Yaroslav Losovyj

Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals

Large-crystal perovskite films The performance of organic-inorganic hybrid perovskite planar solar cells has steadily improved. One outstanding issue is that grain boundaries and defects in polycrystalline films degrade their output. Now, two studies report the growth of millimeter-scale single crystals. Nie et al. grew continuous, pinhole-free, thin iodochloride films with a hot-casting technique and report device efficiencies of 18%. Shi et al. used antisolvent vapor-assisted crystallization to grow millimeter-scale bromide and iodide cubic crystals with charge-carrier diffusion lengths exceeding 10 mm. Science, this issue p. 522, p. 519 Solution processing techniques enable the growth of high-quality, large-area perovskite crystals for solar cells. The fundamental properties and ultimate performance limits of organolead trihalide MAPbX3 (MA = CH3NH3+; X = Br– or I–) perovskites remain obscured by extensive disorder in polycrystalline MAPbX3 films. We report an antisolvent vapor-assisted crystallization approach that enables us to create sizable crack-free MAPbX3 single crystals with volumes exceeding 100 cubic millimeters. These large single crystals enabled a detailed characterization of their optical and charge transport characteristics. We observed exceptionally low trap-state densities on the order of 109 to 1010 per cubic centimeter in MAPbX3 single crystals (comparable to the best photovoltaic-quality silicon) and charge carrier diffusion lengths exceeding 10 micrometers. These results were validated with density functional theory calculations.

Ligand‐Stabilized and Atomically Precise Gold Nanocluster Catalysis: A Case Study for Correlating Fundamental Electronic Properties with Catalysis

We present results from our investigations into correlating the styrene-oxidation catalysis of atomically precise mixed-ligand biicosahedral-structure [Au25(PPh3)10(SC12H25)5Cl2](2+) (Au25-bi) and thiol-stabilized icosahedral core-shell-structure [Au25(SCH2CH2Ph)18](-) (Au25-i) clusters with their electronic and atomic structure by using a combination of synchrotron radiation-based X-ray absorption fine-structure spectroscopy (XAFS) and ultraviolet photoemission spectroscopy (UPS). Compared to bulk Au, XAFS revealed low Au-Au coordination, Au-Au bond contraction and higher d-band vacancies in both the ligand-stabilized Au clusters. The ligands were found not only to act as colloidal stabilizers, but also as d-band electron acceptor for Au atoms. Au25-bi clusters have a higher first-shell Au coordination number than Au25-i, whereas Au25-bi and Au25-i clusters have the same number of Au atoms. The UPS revealed a trend of narrower d-band width, with apparent d-band spin-orbit splitting and higher binding energy of d-band center position for Au25-bi and Au25-i. We propose that the differences in their d-band unoccupied state population are likely to be responsible for differences in their catalytic activity and selectivity. The findings reported herein help to understand the catalysis of atomically precise ligand-stabilized metal clusters by correlating their atomic or electronic properties with catalytic activity.

Adsorption-site-dependent electronic structure of catechol on the anatase TiO2(101) surface.

The adsorption of catechol (1,2-benzendiol) on the anatase TiO(2)(101) surface was studied with synchrotron-based ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). Catechol adsorbs with a unity sticking coefficient and the phenyl ring intact. STM reveals preferred nucleation at step edges and subsurface point defects, followed by 1D growth and the formation of a 2 × 1 superstructure at full coverage. A gap state of ∼1 eV above the valence band maximum is observed for dosages in excess of ∼0.4 Langmuir, but such a state is absent for lower coverages. The formation of the band gap states thus correlates with the adsorption at regular lattice sites and the onset of self-assembled superstructures.

The electronic structure change with Gd doping of HfO2 on silicon

Gd-doped HfO2 films deposited on silicon substrates undergo a crystallographic change from monoclinic to fluorite (cubic) phase with increasing Gd concentrations. The crystallographic phase change is accompanied by a small increase in the valence bandwidth and in the apparent band offset in the surface region. Electrical measurements show pronounced rectification properties for lightly doped Gd:HfO2 films on p-Si and for heavily-doped Gd:HfO2 films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level.

Organotrisulfide: A High Capacity Cathode Material for Rechargeable Lithium Batteries

An organotrisulfide (RSSSR, R is an organic group) has three sulfur atoms which could be involved in multi-electron reduction reactions; therefore it is a promising electrode material for batteries. Herein, we use dimethyl trisulfide (DMTS) as a model compound to study its redox reactions in rechargeable lithium batteries. With the aid of XRD, XPS, and GC-MS analysis, we confirm DMTS could undergo almost a 4 e(-) reduction process in a complete discharge to 1.0 V. The discharge products are primarily LiSCH3 and Li2 S. The lithium cell with DMTS catholyte delivers an initial specific capacity of 720 mAh g(-1) DMTS and retains 82 % of the capacity over 50 cycles at C/10 rate. When the electrolyte/DMTS ratio is 3:1 mL g(-1) , the reversible specific energy for the cell including electrolyte can be 229 Wh kg(-1) . This study shows organotrisulfide is a promising high-capacity cathode material for high-energy rechargeable lithium batteries.

