Light-induced Processes Research Articles

Lack of evidence for phase-only control of retinal photoisomerization in the strict one-photon limit.

The concept of shaping electric fields to steer light-induced processes coherently has fascinated scientists for decades. Despite early theoretical considerations that ruled out one-photon coherent control (CC), several experimental studies reported that molecular responses are sensitive to the shape of the excitation field in the weak-field limit. These observations were largely attributed to the presence of rapid-decay channels, but experimental verification is lacking. Here, we test this hypothesis by investigating the degree of achievable control over the photoisomerization of the retinal protonated Schiff-base in bacteriorhodopsin, isorhodopsin and rhodopsin, all of which exhibit similar chromophores but different isomerization yields and excited-state lifetimes. Irrespective of the system studied, we find no evidence for dissipation-dependent behaviour, nor for any CC in the strict one-photon limit. Our results question the extent to which a photochemical process at ambient conditions can be controlled at the amplitude level, and how the underlying molecular potential-energy surfaces and dynamics may influence this controllability.

Novell Plating Processes for Silicon Based Pixel Detectors

New generation silicon detectors produced in Thuringia (Germany) will play an important role in the future, during the upgrade phase of the Large Hadron Collider (LHC) at the CERN nuclear research center. Such detectors are used for monitoring radioactive reactions during the collision of highly accelerated positron radiations produced in large scale experiments like ATLAS (A Toroidal LHC ApparatuS) or CMS (Compact-Muon-Solenoid). The development of the new detectors aims at a functional improvement that will bring an enhanced resistance against radiation damage. At the same time, the surface reengineering for an increased active surface should address also important cost related aspects. This paper will discuss recent results from the authors' labs on the development of such novel sensors. The aim is a full understanding and coverage of the complete production process for the sensor modules. An important challenge to be addressed is the development of low cost metallisation procedures for the silicon detectors that do not require lithographic steps. Especially promising are extreme small and round surfaces of about 10 µm diameter distributed over the entire detector surface with a contact pitch of 50 µm that would add up to several thousand contacts and a high contact density. Given the special structure of the detector surface, electric current can be generated by incoming electromagnetic radiation, e.g. light. This current can be used in a light-induced metallization process of the front side contacts. We will present results from a light-induced plating process involving nickel and tin. The suitability of this layer system for subsequent bonding processes will also be discussed.

(Invited) Charge Carrier Dynamics and Origin of Instability of Organo-Metal Perovskite Films and Quantum Dots

Organo-metal perovskites are highly promising materials for photovoltaic applications, yet their degradation due to factors such as light, moisture, and oxygen remains a significant problem and the mechanism behind is not yet well understood. We have studied the degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films subjected to light in a dry environment from a structural perspective. The effect of mesoporous titanium dioxide (mp-TiO2) layer, commonly used in device applications, on the degradation is also investigated. The perovskite structural change was investigated using XRD, UV-Vis absorption spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and Fourier Transform Infrared (FT-IR) spectroscopy. The mp-TiO2 plays a significant role in the degradation rate of the perovskite film. Our results indicate that the first step in the light-induced degradation process is the deprotonation of the methylammonium cations, which results in the evaporation of HI and CH3NH2 molecules leaving unreacted PbI2 behind. This is further supported by ultrafast dynamics studies of charge carriers using femtosecond transient absorption spectroscopy. In addition, have synthesized various organo-metal perovskite quantum dots (PQDs) and used them as a model system to understand the origin of the materials instability. We have found the photogenerated electrons and holes can cause material degradation through redox reactions that damage the ammonium ions acting as localized trap sites, which is likely the origin of instability from light. We also found that species that are likely to react with or extract the protons from the ammonium ions will cause degradation, such as water or alcoholic solvents. Therefore, a more acidic environment is desired for better stability. Based on this understanding, we have designed new strategy to passivate the surface of PQDs using branched ligands to improve stability. The new insight gained from our studies is also important for developing new perovskite materials for solar cell and other applications such as LEDs and sensors.

