Hoon-Kyu Shin

Characterization of the electrical and optical properties of viologen devices using chlorophyll a as an electron excimer.

This paper uses self-assembled monolayers (SAMs) on an Au(111) substrate to detect the unique characteristics of viologen molecules using scanning tunneling microscopy (STM), and reports the orientation and surface changes of molecules at the nano level in real-time. In particular, the rectification characteristics of the viologen molecule were observed at the molecular level using scanning tunneling spectroscopy (STS). After verifying the rectification characteristics of viologen molecules, an experiment was carried out to demonstrate the possibility of applying viologen to photodiodes and switching devices by forming a thin film of chlorophyll a on the viologen SAMs using the Langmuir-Blodgett (LB) method. This material mimics the photoinduced electron transport phenomenon in the early stage of photosynthesis in living plants. This study demonstrates the applicability of viologen to bioelectronic photodiodes and switching devices based on photo effects by observing the topography, current sensing, and current-voltage (I-V) characteristics using current-sensing atomic force microscopy (CS-AFM) by introducing light to the AFM-tip/chlorophyll a/viologen/Au(111) substrate structure.

Self-Assembly and Redox Properties of Viologen Derivatives on Au(111) Determined by Cyclic Voltammetry and Scanning Tunneling Microscopy

The self-assembly and redox properties of viologen derivatives [N-methyl-N-di(10-mercaptodecyl)-4,4-bipyridinium; HSC10VC10SH] immobilized on a Au(111) substrate were investigated by ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and cyclic voltammetry. We demonstrate here a novel self-assembly monolayer (SAM) matrix appropriate for the isolation of viologen molecules. An octanethiol was used as the SAM matrix, in which the HSC10VC10SH was inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. We measured the conformational change of the viologen molecular protrusions using STM and investigated the changes in the width and height of the alkyl group, which were due to the change in the polarity of viologen molecules caused by the electron charge.

A study on conformational changes by electron charges in viologen single molecules by using STM.

The topography of self-assembled viologen derivatives (VC(8)SH, VC(10)SH, HSC(8)VC(8)SH, and HSC(10)VC(10)SH) molecules on an octanethiol (C(8)) self-assembled monolayer (SAM) modified gold surface was measured using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We demonstrate here a novel matrix SAM appropriate for isolation of the viologen molecules. The C(8) was used for a matrix SAM, in which the VC(8)SH, VC(10)SH, HSC(8)VC(8)SH, and HSC(10)VC(10)SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights (VC(8)SH: 1.53 nm, VC(10)SH: 2.01 nm, HSC(8)VC(8)SH: 2.71 nm, and HSC(10)VC(10)SH: 3.3 nm) of the molecular protrusions using STM. Also, changes in the central axis of viologen molecules were observed as VC(8)SH (0.5-0.73 nm), VC(10)SH (0.4-0.74 nm), HSC(8)VC(8)SH (0.67-0.84 nm), and HSC(10)VC(10)SH (0.67-0.99 nm), respectively.

Study on tunneling current through barrier height using scanning tunneling microscopy

The electrical properties of viologen derivatives were studied of the tunneling current characteristics on the length of the viologen derivatives using self-assembling techniques and ultra high vacuum scanning tunneling microscopy (UHV-STM). The current-voltage (I-V) and differential conductance (dl/dV-V) characteristics were measured while the electrical properties of the formed monolayer were scanned by using a scanning tunneling spectroscopy (STS). Also, we investigated to study electrons movement which is caused by height variation because we can see the organic materials barrier height and STM tip by organic materials energy gap (Eg). The total effective barrier height Phi of viologen derivatives VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH SAMs were calculated to be 1.076 eV, 1.86 eV, 1.85 eV, 2.28 eV, respectively.

Synthesis of Electroluminescent Materials Based on Phosphine Oxide-Naphthoxazole Structure

In the course of investigation for finding new light-emitting materials based on the phosphine oxide-oxazole structure, 2-(2-(diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (2-PPN) and 2-(4-(diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (4-PPN) were newly synthesized. The chemical structures are characterized by 1H-NMR, 13C-NMR, FT-IR, UV-Vis, elemental analysis (EA), and X-ray photoelectron spectroscopy (XPS). 2-PPN and 4-PPN show a strong blue emission under excitation by UV-lamp of 365nm. The photoluminescence (PL) of 2-PPN and 4-PPN showed a blue emission in chloroform. The electroluminescence (EL) of 2-PPN and 4-PPN devices showed a green emission.

