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Fundamental Limit of the Emission Linewidths of Quantum Dots: An Ab Initio Study of CdSe Nanocrystals
摘要： The emission linewidth of a semiconducting nanocrystal (NC) significantly affects the performance in light-emitting applications but its fundamental limit is still elusive. Herein we analyze exciton-phonon coupling (EPC) from Huang-Rhys (HR) factors using ab initio calculations, and compute emission line shapes of CdSe NCs. When surface traps are absent, acoustic modes are found to dominate EPC. The computed linewidths are mainly determined by the size of NCs, being largely insensitive to the shape and crystal structure. Linewidths obtained in this work are much smaller than most measurements on the homogeneous linewidth, but they are consistent with a CdSe/CdxZn1?xSe (core/shell) NC [Park et al. Nat. Mater. 2019, 18, 249-255]. Based on this comparison, it is concluded that the large linewidths in most experiments originated from internal fields by surface (or interface) traps or quasi-type Ⅱ band alignment that amplify EPC. Thus, the present results on NCs with ideal passivation provide the fundamental minimum of homogeneous linewidths, indicating that only the CdSe/CdxZn1?xSe NC has achieved this limit through well-controlled synthesis of the shell structure. To further verify the role of internal fields, we model NCs with charged surface defects. We find that the internal field significantly increases HR factors and linewidths, in reasonable agreements with experiments on single cores. By revealing the fundamental limit of emission linewidths of quantum dots, this work will pave the way to engineering quantum dots with ultrasharp spectrum.
关键词： nanocrystals,photoluminescence,homogeneous linewidth,exciton-phonon coupling,surface traps,density functional theory,electroluminescence,quantum dots
更新于2025-09-19 17:13:59
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Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr <sub/>3</sub> Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy
摘要： We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 nm to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ~ 10 ps) of ‘warm’ electrons in the vicinity (within ~ 0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1 and increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intra-band transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.
关键词： electron-phonon coupling,hot carriers,nonadiabatic molecular dynamics,Charge-carrier cooling,lead-halide perovskite nanocrystal,excited-states dynamics
更新于2025-09-19 17:13:59
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Transport Through a Quantum Dot with Electron-Phonon Interaction
摘要： We theoretically study the electrical transport properties of a single level quantum dot connected to two normal conducting leads, which is coupled to the lattice vibrations. We determine the current through the quantum dot in two different situations: time-independent and time-averaged. In all situations we consider three cases: when there is no electron-phonon interaction, when the dot electrons interact with optical phonons or when they interact with acoustic phonons. At finite temperatures we take into account the temperature dependence of the chemical potential. We treat the electron-phonon interaction by the canonical transformation method. In the case of electron-longitudinal optical phonon interaction the spectrum contains a subpeak. In the case of electron-acoustic phonon interaction the spectrum is continuous. In the time-averaged situation many parasite peaks appear in the spectrum, due to the external time-modulation.
关键词： optical phonon,electron-phonon interaction,acoustic phonon,electron,quantum dot
更新于2025-09-16 10:30:52
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Observation of a phonon bottleneck in copper-doped colloidal quantum dots
摘要： Hot electrons can dramatically improve the efficiency of solar cells and sensitize energetically-demanding photochemical reactions. Efficient hot electron devices have been hindered by sub-picosecond intraband cooling of hot electrons in typical semiconductors via electron-phonon scattering. Semiconductor quantum dots were predicted to exhibit a "phonon bottleneck" for hot electron relaxation as their quantum-confined electrons would couple very inefficiently to phonons. However, typical cadmium selenide dots still exhibit sub-picosecond hot electron cooling, bypassing the phonon bottleneck possibly via an Auger-like process whereby the excessive energy of the hot electron is transferred to the hole. Here we demonstrate this cooling mechanism can be suppressed in copper-doped cadmium selenide colloidal quantum dots due to femtosecond hole capturing by copper-dopants. As a result, we observe a lifetime of ~8.6 picosecond for 1Pe hot electrons which is more than 30-fold longer than that in same-sized, undoped dots (~0.25 picosecond).
