NANO
LETTERS

Reversible Defect Engineering ofSingle-Walled Carbon Nanotubes Using
Scanning Tunneling Microscopy

2007
Vol. 7, No. 123623-3627

Maxime Berthe,†,| Shoji Yoshida,† Yuta Ebine,† Ken Kanazawa,† Arifumi Okada,†
Atsushi Taninaka,† Osamu Takeuchi,† Nobuyuki Fukui,‡ Hisanori Shinohara,‡
Satoru Suzuki,§ Koji Sumitomo,§ Yoshihiro Kobayashi,§ Bruno Grandidier,|
Didier Stiévenard,| and Hidemi Shigekawa*,†
Institute of Applied Physics, Crest, UniVersity of Tsukuba, Tsukuba, 305-8573, Japan,
Department of Chemistry, Nagoya UniVersity, Nagoya, 464-86,2, Japan, NTT Basic
Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan, andInstitut d’Electronique, de Microélectronique, et de Nanotechnologie, IEMN (CNRS,UMR 8520), Département ISEN, 41 bd Vauban, 59046 Lille Cédex, France
Received July 28, 2007; Revised Manuscript Received October 15, 2007

ABSTRACTThe experimental creation and annihilation of defects on single-walled carbon nanotubes (SWCNT) with the tip of a scanning tunneling microscope
are reported. The technique used to manipulate the wall structure of a nanotube at the atomic scale consists of a voltage ramp applied atconstant tunneling current between the tip and the nanotube adsorbed on a gold substrate. While topographic images show an interference
pattern at the defect position, spatially resolved tunneling spectroscopy reveals the presence of localized states in the band gap of thenanotube. Removal of the defect by the same procedure demonstrates the reversibility of the process. Such a precise control in the local
modification of the nanotube wall opens up new opportunities to tailor SWCNT electronic properties at will.

Owing to their unique aspect ratio, single-walled carbon
nanotubes (SWCNTs) have attracted considerable attention
in the study of transport across one-dimensional systems.1
The way they are wrapped not only determines theirgeometric properties but also their electronic structure:
semiconductive or metallic. Such properties allow SWCNTs
to be used as functional devices like transistors.2 Recent
studies have focused on the engineering of SWCNT properties such as the exploitation of intramolecular junctions3 or
the modification of their band gap by the insertion of
impurities: ions4 or molecules.5 Atomic defects are also
investigated as good candidates for tailoring the electronic
properties of SWCNTs; for example, Lee et al. observed deepand shallow levels associated with vacancy-adatom andpentagon-heptagon defects,6 and Park et al. modulated theelectron transmission probability of a defect by an electricfield.7 Until now, several types of defects, like Stone-Wales
defects,8 vacancies,9 ad-dimers,10 or H-C complex11 could
be introduced on SWCNTs by electron or ion sputtering.9,11,12
But the incorporation of defects in the nanotube generally* Corresponding author. Website: http://dora.ims.tsukuba.ac.jp.
† University of Tsukuba.
‡ Nagoya University.
§ NTT Corporation.
| IEMN (CNRS, UMR 8520).
10.1021/nl071845c CCC: $37.00
Published on Web 11/23/2007© 2007 American Chemical Society

relies on macroscopic processes, which inherently involvethe creation of several defects in the same nanotube with alarge dispersion on their spatial distribution. Therefore no
precise control of the creation or even removal of defects ina single nanotube has been achieved so far.
Because of the nanometer size of SWCNTs, scanningtunneling microscopy (STM) is a quite suitable technique
for the investigation of the SWCNTs properties.13 It has been
widely used to study their electronic structure as well as
identify and characterize individual defects on their surface.14
More remarkably, it can also be used to manipulate SWCNT
by locally cutting the tube for example.15 But to date thistechnique has not yet been proved to tailor the properties of
SWCNTs without destroying them.
In this paper, we describe a method to selectively modify
the electronic properties of semiconductor SWCNTs by thecreation and destruction of point defects on their surface withan STM tip. The fabrication of the defects is detected fromthe measurements of the tip height variation when the sample
voltage is increased to a few volts maintaining the tunnelingcurrent constant. The investigation of the defect nature by
STM topographic images shows the formation of interference
patterns on the nanotube wall, and spatially tunnelingspectroscopic measurements reveal that this pattern is related

