Short Lifetime Research Articles

Measuring the Electronic Bandgap of Carbon Nanotube Networks in Non-Ideal p-n Diodes.

The measurement of the electronic bandgap and exciton binding energy in quasi-one-dimensional materials such as carbon nanotubes is challenging due to many-body effects and strong electron-electron interactions. Unlike bulk semiconductors, where the electronic bandgap is well known, the optical resonance in low-dimensional semiconductors is dominated by excitons, making their electronic bandgap more difficult to measure. In this work, we measure the electronic bandgap of networks of polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWCNTs) using non-ideal p-n diodes. We show that our s-SWCNT networks have a short minority carrier lifetime due to the presence of interface trap states, making the diodes non-ideal. We use the generation and recombination leakage currents from these non-ideal diodes to measure the electronic bandgap and excitonic levels of different polymer-wrapped s-SWCNTs with varying diameters: arc discharge (~1.55 nm), (7,5) (0.83 nm), and (6,5) (0.76 nm). Our values are consistent with theoretical predictions, providing insight into the fundamental properties of networks of s-SWCNTs. The techniques outlined here demonstrate a robust strategy that can be applied to measuring the electronic bandgaps and exciton binding energies of a broad variety of nanoscale and quantum-confined semiconductors, including the most modern nanoscale transistors that rely on nanowire geometries.

An unusual CAr^CNHC- and bidentate NAr^CNHC-cyclometalated Re(I)-NHC complexes with quinoline-functionalized ligands: Syntheses, structures, and blue-luminescent properties

An unusual CAr^CNHC-cyclometalated rhenium(I) tetracarbonyl complexes fac- (CAr^CNHC)Re(CO)4 (13) (the first report) and six NAr^CNHC-cyclometalated rhenium(I) tricarbonyl iodide/chloride complexes with quinoline-functionalized N-heterocyclic carbenes (NHC), fac-Re(CO)3(LNHC)I / fac-Re(CO)3(LNHC)Cl (LNHC = 1-(quinolin-8-yl)-3-methyl-1H-imidazol-2-ylidene (4, 10); 1-(quinolin-8-yl)-4-methyl-1H-1,2,4-triazoline-5-ylidene (5, 11); 1-(quinolin-8-yl)-3-benzyl-1H-imidazol-2-ylidene (6, 12) have been synthesized under controllable reaction conditions and characterized by their 1H NMR, 13C NMR, ESI–MS, IR spectra, and single-crystal X-ray diffraction analysis. X-ray crystallography revealed that the Re(I)-NHC complex 13 is a rare five-member bidentate CAr^CNHC-cyclometalated structure containing the coplanar system of quinoline and imidazole ring with a dihedral angle of 7.6°. The density functional theory (DFT) calculations also indicated that the blue shift of 50 nm for the MLCT absorption band of 13 is in good agreement with the coplanar five-member cyclometalated structure and the Re(I)-NHC complex 10 is a thermodynamic product. Seven Re(I)-NHC complexes exhibit photoluminescence emission in the blue region (410–470 nm) with relatively short decay lifetime on the nanosecond time scale, and high quantum yields of the Re(I)-NHC complexes 5 (Φ = 0.558) and 6 (Φ = 0.629) based-triazole were calculated by using [Ru(bpy)3](PF6)2 in degassed acetonitrile solution (Φstd = 0.062) as standard, due to the coordination of quinoline rings, NHC ligands to Re(I) center.

To Be or Not to Be: Will Virtual Worlds and the Metaverse Gain Lasting Traction?

Since their beginnings, virtual worlds have experienced two major media hypes in their short lifetime—the first in 2003 after the launch of Second Life and the second in 2021, with Mark Zuckerberg announcing his strategy for the Metaverse. Most academic research on virtual worlds emerged between these two peaks of interest. This article delves into the enduring relevance of such research, contrasting the two hype cycles. It analyzes the reasons behind current caution expressed by companies and offers insights into the future trajectory of the Metaverse and which crucial developments are needed for sustained traction in virtual worlds.

Modulating Heavy Atom Effect in Germylene for Persistent Room Temperature Phosphorescence.

