The effect of permanent dipole moments of adsorbates upon I-V characteristics of a bilayer tunneling junction between self-assembled monolayers on an Au(111) substrate and a gold tip.

It is important to understand an electronic property of an interface between an organic material and a metal electrode. In the present work, we measured current-voltage (I-V) curves of self-assembled monolayers (SAMs) on Au(111) using a conducting atomic force microscope (AFM) with chemically modified Au-coated AFM tips. This contact resulted in a bilayer junction between the Au(111) substrate covered with one SAM and the Au-coated tip with the other SAM. An alkanethiol (octanethiol) and benzenemethanethiols with various terminal groups (-H, -CH(3), -Cl, -CF(3)) were used as the adsorbates. The shapes of the I-V curves depended on the terminal groups. This phenomenon was attributed to the change in the work function of gold due to different permanent dipole moments of the terminal groups.

Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation.

Patterned self-assembled monolayers (SAMs) were formed on gold films and observed by friction force microscopy (FFM) and adhesive force mapping with pulsed-force mode atomic force microscopy (PFM-AFM). The substrate gold films were prepared by sputtering gold on flat surfaces of osmium-coated cover glass with surface roughness, Ra, of 0.3 nm. The patterned samples with the CH3 and COOH terminated regions were prepared using the Langmuir-Blodgett (LB) method, partial removal of the LB film by ultrasonication, and SAM formation. The CH3 and COOH terminated regions of the patterned SAMs in air and in water were observed by mapping friction and adhesive forces with FFM and PFM-AFM, respectively, using gold-coated AFM tips chemically modified with a thiol compound terminating in CH3 or COOH. The adhesive forces measured in air increased in the order of CH3/CH3, CH3/COOH (or COOH/CH3) and COOH/COOH, while those in water increased in reverse order. The enormous high adhesive force observed in water for CH3/CH3 was attributed to hydrophobic interaction between the CH3 tip and the CH3 terminated sample surface. With CH3 tip, the lower friction force was observed, however, in water on the CH3 terminated region than on the COOH terminated region. This experimental finding raises a question as to what is the effective normal load in friction measurements in water.

Study of microcontact printed patterns by chemical force microscopy.

Patterned self-assembled monolayers (SAMs) on sputtered gold films prepared by microcontact printing (microCP) were studied by mapping adhesive forces with pulsed-force-mode atomic force microscopy. A stamp for microCP was fabricated by pouring polydimethylsiloxane (PDMS) over a photolithographically prepared master. The patterned SAMs were prepared by two methods. One is called the wet-inking method, in which inking was done by placing a thiol ethanol solution for 30 s on the stamp and then removing the excess ink solution under a stream of nitrogen. The other is called the contact-inking method, in which a pad made of PDMS was dipped overnight in a thiol ethanol solution and then the stamp was placed on the inker pad impregnated with the thiol ethanol solution. The second step for pattern formation was the same for both of the two different microCP methods. Namely, the gold surfaces stamped with alkanethiols were further reacted with a thiol terminating in COOH in ethanol. The resulting patterns with CH3- and COOH-terminated regions were analyzed by imaging the adhesive forces with the chemically modified gold coated AFM tips with a SAM of CH3 or COOH terminal functional groups.

Novel self-assembled monolayers of disulfides with bicyclo[2.2.2]octane moieties of Au(111).

A series of disulfides containing bicyclo[2.2.2]octane moieties have been synthesised and their self-assembled monolayers (SAMs) on Au(111) have been characterized using scanning tunnelling microscopy (STM).

A novel cleaning method of gold-coated atomic force microscope tips for their chemical modification

For chemical modification of gold-coated AFM tips with thiol or sulfide compounds, a new two-step precleaning procedure was studied. The two-step cleaning procedure involves (i) oxidation of organic contaminants on the AFM tips with ozone treatment and (ii) reduction of the oxidized gold surface by immersing the oxidized tip into pure hot ethanol at ca. 65 degrees C. The chemically modified tips prepared from gold-coated AFM tips precleaned by the two-step procedure gave almost the same tip characteristics as those chemically modified immediately after gold vapor deposition in a factory. The present two-step cleaning procedure can be used widely for chemical modification of commercially available gold-coated AFM tips with thiol or disulfide compounds for chemical force microscopy.

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.