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Theoretical Study on Frequency Shift Relationship between Grain-scale/Lattice Constant and Infrared Spectrum for Group Ⅱ Metal Nano-oxides

Author: WuNianChu
Tutor: SunFengJiu
School: Northeastern University
Course: Condensed Matter Physics
Keywords: infrared absorption spectra (IR) frequency shift Nano-MgO Nano-CaO Nano-SrO Density Functional Theory
CLC: TB383.1
Type: Master's thesis
Year: 2009
Downloads: 47
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Abstract


Infrared spectroscopy is widely used in studying infrared absorption charactristic of metal oxide nano-materials. However, study on nano-oxide materials of the change of infrared spectroscopy is still at the exploratory stage. Some people observed that there were both red shift and blue shift of IR frequency of MgO; some people observed red shift and broadening of absorption peak; some people observed red shift and narrowing of absorption peak, there were many seemingly contradictions in these observations. There’re not much researches of Nano-CaO and Nano-SrO in this area. In order to completely solving the constractions of frequency shift and revealing the law of infrared spectrum of the group II of metal Nano-oxides, the IR spectrums of Nano-CaO, Nano-MgO, Nano-SrO were investigated by using the CASTEP package of Materials Studio, a first-principles pseudo-potential approach of the plane wave based upon the Density Functional Theory (DFT).The first theoretical part in this thesis was the simulation of the infrared spectra of magnesia with different lattice constants by using CASTEP. According to the result, we found that there were some characteristic lattice constants and characteristic grain-scales in Nano-MgO. The places of infrared spectra changing obviously always happen to be at the points of characteristic lattice constants. Furthermore, all characteristic lattice constants lie in the range where lattice constants happen to change rapidly with grain sizes changing, and the corresponding grain size range is about 70～110nm (characteristic grain size range). The varieties of frequency shifts aren’t monotonic. However, in some certain rang of lattice constants, blue shift will happen; and in other certain rang of lattice constants, red shift will happen. The type of frequency shifts depends on the range which the lattice constants of the samples lie in. There are some relations between the variety of infrared spectra and the variety.The other theoretical part was the simulation of the infrared spectras of CaO and SrO with different lattice constants by using CASTEP. Then come to the conclusion of the law of lattice constants and infrared spectra frequency shift. According to the result of simulation, we found that Nano-CaO had a characteristic lattice constant (a=0.4812nm). When a<0.4812nm, almost all of the frequency shifts are blue-shifted. When a>0.4812nm, almost all of the frequency shifts are red-shifted. But there will be red shift for IR frequency of Nano-SrO with the increase of lattice constant. Comparing groupⅡmetal nano-oxides, we found that with the increase of atomic weight, IR spectral peak would change simpler from the coexistence of red shift with blue shift to only red shift.

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