Periodic Reporting for period 2 - 2D-TOPSENSE (Tunable optoelectronic devices by strain engineering of 2D semiconductors)
Reporting period: 2019-09-01 to 2021-02-28
While bulk semiconductors tend to break under strains larger than 1.5%, 2D semiconductors (such as MoS2) can withstand deformations of up to 10-20% before rupture. This large breaking strength promises a great potential of 2D semiconductors as ‘straintronic’ materials, whose properties can be adjusted by applying a deformation to their lattice. In fact, recent theoretical works predicted an interesting physical phenomenon: a tensile strain-induced semiconductor-to-metal transition in 2D semiconductors. By tensioning single-layer MoS2 from 0% up to 10%, its electronic band structure is expected to undergo a continuous transition from a wide direct band-gap of 1.8 eV to a metallic behavior. This unprecedented large strain-tunability will undoubtedly have a strong impact in a wide range of optoelectronic applications such as photodetectors whose cut-off wavelength is tuned by varying the applied strain or atomically thin light modulators.
To date, experimental works on strain engineering have been mostly focused on fundamental studies, demonstrating part of the potential of 2D semiconductors in straintronics, but they have failed to exploit strain engineering to add extra functionalities to optoelectronic devices. 2D-TOPSENSE will go beyond the state of the art in straintronics by designing and fabricating optoelectronic devices whose properties and performance can be tuned by means of applying strain. 2D-TOPSENSE will focus on photodetectors with a tunable bandwidth and detectivity, light emitting devices whose emission wavelength can be adjusted, light modulators based on 2D semiconductors such as transition metal dichalcogenides or black phosphorus and solar funnels capable of directing the photogenerated charge carriers towards a specific position.
Purchase of most of the equipment needed to set up a state-of-the-technique laboratory working on 2D optoelectronic devices.
Electrical, gas and lighting installation works has been carried out in our fabrication, characterization and measurement labs.
Design, development and installation of scientific equipment and setups: In this reporting period we have installed several specialised scientific equipment that were not available at the Host institution. Briefly, we have designed and set-up 4 deterministic transfer systems (one of then installed inside an anaerobic chamber), 3 micro-reflectance/transmittance setups with different spectral sensitivity (from 400 nm to 2200 nm), two high-vacuum chambers with electrical and optical ports, one room-temperature probe station, one cryogenic probe station, one scanning photocurrent setup and a small micro-fabrication facility with a maskless projection optical lithography system. See publications:
Design of specialized experimental setups to apply strain: we have designed and implemented several setups to apply different kind of strains to two dimensional materials. In particular we have developed a series of bending apparatus that allows to apply uniaxial strain (both tensile and compressive) at different crystalline orientations and we are currently working on the development of another version of the bending apparatus that allows to apply biaxial strain. We have also developed some setups to apply biaxial strain by exploiting the thermal expansion mismatch between a polymeric substrate and the 2D materials. One of these setups is based on the use of micro-fabricated heaters that allows for a fast actuation speed. See the publications:
During this period, we have even demonstrated the first strain tunable photodetector devices (which was originally planned for the month 39). By using the thermal expansion based straining technique we are able of modifying the performance of photodetectors based on 2D materials (MoS2 and InSe so far) through a user-defined biaxial strain. We have demonstrated how biaxial strain can be used to improve the spectral bandwidth of the photodetectors and we show how this strain-induced change is reversible and can be varied on time. See publications:
Regarding the expected results, we have plans to develop a new straining tool to allow larger strain values in combination with electrical transport measurements. We also plan to start the fabrication of the first strain tunable light emitters and exciton funnel devices. I believe that the demonstration of a light emitter whose central emission wavelength can be adjusted at will by means of a mechanical deformation could have a very strong impact in the science and technology and can trigger future interest in straintronics as an emerging new electronic.