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.     

Formation of ordered self-assembled monolayers by adsorption of octylthiocyanates on Au(111)

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of octythiocyanate (OTC) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature and 50 degrees C. The surface structures and adsorption characteristics of the OTC SAMs on Au(111) were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The STM observation showed that OTC SAMs formed in solution at room temperature have unique surface structures including the formation of ordered and disordered domains, vacancy islands, and structural defects. Moreover, we revealed for the first time that the adsorption of OTC on Au(111) in solution at 50 degrees C led to the formation of SAMs containing small ordered domains, whereas the SAMs formed by vapor deposition at 50 degrees C had long-range ordered domains, which can be described as (radical3 x 2 radical19)R5 degrees structures. XPS measurements of the peaks in the S 2p and N 1s regions for the OTC SAMs showed that vapor deposition is the more effective method as compared to solution deposition for obtaining high-quality SAMs by adsorption of OTC on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.     

Electro-oxidation of glucose at self-assembled monolayers incorporated by copper particles

A novel copper incorporated self-assembled monolayers (SAMs) modified gold electrode (Cu/SAMs) for determination of glucose was developed by electrodepositing Cu particles on the SAMs of hexanethiol. The scanning electron microscopic (SEM) images showed that copper formed orbicular particles of nanosizes on the SAMs, which was much different from the fractal-like particles of copper formed at gold electrode. The Cu/SAMs film electrode exhibited high sensitivity to glucose oxidation and depressed responses towards some interferents of glucose in blood like uric acid and ascorbic acid. Under optimal working conditions, the oxidation current of glucose was proportional to the concentration of glucose in the range from 3.0 μM to 10 mM by amperometry with a low detection limit of 0.7 μM glucose (S/N = 3). This electrode was successfully applied to the determination of glucose in rat blood and the results were satisfactory.     

Fabrication of biomolecular nanostructures by scanning near-field photolithography of oligo(ethylene glycol)-terminated self-assembled monolayers

The UV photo-oxidation of oligo(ethylene glycol) (OEG)-terminated self-assembled monolayers (SAMs) has been studied using static secondary ion mass spectrometry, X-ray photoelectron spectroscopy, contact angle measurement, and friction force microscopy. OEG-terminated SAMs are oxidized to yield sulfonates, but photodegradation of the OEG chain also occurs on a more rapid time scale, yielding degradation products that remain bound to the surface via gold-sulfur bonds. The oxidation of these degradation products is the rate-limiting step in the process. Photopatterning of OEG-terminated SAMs may be accomplished by using a mask and suitable light source or by using scanning near-field photolithography (SNP) in which the mask is replaced by a scanning near-field optical microscope coupled to a UV laser. Using SNP, it is possible to fabricate patterns in SAMs with a full width at half-maximum height (fwhm) as small as 9 nm, which is approximately 15 times smaller than the conventional diffraction limit. SNP-patterned OEG-terminated SAMs may be used to fabricate protein nanopatterns. By adsorbing carboxylic acid-terminated thiols into oxidized regions and converting these to active ester intermediates, it has been possible to fabricate lines of protein molecules with widths of only a few tens of nanometers.     

β-联碳酰基类衍生物有序自组装膜的STM研究

在大气条件下, 利用扫描隧道显微镜研究了四个β-联碳酰基类衍生物在高定向裂解石墨(HOPG)表面的自组装结构. 研究分子的结构中均包含π电子共轭体系和烷基链. 实验研究了分子结构对自组装结构的影响, 并利用分子结构的变化实现了自组装膜结构的调控. 结果表明, 在甲苯溶剂中制备的这些自组装结构均长程有序, 分子间氢键和偶极相互作用是影响自组装膜结构变化的重要因素.     

