X-Ray Lithography of Metal and Semiconductor Nanoparticles

In the last few years, a considerable amount of research has focused on the three-dimensional fabrication of contacts and electronic devices. Most techniques, however, are essentially based on photoreduction, and are limited to noble- and semi-noble metals. We present here a general method that allows patterning of porous matrices in 3D with metal, but also with semiconductor nanoparticles which is of potential relevance for microfabrication applications. In our method, the pore-filling solvent of a sol-gel material is exchanged with a solution of precursors. The precursors are photodissociated and nanoparticles are formed when the monoliths are irradiated. In a series of previous publications we showed that noble metals but also semiconductor quantum dots can be produced with our technique. Here we focus on the Xray variation of our technique and show that monoliths can be patterned with metals and also with semiconductor nanoparticles. The patterns have the same resolution than the masks, i.e., around 10 μm, and extend into the bulk of the monoliths for up to a depth of 12 mm. Our method possesses several attractive features. Sample preparation is very simple; the technique has a bottom-up character; it allows access to a wide number of materials, such as noble metals and II-VI semiconductor materials; and it has a 3D character. With additional developments, our technique could be possibly used to complement more established techniques such as LIGA and multiphoton fabrication techniques which are currently used for 3D microfabrication

In Situ Ru K‑Edge X‑ray Absorption Spectroscopy Study of Methanol Oxidation Mechanisms on Model Submonolayer Ru on Pt Nanoparticle Electrocatalyst

In situ X-ray absorption spectroscopy (XAS) with electrochemical reaction control has enabled a detailed investigation of the mechanism of the methanol electrooxidation by a bimetallic PtRu catalyst. By the use of an original electrodeposition technique, ca. 0.3 monolayer of Ru was deposited on the surface of unsupported Pt nanoparticles (Ru@Pt). The presence of Ru atoms only at the surface of nanoparticles turns a bulk sensitive XAS technique into a surface methodology which permits correlation of the X-ray absorption fine structure at the Ru K-edge to the role of Ru atoms in the methanol oxidation process. In situ XAS spectra of the Ru@Pt nanoparticles were collected at various electrode potentials in background electrolyte, and then in 1 M solution of methanol (CH₃OH) in the same electrolyte. Significant differences in the catalyst state have been revealed between these two environments. In the background electrolyte, Ru gradually oxidizes from mostly metallic to a Ru­(III)/Ru­(IV) mixture at the highest potentials. In the presence of CH₃OH, the Ru oxidation state remains a mixture of metallic and Ru­(III) even at the highest potentials. CO-type species were found adsorbed on Ru atoms at all potentials and coadsorbed with OH species at potentials 0.175 V vs Ag/AgCl and higher with steady number of near neighbors. The same potential correlates to the beginning of methanol oxidation reaction observed electrochemically. Therefore coadsorption of CO and OH groups on Ru atoms appears to be critical in the methanol oxidation process. The need for OH groups for CO removal from Pt sites was previously suggested by the bifunctional CH₃OH oxidation mechanism; however, occupancy of Ru atoms with CO species and direct structural observations of the coadsorption of CO and OH on the same Ru atom is a novel finding resulting from this study. Other changes in the catalyst environment were observed, including decreased Ru–Ru and Ru–Pt bond distances, increased numbers of Ru–Pt near neighbors, and decreased number of Ru–Ru near neighbors

Thermally Driven Interfacial Degradation between Li 7 La 3 Zr 2 O 12 Electrolyte and LiNi 0.6 Mn 0.2 Co 0.2 O 2 Cathode

© 2020 American Chemical Society Solid-state batteries offer higher energy density and enhanced safety compared to the present lithium-ion batteries using liquid electrolytes. A challenge to implement them is the high resistances, especially at the solid electrolyte interface with the cathode. Sintering at elevated temperature is needed in order to get good contact between the ceramic solid electrolyte and oxide cathodes and thus to reduce contact resistances. Many solid electrolyte and cathode materials react to form secondary phases. It is necessary to find out which phases arise as a result of interface sintering and evaluate their effect on electrochemical properties. In this work, we assessed the interfacial reactions between LiNi0.6Mn0.2Co0.2O2 (NMC622) and Li7La3Zr2O12 (LLZO) as a function of temperature in air. We prepared model systems by depositing thin-film NMC622 cathode layers on LLZO pellets. The thin-film cathode approach enabled us to use interface-sensitive techniques such as X-ray absorption spectroscopy in the near-edge as well as the extended regimes and identify the onset of detrimental reactions. We found that the Ni and Co chemical environments change already at moderate temperatures, on-setting from 500 °C and becoming especially prominent at 700 °C. By analyzing spectroscopy results along with X-ray diffraction, we identified Li2CO3, La2Zr2O7, and La(Ni,Co)O3 as the secondary phases that formed at 700 °C. The interfacial resistance for Li transfer, measured by electrochemical impedance spectroscopy, increases significantly upon the onset and evolution of the detected interface chemistry. Our findings suggest that limiting the bonding temperature and avoiding CO2 in the sintering environment can help to remedy the interfacial degradation

