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Atomically Flat, 2D Edge-Directed Self-Assembly of Block Copolymers
No full textNanoscale shape engineering is an essential requirement for the practical use of 2D materials, aiming at precisely customizing optimal structures and properties. In this work, sub-10-nm-scale block copolymer (BCP) self-assembled nanopatterns finely aligned along the atomic edge of 2D flakes, including graphene, MoS2, and h-BN, are exploited for reliable nanopatterning of 2D materials. The underlying mechanism for the alignment of the self-assembled nanodomains is elucidated based on the wetting layer alternation of the BCP film in the presence of intermediate 2D flakes. The resultant highly aligned nanocylinder templates with remarkably low levels of line edge roughness (LER) and line-width roughness (LWR) yield a sub-10-nm-wide graphene nanoribbon (GNR) array with noticeable switching characteristics (on-to-off ratio up to approximate to 6 x 10(4))
Exploring ultrafast flow chemistry by autonomous self-optimizing platform
No full textThe rapid development of novel synthetic routes for pharmaceutical compounds is highly attractive for overcoming pandemic and epidemic-prone diseases like COVID-19. Herein, we report an automated microreactor platform (AMP) with Bayesian optimization (BO) that can autonomously explore the optimal conditions for ultrafast synthesis of biologically active thioquinazolinone. First, AMP operation is successfully demonstrated with full control of quantitative variables, specifically reaction volume, temperature, and flow rate, allowing to sequentially conduct a total of 80 experiments planned by the user. Next, BO enables the AMP to autonomously self-optimize the reaction conditions, demonstrating the high efficiency of the fully automated AMP. The fully automated approach is extended to optimize more complex variables including a categorical variable (i.e. the type of organolithium for synthesis), revealing that phenyllithium (PhLi) gives superior yield for synthesizing thioquinazolinone. In addition, the autonomous AMP is utilized for combinatorial chemistry to sequentially synthesize a library composed of nine types of S-benzylic thioquinazolinone under autonomously optimized conditions within only 20 min
Nanocomposite Engineering of a High???capacity Partially Ordered Cathode for Li???ion Batteries
No full textUnderstanding the local cation order in the crystal structure and its correlation with electrochemical performances has advanced the development of high-energy Mn-rich cathode materials for Li-ion batteries, notably Li- and Mn-rich layered cathodes (LMR, e.g., Li1.2Ni0.13Mn0.54Co0.13O2) that are considered as nanocomposite layered materials with C2/m Li2MnO3-type medium-range order (MRO). Moreover, the Li-transport rate in high-capacity Mn-based disordered rock-salt (DRX) cathodes (e.g., Li1.2Mn0.4Ti0.4O2) was found to be influenced by the short-range order (SRO) of cations, underlining the importance of engineering the local cation order in designing high-energy materials. Herein, we reveal the nanocomposite, heterogeneous nature (like MRO found in LMR) of ultrahigh-capacity partially ordered cathodes (e.g., Li1.68Mn1.6O3.7F0.3) made of distinct domains of spinel-, DRX- and layered-like phases, contrary to conventional single-phase DRX cathodes. This multi-scale understanding of ordering informs engineering the nanocomposite material via Ti doping, altering the intra-particle characteristics to increase the content of the rock-salt phase and heterogeneity within a particle. This strategy markedly improves the reversibility of both Mn- and O-redox processes to enhance the cycling stability of the partially ordered DRX cathodes (nearly ???30% improvement of capacity retention). Our work sheds light on the importance of nanocomposite engineering to develop ultrahigh-performance, low-cost Li-ion cathode materials
Molecular guest exchange and subsequent structural transformation in CH4-CO2 replacement occurring in sH hydrates as revealed by 13C NMR spectroscopy and molecular dynamic simulations
No full textThis study adopted experimental and computational approaches to investigate CH4 - CO2 replacement in the structure H (sH) CH4 + methylcyclopentane (MCP) hydrate for its dual functions of CH4 recovery and CO2 sequestration. Hydrate phase equilibria, 13C NMR spectra, and molecular dynamics (MD) simulations of CH4 + MCP - CO2 replacement were examined and compared with those of CH4 + neohexane (NH) - CO2 replacement to elucidate the molecular guest exchange behaviors in both systems. The structure I (sI) hydrates were ther-modynamically favored in CO2-rich gas mixtures (CH4 + CO2 + MCP systems), and a structural transformation from sH to sI occurred when CO2 composition in the feed gas was higher than 20 %. The 13C NMR spectra indicated that the CO2 molecules preferred to occupy the large (51262) cages of sI and the medium (435663) cages of sH compared to the small (512) cages of both sI and sH during replacement. Following CO2 injection into the sH CH4 + MCP hydrate, the initial sH hydrate transformed to the sI hydrate with CH4 recovery of approximately 78 %. The MD simulations also demonstrated that structural transformation in the CH4 + MCP - CO2 replace-ment would occur at the lower CO2 enclathration ratio compared to the CH4 + NH - CO2 replacement, thereby leading to a lower CO2 concentration in the newly formed sI hydrate. This is the first study to provide both experimental and computational evidence of guest-dependent structural transformation in sH CH4 + liquid hydrocarbons - CO2 replacement
