Structure transition of a C60 monolayer on the Bi(111) surface

The interfacial structures of C60 molecules adsorbed on solid surfaces are essential for a wide range of scientific and technological processes in carbon-based nanodevices. Here, we report structural transitions of the C60 monolayer on the Bi(111) surface studied via low-temperature scanning tunneling microscopy (STM). With an increase in temperature, the structure of the C60 monolayer transforms from local-order structures to a (√93 × √93) R20° superstructure, and then to a (11 × 11) R0° superstructure. Moreover, the individual C60 molecules in different superstructures have different orientations. C60 molecules adopt the 6 : 6 C–C bond and 5 : 6 C–C bond facing-up, mixed orientations, and hexagon facing-up in the local-order structure, (√93 × √93) R20°, and (11 × 11) R0° superstructure, respectively. These results shed important light on the growth mechanism of C60 molecules on solid surfaces.

Introduction C 60 molecule, as a prototypical fullerene molecule, has attracted widespread attention due to its potential in endohedral fullerenes, 1 photovoltaic devices, 2 peapod nanotubes, 3 and singlemolecule transistors. 4 A C 60 monolayer grown on solid surfaces is critical for understanding and controlling the interfacial properties of fullerene-derived electronic and photovoltaic devices. 5,6 STM studies demonstrated that the C 60 monolayer on the solid surface exhibit a variety of lattice orientations such as the "in phase" (2O3 Â 2O3) R30 , 7-13 , (7 Â 7) R0 13 and (O589 Â O589) R14.5 . [13][14][15] The individual molecules of fullerene and fulleride within a single domain display different orientations. In the complex orientational ordering (7 Â 7) R0 structure, a 7-molecule C 60 cluster consists of a central molecule sitting atop of a gold atom and six tilted surrounding molecules. 10 In the unit cell of the (O589 Â O589) R14.5 structure, 49 C 60 molecules adopt 11 different orientations. 14 In the (2O3 Â 2O3) R30 structure, all C 60 molecules are in the same orientation. 12,16 The complex chiral motifs have been observed. 17 In CsnC 60 fulleride lms, orientational ordering appears. 18 Moreover, "bright" and "dim" molecules have been widely found in the C 60 monolayer. 9-17 However, the "dim" molecules in superstructures reported so far arrange irregularly.
The structure of C 60 monolayers grown on the solid surface is not only related to C 60 molecules themselves but also the substrate. In the past reports, there have been a large number of investigation on the C 60 monolayer structures grown on numerous metals or semiconducting substrates, such as Ag, [7][8][9]19,20 Au, [10][11][12][13][14][15][16]21,22 Cu, 23-25 graphene, 26,27 Si, 28,29 Ge, 30 C 60 , 29 or NaCl. 31 However, few reports address the superstructure of C 60 molecules adsorbed on semi-metal substrates. It is found that thin lms of organic molecules grown on a semi-metallic Bi(111) surface shows a lot of interesting phenomena, such as the ordered crystalline layer with the standing-up orientation of pentacene molecules, 32 the chiral self-assembly of rubrene molecules, 33 structural transitions in different monolayers of cobalt phthalocyanine lms, 34 and the Moire' pattern in C 60 thin lms. 35 In this study, we use Bi(111) as the substrate and studied the structure transition of the C 60 monolayer. C 60 molecules were deposited at 100 K form local-order structures. When the deposition temperature increased to room temperature, the local-order structures turn into a long-range ordered (O93 Â O93) R20 superstructure. Aer annealing at 400 K, the ordered superstructure transforms into the (11 Â 11) R0 superstructure. These superstructures are different from the structures of the C 60 monolayer reported so far. Furthermore, the individual C 60 molecules in the local-order structure, (O93 Â O93) R20 and (11 Â 11) R0 superstructure, show the 6 : 6 C-C bond and 5 : 6 C-C bond facing-up, mixed orientations, and hexagon facing-up, respectively. The 6 : 6 (5 : 6) C-C bond indicates the common side of two adjacent hexagons (pentagon and hexagon) in C 60 molecules.

