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The Effect of Boron in Metal Borides and BN – A Theoretical Approach
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Boron, B, has many interesting electronic and structural properties, which makes it an ideal material for technical and industrial needs. The different binary materials that have been in focus in the present thesis, do all include B; metal diborides (MB2), cubic boron nitride (c-BN) forming an interface with diamond, and various phases of BN [cubic (c-BN), hexagonal (h-BN), wurtzitic (w-BN), and rhombohedral (r-BN)]. Density Functional Theory (DFT) methods have been used in studying structural geometries, energetical stabilities, electronic properties, and surface reactivity.

A structural and electronic comparison has been made for various MB2 compounds in planar and puckered structural forms. The resulting MB2 structure was found to correlate to the degree of electron transfer from the metal atom to B. A transfer of more than one electron was observed to induce a planar B structure. This is to be compared with the planar MgB2 structure, for which an electron transfer of two electrons was observed.

The initial nucleation of c-BN onto a diamond substrate has also been focused in the present thesis. This step has experimentally been found to be critical for a phase-pure c-BN thin film growth to occur. The evolution of an interfacial diamond//BN structure was investigated, with the purpose to simulate a layer-by-layer growth of c-BN. The obtained results were found to strongly support the experimental findings, in that there is a need for an extra energy in order to avoid non-cubic phases in the closest vicinity to the substrate. However, the simulations showed that it is possible to diminish this need of extra energy by completely terminating the surface by species like H or F. These calculations also showed that terminated diamond//BN generally show a stronger interfacial bond energy, thereby improving the adhesion to the diamond substrate. The importance with surface termination was not found crucial for thicker BN adlayers.

A combined effect of doping and surface termination was investigated for the various BN allotropes, (using O, C, and Si). The electron induced in c-BN by the O (or C) dopant was observed to move towards the surface B atoms, and thereby creating a more reactive surface. For the upper surface N atoms, doping was observed to create a less reactive N surface. The Si dopants did only show a positive effect on surface reactivity at the B surface sites on both h-BN (001) and r-BN (001) surfaces.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 87 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1124
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-218393ISBN: 978-91-554-8879-6 (print)OAI: oai:DiVA.org:uu-218393DiVA: diva2:695846
Public defence
2014-04-01, Häggsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-03-10 Created: 2014-02-11 Last updated: 2014-04-29
List of papers
1. Structure Determination of the 4d Metal Diborides: A Quantum Mechanical Study
Open this publication in new window or tab >>Structure Determination of the 4d Metal Diborides: A Quantum Mechanical Study
2006 (English)In: The Journal of Physical Chemistry B, Vol. 110, no 11, 5367-5371 p.Article in journal (Refereed) Published
Abstract [en]

Metal diborides (MB2) often have interesting thermal, mechanical, and superconducting properties. MgB2 was put into focus some years ago for its high transition temperature (39 K) in combination with its simple AlB2 structure. The boron structure in MB2 is assumed to be dependent on the electron transfer from the nearby positioned metal atoms. An electronic and structural comparison has been performed here for various initially planar and puckered transition-metal borides, using quantum mechanical density functional theory (DFT) calculations under periodic boundary conditions. In comparison to MgB2, the experimentally planar transition-metal diborides (ZrB2, NbB2, and MoB2) and the experimentally puckered ones (TcB2, RuB2, RhB2, and PdB2) have been examined. The results indicate that the energetic stability generally follows the experimentally obtained results. The metals that are less electronegative than boron donate electrons to boron, which in turn induce planar boron structures (graphitic-like). The metals that prefer to be planar donate more than one electron, while the trend for metals which favor puckered B structures is that they donate less than one electron per metal atom. Two donated electrons per metal atom (or very close to) will result in the most stable AlB2 structure.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-80004 (URN)doi:10.1021/jp056652x (DOI)
Available from: 2006-04-20 Created: 2006-04-20 Last updated: 2014-04-29
2. Initial Growth of BN on Diamond Substrates: A Theoretical Approach
Open this publication in new window or tab >>Initial Growth of BN on Diamond Substrates: A Theoretical Approach
2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 26, 11448-11455 p.Article in journal (Refereed) Published
Abstract [en]

