Co-reporter:Kazuyoshi Kobashi;Ryoma Hayakawa;Toyohiro Chikyow
Advanced Electronic Materials 2017 Volume 3(Issue 8) pp:
Publication Date(Web):2017/08/01
DOI:10.1002/aelm.201700106
Negative differential resistance (NDR) has large potential for versatile device applications, including high-frequency oscillators, memories, fast switches, and multilevel logic circuits. NDRs are observed at heteromaterial interfaces in resonant tunneling diodes or Esaki diodes consisting of compound semiconductors or two-dimensional (2D) atomic thin films. However, these devices suffer from poor peak-to-valley ratios (PVR) at room temperature; a cryogenic temperature is needed to improve the PVR. These negative factors are obstacles to practical applications. Here, a new NDR transistor is proposed, in which a p-n heterojunction of organic semiconductors plays a key role. Well-balanced carrier transport is manipulated at the organic p-n junction to realize outstanding NDR. Experimental and simulation analyses reveal that the observed NDR can be explained by analogy with the shoot-through current mechanism in complementary metal-oxide- semiconductor (CMOS) devices. As a result, the NDR transistor shows large PVRs of up to about 104 even at room temperature.
Co-reporter:Ryoma Hayakawa;Toyohiro Chikyow
Nanoscale (2009-Present) 2017 vol. 9(Issue 31) pp:11297-11302
Publication Date(Web):2017/08/10
DOI:10.1039/C7NR02463K
Quantum molecular devices have a potential for the construction of new data processing architectures that cannot be achieved using current complementary metal–oxide–semiconductor (CMOS) technology. The relevant basic quantum transport properties have been examined by specific methods such as scanning probe and break-junction techniques. However, these methodologies are not compatible with current CMOS applications, and the development of practical molecular devices remains a persistent challenge. Here, we demonstrate a new vertical resonant tunneling transistor for large-scale integration. The transistor channel is comprised of a MOS structure with C60 molecules as quantum dots, and the structure behaves like a double tunnel junction. Notably, the transistors enabled the observation of stepwise drain currents, which originated from resonant tunneling via the discrete molecular orbitals. Applying side-gate voltages produced depletion layers in Si substrates, to achieve effective modulation of the drain currents and obvious peak shifts in the differential conductance curves. Our device configuration thus provides a promising means of integrating molecular functions into future CMOS applications.
Co-reporter:Tohru Tsuruoka, Ryoma Hayakawa, Kazuyoshi Kobashi, Kenji Higashiguchi, Kenji Matsuda, and Yutaka Wakayama
Nano Letters 2016 Volume 16(Issue 12) pp:7474-7480
Publication Date(Web):November 15, 2016
DOI:10.1021/acs.nanolett.6b03162
Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. Here, we demonstrate laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. The main findings are (i) a number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams alternately between the source and drain electrodes, (ii) the conductivity (or resistivity) of the 1D channel can be controlled by the illumination conditions, such as the laser power density and the scan speed, and (iii) it is possible to draw an analogue adder circuit by optically writing 1D channels so that a portion of the channels overlaps and to perform optical summing operations by local laser illumination of the respective channels. These findings will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits, which are not possible with conventional thin film OFETs.Keywords: analogue adder circuit; diarylethene; laser patterning; one-dimensional channel; Organic field-effect transistor; photochromism;
Co-reporter:Ryoma Hayakawa, Matthieu Petit, Kenji Higashiguchi, Kenji Matsuda, Toyohiro Chikyow, Yutaka Wakayama
Organic Electronics 2015 Volume 21() pp:149-154
Publication Date(Web):June 2015
DOI:10.1016/j.orgel.2015.03.011
•We improved the optical switching properties in a diarylethene-channel transistor using interface engineering.•The insertion of 6T thin film was effective for reducing the threshold voltage.•The surface modification of a gate insulator with a PMMA layer suppressed degradation in photoswitching.•We eventually achieved an on/off ratio realized by light irradiation of 103 (105%).Photochromic diarylethene (DAE) molecules were employed as the channel layer of a field-effect transistor, where the drain current was effectively modulated by the reversible phase transition between a semiconductor (closed-ring) and an insulator (open-ring) under ultraviolet or visible light irradiation. Our goal was the further improvement of optical switching properties by interface engineering. First, we reduced the hole injection barrier by introducing an α-sexithiophene (6T) thin film at the interface between the source–drain electrodes and the DAE channel layer. As a result, the threshold voltage of the DAE-FETs was greatly reduced from −64 to −4 V. Second, we improved the optical switching performance by the surface treatment of a SiO2 gate insulator with poly(methyl methacrylate) (PMMA). The drain current was unchanged even after 10 cycles of optical switching in contrast to the rapid degradation found with untreated DAE transistors. The combination of these improvements and interdigitated source–drain electrodes eventually resulted in a light irradiation driven on/off ratio of over 103. Significantly, the light-induced on/off ratio was comparable to that driven by an electrical field, which satisfies the requirement for industrial optical applications. Our findings will provide useful ways of realizing high-performance optical switching transistors.
