H2Pc and pentacene on Cu(110)-(2×1)O: A combined STM and nc-AFM study
Graphical abstract
Introduction
The use of functionalized tips for non-contact atomic force microscopy (nc-AFM) is attracting significant interest for imaging organic semiconductors on surfaces, since nc-AFM is seemingly free of the complexities of interpretation inherent to scanning tunneling microscopy contrast. Pioneering work by Meyer, [1,[2], [7]], Mönig [3,4], and others [[5], [6]] showed that nc-AFM is able to provide precise molecular geometries with exquisite detail, resolving e.g. bonds within a molecule. Some reports have even suggested the possibility to image intermolecular and hydrogen bonds, though the origin of the image contrast is still under discussion [5,8,9]. Much of this work relies on the use of a CO-functionalized tip, where the imaging mechanism is largely a result of the flexibility of the tip, and the resulting tip-sample interactions may in fact be rather complex [9], [10], [11]. This suggests that new types of tip functionalization may shed light on prevailing contrast mechanisms in nc-AFM, thereby revealing the details of molecular adsorption of organic semiconductors on surfaces with increased fidelity.
The desire to find new nc-AFM imaging modalities has led recently to the introduction of CuO tips to probe molecules on surfaces [3,4]. Due to the rigidity of the CuO tip, there is less risk for overestimating bond lengths, for introducing apparent molecular distortions, or for generating artefactual bonding features, as may be the case for CO tips [3,9]. Nevertheless, the suggestion that CuO tips may instead underestimate bond lengths [3,4] indicates that at present a full understanding of molecular adsorption requires a combination of different imaging modalities. Moreover, asymmetries in the tip still influence the appearance of nc-AFM images, and consequently symmetric molecules may appear to be asymmetric.
While the study of molecular adsorption and the origin of scanned probe contrast mechanisms are of fundamental interest, there is also a real need to understand the impact of surface defects such as step edges or domain walls on surface-molecule interactions. Realistic surfaces in practical electronic devices may be highly defective, with many small terraces and steps, and one may expect a significant effect of a surface electronic structure that deviates from that of a perfect single crystalline surface. Some studies on miscut stepped or superlattice surfaces are already highlighting the extent to which the molecular electronic structure is influenced by nanostructure [12], [13], [14]. Other means to controllably manipulate local electronic structure of a surface may stem from creating chemical contrast e.g. due to partial oxidation [15], [16], [17], [18], e.g. by partial oxidation of the Cu(110) surface to form the Cu(110)-(2×1)O striped phase reconstructed surface.
Here, we study H2Pc and pentacene on the Cu(110)-(2 × 1)O striped phase reconstructed surface using STM and nc-AFM with a CuO-functionalized tip. To obtain a better understanding of surface-molecule interactions on such a nanostructured surface, it is necessary to assess to what extent there are different molecular adsorption geometries. We show that the combination of constant current STM with constant height STM, nc-AFM and simulations is able to determine the adsorption geometry and associated changes in electronic structure, a useful step towards revealing the interfacial electronic structure. Our study highlights the necessity for using a combination of different modes of STM and nc-AFM for a full interpretation of images gathered from these methods.
Section snippets
Materials and methods
Cu(110) was cleaned by repeated cycles of Ar+ sputtering followed by annealing. The Cu(110)-(2 × 1)O striped surface was formed by dosing O2 ( = 1.0 × 10−8 Torr) with the Cu(110) sample held at 373 K, followed by annealing to 623 K. Pentacene and H2Pc were both evaporated using home-built, water-cooled Knudsen cells onto the Cu(110)-(2 × 1)O reconstructed surface held at room temperature inside a sample preparation chamber (base pressure 1 × 10−9 Torr). For pentacene, the surface was briefly
Results
Partial oxidation of the Cu(110) surface forms a striped phase of Cu(110)-(2 × 1)O, with periodic arrangement of Cu and CuO domains. Annealing promotes the diffusion of oxygen atoms along the surface, which bond with Cu adatoms and rearrange themselves into CuO domains through substrate-induced strain and electronic repulsions between CuO chains [15,23]. The size of the CuO domains is determined by the amount of oxygen dosed, enabling facile control of the stripe period and width. The
Discussion and conclusions
The studies of H2Pc and pentacene on a nanostructured surface such as the Cu(110)-(2 × 1)O striped-phase reconstruction highlight the importance of using a combination of both constant-current and constant-height STM with nc-AFM using a rigid CuO tip as a necessary, powerful and unambiguous way to distinguish between different scenarios for molecule / surface interactions. In this context, nc-AFM experiments with a CuO tip are particularly useful since they are not subject to strong tip
CRediT authorship contribution statement
Angel Garlant: Methodology, Data curation, Writing - original draft. Bret Maughan: Methodology. Percy Zahl: Methodology. Oliver L.A. Monti: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare no conflict of interest for this work.
Acknowledgments
We acknowledge support by the National Science Foundation under grant # CHE-1565497. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. We would like to thank Nathan Bamberger for help with the electronic structure calculations.
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