Bioelectronically Initiated Cyt P450s Catalysis without NADPH

The catalytic cycle of cytochrome P450s (cyt P450s) features delivery of electrons from NADPH to cyt P450 reductase (CPR) for subsequent delivery to cyt P450 heme. Attempts at bioelectronic initiation of cyt P450s catalysis have been made by replacing the electron donation from expensive NADPH by other electron donnors. 1 To this aim, an electrode-driven drug metabolism biosensors were constructed with a mixed film containing ITO nanoparticles and CYP2C9/CPR-microsomes encapsulated by chitosan (CS) on a GC electrode. The electrochemical response of the enzyme modified electrode to 4-hydroxylation of tolbutamide has been validated, which allowed rapid determination of the affinity parameter Km appat 202.84 μM. The inhibition effect of sulfaphenazole on the CYP2C9-dependent 4-hydroxylation of tolbutamide was also investigated. Quantum dots (QDs), a kind of semiconductor materials, can also act as electron donnors. Based on the photo-induced electron transfer (PET) from QDs to the hemoproteins, a photoelectrochemical biosensor for the light-controlled drug metabolism by cyt P450s was then constructed. As shown in scheme 1, the photosensitized QDs offer electrons demanded in the cyt P450 catalytic process while the electrode at appropriate bias potentials acts as an electron supplier to QDs for neutralizing the VB holes and monitors the occurrence of the light-induced catalytic process. This novel biosensor can be applied in drug discovery and development by monitoring substrate metabolism and enzyme inhibition.

Suppression of Deactivation Processes in Photocatalytic Reduction of CO2 Using Pulsed Light

AbstractThe one‐electron‐reduced (OER) species of rhenium‐based catalysts in the homogeneous photocatalytic reduction of CO2 represents the starting point of light‐induced deactivation processes, which lead to low catalyst activity and productivity. Herein, we report the suppression of these processes using pulsed light. Experimental parameters to avoid the irradiation of the OER species were estimated, leading us to conclude that pulse lengths shorter than 1 ns and repetition rates lower than 33 Hz should be employed. [Re(bpy)(CO)3X] (bpy=2,2′‐bipyridine; X=Cl (1), Br (3)) catalysts were employed in pulsed irradiation experiments using different light sources, pulse lengths and repetition rates. Pulsed irradiation experiments using LEDs revealed that a minimum average photon flux is necessary to enable CO2 conversion. Furthermore, pulsed laser light with a 10 ns pulse length partially prevented lightinduced deactivation processes, whereas efficient suppression was achieved using a 30 ps pulse length.

Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer.

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

Poly(4-vinylphenol) as a new stable and metal-free sensitizer of titania for visible light photocatalysis through ligand-to-metal charge transfer process

In this study, poly (4-vinylphenol) (denoted as PVP) was firstly used as a stable and low-cost sensitizer of titania for visible light photocatalysis. The PVP thin-layer was readily grafted on TiO2 (PVP/TiO2) through surface complexation between the multiple hydroxyl groups of PVP and the surface titanol groups (Ti-OH), via simple dispersion of PVP and TiO2 powders in organic solvent. We have demonstrated that the PVP layer sensitizes TiO2 under visible light through the ligand-to-metal charge transfer (LMCT) mechanism. The PVP/TiO2 (with platinum as co-catalyst) composite exhibited a visible light activity of H2 production in the presence of suitable electron donors, whereas phenol-complexed TiO2 (compared as a control) did not generate H2 in the identical condition. Furthermore, PVP/TiO2 exhibited efficient photocatalytic degradation of organic pollutant (4-cholorophenol) under visible light irradiation. In addition, the photoelectrochemical measurements for PVP/TiO2-coated electrodes (PVP/TiO2/FTO) supported the visible light-induced LMCT process on PVP/TiO2, which involves the direct electron transfer from the highest occupied molecular orbital (HOMO) of PVP to the conduction band of TiO2 as the origin of the visible light photocatalytic activity.

