Berndt, R.

Gimzewski, J.K.

English

Proceedings of the NATO Advanced Research Workshop:  Scanning Probe Microscopies and Molecular Materials , 25.05.1994 - 03.06.1994, Schloss Ringberg, Tegernsee, pp 1-

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PHOTON EMISSION FROM C60 IN A NANOSCOPIC CAVITYR.BERNDTInstitut de Physique ExpérimentaleUniversité de Lausanne1015 LausanneSwitzerlandANDJ.K. GIMZEWSKIIBM Research DivisionZurich Research Laboratory8803 RüschlikonSwitzerlandAbstract. We discuss properties of C60 in an STM tunnel junction probed by elastic and inelastictunneling. In particular, the interaction of the molecule with the metallic nanoscopic cavity comprisedof tip and sample is considered. Molecular resolution in photon emission from C60 monolayers isinterpreted in these terms, and we address the extent to which photon emission from C60 reflectsproperties of the cavity and of the molecule itself. We propose that the properties Of C60 in such anenvironment are further modified with respect to its adsorbed state on a surface.The fascinating observation that photons are emitted from the tunnel junction region of a scanningtunneling microscope (STM) has stimulated experimental and theoretical interest [1]. Ourunderstanding of the processes underlying emission for semiconductors is based on concepts similar tothose known from conventional cathodoluminescence [2, 3]. In the case of metals, inelastic tunnelingand local electromagnetic modes that are confined between tip and sample (tip-induced plasmons(TIP) modes) are involved in photon emission [1, 4, 5]. In the case of molecular systems, only recentlyhas evidence for localized emission from individual molecules been obtained for C60 measured inultrahigh vacuum conditions at low temperatures [6, 7]. In those experiments, molecules separated bya lateral distance of ~1 Å have been resolved. These results differ from recent reports of theobservation of isolated molecules with a near-field optical microscope which resolved molecules orclusters thereof separated by large lateral dimensions [8]. More recently yet, "atomic" resolution hasbeen observed on clean metallic surfaces using photon emission from an STM [5].     In this paper we summarize key observations which strongly suggest that in the extreme geometryof an STM tunneling junction, molecular properties are strongly modified. In particular, we proposethat the enhanced electromagnetic field in the cavity of tip and sample due to TIP modes amplifiesphoton emission from individual molecules.     The electronic properties of C60 are strongly influenced when the molecule is in close proximity toa metal surface. For C60 adsorbed on Au(110), molecules in the first monolayer show a shift of theirlowest occupied molecular orbitals down towards the Fermi energy EF as shown by tunnelingspectroscopy. Quite different properties have been reported for molecules in the second layer [9].Similarly, the proximity of the STM tip above C60 can also induce further shifts as well as abroadening and mixing of molecular orbitals. Consequently, it is expected that the electronicproperties of a molecule in a nanoscopic metallic cavity are different from those of a free molecule orone adsorbed on a metallic surface. [6].2Whereas STM images predominantly reflect the elastic tunneling current, the process of photonemission in STM using metallic tips and substrates is dominated by inelastic tunneling. This inelastictunneling is determined by the electromagnetic field strengths of the TIP modes [4, 5] as shown inFig.1.Conceptually, the introduction of a molecule into this cavity has several implications:1. The electronic structure of the surface is affected.2. The electronic structure of the molecule is strongly perturbed as discussed above.3. The distance from the tip to the surface is increased by the presence of C60.4. The electromagnetic modes and hence the inelastic tunneling probability are modified.Figure 2 displays a molecularly resolved photon emission map of C60 on Au(110). Below we addressthe extent to which the emission and the molecular resolution observed in Fig. 2 are related to TIPmodes or to the molecules themselves.     