Structures and displacement of 1-adamantanethiol self-assembled monolayers on Au{111}

We have designed monolayers with weak intermolecular interactions for use as placeholders in intelligent self- and directed-assembly. We have shown that these 1-adamantanethiolate monolayers are labile with respect to displacement by exposing them to dilute solutions of alkanethiols. These self-assembled monolayers (SAMs) of 1-adamantanethiol on Au{111} were probed using ambient scanning tunneling microscopy (STM), and their assembled order was determined. Solution deposition of the molecules results in a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbor distance of 6.9 +/- 0.4 A. The SAMs exhibit several rotational domains, but lack the protruding domain boundaries typical of alkanethiolate SAMs, and are similarly stable at room temperature. Co-deposition of alkanethiol and 1-adamantanethiol from solution results in alkanethiolate SAMs, except when using extremely low alkanethiol to 1-adamantanethiol concentration ratios. Facile displacement of low interaction strength SAMs can be exploited to enhance patterning using soft nanolithography.     

1-Adamantanethiolate monolayer displacement kinetics follow a universal form

Alkanethiol molecules in solution displace 1-adamantanethiolate self-assembled monolayers on Au{111}, ultimately leading to complete molecular exchange. Specifically, here, fast insertion of n-dodecanethiolate at defects in the original 1-adamantanethiolate monolayer nucleates an island growth phase, which is followed by slow ordering of the n-dodecanethiolate domains into a denser and more crystalline form. Langmuir-based kinetics, which describe alkanethiolate adsorption on bare Au{111}, fail to model this displacement reaction. Instead, a Johnson-Mehl-Avrami-Kolmogorov model of perimeter-dependent island growth yields good agreement with kinetic data obtained by Fourier transform infrared spectrometry over 100-fold variation in n-dodecanethiol concentration. Rescaling the growth rate at each concentration collapses all the data onto a single universal curve, suggesting that displacement is a scale-free process. The rate of displacement varies as the square-root of the n-dodecanethiol concentration across the 0.01-1.0 mM range studied.     

Dynamic double lattice of 1-adamantaneselenolate self-assembled monolayers on Au{111}

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.     

Control of alkanethiolate monolayer structure using vapor-phase annealing

We describe an annealing procedure for self-assembled monolayers (SAMs) that uses vapor-phase molecules to modify the local domain structure. Existing SAMs of decanethiolate on Au{111} were annealed using vapor-phase dodecanethiol molecules, so that the original and newly introduced molecules could be distinguished using scanning tunneling microscopy (STM). Molecules deposited from the vapor phase inserted at existing monolayer defect sites and domain boundaries, and at substrate step edges forming discrete network-like domains. The SAM molecular lattice can be preserved across molecular terrace boundaries between the decanethiolate and dodecanethiolate domains. Candidate molecular electronic component molecules were inserted from solution in the decanethiolate matrix as isolated molecules. These inserted molecules could then be surrounded by dodecanethiolate molecules introduced from the vapor phase, thus demonstrating a method for controlling the local environment of inserted molecules.     

Phase separation of ternary self-assembled monolayers into hydrophobic 1-dodecanethiol domains and electrostatically stabilized hydrophilic domains composed of 2-aminoethanethiol and 2-mercaptoethanesulfonic acid on Au(111)

Ternary self-assembled monolayers (SAM) composed of 2-aminoethanethiol (AET), 2-mercaptoethanesulfonic acid (MES), and 1-dodecanethiol (DDeT) form two types of domains as if it were a two-component SAM: DDeT-rich hydrophobic domains and electrostatically stabilized hydrophilic domains composed of MES and AET on Au(111). MES and AET behave virtually as a single surface-active species. Two distinct reductive desorption peaks in cyclic voltammograms (CV) and binarized images of scanning tunneling microscopy clearly show nanometer scale, yet macroscopically distinguishable, phase separation over a wide range of the mixing ratio of DDeT and MES-AET in the bathing solution. X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups are in the charged states, that is, the sulfonate group and the ammonium group. With decreasing the total concentration of the thiols, the mole fraction of DDeT in the bathing solution at which the surface coverage of MES-AET domains is equal to that of DDeT domains dramatically decreases. This suggests that the adsorption kinetics plays a crucial role in the formation of the domains structure.     

Effect of crystallization rate and colloidal stability on structural rearrangements during growth of colloidal monolayers

The structural rearrangements during growth of colloidal crystals were investigated using a combination of light microscopy and image analysis based on a Delaunay triangulation procedure. We followed the creation and disappearance of square lattice domains during the convection-promoted formation of colloidal monolayers by drying. We found that the concentration of square lattice domains increased with the crystal growth rate and that there is a direct relation between the concentration of square lattice domains formed at the crystal-suspension interface and the lower concentration of these domains in the colloidal monolayer; hence, the degree of rearrangement from square lattice domains to a close-packed triangular structure is not significantly affected by the crystal growth rate for colloidally stable suspensions. The colloidal stability, manipulated by the addition of salt, has a profound influence on the structural features of the growing monolayers. Particles that adhere strongly to each other, and to the substrate, tend to resist rearrangement; hence, the defect density is high in the colloidal monolayers and the structural reorganization of the square lattice domains to the more stable close-packed triangular structure occurred gradually over large distances from the crystal-suspension interface.     

