Nonlinear Spectroscopy Studies On The Dynamics Of Ligand Structures At The Electrolyte-aqueous Interface And On The Surface Of Semiconductor Nanocrystals

Interfaces are ubiquitous in nature.A wide range of chemical,environmental,biological and catalytic processes are carried out at the interface.Understanding the interfacial structure and dynamics from the aspect of microscopic molecular level is of great significance for analyzing the interface processes.Conventional experimental technologies for interface studies lack interface specificity and time resolution(10-12-1015 s),and the development of nonlinear spectroscopy provides a suitable means for understanding the interfacial structural dynamics at the molecular level.Based on this,this dissertation uses the nonlinear spectroscopy,namely the sum frequency generation(SFG)spectroscopy and ultrafast infrared spectroscopy,to study the interfacial microstructure of air/electrolyte aqueous solutions and the structural dynamics of the surface ligands of semiconductor nanocrystals(NCs).The dissertation mainly consists of the following three sections:In the first section:Understanding the interfacial microstructure of aqueous electrolyte solutions is important to marine chemistry,electrochemistry and biological environments.The adsorption behavior and molecular orientation of SCN-anions at the interface of air/aqueous solutions were investigated by SFG spectroscopy.The molecular orientation of SCN-anions at the air/sodium thiocyanate(NasCN)aqueous solution interface was determined to be around 44° from the interface normal by quantitative polarization analysis,and the molecular orientation at the interface remained unchanged with the bulk concentration varying from 0.5 mol/kg-13 mol/kg.Based on the Langmuir adsorption model,the free energy of adsorption of SCN-anion at the air/water interface was determined to be-1.53±0.04 kcal/mol.In addition,a new SFG peak was observed at the air/15 mol/kg NaSCN aqueous solution interface.DFT calculation further indicates that SCN-anions may form an ion clusteing structure at the air/water interface.The subtle Na+and K+counter-cation effects on the interfacial structure of the SCN-anions at the aqueous solution interface were also observed,which showed that ion cooperativity plays an important role in affecting the interfacial structure of ions at the air/water interface.In the second section:The microstructure of surface ligands has vital influence on the synthesis,photoelectric and magnetic properties of semiconductor NCs.Zinc Selenide(ZnSe)NCs with different sizes were synthesized,5.6 nm,6.7 nm and 8.5 nm,respectively.The surface ligands modification of ZnSe NCs was performed by SCNanion using ligand exchanging method.The CN stretching peak of SCN-anions bound to ZnSe NCs was blue shifted from 2055 cm-1 to 2080 cm-1.Further ultrafast infrared spectroscopy experiments showed that the vibrational lifetime of stretching of SCNanions was significantly shorter,indicating that SCN-anions were bound to Zn2+ on the ZnSe NCS surface and were affected by electronic coupling and vibrational coupling.The rotational time constant of SCN-anion surface ligands increased from 6±1 ps?18 ± 2 ps to 32 ± 3 with the increase of ZnSe NCs size,indicating that the ligand molecules were in a confined environment on ZnSe NCs surface.In addition,vibrational energy transfer between ligands on the surface of ZnSe NCs was also observed by isotope labeling method.In the third section:three kinds of lead halide perovskite(CsPbX3,X=Br,Cl/Br,Br/I)NCs were synthesized,and the surface of NCs was modified by SCN-anion with infrared activity.Photoluminescence(PL)spectra showed that the PL intensity increased by 10%,220%and 30%,respectively,after SCN-anion ligands modification,indicating that SCN-anion can passivate surface defects significantly.The PL lifetime decay curves of CsPb(Br0.5Cl0.5)3 NCs after ligand modification showed significant differences.The radiation recombination lifetime of CsPb(Br0.5Cl0.5)3 NCs after ligand modification was much longer,increasing from 7.1 ns to 7.4 ns.However,the transient absorption(TA)spectra showed similar exciton bleaching decay curves after ligands modification,which may be caused by the different contribution of electrons and holes.FTIR spectra showed that SCN-anions on the surface of NCs had three different CN stretching modes,and the vibrational absorption peaks were located at 2018 cm-1,2042 cm-1 and 2165 cm-1,respectively,and the ligands modification mainly replaced the original oleylamine(OAm)ligands on the surface of NCs.Further ultrafast infrared spectroscopic measurement of the vibrational peak at 2042 cm-1 showed that the vibrational lifetime of free SCN-anions in the solution was 2.0 ± 0.3 ps,while that of SCN-anions bound to the surface of NCs was 8±2 ps.These results indicate that SCN-anions interact with the surface of NCs,that is,SCN-anions bound to the surface of NCs in a chemisorption,and may bound to Pb2+on the surface.