pubs.acs.org/NanoLett
Unambiguous Determination of the Adsorption Geometry of a Metal-Organic Complex on a Bulk Insulator Knud La¨mmle,† Thomas Trevethan,‡,§ Alexander Schwarz,*,† Matthew Watkins,‡ Alexander Shluger,‡,§ and Roland Wiesendanger† †
Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany, ‡ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom, and § WPI-AIMR, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan ABSTRACT Individual molecules of Co-Salen, a small chiral paramagnetic metal-organic Schiff base complex, were deposited on NaCl(001) and subsequently imaged with noncontact atomic force microscopy employing Cr coated tips in a cryogenic ultrahigh vacuum environment. Images were obtained in which both the position and orientation of the adsorbed molecules and the atomic structure of the surface are resolved simultaneously, enabling the determination of the exact adsorption site. Density functional theory calculations were used to identify the ionic sublattice resolved with the Cr tip and also to confirm the adsorption site and orientation of the molecule on the surface. These calculations show that the central Co atom of the molecule physisorbs on top of a Cl ion and is aligned along 〈110〉-directions in its lowest energy configuration. In addition, a local energy minimum exists along 〈100〉-directions. Due to the chirality of the molecule, two mirror symmetric configurations rotated by approximately (5° away from these directions are energetically equivalent. The resulting 16 low energy configurations are observed in the experimental images. KEYWORDS Adsorption, single molecule, Co-Salen, NaCl(001), DFT, NC-AFM
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films on such substrates,7-16 systematic high-resolution studies of individual well-separated molecules have not yet been performed. Theoretical calculations17 as well as combined STM/NC-AFM experiments18 indicate that the tipsample separation is significantly smaller during atomic resolution imaging in the latter case. Hence, the interaction of the tip with weakly adsorbed molecules can change their position and configuration or cause the adsorbate to be removed by the tip. This makes atomically resolved NC-AFM imaging of molecules at surfaces technically very challenging. Attempts at determining the chemical adsorption site of a molecule bring further complexities as this requires simultaneously resolving both the adsorbed molecule and the atomic structure of the substrate. This is a challenge for both STM and AFM studies and requires the preparation of stable and sharp tips. In most studies on ionic surfaces, it is assumed that surface material is attached to the tip apex due to collisions with the sample. Such polar tips in fact facilitate atomic resolution on ionic surfaces,17 but such tips can also interact strongly with an adsorbed molecule and thus prevent stable imaging.19,20 To obtain reproducible, highresolution imaging of both the surface and an adsorbed molecule, the tip should interact strongly with surface atoms but not with the adsorbed molecule. Further, to determine an adsorption site from an experimental image one needs to chemically identify the positions of individual surface atoms. Here, one possible strategy is to identify atomically resolved image features, for example, whether a bright spot in an image is Na or Cl, based on experimental observations
he adsorption of organic species at insulating surfaces is central to many fields of surface science, such as catalysis,1 molecular electronics2,3 and molecular magnetism.4 In all of these fields it is crucial to know the adsorption site and geometry to understand the underlying chemistry and physics of the system. To date, most of the experimental studies on individual molecules at surfaces have been performed using scanning tunneling microscopy (STM),5 which requires a well-conducting, and in most cases metallic, substrate. The molecule-substrate interaction is often strong on metal surfaces,6 which leads to a significant distortion of the molecular electronic structure, which in turn alters the magnetic properties of the adsorbed molecule. In addition, many technologically relevant substrates, for example, those used in catalytic or sensor applications, are bulk insulators, on which adsorbates cannot be investigated by STM. Furthermore, in envisaged molecular electronic or spintronic devices only the molecule should carry the current and the spin, respectively, and not the substrate. Therefore, it is very important to be able to explore the properties of molecules on electrically insulating surfaces. To analyze local structures on the atomic and molecular scale on bulk insulators, noncontact atomic force microscopy (NC-AFM) has to be utilized. Although this technique has been successfully employed to investigate organic thin * To whom correspondence should be addressed. E-mail: aschwarz@ physnet.uni-hamburg.de. Received for review: 04/13/2010 Published on Web: 07/07/2010 © 2010 American Chemical Society