It is worth but still challenging to develop the low-valent main group compounds with persistent room temperature phosphorescence (pRTP). Herein, we presented germylene-based persistent phosphors by introduction of low-valent Ge center into chromophore. A novel phosphors CzGe and its series of derivatives, namely CzGeS, CzGeSe, CzGeAu, and CzGeCu, were synthesized. Experiments and theoretical calculations reveal that the pRTP behavior were "turn on" due to the heavy atom effect of germylene. More importantly, the low-valent of oxidation state and structural traits propelled GeCz had a balance between the intersystem crossing and the shortening of lifetime caused by the heavy atoms, resulting the ultralong lifetime of 309 ms and phosphorescent quantum efficiency of 15.84 %, which is remarkable among heavy main group phosphors. This research provides valuable insights to the design of heavy atoms in phosphors and expand the applications of germylene chemistry.

Hot Electrons in a Steady State: Interband vs Intraband Excitation of Plasmonic Gold.

Understanding the dynamics of "hot", highly energetic electrons resulting from nonradiative plasmon decay is crucial for optimizing applications in photocatalysis and energy conversion. This study presents an analysis of electron kinetics within plasmonic metals, focusing on the steady-state behavior during continuous-wave (CW) illumination. Using an inelastic spectroscopy technique, we quantify the temperature and lifetimes of distinct carrier populations during excitation. A significant finding is the monotonic increase in hot electron lifetime with decreases in electronic temperature. We also observe a 1.22× increase in hot electron temperature during intraband excitation compared to interband excitation and a corresponding 2.34× increase in carrier lifetime. The shorter lifetimes during interband excitation are hypothesized to result from direct recombination of nonthermal holes and hot electrons, highlighting steady-state kinetics. Our results help bridge the knowledge gap between ultrafast and steady-state spectroscopies, offering critical insights for optimizing plasmonic applications.

Review of actinide core-level photoemission

Photoelectron spectroscopy allows for the investigation of the electronic structure and chemical bonding of actinide elements and their compounds, providing insights into oxidation states, chemical environments, and electronic configurations. This knowledge can aid in comprehending reactivity, stability, and other properties of actinide materials, which is essential for ensuring safe handling, storage, and disposal in nuclear applications. We have reviewed a number of results in actinide core-level photoemission studies, with a particular focus on x-ray photoemission spectroscopy (XPS) techniques. Actinides, due to their inherent radioactivity, have not been as well studied with XPS as have other segments of the periodic table. Given the inherent safety concerns, equipment requirements, and short isotopic lifetimes associated with actinide research, we outline the strategies and precautions necessary for conducting successful and safe XPS experiments on these elements. Core-level photoemission can be a powerful proven tool for investigating the electronic structure, chemical bonding behaviors, and physical properties of actinides, providing valuable insights into an incredibly complex behavior of these systems. We highlight key findings from recent studies that demonstrate the potential of core-level photoemission in uncovering the unique properties of actinides and their compounds. Finally, we identify current knowledge gaps and future research directions that could enhance our understanding of actinide chemistry and physics.

Unraveling the Photocatalytic Mechanism of N2 Fixation on Single Ruthenium Sites.

Photocatalytic N2 fixation offers promise for ammonia synthesis, yet traditional photocatalysts encounter challenges such as low efficiency and short carrier lifetimes. Atomically precise ligand-metal nanoclusters emerge as a solution to address these issues, but the photophysical mechanism remains elusive. Inspired by the synthesis of Au4Ru2 NCs, we investigate the mechanism behind N2 activation on Au4Ru2, focusing on photoactivity and carrier dynamics. Our results reveal that vibration of the Ru-N bond in the low-frequency domain suppresses the deactivation process leading to a long lifetime of the excited N2. By the strategy of isoelectronic substitution, we identify the single Ru sites as the active sites for N2 activation. Furthermore, these ligand-protected M4Ru2 (M = Au, Ag, Cu) NCs show robust thermal stability in explicit solvation and decent photochemical activity for N2 activation and NH3 production. These findings have significant implications for the optimization of catalysts for sustainable ammonia synthesis.

