Investigation of the Effect of the Passivation on the Charge Transfer from MAPbI3 to Electron Transport Layer Using a Heterodyne Transient Grating Spectroscopic Technique.

We fabricated MAPbI3 perovskite thin films with ZnO on a glass substrate, in which a passivation layer (Phenethylammonium iodide (PEAI); p-methoxyphenethylammonium iodide (CH3O-PEAI); 2-methoxyethylammonium iodide (MEAI)) was inserted between two layers. In order to understand the effect of the insertion of each passivation material on the transfer efficiency of the photo-generated electrons from MAPbI3 to ZnO, we observed the near-field heterodyne transient grating (NF-HD-TG) responses of each film and investigated the component arising from the recombination of the trapped electrons at the ZnO surface. Based on the accelerated recombination between photo-generated holes remaining in the MAPbI3 layer and surface-trapped electrons in ZnO and the increase in the number of the trapped electrons in ZnO when either CH3O-PEAI or PEAI was applied, we successfully revealed that the charge transfer efficiency was enhanced by the insertion of the passivation materials including a benzene ring stabilizing the defect states. Particularly, it was demonstrated that CH3O-PEAI showed the highest increase in the charge transfer efficiency, which could be attributed to the high electron density in the benzene ring, resulting from the existence of the electron donating group, CH3O, and its role in the effective transition from 3D to 2D perovskite phases.

A new strategy for monitoring the charge transfer from perovskite thin films to electron transport layers using a heterodyne transient grating technique.

We fabricated CH3NH3PbI3 (MAPbI3) thin films with and without a ZnO layer and measured the heterodyne transient grating (HD-TG) responses of each film to investigate the charge injection dynamics from MAPbI3 to ZnO, based on the component arising from the recombination of the surface trapped electrons in the ZnO layer with the remaining holes in MAPbI3. In addition, we observed the HD-TG response of the MAPbI3 thin film coated with the ZnO layer in which phenethyl ammonium iodide (PEAI) was inserted between two layers as a passivation layer and confirmed that the charge transfer was enhanced when PEAI existed, based on the increase in the amplitude of the component arising from the recombination and its acceleration.

Development of charge carrier-selective heterodyne transient grating spectroscopic technique and its application in the distinction of surface trap states in hematite.

We developed a new spectroscopic technique, which is named the charge carrier-selective heterodyne transient grating (CS-HD-TG) method, for the distinction of surface trap states existing in photocatalysts and photovoltaic materials by employing a burn laser inducing depletion of the number of surface trapped charge carriers. In addition, as a case study, we measured the heterodyne transient grating responses of hematite under bias conditions with and without the burn laser and revealed that two distinct trap states co-exist at the surface of the hematite film and only one of them could act as the reaction intermediate for the oxygen evolution reaction (OER), which is consistent with former studies.

Photo-controllable rotational motion of cholesteric liquid crystalline droplets in a dispersion system.

A photo-controllable rotational motion was demonstrated for an isolated cholesteric liquid crystalline droplet in a surfactant solution. The droplet showed unidirectional rigid-body rotation with UV light irradiation and the rotational rate could be controlled by the light intensity. Furthermore, the rotational direction could be controlled by the chirality of dopants. The motion was explained by the rotational torque induced by the photo-induced gradient of chemical concentrations and/or temperature inside the droplet from the theory of the Lehmann effect, and the possible mechanisms are discussed.

An initial estimation method using cosine similarity for multivariate curve resolution: application to NMR spectra of chemical mixtures.

Multivariate curve resolution (MCR) has been widely utilized to reveal the constituents of chemicals from multiple spectral data of chemical mixtures. In the MCR calculation, the singular value decomposition (SVD) has been utilized to obtain the initial estimation of the spectra for pure chemicals and they are adjusted to obtain the best fit using the alternating least squares (ALS) algorithm. However, wrong initial estimation by SVD frequently leads to convergence at an incorrect local minimum of the least square error. To overcome this problem, we have developed a robust calculation technique, which utilizes a new initial estimation using cosine similarity, and the following optimization was performed by MCR. The calculation was applied for 1H-NMR mixture spectra of 4 different chemicals, and this methodology could recover the spectra of pure chemicals (>85% consistency) and the concentration profile for each mixture within an accuracy of <10%.

Molecular Orientation Change Nearby Topological Defects Observed by Photo-Induced Polarization/Phase Microscopy.

