Keon-Ho Yoo

Efficiency enhancement of organic light-emitting devices due to the localized surface plasmonic resonant effect of Au nanoparticles embedded in ZnO nanoparticles

Organic light-emitting devices (OLEDs) were fabricated utilizing Au-ZnO nanocomposites (NCs) with various Au nanoparticles (NPs) concentrations synthesized by using a sol-gel process to enhance their current efficiency, and these devices were compared with OLEDs fabricated utilizing ZnO NPs only. The electroluminescence intensity of the OLEDs with Au-ZnO NCs at a wavelength of 535 nm was significantly larger than that of the OLEDs with ZnO NPs by a factor of 1.89. The current efficiency of the OLEDs with 10 wt. % Au-ZnO NCs at 1000 cd/m2 was higher by 16.7 cd/A than that of the OLEDs with ZnO NPs. This increase in the current efficiency of the OLEDs with Au-ZnO NCs was attributed to an enhancement of the out-coupling efficiency due to the existence of the localized surface plasmonic resonance effect induced by the Au NPs that were embedded in the Au-ZnO NCs.

Synthesis and characterization of the first adamantane-based poly(p-phenylenevinylene) derivative: an intelligent plastic for smart electronic displays

Abstract A novel poly( p -phenylenevinylene) (PPV) derivative containing adamantane substituent—poly(2-methoxy-5-adamantaneethyloxy- p -phenylenevinylene) (MAE-PPV) is synthesized through the Gilch route. Adamantane was incorporated into this polymer because of its rigid and spherical structure, which results in reduction of interchain interaction. MAE-PPV is solution processable and shows high photoluminescence intensity. This polymer also demonstrates good electroluminescene properties.

Er3+ photoluminescence from Er-doped amorphous SiOx films prepared by pulsed laser deposition at room temperature: The effects of oxygen concentration

We have fabricated Er-doped amorphous SiOx films by laser ablation of a Si:Er2O3 target in He atmosphere. The photoluminescence intensity at 1.54 μm was highly dependent on the oxygen content in the film, which turned out to be changed significantly by the ambient He pressure. Also, we have adopted time-of-flight quadrupole mass spectroscopy to obtain kinetic energies of ionic species in a plume produced by laser ablation. Si and Er ions do not overlap spatially as they expand toward the Si substrate and Er ions impinge on the preformed SiOx layer.

Magnetotransport, magneto‐optical, and electronic subband studies in InxGa1−xAs/In0.52Al0.48As one‐side‐modulation‐doped asymmetric step quantum wells

Shubnikov–de Haas (SdH), Van der Pauw Hall‐effect, and cyclotron resonance measurements on InxGa1−xAs/In0.52Al0.48As asymmetric step quantum wells were carried out to investigate the electrical properties of a free electron gas and to determine the effective mass of the electron gas, the subband energies, and the wave functions in the InxGa1−xAs quantum wells. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi‐two‐dimensional electron gas in the InxGa1−xAs quantum wells, and the fast Fourier transformation results for the SdH data indicated clearly the occupation by that gas of two subbands in the quantum wells. The electronic effective masses determined from the slopes of the main peak absorption energies as a function of the magnetic field were 0.0477 and 0.053 me for the first excited and the ground subbands, respectively. The electronic subband energies and the wave functions in the InxGa1−xAs/In0.52Al0.48As step quantum wells were calculated by a self‐consistent method taking...

Electrical and optical characterizations of self-assembled quantum dots formed by the atomic layer epitaxy technique

We investigated the electrical and optical properties of InGaAs self-assembled quantum dots grown using the atomic layer epitaxy (ALE) technique. Dots–in–a–well structures were grown by alternately supplying InAs and GaAs sources on an InGaAs layer and covering with another InGaAs layer. Three samples produced with different numbers of cycles of alternate InAs/GaAs supply were characterized by capacitance-voltage and photoluminescence (PL) measurements. For the ten cycle dots–in–a–well structure, a strong zero-dimensional electron confinement was observed even at room temperature. On the other hand, for the five-cycle structure, the PL results indicate that the InGaAs quantum well structure coexists unstably with premature quantum dots. By comparing the results for samples with different numbers of cycles, we suggest that an ALE dots–in–a–well structure can be formed by the aggregation of In and Ga atoms incorporated into the InGaAs quantum well layer when the number of cycles exceeds the critical number ...

Electronic subband studies in In0.52Al0.48As/InxGa1−xAs one‐side‐modulation‐doped asymmetric coupled double quantum wells

Shubnikov–de Haas (SdH) and van der Pauw Hall effect measurements on In0.52Al0.48As/InxGa1−xAs coupled double quantum wells grown by metalorganic chemical vapor deposition have been carried out to investigate the magnetotransport properties of an electron gas and to determine the subband energies and wave functions in the coupled quantum wells. Transmission electron microscopy measurements showed that In0.8Ga0.2As and In0.53Ga0.47As quantum wells were separated by an In0.25Ga0.75As potential barrier in an active region. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi‐two‐dimensional electron gas in the quantum wells. The fast Fourier transformation results for the SdH data clearly indicate the occupation of three subbands in the In0.52Al0.48As/InxGa1−xAs coupled quantum wells. Electron subband energies and wave functions in the quantum wells were calculated by a self‐consistent method taking into account exchange‐correlation effects. The first and second excited subband wave fu...

Examination of EL and PL properties of MCHM-PPV and MEH-PPV: a study towards introduction of a new series of thin film EL devices

Abstract The effect of the side group, employed during synthesis, on the PL (photoluminescence), EL (electroluminescence) profiles and the light intensity of PPV derivatives has been investigated using MEH (methoxy-ethyl-hexyloxy) and MCHM (methoxy-cyclo-hexyl-methyloxy) as side groups. The measurements show that even the most subtle change in molecular structure of PPV has a marked improvement on the EL, PL profiles and the light emission intensity. It has been found that the PL and EL intensities of the MCHM-PPV are higher than those of MEH-PPV. MCHM-PPV also demonstrated higher light emission at lower potentials and currents.

Strain distributions and electronic subband energies of self-assembled CdTe quantum wires grown on ZnTe buffer layers

The structural properties and the shape of self-assembled CdTe/ZnTe quantum wires (QWRs) grown by using molecular beam epitaxy and atomic layer epitaxy were determined by using atomic force microscopy (AFM) measurements, and the interband transitions in the CdTe/ZnTe QWRs were investigated by using temperature-dependent photoluminescence (PL) measurements. The shape of the CdTe/ZnTe QWRs on the basis of the AFM image was modeled to be a half-ellipsoidal cylinder approximately. The temperature-dependent PL spectra showed that the PL peaks corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E1-HH1) shifted to lower energy with increasing temperature. Strain distributions and electronic subband energies at several temperatures were numerically calculated by using a finite-difference method (FDM) with and without taking into account shape-based strain and nonparabolicity effects. The excitonic peak corresponding to (E1-HH1) interband transitions, as det...

Effects of Doping Profile on Characteristics of InAs Quantum Dots

Capacitance-voltage measurements were carried out to investigate the effects of the doping profile on characteristics of self-assembled InAs quantum dots. Using this technique, we observed features of zero-dimensional electron confinement indicating the presence of quantum dots. However, the number of confined states differed depending on the doping profile, and this fact was confirmed by photoluminescence measurements. The equation in the depletion approximation led us to calculate the distribution of carriers as a function of depth from the sample surface, and the results are in agreement with the depth of the quantum dot layer.

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%.