Scanning Probe Microscopy (SPM) is arguably the only tool that is available for direct nanoscale electrical and electrochemical characterization since electrical measurement through a highly localized area necessitates a nanoscale electrode. However, conventional SPM systems are functional only up to ~250°C under atmospheric pressure because significant heat convection from the heating stage to thermally-sensitive system modules disturbs a stable operation^1,2. In operando observations of non-vacuum processes necessitating a much higher temperature, e.g. solid oxide-based electrochemical reactions and phase changes, have been largely limited. To address these problems, we have developed micro-thermal stage (MTS)³ in which the heating element is on the micrometer scale with an area roughly a million times smaller than that of a conventional SPM heating stage. We also prepared SPM probes custom-made to suppress thermal degradation during high temperature operation. At high temperature, metal-coated SPM tips tend to degrade at a faster rate, resulting in erroneous observations. The modified SPM setup was applied to study thermally activated bonding formation and highly localized oxygen reduction reaction kinetics on a solid oxide electrolyte.

For the MTS, Pt is micro-patterned on a silicon chip as the heating element and a thin film-based layer of study are deposited on top of the heating element after electrical passivation. The temperature of the Pt heater can be determined by the measured electrical resistance of the heater because the resistance of a metal linearly increases with temperature⁴. A four-probe measurement scheme is integrated onto the chip for sensing the resistance and corresponding temperature of the heating element. The temperature of the heater is feedback controlled. Due to the small size of the heating element and small sample thickness, the desired temperature can be reached and controlled in milliseconds. To circumvent the degradation issue of Pt-coated tip, an all-metal tip is custom-made from Pt-Ir wire by electrochemical etching method, from which tip radii less than 100 nm can be routinely achieved. The non-etched part of the wire is flattened and polished to mount on an SPM system. After verifying the functionality of the Pt-Ir SPM tip through routine SPM contact mode scanning, we used the modified setup to study localized in-situ oxygen reduction catalysis and ionic transport on a platinum/yttria-stabilized ceria (Pt/YSZ) half cell. The aforementioned modified SPM setup has potential to be used in many other applications, such as in-situ observation of changes in mechanical, electrical and thermal properties at the tip-sample interfaces under high-temperature and high-pressure conditions.

Research supported by the AFOSR (Grant # FA9550-15-1-0256), USA and MoE AcRF Tier 2 (Grant # ARC20/14), Singapore.