Abstract:We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS 2 /WSe 2 and MoSe 2 /MoS 2 heterobilayers as well as twisted MoS 2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30 ~ 40 cm -1 ) that arise from the layerbreathing mode (LBM) vibrations between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and atomically clean interface. Its frequency also evolves systematically with the relative orientation between of the two layers. Our research demonstrates that LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.2

Carbon nanotubes provide a rare access point into the plasmon physics of one-dimensional electronic systems. By assembling purified nanotubes into uniformly sized arrays, we show that they support coherent plasmon resonances, that these plasmons enhance and hybridize with phonons, and that the phonon-plasmon resonances have quality factors as high as 10. Because coherent nanotube plasmonics can strengthen light-matter interactions, it provides a compelling platform for surface-enhanced infrared spectroscopy and tunable, high-performance optical devices at the nanometer scale. Main textPlasmons in carbon nanotubes [1][2][3][4][5] comprise longitudinal charge oscillations coupled to infrared or terahertz optical fields. They can either propagate [3][4][5] or be confined to FabryPérot resonators by reflections at the nanotube ends [6][7][8][9][10] (Fig. 1(a)). Propagation losses are low [3], and the resonant frequencies and absorption coefficients can be controlled via the length [8,10] and doping level [7,9] of the nanotubes. The intense concentration of electromagnetic fields deriving from the nanotubes' one dimensionality allows the plasmons both to confine light to the nanometer scale and to enhance light-matter interactions by Purcell factors that are predicted to be as high as 10 6 [5].At infrared frequencies, nanotube plasmonics could lead to highly sensitive absorption spectroscopy through surface enhanced infrared absorption (SEIRA) [11][12][13]. At terahertz frequencies, it could enable tunable lasers and receivers for use in terabit-per-second wireless communications [14][15][16]. Ultra-broadband nanotube plasmonic circuitry could be naturally integrated with high-performance nanotube transistors.However, nanotube plasmonics has been frustrated by the material quality of nanotube films. In inhomogeneous nanotube films, with a broad distribution of lengths, diameters and/or doping levels, plasmons resonating at different frequencies quickly lose phase coherence with each other, leading to fast dissipation. The quality (Q) factor, which is the quotient of the resonant angular frequency and the dissipation rate, is therefore low. To date, this inhomogeneity has limited observations of nanotube plasmon resonators to the incoherent Q ≪ 1 regime [6][7][8][9][10]. Because dissipation constrains nearly all applications of plasmonics, the demonstration of high Q * Contact: alfalk@us.ibm.com 2 resonators would provide crucial evidence that nanotubes are a technologically viable plasmonic material.In this work, we show that coherent nanotube plasmon and phonon-plasmon resonances can have ensemble Q factors as high as 10. The key to our demonstration is our exceptionally uniform nanotube films, which we develop using Langmuir-Schaeffer techniques [17]. We conservatively estimate that our nanotube resonators confine an electromagnetic field whose free-space wavelength (λ0) is 8 µm to a mode volume (V) of 0.002 µm 3 . With this combination of Q and optical concentration (λ / = 300,000), the Purcell factor by w...

Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) → partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferromagnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2via O2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the Ids-Vds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.