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Venturing Further into the Field of 2D Materials and their Laminated Parent Phases
Linköping University, Department of Physics, Chemistry and Biology, Materials design. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0003-2928-0993
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The field of 2D materials is a relatively young and rapidly growing area within materials science, which is concerned with atomically thin states of matter. Because of their intrinsic 2D morphology, 2D materials have exceptionally high surface to weight or surface to volume ratio. This renders them excellent candidates for surface-sensitive applications such as catalysis and energy storage, which can aid us in the transition to a more sustainable society. 2D materials are also interesting because they show properties intrinsically different from those of their 3D counterparts, expanding the attainable property space within materials science. A 2D material can be synthesised by either a bottom-up or top-down approach. The focus here is on the latter, where the 2D material is derived by either mechanical exfoliation or selective etching of a 3D nanolaminated parent phase. 

A 3D laminate can typically be assigned to one of two types, depending on the type of interlayer bonding: van der Waals (vdW) or chemical bonding. In a vdW bonded phase, the constituent layers are kept together into their 3D form by rather weak vdW forces, while in the latter type, the layers are bound more strongly by chemical interactions (i.e., covalent, ionic and metallic bonds). The first 2D materials were derived from vdW-phases, which can be exfoliated by mechanical methods. In a chemically bound laminated phase, the inter layer bonding is stronger, and more complex methods are required for exfoliation of these phases into 2D. This thesis concerns the computational study and development of novel 2D materials through exploration of 3D nanolaminated structures, assessment of their phase stability, and potential for conversion into 2D. The 2D derivatives are in turn studied through prediction of dynamical stability, termination configuration, and evaluation of electronic properties. 

Paper III and IV each addresses a family of van der Waals structures. The family of 3D materials studied in Paper III was chosen because it was recently demonstrated as possible to use for derivation of so called 2D MX-enes, while the 2D form of NbOCl2, from the family studied in Paper IV, has been shown to exhibit exciting optical properties. Both projects focus on identification of parent 3D materials, their exfoliation from 3D to 2D, and the electronic properties of the studied phases. In each project, a range of different chemical compositions is considered, chosen based on the experimentally known members of the respective families. A 3D structural ground state is predicted for each composition and prototype, and the dynamical stability with respect to lattice vibrations is established for each identified structure. To assure the experimental relevance of each considered 3D phase, the thermodynamical stability of each structure is assessed via the formation enthalpy with respect to competing phases, identifying seven stable structures in Paper III, and 17 in Paper IV, all of which are also found dynamically stable. Evaluation of the exfoliation energy for all these phases indicates that 3D to 2D conversion is possible. The electronic band structure and density of states were evaluated both for the 2D materials –being the primary focus in both projects – and their 3D parent phases. Al-though the bandgap for semiconducting phases is generally increased upon exfoliation, the electronic properties are mostly retained when exfoliating the vdW phases studied in this thesis. 

Paper I, II and V address chemically bonded 3D phases and their 2D derivatives. In these 3D phases, auxiliary atoms are interleaved between the 2D units, which needs to be selectively etched to form the corresponding 2D material. Additionally, new terminating species – so called terminations –may attach to the surfaces of the 2D units exposed during etching. Paper I presents an analysis of bonding characteristics in a group of nanolaminated 3D chemically bonded borides: Mo2SiB2, Ti4MoSiB2, and Ti5SiB2, out of which only the two former are observed experimentally. We identify a peak of antibonding states at the Fermi level for Ti5SiB2 as a reason why full elemental substitution of Mo is not achieved experimentally. Papers II and V instead focus on 2D materials derived from chemical 3D parent phases. They go beyond the 2D transition metal carbides and nitrides (MXenes), which until recently were the only 2D materials synthesised through selective etching. Paper II is a study of possible termination configurations on the first 2D boride Mo4/3B2−xTz – boridene – which is identified as being a conductor or small bandgap semiconductor, depending on the terminating species and specific configuration. 

In Paper V, a computational methodology for simulation of the selective etching process is employed to predict the possibility of etching Y from YM2X2, where the transition metal M and metalloid or nonmetal X are chosen to cover a large compositional space. This results in the prediction of 15 stable 2D structures, out of which nine are not previously investigated. All 2D structures are found to be either metallic or semimetallic. 

