Periodic Reporting for period 1 - 2Exciting (Developing optoelectronics in two-dimensional semiconductors)
Période du rapport: 2021-01-01 au 2022-12-31
Since 2010–2011, the relatively young field of two-dimensional (2D) materials has experienced a new boost, originating from the works on the semiconducting 'cousins' of graphene, that is, atomically thin, or 2D, semiconductors (2DS), which now cover a wide range of band gaps in the visible, UV and IR: Their 2D properties encompass large exciton binding energies and exceptionally strong light-matter interaction. They opened the door to new ways of controlling their electrical and optical properties through the valley degree of freedom (DoF), strain or by creating artificial stacks of 2D materials. All these makes 2DS extremely appealing for optoelectronic applications where conventional semiconductors cannot provide the same performance nor added functionality.
The demand for optical-to-electrical and/or electrical-to-optical transducers is growing exponentially as optics is increasingly adopted for energy-efficient data transmission also on short distances. Displays and light sensors are increasingly being integrated in everyday objects. Optoelectronics is at the core of countless science disciplines and technological applications, and there is a clear roadmap towards achieving not only broader band, faster and higher sensitivity (or brighter) devices, but also to develop devices with new functionalities.
2EXCITING combines complementary expertise to investigate fundamental optoelectronic properties of 2DS and to provide new methods to manipulate them for achieving added functionalities to 2DS-based optoelectronic devices by:
- Bringing together a consortium of 8 leading academic groups that are amongst the key players in the field of 2D materials beyond graphene and 8 companies with diverse skills ranging from knowledge transfer and R&D to soft skills.
- Studying fundamental physics of the light-matter interaction in 2DSs.
- Developing innovative optoelectronic devices such as excitonic switches, strain-actuated optical modulators or valleytronic switches.
- Developing strategies to artificially tailor the optoelectronic properties of 2DS through electrostatic or chemical doping, strain engineering or modifying the twisting angle of stacked layers.
- Training 15 ESRs in the state-of-the-technique experimental and theoretical tools in the field of 2DS-based electronic and optoelectronic devices and related materials preparation, nanoscopy, and optical spectroscopy methods.
- Preparing the ESRs for their professional career in the growing field of 2D materials through the specialized training programme ensured by our complementary consortium.
The Objectives in 2EXCITING are to exploit the photophysical and optoelectronic properties of 2DS, and to manipulate their electronic structure by external means.
The demand for optical-to-electrical and/or electrical-to-optical transducers is growing exponentially as optics is increasingly adopted for energy-efficient data transmission also on short distances. Displays and light sensors are increasingly being integrated in everyday objects. Optoelectronics is at the core of countless science disciplines and technological applications, and there is a clear roadmap towards achieving not only broader band, faster and higher sensitivity (or brighter) devices, but also to develop devices with new functionalities.
2EXCITING combines complementary expertise to investigate fundamental optoelectronic properties of 2DS and to provide new methods to manipulate them for achieving added functionalities to 2DS-based optoelectronic devices by:
- Bringing together a consortium of 8 leading academic groups that are amongst the key players in the field of 2D materials beyond graphene and 8 companies with diverse skills ranging from knowledge transfer and R&D to soft skills.
- Studying fundamental physics of the light-matter interaction in 2DSs.
- Developing innovative optoelectronic devices such as excitonic switches, strain-actuated optical modulators or valleytronic switches.
- Developing strategies to artificially tailor the optoelectronic properties of 2DS through electrostatic or chemical doping, strain engineering or modifying the twisting angle of stacked layers.
- Training 15 ESRs in the state-of-the-technique experimental and theoretical tools in the field of 2DS-based electronic and optoelectronic devices and related materials preparation, nanoscopy, and optical spectroscopy methods.
- Preparing the ESRs for their professional career in the growing field of 2D materials through the specialized training programme ensured by our complementary consortium.
The Objectives in 2EXCITING are to exploit the photophysical and optoelectronic properties of 2DS, and to manipulate their electronic structure by external means.
Sample fabrication, implementing characterization methods and theoretical method developments were performed.
We first synthesized new materials using primarily chemical vapor transport. Besides high-quality known 2DS we achieved two important milestones: We synthesized high-quality hexagonal boron nitride (hBN) and high-k dielectric materials based on heavy elements which, after exfoliation, could be used as FET devices (Fig. 1).
Our new solvent-assisted exfoliation methods are capable of transforming bulk layered materials to a homogeneous 2DS flake distribution which can be directly used for inkjet printing. This was demonstrated for PtSe2 and SnSe.
For characterization methods, we perfectionized low-temperature spectroscopy, in particular using a Raman/photoluminescence microscope.
Methods for the description of 2DS optoelectronics were developed by SCM and will be available in a next release of the AMS software. We created a force field setup that allows the atomistic description of twisted layers of 2DS in so-called moiré cells containing hundred thousands of atoms. Numerous tools for the manipulation of such involved systems were developed -most prominently the hetbuilder to create arbitrary heterostructures (HS) of 2D materials, which is publicly available on github.
In Objective 1 (Photophysics), we studied the correlation between layer orientation and exciton properties in detail, including their dynamic properties. The properties of interlayer excitons are in stark contrast with those in a single layer, which have been investigated to address the valley polarization upon excitation. Another topic was the investigation of gated HS, where an external electric field manipulates the electronics of the heterostructured 2DS and hence its optical properties. We were able to produce a series of different devices based on gated 2DS HS which will be carefully examined in the 2nd project period.
