Tadatake Sato

Nano- and Microdot Array Formation of FeSi2 by Nanosecond Excimer Laser-Induced Forward Transfer

Fabrication of FeSi2 nano- and microdot array was performed by utilizing droplet ejection through nanosecond laser-induced forward transfer (ns-LIFT). An amorphous FeSi2 source film on a transparent support was illuminated from the support by a nanosecond excimer laser pulse patterned into migcrogrid form, resulting in size- and site-controlled deposition of microdot array onto a silicon substrate. Micro-Raman spectroscopy confirmed β-FeSi2 crystalline phase even on unheated substrates. Moreover, the dot size was successfully reduced to approximately 500 nm in diameter, smaller than any previously reported by ns-LIFT. This technique is useful for integrating functional nano- and microdots under atmospheric room-temperature conditions.

Laser-Induced Backside Wet Etching of Sapphire

Sapphire plates were etched with the approach of laser-induced backside wet etching (LIBWE) using neat toluene or saturated pyrene/acetone solutions as the etching medium. Using this approach, flexible patterns on the scale of 4-µm can be fabricated on the surface of the plates. The etching process was observed in situ by a CCD camera, which revealed the deposition of black particles and the formation thereafter of a laser-absorbing film in the etched area. By Raman spectroscopic analysis, the film was characterized to be amorphous carbon resulting from the decomposition of toluene or pyrene upon laser irradiation, which is an essential factor contributing to the etching process.

Etching a Micro-Trench with a Maximum Aspect Ratio of 60 on Silica Glass by Laser-Induced Backside Wet Etching (LIBWE)

We have successfully fabricated a deep micro-trench about 7 µm wide and 420 µm deep on silica glass with a maximum aspect ratio of 60 by laser induced backside wet etching (LIBWE) via KrF laser ablation of a saturated pyrene/acetone solution. The processing time for the microetching was as short as 5 min at a repetition rate of 80 Hz and a fluence of F = 1.0 Jcm-2pulse-1. The etch rate was calculated to be approximately 17 nmpulse-1. The LIBWE method is shown to be very useful for surface microstructuring of silica glass with high aspect ratio and high throughput.

Electrochemical construction of ultrathin film composed of quasi two-dimensional porphyrin polymers

Construction of an ultrathin film composed of porphyrin polymer with quasi two-dimensional (2D) structure on a facet of a gold single crystal as the substrate was attempted by the electrochemical polymerization of zinc tetrakis(oligothienyl) porphyrin. In order to produce uniform thin film the polymerization was performed under the adsorption equilibrium by sweeping the potential, and the total electric charge supplied in the polymerization was controlled toward the formation of the monolayer of porphyrin polymer. We also investigated (i) change in the surface structure of the gold crystal substrate under the influence of the applied potential and (ii) morphologies of the polymers depending on the molecular structures of the monomers.

Laser flash photolysis study on the photoinduced reactions of 3,3′-bridged bithiophenes

Photophysical properties and photoinduced reactions of 3,3′-bridged-2,2′-bithiophenes {dithieno[3,2-b:2′,3′-d]- thiophene, 4H-cyclopenta[2,1-b:3,4-b′]dithiophene, and 4H-dithieno[3,2-b;2′,3′-d]pyrrole (DTP)} and 2,2′-bithiophene (BT) were investigated by observing the transient absorption spectra in the visible and near-IR regions using nanosecond laser flash photolysis. Fluorescence quantum yields for the bridged bithiophenes were low compared with that for BT. An especially low quantum yield for DTP in acetonitrile was attributed to an addition reaction with the solvent. The triplet energy of BT was the lowest amongst the examined bithiophenes, indicating some conformational change in the triplet state. Triplet-energy-transfer reactions at diffusion limited rates were confirmed between bithiophenes and triplet-energy donors or acceptors. Photochemical generation of the radical cations of the bithiophenes was confirmed by the transient absorption spectra, which show good correspondence with those observed in γ-ray radiolysis. It was found that the triplet quenching rates of the electron-transfer reactions were small when the Gibbs energy change for the reaction was −25 kJ mol−1. The observed tendency agreed with the semi-empirical equation of Rehm–Weller. The generated radical cations decay according to a second-order function, indicating deactivation by a back electron-transfer reaction.

