Surface roughness in Langmuir-Blodgett multilayer films studied by AFM and X-ray diffraction

Langmuir-Blodgett multilayer films consisting of up to 100 layers of cadmium-substituted arachidic acid on silicon wafers have been studied. The samples were deposited from a water subphase containing Cd²⁺ ions by using an alternate-layer Langmuir-Blodgett (LB) trough under computer control. Long-range orientational order was observed by atomic force microscopy on the surface of films with a small number of layers. X-ray reflectivity curves showed regularly spaced Bragg peaks arising from multilayer structure. The periodicity of the LB film is in agreement with twice the estimated molecular length, which confirms that the multilayer structure has a bilayer (Y) configuration. We demonstrate that the growth process of the LB films must have been stochastic. Our results are in good agreement with the Eden model of growth.     

X-ray reflectivity and diffraction studies on lipid and lipopolymer Langmuir-Blodgett films under controlled humidity

Synchrotron X-ray reflectivity and grazing incidence diffraction were applied to study the structures of phospholipid Langmuir-Blodgett films under controlled humidity. Distearoylphosphatidylethanolamine (DSPE) and DSPE-PEO lipopolymers, in which a poly(ethylene oxide) chain with n = 8, 17, or 45 EO units is covalently linked to the polar headgroup of DSPE, were used. When the relative humidity was changed from below 2 to 97%, the phosphate headgroup in a DSPE film was hydrated with a concomitant thickness increase of 4.2 A and an electron density decrease. The swelling of the monolayer was found to be a reversible process without any significant roughening of the film. Subtle differences in the film thickness could be detected for DSPE monolayers transferred under various lateral pressures. The degree of lateral ordering of the alkyl chains in DSPE monolayers increased considerably when the humidity was raised. In the case of DSPE-PEO with eight EO units, the hydration of the monolayer was also found to be reversible, but the water uptake was larger due to the presence of the hydrophilic polymer interlayer, which is located between the substrate and the DSPE moieties. Under high humidity, the lipopolymer monolayer with n = 17 exhibited a well-defined, layered structure similar to that of DSPE-PEO with n = 8. In the case of n = 45, however, there are indications for a significant intermixing of the polymer with the DSPE moieties, probably resulting in the formation of hydrophobic nanodomains of closely packed alkyl chains that are immersed into the hydrated polymer.     

Characterization of Langmuir-Blodgett organoclay films using X-ray reflectivity and atomic force microscopy

Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of 35 degrees +/-6 degrees from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites.     

X-ray studies on Langmuir-Blodgett films of novel amphiphilic oligothiophenes

A new type of amphiphilic thiophene, oligo[n-alkyl-3-(3-thienyl) glutaric acid], was synthesised (with n = 8, 12 or 18). These molecules form a stable monolayer at the air/water interface. Successful Langmuir-Blodgett (LB) deposition was achieved by vertical dipping. The LB films of Y-type exhibit well-defined layered structures, as determined by X-ray diffraction measurements, with a regular molecular arrangement within the layers.     

Template-directed patterning using phase-separated Langmuir-Blodgett films

The structures of the mixed Langmuir-Blodgett (LB) films of conventional amphiphiles (CAs) and amphiphilic silane-coupling agents (SCAs) were investigated using IR spectroscopy, atomic force microscopy, and friction force microscopy. By using CAs having hydrogenated alkyl chains and SCAs having perfluorinated alkyl chains, phase-separated structures were formed with domains consisting of CAs surrounded by SCAs. The size and shape of the domains depended strongly on the mixed components, the mixing ratios, and the subphase temperature. In particular, usage of a CA having hydrogenated and perfluorinated portions in the hydrophobic group as one of the components led to the formation of nanothreads. When the phase-separated mixed LB films were heated, SCAs formed covalent bonds with the substrates having silanol groups whereas CAs did not have such ability. Rinsing the heat-treated LB films with ethanol selectively removed CAs with the SCA regions intact, resulting in the fabrication of templates. The structures of the templates reflected those of the original phase-separated LB films. LB transfer of amphiphiles on the templates led to the confinement of the amphiphiles in regions with the size and shape delineated by the templates. These results demonstrate that a variety of amphiphiles can be confined two-dimensionally in a designed manner.     

