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CO-tip manipulation using repulsive interactions
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Nanotechnology
Nanotechnology 29 (2018) 495701 (10pp)

https://doi.org/10.1088/1361-6528/aae0df

CO-tip manipulation using repulsive
interactions
Nana K M Nazriq1, E Minamitani2 and T K Yamada1,3,4
1

Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Department of Materials Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan3
Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,
Japan
2

E-mail: toyoyamada@faculty.chiba-u.jp
Received 20 June 2018, revised 7 September 2018
Accepted for publication 12 September 2018
Published 4 October 2018
Abstract

Understanding the interactions between a tip apex and a target atom or molecule is crucial for the
manipulation of individual molecules with precise control by using scanning tunnelling
microscopy (STM) and atomic force microscopy. Herein, we demonstrate the manipulation oftarget CO molecules on a Cu(111) substrate using a CO-functionalized W tip with atomic-scaleaccuracy. All experiments were performed in a home-built ultra-high vacuum STM system at
5 K. The CO-tip was fabricated by picking up a single CO molecule from a Cu(111) surface. In
contrast to a metal tip, repulsive interactions occur between the CO-tip and the target CO
molecule. This repulsive interaction promises perfect lateral hopping without any vertical
hopping. Hopping events were directly monitored as sudden current drops in the simultaneously
measured I–z curves. A larger barrier height between the CO-tip and the target CO (∼9.5 eV)
was found from the slope of the I–z curve, which decreases the electron tunnelling probability
between the tip and sample. Therefore, electron-driven manipulation cannot be a major trigger
for the CO–CO repulsive manipulation. The CO-tip is able to manipulate only the target CO
molecule, even when another CO molecule was located ∼0.5 nm away. Statistical measurements
revealed that the nearest neighbour atop site is the energetically stable position after hopping.
However, if the CO target has another CO molecule in a neighbouring position (denoted as a‘pair’), the target CO hops more than twice as far. This means that the CO-tip experiences alarger repulsive interaction from the pair. These observations of CO-tip manipulation are usefulfor the design of two-dimensional artiﬁcial molecular networks as well as for developing a better
understanding of catalytic oxidation processes.
Supplementary material for this article is available online
Keywords: carbon mono oxide, manipulation, STM, molecule functionalized tip, repulsive
interaction(Some ﬁgures may appear in colour only in the online journal)
Introduction

have attracted signiﬁcant attention as a powerful tool fordeveloping next-generation 1 nm sized devices and forstudying fundamental single-atom-size physical phenomena.
Various substrates, such as metals, semiconductors, andinsulators, have been tested as a playground for the manipulation of single atoms or molecules, whereas metal tips havebeen mostly used for the manipulation. The interaction (either
attractive and/or repulsive) between a metal atom at the tip
apex and the adsorbed target leads to not only lateral hopping

Manipulation techniques using probe microscopy, namelyscanning tunnelling microscopy (STM) and atomic force
microscopy (AFM) can enable the fabrication of artiﬁcial
nanostructures on a substrate by assembling atoms andmolecules using in situ observations [1–4]. Such techniques
4

Author to whom any correspondence should be addressed.

