Many-body correlations are known to be responsible for a broad range of fascinating physical phenomena, introducing corrections that appear elusive at the mean-field level. An example of this is the Lifshitz transition that occurs as the Fermi surface topology changes when e.g. Coulomb interaction effects break into the picture. In particular, the Fermi velocity renormalization can lead a type-II Weyl semimetal at mean-field level to become a trivial or a type-I Dirac material when correlations are accounted for, which is far from being obvious. In this work we scrutinize the band structure of NiTe_2, a material that features a type-II Dirac point near the Fermi level within the mean-field approach. Including GW-level correlations, our findings showcase anisotropic corrections on the Dirac carrier velocity exceeding 100 % enhancements, underscoring the nuanced influence of electronic interactions in the band structure. We also consider type-II Dirac crossings in PtSe_2 and PtTe_2 and observe that including many-body effects via GW the band topology changes, featuring trivial topology and type-I Dirac crossings, respectively. Our findings highlights the necessity to evaluate the many-body effects on non-trivial bands, contributing essential insights to the broader exploration of many-body correlation effects in type-II Dirac points of condensed-matter systems.

Interfacial engineering has fueled recent development of p-i-n perovskite solar cells (PSCs), with self-assembled monolayer-based hole-transport layers (SAM-HTLs) enabling almost lossless contacts for solution-processed PSCs, resulting in the highest achieved power conversion efficiency (PCE) to date. Substrate interfaces are particularly crucial for the growth and quality of co-evaporated PSCs. However, adoption of SAM-HTLs for co-evaporated perovskite absorbers is complicated by the underexplored interaction of such perovskites with phosphonic acid functional groups. In this work, we highlight how exposed phosphonic acid functional groups impact the initial phase and final bulk crystal structures of co-evaporated perovskites and their resultant PCE. The explored surface interaction is mediated by hydrogen bonding with interfacial iodine, leading to increased formamidinium iodide adsorption, persistent changes in perovskite structure, and stabilization of bulk α-FAPbI3, hypothesized as being due to kinetic trapping. Our results highlight the potential of exploiting substrates to increase control of co-evaporated perovskite growth.

In this work, a generalized force-field methodology for the relaxation of large moire heterostructures is proposed. The force-field parameters are optimized to accurately reproduce the structural degrees of freedom of some computationally manageable cells relaxed using density functional theory. The parameters can then be used to handle large moire systems. We specialize in the case of 2H-phased twisted transition-metal dichalcogenide homo-and heterobilayers using a combination of the Stillinger-Weber intralayer and the Kolmogorov-Crespi interlayer potential. Force-field parameters are developed for all combinations of MX2 for M & ISIN; {Mo, W} and X & ISIN; {S, Se, Te}. The results show agreement within 20 meV in terms of band structure between density functional theory and force-field relaxation. Using the relaxed structures, a simplified and systematic scheme for the extraction of the interlayer moire potential is presented for both R-and H-stacked systems. We show that in-plane and out-of-plane relaxation effects on the moire potential, which is made both deeper and wider after relaxation, are essential. An interpolation based methodology for the calculation of the interlayer binding energy is also proposed. Finally, we show that atomic reconstruction, which is captured by the force-field method, becomes especially prominent for angles below 4 degrees -5 degrees, when there is no mismatch in lattice constant between layers.

Mn2+-doped semiconductor nanocrystals with tuned location and concentration of Mn2+ ions can yield diverse coupling regimes, which can highly influence their optical properties such as emission wavelength and photoluminescence (PL) lifetime. However, investigation on the relationship between the Mn2+ concentration and the optical properties is still challenging because of the complex interactions of Mn2+ ions and the host and between the Mn2+ ions. Here, atomically flat ZnS nanoplatelets (NPLs) with uniform thickness were chosen as matrixes for Mn2+ doping. Using time-resolved (TR) PL spectroscopy and density functional theory (DFT) calculations, a connection between coupling and PL kinetics of Mn2+ ions was established. Moreover, it is found that the Mn2+ ions residing on the surface of a nanostructure produce emissive states and interfere with the change of properties by Mn2+-Mn2+ coupling. In a configuration with suppressed surface contribution to the optical response, we show the underlying physical reasons for double and triple exponential decay by DFT methods. We believe that the presented doping strategy and simulation methodology of the Mn2+-doped ZnS (ZnS:Mn) system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors.

