Dependence of tunneling current through a single molecule of phenylene oligomers on the molecular length

The electrical properties of single phenylene oligomers were studied in terms of the dependence of the tunneling current on the length of the oligomers using self-assembling techniques and scanning tunneling microscopy (STM). It is important to isolate single molecules in an insulating matrix for the measurement of the conductivity of the single molecule. We demonstrate here a novel self-assembled monolayer (SAM) matrix appropriate for isolation of the single molecules. A bicyclo[2.2.2]octane derivative was used for a SAM matrix, in which the single molecules were inserted at molecular lattice defects. The isolated single molecules of phenylene oligomers inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights of the molecular protrusions using STM and estimated the decay constant, beta, of the tunneling current through the single phenylene oligomers using a bilayer tunnel junction model.     

Observation of removal of an Fmoc protecting group by scanning tunneling microscopy

We demonstrate here by imaging successive surface reactions in self-assembled monolayers (SAMs) on Au(1 1 1) at molecular scale with a scanning tunneling microscope (STM): (i) SAM matrices formation with 1-octanethiol on Au(1 1 1) in ethanol, (ii) insertion of N-Fmoc-aminooctanethiol into the SAM matrices in ethanol, and (iii) removal of the Fmoc protecting group with tris(2-aminoethyl)amine (TAEA). The total reaction is formation of SAMs containing a small amount of NH₂ terminated molecules in the CH₃ terminated SAM matrices. After the reaction of the protecting group with TAEA, STM imaging revealed the decrease in heights of the inserted molecules on average. We attributed this observation to removal of the protecting group by taking account of a convolution of electronic and topographic contributions to observed STM heights. Apparent areas of the terminal groups, however, became larger on removal. The increase in the areas was attributed to water adsorption to the NH₂ terminal group under air.     

Characterization of the electrical and optical properties of viologen devices using chlorophyll a as an electron excimer

This paper uses self-assembled monolayers (SAMs) on an Au(111) substrate to detect the unique characteristics of viologen molecules using scanning tunneling microscopy (STM), and reports the orientation and surface changes of molecules at the nanolevel in real-time. In particular, the rectification characteristics of the viologen molecule were observed at the molecular level using scanning tunneling spectroscopy (STS). After verifying the rectification characteristics of viologen molecules, an experiment was carried out to demonstrate the possibility of applying viologen to photodiodes and switching devices by forming a thin film of chlorophyll a on the viologen SAMs using the Langmuir–Blodgett (LB) method. This material mimics the photoinduced electron transport phenomenon in the early stage of photosynthesis in living plants. This study demonstrates the applicability of viologen to bioelectronic photodiodes and switching devices based on photo effects by observing the topography, current sensing, and current–voltage (I–V) characteristics using current-sensing atomic force microscopy (CS-AFM) by introducing light to the AFM-tip/chlorophyll a/viologen/Au(1 1 1) substrate structure.     

Unique domain structure of pi-conjugated tolanethioacetate self-assembled monolayers on Au(111)

Surface structures of tolanethioacetate (TTA) self-assembled monolayers (SAMs) on Au(111), formed in a 0.5mM N,N'-dimethylformamide (DMF) solution, were investigated as a function of solution temperature by scanning tunneling microscopy (STM). The STM study revealed that thioacetyl-terminated TTA molecules on Au(111) did not form ordered SAMs at room temperature and 80 degrees C, whereas the adsorption of TTA molecules on Au(111) at a solution temperature of 50 degrees C led to the formation of two mixed phases: a disordered phase and a unique ordered phase, consisting rod-like domains, which can be described as a ( radical6x radical13)15 degrees superstructure. From this study, it was demonstrated that solution temperature is a key factor controlling the two-dimensional SAM structure of TTA molecules.     

STM observation of a single diarylethene flickering

We report the STM observation of single diarylethene derivatives (DD) embedded into alkylthiol self-assembled monolayers (SAM) on Au(111). Telegraph noise in the data shows that the molecular conductance oscillates between two states. Comparing our results to the ones obtained by other teams observing conductance flickering with systems in the same geometry, we relate the two observed states to the two isomeric configurations of the molecule under study.     

Fabrication of an electrically conductive mixed self-assembled monolayer and its application in an electrochemical immunosensor

Oligophenylethynylene thiol containing carboxylic acid in the tail group as a conducting wire bioreceptor was synthesized, and then its electrical property was investigated from the measurement of scanning tunneling microscopy (STM). Mixed self-assembled monolayer (SAM) consisting of 4-(2-(4-acetylthio)phenyl)ethynyl) benzoic acid (APBA) and butanethiol was fabricated in order to improve the electrical conductivity owing to the molecular orientation. We have examined the molecular orientation and the electrochemical activity of mixed SAM via X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). Especially, the prepared mixed SAM used as a bioreceptor in electrochemical prostate specific antigen (PSA) immunosensor showed higher electrochemical activity than that of the other SAMs.     

