Tsuyoshi Tsujioka

Abstract Estimating the surface glass transition temperature (T_g) of materials is important for making organic devices by vacuum evaporation because the surface T_g affects the interface of layered devices and their performance. The enthalpy state of organic films is related to T_g. Bulk T_g is ordinarily measured by differential scanning calorimetry (DSC), although there is no established method for measuring the surface T_g. The Mg deposition property reflects the surface T_g; no Mg deposition denotes a low surface T_g. In this study, we investigated how annealing affects the bulk and surface T_g of N,N′-Bis-(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) using DSC and the Mg deposition properties. For bulk NPB, enthalpy relaxation occurred after annealing at a temperature below T_g. On the other hand, for vacuum-deposited NPB film, the enthalpy was in a relaxed state just after preparation, although the surface’s enthalpy recovered by annealing at same condition, as identified by the Mg deposition property.

Surfaces and interfaces play an important role in obtaining high-performance organic devices. An essential property of organic films is the surface glass transition temperature (surface-Tg) and many methods for characterizing surface-(Tg) have been studied. We propose a novel method for characterizing surface-Tg based on metal-vapor atom deposition and desorption. We monitored metal-vapor deposition and desorption on organic surfaces using double quartz crystal microbalances. Mg vapor is not deposited on organic surfaces with a low bulk-Tg such as a colorless photochromic diarylethene (DAE) film. This deposition phenomenon is caused by Mg-atom desorption from the surface based on active surface molecular motion. However, Mg deposition began after a certain time of continuous evaporation (deposition-threshold time). The threshold time elongated with increased substrate temperature and elongated dramatically at a substrate temperature several degrees below the bulk-Tg for DAE. The surface molec ular motion becomes active and the metal-atom desorption accelerates as the temperature neared the surface-Tg. Thus a temperature with a dramatic elongation of the threshold time indicates the surface-Tg. This method can be applied to a variety of organic films. (C) 2017 Elsevier B.V. All rights reserved.

Vacuum evaporation, which is widely used to obtain a metal-film coating on substrates and/or targets, results in a large loss of evaporation materials. The authors discovered a selective metalvapor deposition phenomenon in which metal-vapor atoms can be deposited selectively without a shadow mask. The core phenomenon of selective deposition is the desorption of metal-vapor atoms from organic surfaces with a low-glass transition temperature (low-Tg). The authors investigated metal-atom desorption from low-Tg polymer [polydimethylsiloxane (PDMS)] surfaces during vacuum deposition and found that greater than 2-lm-thick PDMS film enables high desorption efficiency. The authors proposed two vacuum deposition methods: metal-vapor transportation and metal-vapor integration. The former method attained reduced substrate surface temperatures during vacuum evaporation and the latter enabled extremely high deposition efficiency and three-dimensional deposition in a one-batch process. These new methods can greatly widen applications of vacuum deposition. (C) 2016 American Vacuum Society.

We propose a basic concept of all-electrically operable organic memory with a photochromic diarylethene (DAE) film based on a transistor structure, in which the DAE memory layer is recordable, erasable, and nondestructively readable by an electrical method. To realize such memory, we investigated each elementary process for recording, erasing, or nondestructive reading by current injection and electrostatic methods for the DAE layer. Both ring-opening and ring-closure isomerization reactions were confirmed for the injection of both carriers (electrons and holes). Hole injection induced ring-opening reaction only. These reaction modes can be utilized in the recording and erasing modes. Since no reactions for electron injection and current modulation based on photoisomerization were observed, electron current injection can be applied to nondestructive readout. (C) 2016 The Japan Society of Applied Physics

Selective metal-vapor deposition signifies that metal-vapor atoms are deposited on a hard organic surface, but not on a soft (low glass transition temperature, low T-g) surface. In this paper, we introduce the origin, extension, and applications of selective metal-vapor deposition. An amorphous photochromic diarylethene film shows light-controlled selective metal-vapor deposition, which is caused by a large T-g change based on photoisomerization, but various organic surfaces, including organic crystal and polymers, can be utilized for achieving selective metal-vapor deposition. Various applications of selective metal-vapor deposition, including cathode patterning of organic light-emitting devices, micro-thin-film fuses, multifunctional diffraction gratings, in-plane electrical bistability for memory devices, and metal-vapor integration, have been demonstrated.

