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New 2D Structural Materials: Carbon−Gallium Nitride (CC−GaN) and
Boron−Gallium Nitride (BN−GaN) Heterostructures Materials Design
Through Density Functional Theory
Article · January 2019
DOI: 10.1021/acsomega.8b03025
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Cite This: ACS Omega 2019, 4, 1722−1728 http://pubs.acs.org/journal/acsodf
New 2D Structural Materials: Carbon−Gallium Nitride (CC−GaN) and
Boron−Gallium Nitride (BN−GaN) HeterostructuresMaterials
Design Through Density Functional Theory
Van W. Elloh,†,‡ Abu Yaya,‡ G. Gebreyesus,§ Piyush Dua,† and Abhishek K. Mishra*,†
†Department of Physics, School of Engineering, University of Petroleum and Energy Studies (UPES), Bidholi via Premnagar,
Dehradun 248007, India
‡Department of Materials Science and Engineering and §Department of Physics, University of Ghana, P.O. Box LG 25, Legon,
Ghana
*S Supporting Information
ABSTRACT: New class of ternary nanohetrostructures have
been proposed by mixing 2D gallium nitride (GaN) with
graphene and 2D hexagonal boron nitride (BN) with an aim
towards desgining innovative 2D materials for applications in
electronics and other industries. The structural stability and
electronic properties of these nanoheterostructures have been
analyzed using first-principles based calculations done in the
framework of density functional theory. Different structure
patterns have been analyzed to identify the most stable
structures. It is found to be more energetically favorable that
the carbon atoms occupy the positions of the nitrogen atoms
in a clustered pattern in CC−GaN heterostructures, whereas
boron doping is preferred in the reverse order, where isolated
BN and GaN layered configurations are preferred in BN−GaN heterostructures. These 2D nanoheterostructures are
energetically favored materials with direct band gap and have potential application in nanoscale semiconducting and nanoscale
optoelectronic devices.
■ INTRODUCTION Graphene is still one of the most exciting topics of research
Simulations based on quantum mechanical treatment of atoms in condensed matter physics as well as in materials science, due
and electrons have a great impact on computational material to its remarkable electronic, mechanical, and thermal proper-9−15
science research. These simulations and modeling techniques ties, in particular as its charge carriers behave like massless
not only provide atomistic-level understanding but also give Dirac particles. Additionally, through its electronic band gap
necessary information to tailor design new materials for future tuning, graphene has found applications in nanoelectronic9,16
technologies. Density functional theory (DFT)-based techni- devices. Energy band gap can be opened in graphene17
ques have found applications in academia as well as di erent through various means, including hydrogenation, interactionff
areas of industrial research. with substrate,
18,19 molecules adsorption,20 and also deposition
A great interest has been shown by semiconductor industry on a latticed-matched substrate, such as SiO2 or hexagonal
1,2 boron nitride (h-BN).21,22on gallium nitride (GaN)-based device structures. Owing to Recently, various attempts have
its large direct band gap3 and high peak velocity,4 GaN been made to fabricate graphene devices by engineering their
23−25
possesses substantial promise for optical devices and high- band gaps through doping. Investigations on doped
23,24
power electronics. Moreover, because of its larger peak graphene nanoribbons indicate that upon doping by N or
electron velocity, larger saturation velocity, and higher thermal B, n-type or p-type semiconducting graphene can possibly be
stability, GaN is an ideal material in high-power, high- obtained, respectively. It has been experimentally established
temperature, and high-frequency electronic applications. that upon N doping of graphene,
23 the Dirac point in the band
Recent research shows that incorporating carbon atoms into structure of graphene tends to move below the Fermi level
the GaN volume strongly alters their electronic and magnetic (EF) and an energy gap appears at high-symmetric K-point.
properties.5,6 This incorporation of carbon in GaN films results Boron nitride is an insulator with strong ionic bond and can be
in an insulating material with resistance >108 Ω.7,8 The GaN
films in their high resistive forms are finding applications as Received: October 31, 2018
insulating buffer layers or substrates in GaN-based device Accepted: December 26, 2018
technologies. Published: January 22, 2019
© 2019 American Chemical Society 1722 DOI: 10.1021/acsomega.8b03025
ACS Omega 2019, 4, 1722−1728
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Figure 1. Simulated structures of CC:GaN: (a) CC:GaN heterostructure in configuration 1 (CC:GaN1), (b) CC:GaN heterostructure in
con guration 2 (CC:GaN2fi ). A 2 × 2 unit cell is shown for the better representation of chemical bonding and structures, where a 32-atom GaN
supercell is doped with C atoms with carbon concentrations of 50% used in the calculations.
