6 research outputs found
Increasing the Rate of Magnesium Intercalation Underneath Epitaxial Graphene on 6H-SiC(0001)
Magnesium intercalated 'quasi-freestanding' bilayer graphene on 6H-SiC(0001)
(Mg-QFSBLG) has many favorable properties (e.g., highly n-type doped,
relatively stable in ambient conditions). However, intercalation of Mg
underneath monolayer graphene is challenging, requiring multiple intercalation
steps. Here, we overcome these challenges and subsequently increase the rate of
Mg intercalation by laser patterning (ablating) the graphene to form
micron-sized discontinuities. We then use low energy electron diffraction to
verify Mg-intercalation and conversion to Mg-QFSBLG, and X-ray photoelectron
spectroscopy to determine the Mg intercalation rate for patterned and
non-patterned samples. By modeling Mg intercalation with the Verhulst equation,
we find that the intercalation rate increase for the patterned sample is
4.51.7. Since the edge length of the patterned sample is 5.2
times that of the non-patterned sample, the model implies that the increased
intercalation rate is proportional to the increase in edge length. Moreover, Mg
intercalation likely begins at graphene discontinuities in pristine samples
(not step edges or flat terraces), where the 2D-like crystal growth of
Mg-silicide proceeds. Our laser patterning technique may enable the rapid
intercalation of other atomic or molecular species, thereby expanding upon the
library of intercalants used to modify the characteristics of graphene, or
other 2D materials and heterostructures.Comment: 24 pages, 4 figure
Freestanding n-Doped Graphene via Intercalation of Calcium and Magnesium into the Buffer Layer - SiC(0001) Interface
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 physical and electronic structure of both Ca- and
Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca
intercalates underneath the buffer layer and bonds to the Si-terminated SiC
surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the
buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene
(Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial
graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC
surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in
Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG).
Ca-intercalation underneath the buffer layer has not been considered in
previous studies of Ca-intercalated epitaxial graphene. Furthermore, 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.Comment: 58 pages, 10 figures, 4 tables. Revised text and figure
Freestanding nDoped Graphene via Intercalation of Calcium and Magnesium into the Buffer LayerSiC(0001) Interface
Exfoliation of Quasi-Stratified Bi<sub>2</sub>S<sub>3</sub> Crystals into Micron-Scale Ultrathin Corrugated Nanosheets
There
is ongoing interest in exploring new two-dimensional materials
and exploiting their functionalities. Here, a top-down approach is
used for developing a new morphology of ultrathin nanosheets from
highly ordered bismuth sulfide crystals. The efficient chemical delamination
method exfoliates the bulk powder into a suspension of corrugated
ultrathin sheets, despite the fact that the Bi<sub>2</sub>S<sub>3</sub> fundamental layers are made of atomically thin ribbons that are
held together by van der Waals forces in two dimensions. Morphological
analyses show that the produced corrugated sheets are as thin as 2.5
nm and can be as large as 20 μm across. Determined atomic ratios
indicate that the exfoliation process introduces sulfur vacancies
into the sheets, with a resulting stoichiometry of Bi<sub>2</sub>S<sub>2.6</sub>. It is hypothesized that the nanoribbons were cross-linked
during the reduction process leading to corrugated sheet formation.
The material is used for preparing field effect devices and was found
to be highly p-doped, which is attributed to the substoichiometry.
These devices show a near-linear response to the elevation of temperature.
The devices demonstrate selective and relatively fast response to
NO<sub>2</sub> gas when tested as gas sensors. This is the first report
showing the possibility of exfoliating planar morphologies of metal
chalcogenide compounds such as orthorhombic Bi<sub>2</sub>S<sub>3</sub>, even if their stratified crystal structures constitute van der
Waals forces within the fundamental planes