Photoinduced Phase Research Articles

Light-Induced Mott-Insulator-to-Metal Phase Transition in Ultrathin Intermediate-Spin Ferromagnetic Perovskite Ruthenates.

Light control of emergent quantum phenomena is a widely used external stimulus for quantum materials. Generally, perovskite strontium ruthenate SrRuO3 has an itinerant ferromagnetism with a low-spin state. However, the phase of intermediate-spin (IS) ferromagnetic metallic state has never been seen. Here, by means of UV-light irradiation, a photocarrier-doping-induced Mott-insulator-to-metal phase transition is shown in a few atomic layers of perovskite IS ferromagnetic SrRuO3- δ . This new metastable IS metallic phase can be reversibly regulated due to the convenient photocharge transfer from SrTiO3 substrates to SrRuO3- δ ultrathin films. These dynamical mean-field theory calculations further verify such photoinduced electronic phase transformation, owing to oxygen vacancies and orbital reconstruction. The optical manipulation of charge-transfer finesse is an alternative pathway toward discovering novel metastable phases in strongly correlated systems and facilitates potential light-controlled device applications in optoelectronics and spintronics.

Orientated crystallization of FA-based perovskite via hydrogen-bonded polymer network for efficient and stable solar cells

Incorporating mixed ion is a frequently used strategy to stabilize black-phase formamidinum lead iodide perovskite for high-efficiency solar cells. However, these devices commonly suffer from photoinduced phase segregation and humidity instability. Herein, we find that the underlying reason is that the mixed halide perovskites generally fail to grow into homogenous and high-crystalline film, due to the multiple pathways of crystal nucleation originating from various intermediate phases in the film-forming process. Therefore, we design a multifunctional fluorinated additive, which restrains the complicated intermediate phases and promotes orientated crystallization of α-phase of perovskite. Furthermore, the additives in-situ polymerize during the perovskite film formation and form a hydrogen-bonded network to stabilize α-phase. Remarkably, the polymerized additives endow a strongly hydrophobic effect to the bare perovskite film against liquid water for 5 min. The unencapsulated devices achieve 24.10% efficiency and maintain >95% of the initial efficiency for 1000 h under continuous sunlight soaking and for 2000 h at air ambient of ~50% humid, respectively.

Pulse-to-pulse ultrafast dynamics of highly photoexcited Ge2Sb2Te5 thin films

Observing ultrafast pulse-to-pulse dynamics of highly photoexcited materials could foster a comprehensive understanding of the initial stage of irreversible photoinduced events, such as phase change, structural deformation, and laser ablation. In this study, using high-repetition-rate single-shot spectroscopy and a laser microscope, the pulse-to-pulse ultrafast dynamics of energy relaxation in Ge2Sb2Te5 thin films are revealed under high-density photoexcitation that induces sequential events involving the crystalline-to-amorphous phase transition, melt and quench processes, and formation of laser-induced periodic surface structures (LIPSSs). Above the threshold excitation density for LIPSS formation, the first excitation pulse induces the transient transmittance change of the crystalline phase in a picosecond timescale, and subsequent pulses provoke the amorphous phase energy relaxation with a long decay time of hundreds of picoseconds. We observed that the subsequent pulses gradually reduce the amplitude and decay time of the transient transmittance, leading to efficient energy relaxation and LIPSS formation in the photoinduced amorphous phase.

Effective Inhibition of Phase Segregation in Wide‐Bandgap Perovskites with Alkali Halides Additives to Improve the Stability of Solar Cells

Wide‐bandgap perovskites have attracted much attention due to their potential application in perovskite‐based tandem solar cells, which can surpass the theoretical efficiency up‐limit of single‐junction solar cells. However, photoinduced phase segregation remains one of the most intractable impediments that deteriorate the operational stability of wide‐bandgap perovskites solar cells. Herein, the effect of a series of alkali halides additives on the photoinduced phase segregation of wide‐bandgap perovskites with the composition of FA0.8Cs0.2Pb(I0.7Br0.3)3 (FA is formamidinium) is systematically studied. By coupling in situ time‐dependent photoluminescence technique, potassium chloride (KCl) is demonstrated to be the best in suppressing the photoinduced phase segregation. The reduced iodine vacancy defects owing to supplemented chloride ions and the coupling of potassium ions with the accumulated iodide ions at the grain boundaries lead to effective suppression phase segregation. As a consequence, the KCl‐modified wide‐bandgap perovskite solar cells present a champion efficiency of 19.34%, and the devices can maintain 93% of the initial efficiency after light soaking for 500 h with maximum power point tracking under 1 sun equivalent white light‐emitting diode illumination, much superior to the reference device without KCl modification only retaining 72% of its initial efficiency.

