Masamichi Fujihira

A possible mechanism for enhanced electrofluorescence emission through triplet–triplet annihilation in organic electroluminescent devices

We demonstrate here that luminance increased more than linearly with an increase in current density of tris(8-hydroxyquinoline) aluminum (Alq3)-based electroluminescent (EL) devices and the EL efficiency reached ∼5 cd A−1 at 250 mA cm−2 when electron and hole injection was well balanced. The luminance–current curves were well fitted with a combination of a linear and a quadratic function of the current. The quadratic component can be attributed to additional singlet excited state (1Alq3*) formation through triplet–triplet (T–T) annihilation of triplet excited states (3Alq3*). The requirement of the well-balanced charge injection implies that the long-lived 3Alq3* was quenched efficiently by energy transfer to excess and colored Alq3−⋅ anion or Alq3+⋅ cation radicals in the emission zone when the charge injection was unbalanced. The short-lived 1Alq3* was not quenched appreciably.

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.

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.

Chemical force microscopy of CH3 and COOH terminal groups in mixed self-assembled monolayers by pulsed-force-mode atomic force microscopy

Abstract Pulsed-force-mode atomic force microscopy (PFM-AFM), with functionalized probe tips, was applied to discrimination of chemical functionalities of a binary system of mixed self-assembled monolayers (SAMs) consisting of CH 3 - and COOH-terminating alkane thiols. PFM-AFM enabled simultaneous imaging surface topography and distribution of adhesive forces between the tip and sample surfaces. Since the adhesive forces were directly related to interaction between the chemical functional groups on the tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with the chemically modified tips to accomplish imaging the sample surface with the chemical sensitivity. The adhesive force mapping by PFM-AFM with the CH 3 -modified tips in pure water clearly discriminated the hydrophobic CH 3 -terminating domains embedded in the COOH-terminating SAM matrix.

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.

A study of topographic effects on chemical force microscopy using adhesive force mapping

Origins of peak broadening in a histogram of measured adhesive forces were studied. The adhesive forces were measured in water by pulsed-force-mode atomic force microscopy. One sample was prepared by a microcontact printing method on a sputtered gold film with fine grains, on which CH(3)- and COOH-terminated regions were produced. Gold surfaces of other samples were chemically modified homogeneously by a self-assembling method in solution. Their surfaces were, however, topographically different, i.e. (i) an Au(111)-terrace-rich gold film prepared by vacuum vapor deposition at high temperature and (ii) sputtered gold films on cover glass with different grain sizes obtained by different deposition time. These sample surfaces and the probe tip surface were all CH(3)-terminated by self-assembled monolayers with CH(3)(CH(2))(19)SH. The main origin of peak broadening in the histogram was the topographic effect. Namely, the change in the grain sizes and the change in multiplicity of contacts between the tip and convexities of the grains resulted in the distribution of the observed adhesive forces.

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.

Study of mixed Langmuir–Blodgett films of hydrocarbon and fluorocarbon amphiphilic compounds by scanning surface potential microscopy and friction force microscopy

Abstract The phase separation and orientation of molecules in mixed Langmuir–Blodgett (LB) films of hydrocarbon (HC) and fluorocarbon (FC) amphiphilic compounds were studied using scanning surface potential microscopy (SSPM) and friction force microscopy (FFM). The LB films were formed with a monolayer on an aqueous subphase containing calcium cations as the counter ions for the common carboxylate anions of the two types of amphiphilic compounds. It was found that the structure of the phase-separated LB films was different from that of previously studied HC–FC mixed LB films complexed with polymer cations. Namely, the structure of the present LB films was a “side-by-side” structure, while that of the previous ones was an “on-top” structure. In addition, it was found from SSPM that differences in surface potentials between the two separated phases were small for all the mixed LB films prepared with various HC/FC molar ratios. In other words, compositions were similar between these two phases: the rich component of one phase was at a slightly higher concentration than that of the other phase. The similarity in the compositions explained the low contrast in friction between these two phases as observed by FFM.

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.

Self-assembled monolayers containing biphenyl derivatives as challenge for nc-AFM

Abstract Mixed self-assembled monolayers (SAMs) of biphenylmethanethiol derivatives with CH 3 and COOH terminal groups and those of 1-decanethiol and a biphenylmethanethiol derivative with a CH 3 or a COOH terminal group were prepared on Au(111) surfaces. The resulting mixed SAMs were imaged by scanning tunneling microscopy (STM). These mixed SAMs can be used to study chemical force microscopy (CFM) by noncontact atomic force microscopy (nc-AFM). Namely, the CH 3 and COOH terminal groups of the SAMs can be recognized by different surface interactions with a surface functional group on a gold-coated AFM tip derivatized with the same thiol compound with a CH 3 or a COOH terminal group. The present result also suggests that these SAMs can be used to study CFM using nc-AFM with simultaneous tunneling current measurement. By this method, for example, a molecular location of a biphenylmethanethiol derivative with a COOH terminal group in an alkanethiol matrix observed by nc-AFM can be confirmed by the tunneling current measurement due to lower tunneling barrier height of the biphenylmethanethiol than that of the alkanethiol derivative.