This thesis describes surface engineering strategies for semiconductor nanomaterials to improve the efficiency of optoelectronic devices. In Chapter 3, we demonstrate the surface construction of InP@ZnSeS quantum dots for a color filter.
Introduction of Thesis
- Introduction to nanomaterials
- Interface/surface engineering strategies for nanomaterials
- Interface/surface engineering for optoelectronic devices
Reproduced with permission from ref. a) Schematic depicting the device structure of an organic light emitting diode (OLED) and the band diagram of the OLED. Reproduced with permission from ref. a) Schematic diagram of self-assembled dopamine monolayer between SnO2 ETL and perovskite, (b) best PCE of devices with different DA treatment times and (c) efficiency histogram among 20 devices.
Bio-Inspired Catecholamine-Derived Surface Modifier for Graphene-Based Organic
- Results and discussion
- Experimental section
UPS spectra of (d) threshold and (e) Fermi edge regions for graphene and pNE-coated graphene. SEM images of PEDOT:PSS coatings on (b) pristine graphene surface and (c–e) pNE-coated graphene with different incubation times.
Thiol-Based Surface Modification of InP Quantum Dots for Color Filter
- Results and discussion
- Experimental section
Sheet resistance of pNE-coated graphene films before and after annealing depending on the incubation time. Spectroscopic analyzes of graphene and pNE-coated graphene without annealing. a) Raman spectra of graphene and pNE-coated graphene (the purple shaded area indicates the defect-related peak).
Highly Emissive Blue Quantum Dots with Superior Thermal Stability via In Situ
Metal halide perovskite materials have been recognized as promising candidates for next-generation color displays because their high photoluminescence quantum yields (PLQYs), narrow full widths at half maximum (FWHM), ease of bandgap tuning, and solution processability meet the ITU standards. R International Telecommunications Union (ITU) Recommendation BT.2020 (Rec. 2020).1-3 Recently, significant progress has been made in the development of near-infrared, red and green perovskite light-emitting diodes (LEDs), where the external quantum efficiency (EQE) exceeds 20%.4–8 However, the efficiency of blue perovskite materials has lagged far behind, with EQEs of 12.3%. One method involves mixed halides that include both Br and Cl anions. Although this is a useful method for bandgap engineering, the easy formation of Cl vacancies is a limitation as it results in a deep trap state within the bandgap.18 –20 These defect sites cause perovskite layer degradation and ion migration, resulting in phase segregation in response on the application of an electric field during device operation.21,22 A second method is to use Br-based two-dimensional perovskite nanoplatelets and exploit advantages of the exciton quantum confinement effect.23–27 In inorganic cesium lead bromide (CsPbBr3 ) nanoplatelets, the emission can be controlled based on the number of [PbBr6]4 layers; However, strong exciton-phonon coupling and a randomly oriented distribution of nanoplatelets result in low-performance LEDs.28 A third strategy for achieving blue-emitting perovskite nanocrystals (NCs) is to reduce the crystal size of a perovskite material so that it is within the quantum confinement regime.9,29–32 CsPbBr3 NCs with sizes in the quantum confinement regime (referred to as quantum dots, QDs) usually suffer from low PLQYs and stability because they are strongly affected by surface defects when the surface area to volume ratio is high.33 In particular, small QDs are easily degraded and undergo aggregation due to their high surface energy, leading to broad emission spectra and poor spectral stability, so that the emission color is sensitive to change to green at high temperatures. In this paper, we report a unique method to improve the PLQY and stability of blue-emitting CsPbBr3 QDs via simultaneous generation of mixed CsPbBr3 QDs and Cs4PbBr6 NCs.
By controlling the reactivity of the precursors, the size of the CsPbBr3 QDs was controlled. We investigated the effects of Cs4PbBr6 NCs on the photophysical properties and thermal stability of CsPbBr3 QDs by observing changes in their morphology and optoelectronic properties. Defective sites of octahedra on the surface of CsPbBr3 QDs were etched with Cs4PbBr6 NCs, resulting in the removal of defects.
This surface reconstruction of CsPbBr3 QDs reduces the defect density and eliminates nonradiative recombination pathways, leading to high efficiency and stability of CsPbBr3 QDs. The mixed NC solution retained 90% of its initial PLQY value during 120 days of storage under ambient conditions, with little change in the emission peak position and FWHM. Spectrally stable and efficient blue LEDs with an EQE of 4.65% at 480 nm based on mixed NCs were achieved.
Results and discussion
We investigated the dependence of the photophysical properties of the as-synthesized NCs on the reaction temperature in the range of 60 °C to 160 °C. This result is consistent with the increase in the intensity of the absorption peaks at 313 nm in the UV-vis spectra. Transmission electron microscopy (TEM) analysis also indicated that CsPbBr3 and Cs4PbBr6 NCs coexisted in the samples, and lowering the temperature reduced the overall size of the NCs (Figures 4.3d-g and 4.4).
