Repository Collection:

Unveiling the Mechanisms in Surface Chemical Reactions of 2D MoS2 for Crystallographic and Chemical Engineering

2025-06-30T12:12:45Z

Title: Unveiling the Mechanisms in Surface Chemical Reactions of 2D MoS2 for Crystallographic and Chemical Engineering Author(s): Younghee Park Abstract: Two-dimensional materials have a layered structure, and all atoms are exposed on the surface. This makes surface chemical reactions one of the most important factors in the study of 2D materials. In particular, in the chemical engineering and crystallographic epitaxial growth of MoS2, which is a representative material of Transition metal dichalcogenides, the surface chemical reaction control plays a large role in controlling the properties of the material. This study focused on investigating the surface chemical reaction of 2H-MoS2 from two perspectives: chemical engineering and crystallographic epitaxial growth. In the chemical engineering, a study was conducted to form a covalent bond directly on the surface of 2H-MoS2 through a photochemical reaction using highly reactive diazonium salt and alkyl halide as reactants, and a reaction mechanism was proposed. In addition, in the crystallographic epitaxial growth of MoS2, a systematic investigation Demonstrated that the growth temperature has a significant effect on the crystallographic orientation of MoS2 during its crystallographic epitaxial growth. The study revealed that the surface terminal group of the sapphire substrate is strongly affected by temperature, which determines the crystallographic orientation selectivity of MoS2. These results are anticipated to be applied to various 2D materials by discovering and clarifying the chemical engineering of the surface chemical reaction of MoS2 and the fundamental mechanism of crystallographic epitaxial growth.

Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy

2025-06-30T12:12:30Z

Title: Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy Author(s): Myungsam Jen Abstract: A chemical reaction refers a series of atomic rearrangements between the reactant and product molecules, where the rearrangements of single chemical bonds occur on the ultrafast time scales of femtosecond and picoseconds. The chemical reactions are generally described by the energy difference between the reactant and products, activation barriers, reaction rates, etc. The reaction rates of a certain chemical reaction can be obtained by experimentally observe the concentration changes of reactant or product molecules. In order to measure the reaction rates of the ultrafast chemical reactions such as intramolecular proton transfer, ultrafast spectroscopic methods are inevitable where ultrashort laser pulses initiate the chemical reactions in the excited state and time-resolved spectroscopy measures the spontaneous concentration changes of reactants and/or products. The reaction dynamics study by the time-resolved spectroscopy would be beneficial in many important applications including artificial photosynthesis, dye-sensitized solar cell, or molecular optoelectronics. Simple photochemical reactions of excited state proton transfers and photoinduced charge transfers with subsequent structural changes in the electron donor or acceptor groups are considered as one of the fundamental processes in many chemical and biological systems. Therefore, the excited-state reaction dynamics probed by time-resolved spectroscopic measurements can be used to understand the ultrafast bond breaking and formation of reactant and product molecules, or the or the instantaneous structural changes accompanying various excited state photophysical processes. In this thesis, transient absorption spectroscopy and femtosecond stimulated Raman spectroscopy (FSRS) has been adopted as the time-resolved electronic and vibrational probes, respectively. Time-resolved electronic and vibrational probes would be complementary to each other in analyzing the ultrafast photophysical and photochemical processes in the excited state. The structural changes of 1,2-dihydroxyanthraquinone (alizarin) in dimethyl sulfoxide (DMSO) solution upon the excited-state intramolecular proton transfer (ESIPT) reaction have been studied by FSRS measurements. Previous time-resolved electronic measurements may provide the detailed excited-state dynamics upon the proton transfer. However, the detailed structural changes of alizarin or related molecules upon the ESIPT has not been reported. The ESIPT dynamics of alizarin of 70-80 fs has been observed from the vibrational modes of ν(C═C) and ν(C═O) in the singlet excited state, where the population changes and opposite peak shifts of these modes occurring with the common 70-80 fs time constant are compatible to the proposed transition state of six-membered ring structure with intramolecular hydrogen bonding between the carbonyl and adjacent hydroxyl group. The ESIPT dynamics of alizarin has been updated by improving the temporal resolution of FSRS measurements with the pulse compression of actinic pump. The vibrational probe of ν(C═C) and ν(C═O) modes in the excited state have provided more detailed ESIPT dynamics (110 fs) of alizarin with the population transfer between the locally-excited (LE) and proton-transferred state, and the coherent oscillation signals in these vibrational probes show that the reaction coordinate of the ESIPT reaction is strongly coupled to several low-frequency vibrational modes of intramolecular hydrogen bonding. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC) are strongly changed upon the ESIPT of the solute alizarin. Although the vibrational modes of DMSO are inseparable from the nonlinear cross-phase modulation and long-lasting hot ground state Raman signals, the ν(S=O) and ν(CSC) modes of DMSO shows instantaneous (60-120 fs) increase in the “free (isolated)” or “aggregated (dimer)” bands indicating the solvation changes of DMSO upon the ESIPT of alizarin. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC), may “probe” the ultrafast chemical reactions of the solute indirectly since these modes are very sensitive to the instantaneous solvation changes resulting from the structural changes of the solute. The photoinduced charge transfer dynamics of curcumin in DMSO solution has also been investigated by FSRS measurements. Curcumin is one of the well-known antioxidants, and shows ultrafast intramolecular charge transfer (ICT) in the excited stat, where the excited state lifetime and fluorescence quantum yield are strongly dependent on solvent polarity and hydrogen bonding with solvent. The vibrational modes of curcumin in the LE and charge-transferred (CT) states are separately observed from the FSRS measurements, where an ultrafast ICT (0.6-0.8 ps) and subsequent vibrational relaxation (6-9 ps) dynamics in the CT state have been retrieved. The ground-state vibrational modes of curcumin, ν8a and ν(C=C,C=O) and the solvent vibrational modes of DMSO, ν(CSC) and ν(S=O) appear strongly coupled to the ICT dynamics of curcumin, which supports the strong solvation interactions including the hydrogen bonding. Especially, the ν(CSC) and ν(S=O) modes of DMSO represent the ultrafast (20-50 fs) dynamics with the hydrogen-bonded species upon the ICT of curcumin. However, further explorations on the detailed spectral changes between the “free” and “hydrogen-bonded” species in the ν(CSC) and ν(S=O) mode of DMSO are required to explain the solvation changes of DMSO in the solvation shells with the ultrafast ICT dynamics of chromophores. Lastly, up-to-date investigations and future directions for the solvation dynamics study with the ultrafast excited-state processes of chromophores are summarized. All the FSRS measurements described in the thesis are based on the stimulated Raman “gain” measurements, where the Raman pump centered at higher frequency and the Raman probe with the lower frequencies complete the stimulated Raman process. Similarly, the stimulated Raman “loss” measurements requires the Raman pump at lower and the Raman probe at higher frequencies. It has been known that the modulations of the Raman probe by the Stokes and anti-Stokes Raman processes in the stimulated Raman “loss” measurements are observed in the opposite contribution while the Stokes and anti-Stokes Raman signals are inseparable in the stimulated Raman “gain” measurements. We propose the possible separation of the long-lasting thermal signals in the solvent vibrational modes of DMSO (observed in the excited-state dynamics of alizarin and curcumin) by combining the stimulated Raman “gain” and “loss” measurements. The “hot” ground-state transitions of the ν(S=O) and ν(CSC) modes of DMSO by the anti-Stokes Raman process can be subtracted with some experimental control between the stimulated Raman “gain” and “loss” measurements. Similarly, the nonlinear cross-phase modulation artifacts can be minimized by the control experiment only with the solvent. Further experimental developments are required, however, for more accurate determination of the solvation dynamics of DMSO, which may indirectly “probe” the ultrafast chemical reaction dynamics of chromophores in the excited state including the proton and charge transfer. In this thesis research, the ultrafast chemical reactions of intramolecular proton and charge transfers have been observed by time-resolved electronic and vibrational spectroscopy. FSRS presents numerous advantages in the reaction dynamics study in the excited state due to its high temporal and spectral resolutions and multimodal vibrational probes in a wide spectral range. The ultrafast chemical reaction dynamics in the excited state can also be probed by the instantaneous changes in the solvent vibrational modes of DMSO including the hydrogen bonding interactions when the strong solvation dynamics exists between the chromophore and solvent molecules.

