Photoluminescence Dynamics Research Articles

Origins of excitation-wavelength-dependent photoluminescence in WS2 quantum dots

We report the photoluminescence studies of pristine and diethylenetriamine-doped (DETA-doped) WS2 quantum dots (QDs) synthesized by pulsed laser ablation. The DETA-doped WS2 QDs revealed a notable improvement of the luminescence quantum yield from 0.1% to 15.2% in comparison to pristine WS2 QDs. On the basis of the photoluminescence (PL) under different excitation wavelengths and the emission-energy dependence of PL dynamics, we suggest that the excitation-wavelength-dependent (excitation-wavelength-independent) PL for pristine (DETA-doped) WS2 QDs is attributed to the recombination of carriers from the localized (delocalized) states.

Emission Kinetics from PbSe Quantum Dots in Glass Matrix at High Excitation Levels

The photoluminescence dynamics of PbSe quantum dots in a glass matrix (PbSe:Glass) is investigated at high excitation levels. Up to 470 meV blue‐shifted emissions are observed, which are assigned to excited‐state emission populated by carrier accumulation. Rapid photoluminescence decays of ∼25–40 ps are observed. Together with a threshold‐like dependence on excitation, even at room temperature, this component is tentatively attributed to amplified spontaneous emission. The absence of stimulated emission from the ground state and the overall higher threshold compared to PbS:Glass suggest that there are more intrinsic restrictions on the use of PbSe:Glass as a laser material than for PbS:Glass.

25pDD-1 GaAs/AIAs多重量子井戸構造における励起子-励起子散乱の発光ダイナミクス(量子井戸・光応答,領域4(半導体,メゾスコピック系・局在))

25pDD-1 GaAs/AIAs多重量子井戸構造における励起子-励起子散乱の発光ダイナミクス(量子井戸・光応答,領域4(半導体,メゾスコピック系・局在))

Ce3+-Doping to Modulate Photoluminescence Kinetics for Efficient CsPbBr3 Nanocrystals Based Light-Emitting Diodes.

Inorganic perovskite CsPbBr3 nanocrystals (NCs) are emerging, highly attractive light emitters with high color purity and good thermal stability for light-emitting diodes (LEDs). Their high photo/electroluminescence efficiencies are very important for fabricating efficient LEDs. Here, we propose a novel strategy to enhance the photo/electroluminescence efficiency of CsPbBr3 NCs through doping of heterovalent Ce3+ ions via a facile hot-injection method. The Ce3+ cation was chosen as the dopant for CsPbBr3 NCs by virtue of its similar ion radius and formation of higher energy level of conduction band with bromine in comparison with the Pb2+ cation to maintain the integrity of perovskite structure without introducing additional trap states. It was found that by increasing the doping amount of Ce3+ in CsPbBr3 NCs to 2.88% (atomic percentage of Ce compared to Pb) the photoluminescence quantum yield (PLQY) of CsPbBr3 NCs reached up to 89%, a factor of 2 increase in comparison with the native, undoped ones. The ultrafast transient absorption and time-resolved photoluminescence (PL) spectroscopy revealed that Ce3+-doping can significantly modulate the PL kinetics to enhance the PL efficiency of doped CsPbBr3 NCs. As a result, the LED device fabricated by adopting Ce3+-doped CsPbBr3 NCs as the emitting layers exhibited a pronounced improvement of electroluminescence with external quantum efficiency (EQE) from 1.6 to 4.4% via Ce3+-doping.

Straining effects in MoS2 monolayer on nanostructured substrates: temperature-dependent photoluminescence and exciton dynamics.

Strain-engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) has great potential to alter their electronic and optical properties. Thus far, experimental studies of the straining effects in 2D TMDs primarily focused on the static property measurements at room temperature. However, low-temperature and temperature-dependence studies are essential in understanding the underlying mechanisms of the unique properties of monolayer TMDs. Herein, the temperature-dependent dynamic properties of laser shock strain-engineered monolayer MoS2 were studied using temperature-dependent photoluminescence (PL) and pump-probe spectroscopy. Both the photoluminescence spectra and exciton dynamics exhibit the differences between the MoS2 monolayers transferred on the flat and nanostructured surfaces by laser shock strain engineering and display a strong temperature dependence. The laser-induced straining effect and temperature-dependent dynamic behavior of MoS2 were studied through molecular dynamics simulation. The observed behaviors can be explained by the thermally induced strain in the monolayer MoS2 due to the mismatching thermal expansion coefficients of the monolayer and the substrate, which are coupled by the van der Waals forces. The ultrafast pump-probe experiments were performed to investigate the effect of strain on the exciton dynamics upon optical excitation. The results from the pump-probe measurements indicate that the effects of strain extend beyond that of the static properties and profoundly influence the valley carrier dynamics. This report extends the understanding of the substrate-induced straining effect to temperature-dependent luminescence behaviors and dynamic behaviors of the TMD materials.

