Langmuir-Blodgett Multilayers Research Articles

Particulate assemblies of CdS and TiO 2 prepared by Langmuir–Blodgett technique with octadecylamine/methylstearate mixed films

Langmuir–Blodgett (LB) technique has been applied to fabrications of various nanoparticulate assemblies such as monolayer and multilayer of CdS particles, in-plane mixed monolayer and alternate multilayer of CdS/TiO 2 particles on the solid substrate. We examined the most favorable conditions for yielding uniform and dense packing of the particles in a 2-dimensional arrangement, followed by structural characterization of the as-deposited particulate layers. As a result, it was found for the CdS particulate films that charge density of cationic Langmuir monolayer markedly influences the amount of particles embedded on the substrate and that multilayer LB deposition enables stepwise control of thickness for the particulate assemblies. For the composite CdS/TiO 2 particulate films, desired in-plane mixing ratio and alternating layer structure of two kinds of particles were achieved, on which basis the possibilities of fabricating a variety of heteroparticulate layers are pointed out. In addition, photocatalytic activities of TiO 2 and CdS/TiO 2 particulate films fabricated by the LB technique were evaluated in terms of decomposition of both oleic acid cast film and stearic acid (SA) LB film by irradiating with UV light. The results exhibited that when the bilayer of TiO 2 particulate film is employed in place of the monolayer film, the photodecomposition rate of SA is sufficiently accelerated and that the alternate CdS/TiO 2 particulate layer is less active for the oxidative decomposition.

Preparation and Characterization of Monolayer and Multilayer Langmuir−Blodgett Films of a Series of 6-O-Alkylcelluloses

Monolayer and multilayer Langmuir-Blodgett (LB) films of 6-O-alkylcelluloses with various chain lengths were prepared and studied. The surface pressure (pi)-area (A) isotherms of 6-O-alkylcelluloses exhibited characteristic behaviors depending on the length of the alkyl chain and temperature. 6-O-Stearylcellulose on the subphase formed a homogeneous monolayer at 10 mN m(-1). By transfer ratio, FT-IR, and contact angle measurements, it was proved that the monolayer of 6-O-stearylcellulose on the water surface was transferred successfully onto a substrate by a vertical dipping method to form a Z-type LB film. The transmission and reflection absorption IR spectrum indicated that the hydrocarbon chains had all-trans rotamers and were oriented nearly perpendicular to the surface in the film. AFM section analysis revealed that the thickness per layer was calculated to be 2.35 nm. These results suggested that the hydrocarbon chains were inclined at an angle of about 25.3 degrees to have high packing density in the alkyl region.

Thin-Film Transistors Based on Langmuir−Blodgett Films of Heteroleptic Bis(phthalocyaninato) Rare Earth Complexes

An ordered molecular assembly of heteroleptic bis(phthalocyaninato) rare earth complexes M(Pc)[Pc(OC8H17)8] [M = Tb, Lu; H2Pc = phthalocyanine; H2Pc(OC8H17)8 = 2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyanine] has been fabricated by the Langmuir-Blodgett (LB) technique and characterized by surface pressure-area isotherms, electronic absorption and polarized electronic absorption spectroscopy, low-angle X-ray diffraction, and atomic force microscopy. The molecular ordering in the LB multilayer film on SiO2 substrate was made into a p-channel field effect transistor (FET), which was generally operated in the enhanced mode. The energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital as well as the energy band diagram can be deduced from the electrochemical measurement results. The charge mobilities of Tb(Pc)[Pc(OC8H17)8] and Lu(Pc)[Pc(OC8H17)8] were calculated to be about 6.4 x 10(-4) and 1.7 x 10(-3) cm2 V(-1) s(-1), respectively.

Inducing Spin Crossover in Metallo-supramolecular Polyelectrolytes through an Amphiphilic Phase Transition

A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic-paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte-amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir-Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals.

Synthesis and Characterization of Amphiphilic Fullerenes and Their Langmuir−Blodgett Films