Evaluating NOx stack plume emissions using a high-resolution atmospheric chemistry model and satellite-derived NO2 columns

Abstract. This paper presents large-eddy simulations with atmospheric chemistry of four large point sources world-wide, focusing on the evaluation of NOx (NO + NO2) emissions with the TROPOspheric Monitoring Instrument (TROPOMI). We implemented a condensed chemistry scheme to investigate how the emitted NOx (95 % as NO) is converted to NO2 in the plume. To use NOx as a proxy for CO2 emission, information about its atmospheric lifetime and the fraction of NOx present as NO2 is required. We find that the chemical evolution of the plumes depends strongly on the amount of NOx that is emitted, as well as on wind speed and direction. For large NOx emissions, the chemistry is pushed in a high-NOx chemical regime over a length of almost 100 km downwind of the stack location. Other plumes with lower NOx emissions show a fast transition to an intermediate-NOx chemical regime, with short NOx lifetimes. Simulated NO2 columns mostly agree within 20 % with TROPOMI, signalling that the emissions used in the model were approximately correct. However, variability in the simulations is large, making a one-to-one comparison difficult. We find that temporal wind speed variations should be accounted for in emission estimation methods. Moreover, results indicate that common assumptions about the NO2 lifetime (≈ 4 h) and NOx:NO2 ratios (≈ 1.3) in simplified methods that estimate emissions from NO2 satellite data need revision.

Enhanced amplified spontaneous emission performance through effective regulation of phase distribution in Ruddlesden-Popper perovskite films.

Ruddlesden-Popper (RP) perovskites promise next-generation gain media for laser devices. However, most RP perovskite lasers are still suffering from inferior performance characteristics, such as inadequate energy transfer, unstable emission, and short lifetime. To address the above problems, high crystalline quality, compact, and smooth PEA2FA2Pb3Br10 films with uniform phase distribution were successfully prepared by ionic liquid (IL) methylammonium acetate (MAAc) in an air environment. Compared with the PEA2FA2Pb3Br10 film prepared by the traditional solvent dimethyl sulfoxide (DMSO), an enhanced amplified spontaneous emission (ASE) with a lower threshold of 58 µJ·cm-2 from the MAAc-treated film was obtained under nanosecond laser excitation. The transient absorption (TA) spectroscopy revealed that a uniform phase distribution and more efficient energy transfer processes were achieved in the PEA2FA2Pb3Br10-MAAc film, leading to an enhanced band-to-band spontaneous emission process. Furthermore, the films exhibited better stability, showing no signs of degradation under the 120 min pulsed laser pumping in air and stability of ASE spectra at even 95% humidity conditions. This study provides an important foundation for achieving high-performance optically pumped lasers based on the unique RP perovskites.

Leveraging Compatible Iridium(III) Complexes to Boost Performance of Green Solvent‐Processed Non‐Fullerene Organic Solar Cells

AbstractIn organic solar cells (OSCs), the short exciton lifetime poses a significant limitation to exciton diffusion and dissociation. Extending exciton lifetime and suppressing recombination are crucial strategies for improving the OSC performance. Herein, an effective approach is proposed by introducing the phosphorescent emitter, tris(2‐(4‐(tert‐butyl)phenyl)‐5‐fluoropyridine)Iridium(III), with long‐lived triplet exciton lifetime in OSCs. This research reveals that the steric structure of fac‐Ir(tBufppy)3 exhibits excellent compatibility with both the donor PM6 and acceptor BTP‐eC9, maintaining efficiencies of over 90% even with a 30% third component loading. Moreover, a 10% addition of fac‐Ir(tBufppy)3 mitigates excessive aggregation in the acceptor BTP‐eC9, optimizing the active layer morphology and improving the fill factor. Transient absorption spectroscopy and transient photoluminescence measurements demonstrate that the introduction of fac‐Ir(tBufppy)3 significantly extends exciton lifetimes and suppresses recombination, which increases the short‐circuit current (JSC). Ultimately, employing the non‐halogenated solvent o‐xylene for processing, an impressive power conversion efficiency (PCE) of 18.54% is achieved in devices based on PM6:10%fac‐Ir(tBufppy)3:BTP‐eC9, surpassing the efficiency of binary PM6:BTP‐eC9 devices (17.41%). This work provides a promising approach to further improve the PCEs in binary OSCs by introducing a phosphorescent iridium(III) complex as the third component.

