoluminescence spectra of the AlN films contain a series of equidistant maxima, the intensities of which decrease with energy. Theoretical analysis has shown that the observed photoluminescence features may be caused by strong electron-phonon interaction (long-range interaction of electrons in the band gap with Al3+ ions in the lattice sites). This interaction presumably leads to appearance of quasi-particles in the band gap of AlN, which are a bound state of an electron with an ion in a crystal lattice site. Such quasi-particles have been called “elions”. The energy of an elion is quantized. An elion quantum is equal to the longitudinal optical phonon energy. The low-energy photoluminescence is based on the elion generation and subsequent annihilation mechanism.

Exploring Anisotropic Carrier Transport in WSe2-WTe2 Superlattices: A Computational Study.

Superlattice structures offer distinct benefits in modern semiconductor technology, enabling the development of a deeper understanding of their sublayer arising from the interfaces. However, the advancement of large-scale applications encounters additional concerns, such as the stability and performance of the superlattice. In this study, we employ density functional theory calculations combined with the Boltzmann transport theory to comprehensively analyze the electronic structural and transport properties of the hexagonal phase of WSe2 and the WSe2-WTe2 superlattice for their applications in carrier transport fields. Previous studies showed that longitudinal acoustics phonon limited carrier mobility determined by deformation potential theory (DPT) often compromises the accuracy and overestimates the relaxation time by 2 orders. Herein, we conduct an in-depth analysis of band structural and transport properties, addressing the aforementioned inconsistency by exclusively incorporating scattering from longitudinal optical phonons to accurately compute mobility using the Fröhlich interaction. Our findings reveal a significant enhancement in mobility for both electrons and holes in the WSe2-WTe2 superlattice, reaching 545 and 476 cm2 V-1 s-1, respectively, compared to 104 and 132 cm2 V-1 s-1 for WSe2, which suggests that this superlattice is a promising material for electronics and transport applications.

Spectroscopy of phononic switching of magnetization in the terahertz gap

All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.

Dependence temperature on effective mass and susceptibility in quantum well-wires

In this study, we investigated the simulation impact of carrier density and temperature on properties such as susceptibility and effective mass of electrons in GaAs/Al x Ga1−x As quantum-well wires, involving the treatment of Coulomb interaction and the interaction between electrons and holes with longitudinal optical phonons (LO-phonon) on an equal basis. The susceptibility and effective mass were calculated and compared with the results when no phonon coupling was considered. Furthermore, the key findings were compared with results from the sum-rule version of the Singwi-Tosi-Land-Sj ö lander (STLS) theory, mean-spherical approximation (MSA) theory, and there is a high level of concordance between this formula and experimental values obtained by other researchers. Our results reveal that carrier concentration and temperature significantly affect susceptibility and effective mass of the quantum well-wires in both cases, with LO-phonon contributions. These results contribute to illuminating how Coulomb interactions and exchange-correlation energies influence the susceptibility and effective mass of GaAs/Al x Ga1−x As across a broad range of temperature and carrier density conditions.

Polarization‐Graded High‐Electron‐Mobility Transistors for Improved Johnson's Figure of Merit

Polarization grading in AlGaN/GaN‐based high‐electron‐mobility transistors (HEMTs) is a promising device design option that can improve linearity, speed, power, and noise performance for use in millimeter‐wave applications. This work investigates the potential of compositionally graded HEMT heterostructures to enhance device breakdown through lateral electric field engineering while maintaining a high device cutoff frequency (fT) due to reduced longitudinal optical (LO) phonon scattering. The impact of polarization grading on electric field profile is compared with conventional gate‐integrated mini‐field plates (mini‐FPs). It is also observed that polarization grading can augment the efficacy of gate‐connected FPs, further enhancing performance. Using physics‐based 2D device simulations, it is demonstrated that polarization engineering via polarization grading enhances breakdown (VBD) while preserving high fT, resulting in a Johnson's figure of merit (JFOM = fT × VBD), that is, ≈2.4× that of a conventional abrupt‐junction HEMT. This improvement represents a significant advancement over the ≈1.25× to ≈2× increase achieved with the use of mini‐FPs alone in HEMTs.

Lifetime and Molecular Coupling in Surface Phonon Polariton Resonators.

Surface phonon polariton (SPhP) modes in polar semiconductors offer a low-loss platform for infrared nanophotonics and sensing. However, the efficient design of polariton-enhanced sensors requires a quantitative understanding of how to engineer the frequency and lifetime of SPhPs in nanophotonic structures. Here, we study organ-pipe resonances in 4H-SiC trenches as a prototype system for infrared sensing. We use a transmission line framework that accounts for the field distribution within the trench, accurately predicting mode frequency and lifetime when compared against finite element method (FEM) electromagnetic calculations. Accounting for the electric field profile across the gap is critical in our model to accurately predict mode frequencies, quality factor (Q factor), and reflectance, outperforming previous circuit models developed in the literature. Beyond structural simulation, our model can provide insights into the frequency ranges in the Reststrahlen band where enhanced sensor activity should be present. The radiative lifetime is significantly enlarged close to the longitudinal optic phonon, restricting sensor efficiency at this wavelength range. This pushes the optimal frequency for sensing closer to the center of the Reststrahlen band than might be naively expected. This model ultimately demonstrates the primary challenge of designing SPhP-based sensors: only a relatively narrow region of the Reststrahlen band offers efficient sensing, guiding future designs for infrared spectroscopy.

