Xiang-Dong Wang

Scanning tunneling microscopy study of fullerenes

Scanning tunneling microscopy investigations of adsorption and film growth of various fullerenes on semiconductor and metal surfaces are reviewed. The fullerenes being studied are C60, C70, C84, Sc@C82 and Y@C82 and the substrates being used for adsorption are Si (111), Si (100), Ge (111), GaAs (110), GaAs (001), Au (111), Au (110), Au (100), Cu (111) and Ag (111) surfaces.

Field ion-scanning tunneling microscopy study of C60 on the Si(100) surface

Field ion-scanning tunneling microscopy was employed to study the monolayer and multilayer adsorption behaviors of the C60 fullerene on the Si(100)2×1 surface. The C60 molecules reside stably in the trough at room temperature without rotation, encompassing the 8 neighbouring dimer-forming surface Si atoms with the nearest neighbour distance of 12 A. For the first and second layers, only local ordering of square and quasi-hexagonal patterns was observed. The orderly Stranski-Krastanov mode island formation with the hexagonal packing was observed above the third layer with its lattice constant of 10.4 A.

Scanning Tunneling Microscopy of C60 on the Si(111)7?7 Surface

Adsorption of C60 molecules on the Si(111)7×7 surface was investigated using a field ion-scanning tunneling microscope. C60 adsorbs preferentially on the faulted half of the 7×7 unit and stays still without rotation at room temperature, implying the reasonably strong interaction with the Si substrate. The internal structure of individual C60 molecules can be understood if we assume that the C=C double bonds are imaged brightly. Unlike the case of its adsorption on the Si(100)2×1 surface, C60 do not form ordered mono/multi layers on the 7×7 surface.

Adsorption of fullerenes on Cu(111) and Ag(111) surfaces

We have studied the initial stage adsorption and film growth of C60, C70, and C60(x)C70(1−x) on Cu(111) and Ag(111) surfaces using field-ion scanning tunneling microscopy. Fullerene molecules are mobile on the terrace of the metal surfaces and initially segregate to the step edges. A well-ordered two-dimensional overlayer forms with a close-packed arrangement upon annealing the fullerene covered surfaces.

Field Ion-Scanning Tunneling Microscopy of Metallofullerenes Adsorbed on the Si(100)2×1 Surface

The Sc@C74 and Sc2@C74 metallofullerenes (MF), adsorbed on the Si(100)2×1 clean surface were successfully studied for the first time by the STM. The STM showed that the MF molecules were imaged to be spherical, quite similar to those of C60, C70, and C84, suggesting that Sc atoms were trapped inside the fullerene cage and that the charge is at least partially delocalized over the sphere of the cage. Statistical analysis of both the diameter and height of MF, compared with those of C60, showed that the diameter of the Sc@C74 and Sc2@C74 cages was 9.5 A in average, larger than that of the pristine fullerenes.

Fullerene (C60) adsorption on Si surfaces

Abstract Field ion-scanning tunneling microscopy (FI-STM) was employed to characterize the adsorption behavior of fullerenes on the Si(111)7 × 7 and Si(100)2 × 1 surfaces. On the Si(111)7 × 7 surface, C 60 adsorbs preferentially on the faulted half of the 7 × 7 unit and stays still without rotation at room temperature, implying a reasonably strong interaction with the Si substrate. The C 60 molecules reside stably also at room temperature without rotation on the Si(100) surface. For the first and second layers on the Si(100) surface, only local ordering of square and quasi-hexagonal patterns was observed. The orderly Stranski-Krastanov mode island formation with hexagonal packing was observed above the third layer on the Si(100)2 × 1 surface.

Geometry of metallofullerenes adsorbed on the Si (100) 2×1 surface studied by scanning tunneling microscopy

Abstract Sc n C 74 ( n =1, 2) and Sc 2 C 84 metallofullerenes (MF), adsorbed on the Si (100) 2×1 surface were studied by scanning tunneling microscopy. The MF molecule images were spherical and randomly distributed, similar to those of C 60 , C 70 and C 84 , but were slightly larger than those of the pristine fullerenes. These results suggest that the Sc atoms were trapped inside fullerenes cages, with a significant charge transfer from the metal atom.

Atomic hydrogen chemisorption on Si(100)(2 X 1) studied by FI-STM

Abstract A field ion-scanning tunneling microscopy study on hydrogen chemisorption on the Si(100)(2 X 1) surface is presented. At low coverages, hydrogen atoms reside singly on top of the dimerised Si atoms, and are imaged brightly. The hydrogen chemisorption induces buckling of dimers, indicating the strong bonding between Si and H atoms. With increasing coverage, both the (2 X 1) monohydride and (1 X 1) dihydride phases were formed. The former is imaged dark compared with the unreacted Si dimers, due to the reduction of the density of electronic states near the Fermi level. Surface etching was also observed. It was found that the corrosion of the surface is modest in the monohydride phase, while during the formation of the dihydride phase, the corrosion becomes significant. The behavior of hydrogen desorption from the dihydride and monohydride phases was investigated as a function of annealing temperature. Our STM results support the mechanism that the desorbing H2 molecules are formed by combination of two hydrogen atoms forming the dihydride phase. Upon annealing at elevated temperatures, the Si overlayer stripes are formed by desorption of hydrogen from the etching products and the rearrangement of the Si atoms.

Scanning tunneling microscope study of the structural transformation of the Si(111)7×7 surface to the Na-induced 3×1 surface

The structure of the Na-induced Si(111) 3×1 surface has been studied using a field-ion-scanning-tunneling-microscope. The STM images showed that the entire Si surface was uniformly covered with a single layer of atomically resolved Na 3×1 structure. Upon annealing, the Na layer peeled off gradually, revealing the underlying Si substrate, which exhibited various intermediate reconstructions such as 2×2, 5×5, and 9×9 before the 7×7 structure was restored at 800°C. Our observations suggest that the top layer of the Si substrate converts to the bulk terminated 1×1 structure upon Na deposition and annealing.

Thermal stability of fullerene (C70) on the Si(100)2×1 surface studied with FI-STM

Abstract We have studied the thermal stability of fullerene C 70 adsorbed on the Si(100)2×1 surface with the field ion-scanning tunneling microscope. The results show that the first layer of C 70 adsorbed considerably strongly due to the existence of Si dangling bonds on the surface. The first layer passivated the surface efficiently so that the multilayer crystalline film could be grown as close-packing array. The C 70 above the first layer can be easily desorbed at temperatures above approximately 250°C, which is similar to that of bulk C 70 . In contrast to that, the first layer remains stable without any evidence of cage breaking or reordering until at temperatures above 1000°C, resulting in well-ordered islands: β-SiC(100)3×2 surface, according to our STM observations. The present work reveals a novel fullerene property on the Si(100)2×1 surface.