The PTCDI-Melamine Supramolecular Network and Solution Processed PTCDI on Hexagonal Boron Nitride

In addition to the canted structure, discussed in chapter 4, PTCDI forms a honeycomb supramolecular network when co-deposited with melamine [5]. Using a solution deposition technique, PTCDI-melamine was deposited onto hBN substrates [46]. Deposition of PTCDI- melamine and the acquisition of high-resolution AFM images was carried out by Dr Vladimir Korolkov. The deposition of PTCDI-melamine was optimised to give full monolayer coverage, used for all such samples in this chapter. From high resolution AFM images, a lattice parameter of 3.5 ± 0.1 nm was extracted, in good agreement with previous STM investigations of the

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same structure on metal and HOPG surfaces [5,6]. The PTCDI-melamine network was deposited onto both flame-annealed hBN on 300 nm SiO2/Si and furnace-cleaned hBN on

chromium substrates. The fabrication steps for both substrates are described in section 3.3.

Figure 5.2.1. A monolayer of the PTCDI-melamine supramolecular network was deposited

onto flame-annealed hBN on SiO2 from solution. The morphology of the PTCDI-melamine

network was determined using AFM (a,b), from which a lattice constant of 3.5 ± 0.1 nm was extracted. A diagram of the arrangement of PTCDI and melamine molecules within the network is also shown (c). Deposition of the PTCDI-melamine network from solution and acquisition of high-resolution AFM images was carried out by Dr Vladimir Korolkov.

Fluorescence spectra of the PTCDI-melamine supramolecular network were measured using the Horiba LabRAM HR spectrometer with a 532 nm excitation laser. In order to prevent sample damage and photo-bleaching, filters were used to decrease the laser power. The PTCDI-melamine films were found to degrade over a few days, thought to be due to exposure to moisture under ambient conditions. Spectra were therefore acquired immediately after deposition from solution in order to reduce the effect of sample degradation.

As with sublimed PTCDI on hBN on 300 nm SiO2/Si, discussed in section 4.3, the lineshape of

the PTCDI-melamine fluorescence was found to vary between hBN flakes of different thickness. Fluorescence spectra of PTCDI-melamine on hBN on 300 nm SiO2/Si were acquired

from a number of flakes, see figure 5.2.2. The height of the underlying hBN flakes was extracted from AFM images and compared to fluorescence spectra. The relative intensity of the fluorescence peaks was found to change with the height of the underlying hBN flake; this was attributed to interference effects. In order to explore this effect further, fluorescence

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spectra were modelled using the transfer matrix model and direct calculations introduced in section 4.3.

Figure 5.2.2. Fluorescence spectroscopy of the PTCDI-melamine network on hBN on 300 nm

SiO2/Si was carried out using a 532 nm excitation laser. Normalised fluorescence spectra are

shown for a range of hBN flake thicknesses, extracted from AFM images and indicated in the legend.

By fitting the fluorescence spectra of PTCDI-melamine to a series of Lorentzian curves, the intensities and positions of the 0-0 (2.235 ± 0.002 eV / 554.8 ± 0.5 nm) and 0-1 (2.082 ± 0.002 eV / 595.4 ± 0.5 nm) peaks of PTCDI-melamine were extracted. In figure 5.2.2, spectra taken from flakes with heights of 35 nm, 43 nm and 64 nm show a broad fluorescence peak between 675 nm and 750 nm, the prominence of this broad peak is explained in terms of optical interference.

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Figure 5.2.3. Fluorescence measurements of the PTCDI-melamine network deposited onto

furnace-cleaned hBN on chromium. Measurements were carried out using a 532 nm excitation laser while the flake heights were extracted from AFM images.

In order to reduce the variation in the fluorescence line shape between hBN flakes, PTCDI- melamine was also deposited onto furnace-cleaned hBN on chromium. Fluorescence spectra of PTCDI-melamine on hBN on chromium were acquired from a range of hBN flakes and are shown plotted in figure 5.2.3. On hBN on chromium, the fluorescence lineshape was found to be more consistent (as observed for sublimed PTCDI, see section 4.3).

On hBN on chromium, the 0-0 and 0-1 peaks of PTCDI-melamine were measured to be (2.245 ± 0.002 eV / 552.3 ± 0.5 nm) and (2.085 ± 0.002 eV / 594.7 ± 0.5 nm) respectively. The ratios between the 0-0 and 0-1 peaks were extracted from fluorescence spectra and modelled using analytical methods. By comparing the fitting parameters of interference models, the ratio between the 0-0 and 0-1 peaks was found to decrease by a factor of 4.5 ± 0.2 between sublimed PTCDI and PTCDI-melamine on hBN on chromium, see section 4.3 and appendix 1.

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Figure 5.2.4. By immersing the PTCDI-melamine supramolecular network on hBN in water,

melamine is removed and insoluble PTCDI molecules re-organise on the surface. AFM was used to acquire images of the resulting PTCDI sample showing the island morphology (a) and molecular packing (b) of PTCDI. A diagram showing the canted structure of PTCDI is also shown (c). Solution processing of PTCDI films and acquisition of high-resolution AFM images was carried out by Dr Vladimir Korolkov.

