Dynamics of vacancy defects in 2-D crystalline monolayers under chiral smectic thin films studied by scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) can be used to image adsorbed organic molecules in real space and real time. The technique seems especially well suited for imaging 2-D crystalline monolayers formed under liquid crystal films. In addition to observing perfect 2-D crystals, STM provides the ability to observe crystal defects, and to observe how these defects evolve over time. In this study two different vacancy defects in 2-D lamellar monolayers of chiral liquid crystal molecules under bulk smectic films were observed in situ. Both vacancies showed dynamic behaviour and an unexpected transport anisotropy.     

Observations of focal conic domains in smectic liquid crystals aligned on patterned self‐assembled monolayers

Patterned Self‐Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of nCBs in their smectic‐A phase were created using microcontact printing of functionalized organothiols on gold films. By patterning the surface with homeotropic and planar aligning SAMs, the location and formation of the focal conic domains (FCDs) can be controlled. Polarizing microscopy was used to study the formation of FCDs in circle, stripe and checkerboard pattern geometries. Fluorescent confocal microscopy (FCM) was used for the first time to measure the eccentricity of FCDs that form along a stripe pattern.     

Adsorption behavior and photoelectric response characteristics of bacteriorhodopsin thin films fabricated by self-assembly technique

The characteristics of bacteriorhodopsin (bR)-based thin films fabricated by self-assembly (SA) technique were investigated. Self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (11-MUA) were spontaneously formed onto a pretreated gold substrate by soaking it into the ethanolic solution of 11-MUA, and used as a template for the adsorption of bR. Using poly- -lysine as a bridging molecule for bR adsorption onto SAMs of 11-MUA, bR-embedded purple membrane fragments were adsorbed by electrostatic attractive force. By ellipsometry and atomic force microscopy, the properties of the prepared bR-based thin films were investigated with the various fabrication conditions, such as bR suspension concentration and pH. An artificial photoreceptor was then fabricated with a sandwich-type structure of ITO/electrolyte gel/bR-based thin films/gold substrate. According to the monochromatic light illumination (560 nm) using Xenon lamp system, photoelectric responses of the fabricated photoreceptor were detected and analyzed. The stability of photoreceptors composed of the bR films fabricated by different technique was also examined over the period of 60 days. It is concluded that the SA technique could be usefully applied to the protein-based thin films preparation for the development of bioelectronic devices.     

Comparison of the anchoring of nematic liquid crystals on self-assembled monolayers formed from semifluorinated thiols and alkanethiols

The anchoring of nematic liquid crystals on self-assembled monolayers (SAMs) formed by the chemisorption of semifluorinated thiols or alkanethiols on gold is compared and contrasted. The planar anchoring of 4-n-pentyl-4-cyanobiphenyl (5CB) observed in the past on SAMs formed from alkanethiols is also observed on SAMs formed from semifluorinated thiols. The azimuthal anchoring of 5CB, however, differs on these two types of surfaces: nematic 5CB anchored on SAMs formed from alkanethiols has a grainy appearance due to the formation of domains with sizes 10 mum whereas 5CB forms large domains ( 100 mum) with diffuse branches emerging from defects of strength 1/2 when anchored on SAMs formed from semifluorinated thiols. Mixed (two-component) SAMs formed from either short and long semifluorinated thiols or short and long alkanethiols cause homeotropic anchoring of 5CB. We discuss these results in light of the known differences in the structure of SAMs formed from alkanethiols and semifluorinated thiols, i.e. the tilt of the chains and conformational freedom (flexibility) of the chains within these SAMs.     

Nematic liquid crystal alignment on chemical patterns

Patterned Self-Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of 6CB and 9CB in their nematic phase were created using microcontact printing of functionalized organothiols on gold films. The effects of a range of different pattern geometries and sizes were investigated, including stripes, circles and checkerboards. Evanescent wave ellipsometry was used to study the orientation of the liquid crystal (LC) on these patterned surfaces during the isotropic-nematic phase transition. Pretransitional growth of a homeotropic layer was observed on 1 µm homeotropic aligning stripes, followed by a homeotropic monodomain state prior to the bulk phase transition. Accompanying Monte Carlo simulations of LCs aligned on nanoscale-patterned surfaces were also performed. These simulations also showed the presence of the homeotropic monodomain state prior to the transition.     

正十八烷醇在HOPG上形成自组装膜的吸附特性

研究了正十八烷醇在高定向热解石墨(HOPG)上形成自组装膜的吸附特性, 正十八烷醇在室温下从溶液中吸附至HOPG上形成整齐定向排列的单层自组装膜. 通过扫描隧道显微镜(STM)、接触角测量和X射线光电子能谱(XPS)分析了正十八烷醇单层自组装膜在HOPG上的结构. 实验结果表明, 正十八烷醇自组装膜在基底上成平铺或直立形态, 由于分子在基底上覆盖程度的不同, 会导致它在基底上排列的方式有所不同.     

