P-polarized Multiple-angle Incidence Resolution Spectrometry Research Articles

Quantitative Analysis of Photochemical Reactions in Pentacene Precursor Films.

On-surface reactions are rapidly gaining attention as a chemical technique for synthesizing organic functional materials, such as graphene nanoribbons and molecular semiconductors. Quantitative analysis of such reactions is essential for fabricating high-quality film structures, but until our recent work using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), no analytical technique is available to quantify the reaction rate. In the present study, the pMAIRS technique is employed to analyze the photochemical reaction from 6,13-dihydro-6,13-ethanopentacene-15,16-dione to pentacene in thin films. The spectral analysis on a pMAIRS principle readily reveals the photoconversion rate accurately without other complicated calculations. Thus, this study underlines that the pMAIRS technique is a powerful tool for quantitative analysis of on-surface reactions, as well as molecular orientation.

Insights on the Molecular Orientation of Oligo(p-phenylene vinylene) Derivatives with Alkyl Chains in Langmuir Films

One of the oligo(p-phenylene vinylene) derivatives having alkyl chains, OPV-1, is known to form characteristic rod-shaped nanoaggregates on the water surface, and thin films with aligned nanoaggregates can be obtained by the Langmuir–Blodgett (LB) technique. Although the molecular aggregation and orientation of the “conjugated rings part” in the LB films have already been investigated, analysis of the detailed molecular structure involving the “alkyl chains” is left behind. In the present study, to reveal the role of the alkyl chains in the molecular aggregation in Langmuir films prior to the LB transfer, infrared (IR) external reflection spectroscopic measurements on the water surface are carried out. It has been revealed that not only the conjugated rings of OPV-1 but also the alkyl chains with an ordered conformation are lying on the water surface. The characteristic orientation is found to be kept on a solid substrate even after the LB transfer by IR p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). In addition, pMAIRS was first employed for analyses of the biaxial orientation in the LB films, and the molecular orientations for the conjugated rings and the alkyl chains were quantitatively evaluated from both in-plane and out-of-plane perspectives.

(Invited) Metalation Effect on Diketopyrrolopyrrole-Tetrabenzoporphyrin Conjugate As p-Type Material in BHJ Organic Photovoltaics

Organic photovoltaics have attracted much attention for the next generation of photovoltaics. Tetrabenzoporphyrin (BP) is a p-type organic semiconductor characterized as the large, rigid p-framework, excellent stability, and good photoabsorption capability. Because of the low solubility, thermal precursor approach is necessary for the deposition of BP by solution process, where a soluble precursor (CP) of BP is spin-coated on the substrate, followed by annealing of CP film to make BP film by in-situ retro-Diels-Alder reaction of CP.[1,2] In 2017, we reported side-chain engineering for efficient photocurrent generation using a series of diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates named Cn-DPP–BP (n = 4, 6, 8 or 10 depending on the length of n-alkyl groups on the DPP unit) as p-type materials in BHJ OPVs in combination with PC61BM as a n-type material by the thermal precursor approach.[3] The comparative study shows that the short-circuit current density largely depends on the length of alkyl substituents, ranging from 0.88 mA cm-2 with C10-DPP–BP to 15.2 mA cm-2 with C4-DPP–BP. C10-DPP–BP in the film is edge-on orientation dominant while that of C4-DPP–BP is face-on orientation dominant.Encouraged with the results, we synthesized a zinc complex of C4-DPP-CP (C4-DPP-ZnCP) as a soluble precursor of C4-DPP-ZnBP to investigate the effect of metalation of the porphyrin cavity for the BHJ OPV performance. BHJ OPV was prepared by thermal precursor approach: spin-coating of a mixed solution of the precursor, C4-DPP-ZnCP, and PC61BM, followed by annealing at 200 ºC to make a blend film of C4-DPP-ZnBP and PC61BM. Interestingly, the energy gap and ionization potential of C4-DPP-ZnBP film and C4-DPP-BP film are similar to each other, but the Jsc of C4-DPP-ZnBP is 1.5 times larger than that of C4-DPP-BP. We will discuss the relationship between film morphology and OPV performance by studying the blend film of C4-DPP-ZnBP/PC61BM by atomic force microscopy (AFM), X-ray diffraction (XRD) analysis and p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS).References H. Yamada, T. Okujima, N. Ono, Chem. Commun. 2008, 2957-2974. H. Yamada, D. Kuzuhara, M. Suzuki, H. Hayashi, N. Aratani, Bull. Chem. Soc. Jpn. 2020, 93, 1234-1267. K. Takahashi, D. Kumagai, N. Yamada, D. Kuzuhara, Y. Yamaguchi, N. Aratani, T. Koganezawa, S. Koshika, N. Yoshimoto, S. Masuo, M. Suzuki, K. Nakayama, H. Yamada, J. Mater. Chem. A 2017, 5, 14003-14011.

