Langmuir-Blodgett Layers Research Articles

Green and solid state reduction of GO monolayers sandwiched between Arachidic acid LB layers

A novel, single step and environment friendly solid state approach for reduction of graphene oxide (GO) monolayers has been demonstrated, in which, arachidic acid/GO/arachidic acid (AA/GO/AA) sandwich structure obtained by Langmuir-Blodgett (LB) technique was heat treated at moderate temperatures to obtain RGO sheets. Heat treatment of AA/GO/AA sandwich structure at 200 °C results in substantial reduction of GO, with concurrent removal of AA molecules. Such developed RGO sheets possess sp2-C content of ∼69 %, O/C ratio of ∼0.17 and significantly reduced I(D)/I(G) ratio of ∼1.1. Ultraviolet photoelectron spectroscopy (UPS) studies on RGO sheets evidenced significant increase in density of states in immediate vicinity of Fermi level and decrease in work function after reduction. Bottom gated field effect transistors fabricated with isolated RGO sheets displayed charge neutrality point at a positive gate voltage, indicating p-type nature, consistent with UPS and electrostatic force microscopy (EFM) measurement results. The RGO sheets obtained by heat treatment of AA/GO/AA sandwich structure exhibited conductivity in the range of 2–7 S/cm and field effect mobility of 0.03–2 cm2/Vs, which are comparable with values reported for RGO sheets obtained by various chemical/thermal reduction procedures. The extent of GO reduction is determined primarily by proximity of AA molecules and found to be unaltered with either escalation of heat treatment temperature or increase of AA content in sandwich structure. The single-step GO reduction approach demonstrated in this work is an effective way for development of RGO monolayers with high structural quality towards graphene-based electronic device applications.

SAW Sensor with Langmuir-Blodgett Layer for Detection of Benzene and its Derivatives

Vapors of benzene and its derivatives are harmful and toxic for human beings and natural environment. Their detection has fundamental importance. For this purpose authors propose surface acoustic wave (SAW) sensor with skeletonized layer deposited by Langmuir-Blodgett (L-B) method. This layer was obtained by depositing a binary equimolar mixture of 5-[[1,3-dioxo-3-[4-(1-oxooctadecyl) phenyl]propyl]amino]–1,3–benzenedicarboxylic acid with cetylamine. The skeletonized sensor layer has been obtained by removing cetylamine. Response of this sensor depends mainly of the electrical dipole momentum of molecule. Among the tested compounds, benzene has a zero dipole moment and gives the smallest sensor response, and nitrobenzene has the largest dipole moment and the sensor reacts most strongly to its vapor.

Stability Monomolecular Properties of Archaeal Tetraether Lipids Layers onto Solid Substrates

This paper reports the recent findings related to the stability properties of tetraether lipid layers. Organizations moleculars of chemical structure modified of Langmuir-Blodgett layers of archael tetraether lipids from the archebacterium Sulfolobus acidocaldarius on the wafer silicon substrates are investigated stable and organized. The behavior of Langmuir-Blodgett layers of chemical structure modified of archaeal tetraether lipids on the wafer silicon substrates is characterization using Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM). The thermodynamics behavior and stability of Langmuir-Blodgett layers of archael tetraether lipids on the wafer silicon substrates are shown. Stability of the lipid membranes is of great importance to a number of biomedical applications, including intravenous drug delivery, biomaterials, and biosensors.

Optimizing SERS performance through aggregation of gold nanorods in Langmuir-Blodgett films

Nanoparticles with a high degree of anisotropy are characterized by large electromagnetic field enhancements at the single-particle level; however, their arrangement into aggregated structures does not lead to significantly better performance. Surprisingly, for nanorods a superior accumulation of hot spots is present at surface coverages above 60%, which ensures a higher efficiency for surface-enhanced Raman spectroscopy (SERS). In this study, we produced pegylated gold nanorods in Langmuir-Blodgett layers in order to optimize SERS performance for the ultrasensitive detection of molecules, by tuning plasmonic coupling and nanoparticle surface coverage. To understand the arrangement of the nanorods in Langmuir and Langmuir-Blodgett layers, we performed spectroscopic and microscopic analysis of the obtained films. Based on the compression isotherms of the nanorods, we proposed the best conditions for nanoplatform design. The Raman enhancement factor was around 2·105 with a deposition surface pressure equal to 12 mN·m−1. Due to the polymer’s presence at the surface of the nanorods and the dewetting process, specific dendritic-like aggregates were observed on the solid substrate. The results obtained indicate the significance of further studies on the arrangement of metallic nanoparticles in monolayers, taking into account the size and shape of the nanoparticles, as well as the type of stabilizing ligands at their surface.

