Alternate Langmuir-Blodgett Films Research Articles

Pyroelectric and dielectric characterisation of alternate layer Langmuir–Blodgett films incorporating ions

Non-centrosymmetric Langmuir–Blodgett (LB) thin films exhibit a temperature-dependent electric polarisation, which is known as the pyroelectric effect. Alternate layer LB films were prepared by transferring polysiloxane alternately with eicosylamine from a subphase of Millipore water (18 MΩ cm − 1 ) incorporating different metallic ions onto a 50 nm thick thermally evaporated aluminium substrate. Pyroelectric and dielectric measurements were performed on LB films with a sandwich structure comprising film. Pyroelectric coefficients of these LB films are measured at several temperatures. The dielectric constant and dielectric losses were obtained to determine the pyroelectric figure of merit values of these LB films, which were calculated in the range of 24.9 to 56.7 μC m − 2 K − 1 .

Pyroelectric figure of merit in alternating hemicyanine/NC Langmuir–Blodgett films incorporating barium ions

Pyroelectric properties in alternating hemicyanine (DAEP)/nitrogencrown (NC) Langmuir–Blodgett (LB) films and effects of incorporating barium cations on pyroelectric behaviors have been investigated. Its pyroelectric coefficient measured is found to be 58 μC m −2 K −1at 300 K. Their dielectric properties have been measured and used to determine the figures of merit in thermal devices. Relative permittivity ( ɛ r) and dielectric loss values (tan δ) of pyroelectric films (in the range of 1–100 KHz) are found to be 2.34–1.96 and 0.08–0.04, respectively. These data give a maximum pyroelectric figure of merit of 150 μC m −2 K −1 in the alternating LB films incorporating barium ions. The effects of different deposition on pyroelectric properties are also discussed in this paper.

Enhancement of ferroelectricity in Langmuir–Blodgett multilayer films of weak-polar organic molecules

Ferroelectricity in Langmuir–Blodgett (LB) films (Z-type and alternated Y-type) of nitrogencrown (NC) and arachidic acid (AA) was investigated by electric polarization measurements and small angle X-ray diffraction (SAXD). For Z-type AA LB multilayer and the AA LB films incorporating barium ions, as well as for Z-type NC LB multilayer and the Y-type NC/AA alternating LB films, hysteresis loops of the samples were observed and confirmed existence of the remnant polarizations in the LB films. Their corresponding remnant polarizations were found to be 15, 180, 450 and 9400 μC m −2, respectively. Therefore, the maximum ratios of remnant polarizations up to 620 (9400/15)-folds were obtained from NC/AA alternating LB films. The enhancement of ferroelectricity in NC/AA alternating LB films was observed for the first time. It was demonstrated that the enhancement of ferroelectricity in the LB films could be created by incorporation of barium ions into the LB film structures and strong interaction due to electron clouds redistribution largely originated from the compact insertion configuration of AA between two chains of NC molecules in interleaving films.

Studies of Langmuir and Langmuir–Blodgett films of NLO-active amphiphilic 1,3-indanedione derivatives

We studied Langmuir and Langmuir–Blodgett (LB) films of several strong nonlinear optical (NLO) active amphiphilic derivatives of 1,3-indanedione-5,6-dicarboxylic acid. The surface pressure–area (π–A) isotherms at the air–water interface were investigated at different temperatures and pH values. Non-centrosymmetric Z-type LB films were deposited. UV–visible spectra indicated a uniform film transfer. The orientation of the molecular transition moments was calculated from the polarized UV–visible spectra. Packing within the LB films was observed by UV–visible absorption and fluorescence spectroscopy. The morphology of the LB films was examined by AFM and compared to ellipsometric measurements, and the nature of the films was studied by contact angle measurements. Significant second harmonic generation (SHG) by the LB films was observed. Alternate layer Y-type LB films of two different dyes were fabricated to achieve enhanced stability and SHG.

Characterisation of Langmuir–Blodgett assemblies of polyalkoxystilbazole complexes of iridium(I) and a cyclic polysiloxane in pyroelectric devices

Studies have been made of alternate layer (ABABA superlattices) multilayer Langmuir–Blodgett (LB) films using a cyclic polysiloxane (A) and an iridium(I) polyalkoxystilbazole metallo-organic complex (B). The electrically polar nature of the molecules studied suggests that they may be suitable candidates for new pyroelectric materials. The pyroelectric coefficient has been measured for alternate layer multilayer LB films built with various number of layers of the iridium(I) complex. We investigate the effect of chain length of the substitution and position of the substitution on the pyroelectric coefficient, capacitance and dielectric loss.

