Theoretical Study Of Molecular Adsorption And Surface-Enhanced Raman Spectra

Surface enhanced Raman spectroscopy(SERS)has high sensitivity and can obtain high-quality Raman signals of surface adsorbed molecules.It can be used for qualitative and quantitative analysis and structural characterization of molecules.It is also a powerful tool to identify molecular fingerprint information.In order to investigate the influence of the adsorption interaction on the SERS spectrum of probing molecules on gold nanostructures,we first used the sulfadiazine(SD)molecule as the probe molecule by using the density functional theory(DFT),and studied the influence of the adsorption configuration and photochemical reaction on the Raman spectrum and SERS spectrum of SD molecule.Secondly,using PMBT as a probe molecule,the effects of adsorption orientation and low energy excited state on SERS spectrum were studied.Finally,the activation mechanism of nitrogen molecule was studied.The specific research results are as follows:(1)We theoretically investigated the relationship between Raman spectra and molecular structures.The configuration and conformation of SD molecule are first calculated.Among four different structures considered here,the most stable configuration is used to do further comprehensive vibrational analysis of the SD molecule.Considering the influence of the solvation on the molecular spectrum,we calculated the solvation effect of the molecule and its three protonated structures.In order to study the adsorption effect,we calculated three different adsorption configurations based on the surface molecule-metallic cluster model.It was found that the molecule was adsorbed by its terminal amino group,and the adsorption characteristic peak comes from the wagging vibration of the terminal amino group,indicating that the wagging vibration of the amino group will shift with the changes of reaction conditions.In SERS measurements,laser can activate molecules adsorbed on metal surfaces and finally induce photochemical reactions.Therefore,we have further studied the coupling reaction compounds of the molecules on the metal surface.The theoretical results support my idea.(2)The mechanism of chemical enhancement of PMBT on metal nanostructures is studied here.At first,the adsorption orientation of PMBT on the surface of Au and Ag clusters is discussed.The results showed that the adsorption of PMBT on Au or Ag mainly adopted an adsorption configuration at the bridge site.When the molecule binded to metal surfaces,there is a local low-lying excited state formed from the Au-S bond.This makes the incident light can match resonant with the local low-lying excited state,leading to the specific Raman peak of PMBT is significantly enhanced.It is also credible to predict the SERS spectrum of PMBT by DFT calculations.(3)The periodic model combined to the density functional theoretical method is used to study the surface properties of the single crystal face of iorn-dopped gold,including the activity and density of states on different single crystal surfaces.According to the adsorption energy of N2 molecule on these single crystal surfaces,the reasonable single crystal surface model are built for further study.Then,the preliminary properties of N2 molecule are explored,including the orbital shape,energy level position,N≡N bond length and bond energy of N2 molecules.Finally,we determined the stable adsorption configuration,free energy,vibrational frequencies and other information for each intermediate on the single crystal surface of the iorn-dopped gold.