Aluminum nitride has an intriguing combination of physical properties, such as enhanced field emission, large optical band gap, high thermal conductivity, large electrical resistivity, as well as a piezoelectric coefficient comparable to that of quartz. These properties make AlN nanostructures suitable for advanced nanoscale electronic and optoelectronic device applications, and have motivated sustained efforts to synthesize AlN nanostructures in various morphologies: wires, nanoparticles, nanotubes, needles, and platelets. While many applications for AlN nanostructures target their use as field emitters in at panel displays, their superior piezoelectric properties and integration compatibility with silicon substrates make them excellent candidates for sensors, actuators, and nano-electromechanical systems(NEMS).
Given the diversity in morphology, cross section, and size resulting from current synthesis methods, the properties of 1-dimensional (1-D) AlN nanostructures that are important for the fabrication and performance of NEMS, in particular mechanical properties, can be rather difficult to quantify. This difficulty is due to size-dependent effects at the nanoscale, large surface-to-volume ratio, and the possible presence of defects acquired during the nanostructure growth process. Furthermore, in the indentation-based methods used to measure the elastic properties of AlN bulk single-crystals the sensing indentation depths are on the order of hundreds nanometers. This means that such methods cannot be used to measure the elastic properties of AlN nanostructures, critical to NEMS design and performance, that have diameters comparable to the typical indentation depths. It is therefore necessary to design procedures to accurately determine the elastic properties of 1-D AlN structures, particularly methods that can locally probe the variation of elastic moduli along an axis and in various locations on a cross-section.
NIST Nanomechanical Properties Group researchers developed a new methodology for determining the radial elastic modulus of a 1-D nanostructure laid on a substrate. The methodology consists in the combination of contact resonance atomic force microscopy with finite element analysis, and was illustrated on facetted AlN nanotubes with triangular cross-sections. By doing precision measurements of the resonance frequencies of the AFM cantilever-probe first in air and then in contact with the AlN
nanotubes, the researchers determined the contact stiffness at different locations on the nanotubes, i.e. on edges, inner surfaces, and outer facets. From the contact stiffness they extracted the indentation modulus and found that this modulus depended strongly on the apex angle of the nanotube, varying from 250 GPa to 400 GPa for indentation on the edges of the nanotubes investigated. The findings indicate that the reduction in size (a requirement for NEMS applications) does not imply a significant distortion of the mechanical properties of AlN NTs compared to those of bulk AlN. It is conceivable, however, that a moderate variation in the mechanical properties of AlN NTs synthesized through the "epitaxial casting" process may be achieved by selecting GaN NWs templates with different cross-sections and different effective diameters.
The research was a featured article in the journal Nanotechnology: "Elastic Moduli of Faceted Aluminum Nitride Nanotubes Measured by Contact Resonance Atomic Force Microscopy," G. Stan, C.V. Ciobanu, T.P. Thayer, G.T. Wang, J.R. Creighton, K.P. Purushotham, L.A. Bendersky, and R.F. Cook, 20 (2009) 035707.