Photoinduced Evolution Research Articles

Photo‐Induced Evolution of Randomly Rough Surfaces of Amorphous Chalcogenide Films

Photoinduced (PI) evolution of statistically rough surfaces of amorphous chalcogenide films As20Se80 at room temperature has been studied by measuring the angular dependence of the intensity of light scattered from a surface illuminated by CW laser (λ = 660 nm). The interpretation of the scattering data based on the resonant scattering theory enables to confirm unequivocally the diffusion mechanism of PI mass transfer. It is detected that the change of the amplitude of a spatial harmonic in the roughness spectra strongly depends on its period ∧ . During illumination, the amplitude increases at ∧ > ∧*, whereas harmonics with ∧ < ∧* decreases by ∧*, which corresponds to zero evolution rate, is found to be 6.7 μm. In accordance with our theoretical prediction, both growth and decrease are exponential with the rates depending on ∧. As the result, the roughness with initial rms height of 50–70 nm transforms into quasiperiodic surface grating with the average amplitude of about 400 nm and average period close to 15 μm. From the kinetics of time variation of the scattered intensity, the PI diffusion coefficient D is calculated. When the laser intensity changes from 5.6 to 14 W cm−2, D is found in the range 1 × 10−13–3.4 × 10−13 m2 s−1.

Photocatalytic activity of dual defect modified graphitic carbon nitride is robust to tautomerism: machine learning assisted ab initio quantum dynamics.

Two-dimensional graphitic carbon nitride (GCN) is a popular metal-free polymer for sustainable energy applications due to its unique structure and semiconductor properties. Dopants and defects are used to tune GCN, and dual defect modified GCN exhibits superior properties and enhanced photocatalytic efficiency in comparison to pristine or single defect GCN. We employ a multistep approach combining time-dependent density functional theory and nonadiabatic molecular dynamics (NAMD) with machine learning (ML) to investigate coupled structural and electronic dynamics in GCN over a nanosecond timescale, comparable to and exceeding the lifetimes of photo-generated charge carriers and photocatalytic events. Although frequent hydrogen hopping transitions occur among four tautomeric structures, the electron-hole separation and recombination processes are only weakly sensitive to the tautomerism. The charge separated state survives for about 10 ps, sufficiently long to enable photocatalysis. The employed ML-NAMD methodology provides insights into rare events that can influence excited state dynamics in the condensed phase and nanoscale materials and extends NAMD simulations from pico- to nanoseconds. The ab initio quantum dynamics simulation provides a detailed atomistic mechanism of photoinduced evolution of charge carriers in GCN and rationalizes how GCN remains photo-catalytically active despite its multiple isomeric and tautomeric forms.

Photoinduced Evolution of Lattice Orthorhombicity and Conceivably Enhanced Ferromagnetism in LaMnO3 Membranes.

Photoinduced Evolution of Lattice Orthorhombicity and Conceivably Enhanced Ferromagnetism in LaMnO3 Membranes.

Photoconductance from the Bent-to-Planar Photocycle between Ground and Excited States in Single-Molecule Junctions.

Single-molecule conductance measurements for 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) may offer unique insight into the bent-to-planar photocycle between the ground and excited states. Herein, we employ DPAC derivative DPAC-SMe as the molecular prototype to fabricate single-molecule junctions using the scanning tunneling microscope break junction technique and explore photoconductance dependence on the excited-state structural/electronic changes. We find up to ∼200% conductance enhancement of DPAC-SMe under continuous 340 nm light irradiation than that without irradiation, while photoconductance disappears in the case where structural evolution of the DPAC-SMe is halted through macrocyclization. The in situ conductance modulation as pulsed 340 nm light irradiation is monitored in the DPAC-SMe-based junctions alone, suggesting that the photoconductance of DPAC-SMe stems from photoinduced intramolecular planarization. Theoretical calculations reveal that the photoinduced structural evolution brings about a significant redistribution of the electron cloud density, which leads to the appearance of Fano resonance, resulting in enhanced conductance through the DPAC-SMe-fabricated junctions. This work provides evidence of bent-to-planar photocycle-induced conductance differences at the single-molecule level, offering a tailored approach for tuning the charge transport characteristics of organic photoelectronic devices.

