Periodic Reporting for period 1 - 2DCHEX (Tuning emission of charged excitons in two-dimensional transition metal dichalcogenide monolayers)
Periodo di rendicontazione: 2021-07-01 al 2023-06-30
Transition metal dichalcogenide (TMD) monolayers are two-dimensional (2D) crystals with a thickness of only a few atoms yet exhibiting extremely rich and extraordinary physical properties. Unlike bulk TMD crystals, their monolayers are semiconductors with a direct electronic band gap, which makes them ideal candidates for optoelectronic applications. Moreover, the monolayer crystal structure does not have an inversion centre, thus providing an additional degree of freedom for encoding and processing of quantum information – so-called valley states in K and -K points of the Brillouin zone. Conveniently, these quantum degrees of freedom can be addressed optically with circularly polarized light constituting a new opto-valleytronic technology. Altogether, TMD-based valleytronic devices are anticipated to bring disruptive innovations that will impact our daily life. However, temperature-dependent parasitic processes may cause valley depolarization and loss of all the remarkable properties at room temperature, narrowing the operation of opto-valleytronic technologies only to the cryogenic temperatures, which severely hamper any practical application in our room-temperature environment.
The ambition of this MSCA Individual Fellowship (2DCHEX - 2D CHarged EXitons) was to exploit charged excitonic states to tackle the valley depolarization and enable the remarkable features at room temperature. This functionality holds great potential for quantum technologies capable of boosting information processing and computation abilities beyond our imagination. In a broader perspective, such devices could replace current classical informational and computing technologies, effectively impacting everyone who uses a computer or a smartphone today. Quantum room-temperature opto-valleytronic devices have the potential to revolutionize the security of our communication and enable exponentially more powerful computational capabilities.
The proposed idea within the 2DCHEX project was to develop an approach for inducing charged excitonic states. Electrochemical charging has been suggested here for the ultimate control of charged excitons and their emission properties, since such an approach provided the strongest electron doping in comparison with optical, electrostatic, chemical, or mechanical doping. Via controlling the charging state of an exciton one could influence the temperature-dependent parasitic processes by boosting the emission rate and charge-screening the valley depolarization.
To conclude, the 2DCHEX action was indeed successful in achieving high and robust valley polarization at room temperature. Moreover, other extraordinary properties of excitons in 0D and 2D systems were explored to purify and control quantum emission at room temperature.
The ambition of this MSCA Individual Fellowship (2DCHEX - 2D CHarged EXitons) was to exploit charged excitonic states to tackle the valley depolarization and enable the remarkable features at room temperature. This functionality holds great potential for quantum technologies capable of boosting information processing and computation abilities beyond our imagination. In a broader perspective, such devices could replace current classical informational and computing technologies, effectively impacting everyone who uses a computer or a smartphone today. Quantum room-temperature opto-valleytronic devices have the potential to revolutionize the security of our communication and enable exponentially more powerful computational capabilities.
The proposed idea within the 2DCHEX project was to develop an approach for inducing charged excitonic states. Electrochemical charging has been suggested here for the ultimate control of charged excitons and their emission properties, since such an approach provided the strongest electron doping in comparison with optical, electrostatic, chemical, or mechanical doping. Via controlling the charging state of an exciton one could influence the temperature-dependent parasitic processes by boosting the emission rate and charge-screening the valley depolarization.
To conclude, the 2DCHEX action was indeed successful in achieving high and robust valley polarization at room temperature. Moreover, other extraordinary properties of excitons in 0D and 2D systems were explored to purify and control quantum emission at room temperature.
The work performed in the 2DCHEX project was centred around the following key aspects:
- Fabrication of TMD monolayers and characterization of their photoluminescence properties. During my secondment at DTU I learned the state-of-the-art techniques of monolayer fabrication and used these skills for fabrication of my own samples at the host institution (mechanical exfoliation, dry transfer, assembly of van-der-Waals heterostructures). I designed and built experimental setups for laser excitation of samples and characterization of generated photoluminescence (spectral properties, polarization response, time dynamics). While testing my setups with a system I am familiar with – a colloidal quantum dot (0D system) – I developed a spectral filtering technique to purify single photon emission at room temperature (I published my results as a first-author paper in Nanoscale (DOI: 10.1039/D2NR04744F).
- Development of electrochemical electron-doping technique to manipulate the exciton charging and its optical response. I designed a custom-built electrochemical cell which could control the formation of charged excitons in various monolayers (WS2, WSe2, MoSe2) and described its operational principle (I discovered that the ionization dynamics of the neutral exciton follows the temperature-dependent Fermi-Dirac distribution, which interestingly allowed me to quantify the position of Fermi level and electron doping densities). I demonstrated that the electrochemical doping provides the highest doping densities among all the doping techniques available for TMD monolayers, which unlocks the remarkable properties of charged excitons at room temperature. Based on my research I published a first-author manuscript in Advanced Optical Materials (DOI: 10.1002/adom.202101305).
- Inducing robust valley polarization of charged excitons. I utilized the developed electrochemical doping technique to control the charged excitons and demonstrated the highest observable to date valley polarization at room temperature. I explained my results via enhancing the recombination rate under exciton charging and charge-screening effects in K and -K valleys (first-author manuscript under preparation).
