Photoluminescence Decay Profiles Research Articles

Temperature dependence of photoluminescence dynamics of self-assembled monolayers of CdSe quantum dots: the influence of the bound-exciton state

We have investigated the temperature dependence of photoluminescence (PL) dynamics of self-assembled monolayers (SAMs) of CdSe quantum dots (QDs). The PL decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behavior due to thermal quenching. Such anomalous temperature dependence of the PL-decay profile is explained using a four-state model which introduces a bound-exciton state into a conventional three-state model consisting of a ground state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state. Furthermore, it is proposed that the radiative decay time of QDs is strongly influenced by the presence or absence of the bound-exciton state.

Optical properties of self‐assembled monolayer of CdSe quantum dots

AbstractThe temperature dependence of absorption, photoluminescence (PL), and PL dynamics of self‐assembled monolayers of CdSe quantum dots was investigated. The PL decay profiles become slower with an increase in temperature, contrary to an ordinary behaviour. We have revealed that such anomalous temperature dependence of the PL‐decay profile below 200 K is explained by a four‐state model consisting of a ground state, a bound‐exciton state, and two free‐exciton states of a lower‐lying dark‐exciton state and a higher‐lying bright‐exciton state (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)

Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck–Condon state, resulting in the field-induced decrease in the population of the emitting state, which induces the quenching of photoluminescence.

Modification in the structural and optical characteristics of InAs quantum dots by manipulating the strain distribution

We report the structural and optical properties of InAs quantum dots (QDs) on GaAs formed by using a modified self-assembled method (MSAM) adopting periodical arsenic interruption. From the atomic force microscopy images, the densities of the InAs QDs and large clusters grown by the conventional self-assembled mode (CSAM) were 7.3 × 1010 /cm2 and 1.9 × 109 /cm2, respectively. However, the density for the MSAM QD sample was significantly increased to 9.5 × 1010 /cm2, and no large clusters were observed at all. Also, the size distribution for the MSAM InAs QDs was improved to 8.9% from that for the CSAM QDs with 13.5%. The improvement in the structural properties of the MSAM InAs QDs can be explained by the modulation in the distribution of the strain field controlled by the growth kinetics of indium migration. Compared to that of the CSAM QDs, the photoluminescence (PL) peak for the MSAM QD sample was red-shifted, and the intensity was enhanced by more than 2.2 times. The PL decay profiles corresponding to the carrier lifetimes of InAs QDs are also discussed in terms of the variations in the structural properties.

Carrier repopulation process for spatially-ordered InAs/InAlGaAs quantum dots

From the transmission electron microscope image, the seven-stacked InAs/InAlGaAs QDs on an InP substrate were spatially ordered instead of usual on-top vertical alignment. The increasing rate of the QD size became almost saturated by increasing the number of layers. The photoluminescence (PL) intensity for the seven-stacked InAs/InAlGaAs QDs was decreased up to 60K and remained almost stable at the temperature range from 60 to 220K. And then, the intensity was again drastically decreased with further increasing temperature. The emission peak was first red-shifted at the ratio of 0.446 meV/K from 20 to 60K. However, the degree of the red-shift in the emission peak from 60 to 220K was decreased at the negligibly small ratio of 0.028 meV/K. Above 220K, the emission peak was again significantly decreased at the ratio of 0.323 meV/K. While increasing the temperature, the carrier lifetimes obtained from the PL decay profiles for the seven-stacked QDs initially enhanced and then, almost stable at a certain tempe...

Photoexcitation, trapping, and recombination processes of theF-type centers in lasing MgO microcrystals

We report results of a detailed temperature dependence of photoluminescence (PL) decay time and continuous emission properties of the $F$ and ${F}^{+}$ centers in recently reported lasing MgO microcrystals [T. Uchino and D. Okutsu, Phys. Rev. Lett. 101, 117401 (2008)]. Thermally induced ionization and carrier trapping play a vital role in the entire emission processes of the $F$-type centers, especially at time scales longer than microseconds. In these time scales the PL decay profiles tend to exhibit a power-law behavior over more than 3\ensuremath{-}4 decades of time, implying thermally activated hopping and tunneling of the trapped electrons. Such delayed PL signals show two maxima in intensity at temperatures of \ensuremath{\sim}130 and \ensuremath{\sim}300 K. This apparently anomalous temperature dependence is also indicative of the thermally stimulated emission processes of the originally photoexcited $F$ and ${F}^{+}$ centers. The PL spectra above lasing threshold exhibit temperature-dependent broadening, demonstrating that the thermal vibrations of the crystal lattice affect the stimulated emission scheme as well. On the basis of these experimental results, a model of the photoexcitation, trapping, and recombination processes of the $F$-type centers is presented.