Ultrafast Electron Transfer Between Dye and Catalyst on a Mesoporous NiO Surface.

The combination of molecular dyes and catalysts with semiconductors into dye-sensitized solar fuel devices (DSSFDs) requires control of efficient interfacial and surface charge transfer between the components. The present study reports on the light-induced electron transfer processes of p-type NiO films cosensitized with coumarin C343 and a bioinspired proton reduction catalyst, [FeFe](mcbdt)(CO)6 (mcbdt = 3-carboxybenzene-1,2-dithiolate). By transient optical spectroscopy we find that ultrafast interfacial electron transfer (τ ≈ 200 fs) from NiO to the excited C343 ("hole injection") is followed by rapid (t1/2 ≈ 10 ps) and efficient surface electron transfer from C343(-) to the coadsorbed [FeFe](mcbdt)(CO)6. The reduced catalyst has a clear spectroscopic signature that persists for several tens of microseconds, before charge recombination with NiO holes occurs. The demonstration of rapid surface electron transfer from dye to catalyst on NiO, and the relatively long lifetime of the resulting charge separated state, suggests the possibility to use these systems for photocathodes on DSSFDs.

External electric field-dependent photoinduced electron transfer of porphyrin–oligothiophene–fullerene triads

In this study, we focus on the impact of external electric fields on the light-induced charge transfer process in porphyrin–oligothiophene–fullerene triads. Density functional theory and visualization techniques were used to analyze the nature of the charge transfer excited states and to calculate the electronic coupling matrix, reorganization energy, free energy change, and other related data. Our simulated results reveal that, for this system, the efficiency of charge transfer is controlled by the external electric field. And the external electric field mainly affects the charge transfer integral. The mechanism of this ultrafast charge separation still needs for more in-depth researches.

Organized Aggregation Makes Insoluble Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-Induced Electron-Transfer Processes.

The consecutive photo-induced electron-transfer (conPET) process found with perylene diimide (PDI) overcomes the limitation of visible-light photocatalysis and sheds light on effective solar energy conversion. By the incorporation of PDI into a metal-organic polymer Zn-PDI, a heterogeneous approach was achieved to tackle the poor solubility and strong tendency to aggregate of PDIs that restricted the exploitation of this outstanding homogeneous process. The interplay between metal-PDI coordination and π···π stacking of the organized PDI arrays in Zn-PDI facilitates the conPET process for the visible light-driven reduction of aryl halides by stabilizing the radical-anion intermediate and catalyst-substrate interacted moiety. These synergistic effects between the PDI arrays and Zn sites further render Zn-PDI photoactivity for fundamental oxidation of benzyl alcohols and amines. The tunable and modular nature of the two-dimensional metal-organic polymers makes the catalyst-embedding strategy promising for the development of ideal photocatalysts toward the better utilization of solar energy.

Activation of coherent lattice phonon following ultrafast molecular spin-state photo-switching: A molecule-to-lattice energy transfer.

We combine ultrafast optical spectroscopy with femtosecond X-ray absorption to study the photo-switching dynamics of the [Fe(PM-AzA)2(NCS)2] spin-crossover molecular solid. The light-induced excited spin-state trapping process switches the molecules from low spin to high spin (HS) states on the sub-picosecond timescale. The change of the electronic state (<50 fs) induces a structural reorganization of the molecule within 160 fs. This transformation is accompanied by coherent molecular vibrations in the HS potential and especially a rapidly damped Fe-ligand breathing mode. The time-resolved studies evidence a delayed activation of coherent optical phonons of the lattice surrounding the photoexcited molecules.

Nanoparticle-Mediated, Light-Induced Phase Separations.

