Few-layer black phosphorous (BP) has emerged as a promising candidate for next-generation nanophotonic and nanoelectronic devices. However, rapid ambient degradation of mechanically exfoliated BP poses challenges in its practical deployment in scalable devices. To date, the strategies employed to protect BP have relied upon preventing its exposure to atmospheric conditions. Here, an approach that allows this sensitive material to remain stable without requiring its isolation from the ambient environment is reported. The method draws inspiration from the unique ability of biological systems to avoid photo-oxidative damage caused by reactive oxygen species. Since BP undergoes similar photo-oxidative degradation, imidazolium-based ionic liquids are employed as quenchers of these damaging species on the BP surface. This chemical sequestration strategy allows BP to remain stable for over 13 weeks, while retaining its key electronic characteristics. This study opens opportunities to practically implement BP and other environmentally sensitive 2D materials for electronic applications.
We
have studied the ambient air oxidation of chemical vapor deposition
(CVD) grown monolayers of the semiconducting transition metal dichalcogenide
(S-TMD) WS2 using optical microscopy, laser scanning confocal
microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic
force microscopy (AFM). Monolayer WS2 exposed to ambient
conditions in the presence of light (typical laboratory ambient light
for weeks or typical PL spectroscopy map) exhibits damage due to oxidation
which can be detected with the LSCM and AFM, though may not be evident
in conventional optical microscopy due to poorer contrast and resolution.
Additionally, this oxidation was not random and was correlated with
“high-symmetry” high intensity edges and red-shifted
areas in the PL spectroscopy map, areas thought to contain a higher
concentration of sulfur vacancies. In contrast, samples kept in ambient
and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence
experiments showed that samples subjected to excitation energies at
or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88
eV) oxidized in as little as 7 days, even for irradiances and fluences
8 and 4 orders of magnitude lower (respectively) than previously reported.
No significant oxidation was observed for 760 nm/1.63 eV light exposure,
which lies below the trion excitation energy in WS2. The
strong wavelength dependence and apparent lack of irradiance dependence
suggests that ambient oxidation of WS2 is initiated by
photon-mediated electronic band transitions, that is, photo-oxidation.
These findings have important implications for prior, present, and
future studies concerning S-TMDs measured, stored, or manipulated
in ambient conditions.
The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, X-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the structure of both Ca-and Mg-intercalated epitaxial graphene on SiC(0001). We find that in contrast to previous studies, Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C-Si bonds and 'freestanding' the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). This situation is similar with the Mg-intercalation of epitaxial graphene on SiC(0001), in which an ordered Mg-terminated reconstruction at the SiC surface is formed, resulting in Mgintercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). We find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable.
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