Perylene tetra-carboxylic diimide (PTCDI) was the first organic molecule investigated on the BN/Rh(111) surface – PTCDI has been described in detail in Section 3.4.2 of Chapter 3 and the PTCDI molecule is shown in Figure 4.6.
Figure 4.6: A perylene tetra-carboxylic diimide molecule.
PTCDI molecules were deposited onto the BN/Rh(111) surface in UHV using a Knudsen cell operating at 355◦C, depositing PTCDI onto the surface at a rate of
∼3×10−3 MLmin−1. After PTCDI deposition, the surface was investigated with an STM in-situ and close-packed PTCDI islands were observed on the surface, as shown in Figure 4.7. Irregular-shaped PTCDI islands were revealed to occupy a large percentage of the BN/Rh(111) surface (as shown in Figure 4.7a) and there were also instances of single and multiple PTCDI molecules trapped in the pores of the corrugated boron nitride monolayer (as shown in Figure 4.7b).
When imaging the PTCDI islands on the BN/Rh(111) surface with STM, the islands were commonly observed to follow the contours of the corrugated h-BN layer (Figure 4.7) as might be expected. Due to the directional nature of the hydrogen-bonding between the PTCDI molecules, monolayer PTCDI islands with
Chapter 4. Boron Nitride Nanomesh on Rh(111) Thin Films 97
Figure 4.7: STM images of PTCDI adsorbed on a BN/Rh(111) surface as a comparison of the PTCDI islands formed. (a) large scan size image with multiple PTCDI islands observed, (b) small scan size image with PTCDI molecules within the islands resolved and revealing both PTCDI molecules aligned with the Moir´e superstructure and molecules misaligned, (c) PTCDI islands that follow the contours of the surface and show varying orientations of neighbouring islands and (d) further PTCDI deposition leading to higher PTCDI
island coverage. Images acquired using an STM supplied by WA Technology, imaging parameters for (a) Vbias= 1.0 V, It= 50 pA, (b) Vbias= -1.7 V, It=
30 pA, (c) Vbias= 2.3 V, It= 50 pA and (d) Vbias= 2.3 V, It= 30 pA.
The deposited PTCDI molecules were commonly observed in the pores formed by the corrugated h-BN monolayer, as shown in Figures 4.7 and 4.8. This was similar to the behaviour of naphthalocyanine deposited on a BN/Rh(111) surface as these molecules were also revealed to be trapped due to their comparable size with the nanomesh pores [14]. The BN/Rh(111) surface has also been found to act as a template for Au nanoparticles [23]. The molecules generally occupied the majority of the pores on the surface even when PTCDI islands were also present on the surface. It was difficult to ascertain the preference between the trapping of molecules in pores or 2D island formation, but the molecules did not appear to fill all of the pores of the nanomesh before 2D islands were formed. 2D islands that were tens of nanometres across were observed whilst up to approximately 30% of the visible nanomesh pores did not contain PTCDI molecules.
The pores trapped both single PTCDI molecules and more than one molecule on an approximately 10:1 ratio respectively. However, the number of PTCDI molecules in each pore was difficult to distinguish due to the intramolecular double lobe contrast of the highest occupied molecular orbital (HOMO) of the PTCDI molecules (as described in Reference [88]) coupled with the limited resolution of the STM. Only exceptional images revealed the exact number of molecules, Figure 4.8d as an example. STM images in Figure 4.8 show frequent manipulation of the PTCDI molecules from one pore to a neighbouring pore on the same axis as the STM tip raster motion. This can produce a mass redistribution of PTCDI molecules within the pores themselves as seen in Figure 4.8, where Figure 4.8b is an STM image acquired several scans after the image in Figure 4.8a, and where many pores that were previously empty have become filled. It should be noted that this effect was not seen in previous naphthalocyanine experiments on the same surface [14].
As previously described, it was possible to control the coverage of the close-packed PTCDI islands by altering the duration of PTCDI deposition – the cover-age increased with deposition time. However, the covercover-age of the PTCDI islands
Chapter 4. Boron Nitride Nanomesh on Rh(111) Thin Films 99
Figure 4.8: STM images of PTCDI adsorbed on a BN/Rh(111) surface where PTCDI molecules are both trapped in pores and formed 2D islands, as shown in
(a) and (b). (c) annealing the sample desorbed the PTCDI molecules, reducing the coverage of the PTCDI islands until only the more strongly bound PTCDI molecules in the pores of the BN/Rh(111) surface were present. (d) examples of
both single and pairs of molecules trapped in the pores. Images acquired using an STM supplied by WA Technology, imaging parameters for (a) Vbias= 1.7 V, It= 30 pA, (b), (d) Vbias= -2.0 V, It= 30 pA and (c) Vbias= 2.0 V, It= 30 pA.
anneals rather than both occupied pores and PTCDI islands in the same areas (as shown in Figures 4.7 and 4.8), although there are indications of clusters of molecules that may have previously been part of a PTCDI island. Figure 4.8d shows a small scan size STM image of the surface after a low temperature anneal, revealing single PTCDI molecules trapped in the h-BN pores as well as two PTCDI molecules in one pore, as a more conclusive example of multiple molecules in one pore. However, the double-lobe contrast usually observed for PTCDI molecules is not resolved and therefore the orientations of the PTCDI molecules are unknown.
Figure 4.9: (a) STM image of PTCDI adsorbed on a BN/Rh(111) surface where PTCDI molecules trapped in the pores of the corrugated monolayer are observed
and (b) schematic of the proposed PTCDI molecule placement in the pores and orientation as observed in (a). Image acquired using an STM supplied by WA
Technology, imaging parameters Vbias= 2.0 V, It= 30 pA.
Figure 4.9a is an STM image of a BN/Rh(111) surface after PTCDI deposition and subsequent annealing to produce PTCDI molecules primarily trapped in the pores of the boron nitride monolayer, many of the PTCDI molecules imaged dis-play a double lobe contrast. Thus, the orientation of the PTCDI molecules trapped in the pores was determined as a random distribution. The PTCDI molecules were frequently orientated with their functional ends directed towards an edge of the
Chapter 4. Boron Nitride Nanomesh on Rh(111) Thin Films 101
h-BN pores, however, they did not show a net orientation. The PTCDI molecules were generally positioned towards the edge of the pores, as shown schematically in Figure 4.9b, where the light blue hexagonal lattice represents the Moir´e super-structure and the pores of the h-BN network, as described in Figure 4.2.
The possibility of a bimolecular hydrogen-bonded network of organic molecules formed on top of the covalently bonded corrugated h-BN monolayer was also ex-plored, by depositing melamine onto the BN/Rh(111) surface after PTCDI de-position. On both Ag-Si(111) √
3 × √
3R30◦ [5] and Au(111) [89] surfaces the molecules formed a honeycomb network with a periodicity of ∼3.5 nm. However, no extended honeycomb networks were observed for PTCDI and (subsequent) melamine deposition experiments on the BN/Rh(111) surface when investigated with in-situ STM.