• Journal of IKEEE
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• v.27 no.1
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• pp.30-37
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• 2023

To improve the efficiency and stability of colloidal quantum dot light-emitting diodes (QD-LEDs), it is essential to achieve charge balance within the QD emissive layer. Zinc oxide (ZnO) is widely used for constructing an electron transport layer in the state-of-the-art QD-LEDs, but spontaneous electron injection from ZnO often results in excessive electrons in QDs that significantly deteriorate the performance of QD-LEDs. In this study, we demonstrated the improved performance of QD-LEDs by modifying the electron injection property of ZnO with self-assembled monolayer (SAM)-treatment. As a result of improved charge balance, the external quantum efficiency and maximum luminance of QD-LEDs with SAM-treatment were improved by 25% and 200%, respectively, compared to the devices without SAM-treatment.

• Applied Chemistry for Engineering
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• v.26 no.3
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• pp.362-365
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• 2015

Silica encapsulated ZnSe quantum dots (QDs) were prepared by employing two microemulsion systems: AOT/water/cyclohexane microemulsions containing ZnSe quantum dots with NP5/water/cyclohexane microemulsions containing tetraethylorthosilicate (TEOS). Using this method, cubic zinc blende nanoparticles (3 nm in diameter) were synthesized and encapsulated by silica nanoparticles (20 nm in diameter). The temperature dependence of photoluminescence (PL) for silica-encapsulated ZnSe QDs was investigated to evaluate this material as a temperature sensor media. The fluorescence emission intensity of silica-encapsulated ZnSe nanoparticles (NPs) was decreased with an increase of ambient temperature over the range from $30^{\circ}C$ to $60^{\circ}C$ and a linear relationship between the temperature and the emission intensity was observed. In addition, the temperature dependence of PL intensity for silica-encapsulated ZnSe NPs showed a reversible pattern on ambient temperature. A reversible temperature dependence of the luminescence combined with its insensitivity toward quenching by oxygen due to silica coating established this material as an attractive media for temperature sensor applications.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.370-370
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• 2011

ZnO 나노 라드 위에 Quantum dot을 형성하고 최종적으로 TiO2를 Atomic Layer Deposition방법으로 증착하여, 그 passivation 효과가 solar cell의 효율에 미친 영향에 대한 실험을 진행하였다. 암모니아 솔루션을 이용한 Hydrothermal 방법으로 수직한 1차원 형태의 ZnO 나노라드를 TCO 기판 위에 성장시킨다. 여기에 잘 알려진 SILAR와 CBD 방법으로 CdS, CdSe 양자점을 증착한다. 그리고 amorphous TiO2로 표면을 덮는 과정을 거치는데, TiO2가 좁은 간격으로 형성된 ZnO라드 구조 위에서 균일하고 정밀하게 증착되도록 하기 위해 Atomic Layer Deposition을 이용하였다. 사용된 precursor는 Titanium isopropoxide와 H2O이며, 실험상에서 0~5 nm 두께의 TiO2 박막을 형성해 보았다. 다양한 분석 방법을 통해 TiO2/QDs/ZnO의 shell-shell-core 구조를 조사했다. (Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS)). 이를 solar cell에 적용하고 I-V curve를 통해 그 효율을 확인하였으며, Electrochemical Impedance Spectroscopy (EIS)를 통해서 재결합 측면에서 나타나는 변화 양상을 확인하였다.

• Korean Journal of Materials Research
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• v.18 no.12
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• pp.640-644
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• 2008

ZnO/ZnS core/shell nanocrystals (${\sim}5-7\;nm$ in diameter) with a size close to the quantum confinement regime were successfully synthesized using polyol and thermolysis. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analyses reveal that they exist in a highly crystalline wurtzite structure. The ZnO/ZnS nanocrystals show significantly enhanced UV-light emission (${\sim}384\;nm$) due to effective surface passivation of the ZnO core, whereas the emission of green light (${\sim}550\;nm$) was almost negligible. They also showed slight photoluminescence (PL) red-shift, which is possibly due to further growth of the ZnO core and/or the extension of the electron wave function to the shell. The ZnO/ZnS core/shell nanocrystals demonstrate strong potential for use as low-cost UV-light emitting devices.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.371-371
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• 2011

본 연구에서는 황화납(PbS)과 황화카드뮴(CdS)을 감응물질로 하는 양자점 감응형 태양전지를 만들고 효율을 측정하였다. Sputter를 이용하여 고진공의 상태에서 산화아연(ZnO) film을 seed layer로 증착한 후 수열합성법으로 ZnO 나노선을 합성한다. 합성된 나노선을 successive ionic layer adsorption and reaction (SILAR) 법으로 PbS, CdS 양자점을 합성하고 이를 주사전자 현미경(SEM), X-선 회절(XRD)을 통해 확인하였다. 또한 PbS와 CdS의 co-sensitizer를 합성하고 diffused reflectance spectra (DRS)를 측정함으로써 넓은 범위의 광흡수도를 확인할 수 있었다. Co-sensitizer의 합성 방법을 달리하여 PbS/CdS를 합성한 후 각각의 효율을 측정해보고, 더 높은 효율을 내기 위한 방안에 대해 고찰하였다.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.3
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• pp.71-74
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• 2019

In this study, we fabricated two standard and inverted quantum dot light emitting diodes (QLEDs) using $TiO_2$ nanoparticles (NPs) with lower electron mobility than ZnO NPs as inorganic electron transport layer to suppress electron injection into the emitting layer. Current density was much higher for the inverted QLEDs than the standard ones. The inverted QLEDs were brighter, but showed low current efficiency due to the high current density. In addition, as the current density was higher, the driving voltage was higher, and the red shift was confirmed in the emission wavelength spectrum. The low current density in the standard structured devices showed that the possibility that $TiO_2$ NPs could suppress the electron injection in the QLEDs.

