Synthesis and Properties of (BiSe)0.97MoSe2: A Heterostructure Containing Both 2H-MoSe2 and 1T-MoSe2

The metastable heterostructure, (BiSe)0.97MoSe2, containing alternating bilayers of BiSe and MoSe2 trilayers was synthesized using the modulated elemental reactant method to determine if charge transfer from BiSe to MoSe2 would stabilize the metallic 1T polymorph of MoSe2. Optimum synthesis conditions were determined by following the structural evolution as a function of temperature. The structure of the product contained distorted rock salt-structured BiSe layers alternating with hexagonal MoSe2 layers. High-angle annular dark field scanning transmission electron microscopy images revealed that two different polymorphs of MoSe2 coexisted in (BiSe)0.97MoSe2. Raman spectroscopy confirmed the presence of 1T MoSe2 layers. X-ray photoelectron spectroscopy (XPS) indicated that there were two different electronic states for both Mo and Bi. The Mo states are consistent with having octahedral and trigonal prismatic coordination of molybdenum as found in the 1T and 2H polymorphs of MoSe2. The two different electronic states for Bi are consistent with the presence of antiphase boundaries in the BiSe layers. Estimating the relative amount of each electronic state from the XPS spectra indicates that the percentage of 1T MoSe2 is about 40%, whereas the amount of Bi3+ in the BiSe is approximately 60%. The measured resistivity increases as temperature is decreased, consistent with an activated conduction mechanism with a small activation energy (∼0.05 eV). The temperature stability and low resistivity of (BiSe)0.97MoSe2 make it potentially interesting as a means of improving electrical contacts to MoSe2.