Silicon carbide nanowires: elastic properties, defects, and surface formations

Abstract

A highly reproducible method of producing SiC nanowires on a large scale is presented, and the average size of SiC nanowires was 30 nm. XRD revealed that the molar yield increased linearly with time. TEM showed a distribution of nanowire sizes that shifted towards larger diameters as sintering time increased. It is known that vapor-liquid-solid reactions involving a metal catalyst play a role in their formation, and there is further evidence that a vapor-solid mechanism contributes as well. The elastic properties of the following SiC morphologies were explored with pressure applied via a diamond anvil cell: 20 nm grains, 50 nm grains, 130 nm grains, and 30 nm nanowires The bulk modulus of nanowires increased by 8%, while that of 20 nm grains increased 30% in comparison to bulk material.^The increased bulk modulus is explained by the core-shell model, where nanoparticles possess one or more distinct regions near the surface with identical crystal symmetry but different interatomic distances. Defects may also affect the bulk modulus, especially in the heavily faulted nanowires. As seen by TEM, planar faults were abundant, and their quantity decreased with decreasing diameter. The extended Convolutional Multiple Whole Profile (eCMWP) analysis was employed to quantitate the defects by XRD. This analysis concluded that twins are the most frequently occurring planar fault with a 2.20% probability of formation, which corresponds to a defect spacing of 38 nm. SiC nanowires are formed with an amorphous outer layer a few nanometers deep. It was concluded that the layer consisted mainly of amorphous SiC, but EDS confirmed that this structure was rich in oxygen. FTIR confirmed the presence of Si-O bands which increased in population with thermal treatment.^The surface of SiC nanowires was modified by etching in HF and HNO3 acids. Silica bands were reduced and functional groups appeared after treatment. XRD found that grain size increased by 186% and dislocations decreased by 91% with treatment by nitric acid. It is proposed that modification of the surface leads to a reduction of surface stresses, thereby increasing the apparent grain size and reducing dislocations.