Photoluminescence of a covalent assembled porphyrin-based monolayer: optical behavior in the presence of O2

The optical O(2) recognition capability of a covalently assembled monolayer (CAM) of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin on silica-based substrates was studied at room temperature by both UV-vis and photoluminescence (PL) measurements. The optical properties of this robust monolayer setup appear to be highly sensitive to the O(2) concentration in N(2). Both UV-vis and PL measurements were used to study the porphyrin-oxygen interactions. The monolayer-based sensor exhibits a short response time and can be restored within seconds. The oxygen-induced luminescence quenching of the monolayer involves both ground and excited states. The proposed mechanism responsible for the luminescence quenching involves different kinds of interactions between the monolayer and O(2).     

Memory effects of monolayers and vesicles formed by the non-ionic surfactant, 2C(18)E(12)

The behaviour of monolayers and bilayers formed by the dialkyl chain non-ionic surfactant, 1,2-di-O-octadecyl-rac-glycerol-3-omega-methoxydodecaethylene glycol (2C(18)E(12)) in water at 297 K has been investigated. Using a surface film balance (or Langmuir trough) the compression-expansion cycle of the 2C(18)E(12) monolayer was found to be reversible when compressed to surface pressures (pi) less than 42 mN m(-1). Compression of 2C(18)E(12) monolayer to pi greater than 42 mN m(-1) above this resulted in a considerable hysteresis upon expansion with the pi remaining high relative to that obtained upon compression, suggesting a time/pressure dependent re-arrangement of 2C(18)E(12) molecules in the film. Morphology of the 2C(18)E(12) monolayer, investigated using Brewster angle microscopy, was also found to depend upon monolayer history. Bright, randomly dispersed domains of 2C(18)E(12) of approximately 5 mum in size were observed during compression of the monolayer to pi less than 42 mN m(-1). At pi of 42 mN m(-1) and above, the surfactant film appeared to be almost completely 'solid-like.' Regardless of the extent of compression of the monolayer film, expansion of the film caused formation of chains or 'necklaces' of individual surfactant domains, with the extent of chain formation dependent upon pressure of compression of the monolayer and the length of time held at that pressure. Irreversible effects on 2C(18)E(12) vesicle size were also seen upon temperature cycling the vesicles through their liquid-crystalline phase transition temperature with vesicles shrinking in size and not returning to their original size upon standing at 298 K for periods of more than 24 h. No comparable hysteresis, time, pressure or temperature effects were observed with the monolayer or vesicles formed by the corresponding phospholipid, disteaorylphosphatidylcholine, under identical conditions. The effects observed with 2C(18)E(12) are attributed to the ability of the polyoxyethylene head group to dehydrate and intrude into the hydrophobic chain region of the mono- and bilayers. These studies have important implications for the use of the vesicles formed by 2C(18)E(12) as drug delivery vehicles.     

X-ray reflectivity and interfacial tension study of the structure and phase behavior of the interface between water and mixed surfactant solutions of CH3(CH2)19OH and CF3(CF2)7(CH2)2OH in hexane

The interface between water and mixed surfactant solutions of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH in hexane was studied with interfacial tension and X-ray reflectivity measurements. Measurements of the tension as a function of temperature for a range of total bulk surfactant concentrations and for three different values of the molal ratio of fluorinated to total surfactant concentration (0.25, 0.28, and 0.5) determined that the interface can be in three different monolayer phases. The interfacial excess entropy determined for these phases suggests that two of the phases are condensed single surfactant monolayers of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH. By studying four different compositions as a function of temperature, X-ray reflectivity was used to determine the structure of these monolayers in all three phases at the liquid-liquid interface. The X-ray reflectivity measurements were analyzed with a layer model to determine the electron density and thickness of the headgroup and tailgroup layers. The reflectivity demonstrates that phases 1 and 2 correspond to an interface fully covered by only one of the surfactants (liquid monolayer of CH(3)(CH(2))(19)OH in phase 1 and a solid condensed monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH in phase 2). This was determined by analysis of the electron density profile as well as by direct comparison to reflectivity studies of the liquid-liquid interface in systems containing only one of the surfactants (plus hexane and water). The liquid monolayer of CH(3)(CH(2))(19)OH undergoes a transition to the solid monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH with increasing temperature. Phase 3 and the transition regions between phases 1 and 2 consist of a mixed monolayer at the interface that contains domains of the two surfactants. In phase 3 the interface also contains gaseous regions that occupy progressively more of the interface as the temperature is increased. The reflectivity determined the coverage of the surfactant domains at the interface. A simple model is presented that predicts the basic features of the domain coverage as a function of temperature for the mixed surfactant system from the behavior of the single surfactant systems.     

