Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO3-SrTiO3 interfaces 1 and measure a pronounced, unexpected response. Pressure of ∼2 GPa reversibly doubles the 2D carrier density ns at 4 K. Along with the increase of ns, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure.

Graphene, being an atomically thin conducting sheet, is a candidate material for gate electrodes in vacuum electronic devices, as it may be traversed by low-energy electrons. The transparency of graphene to electrons with energies between 2 and 40 eV has been measured by using an optimized vacuum-triode setup. The measured graphene transparency equals ∼60% in most of this energy range. Based on these results, nano-patterned sheets of graphene or of related two-dimensional materials are proposed as gate electrodes for ambipolar vacuum devices.

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Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. This perspective article provides an assessment of the potential of TEC systems for space and terrestrial applications in the twenty-first century, overviewing recent advances in the field and identifying key research challenges. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed.Keywords: thermal energy conversion, thermionic energy conversion, thermionic emission, heat engine, electron emission iNTRODUCTiON The direct conversion of heat to electricity without any intermediate steps or moving parts remains one of the most promising, yet challenging, methods of power production. The promise of high conversion efficiency, device simplicity, and robust operation continues to push research and technology development at the cutting edge. Furthermore, because of the wide variety of possible heat sources, ranging from combustion of fossil or other fuels, nuclear reactors, solar heat, or even waste heat from the human body, the applications for thermal-to-electric energy convertors are vast, spanning many orders of magnitude of possible temperature and power ranges.Thermionic energy conversion (TEC) for direct conversion of heat to electrical energy occurs when electrons thermally emit from a hot surface, traverse a gap, and are collected by another surface. This process, starting with thermionic emission, produces a current of electrons that can subsequently drive an electrical load to produce work. Particularly well suited for high-temperature applications, since it was first proposed by Schlichter (1915), thermionic emission has been pursued as a power generation method for over a century Gyftopoulos, 1973, 1979), yet has seldom been realized in either space or terrestrial applications. However, recent advances in material science and nanotechnology as well as our evolving understanding of the underlying physical processes afford new opportunities to develop practical thermionic convertors, reinvigorating the field. Thermionic energy conversion has a long and storied history, particularly in the space programs of the United States and the former Soviet Union. However, while the field was vibrant and much progress was made during the 1960s through 1980s, including space flight demonstrations, TEC has largely been supplanted by alternative energy conversion technologies, specifically thermoelectrics and photovoltaics, in both the public and research community consciences. Much of thermionic-based research tapered off after a 2001 report by the National Research Council titled Thermionics Quo Vadis? An Assessment of the DTRA's Advanced Thermionics Research and Development Program (Na...

Transparency of graphene for low-energy electrons measured in a vacuum-triode setup APL Materials 3, 076106 (2015) Mobile energy converters require, in addition to high conversion efficiency and low cost, a low mass. We propose to utilize thermoelectronic converters that use 2D-materials such as graphene for their gate electrodes. Deriving the ultimate limit for their specific energy output, we show that the positive energy output is likely close to the fundamental limit for any conversion of heat into electric power. These converters may be valuable as electric power sources of spacecraft, and with the addition of vacuum enclosures, for power generation in electric planes and cars.