Influence of alcohol on mechanical and electrical properties of thin organic films

Stability of organic materials properties is essential for further applications and was intensely investigated in last few decades. The aim of this study is to detect the structural changes of dipalmitoyl-phosphatidylcholine (DPPC) monolayer as a model system of organic material under influence of alcohols solutions. As subphases of monolayers (Langmuir films), pure water, ethanol and methanol solutions were used. For detection of changes in charge states of the molecules as well as relation with structural and conformational changes, a contactless method employing Maxwell’s displacement currents (MDC) was used. In DPPC monolayer on the subphase of methanol-water, a gradual absorption of methanol molecules into the layer can appear. In DPPC monolayer on the subphase of ethanol-water adsorption of ethanol molecules on the layer can be observed. Influence of alcohols results in a significant change of mechanical and electrical properties as well as in the stability of thin films. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

十八胺单分子膜的研究

通过测定在纯水和CdCl2 溶液亚相上十八胺单分子膜的平衡、循环π～A等温线及其动态弹性 ,发现在亚相中加入Cd2 + 可以使膜的液态相凝聚性增强 ,固态相凝聚性减弱 .液态的单分子膜在两种亚相上有较好的可回复性 ,而在固态膜中则不然 .这可归因于在水面上十八胺分子间可形成氢键 ,而在CdCl2 水溶液亚相上的十八胺则与Cd2 + 发生配位 ,形成了多配位络合物 ,两种情况下十八胺分子在高膜压区都会发生稳定的聚集 .静、动态弹性的比较表明 ,膜障的振动不利于十八胺分子与Cd2 + 间的配位作用     

Studies on behaviors of dipalmitoylposphatidylcholine and bilirubin in mixed monolayer at the air/water interface

Mixed monolayers of dipalmitoylposphatidylcholine (DPPC) and bilirubin (BR) were prepared on different subphases. The properties of DPPC/BR monolayer, such as collapse pressure (π_coll), limiting area per molecule (A_lim), surface compressibility modulus, free energy (ΔG_mix) and excess free energy (ΔG_ex), were investigated based on the analysis of the surface pressure-area isotherms on pure water. The results showed that DPPC and BR were miscible and formed non-ideal mixed monolayers at the air/water interface. With the molar fraction of BR (X_BR) increasing, the LE-LC coexistence region of DPPC monolayer was eliminated gradually. The DPPC/BR complex (M_D-B) of 1:2 stoichiometry formed as a result of the strong hydrogen bonds between the polar groups of DPPC and BR. The studies of effects of pH values and calcium ions in subphase on the DPPC/BR monolayers showed that the mixed monolayer became expanded on alkali aqueous solution and on 1 mmol/L CaCl₂ aqueous solution. The orientation of DPPC and BR at air/water interface was also discussed.     

Lanthanide ion induced formation of stripes domain structure in phospholipid Langmuir-Blodgett monolayer film observed by atomic force microscopy

Long-range ordered stripes domain structures were observed in Dipalmitoylphosphatidylcholine (DPPC) Langmuir-Blodgett monolayer film which was spread on the subphase of lanthanide ion (Eu³⁺) solution and transferred to a freshly cleaved mica substrate by vertical deposition. This novel phenomenon was discussed in terms of the competitive interaction of dipole-dipole and electrostatic interactions of the DPPC molecules combined with lanthanide ions with those DPPC molecules free of lanthanide ions.     

溴化N,N-二甲基二茂铁基十六烷基甲铵盐的合成、表征和性质研究

本文合成了新型两亲分子溴化N ,N 二甲基二茂铁基十六烷基甲铵盐 (FC1 6 AB) ,并利用UV Vis ,FTIR ,1 HNMR ,ESMS等手段进行表征。测量FC1 6 AB不同浓度的水溶液表面张力 ,表明其具有较好的表面活性。FC1 6 AB单分子膜的 π A曲线反映了FC1 6 AB成膜性能较好 ,亚相中抗衡离子的电荷和半径大小对FC1 6AB单分子膜有较大影响。     

Mixed 2D molecular systems: Mechanic, thermodynamic and dielectric properties

Study of Langmuir monolayers consisting of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC) molecules was done by surface pressure-area isotherms (π-A), the Maxwell displacement current (MDC) measurement, X-ray reflectivity (XRR) and atomic force microscopy (AFM) to investigate the selected mechanic, thermodynamic and dielectric properties based on orientational structure of monolayers. On the base of π-A isotherms analysis we explain the creation of stable structures and found optimal monolayer composition. The dielectric properties represented by MDC generated monolayers were analyzed in terms of excess dipole moment, proposing the effect of dipole-dipole interaction. XRR and AFM results illustrate deposited film structure and molecular ordering.     