In addition, other crystal structures such as CsPb2Br5 were not formed, and the diffraction patterns showed that the crystallinity of ISNCs was improved (Figure 4.6c). We hypothesized that the cause of the high PLQY and good thermal stability of ISNCs was related to the presence of Cs4PbBr6 NCs created with CsPbBr3 QDs. The morphology of NC416 was transformed from hexagonal to truncated diamond and assembled into zigzag shapes (Figure 4.13).
Characterization of the interaction between CsPbBr3 QDs and Cs4PbBr6 NCs. a) UV-vis absorption and (b) PL spectra for different ratios of separately prepared CsPbBr3 QDs (S-QD113) and Cs4PbBr6 NCs (NC416). e) Photographs of the prepared sample (left) and the sample after 12 hours of storage at room conditions, obtained with UV light (right). The PLE ISNC spectrum included a sharp dip centered at around 313 nm, corresponding to Cs4PbBr6. In order to investigate the optical properties of NCs in detail, we measured the temperature-dependent PL of S-QD113 and ISNC samples (Figure 4.16).
Monodisperse Cs4PbBr6 NCs were synthesized according to a previously reported method.49 The Cs oleate precursor and NCs were prepared in air. After cooling the solution to 80 °C, preheated Cs oleate (0.75 mL) was rapidly injected into it. Small CsPbBr3 QDs were synthesized according to a previously reported method with some modifications.48 The synthetic approach was based on hot-injection of the Cs-oleate precursor.
After the complete solubilization of the reaction mixture, the preheated Cs-oleate precursor (0.4 ml) was quickly injected into the reaction mixture at 90 °C. To avoid dissolution of the NC416 layer, the slide was coated twice with methyl acetate at 3000 rpm for 1 min. After peeling off the Kapton tape, S-QD113 was spin-coated at 3000 rpm for 1 minute over the entire substrate.
After dissolving the NCs in toluene and adjusting the concentration to 10 mg mL-1, we observed a change in the PL spectrum of the sample over time in a 120 °C oil bath. TFB and PVK (volume ratio 1:1) were mixed and dissolved in chlorobenzene so that the concentration of the mixture was 3 mg ml. The device performance of the encapsulated LEDs was measured using a Keithley 2400 source meter and spectroradiometer (CS-2000, Konica Minolta) under ambient conditions.
Bright Blue Light Emission from Ni(2+) Ion-Doped CsPbClxBr3-x Perovskite Quantum Dots Enabling Efficient Light-emitting Devices. Thermally stable copper(II)-doped cesium lead halide perovskite quantum dots with strong blue emission. Precise Control of Quantum Confinement in Cesium Lead Halide Perovskite Quantum Dots via Thermodynamic Equilibrium.
Original core-shell structure of cubic CsPbBr3@Amorphous CsPbBrx Perovskite Quantum Dots with a high blue photoluminescence quantum yield of over 80%. Ultralow Trap Density Perovskite Quantum Dots by Acid Etch Driven Ligand Exchange for High Luminance and Stable Pure-Blue Light Emitting Diodes. Ligand-mediated transformation of cesium lead bromide perovskite nanocrystals into lead-depleted Cs4PbBr6 nanocrystals.
CsPbX3 Quantum Dots for Lighting and Displays: Room Temperature Synthesis, Photoluminescence Superiorities, Fundamental Origins, and White Light Emitting Diodes. Spectral and dynamical properties of singlet excitons, biexcitons and trions in cesium-lead-halide perovskite quantum dots. Size dependence of charge carrier dynamics in organometal halide perovskite nanocrystals: Deciphering radiative versus non-radiative components.
먼저, 2010년부터 학생연구원으로 시작해 지금까지 지도해주신 지도교수 박종남 교수님께 진심으로 감사의 말씀을 전하고 싶습니다. 연구뿐만 아니라, 인생 멘토로서 교수님께서 해주신 조언도 고민이 많았던 20대를 헤쳐 나가는 데 큰 도움이 되었습니다. 진로에 대해 고민하며 방황하던 중, 군 복무를 마치고 돌아오자 교수님께서 따뜻하게 맞아주시던 모습을 잊을 수 없습니다.
학생들을 자기 자식처럼 사랑해주시는 선생님을 만나서 영광이었습니다. 교수님들과 의사 선생님들의 격려와 조언은 제가 연구를 완성하고 확장하는 데 원동력이 되었습니다. 말씀하신 내용 잊지 않고 사회에서 중요한 역할을 할 수 있도록 최선을 다하겠습니다.
그리고 졸업식 내내 하루종일 함께 생활해준 MCL 식구들에게도 감사하다는 말씀 전하고 싶습니다. 마지막으로 항상 저를 지지하고 응원해주시는 가족들에게 진심으로 감사의 말씀을 전하고 싶습니다. 모두들 건강하시고 앞으로는 좋은 일들만 일어나길 바랍니다.