56 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
Potential high- superconductivity in YCeH and LaCeH under pressure
Lanthanum, yttrium, and cerium hydrides are the three most well-known
superconducting binary hydrides, which have gained great attention in both
theoretical and experimental studies. Recent studies have shown that ternary
hydrides composed of lanthanum and yttrium can achieve high superconductivity
around 253 K. In this study, we employ the evolutionary-algorithm-based crystal
structure prediction (CSP) method and first-principles calculations to
investigate the stability and superconductivity of ternary hydrides composed of
(Y, Ce) and (La, Ce) under high pressure. Our calculations show that there are
multiple stable phases in Y-Ce-H and La-Ce-H hydrides, among which
-YCeH, -LaCeH, -YCeH, and
-LaCeH possessing H or H clathrate structures
can maintain both of the thermodynamic and dynamic stabilities. In addition, we
also find that these phases also maintain a strong resistance to decomposition
at high temperature. Electron-phonon coupling calculations show that all of
these four phases can exhibit high-temperature superconductivity.
-YCeH is predicted to have a superconducting transition
temperature () as high as 246 K at 350 GPa. The value of
-LaCeH at 250 GPa is about 233 K, which is slightly smaller
than that of -YCeH. However, it is found that
-LaCeH can be stabilized at 200 GPa, making the high-pressure
synthesis of LaCeH easier.Comment: 5 figure
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
Data-driven Exploration of New Pressure-induced Superconductivity in PbBiTe with Two Transition Temperatures
Candidates compounds for new thermoelectric and superconducting materials,
which have narrow band gap and flat bands near band edges, were exhaustively
searched by the high-throughput first-principles calculation from an inorganic
materials database named AtomWork. We focused on PbBiTe which has the
similar electronic band structure and the same crystal structure with those of
a pressure-induced superconductor SnBi2Se4 explored by the same data-driven
approach. The PbBiTe was successfully synthesized as single crystals
using a melt and slow cooling method. The core level X-ray photoelectron
spectroscopy analysis revealed Pb2+, Bi3+ and Te2- valence states in
PbBiTe. The thermoelectric properties of the PbBiTe sample were
measured at ambient pressure and the electrical resistivity was also evaluated
under high pressure using a diamond anvil cell with boron-doped diamond
electrodes. The resistivity decreased with increase of the pressure, and two
pressure-induced superconducting transitions were discovered at 3.4 K under
13.3 GPa and at 8.4 K under 21.7 GPa. The data-driven approach shows promising
power to accelerate the discovery of new thermoelectric and superconducting
materials
Pressure-Induced Superconductivity in Layered Transition-metal Chalcogenides (Zr,Hf)GeTe Explored by Data-driven Approach
Layered transition-metal chalcogenides (Zr,Hf)GeTe were screened out
from database of Atomwork as a candidate for pressure-induced superconductivity
due to their narrow band gap and high density of state near the Fermi level.
The (Zr,Hf)GeTe samples were synthesized in single crystal and then the
compositional ratio, crystal structures, and valence states were investigated
via energy dispersive spectrometry, single crystal X-ray diffraction, and X-ray
photoelectron spectroscopy, respectively. The pressure-induced
superconductivity in both crystals were first time reported by using a diamond
anvil cell with a boron-doped diamond electrode and an undoped diamond
insulating layer. The maximum superconducting transition temperatures of
ZrGeTe and HfGeTe were 6.5 K under 57 GPa and 6.6 K under 60 GPa,
respectively
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