The scanning tunnelling microscope as an operative tool: doing physics and chemistry with single atoms and molecules

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1 /rsta The scanning tunnelling microscope as an operative tool: doing physics and chemistry with single atoms and molecules By Karl-Heinz Rieder 1, Gerhard Meyer 2, Saw-Wai Hla 3, Francesca Moresco 1, Kai F. Braun 1, Karina Morgenstern 1, Jascha Repp 2, Stefan Foelsch 4 and Ludwig Bartels 5 1 Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, Berlin, Germany (rieder@physik.fu-berlin.de) 2 BM Zurich Research Laboratory, Säumerstrasse 4/Postfach, 8803 Rüschlikon, Switzerland 3 Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA 4 Paul-Drude-nstitute, Hausvogteiplatz 5 7, Berlin, Germany 5 Department of Chemistry, University of California Riverside, 900 University Avenue, Riverside, CA , USA Published online 31 March 2004 The scanning tunnelling microscope, initially invented to image surfaces down to the atomic scale, has been further developed in the last few years to an operative tool, with which atoms and molecules can be manipulated at will at low substrate temperatures in different manners to create and investigate artificial structures, whose properties can be investigated employing spectroscopic d/dv measurements. The tunnelling current can be used to selectively break chemical bonds, but also to induce chemical association. These possibilities give rise to startling new opportunities for physical and chemical experiments on the single atom and single molecule level. Here we provide a short overview on recent results obtained with these techniques. Keywords: scanning tunneling microscopy; atom manipulation; artificial structures; molecular electronics; chemical reactions 1. Lateral manipulation and build-up of artificial structures Figure 1 shows an initial example of an artificial structure on atomic scale. A regular triangle had been formed with 51 Ag atoms on Ag(111) by laterally manipulating them into the proper positions with atomic accuracy (Braun & Rieder 2002). n lateral manipulation an adparticle at the surface is moved by the tip along the For films showing the gradual build-up of several artificial surface structures in an atom-by-atom way by lateral manipulation see the homepage of the authors: agrieder/index.html. One contribution of 12 to a Discussion Meeting Organizing atoms: manipulation of matter on the sub- 10 nm scale. 362, c 2004 The Royal Society

2 1208 K.-H. Rieder and others Figure 1. The build-up of a regular triangle consisting of 51 Ag atoms on an Ag(111) surface employing lateral manipulation. substrate surface to the desired place without losing contact with the substrate. This is achieved by bringing the tip very close to the adparticle, so that, besides the ever present van der Waals interactions, chemical forces between the tip and particle also come into play. These forces can be tuned to be large enough to surmount the surface diffusion barriers, so that the adparticle moves along with the tip, if the tip is moved parallel to the surface to the desired end point. t is fascinating that, even at the atomic level, a distinction can be made between different manipulation modes, namely pulling, pushing and sliding (Bartels et al. 1997a). The different modes can be discerned by recording tip-height curves during manipulation in the scanning tunnelling microscope (STM) constant-current mode. Upon pulling, the atoms follow the tip in regular jumps from one adsorption site to the next, due to attractive tip particle forces. By applying larger forces (measured by smaller tunnelling resistivities), a sliding motion is induced, in which the adparticle is trapped under the tip and follows the tip motion continuously, so that the tip-height curve yields a picture of the substrate corrugation. Pulling and sliding are usually applied in the manipulation of metal atoms. n contrast, CO molecules are usually pushed: the molecules move discontinuously from one adsorption site to the next in front of the tip due to repulsive forces. On close-packed surfaces like Cu(111), push-

3 The STM as an operative tool 1209 (a) tip tip CO CO surface surface (b) (c) Figure 2. (a) Schematic of the flipping of a CO molecule upon vertical manipulation from the surface to the tip apex. (b), (c) maging changes obtained for CO molecules with a CO tip. Notice that the image of the oxygen atom is unaffected. The white arrow denotes the CO molecule which was transferred deliberately to the tip. ing is not very reliable, as the particles tend to move to the side of the tip and get lost. Build-up of artificial structures in the pushing mode is eased by a suitable choice of the substrate surface: on Cu(211), CO adsorbs at the upper part of the intrinsic step edges, which act as railway trails upon pushing. With sufficiently stable and nevertheless sharp tips it is even possible to remove native substrate atoms from highly coordinated defect step sites and even from regular step sites of high-index surfaces in a one-by-one manner (Meyer et al. 1996). This ability was used in analytic chemistry on the atomic scale by investigating the monolayer structure formed on Cu(211) upon Pb-evaporation at room temperature: atom by atom removal from an island edge revealed that a surface Cu Pb alloy had formed, although the two metals do not mix in the bulk (Bartels et al. 1998). The sole action of tip particle forces in this manipulation variant, with which a build-up of artificial structures with atomic precision is possible (figure 1), has been proven recently by measuring the threshold curves for currents, for which secure manipulation is obtained, over a wide range of tunnelling voltages (Hla et al. 2003). 2. Vertical manipulation The deliberate vertical transfer of a particle from the surface to the tip and vice versa is called vertical manipulation. Again the tip is brought close to the particle to be transferred, until the force between tip and particle is sufficiently strong that the particle can move with the tip upon its withdrawal. Here the electron current effects can help to transfer the particle in the direction desired: the scanning tunnelling micrographs in figure 2b refer to an image of Cu(111) with several CO molecules and one oxygen atom in the upper left corner. All species are imaged as depressions, with a

