Exciton Recombination Dynamics Research Articles

Probing the Wave Function Delocalization in CdSe/CdS Dot-in-Rod Nanocrystals by Time- and Temperature-Resolved Spectroscopy

Colloidal semiconductor quantum structures allow controlling the strong confinement of charge carriers through material composition and geometry. Besides being a unique platform to study fundamental effects, these materials attracted considerable interest due to their potential in opto-electronic and quantum communication applications. Heteronanostructures like CdSe/CdS offer new prospects to tailor their optical properties as they take advantage of a small conduction band offset allowing tunability of the electron delocalization from type-I toward quasi-type-II. Here, we report on a detailed study of the exciton recombination dynamics in CdSe/CdS heterorods. We observed a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization. Moreover, by increasing the temperature from 70 to 300 K, we observed a considerable increase of the radiative lifetime, clearly demonstrating a reduction of the conduction band offset at higher temperatures. Understanding and controlling electron delocalization in such heterostructures will be pivotal for realizing efficient and low-cost photonic devices.

Role of Surface States in the Exciton Dynamics in CdSe Core and Core/Shell Nanorods

Static and time-resolved photoluminescence (PL) along with transient absorption (TA) measurements have been used to elucidate the relaxation and recombination dynamics of single excitons and multiexcitons in CdSe nanorods with organic ligands and CdSe/CdS core/shell nanorods. The core/shell particles are synthesized from the same 3.5 × 17 nm nanorods as used in the studies on ligated particles. The core/shell particles are studied with and without adsorbed hole acceptors, specifically, hexadecanethiol (HDT) and phenothiazine (PZT). The time-resolved PL and quantum yield results show that following single-exciton photoexcitation, the core nanorods undergo exciton recombination dynamics on several different time scales. The sample is inhomogeneous, and most of these particles are not observed at all. Of the particles that are observed, most undergo exciton quenching by hole transfer to surface states, leaving the electron in the conduction band. The extent of hole trapping depends on the adsorbed ligands, with alkyl amines partially passivating the hole traps. High fluence excitation produces a significant concentration of biexcitons, which undergo Auger recombination on the approximately 100 ps time scale. Very different dynamics are observed in the CdSe/CdS core/shell nanorods. These nanorods exhibit high PL quantum yields (>50%), indicating that most of the surface hole traps have been passivated. High-fluence TA measurements indicate extensive conduction band state filling by multiexcitons. The Auger rate depends on the number of electron−hole pairs and is therefore state dependent. Auger recombination times are approximately 700 ps for the lowest conduction band level, 100 ps for the next level at 780 cm−1, and 13 ps for the π conduction band level at 3650 cm−1. The presence of adsorbed hexadecanethiol or phenothiazine changes the single- and multiexciton dynamics. Hole transfer quenches excitons, resulting in much faster PL decays, with little change in the X0 TA kinetics. These dynamical changes are accompanied by the appearance of a broad absorption of the PTZ+• radical cation, which persists for >1 ns. In the case of adsorbed HDT, the loss of valence band holes results in slower Auger recombination of electrons in the π conduction band level.

Mono- and Biexponential Luminescence Decays of Individual Single-Walled Carbon Nanotubes

We have studied the exciton recombination dynamics of individual (6,4) and (6,5) single-walled carbon nanotubes embedded in aqueous gels or deposited on glass surfaces. CoMoCat nanotubes systematically display short monoexponential photoluminescence (PL) decays presumably due to defects introduced during their synthesis. In contrast HiPco nanotubes can either display mono- or biexponential PL decays depending on the environmental conditions. Transition from bi- to monoexponential decays can be reproduced by a simple three level model taking into account defect-dependent nonradiative decay mechanisms.

Novel Picosecond Time-Resolved Cathodoluminescence to Probe Exciton Recombination Dynamics in GaN and GaN Based Heterostructures

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence

We present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs.

Recombination dynamics of exciton and exciton complexes in single quantum dots

We have studied the relaxation dynamics of excitons and exciton complexes in a single quantum dot using time-resolved photoluminescence. To avoid any interference of carrier diffusion and trapping we have excited the dot with energies below that of the wetting layer. The times extracted from a quantitative analysis are thus related only to the relaxation of carriers inside the dot. Since the dot is rather large it is possible to observe the recombination of carriers from different atomic shells. We observe a retardation of the carrier relaxation increasing excitation power due to the large number of charges filling the dot.

