**Graphene-Boron Nitride Composite: A Material with Advanced Functionalities**

Sumanta Bhandary and Biplab Sanyal

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50729

## **1. Introduction**

The discovery of two dimensional materials is extremely exciting due to their unique properties, resulting from the lowering of dimensionality. Physics in 2D is quite rich (e.g., high temperature superconductivity, fractional quantum Hall effect etc.) and is different from its other dimensional counterparts. A 2D material acts as the bridge between bulk 3D systems and 0D quantum dots or 1D chain materials. This can well be the building block for materials with other dimensions. The discovery of graphene, the 2D allotrope of carbon by Geim and Novoselov [1] made an enormous sensation owing to a plethora of exciting properties. They were awarded the Nobel prize in Physics in 2010. Graphene, an atomically thick C layer, has broken the jinx of impossibility of the formation of a 2D structure at a finite temperature, as argued by Landau *et al.*[2, 3]. The argument that a 2D material is thermodynamically unstable due to the out-of-plane thermal distortion, which is comparable to its bond length, was proven invalid with this discovery. One of the recent interests is to understand this apparent discrepancy by considering rippled structures of graphene at finite temperatures.

Graphene, with its exciting appearance, has won the crowns of the thinnest, the strongest, the most stretchable material along with extremely high electron mobility and thermal conductivity [4]. The linear dispersion curve at the Dirac point gives rise to exciting elementary electronic properties. Electrons in graphene behave like massless Dirac fermions, similar to the relativistic particles in quantum electrodynamics and hence has brought different branches of science together under a truly interdisciplinary platform. At low temperature and high magnetic field, a fascinating phenomenon, called the half integer quantum hall effect, is observed. The relativistic nature of carriers in graphene shows 100% tunneling through a potential barrier by changing its chirality. The phenomenon is known as "Klein-Paradox". Minimum conductivity of a value of conductivity quantum (*e*2/*h* per spin per valley) is measured at zero field, which makes graphene unique. "The CERN on table top" is thus a significant naming of the experiments performed with this fascinating material [5].

An infinite pristine graphene is a semi metal, i. e., a metal with zero band gap [6]. Inversion symmetry provided by *P*6/*mmm* space group results in a band degeneracy at the Dirac points

©2012 Bhandary and Sanyal, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©2012 Bhandary and Sanyal, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 2 Will-be-set-by-IN-TECH 4 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 3

(*K* and *K*� ) in the hexagonal Brillouin zone (BZ). This limits its most anticipated application in electronics as the on-off current ratio becomes too small to be employed in a device. The opening of a band gap is thus essential from electronics point of view retaining a high carrier mobility. Several approaches have been already made by modifying graphene, either chemically [7, 8] or by structural confinement [9–11] to improve its application possibilities, both from theory and experiment. It should be noted that in theoretical studies, the use of density functional theory (DFT) [12] has always played an instrumental role in understanding and predicting the properties of materials, often in a quantitative way.

block is replaced by B-N. In this case, the formula unit will be BC2N. Secondly, a whole area of graphene can be replaced by boron nitride, which makes them phase separated. This we call as phase segregated alloy. The formula unit of phase segregated alloy can change depending on the percentage of doping. The final part of the following section will be devoted to the phase segregated BCN alloys. Apart from those, a distributed alloying is possible with different

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 5

The substitution of C2 with B-N introduces several interesting features. Firstly, B-N bond length is 1.7% bigger that C-C bond but C-B bond is 15% bigger than C-N bond. So, this is going to create intra-layer strain, which is going to affect its stability. Secondly, the difference in electro negativity in B (2.0) and N (3.0) will definitely cause a charge transfer. The orientation of charged pair B-N do have a major contribution in cohesive energy. Thirdly, as mentioned earlier, this will break inversion symmetry in graphene, which brings a significant change in electronic properties. Keeping these in mind, we are now going to discuss stability

In this section we will mainly focus on the stability issues for various BC2N structures [13, 15]. To demonstrate the factors for structural stability, we have chosen five different structures of BC2N (Fig. 1). Let's first have a closer look at structure I. Every C atom has one C, N and B as nearest neighbors while B(N) has two Cs and one N(B) as their nearest neighbors. There is a possibility of all bonds to be relaxed, retaining the hexagonal structure. Stress is thus minimized in this structure, which helps obviously in the stability. In structure II, each C has two C and either one B or N as nearest neighbors. C2 and BN form own striped regions, which lie parallel to each other in this structure. Now C-B bond is much larger that C-C. So this is definitely going to put some internal stress. From the point of view of intra-layer stress, this structure is definitely less stable than the previous one. Looking at structure III, one can see this structure looks similar to structure I but B-N bond orientations are different. Each C atom now has either two N or B and one C in its neighboring position while N(B) has two C and one B (N) as neighbors. This obviously adds some uncompensated strain in the structure. Structure III, thus consists of two parallel C-N and C-B chains and as C-B bond length is much larger than C-N (15%), this mismatch is going to introduce a large strain in the interface. On the other hand in structure I, C-N and C-B are lined up making the structural energy lower compared to structure III. Structure IV does not contain any C-C bond. C-B and C-N chains are lined parallel to each other. Finally in structure V, B-N bonds are placed in such a way that they make 60◦ angle to each other. Both of the last two structures thus have uncompensated

Bond energy is another key factor in stability. When the bond energies are counted, the

*B* − *N*(4.00 *eV*) *> C* − *C*(3.71 *eV*) *> N* − *C*(2.83 *eV*) *> B* − *C*(2.59 *eV*) *> B* − *B*(2.32 *eV*) *> N* − *N*(2.11 *eV*) The maximization of stable bonds like B-N and C-C will thus stabilize the structure as a whole. Now, a structure like II, with a striped pattern of C and B-N chains has maximum number of such bonds. This makes it most stable even though a structural strain is present. In this case bond energy wins over structural stress. For the structures like I and III, number

BN:graphene ratios.

and electronic structure of BC2N.

strain, which increases their structural energies.

ordering of the bonds is the following [13]:

**2. Stability of BC**2**N**

Boron Nitride (BN), on the other hand, can have different forms of structures like bulk hexagonal BN with *sp*2 bond, cubic BN with *sp*3 bond, analogous to graphite and diamond respectively. A 2D sheet with strong *sp*2 bonds can also be derived from it, which resembles its carbon counterpart, graphene. But two different chemical species in the two sublattices of BN forbid the inversion symmetry, which results in the degeneracy lifting at Dirac points in the BZ. Hexagonal BN sheet thus turns out to be an insulator with a band gap of 5.97 eV.

This opens up a possibility of alloying these neighboring elements in the periodic table to form another interesting class of materials. Possibilities are bright and so are the promises. B-N bond length is just 1.7% larger than the C-C bond, which makes them perfect for alloying with minimal internal stress. At the same time, introduction of BN in graphene, breaks the inversion symmetry, which can result in the opening up of a band gap in graphene. On top of that, the electronegativities of B, C, and N are respectively 2.0, 2.5 and 3.0 [13], which means that the charge transfer in different kinds of BCN structures is going to play an interesting role both in stability and electronic properties.

Hexagonal BNC (h-BNC) films have been recently synthesized [14] on a Cu substrate by thermal catalytic chemical vapor deposition method. For the synthesis, ammonia borane (NH3-BH3) and methane were used as precursors for BN and C respectively. In the experimental situation, it is possible to control the relative percentage of C and BN. The interesting point is that the h-BNC films can be lithographically patterned for fabrication of devices. The atomic force microscopy images indicated the formation of 2-3 layers of h-BNC. The structures and compositions of the films were characterized by atomic high resolution transmission electron microscopy and electron energy-loss spectroscopy. Electrical measurements in a four-probe device showed that the electrical conductivity of h-BNC ribbons increased with an increase in the percentage of graphene. The h-BNC field effect transistor showed ambipolar behavior similar to graphene but with reduced carrier mobility of 5-20 cm2V−1s−1. From all these detailed analysis, one could conclude that in h-BNC films, hybridized h-BN and graphene domains were formed with unique electronic properties. Therefore, one can imagine the h-BN domains as extended impurities in the graphene lattice.