Ultrafast Photoluminescence Dynamics of Organic Photosensitizers with Conjugated Linkers Containing Different Heteroatoms

Ultrafast Photoluminescence Dynamics of Organic Photosensitizers with Conjugated Linkers Containing Different Heteroatoms

Superlinear Photoluminescence Dynamics in Plasmon–Quantum-Dot Coupling Systems

Light–matter interaction exploiting plasmons is attracting great interest in terms of a new twist, hot electrons. We designed a basic configuration to couple plasmonic metasurfaces with a layer of quantum dots (QDs) embedded in semiconductors and experimentally investigated the photoluminescence (PL) dynamics in the coupled systems of III–V semiconductor QDs with plasmonic metasurfaces. The QDs of InAs, which emit luminescence at telecom wavelengths near 1300 nm, were grown on GaAs substrates with a molecular-beam-epitaxy technique. The plasmonic metasurfaces were fabricated on top of the GaInAs substrates, using a numerical design for single-layer Au metasurfaces. Here we show that the PL responses through the plasmonic metasurface becomes more active in the coupled systems than those in the QDs without the plasmonic metasurfaces, being superlinear with respect to the excitation laser intensity, even under weak continuous-wave excitation. The superlinear responses are successfully described in a general ...

Direct Measurement of Nanoscale Lateral Carrier Diffusion: Toward Scanning Diffusion Microscopy

A multimode scanning near-field optical microscopy technique that allows the mapping of surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, and PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows measuring diffusion lengths as small as ∼100 nm and their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm2/s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the topmost valence band in an m-plane InGaN quantum well is highly anisotropic.

Changes of the absorption cross section of Si nanocrystals with temperature and distance.

The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We clearly demonstrate that roughly doubling the barrier thickness (from ca. 1 to 2.2 nm) induces a decrease of the ACS by a factor of 1.5. An optimum separation barrier thickness of ca. 1.6 nm is calculated to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by smaller occupation number of phonons and expansion of the band gap of Si NCs at low temperatures. This study clearly shows that the ACS of Si NCs cannot be considered as independent on experimental conditions and sample parameters.

The dependence of relaxation kinetics of photoluminescence from interband transitions in InGaAs/GaAs quantum wells on their distance from an interface with Au

Kinetics of relaxation of photoluminescence from the interband transition between dimensionalquantization levels of electrons and holes in InGaAs/GaAs quantum wells as a function of their distance to an interface with Au is investigated. It is demonstrated that the photoluminescence relaxation time becomes several times shorter when the distance from the quantum well to the interface decreases to several tens of nanometers. It is established that the photoluminescence relaxation time at a shorter wavelength corresponding to a recombination transition between excited states of electrons and holes in the quantum well is shorter than that at a longer wavelength corresponding to a recombination transition between the ground states. A theoretical model explaining this phenomenon is proposed.

Photoluminescence dynamics of direct and indirect excitons in CdTe/ZnTe superlattices with quantum dot layers

Photoluminescence kinetics is measured for spatially direct and indirect excitons in CdTe quantum dots excited by second harmonic radiation pulses of a picosecond Ti : sapphire laser. The width of a ZnTe barrier layer varied in the range of 2 – 12 monolayers (MLs). Since the photoluminescence decay curves are not described by a single exponential dependence, two components are analysed – fast and slow, which can be approximated by exponential functions. It is shown that in the samples with a barrier layer width of 12 and 4 MLs, the decay times for the fast component of a direct exciton and an indirect exciton are 0.4 ns and 3 μs, respectively. The long-term component is observed in both spectra. For a direct exciton, the photoluminescence decay time is 6 ns, and for an indirect exciton it is 22 μs. It is found that the recombination times for direct and indirect excitons in a sample with the barrier layer width of 2 MLs are close: in both cases the decay time for the fast component and slow component is 2 and 80 ns, respectively. Taking into account the tensile strain of the ZnTe layer and origin of the quantum well for light holes in this layer it is assumed that electrons in CdTe quantum dots interact with light holes in a ZnTe layer and generate a quasi-direct exciton because electron wave functions penetrate into the barrier.

