Dea Uk Lee

Mimicking Classical Conditioning Based on a Single Flexible Memristor

The mimicking of classical conditioning, including acquisition, extinction, recovery, and generalization, can be efficiently achieved by using a single flexible memristor. In particular, the experiment of Pavlovs dog is successfully demonstrated. This demonstration paves the way for reproducing advanced neural processes and provides a frontier approach to the design of artificial-intelligence systems with dramatically reduced complexity.

Significant enhancement of the power conversion efficiency for organic photovoltaic cells due to a P3HT pillar layer containing ZnSe quantum dots

High-efficiency organic photovoltaic (OPV) cells utilizing a poly(3-hexylthiophene) (P3HT) pillar layer containing ZnSe quantum dots (QDs) were fabricated by using a mixed solution method. Scanning electron microscopy and high-resolution transmission electron microscopy images showed that the ZnSe QDs were dispersed in the P3HT layer. The power conversion efficiency of the OPV cells with a P3HT pillar layer containing ZnSe QDs was as much as 100% higher than that of the OPV cells with a planar layer due to an enhancement of the photon-harvesting ability of the congregated P3HT particles containing ZnSe QDs and to an increase of the interfacial region for efficient charge transport.

Electrical Properties and Operating Mechanisms of Nonvolatile Organic Memory Devices Fabricated Utilizing Hybrid Poly(N-vinylcarbazole) and C60 Composites

Organic memory devices based on hybrid poly(N-vinylcarbazole) (PVK) and C60 composites were fabricated with a spin-coating method. Atomic force microscopy images showed that the surface of the PVK layer containing the C60 molecules was relatively smooth. Capacitance–voltage (C–V) measurements on the Al/C60 embedded in PVK layer/p-Si(100) device at room temperature showed a clockwise hysteresis with a large flatband voltage shift due to the existence of the C60 molecules in the PVK layer, indicative of charge storage in the embedded C60 molecules. The clockwise direction of the C–V hysteresis for the devices was attributed to carrier tunneling through the PVK layer emitting from the Al gate electrode, and the negative and positive flatband voltage shifts of the C–V curves originated from the holes and electrons captured in the C60 molecules, respectively. Possible operating mechanisms corresponding to writing and erasing processes for the Al/C60 embedded in PVK layer/p-Si (100) device are described on the basis of the C–V results.

Microstructural and optical properties of CdSe/CdS/ZnS core-shell-shell quantum dots.

CdSe/CdS/ZnS core-shell-shell quantum dots (QDs) were synthesized by using a solution process. High-resolution transmission electron microscopy images and energy dispersive spectroscopy profiles confirmed that stoichiometric CdSe/CdS/ZnS core-shell-shell QDs were formed. Ultraviolet-visible absorption and photoluminescence (PL) spectra of CdSe/CdS/ZnS core-shell-shell QDs showed the dominant excitonic transitions from the ground electronic subband to the ground hole subband (1S(e)-1S(3/2)(h)). The PL mechanism is suggested; the carriers generated by the exciting high-energy photons in the shell region are relaxed to the band-edge states of the core region and recombined to emit lower-energy photons. The activation energy of the carriers confined in the CdSe/CdS/ZnS core-shell-shell QDs, as obtained from temperature-dependent PL spectra, was 200 meV. The quantum efficiency of the CdSe/CdS/ZnS core-shell-shell QDs at 300 K was estimated to be approximately 57%.

Effects of CdSe shell layer on the electrical properties of nonvolatile memory devices fabricated utilizing core-shell CdTe-CdSe nanoparticles embedded in a poly(9-vinylcarbazole) layer

Nonvolatile memory devices were fabricated with core-shell CdTe-CdSe nanoparticles embedded in a poly(9-vinylcarbazole) (PVK) layer to investigate the variations in the electrical properties due to a CdSe shell layer. Capacitance-voltage measurements on Al/CdTe nanoparticles embedded in PVK layer/p-Si devices and on Al/core-shell CdTe-CdSe nanoparticles embedded in PVK layer/p-Si devices at 300 K showed hysteresis behaviors with a flatband voltage shift due to the existence of the CdTe and the CdTe-CdSe nanoparticles. Capacitance-time measurements showed that the retention time for devices fabricated utilizing core-shell CdTe-CdSe nanoparticles was larger than that for devices fabricated utilizing CdTe nanoparticles.

