Lateral Fusion of Chemical Vapor Deposited N = 5 Armchair Graphene Nanoribbons

Article abstract of DOI:10.1021/jacs.7b05055

Bottom-up synthesis of low-bandgap graphene nanoribbons with various widths is of great importance for their applications in electronic and optoelectronic devices. Here we demonstrate a synthesis of N = 5 armchair graphene nanoribbons (5-AGNRs) and their lateral fusion into wider AGNRs, by a chemical vapor deposition method. The efficient formation of 10- and 15-AGNRs is revealed by a combination of different spectroscopic methods, including Raman and UV-vis-near-infrared spectroscopy as well as by scanning tunneling microscopy. The degree of fusion and thus the optical and electronic properties of the resulting GNRs can be controlled by the annealing temperature, providing GNR films with optical absorptions up to ～2250 nm.

Full text of DOI:10.1021/jacs.7b05055

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Communication
Lateral Fusion of Chemical Vapor Deposited
N = 5 Armchair Graphene Nanoribbons
Zongping Chen, Hai I. Wang, Nerea Bilbao, Joan Teyssandier, Thorsten Prechtl,
Nicola Cavani, Alexander Tries, Roberto Biagi, Valentina De Renzi, Xinliang Feng,
Mathias Kläui, Steven De Feyter, Mischa Bonn, Akimitsu Narita, and Klaus Müllen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05055 • Publication Date (Web): 26 Jun 2017
Downloaded from http://pubs.acs.org on June 26, 2017
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Lateral Fusion of Chemical Vapor Deposited N = 5 Armchair Gra-
phene Nanoribbons
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Zongping Chen,1 Hai I. Wang,2 Nerea Bilbao,3 Joan Teyssandier,3 Thorsten Prechtl,1,4 Nicola
Cavani,5,6 Alexander Tries,1,2 Roberto Biagi,5,6 Valentina De Renzi,5,6 Xinliang Feng,7 Mathias
Kläui,2 Steven De Feyter,3 Mischa Bonn,*,1 Akimitsu Narita,*,1 and Klaus Müllen*,1,4
1Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
2Institute of Physics, Johannes Gutenberg-University Mainz, Staudingerweg 7, D-55128 Mainz, Germany
3Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven Celestijnenlaan, 200 F, B-3001
Leuven, Belgium
4Institute of Physical Chemistry, Johannes Gutenberg-University Mainz, Duesbergweg 10-14, D-55128 Mainz, Ger-
many
5Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, I-41125 Mo-
dena, Italy
6CNR-NANO, Istituto Nanoscienze, Centro S3, I-41125 Modena, Italy
7Center for Advancing Electronics Dresden and Department of Chemistry and Food Chemistry, Technische Univer-
sität Dresden, Mommsenstrasse 4, D-01062 Dresden, Germany
Supporting Information Placeholder
The bandgap in each subfamily scales inversely with the rib-
ABSTRACT: Bottom-up synthesis of low-bandgap graphene
bon width, while the bandgaps of AGNRs in different sub-
families, but with the same n, follow a trend as: 3n + 2 << 3n
< 3n + 1.
nanoribbons with various widths is of great importance for
their applications in electronic and optoelectronic devices.
Here we demonstrate a synthesis of N = 5 armchair graphene
nanoribbons (5-AGNRs) and their lateral fusion into wider
AGNRs, by a chemical vapor deposition method. The effi-
cient formation of 10- and 15-AGNRs is revealed by a combi-
nation of different spectroscopic methods, including Raman
and UV-Vis-near infrared spectroscopy as well as by scanning
tunneling microscopy. The degree of fusion and thus the
optical and electronic properties of the resulting GNRs can
be controlled by the annealing temperature, providing GNR
films with optical absorptions up to ~2250 nm.
Up to now, AGNRs with atomic number N = 5, 7, 9, 13 have
been synthesized by bottom-up protocols through polymeri-
zation and graphitization of different halogenated monomers
on metal surfaces.4, 7-13 However, synthesis of other AGNRs
has remained elusive due to the difficulty in the design and
synthesis of appropriate monomers that can be efficiently
polymerized and precisely cyclized on the surface. An alter-
native approach is lateral fusion of poly(para-phenylene)
(PPP) chains14 or narrower AGNRs,15-17 which are in-situ pre-
pared on the surface from corresponding monomers. For-
mation of AGNRs with N = 6, 14, 18, and 21 has thus been
observed by thermal annealing of PPP and 7- and 9-AGNRs
at higher temperatures.14-17 Nevertheless, they were mostly
rather randomly and/or occasionally generated, coinciding
with branching of GNRs, and studied only locally by scan-
ning probe microscopy (SPM) techniques. This shortcoming
precluded other characterization at the macroscopic level,
such as by Raman and UV-Vis-near infrared (NIR) absorp-
tion spectroscopy.
