12781
J. Phys. Chem. 1995,99, 12781-12785
Diamond Nucleation on Hexagonal Boron Nitride: An ab Initio Study of Energetics E. Johansson,* K. Larsson, and J.-0. Carlsson Angstrom Consortium for Thin Film Processing, University of Uppsala, P.O. Box 531, S-751 21 Uppsala, Sweden Received: April 6, 1995; In Final Form: June 9, 1 9 9 9
The nucleation of diamond on the zigzag and armchair edge atoms of the basal plane of hexagonal boron nitride (h-BN) has been investigated theoretically by using ab initio molecular orbital theory. The calculations have included the effects of electron correlation by means of second-order Moller-Plesset perturbation theory. Outgrowths corresponding to diamond nuclei are calculated to be energetically more stable than the corresponding growth of graphite nuclei, both on the zigzag edge and the armchair edge of the (001) plane of h-BN. A comparison between the nucleation of diamond on h-BN and on the corresponding graphite edges shows only small energy differences.
* (ioo) edge
Introduction Considerable experimental and theoretical effort has been made during the last decade to obtain a fundamental understanding of the low-temperature, low-pressure gas phase synthesis of diamond. For growth of diamond of high quality, there is a need of detailed understanding at the molecular level of the growth processes occurring on various substrates and surfaces of different crystallographic orientations. Especially, basic knowledge of the nucleation process on different substrate surfaces is essential, since the film growth is strongly dependent on the nucleation. It has been shown that under appropriate growth conditions diamond can be grown on nondiamond substrates. Regardless of the deposition technique chosen, usually polycrystalline and textured films have been grown. Such films are rarely epitaxial with the substrate except when grown on bulk diamond.' However, evidence of hetero-epitaxial growth of diamond on Ni? C U ,c-BN,~-I~ ~ SiC,i4.i5and BeOi6 has been obtained. It is well established that unsaturated aromatic (sp2) compounds and graphite can serve as sites for diamond nucleati~n.'~-~O Preferential nucleation of diamond has been observed along the edges of the graphite basal plane (OOl).9 The diamond (1 11) plane appeared to be in parallel to the (001) graphite basal plane in the investigations by Angus et a1.17 and Li et a1.I8 Moreover, the diamond [ 1101 was shown to be in parallel to the graphite [110] direction.I8 This indicates that graphite might act as a nucleation seed. Angus et a1.I7 have also presented some preliminary theoretical results from semiempirical calculations for the interaction of atomic hydrogen with (100) and (1 10) edges and corners of a graphite sheet forming hydrogenated ring compounds. The hydrogenated ring compounds probably act as precursors for diamond formation at these locations. In a recent paper, the nucleation of diamond on the zigzag (100) and armchair (1 10) edge atoms of the basal (001) graphitic plane has been investigated theoretically by using the ab initio molecular orbital method.2i The stability of the buckled carbon rings formed during the process of diamond nucleation on the different edges was discussed in relation to the graphitic planar forms of carbon. The results of the calculations showed that diamond growth along the (100) and (1 10) edges of the basal plane of graphite is energetically more favorable than continued growth of graphite. @
Abstract published in Advance ACS Abstracts, August 1, 1995.
(110)edge
OB
ON
w (100) edge
Figure 1. An ill_ustration of a h-BN (001) plane, demonstrating the ( 100) edge, the (100) edge, and the ( 1 10) edge, respectively.
