We investigate how a box of water molecules affects the HOMO—LUMO gap of a rectangular graphene nanodot (GND) with two zigzag and two armchair edges, using a combination of first principles and molecular mechanics, and also classical molecular dynamics. A GND is solvated in a periodic box of water molecules, and the HOMO—LUMO gap is computed for some snapshots taken from a molecular dynamics simulation. Although an isolated GND has a semiconductor state with degenerate a and/3 gaps, we find that, in a solvated GND, that degeneracy is broken and the gaps of both spins flavors oscillate following the time fluctuations in strength and direction of the electric field generated by the solvent at the edges. The average electric field generated by the water molecules causes an effect equivalent to applying a uniform electric field of 0.16 V/A computed at the PBE level of theory. In particular, this field is not strong enough to change the GND semiconductor ground state to a half-metallic one in nanodots with dimensions smaller than 2.5 nm, as those studied here. These results can be useful in the design of sensors based on graphene, indicating that important fluctuations in the energy gap can occur if water molecules are present.
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机译：我们结合第一原理和分子力学以及经典分子动力学，研究了一盒水分子如何影响具有两个之字形和两个扶手椅边缘的矩形石墨烯纳米点（GND）的HOMO-LUMO间隙。将GND溶解在周期性的水分子盒中，并为分子动力学模拟中的某些快照计算HOMO-LUMO间隙。尽管隔离的GND的半导体状态具有退化的a和/ 3间隙，但我们发现，在溶剂化的GND中，退化被破坏，并且两种自旋风味的间隙随着强度和电场方向的时间波动而振荡通过边缘的溶剂。由水分子产生的平均电场所产生的效果等同于施加以理论PBE水平计算的0.16 V / A的均匀电场。尤其是，该场的强度不足以将GND半导体基态更改为尺寸小于2.5 nm的纳米点的半金属纳米点，如此处研究的那样。这些结果对于基于石墨烯的传感器设计很有用，表明如果存在水分子，则能隙中可能发生重要的波动。
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