Hot Exciton Relaxation Research Articles

Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics

Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

Surface Ligands Increase Photoexcitation Relaxation Rates in CdSe Quantum Dots

Understanding the pathways of hot exciton relaxation in photoexcited semiconductor nanocrystals, also called quantum dots (QDs), is of paramount importance in multiple energy, electronics and biological applications. An important nonradiative relaxation channel originates from the nonadiabatic (NA) coupling of electronic degrees of freedom to nuclear vibrations, which in QDs depend on the confinement effects and complicated surface chemistry. To elucidate the role of surface ligands in relaxation processes of nanocrystals, we study the dynamics of the NA exciton relaxation in Cd(33)Se(33) semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands using explicit time-dependent modeling. The large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitate exciton relaxation. Our computational results for the ligand contributions to the exciton relaxation and electronic energy-loss in small clusters are further extrapolated to larger quantum dots.

Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

Unraveling the Structure and Dynamics of Excitons in Semiconductor Quantum Dots

The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an "artificial atom" because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies. Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the "excitonics", confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have developed a state-resolved spectroscopic approach which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials. We focus on four main topics of longstanding interest and controversy: (i) hot exciton relaxation dynamics, (ii) multiexcitons, (iii) optical gain, and (iv) exciton-phonon coupling. Using this state-resolved approach, we reconcile long standing controversies related to phenomena such as exciton cooling and exciton-phonon coupling and make surprising new observations related to optical gain and multiexcitons.

Efficient spin depolarization in ZnCdSe spin detector: an important factor limiting optical spin injection efficiency in ZnMnSe∕ZnCdSe spin light-emitting structures

Spin depolarization of a ZnCdSe quantum-well spin detector (SD) in ZnMnSe∕ZnCdSe light-emitting quantum structures is investigated by cw and time-resolved optical orientation spectroscopy. It is shown that spin depolarization is governed by three distinct spin relaxation processes with the corresponding polarization decay times of 850, 30, and <10ps. The dominant and the fastest process is attributed to spin relaxation accompanying energy relaxation of hot excitons (and hot carriers) within the SD, providing evidence that it can be an important source of spin loss, leading to the observed limited efficiency of optical spin injection in the structures.

Non‐classical excitonic transport in quantum wells

AbstractWe report investigations on excitonic transport in ZnSe quantum wells using far‐field nano‐photoluminescence enhanced by a solid immersion lens. The 250‐nm spatial resolution and 5‐ps temporal resolution allow the access to non‐classical transport regimes. From zero‐phonon‐line spectroscopy we deduce the spatial distribution of the PL intensity under different cw excitation condition and its temporal evolution after a pulsed excitation, respectively. The periodic quenching of the transport length as function of excitation excess energy and the nonlinear expansion of the PL spot observed in time‐resolved experiments reveal the dominance of hot‐exciton effects. The experimental data are well modelled by a Monte Carlo simulation. Spatially resolved phonon‐sideband spectroscopy is used to observe directly the coherent transport and the energy relaxation of hot excitons during transport. The coherence length is measured to be 300–400 nm at low temperatures and the excitons are found to remain hot during transport on a length scale of several micrometers.

Highest-order optical phonon-mediated relaxation in CdTe/ZnTe quantum dots

The highest 19th-order longitudinal optical (LO) phonon-mediated relaxation was observed in photoluminescence excitation spectra of CdTe self-assembled quantum dots grown in ZnTe. Hot excitons photoexcited highly in the ZnTe barrier layer are relaxed into the wetting-layer state by emitting multiple LO phonons of the barrier layer successively. Below the wetting-layer state, the LO phonons involved in the relaxation are transformed to those of interfacial ZnxCd1−xTe surrounding CdTe quantum dots. The ZnTe-like and CdTe-like LO phonons of ZnxCd1−xTe and lastly acoustic phonons are emitted in the relaxation into the CdTe dots. The observed main relaxation is the fast relaxation directly into CdTe quantum dots and is not the relaxation through either the wetting-layer quantum well or the band bottom of the ZnTe barrier layer. This observation shows very efficient optical phonon-mediated relaxation of hot excitons excited highly in the ZnTe conduction band through not only the ZnTe extended state but also localized state in the CdTe quantum dots reflecting strong exciton–LO phonon interaction of telluride compounds.

Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

We investigate the energy relaxation of excitons during the real-space transport in ZnSe quantum wells by using microphotoluminescence with spatial resolution enhanced by a solid immersion lens. The spatial evolution of the LO-phonon sideband, originating from the LO-phonon assisted recombination of hot excitons, is measured directly. By calculating the LO-phonon assisted recombination probability, we obtain the nonthermal energy distribution of excitons and observe directly the energy relaxation of hot excitons during their transport. We find the excitons remain hot during their transport on a length scale of several micrometers. Thus, the excitonic transport on this scale cannot be described by classical diffusion.

Cooling Process of Hot Excitons in Ordered Ga0.5In0.5P

Time-resolved photoluminescence (PL) spectroscopy has been performed to investigate hot-exciton relaxation process in long-range ordered Ga0.5 In0.5 P. The cooling process of excitons shows strong order parameter dependence. The rise time of the exciton PL is getting faster with increasing degree of ordering. This indicates that optically created excitons rapidly relax in a fluctuated potential of the partially ordered epitaxial film. The exciton-PL lifetime becomes small with increasing order parameter. Also, the PL of ordered Ga0.5 In0.5 P exhibits a localized exciton feature with an much slower decay. These findings suggest that excitons tend to localize in ordered domain structure and can relax into localized deep centers via the nonradiative recombination pass in the ordered domain ensemble.

Optical properties of nanostructures self-organized in CdSe/ZnSe fractional monolayer superlattices

We report on optical studies of excitonic transfer and localization in a CdSe/ZnSe fractional monolayer superlattice with an embedded deeper ZnCdSe quantum well (QW). Time-resolved selective excitation photoluminescence measurements reveal a two-step nature of the energy relaxation process. The first stage includes hot-exciton cascade relaxation assisted by emission of ZnSe LO phonons, which results in the formation of a narrow nonthermal distribution of free excitons. Different mechanisms compete to destroy the distribution, including optical- and acoustic-phonon-assisted exciton localization by fluctuations of the random potential, as well as tunneling escape of hot free excitons towards the embedded deeper QW in the sample.

Hot exciton relaxation in CdZnSe/ZnSe quantum wells and quantum dots

The dimensionality dependence of hot exciton relaxation has been studied by means of time-resolved photoluminescence and photoluminescence excitation spectroscopy comparing CdZnSe/ZnSe quantum wells and quantum dots. An energetically narrow exciton distribution within the inhomogeneously broadened luminescence band is prepared by low density laser excitation with an excess energy corresponding to an integer number of LO phonons. In quantum wells, lateral migration of excitons within the layer causes a transient redistribution of excitons in energy controlled by acoustic phonon interaction. This is indicated by a spectral broadening and a red shift of the PL signal with time. In contrast, this process is found to be substantially suppressed in quantum dots due to the three-dimensional exciton confinement.

The relaxation of hot excitons by longitudinal optical-phonons in Cd xZn 1−xTe/ZnTe asymmetric coupled double quantum wells

Photoluminescence (PL) and photoluminescence excitation (PLE) measurements on Cd xZn 1−xTe/ZnTe asymmetric coupled double quantum wells were performed to investigate the hot-exciton luminescence phenomena. The PLE spectra indicated the creation of hot excitons and subsequent relaxation by longitudinal optical-phonon emission resulting from the localization of excitons due to fluctuation of the Cd xZn 1−xTe alloy well.

Energy relaxation by multiphonon processes in InAs/GaAs quantum dots

Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD's) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy. We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism. Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved. The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states. Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps. The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation. For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative (>1 ns) and nonradiative (\ensuremath{\approx}100 ps) recombination explaining the absence of a ``phonon bottleneck'' effect in the PL spectra.