Transition metal dichalcogenides monolayers host strongly bounded Coulomb complexes such as exciton and trion due to charge confinement and screening reduction in two dimensions. Biexciton, a bound state of two electrons and two holes, has also been observed in these materials with a binding energy which is one order of magnitude larger than its counterpart in conventional semiconductors. Here, using first principles methods, we address the biexciton in WSe2 monolayer and unravel the important role of the electron-hole exchange interaction in dictating the valley character of biexciton states and their fine structure. In particular, the fundamental biexciton transition which is located between the exciton and trion peaks is shown to have a fine structure of 2.8 meV mainly due to the splitting of the dark exciton state under the intervalley electron-hole exchange interaction. Non equilibrium effects are also addressed and optical fingerprints of non-thermalized biexciton population are discussed. The reduced dimensions of 2D materials make them an ideal platform to realise quantum many-body effects such as the formation of exciton complexes. Here, using first principles calculations the authors investigate biexcitons in WSe2 monolayers and uncover the role electron-hole exchange interaction plays in the valley characteristics of the biexciton.

Tin telluride is a narrow gap semiconductor with promising properties for IR optical applications and topological insulators. We report a convenient colloidal synthesis of quasi-two-dimensional SnTe nanocrystals through the hot-injection method in a non-polar solvent. By introducing the halide alkane 1-bromotetradecane as well as oleic acid and trioctylphosphine, the thickness of two dimensional SnTe nanostripes can be tuned down to 30 nm, while the lateral dimensional can reach 6 microns. The obtained SnTe nanostripes are single-crystalline with a rock-salt crystal structure. The absorption spectra demonstrate pronounced absorption features in the IR range revealing the effect of quantum confinement in such structures.

Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their exciton Bohr radius receive a rapidly increasing attention due to their unique physicochemical properties. These superior properties highly depend on the shape and crystal phase of semiconductor NCs. Here, we demonstrate not only the synthesis of well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and emission, but also the shape/phase transformation between wurtzite (WZ) NPLs and zinc blende (ZB) nanorods (NRs). UV-vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp excitonic peak that is not observed in ZB-ZnS NRs. Besides, the photoluminescence characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak, while ZB-ZnS NRs exhibit a broad collective emission band consisting of four emission peaks. The appearance of excitonic features in the absorption spectra of ZnS NPLs is explained by interband electronic transitions, which is simulated in the framework of atomic effective pseudopotentials (AEP).

A hybrid theory which combines configuration interaction and Green's function approaches is proposed here to treat charged and neutral excitations. The theory is parameter free and reduces to the well-known Bethe-Salpeter equation (BSE) in the case of excitons. However, unlike the BSE, the theory can be applied to calculate any excitation beyond the exciton. As this type of computation is generally time consuming, we show that in the case of Wannier-type excitations, the localization in reciprocal space can be used to reduce the required computation load. We apply our approach to excitons and trions in ${\mathrm{WS}}_{2}$ and ${\mathrm{MoS}}_{2}$ transition metal dichalcogenides monolayers and obtain optical spectra, binding energies, and dark-bright exciton splitting in good agreement with experimental measurements. Moreover, in the case of ${\mathrm{WS}}_{2}$, we have found that the negative trion peak shows a fine structure splitting (FSS) of 15 meV which corresponds to the FSS for an isolated monolayer.

Multilayer graphene with rhombohedral stacking is a promising carbon phase possibly displaying correlated states like magnetism or superconductivity due to the occurrence of a flat surface band at the Fermi level. Recently, flakes of thickness up to 17 layers were tentatively attributed to ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D resonant Raman spectrum of Bernal graphite is not understood. We provide a first principles description of the 2D Raman peak in three and four layers graphene (all stackings) as well as in Bernal, rhombohedral, and an alternation of Bernal and rhombohedral graphite. We give practical prescriptions to identify long range sequences of ABC multilayer graphene. Our work is a prerequisite to experimental nondestructive identification and synthesis of rhombohedral graphite.

We show that the Landau levels in epitaxial graphene in the presence of localized defects are significantly modified compared to those of an ideal system. We report on magnetospectroscopy experiments performed on high-quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated with short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies as the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments.