Influence of alkyl chain length on the structure of dialkyldithiophosphinic Acid self-assembled monolayers on gold

We report the formation and characterization of self-assembled monolayers (SAMs) based on dialkyldithiophosphinic acid adsorbates {[CH(3)(CH(2))(n)](2)P(S)SH (n = 5, 9, 11, 13, 15)} on gold substrates. SAMs were characterized using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact angle measurements, and electrochemical impedance spectroscopy. Data show that there is a roughly 60:40 mixture of bidentate and monodentate adsorbates in each of these SAMs. The presence of monodentate adsorbates is due to the numerous and deep grain boundaries of the underlying gold substrate, which disrupt chelation. Comparing the characterization data of dialkyldithiophosphinic acid SAMs with those of analogous n-alkanethiolate SAMs shows that both SAMs follow a similar trend: The alkyl chains become increasingly organized and crystalline with increasing alkyl chain length. The alkyl groups of dialkyldithiophosphinic acid SAMs, however, are generally less densely packed than those of n-alkanethiolate SAMs. For short alkyl chains (hexyl, decyl, and dodecyl), the significantly lower packing densities cause the alkyl chains to be liquid-like and disorganized. Long-chain dialkyldithiophosphinic acid SAMs are only slightly less crystalline than analogous n-alkanethiolate SAMs.     

Multiscale charge injection and transport properties in self-assembled monolayers of biphenyl thiols with varying torsion angles

This article describes the molecular structure-function relationship for a series of biphenylthiol derivatives with varying torsional degree of freedom in their molecular backbone when self-assembled on gold electrodes. These biphenylthiol molecules chemisorbed on Au exhibit different tilt angles with respect to the surface normal and different packing densities. The charge transport through the biphenylthiol self-assembled monolayers (SAMs) showed a characteristic decay trend with the effective monolayer thickness. Based on parallel pathways model the tunneling decay factor β was estimated to be 0.27??(-1) . The hole mobility of poly(3-hexylthiophene)-based thin-film transistors incorporating a biphenylthiol SAM coating the Au source and drain electrodes revealed a dependence on the injection barrier with the highest occupied molecular orbital (HOMO) level of the semiconductor. The possible role of the resistivity of the SAMs on transistor electrodes on the threshold voltage shift is discussed. The control over the chemical structure, electronic properties, and packing order of the SAMs provides a versatile platform to regulate the charge injection in organic electronic devices.     

Binary self-assembled monolayers of alkanethiols on gold: deposition from solution versus microcontact printing and the study of surface nanobubbles

The coadsorption of alkanethiols on noble metals has been recognized for a long time as a suitable means of affording surfaces with systematically varied wettability and other properties. In this article, we report on a comparative study of the composition of the mixed self-assembled monolayers (SAMs) obtained (i) by the coadsorption of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) from ethanol and chloroform onto gold substrates and (ii) by microcontact printing using poly(dimethyl siloxane) (PDMS) stamps. SAMs prepared by coadsorption from solution showed a preferential adsorption of ODT for both solvents, but this trend was reversed in microcontact-printed SAMs when using chloroform as a solvent, as evidenced by contact angle and Fourier transform infrared (FTIR) spectroscopy measurements. An approximately linear relationship between the static contact angle and the degree of swelling with different solvents was observed, which suggests that the surface composition can be controlled by the interaction of the solvent and the PDMS elastomer. The altered preference is attributed to the different partitioning of the two thiols into solvent-swelled PDMS, as shown by (1)H NMR spectroscopy. Finally, molecularly mixed binary SAMs on ODT and MHDA on template-stripped gold were applied to study the effect of surface nanobubbles on wettability by atomic force microscopy (AFM). With a decreasing macroscopic contact angle measured through water, the nanoscopic contact angle was found to decrease as well.     

A modular molecular photovoltaic system based on phospholipid/alkanethiol hybrid bilayers: photocurrent generation and modulation

Monolayer quantities of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), incorporated with either fullerenes or ruthenium tris(bipyridyl) (Ru(bpy)(3)(2+)) complexes, were formed on ferrocene-terminated C11-alkanethiol self-assembled monolayers (SAMs) through lipid fusion. Thus formed hybrid structures are characterized by quartz crystal microbalance, UV-vis spectroscopy, cyclic voltammetry and impedance analysis. In comparison to lipid monolayers deposited on C12-alkanethiol SAMs, photocurrent generation from these ferrocene-based structures is significantly modulated, displaying attenuated anodic photocurrents and enhanced cathodic photocurrents. While a similar trend was observed for the two photoagents studied, the degree of such modulations was always found to be greater in fullerene-incorporated bilayers. These findings are evaluated in the context of the film structure, energetics of the involved photo(electrochemical) species and cross-membrane electron-transfer processes.     

