Direct observation of potential phase at joining interface between p-MgO and n-MgFe2O4

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.

The detection of various gases is important for constructing safety nets in today's society. For example, one type of well-known detection technology is monitoring systems for gas leaks and flammable gases such as methane that originates from coal oil complexes 1 . Oxide semiconductors are used for these gas detection method 2,3 , wherein the presence of a gas is detected based on a change in resistance caused by the electrical interaction between gas molecules and the oxide semiconductor 4,5 . For gas detection with oxide semiconductors, a linear relationship between the change in electrical resistance and gas concentration is assumed as a rule of thumb. According to assumption, increasing the amount of adsorbed oxygen is key to enhancing the sensor response to gas, or gas sensitivity. Adsorbed oxygen is generated when oxygen in the air takes electrons from the surface of the oxide semiconductor and adsorbs to the surface as negatively charged species (adsorbed oxygen: O − , O 2− ). At the same time, a space charge layer (depletion layer) with a low carrier concentration forms from the surface of the oxide inward 6 . Controlling this depletion layer could allow for tuning of the gas sensitivity.
Oxide semiconductor sensors using a change in a depletion layer enlarged by formation of a p-n junction can be categorized following: the enlargement of depletion layer originates only from p-n junction [7][8][9][10][11][12][13][14][15] ; that from sulfurization on the surface of p-type oxide. Especially, latter contains electronic sensitizing by sulfurization [16][17][18][19][20] , as shown in Table 1. Among the former, the morphology of n-TiO 2 /p-CuO nanowires was effective to enhance sensor response to 1 ppm CO as one of reductant gases when the wall thickness of TiO 2 shell was 60 nm. This result was supported by the experimental fact that its base resistance in air was highest in the wall thickness from 0 to 100 nm. If there is correlation data between the combination of p-n junction and the enlargement of depletion layer, the sensor with high sensing performance could be designed by selecting the optimal combination of p-n junctions without try and errors of experiments.
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). The first visualization of the depletion layer is the measurement of the surface potential of the silicon p-n junction by KPFM 21 . After that, it was reported that the barrier height at the grain boundary of polycrystalline silicon was measured by using KPFM, and the potential barrier height changed depending on the grain boundary character 22 . As a recent visualization technique of the depletion layer, when the forward bias and the reverse bias were applied to the operating semiconductor material (GaAs), the change of the depletion layer width of only 1 nm was measured 23 . Besides, the electron beam induced current (EBIC) realized a visualization of the depletion layer by two-dimensional mapping 24 . However, advanced thinning technique is required to visualize the depletion layer by above techniques.
In this study, it is possible to form heterogeneous junctions by our simple process ( Fig. 1) for preparation of core-shell micro hollow particles. The KPFM measurement sample can also be prepared with a simple method (embedding the particles in a conductive resin and polishing them).
In our previous research, we revealed that composite oxides of MgO-Fe 2 O 3 respond to 10 ppb hydrogen sulfide 25 . It was found that MgO acts as a p-type semiconductor and MgFe 2 O 4 acts as an n-type semiconductor, thereby contributing to an increase in the depletion layer due to the p-n junction. In this study, KPFM measurements clarified the formation of p-n junctions in core-shell microspherical particles with Fe 2 O 3 as the core and MgFe 2 O 4 and MgO as the shells. In addition, microstructural observations indicated that the MgO shell behaves as a semiconductor.

