Abstract
Visualization of the depletion layer is a significant a guideline for the material design of gas sensors. We attempted to measure the potential barrier at the interface of core–shell microspheres composed of p-MgO/n-MgFe2O4/Fe2O3 from the inside out by means of Kelvin probe force microscopy (KPFM) as a first step to visualizing enlargement of the depletion layer. As determined by high-angle annular dark-field scanning transmission electron microscopy, ca. 70% of the microspheres were hollow with a wall thickness of ca. 200 nm. Elemental mapping revealed that the hollow particles were composed of ca. 20 nm of MgO, ca. 80 nm of MgFe2O4, and ca. 100 nm of Fe2O3. A difference of 0.2 V at the p-MgO/n-MgFe2O4 interface was clarified by KPFM measurements of the hollow particles, suggesting that this difference depends on the formation of a p–n junction. The potential barrier enlarged by the formation of a p–n junction was considered to increase the resistance in air (Ra), since the Ra of the core–shell hollow microspheres was higher than that of MgO, Fe2O3, MgO–Fe2O3, and MgO/MgFe2O4/Fe2O3 particles with irregular shapes. Measurement of the potential barrier height by KPFM is a promising potential approach to tuning the gas sensitivity of oxide semiconductors.
Highlights
Visualization of the depletion layer is a significant a guideline for the material design of gas sensors
The X-ray diffraction (XRD) pattern of the obtained core–shell microspherical particles is shown in Supplementary Fig. S1
As determined by Kelvin probe force microscopy (KPFM) measurements of the hollow particles, the difference in potential barrier height at the interface between MgO and M gFe2O4 was approximately 0.2 V. This difference was reflected in the measured resistance in air (Ra) values, suggesting that it was due to the formation of a p–n junction between p-type MgO and n-type MgFe2O4
Summary
Visualization of the depletion layer is a significant a guideline for the material design of gas sensors. The potential barrier enlarged by the formation of a p–n junction was considered to increase the resistance in air (Ra), since the Ra of the core–shell hollow microspheres was higher than that of MgO, Fe2O3, MgO–Fe2O3, and MgO/MgFe2O4/Fe2O3 particles with irregular shapes. Among the former, the morphology of n-TiO2/p-CuO nanowires was effective to enhance sensor response to 1 ppm CO as one of reductant gases when the wall thickness of TiO2 shell was 60 nm. As a first step toward this goal, this study visualized the increase in the depletion layer due to p–n junction formation by measuring the barrier height at the grain boundary from the local surface potential near the grain boundary using a Kelvin probe force microscope (KPFM). It was reported that the barrier height at the grain boundary of polycrystalline silicon was measured by using KPFM, and the potential
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