STM manipulation of a subphthalocyanine double-wheel molecule on Au(111)

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2012 J. Phys.: Condens. Matter 24 404001

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IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 24 (2012) 404001 (6pp) doi:10.1088/0953-8984/24/40/404001

STM manipulation of asubphthalocyanine double-wheel moleculeon Au(111)

Anja Nickel1, Joerg Meyer1, Robin Ohmann1,Henri-Pierre Jacquot de Rouville2, Gwenael Rapenne2,3,Francisco Ample4, Christian Joachim2,4, Gianaurelio Cuniberti1,5 andFrancesca Moresco1

1 Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische UniversitatDresden, D-01062 Dresden, Germany2 CNRS, CEMES & MANA Satellite (Centre d’Elaboration des Materiaux et d’Etudes Structurales),BP 94347, 29 rue J Marvig, F-31055 Toulouse, France3 Universite de Toulouse, UPS, 29 rue J Marvig, F-31055 Toulouse, France4 IMRE, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore5 Division of IT Convergence Engineering, POSTECH, Pohang 790-784, Republic of Korea

E-mail: francesca.moresco@nano.tu-dresden.de

Received 12 April 2012, in final form 4 July 2012Published 12 September 2012Online at stacks.iop.org/JPhysCM/24/404001

AbstractA new class of double-wheel molecules is manipulated on a Au(111) surface by the tip of ascanning tunneling microscope (STM) at low temperature. The double-wheel moleculeconsists of two subphthalocyanine wheels connected by a central rotation carbon axis. Each ofthe subphthalocyanine wheels has a nitrogen tag to monitor its intramolecular rolling duringan STM manipulation sequence. The position of the tag can be followed by STM, allowing usto distinguish between the different lateral movements of the molecule on the surface whenmanipulated by the STM tip.

(Some figures may appear in colour only in the online journal)

1. Introduction

Understanding the motion and the intramolecular mechanicsof a single molecule on a surface is of great importancefor the development of mechanical molecular machines [1].In the last few years, several examples of molecularmechanical machines have been demonstrated includingmolecular gears [2–4], motors [5–8], wheels [9], and differentkinds of nanovehicles [10–13]. Moreover, the manipulationof a molecule with the tip apex of a scanning tunnelingmicroscope (STM) has become a well established techniqueto study the mechanics of a single molecule on a surface [14].

To manipulate one atom or a single molecule using theSTM, different experimental protocols have been developedincluding lateral, vertical, and inelastic tunneling inducedmanipulation [15]. In the first example of a single atom STM

lateral manipulation, Xe atoms were moved one at a time onmetal surfaces [16]. To perform a lateral manipulation, the tipwas first positioned above a Xe atom and its distance to thesurface reduced, thereby reducing the interaction between thetip apex end atom and the Xe. The manipulation process wasperformed in constant current mode and the Xe displaced bythe lateral movement of the tip to the chosen final position. Byreducing the tunneling current intensity, the tip finally retractsto a height characteristic for the normal STM imaging mode.

With lateral manipulation experiments on single atomsor small molecules, different modes of manipulation suchas pushing, pulling and sliding can be distinguished bymeasuring the feedback signal during manipulation [17].In particular, by manipulation in constant current mode,detailed information on the process is obtained from thetip height signal, which reflects the interaction between the

10953-8984/12/404001+06$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

J. Phys.: Condens. Matter 24 (2012) 404001 A Nickel et al

Figure 1. (a) Chemical structure of the double-wheel molecules. (b) Optimized conformation of the double-wheel molecules adsorbed on aAu(111) surface using the semi-empirical ASED+ calculations [22] indicating one among many possible positions of the nitrogen tags.

tip apex and the adsorbate [18]. The STM feedback loopmanipulation signal for molecules is normally more complexthan for single atoms or small molecules like CO, especiallywhen the mechanical internal degrees of freedom of themanipulated molecule play a role. For example, and inaddition to a rigid like lateral-type motion on the surface,intramolecular conformation changes can be induced duringthe manipulation [19]. Manipulation signals for complexmolecules normally do not show the regular periodic modesover long periods observed for single atoms and smallmolecules. Changes from one mode to another duringmanipulation are often observed. In some cases, manipulationsignals do not present any periodicity and the peaks areirregular in intensity, length and shape [20]. Molecular flexureand reorientation of the internal conformations play a majorrole, as well as the reorientation of the molecule relative tothe surface [14, 20].

