Investigation of potassium-intercalated bulkMoS2using transmission electron energy-loss spectroscopy

We have investigated the effect of potassium (K) intercalation on $2H\text{\ensuremath{-}}{\mathrm{MoS}}₂$ using transmission electron energy-loss spectroscopy. For K concentrations up to approximately 0.4, the crystals appear to be inhomogeneous with a mix of structural phases and irregular potassium distribution. Above this intercalation level, ${\mathrm{MoS}}₂$ exhibits a $2a\ifmmode\times\else\texttimes\fi{}2a$ superstructure in the $ab$ plane and unit cell parameters of $a=3.20\phantom{\rule{4pt}{0ex}}\text{\AA{}}$ and $c=8.23\phantom{\rule{4pt}{0ex}}\text{\AA{}}$ indicating a conversion from the $2H$ to the $1{T}^{\ensuremath{'}}$ or $1{T}^{\ensuremath{''}}$ polytypes. The diffraction patterns also show a $\sqrt{3}a\ifmmode\times\else\texttimes\fi{}\sqrt{3}a$ and a much weaker $2\sqrt{3}a\ifmmode\times\else\texttimes\fi{}2\sqrt{3}a$ superstructure that is very likely associated with the ordering of the potassium ions. A semiconductor-to-metal transition occurs signified by the disappearance of the excitonic features from the electron energy-loss spectra and the emergence of a charge carrier plasmon with an unscreened plasmon frequency of 2.78 eV. The plasmon has a positive, quadratic dispersion and appears to be superimposed with an excitation arising from interband transitions. The behavior of the plasmon peak energy positions as a function of potassium concentration shows that potassium stoichiometries of less than $\ensuremath{\sim}0.3$ are thermodynamically unstable while higher stoichiometries up to $\ensuremath{\sim}0.5$ are thermodynamically stable. Potassium concentrations greater than $\ensuremath{\sim}0.5$ lead to the decomposition of ${\mathrm{MoS}}₂$ and the formation of ${\mathrm{K}}₂\mathrm{S}$. The real part of the dielectric function and the optical conductivity of ${\mathrm{K}}_0.41{\mathrm{MoS}}₂$ were derived from the loss spectra via Kramers-Kronig analysis.