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Energy gap opening by crossing drop cast singlelayer graphene nanoribbons
To cite this article: Toyo Kazu Yamada et al 2018 Nanotechnology 29 315705

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Nanotechnology
Nanotechnology 29 (2018) 315705 (10pp)

https://doi.org/10.1088/1361-6528/aac36b

Energy gap opening by crossing drop castsingle-layer graphene nanoribbons
Toyo Kazu Yamada1,2 , Hideto Fukuda1, Taizo Fujiwara3, Polin Liu3,
Kohji Nakamura4, Seiya Kasai5, Amadeo L Vazquez de Parga6,7 andHirofumi Tanaka3
1

Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,
Japan
3
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology (Kyutech),2-4 Hibikino, Wakamatsu, Kitakyushu 808-0196, Japan
4
Department of Physics Engineering, Mie University, 1577, Kurimamachiya-cho Tsu, Mie, 514-8507,
Japan
5
Research Center for Integrated Quantum Electronics, Hokkaido University, North 13, West 8, Sapporo
060-8628, Japan
6
Dep. Fisica de la Materia Condensada and IFIMAC, Universidad Autonoma de Madrid, Cantoblanco,
28049 Madrid, Spain
7
Instituto Madrileño de Estudios Avanzados en Nanociencia IMDEA-Nanociencia, Cantoblanco 28049,
Madrid, Spain
2

E-mail: toyoyamada@faculty.chiba-u.jp
Received 18 January 2018, revised 1 May 2018
Accepted for publication 9 May 2018
Published 1 June 2018
Abstract

Band gap opening of a single-layer graphene nanoribbon (sGNR) sitting on another sGNR,
fabricated by drop casting GNR solution on Au(111) substrate in air, was studied by means ofscanning tunneling microscopy and spectroscopy in an ultra-high vacuum at 78 K and 300 K.
GNRs with a width of ∼45 nm were prepared by unzipping double-walled carbon nanotubes(diameter ∼15 nm) using the ultrasonic method. In contrast to atomically-ﬂat GNRs fabricated
via the bottom-up process, the drop cast sGNRs were buckled on Au(111), i.e., some local pointsof the sGNR are in contact with the substrate (d∼0.5 nm), but other parts ﬂoat (d∼1–3 nm),where d denotes the measured distance between the sGNR and the substrate. In spite of the factthat the nanoribbons were buckled, dI/dV maps conﬁrmed that each buckled sGNR had ametallic character (∼3.5 Go) with considerable uniform local density of states, comparable to a
ﬂat sGNR. However, when two sGNRs crossed each other, the crossed areas showed a band gap
between −50 and +200 meV around the Fermi energy, i.e., the only upper sGNR electronic
property changed from metallic to p-type semiconducting, which was not due to the bending, but
the electronic interactions between the up and down sGNRs.
Supplementary material for this article is available online
Keywords: graphene nanoribbon, scanning tunneling microscopy, cross structure, drop cast,energy gap(Some ﬁgures may appear in colour only in the online journal)[1], while the thickness of the wire for interconnects is typically larger than 30 nm and still has room for downsizing.
With the use of an atomically thin wire while maintaining
high conductance, downsizing the circuits in three dimensions

1. Introduction
State-of-the-art nano-wiring of electronic circuits is presently
based on the 14 nm Fin-FET (ﬁeld effect transistor) process
0957-4484/18/315705+10$33.00

1© 2018 IOP Publishing Ltd Printed in the UKNanotechnology 29 (2018) 315705

T K Yamada et alscanning probe microscopy [15]. We used this GNR liquid,
and therefore the presence of bilayer or multilayer GNRs
could be discarded in our solution. We deposited theresulting nanoribbons placing a drop of the nanoribbon
solution on the Au(111) substrates. The samples were thenintroduced into a home-built UHV setup containing a lowtemperature STM. Using a solution with a low concentration
the drop casting process created a sample with individualsGNRs deposited on Au(111). The adsorbed sGNRs did not
lay completely ﬂat on the surface, some local points of thesGNR were in contact with the substrate (d∼0.5 nm), but
in other parts the sGNRs were quite far away (d∼1–3 nm)
from the substrate, where d denotes distance between theadsorbed sGNR and the substrate, i.e., the sGNRs unzippedfrom the CNTs were buckled. The STS maps clearly showthat each buckled sGNR presents metallic properties (conductance of ∼3.5 Go) with a uniform LDOS, independent of
the buckling. By repeatedly adding drops of the solution on
the substrate, we were able to create a network of intercrossed sGNRs, the most frequent conﬁguration was the
crossing between two sGNRs. Due to the buckling structureon the sGNR, the separation between two sGNRs was
between 1.0–1.5 nm. The STS data showed uniform electronic states on the cross areas with a gap around the Fermi
level of the order of 250 meV, with a lower limit of −50
meV and an upper limit of +200 meV i.e., the electronic
properties of the sGNRs varied from metallic to p-typesemiconducting.

