Time-integrated Photoluminescence Research Articles

Acoustically regulated optical emission dynamics from quantum dot-like emission centers in GaN/InGaN nanowire heterostructures

We report on experimental studies of the effects induced by surface acoustic waves on the optical emission dynamics of GaN/InGaN nanowire quantum dots. We employ stroboscopic optical excitation with either time-integrated or time-resolved photoluminescence detection. In the absence of the acoustic wave, the emission spectra reveal signatures originated from the recombination of neutral exciton and biexciton confined in the probed nanowire quantum dot. When the nanowire is perturbed by the propagating acoustic wave, the embedded quantum dot is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling. Depending on the recombination lifetime of the involved optical transitions, we can resolve acoustically driven radiative processes over time scales defined by the acoustic cycle. At high acoustic amplitudes, we also observe distortions in the transmitted acoustic waveform, which are reflected in the time-dependent spectral response of our sensor quantum dot. In addition, the correlated intensity oscillations observed during temporal decay of the exciton and biexciton emission suggest an effect of the acoustic piezoelectric fields on the quantum dot charge population. The present results are relevant for the dynamic spectral and temporal control of photon emission in III-nitride semiconductor heterostructures.

Long-lived hot-carrier light emission and large blue shift in formamidinium tin triiodide perovskites

A long-lived hot carrier population is critical in order to develop working hot carrier photovoltaic devices with efficiencies exceeding the Shockley–Queisser limit. Here, we report photoluminescence from hot-carriers with unexpectedly long lifetime (a few ns) in formamidinium tin triiodide. An unusual large blue shift of the time-integrated photoluminescence with increasing excitation power (150 meV at 24 K and 75 meV at 293 K) is displayed. On the basis of the analysis of energy-resolved and time-resolved photoluminescence, we posit that these phenomena are associated with slow hot carrier relaxation and state-filling of band edge states. These observations are both important for our understanding of lead-free hybrid perovskites and for an eventual future development of efficient lead-free perovskite photovoltaics.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron–hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Carrier recombination dynamics in electronically coupled multi-layer InAs/GaAs quantum dots

Optical properties of multi-layer InAs/GaAs quantum dots (QDs) with different GaAs spacer layer thicknesses (dGaAs) of 20, 15 and 10nm are investigated by time-integrated and time-resolved photoluminescence (PL) spectroscopy. The energy spacing between ground and excited state is observed to be decreased with decrease in dGaAs and increased with increase in excitation intensity. The rate of carrier capture and energy relaxation in QDs is found to be slower at excitation energy of 3.06eV than that at 1.53eV which is attributed to intervalley scattering in GaAs. The effect of intervalley scattering process in GaAs on decay time is negligible for dGaAs of 20nm and is prominent for dGaAs ≤ 15nm.

The influence of the environment on monolayer tungsten diselenide photoluminescence

We report on a comprehensive experimental analysis of the influence of the environment on the optical properties of monolayer WSe2. Different dielectric-environmental configurations with hBN as buffer and encapsulant were studied at room temperature and 10 K using time-integrated and time-resolved photoluminescence spectroscopy. Furthermore, Raman signal was evaluated to highlight the influence of the surroundings on the monolayer system. Here, linewidth and intensity improvement for 2D excitons as a consequence of hBN-encapsulation is demonstrated, and a change of strain and doping levels in our produced structures are observed. The increase of the exciton radius for hBN-buffered samples in comparison to WSe2 on bare substrates is indicated via an exciton–exciton annihilation study, while hBN as buffer is found to generally shorten the effective radiative lifetime. Furthermore, we report that trions experience a weaker interaction with the WSe2 phonons than excitons, while the coupling to phonons is consistently decreased when WSe2 is capped by hBN. Ultimately, our energy-resolved analysis of the dynamics at 10 K shows that the individual excitonic modes exhibit different photoluminescence decay times. A comparison of the different hBN-WSe2 configurations shows that the shortest measurable time constants, which are on the ps-scale, generally increase when buffering or encapsulating the 2D semiconductor, while sandwiching caused the strongest lifetime increase on these short time scales. In contrast, our monolayer–monolayer heterostructures indicate fast charge/energy transfer (<1 ps) from MoSe2 to WSe2 and a comparably slow radiative recombination of the type-I A excitons depending on the tilt angle between the two lattices.

