Orientation Ordering and Chiral Superstructures in Fullerene Monolayer on Cd (0001)

The structure of C60 thin films grown on Cd (0001) surface has been investigated from submonolayer to second monolayer regimes with a low-temperature scanning tunneling microscopy (STM). There are different C60 domains with various misorientation angles relative to the lattice directions of Cd (0001). In the (2√3 × 2√3) R30° domain, orientational disorder of the individual C60 molecules with either pentagon, hexagon, or 6:6 bond facing up has been observed. However, orientation ordering appeared in the R26° domain such that all the C60 molecules adopt the same orientation with the 6:6 bond facing up. In particular, complex chiral motifs composed of seven C60 molecules with clockwise or anticlockwise handedness have been observed in the R4° and R8° domains, respectively. Scanning tunneling spectroscopy (STS) measurements reveal a reduced HOMO–LOMO gap of 2.1 eV for the C60 molecules adsorbed on Cd (0001) due to the substrate screening and charge transfer from Cd to C60 molecules.

In addition to the different close-packing directions, the orientations of individual molecules within a single domain can also be different. The individual C 60 molecules within a single domain can display multiple orientations such that the orientational ordering can be observed at 78 K [35]. For example, a complex orientational ordering was observed for molecules inside the "in-phase" (R0 • ) domain after room-temperature deposition, where a 7-molecule cluster is composed of a central molecule and six tilted surrounding molecules [18]. In the R14.5 • domain, there are 49 molecules with a number of different orientations, on average, every time the seventh C 60 appeared dim, giving rise to a quasiperiodic 7 × 7 superstructure at 5.7 K [12]. In particular, a chiral superstructure made of 7-molecule pinwheels was identified in C 60 multilayer film on NaCl at 77 K [40] and in K x C 60 monolayer on Au (111) at 7 K [41]. The proposed mechanism for orientational ordering was attributed to the maximized overlapping of neighboring molecular orbitals driven by the superexchange interaction. The latter refers to virtual hopping of an electron from the HOMO of one C 4− 60 to the LUMO of its nearest neighbor to gain an energy proportional to the hopping amplitude and the inverse of HOMO-LUMO gap.
In this paper, we chose the divalent metal Cd (0001) thin films as the substrate. The hexagonal close-packed metal Cd is usually used as an electrode material due to the smaller electronegativity compared with precious metals [42][43][44]. Thus, it would be fundamentally important to study the interfaces structure of C 60 monolayer on Cd (0001) because the C 60 molecules need to be contacted with metallic electrode in the electronic devices. It is found that the C 60 domains exhibit a variety of orientations with respect to the lattice directions of Cd (0001). In the (2 3) R30 • domain, individual C 60 molecules reveal orientational disorder with the pentagon, hexagon, or 6:6 bond facing up. In the R26 • domain, orientation ordering takes place such that all the C 60 molecules adopt the same orientation with the 6:6 bond facing up. More interestingly, the complex chiral motifs composed of seven C 60 molecules with clockwise or anticlockwise handedness have been observed in the R4 • and R8 • domains. STS measurements shows that the C 60 molecules have a reduced HOMO-LOMO gap of 2.1 eV, indicating a significant charge transfer from Cd to C 60 and strong substrate screening effect.

Experiment
The experiments were conducted in an ultra-high vacuum, low-temperature scanning tunneling microscopy (Unisoku USM1500) with a base pressure 1.2 × 10 −10 mbar. The Cd (0001) thin films were grown on the Si (111)-7 × 7 surface at room temperature. The Si (111) substrate was continuously degassed at~870 K for 8 h with subsequent flashing to 1400 K for several seconds. Cd atoms with a purity of 99.998% were thermally evaporated onto the Si (111) substrate from a quartz crucible by controlling the current. Deposition of 15 monolayer (ML) Cd at room temperature results in flat and smooth Cd (0001) films, which exhibit a perfect transparency due to the strong anisotropic electron motion with large lateral effective mass [45]. C 60 were evaporated from a homemade tantalum boat at a rate of 0.4 ML/min with the temperature of 770 K. After the deposition, the sample was transferred into the LT-STM chamber. All the STM images were obtained in constant-current mode at 78 K. All differential conductance dI/dV spectra were acquired using a standard lock-in amplifier with a bias modulation signal of 10 mV at 1999 Hz under open-loop condition.