Impact of graphene-molecular interaction on collective orientation barrier for organic film growth

The temperature-dependent molecular orientation variation of pentacene (PEN) on a graphene-covered substrate (PEN/Gr) was investigated via p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). The temperature regime of the orientation transition of PEN/Gr was different from that of PEN/SiO2. The collective orientation barrier (COB), an energy barrier that molecules need to overcome to form a standing orientation, was estimated via pMAIRS. Consequently, the COB of PEN/Gr was found to be 10 times larger than that of PEN/SiO2. This indicated that the COB is valuable for understanding the effect of substrate interaction on molecular orientation.

Maximizing Conversion of Surface Click Reactions for Versatile Molecular Modification on Metal Oxide Nanowires.

Click reactions (e.g., Huisgen cycloaddition) on metal oxide nanostructures offer a versatile and robust surface molecular modification for various applications because they form strong covalent bonds in a wide range of molecular substrates. This study reports a rational strategy to maximize the conversion rate of surface click reactions on single-crystalline ZnO nanowires by monitoring the reaction progress. p-Polarized multiple-angle incidence resolution spectrometry (pMAIRS) and Fourier-transformed infrared (FT-IR) spectroscopy were employed to monitor the reaction progress of an azide-terminated self-assembled monolayer (SAM) on single-crystalline ZnO nanowires. Although various reaction parameters including the concentration of Cu(I) catalysts, triazolyl ligands, solvents, and target alkynes were systematically examined for the surface click reactions, 10-30% of terminal azide on the nanowire surface remained unreacted. Temperature-dependent FT-IR measurements revealed that such unreacted residual azides deteriorate the thermal stability of the nanowire molecular layer. To overcome this observed conversion limitation of click reactions on nanostructure surfaces, we considered the steric hindrance around the closely packed SAM reaction points, then experimented with dispersing the azide moiety into a methyl-terminated SAM. The mixed-SAM method significantly improved the azide conversion rate to almost 100%. This reaction method enables the construction of spatially patterned molecular surface modifications on metal oxide nanowire arrays without detrimental unreacted azide groups.

Quantitative analysis of stereoscopic molecular orientations in thermally reactive and heterogeneous noncrystalline thin films via variable-temperature infrared pMAIRS and GI-XRD

The quantitative estimation of stereoscopic molecular orientations in semicrystalline polymer thin films with characteristic heterogeneous structures is essential for the development of advanced film materials. Grazing-incidence X-ray diffraction (GI-XRD) is a powerful technique used to evaluate the orientational order of crystalline lattices; however, it provides the orientational order of only ordered regions (crystal and glassy liquid crystal regions). In contrast, infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) can provide the averaged orientational order of overall regions in a film regardless of sample crystallinity. This paper combines variable-temperature (VT) pMAIRS, static GI-XRD and spectroscopic ellipsometry and reports general insight into the molecular orientation mechanism occurring in polymer films revealed through the precise analysis of the structural evolution of linear poly(amic n-alkyl ester), PAE, with different lengths of n-alkyl chains during thermal curing, which involves solvent evaporation and dehydration imidization reactions to form the corresponding polyimide (PI). This combined method clarified that the PAE and PI thin films had a heterogeneous structure consisting of a glassy liquid-crystalline (ordered) region and a nonoriented amorphous region. Furthermore, imidization proceeded faster in the amorphous region than in the ordered (oriented) region. In addition, anomalies of refractive index and birefringence were found by comparing pMAIRS and spectroscopic ellipsometry, although the effect of the side chain length on the molecular orientation of PI was insignificant. The anomalies indicate that an isotropic void structures were generated by the elimination reaction of the side chains and that the volume fraction of voids increased with increasing side chain length. These results demonstrated that the combination of VT-pMAIRS and GI-XRD is a very powerful tool for the in situ structural analyses of noncrystalline (disordered crystal, glassy liquid crystal, and oriented amorphous) thin-film materials ongoing chemical reactions in their components and is inherently applicable to all kinds of interesting polymeric thin-film materials. Thin film aromatic polyimides were quantitatively analyzed by grazing-incidence X-ray diffraction (GI-XRD), variable-temperature infrared p-polarized multiple-angle resolution spectroscopy (VT-pMAIRS), and spectroscopic ellipsometry (SE) methods. Combining VT-pMAIRS and GI-XRD revealed characteristic heterogeneous structures comprising non-oriented amorphous and oriented liquid-crystalline glass regions. Additionally, SE indicated that the film involved anisotropic-shaped void inducing form-birefringence. Such an excess of birefringence can make it difficult to estimate orientational order using birefringence measurement. This methodology presented here will remarkably contribute to general structural analyses of all intriguing thin-film polymers with characteristic heterogeneity.

