Time-resolved Photoluminescence Characterization Research Articles

Construction of 2D/2D S-scheme Bi2MoO6/Zn-TCPP heterojunction via in-situ self-assembly growth strategy to enhance interface effect for efficient photocatalytic hydrogen production

Two-dimensional/two-dimensional (2D/2D) heterojunctions are considered to be an effective strategy for forming strong interface effects and facilitating photogenerated carrier separation. However, it is usually limited by the size mismatch of the materials, even at the expense of its redox capability. Herein, 2D/2D S-scheme heterojunction photocatalyst Bi2MoO6/Zn-TCPP (BMO/ZTP) composed of 2D Bi2MoO6 and 2D Zn-TCPP (TCPP: tetrakis (4-carboxyphenyl) porphyrin) (MOFs) was constructed by in-situ self-assembly growth strategy. The size-compatible 2D/2D composites had abundant surface active sites and strong interactions. In addition, band bending and interfacial electric field (IEF) effect based on S-scheme heterojunction could accelerate the separation and migration of photogenerated carriers in BMO/ZTP. The best hydrogen precipitation rate of the BMO/ZTP was 10900.94 umol·g−1·h−1, which was 38.90 and 3.24 times higher than that of Bi2MoO6 (280.26 umol·g−1·h−1) and Zn-TCPP (3360.34 umol·g−1·h−1), respectively. The results indicated that 2D/2D BMO/ZTP S-scheme heterojunction could enhance the interface effect and retain strong reducing electrons to achieve efficient photocatalytic hydrogen production, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), Tafel curve, electron spin resonance (ESR) and time-resolved photoluminescence (TRPL) characterization and density functional theory (DFT) calculations. This work provided a general strategy for constructing 2D Bi2MoO6 and 2D MOFs S-scheme heterojunctions to enhance interface effects for achieving efficient photocatalytic hydrogen production.

In Situ Fabrication of a 2D/2D MXene/CN Heterojunction for Photothermally Assisted Photocatalytic Sterilization under Visible Light Irradiation.

Constructing an efficient visible light-responsive antibacterial material for water treatment remains a principal goal yet is a huge challenge. Herein, a 2D/2D heterojunction composite with robust interfacial contact, named MXene/CN (MCN), was controllably fabricated by using a urea molecule intercalated into MXene following an in situ calcination method, which can realize the rapid separation and migration of photogenerated carriers under visible light irradiation and significantly improve the carrier concentration of the MXene surface, thus generating more reactive oxygen species. The generation of heat induced by MXene could also increase photogenic electron activity to facilitate the photocatalytic reaction using in situ time-resolved photoluminescence characterization. The visible light-activated germicide exhibits a sterilization efficacy against Escherichia coli of 99.70%, higher than those of pure CN (60.21%) and MXene (31.75%), due to the effect of photothermally assisted photocatalytic treatment. This work is an attempt to construct a visible light-driven antimicrobial material using Schottky junctions achieving photothermally assisted photocatalytic disinfection.

Improvements of the modulation bandwidth and data rate of green-emitting CsPbBr3 perovskite quantum dots for Gbps visible light communication.

CsPbBr3 perovskite quantum dots (PQDs) as promising color conversion materials have been widely used in display and visible light communication (VLC), but most CsPbBr3 PQDs for VLC are randomly selected without optimization. Thereby the exploration of fundamental experimental parameters of QDs is essential to better employ their performance advantages. Herein, we investigated the concentration and solvent effects on photoluminescence (PL) properties and communication performance of CsPbBr3 PQDs. The PL, time-resolved PL characterization and communication measurements of CsPbBr3 PQDs all exhibit concentration dependence, suggesting that there exists an optimum concentration to take advantages of performance merits. CsPbBr3 PQDs with a concentration of 0.5 mg/ml show the shortest carrier lifetime and achieve the highest -3 dB bandwidth (168.03 MHz) as well as the highest data rate (660 Mbps) comparing to other concentrations. And in terms of the optimal concentration, we further explored the approach to improve the communication performance, investigating the effect of polarity solvent on the communication performance of CsPbBr3 PQDs. Original 0.5 mg/ml CsPbBr3 PQDs (1 ml) with 55 μL ethanol added in obtain a higher -3 dB bandwidth of 363.68 MHz improved by ∼116.4% and a record data rate of 1.25 Gbps improved by ∼89.4% but weaker PL emission due to energy transfer. The energy transfer assisted improvement may open up a promising avenue to improve the communication performance of QDs, but more feasible energy transfer path needs to be explored to ensure the stability of QDs.

