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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Dimensionality Control of Self-Assembled Azobenzene Derivatives on a Gold Surface Hyo Won Kim, Jaehoon Jung, Mina Han, JiYeon Ku, Young Kuk, and Yousoo Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11744 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019
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The Journal of Physical Chemistry
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Dimensionality Control of Self-Assembled Azobenzene Derivatives on a Gold Surface Hyo Won Kim†*, Jaehoon Jung‡§, Mina Hanǁ, JiYeon Ku†, Young Kuk⊥ ,Yousoo Kim‡* †Samsung ‡Surface
Advanced Institute of Technology, Suwon 13595, Korea
and Interface Science Laboratory, RIKEN, Wako, Saitama 351-0198 Japan
Department of Chemistry, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Korea
§
ǁDepartment ⊥Center
of Chemistry Education, Kongju National University, Gongju 32588, Korea
for Quantum Nanoscience, Institute for Basic Science, Seoul 03760, Korea
ABSTRACT: Well-defined nanostructures constructed with functional molecules provide a feasible route to realize molecular nanotechnology. Synthesis of selectively interacting molecules is essential to develop nanostructures with desired functionalities and dimensions. Substantial efforts have been devoted to achieve finely controlled supramolecular structures on surfaces using various interactions such as van der Waals (vdW), dipolar, hydrogen boning, and metal-ligand interactions. Yet controlling the dimensions of a supramolecular assembly by changing the strength of the intermolecular vdW interactions, in particular through attaching alkyl chains of different lengths, has not been reported so far. Here, we present the dimensionality control of selfassembled azobenzene derivatives, from one-dimensional chain to two-dimensional island, on a Au(111) surface by exploiting vdW interactions assisted by hydrogen bonding. The designed
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azobenzene derivatives have alkoxy groups with different chain lengths (6, 8, and 10 carbons). Depending on the alkyl chain length, the molecules self-assemble into two different stacking structures, which determine the dimensionality of the superstructures. Furthermore, we demonstrate that the reconstructed herringbone structures of the substrate determine the stacking structure and growth direction at the elbow of the Au(111) surface. Our results provide a new perspective for engineering well-defined nanostructures with functional molecules as well as deeper insights into the mechanism of molecular self-assembly on surfaces.
INTRODUCTION Design and synthesis of molecular machines1,2 have attracted renewed interest since the award of Nobel Prize for Chemistry in 2016. The miniaturized machines can be further linked with other molecules to develop memory devices,3 sensors,4 and solar energy storage systems.5 Building molecular architectures via a bottom-up approach is mainly based on surface-assisted6,7 or solution-mediated8,9 molecular self-assemblies. Compared to the solution-controlled method, molecular self-assembly on a surface offers structurally better defined and more homogeneous nanostructures. The size and shape of the self-assembled molecular structures on a given surface should be controlled to eventually construct desired nanostructures. Yokoyama et al.10 first reported the dimensionality control of self-assembled porphyrin molecules by changing the number and site of the substituents to induce hydrogen bonding between the molecules. The dimensionality control of the assemblies was developed further via stronger intermolecular interactions like covalent bonding11,12 to create robust molecular structures or new materials like graphene nanoribbons.13,14 However, such strong linkages can cause the molecules to lose their original functionalities. Comparatively weak van der Waals (vdWs) interactions, on the other
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The Journal of Physical Chemistry
hand, are appropriate for constructing the assemblies while preserving the functionalities of molecules.15 Although the strength of the vdW interactions could be adjusted by attaching alkyl groups with different chain lengths to molecules16-18, the dimensionality control of the resulting assemblies by varying the chain lengths has never been reported. We studied the dimensionality control of self-assemblies of azobenzene derivative molecules on a Au(111) surface, from one-dimensional (1D) chain to two-dimensional (2D) island, using scanning tunneling microscopy (STM) and first-principles calculations based on the density functional theory (DFT). The azobenzene molecules were attached with alkoxy groups of different chain lengths to adjust the strength of inter-molecular vdW interactions. Varying the length of the attached alkyl chain (hexyloxy, octyloxy, and decyloxy) led to two types of stacking structures distinguished by the alignment of molecules in forming pairs: the molecules orient parallelly (“αstacking”) or anti-parallelly (“β-stacking”). These stacking structures determine the dimensionality of the molecular self-assemblies. The DFT calculations also support the transition of the dimensionality depending on the alkyl chain length. In addition, the molecule-substrate interactions, which are influenced by the reconstructed herringbone structures or molecular adsorption sites, affect determining the stacking structure in case of the intermediate phase formed by the azobenzene derivatives having octyloxy group. The interactions dictate the growth direction of the molecules as well as the stacking structure.
EXPERIMENTAL AND COMPUTATIONAL METHODS The substrates with Au(111) surface were cleaned via repeated cycles of Ar ion sputtering and annealing under ultrahigh vacuum (UHV) before use. We synthesized the azobenzene derivatives
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having alkoxy groups with different chain lengths (hexyloxy, octyloxy, and decyloxy) at position 4 and an ethoxy group at position 4’ of the azobenzene core (Supporting Information). The corresponding derivatives are referred to as Az-C6, Az-C8, and Az-C10, respectively (Figure 1a– c).19-21 The azobenzene derivatives were deposited on the clean Au(111) surface via thermal evaporation from very low coverage to ~ 0.5 monolayer (ML) at 380 K under UHV conditions (See supporting Information Figure S1 for 0.5ML of Az-C6 and Az-C8). The Au(111) substrate was maintained at room temperature during deposition except in case of Figure 5a. The STM measurements were performed the assemblies using a low-temperature STM (Omicron GmbH, base pressure
