Substrate transfer and ex situ characterization of on-surface synthesized graphene nanoribbons

Recent progress in the on-surface synthesis of graphene nanoribbons (GNRs) has given access to atomically precise narrow GNRs with tunable electronic band gaps that makes them excellent candidates for room-temperature switching devices such as field-effect transistors (FET). However, in spite of their exceptional properties, significant challenges remain for GNR processing and characterization. This contribution addresses some of the most important challenges, including GNR fabrication scalability, substrate transfer, long-term stability under ambient conditions and ex situ characterization. We focus on 7- and 9-atom wide armchair graphene nanoribbons (i.e, 7-AGNR; and 9-AGNR) grown on 200 nm Au(111)/mica substrates using a high throughput system. Transfer of both, 7- and 9-AGNRs from their Au growth sub-strate onto various target substrates for additional characterization is accomplished utilizing a polymer-free method that avoids residual contamination. This results in a homogeneous GNR film morphology with very few tears and wrinkles, as examined by atomic force microscopy. Raman spectroscopy indicates no significant degradation of GNR quality upon substrate transfer, and reveals that GNRs have remarkable stability under ambient conditions over a 24-month period. The transferred GNRs are analyzed using multi-wavelength Raman spectroscopy, which provides detailed insight into the wavelength dependence of the width-specific vibrational modes. Finally, we characterize the optical properties of 7- and 9-AGNRs via ultra-violet-visible (UV-Vis) spectroscopyComment: 30 pages, 14 figure

Graphene nanoribbons with mixed cove-cape-zigzag edge structure

A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-cape-cove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics

On-surface synthesis of graphene nanoribbons with zigzag edge topology

Graphene-based nanostructures exhibit a vast range of exciting electronic properties that are absent in extended graphene. For example, quantum confinement in carbon nanotubes and armchair graphene nanoribbons (AGNRs) leads to the opening of substantial electronic band gaps that are directly linked to their structural boundary conditions. Even more intriguing are nanostructures with zigzag edges, which are expected to host spin-polarized electronic edge states and can thus serve as key elements for graphene-based spintronics. The most prominent example is zigzag graphene nanoribbons (ZGNRs) for which the edge states are predicted to couple ferromagnetically along the edge and antiferromagnetically between them. So far, a direct observation of the spin-polarized edge states for specifically designed and controlled zigzag edge topologies has not been achieved. This is mainly due to the limited precision of current top-down approaches, which results in poorly defined edge structures. Bottom-up fabrication approaches, on the other hand, were so far only successfully applied to the growth of AGNRs and related structures. Here, we describe the successful bottom-up synthesis of ZGNRs, which are fabricated by the surface-assisted colligation and cyclodehydrogenation of specifically designed precursor monomers including carbon groups that yield atomically precise zigzag edges. Using scanning tunnelling spectroscopy we prove the existence of edge-localized states with large energy splittings. We expect that the availability of ZGNRs will finally allow the characterization of their predicted spin-related properties such as spin confinement and filtering, and ultimately add the spin degree of freedom to graphene-based circuitry.Comment: 15 pages, 4 figure

Synthese und Charakterisierung neuartiger Graphennanostreifen und Graphenmoleküle mit Armlehnen- und Zickzack-Kanten

