Unlike atomically thin graphene, transition metal dichalcogenide (TMD) monolayers are three-atom thick and can exist in many different polymorphs where metal atom coordination changes from trigonal prismatic (1H phase) to octahedral and distorted octahedral (1T and 1T’ phases).1 This diversity of crystal types provides an additional tool to control the band structure of TMDs nanostructures so that their electronic properties can span in a wide range from semiconducting and metallic to topological insulators, quantum spin Hall insulators and two-dimensional superconductors.2 Thermodynamically favourable phase of WSe2 monolayers is the trigonal prismatic semiconducting 1H phase which can be converted into the metastable octahedral phase as a result of either an electron transfer (i.e. during lithiation) or applied mechanical strain. Indeed, direct formation of 1H structure type of WSe2 via wet chemical approaches or physical deposition techniques has only been reported so far.3 In this work, we report on colloidal synthesis of 1T’ WSe2 nanostructures from tungsten carbonyl precursor in a coordinating solvent oleic acid at 300oC. WSe2 nanostructures demonstrate a well-defined flower-like morphology with atomically thin individual petals reaching 50 nm in lateral size. Furthermore, we found that 1T’ phase of WSe2 can be converted into the semiconducting 1H phase upon annealing at 400oC in argon atmosphere preserving the starting flower-like morphology. The phase conversion was confirmed by high-resolution transmission electron microscopy, Raman and optical spectroscopy. We suggest that formation of the metastable 1T’ phase of WSe2 in a low temperature synthesis from tungsten carbonyl can be caused by an excess of electrons on the metal centres during the growth.
1 Acerce et al, Nature, 2017, 549, 370–373
2 Yang et al, Nat. Phys., 2017, 13, 931–937
3 Barrera et al, J. Mater. Chem. C, 2017, 5, 2859–2864.