自组装单分子膜及其表征方法

自组装单分子膜的研究是近年来十分活跃的研究领域. 随着膜的应用领域的拓展 ,对膜的表征方法不断提出新的要求.本文综述了自组装单分子膜体系的类型和基底表面的 处理方法,着重从电化学、谱学、显微学以及表面润湿性等方面综述了近几年来自组装单分子膜的表征方法研究进展, 并对其发展前景作了展望.     

Strong resistance of oligo(phosphorylcholine) self-assembled monolayers to protein adsorption

In this work, we demonstrate the strong resistance of oligo(phosphorylcholine) (OPC) self-assembled monolayers (SAMs) to protein adsorption and cell adhesion. OPC SAMs were characterized using X-ray photoelectron spectroscopy (XPS), and protein adsorption was measured using a surface plasmon resonance (SPR) sensor. Results are compared with those of phosphorylcholine (PC) SAMs. Despite the existence of negative charge on OPC SAMs and the simple synthesis procedure of OPC thiols, OPC SAMs resist protein adsorption as effectively as or better than PC SAMs formed from highly purified PC thiols. The ease of their preparation and the effectiveness of their function make OPC SAMs an attractive alternative for creating nonfouling surfaces.     

Characterization of self-assembled monolayers of porphyrins bearing multiple thiol-derivatized rigid-rod tethers

A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPS(n) (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPS(n) SAMs were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPS(n) SAMs. (1) The ZnPS(n) molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS(3) and ZnPS(4) SAMs bind to the surface in a more upright orientation than the porphyrins in the ZnPS(1), cis-ZnPS(2), and trans-ZnPS(2) SAMs. The porphyrins in the ZnPS(3) and ZnPS(4) SAMs are also more densely packed than those in the cis-ZnPS(2) and trans-ZnPS(2) SAMs. The packing density of the ZnPS(3) and ZnPS(4) SAMs is similar to that of the ZnPS(1) SAMs, despite the larger size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPS(n) SAMs. The general similarities in the electron-transfer characteristics for all of the SAMs are attributed to the similar binding motif.     

Odd-even effects in charge transport across self-assembled monolayers

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (Ag(TS)) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (～1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga(2)O(3)/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes--the "odd-even effect"--is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J(0). This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation.     

硫醇自组装膜对金电极上吡咯电化学聚合的影响

用电化学聚合法在多种烷基硫醇自组装膜修饰金电极上制备了聚吡咯.通过计时安培法、循环伏安法和交流阻抗技术研究了自组装膜的烷基链长和端基功能团对吡咯聚合过程和性质的影响.当自组装膜较完美时,聚吡咯沉积在自组装膜表面;而当自组装膜有一定缺陷时,吡咯在针孔处成核,然后继续生长并完全覆盖在自组装膜表面.研究结果表明,烷基硫醇的链越短,吡咯聚合越容易;疏水的烷基硫醇自组装膜有利于聚吡咯在电极表面的生长.     

Odd-even effects in self-assembled monolayers of omega-(biphenyl-4-yl)alkanethiols: a first-principles study

Conjugated molecules with a saturated alkyl linker between a thiol docking group and the pi-conjugated core have been shown to form self-assembled monolayers (SAMs) with a high degree of long-range order and uniformity. Additionally, pronounced odd-even effects have been observed in a number of properties characterizing these SAMs. We focus on omega-(biphenyl-4-yl)alkanethiols with n = 0-6 -(CH2)n- units deposited on Au(111) and investigate the microscopic origin of these odd-even effects in terms of the local sulfur-gold bonding geometry by employing first-principles calculations. An additional structural parameter, the torsion angle between the two phenyl rings in the biphenyl moiety, is identified and its relation to the experimentally observed odd-even effects is discussed. More importantly, we address relevant quantities for the application of these SAMs in molecular electronic devices, in particular, the modification of the work function of the underlying metal substrate and the energetic alignment of the molecular orbitals in the SAM with the Fermi level. While no clear trend emerges for the former, we find pronounced odd-even effects for the latter. Furthermore, the insertion of a single methylene unit between the biphenyl core and the thiol appears to largely decouple the valence electronic systems of the pi-conjugated segment and the gold substrate. Our results thus provide a solid theoretical basis for the interface energetics in this important class of systems.     

