Strain Engineering of Light-Emitting Nanodomes

Transition metal dichalcogenides (TMDs) are semiconductors with a layered crystal structure. They are characterized by the formula MX2, where M designates a transition metal (usually Mo or W) and X a chalcogen (S or Se most frequently). Due to their peculiar crystal structure, these materials have initially found application as lubricants, while their potential as materials for advanced optoelectronic applications has initially remained largely unexplored. The successful mechanical exfoliation of graphene suggested however that samples as thin as a single monolayer of TMDs can be readily fabricated. When this was done, it was realized that the properties of these materials undergo a major modification when their thickness is reduced to a single layer. While TMDs are in general indirect band gap semiconductors, their band gap becomes direct when they are thinned down to monolayer thickness. This has a dramatic impact on their optical properties: when samples of a monolayer thickness are produced, they are excellent light emitters, while thicker samples are not emissive.

The introduction of a controlled amount of strain in the material represents a well-established method for tuning the electronic properties of semiconductors. In conventional semiconductors, however, the amount of strain that can be introduced is essentially fixed by the fabrication process and in any case cannot exceed an upper bound of ~1%, imposed by their elastic limit. These limitations can be greatly exceeded in the case of 2D materials, wherein very large, variable stresses can be easily applied. This leads to the possibility of introducing very large strains, both uniaxial and biaxial, thus tuning the electronic properties of these materials controllably over a very broad range. Theoretical predictions suggest that layered materials can withstand strains as high as 10%.

Before the beginning of the action, the host had demonstrated that it is possible to form domes of monolayer thickness in TMDs by irradiating thick (bulk) crystals with low energy protons. The structures produced in this manner are subject to a complex strain distribution, which greatly influences their electronic, optical and vibrational properties. The action, based on these preliminary results, aimed at the investigation of the electronic and optical properties of these micro/nanostructures, and at the control over their properties by making use of the strain fields, both naturally present in the domes and supplied from external sources.

The work planned in this action targeted a better understanding the fundamental properties of layered semiconductors. These materials are currently intensively investigated as potential candidates for advanced optoelectronic devices. This project enabled a better understanding of the electronic and optical properties of these emerging semiconductors in the presence of large strain and represents a preliminary step towards the application of large, reconfigurable stresses to layered materials to actively control their electronic and optical properties. Moreover, a strong effort is currently being produced for the investigation of sources of quantum light for intrinsically secure quantum communications. Strain plays also an important role in the formation of three dimensionally confined states hosted by TMDs, which can be purposefully induced by localized stressors. The work carried out in the framework of this action represents a preliminary step in view of reaching an alternative way of generating these quantum light emitters in layered materials.

The work flow usually began by the fabrication of the samples, which were obtained by low-energy hydrogen irradiation of layered materials. These samples have then been investigated by a variety of spectroscopic techniques, among which the most frequently used was spatially resolved photoluminescence spectroscopy. Raman spectroscopy was also performed systematically on the strained domes. These measurements were performed with the highest possible spatial resolution, to access spatially resolved data on the domes. The fellow took part to a series of measurements performed at the High Magnetic Field Laboratory of the Radboud University in Nijmegen, the Netherlands.

The results obtained during the project have been disseminated during two seminars which the fellow has been invited to give. Additionally, two manuscripts based on the work performed during the action are currently in preparation. The fellow published a popularized article in Italian in a journal edited by his former high school, where he was also invited to attend the presentation of the issue. The fellow contributed to the organization of a workshop, to be held at Sapienza University of Rome in June 2021.

The results obtained during the action enable a better understanding of the exciton physics and of the band structure of TMDs in the presence of strain. In particular, important and novel details concerning the band structure and its modification due to the presence of strain have been discovered following the magnetophotoluminescence experiments performed by the fellow in the framework of the action. Moreover, the fellow has also analyzed in detail the impact of large strains due to the formation of domes on the Raman modes of the layered materials studied. He observed a softening of the Raman modes measured on the strained domes and associated the measured spatial dependence of the Raman frequency to the simulated strain distribution across the microstructure. He also analysed the polarization resolved Raman spectrum in the strained areas of the samples.

Periodic Reporting for period 1 - SELENe (Strain Engineering of Light-Emitting Nanodomes)

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