Time-resolved photoluminescence characterization of InGaAs/GaAs nano-ridges monolithically grown on 300 mm Si substrates

The monolithic growth of III–V materials directly on Si substrates provides a promising integration approach for passive and active silicon photonic integrated circuits but still faces great challenges in crystal quality due to misfit defect formation. Nano-ridge engineering is a new approach that enables the integration of III–V based devices on trench-patterned Si substrates with very high crystal quality. Using selective area growth, the III–V material is deposited into narrow trenches to reduce the dislocation defect density by aspect ratio trapping. The growth is continued out of the trench pattern and a box-shaped III–V nano-ridge is engineered by adjusting the growth parameters. A flat (001) GaAs nano-ridge surface enables the epitaxial integration of a common InGaAs/GaAs multi-quantum-well (MQW) structure as an optical gain medium to build a laser diode. In this study, a clear correlation is found between the photoluminescence (PL) lifetime, extracted from time-resolved photoluminescence (TRPL) measurements, with the InGaAs/GaAs nano-ridge size and defect density, which are both predefined by the nano-ridge related pattern trench width. Through the addition of an InGaP passivation layer, a MQW PL lifetime of up to 800 ps and 1000 ps is measured when pumped at 900 nm (only QWs were excited) and 800 nm (QWs + barrier excited), respectively. The addition of a bottom carrier blocking layer further increases this lifetime to ∼2.5ns (pumped at 800 nm), which clearly demonstrates the high crystal quality of the nano-ridge material. These TRPL measurements not only deliver quick and valuable feedback about the III–V material quality but also provide an important understanding for the heterostructure design and carrier confinement of the nano-ridge laser diode.

Electrochemically controlled CdS@CdSe nanoparticles on ITO@TiO2 dual core–shell nanowires for enhanced photoelectrochemical hydrogen production

Here, we report a novel dual heterostructured photoanode consisting of CdS@CdSe core–shell nanoparticles (NPs) and 1D-structure tin-doped indium oxide (ITO)@TiO2 core–shell nanowires (ITO@TiO2@CdS@CdSe) for highly efficient photoelectrochemical (PEC) hydrogen production. The finely controlled hierarchical core–shell CdS@CdSe sensitization from consecutive electrochemical deposition on the ITO@TiO2 core–shell nanowire has synergistic effects of visible-light utilization and efficient charge transport on the PEC response. The rationally designed dual core–shell heterostructure leads to cascade charge migration throughout the aligned energy band edges with rapid charge extraction through the hierarchical heterostructure of ITO@TiO2@CdS@CdSe, alleviating the crucial charge accumulation. As a result, the dual heterostructured photoanode exhibits a maximum photocurrent density of 20.11 mA/cm2 at 1.23 V vs. the reversible hydrogen electrode (RHE) and a dramatic enhancement in the incident photon-to-current efficiency (IPCE) over the extended absorption spectrum. The time-resolved photoluminescence (TRPL) characterization indicates the realized multiple-band cascade charge migration throughout ITO@TiO2@CdS@CdSe could promote an 8-fold increase in the charge separation efficiency. This rational design of dual-heterojunction-structured photoelectrodes via electrochemical deposition provides a demonstration of modifying conventional light-harvesting photoelectrodes with stagnate solar energy conversion and PEC hydrogen production.