Supramolecular Reorientation During Deposition Onto Metal Surfaces of Quasi-Two-Dimensional Langmuir Monolayers Composed of Bifunctional Amphiphilic, Twisted Perylenes.

Supramolecular dye structures, which are often ruled by π-π interactions between planar chromophores, crucially determine the optoelectronic properties of layers and interfaces. Here, we present the interfacial assembly of perylene monoanhydride and monoimide that do not feature a planar chromophore but contain chlorine substituents in the bay positions to yield twisted chromophores and hence modified π-stacking. The assembly of the twisted perylene monoanhydride and monoimide is driven by their amphiphilicity that ensures proper Langmuir layer formation. The shielding of the hydrophilic segment upon attaching an alkyl chain to the imide moiety yielded a more rigid Langmuir layer, even though the degrees of freedom were increased due to this modification. For the characterization of the Langmuir layer's supramolecular structure, the layers were deposited onto glass, silver, and gold substrates via Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques and were investigated with atomic force microscopy and surface-enhanced resonance Raman spectroscopy (SERRS). From the similarity between all SERR spectra of the LS and LB layers, we concluded that the perylenes have changed their orientation upon LB deposition to bind to the silver surface of the SERRS substrate via sulfur atoms. In the Langmuir layer, the perylenes, which are π-stacked with half of the twisted chromophores, must already be inclined and cannot achieve full parallel alignment because of the twisting-induced steric hindrance. However, upon rotation, the energetically most favorable antiparallel aligned structures can be formed and bind to the SERRS substrate. Thus, we present, to the best of our knowledge, the first fabrication of quasi-two-dimensional films from twisted amphiphilic perylene monoimides and their reassembly during LB deposition. The relation between the molecular structure, supramolecular interfacial assembly, and its adoption during adsorption revealed here is crucial for the fabrication of defined functionalizations of metal surfaces, which is key to the development of organic (opto)electronic devices.

Electrowetting on Langmuir-Blodgett films from epicuticular wax

Langmuir and Langmuir-Blodgett (LB) films were fabricated from epicuticular wax, that had been extracted from Vitis vinifera. For Langmuir films, the influence of subphase temperature on their structure was examined, whereas for LB films, the surface morphology at various temperatures and the effect of the layer number on electrowetting were investigated. Isotherms of surface pressure revealed no well-defined phase transition or collapse on Langmuir films at 22 °C, whereas Brewster angle microscopy showed that even before starting the compression procedure, there are platelets of epicuticular wax floating on the subphase. Those platelets give rise to the Langmuir films of epicuticular wax during compression. In contrast, for higher temperatures (40 °C and 50 °C), a well-defined phase transition was found – without epicuticular wax platelets, with a behavior close to those found in typical fatty acids. For LB films, atomic force microscopy images indicated a decreasing of roughness as a function of the subphase temperature as a result of greater organization of the epicuticular wax film. A significant modulation of about 20 ° (under applied voltage range of 0 to 100 V) was demonstrated of the electrowetting on LB films, which did not depend on the LB layer number. Our results suggest that epicuticular wax LB films have the potential for being applied in electrowetting-based devices. Furthermore, the Langmuir-Blodgett technique is feasible to fabricate films with various compositions focusing on the electrowetting.

The liquid crystal induced J-type aggregation of diketopyrrolopyrrole derivatives in monolayer

