Photochromic Diarylethene Film Research Articles

Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect.

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Feasibility study of a photochromic diarylethene film as a clinical dosimeter for kV X-rays

Radiochromic film (RCF) is commonly used as a measurement tool in radiation therapy and diagnostic X-ray imaging. However, the results are not immediately available. This study establishes the workflow of a new diarylethene (DAE) film dosimetry and investigates the film response versus dose against kV X-rays, with the results immediately available for analysis. The DAE film includes a phosphor layer (BaFCl:Eu), a DAE layer, and a UV cut filter. The workflow of the DAE film dosimetry is as follows: (1) irradiation, where the absorbed doses were between 0.1 and 5.0 Gy at 150 kVp X-ray; (2) scanning with blue excitation light (λ = 465 nm) within 1 h of irradiation to obtain a fluorescence intensity map; and (3) analysis of the relationship between the fluorescence intensity and the absorbed dose, and then comparison between such relationship and that between the optical density and absorbed dose of the RCF. Results show that the fluorescence intensity of the DAE film increased with increasing dose. Moreover, the DAE film had a roughly linear response, while the response of RCF was logarithmic (Pearson's correlation coefficients are 0.979 and 0.843, respectively). The DAE film is feasible as a clinical dosimeter of choice.

Ternary‐Responsive Field‐Effect Transistors and Multilevel Memories Based on Asymmetrically Functionalized Janus Few‐Layer WSe2

AbstractHybrids composed of 2D transition metal dichalcogenides with stimuli‐responsive molecules are prototypical components for the development of multifunctional field‐effect transistors (FETs), whose output currents can be remotely controlled by external inputs. Herein, ternary‐responsive FETs based on a few‐layer WSe2 are realized by decorating the two opposite surfaces of the 2D semiconductor with different stimuli‐responsive molecules in an asymmetric fashion: the bottom surface is interfaced with a photochromic diarylethene film and the top surface with a ferroelectric poly(vinylidene fluoride–trifluoroethylene) layer. This novel Janus ternary device architecture shows superior functional complexity compared with normal mono‐stimuli‐responsive FETs. The synergy between the two molecularly induced effects enables the devices to respond orthogonally to an electric field and light irradiation, with an enhanced output current modulation efficiency of 87%. The 9 ferroelectric and 84 photo‐generated states ensure 756 current levels in a single device. The over 10 cycles of cyclic endurance and more than 1000 h of retention time confirm the reliability of each state, implementing the demand for high‐density non‐volatile memories, as well as enriching the diversification in “More than Moore” technologies.

Nucleation, absorption, or desorption of metal-vapor atoms on amorphous photochromic diarylethene films having a low glass transition temperature

Metal-vapor atoms with high or low intrinsic vapor-pressure were absorbed into or desorbed from the low-Tg colorless photochromic film, respectively.

Elemental isomerization processes for a photochromic diarylethene film based on carrier injection toward all-electrically operable organic memory

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.

Isomerization structure of photochromic diarylethene film based on electrical carrier injection

Photochromic films have attracted interest as an organic semiconductive memory material. Isomerization structures of a photochromic diarylethene (DAE) film induced by electrical carrier injection were investigated and were made clear for the coloring-reaction mode (CRM) and decoloring-reaction mode (DRM). Filaments consisting of the colored molecules in the colorless matrix were generated in the CRM, while random distribution of isomerized molecules occurred in the DRM. The ON-OFF ratio in current in the CRM was larger than that in the DRM. These results would be important for constructing organic semiconductor memory device using DAE.

Selective Metal-vapor Deposition on Organic Surfaces.

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 Tg ) 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 Tg 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.

Noble metal deposition modulation on amorphous photochromic diarylethene film

We investigated deposition modulation by noble metals on a photochromic diarylethene (DAE) film. Differences in color and sheet resistance (Rs) between the metal films deposited on the colored and colorless DAE films were observed. We observed Rs ratios of 106 for Ag, 107 for Cu, and 104 for Au. We found that the color and Rs differences were caused by the size differences in the metal nanocrystals on the deposited films. The difference in Rs 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.

Electrical characterization of photochromic diarylethene films consisting of extraordinarily large crystallites

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 (λ = 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.

Photoinduced reversible changes in the electronic structure of photochromic diarylethene films

The change in the valence electronic structure upon switching of two photochromic diarylethene derivatives, i.e. 1,2-bis(2-methyl-5-p-tolylthiophen-3-yl)cyclopent-1-ene (DAE1) and 1,2-bis(5-(4-hexyloxycarbonylphenyl)-2-methylthiophen-3-yl)cyclopent-1-ene (DAE2), was measured by photoelectron spectroscopy. Switching between open and closed forms was followed in situ upon illumination. The increase of the ionization energy from the closed to the open form was 0.85 eV for DAE1 and 0.80 eV for DAE2. For DAE1, the work function also decreased by 0.25 eV upon switching, which is explained by a decrease of the intrinsic molecular dipole moment and a preferential orientation of molecules in thin films.

Subwavelength nanopatterning of photochromic diarylethene films

The resolution of optical patterning is constrained by the far-field diffraction limit. In this letter, we describe an approach that exploits the unique photo- and electro-chemistry of diarylethene photochromic molecules to overcome this diffraction limit and achieve sub-wavelength nanopatterning.

