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Electron and Spin Transport in Graphene-Based Nanodevices
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is devoted to the multi-scale modeling of electron and spin transport in graphene-based nanodevices. Several devices with fascinating structures and attractive properties have been designed by means of state-of-the-art computational methods, which include ab-initio molecular dynamics (MD) simulations for the geometry, density functional theory (DFT) for the electronic structure, and non-equilibrium Green’s functions (NEGF) for carriers transport properties.

Poly-crystalline graphenes offer ample opportunities to make devices with desirable properties. We have systematically studied a type of poly-crystalline graphene constructed by zigzag and armchair graphene nanoribbons (ZGNR and AGNR). It is found that the choice of the supercells in modeling with periodic boundary conditions (PBC) has strong implications on the electronic and magnetic properties of such hybrid systems. A model with minimal lattice mismatch is obtained, which could be regarded as the most appropriate model for hybrid GNRs. With this model, it is revealed that the hybrid GNR is of ferromagnetism with a high Curie temperature. We have then designed armchair/zigzag graphene nanoribbon heterojunctions (AGNR|ZGNR) with a well-defined conductance oscillation and rectification behavior. It is shown that the resonance or nonresonance of the frontier orbitals between AGNR and ZGNR is the source of the oscillation and the asymmetric structure is the root of the rectification. A high rectification ratio can be achieved by tuning the width of ZGNR to enhance the asymmetric character of transmission function and to minimize the backward current.

The electron transport properties of graphene can be modified by hydrogenation strips (HSs) formed from the absorbed hydrogen atoms. We have designed a new graphene nanoribbon that has zigzag-edged HSs placed at its middle region. It is found that the HS can electrically separate the GNR into sub-GNRs and each HS introduces two spin-polarized conducting edge-like states around the Fermi level. This leads to a significant enhancement of the conductance and the spinpolarization. We have also found that by introducing embedding a short sp3-edged section into the sp2-edged ZGNRs or a short sp2-edged section into the sp3-edged ZGNRs, the orbital symmetry mismatch between these two sections can induce the opening of the conductance energy gap in ZGNRs over a wide energy region. This simple strategy explains many unexplained experimental results and offers a simple strategy to design GNRs with a proper energy gap.

We have also carefully examined the spin-polarization of chiral GNRs with reconstructed (2,1)-edges. It is found that the unsaturated (2,1)-edged chiral GNRs can possess strong current polarizations (nearly 100%) and a striking negative differential resistance (NDR) behavior.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xii, 85 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2013:3
National Category
Theoretical Chemistry
Research subject
SRA - Transport
Identifiers
URN: urn:nbn:se:kth:diva-116568ISBN: 978-91-7501-622-1 (print)OAI: oai:DiVA.org:kth-116568DiVA: diva2:591828
Public defence
2013-02-18, FA32, AlbaNova Universitetscentrum, Roslagstullsbacken, Stockholm, 15:02 (English)
Opponent
Supervisors
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20130123

Available from: 2013-01-23 Created: 2013-01-21 Last updated: 2013-01-23Bibliographically approved
List of papers
1. The Choice of the Supercell Controlling the Properties of Armchair/Zigzag Hybrid Graphene Nanoribbons
Open this publication in new window or tab >>The Choice of the Supercell Controlling the Properties of Armchair/Zigzag Hybrid Graphene Nanoribbons
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(English)Article in journal (Other academic) Submitted
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-116672 (URN)
Note

QS 2013

Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2013-01-23Bibliographically approved
2. Design of Graphene-Nanoribbon Heterojunctions from First Principles
Open this publication in new window or tab >>Design of Graphene-Nanoribbon Heterojunctions from First Principles
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 25, 12616-12624 p.Article in journal (Refereed) Published
Abstract [en]

Graphene nanoribbons with armchair and zigzag edges are known to have very different electronic structure and properties. We show here that the fusion of an armchair and a zigzag graphene-nanoribbon (aGNR vertical bar zGNR) can form heterojunctions with remarkable electron transport properties. First-principles calculations reveal that the heterojunction can be either metallic or semiconducting depending on the width of the nanoribbon. A well-defined oscillation of the zero-bias conductance as a function of the ribbon width is observed, which is originated from the resonance and nonresonance of frontier orbitals between aGNR and zGNR We find that the current/voltage characteristics of the aGNR vertical bar zGNR heterojunction possess pronounced rectification effect, and a high rectification ratio can be achieved by tuning the width of the zGNR to minimize the backward current. The unique properties of the proposed heterojunction could be very useful for manufacturing graphene-based electronic devices.