Photoluminescence decay profiles of exciton‐exciton scattering in a ZnO thin film

AbstractWe have investigated the photoluminescence (PL) dynamics of a ZnO thin film under intense excitation conditions at 10 K. In the ZnO thin film, a PL band due to exciton‐exciton scattering, the so‐called P emission, appears. The optical‐Kerr‐gating method with the ultrashort gating time of 0.6 ps enabled us to obtain precise information of the temporal profiles including the peak energy, the bandwidth and the intensity of the P emission, at various detection energies. We have found that the decay time of the P emission gradually shortens with decreasing detection energy. The change in the decay time can be reasonably attributed to the photon‐like characteristics of the lower polariton branch that is the final state of the exciton‐exciton scattering. Furthermore, we have shown that the decay time of the P emission is phenomenologically determined by the group velocity of the final state although the origin of the long decay time remains to be elucidated (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Layer-by-Layer Assembly of Colloidal CdS and ZnS−CdS Quantum Dots and Improvement of Their Photoluminescence Properties

Thin films of colloidal CdS and ZnS−CdS alloy quantum dots (QDs) were fabricated by layer-by-layer (LBL) self-assembly to synthesize blue luminescent QD thin films. The assembly of negatively charged colloidal QDs and positively charged poly(diallyldimethylammonium chloride) results in QD/polymer multilayers. The relative intensity of the band-edge photoluminescence (PL) to that of the defect-related PL was decreased during the LBL assembly process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH 10, the band-edge PL intensity was recovered. The PL decay profiles before and after the dipping treatment were considerably different. From this fact, we conclude that the enhancement of the band-edge PL intensity originates from the reduction of nonradiative recombination processes at the QD surface.

Synthesis, characterization, and its PL dynamics of colloidal type II CdTe/CdSe nanocrystals

AbstractWe describe our improved synthesis and optical properties of high quality type II CdTe/CdSe nanocrystals (NCs). Specifically, clear shell‐thickness dependences have been observed in the absorption and photoluminescence (PL) spectra and PL decay profiles as well. The magnitude of the lowest absorption band decreases drastically with large redshift as the shell thickness increases. The origin will be discussed on the bases of the model where the spatial configuration of the lowest electron‐hole pair in the NCs changes from that of type I to type II as the shell thickness increases. As for the PL lifetime of the lowest electron‐hole excitations, substantial increase is observed with increasing shell thickness. This can also be understood by considering the spatial configuration; spatial overlap between electron and hole wavefunctions decreases with increasing shell thickness, thus the lifetime increases. As for the NCs with extremely thin shell (∼1 ML; 1 ML = 0.35 nm), the PL lifetime seems much longer than expected. This suggests that the thin shells seem imperfect and work rather a kind of trap sites than layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature Dependence of Photoluminescence Dynamics in Colloidal CdS Quantum Dots

We have investigated the temperature dependence of photoluminescence (PL) dynamics in CdS quantum dots (QDs) prepared by a colloidal method. A size-selective photoetching process and a surface modification technique with a Cd(OH)2 layer enabled us to prepare size-controlled CdS QDs with high PL efficiency. The PL decay profiles became slower with an increase in temperature, contrary to an ordinary behavior. We have revealed that such anomalous temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground-state and two excited states: a lower-lying bound-exciton state and a higher-lying free-exciton state (the “dark-exciton state”) having an optically inactive triplet nature.

Highly efficient preparation of size-controlled CdS quantum dots with high photoluminescence yield

We investigated the influence of methylviologen (MV 2+), which accelerates the photoetching process used for controlling size distribution, on the photoluminescence (PL) properties of colloidal CdS quantum dots (QDs). A combination of the photoetching process and surface modification with a Cd(OH) 2 layer enabled us to prepare size- and surface-controlled CdS QDs. The efficiency of the photoetching process is extremely enhanced by the addition of MV 2+. The temperature dependence of PL decay profiles indicates that MV 2+ does not affect the PL properties of QDs. Consequently, the addition of MV 2+ results in a highly efficient preparation of size-controlled CdS QDs without degradation of the PL efficiency.