Structurally Flexible 2D Spacer for Suppressing the Electron–Phonon Coupling Induced Non-Radiative Decay in Perovskite Solar Cells

HighlightsThe soft 2D material reduces the coupling strength between carriers and longitudinal optical phonons, releasing the mechanical stress of lattice vibration.The power conversion efficiency of rigid devices and flexible devices reaches 25.5% and 23.4%, respectively.

On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids I: Klemens channel formalism and two-phonon DOS

The “Klemens channel” formalism is commonly used in interpretations of experimental results on the lifetime of longitudinal optic (LO) phonons and Fröhlich polarons (FPs) in polar bulk, polycrystalline, and nanostructured solids. Here, in Paper I of this bipartite article, the two-particle vibrational DOS function(s) for spatially confined longitudinal acoustic and LO phonons is implemented at evaluation on FP decay probability via the “Klemens channel” and its lifetime for the case of FPs confined within InAs quantum dots. This approach yields a convincing elucidation of the earlier experimental data. An alternative model based on the idea of diabatic thermally activated excitation of FP charge carriers and consecutive lattice “depolarization” will be discussed in detail and implemented at simulations on FP lifetime in Paper II [V. Ligatchev, “On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids II: Diabatic ionization and depolarization,” AIP Adv. 14, (2024)].

On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids II: Diabatic ionization and depolarization

In Paper I of this bipartite article, the two-particle vibrational DOS functions for spatially confined longitudinal acoustic and longitudinal optic phonons are implemented at evaluation on Fröhlich Polarons (FP) decay probability via the “Klemens channel” and lifetime for FP states confined within InAs quantum dots. Here, an alternative model based on the idea of diabatic thermally activated excitation of the FP charge carrier and consecutive lattice “depolarization” is used for evaluations on the spectral dependence of the phononic and polaronic lifetime(s). Key parameters of the “depolarization” phenomenon are evaluated quantitatively using the “entirely quantum” generalized “displaced harmonic oscillator” framework, which is closely related in spirit to the so-called “Feynman polaron” approximation.

Decisive role of preparation technique on the structural, electrical and magnetic properties of vanadium doped ZnO nanoparticles

1% vanadium doped ZnO powders have been successfully synthesized via two different preparation techniques viz., glycine assisted combustion method and mechanical attrition method to understand their impact on the structure-property relation. X-ray Diffraction study validates the Wurtzite phase formation of ZnO with hexagonal structure. X-ray diffraction peak discloses the marked role of size and strain component. Evidently, the shift of lattice strain with the preparation techniques from compressive to tensile nature show the importance of structure property relations in the ZnO compounds. SEM micrographs depict the formation of densely populated nanograins with networking like structures which is characteristics of combustion synthesized product whereas as top down approach exhibits the presence of coarse grains amidst the broken sea of fine particles. Selected area electron diffraction (SAED) images display ring and bright circular spot pattern with changes in method of preparation, which further clarifies the distinct nature of investigated ZnO:V particles. Vibrational spectral study gives clear evidence on the structural disorder as deduced from the changes associated with longitudinal optical mode phonon scattering. Interestingly, the UV spectral study divulges the increment in optical band gap as well as a strong visible light absorption (about 30 %) for milled ZnO:V over their chemically prepared counterparts. Photoluminescence spectra of milled V doped ZnO upheld the structural disorder taken place with the suppression of Near Band Edge (NBE) transition and the shifting of broad asymmetric defect emission band towards the lower wavelength regime. Impedance study enlightens the crucial role of synthesis method with 5 times enhanced grain boundary resistance along with suppression of grain contribution in the milled samples. Magnetic study highlights the remarkable distinction of preparation route which alters the ferromagnetic ordering in the combustion synthesized samples to conventional diamagnetic signature of the host ZnO in the milled samples.

Deep‐Ultraviolet Luminescence Properties of AlN

High‐resolution, low‐excitation photoluminescence (PL) spectroscopy is performed for unintentionally doped, silicon‐doped, and magnesium‐doped homoepitaxial aluminum nitride (AlN) films, using a wavelength‐tunable high‐repetition‐rate laser. The wavelength‐tunable laser is used to distinguish between the luminescence and scattering signals from AlN. Providing the high‐resolution, low‐excitation PL spectra, the current understanding of the deep‐ultraviolet luminescence properties of AlN is reviewed and potential assignments for the unknown luminescence lines and bands are discussed. Although previous studies have led to a consensus on the origins of some emission peaks and bands such as the neutral silicon donor‐bound exciton transition and free exciton transitions involving longitudinal optical phonons, it is shown that many of the emission peaks are still unidentified. The origins of all the emission peaks should be elucidated to enable control of the electronic and optoelectronic properties of AlN.

