Photoinduced Phase Transition Research Articles

Hot electron injection driven phase transitions

We report on a general mechanism for photo-induced phase transitions. The process relies on the photo-injection of hot electrons from an adjacent metallic layer to trigger the structural dynamics of the materials of interest. This mechanism is demonstrated for the semiconductor-to-metal phase transition of VO${}_{2}$ using a 20 nm Au injection layer. The nature of the phase transition is demonstrated by time-resolved optical transmission measurements, as well as a well defined bias dependence that illustrates that the Au film is the source of nonequilibrium electrons driving the phase transition.

PB13 アントラセン光応答性部位を有するキラル剤の合成とキラル液晶相の光誘起相転移(化学・材料,ポスター発表,2012年日本液晶学会討論会)

PB13 アントラセン光応答性部位を有するキラル剤の合成とキラル液晶相の光誘起相転移(化学・材料,ポスター発表,2012年日本液晶学会討論会)

Photomagnetism in Cyano-Bridged Bimetal Assemblies

The study of photoinduced phase-transition materials has implications for the fields of inorganic chemistry, solid-state chemistry, and materials science. Cyano-bridged bimetal assemblies are promising photomagnetic materials. Because cyano-bridged bimetal assemblies possess various absorption bands in the visible light region, their electronic and spin states can be controlled by visible light irradiation. Moreover, the selection of magnetic metal ions and organic ligands provide a way of controlling spin-spin interactions through a cyano bridge. In this Account, we describe cyano-bridged bimetal assemblies developed in our laboratory. Cu(II)(2)[Mo(IV)(CN)(8)]·8H(2)O (CuMo), Rb(I)Mn(II)[Fe(III)(CN)(6)] (RbMnFe), and Co(II)(3)[W(V)(CN)(8)](2)·(pyrimidine)(4)·6H(2)O (CoW) induce photomagnetism via photoinduced metal-to-metal charge transfers (MM'CT), while Fe(II)(2)[Nb(IV)(CN)(8)]·(4-pyridinealdoxime)(8)·2H(2)O (FeNb) exhibits a photoinduced magnetization via a photoinduced spin crossover. Irradiation with 473 nm light causes the CuMo system to exhibit a spontaneous magnetization with a Curie temperature (T(C)) of 25 K, but irradiation with 532, 785, and 840 nm light reduces the magnetization. In this reversible photomagnetic process, excitation of the MM'CT from Mo(IV) to Cu(II) produces a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)]·8H(2)O (CuMo'). CuMo' returns to CuMo upon irradiation in the reverse-M'MCT band. RbMnFe shows a charge transfer (CT)-induced phase transition from the Mn(II)-Fe(III) phase to the Mn(III)-Fe(II) phase. Irradiation with 532 nm light converts the Mn(III)-Fe(II) phase into the Mn(II)-Fe(III) phase, and we observe photodemagnetization. In contrast, irradiation of the Mn(II)-Fe(III) phase with 410 nm light causes the reverse phase transition. A CT-induced Jahn-Teller distortion is responsible for this visible light-induced reversible photomagnetic effect. In the CoW system, a CT-induced spin transition causes the thermal phase transition from the Co(II)-W(V) phase to the Co(III)-W(IV) phase. Irradiation of the Co(III)-W(IV) phase with 840 nm light causes ferromagnetism with a T(C) of 40 K and magnetic coercive field (H(c)) of 12,000 Oe, but excitation of the back M'MCT (Co(II) → W(V)) with 532 nm light leads to the reverse phase transition. These examples of the photomagnetic effect have occurred by exciting MM'CT bands. In the fields of inorganic chemistry and materials science, researchers have studied extensively the photoinduced phase transitions between low-spin (LS) and high-spin (HS) transition metal ions. Recently, we have observed the first example of photoinduced spin crossover ferromagnetism with a FeNb system (T(C) = 20 K and H(c) = 240 Oe), in which a strong superexchange interaction between photoproduced Fe(II)(HS) and neighboring paramagnetic Nb(IV) operates through a CN bridge. The optical switching magnets described in this Account may lead to novel optical recording technologies such as optomagnetic memories and optical computers.

Sample preparation methods for femtosecond electron diffraction experiments

Exploring the dynamics of Charge density wave system 1T-TaS2 via femtosecond electron diffraction demonstrated the power of this technique for studying ultrafast structural phenomena in strongly correlated electron materials [1]. The results revealed first direct information on the order parameter dynamics of Charge Density Waves as well as on their photo-induced phase transition. A prerequisite to perform such experiments on modern quantum materials is the availability of laterally large (~100 μm) and sufficiently thin (<100 nm) single crystalline samples. Different approaches to reach these specifications have been tried out and their effect on sample integrity has been investigated. Finally, using an ultra-microtome, we were able to prepare 30 nm free standing single crystalline films of 1T-TaS2 with lateral dimensions of 200 μm × 200 μm. We have characterized these films with different techniques for their stoichiometric and crystalline integrity, ensuring no measurable alternation of sample properties. The application of this sample thinning technique is expected to find its use in further structural dynamics studies, as well as in optical time-resolved studies where homogeneous excitation profile and/or data in transmission geometry may be required.

