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Quantum Chemical Studies of Diamond for Energy Related Applications
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. (Prof. Karin Larsson)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Diamond is a unique material with excellent properties. As a result of the development within the area of CVD synthesis, doping and surface functionalization, diamond has become a strong candidate for use in electrochemical, electronic and biomedical applications. In this thesis, theoretical calculations have been used with the purpose to investigate various properties of the diamond surfaces.

The effect of doping elements (N and B) on the stability of different surface terminations with X (where X = H, OH, Oontop or Obridge) has been investigated for a diamond (100) surface. As a result, the adsorption energy for all termination types was shown to decrease from the situation with a non-doped diamond surface, to the scenario with a N- (or B-doped) diamond thin film.. This result was found to correlate well with the changes of the calculated Csurface-X bond lengths. Furthermore, the spin density has been calculated and used to show the local distribution of the unpaired electron, which is the consequence of the introduction of dopants into the diamond slab. As a result, the spin density was found to be localized in the vicinity to the dopants for H- (or OH-) terminated diamond (100) surfaces. On the other hand, a delocalised spin density over the Oadsorbate and Csurface layer for Oontop- and Obridge-terminated surfaces, has also been observed. Moreover, the results of the pDOS calculations indicate the electron donating ability of N, and the hole donating ability of B. The Fermi level was shifted towards the lower conduction band edge for N-doped diamond, and towards the upper edge of the valence band edge for B-doped diamond. Hence, N-doped diamond will render n-type conductivity, and B-doped diamond will show p-type conductivity. In addition, an interesting observation was made for Oontop –terminated diamond surfaces. Localized electron conductivity, involving only this type of termination situation,, was also observed for N- (or B-) doped and completely  Oontop-terminated diamond surfaces.

With the purpose of applying diamond substrates in the formation of epitaxial graphene, the annealing process of an ideal diamond (111) surface has also been simulated in the present work. It was thereby shown that high temperatures (over 2000 K) will be required for the epitaxial formation of graphene ontop of the diamond (111) surface. However, in the presence of hydrogen radicals (by saturating the radical sites in the system), the required temperature was observed to decrease to 1000 K. In addition to these MD simulations, by using an interlayer iron ontop of the diamond (111) surface, the adhesion energies between the graphene and the Fe//diamond slab, as well as the adhesion energy between the graphene//Fe layer and the diamond (111) surface, have been calculated. Thereby, the interaction between the graphene and Fe layer was obtained to be very weak, and of an electrostatic type. On the other hand, the interaction between the Fe interlayer and the diamond substrate was calculated as a moderately strong covalent bond. Moreover, the changes in these interactions, correlating to the changes in the pDOS spectra of graphene, Fe and diamond, gave a tendency of one-dimensional quantum size effect, depending on the thickness of Fe interlayer.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 61 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1231
Keyword [en]
Diamond, surface, graphene, terminations, doping, functionalization
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-245224ISBN: 978-91-554-9173-4 (print)OAI: oai:DiVA.org:uu-245224DiVA: diva2:790821
Public defence
2015-04-17, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
EU, FP7, Seventh Framework Programme, 238201
Available from: 2015-03-20 Created: 2015-02-25 Last updated: 2015-04-17
List of papers
1. A Theoretical Study of the Effect of Dopants on Diamond (100) Surface Stabilization for Different Termination Scenarios
Open this publication in new window or tab >>A Theoretical Study of the Effect of Dopants on Diamond (100) Surface Stabilization for Different Termination Scenarios
2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 5, 2545-2556 p.Article in journal (Refereed) Published
Abstract [en]

The effect of dopants (N or B) on differently terminated diamond (100)-2 × 1 surfaces has in the present study been studied theoretically by using DFT (density functional theory) under periodic boundary conditions. The terminating species, X, include H, OH, Oontop, and Obridge. As a result of geometry optimization, the C–N and C–B bond lengths were calculated to be longer than for the situation with saturated binding conditions (i.e., the situation where N (or B) are binding to three other atoms, instead of four). Moreover, the X–Csurface-dopant angles were observed to decrease for the N-doped and increase for the B-doped senarios. In addition, the atomic charges and bond populations for the region surrounding the dopants were also carefully analyzed in order to compare the surface stabilization situations for non-, N- and B-doped diamond surfaces. For the H-terminated diamond surfaces, the C–H bonds became weakened when substituationally doped with either N or B. For the O-terminated diamond surfaces (i.e., both Oontop, and Obridge), the results showed opposite trends by strengthening (or weakening) the C–O bonds for the N- (or B-) doped system, respectivly. The adsorption energies for the various terminating species were observed to decrease when going from a nondoped to an N-doped situation and finally over to a B-doped situation. This is a result that strongly correlates with the calculated Csurface–X (X = H, OH, Oontop, Obridge) bond lengths. In addition, the effect of surface termination on the diamond surface stabilization energy, was observed to be in the following order: Obridge > Otop > H > OH. This result was valid for both non-, N- and B-doped diamond surfaces. The calculated spin density calculations indicated a local distribution of the unpaired electron in the N- and B-doped systems, respectively. This is a result that showed a strong correlation to the bond lengths surrounding the dopants and to the calculated adsorption energies for the terminating species, X. Moreover, the surface electronic structures (i.e., surface states) for the N- and B-doped systems were calculated and visualized by performing pDOS calculations. The results showed a shift of the Fermi levels for the N- and B-doped situations. As expected, the Fermi level was shifted toward the conduction band for the N-doped surfaces and toward the valence band for the B-doped systems. In addition, the pDOS spectra for the Oontop-termination showed extra states around the Fermi level, which were the result induced by the radical nature of this type of termination species.

