Acta Physico-Chimica Sinica  2017, Vol. 33 Issue (3): 627-632   (1462 KB)    
LT-STM Investigation of the Self-Assembled F16CuPc-Corannulene Binary System on Ag (111) and Grap
GUO Rui1, ZHANG Jialin1,2, ZHAO Songtao3, YU Xiaojiang4, ZHONG Shu1, SUN Shuo2, LI Zhenyu3, CHEN Wei1,2,5,6,*   
1 Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore;
2 Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore;
3 Hefei National Laboratory for Physical Sciences at the Microscale, CAS Centre for Excellence and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, P. R. China;
4 Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, 117603, Singapore;
5 Center for Advanced 2D Materials and Graphene Research Center, National University of Singapore, 3 Science Drive 3, 117546, Singapore;
6 National University of Singapore (Suzhou) Research Institute, Suzhou 215123, Jiangsu Province, P. R. China
Abstract: Corannulene (COR) is considered a promising molecular building block for organic electronics owing to its intriguing geometrical and electronic properties. Intensive research efforts have been devoted to understanding the assembly behavior and electronic structure of COR and its derivatives on various metal surfaces via low-temperature scanning tunneling microscopy (LT-STM). Here we report the formation of binary molecular networks of copper hexadecafluorophthalocyanine (F16CuPc)-COR self-assembled on the highly oriented pyrolytic graphite (HOPG) and Ag (111) substrates. Intermolecular hydrogen bonding between F16CuPc and COR facilitates the formation of binary molecular networks on HOPG and further induces a preference for bowl-down configured COR molecules. This observed configuration preference disappears on Ag (111) substrate, where COR molecules lie on the substrate with their bowl openings pointing up and down randomly. We propose that strong interfacial interactions between the molecule and Ag (111) surface constrain the bowl inversion of the COR molecule, which thus retains its initial configuration upon adsorption.
Key words: Molecular assembly     Binary molecular networks     Corannulene     Low-temperature scanning tunneling microscopy     Intermolecular hydrogen bonding    

1 Introduction

Corannulene (COR) is a bowl-shaped molecule and can be regarded as a fragment of fullerene, as shown in Fig. 1a. Since its first successful synthesis in 19661, COR has attracted intensive attention due to several intriguing properties. With fivefold symmetry, COR provides a unique opportunity to study the symmetry mismatching between adsorbate and substrate, given the incompatibility between the fivefold rotational symmetry of molecule and translational order of the underneath crystal lattice2. The combination of non-planar shape and aromaticity makes COR an interesting system with unique geometry and electronic properties2c. Specific π-π interactions between curved and planar structures give rise to fascinating photoelectric properties3. Buckybowls also serve as ideal hosts to form the host-guest complexes in supramolecular chemistry4. COR has always been regarded as a fragment of C60 molecule for its symmetry and conformation. But considering its high solubility in most common solvents2c, COR can be a better choice than fullerenes as a promising candidate for acceptor materials in organic optoelectronic devices2c. It should be mentioned that Kuvychko et al.5 have recently reported a COR derivative (with electron withdrawing groups) that has a higher electron affinity and thus can be a stronger electron acceptor than the well-studied C60.

Fig. 1 Molecular structures for (a) COR and (b) F16CuPc; (c) STM image of F16CuPc monolayer deposited on HOPG; (d) schematic packing structure for F16CuPc molecule on HOPG; (e, f) STM images of long range-ordered binary molecular networks formed by F16CuPc and COR with a molecular ratio of 1 : 2 on HOPG; (g) schematic packing model for the F16CuPc-COR binary structure on HOPG In figure c: The unit cell is highlighted with a=1.66 nm, b=3.5 nm, θ1=108°. Vtip=1.654 V, 20 nm × 20 nm. In figures e, f: The COR molecules adopting the bowl up and bowl down configurations are indicated by the arrows. The unit cell is highlighted in the image with c=2.87 nm, d=2.17 nm, θ2=114°. Vtip=1.496 V, 20 nm × 20 nm; Vtip=1.496 V, 10 nm × 10 nm