The structure and composition of BN-graphene composite are important issues to consider. As mentioned before, substitution of C in graphene by B and N can give an alloyed BCN configuration. Considering the possibilities of thermodynamic non-equilibrium at the time of growth process, one can think of several ways of alloying. The potential barrier among those individual structures can be quite high and that can keep these relatively high energetic structures stable at room temperature. For example, a huge potential barrier has to be crossed to reach a phase segregated alloy from a normal alloy, which makes normal alloy stable at room temperature. Now, depending on the growth process, different types of alloying are possible. Firstly, one can think of an even mixture of boron nitride and carbon, where one C2

block is replaced by B-N. In this case, the formula unit will be BC2N. Secondly, a whole area of graphene can be replaced by boron nitride, which makes them phase separated. This we call as phase segregated alloy. The formula unit of phase segregated alloy can change depending on the percentage of doping. The final part of the following section will be devoted to the phase segregated BCN alloys. Apart from those, a distributed alloying is possible with different BN:graphene ratios.

The substitution of C2 with B-N introduces several interesting features. Firstly, B-N bond length is 1.7% bigger that C-C bond but C-B bond is 15% bigger than C-N bond. So, this is going to create intra-layer strain, which is going to affect its stability. Secondly, the difference in electro negativity in B (2.0) and N (3.0) will definitely cause a charge transfer. The orientation of charged pair B-N do have a major contribution in cohesive energy. Thirdly, as mentioned earlier, this will break inversion symmetry in graphene, which brings a significant change in electronic properties. Keeping these in mind, we are now going to discuss stability and electronic structure of BC2N.

## **2. Stability of BC**2**N**

2 Will-be-set-by-IN-TECH

in electronics as the on-off current ratio becomes too small to be employed in a device. The opening of a band gap is thus essential from electronics point of view retaining a high carrier mobility. Several approaches have been already made by modifying graphene, either chemically [7, 8] or by structural confinement [9–11] to improve its application possibilities, both from theory and experiment. It should be noted that in theoretical studies, the use of density functional theory (DFT) [12] has always played an instrumental role in understanding

Boron Nitride (BN), on the other hand, can have different forms of structures like bulk hexagonal BN with *sp*2 bond, cubic BN with *sp*3 bond, analogous to graphite and diamond respectively. A 2D sheet with strong *sp*2 bonds can also be derived from it, which resembles its carbon counterpart, graphene. But two different chemical species in the two sublattices of BN forbid the inversion symmetry, which results in the degeneracy lifting at Dirac points in the BZ. Hexagonal BN sheet thus turns out to be an insulator with a band gap of 5.97 eV.

This opens up a possibility of alloying these neighboring elements in the periodic table to form another interesting class of materials. Possibilities are bright and so are the promises. B-N bond length is just 1.7% larger than the C-C bond, which makes them perfect for alloying with minimal internal stress. At the same time, introduction of BN in graphene, breaks the inversion symmetry, which can result in the opening up of a band gap in graphene. On top of that, the electronegativities of B, C, and N are respectively 2.0, 2.5 and 3.0 [13], which means that the charge transfer in different kinds of BCN structures is going to play an interesting role

Hexagonal BNC (h-BNC) films have been recently synthesized [14] on a Cu substrate by thermal catalytic chemical vapor deposition method. For the synthesis, ammonia borane (NH3-BH3) and methane were used as precursors for BN and C respectively. In the experimental situation, it is possible to control the relative percentage of C and BN. The interesting point is that the h-BNC films can be lithographically patterned for fabrication of devices. The atomic force microscopy images indicated the formation of 2-3 layers of h-BNC. The structures and compositions of the films were characterized by atomic high resolution transmission electron microscopy and electron energy-loss spectroscopy. Electrical measurements in a four-probe device showed that the electrical conductivity of h-BNC ribbons increased with an increase in the percentage of graphene. The h-BNC field effect transistor showed ambipolar behavior similar to graphene but with reduced carrier mobility of 5-20 cm2V−1s−1. From all these detailed analysis, one could conclude that in h-BNC films, hybridized h-BN and graphene domains were formed with unique electronic properties. Therefore, one can imagine the h-BN domains as extended impurities in the graphene lattice. The structure and composition of BN-graphene composite are important issues to consider. As mentioned before, substitution of C in graphene by B and N can give an alloyed BCN configuration. Considering the possibilities of thermodynamic non-equilibrium at the time of growth process, one can think of several ways of alloying. The potential barrier among those individual structures can be quite high and that can keep these relatively high energetic structures stable at room temperature. For example, a huge potential barrier has to be crossed to reach a phase segregated alloy from a normal alloy, which makes normal alloy stable at room temperature. Now, depending on the growth process, different types of alloying are possible. Firstly, one can think of an even mixture of boron nitride and carbon, where one C2

and predicting the properties of materials, often in a quantitative way.

both in stability and electronic properties.

) in the hexagonal Brillouin zone (BZ). This limits its most anticipated application

(*K* and *K*�

In this section we will mainly focus on the stability issues for various BC2N structures [13, 15]. To demonstrate the factors for structural stability, we have chosen five different structures of BC2N (Fig. 1). Let's first have a closer look at structure I. Every C atom has one C, N and B as nearest neighbors while B(N) has two Cs and one N(B) as their nearest neighbors. There is a possibility of all bonds to be relaxed, retaining the hexagonal structure. Stress is thus minimized in this structure, which helps obviously in the stability. In structure II, each C has two C and either one B or N as nearest neighbors. C2 and BN form own striped regions, which lie parallel to each other in this structure. Now C-B bond is much larger that C-C. So this is definitely going to put some internal stress. From the point of view of intra-layer stress, this structure is definitely less stable than the previous one. Looking at structure III, one can see this structure looks similar to structure I but B-N bond orientations are different. Each C atom now has either two N or B and one C in its neighboring position while N(B) has two C and one B (N) as neighbors. This obviously adds some uncompensated strain in the structure. Structure III, thus consists of two parallel C-N and C-B chains and as C-B bond length is much larger than C-N (15%), this mismatch is going to introduce a large strain in the interface. On the other hand in structure I, C-N and C-B are lined up making the structural energy lower compared to structure III. Structure IV does not contain any C-C bond. C-B and C-N chains are lined parallel to each other. Finally in structure V, B-N bonds are placed in such a way that they make 60◦ angle to each other. Both of the last two structures thus have uncompensated strain, which increases their structural energies.

Bond energy is another key factor in stability. When the bond energies are counted, the ordering of the bonds is the following [13]:

$$\begin{aligned} B - N(4.00 \, eV) &> \text{C} - \text{C}(3.71 \, eV) > N - \text{C}(2.83 \, eV) > 0\\ B - \text{C}(2.59 \, eV) &> B - B(2.32 \, eV) > N - N(2.11 \, eV) \end{aligned}$$

The maximization of stable bonds like B-N and C-C will thus stabilize the structure as a whole. Now, a structure like II, with a striped pattern of C and B-N chains has maximum number of such bonds. This makes it most stable even though a structural strain is present. In this case bond energy wins over structural stress. For the structures like I and III, number

#### 4 Will-be-set-by-IN-TECH 6 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 5

are also found to be stable over the first kind of alloying, which indicates a transformation due to thermal vibration. Yuge *et al.*[17] with DFT studies and Monte Carlo simulations have

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 7

**Figure 2.** (a) Different steps (A-E) of phase separation process, (b) Swapping of BN and C dimers, (c) Formation energies for different steps shown in (a) for two different paths demonstrated in (b), (d) Activation energy in going from left to right configuration in the initial step of phase segregation. Reprinted with permission from Appl. Phys. Lett. 98, 022101 (2011). Copyright (2011) American

Even though a tendency is indicated, a recent calculation by Lam *et al.*, have shown that this possibility is hindered as the activation energy required for phase segregation is extremely high. As shown in Fig. 2, they have chosen a possible path for phase separation by swapping B-N bond to C-C bonds. This kind of swapping can also happen in two ways (Fig. 2(b)). Calculated formation energies for these two process are shown in Fig. 2(c), which basically demonstrates that the intermediate structures are quite high in energy compared to evenly distributed and phase separated structures. The authors also performed nudged elastic band (NEB) calculations to determine the activation barriers for the first step to occur, i. e. to change a B-N bond to B-C and N-C bonds (Fig. 2(d)). Activation energy required is 1.63eV/atom suggesting that this process can happen only at elevated temperatures. At room temperature that is why the pristine (BN)*m*(C2)*n* should be stable and so are the phase separated ones.

Institute of Physics.

shown a tendency of phase separation between BN and graphene.

**Figure 1.** Crystal structure of different isomers of BC2N. Filled black, pink, and blue circles represent carbon, boron and nitrogen respectively.

of such bonds is equal. In that case, intra-layer stress acts as the deciding factor. Structure IV, on the other hand does not have any C-C or B-N bond but only C-B or C-N. Therefore, the issue of stability is the most prominent here. The number of strong bonds is sufficiently large in structure V for the stabilization despite of 60◦ arrangement of B-N bonds.

