Water-ice Interface and Mechanical-magnetic Coupling in Two Dimensional Materials
Supervisor:guo wan lin
Two-dimensional materials,compared with bulk materials,attracted the attention of researchers for its excellent properties,such as flexibility,transparency,easy to lose heat,and so on.However,two-dimensional materials are very susceptible to the enviroment for their distinctive structures.The gas and water in the air are easy to influence the properties of the two-dimensional materials.As the materials and devices scale down to nanoscale,the coupling between local and external fields can be remarkable that tuning the properties of materials.Studying the coupling between the material and the interface provides a basis for the future applications.We systematicly studied the coupling between the water molecule and the materials at the interface in graphene and phospherene.Besides,we also studied the spin-magnetic coupling.We showed a strain-induced switching between antiferro-and ferro-magnetic states in niobium diselenide and niobium disulphide single layers through systematic first-principles calculations.The findings are briefly concluded as following,(1)We systematically studied the charge transfer between water and functional graphene through first principle calculations,and analyzed the mechanism of water evaporation induced electricity based on the experimental results.Referring to the X ray photoelectron spectroscopy datas in the experiment,we systematically studied the interaction between a water molecule and functional graphene.We considered fuctional groups of C-OH,C-O-C,C=O and O=C-OH.The adsorption energies shows that the water molecule is more like to adsorb on fuctional graphene,implying a relatively strong interaction,in accordance with the experimental results that functional graphene is hydrophilic.We compared the differences of water adsorption sites when water adsorption on the functional groups.The results showed that,the water molecule is combined to the functional group with the van de waals interaction,which is not affected by the adsorption configurations.But the charge transfer is influenced by the adsorption configurations.Compared to water adsoption on graphene,the charge redistribution is larger for water adsorption on functional graphene.Based on the above discussions,we thought the mechanism of the evaporation-induced electricity coming from two parts: 1)the interaction between the water molecules and the carbon layers results a large charge transfer,and the double layer is formed at the interface of water and carbon layers;2)evaporation provides the driving force for water flowing within the porous carbon sheets,inducing the voltage generation.(2)Based on the experimental results,we predicted the stucture of monolayered hexagonal ice with unshared edges on few layered graphene using first principle calculations.Referring to the low-temperature scanning tunneling microscopy analyses,we predicted the stucture of monolayer ice through comprehensive first principles calculations.This monolayered ice is built exclusively from water hexamers without shared edges and can move on and stand stably free from the substrate,in sharp contrast to the substrate lattice or confinement dependent layered ices and hexagonal ice with shared edges.We conducted DFT simulations on ice models with shared edge and unshared edge at the same misorientation angle and the same Moiré periodicity.After optimization,the hexagonal rings in the shared edge model are composed by hydronium and hydroxyl.We also performed quantum molecular dynamics simulations to compare the stability of the two models in the same DFT frame.The results showed that the hexagonal rings become disordered in the model with shared edge,while the unshared edge model can remain in the hexagonal rings,only few water molecules shifting a little from the original position on suspended monolayer graphene.The results show that the unshared edge model is much more stable and can better match to the experimental results.(3)We further studied the interaction between water and phosphorene,obtaining the potential energy surface of water adsorption,and we studied the possible water structures on phosphorene.Water adsorption on phosphorene has fundamental importance to device development from black phosphorus,but the stable adsorption configuration remains in confusion.Here we explore the water adsorption configurations on phosphorene through comprehensive first principle calculations.It is found that water takes an optimal adsorption configuration on phosphorene with one of the hydrogen atoms pointing to the surface of phosphorene.This optimal adsorption configuration is significantly more stable than all the configurations reported previously for the strongest van de Walls interaction between the adsorbed water and the phosphorene compared to other adsorption configurations.It is also shown that water serves as an acceptor in the optimal adsorption configuration,clarifying the confusing results from less stable configurations.Based on the studies of optimal water adsorption on phosphorene,we systematically studied the water diffusion on phosphorene,the water chain and the monolayered ice on phosphorene.The resuslts showed that water diffusion on the surface of phosphrene is not selective.We simulated the possible water chains on phosphorene,and compared the stability both from the energies and the dynamics.The results implied that the A1 configuration is the most stable water chain.The strong hydrogen bonding between the water molecules made it more stable than other configurations.By comparing the rhombic ice and the hexagonal ice on phosphorene,we found that the hexagonal ice is more stable.When applying a 9% compressive strain across the armchair direction of phosphorene,the water molecules on the strained phosphorene get closer together and the ice structure became more stable.(4)Beside studying the interaction at the interface of graphene and phosphorene,we also studied the effects of strain on material properties.We showed that the magnetic states in niobium diselenide and niobium disulphide single layers can be switched from antiferro-magnetic to ferro-magnetic by tensile strain.Two dimensional crystals,befitting nanoscale electronics and spintronics,can benefit strain-tunable applications due to their ultrathin and flexible nature.We show through first-principles calculations that tensile strain can enhance the exchange splitting of spin in niobium diselenide and niobium disulphide single layers.Particularly,a switching from antiferro-to ferro-magnetism is realized through the strain engineering.Under strains lower than 4%,an antiferro-magnetic state with opposite spins aligned on the next-nearest-neighbor rows of niobium atoms is favored in energy due to a superexchange interaction;with higher strains the ground state turns to be ferro-magnetic with a double exchange origin.In contrast,the vanadium diselenide and vanadium disulphide single layers,though with the same trigonal prismatic coordination,remain ferro-magnetic even under compressive strains.