Graphene nanoribbons (GNRs), a new class of quasi-one-
dimensional nanomaterials, exhibit unique electronic prop-
erties, distinct from graphene sheets, such as semiconducting
behavior with non-zero bandgap1-3 and strong excitonic ab-
sorption from UV into infrared range.4-5 As the electronic
band structure of GNRs relies critically on their width and
edge configuration,1-2 it is crucial to synthesize GNRs with
controlled structures. In particular, GNRs with armchair edg-
es are chemically stable and promising for application as
semiconductors.6-7 Depending on the width, armchair GNRs
(N-AGNRs) can be divided into three subfamilies with atom-
ic number N = 3n, 3n + 1, and 3n + 2 (with N the number of
carbon atoms across the width of a GNR, and n an integer).2-3
While most of the previous on-surface syntheses of GNRs
have been carried out under ultrahigh vacuum (UHV) condi-
tions, chemical vapor deposition (CVD) has recently been
proven to enable a high-throughput synthesis of structurally
defined GNRs.4, 8, 10, 18 Here we report a bottom-up synthesis
and lateral fusion of 5-AGNRs by a CVD process. Efficient
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formation of wider AGNRs with N = 10 and 15 is demonstrat-
S2). These values were in excellent agreement with the DFT-
ed by Raman spectroscopy, in addition to SPM. Moreover,
high-resolution electron energy loss spectroscopy (HREELS),
UV-Vis-NIR absorption, and ultrafast photoconductivity
measurements corroborated efficient fusion of 5-AGNRs and
simultaneously displayed extension of the absorption up to
~2250 nm after annealing at higher temperatures.
calculated RBLM peaks for 10-, 15-, and 20-AGNRs.9, 21 Peaks
from wider AGNRs than these fell outside the measurement
window of our instrument. At 400 °C, the 5-AGNRs appeared
to be still dominant as suggested by the ratio of the RBLM
intensity, i.e., I10A/I5A ≈ 0.1 (Figures 2 and S3), and scanning
tunneling microscopy (STM) images (see Figure 3a,b). An-
nealed at 500 °C, the fusion of 5-AGNRs further proceeded
and the RBLM peak of 10-AGNRs started to dominate (I10A/I5A
≈ 1.9). Moreover, after annealing at even higher temperature
of 600 °C, the RBLM of 5-AGNRs almost completely disap-
peared (I10A/I5A ≥ 10.6), demonstrating the high efficiency of
the lateral fusion of 5-AGNRs. It must be noted here that the
ratio of RBLM intensities does not quantitatively reflect the
ratio of the AGNRs, since the RBLM is starkly dependent on
the optical absorption of the GNRs and the excitation wave-
length of the measurement.4, 9 For instance, Raman excita-
tion at 488 nm of the 600 °C sample, known to contain both
10- and 15-AGNRs, displayed significant RBLM Raman inten-
sity only from 15-AGNRs (Figure S4).
We first attempted lateral fusion of 7-AGNR into 14- and
21-AGNRs under CVD conditions (see Figure S1). Although
this fusion process has been reported by SPM studies under
UHV conditions,17 we could not observe wider AGNRs by
Raman spectroscopy even after annealing at 650 °C (Figure
S1). We next considered fusion of 5-AGNRs, expected to be
more mobile on the surface than 7-AGNR owing to the
smaller width. First, efficient CVD synthesis of 5-AGNRs was
established, using an isomeric mixture of 3,9-
dibromoperylene and 3,10-dibromoperylene (DBP) as the
precursor, which was prepared following our previous report
(Figure 1).10, 13, 19 In a typical optimized procedure, DBP was
sublimed at 235 °C, and deposited on an Au/mica substrate
kept at 250 °C to induce homolytic carbon-bromo cleavage
and polymerization of the diradical to form linear poly-
perylenes. Subsequent annealing at 350 °C resulted in the
formation of 5-AGNRs through the surface-assisted intramo-
lecular cyclodehydrogenation.
