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Near Ambient Pressure Scanning Photoelectron Microscopy adds to the widely used photoemission spectroscopy and its chemically selective capability two key features: (i) the possibility to chemically analyse samples in a more realistic environmental, gas pressure condition, and (ii) the capability to investigate a system at the relevant spatial scale. To achieve these goals the approach developed at the ESCA Microscopy beamline at the Elettra Synchrotron facility combines the submicron lateral resolution of a Scanning Photoelectron Microscope with a custom designed Near Ambient Pressure Cell where a gas pressure up to 0.1 mbar is confined inside it around the sample. In this manuscript a review of experiments performed with this unique setup will be presented to illustrate its potentiality in both fundamental and applicative research such as the oxidation reactivity and gas sensitivity of metal oxides and semiconductors. In particular the capability to do operando experiment with this setup opens the possibility to perform investigations with active devices to properly address the real nature of the studied systems, because it can yield to more conclusive results when microscopy and spectroscopy are simultaneously combined in a single technique.

Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for safety and health applications, e.g. explosive, toxic gas alarms, controls for intake into car cabins and monitor for industrial processes. In the past, the sensor community has been studying polycrystalline materials as sensors where the porous and random microstructure of the SMOX does not allow a separation of the phenomena involved in the sensing process. This lead to conduction models that can model and predict the behavior of the overall response, but they were not capable of giving fundamental information regarding the basic mechanisms taking place. The study of epitaxial layers is the definite prove to clarify the different aspects and contributions of the sensing mechanisms that are not possible to do by studying a polycrystalline sample. A detailed analytical model for n and p-type single-crystalline/compact metal oxide gas sensors was developed that directly relates the conductance of the sample with changes in the surface electrostatic potential. Combined DC resistance and work function measurements were used in a compact SnO2 (101) layer in operando conditions that allowed us to check the validity of our model in the region where Boltzmann approximation holds to determine surface and bulk properties of the material.

Integrated, highly sensitive and reversible sensor devices for toxic and hazardous gases in environmental pollution monitoring can be realized with graphene-based materials. Here we show that, single layer graphene grown on SiC can be utilized to implement sensor devices being extremely sensitive towards NO2 showing an n-type response. A second type of sensor with an added NiO layer on top of the single layer graphene changed its response to p-type but did not reduce its sensitivity. We show that the conduction switch from n-type to p-type was not a consequence of an alteration of the graphene layer but is found to be an effect of the NiO layer. We find that the NiO leads to lowering of the Fermi level to a point that a crossing of the Dirac Point in the graphene switched the conduction type. These sensors were tested in the 100 ppb NO2 regime, showing good response and a detection limit extrapolated to be below 1 ppb. This new NiO/graphene/SiC configuration can be an attractive p-type sub-ppb sensor platform for NO2 and related gases.

NiO layers were grown on MgO(100), MgO(110), and MgO(111) substrates by plasma-assisted molecular beam epitaxy under Ni-flux limited growth conditions. Single crystalline growth with a cube-on-cube epitaxial relationship was confirmed by X-ray diffraction measurements for all used growth conditions and substrates except MgO(111). A detailed growth series on MgO(100) was prepared using substrate temperatures ranging from 20 {\deg}C to 900 {\deg}C to investigate the influence on the layer characteristics. Energy-dispersive X-ray spectroscopy indicated close-to-stoichiometric layers with an oxygen content of ~47 at. % and ~50 at. % grown under low and high O-flux, respectively. All NiO layers had a root-mean-square surface roughness below 1 nm, measured by atomic force microscopy, except for rougher layers grown at 900 {\deg}C or using molecular oxygen. Growth at 900 {\deg}C led to a significant diffusion of Mg from the substrate into the film. The relative intensity of the quasi-forbidden one-phonon Raman peak is introduced as a gauge of the crystal quality, indicating the highest layer quality for growth at low oxygen flux and high growth temperature, likely due to the resulting high adatom diffusion length during growth. The optical and electrical properties were investigated by spectroscopic ellipsometry and resistance measurements, respectively. All NiO layers were transparent with an optical bandgap around 3.6 eV and semi-insulating at room temperature. However, changes upon exposure to reducing or oxidizing gases of the resistance of a representative layer at elevated temperature were able to confirm p-type conductivity, highlighting their suitability as a model system for research on oxide-based gas sensing.