纳米NiTiO_3光声光谱研究

PHOTOACOUSTIC SPECTRA STUDIES OF NANOMETER NiTiO 3

纳米钛酸镍粉末从可见光到近红外范围的光声光谱出现了三个强的吸收峰。４４５ｎｍ和５０５ｎｍ附近的吸收峰产生于自由激子和束缚激子的光跃迁，其变化趋势强烈地依赖于颗粒度大小。

The photoacoustic spectra of nanometer powder NiTiO 3 with different grain diameters are studied. The results show that three well-defined absorption peaks are observed in visible to near infrared wavelength range 350￣1200nm. Among them two peaks P 1 and P 2 are located at the short wavelength range around 445nm and 505nm, respectively. It is also shown that, as the decrease of the grain size, the blue shifts and the background increases of both peaks (P 1 and P 2) appear, as well as the peak P 2 widens and the sharp of the peak P 1 decreases. In addition, an absorption band with width about 200nm appears in red to near infrared range with an absorption maximum at 840nm. According to the analyses of the energy levels of the luminant titanutes, the free excitons and trapped excitions are responsible for the the peaks P 1 and P 2. But the absorption band around 840nm is from the dilute impuriies. An important property of the structures of nanoscaled materials is that the surface area occupies large pecentage relative to volume and a lot of electrons are located at surfaces. Because to the coordinations of the surface states are not perfective, there are much more holes, thus the probability of the recombination of the electrons and holes increase,which induces much more exciton density in nanoscaled materials than others. Meanwhile, a lot of defects appear in nanoscaled materials and the number and kinds of the defects increase with the reduction of the grain sizes. On the other hand, the trapped exciton increase makes the relative number of the free excitons decrease. The changes become more apparant with the size reduction, which induces the peak P 1 to be smooth. But due to the energy distribution of the trapped excitons in a wide range induced by the different kinds of defects the peak P 2 widens. Besides, the main reason of the blue shifts of peaks P 1 and P 2 is the energy gaps increase. On the other hand, there is an interaction between the excitons and phonons (lattice vibrations). The lattice vibration and then the interaction becomes weak as the grain size reduces, which also induces the blue shifts of P 1 and P 2. Therefore, the characters of the photoaconstic spectra reflect the quantum size effect and the optical absorptions induced by the surfaces, interfaces, defects and impurities of nanometer materials. Although the peaks P 1 and P 3 can be seen in the optical diffusion spectra of nanoscaled NiTiO 3, the P 2 just can be seen in photoacoustic spectra. It is obvious that the photoacoustic spectroscopy is much more sensitive than the others.