Structural and topographical characteristics of dipalmitoyl phosphatidic acid in Langmuir monolayers

Dipalmitoyl phosphatidic acid (DPPA) monolayers at the air-water interface were studied from surface pressure (Pi)-area (A) isotherms and at the microscopic level with Brewster angle microscopy (BAM) under different conditions of temperature, pH, and ionic strength. BAM images were recorded simultaneously with Pi-A isotherms during the monolayer compression-expansion cycles. DPPA monolayers show a structural polymorphism from the liquid-expanded (LE)-liquid-condensed (LC) transition region at lower surface pressures toward liquid-condensed and solid (S) structures at higher surface pressures. An increase in temperature, pH, or ionic strength provokes an expansion in the monolayer structure. The results obtained from the Pi-A measurements are confirmed by the monolayer topography and relative reflectivity. The measurements of relative reflectivity upon monolayer compression showed an increase in relative monolayer thickness of 1.25 and 3.3 times throughout the full monolayer compression from the liquid-expanded to the liquid-condensed and solid states, respectively.     

Mono- and multi-layer adsorption of an ionic liquid on Au(110)

Ultraviolet photoelectron spectroscopy (UPS), work function measurements, low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been used to study the adsorption and desorption of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [C(2)C(1)Im][Tf(2)N], on the (1×2) clean surface reconstruction of Au(110) in the temperature range 100-674 K. The ionic liquid adsorbed without decomposition, and desorbed without leaving any residue on the surface. For adsorption at room temperature a monolayer of strongly bound ionic liquid was formed with four interface states visible in UP spectra. STM at 100 K showed that the monolayer consisted of well-ordered rows of adsorbed ionic liquid aligned parallel to the close packed rows of surface gold atoms (the [110] direction) with a separation of ×2 (the same as the clean surface reconstruction) between the rows in the orthogonal [001] direction. Multilayer adsorption at room temperature occurred by droplet formation followed by smoothing of the droplets to a layered morphology with time. Heating caused multilayer desorption at temperatures in the 363-383 K range, followed by partial monolayer desorption at 548 K to produce a Au(110)-(1×3) reconstructed surface with sub-monolayer domains of ionic liquid. Desorption of the remaining ionic liquid at 600 K caused the gold surface to reconstruct back to the clean (1×2) reconstruction.     

Proximity effects in monolayer films: kinetic analysis of amide bond formation at the air-water interface using (1)H NMR spectroscopy

