1. No category

Nano Research
Nano Res
DOI 10.1007/s12274-014-0639-0
Stability of BN/metal interfaces in gaseous atmosphere
Yang Yang1, Qiang Fu1(), Mingming Wei1, Hendrik Bluhm2, Xinhe Bao1()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0639-0
http://www.thenanoresearch.com on November 14, 2014
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1
Stability
of
BN/metal
interfaces
in
gaseous
atmosphere
Yang Yang1, Qiang Fu1*, Mingming Wei1, Hendrik
Bluhm2, Xinhe Bao1*
1State
Key Laboratory of Catalysis, Dalian Institute of
Chemical Physics, the Chinese Academy of Sciences,
Dalian 116023, P.R. China
2Chemical
Sciences Division, Lawrence Berkeley
National Laboratory, 1 Cyclotron Road, Berkeley,
California 94720, USA
Oxygen intercalation at BN/Ru(0001) interface occurs at BN islands in 10 -8
Torr O2 and on full BN layer in 0.1 Torr O2, which decouples BN overlayer
from the substrate and simultaneously oxidizes the metal surface.
Provide the authors’ webside if possible.
Author 1, webside 1
Author 2, webside 2
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Stability of BN/metal interfaces in gaseous atmosphere
Yang Yang1, Qiang Fu1(), Mingming Wei1, Hendrik Bluhm2, Xinhe Bao1()
Received: day month year
ABSTRACT
Revised: day month year
Hexagonal boron nitride (h-BN) is often prepared by epitaxial growth on
metals, and stability of the formed BN/metal interfaces in gaseous environment
is a key issue for physico-chemical properties of the BN overlayers. As an
illustration here, the structural change of BN/Ru(0001) interface upon exposure
to O2 has been investigated using in-situ photoemission electron microscopy
and ambient pressure X-ray photoelectron spectroscopy. We demonstrate the
occurrence of oxygen intercalation of the BN overlayers in O 2 atmosphere,
which decouples the BN overlayer from the substrate. Comparative studies in
oxygen intercalation at BN/Ru(0001) and graphene/Ru(0001) surfaces indicate
that the oxygen intercalation of BN overlayers happens more easily than
graphene. This finding will be of importance for future applications of
BN-based devices and materials under ambient conditions.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
h-BN, graphene,
intercalation, Ru(0001),
PEEM, AP-XPS
1
strongly affect physical and chemical properties of
Introduction
BN epitaxial structures. For example, BN has been
Hexagonal boron nitride (h-BN) is an analogue of
regarded as an excellent substrate for growth of
graphene. With the rise of graphene and the related
high-quality graphene layers, which is more
two-dimensional
much
feasible on BN layer supported on a metal surface
attention has been paid to the h-BN structure [1-4].
owing to the transparent catalytic activity of the
Among many h-BN preparation methods, epitaxial
underlying metal [10]. Uosaki et al. found that h-BN
growth on transition metal (TM) is one of the most
monolayer supported on Ni(111) or Au(111)
effective routes, in which BN overlayers with
surfaces can activate O2 in sharp contrast to the
controlled domain size and layer number have been
inert behavior of the free-standing BN, and the
obtained on various TMs, such as Rh(111), Ru(0001),
reactivity of the metal-supported BN layer has been
Pt(111), Cu foil, and Ni foil [3-9]. It has been
attributed to the presence of midgap states induced
demonstrated that the formed BN/TM interfaces
by the metal substrate [11, 12]. Greber and
(2D)
atomic
crystals,
Address correspondence to Qiang Fu, [email protected]; Xinhe Bao, [email protected]
2
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coworkers observed disappearance of the periodic
corrugation of monolayer BN on Rh(111) when a
2
Experimental
layer of hydrogen atoms intercalate at the BN/Rh
interface [13].
