Multiple-angle Incidence Resolution Spectroscopy Research Articles

Ordered arrangement of F4TCNQ anions in three-dimensionally oriented P3HT thin films

An ordered arrangement of electron-accepting molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), in three-dimensionally (3D) oriented poly(3-hexylthiophene) (P3HT) film was clarified. The 3D oriented P3HT thin films prepared by the friction-transfer technique were doped with F4TCNQ by dipping into an acetonitrile solution. The presence of F4TCNQ anions in the 3D oriented P3HT thin films was investigated by polarized ultraviolet/visible/near-infrared absorption spectroscopy, grazing incidence X-ray diffractometry, polarized Fourier transform infrared spectroscopy (FT-IR), and infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS). The F4TCNQ-doped 3D oriented P3HT films showed anisotropic properties in all characterizations. In particular, the anisotropic molecular vibrations from polarized FT-IR and pMAIRS have clearly revealed orientations of polymeric chains and molecular dopant molecules. Considering the results from several independent techniques indicated that F4TCNQ anions in the 3D oriented P3HT were orderly arranged in a 3D manner with respect to the 3D oriented P3HT such that their molecular long-axis parallel to the P3HT backbone, with in-plane molecular orientation. Additionally, the direction of the optical transition moment of the F4TCNQ anion was found to be parallel to the molecular short-axis.

Precise structural analysis of polymer materials using synchrotron X-ray scattering and spectroscopic methods

Polymers can exist in many different conformations and exhibit a variety of unique and complicated hierarchical structures, which are dominated mainly by entropy and sometimes by kinetics rather than thermodynamics due to the intrinsic high viscosity, polydispersity, and so on. The physical properties of polymeric materials are determined not only by their primary structures but also by their higher order (hierarchical) structures. Hence, precise determination of the structures at all spatial scales is essential to control the physical properties of polymeric materials. This focus review details the use of various characterization techniques, including variable-temperature (VT) synchrotron (SR) X-ray scattering, VT SR X-ray diffraction, and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), to determine the structure of fully liquid-crystalline ABA-type triblock copolymers, highly crystalline aromatic polyimides, and thin-film polyimides, respectively. The advantages of these methods are briefly reviewed, and the relationship between the macroscopic physical properties and the inherent microstructure is discussed. Physical properties of polymer materials are deeply related to not only the primary structure but also the higher ordered structure such as periodic structures and molecular orientation. If the target material consists of ordered and non-ordered region, the former structure can be selectively detected by X-ray scattering method, whereas structural information of both the regions is obtained by infrared absorption spectroscopy. Particularly for thin film materials, infrared pMAIRS (p-polarized multiple-angle incidence resolution spectroscopy) and GI-XRD (grazing incidence X-ray diffraction methods are very useful for precise structural analyses.

Conformation change of α-synuclein(61－95) at the air-water interface and quantitative measurement of the tilt angle of the axis of its α-helix by multiple angle incidence resolution spectroscopy

Various techniques have been developed to determine protein’s structure to understand how proteins work. Compared with X-ray crystallography requiring proteins to form single crystal structure and NMR which usually needs long time measurement, surface FT-IR techniques are able to quickly determine the tilt angle (the key information to determine whether the α-helix is transmembrane) of peptides/proteins in a monolayer at the interface (e.g. membranes). Specifically, for α-helical peptides/proteins in membrane, the tilt angle of the axis is one of the key information. In this paper, Multiple Angle Incidence Resolution Spectroscopy (MAIRS), a recently developed surface FTIR technique, was applied for the first time to quantitatively determine the tilt angle of the axis of α-helical model peptide related to α-synuclein (α-syn). α-Syn is a 140-amino-acid presynaptic protein whose aggregation is the hallmark of Parkinson’s disease (PD). It is difficult for α-syn to form a single crystal structure and the primary structure of α-syn constitutes three domains: the N-terminus containing residues 1–60; the nonamyloid component (NAC) which spans residues 61–95 and is highly prone to aggregation; and C-terminus with residues 96–140. Here, the NAC part (i.e., α-syn(61–95)) responsible for the aggregation was found to change its unstructured conformation in aqueous solution to α-helix at the air-water interface by circular dichroism and MAIRS. In addition, the instinct power of MAIRS to quantitatively measure the tilt angle of the axis of α-helical α-syn(61–95) in monolayer was fully exhibited. Therefore, MAIRS is a potential supplemental technique to X-ray crystallography and NMR to determine the structure of membrane peptides/proteins.

