Effect of Interstitial Si on Different Boron Nitride Allotropes - The

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Effect of Interstitial Si on Different Boron Nitride Allotropes Anna Pallas and Karin Larsson* Department of Chemistry − Ångström Laboratory, Uppsala University, Box 523, Uppsala 751 20, Sweden ABSTRACT: Boron nitride (BN) is a very interesting material, being isoelectronic with diamond. It can form the following allotropes; hexagonal (h-BN), cubic (c-BN), wurtzitic (w-BN), and rhombohedral (r-BN). However, there are severe problems with the syntheses of some of these crystalline phases, especially using chemical vapor deposition (CVD) techniques. The underlying reasons for these growth difficulties are of largest importance to investigate more in detail. For each of these crystalline phases, the thin film surface reactivity is one factor that has a major influence on both growth and surface properties. The surface reactivity of specifically the B and N surface sites on these crystalline phases has therefore here been studied by performing first-principle density functional theory (DFT) calculations under periodic conditions. The following surfaces were studied: c-BN (100), h-BN (001), wBN (100), and r-BN (001). The adsorption energy for different surface-terminating species (H, F, and Cl) has been taken as a measure of surface site reactivity. Since experimental studies have shown that Si contamination will improve the possibility for growth of r-BN, the effect of this dopant has also been considered in the present work. The results indicate that the surface reactivities for nonterminated N-sites are more pronounced for the situations with an otherwise completely covered surface by H species (compared to F and Cl). In fact, this was the situation for all BN allotropes (c-, h-, w-, and r-BN). The surface reactivities for nonterminated B-sites showed the same behavior for the w- and r-BN phases. For the cand h-BN surfaces, a nonterminated B surface site on an otherwise H-terminated surface is, however, slightly less reactive compared to the corresponding F-terminated surface. In addition, there were extreme problems to terminate the various surfaces with Cl species. Furthermore, the existence of interstitial Si dopants did only show a positive effect for the reactivity of a bare Bsite on the h- vs r-BN surface (on an otherwise H-terminated surface). However, the calculated stabilization energy showed that it is not possible to Si dope the h-BN surface lattice. Hence, it was only for r-BN that interstitial positioning of Si was found possible, and that also gave a positive effect on the surface reactivity.

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INTRODUCTION Boron nitride (BN) is an interesting binary compound, being isoelectronic to carbon and also existing in the same crystalline phases: hexagonal (h-BN), cubic (c-BN), rhombohedral (rBN), and wurtzite (w-BN) boron nitride. The hexagonal sp2bonded phase consists of planar sheets and is easily synthesized by heating boron oxide with a nitrogen compound.1 The sheets are not staggered as in graphite but stacked because of the polarity mismatch between the N and B atoms, with alternating B and N atoms within the rings.1 Because of the strong covalent bonding between the B and N atoms within the same layer, hBN is an insulator.2 In contrary, the corresponding carbon phase (i.e., graphite) has a metallic conductivity. The h-BN phase exhibits several unique properties such as low density (2.27 g cm−3), good thermal properties, low thermal expansion, chemical inertness, optical reflectance, and nontoxicity.2,3 Moreover, h-BN is stable at room temperature and is used in various applications. Because of the weak van der Waals forces between the sheets, the planes easily slide against each other, and h-BN is therefore used as, e.g., a solid lubricant or as a powder.3,4 Because of its properties like nontoxicity, optical reflectance, and the ability to not absorb moisture from the skin, powdered h-BN is also used in the cosmetic industry.5,6 At high pressures and temperatures (60 GPa and 2000 °C), the hexagonal phase changes into a cubic sp3-bonded phase with the same crystal structure as diamond.1 This cubic phase © 2014 American Chemical Society

has several interesting and extreme properties similar, and even superior, to diamond. The properties include chemical stability and inertness, wear resistance, extreme hardness, large bandgap, low density, thermal conductivity, and high resistance to oxidation.7−13 The oxidation and graphitization temperatures for c-BN are much higher than for diamond, and c-BN can more easily than diamond be n-doped.10−12 It is a material well suited for machining of steel, cast iron, and ferrous alloys, as well as a material well suited for cutting tools and highfrequency electronic devices.7,14,15 There are, however, severe problems connected to the syntheses of c-BN, to be compared with diamond that is relatively easily synthesized using chemical vapor deposition methods. The commonly used techniques for c-BN growth are based on surface bombardment with highly energetic ions.16 These techniques lead to delamination of the film at even low BN thicknesses. The main reason is the combination of (i) different BN phases with amorphous BN close to the substrate, (ii) poor crystalline quality, and (iii) high film stress.10,15−17 By using static high-pressure or dynamic shock methods on already synthesized hexagonal BN, a third form of BN can be synthesized: wurtzite BN (w-BN).18,19 The w-BN structure is Received: March 17, 2014 Revised: July 26, 2014 Published: August 1, 2014 20264

dx.doi.org/10.1021/jp502643j | J. Phys. Chem. C 2014, 118, 20264−20274

The Journal of Physical Chemistry C

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an sp3-bonded metastable phase, with slightly higher lattice energy than the sp3-bonded c-BN.20−22 The w-BN is not used in industrial applications and is, hence, only of an academic interest so far.23 Another polymorph of BN, which also is of an academic interest, is the rhombohedral boron nitride (r-BN) phase. It has an sp 2 -bonded structure, analogous to rhombohedral graphite.24 The r-BN phase can transform to the cubic phase under pressure, but some dynamic and kinetic barriers have then to be overcome.25 The main purpose of the present study is to theoretically investigate the surface reactivity of the B and N surface sites for the four crystalline phases of BN (hexagonal, cubic, rhombohedral, and wurtzite). Moreover, the energetic stabilities of the various surfaces have also been of large interest to study. For these purposes, the energy of adsorption for various terminating species (H, F, or Cl) has carefully been calculated using density functional theory (DFT) under periodic boundary conditions. Experimental observations made by Chubarov et al. showed that the presence of Si in the reaction chamber will favor the formation of r-BN (instead of h-BN).26 Gaseous SiH4 was then added in a small amount to the gas mixture. The effect by the interstitially positioned Si atom has therefore also been investigated for the two hexagonal phases r- and h-BN, respectively. The main purpose was to first highlight the effect on phase stability but also on surface reactivity. Both observables are important for the possibility to grow r-BN and are, hence, important for the future design of experimental r-BN synthesis CVD setups.