Control of single photon emitters in semiconductor nanowires by surface acoustic waves

We report on an experimental study into the effects of surface acoustic waves on the optical emission of dot-in-a-nanowire heterostructures in III-V material systems. Under direct optical excitation, the excitonic energy levels in III-nitride dot-in-a-nanowire heterostructures oscillate at the acoustic frequency, producing a characteristic splitting of the emission lines in the time-integrated photoluminescence spectra. This acoustically induced periodic tuning of the excitonic transition energies is combined with spectral detection filtering and employed as a tool to regulate the temporal output of anti-bunched photons emitted from these nanowire quantum dots. In addition, the acoustic transport of electrons and holes along a III-arsenide nanowire injects the electric charges into an ensemble of quantum dot-like recombination centers that are spatially separated from the optical excitation area. The acoustic population and depopulation mechanism determines the number of carrier recombination events taking place simultaneously in the ensemble, thus allowing control of the anti-bunching degree of the emitted photons. The results presented are relevant for the dynamic control of single photon emission in III-V semiconductor heterostructures.

Separating Bulk and Surface Contributions to Electronic Excited-State Processes in Hybrid Mixed Perovskite Thin Films via Multimodal All-Optical Imaging.

A comprehensive understanding of electronic excited-state phenomena underlying the impressive performance of solution-processed hybrid halide perovskite solar cells requires access to both spatially resolved electronic processes and corresponding sample morphological characteristics. Here, we demonstrate an all-optical multimodal imaging approach that enables us to obtain both electronic excited-state and morphological information on a single optical microscope platform with simultaneous high temporal and spatial resolution. Specifically, images were acquired for the same region of interest in thin films of chloride containing mixed lead halide perovskites (CH3NH3PbI3-xClx) using femtosecond transient absorption, time-integrated photoluminescence, confocal reflectance, and transmission microscopies. Comprehensive image analysis revealed the presence of surface- and bulk-dominated contributions to the various images, which describe either spatially dependent electronic excited-state properties or morphological variations across the probed region of the thin films. These results show that PL probes effectively the species near or at the film surface.

Enhancing redshift phenomenon in time-resolved photoluminescence spectra of AlGaN epilayer**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400101) and Beijing Science and Technology Project, China (Grant No. Z151100003315024).

AlGaN epitaxial layer has been studied by means of temperature-dependent time-integrated photoluminescence (PL) and time-resolved photoluminescence (TRPL). An enhancing redshift phenomenon in TRPL spectra with increasing temperature was observed, and the localized excitons behaved like quasi two-dimensional excitons between 6 K and 90 K. We demonstrated that these behaviors are caused by a change in the carrier dynamics with increasing temperature due to the competition of carriers’ localization and delocalization in the AlGaN alloy.

Triplet harvesting in luminescent Cu(i) complexes by the thermally activated luminescence transition mechanism: impact of the molecular structure

Thermally induced transition from ordinary phosphorescence to delayed fluorescence in two kinds of luminescent copper(i) complexes is comprehensively investigated by using variable-temperature time-integrated and time-resolved photoluminescence measurements as well as model analysis.

Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes.

Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states.

Intrinsic and Extrinsic Stability of Formamidinium Lead Bromide Perovskite Solar Cells Yielding High Photovoltage.

We report on both the intrinsic and the extrinsic stability of a formamidinium lead bromide [CH(NH2)2PbBr3 = FAPbBr3] perovskite solar cell that yields a high photovoltage. The fabrication of FAPbBr3 devices, displaying an outstanding photovoltage of 1.53 V and a power conversion efficiency of over 8%, was realized by modifying the mesoporous TiO2-FAPbBr3 interface using lithium treatment. Reasons for improved photovoltaic performance were revealed by a combination of techniques, including photothermal deflection absorption spectroscopy (PDS), transient-photovoltage and charge-extraction analysis, and time-integrated and time-resolved photoluminescence. With lithium-treated TiO2 films, PDS reveals that the TiO2-FAPbBr3 interface exhibits low energetic disorder, and the emission dynamics showed that electron injection from the conduction band of FAPbBr3 into that of mesoporous TiO2 is faster than for the untreated scaffold. Moreover, compared to the device with pristine TiO2, the charge carrier recombination rate within a device based on lithium-treated TiO2 film is 1 order of magnitude lower. Importantly, the operational stability of perovskites solar cells examined at a maximum power point revealed that the FAPbBr3 material is intrinsically (under nitrogen) as well as extrinsically (in ambient conditions) stable, as the unsealed devices retained over 95% of the initial efficiency under continuous full sun illumination for 150 h in nitrogen and dry air and 80% in 60% relative humidity (T = ∼60 °C). The demonstration of high photovoltage, a record for FAPbBr3, together with robust stability renders our work of practical significance.

Exciton and trion dynamics in atomically thinMoSe2andWSe2: Effect of localization

We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong changes in PL including suppression of the trion PL and decrease of the trion PL life-time, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T~10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion and exciton localization and various temperature-dependent non-radiative processes.

Exciton–phonon interaction in Al0.4Ga0.6N/Al0.53Ga0.47N multiple quantum wells**Project supported by the National Basic Research Program of China (Grant No. 2012CB619306), the Beijing Science and Technology Project, China (Grant No. Z151100003315024), and the National Natural Science Foundation of China (Grant No. 61404132).

