Ab initio vibrational and thermal properties of carbon allotropes: Polycyclic and rectangular networks G.A. Nemnes a,b,⇑ , Camelia Visan b a University of Bucharest, Faculty of Physics, Materials and Devices for Electronics and Optoelectronics Research Center, 077125 Magurele, Ilfov, Romania b Horia Hulubei National Institute for Physics and Nuclear Engineering, 077126 Magurele, Ilfov, Romania article info Article history: Received 18 April 2015 Received in revised form 1 July 2015 Accepted 4 July 2015 Keywords: Graphene allotrope Phononic band structure Nanoribbon Thermal conductance abstract Ab initio investigations of vibrational and thermal properties of carbon allotropes are performed using density functional theory calculations. Networks with different symmetries are investigated – polycyclic and rectangular structures. As infinite two dimensional sheets, the former class of systems present phononic gaps in the terahertz frequency domain, while one of the rectangular structures reveals a pseudogap in the same frequency range. Implications to the thermal transport are analyzed compara- tively for nanoribbon systems, which indicate additional pseudogaps at lower frequencies. These are important for decreasing the thermal conductance, which is essential in the development of efficient thermoelectric devices. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Graphene is a two-dimensional (2D) material with remarkable electronic and thermal properties, which consolidated its status as a prime candidate for high-speed nanoelectronics, sensing devices, energy storage and bio-applications [1]. The linear disper- sion in the electronic band structure of graphene leads to unusual excitations that can be described in terms of massless two-dimensional Dirac fermions. The relativistic nature of carriers in graphene shows perfect chiral tunneling through high and wide potential barriers, a phenomenon known as Klein paradox [2]. This also brings a limitation of using pristine graphene in field effect applications, as the bandgap opening cannot be simply controlled by applying an external electric field. On the other hand, efficient thermoelectric devices require high electrical conductivity, while keeping the thermal conductivity as low as possible. A lot of effort has been devoted to nano-engineering graphene, by introducing atomic scale defects [3] or strain [4], by forming hybrid materials [5,6] and layered structures [7,8] in order to con- trol the electronic and transport properties. Another route investi- gated in recent years is the exploration of other carbon-based 2D networks commonly labeled as graphene allotropes [10]. These are structures which contain non-C 6 polygons and may also include other groups of carbon atoms like, e.g., acetylenic linkages. Networks of this kind are obtained by replacing the hexagons with pentagons and heptagons as in pentaheptites [11,12]. Intermediate structures, containing C 6 hexagons, C 5 pentagons and C 8 are called haeckelites [13]. Graphynes are another group of carbon allotropes, which consist of C 6 hexagons linked by carbon chains [14]. Other related structures are the so-called graphdiynes [15] and super- graphenes [10]. Polycyclic networks containing C 12 dodecagons and C 3 cycles, squarographenes and other rectangular networks have been proposed as possible carbon allotrope candidates. The electronic properties of different carbon allotropes have been investigated using a density-functional-based tight-binding method by Enyashin and Ivanovskii [9,10] and by means of first-principles calculations [16] as in the case of planar C 4 carbon sheets and nanoribbons. The spectroscopic and optical properties of porous graphene structures were investigated by Brunetto et al. [17] and by De La Pierre et al. [18], the latter including IR and Raman spectra. In addition, the vibrational properties were in the focus of a number of studies, like the Raman fingerprints [19], elastic properties [20] and, recently, structure-mediated ther- mal transport [21] was analyzed in graphynes, in carbon allotropes comprised of octagons and pentagons [22] and other carbon allotropes with helical chains of complementary chirality [23]. In this paper we investigate two of the aforementioned classes of carbon allotropes, more specifically structures which belong to the polycyclic and rectangular networks, using ab initio density functional theory (DFT) calculations. The electronic properties are first investigated for the infinite two dimensional sheets and http://dx.doi.org/10.1016/j.commatsci.2015.07.007 0927-0256/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: University of Bucharest, Faculty of Physics, Materials and Devices for Electronics and Optoelectronics Research Center, 077125 Magurele, Ilfov, Romania. Tel.: +40 (0)21 457 4949/157. E-mail address: nemnes@solid.fizica.unibuc.ro (G.A. Nemnes). Computational Materials Science 109 (2015) 14–19 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci