Large enhancement of electrical transport properties of SnS in the out-of-plane direction by n-type doping: a combined ARPES and DFT study
Tin sulfide (SnS) is a promising thermoelectric material with advantages of non-toxicity, abundant resources and low cost of its components. However, its figure of merit (ZT) is lower than that of its analogue SnSe, which was recently revealed to have an unprecedentedly high ZT value. Here, we demonstrate the differences of electronic structures between SnS and SnSe by combining angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). Additionally, by n-type doping with Sb or Bi, our findings reveal novel resonant states near the bottom of the SnS conduction band and a considerable increase of the electron delocalization along the out-of-plane direction. Effectively, Boltzmann transport calculations show that controlled doping with Sb or Bi results in an effective increase of the Seebeck coefficient as well as a remarkably improved normalized electrical conductivity. Thus, together with the lower thermal conductivity along the out-of-plane direction due to the interface scattering from the SnS layered structure, an improved thermoelectric performance may be realized along the out-of-plane direction. These results pave the way towards new opportunities for developing high-performance thermoelectric materials based on electronic structure tuning by atomically controlled n-type doping.