As miniaturization of optoelectronic devices progresses, quantum mechanical effects play an ever increasing role. This is particularly true for devices based on two-dimensional systems, such as semiconducting transition metal dichalcogenides (STMD). In this project, we will develop a quantum mechanical theory of electroluminescence and photocurrent generation in STMD devices, which takes into account the physics of excitons (electron-hole bound excitations) under non-equilibrium conditions.
Some of the most pressing challenges faced by modern society require the processing and transmission of an ever increasing amount of information, the development of new types of sensors, and harvesting of solar energy. The solution to these problems will require the development of a new generation of optoelectronic devices. The field of condensed matter physics will play an essential role in this process.
Understanding the physics of excitons - states that occur in semiconducting or insulating materials, when a valence electron is excited to the conduction band and forms an electron-hole bound pair - will be key to push forward new optoelectronic technology. While the study of excitons has a long history in condensed matter physics, recent scientific and technological advances, in nanoscale fabrication techniques and in the field of two-dimensional materials, have led to a resurgent interest in their study. This is mostly due to the isolation of monolayer STMDs, such as MoS2, MoSe2, WS2 and WSe2. These materials are direct band semiconductors with gaps in the visible range and strong light-matter interaction. These properties make STMDs ideally suited to optoelectronic applications. In fact, several STMD based optoelectronic devices have already been developed. These include photodetectors, light-emitting diodes (LED), solar cells, and single-photon quantum emitters. It turns out that the optical properties of STMDs in the visible range are controlled by strongly bound excitons. Therefore, in order to understand the operation of STMD based devices, one must fully grasp the excitonic physics that governs them.
In STMD based optoelectronic devices, excitons are typically generated by non-equilibrium drives. This occurs in excitonic electroluminescence, where an applied bias generates excitons which then decay radiatively emitting light, and in photocurrent generation, where excitons are optically excited by incident light. Typically, these processes are described in a semi-classical way, using predominantly rate equations. However, the increasing miniaturization of devices implies that coherent quantum mechanical effects should play a significant role in these non-equilibrium excitonic devices. Therefore, in order to take full advantage of STMDs in applications, developments in the quantum non-equilibrium theory of excitons are required.
The aim of this project is to theoretically investigate the coherent generation of excitons under non-equilibrium conditions. We will develop the non-equilibrium quantum theory for coherent excitonic electroluminescence and photocurrent generation in STMD based optoelectronic mesoscopic devices. In particular, the results of this project will contribute to the understanding and guide the development of STMD based electrically driven excitonic single-photon sources and photodetectors
The theoretical understanding of the mechanisms that govern exciton generation and dynamics will play a crucial role in interpreting experimental findings and guide new experimental research of both fundamental and applied nature.
This project is funded by the Portuguese Foundation for Science and Technology under the grant agreement EXPL/FIS-MAC/0953/2021.
Some of the most pressing challenges faced by modern society require the processing and transmission of an ever increasing amount of information, the development of new types of sensors, and harvesting of solar energy. The solution to these problems will require the development of a new generation of optoelectronic devices. The field of condensed matter physics will play an essential role in this process.
Understanding the physics of excitons - states that occur in semiconducting or insulating materials, when a valence electron is excited to the conduction band and forms an electron-hole bound pair - will be key to push forward new optoelectronic technology. While the study of excitons has a long history in condensed matter physics, recent scientific and technological advances, in nanoscale fabrication techniques and in the field of two-dimensional materials, have led to a resurgent interest in their study. This is mostly due to the isolation of monolayer STMDs, such as MoS2, MoSe2, WS2 and WSe2. These materials are direct band semiconductors with gaps in the visible range and strong light-matter interaction. These properties make STMDs ideally suited to optoelectronic applications. In fact, several STMD based optoelectronic devices have already been developed. These include photodetectors, light-emitting diodes (LED), solar cells, and single-photon quantum emitters. It turns out that the optical properties of STMDs in the visible range are controlled by strongly bound excitons. Therefore, in order to understand the operation of STMD based devices, one must fully grasp the excitonic physics that governs them.
In STMD based optoelectronic devices, excitons are typically generated by non-equilibrium drives. This occurs in excitonic electroluminescence, where an applied bias generates excitons which then decay radiatively emitting light, and in photocurrent generation, where excitons are optically excited by incident light. Typically, these processes are described in a semi-classical way, using predominantly rate equations. However, the increasing miniaturization of devices implies that coherent quantum mechanical effects should play a significant role in these non-equilibrium excitonic devices. Therefore, in order to take full advantage of STMDs in applications, developments in the quantum non-equilibrium theory of excitons are required.