The large interest in the development of nanovehicleswhich can be driven by molecular manipulation has recentlystimulated the design and synthesis of molecules that mimicmacroscopic machines, transposing mechanical functions atthe scale of a single molecule [21]. The control of complexfunctions at the molecular scale and the design of appropriatemolecules are, however, still very challenging. Thereforewe study here a simple mechanical device: a double-wheelmolecule to study and control a complete rolling motion at theatomic scale. Recently, the rolling of a similar double-wheelmolecule was shown [9]. The molecule is composed of twotriptycene wheels connected by a C-axle. The rolling could bedemonstrated by observing the manipulation signal. However,because the interpretation of the manipulation signal ofcomplex molecules is in most cases very challenging, we havenow added electronic tags (nitrogen atoms) to the wheels,allowing the detection of the rolling movement from thetopographic STM images of the molecule.

In this paper, we investigated by STM the mechanics ofa boron–subphthalocyanine double-wheel molecule adsorbedon Au(111). Our molecule is composed of two subphthalocya-nine wheels connected by an axis, as shown in figure 1. Each

Figure 2. Schematic idea of the experiment. By rolling themolecule, the position of the tag changes, influencing the apparentheight of the molecule wheels in the STM images.

wheel integrates an electronic tag (a nitrogen atom) whichwas designed to monitor the intramolecular rolling of themolecule. The idea of the experiment is schematically shownin figure 2. In the topographic STM images, the positionof the tag can be determined by measuring the apparentheight of the corresponding wheel. By lateral manipulations,our double-wheel molecule can be moved along the surface,mostly showing STM feedback loop manipulation curveswhich are similar to the curves observed for rigid moleculesand single atoms. In a few cases, a different STM feedbackloop manipulation signature is observed and the rolling of awheel confirmed by a change in the position of the tag.

2. Experimental details

Experiments were performed by scanning tunneling mi-croscopy in ultra-high vacuum (UHV) conditions (basepressure below 1 × 10−10 mbar) at a temperature of 5 K.The STM was kept at low temperature by thermal contactwith a liquid He cryostat and was completely shieldedfrom radiation by a double screen. The Au(111) surfacewas cleaned by several cycles of sputtering and annealingat 450 ◦C followed by flashing to 550 ◦C, forming cleanherringbone reconstructions on the Au(111) surface. The

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Figure 3. Overview STM image showing several adsorbeddouble-wheel molecules with different orientations. Defects andmolecular fragments are also visible on the surface. Tunnelingparameters: V = 0.12 V, I = 20 pA. Image size:13.4 nm× 13.4 nm.

double-wheel molecules were evaporated from a Knudsencell for 2 min at a temperature of about 200 ◦C onto theclean Au(111) surface kept at room temperature. As themolecules are expected to be relatively unstable against heatand light intensity, the evaporation temperature was kept aslow as possible. After evaporation, the sample was transferredimmediately into the STM to minimize thermal and lightinduced dissociation.

Images were taken by STM in constant current mode.Because the molecules are very mobile on the surface,low tunneling current (I = 20–50 pA) and bias voltages

(Vbias = 0.1–0.5 V) were used to image the molecules.The bias voltage was applied to the sample with respect tothe tip. Lateral manipulations were performed in constantcurrent mode and an STM tunneling resistance of some M�corresponding to a tip height of a few A above the surface,was sufficient to move the molecule in a controlled way.

The double-wheel molecules consist of two subphthalo-cyanine molecules connected by a linear 2-carbon (acetylenic)axis. The details of the synthesis process are describedelsewhere [22]. The chemical structure is presented infigure 1 together with the optimized conformation of adouble-wheel molecule adsorbed on a Au(111) surface usingthe semi-empirical ASED+ calculations [23]. The additionalnitrogen on one of the three subunits of a subphthalocyaninewheel can be considered as a tag because it brings π∗

orbitals per wheel in the HOMO–LUMO electronic gap of themolecule. This state can be imaged differently depending onwhether the nitrogen is close or far away from the surface.It was designed to monitor the rolling motion in the STMtopographic images.