Relaxation of hot excitons in inhomogeneously broadenedCdxZn1−xSe/ZnSenanostructures

The dimensionality dependence of hot exciton relaxation was studied by means of photoluminescence (PL) spectroscopy comparing ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}$ quantum wells and quantum dots. In both systems, an efficient population of radiative excitonic ground states via longitudinal-optical (LO) phonon scattering is observed for an excess energy of the exciting laser corresponding to an integer number of LO-phonon energies. Thus, only selected areas of the inhomogeneously broadened systems contribute to the PL spectrum immediately after the exciting laser pulse. This results in a drastical reduction of the PL linewidth. In quantum wells, the redistribution of excitons occurs due to acoustic phonon assisted migration within the ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}$ layer. In contrast, this process is found to be substantially suppressed in quantum dots due to the three-dimensional exciton confinement.

Hot excitons in ZnSe quantum wells

The optical and acoustic phonon assisted relaxation of free hot excitons in ZnSe quantum wells is reflected in photoluminescence excitation spectra. Slow thermalization of excitons on a 100 ps time scale is directly observed in time-resolved studies of luminescence in the spectral region of the phonon sideband.

Intersubband relaxation of hot excitons in GaAs quantum wells

We have experimentally studied the time evolution of the exciton population in a higher subband of GaAs quantum wells, below the free-carrier continuum. The lifetime of the exciton formed by an electron of the lowest subband and a heavy hole of the second subband in GaAs quantum wells is determined by time-resolved luminescence as 130+or-20 ps. This result is consistent with theoretical estimations of intersubband scattering by acoustic phonon emission. We discuss subband transitions of excitons in quantum wells as a new appealing concept for optically pumped coherent sources in the meV range.

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1?x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al x Ga1−xAs multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 A, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.

Optical Properties of ZnCdSe/ZnSSe Strained-Layer Quantum Wells

We have shown the optical properties of ZnCdSe/ZnSSe strained-layer single- and multiple-quantum wells (SQWs and MQWs) fabricated by metalorganic molecular beam expitaxy (MOMBE). Excitonic emission and absorption properties were investigated in detail by means of photoluminescence, photoluminescence excitation and absorption spectral measurements. A multiple longitudinal optical (LO)-phonon emission which is related to the relaxation of hot excitons has been observed in the photoluminescence excitation spectrum in a SQW. The temperature dependence of the emission intensity in the SQWs with three different well widths (10, 50, and 100 Å) shows the quenching of the exciton-emission intensity. For a 10 Å well the dependence shows Arrhenius thermally activated behavior, where the activation energy of about 70 meV is obtained. A theoretical analysis of the temperature dependence of an excitonic linewidth indicates that the exciton-LO phonon coupling constant is estimated to be 35 meV which is larger than that (5 meV) of GaAs/AlGaAs QW. As a result, this effect is the dominant cause of the exciton linewidth broadening.

Energy relaxation of hot two-dimensional excitons in a GaAs quantum well by exciton-phonon interaction.

The power loss of hot two-dimensional excitons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum wells and the corresponding energy relaxation times are calculated in the present work. A simplified variational envelope function is assumed for the 1s excitonic level in an infintely deep well and the exciton distribution is described by an effective exciton temperature. For deformation-potential acoustic-phonon scattering, a simple expression for the power loss derived by Takagahara is used, while for polar-optic-phonon scattering the expression is developed in the present paper. The calculated values indicate that for lattice and exciton temperatures below 150 K, polar-optic-phonon scattering contributes little to the power loss. The energy relaxation time due to the acoustic-phonon scattering is found to be about 3--4 times higher than the experimental estimate and hence the higher-lying states have to be considered to obtain enhanced energy relaxation rate.

Hot-exciton relaxation in CdxZn1-xTe/ZnTe multiple quantum wells.

We have observed the creation of hot excitons at high momentum in ${\mathrm{Cd}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Te multiple quantum wells and subsequent relaxation by LO-phonon emission to give sharp luminescence lines superimposed on a broader background. The difference between the exciton lifetime and dephasing time in these materials allows definite identification of the sharp lines as due to luminescence rather than resonant Raman scattering. The results are consistent with excitons localized in alloy potential fluctuations.