Controlled Manipulation of Gadolinium-Coordinated Supramolecules

Letter pubs.acs.org/NanoLett

Controlled Manipulation of Gadolinium-Coordinated Supramolecules by Low-Temperature Scanning Tunneling Microscopy José I. Urgel, David Ecija,* Willi Auwar̈ ter,* and Johannes V. Barth Physik Department E20, Technische Universität München 85748 Garching, Germany S Supporting Information *

ABSTRACT: Coordination bonding between para-quarterphenyldicarbonitrile linkers and gadolinium on Ag(111) has been exploited to construct pentameric mononuclear supramolecules, consisting of a rare-earth center surrounded by five molecular linkers. By employing a scanning tunneling microscope tip, a manipulation protocol was developed to position individual pentamers on the surface. In addition, the tip was used to extract and replace individual linkers yielding tetrameric, pentameric, nonameric, and dodecameric metallosupramolecular arrangements. These results open new avenues toward advanced nanofabrication methods and rare-earth nanochemistry by combining the versatility of metal−ligand interactions and atomistic manipulation capabilities. KEYWORDS: Scanning tunneling microscopy, manipulation, supramolecules, coordination chemistry, lanthanides

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In addition, elegant approaches revealed the possibility of in situ constructing19,46,47 and laterally manipulating organometallic complexes.46,47 However, to the best of our knowledge, the systematic lateral manipulation and tuning of coordination spheres for supramolecular systems stabilized by metal-ligand interactions is still not addressed, which is surprising taking into account its potential for molecular design on surfaces. In this Letter, we exploit coordination interactions of Gd centers and para-quarterphenyl-dicarbonitrile species to form pentameric supramolecules on Ag(111) and explore the potential of low-temperature STM to manipulate and tailor them. By positioning the STM tip above the metallic center and adjusting the tip−metal distance to enter into an attractive regime, pentamers could be laterally displaced across the surface with high reliability. Moreover, we extended this approach to in situ tailor and translate distinct supramolecular units, including tetramers, pentamers, nonamers, and dodecamers, which highlights its potential to deliberately engineer supramolecular systems stabilized by coordination bonds with exquisite control. These results will pave new avenues to create advanced molecular systems on surfaces. The experiments were performed in a custom designed ultrahigh vacuum system that hosts a CreaTec low-temperature STM (www.lt-stm.com), where the base pressure was below 5 × 10−10 mbar. The Ag(111) substrate was prepared using standard cycles of Ar+ sputtering (800 eV) and subsequent annealing to 723 K for 10 min. All STM images were taken in

upramolecular engineering on surfaces provides versatile strategies for the design of low-dimensional molecular nanoarchitectures.1−4 Particularly promising are coordination chemistry schemes since metal−ligand interactions frequently present an adequate balance between robustness and regularity of the resulting designs.5,6 Recently, we explored complex surface-confined metal−organic networks incorporating cerium f-block metal centers with unusually high coordination numbers7 and thus introduced the potential of lanthanide elements for interfacial nanoscience with prospects for magnetism, photovoltaics, photonics, and catalysis.8−10 To explore the foundations of surface-confined molecular nanosystems and to construct novel molecular devices it is beneficial to explicitly tailor the bottom-up designs at will. Accordingly, low-temperature scanning probe microscopies have been exploited to move atoms and molecules on surfaces.11−21 In addition, taking advantage of the absence of thermal motion at low-temperature, scanning tunneling microscopes (STMs) have been used to trigger conformational changes,22−25 tautomerization and switching,15,18,26−31 chemical reactions,19,31−37 or desorption of adsorbates,38 and recently even to induce the translation of a molecular nanocar across a surface.25 On metal surfaces, manipulation techniques include the use of direct tip-adsorbate forces (lateral or vertical manipulation), electric fields, or tunneling electrons.39−42 Hereby, the standard method of lateral manipulation is based on tuning the tip-adsorbate interaction to a certain regime in which the attractive (or repulsive) forces between tip and target species allows the pulling (or pushing) of the adsorbates across the surface, without desorbing them. By exploiting these methods, it has been possible to laterally manipulate atoms,11,12,14 molecules,13,14 or in very few occasions groups of molecules.16,43−45 © 2014 American Chemical Society

Received: November 29, 2013 Revised: January 17, 2014 Published: January 23, 2014 1369

