2, Free University Berlin, Berlin, , Germany
4, University of Ulm, Ulm, , Germany
3, University of Stuttgart, Stuttgart, , Germany
Many of the states that participate in chemical reactions (e.g. heterogeneous catalysis) or that govern the performance of electronic devices (e.g., interface states of selective contacts of solar cells) have been studied using operando X-ray spectroscopy. Despite the success of X-ray-based techniques, their sensitivity is often not high enough to probe low concentration of such states (<ppm) at, for instance, semiconductor interfaces. Moreover, X-ray spectroscopy is not sensitive to the paramagnetic nature of the states involved in chemical reactions. A complementary spectroscopic technique, that can provide deeper insight into the chemical and electronic nature of above-mentioned states and processes, is electron paramagnetic resonance (EPR). Thanks to the unmatched specificity and its quantitative nature, EPR is amongst the most powerful spectroscopic techniques available today and is gaining significant attention in the research community as an analytical tool in life and materials science. In conventional EPR, the to-be-studied specimen is placed inside a microwave resonator and its paramagnetic states are brought into microwave resonance by sweeping an external magnetic field. Despite its analytical success, up to date only very few operando EPR experiments have been performed. The reason lies in the disadvantage of the resonator approach. It is extremely challenging to design the probe or the chemical reactor for an operando EPR experiment due to (a) the limited probing volume inside the cavity; (b) the reduction in sensitivity when metals or polar liquids are within the probing volume; (c) parasitic signals from the substrates; (d) the bulky nature of the experiment due to the required resonator and electromagnet.
In this lecture, we will present a novel EPR technique referred to as EPR-on-a-chip (EPRoC). EPRoC is no longer restricted by the boundary condition of a resonator and has a three order of magnitude higher spin sensitivity than conventional EPR. The EPRoC sensor is a single coil (or an array of coils) that is scaled-down in size to a few 10-100 µm, depending on application. The sensor and the microwave generating as well as detecting unit are integrated on a small, millimeter-sized silicon chip. Different from conventional EPR, EPRoC is carried out by sweeping the microwave frequency instead of the magnetic field. This enables operation with a constant magnetic field, which at X-band is established by a permanent magnet. Due to its simplicity and compactness, EPRoC can be incorporated in conventional growth reactors, (electro)chemical cells or even the endstation at an X-ray beamline. Here, we will review the recent success of operando EPRoC, discuss the detection principle and demonstrate its superior sensitivity. Finally, we will present first time-domain EPRoC results and a first operando experiment. The potential of EPRoC in combination with X-ray spectroscopy will be highlighted.