X-ray and neutron reflectometry investigation of Langmuir-Blodgett films of cellulose ethers

Langmuir-Blodgett films of a series of cellulose ethers are investigated by X-ray and neutron reflectometry. Two types of samples are considered: simple alkyl ethers of cellulose and derivatives obtained by the alkylation of (2-hydroxypropyl)cellulose (HPC). All of the cellulose ethers form stable monolayers at the air-water interface. Significant differences are, however, found in the surface pressure-area compression isotherms. Ethers prepared from HPC typically exhibit larger limiting molecular areas and higher surface pressures than the corresponding simple cellulose ethers. The ease of monolayer transfer to hydrophobic silicon substrates differs greatly from one type of molecule to another. Successful transfer conditions are found only for ethers that form stable monolayers at sufficiently high surface pressures. Surprisingly, deuterated HPC ethers, prepared for neutron reflectivity measurements, exhibit monolayer properties significantly different from those of their hydrogenated analogues. Although essentially identical limiting molecular areas are found, the surface pressure corresponding to a characteristic plateau transition in the compression isotherm is found to decrease by about 8-10 mN m(-1) upon side chain deuteration. X-ray reflectivity results show a linear increase of film thickness with the number of deposited layers, indicating a regular and reproducible transfer. Observed average layer spacings are, however, significantly smaller than the calculated length of fully extended side chains. Neutron reflectivity curves recorded for composite LB films composed of both deuterated and hydrogenated polymers exhibit regular Keissig fringes, but no Bragg peak. This result indicates that these LB films do not possess an internal periodic structure and the initial layer-by-layer organization is lost by large interlayer diffusion.     

X-ray investigations of Langmuir-Blodgett multilayer films of side-chain liquid crystal copolymers

X-ray reflectivity studies on Langmuir-Blodgett multilayer films of side-chain liquid crystal polymers are reported. The films have a high degree of lamellar order. The layer periodicity is independent of the type of monolayer deposition, implying a reorientation of the side group mesogens following the deposition process. X-ray reflectivity from thin films displays subsidiary maxima permitting a quantitative measure of the change in side-chain density between multilayer and monolayer. A unit cell density profile is calculated for thick films assuming a symmetric unit cell.     

An X-ray reflectometry study of Cd- and Mn-substituted tricosanoate Langmuir-Blodgett films

The structures of Langmuir-Blodgett films of 8 and 14 bilayers of Cd²⁺- and Mn²⁺-substituted tricosanoate have been measured by X-ray reflectometry. The specular reflectivity profiles contain up to 21 orders of Bragg peak, which indicates that there is an almost perfect stacking of the bilayers normal to the surface of the films. These profiles also exhibit a characteristic variation in intensity between the even and odd orders of Bragg peak for all four samples. The lineshape of the off-specular scattering at the positions of the Bragg peaks was found to be Lorentzian. The half-width at half-maximum (HFHM) of these peaks varies according to the square of the longitudinal scattering vector q_z, which confirms the presence of self-affine rough interfaces in these samples. The magnitude of the interface roughness is found to be smallest in the films with the greatest number of layers. This is possibly associated with structural defects, caused by an incomplete transfer on to the substrate of the first few layers of the LB film, which can propagate through subsequent layers causing a phase shift of the bilayer structure whilst still preserving the overall bilayer period.     

Molecular orientation of asphaltenes and PAH model compounds in Langmuir-Blodgett films using sum frequency generation spectroscopy

Asphaltenes are an important class of compounds in crude oil whose surface activity is important for establishing reservoir rock wettability which impacts reservoir drainage. While many phenomenological interfacial studies with crude oils and asphaltenes have been reported, there is very little known about the molecular level interactions between asphaltenes and mineral surfaces. In this study, we analyze Langmuir-Blodgett films of asphaltenes and related model compounds with sum frequency generation (SFG) vibrational spectroscopy. In SFG, the polarization of the input (vis, IR) and output (SFG) beams can be varied, which allows the orientation of different functional groups at the interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen functionality have PAHs oriented transverse to the interface. Computational quantum chemistry is used to support corresponding band assignments, enabling robust determination of functional group orientations.     

