Experimental details and sample preparation

4.2 Experimental details and sample preparation

For FM-AFM measurements the apex of the tip was functionalized with a CO molecule [82] except for the temperature dependent mea- surements. All AFM images were recorded by measuring the frequency shift while scanning in constant-height mode. The bias voltage V is applied to the sample. All spectra were acquired atop of the center of the molecules. While taking differential conductance (dI/dV ) spectra the cantilever oscillation drive signal was switched off, while during FM-AFM data acquisition any bias modulation was switched off. For the experiments at elevated temperatures the temperature depen- dence of the expansion coefficient of the piezo-electrical elements and the sensor’s sensitivity has been calibrated from substrate defect step heights and from simultaneous STM/AFM measurements at various oscillation amplitudes. Upon increasing the temperature from 5.2 to 13.9 K the expansion coefficient of the piezo-electrical elements and the sensor’s sensitivity change by slightly less than 15% and 10%, re- spectively. These temperature dependencies have been accounted for. For more details see section 2.8. The sensor is thermally coupled to the helium bath cryostat by a separate wire and may therefore be colder than the sample.

The molecule under investigation, 1,6,7,12-tetraazaperylene (TAPE)- [162] (for its structure see Figure 4.2) belongs to the bis(α, α0-diimine)- type of ligands. It is a planar D2h-symmetric molecule and it is helpful

to consider that its C-H groups qualify as weak hydrogen bond donors, so that many stabilizing C-H· · · N contacts can be formed through an appropriate 2D assembly (Figure 4.2a).

TAPE molecules were thermally sublimed onto a Ag(111) surface held at low temperature with submonolayer coverage. Subsequent anneal- ing to 280 K resulted in self-assembled islands with a periodic struc- ture, in which each molecule is rotated by roughly Θ = 80◦ with re- spect to its four neighbors, as confirmed by atomically resolved AFM

104° 30° 68° 52° 104° 104° 60° 60° 121° 60° 104° 76° 120° 60° 76° 52° 8° 8° TAPE island TAPE chain [110] [211]

a

b

Figure 4.1: (a) Set of topographic images of Ag(111) after deposition of TAPE

and annealing to 280 K. Directions of some chains are shown as yellow lines for a better visualization, with the measured angles indicated. Inset shows one of the Ag slip-step we used for the surface orientation determination. (b) Model of Ag(111) (atoms are represented as grey circles) and all possible orientations of TAPE islands on the surface. The measured angles in (a) can be rationalized based on the model. Large cells consisting of 8 unit cells are shown.

images (see Figure 4.8c). Due to clearly alternating molecular orien- tations within the layer, it can be described by oblique lattice with a cell containing two molecules. Upon annealing we also observed some linear chain-like molecular assemblies. These are composed of few up to hundreds of monomer units and most likely are connected by coordination bonds with Ag adatoms. Their properties will not be discussed in the framework of this thesis.

4.2 Experimental details and sample preparation Evaluation of the azimuthal orientation of the molecular islands on Ag(111) surface relative to the chains, given in a set of the STM im- ages in Figure 4.1a, reveals that the islands edges adopt angles of 52◦ (8◦, 68◦) with respect to the chains’ directions, whereas the chains are rotated by multiples of 60◦1. The angle between two adjacent molec- ular island edges is 104◦ or 76◦. Apparently, the molecules take up only certain positions with regard to the substrate, because other- wise many different angles would be expected. This also means that unit cell vectors should fit to the substrate lattice. Note that, the existence of a definite adsorption site cannot be taken for granted: a large molecule which averages over few periods of the substrate corru- gation potential is not in general expected to adsorb site-specifically. It is known that on weak-interacting metals e.g. on Au(111), many molecules form incommensurate overlayers, which then overgrow the reconstruction of the surface without being influenced by its complex- ity. On the other hand, on Ag(111) the molecule-substrate interaction is slighty stronger and may have as a result definite adsorption sites. The orientation of both islands and chains in respect to the high- symmetry directions of substrate can be determined from atomically resolved STM images of the bare terraces. As atomic resolution of the closed-packed Ag(111) surface could not be obtained under conven- tional imaging conditions in STM, some areas with slip steps running along the closed packed <110> directions (green arrow in Figure 4.1a) allow us to propose orientation of the molecar island in respect to high-symmetry axes as shown in Figure 4.1b (other regions follow no specific surface direction, and therefore are wavy). Our experiments were complemented by DFT calculations by our collaborators from CFM/MPC, Spain, and Instituto de Física de Rosario, Argentina. Here, we will shortly describe their findings: The unit cell contains two molecules lying flat on the surface defined by the vectors a and b with lenghts 4 D and

√ 172

2 D, respectively, with an angle between them

of 52.4◦. This is in a very good agreement with our measurements.

1

Inside the 2D assembly of the island, molecules experience an identical environment, so they can be viewed as being equal, each appearing as one protrusion (see Figure 4.2a). However, along the edges of the island, the molecules alternate between exposing four C-H groups and their α, α0-diimine side to the outside. The latter molecules, labeled here as type Q, have two lone-pairs of the nitrogen atoms unmasked along the periphery, and exhibit a distinctly larger apparent height in STM images than all other molecules, labeled as type A. Their differ- ent appearance is not related to distinct adsorption sites or in-plane orientations of the molecules relative to the high-symmetry directions of the substrate, but only due to their molecular neighborhood that decides upon which molecule takes the role of A and which of Q type.