Thermal activated carrier transfer between InAs quantum dots in very low density samples

In this work we develop a detailed experimental study of the exciton recombination dynamics as a function of temperature on QD-ensembles and single QDs in two low density samples having 16.5 and 25 dots/μm2. We corroborate at the single QD level the limitation of the exciton recombination time in the smallest QDs of the distribution by thermionic emission (electron emission in transient conditions). A portion of these emitted carriers is retrapped again in other (larger) QDs, but not very distant from those emitting the carriers, because the process is limited by the diffusion length at the considered temperature.

Luminescence of ZnO crystals under surface and bulk excitation regimes

AbstractLuminescence properties of vapor phase grown ZnO single crystals with different content of deep traps were explored under one‐photon and two‐photon excitation. Room‐temperature near bandgap emission of highly excited ZnO crystals was originated by inelastic exciton scattering. Under surface excitation regime, luminescence decay profile was governed by exciton diffusion and bimolecular recombination. At higher excitation, exciton recombination transients were significantly altered by stimulated emission. Two‐photon bulk‐like excitation revealed exciton recombination dynamics related to defect states. Correlation between the luminescence decay rate and the density of deep trap states of variously annealed crystals was observed. Results on crystal optical quality were in line with estimated values of stimulated emission threshold. The largest value of the average luminescence decay time τlu = 1.5 ns and the lowest value of the stimulated emission threshold Iexc = 0.6 mJ/cm2 were obtained for ZnO crystal annealed in vacuum. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface Modifications and Optoelectronic Characterization of TiO2‐Nanoparticles: Design of New Photo‐Electronic Materials

AbstractTiO2 nanoparticles are of great current interest for applications in photo‐electronic materials including light‐energy conversion, artificial photosynthetic systems as well as photocatalysis. The success of these applications relies on the exciton recombination dynamics and visible‐light sensitivity of the TiO2 nanomaterials. Thus, in order to develop the highly efficient photo‐electronic materials absorbing visible light, different low dimensional TiO2 nanostructures such as nanodiscs, nanofibers and nanochains were synthesized, and thereafter their surfaces were modified by incorporating with Sn‐porphyrins and heteropoly acid. The optoelectronic properties of the surface‐modified nanomaterials were investigated with regard to the optical properties and the surface exciton dynamics by using both steady‐state and ultrafast time‐resolved laser spectroscopic techniques including single nanoparticle photoluminescence technique. These results were correlated with the photo‐electronic properties including photocatalytic activities and solar cell efficiencies, indicating that the electron transfer mechanism in the modified nanostructures may be similar to the “Z‐scheme” of the plant photosynthetic system so that both photocatalytic activity and solar cell efficiencies were synergistically enhanced by using two color illumination.

Photoluminescence of Cd xZn 1− xO films grown on sapphire substrate by PLD technique

In this paper, the structural and optical properties of Cd x Zn 1− x O films were studied. The films were deposited by pulse laser deposition (PLD). The Cd concentration was changed in the range from x = 0 to 0.2. The structure of the films was characterized by an atom force microscope (AFM) and X-ray diffraction (XRD). The optical band gap of the films was measured by using transmission and photoluminescence (PL) spectra. The exciton recombination dynamics was investigated by means of the time-resolved photoluminescence (TRPL) spectra of the films at 77 K.

Surface excitonic recombination dynamics in ZnO nanowires

AbstractThe optical properties of ZnO nanowires are strongly influenced by surface effects. For diameters smaller than 200 nm, a surface exciton recombination band is observed. We report on the recombination dynamics of this emission band using low temperature and time resolved photoluminescence spectroscopy. We find a biexponential decay with different decay constants for different emission wavelengths on the band. We present a model to explain the biexponential decay characteristics and report on possibilities to experimentally enhance the relative intensity of the surface exciton emission by embedding the ZnO nanowires into polymer matrices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-Dependent Exciton Recombination Dynamics of CdTe Nanocrystals

Exciton recombination dynamics of zinc-blende CdTe nanocrystals (NCs) have been investigated by using an ionic liquid as a cryogenic matrix. Size and temperature dependence of photoluminescence lifetimes of CdTe NCs were studied by time-resolved spectroscopy. The photoluminescence properties in a low-temperature range were characterized by a simple three-level model where populations of two emitting states in CdTe NCs are determined by Boltzmann distribution. Observation at low temperature suggests that the relaxation pathway is dependent on the population between two kinds of excited states of the CdTe NC generated by an electron−hole exchange interaction in the tightly confined particle-in-a-box system.