Another important issue is the charge transfer as there is a difference between the electronegativities of B, C, and N. As mentioned earlier, N is the most electronegative and B is the least one while C behaves as a neutral atom. This also adds an ionic character in the bond formation. B (N) always gains some +ve (-ve) charges. So, the gain in the electrostatic energy only happens if these +/- charges are situated in an alternative manner. Otherwise the electrostatic repulsion makes the structure unstable. From this point of view, structures II, III, and V are more stable than the other two following the trend shown from bond energies. Thus, as reported by Itoh *et al.*[13], ordering of the structures will be: II*>*V*>*(I,III)*>*IV . Stability of other possible isomers can as well be anticipated with the same arguments.

So far we have talked about the substitutional alloying of BN and graphene, where BN to C2 ratio is 1:1. Another group of structures, which can be formed by alloying BN and graphene is the phase segregated BCN. In this kind of structure, BN (graphene) retains its own phase, separated by graphene (BN). The experimental evidence of these kinds of structures have been shown [14]. The size of the graphene or BN phase has an impact on the stability and electronic properties. Here, the BN:C2 ratio is thus not only 1:1 but can be varied and if varied controllably, one can control the electronic properties such as band gap [16]. Lam *et al.*[16] have shown that, by controlling the graphene phase, one can control the band gap according to the desired values for technological applications. The phase segregated (BN)*m*(C2)*n* alloys are also found to be stable over the first kind of alloying, which indicates a transformation due to thermal vibration. Yuge *et al.*[17] with DFT studies and Monte Carlo simulations have shown a tendency of phase separation between BN and graphene.

4 Will-be-set-by-IN-TECH

 -

**Figure 1.** Crystal structure of different isomers of BC2N. Filled black, pink, and blue circles represent

in structure V for the stabilization despite of 60◦ arrangement of B-N bonds.

other possible isomers can as well be anticipated with the same arguments.

of such bonds is equal. In that case, intra-layer stress acts as the deciding factor. Structure IV, on the other hand does not have any C-C or B-N bond but only C-B or C-N. Therefore, the issue of stability is the most prominent here. The number of strong bonds is sufficiently large

Another important issue is the charge transfer as there is a difference between the electronegativities of B, C, and N. As mentioned earlier, N is the most electronegative and B is the least one while C behaves as a neutral atom. This also adds an ionic character in the bond formation. B (N) always gains some +ve (-ve) charges. So, the gain in the electrostatic energy only happens if these +/- charges are situated in an alternative manner. Otherwise the electrostatic repulsion makes the structure unstable. From this point of view, structures II, III, and V are more stable than the other two following the trend shown from bond energies. Thus, as reported by Itoh *et al.*[13], ordering of the structures will be: II*>*V*>*(I,III)*>*IV . Stability of

So far we have talked about the substitutional alloying of BN and graphene, where BN to C2 ratio is 1:1. Another group of structures, which can be formed by alloying BN and graphene is the phase segregated BCN. In this kind of structure, BN (graphene) retains its own phase, separated by graphene (BN). The experimental evidence of these kinds of structures have been shown [14]. The size of the graphene or BN phase has an impact on the stability and electronic properties. Here, the BN:C2 ratio is thus not only 1:1 but can be varied and if varied controllably, one can control the electronic properties such as band gap [16]. Lam *et al.*[16] have shown that, by controlling the graphene phase, one can control the band gap according to the desired values for technological applications. The phase segregated (BN)*m*(C2)*n* alloys

carbon, boron and nitrogen respectively.

**Figure 2.** (a) Different steps (A-E) of phase separation process, (b) Swapping of BN and C dimers, (c) Formation energies for different steps shown in (a) for two different paths demonstrated in (b), (d) Activation energy in going from left to right configuration in the initial step of phase segregation. Reprinted with permission from Appl. Phys. Lett. 98, 022101 (2011). Copyright (2011) American Institute of Physics.

Even though a tendency is indicated, a recent calculation by Lam *et al.*, have shown that this possibility is hindered as the activation energy required for phase segregation is extremely high. As shown in Fig. 2, they have chosen a possible path for phase separation by swapping B-N bond to C-C bonds. This kind of swapping can also happen in two ways (Fig. 2(b)). Calculated formation energies for these two process are shown in Fig. 2(c), which basically demonstrates that the intermediate structures are quite high in energy compared to evenly distributed and phase separated structures. The authors also performed nudged elastic band (NEB) calculations to determine the activation barriers for the first step to occur, i. e. to change a B-N bond to B-C and N-C bonds (Fig. 2(d)). Activation energy required is 1.63eV/atom suggesting that this process can happen only at elevated temperatures. At room temperature that is why the pristine (BN)*m*(C2)*n* should be stable and so are the phase separated ones.

#### 6 Will-be-set-by-IN-TECH 8 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 7

There can be two different patterns for phase segregated BCN alloys. One is the phase separated island-like and the other one is a striped pattern. The island-like pattern consists of larger graphene-BN interface region than that in the striped pattern. This means that the number of B-C and N-C bonds are less in striped pattern than in an island form. As we have discussed earlier, the maximization of C-C and B-N bonds thus favors a striped pattern [13].

Till now, we have discussed mainly the stability issues of (BN)*m*(C2)*n* with 1:1 ratio and phase separated BCN alloys. A distributed mixture of BN and graphene with different *m* : *n* ratios can also form depending on the growth condition. Different isomeric structures are also possible for a particular *m* : *n* ratio. In the following section we are going to present a DFT study to analyze the stability and electronic properties of (BN)*m*(C2)*n* with different *m* : *n* ratios. Utilizing the concept of aromaticity, the aim is to find out stable isomers for a particular *m* : *n* ratio and also to explore the possibilities of achieving desired electronic properties.

Aromaticity, as extensively used to determine the stability of organic molecules, can provide us a working principle for determining stability of the structures as well. Benzene (C6H6) is the prototype for the organic molecules, which are stabilized by aromaticity. Borazine (B3N3H6), an isoelectric BN analogue of benzene on the other hand has one-third stability of benzene from the point of view of aromaticity [18–21]. This is particularly interesting in (BN)*m*(C2)*n*, as the admixture of two not only changes the electronic property but also affects its stability. To investigate a stable isomer, our first working principle thus is to maximize the carbon hexagons, which essentially mimic benzene rings. A carbon-hexagon again can be surrounded by BN and each hexagon can be kept aloof or all hexagons can form a carbon-pathway. In a carbon pathway, *π*-conjugation is allowed whereas it is hindered in isolated C-hexagons.

To look for reasonable isomers, we consider that the following structural possibilities will not occur. Firstly, a hexagon will not contain B and N in 1 and 3 positions with respect to each other. These kind of structures are described by zwitterionic and biradical resonance structures, which basically result in an odd number of *π*-electrons on two of the Cs in the hexagon (Fig. 3).

**Figure 3.** Schematic representation of zwitterionic and biradical resonance structures. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American Chemical Society.

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 9

We have performed density functional calculations to investigate the isomers of (BN)*m*(C2)*n* [22]. All the structures are optimized with both (Perdew-Burke-Ernzerhof) PBE [23] and (Heyd-Scuseria-Ernzerhof) HSE [24] functionals. The functionals based on local spin density approximation or generalized gradient approximations reproduce the structural parameters reasonably well, whereas the band gaps come out to be much smaller compared to experiments. The reason behind this is the self interaction error. HSE, with a better description of exchange and correlation within hybrid DFT, yields a band gap, which is much closer to

The degree of aromaticity is calculated quantitatively, with a harmonic oscillator model of aromaticity (HOMA) prescribed by Krygowski *et al.*[25]. The HOMA value of an ideal aromatic compound (Benzene) will be 1, whereas the value will be close to zero for non aromatic compounds. Anti-aromatic compound with the least stability will have a negative HOMA value. As mentioned earlier, the aim is to find the important isomers with relatively high stability and reasonable band gaps among (BN)*m*(C2)*n* compounds with *m* : *n* ratios 1:1,

the experimental value.

2:1, 1:3 and 2:3. Let's focus on each type separately.

Hence, a B-N pair should be placed either in 1,4 or 1,2 position in the hexagon with respect to each other. *π*-electrons will thus be distributed over a C-C bond and form a resonance structure. Second kind of structural constraint, that we consider, is the absence of B-B or N-N bonds. As discussed earlier, these kind of bonds result in the lowering of *π*-bonds and thus decreased relative stability of an isomer.