Anomalous uniform domain in a twisted nematic cell constructed from micropatterned surfaces

We have discovered an optically uniform type of domain that occurs in twisted nematic (TN) cells that are constructed from substrates chemically patterned with stripes via microcontact printing of self-assembled monolayers; such domains do not occur in TN cells constructed from uniform substrates. In such a cell, the azimuthal anchoring at the substrates is due to the elastic anisotropy of the liquid crystal rather than the conventional rubbing mechanism. A model is presented that predicts the relative stability of the twisted and anomalous states as a function of the material and design parameters.     

Concentration dependence in microcontact printing of self-assembled monolayers (SAMs) of alkanethiols

This communication discusses the electrochemical assessment of self-assembled monolayers (SAMs) formed via microcontact printing with various concentrations of 1-hexadecanethiol (HDT) ink. At concentrations above 20 mM, the printed SAMs are shown to have very similar qualities to those formed from solution using much longer preparation procedures.     

Patterned monolayer self-assembly programmed by side chain shape: four-component gratings

A molecular recognition strategy based on alkadiyne side chain shape is used to self-assemble a four-component, 1D-patterned monolayer at the solution-HOPG interface. The designed monolayer unit cell contains six molecules and spans 23 nm × 1 nm. The unit cell's internal structure and packing are driven by complementary shapes and lengths of six different alkadiyne side chains. A solution of the four compounds on HOPG self-assembles monolayers (i) comprised, almost entirely, of the intended unit cell, (ii) exhibiting patterned domains spanning 10(4) nm(2), and (iii) which are sufficiently robust that patterned domains survive solvent rinsing and drying. The patterned monolayer affords 1D-feature spacings ranging from 3.3 to 23 nm. The results demonstrate the remarkable selectivity afforded by molecular recognition based on alkadiyne side chain shape and the ability to program highly complex 1D-patterns in self-assembled monolayers.     

Tunable plasmonic response from alkanethiolate-stabilized gold nanoparticle superlattices: evidence of near-field coupling

We report on the self-assembly of large-area, highly ordered 2D superlattices of alkanethiolate-stabilized gold nanoparticles ( approximately 10.5 nm in core diameter) onto quartz substrates with varying lattice constants, which can be controlled by the alkyl chain lengths, ranging from C12 (1-dodecanethiolate), C14 (1-tetradecanethiolate), C16 (1-hexadecanethiolate), to C18 (1-octadecanethiolate). These 2D nanoparticle superlattices exhibit strong collective surface plasmon resonance that is tunable via the near-field coupling of adjacent nanoparticles. The approach presented here provides a unique and viable means of building artificial "plasmonic crystals" with precisely designed optical properties, which can be useful for the emerging fields of plasmonics, such as subwavelength integrated optics.     

Synthesis of thiol-derivatized europium porphyrinic triple-decker sandwich complexes for multibit molecular information storage

The storage of multiple bits of information at the molecular level requires molecules with a large number of distinct oxidation states. Lanthanide triple-decker sandwich molecules employing porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information storage applications. Five triple-decker building blocks have been prepared of the type (phthalocyanine)Eu(phthalocyanine)Eu(porphyrin), each bearing one iodo, one ethyne, or one iodo and one ethyne group attached to the porphyrin unit. Two triple-decker building blocks with different oxidation potentials were derivatized with an S-acetylthiophenyl unit for attachment to an electroactive surface. To explore the preparation of arrays comprised of triple deckers, which may lead to the storage of a larger number of bits, two types of dyads of triple deckers were prepared. An ethyne-linked dyad of triple deckers bearing one S-acetylthiophenyl unit was prepared via repetitive Sonogashira couplings, and a butadiyne-linked dyad was prepared via a modified Glaser coupling. The triple deckers were characterized by absorption spectroscopy, laser-desorption mass spectrometry, and (1)H NMR spectroscopy. The thiol-derivatized triple deckers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. These electrochemical characteristics indicate that these types of molecules are well suited for storing multiple bits of information.     

Sum frequency generation microscopy of microcontact-printed mixed self-assembled monolayers

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.     

