We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 due to the combination of spin-orbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomiclike orbitals corresponding to d and p orbitals of the transition metal (W, Mo) and chalcogenide (S, Se) atoms respectively. In the resulting Hamiltonian, we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyze their contribution to the spin splitting at the high symmetry points. We find that-in contrast to the valence band-both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often n-doped, our findings are important for developments in TMD spintronics.

We study the electronic structure of a heterojunction made of two monolayers of MoS 2 and WS 2 . Our first-principles density functional calculations show that, unlike in the homogeneous bilayers, the heterojunction has an optically active band gap, smaller than the ones of MoS 2 and WS 2 single layers. We find that the optically active states of the maximum valence and minimum conduction bands are localized on opposite monolayers, and thus the lowest energy electron-holes pairs are spatially separated. Our findings portray the MoS 2 -WS 2 bilayer as a prototypical example for band-gap engineering of atomically thin two-dimensional semiconducting heterostructures.

Understanding the interaction between water and ceria surfaces is crucial in many catalytic applications. For the clean CeO2(111) surface, density functional theory (DFT) calculations using different generalized gradient approximations (GGAs) to the exchange-correlation functional and the DFT­(GGA)+U method have found that the most stable configuration is on top of a surface cerium atom. However, they disagree on the nature of the adsorption state, with water molecularly adsorbed with one or two Os–H hydrogen bonds (Os indicates a surface oxygen atom) or as a hydroxyl pair (OsHads–OHads), with only one recent report suggesting that these two structures are very close in energy. In this work, we studied the adsorption of water on CeO2(111) employing different approximations to exchange and correlation within DFT, namely, the Perdew–Burke–Ernzerhof (PBE) GGA, DFT­(PBE)+U, the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional, and van der Waals (vdW) density functionals [DFT­(vdW-DF/vdW-DF2)+U] with optimized exchange functionals (for vdW-DF, optB86b, revPBE, and optPBE; for vdW-DF2, rPW86). All of these methods predict close energies (10–100 meV range) for the two lowest-energy structures, the molecular structure with one Os–H bond and the hydroxyl pair. Our calculations show that these two species should be distinguishable by their infrared (IR) spectra. In particular, a rocking libration at 850 cm–1 could be used as an IR fingerprint to reveal the presence of the molecular structure. We found that the inclusion of vdW interactions increases binding energies by ∼0.18 eV, bringing them closer to the available experimental values.

The chirality of molecular structures is paramount in many phenomena, including enantioselective reactions, molecular self-assembly, biological processes and light or electron-spin polarization. Flat prochiral molecules, which are achiral in the gas phase or solution, can exhibit adsorption-induced chirality when deposited on surfaces. The whole array of such molecular adsorbates is naturally racemic as spontaneous global mirror-symmetry breaking is disfavoured. Here we demonstrate a chemical method of obtaining flat prochiral molecules adsorbed on the solid achiral surface in such a way that a specific adsorbate handedness globally dominates. An optically pure helical precursor is flattened in a cascade of on-surface reactions, which enables chirality transfer. The individual reaction products are identified by high-resolution scanning-probe microscopy. The ultimate formation of globally non-racemic assemblies of flat molecules through stereocontrolled on-surface synthesis allows for chirality to be expressed in as yet unexplored types of organic-inorganic chiral interfaces.

Ferrocene-based molecules are extremely appealing as they offer a prospect of having built-in spin or charge functionality. However, there are only limited number of studies of structural and electronic properties on surfaces so far. We investigated the self-assembly processes of 1,1′-ferrocene dicarboxylic acid molecules (C12H10FeO4) on both metallic (Ag(111), Au(111), and Cu(110)) and insulating (Cu3N/Cu­(110)) surfaces with high-resolution ncAFM/STM, XPS, and NEXAFS. The experimental evidence is corroborated with total energy DFT calculations and ncAFM simulations. The combined experimental and theoretical analysis allows detailed understanding of the unique arrangement and adsorption geometries of the molecules on different substrates, as well as the different chemical stability of the carboxylic (COOH) groups. The molecules on noble (Ag, Au) surfaces show only a weak interaction with the substrate forming a complex self-assembled pattern, driven by weak intermolecular interactions. In contrast, the analysis reveals the carboxylic groups undergo dehydrogenation on the Cu(110) and Cu3N/Cu­(110). As a result, the oxygen atoms form strong chemical bonds to the substrate Cu atoms and impose an orientation on the ferrocene cyclopentadienyl rings perpendicular to the substrate.

Ce3+ sites may play an important role in defining active sites in ceria-based catalysis. Yet, though present at the catalyst surface, their positions have not been experimentally observed. Recently, it has been argued that Ce3+ sites are the preferred location for the binding of Au– species on a highly reduced CeO2(111) surface with subsurface oxygen defects. Hence, Au atoms have been proposed as the ideal marker for the reduced centers. Using density functional theory (DFT) with the HSE06 hybrid functional and the DFT­(PBE)+U approach, we find that the binding of Au– at a hollow site centered at a subsurface oxygen atom, where the 4f → 6s electron transfer occurs from a Ce3+ ion in a deeper layer, is energetically more favorable by 0.36 (0.34) eV with HSE06 (PBE+U) than on-top of a Ce3+ ion in the outermost cerium layer. Au atoms can thus not be taken as position markers for Ce3+ surface sites. The site preference is explained in terms of the reduction of both lattice strain and Coulomb repulsion. Our finding is consistent with the interpretation of the most recent experiments where Au– atom pairs with mean separation of two lattice parameters were observed.

We present the results of computational and experimental studies of the electronic and structural properties of the Pb/Mo͑110͒ adsorption system. Computational part of this work is based on ab initio density-functional calculations. The obtained results provide detailed geometrical properties of different adsorption configurations and show the influence of lead coverage on the adsorption energy at different adsorption sites. We also consider the formation of ordered lead superstructures on the Mo͑110͒ surface predicted by earlier experimental investigations as well as those indicated by our present scanning tunneling microscopy ͑STM͒ measurements. Our STM results show coexistence of two well-ordered surface superstructures in the first lead layer.

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