A two-dimensional honeycomb lattice harbours a pair of inequivalent valleys in the k-space electronic structure, in the vicinities of the vertices of a hexagonal Brillouin zone, K±. It is particularly appealing to exploit this emergent degree of freedom of charge carriers, in what is termed 'valleytronics'. The physics of valleys mimics that of spin, and will make possible devices, analogous to spintronics, such as valley filter and valve, and optoelectronic Hall devices, all very promising for next-generation electronics. The key challenge lies with achieving valley polarization, of which a convincing demonstration in a two-dimensional honeycomb structure remains evasive. Here we show, using first principles calculations, that monolayer molybdenum disulphide is an ideal material for valleytronics, for which valley polarization is achievable via valley-selective circular dichroism arising from its unique symmetry. We also provide experimental evidence by measuring the circularly polarized photoluminescence on monolayer molybdenum disulphide, which shows up to 50% polarization.

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, non-centrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys and close to 90% polarized emission at 4K is observed with 40% polarization remaining at 300K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9T indicating robust, selective valley excitation.

Raman spectroscopy is the prime nondestructive characterization tool for graphene and related layered materials. The shear (C) and layer breathing modes (LBMs) are due to relative motions of the planes, either perpendicular or parallel to their normal. This allows one to directly probe the interlayer interactions in multilayer samples. Graphene and other two-dimensional (2d) crystals can be combined to form various hybrids and heterostructures, creating materials on demand with properties determined by the interlayer interaction. This is the case even for a single material, where multilayer stacks with different relative orientations have different optical and electronic properties. In twisted multilayer graphene there is a significant enhancement of the C modes due to resonance with new optically allowed electronic transitions, determined by the relative orientation of the layers. Here we show that this applies also to the LBMs, which can be now directly measured at room temperature. We find that twisting has a small effect on LBMs, quite different from the case of the C modes. This implies that the periodicity mismatch between two twisted layers mostly affects shear interactions. Our work shows that ultralow-frequency Raman spectroscopy is an ideal tool to uncover the interface coupling of 2d hybrids and heterostructures.

Graphene and other two-dimensional crystals can be combined to form various hybrids and heterostructures, creating materials on demand with properties determined by the interlayer interaction. This is the case even for a single material, where multilayer stacks with different relative orientation have different optical and electronic properties. Probing and understanding the interface coupling is thus of primary importance for fundamental science and applications. Here we study twisted multilayer graphene flakes with multi-wavelength Raman spectroscopy. We find a significant intensity enhancement of the interlayer coupling modes (C peaks) due to resonance with new optically allowed electronic transitions, determined by the relative orientation of the layers. The interlayer coupling results in a Davydov splitting of the C peak in systems consisting of two equivalent graphene multilayers. This allows us to directly quantify the interlayer interaction, which is much smaller compared with Bernalstacked interfaces. This paves the way to the use of Raman spectroscopy to uncover the interface coupling of two-dimensional hybrids and heterostructures.

The quantum confinement in atomic scale and the presence of interlayer coupling in multilayer make the electronic and optical properties of 2D materials (2DMs) be dependent on the layer number (N) from monolayer to multilayer. Optical properties of 2DMs have been widely probed by several optical techniques, such as optical contrast, Rayleigh scattering, Raman spectroscopy, optical absorption, photoluminescence, and second harmonic generation. Here, it is reviewed how optical properties of several typical 2DMs (e.g., monolayer and multilayer graphenes, transition metal dichalcogenides) probed by these optical techniques significantly depend on N. Further, it has been demonstrated how these optical techniques service as fast and non destructive approaches for N counting or thickness determination of these typical 2DM flakes. The corresponding approaches can be extended to the whole 2DM family produced by micromechanical exfoliations, chemical-vapor-deposition growth, or transfer processes on various substrates, which bridges the gap between the characterization and international standardization for thickness determination of 2DM flakes. Figure 3. a) The experimental OC(λ) values of a 10LG flake deposited on SiO 2 /Si (h SiO 2 = 89 nm) using objectives with NA = 0.25, NA = 0.45, and NA = 0.90, respectively. The experimental OC(λ) and the calculated results of 2LG, 3LG, and 4LG for b) hSiO 2 = 89 nm and c) hSiO 2 = 286 nm (c), NA = 0.45. wileyonlinelibrary.com

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Color manipulation of intense multiluminescence from CaZnOS:Mn2+ has been realized by adjusting Mn2+ concentration. Not only the photoluminescence (PL) of Mn2+ emission from 4T1(4G) to 6A1(6S) shows a red shift from yellow to red with increasing Mn2+ concentration, which is in contrast to the fixed PL emission reported by Hintzen et al. (Chem. Mater., 2009), but also mechanoluminescence (ML) and cathodoluminescence (CL) have a similar variation. More attractively, the brightness of multiluminescence is surprisingly intense for all the CaZnOS:Mn2+ with a large-scale Mn2+ doping (0.1–10 mol %). Based on the investigation of crystal field, various spectral results, and PL lifetimes, the red-shift mechanism of multiluminescence reported here has been proposed to arise from the exchange interaction effect of Mn2+ pairs at higher concentrations. In addition to correcting the previous misunderstanding on the emission of CaZnOS:Mn2+, these findings extend the tunable emission window, opening up new opportunities in multifunctional applications of PL, ML, and CL involving multicolor light sources, displays, and stress imaging, especially providing a novel resolution to design ML colors.

An SiO2/Si substrate has been widely used to support two-dimensional (2d) flakes grown by chemical vapor deposition or prepared by micromechanical cleavage. The Raman intensity of the vibration modes of 2d flakes is used to identify the layer number of 2d flakes on the SiO2/Si substrate, however, such an intensity is usually dependent on the flake quality, crystal orientation and laser polarization. Here, we used graphene flakes, a prototype system, to demonstrate how to use the intensity ratio between the Si peak from SiO2/Si substrates underneath graphene flakes and that from bare SiO2/Si substrates for the layer-number identification of graphene flakes up to 100 layers. This technique is robust, fast and nondestructive against sample orientation, laser excitation and the presence of defects in the graphene layers. The effect of relevant experimental parameters on the layer-number identification was discussed in detail, such as the thickness of the SiO2 layer, laser excitation wavelength and numerical aperture of the used objective. This paves the way to use Raman signals from dielectric substrates for layer-number identification of ultrathin flakes of various 2d materials.