Photoinduced Phase Research Articles

Nano-imaging a photoinduced phase transition.

Nanoresolved imaging and ultrafast laser excitation are integrated to probe and control a photoinduced hidden state in a strained manganite film.

Polarization-resolved broadband time-resolved optical spectroscopy for complex materials: application to the case of MoTe2 polytypes.

Time-resolved optical spectroscopy (TR-OS) has emerged as a fundamental spectroscopic tool for probing complex materials, to both investigate ground-state-related properties and trigger phase transitions among different states with peculiar electronic and lattice structures. We describe a versatile approach to perform polarization-resolved TR-OS measurements, by combining broadband detection with the capability to simultaneously probe two orthogonal polarization states. This method allows us to probe, with femtoseconds resolution, the frequency-resolved reflectivity or transmittivity variations along two mutually orthogonal directions, matching the principal axis of the crystal structure of the material under scrutiny. We report on the results obtained by acquiring the polarization-dependent transient reflectivity of two polytypes of the MoTe2 compound, with 2H and 1T' crystal structures. We reveal marked anisotropies in the time-resolved reflectivity signal of 1T'-MoTe2, which are connected to the crystal structure of the compound. Polarization- and time- resolved spectroscopic measurements can thus provide information about the nature and dynamics of both the electronic and crystal lattice subsystems, advancing the comprehension of their inter-dependence, in particular in the case of photoinduced phase transitions; in addition, they provide a broadband measurement of transient polarization rotations.

Tuning band gaps with three halides

Solar Cells Tandem solar cells can boost solar cell efficiency by using two active layers to absorb the solar spectrum more completely, provided that the two cells are current-matched. Inorganic-organic perovskites tuned to the appropriate wide band gap (∼1.7 electron volts) as top cells that contained iodine and bromine or bromine and chlorine have short carrier diffusion lengths and undergo photo-induced phase segregation. Xu et al. now report a method for incorporating chloride that allows for fabrication of stable triple-halide perovskites with a band gap of 1.67 electron volts. Two-terminal tandem silicon solar cells made with this material had a power conversion efficiency of 27%. Science , this issue p. [1097][1] [1]: /lookup/doi/10.1126/science.aaz5074

Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems.

Wide-band gap metal halide perovskites are promising semiconductors to pair with silicon in tandem solar cells to pursue the goal of achieving power conversion efficiency (PCE) greater than 30% at low cost. However, wide-band gap perovskite solar cells have been fundamentally limited by photoinduced phase segregation and low open-circuit voltage. We report efficient 1.67-electron volt wide-band gap perovskite top cells using triple-halide alloys (chlorine, bromine, iodine) to tailor the band gap and stabilize the semiconductor under illumination. We show a factor of 2 increase in photocarrier lifetime and charge-carrier mobility that resulted from enhancing the solubility of chlorine by replacing some of the iodine with bromine to shrink the lattice parameter. We observed a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point (MPP) operation at 60°C. By integrating these top cells with silicon bottom cells, we achieved a PCE of 27% in two-terminal monolithic tandems with an area of 1 square centimeter.

A role of a picosecond strain in an ultrafast optically-driven phase transition in VO2 nanostructures

We report on experimental picosecond acoustic studies of an ultrafast photoinduced insulator-to-metal and structural transition in VO2 nanostructures epitaxially grown on Al2O3 substrates with different orientations. Applying a pump-probe technique with combined excitation of a sample with picosecond strain and femtosecond laser pulses we demonstrate that dynamical strain of moderate amplitude of 0.1% has a pronounced impact on ultrafast photoinduced phase transition in VO2 nanohillocks. This enables novel path for controlling such transitions at picosecond and nanometer scales. Our experiments also allowed characterizing elastic and photo-elastic properties of the photo-induced metallic phase in VO2 and to relate them to the properties of the equilibrium phase. Furthermore, we demonstrate the generation of picosecond strain pulses upon laser-induced excitation of thin epitaxial VO2.

Photoprotected spin Hall effect on graphene with substrate induced Rashba spin–orbit coupling

We propose an experimental realization of the spin Hall effect in graphene by illuminating a graphene sheet on top of a substrate with circularly polarized monochromatic light. The substrate induces a controllable Rashba type spin–orbit coupling which breaks the spin-degeneracy of the Dirac cones but it is gapless. The circularly polarized light induces a gap in the spectrum and turns graphene into a Floquet topological insulator with spin dependent edge states. By analyzing the high and intermediate frequency regimes, we find that in both parameter limits, the spin-Chern number can be tuned by the effective coupling strength of the charge particles to the radiation field and determine the condition for the photoinduced topological phase transition.