Voltammetry and in situ scanning tunnelling spectroscopy of osmium, iron, and ruthenium complexes of 2,2':6',2'-terpyridine covalently linked to Au(111)-electrodes

We have studied self-assembled molecular monolayers (SAMs) of complexes between Os(II)/(III), Fe(II)/(III), and Ru(II)/(III) and a 2,2',6',2'-terpyridine (terpy) derivative linked to Au(111)-electrode surfaces via a 6-acetylthiohexyloxy substituent at the 4'-position of terpy. The complexes were prepared in situ by first linking the terpy ligand to the surface via the S-atom, followed by addition of suitable metal compounds. The metal-terpy SAMs were studied by cyclic voltammetry (CV), and in situ scanning tunnelling microscopy with full electrochemical potential control of substrate and tip (in situ STM). Sharp CV peaks were observed for the Os- and Fe complexes, with interfacial electrochemical electron transfer rate constants of 6-50 s(-1). Well-defined but significantly broader peaks (up to 300 mV) were observed for the Ru-complex. Addition of 2,2'-bipyridine (bipy) towards completion of the metal coordination spheres induced voltammetric sharpening. In situ STM images of single molecular scale strong structural features were observed for the osmium and iron complexes. As expected from the voltammetric patterns, the surface coverage was by far the highest for the Ru-complex which was therefore selected for scanning tunnelling spectroscopy. These correlations displayed a strong peak around the equilibrium potential with systematic shifts with increasing bias voltage, as expected for a sequential two-step in situ ET mechanism.     

多巴胺在2-氨基-5-巯基-[1,3,4]三氮唑自组装膜电极上的准可逆行为研究及分析应用

利用新合成的巯基试剂2-氨基-5-巯基-[1，3，4]三氮唑在金电极表面进行了首次自组装，用电化学法和扫描电子显徽镜对自组装膜电极进行了表征。研究了多巴胺在该自组装膜电极上的电化学行为，发现该自组装膜能有效促进多巴胺在电极与溶液之间的电子传递．表现为二电子传递的准可逆行为，电极反应速率常数为0．105cm／s。该自组装膜电极用于多巴胺注射液含量的测定，结果满意。     

Disorder-order phase change of omega-(N-pyrrolyl)alkanethiol self-assembled monolayers on gold induced by STM scans and thermal activation

Molecular ordering of pyrrolyl-terminated alkanethiol self-assembled monolayers (PyC(n)SH SAMs) on Au(111) substrates (n = 11 or 12) was investigated by scanning tunneling microscopy (STM) and various spectroscopic methods. The SAMs, which were in a disordered state when formed at room temperature, could be ordered either globally by thermal annealing at 70 degrees C, or locally via stimulation with repetitive STM scans. The ordered phase was characterized by small domains of molecular rows formed along 112[combining macron] directional set with an inter-row corrugation period close to 1.44 nm, in which defects were abundant. Based on the experimental results, the molecular arrangement in the ordered PyC(n)SH SAM was proposed to be a (5x radical3)rect structure with a molecular deficiency >or=10%. While mechanical interactions between molecules and scanning probe tips had been pointed out as the major cause of scan-induced phase transformations in other SAM systems, electronic or electrostatic factors were thought to affect considerably the scan-induced ordering process in this SAM system. From comparison of surface molecular coverage between disordered and thermally ordered SAMs of PyC(12)SH, it was inferred that the disorder could be ascribed to both kinetic and thermodynamic factors. The kinetic barrier to the ordered phase was supposed to result from strong dipole-dipole interactions among the pyrrolyl endgroups.     

Ferric ion immobilized on three-dimensional nanoporous gold films modified with self-assembled monolayers for electrochemical detection of hydrogen peroxide

A fast, simple square wave potential method is developed for the fabrication of a three-dimensional (3D) nanoporous gold (NPG) film. The nanostructures are characterized and confirmed by scanning electronic microscopy (SEM) and cyclic voltammetry (CV). The nanostructures modified with self-assembled monolayers (SAMs) are employed as an electrode substrate to immobilize inorganic iron(III) ion. After immobilization, iron(III) ion undergoes an effective direct electron transfer reaction with a pair of well-defined redox peak at -256 ± 10 mV (pH 7.0). The iron(III) ion modified electrode displays the excellent electrocatalytic performance for reduction of hydrogen peroxide, and thus can be used as an electrochemical sensor for detecting hydrogen peroxide with a low detection limit (1.0 × 10(-9) M), a wide linear range (9.0 × 10(-7)~5.0 × 10(-4) M), as well as good stability, selectivity and reproducibility.     