Topological defects in liquid crystals (LCs) have been intensively studied and intentionally generated in an organized way recently because they could control the alignment and motion of LCs. We studied how the topological defects could change the molecular orientation/alignment from the observation of photo-induced orientation change of a photo-responsive LC. The photo-induced dynamics was observed by an LED-induced time-resolved polarization/phase microscopy with white light illumination. From the color image sequence, we found that the molecular orientation change started from the topological defects and the orientation change propagated as a pair of defects and was connected, and further disordering was induced as a next step after the initial orientation change finished.

Optical motion control of liquid crystalline droplets by host-guest molecular interaction.

Photo-induced motion is demonstrated for a photo-responsive dye-doped liquid crystal (LC) droplet in a surfactant solution. The LC droplets started rolling on a substrate during UV irradiation and moved either toward or away from the UV light, depending on the functional groups of the guest dyes. The mechanism is explained by the Marangoni flow caused by the photo-isomerization-induced adsorption and desorption of the dye molecules to and from the LC/solution interfaces.

Lifetime mapping of photo-excited charge carriers by the transient grating imaging technique for nano-particulate semiconductor films.

The transient grating (TG) imaging technique has been developed, where the refractive index change due to the photoexcited charge carriers excited with a stripe patterned light can be visualized. The spatiotemporal imaging of photoexcited charge carriers was demonstrated for a nanoparticulate TiO2 film. In the analytical procedures to map out the time constant distribution, the averaged response of photoexcited carriers in each image was obtained from the Fourier transform of the TG images since the image has a spatial modulation with a stripe pattern of light. The oscillation response due to the acoustic grating, the decay of the surface trapped electrons (until 1 µs), and thermal diffusion (until 100 µs) were observed. In order to obtain the lifetime imaging of the photoexcited electrons, the target time region (0-1 µs) for the response was selected and fitted with an exponential function, and the time constants were mapped out. We found that the time constants showed a wide range of distribution (68-920 ns), dependent on the sample positions.

Origin of optical nonlinearity of photo-responsive liquid crystals revealed by transient grating imaging.

A large optical nonlinearity has been observed for the photo-responsive liquid crystals under the condition that the nematic phase is close to the isotropic condition. The direct observation of the photo-response of a liquid crystal by the time-resolved transient grating phase imaging technique revealed that the optical nonlinearity was caused by the transiently generated phase formed inside the photo-induced isotropic region. A shock-like flow was observed for the formation of the transiently generated phase. Based on the theoretical calculation, we propose that a flow generated at the disordered/ordered interface induced the reorientation of the liquid crystal molecules, thereby generating a larger polarization and ultimately causing the optical nonlinearity.

Optically induced motion of liquid crystalline droplets.

The controlled motion of a liquid crystalline active droplet was demonstrated in a surfactant solution and by irradiation with UV light. The droplet could be induced to roll on a glass substrate toward the UV light source. This was explained by the Marangoni flow induced by the UV-induced desorption of surfactants.

Entropic effects make a more tightly folded conformer of a ß-amino acid less stable: UV-UV hole burning and IR dip spectroscopy of l-ß3-homotryptophan using a laser desorption supersonic jet technique.

UV-UV hole burning and IR dip spectra of l-ß3-homotryptophan were measured by a laser desorption supersonic jet technique as a bottom-up approach to understand the secondary structures of ß-peptides. 14 conformers were found by UV-UV hole burning spectroscopy. The conformers were classified into three groups depending on their hydrogen bonding patterns observed in their conformer-specific IR spectra, and tentatively assigned by comparing with quantum chemical calculations. Group 1 had free OH stretch but no NH2 anti-symmetric stretch vibrational transition and was assigned to NH-π hydrogen bonded structures. Group 2, including the most abundant conformer, showed both free OH and NH2 anti-symmetric stretch vibrations, and belonged to NH-O hydrogen bonded conformations. Group 3 of conformers had hydrogen-bonded OH stretch IR transition and had OH-N hydrogen bonds. The internal hydrogen bond of group 3 is a C6 hydrogen bond due to the additional carbon atom at the ß position and shows a shorter bond length than that of a C5 hydrogen bond. While the OH-N C6 hydrogen bond is stronger than NH-O, the entropic effect prefers the more flexible NH-O hydrogen bonded structure. It is expected that the unnatural C6 hydrogen bond influences the conformations of ß-peptides and builds totally different secondary structures than those of α-peptides.

