Assembly of electrochromic device

The electrochromic cells were assembled in a similar manner as described by Steiner et al.²⁰¹ The devices consisted of two ITO-coated glass slides, one acting as working electrode (glass slide coated with the hybrid film) and the other one as counter electrode, both separated by means of a thermoplastic gasket (Parafilm). After fusing at ~140°C for a few seconds, a platin wire acting as a reference electrode was fixed, and the device was filled with 1 M bis(trifluoromethane)sulfonimide lithium in propylene carbonate and sealed with epoxy glue.

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6 Optical properties of self-organized gold nanorod–polymer hybrid films

Part of this chapter has been published:

Tritschler, U.; Zlotnikov, I.; Keckeis, P.; Schlaad, H.; Cölfen, H. “Optical Properties of Self-Organized Gold Nanorod–Polymer Hybrid Films”. Langmuir 2014, 30, 13781-13790.

6.1 Abstract

High fractions of gold nanorods were locally aligned by means of a polymeric liquid crystalline phase. The gold nanorods constituting > 80 wt% of the thin organic-inorganic composite films form a network with side-by-side and end-to-end combinations. Organization into these network structures was induced by shearing gold nanorod–LC polymer dispersions via spin-coating. The LC polymer is a polyoxazoline functionalized with pendent cholesteryl and carboxyl side groups enabling the polymer to bind to the CTAB stabilizer layer of the gold nanorods via electrostatic interactions, thus forming the glue between organic and inorganic components, and to form a chiral nematic lyotropic phase. The self-assembled locally oriented gold nanorod structuring enables control over collective optical properties due to plasmon resonance coupling, reminiscent of enhanced optical properties of natural biomaterials (Scheme 6.1).

Scheme 6.1: Correlation of optical and structural properties of gold nanorod–LC polymer hybrid films.

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6.2 Introduction and aim

Besides an ingenious hierarchical structuring of mineral crystals within an organic matrix as well as a controlled coupling at the interface between both components, natural organic–

inorganic materials often exhibit well-tuned optical properties.17,25,42,46,270

These remarkable optical properties arise by arranging inorganic crystals, such as calcite (in brittlestars)¹⁹ and aragonite (in nacre),²¹ by means of organic molecules (cf. Chapter 1.2).

Many research groups have worked on the synthesis of these challenging, often CaCO3 based, biomineral structures by using templating techniques^271-277 or via methods involving lithography.^278-283 The main drawback of the obtained materials is often their time-consuming fabrication, but also limitations such as their synthesis yields in small scales, or flat surfaces that are required for lithographic techniques. Nevertheless, striking results were achieved with these methods, amongst others the fabrication of artificial nacre via polymer-mediated mineral growth and layer-by-layer deposition of the organic matrix, leading to the mimicking of nacre’s optical iridescence.¹⁴⁷ Remarkable, CaCO₃ microlens arrays replicating arrays with uniform size and focal length as found in brittlestars were recently produced under ambient conditions without using any template.²⁸⁴

An optically interesting nanoparticle system is gold. These metal particles have already been used for stained-glass panes of churches and cathedrals many centuries ago (Figure 6.1).²⁸⁵

Figure 6.1: Stained-glass pane in the cathedral Notre-Dame (Paris, France).²⁸⁵

The optical properties of small metal nanoparticles (conductors) are mainly attributable to surface plasmons. They are formed when conduction electrons oscillate collectively in resonance with incoming electromagnetic radiation. Due to the electric field of the incident radiation, a dipole is formed in the nanoparticle, and a restoring force tries to compensate it.

Consequently, there is one characteristic frequency (for spherical particles), the resonance frequency, fitting to this electron oscillation within the nanoparticle (Figure 6.2a).^285,286

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Figure 6.2: (a) Interaction of an incident electromagnetic radiation with a spherical gold nanoparticle, leading to the formation of a dipole, which oscillates in-phase with the electric field of the incident light. (b) Transversal and longitudinal oscillations of conduction electrons in a gold nanorod.²⁸⁶

It was found that the electric field increases at the interface and decreases exponentially with increasing distance from the surface (in the nm range). In case of gold, silver or copper, the resonance frequency is in the visible range of the spectrum, which is due to d-d band transitions. The surface plasmon resonance determines the optical properties of particles, predominantly if their sizes are below the wavelength of the incident radiation.^285,286

The resonance frequency depends on several factors, such as size, shape, nature of surrounding medium (related to its refractive index), (organic) shell on the particle surface, and interactions between adjacent particles.^285,286

