Elsevier

Surface Science

Volume 696, June 2020, 121590
Surface Science

H2Pc and pentacene on Cu(110)-(2×1)O: A combined STM and nc-AFM study

https://doi.org/10.1016/j.susc.2020.121590Get rights and content

Highlights

  • Concurrent use of STM and nc-AFM with stiff functionalized tip for full interpretation of substrate/molecular interactions and adsorption geometry.

  • Approach for determining molecular geometries and adsorption sites for π-conjugated molecules with multiple adsorption modalities.

  • CuO tip provides stiff tip for high-resolution nc-AFM imaging.

Abstract

Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) with a CuO tip is used to investigate adsorption of metal-free phthalocyanine (H2Pc) and pentacene on the Cu(110)-(2×1)O striped-phase reconstructed surface. We show that the combination of STM and nc-AFM is necessary to reveal the detailed adsorption geometry, necessary for interpreting the observed STM contrast. Comparison of the nc-AFM images with simulations shows that some of the H2Pc molecules are deformed out-of-plane on this surface, while pentacene retains its molecular geometry regardless of adsorption site. Our work highlights how the stiffness of CuO tips makes it possible to probe the structure of organic semiconductor / metal interfaces.

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 (pO2 = 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.

References (40)

  • J. Kröger et al.

    Self-organization of cobalt-phthalocyanine on a vicinal gold surface revealed by scanning tunnelling microscopy

    Surf. Sci.

    (2007)
  • J. Götzen et al.

    Absence of template induced ordering in organic multilayers: the growth of pentacene on a Cu(221) vicinal surface

    Surf. Sci.

    (2011)
  • L. Gross et al.

    The chemical structure of a molecule resolved by atomic force microscopy

    Science

    (2009)
  • L. Gross et al.

    Bond-order discrimination by atomic force microscopy

    Science

    (2012)
  • H. Mönig et al.

    Submolecular imaging by noncontact atomic force microscopy with an oxygen atom rigidly connected to a metallic probe

    ACS Nano

    (2016)
  • H. Mönig et al.

    Quantitative assessment of intermolecular interactions by atomic force microscopy imaging using copper oxide tips

    Nat. Nanotechnol.

    (2018)
  • J. Zhang et al.

    Real-space identification of intermolecular bonding with atomic force microscopy

    Science

    (2013)
  • D.G. de Oteyza et al.

    Direct imaging of covalent bond structure in single-molecule chemical reactions

    Science

    (2013)
  • L. Gross et al.

    Organic structure determination using atomic-resolution scanning probe microscopy

    Nat. Chem.

    (2010)
  • A.M. Sweetman et al.

    Mapping the force field of a hydrogen-bonded assembly

    Nat. Commun.

    (2014)
  • C.-S. Guo et al.

    Origin of the contrast interpreted as intermolecular and intramolecular bonds in atomic force microscopy images

    J. Phys. Chem. C

    (2015)
  • L. Gross et al.

    High-resolution molecular orbital imaging using a p -wave STM tip

    Phys. Rev. Lett.

    (2011)
  • C.-S. Guo et al.

    High-resolution model for noncontact atomic force microscopy with a flexible molecule on the tip apex

    J. Phys. Chem. C

    (2015)
  • C. Baldacchini et al.

    Symmetry lowering of pentacene molecular states interacting with a Cu surface

    Phys. Rev. B

    (2007)
  • K. Kern et al.

    Long-range spatial self-organization in the adsorbate-induced restructuring of surfaces: Cu/110)-(2×1)0

    Phys. Rev. Lett.

    (1991)
  • B. Feng et al.

    Evidence of silicene in honeycomb structures of silicon on Ag(111)

    Nano Lett.

    (2012)
  • S.M. Mohapatra et al.

    Investigation of the electronic structures and associated properties including hyperfine interactions for halogen-adsorbed silicon surfaces: Fluorine through iodine

    Phys. Rev. B

    (1988)
  • R.D. Schnell et al.

    Electronic properties and bonding sites for chlorine chemisorption on Si(111)-(7×7)

    Phys. Rev. B

    (1985)
  • M. Abadía et al.

    Massive surface reshaping mediated by metal–organic complexes

    J. Phys. Chem. C

    (2014)
  • A. Verdini et al.

    Water formation for the metalation of porphyrin molecules on oxidized Cu(111)

    Chem. - Eur. J.

    (2016)
  • Cited by (4)

    • Adsorption and reaction of an alkyne molecule on diverse oxygen-reconstructed Cu(110) surfaces

      2022, Surface Science
      Citation Excerpt :

      To the best of our knowledge, the adsorption and reaction of alkyne molecules on metal oxide surfaces, which are technologically more relevant, remain largely unexplored. Cu(110) is an intriguing surface since its intrinsic atomic corrugations usually encourage rod-like molecules to be oriented along close-packed [1–10] directions [32,33]. For example, dosing acetylene onto Cu(110) preheated at ∼450 K led to the yield of Cu carbynes inside channels along [1–10] [34].

    View full text