Thermoelectric Effect across the MetalInsulator Domain Walls in VO₂ Microbeams

We report on measurements of Seebeck effect in single-crystal VO2 microbeams across their metal-insulator phase transition. One-dimensionally aligned metal-insulator domain walls were reversibly created and eliminated along single VO2 beams by varying temperature, which allows for accurate extraction of the net contribution to the Seebeck effect from these domain walls. We observed significantly lower Seebeck coefficient in the metal-insulator coexisting regime than predicted by a linear combination of contributions from the insulator and metal domains. This indicates that the net contribution of the domain walls has an opposite sign from that of the insulator and metal phases separately. Possible origins that may be responsible for this unexpected effect were discussed in the context of complications in this correlated electron material. High-performance thermoelectric materials are currently one of the focuses in materials research for energy conversion technologies.1-4 A good thermoelectric material should have a relatively high thermopower (Seebeck coefficient).1,5 Quest for an ever higher Seebeck coefficient has led to material innovations where the electron density of state is engineered by homogeneous doping of single-phase materials.4 Alternatively, interfacing different materials has been proposed as a means to enhance the Seebeck effect from that of the constituent materials alone. For example, in addition to the conventional bulk Seebeck effect, extra thermal transport is expected by thermionic emission of free carriers over an energy barrier that is formed in the Schottky junction between a metal and a semiconductor, or from the band offset between two different semiconductors.6-8 This effect would enhance the thermoelectric performance of the structure beyond that of each single-phase material alone. Indeed, such an enhancement of the overall Seebeck coefficient has been observed in InGaAs/InGaAlAs superlattices embedded with randomly distributed ErAs nanoparticles.9 Recently, Seebeck coefficient was also reported to be enhanced by a single tunneling junction in individual Au nanowires.10 Effect of single or a few Schottky junctions on the Seebeck coefficient has not been experimentally tested, mainly due to the lack of materials suitable for accurate determination of the small change in the Seebeck coefficient. We explore this effect using a special material system, VO2 microbeams, where one or a few Schottky junctions can be reversibly created and eliminated in the plane perpendicular to the current and heat flow direction. This offers a material platform where the thermoelectric effect can be measured from the same specimen with or without the Schottky junction, so that an accurate extraction of the net junction effect becomes