The relative positions of B and N around an all C hexagon is also a key factor that controls the electronic properties. To illustrate the phenomenon, let's consider the following two isomers. As in Fig. 3, the isomer I and isomer II, both have similar chemical configuration. But in Isomer I, B and N are connected to C at position 1 and 4 in the hexagon, which we can call B-ring-N para-arrangement. A donor- acceptor (D-A) interaction is thus established in this kind of structural arrangement. On the other hand in isomer II, B and N are connected to 1*st* & 2*nd* (4*th* & 5*th*) positioned C atoms in the hexagon. Although a D-A interaction occurs between neighboring B and N, B-ring-N interaction is forbidden. The local D-A interaction around a C-hexagon, as shown in Fig. 4, increases the HOMO-LUMO gap whereas N-ring-N (or B-ring-B) para arrangement results in the lowering of the HOMO-LUMO gap.

6 Will-be-set-by-IN-TECH

There can be two different patterns for phase segregated BCN alloys. One is the phase separated island-like and the other one is a striped pattern. The island-like pattern consists of larger graphene-BN interface region than that in the striped pattern. This means that the number of B-C and N-C bonds are less in striped pattern than in an island form. As we have discussed earlier, the maximization of C-C and B-N bonds thus favors a striped pattern [13]. Till now, we have discussed mainly the stability issues of (BN)*m*(C2)*n* with 1:1 ratio and phase separated BCN alloys. A distributed mixture of BN and graphene with different *m* : *n* ratios can also form depending on the growth condition. Different isomeric structures are also possible for a particular *m* : *n* ratio. In the following section we are going to present a DFT study to analyze the stability and electronic properties of (BN)*m*(C2)*n* with different *m* : *n* ratios. Utilizing the concept of aromaticity, the aim is to find out stable isomers for a particular *m* : *n* ratio and also to explore the possibilities of achieving desired electronic properties.

Aromaticity, as extensively used to determine the stability of organic molecules, can provide us a working principle for determining stability of the structures as well. Benzene (C6H6) is the prototype for the organic molecules, which are stabilized by aromaticity. Borazine (B3N3H6), an isoelectric BN analogue of benzene on the other hand has one-third stability of benzene from the point of view of aromaticity [18–21]. This is particularly interesting in (BN)*m*(C2)*n*, as the admixture of two not only changes the electronic property but also affects its stability. To investigate a stable isomer, our first working principle thus is to maximize the carbon hexagons, which essentially mimic benzene rings. A carbon-hexagon again can be surrounded by BN and each hexagon can be kept aloof or all hexagons can form a carbon-pathway. In a carbon pathway, *π*-conjugation is allowed whereas it is hindered in

To look for reasonable isomers, we consider that the following structural possibilities will not occur. Firstly, a hexagon will not contain B and N in 1 and 3 positions with respect to each other. These kind of structures are described by zwitterionic and biradical resonance structures, which basically result in an odd number of *π*-electrons on two of the Cs in the

Hence, a B-N pair should be placed either in 1,4 or 1,2 position in the hexagon with respect to each other. *π*-electrons will thus be distributed over a C-C bond and form a resonance structure. Second kind of structural constraint, that we consider, is the absence of B-B or N-N bonds. As discussed earlier, these kind of bonds result in the lowering of *π*-bonds and thus

The relative positions of B and N around an all C hexagon is also a key factor that controls the electronic properties. To illustrate the phenomenon, let's consider the following two isomers. As in Fig. 3, the isomer I and isomer II, both have similar chemical configuration. But in Isomer I, B and N are connected to C at position 1 and 4 in the hexagon, which we can call B-ring-N para-arrangement. A donor- acceptor (D-A) interaction is thus established in this kind of structural arrangement. On the other hand in isomer II, B and N are connected to 1*st* & 2*nd* (4*th* & 5*th*) positioned C atoms in the hexagon. Although a D-A interaction occurs between neighboring B and N, B-ring-N interaction is forbidden. The local D-A interaction around a C-hexagon, as shown in Fig. 4, increases the HOMO-LUMO gap whereas N-ring-N

(or B-ring-B) para arrangement results in the lowering of the HOMO-LUMO gap.

isolated C-hexagons.

hexagon (Fig. 3).

decreased relative stability of an isomer.

**Figure 3.** Schematic representation of zwitterionic and biradical resonance structures. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American Chemical Society.

We have performed density functional calculations to investigate the isomers of (BN)*m*(C2)*n* [22]. All the structures are optimized with both (Perdew-Burke-Ernzerhof) PBE [23] and (Heyd-Scuseria-Ernzerhof) HSE [24] functionals. The functionals based on local spin density approximation or generalized gradient approximations reproduce the structural parameters reasonably well, whereas the band gaps come out to be much smaller compared to experiments. The reason behind this is the self interaction error. HSE, with a better description of exchange and correlation within hybrid DFT, yields a band gap, which is much closer to the experimental value.

The degree of aromaticity is calculated quantitatively, with a harmonic oscillator model of aromaticity (HOMA) prescribed by Krygowski *et al.*[25]. The HOMA value of an ideal aromatic compound (Benzene) will be 1, whereas the value will be close to zero for non aromatic compounds. Anti-aromatic compound with the least stability will have a negative HOMA value. As mentioned earlier, the aim is to find the important isomers with relatively high stability and reasonable band gaps among (BN)*m*(C2)*n* compounds with *m* : *n* ratios 1:1, 2:1, 1:3 and 2:3. Let's focus on each type separately.

## **2.1. 1:1 h-BN:Graphene (BC**2**N)**

We have considered six isomers for BC2N, among which two structures BC2N-I and BC2N-II consist of all C-hexagon pathways. In the third one,BC2N-III, all C-hexagons are connected linearly as in polyacenes where as the fourth one , BC2N-IV, has disconnected all-C-hexagons. The other two structures, BC2N-V & BC2N-VI do not have any all-C hexagon but BC2N-VI has at least polyacetylene paths whereas BC2N-V has only isolated C-C bonds. Although there stable C-C and B-N bonds but also helps in the *π*-conjugation. The result is reflected in the HOMA values of first two structures, which are 0.842 and 0.888 respectively. This suggests the formation of aromatic benzene like all-C hexagons. The HOMA value of BC2N-III is little less (0.642) but this structure in particular is not stable due to aromaticity rather due to the formation of polyacetylene paths. A slightly lower HOMA value observed in BC2N-I compared to BC2N-II is due to the difference in B-C bond (0.02Å), which leads to a change in

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 11

If we look at the formation energies of BC2N-IV & BC2N-VI, the values are quite close. BC2N-IV consists of completely isolated all-C hexagons. This is the reason of having high aromaticity of 0.88. But at the same time this increases N-C & B-C bonds and restricts *π*-conjugation. Therefore, this structure is less probable thermodynamically. BC2N-VI, which was suggested to be the most stable BC2N structure by Liu *et al.*[15], on the other hand has no aromatic all C-hexagon. But this structure contains all-C polyacetylene paths with C-C bond length 1.42 Å, which explains its low formation energy. BC2N-V is the least stable among all, which has neither all-C hexagon nor polyacetylene C-paths. Obviously most unstable B-C and B-N bonds are maximized here creating an enormous strain in the structure. The presence of only C-C bond of 1.327 Å explains that. These factors make this compound

All these results give us a stand point from where we can judge the thermodynamic stability

(b) The formation of aromatic all C-hexagons also does the same, while this is more effective

(c) There is not much contribution of B-ring-B or B-ring-N arrangement of B and N around poly(para-phenylene) (PPP) path in total energies. But indeed these, as discussed earlier,

Coming to the band gap issue, the first three structures, which are close in energy, have band gaps ranging from 1.6 to 2.3 eV in HSE calculations ( 0.7 to 1.7 eV in PBE). The difference in BC2N-I & BC2N-II comes from the arrangement of B and N around all-C hexagon. As discussed in Fig. 3, D-A interaction increases for para-positions (i. e.1,4 or 2,6), which is observed in BC2N-I. The band gap is 0.5 eV (0.65 eV in PBE), higher than that in BC2N-II, where B and N are oriented in ortho-position (i. e.1,2 or 4,5). Quite obviously, BC2N-III has all-C chain, which resembles a graphene nanoribbon and has the least value of the band gap.

