Density Functional Theory Study On The Adsorption Systems Of The Surface Of The β-Si₃N₄

Recently, along with the fast development of computer, the study of the adsorption of the material surface based on quantum chemical methods get progress. In this paper, the adsorption of theβ–Si₃N₄(0001) and small molecules have been studied by density functional theory at 6-31G^* basis set level: the adsorption of water on the surface ofβ–Si₃N₄(0001); the adsorption of water and alcohol on the relaxation surface ofβ–Si₃N₄(0001) the adsorption of oxygen on the surface ofβ–Si₃N₄(0001).Firstly, Density functional theory (DFT) B3LYP method is used to theoretically investigate the interaction of water withβ–Si₃N₄(0001). The optimization of geometry, the calculation of the adsorption energy and the change of net charge have been performed at 6-31G^* basis set level using cluster at normal temperature and pressure. The influence of temperature,pressure and BH₃ modification on the adsorption model has also been studied. The results indicate that the most stable structure is that H₂O is adsorbed through H atom of H₂O locating on the top of N atom ofβ–Si₃N₄(0001). It is found that when the temperature is 100℃and the pressure is 3×10⁵¹0×10⁵ Pa, the adsorption energy is 189.59 kJ/mol, which is increased 57.19 kJ/mol compared with the adsorption model at normal. The elongate of O-H bond probably leads to the decomposition of H₂O molecule. After theβ–silicon nitride surface (0001) is modified by BH₃, the pre-adsorption decreases the adsorption energy of water.Secondly, Density functional theory (DFT) B3LYP method is used to theoretically investigate the relaxation conformation ofβ–Si₃N₄(0001) and the adsorption conformations of water, methanol, ethanol and glycol on the relaxation surface ofβ–Si₃N₄(0001). For H₂O the most stable structure is that it is adsorbed through the H atom lying above a N3 site of the relaxation surface ofβ–Si₃N₄(0001). For alcohol the most stable structure is that it is adsorbed through the H atom of the hydroxyl lying above the center of Si2 and N3 of the relaxation surface ofβ–Si₃N₄(0001). By comparison between the two adsorbed substances from the adsorption energy, the adsorption bond and the transfer electrons, it prove that the alcohol is easy to be adsorbed on the relaxation surface ofβ–Si₃N₄ (0001).Finally, Density functional theory (DFT) B3LYP method is used to theoretically investigate the interaction of O₂ withβ–Si₃N₄(0001) at 1200℃. All the calculations have been performed at 6-31G^* basis set level using H-saturated cluster. From the total energy minimization, the chemisorption with the center of the molecule lying above a Si site and the molecular axis paralleling to the surface is the most stable. After adsorption the O-O bond is easier to dissociate comparing to the free O₂. The electron transferred from the substrate to the O₂ molecule occupies the O₂ anti-bonding orbital, thus leading to the bond strength weakened, which can be reflected by the elongated O₂ bond length. The change trend of the O-O population and vibrational frequency is consistent with the change of O-O bond length. The significant chemisorption energy and short adsorption bond length indicate that the oxidation occurs on theβ–Si₃N₄(0001) surface at 1200℃more easily. In order to illuminate the effect of the temperature, we also calculate the adsorption system at 1300℃,1400℃,1500℃,1600оC , from the adsorption energy, the adsorption bond and the O-O bond after adsorption, it is conclude that the higher the temperature, the easier the oxidation of the surface ofβ–Si₃N₄(0001).The mechanism of lubrication and oxidation of the silicon nitride is explained through the study of the surface ofβ–Si₃N₄ using quantum chemical methods, which provide the theory direction for experiment.