Hindered surface diffusion of bonded molecular clusters mediated by surface defects

The design of low dimensional materials through surface assisted self-assembly requires a better understanding of the factors that limit and control surface diffusion. We reveal how substrate surface defects hinder the mobility of submonolayer organic adsorbates on a metal surface with the model CuPc/Cu(111) system. Postdeposition annealing bonds CuPc molecules into dendritelike clusters that are often mobile at room temperature. Surface defects on Cu(111) create energetic barriers that prevent CuPc cluster motion on the metal surface. This phenomenon was unveiled by the motion of small clusters that show rigid-body diffusion solely in the available space in between defects. When clusters are sufficiently surrounded by defects, they become completely pinned in place and become immobilized.

Figure 1

(a) Annealed 0.25-ML CuPc on Cu(111) $\left(500\times 500{\AA }^2\right)$. Specific parallel (b) and angular (c) bonding orientations $\left(50\times 50{\AA }^2\right)$. Atomic model (d) and STM simulation (e) of parallel bonding. Atomic model (f) and STM simulation (g) of angular bonding. (h)–(j) Different CuPc-Cu adatom coordination structures $\left(50\times 50{\AA }^2\right)$. STM simulations at +0.9 V. Imaged +0.9 V and 0.5 nA.

Figure 2

0.15-ML CuPc on Cu(111) with postdeposition annealing. (a)–(e) Rotational configurations of a CuPc cluster ($-1.0\mathrm{V}, 0.5\mathrm{nA}, 200\times 160{\AA }^2$). The stable positions are indicated by white and black arrows. Diffusion back and forth between different orientations was observed across 17 sequential images recorded over a 3.5-h period. Translations by another cluster are also indicated by blue arrows. (f)–(h) Interaction between different CuPc clusters across three sequential scans highlighted with red arrows (−1.1 V, 0.5 nA, $250\times 200{\AA }^2$).

Figure 3

Addition of 0.15-ML CuPc without annealing. (a) STM scan of annealed CuPc before additional deposition (−2.0 V, 0.1 nA, $600\times 350{\AA }^2$). (b) STM scan after additional deposition of CuPc (−1.2 V, 0.5 nA, $600\times 350{\AA }^2$). White arrows indicate mobile clusters and black arrows indicate substrate surface defects.

Figure 4

Relative stability of CuPc across different interaction schemes. CuPc adsorption on Cu(111) is used as the reference energy. The energy ranges come from taking the range of adsorption energies from several different geometries (details in Ref. [35]). Negative energies are more stable.

Figure 5

(a)–(c) Sequential scans that track mobile cluster movement. From panel (a) to (b) the topmost cluster moves until it is pinned by surface defects outlined in white. Another cluster, enclosed by defects outlined with blue, disappears in panel (b). From panel (b) to (c) motion of another cluster is hindered by defects outlined in red. All imaged at −1.0 V, 0.4 nA, $300\times 180{\AA }^2$.

Figure 6

Surface defects on Cu(111) before (a) and after (b) a tip change (−1,2 V, 0.2 nA, $120\times 80{\AA }^2$) (c) Relative adsorption energies of CuPc near and on top of S defects. (d)–(f) Three relaxed geometries of CuPc by S on Cu(111).