Formation of langmuir-blodgett films containing two-dimensional monoatomic arrays of rare-earth cations

The work is focused on the preparation and characterisation of rare-earth containing stearic acid Langmuir-Blodgett (LB) films. Stearic acid monolayer behavior on the rare-earth containing aqueous subphase have been analyzed by thermodynamic measurements. The compression isotherm shape changed substantially under varying the anion type at the constant rare-earth cation content in aqueous subphase. The structure of formed multilayer rare-earth containing LB films was studied using small angle X-rays diffraction. The considerable structural defects in multilayer films were found when rare-earth chloride solution was exploited as aqueous subphase, caused presumably by electrostatic interactions at the monolayer surface under high binding of rare-earth cations. The use of monodentate complexones like acetic acid was proposed to form bulk phase quasi-neutral rare-earth complexes. The electroneutral ligand exchange reactions of such complexes with stearic acid monolayer allowed to neutralise monolayer surface under rare-earth cations binding and to form condenced monolayer below the triple-point temperature. The compression of such monolayer to the state of minimal compressibility and subsequent monolayer deposition resulted in the formation of high ordered LB films with high content of adsorbed multivalent rare-earth cations arranged in two-dimensional monoatomic arrays.     

Langmuir膜及LB膜中的金属参与

本文主要介绍了金属离子与Langmuir膜及LB膜相互作用中静电、配位等作用方式及其对膜相态和分子二维排列的影响。在此基础上探讨了Langmuir膜对金属离子的识别与传感。以Langmuir膜和LB膜为二维模板诱导无机盐定向生长作为金属/单分子膜结合的重要应用在文中也进行了讨论。通过举例展示了金属离子参与的Langmuir膜和LB膜催化有机反应的特点。最后对金属参与的Langmuir膜和LB膜在功能化和器件化等方面的研究也作了论述， 并通过介绍金属螯合类脂分子的Langmuir膜在蛋白质等生物大分子界面定向聚集研究中的应用表明了金属参与的Langmuir膜及LB 膜在生命科学研究中的意义。全文贯穿了金属结合调节Langmuir膜和LB膜组装结构以及通过金属结合导入功能基团进行有序组装的思想。     

Films of novel mesogenic molecules at air-water and air-solid interfaces

Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.     

芳香席夫碱衍生物在有序分子膜中的相行为与二维-三维形变

研究了芳香席夫碱的设计合成、界面相行为以及与铜离子的配位过程,并与离位合成的铜复合物的界面组装行为进行了对比.利用表面压-面积等温线、紫外光谱、红外光谱、原子力显微镜等一系列实验方法来详细表征组装膜.结果表明,席夫碱配体与铜复合物均可在气液界面上形成稳定的有序组装膜,并可以转移到固体基片上构筑多层膜.进而,席夫碱配体在纯水界面上表现出奇特的相变.在相变点前后,二维的平膜变化成了三维的纤维状组装结构.然而,对于铜离子亚相上的组装过程却没有出现较大的变化.两者之间明显的差异可归因于在气液界面超分子组装过程中的分子构型以及疏水性能的不同引起的.目前的工作为制备和调控有序组装膜提供了有益探索.     

Atomic Force Microscopy of freely suspended liquid crystal films transferred to octadecyltriethoxysilane self-assembled monolayers

Thin freely suspended films of 4-(2-methylbutyl)phenyl 4'-octylbiphenyl-4-carboxylate (8SI) in the smectic C phase were transferred to self-assembled monolayer coated glass and imaged by atomic force microscopy. Upon rapid cooling to room temperature, the smectic C quenches into a lamellar phase, exhibiting islands and single smectic layer steps. The surface of such films can be imaged with molecular resolution, revealing a partially disordered lattice of fixed molecular positions. Over a period of several days these films undergo a transition to a crystal phase having a different film morphology.     

Atomic Force Microscopy of freely suspended liquid crystal films transferred to octadecyltriethoxysilane self-assembled monolayers

Thin freely suspended films of 4-(2-methylbutyl)phenyl 4'-octylbiphenyl-4-carboxylate (8SI) in the smectic C phase were transferred to self-assembled monolayer coated glass and imaged by atomic force microscopy. Upon rapid cooling to room temperature, the smectic C quenches into a lamellar phase, exhibiting islands and single smectic layer steps. The surface of such films can be imaged with molecular resolution, revealing a partially disordered lattice of fixed molecular positions. Over a period of several days these films undergo a transition to a crystal phase having a different film morphology.     

