Experimental Evidence of Large Bandgap Energy in Atomically Thin AlN

Ultrathin III-nitrides beyond BN, such as GaN and AlN, have attracted much research interest due to their potential applications in 2D optoelectronic devices. Taking advantage of the atomic thickness, the bandgap is expected to be widened in these thin films due to quantum confinement. As a promising intrinsic dielectric and tunneling layer for optoelectronic devices, ultrathin freestanding AlN structures have not been systematically studied, and the band structure still remains in the theoretical description. In this work, atomically thin hexagonal AlN nanotubes with controllable wall thickness have been fabricated via selective thermal evaporating the GaN/AlN core/shell nanowires, where the GaN cores entirely decomposed and are removed from the bottom open end while the robust AlN shells remain and form tubular structures. The bandgap energy of 9.2 +/- 0.1 eV is confirmed through spectrally resolved X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy measurements on AlN nanotubes with the wall thickness of two monolayers.