Photochromic diarylethene-film-based compact all-optical nonlinear joint transform correlator

We construct an all-optical nonlinear joint transform correlator utilizing the rewritable photochromic diarylethene film, which serves as a joint transform power spectrum recorder. The computer simulations indicate that, with the nonlinear response characteristic of the diarylethene film, the SNR and peak sharpness of the correlator can be improved. A compact correlator prototype with a 5-cm focal length is constructed and the experiment results verify that the physical length can be reduced remarkably owing to the high resolution of the diarylethene film.

Light-Controlled Selective Metal Deposition on a Photochromic Diarylethene Film—Toward New Applications in Electronics and Photonics—

Abstract 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.

Selective metal deposition for a structure with a thin intermediate layer on a photochromic diarylethene film

Selective metal Mg deposition was achieved for a structure with the 8-nm-thick intermediate layer of Alq3 on a photochromic diarylethene (DAE) layer. Selective deposition for a structure with the Alq3 intermediate layer was attributed to uncolored DAE molecules exposed on the surface due to the migration and the aggregation of Alq3 molecules on the uncolored DAE layer. Laser spot irradiation enables both isomerization and an annealing effect on samples with an Alq3 intermediate layer, thereby achieving selective Mg deposition. Selective deposition for a structure with an intermediate layer could be important for the preparation of patterned cathodes in the field of organic electronics.

Photoelectron Spectroscopy Study of Photoresponsive Change in Electronic Structure of Amorphous Photochromic Diarylethene Film

The photoresponsive change in the electronic structure of photochromic diarylethene, 1,2-bis(5-(2,4-diphenylphenyl)-2,4-dimethyl-3-thienyl) perfluorocyclopentene (DAE) is observed by photoelectron spectroscopy. UV and visible light irradiation introduce clear differences in the spectral shape of photoelectron spectra of the DAE thin film. In the photoelectron spectrum after UV light irradiation, the highest occupied molecular orbital (HOMO) of the DAE film appears at around 1.5 eV below the Fermi level. In contrast, under visible light irradiation, the HOMO of the DAE film has higher binding energy. The variation of the HOMO peak can be reproduced repeatedly by UV and visible light irradiation. These differences correspond to the difference in the electronic structure between the closed-ring and the open-ring isomer of DAE. The molecular orbitals of the DAE isomers are calculated with the density functional theory, and the results are in good agreement with the photoelectron results. Cyclic voltammetry measurement of the closed-ring isomer shows the first oxidation waves at 0.6 V (vs Ag/Ag+) whereas the open-ring isomer shows the first oxidation wave at 1.1 V. The consistency between the photoelectron results and other methods suggests that photoelectron spectroscopy has a powerful potential to reveal the electronic structure of photochromic molecules.

Photocurrent detection from photochromic diarylethene film

The photocurrent characteristics of a diarylethene (DAE) film were investigated. The photocurrents were attributed to the dissociation of electric carriers from the excited molecules in the DAE film. We demonstrated the on-off property of the photocurrent according to the isomerization state. Results reveal that the short-wavelength light corresponding to the high-energy absorption band of the colored DAE film generated higher carrier∕photon efficiency than did the long-wavelength light corresponding to the low-energy absorption band.

Electrical carrier-injection and transport characteristics of photochromic diarylethene films

Electrical carrier-injection and transport characteristics of photochromic diarylethene thin films were investigated. Injected and transported carriers (injected current) to the diarylethene film were increased with the existence ratio of the closed-ring form, which was one isomerization state of the diarylethene molecule. A dramatic increase was observed at a 12-percent existence ratio of the closed-ring molecules. A lowering effect of the potential barrier, which is caused by a Poole–Frenkel-like effect with a short distance interaction potential for hole transportation by increasing the existence ratio of the closed-ring molecules is proposed as the mechanism behind this phenomenon.

Near-Field Optical Recording of Photochromic Materials Using Bent Cantilever Fiber Probes

Near-field optical recording of photochromic materials is carried out using a scanning near-field optical microscope (SNOM). We constructed a simplified yet fully functional SNOM by installing bent cantilever fiber probes in an atomic force microscope. Photochromic diarylethene films are used as re-writable near-field recording media, and we successfully record erasable recording marks having a minimum of 600 nm width in a writing time as short as 30 ms.

Simple Near-Field Optical Recording Using Bent Cantilever Probes

This paper describes our high-density near-field optical recording using bent cantilever fiber probes installed in an atomic force microscope. We conducted a near-field reading of nano-scale hole patterns with a 100 nm spatial resolution and a 25 µm/s scan speed; this implies a capability of a data reading density of 60 Gb/in 2 with a 0.25 kbps data transfer rate. In addition, we investigated re-writable near-field recording on photochromic diarylethene films. We successfully recorded erasable memory bits having a minimum width of 600 nm in a writing time as short as 30 ms. We found that using a cantilever probe simplifies the setup and operation of the near-field optical recording system and may offer multifunctional recording capabilities.

Carrier Injection/Transport Characteristics of Photochromic Diarylethene Film

Photochromic diarylethenes with electric carrier (hole) controlling ability for nondestructive readout were investigated. The ionization potential of the photostationary colored diarylethene films was around 5.7–5.8 eV, irrespective of differences in their molecular structures. Good hole injection/transport characteristics were observed for the colored films of diarylethenes with triphenylamine groups.