Co-reporter:Yasushi Ishiguro, Michel Frigoli, Ryoma Hayakawa, Toyohiro Chikyow, Yutaka Wakayama
Organic Electronics 2014 Volume 15(Issue 9) pp:1891-1895
Publication Date(Web):September 2014
DOI:10.1016/j.orgel.2014.05.030
•Photochromic molecule doped into polymeric semiconductor matrix induced optical switching in transistor properties.•Photoswitching was attributed to the carrier scattering by the closed-ring isomer.•Thermal stability of the photoswitching was drastically improved by appropriate molecular design.Thermally induced structural change in photoisomerization molecules is a serious obstacle to the development of optically controllable organic field-effect transistors (OFETs). This is because the thermal relaxation of molecular structures degrades photo-induced change in drain current and removes the memory function. To deal with this issue, a naphthopyran (NP) derivative, namely 3,13-dihydro-3-(4-triphenylaminyl)-3,13-diphenylbenzopyrano[5,6-a]carbazole (NP-TPAC) was tested that displays pseudo p-type photochromism at room temperature. The NP-TPAC-doped poly(triarylamine) (PTAA) film exhibited a reversible change in transistor properties; the drain current was reduced by ultraviolet (UV) and returned to its original value by visible (VIS) light irradiation. Importantly, no change in the drain current was observed at room temperature for more than 30 h under dark conditions. This was because the open-ring trans–trans (TT) isomer of NP-TPAC is thermally stable owing to the CH-π interaction and the steric force exerted by the phenyl ring of the carbazole unit onto the double bond responsible for the thermal back reaction. In other words, the thermal stability of photochromism-based optical devices can be greatly improved by adopting an appropriate molecular design.Graphical abstract
Co-reporter:Yutaka Wakayama, Ryoma Hayakawa
Thin Solid Films 2014 Volume 554() pp:2-7
Publication Date(Web):3 March 2014
DOI:10.1016/j.tsf.2013.03.081
•Scanning probe microscope-based evaluation of individual molecules•Nano-gap electrodes for electrical current through single-molecules•Incorporation of molecular functions into Si-based devicesRecent progress on nanoscale molecular devices is reviewed. The advantages of organic molecules are highlighted in terms of their unique features such as quantum effects based on their nanometer size, optical properties originating from photochromism and chemical syntheses for various structural designs. This review covers three topics: a scanning probe microscope-based evaluation of individual molecules, nano-gap electrodes for electrical current through single-molecules and the incorporation of molecular functions into Si-based devices. In particular, the importance of the incorporation of organic molecules into Si-device architecture is emphasized with a view to realizing the large-scale integration of nanoscale molecular devices.
Co-reporter:Nobuya Hiroshiba, Jonathan P. Hill, Ryoma Hayakawa, Katsuhiko Ariga, Kiyoto Matsuishi, Yutaka Wakayama
Thin Solid Films 2014 Volume 554() pp:74-77
Publication Date(Web):3 March 2014
DOI:10.1016/j.tsf.2013.03.082
•Layer-by-layer growth of organic molecules by hot-wall deposition technique.•pn-junction of organic semiconductors with single-molecular scale flatness•Characterization of morphologies by atomic force microscopy and X-ray reflection.We demonstrate a technique for growing fine molecular films on a monolayer scale. We achieve layer-by-layer growth under thermally equilibrium condition by precisely controlling the conditions of an ultra-slow deposition technique. This technique is applicable to various kinds of p-type and n-type organic semiconductors and makes it possible to form a hetero-molecular interface (p-n junction) with molecular level flatness. The technique was used to produce a molecular superlattice, which enables the well-controlled design of energy level alignments in organic semiconductors.
Co-reporter:Hoon-Seok Seo ; Ryoma Hayakawa ; Toyohiro Chikyow
The Journal of Physical Chemistry C 2014 Volume 118(Issue 12) pp:6467-6472
Publication Date(Web):February 28, 2014
DOI:10.1021/jp411386s
We attained multilevel manipulation of resonant tunneling in a metal–insulator–semiconductor (MIS) structure, in which heterogeneous molecules of copper hexadecafluorophthalocyanine (F16CuPc) and copper phthalocyanine (CuPc) were embedded in an insulating layer to form a double-tunneling junction. Resonant tunneling was observed in samples with either molecule, revealing that the molecules worked as intermediate electrodes for the tunneling current. The carrier transport was ascribed to resonant tunneling through the energy levels of individual molecules. That is, the threshold voltage of resonant tunneling can be controlled according to the energy levels of the molecules. We achieved multilevel resonant tunneling operation with binary molecules of F16CuPc and CuPc, in which the carriers were injected into the respective molecules at corresponding threshold voltages. Our findings point to a new multilevel operation of resonant tunneling through organic molecules in a practical MIS device structure.