Cubic boron nitride, c-BN, is a very interesting material due to its extreme properties of which some are comparable, or even superior, to diamond. Unfortunately, there are severe problems with the vapor phase synthesis of c-BN, which makes it very important to investigate the possibility for new growth pathways. The choice of substrate has been experimentally found to be decisive for an ideal growth of c-BN. Diamond is a material that has been found to be a good substrate for growth of c-BN directly onto the substrate. By using quantum mechanical density functional theory (DFT) under periodic boundary conditions, the details in the layer-by-layer formation of BN onto diamond (100) has been investigated in the present study. The prerequisites for an initial growth of c-BN (100) have been studied by adding the alternative B and N layers sequentially and calculating the resulting interfacial binding strengths and geometrical structures. For the situation with one monatomic layer on diamond, the interfacial binding energy was calculated to be strongest for nitrogen heteroepitaxially positioned onto diamond (100). The individual atoms in a monatomic B adlayer did, however, not choose corresponding heteroepitaxial positions. When applying a second atomic layer of c-BN, two different models were initially constructed with different structural alignments with respect to the underlying diamond structure. One model was heterostructurally positioned on top of diamond (100), while the other model had the x-axis of the c-BN lattice aligned with the y-axis of the diamond lattice. For the situation with N attached to the diamond substrate, the heteroepitaxial adlayer structure stayed cubic as a result of the geometry optimization, while the other became amorphous-like. Both of these adlayer structures showed large interfacial binding strengths, but the amorphous-like structure became energetically the most stable one. On the contrary, a two-layer BN structure with B attached to the diamond surface did not show the same characteristics. It was only the initial heteroepitaxial BN lattice that resulted in a stable adlayer structure. It stayed cubic after the geometry optimization, with a large interfacial binding strength. The four-layer BN structure on diamond showed similar features.

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-136025 (URN)10.1021/jp911924g (DOI)000279282200018 ()
Available from: 2010-12-10 Created: 2010-12-09 Last updated: 2014-04-29Bibliographically approved
3. Effect of Terminating Species on the Initial Growth of BN on Diamond Substrates
Open this publication in new window or tab >>Effect of Terminating Species on the Initial Growth of BN on Diamond Substrates
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 7, 3490-3503 p.Article in journal (Refereed) Published
Abstract [en]

The details in the layer-by-layer formation of H (or F)-terminated boron nitride onto diamond (100) have been theoretically studied using ab initio density functional theory under periodic conditions. Cubic boron nitride, c-BN, is a very interesting and promising material due to its extreme properties. However, there are severe problems during the vapor phase synthesis of c-BN because of the formation of noncubic phases in the initial grow steps, why a gentle large area chemical vapor deposition (CVD) deposition is needed. The substrate material has been experimentally shown to be very important for an ideal growth of c-BN in the initial grow process. Diamond is a material that has been found to be a good substrate material for this purpose. By alternating H (or F)-terminated B and N layers, and calculating the resulting interfacial binding strengths and geometrical structures, the initial growth has been studied and compared to earlier theoretical results that have been made without terminating species. Two different structural alignments, with respect to the underlying diamond substrate, were initially constructed. One model was heterostructurally positioned on top of the diamond (100) substrate, while the other model had the x-axis of the c-BN lattice aligned with the y-axis of the diamond lattice. For the situation with a terminated monatomic B layer on top of diamond, the heteroepitaxially built structure was the most energetically favored. This result is just the opposite from the nonterminated situation, where the nonheteroepitaxial structures were favored. The binding energy for the heteroepitaxial terminated monolayer of N was calculated even stronger than the terminated B monolayer. When applying a second atomic layer of c-BN on top of the monatomic B layer, the heteroepitaxially build structure is energetically preferred. For the nonheteroepitaxial growth the adlayer actually bonds to the diamond substrate when H-terminated. Without terminating species the nonheteroepitaxial adlayer did not bind at all. Also for the situation with two atomic adlayer with N closest to the diamond substrate, the heteroepitaxial structure is energetically preferred over the nonheteroepitaxial structure. The opposite is true without any terminating species. When four, six, and eight atom layers are applied, with N atoms closest to the diamond substrate, the heteroepitaxially built structures are favored. The results showed that the terminating species helped to uphold the cubic sp(3)-formation. When using terminating species, the heteroepitaxially built interfaces had generally higher binding strengths than the nonheteroepitaxially built interfaces. While, when not using terminating species, the nonheteroepitaxially built interfaces were preferred instead. The c-BN structure seems to be more easily formed when several BN layers already are formed, and the interfacial binding strength is stabilized.

Keyword
c-BN diamond CVD
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-218379 (URN)10.1021/jp4083213 (DOI)000331861700014 ()
Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
4. Effect by Doping and Surface Termination on c-BN Surface Reactivity; A Theoretical DFT Approach
Open this publication in new window or tab >>Effect by Doping and Surface Termination on c-BN Surface Reactivity; A Theoretical DFT Approach
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 19, 10061-10075 p.Article in journal (Refereed) Published
Keyword
c-BN, DFT, CVD
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-218389 (URN)10.1021/jp412695q (DOI)000336198900024 ()
Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
5. Effect of Interstitial Si on Different Boron Nitride Allotropes
Open this publication in new window or tab >>Effect of Interstitial Si on Different Boron Nitride Allotropes
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 35, 20264-20274 p.Article in journal (Refereed) Published
Abstract [en]

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), w-BN (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 B-site 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.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-218392 (URN)10.1021/jp502643j (DOI)000341337700024 ()
Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved

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