Time-resolved Photoluminescence Imaging Research Articles

Quantifying recombination and charge carrier extraction in halide perovskites via hyperspectral time-resolved photoluminescence imaging

Identifying sources of nonradiative recombination and quantifying charge carrier extraction in halide perovskite solar cells are important in further developing this thin-film technology. Steady-state and time-resolved photoluminescence (TRPL), in combination with analytical modeling, have emerged as non-destructive tools to achieve the desired results. However, the exact location of the recombination and charge carrier extraction losses in devices is often obscured by various competing processes when photoluminescence measurements are analyzed. Here, we show via absolute-photon-calibrated hyperspectral photoluminescence and TRPL imaging how surface passivation and inhomogeneities at interfaces impact the photoluminescence quantum yields and minority carrier lifetimes. Laser illumination from the perovskite and glass/TiO2 sides allows us to disentangle changes in surface recombination velocity from the charge carrier extraction at the electron transport layer. We find that charge extraction is spatially modulated due to an inhomogeneous mesoporous (mp)-TiO2 film thickness. Our results show that the mp-TiO2 layer is not fully optimized since the electronic properties are spatially modified, leading to lateral changes in quasi-Fermi-level splitting, minority carrier lifetime and, consequently, a reduction in open-circuit voltage.

Time-resolved photoluminescence imaging for the mapping of weakly luminescent pigments in paintings

Time-resolved photoluminescence imaging for the mapping of weakly luminescent pigments in paintings

Excitonic transport driven by repulsive dipolar interaction in a van der Waals heterostructure.

Dipolar bosonic gases are currently the focus of intensive research due to their interesting many-body physics in the quantum regime. Their experimental embodiments range from Rydberg atoms to GaAs double quantum wells and van der Waals heterostructures built from transition metal dichalcogenides. Although quantum gases are very dilute, mutual interactions between particles could lead to exotic many-body phenomena such as Bose-Einstein condensation and high-temperature superfluidity. Here, we report the effect of repulsive dipolar interactions on the dynamics of interlayer excitons in the dilute regime. By using spatial and time-resolved photoluminescence imaging, we observe the dynamics of exciton transport, enabling a direct estimation of the exciton mobility. The presence of interactions significantly modifies the diffusive transport of excitons, effectively acting as a source of drift force and enhancing the diffusion coefficient by one order of magnitude. The repulsive dipolar interactions combined with the electrical control of interlayer excitons opens up appealing new perspectives for excitonic devices.

Time-resolved analysis of photoluminescence at a single wavelength for ratiometric and multiplex biosensing and bioimaging

Simultaneous analysis of luminescence signals of multiple probes can improve the accuracy and efficiency of biosensing and bioimaging. Analysis of multiple signals at different wavelengths usually suffers from spectral overlap, possible energy transfer, and difference in detection efficiency. Herein, we reported a polymeric luminescent probe, which was composed of a phenothiazine-based fluorescent compound and a phosphorescent iridium(iii) complex. Both luminophores emitted at around 600 nm but their luminescence lifetimes are 160 times different, allowing time-resolved independent analysis. As the fluorescence was enhanced in response to oxidation by hypochlorite and the phosphorescence was sensitive toward oxygen quenching, a four-dimensional relationship between luminescence intensity, fluorescence/phosphorescence ratio, hypochlorite concentration, and oxygen content was established. In cellular imaging, time-resolved photoluminescence imaging microscopy clearly showed the independent fluorescence response toward hypochlorite and phosphorescence response toward oxygen in separated time intervals. This work opens up a new idea for the development of multiplex biosensing and bioimaging.

An aggregation-induced phosphorescent emission-active iridium(III) complex for fluoride anion imaging in living cells

Fluoride anion plays a crucial role in bone and tooth growth. However, overdose of fluoride anion can also lead to many chronic diseases and there is no specific medicine for fluorosis so far. Hence, the early detection and accurate estimation of fluoride anion intake are important. In this work, an aggregation-induced phosphorescent emission (AIPE)-active iridium(III) complex as fluoride anion probe (Ir-AF) has been developed by incorporating N-(4-hydroxylphenyl)-rhodol protected by tert-butyldiphenylsilyl onto cyclometalating ligand. As Ir-AF reacts with fluoride anion, the nonradiative transitions of Ir-AF would be promoted by enhanced photoinduced electron transfer effect and intramolecular rotations, leading to its phosphorescence quenching dramatically. Thus, Ir-AF has been used for fluoride anion measurement in the aqueous solution by ratiometric readout. Furthermore, with the utilization of the long-lived phosphorescence from Ir-AF, fluoride anion sensing in living cells with high signal-to-noise ratio has been achieved by time-resolved photoluminescence imaging. This is the first report of AIPE-active probe for fluoride anion.