Alkanethiolate self-assembled monolayers as functional spacers to resist protein adsorption upon Au-coated nerve microelectrode

Alkanethiolate self-assembled monolayers (SAMs) of varied chain lengths were adsorbed upon Au-coated nerve microelectrodes and employed as protein-resistant spacers. The microelectrode spiraled as a cuff type can be used for restoring motor function via electrical stimulation on the peripheral nerve system; however, an increase of electrode impedance might occur during implantation. In this work, a thin-film SAMs treatment upon Au/polyimide (PI) surface of the microelectrode provided a hydrophobic characteristic, which retarded protein adsorption at the initial stage and subsequent pileup (or thickening) process. The protein-resistant effect exhibited comparable SAMs of different chain lengths adsorbed upon Au/PI surfaces. The increase of electrode impedance as a function of protein deposition time was mainly correlated with the addition of reactance that was associated with the pileup thickness of the deposited protein. Particularly, the SAMs-modified surface was capable to detach a significant portion of the accumulated protein from the protein-deposited SAMs/Au/PI, whereas the protein-deposited layers exhibited firm adhesion upon Au/PI surface. It is therefore very promising to apply thin-film SAMs adsorbed upon Au-coated surface for bioinvasive devices that have the need of functional electrical stimulations or sensing nerve signals during chronic implantation.     

Selective deposition in multilayer assembly: SAMs as molecular templates

Self-assembled monolayers (SAMs) on surfaces may be used as molecular templates for the selective deposition of polymer multilayer films. SAMs of ω-functionalized alkane thiolates are patterned onto gold surfaces with micron scale features using the microcontact printing method; glass substrates can also be patterned with trichloroalkylsilane SAMs. Patterned polymeric monolayer and multilayer films are adsorbed atop the SAM from dilute polymer solutions using ionic macromolecular self-assembly techniques which have been developed recently. The effects of polymer molecular weight and ionic content, as well as the use of a second SAM in the unpatterned regions to promote selectivity are discussed. Surface roughness, selectivity and other film properties are presented. It is demonstrated that this technique can be used successfully in the patterning of micron scale features with multilayers of low molecular weight upon adsorption from dilute solution.     

Staphylococcus aureus adhesion to self-assembled monolayers: effect of surface chemistry and fibrinogen presence

Staphylococcus aureus adhesion on self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols on transparent gold films has been studied in real time under well-defined flow conditions using a radial flow chamber and an automated videomicroscopy system. SAMs terminated with methyl, hydroxyl, carboxylic acid and tri(ethylene oxide) groups were investigated. SAMs were characterized using contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. Adhesion experiments using the Newman strain of S. aureus were performed on bare monolayers and monolayers pre-incubated with fibrinogen. Adhesion was found to be lowest on the ethylene oxide-bearing surfaces, followed by the hydroxyl surfaces. Adhesion on the carboxylic- and methyl-terminated SAMs was much higher. Bacterial adhesion was higher on the hydrophobic surfaces. Pre-incubation of surfaces with fibrinogen minimized the effect of the surface properties of the substrate. Adhesion was increased on all surfaces when fibrinogen was present and no significant differences were observed between adhesion to the different SAMs. This study showed that surfaces rich in ethylene oxide groups can be effectively used to prevent bacterial adhesion. However, under physiological conditions, most of the substrate properties are masked by the presence of the adsorbed protein layer and the effect of substrate properties on bacteria adhesion under flow is minimal.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4'-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Mixed self-assembled monolayers (SAMs) consisting of methoxy-tri(ethylene glycol)-terminated and alkyl-terminated dimethylchlorosilanes control the non-specific adsorption of proteins at oxidic surfaces

Monolayers from the newly synthesized compound methoxy-tri(ethylene glycol)-undecenyldimethylchlorosilane (CH3O(CH2CH2O)3(CH2)11Si(CH3)2Cl, MeO(EG)3C11DMS) and dodecyldimethylchlorosilane (DDMS), both pure and mixed, were prepared by self-assembly from organic solution in the presence of an organic base. The films obtained were characterized by advancing and receding contact angle measurements and ellipsometry to confirm the formation of self-assembled monolayers (SAMs). The resulting data on the covalently attached dimethylsilanes were compared to known oligo(ethylene glycol) (OEG)-terminated SAM systems based on terminal alkenes, thiolates or trihydrolyzable silanes. The composition of the mixed SAMs was found to depend directly and linearly on the composition of the silanization solution. Enhanced protein repellent properties were found for the SAMs using a variety of proteins, including the Ras Binding Domain (RBD), a protein with high relevance for cancer diagnostics. Roughly a RBD protein monolayer amount was adsorbed to silicon oxide surfaces silanized with DDMS or non-silanized silicon wafers, and in contrast, no RBD was adsorbed to surfaces silanized with MeO(EG)3C11DMS or to mixed monolayers consisting of DDMS and MeO(EG)3C11DMS if the content of OEG-silane overcame a critical content of X(EG) approximately 0.9.     