Fabrication and Red-Color Lasing of Individual Highly Uniform Single-Crystal CdSe Nanobelts

Highly uniform single-crystal CdSe nanobelts were prepared via a simple thermal evaporation route. Photoluminescence (PL) from a single belt indicates that the obtained nanostructures can produce stable and ultrafine red-color lasing under pulsed light excitation. The corresponding PL decay profiles show that the PL lifetime under high excitation is sharply shortened compared to that at low excitation power, which further supports that the PL in the nanobelt cavities can realize resonance and give stimulated emission. Comparative experiments with tapered belts show that the high PL emissions are mainly attributed to the uniform configuration of these nanostructures which can serve as a very effective waveguide cavity to realize high optical gain under high excitation. This kind of CdSe nanostructures may find potential applications in red-light nanolasers.

Photoluminescence properties and energy transfer processes from excitons to Mn2+ ions in Mn2+-doped CdS quantum dots prepared by a reverse-micelle method

We investigated the photoluminescence (PL) properties of Mn2+-doped CdS (CdS:Mn2+) quantum dots (QDs) prepared by a reverse-micelle method. A PL band which originates from the intracation transition, the so-called d-d transition, of tetrahedrally coordinated Mn2+ ions, is clearly observed at ∼2.1eV, indicating that Mn2+ ions are thoroughly doped inside CdS QDs. By surface modification with a Cd(OH)2 layer, the intensity of the Mn2+-related PL band is increased approximately ten times compared with that without modification, and strong enhancement of the band-edge PL band is observed. The decay profile of the band-edge PL in surface-modified CdS:Mn2+ QDs is much faster than that in undoped CdS QDs; this shortened decay time originates from the effective energy transfer process from the excitons of host QDs to Mn2+ ions. The PL decay profiles are quantitatively explained on the basis of a three-level model including both the dark-exciton state and the energy transfer process.

Formation of Trititanate Nanotubes by Non‐Hydrothermal Methods: Optical Properties and Surface‐Exciton Dynamics Studied by Photoluminescence Spectroscopy

AbstractTrititanate (H2Ti3O7) nanotubes were prepared by a non‐hydrothermal digestion of TiO(OH)2 solution with 5 M NaOH, and their optical properties and exciton dynamics were studied by using steady‐state and time‐resolved photoluminescence (PL) spectroscopy. Based on TEM, AFM and XRD measurements, the tubular structure was identified to be the same as that of trititanate crystalline multi‐walled scroll nanotubes prepared hydrothermally, which has intershell d‐spacing of 0.79 nm with inner diameters of 5 nm and the lengths of several tens of nanometers. The UV‐absorption spectrum of trititanate nanotubes showed weak surface‐state absorption at 425 nm as well as the absorption maximum band around 350 nm with onset energy 3.03 eV corresponding to the indirect band gap energy. The steady‐state PL spectra of the trititanate nanotubes showed a band‐edge emission band around 360 nm as well as a broad surface emission band originating from widely dispersed surface‐state energy levels (2.84, 2.65, 2.41 2.21 2.03 eV), exhibiting higher relative intensity ratio of the band‐edge emission to the surface emission as compared to that of TiO2 nanoparticles. These results with PL excitation spectra indicate that the density of the surface states was found to be higher in the nanotubes than in TiO2 nanoparticles. The PL decay profiles were measured by using the time correlated single photon counting system (TCSPC) adopting fs‐Ti‐Sappire laser at 350 nm, and they were analyzed to fit a tri‐exponential equation. The decay times depend on monitoring emission wavelength, and at least four different decay time components (˜28 ps, ˜70 ps, ˜700 ps and ˜4 ns) were resolved, implying the existence of charge‐carriers trapping surface states with different energy levels. The two longer decay times are attributed to the deep‐trap surface states of trititanate nanotubes, which are much longer than those of TiO2 nanoparticles. These results suggest that the nonradiative route to the surface excitons recombination is more dominant in trititanate nanotubes than in titania nanoparticles because of strong coupling of excitons wave functions with lattice phonons.

Subpicosecond time-resolved photoluminescence of thioglycerol-capped CdS nanoparticles in water

Subpicosecond time-resolved photoluminescence (STRPL) of aqueous colloidal solutions of thioglycerol-capped CdS nanoparticles (TG-CdS) was successfully measured by fluorescence up-conversion techniques. In their STRPL obtained within the time range of 30 ps after the excitation, the intensity of photoluminescence (PL) around 500 nm at which their static PL is not noticeable is considerably outstanding compared with that at a longer wavelength region at which PL is attributed to deep trap sites in CdS nanoparticles. From the analysis of the PL decay profile at the wavelength region of 490–550 nm at which PL is mainly attributed to near band edge emission, we observed two fast PL decay components of ca. 1 ps and ca. 10 ps, similar to those already reported for the band edge emission of CdS nanoparticles in a glass matrix. In the present study, former decay component was related to trapping rate of photogenerated holes, following the case already reported for CdS nanoparticles embeded in a glass matrix. However, the origin of the latter PL decay component is not clear at present. It is necessary to study in further detail to elucidate it.

