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.     

Organic monolayers chemisorbed on gold observed by scanning probe microscopy

Scanning force (SFM) and scanning tunneling (STM) microscopies are suitable techniques for the investigation of the structure of organic monolayers. Results are presented on thioalkane monolayers and thiolipid monolayers on gold. Both molecules attach covalently to the gold surface. STM images of the self assembled dodecanethiol layer display the molecular order of the film and reveal the presence of defects at the molecular scale. Moreover, domains and domain boundaries can be distinguished. Thiolipid layers on gold have been observed by SFM. The monolayer separates in solid-analogous star shaped domains and fluid-analogous domains. Imaging under water demonstrates the stability of the layer.     

A new phase of the c(4 x 2) superstructure of alkanethiols grown by vapor phase deposition on gold

A self-assembled monolayer of dodecanethiol is grown onto (111) oriented gold by vacuum phase deposition and studied by ultrahigh vacuum scanning tunneling microscopy (STM). The films consist of domains that exhibit the c(4 x 2) over-structure of the hexagonal (square root of 3 x square root of 3)R30 of alkanethiols on gold. The domain size is only limited by the terrace size of the underlying gold. By higher resolution scans a new phase of the c(4 x 2) structure consisting of four inequivalent molecules that display different heights in the STM images is discovered.     

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.     

Scanning tunneling microscopy and spectroscopy studies of 4-methyl- 4'-(n-mercaptoalkyl)biphenyls on Au(111)-(1x1).

4-methyl-4'-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n=3-6, BPn) monolayers assembled on Au(111)-(1x1) in 1,3,5,-trimethylbenzene (TMB) at various temperatures are studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images reveal that BP3 and BP5 form a (sqrt 3x2sqrt 3) repeating motif superimposed on a temperature-dependent Moire pattern. BP4 and BP6 adlayers are characterized by a coexisting (2sqrt 3x5sqrt 3) majority phase and a temperature-dependent (3xpsqrt 3) minority phase. Assembly at 60 degrees C or 90 degrees C leads to p=5. Compression of the adlayer was found at higher temperatures. Combined with high-resolution structure experiments, the electronic characteristics of BP3 and BP4 self-assembled monolayers (SAMs) were studied by monitoring current-distance (iT-Deltaz) and current-voltage (iT-Ebias) characteristics in TMB employing a gold STM tip|BPn|Au(111)-(1x1) configuration. The semilogarithmic (iT-Deltaz) plots yielded three linear regions in the range 10 pA相似文献   

Long-range self-assembly of a polyunsaturated linear organosilane at the n-tetradecane/Au(111) interface studied by STM

We report on the formation of self-assembled monolayers of 13-(trimethylsilyl)-1-tridecene-6,12-diyne [C13H17-Si(CH3)3], an organosilane derivative with a linear polyunsaturated chain, on Au(111) substrates. Molecular resolution STM images recorded at the liquid-solid interface between gold and tetradecane reveal a long-range and densely packed hexagonal lattice with a ( radical3 x radical3)R30 degrees -like structure commensurate against gold adlattice.     

Versatility of aqueous micellar solutions for self-assembled monolayers engineering

Self-assembly of aliphatic as well as aromatic thiol-terminated molecules was achieved onto a variety of gold surfaces using aqueous micellar solutions. Scanning tunneling microscopy experiments allowed us to demonstrate that the increase in the density of self-assembled monolayers (SAMs) prepared from micellar aqueous solvent compared to that prepared from ethanol directly originates from the decrease in defect density in the SAM (etch pits, domain boundaries) and not from a denser local packing of the molecules. Extending the use of such an aqueous solvent to various conjugated molecules, we report for the first time the insertion of these molecules from an aqueous solution in a dodecanethiol (DT) SAM and the ligand-exchange on the surface of DT stabilized gold nanoparticles deposited as a Langmuir-Blodgett film. Finally, we show that aqueous micellar DT solutions allow the preparation of DT SAMs on gold through a micropatterned resist mask. These results make possible the use of water to deliver molecules on a solid substrate to build molecular devices in a way compatible with lithography requirements in microelectronic processes.     