PLASMON ENERGY LOSSES OF ELECTRONS IN MULTILAYER DIELECTRIC STRUCTURES

The possibilities of engineering energy losses of electrons created in the cascade process of electron-electron scattering during the interaction of multilayer dielectric structures with ionizing radiation are considered. It is shown that the contribution of surface plasmons associated with layer boundaries to electron energy losses is significant only for nanometer layer thicknesses and increases with increasing electron energy. At the same time, surface states associated with longitudinal optical phonons in ionic crystals significantly change energy losses during electron thermalization and can lead to an increase in the efficiency and growth rate of scintillation in nanostructured systems.

Multifunctional Trifluoroborate Additive for Simultaneous Carrier Dynamics Governance and Defects Passivation to Boost Efficiency and Stability of Inverted Perovskite Solar Cells

AbstractThe main obstacles to promoting the commercialization of perovskite solar cells (PSCs) include their record power conversion efficiency (PCE), which still remains below the Shockley–Queisser limit, and poor long‐term stability, attributable to crystallographic defects in perovskite films and open‐circuit voltage (Voc) loss in devices. In this study, potassium (4‐tert‐butoxycarbonylpiperazin‐1‐yl) methyl trifluoroborate (PTFBK) was employed as a multifunctional additive to target and modulate bulk perovskite defects and carrier dynamics of PSCs. Apart from simultaneously passivating anionic and cationic defects, PTFBK could also optimize the energy‐level alignment of devices and weaken the interaction between carriers and longitudinal optical phonons, resulting in a carrier lifetime of greater than 3 μs. Furthermore, it inhibited non‐radiative recombination and improved the crystallization capacity in the target perovskite film. Hence, the target rigid and flexible p‐i‐n PSCs yielded champion PCEs of 24.99 % and 23.48 %, respectively. More importantly, due to hydrogen bonding between formamidinium and fluorine, the target devices exhibited remarkable thermal, humidity, and operational tracking at maximum power point stabilities. The reduced Young's modulus and residual stress in the perovskite layer also provided excellent bending stability for flexible target devices.

Multifunctional Trifluoroborate Additive for Simultaneous Carrier Dynamics Governance and Defects Passivation to Boost Efficiency and Stability of Inverted Perovskite Solar Cells.

The main obstacles to promoting the commercialization of perovskite solar cells (PSCs) include their record power conversion efficiency (PCE), which still remains below the Shockley-Queisser limit, and poor long-term stability, attributable to crystallographic defects in perovskite films and open-circuit voltage (Voc) loss in devices. In this study, potassium (4-tert-butoxycarbonylpiperazin-1-yl) methyl trifluoroborate (PTFBK) was employed as a multifunctional additive to target and modulate bulk perovskite defects and carrier dynamics of PSCs. Apart from simultaneously passivating anionic and cationic defects, PTFBK could also optimize the energy-level alignment of devices and weaken the interaction between carriers and longitudinal optical phonons, resulting in a carrier lifetime of greater than 3 μs. Furthermore, it inhibited non-radiative recombination and improved the crystallization capacity in the target perovskite film. Hence, the target rigid and flexible p-i-n PSCs yielded champion PCEs of 24.99 % and 23.48 %, respectively. More importantly, due to hydrogen bonding between formamidinium and fluorine, the target devices exhibited remarkable thermal, humidity, and operational tracking at maximum power point stabilities. The reduced Young's modulus and residual stress in the perovskite layer also provided excellent bending stability for flexible target devices.

Study of cytotoxic performance of green-synthesized Co doped NiO nanoparticles over human breast cancer cells

The present paper presents a green method for the synthesis of pure and 1, 3, and 5% Co doped NiO nanoparticles (Co-NiO-NP) by using the aqueous extract of Salvadora persica. The synthesized nanoparticles were confirmed through PXRD, UV–Vis, FESEM, EDX, and Raman analyses. The results of PXRD, EDX, and UV–Vis affirmed the existence of well doped cobalt within the nickel oxide structure, which caused an increase in its particle size. According to PXRD finding, NiO have a face-centered cubic (FCC) structure. FESEM images showed pseudo-cubic form for nickel oxide nanoparticles which this morphology changes with doped process. Raman outcome were assigned to the longitudinal optical (LO) phonon mode of NiO. UV–Vis outcomes presented a blue shift in the adsorption peak of NiONP after the doping of cobalt ions into nickel oxide structure. According to the performed MTT test on the toxicity effect of synthesized nanoparticles on human breast cancer (MCF-7) and human umbilical vein endothelial (HUVEC) cells, cobalt doped nanoparticles displayed a higher cytotoxic activity than pure nanoparticles. Therefore, we can approve the applicability of our synthesized nanoparticles in biological implementations and cancer treatment.