In Situ Observation of Photoinduced Halide Segregation in Mixed Halide Perovskite

Mixed–halide perovskites have emerged as a promising candidate for optoelectronics, due to their tunable optical properties. However, photoinduced phase segregation remains an obstacle for stable performance in solar cells. Here, we have conducted both bulk and nanoscale measurements to elucidate the mechanism of halide ion migration that leads to phase segregation. By utilizing Kelvin probe force microscopy (KPFM) under illumination, we have observed the time-evolution of ion migration to and from grain boundaries that agreed with bulk photoluminescence spectra of perovskites with a wide range of band gaps (1.67–1.88 eV), Cs0.15FA0.65MA0.20Pb(IxBr1–x)3 where x = 0.47–0.80. By visualizing the changes of band bending at grain boundaries, we deduce that halide segregation is dominantly caused by iodide ions, given faster ion migration in perovskite materials with a higher iodine content. Further, we verify that the changing rate of band bending at grain boundaries is consistent with the emerging rate of I-rich phase at grain boundaries, suggesting the influence of iodide ion migration toward grain boundaries on I-rich phase transition. This work will help provide insight for interpreting the mechanism of light-halide ion interactions.

Laser-driven first-order spin reorientation and Verwey phase transitions in magnetite Fe3O4 beyond the range of thermodynamic equilibrium

Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opportunities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partial or complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses. Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains undergoing the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.

Electronic Origin of Laser-Induced Ferroelectricity in SrTiO3.

Although ultrafast control of the nonthermally driven ferroelectric transition of paraelectric SrTiO3 was achieved under laser excitation, the underlying mechanism and dynamics of the photoinduced phase transition remain ambiguous. Here, the determinant formation mechanism of ultrafast ferroelectricity in SrTiO3 is traced by nonadiabatic dynamics simulations. That is, the selective excitation of multiple phonons, induced by photoexcited electrons through the strong correlation between electronic excitation and lattice distortion, results in the breaking of the crystal central symmetry and the onset of ferroelectricity. The accompanying population transition between 3dz2 and 3dx2-y2 orbitals excites multiple phonon branches, including the two high-energy longitudinal optical modes, so as to drive the titanium ion away from the center of the oxygen octahedron and generate a metastable ferroelectric phase. Our findings reveal a cooperative electronic and ionic driving mechanism for the laser-induced ferroelectricity that provides new schemes for the optical control of ultrafast quantum states.

Bidirectional photoswitchability in an iron(iii) spin crossover complex: symmetry-breaking and solvent effects.

The impact of solvent on spin crossover (SCO) behaviour is reported in two solvates [Fe(qsal-I)2]NO3·2ROH (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate; R = Me 1 or Et 2) which undergo abrupt and gradual SCO, respectively. A symmetry-breaking phase transition due to spin-state ordering from a [HS] to [HS-LS] state occurs at 210 K in 1, while T1/2 = 250 K for the EtOH solvate, where complete SCO occurs. The MeOH solvate exhibits LIESST and reverse-LIESST from the [HS-LS] state, revealing a hidden [LS] state. Moreover, photocrystallographic studies on 1 at 10 K reveal re-entrant photoinduced phase transitions to a high symmetry [HS] phase when irradiated at 980 nm or a high symmetry [LS] phase after irradiation at 660 nm. This study represents the first example of bidirectional photoswitchability and subsequent symmetry-breaking from a [HS-LS] state in an iron(iii) SCO material.

Electron-hole asymmetry observed in photoinduced phase transition of excitonic insulators

During a photoinduced phase transition from an excitonic insulator to a semimetal, photoinduced energy shift of its hole band is delayed than that of its electron band indicating electron-hole asymmetry.

Photoliquefiable Azobenzene Surfactants toward Solar Thermal Fuels that Upgrade Photon Energy Storage via Molecular Design.