becomes feasible and single-layer graphene nanoribbons(sGNRs) are one of the most promising candidates [2–5].
GNRs can be fabricated by unzipping carbon nanotubes(CNTs), such that their electronic properties, e.g., the electronmean free path and the maximum allowed current density arequite similar to those of CNTs [6–9]. Although several
methods have been reported to fabricate sGNRs on substrates,
such as bottom-up processes [4, 5], GNRs fabricated simply
by unzipping the multi-walled CNTs along the axis have beenused in previous studies [10–12]. Several techniques havebeen reported in the last ﬁve years for unzipping CNTs
through the use of (1) sulfuric acid and potassium permanganate as the oxidizing agent [11]; (2) partially embedding ina polymer ﬁlm and etching by argon plasma [12]; (3)inserting alkali-metal atoms between the concentric cylindersof a multi-walled CNT [13]; (4) catalytic metal nanoparticles;and (5) gas-phase oxidation, where oxygen reacts with preexisting defects on a CNT, forming pits, and further sonication in liquid unzips the CNT [14, 15].
In contrast to these unzipping processes, atomically-ﬂat
sGNRs grown on Au(111) were successfully fabricated bysurface-assisted coupling of molecular precursors into linear
polyphenylenes and their subsequent cyclodehydrogenation(i.e., bottom-up process) [4], where scanning tunnelingmicroscopy and spectroscopy (STM/STS) have been usedas powerful tools to visualize the local density of states(LDOS) of the sGNRs, and to study their dependence withthe chirality of the ribbons as well as the existence of edge
states [5, 16]. From a theoretical point of view thedependence of the electronic properties of the graphene
nanoribbons on the edge’s structure and/or the width has
been addressed [17–20]. The growth of sGNRs has been
explored as a feasible route to create a band gap around the
Fermi level by narrowing the sGNR width (<2 nm) owing toquantization effects; so far, rectiﬁcation has not been
observed [17]. The electronic properties of cross-structuredsGNRs are critical for the development of three-dimensional
Fin-FETs.
In this manuscript, we have studied the morphology andelectronic properties of sGNRs on Au(111) prepared by drop
casting GNR solution by means of STM/STS in an ultra-highvacuum (UHV) at 78 and 300 K. In marked contrast to theatomically-ﬂat sGNRs on Au(111) grown by the bottom-up
process [4], our drop casting sGNRs showed unique properties. We used metallic sGNRs to explore, by means of STM/
STS, the intrinsic properties of the crossing areas between twosGNRs.
We used sGNRs unzipped from a double-walled carbon
nanotube (DWNT) by following the method used in theprevious studies [10–12, 15], i.e., after two hours sonication,
more than 95% of the DWNTs were unzipped, producing
three kinds of GNRs: double-layer GNRs, Y-shaped GNRs
and sGNRs. Markedly, after 16 h sonication, more than 99%of the double-layer GNRs and Y-shaped GNRs split intosGNRs, which was conﬁrmed by Raman spectroscopy and2. Results and discussion
2.1. Imaging of drop casting sGNR on Au(111)The sGNRs were prepared by unzipping DWNTs with carefulsonication in solution such that the CNTs were carefully
opened but not destroyed and thus the width of the resultingsGNRs could be controlled by the diameter of the CNTs
[10–12]. In this work, we used DWNTs with diameters of
5–15 nm, i.e., the widths of the obtained sGNRs were comparable to π times the diameter of the unzipped CNT. Also,
two carbon sheets could be initially separated by 1–2 nm,
while bilayer GNRs have a separation distance of ∼0.335 nm[21]. As a substrate, we used 200 nm thick gold ﬁlms
deposited on mica. The ﬁlms present a (111) termination witha grain size large enough to allow STM measurements in asingle Au(111) crystallite [22].
Figure 1(a) shows a typical image of the surface of theAu(111) on mica, atomically-ﬂat terraces separated by atomic
steps can be observed in the STM image. The inset shows thecorresponding low energy electron diffraction (LEED) pattern, the spots corresponding to the six-fold symmetry appear
as small arcs due to the small rotational disorder of the different Au(111) crystallites. Figure 1(b) shows an atomicallyresolved STM image where the atomic periodicity of theAu(111) surface can be observed as well as the modulation