Photoexcited charge carrier dynamics of interconnected TiO2 nanoparticles: evidence of enhancement of charge separation at anatase-rutile particle interfaces.

The charge carrier kinetics of hydrothermally treated TiO2 nanoparticles, consisting of interconnected anatase and rutile crystallographic forms, was investigated using a heterodyne transient grating technique to obtain direct evidence of the enhancement of charge separation efficiency. We found that surface recombination arising from trapped electrons was retarded, compared with that of P25 TiO2 nanoparticles, with the aid of an increase of particle interfaces. This means that the charge separation efficiency of hydrothermally treated TiO2 nanoparticles is higher than that of P25 TiO2 nanoparticles, to which the enhanced photocatalytic performance of the hydrothermally treated TiO2 nanoparticles could be attributed.

Aggregation-Induced Expansion of Poly-(N-isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique.

The anomalous volume expansion of poly-(N-isopropyl acrylamide) (PNIPAM) solutions was observed during the thermally induced polymer phase transition of aqueous solutions having concentrations in the 3-7 wt % range. The process occurred on a millisecond time scale, and a laser temperature-jump time-resolved technique was used to bring about the process. After heating a solution with a pulse laser exploiting light absorption by dyes added to the solution itself, a phase transition was observed to take place, and the temporal changes associated with it were visualized through the transient grating imaging technique, whereby the solution was heated with a stripe pattern. We found several processes occurring on a millisecond time scale, all of which clearly took place after each PNIPAM molecule had collapsed structurally from a coiled to a globular conformation. During the so-called demixing process, the globular polymers aggregated with each other within 10 ms, and suddenly the polymer phase expanded as aggregation progressed further. After this process, the individual globular polymers reverted to their coiled conformation via hydration during the remixing process. We proposed that solution expansion was caused by the mutual entangling of multiple globular PNIPAM molecules, instead each globule polymer was separated.

Optimization of Experimental Parameters for the Performance of Solid-state Dye-sensitized Solar Cells.

The effects of various sample parameters for solid-state dye sensitized solar cells were studied with carrier dynamics measurements and electrochemical measurements. Although many parameters and processes have been decided based on the experience of researchers, the chemical and physical reasons for the selections have not been clarified. We studied the effect of the generally utilized materials and processing such as the blocking layer, titanium oxide thickness, surface treatment, and the selection of dyes and hole transfer materials. Based on our findings, we were able to rationally optimize the structure of the solid-state dye sensitized solar cells in terms of cell performance or the lifetime of charge carriers.

Unifying the microscopic picture of His-containing turns: from gas phase model peptides to crystallized proteins.

The presence in crystallized proteins of a local anchoring between the side chain of a His residue, located in the central position of a γ- or ß-turn, and its local main chain environment, was assessed by the comparison of protein structures with relevant isolated model peptides. Gas phase laser spectroscopy, combined with relevant quantum chemistry methods, was used to characterize the γ- and ß-turn structures in these model peptides. A conformer-selective NH stretch infrared study provided evidence for the formation in vacuo of two types of short-range H-bonded motifs, labelled ε-6δ and δ-δ7/πH, bridging the His side chain (in its gauche+ rotamer) to the neighbouring NH(i) and CO(i) sites of the backbone; each side chain-backbone motif was found to be specific of the tautomer (ε or δ) adopted by the His side chain in its neutral form. A close comparison between ß- and γ-turns, selected from the Protein Data Bank, and the gas phase models demonstrated that a significant proportion of the gauche+ His rotamer distribution of proteins was well described by the corresponding gas phase H-bonded structures. This is consistent with the persistence of local 6δ and δ7/πH intramolecular interactions in proteins, emphasizing the relevance of gas phase data to secondary structures that are poorly accessible to solvents, e.g., in the case of a specific compact topology (Xxx-His ß-turns). Deviations from the gas phase structures were also observed, mainly in His-Xxx ß-turns, and assigned to solvent accessible turn structures. They were well accounted for by theoretical models of microhydrated turns, in which a few solvent molecules take over the gas phase motifs, constituting a water-mediated local anchoring of the His side chain to the backbone. Finally, the present gas phase benchmark models also pinpointed weaknesses in the protein structure determination by X-ray diffraction analysis; in particular, besides the lack of tautomer information, inaccuracies in the description of imidazole ring flip rotamerism were identified.

Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique.

Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeO x on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeO x coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate.

Local NH-π interactions involving aromatic residues of proteins: influence of backbone conformation and ππ* excitation on the π H-bond strength, as revealed from studies of isolated model peptides.

Conformer-selective IR gas phase spectroscopy and high level quantum chemistry methods have been used to characterise the diversity of local NH-π interactions between the π ring of a phenylalanine aromatic residue and the nearby main chain amide groups. The study of model systems shows how the amide NH stretch vibrational features, in the 3410-3460 cm-1 frequency range, can be used to monitor the strength of these local π H-bonds, which is found to depend on both the backbone conformation and the aromatic side chain orientation. This is rationalized in terms of partial electron transfer between the π cloud and the main chain NH bonds, with the help of analysis tools based on Natural Bonding Orbitals and Non-Covalent Interactions plots. The experimental study, extended to the NH-π interactions when the Phe residue is excited in its first ππ* electronic state, also demonstrates the principle of the ππ* labelling technique, i.e. a selective labelling of those NH bonds in a peptide molecule that are in close contact with an aromatic ring, as an elegant tool for IR spectroscopic assignments. The validation of theoretical predictions against experimental data (frequency change upon excitation) eventually qualifies the use of the CC2 method for the description of the ππ* excited states of systems having a phenyl ring, both in terms of structure, vibrational modes and nature of excited states.

Conformationally resolved spectra of acetaminophen by UV-UV hole burning and IR dip spectroscopy in the gas phase.

Electronic and vibrational spectra of acetaminophen were measured by using UV-UV hole burning (HB) and IR dip spectroscopy. HB spectra show the coexistence of 4 different species, which include two new ones. Low-frequency transitions in the spectra are reproduced by a one-dimensional periodic potential with a free-rotor basis set for the methyl group. From the analysis, we concluded that acetaminophen has two conformers and each conformer gives two independent transitions starting from the most stable 0a(1) and the hot 1e internal rotational levels. It is also found that the HB spectrum of the trans-conformer in the previous report is that from the 1e excited level, while the HB spectrum of the cis-conformer is contaminated by the transitions of the trans-conformer. Potential curves of the methyl rotational motion are determined both in S(0) and S(1).

Solvent-assisted conformational isomerization and the conformationally-pure REMPI spectrum of 3-aminophenol.

3-Aminophenol (3AP) has two conformers, cis and trans, depending on the orientation of the OH group relative to the NH(2) group. While both conformers are found in the jet-cooled spectra of 3AP, only the trans isomer was found in the REMPI spectrum of the 3AP(NH(3))(1) cluster. It was suggested that the cis conformer of the cluster isomerizes to the more stable trans conformer in the ground state during supersonic expansion. Solvent-assisted conformational isomerization (SACI) is believed to drive the population into the more stable trans isomer. SACI also occurs for the 3AP monomer, reducing 50% of the cis/trans ratio when the ammonia concentration in the expansion is higher than 0.1%. Depending on the expansion condition, the cis conformer can be completely depleted. When other solvents were introduced in the expansion, SACI occurred with only certain solvents whose binding energy is higher than the isomerization barrier. SACI can be used as a means to prepare the most stable conformer of gas phase biomolecules.

Microhydration effects on the electronic spectra of protonated polycyclic aromatic hydrocarbons: [naphthalene-(H2O)(n = 1,2)]H+.

Vibrational and electronic spectra of protonated naphthalene (NaphH(+)) microsolvated by one and two water molecules were obtained by photofragmentation spectroscopy. The IR spectrum of the monohydrated species is consistent with a structure with the proton located on the aromatic molecule, NaphH(+)-H(2)O. Similar to isolated NaphH(+), the first electronic transition of NaphH(+)-H(2)O (S(1)) occurs in the visible range near 500 nm. The doubly hydrated species lacks any absorption in the visible range (420-600 nm) but absorbs in the UV range, similar to neutral Naph. This observation is consistent with a structure, in which the proton is located on the water moiety, Naph-(H(2)O)(2)H(+). Ab initio calculations for [Naph-(H(2)O)(n)]H(+) confirm that the excess proton transfers from Naph to the solvent cluster upon attachment of the second water molecule.