Periodic Reporting for period 1 - OPTOvanderWAALS (Optoelectronics with Complex van der Waals Heterostructures)
Reporting period: 2018-04-01 to 2020-03-31
Research on two-dimensional materials has spread to a far-reaching range of fields in the last decade. Due to their interesting and large variety of properties, two-dimensional materials are nowadays investigated as potential candidates in a broad assortment of applications. Among these different research fields, optoelectronics is one the most relevant ones, with numerous proof-of-concept and working devices realized and developed during the past few years. Spanning from photodetectors, solar cells to light emitters, two-dimensional materials seem to be good complementary, or even imponent competitors, to the current semiconductor technology. However, research is still needed to catch up with conventional semiconductors. From fundamental physics to applications, the research community dedicates nowadays high efforts to the understanding of the optical and optoelectronic properties of these materials.
The project OPTOvanderWAALS deals with the fabrication and study of complex van der Waals heterostructures to study inter- and intra-layer excitonic phenomena and use these excitonic effects to fabricate optoelectronic devices. Sustained on three main objectives, OPTOvanderWAALS has been dedicated to the fabrication of high-quality van der Waals heterostructures (Objective I) to study different excitonic states in two-dimensional transition metal dichalcogenides (TMDs), employing them in tunable light emitters. Furthermore, these heterostructures and knowledge on exciton Physics is employed to develop novel photodetection (Objective II) and energy harvesting, or photovoltaic (Objective III), devices.
The project OPTOvanderWAALS deals with the fabrication and study of complex van der Waals heterostructures to study inter- and intra-layer excitonic phenomena and use these excitonic effects to fabricate optoelectronic devices. Sustained on three main objectives, OPTOvanderWAALS has been dedicated to the fabrication of high-quality van der Waals heterostructures (Objective I) to study different excitonic states in two-dimensional transition metal dichalcogenides (TMDs), employing them in tunable light emitters. Furthermore, these heterostructures and knowledge on exciton Physics is employed to develop novel photodetection (Objective II) and energy harvesting, or photovoltaic (Objective III), devices.
The results obtained during the project and possible routes to exploit them are listed here:
- Light emitters: high-quality van der Waals heterostructures have been employed in the development of pulsed and tunable light emitters which also served as platform to study new excitonic phenomena in transition metal dichalcogenides.
- Nonvolatile programmable photodetectors: photodiodes with floating gates based on monolayer WSe2 have been developed during the project. This novel photodetector structure allows customization and storage of responsivity values that can be employed in larger-scale pixel arrays. In fact, it was used as proof-of-concept device in a machine vision neural network.
- Photocurrent generation in single-quantum emitters: work has been carried out during the project to understand the photocurrent generation mechanisms governing in localized states, stablishing a relation between light generation and absorption in such systems.
- Infrared photodetectors: a van der Waals heterostructure formed by franckeite and graphene is studied to enhance the responsivity in infrared photodetectors.
- Doubly-gated photovoltaic cells: a novel photovoltaic cells concept has been developed during the project. Work has been carried out to optimize the light absorption in van der Waals heterostructures and their application in vertical solar cells.
The main route to exploit the results has been their publication in scientific journals, including Nature, Nature Communications, Advanced Optical Materials and Advanced Electronic Materials. In addition, two more manuscripts are currently under review in other journals or in preparation.
Furthermore, the results have been communicated at international conferences to the scientific community. To the broader public the routes of dissemination include the project webpage, social media such as LinkedIn, Twitter or ResearchGate, and national and international online journals and blogs.
- Light emitters: high-quality van der Waals heterostructures have been employed in the development of pulsed and tunable light emitters which also served as platform to study new excitonic phenomena in transition metal dichalcogenides.
- Nonvolatile programmable photodetectors: photodiodes with floating gates based on monolayer WSe2 have been developed during the project. This novel photodetector structure allows customization and storage of responsivity values that can be employed in larger-scale pixel arrays. In fact, it was used as proof-of-concept device in a machine vision neural network.
- Photocurrent generation in single-quantum emitters: work has been carried out during the project to understand the photocurrent generation mechanisms governing in localized states, stablishing a relation between light generation and absorption in such systems.
- Infrared photodetectors: a van der Waals heterostructure formed by franckeite and graphene is studied to enhance the responsivity in infrared photodetectors.
- Doubly-gated photovoltaic cells: a novel photovoltaic cells concept has been developed during the project. Work has been carried out to optimize the light absorption in van der Waals heterostructures and their application in vertical solar cells.
The main route to exploit the results has been their publication in scientific journals, including Nature, Nature Communications, Advanced Optical Materials and Advanced Electronic Materials. In addition, two more manuscripts are currently under review in other journals or in preparation.