Keyword
MOLECULAR RECTIFICATION, ELECTRONIC TRANSPORT, QUANTUM INTERFERENCE, ROOM-TEMPERATURE, RECTIFIERS
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-36238 (URN)10.1021/jp202188t (DOI)000291896000049 ()2-s2.0-79959505713 (Scopus ID)
Note
QC 20110711Available from: 2011-07-11 Created: 2011-07-11 Last updated: 2017-12-11Bibliographically approved
3. Electronic transport through zigzag/armchair graphene nanoribbon heterojunctions
Open this publication in new window or tab >>Electronic transport through zigzag/armchair graphene nanoribbon heterojunctions
2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 9, 095801- p.Article in journal (Refereed) Published
Abstract [en]

The electronic transport properties of a graphene nanoribbon (GNR) are known to be sensitive to its width, edges and defects. We investigate the electronic transport properties of a graphene nanoribbon heterojunction constructed by fusing a zigzag and an armchair graphene nanoribbon (zGNR/aGNR) side by side. First principles results reveal that the heterojunction can be either metallic or semiconducting, depending on the width of the nanoribbons. Intrinsic rectification behaviors have been observed, which are largely sensitive to the connection length between the zGNR and aGNR. The microscopic origins of the rectification behavior have been revealed. We find that the carrier type can alter from electrons to holes with the bias voltage changing from negative to positive; the asymmetrical transmission spectra of electrons and holes induced by the interface defects directly results in the rectification behavior. The results suggest that any methods which can enhance the asymmetry of the transmission spectra between holes and electrons could be used to improve the rectification behavior in the zGNR/aGNR heterojunction. Our findings could be useful for designing graphene based electronic devices.

Keyword
HEXADECYLQUINOLINIUM TRICYANOQUINODIMETHANIDE, ELECTRICAL RECTIFICATION, GOLD ELECTRODES, MONOLAYER, RECTIFIERS, DEFECT
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-91600 (URN)10.1088/0953-8984/24/9/095801 (DOI)000300641000022 ()2-s2.0-84857241091 (Scopus ID)
Note
QC 20120402Available from: 2012-04-02 Created: 2012-03-19 Last updated: 2017-12-07Bibliographically approved
4. Tuning the Electronic Transport Properties of Zigzag Graphene Nanoribbons via Hydrogenation Separators
Open this publication in new window or tab >>Tuning the Electronic Transport Properties of Zigzag Graphene Nanoribbons via Hydrogenation Separators
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 49, 24366-24372 p.Article in journal (Refereed) Published
Abstract [en]

Hydrogenation technique is known to be useful for opening up the band gap and controlling the electronic properties of the graphene. We have demonstrated with first principles calculations that the hydrogenation can be used to make separators to electrically separate zigzag graphene nanoribbons (zGNR) and tune their transport properties. First principles calculations reveal that each hydrogenation separator can introduce two conducting edge-like states into the subbands around the Fermi level, which can greatly enhance the conductance of the system. We find that the zGNRs with hydrogenation separators are still spin polarized; the distributions of spin densities are mainly located along the two edges of the pristine nanoribbon and the borders of the separators. The current polarization shows a nice oscillation behavior as a function of the position of the separator, which originates from the symmetry dependent transport character of the zGNRs. Moreover, we find that the hydrogenation separators can screen the impact of rough edges, which makes rough-edge zGNRs behave like smooth-edge zGNRs. Our findings could be very useful for designing electronic devices based on the hydrogenation of graphene nanoribbons.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-55233 (URN)10.1021/jp208892h (DOI)000297609000041 ()2-s2.0-83455165179 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120109Available from: 2012-01-09 Created: 2012-01-02 Last updated: 2017-12-08Bibliographically approved
5. Conductance gap induced by orbital symmetry mismatch in inhomogeneous hydrogen-terminated zigzag graphene nanoribbons
Open this publication in new window or tab >>Conductance gap induced by orbital symmetry mismatch in inhomogeneous hydrogen-terminated zigzag graphene nanoribbons
Show others...
(English)Article in journal (Other academic) Submitted
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-116671 (URN)
Note

QS 2013

Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2013-01-23Bibliographically approved
6. Strong current polarization and negative differential resistance in chiral graphene nanoribbons with reconstructed (2,1)-edges
Open this publication in new window or tab >>Strong current polarization and negative differential resistance in chiral graphene nanoribbons with reconstructed (2,1)-edges
2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 7, 073101- p.Article in journal (Refereed) Published
Abstract [en]

We investigate electronic structures and transport properties of chiral-graphene-nanoribbons (CGNRs) with reconstructed (2,1)-edges. First principles results reveal that the (2,1)-CGNRs can be either spin non-polarized or polarized, depending on whether the edges are H-saturated or unsaturated. H-saturated systems are semiconductors with width-dependent transmission gap, while unsaturated systems are metallic at non-polarized state or at antiferromagnetic state and they are semiconductors at ferromagnetic state. Moreover, unsaturated systems possess strong current polarizations (nearly 100%) and a striking negative differential resistance behavior (with a peak-to-valley ratio about 105). These remarkable properties suggest the potential application of (2,1)-CGNRs in molectronics and spintronics.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-103977 (URN)10.1063/1.4745506 (DOI)000308263100052 ()2-s2.0-84865409561 (Scopus ID)
Note

QC 20121026

Available from: 2012-10-26 Created: 2012-10-25 Last updated: 2017-12-07Bibliographically approved

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