Die vorliegende Arbeit widmet sich der Synthese und Charakterisierung von Graphenausschnitten in Form von Graphennanostreifen und kleinen Graphenmolekülen. Aufgrund ihrer definierten Form und der Randstruktur weisen diese im Gegensatz zu Graphen eine endliche Bandlücke auf. Im ersten Kapitel wird zunächst die „Bottom-up“-Synthese eines Armlehnen-Graphennanostreifens (AGNR) mit einer Breite von neun Kohlenstoffatomen ausgehend von einem neuartigen o-Terphenyl-Monomer vorgestellt. Die beiden Schlüsselschritte stellten dabei die Polymerisation zum Poly-para-phenylen-vorläufer und dessen anschließende Cyclodehydrierung auf einer Gold(111)-Oberfläche dar. Der resultierende GNR konnte mittels Rastertunnelmikroskopie (STM) und nicht-Kontakt-Rasterkraftmikroskopie (nc-AFM) visualisiert werden und der präzise Einbau eines jeden Atoms des Monomers verifiziert werden. Über Rasterkraftspektroskopie (STS) konnte eine geringe Bandlücke von 1.4 eV festgestellt werden. Ausgehend von diesem neuen Konzept zur Synthese von defektfreien GNRs konnten erstmals Feldeffekttransistoren (FETs) mit hoher Effizienz und hohem An/Aus-Verhältnis realisiert werden, welche die Werte für top-down-GNRs deutlich übertreffen. Weiterhin wurde eine Modellverbindung als kleiner Ausschnitt eines 9-AGNRs synthetisiert und charakterisiert, welche demonstrierte, dass die oxidative Cyclodehydrierung auch im Labormaßstab durchführbar ist. Im zweiten Teil der vorliegenden Arbeit wurde ein Synthesekonzept entwickelt, welches die bisher nicht mögliche Darstellung von Graphennanostreifen mit reinen Zickzack-Kanten (ZGNRs) zum Ziel hatte. Nach erfolgreicher Synthese verschieden substituierter Monomere mit Benzo[m]tetraphen-Grundgerüst konnten diese unter Ultrahochvakuum-Bedingungen zunächst polymerisiert und anschließend Planarisiert werden. Die erhaltenen hochreaktiven ZGNRs wurden ebenfalls via nc-AFM und STM visualisiert und deren Randzustände mit Hilfe von STS charakterisiert. Desweiteren konnte das entwickelte Syntheseprotokoll auf die Herstellung eines HBC-Derivats mit vier K-Regionen angewendet und modifiziert werden. Der erhaltene Ausschnitt eines 6-ZGNRs konnte über Massenspektrometrie, FTIR-, Raman- und UV/Vis-Spektroskopie analysiert und charakterisiert werden.This work is dedicated to the synthesis and characterization of small cutouts of graphene in terms of Graphene-Nanoribbons (GNRs) and Graphene-Molecules. Due to their defined shapes and edge-structures, such building blocks show a finite bad gap in contrast to pristine graphene. In the first chapter, the “Bottom-Up”-synthesis of an Armchair-Graphene-Nanoribbon (AGNR) with a width of nine carbon atoms is introduced, starting from a novel o-terphenyl monomer. The keysteps are the polymerization toward a poly-para-phenylene-precursor and its subsequent cyclodehydrogenation on an Au(111)-surface. The resulting GNR could be visualized via scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Thus, the incorporation of every single atom of the monomer could be verified. A low band gap of 1.4 eV was revealed via scanning tunneling spectroscopy (STS). Starting from this concept for the synthesis of perfect AGNRs, field effect transistors (FETs) with high efficiencies and high on/off-ratio were realized for the first time, outperforming top-down GNRFETs by several magnitudes. Furthermore, a small cutout of 9-AGNRs was synthesized and studied, demonstrating the oxidative cyclodehydrogenation in solution which can be performed in large scale. In the second part, a synthetic concept was developed, allowing the elusive creation of GNRs with pure zigzag edges (ZGNRs). After the successful synthesis of U-shaped benzo[m]tetraphene-monomers with different substitution patterns, it was possible to polymerize those followed by planarization toward 6-ZGNRs under ultrahighvacuum (UHV) -conditions. The received highly reactive ZGNRs could also be visualized via STM and nc-AFM while the localized edge states were revealed for the first time via STS. In addition to that, the developed synthetic protocol was adapted to obtain an HBC-derivative containig four K-regions. The obtained stable cutout of 6-ZGNR was analyzed via mass-spectrometry, FTIR-, Raman-, and UV/Vis-spectroscopy

Revealing the electronic structure of silicon intercalated armchair graphene nanoribbons by scanning tunneling spectroscopy