Dynamic and collective electrochemical responses of tetrathiafulvalene derivative self-assembled monolayers

Electroactive tetrathiafulvalene (TTF)-containing alkanethiol self-assembled monolayers (SAMs) were designed and synthesized to elucidate the relationship between electrochemical responses and film structures. Two TTF derivative molecules having one alkanethiol chain (1) and two alkanethiol chains (2) were utilized to modulate the molecular packing arrangements in the SAMs, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR). SPR measurements in various contacting media demonstrated loose packing of SAM 1 and close packing of SAM 2 due to the different space fillings of the molecules. Two successive one-electron redox waves were observed for both SAMs by cyclic voltammetry. The peak widths of the redox waves were strongly dependent on the oxidation states of the TTF moieties, the packing arrangement of the SAMs, and the contacting medium. We found that TTF-based SAMs exhibited collective electrochemical responses induced by dynamic structural changes, depending on the degree of freedom for the component molecules in the SAMs. These results imply that the molecular design, taking into account the electrochemical responses, extends the available range of molecular-based functionalities in TTF-based SAMs.     

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.     

基底电位对硫醇自组装膜形成的影响

应用电位阶跃法,在不同组装电位下制备金/正十二硫醇自组装单分子膜.交流阻抗谱表征该硫醇膜的电化学性质,发现金基底的电位对硫醇自组装膜的形成有重要影响.在-0.8~-0.4 V的电位区间内,随着组装电位的增加,该自组装膜的致密性、有序性增加,缺陷减少,并于0.4 V时达到最佳.组装电位高于0.4 V,膜的致密性、有序性降低,缺陷增多.本文为硫醇及其衍生物的电位调控组装提供了重要依据.     

Metallic contact formation for molecular electronics: interactions between vapor-deposited metals and self-assembled monolayers of conjugated mono- and dithiols

We present grazing-incidence Fourier transform infrared and AFM data of Au, Al, and Ti vapor-deposited onto self-assembled monolayers (SAMs) of conjugated mono- and dithiols. SAMs of 4,4'-dimercapto-p-quaterphenyl, 4,4"-dimercapto-p-terphenyl, and 4,4'-dimercapto-p-biphenyl have reactive thiols at the SAM/vacuum interface that interact with vapor-deposited Au or Al atoms, preventing metal penetration. Conjugated monothiols lack such metal blocking groups, and metals (Au, Al) can penetrate into their SAMs. Vapor deposition of Ti onto conjugated mono- and dithiol SAMs and onto hexadecanethiol SAMs destroys the monolayers.     

Supramolecular microcontact printing and dip-pen nanolithography on molecular printboards

The transfer of functional molecules onto self-assembled monolayers (SAMs) by means of soft and scanning-probe lithographic techniques-microcontact printing (muCP) and dip-pen nanolithography (DPN), respectively-and the stability of the molecular patterns during competitive rinsing conditions were examined. A series of guests with different valencies were transferred onto beta-cyclodextrin- (beta-CD-) terminated SAMs and onto reference hydroxy-terminated SAMs. Although physical contact was sufficient to generate patterns on both types of SAMs, only molecular patterns of multivalent guests transferred onto the beta-CD SAMs were stable under the rinsing conditions that caused the removal of the same guests from the reference SAMs. The formation of kinetically stable molecular patterns by supramolecular DPN with a lateral resolution of 60 nm exemplifies the use of beta-CD-terminated SAMs as molecular printboards for the selective immobilization of printboard-compatible guests on the nanometer scale through the use of specific, multivalent supramolecular interactions. Electroless deposition of copper on the printboard was shown to occur selectively on the areas patterned with dendrimer-stabilized gold nanoparticles.     

Intermixed terpyridine-functionalized monolayers on gold: nonlinear relationship between terpyridyl density and metal ion coordination properties

Aiming at the functionalization of surfaces with terpyridine anchors for the coordinative deposition of additional layers, mixed self-assembled monolayers (SAMs) were prepared from binary solutions of 12-(2,2':6',2″-terpyridine-4'-yl)dodecane-1-thiol (TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting molecules were analyzed by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition of the (TDT/DT)-SAMs and with it the surface density of terpyridyl groups correlates linearly with the relative concentrations of the two compounds in the solution used for depositing them. In marked contrast, the amount of terpyridine-coordinated Pd(II) ions significantly deviates from this trend with an optimum at a 1:3 ratio of TDT/DT. This indicates a major fraction of the terpyridines in TDT-rich SAMs not to be accessible for Pd(II) ion coordination. In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich SAMs not to be ordered, while they are preferentially upright oriented in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine backfolding in TDT-rich SAMs, while they are located in accessible positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy shows the terpyridyl groups not to have a preferential orientation in this SAM and thus retain enough flexibility to adjust to molecules that are deposited on top of the mixed SAM. In conclusion, the novel SAM does not undergo phase separation and consists predominantly of intermixed phases with adjustable surface density of quite flexible terpyridine anchor groups. The terpyridine-Pd(II) anchors are not only available for a future deposition of the next layer, but the metal ions also represent a sensitive probe for the accessibility of the terpyridyl groups.     

