Characterization of the coatings

In this part I summarize the results of the characterizations of the prepared nanostructured coatings. First the surface morphology and homogeneity was characterized by atomic force microscopy, afterwards the thicknesses of the layers were studied by spectroscopic ellipsometry. Finally, the light-guiding capability of a TNT-coated OWLS chip was examined by OWLS. The results of this section were published in Colloids and Surfaces B: Biointerfaces [T1].

5.1.1. Surface morphology and optical characterization of the coatings

From both of the nanoparticles, a dense and homogeneous coating was required to be prepared to be compatible with the OWLS and ellipsometric measurements. Moreover the thickness of the coatings needed to be between 5 and 15 nm, at which it can be expected to well cover the substrates without increasing the optical thickness of the original waveguiding layer significantly (geometric thickness typically 170–180 nm, with a refractive index of about 1.77) [213] or the gold coated (20-30 nm) substrate.

The TNT and TNP coatings were characterized with atomic force microscopy (AFM) [194], [214], [215] on various substrates in tapping mode (Fig. 5.1 and 5.2). The images were recorded at different parts of the substrate surface, so they clearly showed that both the TNTs and the TNPs covered the whole surface uniformly. The arithmetic average roughness (Ra) and root mean squared roughness (RMS) [216], [217] of the surface were calculated from the AFM data by the Gwyddion 2.37 software [193] based on the following equations:

𝑅_𝑎 = ¹

𝑁∑^𝑁_𝑛=1|𝑟_𝑛|

𝑅𝑀𝑆 = √¹

𝑁∑^𝑁_𝑛=1𝑟_𝑛² Eq. (7),

where N is the number of measured data points and rn is the n^th measured value.

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For the TNT coating Ra was 4.67±0.57 nm and RMS was 5.93±0.69 nm, and for the TNP coating Ra was 2.13±0.36 nm and RMS was 2.84±0.52 nm. The uncoated surfaces were featureless with Ra<1 nm.

Figure 5.1. Representative AFM images of the titanate nanotube coating prepared on a silicon wafer (a), an OWLS sensor chip (b), and a glass slide (c) [T1].

Figure 5.2. AFM image of a titania nanoparticle coating spin-coated on a gold-coated glass substrate. The magnified area is 300 nm × 300 nm [T2].

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5.1.2. Thickness characterization with spectroscopic ellipsometry

I used a spectroscopic ellipsometer in mapping mode for characterizing the thickness of the TNT coatings prepared on small pieces of silicon wafers [218]–[220]. A two-layer optical model was applied in the evaluation of the measured data. On the single-crystalline Si substrate a native oxide layer (the thickness of which was fixed at the value obtained from measurements before the preparation of the TNT layers), and a TNT layer was placed. The optical properties of the TNT layer were described by the Cauchy dispersion relation [221] (Eq. (8)), which is an empirical polynomial approximation for the refractive index function of semiconductors and insulators below the band-gap [222]. Usually, only the first three terms of the summation is used:

𝑛 = 𝐴 + 𝐵/𝜆² + 𝐶/𝜆⁴ Eq. (8), where n is refractive index of the layer, λ is the wavelength, A, B, C are coefficients to be determined by fitting (or known from previous measurements).

The best fit could be achieved when I used a vertical grading (a built-in feature in the CompleteEASE software) of the refractive index within the Cauchy layer of the TNT coating. In this case the graded layer contained 5 sublayers, and the refractive index changed from one sublayer to another in a linear way. The inhomogeneity of the grading, which defines the refractive index changes between the top and the bottom layer, was found to be around 60%.

The ellipsometric maps (Fig. 5.3a) showed that in the central part of the area the thickness of TNT coating is around 9-13 nm. According to our aim to prepare these coatings on OWLS sensor chips, this central part of the surface is the most important, as the 1-mm² sensing zone of the chip is located in the center. The quality of the fit (MSE) (Eq. (2)) is also acceptable at this central area (Fig. 5.3b).

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Figure 5.3. A representative thickness map of a TNT-coated silicon plate (a) and the fit quality (MSE) values of this measurement (b) measured by spectroscopic ellipsometry. The central part of the area is coated by a 9–10 nm thick TNT film, and the quality of the fit is very good (MSE<3) in this area [T1].

The thicknesses of the TNP coatings were also characterized by ellipsometry. The TNP-coated slide was modeled by a three-layer optical model on a BK7 glass substrate. On the glass a 2-nm Cr2O3 layer and a 20 nm or 30 nm thick gold layer were placed. Above them the TNP layer was located which is found to be 10-12 nm thick during the evaluation. It is in agreement with the dynamic light scattering measurements, where diameter of the nanoparticles was determined to be 11.3 nm. In the ellipsometric model the coating was represented by a homogeneous layer of effective medium approximation (EMA) (Eq. (9)) [192] of TiO2 (50-55%) and void (45-50%), and could be perfectly fitted. In addition, the resulted proportion of the volume of void and TiO2 is also in good agreement with the spherical geometry of the TNPs (the volume fraction of a monolayer consists of contiguous spheres is very close to our results). Because of the big difference between the refractive indices of the TNP and the gold layer, the uncertainty of the evaluation is acceptable (0.04% for the TNP thickness, and 0.21% for the EMA% of void).

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5.1.3. Light-guiding capabilities of the TNT-coated OWLS chips

If we intend to apply a TNT-coated OWLS sensor chip in adsorption measurements, it is very important that the coating doesn’t quench the propagation of light. By OWLS measurements, I demonstrated that the resonant incoupling peaks are slightly shifted to higher angles by the coating, but the height of the peaks didn’t change significantly (Fig. 5.4).

Figure 5.4. The typical resonant incoupling peaks of an uncoated and a TNT-coated OWLS sensor chip [T1].

The thickness and the refractive index of the guiding film of the uncoated and TNT-coated sensor chips were calculated from the positions of the resonant peaks [65], [213], [223]. By the coating process the refractive index (nF) decreased from 1.77494 to 1.77217 and the thickness (dF) increased from 175 nm to 183 nm, which is in good agreement with the results of the ellipsometric measurements.