Multi-domain high-resolution platform for integrated spectroscopy and microscopy characterizations

In recent decades, materials science has experienced rapid development and posed increasingly high requirements for the characterizations of structures, properties, and performances. Herein, we report on our recent establishment of a multi-domain (energy, space, time) high-resolution platform for integrated spectroscopy and microscopy characterizations, offering an unprecedented way to analyze materials in terms of spectral (energy) and spatial mapping as well as temporal evolution. We present several proof-of-principle results collected on this platform, including in-situ Raman imaging (high-resolution Raman, polarization Raman, low-wavenumber Raman), time-resolved photoluminescence imaging, and photo-electrical performance imaging. It can be envisioned that our newly established platform would be very powerful and effective in the multi-domain high-resolution characterizations of various materials of photoelectrochemical importance in the near future.

Extending Hypochlorite Sensing from Cells to Elesclomol-Treated Tumors in Vivo by Using a Near-Infrared Dual-Phosphorescent Nanoprobe.

Reactive oxygen species (ROS), when beyond the threshold, can exhaust the capacity of cellular antioxidants and ultimately trigger cell apoptosis in tumor biology. However, the roles of hypochlorite (ClO-) in this process are much less clear compared with those of ROS, and its detection is easily obstructed by tissue penetration and endogenous fluorophores. Herein, we first synthesized a near-infrared (NIR) ratiometric ClO- probe (Ir NP) composed of two kinds of phosphorescent iridium(III) complexes (Ir1 and Ir2) encapsulated with amphiphilic DSPE-mPEG5000. Ir NPs are dual-emissive and show obvious changes in phosphorescence intensity ratios and lifetimes of two emission bands upon exposure to ClO-. During the ClO- detection, ratiometric photoluminescence imaging is much more reliable over the intensity-based one for its self-calibration, while time-resolved photoluminescence imaging (TRPI) could distinguish the phosphorescence with long lifetime of Ir NPs from short-lived autofluorescence of tissues, resulting in the high accuracy of ClO- determination. With NIR emission, a long phosphorescence lifetime, fast response, and excellent biocompatibility, Ir NPs were applied to the detection of ClO- in vitro and in vivo by means of ratiometric phosphorescence imaging and TRPI with high signal-to noise-ratios (SNR). Importantly, we demonstrated the elevated ClO- in elesclomol-stimulated tumors in living mice for the first time, which holds great potential for the visualization of the boost of ClO- in anti-carcinogen-treated tumors and the further investigation of ROS-related oncotherapeutics.

Spatial Inhomogeneity Analysis of Cesium-Rich Wrinkles in Triple-Cation Perovskite

Chemical composition engineering in metal halide perovskite leads to enhanced stability and better transport properties, opening the gate to their integration in competitive photovoltaic tandem devices or LEDs. However, triple-cation perovskites show morphological, chemical, optical, and optoelectronic heterogeneities. In this study, we focus on micrometric spatial inhomogeneities where we observe wrinkle formation in the fabrication process linked to the cesium addition. Electron-dispersive spectroscopy and photoluminescence (PL) hyperspectral imaging designate these morphological features as Cs-rich and N-poor, which underlines the role of long-range chemical migration in the perovskite formation. We also study charge-carrier diffusion and recombination using time-resolved PL imaging under wide-field illumination, which we correlate with 3D drift diffusion modeling. We underline the impact of wrinkles on local optoelectronic properties such as the bandgap opening due to Cs enrichment and the longer charge carrier lifetime due to lower trap density.

Using Ultrafast Responsive Phosphorescent Nanoprobe to Visualize Elevated Peroxynitrite In Vitro and In Vivo via Ratiometric and Time-Resolved Photoluminescence Imaging.