Blue-green emitting ZnS0.75O0.25:Ce3+,x%Tb3+ phosphor with tunable fluorescence lifetime

A series of ZnS0.75O0.25:0.1%Ce3+,x%Tb3+ phosphors were prepared by high temperature solid state reaction method. These phosphors exhibited two mixed phases consisting of hexagonal phase ZnS and hexagonal phase ZnO with the average particle size of 13.83 μm and emitted blue-green light. The luminescence mechanism consisted of Zn vacancy defects, the 5d1 → 2F5/2 radiative transitions of Ce3+, the 5D4 → 7F5 and 5D4 → 7F6 radiative transition of Tb3+ induced by the energy transfer of Ce3+ → Tb3+. An equation for the variation of the fluorescence lifetime of ZnS0.75O0.25:0.1%Ce3+,x%Tb3+ phosphors with concentration of Tb3+ fraction was obtained by exponential fitting. The short fluorescence lifetime could be tuned within the range of 113 μs to 550 μs with the increase of Tb3+ concentration, and the color was tunable from blue to blue-green, which is of important application in the field of high-energy particle detection.

Beyond the First Coordination Sphere─Manipulating the Excited-State Landscape in Iron(II) Chromophores with Protons.

Molecular transition metal chromophores play a central role in light harvesting and energy conversion. Recently, earth-abundant transition-metal-based chromophores have begun to challenge the dominance of platinum group metal complexes in this area. However, the development of new chromophores with optimized photophysical properties is still limited by a lack of synthetic methods, especially with respect to heteroleptic complexes with functional ligands. Here, we demonstrate a facile and efficient method for the combination of strong-field carbenes with the functional 2,2'-bibenzimidazole ligand in a heteroleptic iron(II) chromophore complex. Our approach yields two isomers that differ predominantly in their excited-state lifetimes based on the symmetry of the ligand field. Deprotonation of both isomers leads to a significant red-shift of the metal-to-ligand charge transfer (MLCT) absorption and a shortening of excited-state lifetimes. Femtosecond transient absorption spectroscopy in combination with quantum chemical simulations and resonance Raman spectroscopy reveals the complex relationship between protonation and photophysical properties. Protonation is found to tip the balance between MLCT and metal-centered (MC) excited states in favor of the former. This study showcases the first example of fine-tuning of the excited-state landscape in an iron(II) chromophore through second-sphere manipulations and provides a new perspective to the challenge of excited-state optimizations in 3d transition metal chromophores.

Revealing the Optical Transition Properties of Interlayer Excitons in Defective WS2/WSe2 Heterobilayers.

Manipulation of physical properties in multidimensional tunable moiré superlattice systems is a key focus in nanophotonics, especially for interlayer excitons (IXs) in two-dimensional materials. However, the impact of defects on IXs remains unclear. Here, we thoroughly study the optical properties of WS2/WSe2 heterobilayers with varying defect densities. Low-temperature photoluminescence (PL) characterizations reveal that the low-energy IXs are more susceptible to defects compared to the high-energy IXs. The low-energy IXs also show much faster PL quenching rate with temperature, faster peak width broadening rate with laser power, shorter lifetime, and lower circular polarization compared to the low-energy IXs in the region with fewer defects. These effects are attributed to the combined effects of increased electron scattering, exciton-phonon interactions, and nonradiative channels introduced by the defects. Our findings aid in optimizing moiré superlattice structures.

Positive Effects of Guanidinium Salt Post-Treatment on Multi-Cation Mixed Halide Perovskite Solar Cells.

As one of the most promising photovoltaic technologies, perovskite solar cells (PSCs) exhibit high absorption coefficients, tunable bandgaps, large carrier mobilities, and versatile fabrication techniques. Nevertheless, the commercialization of the technology is hindered by poor material stability, short device lifetimes and the scalability of fabrication techniques. To address these technological drawbacks, various strategies have been explored, with one particularly promising approach involving the formation of a low-dimensional layer on the surface of the three-dimensional perovskite film. In this work, we demonstrate the use of guanidinium tetrafluoroborate, CH6BF4N3, (GATFB) as a post-treatment step to enhance the performance of PSCs. Compared with the control sample, the application of GATFB improves the film surface topology, reduces surface defects, suppresses non-radiative recombination, and optimizes band alignment within the device. These positive effects reduce recombination losses and enhance charge transport in the device, resulting in PSCs with an open-circuit voltage (VOC) of 1.18 V and a power conversion efficiency (PCE) of 19.7%. The results obtained in this work exhibit the potential of integrating low-dimensional structures in PSCs as an effective approach to enhance the overall device performance, providing useful information for further advancement in this rapidly evolving field of photovoltaic technology.