Two-dimensional (2D) self-assembly of functional organic molecules into ordered arrays represents one of the most promising strategies to fabricate functional molecular nanostructures over macroscopic areas6. Modification of metal surfaces with COR and its derivatives has been studied for symmetry mismatch between substrate and adsorbate2a, 7, multi-component packing4, 8, templated assembly8a, 9, interface dipole formation10, as well as 2D phase transitions6c, 11. The assembly behaviors of the single-component molecules with fivefold symmetry on surface are of fundamental interest12. The structure of self-assembled COR monolayer on Cu (111) and Cu (110) has been reported. On Cu (111), each COR molecule adsorbs on either fcc or hcp threefold hollow site with its bowl opening pointing up11b. One of the five hexagonal rings orients parallel to the surface plane and therefore a tilt between molecular bowl with respect to the surface exists. A temperature-controlled reversible phase transition was also observed in this system. It is explained that low temperature constrains the vibration of COR molecules, thus leads to a more effective intermolecular attraction, and finally results in the transition to the phase with higher packing density11b. On Cu (110), a similar quasihexagonal lattice with slightly tilted COR molecules was observed2a. In addition to monolayer, a bilayer bowl-in-bowl stacking structure of COR was also reported on Cu (111) at low temperature13. Each second-layer molecule locates directly above one firstlayer molecule, leading to the formation of a bowl-in-bowl dimer.

In contrast to the intensive studies on single component selfassembly of COR and its derivatives, investigation on multicomponent molecular assembled system consisting of COR is rarely reported. Multicomponent 2D assemblies provide more functionality and tunability for the molecular nanostructures14. Calmettes et al.8a reported binary molecular networks comprising 2, 3, 9, 10, 16, 17, 23, 24-octachlorozinc phthalocyanine (ZnPcCl8) and the COR derivative of 1, 3, 5, 7, 9-penta-tert butylcorannulene (PTBC). In this case, the metastable phase of ZnPcCl8 can be used as a flexible template to realize the controllable insertion of PTBC molecule. By selecting different phases formed by ZnPcCl8, the final bimolecular 2D structure, which resembles the original packing of template, can be regulated. Xiao et al.4 reported a CORC60 buckybowl-buckyball host-guest complexes by depositing C60 onto the ordered monolayer of COR on Cu (110). The concave structure of COR is optimal to realize a"face-to-face"contact with the convex surface of C60 and their complementary electron environments are favorable for binding. Via thermal activation, a strongly bound COR-C60 host-guest system is formed. Delicate balance between various intermolecular and interfacial interactions plays essential role in tailoring these supramolecular structures6b, 8a, 14a, 15.

Herein we report the formation of self-assembled binary molecular networks of COR and copper hexadecafluorophthalocyanine (F16CuPc) on the highly oriented pyrolytic graphite (HOPG) and Ag (111). The geometrical arrangements of the binary system on different substrates were systematically investigated by lowtemperature scanning tunneling microscopy/spectroscopy (LTAbstract STM/STS).

2 Experimental methods

The Ag (111) and HOPG single crystal substrates are purchased from MaTeck Material-Technologie & Kristalle GmbH. The F16CuPc molecules are twice sublimed and purchased from CREAPHYS. Both sample preparation and investigation were performed in an ultrahigh vacuum system at a base pressure around 10-10 mbar (1 mbar=101 Pa). The Ag (111) surface was prepared via repeated cycles of sputtering by Ar+ and then annealing to 750 K. Freshly cleaved HOPG was thoroughly degassed in UHV at 800 K overnight. COR and F16CuPc were thermally evaporated from separate Knudsen cells at 380 and 670 K, respectively, onto the substrate (kept at room temperature).

In-situ STM investigation was carried out in a custom-designed Omicron LT-STM with an electrochemically etched tungsten tip scanning at 77 K. All STM images were obtained under constant current mode with bias voltages applied to the tip. To collect the differential conductance dI/dV (local density of states), a lock-in technique was adopted together with a modulation voltage of 50 mV and a frequency of 625 Hz. When ramping the voltage, the feedback loop was opened16.

3 Results and discussion

F16CuPc, as shown in Fig. 1b was first deposited onto HOPG to form a self-assembled monolayer. STM image (Fig. 1c) clearly reveals a typical close packing structure where F16CuPc molecules lie flat on substrate with their molecular planes parallel to the substrate, arising from the interfacial π-π interaction17. A unit cell with a=1.66 nm, b=3.5 nm, θ1=108° is outlined in Fig. 1c and schematic packing model of one unit cell is shown in Fig. 1d. Two different molecular orientations exist in F16CuPc monolayer on HOPG, which has been concretely analyzed in previous report17. In one unit cell, the orientation of four F16CuPc molecules on the corner is deviated from that of two F16CuPc molecules centered at the b edge. Then COR molecules were evaporated onto the F16CuPc covered HOPG. Co-assembly of F16CuPc and COR, as shown in Fig. 1e and 1f, forms a long range-ordered structure with an intermixing ratio of 1 : 2. Aunit cell with c=2.87 nm, d=2.17 nm, θ2=114° is highlighted in Fig. 1f and the schematic packing model of one unit cell is shown in Fig. 1g. It is noteworthy that in the supramolecular structure, only one orientation of F16CuPc molecule is observed and each F16CuPc molecule is surrounded by 6 COR molecules.

Here we observe two kinds of dots around F16CuPc, including the dots that are brighter and the dots that are slightly darker. We propose both kinds of dots are COR molecules but with different configurations: bowl opening pointing up and pointing down. STS measurements (Fig. 2b) confirm this assumption and reveals the highest occupied molecular orbital (HOMO)-the lowest unoccupied molecular orbital (LUMO) gap of around 3.10 eV, which agrees with the theoretically calculated HOMO-LUMO gap of COR molecule18. The simulated topographic STM images of COR, based on semi empirical extended Hϋckel calculation, have been used to determine the configuration of adsorbed COR by Parschau et al.2a. For bowl up configuration, both the HOMO and LUMO topographic simulated images show a density minimum at the center of the molecule together with a distinct fivefold doughnut shape. On the contrary, for bowl down configuration, both the HOMO and LUMO topographic simulated images show a density maximum at the center of the molecule and the outline of COR molecule is rather vague2a. Hence by comparing the simulated STM image with our high resolution STM results in Fig. 2a, we assign these brighter dots to COR with bowl opening down and darker dots to COR with bowl opening up.

Fig. 2 (a) High resolution STM image of the F16CuPc-COR binary molecular networks on HOPG; (b) dI/dV spectra recorded on the bowl-up and bowl-down COR molecules

It is noticed that in the F16CuPc-COR binary molecular networks on HOPG, COR molecules that adopt bowl-down configuration hold majority. We propose that this configuration preference may arise from the formation of multiple intermolecular hydrogen bonding. As F16CuPc molecules lie flat on the plane, peripheral hydrogen atoms of COR molecule with bowl-down configuration can stand closer to the neighboring F16CuPc, which facilitates the formation of multiple intermolecular hydrogen bonding between neighboring F16CuPc and COR. In this way, the binary supramolecular structure is effectively stabilized and bowldown configuration of COR thus is energetically favorable.

