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Bottom
-
Up Synthesis of Liquid
-
Phase
-
Processable Graphene
Nanoribbons with Near-Infrared Absorption
Journal:
ACS Nano
Manuscript ID:
nn-2014-049014.R2
Manuscript Type:
Article
Date Submitted by the Author:
21-Oct-2014
Complete List of Authors:
Narita, Akimitsu; Max-Planck-Institute for Polymer Research,
Verzhbitskiy, Ivan; Free University Berlin, Department of Physics
Frederickx, Wout; Katholieke Universiteit Leuven, Department of Chemistry
Mali, Kunal; Katholieke Universiteit Leuven, Department of Chemistry
Jensen, Soeren; Max Planck Institute for Polymer Research, Molecular
Spectroscopy
Hansen, Michael Ryan; Max-Planck-Institute for Polymer Research,
Bonn, Mischa; Max Planck Institute for Polymer Research, Molecular
spectroscopy
De Feyter, Steven; KU Leuven, Department of Chemistry
Casiraghi, Cinzia; University of Manchester, School of Chemistry
Feng, Xinliang; Dresden University of Technology, Molecular Functional
Materials
Müllen, Klaus; Max-Planck-Institute for Polymer Research,
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ACS Nano
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Bottom-Up Synthesis of Liquid-Phase-Processable
Graphene Nanoribbons with Near-Infrared
Absorption
Akimitsu Narita,
†
Ivan A. Verzhbitskiy,
‡
Wout Frederickx,
§
Kunal S. Mali,
§
Soeren Alkaersig
Jensen,
†,
∥
Michael Ryan Hansen,
†,
⊥
Mischa Bonn,
†
Steven De Feyter,
§
Cinzia Casiraghi,
‡,¶
Xinliang Feng,*
,†
and Klaus Müllen*
,†
†
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
‡
Department of Physics, Free University Berlin, Arnimalle 14, 14195 Berlin, Germany
§
Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven-
University of Leuven, Celestijnenlaan, 200 F, B-3001 Leuven, Belgium
∥
FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
⊥
Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus
University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark
¶
School of Chemistry, Manchester University, Oxford Road, Manchester, M139PL, United
Kingdom
*Address conrrespondence to muellen@mpip-mainz.mpg.de; feng@mpip-mainz.mpg.de.
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KEYWORDS: graphene nanoribbon · cyclodehydrogenation · Diels–Alder reaction · bandgap
engineering · near-infrared absorption
ABSTRACT. Structurally defined, long (>100 nm), and low-bandgap (~1.2 eV) graphene
nanoribbons (GNRs) were synthesized through a bottom-up approach, enabling GNRs with a
broad absorption spanning into the near-infrared (NIR) region. The chemical identity of GNRs
was validated by IR, Raman, solid-state NMR, and UV–vis–NIR absorption spectroscopy.
Atomic force microscopy revealed well-ordered self-assembled monolayers of uniform GNRs on
a graphite surface upon deposition from the liquid phase. The broad absorption of the low-
bandgap GNRs enables their detailed characterization by Raman and time-resolved terahertz
photoconductivity spectroscopy with excitation at multiple wavelengths, including the NIR
region, which provides further insights into the fundamental physical properties of such graphene
nanostructures.
Graphene nanoribbons (GNRs), nano-strips of graphene, are emerging materials of great
promise, which possess non-zero bandgaps in stark contrast to semi-metallic graphene.
1, 2
The
electronic properties such as the bandgaps of the GNRs critically depend on their width and edge
structures,
3-6
which makes it crucial to precisely control the GNR structures for fundamental
studies as well as for future nanoelectronic applications. GNRs are typically prepared through
“top-down” approaches such as lithographic patterning of graphene sheets
7-9
and unzipping of
carbon nanotubes,
10-12
revealing promising electronic properties of the GNRs. However, these
methods cannot avoid high structural disorder, especially at the edges of the resulting GNRs, and
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often lack uniformity in the width. On the other hand, “bottom-up” chemical synthesis can
provide a variety of structurally well-defined GNRs based on solution-mediated
13-19
or surface-
assisted
20-23
cyclodehydrogenation of tailor-made polyphenylene precursors.
Whereas the GNRs previously prepared through the bottom-up methods were limited by the
short length (<50 nm)
13-15
and/or the necessity of metal surfaces,
20-22
we have very recently
reported a solution synthesis of structurally well-defined GNR 1 with high longitudinal extension
over 200 nm (Figure 1).
16
We have employed an efficient AB-type Diels–Alder polymerization
for the preparation of corresponding polyphenylene precursors with exceptionally large
molecular weights, followed by intramolecular oxidative cyclodehydrogenation. Notably, GNR 1
shows a defined bandgap, in stark contrast to carbon nanotubes (CNTs) that are unavoidably
obtained as a mixture of different diameters and chiralities with a corresponding wide
distribution of bandgaps.
24, 25
Nevertheless, the width of GNR 1 is limited to ~1 nm,
corresponding to a bandgap of ~1.9 eV, with an optical absorption only up to ~650 nm.
16
Figure 1. Structures of GNRs 1 and 2. Geometric dimensions were derived from Merck
Molecular Force Field 94 (MMFF94) calculations.
For the future development of GNR-based electronics and opto-electronics, it is highly
important to tune the GNR width and thus the bandgaps, while maintaining the large longitudinal
extension.
5, 26
Specifically, a smaller bandgap leads to absorption over a wider range of
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wavelengths, possibly including the near-infrared (NIR) region, which opens up possibilities for
various applications, including photovoltaics.
5, 27
Moreover, GNRs with NIR absorption are
desirable to spectroscopically obtain an in-depth understanding on the basic physical properties
of the GNRs. For instance, Raman measurements at multiple wavelengths provide information
on the phononic and electronic dispersions of the GNRs, based on the resonant nature of the
Raman scattering process in GNRs.
28
In this article, we describe the synthesis and
characterization of GNR 2 with an extended width of ~2 nm and lengths exceeding 100 nm
(Figure 1). GNR 2 demonstrates a low and well-defined bandgap of ~1.2 eV with broad optical
absorption, extending into the NIR region, in contrast to the relatively large bandgap of GNR 1.
Such GNRs with NIR absorption allow for in-depth investigations using multi-wavelength
Raman spectroscopy and ultrafast terahertz (THz) photoconductivity measurements.
RESULTS AND DISCUSSION
Bottom-up solution synthesis of GNR 2. In order to synthesize GNR 2 through an efficient
AB-type Diels–Alder polymerization, a laterally extended monomer 11 was designed, with a
cyclopentadienone moiety and an ethynyl group as the conjugated diene and the dienophile,
respectively (Scheme 1). 2-Bromo-5-methylbiphenyl (4) was prepared by selective Suzuki
coupling of 4-bromo-3-iodotoluene (3) with phenylboronic acid at 60 °C, and then brominated
with an excess amount of N-bromosuccinimide (NBS) to give a mixture of 2-bromo-5-
bromomethylbiphenyl (5) and 2-bromo-5-dibromomethylbiphenyl. The crude mixture was
subsequently treated with HPO(OEt)
2
and i-Pr
2
NEt for selective debromination at the benzylic
position
29
to afford 5 in 67 % yield for two steps. Phase-transfer carbonylation of 5 with Fe(CO)
5
provided bis(bromobiphenyl)acetone 6 in 39 % yield, which was reacted with dodecylphenyl
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