Ferroelectricity of weak-polar organic molecules in alternate Langmuir-Blodgett multilayer films

Ferroelectricity in Langmuir-Blodgett (LB) films (Z-type and alternated Y-type) of nitrogencrown (NC) and arachidic acid (AA) was investigated by electric polarization measurements and surface pressure-molecular area π(π-A) isotherm. For Z-type AA LB multilayer and the AA LB films incorporating barium and manganese ions, as well as for Z-type NC LB multilayer and the Y-type NC/AA alternating LB films, typical hysteresis loops of the samples were observed and confirmed existence of the remnant polarizations in the LB films. Their corresponding remnant polarizations were found to be 15, 180, 650, 450 and 9400 μC · m−2, respectively. Therefore, it was demonstrated that the enhancement of ferroelectricity in the LB films could be created by incorporation of barium and manganese ions into the LB film structures and strong interaction between alternating monolayers of the samples. These novel approaches can be employed to create strong ferroelectric LB films to make ferroelectric thin-film device for practical applications.

Optimization of pyroelectric properties in ultrathin organized molecular films

Pyroelectric properties in alternating DAEP/NC Langmuir-Blodgett (LB) films and effects of incorporating the barium cations on pyroelectric behaviours have been investigated.The pyroelectric coefficient measured is found to be 58μCm-2K-1 at 300K.The dielectric properties have been measured and used to determine the figures of merit in thermal devices. Relative permittivity εr and dielectric loss values tanδ of pyroelectric films (in the range of 1kHz—100kHz) are 2.34—1.96 and 0.08—0.04, respectively. The effects of different deposition and incorporated metal ions on pyroelectric properties are also discussed in this paper. It is indicated that the alternating LB films is promising in the field of pyroelectricity and pyroelectric infrared detector.

Stability and Disturbance of Fatty Acid Layers by Polymer in Lead Stearate/Poly(tert-Butyl Methacrylate) Alternating Langmuir—Blodgett Films

The stability of lead stearate layers in alternating Langmuir–Blodgett (LB) films of lead stearate/lead deuterated stearate with and without poly( tert-butyl methacrylate) (PtBMA) layers at each interface was measured from the magnitude of the splitting of the CH2 or CD2 scissoring band in transmission FT-IR spectra. Before annealing, the extent of splitting of the CH2 or CD2 bending mode in the alternating LB film with PtBMA layers was larger than that in the alternating LB film without PtBMA layers. However, the opposite trend was observed after annealing. This suggests that PtBMA does not function as a barrier to interlayer mixing on annealing. The results presented here represent the first report that PtBMA has two important roles: before annealing it stabilizes the fatty acid layers, and after annealing it disturbs these layers. PtBMA is expected to act as a substrate when it is inserted between fatty acid layers. This PtBMA substrate can stabilize the fatty acid layers at room temperature. However, external reflection FT-IR spectra indicate that annealing causes the reorientation of PtBMA. This reorientation of PtBMA could disturb the fatty acid layers.

Studies on the release of polymeric Langmuir–Blodgett multilayers from the solid supports on which they were prepared

It has been shown that polymeric Langmuir–Blodgett (LB) multilayers may be released from the supports on which they were prepared by first thermally evaporating thin films of stearic acid onto the support. The acid can subsequently be washed away, so undercutting the film and releasing it to float to the water surface. The films were redeposited onto fresh silicon wafers or onto glass microscope slides. Under the optical microscope, the films on the glass microscope slides were clear and featureless. By XRD the transferred films had the same or fewer orders of Bragg peaks and the corresponding bilayer spacings were the same or slightly larger than those of the original films. Thus, it appears that most films lose a small amount of their order in the transfer process. By second harmonic generation (SHG) the transferred alternating LB films formed from a poly(4-vinylpyridine) partially quaternised by reaction with n-docosyl bromide and from the poly(4-vinylpyridinium bromide) formed by the zwitterionic polymerisation of 4-vinylpyridine with 4(12-bromododecyloxy)-4′-trifluoromethylazobenzene displayed essentially the same SHG properties as the original films. Moreover, the SHG films could be stacked successfully to give thicker films. Several of the transferred films, but not all, contained traces of stearic acid, which appears to be present as small domains of Y-type layers. In one typical case, where the LB film consisted of 100 layers of a poly(4-vinylpyridine), it was shown that the amount of stearic acid present corresponded to an average of one monolayer.

