Bottom-up solution synthesis of narrow nitrogen-doped graphene nanoribbons

Article abstract of DOI:10.1039/c4cc00885e

Large quantities of narrow graphene nanoribbons with edge-incorporated nitrogen atoms can be synthesized via Yamamoto coupling of molecular precursors containing nitrogen atoms followed by cyclodehydrogenation using Scholl reaction. the Partner Organisati

Full text of DOI:10.1039/c4cc00885e

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Bottom-up solution synthesis of narrow
nitrogen-doped graphene nanoribbons†
Cite this: Chem. Commun., 2014,
50, 4172
Timothy H. Vo,a Mikhail Shekhirev,a Donna A. Kunkel,b François Orange,c
Maxime J.-F. Guinel,cd Axel Endersbe and Alexander Sinitskii*ae
Received 3rd February 2014,
Accepted 5th March 2014
DOI: 10.1039/c4cc00885e
www.rsc.org/chemcomm
Large quantities of narrow graphene nanoribbons with edge-
Electronic properties of GNRs could be further tuned via
incorporated nitrogen atoms can be synthesized via Yamamoto their doping with heteroatoms, such as boron or nitrogen.23
coupling of molecular precursors containing nitrogen atoms followed This possibility has been extensively studied theoretically,24–30 but
by cyclodehydrogenation using Scholl reaction.
only a few experimental attempts to synthesize nitrogen-doped
GNRs (N-GNRs) by bottom-up approaches have been reported.14,20
Graphene nanoribbons (GNRs), narrow strips of graphene with Bronner et al. prepared chevron-like N-GNRs by the surface-assisted
high aspect ratios, attract a great deal of attention because of their approach on Au(111) substrate and performed their spectroscopic
interesting electronic and magnetic properties.1 According to the characterization.14 Kim et al. synthesized N-GNRs with a different
theoretical studies, these properties strongly depend on the width structure in solution via Suzuki coupling of molecular precursors,
of GNRs as well as their edge structure and termination, and are one of which was dibrominated pyrazine, and cyclodehydro-
also very sensitive to the edge disorder.2–5 Thus, it is very important genation of the resulting polymers.20 Here we demonstrate that high-
to precisely control the structure of GNRs on the atomic scale. quality chevron-like N-GNRs could be synthesized via Yamamoto
Numerous top-down fabrication approaches have been developed coupling of molecular precursors containing nitrogen atoms
to produce GNRs from graphite, graphene or carbon nanotubes, as followed by cyclodehydrogenation via Scholl reaction (Fig. 1).
discussed in the recent review papers,6–8 but the resulting ribbons
The structure of the N-GNR synthesized in this work is
typically have relatively large and non-uniform widths and dis- shown in Fig. 1a. Since the unit cell of this ribbon contains
ordered edges. Narrow atomically precise GNRs could be syn- four nitrogen atoms, we refer to it as ‘‘4N-GNR’’. The monomer
thesized via bottom-up approaches that employ decomposition molecule 3 used as a precursor for the 4N-GNR synthesis is not
of molecules inside carbon nanotubes,9,10 or rely on the surface- symmetric relative to the plane perpendicular to the direction
assisted11–14 or solution polymerization8,15–22 of molecular
precursors followed by cyclodehydrogenation of the resulting
polymers. While surface-assisted bottom-up synthesis of GNRs
yields small quantities of ribbons on metallic substrates in
ultrahigh vacuum conditions that are ideal for scanning tunnelling
microscopy (STM) characterization, solution-based approaches are
preferred for the bulk synthesis of GNRs, so these two groups of
methods could be considered as complimentary.
a Department of Chemistry, University of Nebraska – Lincoln, Lincoln, NE 68588,
USA. E-mail: sinitskii@unl.edu
b Department of Physics, University of Nebraska – Lincoln, Lincoln, NE 68588, USA
c Department of Physics and Nanoscopy Facility, University of Puerto Rico,
PO Box 70377, San Juan, Puerto Rico 00936-8377, USA
d Department of Chemistry, University of Puerto Rico, PO Box 70377, San Juan,
Puerto Rico 00936-8377, USA
e Nebraska Center for Materials and Nanoscience, University of Nebraska – Lincoln,
Lincoln, NE 68588, USA
Fig. 1 Synthesis of 4N-GNRs. (a) Schematic of 4N-GNR. (b) Reaction
scheme used in this work: (1) Yamamoto coupling, (2) Scholl reaction;
see text and ESI† for details. (c) Optical photograph of a 4N-GNR powder.
† Electronic supplementary information (ESI) available: Experimental details
(synthesis and characterization). See DOI: 10.1039/c4cc00885e
4172 | Chem. Commun., 2014, 50, 4172--4174
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of coupling (Fig. 1b), so as the result, the positions of nitrogen image of 4N-GNRs that were dispersed in toluene by sonication,
