Extended iron phthalocyanine islands self-assembled on a Ge(001):H surface

Abstract

Self-assembly of iron(II) phthalocyanine (FePc) molecules on a Ge(001):H surface results in monolayer islands extending over hundreds of nanometers and comprising upright-oriented entities. Scanning tunneling spectroscopy reveals a transport gap of 2.70 eV in agreement with other reports regarding isolated FePc molecules. Detailed analysis of single FePc molecules trapped at surface defects indicates that the molecules stay intact upon adsorption and can be manipulated away from surface defects onto a perfectly hydrogenated surface. This allows for their isolation from the germanium surface.

Keywords: hydrogenated semiconductor; iron phthalocyanine (FePc); scanning tunneling microscopy; self-assembly.

Figure 1

Typical STM appearance of a Ge(001):H surface with atomic-scale defects. (a, b) Filled- and (c) empty-state STM images. The yellow dashed circles indicate a single DB, while the red circles mark a DBD. In panel (c), the characteristic “butterfly” image of the DBD is shown. Imaging conditions: bias voltage −2.0 V (a), −0.5 V (b) and +1.5 V (c); tunneling current: 2 pA.

Figure 2

Empty-state STM image of individual FePc molecules and an extended molecular island self-assembled on Ge(001):H. White circles mark individual FePc molecules trapped at surface defects. Yellow circles mark single DBs with a clearly discernible dark halo surrounding them due to single electron charging. Red circles indicate isolated DBDs exhibiting the characteristic “butterfly” appearance. Imaging conditions: bias voltage +2 V, tunneling current 50 pA.

Figure 3

FePc island on a Ge(001):H surface. (a) Filled-state STM image of the island. The white arrow indicates the discontinuity of the FePc island image. (b) Height profile of the FePc island measured along the green line in (a). The apparent height of the island reaches approximately 1.05 nm, which indicates the upright orientation of FePc molecules within the island. (c) Structural scheme of FePc (bottom) with schematic appearance of the STM contrast for upright-oriented molecules within the islands (top view). The differently shaded lobes mimic the contrast variation of the STM appearance due to a slight rotation of the molecules. (d) High-resolution STM image of the CuPc island with clearly visible different domains. One white dashed line indicates the direction of surface-reconstructed rows, while the second white line divides the FePc island into two parts, in which molecular columns are oriented along mirrored directions. Black parallelograms indicate anticipated images of single FePc molecules. Red and blue parallelograms show repeated units of two different domains. The black arrow indicates the place where the island is expanded by one additional FePc column. (e) Magnification of the area marked by a violet dashed rectangle in (d) with unit cells and assignment of the STM appearance of FePc molecules within the island. (f) STM image of the island with clearly noticeable side extension composed of two rows of lobes indicated by the dashed white lines. (g, h) Simplified structural models of different domains indicated in (d) and (e). The variation of lobe contrast mimics differences in the STM contrast. STM imaging conditions: bias voltage −2 V (a, e), tunneling current 100 pA (a, d, e, f).

Figure 4

Schematic illustration of the two observed FePc islands on Ge(001):H (top) along with the molecular columns in the α and β phases of FePc (bottom). Images show top views. The differently colored molecules in the “blue” model correspond to molecules differently visualized in STM measurements.

Figure 5

A single-point STS spectrum acquired on a FePc island. The gap measured with STS reaches approximately 2.70 eV, the dot in the inset shows the lateral position of the tip during STS measurements.

Figure 6

Single FePc molecules trapped at surface defects on a Ge(001):H surface. The white dashed circles indicate the molecule of interest. Molecules visualized in panel (a) are unstable and appear as fuzzy features with clearly discernible lateral displacement events. (a, b) The molecule marked by a white circle is removed from the defect and placed onto a perfectly hydrogenated area. Panel (b) shows the appearance of the molecule during upward scanning. The slow scan direction is marked by a white arrow on the right. (c) High-resolution STM image. The molecule marked by a white circle is located on a perfectly hydrogenated Ge(001):H surface and exhibits the typical appearance of isolated FePc molecules. Different atomic-scale surface defects that can be discerned are marked by dashed circles (i.e., a single DB (yellow) and DBDs (red)). The white dashed rectangles in (a) and (c) indicate the area visualized in (b). Imaging conditions: bias voltage −2.0 V; tunneling current 30 pA.