Furthermore, the results have been communicated at international conferences to the scientific community. To the broader public the routes of dissemination include the project webpage, social media such as LinkedIn, Twitter or ResearchGate, and national and international online journals and blogs.
The project has yielded results that constitute new advances in exciton physics in two-dimensional materials, with the first report of electroluminescence of multi-particle exciton complexes in W-based TMDs, the report of complexes not previously observed before and the additional tunability of the emitted light. Our technique extends and complements gate-dependent PL spectroscopy and will enable further investigations of manybody phenomena in two-dimensional materials. From an applied point of view, our devices may find application as wavelength tunable light emitters or furnish new opportunities for quantum light sources, e.g. by quantum-confinement of electrically induced biexcitons.
In the field of photodetection, we have demonstrated a new concept of programmable photodetector based on monolayer WSe2 in a split-gate self-driven photodetector configuration including charge-storing floating gates. Our photodetectors are capable of retaining configured and fully customizable responsivity values over time with remarkable performance and can be easily programed, erased and rewritten by just varying the value of the control gates. These programmable photodetectors could be employed as building blocks in devices such as pixels arrays or image sensors, solving the problem of unbalanced responsivities in individual pixels, as well as in more complex devices such as neural networks.
Furthermore, we have studied the spectral photocurrent of monolayer WSe2 in a p-n junction configuration, where we observe single-photon emission at locations in which high strain is induced in the monolayer. We find that the photocurrent spectra match very well the photoluminescence, with an enhanced response at the same photon energy as the one of the single-photon emitter. Finally, the dependence of the photocurrent intensity with the drain-source voltage suggests that the photocurrent at single-photon emitter locations, at low electric fields, is governed by a Fowler-Nordheim tunneling process. Our results shed light onto the optoelectronic mechanisms for quantum emitters in two-dimensional materials and provide a basis for further studies towards a deeper understanding of the underlying physics.
In addition, we have studied new combinations of materials for the development of infrared photodetectors with higher responsivity than previously reported ones. Although work is still in progress, the proposed heterostructure based on franckeite and graphene seems as a promising candidate for this research field.
Finally, in the field of photovoltaic energy harvesting devices, we have developed a new concept of vertical photovoltaic cell based on WSe2 with double gates that provides tunable doping distribution in the device. The election of materials and thicknesses is based on the enhancement of the light absorption according to an internal reflection model. We have studied the device performance from both experiments and theory, providing a theoretical model of the device functioning. Furthermore, we have observed that dielectric screening limits the functioning of the device, suggesting that the optimization of the device performance undergoes a reduction of the active material thickness and thickening of the insulating layers. These results set a new route to the design of vertical solar cells based on van der Waals heterostructures and the optimization of their performance.
In the field of photodetection, we have demonstrated a new concept of programmable photodetector based on monolayer WSe2 in a split-gate self-driven photodetector configuration including charge-storing floating gates. Our photodetectors are capable of retaining configured and fully customizable responsivity values over time with remarkable performance and can be easily programed, erased and rewritten by just varying the value of the control gates. These programmable photodetectors could be employed as building blocks in devices such as pixels arrays or image sensors, solving the problem of unbalanced responsivities in individual pixels, as well as in more complex devices such as neural networks.
Furthermore, we have studied the spectral photocurrent of monolayer WSe2 in a p-n junction configuration, where we observe single-photon emission at locations in which high strain is induced in the monolayer. We find that the photocurrent spectra match very well the photoluminescence, with an enhanced response at the same photon energy as the one of the single-photon emitter. Finally, the dependence of the photocurrent intensity with the drain-source voltage suggests that the photocurrent at single-photon emitter locations, at low electric fields, is governed by a Fowler-Nordheim tunneling process. Our results shed light onto the optoelectronic mechanisms for quantum emitters in two-dimensional materials and provide a basis for further studies towards a deeper understanding of the underlying physics.
In addition, we have studied new combinations of materials for the development of infrared photodetectors with higher responsivity than previously reported ones. Although work is still in progress, the proposed heterostructure based on franckeite and graphene seems as a promising candidate for this research field.
Finally, in the field of photovoltaic energy harvesting devices, we have developed a new concept of vertical photovoltaic cell based on WSe2 with double gates that provides tunable doping distribution in the device. The election of materials and thicknesses is based on the enhancement of the light absorption according to an internal reflection model. We have studied the device performance from both experiments and theory, providing a theoretical model of the device functioning. Furthermore, we have observed that dielectric screening limits the functioning of the device, suggesting that the optimization of the device performance undergoes a reduction of the active material thickness and thickening of the insulating layers. These results set a new route to the design of vertical solar cells based on van der Waals heterostructures and the optimization of their performance.