Langmuir-Blodgett Layers Research Articles

Electronic transfer through Langmuir–Blodgett layers of capped platinum nanoparticles: An electrochemical approach

Amine functionalized platinum nanoparticles have been modified by over-grafting two different molecules, 2-thiophenecarbonyl chloride (Pt- 1) and 1-hexyl-4-(4-isothiocyanatophenyl)-bicyclo (2, 2, 2) octane (Pt- 2). Cyclic voltammetry was performed at gold electrodes coated with Langmuir–Blodgett (LB) mixed films of Pt- 1 and Pt- 2 nanoparticles, and behenic acid. From five layers the electrochemical response was essentially provided by the last LB component. The electrochemical responses towards the [Fe(CN) 6] 3−/4− couple were strongly influenced by the nature of the over-grafted molecules: films of Pt- 2 presented an almost complete blocking effect, while films of Pt- 1 allowed the redox reaction to occur on Pt nanoparticles. In order to understand the reasons for such different behaviors we built up hetero-nanostructures by superposing Pt- 1 and Pt- 2 LB layers in different ways, yielding different kinds of “sandwich” structures. The electrochemical response depended on the electrode ending. When Pt- 1 nanoparticles were in the outer layer, in contact with the electrolyte solution, the electrode was electroactive toward the redox probe, while when Pt- 2 layers were in the outer layer no electroactivity was detected. For sandwiches made of Pt- 1, with a variable thickness of an intercalated film of Pt- 2, the electrode response to [Fe(CN) 6] 3−/4− was modulated by the thickness of the inter-layer: the thicker the layer, the lower the response.

Static SIMS for analysis of molecular conformation and orientation

Owing to the sensitivity of Static SIMS to molecular structures and its selectivity to the outermost surface layers, this technique is very well suited to gain information about conformation and orientation of molecules at the surface. The aim of this presentation is to discuss critically and to illustrate these unique features of Static SIMS with selected applications. Firstly, the SIMS information depth and its dependence on the molecular fragment ion size are presented in order to justify the surface selectivity. This has been experimentally determined by looking at the substrate signal attenuation when depositing organic layers in a controlled manner on different materials as was done with Langmuir–Blodgett layers and polyelectrolyte ultra thin films. As illustrated for polymers, the surface selectivity allowed SIMS to be sensitive to tacticity of macrochains and orientation of mobile pendant groups. Based on these performances, Static SIMS has been used to study protein adsorption and highlighted modifications in protein conformation/orientation, depending on the substrate properties. Finally, the limitations of Static SIMS analyses are discussed. Indeed, for sample surfaces non-uniform at a nanoscale, which is beyond the limits of SIMS imaging, the chemical molecular information cannot be sufficient. It has to be complemented by surface morphology information in order to get a correct view of the system. This is illustrated by our recent study of very thin layers of block copolymers and polymer blends, forming nanostructures at the surface.

Investigation of Solid Surfaces Modified by Langmuir−Blodgett Monolayers Using Sum-Frequency Vibrational Spectroscopy and X-ray Photoelectron Spectroscopy

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.

Molecular rectification: stabilised alignment of chevron-shaped dyes in hybrid SAM/LB structures in which the self-assembled monolayer is anionic and the Langmuir–Blodgett layer is cationic

The chevron-shaped dye, N-butyl-2,6-bis-[2-{4-(2-(4-dibutylaminophenyl)-vinyl)-phenyl}-vinyl]-pyridinium iodide, has a hydrophilic apex that is electron-accepting and hydrophobic it-bridged limbs that are electron-donating. It forms non-centrosymmetric monolayers at the air-water interface and when deposited, its LB films exhibit second-harmonic generation and asymmetric current-voltage (I-V) characteristics. However, the behaviour is short lived: molecular reorganisation causes the absorption maximum to shift from ca. 400 to 700 nm with suppression of the SHG and rectification. Long-term stability results when the cationic dye is LB deposited on an anionic surface, for example, a SAM formed via chemisorption of sodium 3-mercapto-1-propanesulfonate on a gold-coated substrate. The hybrid Au/SAM/LB device exhibits asymmetric I-V curves with rectification ratios of ca. 25 at +/- 1 V when investigated by scanning tunnelling spectroscopy and contacted by PtIr probes.

