Light-induced Phase Separation Research Articles

The effects of light irradiation on CsPbIBr2 films

In the present, we investigate the light-induced phase instability and the recovery in light-free for CsPbIBr2 film. Both 585 nm and 695 nm photoluminescence (PL) peaks are detected by illuminating films. Results show that the appearance and the intensity of 585 nm and 695 nm PL peaks are affected by the wavelength, remaining time, and intensity of the illuminating light. Under short time illumination, the 695 nm PL peak may disappear after staying light-free, with light–dark reversible behavior. Under long time illumination, the 695 nm PL peak still exist in light-free and become irreversible behavior, which is caused by the formation of I-rich and Br-rich small grains, destroyed the homogeneous phase structure, and formed new steady states. X-ray diffraction analysis and the first principle calculations proved the mechanism of light-induced phase separation. We argue that CsPbIBr2-based solar cells are difficult to be applied before inhibiting the phase separation of CsPbIBr2.

Kinetics and mechanism of light-induced phase separation in a mixed-halide perovskite

•Utilized in situ cryoelectron-microscopy-cathodoluminescence spectroscopy •Observed the morphological and band-gap evolution of iodide-rich clusters in situ •Performed quantitative studies of the characteristic length scale of phase separation •Phase field modeling prediction is consistent with spinodal decomposition Halide ion phase separation is a barrier to the application of mixed-halide perovskites whereby the presence of large populations of photogenerated or injected carriers causes undesirable changes in the local band gap. We investigate the mechanism of phase separation in CsPbIxBr3-x perovskite single crystals driven by light. The phase separation process and its dynamics are visualized at the nanometer scale at cryogenic temperatures using in situ scanning transmission electron microscopy and cathodoluminescence. Combined with phase field modeling, which accounts for the coupling between electronic and chemical driving forces, our observations point to a spinodal decomposition mechanism in which both the amplitude of composition fluctuation and the characteristic length scale grow non-linearly with time. Our findings provide microscopic insights that can assist in further engineering mixed-halide perovskites either for stability or for intentional programming of the local halide ion composition, opening pathways to a wide range of applications. Halide ion phase separation is a barrier to the application of mixed-halide perovskites whereby the presence of large populations of photogenerated or injected carriers causes undesirable changes in the local band gap. We investigate the mechanism of phase separation in CsPbIxBr3-x perovskite single crystals driven by light. The phase separation process and its dynamics are visualized at the nanometer scale at cryogenic temperatures using in situ scanning transmission electron microscopy and cathodoluminescence. Combined with phase field modeling, which accounts for the coupling between electronic and chemical driving forces, our observations point to a spinodal decomposition mechanism in which both the amplitude of composition fluctuation and the characteristic length scale grow non-linearly with time. Our findings provide microscopic insights that can assist in further engineering mixed-halide perovskites either for stability or for intentional programming of the local halide ion composition, opening pathways to a wide range of applications.

A photoregulatory mechanism of the circadian clock in Arabidopsis.

Cryptochromes (CRYs) are photoreceptors that mediate light regulation of the circadian clock in plants and animals. Here we show that CRYs mediate blue-light regulation of N6-methyladenosine (m6A) modification of more than 10% of messenger RNAs in the Arabidopsis transcriptome, especially those regulated by the circadian clock. CRY2 interacts with three subunits of the METTL3/14-type N6-methyladenosine RNA methyltransferase (m6A writer): MTA, MTB and FIP37. Photo-excited CRY2 undergoes liquid-liquid phase separation (LLPS) to co-condense m6A writer proteins in vivo, without obviously altering the affinity between CRY2 and the writer proteins. mta and cry1cry2 mutants share common defects of a lengthened circadian period, reduced m6A RNA methylation and accelerated degradation of mRNA encoding the core component of the molecular oscillator circadian clock associated 1 (CCA1). These results argue for a photoregulatory mechanism by which light-induced phase separation of CRYs modulates m6A writer activity, mRNA methylation and abundance, and the circadian rhythms in plants.