We have investigated three structures of BCN, which have recently been synthesized [26]. The first one (BCN-I) has aromatic all-C hexagons connected in PPP path whereas the second one (BCN-III) contains all-C hexagon but connected in zigzag polyacene bonds. The final one (BCN-IV) consists of neither all-C hexagon nor a stripe of all-C region. BCN-I & BCN-II are iso-energetic, which is expected and is ∼ 0.5 eV lower than BCN-IV. This again explains the importance of aromatic all-C hexagon and *π*-conjugation. The absence of these and also the increased B-C, N-C bonds make BCN-IV relatively unstable. Another key point in BCN-I & BCN-I is the position of B and N around the hexagon. The stability may not be affected but

of other (BN)*m*(C2)*n* structures with the following working principles in hand:

will affect the band gap, which we will present in the following section.

thermodynamically most unstable among all five structures.

(a) *π*-Conjugation within all C path increases stability.

when hexagons are connected.

**2.2. 2:1 h-BN:Graphene (BCN)**

D-A interaction.

**Figure 4.** Qualitative representation of opening up a band gap and D-A interaction in isomer I and reduction of band gap in isomer II, with molecular orbital diagrams and valence bond representation. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American Chemical Society.

are several other isomers possible, we limit ourselves with these and try to understand the properties with the knowledge of aromaticity and conjugation. Firstly, the first three isomers, among all six are most stable and the relative energies differ by at most 0.15 eV (PBE) and 0.07 eV (HSE). The presence of all C-hexagons connected to each other not only increases the stable C-C and B-N bonds but also helps in the *π*-conjugation. The result is reflected in the HOMA values of first two structures, which are 0.842 and 0.888 respectively. This suggests the formation of aromatic benzene like all-C hexagons. The HOMA value of BC2N-III is little less (0.642) but this structure in particular is not stable due to aromaticity rather due to the formation of polyacetylene paths. A slightly lower HOMA value observed in BC2N-I compared to BC2N-II is due to the difference in B-C bond (0.02Å), which leads to a change in D-A interaction.

If we look at the formation energies of BC2N-IV & BC2N-VI, the values are quite close. BC2N-IV consists of completely isolated all-C hexagons. This is the reason of having high aromaticity of 0.88. But at the same time this increases N-C & B-C bonds and restricts *π*-conjugation. Therefore, this structure is less probable thermodynamically. BC2N-VI, which was suggested to be the most stable BC2N structure by Liu *et al.*[15], on the other hand has no aromatic all C-hexagon. But this structure contains all-C polyacetylene paths with C-C bond length 1.42 Å, which explains its low formation energy. BC2N-V is the least stable among all, which has neither all-C hexagon nor polyacetylene C-paths. Obviously most unstable B-C and B-N bonds are maximized here creating an enormous strain in the structure. The presence of only C-C bond of 1.327 Å explains that. These factors make this compound thermodynamically most unstable among all five structures.

All these results give us a stand point from where we can judge the thermodynamic stability of other (BN)*m*(C2)*n* structures with the following working principles in hand:

Coming to the band gap issue, the first three structures, which are close in energy, have band gaps ranging from 1.6 to 2.3 eV in HSE calculations ( 0.7 to 1.7 eV in PBE). The difference in BC2N-I & BC2N-II comes from the arrangement of B and N around all-C hexagon. As discussed in Fig. 3, D-A interaction increases for para-positions (i. e.1,4 or 2,6), which is observed in BC2N-I. The band gap is 0.5 eV (0.65 eV in PBE), higher than that in BC2N-II, where B and N are oriented in ortho-position (i. e.1,2 or 4,5). Quite obviously, BC2N-III has all-C chain, which resembles a graphene nanoribbon and has the least value of the band gap.

#### **2.2. 2:1 h-BN:Graphene (BCN)**

8 Will-be-set-by-IN-TECH

We have considered six isomers for BC2N, among which two structures BC2N-I and BC2N-II consist of all C-hexagon pathways. In the third one,BC2N-III, all C-hexagons are connected linearly as in polyacenes where as the fourth one , BC2N-IV, has disconnected all-C-hexagons. The other two structures, BC2N-V & BC2N-VI do not have any all-C hexagon but BC2N-VI has at least polyacetylene paths whereas BC2N-V has only isolated C-C bonds. Although there

**Figure 4.** Qualitative representation of opening up a band gap and D-A interaction in isomer I and reduction of band gap in isomer II, with molecular orbital diagrams and valence bond representation. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American

are several other isomers possible, we limit ourselves with these and try to understand the properties with the knowledge of aromaticity and conjugation. Firstly, the first three isomers, among all six are most stable and the relative energies differ by at most 0.15 eV (PBE) and 0.07 eV (HSE). The presence of all C-hexagons connected to each other not only increases the

**2.1. 1:1 h-BN:Graphene (BC**2**N)**

Chemical Society.

We have investigated three structures of BCN, which have recently been synthesized [26]. The first one (BCN-I) has aromatic all-C hexagons connected in PPP path whereas the second one (BCN-III) contains all-C hexagon but connected in zigzag polyacene bonds. The final one (BCN-IV) consists of neither all-C hexagon nor a stripe of all-C region. BCN-I & BCN-II are iso-energetic, which is expected and is ∼ 0.5 eV lower than BCN-IV. This again explains the importance of aromatic all-C hexagon and *π*-conjugation. The absence of these and also the increased B-C, N-C bonds make BCN-IV relatively unstable. Another key point in BCN-I & BCN-I is the position of B and N around the hexagon. The stability may not be affected but

#### 10 Will-be-set-by-IN-TECH 12 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 11

the band gap is definitely changed by this. As expected, from the discussion in Fig. 4, BCN-I has a quite high value of the band gap. Aromaticity is higher in BCN-I (0.846) compared to BCN-III (0.557) , which is also seen in BC2N structures. Iso-energetic BCN-I & BCN-II are equally probable during growth process but with a band gap range 1.3 to 2.7 eV .

**Figure 5.** Six isomers of BC2N are considered for this study. Relative energy (per formula unit) with respect to most stable structures and band gaps (in parentheses) are shown from HSE (normal print) and PBE (italic) calculations. HOMA values of all-C hexagons, obtained from HSE calculations are also provided. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American Chemical Society.

**Figure 6.** The isomers of BCN, BC3N, BC6N. Relative energy (per formula unit) with respect to most stable structures and band gaps (in parentheses) are shown from HSE (normal print) and PBE (italic) calculations. HOMA values of all-C hexagons, obtained from HSE calculations are also provided. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 13

general trend is observed that an increase in the graphene region reduces the band gap. All the lowest energy structures for different compositions of (BN)*m*(C2)*n* have band gaps around 1 eV, which is a desired value for technological applications. One very important thing that we learnt from this study is that the position of B and N around C-ring controls the band gap

Incorporation of magnetism in 2D sp-materials has been an important point of discussion in recent times. The combination of localized moments of 3*d* transition metal atoms and the *sp* electrons of the host 2D lattice can give rise to interesting magnetic properties relevant to nano devices based on the principle of magnetoresistance, for example. Ferromagnetic long-ranged order, half metallicity, large magnetic anisotropy, electric field driven switching of magnetization etc. are being studied for transition metal atoms adsorbed on graphene and 2D BN sheets. Another important point is the adsorption of these species at the interface between BN and graphene. A recent study [27] based on first principles electronic structure calculations has revealed some interesting electronic and magnetic properties of Fe, Co and Ni adatoms adsorbed on a h-BC2N sheet. A hexagonal site at the interface between BN and graphene

Chemical Society.

without affecting the stability.

**3. Functionalization**

## **2.3. 2:3** & **1:3 h-BN:Graphene (BC**3**N** & **BC**6**N)**

We now gradually increase the C-percentage with the anticipation of lowering the band gap because of increased graphene region. Two structures of BC3N and three structures of BC6N have been examined. Both the structures of BC3N consist of aromatic hexagonal all-C rings connected in PPP path. The similarity in HOMA values depicts that picture. The position of B and N around C-ring is different though in BC3N-I and BC3N-II. The para arrangement of B-ring-N results in a large band gap in BC3N-I (2.14 eV) while the ortho arrangement of the same in BC3N-II lowers the value of the band gap. The stability is not affected by that fact as aromaticity and *π*-band formation are quite similar, which make those structures iso-energetic. A similar situation is also seen in BC6N structures. BC6N-I and BC6N-II are iso-energetic mainly due to a similarity in structures. Both of them contain all-C rings connected in PPP path. But in the third one (BC6N-III ), all-C rings are separated, which makes this structure relatively unstable due to restricted *π*-conjugation. The HOMA value is maximum (0.93) in BC6N-III, whereas the values are 0.78 and 0.86 respectively for BC6N-I and BC6N-II. The para arrangement of B-ring-N in BC6N-I and BC6N-III leads to large band gaps (1.58 and 1.34 eV respectively) while ortho-positioning of the same results in a reduced band gap in BC6N-II. Finally, we have summarized the calculated (HSE and PBE) band gaps for all (BN)*m*(C2)*n* isomers (Fig. 7A and Fig. 7B). As expected, the band gaps are increased for HSE functional. Apart from that, both HSE and PBE -level calculations show a similar trend. A 12 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 11 Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 13

**Figure 6.** The isomers of BCN, BC3N, BC6N. Relative energy (per formula unit) with respect to most stable structures and band gaps (in parentheses) are shown from HSE (normal print) and PBE (italic) calculations. HOMA values of all-C hexagons, obtained from HSE calculations are also provided. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011) American Chemical Society.

general trend is observed that an increase in the graphene region reduces the band gap. All the lowest energy structures for different compositions of (BN)*m*(C2)*n* have band gaps around 1 eV, which is a desired value for technological applications. One very important thing that we learnt from this study is that the position of B and N around C-ring controls the band gap without affecting the stability.

