  Through infrared spectroscopy, we systematically study the pressure effect on
electronic structures of few-layer black phosphorus (BP) with layer number
ranging from 2 to 13. We reveal that the pressure-induced shift of optical
transitions exhibits strong layer-dependence. In sharp contrast to the bulk
counterpart which undergoes a semiconductor to semimetal transition under ~1.8
GPa, the bandgap of 2 L increases with increasing pressure until beyond 2 GPa.
Meanwhile, for a sample with a given layer number, the pressure-induced shift
also differs for transitions with different indices. Through the tight-binding
model in conjunction with a Morse potential for the interlayer coupling, this
layer- and transition-index-dependent pressure effect can be fully accounted.
Our study paves a way for versatile van der Waals engineering of
two-dimensional BP.
