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First Principles Studies of Functional Materials Based on Graphene and Organometallics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Graphene is foreseen to be the basis of future electronics owing to its ultra thin structure, extremely high charge carrier mobility,  high thermal conductivity etc., which are expected to overcome the size limitation and heat dissipation problem in silicon based transistors. But these great prospects are hindered by the metallic nature of pristine graphene even at charge neutrality point, which allows to flow current even when a transistor is switched off. A part  of the thesis is dedicated to invoke electronic band gaps in graphene to overcome this problem. The concept of quantum confinement has been employed to tune the band gaps in graphene by  dimensional confinement along with the functionalization of the edges of these confined nanostructures. Thermodynamic stability of the functionalized zigzag edges with hydrogen, fluorine and reconstructed edges has been presented in the thesis. Keeping an eye towards the same goal of band gap opening,  a different route has been considered by admixing insulating hexagonal boron nitride (h-BN) with semimetal graphene. The idea has been implemented in two  dimensional h-BN-graphene composites and three dimensional stacked heterostructures. The study reveals the possibility of tuning band gaps by controlling the admixture. Occurrence of defects in graphene has significant effect on its electronic properties. By random insertion of defects, amorphous graphene is studied, revealing a semi-metal to a metal transition.

The field of molecular electronics and spintronics aims towards device realization at the molecular scale. In this thesis, different aspects of magnetic bistability in organometallic molecules have been explored in order to design  practical spintronics devices. Manipulation of spin states in organometallic molecules, specifically metal porphyrin molecules, is achieved by controlling surface–molecule interaction. It has been shown that by strain engineering in defected graphene, the magnetic state of adsorbed molecules can be changed. The spin crossover between different spin states can also be achieved by chemisorption on magnetic surfaces. A significant part of the thesis demonstrates that the surface-molecule interaction not only changes the spin state of the molecule, but allows to manipulate magnetic anisotropies and spin dipole moments via modified ligand fields. Finally, in collaboration with experimentalists, a practical realization of switching surface–molecule magnetic interactions by external magnetic fields is demonstrated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 90 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1120
Keyword [en]
Graphene, Magnetism, Organometallics, Density functional theory, Electron correlation, Spin switching, Nanoribbons, Exchange interaction, Edge functionalization, Band gap
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-217175ISBN: 978-91-554-8869-7 (print)OAI: oai:DiVA.org:uu-217175DiVA: diva2:692301
Public defence
2014-03-14, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-02-21 Created: 2014-01-30 Last updated: 2014-04-29
List of papers
1. Complex edge effects in zigzag graphene nanoribbons due to hydrogen loading
Open this publication in new window or tab >>Complex edge effects in zigzag graphene nanoribbons due to hydrogen loading
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 16, 165405- p.Article in journal (Refereed) Published
Abstract [en]

We have performed density-functional calculations as well as employed a tight-binding theory, to study the effect of passivation of zigzag graphene nanoribbons (ZGNR) by hydrogen. We show that each edge C atom bonded with 2 H atoms open up a gap and destroys magnetism for small widths of the nanoribbon. However, a re-entrant magnetism accompanied by a metallic electronic structure is observed from eight rows and thicker nanoribbons. The electronic structure and magnetic state are quite complex for this type of termination, with sp(3) bonded edge atoms being nonmagnetic whereas the nearest neighboring atoms are metallic and magnetic. We have also evaluated the phase stability of several thicknesses of ZGNR and demonstrate that sp(3) bonded edge atoms with 2 H atoms at the edge can be stabilized over 1 H atom terminated edge at high temperatures and pressures.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-134336 (URN)10.1103/PhysRevB.82.165405 (DOI)000282422700005 ()
Available from: 2010-11-25 Created: 2010-11-24 Last updated: 2017-12-12Bibliographically approved
2. Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalization
Open this publication in new window or tab >>Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalization
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this work, we report a detailed study of the electronic structure and transport properties of mono- and di-fluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0K indicate that the stability of the nanoribbons increases with the increase in the concentration of di-fluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by single band tight binding as well as density functional tight binding methods. Using non-equilibrium Green functional method, we have calculated the transmission coecients of several mono and di-fluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and di-fuorinated C atoms at the edges.

Keyword
Graphene nanoribbons, DFT, Band gap, Transport
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217155 (URN)
Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2014-04-29
3. 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: 2017-12-07Bibliographically approved
4. Interpolation of Atomically Thin Hexagonal Boron Nitride and Graphene: Electronic Structure and Thermodynamic Stability in Terms of All-Carbon Conjugated Paths and Aromatic Hexagons
Open this publication in new window or tab >>Interpolation of Atomically Thin Hexagonal Boron Nitride and Graphene: Electronic Structure and Thermodynamic Stability in Terms of All-Carbon Conjugated Paths and Aromatic Hexagons
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 20, 10264-10271 p.Article in journal (Refereed) Published
Abstract [en]