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Figure 1. (a), (b) STM images obtained on the atomically-ﬂat Au ﬁlm (∼200 nm thick) on mica at 78 K, which was cleaned and ﬂattened in

UHV by several cycles of Ar+ sputtering and annealing before drop casting the GNR solution. (a) Au atomic terraces (Vs=−1.0 V, I=50
pA, 100×100 nm). LEED pattern in the inset image shows six-fold fcc(111) symmetry. (b) Atomically resolved STM image of fcc-Au(111)(Vs=−70 mV, I=100 pA, 8×8 nm). Bright spheres correspond to single Au atoms, while two brighter lines correspond to the Au(111)reconstruction. (c) STM topographic image of a single-layer graphene nanoribbon (sGNR) on the Au(111) substrate (Vs=−1.5 V, I=50
pA, 475×100 nm). The width of the sGNR is ∼45 nm. (d) Line proﬁles along arrows (1) and (2) in (c). (e) Line proﬁles along arrows (3),(4) and (5) in (c).

due to the herringbone reconstruction [22]. First, we used a
low concentration solution in order to deposit individualsGNRs and characterize their properties. Figure 1(c) shows an
STM image (475×100 nm) showing a single sGNR
deposited on the Au(111) surface, the width of the observedsGNR was of the order 45 nm, as expected from the diameterof the original CNTs. The line proﬁle (1) in ﬁgure 1(d) ismeasured along the arrow (1) in ﬁgure 1(c) and shows asteady increase in height from left to right. The Au(111) ﬁlm
substrate was atomically-ﬂat as shown in ﬁgure 1(a), but theterrace width was rather small: 10–20 nm, therefore about 30
steps exist on the substrate from left to right in the STM
image in ﬁgure 1(c), generating a height difference of ∼6 nm.
A single GNR was gently dropped on this substrate, producing a unique shape. In ﬁgure 1(d), the line proﬁle (2)obtained on top the sGNR shows a different evolution withthe lateral position indicating that the sGNR did not follow
the morphology of the substrate. In fact, sGNR along arrow(2) shows clear buckling. Line proﬁles along the arrows (3)–(5) in ﬁgure 1(c) show the separation distance from thesubstrate (see ﬁgure 1(e)). Clearly, the separation distancechanged at different positions: the shortest length d∼0.6 nm
to the longest length of d∼3 nm, suggesting that the sGNR

was in local contact with the substrate, but in most of theareas was far away from the Au(111) substrate. We also
observed that an sGNR hung on a substrate hole, i.e. an sGNR
was able to ﬂoat from the substrate (see the supplementary
information, ﬁgure S1 is available online at stacks.iop.org/
NANO/29/315705/mmedia.) We can speculate that thisconﬁguration may have been due to the drop casting procedure and the subsequent annealing process, because the graphene nanoribbons synthesized in situ always lay ﬂat on thesurface [4, 5].

