Article pubs.acs.org/JPCC
Determining Adsorption Geometry, Bonding, and Translational Pathways of a Metal−Organic Complex on an Oxide Surface: CoSalen on NiO(001) Alexander Schwarz,*,† David Z. Gao,*,‡ Knud Lam ̈ mle,† Josef Grenz,† Matthew B. Watkins,‡ Alexander L. 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 complex, deposited on NiO(001) were imaged with noncontact atomic force microscopy (NCAFM) using metallic Cr coated tips. Experimentally, we simultaneously resolve both the molecule and the individual surface ions. Images recorded at low temperatures show that the Co-Salen molecules are aligned slightly away from the ⟨110⟩ directions of the surface and that the Co center of the molecule is located above a bright spot in atomically resolved images of the surface. Density functional theory calculations predict that the molecule adsorbs with the central Co atom on top of an oxygen ion and is in its lowest energy configuration aligned either + or −4° away from the ⟨110⟩ directions, dependent on the chirality of the molecule. Combining theoretical predictions and experimental data allows us to identify bright spots in NC-AFM images as oxygen sites on NiO(001) and hence determine the exact adsorption geometry and position of the molecule. Additionally, we observed tip-induced translations of the Co-Salen molecules along ⟨110⟩ directions on the substrate, which corresponds to the lowest energy pathway for diffusion. A comparison of these results with theoretical calculations and previously published experimental data for Co-Salen on the (001) surface of bulk NaCl highlights differences in the character of adsorption of individual molecules and the ensuing growth of Co-Salen thin films on these substrates.
■
INTRODUCTION Understanding the geometric parameters and mechanisms of adsorption and diffusion of organic molecules at insulating surfaces is central to catalysis,1 molecular electronics,2−4 molecular sensors,5 and molecular magnets.6 This is also a necessary step in elucidating the mechanisms of formation and structure of molecular assemblies at surfaces. To date, most experimental studies on individual molecules at surfaces have been performed on conducting and in most cases metallic substrates using scanning tunneling microscopy (STM).7 However, many existing and future applications require exploring the properties of molecules on electrically insulating surfaces. Oxides surfaces are particularly interesting due to their wide range of magnetic, optical, and electronic properties,8 some of which can be tuned.9 The conductivity of bulk wide gap oxides, however, is too low for STM measurements. Studying individual molecules with atomic resolution on such surfaces requires using noncontact atomic force microscopy (NC-AFM).10 Although this technique has been successfully employed to investigate organic thin films on insulating substrates,11−22 systematic high-resolution studies of individual molecules, well-separated at a surface are still rare. One reason © 2012 American Chemical Society
is that organic molecules usually bind weakly to insulating surfaces and are therefore often mobile at room temperature. Moreover, interaction with the tip can change their position and configuration during imaging at short tip−surface separations. Thus, resolving individual molecules and the atomic structure of the substrate simultaneously in order to identify the geometry of adsorption and paths for diffusion is still very challenging. In order to determine the exact adsorption site accurately from an experimental image of an individual molecule, atomic resolution of the substrate must be obtained simultaneously. Furthermore, one needs to assign the atomic scale features in the image to atomic species at the surface. Recently, pentacene and other relatively large molecules have been studied using NC-AFM on patches of ultrathin NaCl layers deposited on Cu(111).23 In this setup, tunneling was still possible, and suitable tips could be prepared on the bare metallic substrate in the STM mode. Certain atomic or molecular species were Received: November 28, 2012 Revised: December 12, 2012 Published: December 14, 2012 1105
dx.doi.org/10.1021/jp311702j | J. Phys. Chem. C 2013, 117, 1105−1112
The Journal of Physical Chemistry C
Article
(MExFM).30,31 Magnetic interactions only weakly affect the geometry, character of binding, and tip induced displacements of Co-Salen on NiO and will be discussed in a separate publication.
