Applications of Ultrafast Spectroscopy in Charge Transfer and Interfacial Reactions Open Access

Liu, Qiliang (Spring 2020)

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Ultrafast lasers have been widely applied in various fields. As a physical method, the ultrafast laser can be utilized to study many chemical processes, such as the kinetics of chemical reactions, chemical reactions at electrode-solution interfaces, and other reactions that happen on an ultrafast time scale. In the last 4 years of my scientific research, I worked on several projects in the group using multiple ultrafast techniques. To study how the photoluminescence of a quantum nanorod is quenched on the single-molecule level, time resolved photoluminescence (TRPL) combined with atomic force microscopy (AFM) was applied to collect emitted photons from nanorods with and without contact to an AFM tip. To investigate chemical reactions in an optical cavity, transient absorption spectroscopy was performed to probe the exciton kinetics of fluorescent dye molecules trapped in a Fabry-Perot cavity. Lastly, to study the behavior of heterogeneous electrocatalyst molecules at an electrode-electrolyte interface, vibrational sum frequency generation (SFG) spectroscopy was performed to probe a monolayer of the catalytic molecules adsorbed on the electrode under a certain potential window. These projects reflect the broad applicability of ultrafast laser spectroscopic techniques on modern physical chemistry research. 

Table of Contents


1. Introduction 1

1.1 Time Resolved Single-Molecule Fluorescence 1

1.1.1 Time-Correlated Single-Photon Counting 1

1.1.2 Atomic Force Microscope 3

1.2 Transient Absorption Spectroscopy 5

1.3 Vibrational Sum Frequency Generation Spectroscopy 6

2. Single Nanorod Fluorescence Quenching by an AFM Probe 9

2.1 Introduction 9

2.1.1 Background 9

2.1.2 Purpose and Expectation 11

2.2 Experiment Methods 12

2.2.1 Synthesis of CdSe/CdS dot-in-rod 12

2.2.2 Sample Preparation 13

2.2.3 Single-Molecule Quenching Experiment Setup 14

2.3 Results and Discussion 18

2.4 Conclusion 29

3. Molecules in Optical Cavity with Light-Matter Strong Coupling 30

3.1 Introduction 30

3.1.1 Background 30

3.1.2 Purpose and Expectation 33

3.2 Experiment Methods 34

3.2.1 Optical Cavity Fabrication 34

3.2.2 Transient Absorption Spectroscopy Measurement 34

3.2.3 Time Resolved Fluorescence Measurement 35

3.3 Results and Discussion 35

3.4 Conclusion 43

4. Investigation of a Novel Rhenium Disulfide Electrocatalyst by Vibrational Sum Frequency Generation Spectroscopy 46

4.1 Introduction 46

4.1.1 Background 47

4.1.2 Purpose and Expectation 48

4.2 Experiment Methods 48

4.2.1 SFG Measurements 48

4.2.2 Orientation Extraction from SFG spectra 50

4.2.3 Sample Preparation 51

4.2.4 Electrochemical SFG Measurements 52

4.2.5 Impedance-Potential Measurements 53

4.2.6 Sample Information 53

4.3 Results and Discussion 54

4.3.1 Homodyne and Heterodyne SFG spectra of ReSS SAM in Air 54

4.3.2 Electrochemical Vibrational Stark Shift 56

4.4 Conclusion 62

5. Summary 63

6. Reference 64

List of Figure and Table


1.1 Diagram of a typical TCSPC module. 3

1.2 A simplified AFM schematic of tapping mode. . 5

1.3 Pump-probe transient absorption spectroscopy diagram. 5

1.4 A typical near IR – visible reflective SFG geometry. 8

2.1 Schematic of the single-molecule correlated AFM and fluorescence setup. 12

2.2 Schematic of the single-molecule fluorescence measurement setup. 15

2.3 a), b) and c) Description of AFM tip and excitation beam focus alignment procedures. d) A closed scan stage wrapped by the cling film with the nitrogen purge system. 18

2.4 a) TEM photography of CdSe/CdS dot-in-rod nanorod; b) Absorption and emission spectrum of CdSe/CdS nanorod. Absorption under 500 nm was from the CdS rod; two weak peaks at 570 nm and 605 nm was CdSe seed absorption. 19