Photoinduced evolution of lattice orthorhombicity and conceivably enhanced ferromagnetism in LaMnO3 membranes

Ultrashort laser pulses have been utilized to dynamically drive phase transitions in correlated quantum materials. Of particular interest is whether phases not achievable in thermal equilibrium can be induced in complex oxides with intricately coupled lattice, electron and spin degrees of freedom. Here, we tracked atomic motions in LaMnO3 following photoexcitation with MeV ultrafast electron diffraction (MeV-UED) technique. We found that the light excited state exhibits numerous signatures different from thermal equilibrium ones, including nearly conserved Bragg intensities, strongly suppressed La cation and oxygen anion displacements, and the long-range lattice orthorhombicity evolution. Furthermore, using first-principles calculations, we predict that the ferromagnetic ordering and conductivity are both enhanced upon laser excitation due to the reduction of the lattice orthorhombicity. This work benefits from recent advance in fabrication of membrane films with high epitaxial quality and in MeV-UED with large momentum space access and high temporal resolution.

Coherent vibrational dynamics of NbO2 film

The high thermodynamic transition temperature (\ensuremath{\approx}1080 K) of niobium dioxide (${\mathrm{NbO}}_{2}$) makes it advantageous in differentiating the effects of electronic excitation and heat accumulation in the photoinduced phase transition. Recently, a photoinduced nonthermal metallization of ${\mathrm{NbO}}_{2}$ film has been claimed [R. Rana, J. M. Klopf, J. Grenzer, H. Schneider, M. Helm, and A. Pashkin, Phys. Rev. B 99, 041102(R) (2019)], but the simultaneous lattice evolution still remains blank. In this paper, photoinduced evolution in lattice vibrations of a crystalline ${\mathrm{NbO}}_{2}$ film has been investigated using broadband coherent phonon spectroscopy. Signatures of two optical phonons at 155 and $185\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$ are identified, and the corresponding lattice vibrations are found to be associated with the motions of Nb-Nb dimers. Pump fluence-dependent measurements reveal an electronic phase transition at a threshold fluence of $\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$, while the lattice structure is preserved. A transient disordering in the lattice vibration is simultaneously observed, but lattice transformation is prevented probably by the large enthalpy barrier of the strong Peierls insulation. The accompanied drastic modulation in optical properties without atomic rearrangement also suggests the potential of ${\mathrm{NbO}}_{2}$ in improving the operation speed of related devices.

Pyridazine N-Oxides as Photoactivatable Surrogates for Reactive Oxygen Species.

A method for the photoinduced evolution of atomic oxygen from pyridazine N-oxides was developed. This underexplored oxygen allotrope mediates arene C-H oxidation within complex, polyfunctional molecules. A water-soluble pyridazine N-oxide was also developed and shown to promote photoinduced DNA cleavage in aqueous solution. Taken together, these studies highlight the utility of pyridazine N-oxides as photoactivatable O(3P) precursors for applications in organic synthesis and chemical biology.

Photoinduced evolution of optical properties and compositions of methoxyphenols by Fe(III)-carboxylates complexes in atmospheric aqueous phase

The changes in optical properties and chemical compositions of methoxyphenols, which acted as an important aromatic compound from the biomass burning, were investigated in the presence of Fe(III)-carboxylates under aqueous phase conditions. The light was confirmed to be a key factor for stimulating the reaction of methoxyphenols and Fe(III)-carboxylates. The photoinduced evolution of optical properties of methoxyphenols was dependent on various factors, including irradiation intensity, types of carboxylates, dissolved oxygen and pH. The changes in the mass absorption efficiency at 306 nm (MAE306) positively relied on irradiation intensity and dissolved oxygen. The acceleration effects of carboxylates on the decreases in MAE306 of methoxyphenols followed the order of oxalate > citrate > malonate. The change amplitude of MAE306 decreased with an increasing pH (3.5–9), while that of the mass absorption efficiency at 364 nm (MAE364) increased with pH ranging from 3.5 to 7. The compositional evolutions of methoxyphenols by the photochemical aging were analyzed with the attenuated total reflection infrared spectroscopy (ATR-IR), confirming the decrease of CO groups and the increase of O–H and C–O groups. The photochemical reaction pathways of methoxyphenols with Fe(III)-carboxylates were proposed according to optical properties and compositions measurements.