Oligothiophene electron donor and electron acceptor for all small molecule organic solar cells with efficiency over 9%
No full textBy using easily accessible oligothiophene donors and acceptor, all-oligothiophene organic solar cells (AOT-OSCs) were realized. The devices fabricated with thiazole-centered donor Tz6T and quaterthiophene-based acceptor A4T-16 exhibited a satisfactory power conversion efficiencies (PCEs) of 9.30 %. Moreover, the devices based on Tz6T/A4T-16 binary demonstrated an excellent industrial figure of merit (i-FoM) of up to 0.168 which represented one of the best cost-effectiveness in all small molecule organic solar cells, suggesting the strong potential of all-oligothiophene combinations in developing OSCs with both low cost and high efficiency
Meso-pore generating P doping for efficient photoelectrochemical water splitting
No full textHematite (Fe2O3) has been widely used as a photoanode in photoelectrochemical water splitting (PEC) for green hydrogen production. Here, for the first time, we investigate how a simple in-situ phosphorus (P) doping strategy improves the overall PEC performance of hematite with a systematic analysis of the various effects on the PEC performance. By introducing enriched FePO4 regions on the Ti-doped FeOOH surface and subsequent high-temperature annealing via P-doping, meso-porous P,Ti co-doped Fe2O3 (P,Ti-Fe2O3) nanorods were fabricated. P,Ti-Fe2O3 exhibited four-fold and two-fold increased BET surface area and electrical active area, respectively, compared to that of Ti-Fe2O3. Benefiting from the nano-structuring and efficient P doping effects [e.g., increased carrier density (Nd=3.48168 ??1020 cm???3), enhanced charge separation (??bulk= 38.7% and ??surface= 79.1%), and steeper band bending (Wd=3.910 nm)], the resulting P,Ti-Fe2O3 photoanode exhibited 94% improved photocurrent density of 2.50 mA cm???2 compared to that of Ti-Fe2O3 (@ 1.23 VRHE) under 1 sun illumination. With the deposition of the NiFeOx cocatalyst, the NiFeOx/P,Ti-Fe2O3 photoanode exhibited excellent photocurrent density of 3.54 mA cm???2 (@ 1.23 VRHE) with a remarkable cathodic shift (180 mV) of the onset potential, marking the highest value among P doped hematite studies. This study suggests a new paradigm of P doped hematite with mesopores and gradient doping properties affordable in a cost-efficient way, achieving an excellent PEC water splitting performance
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No full textDepartment of ChemistryThis thesis was written based on a paper published in a journal in which I participated as an author. It also included many contributions from collaborators and was credited.
Nobel prize winner, Richard Smalley, mentioned 20 years ago about the 10 problems which humanity will face in the future. Among them, energy and environmental issues were currently facing humanity. Due to the current increase in energy consumption around the world, interest in renewable energy was increasing considerably. Natural resources such as oil, coal, and natural gas, which were responsible for most of the world's energy, are limited in quantity and cause environmental pollution such as climate change in the energy production process. On the other hand, renewable energy was a field to be researched because there was no limit to the amount and there was no significant impact on environmental pollution during production. Since solar energy was produced from the sun, it was free of cost and is the most abundant sustainable clean energy, so it was in the spotlight along with wind energy. Among the many ways to convert solar energy into electricity, the most efficient and direct way was to use a solar cell. Starting with the early crystalline silicon solar cell, research on various next-generation solar cells such as perovskite solar cell, dye sensitized solar cell (DSSC), organic photovoltaics (OPVs) was being conducted. Perovskite solar cells were receiving a lot of attention among several next-generation solar cells because of their high efficiency and advantage of fabricating large-area modules through solution processes. Silicon solar cells achieved a power conversion efficiency of 26.1 % over 50 years, but perovskite solar cells achieved a power conversion efficiency of 25.7 % within 10 years. The metal halide perovskite material used in perovskite solar cells had an ABX3 structure and had the characteristics of high absorption coefficient and long carrier lifetime. A typical perovskite solar cell was fabricated in a stacked form in which uniform perovskite layer were placed between a hole transporting layer and an electron transporting layer. In addition to the perovskite layer, my research during Ph.D. program has been conducted on the important charge transporting layer that affects power conversion efficiency of perovskite solar cell.
