32 Figure 3.8, it is possible to define three stages of functionalisation of monolayer graphene based on changes in its Raman spectrum:
Stage I – low levels of functionalisation
o Introduction of disorder-related Raman peaks D, Dʹ, and D+Dʹ.
o ID/IG increases with increasing functionalisation to a maximum value.
o I2D/IG decreases.
Stage II – high levels of functionalisation
o Raman peaks broaden and soften with decreasing LD.
o ID/IG decreases to roughly 1.
o 2D peak begins to disappear.
Stage III – fully-functionalised graphene
o Complete disappearance of Raman spectrum at all frequencies.
The nature of the G peak intensity, and thus ID/IG, to depend on excitation
wavelength (Wang 1990, Cançado 2007) makes an absolute measurement of the average defect spacing LD determined using equation 3.7 alongside a visual inspection
of the spectrum using the criteria above a complete measurement of modification extent in graphene samples (Cançado 2006).
3.4.
Experimental hydrogenated graphene
As discussed in § 2.3, the first hydrogenation of monolayer graphene was achieved by Elias et al., who hydrogenated graphene with atomised hydrogen and recorded electronic properties of hydrogenated graphene and structural properties in the form of electron diffraction patterns and Raman spectra – shown in Figure 3.9 (Elias 2009). Elias noted enhancement of the defect-activated D, Dʹ and D+Dʹ peaks with hydrogenation and were able to enhance them further by preparing samples as a membrane, allowing the hydrogen access to the lattice from both sides. Crucially, the D peak in hydrogenated samples was seen to disappear after a thermal annealing significantly below the graphitisation temperature of carbon – i.e. below temperatures required to structurally anneal graphene – to reproduce the Raman spectrum of
3.4 Experimental hydrogenated graphene
33 pristine graphene (green spectra in Figure 3.9), giving evidence that the changes in the Raman spectrum were due to hydrogen bonded to the lattice.
Figure 3.9: Hydrogenation of graphene by Elias et al. (a) Single-sided hydrogenation of graphene shows D peak and related defect peaks begin to appear in the Raman spectrum. (b) Graphene samples prepared as a membrane (photograph in right inset)
show higher levels of hydrogenation overall and (left inset) after one hour of simultaneous exposure. Blue spectra are graphene after hydrogenation, red and
green spectra are pristine graphene before hydrogenation and after annealing respectively (Elias 2009).
Following Elias et al., the use of atomised hydrogen to make samples of hydrogenated graphene has become widespread in plasma-enhanced CVD chambers (Luo 2009, Burgess 2011, Matis 2012, Balog 2013) or with atomising guns (Balog 2009, Sessi 2009, Balog 2013, Guillemette 2013, Guillemette 2014). A range of recorded hydrogenated graphene Raman spectra by exposure to atomic hydrogen are collected in Figure 3.10.
3.4 Experimental hydrogenated graphene
34
Figure 3.10: Raman spectra of hydrogenated graphene sample from the literature, prepared by (a,b,c) exposing graphene to hydrogen plasma in a plasma enhanced CVD chamber by (Luo 2009), (Matis 2012) and (Burgess 2011) respectively. (d,e) Hydrogenated graphene prepared by bombarding with hydrogen from an atomising
gun (Guillemette 2014).
a
b
c
3.4 Experimental hydrogenated graphene
35 It is striking that the spectra shown in Figure 3.10c at high substrate temperatures resemble very low-LD Stage II functionalised graphene (similar to 20
hours spectrum in Figure 3.8b), and yet Burgess et al. dismiss them as damaged due to their experimental procedures and do not use them for electronic measurements – this conclusion was supported by an inability to thermally anneal away the D peak in a sample with a spectrum resembling high-LD Stage II (9 hour spectrum in in Figure 3.8b)
(Burgess 2011). Their suggestion that above a measured ID/IG value of 2, the spectra
resembles disordered graphene and not hydrogenated graphene allowed the conclusion that an ID/IG = 2 corresponds to a hydrogen coverage of ~10 at. %, based
on calculations by Xiang et al. which suggest that single-sided hydrogenated graphene becomes unstable at 10 at. % coverage (Xiang 2010). Note that spectra in Figure 3.10d for longer exposures resemble high-LD Stage II hydrogenated graphene. These samples
were not subject to any documented annealing attempt (Guillemette 2014).
The attribution of Stage II-like appearance and subsequent inability to anneal away defects due to the physical damage to the graphene by high energy hydrogen atoms is also noted by Luo et al. (Luo 2009). The authors illustrate the sensitivity of their experimental procedure by showing hydrogenated and annealed Raman spectra for graphene subject to hydrogen plasma for 9 and 11 minutes on the same graph, where an 11 minute exposure lead to a defective spectra that was not reversible through annealing.
The explanation offered by both Luo et al. and Burgess et al. is that hydrogen plasmas are able to etch a graphene lattice, which is supported by the observation of CH2 groups in the IR spectrum of single-walled carbon nanotubes using the same
process (Zhang 2006) which form at the edges of etched sites. Both papers suggest a saturation in the hydrogenation extent, beyond which further treatment only causes physical defects, which suggests that hydrogenation by exposure to atomic hydrogen is not a strong candidate for synthesising fully-hydrogenated monolayer graphane. These points, alongside the observation by Nair et al. that higher-stage fluorinated graphene was not seen to be fully reversible by annealing in any case (Nair 2010), suggest that other analysis methods are necessary to gain conclusive insight on graphene functionalised to higher extents, such as direct imaging by STM (Sessi 2009).