  SnSe, a group IV-VI monochalcogenide with layered crystal structure similar
to black phosphorus, has recently attracted extensive interests due to its
excellent thermoelectric properties and potential device applications.
Experimental electronic structure of both the valence and conduction bands is
critical for understanding the effects of hole versus electron doping on the
thermoelectric properties, and to further reveal possible change of the band
gap upon doping. Here, we report the multivalley valence bands with a large
effective mass on semiconducting SnSe crystals and reveal single-valley
conduction bands through electron doping to provide a complete picture of the
thermoelectric physics. Moreover, by electron doping through potassium
deposition, the band gap of SnSe can be widely tuned from 1.2 eV to 0.4 eV,
providing new opportunities for tunable electronic and optoelectronic devices.