BN(94%X) + X → BN(100%X)

(X = H, F, or Cl) (1)

ΔEads = E BN(100%X) − (E BN(94%X) + E X ) (X = H, F, or Cl)

(2)

where EBN(100%X) and EBN(94%X) are the total energies for the 100% and 94% X-terminated BN surface, respectively. These total energies have been obtained from the geometry optimization of the model structures. EX is the total energy for the single atom (H, F, or Cl). The relative stabilization energy for the Si-doped surfaces, relative to the nondoped ones, can be calculated as shown in eqs 3 and 4 BN(0%Si) + Si = BN(0.6%Si)

(3)

ΔEstab = E BN(0.6%Si) − (E BN(0%Si) + ESi)

(4)

where EBN(0%Si) and EBN(0.6%Si) are the total energies for the geometry optimized nondoped BN surface and the Si-doped (to 0.06%) surface, respectively. ESi is the total energy for the single Si atom. Because of the expected strong correlation between bond energy and electron bond populations/atomic charges for polar covalent bonds, further analyses of electron bond populations and degree of electron transfer within the bonds have been made in the present study. To estimate the atomic charges, a portioning of the Mulliken charge to individual atoms was made by projecting the plane wave states onto the localized basis set.33 The electron bond population is an estimation of the density of electrons within the bonds and is thereby also regarded as a measure of the covalent bond strength. A more general covalent bond most often shows a certain degree of polarity (i.e., ionicity). Hence, calculations of atomic charges will give a measure of the ionic contribution to the bond strength. The surface models were constructed as supercells. The c-BN (100) surface includes alternating layers of N and B, with 4 × 4 B (or N) atoms in each layer. Moreover, the supercell models of h-BN (001), w-BN(100), and r-BN(001) surfaces do all include 8 B and 8 N atoms in each layer. In addition, the slab thicknesses are for all models 10 atomic layers. To suppress the artificial charge that occurs from the polarity between the upper and lower layers of BN, all supercell structures were associated with a large vacuum slab thickness of 20 Å. In addition, the bottom dangling bonds were also saturated with H atoms.34 The dangling bond-passivating H atoms, in addition to the two bottom layers, were held fixed during the calculations in order to simulate a continuous bulk structure. The rest of the atoms in the slab were allowed to fully relax during the geometry optimization calculations using the Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm.35 A slab of minimum five layers was in ref 36 observed to be required to avoid interactions between the reconstructed upper and lower surfaces of thin slabs in c-BN.36 The 10-layer slabs used in the present study can therefore be regarded as adequate to use for the present study.

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METHODS In order to study the geometrical structures and electron distributions for the different crystalline BN phases, a density functional theory (DFT) method was used. It was based on an ultrasoft pseudopotential plane-wave approach and available within the Cambridge Sequential Total Energy Package (CASTEP) program from Accelrys, Inc.27−29 By using the spin-polarized generalized gradient approximation (GG(S)APW91), developed by Perdew and Wang, the electronic exchange and correlation corrections were approximated.27 In contradiction to the most simple local desity approximation (LDA), the GG(S)A method describes the density inhomogeneity within the system by using an electron density gradient expansion.30 The GGA methods are generally known to be more accurate than the LDAs. The LDA method is known to underestimate bond lengths and to overbind electrons in a molecule.31 Within the present study, the energy cutoff for the plane-wave kinetic was set to 220.00 eV. The k-point mesh (generated from the Monkhorst−Pack scheme) used for all calculations was set to 2 × 2 × 1, yielding two irreducible kpoints.32 These kinetic cut-off values and k-points were defined after careful test calculations. The H adsorption energies for the H-terminated surfaces of c-BN (100), h-BN (001), w-BN, and r-BN (001) were calculated for various cutoff energies and kpoints. The resulting adsorption energy difference between kpoint mesh values of 2 × 2 × 1 and 1 × 1 × 1 was less than 10%. This was also the situation for the difference in H adsorption energies when using the kinetic cutoff energy values 220.00 and 180.00 eV. The adsorption energies associated with the chemisorption of the last terminating species (i.e., before the surface became 100% terminated; see eq 1) was calculated by using eq 2.

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RESULTS AND DISCUSSION A. General. The BN compound can exist in four different phases: cubic (c-BN), hexagonal (h-BN), rhombohedral (rBN), and wurtzite (w-BN). Of a large interest is to first of all compare the structural stabilities for these various phases and

