František Karlický

We report a systematic ab initio investigation of the excited states and absorption spectra of small mercury clusters (Hg2–5). Benchmarking multi-reference configuration interaction (MR-CISD) and equation-of-motion coupled-cluster (EOM-CCSD) methods shows that EOM-CCSD/aug-cc-PVTZ-PP reproduces atomic excitation energies within ±500 cm−1 of experiment, whereas MR-CISD deviates by about ±1100 cm−1. For the mercury dimer, EOM-CCSD potential energy curves yield absorption bands in excellent agreement with experimental transitions, while asymptotic excitation energies closely match atomic references. Extending this methodology, we present the first systematic ab initio study of absorption spectra of Hg3, Hg4, and Hg5, which display broad bands in the range of 37 000–60 000 cm−1. The consistent agreement with experimental benchmarks and diagnostics confirming single-reference character validates EOM-CCSD as a robust approach for describing excited states and optical spectra of mercury clusters.

Van der Waals heterostructures based on MXenes and transition metal dichalcogenides offer a versatile platform for engineering excitonic and optoelectronic properties. However, accurate prediction of their behavior is non-trivial; we highlight that reliance on standard 1 × 1 models of lattice-mismatched systems introduces artificial strain, creating a significant risk of misattributing computational artifacts to intrinsic interface physics. Here, we present a comprehensive first-principles investigation of Ti ₂ CO ₂ /MoS ₂ and Ti ₃ C ₂ O ₂ /MoS ₂ heterostructures, employing many-body perturbation theory (GW-BSE) to benchmark the computationally efficient TD-HSE06 + Δ methodology for realistic, strain-relaxed Moiré supercells. Our analysis reveals a distinct “quasi-type-I” optoelectronic character for these systems. We show that advanced methods are indispensable for MXene/MoS ₂ heterostructures, as basic electronic structure predictions yield an incomplete picture: although the bands exhibit a type-II staggered alignment with the valence band maximum localized on the MoS ₂ and the conduction band minimum on the MXene, the theoretically lowest-energy interlayer transitions are rendered effectively dark due to negligible wavefunction overlap. Consequently, the optical landscape is dominated by bright intralayer excitons confined within the MXene layer. Therefore, while the type-II band alignment allows for subsequent non-radiative charge separation, the initial direct optical generation and radiative recombination of interlayer excitons are highly suppressed. This unique decoupling suggests these heterostructures can function as robust platforms where the bright optical transitions of the MXene are preserved, while still participating in ultrafast non-radiative interfacial charge dynamics.

The emergence of two-dimensional van der Waals (vdW) heterostructures has revolutionized exciton-based research due to their tunable electronic and optical properties. Here, we present a systematic many-body study of a BP/CrSe₂ vdW heterostructure composed of experimentally realized boron phosphide (BP) and chromium diselenide (CrSe₂) monolayers. We find a robust type-II band alignment across all stacking configurations; however, interlayer excitons emerge selectively depending on the stacking order. The vdW heterostructure exhibits strong exciton binding energy (0.34-0.44 eV, depending on stacking) and nanosecond-scale room-temperature radiative lifetimes, indicating long-lived excitonic states. Additionally, its optical absorption extends from the visible to the near-infrared regime, underscoring its potential for optoelectronic and excitonic applications. Our findings highlight the critical role of stacking-dependent interlayer coupling in controlling the excitonic properties of vdW heterostructures, paving the way for tailored design strategies in next-generation light-harvesting and quantum technologies.

Here, we use a GFN-xTB (Geometries, Frequencies, and Noncovalent interactions Tight Binding) Hamiltonian of the density functional tight binding (DFTB) method to study the structural, electronic, and magnetic properties of Ti₂MC₂ (M = Cr, Mo, and W) and Ti₂MC₂T₂ (T = -F, -O, -OH, and -Cl). By investigating the magnetic ordering, we found that the bare, F, -OH, and Cl-terminated Ti₂MC₂ MXenes prefer an antiferromagnetic (AFM) alignment and show a metallic character. However, the magnetism is suppressed by O termination. This result suggests that magnetism can be enhanced in the presence of F-, OH-, and Cl-surface functional groups. We also investigated the Ti and M vacancy defects. We remark that the M-defect maintains the AFM ordering, whereas a magnetic transition from AFM to ferromagnetic (FM) ordering is shown by the creation of a Ti vacancy. Among the studied double transition-metal MXenes, the Ti-defected systems show the highest magnetic moments, and the magnetism is mainly due to the Ti atoms. Our DFTB investigations allow for efficient exploration of the magnetism in double metal MXenes and the effect of functionalization on the electronic and magnetic properties of the studied materials. The obtained electronic properties suggest that the Ti₂MC₂T₂ monolayers are metals, the same behavior as compared to the majority of monotransition-metal MXenes. These findings can serve as a guide for comprehending the electronic and magnetic properties of double metal MXenes.

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.

Chlorinated hydrocarbons, widely used in industrial applications, pose significant environmental and health risks due to their persistence and toxicity. Conventional remediation methods often face efficiency limitations, necessitating the exploration of innovative materials for effective pollutant degradation. MXenes, a novel class of two-dimensional materials, have emerged as promising candidates for environmental remediation due to their exceptional chemical reactivity and tunable surface properties. This study employs density functional theory to investigate trichloroethylene’s (TCE) adsorption and dechlorination mechanisms on variously terminated MXenes. Our results reveal that Ti₂C(OH)₂ and V₂C(OH)₂ facilitate spontaneous TCE dechlorination. With realistic termination ratios, the reaction barriers of TCE dechlorations are below 1 eV. Remarkably, only with just 17% of –O terminations substituted by –OH, non-defected Ti₂CO₂ demonstrates robust dechlorination, forming dichloroethylene and hydrochloric acid (HCl). Additionally, single terminal vacancy MXenes exhibit highly favorable reaction pathways with barriers as low as 0.1 eV. Beyond TCE, chlorinated hydrocarbons lindane and dichlorodiphenyltrichloroethane (DDT) spontaneously dechlorinate on pristine Ti₂C(OH)₂, underscoring the broad applicability of MXenes in chlorinated hydrocarbon remediation. This work highlights MXenes as scalable, efficient, and versatile materials for addressing persistent organic pollutants, providing critical insights into their mechanisms and potential for practical implementation in environmental cleanup strategies.