WNK1 Regulates Cell Volume via Crowding‐Induced Phase Transitions

When challenged by hypertonicity, dehydrated cells recover their volume though regulatory volume increase (RVI). This process is thought to be initiated by an undefined intracellular sensing and transduction mechanism that responds to macromolecular crowding (Parker JC AJP 1993). In many cell types, RVI is mediated by the bumetanide‐sensitive cotransporter NKCC1, which is phosphorylated and activated by the WNK‐SPAK/OSR1 pathway. Reconciling the mechanism of hypertonic stress‐induced activation of NKCC1 has been challenging, as upstream WNK kinases such as WNK1, though required for bumetanide‐sensitive RVI (Roy A AJP 2015), should be inactivated by the strengthened intracellular chloride concentration caused by cell shrinkage (Piala AT Sci Signaling 2014). We describe our efforts to unravel this paradox using a combination of high‐speed live cell and fluorescence lifetime imaging, correlative light electron microscopy, gene editing, and biochemical approaches in cells. During hyperosmotic cell shrinkage and macromolecular crowding caused by reduced cell water, WNK1 dynamically concentrates into membraneless liquid droplets. These changes in WNK1 localization occur at native levels of expression and are visible within physiologic range (25 mOsm above isotonicity), triggering a cascade of condensed WNK1 autoactivation, droplet fusion, cytosolic exchange, and downstream SPAK/OSR1/NKCC1 phosphorylation within a 15 minute bumetanide‐sensitive RVI time course. When cells are rapidly switched from hypertonic to hypotonic media, the WNK1 condensates quickly dissipate, confirming their reversible nature and association with physiologic changes in cell size and intracellular crowding. An optogenetic screening assay that employs the PHR domain of Cry2 to trigger light‐induced crowding and phase separation revealed that the formation of WNK1 condensates requires its large intrinsically disordered C‐terminus, whose strong regional disorder tendency is highly conserved across evolution despite poor sequence homology. This disorder‐encoded phase behavior allows WNK kinases to react to a crowded cytosol, bypass the ionic strength dilemma, and restore cell volume via mass action. Thus, our findings indicate that in addition to their well‐appreciated role as chloride sensors, WNK kinases are molecular crowding sensors that phase separate to coordinate a conserved hyperosmotic stress response. While this evolutionarily conserved mechanism appears to be critical for WNK kinases to sense cell shrinkage and amplify the NKCC1 RVI signal, we propose that it also impacts diverse WNK‐dependent signaling processes important in mammalian epithelial and neuronal physiology.Support or Funding InformationNIH R01DK098145, R01DK119252, K08DK118211, F32HL142229, P30DK079307

Photophysical properties of halide perovskite CsPb(Br1-xIx)3 thin films and nanowires

Thin films and nanowires based on lead halide perovskites are promising objects for the design of various optoelectronic devices as well as nano- and microlasers. One of the main advantages of such materials is their absorption and photoluminescence spectra tuning across the visible range via the change in their chemical composition, for instance, by substitution of one halide atom (Br) for another one (I) in the crystal lattice of CsPb(Br1-xIx)3. However, this approach gives materials showing unstable photoluminescence behavior caused by light-induced perovskite phase separation under high-intensity excitation at room temperature. In this work, CsPb(Br1-xIx)3 thin films and nanowires are obtained by chemical vapor anion exchange method from their CsPbBr3 counterparts fabricated by improved wet chemical methods. Spontaneous and stimulated emission from the mixed-halide and pristine bromide samples are studied. Tribromide nanowires exhibit lasing with relatively low thresholds (10–100 μJ/cm2) and high Q-factor of the laser mode up to 3500. The temperature dependence of the photoinitiated phase separation in CsPbBr1.5I1.5 samples is investigated, showing that light-induced phase instability of the mixed-halide nanowires can be suppressed at the somewhat higher temperature (250 K) than the value observed for the thin films having a similar chemical composition. The results obtained are important for the optimization of the functioning of optoelectronic devices based on considered perovskite materials.