We report here the synthesis and characterization of three amphiphilic fullerene derivatives and their Langmuir-Blodgett thin films. Two of the C(60) amphiphiles are mono-derivatives with a long alkyl chain terminated with either -COOH (2) or NH(2) (3) as the hydrophilic headgroup, and the third one (5) is designed to bear the same NH(2) group as 3 but with 10 additional hydrophobic alkyl chains grafted on the C(60) sphere (Scheme 1). These amphiphiles form stable, ordered monolayers at the air-water interface. The molecular packing at the air-water interface and the mean area per molecule are determined by pressure isotherms at room temperature. Hysteresis of pressure isotherms of side chain C(60) (5) shows complete reversibility upon compression and decompression, which suggests that side chains on the C(60) sphere inhibit formation of aggregates at the air-water interface. Comparative studies of all three amphiphiles allow us to better determine the interaction between C(60)'s and their self-assembly kinetics at the air-water interface. Monolayers of monoderivatized amphiphiles (2 and 3) were transferred successfully onto quartz substrates as Z-type multilayered Langmuir-Blodgett films, and monolayers of 5 were transferred as Y-type films. Detailed characterization of the multilayer films (Z-type deposition) prepared from amine-terminated C(60) (3) using X-ray and neutron reflectometry reveals staggering of C(60) spheres and a head-to-head (Y-type) structure presumably due to flipping and reattaching of C(60) amphiphiles to the previous underlying C(60) layer.

Self-organization of a sulfonamido-porphyrin in Langmuir monolayers and Langmuir–Blodgett films

Langmuir monolayers (LM) and Langmuir-Blodgett (LB) films of pure lipophilic meso-tetra(4-dodecylaminosulfophenyl)porphyrin (PC12) and mixed with the anionic surfactant sodium hexadecylsulfate (SHS) were studied. The molecular packing and structure of PC12 and PC12-4SHS with variable surface pressure were investigated by surface pressure-area measurements, steady-state absorption, fluorescence emission and anisotropy, as well as by fluorescence lifetime imaging microscopy (FLIM). At low surface pressure, the porphyrin molecules are organized with the rings tilted on the water surface whereas at high surface pressure the porphyrin rings achieve a more perpendicular arrangement. Using the FLIM images a gradual change of aggregates into large "islands" is observed. Different patterns are observed in the pure PC12 multilayer films (n = 3 and 5) with ordered patches superimposed which are not observed in the PC12-4SHS multilayer LB films.

Infrared Spectroscopy Analysis of the Structure of Multilayer Langmuir−Blodgett Films: Effect of Deposition Velocity and pH

Multiple Langmuir-Blodgett (LB) films of arachidic acid were deposited on germanium (Ge) substrates from subphase solutions of 10(-4) M CdCl2 at different pH values and at different deposition speeds. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain information on the molecular order and structure of these multilayer LB films. At pHs higher than the pKa of the fatty acid/cation system, transfers took place only during the downstroke, indicating X-type deposition. At pH = pKa and large deposition speeds, deposition partially failed during the downstroke, resulting in Z-type depositions. Analysis of the infrared spectra indicates that multiple LB films deposited only during the upstroke (Z-type) or during downstrokes (X-type) have a centrosymmetric structure typical of films deposited during the upstroke and downstroke, except for a slight decrease in molecular order and tilt angle as the pH increases (X-type). The centrosymmetric structure indicates that rearrangement of layers takes place between cycles. Experimental evidence of such rearrangement occurring in a fatty acid/divalent cation salt subphase is shown here, and rearrangement alternatives are discussed.

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.

Effect of subphase divalent ions on optical second harmonic properties of hemicyanine Langmuir-Blodgett films interleaved with compatible spacer

The surface pressure–area ( π– A) isotherms of 1.10-bistearyl-4.6.13.15-tetraene-8-nitrogen-crown-6 (NC) monolayers at the air–water interface, the UV–visible absorption spectra and optical nonlinear properties of Y-type hemicyanine/NC interleaving Langmuir-Blodgett (LB) multilayers containing divalent ions in the subphase (such as Cd 2+, Ba 2+ and Co 2+) were investigated in the paper. It was found that the π– A isotherms of the NC monolayer, the peak position of absorption spectra and the optical second harmonic intensity in the alternating LB multilayers strongly depend on the divalent ions in the subphase. The 2 ω resonance effect of molecular hyperpolarizability β was responsible for the change of the observed second order polarizability χ (2) of LB multilayers.

Magnetic Langmuir–Blodgett films of ferritin with different iron loadings

Magnetic Langmuir–Blodgett multilayers of two ferritin molecules 1 and 2, containing, respectively, 4220 and 3062 Fe atoms have been prepared by using the adsorption properties of a 6/1 mixed monolayer of dioctadecyldimethylammonium bromide (DODABr) and methyl stearate (SME). Transfer ratios close to unity were reached giving rise to LB films with a strong red colour. Infrared and UV–vis spectroscopy indicates that ferritin molecules are incorporated within the LB films. Magnetic measurements show that the superparamagnetic properties of these molecules are preserved. Thus, a marked hysteresis loop of magnetisation is obtained for LB films of 1 and 2 with a coercive field of 3400 and 2400 G, respectively.