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work by Zhang et al.37 Their XPS data from experimentally synthesized c-BN showed very low concentrations of F atoms onto the N-rich surface.37 Also, earlier theoretical results showed that the F-terminated surface is less energetically preferred for the following adsorption of growth B precursors onto the N-terminated surface. The F-terminated surface was also shown to be less effective in stabilizing and maintaining the surface sp3 hybridization.38,39 The adsorption energy results are supported by the electron bond populations (Table 2), where the bond population for the N-rich N−H bond (0.72) is remarkably higher than for the N− F bonds (0.26)). The N-rich surface was, however, found not to be possible to terminate with a full coverage of Cl species. The energy of Cl adsorption is +181 kJ/mol. This result indicates an endothermic process and is, hence, not expected to take place. The corresponding B-rich and H-terminated c-BN (100) surface is 2 × 1-reconstructed (Figure 1). This surface was used since earlier theoretical and experimental studies have shown that the 100% H-terminated B-rich surface is 2 × 1 reconstructed.40,41 The energy of H adsorption was found to be −247 kJ/mol. As can be seen in Table 1, also the F adsorption process was found to be exothermic and even more exothermic than the H adsorption process: −296 vs −247 kJ/ mol. These results are probably due to the smaller polarity within the B−H bond compared to the B−F bond, which probably will lead to the stronger B−F bonds (with a higher ionic contribution). The electron bond population values (Table 2), which generally are regarded to be a measure of the covalent bond strength, will strongly support these results. The B−H bond population value is much stronger than the corresponding B−F bond population: 0.97 vs 0.54. As was the situation with the N-rich surface, the B-rich surface was not able to be terminated to 100% with Cl species. The energy of Cl adsorption was also here found to be strongly endothermic: +300 kJ/mol (Table 1). The shortest Cl−Cl distance on the surface for this particular structure is 2.55 Å, while the van der Waals distance for two Cl atoms is 3.50.42,43 As can be seen in Table 3, the Cl atomic charges are positive or almost neutral: +0.23 e and −0.01 e for the N-rich and B-rich surfaces, respectively. These results are quite unexpected because of the electronegativity differences between the Cl, N, and B atoms: 3.16 vs 3.04 and 2.04 (i.e., the B and N atoms should be donating electrons to the Cl atoms). The received result is most probably due to the sterical repulsions between the Cl atoms which will force a partial electron transfer from the Cl atoms to the N (or B) surface atoms. Hence, the Cl atoms will just counteract the presumed electron transfer from the surface atoms, which is a tendency that earlier has been observed for a diamond surface.44 B.1.2. h-BN (001). It was found that the H adsorption process for the h-BN (001) surface on a N surface site is

especially for the most common (001) surface of these phases. These surfaces are most commonly terminated, and the present study has therefore been widened to also include the effect on surface reactivity by calculating the adsorption energies for the most common terminating species: H, F, and Cl. These surface planes, with terminating species, are most important for the probability of a successful synthesis process of the various phases. For this purpose, the surface reactivities for the different phases are of major importance to more thoroughly investigate. Another factor that has turned out to be important for the growth of various materials is the effect by impurities. Chubarov et al.26 have experimentally shown that incorporation of Si into the gas phase will favor the r-BN growth. Hence, the effect of interstitially positioned Si atoms on the r-BN and h-BN structural stability and chemical reactivity has also been included in the present study. The Si atom has an extra electron compared to B (four vs three electrons). It has though one valence electron less than the N atom (four vs five electrons). Moreover, the electronegativity values are 2.04 (B), 1.90 (Si), and 3.04 (N). The electronic structures of the various atoms, in addition to the differences in electronegativity values, are expected to be decisive for both the surface stabilities as well as for the various surface reactivities. B. Structural Geometries and Energetic Surface Stabilization. B.1. BN Surfaces without Interstitial Impurities. B.1.1. c-BN (100). For the c-BN (100) surface, without interstitial atoms, the results in the present study have shown that the most reactive surface is the N-rich H-terminated one (Figure 1). As can be seen in Table 1, the energy of H

Figure 1. Model demonstrating a c-BN (100) surface that has been terminated with H species: (a) N-rich surface and (b) B-rich surface.

adsorption to this specific surface is strongly exothermic: −418 kJ/mol. This adsorption energy is valid for the last H adsorbate on an otherwise completely H-terminated surface. As can be seen in Table 1, the corresponding N-rich Fterminated c-BN (100) surface was not as reactive as the Hterminated one. The energy of F adsorption is −139 kJ/mol for the c-BN surface. These results support previous experimental

Table 1. Adsorption Energies [kJ/mol] for Geometry Optimized Structures ΔEads [kJ/mol] N on top c-BN h-BN w-BN r-BN

B on top

H-terminated

F-terminated

Cl-terminated

H-terminated

F-terminated

Cl-terminated

−418 −463 −398 −596

−139 −243 +8 −303

+181 −91 +78

−247 −534 −527 −521

−296 −666 +13 −348

+300 −337 +37

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Table 2. Electron Bond Populations and Bond Lengths between the Surface Layer Atoms and Terminating Species for Geometry Optimized Structures N−H c-BN h-BN w-BN r-BN

N−F

N−Cl

B−H

B−F

B−Cl

BP

BL [Å]

BP

BL [Å]

BP

BL [Å]

BP

BL [Å]

BP

BL [Å]

BP

BL [Å]

0.72 0.73 0.76 0.72

1.02 1.02 1.03 1.01

0.26 0.27 0.26

1.39 1.40 1.32

0.29 -

1.71 -

0.97 1.04 1.07 1.07

1.23 1.19 1.22 1.19

0.54 0.60 0.57

1.67 1.32 1.30

0.82 -

1.75 -

Table 3. Atomic Charges [e] for the Surface Layer Atoms and Terminating Species for Geometry Optimized Structures Atomic Charge [e] H-terminated c-BN h-BN w-BN r-BN

