  Based on first-principles calculations, we present a quantum confinement
mechanism for the band gaps of blue phosphorene nanoribbons (BPNRs) as a
function of their widths. The BPNRs considered have either armchair or zigzag
shaped edges on both sides with hydrogen saturation. Both the two types of
nanoribbons are shown to be indirect semiconductors. An enhanced energy gap of
around 1 eV can be realized when the width decreases to about 10 ang. The
underlying physics is ascribed to the quantum confinement. More importantly,
the quantum confinement parameters are obtained by fitting the calculated gaps
with respect to their widths. The results show that the quantum confinement in
armchair nanoribbons is stronger than that in zigzag ones. This study provides
an efficient approach to tune the energy gap in BPNRs.