Suppression and Reversion of Light-Induced Phase Separation in Mixed-Halide Perovskites by Oxygen Passivation

Mixed-halide perovskites (MHPs), such as methylammonium lead bromide–iodide, CH3NH3Pb(BrxI1–x)3 (0 < x < 1), are very promising candidates for tandem solar cells and wavelength-tunable light-emitti...

Dielectrophoresis and colloidal phase transitions for ultra-broadband optical limiting

We demonstrate wavelength-independent optical limiting based on colloidal phase transitions induced by the dielectrophoretic force from focused electromagnetic radiation. The focused radiation acts as an optical trap that increases the particle density. The increased density then leads to a colloidal gas-solid phase transition and an aggregate that effectively blocks the incoming radiation when it passes a threshold power. The process is reversible, with the colloidal particles returning to a homogenous distribution after the incoming radiation is removed. We demonstrate the effect using polystyrene nanoparticles mixed with pluronics and polyethylene glycol polymers in low-concentration KCl salt solutions. We observe the light-induced phase separation under confocal fluorescent microscope, and we provide a proof-of-principle demonstration of optical limiting using a 100μm thick colloid cell.

Taking Control of Ion Transport in Halide Perovskite Solar Cells

Lead halide perovskites are mixed electron–ion conductors that support high rates of solid-state ion transport at room temperature, in addition to conventional electron and hole conduction. Mass transport mediated by charged defects is responsible for unusual phenomena such as current–voltage hysteresis in photovoltaic devices, anomalous above-bandgap photovoltages, light-induced lattice expansion and phase separation, self-healing, and rapid chemical conversion between halides. We outline the principles that govern ion transport in perovskite solar cells including intrinsic (point and extended defects) and extrinsic (light, heat, electrical fields, and chemical gradients) factors. These microscopic processes underpin a wide range of reported observations, including photoionic conductivity, and offer valuable directions for both limiting ion transport, where required, and harnessing it to enable new functionality.

Rationalizing the light-induced phase separation of mixed halide organic\u2013inorganic perovskites

Mixed halide hybrid perovskites, CH3NH3Pb(I1−xBrx)3, represent good candidates for low-cost, high efficiency photovoltaic, and light-emitting devices. Their band gaps can be tuned from 1.6 to 2.3 eV, by changing the halide anion identity. Unfortunately, mixed halide perovskites undergo phase separation under illumination. This leads to iodide- and bromide-rich domains along with corresponding changes to the material’s optical/electrical response. Here, using combined spectroscopic measurements and theoretical modeling, we quantitatively rationalize all microscopic processes that occur during phase separation. Our model suggests that the driving force behind phase separation is the bandgap reduction of iodide-rich phases. It additionally explains observed non-linear intensity dependencies, as well as self-limited growth of iodide-rich domains. Most importantly, our model reveals that mixed halide perovskites can be stabilized against phase separation by deliberately engineering carrier diffusion lengths and injected carrier densities.

Forming Spacers in Situ by Photolithography to Mechanically Stabilize Electrofluidic-Based Switchable Optical Elements.

Electro-Fluidic Displays (EFD) have been demonstrated to be an attractive technology for incorporation into portable display devices. EFDs have excellent optical efficiency and fast switching enabling video content. Ensuring mechanical stability of EFD display cells is a key challenge and essential for developing large area as well as flexible displays. Although the electro-optic performance of an EFD, unlike a liquid crystal display (LCD), is insensitive to cell-gap, extreme changes in cell-gap can result in irreversible collapse of the cell. Here we use photolithography to develop spacers to prevent cell-gap collapse and provide the required mechanical stability for EFD devices. The spacer is formed directly on the cover plates (ITO/glass) after cell assembly with UV light induced phase separation polymerization in the illuminated area. Phase separation behavior between polar aqueous solution and polymer is closely related to the solubility of acrylate monomers. In this work, polyethylene glycol diacrylate (PEGDA) as cross-linker, 2-hydroxyethyl acrylate (HEA) and acrylic acid or acrylamide as co-monomers are investigated for fabricating the spacers. PEGDA was added to the mixtures in order to increase the mechanical strength of the spacer. The spacers showed excellent performance for cell-gap control in EFD devices.

