Non-destructive testing of nanorough substrates for biomedicine and optoelectronics applications

Modern methods for measuring the heterogeneities of nanosurfaces are divided into contact and non-contact approaches. Among contact measurement methods, optical methods are distinguished, which are characterized by high measurement accuracy in the nanometer/subnanometer range. It is possible to select stylus profilometers or AFM, but the mechanical damage of the sample imposes restrictions on these methods. Non-contact (optical) methods, like confocal microscopes or correlation-optical (interferometric) methods, are non-destructive. They enable us to achieve a sub-nanometer resolution for measuring height surface roughness parameters, with a limit in the transverse direction by the Abbe diffraction criterion. Regarding testing nanostructured surfaces, such as substrates for touch displays, bioimplants, or membranes of biological cells, sensitive and non-destructive full 3D nanotopography becomes possible. We propose a novel non-destructive method based on the principle of super-resolution STED microscopy, overcoming the diffraction limit to measure both height and spatial roughness parameters and 3D surface reconstruction with nanometer accuracy. As surface probes, luminescent carbon nanoparticles with a size of about 30 nm and a significant dipole moment, distributed over the surface area in an external electric field, are used. The specially designed optical setup using structured optical beams provides a pathway to overcome the diffraction limits imposed by the Abbe resolution criterion, enabling a spatial resolution corresponding to the nanoparticle size. The particle's luminescence intensity linearly depends on the local surface height in a range of inhomogeneities up to 100 nm, which enables the effective testing of nanoroughness with an RMS of about 5-15 nm. The use of AI/ML for surface reconstruction based on irregular coordinate values of luminescence intensity is expected. The proposed approach can be applied for non-destructive surface parameter testing and 3D nanotopography of nanostructured surfaces like cell membranes, titanium substrates for bioimplants, or glass substrates for optoelectronic devices.