Vertical stacking of two-dimensional (2D) homo- and heterostructures are intriguing research objects, as they are essential for fundamental studies and a key towards 2D device applications. It is paramount to understand the interlayer coupling in 2D materials and to find a fast yet precise characteristic signature. In this work, we report on a Raman fingerprint of interlayer coupling in 2D transition metal dichalcogenides (TMDCs). We observed that the out-of-plane $B_{2g}$ vibrational mode is absent when two monolayers form a vertical stack yet remain uncoupled but emerges after strong coupling. Using systematic Raman, photoluminescence (PL), and atomic force microscopy (AFM) studies of WSe${}_2$/WSe${}_2$ homo-bilayers and MoSe${}_2$/WSe${}_2$ hetero-bilayers, we conclude that the $B_{2g}$ vibrational mode is a distinct Raman fingerprint of interlayer coupling in 2D TMDCs. Our results propose an easy, fast, precise, and reliable measure to evaluate the interlayer coupling in 2D TMDCs.

We report large exciton energy tuning (∼62 meV) in WSe₂monolayersviasubstrate induced non-degenerate electron doping.

Twisted 2D bilayer materials are created by artificial stacking of two monolayer crystal networks of 2D materials with a desired twisting angle θ. The material forms a moiré superlattice due to the periodicity of both top and bottom layer crystal structure. The optical properties are modified by lattice reconstruction and phonon renormalization, which makes optical spectroscopy an ideal characterization tool to study novel physics phenomena. Here, we report a Raman investigation on a full period of the twisted bilayer (tB) WSe₂ moiré superlattice (i.e. 0 ° ⩽ θ ⩽ 60^∘). We observe that the intensity ratio of two Raman peaks, B 2 g and E 2 g / A 1 g correlates with the evolution of the moiré period. The Raman intensity ratio as a function of twisting angle follows an exponential profile matching the moiré period with two local maxima at 0^∘ and 60^∘ and a minimum at 30^∘. Using a series of temperature-dependent Raman and photoluminescence measurements as well as ab initio calculations, the intensity ratio I B 2 g / I E 2 g / A 1 g is explained as a signature of lattice dynamics in tB WSe₂ moiré superlattices. By further exploring different material combinations of twisted hetero-bilayers, the results are extended for all kinds of Mo- and W-based transition metal dichalcogenides.

We present and demonstrate a novel method for engineering the radial-variant polarization on the incident field to achieve a needle of transversally polarized field without any pupil filters. We generate a new kind of localized linearly-polarized vector fields with distributions of states of polarization (SoPs) describing by the radius to the power p and explore its tight focusing, nonparaxial focusing, and paraxial focusing properties. By tuning the power p, we obtain the needle-like focal field with hybrid SoPs and give the formula for describing the length of the needle. Experimentally, we systematically investigate both the intensity distributions and the polarization evolution of the optical needle by paraxial focusing the generated vector field. Such an optical needle, which enhances the light-matter interaction, has intriguing applications in optical microma-chining and nonlinear optics.

Flexible temperature sensors are becoming increasingly important these days. In this work, we explore graphene oxide (GO)/poly(vinyl alcohol) (PVA) nanocomposites for potential application in temperature sensors. The influence of the mixing ratio of both materials, the reduction temperature, and passivation on the sensing performance has been investigated. Various spectroscopic techniques revealed the composite structure and atomic composition. These were complemented by semiempirical quantum chemical calculations to investigate rGO and PVA interaction. Scanning electron and atomic force microscopy measurements were carried out to evaluate dispersion and coated film quality. The temperature sensitivity has been evaluated for several composite materials with different compositions in the range from 10 to 80 °C. The results show that a linear temperature behavior can be realized based on rGO/PVA composites with temperature coefficients of resistance (TCR) larger than 1.8% K^-1 and a fast response time of 0.3 s with minimal hysteresis. Furthermore, humidity influence has been investigated in the range from 10% to 80%, and a minor effect is shown. Therefore, we can conclude that rGO/PVA composites have a high potential for excellent passivation-free, humidity-independent, sensitive, and fast response temperature sensors for various applications. The GO reduction is tunable, and PVA improves the rGO/PVA sensor performance by increasing the tunneling effect and band gap energy, consequently improving temperature sensitivity. Additionally, PVA exhibits minimal water absorption, reducing the humidity sensitivity. rGO/PVA maintains its temperature sensitivity during and after several mechanical deformations.

The electromagnetic field confinement and amplification typical of nano-sized metallic objects supporting localized surface plasmon resonances, i.e., light-induced collective electronic oscillations, can significantly strengthen the interaction of light with atomically thin transition metal dichalcogenides. In view of the realization of plasmon-enhanced devices, it is crucial to investigate the effects induced by light confinement within metallic nanostructures on the excitonic properties of these materials at the nanoscale. Here, we exploit tip-enhanced photoluminescence spectroscopy to locally control the excitons of monolayer molybdenum disulfide (MoS₂) coupled with gold nanotriangles in the quantum tunneling regime. The spatial resolution of 10 nm in the tip-enhanced photoluminescence measurements made it possible to image the light-emission related properties of monolayer MoS₂ across one single metallic nanostructure and to investigate the effect of the plasmonic enhancement on its photoluminescence peak. Moreover, by taking advantage of the degree of freedom given by the tuning of the tip-sample distance; it was possible to probe the effect of the plasmonic pico-cavity size on the photoluminescence quenching rate of monolayer MoS₂.