Junggwon Yun

Resistance switching memory devices constructed on plastic with solution-processed titanium oxide

Resistance switching memory devices constructed on flexible plastic substrates via the spin-coating of titanium oxide solution were characterized in this study. The resistance switching memory device exhibited a ratio of the high resistance to low resistance states of more than 102, and this large resistance ratio was maintained even after 104 s. These memory characteristics are comparable to those of resistance switching memory devices based on titanium oxide films deposited on Si substrates. Moreover, the endurance of the flexible memory device investigated by means of a continuous substrate bending test revealed that the ratio of the high resistance to low resistance states was negligibly changed up to two hundred cycles. Its resistance switching characteristics were not degraded by the bending of the substrate, due to its short length channel and the high ductility of the electrode.

The transfer of charge carriers photogenerated in ZnO nanoparticles into a single ZnO nanowire.

The transfer of charge carriers, photogenerated in nanoparticles (NPs), into a single nanowire (NW) is demonstrated in this study by conducting a careful comparison of the optoelectronic characteristics of a ZnO NW with ZnO NPs attached to its surface, a bare ZnO NW and a film of close-packed ZnO NPs. Under the illumination of an above-gap light, the photocurrent taken from the NW with the NPs is remarkably higher in magnitude than that obtained from the bare NW, although the photocurrent is substantially lower for the close-packed NPs. The presence of the absorption band of the NPs in the photoresponse spectrum taken from the NW with the NPs reveals that the transfer of the charge carriers photogenerated in the NPs into the NW dramatically enhances the magnitude of the photocurrent flowing in this NW. Nevertheless, during the transfer, the charge carriers experience trapping and detrapping at the interfaces of the NPs and the NW.

Flexible logic circuits composed of chalcogenide-nanocrystal-based thin film transistors

Complementary NAND and NOR gates composed of p-channel HgTe-nanocrystal (NC) films and n-channel HgSe-NC films were constructed on back-gate patterned plastic substrates. The NAND gate was made of two HgTe-p-channel thin film transistors (TFTs) in parallel and two HgSe-n-channel TFTs in series. The NOR gate was built up with both two HgSe-n-channel TFTs in parallel and two HgTe-p-channel TFTs in series. The mobility and on/off ratio for the p-channel TFTs were estimated to be 0.9 cm(2) V(-1) s(-1) and 10, respectively, and those for the n-channel TFTs were measured to be 1.8 cm(2) V(-1) s(-1) and 10(2), respectively. The NAND and NOR gates were operated with gains of 1.45 and 1.63 and transition widths of 7.8 and 6.2 V, respectively, at room temperature in air. In addition, the operations of the NAND and NOR logics are reproducible for up to 1000 strain cycles.

Dynamic Electrical Characteristics of Low-Power Ring Oscillators Constructed with Inorganic Nanoparticles on Flexible Plastics

In this study, we demonstrate for the first time the low-power and stable performance of a ring oscillator constructed on a flexible plastic with solution-processable inorganic nanoparticles (NPs). Our flexible ring oscillator is composed of three inverters based on n- and p-type inorganic NP thin-film transistors. Each of the component inverters exhibits a gain of ∼80 at a voltage of 5 V. For the ring oscillator, the sine waves are generated with a frequency of up to 12 kHz. The waveforms are undistorted under strained conditions and maintained even after 5000 bending cycles. The frequency and waveform of the output waves obtained from our flexible ring oscillator are analyzed and discussed in detail.

Thermoelectric characteristics of nanocomposites made of HgSe and Ag nanoparticles for flexible thermoelectric devices

We synthesized thermoelectric nanocomposites by mixing HgSe nanoparticles (NPs) and Ag NPs in a solution and investigated the thermoelectric properties of the nanocomposite thin films on flexible plastic substrates. The X-ray diffraction patterns and the X-ray photoelectron spectra of the nanocomposites demonstrate that cation-exchange reactions occurred spontaneously in the mixed solution of HgSe and Ag NPs and that the HgSe NPs were completely converted to Ag2Se when the Ag NP content was 20 vol.%. The maximum power factor and the thermoelectric figure of merit were obtained as 75 μW/mK2 and 0.043 at 300 K, respectively, when the Ag NP content was 10 vol.%, which is 100 times higher than that of HgSe NP thin films. In addition, the mechanical stability of the thermoelectric nanocomposite film was confirmed through repeated bending tests.

Nanowatt power operation of silicon nanowire NAND logic gates on bendable substrates

In this paper, we propose a novel construction of silicon nanowire (SiNW) negative-AND (NAND) logic gates on bendable plastic substrates and describe their electrical characteristics. The NAND logic gates with SiNW channels are capable of operating with a supply voltage as low as 0.8 V, with switching and standby power consumption of approximately 1.1 and 0.068 nW, respectively. Superior electrical characteristics of each SiNW transistor, including steep subthreshold slopes, high Ion/off ratio, and symmetrical threshold voltages, are the major factors that enable nanowatt-range power operation of the logic gates. Moreover, the mechanical bendability of the logic gates indicates that they have good and stable fatigue properties.

A p-n Heterojunction Diode Constructed with A p-Si Nanowire and An n-ZnO Nanoparticle Thin-Film by Dielectrophoresis

Newly-developed fabrication of a p-n heterojunction diode constructed with a p-Si nanowire (NW) and an n-ZnO nanoparticle (NP) thin-film by the dielectrophoresis (DEP) technique is demonstrated in this study. With the bias of 20 Vp-p at the input frequency of 1 MHz, the most efficient assembly of the n-ZnO NPs is shown for the fabrication of the p-n heterojunction diode with a p-Si NW. The p-n heterojunction diode fabricated in this study represents current rectifying characteristics with the turn on voltage of 1.1 V. The diode can be applied to the fabrication of optoelectrical devices such as photodetectors, light-emitting diodes (LEDs), or solar cells based on the high conductivity of the NW and the high surface to volume ratio of the NP thin film.

Resistive Switching Characteristics of Hafnium Oxide Thin Films Sputtered at Room Temperature

In this study, we fabricate resistive switching random access memory (ReRAM) devices constructed with a Al//ITO structure on glass substrates and investigate their memory characteristics. The hafnium oxide thin film used as a resistive switching layer is sputtered at room temperature in a sputtering system with a cooling unit. The Al//ITO device exhibits bipolar resistive switching characteristics, and the ratio of the high resistance (HRS) to low resistance states (LRS) is more than 60. In addition, the resistance ratio maintains even after seconds.

Electrical Characteristics of Cu 2 O-PVP Nanofibers Fabricated by Electrospinning

Hybrid nanofibers made of and polyvinyl pyrrolidone were fabricated by electrospinning on glass substrates. The current magnitude of the -PVP hybrid nanofibers is 10 times larger than that of pure PVP nanofibers. In addition, -PVP nanofibers possess high sensitivity to air at room temperature than pure PVP nanifibers.