Synthesis and Optical Characterization of Carbon Dot from Peels of Dragon Fruit and Pear

Fluorescence materials such as carbon dots are unique and non-toxic materials that can be produced using many methods. In this research, synthesis and optical characterization of carbon dots from the peels of dragon fruit and pear have been done. The synthesis uses bottom-up method using microwave with 20 minutes, 40 minutes, and 80 minutes of heating time. The result of optical characterizations showed that the samples that have been heat treated are carbon dots based of the absorbance in the range of 280-350 nm, and the intensity are decreased in the range of visible and infrared wavelength. There are emission and intensity changes by different heat treatments. From the samples of dragon fruit's peels, the emission wavelength tended to shift toward visible light at the wavelength of 551.51 nm, 507.98 nm and 487.28 nm with the excitation at the wavelength of 420 nm from pulsed pico-laser. While, the emission wavelength from the samples of pears peels were fluctuate in wavelength of 529.09 nm, 507.52 nm and 519.46 nm. Similarly, the time-resolved photoluminescence characterization test showed that there were time changes in each sample of carbon dot. The decay lifetime of carbon dots was fluctuated between 4.5 ns - 5.4 ns for dragon fruit’s peels and about 1.9 ns - 2.4 ns for pear's peels.

Van der Waals epitaxial ZnTe thin film on single-crystalline graphene

Graphene template has long been promoted as a promising host to support van der Waals flexible electronics. However, van der Waals epitaxial growth of conventional semiconductors in planar thin film form on transferred graphene sheets is challenging because the nucleation rate of film species on graphene is significantly low due to the passive surface of graphene. In this work, we demonstrate the epitaxy of zinc-blende ZnTe thin film on single-crystalline graphene supported by an amorphous glass substrate. Given the amorphous nature and no obvious remote epitaxy effect of the glass substrate, this study clearly proves the van der Waals epitaxy of a 3D semiconductor thin film on graphene. X-ray pole figure analysis reveals the existence of two ZnTe epitaxial orientational domains on graphene, a strong X-ray intensity observed from the ZnTe [1¯1¯2] ǁ graphene [10] orientation domain, and a weaker intensity from the ZnTe [1¯1¯2] ǁ graphene [11] orientation domain. Furthermore, this study systematically investigates the optoelectronic properties of this epitaxial ZnTe film on graphene using temperature-dependent Raman spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, and fabrication and characterization of a ZnTe-graphene photodetector. The research suggests an effective approach towards graphene-templated flexible electronics.

Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate

Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.

Time-resolved photoluminescence characterization of oxygen-related defect centers in AlN

Time-resolved photoluminescence (PL) spectroscopy has been employed to investigate the emission characteristics of oxygen-related defects in AlN in the temperature region from 77 to 500 K. Two PL components with different decay constants are observed in the near-ultraviolet to visible regions. One is the PL component with decay time of <10 ns and its peak position shifts to longer wavelengths from ∼350 to ∼500 nm with increasing temperature up to 500 K. This PL component is attributed to the radiative relaxation of photoexcited electrons from the band-edge states to the ground state of the oxygen-related emission centers. In the time region from tens to hundreds of nanoseconds, the second PL component emerges in the wavelength region from 300 to 400 nm. The spectral shape and the decay profiles are hardly dependent on temperature. This temperature-independent PL component most likely results from the transfer of photoexcited electrons from the band-edge states to the localized excited state of the oxygen-related emission centers. These results provide a detailed insight into the radiative relaxation processes of the oxygen-related defect centers in AlN immediately after the photoexcitation process.

Photoluminescence of sol–gel-prepared hafnia

We employed a sol–gel route followed by a thermal treatment (up to 1000 °C) to prepare crystalline (monoclinic) hafnium dioxide. Thorough steady-state and time-resolved photoluminescence characterization of the material within the temperature range of 10–300 K was conducted by using various excitation sources. The most prominent spectroscopic feature of the material was an intense broad emission band centered at 2.3–2.5 eV with an associated excitation band centered at 4.2–4.4 eV (well below the bandgap of monoclinic hafnia). The emission was characterized by an essentially non-exponential, thermally stimulated decay and exposed a marked blue shift with the increase of temperature from 10 to 300 K. Relation of the emission to the intrinsic defects of hafnia is discussed.

