Analysis of electronic structure of self-assembled monolayer and Pt interface by doubly resonant sum frequency generation spectroscopy
[Objective]The charge transfer(CT)process is a crucial step in important chemical reactions such as photovoltaic devices,photocatalytic reactions,and industrial corrosion prevention.The interfacial electronic structure has significant impacts on the CT process,and therefore,developing methods to characterize this structure is of great importance.Sum frequency generation(SFG),known for its intrinsically surface selectivity,is a powerful tool to study interfacial electronic structure.In this study,self-assembled monolayers(SAM)on Pt film were used as a model system.By analyzing the wavelength-dependent SFG spectra,the influence of SAM-metal interactions on the electronic structure of the Pt surface was investigated.[Methods]SFG is a second order nonlinear process.In broadband SFG,a femtosecond infrared(IR)beam and a narrowband picosecond visible(Vis)beam are spatially and temporally overlapped on the sample surface,and the output SFG signal radiates in the phase matching direction.When the IR frequency matches the vibrational modes at the interface,SFG signal will be resonantly enhanced.Additionally,when the Vis or SFG resonate with the electronic transitions,the doubly resonant(DR)condition significantly enhances the SFG signal.By scanning the IR and Vis wavelength,interfacial specific vibrational and electronic spectra can be obtained,respectively.The wavelength-dependent SFG spectra were recorded using a spectrograph equipped with a CCD array detector in the PPP polarization combination(P-SFG,P-Vis and P-IR).The SAM/Pt samples were prepared by soaking the e-beam deposited Pt(100 nm thickness on silicon substrates)in 0.1 mmol/L solutions of p-mercaptobenzonitrile(p-MBN)or 4-isothiocyanato-benzonitril(ITC-BN)in ethanol for 24 h,followed by thorough washing before SFG measurements.[Results]In the SFG spectra of the SAM/Pt samples,the nonresonant response of the Pt surface(NR,broadband feature in the SFG spectra corresponding to the electronic response of the metal surface and roughly representing the incident IR spectral distribution)is significant stronger than that of the air/Pt surface.The NR SFG intensity decays with the delay time(time interval between the IR and Vis pulses)much faster than the resonant SFG bands of the SAM molecules(narrow peaks in the SFG spectra).The Vis wavelengths used in our SFG measurements are far from the electronic transitions of the SAM molecules and Pt.Therefore,the NR SFG enhancement is attributed to the additional absorption of the Vis lights by the newly formed interfacial electronic structure induced by the SAM and Pt surface interaction.We attribute this electronic structure as CT state(CTS)at the SAM/Pt interface.By analyzing the wavelength-dependent SFG intensities of p-MBN/Pt in the range of 510-780 nm(Vis),a CTS absorption band from 510 nm to 620 nm was identified.The coupling between the Vis and the CTS caused the enhancement of the SFG signals with DR-SFG effect.Moreover,changing the anchoring group of the SAM from thiol to isothiocyano group significantly altered the trend of wavelength-dependent SFG signal intensities.Hence,the absorption characteristics of the CTS was closely related to interaction strength of the molecular anchoring group and the Pt substrate.[Conclusions]In summary,the interaction between the SAM and Pt surface induced the formation of the CTS at the SAM/Pt interface,which led to extra absorption of the incident Vis or output SFG,and through the DR-SFG effect,the SFG signal from the Pt substrate surface was enhanced.The absorption characteristics of the CTS were determined by the anchoring groups that bonded the SAM molecules to the Pt surface.These results demonstrated the potential of DR-SFG spectroscopy for analyzing interfacial electronic structures and providing in situ characterization of interfacial electronic structure properties for the design of catalysts,electrodes,and photoelectric devices.
doubly resonant sum frequency generationself-assembled monolayerinterfacial electronic structure
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维普
