Electron paramagnetic resonance (EPR) is the method of choice to investigate and quantify paramagnetic states in e.g. semiconductor devices, proteins, catalysts and molecular nanomagnets. Common EPR spectrometers use microwave (mw) resonators, where the sample is inserted. This design, however, limits the versatility for in situ / operando measurements. Here, we present an improved design of a miniaturised EPR spectrometer, implemented on a single microchip (EPR-on-a-chip). Instead of an mw resonator, an array of coils, each from a voltage-controlled oscillator (VCO), with a diameter of a few hundred micrometer is used simultaneously as mw source and detector, replacing the entire microwave bridge. Similar to EPR microresonators, the filling factor of the EPRoC is high by design, leading to a better absolute spin sensitivity than conventional EPR. The usage of the VCO allows to sweep the microwave frequency, instead of magnetic field as in the conventional EPR, thus enabling operation with a permanent magnet. Due to its compactness, EPRoC can be incorporated into conventional thin-film growth reactors, (electro)chemical cells, batteries or in UHV environments.
Frequency sweeps in combination with the intrinsically high B1 render EPRoC perfect for rapid frequency scan EPR (rsEPRoC) with scan rates up to 2 000 THz/s. Rapid scan EPR can lead to a signal-to-noise improvement especially for samples with long relaxation times, which would otherwise be saturated in continuous wave EPR. We demonstrate the increased sensitivity of rsEPRoC, by investigating a few micrometer thick layers of amorphous silicon (a-Si) on quartz.
In this talk, we will review the recent advances in rsEPRoC, show first results on amorphous silicon samples, and discuss applications that will benefit from the increased sensitivity.