Gabriel, Markus

Möller, Rolf

Stöhr, Meike

Wagner, Thorsten

Weyers, Bastian

English

The binary molecular system of C-60 and copper phthalocyanine(CuPc) molecules has been investigated by scanning tunneling microscopy (STM) at room temperature and at 50 K. As substrate Au(111) was chosen. When C-60 and CuPc molecules are sequentially deposited, it is found that well-ordered domains of both molecules may coexist simultaneously Hence hexagonal ordering of C-60 and quadratic ordering of CuPc is observed side by side but no ordered mixed layer of both molecules or heteroepitaxy from one molecule on the other is found. Instead the boundaries of the CuPc domains are often decorated by C-60 molecules and for a particular choice of parameters, with regard to the film preparation, individual CuPc molecules may adsorb on top of a C-60 layer. The interaction with the underlying C-60 layer permits the molecules to perform a localized, hindered rotation. At room temperature the hopping frequency is so high that only the time average of the rotation is seen by STM while at 50 K the rotation is frozen and the CuPc molecule is trapped in one definite position.</p

Dissertations of the University of Groningen

   University of GroningenBinary molecular layers of C-60 and copper phthalocyanine on Au(111)Stöhr, Meike; Wagner, Thorsten; Gabriel, Markus; Weyers, Bastian; Möller, RolfPublished in:Advanced Functional MaterialsIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Stöhr, M., Wagner, T., Gabriel, M., Weyers, B., & Möller, R. (2001). Binary molecular layers of C-60 andcopper phthalocyanine on Au(111): Self-organized nanostructuring. Advanced Functional Materials, 11(3),175-178.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 11-10-2022Binary Molecular Layers of C60 and Copper Phthalocyanineon Au(111): Self-Organized Nanostructuring**By Meike Stöhr, Thorsten Wagner, Markus Gabriel, Bastian Weyers, and Rolf Möller*1. IntroductionThin organic films have received an increasing interest inthe last years because of the possible application for optical orelectrical devices. For further improvements and a generalunderstanding a detailed analysis of the interaction betweenthe surface and the molecules, as well as the interactionbetween molecules, is required. The development of scanningtunneling microscopy (STM) has improved the possibilities ofinvestigating such films and has even enabled the examinationof individual molecules. The adsorption of organic moleculeson inorganic surfaces has been successfully investigated for abroad variety of systems.[1] The pioneering work has been donefor copper phthalocyanine (CuPc) by Gimzewski et al.[2] andby Lippel et al.[3] They were able to image individual mole-cules with intramolecular resolution. Although one of the ear-liest experiments performed by Ohtani et al.[4] dealt with coad-sorption of benzene and CO on Rh(111), binary molecularlayers have rarely been investigated. Cuberes et al.[5] analyzedC60 on a monolayer of dimethyl-bianthron on Cu(111) andperformed C60 manipulation on an organic layer. Epitaxialgrowth of two planar molecules has been demonstrated bySchmitz-Hübsch et al.[6] for 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on hexabenzocoronene and by Staub etal.[7] for PTCDA on decanethiol.An organic molecule that has been intensely investigated isC60. It has been grown on various metal and semiconductorsurfaces and a broad spectrum of surface techniques has beenapplied. However, so far only few experiments have been car-ried out for the combination of CuPc and C60 molecules on asubstrate. One example in the literature is the testing of phtha-locyanines and C60 for solar cells.[8]The present paper deals with the combination of C60 andCuPc molecules adsorbed on a Au(111) substrate. This is a par-ticularly interesting system for several reasons. First of all, it isknown that both molecules form well-ordered monolayers onthis substrate. The size of the molecules is roughly comparablebut their shape is completely different. While the C60 moleculemay be considered in good approximation as a sphere with adiameter of 0.7 nm, the CuPc molecule is a flat planar mole-cule with the form of a cross and a diameter of about 1.2 nm.In consequence, C60 prefers a hexagonal ordering whereasCuPc usually attains a quadratic ordering when adsorbed on(111) oriented surfaces of noble metals. One of the goals of theinvestigation was to analyze the concerted action