  Energy bandgap largely determines the optical and electronic properties of a
semiconductor. Variable bandgap therefore makes versatile functionality
possible in a single material. In layered material black phosphorus, the
bandgap can be modulated by the number of layers; as a result, few-layer black
phosphorus has discrete bandgap values that are relevant for opto-electronic
applications in the spectral range from red, in monolayer, to mid-infrared in
the bulk limit. Here, we further demonstrate continuous bandgap modulation by
mechanical strain applied through flexible substrates. The strain-modulated
bandgap significantly alters the charge transport in black phosphorus at room
temperature; we for the first time observe a large piezo-resistive effect in
black phosphorus field-effect transistors (FETs). The effect opens up
opportunities for future development of electro-mechanical transducers based on
black phosphorus, and we demonstrate strain gauges constructed from black
phosphorus thin crystals.
