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, Postdoctoral Research Mentor
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
- Site-specific structure at multiple length scales in kagome quantum spin liquid candidates PHYSICAL REVIEW MATERIALS 2020; 4 (12)
Depth-dependent valence stratification driven by oxygen redox in lithium-rich layered oxide.
2020; 11 (1): 6342
Lithium-rich nickel-manganese-cobalt (LirNMC) layered material is a promising cathode for lithium-ion batteries thanks to its large energy density enabled by coexisting cation and anion redox activities. It however suffers from a voltage decay upon cycling, urging for an in-depth understanding of the particle-level structure and chemical complexity. In this work, we investigate the Li1.2Ni0.13Mn0.54Co0.13O2 particles morphologically, compositionally, and chemically in three-dimensions. While the composition is generally uniform throughout the particle, the charging induces a strong depth dependency in transition metal valence. Such a valence stratification phenomenon is attributed to the nature of oxygen redox which is very likely mostly associated with Mn. The depth-dependent chemistry could be modulated by the particles' core-multi-shell morphology, suggesting a structural-chemical interplay. These findings highlight the possibility of introducing a chemical gradient to address the oxygen-loss-induced voltage fade in LirNMC layered materials.
View details for DOI 10.1038/s41467-020-20198-w
View details for PubMedID 33311507
- Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy CHEMICAL SCIENCE 2020; 11 (43): 11801–10
- 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)
Saturation and self-absorption effects in the angle-dependent 2p3d resonant inelastic X-ray scattering spectra of Co3.
Journal of synchrotron radiation
2020; 27 (Pt 4): 979–87
Angle-dependent 2p3d resonant inelastic X-ray scattering spectra of a LaCoO3 single crystal and a 55 nm LaCoO3 film on a SrTiO3 substrate are presented. Theoretical calculation shows that, with ∼20 meV resolved Co 2p3d resonant inelastic X-ray scattering (RIXS), the excited states of the isotropic 1A1g(Oh) ground state are split by 3d spin-orbit coupling, which can be distinguished via their angular dependence. However, strong self-absorption and saturation effects distort the spectra of the LaCoO3 single crystal and limit the observation of small angular dependence. In contrast, the RIXS on 55 nm LaCoO3 shows less self-absorption effects and preserves the angular dependence of the excited states.
View details for DOI 10.1107/S1600577520005123
View details for PubMedID 33566007
View details for PubMedCentralID PMC7336173
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 spin- 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu4(OH)6FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q = 0 magnetic order. Partially Zn-substituted barlowite (Cu3.44Zn0.56(OH)6FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.
View details for PubMedID 33072886
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
Use of Transition Models to Design High Performance TESs for the LCLS-II Soft X-Ray Spectrometer.
IEEE transactions on applied superconductivity : a publication of the IEEE Superconductivity Committee
2019; 29 (5)
We are designing an array of transition-edge sensor (TES) microcalorimeters for a soft X-ray spectrometer at the Linac Coherent Light Source at SLAC National Accelerator Laboratory to coincide with upgrades to the free electron laser facility. The complete spectrometer will have 1000 TES pixels with energy resolution of 0.5 eV full-width at half-maximum (FWHM) for incident energies below 1 keV while maintaining pulse decay-time constants shorter than 100 μs. Historically, TES pixels have often been designed for a particular scientific application via a combination of simple scaling relations and trial-and-error experimentation with device geometry. We have improved upon this process by using our understanding of transition physics to guide TES design. Using the two-fluid approximation of the phase-slip line model for TES resistance, we determine how the geometry and critical temperature of a TES will affect the shape of the transition. We have used these techniques to design sensors with a critical temperature of 55 mK. The best sensors achieve an energy resolution of 0.75 eV FWHM at 1.25 keV. Building upon this result, we show how the next generation of sensors can be designed to reach our goal of 0.5 eV resolution.
View details for DOI 10.1109/tasc.2019.2903032
View details for PubMedID 33456289
View details for PubMedCentralID PMC7808210
- Interconnected Signatures of Quantum Spin Liquid Physics Across the Barlowite Family of Quantum Magnets INT UNION CRYSTALLOGRAPHY. 2019: A122