Objective 2 (Optoelectronic devices) focuses on the device making and understanding the behavior of excitations in 2DS: WSe2 was utilized as standard material to study the exciton formation and transport. By interfacing it with different gates (Cr, Pt etc.) and applying a suitable external electric field it was possible to control the excitation into various valleys in the conduction band.
To increase light-matter interactions, we developed methods to waveguide photons to photodetector devices. The external responsivity of the photodetector, defined as the ratio between the generated photocurrent and the incident optical power, reaches values as high as 1000 A/W at low illumination intensities. Photoresponse times below 1 ms could be achieved by adding an hBN layer between the 2DS and the waveguide.
Objective 3 (Manipulation of electronic structure) focuses on the application of external factors to modify the electronic structure of the 2DS and hence the photophysical and optoelectronic properties. As a particularly interesting way to modify the electronic structure we identified twisted bilayers of 2DS. If two 2DS monolayers are put on top of each other with a small twist angle, large domains of uncommon stacking types form, separated by strain solitons which form a superlattice. The superlattice controls the electronic band structure of the valence band, transforming 2DS MoS2 to a materials with electronic signatures such as graphene or hexagonal boron nitride (Fig. 2).
We first synthesized new materials using primarily chemical vapor transport. Besides high-quality known 2DS we achieved two important milestones: We synthesized high-quality hexagonal boron nitride (hBN) and high-k dielectric materials based on heavy elements which, after exfoliation, could be used as FET devices (Fig. 1).
Our new solvent-assisted exfoliation methods are capable of transforming bulk layered materials to a homogeneous 2DS flake distribution which can be directly used for inkjet printing. This was demonstrated for PtSe2 and SnSe.
For characterization methods, we perfectionized low-temperature spectroscopy, in particular using a Raman/photoluminescence microscope.
Methods for the description of 2DS optoelectronics were developed by SCM and will be available in a next release of the AMS software. We created a force field setup that allows the atomistic description of twisted layers of 2DS in so-called moiré cells containing hundred thousands of atoms. Numerous tools for the manipulation of such involved systems were developed -most prominently the hetbuilder to create arbitrary heterostructures (HS) of 2D materials, which is publicly available on github.
In Objective 1 (Photophysics), we studied the correlation between layer orientation and exciton properties in detail, including their dynamic properties. The properties of interlayer excitons are in stark contrast with those in a single layer, which have been investigated to address the valley polarization upon excitation. Another topic was the investigation of gated HS, where an external electric field manipulates the electronics of the heterostructured 2DS and hence its optical properties. We were able to produce a series of different devices based on gated 2DS HS which will be carefully examined in the 2nd project period.
Objective 2 (Optoelectronic devices) focuses on the device making and understanding the behavior of excitations in 2DS: WSe2 was utilized as standard material to study the exciton formation and transport. By interfacing it with different gates (Cr, Pt etc.) and applying a suitable external electric field it was possible to control the excitation into various valleys in the conduction band.
To increase light-matter interactions, we developed methods to waveguide photons to photodetector devices. The external responsivity of the photodetector, defined as the ratio between the generated photocurrent and the incident optical power, reaches values as high as 1000 A/W at low illumination intensities. Photoresponse times below 1 ms could be achieved by adding an hBN layer between the 2DS and the waveguide.
Objective 3 (Manipulation of electronic structure) focuses on the application of external factors to modify the electronic structure of the 2DS and hence the photophysical and optoelectronic properties. As a particularly interesting way to modify the electronic structure we identified twisted bilayers of 2DS. If two 2DS monolayers are put on top of each other with a small twist angle, large domains of uncommon stacking types form, separated by strain solitons which form a superlattice. The superlattice controls the electronic band structure of the valence band, transforming 2DS MoS2 to a materials with electronic signatures such as graphene or hexagonal boron nitride (Fig. 2).
In the first period of 2Exciting we have paved the road for manufacturing new and interesting 2DS beyond the common family of transition or noble metal dichalcogenides. Our exfoliation techniques have been advanced and directly linked to inkjet printing. Similarly, we have produced a number of complex devices, allowing to couple various 2DS to external factors such as electric fields or very strong laser beams. Methods are readily available to study complex 2DS, such as twisted bilayer materials. In the 2nd period, we will explore all these opportunities in detail, which will be important for oure ESRs to reach their main qualification goal: the successful termination of their doctoral thesis, and their recognition in the community by publishing high-quality papers.
Our research has shown that the field of 2DS is far more complex than anticipated, as coupling different 2DS materials and gate materials to complex devices opens opportunities beyond our expectation when applying for this project. Besides the impact in developing optoelectronics and photophysics, we are also aware of fundamental implications, such as the potential appearance of superconductive states in gated twisted MoS2.
Our research has shown that the field of 2DS is far more complex than anticipated, as coupling different 2DS materials and gate materials to complex devices opens opportunities beyond our expectation when applying for this project. Besides the impact in developing optoelectronics and photophysics, we are also aware of fundamental implications, such as the potential appearance of superconductive states in gated twisted MoS2.