4 1210 K.-H. Rieder and others Figure 3. High-resolution STM image obtained with an unknown species transferred to the tip by vertical manipulation. Noticeably, beside the Ag adatoms of the triangular corral (see figure 1) and the standing-wave pattern, even the substrate atoms of the Ag(111) substrate are visible. tip consisting of metal atoms. The CO molecule designated by an arrow is then picked up with the technique of vertical manipulation and the same area is imaged again. Noticeably, all CO molecules have changed in appearance to protrusions, whereas the oxygen atom remains imaged as a depression (Bartels et al. 1997b). t is obvious that deliberate functionalization of the tip with different molecules may lead to chemical contrast a feature very much desired in STM and understood only very recently on the basis of theoretical calculations using a Green function approach, which allows analysis of different electron current paths and their interference (Nieminen et al. 2004). The vertical transfer of CO is interesting because CO stands upright on metal surfaces, with the carbon atom binding to the substrate. Upon transfer to the tip, the molecule consequently has to turn around. A reliable experimental procedure for transferring single CO molecules was found to require ramping of the tunnelling voltage and simultaneous decrease of the tip molecule distance. nvestigations of the transfer mechanism yielded the following picture. Voltage ramping supplies the minimum tunnelling bias of 2.4 ev required to populate the CO anti-bonding 2π level. As the CO hopping rate depends linearly on the tunnelling current, a oneelectron process is responsible for the excitation. Although only 0.5% of the tunnelling current passes through the 2π orbital and the lifetime of the electrons in this antibonding level is only of the order of femtoseconds, the continuous supply of tunnelling electrons eventually causes release of the CO from the surface. The approach of the tip in the pickup procedure just increases the probability that the molecule is caught at or near the tip apex upon desorption (Bartels et al. 1998).

5 The STM as an operative tool 1211 (a) (a) tip (b) C 6 H 5 C 6 H 5 (b) (b) (c) (c) C 6 H 5 C 6 H 5 (c) (d) (d ) C 6 H 5 C 6 H 5 (e) (e) C 6 H 5 C 6 H 5 C 12 H 10 ( f ) ( f ) C 12 H 10 (g) (g) C 12 H 10 C 12 H 10 Ullmann reaction Figure 4. Steps in the STM-tip-induced single-molecule Ullmann reaction on Cu(111). All reaction steps including dissociation of parent iodobenzene molecules, motion of phenyl reactants and iodine byproducts as well as the association of phenyl reactants to biphenyl have been induced with an iodine-functionalized tip.

6 1212 K.-H. Rieder and others t is highly likely that vertical manipulation also plays a role in obtaining the utmost geometrical resolution possible with the STM: figure 3 shows an edge of the triangle of figure 1. Notice that, beside the large corrugation of ca. 1 Åof the adatoms forming the triangle, the standing waves arising from the surface state electrons are seen with an amplitude of ca. 0.1 Å and the substrate atoms are seen as individual small hills with a corrugation amplitude as small as ca Å. This picture was obtained with an hitherto unidentified surface species, which could be imaged at the surface, manipulated laterally and vertically. Upon transferring the particle to the tip, the astonishing resolution demonstrated in figure 3 was obtained. 3. Doing chemistry with the STM tip: inducing all the steps of a chemical reaction Population of an anti-bonding state is also important in the preparation of reactants in a full chemical reaction induced by the tip (Hla et al. 2000). n the so-called Ullmann reaction, iodine has to be split off from the iodobenzene parent molecules to form the phenyl reactants. Tunnelling electrons again populate temporarily the iodine phenyl anti-bonding level, thus causing the dissociation step (figure 4a, b). Both iodine and phenyl fragments are found on the surface. To bring two phenyls together, lateral manipulation in the pulling mode is employed (in a chemical reactor working at elevated temperatures, this step would be performed by thermal diffusion) (figure 4c, d). At the low temperatures of the Cu(111) substrate, the proximity of the two phenyls is not sufficient to induce the association to biphenyl: if a pulling procedure is applied to the phenyl couple from one end, the phenyl on the rear does not move. Only after injection of electrons the synthesis step is performed, which can be proven by pulling the product from one end and realizing that the entire molecule follows the tip (figure 4e, f). Notice that in figure 4c, d one of the iodine atoms was transferred deliberately to the tip after dissociation of the iodobezene and all the following steps were performed with the iodine functionalized tip. The iodine was finally put back on the surface (figure 4g) (Hla & Rieder 2003). The synthesis of the two phenyls to biphenyl is probably connected by local excitation of vibrational modes in the phenyl groups, enabling the two open bonds to find the proper relative orientation for bond formation. Local excitation of the scissoring and the OH-stretching modes was indeed observed to be responsible in current-induced sideward jumps of water molecules adsorbed on Cu(111) (figure 5a). Furthermore, hydrogen bonds can be formed and broken and thus ice clusters up to the very stable ice hexamer (figure 5b) can be crystallized via the same mechanism (Morgenstern & Rieder 2002). 4. Switching and contacting molecules Manipulation can also be performed into parts of molecules, as demonstrated for the case of a TBPP molecule whose centre porphyrin ring has four legs attached which are perpendicular to the centre ring in the gas phase. On Cu(211), however, the legs lie flat (Moresco et al. 2001b). Using lateral manipulation, a single leg can be trans-