Nanoparticle-Mediated, Light-Induced Phase Separations.

Nanoparticles that both absorb and scatter light, when dispersed in a liquid, absorb optical energy and heat a reduced fluid volume due to the combination of multiple scattering and optical absorption. This can induce a localized liquid-vapor phase change within the reduced volume without the requirement of heating the entire fluid. For binary liquid mixtures, this process results in vaporization of the more volatile component of the mixture. When subsequently condensed, these two steps of vaporization and condensation constitute a distillation process mediated by nanoparticles and driven by optical illumination. Because it does not require the heating of a large volume of fluid, this process requires substantially less energy than traditional distillation using thermal sources. We investigated nanoparticle-mediated, light-induced distillation of ethanol-H2O and 1-propanol-H2O mixtures, using Au-SiO2 nanoshells as the absorber-scatterer nanoparticle and nanoparticle-resonant laser irradiation to drive the process. For ethanol-H2O mixtures, the mole fraction of ethanol obtained in the light-induced process is substantially higher than that obtained by conventional thermal distillation, essentially removing the ethanol-H2O azeotrope that limits conventional distillation. In contrast, for 1-propanol-H2O mixtures the distillate properties resulting from light-induced distillation were very similar to those obtained by thermal distillation. In the 1-propanol-H2O system, a nanoparticle-mediated, light-induced liquid-liquid phase separation was also observed.

A light-induced reversible phase separation and its coupling to a dynamic library of imines

Irradiation of an acetonitrile–water solution of the bis-pyridyl hydrazone 1 and calcium chloride causes a photo-induced phase separation. It is coupled to a covalent library of imines, undergoing constitutional reorganization upon phase separation.

Homeotropically aligning phase separated columnar structures for fabrication of flexible electrooptical devices

A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photo-curable pre-polymer is reported. Vertical polymer columns and micro-fibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LC-substrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electro-optical devices with high contrast coupled with fast response times. This simple single-step technique eliminates the need for the traditional polymer alignment layer pre-deposited on substrates.

Evolution of self-organized spin domains under light in single-crystalline[Fe(ptz)6](BF4)2

We study domain growth upon the light-induced phase separation (LIPS) in the spin-crossover $[\text{Fe}{(\text{ptz})}_{6}]{({\text{BF}}_{4})}_{2}$ single crystal under clarified conditions of intensity of light and temperature. Our primary motivation is to model the relaxation behavior of a spin-crossover system under light in the spinodal regime. At this end, we built a discrete spatiotemporal hierarchy of coupled equations, which can be regarded as an efficient scheme for simulating the phase separation under light as well as for obtaining the equilibrium mean-field solutions of lattice models having complex structures. We found that in the spinodal regime under light, initial homogeneous states self-organize into spin domains after some incubation time. The evolution patterns of the high- and low-spin domains revealed that the self-organization proceeds roughly in two regimes, viz. diffusion of interfaces in the early stage followed by a growth regime. The analysis of the correlation function shows that the characteristic domain size behaves as $R(t)\ensuremath{\sim}{t}^{1/2}$, following the Allen-Cahn law.

Photoinduced phase separation in spin-crossover materials: Numerical simulation of a dynamic photocrystallographic experiment

The dynamics of photoinduced phase transition in spin-crossover materials is studied using numerical simulations of a microscopic two-variable anharmonic model, in close connection with photocrystallographic experiments on the prototype system $[\text{Fe}{(\text{btr})}_{2}{(\text{NCS})}_{2}].{\text{H}}_{2}\text{O}$. A comparative analysis of the simulated diffraction pattern with excitation duration and intensity as variables is performed. Nonlinear dynamics, threshold effect in excitation intensity and light-induced phase separation are modeled and attributed to a strong electron-lattice coupling, mediated by long-range elastic interactions. Photoinduced nucleation and domain growth of the metastable high-spin phase is evidenced and quantitatively explained in the Avrami formalism.