3. Imaging the molecular tag

The adsorption of the double-wheel molecules on Au(111)in the above described conditions leads to a submonolayercoverage, where single molecules are uniformly distributedon both fcc and hcp domains, while defects and molecularfragments occupy the elbows of the herringbone reconstruc-tion (figure 3). No preferential orientation of the molecule wasobserved in the STM images.

In figure 4, STM images of the single molecules and thecorresponding linescans along the wheel axis are presented.The double-wheel molecules typically show two lobescorresponding to the two subphthalocyanine molecule wheels.

Figure 4. STM images and corresponding linescans of a given double-wheel molecule on the Au(111) surface. The molecules show twolobes, each lobe corresponding to a single wheel. Tunneling parameters: (a) and (b) V = 0.08 V, I = 35 pA; (c) and (d)V = 0.01 V, I = 50 pA. Image size: 23× 27 A

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Figure 5. ESQC calculated STM image of the double-wheel molecule adsorbed on an Au(111) surface with one tag up and one tag down.The selected molecular conformation, optimized by ASED+, is also presented. Image size: 2 nm× 2.5 nm, voltage bias 0.12 V andfeedback loop set up current 20 pA. Notice the tip facet effect indicated by the two small bumps before and after the molecular skeleton.The wheels are quite high on the surface and are creating a lateral interaction between the facet of the tip and the molecule introducing atunnel current far before the end atom of the tip apex reaches a wheel.

As one can see, the complete molecule has a total apparentlength of about 15 A. Two slightly different conformations,present in almost equal amounts on the surface, are observed.In the first one (figures 4(a) and (b)) the wheels appear paralleland have an apparent height between 0.7 and 0.9 A. In thesecond case (figures 4(c) and (d)), the double-wheel moleculeis slightly asymmetric and shows an apparent height between0.9 and 1.15 A. For both conformations, we ascribe thedifferent apparent height of the lobes to the different positionof the tag, where the highest wheel corresponds to the tag nearto the upper position and the lowest one to a tag close to thesurface.

To confirm this interpretation, a representative constantcurrent STM image had been calculated using one amongmany of the minimum energy conformations of the double-wheel molecule adsorbed on the Au(111) surface. In thisspecific case, a molecule with a tag up and a tag down wasconsidered. This conformation is shown in figure 5 togetherwith the corresponding calculated STM image. Since the tipapex used in those ESQC calculations are perfect (111)-likefacets with an end gold atom, the calculated image shows astronger internal contrast as compared with the experimentalones. The effect of the N tag is clearly visible in the calculatedimage. As already observed in [2], the electronic effect of theN tag is not exactly localized at the position of the N atomin the molecular structure. The tunneling current captures thetunneling channels introduced by the molecular electronicstates. Those states do not necessary have their maximumweight at the exact position of the N atom.

4. Manipulation and rolling

A chosen double-wheel molecule was manipulated with theSTM tip in constant current mode by pushing it with the tipalong one wheel (figure 2). Because of the high mobility ofthese molecules on the Au(111) surface, a tunneling resistanceof about 4 M� was sufficient to move a double-wheelmolecule along the surface. As shown in the examples offigure 6, the molecule can be rigidly manipulated from one

adsorption site to the next one without any apparent changesin the position of the tag. Depending on the manipulationparameters and on the tip apex shape, feedback loop STMmanipulation curves have been recorded, which are typicalfor a partial pulling (figure 6(a)), pushing (figure 6(b)), orsliding mode (figure 6(c)) of the double-wheel molecule [17].In all cases, those manipulation signals show a periodicpart with a periodicity of about 3 A. This correspondsto the average atomic distance on a Au(111) surface [24]and indicates the rigid jumping of the molecule from oneadsorption site to the next. Before or after the periodicsignal indicating a simple translation, a non-periodic tracein the manipulation curve can be observed. This indicates achange in the adsorption orientation of the molecule on thesurface after the manipulation experiment, as confirmed bythe changed orientation of the molecule in the STM imageafter manipulation. The same manipulation signal is often notmaintained through the whole path and many combinations ofpushing, pulling, sliding, and planar reorientations have beenobserved. In particular, as shown in the example of figure 6(a),after a short pushing manipulation sequence, the molecule ispulled by the tip. On the other hand, in the second example(figure 6(b)), the pushing mode ends with a reorientation ofthe molecule on the surface followed by a short pulling. Infigure 6(c) and after a sliding-like manipulation sequence,the double-wheel molecule rotates laterally and is afterwardspulled by the tip. In all described cases, the apparent height ofthe lobes remains unchanged during a manipulation sequence,as can be quantitatively confirmed by comparing the line scansover the molecule. Those manipulation signals indicate that norolling of the wheels takes place here, as the Au(111) surfaceis generally too flat to facilitate the intramolecular rolling of adouble-wheel molecule.