Scanning Maxwell stress microscopy of photo-induced charge separation in A–S–D triad monolayers

Photo-induced charge separation in mixed Langmuir–Blodgett (LB) films containing oriented A–S–D triads and inert matrix molecules was studied by using scanning Maxwell stress microcopy. A, S, and D stand for electron acceptor, sensitizer, and electron donor moieties of the triad, respectively. The change in surface potentials upon photo-excitation corresponded well with creation and disappearance of photo-induced dipole moments in the LB films due to the photo-induced charge separation in the oriented A–S–D triads followed by charge recombination. The observed lifetime of the separated charges in the LB films, however, was longer by several orders of magnitude than that of isolated A–S–D triads in solution observed by transient absorption spectroscopy. The magnitude of the photo-induced surface potentials, i.e. the content of the separated charges with the longer lifetime decreased drastically with the decrease in the concentration of A–S–D triads in the mixed LB films. Therefore, we concluded that the separated charges with the longer lifetime were created by lateral diffusion of photo-produced anion (electrons) and cation radicals (holes) among neighboring A and D moieties, respectively, in the LB films. We also found that addition of a second donor (D′) monolayer next to a layer of D end moieties of the mixed A–S–D monolayer in multilayered LB films enhanced the photo-induced charge separation with the long lifetime. When light harvesting (H) molecules acting as antenna was mixed with the A–S–D triads in place of the inert matrix molecules, the photo-induced charge separation was observed under irradiation of photons, which were absorbed by the H moieties.

Synthesis and electrical properties of CdS Langmuir–Blodgett multilayers nanoparticles on self-assembled carbon nanotubes

Cadmium arachidate Langmuir–Blodgett multilayers were used to grow semiconducting cadmium sulphide (CdS) nanoclusters on the surface of self-assembled carbon nanotubes (CNTs). The formation of CdS in the multilayers was determined by TEM, infrared and UV–Vis spectroscopy. We suggest that the conductivity type of CdS nanoparticles modified nanotubes can be changed to n-type. Thermally stimulated current measurements confirm that the photoinduced oxygen adsorption process in CdS/CNTs sample is an important mechanism via the dominant role of the surface in the electrical properties. These issues have important technological implications for the practical use of these nanocomposite systems in novel nanostructured devices.

Nanomechanical properties of ordered phthalocyanine Langmuir–Blodgett layers

The mechanical properties of monolayer and multilayer structures of several symmetrically substituted alkoxy copper phthalocyanine, CuPc(OR)8, Langmuir–Blodgett (LB) films were evaluated. One-dimensional compliance of the monolayers was determined from the slopes of the surface pressure–area isotherms. Multilayers were examined using a SPM and a commercial nanoindentation system. The nanomechanical studies show a distinct relationship between the position and length of alkoxy substituents on the Pc macrocycle and the LB film elasticity and hardness. The phthalocyanine complexes with peripheral octabutoxy and octaoctyloxy substituents form stiff LB monolayers and multilayers. The nonperipheral alkoxy derivetized CuPc films were less stiff. For monolayer films, it appears that the extent of π-π interaction determines the strength of the film. In multilayers, significant additional stability can be imparted through interdigitation of long paraffinic chains which play a significant role in determining the interlayer structure of the LB layers.

Flow orientation and super-quadratic dependence of second harmonic intensity on the film thickness in hemicyanine Langmuir–Blodgett multilayers

Abstract Substantial in-plane optical anisotropy in Y-type Langmuir–Blodgett multilayers of a hemicyanine dye interleaved with inert material arachidic acid due to dipping-induced and “substrate”-enhanced molecular orientation was demonstrated by polarized UV-visible absorption and rotation-angle second harmonic generation techniques. We gave the first direct evidence for a super-quadratic enhancement of the second harmonic intensity with increasing film thickness due to the additional in-plane polarization.

The role of Zn in the interplay among Langmuir–Blodgett multilayer and myelin basic protein: a quantitative analysis of XANES spectra

We have performed a quantitative analysis of the X-ray absorption near-edge structure (XANES) spectra at the Zinc K-edge of systems formed by phospholipid Langmuir–Blodgett multilayers (LBMLs) in the presence and in the absence of myelin basic protein (MBP) and in two hydration conditions. These spectra have been analysed by a new procedure called Minuit XANes (MXAN) which is able to perform a quantitative fit of XANES data in terms of structural parameters. By this method, we have been able to correlate the relevant differences between the spectra observed in the XANES range with the coordination changes due to reduction of the space around the Zinc when the level of hydration is lowered and/or the myelin basic protein is added. These spectral differences are peculiar of the XANES energy range, and are not present in the extended X-ray absorption fine structure (EXAFS) energy range where the analysis was previously performed. With this investigation, we give an unambiguous answer to the question of the role of zinc in such complexes by showing that the metal interacts with both the phospholipid heads of the substrate and the myelin basic protein.

