Interfacial Charge Transfer Effect In Metal/semiconductor And Semiconductor/semiconductor Systems Investigated By SERs

As an emerging technology in explorations of interfacial properties,molecular interaction,adsorption behaviors and molecular structure,Surface-enhanced Raman Scattering(SERS)spectroscopy has gain more and more attentions.As is known to all,two mechanisms account for SERS: one is electromagnetic mechanism,which plays a major role in SERS effect;another one is charge-transfer mechanism,which strongly depends on the energy level structure of substrate and molecule.Charge-transfer mechanism is very sensitive to the change of the electron density distribution of the molecule and can indirectly reflect the interaction between the molecules on the substrate surface and the atoms on the substrate surface.Thus,the SERS technique based on charge-transfer mechanism provides a new platform for interfacial charge transfer sensing.In this work,we study the charge-transfer mechanism of metal/semiconductor and semiconductor/semiconductor interface with SERS.To realize our goal,we construct a series of metal/molecule/semiconductor and semiconductor/molecule/semiconductor systems as charge transfer models.In these models,the intermediate molecule acts as a SERS probe.Through the change of charge transfer sensitive band in SERS probe molecules,the interface charge transfer mechanism was studied indirectly.This study is of great significance to deeply understand the mechanism of interfacial charge transfer in practical devices and to promote the application of SERS sensing based on charge transfer mechanism in a wider area.In this thesis,several composite systems have been fabricated and the involved interfacial charge transfer has been investigated with SERS.The main topics and results of this thesis are as follows:1.Charge-Transfer Effect on Surface-Enhanced Raman Scattering(SERS)in an Ordered Ag NPs/4-Mercaptobenzoic Acid/TiO2 System In this work,we fabricate a metal-molecular-semiconductor charge transfer complex composed of Ag NPs,4-mercaptobenzoic acid(MBA)molecule,and atomic level TiO2 by a layer-by-layer self-assembly method.The Ag NPs were fixed on glass first,and then MBA molecules were chemisorbed on the surface of the assembled silver NPs through Ag-S bonds.Then the carboxyl group of MBA molecules reacted with titanium butoxide,forming covalent bonding between the molecules and the Ti(IV)ions on the TiO2 surfaces.That is to say,the order of metal-molecule-semiconductor charge transfer complex constructed in the work is fixed,which provides high order for investigating the CT enhancing mechanism in the metal-molecular-semiconductor system.More notably,unlike the above-mentioned research,the charge transfer system is a two-dimensional nanostructure constructed at the molecular level,where the energy level is relatively fixed.Factoring in the energy of laser light,we can discuss the charge transfer process more accurately.This is the first time that the SERS technology is combined with the UPS in the interfacial charge-transfer investigation,which makes the whole inquiry process more rigorous and provides a new idea for the later exploration of similar phenomena.This work is conducive to investigation of the interfacial CT process in metal-molecular-semiconductor system.2.Charge transfer process at the Ag/MPH/TiO2 interface by SERS: alignment of the Fermi level On the basis of the previous chapter,we fabricate a Ag/molecule/TiO2 assembly over Au NPs and Pt film as models to align the Fermi level of the Ag NPs and to study the interfacial CT effects involved in the Ag/MPH/TiO2 assembly,which further verifies the validity of SERS in CT investigation.Similar to the Ag/MBA/TiO2 system,the preparation procedure of the CT assembly Au/Ag/MPH/TiO2 was based on a layer-by-layer self-assembly approach,which ensures this CT assembly is an ordered two-dimensional nano-structure constructed at the nanoscale level.Compared with MBA molecules,-OH groups in MPH molecules have higher reactivity with tetrabutyl titanate,so we replaced MBA molecules with MPH molecules to ensure that the probe molecule sensitivity to charge transfer process.In this chapter,by changing the energy of laser light,the excitation curves corresponding to the positions of different Fermi levels were explored by SERS and the change of SERS excitation threshold induced by the Ag Fermi level adjustment was observed.We correlate the charge transfer effect in the SERS models with the corresponding level transition,which deepens the understanding of charge-transfer processes in metal-molecule-semiconductor systems and provides new perspectives for the study of charge-transfer processes.Last but not least,the study is also helpful for investigating the CT enhancing mechanism of SERS,where we obtain a more ideal SERS signal and more evident variation in the SERS signal derived from the CT contribution.3.Probing the Interfacial Charge Transfer Process of Uniform ALD Semiconductor-Molecule-Metal Models: A SERS Study In this chapter,based on the traditional chemical assembly method,atomic layer deposition(ALD)and Electron Beam Physical Vapor Deposition(EBPVD)technology are introduced into fabrication process to make the metal/molecular/semiconductor system more orderly and controllable.We fabricated an ordered 2D semiconductor-molecule-metal charge transfer system TiO2/MBA/Ag,TiO2/HfO2/MBA/Ag and TiO2/MBA/Al2O3/Ag on fused silica wafer as SERS charge-transfer models.The MBA molecule works as both a linker between the semiconductor and metal and a SERS probe to reveal how charge transits in our system with SERS spectra.The introduction of 4 nm HfO2 and Al2O3 in the TiO2/HfO2/MBA/Ag and Ti O2/MBA/Al2O3/Ag systems further proves the correlation of change in asymmetric vibration mode peaks of the SERS spectra and the charge transfer process between and TiO2 and Ag,which further promotes the application of SERS technology in exploring interfacial charge transfer.The work in this chapter is of great significance to promote the renewal of photovoltaic model building technology and the application of SERS technology in the charge transfer process of semiconductor-molecule-metal interface.4.SERS as a Probe of Charge-Transfer Process in Coupled Semiconductor Nanoparticle System TiO2/MBA/PbS In this chapter,we applied the SERS technology in exploring dual semiconductor coupling interface charge transfer effect for the first time.We fabricated an ordered coupled semiconductor nanoparticle system composed of Ti O2 NPs,probe molecules and Pb S NPs as a SERS model to study the involved interfacial CT effects.The development of the semiconductor-coupled SERS model TiO2/MBA/Pb S was based on electrostatic interactions described in the literature with only slight modification.Different from the method described previously,we replace the molecule mercaptopropionic acid(MPA)with MBA which functions as both a linker in coupled and a SERS probe to reveal how charge transits in our system.This is the first time that the SERS technique has been used to investigate the dual-semiconductor coupling system.By comparing the changes of the SERS spectra which are more sensitive to the charge transfer,the CT process is discussed.Based on the results of this chapter,it is found that the SERS technique has good sensitivity in the interfacial charge transfer sensing,which paves way for the future application of SERS in interfacial charge transfer.