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.

Simultaneous Analysis of Molecular Orientation and Quantity Change of Constituents in a Thin Film Using pMAIRS.

Spectral analysis using chemometrics is extensively used for quantitative chemical analysis in a mixture, but it works powerfully only when the peak intensity is solely proportional to the quantity of chemical components. In this sense, thin films on a solid substrate are not suitable for chemometric analysis, because the molecular orientation also influences the peak intensity via the surface selection rules. In the present study, this long-term analytical issue has readily been overcome by using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), which has a characteristic that the in-plane (IP) and out-of-plane (OP) vibrational spectra of a thin-film sample are obtained simultaneously in a common ordinate scale. Thanks to this unique power of pMAIRS, the average of the IP and OP spectra annihilates optical anisotropy, yielding an orientation-free spectrum, which enables us to perform the simultaneous quantitative analysis of both quantity change and molecular orientation of the constituents in a thin film. Now, we are ready to examine chemical reactions quantitatively in a thin film.

PMAIRS Analysis on Chain-End Functionalization of Densely Grafted, Concentrated Polymer Brushes

The degree of chain-end functionality in a densely grafted, concentrated polymer brush is a critical parameter. In this study, p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) ...

Two-Dimensional Film Growth of Zinc Tetraphenylporphyrin with the Aid of Solvent Coordination

Abstract Since the performance of an organic film device largely depends on the molecular arrangement in the film, control of the arrangement is crucial. Low molecular-weight organic semiconductor compounds, however, tend to have the edge-on orientation spontaneously due to crystallization on an inert surface. In fact, orientation control of a high-crystallinity compound is often difficult especially on an inert surface. In the present study, a face-on crystalline thin film of zinc tetraphenylporphyrin (ZnTPP) has readily been obtained on an inert surface by using solvent annealing (SA) with an appropriate solvent, formamide (FA). The prepared film is carefully analyzed by using the infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) and grazing incidence X-ray diffraction (GIXD) techniques. The measurements show that an FA molecule coordinated on a zinc atom blocks the stacking interaction sites of the porphyrin ring, and instead the side to side interaction between the C–H group of the porphyrin rings and π orbital of the phenyl rings forms a two-dimensional sheet-like structure realizing face-on orientation.

Alternative Face-on Thin Film Structure of Pentacene

Pentacene attracts a great deal of attention as a basic material used in organic thin-film transistors for many years. Pentacene is known to form a highly ordered structure in a thin film, in which the molecular long axis aligns perpendicularly to the substrate surface, i.e., end-on orientation. On the other hand, the face-on oriented thin film, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO2. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal. In the present study, the face-on orientation grown on a SiO2 surface has first been identified by means of visible and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the multiple techniques readily reveals that the face-on phase is definitely realized as the dominant component. The face-on film is obtained when the film growth is kinetically restricted to be prevented from transforming into the thermodynamically stable structure, i.e., the end-on orientation. This concept is useful for controlling the molecular orientation in general organic semiconductor thin films.