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tip back to the normal imaging conditions. In this procedure, the tip−gadolinium separation was carefully chosen to establish a weak chemical bond between tip and metal in order to translate the supramolecule, which corresponds to tunneling resistances (Rt) between 0.3 and 10 MΩ. Experiments performed at closer distances (Rt < 0.3 MΩ) resulted in desorption of the pentamer, whereas at higher distances (Rt > 20 MΩ) the supramolecules could not be displaced. The success of the lateral manipulation experiments did not depend on the direction of movement of the tip with respect to the close-packed directions of the Ag(111). Following this lateral manipulation protocol (exemplified in Figure 1c,d), individual pentamers could be displaced across the surface. A series of experiments was realized trying to cover distances from 10 to 500 Å in a single process (cf. Supporting Information Figure SI1a), whereby approximately 80% of supramolecules were successfully translated (>400 experiments). In addition, multiple manipulation events could be performed on the same supramolecule, displacing it very long distances (>1000 Å), which highlights the potential of our approach. Before and after each manipulation step, the configuration of the supramolecule and its constituents remains influenced by the substrate atomic lattice, in such a way that of two or three of the linear linkers follow the closepacked directions at the final location. As a result, an overall rotational manipulation of the supramolecule can be achieved, which however was not systematically explored. Nonetheless, our observations bear prospects for the understanding of rotational motions and their molecular-level control regarding complex adsorbed species50−56 or even supramolecular dynamers.57 We notably envision that by detailed investigations focusing on this point insight into elementary steps can be gained, which proved elusive in thermally stimulated processes.57 The nature of the mechanism involved in the lateral manipulation process was investigated. By positioning the STM tip at a lateral distance of 5 Å with respect to the Gadolinium center of the pentameric unit, an attractive displacement of the supramolecule toward the tip was detected (cf. Supporting Information Figure SI2). The histogram of displacement accuracy (cf. Supporting Information Figure SI1b) reveals that in most of the experiments the center of the supramolecule is slightly displaced from the final release location (average value of ∼9 Å), regardless of the distance traveled. This behavior signals an attractive interaction between supramolecule and tip, which mediates the translation of the entire supramolecule across the substrate whereupon the tip is retracted in the last step of lateral manipulation process. These results, together with the long distances covered during the manipulation events, strongly suggest a lateral manipulation mode dominated by attractive interactions. To explore the potential of our approach for tailoring advanced molecular architectures, the lateral manipulation protocol was employed to design a pentagon with five pentamers (cf. Figure 2a−c, and Supporting Information Figure SI3 for tip manipulation trajectories), and the letter-sequence “TUM” (acronym of the Technical University of Munich) with 14 supramolecules (cf. Figure 2d and video in Supporting Information). The individual pentamers were extracted from supramolecular islands composed of pentameric units (cf. Figure 1a and Figure 2a). Because of the interactions between supramolecules, the minimum separation between adjacent

constant-current mode with electrochemically etched tungsten tips and applying a bias (Vbias) to the sample. The supramolecular entities based on Gd-ligand coordination motifs described in this letter were prepared in a two-step process. (1) The molecular linkers (p-NC-(Ph)4-CN-p)48,49 were deposited by organic molecular beam epitaxy (OMBE) from a quartz crucible at T = 503 K onto a clean Ag(111) crystal held at ∼300 K. (2) Subsequently, Gd atoms were evaporated by means of electron beam evaporation onto the sample held at ∼300 K from an outgassed Gd rod. Figure 1a displays the gadolinium-directed assembly of para-quarterphenyl-dicarbonitrile species (p-NC-(Ph)4-CN-p)

Figure 1. Lateral manipulation of a 5-fold Gd-carbonitrile coordinated supramolecule (pentamer) by a STM tip on Ag(111). (a) Highresolution STM image of pentameric supramolecules coexisting with some noncoordinated molecular linkers, obtained by the deposition of p-NC-(Ph)4-CN-p species and Gd on Ag(111) under local stoichiometric ratios (Gd:molecule) of ∼1:5 (Vb = 0.7 V, I = 50 pA). Molecular linkers are imaged as rods, whereas Gd centers appear as bright protrusions. (b) Atomistic model of a pentameric unit shown in (a) formed by five molecular linkers and a Gd atom. (c,d) Highresolution STM images illustrating the lateral manipulation in constant current mode (Rt = 2 MΩ) of a pentameric unit. The green arrow sketched in (c) reveals the path followed by the tip during the lateral manipulation, while the white dashed silhouette in (d) depicts the position of the pentamer before the movement of the supramolecule. (a,c,d) Scanning parameters: Vb = 0.3 V, I = 60 pA. Scale bar: 3 nm.

