Anisotropic polymerization of a long-chain diacetylene derivative Langmuir-Blodgett film on a step-bunched SiO2/Si surface

Alternating facet/terrace nanostructures were fabricated on a SiO2 surface by step-bunching and thermal oxidation of a vicinal Si(111) substrate, and their influence upon the polymerization direction of a long-chain diacetylene derivative monolayer film was investigated by angle-dependent polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the peak intensity of the C 1s-pi transition was stronger when the electric vector plane of the incident X-ray was parallel to the direction of the periodic facet/terrace structures rather than perpendicular to them. On the contrary, a polymer film fabricated on a flat SiO2 surface showed no in-plane anisotropy of the peak intensity. These results indicate that the diacetylene groups in the diacetylene derivative monolayer are preferentially photopolymerized in the direction not across but along the periodic one-dimensional structures on the step-bunched and thermally oxidized SiO2/Si(111) surface.     

原子力显微镜研究APS化单晶硅衬底及单层MD膜表面

原子力显微镜研究ＡＰＳ化单晶硅衬底及单层ＭＤ膜表面张希，高芒来，王力彦，沈家骢（吉林大学化学系，长春，１３００２３）关键词原子力显微镜，ＡＰＳ修饰表面，分子沉积膜以静电相互作用为成膜推动力的各种功能体系分子沉积（ＭＤ）超薄膜已有报道［１￣４］．对ＭＤ...     

Stress in polyimide coatings

The effect of film thickness on in-plane molecular orientation and stress in polyimide films prepared from pyromellitic dianhydride with 4,4′-oxydianline was investigated using a prism coupling technique to measure the refractive index. Film thickness was controlled by varying both solution concentration and spinning conditions. Birefringence, the difference between the in-plane and out-of-plane refractive indices, was used to characterize the in-plane molecular orientation. The observed birefringence is a combination of the birefringence resulting from molecular orientation and the birefringence induced by the residual stress present in the films. The birefringence decreases with increasing film thickness over the range of thicknesses studied (3–20 μm) indicating that the molecular orientation decreases with increasing film thickness. The in-plane coefficient of linear thermal expansion (CTE), controlled by the level of orientation in the film, increases from 18 to 32 × 10^?6/°C over the same thickness range. The birefringence of free-standing films was lower than that of adhered films due to the release of residual stress in the film once the film is removed from the substrate. The residual film stress arises primarily from the mismatch in CTEs between the polyimide film and the substrate to which the film is adhered. Thus, since the film anisotropy decreases with increasing thickness, the film stress increases with increasing thickness. Residual stress calculated by integrating the product of the film modulus and the CTE mismatch assuming temperature-dependent properties is comparable to experimentally measured film stress. Ignoring the temperature dependence of the film properties leads to an overestimation of stress. Moisture uptake was used to study the stress dependence of the optical properties. Moisture uptake increases both the in-plane and out-of-plane refractive indices by equal amounts in free-standing films due to an isotropic increase in the polarizability. In adhered films, an increase in moisture uptake leads to a decrease in the birefringence due to a swelling-induced decrease in the residual film stress. © 1994 John Wiley & Sons, Inc.     

基底界面效应对聚苯乙烯薄膜分子运动行为的影响

本文研究了Si／Si02、Si／Si—H基底与聚苯乙烯（Ps）之间的界面相互作用对Ps薄膜的玻璃化转变及相关力学性能的影响．结果显示,无论何种基底,Ps薄膜的玻璃化转变温度（L）都随其厚度降低而降低．但相同厚度（〈110nm）下,以Si／Si-H为基底时Ps薄膜的瓦比以Si／Si02为基底的PS薄膜高．Si／Si02表面Ps薄膜疋开始下降的临界厚度为110nm,远高于以Si／Si—H为基底时的40nm．对Ps薄膜的膨胀系数和弹性模量进行研究,也得到相似的临界厚度．另外,与Si／Si02基底相比,在Si／Si-H上的Ps薄膜具有更低的膨胀系数以及较高弹性模量．可能原因是Si／Si-H与Ps具有较强的相互作用,限制了该界面分子的运动能力,导致基底／PS界面效应对薄膜分子运动的影响力增强,造成该薄膜瓦的厚度依赖性下降,并呈现出相对较硬的力学特征．     

