Phase Diagram and Photonic Films from HM-CNCs

obtained after at least 1 week of standing. The HM-CNC phase diagram shows more linearity when compared to the phase diagram of C-CNCs (see Figure 3.25), presumably due to the absence of sodium counterions and consequently less gela-tion behavior.

Figure 3.31 – Phase diagram of HM-CNCs. a) Vials with increasing HM-CNC concentrations of 2.82 wt%, 4.61 wt%, 6.11 wt%, 7.51 wt%, 9.14 wt% and 10.06 wt% after standing for at least 1 week. Observed through crossed polarizers. Scale bar indicates 1 cm. b) Resulting phase diagram as measured from a).

Due to the reduced amount of HM-CNCs available connected to low yield and throughput no in-depth characterization of photonic properties from each anisotropic phase was conducted. The HM-CNC concentration was fixed at 2.8 wt%, as it emerges from the synthesis. Nonetheless, even at this concentration, still with a considerable amount of isotropic phase, the photonic properties are remarkable when compared to an isotropic suspension of C-CNCs (where usually inferior figures of merit are observed). Figure 3.32 shows transmission spectra of a HM-CNC film deposited on Glass/ITO and dried with the controlled EISA conditions as described before (4 °C and RH > 90 %).

Figure 3.32 – Optical characterization of a HM-CNC3 film on Glass/ITO. a) and b) Circular HM-CNC3 films visualized through LCPL and RCPL filters in reflec-tion, respectively. Scale bars indicate 3 mm. c) Transmission spectrophotometry through CPL filters in the range of 400 nm≤λ≤800 nm.

Apart from presenting a photonic bandgap situated in the blue/UV region (λ_max = 382 ± 12 nm) spectral width is low (113 ± 5 nm) and CPL distinction high with roughly 40 ± 8% (the last two values were obtained through linear extrapolation). This is connected to a low dispersion in HM-CNC rod length (as observed in Figure 2.16e), which leads to better self-assembly properties when compared to more disperse (in length) CNC suspensions (such as C-CNCs).

3.7.5.1 Brief Concluding Remarks on HM-CNC Photonic Film preparation This Chapter gave a thorough insight over the self-assembly behaviour of CNCs in aqueous suspensions. The chiral nematic order observed in suspension can be preserved upon drying to yield iridescent photonic films with bandgaps in the visible light range. The fabrication of photonic drop-cast films is preferred over membranes as the former requires less material, provides more control over EISA conditions and is faster. It was shown how a short UV-treatment or the deposition of SiO₂ thin-films with masks lead to a precise definition of hydrophilic areas on otherwise hydrophobic substrates, where aqueous CNC suspensions can be drop-cast. Furthermore, precise control over the drying conditions yields films with enhanced and more uniform photonic properties throughout the drop-cast films.

It is however important to keep in mind that deviations from sample prepa-ration can give distinct results from the ones presented here as already pointed out by Parker et al. about the visual appearance of photonic films obtained by CNCs.[45] The list starts with the dispersion and sonication conditions. Devia-tions in sample volume, flask shape, power of the system and duration will influ-ence energy input, which has an impact on hydrodynamic diameter and released ions into the solvent by the CNCs. This consequently shifts the photonic bandgap to longer or shorter wavelengths for higher or lower energies, respectively.[73]

Secondly, during film preparation the cast volume, film shape, surface wettability and ionic strength will influence the development of the chiral nematic phase during drying. Lastly, the drying conditions themselves, such as temperature or humidity, strongly affect long-range ordering and efficient chiral nematic layer orientation and consequently the quality of the final photonic film. Generally one can state that increased drying times (high humidity and low temperatures) lead to the maximization of CPL response and a considerable decrease in coffee-ring effect.

The phase separation properties and the resulting phase diagrams for both CNC types were studied. A linear increase in anisotropic volume fraction was observed for HM-CNCs, whereas C-CNCs showed a tapering off of anisotropic volume fraction for increased concentrations due to gelation. 100% of anisotropy was reached at roughly 6.5 wt% and 9 wt% for C-CNCs and HM-CNCs, respec-tively. This discrepancy can be connected to differences in particle dimensions and surface charge, which influence the excluded volume fraction.

The suspensions studied in this Chapter will be at the basis for photonic films that will be implemented in optoelectronic devices in Chapter 5 and 6. For C-CNC films the anisotropic part of a 5 wt% suspension after 1 week of phase separation will be used. For HM-CNC films a suspension with roughly 3.0 ± 0.2 wt% will be used exactly how it emerges from synthesis after dialysis and sonication. Both films show distinct photonic bandgaps and can thus be used in diverse applications, especially when choosing an adequate semiconductor.

Semiconductors, ubiquitous in microelectronics and thus modern society, come in a variety of electronic and optical band-gaps, ranging from low (such as InAs, E_g= 0.43 eV) over intermediate (Si E_g= 1.11 eV) to high (ZnO E_g= 3.2 eV).[see 80, p. 185] In some cases, they only respond to very specific excitation wavelengths usually for energies above their optical bandgap. Therefore, in order to be able to combine photonic CNC films with semiconductors in optoelectronic devices it is of upmost interest to be able to control the photonic bandgap of the former (as the optical bandgap is an intrinsic property). In other words, one must be able

to have complete control not only over the pitch of the CNC films but also the efficiency of the CPL distinction between LCPL and RCPL (as high as possible) and even spectral width (usually as low as possible). Several works have been devoted to the study and control of the nematic pitch in suspension and dry films where a work of Parker et al. stands out with an overview of the techniques.[45]

The next Chapters will thus investigate the proof-of-concept of the implemen-tation of photonic CNC films into optoelectronic device. Two distinct devices are targeted: a thin-film transistor and a thin-film photodiode. Both are fabri-cated with semiconductors that present a suitable response for both CNC films presented in this Chapter. Where amorphous indium-gallium-zinc-oxide transis-tors respond in the blue/UV region (coinciding with films from HM-CNCs), p-i-n silicon junction photodiodes respond in the green/red region (coinciding with films from C-CNCs).

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