Figure 1. STM image of SWCNT deposited by spin coating on a
freshly cleaned Au(111) surface (sample voltage Vs ) -1 V,
tunneling current setpoint It ) 100 pA, T ) 4 K). Inset: Highresolution STM image of a smaller area partially covered withmolecular species from the solvent (Vs ) 1.0 V, It ) 100 pA, T )4 K).

to the presence of localized states in the band gap of thenanotube. Because the defects are removed by applying asimilar voltage ramp, the controlled formation of localizedstates in the band gap of the nanotube indicates that theatomic scale modification of its electronic structure is fully
reversible, providing new pathways to tailor SWCNT
electronic properties.
The experiments were conducted with an Omicron LTSTM operated at 77 and 4 K on SWCNTs deposited by spin
coating on freshly prepared and flame-annealed Au(111)
samples. We used electrochemically etched PtIr and W tips,
cleaned in ultrahigh vacuum (UHV) by heating and electron
bombardment. HiPCO16 SWCNTs dissolved in chloroform
and 1-2-dichloroethane were used at a concentration of 1
µg/10 mL. Then the solution was ultrasonicated for morethan 1 h to obtain individual nanotubes. After ultrasonication,
one drop was deposited by spin coating on the freshly cleaned
Au(111) sample. The samples were then introduced into
UHV and degassed before the experiments.
Figure 1 shows a large scale STM image obtained after
the deposition of SWCNTs on a Au(111) surface. A SWCNT
is clearly visible on the left of the image, running along theedge of an atomic terrace. We note that the terraces are
partially covered with molecules from the solvent. The
observation of single molecules self-aligned in the facecentered cubic region of the herringbone reconstruction in
the inset of Figure 1 reveals a ternary symmetry, suggesting
the adsorption of chloroform molecules of the solvent during
the deposition of SWCNTs.
After having identified the geometric properties of a CNT
from its wrapping angle and its energy gap (for semiconducting CNTs) or low-conductivity zone (for metallic CNTs)
based on established procedures,17 the STM tip was immobilized on a defect-free location on the SWCNT. The
creation of protrusions on the nanotube wall was thenperformed using the method described in Figure 2. A voltage
ramp was additionally applied on the sample voltage used
to image a SWCNT, as shown by the curve labeled Vs. The
typical sweep time was 0.1 s/V. During the voltage ramp,
the feedback loop was kept closed and as a result, the tipsample distance Z increased to keep the tunneling current It
3624

Figure 2. (a) Experimental scheme used to create a defect on a
SWCNT. The STM tip is first immobilized over a defect-free
position at the surface of a SWCNT. While maintaining the
feedback loop close (It constant), the sample voltage Vs is usuallyramped up to (8 V and both It and the tip height Z variation arerecorded. At the time of the defect creation or annihilation, a sharp
peak occurs on the tunneling current as well as a sudden shift of
the tip height, away from the surface for the creation (continuousline), closer to the surface for the annihilation (dotted line). (b)STM images of a SWCNT before and after the defect creation (top)or annihilation (bottom) along with a portion of the experimental
Z(V) curves showing a sudden shift of the tip height (vertical arrow),
signature of the defect creation or annihilation (Feedback parameters: Vs ) -0.4 V, It ) 200 pA, T ) 4 K).

constant. At certain bias, a sharp step appeared on Z
indicating an additional backward shift of the tip from thenanotube. This step is the response of the piezo controlling
the tip height to a sudden increase in the tunneling current
while the sample voltage increases. After the voltage sweep,
the tip started again scanning the nanotube. Figure 2b shows
the modification of the nanotube when a negative sample
voltage ramped is applied to a defect-free SWCNT. At a
voltage of -5.8 V, an abrupt change in the linear behaviorof the Z(V) curve occurs. After this event, a deformation on
Nano Lett., Vol. 7, No. 12, 2007

Figure 3. Examples of protrusions created by the tip and observedon the nanotube wall in topographic STM images. Protrusions are
divided in (a) type A protrusions associated with adsorbates (Vs )1.0 V, It ) 200 pA, and (b,c) type B protrusions associated withpoint defects. (Vs ) -0.5 V/-0.8 V, It ) 100 pA). Scale bar: 1
nm.