Probing the Interlayer Exciton Physics in a MoS2/MoSe2/MoS2 van der Waals Heterostructure.

Stacking atomic monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of TMD heterostructures should result in the formation of interlayer excitons with long lifetimes and robust valley polarization. However, these features have been observed simultaneously only in MoSe2/WSe2 heterostructures. Here we report on the observation of long-lived interlayer exciton emission in a MoS2/MoSe2/MoS2 trilayer van der Waals heterostructure. The interlayer nature of the observed transition is confirmed by photoluminescence spectroscopy, as well as by analyzing the temporal, excitation power, and temperature dependence of the interlayer emission peak. The observed complex photoluminescence dynamics suggests the presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We show that circularly polarized optical pumping results in long-lived valley polarization of interlayer exciton. Intriguingly, the interlayer exciton photoluminescence has helicity opposite to the excitation. Our results show that through a careful choice of the TMDs forming the van der Waals heterostructure it is possible to control the circular polarization of the interlayer exciton emission.

(Invited) Photophysics of Lead Halide Perovskite Solar Cells

Very recently, lead-halide perovskites have received much attention as a promising class of low-cost and high-performance photovoltaics materials since the first report of all-solid-state perovskite solar cells (SCs) in 2012 [1]. Organic-inorganic hybrid CH3NH3PbI3 is a model material of metal-halide perovskites, and its optical and electronic properties have been insensitively studied. It is known that perovskite CH3NH3PbI3 has excellent optoelectronic properties for SC applications: large optical absorption coefficients, extremely low densities of traps and defects, and long diffusion lengths of carriers. Because the energy conversion efficiencies of SCs depend on the luminescence quantum efficiency of the target semiconductor material [2,3], photoluminescence (PL) spectroscopy is one of the most powerful techniques for characterizing and developing new materials and structures for SC devices. By taking advantage of high-efficiency PL of lead-halide perovskites, we can directly monitor their carrier generation, recombination, and transport dynamics of SCs using time-resolved PL spectroscopy [4]. Here, we summarize the optical properties of solution-processed CH3NH3PbI3 and CH3NH3PbBr3thin films and single crystals and discuss the physical origin of high conversion efficiencies in perovskite SCs. Optical studies on high-quality thin films and single crystals are required for a deep understanding of the intrinsic optoelectronic properties of lead-halide perovskites. We studied photocarrier dynamics of high-quality CH3NH3PbI3 and CH3NH3PbBr3thin films and single crystals using one-photon excited PL (1PL) and two-photon excited PL (2PL) spectroscopy [5-7]. In thick single crystals, the photocarriers are generated at the near-surface region under one-photon excitation and in the interior region under two-photon excitation. The time-integrated 1PL peak energy is higher than the 2PL peak energy. The 1PL peak energy shifts to lower energy with delay time after laser excitation. The 1PL lifetime is shorter than the 2PL lifetime. These dynamical PL behaviors of thick single crystals are different from those of thin films. In thin films, the photocarriers are created homogeneously throughout the sample thickness under one- and two-photon excitation [8]. The spatial distribution of photocarriers determines the PL spectrum and dynamics: The PL dynamics of thick single crystals can be explained by fast carrier diffusion and photon recycling (photon emission and reabsorption) [5-7]. In SC device structures, the perovskite CH3NH3PbI3 layer is sandwiched between the mesoporous TiO2electron-transport layer and the spiro-OMeTAD hole-transport layer. Charge carrier dynamics in the perovskite solar cells were studied by means of PL and photocurrent (PC) imaging spectroscopy. Simultaneous PL and PC imaging measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities [8]. These correlations are a result of the competition between the escape of photocarriers from the perovskite layer and the recombination of photocarriers within the perovskite layer. Moreover, from the excitation-fluence dependence of the PL decay curves in thin films and SCs, we conclude that the carrier injection at the interface determines the device performance of perovskite SC [9,10]. Our systematic PL studies provide an essential information for developing high-performance devices based on metal-halide perovskites. Part of this work was supported by JST-CREST (Grant No. JPMJCR16N3). References M.M. Lee et al, Science 338, 643 (2012). O. D. Miller et al., IEEE J. Photovoltaics 2, 303 (2012). L. Zhu et al., Opt. Express 24, A740 (2016). Y. Kanemitsu, J. Mater. Chem. C 5, 3427 (2017). Y. Yamada et al., J. Am. Chem. Soc. 137, 10456 (2015). T. Yamada et al., Adv. Electron. Mater. 2, 1500290 (2016). T. Yamada et al., Phys. Rev. Applied. 7, 014001 (2017). Y. Yamada et al., J. Am. Chem. Soc. 136, 11610 (2014). D. Yamashita et al., J. Phys. Chem. Lett. 7, 3186 (2016). T. Handa et al., J. Phys. Chem. Lett. 8, 954 (2017). D. M. Tex et al., Phys. Rev. Applied 7, 014019 (2017).