Results and discussion
The X-ray diffraction (XRD) pattern of the obtained core-shell microspherical particles is shown in Supplementary Fig. S1. All patterns were attributed to the Fe 2 O 3 core and MgFe 2 O 4 and MgO shells. Since the diffraction peaks of MgFe 2 O 4 and MgO partially overlap, the abundance was estimated as follows from the relative peak intensity ratio in the ICDD database.
According to Eq. (1), the abundance ratio of the sample after heat treatment (800 °C, 3 h) was approximately 1, indicating the presence of the same amounts of the MgFe 2 O 4 and MgO shell constituents.
The secondary electron image of the core-shell microspherical particles and the MgKα, FeKα, and OKα mapping images are shown in Supplementary Fig. S2. The diameter of the core-shell microspherical particles was approximately 1 μm, and Mg and Fe existed in a spherical shape.
The microstructural observations of the core-shell microspherical particles are shown in Fig. 2. The typical bright-field image shows the presence of hollow spherical particles (Fig. 2a). The abundance of hollow particles was approximately 70%, as indicated by the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images ( Fig. 2b-1,2). Figure 2c is enlarged view of the square area in Fig. 2b-2. Based on the elemental mapping of the shell of the hollow particles (Fig. 2d-f) The KPFM results for the core-shell micro hollow particles are shown in Fig. 3. Since the core-shell particles were mirror-polished after being embedded in a conductive resin, the resin penetrated the hollow particles. The potential barrier of Cu within the conductive resin was adopted as the background potential for measuring the potential barrier height. In Fig. 3, high potential barriers are shown in red, and low are in blue. Based on the line analysis of four potential barrier heights (Fig. 3A-H), the average change in the potential barrier in the shell was approximately 0.2 V. In all four lines, the potential barrier in the inner shell tended to be higher than that in the outer shell. This may reflect the result of p-n junction formation between the p-type MgO and n-type MgFe 2 O 4 . Thus far, in the field of gas sensors, although it has been suggested that a p-n junction will cause the depletion layer to expand, this phenomenon has yet to be visualized.
In general, MgO is considered an insulator because of its high band gap (7.8 eV). However, recent studies have reported that one-dimensional MgO monolayers consist of an aggregate of several nanometers of MgO   Figure 4 shows a high-resolution TEM image of the MgO layer (approximately 20 nm) of the core-shell micro hollow particles in this study. The MgO layer consists of MgO microcrystals of several tens of nanometers, suggesting that the MgO layer in this study behaves as a semiconductor. Supplementary Fig. S3 shows the temperature dependence of the electrical resistance in air (R a ) of spherical Fe 2 O 3 particles, amorphous MgO particles, a mixture of both particles, and the core-shell micro hollow particles. The core-shell micro hollow particles showed high R a values at all temperatures due to the p-n junction effect.
The sensor response of MgO ( Supplementary Fig. S3e) to 250 ppm at 3 ppm H 2 S was added as Fig. S4. The sensor response (S = Rg/Ra) increased upon exposure to H 2 S. Generally, in air, oxygen is adsorbed as a negative charge on the surface of an n-type oxide semiconductor (adsorbed oxygen: O 2− ), and a depletion layer is formed from the surface to the inside. When the atmosphere is switched from air to reducing gas (H 2 S), the adsorbed  By moving them to the layer, the depletion layer is reduced, leading to a reduction in electrical resistance. Since the behavior of the sensor response in Supplementary Fig. S4 showed an increase in resistance, the depletion layer is increasing. Therefore, it was considered that MgO behaved as p-type. In order to investigate the cause of the difference in surface potential due to junction between oxides, the wave of surface potential in the same region as  . This suggests that the enlargement of depletion layer by formation of p-n junction could affect the carrier concentration of n-n junction (MgFe 2 O 4 -Fe 2 O 3 ). From above discussion, it is considered that the KPFM measurement results of this study were effective to characterize electrically the core-shell microsphere particles.

Conclusion
The MgO/MgFe 2 O 4 /Fe 2 O 3 core-shell microsphere particles were prepared by heat treating core-shell microsphere particles with Fe 2 O 3 as the core and MgO as the shell. Approximately 70% of the obtained spherical particles were hollow in structure and comprised, from the outside in, approximately 20 nm of MgO, 80 nm of MgFe 2 O 4 and 100 nm of Fe 2 O 3 from the outer shell. As determined by KPFM measurements of the hollow particles, the difference in potential barrier height at the interface between MgO and MgFe 2 O 4 was approximately 0.2 V. This difference was reflected in the measured R a values, suggesting that it was due to the formation of a p-n junction between p-type MgO and n-type MgFe 2 O 4 . In the development of gas sensors, measuring the potential barrier height with KPFM may lead to tunable gas sensitivity. As one of recent our results, the potential barrier height of joining interface between p-type CuO (Eg: 1.4 eV) and n-type SnO 2 (Eg: 3.7 eV) was higher than that of CuO and SnO 2 . The sensitivity to detection gases could be tuned by selecting combination of p-type and n-type oxides among many oxide semiconductors with different bandgap, based on the magnitude in the resultant potential barrier height by the formation of p-n junction. The estimation of carrier concentration based on the surface potential of each junction of oxides measured by KPFM was clarified to be quite effective for depletion engineering of gas sensing materials.

Methods
Core-shell microspherical particle synthesis. Magnetite (Fe 3 O 4 ) spherical particles with the diameter of ca. 1 µm were obtained by dissolving iron oxyhydroxide in a solvent (ethylene glycol and 9 wt% H 2 O) and hydrothermally treating it at 200 °C for 24 h in an autoclave, as described in our previously published work 33 . An Fe 2 O 3 spherical powder with a particle size of 1 μm was then obtained by heat-treating (800 °C, 3 h). The Fe 2 O 3 spherical particles were immersed in a 6 mol/L sodium hydroxide aqueous solution (alkalization). Subsequently, the alkalized Fe 2 O 3 spherical particles were dissolved in an aqueous magnesium acetate (Mg(CH 3 COO) 2 ) solution and stirred for 30 min. At this time, the molar ratio of Fe:Mg was 4:6. The obtained suspension was centrifuged (5000 rpm, 3 min) and heat-treated (800 °C, 3 h) to obtain core-shell microspherical particles.
Sample for HAADF-STEM observation. The powder sample was dissolved in 10 mL of ethanol to 1 wt%, and a suspension was obtained through ultrasonication. The obtained suspension was dropped onto a Cu