- Development of a novel type of antenna for enhancing the electron interaction with TMD monolayers. As monolayers are atomically thin, the exposed volume to electron beam at normal incidence is extremely small, which severely limits the electron-monolayer interaction. Therefore, I developed an antenna – a gold crystalline nanodisk, which redirects incoming electrons along the monolayer plane, allowing for in-plane excitations. Remarkably, under such an excitation I achieved the highest observable to date photon bunching. I published these results as an equal-contribution author in 2D Materials (DOI: 10.1088/2053-1583/acbf66).
2DCHEX project generated high quality scientific results on understanding and exploitation of charged exciton properties in 0D and 2D systems for quantum information and valleytronic applications. My results were disseminated through publications in scientific journals, conferences, educational seminars at SDU, CNO, POLIMA, and DTU, 2DCHEX website, social media (ResearchGate, LinkedIn, Twitter). For communication with the broader public and industry, I prepared infographics and teasing video material. My results on the induced valley polarization at room temperature could be directly exploited by industry for an implementation of opto-valleytronic devices operating at room temperature.
- Fabrication of TMD monolayers and characterization of their photoluminescence properties. During my secondment at DTU I learned the state-of-the-art techniques of monolayer fabrication and used these skills for fabrication of my own samples at the host institution (mechanical exfoliation, dry transfer, assembly of van-der-Waals heterostructures). I designed and built experimental setups for laser excitation of samples and characterization of generated photoluminescence (spectral properties, polarization response, time dynamics). While testing my setups with a system I am familiar with – a colloidal quantum dot (0D system) – I developed a spectral filtering technique to purify single photon emission at room temperature (I published my results as a first-author paper in Nanoscale (DOI: 10.1039/D2NR04744F).
- Development of electrochemical electron-doping technique to manipulate the exciton charging and its optical response. I designed a custom-built electrochemical cell which could control the formation of charged excitons in various monolayers (WS2, WSe2, MoSe2) and described its operational principle (I discovered that the ionization dynamics of the neutral exciton follows the temperature-dependent Fermi-Dirac distribution, which interestingly allowed me to quantify the position of Fermi level and electron doping densities). I demonstrated that the electrochemical doping provides the highest doping densities among all the doping techniques available for TMD monolayers, which unlocks the remarkable properties of charged excitons at room temperature. Based on my research I published a first-author manuscript in Advanced Optical Materials (DOI: 10.1002/adom.202101305).
- Inducing robust valley polarization of charged excitons. I utilized the developed electrochemical doping technique to control the charged excitons and demonstrated the highest observable to date valley polarization at room temperature. I explained my results via enhancing the recombination rate under exciton charging and charge-screening effects in K and -K valleys (first-author manuscript under preparation).
- Development of a novel type of antenna for enhancing the electron interaction with TMD monolayers. As monolayers are atomically thin, the exposed volume to electron beam at normal incidence is extremely small, which severely limits the electron-monolayer interaction. Therefore, I developed an antenna – a gold crystalline nanodisk, which redirects incoming electrons along the monolayer plane, allowing for in-plane excitations. Remarkably, under such an excitation I achieved the highest observable to date photon bunching. I published these results as an equal-contribution author in 2D Materials (DOI: 10.1088/2053-1583/acbf66).
2DCHEX project generated high quality scientific results on understanding and exploitation of charged exciton properties in 0D and 2D systems for quantum information and valleytronic applications. My results were disseminated through publications in scientific journals, conferences, educational seminars at SDU, CNO, POLIMA, and DTU, 2DCHEX website, social media (ResearchGate, LinkedIn, Twitter). For communication with the broader public and industry, I prepared infographics and teasing video material. My results on the induced valley polarization at room temperature could be directly exploited by industry for an implementation of opto-valleytronic devices operating at room temperature.
There are three key scientific results which go beyond the state of the art: (i) the electrochemical doping method provides the strongest doping among all doping techniques, (ii) the demonstration of high valley polarization at room temperature, which according to the common understanding is limited to cryogenic temperatures, and (iii) development of a novel antenna for increasing electron interaction with 2D materials leading to the photon superbunching.
The success of 2DCHEX action, beyond its scientific achievements, were the establishment of fruitful collaborations with DTU (Denmark), Imperial College London (England) and Ghent University (Belgium). 2DCHEX action brought together experts on theory of 2D materials, photonics, scanning electron microscopy, semiconductor devices, which advanced the fundamental knowledge of 2D physics and facilitated the transfer of knowledge between my international collaborators and colleagues at the host institution. I anticipate that the results can be utilized by industry, which would enable quantum technologies at room temperature and bring it to our daily life by replacing the classical technologies.
The success of 2DCHEX action, beyond its scientific achievements, were the establishment of fruitful collaborations with DTU (Denmark), Imperial College London (England) and Ghent University (Belgium). 2DCHEX action brought together experts on theory of 2D materials, photonics, scanning electron microscopy, semiconductor devices, which advanced the fundamental knowledge of 2D physics and facilitated the transfer of knowledge between my international collaborators and colleagues at the host institution. I anticipate that the results can be utilized by industry, which would enable quantum technologies at room temperature and bring it to our daily life by replacing the classical technologies.