## **3. Functionalization**

10 Will-be-set-by-IN-TECH

the band gap is definitely changed by this. As expected, from the discussion in Fig. 4, BCN-I has a quite high value of the band gap. Aromaticity is higher in BCN-I (0.846) compared to BCN-III (0.557) , which is also seen in BC2N structures. Iso-energetic BCN-I & BCN-II are

**Figure 5.** Six isomers of BC2N are considered for this study. Relative energy (per formula unit) with respect to most stable structures and band gaps (in parentheses) are shown from HSE (normal print) and PBE (italic) calculations. HOMA values of all-C hexagons, obtained from HSE calculations are also provided. Reprinted with permission from J. Phys. Chem. C 115, 10264 (2011). Copyright (2011)

We now gradually increase the C-percentage with the anticipation of lowering the band gap because of increased graphene region. Two structures of BC3N and three structures of BC6N have been examined. Both the structures of BC3N consist of aromatic hexagonal all-C rings connected in PPP path. The similarity in HOMA values depicts that picture. The position of B and N around C-ring is different though in BC3N-I and BC3N-II. The para arrangement of B-ring-N results in a large band gap in BC3N-I (2.14 eV) while the ortho arrangement of the same in BC3N-II lowers the value of the band gap. The stability is not affected by that fact as aromaticity and *π*-band formation are quite similar, which make those structures iso-energetic. A similar situation is also seen in BC6N structures. BC6N-I and BC6N-II are iso-energetic mainly due to a similarity in structures. Both of them contain all-C rings connected in PPP path. But in the third one (BC6N-III ), all-C rings are separated, which makes this structure relatively unstable due to restricted *π*-conjugation. The HOMA value is maximum (0.93) in BC6N-III, whereas the values are 0.78 and 0.86 respectively for BC6N-I and BC6N-II. The para arrangement of B-ring-N in BC6N-I and BC6N-III leads to large band gaps (1.58 and 1.34 eV respectively) while ortho-positioning of the same results in a reduced band gap in BC6N-II. Finally, we have summarized the calculated (HSE and PBE) band gaps for all (BN)*m*(C2)*n* isomers (Fig. 7A and Fig. 7B). As expected, the band gaps are increased for HSE functional. Apart from that, both HSE and PBE -level calculations show a similar trend. A

American Chemical Society.

**2.3. 2:3** & **1:3 h-BN:Graphene (BC**3**N** & **BC**6**N)**

equally probable during growth process but with a band gap range 1.3 to 2.7 eV .

Incorporation of magnetism in 2D sp-materials has been an important point of discussion in recent times. The combination of localized moments of 3*d* transition metal atoms and the *sp* electrons of the host 2D lattice can give rise to interesting magnetic properties relevant to nano devices based on the principle of magnetoresistance, for example. Ferromagnetic long-ranged order, half metallicity, large magnetic anisotropy, electric field driven switching of magnetization etc. are being studied for transition metal atoms adsorbed on graphene and 2D BN sheets. Another important point is the adsorption of these species at the interface between BN and graphene. A recent study [27] based on first principles electronic structure calculations has revealed some interesting electronic and magnetic properties of Fe, Co and Ni adatoms adsorbed on a h-BC2N sheet. A hexagonal site at the interface between BN and graphene

#### 12 Will-be-set-by-IN-TECH 14 Composites and Their Properties Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 13

extreme case, all C atoms of ZGNRs are replaced by B and N atoms and zigzag BN nanoribbons are formed. In the ground state, the two edges are antiferromagnetically coupled and remain so for all dopings. However, the application of an external electric field affects the electronic structure of the nanoribbon giving rise to semiconducting and half-metallic properties. Electric-field induced changes in the magnetic properties are very interesting from a technological point of view. Other related studies [29] based on DFT revealed energetics, electronic structure and magnetism of quantum dots and nanorods of graphene embedded in BN sheet. It was showed that the formation energies and the HOMO-LUMO gaps of quantum

Graphene-Boron Nitride Composite: A Material with Advanced Functionalities 15

Adsorption of gases on 2D materials is an important topic from the technological and environmental points of view. Materials for clean energy are always sought for and in this respect, efficient hydrogen uptake of suitable materials is an important issue. Raidongia *et al.* [26] have studied H2 adsorption on BCN at 77 K and 1 atm. pressure. From their experiments, H2 uptake of 2.6 wt % was observed. Also, CO2 adsorption is very important for environmental issues. In their study, BCN was found to have a very high CO2 uptake of 100 wt % at 195 K and 1 atm. pressure. It should be noted that the uptake is only 58 % by the activated charcoal under identical conditions. At room temperature and 40 bar pressure, the

In a recent theoretical study, Cao *et al.* [30] showed that a zigzag interface between BN and graphene can have a strong capability of adsorbing hydrogen, much stronger than pure graphene, BN or the armchair interface between them. Moreover, the adsorption of hydrogen induces a semiconductor to metal transition. As the mobility of hydrogen on the surface is rather high, the hydrogen atoms can migrate to the zigzag interface and hence will increase the density of hydrogen storage with the added functionality of band gap engineering.

Graphene-BN nanocomposites offer a huge potential in various technological sectors, e.g., nano electronics, gas sensing, hydrogen storage, nanomagnetic storage devices, to name a few. The unique combination of these two materials with different electronic properties, forming a 2D network, offers many possibilities for studying fundamental science and applications for nanotechnology. However, many challenges, both in the domains of experiment and theory, will come in the way. Experimental synthesis of samples of good quality and state-of-the-art characterization techniques to reveal the atomic scale physics will be the issues. From the point of view of theory, one faces difficulties in having a correct description of the band gaps and electronic structures in standard approximations of materials-specific theories. However, with the availability of powerful supercomputing facilities, it is nowadays possible to treat large systems by sophisticated many body theories to have a much better quantitative descriptions. Nevertheless, one may envisage many interesting directions for the applications of these nanocomposites to utilize the interface properties of BN and graphene. One of them is the spin switching properties of organometallics adsorbed at the interface, similar to what has been studied recently [31] for a 2D graphene sheet. The other application can be the adsorption of amino acids [32] at the interface to increase the activity by their immobilization. Hopefully, in near future, we will observe many applications of these nanocomposites, useful

dots vary as 1/√*n*, where *n* is the number of carbon atoms in the dots.

CO2 uptake was found to be 44 wt %.

**4. Summary and outlook**

for the human society.

**Figure 7.** Band gaps of different isomers of (BN)*m*(C2)*n* are plotted against BN density, obtained from both (A) HSE and (B) PBE calculations. Relative energies are given in parentheses.

turns out to be a favorable site for adsorption. The presence of Fe, Co and Ni makes the system a magnetic semiconductor, a magnetic semi-metal and a non magnetic semiconductor respectively. Another interesting observation was that the adatoms are highly mobile on the surface and hence have the possibility to have a clustered configuration among themselves. It is interesting to note that the properties of this hybrid system are tunable in the sense that they can be modified by having different combinations of the width of each subsystem, BN and graphene.

Not only by magnetic adatoms, but by intrinsic edge properties, one can render magnetism in these 2D sheets. By DFT calculations, Dutta *et al.* [28] predicted interesting magnetic properties of H-passivated zigzag nanoribbons (ZGNRs) of various widths, doped by boron and nitrogen, keeping the whole system isoelectronic with C atoms in graphene. In the extreme case, all C atoms of ZGNRs are replaced by B and N atoms and zigzag BN nanoribbons are formed. In the ground state, the two edges are antiferromagnetically coupled and remain so for all dopings. However, the application of an external electric field affects the electronic structure of the nanoribbon giving rise to semiconducting and half-metallic properties. Electric-field induced changes in the magnetic properties are very interesting from a technological point of view. Other related studies [29] based on DFT revealed energetics, electronic structure and magnetism of quantum dots and nanorods of graphene embedded in BN sheet. It was showed that the formation energies and the HOMO-LUMO gaps of quantum dots vary as 1/√*n*, where *n* is the number of carbon atoms in the dots.