Place, publisher, year, edition, pages
Washington, DC: , 2015
Keyword
Diamond, surface terminations, dopants
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-242826 (URN)10.1021/jp511077v (DOI)000349136400035 ()
Projects
MATCON
Funder
EU, FP7, Seventh Framework Programme, MATCON-238201
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2017-12-05Bibliographically approved
2. Formation Conditions for Epitaxial Graphene on Diamond (111) Surfaces
Open this publication in new window or tab >>Formation Conditions for Epitaxial Graphene on Diamond (111) Surfaces
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The phase transformation from a non-terminated diamond (111) surface to graphene has in the present study been simulated by using ab initio MD calculations at different temperatures and under various reaction conditions. For strict vacuum conditions, the graphitization process was observed to start at about 800 K, with a final graphene-like adlayer obtained at 2500 K. The C-C bonds across the interface were found to be broken gradually with an increase in temperature. The resulting graphene-like adlayer at 2500 K was observed to chemisorb to the underlying diamond surface with 33% of the initial C-C bonds, and with a C-C covalent energy value of 3.4 eV. The corresponding DOS spectra showed a p-doped character, as compared with graphene.

When introducing H radicals during the annealing process, a graphene-like adlayer started to be formed at a much lower temperature; 500K. The completeness of the diamond-to-graphene process was found to strongly depend on the concentration of H radicals. When introducing a larger concentration of H radicals into the lattice in the initial part of the annealing process, the formation of a free-standing graphene layer was observed to take place at an even lower H concentration and temperature (1000 K). 

Keyword
Diamond, graphene, graphitization, theory, Molecular Dynamics, DFT
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-242877 (URN)
Projects
MATCON
Funder
EU, FP7, Seventh Framework Programme, MATCON-238201
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2015-04-17
3. A Theoretical Study of Dye Molecules Adsorbed onto Diamond (111) Surfaces
Open this publication in new window or tab >>A Theoretical Study of Dye Molecules Adsorbed onto Diamond (111) Surfaces
2016 (English)In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 213, no 8, 2105-2111 p.Article in journal (Refereed) Published
Abstract [en]

The combinations of different dye molecules adsorbed on 100% H-terminated B-doped diamond (111) surfaces, have been carefully simulated by using DFT under periodic boundary conditions. The dye molecules include C20H13NO3S4, C35H37NO2S3, C34H38OS2, C32H36OS2, and C31H35S3Br. The functional group within these dyes, behaves as an electron acceptor during the sunlight harvesting process. By comparing the upper valence band edge of the diamond surface with the HOMO and LUMO levels of the dyes in an energy diagram, a suitable scheme for a p-type dye sensitized solar cell was constructed. These functionalities were further confirmed by the observation of a partial degree of electron transfer from the diamond surface to the dye molecules. The combination of spectra for the dye molecules showed a wide absorption range from 200nm to 620nm. The effect of B doping on the binding of the dye molecules have furthermore been investigated. Shorter diamond//dye bonds are well correlated with large electron bond populations, and a larger degree of electron transfer. The former is regarded to be a meassure of covalency, and the latter a meassure of ionicity, in the interfacial bond. 

Keyword
B-doped diamond, DSSC, dye, DFT
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-242878 (URN)10.1002/pssa.201600154 (DOI)000385223900014 ()
Projects
MATCON
Funder
EU, FP7, Seventh Framework Programme, MATCON-238201
Note

Title in Thesis list of papers: A Theoretical Study of Dye Molecules Adsorbed onto Diamond (111) Surfaces for the Application of a p-Type Dye Sensitized Solar Cell

Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2017-12-05Bibliographically approved
4. Theoretical Study of the Effect of an Fe Interlayer on the Formation of Graphene on Diamond (111) surface
Open this publication in new window or tab >>Theoretical Study of the Effect of an Fe Interlayer on the Formation of Graphene on Diamond (111) surface
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect by a catalytic Fe interlayer on the formation of graphene onto a diamond (111) substrate, has been studied by using DFT calculations under perodict boundary conditions.  With varying the Fe interlayer thickness from two to five atomic layers, geometry optimized diamond//Fe//graphene multilayer models were obtained. A general result was that the Fe atoms are ontop positioned on both the graphene carbon atoms and on the diamond carbon atoms. Moreover, both the interfacial diamond//Fe and Fe//graphene adhesion energies were calculated and compared. As a result, the interaction between graphene and the iron layer, which was indentified as of an electrostatic nature, was found to be weak (-12.3 to -10.5 kJ/mol  per graphene C atom) and propotional to the thickness of the Fe layer. The thicker the Fe interlayer, the stronger was the adhesion energy. On the contary, the adhesion energy between the diamond substrate and the Fe layer was calculated as much stronger (-124.5 to -109.0 kJ/mol per diamond C atom), and following an inverse correlation. The thicker the Fe interlayer, the weaker is the interfacial adhesion energy. Calculations of electron density differences and partial Density of States (pDOS´s), will further support the results of a quantum size effect of the iron layer. 

Keyword
Graphitization, Graphene, Diamond (111), Fe catalysis, DFT
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-242879 (URN)
Projects
MATCON
Funder
EU, FP7, Seventh Framework Programme, MATCON-238201
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2015-04-17

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