High spatial resolution photoluminescence and Raman spectroscopic measurements of a natural polycrystalline diamond, carbonado

Abstract

Photoluminescence and Raman spectra with high spatial resolution (< 1 μm) were observed on a natural polycrystalline diamond, carbonado. The studied sample is from Central African Republic (CAR) and was polished to optical grade. In addition to observation by optical microscopy, Scanning Near-field Optical Microscopy (SNOM) was applied for spectral and topographic analysis at high spatial resolution less than 300 nm. Mapping of the photoluminescence intensity of carbonado using SNOM indicated that the emission intensity was lower at the grain boundaries and the fringe of pores of carbonado revealed weakest intensity. Photoluminescence spectra of carbonado observed using optical microscopy and SNOM comprised three sharp bands at 504, 575 and 638 nm with their side bands and the color of photoluminescence changed according to its location. Raman spectra measured using optical microscopy revealed that the residual stress locally exists underneath the sample surface and the maximum stress value observed in this study was 0.72 GPa. Additionally, pressure dependence of the 575 nm band was measured using a diamond anvil cell up to 3.5 GPa. The 575 nm band shifted linearly to a lower wavelength with increasing pressure with a gradient of − 0.57 GPa/nm. This relationship was applied to estimations of the stress distribution of carbonados using SNOM. No significant changes in peak positions of photoluminescence spectra were detected on the surface of carbonados. This result suggests that carbonado is well sintered and that the residual stress of carbonado exists locally inside the crystal.