By alternate irradiation with UV and visible light, reversible topographical changes were observed on the microcrystalline surfaces of diarylethene derivatives above the glass transition temperatures (T-g) of their open-ring isomers. For the photogenerated closed-ring isomers, the crystal growth proceeded at the surface softened by molecular movement. This is the self-aggregation of the closed-ring isomers. The photogenerated surface topographies appear depending on the crystal habits of the closed-ring isomers. Reflecting the properties (crystal habits) of diarylethene derivatives, we obtained cubic, needle, and plate-shaped crystals of the closed-ring isomers on the microcrystalline surfaces of the open-ring isomers by irradiation with UV light. Even a derivative having one isopropyl group at a reactive carbon atom, whose closed-ring isomer is thermally unstable, showed photoinduced topographical changes, because the Tg of the open-ring isomer is around ambient temperature.

We report a selective metal-vapor deposition phenomenon based on solvent printing and evaporation on polymer surfaces and propose a method to prepare fine metal patterns using maskless vacuum deposition. Evaporation of the solvent molecules from the surface caused large free volumes between surface polymer chains and resulted in high mobility of the chains, enhancing metal-vapor atom desorption from the surface. This phenomenon was applied to prepare metal patterns on the polymer surface using solvent printing and maskless metal vacuum deposition. Metal patterns with high resolution of micron scale were obtained for various metal species and semiconductor polymer substrates including poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] and poly(3-hexylthiophene-2,5-diyl). (C) 2015 Elsevier B.V. All rights reserved.

Electrical bistability is an essential property for memory devices. We report here the in-plane electrical bistability of photochromic diarylethene (DAE)/Cu composite film, which is prepared by Cu vapor deposition on the DAE surface with a low glass-transition temperature. The low-current level around 10 (8) A was switched to a high-current level of ca. 10 (4) A at a low threshold voltage (Vth) in the first voltage sweep. Once this switching occurred, the high-current level was kept in the second voltage sweep, and electrical bistability was achieved for the in-plane current. Vth was distributed in a wide range of voltages (0.5-10 V), and the colored sample obtained by the UV irradiation showed a relatively higher Vth than the colorless sample. The highest ON-OFF ratio in current was ca. 10(6). The origin of the bistability attributed to the electrical breakdown in the insulated lines that was consisted of DAE in Cu film. The in-plane bistability of the DAE/Cu composite film has good retention time (>60 min) and readout-cycle endurance (>10(6) cycles), indicating that it is suitable for write-once organic semiconductor memory characteristics. (C) 2015 Elsevier B.V. All rights reserved.

Photochromic diarylethenes (DAEs) have gained attention as attractive current switching materials by light irradiation in the organic electronics field. We investigated the hole mobility of amorphous films consisting of three types of DAEs using a space-charge-limited current method and a better chemical structure to achieve high mobility. The hole mobility of open-ring (colorless) DAE having benzothiophene rings substituted with triphenylamine (TPA) as an aryl group was 2 x 10(4) times (2 x 10(-6) cm(2)/V s) larger than that of DAE containing thiophene rings without TPA. When the DAE film was irradiated with ultraviolet (UV) light, the hole mobility decreased temporarily at 4% of closed-ring (colored) isomers and then increased to two-three times of the initial colorless state at 85% of the closed-ring isomers. The temporary decrease in the hole mobility originated in the hole trapping effect of the closed-ring molecules in a matrix consisting of open-ring isomers. (C) 2014 Elsevier B.V. All rights reserved.

