Strain-Relief Patterns and Kagome Lattice in Self-Assembled C₆₀ Thin Films Grown on Cd(0001)

Abstract

We report an ultra-high vacuum low-temperature scanning tunneling microscopy (STM) study of the C₆₀ monolayer grown on Cd(0001). Individual C₆₀ molecules adsorbed on Cd(0001) may exhibit a bright or dim contrast in STM images. When deposited at low temperatures close to 100 K, C₆₀ thin films present a curved structure to release strain due to dominant molecule–substrate interactions. Moreover, edge dislocation appears when two different wavy structures encounter each other, which has seldomly been observed in molecular self-assembly. When growth temperature rose, we found two forms of symmetric kagome lattice superstructures, 2 × 2 and 4 × 4, at room temperature (RT) and 310 K, respectively. The results provide new insight into the growth behavior of C₆₀ films.

1. Introduction

The structures and growth process of C₆₀ monolayer grown on metal surfaces have attracted widespread interest in the past decades due to the unique physical and chemical properties [1,2,3]. In the fullerene family, C₆₀ was the first member to be created and most extensively discussed. C₆₀ molecules form various self-assembled structures when deposited on different types of substrate, such as Au [4,5,6,7,8,9], Ag [10,11,12,13,14], Cu [15,16], Pb [17], Pd [18], P [19], Al [20], graphene [21,22,23,24], WSe₂ [25], Si [26,27,28,29], and Ge [30,31,32]. Many results indicate that C₆₀ molecules are easy to nucleate at the terrace edge and may form a close-packed structure under appropriate conditions, regardless of the types of substrate, such as the 2√3 × 2√3 R30° domain on Au(111) [33,34] and the 4 × 4 superstructure on graphene/Cu(111) [22].

Strain plays a central role in governing the structures of self-assembled thin films when organic molecules are deposited onto solid surfaces [35,36,37,38]. Strain derives from the competition between molecule-molecule interactions and molecule-substrate interactions. Currently, several strain relaxation mechanisms are known, such as step bunching [39], faceting [40], misfit dislocations [41,42], and the formation of periodic domain boundaries [43,44], due to the dominant molecule–substrate interactions of the different layers [45]. In the STM images of the C₆₀ monolayer, the individual C₆₀ molecules appear as a bright or dim contrast [4,5,6,7,8,9,10,11,12,13,14,15,16]. The various arrangements of the bright and dim C₆₀ molecules combined with the multiple molecular orientations lead to the formation of a series of superstructures in the C₆₀ monolayer, such as 2 × 2 [8,25,26], 3 × 3 [5,21], √19 × √19 [28,29], 7 × 7 [4,5,9], and a triple-stripe phase [18]. Moreover, the kagome lattices built from intersecting triangles and hexagons are observed in some C₆₀ monolayers [8,13,23]. It is necessary to study the behavior of bright or dim contrast in kagome lattice when the C₆₀ thin film is under strain.

In divalent metal Cd, there is a Coulomb repulsion between the 4d electrons and conduction band electrons, which leads to electrons with strong anisotropy in the mean free path and heat conduction. Recently, the Cd(0001) surface was shown to be a good substrate for observing and investigating strain phenomena of epitaxial films [46,47].

In this study, we used Cd(0001) thin films grown on Si(111)-7 × 7 as a substrate to explore the interface structures of the C₆₀-Cd system. The STM study demonstrated that the C₆₀ thin films on the Cd(0001) surface presented diversity at different growth temperatures. When the growth temperature was close to 100 K, an unexpected wavy structure driven by compressive strain appeared. Two lines of the molecular arrangements were curved and corresponded to the √3 directions of the Cd(0001) surface. Moreover, an edge dislocation was observed when two neighboring wavy domains with different directions were encountered. When the growth temperature increased, two kinds of symmetric kagome lattice, superstructures (2 × 2 and 4 × 4) were identified in the C₆₀ domains, where individual C₆₀ molecules exhibited a bright or dim contrast.

