Systematic control of the packing density of self-assembled monolayers using bidentate and tridentate chelating alkanethiols

The structural and interfacial properties of self-assembled monolayers (SAMs) on gold derived from the adsorption of a series of 1,1,1-tris(mercaptomethyl)alkanes (i.e., CH3(CH2)mC[CH2SH]3, where m = 9, 11, 13, 15) were investigated. The new SAMs, which possess uniformly low densities of alkyl chains, were characterized by ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy. Additional analysis of the SAMs by X-ray photoelectron spectroscopy permitted a direct calculation of the packing densities of the SAMs on gold. The results as a whole, when compared to those obtained on SAMs generated from normal alkanethiols (CH3(CH2)m+2SH), 2-alkylpropane-1,3-dithiols (CH3(CH2)mCH[CH2SH]2), and 2-alkyl-2-methylpropane-1,3-dithiols (CH3(CH2)mC(CH3)[CH2SH]2) having analogous chain lengths, demonstrate that the 1,1,1-tris(mercaptomethyl)alkanes afford SAMs with alkyl chains having the lowest packing density and least conformational order.     

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.     

Structural effects on the barrier properties of self-assembled monolayers formed from long-chain omega-alkoxy-n-alkanethiols on copper

The adsorption of long-chain omega-alkoxy-n-alkanethiols [CH(3)(CH(2))(p-1)O(CH(2))(m)SH; m = 11, 19, 22; p = 18, 22] onto copper produces self-assembled monolayers (SAMs) that can provide protection against corrosion of the underlying metal substrate. The resulting films are 40-60 A in thickness and are isostructural with SAMs formed on copper from unsubstituted n-alkanethiols. As evidenced by electrochemical impedance spectroscopy (EIS), the barrier properties of these ether-containing SAMs depend on the chain length of the adsorbate and the position of the ethereal unit along the hydrocarbon chain. For SAMs where the ether substitution is farther from the copper surface, the initial coating resistances are similar to those projected for unsubstituted n-alkanethiolate SAMs of similar thickness. For SAMs where the ether substitution is nearer to the copper surface (m = 11), the resistances are significantly less than those for unsubstituted n-alkanethiolate SAMs of similar thickness, reflecting the effect of the molecular structure on the barrier properties of the film. Upon exposure to 1 atm of O(2) at 100% RH, the SAMs become less densely packed as observed by infrared (IR) spectroscopy, and their barrier properties deteriorate as observed by EIS. The rate that the SAMs lose their barrier properties upon exposure to oxidizing conditions is correlated to the strength of intermolecular interactions within the bulk state of the adsorbate.     

新型偶氮苯硫醇衍生物自组装膜的制备与结构表征

自组装单分子膜(ＳＡＭｓ)是近年来引起广泛注意的一种稳定的、二维有序的、致密的有机超薄膜体系,由于其优越的性能,在润滑、吸附、防腐、电化学及微电子等领域中显示出广阔的应用前景[1～4].自组装单分子膜是使用含有各种活性官能团(如-ＣＯＯＨ,-ＳＨ,-Ｓ-Ｓ-,-ＯＨ,-ＣＮ等)的分子,以化学键的形式与相应的基底(如Ａｕ,Ａｇ,Ｃｕ,Ｐｔ,Ｓｉ,Ｍｉｃａ等)相互作用从而自发地形成自组装膜.根据不同的研究或应用目的合理设计组装分子的结构及基底表面,从而得到具有所需功能的自组装单分子膜是近年来界面科学和材料科学等领域研究的热点之一.…     

Study on self-assembled monolayers of functionalized azobenzene thiols on gold: XPS,electrochemical properties,and surface-enhanced Raman spectroscopy