transferred from the surface to the tip apex using standard procedures to improve imaging. Such well-controllable procedures still do not exist for pure NC-AFM measurements on bulk-insulating surfaces. A recent review details many of these scanning probe techniques.23 Co-Salen (Co(C16H14N2O2)) is a complex that is known for its ability to reversibly coordinate molecular oxygen24 and is used in numerous applications in material chemistry and transition metal catalysis.25 Recently we investigated Co-Salen molecules adsorbed on the (001) surface of bulk NaCl with NC-AFM.26 Our study clearly demonstrated that using metallic tips allows one to determine the adsorption sites and the geometry of individual molecules on inert bulk insulating surfaces, such as NaCl(001). Combined with density functional theory (DFT) our results also demonstrated 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 surface images and thus determine the chemical nature of the molecular adsorption site. More generally, we proposed27 that clean metallic tips can be used to directly and unambiguously identify anions in atomically resolved NCAFM images of binary ionic insulators and other potentially more complex surfaces. In this paper we take the next step and study individual CoSalen molecules on a chemically more reactive and magnetically interesting bulk insulator, i.e., the (001) surface of the antiferromagnetic oxide NiO. The (001) surface of NiO is a well-defined charge neutral easy cleavage plane that can be prepared cleanly under ultrahigh vacuum conditions. This material has potential uses as a complement to titanium dioxide in devices such as dye sensitized solar cells,28 because it is one of the relatively few oxide materials that will support p-type conductivity.29 Although NiO has the same crystal structure as NaCl and is an ionic surface as well, adsorption, bonding, and growth are very different in the two systems. Our results demonstrate that Co-Salen binds to the NiO(001) surface with the central Co atoms adsorbing on top of oxygen ions in eight stable orientations with respect to the crystallographic axes of the substrate. This adsorption site is in agreement with the well-known oxygen affinity of Co-Salen, while the number of adsorption geometries reflects the chiral character of the molecule and the 4-fold symmetry of the surface. The relatively large binding energy results in a layer-bylayer thin film growth mode. In addition, we monitored the tipinduced mobility of the adsorbed molecules and compare our findings with our theoretical results regarding barriers for rotation and diffusion of the Co-Salen molecule on NiO(001). Interestingly, observed translational pathways correspond to alignment of the molecular axis with respect to crystallographic directions of the substrate. A comparison with adsorption on NaCl(001) reveals that the molecule exhibits twice as many stable orientations than on NiO(001), similar translational pathways, and a lower binding energy that results in island growth. By combining our experimental image data with theoretical calculations we identify the atomic geometry of adsorption to prove that our Cr tip interacts more strongly with the surface oxygen ions. We note that the (001) surface of NiO exhibits an antiferromegnetic alignment of magnetic moments at the nickel ions and Co-Salen is a spin 1/2 paramagnetic molecule in the gas phase. Therefore, it can be expected that the combined system possess interesting magnetic properties which presents a challenge for magnetic exchange force microscopy
■
EXPERIMENTAL METHODS All low-temperature experiments were performed with a homebuilt ultrahigh vacuum (UHV) force microscope32 at its base temperature (about 8.2 K). Room temperature AFM experiments were performed with the Multiprobe UHV AFM/STM from Omicron (www.omicron.de), Taunusstein, Germany. However, the FM mode of the microscopy was operated with a control unit from Nanonis (www.specs.de), Zürich, Switzerland. The substrate, NiO(001), crystallizes in the rock salt structure with a lattice constant of 417 pm. Predominantly due to strong correlation between d-electrons, NiO is an insulator with a band gap of approximately 4.3 eV.33 Below its Neel temperature of 525 K the spins of Ni atoms in neighboring (111) planes are ordered antiferromagnetically.34 The (001) surface is bulk terminated with a small rumpling.35 NiO(001) substrates were prepared by in situ cleavage of single crystals at a pressure of about 1 × 10−10 hPa and subsequently annealed at about 500 °C to remove residual charges. This procedure resulted in clean surfaces with typically 10 to 50 nm wide terraces separated by steps with the height of one (207 pm) or several interatomic distances. Subsequent