Co-reporter:Yasushi Ishiguro, Ryoma Hayakawa, Toyohiro Chikyow and Yutaka Wakayama
Journal of Materials Chemistry A 2013 vol. 1(Issue 17) pp:3012-3016
Publication Date(Web):08 Mar 2013
DOI:10.1039/C3TC30130C
We demonstrate the optical switching of transistor properties by adopting the photoisomerization of spiropyran (SP) molecules dispersed in a poly(triarylamine) (PTAA) channel layer. The drain current (IDS) was reduced by ultra-violet (UV) light irradiation, while visible (VIS) light irradiation restored the IDS to its original value. The UV and VIS light irradiation brought about the photoisomerization reactions of SP, i.e., an ionic polarized open-ring isomer was induced by UV and a less polarized closed-ring isomer was induced by VIS. To clarify the mechanism of the photoisomerization-induced optical switching, we evaluated the variations in transistor properties, namely IDS, carrier mobility and threshold voltage, when subjected to UV and VIS light irradiation. We found that the ionic polarized open-ring SP isomer worked as a carrier scattering site that effectively suppressed carrier transport in the polymer channel.
Co-reporter:Ryoma Hayakawa, Kenji Higashiguchi, Kenji Matsuda, Toyohiro Chikyow, and Yutaka Wakayama
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 9) pp:3625
Publication Date(Web):April 2, 2013
DOI:10.1021/am400030z
We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 102 (1 × 104%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.Keywords: optical and electrical gates; photochromic channel layer; thin film transistor;
Co-reporter:Yasushi Ishiguro, Ryoma Hayakawa, Takeshi Yasuda, Toyohiro Chikyow, and Yutaka Wakayama
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 19) pp:9726
Publication Date(Web):September 16, 2013
DOI:10.1021/am402833k
We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect.Keywords: dual-gate transistor; optical-memory effect; organic transistor; photochromism; poly(triarylamine); spiropyran;
Co-reporter:Ryoma Hayakawa, Kenji Higashiguchi, Kenji Matsuda, Toyohiro Chikyow, and Yutaka Wakayama
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 21) pp:11371
Publication Date(Web):October 7, 2013
DOI:10.1021/am403616m
We demonstrated optical manipulation of single-electron tunneling (SET) by photoisomerization of diarylethene molecules in a metal–insulator–semiconductor (MIS) structure. Stress is placed on the fact that device operation is realized in the practical device configuration of MIS structure and that it is not achieved in structures based on nanogap electrodes and scanning probe techniques. Namely, this is a basic memory device configuration that has the potential for large-scale integration. In our device, the threshold voltage of SET was clearly modulated as a reversible change in the molecular orbital induced by photoisomerization, indicating that diarylethene molecules worked as optically controllable quantum dots. These findings will allow the integration of photonic functionality into current Si-based memory devices, which is a unique feature of organic molecules that is unobtainable with inorganic materials. Our proposed device therefore has enormous potential for providing a breakthrough in Si technology.Keywords: metal−insulator−semiconductor; optical manipulation; photochromic quantum dot; single-electron tunneling;
Co-reporter:Jianchen Hu, Kendal W. Clark, Ryoma Hayakawa, An-Ping Li, and Yutaka Wakayama
Langmuir 2013 Volume 29(Issue 24) pp:7266-7270
Publication Date(Web):January 8, 2013
DOI:10.1021/la304499k
We report on improved electrical conductivity in poly(3-hexylthiophene) (P3HT)/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) composite nanowires grown using an anodized aluminum oxide (AAO) template. The electrical conductivity of individual nanowire measured by four-probe scanning tunneling microscopy shows that F4-TCNQ molecules are effectively doped into P3HT by capillary force. The resistivity is tuned in the 0.1–10 Ω cm range by changing the F4-TCNQ concentration from 10 to 0.1 wt % and is 2–4 orders of magnitude smaller than that of the corresponding P3HT/F4-TNCQ thin film composites. The AAO template-assisted synthesis approach thus appears to be effective for high chemical doping and for improving the electrical conductivity of the molecular wires.
Co-reporter:Seiichi Takami, Seiichi Furumi, Yasuhiro Shirai, Yoshio Sakka and Yutaka Wakayama
Journal of Materials Chemistry A 2012 vol. 22(Issue 17) pp:8629-8633
Publication Date(Web):28 Feb 2012
DOI:10.1039/C2JM30179B
We demonstrate the effective use of a magnetic field to improve molecular alignment, and the resulting enhancement of the electrical conductivity of organic molecular nanowires. The structures of phthalocyanine nanowires, which are produced in porous alumina templates, are characterized by X-ray diffraction. Careful analyses reveal that the crystal morphologies of the phthalocyanine nanowires, e.g., domain size, columnar alignment and face-to-face spacing, are improved by using a 12 T magnetic field. This is because of the anisotropic magnetic susceptibility of the π-conjugated phthalocyanine macrocycle. Electrical measurements of individual nanowires performed with a multi-probe scanning electron microscope show that such highly ordered molecular packing results in a seventeen-fold increase in conductivity.