Smectic order induced at homeotropically aligned nematic surfaces: a neutron reflection study

Neutron reflection was used to measure the buildup of layers at a solid surface as the smectic phase is approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4-octyl-4'-cyanobiphenyl (8CB), and the solid was silicon with one of five different surface treatments that induce homeotropic alignment: (i) silicon oxide; (ii) a cetyltrimethylammonium bromide coating; (iii) an octadecyltrichlorosilane monolayer; (iv) an n-n-dimethyl-n-octadecyl-3- aminopropyltrimethyloxysilyl chloride monolayer; and (v) a lecithin coating. The development of surface smectic layers in the nematic phase of 8CB was followed by measuring specular reflectivity and monitoring the pseudo-Bragg peak from the layers. The scattering data were processed to remove the scattering from short-ranged smecticlike fluctuations in the bulk nematic phase from the specular reflection. The pseudo-Bragg peak at scattering vector Q approximately 0.2 A(-1) therefore corresponded to the formation of long-range smectic layers at the surface. The amplitude of the smectic density wave decayed with increasing distance from the surface, and the characteristic thickness of this smectic region diverged as the transition temperature was approached. It was found that the characteristic thickness for some of the surface treatments was greater than the correlation length in the bulk nematic. The different surfaces gave different values of the smectic order parameter at the surface. This suggests that the interaction with the surface is significantly different from a "hard wall" which would give the same values of the smectic order parameter and penetration depths similar to the bulk correlation length. Comparison of the different surfaces also suggested that the strength and range of the surface smectic ordering may be varied independently.     

On/off luminescence vapochromic selective sensing of benzene and its methylated derivatives by a trinuclear silver(I) pyrazolate sensor

{[3,5-(CF(3))(2)Pz]Ag}(3) (1) films exhibit selective/reversible sensing of small-organic-molecule (SAM) vapors, which readily switch-on bright-green (benzene or toluene) or bright-blue (mesitylene) luminescence that switches-off upon vapor removal. Vapors of electron-deficient SAMs or non-aromatic solvents did not attain luminescence switching and were not adsorbed.     

Comprehensive investigation of self-assembled monolayer formation on ferromagnetic thin film surfaces

We report a simple, universal method for forming high surface coverage SAMs on ferromagnetic thin (< or =100 nm) films of Ni, Co, and Fe. Unlike previous reports, our technique is broadly applicable to different types of SAMs and surface types. Our data constitutes the first comprehensive examination of SAM formation on three different ferromagnetic surface types using two different surface-binding chemistries (thiol and isocyanide) under three different preparation conditions: (1) SAM formation on electroreduced films using a newly developed electroreduction approach, (2) SAM formation on freshly evaporated surfaces in the glovebox, and (3) SAM formation on films exposed to atmospheric conditions beforehand. The extent of SAM formation for all three conditions was probed by cyclic voltammetry for surfaces functionalized with either (11-thiolundecyl)ferrocene (Fc-(CH2) 11-SH) or (11-isocyanoundecyl)ferrocene (Fc-(CH2) 11-NC). SAM formation was also probed for straight-chain molecules, hexadecanethiol and hexadecaneisocyanide, with contact angle measurements, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy (RAIRS). The results show that high surface coverage SAMs with low surface-oxide content can be achieved for thin, evaporated Ni and Co films using our electroreduction process with thiols. The extent of SAM formation on electroreduced films is comparable to what has been observed for SAMs/Au and to what we observe for SAMs/Ni, Co, and Fe samples prepared in the glovebox.     

