Van der Waals Heterostructures: To Infinity and Beyond

Two weeks ago we witnessed to an event that will probably remain in the history books; I am speaking about the successful test of the Falcon Heavy that puts SpaceX on to the front pages of pretty much all magazines that are even remotely interested in space technology.

To sum up, the episode was classified as an astonishing success, and had a large impact on social media thanks to the eccentric personality of Elon Musk, the founder of SpaceX. In fact, the very first test of a new type of rocket is extremely risky (due to the high possibility that everything will blow up on the launch platform), so no one offered to load a satellite on this run. Instead of simulating the load with concrete weights in the payload capsule, Musk decided to send in to space his own cherry-red Tesla Roadster while playing David Bowie’s “Life on Mars” and with the first SpaceX suit sitting as a driver and astronaut. The images are literally astonishing and look like they were photo edited, but apparently this is one of the times when reality overcomes imaginations. From a practical point of view this is most powerful rocket actually in operation and is extremely cheap (£90 million per launch) and opened a door to a new space race.

 

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a): Image of the Tesla Roadster with the Earth in the background, b): Falcon Heavy on the platform during the final tests c): Falcon Heavy engines, formed by 27 Merlin engines (image credit: SpaceX, http://www.spacex.com/)

 

On the wave of this excitement I think it is interesting to focus our attention on how the research on two dimensional materials and Van der Waals heterostructures can potentially be useful also for space exploration technologies. Two of the most pressing problems that we have to solve to gain easier and broader access to our space neighbourhood are the rocket efficiency to lift heavy weights from the Earth’s surface, and energy harvesting once in space. The first one is currently under heavy development (vivified by the entry on stage of private companies, such as SpaceX, blue origin and Orbital ATK); the second one is actually dominated by multi-junction inorganic solar cells, which gather solar light to produce electricity.

They actually hold the record of efficiency over 45%, but such high efficiency is gained at the cost of increasing complexity and manufacturing price, in fact, each single ones of these elements are composed of several different solar cells, each one of them absorbing in a specific range of wavelengths. Moreover, they need to be extremely engineered to be folded during the launch and fully extended and oriented once that they reach the final destination. Last but not least, there is a growing demand of energy to sustain larger and more complex satellites or even human missions.

 

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On the left schematic structure of a multi-junction solar cell; on the right, image of the solar panel on the international space station (ISS) (image credit NASA https://www.nasa.gov/)

 

This is the scenario in which two dimensional materials could give a contribution, especially transition metal dichalcogenides (TMDs). In fact, they have some desirable properties which make them excellent candidates for solar energy harvesting.

Light absorption: a single layer TMD flake of can absorb an extremely large amount of photons in a significant part of the visible spectrum [1]. This is a key property for a material to be employed for light harvesting devices. Considering that these materials have a sub-nanometre thickness their absorption-to-weight ratio is really promising for photovoltaic applications. Especially in the space sector when the weight is a critical parameter to consider.

Efficient carrier separation: putting in contact two different TMDs will create a heterojunction that has a type II band alignment. This means that the electrons will be confined in one material, while the holes (or electron vacancies) are confined in the other material. In a heterojunction the recombination process (which is exactly the opposite of the photovoltaic effect) is a serious problem to overcome, to convert the light absorbed into electrical current.

 

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Schematic sketch of photovoltaic effect and radiative recombination process in a heterojunction.

 

heterobilayers have an extremely efficient carrier separation, this means that when a photon is absorbed the two carriers (electron and holes) are split in less than 50 femtoseconds in to the two different layers [2]. These properties could be the key to massively increasing the efficiency of the device.

Flexibility: since they are so thin and flexible they can be integrated in many more elements of the spaceships, so instead of having big ships with huge solar panels, they can be also built into the surface of the spaceship modules themselves. This will optimize the light collection efficiently by exploiting the entire useful surface. Also due to their flexibility, new solutions can be developed to compress the solar panels during the launch.

Engineering material and devices: Adopting the same strategy seen for multi-junction solar cells, many isolated heterobilayers can be stacked together to form a similar structure, in which each single pair can be tailored at will to absorb most efficiently in a specific spectral region. This will maximize the energy collected and therefore the power gained, moreover, the overall thickness of the devices is indeed negligible due to the two dimensional nature of these materials.

To conclude, we are at the beginning of a new era of space exploration that can offer unprecedented benefits in countless fields, from global marketing to fundamental research, from resource access to aerospace engineering. In this space race, two dimensional materials can also express their full range of potential and lead to a major breakthrough which will bring us a step closer to the stars.

 

Per aspera ad astra

 

By Alessandro Catanzaro, PhD student at the LDSD group at University of Sheffield, Sheffield, United Kingdom.

 

  1. Bernardi M, Palummo M, Grossman J.C, Nano Lett. 2013; 13(8):3664-3670. doi:10.1021/nl401544y.
  2. Yu Y, Hu S, Su L, et al. 2014. doi:10.1021/nl5038177.
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