The Langmuir and Langmuir-Blodgett techniques have been widely used for nearly a century for the production of molecular thin layers with controlled packing and intermolecular distances. Unfortunately, the Langmuir layers of non-amphiphilic molecules have unstable and unrepeatable parameters. The addition of chemical compounds such as liquid crystals can stabilize and prevent aggregation in the Langmuir layer. The aim of the study was to obtain and characterize Langmuir and Langmuir-Blodgett layers of selected dithiophene diketopyrrolopyrrole (DPP) derivatives and to improve their parameters by forming mixtures with 4‑octyl‑4′‑cyanobiphenyl (8CB). Two DPPs with alkyl chains symmetrically and asymmetrically attached to pyrrole unit were chosen for the investigation. The Langmuir layer of the asymmetrical DPP was unstable over time. For the symmetrically functionalized DPP, after transferring onto solid substrates, agglomerates formation was clearly seen. For DPPs/8CB mixtures, a significant improvement in stability over time was achieved. The confocal microscopy investigation revealed almost complete removal of agglomerate structures in the symmetrical DPP with 8CB. The results proved the miscibility of components at air/water interface and the stabilizing effect of 8CB on DPPs layers. Furthermore, the addition of 8CB forces DPP J-type aggregation in the Langmuir film. Above all, the stable layers with repeatable parameters, made of indistinctly amphiphilic DPP dyes, were shown to form at the air/water interface and on solid substrates.

Langmuir-Blodgett Films from Fluorescently Labelled Phospholipids Deposited on Surface Acoustic Wave Devices

Over the years several new effects in fluorescently labelled Langmuir monolayers or Langmuir-Blodgett (LB) films, deposited on solid support plates, have been reported. Mostly layers from Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD) have been studied. This molecule behaves exactly as the DPPE molecule which is part of biological membranes. So it is expected that DPPE-NBD molecules can serve as an appropriate matrix for implementation of selectively reacting proteins or enzymes while preserving their function. This combination can act as an active layer in biosensor applications for operation in either gas or liquid environments and this molecule could further increase the system sensitivity in such applications. Addition of the chromophore NBD head group also stabilizes the film and allows for multilayer deposition which is not possible for phospholipids. In this study, LB monolayers were deposited on glass substrates at different surface pressures and with or without Cadmium ions in the water subphase. They were studied using fluorescence spectroscopy and Atomic Force Microscopy (AFM). In view of possible biosensor applications, LB layer by layer deposition was performed on Rayleigh Surface Acoustic Wave (SAW) resonant devices working at 440 MHz. These can effectively be used to transduce the biosensor signal from the active layer by providing a mass proportional frequency output. Layers from DPPE-NBD that might also be suitable for possible SAW based biosensor applications were deposited at a surface pressure of 10 mN/m. AFM spectroscopy reveals the phase coexistence of liquid and solid phases as well as higher bilayer nanosized cylinders. Fluorescence intensity was strongly self-quenched at this pressure and fluorescence lifetime spectroscopy shows a complex behaviour with a 3-rd order exponential decay. A frequency downshift of 10 KHz per monolayer was measured with the coated SAW device which results in a deposited mass of about 1 ng/monolayer.

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.

Dynamics of Drop Impact Against Surfaces Covered with Langmuir-Blodgett Layers

The dynamics of sucessive impacts of water droplets against flat glass surfaces covered by Langmuir-Blodgett films of zinc stearate with 1, 3, 5 and 7 layers was investigated. The structure and resistance of monolayers to the impact was evaluated by using fast images of the drop deformation, Brewster angle microscopy (BAM) and contact angle measurements. Eventual disruption (erosion) of the layers was investigated by using sum-frequency vibrational spectroscopy (SFG).

Ordered and ultrathin reduced graphene oxide LB films as hole injection layers for organic light-emitting diode.

In this paper, we demonstrated the utilization of reduced graphene oxide (RGO) Langmuir-Blodgett (LB) films as high performance hole injection layer in organic light-emitting diode (OLED). By using LB technique, the well-ordered and thickness-controlled RGO sheets are incorporated between the organic active layer and the transparent conducting indium tin oxide (ITO), leading to an increase of recombination between electrons and holes. Due to the dramatic increase of hole carrier injection efficiency in RGO LB layer, the device luminance performance is greatly enhanced comparable to devices fabricated with spin-coating RGO and a commercial conducting polymer PEDOT:PSS as the hole transport layer. Furthermore, our results indicate that RGO LB films could be an excellent alternative to commercial PEDOT:PSS as the effective hole transport and electron blocking layer in light-emitting diode devices.