The canted PTCDI phase can be formed by immersing the pre-formed PTCDI-melamine network in ultra-pure water to remove the soluble melamine species. PTCDI films on hBN were fabricated in this way by Dr Vladimir Korolkov, prior to the acquisition of images of the sample using AFM. PTCDI was found to rearrange into monolayer islands on the hBN surface, while high-resolution AFM images of the islands revealed a canted molecular packing structure which was also observed for sublimed PTCDI, see section 4.2. The lattice constants of solution processed PTCDI were found to be 1.5 ± 0.1 nm and 1.7 ± 0.1 nm, in good agreement with values extracted from AFM images of sublimed PTCDI on hBN, see chapter 4, and STM images of PTCDI on gold by Mura et al [44]. The island morphology and molecular packing of solution processed PTCDI are shown in figure 5.2.4.

The fluorescence of solution processed PTCDI on hBN on chromium was found to be comparable to sublimed PTCDI films, see figure 5.2.5. The variation in the fluorescence lineshape of solution processed PTCDI monolayers on hBN on chromium was found to be similar to that of sublimed PTCDI monolayers. Comparing the fluorescence spectra of sublimed PTCDI, solution processed PTCDI and PTCDI-melamine on hBN on chromium substrates, a small red shift of 0.006 ± 0.002 eV was observed between solution-deposited

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and sublimed PTCDI. This difference is attributed to the preparation of the two films, sublimed molecular films can exhibit areas of additional material on top of islands while solution- processed films may also be affected by the presence of excess solvent molecules or residual melamine.

Figure 5.2.5. Normalised fluorescence spectra taken from sublimed PTCDI (blue), solution

processed PTCDI (red) and the PTCDI-melamine supramolecular network (black) on hBN. Measurements were taken from hBN flakes of height 48 nm, 48 nm and 49 nm respectively, with an excitation wavelength of 532 nm.

A red shift of 0.031 ± 0.002 eV was observed between PTCDI-melamine and solution processed PTCDI, much larger than the difference between the 0-0 peaks of sublimed and solution- processed PTCDI monolayers. The similarities between the fluorescence spectra of solution processed and sublimed monolayer PTCDI films suggests that the different deposition routes to the canted structure of PTCDI has little effect on the fluorescence of the film. This is the basis for the assumption that differences in the fluorescence spectra between PTCDI and PTCDI-melamine on hBN are a consequence of the structure of the film, rather than any external factors such as the presence of excess solvent molecules. This assumption is supported further by AFM results, where images of the morphology of all samples have been

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acquired with lattice resolution. A larger red shift, ∆Etot, of 0.342 ± 0.002 eV, was observed

between HND measurements of Me-PTCDI (2.5506 ± 0.0001 eV) and solution deposited PTCDI on hBN (2.214 ± 0.002 eV) [71]. Aggregate 0-0 Peak Position (nm/eV) 0-0 Peak Width (nm/eV) 0-1 Peak Position (nm/eV) PTCDI-melamine (hBN-Cr) 552.3 / 2.245 15.1 / 0.061 594.7 / 2.085

PTCDI-melamine (hBN-SiO2-Si) 554.8 / 2.235 19.5 / 0.079 595.4 / 2.082

PTCDI (hBN-Cr) (Sublimed)

561.5 / 2.208 9.8 / 0.039 602.2 / 2.059

PTCDI (hBN-SiO2-Si)

(Sublimed) 561.4 / 2.208 11.3 / 0.043 606.3 / 2.045 PTCDI (hBN-Cr) (Solution Processed) 560.1 / 2.214 9.6 / 0.038 599.9 / 2.067 Me-PTCDI [71] (HND) 486.2 / 2.550 - -

Table 5.2.1. The position and width of the main and vibronic peaks of solution deposited

PTCDI-melamine, sublimed PTCDI and solution processed PTCDI thin films on hBN are shown for a range of substrates. The 0-0 peak of Me-PTCDI doped helium nano-droplets [71] is shown for comparison.

The fluorescence peak positions and widths of both PTCDI and PTCDI-melamine on hBN are shown in table 5.2.1. Slight variations in the peak positions and peak widths between substrates were attributed to interference effects. This was seen in plots of the peak position, extracted from fitting, against the hBN flake height, see appendix 1. For hBN on 300 nm SiO2/Si

substrates, a small modulation of the peak position in the range of 0.008 eV (2 nm) was observed with changing flake height. All comparisons between the fluorescence of PTCDI and PTCDI-melamine phases on hBN are drawn from hBN on chromium substrates, this is because the lineshape and peak positions of spectra did not vary as much between hBN flakes. In this section, the close packing structure and fluorescence of PTCDI-melamine and solution processed PTCDI have been determined experimentally, these results will be discussed further in section 5.5 and 5.6.

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