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and/or theoretical modeling of the tip-surface interaction. An alternative is to solve the inverse problem: one can use a theoretically predicted adsorption site, along with the image to chemically identify surface features. Recently, pentacene, a relatively large hydrocarbon molecule, has been studied with NC-AFM on patches of ultrathin NaCl layers deposited on Cu(111).21 In this setup, tunneling is still possible and suitable tips could be prepared on the bare metallic substrate in the STM mode using standard procedures to transfer certain atomic or molecular species from the surface to the tip apex.22,23 Such well-controllable procedures currently do not exist for pure NC-AFM measurements, particularly not on bulk-insulating surfaces. The tip functionalized with a CO-molecule in this manner was then used in the NC-AFM mode to image pentacene molecules adsorbed on the NaCl patches with submolecular resolution, but without achieving atomic resolution of the substrate at the same time. In this study, we investigated Co-Salen molecules adsorbed on the (001) surface of bulk NaCl using NC-AFM with a Cr-coated tip. We demonstrate that carefully preparing metal-coated tips allows us to unambiguously identify the surface atoms, to image the molecule on the atomically resolved surface, and hence to identify the surface adsorption site and the adsorption geometry. In view of future experiments with respect to the local magnetic properties of molecules, this is an important milestone. Co-Salen (Co(C16H14N2O2)),24 is a complex that is long known for its ability to reversibly coordinate molecular oxygen25 and is used in numerous applications in material chemistry and transition metal catalysis (see, for example, ref 26 for a recent review). Depending on the metallic species in the center, Salen molecules possess interesting local magnetic properties and our goal is to study them eventuallywithmagneticexchangeforcemicroscopy(MExFM), a novel magnetic sensitive force microscopy-based technique.27,28 Therefore, in this study we aimed at investigating whether a metallic (magnetic) tip would allow us to determine the adsorption sites and geometry of individual CoSalen molecules on the NaCl(001) surface. Previous theoretical modeling has demonstrated that metallic tips are likely to strongly interact with anions on oxide surfaces.29 In this study, we performed density functional theory (DFT) calculations to show that the Cr tip interacts more strongly with Cl ions in the surface, allowing us to unambiguously identify the two different chemical sublattices in the experimental images and in turn determine the nature of the molecular adsorption site. We demonstrate that individual Co-Salen molecules adopt 16 low energy configurations on NaCl(001) in which the central Co atom always adsorbs directly above a Cl ion. The number of configurations reflect the two preferred orientations, that is, along 〈110〉- and 〈100〉-direction, respectively, the 4-fold symmetry of the substrate and the chirality of the molecule. The adsorption site and molecular orientations are also determined with © 2010 American Chemical Society
accurate DFT-based simulations of the surface-molecule interaction, which confirm the experimental observations and show that the electronic structure of the molecule is not significantly perturbed by the surface. The (001) surface of bulk NaCl was chosen as a substrate, since it is a well-known prototypical wide band gap (8.9 eV) ionic insulator. Co-Salen is a paramagnetic metal-organic Schiff base complex, which features a square-planar Co(II) at its center, carrying the spin (S ) 1/2). The central Co atom is coordinatively bonded to two N atoms and two O atoms in a banana-shaped hydrocarbon structure. The free molecule possesses a C2 symmetry and is chiral due to the outof-plane deformation of the -C2H4- bridge. Top and side views of the configuration of the free molecule are displayed in Figure 1a,b, respectively. The arrow in (a) marks the reference axis of the molecule and the direction of its electric dipole moment. All experiments were performed with a home-built lowtemperature ultrahigh vacuum force microscope30 using supersharp Si cantilevers31 with Cr-coated tips (nominal coating thickness: about 4 nm). Clean NaCl(001) substrates were prepared by in situ cleavage of single crystals. Molecule deposition took place directly in the microscope onto the cold substrate (about 30 K).32 Data acquisition was performed in the noncontact mode employing the frequency modulation technique33 at the base temperature of the microscope (about 8.2 K). A balancing voltage was applied between tip and sample to minimize the average long-range electrostatic interaction. More details of the experimental procedures as well as the definition of the imaging parameters are given in the Supporting Information. Figure 1c depicts a typical overview image of the NaCl(001) surface after molecule deposition. The banana-shaped objects can be identified as flat lying Co-Salen molecules. They can be straightforwardly distinguished from contaminations by their characteristic shape and their much larger apparent height. Depending on the mesoscopic tip radius and other imaging conditions, the apparent height of a molecule ranges between 60 to 330 pm above the maximum of the surface scan line. The orientations of the molecules appear to be random, but a systematic analysis of the images revealed 16 clearly distinguishable orientations relative to the underlying lattice. Using atomically resolved NC-AFM data on the bare substrate as a reference, we found that the primary axes of all the molecules are rotated either clockwise or anticlockwise by about (5 ( 2)° away from the 〈100〉- and 〈110〉-directions of the substrate; see Figure 1d. From N ) 87 evaluated molecules, 62 (71%) were oriented approximately along 〈110〉-directions, while only 25 (29%) were oriented approximately along 〈100〉-directions. Moreover, we found that 38 (44%) molecules were rotated clockwise and 49 (56%) anticlockwise with respect to the two principal crystallographic axes of the substrate. In Figure 1e,f, two images with an isolated Co-Salen molecule on the atomically resolved NaCl(001) substrate 2966