0957-4484/18/495701+10$33.00

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N K M Nazriq et al

Results and discussion

on the substrates, but also vertical manipulation at the tip apex
[5, 6]. In addition to these simple interactions, molecularvibrations excited via electron injection from the probe tip
have also been utilised for molecular manipulation [1–3]. A
series of previous investigations on CO molecules adsorbed
on metal surfaces have served as a model system to reveal theelementary steps in catalytic oxidation [7–9]. More recently,
CO molecules adsorbed on metal surfaces have attracted
attention as building blocks for molecular logic gates or
artiﬁcial two-dimensional networks [2, 3, 10–12]. CO molecules have been not only the target of observation, but they
have also been used to functionalise probe tips [5, 13, 14].
CO-functionalised tips have attracted signiﬁcant attention
because they enable extremely high spatial resolution [15].
Such high-resolution imaging is possible with a CO-tip
because of the Pauli repulsion between the closed shellmolecule on the probe and the electronic charge of theadsorbed molecule being probed. In addition, it is expectedthat the force ﬁeld from the CO tip would be different fromthat reported in the case of a bare W tip [5, 13]. Accordingly,
molecular force interactions between the CO tip and a CO
molecule on the substrate have been investigated using AFM[14, 16–20], suggesting that the repulsive interaction tilts the
CO molecule axes standing perpendicular to the metal substrate and tip. This indicates that the force ﬁeld from the CO
tip can be applied for molecular manipulation; however, thispossibility has never been demonstrated.
In this article, we demonstrate CO-tip manipulation using
repulsive interaction between a CO molecule on a Cu(111)substrate and a CO-functionalised W tip. A home-built
ultrahigh vacuum (UHV) cryogenic temperature (5 K) STM
system was used. The gentle touching which results from therepulsive interaction between the CO-tip and the target CO
molecule precludes breaking or picking up the target molecule. In contrast to bare metal tips, this CO tip promises
perfect lateral hopping. Vertical hopping was never observed.
We repeated the approach for the CO-tip to the CO moleculeson the substrate and obtained statistical results showing thatthe closest atop site is the most energetically stable position
after hopping. The CO-tip manipulation enables to hop atarget molecule with atomic-scale accuracy. We directly
monitored the CO-tip lateral manipulation by simultaneously
measuring the I–z curve (tunnelling current as a function of
tip-sample separation). A current drop always occurred when
the target molecule hopped. From the slope of the I–z curve,
the CO-tip was found to have a markedly large barrier height.
The CO-tip manipulation was also performed for a CO-pair(two CO molecules sitting next each other at ∼0.5 nm
separation). The CO-tip successfully manipulated only one
CO molecule in the pair. Statistical measurements for the pairshowed that the hopping distance more than doubled, while
the hopping event occurred at a shorter CO-tip displacement.
The CO tip experiences a larger repulsive interaction from thepair than from the monomer.
CO-tip manipulation is useful for not only enhancing the
spatial resolution of STM/AFM, but also for fabricating a
two-dimensional CO molecular network or developing abetter understanding of catalytic oxidation processes.

All experiments were performed using a home-built UHV
low-temperature STM setup (see supplementary information,ﬁgure S6, available online at stacks.iop.org/NANO/29/
495701/mmedia). A CO-terminated tip was fabricated bypicking up a single CO molecule on a Cu(111) surface at 5 K.
An atomically-ﬂat and clean Cu(111) surface was preparedinside the preparation chamber by repeated Ar+ sputteringand annealing cycles (see supplementary information, ﬁgureS5). Without breaking UHV, the Cu(111) substrate was set