Orientational behavior of thermotropic liquid crystals on surfaces presenting electrostatically bound vesicular stomatitis virus

We report the orientational behavior of nematic phases of 4-cyano-4'-pentylbiphenyl (5CB) on cationic, anionic, and nonionic surfaces before and after contact of these surfaces with solutions containing the negatively charged vesicular stomatitis virus (VSV). The surfaces were prepared on evaporated films of gold by either adsorption of poly-L-lysine (cationic) or formation of self-assembled monolayers (SAMs) from HS(CH2)2SO3- (anionic) or HS(CH2)11(OCH2CH2)4OH (nonionic). Prior to treatment with virus, we measured the initial orientation of 5CB (delta epsilon = epsilon(parallel) - epsilon(perpendicular) > 0) to be parallel to the cationic surfaces (planar anchoring) but perpendicular (homeotropic) after equilibration for 5 days. A similar transition from planar to homeotropic orientation of 5CB was observed on the anionic surfaces. Only planar orientations of 5CB were observed on the nonionic surfaces. Because N-(4-methoxybenzylidene)-4-butylaniline (MBBA, delta epsilon = epsilon(parallel) - epsilon(perpendicular) < 0) exhibited planar alignment on all surfaces, the time-dependent alignment of 5CB on the ionic surfaces is consistent with a dipolar coupling between the 5CB and electrical double layers formed at the ionic interfaces. Treatment ofpoly-L-lysine-coated gold films (cationic) with purified solutions of VSV containing 10(8)-10(10) plaque-forming units per milliliter (pfu/mL) led to the homeotropic alignment of 5CB immediately after contact of 5CB with the surface. In contrast, treatment of anionic surfaces and nonionic surfaces with solutions of VSV containing approximately 10(10) pfu/mL did not cause immediate homeotropic alignment of 5CB. These results and others suggest that homeotropic alignment of 5CB on cationic surfaces treated with VSV of titer > or = 10(8) pfu/mL reflects the presence of virus electrostatically bound to these surfaces.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2?wt?%) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Alkanethiols on platinum: multicomponent self-assembled monolayers

We have studied the formation of self-assembled monolayers (SAMs) of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and contact angle (CA) measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UV/ozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission; however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer), intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate: for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.     

Directed growth of poly(isobenzofuran) films by chemical vapor deposition on patterned self-assembled monolayers as templates

This paper describes a method to direct the formation of microstructures of poly(isobenzofuran) (PIBF) by chemical vapor deposition (CVD) on chemically patterned, reactive, self-assembled monolayers (SAMs) prepared on gold substrates. We examined the growth dependence of PIBF by deposition onto several different SAMs each presenting different surface functional groups, including a carboxylic acid, a phenol, an alcohol, an amine, and a methyl group. Interferometry, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and optical microscopy were used to characterize the PIBF films grown on the various SAMs. Based on the kinetic and the spectroscopic analyses, we suggest that the growth of PIBF is surface-dependent and may follow a cationic polymerization mechanism. Using the cationic polymerization mechanism of PIBF growth, we also prepared patterned SAMs of 11-mercapto-1-undecanol (MUO) or 11-mercaptoundecanoic acid (MUA) by microcontact printing (microCP) on gold substrates as templates, to direct the growth of the PIBF. The directed growth and the formation of microstructures of PIBF with lateral dimensions of 6 microm were investigated using atomic force microscopy (AFM). The average thickness of the microstructures of PIBF films grown on the MUO and the MUA patterns were 400 +/- 40 nm and 490 +/- 40 nm, respectively. SAMs patterned with carboxylic acid salts (Cu2+, Fe2+, or Ag+) derived from MUA led to increases in the average thickness of the microstructures of PIBF by 10%, 12%, or 27%, respectively, relative to that of control templates. The growth dependence of PIBF on the various carboxylic acid salts was also investigated using experimental observations of the growth kinetics and XPS analyses of the relative amount of metal ions present on the template surfaces.     

Nanotribological properties of alkanephosphonic acid self-assembled monolayers on aluminum oxide: effects of fluorination and substrate crystallinity

Two phosphonic acid (PA) self-assembled monolayers (SAMs) are studied on three aluminum oxide surfaces: the C and R crystallographic planes of single crystal alpha-alumina (sapphire) and an amorphous vapor-deposited alumina thin film. SAMs are either fully hydrogenated CH3(CH2)17PO3H2 or semifluorinated CF3(CF2)7(CH2)11PO3H2. Atomic force microscope (AFM) topographic imaging reveals that the deposited films are homogeneous, atomically smooth, and stable for months in the laboratory environment. Static and advancing contact angle measurements agree with previous work on identical or similar films, but receding measurements suggest reduced coverage here. To enable reproducible nanotribology measurements with the AFM, a scanning protocol is developed that leads to a stable configuration of the silicon tip. Adhesion for the semifluorinated films is either comparable to or lower than that for the hydrogenated films, with a dependence on contact history observed. Friction between each film and the tips depends strongly upon the type of molecule, with the fluorinated species exhibiting substantially higher friction. Subtle but reproducible differences in friction are observed for a given SAM depending on the substrate, revealing differences in packing density for the SAMs on the different substrates. Friction is seen to increase linearly with load, a consequence of the tip's penetration into the monolayer.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.