The aim of this project is to theoretically investigate the coherent generation of excitons under non-equilibrium conditions. We will develop the non-equilibrium quantum theory for coherent excitonic electroluminescence and photocurrent generation in STMD based optoelectronic mesoscopic devices. In particular, the results of this project will contribute to the understanding and guide the development of STMD based electrically driven excitonic single-photon sources and photodetectors
The theoretical understanding of the mechanisms that govern exciton generation and dynamics will play a crucial role in interpreting experimental findings and guide new experimental research of both fundamental and applied nature.
This project is funded by the Portuguese Foundation for Science and Technology under the grant agreement EXPL/FIS-MAC/0953/2021.
The availability of reliable single-photon sources is of the utmost importance for the devellopment of new quantum technologies, such as quantum communication protocols. Solid-state emitters are particularly promissing as single-photon sources as they can be easily integrated with electronics or quantum cavities.
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Recently, single-photon emission from localized sources as been observed in several 2D transtion metal dichalcogenides, a family of two-dimensional semiconductors. The emission of quantum light by these materials has been atributed to localized exctions (excited states that can exist em semiconductors that can be understood as effective atoms). However, what leads to the localized of this exctions remains unclear, with crystal defects, edges or lattice deformations all being candidates.
The possibility of localized due to deformation is very appealing as this can be controlled. The goal of this project is to clarify this stuation, by theoretically studying how local lattice deformations give origin to localized exctions in STMD. As such this project has three main objectives:
By shedding light on these questions, this timely project will contribute to the understanding of the exact nature of SPEs in STMDs. Furthermore, the theoretical study of how localized excitons are affected by external perturbations, will guide further experimental exploration and optimization of SPEs in STMDs, an essential step in order
to take technological advantage of them.
This project is funded by the Portuguese Foundation for Science and Technology under the grant agreement CEECIND/02936/2017.
The possibility of localized due to deformation is very appealing as this can be controlled. The goal of this project is to clarify this stuation, by theoretically studying how local lattice deformations give origin to localized exctions in STMD. As such this project has three main objectives:
- Study how regions of localized strain and curvature confine electronic states;
- Characterize the excitorder
to take technological advantage of them.nic states localized in these regions (spectrum, optical selection rules, radiative decay rate, luminescence and optical absorption); - Study how the excitonic states are affected by changes in strain, applied magnetic fields and doping level of the STMD.
By shedding light on these questions, this timely project will contribute to the understanding of the exact nature of SPEs in STMDs. Furthermore, the theoretical study of how localized excitons are affected by external perturbations, will guide further experimental exploration and optimization of SPEs in STMDs, an essential step in order
to take technological advantage of them.
This project is funded by the Portuguese Foundation for Science and Technology under the grant agreement CEECIND/02936/2017.
Vertical Transport and Photoresponse
in van der Waals hybrid structures
Van der Waals (vdW) hybrid structures, new systems formed by stacking layers of two-dimensional crystals on top of each other, are a promising
route towards the tailoring of material properties at will. Understanding the properties of individual layers and how they interact with each other to obtain the desired properties is the main focus of both experimental and theoretical research of the community working in this area. Due to the extreme high quality, atomically sharp, interfaces between different layers in vdW structures, lattice mismatch and the relative alignment between consecutive layers play a fundamental role in determining the properties of the vdW structure, governing the electronic coupling between different layers. |
Graphene – insulator/semiconductor – graphene vdW structures have recently received a lot of attention from the community, due to its potential for applications, having been shown to operate both as transistors and photodetectors. It is not clear however how device operation is affected by lattice mismatch effects.
The aim of this project is to develop theory and models, both analytical and numerical, to describe the vertical current and photocurrent generation, both in the steady state and in the transient regime, of the vdW hybrid structures referred above. Special attention will be given to the effect of lattice mismatch and crystal-momentum conservation in the vertical current flow. The results from this project will be useful in interpreting current and future experimental results and in guiding the design of new vdW devices.
In the duration of this project, Bruno Amorim will work under the supervision of Prof. Eduardo V. Castro in the multidisciplinary environment provided by the Center of Physics and Engineering of Advanced Materials at Instituto Superior Técnico, in Portugal.
This project was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Individual Fellowship Grant Agreement No. 706538.
The aim of this project is to develop theory and models, both analytical and numerical, to describe the vertical current and photocurrent generation, both in the steady state and in the transient regime, of the vdW hybrid structures referred above. Special attention will be given to the effect of lattice mismatch and crystal-momentum conservation in the vertical current flow. The results from this project will be useful in interpreting current and future experimental results and in guiding the design of new vdW devices.
In the duration of this project, Bruno Amorim will work under the supervision of Prof. Eduardo V. Castro in the multidisciplinary environment provided by the Center of Physics and Engineering of Advanced Materials at Instituto Superior Técnico, in Portugal.
This project was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Individual Fellowship Grant Agreement No. 706538.