Results and Discussion
We firstly studied the initial adsorption stage of C 60 molecules on Cd (0001). Figure 1a shows the STM image of an isolated C 60 molecule, which has a faint nodal plane separating the round protrusion into two lobes. The insert in Figure 1a shows the atomic-resolution image of the hexagonal lattices of Cd (0001) thin films grown on Si(111)-7 × 7 The measured lattice constant of Cd (0001) films is 3.0 ± 0.05 Å, very close to the bulk value (2.973 Å) in Cd crystals [44]. Figure 1b shows the nucleation of C 60 molecules around a 2D Cd island. It was found that the C 60 molecules aggregated at the step edge of the Cd island, due to the increased interaction with substrate at these positions. From the zoomed-in image in Figure 1c, anisotropy can be found from the individual C 60 molecules attaching to the Cd island: most molecules show an anisotropic shape due to the tip convolution effect in the fast-scanning direction, while keep the original length (~1 nm) parallel to the step edge. In fact, such a type of step decoration was previously observed in the C 60 nanostructures grown on metal surfaces. The C 60 molecules initially adsorbed at intersections of multiple steps and edges of monatomic steps on narrow terraces and periodic arrays of short chains formed as the coverage is increased [46]. When C 60 molecules attach on Ag (111) island, a complete, single-strand C 60 "necklace" circling the island formed. The decoration, in turn, made the equilibrium island shape round [47]. Shown in Figure 1d is a monolayer island of C 60 with a close-packed hexagonal structure. There is a misorientation angle of 20 • between the lattice directions of C 60 island and Cd (0001) surface. The error limit in the measurement of orientation angle is ±2 • in our experiments. The measured lattice constant is a 1 = 10.2 ± 0.1 Å, larger than the lattice constant (10.02 Å) of the (111) plane in fcc C 60 crystals [48]. The apparent height of C 60 molecules in this island is measured to be 7.0 ± 0.1 Å, at the bias of 2.2 V. Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 10 island: most molecules show an anisotropic shape due to the tip convolution effect in the fastscanning direction, while keep the original length (~1 nm) parallel to the step edge. In fact, such a type of step decoration was previously observed in the C60 nanostructures grown on metal surfaces. The C60 molecules initially adsorbed at intersections of multiple steps and edges of monatomic steps on narrow terraces and periodic arrays of short chains formed as the coverage is increased [46]. When C60 molecules attach on Ag (111) island, a complete, single-strand C60 "necklace" circling the island formed. The decoration, in turn, made the equilibrium island shape round [47]. Shown in Figure 1d is a monolayer island of C60 with a close-packed hexagonal structure. There is a misorientation angle of 20° between the lattice directions of C60 island and Cd (0001) surface. The error limit in the measurement of orientation angle is ±2° in our experiments. The measured lattice constant is 10.2 0.1 , larger than the lattice constant (10.02 Å) of the (111) plane in fcc C60 crystals [48]. The apparent height of C60 molecules in this island is measured to be 7.0 ± 0.1 Å, at the bias of 2.2 V.  When the coverage increased to one monolayer, C 60 molecules form a close-packed (2 Figure 2a. The apparent height of C 60 molecules in this domain is measured to be 9.0 ± 0.1 Å, at the bias of 2.2 V. The measured intermolecular spacing is increased to 10.4 ± 0.1 Å, much larger than the preferred spacing of in C 60 crystals [48]. It implies that all the C 60 molecules in the (2 3) R30 • domain suffer a tensile strain as large as 3.8%, which would make the R30 • domain less stable. Moreover, we noticed that all the C 60 molecules present a uniform height except a few dim molecules. The height difference between the dim and