Two-dimensional hexagonal composite materials (BN)(n)(C-2)(m) (n, m = 1, 2,...), which all are isoelectronic with graphene and hexagonal boron nitride (h-BN), have been studied by density functional theory (DFT) with a focus on the relative energies of different material isomers and their band gaps. The well-established chemical concepts of conjugation and aromaticity were exploited to deduce a rationale for identifying the thermodynamically most stable isomer of the specific composites studied. We find that (BN)(n)(C-2)(m) materials will not adopt structures in which the B, C, and N atoms are finely dispersed in the 2D sheet. Instead, the C atoms and C-C bonds, which provide for improved conjugation when compared to B-N bonds, gather and form all-carbon hexagons and paths; that is, the (BN)(n)(C-2)(m) materials prefer nanostructured distributions. Importantly, there are several isomers of similarly low relative energy for each (BN)(n)(C-2)(m) composite type, but the band gaps for these nearly isoenergetic isomers differ by up to 1.0 eV. This feature in the band gap variation of the most stable few isomers is found for each of the four composites studied and at two different DFT levels. Consequently, the formation of a distinct (BN)(n)(C-2)(m) material isomer with a precise (small) band gap will likely be nontrivial. Therefore, one likely has to invoke nonstandard preparation techniques that exploit nanopatterned h-BN or graphene with voids that can be filled with the complementary all-carbon or boron nitride segments.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-154600 (URN)10.1021/jp2016616 (DOI)000290652200055 ()
Available from: 2011-06-09 Created: 2011-06-08 Last updated: 2017-12-11Bibliographically approved
5. 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: 2017-01-25
6. Disorder-induced metallicity in amorphous graphene
Open this publication in new window or tab >>Disorder-induced metallicity in amorphous graphene
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 20, 205414- p.Article in journal (Refereed) Published
Abstract [en]

We predict a transition to metallicity when a sufficient amount of disorder is induced in graphene. Calculations were performed by means of a first-principles stochastic quench method. The resulting amorphous graphene can be seen as nanopatches of graphene that are connected by a network of disordered small and large carbon rings. The buckling of the lattice is minimal and is a result of averaging of counteracting random in-plane stress forces. The linear response conductance is obtained by a model theory as function of lattice distortions, and results in a similar behavior as the first-principles calculation.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-163665 (URN)10.1103/PhysRevB.84.205414 (DOI)000296911900009 ()
Available from: 2011-12-14 Created: 2011-12-13 Last updated: 2017-12-08Bibliographically approved
7. Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.
Open this publication in new window or tab >>Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The proper description of electronic structure of correlated orbitals in the metal centers of functional metalorganics is a challenging problem. In this letter, we apply density functional theory and exact diagonalization method in a many body approach to study the ground state electronic conguration of iron porphyrin (FeP) molecule. Our study reveals that FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal elds and hybridization of the Fe d-orbitals and ligand N p-states. Moreover, the mechanism of switching between two close lying electronic congurations of Fe-d orbitals is revealed. This hybrid method can generally be applied to properly describe the electronic and related low energy physics of the whole class of correlated metal centered organometallic molecules.

Keyword
Molecular magnet, Porphyrin, DFT, DFT++, Spin crossover
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217168 (URN)
Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2014-04-29
8. Graphene as a Reversible Spin Manipulator of Molecular Magnets
Open this publication in new window or tab >>Graphene as a Reversible Spin Manipulator of Molecular Magnets
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2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 25, 257202- p.Article in journal (Refereed) Published
Abstract [en]

One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S = 1 -> S = 2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-167201 (URN)10.1103/PhysRevLett.107.257202 (DOI)000298133000003 ()
Available from: 2012-01-23 Created: 2012-01-23 Last updated: 2017-12-08Bibliographically approved
9. Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates
Open this publication in new window or tab >>Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates
Show others...
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 2, 024401- p.Article in journal (Refereed) Published
Abstract [en]

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L-2,L-3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S = 2) from an intermediate spin state (S = 1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-204271 (URN)10.1103/PhysRevB.88.024401 (DOI)000321123200001 ()
Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2017-12-06Bibliographically approved
10. Defect controlled magnetism in FeP/graphene/Ni(111)
Open this publication in new window or tab >>Defect controlled magnetism in FeP/graphene/Ni(111)
2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, 3405- p.Article in journal (Refereed) Published
Abstract [en]

Spin switching of organometallic complexes by ferromagnetic surfaces is an important topic in the area of molecular nanospintronics. Moreover, graphene has been shown as a 2D surface for physisorption of molecular magnets and strain engineering on graphene can tune the spin state of an iron porphyrin (FeP) molecule from S = 1 to S = 2. Our ab initio density functional calculations suggest that a pristine graphene layer placed between a Ni(111) surface and FeP yields an extremely weak exchange interaction between FeP and Ni whereas the introduction of defects in graphene shows a variety of ferromagnetic and antiferromagnetic exchange interactions. Moreover, these defects control the easy axes of magnetization, strengths of magnetic anisotropy energies and spin-dipolar contributions. Our study suggests a new way of manipulating molecular magnetism by defects in graphene and hence has the potential to be explored in designing spin qubits to realize logic operations in molecular nanospintronics.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-213899 (URN)10.1038/srep03405 (DOI)000327784400002 ()
Available from: 2014-01-06 Created: 2014-01-05 Last updated: 2017-12-06Bibliographically approved
11. Field-regulated switching of the magnetization of Co-porphyrin on graphene
Open this publication in new window or tab >>Field-regulated switching of the magnetization of Co-porphyrin on graphene
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 14, 144411- p.Article in journal (Refereed) Published
Abstract [en]

Different magnetic coupling mechanisms have been identied for a few monolayers of Co-porphyrin molecules deposited on a graphene covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker inter-molecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic eld strength the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element specic magnetization and field dependence was probed. The nature of the magnetic couplings is identied by means of density functional theory and orbital dependent susceptibilities.

Keyword
Molecular magnet, Porphyrin, Magnetic field, XMCD
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217173 (URN)10.1103/PhysRevB.89.144411 (DOI)000337348300004 ()
Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-12-06Bibliographically approved

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