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.

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 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.

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.