Anatase-to-rutile phase transition of samarium-doped [formula omitted] powder detected via the luminescence of [formula omitted

We employed a sol-gel route to prepare 1% samarium-doped TiO2 nanopowders. Time-resolved photoluminescence (PL) and Raman characterization was performed. After a thermal treatment the powder crystallized in an anatase phase and revealed intense Sm3+ photoluminescence. The emission spectrum of Sm3+ exposed well-resolved crystal-field splitting enabling monitoring of the changes in the local environment.We thoroughly investigated the influence of the annealing treatment (in air) on Sm emission intensity. Annealing up to 800 ∘C led to a systematic enhancement of Sm emission. Annealing at higher temperatures, however, led to a marked weakening of Sm3+ emission and simultaneous appearance of an emission band near 830 nm. Annealing temperatures as high as 1000 ∘C were needed to induce the phase transition from anatase to rutile. It was possible to use Sm3+ as a structural probe revealinge peculiarities of the phase transition.

Low-threshold two-photon pumped ZnO nanowire lasers

We report in this communication the two-photon absorption (TPA)-induced room-temperature lasing performance of ZnO nanowires. Under femtosecond pulse-excitation at lambda = 700 nm in the infrared regime, a remarkably low threshold of 160 microJ/cm(2) was observed for the TPA-induced lasing action, which is of the same order of magnitude as that measured for the linear lasing process. Time-resolved photoluminescence characterization of two-photon pumped ZnO nanowires reveals the presence of a fast decay (3-4 ps) in the stimulated emission as compared to the slow decay (50-70 ps) for the spontaneous emission. The TPA process in ZnO nanowires was characterized with the nonlinear transmission measurement, which uncovers an enhanced TPA coefficient, about 14.7 times larger than that of bulk ZnO samples. The observed TPA enhancement in ZnO nanowires accounts for the low threshold lasing behavior, and has been attributed to the intensified optical field confined within the nanowire waveguides.

Homoepitaxial CVD diamond: Raman and time-resolved PL characterization

In this work, we report on the structural characterization of homoepitaxial Microwave Plasma Enhanced CVD diamond grown onto Ib diamond substrates by varying systematically the methane to hydrogen ratio in the gas mixture (1–7% CH 4). X-ray diffraction, Raman spectroscopy and photoluminescence (PL) have been used to characterize the diamond samples. Raman measurements pointed out the excellent crystalline quality and phase purity of the specimens. PL measurements in the 1.7–2.7 eV energy range have shown completely flat spectra, excluding the presence of nitrogen-related optical centers. Such results show that the homoepitaxial CVD diamond can be grown, at moderate microwave power (720 W), and at growth rates not too low (∼ 1 μm/h) preserving a good quality. Moreover, the homoepitaxial crystals exhibited a strong free-exciton recombination radiation at room temperature even at the highest methane concentration used (7%). Preliminary measurements of the lifetime of the free exciton at room temperature have been also performed. The excitation was produced by a 5 ns pulsed laser irradiation at energies above the diamond band gap. The results have been compared with the structural properties of the samples and correlated with the growth conditions.

Visible luminescence from octadecylsilane monolayers on silica surfaces: Time-resolved photoluminescence characterization

We have found that the adsorption of octadecyltrichlorosilane (OTS) monolayers on nanometer-sized silica particles yields a broad blue photoluminescence (PL) with a time scale of nanoseconds. The observed PL intensity increases after curing at temperatures from ∼100 to ∼300°C, suggesting that condensations between adjacent OTS molecules on the silica surface are related to the PL. The PL decay curve of the cured samples remains unchanged from 77 to 450 K, whereas the time-integrated PL intensity shows a monotonous decrease with increasing temperature. From these experimental results, a model of radiative and nonradiative process associated with the PL is presented.

Ion channeling effects on quantum well intermixing in phosphorus-implanted InGaAsP∕InGaAs∕InP

Photoluminescence, time-resolved photoluminescence, and Raman characterization techniques have been used to study In0.73Ga0.27As0.57P0.43∕In0.53Ga0.47As∕InP single quantum well heterostructure after 20-keV phosphorus ion implantation followed by rapid thermal annealing. The annealing process induces intermixing in the heterostructures and results in the blueshift of the quantum well peak emission. In order to investigate ion channeling effects on this band-gap tuning process, room-temperature implantations have been performed at tilt angles of 0° and 7° with respect to the sample (001)-growth axis. We show that the ion channeling increases the blueshift from 24 to 42 nm, while it reduces both the density of the nonradiative defects within the active layer and the structure disordering. These features are attributed to the nature of the damage generated by channeled ions. The band-gap increase observed in the sample implanted at 0° is consistent with the formation of a compressive strain at the barrier/quantum well interface, whose intensity is measured by Raman spectroscopy.

