Band gap and chemically ordered domain structure of a graphene analogue BxCyNz

Article abstract of DOI:10.1016/j.ssc.2010.09.029

Chemically synthesized few layer graphene analogues of B_xC _yN_z are characterized by aberration corrected transmission electron microscopy and high resolution electron energy loss spectroscopy (HREELS) to determine the local phase, electronic structure and band gap. HREELS band gap studies of a B_xC_yN_z composition reveal absorption edges at 2.08, 3.43 and 6.01 eV, indicating that the B _xC_yN_z structure may consist of domains of different compositions. The K-absorption edge energy position of the individual elements in B_xC_yN_z is determined and compared with h-BN and graphite. An understanding of these experimental findings is developed with complementary first-principles based calculations of the various ordered configurations of B_xC_yN_z.

Full text of DOI:10.1016/j.ssc.2010.09.029

Solid State Communications 150 (2010) 2262–2265
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Band gap and chemically ordered domain structure of a graphene
analogue BxCyNz
K. Venua, S. Kanuria, K. Raidongiab, K.P.S.S. Hembramc, U.V. Waghmarec, R. Dattaa,∗
a International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
b Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
c Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemically synthesized few layer graphene analogues of BxCyNz are characterized by aberration corrected
transmission electron microscopy and high resolution electron energy loss spectroscopy (HREELS) to
determine the local phase, electronic structure and band gap. HREELS band gap studies of a BxCyNz
composition reveal absorption edges at 2.08, 3.43 and 6.01 eV, indicating that the BxCyNz structure may
consist of domains of different compositions. The K-absorption edge energy position of the individual
elements in BxCyNz is determined and compared with h-BN and graphite. An understanding of these
experimental findings is developed with complementary first-principles based calculations of the various
ordered configurations of BxCyNz .
Received 28 July 2010
Received in revised form
20 September 2010
Accepted 22 September 2010
by C.S. Sundar
Available online 26 September 2010
Keywords:
A. BCN
© 2010 Elsevier Ltd. All rights reserved.
A. Semiconductor
D. Band gap
E. EELS
1. Introduction
XPS study provides information on the identification of the bonds
between elements but this is based on curve fitting of the
The quest for new materials and an understanding of their
properties has led to the extensive research in science and
engineering of carbon nano-tubes and graphene [1–3]. With
this motivation, the successful synthesis of graphene analogue
materials out of various layered structures has been reported [4–8].
Among various graphene analogue materials, BxCyNz attracts
particular attention due to the tunability of its semiconducting
band gap with control on its composition. Graphene is a semi-
metal whereas BN is an insulator. BxCyNz , an intermediate mixture
with composition of graphene and BN, possesses direct band gap
semiconducting properties depending on its atomic proportions
(x, y, z) and ordering. Many novel electronic device applications
can be realized by utilizing this new class of materials.
BxCyNz in the two dimensional nano-structure and in other
forms is highly stable and particular interest exists in determining
the arrangement of its atoms or phases and their relationship
with electronic structures. Theoretically, many configurations of
hexagonal BCN with comparable energies (within 1%) have been
predicted, and those consisting of motifs with BN3 interlinked
with NB3 are found to have the lowest energy [4]. The electronic
structure was also reported for BC2N in the single layer compound
for various motifs and the lowest energy structure was found to
be a semiconductor with a gap of ∼1.61 eV [9]. Experimentally,
absorption peak and only suggestive about the formation of any
possible phases. EELS quantification very accurately provides the
relative amounts of individual elements in a very small area
and this information can only approximate in suggesting the
formation of a particular phase. For example, EELS quantification
cannot provide information on whether there is a homogeneous
distribution of B, C, and N atoms or whether the BxCyNz layer
structure is a mixture between graphene and BN. Electron energy
loss near edge structure (ELNES) up to 50 eV energy loss from the
core absorption edge in a transmission electron microscope has the
capability to provide finger printing on the coordination along with
high spatial resolution, but due to the unavailability of standards
it has not been possible to correlate the energy loss spectra with
the exact nature of phases formed in BxCyNz . There was another
report on the theoretical computation of the EEL near edge spectra
of BCN [10]. The calculation was based on single electron molecular
orbital theory. The EELS spectra for h-BN, BC2N, BC3 can serve as
guides to compare the experimental spectra of a given BxCyNz
composition. Recently, the formation of nano size domains of
C doped h-BN and graphene has been reported based on UV
absorption spectra for BxCyNz grown by CVD [11]. Here we report
the experimental determination of band gap edge absorption by
high resolution EELS using a gun monochromator and compare
with the first-principles pseudopotential based density functional
theory (DFT) calculations of BCN, BC2N and BCN2 with various
motifs to understand and predict the presence of phases in a
∗
Corresponding author.
E-mail address: ranjan@jncasr.ac.in (R. Datta).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2010.09.029