Nanoparticles that both absorb and scatter light, when dispersed in a liquid, absorb optical energy and heat a reduced fluid volume due to the combination of multiple scattering and optical absorption. This can induce a localized liquid-vapor phase change within the reduced volume without the requirement of heating the entire fluid. For binary liquid mixtures, this process results in vaporization of the more volatile component of the mixture. When subsequently condensed, these two steps of vaporization and condensation constitute a distillation process mediated by nanoparticles and driven by optical illumination. Because it does not require the heating of a large volume of fluid, this process requires substantially less energy than traditional distillation using thermal sources. We investigated nanoparticle-mediated, light-induced distillation of ethanol-H2O and 1-propanol-H2O mixtures, using Au-SiO2 nanoshells as the absorber-scatterer nanoparticle and nanoparticle-resonant laser irradiation to drive the process. For ethanol-H2O mixtures, the mole fraction of ethanol obtained in the light-induced process is substantially higher than that obtained by conventional thermal distillation, essentially removing the ethanol-H2O azeotrope that limits conventional distillation. In contrast, for 1-propanol-H2O mixtures the distillate properties resulting from light-induced distillation were very similar to those obtained by thermal distillation. In the 1-propanol-H2O system, a nanoparticle-mediated, light-induced liquid-liquid phase separation was also observed.

All-Optical Surface Micropatterning by Electric Field Intensity Gradient

In this report an all-optical photo-induced formation of surface relief gratings is shown. For the surface patterning of As2S3 and As4S1.5Se4.5 films a direct holographic recording setup with a 532 nm wavelength Nd:YAG CW laser light was used. Our investigations have shown that the light-induced mass transfer process strongly depends on the material itself and on the polarization of the light. It has been shown that an electric field intensity gradient has to be obtained to achieve a direct patterning. The evolution of a surface relief in relation to recording parameters and thickness of the sample has been investigated in detail.

Three-dimensional nano-optical assembly of antenna structures with collective near-field coupling

Metallic nanostructures are used for various applications because of their characteristic optical responses that generate strong near-fields. We have theoretically clarified that the configuration of nanoparticles (NPs) and their optical properties can be controlled by optical manipulation. In this study, we greatly improved our previously reported simulation method, and this improvement enables investigation of the stable states of optically trapped NPs in a three-dimensional system. On the basis of this method, the assembly process of rod-shaped silver NPs is concisely clarified, where NPs of a specific shape are selectively assembled even in a mixture of NPs of different shapes. In particular, our simulation method clarifies how a light-induced force drives assembly processes, which is an important subject to consider for extending the degree of freedom of an optical manipulation. Furthermore, as reported in our previous studies, an assembled structure inherits the properties of light used for the optical trapping; e.g., rod-shaped silver NPs are radially connected to each other and assembled into a ring-shaped structure under the irradiation of a radially polarized vector beam with a ring-shaped intensity distribution. The assembled structure is an artificial light-harvesting antenna because it theoretically exhibits an enhancement of near-field and extinction in a broader wavelength range than that of the original NPs composing the antenna and also a response to the non-polarized light such as the sunlight. On the basis of an optically manipulated configuration of NPs and their optical properties, we discuss the selective optical assembly in a three-dimensional simulation.

Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution.

Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four para-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH3-, 4-OCH3-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent 15N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958±0.0013 for 4-OCH3-aniline to 1.0035±0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H+ exchange reactions dominate below and N atom oxidation processes above the pKa value of the substituted aniline's conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways.

Carrier Recombination Dynamics in Sulfur-Doped InP Nanowires.

Measuring lifetime of photogenerated charges in semiconductor nanowires (NW) is important for understanding light-induced processes in these materials and is relevant for their photovoltaic and photodetector applications. In this paper, we investigate the dynamics of photogenerated charge carriers in a series of as-grown InP NW with different levels of sulfur (S) doping. We observe that photoluminescence (PL) decay time as well as integrated PL intensity decreases with increasing S doping. We attribute these observations to hole trapping with the trap density increased due to S-doping level followed by nonradiative recombination of trapped charges. This assignment is proven by observation of the trap saturation in three independent experiments: via excitation power and repetition rate PL lifetime dependencies and by PL pump-probe experiment.

