Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userInvestigation of potassium-intercalated bulk MoS$_2$ using transmission electron energy-loss spectroscopy
138
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
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.
We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by 33-36%. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1%. The introduction of potassium causes the formation of charge carrier plasmons. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.
In the quest for dynamic multimodal probing of a materials structure and functionality, it is critical to be able to quantify the chemical state on the atomic and nanoscale using element specific electronic and structurally sensitive tools such as electron energy loss spectroscopy (EELS). Ultrafast EELF, with combined energy, time, and spatial resolution in a transmission electron microscope, has recently enabled transformative studies of photo excited nanostructure evolution and mapping of evanescent electromagnetic fields. This article aims to describe the state of the art experimental techniques in this emerging field and its major uses and future applications.
Transmission electron microscopy, scanning transmission electron tomography, and electron energy loss spectroscopy were used to characterize three-dimensional artificial Si nanostructures called metalattices, focusing on Si metalattices synthesized by high-pressure confined chemical vapor deposition in 30-nm colloidal silica templates with ~7 and ~12 nm meta-atoms and ~2 nm meta-bonds. The meta-atoms closely replicate the shape of the tetrahedral and octahedral interstitial sites of the face-entered cubic colloidal silica template. Composed of either amorphous or nanocrystalline silicon, the metalattice exhibits long-range order and interconnectivity in two-dimensional micrographs and three-dimensional reconstructions. Electron energy loss spectroscopy provides information on local electronic structure. The Si L2,3 core-loss edge is blue-shifted compared to the onset for bulk Si, with the meta-bonds displaying a larger shift (0.55 eV) than the two types of meta-atoms (0.30 and 0.17 eV). Local density of state calculations using an empirical tight binding method are in reasonable agreement.
The low-energy band structure of few-layer MoS$_2$ is relevant for a large variety of experiments ranging from optics to electronic transport. Its characterization remains challenging due to complex multi band behavior. We investigate the conduction band of dual-gated three-layer MoS$_2$ by means of magnetotransport experiments. The total carrier density is tuned by voltages applied between MoS$_2$ and both top and bottom gate electrodes. For asymmetrically biased top and bottom gates, electrons accumulate in the layer closest to the positively biased electrode. In this way, the three-layer MoS$_2$ can be tuned to behave electronically like a monolayer. In contrast, applying a positive voltage on both gates leads to the occupation of all three layers. Our analysis of the Shubnikov--de Haas oscillations originating from different bands lets us attribute the corresponding carrier densities in the top and bottom layers. We find a twofold Landau level degeneracy for each band, suggesting that the minima of the conduction band lie at the $pm K$ points of the first Brillouin zone. This is in contrast to band structure calculations for zero layer asymmetry, which report minima at the $Q$ points. Even though the interlayer tunnel coupling seems to leave the low-energy conduction band unaffected, we observe scattering of electrons between the outermost layers for zero layer asymmetry. The middle layer remains decoupled due to the spin-valley symmetry, which is inverted for neighboring layers. When the bands of the outermost layers are energetically in resonance, interlayer scattering takes place, leading to an enhanced resistance and to magneto-interband oscillations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
