Abstract
Mid-infrared (IR) ellipsometry of thin films and molecule layers at solid–liquid interfaces has been a challenge because of the absorption of light in water. It has been usually overcome by using configurations utilizing illumination through the solid substrate. However, the access to the solid–liquid interface in a broad spectral range is also challenging due to the limited transparency of most structural materials in the IR wavelength range. In this work, we propose a concept of a microfabricated analysis cell based on an IR-transparent Si membrane with advantages of a robust design, flexible adaptation to existing equipment, small volume, multiple-angle capabilities, broad wavelength range, and opportunities of multilayer applications for adjusted ranges of high sensitivity. The chamber was prepared by 3D micromachining technology utilizing deep reactive ion etching of a silicon-on-insulator wafer and bonded to a polydimethylsiloxane microfluidic injection system resulting in a cell volume of approximately 50 μL. The mechanical stability of the 2 and 5 μm-thick membranes was tested using different “backbone” reinforcement structures. It was proved that the 5 μm-thick membranes are stable at lateral cell sizes of 5 mm by 20 mm. The cell provides good intensity and adjustment capabilities on the stage of a commercial mid-IR ellipsometer. The membrane configuration also provides optical access to the sensing interfaces at a broad range of incident angles, which is a significant advantage in many potential sensing structure configurations, such as plasmonic, multilayer, 2D, or metamaterial applications.
Highlights
In situ mid-infrared (IR) spectroscopy at solid−liquid interfaces requires special configurations such as that of[1] utilizing the “mirage” effect or the attenuated total internal reflection method.[2]
Thereafter, the thickness and the dispersion of n were fixed and used to determine the in situ volume fractions of ambient and PLGA (VPLGA) values. Both the VPLGA = 30% (Figure 2A) and n (Figure 2B) are close to the values reported in ref 20, the dispersion of polymers strongly depends on the preparation and type.[21]
It was shown that flow cells for infrared spectroscopic ellipsometry (IRSE) can be prepared using micromachined SOI wafers
Summary
In situ mid-infrared (IR) spectroscopy at solid−liquid interfaces requires special configurations such as that of[1] utilizing the “mirage” effect or the attenuated total internal reflection method.[2]. The instrumentation of ellipsometry is mature for most configurations, including IRSE.[10] The wavelength range from 1600 to 30,000 nm is already covered by most manufacturers, which extends the range of the deep ultraviolet−visible−near infrared ellipsometry of ≈200−2000 nm. Due to the light absorption of typical (water-based) samples in the IR wavelength range, the access to solid−liquid interfaces is only possible from the substrate side. The applicability of materials is limited because there are only a few that are transparent in such a broad wavelength range (Figure 1). Numerous practical considerations (such as fragility or water solubility) play an important role in selecting a proper material
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