  We present an analytic study, based on the tight-binding approximation, of
strain effects on the electronic bandgap in single-layer black phosphorus. We
obtain an expression for the variation of the bandgap induced by a general
strain type that includes both tension in and out of the plane and shear, and
use this to determine the most efficient strain direction for different strain
types, along which the strongest bandgap manipulation can be achieved. We find
that the strain direction that enables the maximum manipulation of the bandgap
is not necessarily in the armchair or zigzag direction. Instead, to achieve the
strongest bandgap modulation, the direction of the applied mechanical strain is
dependent on the type of applied strain.