Experimental
The experiments were conducted in an ultra-high vacuum lowtemperature scanning tunneling microscope produced by Unisoku. The base pressure was kept at $1.2 Â 10 À10 Torr. An Si(111) substrate was continuously degassed at $870 K for 8 h with subsequent ashing to 1400 K for several seconds. The Bi(111) lm was prepared by depositing 20 monolayers of bismuth atoms on a Si(111)-7 Â 7 surface at room temperature with subsequent annealing at 400 K. 36 C 60 molecules were deposited onto the Bi(111) surface by heating the tantalum cell to 700 K. The growth rate of C 60 molecules was about 0.4 monolayers per minute. All STM images were acquired with a tungsten tip in constant-current mode at liquid nitrogen temperature (78 K).

Results and discussion
First, a small number of C 60 molecules were deposited onto the Bi(111) surface when the substrate was maintained at 100 K.  12 This indicates that there are two stable adsorption orientations of isolated C 60 molecules on the Bi(111) substrate, 6 : 6 C-C bond, and 5 : 6 C-C bond facing-up.
When the coverage increases, C 60 molecules form the closepacked hexagonal structure, as shown in Fig. 2. We noticed that all the C 60 molecules present a uniform height, except a few dim molecules (marked by green dotted circles). The brightness contrast in images stems from the different adsorption sites of C 60 molecules. It is well known that metal surfaces do not behave as rigid templates for the chemisorption of C 60 molecules, but may reconstruct substantially to accommodate the molecules. 37 We speculate that Dim C 60 molecules are located at the vacancies of the Bi(111) substrate, originating from the reconstruction of the Bi(111) surface, similar to C 60 molecules on Au(111) 16 and Cu(111). 38 According to the arrangement of bright and dim molecules, we can see some local-order structures, though there is a lack of long-range ordering. In Fig. 2(a), there is an (11 Â 8) R0 localorder structure (marked by red parallelogram). The lattice directions of (11 Â 8) R0 are along with the directions of Bi(111), and the measured lattice constants are 5.00 AE 0.02 nm and 3.64 AE 0.02 nm, corresponding to 11 and 8 times of the lattice constant of the Bi(111) surface. The lattice directions of Bi(111) were obtained on the surface, which was not covered with C 60 molecules. In another domain, shown in Fig. 2(b), the local-order structure is mixed with three types of structures, namely (11 Â 8) R0 (red quadrilateral), (11 Â 11) R0 (white quadrilateral), and (10 Â 8) R10 (blue quadrilateral). In particular, we noticed that C 60 molecules exhibit almost the same orientation in a single domain, and most of the individual C 60 molecules in the local-order structure adopt two favorite orientations (6 : 6 C-C bond and 5 : 6 C-C bond facing up) as the isolated molecules on Bi(111). For example, most of the molecules shown in Fig. 2(a) present two symmetrical lobes, corresponding to C 60 molecules with a 6 : 6 C-C bond facing up. However, in Fig. 2(b), the molecules present two asymmetric lobes, corresponding to the 5 : 6 C-C bond facing up. We suggest that the formation of a local-order structure is due to the low-temperature growth. Because of the low kinetic energy of C 60 molecules at 100 K, molecular mobility is not high enough to form a long-range ordered superstructure. The C 60 molecules adsorbed on Bi(111) adopt their preferred orientations (6 : 6 C-C bond and 5 : 6 C-C bond facing up), similar to the isolated molecules adsorbed on the substrate. This proves the strong molecule-substrate interaction in the local-order structure.
To investigate the inuence of temperature on the structure, we deposited C 60 molecules on Bi(111) at room temperature. It is found that C 60 molecules aggregate into a hexagonal structure, the same as C 60 molecules in the local-order structure. However, the local-order structures, originating from the dim and bright molecules, turn into a long-range ordered (O93 Â O93) R20 superstructure [ Fig. 3(a)]. This superstructure is different from the structures of the C 60 monolayer reported so far. There is a misorientation angle of 20 between the lattice directions of the C 60 monolayer and the Bi(111) surface. The measured lattice constants of (O93 Â O93) R20 are b 1 ¼ b 2 ¼ 4.38 AE 