Energy Efficient Data Aggregation in Wireless Sensor Networks Using Meta Heuristic Based Feed Forward Back Propagation Neural Network Approach

Sensor nodes are low-cost, low-power, tiny devices that make up the majority of WSNs, or distributed, self-organizing systems. These sensor nodes are able to exchange, perceive, and interpret data. The sensor nodes are equipped with a wide variety of sensors, such as chemical, touch, motion, temperature, and weather sensors. Because of its adaptability, sensors are used in a variety of applications such as automation, tracking, monitoring, and surveillance. Despite the enormous number of sensor applications, WSNs continue to suffer from common challenges like as low memory, slow processing speed, and short network lifetime. The feed forward back propagation neural network mode (FFBPNN) based on meta heuristics aims to create many paths for effective data aggregation in wireless sensor networks. This model handled the process of identifying and selecting the optimum route path. The distributed sensor nodes are utilized to create the various route paths. In this research paper, data aggregation is done using meta-heuristic firefly algorithm that helped in identifying an optimal route from among the found routes. After selecting the operative ideal route choice, the data aggregation procedure practices a rank-based approach to accomplish lower latency and a better packet delivery ratio(PDR). In addition to throughput, simulation was done to improve and measure performance in terms of packet delivery ratio, energy consumption, and end-to-end latency.

An Organophosphorescence Probe with Ultralong Lifetime and Intrinsic Tissue Selectivity for Specific Tumor Imaging and Guided Tumor Surgery.

Organic phosphorescent materials are excellent candidates for use in tumor imaging. However, a systematic comparison of the effects of the intensity, lifetime, and wavelength of phosphorescent emissions on bioimaging performance has not yet been undertaken. In addition, there have been few reports on organic phosphorescent materials that specifically distinguish tumors from normal tissues. This study addresses these gaps and reveals that longer lifetimes effectively increase the signal intensity, whereas longer wavelengths enhance the penetration depth. Conversely, a strong emission intensity with a short lifetime does not necessarily yield robust imaging signals. Building upon these findings, an organo-phosphorescent material with a lifetime of 0.94 s was designed for tumor imaging. Remarkably, the phosphorescent signals of various organic nanoparticles are nearly extinguished in blood-rich organs because of the quenching effect of iron ions. Moreover, for the first time, we demonstrated that iron ions universally quench the phosphorescence of organic room-temperature phosphorescent materials, which is an inherent property of such substances. Leveraging this property, both the normal liver and hepatitis tissues exhibit negligible phosphorescent signals, whereas liver tumors display intense phosphorescence. Therefore, phosphorescent materials, unlike chemiluminescent or fluorescent materials, can exploit this unique inherent property to selectively distinguish liver tumor tissues from normal tissues without additional modifications or treatments.

TADF Blue Emitters with Balanced π-Conjugation─Design, Synthesis, Spectral Characterization, and a Model OLED with 8-(5-(tert-Butyl)-1,3,4-oxadiazol-2-yl)-N,N-bis(4-(tert-butyl)phenyl)dibenzo[b,d]furan-2-amine.

Blue organic light-emitting diodes (OLED) suffer from relatively short lifetimes and a comparatively low lighting efficiency. One of the approaches to improving their characteristics is the development of luminophores with the potential for thermally activated delayed fluorescence (TADF). Herein, a set of donor-spacer-acceptor compounds with potential for TADF are designed, synthesized, and computationally and spectroscopically characterized. The excited state dynamics of the most prospective dye is monitored by time-resolved fluorescence and transient absorption spectroscopy. The experimental data are obtained and processed by a newly developed method and supplemented by quantum chemical calculations. The comprehensive approach allowed rationalization of the complex cascade-type photophysical behavior. The most promising emitter is included in an OLED displaying a blue color with a maximum EQE of 4.9% and negligible efficiency roll-off at higher luminance.