Novel diffraction effects in the combined wide-angle and low-angle X-ray diffraction patterns of solution-grown nylon crystals

It was shown previously that in sedimented mats of solution-grown nylon crystals the 001 wide-angle reflection due to the unit-cell period along the chain and the low-angle reflection due to the lamellar periodicity are both along the same azimuth (meridian) in the x-ray diffraction pattern.¹ Presently in photographs which contain both the wide-angle and low-angle regions a series of new meridional reflections have been observed between the low-angle and 001 reflections and between 001 and 002 reflections. This finding implies that, with the large chemical repeat distances in question, diffraction effects at low and wide angles cannot be considered in isolation as is usual in most polymer crystal studies. In particular, the new reflections promise to provide a direct diffraction approach to the structure of the chain-folded lamellae.     

Formation of ring calcium oxalate patterns induced by domains in DPPC Langmuir-Blodgett films

The ring patterns of calcium oxalate crystals were induced by domains in Langmuir-Blodgett （LB） films of dipalmitoylpho- sphatidylcboline （DPPC）. The result was explained by the defects at the ring boundaries of liquid condensed （LC） and liquid expanded （LE） phases of LB film. These boundaries could provide less free energy and much more nucleating sites for COM crystals.     

Characterization of Langmuir-Blodgett film using differential charging in X-ray photoelectron spectroscopy

Differential charging is often regarded as a problem in X-ray photoelectron spectroscopy (XPS) studies, especially for insulating or partially conducting samples. Neutralization techniques have been developed to circumvent this effect. Instead of neutralizing the positive charge, which is often the technique to obtain good quality data, it is possible to exploit this phenomenon to get useful information about the sample. An attempt is made here to use this differential charging to study the mono- and multilayer Langmuir-Blodgett (LB) films of cadmium arachidate on silicon substrate. The surface potential was probed by measuring XPS line shift with respect to their neutral position and was found to have correlation with the thickness of the films. No differential charging was observed in the monolayer LB film where there was only one layer of cadmium headgroup. Significant differential charging was observed for multilayer films, the total charging as well as the differential charging in these films increase with increasing number of layers. Angle-resolved XPS measurements were performed to obtain additional information about the structure of the films. Charging of the upper layer of the films close to the vacuum interface was found to be less compared to that of the interior. The discrete cadmium layers were found to be more differentially charged compared to the continuous hydrocarbon stacks in the multilayer LB films. Charging of the discrete cadmium layers has been utilized to obtain quantitative information of the multilayer LB films.     

Determination of absolute configuration using X-ray diffraction

Methods for determination of absolute structure using X-ray crystallography are described, with an emphasis on applications for absolute configuration assignment of enantiopure light-atom organic compounds. The ability to distinguish between alternative absolute structures by X-ray crystallography is the result of a physical phenomenon called resonant scattering, which introduces small deviations from the inherent inversion symmetry of single-crystal X-ray diffraction patterns. The magnitude of the effect depends on the elements present in the crystal and the wavelength of the X-rays used to collect the diffraction data, but it is always very weak for crystals of compounds containing no element heavier than oxygen. The precision of absolute structure determination by conventional least squares refinement appears to be unduly pessimistic for light-atom materials. Recent developments based on Bijvoet differences, quotients and Bayesian statistics enable better and more realistic precision to be obtained. The new methods are sensitive to statistical outliers, and techniques for identifying these are summarised.     

Polyoxometalate monolayers in Langmuir-Blodgett films

Langmuir and Langmuir-Blodgett (LB) films of a variety of polyoxometalates of different shapes, sizes, and charges were prepared by taking advantage of the adsorption properties of these polyanions on a positively charged monolayer of an organic surfactant spread on water. Three different aspects were investigated. 1) The electrochemical and electrochromic properties of LB films containing the easily reducible polyoxoanion [P2Mo18O62]6-. Absorbance changes of these LB films deposited onto an ITO substrate have been induced by repeated switching of the applied potential. These changes are due to the formation of the colored reduced forms of the polyanion. Coloration and bleaching of the LB film occur very quickly and are reversible. 2) The preparation of LB films based on magnetic polyoxometalates, such as the Keggin anions, [CoW12O40]6- and [SiMn(OH2)W11O39]6-, or containing magnetic clusters of increasing nuclearities such as [Co4(H2O)2(PW9O34)2]10- and [Co4(H2O)2(P2W15O62)2]16- based on a Co4O16 ferromagnetic cluster, and the polyoxometalates [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- and [Ni9(OH)3-(H2O)6(HPO4)2(PW9O34)3]16- based on a nonanuclear M9O36 cluster. 3) The preparation of LB films of the giant heteropolyoxomolybdate, [Na3(NH4)12][Mo57Fe6(NO)6O174(OH)3-(H2O)24]76 H2O.     