AFM observations of phase transitions in molecularly thin films of a three-ring bent-core compound

Submonolayer thin films of a three-ring bent-core (or banana-shaped) compound, m-bis(4-n-octyloxystyryl)benzene (m-OSB), were vacuum-deposited on a mica surface, and a spontaneous transition from monolayer films to bilayer crystals was observed at room temperature, which was ascribed to the specific molecular shape and polar layered packing of the bent-core molecules [Tang et al. J. Phys. Chem. B 2004, 108 (34), 12921-12926]. The crystal nucleation and growth from the monolayer films as well as the melting phase transition from the bilayer crystals were investigated using atomic force microscopy (AFM). It was shown that after initial nucleation, the crystal growth was achieved through three pathways: direct absorption of molecules from monolayer films, molecular cluster diffusion, and quasi-Ostwald ripening. When annealing the bilayer crystals at elevated temperatures, morphological change from a bilayer to a monolayer was observed, and some new islands with fingerlike patterns were formed during this process, which resulted from a diffusion-controlled growth of the molten molecules. In general, the high-resolution AFM in combination with the molecularly thin m-OSB films provided us with direct visualization of nucleation, crystal growth, melting, and film morphology evolution on the mesoscopic scale, which are of fundamental interest from the theoretical viewpoint and are of central importance for the control of interfacial properties in practical applications.     

Phase behavior of mixed submonolayer films of krypton and xenon on graphite

Using the results of extensive Monte Carlo simulations in the canonical and grand canonical ensembles, we discuss the phase behavior of mixed submonolayer films of krypton and xenon adsorbed on the graphite basal plane. The calculations have been performed using two- and three-dimensional models of the systems studied. It has been demonstrated that out-of-plane motion does not affect the properties of the films as long as the total density is well below the monolayer completion and at moderate temperatures. For the total densities close to the monolayer completion, the promotion of particles to the second layer considerably affects the film properties. Our results are in a reasonable agreement with the available experimental data. The melting point of submonolayer films has been shown to exhibit non-monotonous changes with the film composition, and reaches minimum for the xenon concentration of about 50%. At the temperatures below the melting point, the structure of solid phases depends upon the film composition and the temperature; one can also distinguish commensurate and incommensurate phases. Two-dimensional calculations have demonstrated that for the xenon concentration between about 15% and 65% the adsorbed film exhibits the formation of a superstructure, in which each Xe atom is surrounded by six Kr atoms. This superstructure is stable only at very low temperatures and transforms into the mixed commensurate (√3×√3)R30° phase upon the increase of temperature. Such a superstructure does not appear when a three-dimensional model is used. Grand canonical ensemble calculations allowed us to show that for the xenon concentration of about 3% the phase diagram topology of monolayer films changes from the krypton-like (with incipient triple point) to the xenon-like (with ordinary triple point).     

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

The present review reports on the preparation and atomic-scale characterization of the thinnest possible films of the glass-forming materials silica and germania. To this end state-of-the-art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO₄ (X=Si,Ge) building blocks. A side-by-side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.     

Phase behavior and viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface

A trisilanol polyhedral oligomeric silsesquioxane (POSS), trisilanolcyclohexyl-POSS (TCyP), has recently been reported to undergo a series of phase transitions from traditional Langmuir monolayers to unique rodlike hydrophobic aggregates in multilayer films that are different from "collapsed" morphologies seen in other systems at the air/water interface. This paper focuses on the phase transitions and morphology of films varying in average thickness from monolayers to trilayers and the corresponding viscoelastic properties of trisilanolcyclohexyl-POSS molecules at the air/water interface by means of surface pressure-area per molecule (Pi-A) isotherms, Brewster angle microscopy (BAM), and interfacial stress rheometry (ISR) measurements. The morphology studies by BAM reveal that the TCyP monolayer can collapse into different 3D structures by homogeneous or heterogeneous nucleation mechanisms. For homogeneous nucleation, analysis by Vollhardt et al.'s nucleation and growth model reveals that TCyP collapse is consistent with instantaneous nucleation with hemispherical edge growth at Pi = 3.7 mN.m(-1). Both surface storage (Gs') and loss (Gs") moduli obtained by ISR reveal three different non-Newtonian flow regimes that correlate with phase transitions in the Pi-A isotherms: (A) A viscous liquidlike "monolayer"; (B) a "biphasic regime"between a liquidlike viscous monolayer and a more rigid trilayer; and (C) an elastic solidlike "trilayer". These observations provide interesting insights into collapse mechanisms and structures in Langmuir films.     

