Electron Spin Resonance Investigations on Porous Silicon

1994 ◽  
Vol 358 ◽  
Author(s):  
B.K. Meyer ◽  
D.M. Hofmann ◽  
P. Christmann ◽  
W. Stadler ◽  
A. Nikolov ◽  
...  
Keyword(s):  
Porous Silicon ◽  
Defect Density ◽  
Double Resonance ◽  
Hydrogen Desorption ◽  
Dangling Bonds ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Luminescence Efficiency ◽  
Increased Sensitivity ◽  
Optically Detected

ABSTRACTThe defect properties of as-etched, annealed and thermally oxidized nano-porous Si as well as self supporting macro-porous silicon layers are studied by electron paramagnetic resonance (EPR), photoluminescence (PL), optically detected magnetic resonance (ODMR), and electron nuclear double resonance (ENDOR). The paramagnetic defects observed are dangling bonds closely related to the Pb-center, the Si/SiO2 interfacial defect. In EPR a minimum defect density of 1016 cm−3 is observed for as-etched material, it reaches a maximum of 8 x 1018 cm−3 for samples annealed around 400°C. We quantitatively correlate the defect density with hydrogen desorption data and luminescence efficiency. In the ODMR experiments the same dangling bond centers are observed on the 1.7 eV luminescence band, but with increased sensitivity in the infra-red emission band at 1.15 eV. Electron nuclear double resonance experiments show that the dangling bonds are not solely bonded to Si neighbors but involve hydrogen and fluorine.

The Microscopic Structure Of Shallow Donors In Silicon Carbide

1996 ◽  
Vol 442 ◽  
Author(s):  
J.-M. Spaeth ◽  
S. Greulich-Weber ◽  
M. März ◽  
E. N. Kalabukhova ◽  
S. N. Lukin
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Temperature Dependence ◽  
Double Resonance ◽  
Epr Spectra ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Vacancy Pair ◽  
Temperature Spectra ◽  
Electron Paramagnetic

AbstractThe electronic structure of nitrogen donors in 6H-, 4H- and 3C-SiC is investigated by measuring the nitrogen hyperfine (hf) interactions with electron nuclear double resonance (ENDOR) and the temperature dependence of the hf split electron paramagnetic resonance (EPR) spectra. Superhyperfine (shf) interactions with many shells of 13C and 29Si were measured in 6H-SiC. The hf and shf interactions are discussed in the framework of effective mass theory. The temperature dependence is explained with the thermal occupation of the lowest valley-orbit split A1 and E states. It is proposed that the EPR spectra of P donors observed previously in neutron transmuted 6H-SiC at low temperature (<10K) and high temperature (>60K) are all due to substitutional P donors on the two quasi-cubic and hexagonal Si sites, whereby at low temperature the E state is occupied and at high temperature the A1 state. The low temperature spectra are thus thought not to be due to P-vacancy pair defects as proposed previously.

Optically detected electron nuclear double resonance on the residual donor in GaN

1997 ◽  
Vol 43 (1-3) ◽  
pp. 181-184
Author(s):  
F.K. Koschnick ◽  
K. Michael ◽  
J.-M. Spaeth ◽  
B. Beaumont ◽  
P. Gibart
Keyword(s):  
Double Resonance ◽  
Electron Nuclear Double Resonance ◽  
Optically Detected

Solution Tetrahydrobiopterin Radical vs. the Enzyme-Bound Radical: A Paramagnetic Reconciliation

2021 ◽  
Vol 16 ◽  
Author(s):  
Yaser Nejaty Jahromy
Keyword(s):  
Nitric Oxide ◽  
Density Functional ◽  
Double Resonance ◽  
Nicotinamide Adenine Dinucleotide Phosphate ◽  
Cation Radical ◽  
Spectral Simulation ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Isotropic Hyperfine Coupling ◽  
The 1960S