The kinetics of amide bond formation in a monolayer film has been studied by proton NMR spectroscopy. Compression of a hexadecyl thioester of N-acetyl glycine (1) and a hexadecyl amide of glycine (2) at the air-water interface produces a single dipeptide product (4) that remains at the surface once formed. Extraction of the reaction mixture from the interface, followed by (1)H NMR spectroscopy, provides quantitative data on the rate of product formation. The kinetics of this reaction was examined as a function of surface pressure, subphase pH, and temperature. The monolayer provides an effective molarity for the reaction of approximately 500 M as compared to the bimolecular reaction of 1 and 4 in chloroform solution. The first-order rate constant for the reaction of 1 and 2 in the monolayer is less than 70-fold slower than k(cat) for condensation of the first amide bond in the enzymatic synthesis of the cyclic antibiotic gramicidin S by gramicidin S synthetase. Activation energies of the reaction were extracted from the temperature dependence of the rate constants of the reaction and are 9.9 +/- 1.0 and 2.1 +/- 0.2 kcal/mol for the chloroform solution and monolayer reactions, respectively. The pK(a) of 2 in the monolayer was estimated to be approximately 0.5 pK(a) units lower than that of related amines in solution. The lower pK(a) at the interface as compared to that in solution may be ascribed to increased electrostatic repulsion at the interface relative to solution. The rate of reaction in the monolayer was also followed by monitoring changes in surface area as a function of time. The rate constant for the reaction of 1 and 4 as determined by changes in surface area differs significantly from the rate determined by NMR. The results indicate that measurements of surface area versus time may yield erroneous rate constants for reactions in monolayers.     

Ultrathin chromium oxide films on the W(100) surface

Ultrathin chromium oxide films were prepared on a W(100) surface under ultrahigh-vacuum conditions and investigated in situ by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy electron diffraction. The results show that, at Cr coverage of less than 1 monolayer, CrO2 is formed by oxidizing pre-deposited Cr at 300-320 K in approximately 10(-7) mbar oxygen. However, an increase of temperature causes formation of Cr2O3. At Cr coverage above 1 monolayer, only Cr2O3 is detected.     

Ellipsometric and neutron diffraction study of pentane physisorbed on graphite

High-resolution ellipsometry and neutron diffraction measurements have been used to investigate the structure, growth, and wetting behavior of fluid pentane (n-C(5)H(12)) films adsorbed on graphite substrates. We present isotherms of the thickness of pentane films adsorbed on the basal-plane surfaces of a pyrolytic graphite substrate as a function of the vapor pressure. These isotherms are measured ellipsometrically for temperatures between 130 and 190 K. We also describe neutron diffraction measurements in the temperature range 11-140 K on a deuterated pentane (n-C(5)D(12)) monolayer adsorbed on an exfoliated graphite substrate. Below a temperature of 99 K, the diffraction patterns are consistent with a rectangular centered structure. Above the pentane triple point at 143.5 K, the ellipsometric measurements indicate layer-by-layer adsorption of at least seven fluid pentane layers, each having the same optical thickness. Analysis of the neutron diffraction pattern of a pentane monolayer at a temperature of 130 K is consistent with small clusters having a rectangular-centered structure and an area per molecule of approximately 37 A(2) in coexistence with a fluid monolayer phase. Assuming values of the polarizability tensor from the literature and that the monolayer fluid has the same areal density as that inferred for the coexisting clusters, we calculate an optical thickness of the fluid pentane layers in reasonable agreement with that measured by ellipsometry. We discuss how these results support the previously proposed "footprint reduction" mechanism of alkane monolayer melting. In the hypercritical regime, we show that the layering behavior is consistent with the two-dimensional Ising model and determine the critical temperatures for layers n = 2-5.     

Dynamic viscoelastic properties of spread monostearin monolayer in the presence of glycine

The monostearin monolayer at the air-aqueous interface is more expanded in presence of glycine and at higher temperature from both the surface pressure-area per molecule (pi-A) isotherms and static elasticity-surface pressure (E(s)-pi) curves. The dilational viscoelastic properties of monostearin monolayer spread on the subphase of glycine solution have been determined by the dynamic oscillation method and discussed as a function of surface pressure, temperature, and frequency. At the frequency of 50 mHz, the monostearin monolayer on pure water shows negative dilational viscosity and is viscoelastic at some surface pressures, while the monostearin monolayer in the presence of glycine is nearly elastic over a wide range of surface pressure, especially at 25 degrees C. Both positive and negative loss angle tangent can be deduced as a function of surface pressure. The negative dilational viscosity can be attributed to the phase transitions induced by the propagation of the surface waves during the dynamic oscillation. It can be convinced that the interactions between monostearin and glycine play an important role in the formation and rheological behavior of the monolayer. On the other hand, temperature has effect on the dilational elasticity and the dilational viscosity of the monostearin monolayer in different extents. Furthermore, at the surface pressure of 20 mN/m, the monostearin monolayer on the glycine solution at 18 degrees C is essentially elastic at lower frequency (<100 MHz) and shows viscoelastic behavior at higher frequency. These phenomena should be associated with the complicated monolayer structure and structural reorganization due to the interactions between monostearin and glycine in presence of glycine.     

