Photoinduced Phase Transformations Research Articles

Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction

Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials’ properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.

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.

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.

Photochemically-Induced Phase Segregation of Mixed Halide Perovskite Solar Cells

The emerging low cost perovskite solar cell quickly reached power conversion efficiency (PCE) parity with crystalline Si solar cells [1]. Optional bandgap tuning of the perovskites by partial substitution of I- with Br- expand their application to tandem solar cells and LEDs. The only critical issue remaining to solve is their poor long term stability under operational conditions, specifically photochemical degradation by splitting into I- and Br- rich phases. Sufficient suppression of this process requires a thorough understanding of the underlying phenomena. Here we report the detailed investigation of the photoinduced phase transformation in stoichiometric and non-stoichiometric mixed halide CsPbI3-xBrx. Using in-situ Atomic Force Microscopy, we visualized real-time kinetics of halide phase segregation under illumination. The I-rich phase segregated predominantly along grain boundaries while the grain bulk remained enriched with Br. It was proposed that photogenerated Pb0 and I3 - species are selectively expelled from grain bulk into grain boundary resulting in the phase splitting. This work supports our focus in understanding degradation mechanisms in perovskite materials for next generation solar cells [2-3].[1] Best Research-Cell Efficiency Chart | Photovoltaic Research | NREL, (n.d.). https://www.nrel.gov/pv/cell-efficiency.html (accessed October 12, 2019).[2] Akbulatov, A.; Frolova, L.; Dremova, N.; Zhidkov, I.; Martynenko, V.; Tsarev, S.; Luchkin, S.; Kurmaev, E.; Aldoshin, S.; Stevenson, K. J.; Troshin, P. “Light or Heat: What Is Killing Lead Halide Perovskites Under Solar Cell Operation Conditions?," J. Phys. Chem. Lett. 2020, 11(1), 333-339.[3] Frolova, L. A.; Luchkin, S. Yu; Akbulatov, A. F.; Dremova, N. N.; Stevenson, K. J.; Troshin, P. A. “Efficient and Stable All-Inorganic Perovskite Solar Cells Based on Nonstoichiometric CsxPbI2Brx (x>1) Alloy,” J. Mater. Chem. A 2019 , 7(18), 5314-5323.

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.

Enhanced hydrogen production with photo‐induced phase transformation and cocatalyst loading

In this study, reduced graphene oxide (RGO)-Cd(1 − x)ZnxS nanocomposites have been synthesized with the solvothermal method in one pot. Moreover Pt, Ru, and Rh nanoparticles have been loaded on the RGO-Cd(1 − x)ZnxS nanocomposites as cocatalysts with the aim of increasing the photocatalytic (PC) performance for hydrogen evolution reaction. The structure of Cd(1 − x)ZnxS blend transforms from cubic to hexagonal structure during the PC hydrogen evolution reaction (PCHER) at the room temperature. This photo-induced phase transformation (PIPT) enhances not only the hydrogen evolution rate, but also the stability of the photocatalysts. Interestingly, RGO triggers the PIPT process only during the PCHER under solar light illumination. On the other hand, the loading of Pt, Ru, and Rh cocatalysts do not affect the PIPT process. However, they enhance the PC and photoelectrochemical (PEC) hydrogen production activity of RGO-Cd(1 − x)ZnxS photocatalyst. PEC performance increases about 5.5 times when Pt (5%) and RGO are added to the Cd0.60Zn0.40S catalyst. RGO-Cd0.60Zn0.40S including 1.5% Rh photocatalyst reaches a remarkable PC hydrogen production rate of 135 μmolh−1 with QE of 23.3% at 460 nm. Therefore, Rh cocatalyst appears as a good alternative to Pt.

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.

Ultrafast Ge-Te bond dynamics in a phase-change superlattice

A long-standing question for avant-grade data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved X-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The novel approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process, meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in phase-change materials.

