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Register a new userIdentification of 2H and 3R polytypes of MoS2 layered crystals using photoluminescence spectroscopy
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The excitonic radiative recombination of intercalated Cl2 molecules for two different polytypes 2H-MoS2 and 3R-MoS2 layered crystals are presented. The structure of the excitonic emission is unique and provides a robust experimental signature of crystal polytype investigated. This result is confirmed by X-ray diffraction analysis and DFT electronic band structure calculations. Thus, the bound exciton emission provides a nondestructive fingerprint for the reliable identification of the polytype of MoS2 layered crystals.
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We investigate the electronic structure of the 2H and 3R polytypes of NbS$_2$. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb$_{1+x}$S$_2$ compared to 2H-NbS$_2$, which we attribute to the presence of additional interstitial Nb which act as electron donors. Thus we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2H phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb 4$d_{x^2-y^2,xy}$ character than for the Nb 4$d_{3z^2-r^2}$. Our results provide an experimental framework for interpreting the two-gap superconductivity and latent charge density wave in 2H-NbS$_2$.
We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is instead originating from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate is decreasing the indirect band gap energy by up to ~ 80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.
Transition metal dichalcogenide (TMD) materials have received enormous attention due to their extraodinary optical and electrical properties, among which MoS2 is the most typical one. As thickness increases from monolayer to multilayer, the photoluminescence (PL) of MoS2 is gradually quenched due to the direct-to-indirect band gap transition. How to enhance PL response and decrease the layer dependence in multilayer MoS2 is still a challenging task. In this work, we report, for the first time, simultaneous generation of three PL peaks at around 1.3, 1.4 and 1.8 eV on multilayer MoS2 bubbles. The temperature dependent PL measurements indicate that the two peaks at 1.3 and 1.4 eV are phonon-assisted indirect-gap transitions while the peak at 1.8 eV is the direct-gap transition. Using first-principles calculations, the band structure evolution of multilayer MoS2 under strain is studied, from which the origin of the three PL peaks of MoS2 bubbles is further confirmed. Moreover, PL standing waves are observed in MoS2 bubbles that creates Newton-Ring-like patterns. This work demonstrates that the bubble structure may provide new opportunities for engineering the electronic structure and optical properties of layered materials.
Atomically thin two-dimensional molybdenum disulfide (MoS2) sheets have attracted much attention due to their potential for future electronic applications. They not only present the best planar electrostatic control in a device, but also lend themselves readily for dielectric engineering. In this work, we experimentally investigated the dielectric effect on the Raman and photoluminescence (PL) spectra of monolayer MoS2 by comparing samples with and without HfO2 on top by atomic layer deposition (ALD). Based on considerations of the thermal, doping, strain and dielectric screening influences, it is found that the red shift in the Raman spectrum largely stems from modulation doping of MoS2 by the ALD HfO2, and the red shift in the PL spectrum is most likely due to strain imparted on MoS2 by HfO2. Our work also suggests that due to the intricate dependence of band structure of monolayer MoS2 on strain, one must be cautious to interpret its Raman and PL spectroscopy.
We present first-principles calculations of elastic properties of multilayered two-dimensional crystals such as graphene, h-BN and 2H-MoS2 which shows that their Poissons ratios along out-of-plane direction are negative, near zero and positive, respectively, spanning all possibilities for sign of the ratios. While the in-plane Poissons ratios are all positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as layer stacking structures are shown to determine the sign of their out-of-plane ratios. Thorough investigation of elastic properties as a function of the number of layers for each system is also provided, highlighting their intertwined nature between elastic and electronic properties.
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