0.02 nm, agreeing well with O93 times the lattice constant of Bi(111) (0.45 nm). Fig. 3(b) shows the schematic of the (O93 Â O93) R20 superstructure. Based on the lattice constant of the Bi(111) substrate, the lattice vectors of the (O93 Â O93) R20 superstructure can be expressed as following This ordered superstructure implies two things: rst, the intermolecular interaction is getting stronger than that in the local-order structure prepared at low temperature (100 K). Second, the molecule-substrate interaction is also strong since the orientations of the C 60 superstructure are commensurate with those of the substrate. Furthermore, we can clearly see that individual C 60 molecules adopt various orientations, rather than the favorite orientations as C 60 molecules in the localorder structure. As shown in the high-resolution STM image [ Fig. 3(c)], C 60 molecules in (O93 Â O93) R20 present various shapes, such as two asymmetric lobes (white circle), two symmetrical lobes (yellow circle), and three lobes (blue circle),  corresponding to the 5 : 6 C-C bond, 6 : 6 C-C bond, and hexagon facing up. The diversity of C 60 molecular orientations is due to the enhancement of intermolecular interaction in the (O93 Â O93) R20 superstructure. The intermolecular interaction enables C 60 molecules to overcome the molecule-substrate interaction and adopt other orientations, and then make the (O93 Â O93) R20 superstructure stable.
When annealed at 400 K for about 20 min, C 60 molecules still revealed a hexagonal lattice, while the superstructure transformed from (O93 Â O93) R20 into (11 Â 11) R0 superstructure [ Fig. 4(a)], indicating that (11 Â 11) R0 is more stable than (O93 Â O93) R20 . The lattice directions of (11 Â 11) R0 are along the directions of the Bi(111) substrate, and the lattice constants are c 1 ¼ c 2 ¼ 5.00 AE 0.02 nm [ Fig. 4(b)], corresponding to 11 times of the lattice constant of Bi(111). Fig. 4(d) is the fast Fourier transform (FFT) image of the (11 Â 11) R0 superstructure, where the spots marked by red and green circles correspond to C 60 hexagonal lattices and the (11 Â 11) R0 superstructure. In the FFT image, the spots of the superstructure are clearly visible, though they are dimmer than the spots of C 60 hexagonal lattices, implying that the (11 Â 11) R0 superstructure has long-range order. The schematic model of (11 Â 11) R0 is shown in Fig. 4(c). From STM images, the (11 Â 11) R0 superstructure seems to have the same structure as the reported structure attributed to a Moire' pattern in ref. 36. However, in our experiment, the (11 Â 11) R0 superstructure is transformed from the (O93 Â O93) R20 superstructure and have no relationship with the Moire' pattern. From the close-up view of the (11 Â 11) R0 superstructure in Fig. 4(e), it is found that all the C 60 molecules reveal a unied three-lobe structure, corresponding to the hexagon facing up, different from favorite orientations in the local-order structure and mixed orientations in (O93 Â O93) R20 . With an increase in temperature, the superstructure of the C 60 monolayer changes from local order to long-range order and C 60 molecules are re-orientated. This is because the thermal diffusivities of C 60 molecules and Bi atoms increase with the increase in temperature, which is conducive to the formation of a more orderly and stable superstructure.

Conclusions
In summary, the structure of C 60 molecules on Bi(111) changes with temperature variation. When deposited on the Bi(111) surface at 100 K, C 60 molecules form local-order structures, and the molecules in local-order structures adopt their favorite orientations. As the deposition temperature increases to room temperature, the local-order structures turn into a long-range ordered (O93 Â O93) R20 superstructure. The orientations of C 60 molecules in (O93 Â O93) R20 superstructures are diverse. Aer annealing at 400 K for about 20 min, the C 60 lm exhibits a (11 Â 11) R0 superstructure, and all C 60 molecules in this superstructure take the unied orientation, hexagon facing-up. The appearance of numerous superstructures and the molecular orientations in superstructures is due to the change in the thermal diffusivity of C 60 molecules and Bi atoms at different temperatures.

Conflicts of interest
There are no conicts to declare.