Self-assembled monolayers (SAMs) of 4-acetamino-4'-(4-mercaptobutoxy)azobenzene (CH3)CONH-ph-N=N-ph-O(CH2)(4)SH, abbr. aaAzoC4SH) and 4-mercaptobutoxy azobenzene (ph-N=N-ph-O(CH2)(4)SH, abbr. AzoC4SH) on a gold surface have been studied by X-ray photoelectron spectroscopy (XPS), FT Raman spectroscopy, and electrochemistry. A surface-enhanced Raman scattering (SERS)-active system with a "sandwiched" structure of Ag/R-Azo-C4S-/Au was conveniently obtained by the method of Tollen's test. The relationship between the SERS effect and the structural nature of the system indicates that the enhancement correlates to both the silver islands above and the gold substrate underneath. The redox behaviors of the self-assembly on gold electrodes showed that the SAMs of the two compounds exhibit well-behaved voltammetric responses in a Britton-Robinson buffer corresponding to the irreversible two-electron, two-proton reduction-oxidation of azobenzene. The apparent electron-transfer rate kinetics is very sluggish, and the rate constant k(app) of aaAzoC4SH/Au (1.34 x 10(-6) s(-1)) is lower than that of AzoC4SH/Au (1.63 x 10(-4) s(-1)), which may be attributed to the different spatial restriction of close-packing structures on the conformational change accompanied by electron and proton transfer in the SAMs.     

末端碳链长度对偶氮苯自组装膜结构的影响

The end-group dominated molecular orientation in the azobenzene self-assembled monolayers (SAMs), CnAzoC2SH (n=1-4), on gold was evaluated for the first time by grazing incidence reflection absorption FTIR spectroscopy (RA-FTIR). All these azobenzene SAMs have highly-organized and closely-parked structures, with the molecule tilting away gradually from surface normal direction with the increase of end group alkyl length.     

Isostructural self-assembled monolayers. 2. Methyl 1-(3-mercaptopropyl)-oligo(ethylene oxide)s

The structural order and ordering conditions of the self-assembled monolayers (SAMs) of HSCH2CH2CH2O(EO)xCH3, where EO = CH2CH2O and x = 3-9, on polycrystalline gold (Au) were determined by reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy. For x = 5-7, RAIRS and SE data show that the oligo(ethylene oxide) [OEO] segments adopt the near single phase, 7/2 helical conformation of the folded-chain crystal polymorph of crystalline poly(ethylene oxide), oriented normal to the substrate. These SAMs exhibit OEO segment structure and orientation identical to that found in a previous isostructural series [HS(CH2CH2O)6-8C18H37 SAMs. Vanderah, D. J., et al. Langmuir 2003, 19, 3752] and are anisotropic films for surface science metrology where structure is constant and thickness increases in 0.30 nm increments. In addition, this is the first example of OEO SAMs to attain this highly ordered, helical conformation where the (EO)x segment is separated from the Au-sulfur headgroup by a polymethylene chain. For x = 4, 8, and 9, the SAMs are largely helical but show evidence of nonhelical conformations and establish the upper and lower limits of the isostructural set. For x = 3, the SAMs are largely disordered containing some all-trans conformation. SAM order as a function of immersion time from 100% water and 95% ethanol indicates that the HSCH2CH2CH2O(EO)5-7CH3 SAMs order faster and under a wider range of conditions than omega-alkyl 1-thiaolio(ethylene oxide) [HS(EO)xCH3] SAMs, reported earlier (Vanderah, D. J., et al. Langmuir 2002, 18, 4674 and Vanderah, D. J., et al. Langmuir 2003, 19, 2612).     

Self-assembled monolayers of CH3COS- terminated surfactant-encapsulated polyoxometalate complexes

Self-assembled monolayers (SAMs) on gold surfaces based on three kinds of acetylthio-surfactant-encapsulated polyoxometalate clusters (thio-SECs) terminated with multiple CH(3)COS- groups, (NC(26)H(55)S(CO)CH(3))(6)H(2)[Co(H(2)O)CoW(11)O(39)], (NC(26)H(55)S(CO)CH(3))(13)H(3)[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)], and (NC(26)H(55)S(CO)CH(3))(13)[Fe(4)(H(2)O)(2)(P(2)W(15)O(56))(2)]Br, have been prepared, which is representative of a general methodology to fabricate polyoxometalate-based SAMs. Thio-SECs self-assembled into monolayers on gold surfaces through the hydrolysis of CH(3)COS- groups and the subsequent formation of S-Au bonds, which was confirmed by grazing angle infrared spectroscopy, X-ray photoelectron spectroscopy, and ellipsometric and scanning tunneling microscopy (STM) measurements. Furthermore, the SAMs of the thio-SECs possess closely packed structures, and the local short-range order is clearly observed in the magnified STM image. We have also investigated the electrochemical behavior of SAMs of thio-SECs by cyclic voltammetry in detail, and the redox potential of the original polyoxometalates has been well retained. The electrochemical signals of the SAMs are very weak because of the small moiety of thio-SECs that are electrochemically accessible in the cyclic voltammetry experiments. The polyoxometalate-modified electrodes that we prepared are not only highly ordered in the local short range but also stable in electrochemical cycling because of the multiple S-Au bonds of thio-SECs on the gold substrates that aid in the construction of functional materials such as electrochemical and electrocatalytic devices.     

Self-assembled monolayers derived from a double-chained monothiol having chemically dissimilar chains

The structure and conformation of self-assembled monolayers (SAMs) derived from the adsorption of a specifically designed double-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluoro -2-tetradecylnona-decane-1-thiol ( 2) onto the surface of evaporated gold were examined by ellipsometry, contact angle goniometry, polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The results were compared to those of SAMs generated from normal hexadecanethiol ( 1) and a structurally related single-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluorononadecane-1-thiol ( 3). Collectively, the studies demonstrate that the double-chained adsorbate 2 forms SAMs on gold in which the alkyl chains are less densely packed and less conformationally ordered than those in the SAMs derived from each of the single-chained adsorbates. Furthermore, the fluorocarbon moieties in the SAMs derived from 2 are more tilted from the surface normal than those in the SAMs derived from 3. The low values of contact angle hysteresis suggest, however, that the double-chained adsorbate 2 generates homogeneous monolayer films on the surface of gold.     

新型偶氮苯硫醇衍生物自组装膜的制备与结构表征

Self Assembled Monolayers(SAMs) of a series of mercapto contained azobenzene derivatives with the structure of CnH2n+1AzoO(CH 2)mSH (where n =4,6,8,10,12; with m =3,5 respectively) were prepared and characterized. Wettability measurement of water on the SAMs demonstrates that molecular packing density in the monolayers increases while the alkyl chain in the molecules is lengthened. Both the n and m values have similar contribution to the wetting property of SAMs. The RA IR spectra reveal that the alkyl chains in the SAMs tilt away dramatically from the surface normal direction with the increase in their length. However, the orientation of azobenzene moiety is found to be influenced slightly by the alkyl chain length, which is due to the tenderness of the molecule.     

Novel tripod ligands for prickly self-assembled monolayers

The new tridentate thioether ligands PhSi(CH2SMe)3 (1) and Ph-p-C6H4Si(CH2SMe)3 (2) have been synthesised and used for the preparation of the chelates fac-[W(kappa3-1)(CO)3] and fac-[W(kappa3-2)(CO)3], which were characterised by single-crystal X-ray diffraction. 1 and 2 were used as tripodal adsorbate molecules for the fabrication of self-assembled monolayers (SAMs) on gold. Film formation from solution was investigated in situ by second harmonic generation (SHG) and ellipsometry, which revealed a two-stepped process (fast adsorption, followed by slow film ordering). SAMs of 2 on gold were further investigated by ex situ methods, viz. high-resolution X-ray photoelectron spectroscopy (HRXPS), Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), and scanning tunneling microscopy (STM). The latter two methods indicated dense packing of the tripodal anchor groups on the surface, with a substantially lower density of the biphenyl pricks. HRXPS showed three different binding states of sulfur, including a standard thiolate-type and a coordination-type state.     

In vitro stability study of organosilane self-assemble monolayers and multilayers

The stability of self-assembled monolayers (SAMs) and multilayers formed on silicon surface by amino-terminated silanes and SAMs formed by alkyl and glycidyl terminated silanes were investigated in vitro with saline solution at 37 degrees C for up to 10 days. FTIR and XPS results indicated that amino-terminated SAMs and multilayers are very unstable if the alkyl chain is short ((CH2)3), while stable if the alkyl chain is long ((CH2)11). On the other hand, alkyl-terminated SAMs are very stable regardless of the alkyl chain length, and glycidyl terminated SAM retained approximately 77% of the organosilane molecules after 10 days. Hydrogen bonding between the organosilane monomer and silicon surface and among the organosilane monomers is believed to contribute to the instability of the SAM and multilayer formed by amino-terminated silane with a short alkyl chain ((CH2)3). Therefore, the widely used (3-aminopropyl) trimethoxysilane (APTMS) SAM and multilayer may not be suitable for implantable biomedical applications.     

Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups

Whereas thiols and thioethers are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metal and semiconductor surfaces, their use for tridentate bonding configuration is still questionable. Against this background, novel tridentate thiol ligands, PhSi(CH(2)SH)(3) (PTT) and p-Ph-C(6)H(4)Si(CH(2)SH)(3) (BPTT), were synthesized and used as tripodal adsorbate molecules for the fabrication of SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The PTT and BPTT films were compared with the analogous systems comprised of same tripodal ligands with thioether instead of thiol binding units (anchors). XPS and NEXAFS data suggest that the binding uniformity, packing density, and molecular alignment of the thiol-based ligands in the respective SAMs is superior to their thioether counterparts. In addition, the thiol-based films showed significantly lower levels of contamination. Significantly, the quality of the PTT SAMs on Au(111) was found to be even higher than that of the films formed from the respective monodentate counterpart, benzenethiol. The results obtained allow for making some general conclusions on the specific character of molecular self-assembly in the case of tridentate ligands.     

An Experimental and Theoretical Approach to Investigate the Effect of Chain Length on Aminothiol Adsorption and Assembly on Gold

Despite the numerous studies on the self‐assembled monolayers (SAMs) of alkylthiols on gold, the mechanisms involved, especially the nature and influence of the thiol–gold interface are still under debate. In this work the adsorption of aminothiols on Au(111) surfaces has been studied by using surface IR and X‐ray photoelectron spectroscopy (XPS) as well as by density functional theory (DFT) modeling. Two aminothiols were used, cysteamine (CEA) and mercaptoundecylamine (MUAM), which contain two and eleven carbon atoms, respectively. By combining experimental and theoretical methods, it was possible to draw a molecular picture of the thiol–gold interface. The long‐chain aminothiol produced better ordered SAMs, but, interestingly, the XPS data showed different sulfur binding environments depending on the alkyl chain length; an additional peak at low binding energy was observed upon CEA adsorption, which indicates the presence of sulfur in a different environment. DFT modeling showed that the positions of the sulfur atoms in the SAMs on gold with similar unit cells [(2√3×2√3)R30°] depended on the length of the alkyl chain. Short‐chain alkylthiol SAMs were adsorbed more strongly than long‐chain thiol SAMs and were shown to induce surface reconstruction by extracting atoms from the surface, possibly forming adatom/vacancy combinations that lead to the additional XPS peak. In the case of short alkylthiols, the thiol–gold interface governs the layer, CEA adsorbs strongly, and the mechanism is closer to single‐molecule adsorption than self‐assembly, whereas for long chains, interactions between alkyl chains drive the system to self‐assembly, leading to a higher level of SAM organization and restricting the influence of the sulfur–gold interface.     

Alkyl-thiol Langmuir films on the surface of liquid mercury

The coverage dependent phase behavior of monolayers of alkyl thiols (CH3(CH2)(n-1)SH, denoted as CnSH) on mercury was studied for chain lengths 9 相似文献   

Preparation and electrochemistry of azobenzene self-assembled monolayers on gold--long range tunneling and end-group hydrogen bonding effect

A series of thiol-functionalied azobenzene derivatives (RAzoCnSH: R=H for n=3-6, abbreviated as AzoCnSH; R=CH(3)CONH for n=4, abbreviated as aaAzoC4SH) on gold electrodes were prepared and their self-assembly and electrochemical properties were studied by cyclic voltammetry. They all formed uniform and reproducible self-assembled monolayers (SAMs) on gold and showed well-behaved voltammetric responses in aqueous solution. Both the length of the alkyl chain spacer and the H-bonding of the end acetamino group had effects on the stability and the electrochemical kinetics of the SAMs, and the effect of the H-bonding was dominant. The surface coverage of the SAMs (AzoCnSH) is gradually increased with an increase of the alkyl chain spacer length, whereas the presence of the terminal acetamino group leads to a greater increase of the surface coverage. At a low scan rate, voltammetric responses corresponding to an irreversible two-electron, two-proton reduction/oxidation of the trans-azobenzene redox center were obtained in the range of +300 mV and -800 mV, which exhibited very large peak-to-peak splitting. At a high scan rate of 500 mV/s, two steps of reversible one-electron, one-proton reduction/oxidation corresponding to the cis-isomer in azobenzene-thiol SAMs (n is odd) was clearly observed between +300 and -200 mV. The apparent electron-transfer rate is decreased with increasing distance between the azobenzene redox center and the gold electrode. The existence of the end acetamino group which restricted the conformational change during the redox process also led to a decrease of the standard rate constant, and this restriction effect is more predominant than the distance effect.     