F-terminated

Cl-terminated

F-terminated

Cl-terminated

N

H

N

F

N

Cl

B

H

B

F

B

Cl

−0.76 −0.95 −0.76 −1.0

+0.38 +0.40 +0.41 +0.41

−0.24 −0.41 −0.42

−0.17 −0.20 −0.21

−0.29 -

+0.61 -

+0.07 +0.49 +0.34 +0.03

+0.07 +0.04 −0.02 +0.07

+0.59 +1.08 +1.33

−0.47 −0.47 −0.48

+0.88 -

+0.32 -

somewhat more exothermic than for the corresponding c-BN surface: −463 vs −418 kJ/mol (Table 1). The corresponding energy of H adsorption on a B surface site was even more energetically favorable, −534 vs −463 kJ/mol. The surface structure remained perfectly hexagonal as a result of the geometry optimization process. The energy of F adsorption on a N surface site on an otherwise completely F-covered h-BN surface was found to be −243 kJ/mol. This result implies a more exothermic process than for the corresponding N-rich surface on c-BN (−243 vs −139 kJ/mol). The adsorption process onto the B-surface site on h-BN was shown to be highly reactive. In fact, it was the surface site that was the most reactive of all surfaces investigated. The energy of F adsorption became −666 kJ/ mol (Table 1). Also here, the more reactive B site (compared to the N site) is most probably due to the higher polarity within the B−F bond, as compared to the N−F bond. As can be seen in Table 2, apparently also the covalent contribution was higher with a bond population of 0.60 for the B−F bond, compared to 0.27 for the N−F bond. In addition, the surface structure remained perfectly hexagonal from the geometry optimization process. As can be seen in Table 1, the energy of Cl adsorption onto a N surface site on the h-BN surface was found to be −91 kJ/ mol, which implies a slightly exothermic process. The adsorption process on the corresponding B surface site was on the other hand strongly exothermic, with an energy of −337 kJ/mol. The stronger polarity within the B−Cl bond, compared to N−Cl, most probably leads to this stronger bond. The surface Cl atoms were found to reconstruct to a certain extent as a result of the geometry optimization of the N-rich surface, which led to a longer distance between neighboring Cl species on the surface (Figure 2). This distance is larger than for the c-BN surface, 2.87 vs 2.55 Å. This result will most probably explain the fact that this surface is able to become Clterminated (i.e., the sterical repulsions among the Cl terminators are not that pronounced as for the c-BN surface). In addition, the N and B atoms in the surface layer have moved slightly laterally in the same direction as the terminating Cl species (Figure 2). B.1.3. w-BN (100). The H adsorption process onto a N surface site onto the w-BN (100) surfaces was found to be slightly less exothermic than for the corresponding c-BN (100)

H-terminated

Figure 2. Model demonstrating the geometry optimized h-BN (001) crystalline plane which has been terminated with Cl species: (a) side view perpendicular to the BN planes and (b) side view along the BN planes.

surface: −398 vs −418 kJ/mol (Table 1). The energy of H adsorption for the corresponding B-site is −527 kJ/mol. The B site is, hence, more reactive than the N site. This result is further emphasized by the electron bond population results seen in Table 2, where the bond population for the B−H bond (1.07) is higher than for the corresponding N−H bond (0.76). The adsorption process is also much more reactive than for the corresponding c-BN surface (−527 vs −247 kJ/mol). The surface-terminating H species on top of the w-BN surface was slightly reconstructed to a 2 × 1 reconstruction (see Figure 3). It was not found possible to terminate a w-BN (100) surface to 100% with F species. As can be seen in Table 1, the adsorption energy is +8 (N-site) and +13 (B-site) kJ/mol, which indicates an endothermic process and is, therefore, not expected to take place. As was the situation with the Cl atoms on top of the cubic phase, the most probable explanation is also here the sterical repulsions between the F atoms. The shortest F−F distance on the surface is 2.09 Å, while the van der Waals distance for two F atoms is 2.94 Å.42,43 When trying to fully cover the w-BN (100) surface with Cl atoms, the terminating Cl atoms were found not to bind to the surface at all. Hence, no energy of Cl adsorption was possible to calculate. B.1.4. r-BN (001). As can be seen in Table 1, all calculated energies of adsorption for the r-BN surface were very similar when comparing the N- and B-sites. The H adsorption process 20267



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Figure 3. Model demonstrating the geometry optimized w-BN (100) surface that has been terminated with H species: (a) side view showing the yzprojection, (b) side view showing the xz-projection, and (c) the on-top view.

Figure 4. Model demonstrating the geometry optimized r-BN (001) surface that has been terminated with H species: (a) side view along the BN planes and (b) side view perpendicular to the BN planes.

Figure 5. Model demonstrating the geometry optimized r-BN (001) surface that has been terminated with F species: (a) side view along the BN planes and (b) side view perpendicular to the BN planes.

Figure 6. Model demonstrating an h-BN (001) surface that has been terminated with H species: (a) parallel with the BN planes, with a projection onto the xy plane, and (b) with a projection onto the xy plane. The numbers show the different positions of the interstitially positioned Si atoms.

for the r-BN surface on a N surface site is more exothermic than the corresponding c-BN surface. The energy of adsorption is −596 kJ/mol (compared to −418 kJ/mol for the c-BN surface). The energy of H adsorption on the corresponding B surface site was similar: −521 kJ/mol. The bond population is, however, stronger for the B−H bond compared to the N−H bond (1.07 vs 0.72). However, the ionic contribution is expected to be higher for the N−H bond because of the larger

difference in electronegativity between the N and H atoms (compared to the B and H atoms). The surface structures did also change in the top surface layers. As can be seen in Figure 4, the B surface sites did move. The B atoms released one of their two terminating H species. The B atoms did also try to move closer to form pairwise B−B bridges. This phenomenon has earlier been observed by the present authors when studying the initial growth of c-BN onto diamond substrates.45 However, the 20268



