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Raman spectroscopic determination of hole concentration in undoped GaAsBi
摘要： Raman spectra of the undoped GaAs1-xBix (0<x<0.037) grown on GaAs by molecular beam epitaxy were investigated. With an increase of Bi component, we find that the longitudinal optical phonon-hole-plasmon-coupled (LOPC) mode first appears in the vicinity of the unscreened longitudinal optical (ULO) phonon frequency, and then shifts towards near the transverse optical (TO) phonon frequency. A new vibrational modes (~287 cm-1) between the TO and the ULO phonons was verified by use of low temperature polarized Raman measurement and the corresponding scattering intensities are found to be linearly proportional to the composition of Bi in GaAsBi. The hole concentrations determined by using the LOPC/ULO Raman intensities ratio increase from ~6.5×1016 to ~2.8×1017 cm -3 with Bi content and the measured results are in agreement with Hall measurements. Furthermore, the influence of excitation laser power on the estimation of the hole densities is discussed with the help of power dependent Raman spectroscopy.
关键词： Hall measurements,LO-phonon-plasmon–modes,Raman Spectroscopy,GaAsBi,Hole density
更新于2025-09-23 15:21:21
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Ultrafast relaxation dynamics in bimetallic plasmonic catalysts
摘要： Combining a plasmonic metal, such as gold, with other popular catalysts, such as Ni or Pt, can extend its benefits to many energy-extensive reactions catalyzed by those metals. The efficiency of a plasmon-enhanced catalytic reaction is mainly determined by the light absorption cross section and the photo-excited charge carrier relaxation dynamics of the nanoparticles. We have investigated the charge carrier relaxation dynamics of gold/nickel (Au/Ni) and gold/platinum (Au/Pt) bimetallic nanoparticles. We found that the addition of Ni or Pt to gold can reduce light absorption in gold nanoparticles. However, electron–phonon coupling rates of Au/Ni and Au/Pt nanoparticles are significantly faster than that of pure Au nanoparticles. This is due to the fact that both Ni and Pt possess significantly larger electron–phonon coupling constants and higher densities of states near the Fermi level in comparison with Au. Additionally, the phonon–phonon coupling rate of bimetallic Au/Pt and Au/Ni nanoparticles was significantly different from that of pure gold nanoparticles, due to the acoustic impedance mismatch at the nanoparticle/substrate interface. Our findings provide important insights into the rational design of bimetallic plasmonic catalysts.
关键词： bimetallic nanoparticles,ultrafast relaxation dynamics,electron–phonon coupling,phonon–phonon coupling,plasmonic catalysts
更新于2025-09-23 15:21:01
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Terahertz electromagnetic waves radiated from coherent longitudinal optical (LO) phonons and LO-phonon plasmon coupled modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals
摘要： We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This ?nding indicates that the coupling of the LO phonon with the plasmon is not in?uenced by the surface orientations of the three samples.
关键词： coherent LO-phonon plasmon coupled mode,coherent longitudinal optical (LO) phonon,terahertz time-domain spectroscopy,coherent transverse optical phonon,ultrafast phenomena,Raman scattering effect
更新于2025-09-23 15:21:01
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Phonon-Driven Energy Relaxation in PbS/CdS and PbSe/CdSe Core/Shell Quantum Dots
摘要： We study the impact of the chemical composition on phonon-mediated exciton relaxation in the core/shell quantum dots (QDs), with 1-nm core made of PbX and the monolayer shell made of CdX, where X=S and Se. For this, time-domain non-adiabatic dynamics (NAMD) based on Density Functional Theory (DFT) and Surface Hopping techniques are applied. Simulations reveal twice faster energy relaxation in PbS/CdS than PbSe/CdSe due to dominant couplings to higher-energy optical phonons in structures with sulfur anions. For both QDs, the long-living intermediate states associated with the core-shell interface govern the dynamics. Therefore, a simple exponential model is not appropriate, and the four-state irreversible kinetic model is suggested instead, predicting 0.9 ps and 0.5 ps relaxation rates in PbSe/CdSe and PbS/CdS QDs, respectively. Thus, 2-nm PdSe/CdSe QDs with a single monolayer shell exhibit the phonon-mediated relaxation time sufficient for carrier multiplications to outpace energy dissipation and benefit the solar conversion efficiency.
关键词： Density Functional Theory,phonon-driven energy relaxation,PbSe/CdSe,PbS/CdS,core/shell quantum dots,Surface Hopping techniques,non-adiabatic dynamics
更新于2025-09-23 15:21:01
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Strongly Coupled Coherent Phonons in Single-Layer MoS <sub/>2</sub>
摘要： We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (～20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS2, as a representative semiconducting 1L-Transition Metal Dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A'1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab Initio calculations of the change in 1L-MoS2 band structure induced by the A'1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrates that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process.
