Electronic States of Silicene Allotropes on Ag(111)

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)<i>R</i>13.9°, and (2√3×2√3)<i>R</i>30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes

Electronic States of Silicene Allotropes on Ag(111)

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)<i>R</i>13.9°, and (2√3×2√3)<i>R</i>30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes

Surface Modification of ZnO(0001)–Zn with Phosphonate-Based Self-Assembled Monolayers: Binding Modes, Orientation, and Work Function

We used partially fluorinated alkyl and aromatic phosphonates as model systems with similar molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface. The introduced surface dipole moment allows tailoring the ZnO work function to tune the energy levels at the inorganic–organic interface to organic semiconductors, which should improve the efficiency of charge injection/extraction or exciton dissociation in hybrid electronic devices. By employing a wide range of surface characterization techniques supported by theoretical calculations, we present a detailed picture of the phosphonates’ binding to ZnO, the molecular orientation in the SAM, their packing density, as well as the concomitant work function changes. We show that for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding

Spin Tuning of Electron-Doped Metal–Phthalocyanine Layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate–substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu­(II), Ni­(II), and Fe­(II) ions reduce to Cu­(I), Ni­(I), and Fe­(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state

Giant and Tunable Out-of-Plane Spin Polarization of Topological Antimonene

Topological insulators are bulk insulators with metallic and fully spin-polarized surface states displaying Dirac-like band dispersion. Due to spin-momentum locking, these topological surface states (TSSs) have a predominant in-plane spin polarization in the bulk fundamental gap. Here, we show by spin-resolved photoemission spectroscopy that the TSS of a topological insulator interfaced with an antimonene bilayer exhibits nearly full out-of-plane spin polarization within the substrate gap. We connect this phenomenon to a symmetry-protected band crossing of the spin-polarized surface states. The nearly full out-of-plane spin polarization of the TSS occurs along a continuous path in the energy–momentum space, and the spin polarization within the gap can be reversibly tuned from nearly full out-of-plane to nearly full in-plane by electron doping. These findings pave the way to advanced spintronics applications that exploit the giant out-of-plane spin polarization of TSSs

Physical Delithiation of Epitaxial LiCoO₂ Battery Cathodes as a Platform for Surface Electronic Structure Investigation

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain