The growth, surface composition, and chemical activity of Ni–Au clusters on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), and ...temperature-programmed desorption (TPD), as well as density functional theory (DFT) calculations and ab initio molecular dynamics simulations. STM images of similar coverages of pure Au and pure Ni on TiO2(110) illustrate that Au clusters are larger with lower cluster densities, indicating that Au is more mobile on the surface than Ni. Consequently, bimetallic Ni–Au clusters can be grown by nucleating Au at existing Ni clusters. A sequence of STM images acquired from the same region of the surface after various depositions of Au on Ni seed clusters demonstrates that new clusters of pure Au are not formed on the surface. Furthermore, the size of the existing clusters increases with each Au deposition due to the incorporation of incoming Au atoms. For bimetallic clusters of varying compositions with a total coverage of 0.25 ML, the addition of Ni has a minor effect in suppressing cluster sintering. LEIS studies indicate that the surface of the clusters are Au-rich (85–95% Au) for bulk Au fractions ≥50%. For annealed bimetallic clusters, the presence of Au at the cluster surface does not significantly inhibit the encapsulation of Ni by titania, while surface Au is not encapsulated. TPD investigations of CO desorption show that CO desorbs from pure Ni clusters in a molecular peak at ∼400 K and a recombinant peak at ∼790 K. Although CO does not adsorb onto titania or pure Au clusters at room temperature, significant CO desorption occurs from bimetallic clusters even for surfaces with only a small fraction of Ni at the surface; this result suggests that CO induces the diffusion of Ni to the surface of the clusters. DFT calculations for unsupported Ni1Au121 clusters confirm that in the presence of a CO molecule, the lowest energy structure involves CO bonding to a Ni atom at the surface. In contrast, in the absence of CO, the most stable cluster surface is pure Au with all of the Ni atoms in the interior of the cluster. Ab initio molecular dynamics simulations show that Ni will migrate to the cluster surface at 300 K in the presence of CO, but Ni migration to the surface does not occur even at higher temperatures in the absence of CO.
The structural evolution of Au(n) (n=2, 3, 5, 7, 9, and 13) clusters and the adsorption of organic molecules such as acetone, acetaldehyde, and diethyl ketone on these clusters are studied using a ...density functional method. The detailed study of the adsorption of acetone on the Au(n) clusters reveals two main points. (1) The acetone molecule interacts with one gold atom of the gold clusters via the carbonyl oxygen. (2) This interaction is mediated through back donation mainly from the spd-hybridized orbitals of the interacting gold atom to the oxygen atom of the acetone molecule. In addition, a hydrogen bond is observed between a hydrogen atom of the methyl group and another gold atom (not involved in the bonding with carbonyl oxygen). Interestingly, the authors notice that the geometries of Au(9) and Au(13) undergo a significant flattening due to the adsorption of an acetone molecule. They have also investigated the role of the alkyl chain attached to the carbonyl group in the adsorption process by analyzing the interaction of Au(13) with acetaldehyde and diethyl ketone.
Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au19 and Au20 clusters. Au20 has been reported to be a unique molecule having ...tetrahedral geometry, a large HOMO−LUMO energy gap, and an atomic packing similar to that of the bulk gold (Li, J.; et al. Science 2003, 299, 864). Our results show that the geometry of Au19 is exactly identical with that of Au20 with one missing corner atom (called a vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold clusters exhibit dramatic differences. Au20 undergoes a clear and distinct solid-like to liquid-like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au19 has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving a series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au19 between 650 to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au20 is destroyed around 800 K, and the cluster is clearly in a liquid-like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature that has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.
Novel extended tetrahedral forms of CO
2 have been synthesized recently under high-pressure conditions. We perform
ab initio density functional theory calculations to investigate whether doping with ...Si can extend the stability range of such tetrahedral forms of CO
2 to ambient pressure. Calculations are performed with a simple cubic cell containing eight formula units in a
β
-cristobalite-like structure. Though we find that all the Si
x
C
1−
x
O
2 structures considered by us are thermodynamically unstable with respect to decomposition into the end members at ambient pressures, the energy differences are small, suggesting that it might be possible for such phases to exist in metastable forms. At higher pressures, the heat of formation is found to be negative. The bonding between C and O atoms is more covalent than that between Si and O atoms. We also find indications that some C atoms may prefer three-fold coordination at low pressure.