Anisotropic molecular structure in dip-coated films of linear poly(ethylene imine) studied by infrared multiple-angle incidence resolution spectrometry

Molecular structure in dip-coated films of linear poly(ethylene imine) (LPEI) on a germanium (Ge) substrate in dried and hydrated conditions have been analyzed by infrared multiple-angle incidence resolution spectrometry (IR MAIRS). The MAIRS-IP (in-plane) and -OP (out-of-plane) spectra of the dried film exhibited largely different patterns from each other, which indicated that LPEI molecules had an apparent molecular orientation with respect to the substrate surface. Although the film exhibited no peak in X-ray diffraction patterns, the low-crystallinity film has been found to have highly oriented molecular structure. Many key bands indicated that the molecules were involved in the double-stranded helix structure, which is specific to the anhydrate crystal of LPEI, with nearly perpendicular orientation. The Davydov splitting of the NH stretching vibration mode was readily captured by the IR MAIRS spectra, which also supported the helix standing model. When the film was stored in a humid condition, on the other hand, IR MAIRS spectra revealed that the helix was resolved to be straight chains, but the perpendicular orientation was kept unchanged. In addition, the MAIRS spectra also revealed molecular orientation of the water molecules of crystallization. The unique molecular arrangements are understandable by considering that the stabilization energy in the polymer monolayer directly attached on the substrate surface is minimized by the standing-molecule arrangements.     

Morphology of poly(lactide)-block-poly(dimethylsiloxane)-block-polylactide high-χ triblock copolymer film studied by grazing incidence small-angle X-ray scattering

The morphologies of poly(lactide)-block-poly(dimethylsiloxane)-block-polylactide (PLA-b-PDMS-b-PLA) triblock copolymer films were studied using a combination of grazing-incidence small-angle X-ray scattering, X-ray reflectivity and scanning electron microscopy. This block copolymer is characterized by a high Flory–Huggins interaction parameter which leads to the self-assembly of periodic high-resolution nanodomains. In this article, we performed a detailed analysis of GISAXS patterns, in the frame of the Distorted Wave Born Approximation, in order to determine the morphology of blocks and their spatial arrangement. For a low volume fraction of PLA (17%), a three-dimensional hexagonal lattice of PLA spherical blocks is revealed, while, for a PLA fraction twice larger, in-plane (parallel) PLA lying cylinders adopt a two-dimensional centered rectangular lattice. Moreover, the in-depth electron density profile of the polymer film for the cylindrical morphology was extracted from the XRR data, revealing the presence of interfacial layers at the top surface and at the interface with the Si substrate.     

Properties of β-sitostanol/DPPC monolayers studied with Grazing Incidence X-ray Diffraction (GIXD) and Brewster Angle Microscopy

Although the influence of structurally modified sterols on artificial membranes has been intensively investigated, studies on the properties of stanols, which are saturated analogs of sterols, are very rare. Therefore, we have performed Grazing Incidence X-ray Diffraction (GIXD) experiments aimed at studying in-plane organization of a plant stanol-β-sitostanol monolayer and its mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine - DPPC at the air/water interface. The collected GIXD data, resulting in-plane parameters and BAM images provide information on molecular organization and in-plane ordering of the investigated films. It was found that the lateral organization of β-sitostanol/DPPC monolayers depends on their composition. The oblique structure of the in-plane lattice of tilted hydrophobic region of molecules, found for DPPC film, is maintained at 10 mol% of stanol in the system. However, at 30 and 90 mol% of stanol in the mixture, the arrangement of molecules is hexagonal and they are oriented perpendicularly to the interface. With the addition of stanol the extend of the in-plane order of the monolayers decreases. Moreover, in mixtures the ordered domains consist of both monolayer's components. Additionally, β-sitostanol film is of similar in-plane organization as the corresponding sterol monolayer (β-sitosterol) and stanol induces condensing effect on DPPC.     

