Broadband Sum Frequency Generation Spectroscopy Of Ammonium Pyrrolidine Dithiocarbamate Monolayers Self-Assembled On Gold And Copper Surfaces

The self-assembled monolayers(SAMs)on metallic surfaces play important roles in chemistry,biology,energy science and many other fields.Molecular orientation,electronic configuration and molecule-substrate interaction are significant to the design of SAMs.Broadband Sum Frequency Generation(BB-SFG)is a coherent second order nonlinear technique with surface/interface selectivity and high sensitivity.BB-SFG spectroscopy is an ideal tool for interfacial analysis which can provide tremendous surface information including the chemical structure,molecular orientation,electronic configuration and substrate surface conditions.Vibrational dephasing time(T2)can be used to explore the interactions between the interfacial species and surrounding environment.Therefore,it is significant to investigate the SFG spectral lineshape and vibrational dephasing behavior to understand the interfacial properties of SAMs.Here,the thesis takes the SAMs of APDTC on Au and Cu surfaces as model systems to analyze the impacts of substrate electronic configuration on the spectral lineshape of APDTC resonant bands and investigate the molecule-substrate interaction on the dephasing process with BB-SFG.The main results of this thesis are as follows:1.The spectral lineshape of APDTC resonant bands on Au and Cu surfaces has been studied by BB-SFG.The impacts of substrate electronic configuration on the spectral lineshape were analyzed.The main conclusions are as follows:(1)On Au surface,APDTC resonant SFG bands are dips/valleys(negative peaks)superimposed on the broad SFG band of the substrate,while on Cu surface,the resonant bands are positive peaks.The different lineshape was from different relative phase between the resonant bands and substrate,π for Au surface and 0 for Cu surface.(2)The results of X-ray diffraction(XRD)show that the surface of APDTC/Cu is oxidized,APDTC/Au surface is not oxidized.Therefore,the difference of SFG spectral lineshape may be related to the changes of electronic structure caused by metal oxide.The absolute phase difference between the Cu oxidation layer and Au surface caused lineshape changes of APDTC on Cu and Au surface.(3)The adsorption configuration of APDTC on Au and Cu surfaces:they were both adsorbed through the terminal sulfhydryl groups.However,the adsorption configurations of APDTC on Cu surface is affected by surface oxidation.2.The vibrational dephasing behavior of APDTC on Au and Cu surfaces was studied by time-resolved BB-SFG.The influence of molecule-substrate interaction on the vibrational dephasing was explored.The main conclusions are as follows:(1)The dephasing times(T2)of the methylene stretching vibrations are similar under the same VIS wavelength.This is because the dephasing process mainly depends on the interaction between molecule and substrate,and the methylene groups of the pyrrole ring are under similar influence of the substrate surfaces.In addition,T2 of different vibrations varied with the VIS wavelength similarly.(2)In APDTC/Au,the dephasing time T2 is independent of the VIS wavelength when the sp→sp transition dominant on the Au surface.When the d→sp transition occurs,T2 increases with the increase of VIS wavelength.This result is consistent with previous studies of vibrational dephasing on Au surface.(3)In APDTC/Cu,the dephasing time T2 is proportional to the density of states(DOS)of Cu surface.That is,the dephasing time increases with the increase of DOS.