atoms in the ribbon edge could vary, i.e. the nitrogens could deposited on freshly cleaved mica and dried prior to imaging.
also occupy the positions highlighted with red circles in Fig. 1a Observed in this image are high-aspect-ratio nanobelts that are
instead of the ones shown in the schematic. However, the N : C several mm long. The alignment of 4N-GNR nanobelts likely happens
ratio of 1 : 20 in this ribbon should be the same regardless of in the contact angle between a droplet of 4N-GNR dispersion and a
how the monomer molecules couple with each other.
substrate during the solvent drying. As shown in the height profile
The synthesis and characterization of the precursor molecule 3, measured along the blue line in Fig. 2a, the majority of these
as well as the reactions used to convert monomer 3 to polymer 4 to nanobelts have heights o5 Å, which corresponds to the thickness
4N-GNR 5 (Fig. 1b) are described in detail in the ESI.† About of a single layer or graphene (Fig. 2b). In fact, in multiple AFM
100 mg of 4N-GNRs was obtained in a single reaction (Fig. 1c). images we observed only one structure with a thickness (t) larger
Previously, we have demonstrated that a similar procedure could than 5 Å, which is indicated by the black arrow in Fig. 2a. This
be used to obtain 41 g of chevron-type GNRs (nitrogen-free structure (t B 1 nm) is likely a rare stack of 2–3 nanobelts. Fig. 2c
analogues of 4N-GNRs shown in Fig. 1a) in a single reaction,21 shows an STM image of the fragment of a 4N-GNR nanobelt on
so we believe that the synthesis of 4N-GNRs could also be scaled Au(111), which illustrates the side-by-side arrangement of individual
up to at least a gram scale. Though not soluble in any solvents that ribbons that are over 40 nm long. As we indicated in the previous
we tried, these ribbons could be suspended by sonication in, for study of solution-synthesized chevron-like GNRs, it remains unclear
example, toluene or mesitylene, and then deposited on different if these nanobelts exist in solution or form directly on a substrate by
substrates for microscopic and spectroscopic characterization.
capillary forces during the solvent evaporation.21 4N-GNR nanobelts
It was previously demonstrated that when chevron-type GNRs are were also imaged by transmission electron microscopy (TEM), see
deposited from dispersion to a substrate, they could be found in a Fig. 2d and e. The nanobelts found on the TEM grids are 8 to 20 nm
form of elongated self-assembled nanostructures, referred to as wide, which corresponds to 8–12 4N-GNRs arranged in a side-by-side
‘‘GNR nanobelts’’.21 These nanobelts are only one carbon atom thick fashion, and up to 1 mm long.
and comprise multiple GNRs arranged in a side-by-side fashion.21
Raman spectroscopy is very sensitive to the disorder (edges
We deposited 4N-GNRs on a variety of substrates (mica, Au(111), and other structural defects, chemical functionalization, etc.) in
copper TEM grid with ultrathin carbon support film) and observed carbon materials,31 so we used this method to confirm the high
similar nanobelts. Fig. 2a shows an atomic force microscopy (AFM) structural quality of 4N-GNRs. Fig. 3 shows the Raman spectrum
of 4N-GNRs, where in addition to the most intense lines at 1324
and 1600 cmÀ1 that are typically referred to as D and G bands,
respectively,31 a series of smaller peaks around the D and G lines
could be observed. This fine structure is a direct result of the low
dimensionality of a nanoribbon, and the relative intensities and
positions of the lines are characteristic for each specific polycyclic
aromatic hydrocarbon (PAH).32 4N-GNRs synthesized in this work
are structurally close to chevron-like GNRs, so this is not
surprising that the Raman spectrum of 4N-GNRs is very similar to
that of chevron-like GNRs synthesized by the solution approach.21
Since disordered carbon materials usually show only broad D and
G lines without any fine features, the spectrum shown in Fig. 3
indicates the high structural quality of 4N-GNRs.
Fig. 2 Microscopic characterization of 4N-GNRs. (a) AFM image of 4N-GNRs
deposited on mica. Scale bar is 400 nm. (b) Height profile along the blue line
shown in (a). (c) STM image of 4N-GNRs deposited on a Au(111) single crystal.
Scale bar is 1 nm. (d, e) TEM images of 4N-GNRs. Scale bars are 50 nm.
Fig. 3 Raman spectrum of 4N-GNRs. The inset shows UV-vis-NIR absorption
spectrum of 4N-GNRs suspended in mesitylene by sonication.
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and the NSF through Nebraska MRSEC (DMR-0820521) and
EPSCoR (EPS-1004094).
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4174 | Chem. Commun., 2014, 50, 4172--4174
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