Nearly intrinsic exciton lifetimes in single twin-free GaAs∕AlGaAs core-shell nanowire heterostructures

CW and time-resolved photoluminescence measurements are used to investigate exciton recombination dynamics in GaAs∕AlGaAs heterostructure nanowires grown with a recently developed technique which minimizes twinning. A thin capping layer is deposited to eliminate the possibility of oxidation of the AlGaAs shell as a source of oxygen defects in the GaAs core. We observe exciton lifetimes of ∼1ns, comparable to high quality two-dimensional double heterostructures. These GaAs nanowires allow one to observe state filling and many-body effects resulting from the increased carrier densities accessible with pulsed laser excitation.

Recombination dynamics of localized excitons in AlGaN‐based quantum wells

AbstractThe effect of internal electric field on exciton localization in AlGaN‐based quantum wells (QWs) has been studied by means of time‐resolved photoluminescence (PL) spectroscopy. The observed time‐dependent PL was analyzed using rate equations, which enabled us to obtain the PL lifetime and the localization time of excitons for three QWs with different well‐layer thickness. The localization time became longer with increasing well‐layer thickness. This result indicated that a quantum confined Stark effect caused the reduction in transition probability of excitons from extended to localized states. In addition, the localization time decreased with increasing excitation density and the three QWs indicated almost the same localization time at higher excitation density. Since the alloy composition was the same for the three QWs, the degree of localization due to alloy disorder did not change so much. Therefore, the screening of the internal electric field resulted in the same localization time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiative recombination of excitons in disk-shapedInAs∕InPquantum dots

Recombination dynamics of excitons confined in disk-shaped $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ quantum dots is studied by time-resolved photoluminescence measurements. By comparing the result with that in a homologous ultrathin quantum well, it is revealed that the lateral confinement of excitons suppresses the thermal variation of the radiative recombination lifetime. The oscillator strength of the radiative transition is reduced with the decrease of the disk height, which is attributed to the increasing tunneling into the InP barrier of the exciton wave function.

Exciton, biexciton and trion recombination dynamics in a single quantum dot under selective optical pumping

Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power.

Ultrafast dynamics in ZnO/ZnMgO multiple quantum wells

We have investigated carrier relaxation and exciton recombination dynamics inZnO/ZnMgO multiple quantum wells using femtosecond pump–probe techniques at roomtemperature. For a probe energy above the band gap, the hot carriers exhibit aneffective relaxation by longitudinal optical phonon scattering with a cooling time of700–850 fs. By detecting the emission near the band-gap, a longer decay time of a fewpicoseconds was observed which is attributed to acoustic phonon scattering. Asthe probe energy is decreased further, the decay time continues to increase dueto the transitions of exciton recombination or localized carrier recombination.

Surface and bulk exciton recombination dynamics in GaN freestanding films via one- and two-photon excitations

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one- and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultra-long exciton PL lifetime of 17.2 ns at 295 K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100 ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. The room temperature exciton lifetime of 17.2 ns is the longest ever reported for GaN film. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes.

Recombination dynamics of excitons in Mg0.11Zn0.89O alloy films grown using the high-temperature-annealed self-buffer layer by laser-assisted molecular-beam epitaxy

Recombination dynamics of excitons in Mg0.11Zn0.89O epilayers grown by laser-assisted molecular-beam epitaxy on a ScAlMgO4 substrate were investigated. By using the MgZnO high-temperature-annealed self-buffer layer (HITAB), the value of full width at half maximum of the near-band-edge (NBE) photoluminescence (PL) peak at 3.6eV was decreased from 133to94meV at 293K, and the intensity ratio of the NBE emission to the deep emission band centered around 2.2eV was increased by a factor of 3. Also, the PL lifetime of the NBE peak at 293K under the excitation density of 1μJ∕cm2 was increased from 49to60ps. These results suggest that HITAB gave rise to improved alloy compositional homogeneity and reduced concentration of point defects.

Temperature Dependence of Exciton Recombination in Semiconducting Single-Wall Carbon Nanotubes

We study the excitonic recombination dynamics in an ensemble of (9,4) semiconducting single-wall carbon nanotubes by high-sensitivity time-resolved photoluminescence experiments. Measurements from cryogenic to room temperature allow us to identify two main contributions to the recombination dynamics. The initial fast decay is temperature independent and is attributed to the presence of small residual bundles that create external nonradiative relaxation channels. The slow component shows a strong temperature dependence and is dominated by nonradiative processes down to 40 K. We propose a quantitative phenomenological modeling of the variations of the integrated photoluminescence intensity over the whole temperature range. We show that the luminescence properties of carbon nanotubes at room temperature are not affected by the dark/bright excitonic state coupling.