Time-resolved Photoluminescence Techniques Research Articles

Spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanocrystals

The spin dynamics in chemically synthesized CdSe/CdS core/shell nanocrystals (NCs) are studied by polarization- and time-resolved photoluminescence (PL) techniques in high magnetic fields and at low temperatures. Analysis of the recombination dynamics shows that the emission of thin-shell NCs is contributed by neutral excitons, while for thick-shell NCs it is dominated by charged excitons (trions). The sign of the PL polarization unambiguously demonstrates that these trions are negatively charged. A theoretical model of the PL polarization in an ensemble of randomly oriented NCs describes well magnetic field and time dependences of the PL polarization degree and allows us to determine the hole $g$ factor in CdSe/CdS NCs, ${g}_{h}=\ensuremath{-}0.54$. From direct measurements of the spin relaxation rate dependences on magnetic field and temperature, we identify the mechanism of the negative trion spin relaxation as two-phonon-assisted Raman scattering between the hole spin sublevels mixed by the applied magnetic field.

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550 \(^{\circ }\mathrm{C}\)). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.

Synthesis of blue light-emitting graphene quantum dots and their application in flexible nonvolatile memory

We presented a facile method to prepare graphene quantum dots (GQDs) from double-walled carbon nanotube with blue light emission in a chlorobenzene solution, which enabled the preparation of GQD–polymer hybrid nanocomposite. The wavelength-dependent fluorescent lifetime of the GQDs was investigated by using time-resolved photoluminescence technique. Significantly, nonvolatile rewritable memory effect was observed for the GQD-based nanocomposite, suggesting the promising applications of GQDs in data storage. Moreover, due to the easy solution process, we demonstrated the design and realization of flexible GQD-based memory device. This work may expand the application of GQDs to the portable electronic devices.

Hyper-Branched Phosphorescent Conjugated Polyelectrolytes for Time-Resolved Heparin Sensing

A series of hyper-branched cationic conjugated polyelectrolytes containing different contents of phosphorescent Ir(III) complex has been designed and synthesized successfully. Their photophysical properties in both aqueous solution and solid film are investigated and their morphologies in aqueous solution are observed by TEM. Nanoparticles with the size of 80-100 nm have been formed in aqueous solution through the self-assembly of polymers. An energy transfer process from host polyfluorene to guest Ir(III) complex exists and becomes more efficient in the solid films. Importantly, this series of hyper-branched polymers can be applied as light-up heparin probes with good selectivity, high sensitivity, and naked-eye detection through electrostatic interaction between polymers and heparin. Quantification for heparin in aqueous solution can be realized in the range of 0-44 μM in the buffer solution. Detection limit can reach as low as 50 nM. More meaningfully, time-resolved photoluminescence technique is utilized successfully in the heparin sensing to eliminate the background fluorescence interference effectively and enhance the sensing sensitivity in the complicated media.

Carrier dynamics in modulation-doped InAs/GaAs quantum rings

Carrier dynamics in undoped and modulation-doped self-assembled InAs/GaAs quantum rings was comprehensively examined through time-resolved photoluminescence techniques at room temperature. After photoexcitation, the rates of carrier capture and relaxation in the ground state and excited states were equally fast in both the undoped and charged quantum rings. Instead of cascading down through the excited states, the carriers excited in the undoped quantum rings relaxed through the quasi-continuum states via coupling with acoustical phonons and eventually thermalized to the ground state by emitting longitudinal optical phonons. The dynamics of carrier thermalization was governed by cold carriers in the charged quantum rings. In carrier recombination, the undoped quantum rings had a shorter PL decay time and lower luminescence intensity than the charged quantum rings because radiative recombination was suppressed by defects in the matrix and the absence of cold carriers.

A water-soluble phosphorescent polymer for time-resolved assay and bioimaging of cysteine/homocysteine.

