Resonant surface enhanced Raman scattering by ultra thin Langmuir-Blodgett polydiacetylene polymer films

Using resonant Raman scattering and surface enhanced Raman scattering techniques, changes in the structural and electronic properties of Langmuir-Blodgett (L-B) polydiacetylene films were observed as film thicknesses were increased from one monolayer (or one bilayer) to several bilayers. The L-B films, starting with a single monolayer (or one bilayer) in a disordered “red” phase, were found to change into a mixed phase (ordered “blue” and disordered “red”) as one or more additional bilayers were deposited. This is the first observation of a “disorder to order” transformation in a L-B film. The observed effect is attributed to the ordering brought about by interactions between the initial and subsequent L-B PDA bilayers.     

Probing the local order of single phospholipid membranes using grazing incidence x-ray diffraction

We report the first grazing incidence x-ray diffraction measurements of a single phospholipid bilayer at the solid-liquid interface. Our grazing incidence x-ray diffraction and reflectivity measurements reveal that the lateral ordering in a supported DPPE (1, 2-Dipalmitoyl-sn-Glycero-3-Phosphoethanolamine) bilayer is significantly less than that of an equivalent monolayer at the air-liquid interface. Our findings also indicate that the leaflets of the bilayer are uncoupled in contrast to the scattering from free standing phosphatidylcholine bilayers. The methodology presented can be readily implemented to study more complicated biomembranes and their interaction with proteins.     

Reaction between amorphous Si and crystalline Al in Al/Si and Si/Al bilayers: microstructural and thermodynamic analysis of layer exchange

Aluminium-induced crystallization of amorphous silicon (a-Si) in Al/Si and Si/Al bilayers was studied upon annealing at 250 °C by X-ray diffraction and Auger electron spectroscopy. The Al/a-Si bilayers and a-Si/Al bilayers were prepared by sputter deposition on single-crystal silicon wafers with a silicon-oxide film on top. During the isothermal annealing a layer-exchange process occurred in both types of bilayers. A continuous polycrystalline silicon (poly-Si) film was formed within, and thereby gradually replacing, the initial Al metal layer. The sublayer sequence in the original bilayer influenced the speed of the poly-Si formation and the layer-exchange process. After annealing, the Al fiber texture in the as-deposited bilayers had become stronger, the Al crystallites had grown laterally, and the macrostress in the Al layer had been released. The amorphous Si layer had crystallized into an aggregate of nanocrystals with {111} planes parallel to the surface, with a crystallite size of about 15–25 nm. An extensive analysis of the Gibbs energy change due to annealing showed that the layer exchange may be promoted by the release of elastic energy and grain growth for the Al phase. PACS 05.70.Jk; 61.43.Dq; 68.35.Rh; 61.72.Cc; 68.55.Jk     

Patterning silver nanocubes in monolayers using phase separated lipids as templates

Strategies for assembling silver nanocubes (NCs) into distinct 2D patterns on Langmuir–Blodgett (LB) films are demonstrated using two different lipid mixtures as vehicles: (1) raft mixtures containing 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), sphingomyelin (SPM), and cholesterol in different mole ratios (2:2:1 and 1:1:1) and (2) 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) at a 1:3 mol ratio. Atomic force microscopy was employed to unveil the mechanisms of such pattern formation in the LB film. The results demonstrate that aggregation of NCs into round-like pattern is governed by preferential localization of NCs within the liquid condensed (LC) domains of DOPC/SPM/Cholesterol mixture. Cholesterol was found to govern the size and shape of the rounded islands. On the other hand, incorporation of NCs within the liquid expanded (LE) phase of DPPC/DLPC mixture produced linear-branched chains, oriented normal to the Langmuir film transfer direction. The as engineered patterns of silver NCs exhibited characteristic plasmonic signatures. Our results reveal the potential in assembling plasmonic metal nanoparticles into diverse patterns on solid substrates by exploiting their preferential localization either in LC or LE phase of appropriate lipid mixture in Langmuir film.     

气/液界面自组装金纳米粒子薄膜作为SERS基底检测三聚氰胺

采用气/液界面自组装方法制备金纳米粒子薄膜作为SERS基底,其结构规整、均匀,利用此基底对三聚氰胺实现高灵敏的半定量分析。此SERS基底的制备是直接于水相合成的金纳米粒子中加人正十二硫醇,金纳米粒子通过硫醇修饰后由亲水性转变成疏水性质,在相界面上自组装为致密金纳米粒子单层膜结构。这种SERS基底不仅制备方法简单,而且应用范围广,除了检测三聚氰胺还可以拓展到其他的非极性的分子如多环芳烃等高灵敏的半定量分析。     

Organization of cyanobiphenyl liquid crystal molecules in prewetting films spreading on silicon wafers

We observe prewetting films of 8CB (4'-n-octyl-4-cyanobiphenyl) spreading at room temperature on silicon wafers by ellipsometry and x-ray reflectivity. Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-A-thick film. X-ray reflectivity, performed using a ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using x-ray reflectivity because it induces a modulation of the electron density along the normal to the surface. Furthermore, a study of the ellispometric profile of a drop heated in the nematic phase, where we observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.     

Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

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.     

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.     

Investigations of RhB Langmuir monolayer by Fluorescence Imaging Microscopy

This communication reports the surface pressure vs area per molecule isotherm and Fluorescence Imaging Microscopic studies of the formation of domain structure in the mixed Langmuir monolayer of RhB and Stearic acid (SA) at the air-water interface. Strong repulsive interaction between the unlike components leads to the phase separation and formation of microcrystalline domains at the air-water interface of the Langmuir monolayer. These domains can be directly visualized using fluorescence imaging microscope.     

AFM studies on Langmuir-Blodgett films of cholesterol

The Langmuir monolayer of cholesterol at the air-water interface exhibits a condensed phase in which the cholesterol molecules are aligned normal to the water surface. We have transferred the monolayer from water surface to different substrates by Langmuir-Blodgett (LB) technique and have studied their assembly by atomic force microscope (AFM). Our studies reveal that the aggregation of cholesterol molecules on hydrophobic surfaces leads to interesting structures. The cholesterol molecules assemble into a uniform film, elongated domains and uniformly distributed torus-shaped domains (doughnuts) for one, two and four cycles of deposition, respectively. Beyond four cycles, the molecules adsorb and desorb by an equal amount resulting in no further deposition. The formation of uniformly distributed doughnuts can be attributed to the hydrophobic interaction and reorganization of the molecules due to successive adsorption and desorption during deposition cycles. Our studies on hydrophilic surfaces show that cholesterol cannot form more than one layer of deposition.PACS: 68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces - 68.37.Ps Atomic force microscopy (AFM) - 68.43.Mn Adsorption/desorption kinetics     

Probing the properties of metal-oxide interfaces: silica films on Mo and Ru supports

The influence of metal-oxide interactions on the workfunction and band alignment in thin oxide films is investigated for silica mono- and bilayers grown on Mo(112) and Ru(0001) supports. By analyzing the position of field-emission resonances and the Kelvin-probe signal deduced from conductance and force spectroscopy, we have identified a substantial lowering of the workfunction in the monolayer films, with the oxide bands shifting accordingly. We explain this observation with a stronger coupling and a shorter binding length of the silica monolayer to the metal substrate, which removes the effect of electron spill-out, produces a positive interface dipole and reduces the workfunction of the system. In contrast, the van der Waals bound bilayer film interacts only weakly with the Ru support, conserving the effect of electron spill-out and keeping the workfunction high. Direct evidence for the relevance of interface interactions comes from experiments on buckled silica films, for which regular workfunction modulations are revealed that follow the topographic height of the film above the metal surface.     

ATLEED研究6H-SiC(0001)-(3×3)R30°重构表面

低能电子衍射(LEED)对6H-SiC(0001)-(3×3)R30°表面的研究结果表明,该表面有1/3单层的Si原子吸附在T4空位上与第一个SiC复合层中的三个Si原子键接,它们之间的垂直距离为0.171nm.通过对该表面10个非等价垂直入射衍射束的自动张量低能电子衍射(ATLEED)计算,得到“最佳结构”由于表面SiC复合层堆积顺序不同而产生的三种表面终止状态(surface termination)的混合比例为S1∶S2∶S3=15∶15∶70,理论计算与实验I-V曲线比较得到可靠性因子RVHT=0.165,RP=0.142,表明表面生长符合能量最小化的台阶生长机制.     

A spectroscopic study of water-soluble pyronin B and pyronin Y in Langmuir-Blodgett films mixed with stearic acid

Mono and multilayer of water-soluble pyronin B (PyB) and pyronin Y (PyY) mixed with stearic acid (SA) have been incorporated in Langmuir-Blodgett (LB) films. The surface pressure-area (π-A) isotherm studies pointed out that pure PyB and PyY are incapable of forming stable films at air-water interface and collapsed readily at low surface pressures. However, mixture of PyB or PyY with SA easily formed stable films at the air-water interface and they were easily transferred onto solid substrates. The average area per molecule of mixed films of PyB and PyY at the air-water interface was observed to decrease with increasing concentrations of PyB and PyY. The spectroscopic characteristics of PyB and PyY in chloroform, in SA containing chloroform and in LB films have also been investigated by using absorption and steady-state and time-resolved fluorescence spectroscopy techniques. The morphology of the LB film surfaces has been characterized by using atomic force microscopy (AFM).     