Photoinduced Phase Transition in Strongly Electron-Lattice and Electron–Electron Correlated Molecular Crystals

Strongly electron-lattice- and electron-electron-correlated molecular crystals, such as charge transfer (CT) complexes, are often sensitive to external stimuli, e.g., photoexcitation, due to the cooperative or competitive correlation of various interactions present in the crystals. These crystals are thus productive targets for studying photoinduced phase transitions (PIPTs). Recent advancements in research on the PIPT of CT complexes, especially Et2Me2Sb[Pd(dmit)2]2 and (EDO-TTF)2PF6, are reviewed in this report. The former exhibits a photoinduced insulator-to-insulator phase transition with clearly assigned spectral change. We demonstrate how to find the dynamics of PIPT using this system. The latter exhibits a photoinduced hidden state as an initial PIPT process. Wide energy ranged time-resolved spectroscopy can probe many kinds of photo-absorption processes, i.e., intra-molecular and inter-molecular electron excitations and intramolecular and electron-molecular vibrations. The photoinduced spectral changes in these photo-absorption processes reveal various aspects of the dynamics of PIPT, including electronic structural changes, lattice structural changes, and molecular deformations. The complexities of the dynamics of the latter system were revealed by our measurements.

Photoinduced Phase Transitions in α-, θ-, and κ-type ET Salts: Ultrafast Melting of the Electronic Ordering

Photoinduced phase transitions in organic compounds with strong electron correlation ET [bis(ethylenedithio)-tetrathiafulvalene)-based salts α-(ET)2I3, θ-(ET)2RbZn(SCN)4, κ-(d-ET)2Cu[N(CN)2Br] were discussed based, on time resolved optical pump-probe spectroscopy using ~150 fs mid-infrared pulse, 12 fs near infrared pulse, and sub-picosecond terahertz pulse. (i) In charge-ordered insulators α-(ET)2I3 and θ-(ET)2RbZn(SCN)4, we captured ultrafast snapshots of charge dynamics i.e., immediate (ca. 15 fs) generation of a microscopic metallic state (or equivalently the microscopic melting of the charge order) which is driven by the coherent oscillation (period; 18 fs) of correlated electrons. Subsequently, condensation of the microscopic metallic state to the macroscopic scale occurs in α-(ET)2I3. However, in θ-(ET)2RbZn(SCN)4, such condensation is prevented by the large potential barrier reflecting the structural difference between the insulator and metal; (ii) In a Dimer–Mott insulator κ-(d-ET)2Cu[N(CN)2Br], photogeneration of the metallic state rises during ca. 1 ps that is much slower than the melting of charge order, because the photoinduced insulator to metal transition is driven by the intradimer molecular displacement in the dimer Mott insulator. The ultrafast dynamics of photoinduced insulator–metal transitions depend strongly on the molecular arrangement, reflecting various competing phases in the ET sheets.

Time-resolved near-edge x-ray absorption fine structure spectroscopy on photo-induced phase transitions using a tabletop soft-x-ray spectrometer

We present a table-top soft-x-ray spectrometer for the wavelength range λ = 1-5 nm based on a stable laser-driven x-ray source, making use of a gas-puff target. With this setup, optical light-pump/soft-x-ray probe near-edge x-ray absorption fine structure (NEXAFS) experiments with a temporal resolution of about 230 ps are feasible. Pump-probe NEXAFS measurements were carried out in the "water-window" region (2.28 nm-4.36 nm) on the manganite Pr(0.7)Ca(0.3)MnO(3), investigating diminutive changes of the oxygen K edge that derive from an optically induced phase transition. The results show the practicability of the table-top soft-x-ray spectrometer on demanding investigations so far exclusively conducted at synchrotron radiation sources.