PEEM/LEEM experiments were carried out in an
The important role of the BN/substrate interfaces
Elmitec ultrahigh vacuum (UHV) system, which
calls for more attention to study the stability of the
consists of a preparation chamber, a main chamber
interface structure. BN/metal systems are often used
with
in gaseous atmospheres. Molecules may diffuse into
ultraviolet laser source (λ = 177.3 nm) [27]. The
the BN/metal interfaces, which intercalate the BN
imaging system contains a hemispherical analyzer,
overlayers and change the interface structure [13].
an aberration corrector, a field emission electron
Moreover, the BN-substrate interaction can be
source, and other electron lens. Ru(0001) surface
tuned by the intercalation, which could affect the
was cleaned by repeated cycles of Ar+ sputtering
physico-chemical properties of BN overlayers [14,
(2.0 kV, 7  10-6 Torr Ar), heating in O2 (550 oC, 5 
15]. In the past few years, much work has been
10-7 Torr O2), and annealing in UHV to 1300 oC by
devoted to the intercalation of graphene overlayers
electron
grown on metals and SiC surfaces [15-27]. It has
structure was obtained on the clean Ru(0001)
been shown that molecules such as CO, O2, and
surface using ammonia borane (BH3NH3) as the
H2O can be trapped between the graphene
precursor. The growth details are as follows: the
overlayers
gaseous
clean Ru(0001) surface was kept at a temperature
environments, and novel physics and chemistry
between 630 and 700 oC. BH3NH3 powder was
under the graphene covers have been revealed
placed in a steel container, which was heated to
[21-26]. In contrast to the extensive studies in the
around 100 oC. Vapor produced in the container
intercalation
of
was allowed to leak into the main chamber using a
BN/metal interfaces with gases has not been well
high-precision leakage valve, and pressure in the
explored. Particularly, how stable are the epitaxial
chamber was in the range of 10-8 Torr. The growth
BN structures under gaseous conditions remains
process was dynamically imaged by LEEM or
unclear.
PEEM, and the BN coverage can be controlled by
and
of
the
substrates
graphene,
the
in
interaction
imaging
beam
components,
and
bombardment.
a
vacuum
Monolayer
BN
In this work, we investigated oxygen intercalation
the dosing time. Graphene was deposited on the
of BN overlayers grown on Ru(0001) in O2 using
Ru(0001) surface using the same process reported
photoemission electron microscopy (PEEM)/low
previously [27].
energy electron microscopy (LEEM) and ambient
atmosphere
(AP-XPS).
X-ray
Our
photoelectron
spectroscopy
in-situ characterization
AP-XPS
investigations
were
performed
at
beamline 11.0.2 at the Advanced Light Source, using
results
an ambient pressure XPS end-station. BN overlayers
confirm the diffusion of oxygen atoms into the
were grown on the clean Ru(0001) with a similar
BN/Ru(0001) interface and the decoupling of the BN
recipe but using borazine (H6B3N3) as the precursor.
overlayers from the substrate surface. Comparative
After the BN deposition, high purity O2 was
studies in the oxygen intercalation at BN/Ru(0001)
exposed to the BN/Ru(0001) surfaces via backfilling
and
a
the analysis chamber. The B 1s, N 1s, and Ru 3d
difference in the intercalation kinetics at the two
spectra were acquired with a photon energy of 495,
interfaces.
710, and 465 eV, respectively. A graphene/Ru(0001)
graphene/Ru(0001)
surfaces
indicate
surface was subjected to the same O2 exposure, and
C 1s + Ru 3d spectra were recorded with a photon
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energy of 390 eV. The Fermi edges were measured
each measurement was carried out at a new sample
at each photon energy, which were used to calibrate
position to avoid potential beam damage [26].
the binding energy positions. In O2 atmospheres
3
Results
Figure 1 (a-c) Selected LEEM images during BN growth on Ru(0001)at 640 oC at the growth time of 80, 200, and 320 s, respectively.
Start voltage (STV) = 12.8 V. The pressure in the main chamber was increased gradually from 2.2  10-8 to 7.1  10-8 Torr in the
growth process. 0 s is taken as the appearance of BN islands. The nucleation sites are marked by red dots in each image. The step
direction is indicated by the yellow arrow in (a). (d) A PEEM image of a BN overlayer with a high coverage. (e) A PEEM image of a
full BN layer. A few domain boundaries are labeled by red arrows. (f) -LEED pattern from the BN monolayer (46 eV). LEED spot
for Ru and BN are indicated by yellow and red arrows, respectively.
Monolayer BN islands were grown on the clean
the uphill growth, which is similar to graphene
Ru(0001) surface, which was monitored by in-situ
growth on Ru(0001) [27, 28]. The downhill growth
LEEM. Both the substrate temperature and the
of graphene on Ru(0001) has been attributed to its
chamber pressure can be used to control the density
strong interaction with the Ru surface, and so it is
of nucleation sites and island growth rate. A typical
for BN. Depending on the growth time, monolayer
growth process was illustrated in Figure 1a-1c, and
BN islands with the size of micrometers (see Figure
the corresponding LEEM video can be found in
1d) or a full BN layer (see Figure 1e) can be
video S1 in the ESM. Two growth features can be
obtained. At the full BN layer domain boundaries
clearly identified. First, the growth rate along steps
were clearly observed, as indicated by arrows in
is larger than that across steps, producing BN strips.