Rational Method of Monitoring Molecular Transformations on Metal-Oxide Nanowire Surfaces.

Metal-oxide nanowires have demonstrated excellent capability in the electrical detection of various molecules based on their material robustness in liquid and air environments. Although the surface structure of the nanowires essentially determines their interaction with adsorbed molecules, understanding the correlation between an oxide nanowire surface and an adsorbed molecule is still a major challenge. Herein, we propose a rational methodology to obtain this information for low-density molecules adsorbed on metal oxide nanowire surfaces by employing infrared p-polarized multiple-angle incidence resolution spectroscopy and temperature-programmed desorption/gas chromatography-mass spectrometry. As a model system, we studied the surface chemical transformation of an aldehyde (nonanal, a cancer biomarker in breath) on single-crystalline ZnO nanowires. We found that a slight surface reconstruction, induced by the thermal pretreatment, determines the surface chemical reactivity of nonanal. The present results show that the observed surface reaction trend can be interpreted in terms of the density of Zn ions exposed on the nanowire surface and of their corresponding spatial arrangement on the surface, which promotes the reaction between neighboring adsorbed molecules. The proposed methodology will support a better understanding of complex molecular transformations on various nanostructured metal-oxide surfaces.

Control of Molecular Orientation in Naphthalene Diimides

Lead halide perovskite solar cells represent attractive cost-effective and solution-processable photovoltaics. At present, the record-high performance perovskite solar cells are based on an n-i-p device configuration, in which metal oxides, such as TiO2, are generally utilized as electron transporting layer. Deposition of metal oxides usually requires high temperature sintering process, which prevents mass production and application to flexible devices on plastic substrates. Here, we focused on the naphthalene diimide (NDI) skeleton as alternative transparent n-type materials. By introducing hydrogen atoms at the imide moiety (NDI-H), we envisioned the achievement of face-on orientation on the substrates, which is preferable for vertical charge transport, by the formation of intermolecular N–H···O hydrogen bondings. To realize both the solution-processability and the durability to the deposition of the upper layers, we designed NDI-Boc having tert-butoxycarbonyl (Boc) solubilizing groups at the imide moiety, which can be cleaved by simple thermal annealing. Infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) not only confirmed the cleavage of the Boc group, but also revealed that the molecular orientation of NDI changed from edge-on to face-on during the cleavage process. Furthermore, NDI-H film works as electron transporting layer in n-i-p type planer perovskite solar cells with the power conversion efficiency of 10.7%.

Impact of Kinetically Restricted Structure on Thermal Conversion of Zinc Tetraphenylporphyrin Thin Films to the Triclinic and Monoclinic Phases

The powerful combination of p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) and grazing incidence X-ray diffraction (GIXD) is applied to the structural characterization of zinc tetraphenylporphyrin (ZnTPP) in vapor-deposited films as a function of the deposition rate. The deposition rate is revealed to have an impact on the initial film structure and its conversion by thermal annealing. The pMAIRS spectra reveal that a fast deposition rate yields a kinetically restricted amorphous film of ZnTPP having a “face-on orientation”, which is readily discriminated from another “randomly oriented” amorphous film generated at a slow deposition rate. In addition, the GIXD patterns reveal that the film grown at a slow deposition rate involves a minor component of triclinic crystallites. The different initial film structure significantly influences the thermal conversion of ZnTPP films. The randomly oriented amorphous aggregates with the triclinic crystallite seeds are converted to the thermodyna...

Understanding device-structure-induced variations in open-circuit voltage for organic photovoltaics.