Adsorption of gases on 2D materials is an important topic from the technological and environmental points of view. Materials for clean energy are always sought for and in this respect, efficient hydrogen uptake of suitable materials is an important issue. Raidongia *et al.* [26] have studied H2 adsorption on BCN at 77 K and 1 atm. pressure. From their experiments, H2 uptake of 2.6 wt % was observed. Also, CO2 adsorption is very important for environmental issues. In their study, BCN was found to have a very high CO2 uptake of 100 wt % at 195 K and 1 atm. pressure. It should be noted that the uptake is only 58 % by the activated charcoal under identical conditions. At room temperature and 40 bar pressure, the CO2 uptake was found to be 44 wt %.

In a recent theoretical study, Cao *et al.* [30] showed that a zigzag interface between BN and graphene can have a strong capability of adsorbing hydrogen, much stronger than pure graphene, BN or the armchair interface between them. Moreover, the adsorption of hydrogen induces a semiconductor to metal transition. As the mobility of hydrogen on the surface is rather high, the hydrogen atoms can migrate to the zigzag interface and hence will increase the density of hydrogen storage with the added functionality of band gap engineering.

## **4. Summary and outlook**

12 Will-be-set-by-IN-TECH

20 30 40 50 60 70 BN Content (%)

BC BCN 3

(0.11)

(0.56) (1.53)

(0.33)

(0.0)

(0.07)

20 30 40 50 60 70 BN Content (%)

(0.03)

**Figure 7.** Band gaps of different isomers of (BN)*m*(C2)*n* are plotted against BN density, obtained from

turns out to be a favorable site for adsorption. The presence of Fe, Co and Ni makes the system a magnetic semiconductor, a magnetic semi-metal and a non magnetic semiconductor respectively. Another interesting observation was that the adatoms are highly mobile on the surface and hence have the possibility to have a clustered configuration among themselves. It is interesting to note that the properties of this hybrid system are tunable in the sense that they can be modified by having different combinations of the width of each subsystem, BN

Not only by magnetic adatoms, but by intrinsic edge properties, one can render magnetism in these 2D sheets. By DFT calculations, Dutta *et al.* [28] predicted interesting magnetic properties of H-passivated zigzag nanoribbons (ZGNRs) of various widths, doped by boron and nitrogen, keeping the whole system isoelectronic with C atoms in graphene. In the

(0.0)

(0.0) (0.07)

(0.06) (0.34)

(0.42) (1.61)

N BC2

(0.0)

(0.0)

(0.51)

(0.05)

(0.01) (0.54)

N BCN

0 0.5 1 1.5 2 2.5 3 3.5

0

0.5

1

1.5

Band Gap in GGA (eV)

and graphene.

2

2.5

(0.0)

N

B)

(0.0)

both (A) HSE and (B) PBE calculations. Relative energies are given in parentheses.

(0.08) (1.29)

(0.07) (1.34)

BC6

A)

(0.0)

N BC3

(0.07)

N BC2 BC N 6

Band Gap in HSE (eV)

Graphene-BN nanocomposites offer a huge potential in various technological sectors, e.g., nano electronics, gas sensing, hydrogen storage, nanomagnetic storage devices, to name a few. The unique combination of these two materials with different electronic properties, forming a 2D network, offers many possibilities for studying fundamental science and applications for nanotechnology. However, many challenges, both in the domains of experiment and theory, will come in the way. Experimental synthesis of samples of good quality and state-of-the-art characterization techniques to reveal the atomic scale physics will be the issues. From the point of view of theory, one faces difficulties in having a correct description of the band gaps and electronic structures in standard approximations of materials-specific theories. However, with the availability of powerful supercomputing facilities, it is nowadays possible to treat large systems by sophisticated many body theories to have a much better quantitative descriptions. Nevertheless, one may envisage many interesting directions for the applications of these nanocomposites to utilize the interface properties of BN and graphene. One of them is the spin switching properties of organometallics adsorbed at the interface, similar to what has been studied recently [31] for a 2D graphene sheet. The other application can be the adsorption of amino acids [32] at the interface to increase the activity by their immobilization. Hopefully, in near future, we will observe many applications of these nanocomposites, useful for the human society.

14 Will-be-set-by-IN-TECH 16 Composites and Their Properties **Chapter 2**

## **Author details**

Sumanta Bhandary and Biplab Sanyal *Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden*

## **5. References**

© 2012 Yan et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Yan et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Graphene Nanocomposites**

Mingchao Wang, Cheng Yan and Lin Ma

Additional information is available at the end of the chapter

Graphene, one of the allotropes (diamond, carbon nanotube, and fullerene) of carbon, is a monolayer of honeycomb lattice of carbon atomsdiscovered in 2004. The Nobel Prize in Physics 2010 was awarded to Andre Geim and Konstantin Novoselov for their ground breaking experiments on the two-dimensional graphene [1]. Since its discovery, the research communities have shown a lot of interest in this novel material owing to its unique properties. As shown in Figure 1, the number of publications on graphene has dramatically increased in recent years. It has been confirmed that graphene possesses very peculiar electrical properties such as anomalous quantum hall effect, and high electron mobility at room temperature (250000 cm2/Vs). Graphene is also one of the stiffest (modulus ~1 TPa) and strongest (strength ~100 GPa) materials. In addition, it has exceptional thermal conductivity (5000 Wm-1K-1). Based on these exceptional properties, graphene has found its applications in various fields such as field effect devices, sensors, electrodes, solar cells, energy storage devices and nanocomposites. Only adding 1 volume per cent graphene into polymer (e.g. polystyrene), the nanocomposite has a conductivity of ~0.1 Sm-1[2], sufficient for many electrical applications. Significant improvement in strength, fracture toughness and fatigue strength has also been achieved in these nanocomposites [3-5]. Therefore, graphene-polymer nanocomposites have demonstrated a great potential to serve as next generation functional

Relatively, limited research has been conducted to understand the intrinsic structureproperty relationship in graphene based composites such as graphene-polymer nanocomposites. The mechanical property enhancement observed in graphene-polymer nanocomposites is generally attributed to the high specific surface area, excellent mechanical properties of graphene, and its capacity to deflect crack growth in a far more effectively way than one-dimensional (e.g. nanotube) and zero-dimensional (e.g. nanoparticle) fillers [5]. On the other hand, the graphene sheets or thin platelets dispersed in polymer matrix may create wavy or wrinkled structures that tend to unfold rather than stretch under applied loading.

http://dx.doi.org/10.5772/50840

**1. Introduction**

or structural materials.


*Optical Nonlinearities in Glasses DOI: http://dx.doi.org/10.5772/intechopen.101774*

example, the application of both heat and electric fields (thermal poling) gives rise to SHG. Since χ(2) is not physically possible in a centrosymmetric material, the creation of an axial symmetry under thermal poling has been demonstrated to be effective to introduce second-order nonlinearity properties [29]. Another route to create an optical SHG is by the introduction of optical non-linear nanocrystals within a glass matrix. Although thermal poling is an efficient way to induce SHG in silicate glasses, χ(2) also appeared after glass heat treatments to precipitate crystallites of non-centrosymmetric compounds [39]. This strategy gives rise to transparent crystallized glasses (glass-ceramics). Nevertheless, more research is necessary to clarify some aspects, for instance, whether the thermal poling approach is effectively the best choice for raising SHG.

In the glass transparency region, which is found between the ionic (vibrational) and the electronic excitation interactions and where no permanent electric dipoles are present, the light frequency is too high for the ionic polarizability to follow the E field oscillations and too low to resonate with the electronic excitations [10]. Still, multiphoton processes may occur. For example, the probability of two-photon absorption is proportional to the square of the E field intensity [10].


**16. Safe biomaterial**

12 The Complex World of Polysaccharides

quality will also influence its biocompatibility [69.70].

responses to biomaterial implants [76-81].