Characterization of self-assembled monolayers of thiols on Au(111)

Self-assembled monolayers (SAMs) of n-butanethiol, n-dodecanethiol and their equimolar mixture on Au(111) were prepared and characterized by ellipsometry, contact angle measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results revealed that these SAMs are oriented ultrathin films with the thickness of nanometer scale, and the SAMs were influenced by the molecular chain length, the lattice orientation and cleanliness of the substrates. The surface of the longer chain SAM is hydrophobic. The thicknesses of three SAMs of n-butanethiol, n-dodecanethiol and their mixture revealed by ellipsometry and XPS are about 0.59 - 0.67nm, 1.60- 1.69 nm and 1.23 - 1.32nm, respectively. AFM images further demonstrated that the SAM formed by the mixture has some microdomains with two different thicknesses.     

Catalytic probe lithography: catalyst-functionalized scanning probes as nanopens for nanofabrication on self-assembled monolayers

This article describes the use of scanning catalytic probe lithography for nanofabrication of patterns on self-assembled monolayers (SAMs) of reactive adsorbates. Catalytic writing was carried out by scanning over bis(omega-tert-butyldimethyl-siloxyundecyl)disulfide SAMs using 2-mercapto-5-benzimidazole sulfonic acid-functionalized gold-coated AFM tips. The acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas, and thus patterned surfaces were created. Diffusion effects arising from the use of an ink were excluded in these type of experiments, and therefore structures with well-defined shapes and sizes were produced. The smallest lines drawn by this technique were about 25 nm wide, corresponding to the actual contact area of the tip. Lateral force microscopy studies performed on different SAMs helped to clarify the nature and cause of the friction contrasts observed by AFM. Dendritic wedges with thiol functions inserted into the catalytically written areas, thus enhancing the height contrast. The created patterns open possibilities to build 3D nanostructures.     

Tunable magnetic properties of nanoparticle two-dimensional assemblies addressed by mixed self-assembled monolayers

Assemblies of magnetic nanoparticles (NPs) are intensively studied due to their high potential applications in spintronic, magnetic and magneto-electronic. The fine control over NP density, interdistance, and spatial arrangement onto substrates is of key importance to govern the magnetic properties through dipolar interactions. In this study, magnetic iron oxide NPs have been assembled on surfaces patterned with self-assembled monolayers (SAMs) of mixed organic molecules. The modification of the molar ratio between coadsorbed 11-mercaptoundecanoic acid (MUA) and mercaptododecane (MDD) on gold substrates is shown to control the size of NPs domains and thus to modulate the characteristic magnetic properties of the assemblies. Moreover, NPs can be used to indirectly probe the structure of SAMs in domains at the nanometer scale.     

Electroactive self-assembled monolayers of unique geometric structures by using rigid norbornylogous bridges

Herein, we describe the synthesis of straight (S) and L-shaped (L) norbornylogous bridges (NBs) with an anthraquinone moiety at the distal end as the redox-active head group and two thiol feet at the proximal end, by which the molecules assemble on gold surfaces. The NB molecules were shown to form self-assembled monolayers (SAMs) with a well-behaved surface redox process. The SAMs were characterized by using in situ IR spectroscopy, cyclic voltammetry, scanning tunnelling microscopy and electrochemical impedance spectroscopy. The surface selection rules associated with the IR band intensities allowed the estimation of the position of the anthraquinone moiety with respect to the surface and the tilt of the bridge with respect to the surface normal, both in pure and diluted monolayers. It is shown that the S- and L-NBs hold the plane of the anthraquinone moiety close to the surface normal or the surface tangent, respectively. Neither NB molecule changes its orientation if spaced by diluents on the surface. The difference in the structure of the S- and L-NB SAMs provides a suitable framework for the investigation of factors that govern electron transfer of anthraquinone moieties across self-assembled monolayers with limited structural ambiguity, as compared with the commonly used structurally flexible alkanethiol monolayers.     