Growth and Raman spectroscopic characterization of As 4S 4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As 4S 4 (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS 2 solvent. Results show that single crystals of the As 4S 4 (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As 4S 4 (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As 4S 4 (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å 3, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.

Photoinduced Phase Transformations in Boron Nitride: New Polytypic Forms of sp3-Bonded (6H- and 30H-) BN

New sp3-bonded polytypic forms of boron nitride (BN), namely, 6H-BN and 30H-BN, were prepared by plasma-assisted chemical vapor deposition (CVD) with an excimer laser at 193 nm being irradiated on the growing film surface. Only the 6H-BN was formed by postdeposition laser irradiation (PDL) of sp2-bonded BN precursor films prepared by plain plasma-assisted CVD. The PDL demonstrated direct photoinduced phase transformation from sp2-bonded BN into denser sp3-bonded BN here. Typical lattice constants a and c for 6H-BN determined by X-ray diffraction were 2.501 A and 12.45 A, respectively, while those for 30H-BN were 2.538 A and 62.61 A, respectively. The polytypic structures were analyzed in terms of “hexagonality” H and “close-packing” index D, and the “metastability” ΔE estimated by the first principles calculations. Linear relationships were found among the H, D, and ΔE for the polytypes of BN, AlN, and SiC, whose behavior proved to depend on the degree of ionicity in their iono-covalent bonds. The growth ...

Light-controllable photoresponsive liquid-crystal photonic crystal fiber

We have developed all-optical modulation of photonic crystal fiber (PCF) by infiltrating photoresponsive liquid crystal (LC) into the voids within the cladding structure. The photo-induced phase transformation of the photoresponsive LC modulates the effective refractive index of the photoresponsive LC-filled cladding, thereby creating an environment of modifiable total internal reflection that tunes the output intensity of guided light upon the stimulus of optical field. The modulation range for the 632 nm wavelength is 10 dB and the response time for switching is less than 1 second by manually obstructing the pumping light path. In addition to altering the power of the pumping laser to actively tune the output intensity, the polarization direction of the pumping laser can also tune the output intensity by 5 dB.

Optical recording in Rb loaded-porous glass by reversible photoinduced phase transformations

We report reversible phase transformations in Rb loaded-porous glass irradiated with weak laser light which allow us to realize image storage on it. The effect is due to photo-induced changes of Rb distribution inside the glass pores, where atomic photodetachment and confinement produce either formation or evaporation of Rb nanoclusters. These processes depend on light frequency and intensity making controllable by light the porous glass transparency. We demonstrate that porous glass doped with Rb can be used as a support to record a light pulse for a long time as well as to remember the order of light colors in an illumination sequence.

Synthesis and photochemical properties of HTaWO6/ (Pt, TiO2) nanocomposite under visible light irradiation

A new layered nanocomposite, HTaWO6/(Pt, TiO2) was synthesized using n-type semi-conductor HTaWO6 as a host material. HTaWO6 and HTaWO6/TiO2were white, while both HTaWO6/Pt and HTaWO6/(Pt, TiO2) were yellow. The yellow color might be attributed to H1-xTaWO6-x/2 formed by the photo-induced phase transformation promoted by Pt. Although HTaWO6/Pt showed absorption in visible light region (λ > 400 nm), the hydrogen evolution activity was negligibly small. On the other hand HTaWO6/(Pt, TiO2) showed excellent photocatalytic activities even under visible light irradiation. The sample containing rutile type titania such as TiO2(P-25) also showed hydrogen evolution activity, but the activity was smaller than that of HTaWO6/(Pt, TiO2). These results suggested that rutile type titania, which can be excited by visible light of wavelength less than 413 nm, played an important role in the visible light-induced photocatalytic activity. The improvement of the hydrogen evolution activity of rutile type titania by intercalating into HTaWO6/Pt may be due to the depression of the recombination of photoinduced electrons and holes by the heterogeneous electron transfer from rutile type TiO2 to HTaWO6/Pt.