Room Temperature Steady State and Time Resolved PL Characterization of Ion Irradiation Induced Defects in 6H-SiC

Photoluminescence (PL) and time resolved PL are well established as important experimental techniques to study electronic properties of SiC. We studied the influence of ionimplantation on the photoluminescence peak at 423nm and the variation of the minority carrier lifetime (MCL).

Low-temperature time-resolved photoluminescence characterization of 3,4,9,10-perylene tetracarboxylic dianhydride crystals

In the present work, we investigate the recombination dynamics in $\ensuremath{\alpha}\ensuremath{-}\mathrm{PTCDA}$ (perylene tetracarboxylic dianhydride) crystals with time-resolved photoluminescence (PL) techniques. From a data analysis based on three different recombination channels, we assign two decay times of ${\ensuremath{\tau}}_{s}=(33.5\ifmmode\pm\else\textpm\fi{}2)\mathrm{ns}$ and ${\ensuremath{\tau}}_{m}=(12.7\ifmmode\pm\else\textpm\fi{}0.4)\mathrm{ns}$ to radiative decay, while a faster component of ${\ensuremath{\tau}}_{f}=(3\ifmmode\pm\else\textpm\fi{}1)\mathrm{ns}$ is likely to have strong nonradiative contributions. Our findings are compared to recent investigations of the dispersion of Frenkel excitons and calculated radiative recombination rates for PL out of the minimum of the Frenkel exciton dispersion.

Time-resolved photoluminescence characterization of nm-sized silicon crystallites in SiO 2

Time-resolved photoluminescence (PL) decays have been measured for silicon nanocrystallites embedded in silicon dioxide. The nanocrystallites were formed by implanting Si ions at various doses into thermally grown SiO 2 films, followed by thermal annealing at 1100 °C. Silicon dioxide films of various thickness in the range 100–1000 Å were used. The results show that PL could be detected even for films as thin as 100 Å. The spectral and time-dependence characteristics are very similar to those of porous Si, exhibiting a clear stretched exponential behavior. The extracted parameters τ and β are, however, somewhat higher in value for the Si nanocrystallites. This may tentatively be attributed to a high degree of passivation of the Si nano crystallites in the oxide matrix. An observed spectral redshift for thinner oxides and higher implantation doses may be explained considering the excess Si concentration along the implantation profile. The stretched exponential decay suggests that carrier trapping and diffusion occurs between adjacent nanocrystallites, most likely in the form of excitons. © 1997 Elsevier Science S.A. All rights reserved.

Time-resolved photoluminescence characterization of GaAs and AlGaAs on Si substrate grown by MOCVD

Abstract GaAs layers and AlGaAs solar cells grown on Si substrates have been characterized by time-resolved photoluminescence (TRP). The effects of the growth temperature, the strained layer superlattice (SLS), and the thermal cycle annealing (TCA) on the TRP characteristics are discussed. The minority carrier lifetime is increased with increasing growth temperature and by using SLS + TCA. The longest minority carrier lifetime of GaAs on Si obtained in this study is 0.50 ns.

Picosecond time-resolved photoluminescence characterization of a-SiC:H films prepared by electron cyclotron resonance plasma

Microwave excited electron cyclotron resonance (ECR) plasmas were used to produce five a-SiC:H films of varying relative carbon content. This was achieved by adjusting the relative hydrogen gas/organic liquid source flow rates and resulted in optical gaps between 2.3eV and 3.3eV. A time-correlated single photon counting technique incorporating a picosecond laser was used to acquire the fast photoluminescence decay emitted at several wavelengths for each sample. A lifetime distribution analysis of the photoluminescence decay reveals the presence of up to four distinct components. These resolved decay times range from 80ps to over 20ns. Each component decay has its own emission spectrum. The blue component has the fastest and the red the slowest decay. We present the variation of photoluminescence lifetime distributions with deposition parameters. We also present continuous wave (cw) and absorption photoluminescence spectra.