Molecular Orientation Change in Naphthalene Diimide Thin Films Induced by Removal of Thermally Cleavable Substituents

The potential of naphthalene-1,8:4,5-tetracarboxylic diimide (NDI-H) as a transparent electron-transporting material was examined. A soluble precursor was designed and synthesized having two tert-butoxycarbonyl solubilizing substituents at the imide moieties. This precursor molecule, NDI-Boc, is converted to the hydrogen-substituted NDI-H by heating the spin-coated precursor films. The molecular orientation during the thermal conversion of NDI-Boc to NDI-H was examined using two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and p-polarized multiple-angle incidence resolution spectrometry. It was revealed that, driven by the formation of intermolecular hydrogen bonds, the molecular orientation changes from tilted edge-on to face-on orientation. In situ 2D-GIXD measurements confirmed that the change of molecular orientation is simultaneously caused by the cleaving of the Boc substituents. Time-resolved microwave conductivity measurements were used to show that the resulting NDI-H film has anisot...

Analysis of the Molecular Orientation of Poly(3-hexylthiophene) on Silicon Treated with Silane Coupling Agents, by Infrared p-Polarized Multiple-angle Incidence Resolution Spectrometry

The molecular orientation of poly(3-hexylthiophene) (P3HT) thin film on silicon substrate treated with silane coupling agents was evaluated by infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). It was found that the orientation is altered by interaction between the end group of the silane coupling agent and the P3HT ring. The amino group forming the end group of the silane coupling agent strongly influences the P3HT molecular orientation, causing it to shift to a more perpendicular direction with respect to the substrate.

Controlling Mechanism of Molecular Orientation of Poly(3-alkylthiophene) in a Thin Film Revealed by Using pMAIRS

A face-on oriented thin film of poly(3-hexylthiophene) (P3HT) is suitable for an organic semiconductor layer in a photovoltaic device, and thus analysis of the film structure in terms of molecular orientation is crucial. Although the face-on film often has a poor crystallinity, diffraction techniques have long been employed for the structural analysis, and only very minor crystal parts have been discussed. In our previous study, P3HT was revealed to have a uniquely oriented structure even in an amorphous film by using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), which is powerful to analyze the molecular orientation, crystallinity, and the conjugation length of P3HT. With the best use of the potential of pMAIRS, in the present study, the controlling mechanism of the molecular orientation of P3HT correlated with the crystallinity in a spin-coated thin film is revealed. As a result, by employing a high-spin speed at 8000 rpm, a highly face-on oriented thin film having a very low cr...

Fringe and Noise Reductions of pMAIRS Spectra Using Principal Component Analysis.

Infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a promising analytical tool for revealing the molecular orientation quantitatively of each chemical group in a thin film even with surface roughness. The spectra are often disturbed by noise and fringe, however, due to the multiple reflections in the substrate and the film, which makes the quantitative analysis very difficult. Therefore, improvement of the signal to noise (SN) ratio of the spectra is expected. Principal component analysis (PCA), in the present study, is first applied to the noise reduction for pMAIRS spectra of a poly(3-hexylthiophene) spin-coated thin film by employing the spin-speed as the experimental parameter. As a result, high quality pMAIRS spectra are readily obtained, with which highly reliable quantitative discussion is made.

Accurate Molecular Orientation Analysis Using Infrared p-Polarized Multiple-Angle Incidence Resolution Spectrometry (pMAIRS) Considering the Refractive Index of the Thin Film Sample.

Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a powerful tool for analyzing the molecular orientation in an organic thin film. In particular, pMAIRS works powerfully for a thin film with a highly rough surface irrespective of degree of the crystallinity. Recently, the optimal experimental condition has comprehensively been revealed, with which the accuracy of the analytical results has largely been improved. Regardless, some unresolved matters still remain. A structurally isotropic sample, for example, yields different peak intensities in the in-plane and out-of-plane spectra. In the present study, this effect is shown to be due to the refractive index of the sample film and a correction factor has been developed using rigorous theoretical methods. As a result, with the use of the correction factor, organic materials having atypical refractive indices such as perfluoroalkyl compounds ( n = 1.35) and fullerene ( n = 1.83) can be analyzed with high accuracy comparable to a compound having a normal refractive index of approximately 1.55. With this improved technique, we are also ready for discriminating an isotropic structure from an oriented sample having the magic angle of 54.7°.

Comprehensive Understanding of Structure-Controlling Factors of a Zinc Tetraphenylporphyrin Thin Film Using pMAIRS and GIXD Techniques.