We investigated deposition modulation by noble metals on a photochromic diarylethene (DAE) film. Differences in color and sheet resistance (R-s) between the metal films deposited on the colored and colorless DAE films were observed. We observed R-s ratios of 10(6) for Ag, 10(7) for Cu, and 10(4) for Au. We found that the color and R-s differences were caused by the size differences in the metal nanocrystals on the deposited films. The difference in R-s was increased further after ozone oxidation treatment of the samples. We also demonstrated that this phenomenon can be applied to wiring in electronics and the crosstalk between adjacent wiring can be reduced by ozone oxidation treatment. (C) 2014 The Japan Society of Applied Physics

Selective metal deposition signifies that metal vapor atoms are deposited on a hard organic surface but not on a soft (low glass transition temperature) surface. This paper introduces the origin, extension, and device applications of selective metal deposition. A photochromic surface shows the light-controlled selective metal deposition phenomenon, but the surface material is not essential to it; various organic surfaces including organic crystal and polymer surfaces can be used. We demonstrate the applications of selective metal deposition, including cathode patterning for organic light-emitting devices, micro thin-film fuses, and multifunctional diffraction grating. Selective metal deposition can be applied to a variety of electric and optic devices.

Photochromic diarylethene crystal films are promising candidates for future organic memory applications but this has not yet been studied. We found that a specific molecule of photochromic diarylethenes (DAEs) formed a crystal film that consisted of extraordinary large crystallites with a 1 cm scale domain. We investigated the carrier injection/transport characteristics in the DAE crystal film and found that photocontrolled current switching using UV (lambda = 365 nm) irradiation showed a large on- off ratio of 500, which was 50- times that of the amorphous film. Positive temperature dependence of the current was observed for the initial colorless crystal film, whereas negative dependence for the colored and decolored films was observed. The current level of the decolored sample was lower than that of the initial colorless state and the difference in the current levels was based on the change from the crystal to the amorphous state. The results are essential for achieving organic memory applications with DAE crystals.

The sheet resistance (R-s) of a structure prepared by Ag vapor deposition on an organic photochromic diarylethene (DAE) surface was modulated at a substrate temperature (T-sub) of 50 degrees C. The R-s of a Ag film on the uncolored DAE surface was over 10(6) times higher than that on a colored DAE surface. Such a Ag film with a high R-s was formed by a structure consisting of small Ag nanocrystals. We propose a thin-film micro-fuse (mu-fuse) with a novel structure based on Ag deposition modulation on the DAE surface. The mu-fuse with 700 mu m size blew at a current level of 350 mA at 4.5 V. (c) 2013 The Japan Society of Applied Physics

We report a novel function of a polymer coffee-ring surface called metal-vapor deposition modulation where metal-vapor atoms are deposited on the center of the polymer coffee-ring surface using vacuum evaporation but not on the ring-shaped areas at the edge. The essential factor of the metal-deposition modulation is the difference of polymer chain mobility between the center and the ring. In the ring, the solvent evaporation process produces large spaces among surface polymer chains, resulting in high mobility of polymer chains. The desorption of metal vapor atoms from the surface was caused by the high mobility of the polymer chains. This result not only provides new knowledge about the fundamentals of a polymer surface but also suggests a novel technology for engineering and industrial fields using metallic coating technology on polymer surfaces. © 2013 The Royal Society of Chemistry.

We identified the mechanism of the formation of needle-shaped microcrystals on which the contact angle of a water droplet exceeds 170 degrees [Nishikawa, N. et al. Langmuir, 2012, 28, 17817-17824]. The standing needle-shaped crystal of the closed-ring isomer of a diarylethene 3c grew at a much lower temperature than the eutectic temperature by irradiation of UV light on the thin films of the open-ring isomer 3o, due to the epitaxial growth of the 013 plane of 3c over the 110 plane of the crystal lattice of 3o in the subphase. Therefore, the new crystal-growth mechanism triggered by the photoisomerization does not require special inorganic single-crystal substrates and may be called self-epitaxial crystal growth. The needle-shaped crystals appeared well-ordered and stood inclined at an angle of about 60 degrees to the surface. Consequently, the photo-induced rough surface show not only the superhydrophobic lotus effect, but also the antireflection moth-eye effect, and these effects were switchable by alternate irradiation with UV and visible light.

We report selective metal deposition on a phase-separated polymer blend surface. A polymer blend film consisting of polystyrene (PS) and a polystyrene-block-polybutadiene copolymer (PS-BR) was annealed, and a micro-phase-separated film was obtained. Pb was evaporated onto the phase-separated surface without an evaporation mask and was selectively deposited on the PS phase but not on the PS-BR phase. We achieved fine metal patterns corresponding to the microphase separation. This result suggests a novel method of preparing fine metal patterns for electronics and photonics. © 2013 The Japan Society of Applied.