The first topic of my research was on a new carbazole-based hole transporting material. I synthesized SFXDAnCBZ, which is a new carbazole-based spiro[fluorene-9,9`-xanthene] (SFX) derivative, where the central core and end-cap units consist of SFX and N3,N6-bis(di-4-anisylamino)-9H-carbazole (DAnCBZ),respectively, as an efficient and low-cost HTM for PSCs with my co-worker. SFXDAnCBZ could be easily synthesized compared to Spiro-OMeTAD and reduce the corresponding fabrication costs. The glass transition temperatures of SFXDAnCBZ and Spiro-OMeTAD was similar and the HOMO energy level of SFXDAnCBZ was higher than that of the Spiro-OMeTAD. PL quenching results and the emission lifetime characteristics of SFXDAnCBZ and Spiro-OMeTAD were similar. We achieved a high PCE value of 20.87 % under 1 sun illumination (100 mW cm-2) for a small cell
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with the surface area of 0.094 cm2 by optimizing the concentration of SFDAnCBZ. This work was published in ACS applied materials & interfaces. The second topic was about interfaces between perovskite layer and electron transporting layer. A typical perovskite solar cell (PSC) composes of six main interfaces including between the perovskite layer and the charge transporting layer, between the charge transporting layer and the electrode and the perovskite grain boundaries. In particular, the interface between the perovskite layer and the charge transporting layer contains hundreds of times higher trap density. This trap density had a significant effect on the power conversion efficiency of perovskite solar cells. And as a proof of this, one of the technologies that recently increased the efficiency of perovskite solar cells is the passivation of the surface of perovskite layer with 2D-perovskite materials. However, passivating the perovskite surface that touches the electron transporting layer was hard because perovskite solution could dissolve the surface treatment agents. so far, we conducted a study on the passivation of interfacial deep-level defects that degrade the power conversion efficiency of devices. The 2 nm thick FASnClx interlayer was introduced between the SnO2 electron transport layer and the halide perovskite layer by bonding Cl-bonded SnO2 with a Cl-containing perovskite precursor. The coherent interlayer formed at the interface between the SnO2 electrode, and the perovskite film has fast charge extraction and minimal charge recombination. This coherent interlayer made it possible to fabricate perovskite solar cells with a certified PCE of 25.5% at standard air mass 1.5 global illumination. This work was published in Nature. Currently, a new metal phthalocyanine hole transporting materials has been synthesized, research on improving the stability and efficiency of perovskite solar cells and semi-transparent solar cells that could be sued on exterior walls of buildings, vehicle roofs, or windows are in progress.clos
Design and Analysis of P-well/Deep N-well SPAD for LiDAR Sensor
No full textDepartment of Electrical Engineeringclos
DESIGNING COVALENT ORGANIC FRAMEWORKS FOR LITHIUM METAL ANODE-BASED BATTERIES
No full textSchool of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))Lithium-ion batteries (LIBs) have become a significant part of our daily life use, providing electricity on the laptops and mobiles that have changed the modern society and now they are converting the transportation mechanism with electric vehicles. They are also expected to be helpful for replacing a method for electricity generation from fossil fuel to renewable energy sources like wind, sea and solar, providing much cleaner and more sustainable energy storage system (ESS).
Despite the remarkable increase in demands, however, the energy density of commercial LIBs couldn???t keep up the growth in sales. Among various alternatives, Li metal is considered as a promising anode material due to its high theoretical specific capacity (~3860???mAh g???1) and lowest negative electrochemical potential (???3.04???V vs. standard hydrogen electrode). Unlike the intercalation of Li+ in layered graphite anode, the Li metal anode shows conversion between Li metal and Li ions showing host-less Li storage behavior. Until now, extensive efforts have been made to realize high energy density Li metal anode-based batteries. However, the issues caused by the growth of Li dendrites during Li deposition and low Coulombic efficiency (CE) is the major reason why practical availability of Li metal was obstructed.
The objective of this thesis if focused on designing the target-specific materials to overcome the issues of Li metal anode-based batteries with covalent organic frameworks (COFs). COFs, which are characterized by lightweight elements, tunable chemical nature, and robust chemical/thermal stability, provide opportunities to construct target-specific materials for each battery system. In Chapter 2, we represented a solvent-free, solid-state single Li+ conducting COF with directional ion conduction pathways through the ordered pores. Removal of solvent and anion was critical to stabilize the Li metal anode, which also provided a new path for COF-based electrolyte. In Chapter 3, we designed COFs for not only anode, but cathode which needs to be considered since battery is orchestrated by crosstalk of anode and cathode. The solution to resolve issues on both cathode and anode was represented on Chapter 3, and the designed COFs presented enhanced cyclability at high-mass-loading of cathode. Therefore, importance of targeting the functionality required on batteries and customizing the molecular structures of COFs will be suggested in this thesis.clos
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No full textDepartment of Chemistryclos