Measurements of stretch lengths of gold mono-atomic wires covered with 1,6-hexanedithiol in 0.1 M NaClO4 with an electrochemical scanning tunneling microscope

Stretch lengths of pure gold mono-atomic wires have been studied recently with an electrochemical scanning tunneling microscope (STM). Here, we will report a study of stretch lengths of gold mono-atomic wires with and without 1,6-hexanedithiol (HDT) using the STM break-junction method. First, the stretch length was measured as a function of electrode potentials of a bare Au(111) substrate and a gold STM tip in a 0.1M NaClO(4) aqueous solution. Second, a self-assembled monolayer (SAM) was fabricated on an Au(111) substrate by dipping the substrate into a 1mM HDT ethanol solution. At last, we measured the stretch length of gold mono-atomic wires on a substrate covered with the SAM in place of the bare Au(111) substrate. We compared the electrode potential dependence of the stretch lengths of gold mono-atomic wires covered with and without HDT. We will discuss the effect of the electrode potential on the stretch lengths by taking account of electrocapillarity of gold mono-atomic wires.     

Self-assembled monolayer on diamond-like carbon surface: formation and friction measurements

We have investigated self-assembled monolayers (SAMs) of heptadecafluoro-1,1,2,2-tetradecyltrietoxysilane (FTE) on diamond-like carbon (DLC) surfaces formed by a simple immersing process. SAM formation on DLC surfaces was verified by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy (XPS). Water and hexadecane contact angles increased gradually with immersing time and saturated at about 110 and 70 degrees, respectively. Ellipsometric measurements showed that the film thickness was 1.4 to 1.6 nm, which corresponded reasonably to the thickness of FTE monolayer. XPS data showed the presence of FTE molecules on the DLC surface. These results ensured the SAM formation of FTE molecules on the DLC surface.We further measured and compared the friction of unlubricated, SAM coated and 2 nm thick perfluoropolyether (PFPE) coated DLC surfaces using lateral force microscopy (LFM) as functions of the applied load and the sliding velocity. The SAM coated DLC surfaces showed lower friction than the unlubricated DLC surfaces and the friction coefficient decreased by about 15% compared to the unlubricated DLC surfaces. Scratch tests revealed that the critical load of the DLC film increased due to the SAM deposition. These results are attributed to the hydrophobic nature of the SAM coated surface. On the other hand, even though the water contact angle of the SAM coated surface was larger than the 2 nm thick PFPE coated surface, the friction of the SAM coated surface was larger than that of the PFPE coated surface. Also, the critical load of the SAM coated DLC surface in scratch test was lower than the PFPE coated surface. These results indicate that the hydrophobic nature of the surface is not the only factor which determines the friction characteristics in the nano-lubricating system, and it is attributed to the mobile characteristic of PFPE lubricant.     

Detection of C-reactive protein on a functional poly(thiophene) self-assembled monolayer using surface plasmon resonance

The preparation of a new poly(thiophene) with pendant N-hydroxysuccinimide ester groups and its application to immobilization of biomolecules are reported. A thiophene derivative of N-hydroxysuccinimide ester was polymerized with FeCl(3) in chloroform and the resulting poly(thiophene) was characterized by nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR), and gel permeation chromatography (GPC). This polymer reacts with amine-bearing molecules to yield new poly(thiophene) derivatives and the specific interactions at the side groups could be detected. Thus, a self-assembled monolayer (SAM) using the polymer was formed on a gold-coated quartz cell and anti-C-reactive protein (anti-CRP) was immobilized. The binding behavior of CRP on the surface was monitored by use of a surface plasmon resonance (SPR) sensor system.     

Fast displacement and structural transition of cyclohexanethiol self-assembled monolayers by octanethiols on Au (111)

Displacement of cyclohexanethiols (CHTs) self-assembled monolayers (SAMs) by octanethiols on Au (111) (OTs) was examined by scanning tunneling microscopy (STM) and contact angle measurements. We revealed that the fast displacement of CHT by OT takes place within a few minutes, and then displacement proceeds slowly to form the closely packed OT SAMs. The main driving force for the unusual fast displacement is due to the large increase of chemical interactions between the sulfur and gold atoms, and the van der Waals interactions between alkyl chains after displacement of CHT by OT. STM imaging clearly demonstrated the structural transitions from the (5x2 radical10)R48 degrees structure of CHT SAMs to the ( radical3x radical3)R30 degrees or c(4x2) structures of OT SAMs via an intermediate phase that were often observed during alkanethiol SAM growth on gold. In this study, we found that CHT SAMs can be used as a new transient layer for the fabrication of nanostructures on a surface.     