2. Results and Discussion

2.1. An Individual C₆₀ Seven-Molecule Cluster at Two Bias Voltage

When a small amount of C₆₀ molecules was deposited on the Cd(0001) surface at 100 K, they formed individual small clusters. Figure 1A shows a typical C₆₀ heptamer (seven-molecule cluster) with one central C₆₀ molecule surrounded by six peripheral molecules. At the bias of 1.2 V, except for the central molecule with a dim protrusion, the other six peripheral C₆₀ molecules revealed a similar bright contrast. When the bias was reduced to 0.5 V, the upper-right C₆₀ molecule marked by the arrow became dim and revealed a two-lobe shape (Figure 1B). We speculate that the main mechanism for the contrast changes of this molecule arises from the modification of the molecular orientation during low-bias scanning. To the best of our knowledge, such forms of isolated C₆₀ heptamers were not reported in previous experiments.

2.2. Wavy Structure of the C₆₀ Submonolayer Appears at 100 K

As molecule coverage increased, an unexpected wavy structure of the C₆₀ submonolayer appeared. Figure 2A is the STM image of a wavy domain where the C₆₀ molecules in the (120) direction present a wavy arrangement while the molecules in the (1$\overline{1}$0) direction are arranged in a straight line. Both directions correspond to the √3 directions of the Cd(0001) surface. The three rows marked with red curves demonstrate the wavy arrangement clearly. Figure 2B displays another wavy domain and the direction of straight alignment has an angle of 60 degrees with respect to Figure 2A. The intermolecular spacing is 9.8 ± 0.1 Å in the straight rows and 9.4 ± 0.1 Å in the wavy rows. Both are apparently smaller than the preferred spacing (10.02 Å) of the (111) plane in fcc C₆₀ crystals [48], indicating a 2% compressive strain in straight molecular rows, and a 6.3% compressive strain in wavy molecular rows.

When C₆₀ submonolayers were subjected to external forces, C₆₀ molecules correspond with stable orientations to attach to the substrate. However, it was not enough to release the stress by orientation and a wavy structure appeared. Driven by the compressive stress, the individual C₆₀ molecules deviated from linear arrangement, and the molecular rows became wavy in order to release the strain. In early studies, when the molecule-substrate force worked as a dominant role, slightly curved stripes were also observed in C₆₀ films deposited on an Si(111) 4 × 1 In surface [49]. In addition to unidirectional wavy structures, we observed another strain relaxation pattern through the formation of edge dislocations in the C₆₀ monolayer.

2.3. High-Resolution Topological Graph of the Edge Dislocation

Figure 3A shows another area of the wavy structures comprising two neighboring wavy domains (I and II) with different directions for the straight alignments. The straight arrangements and wavy arrangements in Domain I and II are marked with blue curves and red lines, respectively. The straight arrangement in Domain II is in the (120) direction, while the straight arrangement in Domain I is in the (210) direction. When these domains encounter each other, an edge dislocation appears at the domain boundaries, tagged with white dotted lines. To the best of our knowledge, edge dislocation is seldomly precisely observed in molecular self-assembly. A similar phenomenon was achieved in the work of Klyachko, who found the edge dislocation in the third layer of C₆₀ films grown on Ge(100) [31]. Unfortunately, the detail of the edge dislocation was hard to obtain due to the absence of a high-resolution STM diagram.

Figure 3B depicts the close-up view of the edge dislocation. Represented by black dotted lines, Domain I has an extra C₆₀ row compared to Domain II. The same situation also occurs along the purple dotted lines. This observation indicates that the edge dislocation may occur in the monolayer regime and provides an intuitive STM diagram for the study of edge dislocations. Moreover, we noticed that this type of dislocation is absent when Cd(0001) substrate is kept at RT during C₆₀ deposition.

