Ozturk 1..8 - Science Advances

RESEARCH ARTICLE
MATERIALS ENGINEERING
Atomically thin layers of B–N–C–O with
tunable composition
2015 © The Authors, some rights reserved;
exclusive licensee American Association for
the Advancement of Science. Distributed
under a Creative Commons Attribution
NonCommercial License 4.0 (CC BY-NC).
10.1126/sciadv.1500094
Birol Ozturk,1,2,3 Andres de-Luna-Bugallo,1,4 Eugen Panaitescu,1 Ann N. Chiaramonti,5
Fangze Liu,1 Anthony Vargas,1 Xueping Jiang,6 Neerav Kharche,7 Ozgur Yavuzcetin,8
Majed Alnaji,9 Matthew J. Ford,10 Jay Lok,11 Yongyi Zhao,1 Nicholas King,1 Nibir K. Dhar,12
Madan Dubey,13 Saroj K. Nayak,14 Srinivas Sridhar,1,2 Swastik Kar1,15*
INTRODUCTION
Alloy formation is a powerful technology, one that has enabled a variety of semiconductors to be used in an array of tunable optics and
electronics (1, 2). In recent times, atomically thin two-dimensional
(2D) semiconductors have become extremely important for both fundamental science and technology owing to their unique electronic, optical, and mechanical properties (3–9). Hence, the possibility of alloy
formation in 2D materials is a topic of significant interest because
composition-tunable 2D materials are expected to show a rich variety
of tunable electronic, optical, and magnetic properties in an atomically
thin layer. So far, the most promising approach for developing
composition-tunable 2D materials has been chemical vapor deposition
(CVD) synthesis of hybrid domains of boron, nitrogen, and carbon in
an atomically thin plane (10–16). This B–N–C system allows the substitution of C atoms of graphene by B, N, and/or hexagonal BN (h-BN),
and enables the growth of atomically thin alloys with band gaps ranging
from 0 eV (graphene) to 5.2 eV (h-BN).
1
Department of Physics, Northeastern University, Boston, MA 02115, USA. 2Electronic Materials Research Institute, Northeastern University, Boston, MA 02115, USA. 3Department
of Physics and Engineering Physics, Morgan State University, Baltimore, MD 21251, USA.
4
Cinvestav Unidad Querétaro, Querétaro, Qro. 76230, Mexico. 5National Institute of
Standards and Technology, Boulder, CO 80305, USA. 6Department of Physics, Applied
Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. 7Chemistry
Department, Brookhaven National Laboratory, Upton, NY 11973, USA. 8Department of Physics,
University of Wisconsin–Whitewater, Whitewater, WI 53190, USA. 9Department of Mechanical
and Industrial Engineering, Northeastern University, Boston, MA 02115, USA. 10Department of
Chemical Engineering, Northeastern University, Boston, MA 02115, USA. 11College of Computer
and Information Science, Northeastern University, Boston, MA 02115, USA. 12Night Visions
Electronic Sensors Directorate, Fort Belvoir, VA 22060, USA. 13Sensors and Electron Devices
Directorate, U.S. Army Research Laboratory, Adelphi, MD 20783, USA. 14School of Basic Sciences,
Indian Institute of Technology Bhubaneswar, Odisha 751013, India. 15George J. Kostas Research
Institute for Homeland Security, Northeastern University, Burlington, MA 01803, USA.
*Corresponding author. E-mail: s.kar@neu.edu
Ozturk et al. Sci. Adv. 2015;1:e1500094
31 July 2015
In all relevant reports, the possible role of oxygen, whose presence
in small quantities is inevitable in CVD chambers, has not received any
attention (11, 17, 18). Oxygen has been proposed to cause C-species
oxidation and, hence, etching of carbon (19). In the context of BNC
growth, this is significant, because in a recent report, Liu et al. have shown
that any etched edge of graphene can, in principle, be a template for
2D heteroepitaxial growth (that is, growth from a 1D edge) of h-BN
species (13). Because oxygen is always present during the synthesis of
graphene, either as an adsorbed impurity on Cu substrates (18) or in
the surrounding gaseous environment, the question arises whether simultaneous growth of BN species in graphene can be influenced by controlling the oxygen content in the CVD chamber. Moreover, Krivanek et al.