low-density molecule deposition onto the cold substrate (about 27 K) took place directly in the low-temperature microscope kept in a cryogenic environment with a miniaturized crucible loaded temporarily into the cantilever stage.36 To study growth, larger amounts were deposited and investigated at room temperature. High resolution AFM measurements were performed in the noncontact mode (NC-AFM) using the frequency modulation technique (FM-AFM).37 In this mode of operation the cantilever self-oscillates with constant amplitude A0 at its resonance frequency f 0. Forces between tip and sample shift the actual cantilever frequency f by Δf = f − f 0. To image in the noncontact regime, where attractive forces dominate, a suitable negative Δf was chosen as set-point for the z-regulator. Scanning the tip line by line across the (x,y) sample surface while Δf is kept constant provides a z(x,y) map of constant tip−sample interaction, i.e., the topography. In atomically resolved images the topography reflects variations in the magnitude of the short-ranged forces between the foremost tip atom and the surface atom underneath. On polar surfaces metallic tips exhibit a stronger interaction with surface anions, i.e., Cl on NaCl(001).27 To unambiguously identify the atomic species, the tips of supersharp Si cantilevers (2 nm nominal tip radius, purchased from Nanosensors) were therefore coated in situ with a few nanometers of Cr, as it sticks well to the oxide layer present on the Si tip. To determine the contact potential difference between tip and sample, we recorded Δf(Ubias)curves and applied the voltage UCPD measured at the apex of the parabola to minimize the average long-range electrostatic interaction. These curves were also used to characterize the metal coated tips.27
■
THEORETICAL METHODS
Density Functional Theory (DFT) simulations were performed using the CP2K code and the mixed Gaussian and plane waves basis set.38,39 These include geometry minimization and diffusion and rotational barrier calculations. The density of 1106
dx.doi.org/10.1021/jp311702j | J. Phys. Chem. C 2013, 117, 1105−1112
The Journal of Physical Chemistry C
Article
Salen molecules on the atomically resolved NiO(001) substrate (corrugation amplitude: about 15 pm) is shown in Figure 1.
states of NiO(001) was also examined in the presence and absence of the Co-Salen molecule. To describe the interactions between Co-Salen and NiO(001), the B3LYP40,41 hybrid functional was selected. Initial calculations of bulk NiO using the GGA approximation and the PBE functional produced a band gap of 0.65 eV, which may result in unphysical charge transfer. Using the B3LYP hybrid functional we obtained a much more reasonable band gap of 3.6 eV compared to the experimental value of 4.3 eV.33 Previous calculations of CoSalen on NaCl were performed with a 6-31G basis on Co, 631G(d) basis on other heavy atoms, and 6-31G basis on hydrogen, and produced reasonable results with a basis set superposition error (BSSE) of 0.16 eV.26 Prior work on the TiO242 system suggests using the 86−411 CRYSTAL basis set for the oxide and 6-31G for the molecule as an initial starting point. This treatment of our system, however, results in a large BSSE43,44 of 1.1 eV with uncorrected adsorption energy of only 1.2 eV. Large BSSE values have indeed been observed in the past for the NiO system, and the use of extensive basis sets was needed to reduce the error to a manageable level45 and obtain a more accurate adsorption energy. In order to reduce BSSE, the MOLOPT46 basis set was employed. With this basis set the band gap of the NiO(001) surface was calculated to be 3.6 eV with a negligible rumpling of less than 1% and a surface Ni−O bond length deviation in agreement with experiment.47 These results indicate that this basis set is able to produce accurate surface properties while reducing the BSSE of the molecule surface interaction to less than 0.1 eV. The expense of using a hybrid functional combined with a need to minimize basis set superposition error (BSSE) by using an extensive basis set was mitigated by employing the auxiliary density matrix method.48 The auxiliary density matrix method employs an auxiliary basis set to greatly improve the efficiency of calculating Hartree−Fock exchange when using hybrid functionals. We employed an auxiliary basis set to calculate the electron repulsion integrals consisting of three uncontracted Gaussians for each angular momentum channel. Basis sets for all elements except Ni and Co were taken from the standard CP2K distribution (pFIT348). Ni and Co auxiliary basis sets were fitted to a small set of test molecules - it was confirmed that this approximation affected total energies of the small systems by