Nanoscale Biomolecular Structures on Self‐Assembled Monolayers Generated from Modular Pegylated Disulfides

A solid‐phase synthetic strategy was developed that uses modular building blocks to prepare symmetric oligo(ethylene glycol)‐terminated disulfides with a variety of lengths and terminal functionalities. The modular disulfides, composed of alkyl amino groups linked by an amide group to oligoethylene chains were used to generate self‐assembled monolayers (SAMs), which were characterised to determine their applicability for biomolecular applications. X‐ray photoelectron spectroscopy (XPS) of the SAMs obtained from these molecules demonstrated improved stability towards displacement by 16‐hexadecanethiol, while surface plasmon resonance (SPR) analyses of SAMs prepared with the hydroxy‐terminated oligoethylene disulfide showed equal resistance to non‐specific protein adsorption in comparison to 11‐mercaptoundecyl tri(ethylene glycol). SAMs made from these adsorbates were amenable to nanoscale patterning by scanning near‐field photolithography (SNP), facilitating the fabrication of nanopatterned, protein‐functionalised surfaces. Such SAMs may be further developed for bionanotechnology applications such as the fabrication of nanoscale biological arrays and sensor devices.     

Electrochemical Studies of Chemically Modified Nanometer‐Sized Electrodes

Self‐assembled monolayers (SAMs) of 4‐aminothiophenol (4‐ATP) has been successfully deposited onto nanometer‐sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4‐ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer‐sphere electron transfer reaction on the 4‐ATP SAMs modified nanometer‐sized electrode has also been studied. The 4‐ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4‐ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.     

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.     

Surface microscopic structure and electrochemical rectification of a branched alkanethiol self-assembled monolayer

The tert-butanethiol self-assembled monolayers (SAMs) on Au(111) surfaces were prepared from various solvents and investigated by a combination of scanning tunneling microscopy (STM) and electrochemistry in aqueous environments. High-resolution STM images reveal a (radical(7) x radical(7))R19 degrees surface lattice structure, in contrast with the conventional lattice (radical(3) x radical(3))R30 degrees structure for straight-chain alkanethiol SAMs. Interestingly, such a branched monolayer shows electrochemical rectification toward redox probes. We suggest that electrochemical rectification could be a general characteristic of short-chain branched alkanthiol SAMs, and originate in localized electronic effects.     

An STM study on nonionic fluorosurfactant zonyl FSN self-assembly on Au(111): large domains, few defects, and good stability

Nonionic Fluorosurfactant Zonyl FSN self-assembly on Au(111) is investigated with scanning tunneling microscopy under ambient conditions. STM reveals that the FSN forms SAMs on Au(l11) with very large domain size and almost no defects. A (mean square root of 3 x mean square root of 3)R3 degree arrangement of the FSN SAM on Au(111) is observed. The SAMs show excellent chemical stability and last for at least a month in atmospheric conditions. The structure and stability of the FSN SAMs are compared with those of alkanethiols SAMs. It is expected that FSN may serve as a new kind of molecule to form SAMs for surface modification, which would benefit wider applications for various purposes.     

Structure-dependent charge transport and storage in self-assembled monolayers of compounds of interest in molecular electronics: effects of tip material, headgroup, and surface concentration

The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. Peak-shaped i-V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i-V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, V(TH), for electric conduction. For a SAM of 2',5'-dinitro-4,4'-bis(phenylethynyl)-1-benzenethiolate, VII, V(TH) was nearly independent of the tip material [Ir, Pt, Ir-Pt (20-80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (beta values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both V(TH) and beta of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the "memory" effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.     

Combined STM and FTIR characterization of terphenylalkanethiol monolayers on Au(111): effect of alkyl chain length and deposition temperature

Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n = 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS). When the SAMs were prepared at room temperature (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both molecular orientation and packing density. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2 square root(3) x square root(3))R30 degree molecular arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5 square root(3) x 3) rectangular unit cell was found. The packing density for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepared at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing density. In the case of TP2, instead of the (2 square root(3) x square root(3))R30 degree structure formed at room temperature, a c(5 square root(3) x 3) structure was observed. For TP6 SAMs, the room-temperature c(5 square root(3) x 3) structure was replaced by a (6 square root(3) x 2 square root(3))R30 degree structure.