ZnO/γ-Al2O3体系的状态和表面结构的研究(Ⅰ)

本文用X射线衍射和X射线光电子能谱法研究了用浸渍法制取的ZnO/γ-Al_2O_3样品。实验表明, ZnO 极易与γ-Al_2O_3发生固相反应, 经240 ℃烘烤就可以观察到反应的进行。样品的结构和形态依其组成和焙烧条件而异。氧化锌含量较低的样品由缺锌铝酸锌构成, 基表相Zn浓度取决于样品组成和焙烧温度。氧化锌含量较高的样品分两种情况: 低温焙烧的样品由Zn_xAl_2O_(3+x)(x<1)和晶相ZnO构成; 高温焙烧的样品可生成化学计量比的ZnAl_2O_4, 其上还单层分散着一层ZnO。仅当ZnO/ZnAl_2O_4中ZnO的含量超过此分散量时, 才有晶相ZnO析出。     

CrO3在γ-Al2O3和SiO2载体表面的分散状态

用X光衍射(XRD)方法研究了CrO_3/SiO_2和CrO_3/γ-Al_2O_3体系。用相定量外推法测定活性组份在载体表面的最大分散量。在干燥气氛中将CrO_3与载体混合, 并在低于CrO_3熔点的温度下烘烤制备样品, 实验得到CrO_3在SiO_2或者γ-Al_2O_3表面的最大分散量都随温度的升高而增大。CrO_3在SiO_2表面的最大分散量由101 ℃的0.27gCrO_3/g SiO_2到170 ℃的0.38g CrO_3/g SiO_2; CrO_3在γ-Al_2O_3表面的最大分散量由120 ℃的0.22g CrO_3/g γ-Al_2O_3到171 ℃的0.42g CrO_3/g γ-Al_2O_3。CrO_3在SiO_2或γ-Al_2O_3表面的最大分散量超过密置单层量, 可由易聚合形成同多酸根来解释。     

硅表面含二茂铁基团自组装单分子膜制备及电流-电压关系测量

合成了一种头基为二茂铁基团的长链烯烃分子(Fc-CO-NH-(CH₂)₉-CH=CH₂) (FcUA)，并用红外、核磁、质谱等手段对其进行了表征。用微波引发将该化合物嫁接到平面硅的表面，并用X射线光电子能谱、反射红外光谱、原子力显微镜表征了这一过程。最后，通过循环伏安电化学法和电敏感原子力显微镜的导电模式对其进行电学性质测试。结果表明这层单分子膜可以提高硅片的介电常数。同时还观察到了一种不稳定的类似负微分电流现象。     

单层 FeO 薄膜表面周期性氧缺陷结构的形成

 采用扫描隧道显微镜和 X 射线光电子能谱对含有次表层 Fe 的 Pt 表皮结构 (Pt skin), 即 0.4 ML Fe 的 Pt/Fe/Pt(111) 表面, 在 1.1 × 10?7 kPa 氧气气氛退火过程中的变化进行了研究. 结果表明, 当退火温度为 600 K 时, 氧气在 Pt/Fe/Pt(111) 表面上解离吸附并诱导表面局域结构的重构; 750 K 时次表层 Fe 可以扩散到表面并被氧化; 当升至 850 K 时, 在样品表面形成单层 FeO 结构, 并且 FeO 表面具有周期性的缺陷. 这种缺陷是由于单层 FeO 薄膜的摩尔条纹单胞中 fcc 位上一个或多个氧原子缺失形成的, 其中多原子空位被确定为缺失 6 个氧原子所致. FeO 表面缺陷结构的研究为理解 Fe-Pt 催化剂在氧化气氛中的结构稳定性以及构造表面活性位提供一定的基础.     