normal C 60 molecules is~0.6 Å. In particular, some dim molecules aggregate into a wire-like region, indicating the attractive force among the dim molecules. Figure 2b is the high-resolution STM image of the uniform R30 • domain, where C 60 molecules with different molecular orientations can be identified. The individual molecules marked by red, green, and yellow circles exhibit a round protrusion with a small hole (like a doughnut), three lobes like a clover, and two parallel lobes corresponding to the C 60 molecules with a pentagon, hexagon, and 6:6 bond facing up, respectively [13,40]. We noticed that such types of orientational disorder also appeared in the R30 • domains of C 60 grown on Au (111) [13,18,[21][22][23] or Ge (111) surfaces [39]. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 10 When the coverage increased to one monolayer, C60 molecules form a close-packed (2√3 × 2√3) R30° domain as shown in Figure 2a. The apparent height of C60 molecules in this domain is measured to be 9.0 ± 0.1 Å, at the bias of 2.2 V. The measured intermolecular spacing is increased to 10.4 ± 0.1 Å, much larger than the preferred spacing of in C60 crystals [48]. It implies that all the C60 molecules in the (2√3 × 2√3) R30° domain suffer a tensile strain as large as 3.8%, which would make the R30° domain less stable. Moreover, we noticed that all the C60 molecules present a uniform height except a few dim molecules. The height difference between the dim and normal C60 molecules is ~0.6 Å. In particular, some dim molecules aggregate into a wire-like region, indicating the attractive force among the dim molecules. Figure 2b is the high-resolution STM image of the uniform R30° domain, where C60 molecules with different molecular orientations can be identified. The individual molecules marked by red, green, and yellow circles exhibit a round protrusion with a small hole (like a doughnut), three lobes like a clover, and two parallel lobes corresponding to the C60 molecules with a pentagon, hexagon, and 6:6 bond facing up, respectively [13,40]. We noticed that such types of orientational disorder also appeared in the R30° domains of C60 grown on Au (111) [13,18,[21][22][23] or Ge (111) surfaces [39]. When the lattice direction of C60 monolayer deviates from the Cd (0001) lattice for 26° angle, all C60 molecules adopt the same orientation, forming a homogeneous orientation domain. It is observed that from the empty-state STM image at 1.4 V (Figure 3a), all the C60 molecules present a two-lobe structure oriented at the same direction, similar to the motifs marked by yellow circle in Figure 2b. It means that all the molecules in R26° domain adopt the 6:6 bond facing-up orientation, and are adsorbed at the equivalent sites of the Cd (0001). The apparent height of C60 molecules in this domain is measured to be 8.7 ± 0.1 Å at the bias of −2.8 V. The intermolecular spacing is 10 ± 0.2 Å, slightly smaller than that in the C60 island of Figure 1d. Although a similar phenomenon was observed previously in the close-packed C60 monolayer grown on Au (111) [21], the bias-dependent behaviors of the homogeneous domain has not been studied yet. Here, we have made investigations of the variation of C60 molecular contrast with the bias voltage from 1.4 to −2.0 V in the R26° domain. As shown in Figure 3b, when the bias is reduced to 0.6 V, the contrast of each molecule shows four small When the lattice direction of C 60 monolayer deviates from the Cd (0001) lattice for 26 • angle, all C 60 molecules adopt the same orientation, forming a homogeneous orientation domain. It is observed that from the empty-state STM image at 1.4 V (Figure 3a), all the C 60 molecules present a two-lobe structure oriented at the same direction, similar to the motifs marked by yellow circle in Figure 2b. It means that all the molecules in R26 • domain adopt the 6:6 bond facing-up orientation, and are adsorbed at the equivalent