Figure 2. Simulated structures of BN:GaN: (a) BN:GaN heterostructure in con 1figuration 1 (BN:GaN ), (b) BN:GaN heterostructure in
configuration 2 (BN:GaN2). A 2 × 2 unit cell is shown for the better representation of chemical bonding and structures, where a 32-atom GaN
supercell is doped with B atoms with boron concentrations of 25% used in the calculations.
synthesized in hexagonal close-packed structure,26 similar to changes. Next, we describe the electronic properties, including
graphite, with lattice constants very close to those of graphite. band structure and density-of-states (DOS) plots, and explain
This auxiliary closeness among graphite and hexagonal boron the band gap changes due to incorporation of CC and BN
nitride (h-BN) drove a few examinations to make multilayers layers in 2D GaN sheet. We conclude the work in the
of these materials, which can have semiconducting properties Conclusions section, whereas computational and other
with a small band gap. technical details of the calculations are given at the end in
A simple metal−insulator−n-type-GaN diode produces blue Computational Methodology section.
electroluminescence,27 and consequently, GaN has been
studied as a suitable material in optoelectronic devices as RESULTS AND DISCUSSION
laser and light-emitting diodes. However, the concentration of ■
free electrons is too large (1018−1020 cm−3) for the fabrication Structure and Stability. In this section, we discuss the
of a p-type crystal to be successful. Nitrogen vacancies have structures and different configurations of heteronanostructures
been found as major source of free electrons rather than any investigated in the present work. First, we investigated
unknown donor impurities.28 In this work, we performed first- CC:GaN hetrostructures by replacing gallium nitride (GaN)
principles electronic structure calculations based on density layers with carbon carbon (CC) layers in a 2D GaN sheet of
functional theory approximation to investigate the effects of 32 atoms. We tried two different configurations: (a) in the first
1
the incorporation of CC and BN layers in the GaN two- configuration (CC-GaN ), we replaced single gallium nitride
dimensional (2D) sheet on its structural and electronic layers by single layers of carbon atoms in alternative patterns
properties. We have investigated two different patterns of (Figure 1a,b); in the second configuration (CC-GaN2), we
heterostructures: first, in the GaN 2D structure, we replaced replaced two adjacent GaN layers by two CC layers (Figure
two adjacent gallium nitride layers by two layers of carbon 1b), with a carbon concentration of 50% in each case.
atoms in alternative patterns, and in the second configuration, Similarly, we modeled the BN:GaN nanoheterostructures and
single gallium nitride layers are replaced by single carbon− considered two different configurations of alternate BN−GaN
carbon layer in alternate patterns. Similar configurations are layers (BN-GaN1)(Figure 2a) and two alternate BN−BN and
investigated by replacing GaN layers by BN layers, keeping GaN−GaN layers (BN-GaN2)(Figure 2b), with a BN layer
boron concentration of 25% in both cases. The paper is concentration of 50% in each case.
organized as follows: Under Results and Discussion section We evaluated the stability of the CC:GaN nanoheteros-
(Structure and Stabilty sub-section), we first define the tructures by calculating the formation energy of each
hetrostructures and their different configurations and discuss substitution to find the energetically most favorable config-
their stability on the basis of fomation energies and structural urations, as
1723 DOI: 10.1021/acsomega.8b03025
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ΔE = ECC:GaN − NCEC − NGaEGa − NNEN (1) GaN structures, the supercell shrinks and the volume of the
simulated 32-atom GaN supercell decreases from 981.09 to
where ECC:GaN is the total ground-state energy of the CC−GaN 825.70 Å3 and to 776.86 Å3 for most stable CC:GaN and
nanoheterostructure; EC, EGa, and EN are the ground-state BN:GaN structures, respectively.