Molecular orientation in electrodeposited polypyrrole films

The molecular arrangement in electrodeposited polypyrrole films was studied by means of linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra measured at the K absorption edges of carbon and nitrogen. It has been found that the change of the exciting radiation incidence from normal to grazing leads to an increase in the intensity of π*-related resonances with simultaneous decrease in the intensity of σ*-related resonances in the spectra. Similar changes in the spectra measured for both absorption edges indicate a pronounced conjugation of π-bonds in the polypyrrole chains in the grown films. Preferential in-plane orientation of pyrrole rings relative to the substrate surface is observed for all the deposited films. The linear dichroism is more pronounced at the initial stages of deposition (2D growth) than at later stages characterized by “cauliflower”-like morphology of the grown film.     

Direct observation of liquid crystal phases under nanoconfinement: A grazing incidence X-ray diffraction study

We have controlled the molecular orientation of nematic and smectic A liquid crystal (LC) phases in a porous anodic aluminium oxide (AAO) film, in which the pore diameter was varied from 20 nm to 100 nm. Surface anchoring to induce planar and homeotropic molecular arrangement was controlled by chemical modification of the AAO inner surface. Direct observation of the molecular orientation of LC phases was performed using grazing incidence X-ray diffraction technique, showing in-plane and out-of-plane molecular orientation, and the corresponding layer orientation. The systematic investigation of LC phases under nanoconfinement will be useful to design various kinds of physicochemical environments to control the orientation of other soft matters.     

Orientation of the alkyl side chains and glucopyranose rings in Langmuir–Blodgett films of a regioselectively substituted cellulose ether

An arrangement for the long alkyl side chains in a Langmuir–Blodgett (LB) film from regioselectively alkylated 2,3-di-O-octadecylcellulose on an Au-coated substrate was investigated using Fourier transform infrared reflection absorbance spectroscopy. The IR results indicated that the hydrophobic long alkyl side chains were forced to be repellent to the surface of water, resulting in deposition on the substrate, with it being tilted in the vertical direction, and further formed both hexagonal and orthorhombic crystalline structures up to 10 layers in the LB film. In addition, molecular modeling with CAChe software indicated that the optimized assembly of the side chains was supposed to own a vertical arrangement against the substrate surface after compression of the monolayer. This means that the conformation of 2₁ screw of the cellulose molecular chain may be altered by the compression to have an unusual conformation by a different φ–ψ dihedral angle such as onefold axis without a symmetry element.     

Effect of molecular weight and substrate on silicone segregation from UV resin at plasma polymerized mold interface

Pristine‐, poly(octafluorotoluene)‐ (POFT), and polyacetylene (PAc)‐coated Si wafers were used as substrates for the study of segregation of silicone diacrylate (SA) from a formulation containing other oligomeric and monomeric acrylates. POFT and PAc were microwave plasma polymerized on the Si wafers. Three SAs with molecular weights ranging from 700 to 6000 Da were synthesized and characterized. Formulations with 2 wt % SA were ultraviolet cured on the silicon wafers. The surface composition of formulation‐substrate side of the cured film was analyzed with X‐ray photoelectron spectroscopy, and the depth profile was analyzed with time of flight‐secondary ion mass spectrometry. The analysis results indicated that SA aggregated on all three types of Si surfaces. However, SA segregation is highest on the low surface energy POFT‐coated Si substrate, and low‐molecular‐weight SA is favorable to the segregation. For high‐molecular‐weight SA, the different Si substrates do not affect the degree of aggregation at the formulation‐substrate interface. The observed SA aggregation trend can be predicted by the Gibbs‐adsorption equation correlated to the resin surface tension and contact angle on substrates. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 442–450, 2010     

Anisotropic optical properties in electroluminescent conjugated polymers based on grazing angle photoluminescence measurements

