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A combined theoretical-experimental investigation of paramagnetic centres in chemically exfoliated graphene nanoribbons
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Image of FIG. 1.
FIG. 1.

Schematic for processing GNRs by oxidation of multi-wall nanotubes, centrifuging the solution and separating the supernatant and sediment, dip coating the solutions onto a Si/SiO substrate, and vacuum filtrating the solutions onto MCE filter membranes.

Image of FIG. 2.
FIG. 2.

Samples produced by vacuum filtration onto a MCE filter membrane after centrifuging for all seven oxidation temperatures and (below) schematic of the structural change in the samples with changing oxidation temperature.

Image of FIG. 3.
FIG. 3.

(a) Scanning electron micrograph of pristine MWNTs dip-coated on a SiO coated Si (1,0,0) substrate; (b) corresponding micrographs of GNRs obtained at 40 °C, with a partially unzipped MWNT and (c) with a shorter, completely unzipped, GNR obtained at 80 °C; (d) entangled GNRs from the same suspension reported in panel (c), which is representative of our vacuum-filtrated GNR-based thin films.

Image of FIG. 4.
FIG. 4.

(a) Raman spectra of pristine MWNTs (blue) and GNRs (red) obtained from surfactant oxidated at 80 °C. The single 2D peak is consistent with abundant single or few-layer graphene sheets in GNR at the opposite of multiple rolled sheets in MWNTs (which give rise of a composite 2D peak in the blue spectrum) (b) Detail of the Raman spectrum and Lorentzian fit of the peaks (D, G, G, and D′). The inset indicates the position of the G and G peaks as a function of the laser power density on the sample. The degeneracy removal of the G-peak at increasing temperature is also an indication of GNRs.

Image of FIG. 5.
FIG. 5.

Atomic force height micrographs of our samples: (a) MWNTs and (b) GNRs obtained from surfactant oxidated at 80 °C as in Figure 3 . (c) The different height profiles for MWNTs and GNRs obtained through the cross sections drawn in panels (a) and (b) are shown.

Image of FIG. 6.
FIG. 6.

Atomic force phase micrographs obtained from samples oxidated at (a) 40 °C, (b) 80 °C, and (c) 90 °C. Length distribution of nanoribbons at (d) 40 °C and (e) 80 °C. (f) Electron dispersive X-ray spectrum for the sample prepared at 90 °C with no ribbon features.

Image of FIG. 7.
FIG. 7.

ESR spectra of samples processed at of 20 °C, 40 °C, and 60 °C for (a) supernatant and (b) sediment samples. (c) Low-temperature (−160 °C) ESR spectrum of the supernatant sample oxidated at 80 °C, with two different types of paramagnetic centres.

Image of FIG. 8.
FIG. 8.

Intensity of the ESR signal vs. measurement temperature for the supernatant at oxidation temperatures of (a) 20 °C, (b) 80 °C, and (c), 110 °C, and sediment at oxidation temperatures of (d) 20 °C, (e) 80 °C, and (f) 110 °C.

Image of FIG. 9.
FIG. 9.

Density of states for (a) zigzag and (b) armchair nanoribbons with a length/width aspect ratio of 11, calculated using the Hückel tight-binding method.

Image of FIG. 10.
FIG. 10.

Fraction of Fermi states in the range of ± for both armchair and zigzag nanoribbons plotted in terms of the aspect ratio.

Image of FIG. 11.
FIG. 11.

(a) GNR terminated with C-O bonds (in red) with a interaction energy, , different from the C-C bond interaction energy, , and (b) formation energy of GNRs as in panel (a) as a function of the ratio. Even for the smallest GNR, the formation energy for armchair and zigzag edges increases with the same rate, making oxidation irrelevant to the defect formation energy.

Image of FIG. 12.
FIG. 12.

Vacancy-type defect localized near the centre of an armchair GNR. The bond reconstruction and the localized paramagnetic centre arising from it are shown in red.

Image of FIG. 13.
FIG. 13.

(a) Formation energy of zigzag edges (triangles) and (b) vacancies (circles and squares) vs. length/width aspect ratio. (b) Total fraction of Fermi states from both armchair and zigzag structures vs. aspect ratio. Spin density of the (c) sediment and (d) supernatant as a function of processing temperature.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A combined theoretical-experimental investigation of paramagnetic centres in chemically exfoliated graphene nanoribbons