Toward the growth of an aligned single-layer MoS2 film

Molybdenum disulfide (molybdenite) monolayer islands and flakes have been grown on a copper surface at comparatively low temperature and mild conditions through sulfur loading of the substrate using thiophenol (benzenethiol) followed by the evaporation of Mo atoms and annealing. The MoS(2) islands show a regular Moire? pattern in scanning tunneling microscopy, attesting to their atomic ordering and high quality. They are all aligned with the substrate high-symmetry directions providing for rotational-domain-free monolayer growth.     

Wetting and freezing of hexadecane on an aqueous surfactant solution: triple point in a 2-D film

Wetting of water by hexadecane has been investigated by ellipsometry as a function of the concentration of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) in the aqueous phase and temperature. Three phases are identified: a 2-D gas of hexadecane molecules and DTAB molecules, a 2-D liquid comprising a mixed monolayer of hexadecane and DTAB, and a 2-D 'solid' phase. Evidence is presented to support the hypothesis that the liquid-solid phase transition is actually a wetting transition in which a surface-frozen layer of pure hexadecane wets the liquid-like mixed monolayer of hexadecane and DTAB. The triple point, at which the three phases coexist, is located at a temperature of 17.3 degrees C and DTAB concentration of 0.75 mmol kg (-1). The slopes of the three phase boundaries are analyzed thermodynamically.     

Fatty-acid monolayers at the nematic/water interface: phases and liquid-crystal alignment

The two-dimensional (2D) phases of fatty-acid monolayers (hexadecanoic, octadecanoic, eicosanoic, and docosanoic acids) have been studied at the interface of a nematic liquid crystal (LC) and water. When observed between crossed polarizers, the LC responds to monolayer structure owing to mesoscopic alignment of the LC by the adsorbed molecules. Similar to Langmuir monolayers at the air/water interface, the adsorbed monolayer at the nematic/water interface displays distinct thermodynamic phases. Observed are a 2D gas, isotropic liquid, and two condensed mesophases, each with a characteristic anchoring of the LC zenithal tilt and azimuth. By varying the monolayer temperature and surface concentration we observe reversible first-order phase transitions from vapor to liquid and from liquid to condensed. A temperature-dependent transition between two condensed phases appears to be a reversible swiveling transition in the tilt azimuth of the monolayer. Similar to monolayers at the air/water interface, the temperature of the gas/liquid/condensed triple-point temperature increased by about 10 degrees C for a two methylene group increase in chain length. However, the absolute value of the triple-point temperatures are depressed by about 40 degrees C compared to those of analogous monolayers at the air/water interface. We also observe a direct influence by the LC layer on the mesoscopic and macroscopic structure of the monolayer by analyzing the shapes and internal textures of gas domains in coexistence with a 2D liquid. An effective anisotropic line tension arises from elastic forces owing to deformation of the nematic director across phase boundaries. This results in the deformation of the domain from circular to elongated, with a distinct singularity. The LC elastic energy also gives rise to transition zones displaying mesoscopic realignment of the director tilt or azimuth between adjacent regions with a sudden change in anchoring.     

