Multi-wavelength Raman Spectroscopy of Nanodiamond Particles
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1039-P15-03
Multi-wavelength Raman Spectroscopy of Nanodiamond Particles Paul William May, Philip Overton, James A Smith, and Keith N Rosser School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom ABSTRACT We have used Raman spectroscopy with 3 different laser excitation wavelengths (near infrared: 785 nm, green 514 nm, and ultraviolet 325 nm) to study diamond particles as a function of particle size, ranging from 5 nm to 100’s of µm. We find that the position of the 1332 cm-1 diamond line varies with particle size as a direct result of heating by the laser. This effect is more significant for lower wavelengths, probably as a result of the increased absorbance by nanodiamond particles in the UV. INTRODUCTION Laser Raman spectroscopy is a powerful technique used to study various forms of carbon films, including diamond, diamondlike carbon, and nanodiamond particles [1]. For diamond particles of size >100 nm, the Raman spectra usually consist only of the zone-centre phonon line at 1332 cm-1 corresponding to symmetric vibrations of the sp3 diamond lattice. However, for smaller diamond particles, Zhao et al [2] showed that the intense laser beam focused onto a small volume particle causes significant heating, and this shifts the diamond line to lower wavenumbers. Thus, the observed Raman line position is dependent upon laser power. Estimates for the temperature reached by the particles can be obtained by using results from Liu et al. [3] and Borer et al. [4], who independently measured the position of the diamond Raman line from a single crystal diamond sample as a function of measured temperature, as shown in Fig.1. Both sets of data can be fitted to an equation of the form:
∆ω =
−A (e B / T − 1)
(1)
where A and B are constant fitting parameters and ∆ω is the shift in peak position from a starting value of ω0 = 1332.7 cm-1 at 100 K. For Liu et al’s data, we found that values of A = 10 cm-1 and B = 1222.1 K-1, and for Borer et al’s data A = 11.5 cm-1 and B = 1527.6 K-1, gave the best fits to the experimental data. The two curves are reasonably consistent, which shows that the observed shifts down to ~1323 cm-1 occur when the diamond approaches temperatures ~750-900 K, which is close to the temperature at which diamond oxidises to graphite in air. Thus Raman may not be a completely non-destructive analysis tool for diamond unless low power lasers are employed. Other data for variation of the diamond Raman line with temperature include [5,6,7,8,9].
1334 1333 1332 1331
Raman shift / cm-1
1330 1329 1328 1327 1326 1325 1324 1323 1322 1321 1320 1319 100
200
300
400
500
600
700
800
900 1000 1100
T /K
Figure 1. Values of Raman diamond line position from ref.[3] (red triangles) and ref.[ 4] (green squares) at different temperatures. The lines are fits from Equation (1) in the text.
Recently ultra-dispersed diamond (UDD) or detonation nanodiamond particles have become available, with particles sizes down to 5 nm. For UDD, Yushin et al [10] showed that the diamond line is n
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