High Pressure Ultrafast Spectroscopy Study Of Typical Photoelectric Functional Materials

The invention of diamond anvil cells makes it more convenient to investigate the structures and properties of materials in extreme condition, which greatly promotes the further development of high pressure science. Transient absorption spectroscopy technique can be used to detect the transient process of interaction between light and matter, which is a powerful means to study the excited-state dynamical evolution and transient products in the process of photophysical and photochemical. The combination of diamond anvil cells technique with transient absorption spectroscopy technique is expected to open a new window to observe and investigate the transient photophysical process in extreme condition, such as electronic transition, energy transfer, the formation of new species and its evolution with time, etc.We successfully built a high-pressure femtosecond transient absorption spectroscopy system on the basis of diamond anvil cells and transient absorption spectra technique. The characteristics of this system are using the coaxial pump-probe technique to detect the change of the particle number on different energy level under different pressures; using a microscope objective in front of the sample to make the pump and probe beam focus on the sample with a few microns of laser spot, assuring the precision and reliability of the data collected in situ high pressure measurement; using an optical microscope to monitor the diamond anvil cell in real time. The time resolution was achieved by adjusting the relative time delay between the pump and probe beam. In addition, the control software was wrote for time resolution and data acquisition. This high-pressure femtosecond transient absorption spectroscopy system can be used to study the ultrafast photophysical processes of materials under different pressures and provide an effective method for further study of structures and properties of materials under the extreme condition.After the completion of high-pressure femtosecond transient absorption spectroscopy system, the ultrafast photophysical processes of several typical photoelectric functional materials, such as organic conjugated polymer PDPP-F film and norganic monolayer and bilayer MoS₂, were investigated under different pressures using this current system. Then the relationship between structures and physical properties of materials as well as the high pressure effect on the structures and physical properties of materials were illustrated on the molecular level.The photophysical properties of conjugated polymer poly{2,7’-9,9-dioctyl fluorene-alt-5-dieth-ylhexyl-3,6-bis（5-bromothiophen-2-yl）pyrrolo[3,4-c]pyrrole-1,4-dione}（PDPP-F） film was investigated under different pressures using high-pressure femtosecond transient absorption spectroscopy technology, and the pressure effects on the relaxation dynamics of excitons in PDPP-F film were verified. Under ambient pressure, it was found that the excitons in PDPP-F film exhibited only a slow relaxation process, and the corresponding lifetime of exciton relaxation was 172 ps, the slow relaxation process was assigned to the free exciton relaxation. However, the exciton relaxation process became complex after high pressure is applied. A fast relaxation process followed by a slow relaxation process could be recognized, indicating that multiple species participated in the relaxation process under high pressures. The pressure-dependent fast relaxation process was attributed to exciton-exciton annihilation（EEA）, and the slow relaxation process was assigned to the diffusion process of free excitons. The scale of exciton delocalization can be quantitatively estimated according to the percentage of EEA. The delocalization extent of excitons was found to be significantly enhanced with pressure, and the scale of exciton delocalization at 30 GPa was tripled compared with the circumstance at 1 atm. It was also found that both the diffusion speed and the lifetime of excitons increased with pressure, which means that the diffusion length of excitons increased with pressure. This work has important implications for understanding the photophysical property of polymers and improving the efficiency of polymer solar cells.The comparative study on the structure- and layer-dependent photophysical properties of monolayer and bilayer MoS₂ samples were carried out by femtosecond transient absorption system. First of all, the monolayer and bilayer MoS₂ films were deposited on the surface of quartz substrate by means of chemical vapor deposition（CVD）, and the Raman spectroscopy was recorded to identify the layer numbers of the as-prepared MoS₂ sample. Besides, the Raman spectra, steady-state absorption spectra, steady-state photoluminescence（PL） spectra and transient absorption spectroscopy of monolayer and bilayer MoS₂ samples were also investigated respectively. The results reveal that both of monolayer and bilayer MoS₂ samples had A and B exciton absorption. As for the bilayer MoS₂, the absorption peaks of A and B exciton had a slight redshift compared with monolayer MoS₂, which should be attribute to the decrease of quantum confinement effect. After comparing the PL spectra of monolayer and bilayer MoS₂ samples with the absorption spectra, we found that the two PL peaks in monolayer and bilayer MoS₂ were both corresponding to the resonance absorption of A and B exciton. Therefore, the observed PL spectra of monolayer and bilayer MoS₂ were believed to be related to the direct transition luminescence. Interestingly, with the reducing of the layer number of MoS₂ samples, the intensity of PL signal was enhanced, which shoud attribute to the change from indirect band-gap semiconductor（bilayer MoS₂） to direct band-gap semiconductor（monolayer MoS₂）. In addition, from the transient absorption spectra of MoS₂ samples, we found that the exciton decay dynamics of bilayer MoS₂ was faster than that of monolayer MoS₂, suggesting that bilayer MoS₂ sample had faster exciton recombination speed. In the view of photophysical properties, we think that the faster exciton recombination was related to the faster rate of non-radiative transition in bilayer MoS₂. From the perspective of semiconductor characteristic, we think this is because the bilayer MoS₂ had smaller exciton binding energy. Moreover, the bilayer MoS₂, as an indirect band-gap semiconductor, has an additional radiative transition route from the bottom of the conduction band at K points to the top of the valence band at Γ points in brillouin zone. Furthermore, the bilayer MoS₂ has an interlayer coupling effect, leading to the enhancement of excitons vibration. All of these factors will promote the exciton recombination, thus it is believed that the interlayer coupling played an important role in promoting the exciton recombination. In addition, the excited state relaxation dynamics process of MoS₂ under different pressures have been investigated, and the corresponding pressure effects on ultrafast photophysical properties have been well studied. The studies on the layer-dependent photophysical properties and semiconductor characteristics here are expected to provide important basis and guidance for the preparation of photoelectric devices and semiconductor devices based on MoS₂ as well as the research on other transition metal sulfide. Key words:...