Induced Collapse in the Formation of Alternating Langmuir–Blodgett Films

Two compounds whose molecules have large dipole moments, strereoregular perfluorinated alkylmethacrylate-co-methacrylic acid (A) and p-octadecylaminoazobenzene-p"-sulfamide (B), were used to assemble Langmuir–Blodgett polar films by the alternating method to achieve high polarization. According to estimates, the dipole moment of the dimeric unit of copolymer A is equal to about 4 D, the dipole moment of compound B, to 12 D. Unusually high (several units) transfer ratio was observed for the monolayer B when preparing the structures of (AB)n type. The monolayers were stable during their formation process. Upon the pause of a substrate when passing through the monolayer B, the monolayer area remained strictly constant. According to the model proposed, the monolayer looses its stability and locally collapses in the meniscus zone at the contact line of the monolayer B and substrate due to an increase in the electrostatic repulsion between the dipoles of molecules B. The validity of this model is confirmed by the data of small-angle X-ray diffraction and atomic force microscopy for various alternating Langmuir–Blodgett films.

Vectorial photoinduced electron transfer in alternating Langmuir–Blodgett films of phytochlorin–[60]fullerene dyad and regioregular poly(3-hexylthiophene)

Alternating Langmuir–Blodgett (LB) bilayer structures, consisting of a donor–acceptor (DA) layer of a phytochlorin–fullerene (PF) dyad and a layer of a regioregular poly(3-hexylthiophene) (PHT) polymer were used to study interlayer vectorial photoinduced electron transfer (VPET). As the dyad PF undergoes, under light illumination, an intramolecular ET from the phytochlorin to the fullerene moiety an intralayer VPET takes place in the LB monolayer. When PF was deposited on the PHT layer and excited the second ET took place from the PHT layer to the phytochlorin cation. Thus the PHT layer can act as a secondary electron donor and accompany the primary photoinduced electron transfer in the PF layer by a spontaneous interlayer electron transfer. Important characteristic properties of the VPET bilayer are the longer distance of charge separation (CS) and the longer lifetime of the charge separated state (lifetime from microsecond to second) as compared to VPET of the PF monolayer alone (where the lifetime of CS state was ≈30 ns). The CT measurements were carried out for different molecular orientations and film structures. Models for the multistep photochemical reactions are discussed.

Structure and thermal stability of cadmium arachidate/poly( tert-butyl methacrylate) alternating Langmuir–Blodgett film

Layer structure and thermal stability of alternating Langmuir–Blodgett (LB) films of cadmium arachidate (CdA 2) and poly( tert-butyl methacrylate) (PtBMA) was investigated by X-ray reflection and in-plane diffraction study. The LB films were deposited on hydrophobically pre-treated silicon wafer surface and the polymer layers are inserted between CdA 2 bilayers. Layer spacing of the alternating LB film increases linearly by 9.8 Å per PtBMA layer, which is consistent with the single layer spacing of homo-PtBMA LB film. Lateral lattice structure of CdA 2 in alternating LB films maintains its hexagonal packing structure as found in CdA 2 monolayers while the multilayer CdA 2 LB film is known to change its structure from hexagonal to orthorhombic lattice. Therefore, the polymer layers inserted between CdA 2 bilayers in the alternating LB films effectively prevent the CdA 2 film not only from the growth of defect but also from the formation of three-dimensional crystallites of CdA 2. In addition, the hexagonal packing structure of CdA 2 bilayer was maintained up to 90°C while the molecular area increases slightly.

X-ray Diffraction Study of Cadmium Arachidate/Poly(tert-butyl methacrylate) Alternating Langmuir−Blodgett Film