the nanotube wall was clearly visible at the position wherethe voltage ramp was performed.
Similarly, applying a voltage ramp at constant current on
a protrusion created by the tip leads to the annihilation of
the protrusion and restores the defect-free structure of thenanotube wall, as shown in Figure 2. We note that the voltage
threshold required to modify the nanotube varies and thatmultiple events can sometimes occur in a single voltage
ramp, indicating both the creation and removal reactions
during the same voltage ramp.
The protrusions, which are created by this method, exhibit
different shapes that are visible in Figure 3. They can bedivided in two categories: protrusions that show an unperturbed structure of the nanotube lattice surrounding the
protrusion (type A) and protrusions which involve strong
distortion of the nanotube lattice (type B). An example oftype A protrusion is seen in Figure 3a, and we associated itwith the adsorption of a molecule onto the wall of thenanotube.18 This molecule may diffuse from the Au(111)
surface to adsorb on the nanotube or may have been picked
up by the tip during scanning and is released on the nanotube
during the voltage ramp. In contrast, type B protrusions show
a lower contrast (see Supporting Information, Figure S4),
and the protrusions resemble those generally attributed topoint defects in the nanotube.6,14 They are found either on
top of the nanotube (Figure 3b) or on the sidewall (Figure3c). As the interference pattern observed for these protrusions
suggests an electronic origin with the expectation of a strong
modification of the local density of states at the position of
the defect, we focus the rest of the paper onto type B
protrusions.
Nano Lett., Vol. 7, No. 12, 2007

Figure 4. (a) From left to right: series of successive STM
topography images (5 × 16 nm2, Vs ) -0.4 V; It ) 200 pA) taken
at T ) 4 K at the same place on a (9,5) CNT before the creationof a defect, during the creation of the defect, with the created defect,
during the removal of the defect, and after the annihilation of thedefect. The scan direction for each image is from right to left. The
dashed lines and the arrows indicate, respectively, when and wherethe voltage ramp was applied during scanning. (b) Portion of Z(V)curves showing the characteristic step, indicated by a vertical arrow,and related to the local modification of the SWCNT. The upper
curve corresponds to the voltage ramp used for the creation of thedefect, and the lower one is for its annihilation.

Central to the work reported here is our ability to createand subsequently remove a type B protrusion. An example
is given in Figure 4, where a defect-free nanotube is firstimaged and the voltage ramp is then turned on (upper curvein Figure 4b). After the voltage is reset to its initial value, aprotrusion is visible in the middle of the nanotube. By
applying a second voltage ramp to the nanotube, theprotrusion disappears. Although the characteristic step in theZ(V) curve does not occur at the same voltage used for thecreation of the protrusion (lower curve in Figure 4b), thenanotube recovers its initial crystallographic structure, as
shown in the last STM image of Figure 4a. Interestingly,
the example in Figure 4 illustrates that the creation or
removal of a defect does not always occur at the position
3625

Figure 5. (a) dI/dV image acquired at Vs ) + 0.4 V in the regionof the defect shown in Figure 4. The image is rotated by 90° withrespect to the STM images of Figure 4. Feedback parameters Vs )-0.9 V, It ) 200 pA, T ) 4 K. Image size: 10 nm × 5 nm. (b)Spatial variation of dI/dV as a function of the sample voltage
acquired along the horizontal line in panel a. The four horizontal
dashed lines indicate characteristic energy positions in the dI/dV
curves: the upper and lower ones correspond to the band gap of
the nanotube and the line in between corresponding to the defectstates (areas 1, 2, and 3). (c) dI/dV curves corresponding respectively
to the four vertical dashed lines running through the defect states(I-III from left to right) and a defect-free region (IV, upmost to
the right) in panels a,b.