Emerging cocatalysts in TiO2-based photocatalysts for light-driven catalytic hydrogen evolution: Progress and perspectives

TiO2-based photocatalysts have been maintained as the most prominent candidate for solar-driven hydrogen (H2) evolution over the past decades. However, poor separation of generated electron-hole pairs has been considered the bottle-neck issue, restricting the TiO2 activity. Coupling a TiO2 photocatalyst to cocatalyst(s) turns out to be the ideal strategy to suppress the charge recombination and offer robust active centres, boosting the H2 evolution performance. This review aims at providing the frontier investigations of cocatalysts-integrated TiO2 for photo-induced H2 evolution. Four types of cutting-edge development of cocatalysts, including metal (noble metal, non-noble metal, bimetallic), metal sulfides and metal phosphides, 2D-MXenes, and dual materials-based cocatalysts, have been successfully highlighted and discussed. The systematically provided cocatalysts, which remarkably promote the charge separation and facilitate the surface reactions, bring out a roadmap to inspire the preparation of superior TiO2-based materials for H2 evolution shortly. We expect this review could provide enriched information to tailor the TiO2 supported active sites of cocatalysts for highly photo-induced H2 evolution.

Photonic Materials: On the Molecular Mechanism of a Photo‐Responsive Phase Change Memory (Adv. Theory Simul. 5/2021)

A population swapping genetic algorithm is introduced in article 2100017 by Saeed Amirjalayer to efficiently develop an atomistic potential describing the out-of-equilibrium dynamics of an azobenzene-based phase change material. Based on this approach, a balanced representation of the energy landscape is ensured. Thus, the photo-induced structural evolution during the phase transformation of the periodic matrix is captured at the atomic level, providing new insight into molecular-based photonic materials.

Mimicking Functions of Native Enzymes or Photosynthetic Reaction Centers by Nucleoapzymes and Photonucleoapzymes.

The covalent linkage of catalytic units to aptamer sequence-specific nucleic acids exhibiting selective binding affinities for substrates leads to functional scaffolds mimicking native enzymes, nucleoapzymes. The binding of the substrates to the aptamer and their structural orientation with respect to the catalytic units duplicate the functions of the active center of enzymes. The possibility of linking the catalytic sites directly, or through spacer units, to the 5′-end, 3′-end, and middle positions of the aptamers allows the design of nucleoapzyme libraries, revealing structure–functions diversities, and these can be modeled by molecular dynamics simulations. Catalytic sites integrated into nucleoapzymes include DNAzymes, transition metal complexes, and organic ligands. Catalytic transformations driven by nucleoapzymes are exemplified by the oxidation of dopamine or l-arginine, hydroxylation of tyrosine to l-DOPA, hydrolysis of ATP, and cholic acid-modified esters. The covalent linkage of photosensitizers to the tyrosinamide aptamer leads to a photonucleoapzyme scaffold that binds the N-methyl-N′-(3-aminopropane)-4,4′-bipyridinium-functionalized tyrosinamide to the aptamer. By linking the photosensitizer directly, or through a spacer bridge to the 5′-end or 3′-end of the aptamer, we demonstrate a library of supramolecular photosensitizer/electron acceptor photonucleoapzymes mimicking the functions of photosystem I in the photosynthetic apparatus. The photonucleoapzymes catalyze the photoinduced generation of NADPH, in the presence of ferredoxin-NADP+-reductase (FNR), or the photoinduced H2 evolution catalyzed by Pt nanoparticles. The future prospects of nucleoapzymes and photonucleoapzymes are discussed.

