One-layer Langmuir-Blodgett Research Articles

Control of indium tin oxide anode work function modified using Langmuir-Blodgett monolayer for high-efficiency organic photovoltaics

The use of Langmuir-Blodgett (LB) monolayers to modify the indium tin oxide (ITO) work function and thus improve the performance of zinc phthalocyanine (ZnPc)/fullerene (C60)-based and boron subphthalocyanine chloride (SubPc)/C60-based small molecule organic photovoltaic devices (OPVs) was examined. In general, LB precursor compounds contain one or more long alkyl chain substituents that can act as spacers to prevent electrical contact with adjoining electrode surfaces. As one example of such a compound, arachidic acid (CH3(CH2)18COOH) was inserted in the forms of one-layer, three-layer or five-layer LB films between the anode ITO layer and the p-type layer in ZnPc-C60-based OPVs to investigate the effects of the long alkyl chain group when it acts as an electrically insulating spacer. The short-circuit current density (Jsc) values of the OPVs with the three- and five-layer inserts (1.78 mA·cm−2 and 0.61 mA·cm−2, respectively) were reduced dramatically, whereas the Jsc value for the OPV with the single-layer insertion (2.88 mA·cm−2) was comparable to that of the OPV without any insert (3.14 mA·cm-2). The ITO work function was shifted positively by LB deposition of a surfactant compound, C9F19C2H4-O-C2H4-COOH (PFECA), which contained a fluorinated head group. This positive effect was maintained even after formation of an upper p-type organic layer. The Jsc and open-circuit voltage (Voc) of the SubPc-C60-based OPV with the LB-modified ITO layers were effectively enhanced. As a result, a 42% increase in device efficiency was achieved.

Controlling the Structural and Electrical Properties of Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films

The preparation, characterization and electrical properties of Langmuir-Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4'-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good-quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X-ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one-layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently -COO-Au junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na(+) counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H-bonds with neighbouring molecules give rise to a supramolecular structure. STM-based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single-molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in -COO-Au molecular junctions, but also in terms of the presence of lateral H-bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.

Preparation and characterization of mixed monolayers and Langmuir−Blodgett films of merocyanine 540/octadecylamine mixture

Mixed monolayer of merocyanine 540 (MC540) dye and octadecylamine (ODA) at the air/water interface has been prepared using the co-spreading method. The pressure–area (π–A) isotherm studies revealed that the mixtures of MC540/ODA at a different ratio formed a stable monolayer at the air/water interface and these floating layers were easily transferred onto hydrophilic substrates as the Y-type Langmuir–Blodgett (LB) film. The specific area per molecule in the π–A isotherm of the mixed monolayer of MC540/ODA was larger than that of the pristine ODA. The area can also get increased up to 40mN/m surface pressure by the increase of the dye concentration in the mixtures. The LB films of MC540/ODA mixture were formed at 30mN/m surface pressure by transferring the mixed monolayer at the air/water interface on hydrophilic glass substrate via vertical dip-coating as mono- and multilayer films. The photophysical properties of MC540 in chloroform and LB films have been investigated using the absorption, steady-state and time-resolved fluorescence spectroscopy techniques. H-aggregate formation of MC540 both in chloroform and LB film were concluded from the spectroscopic results. The morphology of the one-layer mixed LB film of MC540/ODA on the glass substrate has been characterized by AFM (atomic force microscopy). The non-contact mode AFM image of the one-layer mixed LB film showed that the film surface consisted of MC540/ODA nanoclusters. In conclusion, our results contribute to understanding the strong interaction between MC540 and ODA at the air/water interface, and show the effects of some parameters on the mixed LB films of MC540.

Metal–Molecule–Metal Junctions in Langmuir–Blodgett Films Using a New Linker: Trimethylsilane

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.

Preparation of Ordered Films Containing a Phenylene Ethynylene Oligomer by the Langmuir−Blodgett Technique

This paper reports the fabrication and characterization of Langmuir and Langmuir-Blodgett (LB) films incorporating an oligo(phenylene-ethynylene) (OPE) derivative, namely, 4-[4-(4-hexyloxyphenylethynyl)-phenylethynyl]-benzoic acid (HBPEB). Conditions appropriate for deposition of monolayers of HBPEB at the air-water interface have been established and the resulting Langmuir films characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. The Langmuir films are readily transferred onto solid substrates, and one-layer LB films transferred at several surface pressures onto mica substrates have been analyzed by means of atomic force microscopy, from which it can be concluded that 14 mN/m is an optimum surface pressure of transference, giving well-ordered homogeneous films without three-dimensional defects and a low surface roughness. The optical and emissive properties of the LB films have been determined with significant blue-shifted absorption spectra indicating formation of two-dimensional H aggregates and a Stokes shift illustrating the effects of the solid-like environment on the molecular chromophore.