2.2. Electronic properties of drop casting sGNRs

By means of STS we studied the electronic properties ofindividual sGNRs. Figure 2(a) shows the spectroscopy results
measured on individual sGNRs, the dI/dV curves shown are
obtained at the A, B, C, D, E, and F areas in ﬁgure 4(a). It
should be noted that all curves have identical shapes andfeatures irrespective of the position they were measured on
the buckled sGNRs. In the vicinity of the Fermi energy(Vs∼0), dI/dV increased linearly for both negative andpositive voltages and did not reach zero at zero energy,
indicating a metallic behavior as expected from the width3

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T K Yamada et al

Figure 2. (a) dI/dV curves, proportional to the LDOS, obtained on the sGNR adsorbed on Au substrate at the areas marked by A, B, C, D, E,

and F in ﬁgure 4(a). (b) Calculated LDOS of sGNR with a width of N=19 and N = inﬁnity (see the supplementary information, ﬁgure S3).
(c) STM conductance measurement through an sGNR at 78 K. z denotes the distance of the tip approaching toward the sGNR. Before
approaching the STM W tip towards the sGNR on the Au substrate, the tip height position was set as Vs=+10 mV, I=150 pA. The blueline shows conductance during approach. From −180 pm to −220 pm, we can observe big jumps of the conductance, and around −300 pm,
the conductance becomes constant. When we retracted the tip, the conductance was constant (∼3.5 Go), indicating that the tip is still incontact with the sGNR and the tip started to pick up the sGNR. The conductance through the partially lifted sGNR was ∼3.5 Go.

[17–20]. The calculated LDOS of a graphene sheet(N=inﬁnity) and that of an sGNR with a width of N=19,
where N represents the number of atoms included in the GNR
width, are also shown in ﬁgure 2(b). Although the ﬁnite size
effect in the sGNR, as seen in that of N=19, may haveopened an energy gap at the Fermi level, this happened forwidths much smaller than the widths of the nanoribbons
deposited in our experiments, and therefore, the experimentally observed dI/dV curves agree well with the behavior
expected from the theoretical calculations. In order to test if
the observed metallic behavior in the STS spectra was an
intrinsic property of the deposited sGNRs or, on the contrary,
we measured the metallic character of the Au(111) substrate
through the graphene nanoribbon; we measured the electricalconductance along the nanoribbons’ axis in contact with the
STM tip at one end of the sGNRs and lifted them from thesurface [23, 24]. The experimental procedure was as follows,

we approached the W tip towards the sample to makedirect contact with the sGNRs avoiding plastic deformationof the sample. During the approach-retract process, thecurrent was measured as a function of the tip-sample
separation (I–z curve) while the bias voltage was ﬁxed at+10 mV. The results are shown in ﬁgure 2(c), during theapproach of the STM tip towards the sGNR, the tunnelingcurrent increased exponentially until the point of contact, at
this point the current suddenly jumped and a constant value of
conductance of ∼3.5 Go was obtained, a further tip approach
did not change this value. It is worth mentioning that thisvalue is comparable to the conductance of a 200 nm-long (and14 nm-wide) sGNR (∼3 Go) [14]. We measured the STM
images of the contacted sGNRs before and after the contact tocheck the morphology and no changes were observed. The
conductance measurements are clear evidence of the metallic
character of the deposited sGNRs on the Au(111) substrate.
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Figure 3. (a) STM image of cross bridging structures of sGNRs (Vs=−1.5 V, I=40 pA, 130×180 nm). Three sGNRs are stacked. (b) A

simple sketch of three sGNRs in (a), named A, B, and C. Dashed lines are guides to see each sGNR. The cross bridging area is marked as
A+B. (c), (d) Line proﬁles along the arrows (1) and (2) in (a).

map at +30 mV (ﬁgure 4(b)), respectively. Since the sGNR
surface was not ﬂat, the line proﬁle in ﬁgure 4(c) showsheight variations within GNR 3. At the position where GNR 3
crossed with GNR 5, the height changed by ∼1.0 nm, which
agrees with the average distance measured in the otheroverlapping areas (see ﬁgure 3). On the other hand, the line
proﬁle in the dI/dV map along the same path gives us interesting information (ﬁgure 4(d)). In spite of the fact that the
topographic height varied along GNR 3, the dI/dV values
remain constant around 0.20±0.05 nA V−1, i.e., the LDOS
inside the GNR are quite uniform, independent of themorphology variation. The dI/dV curves obtained at the A–F
areas in ﬁgure 4(a) are shown in ﬁgure 2(a). It should be
noted that all curves show similar features, i.e., in the vicinityof the Fermi energy (Vs∼0), dI/dV increases linearly forboth negative and positive voltages, meaning that the LDOS
at different positions on the sGNRs is almost the same.
Markedly, at the position where the two sGNRs overlap, thedI/dV signal rapidly dropped down to ∼0.0 nA/V, andremained constant in all the crossing areas. The edge of thecrossing area is focused on in ﬁgure 4(e), where the heightvariation of 1.0 nm takes place in Δ∼7 nm distance, however, the dI/dV signal drops within Δ∼3 nm, which indicates that the dI/dV drop was not due to the GNR bending,but the electronic interactions between the up and downsGNRs. Figure 4(f) shows the dI/dV curve obtained at thecrossed area (G1 in ﬁgure 4(a)), it presents dI/dV values of∼0.00 nA/V around the Fermi energy (red curve). The blue
curve in ﬁgure 4(e) was obtained from the crossed sGNR areaat G2 in ﬁgure 4(a) where GNRs 2 and 6 cross each other. It
should be noted that G1 and G2 crossing areas showed
exactly the same dI/dV curve, i.e., different crossed sGNRs
had the same LDOS and gap, suggesting that there was nocrossing angle dependence between two sGNRs, further
suggesting that random stacking sGNRs may have a gap. Justby crossing, the sGNRs changed their electronic property
from metallic to semiconducting.