Enhancement of the power conversion efficiency for inverted organic photovoltaic devices due to the localized surface plasmonic resonant effect of Au nanoparticles embedded in ZnO nanoparticles

The absorption spectra and input photon-to-converted current efficiency curves showed that Au nanoparticles increased the plasmonic broadband light absorption, thereby enhancing the short-circuit current density of the inverted organic photovoltaic (OPV) cells with a Au–ZnO nanocomposite electron transport layer (ETL). The power conversion efficiency of the inverted OPV cell fabricated with a Au–ZnO nanocomposite ETL was higher by 40% than that of the inverted OPV cell fabricated with a ZnO nanoparticle ETL, which could be attributed to the enhanced photon absorption in the active layer due to the localized surface plasmonic resonance of the Au nanoparticles.

Optical and electronic properties of type-II CdSe/CdS core–shell quantum dots

CdSe/CdS core–shell quantum dots (QDs) were synthesized using a facile method in aqueous phase. X-ray diffraction pattern, high-resolution transmission electron microscopy images, and energy dispersive spectroscopy profiles showed that stoichiometric CdSe/CdS QDs were formed. Temperature-dependent photoluminescence spectra showed that the activation energy of CdSe/CdS core–shell QDs was 15 meV. The potential profiles and interband transition energies of the strained type-II CdSe/CdS core–shell QDs were calculated. The calculated interband transition energies slightly decreased from 2.061 to 2.007 eV when the shell thickness increased from 10 to 17 A. The theoretical interband transition energy of 2.007 eV was in reasonable agreement with the photoluminescence excitonic transition energy of 1.98 eV.

Electron-beam-induced formation mechanisms for Ti2O3–SiO2 composite nanofibers

Anatase TiO2 nanoparticles with high crystallinity were embedded in the SiO2 matrix by electrospining and calcining. As-calcined TiO2–SiO2 nanofibers were transformed into Ti2O3–SiO2 nanofibers owing to in situ electron-beam irradiation in a transmission electron microscope. The microstructural properties and the mechanisms of electron-beam-induced formation of Ti2O3–SiO2 composite nanofibers were described on the basis of the obtained high-resolution transmission electron microscopy images, fast-Fourier-transformed patterns, and energy dispersive spectroscopy profiles.

1.55 µm Wavelength Strain-Compensated InxGa1-xAs/InyAl1-yAs Electroabsorption Modulators with High Extinction Ratio and Low Polarization-Dependent Loss

Photocurrent (PC) measurements were carried out to investigate the excitonic transitions in InxGa1-xAs/InyAl1-yAs multiple quantum wells with and without an applied electric field. Transmission electron microscopy showed that high-quality 11-period strain-compensated In0.64Ga0.36As/In0.47Al0.53As electroabsorption modulator structures with high-quality heterointerfaces were grown by molecular beam epitaxy. The results for the PC data at 300 K for several applied electric fields showed that many excitonic transitions shifted to longer wavelengths as the applied electric field increased. The calculated value of the interband transitions from the first electronic state to the first heavy-hole state were in qualitative agreement with those obtained from the PC measurements. The maximum extinction ratio at a wanelength of 1.55 µm under an applied voltage of -1.5 V was 14.3 dB, the polarization-dependent loss at an extinction ratio of 14.3 dB was less than 0.5 dB, and the coupling losses were below 1.5 dB per facet at 1.55 µm. These results indicate that the electroabsorption modulators fabricated utilizing srain-compensated In0.64Ga0.36As/In0.47Al0.53As multiple quantum wells hold promise for high-efficiency devices in the 1.55-µm spectral range.