have shown, using annular dark-field electron microscopy, that oxygen
can enter the BNC honeycomb lattice by forming in-plane covalent
bonds with B and C (20). It is rather remarkable that oxygen, a
group VI element, can enter the hexagonal sp2-type lattice of graphene
and h-BN. Hence, it is also a matter of immense fundamental interest
whether oxygen can be controllably introduced into the BNC system
and whether an atomically thin quaternary material, that is, BNCO, can
be stably synthesized with tunable composition. Here, we show that B,
N, C, and O atoms can indeed form a single-atomic-layer, graphenelike honeycomb lattice (which we call 2D-BNCO), resulting in the first
demonstration of a purely 2D quaternary alloy.
RESULTS AND DISCUSSION
BNCO samples were fabricated on Cu foils in a low-pressure CVD system (see Materials and Methods) with CH4 as the carbon source and
powdered NH3BH3 as the common source for B and N. To analyze
the effect of oxygen content, we synthesized a set of eight samples by
introducing Ar/O2 (O2 mole fraction of 10−3) into the CVD chamber
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In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a
composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials.
The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp2-type 2D-BNC lattice is
an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically
thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density
functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area
coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated
electronic transport with mobility exceeding 500 cm2 V−1 s−1, and Arrhenius-like activated temperature
dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < Eg < 0.8 eV) band gaps in their density of
states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for
nanoelectronics and spintronic applications in an atomically thin platform.
RESEARCH ARTICLE
A
B
tary Materials). For convenience, we term the brighter regions “domains,”
because they are compositionally different from the surrounding matrix
and have several interesting properties. First, the sharp visual contrast
between the domains and the surrounding regions (matrix) is only
visible when the SEM images are collected from samples on Cu substrates. Upon being transferred to insulating surfaces such as SiO2, the
contrast difference completely vanishes. This observation, along with
the homogeneous nature of the TEM image in Fig. 1A and similar SAED
diffraction patterns obtained everywhere in the samples, suggests that
the two regions, the “bright” domains and the “dark” matrix, have differing electrical conductivities but are structurally similar.
Second, when sufficiently isolated from each other, the domains
appear to have very well defined shapes, mostly six-sided and with sharp,
angular edges as seen in Fig. 1D. Liu et al. demonstrated the growth of
h-BN domains from the graphene edges in a two-step process (13). In
our “single-step” process, we observed domain geometry and sizes that
look strikingly similar to those of Liu et al., although as shown later,
in our case, the chemical composition of the domains is different. We
believe that introduction of oxygen during the synthesis process collapses
the two-step process of Liu et al. into a single step either by slowing
down or terminating the growth, or by etching away carbon-rich regions
and introducing a richer combination of 2D-BNCO in their place.
Third, the relative coverage of these domains over the entire
millimeter-scale sample could be systematically controlled between 0
C
D
E
100
O1s
80
60
40
20
0
C1s
B1s
N1s
O2 source = 1000 ppm O2 in UHP Ar
0
2
4
6
200
8
Ar/O2 flow rate (sccm)
F
300
400
500
Binding energy (eV)
5 µm
Oxygen %
Carbon %
80
Relative atomic
concentration (%)
Domains region
Graphene region
XPS intensity (a.u.)
% Area coverage of sample
10 µm
60
40
20
0
G
0
25
50
75
Domain % area coverage
Fig. 1. Atomically thin 2D-BNCO sheets. (A) A bright-field TEM image of a typical BNCO sample transferred onto a lacey carbon-coated TEM grid.
(B) Typical SAED pattern from various locations demonstrating honeycomb-like lattice structure in monolayer or rotated bilayer forms. (C and D) SEM
images of a BNCO sample as grown on Cu substrate. The samples shown here were grown with different oxygen flow rates in the CVD chamber. See the
Supplementary Materials, section S1, for details of sample synthesis. (E) Percent surface coverage of domains and graphitic areas as extracted from the
analysis of SEM images, as a function of O2 flow rate into the chamber. (F) Typical XPS survey scan from BNCO-12 sample. The B1s and N1s peaks (regions
marked by arrows) are only visible in high-resolution detailed scans (see the Supplementary Materials, section S4). (G) Relative occurrence of oxygen
atoms as a function of the fractional coverage of the samples by domains, providing direct evidence of the presence of oxygen in the domain areas.
