Mi Yeon Cho

High-detectivity multilayer MoS(2) phototransistors with spectral response from ultraviolet to infrared.

Phototransistors based on multilayer MoS(2) crystals are demonstrated with a wider spectral response and higher photoresponsivity than single-layer MoS(2) phototransistors. Multilayer MoS(2) phototransistors further exhibit high room temperature mobilities (>70 cm(2) V(-1) s(-1) ), near-ideal subthreshold swings (~70 mV decade(-1) ), low operating gate biases (<5 V), and negligible shifts in the threshold voltages during illumination.

Poly(3-hexylthiophene)/multiwalled carbon hybrid coaxial nanotubes: nanoscale rectification and photovoltaic characteristics.

We fabricate hybrid coaxial nanotubes (NTs) of multiwalled carbon nanotubes (MWCNTs) coated with light-emitting poly(3-hexylthiophene) (P3HT). The p-type P3HT material with a thickness of approximately 20 nm is electrochemically deposited onto the surface of the MWCNT. The formation of hybrid coaxial NTs of the P3HT/MWCNT is confirmed by a transmission electron microscope, FT-IR, and Raman spectra. The optical and structural properties of the hybrid NTs are characterized using ultraviolet and visible absorption, Raman, and photoluminescence (PL) spectra where, it is shown that the PL intensity of the P3HT materials decreases after the hybridization with the MWCNTs. The current-voltage (I-V) characteristics of the outer P3HT single NT show the semiconducting behavior, while ohmic behavior is observed for the inner single MWCNT. The I-V characteristics of the hybrid junction between the outer P3HT NT and the inner MWCNT, for the hybrid single NT, exhibit the characteristics of a diode (i.e., rectification), whose efficiency is clearly enhanced with light irradiation. The rectification effect of the hybrid single NT has been analyzed in terms of charge tunneling models. The quasi-photovoltaic effect is also observed at low bias for the P3HT/MWCNT hybrid single NT.

Fabrication and magnetic characteristics of hybrid double walled nanotube of ferromagnetic nickel encapsulated conducting polypyrrole

This letter is a report on hybrid double walled nanotubes (HDWNTs) of crystalline ferromagnetic nickel (Ni) nanotubes encapsulated conducting polypyrrole (PPy) nanotubes through a sequentially electrochemical synthetic method. Ferromagnetic Ni nanotubes were fabricated by an electrochemical deposition method outside the wall of the conducting PPy nanotubes. The formation and structure of HDWNTs of conducting PPy nanotubes and ferromagnetic Ni nanotubes were confirmed by transmission electron microscopy, high-resolution transmission electron microscopy, and elementary analysis. From the angular dependences of the magnetic hysteresis curves of the HDWNTs, the authors observed that the Ni nanotubes of the HDWNT systems had an anisotropic ferromagnetic nature with the maximum of coercivity and remanent-saturation magnetization when applying a magnetic field along the parallel direction of the tubes.

Photoresponsive characteristics and hysteresis of soluble 6,13-bis(triisopropyl-silylethynyl)-pentacene-based organic thin film transistors with and without annealing

We report on the photoresponsive characteristics and hysteresis of soluble 6,13-bis(triisopropyl-silylethynyl; TIPS)-pentacene-based organic thin film transistors (OTFTs) with and without an annealing process. Under incident light, the hysteresis and memory effect increased for the annealed devices through photoinduced electron trapping and recombination at the gate-dielectric-semiconductor interface. To investigate the cause of the photoenhanced memory effect and hysteresis, we estimated the trapped electron densities from the threshold shift, while applying a gate bias to the annealed OTFTs under both dark and light conditions. A comparison of the characteristics of OTFT devices that are prepared under various conditions suggests that deep electron traps might be caused by the appearance of a domain boundary (i.e., cracks) in the annealed TIPS-pentacene film during the annealing process, which showed stable photoresponsive characteristics but relatively lower mobility.

Electrical characteristics of organic perylene single-crystal-based field-effect transistors

We report on the fabrication of organic field-effect transistors (OFETs) using perylene single crystal as the active material and their electrical characteristics. Perylene single crystals were directly grown from perylene powder in a furnace using a relatively short growth time of 1–3 h. The crystalline structure of the perylene single crystals was characterized by means of a single-crystal x-ray diffractometer. In order to place the perylene single crystal onto the Au electrodes of the field-effect transistor, a polymethlymethacrylate thin layer was spin-coated on top of the crystal surface. The OFETs fabricated using the perylene single crystal showed a typical p-type operating mode. The field-effect mobility of the perylene crystal based OFETs was measured to be ∼9.62×10−4 cm2/V s at room temperature. The anisotropy of the mobility implying the existence of different mobilities when applying currents in different directions was observed for the OFETs, and the existence of traps in the perylene crystal...