Grazing angle photoluminescence (GPL) originates from a waveguided light emitted at grazing angle to the substrate due to the total internal reflections, and the light emission is polarized with enhanced intensity at selective mode wavelength. GPL measurements reveal the optical anisotropy of luminescent conjugated polymers, in particular, the alignment of emitting dipoles from which emission occurs, in contrast to spectroscopic ellipsometry measurements that give the anisotropy in the absorption. Based on the GPL emission intensities and spectra, we investigate the anisotropic optical properties in electroluminescent poly(9,9'-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer thin films of different molecular weights (M(n) = 9-255 kg/mol), both in the pristine and annealed states. The optical anisotropy in F8BT films generally increases with molecular weight, suggesting that higher molecular weight polymers with longer chains are more likely to lie in-plane to the substrate. Upon annealing, high molecular weight F8BT films show even a higher degree of anisotropy, in contrast to low molecular weight F8BT films that become more isotropic. Annealing causes the polymer chains to rearrange and adopt a configuration in which the interchain exciton migration to better ordered low energy (LE) emissive states is strongly suppressed. We observe that the emissive states in F8BT are strongly affected by the local polymer chain arrangement, producing the less ordered high energy (HE) emissive states near the substrate interface where there is a higher degree of chain disorder and the LE states in the bulk of the film. When spin coated onto a quartz substrate precoated with a poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) layer, films of F8BT show severe luminescence quenching near the PEDOT:PSS interface for both the LE and HE emissive states, but a selective quenching of the LE states in the bulk of the film. These observations have important implications for fabricating efficient electronic devices using conjugated polymers as an active material, since the performance of these devices will strongly depend on anisotropic optical properties of electroluminescent conjugated polymers.     

In-plane orientation controlled by a rotating disc in Langmuir and Langmuir-Blodgett films of a merocyanine dye

When a Langmuir film consists of mesoscopic domains having an elongated shape, stretching of the film due to Langmuir-Blodgett (LB) deposition and shearing due to a rotating disc can induce in-plane orientations of these domains. When the former governs the phenomenon, the domains are aligned perpendicular to the substrate surface (just before the transfer). When the latter governs the phenomenon, the domains are aligned along the concentric flow lines. We have studied the more complicated intermediate case by using two different molecular systems. It has been demonstrated that in LB films fabricated under such conditions, the orientation of the domains depends on the position on the substrate and the deposition type. Furthermore, monolayers deposited during the upward and downward strokes of the substrate become non-equivalent.     

Structure and topography of molecular assemblies on solid substrates by infrared spectroscopy and scanning tunneling microscopy

Recently we have combined infrared spectroscopy and atomic resolution scanning tunneling microscopy (STM) to probe the local structure and intermolecular arrangement of molecules within thin films. IR spectroscopy provides spatially averaged information about orientation of the molecules with respect to the surface and about intermolecular arrangement within the crystallographic unit cell. STM data yields a local picture of molecular packing within the film. The requirements of an atomically flat (over distances of hundreds of angstroms) conducting substrate for the STM are fulfilled by an epitaxially grown film of gold on a cleaved mica substrate which also provides a good infrared reflective surface, enabling IR and STM measurements on identical samples. Systems investigated include Langmuir-Blodgett films of cadmium arachidate and self-assembled films of octadecyltrichlorosilane.     

预沉积Ge对Si(111)衬底上SSMBE外延生长SiC薄膜的影响

利用固源分子束外延(SSMBE)生长技术, 在Si(111)衬底上预沉积不同厚度(0、0.2、1 nm)Ge, 在衬底温度900 ℃, 生长SiC单晶薄膜. 利用反射式高能电子衍射仪(RHEED)、原子力显微镜(AFM)和傅立叶变换红外光谱(FTIR)等实验技术, 对生长的样品进行了研究. 结果表明, 预沉积少量Ge(0.2 nm)的样品, SiC薄膜表面没有孔洞存在, AFM显示表面比较平整, 粗糙度比较小, FTIR结果表明薄膜内应力比较小. 这说明少量Ge的预沉积抑制了孔洞的形成, 避免衬底Si扩散, 因而SiC薄膜的质量比较好. 没有预沉积Ge的薄膜, 结晶质量比较差, SiC薄膜表面有孔洞且有Si存在. 然而预沉积过量Ge (1 nm) 的样品, 由于Ge的岛状生长,导致生长的SiC表面粗糙度变大, 结晶质量变差, 甚至导致多晶产生.     