Photophysical properties of bis(2,2′-bithiophene-5-yl)benzenes

The synthesis of three kinds of bis(2,2′-bithiophene-5-yl)benzenes (1,2-, 1,3- and 1,4-bis(2,2′-bithiophene-5-yl)benzene; o-PhBT2, m-PhBT2 and p-PhBT2) and analyses of their photophysical properties are reported. The electronic structures were also studied by the molecular orbital (MO) method. It was found that the extension of π-conjugation varies with the substitution mode (o-, m-, or p-), which in turn influences the absorption, fluorescence and transient absorption spectra. The unique behaviour of o-PhBT2 was interpreted in terms of its lowest unoccupied MO (LUMO) pattern, in which π-orbitals on the adjacent sulfur atoms in each bithienyl moiety can have considerable overlap.

Dual photoluminescence of polythiophene thin films

Abstract Transient photoluminescence of electrochemically prepared thin films of polybithiophene and polyterthiophene was analyzed. It was observed for the first time that time-integrated spectra exhibited dual photoluminescence. In the time interval right after the excitation, a short-lifetime emission corresponding to the bandgap energy was observed, whose lifetime was estimated to be several tens of ps. Moreover, another emission of smaller energy than the bandgap was observed in the following time interval. This emission had relatively longer lifetime: several hundred ps. These results indicated that at least two types of excitons were formed in the films. Both of these could be the self-trapped excitons generated at the recombination sites with different local environments. Polyterthiophene contains many microcrystalline regions in comparison with polybithiophene. Enhancement of the longer-lifetime photoluminescence (PL) component in polyterthiophene suggests that the exciton with longer lifetime is generated in the microcrystalline region.

Photoluminescence quenching in oligothiophene single crystal

A single crystal of stereo-regularly substituted dihexylsexithiophene (DH6T) was synthesized. The transient photoluminescence decay curve of the crystal was non-exponential as in amorphous polythiophene. This non-exponential decay curve could be successfully described by stretched exponential decay kinetics, which represent the exciton diffusing to the one-dimensionally dispersed quenching centers. The generation of radical species by photoirradiation was confirmed by light-induced ESR measurement. It is probable that these radical species act as the quenching center for photoluminescent excitons.

Benzdiynes revisited: ab initio and density functional theory

Ab initio and density functional theory (DFT) studies were performed on three isomers of tetradehydrobenzene (benzdiynes). Four different density functionals (BPW91, BLYP, B3LYP, and B1LYP) and two higher levels of theory [QCISD and CCSD(T)] incorporating basis sets up to Dunnings correlation‐consistent polarized valence triple‐ζ (cc‐pVTZ) were utilized for this purpose. Stability tests showed that more stable solutions were available for 1,4‐benzdiyne with unrestricted than with restricted DFT, while solutions obtained with later descriptions of 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were stable. UB3LYP provided better geometry for 1,4‐benzdiyne. Unlike in an earlier study, the energy difference between 1,2,3,5‐tetradehydrobenzene and 1,4‐benzdiyne calculated with the B3LYP functional coincided well with that calculated at CCSD(T): 13.07 and 14.32 kcal/mol at the B3LYP/cc‐pVTZ and CCSD(T)/cc‐pVTZ levels, respectively. 1,2,3,5‐Tetradehydrobenzene was 8 kcal/mol more stable than 1,3‐benzdiyne at the CCSD(T)/cc‐pVTZ level. The heats of formation of benzdiynes were obtained by using the G2, CBS‐Q, and CBS‐QB3 methodologies. The heats of formation calculated for 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were 208.6 and 197.9 kcal/mol, respectively, at the CBS‐QB3 level of theory.

Synthesis and photophysical properties of 1,3-di(oligothienyl)propane

Abstract Three kinds of 1,3-di(oligothienyl)propanes were synthesized. Two of them are homogeneous dimers composed of bithienyls or terthienyls and the third, a heterogeneous dimer in which bithienyl and terthienyl units are linked. It has been found that in the homogeneous dimers there is little electronic interaction between two oligothienyl units both in the ground and the singlet excited states. On the other hand, it has become clear that in the heterogeneous dimer an effective energy transfer from the bithienyl to the terthienyl unit takes place.