71 research outputs found
THz Electron Paramagnetic Resonance / THz Spectroscopy at BESSY II
The THz beamline at BESSY II employs high power broadband femto- to picosecond long THz pulses for magneto-optical THz and FIR studies. A newly designed set-up exploits the unique properties of ultrashort THz pulses generated by laser-energy modulation of electron bunches in the storage ring or alternatively from compressed electron bunches. Experiments from 0.15 to 5 THz (~ 5 â 150 cm-1) may be conducted at a user station equipped with a fully evacuated high resolution FTIR spectrometer (0.0063 cm-1), lHe cooled bolometer detectors, a THz TDS set-up and different sample environments, including a superconducting high field magnet (+11 T - 11T) with variable temperature insert (1.5 K â 300 K), a sample cryostat and a THz attenuated total reflection chamber. Â Main applications are Frequency Domain Fourier transform THz-Electron Paramagnetic Resonance (FD-FT THz-EPR), THz-FTIR spectroscopy and optical pump - THz probe time domain spectroscopy (TDS), with sub-ps time resolution
High Resolution Microimaging with Pulsed Electrically-Detected Magnetic Resonance
The investigation of paramagnetic species (such as point defects, dopants,
and impurities) in solid-state electronic devices is significant because of
their effect on device performance. Conventionally, these species are detected
and imaged using the electron spin resonance (ESR) technique. In many
instances, ESR is not sensitive enough to deal with miniature devices having
small numbers of paramagnetic species and high spatial heterogeneity. This
limitation can in principle be overcome by employing a more sensitive method
called electrically-detected magnetic resonance, which is based on measuring
the effect of paramagnetic species on the electric current of the device while
inducing electron spin-flip transitions. However, up until now, measurement of
the current of the device could not reveal the spatial heterogeneity of its
paramagnetic species. We provide here, for the first time, high resolution
microimages of paramagnetic species in operating solar cells obtained through
electrically-detected magnetic resonance. The method is based on unique
microwave pulse sequences for excitation and detection of the electrical signal
under a static magnetic field and powerful pulsed magnetic field gradients that
spatially encode the electrical current of the sample. The approach developed
here can be widely used in the nondestructive three-dimensional inspection and
characterization of paramagnetic species in a variety of electronic devices.Comment: 19 pages, 4 figures +S
Thermosensitive Cu2O-PNIPAM core-shell nanoreactors with tunable photocatalytic activity
We report a facile and novel method for the fabrication of Cu2O@PNIPAM
core-shell nanoreactors using Cu2O nanocubes as the core. The PNIPAM shell not
only effectively protects the Cu2O nanocubes from oxidation, but also improves
the colloidal stability of the system. The Cu2O@PNIPAM core-shell microgels can
work efficiently as photocatalyst for the decomposition of methyl orange under
visible light. A significant enhancement in the catalytic activity has been
observed for the core-shell microgels compared with the pure Cu2O nanocubes.
