Skip to main content

Unlocking new physics in controllably strained two-dimensional materials

Periodic Reporting for period 4 - Strained2DMaterials (Unlocking new physics in controllably strained two-dimensional materials)

Reporting period: 2020-05-01 to 2022-04-30

The overarching goal of Strained2DMaterials is to use strain engineering as an enabling tool to study previously inaccessible or hard-to-study phenomena in two-dimensional atomic crystals (e.g.: graphene, bilayer graphene, and monolayer transition metal dichalcogenides). The main aims are: (i) to probe the effects of commonly neglected flexural phonons on electrical transport, thermal conductivity, and mechanical properties of suspended 2D materials. (ii) to study the roles of non-uniform strain fields (iii) to develop the techniques to control and tune excitons in TMDC material using uniform and non-uniform strain fields.
On the technological side, we developed several new approaches to apply mechanical strain to 2D materials. We realized new approaches to apply either large uniforms strain or strongly spatially inhomogeneous strain at low temperate. These new techniques allowed us to explore previously unexplored regions of the phase space of strain-dependent physics in 2D materials and to discover multiple new phenomena. First, we discovered strain-related renormalization of mechanical constant (Young modulus, Poisson’s ratio, etc…) of 2D materials. We showed that 2D materials are more akin, mechanically, to soft matter studied in biology than to hard solids. Second, we showed that inhomogeneous strain field acts as a driving force on normally neutral excitons, electron/hole complexes induced by light in 2D semiconductors. We showed that in presence of this force neutral excitons are converted into charged excitons (Obj. 4,6). This discovery is critical for the operation of single quantum emitters based on 2D materials. Finally, we explored non-linear couplings between various vibrational modes in resonators based on 2D materials. We showed that this coupling leads to mode hybridization, nanomechanical squeezing, and upconversion. We showed these properties can be used to realize all-mechanical optical spectroscopic characterization of 2D materials.

To enable progress along these main directions, we had to improve the quality of 2D materials (2DMs) significantly. Specifically, we found approaches to characterize the presence of defects, and scattering channels, and improve the gating efficiency of 2DMs. Specifically, we demonstrated the use of excitonic resonance to characterize the presence of surface disorder, the effect of defects on spin transport across 2DM interfaces, showed the approach to generate record electric fields up to 3V/nm in a solid, and understood the effect of disorder on dielectric screening. The work was disseminated in scientific publications, press-releases, and popular articles.
The main achievements of the project :

1) Discovery of a non-linear Hooke's law in crumpled graphene
2) Experimental discovery of exciton/trion funneling in non-uniformly strained TMDCs and single quantum emitters in non-uniformly strained hBN
3) Development of NEMS devices based on coupled SiNx/graphene hybrids: ultrasensitive mechanical amplifiers and devices with tunable nonlinearities
4) Development of approaches to non-uniform strain engineering
5) Development of approaches to chemically engineer 2D materials
6) Demonstration of strain-controlled 2D "phononic crystals"
7) Development of techniques to probe surface contamination of 2D materials, towards the creation of devices with well-characterized mechanical properties.
8) The demonstration of techniques for induction of extreme electrical fields, the study of physical phenomena at these fields
Strain Engineering of 2D materials