关键词： phonon bottleneck,electron-phonon scattering,hot electrons,colloidal quantum dots,copper-doped
更新于2025-09-16 10:30:52
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Coherent Coupled-Mode Phonon Emission in a Photoexcited Charge-Density-Wave System
摘要： Photoexcitation the quasi-one-dimensional of charge-density-wave (CDW) system, blue bronze (K0.3MoO3), with near-infrared femtosecond pulses, results in both a low-frequency, broadband THz surface emission from the photo-Dember effect, and a narrow-band emission from a TO phonon doublet centered at 1.5 THz below ~100 K in the CDW state. The polarization dependence of the phonon emission leads us to propose a surface-field/quadrupole excitation mechanism, distinct from the typical mechanisms for phonon emission in other materials. A coupled-oscillator model with a transient suppression of the mode coupling can reproduce the detailed temporal and spectral characteristics of the emission.
关键词： charge-density-wave,photo-Dember effect,coupled-oscillator model,THz emission,coherent phonon emission
更新于2025-09-16 10:30:52
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Intraband Ultrafast Terahertz Conductivity Dynamics in Graphene
摘要： The cooling mechanism is crucial for nanoelectronics, which determines the response time limited for the devices. The cooling process after optical pump in graphene, which is a well-known material with ultrahigh carrier mobility, is therefore an urgent research topic. Ultrafast thermalization and relaxation process in a monolayer graphene film grown on a quartz substrate is investigated by femtosecond laser pump, terahertz probe induced negative technique. A typical photo found, and the electron-phonon-impurity supercollision process is determined to about 2.8 picosecond at different pump photon energy and pump fluence. The competition between the impurity supercollision and the interband-intraband scattering in graphene could give new insight into the design of the application of graphene photonic devices.
关键词： ultrafast dynamics,cooling mechanism,electron-phonon-impurity supercollision,terahertz conductivity,graphene
更新于2025-09-16 10:30:52
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - High power THz quantum cascade laser and its demonstration in high resolution holographic imaging
摘要： Fabrication of high power terahertz quantum cascade laser (THz QCL) based upon the resonant-phonon active region design and semi-insulating surface-plasmon waveguide with frequency at 4.4 THz and its demonstration in digital holographic imaging are presented. The maximum peak output power is 95mW at 10K in pulsed mode. The maximum working temperature is 80 K. Interestingly, by changing the injection current, the transverse mode of the laser can be switched between TM00 mode and TM01 mode. With such THz QCL as the light source, the lateral resolution of a home-built digital holographic imaging system can reach 80μm.
关键词： holographic imaging,high power,resonant-phonon,semi-insulating surface-plasmon waveguide,THz quantum cascade laser
更新于2025-09-16 10:30:52
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Light emission properties in a double quantum dot molecule immersed in a cavity: Phonon-assisted tunneling
摘要： Two main mechanisms dictate the tunneling process in a double quantum dot: overlap of excited wave functions, effectively described as a tunneling rate, and phonon-assisted tunneling. In this paper, we study different regimes of tunneling that arise from the competition between these two mechanisms in a double quantum dot molecule immersed in a unimodal optical cavity. We show how such regimes affect the mean number of excitations in each quantum dot and in the cavity, the spectroscopic resolution and emission peaks of the photoluminescence spectrum, and the second-order coherence function which is an indicator of the quantumness of emitted light from the cavity.
关键词： Double quantum dot,Light-matter interaction,Second-order coherence function,Phonon-assisted tunneling
更新于2025-09-16 10:30:52
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Investigation of Phonon Scattering on the Tunable Mechanisms of Terahertz Graphene Metamaterials
摘要： The in?uences of di?erent kinds of phonon scatterings (i.e., acoustic (AC) phonon, impurity, and longitudinal optical (LO) phonon scatterings) on the tunable propagation properties of graphene metamaterials structures have been investigated, also including the e?ects of graphene pattern structures, Fermi levels, and operation frequencies. The results manifested that, at room temperature, AC phonon scattering dominated, while with the increase in temperature, the LO phonon scattering increased signi?cantly and played a dominate role if temperature goes beyond 600 K. Due to the phonon scatterings, the resonant properties of the graphene metamaterial structure indicated an optimum value (about 0.5–0.8 eV) with the increase in Fermi level, which were di?erent from the existing results. The results are very helpful to understand the tunable mechanisms of graphene functional devices, sensors, modulators, and antennas.
关键词： terahertz,metamaterials,phonon scatterings,graphene
更新于2025-09-16 10:30:52
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Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models
摘要： n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.
关键词： silicon–germanium heterostructures,intersubband photoluminescence,optical pumping,intersubband transitions,electron–phonon interaction,infrared spectroscopy,quantum wells,free electron laser,terahertz quantum cascade laser
更新于2025-09-16 10:30:52