We investigate the structural influences on the device performance, especially on open-circuit voltage (V(OC)) in squaraine (SQ)/fullerene (C60) bilayer cells. Simply changing the SQ thickness could lead to 40% variation in V(OC) from 0.62 to 0.86 V. The ionization potential (IP) of SQ films and recombination at the anode surface as well as donor/acceptor (D/A) interface sensitively vary with film thicknesses, which account for the shifts in V(OC). The anode recombination can be effectively suppressed by preventing direct contact between C60 and the anode with a buffer layer, delivering an elevated V(OC). Through polarized infrared-multiple-angle incidence resolution spectroscopy measurement, the molecular structure of SQ films is found to gradually evolve from lying-down on indium-tin oxide substrates with noncentrosymmetric orientation at low thicknesses to random structure at high thicknesses. The different molecular orientation may yield different strengths of electronic coupling, which affects the charge-carrier recombination and thus V(OC). Moreover, the oriented SQ films would spontaneously compose aligned dipole moments at the D/A interface because of the strong dipolar effects in SQ molecules identified by density functional theory calculations, whereas no aligned interfacial dipole moment exists in the random structure. The resulting interfacial dipole moments would form an electric field at the D/A interface, leading to variations in the IP and thus impacting V(OC). Our findings demonstrate that V(OC) in organic photovoltaic cells is critically associated with the molecular orientation that affects the charge-carrier recombination and interfacial dipole alignment, which should be seriously taken into consideration for the design of organic molecules and optimization of the cell efficiency.

A new schematic for poly(3-alkylthiophene) in an amorphous film studied using a novel structural index in infrared spectroscopy.

The molecular structure of poly(3-alkylthiophene-2,5-diyl) in an amorphous film reveals that the short axis of the thiophene ring is kept highly oriented parallel to the substrate, whereas the long axis along the polymer chain is largely disordered. This is unveiled by infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS), achieved by analyzing the orientation angles of three mutually orthogonal vibrational modes localized on the thiophene ring with the aid of a newly developed structural index. This new analytical technique is useful irrespective of the crystallinity of the thin film. As a result, the intrinsic chemical parameters controlling the molecular orientation are understood in a unified manner, and the reason that the hexyl group gives the best results for a photovoltaic cell is also revealed.

X-ray diffraction and infrared multiple-angle incidence resolution spectroscopic studies on the crystal structure and molecular orientation of zinc-porphyrin thin films on a SiO2/Si substrate

The crystal structure and molecular orientation of zinc-octaethylporphyrin [Zn(OEP)] thin films deposited on a SiO2/Si(111) substrate have been investigated using x-ray diffraction (XRD) and infrared multiple-angle incidence resolution spectroscopy (MAIRS), respectively. XRD results show that the Zn(OEP) thin films have two kinds of crystallites with a diffraction plane (2θ=7.9°) parallel to the substrate and with that (2θ=7.1°) inclined mainly by 60°–70° to the substrate. Comparison with simulated XRD results of the Zn(OEP) single crystal indicates that the diffraction angle of 2θ=7.9° is consistent with that of the (011¯) plane of the single crystal, while the angle of 2θ=7.1° is slightly different from that of 7.3° corresponding to the (010) plane of the single crystal. On the other hand, MAIRS results demonstrate that Zn(OEP) molecules in the crystallite corresponding to 2θ=7.9° are inclined by 52.2±0.9° to the substrate, which is in a good agreement with that of ca. 50° estimated from the Zn(OEP) single crystal structure. In addition, these crystalline sizes are estimated to be approximately 20 nm and independent of film thickness. Accordingly, it can be concluded from the XRD and MAIRS results that the Zn(OEP) films have two kinds of grains. One has the single crystal structure, in which Zn(OEP) molecules are inclined by 52.2°±0.9° to the substrate. On the other hand, the other has a crystal structure close to the single crystal, in which Zn(OEP) molecules would be inclined by 10°–20° to the substrate.

Advanced Multiple-Angle Incidence Resolution Spectrometry for Thin-Layer Analysis on a Low-Refractive-Index Substrate

A long-time issue that thin films supported on a substrate with low refractive index cannot be subjected to multiple-angle incidence resolution spectrometry (MAIRS) has been resolved, and an advanced MAIRS technique has been developed. Infrared multiple-angle incidence resolution spectroscopy (IR MAIRS) is being recognized as a powerful spectroscopic tool for revealing molecular orientation in thin films or molecular adsorbates. MAIRS has been, however, employed with only IR spectroscopy thus far, since MAIRS requires high refractive index substrates such as germanium and silicon, and shorter wavelength regions have no appropriate substrates. Although one of the most commonly useful substrates for IR spectroscopy is CaF2, unfortunately it cannot be employed for IR MAIRS because of the low refractive index. To overcome this experimental limitation, the principle of MAIRS has been studied theoretically, and it has been found that polarization effects on transmission intensity is the main cause that disturbs a proper MAIRS analysis. As a result, now it is possible to employ IR MAIRS for analysis of thin films on CaF2 by removing the s-polarization and by use of the advanced MAIRS algorithm.

