Laser power effects on the Raman spectrum of isolated diamond chemical vapor deposition particles

In the present work the effect of incident laser power on the Raman spectra of diamond isolated particles and continuous films deposited on silicon and glassy carbon (GC) substrates by the chemical vapor deposition method is investigated. It is shown that the Raman line position measured for diamond particles shifts to lower wave numbers as a function of incident laser power. These shifts were most drastic for single particles deposited on GC that were examined using a Raman microprobe. In this case the diamond peak displayed a negative shift of ∼18 cm−1 when the laser power output was increased from 1 to 15 mW. The laser beam diameter was ∼2 μm and the diamond particle measured was 3–6 μm in diameter. Micro-Raman measurements of diamond particles deposited on a silicon substrate or continuous diamond films on GC display very small changes in the diamond Raman peak wavelength for the same laser power range. From our studies it is concluded that the negative shift of the Raman peak position is caused by laser-induced local heating of the irradiated diamond particles. The temperature under the laser spot was calculated from the intensity ratio of Stokes to anti-Stokes Raman lines measured as a function of laser power output. The Raman peak wavelength calculated for each temperature showed excellent agreement with our experimental results. The local temperature of an isolated diamond crystal on GC rises to ∼1000 K at 15 mW laser power output, whereas the temperature change of the continuous film on GC and of a single particle on silicon was in the 0–30 K range above room temperature for the same laser power output range. This difference in heating is explained on the basis of efficient heat dissipation through a large contact area between the deposited particles and the substrate surface in the case of single particles deposited on silicon or through grain boundaries in the case of the continuous film on GC. The inefficient heat dissipation from the isolated diamond particles on GC is related to the small contact area between the diamond crystals and the GC substrate as a result of etching during the deposition process and possibly to the presence of an amorphous component in the diamond crystals deposited.