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FIGURE 1. Top (a) and side (b) of the free Co-Salen molecule. It is a square-planar metal-organic paramagnetic Schiff base complex with a central Co-atom (S ) 1/2). The tilted -C2H4- bridge results in a C2 symmetry. The arrow marks the reference axis. (c) Overview NC-AFM image after deposition of Co-Salen molecules on NaCl(001). The banana-shaped Co-Salen molecules can be clearly distinguished from adsorbates. Imaging parameters: f0 ) 186 kHz, cz ) 143.6 N/m, A0 ) 5 nm, ∆f ) -0.66 Hz, Ubias ) -0.85 V. (d) Two individual Co-Salen molecules. The angle between their axis is approximately (5° away from the [11¯0]-direction. Analyzing 87 molecules in this way revealed 16 different orientations of the molecule with respect to the principal crystallographic axis of the substrate, compared with dotted lines relative to the crystalographic axes inset. Imaging parameters: f0 ) 187 kHz, cz ) 145.3 N/m, A0 ) 5 nm, ∆f ) -1.31 Hz, Ubias ) 0 V. (e) and (f) represent high-resolution NC-AFM images showing a single molecule and atomic resolution on the substrate simultaneously using a Cr coated tip. As visualized by the grid, the center of the Co-Salen molecule is above a maximum in both cases. Since the molecule appears larger in (f), the tip apex in (e) seems to be sharper. Imaging parameters: (e) f0 ) 189 kHz, cz ) 148.7 N/m, A0 ) 5 nm, ∆f ) -5.4 Hz, Ubias ) 5 V; (f) f0 ) 187 kHz, cz ) 145.4 N/m, A0 ) 5 nm, ∆f ) -2 Hz, Ubias ) -0.25 V. © 2010 American Chemical Society
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recorded with two different Cr coated tips are displayed. Atomic corrugation, apparent height and length of the molecule are 8 pm, 60 pm, and 1.2 nm in panel e and 25 pm, 330 pm, and 1.6 nm in panel f. The larger atomic corrugation and apparent height in panel f indicates that the tip was approached closer to the surface than in panel e. Furthermore, the larger apparent lateral dimensions of the molecule in panel f and its less well expressed banana shape suggests a larger mesoscopic tip radius. As reported in several other publications, only one sublattice is imaged as maxima on binary ionic surfaces.34-37 To accurately identify the adsorption site, the lattice periodicity is superimposed over the image where it is clear that the center of the molecule is directly over a surface site corresponding to a maximum in the surface corrugation. Note that in both images the largest contrast is not observed above the center, but somewhat shifted away from it, a feature, which we observed quite often. It should be noted that not all tips were able to produce this kind of high-resolution data. For many tips, large negative ∆f set-points, which must be adjusted to achieve atomic resolution on the substrate, frequently resulted in a tipinduced translation or rotation of adsorbed molecules. While undesirable for imaging this could be used for a controlled manipulation of the molecules on the surface. However, this was not the main focus of our investigation. At too large negative ∆f set-points, instabilities of the tip-surface gap occurred and stable imaging became impossible. On binary ionic surfaces like NaCl(001), the ion type that interacts more strongly with the tip, and hence is imaged as maxima in images, directly depends on the atomic species present at the tip apex.29 Characterizing tips is therefore crucial for our understanding of the images. The tips of the Si cantilevers used are covered by an oxide layer. After in situ evaporation of Cr onto the cantilever and before and after taking the data shown in Figure 1e, we recorded ∆f(Ubias) curves to check whether the tip is indeed metallic and hence Cr-terminated at its apex. For the two different tips used to record Figure 1e,f, the curves were smooth without jumps and look identical independent of the sweeping direction (no hysteretic behavior). Jumps would indicate charge reconfigurations due to tunneling processes at the tip apex, while hysteresis can be attributed to slow charge relaxation processes. Both effects occur on tip apexes, which are either not metallic at all or on which the metallic film is not continuous.38 Hence, we can infer that the tip apex is most likely Cr-terminated. To determine the origin of the atomic scale contrast on the bare substrate, we performed periodic DFT simulations to investigate the interaction of a Cr tip with the NaCl(001) surface. A simple Cr tip model was constructed from the body-centered cubic bulk structure forming a 4 layer pyramid; compare with inset in Figure 2. The technical details of these calculations are included in the Supporting Information. Force-distance curves calculated for the tip apex atom © 2010 American Chemical Society
FIGURE 2. Simulated force-distance curves for the Cr tip interacting with the NaCl(001) surface. A side-view of the structure of the Cr tip is shown as an inset. Since the attractive forces are larger above the Cl ions, they are imaged as maxima in constant ∆f atomic resolution NC-AFM images.
