12 research outputs found
The Small Unit Cell Reconstructions of SrTiO3 (111)
We analyze the basic structural units of simple reconstructions of the (111)
surface of SrTiO3 using density functional calculations. The prime focus is to
answer three questions: what is the most appropriate functional to use; how
accurate are the energies; what are the dominant low-energy structures and
where do they lie on the surface phase diagram. Using test calculations of
representative small molecules we compare conventional GGA with higher-order
methods such as the TPSS meta-GGA and on-site hybrid methods PBE0 and TPSSh,
the later being the most accurate. There are large effects due to reduction of
the metal d oxygen sp hybridization when using the hybrid methods which are
equivalent to a dynamical GGA+U, which leads to rather substantial improvements
in the atomization energies of simple calibration molecules, even though the
d-electron density for titanium compounds is rather small. By comparing the
errors of the different methods we are able to generate an estimate of the
theoretical error, which is about 0.25eV per 1x1 unit cell, with changes of
0.5-1.0 eV per 1x1 cell with the more accurate method relative to conventional
GGA. An analysis of the plausible structures reveals an unusual low-energy
TiO2-rich configuration with an unexpected distorted trigonal biprismatic
structure. This structure can act as a template for layers of either TiO or
Ti2O3, consistent with experimental results as well as, in principle, Magnelli
phases. The results also suggest that both the fracture surface and the
stoichiometric SrTiO3 (111) surface should spontaneously disproportionate into
SrO and TiO2 rich domains, and show that there are still surprises to be found
for polar oxide surfaces.Comment: 14 pages, 4 Figure
Epitaxial (111) Films of Cu, Ni, and Cu_y_2_3$(0001) for Graphene Growth by Chemical Vapor Deposition
Films of (111)-textured Cu, Ni, and CuNi were evaluated as substrates
for chemical vapor deposition of graphene. A metal thickness of 400 nm to 700
nm was sputtered onto a substrate of AlO(0001) at temperatures
of 250 C to 650 C. The films were then annealed at 1000 C in a tube furnace.
X-ray and electron backscatter diffraction measurements showed all films have
(111) texture but have grains with in-plane orientations differing by
. The in-plane epitaxial relationship for all films was
||. Reactive sputtering of Al in
O before metal deposition resulted in a single in-plane orientation over 97
% of the Ni film but had no significant effect on the Cu grain structure.
Transmission electron microscopy showed a clean Ni/AlO interface,
confirmed the epitaxial relationship, and showed that formation of the
twin grains was associated with features on the AlO
surface. Increasing total pressure and Cu vapor pressure during annealing
decreased the roughness of Cu and and CuNi films. Graphene grown on the
Ni(111) films was more uniform than that grown on polycrystalline Ni/SiO
films, but still showed thickness variations on a much smaller length scale
than the distance between grains
Behavior of molecules and molecular ions near a field emitter
The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions
Gold Nanoparticle Quantitation by Whole Cell Tomography
Many proposed biomedical applications for engineered gold nanoparticles require their incorporation by mammalian cells in specific numbers and locations. Here, the number of gold nanoparticles inside of individual mammalian stem cells was characterized using fast focused ion beam–scanning electron microscopy based tomography. Enhanced optical microscopy was used to provide a multiscale map of the <i>in vitro</i> sample, which allows cells of interest to be identified within their local environment. Cells were then serially sectioned using a gallium ion beam and imaged using a scanning electron beam. To confirm the accuracy of single cross sections, nanoparticles in similar cross sections were imaged using transmission electron microscopy and scanning helium ion microscopy. Complete tomographic series were then used to count the nanoparticles inside of each cell and measure their spatial distribution. We investigated the influence of slice thickness on counting single particles and clusters as well as nanoparticle packing within clusters. For 60 nm citrate stabilized particles, the nanoparticle cluster packing volume is 2.15 ± 0.20 times the volume of the bare gold nanoparticles
Stability and phase transfer of catalytically active platinum nanoparticle suspensions
In this work, we present a robust synthesis protocol for platinum nanoparticles that yields a monomodal dispersion of particles that are approximately 100 nm in diameter. We determine that these particles are actually agglomerates of much smaller particles, creating a “raspberry” morphology. We demonstrate that these agglomerates are stable at room temperature for at least 8 weeks by dynamic light scattering. Furthermore, we demonstrate consistent electrocatalytic activity for methanol oxidation. Finally, we quantitatively explore the relationship between dispersion solvent and particle agglomeration; specifically, particles are found to agglomerate abruptly as solvent polarity decreases