on Ag(111) following a local stoichiometry (Gd/molecule) of ∼1:5, where linker units are imaged as rods and Gd centers as bright protrusions. Under these stoichiometric conditions the formation of pentameric supramolecular units (pentamers) comprising five linkers coordinated to a Gd center is prevalent. The influence of the substrate is manifested by the alignment of two or three of the molecular linkers constituting the supramolecule with the close-packed directions of Ag(111). In order to design artificial architectures based on the pentameric supramolecules, we have performed lateral manipulation14,40 of the pentamers (cf. figure captions for experimental details). Our manipulation experiments involve three steps: (1) Vertical approach of the STM tip toward the Gadolinium center of the pentamer to increase the tip-supramolecule interaction; 2) movement of the tip along the surface to the desired final location displacing the pentamer; and (3) retraction of the STM 1370

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Figure 2. Design of nanostructures by lateral manipulation of pentameric supramolecules with an STM tip on Ag(111). (a−c) Highresolution STM images of the formation of a pentagon by displacing five pentamers. (d) Design of the letter sequence TUM (acronym of the Technical University of Munich) by lateral manipulation of 14 pentameric entities. (a−d) For the lateral manipulation, the tunneling resistance (Rt) was chosen in the range of 0.3−0.6 MΩ. Scanning parameters: (a−c) Vb = 0.7 V, I = 50 pA; (d) Vb = 0.7 V, I = 80 pA. (a−c) Scale bar = 3 nm.

pentameric entities providing structural stability of the resulting design was found to be ∼35 Å. Remarkably, the bonding of the f-block elements is predominantly ionic in nature, giving rise to a lack of rigid coordination, which together with the big size of the lanthanide ions enables high coordination numbers and unusual coordinative topologies. As a result, the coordination chemistry of these inner transition metals allows more geometrical possibilities compared to the rigid polyhedra of bonding in d-block elements governed by the electrons in the d-orbitals.58 Regarding the capabilities of scanning tunneling microscopy for in situ synthesis in a 2D environment,19,35−37,46 we initially used the STM tip to detach single molecular linkers from the pentamers, creating 4-fold coordinated supramolecules. Figure 3a,b illustrates the successful transformation of a pentameric entity into a tetramer, by pulling out one linker species from the supramolecule, presumably resulting in the desorption of the missing molecule. These tetramers could be displaced across the surface under similar manipulation conditions as the pentamers (cf. Figure 3b−d). In addition, the tetramers could be transformed back into pentamers by displacing a para-quarterphenyl-dicarbonitrile species into the Gd sphere, and subsequently laterally manipulated as before (cf. Figure 3f−h), which highlights the potential of the in situ chemistry of the lanthanide elements. The addition of further organic linkers to the Gd sphere was not possible, probably due to steric limitations of the coordination environment. In order to analyze the limits of this in situ chemical procedure and the robustness of the Gd coordination sphere, nonameric and dodecameric supramolecules were tailored by lateral manipulation of pentamers. Figure 4a−d shows the formation of a nonameric unit (consisting of nine linkers and two Gd centers) by merging one pentamer into another, which entails

Figure 3. High-resolution STM images of the in situ tailoring and displacement of Gd-carbonitrile metal−organic supramolecules. (a,b) Detachment of one p-NC-(Ph)4-CN-p species from a pentameric supramolecule by the lateral manipulation of one constituting linker (Rt = 0.4 MΩ), which gives rise to the formation and displacement of a 4-fold Gd-carbonitrile coordinated supramolecule (tetramer). (b−d) Lateral manipulation of the tetrameric species (Rt = 0.4 MΩ). (a−d) Scanning parameters: Vb = 0.7 V, I = 58 pA. (e,f) Removal of one p-NC-(Ph)4-CN-p molecule from a pentameric supramolecule by applying the lateral manipulation procedure describe in the text (Rt = 6 MΩ), which results in the creation and displacement of a 4-fold Gd-coordinated supramolecule. (f,g) A p-NC-(Ph)4-CN-p linker located in the vicinity of a tetramer is fused to the 4-fold species recovering the pentameric supramolecule. (g,h) Lateral manipulation of the pentamer (Rt = 6 MΩ). (e−h) Scanning parameters Vb = 0.9 V, I = 65 pA. Blue arrows indicate the path of the STM tip during a lateral manipulation process giving rise to the formation of a tetramer (a,e) or a pentamer (f), whereas green arrows display the path of the tip during an ordinary lateral manipulation procedure keeping the coordination number of the Gd sphere (b−d, g−h). (a−h) Scale bar = 3 nm.