Light-induced liquid crystallinity

Liquid crystals are traditionally classified as thermotropic, lyotropic or polymeric, based on the stimulus that governs the organization and order of the molecular system. The most widely known and applied class of liquid crystals are a subset of thermotropic liquid crystals known as calamitic, in which adding heat can result in phase transitions from or into the nematic, cholesteric and smectic mesophases. Photoresponsive liquid-crystal materials and mixtures can undergo isothermal phase transitions if light affects the order parameter of the system within a mesophase sufficiently. In nearly all previous examinations, light exposure of photoresponsive liquid-crystal materials and mixtures resulted in order-decreasing photo-induced isothermal phase transitions. Under specialized conditions, an increase in order with light exposure has been reported, despite the tendency of the photoresponsive liquid-crystal system to reduce order in the exposed state. A direct, photo-induced transition from the isotropic to the nematic phase has been observed in a mixture of spiropyran molecules and a nematic liquid crystal. Here we report a class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions. Appropriate functionalization of the naphthopyran molecules leads to an exceedingly large order parameter in the open form, which results in a clear to strongly absorbing dichroic state. The increase in order with light exposure has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting. The large, photo-induced dichroism exhibited by the material system has been long sought in ophthalmic applications such as photochromic and polarized variable transmission sunglasses.

Probing the metal–insulator phase transition in the (DMEDO-EBDT)2PF6 single crystal by optical measurements

The temperature and polarization dependence of the optical reflectivity spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT)2PF6, have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (Eg > 0.1 eV) and transfer of the spectral weight accompanied by the metal–insulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase.

Photoinduced phase transition behaviours of the liquid crystal blue phase doped with azobenzene bent-shaped molecules

A kind of azobenzene-based bent-shaped molecule is doped into a chiral nematic liquid crystal, so as to induce a 25°C blue phase range. As the sample is irradiated by 365 nm ultraviolet radiation, phase transitions from the blue phase to the chiral nematic, and then to the isotropic state are observed, because of the trans–cis isomerisation of the azobenzene moiety; on the other hand, the cis–trans isomerisation of azobenzene leads to the recovery from the isotropic to the blue phase. The transition time from the blue phase to the isotropic is about ∼1–2 minutes, and the time for the recovery is about 2 hours; these times are significantly faster than that of conventional materials. It is also found that the transition time from the blue phase to the isotropic is extended with increase of the material thickness and the weakening of ultraviolet intensity; however, no distinct effects on the time of the reverse process are seen. In addition, a stripe pattern is fabricated by photomask exposure, and such results indicate prospects for application of this material in light-driven displays and information storage.

Reversible Phase Transition and Rapid Switching of Azobenzene-Doped Cholesteric Liquid Crystals with a Single Laser

We demonstrate the photoinduced phase transition and reversible switching of a cholesteric liquid crystal (CLC) doped with a high concentration of azobenzene LC (azo-LC). The photoinduced isotropic (PHI) state (light “ON”) is created by the irradiation of a 405 nm laser, which generates cis-isomers that destabilize the cholesteric phase and make the LC transmissive. The response time is reduced by increasing the concentration of the doped azo-LC (<10 sec with 15 wt% azo-LC doped sample). A second light source of longer wavelength can be used to initiate a cis-to-trans photoisomerization of azobenzene. However, when a second light source is not used, the isotropic-to-cholesteric phase transition typically takes a long period of time (> 2 hr) for the cis-to-trans thermal isomerization reaction. We demonstrate a rapid (<10 sec) isotropic-to-cholesteric phase transition in a 10 wt% azo-LC doped sample without the use of a second light source. The phototunable cholesteric-isotropic phase transitions of azo-LC doped CLCs are reversible and repeatable. LC alignment is unnecessary in our highly concentrated azo-LC doped CLC recipes. Our study advances the research of optically switchable photonic devices based on LC materials.

Roles of molecular vibrations in photoinduced insulator‐to‐metal and neutral‐to‐ionic transitions

AbstractThe roles of molecular vibrations during photoinduced phase transitions in strongly correlated electron systems are theoretically studied on the basis of the time evolution of the exact many‐electron and many‐electron–phonon wave functions in extended Holstein–Peierls–Hubbard models. For the transition from a Coulomb‐driven charge‐ordered insulator to a metal, molecular vibrations interfere with charge transfers between neighboring molecules. For the transition from a neutral paraelectric state to an ionic ferroelectric state, they enhance the amplitude of ionicity modulation. They stabilize the initial phase in both cases, but their quantum nature is pronounced only in the former case.

Photoinduced Magnetization with a High Curie Temperature and a Large Coercive Field in a Co‐W Bimetallic Assembly

AbstractThe crystal structure, magnetic properties, and temperature‐ and photoinduced phase transition of [{CoII(4‐methylpyridine)(pyrimidine)}2{CoII(H2O)2}{WV(CN)8}2]·4H2O are described. In this compound, a temperature‐induced phase transition from the CoII(S= 3/2)‐NC‐WV(S= 1/2) [high‐temperature (HT)] phase to the CoIII(S= 0)‐NC‐WIV(S= 0) [low temperature (LT)] phase is observed due to a charge‐transfer‐induced spin transition. When the LT phase is irradiated with 785 nm light, ferromagnetism with a high Curie temperature (TC) of 48 K and a gigantic magnetic coercive field (Hc) of 27 000 Oe are observed. TheseTCandHcvalues are the highest in photoinduced magnetization systems. The LT phase is optically converted to the photoinduced phase, which has a similar valence state as the HT phase due to the optically induced charge‐transfer‐induced spin transition.

