Thermoelectrics, in particular solid-state conversion of heat to electricity, is expected to be a key energy harvesting technology to power ubiquitous sensors and wearable devices in the future. A comprehensive review is given on the principles and advances in the development of thermoelectric materials suitable for energy harvesting power generation, ranging from organic and hybrid organic-inorganic to inorganic materials. Examples of design and applications are also presented.

ZnO/Mg 0.2 Zn 0.8 O superlattices with a band-gap offset of about 0.5 eV were epitaxially grown by laser molecular-beam epitaxy on a sapphire͑0001͒ substrate using a ZnO buffer layer. The superlattice structure with a period ranging from 8 to 18 nm was clearly verified by cross-sectional transmission electron microscopy, Auger depth profile, and x-ray diffraction. As the well layer thickness decreased below 5 nm, the photoluminescence peak and absorption edge in the photoluminescence excitation spectra showed a blueshift, indicating a quantum-size effect.

We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfO x buffer layer between ZnO channel and amorphous silicon–nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2·V-1·s-1 for devices with maximum process temperature of 300°C. The process temperature can be reduced to 150°C without much degrading the performance, showing the possibility of the use of polymer substrate.

In situ thickness-dependent photoemission spectroscopy (PES) has been performed on SrRuO3 (SRO) layers deposited on SrTiO3 substrates to study the structure-induced evolution of the electronic structure. The PES spectra showing the existence of two critical film thicknesses reveal that a metal-insulator transition occurs at a film thickness of 4–5 monolayers (ML) and the evolution of Ru 4d-derived states around the Fermi level (EF) saturates at about 15 ML. The observed spectral behavior well matches the electric and magnetic properties and thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth-mode transition in SRO films.

We have examined the thermal stability of wurtzite-phase Mg x Zn 1Ϫx O alloy films and ZnO/Mg x Zn 1Ϫx O bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg 0.23 Zn 0.78 O film was annealed, the segregation of MgO started at 850°C and the band gap was reduced to the value of that for an xϭ0.15 film after annealing at 1000°C. Mg 0.15 Zn 0.85 O films showed no change of the band gap even after annealing at 1000°C. Therefore, we conclude that the thermodynamic solubility limit of MgO in Mg x Zn 1Ϫx O epitaxial film is about xϭ0.15. The thermal diffusion of Mg across the Mg x Zn 1Ϫx O/ZnO interface was observed only after annealing above 700°C. Unlike other II-VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures.

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Well-defined metal-insulator-metal trilayered structures composed of epitaxial Pr0.7Ca0.3MnO3 insulator layers, epitaxial LaNiO3 bottom electrodes, and Al metal top electrodes were fabricated on LaAlO3 (100) substrates. The I-V characteristics of the trilayer structures show electric-field-induced resistance switching. The resistance switching ratio of the heterostructures was up to 100 when positive and negative pulsed voltages were applied. Detailed I-V analysis indicates the importance of both trap-controlled space-charge-limited current and Poole–Frenkel effect in resistance switching at Al∕Pr0.7Ca0.3MnO3 interfaces.

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