Samuel Froeschke

Van der Waals layered rhodium chalcochlorides demonstrate a unique interplay between the structure of the layers and the size of the anions. While RhSeCl has a 2D Janus structure with Se and Cl on opposite sides of the layers, RhTeCl forms a non-Janus structure with alternating stripes of Cl and Te. Here, we study the lattice dynamics of these semiconductors by a combination of far-infrared and Raman spectroscopy and DFT phonon calculations. Polarization-dependent measurements enabled the complete assignment of all Raman-active modes of both compounds. The resonance effects in Raman scattering were addressed by using multiple excitation wavelengths and a tunable dye laser between 457 and 785 nm. Raman excitation profiles appeared to be more sensitive to optical excitations than direct measurements of excitation spectra by diffuse reflectance spectroscopy. Finally, variable-temperature Raman measurements between 10 and 300 K revealed that the optical phonons of RhSeCl are weakly susceptible to temperature variations, whereas the temperature dependence of RhTeCl is more pronounced.

Substrate-mediated CVT growth enables highly crystalline 3R-TaSe ₂ nanocrystals exhibiting charge density wave behavior at ∼100 K and superconductivity below 2.2 K.

Synthesis of high crystalline CrCl ₃ /MoS ₂ van der Waals heterostructures via chemical vapor transport approach.

The structural and electrical properties of thin nanocrystals of the 3D topological insulator BiSb(Te _{1− y} Se _y ) ₃ ( y = 0, 0.01, 0.02, …, 0.09) have been investigated.

A ZnO-Graphene oxide nanocomposite (Z-G) was prepared in order to exploit the biomedical features of each component in a single anticancer material. This was achieved by means of an environmentally friendly synthesis, taking place at a low temperature and without the involvement of toxic reagents. The product was physicochemically characterized. The ZnO-to-GO ratio was determined through thermogravimetric analysis, while scanning electron microscopy and transmission electron microscopy were used to provide insight into the morphology of the nanocomposite. Using energy-dispersive X-ray spectroscopy, it was possible to confirm that the graphene flakes were homogeneously coated with ZnO. The crystallite size of the ZnO nanoparticles in the new composite was determined using X-ray powder diffraction. The capacity of Z-G to enhance the toxicity of the anticancer drug Paclitaxel towards breast cancer cells was assessed via a cell viability study, showing the remarkable anticancer activity of the obtained system. Such results support the potential use of Z-G as an anticancer agent in combination with a common chemotherapeutic like Paclitaxel, leading to new chemotherapeutic formulations.

Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl₃, 2D van der Waals material with covalently bonded Mo₂ dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl₃ were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and (Density Functional Theory) DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, PL, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d³–d³ interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm⁻¹ when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo₂ dimer (σ → σ*). The deeper understanding of the metal–metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.

Zinc oxide/Curcumin (Zn(CUR)O) nanocomposites were prepared via hydrothermal treatment of Zn(NO₃)₂ in the presence of hexamethylenetetramine as a stabilizing agent and CUR as a bioactive element. Three ZnO : CUR ratios were investigated, namely 57 : 43 (Zn(CUR)O-A), 60 : 40 (Zn(CUR)O-B) and 81 : 19 (Zn(CUR)O-C), as assessed by thermogravimetric analyses, with an average hydrodynamic diameter of nanoaggregates in the range of 223 to 361 nm. The interaction of CUR with ZnO via hydroxyl and ketoenol groups (as proved by X-ray photoelectron spectroscopy analyses) was found to significantly modify the key properties of ZnO nanoparticles with the obtainment of a bilobed shape (as shown by scanning electron microscopy), and influenced the growth process of the composite nanoparticles as indicated by the varying particle sizes determined by powder X-ray diffraction. The efficacy of Zn(CUR)O as anticancer agents was evaluated on MCF-7 and MDA-MB-231 cancer cells, obtaining a synergistic activity with a cell viability depending on the CUR amount within the nanocomposite. Finally, the determination of reactive oxygen species production in the presence of Zn(CUR)O was used as a preliminary evaluation of the mechanism of action of the nanocomposites.

In the search for novel 2D materials with potentially valuable properties, such as a tunable band gap for optoelectronic or catalytic applications, solid solutions hold the potential to significantly expand the inventory of available 2D nanomaterials. In this study, we present for the first time the synthesis of such 2D rhodium trihalide solid solutions: RhBr _x Cl_3−x and RhBr _x I_3−x . We use thermodynamic simulations and simultaneous thermal analysis to predict conditions for their rational synthesis and to investigate suitable chemical vapor transport (CVT) parameters for these solid solutions. The evolution of the lattice parameters was investigated by powder x-ray diffraction, showing an isostructural relationship of the synthesized compounds and only minor deviation from Vegard’s law. The optical band gap of these materials can be tuned in an energy range from 1.5 eV (RhCl₃) to 1.2 eV (RhI₃) by choosing the composition of the solid solution, while the samples also exhibit photoluminescence in similar energy ranges. Ultimately, the successful deposition of bulk as well as ultrathin 2D nanocrystals of RhBr _x Cl_3−x by CVT from 925 °C to 850 °C is shown, where the composition of the deposited crystals is precisely controlled by the choice of the starting composition and the initial amount of material. The high quality of the obtained nanocrystals is confirmed by atomic force microscopy, high resolution transmission electron microscopy and selected area electron diffraction. For RhBr _x I_3−x , the CVT from 900 °C to 825 °C is more difficult and has only been practically demonstrated for an exemplary case. According to the observed properties, these novel solid solutions and nanocrystals show a great potential for an application in optoelectronic devices.