Purpose
The purpose of this study was to investigate daily repositioning accuracy by analyzing inter‐ and intra‐fractional uncertainties associated with patients treated for intracranial or base of ...skull tumors in a compact proton therapy system with 6 degrees of freedom (DOF) robotic couch and a thermoplastic head mask indexed to a base of skull (BoS) frame.
Materials and methods
Daily orthogonal kV alignment images at setup position before and after daily treatments were analyzed for 33 patients. The system was composed of a new type of thermoplastic mask, a bite block, and carbon‐fiber BoS couch‐top insert specifically designed for proton therapy treatments. The correctional shifts in robotic treatment table with 6 DOF were evaluated and recorded based on over 1500 planar kV image pairs. Correctional shifts for patients with and without bite blocks were compared.
Results
Systematic and random errors were evaluated for all 6 DOF coordinates available for daily vector corrections. Uncertainties associated with geometrical errors and their sources, in addition to robustness analysis of various combinations of immobilization components were presented.
Conclusions
Analysis of 644 fractions including patients with and without a bite block shows that the BoS immobilization system is capable of maintaining intra‐fraction localization with submillimeter accuracy (in nearly 83%, 86%, 95% of cases along SI, LAT, and PA, respectively) in translational coordinates and subdegree precision (in 98.85%, 98.85%, and 96.4% of cases for roll, pitch, and yaw respectively) in rotational coordinates. The system overall fares better in intra‐fraction localization precision compared to previously reported particle therapy immobilization systems. The use of a mask‐attached type bite block has marginal impact on inter‐ or intra‐fraction uncertainties compared to no bite block.
The purpose of this work is to validate the application of a deep learning-based method for pelvic synthetic CT (sCT) generation that can be used for prostate proton beam therapy treatment planning. ...We propose to integrate dense block minimization into 3D cycle-consistent generative adversarial networks (cycleGAN) framework to effectively learn the nonlinear mapping between MRI and CT pairs. A cohort of 17 patients with co-registered CT and MR pairs were used to test the deep learning-based sCT generation method by leave-one-out cross-validation. Image quality between the sCT and CT images, gamma analysis passing rate, dose-volume metrics, distal range displacement, and the individual pencil beam Bragg peak shift between sCT- and CT-based proton plans were evaluated. The average mean absolute error (MAE) was 51.32 ± 16.91 HU. The relative differences of the statistics of the PTV dose-volume histogram (DVH) metrics in between sCT and CT were generally less than 1%. Mean values of dose difference, absolute dose difference (in percent of the prescribed dose) were -0.07% ± 0.07% and 0.23% ± 0.08%. Mean gamma analysis pass rate of 1 mm/1%, 2 mm/2%, 3 mm/3% criteria with 10% dose threshold were 92.39% ± 5.97%, 97.95% ± 2.95% and 98.97% ± 1.62% respectively. The median, mean and standard deviation of absolute maximum range differences were 0.09 cm and 0.23 ± 0.25 cm. The median and mean Bragg peak shifts among the 17 patients were 0.09 cm and 0.18 ± 0.07 cm. The image similarity, dosimetric and distal range agreement between sCT and original CT suggests the feasibility of further development of an MRI-only workflow for prostate proton radiotherapy.
We have carried out scalar relativistic density functional theory calculations within the projector augmented wave scheme and the pseudopotential approach, to examine the effect of ligands on the ...geometric and electronic structure of four Au13 isomers: planar, flake, cuboctahedral, and icosahedral clusters. We find, in agreement with previous theoretical calculations, that for the clean cluster the planar geometry has the lowest total energy while the icosahedral and cuboctahedral structures undergo Jahn−Teller distortion. On the other hand, when ligated by phosphines, the icosahedron is found to assume the lowest total energy. The rationale for the stabilization of the icosahedron in the ligated Au13 cluster is traced to the ligand-induced charge transfer from the surface Au−Au to Au-ligand bonds leading to the formation of a strong Au-ligand covalent bond and introduction of a compressive strain which further weakens the Au−Au bonds.
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