CARBON 90 (2015) 34– 43 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon Self-assembly of fullerenes and graphene flake: A molecular dynamics study Jia-wei Feng a, Hong-ming Ding a, Yu-qiang Ma a,b,* a Collaborative Innovation Center of Advanced Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China b Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China ARTICLE INFO ABSTRACT Article history: The interaction between graphene and other materials is of great importance for its poten- Received 9 January 2015 tial applications in nanotechnology. In this paper, by using all atom molecular dynamics Accepted 27 March 2015 simulations, we systematically investigate the self-assembly of fullerenes (diameter from Available online 3 April 2015 0:7to2:3 nm) and graphene at room temperature (300 K). It is found that single fullerene can be wrapped by graphene nanoribbon (GNR) due to the van der Waals interaction between them. However, if the GNR is wide enough, the fullerene will only bind to the sur- face of GNR. To overcome the bending energy of wide GNR, we further use multiple fullere- nes, and find that they can self-assemble into various structures. Importantly, fullerenes show dramatically different behaviors as the size changes. Giant fullerenes can work together to scroll a very large graphene from the corner, while this effect is weakened or even disappears in the cases of smaller ones. Finally, we also find that the wrapping process can be completely retarded by adding a substrate below the graphene flake. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction attracted a lot of interest because of its remarkable structure [15–18]. GNS not only has some similar mechanical and elec- Graphene, as a two-dimensional thin film material, has trical properties with carbon nanotube (CNT) (e.g., water received great attention due to its unique electrical and transportation behaviors [19–21]), but also represents unique mechanical properties since it was firstly exfoliated from features such as tunable inner space [22]. A report by the bulk graphite [1]. With the development of nanotechnol- Buehler et al. has shown that graphene can be assembled into ogy, various experimental methods have been applied to fab- nanoscroll under high aspect ratio to minimize the surface ricate graphene with desired geometry [2–5]. Besides, the energy [16]. Moreover, graphene can be guided to scroll on a physical and chemical properties of graphene can be easily CNT, forming a stable core–shell nanostructure [23,24]. controlled [6–8], which makes graphene a promising candi- Other rod-like materials such as silicon nanowire, iron nano- date for real applications such as nanoelectronic devices [9], wire, etc. can also activate the scrolling of graphene [25,26]. biofunctional materials [10,11], solar cells [12] and sea water Recently, molecular dynamics simulations even show that a desalination [13,14]. drop of water or ionic liquid can fold graphene into different However, planar graphene is unstable and can be kinds of nanostructures [27–29]. Experimentally, the encap- deformed into graphene nanoscroll (GNS), which has also sulation of metal nanoparticle by graphene has also been * Corresponding author at: Collaborative Innovation Center of Advanced Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China. E-mail address: [email protected] (Y.-q. Ma). http://dx.doi.org/10.1016/j.carbon.2015.03.071 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved. CARBON 90 (2015) 34– 43 35 observed [30,31]. The authors claimed that the interaction than 1 Â 10À4 eV or the 2-norm of the global force vector is between metal nanoparticle and oxygenated defects on gra- less than 1 Â 10À6 eV=A),˚ followed by an MD run until equili- phene accounts for this phenomenon. However, nanoparticle brated (total energy and structure of the system keep is very different from one-dimensional nanorods above. So it unchanged for 500 ps). is still unclear how nanoparticles roll up graphene and make itself encapsulated. 