On the importance of purity for the formation of self-assembled monolayers from thiocyanates

Assembly of dodecyl thiocyanate (C12SCN) from ethanol solution onto Au(111)/mica substrates was investigated by scanning tunneling microscopy (STM), near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), and infrared reflection-absorption spectroscopy (IRRAS). Contrary to previous reports, thiolate monolayers formed by cleavage of the S-CN bond can be obtained whose quality is at least as good as that of self-assembled monolayers (SAMs) formed directly from the thiol analogue of C12SCN, dodecanethiol (C12SH). However, the achievable quality is strikingly dependent on the purity of the thiocyanate with even low levels of contamination impeding the formation of structurally well-defined monolayers.     

Molecular-level approach to inhibit degradations of alkanethiol self-assembled monolayers in aqueous media

A molecular-level approach is developed to prevent or inhibit the degradation processes of alkanethiol self-assembled monolayers (SAMs). Previous studies revealed two degradation pathways: direct desorption and oxidation-desorption. By use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in situ and time-dependent imaging reveals and confirms that degradations of alkanethiol SAMs on gold mainly initiate at defect sites, such as domain boundaries and vacancy islands, and then propagate into the ordered domains. Our approach targets at attaching small molecules with preferred adhesion to the defects. The best candidates are aqueous media containing a small amount of amphiphilic surfactant molecules, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). High-resolution studies demonstrate that DMSO and DMF molecules attach to SAM surfaces and more favorably at defect sites, forming relatively stable adsorbates. This attachment increases the activation energy sufficiently to inhibit both degradation pathways. The robustness of this approach has been investigated as a function of surfactant concentration, solution temperature, and the stirring condition. Molecular-level mechanisms and energetics for degradation inhibition of SAMs are also discussed in detail.     

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.     

Competition between C-C and C-H insertion in prototype transition metal-hydrocarbon reactions

The competition between C-C and C-H insertion in model transition-metal reactions with cyclopropane and propene (C3H6) was studied as a function of total energy. Insertion of neutral transition metal atoms M (= Y, Zr, Nb, and Mo*) into the C-C bonds of cyclopropane led to formation of MCH2 + C2H4, whereas C-H insertion produced MC3H4 + H2. The measured product branching ratios verify the relative potential energy barrier heights for C-C and C-H insertion predicted by ab initio calculations.     

Kinetics of exchange of alkanethiol monolayers self-assembled on polycrystalline gold

We report on the exchange between a hydrophilic thiol (11-mercapto-1-undecanol) in a liquid or gas phase and a hydrophobic thiol (dodecanethiol) of similar length self-assembled on a polycrystalline gold surface for a wide range of temperatures and times. The molecular composition of the mixed monolayers is determined by the static water contact angle and X-ray photoelectron spectroscopy measurements. Atomic force microscopy in lateral force mode is used to characterize the molecular domains at the nanometer level. The exchange first occurs rapidly at the gold grain boundaries, with an activation energy of about 66 +/- 4 kJ/mol. Then, boundaries of ordered thiol domains are progressively replaced, and the exchange is slowed because only regions of increasing perfection are left untouched. Higher temperatures lead to faster kinetics of replacement and the removal of larger amounts of the original thiol. No significant difference could be detected between exchange occurring in an ethanol solution or in the gas phase, and the initial rate of exchange was found to be similar for the displacement of dodecanethiol by 11-mercapto-1-undecanol molecules and for the converse displacement.     

Self-assembled monolayers on Pt(111): molecular packing structure and strain effects observed by scanning tunneling microscopy

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.     

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.     

Structural characterization of aldehyde-terminated self-assembled monolayers

The structure of aldehyde-terminated alkanethiol self-assembled monolayers (SAMs) on Au(111) is investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. For the first time, the structures of aldehyde-terminated SAMs are revealed with molecular resolution. SAMs of 11-mercapto-1-undecanal exhibit the basic (radical3xradical3)R30 degrees periodicity and form various c(4x2) superstructures upon annealing. In conjunction with DFT studies, the models of the (radical3xradical3)R30 degrees and the c(4x2) superstructures are constructed. In comparison with alkanethiol SAMs, the introduction of aldehyde-termini results in smaller domain size, lower degree of long-range order, large coverage of disordered areas, and higher density of missing molecules and other point defects within domains of closely packed molecules. The origin of these structural differences is mainly attributed to the strong dipole-dipole interactions among the aldehyde termini.     