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Furthermore, the G peak shows an interesting behavior,
accompanying the lateral fusion. The G peak of 5-AGNRs
prepared at 350 °C splits into two peaks, namely a main G–
peak at 1563 cm−1 and a weak G+ peak as a shoulder at 1596
cm−1, which originates from the frequency splitting of the
longitudinal and transverse optical modes in AGNRs (corre-
sponding to the E2g mode in graphene).22 For increasing an-
nealing temperatures, the intensity of the G+ peak increased
while that of the G– peak decreased. The G+ peak was more
intense after annealing at 500 °C and then became dominant
at 600 °C while G– was attenuated to a tiny shoulder peak,
which further corroborated the high efficiency of the lateral
fusion of 5-AGNRs.
Besides Raman spectroscopy, HREELS provides further ev-
idence for the temperature-dependent GNR fusion (see Fig-
ure S5). The intensity of a so-called DUO C-H out of plane
(opla) bending peak,23 characteristic of the armchair edges of
5-AGNRs decreased with increasing annealing temperature,
in line with the disappearance of the armchair edges at one
side of the GNRs upon the lateral fusion. The emergence of a
new SOLO opla bending peak also supports the formation of
wider GNRs, since the SOLO C-H is present only in fused
GNRs (see Figure S5). Additionally, the very weak C-O-C
peak in HREELS and very small O1s signal in X-ray photoe-
lectron spectroscopy (XPS) prove the suppressed oxidation
and high-quality of thus-prepared GNR samples (Figure S6).8
Figure 1. CVD growth of N = 5 armchair GNRs (5-AGNRs) and
their lateral fusion to wider GNRs.
While SPM can only provide very local information, which
might not be representative of the majority of the sample,
Raman spectroscopy is a very strong method to macroscopi-
cally characterize GNRs.4, 7-9, 20-21 The resulting 5-AGNRs were
first characterized by Raman spectroscopy with laser excita-
tion at 785 nm (Figures 2, S2, and S3). The spectra reveal four
main peaks at 1564, 1340, 1220, and 532 cm−1, which can be
assigned to G, D, edge C−H, and radial breathing-like mode
(RBLM) peaks, respectively.7, 20 The RBLM of GNRs is width-
specific and varies strongly with GNR width.21 The sharp and
intense RBLM peak at ~532 cm−1 can unambiguously be as-
signed to 5-AGNRs based on the value simulated by density
functional theory (DFT).21 Lateral fusion of as-formed 5-
AGNRs was accomplished by annealing at higher tempera-
tures. Notably, when the 5-AGNRs were annealed at 400 °C,
wider GNRs started to form, as evidenced by the appearance
of new RBLM peaks at 285, 188, and 122 cm−1 (Figures 2 and
The GNR samples were next characterized by STM, reveal-
ing small organized domains of lamellar features for samples
annealed at 350 and 400 °C (Figures 3 and S7). The periodici-
ty of the lamellar features is in the range 0.7 – 0.8 nm, which
is very close to the expected width of 5-AGNRs (0.9 nm),
verifying their formation under these growth conditions. The
fusion of 5-AGNRs into broader GNRs could be confirmed on
samples that had been annealed at higher temperatures. For
example, Figure 3d displays lamellar features of different
widths for a sample annealed at 600 °C. The widths obtained
from STM images correspond to 5-AGNRs (~0.9 nm), 10-
AGNRs (~1.3 nm), and 15-AGNRs (~1.8 nm), based on the
expected widths of 0.9, 1.5, and 2.1 nm, respectively.
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fined, the intense blue-shifted absorption maxima suggest
that the main component is 10-AGNRs.
20
15
600
500
400
350
°
°
°
°
C
C
C
C
10
D
To obtain further insight into the lateral fusion of the
AGNRs, we have investigated the photoconductivity of the
samples annealed at different temperatures, using optical-
pump, Terahertz-probe (OPTP) spectroscopy. Terahertz
(THz) absorption (∆E) by photogenerated free charges in the
samples is proportional to the real optical conductivity Re(σ),
which can be further correlated to the intrinsic carrier mobil-
ity in GNRs by: Re(σ) = n*e*µ, where n is the carrier density
and µ the mobility. To enable a qualitative comparison of
carrier mobility µ among different GNR structures, we report
the conductivity of the samples for the same carrier density
n. This is achieved by exciting all samples by the identical
incident photon-density (2.2*1014 cm–2) and normalizing the
conductivity to the optical density (OD) of each sample.
C-H
+
G
-
5
G
9
10
11
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13
14
15
16
17
400
800
1200
Raman shift (cm-1)
1600
2000
Figure 2. Raman spectra of AGNRs annealed at different
temperature. The numbers 5, 10, etc. indicate the position of
the RBLM peaks of respective AGNRs (excitation wavelength:
785 nm).