Langmuir单分子膜的动态稳定性:

应用KSV LB－5000型Langmuir膜天平的VISCOS模式,对硬脂酸和19,21－二炔二十二碳酸（DDA）单分子膜在有无亚相离子、不同目标膜坟、不同膜障振动频率和不同温度下的动态稳定性作了研究。实验表明,单分子膜的目标膜压越低,膜障交变振动的频率越低,单分子膜越稳定,亚相中加有金属正离子对单分子膜的动态稳定性也有利。     

Effects of Calcium Ions on Thermodynamic Properties of Mixed Bilirubin/Cholesterol Monolayers

通过?-A等温线研究了含钙离子浓度不同的水溶液亚相上胆红素/胆固醇混合单分子膜的性质．在此基础上,采用数学方法计算了不同亚相上单分子膜的平均单分子面积、崩溃压、表面压缩系数、超额单分子面积、混合自由能和超额自由能．结果表明,在钙离子溶液亚相上混合单分子膜的结构发生扩张,并且在界面上形成非理想混合单分子膜．与理想混合时相比,这种混合单分子膜在超额单分子面积上表现为明显的正偏差．在纯水亚相上,胆红素与胆固醇的摩尔比为3:2时混合单分子膜为热力学最稳定体系．但是在钙离子浓度逐渐增大的亚相中,这种摩尔比的混合单分子膜热力学稳定性被破坏,即混合单分子膜中的不同组分间自发混合能力下降,这是由于受到钙离子与胆红素和胆固醇之间配位作用的影响．此外,在钙离子溶液亚相中,在较低的表面压下制备出的混合膜具有更好的热力学稳定性．     

Surface nano/micro functionalization of PMMA thin films by 157 nm irradiation for sensing applications

Laser irradiation at 157 nm of polymethylmethacrylate (PMMA) thin films induces major variations of polymer film thicknesses from sorption (absorption/desorption) of methanol and ethanol analytes in the gas phase as much as 400%, in comparison to the film thickness variation of the non-irradiated areas. The structural changes of irradiated areas involve scission of polymeric chains, cross-linking and formation of new bonds. In addition, 157 nm induces surface and volume morphological changes in the nano/micro domain, with different shapes, depending on the irradiation conditions. The reversibility of the sorption processes suggests that the polymer swelling has its origin at the tendency of the system to increase its volume during sorption. The internal forces from sorption are higher than the weak dipole interactions between the polymer and the analytes and they are amplified following 157 nm irradiation. A simple qualitative model explains adequately the experimental results. 157 nm laser treatment forms the basis to engineer a novel class of polymer sensor arrays with enhanced detection efficiency of liquid/gas analytes.     

Synthesis of graphene from naphthalene molecules on the surface of a Langmuir monolayer

This paper considers some approaches to the technology of the synthesis of a graphene monolayer at a phase interface. A surfactant monolayer on an aqueous subphase is proposed as the substrate for graphene synthesis. A monolayer is formed by the Langmuir–Blodgett method. Simple polyaromatic molecules, in particular, naphthalene, are considered as the basic substance for the synthesis of graphene. Arachidic acid is used as the basic surfactant molecule. To confirm the possibility of synthesizing graphene by the mentioned method, both experimental and theoretical studies are performed. In the course of experiemnts, it is shown that naphthalene molecules are pressed into the space above arachidic acid molecules upon the compression of monolayer of arachidic acid–naphthalene mixtures (such an assumption is made due to the characteristic value of the surface areas attributed to different phases of the monolayer and also to its characteristic parameters). The formation of a layer of naphthalene molecules on the surface of a monolayer is modeled by the molecular dynamics method (Amber potential). Different variants of the initial distribution of molecules are considered.     

Temperature effect on thin lipid film elasticity and phase separation: insights from Langmuir monolayer and fluorescence microscopy techniques

Langmuir monolayer pressure isotherms and compressibility modulus measurements of phospholipid mixtures in several Langmuir monolayer systems at the air/water interface were investigated in this study. The ultimate aim was to carry out a comparison of the elasticity modulus for monolayers with different mixtures of l,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chicken egg yolk sphingomyelin (eSM), in the presence/absence of cholesterol (Chol). In particular, we were able to propose that the leading force beyond the phase separation into liquid expanded (LE-) and liquid condensed (LC-) phases emerges from the increasing barrier to incorporate DOPC molecules into a highly ordered LC-phase. In addition, our findings suggest that DOPC lipid molecules have a priority to incorporate in a disordered LE-phase, while DPPC and eSM prefer the ordered one. Also, Chol seems to split almost equally into both phases, indicating that Chol has no priority for either phase and there are no particular interactions between Chol and saturated lipid molecules.     

