Strain Fields at the Micro-Scale in Two-Dimensional Materials

The variegated family of two-dimensional (2D) materials comprises crystals that have attracted great interest for their exceptional characteristics. Among them, hBN is a thermally stable and mechanically robust insulator, and semiconducting transition-metal dichalcogenides (TMDs) possess alluring optoelectronic and spin properties when reduced to the single layer. In particular, TMD monolayers are characterised by a direct bandgap, resulting in an efficient light emission in the visible/infrared range, which renders them appealing for optoelectronic devices. Furthermore, a strong spin-orbit coupling makes them interesting candidates for valley- and spin-tronics. Indeed, the inherent plane-confined nature of these materials -coupled to their exceptional mechanical flexibility and robustness- makes them highly sensitive to external stimuli. Methods to tailor their unique properties on demand have been thus sought after, and protocols based on controllable external perturbations such as mechanical deformations have shown promise in this respect.<br/><br/>Here, we explore new strategies to tune the peculiar properties of 2D TMDs and hBN by mechanical deformations.<br/><br/>We show how low-energy hydrogen-ion irradiation of bulk flakes can be exploited to engender localised strains. More specifically, the hydrogen ions penetrate through one or a few layers, leading to the production and accumulation of molecular hydrogen in the first interlayer region. The trapped gas coalesces, leading to a local blistering of the material, and thus to the formation of few-layer-thick micro/nano-bubbles [1,2].<br/><br/>The bubbles host complex strain fields that reach high values. Raman measurements allowed us to characterise experimentally the strain and its effect on the vibrational properties of the materials [2,3]. An analytical model allows us to calculate the strain distribution, in agreement with the experiments. The model further allows us to obtain precious information on the adhesion energy between the bubble and the substrate, and the bubbles can thus be exploited to measure the adhesion energy of a variety of van der Waals materials [3]. Furthermore, nano-indentation measurements permitted us to probe the elastic properties of the material and highlight their exceptional flexibility [4].<br/><br/>These bubbles are durable and incredibly robust, and in TMDs the bubble thickness is just one layer. In turn, TMD bubbles behave as efficient light emitters. The high fields they host cause dramatic changes in the TMD opto-electronic properties, and photoluminescence steady-state and time-resolved studies enabled the characterisation of the strain-induced band-structure modifications and revealed intriguing phenomena, such as a strain-induced direct-to-indirect bandgap crossover [5]. Our optical characterisation further allowed to pinpoint hybridization phenomena between direct and indirect excitons [6].<br/><br/>Our results unveil unprecedented information on the strain effects on TMDs and hBN, the understanding of which represents an essential step towards their integration into flexible electronic devices.<br/>[1] D. Tedeschi, E. Blundo, et al., Adv. Mater. 31, 1903795 (2019).<br/>[2] E. Blundo et al., Nano Lett. 22, 1525 (2022).<br/>[3] E. Blundo et al., Phys. Rev. Lett. 124, 046101 (2021).<br/>[4] C. Di Giorgio, E. Blundo et al., ACS Appl. Mater. Interfaces 13, 48228 (2021).<br/>[5] E. Blundo et al., Phys. Rev. Res. 2, 012024 (2020).<br/>[6] E. Blundo et al., Phys. Rev. Lett. (2022), in press.