铝表面自组装端羟基聚酯胺膜的XPS和STM表征

制备了端羟基聚酯胺(HTP)在铝基片上的自组装膜并进行了XPS和STM表征,确认了HTP在铝基片上的吸附组装;由XPS的吸附时间扫描,自组装单分子膜(SAMs)随浸泡时间的延长而增厚,在2 h后达到平衡。由HTP在铝基片的STM三维图可以看到HTP自组装膜以团块形式组成,其间包含孔洞缺陷。提出了铝基片上HTP–SAMs的多羟基结构与膜上孔洞的共同作用是提高环氧类涂层附着力的主要原因。     

Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers

We present a quantitative study of the nanoscale frictional properties of one-component (pure) and two-component (mixed) alkylsilane self-assembled monolayers (SAMs). The load and velocity dependence of the friction force was measured in air and ethanol using lateral force microscopy (LFM). It was observed that for SAMs with well-ordered structure (pure SAMs and mixed SAMs composed of two long chain molecules) friction depends nonlinearly on load, at low loads, both in air and in ethanol. These observations are consistent with the low-load contact area predictions of the Johnson-Kendall-Roberts (JKR) theory, indicating that for well-ordered SAMs friction force is proportional to contact area and that the true contact area is determined by elastic deformation of the SAM by the LFM probe. In ambient air, the magnitude of the friction force measured using mixed SAMs is found to be similar to that obtained using pure SAMs at the same external load. Changing the medium to ethanol, however, leads to dramatically lower friction in the mixed SAMs. An analysis of the friction data using a thermally activated Eyring model that takes into account the monolayer viscoelasticity suggests that the better friction properties of the mixed SAMs are a consequence of greater disorder and higher molecular mobility in the outer layer/canopy. These findings indicate that multi-tiered SAM coatings comprising a highly ordered underlayer and a disordered, mobile canopy can provide the basis for low-friction coatings for small mechanical systems.     

Octadecyltrichlorosilane (OTS): a resist for OMCVD gold nanoparticle growth

One application of octadecyltrichlorosilane (OTS) self‐assembled monolayers (SAMs) is its use as thin film resists. In this work, we demonstrated that OTS SAMs can be reliable resists for organo‐metallic chemical vapor deposition (OMCVD) grown gold nanoparticles (Au NPs). In optical sensing applications based on Au NPs, one candidate system consists of patterned OTS SAMs and precisely grown OMCVD Au NPs for achieving a high sensitivity. As an initial step, the OTS SAMs need to perfectly resist the OMCVD Au NP growth. Hence the optimized formation of the OTS SAMs affected by different assembly times and baking temperatures was studied by contact angle, ellipsometry, XPS, SEM, and atomic force microscopy (AFM). To demonstrate the ability of the OTS SAMs to resist OMCVD Au NP growth, the OMCVD process was carried out on two sets of samples: OTS SAMs fabricated under optimized conditions on one set and the other set without OTS SAMs. High‐resolution XPS, RBS, SEM, and ultraviolet‐visible (UV‐Vis) spectroscopy were applied to study the growth of Au NPs on the samples with and without OTS SAM resists. Copyright © 2009 John Wiley & Sons, Ltd.     

Electroactive self-assembled monolayers on gold via bipodal dithiazepane anchoring groups

Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized. Self-assembled monolayers (SAMs) of these ferrocene derivatives have been studied by X-ray photoelectron spectroscopy, ellipsometry, and cyclic voltammetry. It has been shown by XPS that monolayers of the dithiazepane-anchored molecules on gold electrodes contain gold-thiolate species. Cyclic voltammetry of the SAMs were characteristic of stable electroactive monolayers even for single-component SAMs of 1 and 2, with the more ideal responses recorded for the two-component SAMs diluted with undecanethiol. The small variation in peak splittings at progressively higher scan rates in these SAMs makes dithiazepane-bridged redox species promising candidates for further studies on molecular wires with bipodal anchoring.