Lateral diffusion in equimolar mixtures of natural sphingomyelins with dioleoylphosphatidylcholine

Cellular membranes of mammals are composed of a complex assembly of diverse phospholipids. Sphingomyelin (SM) and phosphatidylcholine (PC) are important lipids of eukaryotic cellular membranes and neuronal tissues, and presumably participate in the formation of membrane domains, known as "rafts," through intermolecular interaction and lateral microphase decomposition. In these two-dimensional membrane systems, lateral diffusion of lipids is an essential dynamic factor, which might even be indicative of lipid phase separation process. Here, we used pulsed field gradient nuclear magnetic resonance to study lateral diffusion of lipid components in macroscopically oriented bilayers composed of equimolar mixtures of natural SMs of egg yolk, bovine brain, bovine milk and dipalmitoylphosphatidylcholine (DPPC) with dioleoylphosphatidylcholine (DOPC). In addition, differential scanning calorimetry was used as a complementary technique to characterize the phase state of the lipid bilayers. In fully liquid bilayers, the lateral diffusion coefficients in both DOPC/DPPC and DOPC/SM systems exhibit mean values of the pure bilayers. For DOPC/SM bilayer system, this behavior can be explained by a model where most SM molecules form short-lived lateral domains with preferential SM-SM interactions occurring within them. However, for bilayers in the presence of their low-temperature gel phase, lateral diffusion becomes complicated and cannot simply be understood solely by a simple change in the liquid phase decomposition.     

Influence of boron on the initial stages of Si molecular beam epitaxy on Si(1 1 1) studied by reflection high-energy electron diffraction

The influence of boron as a function of coverage on molecular beam epitaxial Si growth on Si(1 1 1) surface was systematically studied by reflection high-energy electron diffraction. At boron coverage above 0.3 monolayer regular oscillations occur with a period typically for two-bilayers, whereas at lower boron coverage a transient behaviour with irregular intensity oscillations was observed in the initial growth stages. This behaviour can be attributed to a superposition of a bilayer and a two-bilayer dominated growth mode. The appearance of these two growth modes is discussed in terms of an initial surface defect-induced nucleation of bilayer-high Si islands and the formation of two-bilayers-high Si islands on top of the van der Waals like boron-covered surface, respectively. We suggest that these surface defects are of intrinsic nature, and their number only depends on the amount of boron at the surface. The oscillations become regular during further Si deposition with a bilayer period, indicating a diminishing influence of the layer/substrate interface on the growth processes.     

拉曼光谱研究人参皂苷Rb1与DPPC双层膜的作用

为深入了解人参皂苷的分子药理学特性,阐明人参皂苷与细胞膜的作用机制,利用拉曼光谱从分子水平研究了不同浓度人参皂苷Rb1与DPPC(二棕榈酰磷脂酸胆碱)双层膜的作用.结果表明,人参皂苷Rb1没有改变DPPC的极性头部O-C-C-N+的稳定构象,极性头仍然平行于膜表面.并且,拉曼峰值比I1096/I1126/1096/I1062和I2848/I288/0随着药物浓度的增加而相应的变大,说明Rbl增加了烃链的无序度,增强了双层膜的流动性.由此推测该药物与DPPC的作用可能由于皂苷分子内及分子间的氢键与磷脂双层膜的极性头部相作用而停留在膜的表面.     

鞘磷脂与胆固醇相互作用的拉曼光谱研究

利用ＦＴ Ｒａｍａｎ光谱研究了不同浓度的胆固醇对鞘磷脂质脂体的作用。结果表明,胆固醇没有改变鞘磷脂的Ｏ Ｃ Ｃ Ｎ＋骨架构象,极性头仍然平行膜表面。Ｃ Ｃ骨架振动强度比Ｉ１０８６／Ｉ１０６４说明,随着胆固醇浓度上升,鞘磷酸分子内部旁式构象也逐渐增多。从Ｉ２８４７／Ｉ２８８０强度比看,胆固醇具有使鞘磷脂脂双层有序性上升,流动性降低的作用,但这种作用随胆固醇浓度上升而逐渐减弱。     

Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.     

Origins of interdiffusion, crystallization and layer exchange in crystalline Al/amorphous Si layer systems

Aluminium-induced crystallization of amorphous silicon (a-Si) in Al/a-Si and a-Si/Al bilayers was studied upon annealing at low temperatures between 165 and 250 °C, by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Upon annealing the inward diffusion of Si along grain boundaries in Al takes place, followed by crystallization of this diffused Si. Continuous annealing leads to (more or less) layer exchange in both types of bilayers. The change in bulk energy of the Al phase (release of macrostress and microstrain, increase of grain size) promotes the occurrence of layer exchange, whereas changes in surface and interface energies counteract the layer exchange.