On Au(110) surfaces, C60 appears as a protrusion of typically 4-5 Å, and molecular structure withinthese protrusions has been observed [10].Figure 1. Schematic diagram of a tip-induced plasmon mode. Plus and minus signs represent a snapshot of theoscillating electron density of this approximately dipolar mode. The mode is affected by geometric parameters(distance of tip and sample, tip geometry represented by a radius) and the dielectric functions of tip and sample.Figure 2. Photon intensity map of a C60 monolayer on a Au(110) single-crystal surface. Individual moleculesseparated by ~10 Å are clearly resolved.3Calculations show that C60 increases the elastic tunneling by approximately six orders of magnitudecompared to the equivalent junction without C60 [11]. This goes beyond a mere modification of themetallic density of states as predicted and observed for small adsorbates on metals [12, 13]. Hence themolecule plays an active and determining role in the elastic tunneling current. This strongly suggeststhat the inelastic tunneling channels are also dominated by molecular properties. The inelastictunneling and subsequent photon emission from TIP modes on a clean Au surface is stronglydistance-dependent [5]. This factor reduces emission from such a mode on C60-covered areas and,similarly, hints that there is a molecular source of the photon emission. The observation that thedetected emission is most intense when the STM tip is centered above a molecule provides furtherevidence of an active role of the molecule [6].     It is therefore unlikely that the observation of molecular resolution in photon emission from C60 onAu(110) shown in Fig. 2 is directly related to a tip-Au TIP mode. Instead, the emission involves theC60 molecules. Conventional molecular fluorescence from molecules is, however, strongly damped bytheir proximity to metal surfaces [14]. Little is known, however, about the properties of moleculesstressed by their interactions in a nanoscopic cavity. However, one dramatic manifestation of theirunusual behavior in such an environment is the surface-enhanced Raman effect (SERS) [14],in whichthe Raman signal from molecules in nanoscopic metallic cavities is increased by several orders ofmagnitude. An important contribution to this enhancement is made by the increased electromagneticfield in these cavities [14]. It should be noted that the explanation of SERS also involves chargetransfer between the molecules and the surface [14]. In the present case of C60 on Au, charge transferhas indeed been observed [15]. We are currently investigating other organic molecular species toassess the generality of these concepts [16].     It is becoming quite clear both for elastic and inelastic tunneling that molecules exhibit radicallydifferent properties in nanoscopic cavities such as STM. This may partially explain why moleculessuch as hydrocarbons, which are insulators, can be imaged with STM [17]. Our observations of photonemission clearly resolved at the molecular level from C60 species separated by ~10 Å providepersuasive evidence that new inelastic tunneling channels are opened, as are the elastic ones by newproperties induced by the environment of the STM experiment. This has important implications.Significant changes can be induced in molecules by the proximity of a tip. As a consequence, thefunctionality of a molecule may be altered as well, analogous to changes in functionality by chemicalmodifications.References1. Coombs, J.H., Gimzewski, J.K., Reihl, B., Sass, J.K. and Schlittler, R.R. (1988) J. Microscopy 152,325.2. Berndt, R. and Gimzewski, J.K. (1992) Phys. Rev. B 45, 14095.3. Abraham, D.L., Veider, A., Schönenberger, Ch., Meier, H.P., Arent, D.J. and Alvarado, S.F. (1990)App. Phys. Lett. 56, 1564.4. Berndt, R., Gimzewski, J.K. and Johansson, P. (1991) Phys. Rev. Lett. 67, 3796; (1993) Phys. Rev.Lett. 71, 3493.5. Berndt, R., Gaisch, R., Gimzewski, J.K., Reihl, B., Schlittler, R.R., Schneider, W.D. and Tschudy,M., submitted.6. Berndt, R., Gaisch, R., Gimzewski, J.K., Reihl, B., Schlittler, R.R., Schneider, W.D. and Tschudy,M. (1993) Science 262, 1425.7. Berndt, R., Gaisch, R., Gimzewski, J.K., Reihl, B., Schlittler, R.R., Schneider, W.D. and Tschudy,4M. (1993) Appl. Phys. A 57, 513; (1994) Surf. Sci. 307-309, 1033.8. Betzig, E. and Chichester, R.J. (1993) Science 262, 1422.9. Gimzewski, J.K., Modesti, S. and Schlittler, R.R. (1994) Phys. Rev. Lett. 72, 1036.10. Gaisch, R., Berndt, R., Gimzewski, J.K., Reihl, B., Schlittler, R.R., Schneider, W.D.  and Tschudy, M. (1993) Appl. Phys. A  57, 207; J. Vac. Sci. Technol., in print.11. Joachim, C. and Gimzewski, J.K., in these proceedings.12. Lang, N.D. (1987) Phys. Rev. Lett. 58, 45.13. Berndt, R., Gimzewski, J.K. and Schlittler, R.R. (1994)  Surf. Sci. 310, 85.14. Otto, A., Mrozek, I., Grabhorn, H. and Akemann, W. (1992) J. Phys.: Condens. Matter 4, 1143;  Pettinger, B. (1992) Adsorption of Molecules at Metal Electrodes, J. Lipkowski and P.N. Ross      (eds.), Frontiers in Electrochemistry Series Vol. 1, VCH Publishers, New York, p. 285.15. Modesti, S., Cerasari, S. and Rudolf, P. (1993) Phys. Rev. Lett. 71, 2469.16. Böhringer, M., and Berndt, R., unpublished.17. Joachim, C., Gimzewski, J.K., Schlittler, R.R. and Chavy, C., submitted.

PHOTON EMISSION FROM C60 IN A NANOSCOPIC CAVITY

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PHOTON EMISSION FROM C60 IN A NANOSCOPIC CAVITY

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