• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.332-332
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• 2012

분자선 에피탁시(molecular beam epitaxy)를 이용하여 GaAs 기판에 성장한 InAs 양자점(QDs: quantum dots)은 성장 온도, 압력, As/In의 공급비 등의 성장 조건에 따라 다른 변수(parameter)를 갖는다. 따라서 성장변수에 따라 양자점의 모양과 크기, 밀도가 달라져 균일한 양자점 형성에 어려움이 있어 많은 연구가 진행되고 있다. 예를 들면 In-interruption 법으로 성장한 양자점의 특성이 S-K mode (Stranski-Krastanov mode)로 성장한 양자점에 비해 광학적 특성이 향상되었다. 본 연구에서는 In pre-deposition (IPD) 법으로 성장한 InAs/GaAs 양자점의 광학적 특성을 PL(photoluminescence)와 TRPL (time-resolved PL)을 이용하여 분석하였다. InAs QDs 시료들은 In과 As 공급시간을 각각 1초와 19초 (QD1), 2초와 18초 (QD2), 3초와 17초 (QD3)로 조절하여 성장하였으며, In이 공급되는 시간 동안 As shutter를 차단하여 As 공급을 중단하였다. In과 As의 차단 없이 S-K mode로 성장한 시료를 기준시료로 사용하였다 (QD0). AFM (atomic force microscope) 측정결과, In 공급시간이 1초에서 2초로 증가할 때, 양자점의 밀도와 종횡비(aspect ratio)가 증가하였고, 양자점의 균일도가 증가하였다. 그러나 QD3 시료는 QD1 시료에 비해 밀도와 종횡비, 균일도가 감소하였다. 10 K에서 PL 피크는 In 공급 시간이 증가할 때, 970 nm에서 1020 nm로 적색편이 하였고 반치폭 (FWHM: full width at half maximum)은 75 meV에서 85 meV로 증가하였다. QD2 시료의 PL 피크 에너지가 가장 낮았고, 가장 강한 PL 세기를 보였다. IPD 시간이 증가함에 따라 PL 피크에서 측정한 PL 소멸은 점차 빨라졌다. IPD 기법으로 성장한 양자점의 빠른 PL 소멸은 양자점 밀도와 종횡비 향상에 의한 파동함수 중첩의 증가와 구속 에너지 증가에 의한 것으로 설명된다.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.146-146
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• 2015

We report the enhanced performance of poly(N-vinylcarbozole) (PVK)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based quantum-dot light-emitting diodes by inserting the polyaniline:poly (p-styrenesulfonic acid) (PANI:PSS) interlayer. The QD-LED with PANI:PSS interlayer exhibited a higher luminance and luminous current efficiency than that without PANI:PSS. Ultraviolet photoelectron spectroscopy results exhibited different electronic energy alignments of QD-LEDs with/without the PANI:PSS interlayer. By inserting the PANI:PSS interlayer, the hole-injection barrier at the QD layer/PVK interface was reduced from 1.45 to 1.23 eV via the energy level down-shift of the PVK layer. The reduced barrier height alleviated the interface carrier charging responsible for the deterioration of the current and luminance efficiency. This suggests that the insertion of PANI:PSS interlayer in QD-LEDs contributed to (i) increase the p-type conductivity and (ii) reduce the hole barrier height of QDs/PVK, which are critical factors leading to improve the efficiency of QD-LEDs.

• Journal of the Korean Vacuum Society
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• v.15 no.3
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• pp.301-307
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• 2006

We have investigated optical properties of p-modulation doped In(Ga)As quantum dots (QDs) on InP substrate with a comparison with the undoped QDs. Photoluminscence (PL) intensity of doped QDs at 10 K was about 10 times weaker than that of undoped QD sample. The decay time of doped QD sample at its PL peak, obtained from the time-resolved PL (TR-PL) experiment at 10 K, was very fast compared to that of undoped sample. We interpret that this fast decay time of the doped QD sample comes from the addition of non-radiative recombination paths, which are originated from the doping-related defects.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.541-541
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• 2012

InP quantum dots capped by myristic acid (InP-MA QDs) were synthesized by a typical hot injection method using MA as stablizing agent. The current density across the InP-MA QDs thin film which was fabricated by spin-coating method is about $10^{-4}A/cm^2$ at the electric field of 0.1 MV/cm from I-V measurement on a metal-insulator-metal (MIM) device. The low conductivity of the InP-MA QDs thin film is interpreted as due to the long interdistances among the dots governed by the MA molecules. Therefore, replacing the MA with thioacetic acid (TAA) by biphasic ligand exchange was conducted in order to obtain TAA capped InP QDs (InP-TAA). InP-TAA QDs were designed due to: 1) the TAA is very short molecule; 2) the thiolate groups on the surface of the InP-TAA QDs are expected to undergo condensation reaction upon thermal annealing which connects the QDs within the QD thin film through a very short linker -S-; and 3) TAA provides better passivation to the QDs both in the solution and thin film states which minimizing the effect of surface trapping states.

• Journal of Sensor Science and Technology
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• v.31 no.4
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• pp.249-254
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• 2022

To achieve high-performance colloidal quantum dot light-emitting diodes (QD-LEDs), the use of a densely packed QD film is crucial to prevent the formation of leakage current pathways and increase in interface resistance. Spin coating is the most common method to deposit QDs; however, this method often produces pinholes that can act as short-circuit paths within devices. Since state-of-the-art QD-LEDs typically employ mono- or bi-layer QDs as an emissive layer because of their low conductivities, the use of a densely packed and pinhole-free QD film is essential. Herein, we introduce the Langmuir-Blodgett (LB) technique as a deposition method for the fabricate densely packed QD films in QD-LEDs. The LB technique successfully transfers a highly dense monolayer of QDs onto the substrate, and multilayer deposition is performed by repeating the transfer process. To validate the comparability of the LB technique with the standard QD-LED fabrication process, we fabricate and compare the performance of LB-based QD-LEDs to that of the spin-coating-based device. Owing to the non-destructiveness of the LB technique, the electroluminescence efficiency of the LB-based QD-LEDs is similar to that of the standard spin coating-based device. Thus, the LB technique is promising for use in optoelectronic applications.

