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Effect of Substrate on Bottom-Up Fabrication and Electronic Properties of Graphene Nanoribbons
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems.

Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110).

It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible.  It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 101 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1385
Keyword [en]
graphene nanoribbons, bottom-up, substrate, metal contact, electronic structure, electron doping, PES, ARPES, NEXAFS, STM
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics Other Physics Topics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-295884ISBN: 978-91-554-9610-4OAI: oai:DiVA.org:uu-295884DiVA: diva2:936003
Public defence
2016-09-23, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2016-08-16 Created: 2016-06-10 Last updated: 2016-08-25
List of papers
1. Effect of Substrate Chemistry on the Bottom-Up Fabrication of Graphene Nanoribbons: Combined Core-Level Spectroscopy and STM Study
Open this publication in new window or tab >>Effect of Substrate Chemistry on the Bottom-Up Fabrication of Graphene Nanoribbons: Combined Core-Level Spectroscopy and STM Study
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 23, 12532-12540 p.Article in journal (Refereed) Published
Abstract [en]

Atomically precise graphene nanoribbons (GNRs) can be fabricated via thermally induced polymerization of halogen containing molecular precursors on metal surfaces. In this paper the effect of substrate reactivity on the growth and structure of armchair GNRs (AGNRs) grown on inert Au(111) and active Cu(111) surfaces has been systematically studied by a combination of core-level X-ray spectroscopies and scanning tunneling microscopy. It is demonstrated that the activation threshold for the dehalogenation process decreases with increasing catalytic activity of the substrate. At room temperature the 10,10'-dibromo-9,9'-bianthracene (DBBA) precursor molecules on Au(111) remain intact, while on Cu(111) a complete surface-assisted dehalogenation takes place. Dehalogenation of precursor molecules on Au(111) only starts at around 80 degrees C and completes at 200 degrees C, leading to the formation of linear polymer chains. On Cu(111) tilted polymer chains appear readily at room temperature or slightly elevated temperatures. Annealing of the DBBA/Cu(111) above 100 degrees C leads to intramolecular cyclodehydrogenation and formation of flat AGNRs at 200 degrees C, while on the Au(111) surface the formation of GNRs takes place only at around 400 degrees C. In STM, nanoribbons have significantly reduced apparent height on Cu(111) as compared to Au(111), 70 +/- 11 pm versus 172 +/- 14 pm, independently of the bias voltage. Moreover, an alignment of GNRs along low-index crystallographic directions of the substrate is evident for Cu(111), while on Au(111) it is more random. Elevating the Cu(111) substrate temperature above 400 degrees C results in a dehydrogenation and subsequent decomposition of GNRs; at 750 degrees C the dehydrogenated carbon species self-organize in graphene islands. In general, our data provide evidence for a significant influence of substrate reactivity on the growth dynamics of GNRs.

National Category
Physical Chemistry Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-228705 (URN)10.1021/jp502215m (DOI)000337497400051 ()
Funder
Swedish Research CouncilSwedish Energy AgencyEU, European Research Council, 321319
Available from: 2014-07-21 Created: 2014-07-21 Last updated: 2016-08-25
2. Comment on "Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity"
Open this publication in new window or tab >>Comment on "Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity"
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2015 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 4, 3399-3403 p.Article in journal, Letter (Refereed) Published
National Category
Condensed Matter Physics Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-257309 (URN)10.1021/nn506439a (DOI)000353867000002 ()25916424 (PubMedID)
Available from: 2015-07-02 Created: 2015-07-01 Last updated: 2016-08-25
3. From Graphene Nanoribbons on Cu(111) to Nanographene on Cu(110): Critical Role of Substrate Structure in the Bottom-Up Fabrication Strategy
Open this publication in new window or tab >>From Graphene Nanoribbons on Cu(111) to Nanographene on Cu(110): Critical Role of Substrate Structure in the Bottom-Up Fabrication Strategy
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2015 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 9, 8997-9011 p.Article in journal (Refereed) Published
Abstract [en]