sites of the Cd (0001). The apparent height of C 60 molecules in this domain is measured to be 8.7 ± 0.1 Å at the bias of −2.8 V. The intermolecular spacing is 10 ± 0.2 Å, slightly smaller than that in the C 60 island of Figure 1d. Although a similar phenomenon was observed previously in the close-packed C 60 monolayer grown on Au (111) [21], the bias-dependent behaviors of the homogeneous domain has not been studied yet. Here, we have made investigations of the variation of C 60 molecular contrast with the bias voltage from 1.4 to −2.0 V in the R26 • domain. As shown in Figure 3b, when the bias is reduced to 0.6 V, the contrast of each molecule shows four small protrusions: three bright and one dim. In the filled state STM image at −1.4 V (Figure 3c), all the C 60 molecules present a bright protrusion but with a small off-center hole, resembling the contrast of C 60 molecule with a carbon atom at the top [35,49]. When the bias voltage is increased to −2.0 V, the off-center hole becomes smaller and the bright protrusion becomes larger (Figure 3d). It is well known that STM image yields a spatial map of the local density of states (DOS) for the adsorbates on metal surfaces. Thus, the bias-dependent STM images mentioned above can be attributed to the DOS variation of C 60 molecules with energy. However, the possibility of orientation modification driven by the tip electric field cannot be safely excluded under the successive STM scanning. Previously, the tip-induced rotation was reported in the unstable C 60 molecules adsorbed on graphene/Cu (111), where tip electron field or tunneling electron might play an important role for the reorientation [32]. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 10 protrusions: three bright and one dim. In the filled state STM image at −1.4 V (Figure 3c), all the C60 molecules present a bright protrusion but with a small off-center hole, resembling the contrast of C60 molecule with a carbon atom at the top [35,49]. When the bias voltage is increased to −2.0 V, the offcenter hole becomes smaller and the bright protrusion becomes larger (Figure 3d). It is well known that STM image yields a spatial map of the local density of states (DOS) for the adsorbates on metal surfaces. Thus, the bias-dependent STM images mentioned above can be attributed to the DOS variation of C60 molecules with energy. However, the possibility of orientation modification driven by the tip electric field cannot be safely excluded under the successive STM scanning. Previously, the tip-induced rotation was reported in the unstable C60 molecules adsorbed on graphene/Cu (111), where tip electron field or tunneling electron might play an important role for the reorientation [32]. Beside the homogeneous orientation, we also found a local and complex orientational ordering in the C60 monolayer domain. As shown in Figure 4a, four chiral motifs composed of 7 C60 molecules appeared in the R (−4°) domain. The central C60 molecule presents three lobe protrusions, the other six tilted surrounding molecules appear as two-lobe motifs with a nodal plane aligned at different directions, constituting a pinwheel pattern with clockwise handedness. Figure 4b shows a C60 domain with a misorientation angle of +8° relative to the lattice directions of Cd (0001). There are eleven chiral pinwheels with anticlockwise handedness in this domain. Such types of 7-molecule clusters resemble the C60 clusters appeared in the R0° domain grown on Au (111) [18], but the chiral feature appeared in the present domain. To some extent, it is more like the chiral motifs appeared in the C60 multilayer on NaCl [40] or K4+δ C60 monolayer on Au (111) surface [41], which was attributed to overlap of Beside the homogeneous orientation, we also found a local and complex orientational ordering in the C 60 monolayer domain. As shown in Figure 4a, four chiral motifs composed of 7 C 60 molecules appeared