Most importantly, the photocatalytic activity of the Cu2O nanocubes can be
further tuned by the thermosensitive PNIPAM shell, as rationalized by our
recent theory.Comment: 8 pages, 6 figures (Supporting Information included: 11 pages, 10
figures
a FD-FT THz-EPR study
A combined X-band and frequency-domain Fourier-transform THz electron
paramagnetic resonance (FD-FT THz-EPR) approach has been employed to determine
heme Fe(III) S = 5/2 zero-field splitting (ZFS) parameters of frozen metHb and
metMb solutions, both with fluoro and aquo ligands. Frequency-domain EPR
measurements have been carried out by an improved synchrotron-based FD-FT THz-
EPR spectrometer. ZFS has been determined by field dependence of spin
transitions within the mS = ±1/2 manifold, for all four protein systems, and
by zero-field spin transitions between mS = ±1/2 and mS = ±3/2 levels, for
metHb and metMb flouro-states. FD-FT THz-EPR data were simulated with a novel
numerical routine based on Easyspin, which allows now for direct comparison of
EPR spectra in field and frequency domain. We found purely axial ZFSs of D =
5.0(1) cmâ1 (flouro-metMb), D = 9.2(4) cmâ1 (aquo-metMb), D = 5.1(1) cmâ1
(flouro-metHB) and D = 10.4(2) cmâ1 (aquo-metHb)
Direct Detection of Photoinduced Charge Transfer Complexes in Polymer:Fullerene Blends
We report transient electron paramagnetic resonance (trEPR) measurements with
sub-microsecond time resolution performed on a P3HT:PCBM blend at low
temperature. The trEPR spectrum immediately following photoexcitation reveals
signatures of spin-correlated polaron pairs. The pair partners (positive
polarons in P3HT and negative polarons in PCBM) can be identified by their
characteristic g-values. The fact that the polaron pair states exhibit strong
non-Boltzmann population unambiguously shows that the constituents of each pair
are geminate, i.e. originate from one exciton. We demonstrate that coupled
polaron pairs are present even several microseconds after charge transfer and
suggest that they embody the intermediate charge transfer complexes which form
at the donor/acceptor interface and mediate the conversion from excitons into
free charge carriers
Lock-in detection for pulsed electrically detected magnetic resonance
We show that in pulsed electrically detected magnetic resonance (pEDMR)
signal modulation in combination with a lock-in detection scheme can reduce the
low-frequency noise level by one order of magnitude and in addition removes the
microwave-induced non-resonant background. This is exemplarily demonstrated for
spin-echo measurements in phosphorus-doped Silicon. The modulation of the
signal is achieved by cycling the phase of the projection pulse used in pEDMR
for the read-out of the spin state.Comment: 4 pages, 2 figure
Rapid-scan electron paramagnetic resonance using an EPR-on-a-Chip sensor
Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we present an improved design of a miniaturized EPR spectrometer implemented on a silicon microchip (EPR-on-a-chip, EPRoC). In place of a microwave resonator, EPRoC uses an array of injection-locked voltage-controlled oscillators (VCOs), each incorporating a 200 ÎŒm diameter coil, as a combined microwave source and detector. The individual miniaturized VCO elements provide an excellent spin sensitivity reported to be about 4 Ă 109spins/âHz, which is extended by the array over a larger area for improved concentration sensitivity. A striking advantage of this design is the possibility to sweep the MW frequency instead of the magnetic field, which allows the use of smaller, permanent magnets instead of the bulky and powerhungry electromagnets required for field-swept EPR. Here, we report rapid scan EPR (RS-EPRoC) experiments performed by sweeping the frequency of the EPRoC VCO array. RS-EPRoC spectra demonstrate an improved SNR by approximately two orders of magnitude for similar signal acquisition times compared to continuous wave (CW-EPRoC) methods, which may improve the absolute spin and concentration sensitivity of EPR-on-a-Chip at 14 GHz to about 6 Ă 107 spins/âHz and 3.6 nMââHz, respectively
Easy-plane to easy-axis anisotropy switching in a Co(ii) single-ion magnet triggered by the diamagnetic lattice
Single ion magnets SIMs with large magnetic anisotropy are promising candidates for realization of single molecule based magnetic memory and qubits. Creation of materials with magnetically uncoupled spatially separated SIMs requires dilution in a diamagnetic matrix. Herein, we report that progressive dilution of paramagnetic Co II by diamagnetic Zn II in the SIM [CoxZn 1 amp; 8722;x piv 2 2 NH2 Py 2], x 1 0 beyond a threshold of 50 reveals an abrupt structural change, where the distorted tetrahedral Zn coordination structure is superimposed on the remaining Co ions, which were initially in a distorted octahedral environment. Dilution induced structure modification switches the magnetic anisotropy from easy plane D 36.7 cm amp; 8722;1 to easy axis type D amp; 8722;23.9 cm amp; 8722;1 , accompanied by a fivefold increase of the magnetic relaxation time at 2 K. Changes of the static and dynamic magnetic properties are monitored by electron paramagnetic resonance spectroscopy and AC susceptibility measurements. Complementary quantum chemical ab initio calculations quantify the influence of structural changes on the electronic structure and the magnetic anisotropy. Thus, magnetic dilution hits two goals at once, the creation of isolated magnetic centres and an improvement of their SIM propertie
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