Electrical properties of organic light-emitting diodes using PECCP Langmuir–Blodgett films

Organic light-emitting diodes were fabricated using Langmuir–Blodgett (LB) technique with new emissive organic material. This new emissive organic material was synthesized and called PECCP [poly(3,6- N-2-ethylhexyl carbazolyl cyanoterephthalidene)], which has strong electron donor group and electron acceptor group in main chain repeated unit. This material has good solubility in common organic solvent and has a good stability in air. We made PECCP LB films and investigated current–voltage ( I– V) characteristics in ITO/PECCP LB/Al and ITO/PECCP LB/Alq 3/Al structures, and the Alq 3 was used as an electron transport layer. We have observed that the peak in electroluminescent spectrum of ITO/PECCP LB/Alq 3/Al device shifts to lower wavelength as the number of LB layer increases. The electroluminescent peaks are at about 507 nm, 505 nm, 503 nm, and 491 nm when the number of LB layers are 5, 7, 9, and 11 layers, respectively. From the I– V characteristics, the barrier height was obtained using Fowler–Nordheim analysis and it is in the range of 0.18 eV–0.26 eV.

On the origin of multilevel conductance and memory in ultrathin organic films

Conductivity of certain organic molecules switch to a high-state via electroreduction. Different high-states or multilevel conductivity in organics has been due to different density of high-conducting molecules in a device. We have studied how the population distribution of reduced molecules changes in achieving different conductivity levels. In devices based on a few molecular layers, we have observed that the number of conductivity levels can exceed the number of Langmuir–Blodgett layers. The results showed that the distribution of high-conducting molecules did not increase layer-by-layer, but throughout the volume of the device enabling large number of conductivity levels for higher level (multibit) applications.

Fluorescence in the presence of metallic hole arrays

We show that the transmission of light through metallic hole arrays supported by a glass substrate can be tuned by depositing a controlled number of Langmuir–Blodgett layers on top of the hole array. Enhanced transmission is achieved when the number of overlayers is such that the surface plasmon-polariton modes on the two sides of the metal hole array have matched wavevectors. Dye molecules introduced into some of these overlayers allow us to explore the relationship between molecular fluorescence and the transmission properties of the structure, through measurement of the fluorescence lifetime of the molecules. We find there to be little change in the fluorescence lifetime between enhanced and non-enhanced transmission regimes and offer an explanation of our findings in terms of changes in the photonic mode density.

Influence of Langmuir–Blodgett nanolayers on microbial adhesion

Langmuir–Blodgett (LB) nanolayers of alkyl hydroxamic and phosphonic acids on glass, iron and copper surfaces were studied in the presence of different, corrosion relevant micro-organisms. The quality of monomolecular and multimolecular LB layers was characterised by dynamic contact angle measurement, atomic force microscopy and the sum frequency vibration technique. The inhibition efficiency of microbial adhesion and biofilm formation, which was analysed by the surface visualising technique, epifluorescence microscopy and microbiological methods, increased with layer thickness. There were some differences in the blocking activity of the two amphiphiles (octadecylphosphonic acid C18P and octadecanoyl hydroxamic acid C18N) applied in LB films. Their repellent activity did not differ in cooling water but, in the presence of Acidithiobacillus ferrooxidans, the C18N nanolayer inhibited the biofilm formation much better.

A Study on the Electrical Physical Properties of Organic Thin Films for Manufacture in Power Device

Monolayers of lipids on a water surface have attracted much interest as models of biological membranes, but also as precursors of multilayer systems promising many technical applications. Until now, many methodologies have been developed in order to gain a better understanding of the relationship between the structure and function of the monolayers. Maxwell displacement current (MDC) measurement has been employed to study the dielectric property of Langmuirfilms. MDC flowing across monolayers is analyzed using a rod-like molecular model. A linear relationship between the monolayer compression speed a and the molecular area Am. Compression speed a was about 30, 40, and 50 mm/min. Langmuir-Blodgett(LB) layers of Arachidic acid deposited by LB method were deposited onto slide glass as Y-type film. The structure of manufactured device is Aul Arachidic acid! AI, the number of accumulated layers are <TEX>$9{\sim}21$</TEX>. Also, we then examined of the Metal-Insulator-Metal(MIM) device by means of I-V. The I-V characteristics of the device are measured from -3 to+3 V. The insulation property of a thin film is better as the distance between electrodes is larger.

