From Point Defects in Graphene to Two-Dimensional Amorphous Carbon

While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material has not been reported before. Here, using electron irradiation we create an $s{p}^2$-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step by step by nucleation and growth of low-energy multivacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.

Figure 1

(a) Amorphous two-dimensional $s{p}^2$-bonded carbon membrane created by a high-dose exposure of graphene to 100 keV electron irradiation in an HRTEM. The blue colored area is a single-layer carbon structure. Scale bar is 1 nm. (b) Fourier transforms (power spectra) from HRTEM images of the initial graphene configuration (top), an intermediate configuration (center), and of the amorphous 2D carbon (bottom).

Figure 2

Elementary defects and frequently observed defect transformations under irradiation. Atomic bonds are superimposed on the defected areas in the bottom row. Creation of the defects can be explained by atom ejection and reorganization of bonds via bond rotation. (a) Stone-Wales defect, (b) defect-free graphene, (c) $V_1(5\mathrm{\text{−}}9)$ single vacancy, (d) $V_2(5\mathrm{\text{−}}8\mathrm{\text{−}}5)$ divacancy, (e) $V_2(555\mathrm{\text{−}}777)$ divacancy, (f) $V_2(5555\mathrm{\text{−}}6\mathrm{\text{−}}7777)$ divacancy. Scale bar is 1 nm.

Figure 3

(a)–(d) Electron beam driven divacancy migration observed at 80 keV. In (e), the changes in the bond configuration required for allowing the migration of a divacancy are shown. Transformation $V_2(5\mathrm{\text{−}}8\mathrm{\text{−}}5)\to 2\times (5\mathrm{\text{−}}7)$ is initiated by rotating bond A, and $V_2(5\mathrm{\text{−}}8\mathrm{\text{−}}5)\to V_2(555\mathrm{\text{−}}777)$ by rotating bond B. Rotating bonds C and D will lead to the final $V_2(5\mathrm{\text{−}}8\mathrm{\text{−}}5)$ structures. In the first case the defect has moved by ${\mathbf{a}}_1\mathrm{\text{−}}{\mathbf{a}}_2$, and in the second case it has rotated by 60° around pentagon 2. The original TEM images without overlays for panels (a)–(d) are presented in Ref. 33.

Figure 4

Formation of rotated-hexagon kernels in multivacancy structures under a 100 keV electron beam. Scale bar is 1nm. The original TEM images without overlays are presented in Ref. 33.

Figure 5

dealized rotated-hexagon defects (a) formed from one, two, and three $V_2(5555\mathrm{\text{−}}6\mathrm{\text{−}}7777)$ divacancy defects, as optimized with DFT calculations. Electronic density of states (b) for pristine graphene and the structures presented in panel (a).