Co-reporter:Ryoma Hayakawa;Nobuya Hiroshiba;Toyohiro Chikyow
Advanced Functional Materials 2011 Volume 21( Issue 15) pp:2933-2937
Publication Date(Web):
DOI:10.1002/adfm.201100220
Abstract
A sigle-electron tunneling (SET) in a metal-insulator-semiconductor (MIS) structure is demonstrated, in which C60 and copper phthalocyanine (CuPc) molecules are embedded as quantum dots in the insulator layer. The SET is found to originate from resonant tunneling via the energy levels of the embedded molecules, (e.g., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)). These findings show that the threshold voltages for SET are tunable according to the energy levels of the molecules. Furthermore, SET is observable even near room temperature. The results suggest, together with the fact that these properties are demonstrated in a practical device configuration, that the integration of molecular dots into the Si-MIS structure has considerable potential for achieving novel SET devices. Moreover, the attempt allows large-scale integration of individual molecular functionalities.
Co-reporter:N. Hiroshiba, R. Hayakawa, T. Chikyow, Y. Yamashita, H. Yoshikawa, K. Kobayashi, K. Morimoto, K. Matsuishi and Y. Wakayama
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 13) pp:6280-6285
Publication Date(Web):28 Feb 2011
DOI:10.1039/C0CP02663H
Photo-induced carrier processes at the heteromolecular interface of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) and quaterrylene (QT) on a molecular scale were examined by optical and photoelectron spectroscopy. The energy level alignments of the molecules were determined by X-ray photoelectron spectroscopy and the optical absorption spectra for detailed investigation of the photo-induced carrier process were analysed. A reduction in photoluminescence from PTCDI-C8 on QT was observed, clearly demonstrating that the excitons generated in the PTCDI-C8 layer are effectively dissociated at the heteromolecular interface. One important factor inducing this effective charge dissociation is the highly ordered molecular packing, which acts to increase the exciton diffusion length. Moreover, a specific increase in the photoluminescence excitation spectrum was observed around 3 eV, indicating that simultaneous exciton generation in both the QT and PTCDI-C8 layers effectively suppresses such charge dissociation of the excitons. In other words, the existence of excitons in each molecule at the heteromolecular interface and HOMO–LUMO level alignment at the interface play an essential role in charge dissociation. Our results provide a striking insight into intermolecular interactions in the carrier process at the heteromolecular interface such as exciton generation, the recombination and dissociation processes, and the photovoltaic effect in organic semiconductors.
Co-reporter:Nobuya Hiroshiba, Ryoma Hayakawa, Toyohiro Chikyow, Kiyoto Matsuishi, Yutaka Wakayama
Organic Electronics 2011 Volume 12(Issue 8) pp:1336-1340
Publication Date(Web):August 2011
DOI:10.1016/j.orgel.2011.05.001
We examined the transistor properties of an N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)/quaterrylene (QT) heteromolecular layer, in which the highly ordered molecular layers were stacked on the monolayer level. Ambipolar behavior was clearly observed when behaving as a field effect transistor (FET). The field effect mobilities and threshold voltages were evaluated to gain an insight into the injection and transport of the carriers. The obtained carrier mobilities for holes and electrons were calculated to be 3.6 × 10−3 and 2.7 × 10−2 cm2 V−1 s−1, respectively. These values are comparable to those of the respective FET behaviors of QT (p-channel) and PTCDI-C8 layers (n-channel), indicating that a well-defined heteromolecular interface is an essential factor for improving carrier transport in ambipolar FETs. On the other hand, in the PTCDI-C8 layer, a marked shift from 5 to 61.6 V was observed in the threshold voltage for electron transport. We concluded this drastic change in threshold voltage to be due to the presence of hole carriers in the underlying QT layers. This finding demonstrates the potential to manipulate threshold voltage according to the electronic states of the underling layer without disturbing carrier transport.Graphical abstractHighlights► Transistor properties of an PTCDI-C8/QT heteromolecular layer were investigated. ► The highly ordered PTCDI-C8/QT layers were stacked on the monolayer level. ► The field effect mobilities and threshold voltages were evaluated to gain an insight. ► A marked shift from 5 to 61.6 V was observed in the Vth for electron transport. ► Our finding demonstrates the potential to manipulate Vth in layer stacked FET.