Morphology of compressed dipalmitoyl phosphatidylcholine monolayers investigated by atomic force microscopy

The effectiveness of a substitute of natural lung surfactants on replacement therapy strongly depends on the stability of the monolayer of those substitute molecules. An atomic force microscope is utilized to investigate the microstructure of the films of the major components of natural lung surfactants, dipalmitoyl phosphatidylcholine—DPPC, which are transferred to mica substrates by the Langmuir-Blodgett film technique. A concave deformation structure was first observed for DPPC in solid phase. The depth of the concave domain was about 6 nm and was remarkably uniform. For a collapsed DPPC monolayer, the surface film consists of a granular convex multilayer structure and a disc-like concave structure. Dynamic cyclic compression-expansion experiments indicate that the formation of the concave domain is a reversible process while the process for convex multilayer formation is irreversible. This gives direct evidence that convex grain is the collapsed structure of DPPC monolayer and the concave shallow disc corresponds to the elastic deformation of a DPPC solid film. Results of atomic force microscopy indicate that the nucleation and growth model instead of the fracture model can better describe the collapse behavior of a DPPC monolayer.     

First-principles calculations of magnetic and electronic structures around surfaces and interfaces of magnetic multi-layered structure

Electronic and magnetic structures of ferromagnetic (FM)/non-magnetic (NM) and FM/antiferromagnetic (AF) bi-layer systems are calculated by the first principles approach. For the FM/NM system, we focus on the Co/Cu multi-layered structure whose interfacial layer is assumed to have a mixed composition of Co and Cu atoms, and show a possibility that Co atoms at the interface play a significant role as the spin-dependent scattering potentials. In the FM/AF system, we consider Fe or Co monolayer as FM layer and MnNi as AF layers. It is predicted that the Mn moments adjacent to FM layer are forced to align the FM moments, and those of under layer go gradually to anti-parallel alignment as in the bulk MnNi.     

Cells anchored upon a thin organic film with different nano-mechanical properties

By applying dynamic contact module and particular measurement of phase angles, harmonic contact stiffness (S) along with the measured displacement (D) of different self-assembled monolayers (SAMs) adsorbed on Au can be distinguished. The relatively ordered and hydrophobic ODT and DDT molecules adsorbed on Au form high contact stiffness, which are presumably unfavorable substrates for a cell to adhere upon. Short-chain MUA molecules adsorbed on Au provides a hydrophilic characteristic with a relatively low contact stiffness, which may significantly promote cell adhesion. It is, therefore, estimated that the behavior of a cell adhered on SAMs/Au is correlated not only with their outermost chemical species but also with a proper dS/dD matrix acting as a cushion.     

Self-assembled monolayers of radical molecules physisorbed on HOPG(0 0 0 1) substrate studied by scanning tunnelling microscopy and electron paramagnetic resonance techniques

In this paper, we present a combined STM and EPR study on the adsorption and self-organization of monolayers formed from 2-(14-Carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxy (16DS) and 4′,4′-Dimethylspiro(5α-cholestane-3,2′-oxazolidin)-3′-yloxy (CSL) adsorbed on a highly oriented pyrolitic graphite HOPG(0 0 0 1) substrate. Both 16DS and CSL molecules are persistent free radicals containing a paramagnetic doxyl group. The STM measurements of 16DS on HOPG(0 0 0 1) were performed at the liquid-solid interface while the studies of CSL on HOPG(0 0 0 1) were carried out under ultrahigh vacuum conditions. It was found that the 16DS molecules on the HOPG(0 0 0 1) surface form a highly-ordered monolayer with a domain structure. The high-resolution STM images show structural details of 16DS molecules on HOPG(0 0 0 1) revealing the paramagnetic doxyl group. In contrast, CSL molecules on HOPG(0 0 0 1) form a well-ordered monolayer without domain structure. The EPR results indicate that both compounds deposited on HOPG(0 0 0 1) substrate are not reduced and retain their paramagnetic character. We believe that the molecular systems described can be used in single spin detection experiments using an electron spin noise-scanning tunnelling microscopy (ESN-STM) technique. In particular, the possibility of obtaining contrast spin signals from the paramagnetic and diamagnetic parts of molecules increases the significance of our results.     

