The sky is not the limit for 2D material space technology
For space missions, mass costs. When every kilo launched costs USD$25000 just to get it to a near-Earth orbital, the low size, mass and power consumption of 2D material devices can start to look very attractive. But the onslaught of radiation involved in space missions also leaves devices prone to damage so that like the astronauts in manned space missions, device high performance needs to be matched with high resilience.
Reporting in Nature Communications Tobias Vogl and Ping Koy Lam at the Australian National University’s Centre for Quantum Computation and Communication Technology, alongside colleagues at the University, present the results of indepth investigations of the effects of the radiation in earth’s atmosphere on various 2D material devices. “There is a global quest to find smaller, lighter and more efficient materials for space devices,” Lam tells Physics World. “In this work we show how atomically thin 2D materials can be used to build space compatible devices. This work is an example of how quantum technology can be used to enhance space instrumentations.” Not only do the results confirm that these devices can withstand radiation exposure far greater than the levels likely during low Earth orbital space missions, but they also observe defect healing effects in some of the devices, which may find applications in compact radiation dosimeter or radiation detector applications. Radiation resilience Vogl, Lam and colleagues focused their study on single-photon sources based on defects in hexagonal boron nitride, field-effect transistors based on monolayer MoS2 and WSe2, and finally transition metal dichalcogenide monolayers in their native state. They exposed the structures to the most common radiation in low Earth orbitals: proton, electron and gamma radiation. The Earth’s magnetic field traps high-energy electrons and protons in trajectories oscillating between both magnetic poles called the Van Allen belts. Although this protects the Earth’s surface from radiation from solar wind and cosmic particles it gives rise to high levels of exposure to space craft orbiting through these belts. Read more Can nanomaterials take on the extremes of space? To expose the structures to protons they used a 1.7 MV tandem accelerator. Although the fluence was much higher than the exposure in space according to the atmospheric radiation levels calculated from the European Space Agency Space Environment Information System (SPENVIS) software, neither photoluminescence nor carrier lifetime measurements revealed any damage, and both the field effect transistors and single-photon sources remained unaffected. Using a scanning electron microscope to expose the structures to electron radiation also caused no damage according to optical and electronic measurements, despite the fluence exceeding atmospheric levels by three orders of magnitude. Radiation healing For the gamma radiation tests the researchers used isotope 22Na, the isotope they had access to that emits radiation most similar to the 60Co predominantly used for space qualification. These experiments inadvertently exposed the structures to radiation levels equivalent to 2170 years at 500 km above the polar caps instead of 4 years as intended. Yet still the structures were mostly unchanged by gamma radiation exposure, with the exception of the WS2 monolayers, where photoluminescence and carrier lifetimes actually increased. “We initially expected that the high-energy space radiation would only very little, if at all interact with these thin nanomaterials. Being able to observe any changes was thus very surprising for us and initially it was hard to believe,” says Vogl, adding. “A material getting stronger after irradiation with gamma rays – that reminds me of the Hulk.” Read more Can nanomaterials tool up space missions? The researchers attribute the increased photoluminescence to the healing of sulphur vacancies induced by γ-radiation. In their report they outline a mechanism based on a process similar to Compton scattering where the γ-rays, dissociate atmospheric oxygen, which then chemically reacts with the vacancies. Measurements of diminished defect emission following irradiation, as well as comparison of experiments in vacuum and in air support their explanation. They also add that the low-temperature and vacuum conditions used for the experiments confirm the thermal and vacuum cycling resilience of the structures. The researchers later extended their simulations to study the effects at higher geostationary orbitals, which require altitudes of more than 35,000 km compared with around 2000 km in a low Earth orbit. Again, they found the devices and structures remained resilient to radiation damage. “For the future we already integrated an experiment based on the 2D material single-photon source on a pico-class satellite platform (1U CubeSat),” says Vogl. “We received great interest in this direction and will be able to fly with our 2D material experiment in the near future.” Full details are reported in Nature Communications.