Measurements of stretch lengths of gold mono-atomic wires covered with 1,6-hexanedithiol in 0.1 M NaClO4 with an electrochemical scanning tunneling microscope

Stretch lengths of pure gold mono-atomic wires have been studied recently with an electrochemical scanning tunneling microscope (STM). Here, we will report a study of stretch lengths of gold mono-atomic wires with and without 1,6-hexanedithiol (HDT) using the STM break-junction method. First, the stretch length was measured as a function of electrode potentials of a bare Au(1 1 1) substrate and a gold STM tip in a 0.1 M NaClO₄ aqueous solution. Second, a self-assembled monolayer (SAM) was fabricated on an Au(1 1 1) substrate by dipping the substrate into a 1 mM HDT ethanol solution. At last, we measured the stretch length of gold mono-atomic wires on a substrate covered with the SAM in place of the bare Au(1 1 1) substrate. We compared the electrode potential dependence of the stretch lengths of gold mono-atomic wires covered with and without HDT. We will discuss the effect of the electrode potential on the stretch lengths by taking account of electrocapillarity of gold mono-atomic wires.     

Binding behavior of CRP and anti-CRP antibody analyzed with SPR and AFM measurement

Atomic force microscope (AFM) was exploited to take picture of the molecular topology of C-reactive protein (CRP) in phosphate-buffered saline (PBS) solution. An explicit molecular image of CRP demonstrated a pentagonal structure composed of five subunits. Dimensions of the doughnut-shaped CRP molecule measured by AFM were about 25nm in outside diameter and 10nm in central pore diameter, and the height of CRP molecule was about 4nm which was comparable to the value determined by X-ray crystallography. Bis(N-succinimido)-11,11'-dithiobis (undecyl succinate) (DSNHS) was synthesized for use as a linker for immobilizing anti-CRP antibody (anti-CRP) onto the gold surface of a surface plasmon resonance (SPR) sensor chip. DSNHS formed self-assembled monolayer (SAM) on the gold surface. By use of an AFM tip, a pattern of ditch was engraved within the SAM of DSNHS, and anti-CRP was immobilized on the engraved SAM through replacement of N-hydroxysuccinimide group on the outside surface of DSNHS by the amine group of anti-CRP. Formation of CRP/anti-CRP complex on the gold surface of SPR sensor chip was clearly demonstrated by measuring SPR angle shift. A consecutive series of SAM, SAM/anti-CRP, and SAM/anti-CRP/CRP complexes was generated on a SPR sensor chip, and the changes in depth of the ditch were monitored by taking AFM images of the complexes. Comparative analysis of the depth differences indicates that binding of CRP to anti-CRP occurs in a planar mode.     

STM of metal embedded and coated DNA and DNA–protein complexes

Bare and Pt/Ir/C-coated DNA has been analysed using scanning tunnelling microscopy (STM). To achieve reproducible imaging of bare DNA on mica ethanol/air-dried molecules were embedded in Pt/C. By peeling the metal film off the mica, the previously mica-exposed side of the Pt/C-film with the embedded DNA molecules was accessible for STM analysis. By applying this replica/anchoring technique only hollow trenches in the metal film, and not the DNA itself, could be visualized. The gaps averaged 3.1 nm (± 0.9 nm) wide and 1 nm (± 0.5 nm) deep. Using scanning force microscopy it could be confirmed that the DNA remained in the Pt/C film during the peel-off procedure. For STM, DNA fragments were also coated with 0.7–1 nm Pt/Ir/C. Owing to the high Z-resolution the STM samples were coated at a high elevation angle (65°), thereby minimizing the problem of self-shadowing. Coating by Pt/Ir/C allowed routine imaging and quantitative analysis of both ethanol/air- and freeze-dried DNA under atmospheric conditions. After ethanol/air drying measured values for DNA width and height were 5.1 nm (± 1.8 nm) and 0.9 nm (± 0.2 nm), respectively. Freeze-dried DNA averaged 4.2 nm (± 1.3 nm) wide and 1.1 nm (± 0.1 nm) high. A Pt/Ir/C-coating was also applied to visualize DNA–protein interaction using STM.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.     

Electrochemical immunosensor for prostate-specific antigen using self-assembled oligophenylethynylenethiol monolayer containing dendrimer

Electrochemical immunosensor for prostate-specific antigen (PSA) was fabricated with a self-assembled 4-(2-(4-(acetylthio)phenyl)ethynyl)benzoic acid (APBA) as a bioreceptor. In order to enhance the electrochemical activity of PSA detection, poly(amidoamine) dendrimer was linked on the APBA self-assembled monolayer (SAM). The formation and electrical properties of the SAMs were investigated by surface plasmon resonance and cyclic voltammetry, respectively. The surface morphology of PSA sandwich complex onto the APBA SAM was studied by atomic force microscopy.     

