Activation of 2D polymerisation with atomic quantum clusters on insulating and metal surfaces

Session

Atomic and Molecular Resolution Phenomena via AFM, STM and Scanning Probes

Authors

Alessio Quadrelli (1), Leonardo Forcieri (1), Qingqing Wu (1), Songjun Hou (1), Barry Mangham (3), Neil Champness (2), David Buceta (4), Manuel Arturo Lopez-Quintela (4), Colin Lambert (1), Samuel Paul Jarvis (1)

Affiliations

1. Lancaster University
2. University of Birmingham
3. University of Nottingham
4. University of Santiago de Compostela

Keywords

molecules, 2D materials, 2D polymers, on-surface polymerisation, high-resolution AFM, simulations,DFT, x-ray photoelectron spectroscopy, atomic quantum clusters, catalysis, insulating surfaces, metal surfaces

Abstract text

On-surface polymerisation is routinely used to produce a wide variety of covalently stabilised 1D and 2D molecular structures at surfaces¹. Despite rapid progress in this field, the fabrication of these polymers on non-metal surfaces remains comparatively slow, particularly using thermal methods. Without a catalyst, precursor molecules sooner desorb from insulating surfaces before they can polymerise, greatly limiting our ability to prepare surface polymers on non-metal substrates where their properties could be better exploited. 

Here we report on-surface 2D-polymerisation on insulating and metal substrates using catalytically active copper atomic quantum clusters (AQCs). These unusual AQCs consist of just five copper atoms stabilised by an oxide layer² (Cu₅[O₂]_n), which we characterise using a combination of AFM, XPS, and normal incidence X-ray standing wave (NIXSW). Following this, we report the polymerisation of tetra (4-bromophenyl) porphyrin (Br₄TPP) (Figure 1a) on mica and Au(111) substrates. Temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) measurements show that Cu₅[O₂]_n clusters substantially reduce the activation temperature for polymerisation, resulting in a polymer layer that survives well beyond the desorption temperature of the single monomers (Figure 1b,c). Polymer formation is supported by high-resolution AFM in ambient conditions (Figure 1d) and by extensive density functional theory (DFT) and climbing image nudged elastic band (CI-NEB) calculations which reveal a strong dependence between the catalytic activity of Cu₅[O₂]_n AQCs and their stabilizing oxygen layer. 

These results provide a new route for activating on-surface polymerisation on insulating surfaces that could be extended to other surfaces such as SiO₂ and TiO₂. Furthermore, we establish ambient condition AFM as a tool to investigate the reaction products in systems inaccessible by Scanning Tunnelling Microscopy. 

Figure 1. (a) Schematic of Br₄TPP polymerisation catalysed by Cu₅[O₂]_n. TP-XPS plot of the Br 3p region for Br₄TPP on mica (b) without and (c) with Cu₅[O₂]_n atomic clusters, respectively.   The addition of Cu₅[O₂]_n reduces the dehalogenation temperature by ΔT=120±30 °C. (d) High-resolution AFM image of Br₄TPP polymer on mica prepared with Cu₅[O₂]_n clusters acquired in ambient (scalebar equals 10 nm).

References

1. Grill, L. & Hecht, S. Covalent on-surface polymerization. Nat. Chem. 12, 115–130 (2020).

2. Huseyinova, S. et al. Synthesis of Highly Stable Surfactant-free Cu5 Clusters in Water. J. Phys. Chem. C 120, 15902–15908 (2016).