CaF2 Plate Research Articles

(Invited) Amplified Spontaneous Emission Dynamics of a Lead Halide Perovskite Crystal as Revealed by Femtosecond Transient Absorption Microscopy

Organic-inorganic lead halide perovskite materials (e.g. CH3NH3PbX3, CsPbX3, X=Cl, Br, I) have various attractive properties not only for solar cells but also for LED and nanoscale lasers because of their wavelength tunability and low lasing threshold. Such an efficient lasing is indispensable for their applications, and the essential needs are miniaturization and low threshold. While this material was reported in 2009 for use in solar cells, it is also expected to be used in light-emitting and laser devices due to its easy tunability of emission wavelength and low-cost fabrication. However, the low-threshold lasing mechanism of lead halide perovskites has not been established due to various theories on the interaction state between light and carriers generated in the cavity at the initial stage and the carrier-carrier interaction state, which has greatly hindered progress in improving performance. In the present study, we have measured transient absorption spectra of a single CH₃NH₃PbBr₃ microcrystal using a femtosecond transient absorption microscopy to reveal the initial carrier process that induces nonlinear luminescence[1].CH3NH3PbBr3 crystals were prepared by a previously reported method. Aqueous lead(II) acetate solution (100 mg/mL) was dropped onto a glass slide and dried at 60°C for 30 min to obtain a film of lead acetate. Microcrystals of CH3NH3PbBr3 were then obtained by immersing the film in methylammonium bromide solution (5 mg/mL) with isopropanol as a solvent for 20 hours at room temperature in air.For transient absorption measurements, a regeneratively amplified femtosecond pulse (Spectra-Physics, Solstice, 795 nm, 1 kHz) was split in two; the second harmonic of one beam was used as the excitation light, and the white light generated with the other beam by introducing it into an optical parametric amplifier (Light Conversion, TOPAS) and focusing the obtained signal light (1.3 mm) into a CaF2 plate was used as the probe light.Fig. 1(a) shows the transient absorption spectra of lead halide perovskite excited at the intensities over the lasing threshold. The spectral signals that were not observed before the time origin (-1 ps) were significantly observed with modulated spectral shape after the time origin (≥0 fs). The spectral bleaching around 540 nm indicates carrier generation by the excitation light, and the relaxation of the bleaching indicates the recombination of electrons and holes. Furthermore, at around 557 nm, a bleaching signal appeared as shown in Fig. 1(b), that was not represented in the transient absorption spectrum excited at the intensity below the lasing threshold. This signal is suggested to be due to amplified stimulated emission. Spectral modulation is often observed as interference of light propagating in thin-film systems. The thickness of the perovskite crystals is around 200-300 nm, which is suitable to observe the optical interference. However, generally, this interference tends to become lower in frequency as one observes longer wavelengths, which is essentially different from the modulation observed here. The spectra modulation appeared immediately after the carrier was generated, and the modulation continued for up to 1ns. These results suggest that the modulation is caused by the electric field effect of the generated carriers on the electron spectrum. Franz-Keldysh’s theory is often used for spectral modulation in the presence of an electric field. In reflectance transient absorption spectroscopy, the electric field modulated spectrum has been explained in the GaInP2 system by Franz-Keldysh’s theory in systems where charges are generated on the semiconductor surfaces[2]. These results indicate the spectral modulation occurred from the charges separated on the surface of the crystal. We will discuss the mechanism of amplified spontaneous emission and low threshold lasing dynamics at the conference site. Funding: This study was partly supported by KAKENHI (Grant Number JP22H04755, JP19K05190) and JST FOREST (JPMJFR213G).

Spectral and Photochemical Properties of Rhodobacter sphaeroides R-26 Reaction Center Films in Vacuum.