Indium–chlorine and gallium–chlorine tetrasubstituted phthalocyanines in a bulk system, Langmuir monolayers and Langmuir–Blodgett nanolayers – Spectroscopic investigations

The paper deals with spectroscopic characterization of metallic phthalocyanines (Pc’s) (indium and gallium) complexed with chlorine and substituted with four benzyloxyphenoxy peripheral groups in bulk systems, 2D Langmuir monolayers and Langmuir–Blodgett nanolayers. An influence of the molecular structure of dyes (the presence of metal and of substitutes attached to the phthalocyanine macroring) on the in situ measurements of light absorption is reported. Molecular arrangement of the phthalocyanine molecular skeleton in the Langmuir monolayers on water substrate and in the Langmuir–Blodgett nanolayers is evaluated. A comparison of the light absorption spectra of the phthalocyanine monolayers with the spectra of the dyes in solution supports the existence of dye aggregates in the monolayer. It was shown that the type of dye aggregates (oblique and H types) depends markedly on the dye molecular structures. The NIR-IR, IR reflection–absorption and Raman spectra are also monitored for Langmuir–Blodgett nanolayers in non-polarized and polarized light. It was shown that the dye molecules in the Langmuir–Blodgett layers are oriented nearly vertically with respect to a gold substrate.

The Role of Langmuir-Blodgett (Lb) Protein Thin Film in Protein Crystal Growth by Lb Nanotemplate and Robot

The crystal growth of biological macromolecules is a very complicated process involving numerous parameters, such as pressure, pH, micro-gravity, photochemical and mechanical induction of nucleation. The utilization of Langmuir–Blodgett (LB) thin film as a template plays an important role in order to produce well-diffracting, stable and radiation-resistant protein crystals. LB nanotemplate crystallization method is a modification of the classical hanging drop vapor diffusion method. We tested four standard proteins, changing different parameters such as protein and salt concentration, number of LB layers etc., in order to verify the real role of LB template. Our results suggest that LB template essentially acts in two way on crystallization: in nucleation and in crystal growth. The possible explanation of these effects is also discussed.

Nanoparticle‐Imprinted Polymers for Size‐Selective Recognition of Nanoparticles

Citrate-stabilized gold nanoparticles 15 nm and 33 nm in diameter were transferred concomitantly with a monolayer of positively charged polyaniline by Langmuir-Blodgett transfer at pH 5 onto a conducting indium-doped tin oxide (ITO) support. Films consisting of one to three layers of polyaniline with thicknesses of 1-3 nm were prepared and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. After electro-oxidation of the Au nanoparticles in 0.1 M KCl, cavities were left behind in the film that could be analyzed by SEM. These cavities were able to recapture analyte nanoparticles from a solution of pH 10 and showed size-exclusion properties. The amount of nanoparticles taken up by the cavities was conveniently analyzed by measuring the charge associated with the electro-oxidation of these particles in 0.1 M KCl after the film had been rinsed with water. The size-exclusion properties improved with the number of Langmuir-Blodgett layers transferred.

Spectroscopic properties and orientation of molecules in Langmuir–Blodgett layers of selected functionalized fullerenes

Vibrational properties of two fullerene derivatives: C60TZ-OT-5 (1) and C60TH-3HX (2) have been studied using infrared absorption and Raman scattering spectroscopies. Additionally, quantum chemical calculations of the equilibrium geometry and normal mode vibrations of these functionalized fullerenes were performed. It was stated that despite of distinct structural differences between the investigated molecules, their experimental spectra are quite similar and correspond well with the calculated ones. The orientation of the molecules in the Langmuir–Blodgett films was evaluated.

Estimation of contact angle for hydrophobic silica nanoparticles in their hexagonally ordered layer

Wetting properties of bilayered nanostructured coatings were studied in this work. Coatings were prepared by stratifying a compact silica sol–gel (SG) film and a Langmuir–Blodgett (LB) layer of differently sized (84, 131 and 227 nm) Stöber silica particles onto glass substrates. Joint high temperature annealing of the layers increased the mechanical stability of the particulate film facilitating its further investigations.Regular structure of LB layers allowed us to carry out wetting model investigations. Close-packed arrangement of particles was reinforced by optical measurements. Transmittance spectra of LB films were taken and evaluated by a theoretical model. Resulting effective refractive index and film thickness values indicated a good agreement between the real and the supposed (hexagonally close packed, monodisperse spherical particles) layer structure. It was confirmed by field emission scanning electron microscopy (FESEM) images, too.Advancing and receding water contact angles (CA) on differently (with mono- and bifunctional chlorosilanes) hydrophobized SG and combined films were measured applying the sessile drop method. SG films showed approximately the same CA-s after different silylation processes without considerable contact angle hysteresis (CAH). In case of combined films ca. 30° higher advancing CA-s were measured at stronger silylation and only the milder silylation resulted in significant CAH. It was explained by the surface heterogeneity of constituent particles. Layers of differently sized particles showed the same wetting properties in all cases in agreement to the Cassie–Baxter equation which was used for the estimation of CA values for the individual silica particles (88–90° and 105–106° depending on the silylation conditions).