Interaction of small amounts of bovine serum albumin with phospholipid monolayers investigated by surface pressure and atomic force microscopy

The influence of small amounts of bovine serum albumin (BSA) (nM concentration) on the lateral organization of phospholipid monolayers at the air–water interface and transferred onto solid substrates as one-layer Langmuir–Blodgett (LB) films was investigated. The kinetics of adsorption of BSA onto the phospholipid monolayers was monitored with surface pressure isotherms in a Langmuir trough, for the zwitterionic dipalmitoylphosphatidyl ethanolamine ( N , N -dimethyl-PE) and the anionic dimyristoylphosphatidic acid (DMPA). A monolayer of N , N -dimethyl-PE or DMPA incorporating BSA was transferred onto a solid substrate using the Langmuir–Blodgett technique. Atomic force microscopy (AFM) images of one-layer LB films displayed protein–phospholipid domains, whose morphology was characterized using dynamic scaling theories to calculate roughness exponents. For DMPA–BSA films the surface is characteristic of self-affine fractals, which may be described with the Kardar–Parisi–Zhang (KPZ) equation. On the other hand, for N , N -dimethyl-PE–BSA films, the results indicate a relatively flat surface within the globule. The height profile and the number and size of globules varied with the type of phospholipid. The overall results, from kinetics of adsorption on Langmuir monolayers and surface morphology in LB films, could be interpreted in terms of the higher affinity of BSA to the anionic DMPA than to the zwitterionic N , N -dimethyl-PE. Furthermore, the effects from such small amounts of BSA in the monolayer point to a cooperative response of DMPA and N , N -dimethyl-PE monolayers to the protein.

Photochemical reaction, acidichromism, and supramolecular nanoarchitectures in the Langmuir-Blodgett films of an amphiphilic styrylquinoxaline derivative.

An amphiphilic styrylquinoxaline derivative, 3-(4-(hexadecyloxy)styryl)quinoxalin-2(1H)-one (SQC16), was newly synthesized to investigate their photochemical and gas responsive properties in organized molecular films. It was observed that SQC16 can spread as a monolayer on the subphases with various pH values and be subsequently transferred onto solid substrates. While SQC16 showed predominantly reversible trans-cis photoisomerization in methanol solution, it showed both photoisomerization and photodimerization in Langmuir-Blodgett (LB) films. Photodimerization was only observed in the LB film due to the face-to-face arrangement of the functional headgroup in the LB film, and the process was irreversible. In addition, the LB film showed acidichromism, i.e., when the film was exposed to HCl gas its color changed from yellow to red, and the color could be recovered after exposure to NH(3) gas. The process was reversible and could be repeated many times. An interesting surface morphology of the SQC16 LB film was revealed. It was observed that SQC16 can form nanowire architecture in the transferred one-layer LB film. This morphology can be changed upon photoirradiation or in gas reactions. Through the atomic force microscopy measurements it was suggested that the photodimerization predominantly occurred from the nanowire structures, while during the acidichromism the reaction occurred preferentially in the flat region. X-ray diffraction studies revealed that while layer distance showed a slight change for the LB film during acidichromism and photoreaction, the layer structure of SQC16 LB film was retained.

Infrared and Ultraviolet−Visible Spectroscopic Studies on Premelting Behavior of One- and Eleven-Layer Langmuir−Blodgett Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane

Premelting behavior of one- and eleven-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) has been investigated by infrared (IR) and ultraviolet-visible (UV-vis) spectroscopies. It has been suggested from the temperature-dependent variations in the CH 2 scissoring band region of the one-layer LB film of pentadecyl-TCNQ that the packing of the alkyl chains is changed to the hexagonal state from the highly packed state around 60 °C. A CH 2 symmetric stretching band near 2850 cm - 1 of the pentadecyl-TCNQ LB film shows an apparent shift with temperature, indicating that the alkyl chain undergoes premelting phase transitions near 60 and 90 °C. It seems likely that the thermal movement by the alkyl chains in the hexagonal state plays the role of the trigger for the melting phenomenon. The IR and UV-vis studies of the premelting behavior of the pentadecyl-TCNQ LB film suggest that there are one intermediate state between 60 and 90 °C and another intermediate state between 90 and 120 °C. The comparison among the temperature-dependent profiles of the peak maximum position of the CH 2 symmetric stretching band of one-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ suggests that all the one-layer LB films have two intermediate states between the solid and liquid states and that the longer alkyl chain assumes the more ordered conformation due to van der Waals attraction force among the alkyl chains. The eleven-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ show the temperature-dependent profiles different from those of the one-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ. The temperatures for the formation of the second intermediate state of the eleven-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TNCQ are lower than those of the corresponding one-layer LB films, probably because of the layer-layer interaction in the eleven-layer LB films.

