| AVS 66th International Symposium & Exhibition | |
| 2D Materials | Monday Sessions |
| Session 2D+AP+EM+MI+MN+NS+PS+TF-MoA |
| Session: | Nanostructures including Heterostructures and Patterning of 2D Materials |
| Presenter: | Jun Nakamura, The University of Electro-Communications (UEC-Tokyo), Japan |
| Authors: | M. Ushirozako, The University of Electro-Communications (UEC-Tokyo), Japan H. Matsuyama, The University of Electro-Communications (UEC-Tokyo), Japan A. Akaishi, The University of Electro-Communications (UEC-Tokyo), Japan J.N. Nakamura, The University of Electro-Communications (UEC-Tokyo), Japan |
| Correspondent: | Click to Email |
Recently, nano-scale graphene nanoflakes (GNFs) have attracted great attention as one of the promising materials for electronics and spintronics. Kim et al. have successfully fabricated GNFs with various sizes up to 35 nm and have reported that the photoluminescence property of GNFs depends on the size and the edge shape [1]. From the view point of the structural stability of GNFs, we have not yet acquired the systematic comprehension with regard to effects of shapes and sizes of GNFs on the stability. In the present study, we have examined how the stability of GNFs is dominated by the edge shape and the size of GNFs, using first-principles calculations within the density functional theory.
In order to evaluate the stability of GNFs, we calculated the edge formation energy. First, we consider GNFs with the six-fold symmetry (D6h) and classify them into zigzag GNFs (ZZGNFs) and armchair GNFs (ACGNFs). ACGNFs have two subtypes, AC(1) and AC(2), depending on whether carbon atoms are just at the corner of the outermost envelope hexagon of GNFs. We define the edge purity as the ratio of the number of carbon atoms at the edge unambiguously regarded as the armchair to the total number of edge atoms. The purity of AC(1) is higher than that of AC(2). The chemical formulae associated with ZZ, AC(1), and AC(2) are C6n2H6n, C18n2-18n+6H12n-6, C18n2-30n+12H12n-12, respectively. In addition, we also evaluate the structural stabilities of triangular and rhombus GNFs.
We calculated the edge formation energy of the GNFs having up to 1200 carbon atoms as a function of the number of edge carbon atoms [3]. The formation energy of ZZGNFs is higher than that of ACGNFs irrespective of the size of GNFs. This instability of ZZGNFs is attributed to the presence of the so-called edge state. Indeed, it has also been shown that the formation energy of the zigzag graphene nanoribbon is higher than that of the armchair one [4]. It is noted that AC(2) is slightly more stable than AC(1), whereas the purity of AC(2) is lower than that of AC(1). Such peculiar stabilization can be reasonably explained in terms of the aromaticity of GNFs. The Nucleus Independent Chemical Shifts (NICS) values, which is averaged for the six-membered rings in GNFs, for AC(2) are lower than those for AC(1). This means AC(2) is more aromatic than AC(1). We will discuss the quantitative relationship between the stability and the aromaticity of GNFS.
[1] S. Kim et al., ACS Nano, 6, 9, 8203 (2012)
[2] W. Hu et al., J. Chem. Phys. 141, 214704 (2014)
[3] A. Akaishi, M. Ushirozako, H.Matsuyama, andJ.Nakamura, Jpn.J.Appl.Phys. 57, 0102BA(2018)
[4] S. Okada. Phys. Rev. B, 77, 041408 (2008)