Using absorption spectroscopy in the visible/near-IR and mid-IR regions, spectral and photochemical properties of isolated reaction centers (RCs) from Rhodobacter sphaeroides R-26 were studied in dried films on the inorganic support surface (quartz or CaF2 plates) under vacuum dehydration conditions (10-2 or 7·10-5 mm Hg). Three detergents, N,N-dimethyldodecylamine N-oxide (LDAO), Triton X-100 (TX100), and n-dodecyl-β-D-maltoside (DM), were tested for their ability to stabilize the RC-detergent complexes in the vacuum-dried state. It was shown that in the presence of LDAO, RC complexes underwent destruction in vacuum. In contrast, DM provided an environment that minimized irreversible disruptive changes in the RCs in vacuum. The effects of vacuum dehydration on the RC-DM films included a small increase in the content of α-helices in the RC protein, a short-wavelength reversible shift in the optical transitions of pigments, and minor changes in the electronic structure of the P+ dimer. The films retained their photochemical activity upon excitation with high-intensity light (200 mW/cm2). TX100 also helped to maintain spectral and functional properties of the RCs in vacuum; however, in this case, the stabilizing effect was less pronounced than in the presence of DM, especially, at high detergent concentrations. The results are discussed within the framework of a model suggesting that the detergent-protein interactions and the properties of detergent micelles play a dominant role in maintaining the structure of the RCs upon vacuum dehydration of the RC complexes. The obtained data can be useful for developing hybrid photoconverting systems based on bacterial RCs.

The nature of hydrogen-bonding interactions in nonsteroidal anti-inflammatory drugs revealed by polarized IR spectroscopy

The influence of hydrogen-bonding interactions in the solid phase on the IR spectroscopic pattern of the νOH band of nonsteroidal anti-inflammatory drugs (NSAIDs) was studied experimentally by IR spectroscopy with the use of polarized light at two temperatures (293K and 77K) and in isotopic dilution. The neat and deuterated crystals of (S)-naproxen ((S)-NPX), (R)-flurbiprofen ((R)-FBP), (RS)-flurbiprofen ((RS)-FBP) and (RS)-ketoprofen ((RS)-KTP) were obtained by melt crystallization between the two squeezed CaF2 plates. The vibrational spectra of selected α-aryl propionic acid derivatives (2APAs) reflected the characteristics of their hydrogen-bond networks, i.e., 2APAs were characterized by the chain ((S)-NPX, (R)-FBP) and by dimeric ((RS)-FBP, (RS)-KTP) arrangement of hydrogen bonds in the crystal lattice. Spectroscopic results showed that the interchain (through-space) exciton coupling, between two laterally-spaced hydrogen bonds, dominates in the crystals of four NSAIDs. The same exciton coupled hydrogen bonds were also responsible for the H/D isotopic recognition mechanism in the crystalline spectra of deuterated 2APAs. The presented spectral results may help to predict the hydrogen bond motifs in the crystalline NSAIDs, which structures are not yet known, based on their IR spectra of hydrogen bond in the crystals.

Convenient ultra-broadband femtosecond optical gating utilizing transient beam deflection effect.

A simple but robust ultra-broadband femtosecond optical gating method utilizing transient beam deflection effect is demonstrated with direct CCD imaging of the distorted single-color probe and the measurement of the chirp structure of a white light continuum generated from a CaF₂ plate. The non-collinear configured beam deflection gating technique not only preserves all the advantages of the previous optical Kerr lens based gating methods, such as having no phase matching conditions, little dependence on probe intensity or special nonlinear media, and no requirements on the pump-probe polarization relationship, but it also extends the measurable probe bandwidth. Meanwhile, it is also proved that the current gating technique is easy-aligned, free from the influence of the pump-probe pulse-front mismatch and the probe beam profile, which is much convenient for the characterization of ultra-broadband light pulses in the applications of ultrafast spectroscopy.