Photoresponsive phase change materials (PPCMs) are capable of storing photon and heat energy simultaneously and releasing the stored energy as heat in a controllable way. While, the azobenzene-based PPCMs exhibit a contradiction between gravimetric energy storage density and photoinduced phase change. Here, a type of azobenzene surfactants with balance between molecular free volume and intermolecular interaction is designed in molecular level, which can address the coharvest of photon energy and low-grade heat energy at room temperature. Such PPCMs gain the total gravimetric energy density up to 131.18 J g-1 by charging solid sample and 160.50 J g-1 by charging solution. Notably, the molar isomerization enthalpy upgrades by a factor of up to 2.4 compared to azobenzene. The working mechanism is explained by the computational studies. All the stored energy can release out as heat under Vis light, causing a fast surface temperature rise. This study demonstrates a new molecular designing strategy for developing azobenzene-based PPCMs with high gravimetric energy density by improving the photon energy storage.

Photoinduced phase transitions in nanogel particles for reversible CO2 capture

Solar energy has been utilized to strip CO2 for saving the CO2 capture energy consumption. However, a large temperature swing is necessary for higher regeneration efficiency, which also will result in a serious materials degradation. Herein, we report a photo triggered phase transition strategy for reversible capture CO2 using amine containing nanogel particles-based solvents with a small temperature swing. The composite absorbents were prepared by combing N-[3-(dimethylamino)propyl]methacrylamide containing nanogel particles with polydopamine. The phenomenon of photoinduced phase transition was observed in both nanogel particles-based solvents and the composite absorbents. Although the nanogel particles can release most of CO2 undergo a light irradiation, the composite absorbents present a more faster desorption rate than the former, which can be explained by an enhanced light absorbability and photothermal conversion efficiency of the latter. An average CO2 regeneration efficiency can reach up to 92.45% in five CO2 uptake and release cycles, indicating that the photo triggered phase transition can be used to develop energy efficient CO2 capture technology.

Predicted Novel Type of Photoinduced Topological Phase Transition Accompanied by Collision and Collapse of Dirac-cone Pair in Organic Salt α-(BEDT-TTF)2I3

Photoinduced topological phase transitions in the Dirac-electron systems have attracted intensive research interest since its theoretical prediction in graphene, where the application of circularly polarized light opens a gap at the Dirac points and renders the system a topologically nontrivial Chern insulator phase through breaking the time-reversal symmetry. However, most of the previously studied phenomena in two-dimensional Dirac systems are basically based on the same physicical mechanism, i.e., the gap opening in the Dirac electron bands with circularly polarized light, and it is lacking in variety of the physics. In this paper, we theoretically predict a novel type of photoinduced topological phase transition accompanied by collision and collapse of gapped Dirac points in the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$. By constructing the Floquet theory for this compound, we demonstrate that the irradiation of elliptically polarized light causes collision of the Dirac points through the photoinduced band deformation and their collapse, which eventually results in the topological phase transition from a topological semimetal with gapped Dirac cones to a normal insulator when the elliptical axis is oriented at a specfic angle with respect to the crystallographic axes. We argue that this novel photoinduced phase transition can be experimentally detected by the measurement of Hall conductivity. The present work enriches the fundamental physics of photoinduced topological phase transitions and thus contribute to development of this rapidly growing research field.

Ultrafast photo-induced dynamics of $$\hbox {V}_{2} \hbox {O}_{3}$$ thin films under hydrostatic pressure

Controlling material properties with light pulses represents one of the next great challenge in material science. To achieve this goal, one needs to impact the material on the relevant time scale for controlling electronic and phononic properties. Targeting this challenge becomes possible thanks to the emerging field of photo-induced phase transition when a femtosecond light pulse interacts with the system. Along this line, we demonstrate the possibility to perform femtosecond optical spectroscopy under control thermodynamical environment. This new experimental setup includes a very precise pressure control between 1 and 6 kbar. These experimental results clearly demonstrate an effect of hydrostatic pressure onto the out-of-equilibrium photo-response of \(\hbox {V}_{2} \hbox {O}_{3}\). The main feature is a blue shift of the Brillouin frequency that correlates with an increasing speed of sound.