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sum of the theoretical covalent radius for the B and N atoms (0.88 vs 0.73 Å), there is an interatomic distance of 1.67 Å which is too small a Si atom to be interstitial positioned. Hence, it is obvious that the Si atom prefers to move out toward the middle of the sheets. When trying to chemisorb H atoms onto a N-rich h-BN surface, the Si atom was found to desorb from the surface as a result of the geometry optimization process. The H adsorption process for the B-rich h-BN surface with an interstitial Si was, though, possible to calculate, and the process was highly exothermic. The energy of H adsorption was −559 kJ/mol. This adsorption process is only slightly more exothermic compared to the nondoped surface (−559 vs −534 kJ/mol). The electron bond population values for the B−H bond (in the vicinity of Si) were shown to slightly decrease to 1.0 (Sidoped) from a value of 1.1 for the nondoped situation. This increase was accompanied by a minor bond lengthening of 0.05 Å. Since the electron bond population values, which generally are regarded to be a measure of the covalency in a bond, show a less strong value, this result implies that there is most probably a larger percentage of an ionic B−H contribution within the Sidoped surface. This result is supported by the atomic charges, which are −0.1 (Si-doped) vs +0.5 e (nondoped) for the B atom and +0.1 (Si-doped) and +0.0 e (nondoped) for the H atom (Table 3). The relative stabilization energy for a Si-doped lattice, versus a nondoped one, was calculated to become +8 kJ/mol. Hence, there is a probability that this Si-doped phase exists in equilibrium since the energy of stabilization is so close to zero. The extra electrons introduced into the system by the interstitial Si atom were observed to stay in the vicinity of Si. The bond population values for the N−B bonds close to the Si atom increased from 0.57 (for the h-BN without impurities) to 0.60 (for the h-BN with an interstitial Si atom). These values are furthermore strongly supported by the electron density difference maps, where it is shown that the electron density is somewhat more pronounced between the N and B atoms surrounding the Si (as compared with the other N−B bonds in the structure (Figure 8)). When studying the bonds with the Si atom, the Si−B bond had a bond population of 0.49, while the Si−N bonds showed negative values, i.e., antibonding, of −0.06. This observation is strongly supported by the atomic charge calculations, which show that the extra electron density induced by Si will move toward the B atom that is binding to the Si

surface B atoms did not make a bond to each other, which is why there is a double bond to the N atom beneath. The F adsorption process onto a N surface site for the r-BN surface was found to be less exothermic than the corresponding H-terminated r-BN surface. The energy of adsorption was found to be −303 kJ/mol for the N surface site and −348 kJ/ mol for the B surface site. The surface structure has also here changed as a result of the geometry optimization process (Figure 5). The three top atomic layers have moved laterally, and one of the N-binding F atoms has been released and binded to the adjacent B atom. Either the N or B surface sites were found to be able to be fully Cl terminate. As can be seen in Table 1, the energies of Cl adsorptions are +78 and +37 kJ/mol for the N and B surface sites, respectively. These results indicate an endothermic process and are hence not expected to take place. B.2. h- and r-BN Surfaces with a Si Interstitial Impurity. B.2.1. h-BN (001). As explained earlier in the Introduction, the experimental results by Chubarov et al.26 have showed that incorporation of Si into the gas phase will favor the r-BN growth. The effect by the presence of Si in the lattice, on the BN structures and surface reactivity, has therefore been included in the present study for the various BN allotropes. The Si atom has been interstitially positioned within the geometry optimized H-terminated h-BN (001) surface in seven different positions, and for each position a new geometry optimization took place (Figure 6). It was only for positions 1, 6, and 7 that the Si atom stayed in the surface or made it possible to calculate the adsorption energy after the geometry optimization. For all the other calculated surfaces (with Si in positions 2, 3, 4, and 5), the Si atoms became released from the surface. As a result of the geometry optimization for the h-BN (001) surface with Si interstitially in position 1, the Si atom moved out in between hexagonal sheets (almost to start position number 2 (see Figure 7)). Thereafter, this Si atom continued to

Figure 7. Geometry optimized structures showing h-BN (001) surfaces that have been terminated with H species and with an interstitial Si atom that has moved from position 1: (a) side view and (b) on-top orientation.

move up toward the upper surface N/B layer. As can be seen in Figure 7, the B (or N) and H atoms (all surrounding the Si atom) did all move laterally away from the Si atom. This result is most probable due to steric repulsions between the Si atoms and these surrounding atoms. In addition to these physical effects, this is further confirmed by the electron bond population results which are showing negative, i.e., antibonding, values for the Si−N bonds (−0.06). The shortest obtained geometrical distance between the B and N atoms (surrounding the interstitial Si in position 1) is 2.88 Å. When subtracting the

Figure 8. For a Si-doped h-BN (001) surface with a H-terminated upper layer. A slice through an electron density difference (Δρ) map has been shown for a cross-section (being perpendicular to the surface in parallel with the yz plane intersecting the sheets). 20269



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charge calculations show that the surface H atoms are more positive (+0.42 e) for the N atom positioned just behind the Si atom but less positive for the two H atoms on each side of Si (+0.39 e), as compared to the nondoped h-BN (+0.40 e) (Table 3). Also here it has been shown that the presence of Si in the system will have an effect on the bond strengths on the surrounding N and B neighbors. The presence of extra electrons induced in the system by the Si atom seems to stay in the vicinity of Si (Table 3). The bond population values for the B−N bonds behind the Si atom have increased from 0.88 (nondoped) to 0.92 (Si doped). These values are, furthermore, supported by the electron density difference maps, where it is shown that the electron density is somewhat more pronounced between the N and the B atoms behind the Si (when compared with the other N−B bonds in the structure (Figure 10)). The

atom (Table 3). This B atom will have a more pronounced atomic charge of −0.05 e, as compared with the values of +0.49 e for the nondoped situation. For the Si doping situation, the N atom closest to Si is slightly less negative (−0.78 e), compared to nondoped scenario (−0.82 e). Further away for the Si dopant, there was no observed change in atomic charge due to doping. For h-BN with Si interstitially in position 6, the Si atom was observed to stay within the lattice (just as for position 1). However, the Si atom moved out to the middle of the sheets, just as the initial movement for Si in position 1 (see Figure 9).