in the R (−4 • ) domain. The central C 60 molecule presents three lobe protrusions, the other six tilted surrounding molecules appear as two-lobe motifs with a nodal plane aligned at different directions, constituting a pinwheel pattern with clockwise handedness. Figure 4b shows a C 60 domain with a misorientation angle of +8 • relative to the lattice directions of Cd (0001). There are eleven chiral pinwheels with anticlockwise handedness in this domain. Such types of 7-molecule clusters resemble the C 60 clusters appeared in the R0 • domain grown on Au (111) [18], but the chiral feature appeared in the present domain. To some extent, it is more like the chiral motifs appeared in the C 60 multilayer on NaCl [40] or K 4+δ C 60 monolayer on Au (111) surface [41], which was attributed to overlap of neighboring molecular orbitals due to the superexchange interaction. The latter refers to virtual hopping of an electron from the HOMO of one C 4− 60 to the LUMO of its nearest neighbor to gain an energy proportional to the hopping amplitude and the inverse of HOMO-LUMO gap. Both the apparent heights of C 60 molecules in these two domains are 9.5 ± 0.1 Å. Furthermore, we found the C 60 molecules in above two domains reveal the same intermolecular distance as that in Figure 3, i.e., 10 ± 0.2 Å. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 10 neighboring molecular orbitals due to the superexchange interaction. The latter refers to virtual hopping of an electron from the HOMO of one C to the LUMO of its nearest neighbor to gain an energy proportional to the hopping amplitude and the inverse of HOMO-LUMO gap. Both the apparent heights of C60 molecules in these two domains are 9.5 ± 0.1 Å. Furthermore, we found the C60 molecules in above two domains reveal the same intermolecular distance as that in Figure 3, i.e., 10 ± 0.2 Å.  Figure 5 shows a scanning tunneling spectra (STS) acquired on top of a C60 molecule at 77 K in the (2√3 × 2√3) R30° domain. The measured highest occupied molecular orbital (HOMO) is located at −1.8 eV, while the lowest unoccupied molecular orbital (LUMO) appears at 0.32 eV. Thus the HOMO-LUMO gap is 2.12 eV, which is much smaller than that (4.9 eV) of a free C60 molecule in gas state, but comparable to that (2.7 eV) of C60 on Au (111) and (2.3 eV) of C60 on Ag (100) [5,13]. Compared with the LUMOs of C60 on noble metal surfaces [0.84 eV for Au (111), 0.49 eV for Ag (100), and 0.43 eV for Cu (100)], the lower position of C60 LUMO (0.32 eV) on Cd (0001) reflect the stronger reactivity of Cd substrate [4,5,13]. Moreover, the effect of substrate screening may also reduce the HOMO-LUMO gap, through reducing the intramolecular Coulomb repulsion [50]. In addition to the peaks corresponding to HOMO and LUMO, there are also small peaks appeared in the gap, which result from the surface states or quantum well state in the Cd (0001) films [45].  Figure 5 shows a scanning tunneling spectra (STS) acquired on top of a C 60 molecule at 77 K in the (2 The measured highest occupied molecular orbital (HOMO) is located at −1.8 eV, while the lowest unoccupied molecular orbital (LUMO) appears at 0.32 eV. Thus the HOMO-LUMO gap is 2.12 eV, which is much smaller than that (4.9 eV) of a free C 60 molecule in gas state, but comparable to that (2.7 eV) of C 60 on Au (111) and (2.3 eV) of C 60 on Ag (100) [5,13]. Compared with the LUMOs of C 60 on noble metal surfaces [0.84 eV for Au (111), 0.49 eV for Ag (100), and 0.43 eV for Cu (100)], the lower position of C 60 LUMO (0.32 eV) on Cd (0001) reflect the stronger reactivity of Cd substrate [4,5,13]. Moreover, the effect of substrate screening may also reduce the HOMO-LUMO gap, through reducing the intramolecular Coulomb repulsion [50]. In addition to the peaks corresponding to HOMO and LUMO, there are also small peaks appeared in the gap, which result from the surface states or quantum well state in the Cd (0001) films [45].
