Photoinduced Phase Research Articles

Photochemical liquefaction and softening in molecular materials, polymers, and related compounds

Recent developments in materials showing photo-induced liquefaction and softening, including molecular materials, gels, and polymers, are described. A macrocyclic azobenzene molecule that has long alkoxy chains was first reported to show a photo-induced solid–liquid phase transition at room temperature through trans–cis photoisomerization. The structure–property relationship for understanding the photo-induced phase transition has since been investigated by using model compounds with a simple molecular design. Such azobenzene-based materials also exhibited dynamic motion on glass and water surfaces; these motions are driven by continuous liquefaction/crystallization and dissolution of the liquefied material. Photo-induced liquefaction has been further developed in middle molecular and polymeric materials. The effects of the alkyl chain length, functional groups, and number of azobenzene units on the photo-induced behavior have been extensively investigated. In addition, by using photoresponsive gelators, the photo-induced quasi-solid–liquid (gel–sol) phase transition of gels was accomplished. Moreover, very recently, a photoresponsive plasticizer that can plasticize “photo-inactive” polymers by light irradiation has been proposed. This review focuses on the recent developments on the photo-induced liquefaction and softening of azobenzene-based materials. The trans–cis photoisomerization of azobenzene units incorporated in molecular materials, polymers, and gels induces a variety of state changes such as solid–liquid, gel–sol, and glass–rubber transitions of those materials isothermally. Several novel applications such as reusable and environmentally friendly photoresists, reworkable adhesives, and self-healing materials can be expected using the photochemical liquefaction and softening.

Critical fluctuations upon photoinduced phase transition in manganite strips

Light, acting as an external stimulus to induce various intriguing phenomena ranging from photovoltaics to photoinduced catalysis, exerts prominent effects in strongly correlated systems. It would be of particular interest to investigate photon-induced emerging phenomena in spatially confined strongly correlated systems, which are important for applications of these materials in future electronic devices. Colossal magnetoresistive manganites materials offer an ideal platform for such study due to their sensitivity to photo-excitation. Here, we fabricated 900 nm wide La0.325Pr0.3Ca0.375MnO3 strips, whose width is comparable to the size of the electronic phase separation (EPS) domains in this system. We observed the photoinduced critical fluctuations in the strips, where abrupt resistivity jumps occurred upon photoinduced phase transition depending sensitively on the light intensity. Based on the microscopic views of the EPS domains under photoexcitation, we conclude that such photo-induced resistivity fluctuations originate from the photoinduced phase fluctuations of individual EPS domains when their size becomes comparable to the strip width.

Nonequilibrium Phase Precursors during a Photoexcited Insulator-to-Metal Transition in V_{2}O_{3}.

Here, we photoinduce and directly observe with x-ray scattering an ultrafast enhancement of the structural long-range order in the archetypal Mott system V_{2}O_{3}. Despite the ultrafast increase in crystal symmetry, the change of unit cell volume occurs an order of magnitude slower and coincides with the insulator-to-metal transition. The decoupling between the two structural responses in the time domain highlights the existence of a transient photoinduced precursor phase, which is distinct from the two structural phases present in equilibrium. X-ray nanoscopy reveals that acoustic phonons trapped in nanoscale twin domains govern the dynamics of the ultrafast transition into the precursor phase, while nucleation and growth of metallic domains dictate the duration of the slower transition into the metallic phase. The enhancement of the long-range order before completion of the electronic transition demonstrates the critical role the nonequilibrium structural phases play during electronic phase transitions in correlated electrons systems.

Topological phase transitions and quantum Hall effect in the graphene family

Monolayer staggered materials of the graphene family present intrinsic spin-orbit coupling and can be driven through several topological phase transitions using external circularly polarized lasers, and static electric or magnetic fields. We show how topological features arising from photo-induced phase transitions and the quantum Hall effect coexist in these materials, and simultaneously impact their Hall conductivity through their corresponding charge Chern numbers. We also show that the spectral response of the longitudinal conductivity contains signatures about the various phase transition boundaries, that the transverse conductivity encodes information about the topology of the band structure, and that both present resonant peaks which can be unequivocally associated to one of the four inequivalent Dirac cones present in these materials. This complex optoelectronic response can be probed with straightforward Faraday rotation experiments, allowing the study of the crossroads between quantum Hall physics, spintronics, and valleytronics.