2.3. Crossing sGNRs by drop cast

By repeating the drop casting on the same substrate, we
increased the number of deposited sGNRs and therefore
increased the number of sGNRs crossing each other.
Figure 3(a) shows the area where three sGNRs (named A, B
and C) overlapped. Figure 3(b) is a simple sketch to identify
the sGNRs’ positions. By comparing ﬁgures 3(a) and (b), we
can see that sGNR A and B crossed. The crossing area ismarked as A+B. The line proﬁles along arrow (1) and (2) inﬁgure 3(a) show that the distance between the two sGNRs
was always around 1.0–1.5 nm (see ﬁgures 3(c) and (d)), so
the sGNRs did not lay ﬂat one on top of the other as aconsequence of the deposition method. (A wide scan range
STM image is shown in the supplementary information,ﬁgure S2, where more than 20 sGNRs are stacked.)
Figure 4(a) shows an STM topographic image (200×
180 nm) acquired in another area showing six sGNRs which
are marked by numbers: 1–6. There are four sGNRs (1–4)
running from left to right and another two sGNRs (5 and 6)
running from the bottom to the top of the image. With thisconﬁguration GNRs 5 and 6 partially crossed with GNRs 1–4.
The crossed areas are marked by G1 and G2. Simultaneously
to the topographic image we measured at every pixel of theimage the dI/dV curve. Figure 4(b) shows the dI/dV map at+30 meV. The brighter color denotes higher dI/dV [nA/V],
i.e. a higher LDOS. By comparing ﬁgures 4(a) and (b), we
conclude that sGNRs 1–4 show uniform signal in the dI/dV
map, which conﬁrms that monolayer sGNRs present a uniform LDOS as long as they are deposited on the Au(111)substrate, as discussed before. However, in all the areas where
two sGNRs cross the dI/dV signal is smaller and the areas
appear darker in the maps, i.e., there is a clear reduction in the
LDOS of the nanoribbons when they are deposited on top of
another nanoribbon in comparison to the ones deposited on
Au(111). Figures 4(c) and (d) show the line proﬁle along thearrows in the topographic image (ﬁgure 4(a)) and the dI/dV
5

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T K Yamada et al

Figure 4. (a) STM topographic image of sGNRs on the Au(111) substrate (Vs=−1.0 V, I=500 pA, 200×180 nm). Six sGNRs areobserved. Four sGNRs marked by 1, 2, 3, and 4 run from left to right, while 5 and 6 run from the bottom to the top of the image. Areas
marked with G1 and G2 indicates zones where the two GNRs overlap. (b) dI/dV map at +30 mV obtained at the same area as (a). Brighter
color denotes high density of states. (c) Line proﬁle along the arrow in (a). The crossing sGNRs show a height difference of ∼1.0 nm. (d)Line proﬁle in the dI/dV map along the arrow shown in (b). (e) Enlarged plots in (c) and (d). (f) dI/dV curves obtained at 300 K on the sGNR
(black line) and on the area G marked in (a). Red and blue curves are obtained from the areas marked by G1 and G2 in panel (a), respectively.

