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Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room-temperature photoluminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trapped-carrier-induced Stark shifts. Large spectral jumpsreaching up to 100 nmfollowed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional photochemical changes possibly taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresce on green illumination, suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the crystal defect at play.

Surface-enhanced Raman scattering (SERS) is commonly associated with noble metal substrates. However, over the years modest Raman enhancements (<104) have also been observed in semiconductor substrates. This enhancement stems predominantly from the excitonic resonance of the semiconductors. The use of two-dimensional semiconductors with large excitonic oscillator strength provides an attractive pathway to further enhance this effect. Here we report for the first time a >3 × 105 enhancement in SERS signal from an organic molecule (4-mercaptopyridine) placed in the near field of a two-dimensional semiconductor molybdenum disulfide (MoS2) monolayer. This large enhancement in the SERS signal is attributed to the charge transfer (CT) state formed at the interface of the 2D semiconductor and organic molecule and is found to occur when the excitation source is chosen to be in resonance with the CT state. This approach provides a new strategy for carrying out SERS experiments on molecules with very weak Raman signatures without the need for nanopatterning.

The low quantum yield observed in two-dimensional semiconductors of transition metal dichalcogenides (TMDs) has motivated the quest for approaches that can enhance the light emission from these systems. Here, we demonstrate broadband enhancement of spontaneous emission and increase in Raman signature from archetype two-dimensional semiconductors: molybdenum disulfide (MoS2) and tungsten disulfide (WS2) by placing the monolayers in the near field of a photonic hypercrystal having hyperbolic dispersion. Hypercrystals are characterized by a large broadband photonic density of states due to hyperbolic dispersion while having enhanced light in/out coupling by a subwavelength photonic crystal lattice. This dual advantage is exploited here to enhance the light emission from the 2D TMDs and can be utilized for developing light emitters and solar cells using two-dimensional semiconductors.

Interaction of a trapped charge with a point defectLet us consider a sheet of h-BN of thickness ݀ (see Fig. S1a); h-BN is a strongly anisotropic material with static dielectric constants ߝ // = 5.0 and ߝ ୄ = 7.0, respectively parallel and perpendicular to the sheet plane. For the sake of simplicity, let us take the value averaged over the three spatial directions: ߝ = 5.7. Let us place a point charge ‫ݍ‬ in the middle of the plane and let us calculate the electric field in the same mid-plane a distance ‫ݎ‬ away from the point charge. Due to symmetry, the electric field is radially symmetric. It can be found using the method of images, and it is given by (in atomic units) ‫ܨ‬ሺ‫ݎ‬ሻ = ‫ݍ‬ ‫ݎߝ‬ ଶ ൝1 + 2 ߛ ሾ1 + ሺ݀ ‫ݎ‬

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