Charge and Structural Dynamics in Photoinduced Phase Transition of (EDO-TTF)2PF6 Examined by Picosecond Time-Resolved Vibrational Spectroscopy

Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF)2PF6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83–1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300–1700 cm–1, where charge- and structure-sensitive C═C stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation.

Growth Dynamics of Photoinduced Phase Domain in Cyano-Complex Studied by Boundary Sensitive Raman Spectroscopy

Rubidium manganese hexacyanoferrate exhibits charge transfer phase transition from high temperature phase to low temperature phase at 230 K. This phase transition can also be triggered by light irradiation resonantly inducing charge transfer from Mn to Fe. In the present study, boundary sensitive Raman spectroscopy was performed for Rb0.94Mn[Fe(CN)6]0.98·0.2H2O in both cases of photoinduced and thermal phase transition. Since the frequencies of C≡N stretching vibration modes are very sensitive to the valence states of the adjacent metal ions, we can quantify the distribution of not only high and low temperature phase but also boundary configurations from the observed spectra. We obtained the time evolution of the fraction ratios of the valence states from the observed peak areas. In the case of photoinduced situation, the boundary increases up to 15% when high temperature phase diminishes to 55% of the initial fraction. This is quite different from the result in thermal phase transition where the boundary is created only 0.8% at the same high temperature phase fraction value. We conclude that many small domains are preferably created in photoinduced phase transition since the ratio of boundary is large, while large domains grow in thermal phase transition.

Quantum and Spontaneous Breakage of Translational Symmetry due to Light Excitation in Photo-Induced Structural Phase Transitions

Real time quantum dynamics of the spontaneous translational symmetry breakage due to light excitation in the early stage of photo-induced structural phase transitions is reviewed under the guide of the Toyozawa theory, which is in exact compliance with the conservation law of the total momentum. At the Franck–Condon state, an electronic excitation just created by a visible photon is in a plane wave state, extended all over the crystal. While, after the lattice relaxation having been completed, it is localized as a new excitation. So, is there the shrinkage of the excitation wave function? 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 the breakage of translational symmetry.

Photoinduced Phase Transition in Pr0.5Ca0.5CoO3Studied by Sequential Pulse Excitations

Photoinduced Phase Transition in Pr_0.5Ca_0.5CoO₃Studied by Sequential Pulse Excitations

PIPT from the Beginning to Future

The birth of the field of photoinduced phase transitions was strongly influenced by the conceptual viewpoint expressed by Professor Y. Toyozawa on the condensation of relaxed excitons. Since this first period, twenty years ago, this young field has been expanding rapidly along a diversity of directions. Nowadays, it goes hand in hand with the challenges of today's science: emergence, nonlinearity, coherence, far away from equilibrium, for example. The control of the functionality of a material via photoexcited states poses many new fundamental questions. Some of them will be overviewed: (i) the nature of the control parameters and the nature of the relevant collective variables, especially the order parameters, which characterize the evolution of the system, (ii) the difference between photoinduced transformations under continuous light irradiation and those resulting from an ultrashort laser pulse, (iii) the physical mechanisms of ultrafast photoinduced phase transitions from the formation and proliferation of phototransformed entities to the softening of a collective mode.

Photoinduced Ionic to Neutral Phase Transition in TTF-CA Studied by Time-Resolved Infrared Vibrational Spectroscopy

We studied the photoinduced ionic to neutral phase transition in tetrathiafulvalene-p-chloranil (TTF-CA) using time-resolved infrared vibrational spectroscopy. By monitoring the charge sensitive b1uν10 band of CA after photoexcitation by a 1.55 eV pulse, we found that the photoinduced N-phase has the same charge of the N-phase in thermal equilibrium and there is no charge variation during the photoinduced phase transition. We also found that the photoinduced N-phase grows through at least two steps over 300 ps.

Slow Dynamics of the Photoinduced Phase Transition in Pd(dmit)2Salts (dmit = 1,3-dithiol-2-thione-4,5-dithiolate)

Slow Dynamics of the Photoinduced Phase Transition in Pd(dmit)₂Salts (dmit = 1,3-dithiol-2-thione-4,5-dithiolate)