Light Emission from Self‐Assembled and Laser‐Crystallized Chalcogenide Metasurface

AbstractSubwavelength periodic confinement can collectively and selectively enhance local light intensity and enable control over the photoinduced phase transformations at the nanometer scale. Standard nanofabrication process can result in geometrical and compositional inhomogeneities in optical phase change materials, especially chalcogenides, as those materials exhibit poor chemical and thermal stability. Here the self‐assembled planar chalcogenide nanostructured array is demonstrated with resonance‐enhanced light emission to create an all‐dielectric optical metasurface, by taking advantage of the fluid properties associated with solution‐processed films. A patterned silicon membrane serves as a template for shaping the chalcogenide metasurface structure. Solution‐processed arsenic sulfide metasurface structures are self‐assembled in the suspended 250 nm silicon membrane templates. The periodic nanostructure dramatically manifests the local light–matter interaction such as absorption of incident photons, Raman emission, and photoluminescence. Also, the thermal distribution is modified by the boundaries and thus the photothermal crystallization process, leading to the formation of anisotropic nanoemitters within the field enhancement area. This hybrid structure shows wavelength‐selective anisotropic photoluminescence, which is a characteristic behavior of the collective response of the resonant‐guided modes in a periodic nanostructure. The resonance‐enhanced Purcell effect can manifest the quantum efficiency of localized light emission.

Revealing the Photocharge-Transfer Mechanism at Manganite-Buffered LaAlO3/SrTiO3 Interfaces by Giant Photoresponse

The photoinduced phase transition at complex oxides remains one of the very important issues because of the emergent physics and potential applications. In particular, the mechanism of charge transfer at interfaces under irradiation is challenging. Herein, the photoinduced properties of manganite-buffered LaAlO3/SrTiO3 interfaces with different thicknesses of the buffer layer are systematically investigated. The giant photoresponse is observed, and its relative change in resistance is about 6.24 × 106% at T = 20 K for the sample with a buffer layer thickness of 4.8 nm. Moreover, the transition temperature is enhanced by increasing the thickness of the buffer layer. More importantly, the dead layer effect at the interfaces has been suppressed by using light. All these results are attributed to the charge transfer because of the octahedral tilting at low temperatures and provide a new kind of oxide-based optical devices, such as ultraviolet detectors. This piece of work will pave the way toward two-dimensional electron gas-based optoelectronic devices.

Strong Terahertz Radiation via Rapid Polarization Reduction in Photoinduced Ionic-To-Neutral Transition in Tetrathiafulvalene-p-Chloranil.

Terahertz lights are usually generated through the optical rectification process within a femtosecond laser pulse in noncentrosymmetric materials. Here, we report a new generation mechanism of terahertz lights based upon a photoinduced phase transition, in which an electronic structure is rapidly changed by a photoirradiation. When a ferroelectric organic molecular compound, tetrathiafulvalene-p-chloranil, is excited by a femtosecond laser pulse, the ionic-to-neutral transition is driven and simultaneously a strong terahertz radiation is produced. By analyzing the terahertz electric-field waveforms and their dependence on the polarization direction of the incident laser pulse, we demonstrate that the terahertz radiation originates from the ultrafast decrease of the spontaneous polarization in the photoinduced ionic-to-neutral transition. The efficiency of the observed terahertz radiation via the photoinduced phase transition mechanism is found to be much higher than that via the optical rectification in the same material and in a typical terahertz emitter, ZnTe.

Photoinduced topological phase transition and optical conductivity of black phosphorene

We theoretically study the photoinduced topological phase transition of black phosphorene induced by laser light with moderate intensity, which can satisfy the experimentally realistic requirement to preserve the quality of the sample. By deriving the effective Floquet Hamiltonian in terms of pseudospin $S=1/2$ degrees of freedom, we calculate the Chern number and the optical conductivity of the system with varying laser frequency $\mathrm{\ensuremath{\Omega}}$. As one can expect from the photon-assisted transport, the longitudinal optical conductivity has a threshold frequency at $\mathrm{\ensuremath{\Omega}}=\mathrm{\ensuremath{\Delta}}/\ensuremath{\hbar}$, with $\mathrm{\ensuremath{\Delta}}$ being the band gap of black phosphorene. Unlike the longitudinal optical conductivity, the optical Hall conductivity sharply increases when $\ensuremath{\hbar}\mathrm{\ensuremath{\Omega}}$ goes beyond one-half of the band gap $\mathrm{\ensuremath{\Delta}}/2$. We also show that the Chern number changes from trivial to nontrivial upon increasing frequency $\mathrm{\ensuremath{\Omega}}$ beyond $\ensuremath{\hbar}\mathrm{\ensuremath{\Omega}}=\mathrm{\ensuremath{\Delta}}/2$.

