6 research outputs found
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 LiCoO<sub>2</sub> 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