The layer structure of alternating Langmuir−Blodgett (LB) films of cadmium arachidate (CdAA2) and poly(tert-butyl methacrylate) (PtBMA) was investigated by an X-ray reflection and in-plane diffraction study. The LB films were deposited on a silicon wafer surface pretreated hydrophobically by transferring a behenic acid monolayer, and two or four PtBMA layers are inserted between each CdAA2 bilayer. While CdAA2 homo-LB films are known to show gradual deterioration of the transfer ratio as the number of accumulated layers increases, the alternating LB films maintain the transfer ratio near unity up to 32 layers. The layer spacing of the alternating LB films determined from the position of the Bragg diffraction peaks increases linearly by 9.9 Å per PtBMA layer. The lateral lattice structure of CdAA2 in alternating LB films maintains its hexagonal packing structure, as found in CdAA2 monolayers, while the multilayer CdAA2 LB film is known to change its structure from hexagonal to orthorhombic lattice. The spacing of the hexagonally packed CdAA2 was 4.15 Å, and the mean molecular area of CdAA2 was 19.9 Å2, which is in good agreement with the value found for CdAA2 monolayers at the air/water interface. Therefore, it appears that the polymer layers inserted between CdAA2 bilayers in the alternating LB films effectively protect the CdAA2 film not only from the growth of defects but also from the formation of three-dimensional crystallites of CdAA2, which degrade the optical clarity of the LB film. The thermal stability of the alternating LB film was investigated by X-ray reflectivity over the temperature range 34−106 °C. The layer structure of the alternating LB film starts to phase-segregate around 70 °C, which is over 60 °C lower than the glass transition temperature of the bulk polymer. At higher temperature, Bragg peaks of the CdAA2 bilayer spacing appear, indicating possible formation of a CdAA2 crystal domain due to dewetting of the inserted polymer layers.

NLO properties of polymeric Langmuir-Blodgett films of sulfonamide-substituted azobenzenes

The behaviour at the air/water interface of polymeric materials containing sulfonamide-substituted azobenzene chromophores has been investigated using surface pressure versus area measurements. Langmuir-Blodgett (LB) films of the polymers have been built up and then studied using X-ray diffraction. The polymer which gave the best organised LB film has been used to prepare an alternating Y-type multilayer structure with a polymer inactive in non-linear optics. The obtained film has a well-ordered polymeric LB structure giving three Bragg peaks. A series of films with various numbers of bilayers within the range from 2 to 80 has been prepared and the second harmonic generating properties of the films investigated. The measured intensity of the SH signal produced from incident light of wavelength 1064 nm was proportional to the square of the number of bilayers. The alternating LB films exhibited a non-linear susceptibility of up to 24 pm V –1 corresponding to a chromophore non-linear vector susceptibility of magnitude χ zzz (2) =170 pm V –1 .

Ordered Langmuir-Blodgett multilayers of copper phthalocyanine derivatives

Copper(II) phthalocyanine derivatives with four alkoxycarbonyl chains as substituents were synthesized. Langmuir-Blodgett (LB) films of the two following compounds were deposited: Cu(II) [tetrakis(3,3-dimethyl-1-butoxycarbonyl)] phthalocyanine and Cu(II) [tetrakis( n-butoxycarbonyl)] phthalocyanine. Moreover, alternate LB films of the two compounds and LB films from a 1:1 mixture of them were prepared. Monolayers were spread onto the water surface from different solutions and were transferred onto quartz slides using the vertical lifting method. The deposition parameters and processes are described, and a UV-visible spectroscopic characterization of these compounds in different solvents and of the related LB films is presented. All isotherms and optical data are consistent with the molecules being stacked nearly edge-on to both the water surface and the substrate. Moreover, after deposition, the molecular faces exhibit a preferred orientation perpendicular to the dipping direction. Molecular order in the films was explored by polarized electronic absorption spectroscopy. Optical characterization of our LB films indicates the presence of some degree of molecular aggregation even in the multilayers. Finally, the results from a preliminary transmission electron microscopy study on the LB films are presented.

Preparation and Organized Assembly of Nanoparticulate TiO2–Stearate Alternating Langmuir–Blodgett Films

Nanoparticulate TiO2–stearate Langmuir–Blodgett-type monolayers and multilayers were directly obtained by using TiO2hydrosol as the subphase. The surface pressure-versus-surface area isotherms showed that the monolayer could be compressed to a mean molecular area of 0.25 nm2. The monolayer was transferred onto a CaF2or Si substrate at a dipping speed of 18 cm/min and surface pressure of 25 mN/m. It exhibited Y-type multilayer films over the range investigated (1–30 layers) with a transfer ratio of 1.0 ± 0.1. FTIR transmission spectra and linear infrared dichroic spectra provided evidence of the formation of a stable organic/inorganic alternating multilayer with nanoparticulate TiO2between the organic monolayers. There is only slight absorption in the range of the C;equals;O stretching vibration band of the carboxylic group with the 3- and 6-nm TiO2nanoparticles. This means that TiO2particles are packed closely; i.e., the surface coverage is very high. A possible structure consists of layers of nanoparticles in close-packed form with two stearate ion layers inbetween. But it can be seen that there are some carboxylic groups (1700 cm−1) not connected to the 20-nm TiO2nanoparticle. Electron microscopic (TEM) images of TiO2–stearate monolayers show that high-coverage monolayers were obtained when the surface pressure increased to 25 mN/m and the dipping speed to 18 cm/min.