Macroscopic Alignment of Graphene Stacks by Langmuir−Blodgett Deposition of Amphiphilic Hexabenzocoronenes

We present structural studies of Langmuir (L) and Langmuir-Blodgett (LB) films of new amphiphilic hexa-peri-hexabenzocoronene (HBC) discotics, carrying five branched alkyl side chains and one polar group. The polar group is either a carboxylic acid moiety or an electron acceptor moiety (anthraquinone). Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity, both utilizing synchrotron radiation, show that these amphiphilic HBCs form well-defined Langmuir monolayers at the air-water interface, with a pi-stacked columnar structure where the HBC cores are rotated around the surface normal and tilted relative to the water surface. The intercolumnar distance is 20 A. The HBCs are confined to a layer lying on top of the layer of polar groups that are in contact with the water subphase. Efficient transfer of the monolayer of the anthraquinone-substituted HBC derivative to hydrophobic quartz substrates by vertical dipping gave well-defined multilayer Y-type LB films. Polarized optical spectroscopy, GIXD, and X-ray reflectivity measurements show that the LB films consist of at least two phases. Heating the films results in an irreversible rearrangement to a single macroscopically aligned phase of hexagonally packed columns oriented along the dipping direction with disk planes perpendicular to the columnar axes and stacked in a cofacial manner. This phase transition is analogous to the reversible transition observed in the bulk material.

Synthesis of surfactant encapsulated nickel hexacyanoferrate nanoparticles and deposition of their Langmuir–Blodgett film

Sodium hexametaphosphate (HMP) stabilized nickel hexacyanoferrate (NiHCF) nanoparticles were prepared in aqueous solution and were successfully extracted into an organic phase using cetyltrimethylammonium bromide (CTAB) as the surfactant. Dynamic Light Scattering (DLS) studies suggest that the average size of the nanoparticles is retained during the extraction process from the aqueous to the organic phase. X-Ray diffraction, cyclic voltammetry, IR spectroscopy and magnetic measurements carried on the organic phase shows specific signatures of the presence of the surfactant encapsulated NiHCF nanoparticles. Transmission Electron Microscopy (TEM) measurements show that the average size of these surfactant encapsulated nanoparticles in the organic phase is about 22 nm, and as has been suggested by DLS studies, it does not change with respect to repeated evaporation and re-extraction processes of the organic phase. Pressure–area isotherms of the organic phase in a Langmuir–Blodgett (LB) trough, with water as subphase, indicate stable monolayer formation of the surfactant-encapsulated NiHCF nanoparticles at the air–water interface. Multi-layered deposition of the surfactant-encapsulated nanoparticles onto an indium tin oxide (ITO) coated glass slide could also be carried out using the LB technique. Cyclic voltammetry studies on these LB multilayers confirm regular and systematic transfer of the NiHCF nanoparticles on the ITO substrate. The method described here is the first of its kind with respect to the synthesis of surfactant encapsulated molecular magnetic nanoparticles and subsequent deposition of their LB films.

Thickness Dependence of Ferroelectricity in Hemic yanine Langmuir-Blodgett Multilayer Films

Thickness Dependence of Ferroelectricity in Hemic yanine Langmuir-Blodgett Multilayer Films

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.

A novel NO/sub 2/ gas sensor based on bis[phthalocyaninato] samarium complex/silicon hybrid charge-flow transistor

A new kind of sandwich-like bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] samarium complex Sm[Pc/sup */]/sub 2/(Pc/sup */=Pc(OC/sub 8/H/sub 17/)/sub 8/) is used as film-forming material. Pure Sm[Pc/sup */]/sub 2/ and mixture of Sm[Pc/sup */]/sub 2/ and octadecanol(OA) deposited from both pure water and 10/sup -4/M Cd/sup 2+/ subphases are investigated. It is found that a mixture of 1:3 Sm[Pc/sup */]/sub 2/:OA forms an excellent material for the fabrication of the gas-sensing Langmuir-Blodgett (LB) film by studying the film-forming characteristics. A new gas sensor has been fabricated by incorporating the multilayer LB film into the gate electrode of a metal-oxide-semiconductor field effect transistor, forming an array of charge-flow transistor. On the application of a gate voltage (V/sub GS/), greater than the threshold voltage (V/sub TH/), a delay was observed in the response of the drain current. This is due to the time taken for the resistive gas-sensing film to charge up to V/sub GS/. This delay characteristic was found to depend on the concentration of NO/sub 2/. Results are presented showing that the device can detect reversibly the concentration of NO/sub 2/ gas down to 5 ppm at room temperature.