located directly above Na and Cl lattice positions are shown in Figure 2. These force curves demonstrate clearly that the attractive tip-surface interaction above Cl ions in the surface is significantly stronger than the interaction above Na ions. This main finding is also true for other orientations of the tip with respect to the surface. Since a stronger tip-surface attraction leads to a larger negative ∆f, this shows that the Cl ions will be imaged as maxima in a constant ∆f image. By implication, the center of the molecule, that is, the central Co atom, as imaged in Figure 1e,f, is above the surface Cl ion. To further check the validity of this preliminary conclusion and to confirm the adsorption site, we investigated the interaction of a single molecule with the surface and possible stable configurations of the molecule on the surface. We performed a series of embedded cluster density functional theorycalculations,employingtheB3LYPhybridfunctional.39,40 It has been previously shown that the B3LYP functional is able to correctly reproduce the electronic structure of both this molecule41,42 and the NaCl (001) surface.43 In these calculations, the Co-Salen molecule is treated using a 6-311G basis on Co, 6-31G(d) basis on all other heavy atoms and a 6-31G basis on hydrogens. The optimized structure of the free molecule in the S ) 1/2 state is shown in Figure 1a,b. Here, all of the spin density is localized on the Co atom. The S ) 3/2 state of the molecule is 0.28 eV higher in energy than the S ) 1/2 state. The NaCl surface is treated using an embedding scheme, which is described in detail in the Supporting Information. To include the effect of the van der Waals interaction between the molecule and the surface, which is not accounted for effectively within standard DFT methods and can be a significant contributor to surface binding in systems such as this, we included damped pairwise London terms between the atoms of the molecule and the surface, based on the scheme described in detail in ref 44. 2968
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FIGURE 3. (a) Top view of the configuration of the molecule adsorbed in its lowest energy configuration with the primary molecular axis (black line) and [110] surface axis (dashed line) shown. (b) Side view of the molecule viewed along the [110] direction. (c) Top view of the configuration of the molecule adsorbed in the alternate configuration (a local energy minimum) with the primary molecular axis (black line) and [100] surface axis (dashed line) shown. (d) Side view of the molecule viewed along the [010] direction.
The Co-Salen molecule is initially placed above the NaCl(001) surface with the plane of the molecular board parallel to the surface plane, separated by 0.4 nm. This configuration is suggested by the shape of the molecule when imaged with NC-AFM (see Figure 1). To find all stable configurations of the molecule on the surface, we then tried several initial guesses with the axis of the molecule (compare with Figure 1a) parallel to the [100]-direction and at 5° intervals from here until the axis of the molecule was aligned along the [010]-direction. The molecule was located with the Co atom directly above a Cl ion in the surface, directly above a Na ion, and in the bridge position between two neighboring Cl atoms. For each of these initial configurations, the structure was optimized to minimize the systems total energy. All atoms in the NaCl quantum cluster as well as several hundred classical NaCl ions were allowed to relax. The lowest energy configuration was found for the molecule orientated laterally 5° from the [110] direction with the Co atom directly above a Cl ion in the surface. The relaxed structure is shown in Figure 3a,b, where the orientation of the molecule due to the interaction with the surface has slightly changed (