A water-soluble phosphorescent bioprobe was successfully developed by introducing an iridium(iii) complex as a phosphorescent signaling unit with poly(N-isopropylacrylamide) (PNIPAM) as the stimuli-responsive backbone. The probe was used for the effective detection of cysteine (Cys)/homocysteine (Hcy) and temperature based on changes in the phosphorescence signal. The design principle was based on the fact that the aldehyde groups in the cyclometalated ligands of the iridium(iii) complex moiety can react with the β- or γ-aminothiol group to form thiazolidine or thiazinane, respectively, resulting in a phosphorescence change in the iridium(iii) complex, thereby facilitating the detection of Cys and Hcy. Moreover, a phosphorescent hydrogel based on this probe was formed upon cross-linking and was then used as a quasi-solid sensing system for detecting Cys and Hcy. Furthermore, by using a time-resolved photoluminescence technique, the probe can detect Hcy in the presence of intense background fluorescence. In addition, phase changes in temperature-responsive PNIPAM can result in a switch of microenvironment between hydrophilicity and hydrophobicity, to which the phosphorescent emission of the iridium(iii) complex is very sensitive. This bioprobe integrates water solubility, biocompatibility, and sensing capability into one system, which is advantageous for biological applications. Further investigation of the application of the bioprobe for living-cell imaging confirmed that the probe is membrane permeable and is capable of detecting Cys in living cells with notable phosphorescence enhancement. Fluorescence lifetime imaging microscopy is successfully applied for sensing and bioimaging of intracellular Cys in the presence of short-lived background fluorescence.

Bimodal luminescence behavior of spatially-ordered seven-stacked InAs/InAlGaAs quantum dots

Seven-stacked InAs/InAlGaAs quantum dots (QDs) grown on InP substrates have been studied using photoluminescence (PL) and time-resolved PL techniques, and the results show a bimodal luminescence behavior. The seven-stacked QD sample shows a redshifted PL emission energy, a broad PL linewidth, and a strong temperature dependent PL compared with the single layer QD sample. The decrease of the PL decay time with increasing emission wavelength has been observed in the seven-stacked sample, but not in the single layer sample. All of these results are attributed to the bimodal size distribution of QDs in the seven-stacked sample, which originates from different sizes of spatially ordered QDs formed on different layers.

Fluorescence Dynamics in BSA-Protected Au25Nanoclusters

Fluorescent Au25 nanoclusters recently have drawn considerable research interest due to their unique properties and potential applications. Despite significant advances in their synthesis methods and application development, the origin of the fluorescence and underlying mechanism still remain unclear. In this work we investigate the fluorescence dynamics in BSA-protected Au25 nanoclusters by time-resolved photoluminescence and transient absorption techniques covering picosecond to microsecond time scales. We demonstrate here that the red fluorescence consists of both prompt fluorescence and thermally activated delayed fluorescence, and the latter is more dominant. A small energy gap of 30 meV between the triplet and the singlet states was determined from our temperature-dependent time-resolved fluorescence measurement. Moreover, we elucidate that the absorption band at 2.34 eV corresponds to the HOMO–LUMO transition in this system due to the interaction between Au25 NCs and BSA. We also show that an effective relaxation pathway exists from the higher excited state to the LUMO.

Photophysics and Dynamics of Surface Plasmon Polaritons-Mediated Energy Transfer in the Presence of an Applied Electric Field

The possibility to transfer energy between molecular excitons across a metal film up to 150 nm thick represents a very attractive solution to control and improve the performances of thin optoeletronic devices. This process involves the presence of coupled surface plasmon polaritons (SPPs) at the two dielectric-metal interfaces, capable of mediating the interactions between donor and acceptor, located on opposite sides of the metal film. In this Article, the photophysics and the dynamics of an efficient SPP-mediated energy transfer between a suitable dye and a conjugated polymer is characterized by means of steady-state and time-resolved photoluminescence techniques. The process is studied in model multilayer structures (donor/metal/acceptor) as well as in electrically pumped heterostructures (donor/metal cathode/acceptor/anode), to verify the effects of applied electric fields on the efficiency and the dynamics of SPP-mediated energy transfer. A striking enhancement of the overall luminescence was recorded in a particular range of applied bias, suggesting the presence of cooperative effects between optical and electrical stimulations.

Narrow band gap (1 eV) InGaAsSbN solar cells grown by metalorganic vapor phase epitaxy

Heterojunction solar cell structures employing InGaAsSbN (Eg ∼ 1 eV) base regions are grown lattice-matched to GaAs substrates using metalorganic vapor phase epitaxy. Room temperature (RT) photoluminescence (PL) measurements indicate a peak spectral emission at 1.04 eV and carrier lifetimes of 471–576 ps are measured at RT from these structures using time-resolved PL techniques. Fabricated devices without anti-reflection coating demonstrate a peak efficiency of 4.58% under AM1.5 direct illumination. Solar cells with a 250 nm-thick InGaAsSbN base layer exhibit a 17% improvement in open circuit voltage (Voc), 14% improvement in fill factor, and 12% improvement in efficiency over the cells with a thicker (500 nm-thick) base layer.

