Rieko Ueda

Device characteristics of short-channel polymer field-effect transistors

The influence of channel materials on the electrical characteristics of organic field-effect transistors (OFETs) with short-channel lengths ranging from 1 μm to 30 nm is investigated using polymer semiconductors. The current-voltage characteristics of short-channel OFETs strongly depend on the electrode/organic semiconductor contacts, and the parabolic output current due to space-charge limited current can be reduced by increasing the ionization potential of organic semiconductors. Transistor operations with a high on/off ratio over 103 are achieved in OFETs with 30 nm length channels.

A photoresponsive single electron transistor prepared from oligothiophene molecules and gold nanoparticles in a nanogap electrode

Gold nanoparticle-oligothiophene pentamer networks were prepared in a nanogap electrode and their photoresponsive and conductive properties were measured. Coulomb diamond appeared in the differential conductance map of the device at cryogenic temperatures, indicating that the device worked as a single electron transistor. Upon irradiation with UV light, the current showed discontinuous changes. The I-VSD curve and differential conductance mapping before and after irradiation showed that the abrupt changes in current can be explained by a shift in the potential of the Coulomb island.

Photoresponses in Gold Nanoparticle Single-Electron Transistors with Molecular Floating Gates

We have proposed a simple method of activating advanced functions in single-electron transistors (SETs) based on the specific properties of individual molecules. As a prototype, we fabricated a copper phthalocyanine (CuPc)-doped SET. The device consists of a gold-nanoparticle (GNP)-based SET doped with CuPc as a photoresponsive floating gate. In this paper, we report the details of the photoresponses of the CuPc-doped SET, such as conductance switching, sensitivity to the wavelength of the incident light, and multiple induced states.

Two-photon excited fluorescence from CdSe quantum dots on SiN photonic crystals

Two-photon excited fluorescence from CdSe quantum dots on a two-dimensional SiN photonic crystal surface is investigated by using a femtosecond laser. By using a photonic crystal, a 90-fold enhancement in the two-photon excited fluorescence in the vertical direction is achieved. This is the highest enhancement achieved so far in the two-photon excited fluorescence in the vertical direction. The mechanism of the enhancement for two-photon excited fluorescence from quantum dots on photonic crystals is analyzed.

Controlled Spontaneous Emission of Single Molecules in a Two-Dimensional Photonic Band Gap

We have established a new platform to control the rate of spontaneous emission (SE) of organic molecules in the visible-light region using a combination of a two-dimensional (2D) photonic crystal (PC) slab made of TiO(2) and a single-molecule measurement method. The SE from single molecules of a perylenediimide derivative was effectively inhibited via a radiation field controlled by the 2D PC slab, which has a photonic band gap (PBG) for transverse-electric (TE)-polarized light. The fluorescence lifetimes of the single molecules were extended up to 5.5 times (28.6 ns) by the PBG effect. This result appears to be the first demonstration of drastic lifetime elongation for single molecules due to a PBG effect.

Photoinduced conductance switching in a dye-doped gold nanoparticle transistor

Photoinduced conductance switching was demonstrated in a copper phthalocyanine (CuPc)-doped gold nanoparticle (GNP) transistor formed in a nanogap electrode with a back gate structure. Two specific states were reversibly induced in conductance of the CuPc-doped devices by light irradiation and applied voltages. The probability of occurrence of conductance switching decreased with a reduction in the number of incident photons. In addition, conductance switching was not observed in the devices before CuPc doping. Conductance switching originates from change in the local potential of GNPs, possibly induced by a charge-state bistability of an individual CuPc molecule worked as a floating gate.

Maskless fabrication of nanogap electrodes by using Ga-focused ion beam etching

We develop maskless fabrication methods using sputter etching with Ga-focused ion beams (FIBs) to obtain nanogap electrodes with high reproducibility. This method is based on in situ monitoring of etching steps by measuring current through patterned electrode films. The etching steps are terminated electrically at a predetermined current level. In the present experiment, 30-keV Ga FIBs with beam size of ~12 nm is irradiated, and the effect of film structures and monitoring current is investigated to obtain reliable fabrication methods. We find that electrode gaps much narrower than the beam size can be reproducibly fabricated. The controllability of the fabrication steps is significantly improved by using triple-layered films consisting of a thin Ti top, an Au electrode, and a bottom Ti adhesion layer. The minimum gap width achieved is ~3 nm, and the fabrication yield reached ~90% for ~3- to 6-nm-wide gaps. Most of the fabricated nanogap electrodes show high insulating resistance ranging from 1 G to 1 T.

Dynamics of spontaneous emission from SiN with two-dimensional photonic crystals

We investigated the dynamics of spontaneous emission from a photonic crystal etched into a SiN slab. After fitting the decay curves of the emission to double exponential functions, we divided the dynamic process of the spontaneous emission into a fast process and a slow process. It was observed that the presence of the photonic crystal increased the proportion of the fast decay component, and consequently, the emission rate and time-integrated emission intensity were also enhanced. These enhancements were a result of the coupling of the guide modes to the leaky modes of the photonic crystal slab waveguide.

Fabrication of two-dimensional Ta2O5 photonic crystal slabs with ultra-low background emission toward highly sensitive fluorescence spectroscopy

A two-dimensional tantalum pentoxide (Ta2O5) photonic crystal (PC) slab with low-background emission was fabricated and a 12-fold enhancement of fluorescence from the organic dyes of perylene diimide adsorbed on the surface of the PCs was observed. The background emissions of the Ta2O5 substrates with and without the PCs after thermal annealing at 600°C with oxygen gas were comparable to that of a well-cleaned cover glass. This is to date the lowest level of background emissions of two-dimensional PCs using materials with a high refractive index (>2). The results reported here provide new insights into the fabrication of the photonic devices that enable highly sensitive fluorescence microscopy or optical detections.

Fowler-nordheim tunneling in electromigrated break junctions with porphyrin molecules

We measured the current–voltage (I–V) curves of 158 electromigrated break junctions with a Co(II) complex of tetraphenylporphyrin derivative (CoBSTBPP) at a temperature of 11 K within the applied bias ≤1.0 V. Symmetry I–V curves were observed in 18.4% of the measured junctions, and we found that Fowler–Nordheim tunneling is dominant in most of these junctions. The tunneling barrier height estimated from these I–V curves was smaller than 0.5 eV, which could be explained by considering the energy levels in CoBSTBPP and the Fermi level of an electrode. We conclude that CoBSTBPP forms the small tunneling barrier height in the molecular junctions.