Quantum size effects on vanadium nanoparticles

Vanadium nanoparticles of two sizes were fabricated by pulse laser deposition. Transmission electron microscopy (TEM) images revealed that the diameters of the particles two samples were 2.5 and 4 nm. X-ray diffraction was used to determine the purity and lattice constant. Both samples were in the cubic 1-3-m phase. The lattice constants increased with diameter: the 4- and 2.5-nm samples had constants that were 0.42% and 0.55% larger than the lattice constant of the bulk, respectively. Superconductivity behaviors were determined by experiments on magnetic susceptibility. No Meissner effect was observed in the 2.5-nm nanoparticles. The T_c of the 4-nm nanoparticles was about 5.4 K, and accompanied a H_c of about 500 Oe. The critical diameter of vanadium nanoparticles for superconductivity can be estimated using Kubo theory^1,2. The calculated critical diameter of the Kubo gap for superconductivity (∼1.6 meV) is about 3 nm. This fact is believed to be the main explanation of the lack of superconductivity in 2.5-nm nanoparticles.