We report a systematic study of the etching of MoSs crystals by using XeF2 as a gaseous reactant. By controlling the etching process, monolayer MoS2 with uniform morphology can be obtained. The Raman ...and photoluminescence spectra of the resulting material were similar to those of exfoliated MoS2. Utilizing this strategy, different patterns such as a Hall bar structure and a hexagonal array can be realized. Furthermore, the etching mechanism was studied by introducing graphene as an etching mask. We believe our technique opens an easy and controllable way of etching MoS2, which can be used to fabricate complex nanostructures, such as nanoribbons, quantum dots, and transistor structures. This etching process using XeF2 can also be extended to other interesting two-dimensional crystals.
We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show ...that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photothermoelectric effect. Consistent with the photocurrent study, infrared nanoimaging experiments reveal optical conductivity features dominated by twist-angle-dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of Moiré superlattices in other van der Waals heterostructures.
Natural hyperbolic materials with dielectric permittivities of opposite signs along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the ...diffraction limit. Confined electromagnetic waves coupled to phonons in hyperbolic dielectrics including hexagonal boron nitride (hBN) and α-MoO3 are referred to as hyperbolic phonon polaritons (HPPs). HPP dissipation at ambient conditions is substantial, and its fundamental limits remain unexplored. Here, we exploit cryogenic nanoinfrared imaging to investigate propagating HPPs in isotopically pure hBN and naturally abundant α-MoO3 crystals. Close to liquid-nitrogen temperatures, losses for HPPs in isotopic hBN drop significantly, resulting in propagation lengths in excess of 8 μm, with lifetimes exceeding 5 ps, thereby surpassing prior reports on such highly confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in HPP damping and will be instrumental in mitigating such losses for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.
Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. Here, ...we critically assess an experimental approach using near-field imaging to probe HCP in Bi
Sr
CaCu
O
(Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that inherently weak HCP features in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene/hexagonal boron nitride (hBN)/Bi-2212 heterostructures. This enhancement arises from our multilayered structures effectively acting as plasmonic cavities capable of altering collective modes of a layered superconductor by modifying its electromagnetic environment. The degree of enhancement can be selectively controlled by tuning the insulating spacer thickness with atomic precision. Finally, we verify the expected renormalization of room-temperature graphene SPP using near-field infrared imaging. Our modeling, augmented with data, attests to the validity of our approach for probing HCP modes in cuprate superconductors.
Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as ...revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible Li, H. ; Nano Lett. 2020, 20, 3106−3112 . In this work, we fully characterize two approaches to realizing nano-optics compatible top gates: bilayer MoS2 and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.
Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. In ...this study, we critically assess an experimental approach using near-field imaging to probe HCP in Bi2Sr2CaCu2O8+x (Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that inherently weak HCP features in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene/hexagonal boron nitride (hBN)/Bi-2212 heterostructures. This enhancement arises from our multilayered structures effectively acting as plasmonic cavities capable of altering collective modes of a layered superconductor by modifying its electromagnetic environment. The degree of enhancement can be selectively controlled by tuning the insulating spacer thickness with atomic precision. Finally, we verify the expected renormalization of room-temperature graphene SPP using near-field infrared imaging. Our modeling, augmented with data, attests to the validity of our approach for probing HCP modes in cuprate superconductors.
The variety of correlated phenomena in moiré systems is incredibly rich, spanning effects such as superconductivity, a generalized form of ferromagnetism, or even charge fractionalization. This wide ...range of quantum phenomena is partly enabled by the large number of internal degrees of freedom in these systems, such as the valley and spin degrees of freedom, which interplay decides the precise nature of the ground state. Identifying the microscopic nature of the correlated states in the moiré systems is, however, challenging, as it relies on interpreting transport behavior or scanning-tunneling microscopy measurements. Here we show how the real-space structure of collective charge oscillations of the correlated orders can directly encode information about the structure of the correlated state, focusing in particular on the problem of generalized Wigner crystals in moiré transition metal dichalcogenides. Our analysis builds upon our earlier result 10.1126/sciadv.adg3262 that the presence of a generalized Wigner crystal modifies the plasmon spectrum of the system, giving rise to new collective modes. We focus on scanning near-field optical microscopy technique (SNOM), fundamentally a charge-sensing-based method, and introduce a regime under which SNOM can operate as a probe of the spin degree of freedom.
Using scanning near-field optical microscopy at cryogenic temperatures, we explored the first-order metal-insulator transition of exfoliated 1T−TaS2 microcrystals on a SiO2/Si substrate. We clearly ...observed spatially separated metallic and insulating states during the transition between commensurate and nearly commensurate charge-density-wave phases. The capability to probe electrodynamics on nanometer length scales revealed temperature-dependent electronic properties of the insulating and metallic regions near the transition temperature. At fixed temperature, a remarkably broad spatial boundary between insulating and metallic regions was observed, across which the nano-optical signal smoothly evolved over a length scale of several hundred nanometers. To understand these observations, we performed Ginzburg-Landau calculations to determine the charge-density-wave structure of the domain boundary, which revealed the existence of an intermediate electronic phase with unique properties distinct from the bulk thermodynamic phases.
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