Strain Engineering of Exciton/Quantum Emission

Strain in a crystal changes the bond strengths between atoms and therefore modulates various properties arising from the periodic crystalline structures, such as electronic band structures, phonon band structures, and spin state. Strain has been implemented into CMOS technology, and have proved to be powerful method to enhance the transistor performance by 10s-of % from merely ≈1 % of strain. 

Unlike silicon or traditional semiconductors, 2D materials are composed of atomic layers weakly held together by van der Waals (vdW) force. The layered structure allows 2DMs to withstand an extreme (10+ %) level of mechanical strain without breaking covalent bonds, making them an optimal system for strain engineering. For instance, strain in the in-plane direction can easily modulate 100s-of meV of bandgap in monolayer transition metal dichalcogenides (TMDCs). Moreover, strains applied to 2D system in the out-of-plane direction modulate the vdW interaction between the 2DM and the substrate or between different 2D atomic layers. Since the vdW bond is orders of magnitude weaker than the in-plane covalent bonds, the out-of-plane strain is far more efficient in modulating the 2D materials and heterostructures' properties. 

Strain can be controllably applied to 2DMs using various approaches. In-plane strain is widely applied by bending or stretching a flexible substrate on which 2D materials are placed. Placing 2D materials on a topographically uneven substrate also produces spatially-predefined strain to the atomic layer. Although being less controllable, changing the temperature of 2D materials-substrate stacks results in in-plane strain due to the thermal expansion coefficient mismatch. Out-of-plane strain can be applied using a diamond anvil cell (DAC) apparatus, where two diamond anvils compress the sample in all directions (i.e., applying hydrostatic pressure). Optical measurements such as photoluminescence spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy, as well as electrical conductivity measurements, can be carried out to characterize in situ strain-dependent material properties.