Gate-field dependent photosensitivity of soluble 1,2,4,5-tetra(5′-hexyl-[2,2′]terthiophenyl-5-vinyl)-benzene based organic thin film transistors

The photoresponsive current-voltage characteristics of organic thin film transistors (OTFTs) have been studied as a function of gate-bias. For the active layer of the OTFTs, soluble 1,2,4,5-tetra(5′-hexyl-[2,2′]terthiophenyl-5-vinyl)-benzene materials have been used, and the thickness of the active layers varied. The photosensitivity of the OTFTs was controlled through both gate-bias (Vg) and incident light power. With increasing incident light power, the photosensitivity decreased during the source-drain current actively varied with Vg [i.e., at on-state with accumulated hole channel (Vg<Vonset)], while it increased for Vg≥Vonset (at off-state without accumulated hole channel). These variations are caused by two kinds of photocurrent mechanisms: one based on the photovoltaic effect for Vg<Vonset and another based on the photoconductive effect for Vg≥Vonset. The maximum photosensitivity of OTFTs was found to be approximately 40 times higher in the on-state than in the off-state due to the contribution of ...

p-type semiconducting dendrimers bearing thiophenyl peripheral moieties for organic field effect transistors

First generation thiophene-labeled conjugated dendrimers have been synthesized through the Heck coupling reaction between the 5-hexyl-5-propenyl thiophene derivative and 1, 2, 4, 5-tetrabromo-benzene as the core. They display a p-type semiconducting behavior and a considerably better solubility than the linear conjugated oligothiophene. Two crystalline dendrimers such as 4(HPBT)-benzene and 4(HPTT)-benzene showed carrier mobilities as high as 6.2×10−3 and 2.4×10−4cm2V−1s−1, respectively.

Electrical Characteristics of Poly(ethylene oxide)-urea Complex Films

The electrical characteristics of complex films composed of poly(ethylene oxide) (PEO) and urea as a function of the urea concentration were examined in this study. Moreover, their structural characteristics were also compared. Depending on the urea concentration, the structural phases were classified as PEO+-phase composite, -phase+-phase composites, or -phase composite+urea. At urea concentrations below ~0.064 M, the -phase was dominant in the complex film. Moreover, the conductance increased rapidly with an increase in the urea concentration. For urea concentrations ranging from ~0.064 to ~0.25 M, the -phase was gradually substituted by the -phase. As the film was composed entirely of the -phase at urea concentrations greater than ~0.25 M, its conductance was decreased. In this study, the electrical characteristics observed for the different phases are analyzed and discussed.

Synthesis and Properties of Soluble Organic p-Type Semiconductor based on TIPS-anthracene Moiety

New multi-branched molecules have been synthesized through Horner-Emmons reactions using 10-((triisopropylsilyl)ethynyl) anthracene-9-carbaldehyde as dendrons and octaethyl benzene-1,2,4,5-tetrayltetrakis(methylene) tetraphosphonate as a reactive core unit; these molecules have been fully characterized. Two anthracene-based star-shaped molecules exhibit good solubility in common organic solvents and good self-film forming properties. The four-armed molecule, 4(TIPS-ant)-benzene is intrinsically crystalline as they exhibit well-defined X-ray diffraction patterns from relatively uniform orientations of molecules. According to the AFM analysis, 4(TIPS-ant)-benzene helps in attaining good network interconnection of the carrier transport channel, which is responsible for the charge carrier mobility in solution-processed organic semiconductors for organic thin film transistor (OTFT).

Copper-phthalocyanine based organic thin film transistor

Copper-phthalocyanine based organic thin film transistors (OTFTs) were fabricated. Cu-Pc thin films as an active layer were deposited onto the SiO2 layer as a gate insulator by using organic molecular beam deposition system. The OTFT devices showed p-type characteristics with field-effect mobility and threshold voltage as 1.22x10-3 cm2/Vsand -40 V, respectively. The activation energy was estimated to be 0.073 eV, through the temperature dependence of carrier mobility.