Miniaturizing spectrometers: Spectroscopy is a ubiquitous characterization tool spanning most scientific and many industrial disciplines. Most handheld spectrometers are based on tabletop optical components, which limits the scale to which these spectrometers can be shrunk. To address the desire for miniaturized spectrometers with a micrometer-scale (and smaller) footprint, Yang et al. developed such a microspectrometer based on single, compositionally engineered nanowire. This result is a practical step forward for the use of other light-sensitive nanomaterials for such ultra-miniaturized spectroscopy platforms. Science , this issue p. 1017

Van der Waals materials possess an innate layer degree of freedom and thus are excellent candidates for exploring emergent two-dimensional ferroelectricity induced by interlayer translation. However, despite being theoretically predicted, experimental realization of this type of ferroelectricity is scarce at the current stage. Here, we demonstrate robust sliding ferroelectricity in semiconducting $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$ multilayers via a combined study of theory and experiment. Room-temperature vertical ferroelectricity is observed in two-dimensional $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$ with layer number $N\ge 2$. The electric polarization stems from the uncompensated charge transfer between layers and can be switched by interlayer sliding. For bilayer $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$, the ferroelectric transition temperature is estimated to be $\sim 405\mathrm{K}$ from the second harmonic generation measurements. Our results highlight the importance of interlayer engineering in the realization of atomic-scale ferroelectricity.

Monolayer molybdenum disulphide (MoS₂) has attracted much attention, due to its attractive properties, such as two-dimensional properties, direct bandgap, valley-selective circular dichroism and valley Hall effect. However, some of its fundamental physical parameters, e.g. refractive index, have not been studied in detail because of measurement difficulties. In this work, we have synthesized highly crystalline monolayer MoS₂ on SiO₂/Si substrates via chemical vapor deposition (CVD) method and devised a method to measure their optical contrast spectra. Using these contrast spectra, we extracted the complex refractive index spectrum of monolayer MoS₂ in the wavelength range of 400 nm to 750 nm. We have analyzed the pronounced difference between the obtained complex refractive index spectrum and that of bulk MoS₂. The method presented here is effective for two-dimensional materials of small size. Furthermore, we have calculated the color contour plots of the contrast as a function of both SiO₂ thickness and incident light wavelength for monolayer MoS₂ using the obtained refractive index spectrum. These plots are useful for both fundamental study and device application.