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Interestingly, the 5-, 10-, and 15-AGNRs belong to different
sub-groups (3n + 2, 3n + 1, and 3n, respectively) of AGNRs
and therefore possess distinctly different band structures. To
study their optical properties by UV-Vis-NIR spectroscopy,
we have transferred multiple layers of GNR films prepared at
350, 400, 500, and 600 °C onto fused silica substrates. Nota-
bly, the 5-AGNR sample annealed at 350 °C displays an ab-
sorption maximum at ~900 nm in the IR region and an ab-
sorption onset at ~1400 nm, suggesting an optical bandgap of
~0.9 eV (Figure 4a). Furthermore, the absorption maximum
of the sample annealed at 400 °C redshifts to ~1000 nm, with
an extended absorption with onset at ~1500 nm correspond-
ing to a lowered optical bandgap of ~0.8 eV. This observation
suggests that the 5-AGNRs prepared at 400 °C are longer
than those at 350 °C, which could be due to end-end cou-
pling of as-formed 5-AGNRs and/or higher efficiency of the
cyclodehydrogenation at 400 °C, given some open bonds
were present after annealing at 350 °C. It should be noted
that the observed optical bandgap of ~0.8 eV is still larger
than the theoretical optical transition at 0.48 eV based on
the GW-BSE method,24 indicating that the obtained 5-
AGNRs are not yet long enough to have the properties of
infinite 5-AGNRs.13
Figure 3. a, b, STM images of the sample prepared at 400 °C
showing 5-AGNR domains covering the surface of Au
(111)/mica (Iset = 100 pA, Vbias = −0.2 V and Iset = 200 pA, Vbias
=
−0.65 V respectively). c, Line profile along the dotted line in
panel (b) showing a periodicity of 0.8 nm for the lamellae. d,
STM image showing the formation of broader GNRs at
600 °C (Iset = 200 pA, Vbias = −0.4 V). e, Molecular model de-
picting the lamellar assembly of 5-AGNRs.
In contrast, the GNR sample annealed at 500 °C exhibits a
largely distinct spectrum with a substantially lower relative
intensity at ~1000 nm from 5-GNRs, and two dominant ab-
sorption peaks at ~650 nm (~1.91 eV) and ~750 nm (~1.65 eV).
These two peaks can be assigned to the 10-AGNRs with a
calculated optical transitions of 1.51 and 1.87 eV based on the
GW-BSE method,25 again supporting the conclusion of effi-
cient lateral fusion at this temperature. Finally, the sample at
600 °C shows emergence of very broad absorption features
extending into the infrared regime up to ~2250 nm, which
can tentatively be attributed to wider GNRs including (i) 15-
AGNRs with GW-BSE-calculated optical transition of 0.73
eV;26 and (ii) 20-AGNRs that belong to the N = 3n + 2 family
and are expected to have an optical bandgap < 0.5 eV. There
is also a possibility that 5-AGNRs longer than those at 400 °C
are present, considering the GW-BSE-calculated optical tran-
sition of 0.48 eV.24 Although this sample is relatively unde-
Figure 4. a, UV-Vis-NIR absorption spectra of AGNRs sup-
ported by fused silica substrates. The spectra are normalized
to the number of GNR layers (Nlayer); b, THz photoconductiv-
ity of AGNRs. The dynamics are normalized to the optical
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density of the samples. The measurement error, originating
from inhomogeneity of the sample, is ~10% by measuring 5
different spots on the sample annealed at 600 °C.
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Corresponding Author
Narita, A.; Müllen, K.; Kharche, N.; Meunier, V.; Fasel, R.; Ruffieux, P.
Nano Lett. 2017, 17, 2197-2203.
*bonn@mpip-mainz.mpg.de
*narita@mpip-mainz.mpg.de
*muellen@mpip-mainz.mpg.de
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This work was financially supported by the DFG (Priority
Program Graphene SPP 1459, KL1811), the Max Planck Society,
the ERC Starting Grant MASPIC (No. ERC-2007-StG 208162),
EC through the Graphene Flagship and the Seventh Frame-
work Programme within the project Moquas (FP7 FET-ICT-
2013-10 610449), the Office of Naval Research BRC program,
and the State Research Centre for Innovative and Emerging
Materials (CINEMA). We acknowledge Xiaoyu Jia for his
support in the absorption spectroscopy measurements.
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5-AGNRs
20-AGNRs
10-AGNRs
600
500
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350
°
°
°
°
C
C
C
C
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5-AGNRs
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Fusing into
wider GNRs
15-AGNRs
400–600 °C
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10-AGNRs
Raman shift (cm-1
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