into the 5 K STM stage located inside the analytical UHV
chamber. CO molecules were adsorbed inside the STM. A
chemically etched W tip cleaned in the UHV preparationchamber via proper ﬂashing was used [21]. We successfully
manipulated CO molecules on Cu(111) using the W tips (seesupplementary information, ﬁgure S5). Then, we conﬁrmed
that, because of attractive interactions between the W tip apex
and the CO molecule, the W tip had a 50% chance of picking
up a target CO molecule, thus producing a CO-tip. Voltage
pulses have also been used to pick up CO molecules on a W
tip [6, 14, 18, 19, 22], but here, simply a use of the attractive
interaction is enough to produce a CO-tip. The single COmolecules on the metal substrate were observed in the STM
images as dark spots by using the metal W tip; however,the STM images obtained using the CO tip showed brightprotrusions with halo-like depressions (see supplementary
information, ﬁgure S1).
Figure 1 shows a demonstration of STM vertical manipulation of a single CO molecule on Cu(111). In ﬁgure 1(a),thirteen molecules (Nos. 1–13) can be observed as dark spots
when scanned with a clean W-tip. We recorded the STM image
in ﬁgure 1(b) by starting the scan of ﬁgure 1(a) from the top
with a clean W tip. When the W tips came to the molecule No.
2 position, the tip was made to approach the surface. Previous
AFM and density functional theory (DFT) studies showed that
attractive forces are dominant, which explains the jump of the
CO molecule from the Cu(111) substrate to the W tip apex
(vertical hopping) [16, 18, 19, 23–29]. Then, we restarted
the scan. Molecule No. 2 could no longer be seen, while theobserved contrast of molecules No. 3-13 changed. Now themolecules could be observed as bright protrusions with haloshaped depressions. The line proﬁles (shown in supplementary
information, ﬁgure S1) show that the protrusions have a heightof 8.0±2.0 pm and a diameter of 1.0±0.2 nm. Theseobservations provide clear indication that the tip now has a CO
molecule at its apex.
Because of the attractive interaction between CO andmetal surfaces, we can also return the CO molecule to theCu(111) substrate from the W tip apex. We approached the tip
near the lower left of molecule No. 7 and this caused molecule No. 2 (dotted circle) to be transferred back to theCu(111) surface, leaving the tip as bare W again. Figure 1(c)shows this result, where the image is the same as that inﬁgure 1(a) and the CO molecules can be observed as dark
spots. Sphere models of the vertical manipulation process areshown in ﬁgure 1(d).
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Figure 1. Fabrication of the CO-terminated W tip. (a) STM image scanned with a clean W tip. A total of 13 single molecules are observed as
dark spots (Vs=0.47 V, I=0.7 nA). (b), (c) STM images obtained from the same area as (a) (Vs=0.47 V, I=0.7 nA). (b) The W tip
picked up CO molecule No. 2; thus, the observed contrast changed for molecules No. 3-13. (c) Molecule No. 2 adsorbed on the W tip apex
was dropped back onto the substrate, so this STM image was obtained with a bare W tip again. (d) Sphere models depicting the verticalmanipulation process. (e) Total energy variation as a function of tip apex displacement obtained by DFT calculation. (f) I–z spectroscopy
curve (on log scale) during the approach. The red circles show the approaching direction of the W-tip and the black circles correspond to theretracting direction of the tip with CO attached at the apex. Insets show schematic diagrams during the approaching process. The decay rate(κ) was obtained by the least squares ﬁtting of I=I0 exp[−κz] to the experimentally obtained I–z data points: κ=2.220±0.013 Å−1 forapproaching and κ=2.286±0.010 Å−1 for retracting. (g) Measured barrier height between the CO-tip and the Cu(111) substrate. Work
function of Cu(111) (4.55 eV) and W(110) (4.80 eV) are shown [41]. EF and VL denote the Fermi energy and vacuum level, respectively.