Peroxynitrite (ONOO- ), a potent biological oxidant, which has a short half-life in physiological conditions, is related to many diseases. Accurate peroxynitrite determination with superior selectivity and sensitivity is important for understanding biological roles of peroxynitrite in different health and disease tissues. Autofluorescence is an inevitable interference in luminescence biodetection and bioimaging, which often reduces signal-to-noise ratio during detection. In this work, a phosphorescent peroxynitrite nanoprobe (MSN-ONOO) which displays two emission bands is prepared by immobilizing two long-lived phosphorescent iridium(III) complexes that are peroxynitrite-activable and -inert, respectively, into water-dispersible mesoporous silica nanoparticles. Owing to the fast response rate, excellent sensitivity and outstanding selectivity of the nanoprobe toward peroxynitrite, it is further used for peroxynitrite determination in vitro and in vivo via ratiometric photoluminescence imaging. More notably, taking advantage of the long-lived phosphorescence of MSN-ONOO, in vivo elevated peroxynitrite is imaged with diminished autofluorescence interference and improved signal-to-noise ratio via time-resolved photoluminescence imaging. As far as it is known, this is the first time for endogenous peroxynitrite detection in vivo via the time-resolved photoluminescence imaging. Furthermore, the production of peroxynitrite in inflamed tissues is visualized.

Raman mapping and time-resolved photoluminescence imaging for the analysis of a cross-section from a modern gypsum sculpture

This work proposes a non-destructive multi-analytical approach, based on Raman and photoluminescence mapping to characterize the constituent materials and the stratigraphy of a multi-layer sample from a 19th century sculpture by Pompeo Marchesi. The study exploits a macro-Raman device, capable of mapping heterogeneities on a scale from sub mm2 to cm2 without the need for refocusing. The combination of the macro-Raman probe with a more conventional micro-probe improves the quality of Raman spectra and allows the material identification with a high level of confidence. Time-resolved photoluminescence microscopy allows the mapping of photoluminescent centres in the micro-sample, ascribed to crystal defects in inorganic materials. The proposed combination of optical analytical techniques provides an in-depth characterization of the constituent materials of the sample, yielding insights on its conservation history. The protocol could be potentially extended to the analysis of cross-sections of paintings.

Dual wavelength excitation for the time-resolved photoluminescence imaging of painted ancient Egyptian objects

The scientific imaging of works of art is crucial for the assessment of the presence and distribution of pigments and other materials on surfaces. It is known that some ancient pigments are luminescent: these include pink red-lakes and the blue and purple pigments Egyptian Blue (CaCuSi4O10), Han blue (BaCuSi4O10) and Han purple (BaCuSi2O6). Indeed, the unique near-infrared luminescence emission of Egyptian blue allows the imaging of its distribution on surfaces. We focus on the imaging of the time-resolved photoluminescence of ancient Egyptian objects in the Burri Collection from the Civic Museum of Crema and of the Cremasco (Italy). Time-resolved photoluminescence images have been acquired using excitation at 355 nm for detecting the ns-emission of red lakes and binding media; by employing 532 nm excitation Egyptian blue is probed, and the spatial distribution of its long-lived microsecond emission is imaged. For the first time we provide data on the photoluminescence lifetime of Egyptian blue directly from objects. Moreover, we demonstrate that the use of a pulsed laser emitting at two different wavelengths increases the effectiveness of the lifetime imaging technique for mapping the presence of emissions from pigments on painted surfaces. Laser-induced luminescence spectra from different areas of the objects and traditional digital imaging, using led-based lamps, long pass filters and a commercial photographic camera, complement the results from photoluminescence lifetime imaging. We demonstrate the versatility of a new instrumental setup, capable of recording decay emission kinetics with lifetimes from nanosecond to microseconds. While the combined wavelength approach for the imaging of emissions from different materials has been demonstrated for the study of ancient Egyptian pigments (both organic and inorganic), the method could be extended to the analysis of modern pigments and paintings.

Ultrahigh exciton diffusion in intrinsic diamond

We observe exceptionally high diffusion of excitons in diamond using time-resolved photoluminescence imaging. The diffusivity is found to increase with decreasing temperature more rapidly than well-established predictions. The highest diffusion constant, $9.2\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\text{s}$, measured for thermalized excitons is 150 times that recorded previously in diamond. We elucidate the momentum relaxation mechanisms determining transport: intraband and interband scattering by acoustic phonons in the exciton fine-structure levels. The enhanced diffusivity above 100 K is explained by a free-carrier contribution.

