48 research outputs found
Absence of stable atomic structure in fluorinated graphene
Based on the results of first-principles calculations we demonstrate that
significant distortion of graphene sheets caused by adsorption of fluorine
atoms leads to the formation of metastable patterns for which the next step of
fluorination is considerably less energetically favorable. Existence of these
stable patterns oriented along the armchair direction makes possible the
synthesis of various CFx structures. The combination of strong distortion of
the nonfluorinated graphene sheet with the doping caused by the polar nature of
C-F bonds reduces the energy cost of migration and the energy of migration
barriers, making possible the migration of fluorine atoms on the graphene
surface as well as transformation of the shapes of fluorinated areas. The
decreasing energy cost of migration with increasing fluorine content also leads
to increasing numbers of single fluorine adatoms, which could be the source of
magnetic moments.Comment: 16 pages, 6 figures (one figure added), accepted in PCC
Atomic and electronic structure of nitrogen- and boron-doped phosphorene
First principle modeling of nitrogen- and boron-doped phosphorene
demonstrates the tendency toward formation of highly ordered structures.
Nitrogen doping leads to the formation of -N-P-P-P-N- lines. Further
transformation to -P-N-P-N- lines across the chains of phosphorene occurs with
increasing band gap and increasing nitrogen concentration, which coincides with
the decreasing chemical activity of N-doped phosphorene. In contrast to the
case of nitrogen, boron atoms prefer to form -B-B- pairs with the further
formation of -P-P-B-B-P-P- patterns along the phosphorene chains. The low
concentration of boron dopants converts the phosphorene from a semiconductor
into a semimetal with the simultaneous enhancement of its chemical activity.
Co-doping of phosphorene by both boron and nitrogen starts from the formation
of -B-N- pairs, which provide flat bands and the further transformation of
these pairs to hexagonal BN lines and ribbons across the phosphorene chains.Comment: 21 pages, 8 figures, 2 tables, to appear at PCC
Origin of Anomalous Water Permeation through Graphene Oxide Membrane
Water inside the low dimensional carbon structures has been considered
seriously owing to fundamental interest in its flow and structures as well as
its practical impact. Recently, the anomalous perfect penetration of water
through graphene oxide membrane was demonstrated although the membrane was
impenetrable for other liquids and even gases. The unusual auxetic behavior of
graphene oxide in the presence of water was also reported. Here, based on
first-principles calculations, we establish atomistic models for hybrid systems
composed of water and graphene oxides revealing the anomalous water behavior
inside the stacked graphene oxides. We show that formation of hexagonal ice
bilayer in between the flakes as well as melting transition of ice at the edges
of flakes are crucial to realize the perfect water permeation across the whole
stacked structures. The distance between adjacent layers that can be controlled
either by oxygen reduction process or pressure is shown to determine the water
flow thus highlighting a unique water dynamics in randomly connected
two-dimensional spaces.Comment: 5 pages, 4 figures, to appear in Nano Letter
Inducing and Optimizing Magnetism in Graphene Nanomesh
Using first-principles calculations, we explore the electronic and magnetic
properties of graphene nanomesh (GNM), a regular network of large vacancies,
produced either by lithography or nanoimprint. When removing an equal number of
A and B sites of the graphene bipartite lattice, the nanomesh made mostly of
zigzag (armchair) type edges exhibit antiferromagnetic (spin unpolarized)
states. In contrast, in situation of sublattice symmetry breaking, stable
ferri(o)magnetic states are obtained. For hydrogen-passivated nanomesh, the
formation energy is dramatically decreased, and ground state is found to
strongly depend on the vacancies shape and size. For triangular shaped holes,
the obtained net magnetic moments increase with the number difference of
removed A and B sites in agreement with Lieb's theorem for even A+B. For odd
A+B triangular meshes and all cases of non-triangular nanomeshes including the
one with even A+B, Lieb's theorem does not hold anymore which can be partially
attributed to introduction of armchair edges. In addition, large triangular
shaped GNM could be as robust as non-triangular GNMs, providing possible
solution to overcome one of crucial challenges for the sp-magnetism. Finally,
significant exchange splitting values as large as eV can be obtained
for highly asymmetric structures evidencing the potential of GNM for room
temperature carbon based spintronics. These results demonstrate that a turn