Figure 1e. Extended defects such as pinholes and
Second, the downhill growth is much faster than
vacancies exist in the domain boundaries, which
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Nano Res.
should affect the stability of the full BN layer in
of the BN/Ru(0001) surface observed by scanning
gaseous atmosphere as discussed later.
tunneling
microscope
[30].
Besides,
-LEED
A typical micro-region LEED (-LEED) pattern
investigation indicates that all BN islands have the
acquired from the as-prepared monolayer BN
same orientation, suggesting the single crystallinity
islands was displayed in Figure 1f, which shows the
of the epitaxial BN overlayers on Ru(0001). The
presence of a coincidence structure with 13×13 h-BN
growth behavior and structural feature of BN
unit cells matched to 12×12 units of Ru(0001) [29].
indicate
This result is also in agreement with moiré patterns
overlayers and Ru(0001) surface [31].
the
strong
interaction
between
BN
Figure 2 (a-d) Selected PEEM images during the O-intercalation of BN islands in 5  10-8 Torr O2 at 180 oC at 0, 108, 216, and 324 s,
respectively. The width of the un-intercalated part of a BN strip is measured along the yellow line as shown in (a-d). (e) (0, 0) spot of
a LEED pattern (29 eV) of the pristine BN surface and (f) that after the O-intercalation. (g) I-V curves recorded from a pristine BN
island and the O-intercalated BN island. The change in the surface work function has been marked in the figure.
The BN/Ru(0001) surfaces were then exposed to
ESM), which indicates that the intercalation process
O2, and in-situ PEEM was used to study the oxygen
is reaction limited and a reaction rate can be
adsorption process considering that PEEM is more
derived accordingly [32].
sensitive to O2 exposure than LEEM (see video S2
Accompanied with the change in the PEEM
and S3 in the ESM). As shown in Figure 2a-2d, the
image, the high-order diffraction spots around the
image contrast of most BN islands starts to change
(0, 0) spot were strongly attenuated (Figures 2e and
when heating the surface in 5 10-8 Torr O2 at 180 oC.
2f). Thus, we infer that the strong interaction
Each island gets darkened from the edge to the
between BN and Ru(0001) has been significantly
center, which indicates an increase in the surface
weakened, which is similar to Pb, Ni, and O
work function. From PEEM videos, intercalation
intercalation at the graphene/Ru(0001) interface [24,
rates (v) can be derived by v = -dL(t)/dt, where L(t)
27, 33]. In addition, -LEED patterns acquired from
is the width of the un-intercalated part of a BN strip
the O2-exposed BN islands show a clear (2  2)
at the reaction time of t along the yellow line
structure, which is due to oxygen adsorption on
marked in Figure 2a-2d. The measured L(t) values
Ru(0001) [34] (Figure S5b in the ESM).
show a linear dependence on t (see Figure S4 in the
LEEM intensity vs. voltage curves (I-V curves)
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Nano Res.
from the BN islands also present significant changes
surface. The dip structures in the I-V curves are
after the O2 exposure. First, an increase in the
often used as a fingerprint for the layer structure of
surface work function by 1.78 eV (see Figure 2g) can
graphene and BN, and the dip number increases by
be observed by measuring the shift of the energy
one in case that the layer number increases by one
position in which the reflected intensity decreases
or the overlayer has been decoupled from the
by 10% [33, 35-37]. This result is consistent with the
substrate via an interfacial intercalation [33, 38, 39].
darkening of the PEEM images when subjected to
Overall, based on the results from PEEM, -LEED,
O2 exposure. Second, an increase in the dip number
and I-V curves we conclude that the oxygen
by one has been observed in the curve after the O2
intercalation happens at the BN/Ru(0001) interface
exposure. More specifically, the original dip at 7.1
in 5  10-8 Torr O2 at 180 oC, which decouples the BN
eV from the pristine BN structure splits into two
overlayer from the Ru(0001) surface.
dips at 5.9 and 10.3 eV at the intercalated BN
Figure 3 (a-c) B 1s, N 1s, and Ru 3d5/2 XPS spectra of a 0.5 ML BN/Ru(0001) surface kept in 5  10-8 Torr O2 with temperature
increased from 50 to 400 oC. The bottom lines are from the as-prepared BN/Ru(0001) surface.
