  The inherently low photoluminescence (PL) yields in as prepared transition
metal dichalcogenide (TMD) monolayers are broadly accepted to be the result of
atomic vacancies (i.e. defects) and uncontrolled doping, which give rise to
non-radiative exciton decay pathways. To date, a number of chemical passivation
schemes have been successfully developed to improve PL in sulphur based TMDs
i.e. molybdenum disulphide (MoS2) and tungsten disulphide (WS2) monolayers.
Reports on solution based chemical passivation schemes for improving PL yields
in selenium (Se) based TMDs are lacking I comparison, with only one known study
that uses hydrobromic acid vapour to improve PL in chemical vapour deposited
(CVD) Molybdenum diselenide (MoSe2). Here, we demonstrate that treatment with
oleic acid (OA) provides a simple wet chemical passivation method for monolayer
MoSe2, enhancing PL yield by an average of 58 fold, while also enhancing
spectral uniformity across the material and reducing emission linewidth.
Excitation intensity dependent PL reveals trap-free PL dynamics dominated by
neutral exciton recombination. Time-resolved PL (TRPL) studies reveal
significantly increased PL lifetimes, with pump intensity dependent TRPL
measurements also confirming trap free PL dynamics in OA treated MoSe2. Field
effect transistors show reduced charge trap density and improved on-off ratios
after treatment with OA. These results indicate defect passivation by OA, which
we hypothesise act as ligands, passivating chalcogen defects through oleate
coordination to Mo dangling bonds.
