Barbora Vénosová

In this study, we explore the influence of surface functionalization and quantum dot size on the electronic and optical properties of MXene quantum dots (MXQDs) using time-dependent density functional theory. Our findings demonstrate that surface terminations have a significant influence on the electronic and optical properties of Ti₂CT₂ (T = O, F, OH) MXQDs. Different functional groups induce notable shifts in both the energy gap and the absorption spectrum. The oxygen termination yields the highest stability and the largest energy gap, while hydroxyl and fluorine terminations shift absorption toward the visible and near-infrared regions. The study also reveals a pronounced blue shift in the absorption spectrum as the Ti₂CO₂ QDs shrink. In addition, small quantum dots with a lateral size of ∼1–2 nm exhibit strong quantum coupling effects, accompanied by an increase in exciton binding energy with delocalization across the quantum dot. The binding energy of the first exciton in MXQDs can achieve up to 75% of its energy gap (in corresponding 2D materials, the typical value is around 25%), which critically influences (increases and shifts) optical absorption.

The magnetic properties of Ti₂C MXenes can be tuned by varying the coverage and patterns of oxygen and fluorine terminations, with potential applications in electronic nanodevices.

In the last six years, the synthesis of MXene-based quantum dots (MXQDs) has gained widespread attention. Due to the quantum confinement effect, it is possible to significantly improve their properties compared to 2D counterparts, such as higher chemical stability and better electronic and optical properties. However, despite the growing interest in their properties, much remains unexplored. One of the biggest challenges is to study in more detail the structure of quantum dots, in particular, their edge functionalization and its effect on their properties. In this paper, the structural stability and electronic and magnetic properties of Ti₂CO₂ QDs based on different lateral dimensions and edge functionalization (–O, –F, and –OH) are investigated using density functional theory. The study shows that the energy gap of Ti₂CO₂–O QDs decreases with increasing lateral size for both nonmagnetic (spin-unpolarized, close shell) and magnetic (spin-polarized, open shell) cases. Furthermore, the magnetic behavior of quantum dots was revealed by shrinking from 2D Ti₂CO₂ to 0D Ti₂CO₂ QDs with lateral dimensions below 1.4 nm. The binding energy confirms the stability of all three types of edge functionalization, while the most stable structure was observed under fully saturated edge oxygenation. Moreover, it was also found that the spin density distribution and the energy gap of Ti₂CO₂–X QDs (X = O, F, and OH) are both dependent on the type of atom saturation. Size and edge confinement modeling has been demonstrated to be an effective tool for tuning the electronic and magnetic properties of MXQDs. Moreover, the observed enhanced spin polarization together with tunable magnetic properties makes the ultrafine Ti₂CO₂–X QDs promising candidates for spintronic applications.

High-resolution X-ray diffraction experiments, theoretical calculations and atom-specific X-ray absorption experiments were used to investigate two nickel complexes, (MePh ₃ P) ₂ [Ni ^II (bdtCl ₂ ) ₂ ]·2(CH ₃ ) ₂ SO [complex (1)] and (MePh ₃ P)[Ni ^III (bdtCl ₂ ) ₂ ] [complex (2)]. Combining the techniques of nickel K - and sulfur K -edge X-ray absorption spectroscopy with high-resolution X-ray charge density modeling, together with theoretical calculations, the actual oxidation states of the central Ni atoms in these two complexes are investigated. Ni ions in two complexes are clearly in different oxidation states: the Ni ion of complex (1) is formally Ni ^II ; that of complex (2) should be formally Ni ^III , yet it is best described as a combination of Ni ²⁺ and Ni ³⁺ , due to the involvement of the non-innocent ligand in the Ni— L bond. A detailed description of Ni—S bond character (σ,π) is presented.

Two 15-membered octaazamacrocyclic nickel(II) complexes are investigated by theoretical methods to shed light on their affinity forwards binding and reducing CO₂. In the first complex ¹[Ni^IIL]⁰, the octaazamacrocyclic ligand is grossly unsaturated (π-conjugated), while in the second ¹[Ni^IILH]²⁺ one, the macrocycle is saturated with hydrogens. One and two-electron reductions are described using Mulliken population analysis, quantum theory of atoms in molecules, localized orbitals, and domain averaged fermi holes, including the characterization of the Ni-C_CO2 bond and the oxidation state of the central Ni atom. It was found that in the [NiLH] complex, the central atom is reduced to Ni⁰ and/or Ni^I and is thus able to bind CO₂ via a single σ bond. In addition, the two-electron reduced ³[NiL]²⁻ species also shows an affinity forwards CO₂.

The structure of 4-methyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]thiazole-2(3H)-thione (MTTOTHP) was investigated using X-ray diffraction and computational chemistry methods for determining properties of the nitrogen—oxygen bond, which is the least stable entity upon photochemical excitation. Experimentally measured structure factors have been used to determine and characterize charge density via the multipole model (MM) and the maximum entropy method (MEM). Theoretical investigation of the electron density and the electronic structure has been performed in the finite basis set density functional theory (DFT) framework. Quantum Theory of Atoms In Molecules (QTAIM), deformation densities and Laplacians maps have been used to compare theoretical and experimental results. MM experimental results and predictions from theory differ with respect to the sign and/or magnitude of the Laplacian at the N—O bond critical point (BCP), depending on the treatment ofnvalues of the MM radial functions. Such Laplacian differences in the N—O bond case are discussed with respect to a lack of flexibility in the MM radial functions also reported by Rykounovet al.[Acta Cryst.(2011), B67, 425–436]. BCP Hessian eigenvalues show qualitatively matching results between MM and DFT. In addition, the theoretical analysis used domain-averaged fermi holes (DAFH), natural bond orbital (NBO) analysis and localized (LOC) orbitals to characterize the N—O bond as a single σ bond with marginal π character. Hirshfeld atom refinement (HAR) has been employed to compare to the MM refinement results and/or neutron dataset C—H bond lengths and to crystal or single molecule geometry optimizations, including considerations of anisotropy of H atoms. Our findings help to understand properties of molecules likeMTTOTHPas progenitors of free oxygen radicals.