The oxygen intercalation was further studied by
interaction
with
while
prepared by exposing the clean Ru(0001) surface to
chemisorbed BN sites (strong interaction with Ru)
3  10 Torr borazine at 780 C for 7 min. Then, this
[31, 40]. An obvious change in the N 1s and B 1s
sample was exposed to 5  10 Torr O2 and heated
spectra can be observed upon heating at 100 oC, in
stepwise from room temperature to 400 oC. XPS B 1s,
which a shoulder peak at much lower binding
N 1s, and Ru 3d spectra were in-situ recorded at the
energy appears in each spectrum (Figure 3a and 3b).
indicated heating temperatures (Figure 3). On the
The newly appeared low-energy peaks have been
as-prepared 0.5 ML BN/Ru(0001) surface, both N 1s
gradually strengthened and keep on shifting to
and B 1s spectra are asymmetrical. It has been
lower binding energy positions with the increasing
shown that the low-energy components are from B
heating temperature. At 400 oC the overall binding
and N atoms in physisorbed BN sites (weak
energy shifts are -1.88 eV (190.94 vs. 189.06 eV) and
-8
to
high-energy
components
o
attributed
the
AP-XPS. A 0.5 ML BN/Ru(0001) surface was
-8
are
Ru),
those
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in
the
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-2.44 eV (398.99 vs. 396.55 eV) for B 1s and N 1s,
surface region and the interfacial Ru atoms under
respectively, and the spectra are more symmetric.
the cover of BN, almost disappears. Both surface
The strong shifts of B 1s and N 1s peaks to lower
and interfacial Ru atoms have been saturated by
binding energy positions clearly demonstrate the
surface adsorbed oxygen atoms, shifting the
decoupling of the BN structure from the Ru(0001)
binding energy to higher values [41, 42]. At the
surface
The
same time, we observed a new shoulder peak at
O-intercalated BN layer is now quasi free-standing,
280.33 eV from Ru-O bonding [43]. These AP-XPS
and all B and N atoms should have the same
results are consistent with the PEEM/LEEM data,
chemical environment thus presenting symmetrical
suggesting the onset of oxygen intercalation of the
N 1s and B 1s peaks.
BN overlayer in 5  10-8 Torr O2 and above 100 oC. It
via
the
oxygen
intercalation.
In the Ru 3d5/2 spectra, obvious change was also
should be noted that oxygen intercalation at the
observed when heating the sample in the O2
graphene/Ru(0001) surface happens at much higher
atmosphere at 100 C. In such a case the shoulder
temperatures under
peak at low binding energy position (279.72 eV),
condition, for instance above 300 C [23, 24] (also
which is from the surface Ru atoms at the bared Ru
see below).
o
the
similar
O2
exposure
o
Figure 4 (a-c) Selected PEEM images of oxygen intercalation of BN islands in 5  10-8 Torr O2 at 200 oC at the reaction time of 0, 40,
and 90 s, respectively. (d-f) Selected PEEM images of oxygen intercalation of BN islands in 5  10-8 Torr O2 at 160 oC at the reaction
time of 0, 510, and 1050 s, respectively. (g) An Arrhenius plot of oxygen intercalation rate with intercalation temperature on the
BN/Ru(0001) surface.
In
the
following
a
kinetic
study
in
the
all BN strips at 160 oC. The intercalation rates at
intercalation reaction was carried out using in-situ
different temperatures were obtained using the
PEEM. The intercalation processes were conducted
similar process as shown in Figure 2a-2d.
in 5  10 Torr O2 at various temperatures. Selected
-8
From
the
Arrhenius
equation
we
have
images of two PEEM videos of the intercalation
intercalation rate constant of k = Aexp(-Ea/RT). For
processes at 200 and 160 C were shown in Figure 4.
simplicity the pre-exponential factor (A) and overall
The intercalation at 200 C has been completed in
intercalation activation energy (Ea) are thought to be
less than 2 min, while it takes 20 min to intercalate
invariant with temperature (T) in the investigated
o
o
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Nano Res.
temperature range and for the fixed O2 pressure. An
graphene islands results in darkening of PEEM
Arrhenius plot of the intercalation rate with the
images and disappearance of the superstructure in
reciprocal of intercalation temperature (1/T) was
LEED patterns (Figure 5 and Figure S6 in the ESM).