Analysis of Structurally Heterogeneous Langmuir−Blodgett Films of Folded/Unfolded Long-Chain Molecules by Infrared Multiple-Angle Incidence Resolution Spectroscopy

Infrared multiple-angle incidence resolution spectroscopy (IR MAIRS) has been employed for the structural analysis of a heterogeneous multilayer Langmuir-Blodgett (LB) film of ketomycolic acid (keto-MA). Keto-MA was chosen for the study because the molecule changes its conformation largely, which depends on the layer structure of the LB film: the long alkyl chain with a keto group remains folded when it has no counter layer for interdigitation. IR MAIRS is a surface analytical technique that was originally developed to analyze anisotropic structure in a thin film prepared on an IR transparent material. In the present study, however, a newly found characteristic of MAIRS has been used to discuss the molecular conformation and the interdigitation architecture between two adjacent monolayers. As expected theoretically, the molecular folding was clearly detected as "IR MAIRS shift". Subtraction of the IR MAIRS spectra of single- and triple-monolayer LB film of keto-MA yielded unambiguous results for the interdigitated double layer in the triple-monolayer LB film, and the layer sequence was also revealed.

Measurements of a monolayer Langmuir–Blodgett film on a thin glass plate by infrared multiple-angle incidence resolution spectroscopy

The infrared analysis of ultrathin films on a thin glass slide often has a problem that optical interference fringes appear strongly on the absorption spectra due to multiple reflections in the glass. In the present study, the infrared multiple-angle incidence resolution spectrometry (MAIRS) has been employed to remove the fringes from the spectra of single- and five-monolayer Langmuir–Blodgett (LB) films of cadmium stearate deposited on a thin glass plate. As a result, it is preliminarily suggested that an ultrathin film on a thin glass substrate can be successfully analyzed by FT-IR spectroscopy.

Analysis of hydrogen-terminated Si(1 1 1) surface by infrared multiple-angle incidence resolution spectroscopy

The infrared multiple-angle incidence resolution spectroscopy (MAIRS) has been employed to analyze the hydrogen-terminated Si(1 1 1) 1 × 1–H surface. The new technique of infrared MAIRS provides two absorption spectra simultaneously on an identical sample, which separately reveal the surface-parallel and surface-normal vibration spectra. It has been exhibited in the present study that the infrared MAIRS technique is powerful to discuss some infrared key bands in terms of molecular orientation concerned with Si–H vibrational modes on the Si(1 1 1) 1 × 1–H surface with the use of the two spectra.

Fibril-Like Aggregate Formation of Peptide Carboxylate Langmuir Films Analyzed by Surface Pressure, Surface Dipole Moment, and Infrared Spectroscopy

The fibril formation process of a synthetic peptidolipid compound in a Langmuir monolayer at the air-water interface has been analyzed by surface pressure and surface dipole moment-area isotherms, followed by infrared spectral analysis of related Langmuir-Blodgett films. Thus far, the analysis of randomly oriented molecular assemblies has been a difficult matter, especially for spectroscopic measurements. In the present study, the Langmuir film isotherms were discussed in detail, and they have readily been correlated to the infrared spectra. For the spectral analysis, infrared multiple-angle incidence resolution spectroscopy (MAIRS) was employed, which was compared to the results by conventional techniques. Since the peptide assemblies greatly responded to a metal surface, the reflection-absorption technique was not useful for our analysis. Instead, MAIRS was found to be powerful to reveal the anisotropic structure of the Langmuir films, and a disordered molecular architecture has been revealed via the molecular orientation analysis. As a result, the fibril-like aggregation formation process during the monolayer compression, which was suggested by previous topographical study, has been found to be due to the stiff domain formation in the Langmuir films.