Salt Dependent Stability of Stearic Acid Langmuir–Blodgett Films Exposed to Aqueous Electrolytes

We use contact angle goniometry, imaging ellipsometry, and atomic force microscopy to study the stability and wettability of Langmuir-Blodgett (LB) monolayers of stearic acid on silica substrates, upon drying and exposure to aqueous solutions of varying salinity. The influences of Ca(2+) and Na(+) ions are compared by varying their concentrations, both in the subphase before the LB transfer, and in the droplets to which the dried LB layers are exposed. Ca(2+) ions in the subphase are found to enhance the stability, leading to contact angles up to 100°, as compared to less than 5° for Na(+). Consistent with the macroscopic wettability, AFM images show almost intact films with few holes exposing bare substrate when prepared in the presence of Ca(2+), while subphases containing Na(+) result in large areas of bare substrate after exposure to aqueous drops. The observations on varying the composition of the droplets corroborate the stabilizing effect of Ca(2+). We attribute these findings to the cation-bridging ability of Ca(2+) ions, which can bind the negatively charged stearate groups to the negatively charged substrates. We discuss the relevance of our findings in the context of enhanced oil recovery.

Graphene Oxide as a Monoatomic Blocking Layer

Monolayer graphene oxide (mGO) is shown to effectively protect molecular thin films from reorganization and function as an atomically thin barrier for vapor-deposited Ti/Al metal top electrodes. Fragile organic Langmuir-Blodgett (LB) films of C(22) fatty acid cadmium salts (cadmium(II) behenate) were covered by a compressed mosaic LB film of mGO flakes. These hybrid LB films were examined with atomic force microscopy (AFM) and X-ray reflectivity, both with and without the metal top electrodes. While the AFM enabled surface and morphology analysis, the X-ray reflectivity allowed for a detailed structural depth profiling of the organic film and mGO layer below the metal top layers. The structure of the mGO-protected LB films was found to be perfectly preserved; in contrast, it has previously been shown that metal deposition completely destroys the first two LB layers of unprotected films. This study provides clear evidence of the efficient protection offered by a single atomic layer of GO.

Enhanced Photoluminescence of ZnO Langmuir–Blodgett Films on Gold-Coated Substrates by Plasmonic Coupling

The interaction between a nanostructured metal and a semiconductor can lead to advantageous enhancement of the emission properties of the semiconductor, which is important for light-emitting diode applications. Herein, we present a study of the interaction between ZnO nanoparticles (3 nm) and two types of gold nanostructures: 10-nm particles and 50-nm-thick films. Photoluminescence emission measurements were performed to characterize the effects of the gold on the emission properties of ZnO and to study the dependence of the interaction on the structure of Au. Stearic acid Langmuir–Blodgett layers were introduced onto the film to control the distance between the noble metal and the semiconductor. The largest (12-fold) enhancement in the emission intensity due to plasmonic coupling was observed for a continuous Au film with one spacer layer, that is, a 2.2-nm distance between the metal and the ZnO.

Layers of Polyaniline Nanotubes Deposited by Langmuir–Blodgett Method

The Langmuir–Blodgett (LB) method has long been used for the preparation of ultrathin films, offering the unique control over the film thickness and the molecular orientation. Here, we report on the deposition and the characterization of LB layers of polyaniline (PANI) nanotubes on solid supports. Stable suspension of the nanotubes in chloroform was obtained by adding sodium dodecyl sulfate (SDS). The nanotubes to SDS ratio is important for the stability of the suspension. Langmuir films were spread at the water/air interface and transferred on gold or ITO glass substrate. The electroactivity of obtained films was investigated using cyclic voltammetry. The degree of assembly of the PANI nanotubes in thin films was characterized by optical microscopy and scanning electron microscopy (SEM). To evaluate the molecular structure and the PANI oxidation state, UV–vis and Raman mapping were used. The PANI nanotube films on the gold support are electroactive with cyclic voltammetry responses closely resembling typ...