Self-assembled spiral nanoarchitecture and supramolecular chirality in Langmuir-Blodgett films of an achiral amphiphilic barbituric acid.

An amphiphilic barbituric acid derivative was found to form stable monolayers showing a clear phase transition at the air/water interface. It is interesting to find that the deposited Langmuir-Blodgett (LB) films of the compound showed circular dichroism (CD) although the molecule itself was achiral. AFM measurements on the transferred one-layer LB film revealed that spiral nanoarchitectures were formed. It was further found that the supramolecular chirality of the LB films was related to symmetry breaking at the interface. Hydrogen bonding and the pi-pi stacking between the neighboring molecules resulted in chiral fibers which formed the spiral structures. To the best of our knowledge, this is the first report on the chirality of the molecular assemblies and spiral nanostructures formed through the air/water interface by achiral molecules.

Quartz crystal microbalance and electrochemical studies on the electrode modified by layer-by-layer multilayers of viologen polyelectrolytes

Cationic poly(viologen) derivatives [(V 2+) m ] and anionic polymer [poly(styrenesulfonic acid- co-maleic acid), sodium salt] were assembled on electrode surfaces by layer-by-layer (LBL) and Langmuir–Blodgett (LB) methods. The assembling process was in situ investigated by quartz crystal microbalance (QCM) technique. Based on the frequency change, the average surface coverage of poly(butylviologen) in the LBL multilayers is in the range of 1.6∼3.3×10 −10 mol/cm 2. The mass change for one-layer LB film of poly(dodecylviologen)-anionic polymer is about 44, 96 and 263 ng/cm 2 for the films deposited at 5, 10 and 15 mN/m, respectively. The assembled LBL and LB multilayers show well reversible redox characteristics of the viologen. For poly(viologen) with short alkyl chains, the cyclic voltammograms (CVs) show two redox couples of (V 2+) m ↔ (V +) m and (V +) m ↔ (0.5(V +) 2) m when the last layer is poly(viologen), or one redox couple of (V 2+) m ↔ (V +) m when the last layer is anionic polymer. While for poly(viologen) with long alkyl chains, the CVs show only one couple of (V 2+) m ↔ (V +) m . The LBL multilayers show much higher stability than the self-assembled monolayers and LB films in electrolyte solution.

Langmuir and Langmuir-Blodgett films of organophosphorus acid anhydrolase.

In this paper, we describe the preparation and characterization of Langmuir and Langmuir-Blodgett (LB) monolayers of the enzyme organophosphorus acid anhydrolase (OPAA). Langmuir films of OPAA were characterized on different subphases, such as phosphate, ammonium carbonate, and bis-tris-propane buffers. Monolayers at the air-water interface were characterized by measuring the surface pressure and surface potential-area isotherms. In situ UV-vis absorption spectra were also recorded from the Langmuir monolayers. The enzyme activity at the air-water interface was tested by the addition of diisopropylfluorophosphate (DFP) to the subphase. LB films of OPAA were transferred to mica substrates to be studied by atomic force microscopy. Finally, a one-layer LB film of OPAA labeled with a fluorescent probe, fluorescein isothiocyanate (FITC), was deposited onto a quartz slide to be tested as sensor for DFP. The clear, pronounced response and the stability of the LB film as a DFP sensor show the potential of this system as a biosensor.

Development of fluorescent film sensors for the detection of divalent copper.

Monolayers of several peptide lipids at air-water and air-solid interfaces were prepared using Langmuir and Langmuir-Blodgett (LB) film techniques, and tested as fluorescent sensors for copper ions in aqueous phase. In one method, both the ionophore and the fluorophore were in the same molecule (lipid A), so intramolecular interaction was responsible for the fluorescence quenching of monolayers of this lipid. In the other method, ionophore and fluorophore were located on two different molecules (lipids B and C) so the intramolecular coupling does not exist; instead the fluorescence quenching was realized by a through-space interaction mechanism. Several experimental techniques, including pi-A isotherm, epifluorescence microscopy, and absorption and emission spectroscopies were used to study the different characteristics of copper ion effect on the properties of the lipid monolayers. Additionally, the fluorescence quenching properties of the Langmuir monolayers were found to be transferred to the one-layer LB films. On LB films, the fluorescence response presented a clear selectivity for copper ions in comparison with several other transition metal ions. Further, an excellent reversibility was observed: the fluorescence was switched OFF by immersing the solid substrate in copper ion solution and ON by washing with HCl solution. The intermolecular approach used here seems to be a very flexible and general method to design surface-oriented fluorescent sensors to meet different analytic purposes.