While the size of (spherical) particles is usually of rather minor importance, the particle shape can significantly influence the resonance frequency. For anisotropic, e.g. rod-shaped, particles, the resonance frequency depends on the direction of the electric field of the incident electromagnetic radiation relative to the particle orientation. For nanorods, two resonance modes arise: a longitudinal surface plasmon resonance (LSPR) and a transverse surface plasmon resonance (TSPR), attributable to the resonance parallel and perpendicular to the long axis of the rods, respectively (Figure 6.2b). The LSPR appears at higher wavelength (i.e.

lower energies) and exhibits a higher absorption coefficient, compared to the TSPR.^285,286 The absorption spectrum of nanorods can be modelled by using the Mie theory. This theory, however, is limited to systems with relatively simple geometrical complexity. In contrast, the discrete dipole approximation method allows reproducing the absorption response of geometrically more complex structures. This method is based on dividing elements into small subunits, which interact via dipole-dipole interactions.²⁸⁶

The tunable, aspect-ratio dependent longitudinal surface plasmon resonance (LSPR) and, to a lower extent, transversal surface plasmon resonance (TSPR) of gold nanorods make them

121 interesting for the design of nanostructured materials with specific, orientation-dependent optical properties.²⁸⁷ Ensembles of gold nanorods may possess collective optical properties differing from the optical properties of individual gold nanorods and bulk samples. An important tool to control the structure and optical properties of gold nanorods is the self-assembly of the gold nanoparticles.²⁸⁸ The anisotropic shape of high aspect-ratio gold nanorods allows them to form liquid crystalline (LC) phases in the presence of certain amounts of the stabilizing agent cetyltrimethylammonium bromide (CTAB) in concentrated aqueous dispersions.^289,290 Alignment of CTAB-coated gold nanorods with lower aspect ratios of < 3 on the higher nanometer length scale was only achieved when working under strict conditions allowing a uniform local concentration of gold particles during slow and controlled evaporation of a concentrated, aqueous gold nanorod dispersion until complete dryness.²⁹¹ The effect of the optical properties of gold nanorods in terms of the change of their LSPR and TSPR depending on the plasmon resonance coupling with different geometries as well as the number and distance of participating nanorods were studied.286,292,293

Orientation dependent optical properties of gold nanorods dispersed in high volume fractions of PVA polymer films were studied by van der Zande et al.²⁹⁴, Pérez-Juste et al.²⁹⁵ and Murphy et al.²⁹⁶ For example, the intensity of the absorbance of both plasmon resonances of aligned gold nanorods within a poly(vinyl alcohol) film was varied by varying the polarization angle of the incoming light.²⁹⁵ Color switching behavior of gold–polysiloxane composites was reported by Uhlenhaut et al.²⁹⁷ Self-assembly and consequently optical properties of gold nanorods embedded in polymer films, prepared by spin-coating, were tuned by controlling their spacing and orientation via changing the volume fraction of the gold particles (1-16%).²⁹⁸ Resulting structure-optical property relationships of polymer films containing gold nanorods with aspect ratios of ca. 3-4 were reported to be dependent on the gold nanorod volume fraction (1.2-3.7%) as well as the film thickness due to self-assembly and percolation of the nanoparticles within the polymer film (Figure 6.3).²⁹⁹

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Figure 6.3: Optical properties of gold nanorods embedded in a poly(2-vinyl pyridine) film were correlated with their local orientation, forming side assemblies and percolated networks with end-to-end and side-by-side linkages. For example, end-to-end and side-by-side-by-side-by-side alignments of gold nanorods induce red-shifts and blue-shifts of the LSPR, respectively (see arrows).²⁹⁹

Addressing the hierarchical structuring of biomaterials as well as the glue between organic and inorganic components, a synthesis concept based on LC formation of both organic and inorganic components via a one-step self-organization was established (cf. Chapters 4 and 5).

To this end, polyoxazolines with pendant cholesteryl units, enabling the polymer to form lyotropic phases, and different charged/polar units, allowing the polymer to bind to nanoparticles via Coulomb interaction or hydrogen bridges, were used. Due to the anisotropic shape of the nanoparticles, such as Laponite and V2O5, the inorganic phase is also able to form LC phases, and hierarchically organized composite materials structured on several levels were synthesized. In this chapter, the one-step self-organization procedure was transferred to the synthesis of gold nanorod–LC polymer composite films with high inorganic fractions. By applying structural concepts derived from biological tissues, it was aimed for influencing and tuning of collective optical properties of gold nanorods, expressed in a shift of the maxima (LSPR and TSPR) in the absorption spectra.

6.3 Gold nanorod–LC polymer hybrid particles: Investigation of binding