Periodic Reporting for period 1 - IXIXions (Interlayer exciton interactions and their many-body physics)
Período documentado: 2023-04-01 hasta 2025-03-31
Since the discovery of graphene in 2004 the field of 2-dimensional (2D) materials has become a prime discipline in physics research. The realization that 2D-materials are stable and accessible materials has sparked the curiosity and creativity of many scholars. The rich choice of available materials that can be thinned down to the monolayer limit, the fact that the properties of a monolayer can be locally altered because of their sensitivity to the direct environment and the option to stack layers of different materials on top of each other creates endless opportunities for new physics. Examples of game changers enabled by 2D materials are Moiré physics, Hubbard model simulators and the observation of the quantum anomalous Hall effect.
The potential of 2D-materials has also caught the attention of the industry, where several key semiconductor companies are actively investigating how to capitalize on the strong light absorption and photoluminescent properties of these materials for advanced, low-power devices. Here, transition metal dichalcogenides (TMDs) like MoSe2 and WSe2 have emerged as key players. Unlike graphene, TMDs are semiconductors with direct band gaps, making them ideal for optoelectronic applications such as photodetectors and solar cells. What makes them particularly interesting is their spin-chirality coupling, meaning that one can directly address a specific spin state optically by choosing the polarization of the interaction light. Also, for electrical applications they hold great promise and are for instance added to the logic scaling roadmap of IMEC.
In a semiconductor material an electron can be excited to a higher energy state when it absorbs a light particle, better known as a photon, leaving behind a void in the sea of electrons which we refer to as a hole. An electron and hole can bind to each other, forming a quasi-particle called an exciton. The physics of 2D-materials is dominated by excitons, that bind together so strongly that they easily prevail up to room temperature. This in contrast to the established industry standard materials such as Silicon and Indium-Phosphide where excitons already fall apart at cryogenic temperatures and the physics is dominated by free electrons and free holes. On the one hand, monolayer TMDs have emerged as an ideal platform to investigate exciton physics thanks to their huge exciton binding energies, while on the other hand a detailed understanding and control of excitons is of paramount importance to push these materials to the industry. This MSCA has contributed to both the understanding of exciton physics TMDs as well as the technological development to push them to an industry application.
Starting with the fundamental progress, we need to consider that excitons in TMDs become positively (negatively) charged when an excess of free holes (electrons) resides in the TMD. Due to the rich electronic band structure of TMDs more than one additional hole (electron) can bind to the exciton giving rise to a plethora of charged excitons. By tuning the charge density, the species of charged exciton can be selected, where excitons with up to three additional bound electrons have already been observed. In this MSCA devices have pushed the reachable charge density to a new limit and a new charged particle has been observed. According to the model that we use this particle might have bound to up to nine additional electrons. This observation does not only add to the set of observed particles but also allows to scrutinize existing exciton models and widen the overall understanding of excitons in this promising material class.
Second, in this action, a novel yet simple device design to trap individual excitons in MoSe2 has been simulated, realized, and tested. A trap for individual excitons is also known as a quantum dot and is a key technological ingredient to create single-photon sources. Single photon sources are needed for numerous applications in quantum technologies, including quantum communication, quantum computing, and quantum sensing.
The potential of 2D-materials has also caught the attention of the industry, where several key semiconductor companies are actively investigating how to capitalize on the strong light absorption and photoluminescent properties of these materials for advanced, low-power devices. Here, transition metal dichalcogenides (TMDs) like MoSe2 and WSe2 have emerged as key players. Unlike graphene, TMDs are semiconductors with direct band gaps, making them ideal for optoelectronic applications such as photodetectors and solar cells. What makes them particularly interesting is their spin-chirality coupling, meaning that one can directly address a specific spin state optically by choosing the polarization of the interaction light. Also, for electrical applications they hold great promise and are for instance added to the logic scaling roadmap of IMEC.
In a semiconductor material an electron can be excited to a higher energy state when it absorbs a light particle, better known as a photon, leaving behind a void in the sea of electrons which we refer to as a hole. An electron and hole can bind to each other, forming a quasi-particle called an exciton. The physics of 2D-materials is dominated by excitons, that bind together so strongly that they easily prevail up to room temperature. This in contrast to the established industry standard materials such as Silicon and Indium-Phosphide where excitons already fall apart at cryogenic temperatures and the physics is dominated by free electrons and free holes. On the one hand, monolayer TMDs have emerged as an ideal platform to investigate exciton physics thanks to their huge exciton binding energies, while on the other hand a detailed understanding and control of excitons is of paramount importance to push these materials to the industry. This MSCA has contributed to both the understanding of exciton physics TMDs as well as the technological development to push them to an industry application.
Starting with the fundamental progress, we need to consider that excitons in TMDs become positively (negatively) charged when an excess of free holes (electrons) resides in the TMD. Due to the rich electronic band structure of TMDs more than one additional hole (electron) can bind to the exciton giving rise to a plethora of charged excitons. By tuning the charge density, the species of charged exciton can be selected, where excitons with up to three additional bound electrons have already been observed. In this MSCA devices have pushed the reachable charge density to a new limit and a new charged particle has been observed. According to the model that we use this particle might have bound to up to nine additional electrons. This observation does not only add to the set of observed particles but also allows to scrutinize existing exciton models and widen the overall understanding of excitons in this promising material class.
Second, in this action, a novel yet simple device design to trap individual excitons in MoSe2 has been simulated, realized, and tested. A trap for individual excitons is also known as a quantum dot and is a key technological ingredient to create single-photon sources. Single photon sources are needed for numerous applications in quantum technologies, including quantum communication, quantum computing, and quantum sensing.