Defect-related luminescence in InAlAs on InP grown by molecular beam epitaxy

The photoluminescence (PL) of InAlAs grown on InP has been studied in a wide range of temperatures and excitation intensities. A novel emission ascribed to the presence of defects has been found by about 120–180 meV below the near band edge (NBE) line. The novel wide PL band is observed in the spectra only in a limited range of temperatures of 50–160 K, and is seen neither at liquid helium nor at room temperatures. The analysis of the PL behaviour with sample temperature and excitation power together with non-stationary PL kinetics allows us to conclude that both the NBE PL and the novel PL band are controlled by transitions via states of band tails formed due to alloy disorder in these films. The NBE PL is caused by recombination of carriers in the band tails, while deep levels related to defects and located in the same regions as the deepest band tail states are supposedly involved in the defect-related PL transitions. We demonstrate that no defect-related PL is found in the spectra if quasi-stoichiometric growth conditions were used during film growth, which resulted in a PL efficiency by about 1–2 orders of magnitude greater than that of samples grown under more common As-rich conditions.

Study of the photoluminescence kinetics of heterogeneous nanostructured Pb0.30Cd0.70I2 solid solutions

The study of photoluminescence (PL) kinetics for strongly heterogeneous materials such as nanostructured scintillator Pb0.30Cd0.70I2 solid solutions was carried out. Fitting the obtained results by Kohlrausch-Williams-Watts exponential function allows us to obtain the effective lifetime and the lifetime distribution for the PL decay. At the same time, fitting by a sum (about several hundreds) of single exponential functions enables us to determine the lifetime distribution associated with different radiative recombination processes characteristic of the investigated novel semiconductor materials. It was found that the radiative recombination of free electrons and holes associated with the deep surface acceptor centers is a fast process. The obtained results open a way to optimize defect structure of the investigated semiconductor materials in order to obtain high-sensitive fast non-cooled radiation detector material for X- and γ-rays suitable for biomedical and industrial imaging applications.

Resonant photoluminescence and dynamics of a hybrid Mn hole spin in a positively charged magnetic quantum dot

We analyze, through resonant photoluminescence, the spin dynamics of an individual magnetic atom (Mn) coupled to a hole in a semiconductor quantum dot. The hybrid Mn-hole spin and the positively charged exciton in a CdTe/ZnTe quantum dot forms an ensemble of $\Lambda$ systems which can be addressed optically. Auto-correlation of the resonant photoluminescence and resonant optical pumping experiments are used to study the spin relaxation channels in this multilevel spin system. We identified for the hybrid Mn-hole spin an efficient relaxation channel driven by the interplay of the Mn-hole exchange interaction and the coupling to acoustic phonons. We also show that the optical $\Lambda$ systems are connected through inefficient spin-flips than can be enhanced under weak transverse magnetic field. The dynamics of the resonant photoluminescence in a p-doped magnetic quantum dot is well described by a complete rate equation model. Our results suggest that long lived hybrid Mn-hole spin could be obtained in quantum dot systems with large heavy-hole/light-hole splitting.

The effect of Auger recombination on the nonequilibrium carrier recombination rate in the InGaAsSb/AlGaAsSb quantum wells

Time dependencies of interband photoluminescence are studied in InGaAsSb/AlGaAsSb quantum well structures with various barrier materials and quantum well widths. The experimentally determined dependencies of photoluminescence intensity on the optical pumping level are compared with calculated dependencies of photoluminescence intensity on the nonequilibrium carrier concentration. Time of charge carrier trapping into quantum wells and the energy relaxation time related to the polar optical phonon emission were obtained from analysis of the photoluminescence dynamics at different optical pumping levels. The recombination rates with respect to nonradiative Shockley–Read–Hall and Auger recombination were determined. It is shown that, at certain parameter sets, resonant Auger recombination can exist in InGaAsSb/AlGaAsSb quantum well structures, which should results in the decrease of the concentration of carriers taking part in the radiative recombination.

