Using a multiscale computational scheme, we study the trends in distribution and composition of the surface functional groups −O, −OH, and −F on two-dimensional (2D) transition metal carbides and nitrides (MXenes). We consider Ti₂N, Ti₄N₃, Nb₂C, Nb₄C₃, Ti₂C, and Ti₃C₂ to explore MXenes with different chemistry and different number of atomic layers. Using a combination of cluster expansion, Monte Carlo, and density functional theory methods, we study the distribution and composition of functional groups at experimentally relevant conditions. We show that mixtures of functional groups are favorable on all studied MXene surfaces. The distribution of functional groups appears to be largely independent of the type of metal, carbon, or nitrogen species and/or number of atomic layers in the MXene. We further show that some properties (e.g., the work function) strongly depend on the surface composition, while others, for example, the electric conductivity, exhibit only a weak dependence.

MXenes are a new rapidly developing class of two-dimensional materials with suitable properties for a broad range of applications. It has been shown that during synthesis of these materials the surfaces are usually functionalized by O, OH, and F and further suggested that controlling the surface allows controlling the material properties. However, a proper understanding of the surface structure is still missing, with a significant discrepancy between computational and experimental studies. Experiments consistently show formation of surfaces with mixed terminations, whereas computational studies point toward pure terminated surfaces. Here, we explain the formation of mixed functionalization on the surface of titanium-based two-dimensional carbides, Ti₂C and Ti₃C₂, using a multiscale modeling scheme. Our scheme is based on calculating Gibbs free energy of formation by a combination of electronic structure calculations with cluster expansion and Monte Carlo simulations. Our calculations show formation of mixtures of O, OH, and F on the surface with the composition depending on pH, temperature, and the work function. On the other hand, our results also suggest a limited stable range of compositions, which challenges the paradigm of a high tunability of MXene properties.

The efficiency of Cu(In,Ga)Se₂ (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe₂. First, the effects of the AM impurities in CuInSe₂ have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only for Li, Na, and K, whereas all the AM atoms can diffuse with the help of Cu vacancies. The competition between these two mechanisms strongly depends on the concentration of Cu vacancies. We also discuss how AM atoms can contribute to increasing Cu-depleted regions. Second, AM atoms can form secondary phases with Se and In atoms. We suggest a mechanism for the secondary phase formation following the PDT process. On the basis of the calculated reaction enthalpies and migration considerations, we find that mixed phases are more likely in the case of LiInSe₂ and NaInSe₂, whereas formation of secondary phases is expected for KInSe₂, RbInSe₂, and CsInSe₂. We discuss our findings in the light of experimental results obtained for AM treatments. The secondary phases have large energy band gaps and improve the morphology of the buffer surface by enabling a favorable band alignment, which can improve the electrical properties of the device. Moreover, they can also passivate the surface by forming a diffusion barrier. Overall, our work points to different roles played by the light and heavy AM atoms and suggests that both types may be needed to maximize their benefits on the solar cell performance.