Toward mixed-halide perovskites: insight into photo-induced anion phase segregation

A review of the photo-induced anion phase segregation in mixed-halide perovskites.

О фотоиндуцированной гетероструктуре в пленке двуокиси ванадия

A non-thermal photoinduced semiconductor-metal phase transition occurring over a time Δt <1ps in a vanadium dioxide film on an aluminum substrate is theoretically investigated. It is shown that under the action of a short laser pulse in the VO2 film a structure of metal and semiconductor layers parallel to the substrate is formed. The dependence of the layer thickness on the energy density W of the laser pulse is obtained. A diagram is constructed that determines the number of layers depending on the energy density W of the laser pulse and the thickness a of the film. A comparison with experimental data is made.

Photo-induced actuator using temperature and light dual responsive azobenzene containing ion gel in ionic liquid

A series of well-defined random copolymers comprising butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) (P(AzoMA-r-BA)) are prepared successfully by reversible addition fragmentation chain transfer (RAFT) polymerization. P(AzoMA-r-BA)s show LCST-type phase transition in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfone)amide ([C4mim][NTf2]). LCST depends on the photoisomerization state of azobenzene, as well as on the AzoMA composition in the random copolymers. LCST of (P(cis-AzoMA-r-BA) is significantly higher than that of P(trans-AzoMA-r-BA), because cis-AzoMA and trans-AzoMA behave as solvato-philic and solvato-phobic comonomer, respectively. At a bistable temperature, photo-induced phase separation is completely reversible. Secondly, based on this phenomenon, a thermo-/photo- responsive ion gel (BA-AzoMA ion gel) can be prepared by free radical polymerization of BA and AzoMA using ethylene glycol dimethacrylate (EGDMA) as crosslinker in [C4mim][NTf2]. BA-AzoMA ion gel shows high temperature contraction and low temperature expansion behavior, due to LCST-type phase behavior of polymer system consisting of AzoMA and BA. Contraction temperature of BA-AzoMA ion gel depends on photoisomerization state of the azobenzene group in polymers. At a bistable temperature, photo-induced expansion-contraction is reversible. Finally, a Photo-induced actuator can be realized using BA-AzoMA ion gel at suitable temperatures, featuring reversible bending by alternate irradiating with visible and UV light. The bending behavior is also demonstrated.

Ultrathin Nanosheets of Oxo‐functionalized Graphene Inhibit the Ion Migration in Perovskite Solar Cells

AbstractMixed cation/halide perovskites have led to a significant increase in the efficiency and stability of perovskite solar cells. However, mobile ionic defects inevitably exacerbate the photoinduced phase segregation and self‐decomposition of the crystal structure. Herein, ultrathin 2D nanosheets of oxo‐functionalized graphene/dodecylamine (oxo‐G/DA) are used to solve ion migration in cesium (Cs)‐formamidinium (FA)‐methylammonium (MA) triple‐cation‐based perovskites. Based on the superconducting carbon skeleton and functional groups that provide lone pairs of electrons on it, the ultrathin 2D network structure can fit tightly on the crystals and wrap them, isolating them, and thus reducing the migration of ions within the built‐in electric field of the perovskite film. As evidence of the formation of sharp crystals with different orientation within the perovskite film, moiré fringes are observed in transmission electron microscopy. Thus, a champion device with a power conversion efficiency (PCE) of 21.1% (the efficiency distribution is 18.8 ± 1.7%) and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability, is reported. This work provides a simple method for the improvement of the structural stability of perovskite in solar cells.

Structural Origins of Light-Induced Phase Segregation in Organic-Inorganic Halide Perovskite Photovoltaic Materials

<h2>Summary</h2> Organic-inorganic metal-halide perovskite materials offer a promising route to reducing the dollars-per-watt cost of solar energy due to their good optoelectronic properties and facile, scalable processing. Compositional tuning allows for the preparation of absorbers with band gaps tailor-made for specific tandem and single-junction applications, but photoinduced phase segregation in mixed-halide materials leads to the formation of low-band-gap regions that reduce the voltage of devices. This work explores the structural origins of photoinduced phase segregation in FA_yCs_1−yPb(Br_xI_1−x)₃ perovskite alloys. We use synchrotron X-ray diffraction to map the solvus between the cubic and cubic-tetragonal mixed-phase region and time-dependent photoluminescence to assess stability under illumination. We show that the correlation between crystallographic phase and phase-segregation behavior is imperfect, so phase is not the sole determinant of optical stability. Instead, we consider several possible mechanisms that could underlie the dependence of optical stability on perovskite composition.