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and varying its flow rate. These samples, synthesized using flow rates
of 0, 2, 4…14 standard cm3/min (sccm), were correspondingly labeled
BNCO-0, BNCO-2…BNCO-14. For samples synthesized using flow
rates between 0 and 8 sccm, we found reproducible results and systematic correlations between various investigated material properties.
Beyond a flow rate of 8 sccm, the system became unstable, with fluctuating and irreproducible results, most possibly due to the uncontrolled
oxidation of the samples. Figure 1A shows a transmission electron microscopy (TEM) image of a typical sample, which was found to be
mostly monolayer and bilayer in nature. Figure 1B shows typical
selected-area electron diffraction (SAED) patterns from various locations,
revealing the ubiquitous sixfold honeycomb symmetry of the crystal
lattice. Analysis of SAED results suggests the presence of monolayer,
possible Bernal-stacked bilayer, and some turbostratic bilayer regions, as
exemplified in the four diffraction patterns shown. SAED analysis of monolayer samples reveals a nearest-neighbor lattice separation of 0.21 nm.
From these, we conclude that the samples are atomically thin (mostly
mono- or bilayered), with a graphene-like honeycomb lattice structure.
Although the structure appears to be highly homogeneous in the
TEM images, closer inspection of the samples using scanning electron
microscopy (SEM) provides a clear indication of the presence of two
distinct phases, characterized by contrast between brighter and darker
regions, as seen in the two samples shown in Fig. 1 (C and D), which
were synthesized under different growth conditions (see the Supplemen-
RESEARCH ARTICLE
Hydrocarbon (including B,N)
derivatives (~532 eV)
C
30
20
O=N
(530.7 eV)
30
O-Cu
(527.6 eV)
530
535
C 1s
C=C
(284.5 eV)
% of C atoms in C-O bonds
20
10
60
Binding energy (eV)
% of N atoms in N-C bonds
% of amine groups
in N1s spectrum
50
40
20
% of C atoms in C-B bonds
0
15
% of N atoms in N-B bonds
% of O atoms in O-Cu bonds
10
10
5
290
% of C atoms in C-N bonds
10
C-O
(286.2 eV)
285
15
20
C-N
(288.7 eV)
280
% of N atoms in N-O bonds
40
C defect
(285.5 eV)
C-B
(282.8 eV)
30
20
(~533 eV)
Binding energy (eV)
XPS intensity (a.u.)
% of O atoms in O-C bonds
D 40
40
525
B
% of O atoms in C-O bonds
60
% Concentration
XPS intensity (a.u.)
O 1s
% Concentration
A
regions). Hence, the question arises: What fraction of each observed
element originates from the domains? To address this question, in
Fig. 1G, the relative concentrations of oxygen atoms and carbon atoms
are plotted against fractional domain coverage (for the set of samples
that were examined in this study). We see a clear correlation between
the domain coverage and the relative concentration of oxygen atoms,
and especially at higher surface coverage of domains, the relation is
nearly linear. This correlation implies that the XPS-observed oxygen is
present predominantly in the domains. Detailed peak analysis (see the
Supplementary Materials) shows that the oxygen atoms are bonded
with B, N, and C atoms, and we next present some of the salient features
of the chemical bonding of the oxygen and other atoms in the system.
