Forces between thiolate-modified gold surfaces in a melt of end-functionalized polymers

To better understand surface forces across polymer melts, we measured the force between two chemically well-defined solid surfaces in a melt of polymer chains with a functional end group. As for surfaces, we used self-assembed monolayers (SAMs) of alkyl thiols with different end groups (methyl, amino, and hydroxyl) on gold. The polymer was a hydroxyl-terminated polyisoprene. To measure the force, an atomic force microscope was used. Between methyl-terminated SAMs, a weak and short-range repulsion was detected. Between hydroxyl or amino-terminated SAMs, a strong and long-range repulsion was observed up to distances of 16 nm. This indicates that the hydroxyl group of the polymer binds to the hydroxyl or amino groups of the SAMs. It forms a brush-like structure, which leads to steric repulsion. On amino-terminated SAMs, force-versus-distance curves on approach and retraction were monotonically repulsive and reversible. With hydroxyl-terminated SAMs, a jump was observed on approach when the load exceeded a certain threshold. On retraction, an adhesion had to be overcome. We interpret the jump as a rupture of the polymer layer. It indicates that the kinetics of bond and brush formation is faster on OH-SAMs than on NH2-SAMs.     

A Chiral Self‐Assembled Monolayer Derived from a Resolving Agent and its Performance as a Crystallization Template for an Organic Compound from Organic Solvents

A new chiral nonracemic thiol derived from a popular acidic resolving agent that incorporates a cyclic disubstituted phosphate group (phencyphos) has been prepared in enantiomerically pure form. The stereochemistry and absolute configuration were established by performing a single‐crystal X‐ray structural analysis of a synthetic intermediate. The thiol compound was used for the preparation of self‐assembled monolayers (SAMs) on both monocrystalline and polycrystalline metallic gold, which have very different surface roughness. The monolayers were used to promote the nucleation and growth of crystals from nonaqueous solutions of an organic molecule (the parent phencyphos) of similar structure to the compound present in the monolayer. The template layers influence the nucleation and growth of the phencyphos crystals despite the lack of two‐dimensional order in the surfaces. Heterogeneous nucleation of phencyphos takes place upon evaporation of either CHCl₃ or isopropanol solutions of the compound on the SAM surfaces, where the evaporation rate merely influences the size and homogeneity of the crystals. The roughness of the surface also plays an important role; the polycrystalline gold produces more homogeneous samples because of the greater number of nucleation sites. Clear evidence for nucleation and growth on the surfaces is shown by scanning electron microscopy. The variation in crystal form achieved by using different surfaces and solvents suggests that the layers are applicable for the preparation of organic crystals from organic solutions.     

Directed electroless growth of metal nanostructures on patterned self-assembled monolayers

The directed placement of Cu nanostructures on surfaces has been studied using a combination of scanning probe lithography and electroless metal deposition onto nanopatterned SAMs of 16-mercaptohexadecanoic acid (16-MHA) on Au. In situ studies using nanoscale molecular gradients reveal how controlling the areal density of the 16-MHA molecules dictates the nucleation and growth of the metal nanostructures. The influence of controlling pattern line spacing and tip path on pattern feature fidelity is also discussed.     

Monolayers of twisted binaphthyls for aromatic crystallization at low nucleation densities and high growth rates

Monolayers of 1,1'-bi-2-naphthol (BN) derivatives, of which the two naphthalene rings are twisted along the carbon(1)-carbon(1') single bond, were studied for their conformational effect on the growth of pentacene crystals on their monolayer surface. BN monolayers with H and Br at 6,6'-positions (H-BN and Br-BN) were prepared by immersion-coating in toluene solution of the corresponding BNSiCl2. Pentacene was thermally evaporated onto the H-BN and Br-BN monolayers, silica, octadecylsilyl (ODTS) SAM, and a micropattern of H-BN and ODTS SAM. Pentacene crystals were also grown on the SAMs of 1-naphthylsilyl(NPh), phenylsilyl(Ph), and diphenylsilyl (DPh) groups, which are aromatic and have contact angle values similar to those of the the BN monolayers. AFM images of the crystals at the early stage of growth indicated that the BN monolayers suppressed the nucleation while facilitating the growth of nuclei to larger crystals. The low nucleation density and high growth rate are accounted for by the amorphous nature of the twisted BN monolayer surface where the intermolecular interaction between neighboring adsorbates is likely to be suppressed. The results offer new insights into designing surfaces for controlling the crystallization kinetics of organic materials.     

Applications of self-assembled monolayers for biomolecular electronics

Preparation and characterization of ordered ultrathin organic films (a few nanometers to several hundred nanometers) has recently attracted considerable attention because of the possibility of controlling order and interactions at the molecular level and has triggered several innovative applications ranging from molecular electronics to tribology. Monomolecular films prepared by self-assembly are attractive for several exciting applications because of the unique possibility of making the selection of different types of terminal functional groups as well as length scales more flexible. The present article discusses various applications of self-assembled monolayers (SAMs) in molecular electronics ranging from biosensors to optoelectronic devices with specific examples. Similarly, SAMs and multilayers of bifunctional molecules on polycrystalline substrates can be effectively used to carry out specific reactions between pendent functionalities and solution or gaseous species to produce new hybrid materials for devices such as molecular diodes. The importance of SAMs in controlling nucleation and growth is also illustrated using biomimetic synthesis of ceramic thin films (biomineralization) of zirconia.     

Inorganic semiconductor nanowires: rational growth, assembly, and novel properties

Rationally controlled growth of inorganic semiconductor nanowires is important for their applications in nanoscale electronics and photonics. In this article, we discuss the rational growth, physical properties, and integration of nanowires based on the results from the authors' laboratory. The composition, diameter, growth position, and orientation of the nanowires are controlled based on the vapor-solid-liquid (VLS) crystal growth mechanism. The thermal stability and optical properties of these semiconductor nanowires are investigated. Particularly, ZnO nanowires with well-defined end surfaces can function as room-temperature ultraviolet nanolasers. In addition, a novel microfluidic-assisted nanowire integration (MANI) process was developed for the hierarchical assembly of nanowire building blocks into functional devices and systems.     

Facile solution route to vertically aligned, selective growth of ZnO nanostructure arrays

For any future cost-effective applications of inorganic nanostructures, in particular, hybrid photovoltaic cells, it is essential that these inorganic nanomaterials be solution processable and selectively printable. This letter reports the selective growth of single-crystal ZnO nanostructures based on the microcontact printing of an inorganic nanocrystal seeding film. The pattern-transfer quality is dependent on the concentration of the inking solution. Variable yet controllable anisotropic growth of ZnO nanowires has been demonstrated on the transferred patterns of ZnO nanocrystal films. The patterning and growth of these highly ordered arrays of ZnO nanostructures employ a simple soft lithography technique and mild reaction conditions at low temperature and in the absence of harmful organic additives.     

Expedient generation of patterned surface aldehydes by microfluidic oxidation for chemoselective immobilization of ligands and cells

An expedient and inexpensive method to generate patterned aldehydes on self-assembled monolayers (SAMs) of alkanethiolates on gold with control of density for subsequent chemoselective immobilization from commercially available starting materials has been developed. Utilizing microfluidic cassettes, primary alcohol oxidation of tetra(ethylene glycol) undecane thiol and 11-mercapto-1-undecanol SAMs was performed directly on the surface generating patterned aldehyde groups with pyridinium chlorochromate. The precise density of surface aldehydes generated can be controlled and characterized by electrochemistry. For biological applications, fibroblast cells were seeded on patterned surfaces presenting biospecifc cell adhesive (Arg-Glyc-Asp) RGD peptides.     

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.     

Frictional dynamics of fluorine-terminated alkanethiol self-assembled monolayers

The frictional dynamics of fluorine-terminated alkanethiol (S(CH2)8CF3) self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. The simulations treat the interactions between two SAMs on flat surfaces. The structure and frictional behavior are investigated as a function of applied pressure (200 MPa to 1 GPa) for a shear velocity of 2 m/s and compared to methyl-terminated alkanethiol SAMs. The maximum adhesive pressure between the SAMs is 220 MPa for both end groups. In agreement with experiments on the molecular scale, the shear stress and the coefficient of friction for CF3-terminated alkanethiols are larger than for CH3-terminated alkanethiols. The main source for the difference is primarily the tighter packing of the fluorinated terminal group resulting in a higher degree of order. The molecular scale coefficient of friction is correlated with the degree of order among all the systems.     

Acid-base properties and zeta potentials of self-assembled monolayers obtained via in situ transformations

Siloxane-anchored, self-assembled monolayers (SAMs) on single crystal Si were prepared with a variety of surface functional groups using a single commercially available surfactant (1-bromo-11-(trichlorosilyl)undecane) followed by in situ transformations. Polar (thioacetate and thiol), nonpolar (methyl), acidic (sulfonic and carboxylic), basic (various amines), and ionic (alkylammonium) surface functionalities were prepared. For primary amine and sulfonate surfaces, the degree of surface charge as a function of pH was determined ex situ using X-ray photoelectron spectroscopy (XPS). Sulfonate SAMs exhibited much higher effective pKa (approximately 2) than dilute sulfonic acid (-5 to -6), and amine SAMs exhibited much lower pKa (approximately 3) than dilute organic amines (approximately 10). This is attributed to the stabilization of nonionized groups by adjacent ionized groups in the SAM. Zeta potentials of these SAMs as a function of pH were consistent with the XPS results and indicated that ionizable SAM surfaces can generate surface potentials much higher than those of nonionic SAMs (thioacetate, methyl) and typical oxide surfaces.     

Formation mechanisms of gold-zinc oxide hexagonal nanopyramids by heterogeneous nucleation using microwave synthesis

This work reports the development of a fast and simple "one-pot" route for the synthesis of hybrid Au-ZnO hexagonal nanopyramids by sequential homogeneous-heterogeneous nucleation steps involving both Au and Zn ions using microwave irradiation (MWI). The rapid decomposition of zinc acetate by MWI in the presence of a mixture of oleic acid (OAc) and oleylamine (OAm) results in the formation of hexagonal ZnO nanopyramids. In the presence of Au ions, the initially formed Au nanocrystals act as heterogeneous nuclei for the nucleation and growth of the ZnO nanopyramids. The Au nanoparticles promote the heterogeneous nucleation of ZnO and the formation of the hexagonal base of the ZnO nanopyramids. Using preformed Au nanoparticles instead of Au ions results in a narrow size distribution of uniform Au-ZnO nanopyramids, each consisting of a gold nanoparticle embedded in the center of the hexagonal base of the ZnO nanopyramid. We study the factors that control the nucleation and growth of these complex structures, and provide new insights into the stepwise homogeneous-heterogeneous mechanism and the conventional heterogeneous nucleation on preformed Au nanoparticles. The formation of the hetero nanostructures Au-ZnO nanopyramids is strongly dependent on the molar ratios of OAc to OAm. The presence of OAc with a considerable dipole moment results in strong electrostatic interaction with the polar surfaces of the growing ZnO nanocrystals thus resulting in slowing the growth rate of the polar planes and allowing the formation of well-developed facets. In the absence of Au nanoparticles, a high concentration of zinc acetate and longer MWI times are required for the production of the nanopyramids. The gold nanoparticles could provide the metallic contact points within the hybrid nanopyramids which could facilitate the bottom-up assembly of Au-ZnO devices. Furthermore, the Au-ZnO nanopyramids could have improved performance in solar energy conversion and photocatalysis.     