given in Figure 4g, from which Ea for the oxygen
However, the occurrence of oxygen intercalation of
intercalation of BN/Ru(0001) interface was derived
graphene islands needs higher temperatures than
to be 0.88 eV. Previous works have shown that
that at the BN/Ru(0001) surface under the same O2
oxygen intercalation occurs at graphene/Ru(0001)
exposure condition. For instance, complete oxygen
surface at O2 atmosphere [23, 24]. Here, a
intercalation of graphene islands was observed at
comparative study has been performed to reveal the
303 oC when heating in 5  10-8 Torr O2 for 6 min
difference in the intercalation reaction at BN and
(Figure 5a-5c), and only a small part of graphene
graphene islands. Sub-monolayer graphene layers
islands were intercalated after reaction at 281 oC for
grown on Ru(0001) were exposed to 5  10-8 Torr O2
11 min (Figure 5d-5f). The intercalation reaction has
at
also
been performed at various temperatures, and the
investigated by in-situ PEEM. Similar to the
reaction activation energy was obtained by the
previous findings, oxygen intercalation of the
similar way, which was around 1.11 eV (Figure 5g).
various
temperatures,
which
were
Figure 5 (a-c) Selected PEEM images during oxygen intercalation of graphene islands in 5  10-8 Torr O2 at 303 oC at the reaction
time of 0, 170, and 340 s, respectively. (d-f) Selected PEEM images during oxygen intercalation of graphene islands in 5  10-8 Torr
O2 at 281 oC at the reaction time of 0, 340, and 700 s, respectively. (g) An Arrhenius plot of oxygen intercalation rate with
intercalation temperature on the graphene/Ru(0001) surface.
Having demonstrated the occurrence of oxygen
(Figure S7 in the ESM). The full BN layer was then
intercalation at BN islands, we further study the
exposed to higher pressure O2, for instance 0.1 Torr
oxygen intercalation at the full BN layer. The full
O2. In-situ AP-XPS B 1s, N 1s, and Ru 3d spectra
BN overlayer was grown on the Ru(0001) surface
were recorded at various temperatures. As seen in
with prolonged growth time. Oxygen intercalation
Figure 6, the B 1s binding energy position decreases
at the 1 ML BN/Ru(0001) surface has been
from 190.88 to 190.66 eV and the N 1s peak shifts
attempted in 1  10
Torr O2 at 190 C, but no
from 398.93 to 398.46 eV after the O2 exposure at
evidence of the oxygen intercalation was observed
room temperature, indicating that the full-layer was
after an overall exposure of 1400 Langmuir O2
partially decoupled from the substrate. Moreover,
-6
o
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the Ru 3d5/2 surface component located at 279.72 eV
surface.
has been strongly attenuated. These results suggest
For comparison, a full graphene layer grown on
that oxygen has intercalated between the full BN
Ru(0001) was also exposed to 0.1 Torr O2 at different
layer
room
temperatures. Ru 3d + C 1s spectra indicate that no
temperature. With a further increase of the
change was observed at temperatures below 150 oC.
annealing temperature both B 1s and N 1s peaks
At 200 oC both the C 1s component at 285.27 eV
keep on shifting to lower binding energy positions
from C atoms interacting strongly with Ru(0001)
and finally get stabilized at 189.51 and 396.95 eV,
(noted as C1) and that at 284.55 eV attributed to C
respectively, at 300 oC. At the same time, a new peak
atoms interacting weakly with Ru (noted as C2)
at 280.94 eV appears in the Ru 3d5/2 spectra, which
shift to 283. 88 eV (noted as C3), which are assigned
are from the RuOx species [43]. This result indicates
to C atoms free from interaction with Ru(0001) [44,
that the oxygen intercalation of BN overlayers has
45]. Therefore, the complete oxygen intercalation of
been accompanied by the oxidation of Ru(0001)
the graphene layer occurs at 200 oC.
and
Ru(0001)
surface
even
at
Figure 6 (a-c) B 1s, N 1s, and Ru 3d spectra of the 1 ML BN/Ru(0001) surface kept in 0.1 Torr O 2 with temperature increased from
30 to 350 oC, respectively. (d) Ru 3d + C 1s spectra of the 1 ML graphene/Ru(0001) surface kept in 0.1 Torr O2 at temperature from
30 to 350 oC. The bottom curves are from the as-prepared BN/Ru(0001) and graphene/Ru(0001) surfaces.
