PCCP View Article Online PAPER View Journal | View Issue Fullerenes generated from porous structures† a b b Cite this: Phys. Chem. Chem. Phys., Ricardo Paupitz,* Chad E. Junkermeier, Adri C. T. van Duin and c 2014, 16, 25515 Paulo S. Branicio A class of macromolecules based on the architecture of the well-known fullerenes is theoretically investigated. The building blocks used to geometrically construct these molecules are the two dimensional structures: porous graphene and biphenylene-carbon. Density functional-based tight binding methods as well as reactive molecular dynamics methods are applied to study the electronic and structural properties of these molecules. Our calculations predict that these structures can be stable up to temperatures of 2500 K. Received 7th August 2014, The atomization energies of carbon structures are predicted to be in the range of 0.45 eV per atom to Accepted 2nd October 2014 12.11 eV per atom (values relative to the C60 fullerene), while the hexagonal boron nitride analogues DOI: 10.1039/c4cp03529a have atomization energies between À0.17 eV per atom and 12.01 eV per atom (compared to the B12N12 fullerene). Due to their high porosity, these structures may be good candidates for gas storage and/or www.rsc.org/pccp molecular encapsulation. 1 Introduction we included in the present study not only carbon-based struc- tures, but also their hBN counterparts. In the last few decades, materials scientists have thoroughly Theoretical studies have proposed super structures based on investigated the properties of many carbon-based structures. This fullerene,18,19 nanotube20 or graphene21 architectures. These quest resulted in the experimental verification of fullerenes,1 superstructures often mimic the pattern of one of the well- carbon nanotubes (CNT),2 and graphene.3,4 Further effort has known carbon allotropes but use another structure as the been put into understanding changes in the mechanical, building block. For example, Coluci et al. used single walled electrical, and chemical properties due to defects,5 adsor- CNTs linked together with y-like junctions to form a super bates,6–9 and electromagnetic fields,10 for instance. Also, the nanotube.20 These structures can present interesting properties, Published on 03 October 2014. Downloaded by UNESP-ARAIQ 20/02/2015 16:35:56. synthesis of these carbon based materials has led to great for instance, mechanical stability at high temperatures, and can interest in inorganic analogues, such as silicene,11,12 germa- be promising alternatives in the development of highly porous, nene, and hexagonal boron nitride13 (hBN), which are, respec- flexible, and high strength materials. Calculations regarding the tively, the silicon-based, germanium-based, and boron nitride electronic properties of some of these materials have shown the analogues of graphene. In this context, the latter is of special possibility of metallic or semiconducting behavior depending on interest, due to the fact that hBN presents the same honeycomb the specific construction of the superstructure.22 morphology with almost identical bond lengths and cell para- Well before the experimental verification of graphene, theorists meters as its carbon counterpart. These similarities point to the were predicting alternative two-dimensional carbon structures. possibility of synthesis of several boron nitrite nanostructures These structures have the potential to overcome some of the equivalent to known carbon nanostructures.14–17 Although these inherent limitations of graphene (i.e. zero band gap). Baughman compounds are geometrically similar, their electronic properties et al. studied many such structures comparing their mechanical are quite different leading to interesting contrasts, for instance, properties and formation energies predicting that graphyne (also the zero-width band gap of graphene compared to the wide called 6,6,6-graphyne or g-graphyne) would be one of the easiest band gap of hBN. Taking into account these interesting facts, to synthesize.23 Recently, a new synthesis route, using dehydro- genation of porous graphene, has been suggested such that a Departamento de Fı´sica, IGCE, Univ Estadual Paulista, UNESP, 13506-900, biphenylene-carbon (BPC) could be synthesized conveniently 24,25 Rio Claro, SP, Brazil. E-mail: [email protected]; Tel: +55 019 3526 9156 as well. b Department of Mechanical and Nuclear Engineering, The Pennsylvania State Alternatively, the discovery of the C60 molecule generated a University, University Park, PA 16802, USA tremendous expectation that new materials with novel proper- c Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ties could be synthesized with fullerene-like structures as a † Electronic supplementary information (ESI) available. See DOI: 10.1039/ building unit. However, the synthesis of these structures has c4cp03529a always been challenging. Nonetheless, steady progress has been This journal is © the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 16, 25515--25522 | 25515 View