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When doped into a certain range of charge carrier concentrations, MoS2 departs from its pristine semiconducting character to become a strongly correlated material characterized by exotic phenomena such as charge density waves or superconductivity. However, the required doping levels are typically achieved using ionic‐liquid gating or air‐sensitive alkali‐ion intercalation, which are not compatible with standard device fabrication processes. Here, the emergence of superconductivity and a charge density wave phase in air‐stable organic cation intercalated MoS2 crystals are reported. By selecting two different molecular guests, it is shown that these correlated electronic phases depend dramatically on the intercalated cation, demonstrating the potential of organic ion intercalation to finely tune the properties of 2D materials. Moreover, it is found that a fully developed zero‐resistance state is not reached in few‐nm‐thick flakes, indicating the presence of 3D superconductive paths that are severed by the mechanical exfoliation. This behavior is ascribed to an inhomogeneous charge carrier distribution, which is probed at the nanoscale using scanning near‐field optical microscopy. The results establish organic‐ion intercalated MoS2 as a platform to study the emergence and modulation of correlated electronic phases. [ABSTRACT FROM AUTHOR]

Here, we propose a method to determine the thickness of the most common transition metal dichalcogenides (TMDCs) placed on the surface of transparent stamps, used for the deterministic placement of two-dimensional materials, by analyzing the red, green and blue channels of transmission-mode optical microscopy images of the samples. In particular, the blue channel transmittance shows a large and monotonic thickness dependence, making it a very convenient probe of the flake thickness. The method proves to be robust given the small flake-to-flake variation and the insensitivity to doping changes of MoS2. We also tested the method for MoSe2, WS2 and WSe2. These results provide a reference guide to identify the number of layers of this family of materials on transparent substrates only using optical microscopy. [ABSTRACT FROM AUTHOR]

When doped into a certain range of charge carrier concentrations, MoS2 departs from its pristine semiconducting character to become a strongly correlated material characterized by exotic phenomena such as charge density waves or superconductivity. However, the required doping levels are typically achieved using ionic‐liquid gating or air‐sensitive alkali‐ion intercalation, which are not compatible with standard device fabrication processes. Here, the emergence of superconductivity and a charge density wave phase in air‐stable organic cation intercalated MoS2 crystals are reported. By selecting two different molecular guests, it is shown that these correlated electronic phases depend dramatically on the intercalated cation, demonstrating the potential of organic ion intercalation to finely tune the properties of 2D materials. Moreover, it is found that a fully developed zero‐resistance state is not reached in few‐nm‐thick flakes, indicating the presence of 3D superconductive paths that are severed by the mechanical exfoliation. This behavior is ascribed to an inhomogeneous charge carrier distribution, which is probed at the nanoscale using scanning near‐field optical microscopy. The results establish organic‐ion intercalated MoS2 as a platform to study the emergence and modulation of correlated electronic phases. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is a powerful analytical method widely used in many fields of science and applications. However, one of the inherent issues of this method is a low signal-to-noise ratio for ultrathin and two-dimensional (2D) materials. To overcome this problem, techniques like surface-enhanced Raman spectroscopy (SERS) that rely on nanometer scale metallic particles are commonly employed. Here, we demonstrate a different approach that is based on a microcavity structure consisting of a hexagonal boron nitride (h-BN) membrane spanning over an air-filled trench in germanium. In this structure, the h-BN membrane is an integral part of the cavity and, at the same time, shows an about 10-fold, polarisation-dependent h-BN Raman signal enhancement. With h-BN being transparent, flat, and chemically robust, it provides an excellent interface between the cavity and adjacent materials. We show that the Raman enhancement is also present for graphene layers transferred on top of the h-BN membrane, which proves that our approach can be extended to van der Waals heterostructures. The observed polarisation and position-dependent enhancements are in very good agreement with numerical simulations of the electric field intensity of the cavity. These results, together with the presented facile h-BN membrane fabrication process, which does not require any lithographic methods, open up new possibilities for enhancing Raman signals of 2D crystals without the need for metal particles.

We report the development and characterization of a detection technique for scattering-type scanning near-field optical microscopy (s-SNOM) that enables near-field amplitude and phase imaging at two or more wavelengths simultaneously. To this end, we introduce multispectral pseudoheterodyne (PSH) interferometry, where infrared lasers are combined to form a beam with a discrete spectrum of laser lines and a time-multiplexing scheme is employed to allow for the use of a single infrared detector. We first describe and validate the implementation of multispectral PSH into a commercial s-SNOM instrument. We then demonstrate its application for the real-time correction of the negative phase contrast (NPC), which provides reliable imaging of weak IR absorption at the nanoscale. We anticipate that multispectral PSH could improve data throughput, reduce effects of sample and interferometer drift, and help to establish multicolor s-SNOM imaging as a regular imaging modality, which could be particularly interesting as new infrared light sources become available.

Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale optical mapping of manifold material properties. It is based on interferometric recording of the light scattered at a scanning probe tip. For dielectric samples such as biological materials or polymers, the near-field amplitude and phase signals of the scattered field reveal the local reflectivity and absorption, respectively. Importantly, absorption in s-SNOM imaging corresponds to a positive phase contrast relative to a non-absorbing reference sample. Here, we describe that in certain conditions (weakly or non- absorbing material placed on a highly reflective substrate), a slight negative phase contrast may be observed, which can hinder the recognition of materials exhibiting a weak infrared absorption. We first document this effect and explore its origin using representative test samples. We then demonstrate straightforward simple correction methods that remove the negative phase contrast and that allow for the identification of weak absorption contrasts.

Atomically thin semiconducting transition metal dichalcogenides (TMDCs) have unique mechanical and optical properties. They are extremely flexible and exhibit a strong optical absorption at their excitonic resonances. Excitons in TMDC monolayers are strongly influenced by mechanical strain. Their energy shifts and even their line widths change. In bilayers, intralayer excitons with electrons and holes residing in the same layer also shift their energy with the applied strain. Recently, interlayer excitons with electrons and holes in different layers have been observed in bilayer MoS 2 at room temperature. Here, we report on the behavior of interlayer excitons in bilayer MoS 2 under uniaxial tensile strain of up to 1.6%. By recording the differential transmission spectra for different strain values, we derive a gauge factor of -47 meV per % for the energy shift of the interlayer exciton, which is similar to -49 meV per % for the intralayer A and B excitons. Our finding confirms the origin of the interlayer exciton at the K point in the Brillouin zone, with the electron located in one layer and the hole delocalized over two layers. Furthermore, our work paves the way for future straintronic devices based on interlayer excitons.