Young Investigator Group Nanoscale Solid-Liquid Interfaces

Ellipsometry

Multiscale ellipsometric and polarimetric methods use polarized light to probe and manipulate the sample properties. We develop and apply these methods for the investigation of functional layers and thin films, surfaces, heterogeneous structures, and 2D materials in various environments ranging from ultra-high vacuum to atmospheric and liquid conditions.

We employ a wide array of optical models, layer calculations, and simulations that facilitate the interpretation of polarimetric and ellipsometric measurements, and provide access to the chemical and physical material properties, even of structured, anisotropic, and depolarizing surfaces. Accessible characteristic material properties are, for example, conductivity, film thicknesses, composition, and dielectric functions (optical constants).

References

• Furchner A. et al., "Ti₃C₂T_x MXene Thin Films and Intercalated Species Characterized by IR-to-UV Broadband Ellipsometry", J. Phys. Chem. C 2025, 129 (1), 500–507.

IR Polarimetry

In-situ IR ellipsometry

In-situ IR ellipsometry (developed in-house) is particularly well-suited for the study of interfaces in liquid and defined environmental conditions. Liquid and electrochemical flow-cells can be used for monitoring and detailed studies of chemical and physical variations at interfaces and in thin films (e.g., dissociation and swelling). Combined cells for studies by IR polarimetry and Reflection Anisotropy Spectroscopy (RAS) are available. Operando studies are possible during functionalization, adsorption processes, or during external manipulations, for exmaple, by temperature or pH. Submonolayer variations can be analyzed for spot sizes down to mm².

References:

• Hinrichs et al 2024, “Structure and chemical analysis in thin films by in situ IR ellipsometry”, (DOI: 10.1016/B978-0-323-85669-0.00019-2)
• Nguyen et al. 2023, "Molecularly imprinted co-polymer for class-selective electrochemical detection of macrolide antibiotics in aqueous media" (DOI: 10.1016/j.snb.2022.132768)
• Furchner et al. 2017, "Molecular Interactions and Hydration States of Ultrathin Functional Films at the Solid–Liquid Interface" (DOI: 10.1021/acs.analchem.7b00208)
• Asheghali et al. 2020, "Enhanced neuronal differentiation of neural stem cells with mechanically enhanced touch-spun nanofibrous scaffolds" (DOI: 10.1016/j.nano.2020.102152)

IR Mueller matrix polarimetry

IR Mueller Matrix polarimetry (developed in-house) is particularly well-suited for examining the optical properties of complex systems, ranging from isotropic, anisotropic, or optically active samples to depolarizing materials. Being the world’s most sensitive IR MM polarimeter, it facilitates studies of optical properties at high sensitivity, with applicability to thin films. Measurements can be performed both in reflection and transmission down to mm² spot sizes. Particularly, it enables the determination of layer thicknesses and direction dependent dielectric functions. In combination with optical simulations, detailed information can be gathered on chemical and physical structure properties, such as molecular orientations in polymers, doping properties of conductive films, or dielectric function of 2D materials. 

References:

• Furchner et al. 2025, "Ti3C2Tx MXene Thin Films and Intercalated Species Characterized by IR-to-UV Broadband Ellipsometry" (DOI: 10.1021/acs.jpcc.4c06906)
• Hinrichs et al. 2023, "Mid-infrared dual-comb polarimetry of anisotropic samples" (DOI: 10.1002/ntls.20220056)
• Furchner et al. 2020, "Ultrasensitive broadband infrared 4 × 4 Mueller-matrix ellipsometry for studies of depolarizing and anisotropic thin films" (DOI: 10.1116/1.5129800)

IR laser ellipsometry

IR laser ellipsometry was developed in cooperation with Sentech Instruments (EFRE 1.8/13). Compared to classical Fourier-transform IR polarimetry, high sensitivity can be achieved by IR laser ellipsometry simultaneously for high spatial (< 150 µm) and spectral resolutions (0.5 cm^–1). Measurement times for pulsed or modulated laser sources reduce to the millisecond and microsecond range (< 400 ms spectrally, or 20 µs at single wavelength) when a single-shot concept is involved.

 References:

• Furchner et al. 2022, "Multi-timescale infrared quantum cascade laser ellipsometry" (DOI: 10.1364/OL.457688)
• Furchner et al. 2022, "Mid-infrared laser ellipsometry – a new era beyond FTIR" (DOI: 10.1515/aot-2022-0013)

Multi-timescale IR QCL ellipsometry, featuring long-term stable single-shot Ψ and ∆ measurements of individual pulses at the laser repetition rate. Adapted from Furchner et al, Optics Letters, 2022, 47, 2834.

Imaging Ellipsometry

Imaging NIR-VIS-UV Ellipsometry

The EP4 spectroscopic imaging ellipsometer (Park Systems) enables comprehensive laterally highly resolved optical characterizations of microstructures and heterogenous thin films at the microscale. Being able to probe samples such as MXene flakes and other 2D materials with sub-10 μm spatial resolution is crucial to improve and tailor material and thin-film synthesis towards electrochemical applications. Acquiring microscopic images with enhanced polarization and ellipsometric contrast allows the identification of regions of interests for in-depth ellipsometric measurements, providing access to the sample’s intrinsic optical properties (refractive and absorptive index) as well as its structural and material properties such as layer thicknesses (≈10 µm down to 0.1 nm), porosity, inhomogeneities, gradients and mixed-phase compositions. The ellipsometer covers the spectral range from 360 nm to 1000 nm. The range can be extended into the UV (down to 250 nm) for wide-bandgap semiconductors and metal-like 2D materials, and into the NIR (up to 1700 nm) for the characterization of plasmons and vibrational signatures.

Microscopic IR polarimetry

Microscopic IR polarimetry (BRUKER Hyperion 3000) is well-suited to examine 2D materials, metamaterials, hybrid materials, and thin organic and inorganic layers. Mapping at lateral resolutions of about 100 µm resolution is possible to study lateral inhomogeneities and anisotropies, both in reflection and transmission geometries. Using a grazing incidence objective in reflection, thin films down to monolayers can be measured. Microfluidic equipment can be attached for studies at solid/liquid interfaces. A temperature cell for detailed studies of temperature dependent behaviours from –100°C to +300°C or higher is available. 

References:

• Hinrichs et al. 2023, "Infrared and Raman spectroscopic analysis of functionalized graphene" (DOI: 10.1002/appl.202200054)
• Kratz et al. 2018, "Nanoliter Sensing for Infrared Bioanalytics" (DOI: 10.1021/acssensors.7b00902)

Polarization dependent AFM-IR

The polarization dependent AFM-IR (BRUKER Anasys NanoIR2) probes the photothermal expansion of samples in contact-mode at resolutions down to about 30 nm lateral resolution. Depending on the kind of sample, the measured signal can be correlated with the spectral absorption and the chemical structure of the studied material. Polarization dependent measurements enable one to identify anisotropies such as molecular ordering during the aggregation of molecules or of crystals in minerals.