Inducement Synthesis Of Nanocrystalline SnO₂ Thin Films By Functionalized SAMs

A new thin film synthetic method was investigated involving preparation of nanocrystalline thin films at low temperature in an aqueous solution. By using the biomimetic systhesis technique and the liquid deposition technique, the nanocrystalline tin oxide thin films were deposited on sulfonated self-assembled organic layers on the silicon and glass substrates at 80oC respectively. The as-deposition films on the SAMs were uniform, dense, with excellent adherence and controlled thickness.Firstly, the growth behavior and structure of self-assembled monolayers (SAMs) formed from 3-mercaptopropyltrimethoxysilane (MPS, (CH3O)3SiCH2CH2CH2SH) on hydroxylated wafers was investigated in the present study. The effects of the alkoxysilane concentration and immersion intervals on the monolayers' formation were characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurement and Atomic force microscopy (AFM). The results suggested that the effect of solution concentration and immersion time on the structure of MPS self-assembled layers on hydroxylated surface was significant. With both low MPS concentration and suitable immersion time, a well-ordered and closely self-assembled MPS layers could be prepared on the hydroxylated surface. The in situ oxidization of the –SH groups to the desired –SO3H groups were carried out by dipping the substrates into the solution of H2O2/HAc. The results of XPS and contact angle measurement revealed that the oxidation of the –SH groups was complete.The short-chain alkoxysilane self-assembled monolayers (SAMs) with sulfonate functional groups on the silicon and glass wafers was firstly used as substrates for the deposition of SnO2 nano-crystalline thin films from aqueous solutions. The structure, phase composition and surface morphology of the deposited tin oxide films was determined via XPS, X-ray diffraction (XRD) and scanning electronic microscope (SEM). The results indicated that uniform SnO2 thin films with densely packed rutile crystallites formed onto the sulfonated SAMs by hydrolysis of SnCl4 acidic solutions at low temperature with an average growth rate of 4 nm/h. The organic SAMs, which promoted the heterogeneous nucleation and deposition of the precursor solution onto the substate surface, had been confirmed an effective inducement for the crystal and growth of the oxide films. These SAMs-oriented SnO2 thin films showed a preferable resistance-temperature relation and high visible transmittance properties, which indicated that these SnO2 thin films were promising materials for application as gas sensors or transparent conductive coatings.