Highly ordered surface structure of large-scale porphyrin aggregates assembled from protonated TPP and water

Large-scale aggregates assembled from protonated meso-tetraphenylporphine (TPP) dimers and water have been investigated by IR and resonance Raman spectroscopy and also by scanning electron microscopy (SEM). It was found that the properties of water confined in the aggregates depend on the physical state of the support. When the aggregates were deposited on a solid CaF2 plate, they showed properties consistent with a quasi-crystalline structure. But when the aggregates were dispersed in oil, their IR characteristics were different; the vibration bands of the confined water were like those of water in liquid state. A doublet at about 1000cm−1, components of which have been attributed to specific vibrations of H3O+ and H2O bound in the structure of water-porphyrin dimeric complex, was found in IR and resonance Raman spectra (λex=441.6nm) of protonated TPP aggregates. This doublet indicates the hydrogen ion involving in the vibrational system of water-porphyrin dimeric complex with hydrogen bonding by similar way as in so-called Zundel cation. The resonance Raman spectrum shows evidence for proton sharing between protonated water dimer and N groups of the pyrrole rings. SEM results indicate that the large-scale aggregates of the protonated porphyrin possess highly ordered structure, are only observed when using extremely pure water.

Generation of broadband multicolor femtosecond laser pulses by using cascading four-wave mixing in a CaF2 plate

Femtosecond laser pulses with new properties have been required for the great progress of femtosecond laser spectroscopy experiments. For example, simultaneous multiple ultrashort femtosecond laser pulses with different frequencies are needed in multicolor pump-probe experiments. Here, we have designed a much more simple and compact system for multicolor femtosecond pulse generation by using cascaded four-wave mixing in a 0.5 mm thick CaF2 plate. Multicolor femtosecond pulses with a sub-10 fs Fourier transform limit pulse width have been obtained by using two oppositely chirped incident pulses. These ultrashort multicolor femtosecond pulses will be used as a new laser source in femtosecond laser spectroscopy.

Generation of sub-20-fs deep-ultraviolet pulses by using chirped-pulse four-wave mixing in CaF_2 plate

Sub-20-fs deep ultraviolet (DUV) pulses are generated by using nondegenerate, chirped-pulse four-wave mixing of the fundamental and second-harmonic pulses from a commercial Ti:sapphire amplifier in a CaF(2) plate. The energy of the DUV pulses is 3.8 μJ, with a conversion efficiency from total pump energy to DUV of ~3.8%. The DUV pulse is compressed using a pre-chirp, introduced via a fused silica window in the fundamental beam. The central wavelength of the DUV spectrum can be tuned from 257 to 277 nm by adjusting the cross angle between the two pump beams. The spectrum can reach a width of 16.8 nm, which can support a pulse duration of 8.7 fs.

Carrier Recombination-Incited Substrate Vibrations after Pulsed UV-Laser Photolysis of TiO2 Thin Single-Crystal Plate and Nanoparticle Films

Photo-induced carrier generation and recombination have been regarded as important steps in understanding the photocatalytic reactions on the surfaces of semiconductors such as TiO2. During the investigation of a photocatalytic water-splitting reaction on the surface of semiconductor TiO2 (rutile) single-crystal plate and sintered-nanoparticle (anatase) films coated on a CaF2 plate, a pulsed 355 nm laser was used for band-gap excitation and a continuous-working, mid-infrared laser as the probe to trace the kinetics of the photogenerated electrons. Fast oscillations with periods of 10-50 μs were observed. We demonstrated experimentally that these oscillations are neither from the nonlinear carrier recombination dynamics nor surface chemical reactions; instead, they are caused by the thermal-induced flexural vibrations initiated by the fast carrier recombination in the light-absorbing area. The observed oscillations can be well accounted for by thermal stress-induced flexural vibrations of the substrates due to rapid charge recombination, and the observed oscillation frequencies match the flexural vibration frequencies predicted by the Kirchhoff-Love thin-plate theory. Light-interference effect caused by the beam reflecting off the surfaces of the substrate and windows, which can amplify the oscillation signals, is also discussed. Several approaches are proposed to minimize or eliminate the interference from the substrate vibration in the transient kinetic measurement when a coherent laser is used as the probe beam.

