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Tunable optoelectronic devices by strain engineering of 2D semiconductors

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

The goal of 2D-TOPSENSE is to exploit the remarkable stretchability of two-dimensional semiconductors to fabricate optoelectronic devices where strain is used as an external knob to tune their properties.

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.
During this reporting period (month 1 to month 30) we have:

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:
https://doi.org/10.1088/2515-7639/ab6a72
https://doi.org/10.1088/2053-1583/ab72d6
https://doi.org/10.1088/2515-7639/ab8781

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:
https://doi.org/10.1007/s12274-020-2918-2
https://doi.org/10.1088/2515-7639/ab4432
https://doi.org/10.1021/acs.nanolett.0c01706

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:
https://doi.org/10.1016/j.mattod.2019.04.019
https://doi.org/10.1002/advs.202001645
Within 2D-TOPSENSE we have push forward the knowledge frontier in the field of strain engineering in 2D materials. With respect to the development of experimental techniques and tools we have developed a scientific apparatus that allow to apply uniaxial strain along different crystalline orientations which is beyond the state-of-the-technique (article in preparation). Also, we have developed micro-heater actuators that allow to apply biaxial strain to 2D materials (https://doi.org/10.1021/acs.nanolett.0c01706). Importantly these micro-heaters actuators allow to modulate the strain 100 times faster than the previous techniques reported in the literature. But if we have to stress on some result that has gone far beyond the state of the art I would highlight our recent works demonstrating the operation of strain tunable photodetector devices based on MoS2 and InSe where biaxial strain is used to modify the spectral bandwidth of the devices (https://doi.org/10.1016/j.mattod.2019.04.019 and https://doi.org/10.1002/advs.202001645). These works are the first experimental demonstration of a functional straintronic device where the external strain, applied at will by the user, can modify the output characteristics of the electronic component. Up to know most of the strain engineering works focused on static strain or dummy strain tunable devices where strain only modified the resistance of the devices but didn’t change any function of the device.

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.
Microheater thermal expansion actuator to controllably apply biaxial strain to atomically thin MoS2
Giant piezoresistive effect and strong bandgap tunability in ultrathin InSe upon biaxial strain
Artistic impression of a strain tunable single-layer MoS2 photodetector