To do high-quality science, great emphasis is put on having state-of-the-art measurement techniques and the highest sample quality. During the course of this project, two measurement setups have been developed. The first is a confocal microscope integrated with a closed-cycle helium cryostat with direct current electrical connections, providing excellent thermal and mechanical stability. It allows us to do state-of-the-art photoluminescence and reflection contrast experiments on our samples. The second is a similar optical setup, but now built free-space into a dilution refrigerator where samples can be investigated at millikelvin temperatures and under a magnetic field.
For this work, two types of samples have been developed. The first one being charge-tunable devices of hBN encapsulated TMD monolayers, with a top and bottom gate consisting of few-layer graphene. These samples have been optimized for cleanliness of flakes and thin hBN such that they can access previously uncharted charge doping regimes to study many-body complexes. The second set of samples is built upon the technology of the first set. An added technological complexity is spatial control, required to carefully align atomically sharp edges of top and bottom gates over a monolayer TMD to realize a novel sample architecture that will allow us to capture single excitons. These technological challenges have been solved using state-of-the-art stacking techniques and electron beam lithography, and suitable samples were realized.
Our best charge-tunable device was measured in a reflection contrast experiment and a photoluminescence experiment. We measured a dataset with a state-of-the-art linewidth for the observed optical features, and achieved a doping density far past values reported in the literature. This extreme condition allowed us to observe a new, undescribed type of exciton consisting of up to 20 bodies. Subsequently, the sample has been measured in an 8 T magnetic field, which led us to demonstrate an ultrafast scattering process between two valleys in the bandstructure of WSe2.
In our top cross-gated device, we have reproduced one-dimensional confinement of neutral excitons in agreement with literature reports. We have verified the consistency with both detailed gate-voltage-dependent measurements and spatial measurements using reflection contrast and photoluminescence experiments. In addition, we have observed the first indications of zero-dimensional confined states in photoluminescence experiments on the crossing point of our top and bottom gate. We produced gate-voltage-dependent and spatial-dependent datasets, which are both consistent with zero dimensionally confined neutral excitons. The experiment that will conclusively prove that zero-dimensional confinement is achieved (second-order correlation measurements) is still to be carried out.
For this work, two types of samples have been developed. The first one being charge-tunable devices of hBN encapsulated TMD monolayers, with a top and bottom gate consisting of few-layer graphene. These samples have been optimized for cleanliness of flakes and thin hBN such that they can access previously uncharted charge doping regimes to study many-body complexes. The second set of samples is built upon the technology of the first set. An added technological complexity is spatial control, required to carefully align atomically sharp edges of top and bottom gates over a monolayer TMD to realize a novel sample architecture that will allow us to capture single excitons. These technological challenges have been solved using state-of-the-art stacking techniques and electron beam lithography, and suitable samples were realized.
Our best charge-tunable device was measured in a reflection contrast experiment and a photoluminescence experiment. We measured a dataset with a state-of-the-art linewidth for the observed optical features, and achieved a doping density far past values reported in the literature. This extreme condition allowed us to observe a new, undescribed type of exciton consisting of up to 20 bodies. Subsequently, the sample has been measured in an 8 T magnetic field, which led us to demonstrate an ultrafast scattering process between two valleys in the bandstructure of WSe2.
In our top cross-gated device, we have reproduced one-dimensional confinement of neutral excitons in agreement with literature reports. We have verified the consistency with both detailed gate-voltage-dependent measurements and spatial measurements using reflection contrast and photoluminescence experiments. In addition, we have observed the first indications of zero-dimensional confined states in photoluminescence experiments on the crossing point of our top and bottom gate. We produced gate-voltage-dependent and spatial-dependent datasets, which are both consistent with zero dimensionally confined neutral excitons. The experiment that will conclusively prove that zero-dimensional confinement is achieved (second-order correlation measurements) is still to be carried out.
This action has produced two main results: first, the observation of a new many-body exciton in a WSe2 monolayer and second, the first indications of zero-dimensional confinement of neutral excitons in a MoSe2 monolayer using a novel gating design.
Observing a new excitonic particle provides additional insights into the nature of a material and its applications. It offers new grounds to test the current theories describing exciton physics and allows to expand upon them. In addition, we created a recipe to reach extreme carrier densities in two-dimensional materials, which can be readily applied to other upcoming van der Waals materials.
Quantum confining neutral excitons in a two-dimensional material in a deterministic way is still an outstanding problem to unlock the full potential of TMD materials. We propose a new, simple device design that allows to quantum confine single neutral excitons and in doing so making a single photon emitter in a TMD. Our measurements show the first indications of the functionality of this device. Further research is needed to unambiguously verify the viability of our approach. We do, however, provide a clear path towards proof of principle, altering the design to be electrical driven and therefore industry-relevant, and finally commercializing the idea.
Observing a new excitonic particle provides additional insights into the nature of a material and its applications. It offers new grounds to test the current theories describing exciton physics and allows to expand upon them. In addition, we created a recipe to reach extreme carrier densities in two-dimensional materials, which can be readily applied to other upcoming van der Waals materials.
Quantum confining neutral excitons in a two-dimensional material in a deterministic way is still an outstanding problem to unlock the full potential of TMD materials. We propose a new, simple device design that allows to quantum confine single neutral excitons and in doing so making a single photon emitter in a TMD. Our measurements show the first indications of the functionality of this device. Further research is needed to unambiguously verify the viability of our approach. We do, however, provide a clear path towards proof of principle, altering the design to be electrical driven and therefore industry-relevant, and finally commercializing the idea.