The Conserved His-144 in the PsbP Protein Is Important for the Interaction between the PsbP N-terminus and the Cyt b559 Subunit of Photosystem II

The PsbP protein regulates the binding properties of Ca(2+) and Cl(-), and stabilizes the Mn cluster of photosystem II (PSII); however, the binding site and topology in PSII have yet to be clarified. Here we report that the structure around His-144 and Asp-165 in PsbP, which is suggested to be a metal binding site, has a crucial role for the functional interaction between PsbP and PSII. The mutated PsbP-H144A protein exhibits reduced ability to retain Cl(-) anions in PSII, whereas the D165V mutation does not affect PsbP function. Interestingly, H144A/D165V double mutation suppresses the effect of H144A mutation, suggesting that these residues have a role other than metal binding. FTIR difference spectroscopy suggests that H144A/D165V restores proper interaction with PSII and induces the conformational change around the Mn cluster during the S(1)/S(2) transition. Cross-linking experiments show that the H144A mutation affects the direct interaction between PsbP and the Cyt b(559) α subunit of PSII (the PsbE protein). However, this interaction is restored in the H144A/D165V mutant. In the PsbP structure, His-144 and Asp-165 form a salt bridge. H144A mutation is likely to disrupt this bridge and liberate Asp-165, inhibiting the proper PsbP-PSII interaction. Finally, mass spectrometric analysis has identified the cross-linked sites of PsbP and PsbE as Ala-1 and Glu-57, respectively. Therefore His-144, in the C-terminal domain of PsbP, plays a crucial role in maintaining proper N terminus interaction. These data provide important information about the binding characteristics of PsbP in green plant PSII.

Polarization properties of SC generated in CaF2

Polarization of SC generated by 80fs pulses of Ti:Sapphire laser strongly depends on the orientation of CaF2 plate with respect to the plane of polarization of the pump light. Intensity modulation of the broad blue-shifted wing of SC against crystal rotation for the polarization parallel and orthogonal to the polarization of the pump beam is observed with CaF2 and Sapphire. Observed modulation is due to evolution of the polarization state along the propagation distance in crystal.

Reactivity between Biphenyl and Precursor of Solvated Electrons in Tetrahydrofuran Measured by Picosecond Pulse Radiolysis in Near-Ultraviolet, Visible, and Infrared

The initial decrease of solvated electrons in tetrahydrofuran (THF) upon addition of biphenyl was investigated by picosecond pulse radiolysis. Transient absorption spectra derived from the biphenyl radical anion (centered at 408 and 655 nm) and solvated electrons of THF (infrared) were successfully measured in the wavelength region from 400 to 900 nm by the extension of a femtosecond continuum probe light to near-ultraviolet using a second harmonic generation of Ti:sapphire laser and a CaF2 plate. From the analysis of kinetic traces at 1300 nm considering the overlap of primary solvated electrons and partial biphenyl radical anion, C37, which is defined by the solute concentration to reduce the initial yield of solvated electrons to 1/e, was found to be 87 +/- 3 mM. The rate constant of solvated electrons with biphenyl was determined as 5.8 +/- 0.3 x 10(10) M(-1) s(-1). We demonstrate that the kinetic traces at both 408 nm mainly due to biphenyl radical anion and 1300 nm mainly due to solvated electrons are reproduced with high accuracy and consistency by a simple kinetic analysis. Much higher concentrations of biphenyl (up to 2 M) were examined, showing further increase of the initial yield of biphenyl radical anion accompanying a fast decay component. This observation is discussed in terms of geminate ion recombination, scavenging, delayed geminate ion recombination, and direct ionization of biphenyl at high concentration.

Stroboscopic Picosecond Pulse Radiolysis Using Near-Ultraviolet-Enhanced Femtosecond Continuum Generated by CaF2

For investigations of ultrafast reactions induced by radiation in the short-wavelength region, a near-ultraviolet-enhanced continuum ranging from ca. 390 to 600 nm was generated by focusing the second-harmonic generation (400 nm) of a femtosecond Ti:sapphire laser into a CaF2 plate and applied to stroboscopic picosecond pulse radiolysis. By utilizing a double-pulse detection scheme, the fluctuation between the signal and reference pulses was improved to 2–12% of the standard deviation. The capability of the system in the wavelength region from 400 to 900 nm is demonstrated by measuring transient photoabsorption spectra in water and in tetrahydrofuran solution of biphenyl, where these spectra are attributed to hydrated electrons and the overlap between biphenyl radical anions and the solvated electrons of tetrahydrofuran, respectively.

