Speaker: Lili Liu
Beijing Computational Science Research Center
Abstract:Graphene, a carbon-based compound has attracted considerable attention according to its unique electronic, thermal, and mechanical properties. But different from the graphite, single-layer graphene flakes were early predicted to be inherently unstable. In this case, we studied the bond alternation patterns (BLA) on the high symmetry hexagonal graphene flakes (HGFs) to compare with the uniformgraphite structure usingSelf-Consistent Charge Density-Functional Tight -Binding (SCC-DFTB) method. First we performed benchmark studies on systems up to the size of about 100 atoms at the HF, BLYP, B3LYP, MP2 and SCC-DFTB methods. We found that SCC-DFTB method reproduces BLA patterns of DFT and ab initio methods optimized structures well and shows excellent agreement with B3LYP/SVP and MP2/SVP optimized geometries. Then we analyzed the BLA patterns on three series of HGFs (zigzag, armchair and chiral edge), which was calculated using SCC-DFTB method. It was found that zigzag HGFs alternate between Clar and Kekule patterns, quickly lose BLA patterns in their centers, show faster convergence to graphite, and armchair HGFs show persistent Clar structures, even in very large flakes with more than 4000 atoms. Chiral HGFs exhibit intermediate BLA patterns between zigzag and armchair. Therefore, we show that the geometrical transition from finite-size hydrogen-terminated graphene to infinite graphite depends strongly on the edge structures.
In addition, In order to identify the thermodynamically most favorable configuration of a monovacancy defect in finite-size graphene flakes, we studied molecular and electronic structures of the planar 5/9 and non-planar spiro monovacancy isomers whose relative stabilities are determined by their distance from the graphene edge using B3LYP/6-31G(d), GGA-PBE/DZP, SCC-DFTB with finite electronic temperature and SDFTB (DFTB without and with spin-polarization included) methods. Different from previous describing that graphene monovacancies only as a magnetic 5/9-isomer with lowered symmetry, we found that the non-planar spiro-isomer is the most stable structure for monovacancies when the defect is close to the graphene flake periphery (d < 7 Å). We also performed high-temperature thermal annealing using quantum chemical molecular dynamics simulations based on the SCC-DFTB method, and found that an interior monovacancy defect is subject to migration towards the outermost periphery of a graphene flake, indicating an efficient route for defect healing. The associated dihedral angles and considerable room-temperature stability make the spiro-isomer an ideal structural building block for the design and synthesis of carbon spiral helixes. Our results predict a family of novel carbon architectures that can be derived from graphene monovacancies.
Date&Time: June 1, 2012 14:30 - 15:30
Location: 606 Conference Room