First-principles thermodynamics study of phase stability in inorganic halide perovskite solid solutions

Halide substitution gives rise to a tunable band gap as a function of composition in halide perovskite materials. However, photoinduced phase segregation, observed at room temperature in mixed halide $A\text{Pb}{({\mathrm{I}}_{x}{\mathrm{Br}}_{1\ensuremath{-}x})}_{3}$ systems, limits open circuit voltages and decreases photovoltaic device efficiencies. We investigate equilibrium phase stability of orthorhombic $Pnma\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-phase $\text{Cs}M{({X}_{x}{Y}_{1\ensuremath{-}x})}_{3}$ perovskites where $M$ is Pb or Sn, and $X$ and $Y$ are Br, Cl, or I. Finite-temperature phase diagrams are constructed using a cluster expansion effective Hamiltonian parameterized from first-principles density-functional-theory calculations. Solid solution phases for $\text{Cs}M{({\mathrm{I}}_{x}{\mathrm{Br}}_{1\ensuremath{-}x})}_{3}$ and $\text{Cs}M{({\mathrm{Br}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x})}_{3}$ are predicted to be stable well below room temperature while $\text{Cs}M{({\mathrm{I}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x})}_{3}$ systems have miscibility gaps that extend above 400 K. The height of the miscibility gap correlates with the difference in volume between end members. Also layered ground states are found on the convex hull at $x=2/3$ for ${\mathrm{CsSnBr}}_{2}\mathrm{Cl},{\mathrm{CsPbI}}_{2}\mathrm{Br}$, and ${\mathrm{CsPbBrCl}}_{2}$. The impact of these ground states on the finite temperature phase diagram is discussed in the context of the experimentally observed photoinduced phase segregation.

Photoinduced nonequilibrium dynamics in charge ordered materials

We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter and the periodic lattice distortion during CO melting, which becomes more energy efficient with lower photon energy. The cooling rate of the electron system correlates with the CO gap dynamics, responsible for an order of magnitude decrease in the cooling rate upon the gap reopening. We also find that the time-dependent frequency of coherent oscillation reflects the dynamics of the energy landscape, such as transition between single-well and double-well, which sensitively depends on the photon energy and the pump fluence. The results demonstrate the intricate nonequilibrium dynamics in CO materials.

Photoinduced Topological Phase Transitions in Topological Magnon Insulators

Topological magnon insulators are the bosonic analogs of electronic topological insulators. They are manifested in magnetic materials with topologically nontrivial magnon bands as realized experimentally in a quasi-two-dimensional (quasi-2D) kagomé ferromagnet Cu(1–3, bdc), and they also possess protected magnon edge modes. These topological magnetic materials can transport heat as well as spin currents, hence they can be useful for spintronic applications. Moreover, as magnons are charge-neutral spin-1 bosonic quasiparticles with a magnetic dipole moment, topological magnon materials can also interact with electromagnetic fields through the Aharonov-Casher effect. In this report, we study photoinduced topological phase transitions in intrinsic topological magnon insulators in the kagomé ferromagnets. Using magnonic Floquet-Bloch theory, we show that by varying the light intensity, periodically driven intrinsic topological magnetic materials can be manipulated into different topological phases with different sign of the Berry curvatures and the thermal Hall conductivity. We further show that, under certain conditions, periodically driven gapped topological magnon insulators can also be tuned to synthetic gapless topological magnon semimetals with Dirac-Weyl magnon cones. We envision that this work will pave the way for interesting new potential practical applications in topological magnetic materials.

Pressure and Photoinduced Phase Transitions of Elemental Sulfur Confined by Open-End Single-Wall Carbon Nanohorns

Confined molecules in tubular nanospaces of nanocarbons, for example, carbon nanotubes and nanohorns (CNHs), lead to extraordinary behavior and properties different from their bulk analogues. Here, we investigated the confinement effect of CNHs on high-pressure elastic and vibrational properties of sulfur via the diamond anvil cell technique. X-ray diffraction measurements up to 40 GPa demonstrate two phase transitions of S-I → amorphous → S-II. A fit of equation of state yields a bulk modulus of ∼24.8 GPa, about 70% higher than that of soft sulfur. Different from previous Raman studies, laser with red light wavelength (694.8 nm) and high laser density (∼2 mW μm–2) was employed under the threshold for generating C–S bonds. We observed a similar photoinduced transition of S-I to amorphous sulfur at 4–6 GPa compared to the results taken from blue and green light excitation and low laser density (e.g., <28 μW μm–2), showing enhanced photothermal stability of sulfur by the aid of single-wall CNHs.

Local Observation of Phase Segregation in Mixed-Halide Perovskite

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

Real Time Quantum Dynamics of Spontaneous Translational Symmetry Breakage in the Early Stage of Photo-Induced Structural Phase Transitions

Real time quantum dynamics of the spontaneous translational symmetry breakage in the early stage of photo-induced structural phase transitions is reviewed and supplementally explained, under the guide of the Toyozawa theory, which is exactly in compliance with the conservation laws of the total momentum and energy. At the Franck-Condon state, an electronic excitation just created by a visible light, is in a plane wave state, which is extended all over the crystal. While, after the lattice relaxation having been completed, it is localized around a certain lattice site of the crystal, as a new excitation. Is there a sudden shrinkage of the excitation wave function, in between? No! The wave function never shrinks, but only the spatial (or inter lattice-site) quantum coherence (interference) of the excitation disappears, as the lattice relaxation proceeds. This is nothing but the spontaneous breakage of translational symmetry.

Modulation of terahertz emission in time-domain waveform via a photoinduced phase transition in a charge ordered organic ferroelectric

The time-domain waveform of the terahertz (THz) electric field emitted from the charge-ordered organic ferroelectrics α − (ET)2I3 [ET:bis(ethylenedithio)-tetrathiafulvalene)] can be significantly modulated depending on incident fluence of femtosecond pulses. Weak irradiation triggers the emission, showing ∼1 THz oscillation in the time-domain waveform with more than 5 oscillation cycles and a corresponding narrow spectral shape. In contrast, strong irradiation almost completely suppresses the trailing portion of the waveform after the first cycle, resulting in over 3 times broader bandwidth. As revealed by time-resolved experiments, this modulation arises from the cooperative nature of photoinduced melting of the charge order accompanying quenching of infrared-activity of the intermolecular vibrations which strongly couple with the emission.

Measurements of photorefractive birefringence on the different propagation directions in electro-optic crystals

A single-beam pump-probe method based on a dual-channel phase measurement system was adopted to measure photo-induced phase variations caused by the photorefractive effect. This study systematically studied the dependence of photorefractive birefringence on the propagation directions of probing lights in various electro-optic crystals at visible wavelengths.

Robustness of the charge-ordered phases in IrTe2 against photoexcitation

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occuring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches timescales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved.

Compositional Engineering for Efficient Wide Band Gap Perovskites with Improved Stability to Photoinduced Phase Segregation

Metal halide perovskites are attractive candidates for the wide band gap absorber in tandem solar cells. While their band gap can be tuned by partial halide substitution, mixed halide perovskites often have lower open-circuit voltage than would be expected and experience photoinduced trap formation caused by halide segregation. We investigate solar cell performance and photostability across a compositional space of formamidinium (FA) and cesium (Cs) at the A-site at various halide compositions and show that using more Cs at the A-site rather than more Br at the X-site to raise band gap is more ideal as it improves both VOC and photostability. We develop band gap maps and design criteria for the selection of perovskite compositions within the CsxFA1–xPb(BryI1–y)3, space. With this, we identify perovskites with tandem-relevant band gaps of 1.68 and 1.75 eV that demonstrate high device efficiencies of 17.4 and 16.3%, respectively, and significantly improved photostability compared to that of the higher Br-co...

Surface relief and refractive index gratings patterned in chalcogenide glasses and studied by off-axis digital holography.

Surface relief gratings and refractive index gratings are formed by direct holographic recording in amorphous chalcogenide nanomultilayer structures As2S3-Se and thin films As2S3. The evolution of the grating parameters, such as the modulation of refractive index and relief depth in dependence of the holographic exposure, is investigated. Off-axis digital holographic microscopy is applied for the measurement of the photoinduced phase gratings. For the high-accuracy reconstruction of the wavefront (amplitude and phase) transmitted by the fabricated gratings, we used a computational technique based on the sparse modeling of phase and amplitude. Both topography and refractive index maps of recorded gratings are revealed. Their separated contribution in diffraction efficiency is estimated.

Convenient and Robust Route to Photoswitchable Hierarchical Liquid Crystal Polymer Stripes via Flow-Enabled Self-Assembly.

Hierarchically arranged stripes of photoswitchable liquid crystal polymers (LCPs) containing azobenzene moieties were conveniently crafted via a flow-enabled self-assembly (FESA). Interestingly, by subjecting a drop of LCP solution to dry in a restricted geometry comprising two nearly parallel plates with a stationary upper plate and a movable lower plate that programmably traveled in a "stop-and-move" manner during the FESA process, photoswitchable LCP stripes were yielded, displaying two modes of deposition, namely, periodic primary stripes of large dimensions and regularly spaced secondary stripes of small dimensions situated between adjacent primary stripes (i.e., forming hierarchical LCP stripes). Notably, these hierarchical azobenzene moieties-containing stripes demonstrated sequential photoinduced reversible phase transition (i.e., photoswitching) due to the thickness difference between primary and secondary stripes. A UV light-induced expansion effect was observed on the LCP stripes. Clearly, such rapid creation of hierarchical stripes by FESA represents a robust means of organizing polymers, nanoparticles, colloids, DNA, etc. into complex yet ordered patterns over a large area in a simple and controllable manner for potential use in surface relief grating, photoactuators, photoswitchable devices, antifake labels, etc.