Entangled Azobenzene‐Containing Polymers with Photoinduced Reversible Solid‐to‐Liquid Transitions for Healable and Reprocessable Photoactuators

AbstractPhotoactuators based on liquid crystal elastomers or networks are smart materials that show photoinduced motions. However, their crosslinked networks make their repair or reprocessing difficult. Here, a healable and reprocessable photoactuator is fabricated using entangled high‐molecular‐weight azobenzene‐containing polymers (azopolymers) that are non‐crosslinked. A series of linear liquid crystal azopolymers with different molecular weights are synthesized. The low‐molecular‐weight azopolymers (5–53 kg mol−1) cannot form freestanding photoactuators because their polymer chains lack entanglements, which makes them hard and brittle. In contrast, flexible and stretchable actuators are fabricated using high‐molecular‐weight azopolymers (80–100 kg mol−1) that exhibit good processability because of the polymer chain entanglements. The azopolymer photoactuators show photoinduced bending based on photoinduced trans–cis isomerization of the azopolymers on the irradiated side. The experiments show not only photoinduced phase transitions or changes in the order parameters but also photoinduced solid‐to‐liquid transition of the azopolymers resulting in shape changes and mechanical responses. Thus, photoinduced solid‐to‐liquid transition is a new mechanism for the design of photoactuators. Moreover, the azopolymer photoactuators are healable and reprocessable via solution processing or light irradiation. Healability and reprocessability prolong lifetimes of photoactuators are important for materials reusage and recycling, and represent a new strategy for the preparation of smart materials.

A Bright Future for Liquid Functional Materials?

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials.

Poly(2‐isopropyl‐2‐oxazoline) Bearing a Spiropyran Side‐Chain: Synthesis and Evaluation of Photo and Temperature‐Responsive Properties

AbstractPoly(2‐isopropyl‐2‐oxazoline)s with varied loading of the spiropyran side‐chain are synthesized, and their photochemical and thermosensitive properties are characterized in aqueous solution. The absorption spectrum of the sample aqueous solution (0.1 wt%) indicates that 7.8 mol% of the pendant groups are in the zwitterionic merocyanine form. The introduction of spiropyran groups causes a marked decrease in the cloud point of the aqueous solution even when the pendant groups contain the zwitterionic merocyanine form. The cloud point of the aqueous solution of the samples in the spiropyran form is slightly (less than 1 °C) lower than that bearing the merocyanine group. Moreover, the photoinduced phase transition is demonstrated at an optimum temperature for the spiropyran‐bearing poly(2‐isopropyl‐2‐oxazoline). This material can be made applicable to a light‐driven drug releasing system by introducing a hydrophobic group to form micelles and increasing the photoinduced change of the cloud point.

Dynamically Tunable, Macroscopic Molecular Networks Enabled by Cellular Synthesis of 4-Arm Star-like Proteins

Summary Design of proteins with nonlinear topologies has emerged as a nascent branch of protein engineering, but significant applications remain to be seen. Here, we demonstrate the cellular synthesis of (SpyCatcher)4GFP, a 4-arm star-like protein enabled by spontaneous split GFP reconstitution, which further led to the creation of various protein networks exhibiting tunable mechanics and suitability for cell encapsulation. A derivative 4-arm star-like protein, (CarHC)4GFP, resulting from the conjugation of (SpyCatcher)4GFP with the SpyTag-fusion CarHC photoreceptors, can undergo rapid sol-gel and gel-sol transitions in response to AdoB12 and light, respectively. The chemo- and photo-induced phase transitions enabled encapsulation and controlled release of protein molecules such as the biofilm-degrading glycosyl hydrolase PslG, a potential agent for combatting multidrug-resistant bacterial species in chronic infections. The creation of those uncommon protein architectures promises great opportunities for materials biology and “smart” therapeutic delivery.

Molybdenum Disulfide Nanosheet/Quantum Dot Dynamic Memristive Structure Driven by Photoinduced Phase Transition.

MoS2 2D nanosheets (NS) with intercalated 0D quantum dots (QDs) represent promising structures for creating low-dimensional (LD) resistive memory devices. Nonvolatile memristors based 2D materials demonstrate low power consumption and ultrahigh density. Here, the observation of a photoinduced phase transition in the 2D NS/0D QDs MoS2 structure providing dynamic resistive memory is reported. The resistive switching of the MoS2 NS/QD structure is observed in an electric field and can be controlled through local QD excitations. Photoexcitation of the LD structure at different laser power densities leads to a reversible MoS2 2H-1T phase transition and demonstrates the potential of the LD structure for implementing a new dynamic ultrafast photoresistive memory. The dynamic LD photomemristive structure is attractive for real-time pattern recognition and photoconfiguration of artificial neural networks in a wide spectral range of sensitivity provided by QDs.