energies of carbon, gallium, and nitrogen atoms, respectively; In two CC:GaN configurations investigated, we observe that
and NC, NGa, and NN are the number of carbon, gallium, and C−C bonds and Ga−N bonds are of slightly smaller length in
nitrogen atoms in the heterostructure, respectively. the second configuration (1.505 Ǻ in CC:GaN2) compared to
Similarly, we calculated the formation energy of BN:GaN that in the first configuration (1.515 Ǻ in CC:GaN1),
nanoheterostructures as indicating more stronger bonding in the second configuration
ΔE = EBN:GaN − NBEB − NGaEGa − NNEN and preferring bigger clusters of Ga−N and C−C chains over
isolated CC and GaN chains in graphene and gallium nitride
with EBN:GaN as the ground-state energy of the BN:GaN heterostructures (Table 3). However, in BN:GaN, shorter
nanoheterostructure; EB, EGa, and EN as the ground-state
energies of isolated boron, gallium, and nitrogen atoms,
respectively; and N , N Table 3. Optimized Bond Lengths of the GaN, Graphene, h-B Ga, and NN are the number of boron,
gallium, and nitrogen atoms in the heterostructure, respec- BN, and Their Heterostructures
tively. nanohetrosructure bond lengths (Å)
The calculated formation energies per atom for different Ga−N
hetrostructures is tabulated in Table 1, and it can be seen that GaN 1.855
C−C
Table 1. Formation Energy Per Atom Calculated Using Eq 1 CC 1.426
for Ternary CxGa1−xN and BxGa1−xN (x = 0.25, 0.50) B−N
Compounds BN 1.451
C−C C−Ga N − C Ga−N
nanoheterostructure % doping of C/B atoms formation energy (eV)
CC:GaN1 1.515 1.893 1.362 1.811
CC:GaN1 0.50 −6.8989
CC:GaN2 1.505 1.906 1.360 1.775
CC:GaN2 0.50 −6.9634
B−N Ga−N
BN:GaN1 0.25 −6.9671
BN:GaN1 1.554 1.790
BN:GaN2 0.25 −6.9527
BN:GaN2 1.560 1.845
for all concentrations, the formation energy is lower when a bond lengths and hence stronger bonds are found to be
carbon atom or a boron atom occupies the position of a indicative of more stability of segregated layered structures of
gallium atom; therefore, it is energetically more favorable that BN−GaN. Moreover, Ga−N bonds are found to be more
carbon or boron atoms occupy the positions of gallium atoms stiffer in CC:GaN and BN:GaN structures than in pristine
in the GaN 2D structure. Additionally, all of the new 2D GaN 2D structure, resulting in their stability and rigidity due to
nanoheterostructures have negative formation energies, which a strong covalent bonding character.
shows that these Gr:GaN and BN:GaN structures are Electronic Properties. The electronic band structures at
thermodynamically stable. Further, we calculated the formation high-symmetry k-points for both CC:GaN and BN:GaN
energy per atom for GaN sheet and found the value to be
− heterostructures are shown in Figures 3 and 4. The band gap5.720 eV, which is greater than the formation energies per of GaN 2D sheet is found to be 2.285 eV (Figure S4). The
atom (Table 1) for all of the new CC:GaN and BN:GaN calculated band gaps of CC:GaN heterostructures (Figure
structures designed, indicating new heterostructures to be 3a,b) are 1.92 and 1.70 eV for CC:GaN1 and CC:GaN2,
more stable. respectively, while for two BN:GaN heterostructures (25% B
Our calculations indicate that in CC:GaN heterostructures, concentration, Figure 4a,b), the band gaps are found to be 2.44
clustering of CC chains and GaN chains is favored over and 2.14 eV, respectively, for BN:GaN1 and BN:GaN2
scattering of CC and GaN chains within the ternary structure,
2 heterostructures. Hence, incorporation of graphene CC layerswith clustered configurations (CC:GaN ) being more stable in GaN 2D structure lowers the electronic band gap, whereas
compared to isolated chains configuration (CC:GaN1). In incorporation of BN layers increases the band gap of GaN 2D
contrast to CC:GaN, clustering of chains is not favored in structure. The gaps occur due to mixed hybridization of
BN:GaN configurations and GaN and BN single isolated
1 valence states of Ga and N with that of C atoms and due tolayered structure is more stable (BN:GaN ), as depicted in mixed hybridization of valence states of Ga and N with that of
Table 1, reporting the formation energies. From Table 2, we B atoms in their respective compounds. The calculated values
observe that as a result of incorporating CC and BN layers in of band gap are presented in Table 2.