We have also studied the structure of the second C 60 layer. Shown in Figure 6a is a C 60 trimer that appeared on top of the first monolayer. It was found that all the three molecules in the second layer appeared at the atop position of the first C 60 monolayer. Figure 6b shows a second layer island containing thirteen molecules arranged in a triangular pattern. Similar to the C 60 trimer in Figure 6a, all thirteen C 60 molecules also occupy the atop position of the first C 60 layer, indicating the C 60 molecules of second layer adopt the same hexagonal lattice as the first layer. Furthermore, we noticed that the intermolecular spacing for the peripheral molecules is slightly larger than that of the internal molecules. This is different from the second layer of C 60 molecules on Au (111) that C 60 molecules occupy the three-fold hollow sites of the first layer [46]. It is also different from the simple cubic lattice structure of C 60 bulk below 249 K, or the face-centered cubic structure of C 60 bulk above 249 K [51]. This phenomenon implies that the charge-transfer induced strong Coulomb interaction among fullerene molecules exist not only in the lateral directions, but also in the normal direction of the Cd substrate. We have also studied the structure of the second C60 layer. Shown in Figure 6a is a C60 trimer that appeared on top of the first monolayer. It was found that all the three molecules in the second layer appeared at the atop position of the first C60 monolayer. Figure 6b shows a second layer island containing thirteen molecules arranged in a triangular pattern. Similar to the C60 trimer in Figure 6a, all thirteen C60 molecules also occupy the atop position of the first C60 layer, indicating the C60 molecules of second layer adopt the same hexagonal lattice as the first layer. Furthermore, we noticed that the intermolecular spacing for the peripheral molecules is slightly larger than that of the internal molecules. This is different from the second layer of C60 molecules on Au (111) that C60 molecules occupy the three-fold hollow sites of the first layer [46]. It is also different from the simple cubic lattice structure of C60 bulk below 249 K, or the face-centered cubic structure of C60 bulk above 249 K [51]. This phenomenon implies that the charge-transfer induced strong Coulomb interaction among fullerene molecules exist not only in the lateral directions, but also in the normal direction of the Cd substrate.   We have also studied the structure of the second C60 layer. Shown in Figure 6a is a C60 trimer that appeared on top of the first monolayer. It was found that all the three molecules in the second layer appeared at the atop position of the first C60 monolayer. Figure 6b shows a second layer island containing thirteen molecules arranged in a triangular pattern. Similar to the C60 trimer in Figure 6a, all thirteen C60 molecules also occupy the atop position of the first C60 layer, indicating the C60 molecules of second layer adopt the same hexagonal lattice as the first layer. Furthermore, we noticed that the intermolecular spacing for the peripheral molecules is slightly larger than that of the internal molecules. This is different from the second layer of C60 molecules on Au (111) that C60 molecules occupy the three-fold hollow sites of the first layer [46]. It is also different from the simple cubic lattice structure of C60 bulk below 249 K, or the face-centered cubic structure of C60 bulk above 249 K [51]. This phenomenon implies that the charge-transfer induced strong Coulomb interaction among fullerene molecules exist not only in the lateral directions, but also in the normal direction of the Cd substrate.

Conclusions
In summary, C 60 molecules deposited on the Cd (0001) surface form a variety of domains with different misorientation angles with respect to the lattice directions of Cd (0001). Both orientation disorder and orientation ordering have been observed in the C 60 domains. In the (2 3) R 30 • domain, orientational disorder of the individual C 60 molecules with either pentagon, hexagon, or 6:6 bond facing up has been observed. Orientation ordering appeared in the R26 • domain such that all the C 60 molecules adopt the same orientation with the 6:6 bond facing up. With the bias variation, possible orientation change of the C 60 molecules takes place due to the influence from tip electrical field. In particular, complex chiral motifs composed seven C 60 molecules with clockwise or anticlockwise handedness have been observed in the R4 • and R8 • domains, respectively. Due to the substrate screening and charge transfer from Cd to C 60 molecules, the C 60 molecules reveal a reduced HOMO-LOMO gap of 2.1 eV, which results in the strong Coulomb interaction among fullerene molecules in lateral and longitude directions. The orientational order and chiral superstructures found in present work provide essential information for understanding the C 60 -metal interaction, and are of great relevance to the carbon-based nanodevices and nanomaterials.