Strain induced bandgap engineering in multiferroic CuO nanoparticles: Competing micro-strain and geometrical size in nanometer scales

The phase pure monoclinic copper oxide (CuO) nanoparticles (NPs) are synthesized, and the observed strain strongly affects bandgap, leading to its reduction from 3.62 eV to 3.00 eV (±0.02 eV) by reducing the crystallite size from 92 to 37 nm (±5 nm), in contrast to the geometrical size effect, where bandgap (Eg) increases with decreasing particle size. This exciting characteristic is attributed to the dominant micro-strain effect over the size on electronic properties of CuO nanoparticle systems, proving a new way of strain mediated bandgap engineering in NP systems.