To further analyze the nature of interactions and origin of
Table 2. Optimized Lattice Parameters, Cell Volume, and gaps in these nanoheterostructures, we have plotted the DOS
Energy Band Gap of Different Heterostructures and projected DOS (p-DOS) of CC:GaN heterostructures in
Figures 5 and 6 and for BN:GaN heterostructures in Figures 7
nanohetrosructure a (Å) b (Å) c (Å) V (Å3) Eg (eV) and 8, respectively. On visualization of atomic orbitals, we note
GaN 12.852 12.856 6.889 981.09 2.285 that the DOS near the gaps is essentially of pz character
CC:GaN1 10.851 11.586 7.418 824.09 1.92 occurring from the antibonding and bonding Ga and N states
CC:GaN2 10.840 11.544 7.461 825.70 1.70 hybridizing with those of pz states of carbon. Similar
BN:GaN1 11.168 11.567 6.866 776.86 2.44 hybridization of states is observed due to boron atoms in
BN:GaN2 11.178 11.579 6.580 745.74 2.14 BN−GaN heterostructures. The position of the energy gap due
1724 DOI: 10.1021/acsomega.8b03025
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Figure 3. Calculated band structures at high-symmetry k-points for C-doped GaN: (a) CC:GaN1 configuration and (b) CC:GaN2 configuration.
Figure 4. Calculated band structures at high-symmetry k-points for B-doped GaN: (a) BN:GaN1 configuration and (b) BN:GaN2 configuration.
Figure 5. Electronic density of states (DOS) for C-doped GaN: (a) CC:GaN1 configuration and (b) CC:GaN2 configuration, with fermi energy
level set at zero.
to such hybridization is very crucial such that upon doping of ■ CONCLUSIONS
GaN by different coverage or configurations of C or B, the In this article, we describe a successful example of the
energy gap occurs at the EF. For the ternary structures, the application of DFT to design new structural materials. We used
calculated band gaps are given in Table 2. From the electronic DFT calculations to investigate the ground-state structural and
electronic properties of new 2D heterostructures of 2D gallium
band structure plots analysis, we observe that these structures nitride, graphene, and h-boron nitride. We discussed the
are direct band gap materials with band gap values of 1.92, stability through calculation of formation energies of different
1.70, 2.44, and 2.14 eV, with the valance band and conduction heterostructures, investigating two different doping patterns for
band to be located at the γ k-point. A comparison with the both the CC:GaN and BN:GaN heterostructures. Similar to
borocarbonitrides, these new 2D materials have tunable band
band structures of graphene and GaN with the new 2D gap and may find applications in various fields. These results
structures shows that new bands are formed due to may provide guidance in practical engineering applications,
hybridization of states in the new 2D ternary compounds. especially to tune the band gap in 2D materials. Finally, our
1725 DOI: 10.1021/acsomega.8b03025
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Figure 6. Projected density of states (p-DOS) for C-doped GaN: (a) CC:GaN1 configuration and (b) CC:GaN2 configuration, with fermi energy
level set at zero.
Figure 7. Electronic density of states (DOS) for B-doped GaN: (a) BN:GaN1 configuration and (b) BN:GaN2 configuration, with fermi energy
level set at zero.
Figure 8. Projected density of states (p-DOS) for B-doped GaN: (a) BN:GaN1 configuration and (b) BN:GaN2 configuration, with fermi energy
level set at zero.
results are useful to provide an explanation of the formation of configurations. Our case study could provide some guidelines
hybridized 2D nanomaterials. This new form of hybridized 2D for the industrialists and academic scientists in the develop-
material facilitates the development of band gap engineering ment of new materials.
and applications, in particular, in nanoelectronics and nano-
optics. To generalize, we would like to emphasize that DFT ■ COMPUTATIONAL METHODS
calculations should not only be considered useful for studying The first-principles calculations performed in the present work
some selected cases, i.e., compounds with a given composition use generalized gradient approximation (GGA) of density
as in the present case, but they are very useful to produce a functional theory (DFT) through Quantum ESPRESSO
database for searching a large space of possible structural package.29 The calculations are based on ultrasoft pseudopo-
1726 DOI: 10.1021/acsomega.8b03025
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1728 DOI: 10.1021/acsomega.8b03025
ACS Omega 2019, 4, 1722−1728
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