Proceso de Fabricación en la Planta

The presence of alkyl side-chains was known from previous studies of DP-PTCDI to enhance the stability of a chiral molecular trimer vertex (illustrated in Figure 5.20) as compared with the linear hydrogen-bonded PTCDI-PTCDI junctions [104]. A comparison of Figures 5.14, 5.21 and 5.26 indicates a progres-sively enhanced stabilisation of a trimer vertex as the alkyl chain length increased from zero length (PTCDI) through to the longest chain (DB-CTCDI). This led to a change in morphology from the rows, for PTCDI, through to the honey-comb DB-CTCDI, with the DP-PTCDI being considered as an intermediate case where linear segments co-existed with junctions of three molecules. The junctions formed and their positions relative to the Moir´e superstructure are summarised in Figure 5.30 for each of the three molecules.

The enhanced stability of the trimer vertex for molecules with longer alkane chains was supported by calculations performed using density functional theory (DFT) – all DFT calculations detailed in this thesis were performed by N. Smith and A. Saywell. The molecules were modelled in the gas phase, with the substrate not included in the calculations. Further details on the DFT calculations can be found in the Supplementary Material of Reference [102]. The results of the DFT calculations are shown in Table 5.1, which reveals that the greatest difference between trimer and dimer binding energies was predicted for DB-CTCDI. These data also indicate that, in the gas phase, the trimer was more stable for DP-PTCDI and even for PTCDI. Overall the stability of the trimer junction increased as the alkyl chain length increased, which was observed in the STM images.

Also tabulated in Table 5.1 are the calculated separations of the intermolecular junctions. For dimers formed from all of the molecules investigated the calculated equilibrium separation was very close (within 0.03 nm) to am/2 (the observed spacing of molecules in the commensurate rows was 1.47±0.03 nm). However, the predicted separation for intermolecular trimer junctions (shown in Table 5.1) was significantly lower (by up to 0.1 nm) than their expected separation in an extended array, a_m/√

3 (1.70 nm). Thus, it is believed that these junctions were strained in an extended honeycomb array, leading to a reduced binding energy, and accounting for the stability of dimer rows for the two molecules PTCDI and DP-PTCDI, whilst only the molecule with the strongest predicted trimer junc-tion, DB-CTCDI, formed a honeycomb array. There were also clear gaps between

Figure 5.30: Schematics of the PTCDI dimer junction (a), the resulting PTCDI commensurate rows (b), the DP-PTCDI dimer junction (c), the DP-PTCDI trimer junction (d), the DB-CTCDI trimer junction (e) and the positioning of

the DB-CTCDI molecules (f), all formed on the Moir´e superstructure of the g/Rh(111) surface. The distance between molecules for dimer bonding, d, and

distance from the centre of the trimer vertex to the centre of a molecule, r, is also shown.

Chapter 5. Graphene Formation on Rh(111) Thin Films 137

Table 5.1: Binding energies for PTCDI, DP-PTCDI and DB-CTCDI dimer (E^d) and trimer (E^t) junctions calculated using DFT. The dimer and trimer lengths, d and r (as depicted in Figure 5.30), are also calculated. The difference in energies

for the dimer and trimer junction for each molecule are also shown.

some of the DB-CTCDI trimers that formed the honeycomb networks, shown in Figure 5.29, further evidence of strain arising from a small mismatch between su-perstructure and molecular dimensions for the trimer array. This may also account for how molecules were able to sit in neighbouring rows but did not appear to be hydrogen-bonded.

The van der Waals contributions to the interaction energies of the proposed dimer and trimer junctions – that were calculated using DFT – for the three different molecules were also investigated and are discussed in the Supplementary Information of Reference [102]. Briefly, the van der Waals interaction led to slightly higher energies (∼0.2 eV) for trimer junctions involving molecules with side chains, with an increase in energy as the side chain length increased. Thus the trends observed in the DFT results in Table 5.1 – where only hydrogen-bonding energies were taken into account – were further enhanced by the van der Waals interaction.

The STM images of the three adsorbed molecules on the g/Rh(111) surface clearly show that the adsorbed molecules experienced a local potential due to the graphene superstructure which was sufficiently strong to inhibit the formation of two-dimensional islands. The origin of this potential has recently been discussed by Brugger et al. [29] who showed that variations in work function for both gra-phene and boron nitride monolayers on Ru(0001) led to a periodic potential – also discussed by Dil et.al. [26] for the BN/Rh(111) surface. For graphene on Ru(0001)

compliant to allow the molecules to relax and adopt a local configuration, which was controlled by interactions with molecules trapped in neighbouring energy min-ima, so that extended, connected structures were formed. The formation of the molecular rows was determined by intermolecular interactions but the placement and separation of the rows was guided by the underlying level of organisation of the Moir´e pattern. The control of row separation is shown in Figure 5.14 and more clearly for a higher molecular coverage in Figure 5.17 (the coverage was close to two molecules per unit cell of the Moir´e superstructure) where it was clearly observed that the surface was covered with regularly spaced molecular rows separated by

√3am/2.

This insight also raises interesting questions in relation to graphene electronics.

Recent papers have demonstrated that molecules can act as molecular dopants [105]

and also that the superstructure arising from a Moir´e pattern of graphene grown on Ir(111) gives rise to the formation of a band-gap [106] raising interesting con-nections with the self-assembled molecular structures detailed here. The interplay between the characteristic dimensions arising from graphene growth and molecular ordering offer many possible routes for further investigation related to both the electronic properties of graphene as well as the formation of complex self-assembled structures.