of these twodifferent ordering schemes, with six and fourfold symmetry,and to find out if mixed layer or epitaxial growth of one mole-cule on a layer of the other occurs.2. ResultsFirstly, the preparation of monolayers of either C60 or CuPcon Au(111) was studied. Figure 1 shows the typical appearanceof a C60 layer at a coverage of slightly less than one monolayer.Large domains with perfect hexagonal packing are formed inagreement with findings previously reported (see e.g., Altmanand Colton[9]). For the given bias voltage V = ±1.85 V at the tipsome of the C60 molecules appear brighter than the majority ofthe molecules. It has been proposed that this is due to a differ-ent orientation of the C60 relative to the substrate.[10] The slightfuzziness at the boundary indicates that the molecules at theedges are somewhat mobile.±[*] Prof. R. Möller, M. Stöhr, T. Wagner, M. Gabriel, B. WeyersUniversität EssenFachbereich PhysikD-45117 Essen (Germany)E-mail: rolf.moeller@uni-essen.de[**] This work has been supported by the German Science Foundation (SFB452) and by the state of ªNordrhein-Westfalenº under the title of ªLandes-initiative Nanowissenschaftenº.The binary molecular system of C60 and copper phthalocyanine(CuPc) molecules has been investigated by scanning tunnelingmicroscopy (STM) at room temperature and at 50 K. As substrate Au(111) was chosen. When C60 and CuPc molecules aresequentially deposited, it is found that well-ordered domains of both molecules may coexist simultaneously. Hence hexagonalordering of C60 and quadratic ordering of CuPc is observed side by side but no ordered mixed layer of both molecules orheteroepitaxy from one molecule on the other is found. Instead the boundaries of the CuPc domains are often decorated byC60 molecules and for a particular choice of parameters, with regard to the film preparation, individual CuPc molecules may ad-sorb on top of a C60 layer. The interaction with the underlying C60 layer permits the molecules to perform a localized, hinderedrotation. At room temperature the hopping frequency is so high that only the time average of the rotation is seen by STM whileat 50 K the rotation is frozen and the CuPc molecule is trapped in one definite position.Adv. Funct. Mater. 2001, 11, No. 3, June Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1616-301X/01/0306-0175 $ 17.50+.50/0 175FULLPAPERIn contrast to the C60 molecule, CuPc on Au(111) prefers anordering with a quadratic unit cell.[11] Figure 2 displays the typ-ical structure of CuPc at a coverage of about one monolayer.The planar CuPc molecules lie flat on the surface and theapparent height is about 0.3 nm. It can clearly be seen that theunderlying ªherringboneº reconstruction of the Au(111) is notlifted.Fig. 2. Monolayer of CuPc on Au(111), the area is 23  21 nm2, the tunneling cur-rent is 9 pA at ±2.1 V tip bias.Both C60 and CuPc show a commensurate superstructure onAu(111), which indicates that the interaction between the mol-ecules and the substrate is stronger than van der Waals bondsbetween the molecules. In the case of C60 the reconstruction ofthe Au substrate can be lifted as can be seen in Figure 1. Adetailed description of this phenomenon is given by Gimzewskiet al.[12]Figure 3 displays the combination of both molecules. Theupper left corner exhibits the quadratic superstructure of CuPc.It can be seen that one axis of the unit cell is parallel to a stepline of the Au(111) substrate running from the left to the rightof the image. The latter coincides with one axis of the unit cellof the Au(111) substrate. To the lower right the hexagonalordering of C60 is found, which is also aligned with one axis ofthe unit cell to the same direction. Hence the hexagonal unitcell of the superstructure of the C60 molecules and the quadrat-ic one of the CuPc molecules may share a common axis, whichis induced by the substrate.Figure 4 shows a smaller area of the same sample. The intra-molecular structure of the CuPc molecule is now clearly visible,e.g., in the lower part of the figure. Two different superstruc-tures can be distinguished. In between isolated CuPc are foundforming a dense but irregular pattern, e.g., in the middle on theright side. Although no C60 is found within a CuPc domain,they may be observed at domain boundaries of ordered areasof CuPc. The size of the observed protrusions seems to beFig. 