2.5 a) AFM image of a single CdSe/CdS dot-in-rod nanorod. b) Large size AFM image for alignment. c) Correlated PL image with b). 20

2.6 a) Integrated PL intensity and PL lifetime trajectories. Free state is marked black, the contact state was marked red. Line indicted PL intensity trajectories while dots indicated PL lifetime trajectories. b) PL decay curve at free and contact state at seed and rod tip location. c) The corresponding points markers in AFM image, associated with contact position. 21

2.7 a) Integrated mean PL intensity and lifetime trajectory. b) Normalized PL intensity trajectory. c) Normalized PL lifetime trajectory. d) The CdSe/CdS nanorod AFM image after first scan with Pt contact, before second backward scan. e) The degraded nanorod AFM image. f) Diagram of the normal shape of the nanorod. g) Diagram of the degraded shape of the nanorod. h) PL intensity and lifetime trajectory when degradation happened. Blinking behaviors increased remarkably. 23

2.8 a) Simulation model. b) Simulated PL lifetime trajectory with respect to different Pt contact position with different electron trapping rate kpt. Seed was set at 25 nm. kpt = 0 means free state. c) single exponential decay with different kpt. 25

2.9 a-d) Arshad’s work on this project. e-h) Banin’s group published results. 28

3.1 Interpretation of hybrid light-matter states. 31

3.2 Photonic cavities forms: a) Fabry-Perot cavities, b) resonators, c) Bragg gratings or photonic crystals; Plasmonic cavities forms: d) trapped standing wave or surface plasmon, e) and f) localized surface plasmon, g) nano antenna. 31

3.3 Diagram of the sandwich structure molecules in optical cavity. 34

3.4 a). Extinction curves of different single layer sliver on glass. b) Transmission curves of optical cavity by spin-coating different concentration PVA solution. 37

3.5 a) R6G absorption spectrum in water. b) Transmission curves of 5 mM R6G in optical cavity. The cavity spacing distance was determined by a series concentrations of PVA solution. c) The PL decay curves of the samples in b). d) Cavities made by 25 mm  25 mm glass substrate with same conditions as in b). All the samples were off-resonance. 39

3.6 a) Optical cavities fabricated with 4.5% PVA solution and various concentration of R6G from empty cavity to 5 mM. b) Linear relationship between Rabi splitting and sqrt of R6G concentration. 40

3.7 TAS of the R6G in cavities. a), b), c), d), e), f), g) 500 nm pump; h), i), j), k), l), m), n) 550 nm pump. The spectral shape changed with the shift of cavity transmission. 44

3.8 a) TAS of R6G molecules in 4.5% PVA polymers film outside of the cavity, pumped by 500 nm. b) TAS of free R6G molecules in ethanol, pumped by 500 nm. 45

4.1 Experimental setup scheme of high repetition rate sum frequency generation spectroscopy. 49

4.2 Diagram of electrochemical SFG in chamber cell geometry. 52

4.3 a) Re(SSbpy)(CO)3Cl molecule structure. b) FTIR spectrum of ReSS in KBr pellet. c) UV-Vis absorption spectrum of ReSS in acetonitrile. 54

4.4 a) Scheme of SFG measurement in the air. b) homodyne SFG spectrum with fitting result (top) and heterodyne SFG spectrum (bottom). 55

4.5 a) Cyclic voltammogram associated with potential dependent SFG scans. b) First cycle potential dependent SFG scans. 56

4.6 a), b) and c) are spectra, frequency and amplitude fitting results of second cycle potential dependent SFG respectively. d), e) and f) are spectra, frequency and amplitude fitting results of second cycle potential dependent SFG respectively. 58

4.7 a) Fitting results of non-resonant signal in 2nd cycle of electrochemical SFG scans. b) Fitting results of non-resonant signal in 3rd cycle of electrochemical SFG scans. c) Capacitance-potential curves. 59


4.1 Fresnel Factors of homodyne (IR AOI = 40°) and heterodyne (IR AOI = 55°) SFG. 51

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