The Study Of Potassium Atom Adsorption On Silicene

In recent decades,with the development of semiconductor industry,the graphene-like two-dimensional materials such as silicene,germanene etc.are attracting more and more interests due to their unique structural and electrical properties.Silicene,a monolayer structure of silicon,has low buckled honeycomb structure and many intriguing physical properties.Because of the larger gap induced by stronger spin-orbit coupling,silicene is expected to realize the Quantum Spin Hall effect and other topological quantum phase transition.The sp²-sp³hybridization between Si-Si atoms in silicene also makes it more easily functionalized by adsorption of foreign adatoms.Alkali metals,which have simple hydrogen-like electronic structure,are ideal candidates to study bonding mechanisms of the adatom-surface system.Theoretical investigations have predicted that the adsorption of alkali metal atoms is an effective way to modify the electronic properties of silicene and realize the functionalization of silicene for future applications.For example,alkali metal adatoms will bind strongly to the hollow site of free-standing silicene with an approximately ideal ionic bonding.As the binding energy（E_b）is larger than the cohesive energy（E_c）,it is expected to be a large possibility for potassium atoms to form a uniform and stable coverage on top side of silicene and greatly improve the hydrogen storage capacity of silicene.The band gap size of silicene may also be controlled by changing the coverage of alkali metal adatoms.However,up to now,there is still no experimental progress in the adsorption of alkali metals on silicene/Ag（111）system.In this thesis,we study the adsorption of potassium atoms on the silicene/Ag（111）system by low-temperature scanning tunneling microscopy/spectroscopy combining with the first principle calculations.The main conclusions are as follows:（1）When the substrate temperature is higher than room temperature（300K-510K）,potassium atoms present a selective adsorption behavior on silicene-（4×4）.They prefer to adsorb on silicene-（4×4）stable phase and form a disordered dispersed structure.High temperature annealing or applying voltage pulse from the STM tip will easily desorb.In contrast,nearly no potassium atoms adsorption happens on silicene-（2√3×2√3）phase and（√13×√13）phase in such condition.（2）At a low substrate temperature（80K-200K）,potassium atoms experience a structural phase transition on silicene-（4×4）with the increasing coverage.When the K coverage is lower than 0.04ML（Θ≤0.04ML）,the same dispersed phase is formed.Increasing K coverage to the range between 0.04ML and 0.17ML（0.04ML<Θ≤0.17 ML）,the dispersed phase transforms into an ordered and stable closely packed potassium layer.The period of potassium layer is the same as silicene-（4×4）.After annealing to a certain temperature,potassium atoms will totally desorb,returning the surface back to intact silicene-（4×4）structure,which indicates a reversible adsorption process for K/silicene system.（3）At the low substrate temperature（80K-200K）,potassium atoms form a layer with hexagonal honeycomb-like structure on silicene-（2√3×2√3）phase.Comparing with the long-range ordered structure of K on silicene-（4×4）,the potassium layer presents a characteristic structure with short-range order but long-range disorder.Besides,the potassium atoms are not so stable on surface.The adsorption sites are easily changed in a continuous scanning.It is also a reversible process for K adsorption on silicene-（2√3×2√3）phase.（4）At the same low substrate temperature（80K-200K）,there are mainly two types of potassium structure forming on silicene-（√13×√13）phases:a"short line"structure and a"triangle"structure.As the K layers on silcene-（2√3×2√3）phase and（√13×√13）phases show much more complexity than that on（4×4）phase,the formation mechanism of K atoms on both phases are still analyzing.More detailed structural information of silicene phases and the interaction between K atoms and silicene will be revealed with the combination of DFT calculations.It is found that K atoms show the same reversible behavior on every monolayer silicene phases.（5）To clarify the interaction mechanism between K atoms and silicene,we perform DFT calculations on K/silicene-（4×4）system.Experimentally,potassium atoms around the defects and boundaries are movable during a continuous STM scanning period.Besides,the fact of different ordered K domains coexisting on surface indicates not only one adsorption site.Combining with DFT calculations,we find that potassium atoms prefer to adsorb on the bridge site of silicene-（4×4）unit cell,and there are three metastable adsorption sites B₁,B₂and B₃.Due to the similar binding energy,potassium atoms are easily jumping between different adsorption sites by thermal disturbance or tip interaction.The DFT results agree well with our STM observations.（6）From the STS results on the closely packed K layer on silicene-（4×4）,we find a well defined V-shape density of states,which is a typical trait of two dimensional Dirac Fermion materials.DFT calculations show a charge transfer from K atoms to silicene and forming ionic bonding,the main contribution of DOS around fermi level comes from silicene.Therefore,we suspect that the interaction between K atoms and silicene is rather weak,the adsorption of K atoms on the surface of silicene may weaken the interaction between silicene and substrate,so the V-shape DOS character represents the intrinsic electronic properties of silicene-（4×4）rather than K layers.