Dissolution Dynamics Of Sodium Chloride Decoupling Layers On Au(111) Surface

The preparation of oxide surfaces and insulating films can find more potential applications in the fields of nanotechnology and nanoelectronics.Sodium chloride films stand out among insulating films due to their relatively flat and inert surfaces.Na Cl films with metal surface thicknesses of less than 1 nm have electron densities that do not vanish,so these surfaces can be characterized by scanning tunneling microscopy（STM）.One of the most important prerequisites for realizing surface observations is the ability to grow stable atomic-scale Na Cl films on metal surfaces with well-characterized and ordered geometries.However,in the ultra-high vacuum environment,the sodium chloride film will undergo a dissolution process due to the presence of water molecules,which will affect the observation of the surface.Therefore,by understanding the dissolution mechanism of Na Cl films,we can gain general insights related to this process and provide more references for the application and design of Na Cl as insulating films and decoupling layers.In this paper,we observed the dissolution dynamics process of bilayer Na Cl islands on the Au（111）surface at the atomic scale using low-temperature scanning tunneling microscopy,and used sodium chloride as a decoupling layer on the metal surface to study the adsorption properties of C₆₀molecules.The main contents are as follows:Deposit water molecules through the reagent tube onto the Na Cl surface.It is found that water molecules preferentially combine with Na⁺to form Na⁺hydrates accompanied by Na⁺vacancy defects.Scanning tunneling spectroscopy（STS）measurements at the Na⁺vacancy defect revealed a shift in the characteristic peak position,which is due to the change of the work function here due to the lack of electrons on the defect.By exposing the samples to water molecules for extended periods of time,the migration of Na Cl surface defects and the presence of hydrates were observed.Through density functional theory（DFT）calculations,we confirmed that the hydrate is the most energetically stable Na⁺·4H₂O and its diffusion barrier.Furthermore,competition between ion-ion and ion-water interactions can lead to the migration of Na⁺vacancy defects on the surface.Using Na Cl as the decoupling layer,the adsorption of C₆₀molecules on Na Cl islands on metal surfaces at different temperatures was studied from the perspective of thermodynamics.It was found that when the metal substrate was at room temperature,C₆₀molecules were more preferred to adsorb on the step edges of the metal and Na Cl islands in the form of islands.When the metal substrate was at low temperature,C₆₀molecules tend to be adsorbed in the form of clusters on the step edges of Na Cl islands.When the Na Cl island was deposited under the heating state of the substrate,and the same substrate was at low temperature,we found a small amount of C₆₀molecules in the center of the Na Cl island.This shows that Na Cl films with different thickness characteristics can provide different decoupling strengths from the substrate,which makes this system an ideal system for studying the electronic properties of metal clusters or molecules.