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Influence of defects and impurities on the properties of 2D materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-3161-4326
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Graphene, the thinnest material with a stable 2D structure, is a potential alternative for silicon-based electronics. However, zero band gap of graphene causes a poor on-off ratio of current thus making it unsuitable for logic operations. This problem prompted scientists to find other suitable 2D materials. Creating vacancy defects or synthesizing hybrid 2D planar interfaces with other 2D materials, is also quite promising for modifying graphene properties. Experimental productions of these materials lead to the formation of possible defects and impurities with significant influence in device properties. Hence, a detailed understanding of the effects of impurities and defects on the properties of 2D systems is quite important.

In this thesis, detailed studies have been done on the effects of impurities and defects on graphene, hybrid graphene/h-BN and graphene/graphane structures, silicene and transition metal dichalcogenides (TMDs) by ab-initio density functional theory (DFT). We have also looked into the possibilities of realizing magnetic nanostructures, trapped at the vacancy defects in graphene, at the reconstructed edges of graphene nanoribbons, at the planar hybrid h-BN graphene structures, and in graphene/graphane interfaces. A thorough investigation of diffusion of Fe adatoms and clusters by ab-initio molecular dynamics simulations have been carried out along with the study of their magnetic properties. It has been shown that the formation of Fe clusters at the vacancy sites is quite robust. We have also demonstrated that the quasiperiodic 3D heterostructures of graphene and h-BN are more stable than their regular counterpart and certain configurations can open up a band gap. Using our extensive studies on defects, we have shown that defect states occur in the gap region of TMDs and they have a strong signature in optical absorption spectra. Defects in silicene and graphene cause an increase in scattering and hence an increase in local currents, which may be detrimental for electronic devices. Last but not the least, defects in graphene can also be used to facilitate gas sensing of molecules as well as and local site selective fluorination.  

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 100 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1432
Keyword [en]
2D Materials, Defects on 2D materials, Impurities on 2D materials
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-300970ISBN: 978-91-554-9699-9OAI: oai:DiVA.org:uu-300970DiVA: diva2:974165
Public defence
2016-11-11, Polhemsalen Ång/10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2016-10-19 Created: 2016-08-16 Last updated: 2016-11-03
List of papers
1. Magnetic impurities in graphane with dehydrogenated channels
Open this publication in new window or tab >>Magnetic impurities in graphane with dehydrogenated channels
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 15, 155426- p.Article in journal (Refereed) Published
Abstract [en]

We have investigated the electronic and magnetic response of a single Fe atom and a pair of interacting Fe atoms placed in patterned dehydrogenated channels in graphane within the framework of density functional theory. We have considered two channels: "armchair" and "zigzag" channels. Fully relaxed calculations have been carried out for three different channel widths. Our calculations reveal that the response to the magnetic impurities is very different for these two channels. We have also shown that one can stabilize magnetic impurities (Fe in the present case) along the channels of bare carbon atoms, giving rise to a magnetic insulator or a spin gapless semiconductor. Our calculations with spin-orbit coupling shows a large in-plane magnetic anisotropy energy for the case of the armchair channel. The magnetic exchange coupling between two Fe atoms placed in the semiconducting channel with an armchair edge is very weakly ferromagnetic whereas a fairly strong ferromagnetic coupling is observed for reasonable separations between Fe atoms in the zigzag-edged metallic channel with the coupling mediated by the bare carbon atoms. The possibility of realizing an ultrathin device with interesting magnetic properties is discussed.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-172672 (URN)10.1103/PhysRevB.85.155426 (DOI)000302697400007 ()
Available from: 2012-04-12 Created: 2012-04-12 Last updated: 2016-09-25Bibliographically approved
2. Functionalization of edge reconstructed graphene nanoribbons by H and Fe: A density functional study
Open this publication in new window or tab >>Functionalization of edge reconstructed graphene nanoribbons by H and Fe: A density functional study
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2012 (English)In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 152, no 18, 1719-1724 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we have studied functionalization of 5-7 edge-reconstructed graphene nanoribbons by ab initio density functional calculations. Our studies show that hydrogenation at the reconstructed edges is favorable in contrast to the case of unreconstructed 6-6 zigzag edges, in agreement with previous theoretical results. Thermodynamical calculations reveal the relative stability of single and dihydro-genated edges under different temperatures and chemical potential of hydrogen gas. From phonon calculations, we find that the lowest optical phonon modes are hardened due to 5-7 edge reconstruction compared to the 6-6 unreconstructed hydrogenated edges. Finally, edge functionalization by Fe atoms reveals a dimerized Fe chain structure along the edges. The magnetic exchange coupling across the edges varies between ferromagnetic and antiferromagnetic ones with the variation of the width of the nanoribbons.