Design of ruthenium/iron oxide nanoparticle mixtures for hydrogenation of nitrobenzene

Here we report novel catalysts for nitrobenzene hydrogenation based on Ru/RuO2 nanoparticles (NPs) and including iron oxide NPs, allowing magnetic recovery. The solvent type, reaction temperature, and the size and composition of initial iron oxide NPs are demonstrated to be the control factors determining synthesis outcomes including the degree of NP aggregation and catalytic properties. A complete characterization of the catalysts using transmission electron microscopy (TEM), X-ray powder diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and energy dispersive x-ray spectroscopy (EDS) allowed assessment of the structure–property relationships. It is revealed that coexistence of the Ru/RuO2 and iron oxide NPs in the catalyst as well as the proximity of two different NP types lead to significantly higher aniline yields and reaction rates. The catalytic properties are also influenced by the type of iron oxide NPs present in the catalytic samples.

Fabrication of magnetically recoverable catalysts based on mixtures of Pd and iron oxide nanoparticles for hydrogenation of alkyne alcohols.

We report a novel method for development of magnetically recoverable catalysts prepared by thermal decomposition of palladium acetylacetonate in the presence of iron oxide nanoparticles (NPs). Depending on conditions, the reaction results either in a dispersed mixture of Pd and iron oxide NPs or in their aggregates. It was demonstrated that the Pd loading, reaction temperature, solvent, and iron oxide NP size and composition are crucial to control the reaction product including the degree of aggregation of Pd and iron oxide NPs, and the catalyst properties. The aggregation controlled by polarization and magnetic forces allows faster magnetic separation, yet the aggregate sizes do not exceed a few hundred nanometers, making them suitable for various catalytic applications. These NP mixtures were studied in a selective hydrogenation of 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol, demonstrating clear differences in catalytic behavior depending on the catalyst structure. In addition, one of the catalysts was also tested in hydrogenation of 3-methyl-1-pentyn-3-ol and 3-methyl-1-nonyn-3-ol, indicating some specificity of the catalyst toward different alkyne alcohols.

Surface photovoltage effects on the isomeric semiconductors of boron-carbide

During exposure to synchrotron radiation, closo 1,7-dicarbadodecaborane (metacarborane) and closo 1,2-dicarbadodecaborane (orthocarborane) decompose, and are accompanied by increasingly evident photoemission surface photovoltage effects. We show that metacarborane and orthocarborane form self-doped n-type and p-type boron-carbides, respectively. Surface photovoltage effects dominate the photoemission final state, not the changes in electronic structure due to decomposition.

Ru-Containing Magnetically Recoverable Catalysts: A Sustainable Pathway from Cellulose to Ethylene and Propylene Glycols.

Biomass processing to value-added chemicals and biofuels received considerable attention due to the renewable nature of the precursors. Here, we report the development of Ru-containing magnetically recoverable catalysts for cellulose hydrogenolysis to low alcohols, ethylene glycol (EG) and propylene glycol (PG). The catalysts are synthesized by incorporation of magnetite nanoparticles (NPs) in mesoporous silica pores followed by formation of 2 nm Ru NPs. The latter are obtained by thermal decomposition of ruthenium acetylacetonate in the pores. The catalysts showed excellent activities and selectivities at 100% cellulose conversion, exceeding those for the commercial Ru/C. High selectivities as well as activities are attributed to the influence of Fe3O4 on the Ru(0)/Ru(4+) NPs. A facile synthetic protocol, easy magnetic separation, and stability of the catalyst performance after magnetic recovery make these catalysts promising for industrial applications.

Magnetism of Cr-doped diamond-like carbon

Chromium-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated DLC alloys were synthesized by plasma-assisted vapor deposition and investigated by x-ray absorption near edge structure spectroscopy, extended x-ray absorption fine structure, and superconducting quantum interference device (SQUID) magnetometry. Structural and magnetic properties of the doped and alloy materials were investigated as a function of the Cr concentration (0.1–20 at. %). Toward the upper end of the concentration range, Cr precipitates in the form of chromium carbide (Cr3C2) nanoclusters. For low Cr concentrations, the systems are ferromagnetic at very low temperatures, whereas the chromium carbide clusters appear to be antiferromagnetic with uncompensated spins at the surface. Cr-DLC films and alloys with various Cr concentrations are used to make heterojunctions on silicon, and the produced diodes are investigated by I-V measurements. The heterojunctions exhibit negative magnetoresistance that saturate...