We have investigated the effect of potassium (K) intercalation on $2H$-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, MoS$_2$ exhibits a $2a times 2a$ superstructure in the $ab$ plane and unit cell parameters of a = 3.20 $unicode{x212B}$ and c = 8.23 $unicode{x212B}$ indicating a conversion from the $2H$ to the $1T$ or $1T$ polytypes. The diffraction patterns also show a $sqrt{3}a times sqrt{3}a$ and a much weaker $2sqrt{3}a times 2sqrt{3}a$ 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 $sim 0.3$ are thermodynamically unstable while higher stoichiometries up to $sim 0.5$ are thermodynamically stable. Potassium concentrations greater than $sim 0.5$ lead to the decomposition of MoS$_2$ and the formation of K$_2$S. The real part of the dielectric function and the optical conductivity of K$_{0.41}$MoS$_2$ were derived from the loss spectra via Kramers-Kronig analysis.
By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in $1T$-HfS$_2$. The results differ significantly from the properties of the more scrut
inized group VI semiconducting transition metal dichalcogenides such as MoS$_2$ and WSe$_2$. The measurements revealed a parabolic exciton dispersion for finite momentum $textbf{q}$ parallel to the $Gamma$K direction which allowed the determination of the effective exciton mass. The dispersion decreases monotonically for momentum exchanges parallel to the $Gamma$M high symmetry line. To gain further insight into the excitation mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The results matched the experimental loss spectra closely, thereby confirming the excitonic nature of the observed transitions, and produced the momentumdependent binding energies. The simulations also demonstrated that the excitonic transitions for $textbf{q}$ || $Gamma$M occur exactly along that particular high symmetry line. For $textbf{q}$ || $Gamma$K on the other hand, the excitations traverse the Brillouin zone crossing various high symmetry lines. A particular interesting aspect of our findings was that the calculation of the electron probability density revealed that the exciton assumes a six-pointed star-like shape along the real space crystal planes indicating a mixed Frenkel-Wannier character.
We have investigated indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the $Gamma$K and $Gamma$M directions of
the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K$_{mathrm{v}}$ and conduction band $Lambda_{mathrm{c}}$ points, the $Gamma_{mathrm{v}}$ and K$_{mathrm{c}}$ points as well as adjacent K$_{mathrm{v}}$ and K$^{prime}_textrm{c}$ points. The energy-momentum dispersions for the K$_{mathrm{v}}$-$Lambda_{mathrm{c}}$, $Gamma_{mathrm{v}}$-K$_{mathrm{c}}$ and K$_{mathrm{v1}}$-K$^{prime}_textrm{c}$ excitons along the $Gamma$K line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K$_mathrm{v1}$-K$^{prime}_textrm{c}$ transition follows a more linear dispersion relationship.
Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound 2H-tantalum-diselenide. We observe an unususal doping
dependence of the plasmon dispersion, which even changes sign upon alkali addition. In contrast to the continous energy shift of the plasmon position upon doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can only be explained in a picture, where complex phenomena are taken into account including the suppression of the CDW upon doping as well as the interplay of the CDW and the plasma resonance.
We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coh
erently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.
We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the
plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as well as the related system 2H-NbS2, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations.
