Theoretical Study On The Antioxygenic Property Of Germanene And The Regulation Of Its Electronic Structure

The discovery of graphene and a series of outstanding achievements in its subsequent researches have greatly promoted the development of other two-dimensional?2D?layered materials composed of elements other than carbon.As one of the typical representatives,germanene and its derivatives have also attracted tremendous attention in science and technology due to its novel properties.For its practical applications,the stability of germanene and its derivatives in atmospheric environment is becoming an inevitably primary issue.Moreover,tuning the electronic structure and the physicochemical properties of germanene has been hot in research to meet the needs of functional materials and electronic devices in the future,which is of greatly scientific importance for correlative experiments and its potential applications.Aiming at the stability of germanene and its derivatives in atmospheric environment as well as the tunable physicochemical properties,we carry out the theoretical calculations by Vienna Ab-initio Simulation Package?VASP?based on density functional theory?DFT?in this thesis.Firstly,we thoroughly study the stability of germanene and its derivatives in oxygen and then we transversely compare their stabilities with other 2D materials,such as graphene,silicene,and silicane.Thereafter,we introduce a simulation model of Al?111?supported germanene based on experimental results to calculate its electronic structure and tune its physicochemical properties.Finally,we investigate the adsorption behaviors of some small molecules on germanene.Further,we attempt to tune their band structures by utilizing external electric field for its potential applications as a gas sensor.The main conclusions in this thesis are summarized as follows1.The study on the stability of germanene in oxygen shows that oxygen molecule can spontaneously adsorb on germanene surface,exhibiting the chemisorption characteristics.This further demonstrates the instability of germanene in oxygen from the thermodynamic point of view.Overcoming an energy barrier of 0.57 eV,oxygen molecule can be dissociated into two O atoms which can strongly bond with two neighboring Ge atoms.The dissociation process corresponds to a typically exothermal reaction.Moreover,the migration of O₂-dissociation-induced O atom on germanene is very difficult because of the energy barrier of 1.28/1.34 eV for O atom migration and the strong Ge-O bonding,resulting in form Ge-O compounds ultimately.Furthermore,by comparing with the dissociation of oxygen and migration of O atom on graphene,silicene and germanene,respectively,we find that the stabilization of graphene in oxygen is highest while that of silicene is lowest.2.The investigation of oxygen adsorption and dissociation on germanane surface indicates that oxygen molecule can physically adsorb on germanane surface,which is significantly different from the spontaneously chemisorption of oxygen on germanene.On the other hand,the energy barrier for oxygen dissociation is 0.50 eV.Considering two key factors mentioned above,it implies that gemanane exhibits an inert characteristic in oxygen which agrees well with the previously experimental results.Once oxygen molecule is dissociated into two O atoms,the dissociated O atoms are more easily bonded with H atoms on germanene surface to form hydroxyl,which is quite different to the case of germanene.Due to the powerful O-H bonding,the migration of O atom becomes more difficult on germanane than that on germanene.Meanwhile,combining the reverse reaction barrier of up to⁶ eV,desorption of O atom on germanene is impossible,leading to oxidation of germanene finally.This is also in accordance with the previous experiment.3.A continuous germanene layer grown on the Al?111?surface has recently been achieved in experiment.Therefore,we investigate its structural,electronic,and hydrogenation-induced properties through first-principles calculations.We find that despite having a different lattice structure from its free-standing form,germanene on Al?111?still possesses Dirac points at high-symmetry K and K'points.More importantly,there exist another three pairs of Dirac points?D points?on the K?K'?-M high-symmetry lines,which have highly anisotropic dispersions due to the reduced symmetry.For D points,the Fermi velocity along K-M??2.8×10⁵ m/s?is almost twice as large as the velocity along the perpendicular direction??1.6×10⁵m/s?.Moreover,a small gap?4 meV is opened at the original Dirac point when spin-orbit coupling is included.Hydrogenation of the germanene layer strongly affects its structural and electronic properties.Particularly,when not fully hydrogenated,ferromagnetism can be induced due to unpaired local orbitals from the unsaturated Ge atoms.Remarkably,we discover that the one-side semihydrogenated germanene turns out to be a two-dimensional half-semimetal,representing a novel state of matter that is simultaneously a half-metal and a semimetal.4.To explore the potential applications of germanene for gas sensor in the future,we study the adsorption of small molecules?SMs?on germanene,such as methane,ammonia and so on.The molecular adsorption-induced intrinsic electric field by small molecules?SMs?breaks the symmetry of two sublattices in germanene,leading to band gap opening.By using?electron model,we illustrate the physics essence therein.Further,the electronic properties of methane/germanene and ammonia/germanene systems under the external electric field are studied.A wide range of linearly tunable band gap is realized,which is merely determined by the strength of composited electric field despite of its direction.More importantly,the band gap of germanene/SMs can be closed and reopened at a critical electric field.On the other hand,the mechanism of charge transfer between germanene and SMs under the external electric field is revealed by equivalent capacitor model to explain the tunable characteristics of charge transfer.The composited field-induced charge transfer could be promoted when external electric field and molecular adsorption-induced internal electric field have the same direction.Otherwise,the charge transfer will be inhibited.Particularly,the tunable electronic properties of ammonia/germanene are sensitive to the concentration of molecular adsorption under electric field,representing a multiple-effect that would significantly enhance the performance of germanene for electronic devices in the future.