In their recent letter, Teweldebrhan and Balandin1 reported modification of graphene properties due to electron-beam irradiation at energies of 5–20 keV from a scanning electron microscope (SEM). After irradiation, they observe the appearance of a strong disorder D peak at around 1345cm1 and a drastic increase in electrical resistance. They state that their findings follow the known stages of amorphization of the crystalline graphite lattice into the nanocrystalline state and then, upon further irradiation, into the amorphous state. In this comment, we would like to point out that the appearance of the D peak and increase in resistance with electron irradiation reported in Ref. 1 results from reversible hydrogenation of graphene into graphane or partially hydrogenated graphene,2,3 instead of amorphization. Graphane is graphene with an H atom attached to each C atom above and below the C atom in an alternating manner, and is a stable crystalline structure.2,4 Partially hydrogenated graphene has H atoms attached to C atoms on only one side of the graphene layer, and is stabilized by ripples in graphene that facilitate sp3 bonding with H.2,3 The crucial observation we have made that led us to this conclusion is that the D peak that appears after irradiation almost completely disappears after the sample is annealed at low temperatures of 270°C for 30 min. This reversible appearance of the D peak is the characteristic signature of graphane and partially-hydrogenated graphene.2,3

In our experiments, graphene samples were prepared in air at 50% relative humidity and room temperature by mechanical exfoliation of highly oriented-pyrolitic-graphite onto a 300 nm thick SiO2 layer grown on a Si wafer.5 Graphene samples were identified using optical microscopy,5 and micro-Raman spectroscopy that can be used to distinguish single-layer graphene from multi-layer graphene samples by inspection of the full-width-at-half-maximum (FWHM) of the overtone of the D (2D) peak at about 2700cm1.6 The FWHM is 30cm1 for graphene and 60cm1 for multilayer samples. The micro-Raman system used was a Thermo Electron Almega XR with a spot size of 0.6μm2, a laser power of 6 mW and a spectral resolution of 2cm1. The electron irradiation was carried out using a high-resolution FEI Nova 200 NanoLab SEM system with a sample chamber at a pressure of 106Torr, and also an electron gun in an ultrahigh vacuum (UHV) chamber at a pressure <1010Torr. Postannealing of irradiated samples was carried out using a tube furnace at a pressure of 104Torr.

Figure 1(a) shows micro-Raman spectroscopy data of a pristine graphene sample that has not been irradiated. Figure 1(b) shows data of the same sample shown in Fig. 1(a) after irradiation for 2 min at an energy of 30 keV using a beam current of 9.5 nA and scanning area of 1.2×103μm2. The resulting electron dosage on the graphene sample was 5.2×1017e/cm2. In Fig. 1(b), we observe a strong and sharp D peak similar to that reported in Ref. 1. In Fig. 1(b), the ratio of the integrated intensity of the D peak, ID, to the integrated intensity of the G peak at about 1580cm1, IG, is ID/IG=2.7. This value is similar to the literature value for graphane of about three.2 Figure 1(b) also shows D and (D+D) peaks, which like the D peak, require a defect for activation.2 A decrease is observed in I2D/IG, where I2D is the integrated intensity of the 2D peak at about 2660cm1. Also, a slight broadening of the 2D peak is seen in comparison to that of the pristine sample shown in Fig. 1(a). All of these effects are seen in hydrogenated graphene.2 After post annealing the sample shown in Fig. 1(b) at 270°C for 30 min, the D peak almost completely disappeared, as shown in Fig. 1(c). The nearly complete disappearance of the D peak after annealing at temperatures of 100400°C is a unique characteristic of graphane and partially hydrogenated graphene.2,3 These temperatures are significantly lower than those for removal of lattice damage in graphite that are typically >1000°C. Therefore, we propose that the D peak observed after irradiation is due to hydrogenation instead of amorphization. We note that graphane and partially hydrogenated graphene were shown to be insulating,2,3 consistent with the findings of high resistance reported in Ref. 1.

FIG. 1.