Kinetics for the collapse of trilayer liquid-crystalline disks from a monolayer at an air-water interface

Unlike surfactants considered in previous studies, when phosphatidylcholine (PC) monolayers collapse at constant surface tension to form a 3D bulk phase, surface area decreases at rates that slow. The different kinetics could result from collapse by a distinct mechanism. Rather than the transfer of molecules all along the interface between the monolayer and bulk phase, PC films can collapse by the folding and subsequent sliding of a bilayer over the monolayer. By this mechanism, molecules can transfer to collapsed trilayers through a locus of constant size. In this article, we use the theory of nucleation and growth to show analytically that during collapse, the area can decrease at rates that decelerate when each individual structure grows through a region of fixed dimensions. We also show that binary films of 30% dihydrocholesterol (dchol) and dipalmitoyl phosphatidylcholine (DPPC), which have previously been shown to form a homogeneous monolayer from which trilayer disks grow through a point, collapse with rates of area decay that slow, in agreement with our analytical expressions.     

Thin films of discotic liquid crystals and their applications

ABSTRACT

Discotic liquid crystals (DLCs) are considered as fascinating systems due to their unique property of self-assembly to yield different columnar structures. DLCs are organic semiconductors and create pathways for the development of numerous optical and electrical devices. The thin films of DLCs can be considered as low dimensional system which can exhibit remarkable optical and physical properties. In this article, we present a review on ultrathin films of some interesting DLC molecules at air–water and air–solid interfaces. The Langmuir monolayer and Langmuir–Blodgett films of DLC molecules are extensively studied. The ultrathin films of DLC molecules can yield highly anisotropic layer wherein the molecular orientation and aggregation can have large impact on the physicochemical properties of the film. Different surface phases with different molecular orientations as function of surface density and temperature can be obtained by forming the Langmuir monolayer of the DLC molecules at the air–water interface. The Langmuir monolayer in a particular phase can be deposited onto the active area of a device layer-by-layer by employing a highly controlled Langmuir–Blodgett technique. Here, we report some interesting results related on molecular orientation of the DLC molecules at different interfaces. Such aggregation of DLC molecules in ultrathin films may find applications in thin film-based electro-optical devices.     

在半金属表面上应力诱导的红荧烯晶态薄膜

报道了在Bi（001）衬底上从第一层开始形成的红荧烯晶体薄膜.随着覆盖度的增加,红荧烯薄膜的结构会发生从自组装到混合相的转变.在混合相中存在着红荧烯的晶体畴和分子自组装形成的畴壁.特别是,我们在这种晶体畴中发现了Kurdjunov-Sachs（KS）转动外延和巨大的压缩应力.随着覆盖度的增加,红荧烯薄膜会按照逐层生长的模式进行,直至第四个分子单层.我们认为,转动外延过程中所产生的巨大压缩应力是导致晶态红荧烯薄膜的形成原因.     

Ultrathin films of functionalised single-walled carbon nanotubes: a potential bio-sensing platform

ABSTRACT

The Langmuir monolayer is a special class of lyotropic liquid crystalline system wherein phase transition essentially depends on surface density, temperature and ion-content in the aqueous medium. The variety of surface phases can be transferred onto devices by the Langmuir–Blodgett (LB) technique. The Langmuir monolayer of pristine single-walled carbon nanotubes (SWCNTs) exhibited gas and liquid-like phases. The LB film of SWCNTs shows target surface pressure dependent interesting morphologies. The methane gas sensing using parallel alignment of SWCNTs was found to be better than that of randomly oriented SWCNTs. The SWCNTs can be functionalised chemically to enhance the ease of film processability and affinity towards analytes. These are essential parameters for the development of a sensor. In this article, we present our work on Langmuir monolayer and LB films of octadecylamine functionalised SWCNTs (ODACNTs) and its sensing application towards bio-analytes, e.g. L-aspartic acid and bovine serum albumin. The sensing performance of LB film of ODACNTs was compared with that of spin-coated films of ODACNTs. The sensing performance of LB films of ODACNTs indicated a potential platform for bio-sensing application.     

The influence of ageing in the kinetics of the electroreduction of gold oxide films of the order of a monolayer thickness

Gold oxide films of the order of a monolayer thickness are electroreduced under different potentiodynamic conditions. The electrochemical process is coupled to a chemical dissolution of the film and a solid phase transformation. Under controlled ageing the kinetics of the electroreduction reaction obey simple laws which are derived from reaction models already discussed in previous publications.     

