High
light absorption (∼15%) and strong photoluminescence
(PL) emission in monolayer (1L) transition metal dichalcogenides (TMDs)
make them ideal candidates for optoelectronic device applications.
Competing interlayer charge transfer (CT) and energy transfer (ET)
processes control the photocarrier relaxation pathways in TMD heterostructures
(HSs). In TMDs, long-distance ET can survive up to several tens of
nm, unlike the CT process. Our experiment shows that an efficient
ET occurs from the 1Ls WSe2-to-MoS2 with an
interlayer hexagonal boron nitride (hBN), due to the resonant overlapping
of the high-lying excitonic states between the two TMDs, resulting
in enhanced HS MoS2 PL emission. This type of unconventional
ET from the lower-to-higher optical bandgap material
is not typical in the TMD HSs. With increasing temperature, the ET
process becomes weaker due to the increased electron–phonon
scattering, destroying the enhanced MoS2 emission. Our
work provides new insight into the long-distance ET process and its
effect on the photocarrier relaxation pathways.
Photoluminescence from bulk HfS2 grown by the chemical vapor transport method is reported. A series of emission lines is apparent at low temperature in the energy range of 1.4–1.5 eV. Two groups of the observed excitonic transitions followed by their replicas involving acoustic and optical phonons are distinguished using classical intensity correlation analysis. The emission is attributed to the recombination of excitons bound to iodine (I2) molecules intercalated between layers of HfS2. The I2 molecules are introduced to the crystal during the growth as halogen transport agents in the growth process. Their presence in the crystal is confirmed by secondary ion mass spectroscopy.
It is known that Sb 2 Se 3 does not exhibit topological insulator behavior due to its orthorhombic structure. The introduction of a small amount of bismuth and tellurium may change its structure to hexagonal, leading to a stable topological insulator compound. We report here the synthesis and the structural, chemical, and electronic properties of the topological insulator BiSbSe 2.5 Te 0.5 . Combining X-ray and electron diffraction measurements, we demonstrate the formation of this stable quaternary hexagonal single crystal. We used X-ray photoelectron spectroscopy to determine quantitatively the exact chemical composition of the sample. The topological insulating behavior is similar to that of other bismuth chalcogenides, as probed by angle-resolved photoemission spectroscopy. A p-type doping, leading to a 0.15 eV shift of the Fermi level was found. This value compensates the intrinsically n-type doping produced by selenium vacancies. We also found a smaller effective mass and a higher electron group velocity for the electrons in the topological states compared with Bi 2 Se 3 .
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