Graphene nanoengineering and the inverse Stone-Thrower-Wales defect

We analyze a fundamental building block for monolithic nanoengineering on graphene: the Inverse-Stone-Thrower-Wales (ISTW) defect. The ISTW is formed from a pair of joined pentagonal carbon rings placed between a pair of heptagonal rings; the well-known Stone-Thrower-Wales defect is the same arrangement, but with the heptagonal rather than pentagonal rings joined. When removed and passivated with hydrogen, the structure constitutes a molecule, diazulene, which may be viewed as the result of an ad-dimer defect on anthracene. Embedding diazulene in the honeycomb lattice, we study the effect of ad-dimers on planar graphene. Because the ISTW defect has yet to be experimentally identified, we examine several synthesis routes and find one for which the barrier is only slightly higher than that associated with adatom hopping on graphene. ISTW and STW defects may be viewed as fundamental building blocks for monolithic structures on graphene. We show how to construct extended defect domains on the surface of graphene in the form of blisters, bubbles, and ridges on a length scale as small as $2\text{Å}\times 7\text{Å}$. Our primary tool in these studies is density functional theory.

Figure 1

(Color online) (a),(b) Stone-Thrower-Wales and (c),(d) ad-dimer defects on carbon nanotubes. The defects are shown with achiral (a),(c) and chiral (b),(d) CNTs.

Figure 2

(Color online) (a),(b) Top and perspective views of an ISTW defect. (c),(d) Pi-bonding is shown as lobes above and below a hydrogen-passivated ISTW defect: (c) isosurface (0.03) of electron density for the highest occupied molecular orbital; (d) electron density cross section, centered on the adatoms, of the highest occupied molecular orbital. The units of both isosurface and cross section are $\text{electrons}/{\text{Å}}^3$.

Figure 3

(Color online) Geometric explanation for the out-of-plane distortion of an ISTW defect. (a) ISTW defect without graphene has planar ground state and constitutes a new molecule, (b) addition of 6 of the 10 carbon rings required to encircle the ISTW defect also has a planar ground state, (c) addition of the remaining carbon rings pulls the left and right sides together at both top and bottom, and this causes out-of-plane distortion, (d) side view of image in (c).

Figure 4

(Color online) ISTW defect on periodic domain with 182 carbon atoms showing (a) edge view in which adatoms are aligned, (b) edge view in transverse to adatom alignment, and (c) perspective view. The black lines indicate the boundaries of the cell.

Figure 5

(Color online) Side (a) and perspective (b) views of ISTW defect on a freestanding, hydrogen-passivated graphene sheet with 182 carbon atoms.

Figure 6

(Color online) ISTW defect on periodic domain with 182 carbon atoms showing (a) electrostatic potential well of 1.36 V atop the defect, (b) electron density isosurface of the difference in electron density between charged and uncharged systems, (Blue indicates a value of $0.002\text{electrons}/{\text{Å}}^3$ while yellow is the negative of this. The integral of this density field is equal to unity), and (c) electrostatic potential in a plane $1.2\text{Å}$ above the defect tangent plane of its rising right surface. (The perspective is from beneath the defect in order to clearly show the relation of the extrema to the carbon nuclei.)

Figure 7

(Color online) Synthesis of an ISTW defect by meeting of two adatoms. The top adatom takes two hops toward the other via bridge sites from a position (a) in which the adatom interaction is negligible. The hopping barriers are 0.57 and 0.44 eV, respectively. The second hop causes a significant restructuring with a 6.89 eV drop in energy. The result is an ISTW defect (c, d).

Figure 8

(Color online) The first adatom hops to an STW defect in the synthesis of an ISTW-STW blister: (a)–(c) correspond to A1–A3 in Fig. 10a. (d) Side perspective view of (c).

Figure 9

(Color online) The second adatom hops to an STW defect in the synthesis of an ISTW-STW blister, following from Fig. 8: (a)–(d) correspond to B1–B4 in Fig. 10b.

Figure 10

Reaction path for creation of an ISTW-STW blister by adding adatoms to an STW defect, corresponding to Figs. 8, 9. (a) Path A1–A3 tracks the hopping of the first adatom to the STW defect (Fig. 8). (b) Path B1–B4 tracks the hopping of the second adatom (Fig. 9).

Figure 11

(Color online) Linear arrangements of ISTW defects: (a)–(b) close spacing of two defects; (c)–(d) wider spacing of two defects; (e)–(f) a monolithic ridge of defects that could be used to influence the direction of charge transport as a nanowire in monolithic graphene-based electronics

Figure 12

(Color online) Nonlinear arrangements of ISTW defects can be used to create a closed contour which results in a raised graphene bubble. Top view.

Figure 13

(Color online) Graphene bubble: side perspective view of Fig. 12.