2-(2,3,4,5,6-Pentafluorophenyl)-1H-benzo[d]imidazole, a fluorine-rich building block for the preparation of conjugated polymer donors for organic solar cell applications

We have introduced the pentafluorophenyl substituted benzimidazole as a building block of conjugated polymers for optoelectronic applications. We present the synthesis of a copolymer bearing fluorene as the comonomer. A detailed characterization of the novel material including structural, electrochemical and optical (using absorption, emission and time-resolved photoluminescence techniques) properties as well as computational modeling is described. The polymer exhibits highly efficient photoluminescence quenching when blended with fullerene derivatives (PCBM). The experimental results are compared with the alternating fluorene copolymer (APFO-3) in order to identify structure and property relations with the novel synthesized conjugated polymer. Early photovoltaic performance data are presented and compared with the well established APFO-3 : PCBM material system.

Charge Transfer in the Heterointerfaces of CdS/CdSe Cosensitized TiO2 Photoelectrode

One of the key issues affecting the performance of solar cells is the behavior of carrier transfer. In this work, the time-resolved photoluminescence (TRPL) technique was utilized to investigate the electron transfer at the CdS/CdSe, TiO2/CdS, and TiO2/CdSe heterointerfaces. By varying the excitation wavelengths, photoluminescence lifetimes of CdSe and CdS in TiO2/CdSe, TiO2/CdS, TiO2/CdS/CdSe, and TiO2/CdSe/CdS photoelectrodes were measured. The results show that, for the single sensitizer electrodes (TiO2/CdS, TiO2/CdSe), the average PL lifetime of CdS (0.69 ns) is shorter than CdSe (0.99 ns), suggesting that CdS has higher electron transfer rate toward TiO2 compared with CdSe. For the TiO2/CdSe/CdS electrode, the PL lifetime of CdSe exhibits an excitation-wavelength-dependent behavior. A shorter excitation wavelength leads to a longer PL lifetime of CdSe. This additional long lifetime is ascribed to the rapid carrier transfer from the photoexcited carriers in CdS layer into the CdSe layer. On the contrary, the PL lifetime of CdSe is independent of the excitation wavelength in the TiO2/CdS/CdSe electrode, indicating that the excited electrons in the CdS layer did not inject into the CdSe layer. This observation confirms that the charge transfer from the cosensitizers toward the TiO2 is much more efficient in the TiO2/CdS/CdSe electrode rather than in the TiO2/CdSe/CdS electrode.

Charge transfer and retention in directly coupled Au-CdSe nanohybrids

The energy and charge transfer dynamics of directly coupled Au-CdSe hybrid nanocrystals have been studied using time-resolved photoluminescence (PL) techniques. The PL of such nanohybrids was found to be quenched dramatically compared to that of both CdSe quantum dots and mixtures of CdSe quantum dots with Au nanoparticles. Fluorescence decay curves of the Au-CdSe nanohybrids show three distinct decay channels with the fastest one associated with the transfer of electrons from the CdSe portion to the Au portion. The holes on the CdSe portion created by such charge transfer were then quickly taken away by the solution, while the electrons on the Au portion slowly leaked into the solution as well, thus serving as a reductant for redox reactions. Using a model reaction based on the reduction of methylene blue by the leaking electrons, our photocatalytic experiments indicate that the electrons can be temporarily retained in the Au portion (most likely at the Au-capping agent interface) for a dramatically long timescale, up to 100 min. Finally, by merging all of the observations in the time-resolved PL measurements, we were able to figure out a relatively complete picture of charge transfer and retention in the Au-CdSe nanohybrids. This picture is expected to guide researchers in designing modern photocatalysts and solar cells constructed from nanoscale metal-semiconductor hybrids.

High current-induced degradation of AlGaN ultraviolet light emitting diodes

Degradation under high current stress of AlGaN quantum well based light emitting diodes emitting at 285 and 310 nm has been studied using electroluminescence, time-resolved photoluminescence and current-voltage experimental techniques. The measurements have revealed that during aging decrease of the emission intensity is accompanied by increase of the tunneling current, increase of the nitrogen vacancy concentration and partial compensation of the p-doping. The main role in the device degradation has been ascribed to formation of tunneling conductivity channels, probably, via activation of the closed core screw dislocations with the help of nitrogen vacancies. Carrier lifetimes in the quantum wells and the p-cladding were found to be unaffected by the aging process, suggesting that the nonradiative recombination has a lesser influence on the device degradation.