Figure 2 (A and B) shows examples of typical high-resolution XPS
scans of the O1s and C1s spectra, each fitted to several identifiable
peaks representing known chemical shifts. In the O1s spectrum, the
most significant contribution came from a peak at 532 eV identified
with O–C bonds (21). A red-shifted O–N peak at 530.7 eV (22) and a
blue-shifted O–B (23) peak were also identified in all samples. A very
295
0
2
4
6
Ar/O2 flow rate (sccm)
8
0
0
2
4
6
8
Ar/O2 flow rate (sccm)
Fig. 2. Chemical composition and structure of the 2D-BNCO samples through XPS studies. (A) Typical high-resolution O1s XPS spectrum. The
oxygen atoms were mostly found to attach with N-, B-, and C-based moieties that could include B and N (see text). Direct bond formation with Cu
(the growth substrate) was almost negligible in all samples. a.u., arbitrary unit. (B) Typical high-resolution C1s spectrum, which could be resolved into
a number of peaks associated with known bonds –sp2 (graphene-like) carbon, as well as bonds with B, N, and O entities. (C and D) Specific bonds whose
relative occurrences were found to (C) grow or (D) decrease/remain negligible as the oxygen flow rate was increased during CVD growth of samples.
Specifically, some of the flow rate dependences seen in (C) appear to have a reasonable similarity to the flow rate dependence of domain area coverage
as shown in Fig. 1E, implying that these bonds were instrumental in the formation of these domains. Similarly, the bonds that decreased or remained
negligibly low as seen in (D) could be less associated with the 2D-BNCO domains.
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and 60% by changing the oxygen content during CVD growth (see the
Supplementary Materials for measurement details). Figure 1E shows
the percentage coverage of domains as progressively more oxygen is
introduced into the CVD chamber. Starting from a near-complete absence of the domains at 0 sccm Ar/O2, there is a clear overall increase
in the surface coverage of the domains, although the dependence is
nonmonotonic.
Figure 1F shows an x-ray photoelectron spectroscopy (XPS) survey
scan performed on a typical sample (BNCO-12). Peaks corresponding
to C1s and O1s are immediately visible in the survey, whereas weak
peaks corresponding to B1s and N1s are found only in high-resolution
scans of the regions indicated with red arrows (see the Supplementary
Materials). In all samples between 0 and 8 sccm, the B1s peaks were
too weak for any quantitative analysis, and the presence of B was confirmed through analysis of the N peak and Raman spectroscopy. Because the XPS beam size is much larger (400 mm) than the domains,
it was not possible to obtain XPS for individual domains (that is, the
XPS survey scan contains contributions from both domain and matrix
RESEARCH ARTICLE
C
D
Bilayer region
Monolayer
region
D
Monolayer region
G
1400
E
1600
2600
2700
Raman shift (cm−1)
F
G
1µm
D peak
−1
1353 cm
h-BN peak
−1
1367 cm
B-C peak
−1
1330 cm
G'
D
1 µm
From monolayer region
From bilayer region (h-BN rich)
G'
Intensity (a.u.)
Bilayer
region
Intensity (a.u.)
G
h-BN-rich layer
Graphene & h-BN rich regions
2800 1300
1350
1400
−1
Raman shift (cm )
H
80
Percentage occurence (%)
SiO2 B
A
from the C1s spectrum) varied with the increasing percentage coverage of the domain regions in the samples (Fig. 1E). Although less conspicuous, there is a corresponding correlation between the growth of
bonds between nitrogen and carbon (that is, both N–C, obtained from
the N1s spectrum as shown in the Supplementary Materials, and C–N,
obtained from the C1s spectrum) as a function of domain size. Analysis of the N1s and O1s peaks show that the relative number of bonds
that participated in the N–B (in h-BN form), N–O, and B–O bonds
changed nonmonotonically as the oxygen content in the CVD chamber was increased. There was an overall increase in B–N bonds and
decrease in N–O bonds, which implied that the presence of O in the
chamber facilitated the relative growth of h-BN while sacrificing the
less-preferred N–O bonds. Other N moieties such as amine groups decreased as a function of oxygen inclusion into the chamber, whereas
there was a remarkable suppression of C–B bonds over the entire range
of samples synthesized, indicating that C–B is not a preferred bonding
configuration for this alloy system. As a result of the above analysis,
we are able to conclude that as more oxygen was introduced into the
CVD chamber, (i) the percentage coverage of the domains increased,
(ii) the percentage of O atoms in the samples increased whereas the percentage of C atoms decreased, and (iii) the formation of domain regions
is favored by the presence of O–C, N–C, and B–N bonds.
