Unique structure and surface-related elastic modulus of alumina nanobelts

Single-crystalline alpha-Al2O3 nanobelts were synthesized by high-temperature chemical vapor deposition in a high-purity H-2 atmosphere. The crystalline planes for the upper and side surfaces of the nanobelts were (11 (2) over bar0) and (1 (1) over bar 04), and the orientations along height, length and width directions were [11 (2) over bar0], [(2) over bar 201] and [5 (5) over bar 04], respectively. The formation of such a unique structure was dependent on the strong reducing atmosphere used in the growth process, and the deactivation of the {11 (2) over bar0} plane by hydrogen could be the primary cause. The elastic modulus of the nanobelts was measured using a thermal resonance method. The moduli for the nanobelts were about 320 GPa for thicknesses above 40 nm, and slightly increased to 356 GPa as the thickness decreased to 31 nm. The slightly low modulus values compared to the theoretical value of 371 GPa is attributed to oxygen vacancies within the nanobelts, while the increase in modulus with decreased thickness comes from the stiffening effect caused by surface relaxation.