Phosphorescence Quenching of Dyes Adsorbed to Silica Thin-Layer Chromatography Plates

We describe photoluminescence (PL) quenching experiments by oxygen for a series of transition metal dyes adsorbed to commercial silica thin-layer chromatography plates. These TLC plates have been used by others as rapidly responding PL pressure sensors. The quenching kinetics show interesting differences from the behavior of the same dyes adsorbed to the more well-defined surfaces of SBA-15 mesoporous silica particles adsorbed to a thin layer-by-layer polymer film as reported in Chem. Mater. 2005, 17, 3160. The pore size and the pore size distribution are much larger for the TLC silica than for the mesoporous silica. On the TLC silica, the dye PL decay profiles show smaller deviations from an exponential form. One sees larger differences between the intensity and lifetime Stern-Volmer plots, and most surprisingly, the dyes on the surface of the TLC silica are about three times more sensitive to quenching by oxygen.

Temperature-dependent photoluminescence in meso-porous MCM nanotubes

Temperature-dependent photoluminescence (PL) was exploited to investigate the mechanism of luminescence of MCM (Mobil Composition of Matter)-41 and MCM-48 nanotubes. The PL intensity has a maximum around 40 K. Localization of the carriers involved in the radiative recombination was deduced from the PL decay profiles at various energies. A model based on competition between radiative recombination of localized carriers and nonradiative recombination is suggested to explain the temperature-dependence of PL intensity. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Nano-size Indium Cluster in InGaN/GaN Multiple QuantumWells with High Indium Composition

AbstractWe report the effect of strain-induced indium clustering on the emission properties of InGaN/GaN multiple quantum wells grown with high indium composition by MOCVD. Nanosize indium clustering confirmed by high-resolution transmission electron microscopy results in the redshift of the emission peak and the increase of the integrated photoluminescence (PL) intensity. We found that strong carrier localization in indium clustering induces the increases of the activation energy of PL integrated intensity and the temperature independence of PL decay profiles. All these observations suggest structurally and optically that the improved emission properties in the InGaN/GaN multiple quantum well with high indium composition are associated with the localized states in the nano-size indium cluster.

Influence of strain-induced indium clustering on characteristics of InGaN/GaN multiple quantum wells with high indium composition

We report the effect of strain-induced indium clustering on the emission properties of InGaN/GaN multiple quantum wells grown with high indium composition by metalorganic chemical vapor deposition. Indium clustering confirmed by high-resolution transmission electron microscopy results in the redshift of the emission peak and the increase of the integrated photoluminescence (PL) intensity. We found that strong carrier localization in indium clustering induces the increases of the activation energy of PL integrated intensity, the temperature independence of PL decay profiles, and the intensity fluctuation of the cathodoluminescence images. All these observations suggest structurally and optically that the improved emission properties in the InGaN/GaN multiple quantum well with high indium composition are associated with the localized states in the strain-induced indium cluster.

Investigation of the Charge Complex Formation of Novel Silicon-Based Copolymers Containing Carbazole Unit by Time-Resoved Laser Spectroscopic Technique in the Electronic Excited State

SiPhPVK has unusual photophysical characteristics that are attributed to the formation of charge transfer complex in the excited state. The photoluminescence (PL) and excitation spectra for SiPhPVK at emission wavelengths of 440 and 520 nm were compared with those for SiHMPVK. Two excitation spectra of SiPhPVK monitored at two probes were observed different obviously. However, the excitation spectra of SiHMPVK obtained at two different probes showed the same spectral feature. In addition, upon photoexcitation with 300 nm, the PL life time of SiPhPVK became longer with increasing the probe wavelength. Experimental data suggest that the electron transfer can occur from carbarzole to SiPh unit of SiPhPVK to generate the stabilized excited state. However, the PL decay profiles of SiHMPVK exhibited almost same at all probes with photoexcitation of 325 nm. Therefore, the PL emission of SiHMPVK at 325 mainly results from carbazole segments.