Shaped and Feedback-Controlled Excitation of Single Molecules in the Weak-Field Limit.

Coherent control uses tailored femtosecond pulse shapes to influence quantum pathways and drive a light-induced process toward a specific outcome. There has been a long-standing debate whether the absorption properties or the probability for population to remain in an excited state of a molecule can be influenced by the pulse shape, even if only a single photon is absorbed. Most such experiments are performed on many molecules simultaneously, so that ensemble averaging reduces the access to quantum effects. Here, we demonstrate systematic coherent control experiments on the fluorescence intensity of a single molecule in the weak-field limit. We demonstrate that a delay scan of interfering pulses reproduces the excitation spectrum of the molecule upon Fourier transformation, but that the spectral phase of a pulse sequence does not affect the transition probability. We generalize this result to arbitrary pulse shapes by performing the first closed-loop coherent control experiments on a single molecule.

Highly Efficient Thermal and Light-Induced Spin-State Switching of an Fe(II) Complex in Direct Contact with a Solid Surface.

Spin crossover (SCO) complexes possess a bistable spin state that reacts sensitively to changes in temperature or excitation with light. These effects have been well investigated in solids and solutions, while technological applications require the immobilization and contacting of the molecules at surfaces, which often results in the suppression of the SCO. We report on the thermal and light-induced SCO of [Fe(bpz)2phen] molecules in direct contact with a highly oriented pyrolytic graphite surface. We are able to switch on the magnetic moment of the molecules by illumination with green light at T = 6 K, and off by increasing the temperature to 65 K. The light-induced switching process is highly efficient leading to a complete spin conversion from the low-spin to the high-spin state within a submonolayer of molecules. [Fe(bpz)2phen] complexes immobilized on weakly interacting graphite substrates are thus promising candidates to realize the vision of an optically controlled molecular logic unit for spintronic devices.

Light-Induced Processes in Plasmonic Gold/TiO2 Photocatalysts Studied by Electron Paramagnetic Resonance

X-band and W-band continuous-wave (CW) electron paramagnetic resonance (EPR) was used to study in situ light-induced (LI) mechanisms in commercial P90 titania (90 % anatase/10 % rutile) compared to plasmon-enhanced Au-P90 photocatalyst. These materials were excited using UV and 532 nm visible light to generate different excitation states and distinguish pure charge separation from plasmon-assisted resonance processes. Up to nine different photoinduced species of trapped electrons and holes were identified. LI CW EPR of P90 is presented for the first time, showing a UV excitation response similar to the well-known mixed-phase P25 titania. It is shown that incorporation of Au nanoparticles in Au-P90 and formation of a Schottky junction affects the charge separation state of the catalyst under UV light. Moreover, Au impregnation activated P90 through plasmon ‘hot’ electron injection under visible light excitation (“plasmonic sensitization effect”). In general, EPR proved to be crucial to determine the different photoexciation paths and reactions that regulate plasmonic photocatalysis.

Visualization of Spatial–Temporal Evolution of Light-Induced Refractive Index in Mn:Fe:KTN Co-Doped Crystal Based on Digital Holographic Interferometry

The dynamic refractive-index-change behavior of the light-induced process in an Mn:Fe:KTN crystal illuminated by a focused light sheet can be observed experimentally by digital holographic interferometry. By numerically retrieving a series of sequential phase maps from recording digital holograms, the spatial and temporal evolution of the light-induced refractive-index-change distribution inside the material is visualized in situ and monitored in a quantitative and in full field way. With this technique, the effect of recording parameters, such as writing laser power and polarization, bias voltage, temperature, and writing time, on the Mn:Fe:KTN crystal in the photorefractive effect can be explored. Therefore, optimized recording parameters will be achieved according to the dynamic behavior. The method provides an access to explore the evolution of the photorefractive (PR) effect of electrooptic crystal under various situations.