Hexagonal boron nitride (h-BN) has a structure which is almost identical to the graphite structure but where every other carbon atom has been replaced by a boron atom and a nitrogen atom, respectively. Hence, the (001) plane of h-BN is crystallographically very similar to the basal plane of graphite. The bond length m - ~of h-BN is 1.45 A, which is comparable to that of graphite (rc-c = 1.42 A) and the zigzag as well as the armchair edge atoms in the (001) plane of h-BN are expected to be favorable sites for diamond nucleation. Few experimental studies of diamond growth on hexagonal boron nitride have been performed. Unfortunately all of these experiments have been made on polycrystalline substrates. A nucleus density of 1 x lo4 nuclei/cm2 was obtained after 3 h of diamond deposition using the hot-filament technique.22 Lindlbauer et ~ 1 reported . ~ good ~ adherence of diamond films grown on h-BN. Their nucleus density reported was 5 x lo5 nuclei/cm2 after 5 h of deposition (hot-filament technique). The purpose of the present work is to make a comparative structural and energetic investigation of diamond nucleation on the three different edges of the h-BN (001) plane, using a cluster approach and ab initio molecular orbital theory. The three different edges include the two zigzag edges (boron atoms on the (100) edge and nitrogen atoms on the (TOO) edge) and the armchair edge (a combination of boron and nitrogen atoms on the (1 10) edge). The three edges are illustrated in Figure 1. The stability of the buckled carbon rings formed during the process of diamond nucleation on the different types and configurations of the edge atoms on the (001) h-BN plane will be presented and discussed in relation to each other and the graphitic planar forms of carbon. The results will be compared structurally and energetically with the nucleation of diamond on the zigzag (100) and armchair (1 10) edges on the basal (001) plane of graphite. No analysis to enable more conclusive predictions regarding preferred reaction pathways will be made.
0022-3654/95/2099-1278 1 $09.Oo/O 0 1995 American Chemical Society
Johansson et al.
12782 J. Phys. Chem., Vol. 99, No. 34, 1995 Method The nucleation of diamond on the three different types and configurations of edge atoms of the hexagonal BN (001) plane has been investigated within ab initio molecular orbital theory using the program system GAUSSIAN92.24 The present work includes calculations of the total electronic energies for two different types of outgrowths from the three edges: partially saturated carbon ring systems and the plane graphitic counterparts, respectively. A split-valence basis set with polarization functions (6-31G**) was used. The valence electrons are in this basis set expanded in terms of two contracted basis functions. One of these contracted basis functions is constructed from three primitive Gaussians, and the other is constructed from one primitive Gaussian. The innershell atomic orbitals are represented by single basis functions which are contractions of six primitive Gaussians. These splitvalence basis sets are able to give an improved description of expansion or contraction of the valence shell in response to differing molecular environments. A more flexible basis is then obtained by adding polarization functions (p to H and d to C).25 The primary deficiency of Hartree-Fock theory is the inadequate treatment of the instantaneous correlation between motions of electrons. The main part of the correlation energy comes from electrons with opposite spins. These effects are hence of special importance if compounds with different numbers of electroh pairs (bonds) are to be investigated and energetically compared. Since the partially saturated carbon ring systems and the planar graphitic counterparts in the present study contain different numbers and different types of bonds, electron correlation treatment has been included in the investigation by using the second-order Moller-Plesset (ME)perturbation theory.25 A necessary condition for obtaining a good description of the electronic state close to the region of diamond nucleation is primarily to choose a model cluster (template) describing the hexagonal BN well. Geometry optimization is also of vital importance. Hence, it is desirable to be able to investigate the effect of template size and geometry optimization on the energetics of diamond nucleation for the different edges (loo), (TOO), and (1 10). In practice, however, it is not economically feasible to use a larger template size and/or a more extended geometry optimization for the type of basis set (6-31G**) used in calculations based on the a6 initio molecular orbital method. Instead, the computationally less demanding LDA method (local density approximation) is useful, by using the Dmol program system and the numerical dnp basis set. This method is capable of providing information on bond lengths and bond angles within the 1% range compared with experiment for metallic systems, semiconductors, and covalently bonded molecules.26 On the other hand, binding energies are found to be consistently too large by about 40 kJ/mol for a specific chemical system (e.g., hydrocarbons).26 Hence, due to the more or less constant overestimation in binding energy, it will be possible to regard relative binding energies (especially relative energetics of diamond nucleation) to be reliable when major effects of template sizes and/or geometry optimization on the energetics of diamond nucleation are investigated. As a check, calculations of the energy of diamond nucleation on the (100) edge resulted in a difference of 31 kJ/mol when the results of the two quantum mechanical calculations are compared. The difference in the number of bonds between the planar and the buckled forms of outgrowths from the (100) edge is only one. On the other hand, there is a difference of two bonds between the corresponding ring systems on the (1 10) edge of h-BN. Hence, the difference in energy of diamond nucleation on this latter type of edge
OH
.C OB ON
Figure 2. An illustration of diamond nucleation (a) and a continued growth of graphite (b) on the template modeling the (100) edge of the h-BN (001) plane.