Directed, selective insertion of single molecules into patterned self-assembled monolayers of alkanethiols with different chain lengths

Pd(ii) pincer adsorbate molecules (1) were inserted into self-assembled monolayers (SAMs) of alkanethiols with different chain lengths (C(8) to C(18)) on annealed gold substrates. Their presence was brought to expression by reaction of with Au nanoclusters bearing phosphine moieties (2). The surface-confined Au nanoclusters were observed only on the shorter chain SAMs (C(8)SH to C(16)SH) and not on C(18)SH SAMs. This is attributed to the longer chain length of C(18)SH preventing the insertion of pincer molecules. Microcontact printing (microCP) with C(18)SH on unannealed gold substrates and the subsequent immersion of the substrates into C(8)SH, C(10)SH, C(12)SH, or C(16)SH solutions, yielded a series of patterned SAMs that have areas of thiols of different chain lengths. Insertion of 1 followed by expression using 2, or insertion of 3 showed inserted molecules only in the shorter chain SAM areas. The absolute particle densities in the former case were higher than on the corresponding homogeneous SAMs on annealed substrates, probably due to larger numbers of defects in the SAMs on unannealed substrates.     

偶氮苯衍生物自组装单分子膜中的分子取响

利用反射红外光谱研究了金表面一系列具有不同碳链长度的偶氮苯巯基衍生物的自组装单分子膜.通过对比各向同性样品的透射谱和单分子膜的反射谱中各个吸收峰强度,定量地研究了分子中各部分的取向与分子结构的关系.我们分别提出了烷基链和偶氮基团取向计算的方法,利用该方法成功地求得了分子中各部分在膜的倾角.结果显示,当分子中烷基链长度增大时,碳链和偶氮苯基团相对于法线的倾斜逐渐加剧.这种倾角的变化归因于分子中碳链间范德华引力增大时,引起分子逐渐倾斜以达到最佳的范德华接触.同时研究发现,烷基链和偶氮基团受碳长度变化的影响并不相同.当分子中亚甲基数目增多时,烷基链的倾角迅速增大而偶氮苯倾角的增大则相对缓慢,这反映了它们在空间需求和本身刚性上的不同。     

含酰胺结构的巯基自组装膜的设计与结构表征

提出了一种简便通用的合成巯基化合物的途径，以分子中的羧基CO2H为起始基团，与2-流基乙胺的氨基选择性缩合；合成了一系列具有RC（O）NHCH2CH2SH（R分别为偶氯苯衍生物，双炔衍生物及直链烷基）结构的化合物，并用接触角测量，电化学和掠角反射红外光谱（GIR-IR）等手段对这些化合物在金表面形成的自组装单分子膜进行了表征。发现4-（N－（2‘-巯基已基））酰胺偶氮苯的自组装膜表现出良好的电活性，电化学测定表面浓度为4．21×10(－10)mol·cm(-2)．当R为烷基链时，随烷基链的增长，膜的致密度与有序度增加GIR-IR证明在自组装腹中CH3（CH2）6C（O）NHCH2CH2SH的C＝O和N－H键与Au表面平行，分子轴线与Au表面近似垂直．     

Competition as a design concept: polymorphism in self-assembled monolayers of biphenyl-based thiols

Self-assembled monolayers (SAMs) of two omega-(4'-methylbiphenyl-4-yl)alkanethiols (CH(3)(C(6)H(4))(2)(CH(2))(n)SH, BPn, n = 4, 6) on Au(111) substrates, prepared from solution at room temperature and subsequently annealed at temperatures up to 493 K under a nitrogen atmosphere, were studied using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is observed, the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in density and stability. Upon annealing, BP4 and BP6 transform into a beta-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diameter. Exchange experiments, monitored by contact angle measurement, reveal that the beta-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.