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Introduction
Density Functional Theory for Better Understanding Raman Spectroscopy Data of Surface Carbon on Bulk Niobium
A. Denchfield1, J. Zasadzinski1, 2, J. Li1
1Physics or Chemistry Department, Illinois Institute of Technology, Chicago, Illinois 60616, USA
2Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
Raman microscopy/spectroscopy measurements on Niobium processed
according to the recipes for Superconducting Radio Frequency (SRF)
cavities show surface patches with relatively thick layers of graphitic
carbon1. Sharp Raman peaks are consistent with graphene or graphite while
other broader peaks appear to be disordered or amorphous carbon. Other
Raman peaks are consistent with C-H modes from aromatic hydrocarbons.
The purpose of this work is to develop a theoretical framework for
understanding the Raman data. The goal is to use Density Functional
Theory (DFT) to determine the Raman active vibrational modes of
graphene, graphite and amorphous carbon, as well as hydrogen
terminated carbon, and to compare with experiment.
Abstract
A model of hydrogen-terminated carbon is developed for
interpreting carbon-containing systems, such as when Niobium processed
for SRF cavities develops layers of graphitic carbon on the surface. A com-
putational comparative study on benzene, naphthalene, and larger aromatic
hydrocarbons was performed, giving rise to physical interpretations of ob-
served Raman active modes for hydrocarbons.
References
1C. Cao, R. Tao, et. Al. Phys. Rev. B 91, 094302 (2015)
2Makarova, T., Riccò, et al. (2008), Phys. Status Solidi B, 245: 2082–2085.
3P. Larkin et al, (Elsevier, Amsterdam, 2011), p. 149.
4James R. Scherer, Spectrochimica Acta, V. 21, Is. 2, 1965, Pages 321-339
5Castiglioni, C. and Mapelli, C. and Negri, F. and Zerbi, G., J. Chem.
Phys., 114, 963-974 (2001)
6Tommasini, M., Zerbi, G., Chem. Eng. Trans. 22: 263-268 (2010)
Raman Spectroscopy of Niobium
Raman spectra of processed Nb for SRF cavities often display micron-sized
patches of graphitic carbon1, as shown in the two figures below.
Figure 1
Raman Spectroscopy of Nb Surfaces
The modes of pit 14 also have graphitic peaks at 1063cm-1, 1129cm-1,
1300cm-1, 1442cm-1, 1582cm-1, 1611cm-1, and 2600cm-1. This pattern is
consistent with a combination of the active modes of stearic acid3 and
defect-ridden graphene2.
Computational Methods
Density Functional Theory (DFT) is a quantum mechanical modelling
method that can effectively handle many-body systems such as
molecules. It does this by treating the nuclei of the atoms in the system as
fixed, and then using functionals of the spatially dependent electron density.
Calculations were performed at the PBE0 level of theory with the
cc-pvdz basis set on various polycyclic aromatic hydrocarbons (PAHs).
Examples of Benzene and Coronene are shown below.
Figure 4
Computed Raman Modes of Benzene
A major peak is computed at 3229 cm-1, which is known to be the C-H interactions.
There are minor peaks at 1035cm-1, 1189cm-1, and 1674cm-1. The latter two were
originally attributed to D4 and D’ peaks, respectively2. The origin of the 1035cm-1
and D4 peaks are in question.
Figure 5
Computed Raman Modes of Coronene
Once again, a large frequency (at 3215cm-1) is computed. In the other aromatic
hydrocarbons studied, this peak was found in all of them, between 3200-3250cm-1.
Another peak is at 1428cm-1; this peak is found in pit 14 (Fig. 1). Gabedit allows
the opportunity to visualize what molecular movement is responsible for this peak,
as shown in Fig. 2 on the left. There is another peak at 1670cm-1, attributed to the G
peak. Minor peaks at 1057cm-1, 1245cm-1 and 1507cm-1, like stearic acid (Fig. 3).
Figure 2
Molecular movement responsible
for 3215cm-1 peak for Coronene
Results and Discussion
Conclusions and Future Work
Gabedit has allowed visualization of modes and therefore the ability to dis-
tinguish between C-C and C-H interactions. For example, there is
significant C-H bending going on in the vibrational modes seen from
1050cm-1 to 1480cm-1 (such as in Fig. 6 above) for the modeled molecules.
The interpretation is that due to the small size and particular shape of the
molecules modeled, the C-C stretching that is responsible for the D region
peaks can be modified by C-H wagging vibrations on the edge, as found in
a similar study done by Ref. [6].
Gabedit has also helped visualize the C-H stretching modes at near
3000cm-1, as seen in Fig. 2.
A comparison can also be made between these modeled molecules,
experimental evidence (such as in Fig. 1), and experimental results for
stearic acid. On first glance, the peaks in the D region for these PAHs,
experimental results, and stearic acid are very similar, such as if one
compares Fig. 1, 3 and 5. Because the computed results give more insight
into small PAHs, it can be determined that while pit 14’s spectrum appears
very similar to stearic acid, it is actually composed of PAHs like Coronene.
The D-modes in Coronene appear as Raman active; this is contradictory
with current knowledge of graphene’s Raman activity. It is suggested that
the D-modes in Coronene are appearing as Raman active due to its edge
defects - in the form of C-H bonds. It is expected that as PAH molecule
size increases, the D-mode computed will be more consistent with
experiment, i.e. the computed peak will steadily decrease.
- Computations and visualization aids (Gabedit) readily confirm that the
Raman peaks at 3000cm-1 and above are C-H stretching modes
- While it is expected that the D-band mode should not be Raman active
for pristine graphene, the vibrational mode itself is not found at 1350cm-1
either; instead, it is split up into various split D-bands, in a manner quite
similar to Ref. [2].
- PAHs with sizes on the order of Coronene and below can have their
Raman spectra in the D band influenced by C-H wagging modes.
- C-H wagging modes can also influence the Raman results
- It is speculated that the D-mode in small PAHs is Raman active due to
the C-H bonds (‘defects’) on their edges.
- With a model comprised of the above observations, it can be concluded
that pit 14 in Fig. 1 is composed of small PAHs, in contrast to pit 15,
which is more likely composed of larger crystals of amorphous carbon.
Figure 3
Raman active modes of stearic acid,
bent Nb foil and their assignments3
Figure 6
Molecular movement of Coronene at its
1428cm-1 peak, visualized by Gabedit.
Although this motion’s computed Raman
intensity is quite large relative to the other
intensities computed, this C-C double bond
interaction is not in the experimental
literature for graphene nor Coronene. Due to
the direction of the outer atoms’ movement,
however, it is taken to be one of the various
D-modes2, just displaced.
G peak
1580cm-
D-peak, 1350cm-1
2D peak, 2700cm-1
