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 jumpsreaching up to 100 nmfollowed
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.
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