Investigation of potassium-intercalated bulk MoS2 using transmission electron energy-loss spectroscopy
Abstract
We have investigated the effect of potassium (K) intercalation on 2H-MoS2 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, MoS2 exhibits a 2a × 2a superstructure in the ab plane and unit cell parameters of a = 3.20 x212B and c = 8.23 x212B indicating a conversion from the 2H to the 1T' or 1T'' polytypes. The diffraction patterns also show a 3a × 3a and a much weaker 23a × 23a 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 0.3 are thermodynamically unstable while higher stoichiometries up to 0.5 are thermodynamically stable. Potassium concentrations greater than 0.5 lead to the decomposition of MoS2 and the formation of K2S. The real part of the dielectric function and the optical conductivity of K0.41MoS2 were derived from the loss spectra via Kramers-Kronig analysis.