Self-assembly of alkanols on Au(111) surfaces

Self-assembled monolayers (SAMs) of alkanols (1-C(N)H(2N+1)OH) with varying carbon-chain lengths (N = 10-30) have been systematically studied by means of scanning tunneling microscopy (STM) at the interfaces between alkanol solutions (or liquids) and Au(111) surfaces. The carbon skeletons were found to lie flat on the surfaces. This orientation is consistent with SAMs of alkanols on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces, and also with alkanes on reconstructed Au(111) surfaces. This result differs from a prior report, which claimed that 1-decanol molecules (N = 10) stood on their ends with the OH polar groups facing the gold substrate. Compared to alkanes, the replacement of one terminal CH3 group with an OH group introduces new bonding features for alkanols owing to the feasibility of forming hydrogen bonds. While SAMs of long-chain alkanols (N > 18) resemble those of alkanes, in which the aliphatic chains make a greater contribution, hydrogen bonding plays a more important role in the formation of SAMs of short-chain alkanols. Thus, in addition to the titled lamellar structure, a herringbone-like structure, seldom seen in SAMs of alkanes, is dominant in alkanol SAMs for values of N < 18. The odd-even effect present in alkane SAMs is also present in alkanol SAMs. Thus, the odd N alkanols (alkanols with an odd number of carbon atoms) adopt perpendicular lamellar structures owing to the favorable interactions of the CH3 terminal groups, similar to the result observed for odd alkanes. In contrast to alkanes on Au(111) surfaces, for which no SAMs on an unreconstructed gold substrate were observed, alkanols are capable of forming SAMs on either the reconstructed or the unreconstructed gold surfaces. Structural models for the packing of alkanol molecules on Au(111) surfaces have been proposed, which successfully explain these experimental observations.     

Structural development of mercaptophenol self-assembled monolayers and the overlying mineral phase during templated CaCO3 crystallization from a transient amorphous film

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.     

Preparation of ZnO Nanowires and Study of Surface‐adsorbate Interaction by Fourier‐Transform Infrared Spectroscopy

To study the surface‐adsorbate properties of ZnO nanowires, a hydrothermal method was modified to grow ZnO nanowires directly on ZnSe, which were then characterized by attenuated total reflection infrared (ATR‐IR) spectroscopy. To prepare ZnO nanowires directly on ATR sensing element of ZnSe, ZnO seed layers were first formed by annealing of ZnO seeds on ZnSe surfaces. The ZnO seed layers then were exposed to growth solution, forming ZnO nanowires directly on the ATR crystals. The interaction properties of the resulting surfaces were studied by an ATR‐IR method. The diameter, length and distribution of the ZnO nanowires can be tuned by adjusting the growth conditions, particularly the growing time and the concentrations of reagents. Two surfaces, namely Zn‐rich and Zn‐O ion‐pair surfaces were studied in detail for their adsorption properties toward compounds bearing different functional groups. By examination of several volatile organic compounds (VOCs), it was found that the Zn‐rich surface is less selective and interacts with compounds bearing the functional groups of amino and hydroxyl. The Zn‐O ion‐pair surface is more selective and a much stronger interaction was observed with non‐aromatic amino compounds. These results indicate that the improving of the selectivity of a ZnO‐based sensing device can be achieved by tuning the surface structure of the ZnO nanomaterials.     

The epitaxial growth of cholesterol crystals from bile solutions on calcite substrates

Epitaxial relationships between the surfaces of inorganic and bioorganic crystals can be an important factor in crystal nucleation and growth processes in a variety of biological environments. Crystalline cholesterol monohydrate (ChM), a constituent of both gallstone and atherosclerotic plaques, is often found in association with assorted mineral phases. Using in situ atomic force microscopy (AFM) and well-characterized model bile solutions, the nucleation and epitaxial growth of ChM on calcite (104) surfaces in real-time is demonstrated. The growth rates of individual cholesterol islands formed on calcite substrates were determined at physiological temperatures. Evidence of Ostwald's ripening was also observed under these experimental conditions. The energetics of various (104) calcite/(001) ChM interfaces were calculated to determine the most stable interfacial structure. These simulations suggest that the interface is fully hydrated and that cholesterol hydroxyl groups are preferentially positioned above carbonate ions in the calcite surface. This combination of experimental and theoretical work provides a clearer picture of how preexisting mineral seeds might provide a viable growth template that can reduce the energetic barrier to cholesterol nucleation under some physiological conditions.     