Ultrathin films of perfluoropolyether-grafted polysiloxanes chemisorbed via alkylthiolate anchors to gold surfaces

Polysiloxanes grafted with both perfluoropolyether and alkyldisulfide side chains were synthesized and chemisorbed from dilute solutions to fresh gold surfaces. Polymer monolayer films form spontaneously from disulfide-gold mediated interfacial attachment, yielding highly hydrophobic films approximately 30 A thick. X-ray photoelectron spectroscopy measurements show that perfluoropolyether segments enrich the outer interface of these polymer films, consistent with partial phase segregation of perfluorospecies near the film interface. Time-of-flight secondary ion mass spectrometry of these films supports the presence of a perfluoropolyether-rich overlayer. Polarized grazing incidence reflection FTIR and NEXAFS spectra show no evidence for a consistent film anisotropy or off-vertical chain organization in these films, in contrast to recent observations with perfluoroalkyl-grafted polymer thin film analogues.     

Influence of single-wall carbon nanotubes on Langmuir–Blodgett films of ferroelectric liquid crystals as studied by atomic force microscopy

Langmuir–Blodgett films of ferroelectric liquid crystal (LC) doped with low concentration of single-wall carbon nanotubes have been prepared and characterised. Pressure–area isotherms show that the films are stable and have good spreading properties. The interaction between nanotubes and LC molecules in the monolayer was increased during barrier compression, resulting in increased surface pressure. We observed phase change with increasing nanotube concentration in ferroelectric LC matrix. Atomic force microscopy profiles indicate uniform deposition of material on single crystal silicon wafer.     

Control of CaCO3 crystallization by demixing of monolayers

In this paper we describe how to template a demixed monolayer into a spatially patterned inorganic replica. For this purpose a new amphiphilic monomer was synthesized which can be polymerized both in solution and in the monolayer of a Langmuir-Blodgett (LB) trough. Since it inhibits the crystallization of CaCO3, it can be used--in combination with stearic acid (nucleation-promotor)--to control CaCO3 crystals formed under the monolayer. Investigations of the two-component monolayer (Langmuir isotherms and AFM measurements of transferred films) show--in the biphasic region--demixing in solid analogue stearic acid domains and the liquid analogue phase of the monomer. Crystallization of CaCO3 starts under the stearic acid domains whose size varies from less than 100 nm to several tens of micrometers. The addition of poly(acrylic acid) into the subphase hinders the three-dimensional growth of CaCO3 crystals from the monolayer into the solution. Thus, it becomes possible to transfer the pattern of the demixed domains into an inorganic replica of CaCO3.     

Langmuir monolayers based on rigid wedge-shaped dendrons of benzenesulfonic acid

The structure formation of wedge-shaped monodendrons based on symmetric benzenesulfonic acid with different lengths of peripheral alkyl chains was studied in Langmuir monolayers and Langmuir–Blodgett (LB) films. A phase transition from the liquid-expanded state to the liquid-condensed state was observed on compression of the Langmuir monolayers of the dendrons containing dodecyl lateral chains. The transition is accompanied by the formation of star-shaped aggregates visualized by Brewster angle microscopy. The three-layer LB transfer results in the reorganization of the monolayer into regions of bi-, tetra-, and hexalayers on a solid substrate with a low coverage of the surface. Homogeneous liquid-condensed mono layers are formed for the dendrons with hexa- and octadecyl chains, and the film thickness achieved by the LB transfer corresponds to the monolayer alignment of the molecules with the surface coverage up to 90%. It was determined that varying the alkyl length of wedge-shaped dendrones based on symmetric benzenesulfonic acid leads to a change in phase behavior of Langmuir monolayers as well as Langmuir–Blodgett films formed by them.     

Monolayer organic field-effect transistors

Monolayer organic field-effect transistors(OFETs) are attracting worldwide interest in device physics and novel applications due to their ultrathin active layer for two-dimensional charge transport. The monolayer films are generally prepared by thermal evaporation, the Langmuir technique or self-assembly process, etc., but their electrical performance is relatively lower than corresponding thick films. From 2011, the performance of monolayer OFETs has been boosted by using the monolayer molecular crystals(MMCs) as active channels, which opened up a new era for monolayer OFETs. In this review, recent progress of monolayer OFETs, including the preparation of monolayer films, their OFET performance and applications are summarized.Finally, perspectives of monolayer OFETs in the near future are also discussed.