CONDUCTAS INADECUADAS

For the DTE-conductance switch, the goal is ultimately to connect it from both sides with molec- ular wires. This is already programmed into the monomer unit discussed above, with the bromine intended to supply appropriate connection sites, according to the on-surface synthesis approach described in ch. 3.1. The switches equipped with two units of fluorene (labeled ”Fl2DTE” in

Fig. 4.8) represent an intermediate step in which the switching unit is connected from both sides to the shortest possible wire of the above type. The study of these molecules yields important insight because it circumvents complications in the linking process (cf. ch. 4.3) and can therefore serve as a reference system. The monomers furthermore don’t contain halogens that in the previ- ous section precluded the recording of differential conductance beyond the bias voltage sufficient to dissociate the carbon-bromine bond.

d

c

b

a

Figure 4.15: STM images of Fl2DTE(o) on Au(111). (a) one linear and two angled conformers, (b) one symmetric angled conformer, (c) and (d) differently ordered islands. Some of the molecules are highlighted by ’V’s. The border of a single molecule is outlined by the dashed line in (c) and (d). Image size 6 × 11 nm2

(a), 6 × 4.5 nm2

(b), 12 × 16 nm2

(c) and 17 × 17 nm2

(d). Feedback parameters for all: 500 mV, 0.1 nA.

The molecules were evaporated at a temperature of 205◦_{C at a rate of rate 0.4 ML/min. Imag-}

ing the molecules on the Au(111) surface, the central part appears like the one described for Br2DTE (o), displaying either the angled conformation with the bright central lobe or the linear

structure with intramolecular contrast, as shown in Fig. 4.15 (a). In addition, the molecules possess a bright protrusion on both ends which can be identified with the fluorene groups. The angled conformer can be transformed into a linear one and vice versa by means of manipulation with the STM tip. Furthermore, similar to the case of DBTF [56], the angle of the molecule is altered by rotation of the fluorene around the σ-bond connecting it to the phenyl group. This is most apparent by the difference in angles that is observed in the angled conformers. A symmetric angled conformation is observed, albeit with a very low frequency (less than 1 % in over 400 molecules). It can however be produced by lateral manipulation, as the example shown in Fig. 4.15 (b). The appearance of this conformer is consistent with the one sketched in Fig. 4.9 (b). While most molecules appear intact, some fragmentation has taken place and some of the monomers are found missing fluorene legs. The amount of fragments is around 10 % of the observed units. Considering that such a dissociation produces at least two fragments one can conclude that the amount of damaged molecules is closer to 5 %. These might give rise to a mobile species that is periodically visible between the static Fl2DTE islands. Thus, mostly intact

deposition of these minimally contacted switches is possible. The angled Fl2DTE molecules form

islands of limited order on the fcc regions of the herringbone reconstruction, such as the ones shown in Fig. 4.15 (c) and (d). While the island in (c) derives from the typical zigzag stacking (cf. the three units at the lower right corner) and displays open pores, the island in (d) appears close-packed with a windmill-like motif as highlighted by the arrow heads. Interestingly, there

doesn’t appear to be any difference in the appearance and arrangement between preparations of the open- and closed-ring isomers. This behavior is in partial accordance with the results for the brominated switches, in that the preparation of Br2DTE (c) would yield monomers displaying

the angled conformation that is only compatible with the ring-open isomer. On the other hand, a flat and featureless species such as the ones in the double row shown in Fig. 4.14 has not been observed for neither form of Fl2DTE.

bias voltage (mV)

0

1000

2000

3000

-1000

-2000

-3000

dif

ferential conductance (arb. u.)

tunneling current (nA)

0.5

0

-0.5

-1.0

-1.5

-2.0

HOMO

LUMO

Figure 4.16: Tunneling current (blue) and differential conductance signal (red) recorded over a linear monomer of Fl2DTE (raw data, lock-in modulation amplitude = 10 mV, frequency = 723 Hz). The inset shows the STM image recorded beforehand (image size 7 × 7 nm2

). The position of the tip during the spectroscopy measurement is indicated by the cross. HOMO and LUMO are indicated by dashed lines.

The identification of the angled conformers with the open-ring switch is straight forward for geometric reasons and is corroborated by the interconvertibility into the linear form by lateral manipulation. The assessment of the linear species on the other hand is more complicated, because its shape is compatible with both the open- and closed-ring forms. As will be shown, the recording of differential conduction (dI/dV) traces allows the unambiguous assignment of the respective state to a particular monomer. A characteristic curve recorded on the center of a linear Fl2DTE is shown in Fig. 4.16. The states of the linear conformers are found at

EHOMO,lin= (−1.25 ± 0.05) eV and ELUMO,lin= (+2.5 ± 0.1) eV, whereas the angled conformers

display theirs at EHOMO,ang= (−1.12 ± 0.08) eV and ELUMO,ang= (+2.5 ± 0.2) eV. Thus, the

surprising and central result is that the features are the same for spectra taken on the linear as well as the angled conformers. Furthermore, they are identical for the preparations of open- and closed-ring Fl2DTE. Since the angled conformer is incompatible with the closed species and

since from experiments in solution the cyclization is expected to lead to a considerable change in the HOMO-LUMO gap, it can be concluded that the observed isomer of Fl2DTE is always

the ring-open form. Thus, it seems to be the case that the isomers do not stay in the closed-ring form, but are modified in the evaporation process, similar to what was observed for Br2DTE (c).

The understanding of the appearance and more importantly the spectroscopic signature of the monomers constitute an important prerequisite for gaining control of the switching functionality of the DTE. While the features in the spectrum shown in Fig. 4.16 reflect read-out of the open (”Off”) state, it remains to be seen if the closed (”On”) state can be produced and if its properties differ sufficiently from its isomer. However, before addressing this issue I will turn to the coupling of the monomers to covalently bonded structures, since the ultimate goal for the switches is to include them into a network mimicking a circuit-like architecture. The incorporation of functional units into a network has not been demonstrated so far. Most importantly, the properties of the switching center must not be adversely affected by the connection to not suppress its functionality.