Comparative performance analysis of 2D and 3D gamma metrics for patient specific QA in VMAT using Octavius 4D with 2D-Array 1500.

INTRODUCTION: Gamma pass percentage (GPP) is the predominant metric used for Patient Specific Quality Assurance (PSQA) in radiation therapy. The dimensionality of the measurement geometry in PSQA has evolved from 2D planar to 3D planar, and presently to state-of-the-art 3D volumetric geometry. We aim to critically examine the performance of the three-dimensional gammas vis-à-vis the older gamma metrics of lower dimensionality to determine their mutual fungibility in PSQA, using clinically approved Volumetric Arc Therapy (VMAT) plans. METHODS AND MATERIALS: Gamma pass percentages derived from PSQA for VMAT plans using Octavius 4D phantom with 2D-Array 1500 and its proprietary software were recorded. 2D planar, 3D planar, and 3D volumetric gamma pass percentages were retrospectively extracted for multiple treatment plans at three sites, using three acceptance limits, and for two modes of normalization. The differences in mean pass percentages, and the pairwise correlation between geometries were calculated within limits of statistical significance. RESULTS: A significant increase in mean pass rates was observed from 2D planar to 3D planar geometries. The difference was less pronounced from 3D planar to 3D volumetric. 2D planar v/s 3D planar showed a significant degree of correlation among themselves, which was not seen against most of the 3D volumetric pass rates. CONCLUSION: The mean gamma pass rates show conclusive evidence of the benefits of shifting from 2D planar to higher dimensions measurement geometries, but the benefits of using 3D volumetric compared to 3D planar is not always unequivocal. The correlations show mixed results regarding the interdependence of pass percentages at different geometries.

Large Perovskite Grain Growth in Low-Temperature Solution-Processed Planar p-i-n Solar Cells by Sodium Addition.

Thin-film p-i-n type planar heterojunction perovskite solar cells have the advantage of full low temperature solution processability and can, therefore, be adopted in roll-to-roll production and flexible devices. One of the main challenges with these devices, however, is the ability to finely control the film morphology during the deposition and crystallization of the perovskite layer. Processes suitable for optimization of the perovskite layer film morphology with large grains are highly desirable for reduced recombination of charge carriers. Here, we show how uniform thin films with micron size perovskite grains can be made through the use of a controlled amount of sodium ions in the precursor solution. Large micrometer-size CH3NH3PbI3 perovskite grains are formed during low-temperature thin-film growth by adding sodium ions to the PbI2 precursor solution in a two-step interdiffusion process. By adjusting additive concentration, film morphologies were optimized and the fabricated p-i-n planar perovskite-PCBM solar cells showed improved power conversion efficiences (an average of 3-4% absolute efficiency enhancement) compared to the nonsodium based devices. Overall, the additive enhanced grain growth process helped to reach a high 14.2% solar cell device efficiency with low hysteresis. This method of grain growth is quite general and provides a facile way to fabricate large-grained CH3NH3PbI3 on any arbitrary surface by an all solution-processed route.

Tandem Solar Cells from Accessible Low Band-Gap Polymers Using an Efficient Interconnecting Layer.

Tandem solar cell architectures are designed to improve device photoresponse by enabling the capture of wider range of solar spectrum as compared to single-junction device. However, the practical realization of this concept in bulk-heterojunction polymer systems requires the judicious design of a transparent interconnecting layer compatible with both polymers. Moreover, the polymers selected should be readily synthesized at large scale (>1 kg) and high performance. In this work, we demonstrate a novel tandem polymer solar cell that combines low band gap poly isoindigo [P(T3-iI)-2], which is easily synthesized in kilogram quantities, with a novel Cr/MoO3 interconnecting layer. Cr/MoO3 is shown to be greater than 80% transparent above 375 nm and an efficient interconnecting layer for P(T3-iI)-2 and PCDTBT, leading to 6% power conversion efficiencies under AM 1.5G illumination. These results serve to extend the range of interconnecting layer materials for tandem cell fabrication by establishing, for the first time, that a thin, evaporated layer of Cr/MoO3 can work as an effective interconnecting layer in a tandem polymer solar cells made with scalable photoactive materials.

Biomimetic multifunctional porous chalcogels as solar fuel catalysts.

Biological systems that can capture and store solar energy are rich in a variety of chemical functionalities, incorporating light-harvesting components, electron-transfer cofactors, and redox-active catalysts into one supramolecule. Any artificial mimic of such systems designed for solar fuels production will require the integration of complex subunits into a larger architecture. We present porous chalcogenide frameworks that can contain both immobilized redox-active Fe(4)S(4) clusters and light-harvesting photoredox dye molecules in close proximity. These multifunctional gels are shown to electrocatalytically reduce protons and carbon disulfide. In addition, incorporation of a photoredox agent into the chalcogels is shown to photochemically produce hydrogen. The gels have a high degree of synthetic flexibility, which should allow for a wide range of light-driven processes relevant to the production of solar fuels.

