Surface-Enhanced Raman Spectroscopy and Artificial Neural Networks for Detection of MXene Flakes' Surface Terminations

The properties of MXene flakes, a new class of two-dimensional materials, are strictly determined by their surface termination. The most common termination groups are oxygen-containing (=O or -OH) and fluorine (-F), and their relative ratio is closely related to flake stability and catalytic activity. The surface termination can vary significantly among MXene flakes depending on the preparation route and is commonly determined after flake preparation by using X-ray photoelectron spectroscopy (XPS). In this paper, as an alternative approach, we propose the combination of surface-enhanced Raman spectroscopy (SERS) and artificial neural networks (ANN) for the precise and reliable determination of MXene flakes' (Ti3C2Tx) surface chemistry. Ti3C2Tx flakes were independently prepared by three scientific groups and subsequently measured using three different Raman spectrometers, employing resonant excitation wavelengths. Manual analysis of the SERS spectra did not enable accurate determination of the flake surface termination. However, the combined SERS-ANN approach allowed us to determine the surface termination with a high accuracy. The reliability of the method was verified by using a series of independently prepared samples. We also paid special attention to how the results of the SERS-ANN method are affected by the flake stability and differences in the conditions of flake preparation and Raman measurements. This way, we have developed a universal technique that is independent of the above-mentioned parameters, providing the results with accuracy similar to XPS, but enhanced in terms of analysis time and simplicity.

Plasmon assisted Ti3C2Tx grafting and surface termination tuning for enhancement of flake stability and humidity sensing performance

Humidity sensors play a critical role in monitoring human activities, environmental health, food processing and storage, and many other fields. Recently, some 2D materials, particularly MXenes, have been considered as promising candidates for creating humidity sensors because of their high surface area, surface-to-bulk ratio, and excellent conductivity, arising from the high concentration and mobility of free electrons. In this work, we propose the plasmon-assisted surface modification and termination tuning of common MXene (Ti3C2Tx) to enhance their response to humidity and increase their stability against oxidation. Hydrophobic (-C6H4-CF3) and hydrophilic (-C6H4-COOH) chemical moieties were covalently grafted to the Ti3C2Tx surface using plasmon-mediated diazonium chemistry. In situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements, performed at different humidity levels indicate that surface modification significantly affects penetration of water molecules in Ti3C2Tx films. As a result, the sensitivity of the flakes to the presence of water molecules was significantly altered. Additionally, proposed surface grafting commonly proceeds on the less stable MXene surface sites, where flake oxidation commonly initiates. As a result of the modification, such "weak" and more chemically active sites were blocked and Ti3C2Tx stability was significantly enhanced.

On the Dynamics of Transient Plasmas Generated by Nanosecond Laser Ablation of Several Metals

The dynamics of transient plasma generated by UV ns-laser ablation of selected metals (Co, Cu, Ag, Bi) were investigated by the Langmuir Probe method in angle- and time-resolved modes. Multiple ionic and electronic structures were seen for all plasmas with some corresponding to anions or nanoparticle-dominated structures. The addition of an Ar atmosphere energetically confined the plasma and increased the charge density by several orders of magnitude. For pressure ranges exceeding 0.5 Pa fast ions were generated in the plasma as a result of Ar ionization and acceleration in the double layer defining the front of the plasma plume. Several correlations between the target nature plasma properties were attempted. The individual plasma structure expansion velocity increases with the melting point and decreases with the atomic mass while the corresponding charged particle densities decrease with the melting point, evidencing the relationship between the volatility of the sample and the overall abated mass.