Micro-Raman spectra of graphene. All the curves are normalized to have the same G peak intensity. (a) Pristine graphene sample that has not been irradiated. (b) Sample shown in (a) after irradiation at 30 keV to a dosage of 5.2×1017e/cm2. A large and sharp D peak is observed with ID/IG=2.7 indicating graphane has been formed. (c) Same sample shown in (b) after postannealing at 270°C for 30 min. No D peak is observable indicating the sample has reverted back to its original state. (d) Graphene sample that is preannealed in UHV at 590°C for 1 h and then irradiated at a dosage of 7.4×1015e/cm2 at 5 keV. A small D peak having ID/IG=0.07 is observed, indicating that little or no hydrogenation occurred.

FIG. 1.

Micro-Raman spectra of graphene. All the curves are normalized to have the same G peak intensity. (a) Pristine graphene sample that has not been irradiated. (b) Sample shown in (a) after irradiation at 30 keV to a dosage of 5.2×1017e/cm2. A large and sharp D peak is observed with ID/IG=2.7 indicating graphane has been formed. (c) Same sample shown in (b) after postannealing at 270°C for 30 min. No D peak is observable indicating the sample has reverted back to its original state. (d) Graphene sample that is preannealed in UHV at 590°C for 1 h and then irradiated at a dosage of 7.4×1015e/cm2 at 5 keV. A small D peak having ID/IG=0.07 is observed, indicating that little or no hydrogenation occurred.

Close modal

We observed the appearance of a reversible D peak by irradiating samples over a range of parameters: with the SEM at energies from 2 to 30 keV and dosages from 7.8×1015 to 7.8×1018e/cm2, and with the electron gun in UHV at energies of 100 eV–5 keV and dosages of 5.4×1015 to 1.8×1016e/cm2. In addition, we annealed graphene samples at 590°C for 1 h in the UHV chamber to remove adsorbates. The samples were then irradiated in situ in UHV with the electron gun at an energy of 5 keV and dosage of 7.5×1015e/cm2. As shown in Fig. 1(d), these samples exhibited values of ID/IG of about 0.07, indicating that little or no hydrogenation occurred. Therefore, we propose that hydrogenation is due to adsorbates such as H2O on graphene from which H is dissociated by the incident electrons.

Figure 2(a) shows micro-Raman spectroscopy data for a pristine double-layer (DL) graphene sample that has not been irradiated. Figure 2(b) shows data for the same sample after irradiation using the SEM at an energy of 30 keV and dosage of 2.6×1018e/cm2. A small D peak with ID/IG=0.27 is observed. This result is also consistent with hydrogenation, since one would expect ID/IG for DL samples to be within a factor of a half of ID/IG for graphene if the D peak were due to amorphization. This is because one expects at least the top layer of a DL sample to be amorphizised. The small D peak in DL samples has been attributed to less partial hydrogenation due to the significantly fewer number of ripples in DL versus single-layer graphene samples.2,3 As shown in Fig. 2(c), the D peak for DL graphene also disappears after postannealing.

FIG. 2.

Micro-Raman spectra of DL graphene. (a) Pristine DL sample that has not been irradiated. (b) Sample shown in (a) irradiated at 30 keV to a dosage of 2.6×1018e/cm2. A D peak is observed with ID/IG=0.27, indicating significantly less hydrogenation than for single-layer graphene samples. (c) Same sample in (b) after postannealing at 270°C for 30 min, showing that a D peak is not observable.

FIG. 2.

Micro-Raman spectra of DL graphene. (a) Pristine DL sample that has not been irradiated. (b) Sample shown in (a) irradiated at 30 keV to a dosage of 2.6×1018e/cm2. A D peak is observed with ID/IG=0.27, indicating significantly less hydrogenation than for single-layer graphene samples. (c) Same sample in (b) after postannealing at 270°C for 30 min, showing that a D peak is not observable.

Close modal

In summary, we claim that the appearance of the D peak in graphene after electron irradiation is due to hydrogenation instead of amorphization as reported in Ref. 1. This conclusion is primarily based on the almost complete disappearance of the D peak after annealing the sample at temperatures of 270°C for 30 min. In addition, our conclusion is supported by a decrease in I2D/IG and slight broadening of the FWHM of the 2D peak, and a significantly reduced value of ID/IG for double layer samples after irradiation.

This work was supported by the Faculty Research Grant Program and the Center for Advanced Research and Technology of the University of North Texas. We thank Joshua Wahrmund for useful discussions.

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