Structure and diffusive properties of fluid-filled grain boundaries: An in-situ study using infrared (micro) spectroscopy

The rate of numerous diagenetic, deformation and metamorphic processes in crustal rock systems ultimately depends on the structure and diffusive properties of water-bearing grain boundaries. We present the first in-situ spectroscopic study of the nature of aqueous films present in mineral interfaces undergoing stress-induced dissolution or ‘pressure solution’. Using infrared micro-spectroscopy, we show that during active pressure dissolution of the (111) plane of halite (NaCl) against a CaF2 plate, the confined intercrystalline liquid has an average thickness of 85–185 nm and occupies a rough, non-equilibrium grain boundary structure. The hydrogen bonding of the water within the grain boundary film is modified due to increased polymerization towards ice-like (hard) water. Simultaneous pressure solution rate measurements show that diffusion within the grain boundary fluid is about one order of magnitude slower than in bulk solution. During active pressure solution of the (100) plane of halite, a similar non-equilibrium grain boundary structure develops but the average thickness of the confined water film is only ∼40 nm and no evidence is found for modification of the hydrogen bonding of the water. The difference in hydrogen bonding between the two orientations studied is attributed to differences in surface charge distribution on the (111) and neutral (100) interfaces in NaCl. The difference in mean film thickness is attributed to differences in crystallographically controlled roughness of the dissolving NaCl Surface. If similar grain boundary thickness and diffusivity effects occur in other rock forming minerals, pressure solution in such systems will tend to be interface reaction controlled and may be capable of producing significant seismic anisotropy.

Theoretical and experimental studies of tunable ultraviolet-blue femtosecond pulses in a 405-nm pumped type I β-BaB_2O_4 noncollinear optical parametric amplifier and cascading sum-frequency generation

Femtosecond laser pulses that are tunable from 380 to 460 nm were directly generated from a 405-nm-pumped type I β-BaB2O4 noncollinear optical parametric amplifier (NOPA). A white-light supercontinuum from a CaF2 plate excited by 810-nm pulses was seeded into the NOPA. A theoretical analysis showed that the near-UV-to-blue radiation generated is attributable to cascaded sum-frequency generation (SFG) of the output of the NOPA and the residual fundamental beam at 810 nm. One can tune the blue–UV light by either changing the orientation of the NOPA crystal or adjusting the angle between the pump and the seeding pulse. The optical conversion efficiency from the 405-nm pump to the tunable SFG radiation is more than 5%.

Novel Method for Preparing a Mixed-Stack Charge-Transfer Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine: The Use of a Mixed Langmuir−Blodgett Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and Stearic Acid as Starting Material for the Fabrication of the Charge-Transfer Film

The present paper reports a novel method for preparing a mixed-stack charge-transfer (CT) film having 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) as an electron acceptor and the possibility of the control of molecular orientation in the CT film. A seven-layer mixed Langmuir−Blodgett (LB) film of C18TCNQ and stearic acid is deposited onto a CaF2 plate or a gold-evaporated glass slide at the surface pressure of 15 mN m-1. Dipping the mixed LB film into a petroleum ether solution of 3,3‘,5,5‘-tetramethylbenzidine (TMB) results in the formation of a mixed-stack CT film of TMB·C18TCNQ. Upon the formation of the CT film, the dilution molecules, namely stearic acid, are removed completely from the film. The appearance of a broad absorption band in the near-infrared region and the frequency shift of the CN stretching band (2208 cm-1) verify that the CT complex film of TMB·C18TCNQ is formed by the TMB doping. IR spectra of the CT films do not show any bands assignable to stearic acid, suggesting that stearic acid molecules are removed from the thin films after the dipping. The band at 3496 cm-1 due to the antisymmetric stretching mode of the NH2 group does not appear in the IR transmission spectrum. Thus, it seems that the chromophore plane of TMB component is nearly perpendicular to the substrate surface with its long axis lying on the substrate in the CT films. The molecular structure and orientation in the CT films of TMB·C18TCNQ prepared by the present method and those fabricated by dipping the LB films of C18TCNQ alone into the TMB solution have been compared. The comparison shows that the orientation of the chromophore planes of both donor and acceptor components and that of the alkyl chain of C18TCNQ in the CT films are controlled mainly by the doping process.