The exciton–phonon interaction in Al0.4Ga0.6N/Al0.53Ga0.47N multiple quantum wells (MQWs) is studied by deep-ultraviolet time-integrated and time-resolved photoluminescence (PL). Up to four longitudinal-optical (LO) phonon replicas of exciton recombination are observed, indicating the strong coupling of excitons with LO phonons in the MQWs. Moreover, the exciton–phonon coupling strength in the MQWs is quantified by the Huang–Rhys factor, and it keeps almost constant in a temperature range from 10 K to 120 K. This result can be explained in terms of effects of fluctuations in the well thickness in the MQWs and the temperature on the exciton–phonon interaction.

Exciton emission from hybrid organic and plasmonic polytype InP nanowire heterostructures

We investigate the emission of excitons in bare, hybrid organic, and metal coated polytype wurtzite/zincblende (WZ/ZB) InP nanowire (NW) heterostructures by intensity- and temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). TI PL measurements at 20 K reveal two strong emission bands at ∼1.48 and ∼1.44 eV that are attributed to non-thermalized weakly and deeply localized indirect WZ/ZB excitons due to randomly distributed short WZ and ZB segments. The PL yield of both bands increases when the NWs are covered with an Alq3 layer which is attributed to surface charge passivation. In metal coated NWs the weakly localized indirect WZ/ZB exciton emission is significantly reduced while the strongly localized indirect WZ/ZB band is less affected. The observed PL quenching is attributed to radiationless Förster energy-transfer from NW excitons to plasmon oscillations in the deposited metal. TR PL investigations support this interpretation revealing enhanced PL lifetimes in Alq3 coated NWs compared to uncovered NWs. The lifetime of weakly trapped indirect excitons is shortest in metal coated NWs due to Förster energy-transfer while the dynamics of strongly localized indirect WZ/ZB excitons is less affected because of the small dipole-moment of these transitions.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Ultrafast electron trapping in ligand-exchanged quantum dot assemblies.

We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature optical properties of CdSe quantum dot solids after exchanging native aliphatic ligands for thiocyanate and subsequent thermal annealing. In contrast to trends established at room temperature, our data show that at low temperature the band-edge absorptive bleach is dominated by 1S3/2h hole occupation in the quantum dot interior. We find that our ligand treatments, which bring enhanced interparticle coupling, lead to faster surface state electron trapping, a greater proportion of surface-related photoluminescence, and decreased band-edge photoluminescence lifetimes.

Electrochemically grafted polypyrrole changes photoluminescence of electronic states inside nanocrystalline diamond

Hybrid diamond-organic interfaces are considered attractive for diverse applications ranging from electronics and energy conversion to medicine. Here we use time-resolved and time-integrated photoluminescence spectroscopy in visible spectral range (380–700 nm) to study electronic processes in H-terminated nanocrystalline diamond films (NCD) with 150 nm thin, electrochemically deposited polypyrrole (PPy) layer. We observe changes in dynamics of NCD photoluminescence as well as in its time-integrated spectra after polymer deposition. The effect is reversible. We propose a model where the PPy layer on the NCD surface promotes spatial separation of photo-generated charge carriers both in non-diamond carbon phase and in bulk diamond. By comparing different NCD thicknesses we show that the effect goes as much as 200 nm deep inside the NCD film.

Investigation of GaAs/AlGaAs quantum cascade structures by optical methods based on hot luminescence in the near-infrared range

The time-resolved photoluminescence of GaAs/AlGaAs quantum-cascade structures under intense pulse excitation is studied. Aside from optical transitions between the ground electron and hole states of a system of two tunnel-coupled quantum wells, the photoluminescence spectrum at short times after the excitation pulse exhibits features corresponding to transitions between the excited states of these wells, which are not observed in time-integrated photoluminescence spectra. It is shown that, due to a high pump level, the electron gas is initially strongly heated, which makes it possible to observe band-to-band transitions between both the ground and excited states. Nonequilibrium carriers cool down with a characteristic relaxation time of ∼125 ps.

Anomalous temperature-dependent photoluminescence peak energy in InAlN alloys

InAlN has been studied by means of temperature-dependent time-integrated photoluminescence and time-resolved photoluminescence. The variation of PL peak energy did not follow the behavior predicted by Varshni formula, and a faster redshift with increasing temperature was observed. We used a model that took account of the thermal activation and thermal transfer of localized excitons to describe and explain the observed behavior. A good fitting to the experiment result is obtained. We believe the anomalous temperature dependence of PL peak energy shift can be attributed to the temperature-dependent redistribution of localized excitons induced by thermal activation and thermal transfer in the strongly localized states. V-shaped defects are thought to be a major factor causing the strong localized states in our In0.153Al0.847N sample.

Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.