where the voltage sweep is applied, but a few nanometerapart. In that case, the annihilation of the defects might takeplace with a backward step of the Z piezo, instead of adecrease of the tip-sample distance as shown in Figure 2b.
Because the lattice distortion is likely to be caused by alocal modification of the nanotube electronic structure,spatially resolved spectroscopic measurements were performed on the nanotube shown in Figure 4 between thecreation and annihilation processes. Figure 5a shows thedifferential conductance imaged at 0.4 V in the area of thedefect, where an interference pattern is clearly seen wrapping
the nanotube, but spatially localized along the main axis of
the nanotube. To get a better insight into the local variationof the SWCNT electronic structure, the variations of thedifferential conductance are plotted as a function of thevoltage along a line running through the defect. As seen in
Figure 5b, while the band gap of the nanotube is measured
to be 1.13 eV in the defect-free region deduced from mostexternal horizontal dashed lines, the dI/dV profile measured
on the defect clearly shows intense signal in the band gap
region of the nanotube at energies of at -0.45 and +0.26
eV. Those peaks, visible in Figure 5c, are quite localizedalong the main axis of the nanotube and are consistent withthe position of the deep levels generally found for point
defects related to vacancy-adatom pair or adatom-adatom
complex in a nanotube.6,19,20 Although theoretical support isrequired to determine the precise nature of the defect created3626

Figure 6. Distribution of the voltage thresholds for (a) creationand (b) annihilation events.

on this (9,5) nanotube, the spectroscopic measurements
indicate that the creation of a type B protrusions causes a
significant modification of the nanotube electronic structure,spatially localized at the location of the defect.
We have observed that the type B protrusions are stablefor days if we do not apply a second voltage sweep to thenanotube and that it did not disappear due to thermal
fluctuations event at room temperature. To further understandthe creation and annihilation reaction discussed above, we
examined the bias-voltage dependence of the reaction rates.
For this purpose, the voltage ramps were applied on many
nanotubes with different STM tips. The sample voltage was
swept from (1.0 to (8.0 V with increasing absolute valueof Vs in 1.2 s. During the voltage ramp, the tip-sample
distance was feedback controlled with the reference tunnel
current at 1.0 nA. All experiments were performed at 77 K.
The histograms of the sample voltage thresholds at which
the creation or annihilation events occur are shown in Figure
6.
The distribution of the voltage thresholds for the creation
events does not mark a clear onset, although the creation ofa defect is lower for smaller voltages. Conversely, theprobability to produce an annihilation event is the highest
at small voltages indicating therefore that the energy barrierheight for the annihilation process is lower than the one forthe creation process. Futhermore, it is worth pointing outthat the measured voltage thresholds are smaller comparedwith the barrier energy for the removal of a carbon atomfrom a SWCNT, which was estimated to be 10 to 20 eV.21
Usually, for an accelerated electron to give this amount ofenergy to a CNT, electrons must be accelerated up to 100
keV.21 In contrast, the amount of energy needed to rotate a
C-C bond in a nanotube wall or to create a point defectinduced by a hydrogen ion is found to be compatible withthe range of voltage thresholds shown in Figure 6.22,23
Although we cannot rule out that hydrogen ions may be
produced by the voltage ramp due to their desorption from
Nano Lett., Vol. 7, No. 12, 2007

molecular species in the vicinity of the nanotube and theirfurther adsorption on the nanotube wall, the reproducibilityof the defect shape for several successive creation eventson the same nanotube favors the formation of defects related
to the rotation of C-C bonds (see Supporting Information).
Variability of the threshold voltages is also consistent withthe variation of the energy barrier to create or annihilate point
defects induced by different rotation of C-C bonds, whose
formation energy depends on the helicity of the nanotubes.24
In a conclusion, we propose a method to form point defects
on the wall of a nanotube based on the application of a
voltage ramp while the tunneling current is kept constant
between the STM tip and the nanotube. The defects show
localized states in the band gap of the nanotube, andspectroscopic studies as well as the analysis of the energetic
distribution for the defect formation suggest the creation of
defects induced by the rotation of C-C bonds, such as StoneWales defects. Remarkably, the defects are removed by thesame methods, allowing the nanotube wall to recover its
initial structure. Such a reversible manipulation of thenanotube wall should open the way to create confined regions
within SWCNTs.
Acknowledgment. This work was supported in part by
a Grant-in-Aid for Scientific research from the Ministry of
Education, Culture, Sports, Science, and Technology ofJapan.
Supporting Information Available: Figures showing
different sequences of creation and annihilation events. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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