GNS synthesis and microscopic analysis

The synthesis of large quantities of GNS is achieved through a combination of chemical vapor deposition (CVD) graphene and appropriate mild synthesis conditions, namely aqueous formic acid at a temperature of 45 °C as shown in Fig. 1a. The rolling process of graphene results in more than 100 µm long GNS as it is observed in the light optical image (Fig. 1b) and the histogram gives an average length of 57 ± 12 μm (Fig. 1c). From the evaluation of the light optical microscope image we measure that the surface coverage of the remaining graphene flakes on the surface is less than 5%, i.e., that the yield of the GNS synthesis method is over 95%. The AFM image in Fig. 1d demonstrates the formation of a long tubular structure. From the evaluation of AFM images of a large number of GNSs, we obtain an average height of 120 ± 35 nm shown in Fig. 1e. Combining the statistics of the heights and lengths, the GNS has an average aspect ratio of 500, placing it into the class of ultrahigh aspect ratio structures with a ratio of more than 30018,19. In order to investigate the inner structure of graphene nanoscrolls, transmission electron microscopy (TEM) cross-section is carried out and the corresponding high resolution TEM image is shown in Fig. 1f (also see Supplementary Note 1). Along with the GNS shell, the adjacent layer distances are measured and the average graphene layer spacing of 0.39 ± 0.03 nm is obtained (Fig. 1g), which is slightly larger than the graphite c-plane distance and matches well with reported values12,14.

Fig. 1 Graphene nanoscroll (GNS) synthesis and characterizations. a A chemical vapor deposition grown graphene sample on SiO 2 (0.5 × 0.5 cm) was placed in a glass vial containing a freshly prepared 1/1 (v/v) HCOOH/H 2 O mixture, in a way that the graphene sample was fully immersed in the aqueous HCOOH mixture. The vial was sealed with a rubber septum and degassed with Ar gas, then placed in an oil bath with a regulated temperature of 45 °C for 24 h. After this period, the graphene sample was repeatedly washed with water (to remove HCOOH) and then with acetone, and finally dried using a light stream of Ar gas. b Light optical image of the graphene nanoscroll (GNS) on SiO 2 substrate, where the lines are the GNS and the background is the SiO 2 substrate. c Length distribution diagram based on random 100 GNS. d AFM image of the GNS with a height of 125 nm. e Height distribution based on random 50 GNS. f A zoomed-in cross-section of a 125 nm high GNS, as well as (g) the statistic adjacent layer distance histogram (random 150 counts) Full size image

Raman scattering and X-ray photoelectron spectroscopy

The comparison of the Raman spectra of pristine graphene20,21 and GNS (shown in Fig. 2a) demonstrates that the GNS has clearly maintained the graphene signature with the sharp 2D band (at ~2700 cm−1 with FWHM ~30 cm−1) and the intensity ratio of more than three between the 2D band and G band (at ~1580 cm−1). The sharp 2D band in GNS together with the absence of 2D band splitting reveals that the inner graphene layers lack π-stacking. The sharp 2D band signature is similar to that of monolayer graphene. Thus, the GNS in this work is significantly different from conventionally fabricated carbon nanoscrolls (CNS)22,23. The reason of the 2D band non-splitting phenomenon is mainly due to the deviation in the orientation from Bernal stacking24, which has been observed during the deformation of multilayer graphene25. The increased c-plane spacing as compared to graphite could contribute to the observation of the graphene signature in the Raman spectra. The D band at 1350 cm−1, normally indicating the presence of defects, is negligible in pristine graphene and has a slight increase in GNS. The G band, generated from the activation of the E 2g mode at the Γ points, shows a ~10 cm−1 low frequency shift in GNSs compared to pristine graphene (inserted graph in Fig. 2a), indicating a ~0.7%26 in-plane strain in the GNS layers. X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical modification of the pristine graphene immersed in HCOOH/H 2 O. The result as shown in Fig. 2b clearly shows that, before and after scrolling up, all peak areas, which correspond to different bonds, of pristine graphene and GNS remain nearly the same. This indicates that the synthesis mechanism is mainly a physical intercalation process and not a chemical reaction.

Fig. 2 Raman and X-ray photoelectron spectroscopy (XPS) characterization of the graphene nanoscroll (GNS). a Raman spectra comparison of pristine graphene and GNS. After spirally wrapped, the GNS shows the monolayer graphene featured spectra: sharp 2D band and I 2D /I G > 3. (Inserted plot shows the left-shift of G band after scrolling up). b XPS of the C 1s peaks shows no chemical reaction observed during the fabrication, meaning that physical intercalation and rolling up should be the main synthesis mechanism Full size image

Adhesion measurements

Atomic force microscope based adhesion measurements are employed to investigate the adhesion properties of the GNS27,28,29,30,31,32. This method is based on the nanoindentation of a sharp AFM tip made of silicon into a surface, in which the force-displacement curves shown in Fig. 3a can be recorded and the graphic illustration of different phases is shown in Fig. 3b. In our experiments, the AFM based adhesion measurements are carried out on the GNSs with the height range of 120–130 nm. Comparing the force-displacement curves, GNS displays a 2.5-fold stronger adhesion force than pristine graphene for the same setpoint as used for the measurement on graphene, which is visible in phase iv of Fig. 3a. It is noted that the force curve has a slow recovery after the jump-off point, and it could be explained by the possible presence of capillary force33 even if the experiments were carried out in a low humidity level of 20–27%. According to the work of Jiang et al.34, the humidity level of less than 30% plays a limited role in the adhesion force measurements by using AFM technique, thus the obtained adhesion forces in our experiments should be reliable even if the low capillary force still exhibits an uncertainty. Another explanation of the slow recovery phenomenon is the possible delamination of the GNS layers, which needs to be further studied.

Fig. 3 Adhesion property measurement of the graphene nanoscroll (GNS) and monolayer graphene. a Typical force-displacement curve comparison of the GNS (red) and the monolayer graphene (blue). The displacement represents the movement of the piezo, where the displacement of zero is chosen at the highest load force. The indentation consists of five main phases shown in (b): (1) tip approaching towards the sample. (2) tip contacting and deforming the sample. In this phase, the force is repulsive. (3) tip retracting and the samples starts to recovery. At first, the force shows repulsive force and after the recovery of the sample, it turns to an attractive force. (4) tip jumping out of the sample. The jumping point refers to the maximum adhesion force between the sample and the tip. (5) tip retracting backwards the sample. c Adhesion energies between the AFM tip and the GNS and monolayer graphene with standard deviation as error bar. The measurements of the GNS are carried out on 20 different nanoscrolls with the height 120–130 nm Full size image

The Derjaguin-Muller–Toporov (DMT) model35 is employed to determine the adhesion energy and can be expressed as (Supplementary Note 2):

$$F_{\mathrm{{ad}}} = - 2\pi R\it{\Gamma}$$ (1)