2.4. Two Regular Domains in R26° and R33°

When growth temperature is raised to RT, two types of domain appear in the C₆₀ monolayer. Figure 4A shows the topography of a R26° domain, in which all the C₆₀ molecules adopt the same orientation. Each molecule reveals a two-lobe contrast corresponding to the C₆₀ orientation with a 6:6 bond facing upward, similar to the C₆₀ molecule in Au(111) and graphene [4,22]. The molecules in this domain exhibit a hexagonal lattice constant a₁ = 10.0 ± 0.1 Å, close to the same value (10.02 Å) in C₆₀ crystals [48]. This means that there is almost no strain when C₆₀ molecules adopt the same orientation. Figure 4B shows a R33° domain that is composed of symmetric kagome lattices. Inside this domain, each dim C₆₀ molecule is surrounded by six bright molecules, constituting a head-to-head arrangement of hexagonal rings. In other words, the C₆₀ monolayer is composed of two types of molecular rows: In row-b, all the molecules show a bright protrusion, while in row-a, one bright and one dim C₆₀ molecule alternately appear. As a result, the bright and dim molecules constitute a 2 × 2 superstructure, with each unit cell containing four molecules. The measured intermolecular distance is a₂ = 10.2 ± 0.1 Å, indicating a 2% tensile strain in the R33° domain. These types of symmetric kagome lattices also exist in the reported C₆₀ domains in Au(111) [8,23].

2.5. A larger 4 × 4 Superstructure of Kagome Lattice in an R44° Domain

When the growth temperature rises to 310 K, the C₆₀ monolayer presents a larger kagome lattice with a 4 × 4 superstructure, shown in Figure 5A. The intermolecular distance is a₃ = 10.5 ± 0.1 Å, much larger than the value (10.02 Å) in C₆₀ crystals. The tensile strain is as large as 5%. These properties indicate that the packing density of C₆₀ molecules decreases with an increase in growth temperature due to thermal expansion of the C₆₀ layer. As displayed in Figure 5B, the six trigonal regions distribute symmetrically around the central hexagon, constituting the kagome lattice. Each kagome unit cell contains sixteen molecules: a C₆₀ heptamer located at the inner hexagon and nine peripheral molecules. The heptamer consists of a central dim molecule and six bright surrounding molecules. We noticed that the C₆₀ monolayer in this domain is composed of three types of molecular rows: all C₆₀ molecules in row-a show a dim contrast; two dim molecules and two bright molecules are alternately arranged in row-b; one bright and one dim molecule appear alternately in row-c.

3. Materials and Methods

The experiments were performed in an ultra-high vacuum low-temperature scanning tunneling microscopy (Unisoku, Japan) with a base pressure close to 1.5 × 10⁻¹⁰ Torr. A clean surface of Si(111)-7 × 7 was obtained through thermal flashing to 1550 K after uninterrupted degassing at 820 K for more than 7 hours. The flat and smooth Cd(0001) thin film was treated by depositing 15-20 monolayers onto the surface of Si-7 × 7. C₆₀ molecules were evaporated from a homemade Tantalum boat at a rate of 0.4 ML/min onto the Cd(0001) thin films. After the process was completed, C₆₀/Cd/Si was transferred into the STM observation chamber. The entire data were obtained in a constant current mode at 78 K (liquid nitrogen temperature).

4. Conclusions

In summary, our research showed the morphology of C₆₀ molecules on Cd(0001) substrate in three different growth temperatures (100 K, RT, and 310 K). STM studies demonstrated that C₆₀ molecules are bright or dim in the thin film. At 100 K, we found the isolated C₆₀ seven-molecule cluster, which indicates that the C₆₀ heptamer can exist alone. As the molecule coverage increased, an unexpected wavy structure appeared, which suggests the C₆₀ submonolayer was subjected to a large compressive strain. As a normal reaction, when under large external stress, the film of atoms or molecules may present larger bulk density or form superstructures. In this study, the arrangement of C₆₀ curved instead of the aforementioned action. Additionally, an edge dislocation was captured when two different wavy structures encountered each other, which may develop a new phenomenon to study edge dislocation. Two types of symmetric kagome lattices were found at elevated growth temperatures, the 2 × 2 superstructure in the R33°domain with a 2% tensile strain at RT, and the 4 × 4 superstructure in the R44° domain with a 5% tensile strain at 310 K. Our results provide a new routine to fabricate and investigate pressure-resistant materials.