Metal-molecule-metal junctions in Langmuir-Blodgett films using a new linker: trimethylsilane

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.     

Electron diffraction structural study of thin films of a charge transfer complex prepared by the Langmuir-Blodgett method

Structural information on thin Langmuir-Blodgett films of a charge transfer complex (N-docosylpyridinium TCNQ) is obtained by transmission and reflection electron diffraction. The data obtained complete the information supplied by other methods and cast a new light on some controversial points. In the precursor phase, the first layer has a specific structure in which TCNQ molecules with their faces normal to the substrate, display a long-range one-dimensional order. On subsequent layers, the TCNQ tend progressively to lie flat on the substrate, but in the conducting phase, the TCNQ molecules change their position again and are now upright on their shorter edge, grouped by pairs (dimers). To take into account all the reflections of the electron diffraction patterns obtained on the precursor phase, an original monoclinic cell is proposed with the parameters a = 13.85 Å, b = 14.65 Å, γ= 135°. An explanation of the 3D character of the layers on one hand and the partial disorder of the aliphatic chains on the outer surface on the other hand is suggested. The crystal size in the precursor phase depends particularly on the nature of the substrate and a tendency to epitaxial behaviour is detected. The structural results deduced from electron diffraction studies of the conducting phase are in good agreement with those obtained by X-rays.     

X-ray diffraction method for the investigation of the oxidation kinetics in thin iron films

Summary The time-dependent decrease of the intensity of X-ray diffraction peaks due to the interaction of the investigated metals with gases can be measured in situ with the help of a recently developed ultrahigh-vaccum diffraction chamber. We have used this method to study the oxidation kinetics of about 450 nm thick iron films. The activation energy obtained from the temperature dependence of the kinetics at a relatively high oxygen exposure corresponds to the well-known cation vacancy diffusion while distinctly smaller values are found for short duration of exposure. This result leads to the conclusion that grain boundary diffusion becomes effective in the films at the early stages of the oxidation process.

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Röntgenographisches Verfahren zur Untersuchung der Oxidationskinetik in dünnen Eisenschichten

Zusammenfassung Die zeitliche Änderung der Intensität von Röntgenbeugungsreflexen, die von der Wechselwirkung der untersuchten Metalle mit Gasen herrührt, kann in situ mit Hilfe einer kürzlich entwickelten Ultrahochvakuum-Röntgenbeugungskammer gemessen werden. Wir haben diese Methode zum Studium der Oxidationskinetik ca. 450 nm dicker Eisenfilme herangezogen. Die Aktivierungsenergie, die man aus der Temperaturabhängigkeit der Kinetik bei relativ hohen Einwirkungszeiten des Sauerstoffs erhält, entspricht der wohlbekannten Kationenleerstellendiffusion, während deutlich kleinere Werte für kurze Einwirkungszeiten gefunden werden. Dieses Ergebnis legt den Schluß nahe, daß zu Beginn des Oxidationsprozesses eine Korngrenzendiffusion wirksam wird.

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Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Thanks are due to Mrs Schuster of the Institute of Material Science I of our University for preparing the SEM micrographs.     

Langmuir-Blodgett films of diazobenzene molecules

Langmuir-Blodgett (LB) films from diazobenzene Sudan III have been investigated using surface potential measurements as a function of number of layers and deposition pressures, with the surface potential data being related to molecular dipole moments obtained from theoretical electronic structure calculations. The surface potential increased with the number of layers for SIII LB films, and then tended to saturate. Results from density functional theory (DFT) and UV-vis spectroscopy indicated that the increase is due to addition of layers with oriented molecular dipoles, with the saturation tendency being attributed to a decrease in the amount of material deposited in each layer. The surface potential increased with the surface pressure used for deposition, probably owing to a higher contribution from the vertical component of the dipole moment as a closer molecular packing, which is associated with decreasing conformational entropy, was reached.     

大黄游离羟基蒽醌的TG—DTA和XRD谱图特征分析

中药大黄属蓼科植物,性寒味苦,是中医临床常用药物,具有攻坚破积、活血化瘀、泻火凉血、清热解毒及抗菌、抗炎、调节免疫、抗肿瘤、止血和保健等作用[1].