An In-situ Transmission X-ray Microscope to Observe the Dewetting Process of Au Thin Films induced by Nanosecond Pulsed Laser Irradiation

We coupled an in-situ transmission X-ray microscope (TXM) with a Nd:YAG pulsed laser with a 532-nm wavelength and a 6-ns pulse width. This system was employed to investigate the photo-induced morphological evolution in Au thin films during shot-by-shot irradiation by a single laser pulse. Based on results for Au thin films with thicknesses of 10 and 20 nm, we successfully demonstrated the in-situ TXM system to be a powerful tool for imaging the photo-induced transition from metal thin films to metal nanoparticles. We also found that the sequence of steps leading to this transition depended critically on the thickness of the Au films.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase.

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity. NBP is a well-folded and stable construct displaying native-like unfolding behavior. Spectroscopic and computational studies indicated that the NBP bound nickel in a distorted square planar geometry that validated the design. Ni(II)-NBP is active for photo-induced H2 evolution following a reductive quenching mechanism. Ni(II)-NBP catalyzed H+ reduction to H2 gas electrochemically as well. Analysis of the catalytic voltammograms established a proton-coupled electron transfer (PCET) mechanism. Electrolysis studies confirmed H2 evolution with quantitative Faradaic yields. Our studies demonstrate an important scope of rational metalloprotein design that allows imparting functions into protein scaffolds that have natively not evolved to possess the same function of the target metalloprotein constructs.

2D-montmorillonite-dispersed g-C3N4/TiO2 2D/0Dnanocomposite for enhanced photo-induced H2 evolution from glycerol-water mixture

Montmorillonite (MMT) dispersed g-C3N4/TiO2 hybrid nanocomposite for enhanced photo-catalytic hydrogen production from glycerol-water mixture has been investigated. The newly designed composite photo-catalysts were fabricated through a sol-gel assisted hydrothermal method and were characterized by XRD, XPS, SEM, EDX, TEM, FTIR, UV–Vis, Raman and PL spectroscopy. Well-designed g-C3N4/MMT/TiO2 heterojunction composite was obtained with 2D MMT structure, which promoted both visible light absorption and hindered charges recombination rate. The modification of 2D/0D g-C3N4/TiO2 heterojunction with 2D MMT sheets enhances H2 production due to MMT works as a mediator for effective charges trapping and transportation within the composite structure. The g-C3N4/MMT/TiO2 photo-catalyst exhibits highest H2 production of 4425 ppm h−1 g−1 at pH 7.0, which was 2.12 times higher than the pure TiO2 (2085 ppm h−1 g−1). In addition, increasing catalyst loading promotes more H2 evolution and among the different sacrificial reagents, glycerol-water mixture gave highest H2 production due to the presence of α-hydrogen atoms attached to carbon atoms. The enhanced photo-catalytic efficiency can be attributed to synergistic effect of MMT with g-C3N4/TiO2 heterojunction composite, appropriate band structure and transportation of electrons–holes with their hindered recombination rate. These composite catalysts exhibited excellent photo-catalytic stability for H2 production in cyclic runs. Possible reaction mechanism for hydrogen production over g-C3N4/MMT/TiO2 composite has been explained based on the experimental results. The finding of this work would be fruitful for hydrogen production applications with all sustainable systems.

Photoinduced discommensuration of the commensurate charge-density wave phase in 1T−TaS2

The dynamics induced by femtosecond-laser excitation of the commensurate phase of the charge-density wave (CDW) in $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ have been studied using both time-resolved electron diffraction and the time-resolved spectroscopy of coherent-phonon dynamics. Electron diffraction results show that the commensurate CDW phase is transformed into a new phase with CDW order that is similar to the nearly commensurate phase with threshold-type transition rates; the threshold excitation density of 0.2 per 13 Ta atoms is evaluated. Coherent-phonon spectroscopy results show that, together with the amplitude mode of CDW with a frequency of 2.41 THz, two other modes with frequencies of 2.34 and 2.07 THz are excited in the photoexcited commensurate CDW phase over a timescale of several tens of picoseconds after excitation. Spectroscopic, temporal, and excitation-intensity dependent characteristics of the three coherent phonons reveal that a photoinduced decomposition of the commensurate CDW order into an ensemble of domains with different CDW orders is induced before the CDW-phase transition occurs. The physics underlying the photoinduced decomposition and evolution into discommensurations responsible for the CDW-order transformation are discussed.

