Downshift of Raman peak in diamond powders
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Downshift of Raman peak in diamond powders Xing-Zhong Zhao, K. A. Cherian, Rustum Roy, and William B. White Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 27 August 1996; accepted 28 July 1997)
Results are presented on the influence of the size of diamond powders and the laser power on the main Raman line. These results show conclusively that there is a consistent and systematic, reversible, downshift with both decrease of powder size, and increase of power. The shift can be explained by local heating of about 500 ±C in the extreme case. Its significance applies to interpretation of the alleged “downshifting” of the 1332 cm21 line in all diamond research. In the future, the grain size, the thermal contact, and the beam power must be carefully monitored in reporting and interpreting any frequency shifts.
I. INTRODUCTION
Success in growing diamond thin films using chemical vapor deposition (CVD) methods has stimulated enormous interest in the unique combination of properties of diamond for new technological applications.1 Raman spectroscopy is among the most important characterization techniques for diamond. Diamond belongs to the face-centered cubic lattice with space group Fd3m. There are two carbon atoms in the primitive unit cell, resulting in a single triply degenerate, first-order zonecenter optical phonon with symmetry of T2g . From the selection rules of factor group Oh , this mode is Raman active only, and the diamond structure has no first-order infrared absorption. The 1332 cm21 Raman line has become the fingerprint for diamond’s presence. Several factors, such as variation of 13 C : 12 C in diamond,2 different structure and substrate,3 and different gas pressure in CVD deposition,4 are known that cause a shift of the 1332 cm21 diamond Raman line both upward and downward. The temperature dependence of the shift of the characteristic diamond Raman peaks for both homoepitaxial CVD diamond films and natural type IIa diamond has also been studied by several groups over the temperature range 300–1200 K, respectively.5,6 The 1332 cm21 diamond Raman line shifts downward continuously with the increase of temperature in both synthetic and natural diamond. Andreyev et al.7 also studied this downshifting of the 1332 cm21 diamond line with beam intensity in the case of powders. In addition, all of these factors mentioned above might be active simultaneously and make the interpretation of Raman shifts even more confusing. In our research on hydrothermal synthesis of diamond,8–11 pure diamond powders were used as seeds in the mix of starting material for homoepitaxial growth in the supercritical liquid. It was crucial for us to make a distinction between the original diamond seeds and the diamond grown after hydrothermal runs. Therefore, 1974
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 7, Jul 1998
Downloaded: 07 Apr 2015
it became critical to study the effects of the size of pu
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