Having inspected the chemical bonding in the domains, we now
investigate a few morphological characteristics of these 2D materials.
Figure 3 (A and B) shows SEM and atomic force microscopy (AFM)
h-BN region
Graphene region
70
60
50
40
30
20
10
0
2
4
6
8
Ar/O 2 flow rate (sccm)
Fig. 3. Structural, morphological, and Raman analyses of the 2D-BNCO samples. (A and B) SEM (A) and AFM (B) images of the same location of a
2D-BNCO sample mechanically transferred onto a SiO2 (300 nm)/Si substrate. The 2D-BNCO domains could not be “seen” in the SEM images after they
were transferred to the insulating silica substrate. AFM images confirmed that the “darker” contrast seen in (A) is due to the presence of a second layer.
The thinner layer was found to have a height of 0.6 nm with respect to the SiO2 substrate, indicating a possible monolayer region. (C) Typical Raman
spectra obtained from the monolayer and bilayer regions showing peaks that could be identified with the G, G′, and D peaks of graphene. Decomposed
G′ peaks could be fitted with one or four peaks confirming their mono- and bilayer natures, respectively. (D) Typical composition of the Raman peaks
about 1355 cm−1 showing the presence of the D peak, along with h-BN and B–C bonds only in the bilayer area. (E to G) Optical image (E), G peak (F), and
h-BN peak (G) Raman maps of a 2D-BNCO domain and the surrounding 2D-BNC matrix from sample BNCO-3 on SiO2 (300 nm)/Si substrate. (H) Percentage
occurrence of regions containing h-BN (as obtained from Raman spectra from 30 to 35 randomly selected regions) and h-BN–free graphene-rich regions,
as a function of increasing oxygen content.
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small contribution to the overall oxygen content comes from the Cu
surface oxide (24), as indicated by the nearly negligible size of the
O–Cu peak. The above observations imply that the O atoms are primarily attached to the B, N, and C atoms in the sample and not to the
substrate. The C1s spectrum can be decomposed into several known
peaks as seen in Fig. 2B. In addition to a peak due to the graphitic C–C
bonds and a defect-induced peak (25–27), the C atoms were found to
be attached to oxygen (11, 25), to nitrogen (28), and, in small parts, to
boron (29). Together with the TEM structure analysis, the XPS results
suggest that B, N, C, and O atoms are chemically bonded with each
other in a honeycomb lattice with lattice spacing similar to graphene or
h-BN. To obtain a better understanding of whether these bonds lie within the domain or matrix regions of our samples, the percentages of each
bond type (for example, O–C, O–B, O–N, or O–Cu) for a given element (for example, O) were obtained as a function of O2 flow rate
during synthesis. Figure 2C shows the variation of all bonds whose relative occurrence increased with increasing oxygen flow (implying their
growing dominance in the domain regions), and Fig. 2D shows the
relative concentrations of bonds whose occurrence decreased (implying their possible presence in the matrix region) or remained insignificantly low (implying insignificant presence in either domain or matrix)
with the same. The most striking similarity was found between the
manner in which (i) the percentage of total oxygen bonds attached
to C atoms (O–C, obtained from the O1s spectrum), and (ii) the percentage of C bonds that were attached to O atoms (C–O, obtained
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with an increasing number of O atoms around an h-BN hexagon embedded in the graphene matrix, as shown in Fig. 4A (a to c). The technical details and the metastable structures found in the DFT calculations
are discussed in the Supplementary Materials. The stability of these
patches was verified in the DFT calculations by confirming the return
of intentionally out-of-plane displaced atoms back into the plane. In
particular, the atomic arrangements of Fig. 4A (c) have no B–C bonds,
and hence, the rather low percentage of B–C bonds seen in our XPS
measurements could imply that patches of samples such as the one
seen in Fig. 4A (c) may be prevalent in the system. The remaining
bonding configurations are consistent with all the experimental (TEM,
XPS, and Raman) observations, implying that energetically stable
in-plane oxygen inclusion into the BNCO system may be possible through
B–O–C bonds at the periphery of h-BN domains.