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4
Discussions
The
above
in-situ
characterization
results
intercalation reaction at graphene and BN islands
demonstrate that the oxygen intercalation of a full
can be more clearly demonstrated by the oxygen
BN layer necessitates a higher O2 pressure than that
intercalation experiment over BN and graphene
on a sub-monolayer BN overlayer. As discussed in
hybrid structures. First, BN islands formed on the
our previous works, intercalation of molecules at
Ru(0001) surface, and then the surface was exposed
the graphene/metal interfaces needs extended
to 4  10-8 Torr C2H4. Graphene nucleates at each BN
defects of graphene, such as island edges, domain
island
boundaries, and large vacancies [21, 33]. Here, we
eventually surrounds the BN islands (Figure 7a).
suggest that the same channels should be active for
The surface (noted as BN@graphene/Ru(0001)) was
the oxygen intercalation of BN overlayers. At
exposed to O2 with pressure ranged from 5  10-7 to
submonolayer BN/Ru(0001) surface BN island
1  10-6 Torr. At 200 C no change was observed at
edges provide the main channels for oxygen
the hybrid structure, although oxygen intercalation
intercalation. In contrast, oxygen atoms have to
already happens at pure BN islands under the same
diffuse through BN domain boundaries into the
condition. Oxygen starts to intercalate the narrow
BN/Ru interface on the full BN/Ru(0001) surface, in
part of the outer graphene ring at 330 C (marked
which a higher barrier may be present.
by arrow in Figure 7b) and subsequently the inner
Moreover, we observed that oxygen intercalation
edge,
which
continues
to
grow
and
BN island has been intercalated by oxygen quickly
of BN overlayers on Ru(0001) happens at relatively
(Figure
low temperatures, i.e. around room temperature in
graphene@BN/Ru(0001) surface, in which graphene
0.1 Torr O2. In contrast, 200 C is needed to drive the
islands formed first and then BN was grown along
oxygen intercalation at the graphene/Ru(0001)
the graphene island edges. The outer BN ring
interface in 0.1 Torr O2. In 5  10
Torr O2, BN
structures can be intercalated by oxygen in 5  10-8
islands can be intercalated at 160 oC while no
Torr O2 at 200 C, while the inner graphene islands
obvious intercalation of graphene islands has been
still keep intact (Figure 7e-7h).
o
observed till 280
o
-8
7c,
7d).
We
also
prepared
a
C. The difference in the
Figure 7 (a-d) Selected PEEM images during O2 intercalation of a BN@graphene/Ru(0001) surface at about 330 oC at 0, 340, 420,
and 500 s, respectively. O2 pressure was in the range between 5 × 10-7 and 1.6 × 10-6 Torr. An open channel for O-intercalation of the
BN inner island is indicated by a red arrow in (c). (e-h) Selected PEEM images during O2 intercalation of a graphene@BN/Ru(0001)
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surface at 200 oC at 0, 160, 240, and 400 s, respectively. O2 pressure was 5 × 10-8 Torr. In both (a) and (e) images, BN and graphene
regions are marked with red and yellow spot, respectively.
The above results reveal two facts. First, the
the degree of oxygen intercalation of BN varies with
island edges are important for oxygen intercalation,
the O2 pressure and intercalation temperature,
particularly for oxygen intercalation of BN islands.
which can be used as a convenient way to tune the
As shown in Figures 6 and 7, for the full BN layer
interaction strength of BN with the substrate and
without open edges and for the BN islands with
modulate the electronic structure of the BN layer.
edges saturated by graphene rings much harsher
condition is required to intercalate the interface.
Second, the levitation of the graphene island edges
5
Conclusions
is more difficult than BN. At the graphene edge,
We present clear evidences for oxygen intercalation
carbon atoms are unsaturated and tend to bond
of BN overlayers grown on Ru(0001) upon exposure
with surface metal atoms. It is the disruption of the
of the BN/Ru(0001) surface to O2 atmosphere. The
C-metal
that causes the
oxygen intercalation of BN islands occurs around
intercalation barrier. In contrast, we infer that the
bond
at the
150 oC in 5  10-8 Torr O2. In contrast, high pressure
bonding of B and N edge atoms with metal surface
O2 is needed to intercalate the full BN layer, which
atoms may not be so strong, and thus the levitation
starts to happen around room temperature in 0.1
of BN sheets from the metal surface becomes more
Torr O2. The diffusion of oxygen atoms at the
facile, which can be attributed to the observed
BN/Ru(0001) interface results in the decoupling of
lower
the
BN overlayers from the Ru substrate, as manifested
BN/Ru(0001) interface than the graphene/Ru(0001)
by the disappearance of the moiré patterns and
interface.
