I am a graduate student in the Department of Physics at Stanford University. As part of the research group of Dr. Kent Irwin, I focus on the application of superconducting detectors to X-ray spectroscopy.
Working at the Stanford Synchrotron Radiation Lightsource (SSRL), I operate a 240 pixel transition-edge sensor (TES) array in support of a diverse user program at beamline 10-1. TES devices have emerged in the soft X-ray regime as moderate-resolution, high-throughput spectrometers that are particularly suited to measure dilute and damage-sensitive samples. My role as an instrumentation scientist has focused on fast data processing, instrument calibration, and ease-of-use for users.
My research into spectroscopy focuses on using partial-fluorescence-yield X-ray absorption spectroscopy to probe electronic structure in transition-metal complexes. Transition metals play a critical role in proteins such as hemoglobin and photosystem-II, catalysts, and batteries. In all of these systems, metals have a powerful ability to change oxidation states, store energy, and shuttle electrons around. X-ray spectroscopy allows us to directly probe the properties of transition metals that make them so useful for chemistry and biology.
Kent Irwin, Doctoral Dissertation Advisor (AC)
Soft X-ray spectroscopy with transition-edge sensors at Stanford Synchrotron Radiation Lightsource beamline 10-1.
The Review of scientific instruments
2019; 90 (11): 113101
We present results obtained with a new soft X-ray spectrometer based on transition-edge sensors (TESs) composed of Mo/Cu bilayers coupled to bismuth absorbers. This spectrometer simultaneously provides excellent energy resolution, high detection efficiency, and broadband spectral coverage. The new spectrometer is optimized for incident X-ray energies below 2 keV. Each pixel serves as both a highly sensitive calorimeter and an X-ray absorber with near unity quantum efficiency. We have commissioned this 240-pixel TES spectrometer at the Stanford Synchrotron Radiation Lightsource beamline 10-1 (BL 10-1) and used it to probe the local electronic structure of sample materials with unprecedented sensitivity in the soft X-ray regime. As mounted, the TES spectrometer has a maximum detection solid angle of 2 × 10-3 sr. The energy resolution of all pixels combined is 1.5 eV full width at half maximum at 500 eV. We describe the performance of the TES spectrometer in terms of its energy resolution and count-rate capability and demonstrate its utility as a high throughput detector for synchrotron-based X-ray spectroscopy. Results from initial X-ray emission spectroscopy and resonant inelastic X-ray scattering experiments obtained with the spectrometer are presented.
View details for DOI 10.1063/1.5119155
View details for PubMedID 31779391
L-edge spectroscopy of dilute, radiation-sensitive systems using a transition-edge-sensor array
JOURNAL OF CHEMICAL PHYSICS
2017; 147 (21): 214201
We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000 eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by grating spectrometers. These results show that soft-X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry, and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.
View details for PubMedID 29221417
View details for PubMedCentralID PMC5720893
- X-ray absorption spectroscopy of organic sulfoxides RSC ADVANCES 2020; 10 (44): 26229–38
- Saturation and self-absorption effects in the angle-dependent 2p3d resonant inelastic X-ray scattering spectra of Co3+ JOURNAL OF SYNCHROTRON RADIATION 2020; 27: 979–87
[(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-: Foundations toward the Development of Trifluoromethylations at Unsupported Nickel.
Nickel anions [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2- were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3 to [(dme)NiBr2] in the presence of the [PPh4]+ counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-, these complexes are better physically described as d9 metal complexes. [(MeCN)Ni(CF3)3]- is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2- salt, revealing the broader potential of [(MeCN)Ni(CF3)3]- in the development of "ligandless" trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF3)3]- with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh4]2[Ni(CF3)4(SO4)], which was derived through the two-electron oxidation of [Ni(CF3)4]2-.
View details for DOI 10.1021/acs.inorgchem.0c01020
View details for PubMedID 32573210
- Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers CHEMICAL SCIENCE 2020; 11 (17): 4360–73
- Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states NPJ QUANTUM MATERIALS 2020; 5 (1)
The Myth of d8 Copper(III).
Journal of the American Chemical Society
Seventeen Cu complexes with formal oxidation states ranging from CuI to CuIII are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally CuIII species is extremely covalent, resulting in Cu K-edge and L2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L2,3-edge data reveals that all formally CuIII species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d8 description for these formally CuIII species is inaccurate. This study provides an alternative explanation for the competence of formally CuIII species in transformations that are traditionally described as metal-centered, 2-electron CuI/CuIII redox processes.
View details for DOI 10.1021/jacs.9b09016
View details for PubMedID 31710466
- Advanced X-ray Scattering and Spectroscopy Characterization of an Antisoiling Coating for Solar Module Glass ACS APPLIED ENERGY MATERIALS 2019; 2 (11): 7870–78
Synthesis of a copper-supported triplet nitrene complex pertinent to copper-catalyzed amination.
Science (New York, N.Y.)
2019; 365 (6458): 1138–43
Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper-nitrenoid bonds [1.745(2) to 1.759(2) angstroms]. X-ray absorption spectroscopy and quantum chemistry calculations reveal a predominantly triplet nitrene adduct bound to copper(I), as opposed to copper(II) or copper(III) assignments, indicating the absence of a copper-nitrogen multiple-bond character. Employing electron-deficient aryl azides renders the copper nitrene species competent for alkane amination and alkene aziridination, lending further credence to the intermediacy of this species in proposed nitrene-transfer mechanisms.
View details for DOI 10.1126/science.aax4423
View details for PubMedID 31515388
- Two-Level Switches for Advanced Time-Division Multiplexing IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2019; 29 (5)
- In-situ functionalization of tetrahedral amorphous carbon by filtered cathodic arc deposition AIP ADVANCES 2019; 9 (8)
- Use of Transition Models to Design High Performance TESs for the LCLS-II Soft X-Ray Spectrometer IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2019; 29 (5)
- High-Throughput, DC-Parametric Evaluation of Flux-Activated-Switch-Based TDM and CDM SQUID Multiplexers IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2019; 29 (5)
- Hybrid X-ray Spectroscopy-Based Approach To Acquire Chemical and Structural Information of Single-Walled Carbon Nanotubes with Superior Sensitivity JOURNAL OF PHYSICAL CHEMISTRY C 2019; 123 (10): 6114–20
Surface-to-Bulk Redox Coupling through Thermally Driven Li Redistribution in Li- and Mn-Rich Layered Cathode Materials.
Journal of the American Chemical Society
Li- and Mn-rich (LMR) layered cathode materials have demonstrated impressive capacity and specific energy density thanks to their intertwined redox centers including transition metal cations and oxygen anions. Although tremendous efforts have been devoted to the investigation of the electrochemically driven redox evolution in LMR cathode at ambient temperature, their behavior under a mildly elevated temperature (up to ∼100 °C), with or without electrochemical driving force, remains largely unexplored. Here we show a systematic study of the thermally driven surface-to-bulk redox coupling effect in charged Li1.2Ni0.15Co0.1Mn0.55O2. We for the first time observed a charge transfer between the bulk oxygen anions and the surface transition metal cations under ∼100 °C, which is attributed to the thermally driven redistribution of Li ions. This finding highlights the nonequilibrium state and dynamic nature of the LMR material at deeply delithiated state upon a mild temperature perturbation.
View details for DOI 10.1021/jacs.9b05349
View details for PubMedID 31287957
- Interconnected Signatures of Quantum Spin Liquid Physics Across the Barlowite Family of Quantum Magnets INT UNION CRYSTALLOGRAPHY. 2019: A122