K. Venu et al. / Solid State Communications 150 (2010) 2262–2265
2263
(a) BK
(b) NK
180
190
200
210
220
230
240
390
400
410
420
430
440
450
Energy loss (eV)
Energy loss (eV)
Fig. 1. Electron energy loss near edge structures for both BCN and BN at the (a) B K and (b) N K edge. Extra features are marked by A/B at the B K edge structure.
Table 1
chemically synthesized BxCyNz composition. The experimental
Experimental K absorption edge energy positions of a few layers BxCyNz and BN
electron energy loss near edge structure and K-edge energy
absorption values of the BxCyNz sample are reported and the latter
are interpreted through comparison with h-BN layer structure and
graphite.
determined by high resolution EELS.
K edge energy
BCN (eV)
BN (eV)
B K (π∗∗)
N K (π∗ )
C K (π∗)
188.91
397
280.20
195.20
403.16
287.80
190.86
401.53
–
197.61
408.67
–
2. Experiment
B K (σ ∗
)
N K (σ∗
C K (σ
)
)
Graphene analogues of BxCyNz and BN were synthesized by
chemical synthesis route using a mixture of urea, boric acid and
activated charcoal for BxCyNz , urea and boric acid for BN. The
mixture was then heated at ∼900 °C [4]. The formation of a
number of layers can be controlled by controlling the amount of
urea in the reaction mixture and it was found that the number
of layers decreases with the increasing amount of urea in the
mixture [6]. The EELS data of BN is also presented to compare
with the BxCyNz . HRTEM imaging was performed in an aberration
Table 2
EELS quantification and possible phase formation in the structure from different
areas.
Area
EELS quantification
Possible phases
A1
A2
A3
A4
A5
A6
B3.3CN5.8
B0.9CN1.8
B0.9CN1.8
B2.8CN4.0
B2.9CN6.2
B1.6CN1.9
Domains of BN and low C doped BN
Mostly low C doped BN
Mostly low C doped BN
corrected FEI TITAN3TM 80–300 kV microscope. The images were
acquired under a negative spherical aberration coefficient (Cs) of
about 30 µm along with a small positive defocus (1f ) value of
about 8 nm (bright atom image). The band gap absorption and
electron energy loss near edge structure study were performed
using a gun monochromator at 300 kV with an energy resolution
better than 0.2 eV (measured from FWHM of zero loss peak). The
K-edge absorption energy for various elements was determined by
calibrating the spectra with respect to the zero loss peak position.
Domains of BN and low C doped BN
Domains of BN and low C doped BN
Mostly low C doped BN
for BCN is ∼397.0 eV and for BN it is ∼401.5 eV. The carbon K edge
for BCN is determined to be 280.2 eV and this value for graphite
is 284.3 eV. These results suggest that this BxCyNz sample is differ-
ent from h-BN and graphite. However, we are unable to predict the
formation of different phases present in the sample. Our absorp-
tion edge value for BN is a little different from literature BN values
(191.4 and 398.9 eV for B and N respectively) and the reason is the
reduced dimension in one direction compared to the bulk form of
BN [10]. The separation between π∗ and σ∗ of the B K and N K edge
was found to be the same (7.2 and 6.3 eV respectively) for both BCN
and BN.
3. Results and discussion
Transmission electron microscopy images have already con-
firmed the formation of a few layers of BxCyNz and BN and the
hexagonal arrangement of atoms [4,5]. Although it is now possi-
ble to characterize a few layers of BN [12] and similar materials
by atomic resolution phase contrast imaging, for chemically syn-
thesized buckled sample atomic resolution imaging is very diffi-
cult. As mentioned before, XPS is routinely employed to deduce
the nature of chemical bonding environments between the ele-
ments in BCN by a peak fitting procedure of the absorption edge.
Mannan et al. [13] reported the formation of B–N, B–C, C–C, N–C
types of bonding in their BCN film produced by microwave plasma
CVD. From XPS, the presence of four different sp2 hybrid configu-
rations (BN3, BN2C, BNC2, BC3) around the B atom was suggested.
XPS study of our sample revealed close to equal distribution of the
elements in BxCyNz [4]. We have used high resolution EELS for de-
termining accurately the K-edge absorption energy value for dif-
ferent elements of two dimensional layers both in BxCyNz and BN.
Table 1 summarizes the absorption edge energy positions for both
π∗ and σ∗ absorption edges. The B π∗K edge of BCN appears at
∼188.9 eV whereas for BN it is at ∼190.9. Similarly, the N π∗K edge
EELS quantification was carried out to find the elemental
distribution and average composition of BxCyNz . We found either
C deficient some form of BxCyNz in most of the regions or
almost equal proportions of BxCyNz in some areas. Table 2 lists
the composition data from different regions of samples and
approximate average chemical phases. It seems that we have the
BCN2 phase in many areas. However, the presence of band edge
absorption in low loss EEL spectra rules out the presence of any
BCN2 phase after comparing with theoretical calculations and is
discussed later.
The EEL spectra show strong π∗ peaks apart from σ∗ which
is a signature of a two dimensional graphene-like hexagonal
structure (sp2 hybridization). The EELS near edge structure
provides information about the density of unoccupied states in
the conduction band and is very useful to get information about
coordination fingerprinting. One can note the difference in the
near edge structure between BxCyNz and BN in Fig. 1. The peak

