2,014 research outputs found
Adsorption of common solvent molecules on graphene and MoS from first-principles
Solvents are an essential element in the production and processing of
two-dimensional (2D) materials. For example, the liquid phase exfoliation of
layered materials requires a solvent to prevent the resulting monolayers from
re-aggregating, while solutions of functional atoms and molecules are routinely
used to modify the properties of the layers. It is generally assumed that these
solvents do not interact strongly with the layer and so their effects can be
neglected. Yet experimental evidence has suggested that explicit atomic-scale
interactions between the solvent and layered material may play a crucial role
in exfoliation and cause unintended electronic changes in the layer. Little is
known about the precise nature of the interaction between the solvent molecules
and the 2D layer. Here, we use density functional theory calculations to
determine the adsorption configuration and binding energy of a variety of
common solvent molecules, both polar and non-polar, on two of the most popular
2D materials, namely graphene and MoS. We show that these molecules are
physisorbed on the surface with negligible charge transferred between them. We
find that the adsorption strength of the different molecules is independent of
the polar nature of the solvent. However, we show the molecules induce a
significant charge rearrangement at the interface after adsorption as a result
of polar bonds in the molecule.Comment: 8 pages, 6 figure
Large bias-dependent magnetoresistance in all-oxide magnetic tunnel junctions with a ferroelectric barrier
All-oxide magnetic tunnel junctions (MTJs) incorporating functional materials
as insulating barriers have the potential of becoming the founding technology
for novel multi-functional devices. We investigate, by first-principles density
functional theory, the bias-dependent transport properties of an all-oxide
SrRuO3/BaTiO3/SrRuO3 MTJ. This incorporates a BaTiO3 barrier which can be found
either in a non-ferroic or in a ferroelectric state. In such an MTJ not only
can the tunneling magnetoresistance reach enormous values, but also, for
certain voltages, its sign can be changed by altering the barrier electric
state. These findings pave the way for a new generation of
electrically-controlled magnetic sensors.Comment: 4 pages, 5 figure
The role of solvent interfacial structural ordering in maintaining stable graphene dispersions
Liquid phase exfoliation (LPE) is the most promising method for the low-cost,
scalable production of two-dimensional nanosheets from their bulk counterparts.
Extensive exfoliation occurs in most solvents due to the huge amount of energy
introduced by sonication or shear mixing. However, the subsequent dispersion is
not always stable, with extensive reaggregation occurring in some solvents.
Identifying the optimal solvent for a particular layered material is difficult
and requires a fundamental understanding of the mechanism involved in
maintaining a stable dispersion. Here, we use molecular dynamics calculations
to show that when graphene is immersed in a solvent, distinct solvation layers
are formed irrespective of the choice of solvent and their formation is
energetically favourable for all considered solvents. However, energetic
considerations such as these do not explain the experimental solvent-dependence
of the dispersion concentration. Instead, we find that solvents with high
diffusion coefficients parallel to the graphene layer result in the lowest
experimental concentration of graphene in solution. This can be explained by
the enhanced ease of reaggregation in these solvents. Solvents with smaller
diffusion coefficients result in higher experimental graphene concentrations as
reaggregation is prevented. In the low diffusion limit, however, this
relationship breaks down. We suggest that here the concentration of graphene in
solution depends primarily on the separation efficiency of the initial
exfoliation step. Based on this, we predict that the concentration of
exfoliated graphene in solvents such as benzaldehyde and quinoline, which have
low diffusion constants, can be increased dramatically by careful tuning of the
experimental sonication parameters
Structural and electronic properties of Li intercalated graphene on SiC(0001)
We investigate the structural and electronic properties of Li-intercalated
monolayer graphene on SiC(0001) using combined angle-resolved photoemission
spectroscopy and first-principles density functional theory. Li intercalates at
room temperature both at the interface between the buffer layer and SiC and
between the two carbon layers. The graphene is strongly -doped due to charge
transfer from the Li atoms and two -bands are visible at the
-point. After heating the sample to 300C, these -bands
become sharp and have a distinctly different dispersion to that of
Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate
between the two carbon layers with an ordered structure, similar to that of
bulk LiC. An AA-stacking of these two layers becomes energetically
favourable. The -bands around the -point closely resemble the
calculated band structure of a CLiC system, where the intercalated Li
atoms impose a super-potential on the graphene electronic structure that opens
pseudo-gaps at the Dirac points of the two -cones.Comment: 9 pages, 7 figure
High Cyanobacterial Abundance in Three Northeastern Gulf of Mexico Estuaries
Aquatic phytoplankton comprise a wide variety of taxa spanning more than 2 orders of magnitude in size, yet studies of estuarine phytoplankton often overlook the picoplankton, particularly chroococcoid cyanobacteria (cf. Synechococcus). Three Gulf of Mexico estuaries (Apalachicola Bay, FL; Pensacola Bay, FL; Weeks Bay, AL) were sampled during summer and fall 2001 to quantify cyanobacterial abundance, to examine how cyanobacterial abundance varied with hydrographic and nutrient distributions, and to estimate the contribution of cyanobacteria to the bulk phytoplankton community. Cyanobacterial abundances in all 3 estuaries were high, averaging 0.59 ± 0.76 X 109 L–1 in Apalachicola Bay, 1.7 ± 1.2 X 109 L–1 in Pensacola Bay and 2.4 ± 1.9 X 109 L–1 in Weeks Bay (mean ± standard deviation). Peak abundances typically occurred in the oligohaline zone (low salinity estuarine zone) during the summer. Freshwater sites had nearly undetectable abundances, and marine sites had abundances several-fold lower than the oligohaline zone. When converted to equivalent chlorophyll a concentrations, cyanobacteria comprised a large fraction of the total phytoplankton biomass, at times approaching 100% in all 3 systems. These observations clearly indicate a cyanobacterial community of estuarine origin that can make up a large proportion of phytoplankton biomass
- …