Ultrathin-film growth of para-sexiphenyl (I): submonolayer thin-film growth as a function of the substrate temperature

The para-sexiphenyl (p-6P) monolayer film induces weak epitaxy growth (WEG) of disk-like organic semiconductors, and their charge mobilities are increased dramatically to the level of the corresponding single crystals [Wang et al., Adv. Mater. 2007, 19, 2168]. The growth behavior and morphology of p-6P monolayer film play decisive roles on WEG. Here, we investigated the growth behavior of p-6P submonolayer film as a function of the substrate temperature. Its growth exhibited two different mechanisms at high and low substrate temperature. At high substrate temperature (>60 degrees C), the mechanism of diffusion-limited aggregation controlled the growth of submonolayer thin film with fractal islands, whereas at low substrate temperature (< or =60 degrees C), the submonolayer thin film was composed of the compact islands. Its growth exhibited another growth mechanism in which the stable compact islands were formed by dissociation and reorganization of the metastable disordered film. The substrate temperature of about 180 degrees C may be optimal to fabricate high-quality p-6P monolayer film with large-size domains and low saturated island density of about 0.018 microm(-2).     

Supercritical self-assembled monolayer growth

The growth of octadecyltrimethylammonium bromide (C(18)TAB) monolayers on mica was investigated using atomic force microscopy and infrared spectroscopy. A critical temperature was identified below which the monolayer formed via an "islanding" mechanism, that is, nucleation and growth of densely packed two-dimensional (2D) islands within a matrix of a disordered dilute phase. However, above the critical temperature, there was no coexistence of 2D phases during film formation. Instead, the monolayer gradually became better ordered, remaining laterally homogeneous throughout. We show that this corresponds to a critical point in a 2D phase diagram of the monolayer. Additional evidence is provided by the in situ observation of 2D phase separation upon cooling an incomplete monolayer from the one-phase to the two-phase region. The lack of coexisting domains (and domain boundaries) during growth above the critical point provides a possible route for the preparation of essentially defect-free monolayers.     

Influence of temperature on microdomain organization of mixed cationic-zwitterionic lipidic monolayers at the air-water interface

The thermodynamic behavior of mixed DOTAP-DPPC monolayers at the air-water interface has been investigated in the temperature range from 15 to 45 degrees C, covering the temperature interval where the thermotropic phase transition of DPPC, from solid-like to liquid-like, takes place. Based on the regular solution theory, the miscibility of the two lipids in the mixed monolayer was evaluated in terms of the excess Gibbs free energy of mixing DeltaG(ex), activity coefficients f(1) and f(2) and interaction parameter omega between the two lipids. The mixed DOTAP-DPPC film was found to have positive deviations from ideality at low DOTAP mole fractions, indicating a phase-separated binary mixture. This effect depends on the temperature and is largely conditioned by the structural chain conformation of the DPPC lipid monolayer. The thermodynamic parameters associated to the stability and the miscibility of these two lipids in a monolayer structure have been discussed in the light of the phase diagram of the DOTAP-DPPC aqueous mixtures obtained from differential scanning calorimetry measurements. The correlation between the temperature behavior of DOTAP-DPPC monolayers and their bulk aqueous mixtures has been briefly discussed.     

The study of the interaction of a model alpha-helical peptide with lipid bilayers and monolayers

We studied the interaction of the alpha-helical peptide acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)) with tethered bilayer lipid membranes (tBLM) and lipid monolayers formed at an air-water interface. The interaction of L(24) with tBLM resulted in adsorption of the peptide to the surface of the bilayer, characterized by a binding constant K(c)=2.4+/-0.6 microM(-1). The peptide L(24) an induced decrease of the elasticity modulus of the tBLM in a direction perpendicular to the membrane surface, E(radial). The decrease of E(radial) with increasing peptide concentration can be connected with a disordering effect of the peptide to the tBLM structure. The pure peptide formed a stable monolayer at the air/water interface. The pressure-area isotherms were characterized by a transition of the peptide monolayer, which probably corresponds of the partial intercalation of the alpha-helixes at higher surface pressure. Interaction of the peptide molecules with lipid monolayers resulted in an increase of the mean molecular area of phospholipids both in the gel and liquid crystalline states. With increasing peptide concentration, the temperature of the phase transition of the monolayer shifted toward lower temperatures. The analysis showed that the peptide-lipid monolayer is not an ideally miscible system and that the peptide molecules form aggregates in the monolayer.