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K. Venu et al. / Solid State Communications 150 (2010) 2262–2265
Table 3
Calculated band gap energy and stability of the BC2N phase for various motif
configurations.
Structure of
BC2N
Lattice constant
(Å)
Ecohesive
(eV/atom)
Band gap
(eV)
1
2
3
4
5
5.01
5.02
5.01
5.02
5.02
8.28
8.37
8.58
8.31
8.05
1.95
Metallic
1.45
0.55
0.46
Table 4
Calculated band gap energy and stability of the BCN2 phase for various motif
configurations.
2
4
6
8
10
12
Energy loss (eV)
Structure of
BCN2
Lattice constant
(Å)
Ecohesive
(eV/atom)
Band gap
(eV)
Fig. 2. Band gap absorption edges at 2.25, 3.35 and 6.01 eV showing the formation
1
2
3
4
5
4.94
4.94
5.02
4.94
5.03
7.93
7.95
7.34
7.86
7.56
Metallic
Metallic
Metallic
Metallic
Metallic
of domain structure in BCN.
A/B is more pronounced/weak in BxCyNz compared to BN. No clear
difference is observed for the N K near edge structure.
We have performed a band gap absorption edge measurement
using high resolution EELS.
A gun monochromator with a
150 Ry. A monolayer with an eight atom unit cell was considered
in the calculation. Integration over the Brillouin zone was sampled
with a Monkhorst–Pack scheme [17] with a 12 × 15 × 1 mesh
of k points and the occupation numbers were smeared using a
Methfessel–Paxton scheme [18] with a broadening of 0.01 Ry.
The structures are allowed to relax fully to minimize energy until
the force on each atom is less than 0.001 Ry/bohr. The lattice
constants of five different configurations (see Fig. 3) vary within
1% of each other and the smallest one is 5.01 Å for BC2N and 4.94
Å for BCN2. The details of the BCN structure have been reported
earlier [4]. The cohesive energy of the lowest energy structure for
BC2N is 8.58 eV/atom and that of BCN2 is 7.95 eV/atom, which are
smaller than BCN (8.61 eV/atom), graphene (9.16 eV/atom) and
BC4N (8.93 eV/atom). The cohesive energies of other structural
arrangements of BC2N and BCN2 are given in the Tables 3 and 4.
The most stable structures of BCN and BC2N are semiconductors
with band gap energies of 1.8 eV and 1.45 eV respectively. For
five different configurations, BCN2 appeared to be metallic which
is due to the presence of extra N atoms and elements in the
structure. Fig. 4 shows the electronic density of states of BC2N
and BCN2 for the lowest energy configurations, indicating their
semiconducting and metallic characteristics. These calculations
helped us in ruling out the formation of any BCN2 phase in the
BCN alloy and confirming the domain structure from both band gap
absorption edge measurement and EELS quantification.
resolution better than 0.18 eV was used to determine the
band edge absorption in our chemically synthesized samples.
Theoretical calculation predicted the direct band gap energy of
1.8 eV for BCN with interlinked BN3 and NB3 motifs, which has
the lowest energy structure. We have obtained the band edge
absorption energy (measured at midpoint of the edge, see Fig. 2)
at 2.08 (±0.2), 3.43 (±0.2) and 6.01 eV in most of the regions.
This suggests that the sample consists of domains of h-BN and two
different types of slightly C-doped h-BN with little N rich condition.
Therefore one could decompose the atomic percentage obtained by
EELS quantification into combinations of h-BN and two different
levels of C doped h-BN domain structure. This further confirms the
nature of the BxCyNz two dimensional layers consisting of domains
of different compositions. The composition of domains may be
dependent on the synthesis route.
We have used plane wave self consistent field (PWSCF)
implementation of density functional theory (DFT) [14], with a
generalized gradient approximation (GGA) [15] to exchange the
correlation energy of electrons and ultrasoft pseudopotentials [16]
to represent the interaction between the ionic cores and valence
electrons. We determine the electronic band structure for various
possible phases of monolayer BxCyNz , namely BCN, BC2N and
BCN2 with different atomic configurations (or motifs). Kohn–Sham
wave functions were represented with a plane wave basis with
an energy cutoff of 25 Ry and charge density with a cutoff of
a
b
Fig. 3. Various atomic configurations of (a) BC2N, and (b) BCN2.

K. Venu et al. / Solid State Communications 150 (2010) 2262–2265
2265
Acknowledgement
One of the authors (Hembram) is thankful to DRDO, Govt. of
India for financial support.
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4. Conclusions
In summary, we have shown from the band gap absorption edge
that h-BN and two different low C doped h-BN phases have been
formed in a BxCyNz sample synthesized by a chemical route. Band
gap values of 2.03 eV and 3.43 eV have been obtained for two
low C doped h-BN domains. The K-edge energy absorption values
indicate that bonding between the atomic pairs of B, C and N has
been formed.