Role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on steel: An XPS study

The role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on mild steel (E24) surfaces was investigated by XPS. Fe 2p_3/2 and O 1s spectra show that the oxide/hydroxide layer developed on the steel surface, immersed in the diphosphonate solution (7 ≤ pH ≤ 13, without Ca²⁺) or in a filtered cement solution (pH 13, 15.38 mmol l⁻¹ of Ca²⁺), consists of Fe₂O₃, covered by a very thin layer of FeOOH (goethite). The total thickness of the oxide/hydroxide layer is ∼3 nm and is independent of the pH and the presence/absence of Ca²⁺. In the absence of Ca²⁺ ions, the N 1s and P 2p spectra reveal that the adsorption of the diphosphonate on the outer layer of FeOOH takes place only for pH lower than the zero charge pH of goethite (7.55). At pH 7, the adsorbed diphosphonate layer is continuous and its equivalent thickness is ∼24 Å (monolayer). In the presence of Ca²⁺ ions, the C 1s and Ca 2p signals indicate that calcium is present on the steel surface as calcium phosphonate (and Ca(OH)₂, in very small amount). The adsorption of the diphosphonate molecules on the steel surface is promoted in alkaline solution (pH > 7.55) by the doubly charged Ca²⁺ ions that bridge the O⁻ of goethite and the P-O⁻ groups of the diphosphonate molecules. The measured values for the Ca/P intensity ratio are in the range 0.75-1, which suggests that the diphosphonate molecules are adsorbed on steel forming a polymer cross-linked by calcium ions through their phosphono groups. In the presence of Ca²⁺ ions in alkaline solution, the adsorbed diphosphonate layer is discontinuous and the surface coverage is found to be ∼34%.     

Melting mechanism of monolayers adsorbed in cylindrical pores: An influence of the pore wall roughness

We have analyzed the mechanism of melting of layers adsorbed in cylindrical pores of porous materials. The goal was to understand the melting mechanism of simple fluids adsorbed in pores with heterogeneous wall surface. The studied system was a monolayer of methane molecules adsorbed in MCM-41 pore of diameter d = 4 nm. Both experimental (neutron scattering) and simulation (Monte Carlo) results proved extremely strong influence of the wall roughness on the melting mechanism. The most striking difference between melting on smooth and rough surfaces was in the temperatures of the transition. The transformation between solid-like and liquid-like monolayer phases adsorbed on a rough surface was observed in a very large temperature range and the solid like properties were observed even above the bulk methane melting temperature.     

Growth of pentacene on Ag(1 1 1) surface: A NEXAFS study

Thin films of pentacene (C₂₂H₁₄) have become widely used in the field of organic electronics. Here films of C₂₂H₁₄ of thickness ranging from submonolayer to multilayer were thermally deposited on Ag(1 1 1) surface. The determination of molecular geometry in pentacene films on Ag(1 1 1) studied by X-ray absorption at different stages of growth up to one monolayer is presented.XAS spectra at the C K-edge were collected as a function of the direction of the electric field at the surface. The different features of the spectra were assigned to resonances related to the various molecular unoccupied states by the comparison with the absorption coefficient of the pentacene gas phase. The transitions involving antibonding π states show a pronounced angular dependence for all the measured coverages, from submonolayer to multilayer. The spectra analysis indicates a nearly planar chemisorption of the first pentacene layer with a tilt angle of 10°.     

Adsorption mode of the chiral modifier cinchonidine on Au(1 1 1)

The adsorption of the chiral modifier cinchonidine on Au(l 1 1) in UHV has been studied by means of TPD, LEED and XPS. In the monolayer the molecule is bound via nitrogen lone pair electrons of its quinoline part rather than via the π-system of this aromatic moiety. Intact molecular desorption is only observed for the multilayers. Decomposition in the first monolayer upon heating occurs above 400 K, indicating a stronger interaction in the monolayer. No long-range ordered structures were observed via LEED. Long-time exposure leads to rearrangement and further stabilization of the first molecular monolayer. Quinoline is bound to gold via the nitrogen lone pair as well. The binding energy of 9.6 kcal/mol is characteristic for physisorption.     

Polymeric membrane studied using slow positron beam

A radioisotope slow positron beam has been built at the Chung Yuan Christian University in Taiwan for the research and development in membrane science and technology. Doppler broadening energy spectra and positron annihilation lifetime have been measured as a function of positron energy up to 30 keV in a polyamide membrane prepared by the interfacial polymerization between triethylenetetraamine (TETA) and trimesoyl chloride (TMC) on modified porous polyacrylonitrile (PAN) asymmetric membrane. The multilayer structures and free-volume depth profile for this asymmetric membrane system are obtained. Positron annihilation spectroscopy coupled with a slow beam could provide new information about size selectivity of transporting molecules and guidance for molecular designs in polymeric membranes.