Co-reporter:Yasuhiro Shirai;Seiichi Takami;Soesan Lasmono;Hideo Iwai;Toyohiro Chikyow
Journal of Polymer Science Part B: Polymer Physics 2011 Volume 49( Issue 24) pp:1762-1768
Publication Date(Web):
DOI:10.1002/polb.22376
Abstract
Polymeric nanowires of poly(3,4-ethylenedioxythiophene) (PEDOT) are electrochemically synthesized using porous anodic alumina oxide (AAO) membranes as templates. Four-point resistivity measurements on more than 100 PEDOT nanowires with different diameters (50–250 nm) reveal a statistically significant size-dependent phenomenon in which the nanowires with a smaller diameter exhibit higher conductivity. Structural characterization with Raman spectroscopy and doping level estimation with energy-dispersive X-ray spectrometry and X-ray photoelectron spectroscopy indicate that the observed conductivity enhancement can be attributed to improved carrier mobility in PEDOT nanowires having an elongated conjugation structure because of the effect of the AAO template. From the estimated doping levels (∼5%) and conductivity data (∼100 S/cm), it is found that the carrier mobility reach 2.0 cm2/V s for the nanowire with the smallest diameter, as compared with 4.0 × 10−4 cm2/V s for a bulk PEDOT film. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011
Co-reporter:Yutaka Wakayama, Dimas G. de Oteyza, Juan M. Garcia-Lastra, and Duncan J. Mowbray
ACS Nano 2011 Volume 5(Issue 1) pp:581
Publication Date(Web):December 13, 2010
DOI:10.1021/nn102887x
Various phases of binary molecular assemblies of perfluorinated Cu-phthalocyanine (F16CuPc) and pentacene were examined using scanning tunneling microscopy (STM). Alloying, solid solutions, phase separation, and segregation were observed in assemblies on monolayers according to the mixture ratios. The main driving force behind such molecular blending is CH−F hydrogen bonds. Lattice matching and molecular symmetry are other factors that determine the assembly configuration. A detailed understanding of such solid-state reactions provides a guideline to the construction of multilayered binary assemblies, where intermixing between molecules takes place when multiple layers are stacked.Keywords: hydrogen bonds; molecular alloy; molecular solid solution; scanning tunneling microscopy; self-assembly
Co-reporter:Matthieu Petit, Ryoma Hayakawa, Toyohiro Chikyow, Jonathan P. Hill, Katsuhiko Ariga, Yutaka Wakayama
Organic Electronics 2009 Volume 10(Issue 6) pp:1187-1190
Publication Date(Web):September 2009
DOI:10.1016/j.orgel.2009.05.026
Organic thin film transistors (OTFT) based on N,N′-Bis(n-pentyl)terrylene-3,4:11,12-tetracarboxylic diimide (TTCDI-5C) with Al or Au top-contact electrodes were deposited on SiO2 (200 nm)/p-Si (0 0 1) substrates. Carrier mobility was examined as a function of temperature in the range from 50 to 310 K. Two distinct carrier transfer behaviours were observed: temperature independent behaviour below 150 K and thermally activated behaviour above 150 K. Activation energies presented values of 85–130 meV depending on the metal electrodes (Au, Al), which can be attributed to the carrier traps at the interface and the energy-level offset between the lowest unoccupied molecular orbital (LUMO) and the work functions of the respective metals.
Co-reporter:Nobuya Hiroshiba, Ryoma Hayakawa, Matthieu Petit, Toyohiro Chikyow, Kiyoto Matsuishi, Yutaka Wakayama
Organic Electronics 2009 Volume 10(Issue 5) pp:1032-1036
Publication Date(Web):August 2009
DOI:10.1016/j.orgel.2009.05.010
A molecular superlattice consisting of alternate layers of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) and quaterrylene was prepared by using an ultra-slow deposition technique. Film growth under equilibrium conditions with precise optimization of the substrate temperature enabled the layer-by-layer stacking of hetero-molecules at a single-layer level. The morphology of the films and the orientation of the molecules in each layer were analyzed by atomic force microscopy (AFM) and an X-ray reflection (XRR) technique.
Co-reporter:Ryoma Hayakawa, XueNa Zhang, Helmut Dosch, Nobuya Hiroshiba, Toyohiro Chikyow and Yutaka Wakayama
The Journal of Physical Chemistry C 2009 Volume 113(Issue 6) pp:2197-2199
Publication Date(Web):2017-2-22
DOI:10.1021/jp809556p
The growth process of quaterrylene thin films was examined by atomic force microscopy and in-plane and out-of-plane X-ray diffraction (XRD). Quaterrylene thin films on the SiO2 surface exhibit the Stranski−Krastanov (SK) growth mode; the films initially formed two-dimensional (2D) layers (<4 monolayers (ML)) followed by three-dimensional (3D) island growth. Grazing incidence X-ray diffraction (GIXD) measurement revealed that the first few layers were subjected to compressive stress along the b axis of the unit cell, resulting in expansion of the a- and c-lattice constants. This result revealed that compressive stress generated in the 2D layers drives the change of the growth mode to 3D. An understanding of the transition mechanism in early growth has important implications for the improvement of carrier transport in organic field-effect transistors.
Co-reporter:Tobias N. Krauss;Esther Barrena Dr.;Helmut Dosch Dr. Dr.