Pt Nanocrystals: Shape Control and Langmuir−Blodgett Monolayer Formation

We report the synthesis of monodisperse Pt nanocrystals with three different shapes-cubes, cuboctahedra, and octahedra, selectively, with similar sizes of 9-10 nm by a modified polyol process. We found that addition of silver ion enhances the crystal growth rate along 100, and essentially determines the shape and surface structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by using the Langmuir-Blodgett (LB) method. The particles were evenly distributed on the entire substrate, and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the structural uniformity of the metal nanocrystals.

Characterization of interfacial effects in organic macrocycles Langmuir and Langmuir–Blodgett layers studied by surface potential and FT–IR spectroscopy examination

Surface potential (SP) examination and FT–IR (infrared) reflection–absorption spectra were used for the characterization of interfacial effects in organic dye thin layer on solid substrates. Surface potentials of magnesium tetranaphtylporphyrin (MgTNP), magnesium or lead phthalocyanine monolayer on the water subphase were measured and the dipole moments of the investigated dyes were evaluated (1.07 D, 0.52 D and 0.31 D for MgTNP, MgPc and PbPc, respectively). The differences between the dipolar moment values were attributed to the differences between porphyrin and phthalocyanines molecular structures and to the differences in metal electronegativity and metal ion distortion in the molecular frame. Also asymmetry in the covalent linkage and coordination bonding in the center of the molecular skeleton and the differences in polarisablility of the dye molecules as sources of the difference in the dipolar moment values were taken into consideration. The FT–IR reflection–absorption spectra were used for the characterization of the magnesium phthalocyanines Langmuir–Blodgett (LB) layer formed on semiconducting (In 2O 3) or Au substrates. The modification of the IR spectra upon Langmuir–Blodgett dye layer deposition was attributed to the redistribution of electrons at the semiconducting (metallic)/dye layer interface and to different substrate morphology. The difference in the band splitting for dye on Au and In 2O 3 was related to the smaller amount of charge transferred to the dye film from In 2O 3 than from Au substrate and to the changes in the topology of the different substrates after coating with the dye layer.

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.

Langmuir–Blodgett films incorporating redox mediators for molecular recognition of NADH

We focused our studies on the fabrication of nano-organized systems incorporating redox mediators for NADH. We fabricated gold-modified electrodes of phospholipid layers containing a redox mediator for NADH by means of the Langmuir–Blodgett (LB) technique. We chose tetramethylbenzidine (TMB) which is known to decrease the very large overvoltage involved in the direct oxidation of NADH. The electrochemical characterisation of the films allowed to determine the presence and the electrochemical activity of the mediator in the LB layers. The response of the sensing layer in solutions containing NADH was investigated by means of cyclic voltammetry and fluorescence spectroscopy. We observed that NADH has a strong affinity with the film, and it interacts with the TMB-containing phospholipid layers. In fact, we found that TMB molecules mediate the NADH oxidation reaction, even if the surrounding matrix relatively hinders their mobility.

Organic light-emitting diodes with improved hole-electron balance by using molecular layers of phthalocyanine to modify the anode surface

By using the Langmuir–Blodgett (LB) technique, well-defined molecular layers of 2,9,16-tri(tert-butyl)-23-(10-hydroxydecyloxy)phthalocyanine were fabricated. These LB films have been employed to modify an anode surface of organic light-emitting devices. The insertion of LB films between an indium tin oxide (ITO) and hole-transport layers leads to an increase in device efficiency as a result of an improvement of the balanced carrier injection. An external quantum efficiency of 0.88% and a brightness of 10840 cd/m2 for a device with a structure of ITO/two LB layers/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/tris(8-hydroquinolinato)aluminium (Alq3)/Al were obtained, while the external quantum efficiency and brightness for the device without LB layers of ITO/TPD/Alq3/Al were 0.51% and 5744 cd/m2, respectively.

Optimal deposition condition of chlorophyll a Langmuir–Blodgett film

The optimal deposition condition of chlorophyll a (Chl a) Langmuir–Blodgett (LB) film were investigated based on π– a isotherms, absorption spectrum and scanning tunneling microscopy (STM) observation. The π– a isotherms of Chl a LB film at various air temperatures and subphase pHs were obtained. Optimum temperature and pH for dense packing of Chl a LB layers are 25 °C and 7.0, respectively. The formation of J- and H-aggregations of the ordered chlorophyll a molecules was confirmed by absorption spectrum measurements. The well-ordered structure of Chl a LB layer was observed by STM. Thus, the densely packed Chl a LB layer could be fabricated by the proposed optimal deposition condition.