We also grew the same F16CuPc-COR binary system on Ag (111) to compare the co-assembly structures on different substrates. Ag (111) has shown much stronger molecule-substrate interactions for various organic adsorbates19, compared with HOPG. Hence we were able to grow a monolayer of COR onto Ag (111). A large scale and the corresponding close up STM images of COR on Ag (111) are shown in Fig. 3(a, b) with a unit cell outlined (e=1.02 nm, f=1.17 nm, θ3=73°). Each COR molecule is shared by four unit cells (Fig. 3c). Likewise, we observe brighter and slightly darker dots in the STM image of COR monolayer. Careful inspection of high-resolution STM (Fig. 3d) confirms the co-existence of COR molecules with different configurations. Herein the brighter dot obviously has an intensity minimum in the center. Hence by using the aforementioned comparison of high-resolution STM images with simulated results, these brighter dots should be assigned to COR molecules with bowls opening up and the darker and vague dots should be COR molecules with bowls opening down.

Fig. 3 (a) Large scale STM image of COR monolayer deposited on Ag (111); (b) close up STM image of COR on Ag (111); (c) proposed schematic packing model for COR on Ag (111); (d) high resolution STM image of COR on Ag (111) with the bowl-up and bowl-down configurations indicated by red arrows In figure a: Vtip=1.0 V; 60 nm × 60 nm. In figure b: The unit cell is highlighted by the red rhombus with e=1.02 nm, f=1.17 nm, θ3=73°; Vtip=1.0 V, 10 nm × 10 nm. In figure d: Vtip=-1.5 V; 6 nm × 6 nm

To further confirm our assignment, a comparison of the brighter dots on HOPG and Ag (111) under high-resolution STM is shown in Fig. 4. It is obvious that in Fig. 4a, the COR molecule with bowlup configuration possesses a hollow center with a rough pentagonal doughnut shape, which is consistent with features of the simulated bowl-up COR. While in Fig. 4b, the COR molecule accounted as bowl-down configuration is more protruding in the center and the molecule shape is obscure, which also resembles simulated bowl-down topography. We noted that on Ag (111) substrate, the configuration preference of COR disappears: both bowl-up and bowl-down COR exist in almost equal amount. In other words, the adoption of bowl-up or bowl-down configuration is random. We suggest that the strong COR-Ag (111) interfacial interaction constrains the movement and bowl inversion of COR molecules. Once adsorbed on Ag (111), COR molecule could only retain its initial configuration and therefore both configurations have equal chance to appear.

Fig. 4 Comparison of the bighter dots under high-resolution STM (a) COR molecule with bowl-up configuration on Ag (111); (b) COR molecule with bowl-down configuration on HOPG

Co-assembly monolayer of F16CuPc and COR on Ag (111) was also prepared by further evaporating F16CuPc molecules onto the COR covered substrate. STM image reveals the long range-ordered binary molecular networks with a molecular ratio of 1 : 4. The unit cell is outlined in Fig. 5a with features including g=h=2.73 nm and θ4=100°. Corresponding schematic packing model for the binary structure is shown in Fig. 5b. All the F16CuPc molecules lie in the same orientation and each F16CuPc molecule is surrounded by 8 COR molecules.

Fig. 5 (a) High resolution STM image of F16CuPc-COR binary molecular networks on Ag (111) with a molecular ratio of 1 : 4; (b) schematic packing model for the F16CuPc-COR binary structure on Ag (111) In figure a: The unit cell is highlighted by the red rhombus with g=h=2.73 nm, θ4=100°. Vtip=1.26 V, 10 nm × 10 nm

4 Conclusions

In summary, we have investigated the binary supramolecular structure of F16CuPc-COR monolayer assembled on HOPG andAg (111) substrates. The formation of multiple intermolecular hydrogen bonding between F16CuPc and COR could result in a preferred bowl-down configuration for COR molecules on the weakly interacting HOPG. In contrast, this configuration preference disappears on Ag (111) substrate where the adoption of bowl-up or bowl-down configuration is random, resulting from the strong molecule-substrate interactions. Our work would further reinforce the modification of surface with binary molecular networks consisting of COR and its derivatives.

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