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.

Thermal Behavior of One-Layer Langmuir-Blodgett Films of 2-Pentadecyl-7,7,8,8-Tetracyanoquinodimethane Studied by Ultraviolet-Visible and Infrared Spectroscopies

Thermal Behavior of one-layer Langmuir-Blodgett (LB) films of 2-pentadecyl-7,7,8,8-tetra-cyanoquinodimethane (pentadecyl-TCNQ) has been investigated by use of infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopies. An IR transmission spectrum of the film shows gradual temperature-dependent changes below the melting point of the film, and then gives dramatic spectral changes near the melting point. Temperature-dependent UV-Vis spectral changes of the film indicate that the morphological changes occur in domains of the film, and that absorption bands in the 320–560 nm region arising from the π-π* transition of the chromophore consist of at least three components due to different forms of chromophore aggregation. More active state of the alkyl-chain, hexagonal state, seems to be a trigger for the morphological changes in the domains.

An Infrared Study of Molecular Orientation and Structure in One-Layer Langmuir−Blodgett Films of Octadecyldimethylamine Oxide and Dioctadecyldimethylammonium Chloride: Dependence of the Structures of the Langmuir−Blodgett Films on Substrates, Aging, and pH of Water Subphase

Molecular orientation and structure in one-layer Langmuir−Blodgett (LB) films of octadecyldimethylamine oxide (C18DAO) and dioctadecyldimethylammonium chloride (2C18DAC) were investigated by use of infrared (IR) transmission, reflection−absorption (RA), and attenuated total reflectance (ATR) spectroscopy. The IR measurements suggest that the alkyl chains of the one-layer LB film of 2C18DAC on a gold-evaporated glass slide are highly ordered, whereas those of the C18DAO LB film have some gauche conformers. The molecular orientation and structure in both LB films show significant substrate dependence. Probably, the headgroup−substrate interaction controls them. Aging effects on the LB and Langmuir (L) films on an air−substrate interface were studied for the two kinds of amphiphiles. The alkyl chains of C18DAO in the LB film on the gold-evaporated glass slide become more ordered with time whereas the reverse trend is observed for those of 2C18DAC in the film. The effects on the L films were investigated by measuring IR spectra of the LB films prepared from the L films at various stages. The alkyl chains are less ordered in the LB films of C18DAO and 2C18DAC prepared from the aged L films. It is also noted that the molecular orientation and structure in the films of both compounds show strong pH dependence, presumably because of the strong interaction between the headgroups and substrates.

Surface-enhanced Raman scattering study of organic thin films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane and its charge transfer films doped with 3,3′,5,5′-tetramethylbenzidine

Raman, surface-enhanced Raman scattering (SERS) and ultraviolet–visible–near infrared (UV–vis–NIR) spectra have been measured for Langmuir–Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) and mixed-stack charge transfer (CT) films of octadecyl-TCNQ doped with 3,3′,5,5′-tetramethylbenzidine (TMB) deposited on CaF 2 plates and Au-evaporated glass slides. The 488.0 nm-excited Raman spectra of one-layer LB and CT films of octadecyl-TCNQ deposited on the Au-evaporated glass slides show a high signal-to-noise ratio owing to surface-enhanced Raman scattering (SERS). These Raman spectra are resonanced with absorption bands arising from intramolecular excitations of TCNQ molecules. Thus, we suggest that the 488.0 nm-excited Raman spectra of the LB and CT films deposited on the Au-evaporated glass slides are surface-enhanced resonance Raman (SERRS) spectra. The UV–vis–NIR spectrum of the CT film on the Au-evaporated glass slide is largely different from that of the film on the CaF 2 plate. However, their Raman spectra are very close to each other, although the SERRS spectrum of the CT film on the Au-evaporated glass slide is more enhanced than the resonance Raman spectrum of the film on the CaF 2 plate. It seems that the structure of octadecyl-TCNQ and TMB changes little with the substrates, but that the mechanism of CT is perturbed by the interaction between the first layer and the substrate.

