Self-organized Arrays Of Cd Nanocluster On Si(111)-7×7 Surface

Nano-technology has attracted the in-depth research of the world due to its wide application value.Nanotechnology is a technology that studies the characteristics and interaction of matter in the range of 1 to 100 nm to build some nanomaterials and nanostructures with specific function,which makes human ability to explore the material world deep into the atomic filed.Due to the reduced dimensions and size reduction,lowdimensional nanostructures exhibit many interesting physical and chemical phenomena,and have huge potential technology applications.Nanocluster is the relatively stable aggregates consisting of several or even thousands of atoms,molecules or ions by physical and chemical ways.It presents many singularities that are different from individual molecule or atom,and the macroscopic substance.Therefore,nanocluster is seen as a new level of material structure between atomic and macroscopic substance.With the development of nanoscience and nanotechnology,the preparation and characterization of low-dimensional nanostructures become more and more important.Scanning tunneling microscope(STM)is an advanced surface analysis equipment,and we can observe the structure and arrangement of surface atoms by using it.Especially,STM doesn't damage surface of the sample.It has a wide range of applications in many fields.In addition,the ultra-high vacuum system used in scanning tunneling microscopes can reduce the interference of other microscopic particles and further ensure the accuracy of the experimental data.In this dissertation,we perform the ultrahigh vacuum low temperature scanning tunneling microscopy(STM)study of the self-organized Cd nanoclusters on the Si(111)-7×7 surface.Our study starts with low coverage of Cd on Si(111)-7×7 surface at the elevated substrate temperature of ~ 150?.It is found that,at low coverage,Cd atoms adsorb in the Si(111)-7×7 HUCs and forms nanoclusters.Further observation indicates that the Cd clusters are not distributed uniformly on the Si(111)-7×7 surface,instead they are assembled into various nanostructures such as cluster dimers(pairs),trimers,hexamers,as well as long zigzagged chains,although the clusters still reside in their own half-unit cells of Si(111)-7×7.To our surprised,the Cd nanoclusters don't show obvious preference for the FHUCs,i.e.,both the faulted and unfaulted half-unit cells of Si(111)-7×7 have been equally occupied by the Cd clusters,distinct from the reported metal clusters with a high preference to FHUCs such as Na,Pb and Zn nanoclusters.The results show that the brightness of the clusters in the image under low bias is small and even disappear,indicating that the Cd nanoclusters have unique electronic properties.Compared to the positive and negative bias STM images,another obvious feature is that the Cd cluster has a great effect on the three adjacent half unit cells,where the three closest center Si adatoms are invisible in the filled-states STM images,while in the empty-state STM images they reappear.Considered the difference of electronegativity between the Cd and Si,it suggests that the charge transfer occurs between them.It is to be noted that,the substrate is kept at an elevated temperature of ~ 150?.If the Si(111)substrate is kept at room temperature during Cd depositing,the resultant Cd clusters would be irregular and disordered.In addition,by controlling the deposition amount of Cd,highly-ordered arrays of Cd nanoclusters with six fold symmetry was been fabricated on the Si(111)-7×7 surface.