Background: Nitric oxide synthase (NOS) catalyzes the formation of nitric oxide (NO) and citrulline from L-arginine, dioxygen (O2), and nicotinamide adenine dinucleotide phosphate (NADPH) in a two-step reaction, with the enzyme-bound intermediate Nω-hydroxy-L-arginine (NHA). Previous electron paramagnetic resonance (EPR) studies of NOS reaction have shown that (6R, 1'R, 2'S)-6-(l',2'-dihydroxypropyl)-5,6,7,8-tetrahydropterin (H4B) acts as a single electron donor in both steps of the reaction, resulting in the transient generation of a tetrahydropterin cation radical (H4B•+). Results: H4B•+ can also be chemically generated in strongly acidic solutions. EPR studies of chemically generated H4B•+ and similar pterin radicals date back to the 1960s. However, the reported paramagnetic parameters of H4B•+ in NOS do not seem to match the corresponding reported parameters for either H4B•+ or other pterin centered radicals chemically generated in solution. In particular, the rather isotropic hyperfine coupling of ca. 45 MHz for 1H6 of H4B•+ in NOS is at least 15 MHz larger than that of H4B•+ or any other previously studies pterin solution radical. In the work reported here, a combination of 9.5 - 9.8 GHz contentious wave (cw-) EPR, 34GHz 1H electron nuclear double resonance (ENDOR), spectral simulation and Density Functional Theory (DFT) calculations were used to investigate this seeming discrepancy. Conclusion: We demonstrated that the differences in the paramagnetic parameters of the chemically generated H4B radicals in solutions and those of the H4B radicals in NOS are consistent with the presence of two different conformers of the same cation radical in the two media.

scholarly journals The binuclear cluster of [FeFe] hydrogenase is formed with sulfur donated by cysteine of an [Fe(Cys)(CO)2(CN)] organometallic precursor

2019 ◽  
Vol 116 (42) ◽  
pp. 20850-20855 ◽  
Author(s):  
Guodong Rao ◽  
Scott A. Pattenaude ◽  
Katherine Alwan ◽  
Ninian J. Blackburn ◽  
R. David Britt ◽  
...  
Keyword(s):  
Double Resonance ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Carbon And Nitrogen ◽  
Binuclear Cluster ◽  
Organometallic Precursor ◽  
Pulse Epr ◽  
X Ray Absorption ◽  
Electron Paramagnetic

The enzyme [FeFe]-hydrogenase (HydA1) contains a unique 6-iron cofactor, the H-cluster, that has unusual ligands to an Fe–Fe binuclear subcluster: CN−, CO, and an azadithiolate (adt) ligand that provides 2 S bridges between the 2 Fe atoms. In cells, the H-cluster is assembled by a collection of 3 maturases: HydE and HydF, whose roles aren’t fully understood, and HydG, which has been shown to construct a [Fe(Cys)(CO)2(CN)] organometallic precursor to the binuclear cluster. Here, we report the in vitro assembly of the H-cluster in the absence of HydG, which is functionally replaced by adding a synthetic [Fe(Cys)(CO)2(CN)] carrier in the maturation reaction. The synthetic carrier and the HydG-generated analog exhibit similar infrared spectra. The carrier allows HydG-free maturation to HydA1, whose activity matches that of the native enzyme. Maturation with 13CN-containing carrier affords 13CN-labeled enzyme as verified by electron paramagnetic resonance (EPR)/electron nuclear double-resonance spectra. This synthetic surrogate approach complements existing biochemical strategies and greatly facilitates the understanding of pathways involved in the assembly of the H-cluster. As an immediate demonstration, we clarify that Cys is not the source of the carbon and nitrogen atoms in the adt ligand using pulse EPR to target the magnetic couplings introduced via a 13C3,15N-Cys–labeled synthetic carrier. Parallel mass-spectrometry experiments show that the Cys backbone is converted to pyruvate, consistent with a cysteine role in donating S in forming the adt bridge. This mechanistic scenario is confirmed via maturation with a seleno-Cys carrier to form HydA1–Se, where the incorporation of Se was characterized by extended X-ray absorption fine structure spectroscopy.

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