Leucine Fastener Formation Mechanism between Peptide β-Sheets in a Monolayer Studied by Infrared Multiple-Angle Incidence Resolution Spectroscopy

A leucine zipper and a leucine fastener formed between peptide molecules have been hypothetical models of a molecular association via interdigitation. As a molecular interaction mechanism, a "leucine zipper" with the aid of an alpha-helix backbone in biological peptides is believed to play an important role in the molecular association, but no experimental evidence to prove the zipping has been reported thus far. In the same fashion, a "leucine fastener" built on the structure of peptide beta-sheets has never been experimentally captured either. In the present study, very fine changes of molecular stress and orientation in monolayers of a synthesized tetraleucine-containing amphiphile before and after the molecular interdigitation have readily been detected by infrared multiple-angle incidence resolution spectroscopy, which was recently developed for the analysis of structural anisotropy in thin materials. It has been suggested that the conventional molecular orientation model of the leucine fastener should be modified, and the backbone structure (parallel beta-sheet in the present study) plays a necessary role for the interlock of the leucine fastener.

Cartesian-structure analysis in cast films by advanced infrared multiple-angle incidence resolution spectroscopy.

Multiple-angle incidence resolution spectroscopy (MAIRS) has been improved to be an advanced algorithm so that the Cartesian structure in organic thin films can be analyzed. The infrared MAIRS technique was originally proposed as a totally new spectroscopic technique to reveal structural anisotropy in thin films on an infrared-transparent substrate, which yields both in-plane- (IP; X and Y) and out-of-plane (OP; Z)-mode spectra from an identical sample. Since this technique employs an analytical concept based on a signal decomposition of light intensity (not absorbance spectra), the algorithm intrinsically has high potential for further development. In the present study, the theoretically deduced matrix that correlates the light intensity to the angle of incidence has been modified to further decompose the IP-mode spectrum into X and Y components. As a result, anisotropic measurements of infrared spectra of thin film have become possible for the X, Y, and Z directions (Cartesian coordinate) simultaneously. With this advanced algorithm, the Cartesian structural changes in a cast film prepared on a germanium substrate have readily been analyzed, and a change from the biaxial to the uniaxial film structure with aging has spectroscopically been revealed.

Spectroscopic Study of Surface Recovery of Germanium Substrate for Langmuir-Blodgett Films by Infrared Multiple-Angle Incidence Resolution Spectrometry

Surface recovery of germanium substrate for infrared spectroscopic studies of Langmuir-Blodgett (LB) films has been investigated by infrared multiple-angle incidence resolution spectroscopy (MAIRS). A Gesubstrate is, in general, repeatedly used for fabrication of LB films for infrared spectroscopic studies. For the recycling process, the substrate covered by LB films is washed by sonication in organic solvents and pure water. This cleaning generally works well for ordinary analytical use, especially for thick samples, such as polymer and liquid materials. Nonetheless, in the present study, the cleaning by sonication only has been found to be inappropriate for the measurements of monolayer-level samples. The infrared MAIRS analysis has revealed that the 5-monolayer cadmium stearate LB film deposited on a fresh Ge substrate has a molecular tilt angle of ca. 13° from the surface normal, while that deposited on a recycled substrate by sonication has an angle of ca. 30°. Infrared MAIRS analysis has experimentally proved that the ideal recovery of the Ge surface can be achieved by cleaning the substrate with chemical rinse with the use of acid solution.

New Monolayer Architecture Constructed by Competitive Hydrogen-Bonding Force and Compression Pressure Characterized by Infrared Multiple-Angle Incidence Resolution Spectroscopy

A unique monolayer that has a novel film architecture has been constructed by controlling a balance of hydrogen-bonding force and monolayer compression pressure, and the new film architecture has been characterized by infrared multiple-angle incidence resolution spectroscopy (MAIRS) that has recently been developed. The compound synthesized for the study consists of a single hydrocarbon chain and a headgroup that has three trans amide moieties. The limiting molecular area of the monolayer isotherm was approximately double that typically obtained for a single alkanoic-acid chain molecule, which suggested unusual film architecture. Then, the monolayer was transferred on a germanium substrate by the Langmuir−Blodgett film technique, and it was subjected to the infrared MAIRS analysis. The in-plane and out-of-plane mode spectra of MAIRS clearly suggested a flat alignment of interdigitated hydrogen-bonding networks and standing hydrocarbon chains with a disordered kink, which were totally consistent with a corresponding infrared reflection−absorption spectrum and the results of the isotherm measurement. The study suggests that the strongly correlated physical parameters are useful to develop a new film architecture.