negative shifts of B 1s and N 1s binding energy by
intercalation
edge
activation
barrier
at
Finally, we here confirm that the BN/Ru(0001)
1.88 and 2.44 eV, respectively. Comparative studies
interface is not stable enough under near ambient
in the oxygen intercalation of BN and graphene
conditions,
other
layers indicate that oxygen intercalation of BN
graphene/metal and BN/metal interfaces [45, 46]. If
presents smaller activation energy than that of
the
graphene.
which
stability
of
may
the
be
true
BN/solid
for
interfaces
is
indispensable for applications of the BN-based
materials and devices, effective protection of the
interfaces should be made. Moreover, we find that
Acknowledgements
This work was financially supported by the National
Natural Science Foundation of China (No. 21222305,
No. 21373208, and No. 21033009), Ministry of Science
and Technology of China (No. 2011CB932704 and No.
2013CB834603), and the Key Research Program of the
Chinese Academy of Sciences. The ALS and the MES
beamline 11.0.2 are supported by the Director, Office
of Science, Office of Basic Energy Sciences, Division
of Chemical Sciences, Geosciences, and Biosciences
and Materials Sciences Division of the US
Department of Energy at the Lawrence Berkeley
National
Laboratory
under
Contract
No.
DE-AC02-05CH11231.
Electronic Supplementary Material: Supplementary
material (growth and intercalation video of BN S1-S3
and related LEED and I-V curves Figure S4-S7) is
available in the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-* .
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11
Nano Res.
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Electronic Supplementary Material
Stability of BN/metal interfaces in gaseous atmosphere
Yang Yang1, Qiang Fu1(), Mingming Wei1, Hendrik Bluhm2, Xinhe Bao1()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Video S1 LEEM video of BN growth on Ru(0001) at 640 oC. FoV = 50 m, STV = 12.8 V. The pressure in the
main chamber increased gradually from 2.2  10-8 to 7.1  10-8 Torr in the growth process.
http://fruit.dicp.ac.cn/images/yy-S1.avi
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Nano Res.
Video S2 PEEM video of O2 intercalation of BN islands grown on Ru(0001) in 5  10-8 Torr O2 at 180 oC. FoV =
50 m. http://fruit.dicp.ac.cn/images/yy-S2.avi
Video S3 LEEM video of O2 intercalation of BN islands grown on Ru(0001) in 5  10-8 Torr O2 at 180 oC. FoV =
20 m. STV = 2.3 V. http://fruit.dicp.ac.cn/images/yy-S3.avi
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Nano Res.
Figure S4 The width of the un-intercalated part of a BN strip (L(t)) at different reaction time (t) along the
orange line marked in Figure 2a-2d. The data were acquired from video S2 and Figure 2a-2d. A fitting result
was also given, which shows a good linear correlation.
(a)
(b)
Figure S5 LEED patterns (36 eV) of the pristine BN overlayer (a) and the O-intercalated BN overlayer (b). In
image (b), a clear (2  2) diffraction pattern can be resolved, which is from oxygen adsorption on the Ru(0001)
surface.
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Nano Res.
(a)
(b)
Figure S6 LEEM images of a graphene island (a) before exposure to 5  10-8 Torr O2 (STV = 12.5 V) and (b)
after exposure to 5  10-8 Torr O2 at 300 oC (STV = 12.7 V). Insets are corresponding LEED patterns (36 eV),
which shows the disappearance of the superstructure from the Moire pattern after the oxygen intercalation.
The appearance of the wrinkles at the graphene island as shown in image (b) also indicates the occurrence of
the oxygen intercalation.
(a)
(b)
(c)
Figure S7 PEEM images of a full BN layer (a) before and (b) after an exposure of about 1400 L O2 at 190 oC.
Insets are corresponding LEED patterns (36 eV). (c) Corresponding I-V curves of the BN surface before and
after the O2 exposure. The two curves almost overlap, which means that no big change happens at the BN/Ru
interface.
Address correspondence to Qiang Fu, [email protected]; Xinhe Bao, [email protected]
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