Structural study on the alternating Langmuir—Blodgett films deposited with two complementary barbituric acid and melamine derivatives

Alternating Langmuir-Blodgett films were deposited with two complementary molecules: 5-(4- N,N-dioctadecyl aminobenzylidene)-2,4,6-(1H,3H)-pyrimidinetrione (B) and 4-amino-2,6-didodecylamino-1,3,5-triazine (M). Pressure-area isotherms were recorded. The transfer behavior of the alternating films was compared with that of the separate films of B. X-ray diffraction measurements confirm that the alternating films have a genuinely alternating structure: MBMB…MB. Transmission FTIR measurements reveal that in the separate films B is exclusively self-aggregated through the hydrogen bonding between the C=O and N−H on the barbituric ring of B, and in the alternating films new hydrogen bonds are formed between B and M at the expense of those of the initial self-aggregation. Polarized attenuated total reflectance Fourier transform infrared spectroscopy measurements show that in the alternating films the hydrocarbon chains stand almost vertically, in comparison with the separate films of B as well as of M. It is found that in the alternating films the 2-position carbonyl on the barbituric ring of B is perpendicular to the film plane. It is concluded that the interlayer hydrogen bonding between B and M gives rise to a better film structure.

Synthesis of long alkyl chain substituted TTF and TCNQ derivatives and conductivities of their Langmuir-Blodgett films

2-Octadecyl-7,7,8,8-tetracyanoquinodimethane (C 18TCNQ) and 2,3-dimethylthio-6,7-dihaxadecylthiotetrathiafulvalene (C 1TTFC 16) were synthesized and confirmed by elemental analysis, IR, 1H NMR and mass spectrometry. The molecules of C 18TCNQ can form a stable monolayer at the air-water interface. Anisotropy of conductivity was observed in the Langmuir-Blodgett (LB) films. The maximum conductivity of the alternating LB films of C 18TCNQ and C 1TTFC 16 (1:1) is 1.5 × 10 2 S cm −1 after iodine doping.

Scanning probe microscopies for molecular photodiodes

Photo-electric conversion molecular devices, i.e. molecular photodiodes based upon the charge separation mechanism of the primary process of natural photosynthesis, have been studied in our laboratory. Amphiphilic A-S-D triad molecules can be oriented unidirectionally in monolayers at the air-water interface. The resulting oriented monolayer assemblies are fabricated into thin films on electrode surfaces by the Langmuir-Blodgett (LB) technique. The A-S-D triads for charge separation together with light harvesting antenna molecules can be assembled into sub-micron island domains by making use of phase separation of mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles. The structure and properties of the domains of HC-FC mixed monolayers were studied by various scanning probe microscopies (SPMs) such as atomic force microscopy, friction force microscopy, scanning surface potential microscopy (SSPM), and scanning near-field optical/atomic force microscopy. Some SPMs can be used not only to characterize the LB films, but also to drive the molecular devices. The photo-induced charge separation in unidirectionally oriented triad molecules embedded in alternate LB films was observed by SSPM.

Preparation of nanoparticulate Fe2O3—polymaleic monoester alternating Langmuir—Blodgett films with functional organic hydrophobic part

An organic functional group was introduced into the inorganic nanoparticle—organic alternating LB films by the use of the polymaleic monoester with a functional lateral chain ▪ to replace a fatty acid. Using the polymer (PMANN), the distance between adjacent nanoparticles in the LB films is apparently decreased and the distribution of the nanoparticles in the monolayer is more homegeneous than that in the nanoparticulate Fe2O3—stearate monolayer. All these phenomena are ascribed to the carboxylate groups of a polymer molecule bound to two or more nanoparticles. The photovoltage response of n-type (p-type) silicon coated with a monolayer of nanoparticulate Fe2O3—PMANN LB films increases (decreases) for two orders.