ChemPhysChem 2009 Volume 10( Issue 14) pp:2445-2448
Publication Date(Web):
DOI:10.1002/cphc.200900408
Abstract
A crystalline nanoporous molecular network was tailored by supramolecular assembly of pentacene and F16CuPc on Cu(100). The structure and self-assembly mechanisms of the pure and binary layers were analyzed by STM. F16CuPc films and mixed layers of pentacene/F16CuPc in a ratio of 2:1 show two enantiomorphic chiral domains with high structural order in contrast to pentacene which exhibits no long-range order in pure films. A model of the epitaxial relationship on Cu(100) is given, which suggests CF⋅⋅⋅H bonding as a possible driving force for the bimolecular self-assembly in addition to the still strong interaction between the substrate and the organic bilayer.
Co-reporter:Nobuya Hiroshiba, Ryoma Hayakawa, Matthieu Petit, Toyohiro Chikyow, Kiyoto Matsuishi, Yutaka Wakayama
Thin Solid Films 2009 Volume 518(Issue 2) pp:441-443
Publication Date(Web):30 November 2009
DOI:10.1016/j.tsf.2009.07.050
We examined the film morphologies and transistor properties of hetero-molecular bilayer consisting of N, N’-dioctyl-3, 4, 9, 10-perylenedicarboximide (PTCDI-C8) and quaterrylene. First, the structure and carrier conduction of PTCDI-C8 films were studied, followed by an analysis of the carrier accumulation process in a PTCDI-C8/quaterrylene hetero-bilayer transistor. Based on the displacement current measurement (DCM), we stress the potential of the hetero-bilayer for tuning carrier accumulation like carrier doping techniques in field-effect transistors.
Co-reporter:Ryoma Hayakawa, Nobuya Hiroshiba, Toyohiro Chikyow, Yutaka Wakayama
Thin Solid Films 2009 Volume 518(Issue 2) pp:437-440
Publication Date(Web):30 November 2009
DOI:10.1016/j.tsf.2009.07.065
Quaterrylene field-effect transistors (FETs) were formed on a silicon oxide (SiO2) layer and on an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM). To elucidate the transport mechanisms in the respective devices, we examined the dependence of carrier mobility on film thickness and temperature. On the OTS surface, a marked increase in the carrier mobility was observed in the initial layers, indicating that the accumulated carriers were distributed closer to the interface than were those on the SiO2 surface. Moreover, the carrier transport in the respective devices exhibited distinct behaviors in the low temperature range, particularly in the initial layers. On the SiO2 surface the carrier mobility depended strongly on temperature; the value drastically declined with the decreasing temperature from 300 K down to 60 K. On the OTS surface, the carrier mobility showed temperature-independent transport below 210 K. This maintenance of the carrier transport at low temperatures was caused by the termination of the trap-state density near the interface. These results clearly reveal that the OTS treatment effectively helped improve the interface properties because of a reduction in the density of the carrier traps, dramatically facilitating the carrier transport in the initial layers.
Co-reporter:Yutaka Wakayama, Ryoma Hayakawa, Toyohiro Chikyow, Shinichi Machida, Tomonobu Nakayama, Stefan Egger, Dimas G. de Oteyza, Helmut Dosch and Kenji Kobayashi
Nano Letters 2008 Volume 8(Issue 10) pp:3273-3277
Publication Date(Web):September 9, 2008
DOI:10.1021/nl801649v
We demonstrate a comprehensive study of self-assembled molecular nanowire, including molecular design, one-dimensional crystal growth, resistivity measurement of individual wire, and application to a field-effect transistor. Appropriate molecular design and control of interfacial interactions lead to single crystalline wire growth with an extensive π-stacking motif. Resistivity measurements of an individual molecular wire indicate that these structural features are advantageous for electrical transport. Finally, field-effect transistors with single- and double-wire channels were fabricated to give some indication of the potential application of the molecular wires.
Co-reporter:Ryoma Hayakawa, Matthieu Petit, Yutaka Wakayama, Toyohiro Chikyow
Organic Electronics 2007 Volume 8(Issue 5) pp:631-634
Publication Date(Web):October 2007
DOI:10.1016/j.orgel.2007.07.007
Quaterrylene molecules, which have a planar and highly π-conjugated chemical structure, were deposited on a SiO2 surface, and their thin film structures, including surface morphology and molecular orientation, were examined by atomic force microscopy (AFM) and X-ray diffractometry (XRD). AFM observations revealed the grain size and surface roughness to be closely dependent on the substrate temperature in the range from 27 °C to 200 °C. Particularly at a substrate temperature of 140 °C, grain sizes of up to 6 μm and low surface roughness of 1.67 nm were successfully obtained in the 8 ML-thick film. XRD measurements of the quaterrylene thin film revealed (0 0 l) Bragg reflections, corresponding to a spacing of 1.89 nm. This value coincides with the average height of the terraces of the stepped structure observed in the AFM images. These results clearly demonstrate the quaterrylene molecules to have an upright orientation and that thin films grow as layered structures on the surface. From the full width of half maximum (FWHM) of the XRD rocking curve, the degree of alignment of the molecular planes (mosaicity) was estimated to be 0.09°, which shows that the film has a highly ordered structure.