Figure 3. An illustration of diamond nucleation (a) _and a continued growth of graphite (b) on the template modeling the (100) edge of the h-BN (001) plane.
(about 100 kJ/mol) for the two different theoretical methods used is about twice the estimated value of 40 kJ/mol presented by Wimmer et ~ 1 . ~ ~ Results and Discussion A. Nucleation on the Zigzag Edges of the Basal Plane. The assumed mechanism in the present work for diamond nucleation on the zigzag edge of the basal plane of h-BN involves a combination of hydrogen abstraction steps and hydrocarbon adsorption steps. These individual steps will then result in a completion of a buckled six-membered ring containing three sp3-hybridizedcarbon atoms. The three other members of this partially saturated outgrowth from the zigzag edge atoms of h-BN are sp2-hybridizedboron and nitrogen atoms (Figures 2 and 3). On the boron-rich (100) edge, two boron atoms are bonded to the carbon atoms and a nitrogen atom completes the ring (Figure 2). On the nitrogen-rich (700)edge, two nitrogen atoms are bonded to the carbon atoms and a boron atom completes the ring (Figure 3). The corresponding graphitic ring systems are also shown in Figures 2 and 3. In these graphitic counterparts, two of the three added carbon atoms are sp2hybridized while the third is sp3-hybridized. However, the results of the geometrical optimization shows that the whole ring system becomes planar as in hexagonal boron nitride. Hence the surrounding sp2-hybridized carbon atoms “force” the sp3hybridized carbon atom to lie in the plane of the surrounding ring system. An alternative approach would have been to choose an sp2-hybridized carbon radical instead of the sp3hybridized carbon atom. However, the possibility for this sp2hybridized carbon radical to undergo an H adsorption reaction, resulting in an sp3-hybridized carbon atom, is very large in the growth environment in a hot-filament reactor. All geometrical parameters of the different types of outgrowths from the edge atoms of the hexagonal BN (001) plane were, with a few exceptions, allowed to be relaxed in the field of a fixed structure of the template modeling the hexagonal BN (001) plane. The relative total energies for the buckled and planar ring systems are given in Table 1. The relative energy is balanced by the total energy of two hydrogen atoms since the buckled form of the ring systems contains two more hydrogen atoms than the corresponding planar counterparts. As can be seen in Table 1, the growth of the partially saturated ring systems on the boron- and nitrogen-rich edges are energetically favored by 562 and 574 kJ/mol, respectively, at the MP2/6-31G** level of theory. These partially saturated outgrowths are assumed to
J. Phys. Chem., Vol. 99,No. 34, 1995 12783
Diamond Nucleation on Hexagonal Boron Nitride TABLE 1: Nucleation of Diamond on the Zigzag Edges of the (001) Plane of h-BN and Graphite and the Relative Total Energies for the Buckled Ring System and Their Planar Counterparts
planar ring, system BsNsC3Hio BsNsCjHio (100) CI3HlO B ~ N ~ C ~(100) HM
buckled ring system BsNsC3Hi2 (loo) BsNsCjHi2 (100) CisH12 B9N9C3Hi6 (100)
MP2/6-3 1 G** (BSSE) 562 (544)
LDN dnp 594
574
568 5 84
act as precursors for diamond formation. Moreover, the relative energies for diamond nucleation on the (100) and (TOO) edges of h-BN are very similar to the relative energy for a corresponding diamond nucleation on graphite (562, 574, and 568 kJ/mol, respectively). A complication in these calculations is that the basis functions located on the extra two H atoms in the buckled ring formation partly overlap the rest of the ring system and vice versa. The effect of this “basis set superposition error” (BSSE)27.28is, hence, that the differences in energy of the planar and buckled ring formations will be too large. More correct numerical values of the relative energies of the different edges will be obtained by subtraction of calculated “counterpoise correction^"^^-^* from the total binding energy. Corrections for BSSE were only made for the boron-rich zigzag edge in the present investigation. This is due to the facts that (1) corrections for BSSE are expected to be very similar for the different edges of the basal planes of h-BN and graphite and (2) these particular calculations are extremely time-consuming. Furthermore, it is expected that corrections for BSSE will be reasonably small for the particular large basis set 6-31G** used in the present investigation. As can be seen in Table 1, the “counterpoise correction” for the (100) edge on the basal plane of h-BN is as small as 