from 0-dimensional graphene nanoflakes throughout 1-dimensional graphene
nanoribbons with zigzag edges to GNM breaks localization of unpaired electrons
and provides deviation from the rules based on Lieb's theorem. Such
delocalization of the electrons leads the switch of the ground state of system
from antiferromagnetic narrow gap insulator discussed for graphene nanoribons
to ferromagnetic or nonmagnetic metal.Comment: 7 pages, 5 figures, 1 tabl
A Computational Investigation of the Catalytic Properties of Graphene Oxide: Exploring Mechanisms Using DFT Methods
Here we describe a computational study undertaken in an effort to elucidate
the reaction mechanisms behind the experimentally observed oxidations and
hydrations catalyzed by graphene oxide (GO). Using the oxidation of benzyl
alcohol to benzaldehyde as a model reaction, density functional theory (DFT)
calculations revealed that this reactivity stemmed from the transfer of
hydrogen atoms from the organic molecule to the GO surface. In particular,
neighbouring epoxide groups decorating GO's basal plane were ring-opened,
resulting in the formation of diols, followed by dehydration. Consistent with
the experimentally-observed dependence of this chemistry on molecular oxygen,
our calculations revealed that the partially reduced catalyst was able to be
recharged by molecular oxygen, allowing for catalyst turnover. Functional
group-free carbon materials, such as graphite, were calculated to have
substantially higher reaction barriers, indicating that the high chemical
potential and rich functionality of GO are necessary for the observed
reactivity.Comment: 5 two column pages, 4 figures, stability of reduced graphene oxide
also discussed, accepted to ChemCatChe
Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene
Atomic resolution high angle annular dark field imaging of suspended,
single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms
had been deposited was carried out in an aberration corrected scanning
transmission electron microscope. In combination with electron energy loss
spectroscopy, employed to identify individual impurity atoms, it was shown that
nano-scale holes were etched into graphene, initiated at sites where single
atoms of all the metal species except for gold come into close contact with the
graphene. The e-beam scanning process is instrumental in promoting metal atoms
from clusters formed during the original metal deposition process onto the
clean graphene surface, where they initiate the hole-forming process. Our
observations are discussed in the light of calculations in the literature,
predicting a much lowered vacancy formation in graphene when metal ad-atoms are
present. The requirement and importance of oxygen atoms in this process,
although not predicted by such previous calculations, is also discussed,
following our observations of hole formation in pristine graphene in the
presence of Si-impurity atoms, supported by new calculations which predict a
dramatic decrease of the vacancy formation energy, when SiOx molecules are
present.Comment: final version accepted in ACS Nano + supplementary info. 22+6 pages,
4+5 figure
Charge Transfer Induced Molecular Hole Doping into Thin Film of Metal-Organic-Frameworks
Despite the highly porous nature with significantly large surface area, metal
organic frameworks (MOFs) can be hardly used in electronic, and optoelectronic
devices due to their extremely poor electrical conductivity. Therefore, the
study of MOF thin films that require electron transport or conductivity in
combination with the everlasting porosity is highly desirable. In the present
work, thin films of Co3(NDC)3DMF4 MOFs with improved electronic conductivity
are synthesized using layer-by-layer and doctor blade coating techniques
followed by iodine doping. The as-prepared and doped films are characterized
using FE-SEM, EDX, UV/Visible spectroscopy, XPS, current-voltage measurement,
photoluminescence spectroscopy, cyclic voltammetry, and incident photon to
current efficiency measurements. In addition, the electronic and semiconductor
property of the MOF films are characterized using Hall Effect measurement,
which reveals that in contrast to the insulator behavior of the as-prepared
MOFs, the iodine doped MOFs behave as a p-type semiconductor. This is caused by
charge transfer induced hole doping into the frameworks. The observed charge
transfer induced hole doping phenomenon is also confirmed by calculating the
densities of states of the as-prepared and iodine doped MOFs based on density
functional theory. Photoluminescence spectroscopy demonstrate an efficient
interfacial charge transfer between TiO2 and iodine doped MOFs, which can be
applied to harvest solar radiations.Comment: Main paper (19 pages, 6 figures) and supplementary information (15
pages, 10 figures), accepted in ACS Appl. Materials & Interface