Interaction Between C₆₀and H₂O And Semi-metal Substrate

H₂O and C₆₀ are the two most typical inorganic and organic molecules,respectively.The former can participate in various physical,chemical and life processes,and has important application backgrounds in the fields of biology,environment,catalysis,materials and lubrication;the latter has a highly symmetrical cage structure,is the cornerstone of carbon-based nanomaterials,and has potential application value in new nanoelectronic devices.Past studies have found that water molecules will form various self-assembled nanostructures and two-dimensional films on the solid surface.Especially on precious metal substrates,water molecules will form a layer of hexagonal honeycomb structure similar to graphene.The basic unit of the honeycomb structure is a water hexamer connected by hydrogen bonds,and two adjacent hexamers are connected in a co-edge form.In the reported C₆₀ molecular film,it was found that C₆₀ molecules will be arranged along different substrate lattice directions to form a series of molecular monolayers with different structures.On the other hand,the C₆₀ molecules themselves in the molecular monolayer also exhibit different molecular orientations(hexagons facing up,pentagons facing up,carbon atoms facing up,6:6 bonds facing up,or 5:6 bond facing up)to form a superstructure with various orientations.Ultra-high vacuum low-temperature scanning tunneling microscope(STM)is an advanced surface analysis tool that can characterize the structure of thin films deposited on solid surfaces in situ on the atomic scale.It can not only provide high-resolution real-space images,but also detect the electronic density of states information of the film,and even use STM tip to manipulate molecules or atoms to construct novel artificial nanostructures.In this paper,STM was used to study the self-assembly and film growth of water molecules on the graphite surface,and the adsorption structure and two-dimensional self-assembly of C₆₀ molecules on the Bi(111)surface.(1)Novel single layer of ice.Using low-temperature growth and post-annealing methods,we have grown a new type of hexagonal single-layer ice on the graphite surface.Unlike hexagonal ice grown on metal substrates,this single layer of ice is composed of cyclic water hexamers without sharing edges.Due to the weaker molecular-substrate interaction,this single layer of ice can have a variety of angles with the substrate lattice,so that moirépatterns of different periods can be observed in the STM images.In addition,the structure of the single layer of ice is very stable and can cross the steps of the substrate without breaking.It is a good two-dimensional molecular material.In addition,first-principles calculations show that the single layer of ice is assembled from a large number of water hexamers in a close-packed manner,and the stability of its structure comes from the maximization of the number of hydrogen bonds in the layer.(2)Structure transition of a C₆₀ monolayer.In order to reduce the interaction between C₆₀ and the substrate,we choose the relatively inert semi-metal Bi(111)surface as the substrate.Under low coverage,we have found that a single C₆₀ molecule has two different orientations.As the coverage increases,the C₆₀ molecules grown at low temperature forms a local-order structure.When the temperature of the substrate rises to room temperature,the local-order structure turn into a long-range ordered(?93×?93)R20°phase.Continue to heat up to 400 K,we find that the C₆₀ film has undergone a structural phase change from(?93×?93)R20°phase to(11×11)R0°phase.The former exhibits multiple molecular orientations(6:6 bonds facing up,5:6 bonds facing up and hexagons facing up),while the latter has a uniform molecular orientation hexagons facing up.We believe that the structural transition and molecular uniform orientation can be attributed to the enhanced effect of temperature-induced molecular diffusion.