Bottom-up strategies can be effectively implemented for the fabrication of atomically precise graphene nanoribbons. Recently, using 10,10'-dibromo-9,9'-bianthracene (DBBA) as a molecular precursor to grow armchair nanoribbons on Au(111) and Cu(111), we have shown that substrate activity considerably affects the dynamics of ribbon formation, nonetheless without significant modifications in the growth mechanism. In this paper we compare the on-surface reaction pathways for DBBA molecules on Cu(111) and Cu(110). Evolution of both systems has been studied via a combination of core-level X-ray spectroscopies, scanning tunneling microscopy, and theoretical calculations. Experimental and theoretical results reveal a significant increase in reactivity for the open and anisotropic Cu(110) surface in comparison with the close-packed Cu(111). This increased reactivity results in a predominance of the molecular substrate interaction over the intermolecular one, which has a critical impact on the transformations of DBBA on Cu(110). Unlike DBBA on Cu(111), the Ullmann coupling cannot be realized for DBBA/Cu(110) and the growth of nanoribbons via this mechanism is blocked. Instead, annealing of DBBA on Cu(110) at 250 degrees C results in the formation of a new structure: quasi-zero-dimensional flat nanographenes. Each nanographene unit has dehydrogenated zigzag edges bonded to the underlying Cu rows and oriented with the hydrogen-terminated armchair edge parallel to the [1-10] direction. Strong bonding of nanographene to the substrate manifests itself in a high adsorption energy of -12.7 eV and significant charge transfer of 3.46e from the copper surface. Nanographene units coordinated with bromine adatoms are able to arrange in highly regular arrays potentially suitable for nanotemplating.

Keyword
graphene nanoribbons, nanographene, Ullmann reaction, photoemission, X-ray absorption, STM, DFT
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-265918 (URN)10.1021/acsnano.5b03280 (DOI)000361935800037 ()26301684 (PubMedID)
Funder
Swedish Research CouncilEU, European Research Council, 321319
Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2016-08-25
4. On-Surface Growth of Graphene Nanoribbons via Organometallic Intermediates
Open this publication in new window or tab >>On-Surface Growth of Graphene Nanoribbons via Organometallic Intermediates
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(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-295865 (URN)
Funder
EU, European Research Council, 321319Swedish Research Council
Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2016-08-25
5. Effect of Electron Injection in Copper-Contacted Graphene Nanoribbons
Open this publication in new window or tab >>Effect of Electron Injection in Copper-Contacted Graphene Nanoribbons
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2016 (English)In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000Article in journal (Refereed) Accepted
Keyword
graphene nanoribbons, bottom-up method, copper intercalation, charge injection, ARPES, Scanning tunneling microscopy
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:nbn:se:uu:diva-295867 (URN)10.1007/s12274-016-1162-2 (DOI)
Funder
Swedish Energy AgencySwedish Research CouncilEU, European Research Council, 321319
Note

For practical electronic device applications of graphene nanoribbons (GNR) it is essential to implement abrupt and well-defined contacts between the ribbon and the adjacent metal lead. By analogy with graphene, these contacts can induce electron or hole doping, which may significantly affect the I/V characteristics of the device. Copper is among the most popular metals of choice in selecting contact materials. In this paper we investigate the effect of in situ intercalation of copper on the electronic structure of atomically precise,  spatially aligned armchair GNRs of width N = 7 (7-AGNRs) fabricated via a bottom-up method on the Au(788) surface. Scanning tunneling microscopy data reveal that the complete intercalation of about one monolayer of copper under 7-AGNRs can be facilitated by gentle annealing of the sample at 80⁰C. Angle-resolved photoemission spectroscopy (ARPES) data clearly reflect the one-dimensional character of the 7-AGNR band dispersion before and after intercalation. Moreover, ARPES and core-level photoemission results show that intercalation of copper leads to significant electron injection into the nanoribbons, which causes a pronounced downshift of the GNR´s valence and conduction bands with respect to the Fermi energy (ΔE~ 0.5 eV). As demonstrated by ARPES and x-ray absorption spectroscopy measurements the effect of copper intercalation is restricted to n-doping only, without considerable modification of the band structure of the GNRs. Post-annealing of the 7-AGNRs/Cu/Au(788) system at 200⁰C activates the diffusion of copper into the gold and the formation of a Cu-rich surface gold layer. Alloying of intercalated copper leads to a recovery of the initial position of GNR-related bands with respect to EF, thus proving tunability of the induced n-doping.

Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2016-08-25
6. Giant Renormalization of the Quasiparticle Dispersion in Degenerately Doped Graphene Nanoribbons
Open this publication in new window or tab >>Giant Renormalization of the Quasiparticle Dispersion in Degenerately Doped Graphene Nanoribbons
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(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-295869 (URN)
Funder
EU, European Research Council
Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2016-08-25

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