Strain engineering of photo-induced phase transformations in Prussian blue analogue heterostructures.

Heterostructures based on Prussian blue analogues (PBA) combining photo- and magneto-striction have shown a large potential for the development of light-induced magnetization switching. However, studies of the microscopic parameters that control the transfer of the mechanical stresses across the interface and their propagation in the magnetic material are still too scarce to efficiently improve the elastic coupling. Here, this coupling strength is tentatively controlled by strain engineering in heteroepitaxial PBA core-shell heterostructures involving the same Rb0.5Co[Fe(CN)6]0.8·zH2O photostrictive core and isostructural shells of similar thickness and variable mismatch with the core lattice. The shell deformation and the optical electron transfer at the origin of photostriction are monitored by combined in situ and real time synchrotron X-ray powder diffraction and X-ray absorption spectroscopy under visible light irradiation. These experiments show that rather large strains, up to +0.9%, are developed within the shell in response to the tensile stresses associated with the expansion of the core lattice upon illumination. The shell behavior is, however, complex, with contributions in dilatation, in compression or unchanged. We show that a tailored photo-response in terms of strain amplitude and kinetics with potential applications for a magnetic manipulation using light requires a trade-off between the quality of the interface (which needs a small lattice mismatch i.e. a small a-cubic parameter for the shell) and the shell rigidity (decreased for a large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will show a decrease in its mechanical retroaction on the photo-switching properties of the core particles.

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode.

Solar cells based on methylammonium lead triiodide (MAPbI3) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long‐term stability of MAPbI3‐based solar cells has yet to be achieved. Besides the well‐known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current–voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI3. In addition to Au, many other metals have been used as electrodes in MAPbI3 solar cells. However, how the external metal impurities introduced by electrodes affect the long‐term stability of MAPbI3 solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI3 based on first‐principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI3 and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI3 while having low resistivities and suitable work functions for carrier extraction.

Theoretical Phase Diagram for the Room-Temperature Electron-Hole Liquid in Photoexcited Quasi-Two-Dimensional Monolayer MoS2.

Strong correlations between electrons and holes can drive the existence of an electron-hole liquid (EHL) state, typically at high carrier densities and low temperatures. The recent emergence of quasi-two-dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) provides ideal systems to explore the EHL state since ineffective screening of the out-of-plane field lines in these quasi-2D systems allows for stronger charge carrier correlations in contrast to conventional 3D bulk semiconductors and enables the existence of the EHL at high temperatures. Here we construct the phase diagram for the photoinduced first-order phase transition from a plasma of electron-hole pairs to a correlated EHL state in suspended monolayer MoS2. We show that the quasi-2D nature of monolayer TMDCs and the ineffective screening of the out-of-plane field lines allow for this phase transition to occur at and above room temperature, thereby opening avenues for studying many-body phenomena without the constraint of cryogenics.

Nanosegregated Chiral Materials with Self-Assembled Hierarchical Mesophases: Effect of Thermotropic and Photoinduced Polymorphism in Rodlike Molecules.

Supramolecular chirality in a binary mixture of achiral bent-core (BC) and achiral rodlike mesogens was observed. Three different nanosegregated mesophases were determined in the binary system, and meaningful changes in circular dichroism (CD) were detected near the phase-transition temperatures of the rodlike mesogens. The highest CD intensity in the binary system was noted in the nanosegregated mesophase, in which the BC mesogens were in the helical nanofilament (HNF) phase and the rodlike mesogens were in the smectic A phase. The supramolecular chirality in the binary mixture was attributed to the self-assembled hierarchical chiral superstructures. Based on the experimental results, plausible scenarios for the chiral superstructures of the rodlike molecules embedded in the HNF networks are suggested. In addition, a system comprising BC and rodlike molecules doped with a photoresponsive compound exhibited remarkable photoswitching of CD intensity. According to the isothermal photoinduced phase transition of the embedded molecules in BC molecular HNFs, the observed CD intensities can be dynamically and reversibly modulated. Such a material with easily controllable functionality is of considerable interest in the field of materials science.