Current Nanoscience, 2012, 8, 89-96 89 Mechanical Properties of Graphene Nanobuds: A Molecular Dynamics Study Yongping Zheng 1* , Lanqing Xu 1,2 , Zheyong Fan 2 , Ning Wei 2 , Yu Lu 1 and Zhigao Huang 1* 1 School of Physics and OptoElectronics Technology, Fujian Normal University, Fuzhou 350007, People’s Republic of China; 2 Department of Physics, Institute of Theoretical Physics and Astrophysics, and Fujian Key Lab of Semiconductor Materials and Ap- plications, Xiamen University, Xiamen 361005, People’s Republic of China Abstract: Carbon-based hybrid nanostructures are believed to entail certain advantages of their parent low-dimensional materials and would serve as building blocks to bridge the nanoscale geometry to the large-scale application requirements. Fullerene, carbon nanotubes and graphene are suggested as ideal ‘building blocks’ for this kind of bottom-up strategy. In this work a series of hybrid gra- phene/fullerene architectures, termed graphene nanobuds, are constructed by attaching or fusing C60 molecules on a defect graphene sheet and the mechanical properties are investigated through molecular dynamics simulations. The elastic moduli are observed to degrade by a certain amount but are still rather high. The obtained Young’s moduli are of several hundred GPas and the tensile strengths are above 50 GPa. Furthermore, the intriguing feature of the nearly linear stress-strain response could attract intense follow-up investigations and could be utilized in various application branches such as position sensing. Keywords: Mechanical properties, fracture, nanostructure, fullerene, graphene, molecular dynamics simulation. 1. INTRODUCTION Low-dimensional carbon-based nanostructures with a perfect honeycomb lattice, namely, fullerene, carbon nanotubes (CNTs), and graphene, have attracted considerable attentions on the theo- retical research and the potential applications in material science owing to their exotic physical and chemical properties [1-12]. Among them the two dimensional monolayer graphene is especially attractive because numerous fascinating physical properties such as a massless Dirac electronic structure, extraordinarily high stiffness and strength, ultrahigh thermal conductivity, tunable chemical reac- tivity have been reported, which have elicited a large number of novel applications [13-17]. To bridge the scale from their nanos- tructural geometry to the requirements critical for meso- and mac- roscale device applications and to tune the functional properties through structure engineering, the combinations of CNT, fullerene and graphene are proposed, which further expands graphene's chemical, biomedical, mechanical, and electronic diversity [18-23]. The first hybrid low-dimensional carbon nanostructure successfully fabricated in the laboratory is the carbon nanopeapod, in which fullerenes are assembled as a chain in a CNT [24]. Another experi- mentally produced hybrid carbon nanostructure is carbon nanobuds, in which C 60 buckyballs are covalently bonded to the outer side- walls of a CNT [25]. Consequently, nanobuds exhibit properties of both CNTs and fullerenes. Moreover, the high reactivity of the attached fullerene molecules provides a versatile platform for the hybrid nanomaterial to be easily functionalized through known chemical modification. Recently, the periodically rippled graphene with fascinating electronic properties has been grown in the labora- tory [26]. The structural and electronic properties of a similar hy- brid material, periodic graphene nanobuds, were investigated theo- retically; and the results indicate that the graphene nanobuds can be either semiconductor or semimetallic, depending on the pattern of chemical bonding between C 60 and grapheme [27]. The conic Dirac points can be still preserved in some type of graphene nanobuds. The diversity in electronic structures renders the graphene nano- buds a promising candidate for nanoelectronic applications. One of the outstanding features of graphene is its extraordinary mechanical strength. Various experimental measurements as well as *Address correspondence to these authors at the School of Physics and OptoElectronics Technology, Fujian Normal University, Fuzhou 350007, People’s Republic of China; Tel: +86-0591-22867399; Fax: +86-0591- 83465373; E-mail: zyp@fjnu.edu.cn, zghuang@fjnu.edu.cn theoretical investigations in the open literature have shown that pristine graphene is strong and stiff, with Young’s modulus of around 1 TPa and fracture strength above 100 GPa [11, 28-35]. Meanwhile, the hollow structure of fullerene which consists of atoms densely packed along a closed surface manifests itself in dynamic properties of molecules, conferring so much the excellent elasticity as shells to the composite. In carbon nanobuds, the at- tached fullerenes could be used as molecular anchors to prevent slipping of nanotubes in various composite materials, thus modi- fying the composite’s mechanical properties [25]. In graphene nanobuds, the attached or fused fullerenes could form junctions with the graphene sheet. Intramolecular junctions have been found to influence the electronic structure and thermal transport signifi- cantly [36, 37]. But how the mechanical performance could be af- fected by the graphene/fullerene junctions is still unknown. The recent work by Lusk et al. suggested that graphene nanobuds could be constructed by introducing inverse Stone-Wales defects or haeckelites to a graphene sheet [38], and the periodic graphene ripples fabricated in the laboratory [26] further confirmed that gra- phene nanobuds is a working material feasible of manufacture. Therefore it is a matter of practical importance and theoretical in- terest to find the elastic responses of graphene nanobuds, as the mechanical stability is of vital importance in a wide range of nanoscale applications. In this work we constructed a series of hybrid carbon nanostruc- tures, termed graphene nanobuds, by attaching or fusing fullerene buckyballs on pored graphene sheet. The mechanical properties are then computed using these atomistic models via molecular dynam- ics (MD) methods to help understand how the mechanical behaviors are influenced or indeed governed by the micro structure conver- sions and junctions. 2. MODELS AND COMPUTATION METHODS 2.1. Models Two prototype building blocks of the graphene nanobuds based on hybrid C 60 molecules and a graphene monolayer have been cre- ated. The first type is similar to the experimentally synthesized carbon nanobuds where fullerenes are covalently attached to the outer sidewalls of the underlying nanotubes. Several possible ways of chemical bonding can arise, and the most stable one is found to be the [6+6] cycloaddition, where a hexagonal ring of C 60 is con- nected to a hexagonal ring of graphene sheet and forms six C-C 1 7 - /12 $58.00+.00 © 2012 Bentham Science Publishers