Charge TransferTransition metal dichalcogenides (TMDCs) belong to the class of various 2D materials which in bulk form are composed of strongly bonded layers with week van der Waals force among them, [1][2][3] allowing exfoliation into atomically thin layers whose properties can be distinct from their bulk counterparts. [1,4,5] TMDCs show a wide range of electronic, optical, mechanical, chemical, and thermal properties which have been studied for decades. [4] Intercalation of TMDCs is one important field in the chemistry of inclusion compounds. With guest materials such as alkali metals intercalated in the host ones, the structure Figure 4. Schematic illustration of the phase transition. a) structure of 2H-Na 0.5 MoS 2 and valence state of Mo. b) Schematic showing charge transfer between Mo 3+ and Mo 4+ and the charge transfer induced phase transition from 2H to 1T. www.advancedsciencenews.com
Among 2D/layered semiconductors,
group IV monochalcogenides such
as SnS(e) and GeS(e) have attracted attention as phosphorene/black
phosphorus analogues with anisotropic structures and predicted unusual
properties. In contrast to SnS, for which bottom-up synthesis has
been reported, few-layer GeS has been realized primarily via exfoliation from bulk crystals. Here, we report the synthesis of
large (up to >20 μm), faceted single crystalline GeS flakes
with anisotropic properties using a vapor transport process. In situ electron microscopy is used to identify the thermal
stability and sublimation pathways, and demonstrates that the GeS
flakes are self-encapsulated in a thin, sulfur-rich amorphous GeS
x
shell during growth. The shell provides
exceptional chemical stability to the layered GeS core. In contrast
to exfoliated GeS, which rapidly degrades during exposure to air,
the synthesized GeS–GeS
x
core–shell
structures show no signs of chemical attack and remain unchanged in
air for extended time periods. Measurements of the optoelectronic
properties by photoluminescence spectroscopy show a tunable bandgap
due to out-of-plane quantum confinement in flakes with thickness below
100 nm. Cathodoluminescence (CL) spectroscopy with nanoscale excitation
provides evidence for interfacial charge transfer due to a type II
heterojunction between the crystalline core and amorphous shell. Measurements
by locally excited CL yield a minority carrier (electron) diffusion
length in the p-type GeS core
=
0.27 μm, on par with diffusion
lengths in the highest-quality layered chalcogenide semiconductors.
In situ TEM studies unravel a mechanistically distinct two-stage discharge mechanism for graphite fluoride (CFx) nanosheets upon electrochemical lithiation.
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