Chalcogels: porous metal-chalcogenide networks from main-group metal ions. Effect of surface polarizability on selectivity in gas separation.

We report the synthesis of metal-chalcogenide gels and aerogels from anionic chalcogenide clusters and linking metal ions. Metal ions such as Sb(3+) and Sn(2+), respectively chelated with tartrate and acetate ligands, react in solution with the chalcogenide clusters to form extended polymeric networks that exhibit gelation phenomena. Chalcogenide cluster anions with different charge densities, such as [Sn(2)S(6)](4-) and [SnS(4)](4-), were employed. In situ rheological measurements during gelation showed that a higher charge density on the chalcogenide cluster favors formation of a rigid gel network. Aerogels obtained from the gels after supercritical drying have BET surface areas from 114 to 368 m(2)/g. Electron microscopy images coupled with nitrogen adsorption measurements showed the pores are micro (below 2 nm), meso (2-50 nm), and macro (above 50 nm) regions. These chalcogels possess band gaps in the range of 1.00-2.00 eV and selectively adsorb polarizable gases. A 2-fold increase in selectivity toward CO(2)/C(2)H(6) over H(2) was observed for the Pt/Sb/Ge(4)Se(10)-containing aerogel compared to aerogel containing Pt(2)Ge(4)S(10). The experimental results suggest that high selectivity in gas adsorption is achievable with high-surface-area chalcogenide materials containing heavy polarizable elements.

Spongy chalcogels of non-platinum metals act as effective hydrodesulfurization catalysts.

Aerogels are low-density porous materials, made mostly of air, for which hundreds of applications have been found in recent years. Inorganic oxide-based aerogels have been known for a long time, carbon aerogels were discovered in the early 1990s and sulfur- and selenium-based aerogels (chalcogels) are the most recent additions to this family. Here we present new aerogels made of Co(Ni)-Mo(W)-S networks with extremely large surface areas and porosity. These systems are formed by the coordinative reactions of (MoS(4))(2-) and (WS(4))(2-) with Co(2+) and Ni(2+) salts in non-aqueous solvents. We show that these low-density sponge-like networks can absorb conjugated organic molecules and mercury ions, and preferentially adsorb CO(2) over H(2), which illustrates their high potential as gas-separation media. The chalcogels are shown to be twice as active as the conventional sulfided Co-Mo/Al(2)O(3) catalyst for the hydrodesulfurization of thiophene.

Importance of solution equilibria in the directed assembly of metal chalcogenide mesostructures.

We describe the new nanostructured Pt/Ge/Se materials prepared from the molecular units [Ge2Se6](4-) and [GeSe4](4-) and linking Pt(2+) ions in the presence of surfactant micelles. X-ray diffraction coupled with transmission electron microscopy images reveals hexagonal pore symmetry. The solvent dependence and solution speciation of these building blocks were investigated by means of multinuclear NMR spectroscopy and by fast atom bombardment (FAB) mass spectroscopy and it is shown that rapid exchange equilibrium is reached between species like [Ge4Se10](4-), [Ge2Se6](4-), and [GeSe4](4-) in both water and formamide. This results in multiple Ge/Se anions being incorporated in the mesostructured materials which is supported by Raman and IR spectroscopic data. It is likely that the presence of multiple building units both in water and formamide solutions favors the assembly of mesostructured metal chalcogenides with good pore order. Systematic variation of both surfactant headgroup and chain length modulates the optoelectronic properties of the mesostructures. The Pt/Ge/Se materials show sharp band gap transitions in the range of 1.24-1.97 eV. Finally, the materials exhibit reversible ion-exchange properties and a marked inorganic framework flexibility that enables a contraction-expansion process in response to the exchange. The Pt/Ge/Se framework possesses a very high surface area as estimated by small-angle X-ray scattering techniques.

Porous semiconducting gels and aerogels from chalcogenide clusters.

Inorganic porous materials are being developed for use as molecular sieves, ion exchangers, and catalysts, but most are oxides. We show that various sulfide and selenide clusters, when bound to metal ions, yield gels having porous frameworks. These gels are transformed to aerogels after supercritical drying with carbon dioxide. The aerogels have high internal surface area (up to 327 square meters per gram) and broad pore size distribution, depending on the precursors used. The pores of these sulfide and selenide materials preferentially absorb heavy metals. These materials have narrow energy gaps (between 0.2 and 2.0 electron volts) and low densities, and they may be useful in optoelectronics, as photocatalysts, or in the removal of heavy metals from water.