The DFT calculations reveal extremely interesting local structural
and magnetic properties of these patches. In agreement with our experimental finding, all three nano patches are planar in the sense that
the hexagonal bonding is preserved. As shown in the side views in Fig.
4B, the pair of B atoms, closest to the O atoms, are slightly displaced out
of plane (by 0.26 to 0.67 Å). This result highlights the fact that the O
atoms can indeed be stably introduced into the plane of a graphenelike honeycomb lattice at the cost of a small out-of-plane distortion of
the B–O bonds. Furthermore, spin-polarized DFT calculations reveal
another rather surprising result: The differential spin density (Dr =
r↑ − r↓) is localized on the B atoms, which are bonded to the O atoms
(and which exhibit the largest out-of-plane displacement), implying
that these B atoms carry non-zero magnetic moments. The possible
magnetization of a 2D system composed of nonmagnetic elements
and without the addition of any defects is an extremely intriguing result. Here, the localized magnetic moments originate from the fact that
the BO2 motif has an odd number of electrons. As seen from the spinselective density of state (DOS) plots in Fig. 4C, all nano patches exhibit sizable band gaps exceeding 0.5 eV. The projected DOS (PDOS)
analysis of all three structures shows that the states near the valence
and conduction band edges are mainly composed of pz orbitals, with B
pz orbitals dominating over pz orbitals of C, N, and O. The DFT predictions for bond lengths, magnetic moments, and electronic band
gaps are tabulated in Fig. 4D. The possible magnetic nature of these 2D
sheets along with their semiconducting behavior (see the Supplementary
Materials) gives rise to the intriguing possibility that controlled structures
of 2D-BNCO sheets may be atomically thin diluted magnetic semiconductors, which are key spintronic materials (32). Furthermore, these DFTpredicted ground states are more stable than the possibly nonmagnetic
metastable states (see the Supplementary Materials) by ~200 meV, implying that they may be magnetic even at room temperature (~25 meV).
In conclusion, we have synthesized and characterized atomically
thin layers of 2D-BNCO, a quaternary system that allows for the incorporation of oxygen into a graphene-like honeycomb lattice. Under
suitable conditions, isolated 2D-BNCO domains grow within a graphenerich 2D-BNC matrix with well-defined shapes and sharp edges, which
suggests that these domains grow hetero-epitaxially from the 1D edges
of graphene-rich monolayer 2D-BNC in a manner similar to how 2D
h-BN can grow from 1D etched edges of graphene (13). We believe
that oxygen plays an important role in partially etching or limiting the
monolayer graphene-rich 2D-BNC edge growth, providing active 1D
edges where 2D-BNCO can continue to grow. It also appears that oxygen
promotes the increased presence of h-BN rings in the 2D-BNCO crystal.
Detailed investigation of synthesis as a function of growth durations,
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images of the same area on a sample mechanically transferred onto an
SiO2/Si substrate. This area, which shows the presence of regions with
different numbers of layers, was chosen to investigate the layer thickness of these materials. Unlike the SEM images obtained from as- grown
samples on Cu, the SEM and AFM images of the samples transferred
onto SiO2/Si do not show any contrast between the domain and matrix
regions (the contrast in these images arises from the presence of regions with different numbers of layers). The AFM topographic image
(Fig. 3B) reveals that the height difference between these two regions is
≈0.5 nm, which is similar to the second-layer thickness of bilayer
graphene samples. The chemical and structural nature of these layers
could be verified by analyzing peaks occurring in Raman spectra measured from each of these layers, as shown in Fig. 3C. The most distinct
spectral signal was from the G and G′ peaks of graphitic carbon, located roughly at 1580 and 2700 cm−1, respectively, implying that both
regions contained graphitic (−sp2 C=C) carbon. A detailed line shape
analysis of the G′ bands confirms that the thinner region is monolayer
in nature (the G′ band was fitted to a single Lorentzian), whereas the
thicker layer is bilayer (the G′ band was fitted to a sum of four
Lorentzians) (30), which is consistent with both TEM and AFM measurements. Further information could be obtained by deconvolving
the spectral shape between 1300 and 1400 cm−1 (Fig. 3D). A graphitic
defect-induced D band occurs near 1350 cm−1 in the monolayer region, whereas contributions from the vibrational modes of B–C bonds
(31) (relatively weaker) and h-BN (12, 15) are identified in the bilayer
region. In general, it was found that the second layers were richer in
h-BN content. To establish this, we obtained Raman spectral maps of
a sample area that encompasses a two-layer region, as shown in the
digital optical image in Fig. 3E, where a second layer is seen to have
grown over the first (darker green). Figure 3F shows a map of the G
band (1580 cm−1) intensity, measured with a ≈350-nm spatial resolution, which illustrates the non-uniform distribution of graphitic carbon in the sample. Figure 3G shows a map of the h-BN (1367 cm−1)
intensity, which correlates strongly with the second layer. A second region of the sample, indicated by arrows, was also found to be h-BN–
rich (and less populated by graphitic carbon, as seen in Fig. 3F). Note
that the contrast seen in the Raman spectral maps is not reproduced
in the optical image of Fig. 3E. These investigations reveal that of the
various bonding configurations obtained via XPS measurements, some
are arranged in graphene-like carbon bonds, a small quantity of B–C
bonds, and h-BN bonds.