A simple method for the removal of thiols on gold surfaces using an NH4OH-H2O2-H2O solution

The removal behavior of self-assembled monolayers (SAMs) of thiol molecules on a gold substrate by an NH(4)OH-H(2)O(2)-H(2)O solution was studied using attenuated total reflectance infrared spectroscopy (ATR-IR) and atomic force microscopy (AFM). Furthermore, the impact of the concentration of NH(4)OH and H(2)O(2) in the solution and reaction temperature on the SAM removal rate and efficiency was explored. The SAM removal rate and efficiency were significantly influenced by the concentration of NH(4)OH rather than H(2)O(2). The solution containing the 2 : 1 molar ratio of NH(4)OH : H(2)O(2) among three different solutions showed the highest removal rate and efficiency in the removal of 11-mercapto-1-undecanol. The increase in the reaction temperature resulted in the enhancement on the SAM removal rate, but it led to the fast delamination of the gold layer. These results may be useful in the regeneration of sensor surfaces relying on gold/thiol chemistry.     

Achieving precision and reproducibility for writing patterns of n-alkanethiol self-assembled monolayers with automated nanografting

Nanografting is a high-precision approach for scanning probe lithography, which provides unique advantages and capabilities for rapidly writing arrays of nanopatterns of thiol self-assembled monolayers (SAMs). Nanografting is accomplished by force- induced displacement of molecules of a matrix SAM, followed immediately by the self-assembly of n-alkanethiol ink molecules from solution. The feedback loop used to control the atomic force microscope tip position and displacement enables exquisite control of forces applied to the surface, ranging from pico to nanonewtons. To achieve high-resolution writing at the nanoscale, the writing speed, direction, and applied force need to be optimized. There are strategies for programing the tip translation, which will improve the uniformity, alignment, and geometries of nanopatterns written using open-loop feedback control. This article addresses the mechanics of automated nanografting and demonstrates results for various writing strategies when nanografting patterns of n-alkanethiol SAMs.     

Adlayer structure of octa-alkoxy-substituted copper(II) phthalocyanine on Au(111) by electrochemical scanning tunneling microscopy

Electrochemical scanning tunneling microscopy (ECSTM) has been used to examine the adlayer of octa-alkoxy-substituted copper(II) phthalocyanines (CuPc(OC(8)H(17))(8)) on Au(111) in 0.1 M HClO(4), where the molecular adlayer was prepared by spontaneous adsorption from a benzene solution containing this molecule. Topography STM scans revealed long-range ordered, interweaved arrays of CuPc(OC(8)H(17))(8) with coexistent rectangular and hexagonal symmetries. High-quality STM molecular resolution yielded the internal molecular structure and the orientation of CuPc(OC(8)H(17))(8) admolecules. These STM results could shed insight into the method of generating ordered molecular assemblies of phthalocyanine molecules with long-chained substitutes on metal surface.     

Tribological properties of self-assembled monolayers on Au,SiOx and Si surfaces

Using interfacial force microscopy (IFM), the tribological properties of self-assembled monolayers (SAM) on Si surfaces produced by a new chemical strategy are investigated and compared to those of “classical” SAM systems, which include alkanethiols on Au and alkylsilanes on SiO_x. The new SAM films are prepared by depositing n-alkyl chains with OH-terminations onto Cl-terminated Si substrates. The chemical nature of the actual lubricating molecules, n-dodecyl, is kept constant in all three thin film systems for direct comparison and similarities and differences in tribological properties are observed. The adhesion strength is virtually identical for all three systems; however, frictional properties differ due to differences in film packing. Differences in the chemical bonds that attach the lubricant molecules to the substrate are also discussed as they influence variations in film wear and durability. It is demonstrated that the new SAM films are capable of controlling the friction and adhesion of Si surfaces equally well as the classical SAMs and are potentially more reproducible and more durable.     

Surface modification of an organosilane self-assembled monolayer on silicon substrates using atomic force microscopy: scanning probe electrochemistry toward nanolithography

Organosilane self-assembled monolayers (SAMs) have been applied to resist materials for nanolithography based on scanning probe microscopy. An organosilane SAM was prepared on Si substrates from a precursor, that is octadecyltrimethoxysilane. Using an atomic force microscope with a conductive probe, current was injected from the probe into the SAM-covered Si substrate so that the SAM was locally degraded at the probe-contacting point. Nanoscale patterns drawn on the SAM was clearly imaged by lateral force microscopy. The patterning could be conducted in air while, in vacuum at the order of 10(-6) Torr, no detectable patterns were fabricated. The presence of adsorbed water at the probe/sample junction was confirmed to be crucial for the patterning of the SAM/Si. Its mechanism was, thus, ascribed to electrochemical reactions of both the SAM and Si with adsorbed water.