154 research outputs found
Electronic Structures of N-doped Graphene with Native Point Defects
Nitrogen doping in graphene has important implications in graphene-based
devices and catalysts. We have performed the density functional theory
calculations to study the electronic structures of N-doped graphene with
vacancies and Stone-Wales defect. Our results show that monovacancies in
graphene act as hole dopants and that two substitutional N dopants are needed
to compensate for the hole introduced by a monovacancy. On the other hand,
divacancy does not produce any free carriers. Interestingly, a single N dopant
at divacancy acts as an acceptor rather than a donor. The interference between
native point defect and N dopant strongly modifies the role of N doping
regarding the free carrier production in the bulk pi bands. For some of the
defects and N dopant-defect complexes, localized defect pi states are partially
occupied. Discussion on the possibility of spin polarization in such cases is
given. We also present qualitative arguments on the electronic structures based
on the local bond picture. We have analyzed the 1s-related x-ray photoemission
and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales
defect in connection with the experimental ones. We also discuss characteristic
scanning tunneling microscope (STM) images originating from the electronic and
structural modifications by the N dopant-defect complexes. STM imaging for
small negative bias voltage will provide important information about possible
active sites for oxygen reduction reaction.Comment: 40 pages, 2 tables, 16 figures. The analysis of Clar sextets is
added. This version is published on PHYSICAL REVIEW B 87, 165401(2013
Interplay between Nitrogen Dopants and Native Point Defects in Graphene
To understand the interaction between nitrogen dopants and native point
defects in graphene, we have studied the energetic stability of N-doped
graphene with vacancies and Stone-Wales (SW) defect by performing the density
functional theory calculations. Our results show that N substitution
energetically prefers to occur at the carbon atoms near the defects, especially
for those sites with larger bond shortening, indicating that the defect-induced
strain plays an important role in the stability of N dopants in defective
graphene. In the presence of monovacancy, the most stable position for N dopant
is the pyridinelike configuration, while for other point defects studied (SW
defect and divacancies) N prefers a site in the pentagonal ring. The effect of
native point defects on N dopants is quite strong: While the N doping is
endothermic in defect-free graphene, it becomes exothermic for defective
graphene. Our results imply that the native point defect and N dopant attract
each other, i.e., cooperative effect, which means that substitutional N dopants
would increase the probability of point defect generation and vice versa. Our
findings are supported by recent experimental studies on the N doping of
graphene. Furthermore we point out possibilities of aggregation of multiple N
dopants near native point defects. Finally we make brief comments on the effect
of Fe adsorption on the stability of N dopant aggregation.Comment: 10 pages, 5 figures. Figure 4(g) and Figure 5 are corrected. One
additional table is added. This is the final version for publicatio
Atomic-scale characterization of nitrogen-doped graphite: Effects of dopant nitrogen on the local electronic structure of the surrounding carbon atoms
We report the local atomic and electronic structure of a nitrogen-doped
graphite surface by scanning tunnelling microscopy, scanning tunnelling
spectroscopy, X-ray photoelectron spectroscopy, and first-principles
calculations. The nitrogen-doped graphite was prepared by nitrogen ion
bombardment followed by thermal annealing. Two types of nitrogen species were
identified at the atomic level: pyridinic-N (N bonded to two C nearest
neighbours) and graphitic-N (N bonded to three C nearest neighbours). Distinct
electronic states of localized {\pi} states were found to appear in the
occupied and unoccupied regions near the Fermi level at the carbon atoms around
pyridinic-N and graphitic-N species, respectively. The origin of these states
is discussed based on the experimental results and theoretical simulations.Comment: 6 Pages, with larger figure
Electronic Structure of the Novel Filled Skutterudite PrPt<sub>4</sub>Ge<sub>12</sub> Superconductor
We have performed soft x-ray photoemission spectroscopy (SXPES) and resonant photoemission spectroscopy (RPES) of the filled skutterudite superconductor PrPt4Ge12 in order to study the electronic structure of valence band and the character of Pr 4f. SXPES of PrPt4Ge12 measured with 1200 eV photon energy, where spectral contribution of Pr 4f is negligible, was found nearly identical with that of LaPt4Ge12, indicating similarity of Pt–Ge derived electronic states of the two compounds. Good correspondence with band calculations allows us to ascribe the dominant Ge 4p character of the density of states at the Fermi level (EF). Pr 3d → 4f RPES shows that, although Pr 4f electrons in PrPt4Ge12 are not as strongly hybridized with conduction electrons near EF as in PrFe4P12, there are finite Pr 4f contribution to the states near EF in PrPt4Ge12. These PES results give the information of fundamental electronic structure for understanding the physical properties of the novel filled skutterudite superconductor PrPt4Ge12
Epitaxially Stabilized EuMoO3: A New Itinerant Ferromagnet
Synthesizing metastable phase often opens new functions in materials but is a
challenging topic. Thin film techniques have advantages to form materials which
do not exist in nature since nonequilibrium processes are frequently utilized.
In this study, we successfully synthesize epitaxially stabilized new compound
of perovskite Eu2+Mo4+O3 as a thin film form by a pulsed laser deposition.
Analogous perovskite SrMoO3 is a highly conducting paramagnetic material, but
Eu2+ and Mo4+ are not compatible in equilibrium and previous study found more
stable pyrochlore Eu23+Mo24+O7 prefers to form. By using isostructural
perovskite substrates, the gain of the interface energy between the film and
the substrate stabilizes the matastable EuMoO3 phase. This compound exhibits
high conductivity and large magnetic moment, originating from Mo 4d2 electrons
and Eu 4f7 electrons, respectively. Our result indi-cates the epitaxial
stabilization is effective not only to stabilize crystallographic structures
but also to from a new compound which contains unstable combinations of ionic
valences in bulk form.Comment: 7 pages, 9 figure
Digging up bulk band dispersion buried under a passivation layer
Atomically controlled crystal growth of thin films has established
foundations of nanotechnology aimed at the development of advanced functional
devices. Crystallization under non-equilibrium conditions allows engineering of
new materials with their atomically-flat interfaces in the heterostructures
exhibiting novel physical properties. From a fundamental point of view,
knowledge of the electronic structures of thin films and their interfaces is
indispensable to understand the origins of their functionality which further
evolves into realistic device application. In view of extreme surface
sensitivity of the conventional vacuum-ultraviolet (VUV) angle-resolved
photoemission spectroscopy (ARPES), with a probing depth of several angstroms,
experiments on thin films have to use sophisticated in-situ sample transfer
systems to avoid surface contamination. In this Letter, we put forward a method
to circumvent these difficulties using soft X-ray (SX) ARPES. A GaAs:Be thin
film in our samples was protected by an amorphous As layer with an thickness of
nm exceeding the probing depth of the VUV photoemission with photon
energy around 100 eV. The increase of the probing depth with increasing
towards the SX region has clearly exposed the bulk band dispersion
without any surface treatment. Any contributions from potential interface
states between the thin film and the amorphous capping layer has been below the
detection limit. Our results demonstrate that SX-ARPES enables the observation
of coherent three-dimensional band dispersion of buried heterostructure layers
through an amorphous capping layer, breaking through the necessity of surface
cleaning of thin film samples. Thereby, this opens new frontiers in diagnostics
of authentic momentum-resolved electronic structure of protected thin-film
heterostructures.Comment: 5 pages, 3 figure
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