An interesting observation is the behavior of h-BN–rich regions as
the oxygen content of the CVD chamber was increased. Figure 3H
shows the fraction of the randomly selected regions that show h-BN
peaks, as well as the fraction of regions that do not, as a function of increasing oxygen content in the system. An overall increase in the fractional occurrence of h-BN regions in the samples is seen as a function
of increasing oxygen, which implies that the presence of oxygen promotes the formation of h-BN domains within 2D-BNCO layers, in
agreement with our XPS analysis results.
The observations reported in this study suggest that the four constituent elements of 2D-BNCO are present in a number of different bonding configurations. At the macroscale, the samples can be viewed as a
patchwork of nanoscale domains of B, N, C, and O atoms bonded
together in a honeycomb lattice with energetically stable configurations.
To gain further insight into their possible structural and electronic
properties, we have used ab initio density functional theory (DFT) to
investigate three representative nano patch configurations of 2D-BNCO
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A
B
C
preferably under in situ probes, could potentially provide a clear picture regarding the exact nature of 2D-BNCO domain growth. The
possibility that oxygen, a group VI element, can be controllably introduced into an sp2-type hexagonal lattice with only small out-of-plane
distortions is by itself a fundamentally exciting discovery. In addition,
our DFT results predict that in their most stable configurations, the
boron atoms closest to oxygen atoms have non-zero differential spin
density, rendering 2D-BNCO nano patches with sizable magnetic moments. Magnetism in carbon-based materials (33, 34) is a topic of great
importance, because such materials are expected to provide lightweight
and low-cost alternatives to conventional magnets. In this context, the
possibility of magnetic 2D-BNCO sheets, which are also semiconducting with variable band gap values, could have exciting potential
for fundamental investigations and applications in spin-based electronics.
Ozturk et al. Sci. Adv. 2015;1:e1500094
31 July 2015
Furthermore, because oxygen is a critical ingredient in many important
classes of materials such as superconductors, electroceramics, magnetoresistors, and solid-state catalysts, inclusion of oxygen into the 2D-BNC
lattice could potentially lead to new classes of 2D materials. We believe
that this is a big step toward designing next-generation atomically thin
functional alloys that may find applications in a number of technologies.
MATERIALS AND METHODS
Conventional CVD chambers operating under low pressure always
contain oxygen as an ambient element. Although this oxygen rarely
enters the graphitic lattice during pure graphene synthesis, it has been
suggested that the presence of nitrogen can promote oxygen inclusion
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Fig. 4. DFT-computed electronic properties of representative atomic configurations of 2D-BNCO nano patches. (A) (a to c) Optimized structures of
distinct stable configurations of model BNCO structures including the spatial distribution of the differential spin density Dr = r↑ − r↓. Regions of |Dr| > 0
were found to be associated with the local BO2 motifs, which leads to non-zero magnetic moments. (B) Side views showing the out-of-plane displacement
of B atoms closest to O atoms. (C) DOS of the BNCO units. The Fermi level is aligned with the top of the valence band. The lines in red and blue correspond
to opposite spin polarizations. (D) A table of bond lengths, electronic band gap, and average magnetic moment on the B atoms closest to the O atoms.
