ANNUAL REPORT - MTA SzFKI

Raman spectroscopy is widely used for the characterization of these materials. However it
is rather difficult to obtain specific information on the bonding configuration of only the
grain boundaries from Raman spectra of NCD thin films. These spectra give averaged
information about the bonding configuration of all carbon atoms falling into the excitation
volume being a part of the diamond crystallites or the surrounding amorphous matrix.
Additionally the resonance enhancement of the scattering in the amorphous matrix causes
the intense bands of this phase to overlap the scattering contribution of such minor parts of
the structure, like the grain boundaries.
To overcome these difficulties and to obtain information on the bonding configuration in
grain boundaries from the Raman spectrum of NCD films we have lowered the excitation
volume so that the excitation took place only on a few crystallites. Additionally the
excitation energy was shifted in order to decrease the resonant Raman scattering. This way
the characteristic vibrations from grain boundaries of NCD thin films of different grain
size were detected successfully (Fig. 1) and identified as characteristic bands of aromatic
hydrocarbons and sp 3 CH 2 groups.
A100
Intensity [a.u.]
A100
A50
Intensity [a.u.]
A50
A40
A40
A25
A25
800 1000 1200 1400 1600 1800 2000
Raman shift [cm -1 ]
800 1000 1200 1400 1600 1800 2000
Raman shift [cm -1 ]
Fig. 1. 488 nm (a) and 785 nm (b) excited Raman spectra of ultra-nanocrystalline diamond films of different
grain size: A25, A40, A50 and A100 samples have grain sizes between 3-50 nm, 50-100 nm, 100-150 and
200-300 nm respectively.
The above described method cannot be used for UNCD films having grain sizes below 50
nm, since excitation beam diameter is limited by the diffraction. In the case of these layers
we have used Surface Enhanced Raman Scattering (SERS) to detect characteristic
vibrations from grain boundaries. Putting SERS-active gold nanoparticles (average size is
20 nm) in contact with the UNCD film, the excitation volume from where the SERS signal
arises is limited to a few tens nm surrounding of these nanoparticles. As a result, we were
able to detect characteristic vibrations of the grain boundaries of UNCD thin films too.
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