Keyword
Graphene, Functionalization, Magnetism, Phonons, Density functional theory
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-177362 (URN)10.1016/j.ssc.2012.06.028 (DOI)000308776600003 ()
Available from: 2012-07-11 Created: 2012-07-11 Last updated: 2016-09-25Bibliographically approved
3. Designing Fe Nanostructures at Graphene/h-BN Interfaces
Open this publication in new window or tab >>Designing Fe Nanostructures at Graphene/h-BN Interfaces
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 42, 21763-21771 p.Article in journal (Refereed) Published
Abstract [en]

Tailor-made magnetic nanostructures offer a variety of functionalities useful for technological applications. In this work, we explore the possibilities of realizing Fe nanostructuresat the interfaces of 2D graphene and h-BN by ab initio density functional calculations. With the aid of ab initio Born-Oppenheimer molecular dynamics simulations and diffusion barriers calculated by the nudged elastic band method, we find that (i) diffusion barriers of Fe on BN are much smaller than those on graphene, (ii) the Fe adatoms form clusters within a short time interval (similar to 2.1 ps), and (iii) Fe clusters diffuse easily across the C-N interface but become immobile at the C-B interface. The calculated magnetic exchange coupling between Fe clustersat C-B interfaces varies nonmonotonically as a function of the width of BN separating thegraphene parts. One may envisage design of magnetic nanostnictures at the C-B interface of 2Dgraphene/h-BN hybrids to realize interesting applications related to spintronics.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-208999 (URN)10.1021/jp405346r (DOI)000326260000016 ()
Funder
Knut and Alice Wallenberg Foundation
Available from: 2013-10-14 Created: 2013-10-14 Last updated: 2016-09-25Bibliographically approved
4. Quasiperiodic van der Waals heterostructures of graphene and h-BN
Open this publication in new window or tab >>Quasiperiodic van der Waals heterostructures of graphene and h-BN
(English)Manuscript (preprint) (Other academic)
Keyword
Graphene, Boron nitride, Heterostructure, Fibonacci sequence, Band gap
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217159 (URN)
Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2016-09-25
5. Fen (n=1–6) clusters chemisorbed on vacancy defects in graphene: Stability, spin-dipole moment, and magnetic anisotropy
Open this publication in new window or tab >>Fen (n=1–6) clusters chemisorbed on vacancy defects in graphene: Stability, spin-dipole moment, and magnetic anisotropy
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, 205411- p.Article in journal (Refereed) Published
Abstract [en]

In this work, we have studied the chemical and magnetic interactions of Fen (n = 1–6) clusters with vacancy defects (monovacancy to correlated vacancies with six missing C atoms) in a graphene sheet by ab initio density functional calculations combined with Hubbard U corrections for correlated Fe-d electrons. It is found that the vacancy formation energies are lowered in the presence of Fe, indicating an easier destruction of the graphene sheet. Due to strong chemical interactions between Fe clusters and vacancies, a complex distribution of magnetic moments appear on the distorted Fe clusters which results in reduced averaged magnetic moments compared to the free clusters. In addition to that, we have calculated spin-dipole moments and magnetic anisotropy energies. The calculated spin-dipole moments arising from anisotropic spin density distributions vary between positive and negative values, yielding increased or decreased effective moments. Depending on the cluster geometry, the easy axis of magnetization of the Fe clusters shows in-plane or out-of-plane behavior.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-224610 (URN)10.1103/PhysRevB.89.205411 (DOI)000341351300003 ()
Funder
Carl Tryggers foundation EU, European Research Council
Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2016-09-25Bibliographically approved
6. Systematic study of structural, electronic, and optical properties of atomic-scale defects in the two-dimensional transition metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te)
Open this publication in new window or tab >>Systematic study of structural, electronic, and optical properties of atomic-scale defects in the two-dimensional transition metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te)
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 23, 235408Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-269013 (URN)10.1103/PhysRevB.92.235408 (DOI)000366086300009 ()
Funder
Knut and Alice Wallenberg FoundationCarl Tryggers foundation Swedish Research Council
Available from: 2015-12-12 Created: 2015-12-12 Last updated: 2016-09-25
7. Energetic stability, STM fingerprints and electronic transport properties of defects in graphene and silicene
Open this publication in new window or tab >>Energetic stability, STM fingerprints and electronic transport properties of defects in graphene and silicene
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2016 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 8, 6702-6708 p.Article in journal (Refereed) Published
Abstract [en]

