Comparing cluster and slab model geometries from density functional theory calculations of silicon(100)-2x1 surfaces using low-energy electron diffraction.

摘要

Experimental and theoretical studies of Si(100)-2x1 surfaces to compare cluster and slab models using computational low-energy electron diffraction intensity-voltage (LEED-IV) data are presented. The experiments were performed in an ultra-high vacuum chamber using LEED and Auger electron spectroscopy (AES). The computational studies were done using Gaussian'03 to model the Si(100)-2x1 surfaces using a cluster approach, the Car-Parinello Molecular Dynamics (CPMD) package for slab models, and Symmetrized Automated Tensor LEED (SATLEED) for theoretical LEED-IV data.; The experimental LEED studies were performed on the clean Si(100)-2x1 surface and this surface with the following adsorbates: ammonia, methylamine, dimethylamine, trimethylamine, ethylene, and methanol. AES was used to check for cleanliness before the experiment. Experimental LEED-IV images show the 2x1 periodicity of the Si(100) was maintained for all these adsorbate covered surfaces after a dose of one Langmuir at room temperature. LEED-IV data were retrieved from the images, and the corrected intensities were plotted against beam energy (in electron volts).; There is an overall Pendry R-factor of 0.36 between two experimental data sets that have a good match between LEED-IV plots obtained a clean sample, demonstrating the limit of reproducibility in the data.; The computational models were used to generate at least two sets of optimized structures for all the surfaces under study.; Using coordinates from the optimized structures, the minimum Pendry r-factors were collected for each model. The overall Pendry R-factor, RPE = 0.31, for the clean surface shows cluster model used provided a more reliable representation of the surface than the slab models, obtained from the density functional theory methods implemented in CPMD (RPE = 0.40). Cluster models for clean, NH3, N(CH3)3 and CH 3OH covered surfaces appear to provide better representations than the slab; however for the Si(100)-NH2CH3 and Si(100)-NH(CH 3)2 surfaces, the slab model appeared to give slightly better agreement. For ethylene the LEED-IV computations were not conclusive. This requires further investigation with different experimental data sets and remodelling the Si(100)-2x1-C2H4 surface.