IR and AFM Studies on Aging Effects of One-layer Langmuir-Blodgett Films of 2-Alkyl-7,7,8,8-tetracyanoquinodimethane

Aging Effects of one-layer Langmuir-Blodgett (LB) films of 2-alkyl-7,7,8,8-tetracyanoquinodimethane (alkyl-TCNQ) have been investigated by means of infrared (IR) and atomic force microscopy (AFM). Time-dependent infrared (IR) reflection-absorption (RA) spectral changes of a one-layer LB film of pentadecyl-TCNQ on a gold-evaporated glass slide indicate that the alkyl chain becomes more tilted with respect to the surface normal while the TCNQ plane becomes more perpendicular with time. Similar time-dependent changes in the molecular orientation were observed also for a one-layer LB film of pentadecyl-TCNQ on a CaF2 plate and for one-layer LB films of dodecyl-TCNQ on the gold-evaporated glass slide and CaF2 plate. However, one-layer LB films of octadecyl-TCNQ did not show a significant change in the molecular orientation. Time-dependent changes in the AFM image of a one-layer LB film of pentadecyl-TCNQ on mica show that the thickness of the LB film varies from 4.6 to 8.2 nm during the time course from 15 to 180 minutes after film deposition.

Thermal behavior of Langmuir–Blodgett films of 5-(4- N-octadecylpyridyl)-10,15,20-tri- p-tolylporphyrin studied by ultraviolet-visible and infrared spectroscopies

Thermal behavior of Langmuir–Blodgett (LB) films of 5-(4- N-octadecylpyridyl)-10,15,20-tri- p-tolylporphyrin (porphyrin 338a) was investigated by use of ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. With increasing temperature from 30°C to 170°C, Soret bands of one- and nine-layer LB films of porphyrin 338a on CaF 2 substrates show a blue shift by 6 and 2 nm, respectively. In contrast, a Q(x)(0,0) band shows a red shift by 10 and 5 nm, respectively, for the one- and nine-layer LB films. The one-layer LB film shows more significant shifts of the Soret and Q(x)(0,0) bands than the nine-layer film, indicating different thermal behavior of the porphyrin aggregates, which may be induced by different effects of interaction between the substrate and the first monolayer, and by the existence and nonexistence of interlayer interaction. In the IR spectra of a nine-layer LB film of porphyrin 338a on a CaF 2 plate, the frequencies of CH 2 antisymmetric and symmetric stretching bands increase significantly with temperature, suggesting the increase in gauche conformers in the hydrocarbon chain of the porphyrin 338a. In contrast, the frequencies of bands arising from the porphyrin macrocycle and attached pyridyl groups decrease gradually with the increase in temperature. These observations indicate deformation of porphyrin core and changes in coplanarity of the pyridyl groups with the porphyrin plane.

Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 2. Annealing Effects on the Films Investigated by Atomic Force Microscopy and Ultraviolet−Visible and Infrared Spectroscopies

The annealing effects on Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (abbreviated as octadecyl-TCNQ) have been investigated by atomic force microscopy (AFM) and ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. An AFM image of a one-layer LB film of octadecyl-TCNQ scanned at room temperature shows that the film consists of numerous platelike microcrystal domains. A periodic structure of octadecyl-TCNQ molecules with a period of 0.85 nm can be observed inside these domains. After the annealing, cooling down the film from 130 to 30 °C, some of these domains lie on top of one another, keeping their original morphology. The rest seem to melt into a globular bulk sample. In the case of an 11-layer film, it can be seen from its AFM images that a number of domains with smaller size cover the substrate and that they completely melt after the annealing cycle. The UV−vis spectra of the one- and 11-layer films measured before and after the annealing show that the stacked...

Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 1. Order−Disorder Transitions in the Films Investigated by Ultraviolet−Visible and Infrared Spectroscopies

Dependences of thermal behaviors on the number of layers in Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) have been investigated for both morphological and molecular structural aspects by using ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. The temperature-dependent morphological changes have been monitored by the base-line shift in the UV−vis spectra of the LB films. The changes in the base-line in the 450−600 nm region suggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the domains in the seven- and 11-layer films are stable until 80 °C and undergo rather abrupt collapse between 80 and 90 °C. These differences in the thermal behaviors of the domain structure between the one-layer film and the seven- and 11-layer films may be attributed to the differences in the film thickness, the effect of the interaction between the first layer and a CaF2 substrate, and the longitudinal interaction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order−disorder transition temperature of the one-layer LB film is lower by about 5 °C than that of the 11-layer film, which is close to the chain melting point of the bulk material. Of particular interest is that the domain structure of the one-layer film shows the gradual temperature change but that the molecular orientation and structure in the domains experience the clear order−disorder transition.