Novel two-dimensional materials such as graphene and silicene have been heralded as possibly revolutionary in future nanoelectronics. High mobilities, and in the case of silicene, its seemingly natural integration with current electronics could make them the materials of next-generation devices. Defects in these systems, however, are unavoidable particularly in large-scale fabrication. Here we combine density functional theory and the non-equilibrium Green’s function method to simulate the structural, electronic and transport properties of different defects in graphene and silicene. We show that defects are much more easily formed in silicene, compared to graphene. We also show that, although qualitatively similar, the effects of different defects occur closer to the Dirac point in silicene, and identifying them using scanning tunneling microscopy is more difficult particularly due to buckling. This could be overcome by performing direct source/drain measurements. Finally we show that the presence of defects leads to an increase in local current from which it follows that they not only contribute to scattering, but are also a source of heating.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-271340 (URN)10.1039/C5RA23052G (DOI)000368858000083 ()
Funder
Carl Tryggers foundation Swedish Research Council, 621-2009-3628Knut and Alice Wallenberg Foundation
Available from: 2016-01-07 Created: 2016-01-07 Last updated: 2016-09-25Bibliographically approved
8. Improved gas sensing activity in structurally defected bilayer graphene
Open this publication in new window or tab >>Improved gas sensing activity in structurally defected bilayer graphene
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2012 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 50, 50550- p.Article in journal (Refereed) Published
Abstract [en]

Graphene is a two-dimensional material with a capability of gas sensing, which is here shown to be drastically improved by inducing gentle disorder in the lattice. We report that by using a focused ion beam technique, controlled disorder can be introduced into the graphene structure through Ga + ion irradiation. This disorder leads to an increase in the electrical response of graphene to NO 2 gas molecules by a factor of three in an ambient environment (air). Ab initio density functional calculations indicate that NO 2 molecules bind strongly to Stone–Wales defects, where they modify electronic states close to the Fermi level, which in turn influence the transport properties. The demonstrated gas sensor, utilizing structurally defected graphene, shows faster response, higher conductivity changes and thus higher sensitivity to NO 2 as compared to pristine graphene.

National Category
Atom and Molecular Physics and Optics Engineering and Technology
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-186253 (URN)10.1088/0957-4484/23/50/505501 (DOI)000311855300014 ()
Available from: 2012-11-28 Created: 2012-11-28 Last updated: 2016-09-25
9. Site-selective local fluorination of graphene induced by focused ion beam irradiation
Open this publication in new window or tab >>Site-selective local fluorination of graphene induced by focused ion beam irradiation
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2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, 19719Article in journal (Refereed) Published
Abstract [en]

The functionalization of graphene remains an important challenge for numerous applications expected by this fascinating material. To keep advantageous properties of graphene after modification or functionalization of its structure, local approaches are a promising road. A novel technique is reported here that allows precise site-selective fluorination of graphene. The basic idea of this approach consists in the local radicalization of graphene by focused ion beam (FIB) irradiation and simultaneous introduction of XeF2 gas. A systematic series of experiments were carried out to outline the relation between inserted defect creation and the fluorination process. Based on a subsequent X-ray photoelectron spectroscopy (XPS) analysis, a 6-fold increase of the fluorine concentration on graphene under simultaneous irradiation was observed when compared to fluorination under normal conditions. The fluorine atoms are predominately localized at the defects as indicated from scanning tunneling microscopy (STM). The experimental findings are confirmed by density functional theory which predicts a strong increase of the binding energy of fluorine atoms when bound to the defect sites. The developed technique allows for local fluorination of graphene without using resists and has potential to be a general enabler of site-selective functionalization of graphene using a wide range of gases.

Keyword
graphene, site-selective, fluorination, focused ion beam
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-268033 (URN)10.1038/srep19719 (DOI)000368926300001 ()26822900 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation
Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2016-09-25Bibliographically approved

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