The performance of an organic electronic device is significantly influenced by the anisotropic molecular structure in the film, which has long been difficult to predict especially for a solution process. In the present study, a zinc tetraphenylporphyrin (ZnTPP) thin film prepared by a solution process was chosen to comprehensively explore the molecular-arrangement mechanism as a function of representative film-preparation parameters: solvent, film-preparation technique, and thermal annealing. The anisotropic structure was first analyzed by using a combination of infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) and grazing incidence X-ray diffraction (GIXD), which readily revealed the molecular orientation and crystal structure, respectively. As a result, the real dominant factor was found to be the evaporation time of the solvent that determines the initial two different molecular arrangements, types-I and -II, while the thermal annealing was found to play an additional role of improving the molecular order. The correlation between the molecular orientation and the crystal structure was also revealed through the individual orientation analysis of the porphyrin and phenyl rings.

Optimal Experimental Condition of IR pMAIRS Calibrated by Using an Optically Isotropic Thin Film Exhibiting the Berreman Effect.

Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a useful spectroscopic tool for revealing the molecular anisotropic structure in a thin film, which is used for the molecular orientation analysis of many functionalized organic thin films. Infrared pMAIRS provides both in-plane (IP) and out-of-plane (OP) vibrational mode spectra, which are influenced by the choice of the angles of incidence, i.e., angle set. To obtain quantitatively reliable pMAIRS spectra, therefore, the optimal angle set must be revealed. In a former study, an optimization study was carried out on a silicon substrate by using the band intensity ratio of the IP and OP spectra of highly oriented molecules in a thin film, which has a problem that the optimized results cannot be used for another substrate. In the present study, a totally new idea using an optically isotropic thin film as a standard sample is proposed to comprehensively explore the optimal angle set on various substrates: the band shift due to the Berreman effect of a strongly absorbing compound is used, instead of the band intensity. This new approach makes the pMAIRS calibration for various substrates a much easier task. With the optimal angle set, the molecular orientation angle in the film calculated by the pMAIRS spectra is also found to be reliable quantitatively. This technique opens a user-friendly way to a reliable molecular orientation analysis in an ultrathin film using IR pMAIRS.

Interfacial Structure and Proton Conductivity of Nafion at the Pt-Deposited Surface.

Understanding the Nafion-Pt interface structure is important because fuel cell reactions occur at the three-phase boundary. Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique was used to investigate the in-plane (IP) and out-of-plane (OP) spectra in the identical substrate. Our previous study revealed that the proton conductivity of the Nafion thin films decreased at the MgO and SiO2 surfaces. We proposed that the origin for the lower proton conductivity can be derived from the highly oriented structure at the interface. However, the interface structure of the Nafion-Pt interface remains unclear. In this study, Nafion thin films were prepared by spin-coating on a Pt-deposited MgO substrates. The IP spectrum exhibited a well-known spectrum, but the OP spectrum was quite differed considerably from the IP spectrum. Furthermore, thickness dependence of the degree of orientation for this OP band was observed at the Nafion-Pt interface. This OP band can be assigned as the vibration mode of the mixture of the CF2 and sulfonic acid groups. At the low-RH region, proton conductivity of the Nafion thin film on the Pt-deposited surface was 1 order of magnitude higher than that on the SiO2 surface. Furthermore, the activation energy was 0.4-0.5 eV, which is lower than that of the SiO2 surface. These results, which suggest that the Pt surface influenced the proton transport property of Nafion thin film, can contribute to understand the relationship between the proton transport property and thin film structure on the Pt-deposited surface at the three-phase boundary for fuel cells.

An Origin of Complicated Infrared Spectra of Perfluoroalkyl Compounds Involving a Normal Alkyl Group

Abstract Perfluoroalkyl (Rf) compounds containing a normal alkyl group often yield highly complicated infrared (IR) spectra especially in the C–F stretching vibration (νC–F) region. To reveal the reason behind this, the IR p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is employed to measure a monolayer of CF3(CF2)9(CH2)3COOH deposited on a silicon substrate. The compound is known to spontaneously aggregate to form a molecular assembly with the closest packing, in which the molecules are oriented perpendicular to the substrate. The IR pMAIRS spectra apparently prove that the complexity of the νC–F region is due to the normal alkyl part directly connected to the Rf group because the carbons in the Rf group are vibrated as a coupled oscillator, and the oscillation of the alkyl part propagates to the Rf part along the molecular axis.