Synthesis of phosphine oxides with tris(benzoxazole/benzothiazole) moieties and their OLED characteristics

ABSTRACT We have developed a series of phosphine oxides with heterocycles for the light-emitting materials of various colors. In this study, we report novel phosphine oxides with tris(benzoxazole/benzothiazole) moieties, such as (4-(benzo[d]thiazol-2-yl)phenyl)phosphine oxide (4-TBTPO) and tris(4-(benzo[d]oxazol-2-yl)phenyl)phosphine oxide (4-TBOPO). The cyclization reactions were carried out from 4,4′,4″-phosphoryltribenzoic acid and 2-aminobenzenethiol or 2-aminophenol with tetrabutylammonium bromide and triphenyl phosphite as catalysts. The structures and thermal properties were investigated by 1H-NMR, 13C-NMR, FT-IR, UV-Vis, TGA and DSC. The OLED devices of ITO/MoOx(30 nm)/NPB(600 nm)/4-TBTPO or 4-TBOPO(200 nm)/Alq3(300 nm)/Liq(10 nm)/ Al(120 nm) showed yellow-green emissions at (0.429∼0.529) and (0.406∼0.563) of CIE color coordinates, respectively.

Synthesis and OLED Properties of Zinc Complexes Based on Quinaldic Acid

Metal-chelate materials based on polycyclic aromatic ligands are attractive for tuning emission colors of organic light-emitting diodes (OLED). In this work, quinaldic acid (QA) and 8-hydroxyquinoline (HQ) were employed as organic ligands and the corresponding zinc complexes (Zn(HQA), Zn(QA), Zn(HQ)) were synthesized. The structures of Zn(HQA), Zn(QA), and Zn(HQ) were determined by 1H-NMR, 13C-NMR, FT-IR, UV-Vis and XPS. Zinc complexes showed thermal stability up to 350°C under nitrogen flow by TGA. The photoluminescence (PL) was measured from DMSO solution. The Zn(HQA) emitted a yellow light around 550 nm, while Zn(HQ) emitted a green light around 500 nm. Electroluminescence (EL) device having the structure of ITO/NPB/Zn(HQA)/Liq/Al was fabricated. The device emitted a yellow light around 550 nm. The maximum luminance of 190 cd/m2 and current of 50 mA/cm2 were observed at 15 V in the Zn(HQA)-containing device.

Morphology Observation and J-Aggregation Characteristics of Merocyanine Dye with Arachidic Acid LB Films

In order to confirm the application possibility to the molecular electronic device, the morphological property of arachidic acid (AA) was investigated. We have determined morphology by BAM and AFM. π-A curves investigated surface pressure of this LB film from liquid to solid state ranged between 40 to 45 mN/m. BAM images investigated the different states of AA LB film. When the surface pressure reaches at 40 mN/m, the monolayer was deposited onto the hydrophilic glass substrates by Y-type deposition. We investigated morphological property of arachidic acid by AFM. As a result, we obtained the frequency characteristic and morphology of LB films from controlling the deposited layers. In this paper, I review this subject mainly based on our recent results in merocyanine dye. LB films of merocyanine dye, mixed with arachidic acid, exhibit J-aggregate formation and have been serving as typical systems in revealing the physical and structural aspects of nano-sized molecular aggregates constructed as muiltilayers.

Measurement of Tunneling Current of Self-Assembled Viologen Derivative Molecules Using Scanning Tunneling Microscopy

The electrical properties of viologen derivatives with various lengths were studied using self-assembly and ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). In addition, in this study, we produced the vertical junction structure of an STM tip/molecule/Au(111) substrate using an STM tip. We used Au(111) as the base substrate. Also, the current–voltage (I–V) and differential conductance (dI/dV–V) characteristics were measured while the electrical properties of the self-assembled viologen derivatives were scanned by scanning tunneling spectroscopy (STS). We investigated electron movement, which is caused by height variation, because we can observe height variation between the barrier height of organic materials and the STM tip from the energy gap (Eg) of organic materials. The effective barrier heights of viologen derivative N-methyl-N-(8-mercaptooctyl)-4,4-bipyridinium (VC8SH), N-methyl-N-(10-mercaptodecyl)-4,4-bipyridinium (VC10SH), N-methyl-N-di(8-mercaptooctyl)-4,4-bipyridinium (HSC8VC8SH), and N-methyl-N-di(10-mercaptodecyl)-4,4-bipyridinium (HSC10VC10SH) self-assembled monolayers (SAMs) were calculated from the decay constant (β) to be 0.81, 0.46, 0.28, and 0.26 A-1, respectively.

Electrical properties and molecular behavior of functionalized dendrimer monolayers

The dendrimer with azobenzene group compared trans form and cis form at electrical properties. By irradiation of 365 [nm] light, the trans state current of G4-48Azo dendrimer was increased, which was originated by the photoisomerization process of the azobenzene group on the periphery from trans to cis form. All of the conductivities were ohmic, there is no evidence of polarization on switching the current direction, and the conductivity values remained constant for several hours. This demonstrates that the conductivity is electronic, not ionic. This suggests that optical behavior and conductivity change are affected by the functional group and symmetric chain.