The Study of Interface Dynamics and Surface Termination Effect Realizing Large-Scale Epitaxial Growth of 2D MoS2

2025-06-30T12:11:52Z

Title: The Study of Interface Dynamics and Surface Termination Effect Realizing Large-Scale Epitaxial Growth of 2D MoS2 Author(s): 안채현 Abstract: Two-dimensional (2D) molybdenum disulfide (MoS2) shows a wide range of application possibilities due to the unique properties exhibited at the monolayer thickness. Therefore, many efforts have been performed to grow 2D MoS2. However, different growth results attributed to the various parameters still remain a challenge for its applications. In this dissertation, I focused on interfacial correlations to analyze the variations of growth and characteristics of MoS2. Especially, I observed the different growth behaviors exhibited by controlling the interfaces in chemical vapor deposition method. Chapter 1 introduces the overall characteristics exhibited at the 2D MoS2 including crystal structure and various properties. Furthermore, the various synthesis methods that have been applied to obtain 2D MoS2 is introduced. Chapter 2 investigates the influence of interaction between interface and precursors by utilizing the inorganic molecular precursor, MoOCl4 and H2S. Chapter 3 explores the effect of interfacial water existed on the surface, where it affects the optical properties of 2D MoS2. Chapter 4 investigates the effects of surface termination of sapphire depending on the temperature and growth environment, where MoS2 shows different crystallinity and grain size. These findings offer deeper insight into the interfacial dynamics between MoS2 and various interface structures, highlighting promising directions for optimizing interfacial structures. Deliberated investigation the growth behavior and properties of MoS2 relative to interfacial states will likely contribute significantly to advancing applications across a range of 2D materials beyond 2D MoS2.

The Research for the Cathode Catalyst Layer to Enhance the Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells

2025-06-30T12:11:26Z

Title: The Research for the Cathode Catalyst Layer to Enhance the Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells Author(s): Do-Hyung Kim Abstract: This thesis describes the analysis of the cathode catalyst layer (CCL) of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). This study examines the resistance analysis of the commercial cathode by the distribution of relaxation time (DRT) method of electrode half-cell. The five peaks were observed in the DRT plot separate the resistances by frequency: Peak 1and Peak 2 (0.1 – 102 Hz) represent the charge transfer resistance of oxygen reduction reaction, and Peak 3 (102 – 103 Hz) represents the resistance for proton conductive loss. Peak 0 (< 0.1 Hz) appears due to produced H2O between electrolyte and cathode GDE and Peak 4 (103-104 Hz) appeared from the MEA is assigned to the anode reaction. The DRT analysis indicated that the phosphoric acids in the CCL of the HT-PEMFC enhance proton conduction to improve reaction kinetics (positive effect) bu poison the Pt electrocatalyst and limit the oxygen transport (negative effect). To mitigate the mass transport issue, hydrophobic polymers such as polytetrafluoroethylene (PTFE) are used in the CCL using polyvinylidene fluoride (PVDF) as a binder. Microscopy and contact angle analysis indicated that the PTFE dispersed evenly on the carbon surface and CCL became mor hydrophobic as PTFE content increased. A maximum MEA performance was dbtaied with 6% PTFE. Since the hydrophobic PTFE repels the phosphoric acid within the CCL, the additive including the PTFE made the oxygen passage. Finally, polymers containing phosphonic acid groups were used as a binder. The cathode using the phosphonic acid polymers showed lower performance than the cathode using the PVDF suggesting the hydrophobicity is the predominating factor for HT-PEMFC performance. This is because the commercial membrane used in this research has a high amount of phosphoric acid. This research suggests that advanced membranes with a lower amount of phosphoric acid may perform better with phosphonic acid ionomers.