Co-reporter:Yutaka Wakayama, Jonathan P. Hill, Katsuhiko Ariga
Surface Science 2007 Volume 601(Issue 18) pp:3984-3987
Publication Date(Web):15 September 2007
DOI:10.1016/j.susc.2007.04.068
Dynamic processes of molecular assembly on a metal surface were examined using scanning tunneling microscopy (STM). Molecules of a porphyrin derivative were deposited on a Cu(1 1 1) surface and were found to be highly mobile at room temperature. The real-time STM observation enabled visualization of molecular activity such as surface diffusion, domain formation and phase transition. The high mobility of the molecules caused build-up and break-down of molecular domains. Metastability of the molecular assembly caused various domain formations with different molecular alignments, including square and hexagonal motifs. A phase transition from a hexagonal to a square domain structure was successfully observed by sequential STM imaging.
Co-reporter:Y. Wakayama, T. Mitsui, T. Onodera, H. Oikawa, H. Nakanishi
Chemical Physics Letters 2006 Volume 417(4–6) pp:503-508
Publication Date(Web):10 January 2006
DOI:10.1016/j.cplett.2005.10.085
Abstract
The growth process of molecular nanocrystals on a solid surface was explored through the application of a vacuum evaporation technique. The combination of ultra rapid evaporation, a patterned intended surface, and a perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) buffer layer was found to be effective in producing small perylene nanocrystals (ca. 100 nm) at a high density (13.6 μm−2). The prepared perylene nanocrystals exhibited characteristic fluorescence properties. The fluorescence spectra contained luminescence peaks attributed to a free exciton and self-trapped exciton, which were the result of nanometer-scale size effects.
Co-reporter:T. Sekiguchi, Y. Wakayama, S. Yokoyama, T. Kamikado, S. Mashiko
Thin Solid Films 2004 Volumes 464–465() pp:393-397
Publication Date(Web):October 2004
DOI:10.1016/j.tsf.2004.06.084
The adsorbed structures and assembly behavior of porphyrin derivatives on Cu substrates were studied using low-temperature scanning tunneling microscopy (STM). The STM images showed that a porphyrin molecule with four tert-butyl-phenyl groups (H2-TBPP) forms twin domains of a square superstructure. The deformation of H2-TBPP required to explain the STM images forms a pair of corresponding chiral conformations. The conformation determines which of the twin domains is formed. In contrast, porphyrin molecules with two tert-butyl-phenyl and two vinyl–phenyl groups (H2-TBPVPP) showed no form of ordered assembly due to the strong interaction between the vinyl–phenyl and substrate. STM observation showed that cis- and trans-H2-TBPVPP undergo different conformational changes, leading to mirror and two-fold symmetry, respectively.
Co-reporter:Y. Wakayama, L.V. Sokolov, N. Zakharov, P. Werner, U. Gösele
Applied Surface Science 2003 Volume 216(1–4) pp:419-423
Publication Date(Web):30 June 2003
DOI:10.1016/S0169-4332(03)00385-4
Abstract
In order to produce dome-shaped Ge dots with small size and high density, C submonolayers (C-SMLs) were incorporated at the interface between Ge wetting layers and Ge dots. The C atoms are considered to induce a local strain field by forming Ge–C bonding. Such strain field enhanced dome formation even at low temperature (<500 °C). Optimization of experimental conditions enabled precise control of the Ge dome size in the range of 30–40 nm with the density of 1010 cm−2. The Ge domes thus prepared exhibited intensive photoluminescence (PL) compared to those prepared by a conventional self-assembling technique.
Co-reporter:Yutaka Wakayama, Gerhard Gerth, Peter Werner, Leonid V. Sokolov
Surface Science 2001 Volume 493(1–3) pp:399-404
Publication Date(Web):1 November 2001
DOI:10.1016/S0039-6028(01)01245-6
The effect of a small amount of C atoms on the Ge dot morphology on Si(0 0 1) has been investigated. Submonolayers of carbon were deposited on Ge wetting layers to modify the subsequent Ge dot growth mode. AFM studies revealed that the C layer has two main effects on the Ge dot growth, which were to promote a structural transition from huts to domes and to initiate three-dimensional growth even on a thin wetting layer. The results indicated that the C atoms, which were localized at the interface between the Ge wetting layer and the Ge dot, induced a strain field and destabilized the hut structure. As a result, Ge domes could be grown with the help of the C atoms at relatively low temperature, suggesting a possibility to produce small quantum dots with high number density as well as size uniformity.