Spectroscopic properties of Langmuir–Blodgett films containing a potential-sensitive dye

This paper reports the incorporation of a potential-sensitive fluorescent dye, Di-8-ANEPPS, in Langmuir–Blodgett (LB) films mixed with amphiphiles of different nature. We used dihexadecylphosphate (DHP) and octadecyltrimethylammonium bromide (OTMA) as lipid matrices. Surface pressure versus molecular area isotherms reveal the presence of small interactions between the components in the mixture, resulting in the formation of microcrystallites and aggregates in the LB films. The optical characteristics of the mono- and multilayers were evaluated by UV–VIS absorbance and fluorescence emission spectroscopy and they were controlled for different mixing ratio and number of LB layers.

Emission Light and Multiple Surface Plasmon Excitations at Prism/Ag/Merocyanine Langmuir-Blodgett Films

Emission light through a prism has been investigated for prism/silver (Ag) thin film/merocyanine (MC) Langmuir-Blodgett (LB) films in the Kretschmann configuration. Emission light was observed by means of direct irradiation of a laser beam from air to the MC LB films, that is, reverse irradiation. The emission properties coincided with resonant conditions of surface plasmon (SP) excitations in the Kretschmann configuration. Emission due to SP excitations depended upon the position of the MC LB film in the LB hetero films of arachidic acid (C20)/MC/C20 on the Ag thin film. The strongest emission was observed at some separation of the MC LB layer from the metal surface. Emission properties were also investigated for the hetero LB films of MC and C20 on aluminum (Al) thin film and for the MC LB films with various thicknesses. It is thought that the phenomenon is useful for application to new devices utilizing multiple SP excitations.

Langmuir–Blodgett layers of metal complexes of 4,4′-bis-(1-butyl-pentyl)-[2,2′]bipyridinyl-6,6′-diamine

The amphiphilic ligand 4,4′-bis-(1-butyl-pentyl)-[2,2′]bipyridinyl-6,6′-diamine (BBPBPDA) was synthesized by the Chichibabin reaction. It formed stable monolayers on the water surface which could be transferred onto solid substrates to construct Langmuir–Blodgett (LB) multilayers. The experimental data obtained by monolayer investigations on the water surface and X-ray investigations on multilayers were compared with the results of molecular modeling. In the presence of transition metal ions, BBPBPDA formed metal complexes with 1:1 stoichiometry. Here, the investigations were focused on complex formation with Cu 2+ and the construction of LB multilayers thereof. This was possible by transferring a monolayer of the copper complex formed by an interfacial reaction of the ligand monolayer on an aqueous subphase with dissolved Cu 2+ ions to solid substrates as well as by exposing an uncomplexed ligand LB layer to a copper ion containing solution.

Interaction of Langmuir–Blodgett Layers Bearing Amino Groups with Sulfur Dioxide

The adsorption of SO2 on thin Langmuir–Blodgett layers of long-chain molecules containing tertiary amino groups is studied by the polarization method. The study was performed at room temperature using 10- to 25-nm-thick layers in the SO2 concentration range of 100–3000 mg/m3. It is shown that the amount of the adsorbed sulfur dioxide is proportional to its concentration in the gas phase.

Uncertainty and Invariants of Inverse Problem Solutions for a First-Order Reaction

The problem of uncertainty of inverse problem solution for a first-order reaction with rate constant distribution of reactive states is considered. Although every distribution determines the reaction kinetics unambiguously, the inverse problem solving recovers a set of distributions in accordance with the same kinetic data. A choice of one of them is shown to be impossible in principle because all distributions give the same description of the reaction kinetics, and uncertainty of the solution remains even if the accuracy and number of measurements are increased many times. A method for the calculation of a set of distributions is proposed. The method can be applied to systems with both discrete and continuous sets of states. The distributions belonging to the set of inverse problem solutions are shown to have invariants, which can be determined from the initial moments of normalized distributions or directly from data on the reaction kinetics. The physical meaning of the invariants is considered as statistical rate constants for states with high, medium, and low reactivities that characterize the width of the true distribution and the degree of its asymmetry. The application of the invariants is considered for particular examples of the fluorescence decay of Rhodamine 6G in porous glasses and of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin in the Langmuir–Blodgett layers. Despite uncertainty of the inverse problem solution, the application of the invariants gives rise to unambiguous characteristics of the reactivity of compounds in reactions with distributed parameters.