Figure 9. Geometry optimized structure showing a h-BN (001) surface that has been terminated with H species and that includes an interstitial Si atom that has moved from position 6: (a) side view and (b) on-top orientation.

For the same reason as the situation with the initial position 1, the diameter of the Si atom is too big for the open BN ring diameter which makes it obvious that the Si atom prefers to move out toward the middle of the sheets. Also here, the atoms surrounding the Si atom did move laterally away from the dopant. This result is also here confirmed by negative Si−N bond population values of −0.20. When the Si atom has been interstitially positioned at position 6 within the h-BN (001) surface, the adsorption process of H has been found to be exothermic. Compared to the nondoped situation, both the N and B surface sites are calculated to be less reactive. The H adsorption energies for a nonterminated B and N surface site are for an otherwise completely H-terminated surface: −307 vs −479 kJ/mol (Sidoped) and −463 vs −534 kJ/mol (nondoped). Hence, the interstitial Si atom will not improve the adsorption energy on this specific h-BN surface. It should be stressed that this specific h-BN surface was the only Si-doped one for which it was possible to calculate the energy of H adsorption for both the N and B surface sites. However, the calculated relative stabilization energy for this Si-doped surface versus a nondoped one was +364 kJ/mol which shows that extra energy is needed for it to be formed. The less strong H adsorption energy for the Si-doped Nsurface site is supported by the calculated electron bond population values for the N−H bond (in the vicinity of Si). These values were shown to slightly decrease to 0.70 (Sidoped) from a value of 0.73 (nondoped). This decrease was accompanied by a minor bond lengthening of 0.05 Å. The less strong H adsorption energy for the Si-doped B-surface site is also supported by a B−H bond lengthening of 0.06 Å. The electron bond population values for the B−H bond (in the vicinity of Si), were, however, identical for the Si-doped and nondoped scenarios, showing that the covalent contribution has not been changed by the Si dopant. Furthermore, the atomic

Figure 10. Model of a Si-doped h-BN (001) surface with a Hterminated upper layer. A slice through an electron density difference (Δρ) map has been shown for a cross-section in parallel with the zy plane and perpendicular to the sheets.

corresponding bond length was decreased by 0.04 Å. The Si atom is binding to two B atoms and one N atom, as can be seen in Figure 10. The Si−B bond populations are 0.71, while the Si−N bond population is 0.52. The Si-binding B atom is no longer bonding to the N atom beneath but has instead moved up toward the surface (see Figure 10). These observations are further supported by the atomic charge calculations, which show that the extra electron density will move toward the B and N atoms that bind to the Si atom (Table 3). This B atom will have a more pronounced atomic charge of +0.38 e, as compared to +0.85 e for the nondoped h-BN. The Si-binding N atom is slightly more negatively charged for the Si-doped surface (−0.90 e), as compared to the nondoped one (−0.82). Further down in the lattice, the atomic charges remained the same. The resulting atomic charge of the Si atom is +0.80 e. As can be seen in Figure 11, also the interstitial Si atom within the h-BN surface which was initially positioned in position 7 was observed to stay within the lattice. The Si atom did also here move out in between two hexagonal sheets as a result of the geometry optimization process. The distance between the B and N atoms surrounding the Si atom in position 7 is 1.45 Å (the covalent N−Si−B distance is 3.93 Å), which also here shows that this position will be too small for the Si atom. The surrounding atoms have therefore moved laterally away from the Si atom, which is confirmed by negative (antibinding) Si−N bond population values of −0.17. 20270



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though retained the same charge as without any Si impurity (+0.80 e). The Si-binding N atom has, though, a somewhat less negative atomic charge than the h-BN without any Si impurity: −0.73 e (Si-doped) vs 0.82 e (nondoped). This is also the situation for the N atoms in one layer further down in the lattice (−0.77 vs −0.81 e) and one layer up in the lattice (−0.91 vs −0.95 e). The N atoms further down in the h-BN seem to be unchanged. The resulting atomic charge of the Si atom is +0.41 e. The Si atom is binding to a neighboring B atom with an electron bond population value of 0.38. The bond population to a neighboring N atom is, however, negative (−0.29) and, hence, antibonding. However, the surrounding bond populations are for this situation a bit different compared to the scenarios with Si in positions 1 and 6, respectively. The bond population between the two Si-binding B and N atoms is 0.72, which is somewhat lower than for the nondoped h-BN lattice (0.88). The corresponding bond length is also 0.07 Å longer. The N−B bond which points up from the Si-binding N atom shows also a slightly lower bond population value, as compared to h-BN without any Si impurity (0.83 vs 0.88). The corresponding bond length is 0.05 Å longer. The N−B bond population (N being in the surface) is almost the same (0.87 vs 0.88), with a corresponding bond lengthening of only 0.01 Å. The bond population for the B−N bond that is pointing downward from the Si-bonded B atom is also somewhat smaller (0.79 vs 0.88). The corresponding bond lengths are also here somewhat longer (by 0.05 Å). Further down in the h-BN lattice, the bond populations remained the same as for the nondoped surfaces. As a conclusion, the existence of Si dopants in the lattice seems quite difficult to achieve for the h-BN surface. The Si atom prefers to move up toward the surface and desorbs from the h-BN lattice. The existence of interstitial Si dopants in the lattice only showed a positive effect for the nonterminated site reactivity for an otherwise H-terminated B-surface site on the hBN surface. All the calculated stabilization energies were positive (relative to the nondoped surface), and hence, the Sidoped structure can only be formed by introducing energy to the system. B.2.2. r-BN (001). The Si atom has been interstitially positioned within the geometry optimized H-terminated r-BN (001) surface in seven different positions, and for each position a new geometry optimization took place (Figure 13). It was only for position 6 that the Si atom stayed in the surface or for which it was possible to calculate the adsorption energy. For all the other calculated surfaces (with Si in positions 1, 2, 3, 4, 5, and 7), the Si atoms became released from the surface. The r-BN (001) surface with interstitial Si also showed an even more pronounced B−B interaction on the surface. This was confirmed by a stronger electron bond population of 0.92 (Si-doped) vs 0.89 (nondoped). This increase was accompanied by a minor bond shortening of about 0.05 Å when going from a nondoped to a Si-doped lattice. When the r-BN (001) surface is Si-doped with the Si atom in position 6, the adsorption process of H has been found to be exothermic. Compared to the nondoped situation, the Nsurface site is calculated to be less reactive. The H adsorption energy is −418 vs −596 kJ/mol; i.e., the interstitial Si atom will not improve the adsorption energy onto the N-site on the r-BN surface. The B-surface site is, however, calculated to be somewhat more reactive, with H adsorption energy values of −545 (Si-doped) vs −521 kJ/mol (nondoped). Hence, the

