Preparation and characterization of atomically flat and ordered silica films on a Pd(100) surface
Introduction
Silicon dioxide (SiO2) is extensively used as a catalyst support. For surface scientists it is highly desirable to develop model catalysts consisting of metal clusters or nanoparticles supported on the surfaces of SiO2. However, the insulating properties of bulk silica cause many experimental difficulties, such as surface charging, sample mounting, sample heating and cooling.
In order to circumvent these difficulties and to explore micro-processes on a silica surface (such as nanoparticle growth, surface chemical reaction, and thus induced structure change, etc. [1], [2], [3], [4], [5], [6]) using surface analysis techniques, several methods have been recently developed to synthesize ultrathin SiO2 films, among which two methods are frequently used. One is to directly expose single crystal silicon surfaces to oxygen, as in the case of Si(111)-7 × 7 [1], [2], [3] and Si(100) [4], [5]; the other is to deposit silicon in an oxygen atmosphere or to oxidize a silicon layer on a metal substrate, such as Mo(110) [6], [7], [8], Mo(100) [9], Mo(112) [10], [11], [12], [13], [14], and Ni(111) [15]. Numerous studies have been performed on the preparation and characterization of thin SiO2 films; while a few works are on the growth of ordered SiO2 films. Freund's group and Goodman's group have prepared monolayer crystalline silica films on Mo(112) surfaces. The typical recipe for the growth of SiO2 on Mo(112) consisted of repeated cycles of depositing one-half monolayer of silicon onto a Mo(112) surface at room temperature (RT) followed by oxidation at 800 K. The resulting SiO2 films were subsequently annealed in four steps, which took 15 min each in an O2 background of 1 × 10−3 Pa with the temperature ranging from 1100 to 1250 K [12], [13]. Kundu and Murata reported growth of a 4.0 nm thick crystalline SiO2 film on a Ni(111) surface [15]. Si was deposited on a clean Ni(111) surface at RT with 3 nm thickness followed by oxidation in the presence of atomic hydrogen for 1 h at a substrate temperature of 620 K. Finally the sample was annealed at 1100 K in an ambient O2 atmosphere of 2.6 × 10−5 Pa for 10 min.
In this paper, we report a system for easily growing atomically flat and well-ordered silica films. In our growing system, Pd(100) was chosen as a substrate, and silica films were grown by depositing silicon onto Pd(100) in the presence of O2. The morphologies, electronic properties, and thermal stabilities of films have been investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy loss spectroscopy (HREELS), and ion scattering spectroscopy (ISS).
Section snippets
Experimental details
The experiments were carried out in an Omicron multiprobe surface analysis system with a base pressure of below 3.0 ×10−8 Pa. The system consists of three ultrahigh vacuum chambers: preparation, spectroscopic and microscopic chambers. The preparation chamber is equipped with a silicon evaporator and an ion-sputtering gun for sample cleaning. The spectroscopic chamber is installed with XPS, UPS, ISS, and HREELS (LK-ELS5000). The microscopic chamber is equipped with STM (Omicron
Results and discussion
The thickness of silica films was controlled by the deposition time of Si in O2. A simple inelastic attenuation model was applied to estimate the film thickness, d, using the relation oft is the deposition time of Si in O2, r is the growth rate, I0 and I are the XPS intensity of Pd 3d core level from bare Pd(100) and Pd(100) after deposition of SiO2, α (63°) is the take-off angle with respect to the surface normal, λ is the electron inelastic mean free path. In our work, λ
Conclusions
We demonstrated a method of preparation of ultrathin silica films on a Pd(100) surface by depositing silicon in the 1 × 10−3 Pa oxygen pressure at 500 K. SiO2 films with different thicknesses (0.4–6.5 nm) have been prepared. The 2.8 nm silica film shows smooth morphology, stoichiometry, and well-ordered structure in a long range. The lattice structure of the SiO2 film is closely related with the Pd substrate structure, with a lattice constant of 3.6 ± 0.2 Å, similar to times of the Pd(100)
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 90206036 and No. 20573107). We acknowledge Dr. Zhen Song, Prof. Qinlin Guo, and Dr. Xiulian Pan with pleasure for helpful discussions and critically reading the manuscript.
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