**15. Biodegradability**

the physical and chemical properties of the chitosan in order to improve its medicinal

The excellent biological properties of chitosan can be potentially improved with a variety of additional chemicals such as polyethylene glycol and carboxymethyl N-acyl groups in order to produce biocompatible chitosan derivatives for use as wound dressings [72]. Chitosan's positive surface charge enables it to effectively support cell growth [73]. Chitosan-gelatin sponge wound dressing had demonstrated a superior antibacterial effect. Additionally, chitosan gelatin sponge allowed the wound site to contract markedly and shortened the wound healing time, as compared with sterile Vaseline gauze [74]. Widely used surface modification techniques include coating, oxidation by low temperature plasma for better printing and adhesion and surfactant addition for antistatic. Blends are often used to improve tensile properties and to provide a stronger structural component for separation media that supports the active polymer. The physical properties of a polymer can also be altered by introducing a second polymer that improves the properties of the original polymer in certain aspects, such as hydrophobility, lowered melt temperature, raised glass transition temperature, etc [75]. A thorough understanding of cell and proteins interactions with artificial surfaces is of importance to design suitable implant surfaces and substrates. The surface properties of newly synthesized biomedical grade chitosan derivatives, including surface composition, wettability, domain composition, surface oxidation, surface charge and morphology, may influence protein adsorption and subsequently, the cellular

The claim "biodegradable" is often associated with environmentally friendly products. It is defined as being able to be broken down by natural processes, into more basic components.

An important aspect in the use of polymers as drug delivery systems is their metabolic fate in the body or biodegradation. In the case of the systemic absorption of hydrophilic polymers such as chitosan, they should have a suitable Mw for renal clearance. If the administered polymer's size is larger than this, then the polymer should undergo degradation. Biodegradation (chemical or enzymatic) provides fragments suitable for renal clearance. Chemical degradation in this case refers to acid catalysed degradation i.e. in the stomach. Enzymatically, chitosan can be degraded by enzymes able to hydrolyse glucosamine–glucosamine, glucosamine–N- acetyl-glucosamine and N-acetyl-glucosamine– N-acetyl- glucosamine linkages [83]. Even though depolymerisation through oxidation– reduction reaction [84] and free radical degradation [85] of chitosan have been reported

Chitosan is thought to be degraded in vertebrates predominantly by lysozyme and by bacterial enzymes in the colon [83, 86]. However, eight human chitinases (in the glycoside hydrolase 18 family) have been identified, three of which have shown enzymatic activity

Products are usually broken down by bacteria, fungi or other simple organisms [82].

these are unlikely to play a significant role in the in vivo degradation.

Chitosan is a potentially biologically compatible material that is chemically versatile (–NH2 groups and various Mw). These two basic properties have been used by drug delivery and tissue engineering to create a great amount of formulations and scaffolds that show promise in healthcare. It is approved for dietary applications in Japan, Italy and Finland [91] and it has been approved by the FDA for use in wound dressings [92] but is not approved for any product in drug delivery. The term "Chitosan" represents a large group of structurally different chemical entities that may show different biodistribution, biodegradation and toxicological profiles.

The formulation of chitosan with a drug may alter the pharmacokinetic and biodistribution profile.The balancing, or reduction, of the positive charges on the chitosan molecule has effects on its interaction with cells and the microenvironment, often leading to decreased uptake and a decrease in toxicity. The modifications made to chitosan could make it more or less toxic and any residual reactants should be carefully removed. In addition, the route of administration determines the uptake, concentration, contact time and cell types affected.

**Table 1.** Toxicity of chitosan and chitosan derivatives Table taken from [94]

14 The Complex World of Polysaccharides

95% DD, 18.7 kDa Steric acid conjugation micelle

95% DD, 18.7 kDa teric acid conjugation and

entrapment in micelle

97% DD, 65 kDa N-octyl-O-sulphate Invitro, primary rat

**Modification Assessment IC50**

hepatocytes

None, glutamic acid salt 0.56±0.10, 0.48±0.07,

None, Lactic acid salt 0.38±0.13, 0.31±0.06,

None, hydrochloride salt 0.23±0.13, 0.22±0.06,

None, lactic acid salt In vitro B16F10 cells 2.00±0.18 mg/ml

None, glutamic acid salt In vitro B16F10 cells 2.47±0.14 mg/ml

None, lactic acid salt In vitro B16F10 cells 1.73±1.39 mg/ml

In vitro, MCF7 and COS7 cells, 6 h & 24 h

pH 6.2

None, aspartic acid salt In vitro, Caco-2 cells,

78% DD, <50 kDa None, lactic acid salt In vitro B16F10 cells 2.50 mg/ml

100% DD, 152 kDa Glycol chitosan In vitro B16F10 cells 2.47±0.15 mg/ml

None, hydrochloric acid

None, hydrochloric acid

chitosan, chloride salt

salt

salt

100% DD, 3–6 kDa 20, 44, 55% Trimethyl

100% DD, 3–6 kDa 94% Trimethyl chitosan,

100% DD, 3–6 kDa 94% Trimethyl chitosan,

100% DD, 100 kDa 36% Trimethyl chitosan, chloride sal

100% DD, 100 kDa 36% Trimethyl chitosan, chloride sal

chloride salt

chloride salt

In vitro ,A549 cells 369±27 μg/ml

In vitro ,A549 cells 234±9 μg/ml

>200 mg/ml

mg/ml

mg/ml

mg/ml

mg/ml

>10 mg/ml

In vitro B16F10 cells 2.24±0.16 mg/ml

In vitro B16F10 cells 0.21±0.04 mg/ml

In vitro, MCF7, 6 h 1.402±0.210 mg/ml

In vitro, COS7, 6 h 2.207±0.381 mg/ml

In vitro, MCF7, 6 h 0.823±0.324 mg/ml

In vitro, COS7, 6 h >10 mg/ml

0.67±0.24, 0.61±0.10, 0.65±0.20, 0.72±0.16

0.35±0.06, 0.46±0.06

0.34±0.04, 0.37±0.08

0.27±0.08, 0.23±0.08

**Chitosan details (DD, MW)**

87% DD, 20, 45, 200, 460 kDa

87% DD, 20, 45, 200, 460 kDa

87% DD, 20, 45, 200, 460 kDa

87% DD, 20, 45, 200, 460 kDa

82% DD, 150–170

>80% DD, 60–90

77% DD, 180–230

85% DD, 60–90

81% DD, 100–130

kDa

kDa

kDa

kDa

kDa

In a series of articles are described the effects of chitosans with differing molecular weights and degree of deacetylation in vitro and in vivo. Toxicity was found to be degree of deacetylation and molecular weight dependent. At high DD the toxicity is related to the molecular weight and the concentration, at lower DD toxicity is less pronounced and less related to the molecular weight [93].

A summary of toxicities of chitosan and derivatives assessed through IC50 values is presented in the next table [94].

From this table it can be gathered that most chitosans (and derivatives) are not significantly toxic compared to a toxic polymer such as polyethylenimine [94].

It appears that the toxicity of chitosan is related to the charge density of the molecule, toxicity increases with increasing density. It appears that there is a threshold level below which there are too few contact points between a molecule and the cell components to produce a significantly toxic effect. This balance is between 40 and 60% DD, or degree of trimethylation, although any sufficiently small chitosan (<10 kDa) is not appreciably toxic. Modifications that do not increase the charge on the molecule seem to have little effect on the toxicity beyond that of the native molecule [94].


**2. Early histological backgrounds of "scientifically approach to nutrition"**

For more than 200 years, the fibre in plant foods has been known by animal nutritionists to have significant effects on digestion. Its role in human nutrition began to be investigated towards the end of the nineteenth century. Denis Burkitt (1911–1993), a surgeon, brought back to the United Kingdom from Africa ideas from a range of disciplines along with his own observations to propose a radical view of fibre in human health. Later, Burkitt met with three physicians in London, Cleave, Campbell, and Trowell, who collected observations on fibre consumption and the distribution of different diseases in Africa. Trowell worked in London at St. Thomas' Hospital and had visited Uganda in 1948, 1958, and 1970, and Kenya in 1929. He observed that the distribution of different diseases changed along with changes in the population during the observed periods. Namely, the numbers of different diseases increased in people who came from Western countries, whereas the numbers remained unchanged in the African populations. In addition, he noted the emergence of entirely new diseases like diabetes mellitus, hypertension, and malignant conditions in the population coming from Europe.

Burkitt built on the work of Cleave, Campbell, and Trowell to develop the "fibre hypothesis" [2, 3], the main point of which is that fibre consumption decreases the risks of obesity, diabetes, dental caries, various vascular disorders, large bowel cancer, appendicitis, and diverticulosis, particularly in the UK population. This hypothesis was considered groundbreaking at the time, and spurred further research (e.g., "Dietary Fibre in Europe. Current State of Knowledge on Definitions, Sources, Recommendations, Intakes and Relationships to Health." Nutrition Research Reviews. 2017;**30**:149-190).