  Nanometer-scale graphene objects are attracting much research interest
because of newly emerging properties originating from quantum confinement
effects. We present Raman spectroscopy studies of graphene nanoribbons (GNRs)
which are known to have nonzero electronic bandgap. GNRs of width ranging from
15 nm to 100 nm have been prepared by e-beam lithographic patterning of
mechanically exfoliated graphene followed by oxygen plasma etching. Raman
spectra of narrow GNRs can be characterized by upshifted G band and prominent
disorder-related D band originating from scattering at ribbon edges. The D-to-G
band intensity ratio generally increases with decreasing ribbon width. However,
its decrease for width < 25 nm, partly attributed to amorphization at the
edges, provides a valuable experimental estimate on D mode relaxation length of
<5 nm. The upshift in the G band of the narrowest GNRs can be attributed to
confinement effect or chemical doping by functional groups on the GNR edges.
Notably, GNRs are much more susceptible to photothermal effects resulting in
reversible hole doping caused by atmospheric oxygen than bulk graphene sheets.
Finally we show that the 2D band is still a reliable marker in determining the
number of layers of GNRs despite its significant broadening for very narrow
GNRs.