RESEARCH ARTICLE
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/1/6/e1500094/DC1
Text
Fig. S1. CVD setup for BNCO growth.
Fig. S2. Morphology of growth substrate (copper) with and without boron and nitrogen
source.
Fig. S3. Morphological characterizations of sample and substrate after growth.
Fig. S4. XPS analysis of the chemical composition of the 2D-BNCO samples.
Fig. S5. 2D-BNCO surface coverage analysis.
Fig. S6. Detailed Raman spectroscopic analysis.
Fig. S7. DFT-computed electronic properties of metastable structures of C38(BN)3B2O4 and
C35(BN)3B3O6 nano patches.
Fig. S8. Source-drain current as a function of back gate voltage measured from BNCO-6 sample.
Fig. S9. Absorption spectroscopy studies.
Fig. S10. Variation of resistance with temperature in a typical BNCO sample, demonstrating
semiconducting behavior with a negative temperature coefficient of resistance (TCR).
Fig. S11. Temperature dependence of resistance.
Fig. S12. Phototransmittance characteristics of 2D-BNCO.
Table S1. Calculated TCR values for BNCO samples.
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into the lattice (35). In our synthesis method, carbon was sourced from
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Cleaned copper foils were placed in a quartz tube inside a furnace,
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(10 mtorr). Hydrogen (H2) was introduced as carrying gas at a controlled flow rate of 10 sccm, with the equilibrium pressure reaching
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RESEARCH ARTICLE
Funding: We acknowledge financial support from ARL/DARPA (U.S. Army Research Laboratory/Advanced Research Projects Agency) contract number ARMY/W911NF-11-2-0080 for the
Ozturk et al. Sci. Adv. 2015;1:e1500094
31 July 2015
experimental aspects of this work. S.K. also acknowledges partial financial support from
NSF-ECCS-1351424 (for. F.L.), NSF ECCS 1202376 (for M.F., J.L., and cost of publication), as well
as U.S. Army Grant, W911NF-10-2-0098, sub-award 15-215456-03-00 (for A.V.). N.K., X.J., and S.K.N.
would like to thank the Interconnect Focus Center (MARCO program), State of New York, the
NSF Petascale Simulations and Analysis (PetaApps) program (grant no. 0749140), and an anonymous gift from Rensselaer for the support. Computing resources of the Computational Center
for Nanotechnology Innovations at Rensselaer, partly funded by the State of New York, have
been used for this work. This work is a partial contribution of the National Institute of Standards
and Technology, an agency of the U.S. government. Hence, it is not subject to copyright in the
United States. Author contributions: B.O. and S.K. designed the study, developed the methodology, performed the analysis, and wrote the manuscript. B.O., A.d.L.B., E.P., A.N.C., F.L., A.V.,
O.Y., M.A., M.J.F., J.L., Y.Z., and N.K. collected the data. X.J., N.K., and S.K.N. performed DFT
calculations. N.K.D., M.D., S.S. and S.K. provided direction and guidance and participated in writing
and reviewing the manuscript. Competing interests: The authors declare that they have no
competing interests.
Submitted 24 January 2015
Accepted 14 May 2015
Published 31 July 2015
10.1126/sciadv.1500094
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Atomically thin layers of B−N−C−O with tunable composition
Birol Ozturk, Andres de-Luna-Bugallo, Eugen Panaitescu, Ann N.
Chiaramonti, Fangze Liu, Anthony Vargas, Xueping Jiang, Neerav
Kharche, Ozgur Yavuzcetin, Majed Alnaji, Matthew J. Ford, Jay
Lok, Yongyi Zhao, Nicholas King, Nibir K. Dhar, Madan Dubey,
Saroj K. Nayak, Srinivas Sridhar and Swastik Kar (July 31, 2015)
Sci Adv 2015, 1:.
doi: 10.1126/sciadv.1500094
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