Adsorption And Self-assembly Of The Water Molecules On The Cd(0001)surface

Water,the most important part of living organisms,is known as the source of life.Under ambient conditions,almost all solid surfaces are covered with thin water films.The ubiquitous nature of water-solid interfaces manifests that they are closely associated with our daily lives and technical phenomena.Due to this broad importance and the urgent need for clean water,renewable energy and an in-depth understanding of environmental chemical processes,extensive investigations of the nature of the interfacial water have been carried out.Scanning tunneling microscope(STM),as an advanced surface analysis facilities,can not only observe the morphology,adsorption behavior,dynamic diffusion process,molecular orbital of water molecules on a solid surface,but also can manipulate individual atoms or molecules.In this paper,we study the initial stage of water adsorption and nucleation on a Cd(0001)surface,through a combination of low temperature scanning tunnelling microscopy(STM)experiments and first-principles density-functional theory(DFT)calculation.Water is found to aggregate into various clusters with the dominant of water dimers.Particularly,the ice nanoflakes are built exclusively from water dimers.At the temperature of liquid helium(4.7 K),we observed single water molecules and their LUMO orbitals adsorbing on different positions of Cd.Combined with the first-principles density-functional theory,we calculated the adsorption energy and the electric dipole moment at the different adsorption sites,and found that the adsorption energy of water molecules adsorbed at the bridge site is higher than the top carrying a larger dipole moment.Thus,the electric field from a STM tip may affect the azimuthal orientation of the monomer such that changing from a stable state(top)to a metastable state(bridge).Furthermore,compare to the water monomer in the gas phase,the dipole moment of adsorptive water monomer has a significant increasing,which means that there is a obvious charge transfer between the Cd substrate and the water molecules.At the temperature of liquid nitrogen(77K),water molecules aggregate into various clusters with the dominant of water dimers.Particularly,the ice nanoflakes are built exclusively from water dimers.In an individual water dimer,two water molecules are found to adsorb on the top site,and have stronger hydrogen bonds than the free dimer.The shorter O-O distance(2.76 A,compared with 2.86 A for free dimer)suggests that the hydrogen bond in the adsorbed dimer is enhanced.In addition,driven by the electric field from a STM tip,the water dimer can detach from one cluster,and then adsorbed to one side of the other cluster,and the water dimer do not rotate during the desorption and adsorption.In the interior of the ice nanoflakes composed of water dimers,the dimers can not only diffuse but also rotate.DFT calculations demonstrate that the ultrahigh stability of water dimers result from the substrate-induced enhancement of dipole moment of water.