Carbon three-dimensional architecture formed by intersectional collision of graphene patches

Graphite is the most stable form of carbon under room temperature and atmospheric pressure, and consists of two-dimensional honeycomb lattices with intralayer $s{p}^2$ bonding and rather weak van der Waals like interlayer interaction. When we supply gaseous small carbonic molecules such as methane to a patch of graphene, the patch will grow into graphite. Now, let us imagine a slightly different situation. Is a layered structure of graphite always formed, when we supply not methane molecules but another graphene patch? The answer from our computer simulations is “No.” Some graphene patches collide in parallel, but others at right angles, which result in a formation of junction structures (graphitic Y junctions). These junction structures are different from those of common sense for graphitic materials. Performing density functional calculations, we found that the reaction barrier height required for the formation of graphitic Y junctions are almost zero, and the binding energies per bond for each structure are $\sim 1\mathrm{eV}$. Furthermore, tight-binding molecular dynamics simulations showed high thermal stability and high formation probabilities for these junction structures. As applications of graphitic Y junctions, we will present two interesting structures, where we focus on the magnetic properties of junction structures and nanotube T-junction structures which are different from conventional models. We expect that graphitic Y junctions might be hidden in graphitic soot and not characterized yet in experiment. 3D architectures constructed from those unit structures are expected to have various applications with lightweight, ferromagnet, high molecular storage and high thermal conductor.

Figure 1

(Color online) Schematic view of GYJ $A$, $B$, and $C$ which are three-dimensional junction structures of $s{p}^2$ network, where red and gray spheres represent carbon atoms. Each structure can be divided into two graphene sheets, where one graphene sheet (red spheres) is attaching the armchair or zigzag edge to the surface of the other (gray spheres). For the GYJ $C$, the target graphene sheet has a line of defect array with 5–8 membered rings.

Figure 2

(a) The potential energy curves for GYJ $A$ (rectangles), $B$ (triangles), and $C$ (circles) formation as a function of reaction coordinate. The reaction coordinate is normalized by the length along the minimum energy path in the configuration space. The origin of potential energy is taken as that of initial configuration. (b) The cross-sectional views of atomic structures correspond to the formation steps indicated by the arrows in Fig. 2a.

Figure 3

Snapshots of molecular dynamics simulations for the formation of GYJ $A$ from a collision of a graphene sheet and a graphitic ribbon (a) at right angles, (b) with $1\mathrm{\AA }$ shift perpendicular to the ribbon surface, (c) with 40° tilt angle, and (d) with 15° of azimuthal angle rotation. (e) GYJ formation from the reaction of zigzag edge and graphene surface with 30° of azimuthal angle.

Figure 4

(Color online) Formation probabilities for collisions of a graphene sheet and a graphitic ribbon or a patch as a function of (a) tilt angle and (b) azimuthal angle. Open circle (red lines), rectangle (blue lines), and triangle (green lines) represent formation probabilities for GYJ $A$, $B$, and $C$ junctions, respectively. Solid and dotted lines correspond to $600\mathrm{K}$ and $1000\mathrm{K}$ of initial given temperatures.

Figure 5

(Color online) (a) The unit structure for a new 3D architecture formed of GYJ $B$ and $C$. The letters on each sphere classify the “A” site and “B” site. (b) Total spin density isosurfaces of the new architecture. Red isosurfaces represent majority spins density, and blue ones minority.

Figure 6

(Color online) Resulting structures for the coalescence of nanotubes (17,0) sidewall (blue spheres) with termination free (a) (5,5) edge and (b) (9,0) edge (gray spheres). Fourfold atoms at the junction, which originally belong to the (17,0) nanotube, are indicated by the red spheres.