Kinetics of photo-induced phase transition and relaxation in the spin-crossover complexes [FexZn1-x(phen)2(NCS)2], influence of metal dilution

The kinetics of light-induced spin transition and relaxation in [FexZn1-x(phen)2(NCS)2] (phen=1,10-phenanthroline) has been investigated from time/temperature dependent x-ray powder diffraction with in situ optical excitation. We show that the phase transformation is driven by a heterogeneous nucleation and growth mechanism with phase separation, in both the thermally induced and light-induced regimes. The high spin to low spin isothermal relaxation curves strongly differ from first-order kinetics, and are interpreted using the Kolmogorov-Johnson-Mehl-Avrami model of phase transformation, from which the activation energy to domain growth is derived. Dilution tends to increase the activation energy, and slows down the whole relaxation process. Non-linear photo-induced kinetics as a function of laser power is evidenced, resulting from a light-induced phase separation process; a laser power threshold effect is pointed out. It is found that dilution speeds up the photo-conversion kinetics and reduces the laser power threshold.

Light-induced phase separation (LIPS) in [Fe(ptz)6](BF4)2 spin-crossover single crystals: Experimental data revisited through optical microscope investigation

We discuss the available experimental data for light-induced phase separation (LIPS) in the spin-crossover crystal [Fe(ptz)6](BF4)2. They are found in qualitative agreement with a spinodal instability process decribed by a macroscopic mean-field master equation. Sizable discrepancies with the model are discussed in terms of diffusion of light due to structural transformations of the crystal.

Light-induced phase separation (LIPS) into like-spin phases observed by Laue neutron diffraction on a single crystal of [Fe(ptz)6](BF4)2

Abstract We have followed the photo-transformation, the light-induced thermal hysteresis, and the light-induced phase separation (LIPS) into like-spin phases on a [Fe(ptz)6](BF4)2 single crystal, in the quenched rhombohedral phase, through the analysis of neutron Laue diffraction patterns. In agreement with the analysis of previous magnetic measurements, phase separation was observed in the close vicinity of the static light-induced instability, but not otherwise. The positions of the Laue spots were analysed in terms of variation in the a/c lattice-parameter ratio which is well correlated with the variation of the high-spin fraction predicted by a simple homogeneous mean-field model. Using a mapped version of the same model, we analyzed the line shapes of the Laue spots during photo-excitation and relaxation experiments so as to determine the width of the distribution of irradiation intensities, a crucial parameter in phase separation. The predictions of the mapped model failed to reproduce the rapid initial kinetics of the phase separation process, but were in agreement with the further evolution. These features are attributed to a nucleation and growth process, for which the crossover from microscopic to macroscopic regimes has been observed.

An optical microscope study of photo-switching and relaxation in single crystals of the spin transition solid [Fe(ptz) 6](BF 4) 2, with image processing

We report on the use of optical microscopy for investigating quantitatively the thermal and optical switching of spin states. The sample was [Fe(ptz) 6](BF 4) 2 single crystal in the thermally quenched structure (rhombohedral). We followed the photo-excitation from the stable LS state to the metastable HS state, the subsequent thermally activated relaxation and the light-induced thermal hysteresis. We evidenced the presence of temperature gradients, with larger differences between the crystal support and the cold finger of the cryostat, and observed the development of cracks in the crystal at crucial stages of the spin state transformations, associated with an irreversible increase in diffusion of light. We used the green and red optical densities for monitoring, respectively, the variations of the spin fractions and of diffusion of light. We processed the images so as to blur out the digital character of the data and make possible establishing a temperature map of the crystal in the photo-stationary state. We also discussed the conditions required for visualising the recently observed light-induced phase separation into like-spin domains, which are far form reached at the present state of art.

Analysis of the Phase Separation Process in UV Cured Polymer Dispersed Liquid Crystals for Optical Applications

We have examined the morphology of UV cured PDLCs made by the standard mixture NOA65/E7, for different liquid crystal content, UV dose and intensity and exposure time. The distribution function for the liquid crystal droplets size has been determined in order to provide much understanding in the UV light-induced phase separation process.