In this thesis, several different computational tools are used to predict and study laminated 3D phases and their corresponding 2D derivatives, assessing their properties considering both purely hypothetical and experimentally realised structures. Experimental relevance is central to all calculations, either by complementing already established experimental results, or by rigorous assessment of thermodynamical and dynamical stability to estimate the potential for experimental synthesis. The thesis expands our knowledge of 3D laminated phases and their 2D derivatives, and identifies several new phases which are likely possible to synthesise. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2024. , p. 56
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2376
Keywords [en]
Density Functional Theory, Bonding analysis, Themodynamical stability, Electronic band structure, Computational screening, Dynamical stability, Nanolaminate, MXene, 2D material, Boridene, Chemical exfoliation, Selective etching
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-201557DOI: 10.3384/9789180755443ISBN: 9789180755436 (print)ISBN: 9789180755443 (electronic)OAI: oai:DiVA.org:liu-201557DiVA, id: diva2:1843859
Public defence
2024-04-12, Ada Lovelace, B-building, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Note

Financial support from the Swedish Research Council, VR (grant number 2019-05047 and 2022-06099), the Göran Gustavsson Foundation for Research in Natural Sciences and Medicine, and the Knut and Alice Wal-lenberg (KAW) foundation has made this work possible. Simulations has been carried out using resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) and Swedish National Infrastructure for Computing (SNIC), using the NSC, PDC and HPC2N computer clusters.

Available from: 2024-03-12 Created: 2024-03-12 Last updated: 2024-03-12Bibliographically approved
List of papers
1. Investigation of out-of-plane ordered Ti4MoSiB2 from first principles
Open this publication in new window or tab >>Investigation of out-of-plane ordered Ti4MoSiB2 from first principles
2022 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 34, no 18, article id 185501Article in journal (Refereed) Published
Abstract [en]

The laminated ternary boride Mo5SiB2 of T2 structure have two symmetrically inequivalent metallic sites, 16l and 4c, being occupied in a 4:1 ratio. The phase was recently shown to be stable for 80% substitution of Mo for Ti, at the majority site, forming an out-of-plane chemically ordered quaternary boride: Ti4MoSiB2. Considering that the hypothetical Ti5SiB2 is theoretically predicted as not stable, a key difference in bonding characteristics is indicated for full substitution of Mo for Ti at the metallic sites. To explore the origin of formation of Ti4MoSiB2, we here investigate the electronic properties and bonding characteristics of Mo5SiB2, Ti4MoSiB2 and Ti5SiB2 through their density of states, projected crystal orbital Hamilton population (pCOHP), Bader charge partitioning and second order force constants. The bond between the two different metallic sites is found to be key to the stability of the compounds, evident from the pCOHP of this bond showing a peak of bonding states close to the Fermi level, which is completely filled in Mo5SiB2 and Ti4MoSiB2, while only partially filled in Ti5SiB2. Furthermore, the lower electronegativity of Ti compared to Mo results in charge accumulation at the Si and B sites, which coincides with a reduced bond strength in Ti5SiB2 compared to Mo5SiB2 and Ti4MoSiB2. Bandstructure calculations show that all three structures are metallic. The calculated mechanical and elastic properties show reduced bulk (B) and elastic (E) moduli when introducing Ti in Mo5SiB2, from 279 and 365 GPa to 176 and 258 GPa, respectively. The Pugh criteria indicates also a slight reduction in ductility, with a G/B ratio increasing from 0.51 to 0.59.

Place, publisher, year, edition, pages
IOP Publishing Ltd, 2022
Keywords
chemical bonding; MAB; electronic structure; mechanical properties; quaternary laminated boride; transition metal boride; T2 phase
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:liu:diva-183752 (URN)10.1088/1361-648X/ac51fe (DOI)000763069100001 ()35120334 (PubMedID)
Note

Funding Agencies|Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2020.0033]; Swedish Research councilSwedish Research CouncilEuropean Commission [2019-04233, 2018-05973]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

Available from: 2022-03-23 Created: 2022-03-23 Last updated: 2024-03-12
2. Investigation of 2D Boridene from First Principles and Experiments
Open this publication in new window or tab >>Investigation of 2D Boridene from First Principles and Experiments
Show others...
2022 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 32, no 14, article id 2109060Article in journal (Refereed) Published
Abstract [en]

Recently, a 2D metal boride - boridene - has been experimentally realized in the form of single-layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2. The chemical formula of the boridene was suggested to be Mo4/3B2-xTz, where Tz denotes surface terminations. Here, the termination composition and material properties of Mo4/3B2-xTz from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo4/3B2-xTz is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo1.33B1.9O0.3(OH)1.5F0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo4/3B2-xTz shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.

Place, publisher, year, edition, pages
Wiley, 2022
Keywords
Boridene, Electronic structure, HER, MBene, Surface terminations
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-182697 (URN)10.1002/adfm.202109060 (DOI)
Note

Funding agencies: The Knut and Alice Wallenberg Foundation (KAW 2020.0033), The Swedish Foundation for Strategic Research (EM16-0004 and ARC19-0026), The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No 2009 00971), The Swedish Research Council (no. 2018-03927 and 2019-04233). The calculations were carried out using supercomputer resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC) and the PDC Center for high-performance computing partially funded by the Swedish Research Council through grant agreement no. 2018-05973.

Available from: 2022-02-03 Created: 2022-02-03 Last updated: 2024-03-12Bibliographically approved

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