Figure 11. Geometry optimized structures showing the H-terminated h-BN (001) surface with an interstitial Si atom that has moved from position 7: (a) side view and (b) on-top orientation.

For Si interstitially positioned at position 7 within the h-BN (001) surface, the energy of H adsorption onto a N-surface site was unfortunately not possible to calculate. The Si atom was found to desorb from the surface as a result of the geometry optimization process. The H adsorption process onto a Bsurface site for the h-BN surface was, on the other hand, found to be highly exothermic. The energy of H adsorption was found to be −562 kJ/mol. This adsorption process is only slightly more exothermic than for the nondoped surface: −534 kJ/mol. The relative stabilization energy for a Si-doped surface versus a nondoped one was though positive (+364 kJ/mol), and as was the situation with Si in position 6, extra energy is needed for the doped lattice to be formed. The electron bond population values for the B−H bond in the vicinity of Si were found to be identical for the Si-doped and nondoped scenarios, showing that the covalent contribution has not been changed by the Si dopant. Also here it has been shown that the presence of Si in the system will have an effect on the bond strengths for the surrounding N and B neighbors. The presence of extra electrons induced in the system by the Si atom seems to stay in the vicinity of Si. These results are supported by the spin density maps (Figure 12), where the spin density is

Figure 12. Spin density maps for a H-terminated h-BN (001) surface with Si interstitially that has moved from position 7: (a) side view and (b) top view.

surrounding the Si atom and the atoms positioned closest to the Si atom. These observations are further supported by the atomic charges, where the B atom (binding to the Si atom) has an electron charge of +0.51 e, as compared to +0.80 for the hBN without any Si impurity (Table 3). Also the other B atom (on the other side of Si) shows a lower charge of +0.60 e. The B atom in the second atomic layer, being bonded to the Sibinding N atom, also shows a somewhat decreased charge of +0.69 e. The B atoms further down in the h-BN lattice have 20271



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Figure 13. Model demonstrating an r-BN (001) surface that has been terminated with H species: (a) parallel with the BN planes, (b) with a projection onto the xy plane and intersecting the B-rich surface sheet, and (c) with a projection onto the xy plane and intersecting the N-rich surface sheet. The numbers show the different positions of the interstitially positioned Si atoms.

interstitial Si atom will slightly improve the adsorption energy onto the B-site on the r-BN surface. The less strong H adsorption energy for the Si-doped Nsurface site is supported by electron bond population values for the N−H bond (in the vicinity of Si), which was shown to slightly decrease to 0.72 (Si-doped) from a value of 0.73 (nondoped). The corresponding bond length was, however, identical for the Si-doped and nondoped scenarios. The electron bond population values for the B−H bond (in the vicinity of Si) was also found to slightly decrease to 1.01 (Sidoped) from a value of 1.03 (nondoped). This decrease was accompanied by a minor bond lengthening of 0.01 Å. Since the electron bond population values, which generally are regarded to be a measure of the covalency in a bond, show a less strong value, this result implies that there is most probably a larger percentage of an ionic B−H contribution within the Si-doped surface. This result is supported by the atomic charges, which are +0.16 (Si-doped) vs +0.02 e (nondoped) for the B atom and +0.02 (Si-doped) and +0.06 e (nondoped) for the H atom. The atomic charges of the surface N atoms closest to the Si atom have, though, remained the same (−0.95 e). However, its terminating H atom shows a more positive charge (+0.41), as compared with the nondoped r-BN surface (+0.39 e). These results imply that the Si-doped surface has received a B−H bond that is less covalent (i.e., with a higher percentage of ionic contribution) as compared to the nondoped r-BN surface. The calculated relative stabilization energy (vs the nondoped structure) was −100 kJ/mol, and hence, this phase might most probably exist at equilibrium. For r-BN (001), with Si interstitially positioned in position 6, the Si atom moved out between the BN sheets and thereafter continued down into the r-BN lattice (as can be seen in Figure 14). The initial distance between the N and B atom is only 1.46 Å at the r-BN surface (nearby position 6), which should be compared with the covalent radius of Si (1.16 Å).46 The atoms on the adjacent side of the Si atom have therefore moved

Figure 14. Geometry optimized structures showing the r-BN (001) surface that has been terminated with H species and with an interstitial Si atom that has moved from position 6 (side view).