• Transactions of the Korean hydrogen and new energy society
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• v.24 no.2
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• pp.107-112
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• 2013

The ZnO is able to produce hydrogen from water however it can only absorb ultraviolet region due to its 3.37eV of wide band gap. Therefore efficiency of solar hydrogen production is low. In this work we report investigation results of improved visible light photo-catalytic properties of CdSe quantum dots(QDs) sensitized ZnO nanorod heterostructures. Hydrothermally vertically grown ZnO nanorod arrays on FTO glass were sensitized with CdSe by using SILAR(successive ionic layer adsorption and reaction) method. Morphology of grown ZnO and CdSe sensitized ZnO nanorods had been investigated by FE-SEM that shows length of $2.0{\mu}m$, diameter of 120~150nm nanorod respectively. Photocatalytic measurements revealed that heterostructured samples show improved photocurrent density under the visible light illumination. Improved visible light performance of the heterostructures is resulting from narrow band gap of the CdSe and its favorable conduction band positions relative to potentials of ZnO band and water redox reaction.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.06a
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• pp.387-387
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• 2007

기능성 나노소자를 구현할 수 있는 나노 소재로 0차원 구조의 양자점(quantum dot)과 1차원 구조의 양자선 및 나노선(nanorod)이 제안되고 있다. 나노선의 경우 나노스케일의 dimension, 앙자 제한 효과, 탁월한 결정성, self-assembly, internal stress등 기존의 벌크형 소재에서 발견할 수 없는 새로운 기능성이 나타나고 있어서 바이오, 에너지, 구조, 전자, 센서 등의 분야에서 활용되고 있다. 현재 국내외적으로 널리 연구되고 있는 나노선으로는 Si 및 Ge, $SnO_2$, SiC, ZnO 등이 있으며 특히, ZnO는 우수한 물리적 전기적 특성과 함께 나노선으로의 합성이 비교적 쉬워 주목받고 있는 재료이다. ZnO의 합성방법으로는 thermal CVD, MOCVD, PLD, wet-chemistry 등 다양한 방법이 사용되고 있다. 특히 MOCVD 법은 수직 정렬된 ZnO 나노막대를 합성하기가 매우 용이하다. 본 실험에서는 자체개발된 MOCVD 장비를 이용한 일차원 ZnO 나노선을 성장하였다. 이러한 ZnO 나노선의 성장은 사파이어 기판과 실리콘 기판 위에서 이루어졌으며 기판의 종류와 격자상수 불일도에 따른 상이한 성장과정을 온도에 따른 나노선 성장에서 관찰할 수 있었다. 사파이어 기판의 경우, 240도의 온도에서는 박막형상을 지닌 ZnO가 온도가 320도 이상으로 상승하면서 나노선으로 변함을 보였고, 실리콘 기판의 경우 380도 이상에서 기울기률 가진 나노선을 관찰하였으며, 420도에서는 나노선을 관찰 할 수 없었다. 또한 PL 장비를 이용한 PL 강도와 성장과정을 연관하여 생각하였을 때, 나노선의 기물기가 PL 강도비과 연관성을 가진다는 것을 측정을 통해 확인하였다.

• Journal of Sensor Science and Technology
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• v.31 no.2
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• pp.125-130
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• 2022

We describe the importance of zinc oxide nanoparticle (ZnO NP) concentration in the enhancement of electrical properties in a lead sulfide quantum dot (PbS QD)-based shortwave infrared (SWIR) photodetector. ZnO NPs were synthesized using the sol-gel method. The concentration of the ZnO NPs was controlled as 20, 30 and 40 mg/mL in this study. Note that the ZnO NPs layer is commonly used as an electron transport layer in PbS QDs SWIR photodetectors. The photo-to-dark ratio, which is an important parameter of a photodetector, was intensively examined to evaluate the electrical performance. The 20 mg/mL condition of ZnO NPs exhibited the highest photo-to-dark ratio value of 5 at -1 V, compared with 1.8 and 0.4 for 30 mg/mL and 40 mg/mL, respectively. This resulted because the electron mobility decreased when the concentration of ZnO NPs was higher than the optimized value. Based on our results, the concentration of ZnO NPs was observed to play an important role in the electrical performance of the PbS QDs SWIR photodetector.

• Journal of Sensor Science and Technology
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• v.32 no.6
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• pp.455-461
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• 2023

We investigated the electrical characteristics of ZnO nanoparticles (NPs) with air exposure that is a widely used electron transport layer for quantum dot light-emitting diodes (QLEDs). Upon air exposure, we observed changes in the density of states (DOS) of the trap levels of ZnO NPs. In particular, with air exposure, the concentration of deep trap energy levels in ZnO NPs decreased and electron mobility significantly improved. Consequently, the air-exposed ZnO reduced leakage current by approximately one order of magnitude and enhanced the external quantum efficiency at the low driving voltage region of the QLED. In addition, based on the excellent conductivity properties, high-brightness QLEDs could be achieved.