18 kJ/mol at the MP2/6-31G** level of theory (a decrease of only about 3%). A large h-BN template, representing the (100) edge of the (001) plane, has been used in investigating the influence of template size on the difference in energy for a diamond nucleation and a continued graphitic growth at the boron-rich edge. The enlargement was made by replacing the hydrogen atoms on the two (1 10) edges of the smaller two-ring system (Figure 2) with B2N2& clusters, resulting in a h-BN template consisting of a nonbranched four-ring system. In practice, however, it is not possible to use the large basis set 6-31G** together with a template of this larger size when calculations at the MP2 level of theory are performed. Instead, the computationally less demanding quantum mechanical LDA method was used to study the trends in relative energy when the size of the template modeling the h-BN (001) plane is varied. The results of the LDA calculations showed that a decrease in relative energy by less than 2% (10 kJ/mol) will be obtained when the size of the template is increased by almost a factor of 2 (Table 1). The results in Table 1 indicate a very weak tendency for a specific order of the relative energies. However, this order will probably be changed due to different circumstances. For instance, the obtained correction for BSSE is small, but the numerical value of 18 kJ/mol is larger than the differences in relative energy for all the zigzag edges studied in the present investigation. Furthermore, the effect of increasing the template size by almost a factor of 2 resulted in a decrease in relative energy of 10 kJ/mol for the (100) edge, which is numerically comparable with the differences in relative energies. The present results thus support the conclusion that the probability
Figure 4. An illustration of sp3 carbon nucleation on the (1 10) edge of the h-BN (100) plane. The chair conformation (a) corresponds to diamond nucleation while the boat conformation (b) corresponds to nucleation of lonsdalite. The planar conformation (c) corresponds to a continued growth of graphite.
for diamond nucleation is almost identical for the different types of zigzag edges on the basal planes of h-BN and graphite. B. Nucleation on the Armchair Edge of the Basal Plane. The mechanism of diamond nucleation on the armchair edge of the h-BN (001) plane is assumed to be similar to the mechanism of diamond nucleation on the corresponding zigzag edge, involving a combination of hydrogen abstraction steps and hydrocarbon adsorption steps. These different steps are then assumed to result in a completion of a six-membered ring containing two sp3-hybridized carbon atoms. The other four members in the ring consist of two boron atoms and two nitrogen atoms. The boron atom and nitrogen atom that are bonded to the carbon atoms in the outgrowths are saturated (sp3hybridized) with hydrogen atoms. The remaining boron and nitrogen atoms in the ring formation are sp2-hybridized. The geometrical parameters of the saturated boron and nitrogen atoms (including the hydrogens) were, in addition to the different type of outgrowths from the edge atoms, allowed to be relaxed in the field of a fixed structure of the template modeling the h-BN (001) plane. There are two possible conformations of the partially saturated ring formation: a chair conformation (Figure 4a) and a boat conformation (Figure 4b). The chair conformation corresponds to a diamond nucleus while the boat conformation corresponds to a lonsdalite nucleus. As can be seen in Figure 4c, the corresponding planar ring formation consists of solely sp2hybridized atoms. The differences in total energy for the planar and buckled (chair/boat) types of outgrowths from the (1 10) edge of the h-BN (001) plane are presented in Table 2. These relative energies are balanced by the total energy of four hydrogen atoms since the buckled ring formation contains four more hydrogen atoms than the corresponding planar ring formation. The result shows that all diamond outgrowths are energetically more stable than their graphitic counterparts by almost 700 nkJ/molat the MP2/ 6-31G** level of theory. The relative energy of the chair formation of outgrowth is numerically almost as large as the relative energy of the corresponding outgrowth on the (1 10) edge of the basal plane of graphite (662 and 667 kJ/mol, respectively) (Table 2). Hence, the probability for diamond nucleation on the (1 10) edges of the basal planes of the substrates graphite and h-BN is almost identical. Furthermore,
12784 J. Phys. Chem., Vol. 99, No. 34, 1995
Johansson et al.
