Understanding Of Carrier Dynamics Research Articles

Carrier tunneling and transport in coupled quantum wells: Modeling and experimental verification

We propose an approach to elucidate carrier dynamics by developing a robust rate equation model capable of explaining carrier dynamics in a dual-well system. To experimentally validate the accuracy and reliability of our model, we utilized a combination of time-resolved photoluminescence and spatially resolved cathodoluminescence measurements. The integration of these complementary techniques allowed us to thoroughly investigate the spectral characteristics in both wells, revealing distinct features attributed to carrier tunneling and transport effects and verifying the predictive ability of the model. This validation process not only confirms the model's accuracy under both coupled and uncoupled conditions but also highlights its potential applicability to quantum-well-based semiconductor lasers. Overall, our study not only advances our understanding of carrier dynamics in quantum wells but also introduces a versatile and reliable tool for designing and simulating ultrafast semiconductor lasers.

Ultrafast Electron Transfer in InP/ZnSe/ZnS Quantum Dots for Photocatalytic Hydrogen Evolution.

InP/ZnS core/shell quantum dots have shown extraordinary application potential in photocatalysis. In this work, we demonstrated by ultrafast spectroscopy that the electron transfer ability of InP/ZnSe/ZnS core/shell/shell quantum dots was better than that of InP/ZnS quantum dots, because the introduction of ZnSe midshell resulted in improved passivation and greater exciton delocalization. The temperature-dependent PL spectra indicate that the exciton-phonon coupling strength and exciton binding energy of InP/ZnSe/ZnS quantum dots are smaller than those of InP/ZnS quantum dots. Further photocatalytic hydrogen evolution testing revealed that the photocatalytic activity of InP/ZnSe/ZnS quantum dots was significantly higher than that of InP/ZnS quantum dots, and InP/ZnSe/ZnS quantum dots even demonstrated improved stability. This research deepened our understanding of carrier dynamics and charge separation of InP/ZnSe/ZnS quantum dots, especially highlighting the application potential of InP/ZnSe/ZnS quantum dots in photocatalytic hydrogen evolution.

Coupled valence carrier and core-exciton dynamics in WS2 probed by few-femtosecond extreme ultraviolet transient absorption spectroscopy

Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with optical 500-1000 nm supercontinuum and broadband XUV pulses (30-50 eV), simultaneously probes dynamics of photoexcited carriers in WS$_{2}$ at the W O$_3$ edge (37-45 eV) and carrier-induced modifications of core-exciton absorption at the W N$_{6,7}$ edge (32-37 eV). Access to continuous core-to-conduction band absorption features and discrete core-exciton transitions in the same XUV spectral region in a semiconductor provides a novel means to investigate the effect of carrier excitation on core-exciton dynamics. The core-level transient absorption spectra, measured with either pulse arriving first to explore both core-level and valence carrier dynamics, reveal that core-exciton transitions are strongly influenced by the photoexcited carriers. A $1.2\pm0.3$ ps hole-phonon relaxation time and a $3.1\pm0.4$ ps carrier recombination time are extracted from the XUV transient absorption spectra from the core-to-conduction band transitions at the W O$_{3}$ edge. Global fitting of the transient absorption signal at the W N$_{6,7}$ edge yields $\sim 10$ fs coherence lifetimes of core-exciton states and reveals that the photoexcited carriers, which alter the electronic screening and band filling, are the dominant contributor to the spectral modifications of core-excitons and direct field-induced changes play a minor role. This work provides a first look at the modulations of core-exciton states by photoexcited carriers and advances our understanding of carrier dynamics in metal dichalcogenides.

Ultrafast dynamics of photoinduced terahertz electron-hole plasma waves in semiconductor junctions

The ultrafast transport processes of electrons and holes excited by femtosecond laser pulses are essential for a better understanding of carrier dynamics in semiconductors. Transient optical techniques are commonly used to investigate the carrier excitation and transport in these materials. Recently, ultrafast scanning electron microscopy has been employed to elucidate the ballistic dynamics of such hot carriers near a $p\text{\ensuremath{-}}n$ junction. We provide here a model not only to explain the previously reported superfast expansion of electron-hole clouds but also to predict plasma wave behavior at high laser fluences. In addition to a bipolar junction, we also investigate the well structure of a $p\text{\ensuremath{-}}n\text{\ensuremath{-}}p$ junction sandwiched by insulating layers. By confining hot carriers inside junctions, these carriers begin to oscillate as a result of Coulomb interactions among carriers and with the intrinsic field at the depletion zone, leading to the occurrence of terahertz-scale plasma waves for potential applications as pulsed terahertz light sources.

Carrier dynamics in hybrid nanostructure with electronic coupling from an InGaAs quantum well to InAs quantum dots

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In0.15Ga0.85As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures.

Origins of photoluminescence decay kinetics in CdTe colloidal quantum dots.

Recent experimental studies have identified at least two nonradiative components in the fluorescence decay of solutions of CdTe colloidal quantum dots (CQDs). The lifetimes reported by different groups, however, differed by orders of magnitude, raising the question of whether different types of traps were at play in the different samples and experimental conditions and even whether different types of charge carriers were involved in the different trapping processes. Considering that the use of these nanomaterials in biology, optoelectronics, photonics, and photovoltaics is becoming widespread, such a gap in our understanding of carrier dynamics in these systems needs addressing. This is what we do here. Using the state-of-the-art atomistic semiempirical pseudopotential method, we calculate trapping times and nonradiative population decay curves for different CQD sizes considering up to 268 surface traps. We show that the seemingly discrepant experimental results are consistent with the trapping of the hole at unsaturated Te bonds on the dot surface in the presence of different dielectric environments. In particular, the observed increase in the trapping times following air exposure is attributed to the formation of an oxide shell on the dot surface, which increases the dielectric constant of the dot environment. Two types of traps are identified, depending on whether the unsaturated bond is single (type I) or part of a pair of dangling bonds on the same Te atom (type II). The energy landscape relative to transitions to these traps is found to be markedly different in the two cases. As a consequence, the trapping times associated with the different types of traps exhibit a strikingly contrasting sensitivity to variations in the dot environment. Based on these characteristics, we predict the presence of a sub-nanosecond component in all photoluminescence decay curves of CdTe CQDs in the size range considered here if both trap types are present. The absence of such a component is attributed to the suppression of type I traps.

ChemInform Abstract: Femtosecond Studies of Electron Dynamics at Interfaces

A delocalized electron at a metal-dielectric interface interacts with the adlayer and spatially localizes or self-traps on the femtosecond time scale into what is termed a small polaron. The dynamics can be studied by two-photon photoemission. Theoretical and experimental analyses reveal the interaction energy and the lattice vibrational mode that mediates electron localization. These results contribute to a fundamental understanding of electron behavior in weakly bonded solids and can lead to a better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

Femtosecond studies of electron dynamics at interfaces.

A delocalized electron at a metal-dielectric interface interacts with the adlayer and spatially localizes or self-traps on the femtosecond time scale into what is termed a small polaron. The dynamics can be studied by two-photon photoemission. Theoretical and experimental analyses reveal the interaction energy and the lattice vibrational mode that mediates electron localization. These results contribute to a fundamental understanding of electron behavior in weakly bonded solids and can lead to a better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

Femtosecond dynamics of electron localization at interfaces

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.