Electrical performance enhancement by resonant states in PbSe single-crystal growth via Bi-flux method

This paper reports the electrical transmission properties of single-crystal PbSe samples prepared via Bi-flux method based on a stoichiometric ratio of Pb:Se:Bi = 1:1: x (x = 2.5, 3, 3.5, 4). Results show that all samples adopt an NaCl structure with the space group Fm (3) over barm, with no cracks or micropores; the Bi atom substitutes for Pb consistently. All samples prepared by this method demonstrate n-type conduction and carrier concentrations varying from 2.48 and 4.21 x 10(18) cm(-3), which far exceeds that of the polycrystalline sample. Transmission electron microscopy (TEM) and corresponding area electron diffraction (SAED) further verify that the prepared sample has a typical single-crystal structure. Electronic band structure calculations indicate that the band gap of Pb32Se32 is 0.43 eV. After replacing a Pb by Bi atoms, the band gap is invariable and a resonant energy level is introduced into the band gap, which enhances the Seebeck coefficient. The power factor of the sample x = 3.5 reaches 3.6 x 10(-3) W m(-1) K-2 at 550 K, which is approximately 50% higher than that of the polycrystalline sample (1.8 x 10(-3) W m(-1) K-2) and can be attributed to the introduction of resonant states in the energy band for Bi replacing Pb.