Self-build of C60-C70 molecular heterojunctions on highly oriented pyrolytic graphite
Graphical abstract
Introduction
Carbon-based materials are the most studied and exploited due to the versatile ways of bonding between the carbon atoms [1], [2], [3]. Highly oriented pyrolytic graphite (HOPG), one of the allotropies of carbon, is a useful solid-state material consisting of large numbers of regularly stacked graphene layers, often used as a subtract in the study of surface science due to its stable, renewable, and an atomically flat surface. Although HOPG, carbon nanotubes, and fullerene molecules are relatively new materials, they have also been extensively studied. The preparation of carbon clusters using laser evaporation, Cn, which is n ranges from 2 to 200, has also been extensively studied [2], [4]. Among these carbon clusters, C60 and C70 were investigated for the first time due to their abundance and higher stability [5]. The long axis of C70 is about 1.07 times the diameter of C60 [6], [7], both these two molecules can form single crystals with the fcc structure at room temperature (RT). Thus, it is expected that the two molecules can be mixed to form a binary solid similar to the formation of binary colloidal lattices [8], [9]. Binary mixtures of C60 and C70 in solid solutions have been studied and found that mixing of the molecules can suppress the orientational transition temperatures of the whole system [10], [11]. When deposited, both C70 and C60 onto HOPG they form close-packed molecular layers [12], [13], [14], [15]. The growth follows approximately the layer-by-layer growth mode and so this is called two-dimensional layered materials (2DLMs) [16]. The interaction of layers is van der Waals interactions rather than chemical bonding or a strong hybridization with delocalized states, such a C60 on Au(1 1 1) or PTCDA on conductive n-doped ZnO, which allows us to produce a vertical/lateral heterojunctions [17], [18], [19], [20], [21], [22]. A large number of conjugated π bonds exist in these organic molecules, makes it possible to grow an inorganic material on its surface and form an organic–inorganic van der Waals heterojunctions due to van der Waals interactions [23]. Moreover, the strong hybridization with localized states in a proper substrate or organic molecules, makes these fullerene molecules are promising material exploring and fabricate organic electronics device or single-molecule devices [19], [20], [23], [24], [25], [26]. Besides the similarities between C60 and C70, there are clear differences in their geometric structures. While C60 has a spherical shape with an icosahedral symmetry [27], C70 adopts an ellipsoidal shape due to the extra ten carbon atoms which creates a more complex carbon–carbon bonding. Stacking a C60 layer on a C70 layer or vice versa introduces strain at the interface. It has been found that the C70 molecules can take a particular orientation in order to reduce the interfacial strain, however, the impact of strain and regulatory on the packaging structure still undiscovered [27], [28]. Van der Waals Heterostructures, such as the system of C60-C70/Graphene, C60-C70/hBN, and C60-C70/Transition metal dichalcogenide are promising materials for the fabrication of C60-C70 molecular heterojunctions [29], [30], [31].
The STM is a versatile tool to study molecular heterojunctions due to its ability to image surfaces in real space with atomic resolution. The molecular layers of fullerenes were prepared on various substrates, such as Ag, Cu, Au, Al [32], [33]; MoS2 [34], [35], graphene [36] and their heterostructures [15], [37] and the molecular orientation and structures were revealed by STM at various temperature. The C60/C70 heterostructures on graphene surfaces have been rarely studied; there are still open questions such as the formation of C60 and C70 layer as a function of coverage and elevated temperature. In this paper, we focus on these open questions and report the findings from a recent study of the intermixing of C60 and C70 on a HOPG surface under ultra-high-vacuum (UHV) conditions in STM. The growth process of C60-C70 molecular heterojunctions will be discussed with different deposition sequences/ratio. Furthermore, the effect of elevated temperature on the C60-C70 heterojunction results in self-organize layers and creates a new type of molecular Van der Waals heterostructure.
Section snippets
Experimental
The experiments were performed using a room temperature Beetle-type 400 RHK and Omicron variable-temperature STM systems. HOPG (purchased from Goodfellow, 99.99% purity) was used as a substrate and it was annealed before fullerene depositions in UHV at 475 K for 30 min to remove surface contaminations. C60 and C70 molecules (purchased from MER, 99.5% purity) were deposited onto the HOPG substrate using home-built effusion cells with a rate of 0.12 and 0.10 ML/min, respectively while keeping the
Deposition of 1.2 ML C60 (and C70) on HOPG
Fig. 1(a) shows an STM image of the first sample with 1.2 ML C60 deposition on HOPG at room temperature. The image shows the formation of the first layer with the C60 array and the second layer with dendritic-island structures. We observed two incomplete layers of C60 island formations. The first layer formed directly above HOPG that covers ~78.6% of the substrate. The second layer of C60 island forms on the top of the first layer. Although the total coverage is greater than one monolayer,
Conclusions
In summary, the van der Waals heterojunction between C60 and C70 molecular layers was studied at different coverages/sequences and temperatures. Adding C70 to an existing C60 layer or vice versa is a possible preparation method for the van der Waals heterojunction applications. When C70 molecules are added to a C60 layer, C70 molecules occupy the three-fold hollow site and form a lattice-matched interface. The lattice match forces the C70 molecules to take a fixed upright configuration with a
CRediT authorship contribution statement
Lu’an Guo: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Yitao Wang: Formal analysis, Investigation. Dogan Kaya: Formal analysis, Investigation. Zhiming Wang: Co-Supervision, Review, Funding acquisition. Min Zhang: Co-Supervision, Writing – Review, Funding acquisition. Quanmin Guo: Main Supervision, Conceptualization, Methodology, Writing - Review & Editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Chinese Scholarship Council for providing a scholarship to Lu’an Guo. Special thanks to Dogan Kaya who generously volunteered his time and expertise during the revision. The authors also acknowledge the support from the National Key Research and Development Program (No. 2019YFB2203400), UESTC Shared Research Facilities of Electromagnetic Wave and Matter Interaction (Y0301901290100201) and Young Scientists Fund of the Basic and Applied Basic Research of Guangdong (2019A1515110021).
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