7 The STM as an operative tool (a) N U (mv) (b) Figure 5. Current-induced manipulation of water molecules via excitation of vibrational modes, whose energy is transferred into irregular lateral jumps, upon which water dimers, trimers and even the specially stable water hexamers can be formed. (a) The histogram of events peaks at the energies of the scissoring (ca. 220 mev) and O H-stretching (ca. 430 mev) modes. (b) mage of water hexamers formed. A single water monomer can be seen at the bottom left. formed into an almost perpendicular conformation and the leg can be pushed back into the flat position again with the tip. As the perpendicular and parallel conformations exhibit conductivities of different orders of magnitude, these experiments open up the possibility of a molecular switch, in which a mechanical action causes switching from conducting to nonconducting behaviour (Moresco et al. 2001a). A related molecule with a long polyaromatic conducting board and the same legs, called lander, has been used in a recent study concerning the problem of contacting molecular wires with atomic precision. t may be shown that, upon moving the molecule with the legs parallel towards a step edge, the standing-wave pattern of the Cu(111) substrate did not change (figure 6b), whereas, when the molecule was moved with its legs perpendicular to the step, the board connected to the step and an electron diffraction pattern characteristic for the contact area was observed (figure 6c) (Moresco et al. 2003).

8 1214 K.-H. Rieder and others 5. Doing physics with artificial structures: measuring electron lifetimes inside a quantum corral Adparticles arranged in a closed geometry such as that shown in figure 1 act as partial confinement for electrons and can be used to determine the electron lifetime (Braun & Rieder 2002). The electrons of the surface state present on the Ag(111) surface are scattered by these Ag adatoms, resulting in a complex interference pattern (figure 1). Energy resolved data taken in the spectroscopic d/dv mode allow measurements of the interference patterns as a function of energy. Calculations of the wave patterns were performed based on a multiple scattering approach taking into account the phase-relaxation lengths of the electrons, which reflect scattering events inside the triangle influencing their phase coherence and can be directly converted into electron lifetimes. nside the triangle, electron electron and electron phonon scattering determine the electron lifetime and the spatial decay of the interference pattern. The evaluations for the electron lifetimes as function of energy clearly show a sharp maximum at the Fermi energy, in accordance with Fermi liquid theory for a twodimensional electron gas due to a decreased phase-space for electron electron and electron phonon scattering. Furthermore, two pronounced edge-like features show up in the data at +65 mev and +300 mev, whereby the latter can be attributed to the transition of the surface state into a surface resonance. The former can be interpreted by a change of the scattering probability as the electron energy becomes smaller than the surface state binding energy of 65 mev. Finally, the additional fine structure observed may be attributed to the geometrical influence of the triangle. 6. Outlook The results described above relate to many diverse aspects of physics and chemistry on the atomic and nanoscales. Measuring tip-height curves in constant-current, or current curves in constant-height mode, during lateral manipulation reveals the internal motion of the entities and thus allows new insights into nanomechanics. The fact that small structures can be assembled or taken apart yields important routes to synthetic and analytic chemistry on the atomic scale. The possibility of taking atoms out of the substrate from defect or intrinsic surface steps can be used to structure the surface itself, with the possibility of also including layers deeper than the topmost one. Thus, the build-up of three-dimensional nanostructures also appears to be possible, especially by combining lateral and vertical single-particle manipulation. The use of artificially created adatom hole pairs as binary units with writing, reading and rewriting possibilities certainly would yield the utmost possible surface storage density. Artificial structures on the surface built either with native substrate atoms or adsorbed species in an atom/molecule-by-atom/molecule way (figure 1) can be investigated in situ with respect to their physical properties employing scanning tunnelling spectroscopy. mportant progress for nanoelectronics can be expected from the ability to modify internal molecule conformations with the tip and to contact them to leads with atomic precision (figure 6). The successful induction of all steps of a complex chemical reaction using force, current and field effects (figures 4 and 5) raises the hope that, by taking different parent molecules apart and welding the dissociation products together at will, it will be possible to synthesize new molecules, whose properties can be again investigated by tunnelling spectroscopy.