Quantum Dots by Ultraviolet and X-ray Lithography

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented

First experimental demonstration of time-resolved X-ray measurements with next-generation fast-timing MCP-PMT

We report the first successful time-resolved X-ray measurements at the APS 10-ID-B beamline by coupling ultrafast scintillators with photodetectors. Multiple scintillator and sensor pairs (LYSO, plastic scintillator, dynode PMTs, MCP-PMT), as well as a standalone detector (diamond), were tested to demonstrate the time-resolved measurements using hard X-rays at energies of 20 keV and above. The experimental results show that a number of choices exist for time-resolved high-energy X-ray beam measurement. Notably, by gating the photocathode, the Argonne fabricated fast-timing microchannel plate photomultiplier and LYSO crystal pair achieved fast signal detection with a rise time of ~6 ns and a decay time of ~33 ns. These experiments pave the way towards ultrafast imaging technologies using hard X-rays for many applications

First experimental demonstration of time-resolved X-ray measurements with next-generation fast-timing MCP-PMT

We report the first successful time-resolved X-ray measurements at the APS 10-ID-B beamline by coupling ultrafast scintillators with photodetectors. Multiple scintillator and sensor pairs (LYSO, plastic scintillator, dynode PMTs, MCP-PMT), as well as a standalone detector (diamond), were tested to demonstrate the time-resolved measurements using hard X-rays at energies of 20 keV and above. The experimental results show that a number of choices exist for time-resolved high-energy X-ray beam measurement. Notably, by gating the photocathode, the Argonne fabricated fast-timing microchannel plate photomultiplier and LYSO crystal pair achieved fast signal detection with a rise time of ~6 ns and a decay time of ~33 ns. These experiments pave the way towards ultrafast imaging technologies using hard X-rays for many applications

Temperature Dependence of Aliovalent-Vanadium Doping in LiFePO₄ Cathodes

Vanadium-doped olivine LiFePO₄ cathode materials have been synthesized by a low-temperature microwave-assisted solvothermal (MW-ST) method at ≤300 °C. The samples have been extensively characterized by neutron/X-ray powder diffraction, infrared and Raman spectroscopy, elemental analysis, electron microscopy, and electrochemical techniques. The compositions of the as-synthesized materials were found to be LiFe_{1–3<i>x</i>/2}V_<i>x</i>□_<i>x</i>/2PO₄ (0 ≤ <i>x</i> ≤ 0.2) with the presence of a small number of lithium vacancies (□) charge-compensated by V⁴⁺, not Fe³⁺, leading to an average oxidation state of ∼3.2+ for vanadium. The vacancies on the Fe site likely provide an additional conduction pathway for Li⁺ ions to transfer between neighboring 1D conduction channels along the crystallographic <i>b</i> axis. Heating the pristine 15% V-doped sample in inert or reducing atmospheres led to a loss of vanadium from the olivine lattice with the concomitant formation of a Li₃V₂(PO₄)₃ impurity phase; after phase segregation, a partially V-doped olivine phase remained. For comparison, V-doped samples were also synthesized by conventional ball milling and heating, but only ∼10% V could be accommodated in the olivine lattice in agreement with previous studies. The higher degree of doping realized with the MW-ST samples demonstrates the temperature dependence of the aliovalent-vanadium doping in LiFePO₄

Initial assessment of multilayer silicon detectors for hard X-ray imaging

Silicon detectors with lower material budget, ultrafast readout and radiation hardness are under developments. These unique features make pixelized silicon sensors a good option for hard X-ray imaging. To verify the performance of spatial resolution and energy sensitivity of silicon sensors to hard X-rays, a two layer setup of Pixelink PL-D725MU sensors has been tested at the Argonne National Laboratory Advanced Photon Source (APS) ID-10 sector with 29.2 keV high photon flux (4.5×10^8 to 4.5×10^(10) photons per second) X-rays. Better than 3 μm spatial resolution and clear energy characterization have been achieved by both layers. Commercial CMOS sensors with superior spatial resolution could be used for phase contrast imaging in current synchrotrons. These studies pave the path for future multilayer ultrafast silicon sensor development with ns to sub-ns readout speeds in hard X-ray imaging at synchrotrons and XFEL beamlines