TABLE 2: Nucleation of Diamond on the Armchair Edge of the (001) Plane of h-BN and Graphite and the Relative Total Energies for the Buckled Ring Systems and Their Planar Counterparts
planar ring system B~N~CZH (1I10) O B7N7C2Hio (1 10) Ci6Hio B ~ N ~ C (110) ~HI
buckled ring system
MP2/ 6-31G**
B ~ N ~ C ~(1H10) I~" 689 B ~ N ~ C (1~ 10) H I ~ ~ 662 Ci6Hi4' 661 ~B9N9C2Hibfl (110)
LDN dnp
792 758 819
Boat formation. Chair formation.
a comparison of the relative energies for the chair and boat conformations shows that the boat conformation of the outgrowth is slightly more stable than the chair conformation (689 and 662 kJ/mol, respectively). However, different circumstances may influence this order in relative energies. As discussed in section A of Results and Discussion, both the corrections for BSSE for the system under investigation and the influence of varying template sizes on the relative energies are numerically comparable to the difference in relative energies in Table 2. The influence of a larger template, modeling the h-BN (001) plane, on the relative energy for a boat formation of the partially saturated outgrowth on the armchair edge atoms has been investigated using the quantum mechanical LDA method. The enlargement of the template was made by replacing two hydrogens with a BZNZHZ ring formation, resulting in a h-BN template consisting of a nonbranched fourring system. The results of the LDA calculations showed that an increase of about 3% (27 kJ/mol) in the relative energy was obtained when the template was enlarged. C. General Discussion. The results of the quantum mechanical calculations have shown that, in a hydrogen-rich environment (Le., large excess of hydrogen in the reaction mixture of hydrogen and hydrocarbon), the outgrowths acting as diamond embryos are energetically more stable than the corresponding graphite nuclei, both on the zigzag and on the armchair edge atoms of the basal plane of h-BN. The contribution of the bond energies of the balancing hydrogen atoms to the relative energy has been studied in a former theoretical investigation regarding diamond nucleation on the zigzag and armchair edge atoms on the basal plane of graphite.2' The numerical value of the CH bond strength was calculated to be 391 kJ/mol, which is the average value obtained when two hydrogen atoms are simultaneously added to two neighboring carbon radical atoms. Hence, the creation of CH bonds is the driving force for the formation of the diamond nuclei. However, different numbers of hydrogen atoms (two and four) have been used in the present calculations in balancing the relative energies of the diamond outgrowths on the zigzag and on the armchair edge atoms on the h-BN (001) plane, respectively. In the case of the diamond nucleation on the armchair edge atoms, two of the four balancing hydrogen atoms are bonded to a boron and nitrogen atom, respectively (Figure 4). However, this number of balancing hydrogens is the minimum required to form buckled rings on the different edges of the h-BN (001) plane. The different number of balancing hydrogens for the two edges makes it difficult to compare the relative energies for diamond nucleation on the zigzag and armchair edges on the h-BN (001) plane. On the other hand, the different zigzag edges on the basal planes of h-BN (001) and graphite are crystallographically very similar, allowing for a comparison of the relative energy of diamond nucleation on these edges. This is also the situation for the armchair edges on the basal
planes of h-BN and graphite. Hence, in the present investigation a comparison of the relative energy has only been made for the corresponding edges of the substrates h-BN and graphite. The diamond nucleation on the armchair edge of h-BN has been compared with the diamond nucleation on the armchair edge of graphite. Also, the diamond nucleations on the zigzag edges of h-BN ((100) and (700)) have been comparded with each other, and with the diamond nucleation on the zigzag edge of graphite. Any conclusive predictions regarding the nucleation process have not been made in the present investigation, since such information is not easily obtained at the level of theory employed. The feasibility of a specific reaction mechanism (e.g., nucleation mechanisms) cannot be judged on its energetics alone. Conceivable transition states must be identified, and the effect of entropy has to be considered. A continued growth of diamond, originating from the buckled outgrowths from the different edges of the h-BN (001) plane, is assumed to generate the following epitaxial relationship with the h-BN substrate. On the (1 10) edge of the h-BN substrate, the (1 11) plane of diamond