Charles James Titus

We present a new instrument for soft x-ray absorption and emission spectroscopy based on the latest generation of transition edge sensor array optimized for soft x-ray detection, built on microwave superconducting quantum interference device multiplexing readout, and cooled with a dilution refrigerator. Its extreme collecting efficiency enables spectroscopy measurements on dilute systems. It also enables shorter acquisition time and large energy-range spectra. We describe the design and operation of the spectrometer and characterize its performance in terms of energy resolution, photon collecting efficiency, and stability. A resolution of 0.7–1.8 eV in the energy range 260–900 eV with a high detection rate of up to 10 000 photons per second across the array is achieved with minimal performance degradation. In addition, the use of the dilution refrigerator to cool the detector results in a robust and stable energy calibration over time. The spectrometer is attached to a dedicated ultra-high vacuum sample chamber equipped with a fully motorized sample cryostat for experiments at temperatures between 10 and 300 K, multi-sample mounting, and electric field-gated devices. The spectrometer-sample chamber is installed at a beamline providing full polarization control. We demonstrate the capabilities of the setup using two representative examples of XES on extremely low-concentration systems, namely monolayer hexagonal boron nitride and the K3[Fe(CN)6] molecular system at sub-millimolar concentration.

High-valent Fe ^IV =O species are common intermediates in biological and artificial catalysts. Heme and nonheme S=1 Fe ^IV =O sites have been synthesized and studied for decades but little quantitative experimental comparison of their electronic structures has been available, due to the lack of direct methods focused on the iron. This study allows a rigorous determination of the electronic structure of a nonheme Fe ^IV =O center and its comparison to an Fe ^IV =O heme site using 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy (XAS). Further, variable temperature magnetic circular dichroism (VT-MCD) of the ligand field transitions, combined with nuclear resonance vibrational spectroscopy of the two S=1 Fe ^IV =O systems show that the equatorial ligand field decreases from a nonheme to a heme Fe ^IV =O site. Alternatively, RIXS and Fe L-edge XAS combined with MCD show that the Fe d π orbitals are unperturbed in the Fe ^IV =O heme relative to the nonheme site because the strong axial Fe-O bond uncouples the Fe d π orbitals from the porphyrin π -system. As a consequence, the thermodynamics and kinetics of the H-atom abstraction reactions are actually very similar for heme compound II and nonheme Fe ^IV =O active sites.

Surface chemistry of materials that host quantum bits such as diamond is an important avenue of exploration as quantum computation and quantum sensing platforms mature. Interfacing diamond in general and nanoscale diamond (ND) in particular with silica is a potential route to integrate room temperature quantum bits into photonic devices, fiber optics, cells, or tissues with flexible functionalization chemistry. While silica growth on ND cores has been used successfully for quantum sensing and biolabeling, the surface mechanism to initiate growth was unknown. This report describes the surface chemistry responsible for silica bond formation on diamond and uses X-ray absorption spectroscopy (XAS) to probe the diamond surface chemistry and its electronic structure with increasing silica thickness. A modified Stöber (Cigler) method was used to synthesize 2-35 nm thick shells of SiO₂ onto carboxylic acid-rich ND cores. The diamond morphology, surface, and electronic structure were characterized by overlapping techniques including electron microscopy. Importantly, we discovered that SiO₂ growth on carboxylated NDs eliminates the presence of carboxylic acids and that basic ethanolic solutions convert the ND surface to an alcohol-rich surface prior to silica growth. The data supports a mechanism that alcohols on the ND surface generate silyl-ether (ND-O-Si-(OH)₃) bonds due to rehydroxylation by ammonium hydroxide in ethanol. The suppression of the diamond electronic structure as a function of SiO₂ thickness was observed for the first time, and a maximum probing depth of ∼14 nm was calculated. XAS spectra based on the Auger electron escape depth was modeled using the NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to support our experimental results. Additionally, resonant inelastic X-ray scattering (RIXS) maps produced by the transition edge sensor reinforces the chemical analysis provided by XAS. Researchers using diamond or high-pressure high temperature (HPHT) NDs and other exotic materials (e.g., silicon carbide or cubic-boron nitride) for quantum sensing applications may exploit these results to design new layered or core-shell quantum sensors by forming covalent bonds via surface alcohol groups.

Surface chemistry of materials that host quantum bits such as diamond are an important avenue of exploration as quantum computation and quantum sensing platforms mature. Interfacing diamond in general, and nanoscale diamond (ND) in particular with silica is a potential route to integrate the quantum bit into a photonic device, fiber optic, cells or tissues with flexible functionalization chemistry. While silica growth on ND cores has been used successfully for quantum sensing and biolabeling, the surface mechanism to initiate growth was unknown. This report describes the surface chemistry responsible for silica bond formation on diamond and uses X-ray absorption spectroscopy (XAS) to probe the diamond surface chemistry and its electronic structure with increasing silica thickness. A modified Stöber (Cigler) method was used to synthesize 2–35 nm thick shells of SiO2 onto carboxylic acid rich ND cores and the diamond features and surface structure were characterized by overlapping techniques including electron microscopy. Importantly, we discovered that SiO2 growth on carboxylated NDs eliminates the presence of carboxylic acids and that basic ethanolic solutions converts the ND surface to an alcohol-rich surface prior to silica growth. The data supports a mechanism that alcohols on the ND surface generate silyl-ether (ND-O-Si-(OH)3) bonds due to rehydroxylation by ammonium hydroxide in ethanol. Additionally, resonant inelastic X-ray scattering (RIXS) maps produced by the transition edge sensor supports the chemical analysis provided by XAS. The suppression of the diamond electronic structure as a function of SiO2 thickness was observed, and the Auger electron escape depth was modeled using the NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to support our experimental results. Researchers using high-pressure high temperature (HPHT) NDs or any alcohol-terminated material (metal oxides, oxidized silicon carbide or cubic-boron nitride) for quantum sensing applications may exploit these results to design new core-shell quantum sensors with base-catalyzed reactions and metal oxide precursors.

Reduction of nitrite anions (NO₂⁻) to nitric oxide (NO), nitrous oxide (N₂O) and ultimately dinitrogen (N₂) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (−0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO₂²⁻), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N₂O) and nitrite (NO₂⁻). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO₂⁻ to NO.