Study On Multilayer Graphene And Its Adsorption For Hydrogen Atom

Since 2004,K.s.Novoselov and A.K.Geim,the Professor of the University of Manchester in the United Kingdom made single-layer graphene by mechanical exfoliation method,one-layer,bilayer,trilayer and few layers graphene has attracted extensive research and attention due to its novel physical and chemical properties(topology,unconventional superconductivity,magic angle,optics,catalysis,etc).Previous studies by Xue Qikun’s team have shown that in nano scale,ultra-thin metal materials stacked by multiple atomic layers,due to quantum size effect,some physical parameters will present unique quantum oscillation phenomenon different from the bulk.In this paper,we will study the quantum size effect in multilayer graphene system based on first principles calculations.In order to compare the results,we choose PBE,DFT-D2,vd W-DF and opt PBE-vd W to calculate the energy of multilayer graphene.It is found that the average monolayer energy of the last three kinds of functional considering van de Waals effect decays with the increasing layers,but the decay will finally stabilize to a fixed value,however the PBE functional has no such phenomenon.At the same time,the interlayer distance of multilayer graphene after ion relaxation is also calculated.The result by PBE functional method fluctuates greatly than bulk graphite,and by DFT-D2 functional is slightly smaller while by vd W-DF functional is slightly larger,and by opt PBE-vd W functional is almost unchanged.In addition,the Femi energy and Fermi wavelengths are calculated.It is found that with the increase of graphene layers,quantum size effect of Fermi wavelength tuned by the interlayer distance will appear.The half wavelength of Fermi wavelength will be closer to the interlayer distance of multilayer graphene.In addition,the calculated results also show that the density of states at the Fermi surface is 0 at 0 K,and the behavior near the Fermi surface is linear,which has almost no change with the increase of the number of graphene layers.At about 200 K,the electron density of states oscillates in odd and even layers with a period of two layers.For multilayer graphene,affected by hydrogen atom with the increasing layers,the energy gap of the multilayer graphene is changed from semimetal to semiconductor by adding the hydrogen atom to the top site,and the energy gap is opened about 1.5ev.the electron density near the Fermi surface decreases with the increase of the number of layers.The hollow and bridge site are still semi-metal.The adsorption energy will oscillate with the increasing layers except for the opt PBE-vd W functional.We tend to think that this is the result of quantum size effect.As a comparison,the characteristics of adsorption energy in the case of weak adsorption(water molecule)are also studied.