Dong-Jin Yun

Resistive switching characteristics of ZnO thin film grown on stainless steel for flexible nonvolatile memory devices

This paper reports a resistive switching device of Au/ZnO/stainless steel (SS) and its applicability as a flexible resistive random access memory (ReRAM). The Au/ZnO/SS structure was fabricated by radio frequency sputtering deposition of a ZnO thin film on the SS substrate. The fabricated device showed stable unipolar and bipolar resistive switching behaviors with reliable switching responses over 100 cycles. The device performance was not degraded upon bending, which indicates high potential for flexible ReRAM applications.

Multiwall Carbon Nanotube and Poly(3,4-ethylenedioxythiophene): Polystyrene Sulfonate (PEDOT:PSS) Composite Films for Transistor and Inverter Devices

Highly conductive multiwalled carbon nanotube (MWNT)/Poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared by spin coating a mixture solution. The solution was prepared by dispersing MWNT in the PEDOT:PSS solution in water using ultrasonication without any oxidation process. The effect of the MWNT loading in the solution on the film properties such as surface roughness, work function, surface energy, optical transparency, and conductivity was studied. The conductivity of MWNT/PEDOT:PSS composite film was increased with higher MWNT loading and the high conductivity of MWNT/PEDOT:PSS films enabled them to be used as a source/drain electrode in organic thin film transistor (OTFT). The pentacene TFT with MWNT/PEDOT:PSS S/D electrode showed much higher performance with mobility about 0.2 cm²/(V s) and on/off ratio about 5 × 10⁵ compared to that with PEDOT:PSS S/D electrode (∼0.05 cm²/(V s), 1 × 10⁵). The complementary inverters exhibited excellent characteristics, including high gain value of about 30.

Self-assembled foam-like graphene networks formed through nucleate boiling

Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400°C) SFG film were 11.8 S·cm–1 and 91.2 Ω□−1, respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (~10 S·cm–1). The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs).

Metal–semiconductor contact in organic thin film transistors

Optimization of the interface between the source/drain electrode and the organic semiconductor is one of the important factors for organic thin film transistor (OTFT) performance along with the insulator–semiconductor interface. In this Feature Article, items needed for the optimization of the metal–semiconductor interface and methods to measure interface properties are reviewed. Various electrode materials are compared in terms of the work function, contact resistance and the pentacene OTFT performance. The effect of surface modification is also discussed. Experimental data obtained in LAMP is introduced to show the effect of the various material properties more clearly.

Composite Films of Oxidized Multiwall Carbon Nanotube and Poly(3,4-ethylenedioxythiophene): Polystyrene Sulfonate (PEDOT:PSS) As a Contact Electrode for Transistor and Inverter Devices

Composite films of multiwall carbon nanotube (MWNT)/poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) were prepared by spin-coating a mixture solution. The effect of the MWNT loading and the MWNT oxidation, with acid solution or ultraviolet (UV)-ozone treatment, on the film properties such as surface roughness, work function, surface energy, optical transparency and conductivity were studied. Also pentacene thin film transistors and inverters were made with these composite films as a contact metal and the device characteristics were measured. The oxidation of MWNT reduced the conductivity of MWNT/PEDOT:PSS composite film but increased the work function and transparency. UV-ozone treated MWNT/PEDOT:PSS composite film showed higher conductivity (14000 Ω/□) and work function (4.9 eV) than acid-oxidized MWNT/PEDOT:PSS composite film and showed better performance as a source/drain electrode in organic thin film transistor (OTFT) than other types of MWNT/PEDOT:PSS composite films. Hole injection barrier of the UV-ozone treated MWNT/PEDOT:PSS composite film with pentacene was significantly lower than any other films because of the higher work function.

The fabrication of highly uniform ZnO/CdS core/shell structures using a spin-coating-based successive ion layer adsorption and reaction method.

We developed a successive ion layer adsorption and reaction method based on spin-coating (spin-SILAR) and applied the method to the fabrication of highly uniform ZnO/CdS core/shell nanowire arrays. Because the adsorption, reaction, and rinsing steps occur simultaneously during spin-coating, the spin-SILAR method does not require rinsing steps between the alternating ion adsorption steps, making the growth process simpler and faster than conventional SILAR methods based on dip-coating (dip-SILAR). The ZnO/CdS core/shell nanowire arrays prepared by spin-SILAR had a denser and more uniform structure than those prepared by dip-SILAR, resulting in the higher power efficiency for use in photoelectrochemical cells.