3. Results and discussion Here, by using molecular dynamics simulations, we choose fullerene as a prototype to investigate the self-assembly of 3.1. Self-assembly of one fullerene and GNR nanoparticles and graphene. Actually, the fullerene–graphene composite was found to exhibit photoinduced electron trans- Our simulation results show that fullerene with appropriate fer, which indicates that the composite can be a candidate for size can activate the scrolling of GNR. As illustrated in fabrication of all-carbon solar cells [7,32]. Fullerenes with dif- Fig. 1a, one C540 is firstly put at one end of the GNR with a ferent numbers of atoms (from 60 to 540) are used to mimic width of 3:6 nm. After energy minimization, we observe that the nanoparticles with different sizes (diameter from 0.7 to the C540 exhibits an icosahedral shape (Fig. 1a, 0 ps), which is : 2 3 nm). As we will show below, a single large fullerene can in accordance with previous report [40] and validates the be wrapped by graphene nanoribbon (GNR), forming a core– choice of force field. In the beginning, the C540 firstly binds shell structure, and the self-assembly of two fullerenes and to the surface of GNR and diffuses to the corner. Then, the GNR shows more versatile configurations. Further, the effects corner of GNR gradually adsorbs to the surface of C540 (from of fullerene size and number on the assembly will also be 10 to 30 ps). After that, the GNR wraps around the fullerene investigated in details. and finally reaches a fully wrapped state (we name it as core–shell structure) at 87 ps. Such core–shell structure 2. Modeling and methods remains unchanged until the end of the simulation, which implies that it’s quite stable. Here, we consider a system comprised of one graphene flake In order to elucidate the driving force of this phenomenon, and one or more fullerenes. Fullerenes with different sizes we calculate the total potential energy (Etotal), the energy of the single GNR (EGNR), the energy of fullerene (EFUL) and the (i.e., C60,C180,C240,C320 and C540) are chosen. Besides, since graphene is an isotropic elastic material, the choice of edge interaction energy between GNR and fullerene (Einter). The shape doesn’t affect its mechanical properties [33]. For the interaction energy is defined as sake of simplicity, we chose armchair-edged graphene flakes Einter ¼ Etotal À EGNR À EFUL: ð1Þ along X direction unless otherwise stated. Zigzag-edged gra- As shown in Fig. 1b, the total energy keeps nearly constant phene will also be briefly studied for comparison. Hydrogen during the first 50 ps. And we can observe a slight increase atoms are covalently bonded to the edges of graphene flake of the E (Fig. 1c) and a slight decrease of the E to stabilize the carbon atoms with dangling bonds. All GNR inter (Fig. 1d). We don’t show the E because the energy of fuller- together, three types of systems are constructed. The first FUL ene doesn’t change (i.e., it behaves like a rigid nanoparticle) one contains single fullerene and graphene, with the size of during the whole simulation time. As shown in Fig. 1a, in graphene ranging from 0:4 Â 8:5nm2 to 6:0 Â 8:5nm2. The the beginning, the fullerene seeks an appropriate config- second one is almost the same as the first one, except that uration (i.e., binding to the corner) for the following wrapping we include two fullerenes with the same type in the system process. If the fullerene bends the GNR from the edge, there and elongate the GNR to 16:0 nm. The third one contains mul- will be a sharp increase of EGNR compared to bending from tiple fullerenes (three to six) and one graphene flake with size the corner which results from the increase of the bending from 0:4 Â 16:0nm2 to 16:0 Â 16:0nm2. energy. The bending energy (Ebend) is given by [16] All the simulations are carried out by using the LAMMPS Z 1 package [34]. The adaptive intermolecular reactive empirical E ¼ Dj2ðx; yÞdxdy; ð2Þ bend 2 bond-order (AIREBO) potential function is applied to describe A the bonded and nonbonded interactions [35]. This potential where D is bending rigidity, j is the mean curvature at ðx; yÞ has been proven to be one of the most successful potential and the integral is calculated over the bending area. It should functions to describe the intra- and intermolecular interac- be noted that the energy change of an elastic membrane (gra- tions in hydrocarbon systems [36–38]. Simulations are per- phene) mainly comes from two aspects, i.e., the elastic energy formed in the NVT ensemble at 300 K using the Nose´- and the bending energy. Here, the change of the EGNR mainly Hoover thermostat [39] with a damping constant of 0:1 ps. comes from the bending energy [16]. So it is more energy- We also use 1 K and 500 K to investigate the temperature favorable to initialize the wrapping from the corner. Since dependence. The periodic boundary conditions are applied the structure doesn’t change much in the beginning (see in all directions and the vacuum separations between gra- Fig. 1a), the total energy keeps unchanged during this period. phene flake and box edge are at least 2 nm (i.e., at least Then, after 50 ps, when the GNR wraps around the fullerene, 4 nm between graphene flake and its mirror). The time step we observe a sharp increase of the EGNR (induced by the bend- is 1 fs and data are collected every 1 ps.