Molecular Orientation and Phase-Transition Behavior of Langmuir−Blodgett and Casting Films of Reversed Duckweed Polymer ES-1 Studied by Infrared Spectroscopy

A kind of amphiphilic polymer, ES-1, is transferred onto solid substrates as so-called “reversed duckweed” polymeric Langmuir−Blodgett (LB) films. Their orientations, structures, and phase-transition behaviors were studied in detail by infrared (IR) transmission and reflection−absorption (RA) spectroscopy. The IR studies have revealed that the alkyl chains are nearly perpendicular to the film surface and assume highly ordered conformation. The first monolayer has a different orientation and arrangement from other layers in a multilayer ES-1 LB film deposited on a Au-evaporated glass slide because of the interaction between the gold surface and hydrophilic headgroup of ES-1. IR−RA spectra of an ES-1 LB film on a Au-evaporated glass slide at evaporated temperatures have shown that the order−disorder phase transition of the LB film takes place at about 60 °C after the clear pretransitional alterations. The packing density of alkyl chains decreases gradually with temperature after the order−disorder transitions in the LB films. The alkyl chains in ES-1 casting films on a CaF2 plate and a Au-evaporated glass slide are tilted considerably. The studies on the phase-transition behavior have revealed that the two casting films give rise to the order−disorder transition at about 75 °C after undergoing the pretransitional changes. For the casting film of ES-1 on the Au-evaporated glass slide, there is a phenomenon of reorientation that the alkyl chains change from more tilted orientation at room temperature to more perpendicular orientation at higher temperature before the order−disorder transition takes place and turn tilted again above the transition temperature.

On-line fourier transform infrared detection in capillary electrophoresis.

The coupling of Fourier transform infrared (FT-IR) spectroscopy as a new on-line detection principle in capillary electrophoresis (CE) is presented. To overcome the problem of total IR absorption by the fused-silica capillaries that are normally employed in CE separations, a micromachined IR-transparent flow cell was constructed. The cell consists of two IR-transparent CaF2 plates separated by a polymer coating and a titanium layer producing an IR detection window, 150 microm wide and 2 mm long, with a path length of 15 microm. The IR beam was focused on the detection window using an off-axis parabolic mirror in an optical device (made in-house) attached to an external optical port of the spectrometer. The connections between the fused-silica capillaries and the flow cell were made by a small O-ring of UV-curing epoxy adhesive on the sharply cut ends of the capillaries, allowing the capillaries to be easily replaced. Aqueous solutions comprising mixtures of adenosine, guanosine, and adenosine monophosphate were used to test the system's performance. Conventional on-line UV detection was employed to obtain reference measurements of analytes after the IR detection flow cell. The limit of FT-IR detection for all analytes (in absolute amounts) was in the nano- to picogram range corresponding to concentrations in the low-millimolar range.

White Lights, Broken Molecules

CHEMISTRY Excitation of molecules with ultrashort laser pulses has revealed many insights into chemical dynamics, but some processes are more readily stimulated with continuum, or white light, pulses that span a broader frequency range. Muller et al. fired 80-femtosecond, 800-nanometer-wavelength pulses through a 2-millimeter-thick CaF2 plate into toluene vapor. Increasing the pulse intensity broadened the pulses through self-phase modulation, and these broadened pulses fragmented the toluene molecules. Higher intensity monochromatic pulses left the molecules intact. The authors conclude that the broadened pulses stimulated Raman-active carbon-carbon bonds to such an extent that the backbone framework shattered. — PDS Phys. Rev. Lett . 88 , 023001 (2002).