In situ ultraviolet photoemission spectroscopy measurement of the pentacene-RuO2/Ti contact energy structure

In situ ultraviolet photoemission spectroscopy was used during the pentacene layer growth on ruthenium and ruthenium oxide films to measure the energy barrier in the metal-semiconductor contact. The measurement showed that ruthenium oxide film formed lower hole-injection barrier with pentacene than ruthenium or gold metal film due to its high work function of 4.92 eV and low resistivity of ∼350 μΩ cm. Pentacene thin film transistor with ruthenium oxide electrode showed higher mobility of 0.435 cm2/V s and on-off ratio of 106 than ruthenium (μ: 0.205 cm2/V s, on/off ratio: 106) or gold electrode (μ: 0.338 cm2/V s, on/off ratio: 106) of the same structure.

Pentacene Thin-Film Transistor with NiO x as a Source/Drain Electrode Deposited with Sputtering

1,2 But the source‐drain S/D contacts in OTFTs are not easily optimized by conventional processes, such as semiconductor doping or metal alloying. Interface properties between the S/D electrode and semiconductor are critical factors for TFT performance. To reduce the hole injection barrier from the difference in the work function of the S/D electrode and the highest occupied molecular orbital HOMO level of the semiconductor, it is important to choose a S/D electrode which has a work function close to the HOMO level of the organic semiconductor. As a contact metal, gold Au, silver, calcium, and indium tin oxide were studied. 2-5 Until now, Au has been the most widely used S/D electrode in pentacene TFT because it has a work function of 5.0 eV, which is close to the HOMO level of pentacene, 5.2 eV. Au may not be preferred in a real application, because as a noble metal, it is expensive and difficult to etch for patterning. Recently, it has been reported that high-performance pentacene TFTs could be made with NiOx S/D electrode deposited using a thermal evaporator. 6-8 Up to now, there has been no information on the properties of NiOx film properties as a function of the film composition x as a S/D electrode. It is easier to control the film composition with sputtering by controlling the oxygen flow rate than by thermal evaporation. In this study, radio frequency rf sputtering was used to obtain the nickeloxide film starting from the nickel-metal Ni metal target, and the effect of the composition on the performance of the pentacene TFTs was studied. The film composition was controlled with sputtering by varying the O2/Ar flow ratio. Film properties such as resistivity, fraction of the oxidation states of Ni 0, +2, +3, and the work function were measured. Also, pentacene TFTs using NiOx as S/D electrodes were made, and their performance was compared with an Au/Ti electrode. In our experiment, it is not easy to optimize all the processes and the materials involved in the TFT manufacturing, so the device performance cannot directly be compared with other research results with the highest mobility. We made the same TFT with gold and compared the performance. Gold is the most widely used metal, and the comparison can give a clear idea about the NiOx material.

Metallic conduction induced by direct anion site doping in layered SnSe2.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020 cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm−1 from ~1.7 S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019 cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

Fabrication of high-performance composite electrodes composed of multiwalled carbon nanotubes and glycerol-doped poly(3,4-ethylenedioxythiophene):polystyrene sulfonate for use in organic devices

In this study, composite films composed of highly conductive multiwalled carbon nanotubes (MWCNTs) and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) were fabricated via additional organic-compound doping. The effects of glycerol (GL) or dimethyl sulfoxide (DMSO) doping on the film properties, such as surface roughness, work function (Φ), and conductivity, were studied for both PEDOT:PSS and MWCNT/PEDOT:PSS composite systems. The interactive couplings between the PEDOT and PSS molecules became disordered upon doping with GL or DMSO, which altered the conjugated structure between the PEDOT and PSS chains. Therefore, the electrical conductivity of the PEDOT:PSS and MWCNT/PEDOT:PSS films was enhanced by the addition of GL or DMSO molecules. The GL-doped PEDOT:PSS (PEGL) and ultraviolet (UV)-oxidized MWCNT/PEDOT:PSS (0.2-UVGL) films exhibited comparable work functions (PEGL = 4.87 eV, 0.2-UVGL = 5.0 eV). They also had lower sheet resistances (Rs; PEGL = 806.7 ± 50 Ω □−1, 0.2-UVGL = 613 ± 120 Ω □−1) as compared to those of the undoped PEDOT:PSS (Φ = 4.92 eV, Rs = 1.03 ± 0.10 MΩ □−1) and MWCNT/PEDOT:PSS composites (Φ = 4.7 eV, Rs = 2184 ± 244 Ω □−1). Furthermore, because of these excellent electrical properties, the doped MWCNT/PEDOT:PSS films showed great potential as the source/drain electrode in an organic thin-film transistor and as the catalytic counter electrode in a dye-sensitized solar cell. In conclusion, devices with 0.2-UVGL electrodes performed better than the corresponding devices with other MWCNT/PEDOT:PSS composite electrodes and the device characteristics were comparable to that of standard devices with platinum/fluorine-doped tin oxide electrodes.