Ultra-fast photoluminescence in fused silica surface flaws susceptible to laser damage

ABSTRACTUsing high-sensitivity confocal time-resolved photoluminescence (PL) techniques, we found an ultrafast PL (40 ps-5 ns) from impurity-free surface flaws on fused silica. This PL is excited by the single-photon absorption of sub-band gap light. Regions which exhibit this PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5 eV nanosecond-scale laser pulses. Very high defect densities are needed to explain the damage thresholds observed. For such high defect densities, significant interactions between defects may strongly affect the temporal characteristics of the emission of electronic excitations. We propose that the distribution in lifetimes observed is not simply due to a large variety of defect states, but due to a variety of energy transfer interactions between defect states.

Near infrared broadband emission of In0.35Ga0.65As quantum dots on high index GaAs surfaces

The morphology and optical properties of In(0.35)Ga(0.65)As/GaAs quantum dots (QDs) grown on (210), (311)A, (711)A, (731) and (100) substrates are investigated. QDs formed on (210) and (731) oriented substrates are grown by molecular beam epitaxy. Regular QDs are observed on (100), (311)A, and (711)A. Randomly distributed QDs and comet-shaped QDs form on (210) and (731) substrates, respectively. A high density of QDs on the order of 10(11) cm(-2) are obtained from (711)A. The optical measurement shows a spectrum linewidth (FWHM = 74.3 nm) of QDs on GaAs (210) three times wider than GaAs (100) substrate. Long exciton decay times, over 1 ns, are also measured by time-resolved photoluminescence technique for all samples. Our results demonstrate the potential for QDs on GaAs high index substrates for wideband applications.

Optical properties of light-hole excitons in GaN epilayers

Optical properties of light-hole free exciton (FXB) in GaN epilayers were investigated by using near-resonance photoluminescence (PL) and time-resolved PL techniques. In contrast to the case of off-resonance PL where only heavy-hole free excitons (FXA) have strong response, FXB band can be well resolved in the near-resonance PL spectra. The variable-temperature near-resonance PL spectra show that the linewidth of FXB broadens faster than the FXA with increasing temperature. Moreover, the luminescence lifetime of FXB is found to be shorter than that of FXA.

Ultrafast electrical charging and discharging of a single InGaAs quantum dot

We report on ultrafast control of the charge state of a single InGaAs quantum dot in a charge-tunable p-i-n diode structure. Focused ion beam etching is employed to decrease the capacitance of the device to enable radio frequency operation. A time-resolved photoluminescence technique is demonstrated that allows monitoring the charge state with a time-resolution which is limited only by the radiative lifetime of the charged and neutral exciton, respectively. Experimental data show that the charge state can be manipulated on time scales shorter than the radiative lifetime of approximately 1.4 ns.

Dynamical control of Mn spin-system cooling by photogenerated carriers in a (Zn,Mn)Se/BeTe heterostructure

The magnetization dynamics of the Mn spin system in an undoped (Zn,Mn)Se/BeTe type-II quantum well was studied by a time-resolved pump-probe photoluminescence technique. The Mn spin temperature was evaluated from the giant Zeeman shift of the exciton line in an external magnetic field of 3 T. The relaxation dynamics of the Mn spin temperature to the equilibrium temperature of the phonon bath after the pump-laser-pulse heating can be accelerated by the presence of free electrons. These electrons, generated by a control laser pulse, mediate the spin and energy transfer from the Mn spin system to the lattice and bypass the relatively slow direct spin-lattice relaxation of the Mn ions.

Temperature-sensing behavior of Eu:BaB 4O 7 nanocrystals

Nanoscale Eu:BaB 4O 7 crystals were synthesized in a barium borate glass host, and the temperature-sensing behavior of these crystals was demonstrated over the temperature range of 10–673 K using fluorescence intensity ratio and time-resolved photoluminescence techniques. It was concluded that temperature measurement based on rise time variation is sensitive in the low temperature region (<250 K), while decay time variation is found to be sensitive in the high temperature region (>250 K).