By alternate irradiation with UV and visible light, reversible topographical changes were observed. Step and valley formation while keeping the crystal confinement was observed below the glass transition temperature (T-g), while crystal growth via a softened state was observed above the T-g on the same diarylethene crystalline surface.

A superhydrophobic surface on which the contact angle of a water droplet exceeds 170 degrees was reversibly produced by alternate irradiation with UV and visible light. Superhydrophobicity is due to the formation of densely generated submicrometer sized needle-shaped crystals (less than 0.2-0.3 mu m diameter and 2.2-2.5 mu m long) at 30 degrees C, which is much lower than the eutectic temperature of either isomers of the diarylethene. Below the eutectic temperature, the generated crystals were much smaller than those generated above the eutectic temperature. These smaller crystals more effectively enhanced the superhydrophobicity.

We report light-controlled selective Pb deposition on a photochromic surface. Pb evaporation onto the photochromic surface with isomerization patterns at room temperature (24 degrees C) generated no selective deposition; however a slight increase of the substrate temperature around 10-15 degrees C enabled the perfect selective Pb deposition, that is, Pb was deposited on the colored surface but not on the uncolored surface. A preparation of thin-film microfuse (mu-fuse) based on selective Pb deposition was demonstrated using laser scanning and maskless Pb evaporation. The mu-fuse showed a highly sensitive interrupt current level. (C) 2012 The Japan Society of Applied Physics

Metal-vapor deposition modulation on soft polymer surfaces and its applications are reported. A soft viscous surface of poly(dimethylsiloxane) (PDMS) with a glass transition temperature (T-g) of -123 degrees C showed perfect desorption at room temperature for many kinds of metal vapor. Metal-vapor deposition modulation on PDMS surfaces was applied to the Ca-cathode patterning of an organic light-emitting device, the preparation of thin-film Pb fuse, and the Mg vapor transportation by a pipeline, indicating great potential in various fields of basic research, engineering, and industry. (C) 2012 The Japan Society of Applied Physics

We introduce photoreprogrammable dual-function gratings, which show different diffraction in transmission and reflection. Parallel metal lines on the photochromic diarylethene (DAE) surface were prepared by selective metal deposition, enabling one-dimensional diffraction in transmission and reflection. Then, another isomerization pattern was programmed on the DAE layer by light irradiation. This grating generated unique diffraction: onedimensional diffraction in the reflection but two-dimensional diffraction in the transmission, and a dual-function grating was achieved. This programmed state on the DAE layer could be erased by UV irradiation and could be reprogrammed to generate different diffraction by visible laser scanning. This result shows the great potential of the photoreprogrammable dual-function grating as a key element in various future optical devices and systems. (C) 2011 Optical Society of America

Diarylethene derivatives, 1c and 1d, having carrier mobilization sites have been synthesized. Their photochromic properties are examined both in solution and in thin film. Current voltage characteristics at photostationary state of 1c and 1d showed drastic current increase compared with the corresponding open-ring isomers 1c and 1d, respectively.

A diarylethene (DAE) study using thermodynamical physical chemistry, elemental fractal analysis, and quantum chemistry is presented. Attention is focused on the ways the polymer environment affects DAE photochromism and on the ways that DAE photochromism affects surfaces. Non-constant quantum yields in single-molecule measurements, selective metal deposition, and a super-water-repellent fractal surface are discussed after a short summary of the latest experimental results concerning photochromism in DAE molecules. (C) 2011 Elsevier B.V. All rights reserved.

Dual-functional diffraction gratings, which show different diffractions in transmission and reflection, are proposed as an essential optical element. We utilize a flattening effect of photochromic diarylethene (DAE) in the low glass transition temperature state and selective metal deposition to prepare the gratings. The uncolored lines of the colored DAE layer on a grating substrate with grooves were formed by red-laser scanning, and Mg evaporation onto the substrate without a shadow mask enabled Mg-line formation at right angles to the grooves. After being stored at 27 degrees C for 500 min, the sample showed two-dimensional diffraction in reflection but one-dimensional diffraction in transmission, meaning that a dual-functional grating was achieved. This result shows great potential as a key element for various future optical devices and systems. (C) 2011 Optical Society of America

