Nonlinear Spectroscopic Study On Surfcae Molecular Structures Of Mesoscopic-Scale Materials And Vibrational Coupling Behavior

Micrometer/nanometer is an important geometric scale.There are various such examples in nature and human society,for example,cells,micelles,metal nanoparticles and liposomes.For mesoscopic-scale（micrometer（?m）or nanometer（nm））materials,there exist surface effect,small size effect,and microscopic quantum effect,which could affect their performance.Due to the surface features（such as high surface area,easy to be functionalized,etc.）,these mesoscopic-scale materials can widely be used in many fields,like biomedicine,chemical catalysis and materials engineering.Especially in the area of biomedicine,a variety of photothermal,photodynamic and targeted nanomedicines have been developed,involving many physicochemical processes such as transportation,signal transduction,adsorption and desorption,which can be used for diseases diagnosis and treatment.Therefore,it is very critical to reveal the surface structures of these materials.Mesoscopic-scale materials are irregular and have complex curved surfaces.In the past decades,a series of characterization techniques have been used to study the mesoscopic-scale materials to obtain the structural information,such as scanning electron microscope（SEM）,transmission electron microscopy（TEM）,x-ray diffraction（XRD）,attenuated total reflectance-fourier transform infrared spectroscopy（ATR-FTIR）and surface enhanced raman scattering（SERS）,etc.However,these traditional characterization techniques are not the ideal techniques to characterize the surfaces or interfaces of the mesoscopic materials because of the lack of surface and interfacial selectivity.In recent years,sum frequency generation（SFG）vibrational spectroscopy,as a second-order nonlinear optical technique,has rapidly been developed and used as a powerful characterization method to probe the surface properties of the mesoscopic-scale materials due to its intrinsic ultra-high monolayer or sub-monolayer sensitivity and surface/interfacial selectivity.It could provide the chemical composition,molecular orientation,orientation distribution and dynamic processes of surface/interfacial molecules non-destructively in real time and in situ.In this thesis,the picosecond（ps）and femtosecond（fs）SFG vibrational spectroscopy techniques were used to study the irregular and complex surfaces/interfaces of the mesoscopic-scale materials and the self-assembled monolayer on the metal substrate.The new concept of total internal reflection SFG（TIR-SFG）using evanescent waves was introduced to detect the complex surfaces of the mesoscopic-scale materials.First,the surface molecular structural information of the polymer particles like polyethylene（PEO）,poly（methyl methacrylate）（PMMA）and poly（benzyl methacrylate）（PBenMA）were detected before and after grinding.Compared with PEO and PMMA,the orientation of phenyl groups in the branched PBenMA changed,indicating that the grinding process changed the surface molecular structures to some extent.Second,for the polystyrene（PS）,PMMA and PBenMA particle samples,the SFG spectra features of spin-coated polymer films are different,indicating that the surface molecular structures were different.Third,for PS and PS-COOH nanospheres with different diameters,the size effect affects the arrangement of surface molecular groups.The above studies demonstrated that the generality of the TIR-SFG method,which can be extended to study the surfaces of all mesocopic-scale materials.Also,the dynamic restructuring processes of the irregularly shaped materials’curved surfaces were studied qualitatively and semi-quantitatively at the molecular level.For the carboxylated multi-walled carbon nanotubes（c-MWCNTs）,the molecular structures of the surface carboxylic functional groups were significantly different in non-polar environments(air,decahydronaphthalene(C₁₀H₁₈))in comparison to those in the polar environment（H₂O）.It was found that in the non-polar environments,the carboxylic groups arranged randomly on the surface of the MWCNTs,resulting in no SFG vibrational signals.While in the polar environment,the carboxylic groups were induced to extend out of the MWCNTs’surfaces by the polar/polar interaction,resulting in detectable SFG vibrational signals.Furthermore,the method of time-dependent SFG signal variation was used to trace the adsorption or desorption dynamics of the toluene molecules on the activated carbon surfaces in real time and in situ.The apparent adsorption/desorption coefficients could be calculated in semi-quantitatively.The study was also extended to the femtosecond time scale besides the above-mentioned geometric scale.Using both the frequency-domain and time-domain measurements of the fs-SFG,the coupling behaviors of aromatic thiol molecules（such as Thiophenl（TP）,Benzyl Mercaptan（BMP）,2-phenylethanethiol（PET））which self-assembled on the gold film substrate could be traced.The coupling behavior between the excited surface free electrons and phenyl ring on the gold surface could be regulated by the methylene group（s）between the phenyl ring and the thiol group.Meanwhile,trichlorophenylsilane（TPS）molecules self-assembled on the silica surface were also detected as a control.Using the linear and quantum beat fitting methods,for TP,there is no methylene group as the insulating barrier,the excited surface free electrons could easily be coupled with the phenyl ring vibration.So it had two kinds of damping modes,rapid damping of surface electrons and coupled phenyl C-H stretching vibration.However,the bridged methylene group（s）in the BMP and PET can substantially affect the coupling between the excited surface free electrons and the phenyl C-H vibration.Therefore,there were three modes,damping of surface free electrons,coupled and uncoupled phenyl C-H stretching vibrational modes.For TPS on silica,because there was no such excited surface free electrons,only the non-resonant phase and the uncoupled mode could be detected.In summary,this thesis demonstrated that TIR-SFG is a powerful technique to study the irregular surfaces of mesoscopic-scale materials,and fs-SFG is a powerful tool to study the coupling process between the molecular vibration and surface excited free electrons.Such experimented studies are useful to understand the physical,chemical,and biological effects of the material surfaces at the molecular level,which can be correlated to the macroscopic properties.Meanwhile,the above-mentioned geometric scale and time scale investigations also promote the application of SFG spectroscopy to a broad range of research fileds.