Immobilization of methylviologen between well-ordered multilayers of manganese oxide during their electrochemical assembly

Methylviologen dications (MV2+) were immobilized between layers of manganese oxide during their electrochemical assembly by an anodic route in a homogeneous aqueous Mn2+ solution. This approach yielded a well-ordered multilayer film on a platinum substrate as a result of dense packing of planar MV2+ molecules to stabilize the layered framework. A grazing angle in-plane X-ray diffraction study revealed that the manganese oxide sheets and the molecular planes of inserted MV2+ ions are oriented parallel to the electrode surface. Cyclic voltammetry of the product film indicated an electron transfer from the underlying Pt substrate to inserted methylviologen ions through the manganese oxide sheets.     

Organic Thin Film Transistors Based on 2,3‐Dimethylpentacene and 2,3‐Dimethyltetracene

2,3‐Dimethylpentacene (DMP) and 2,3‐dimethyltetracene (DMT) were synthesized, characterized and employed as the channel material in the fabrication of thin‐film transistors. The two methyl groups increase the chemical stability of the compounds versus the pristine acene analogues. The crystals maintain herringbone‐like molecular packing, whereas the weak dipole associated with the unsymmetrical molecule induces an anti‐parallel alignment among the neighbors. This structural motif favors layered film growth on SiO₂/Si surface. Thin film transistors prepared on SiO₂/Si and n‐nonyltrichlorosilane‐modified SiO₂/Si at different substrate temperatures were compared. DMP‐based transistors prepared on rubbed n‐nonyltrichlorosilane‐modified SiO₂/Si substrate gave the highest field‐effect mobility of 0.46 cm²/Vs, whereas DMT‐based transistor gave a mobility of 0.028 cm²/Vs.     

Sol–gel and transfer technique for fabricating dual ceramic thin film patterns on plastics

A technique that realizes alternating, two kinds of ceramic ribbons on plastics is proposed. An Si(100) plate with periodic trenches coated with a polyimide (PI)-polyvinylpyrrolidone (PVP) mixture layer was used as the mother substrate. An indium–tin–oxide (ITO) thin film was deposited on the mother substrate by the sol–gel method with a firing process, followed by transferring it to a polycarbonate (PC) substrate by melting the PC surface in contact with the ITO thin film. This resulted in patterned ITO ribbons on the PC substrate where the ridges of PC were formed between the ribbons. A ZnO thin film was prepared on a flat Si(100) substrate coated with a PI-PVP mixture layer, followed by firing, and then was transferred on the PC ridges in the same manner, resulting in alternating ITO and ZnO ribbons on the PC substrate. The technique proposed here allows dense, alternating ceramic ribbons to be fabricated on plastics irrespective of the combinations of ceramics and plastics.     

Crystal structure of friction-transferred poly(2,5-dioctyloxy-1,4-phenylenevinylene)

Poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOPPV) was found to form a highly oriented film by a friction-transfer technique. Structural investigation of friction-transferred DOPPV was studied by means of polarized ultraviolet-visible (UV-vis) absorption spectroscopy, polarized photoluminescence (PL) spectroscopy, and synchrotron-sourced grazing incident X-ray diffraction (GIXD) analysis. The polarized UV-vis absorption and PL spectra indicate clear axial alignment. DOPPV backbones in friction-transferred film are highly aligned along the drawing direction of the friction-transfer. Further information of the molecular arrangement in friction-transferred DOPPV film was investigated by both the out-of-plane and the in-plane GIXD analyses with synchrotron source. The DOPPV molecules in friction-transferred films were perfectly arranged three-dimensionally: the backbones aligned along the drawing direction of friction-transfer, the alkyl side chains lay in the film plane, and the planar backbones were arranged parallel to the film surface. Additionally, two neighboring DOPPV molecules along the direction of inter-backbones separation by alkyl side chains were found to be shifted with respect to one another by the mean distance of half of a monomeric repeat.