Light-Induced Fluorescence Modulation of Quantum Dot-Crystal Violet Conjugates: Stochastic Off–On–Off Cycles for Multicolor Patterning and Super-Resolution

Photoswitching or modulation of quantum dots (QDs) can be promising for many fields that include display, memory, and super-resolution imaging. However, such modulations have mostly relied on photomodulations of conjugated molecules in QD vicinity, which typically require high power of high energy photons at UV. We report a visible light-induced facile modulation route for QD-dye conjugates. QD crystal violets conjugates (QD-CVs) were prepared and the crystal violet (CV) molecules on QD quenched the fluorescence efficiently. The fluorescence of QD-CVs showed a single cycle of emission burst as they go through three stages of (i) initially quenched "off" to (ii) photoactivated "on" as the result of chemical change of CVs induced by photoelectrons from QD and (iii) back to photodarkened "off" by radical-associated reactions. Multicolor on-demand photopatterning was demonstrated using QD-CV solid films. QD-CVs were introduced into cells, and excitation with visible light yielded photomodulation from "off" to "on" and "off" by nearly ten fold. Individual photoluminescence dynamics of QD-CVs was investigated using fluorescence correlation spectroscopy and single QD emission analysis, which revealed temporally stochastic photoactivations and photodarkenings. Exploiting the stochastic fluorescence burst of QD-CVs, simultaneous multicolor super-resolution localizations were demonstrated.

Synthesis and time-resolved photoluminescence of SnO2 nanorods

A solvo-thermal method is employed to synthesize SnO2 nanorods and the Fourier transformed infrared spectroscopic analysis confirms the formation of SnO bond. The X-ray diffraction analysis suggests that SnO2 nanorods exist in tetragonal rutile crystal structure phase. Transmission electron microscopy images show the formation of nanorods with an average diameter ∼10–15 nm and length 35–50 nm. The deconvoluted photoluminescence spectrum suggests the existence of three distinct origins of photoluminescence, which peaks at photon energies of ∼423 nm (2.93 eV), ∼470 nm (2.64 eV) and 480 nm (2.58 eV). The measured photoluminescence kinetics is best described by a tri-exponential decay model suggesting that the photoluminescence occurs from three distinct channels with time constants 1.31 ns, 4.89 ns and 13.24 ns. These studies suggest that SnO2 nanorods synthesized by solvo-thermal method at mild conditions can be used for luminescent device applications. The long lived emission of SnO2 nanorods in the visible region make them suitable candidate as an active materials for many opto-electronic devices such as light emitting diodes and solar cells.

"Supertrap" at Work: Extremely Efficient Nonradiative Recombination Channels in MAPbI3 Perovskites Revealed by Luminescence Super-Resolution Imaging and Spectroscopy.

Organo-metal halide perovskites are some of the most promising materials for the new generation of low-cost photovoltaic and light-emitting devices. Their solution processability is a beneficial trait, although it leads to a spatial inhomogeneity of perovskite films with a variation of the trap state density at the nanoscale. Comprehending their properties using traditional spectroscopy therefore becomes difficult, calling for a combination with microscopy in order to see beyond the ensemble-averaged response. We studied photoluminescence (PL) blinking of micrometer-sized individual methylammonium lead iodide (MAPbI3) perovskite polycrystals, as well as monocrystalline microrods up to 10 μm long. We correlated their PL dynamics with structure employing scanning electron and optical super-resolution microscopy. Combining super-resolution localization imaging and super-resolution optical fluctuation imaging (SOFI), we could detect and quantify preferential emitting regions in polycrystals exhibiting different types of blinking. We propose that blinking in MAPbI3 occurs by the activation/passivation of a "supertrap" which presumably is a donor-acceptor pair able to trap both electrons and holes. As such, nonradiative recombination via supertraps, in spite being present at a rather low concentrations (1012-1015 cm-3), is much more efficient than via all other defect states present in the material at higher concentrations (1016-1018 cm-3). We speculate that activation/deactivation of a supertrap occurs by its temporary dissociation into free donor and acceptor impurities. We found that supertraps are most efficient in structurally homogeneous and large MAPbI3 crystals where carrier diffusion is efficient, which may therefore pose limitations on the efficiency of perovskite-based devices.