appears brighter. These variations in the differential conductance maps can be clearly explained by the dI/dV plot on alog-scale shown in ﬁgure 5(b). White and red dots are thedI/dV plots obtained at the sGNR and the crossed sGNRs,
respectively. Around −650 meV, both the sGNR and crossedsGNRs have a similar conductance of ∼0.5 nA/V, then from−650 meV to −50 meV, the conductance of the crossedsGNRs drastically decays below 0.01 nA/V. Above+200 meV, again the conductance of the crossed sGNRs
starts to increase rapidly and above ∼+600 meV, theIn order to characterize with precision the energy positionof the gap we measured dI/dV curves with high energy
resolution. Figure 5(a) shows dI/dV maps at −697, +10,+313, and +717 mV obtained at the boundary between thesGNR and the crossed GNRs. At −697 mV, it is hard to see
the difference in the map, meaning the sGNR and the crossedsGNRs had a comparable conductance, however, at +10 mV,
the crossed sGNRs show clear dark contrast, while the contrast becomes weaker with an increase in the bias (see the map
at +313 mV), and ﬁnally at +717 mV even the crossed area
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There are a couple of mechanisms that could explain
the presence of a gap in the LDOS of the upper GNRs in theoverlapping areas. Botello-Mendez et al calculated that thestacking angle between individual GNRs plays a central
role in dictating the probability of transmission of the electrons from one ribbon to the other [21]. In fact, gaps were
found in perpendicular conductance at crosses with different
orientations. Initially one can think that our STM/STS results
agree with these theoretical predictions; however, a detailed
analysis of our results indicates that the presence of a gap in
the differential conductance must have a different origin. On
the one hand, in our experiments, the gap width in the conductance does not depend on the angle between the crossing
ribbons, as opposed to the calculations. On the other hand,
our experiments measured the tunnel probability when the
STM tip was located on top one of the crosses of the graphene
nanoribbons and not directly the probability of transmissionof an electron from the upper to the lower nanoribbon. The
GNRs used in this study are metallic, as demonstrated above,and therefore even if the transmission of electrons between
the upper and lower ribbon is prohibited, we should not see a
gap in the differential tunneling conductance measured bymeans of STM since the electrons can reach the Au(111) bymoving through the top ribbon.
An alternative explanation for our experimental results
is the existence of a gap in the electronic structure of graphene bilayers. It has been shown that breaking the inversion
symmetry in graphene bilayers by applying an electric ﬁeld
perpendicular to the plane of the graphene bilayer opens agap [25, 26]. A similar way of getting the same results is
growing or depositing the bilayer on a substrate, which
modiﬁes the density of carriers differently in one of thelayers of the bilayer [9, 27]. In all cases gaps observed are of
the order 250 mV. Figures 5(c) and (d) show schematics ofelectron transfer between the tip and the sample when
positive (+200 mV) and negative (−50 mV) bias voltages
are applied. At the crossed structure, upper and lower sGNR
layers were stacked. We know that the lower sGNR layer
has a metallic property, thus electrons ﬁll up to the Fermi
energy (EF). However, the situation of the upper sGNR layer
is totally different; from the STS results in ﬁgures 5(a) and(b), we know that the LDOS of the upper sGNR layer has agap. In ﬁgures 5(c) and (d), blue and red lines denote thevalence band edge (Ev) and the conduction band edge (Ec) of
the upper sGNR layer, respectively. When a positive bias
was applied to the sample, electrons from the tip Fermi
energy tunneled into the unoccupied states of the sample
substrate through the conduction band of the upper sGNR
layer (see ﬁgure 5(c)). Reversely, when a negative bias wasapplied to the sample, electrons tunneled from the Fermi
energy of the substrate to the tip unoccupied states throughthe valence band of the upper sGNR layer. Considering thatFermi levels in the lower GNR and the Au substrate were in
equilibrium whereas the upper sGNRs was electrically isolated from the lower one by 1.0 nm spacing, the verticalelectric ﬁeld of around 400 MV m−1 could have been caused
between the two sGNRs layers by the difference in work
functions of Au and the sGNR; 5.1 eV for Au and 4.7 eV for