Co-reporter:Yasushi Ishiguro ; Ryoma Hayakawa ; Toyohiro Chikyow
ACS Applied Materials & Interfaces () pp:
Publication Date(Web):
DOI:10.1021/am501884q
We produced an optically controllable dual-gate organic field-effect transistor by a simple one-step spin-coating of a mixed solution of photochromic spiropyran (SP) and poly(3-hexylthiophene) (P3HT). Postannealing enhanced polymer chain ordering of P3HT to induce phase separation into an SP-rich lower layer and an SP-free upper layer. These layers worked independently as transistor channels with distinct optical responsivity. The top channel was optically inactive, but the bottom channel was optically active, because of the photoisomerization of SP. These results demonstrate the potential of our technique to produce a multifunctional photoactive organic transistor by a simple process.
Co-reporter:N. Hiroshiba, R. Hayakawa, T. Chikyow, Y. Yamashita, H. Yoshikawa, K. Kobayashi, K. Morimoto, K. Matsuishi and Y. Wakayama
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 13) pp:NaN6285-6285
Publication Date(Web):2011/02/28
DOI:10.1039/C0CP02663H
Photo-induced carrier processes at the heteromolecular interface of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) and quaterrylene (QT) on a molecular scale were examined by optical and photoelectron spectroscopy. The energy level alignments of the molecules were determined by X-ray photoelectron spectroscopy and the optical absorption spectra for detailed investigation of the photo-induced carrier process were analysed. A reduction in photoluminescence from PTCDI-C8 on QT was observed, clearly demonstrating that the excitons generated in the PTCDI-C8 layer are effectively dissociated at the heteromolecular interface. One important factor inducing this effective charge dissociation is the highly ordered molecular packing, which acts to increase the exciton diffusion length. Moreover, a specific increase in the photoluminescence excitation spectrum was observed around 3 eV, indicating that simultaneous exciton generation in both the QT and PTCDI-C8 layers effectively suppresses such charge dissociation of the excitons. In other words, the existence of excitons in each molecule at the heteromolecular interface and HOMO–LUMO level alignment at the interface play an essential role in charge dissociation. Our results provide a striking insight into intermolecular interactions in the carrier process at the heteromolecular interface such as exciton generation, the recombination and dissociation processes, and the photovoltaic effect in organic semiconductors.
Co-reporter:Yasushi Ishiguro, Ryoma Hayakawa, Toyohiro Chikyow and Yutaka Wakayama
Journal of Materials Chemistry A 2013 - vol. 1(Issue 17) pp:NaN3016-3016
Publication Date(Web):2013/03/08
DOI:10.1039/C3TC30130C
We demonstrate the optical switching of transistor properties by adopting the photoisomerization of spiropyran (SP) molecules dispersed in a poly(triarylamine) (PTAA) channel layer. The drain current (IDS) was reduced by ultra-violet (UV) light irradiation, while visible (VIS) light irradiation restored the IDS to its original value. The UV and VIS light irradiation brought about the photoisomerization reactions of SP, i.e., an ionic polarized open-ring isomer was induced by UV and a less polarized closed-ring isomer was induced by VIS. To clarify the mechanism of the photoisomerization-induced optical switching, we evaluated the variations in transistor properties, namely IDS, carrier mobility and threshold voltage, when subjected to UV and VIS light irradiation. We found that the ionic polarized open-ring SP isomer worked as a carrier scattering site that effectively suppressed carrier transport in the polymer channel.
Co-reporter:Seiichi Takami, Seiichi Furumi, Yasuhiro Shirai, Yoshio Sakka and Yutaka Wakayama
Journal of Materials Chemistry A 2012 - vol. 22(Issue 17) pp:NaN8633-8633
Publication Date(Web):2012/02/28
DOI:10.1039/C2JM30179B
We demonstrate the effective use of a magnetic field to improve molecular alignment, and the resulting enhancement of the electrical conductivity of organic molecular nanowires. The structures of phthalocyanine nanowires, which are produced in porous alumina templates, are characterized by X-ray diffraction. Careful analyses reveal that the crystal morphologies of the phthalocyanine nanowires, e.g., domain size, columnar alignment and face-to-face spacing, are improved by using a 12 T magnetic field. This is because of the anisotropic magnetic susceptibility of the π-conjugated phthalocyanine macrocycle. Electrical measurements of individual nanowires performed with a multi-probe scanning electron microscope show that such highly ordered molecular packing results in a seventeen-fold increase in conductivity.
![Poly[[2,2'-bithiophene]-5,5'-diyl(9,9-dioctyl-9H-fluorene-2,7-diyl)]](http://img.cochemist.com/ccimg/210400/210347-56-1.png)
![Poly[[2,2'-bithiophene]-5,5'-diyl(9,9-dioctyl-9H-fluorene-2,7-diyl)]](http://img.cochemist.com/ccimg/210400/210347-56-1_b.png)
![Anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone, 2,9-dioctyl-](/data/chemimg/709500/78151-58-3.png)
![Anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone, 2,9-dioctyl-](/data/chemimg/709500/78151-58-3_b.png)