The electronic properties of graphene nanoribbons grown on metal substrates are significantly masked by the ones of the supporting metal surface. Here, we introduce a novel approach to access the frontier states of armchair graphene nanoribbons (AGNRs). The in situ intercalation of Si at the AGNR/Au(111) interface through surface alloying suppresses the strong contribution of the Au(111) surface state and allows for an unambiguous determination of the frontier electronic states of both wide and narrow band gap AGNRs. First-principles calculations provide insight into substrate induced screening effects, which result in a width-dependent band gap reduction for substrate-supported AGNRs. The strategy reported here provides a unique opportunity to elucidate the electronic properties of various kinds of graphene nanomaterials supported on metal substrates.This work has been supported by the Swiss National Science Foundation, the Office of Naval Research BRC program, and the European Commission Graphene Flagship (No. CNECT-ICT-604391). C.S.S. is grateful to Ministerio de Economía y Competitividad for financial support via the Juan de la Cierva Incorporación Grant (IJCI-2014-19291, cofunded by the European Investment Bank)

Hexa-peri-benzocoronene with two extra K-regions in an ortho-configuration

There are three possible isomers for hexa-peri-hexabenzocoronene (HBC) with two extra K-regions, but only two of them have been reported, namely with the meta- and para-configurations. Herein, we describe the synthesis of HBC 4 with two extra K-regions in the ortho-configuration, forming a longer zigzag edge compared with the other two isomers. The structure of 4 was validated by laser desorption/ionization time-of-flight mass analysis and nuclear magnetic resonance spectra, as well as Raman and infrared spectroscopies supported by density functional theory calculations. The optical properties of 4 were investigated by UV/vis absorption and ultrafast transient absorption spectroscopy. Together with the analysis of aromaticity, the influence of the zigzag edge on the π-conjugation pathway and HOMO–LUMO gaps of the three isomers were investigated

Adding Four Extra K-Regions to Hexa-peri-hexabenzocoronene

A multistep synthesis of hexa-perihexabenzocoronene (HBC) with four additional K-regions was developed through a precursor based on two benzotetraphene units bridged with p-phenylene, featuring preinstalled zigzag moieties. Characterization by laser desorption/ ionization time-of-flight mass spectrometry, Raman and IR spectroscopy, and scanning tunneling microscopy unambiguously validated the successful formation of this novel zigzag edge-rich HBC derivative. STM imaging of its monolayers revealed large-area, defect-free adlayers. The optical properties of the modified HBC were investigated by UV/visible absorption spectroscopy.status: publishe

Adding Four Extra K-Regions to Hexa-peri-hexabenzocoronene

A multistep synthesis of hexa-peri-hexabenzocoronene (HBC) with four additional K-regions was developed through a precursor based on two benzotetraphene units bridged with p-phenylene, featuring preinstalled zigzag moieties. Characterization by laser desorption/ionization time-of-flight mass spectrometry, Raman and IR spectroscopy, and scanning tunneling microscopy unambiguously validated the successful formation of this novel zigzag edge-rich HBC derivative. STM imaging of its monolayers revealed large-area, defect-free adlayers. The optical properties of the modified HBC were investigated by UV/visible absorption spectroscopy

Optical Investigation of On-Surface Synthesized Armchair Graphene Nanoribbons

International audienceOn-surface synthesized N ¼ 9-armchair graphene nanoribbons (AGNRs) are investigated by Raman spectroscopy and AFM/micro photoluminescence measurements. In order to perform the optical experiments, the AGNR film is transferred on a glass substrate through a non-membrane method. The Raman spectroscopy shows the radial breathing-like mode characteristic of the 1D nature of GNRs, proving the efficiency of the transfer method. Then, the results of combined AFM/micro photoluminescence measurements are discussed. First, the observation of high-order Raman lines suggests the 1D nature of the electron–phonon coupling in GNR, similar to the case of carbon nanotubes. Secondly, the origin of the broad luminescence line is discussed in comparison with the predicted gap energy of the 9-AGNR. Due to its width and energy range, the emission is interpreted as arising from defect sites, missing phenyl rings for instance, that occur during the synthesis of these specific armchair nanoribbons