An Organophosphorescence Probe with Ultralong Lifetime and Intrinsic Tissue Selectivity for Specific Tumor Imaging and Guided Tumor Surgery

AbstractOrganic phosphorescent materials are excellent candidates for use in tumor imaging. However, a systematic comparison of the effects of the intensity, lifetime, and wavelength of phosphorescent emissions on bioimaging performance has not yet been undertaken. In addition, there have been few reports on organic phosphorescent materials that specifically distinguish tumors from normal tissues. This study addresses these gaps and reveals that longer lifetimes effectively increase the signal intensity, whereas longer wavelengths enhance the penetration depth. Conversely, a strong emission intensity with a short lifetime does not necessarily yield robust imaging signals. Building upon these findings, an organo‐phosphorescent material with a lifetime of 0.94 s was designed for tumor imaging. Remarkably, the phosphorescent signals of various organic nanoparticles are nearly extinguished in blood‐rich organs because of the quenching effect of iron ions. Moreover, for the first time, we demonstrated that iron ions universally quench the phosphorescence of organic room‐temperature phosphorescent materials, which is an inherent property of such substances. Leveraging this property, both the normal liver and hepatitis tissues exhibit negligible phosphorescent signals, whereas liver tumors display intense phosphorescence. Therefore, phosphorescent materials, unlike chemiluminescent or fluorescent materials, can exploit this unique inherent property to selectively distinguish liver tumor tissues from normal tissues without additional modifications or treatments.

Approaching the low thermal conductivity in layered oxychalcogenide Bi2-xPrxO2Se (0 ≤ x ≤ 0.15) via mass and strain field fluctuatiocn for thermoelectric application

Bi2O2Se, a layered oxychalcogenide has gained much attention among the conventional oxides and chalcogenides based thermoelectric materials because of its high stability and less toxicity. Herein, Bi2-xPrxO2Se (0 ≤ x ≤ 0.15) is synthesized by ball milling followed by hot press densification technique, the x represents the replacement of Bi with Pr in atomic%. The influence of isovalent Pr substitution on the structural and thermal transport properties of Bi2O2Se were studied. The formation of tetragonal crystal structure with space group I4/mmm is confirmed by X ray diffraction (XRD) Pattern. HRTEM micrographs reveals the crystalline nature and lattice defects in the synthesized composition. The influence of 10 at % Pr substitution at Bi site reduces the thermal conductivity to 0.276 Wm−1K−1 at 653 K, originated from short phonon lifetime due to the effect of mass and strain field fluctuation. This result exhibits that the incorporation of Pr substitution in Bi2O2Se can dramatically enhance the thermoelectric performance of Bi2O2Se ceramic by reducing thermal conductivity.

Multispectral label-free in vivo cellular imaging of human retinal pigment epithelium using adaptive optics fluorescence lifetime ophthalmoscopy improves feasibility for low emission analysis and increases sensitivity for detecting changes with age and eccentricity.

Adaptive optics fluorescence lifetime ophthalmoscopy (AOFLIO) provides a label-free approach to observe functional and molecular changes at cellular scale in vivo. Adding multispectral capabilities improves interpretation of lifetime fluctuations due to individual fluorophores in the retinal pigment epithelium (RPE). To quantify the cellular-scale changes in autofluorescence with age and eccentricity due to variations in lipofuscin, melanin, and melanolipofuscin in RPE using multispectral AOFLIO. AOFLIO was performed on six subjects at seven eccentricities. Four imaging channels ( ) were used: 473/SSC, 473/LSC, 532/LSC, and 765/NIR. Cells were segmented and the timing signals of each pixel in a cell were combined into a single histogram, which were then used to compute the lifetime and phasor parameters. An ANOVA was performed to investigate eccentricity and spectral effects on each parameter. A repeatability analysis revealed change in lifetime parameters in repeat visits for 532/LSC. The 765/NIR and 532/LSC had eccentricity and age effects similar to previous reports. The 473/LSC had a change in eccentricity with mean lifetime and a phasor component. Both the 473/LSC and 473/SSC had changes in eccentricity in the short lifetime component and its relative contribution. The 473/SSC had no trend in eccentricity in phasor. The comparison across the four channels showed differences in lifetime and phasor parameters. Multispectral AOFLIO can provide a more comprehensive picture of changes with age and eccentricity. These results indicate that cell segmentation has the potential to allow investigations in low-photon scenarios such as in older or diseased subjects with the co-capture of an NIR channel (such as 765/NIR) with the desired spectral channel. This work represents the first multispectral, cellular-scale fluorescence lifetime comparison in vivo in the human RPE and may be a useful method for tracking diseases.