Preparation and Structural Characterization of Mixed-Stack Charge-Transfer Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and N,N,N‘,N‘-Tetramethylbenzidine

One-, three-, and seven-layer Langmuir−Blodgett (LB) films of a mixed-stack charge-transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and N,N,N‘,N‘-tetramethylbenzidine (N-TMB) were successfully deposited onto CaF2 plates and gold-evaporated glass slides at the surface pressure of 10 mN m-1. This is the first time that the mixed-stack CT films of N-TMB·C18TCNQ have been prepared by the LB technique. Ultraviolet−visible-near-infrared (UV−vis-NIR) and infrared spectroscopies have been employed to investigate the formation, degree of charge transfer, molecular orientation, and structure of the CT complex in the LB films. The appearance of a broad absorption band with its center at about 1750 nm reveals that the CT complex in the LB films is in the mixed-stack form. The frequency of a C⋮N stretching band of C18TCNQ in the CT complex indicates that the degree of charge transfer (ρ) is about 0.24. A comparison of the infrared band intensities between the transmission and RA spectra ...

Aging Effects on Molecular Orientation, Structure, and Morphology in Langmuir−Blodgett Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane Studied by Infrared and Ultraviolet−Visible Spectroscopy and Atomic Force Microscopy

Time-dependent changes in molecular orientation, structure, and morphology in one-layer Langmuir−Blodgett (LB) films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) have been investigated by means of infrared and ultraviolet and visible (UV−vis) spectroscopy and atomic force microscopy (AFM). The intensities of bands at 2920 and 2850 cm-1 due to CH2 antisymmetric and symmetric stretching modes of the alkyl chain and those of bands at 2222, 1547, and 1529 cm-1 assigned to C⋮N, CC, and CC stretching modes of the TCNQ chromophore increase significantly with time in the infrared reflection−absorption (RA) spectrum of a one-layer LB film of pentadecyl-TCNQ after the film deposition on a gold-evaporated glass slide. These observations indicate that the alkyl chain becomes more tilted with respect to the surface normal during the time course and the TCNQ plane becomes more perpendicular with respect to the substrate surface. The time-dependent changes are also observed for one-layer LB films of dodecyl-TCNQ, but the changes are much smaller than those for the films of pentadecyl-TCNQ. One-layer LB films of octadecyl-TCNQ do not show appreciable infrared spectral changes with time. A UV−vis spectrum in the 350−450 nm region of the one-layer LB film of pentadecyl-TCNQ on a CaF2 plate consists of two bands due to the stacked and aggregation forms of the TCNQ chromophore. As a function of time, the intensity of the band due to the aggregation form increases while that of the band due to the stacked form decreases, indicating that some of the stacked TCNQ chromophores change into the aggregation with the change in the molecular orientation. The increase in baseline is also noted in the UV−vis spectrum. The baseline change may be due to the partition of the large domains of the one-layer film into small domains with the size of ∼1 μm. Time-dependent measurements of the AFM image of a one-layer LB film of pentadecyl-TCNQ on mica have revealed that the thickness of the film changes from 4.6 to 8.2 nm and that the flat and uniformed structure of the film changes into the condensed and accumulated structure during the time course from 15 to 180 min after the film deposition. Aged one-layer LB films of dodecyl- and octadecyl-TCNQ are similar to the aged and fresh one-layer LB films of pentadecyl-TCNQ, respectively, in terms of both the film morphology and thickness. On the basis of the results of infrared and UV−vis spectroscopy and AFM, we propose a model for the time-dependent morphological and structural changes in the one-layer LB films of the alkyl-TCNQ. The differences in the aging effects among the one-layer LB films of the three kinds of alkyl-TCNQ may be concerned with the differences in the strength of the hydrophobic interaction between the interdigitated alkyl chains and in the degree of three-dimensional microcrystal growth of the one-layer LB films.