Ordered Array Of CoPc-vacancies Filled With Single-molecule Rotors

In the past decade,strain attracted considerable interest because it plays an important role in controlling the structure and morphology of epitaxial films.Several strain relaxation mechanisms have been found such as step bunching,faceting,and misfit dislocations.In addition,there is an unusual mechanism that strain is released through the formation of periodic domain boundaries(discommensuration).For example,strain is relieved through the vacancy-like craters at the domain boundaries of Si(111)-5.5×5.5-Cu.In fact,discommensuration takes place when the adsorbate-adsorbate interaction is comparable to the adsorbate-substrate interaction.When the former is dominant,the adsorbates will adopt the incommensurate lattices that are incoherent with the substrate.When the latter is dominant,the adsorbates will adopt commensurate lattices that are coherent with the substrates.The discommensurate structure is neither commensurate nor incommensurate because it consists of periodic domain boundaries.Strain is built in the domains and is released at domain boundaries.If the adsorbate is compressed in the domains,it expands in the domain boundary and vice versa.Nevertheless,all the reported discommensurate structures were limited to the inorganic adsorbates such Cu,In,and Ga films grown on Si(111)or Ge(111).To our knowledge,there has been so far no report on the formation of organic discommensurate thin films.So it remains unclear whether the organic molecules can form the discommensurate structure or not.If yes,what kind of new features will be presented by the organic molecular films,which usually have large unit cells and numerous degrees of freedom?Metal phthalocyanine(MPc)is a model system to study the interaction between metal-organic complexes and solid surfaces.There have been many studies on the adsorption and self-assembly of MPcs on various solid surfaces such as Au,Ag,Cu,Pb,Bi,or graphene.Low temperature scanning tunneling microscopy(LT-STM)is considered to be a very beneficial characterization technique in the research field of nanometer science and the technology.It can be invariably used to characterize the topographic image of nanostructures at the atomic scale.At the same time,it can deeply explore the information of electronic structure combined with scanning tunneling spectrum(STS)function.In this paper,we use LT-STM to study the growth of cobalt phthalocyanine(CoPc)molecules on Cd(0001).We report the highly ordered array of CoPc-vacancies and the single-molecule rotors inside the vacancies.When CoPc molecules are deposited on Cd(0001)at low-temperature,three types of molecular vacancies appeared randomly in the CoPc monolayer.Annealing the sample to higher temperature leads to the spontaneous phase separation and self-organized arrangement of the vacancies.Highly ordered arrays of two-mo lecule vacancies and single-molecule vacancies have been obtained.In particular,there is a rotating CoPc molecule inside each single-molecule vacancy,which constitutes the array of singlemolecule rotors.These results provide a new routine to fabricate the nano-machines on a large scale.