Figure 5. (a) dI/dV maps at −697, +10, +313, and +717 mV obtained
from the crossed sGNRs areas. Brighter areas denote higher
conductance. (b) Differential conductance (dI/dV ) plot on a log-scale as
a function of energy, where zero corresponds to the Fermi energy, andpositive and negative energy denote unoccupied and occupied states,
respectively. White and red dots show dI/dV values obtained on thesGNR area and the crossed sGNRs area, respectively. Two dashed lines
denote the region where dI/dV values are lower than 0.002 nA V−1,
suggesting a band gap. (c), (d) Scheme showing the electron transfer
between the tip and the sample at the crossed sGNRs areas. From the left
side, the lower sGNR/Au(111) substrate, the upper sGNR, and the W
tip energy diagrams are shown. Bias voltage is applied between thesubstrate and the tip. The substrate and the upper sGNR are separated by∼1.2 nm, and the W tip and the upper sGNR are separated by ∼1 nm. In(c), a positive bias voltage of +200 mV and, in (d), a negative bias
voltage of −50 mV is applied. Ec and Ev denote conduction and valenceband edge, respectively.

conductance of the crossed sGNRs becomes higher than thatof the sGNR. From this plot, if we deﬁne the dI/dV values
lower than 0.002 nA/V as insulating, we can say that thecrossed sGNRs have a gap between −50 and +200 meVaround the Fermi energy (an energy gap width ofEg=250 meV), i.e., the crossed sGNRs have p-type semiconducting characteristics.
7

Nanotechnology 29 (2018) 315705

T K Yamada et algraphene [28]. It is worth mentioning that the STM tip
potential hardly inﬂuenced the electric ﬁeld in the sGNRs
due to the presence of the metallic substrate. In fact, we did
not ﬁnd any dependence of the measured width of theenergy gap with the tunneling current used during themeasurements.

3.3. Au(111)/mica substrates

Commercial Au ﬁlms (∼200 nm thick) deposited on mica
were used as substrates. The observation of atomically-ﬂat
sGNRs required atomically-ﬂat substrates. In the present
study, we used two methods: (1) the Au/mica was heated in
air by ﬂame for 5 s (we used the top of the orange-ﬂame,
C4H10 gas). (2) the Au/mica was sputtered by Ar+ (+1 kV,
10 min) and annealed (853 K) in the UHV preparationchamber. We repeated these cycles ﬁve times (too much
sputtering removes the entire Au ﬁlm). LEED located inside
the preparation chamber showed six-fold fcc-Au(111) spots.
STM topographic images showed atomically-ﬂat terraces.

3. Materials and methods
3.1. STM setup

All experimental studies were performed with home-built
UHV-STM equipment [29], which consists of introduction,
preparation, and analytical chambers. The scanning head ofthe equipment was located inside the analytical chamber andwas cooled down to low temperatures with a cryostat. We
achieved 78 K by ﬁlling liquid nitrogen (LN2) in the cryostat(4 liters of LN2 remained at 78 K for more than 80 h). W tips
were fabricated by chemical etching from a commercial
polycrystalline W wire (99.9% purity) with KOH aq. under
air. The etched W tip apices were quickly checked by SEM(Technex Tiny-SEM, ∼10−3 Pa). Sharp tips were subsequently introduced into the UHV-STM setup. In the preparation chamber, the tip apex was properly annealed byelectron bombardment (10–30 W, 10 s, once) to remove oxide
layers coating the apex [30], then without breaking UHV, thetip was set into the STM.

3.4. GNR preparation

First, the atomically-ﬂat Au(111) substrates were conﬁrmed
in our STM setup, and then the substrate was taken out of the
setup. GNRs were produced by unzipping the DWNTs inliquid [10–12, 15]. Under sonication (37 kHz, 600 W) for 2 h,more than 95% of the DWNTs were unzipped, producing
double-layer GNRs (∼20%), Y-shaped GNRs (∼30%), andsGNRs (∼45%) (see the supplementary information in ref.
[15]). Further sonication for more than 16 h split more than
99% of the double-layer GNRs and Y-shaped GNRs intosGNRs. We used this solution in this study. The sGNR liquid
was drop cast at the center of the substrate under air. The sizeof the substrate was 10×5 mm2 with a thickness of∼0.5 mm. The sample was then annealed up to 623 K for 3 h
in air to remove poly (m-phenylene vinylene-co-2,5-dioctoxyp-phenylene) (PmPV). Again, the sample was set into the
UHV setup, and annealed in the preparation chamber at 853 K
for 10–30 min. The sample was transferred to the analyticalchamber and we performed the STM/STS measurements.