Electrochemical factors controlling the patterning of metals on SAM-coated substrates

Alkanethiol self-assembled monolayers (SAMs) have been used in electrochemical microfabrication processes. The reductive desorption potential of alkanethiol SAMs, Edes, can be comparable to, greater than, or less than the metal reduction potential during electrodeposition, Emet. As a result, the SAM layer can passivate the surface or desorb simultaneously with metal deposition. We show that these electrochemical traits can be combined with a rastering microjet electrode to pattern SAMs directly and create patterned metal films without lithography steps. For the case of copper deposition on 1-octanethiol (OT)- and 1-dodecanethiol (DT)-coated substrates, Edes is significantly negative of Emet, resulting in high-resolution metal patterns with poor nucleation and poor adhesion to the substrate. However, nickel patterns deposited on 1-butanethiol (BT), OT, and DT have traits similar to bare gold (excellent nucleation and adhesion) because Edes is positive of Emet. Substrates with SAMs also suppress adventitious chemistries that occur distant from the rastering microjet electrode, such as oxygen reduction, making samples more corrosion resistant and improving the overall patterning process that we call electrochemical printing.     

籽晶层制备方式对氧化锌纳米棒阵列的影响

采用3种不同的方式制备ZnO薄膜籽晶层:旋涂、喷雾热解和脉冲激光沉积。对于每一种制备方式,其薄膜的晶体结构、形貌、表面粗糙度等性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)和原子力显微镜(AFM)进行了表征。之后,通过水热合成方法,在3种籽晶层衬底上制备得到具有不同结构和形貌特征的ZnO纳米棒阵列。结果表明,ZnO纳米棒生长和籽晶层制备方式具有极强的相关性。最后,对两者相关性的生长机理进行了解释。     

Reactivity of acetylenyl-terminated self-assembled monolayers on gold: triazole formation

We report the reactivity of acetylenyl-terminated self-assembled monolayers (SAMs) on gold toward "click" chemistry, Huisgen 1,3-dipolar addition, leading to the formation of triazoles. After the formation of acetylenyl-terminated SAMs, the triazole formation was performed on the SAMs and the reaction was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and contact angle goniometry. "Click" chemistry has offered a versatile strategy for the functionalization in solution chemistry with mild reaction conditions and a high compatibility in functional groups, and our result shows that the reaction could be applied to acetylenyl-terminated SAMs for the introduction of useful functional groups to the surfaces.     

Investigation of the mechanism of nickel electroless deposition on functionalized self-assembled monolayers

We have investigated the seedless electroless deposition (ELD) of Ni on functionalized self-assembled monolayers (SAMs) using scanning electron and optical microscopies, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. For all SAMs studied, the Ni deposition rate is dependent on the bath pH, deposition temperature, and complexing agent. In contrast to the physical vapor deposition of Ni on SAMs, electrolessly deposited Ni does not penetrate through the SAM. This behavior indicates that ELD is a suitable technique for the deposition of low-to-moderate reactivity on organic thin films. We demonstrate that Ni can be selectively deposited on SAMs using two different methods. First, selectivity can be imparted by the formation of Ni(II)-surface complexes. As a demonstration, we selectively deposited Ni on the -COOH terminated SAM areas of patterned -COOH/-CH(3) or -COOH/-OH terminated SAMs. Here, Ni(2+) ions form Ni(2+)-carboxylate complexes with the -COOH terminal group, which comprise the nucleation sites for subsequent metal deposition. Second, we demonstrate that nickel is selectively deposited on the -CH(3) terminated SAM areas of a patterned -OH/-CH(3) terminated SAM. In this case, the Ni(2+) ion does not specifically interact with the -CH(3) terminal group. Rather, selectivity is imparted by the interaction of the reductant, dimethylamine borane (DMAB), with the -OH and -CH(3) terminal groups.