Comparative Behavior of Aromatic Disulfide and Diselenide Monolayers on Polycrystalline Gold Films Using Cyclic Voltammetry, STM, and Quartz Crystal Microbalance

A comparative investigation of the self-assembled monolayers of diphenyl disulfide (DDS), diphenyl diselenide (DDSe), and naphthalene disulfide (NDS) on polycrystalline gold films using STM, QCM, and electrochemical techniques is presented. The geometric constraint imposed by the rigid naphthalene ring for NDS inhibits the cleavage of the S-S bond, thus adversely affecting the monolayer organization and stability relative to the monolayers formed with DDS and DDSe. A comparative analysis using techniques like cyclic voltammetry and quartz-crystal microbalance indicates that, for DDS, the facile cleavage of the S-S bond leads to strong binding of the adsorbate molecules at the preferred surface sites, resulting in a rather well-organized self-assembled structure. The STM pattern of NDS reveals a periodic domain (i.e., less than 10 nm in size) while no such small domains are seen in the case of DDS and DDSe due to the orientational flexibility of the rings. Copyright 2001 Academic Press.     

Reaction of 5-40 eV ions with self-assembled monolayers

The reaction of 5-40 eV O(+) and Ne(+) ions with alkanethiolate and semifluorinated alkanethiolate self-assembled monolayers (SAMs) is studied under ultrahigh vacuum (UHV) conditions. Whereas Ne(+) simply sputters fragments from the surface, O(+) can also abstract surface atoms and break C-C bonds in both the hydrocarbon and fluorocarbon SAM chains. Isotopic labeling experiments reveal that O(+) initially abstracts hydrogen atoms from the outermost two carbon atoms on an alkanethiolate SAM chain. However, the position of the isotopic label quickly becomes scrambled along the chain as the SAM is damaged through continuous ion bombardment. Scanning tunneling microscopy (STM) monitors changes in the SAM conformational structure at various stages during 5 eV ion bombardment. STM images indicate that O(+) reacts less efficiently with dodecanethiolate molecules packed internally within a structural domain than it does with molecules adsorbed at domain boundaries or near defect sites. STM images recorded after Ne(+) bombardment suggest that Ne(+) attacks the SAM exclusively near the domain boundaries. Taken collectively, these experiments advance our understanding of the degradation pathways suffered by polymeric satellite materials in the low-earth orbit (LEO) space environment.     

Observation of hierarchical chiral structures in 8-nitrospiropyran monolayers

The adsorption and self-organization of racemic mixture of 8-nitrospiropyran (SP8) molecules on Au(111) surfaces was studied by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). The SP8 enantiomers, in spite of their low-symmetric and nonplanar molecular structures, formed well-ordered monolayers on Au(111). In the monolayers, we found two types of enantiomorphous, i.e., mirror-imaged, 2D chiral domains, denoted as lambda and delta phases. Both phases consist of periodically packed chiral quatrefoils. In the lambda domain, the quatrefoils are counterclockwise folded, while in the delta domain, the quatrefoils are clockwise folded. High-resolution STM images revealed that each chiral quatrefoil contains four heterochiral dimers and that each dimer is composed of two antiparallelly packed homochiral SP8 molecules. Therefore both of the two mirror-imaged 2D chiral structures are not chirally pure but racemic 2D crystals. A domain boundary, which serves as the glide reflection line between a lambda domain and a delta domain, was also observed along the [11] direction of the Au(111) substrate.     

金电极上L-半胱氨酸短链分子自组装单层的电化学性能和拉曼光谱研究

用循环伏安法分别测定了金电极表面L-半胱氨酸(L-Cys)和十二硫醇自组装单分子层的电化学行为, 实验发现虽然单层结构排列致密, 但并不能有效地阻碍铁氰化钾与电极间异相电子转移过程, 同时观察到十二烷基硫醇自组装层能较好地阻碍电子转移作用. 运用表面增强拉曼散射光谱技术, 以十二烷基硫醇作为缺陷探针, 从分子水平上证实了L-半胱氨酸自组装单层的稳定性和致密性.     

Formation of highly ordered and orientated gold islands: effect of immersion time on the molecular adlayer structure of pentafluorobenzenethiols (PFBT) SAMs on Au(111)

Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the [11(-)0] substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, β, γ, δ, and ε were found. The α-, β-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→β→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.