关键词： exciton-phonon interaction,Coherent phonons,transient absorption spectroscopy,transition metal dichalcogenides,ab-initio calculation
更新于2025-09-23 15:21:01
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Self-assembled InAs/InGaAsP/InP quantum dots: Intraband relaxation impacted by ultrathin GaP sublayer
摘要： The influence of an ultrathin GaP (or GaAs) sublayer on the nonradiative intraband relaxation in InAs/InGaAsP/InP quantum dots (QDs) is investigated. It is found that, based on our studies, the QDs with some heights (e.g., 1.5 nm) and GaP sublayer thicknesses (e.g., 1.03 monolayers) present the first excited state (ES) with higher state degeneracy with respect to ground state (GS), which suggests that the Auger relaxation is triggered more easily. We also find that the energy difference of the ES and GS decreases with increasing sublayer thickness, which suggests that the electron–phonon interaction is affected. This work further presents a study of intraband relaxation for an InAs/InP QD with a GaP or GaAs sublayer. It is found that there is a critical thickness of the GaP sublayer: When the sublayer is less than the critical thickness, the intraband relaxation is only determined by one-longitudinal optical (LO) phonon or two-LO phonons, which is dependent on QD heights. However, with the GaAs sublayer, QDs do not have the above feature. This finding may be helpful for designing and optimizing high-speed QD devices.
关键词： quantum dots,electron–phonon interaction,InAs/InGaAsP/InP,intraband relaxation,GaP sublayer
更新于2025-09-23 15:21:01
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Determination of Phonon Deformation Potentials in Carbon-Doped Silicon
摘要： Carbon-doped silicon (Si:C) is used as a stressor to the channel of n-type metal oxide semiconductor field-effect-transistor (MOSFET). In this study, we determined PDPs, phonon deformation potentials, (p and q) of Si:C to realize evaluation of anisotropic biaxial stress by using water-immersion Raman spectroscopy. Raman peak shift in longitudinal and transverse optical (LO and TO) phonon modes were measured by water-immersion Raman spectroscopy. Furthermore, in-plane and out-of-plane strain were measured by X-ray diffraction (XRD). Based on the Raman shift and the in-plane strain, p and q in each sample were derived. As a result, it took a different value from pure-Si, and the PDPs tended to increase as the C concentration increased.
关键词： phonon deformation potentials,strain evaluation,Carbon-doped silicon,X-ray diffraction,water-immersion Raman spectroscopy
更新于2025-09-23 15:21:01
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Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (X= S, Se) monolayers
摘要： Although CdX (X= S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity (1.08 W/mK (0.75 W/mK)) and high p-type Seebeck coefficient (1300 μV/K (850 μV/K)) in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit (ZT) is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of 2D tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98 % (89.22 %)] and optical modes [8.02 % (10.78 %)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km/s (3 km/s)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (~ 4-5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along the ?? direction causes out-of-plane piezoelectricity. |d33| ~ 21.6 pm/V, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young’s modulus ~ 41 N/m (31 N/m) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N/m) and TcTe2 (34 N/m), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (~ 500 cm2V-1s-1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.
关键词： thermoelectrics,flexible nanoelectronics,phonon transport,CdX monolayers,piezoelectricity
更新于2025-09-23 15:21:01
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Transiently changing shape of the photon number distribution in a quantum-dota??cavity system driven by chirped laser pulses
摘要： We have simulated the time evolution of the photon number distribution in a semiconductor quantum-dot–microcavity system driven by chirped laser pulses and compare with unchirped results. When phonon interactions with the dot are disregarded—thus corresponding to the limit of atomic cavity systems—chirped pulses generate photon number distributions that change their shape drastically in the course of time. Phonons have a strong and qualitative impact on the photon statistics. The asymmetry between phonon absorption and emission destroys the symmetry of the photon distributions obtained for positive and negative chirps. While for negative chirps transient distributions resembling thermal ones are observed, for positive chirps the photon number distribution still resembles its phonon-free counterpart but with overall smoother shapes. In sharp contrast, using unchirped pulses of the same pulse area and duration wave packets are found that move up and down the Jaynes-Cummings ladder with a bell shape that changes little in time. For shorter pulses and lower driving strength Rabi-like oscillations occur between low photon number states. For all considered excitation conditions transitions between sub- and super-Poissonian statistics are found at certain times. For resonant driving with low intensity the Mandel parameter oscillates and is mostly negative, which indicates a nonclassical state in the cavity ?eld. Finally, we show that it is possible that the Mandel parameter dynamically approaches zero and still the photon distribution exhibits two maxima and thus is far from being a Poissonian.
关键词： Mandel parameter,photon number distribution,chirped laser pulses,quantum-dot–cavity system,phonon interactions
更新于2025-09-23 15:21:01
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Remote Phononic Effects in Epitaxial Ruddlesden-Popper Halide Perovskites
摘要： Despite their weak nature, van der Waals (vdW) interactions have been shown to effectively control the optoelectronic and vibrational properties of layered materials. However, how vdW effects exist in Ruddlesden-Popper layered halide perovskites remains unclear. Here we reveal the role of interlayer vdW force in Ruddlesden-Popper perovskite in regulating phase transition kinetics and carrier dynamics, based on high-quality epitaxial single crystalline (C4H9NH3)2PbI4 flakes with controlled dimensions. Both substrate-perovskite epitaxial interaction and interlayer vdW interaction play significant roles in suppressing the structural phase transition. With reducing flake thickness from ~100 nm to ~20 nm, electron-phonon coupling strength decreases by ~30%, suggesting the ineffectiveness of phonon confinement of the natural quantum wells. Therefore, the conventional understanding that vdW perovskite is equivalent to a multiple quantum well has to be substantially amended due to significant nonlocal phononic effects in the layered crystal where intralayer interaction is not drastically different from the interlayer force.
关键词： phase transition,electron-phonon coupling,Ruddlesden-Popper perovskites,epitaxial growth,van der Waals interactions
更新于2025-09-23 15:21:01