Raman and Eels Studies on Nanocrystalline Diamond Prepared in a Low Pressure Inductively Coupled Plasma
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RAMAN AND EELS STUDIES ON NANOCRYSTALLINE DIAMOND PREPARED IN A LOW PRESSURE INDUCTIVELY COUPLED PLASMA KATSUYUKI OKADA, KOJI KIMOTO, SHOJIRO KOMATSU, and SEIICHIRO MATSUMOTO Advanced Materials Laboratory, National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan [email protected] ABSTRACT Nanocrystalline diamonds with several hundred nm in diameter have been prepared in a 13.56 MHz low pressure inductively coupled CH4/H2 or CH4/CO/H2 plasma. The bonding structures were investigated by Raman spectroscopy and electron energy loss spectroscopy (EELS). Visible (514 nm) and UV (325, 244 nm) excited Raman spectra with CO additive exhibit peaks at 1150 cm-1 assigned to sp3 bonding and at 1332 cm-1 due to zone center optical phonon mode of diamond, respectively. It indicates that the UV excitations are possibly sufficient to excite the state of both sp2- and sp3-bonded carbon. The high resolution EELS (HREELS) spectra with CO additive show peaks at 1100 cm-1 assigned to C-C stretching vibration of sp3 bonding and at 700 cm-1 corresponding to the bending vibration of sp3 bonding. It is qualitatively agreement with the Raman spectra. Furthermore the EELS spectrum without CO additive exhibits two peaks at 284 eV and at 292 eV corresponding to * states and * states, respectively, and is similar to that of graphite rather than that of sp2-rich amorphous carbon. The EELS spectrum with CO additive, on the other hand, shows a peak at 292 eV due to * states and is similar to that of diamond. A slight peak appears at 285 eV corresponding to * states. It consequently implies that the particles almost consist of sp3 bondings and that the small amount of sp2 bondings are considered to exist in grain boundaries. The EESL spectra are consistent with the results of Raman scattering and HREELS. INTRODUCTION Amorphous and nanostructured carbon materials have attracted considerable attention in the last twenty years since the chemical vapor deposition of diamond was developed, followed by fullerenes and carbon nanotubes. From applied perspectives, they are being extensively studied for electron-emitting elements, cold-cathode sources, and ultrahard tribological coatings, etc. From fundamental perspectives, on the other hand, the structure of these materials contains both three-fold coordinated (sp2-bonded) and four-fold coordinated (sp3-bonded) carbon atoms. The phonon density of state (PDOS) and the fraction of sp3 bondings were quantitatively measured by Raman spectroscopy1,2 and electron energy loss spectroscopy (EELS).3 Nanocrystalline diamond films also have drawn remarkable attention4 because they have a low coefficient of friction and low electron emission threshold voltage. The small grain size (approximately 5-100 nm) gives films with valuable tribology and field-emission properties,5 being compared with those of conventional polycrystalline diamond films. We have tried to prepare diamond films in a 13.56 MHz low pressure inductively coupled plasma (ICP).6 The resultant deposits were found to be
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