will be parallel to the (001) plane of h-BN. Moreover, the diamond [ 1101 direction will be parallel to the [110] direction of h-BN. Diamond growth on the (100) and (700) edges of the h-BN (001) plane is assumed to give the same epitaxial relationship. Unfortunately, the lack of experimental investigations regarding diamond deposition on single-crystal h-BN makes it impossible to compare the proposed epitaxial relationship with experimental data. However, the proposed relationship seems to be reasonable, since earlier experimental investigations regarding diamond nucleation on single-crystal graphite (being crystallographically very similar to h-BN) resulted in the corresponding epitaxial re1ati0nship.l~ The probability for diamond nucleation on the central region of the basal plane of h-BN has not been investigated in the present work. A nucleation process of this type has to be initiated by an adsorption of a gas species on the (001) terrace of h-BN. Mehandru et aLZ9have shown that the probability for adsorption of hydrogen atoms on the basal plane of graphite is much less probable to occur compared to the corresponding adsorption of hydrogen atoms on the edge atoms (100) and (1 lo), respectively. Hence, there is a small probability for nucleation of diamond to occur to a larger extent on the basal plane of graphite, and due to the crystallographic similarities between graphite and h-BN, this is also assumed to be the situation for the nucleation of diamond on the (001) plane of h-BN. Summary The nucleation of diamond on the zigzag edges ((100) and (100))and on the armchair edge (1 10) and the basal (001) plane of hexagonal boron nitride has been investigated theoretically, using a cluster approach and mainly the ab initio MO method. The total energy of the different outgrowths from the edge atoms have been related to the corresponding graphitic (planar) counterparts. The results have been compared to similar quantum mechanical calculations on the nucleation of diamond on the corresponding zigzag (100) and armchair (110) edges on the basal (001) plane of graphite. The result of the calculations shows that, in a hydrogen-rich environment, a diamond growth along both the armchair and the zigzag edges of the h-BN (001) plane is energetically more favorable than a corresponding growth of graphite. These results are numerically very similar to the results of the corresponding diamond nucleation on the zigzag and armchair edges of the basal plane of graphite. The relative energies for diamond
Diamond Nucleation on Hexagonal Boron Nitride nucleation on the (100) and (TOO)edges of h-BN are very similar to each other and to the relative energy for a corresponding diamond nucleation on the zigzag edge of graphite (562, 574, and 568 kJ/mol, respectively). The relative energy for the chair formation (corresponding to a diamond nucleus) on the (110) edge of the basal plane of h-BN is numerically almost identical to the relative energy of the corresponding outgrowth of the (110) edge of the basal plane of graphite (662 and 667 kJ/mol, respectively). On the other hand, the relative energy of the boat conformation of the outgrowth on the (110) edge atoms (corresponding to a lonsdalite nucleus) is slightly larger than the relative energy for a corresponding diamond nucleation (689 and 662 kJ/mol, respectively). Further analysis is needed, however, to enable more conclusive predictions about preferred reaction pathways. The present theoretical investigation supports the conclusion that diamond nucleation is almost equally preferable on boron, nitrogen, and carbon atoms, respectively, for the present type of crystallographic system. Acknowledgment. Financial support provided by the Angstrom Consortium is gratefully acknowledged. The calculations were performed on the CRAY Y-MP computer of the National Supercomputer Centre (NSC) in Linkoping. References and Notes (1) Shiomi, H.; Tanabe, K.; Nishibayashi, Y.; Fujimori, N. Jpn. J . Appl. Phys. 1990, 29, 34. (2) Narayan, J.; Srivatsa, A. R.; Peters, M.; Yokota, S.; Rrtvi, K. V. Appl. Phys. Lett. 1988, 53, 1823. (3) Williams, B. E.; Glass, J. T. J . Mater. Res. 1989, 4 , 373. (4) Jeng, D. G.; Tuan, H. S.; Salat, R. F.; Fricano, G. J. Appl. Phys. Lett. 1990, 20, 1868. (5) Yang, P. C.; Zhu, W.; Glass, J. T. J . Mater. Res. 1993, 8, 1773. (6) Prins, J. F. In Proceedings of the 2nd International Conference on New Diamond Science and Technology; Messier, R., Glass, J. T., Butler, J. E., Roy, R., Eds.; Material Research Society: Pittsburgh, PA, 1991; p 386. (7) Haubner, R. Refrac. Metals Hard Mater. 1990, 9 , 70. (8) Koizumi, S.; Murakami, T.; Inuzuka, T.; Suzuki, K. Appl. Phys. Lett. 1990, 57, 563.
