Preparation Of Noble Metal Film On Silicon Substrate, Morphological Control And Its Property Study

This thesis describes the preparation of noble metal films on silicon substrates, morphological control and the studies of their properties such as optical,electrical and catalytic properties.The properties of the films and metal deposition mechanism were investigated by using electrochemical methods involving cyclic voltammetry(CV),an electrochemical direct current polarization method and open circuit potential-time (Ocp-t)technique,scanning electron microscopy(SEM),atomic force microscopy (AFM),X-ray photoelectron spectroscopy(XPS),X-ray diffraction measurement (XRD),Raman spectroscopic analysis,etc.Some important results obtained are described as follows:1.The microcontamination process of silver onto p-type crystalline silicon(111)in a solution of 0.01 mol/L AgNO₃ at room temperature was investigated by studying the anodic stripping behavior using cyclic voltammetry(CV).This result shows that the rate of Ag deposition is rapid and that deposition is almost fully accomplished within 1 s.Calculating the surface coverage(F)for 1 s,10 min,or 1 h immersion based on the CV curves demonstrated that the silver layer was only a monolayer.(See Chapter 2)2.Electroless silver deposition onto p-silicon(111)from 0.005 mol/L AgNO₃ solutions with different HF concentration was investigated by using an electrochemical direct current polarization method and open circuit potential-time (Ocp-t)technique.The fact that three-dimensional(3D)growth of silver onto silicon is favored with increasing the HF concentration was ascribed to the drop of the surface energy and approved by the two electrochemical methods and atomic force microscopy(AFM).The drop slope of open circuit potential,K_{-ΔE(OCP)/t},was educed from the mixed-potential theory.K_{-ΔE(OCP)/t}as well as the deposition rate determined by an inductively coupled plasma atomic emission spectrometry(ICP-AES), increased with the HF concentration,yet was not a linear function.Results were explained by the stress generation and relaxation mechanisms.(See Chapter 2)3.Via electroless metal deposition,well-defined silver dendrites and thin porous silicon(por-Si)layers were simultaneously prepared in ammonia fluoride solution containing AgNO₃ at 50℃.A self-assembled localized microscopic electrochemical cell model and a diffusion-limited aggregation mode were used to explain the growth of silver dendrites.The formation of silver dendritic nanostructures derives from the continuous aggregation growth of small particles on a layer of silver nanoparticles or nanoclusters(Volmer-Weber layer).Thin and homogeneous nanostructure por-Si layers displayed visible light-emission properties at room temperature.The investigation of the surface-enhanced Raman scattering(SERS)revealed that the film of silver dendrites on por-Si was an excellent substrate with significant enhancement effect.(See Chapter 3)4.Hierarchical silver dendritic nanostructures were prepared at room temperature via a simple replacement reaction by dropping a droplet of HF-AgNO₃ solution on silicon wafers.The morphological diversity of the resulting dendritic structures was observed.Open circuit potential-time(Ocp-t)curve was used to record in situ the variation of the open circuit potentials of silicon/electrolyte,which reflected the structural evolution process.The crystal structures of the deposits were characterized by X-ray diffraction(XRD).Results were interpreted in term of the theory that orientation selection in dendritic evolution is determined by the anisotropy of the solid-liquid interfacial energy and the oriented attachment-based aggregation mechanism.The silver dendrites show significant SERS effect as probed with Rhodamine B(RB).The ease of the approach to prepare SERS-active substrates makes it very promising in sensing and detection oriented applications.(See Chapter 3)5.Gold films with well-defined dendritic nanostructure on a silicon substrate were firstly fabricated by using electroless deposition technique from aqueous HF solution containing HAuCI4 at 50℃.And then the gold films were self-assembled with 2-naphthalenethiol.Using potassium ferricyanide as an electron transfer probe,the electrochemical behavior of the resulting monolayers showed that 2-naphthalenethiol self-assembled monolayers(SAMs)failed to block the electron transfer(ET)between the gold film and potassium ferricyanide.Electrochemical desorption of SAMs in 0.5 mol/L NaOH solution revealed that 2-naphthalenethiol molecules were adsorbed on gold films by chemisorption resulting in forming Au-S bonding.Surface enhanced Raman scattering spectroscopic observations confirmed that the SAMs adsorbed on Au dendrites had significant enhancement effect.(See Chapter 3)6.Volmer-Weber(VW)film and dendritic nanostructure film of W-doped Ag were prepared by using electroless deposition from hydrofluoride solution.Compared with the deposition of the corresponding structure of only silver,the results show that the growth of silver is leading and can not be changed essentially by tungstate ions in the Ag-W binary system.A doping mode of W element was proposed,i.e.,the doping of W may occur during silver deposition through chemisorption—chemical bonding of oxygen atoms of tungstate dimer with silver.Cyclic voltammetry was employed to determine the chemical bonding energy between silver and oxygen.Annealing at high temperature in air revealed that the Ag-W composites had good anti-corrosion in air. (See Chapter 4)7.Pd/WO₃ composite film on indium tin oxide(ITO)glass was prepared by electroplating in a solution of 50 mmol/L tungsten-peroxo complex+5 wt.%SDS with 5 mmol/L PdCl₂.The structure and composition of the synthesized Pd/WO₃ composites were characterized by scanning electron microscopy combined with energy dispersion spectroscopy,X-ray photoelectron spectroscopy and X-ray diffraction.The promotional effect of WO₃ for the electrocatalytic activity for hydrazine oxidation was investigated.The peak current of Pd/WO₃ ITO glass electrode reached up to 106μA,which was 1.7 times as high as that of a Pd-ITO glass electrode(62μA)under the same working conditions.It might be mainly attributed to the good dispersion of palladium nanoparticles on WO₃ supports.(See Chapter 5)8.An electrodeposition process using cyclic voltammetry(CV)for preparing Ag-W films on p-type silicon(100)wafers is described.The electrochemical behaviors and microstructural properties of Ag-W deposits were compared in different concentration of sodium dodecylsulfate(SDS).It was found that SDS functioned as precipitation action to silver ions and promoted strongly the microstructure of Ag-W films as its concentration amounted to 5 wt.%.X-ray electronspectroscopy(XPS)investigation demonstrated that the concentration of tungsten was 3.4%and the O/W atom ratio was about 3.0 for Ag-W deposits.XPS and X-ray diffraction(XRD)measurements certified that the silver coating was not corroded at temperatures up to 350℃in air.Finally,the resistivity of the films with the CV cycle number was analyzed.(See Chapter 5)...