Photoinduced reversible phase transition in a phenothiazine-based metal-organic framework

Photoinduced phase transitions in metal-organic frameworks are provoked by structural changes of photoresponsive linkers within the framework under light irradiation. These transitions are rare but fundamentally important as they can bring about light-switchability on a variety of properties of the materials. In this work, phenothiazine as a photoresponsive unit with distinctive photochemical properties is incorporated into a Zn-based metal-organic framework, PCN-401. The structural characterization of PCN-401 revealed a reversible structural transition upon light irradiation. The mechanisms behind the photoinduced phase transition are studied systematically by spectrometric methods and structural stability characterization. Our mechanistic studies successfully showcased how the phenothiazine unit in the framework undergoes structural transformation under light irradiation and how the reversible phase transition leads to the property changes. The findings have emphasized the significance of phenothiazine in photoresponsive materials and can serve as inspiration for the design and understandings of next-generation photoresponsive metal-organic materials. • PCN-401 can go through a light-induced reversible phase transition • The oxygen content can tune the nucleation of PCN-401 • The CO 2 adsorption changes drastically as it transitions between phases • Inter-ligand electron transfer under light was revealed as the mechanism Xiao et al. report a photoresponsive MOF, PCN-401, which undergoes a structural change and radical formation upon light radiation. The photoresponsive behavior, caused by electron transfer among the phenothiazine-based ligands within the framework, brings about drastic property changes.

Photoinduced charge-density wave phase in 1T-TaS2: growth and coarsening mechanisms

Photoinduced charge-density wave phase in 1T-TaS₂: growth and coarsening mechanisms

Factors influencing halide vacancy transport in perovskite solar cells

Ion migration in inorganic and hybrid organic-inorganic metal halide perovskites causes unusual phenomena in photovoltaic devices, such as current-voltage hysteresis and photoinduced phase transformations. It is now well established that metal halide perovskites are mixed ionic-electronic conductors and halide ions are mobile at room temperature. The effect of various physical stimuli like light, heat, pressure, and applied bias can affect the distribution and movement of ions. However, the impact of each stimulus is not understood as the measured response includes a combination of factors. We explore, using first-principles density functional theory (DFT), halide migration mediated by vacancies in model cubic phases of CsPbX3 (X = Cl, Br, I) and systematically control the electronic, chemical, and mechanical conditions. We assess the potential energy landscape for ion migration and how device-relevant conditions and compositional engineering can influence the physical behaviour of halide perovskites.

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order.

Nonequilibrium hidden states provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the origin of these states allows the exploration of far-from-equilibrium thermodynamics and the development of optoelectronic devices with on-demand photoresponses. However, mapping the ultrafast formation of a long-lived hidden phase remains a longstanding challenge since the initial state is not recovered rapidly. Here, using state-of-the-art single-shot spectroscopy techniques, we present a direct ultrafast visualization of the photoinduced phase transition to both transient and long-lived hidden states in an electronic crystal, 1T-TaS2, and demonstrate a commonality in their microscopic pathways, driven by the collapse of charge order. We present a theory of fluctuation-dominated process that helps explain the nature of the metastable state. Our results shed light on the origin of this elusive state and pave the way for the discovery of other exotic phases of matter.

Dynamical Phase Transitions in the Photodriven Charge-Ordered Dirac-Electron System.

We study photoinduced phase transitions and charge dynamics in the interacting Dirac-electron system with a charge-ordered ground state theoretically by taking an organic salt α-(BEDT-TTF)_{2}I_{3}. By analyzing the extended Hubbard model for this compound using a combined method of numerical simulations based on the time-dependent Schrödinger equation and the Floquet theory, we observe successive dynamical phase transitions from the charge-ordered insulator to a gapless Dirac semimetal and, eventually, to a Chern insulator phase under irradiation with circularly polarized light. These phase transitions occur as a consequence of two major effects of circularly polarized light, i.e., closing of the charge gap through melting the charge order and opening of the topological gap by breaking the time-reversal symmetry at the Dirac points. We demonstrate that these photoinduced phenomena are governed by charge dynamics of driven correlated Dirac electrons.

Unifying the order and disorder dynamics in photoexcited VO2

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory method, here we track the motion of individual vanadium (V) ions during PIPT in VO2 and uncover that their coherent or disordered dynamics can be manipulated by tuning the laser fluence. We find that the photoexcited holes generate a force on each V-V dimer to drive their collective coherent motion, in competing with the thermal-induced vibrations. If the laser fluence is so weak that the photoexcited hole density is too low to drive the phase transition alone, the PIPT is a disordered process due to the interference of thermal phonons. We also reveal that the photoexcited holes populated by the V-V dimerized bonding states will become saturated if the laser fluence is too strong, limiting the timescale of photoinduced phase transition.

Ultrafast Carrier-Coupled Interlayer Contraction, Coherent Intralayer Motions, and Phonon Thermalization Dynamics of Black Phosphorus.

Black phosphorus (bP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations occur concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and bP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early time out-of-plane dynamics reported here have important implications for single- and few-layer bP and other vdW materials with strong electronic-lattice correlations.