1. Monolayer of C60 on Au(111), the area is 47  44 nm2, the tunneling cur-rent is 13 pA at ±1.85 V tip bias. Fig. 3. A mixed layer of CuPc and C60 at 50 K; the area is 93  86 nm2, the tun-neling current is 10 pA at ±2.1 V tip bias, the STM data have been partiallyshaded to enhance the visibility of small details, the quadratic ordering of theCuPc is visible at the upper left, and to the lower right the hexagonal ordering ofthe C60 molecules can be seen.Fig. 4. Smaller area of a mixed layer of CuPc and C60 at 50 K; the area is 47 47 nm2, the tunneling current is 24 pA at 1.8 V tip bias, the STM data have beenpartially shaded to enhance the visibility of small details.176 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1616-301X/01/0306-0176 $ 17.50+.50/0 Adv. Funct. Mater. 2001, 11, No. 3, JuneFULLPAPERM. Stöhr et al./Binary Molecular Layers of C60 and Copper Phthalocyanine on Au(111)much larger than the one of a molecule that is incorporated inan ordered area. However, this is an effect that is due to theªconvolutionº of the investigated structure with the shape ofthe tunneling tip.When the preparation is slightly modified, by increasingthe amount of CuPc or by evaporating CuPc, new featuresmay be observed. Figure 5 shows a large area of the corre-sponding molecular layer. To the lower right a domain ofCuPc is found. Its edge is decorated by C60 molecules. Theytend to form linear clusters of 3±5 molecules. The mostastonishing features are the rings that are found on top ofthe C60 domain. Since they can only be found if CuPc is pres-ent, they have to be attributed to the CuPc molecule. How-ever, the topography seems to contradict the fourfold sym-metry of the CuPc molecule. Figure 6 shows another islandof C60 molecules, where three of these rings are present. Thearea surrounding the island seems to be rather disordered,nevertheless, a closer look reveals a partially ordered area ofCuPc.Fig. 6. Another C60 island on the same sample; the area is 65  56 nm2, the tun-neling current is 8 pA at 1.4 V tip bias.The structure of one of those rings is shown in Figure 7,which displays a high resolution scan of the C60 area fromFigure 6. It clearly reveals the sixfold symmetry of the struc-ture. Even if one takes into account that the size of a protru-sion is always overestimated by a scanning probe microscope,the structure is too large to be a CuPc molecule that rests onthe surface. To explain the observed structure we have triedto find an energetically favorable orientation by simple geo-metrical arguments. According to these arguments it is ratherunlikely that a CuPc molecule would occupy a position cen-tered on one C60 molecule because this would be unfavor-able for two of the four lobes of the CuPc molecule. It ismuch more probable that it occupies an asymmetric placewith one of the five-membered rings consisting of four car-bon atoms and one nitrogen atom centered on one C60 mole-cule. This geometry is indeed found at low temperature ascan be seen in Figure 8. Due to the symmetry of the C60layer there are six equivalent orientations of this type. If oneassumes that the potential barrier is low enough[13] the mole-cule may hop between the six positions at a very high rateperforming a hindered eccentric rotation. Since the scan mo-tion of the tip of the STM is several orders of magnitudeslower than the molecular motion, only the time average isseen in the topographic image.Fig. 8. Individual CuPc molecule at about 50 K; the area is 22  22 nm2, the tun-neling current is 13 pA at 2 V tip bias.Fig. 5. Mixed layer of C60 and CuPc for a slightly different preparation; the areais 87  80 nm2, the tunneling current is 9 pA at 2.1 V tip bias, the domain bound-ary of the CuPc (lower right) is decorated by C60 molecules, on top of the C60island individual CuPc molecules are found.Fig. 7. Individual CuPc molecule on C60 island of Figure 6, the area is 5.4 5.2 nm2, the tunneling current is 8 pA at 1.5 V tip bias.Adv. Funct. Mater. 2001, 11, No. 3, June Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1616-301X/01/0306-0177 $ 17.50+.50/0 177FULLPAPERM. Stöhr et al./Binary Molecular Layers of C60 and Copper Phthalocyanine on Au(111)3. ConclusionAs an example of a binary system, the combination of C60and CuPc molecules deposited on Au(111) has been investi-gated at a coverage of about one monolayer. After thermalannealing well-ordered structures are observed, which revealthat a demixing occurs, and the molecules mainly form separat-ed domains. Using a substrate with two types of surfaces, whichdiffer in their affinity to the molecules, a well-defined coverageof nanostructures with either a hexagonal or a quadratic super-structure may be achieved. They may serve as a template forfurther processing. Possible substrates for this type of prepara-tion may be vicinal surfaces.