centre of H2Pc. (see supplementary information, ﬁgures
S1(g), (h))
Figure 1(f) shows I–z curves during the pick-up process.
The primary vertical axis denotes the current detected by thetip (log scale) and the x-axis denotes the relative tip-sample
distance. The conductance was calculated using the relationship G=I/V in units of the quantum conductance factor,
Go=2e2/h. The initial position of the tip was set as z=0 pm

Such STM imaging with a CO-tip has also attracted
much attention as a method for visualising the atomic-scalechemical structure of a single organic molecule [6, 18]. We
tested the CO-tip for a single metal-free phthalocyanine
molecule (H2Pc) and observed extra structures which were
not possible to observe with a bare W tip, i.e., a W tip showed
only four bright spots, while the CO-tip showed two-foldsymmetry in the molecule, referring to two H atoms at the3Nanotechnology 29 (2018) 495701

N K M Nazriq et alshown in ﬁgures 2(a), (b) demonstrate that the CO tip is able
to repulsively manipulate surface molecules with atomic-scaleaccuracy. The interactions between the CO tip and the COmolecule on the substrate during this approaching process canbe investigated by simultaneous monitoring of the tunnellingcurrent (I) as a function of the tip apex displacement (z).
Figure 2(d) shows the obtained I–z curves.
When the tip started to move toward the target moleculefrom the initial position (z=0 Å), the tunnelling current
increased exponentially, I∝I0 exp[−κ(S0+z)], where κ, S0,
and z denote the decay rate, initial tip-sample separation, and tip
apex displacement, respectively (here plotted on a log scale).
During the exponential increases, both CO molecules on the tip
and substrate remain at their original positions. However,at approximately z=−1.60±0.15 Å, we observed a current
drop (∼40 nA down). Below z<−1.65 Å, the current
increased exponentially again. By comparing our data withprevious AFM results [14, 18, 19], both CO molecules on the
tip and substrate could start to tilt before hopping when facing
opposite to each other. It should be noted that the wavy noise in
the I–z curves increased compared to ﬁgure 1(f); this increased
noise could represent instability due to repulsive interactions
between the CO-tip and the CO single molecules on theCu(111) surface. Further moving the CO tip toward the COtarget enhanced the repulsive force. When the repulsive force
overcame the adsorption energy of the CO molecule on thesubstrate, lateral hopping occurred. The drop in the tunnellingcurrent in ﬁgure 2(d) is a sign of lateral CO hopping. Since theI–z curve was measured under feedback-off conditions, if themolecule below the tip was suddenly moved, the tunnellingcurrent decreased owing to an increase in the tip-sample
separation based on the CO molecule height (ΔS), i.e.,
I∝exp[−κ(S0+z)] changed to exp[−κ(S0+ΔS+z)]. Once
the CO molecule on the Cu(111) surface was moved laterally,
the CO-tip detected the tunnelling current from the substrate.
The decay rate was obtained from the slopes of the I–z curves̃(see ﬁgure 2(d)). The decay rate changed before (κ3.16
Å−1)
−1
̃
and after (κ2.47
Å ) the CO lateral manipulation, i.e., thebarrier height between the CO-tip and the CO molecule isf∼9.49 eV (see ﬁgure 2(e)): almost two times larger than thebarrier height between the CO-tip and the bare Cu(111) surface,
as shown in ﬁgure 1(g) (∼4.97 eV). Such a large barrier heightsuppresses the electron tunnelling probability between tip andsample. Thus, electron-driven manipulation cannot be a major
trigger for the CO-tip manipulation of the CO molecule.
Although the W tip can cause the CO molecule to hop laterally
on metal substrates by electron injection at ∼50 mV [31–33],
the CO-tip did not cause electron-driven hopping at 50 mV (seesupplementary information, ﬁgure S2). This could be due tofewer electrons being injected from the CO-tip.
The DFT calculations in ﬁgure 2(c) support the above
explanation of the experimental results shown in ﬁgure 2. The
total energy of the CO molecules on the tip and substrate wascalculated as a function of the distance between the tip andsubstrate. Previous AFM and DFT studies found three kindsof interactions between the CO-tip and CO molecules onsubstrates [15, 19, 20]: (1) short-range chemical interaction
including Pauli repulsive interaction [19], (2) attractive van derduring the tunnelling feedback. The substrate bias was set to+50 mV. The W-tip approached the CO molecule from theinitial position z=0 Å. During the approach, the current
increased exponentially owing to tunnelling electron transmission (red circles in ﬁgure 1(d); here shown in linear-log
scale). However, at z=−2.40 Å, the conductance abruptly
jumped from 200 to 600 nA. Then, the tip was retracted backto the original position (from z=−2.40 Å to z=0 Å) (black
circles in ﬁgure 1(f)). The I–z curves in ﬁgure 1(f) weresuccessfully ﬁtted to the function I=I0 exp[−κz] [30]. The
barrier height, f, between the tip and the sample was obtained
from the decay rate, κ =√(32π2mf)/h, where m and h denotethe electron mass and Planck’s constant, respectively. Thẽ
decay rates before and after the hopping were κ2.22
Å−1−1̃and κ2.29 Å , respectively. The obtained barrier height
between the W-tip and the CO/Cu(111) was 4.69 eV andthe barrier height between the CO-tip and the Cu(111) was
4.97 eV (see ﬁgure 1(g)). Since both systems are symmetric,
the barrier height was almost constant.
Further, the I–z measurements in ﬁgure 1(f) show theconductance through the CO single molecule to be
0.10±0.05 Go, which is in good agreement with previous
STM conductance results [19].
The total energy of the metal-tip and the CO molecule
adsorbed on Cu(111) was calculated as a function of the
distance between the tip and the substrate (see ﬁgure 1(e)).
Starting from the initial position (z=0), the total energy
decreased, indicating that attractive interactions dominate inthis system, producing a jump of the CO from the Cu(111)substrate to the W-tip apex even without an applied voltage
pulse.