• Current Optics and Photonics
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• v.4 no.3
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• pp.161-173
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• 2020

Colloidal quantum dots (QDs) have gained tremendous attention as a key material for highly advanced display technologies. The performance of QD light-emitting diodes (QLEDs) has improved significantly over the past two decades, owing to notable progress in both material development and device engineering. The brightness of QLEDs has improved by more than three orders of magnitude from that of early-stage devices, and has attained a value in the range of traditional inorganic LEDs. The emergence of high-luminance (HL) QLEDs has induced fresh demands to incorporate the unique features of QDs into a wide range of display applications, beyond indoor and mobile displays. Therefore it is necessary to assess the present status and prospects of HL-QLEDs, to expand the application domain of QD-based light sources. As part of this study, we review recent advances in HL-QLEDs. In particular, based on reports of brightness exceeding 10⁵ cd/㎡, we have summarized the major approaches toward achieving high brightness in QLEDs, in terms of material development and device engineering. Furthermore, we briefly introduce the recent progress achieved toward QD laser diodes, being the next step in the development of HL-QLEDs. This review provides general guidelines for achieving HL-QLEDs, and reveals the high potential of QDs as a universal material solution that can enable realization of a wide range of display applications.

• Korean Journal of Materials Research
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• v.23 no.8
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• pp.430-434
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• 2013

Quantum dots(QDs) with their tunable luminescence properties are uniquely suited for use as lumophores in light emitting device. We investigate the microstructural effect on the electroluminescence(EL). Here we report the use of inorganic semiconductors as robust charge transport layers, and demonstrate devices with light emission. We chose mechanically smooth and compositionally amorphous films to prevent electrical shorts. We grew semiconducting oxide films with low free-carrier concentrations to minimize quenching of the QD EL. The hole transport layer(HTL) and electron transport layer(ETL) were chosen to have carrier concentrations and energy-band offsets similar to the QDs so that electron and hole injection into the QD layer was balanced. For the ETL and the HTL, we selected a 40-nm-thick $ZnSnO_x$ with a resistivity of $10{\Omega}{\cdot}cm$, which show bright and uniform emission at a 10 V applied bias. Light emitting uniformity was improved by reducing the rpm of QD spin coating.At a QD concentration of 15.0 mg/mL, we observed bright and uniform electroluminescence at a 12 V applied bias. The significant decrease in QD luminescence can be attributed to the non-uniform QD layers. This suggests that we should control the interface between QD layers and charge transport layers to improve the electroluminescence.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.191-191
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• 2012

InP quantum dot (QD) - organosilicon nanocomposites were synthesized and their photoluminescence quenching was mainly investigated because of their applicability to white LEDs (light emitting diodes). The as-synthesized InP QDs which were capped with myristic acid (MA) were incompatible with typical silicone encapsulants. Post ligand exchange the MA with a new ligand, 3-aminopropyldimethylsilane (APDMS), resulted in soluble InP QDs bearing Si-H groups on their surface (InP-APDMS) which allow embedding the QDs into vinyl-functionalized silicones through direct chemical bonding, overcoming the phase separation problem. However, the ligand exchange from MA to APDMS caused a significant decrease in the photoluminescent efficiency which is interpreted by ligand induced surface corrosion relying on theoretical calculations. The InP-APDMS QDs were cross-linked by 1,4-divinyltetramethylsilylethane (DVMSE) molecules via hydrosilylation reaction. As the InP-organosilicon nanocomposite grew, its UV-vis absorbance was increased and at the same time, the PL spectrum was red-shifted and, very interestingly, the PL was quenched gradually. Three PL quenching mechanisms are regarded as strong candidates for the PL quenching of the QD nano-composites, namely the scattering effect, Forster resonance energy transfer (FRET) and cross-linker tension preventing the QD's surface relaxation.

• Journal of Powder Materials
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• v.19 no.6
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• pp.442-445
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• 2012

The quantum dots (QD) have unique electrical and optical properties due to quantum dot confinement effect. The optical properties of QDs are decided by various synthesis conditions. In a prior QDs study, a study on the QDs size with synthesis condition such as synthesis time and temperature is being extensively researched. However, the research on QDs size with composition ratio has hitherto received scant attention. In order to evaluate the ratio dependence of CdSe crystal, synthesis ratio of Se precursor is changed from 16.7 mol%Se to 44 mol%Se. As the increasing Se ratio, the band gap was increased. This is caused by red shift of emission. We confirmed optical property of CdSe QDs with composition ratio.

• Journal of the Microelectronics and Packaging Society
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• v.28 no.4
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• pp.69-72
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• 2021

Various studies for QLEDs using inkjet printing has been actively conducted. Multilayers in QLEDs need an orthogonal process inevitably using different solvents and it makes the inkjet printing process more difficult and expensive. Therefore, coating two layers in a single process can reduce the fabrication step, resulting in the process time. In this study, we fabricated QLEDs of standard structure using a mixture of emission layer and hole transport layer. The mixed layer was fabricated by dissolving TFB and QDs in chlorobenzene, and the maximum luminance of the device was 45,850 cd/m². It shows the bright future of the electroluminescence devices applied with inkjet printing process.