Article Online Paper PCCP made for the identification of synthesis routes for closed cage The porous hBN analogs of C20 and C60 resulted in systems structures, e.g. fullerenes and heterofullerenes. Ray et al. reported that had B–B or N–N bonds and were thus not included in this the synthesis of silicon-doped heterofullerenes in the carbon-rich work since their geometry precludes consistent closed shell limit based on quenching of SiC plasma, generated by laser construction. The considered structures and architectures are vaporization, with a high pressure helium burst promoting shown in Table 1. The second and third rows are fullerenes based growth of SiC heterofullerenes and ejection.26 Hultman et al. on porous graphene and graphenylene,38 while the last two reported the generation of CN aza-fullerenes consisting of cross- rows contain the hBN analogues, porous hBN and inorganic linked nano-onions of C and N by magnetron sputtering.27 Narita graphenylene.39 Each molecule was then optimized, the electronic et al. reported the synthesis of hBN fullerenes by an arc-melting properties computed, and the range of temperature at which of a combination of B rich metal powders and catalytic metals they are stable in the gas phase was estimated. 28 (Nb, Y, and Sc) in a N2/Ar atmosphere. Nakamura et al. reported the synthesis of heterofullerenes by a direct hBN substitution 29 reaction of C60 under irradiation using a KrF excimer laser. 2 Methodology Melinon et al. discussed the theoretical principles and the experimental work on SiC cage-like clusters.30 Recently, the use Geometry optimizations and electronic structure calculations of a laser-vaporization cluster source to achieve the synthesis of of all the molecules in Fig. 1 were performed using DFTB in its 31 the all-boron fullerene, namely B40, was reported by Zhai et al. self consistent charge (SCC)-DFTB version as implemented in The use of heated metallic surfaces to promote dehydrogenation the DFTB+ code.33 DFTB methodology was conceived to benefit of aromatic precursors as an alternative route for the generation from both the low computational cost of tight binding methods of fullerene structures was also reported.32 The identification of and of the DFT capability to describe a wide range of phenomena these new synthesis routes points to the need to investigate for several classes of molecules and compounds.40 This metho- theoretically the structures and superstructures created from dology is based on a second-order expansion of the Kohn–Sham BPC and related building blocks. energy defined in DFT and has been used successfully to describe Using a combination of density functional tight binding a great variety of systems, which include organic molecules, methodology (DFTB)33 and a reactive force field34 we investi- hydrocarbons, and systems of interest for materials scientists, gate the electronic and structural properties of the closed shell such as solids and surfaces.33,41 It is known that energy and gap molecules based on the architectures of well known fullerenes values obtained with this methodology usually are underesti- in which the building blocks are the unit cells of porous mated, since it considers only valence electrons.42 For this graphene and BPC, as well as their hBN structural analogs.35 reason, DFTB+ is particularly useful when the interest is to obtain The porous graphene and BPC unit cells are shown in Fig. 1. As a relative energy values or to explain trends in a series of large starting point we used the architectures of some of the simplest atomic structures. The particular parameterization used in the fullerenes found in the literature,1,36 these are C20, C24, C36, present work was already tested in other systems of interest for C48 and C60 shown in the first row of Table 1. For each carbon– materials science.43 Inthecaseofgeometryoptimizations,a carbon bond in one of these structures we substituted one of the nonperiodic system was considered and a conjugate gradient Published on 03 October 2014. Downloaded by UNESP-ARAIQ 20/02/2015 16:35:56. unit cells shown in the first column of Table 1, we were able to algorithm was applied during the search for the minimal energy construct several ‘‘closed shell’’ molecules. These unit cells were configuration. As convergence criteria for the geometrical search chosen for their symmetry and for the fact that each one was and for the SCC iterations we adopted a maximum force difference either predicted and/or already synthesized in two-dimensional of 10À5 and a maximum tolerance of 10À4 respectively. sheets. Some of these structures were already proposed elsewhere.37 Since our interest was to understand the behavior of these molecules with respect to temperature, a stability study was performed using ReaxFF.34 ReaxFF is a reactive force field method developed by van Duin, Goddard III and co-workers, which allows the calculation of chemical reactions.44,45 ReaxFF has some characteristics that are similar to those found in standard non-reactive force fields.