In a few cases, however, we have succeeded in rolling adouble-wheel molecule on the Au(111) surface over a shortpath. In those cases, as shown in the example of figure 7, themanipulation curves present wider non-regular peaks and theapparent height of one lobe is changed after the manipulation.As can be clearly seen in the line scans recorded over thewheels before and after a manipulation (figure 7(b)), the two

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Figure 6. Examples of lateral manipulation of a double-wheel molecule. Upper panel: STM images before the manipulation; central panel:STM images after the manipulation; lower panel: manipulation curves as described in the text. The arrows indicate the manipulation path.Different manipulation modes could be observed: (a) pulling mode, (b) pushing mode, and (c) sliding mode. The small fragments on thesurface can be used as reference for the movement of the molecule. The bright structures in the background are due to the herringbonereconstruction of the Au(111) surface. Tunneling parameters: (a) and (b) V = 0.08 V, I = 35 pA; (c) V = 0.01 V, I = 50 pA. Tunnelingresistance by lateral manipulation R = 6 M�. Image size: 60× 36 A

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Figure 7. Example of lateral manipulation with rolling of a double-wheel molecule. (a) STM image before the manipulation (the blackarrow indicates the manipulation path); (b) STM image after the manipulation; (c) linescans over the line indicated by green and blue arrowsin (a) and (b) showing the change in apparent height of a wheel after the manipulation; (d) corresponding manipulation curve. Tunneling:parameters: V = 0.01 V, I = 50 pA. Tunneling resistance by lateral manipulation: R = 4 M�. Image size: 37× 35 A

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lobes of the molecule have the same apparent height beforethe manipulation, while one of the lobes appears about 6 pmhigher than the other after the manipulation. This changein the apparent height of the molecule in the STM imagerepresents the signature of an intramolecular rolling of oneof the wheels, with the consequent change in the positionof the tag to the upper position. However, less informationcan be extracted from the manipulation curve in this case(figure 7(d)), because a planar rotation of the molecule onthe surface accompanies the rolling of the molecule, makinga straightforward interpretation of the STM feedback loopsignal difficult. In the example of figure 7, after a rolling andthen a possible reorientation on the surface, the molecule isfinally pulled by the tip, jumping over a further two adsorptionsites, as shown in the last part of the manipulation curve(figure 7(d)).

5. Conclusions

In the present investigation we have reported for the first timethe lateral manipulation of double-wheel molecules equippedwith N tags on Au(111) by STM at low temperature. Wehave observed that in most cases the molecule is movedrigidly on the surface, without rolling its wheels. In a fewcases, the rolling of a wheel is observed, as concludedfrom the change in the position of the tag on one of thetwo wheels. This was monitored by measuring the apparentheight of the corresponding wheel in the STM images.Rolling experiments on more corrugated surfaces shouldnow be undertaken to favor intramolecular rotation, and newnanovehicles are being designed, equipped with this newgeneration of subphthalocyanine molecule wheels.

Acknowledgments

This work is funded by the ICT-FET Integrated ProjectAtMol, the European Union (ERDF), the Free State of Saxonyvia the ESF project 080942409 InnovaSens and TP A2(‘MolFunc’) of the cluster of excellence ‘European Centerfor Emerging Materials and Processes Dresden’ (ECEMP).We are grateful to the CNRS, the University Paul Sabatierof Toulouse and the ANR P3N (AUTOMOL project n◦ANR09-NANO-040) for financial support. H-P J de Rouville

thanks the French Ministry of National Education for a PhDFellowship.

The authors thank Cormac Toher for fruitful discussions.

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