Thin Films Formed Through Dip Coating

Thin films prepared with 100, 206, 300 and 400 layers of PAA and surfactant exfoliated graphene were constructed on a variety of surfaces using dip coating. Optical microscopy and surface profilometery were used to confirm the deposition and film

0 1 2 3 4 5 6 7 8 0 2 4 6 8 10 12 14 Sa u e rb re y m ass of f ilm (m g/m 2) Number of Layers PEI PAA Graphene pH Unadjusted Rinse pH 2 Rinse

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coverage of the dip-coated samples (See Appendix, §A.2.4). Spectroscopic measurements were then performed on the samples in an effort to confirm that the graphene particles were retained during the dip coating process and if multilayer thin films were also successfully constructed. The UV-Vis spectrum of a sample comprised of a single layer of PEI and 400 graphene/PAA layers was measured using a Cary 5000 UV-Vis spectrophotometer over the wavelength range of 200 - 800 nm (See Appendix, §A.2.5).

The dip coated thin films were also characterised using Raman spectroscopy. Raman spectra were recorded for the samples on silicon wafer using a Horiba Jobin Yvon Raman system, with 633 nm excitation laser. Raman spectroscopy measurements were performed on samples consisting of between 100 and 400 layers of PAA and graphene adsorbed onto silicon substrates (Figure 6.6). These measurements were unable to be performed on the thin films prepared using with QCM, due to the small number of adsorbed layers which results in a low signal-to-noise ratio. The spectra of the samples show three main peaks located at approximately 1332, 1579 and 2664 cm-1, named the D, G and 2D peaks. These peaks indicate the presence of graphitic material in the sample and are associated with sp2 carbon breathing modes (indicative of the proportion of sp3 carbon in the sample), in plane bond stretching between sp2 carbon atoms (indicative of the proportion of sp2 carbon), and the number of graphene layers in a sample.234 The ratio of the peak intensity between the D and G peaks in each sample is consistent with the presence of defect-free graphene sheets. Furthermore, the single 2D peak at 2664 cm-1 is symmetrical and down shifted from 2700 cm-1, consistent with the presence of single and few-layer graphene6, 123.

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Figure 6.6: Raman spectrum of multilayer films consisting of a single layer of PEI deposited at pH 4, followed by alternating layers of PAA and Graphene at pH 2, prepared through dip

coating. Inset shows the Raman peak intensity at 1580 cm-1 as a function of the number of

adsorbed layers.

The Raman spectra of the dip coated thin films also provides further information regarding the growth process of the films, and therefore yields evidence of their internal structuring. Concentrations of nanoparticles have been shown previously to be directly related to the peak intensity of its corresponding Raman spectra.235 In particular, highly stratified multilayer films containing 2D materials and polyelectrolyte have been shown to exhibit a linear relationship between the peak intensity of the material and the number of layers in the film.236 The inset in Figure 6.6 shows the relationship between the peak intensity from Raman spectra of the samples, and the number of layers deposited on the substrate. This relationship is linear in nature, which shows the adsorption of the PAA and surfactant stabilised graphene forms discrete layers in the multilayer film.

The QCM measurements and Raman spectra presented in Figure 6.2 and Figure 6.6 suggest two distinct film growth profiles, despite the same chemical species used in the

-50 0 50 100 150 200 250 300 1200 1700 2200 2700 In te n si ty (a .u .) Raman shift (cm-1₎ 0 layers (Substrate) 101 layers 207 layers 301 layers 401 layers R² = 0.9996 -100 0 100 200 300 0 100 200 300 400 In te n si ty (a.u .) Number of Layers

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construction of the films. The difference in observed film growth between the two characterization techniques could be the result of the different deposition methods used to prepare the samples. Flow cell properties and shear effects may influence film assembly to such a degree that the two multilayer systems demonstrate distinct film growth regimes. Polyelectrolytes deposited through the LbL process have been shown previously to reorient upon drying237, possibly affecting the film growth profile.

Alternatively, the two distinct growth patterns observed from QCM and Raman spectra measurements may form different parts of a single growth pattern, a phenomena known as superlinearity. Superlinear growth profiles are characterised by a transition from exponential to linear growth193 at large numbers of deposited layers and have been previously observed in a number of polyelectrolyte multilayer systems193, 206, 238, 239, including PAA/PEO multilayer films. A model for the process was proposed by Hübsch et al.232 and later by Salomäki et al.197. During the initial stages of this process, the adsorption of alternating layers of chemical species proceeds exponentially. Like systems that demonstrate purely exponential growth, this phase requires constant diffusion of one of the species through the multilayer towards the substrate surface, creating a diffuse zone in which the structure of the multilayer is homogeneous. As the number of adsorbed layers increases, eventually diffusion of the more mobile species decreases due to rearrangement of the chemical species in order to promote the most favourable interactions in the film. As a result, the area nearest to the substrate surface undergoes restructuring to form distinct, impenetrable layers. When diffusion of the species entering and leaving the diffusion zone reaches equilibrium, the diffusion zone begins to extend away from the substrate as the number of distinct layers increases. The growth rate of the multilayer film is dependent only on the amount of chemical species that reach the restructured zone, causing linear growth to dominate at higher numbers of

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adsorbed bilayers. The two growth profiles resulting from Raman and QCM data in the current study supports this proposed method of film growth. Interestingly, if the superlinear profile applies to the films presented here, this implies that the internal structure of the film varies with the number of adsorbed layers.

Indeed, the superlinear growth mechanism supports details of the QCM data shown in Figure 6.3 which are not adequately addressed by the exponential growth mechanism alone. In particular, the superlinear growth mechanism is consistent with the linear change in mass of the film when graphene is adsorbed (Figure 6.3), as well as the loss of mass from the film when PAA is adsorbed during the initial stages of film formation. Assuming a superlinear growth regime and diffusion of PAA, it is expected that fewer hydrogen bonding interactions are available to the PAA close to the substrate upon successive deposition steps of PAA and consequently, an increasingly higher PAA concentration is likely in the upper most layers of the film during the initial stages of film formation. This could be the cause of the observed linear increase in the change of the mass of the film upon the addition of graphene, resulting from an increase in the number of hydrogen bonds possible between the PEO and PAA at the outermost layers. The superlinear growth mechanism also explains the increasing ability for PAA to collapse the PEO hydration, which appears as a loss of mass from the film and increased film rigidity (Figure 6.3 and Figure 6.4). As more graphene is adsorbed onto the film, more entrained water is likely to be present in the graphene layers due to hydrogen bonding with the PEO groups and therefore greater loss in mass is experienced upon each addition of PAA. As the diffusion of PAA into the film and the restructuring zone reaches equilibrium, the concentration of PAA in the outermost layers of the film are expected to remain constant, resulting in a linear growth pattern

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The switch between exponential and linear growth in a superlinear growth regime typically occurs when only a few bilayers have been adsorbed onto the substrate. It is currently understood to coincide with the point at which the rate of diffusion of molecules from out of the film reaches equilibrium with the rate of diffusion of molecules into the film. Transition between the linear and exponential regimes is dependent on the strength of the polyelectrolytes involved, but literature values indicate this changeover point commonly occurs when between 14 and 36 layers have been deposited.193, 206, 239 This range agrees with growth profiles obtained from the QCM and Raman peak intensity data presented in this study, which indicates the switch between exponential and linear film growth took place between 20 and 100 layers of PAA and graphene.