the release of a para-quarterphenyl-dicarbonitrile species and the creation of a supramolecular nonamer. Figure 4e−n 1371

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Figure 4. In situ fabrication of nonameric and dodecameric species by the lateral manipulation of self-assembled Gd-carbonitrile coordinated supramolecules on Ag(111). High-resolution STM images (a,b) and schemes (c,d) of the formation of a nonameric supramolecule by merging one pentamer into another. (a,b) Scanning parameters: Vb = 0.85 V, I = 60 pA. Tunneling resistance during lateral manipulation: Rt = 1.1 MΩ. Highresolution STM images (e−i) and schemes (j−n) of the design of a dodecameric supramolecule by the lateral manipulation (Rt = 6 MΩ) of a pentamer into a nonameric species giving rise to a triskaidecameric species, which is displaced across the surface to release a p-NC-(Ph)4-CN-p unit, resulting in a dodecamer (f−i, k−n). (e−i) Scanning parameters: Vb = 0.9 V, I = 65 pA; tunneling resistance during lateral manipulation: Rt = 6 MΩ. Blue and green arrows display the path followed by the tip during the lateral manipulation resulting in the synthesis of novel species (blue) and the displacement of the supramolecular entities (green), respectively. The curved red arrows illustrate the detachment of a p-NC-(Ph)4-CN-p species. Molecular linkers singly coordinated to Gd are depicted in green, whereas those double coordinated are represented in maroon. Scale bar: 3 nm.

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addresses the in situ formation of a dodecamer by exploiting a lateral manipulation protocol consisting in crashing one pentamer into a nonamer, producing a triskaidecameric supramolecule (composed of thirteen linkers and three Gd centers) and subsequently displacing it across the surface to induce conformational changes that lead to the formation of a dodecameric supramolecule after the detachment of one para-quarterphenyl-dicarbonitrile species. During this process, the nature of the lanthanide coordination sphere is of special relevance, as it permits (i) enough molecular space to induce conformational changes, and (ii) certain lability to promote the formation of new bonds under pertinent manipulation conditions. Our findings demonstrate that by providing appropriate linkers, we can systematically fine-tune the coordination spheres of surface confined rare-earth centers, for instance, to explore the magnetic coupling at the supramolecular level.59 In summary, we introduced novel protocols to manipulate and design molecular nanoarchitectures on surfaces, by exploiting the potential of lanthanide coordination chemistry to create gadolinium−carbonitrile coordinated supramolecules, and the versatility of scanning tunneling microscopy to deliberately position them. In particular, mononuclear selfassembled pentamers, built up by the 5-fold coordination of para-quarterphenyl-dicarbonitrile species and Gd, were laterally displaced in a constant current mode dominated by attractive tip-supramolecule interactions. In addition, the lanthanide coordination sphere of the pentameric nodes was tailored and molecular bridges between different Gd-centers were established by the STM tip. The afforded multicenter metallosupramolecular arrangements, together with the lateral manipulation capabilities, provide versatile prospects for the in situ design of advanced surface-confined molecular architectures incorporating the functionalities of the lanthanides elements, thus presenting novel opportunities in different areas such as organic electronics, nanomagnetism, and catalysis.

ASSOCIATED CONTENT

S Supporting Information *

Video of high-resolution STM images displaying the writing of the letter sequence “TUM” by lateral manipulation of 14 pentameric supramolecules. Histograms of the manipulation events versus traveled distance and deviation distance, respectively. STM images of the attraction of a pentameric suprameric supramolecule toward the STM tip. Tip manipulation trajectories during the design of a pentagon. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (D.E.) [email protected]. *E-mail: (W.A.) [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Mario Ruben and Dr. Svetlana Klyatskaya for the synthesis of the molecular compounds. Work was supported by the European Research Council Advanced Grant MolArt (Grant 247299) and the Technische Universität München-Institute for Advanced Study.

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