[14]A very peculiar behavior has been observed for individualCuPc molecules adsorbed onto an array of hexagonallyordered C60 molecules. At room temperature they performan eccentric rotation around one central C60 molecule. Fromthe observed topography it can be concluded that thismotion is not continuous but results from the fast hoppingbetween six equivalent positions. The observation gives riseto a speculation of using this system as a ªmolecularbearingº.[15]4. ExperimentalThe experiments were performed in an ultra-high vacuum (UHV) system con-sisting of a preparation chamber and an analysis chamber, in which the STM issituated. The preparation of the samples includes several steps. First a (111) ori-ented gold film was epitaxially grown onto a mica substrate heated to 600 K asdescribed, e.g., in [16]. The mixed layers of CuPc and C60 were prepared eitherby depositing the C60 prior to the CuPc or by the inverse sequence. Surprisinglyboth methods lead to nearly the same findings if the surface is thermallyannealed. The following description applies to the first case. The equivalent ofsomewhat less than one monolayer of C60 was deposited by thermal evaporation.These films were annealed using a linear temperature increase of 1 K/s up to600 K. This mainly yielded an ordered monolayer of C60. In a final step a fractionof a monolayer of CuPc molecules was deposited. Again the samples wereannealed, this time to 500 K.The STM, which was used for the experiments, is a slightly modified commer-cial variable-temperature STM (Omicron). The results presented in this publica-tion were obtained at room temperature and at 50 K. The tunneling tips wereprepared by electrochemical etching of a tungsten wire. The tips were furtherprocessed in situ by sputtering with Ar+ ions and by field emission shortly beforethe experiment. The STM was operated at low tunneling currents in the range of3 to 50 pA. The sample bias voltage was varied in the range from ±2.5 V to+2.5 V.Received: February 25, 2001±[1] J. K. Gimzewski, T. A. Jung, M. T. Cuberes, R. R. Schlittler, Surf. Sci. 1997,386, 101.[2] J. K Gimzewski, E. Stoll, R. R. Schlittler, Surf. Sci. 1987, 181, 267.[3] P. H. Lippel, R. J. Wilson, M. D. Miller, C. Wöll, S. Chiang, Phys. Rev. Lett.1989, 62(2), 171.[4] H. Ohtani, R. J. Wilson, S. Chiang, C. M. Mate, Phys. Rev. Lett. 1988, 60,2398.[5] M. T. Cuberes, R. R. Schlittler, J. K. Gimzewski, Surf. Sci. 1997, 371, L231.[6] T. Schmitz-Hübsch, F. Sellam, R. Staub, M. Törker, T. Fritz, C. Kübel,K. Müllen, K. Leo, Surf. Sci. 2000, 445, 358.[7] R. Staub, M. Toerker, T. Fritz, T. Schmitz-Hübsch, F. Sellam, K. Leo, Surf.Sci. 2000, 445, 368.[8] K. Murata, S. Ito, K. Takahashi, B. Hoffman, Appl. Phys. Lett. 1996, 68,427.[9] E. I. Altman, R. J. Colton, Phys. Rev. B 1993, 48, 18 244.[10] L. Cepek, L. Giovanelli, M. Sancrotti, C. Costantini, C. Boragno, U. Valbu-sa, Surf. Sci. 2000, 454, 766.[11] T. Fritz, M. Hara, W. Knoll, H. Sasabe, Mol. Cryst. Liq. Cryst. 1994, 252±253, 561.[12] J. K. Gimzewski, S. Modesti, C. Gerber, R. R. Schlittler, Chem. Phys. Lett.1993, 213, 401.[13] A rough estimate of the activation energy can be obtained by fitting theArrhenius equation assuming: 1) that there is no hopping at 50 K, 2) thatthe tunneling current noise has a typical frequency of about 500 Hz at150 K, and 3) that the hopping rate is larger than 105 Hz at room tempera-ture.[14] H. Fuchs, private communication.[15] J. K. Gimzewski, C. Joachim, R. R. Schlittler, V. Langlais, H. Tang, I. Jo-hannsen, Science 1998, 281, 531.[16] A. A. Baski, H. Fuchs, Surf. Sci. 1994, 313, 275.______________________178 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1616-301X/01/0306-0178 $ 17.50+.50/0 Adv. Funct. Mater. 2001, 11, No. 3, JuneFULLPAPERM. Stöhr et al./Binary Molecular Layers of C60 and Copper Phthalocyanine on Au(111)

Binary molecular layers of C-60 and copper phthalocyanine on Au(111):Self-organized nanostructuring

https://pure.rug.nl/ws/files/3094486/2001AdvFunctMaterStohr.pdf

Binary molecular layers of C-60 and copper phthalocyanine on Au(111):Self-organized nanostructuring

Abstract

Similar works

Full text

Available Versions

Dissertations of the University of Groningen