9 The STM as an operative tool 1215 (a) (b) (c) Figure 6. Models and STM images of Lander molecules on a Cu(111) surface. (a) A single molecule on a flat terrace. Notice the circular standing-wave pattern. (b) The molecule is manipulated so that the central board is parallel to a step edge. The standing-wave pattern at the upper part of the step remains unaffected. (c) The molecule is manipulated so that the board is perpendicular to the step edge. The interference pattern at the upper edge of the terrace is determined by the area of contact between the molecule and the upper part of the step and indicates electrical contact. This dream of molecular surgery is certainly one of the most exciting, but also one of the most ambitious, challenges for the future. Because controlled atomic manipulation allows the design of arbitrary scattering geometries, on the basis of a deeper understanding of the electron lifetimes it should become possible even to engineer these lifetimes, which are a key quantity in quantum computing and quantum transportation. A further important present goal is to transfer all the possibilities outlined here to technologically important substrates like insulators; as for sufficiently thin insulator films on metallic substrates, the use of the STM is still possible (Repp et al. 2001; Foelsch et al. 2002). The recent observation of electron refraction at the edge of an NaCl film on Cu(111), where a surface-localized metal state and an interfacial state localized between metal and ionic film merge (Repp et al. 2004), even gives rise to speculations on the possibility to build devices based on surfaceelectron optics. t should not be forgotten that the ability to manipulate individual atoms and molecules can also have important consequences for high-precision measurements on surfaces, as demonstrated by the experimental determination of the surface-state-mediated oscillatory interaction between adsorbates (Repp et al. 2000). The possibility of obtaining chemical resolution in STM imaging with deliberately functionalized tips prepared by vertical manipulation (figure 2) also very much deserves further investigation. This holds also for the achievement of STM

10 1216 K.-H. Rieder and others images with extremely high resolution (figure 3). Manipulation possibilities at room temperature as well as on the liquid solid interface constitute further experimental challenges which it will be very worthwhile to pursue. References Bartels, L., Meyer, G. & Rieder, K. H. 1997a Phys. Rev. Lett. 79, 697. Bartels, L., Meyer, G. & Rieder, K. H. 1997b Appl. Phys. Lett. 71, 213. Bartels, L., Meyer, G., Rieder, K. H., Velic, D., Knoesel, E., Hotzel, A., Wolf, M. & Ertl, G Phys. Rev. Lett. 80, Braun, K. F. & Rieder, K. H Phys. Rev. Lett. 88, Foelsch, S., Riemann, A., Repp, J., Meyer, G. & Rieder, K. H Phys. Rev. B 66, Hla, S. W. & Rieder, K. H Acc. Chem. Res. 54, 307. Hla, S. W., Bartels, L., Meyer, G. & Rieder, K. H Phys. Rev. Lett. 85, Hla, S. W., Braun, K. F. & Rieder, K. H Phys. Rev. B 67, Meyer, G., Zoephel, S. & Rieder, K. H Phys. Rev. Lett. 77, Moresco, F., Meyer, G., Rieder, K. H., Tang, H., Gourdon, A. & Joachim, C. 2001a Phys. Rev. Lett. 86, 672. Moresco, F., Meyer, G., Rieder, K. H., Tang, H., Gourdon, A. & Joachim, C. 2001b Phys. Rev. Lett. 87, Moresco, F., Gross, L., Alemani, M., Rieder, K. H., Tang, H., Gourdon, A. & Joachim, C Phys. Rev. Lett. 91, Morgenstern, K. & Rieder, K. H J. Chem. Phys. 116, Nieminen, J., Niemi, E. & Rieder, K. H Surf. Sci. 552, 247. Repp, J., Moresco, F., Meyer, G., Rieder, K. H., Hyldgaard, P. & Persson, M Phys. Rev. Lett. 85, Repp, J., Foelsch, S., Meyer, G. & Rieder, K. H Phys. Rev. Lett. 86, 672. Repp, J., Meyer, G. & Rieder, K. H Phys. Rev. Lett. 92,