We report on metal deposition selectivity on a photochromic diarylethene (DAE) surface in intermediate vacuum. Selective deposition means that metal vapor is deposited on colored DAE but not on uncolored. Mg was evaporated on the DAE surfaces at various pressures with the introduction of inert gas. Mg was easy to deposit on the uncolored DAE surface in intermediate vacuum (2 x 10(-1) Torr). The easy-deposition effect in intermediate vacuum was explained by the re-adsorption of Mg atoms desorbed from the DAE surface. Mg cathode patterning of an organic light emitting device on the basis of selective deposition in the intermediate vacuum was successfully demonstrated. (C) 2011 The Japan Society of Applied Physics

Photocurrent switching for nondestructive readout of photochromic memory is reported. The photoisomerization of a diarylethene (DAE) molecule in a memory layer switched the photocurrent generated by laser light irradiation. The switching is based on an ionization potential change of the DAE molecules. Excellent ON-OFF ratio of photocurrent was achieved by the irradiation at 780 nm. When the pulsed laser light with a wavelength of 780 nm was repeatedly irradiated to the colored (recorded) and uncolored (initial) memory states, no change of the photocurrent signal levels was observed even after 800x103 cycles, indicating a successful demonstration of the nondestructive readout. © 2011 OSA.

In organic electronics, organic crystals are essential materials, and metal deposition on organic surfaces is an important process. We report light-controlled metal deposition on photosensitive organic crystals. Light irradiation to the photosensitive diarylethene (DAE) crystal surface generated another isomer in the crystal lattice and made the surface soft, which caused the Mg-undeposition effect. The light-controlled selective Mg deposition can be applied to fine metal patterning by laser scanning and maskless evaporation. Fine parallel and grid Mg patterning on the DAE crystal surface are demonstrated successfully. This method is promising for fine electrode patterning of various electronics devices with an organic crystal.

We report on photocurrent switching based on photoisomerization for the nondestructive readout of photochromic optical memory. The photoisomerization of a diarylethene (DAE) memory layer switched the photocurrent generated in a light-absorbing phthalocyanine layer upon irradiation of a laser light. This switching is based on the ionization potential change of the DAE molecules. Switching characteristics of the photocurrent were investigated for the laser light with a wavelength of 410 nm, 630 nm, or 780 nm. Excellent on-off ratios of the photocurrent were achieved by irradiation at 630 nm and 780 nm. When the pulsed laser light with a wavelength of 780 nm was repeatedly irradiated to the colored and uncolored memory devices, no change of the photocurrent signal levels was observed, even after 8 x 10(5) cycles, indicating a successful demonstration of the nondestructive readout. (C) 2010 Optical Society of America

Selective metal deposition controlled by light irradiation was achieved for Zn, Mg, and Mn on a photochromic diarylethene (DAE) film; metal vapor atoms were deposited only on the colored film but not on the uncolored film. The deposition properties of the metals were strongly dependent on the deposition rates. Fine metal patterns were produced by laser scanning and a conventional vacuum-evaporation method without a shadow mask. Multiple-metal pattern preparation of Zn, Mg, and Mn via a maskless evaporation in a single vacuum process was also demonstrated.

Recent progress in electrical properties of photochromic molecules is reviewed. A typical application of the properties is current switching based on the changes in ionization potential or carrier mobility induced by the photoisomerization of photochromic molecules. Carrier injection-type molecular memories have also attracted wide interest because they are a promising candidate for organic semiconductor devices in the field of organic electronics. Various new applications are proposed using photo-induced electrical, as well as optical, property changes of photochromic molecules. (C) 2010 Elsevier B.V. All rights reserved.

Electrical carrier separation from a photoexcited photochromic molecule could be a promising method for controlling the photosensitivity of such a molecule. We report an efficient carrier separation from a photochromic diarylethene (DAE) molecule by adopting a device structure with a heterojunction consisting of an n-type diarylethene layer and a p-type layer of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB). A photocurrent originating from carrier separation for the colored photostationary state was observed, even at zero applied voltage. The efficient carrier separation occurred at the interface between the DAE and NPB layers and was the result of the internal electrical field induced by the pn heterojunction.