Large Photo-Thermal Effect in Sub-40 nm h-BN Nanostructures Patterned Via High-Resolution Ion Beam

The controlled nanoscale patterning of 2D materials is a promising approach for engineering their optical, thermal, and mechanical properties. Herein, we demonstrate nanoscale patterning of hexagonal boron nitride (h-BN) via both helium and neon ion beams and the resulting photo-thermal effects. We fabricate a grating with a 35 nm pitch, individual structure sizes down to 20 nm, and nanostructures with different aspect ratios. Raman spectroscopy is used to study the defects induced by the ion beam and is correlated to scanning probe microscopy. Photo-thermal and scanning near-field optical microscopy measure the near-field absorption and scattering. These measurements reveal a large photo-thermal expansion of nanostructured h-BN that is dependent on the height to width aspect ratio. This effect is attributed to the large anisotropy of the thermal expansion coefficients of h-BN. For the nanostructures with the largest photo-thermal expansion (absorption), the Raman signal for the h-BN phonon mode at 1368 cm^-1 is reduced 49 times while the photo-thermal expansion is increased 11 times, indicating that the nanostructuring significantly modifies the h-BN. Photo-thermal expansion should be present in other vdW materials and may lead to nanomechanical switches driven by light.