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(10) Yoshikawa, M.; Ishida, H.; Ishitani, A.; Koizumi, S.; Inuzuka, T. Appl. Phys. Lett. 1991, 58, 1387. (11) Inuzuka, T.; Koizumi, S.; Suzuki,K. Diamond Relat. Mater. 1992, 1, 175. (12) Grot, S. A.; Hatfield, C. W.; Goldenblat, G. S.; Badzian, A. R.; Badzian, T. Appl. Phys. Lett. 1991, 58, 1542. (13) Argritia, A.; Angus, J. C.; Ma, J. S.; Wang, L.; Pirouz, P.; Lambrecht, W. R. L. J . Mater. Res. 1994, 9, 1849. (14) Wang, W.; Liao, K.; Gao, J. Phys. Status SolidiA 1991, 128, K83. (15) Stoner, B. R.; Glass, J. T. Appl. Phys. Lett. 1992, 60, 698. (16) Argritia, A.; Angus, J. C.; Wang, L.; Niry, X.I.; Pirouz, P. J. Appl. Phys. 1993, 73, 4305. (17) Angus, J. C.; Li, Z.; Sunkara, M.; Gat, R.; Anderson, A.; Mehandm, S. P.; Geiss, M. W. In Proceedings of the 2nd International Symposium on Diamond Materials; Purdes, A. J., et al., Eds.; Electrochemical Society: Pennington, NJ, 1991; Vol. 91-8. p 125. (18) Li, Z.; Wang, L.; Suzuki, T.; Argoitia, A.; Piroutz, P.; Angus, J. C. J. Appl. Phys. 1993, 73, 7 11. (19) Johansson, E.; Norekrans, A.-S.; Carlsson, J.-0. Diamond Relat. Mater. 1993, 2 , 383. (20) Debroy, J. J.; Pantano, G. G.; Yarbrough, W. A. J . Appl. Phys. 1992, 72, 3136. (21) Larsson, K.; Carlsson, J.-0.; Lunell, S. J . Phys. Chem. 1994, 98, 5019. (22) Johansson, E. Unpublished results. (23) Lindlbauer, A.; Haubner, R.; Lux, B. Wear 1992, 259, 67. (24) Frish, M. J.; Head-Gordon, M.; Trucks, G. W.; Foreman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A,; Binkley, C.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. GAUSSIAN92; Gaussian Inc.: Pittsburgh, PA, 1990. (25) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. A b Initio Molecular Theory; Wiley: New York, 1986. (26) Wimmer, E.; Freeman, A. J.; Fu, C.-L.; Cao, P.-L.; Chou, S.-H.; Delley, B. Supercomputer Research in Chemistry and Chemical Engineering; Jensen, K. F., Truhlar, D. G., Eds.; ACS Symposium Series 353; American Chemical Society; Washington, DC, 1987; pp 49-68. (27) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (28) Schwente, D. W.; Truhlar, D. G. J . Chem. Phys. 1985, 82, 2418. (29) Mehandru, S. P.; Anderson, A. B.; Angus, J. C. J. Chem. Phys. 1992, 96, 10978.
JF950976B
