*2.4. Surface Topography*

To determine the surface morphology of the coatings, a Bruker multimode AFM microscope, equipped with a Nanoscope V controller, was used. This enabled a collection of images 10 × 10 μm<sup>2</sup> in size. The MicroMash OTESPA type of probe had the following parameters: nominal probe radius of 7 nm, elasticity constant of 26 N/m and resonance sampling frequency of 300 kHz. All data acquisition was performed with the help of Brüker Nanoscope 7.3 software with the images being processed using the Bruker Nanoscope Analysis 1.5 application. Each measurement series was conducted on three different coatings deposited under identical conditions, with the arithmetic mean and standard deviation values being calculated from the results acquired.

#### *2.5. XPS Spectroscopic Analysis of Elemental Composition and Chemical Structure*

XPS analysis was conducted with the help of Kratos AXIS Ultra XPS spectrometer equipped with a monochromatic Al Kα source of X-rays of excitation energy of 1486.6 eV. Spectra were collected from an area of 300 × 700 μm2, with the anode power amounting to 150 W. Transfer energy of a semicircular analyzer was equal 20 eV. Due to a non-conductive character of the sample surface, an additional charge neutralizer was applied. All narrow chemical shifts for each element were calibrated with respect to the most intensive spectrum component, i.e., Si (2p) maximum positioned at 101.8 eV and assigned to a Si-N bond. Sample etching was carried out down to different depths at nine different sites of the coating. The following were the etching parameters: beam energy of 4 keV, etching time of 90 s, etching rate of 26.706 × 0.85 nm<sup>2</sup>/min, surface area etched of 2 × 2 mm<sup>2</sup> and surface area analyzed of 200 × 200 μm2.

#### *2.6. FTIR Analysis of Chemical Bonding*

FTIR Analysis was carried out with the help of ThermoScientific model Nicolet iS50 FTIR spectrometer, working in absorbance mode and using a MCT/B beam splitter. Spectra were collected within the range of 4000 to 400 cm<sup>−</sup>1, with a resolution of 4 cm<sup>−</sup>1. Several scans in a single measurement cycle amounted to 128.

#### *2.7. Design of a "Cold Mirror" Type of a Gradient Interference Filter*

The gradient interference system was simulated with the help of the TFCalcTM 3.5 software from Software Spectra. The material database was updated with the optical properties of the layers obtained from HMDS mixed with pure oxygen or nitrogen, which had refractive indices of 1.65 and 2.22, respectively. In the next step, the database was enriched with materials obtained for intermediate mixtures of reactive gases. A complete set of materials served as an input to create the basic elements of a stack formula with a gradual change of optical properties. Such a package can be used for more complex systems by repeating and setting common optical thickness values. Filter design can be further developed and optimized by setting continuous or discrete target values for optical properties of the filter.

In the present work, a basic stack of 10 multiplications was designed without adaptations on the single layer level. Environmental details used for simulation, such as illumination, detector and medium properties, and light incident angle, were aligned with ThermoScientific ™ Evolution 220 UV-Vis equipment, which was used to measure the optical properties of a real filter. The filter designed belongs to the category of edge filters. A longpass (LP) filter is an optical interference filter that attenuates shorter wavelengths and transmits (passes) longer wavelengths over the active range of the target spectrum (ultraviolet, visible, or infrared). Such filters are known as the "cold mirror" type. Longpass filters (also referred to as edge filters) have a very sharp slope and are described by the cut-on wavelength at 50 percent of the peak transmission. In the designed filter, its value was set at 290 nm.

#### *2.8. Ihe Cross-Section of the "Cold Mirror" Gradient Optical Filter*

For the assessment of the film surface morphology, a Carl Zeiss ULTRAPlus scanning electron microscope (SEM), supported with the SmartSEM software, was used. The microscope was equipped with an FEG-type cathode, allowing observations to be made at low values of accelerating voltage of 0.5–30 kV. Surface charge scattering was performed using a Carl Zeiss Car Charge Compensator mechanism. For the magnification of 60,000 times, an accelerating voltage of 2.5 kV at the distance of 6 mm was applied.