19F fast magic-angle spinning NMR studies of perfluoroalkanoic acid self-assembled monolayers

The bonding and dynamic properties of perfluoroalkanoic acid self-assembled monolayers (SAMs) on zirconia and titania powders were characterized by Fourier transform infrared and solid-state 19F magic-angle spinning NMR spectroscopy. The perfluoro fatty acids investigated included C(n)F(2n+1)CO2H, n = 7, 13, 15 and 17. The acids bind to both metal oxides via ionic carboxylate bonds, but complete monolayers are only formed on the zirconia. The shift of the CF3 group from -83 ppm in the bulk state to -85 ppm for the adsorbed monolayers is assigned to CF3 groups located at the air/monolayer interface. With the exception of the perfluorooctanoic acid, 19F spin lattice relaxation measurements indicate that the fluorocarbon chains of the adsorbed acids, even in the case of densely packed monolayers, are significantly more mobile than those in the bulk state. The motions associated with the enhanced mobility of the adsorbed acids are proposed to involve reorientations along the long chain axes. No evidence for chain melting in the fluorocarbon SAMs is found for temperatures well above the melting temperature of the bulk acids.     

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.     

Covalent grafting nitrophenyl group on Au surface via click reaction: Assembling process and electrochemical behaviors

In this paper, we described a simple and facile method for the covalent functionalization of Au surface with nitrophenyl group via stepwise strategy. Structurally well-defined azide-terminated organic self-assembled monolayers (SAMs) were formed on Au surface from a commercially available mixture solution of azidoundecanethiol and dilute thiol. Subsequent, derivatization of the azide-terminated monolayers was conducted in aqueous environments with ethynyl nitrobenzene via a selective, reliable, robust click reaction. By this way, the nitrophenyl group was covalently and quantitatively grafted on Au surface, which was confirmed by Raman Spectrometry and electrochemical methods. These results demonstrated the efficiency of using click chemistry in assembling covalently linked nanostructures.     

Self-organization of a wedge-shaped surfactant in monolayers and multilayers

The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at approximately 10 degrees C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are approximately 0.5 nm thinner than the LC regions. AFM images were also obtained of self-assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self-assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.     

Structure and conductance of aromatic and aliphatic dithioacetamide monolayers on Au(111)

The structure and electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides (1,4-bis(mercaptoacetamido)benzene and 1,4-bis(mercaptoacetamido)cyclohexane) and of mixed dithioacetamide/alkanethiol monolayers are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and contact angle goniometry. Both dithioacetamides are found to pack densely on Au(111), however the monolayers are poorly ordered as a result of hydrogen bond formation between the amide groups. The coassembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers. By coassembly, islands of dithioacetamides in a dodecanethiol matrix can only be obtained at a low dithioacetamide/dodecanethiol concentration ratio in solution (1/10) and by thermal annealing of the resulting monolayers. Small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles and molecular lengths resulting from density functional theory (DFT) calculations. The difference in the tunneling decay constant beta measured for aromatic dithioacetamides (beta = 0.74-0.76/A) and for aliphatic dithioacetamides (beta = 0.84-0.91/A) highlights the influence of the conjugation within the cyclic core on molecular conductance.     

Attachment of ferrocene nanotubes on beta-cyclodextrin self-assembled monolayers with molecular recognitions

Ferrocene nanotubes were fabricated by binding carboxylic acid-derivatized ferrocenes onto template peptide nanotubes via hydrogen bonding. When these ferrocene-functionalized nanotubes were incubated with beta-cyclodextrin (beta-CD) self-assembled monolayers (SAMs) coated on patterned Au substrates in solution, the ferrocene nanotubes recognized and attached onto the beta-CD SAMs via host-guest molecular recognition. The ferrocene nanotubes were also observed to recognize the certain cavity size of CD. The attachment/detachment of nanotubes on the beta-CD SAMs was controlled electrochemically by tuning the redox states of ferrocene nanotubes. This electric field-responsive building block may be applied to build nanometer-sized switching components in electronics and sensors.     

Coating of titanium implant materials with thin polymeric films for binding the signaling protein BMP2

A fast and simple approach for immobilization using copolymers as interlayers is reported. The synthesized copolymers form stable self-assembled layers on implant materials like, e.g., titanium in a simple coating/drying/washing sequence and have functional groups which can bind proteins from an aqueous solution. The copolymer films have been characterized via ellipsometry and contact angle measurements and were tested for biocompatibility. An immunoassay was used to determine the amount of BMP2 and demonstrated an approximately 10-fold increase as compared to previously used self-assembled monolayers. A BMP2-responsive cell line with luciferase detection was used to determine the biological activity of the bound signaling protein.     

Halogen bonds as stabilizing interactions in a chiral self-assembled molecular monolayer

We report the formation of highly-ordered self-assembled monolayers of an achiral organic semiconductor molecule. STM results show spontaneous formation of very large single domains of ordered chiral monolayers. DFT calculations support the identification of halogen bonds as the primary interactions that steer molecular self-assembly, leading to organizational chirality.