Rate-limiting steps in the dark-to-light transition of Photosystem II - revealed by chlorophyll-a fluorescence induction

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Atomistic Simulations of Charge Separation at a Nanohybrid Interface: Relevance of Photoinduced Initial State Preparation.

Charge separation kinetics at a nanohybrid interface are investigated in their dependence on ultrafast optical excitation. A prototypical organic/inorganic interface is considered. It is formed by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocluster of 3783 atoms. A first principle parametrized Hamiltonian is employed, and the photoinduced subpicosecond evolution of Frenkel-excitons in the organic part is analyzed besides the formation of charge separated states across the interface. The interface absorption spectrum is calculated. Together, the data indicate that the charge separation is based on the direct excitation of the charge separated states but also on the migration of created Frenkel excitons to the interface with subsequent decay. Further, the photoinduced interface dynamics are compared with data resulting from direct set-ups of an initially excited state. Mostly such set-ups lead to substantially different charge separation processes.

Significance of Hydroxyl Radical in Photoinduced Oxygen Evolution in Water on Monoclinic Bismuth Vanadate

Understanding the details of the mechanism of water photo-oxidation on monoclinic bismuth vanadate (m-BiVO4) is intriguing for efficient oxygen (O2) evolution under solar light. Hydroxyl (OH) radical was confirmed in a photocatalytic system of m-BiVO4 powders. OH radical is highly reactive to dimerize into hydrogen peroxide (H2O2), which is further oxidized into O2 molecule. Therefore, O2 evolution from OH radicals could be one of the plausible mechanisms. In the present study, to clarify the significance of O2 evolution from OH radicals, O2 molecules and OH radicals were generated by photoelectrochemical water oxidation on m-BiVO4 electrode. The amounts of O2 molecules and OH radicals were quantified by using an oxygen meter and a fluorescence probe method, respectively. The current efficiency of the photocurrent for the formation of OH radicals (∼0.06%) was much smaller than that for O2 evolution (∼100%), indicating that O2 evolution from OH radicals was not significant on m-BiVO4.

A Water-Soluble Copper-Polypyridine Complex as a Homogeneous Catalyst for both Photo-Induced and Electrocatalytic O2 Evolution.

The water-soluble polypyridine copper complex [Cu(F3TPA)(ClO4)2] [1; F3TPA=tris(2-fluoro-6-pyridylmethyl)amine] catalyzes water oxidation in a pH 8.5 borate buffer at a relatively low overpotential of 610 mV. Assisted by photosensitizer and an electron acceptor, 1 also exhibits activity as a homogeneous catalyst for photo-induced O2 evolution with a maximum turnover frequency (TOF) of (1.58 ± 0.03) × 10(-1) s(-1) and a maximum turnover number (TON) of 11.61 ± 0.23. In comparison, the reference [Cu(TPA)(ClO4)2] [TPA=tris(2-pyridylmethyl)amine] displayed almost no activity under either set of conditions, implying the crucial role of the ligand in determining the behavior of the catalyst. Experimental evidence indicate the molecular catalytic nature of 1, leading to a potentially practical strategy to apply the copper complex in a photoelectrochemical device for water oxidation.

Water Oxidation Catalyzed by a Dinuclear Cobalt–Polypyridine Complex

AbstractThe dinuclear Co complex [(TPA)Co(μ‐OH)(μ‐O2)Co(TPA)](ClO4)3 (1, TPA=tris(2‐pyridylmethyl)amine) catalyzes the oxidation of water. In the presence of [Ru(bpy)3]2+ and S2O82−, photoinduced oxygen evolution can be observed with a turnover frequency (TOF) of 1.4±0.1 mol(O2) mol(1)−1 s−1 and a maximal turnover number (TON) of 58±5 mol(O2) mol(1)−1. The complex is shown to act as a molecular and homogeneous catalyst and a mechanism is proposed based on the combination of EPR data and light‐driven O2 evolution kinetics.