Probing photoinduced phase transition in a charge-transfer molecular crystal by 100 picosecond X-ray diffraction

Watching with an atomic resolution at structural changes as fast or ultra-fast photoinduced physical processes take place benefits from recent progresses in time-resolved X-ray diffraction. Molecular materials where electronic and structural changes are strongly coupled are model systems to perform such time-resolved crystallography studies. We report the structural investigation of photoinduced phase transformations between ionic and neutral states in an organic charge-transfer molecular material, using 100 picosecond (ps) synchrotron pulses. This light-induced phenomenon, triggered by an ultra-short optical pulse from a femtosecond laser, occurs by virtue of intrinsic cooperativity. Since electronic and structural changes are strongly coupled, it is of fundamental interest to perform time-resolved X-ray diffraction to obtain information at the atomic scale. We also discuss the problem of coexistence of phases and the interest of future investigations in faster timescale.

One-dimensionality and neutral-to-ionic transition of some mixed-stack charge-transfer complexes

Mixed-stack charge-transfer (CT) complexes undergoing the neutral-ionic (N-I) phase transition are molecular materials formed of stacks where electron donor (D) and acceptor (A) molecules regularly alternate. In the N phase, the CT is low and molecules are situated on inversion centers, while in the I phase, the increase of CT is accompanied by a lattice distortion (dimerization process and symmetry breaking). The one-dimensional (1D) architecture triggers the chain multistability by stabilizing lattice-relaxed (LR)-CT excitations ...D° A° D° A° $(D^+A^-)(D^+A^-)(D^+A^-)$ Do A" D° A° D°... These 1D nano-scale objects are at the heart of the equilibrium N-I phase transition and govern the fascinating physical properties such as giant dielectric response or photo-induced phase transformations. In this contribution, the 1D character of these critical excitations will be demonstrated by direct observation using high resolution X-Ray diffraction.

Thermodynamics of Molecular Chains with a Local Asymmetric Double-Well Potential

The thermodynamics of one dimensional chains with asymmetric double-well on-site potential is investigated. Pseudo-Schrödinger equation resulting from the transfer integral method is solved numerically and in terms of semiclassical approximation. This allows exact calculation and qualitative interpretation of the partition function, free energy and the specific heat function. The specific heat exhibits a characteristic hump similar to the one observed in the structures with a symmetric double-well potential. In some systems, another extremely narrow and extremely high, peak might be developed indicating an entropy driven phase transformation. Comparison between this highly simplified model and more practical approaches to the phenomena of photoinduced phase transformations is discussed underlying the possibility of manifestation of quantum effects in some (spin-crossover) systems.

Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition; the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition; optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results.

Cooperative electron transfer at thermo- and photo-induced neutral-ionic phase transformations

Cooperative electron transfer at thermo- and photo-induced neutral-ionic phase transformations

Photoinduced Phase Transformation in Polythiophene

The thermally restorable photoswitching between the spectroscopically distinct two phases has been observed for thin films of poly(3-alkylthiophene) with regioregular alkyl sidegroups. The primary process of the photoinduced phase transition is completed within a duration of the exciting laser pulse (5 ns), in which photogenerated electron-hole pairs on the polymer backbone with the density above a critical value appear to trigger a self-activated process for the phase conversion. Such a fast photoswitching of the electronic and/or structural state may be applied as well to many other conjugated polymers with optoelectronic functionality.

Photoinduced phase transformation in Y-Ba-Cu-O ceramics

Investigations of the photoinduced phenomena in Y-Ba-Cu-O ceramics have been performed using time-resolved measurements of the photovoltage, photocurrent, photoresistivity and optical second harmonic generation. A time delay shift between the non-linear optical response and photocurrent was revealed. Application of external magnetic fields of up to 1 T leads to an essential decrease of the second harmonic generation response. Different approaches for explaining the observed data are discussed.