3.2. STM spectroscopy

All STM topographic images were obtained in constant-current mode using a feed-back loop. With the use of STM
spectroscopy, the LDOS of each atomic position could bedirectly obtained [29, 31–33]. The tunneling current as afunction of the sample bias voltage (I–V curve) was measured
at each pixel position in the STM topographic image. The
differential conductance (dI/dV) was obtained by numericaldifferentiation of the I–V curve. Since the experimentallyobtained dI/dV is proportional to the sample LDOS [29,
31–34], the zero voltage corresponded to the Fermi energy
and dI/dV in the positive (negative) voltage range denoted theunoccupied (occupied) state. Conductance through the sample
could also be measured by approaching the W tip to thesample to make direct contact without plastic deformation of
the sample [23, 24]. During the approach-retract process, thecurrent was measured as a function of the tip-sample
separation (I–z curve). We set the constant voltage at +10 mV
during the I–z measurement. Typically, the tip detected thetunneling current increasing exponentially when the tip started to approach the sample. The current (and thus the conductance) was drastically increased by a factor of more than
10 when the tip made contact with the sample. Further
approaching the sample did not signiﬁcantly increase thecurrent. Here, we used Go=2e2/h∼1/12.9 kΩ [23, 24],
where e is electron charge, and h is Planck’s constant.

3.5. Theoretical calculations

As a theoretical model of the GNR, we adopted an armchair
ribbon with a width of N atomic rows of carbon, where bothedges were terminated by hydrogens to remove the effects ofthe dangling bonds (see the supplementary information, ﬁgureS3). The bond length was taken to be the experimental valueof graphite, 0.142 nm. Calculations were performed based on
the local spin density approximation using the full-potential
linearized augmented plane-wave method with a single slab
geometry [35–37]. Linearized augmented plane-wave functions with a cutoff of |k+G|<5.0 a.u.−1 and mufﬁn-tin (MT)sphere radii of 1.2 and 0.8 a.u., respectively, were used for the
C and H atoms, where the angular momentum expansions
inside the MT spheres were truncated at l, 6 and 4 for thewave functions, charge density, and potential.

4. Conclusion
In this study, single-layer metallic graphene nanoribbons
unzipped from double-walled carbon nanotubes with a sonication method were deposited on Au(111) by drop casting thesGNR solution in air with subsequent annealing (853 K) in
8

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UHV. In contrast to the bottom-up CVD process, the drop
cast sGNRs were buckled, i.e., some local points of the sGNR
were in contact with the substrate (d∼0.5 nm), but otherparts were further apart (d∼1–3 nm). The STS maps clearly
showed that single buckled sGNRs on Au(111) have metallic
properties (conductance of ∼3.5 Go) with considerable uniform LDOS, independent of topographic variations. On thecontrary, the areas where two sGNRs overlapped show auniform electronic structure with an energy gap between −50
and +200 meV around the Fermi energy, i.e., the electronic
properties of the sGNRs varied from metallic to p-typesemiconducting, which was not due to the bending, but theelectronic interactions between the up and down sGNRs.
These ﬁndings in our STM/STS studies of the crossedsGNRs could be useful with regard to future architecture ofcross bridging array structures in nanodevices using sGNRs.

Acknowledgments
This work was supported by JSPS KAKENHI Grant numbers
23681018, 25110011, 15H03531, 25110002, 25110013 and
16K05415, the Asahi Glass Foundation, and the Spanish
Ministry of Economy and Competitiveness through Grants
FIS2015-67367-C2-1-P and the Comunidad de Madrid
through Grant MAD2D-CM. We thank Dr L Gerhard, Dr E
Inami, and Mr N M K Nazriq for their careful reading of ourmanuscript, and Mr M Shimasaki for his support of theAu(111) substrate preparation. Computations were performedat the Research Institute for Information Technology, Kyushu
University, and Supercomputer Center, Institute for Solid
State Physics, University of Tokyo.

ORCID iDs
Toyo Kazu Yamada
5185-6472

https://orcid.org/0000-0001-

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