laterally away from the Si atom (due to sterical hindrances). This result is confirmed by the electron bond population results which are negative (i.e., antibonding): −0.13 (Si−N) and −0.09 (Si−B). In this situation, also the B atom behind the Si atom has moved away from the Si atom, which is confirmed by a negative bond population value of −0.32. As can be seen in Figure 14, this will further make the adjacent N atom in the sheet behind the B atom move closer to the B atom. This B−N bond has a bond population of 0.44, with a corresponding distance of 1.69 Å. The Si atom is binding to three N atoms, with bond populations of 0.23 (downward from the Si atom) and 0.10 (upward from the Si atom). The B−N bonds behind the Si atom show a less strong bond population compared to the nondoped r-BN surface: 0.74 and 0.76 (Si-doped) vs 0.89 and 0.91 (nondoped). The corresponding bond lengths are also longer: 0.13 and 0.12 Å. These values are furthermore strongly supported by the electron density difference maps, where it is shown that the electron density is somewhat less pronounced between the N and B atoms surrounding the Si (as compared with the other N−B bonds in the structure (Figure 15)). The N−B bonds further up in the lattice do also show less strong 20272



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phase-pure BN growth to occur as well as for a designed surface chemical process to take place. The calculated results show that the N-surface sites, for all BN allotropes, will give a more exothermic H-adsorption process (as compared with the F or Cl adsorption processes). On the corresponding B-surface sites, this is true for the w- and r-BN surfaces. The B-rich c-BN surface and the B-site on the hBN surface will give slightly more exothermic F-adsorption processes (as compared to the corresponding H-adsorption processes). The process of F adsorption onto the w-BN surface was furthermore found to be endothermic and will thereby not be able to take place (with 100% surface coverage). The process of Cl adsorption was found to be endothermic for both the c-BN and r-BN surfaces. The Cl termination onto the wBN surface was found to be highly unstable, and the Cl atoms will automatically desorb from the surface. The h-BN surface was, in fact, the only surface within this study that resulted in an exothermic Cl adsorption process. It was found difficult to dope the h-BN (and r-BN) surfaces with interstitial Si atoms. The Si atom will desorb from most of the tested h- and r-BN surfaces. For the h-BN surface, the Si atom stayed within three out of seven tested positions. The results showed that only for one of these surfaces the energy of H adsorption was able to calculate for both the B- and Nsurface sites. The resulting processes of H adsorption were less exothermic than the nondoped surface. The other two h-BN surfaces resulted in processes of H adsorption slightly more exothermic than for the corresponding nondoped surfaces on the B-surface sites, while the Si atom desorbed from the surface on the N-sites. For the Si-doped r-BN surface, the Si atom stayed within the surface. The Si atom only showed a positive effect for the nonterminated site reactivity for an otherwise Hterminated B surface site; i.e., the process of H adsorption was only slightly more exothermic than for the corresponding nondoped surface. On the corresponding N surface site the H adsorption was less exothermic than the nondoped surface. Finally, the calculated stabilization energies were only energetically favored (relative the nondoped system) for the r-BN surface, and hence, there is a probability for this phase to exist in equilibrium. The Si-doped h-BN structure seems to only be possible to be formed by introducing energy to the system.

Figure 15. Slice through electron density difference (Δρ) maps for a Si-doped r-BN (001) surface with a H-terminated upper layer in parallel with the xy plane and intersecting the sheets.

bond population values compared to the nondoping situation (0.81 vs 0.89). Further down in the lattice (from the Si atom), the result is the opposite: the N−B bond populations are larger compared to the nondoping situation (1.25 vs 1.04) with a decrease in bond lengths (of 0.05 Å). The atomic charge of the B atom behind the Si atom has become lowered, as compared to the nondoped r-BN surface: +0.56 e (Si-doped) vs +0.84 e (nondoped). This implies that the B atom has received extra electrons. This extra electron density is most probably originating from the adjacent N atoms, which instead has increased their atomic charges from −0.81 (Si-doped) e to −0.79 e. The Si atom has an atomic charge of +0.98 e. As a conclusion, the existence of Si dopants in the lattice seems quite difficult to achieve also for the r-BN surface. The Si atom prefers to move up toward the surface and thereafter to desorb from the r-BN lattice. The existence of interstitial Si dopants in the lattice will only show a positive effect for the nonterminated site reactivity for an otherwise H-terminated B surface site on the r-BN surface. The calculated stabilization energy was negative, and hence, the probability of this phase to occur in equilibrium should be high. Another observed difference between the Si-doped h-BN and r-BN surfaces is that the Si atom moves up toward the surface for h-BN. For one of the Si positions within the r-BN surfaces, the Si atom was observed to instead move down into the lattice. This result may be due to the difference in the stacking between the hexagonal and rhombohedral structures, which can be seen as two different stacking sequences of the same basal plane.47

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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SUMMARY AND CONCLUSIONS In the present study, the process of H, F, and Cl adsorption on the c-, h-, w-, and r-BN (001) surfaces has been theoretically investigated by performing DFT calculations under periodic boundary conditions. In addition, also the process of Hadsorption onto the h- and r-BN surfaces, with one Si atom interstitially positioned into several different surface near positions, has been included in the present study. The main purpose was to get a deeper knowledge about the effects from the different terminating species on the surface reactivity of various BN surfaces (with or without an interstitial Si atom). The effect from the interstitial Si atom on the surface reactivity was examined by studying the effect of the adsorption energies, bond populations, and atomic charges, respectively. These effects are, thereby, very important for both the possibility for a

ACKNOWLEDGMENTS The computational results were obtained using the software programs from Accelrys, Inc. (first-principle calculations were done with the CASTEP program within the Cerius2 program package).

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REFERENCES

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