Time-resolved photoluminescence imaging with electronic shuttering using an image intensifier unit

In contrast to traditional steady-state photoluminescence imaging (PLI), time-resolved photoluminescence imaging (TR-PLI) allows for a calibration-free measurement of the effective transient minority charge carrier lifetime τeff in a silicon sample. For transient photoluminescence measurements, the illumination source as well as the camera signal have to be modulated on a time-scale in the order of τeff. Different approaches for camera signal modulation have been presented, including the use of a complementary metal-oxide-semiconductor (CMOS) camera or a rotating shutter wheel. In this work, the use of an InGaAs-based image intensifier unit as a fast optical shutter for TR-PLI was evaluated. Due to the fast switching times of the image intensifier, effective lifetimes down to 1/50 of the modulation period could be resolved reliably. Measurements under different illumination conditions allow for an injection-dependant analysis of τeff and comparison to photoconductance decay measurements.

Minority charge carrier lifetime mapping of crystalline silicon wafers by time-resolved photoluminescence imaging

A camera-based method to record spatially and time-resolved photoluminescence images of crystalline silicon wafers was developed. The camera signal is modulated by a rotating shutter wheel, allowing for a wide range of camera types to be used for the measurement and easy integration into existing photoluminescence setups. The temporal resolution is sufficient to record the decay curve of photoexcited charge carriers in surface-passivated silicon wafers. A transient measurement of minority carrier lifetimes down to less than 10 μs can be obtained for each pixel individually, without the need for any external calibration.

Two-body decay of thermalized excitons inCu2O

We have examined the decay of thermalized excitons in cuprous oxide $({\mathrm{Cu}}_{2}\mathrm{O})$ and determined their lifetime against two-body decay (i.e., Auger recombination). The experiments are conducted at $T=70 \mathrm{K}$ with near-resonant picosecond excitation to ensure a thermal equilibrium between orthoexcitons, paraexcitons, and the crystal lattice. Time-resolved spectroscopy reveals the gas reaching equilibrium with the lattice temperature in 0.5 ns. The wavelength of the excitation photons and the spatial distribution of the laser beam are selected to produce a well defined spatial distribution of excitons. Time-resolved photoluminescence imaging measures the diffusion of excitons. From absolute measurements of the gas volume and the luminescence intensity, we determine the instantaneous gas density. At high excitation levels, a rapid nonexponential decay of the excitonic gas is observed. The decay curve is well explained by assuming that the local exciton density is governed by the rate equation $dn/dt=\ensuremath{-}{\mathrm{An}}^{2}\ensuremath{-}n/\ensuremath{\tau},$ with an Auger constant $A=0.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16} {\mathrm{cm}}^{3}/\mathrm{n}\mathrm{s}$ and a residual decay time $\ensuremath{\tau}=300 \mathrm{ns}.$ This value of the Auger constant is comparable to that estimated previously for a nonthermalized exciton gas at a lattice temperature of 2 K, indicating that the Auger lifetime of an exciton is only weakly dependent on its kinetic energy. The Auger process characterized here defines the practical limits for exciton densities in cuprous oxide.

Time-dependent heterointerfacial band bending and quasi-two-dimensional excitonic transport in GaAs structures

Using a time-resolved photoluminescence imaging technique with high spectral, temporal, and spatial resolution, we have directly measured the time- and carrier-density-dependent heterointerfacial band bending in GaAs/Al{sub x}Ga{sub 1{minus}x}As structures, and its influence on quasi-two-dimensional exciton transport. We find the spatially nonuniform band bending induced by laser excitation decays to its static, uniform band bending in {approximately}20thinspns. Further, we find that the observed transport is initially {open_quotes}driven{close_quotes} by a force originating from the spatially nonuniform band bending arising from photoexcited carrier screening of the built-in heterointerface field and is of very high mobility. This transport becomes diffusive asymptotically in time. {copyright} {ital 1998} {ital The American Physical Society}

Optical studies of heterointerfacial growth interrupts in type-II GaAs/AlAs superlattices by time resolved photoluminescence imaging

We have quantitatively characterized the heterointerfaces in type-II GaAs/AlAs short-period structures with various types of growth interrupts. The quality of the heterointerfaces is governed by two factors: the magnitude of the potential fluctuations or interface roughness and the density of nonradiative defect centers incorporated at the heterointerfaces during growth. We have quantified these aspects of the heterointerfaces through photoluminescence (PL), PL time-decay, and time-resolved PL-imaging measurements. We find that growth interrupts at the normal heterointerfaces significantly improves the quality of the interfaces over growth with no interrupts in regard to the interface roughness. Further, the transport of the excitons along the heterointerfaces may be explained by interface roughness induced scattering. We also find that growth interrupts at only the normal interfaces substantially reduces the nonradiative trap densities at the heterointerfaces.