CO-tip manipulation of CO-monomer
We fabricated a CO-tip by using the attractive interactions
between the W tip and the CO molecule as shown in ﬁgure 1.
Here, we demonstrated what happens when the CO tip
approaches a target CO molecule on the substrate. Figure 2(a)shows an image of two single CO molecules on Cu(111)
obtained by using the CO-tip. Hexagonal lines showing thefcc(111) atomic lattice, as obtained from an atomicallyresolved Cu(111) STM image (see the inset), are overlaid on
this image as a visual guide. CO molecules on Cu(111) arelocated at the energetically stable atop site positions.
Now, the CO tip was approached toward the target CO
molecule on Cu(111) (black circle in ﬁgure 2(a)). The tip
apex was moved ∼2 Å toward the molecule and then moved
back to the initial position. Subsequently, the same area wasscanned as that shown in ﬁgure 2(b). Again, two CO molecules were observed; however, it can be observed that theapproached target CO molecule hopped approximately twotimes the atomic distance (2d = 2a / 2 , where a denotes thelattice constant of fcc-Cu: 0.361 nm) along the [011] direction
to a bridge site.
We repeated the same approaching process toward thesame target molecule and obtained similar results. Only theapproached target molecule moved. Consecutive STM images
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Figure 2. CO-tip manipulation of a CO-monomer. (a), (b) STM topography images of two single CO molecules adsorbed on Cu(111)

obtained by the CO-tip (Vs=+500 mV, I=500 pA). The overlaid black lines portray the Cu(111) fcc lattice as a guide for the eyes byusing the Cu atomic image in the inset. In (a) the tip approached at the black circle position. In (b), the hopping CO is marked by a red circle.
(c) Theoretical model and DFT calculation of the total energy of a system including a CO-tip. The total energy as a function of tip-sample
distance shows the presence of repulsive interactions. Note that the total energy difference in this ﬁgure does not correspond to the energy
barrier of the hopping of CO molecules. (d) I–z spectroscopy curve. Blue-red squares were measured between (a) and (b). A lateral hopping
event occurred at the current drop position. The decay rate (κ) was obtained by the least squares ﬁtting of I=I0 exp[−κz] to theexperimentally obtained I–z data points: κ=3.157±0.024 Å−1 for approaching and κ=2.469±0.011 Å−1 for retracting. (e) Measured
barrier height between the CO-tip and the CO monomer on Cu(111). Work function of Cu(111) (4.55 eV) and W(110) (4.80 eV) are shown[41]. EF and VL denote the Fermi energy and vacuum level, respectively.

Waals (vdW) interaction at a larger tip-sample separation, and(3) the electrostatic dipole interaction by the CO molecules at alarger tip-sample separation [15, 19, 20]. Our DFT calculations
include short-range force and electrostatic potential contribution. But vdW interaction is not included. It is difﬁcult todistinguish the short-rang atomic force and the electrostatic
potential from this calculation results itself. In what follows,
we discuss which contribution is dominant in the CO molecule
hopping event from the tip-sample separation.
In the STM measurement, the tip-sample separation, S,
can be estimated from tunnelling resistance. The I–z curves inﬁgures 1–2 were measured at a typical setpoint of Vs =0.5 V
and I=0.5 nA (R=109 Ω). When both the tip and sample
are metal, R=109 Ω corresponds to S∼8 Å. The tip-sample
separation can be estimated as follows. Resistance at thequantum point contact is R=12.9 kΩ ∼104 Ω. At this point
contact, the tip-sample separation corresponds to distance
between the sample surface atom and the tip surface atom, i.e.
S∼3 Å. One order of magnitude increase in the resistanceroughly corresponds to an increase of the tip-sample separation by ∼1 Å (R∝exp [κS]), thus the tip-sample separation
at R=109 Ω is simply estimated as S ∼3+5=8 Å.
However, the contact resistance increased from 104 Ω to R
∼105 Ω by using the CO-tip and the Cu(111) surface (seeﬁgure 1(f)). Therefore, the tip-sample separation at the typical
setpoint decreased to S ∼3+4=7 Å. Further, for the COtip and the target CO molecule case, the resistance just before
the hopping was R ∼106 Ω (see ﬁgure 2(d)). If we assume thisis close to the contact, the tip-sample separation at the typical
setpoint decreased to S ∼3+3=6 Å. Since the hopping

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ﬁgure 3(c), where CO molecules are located at the surface
position (z=0). In the vacuum region (z>1 Å), thepotential is constant, but it decays rapidly below the surface(z<−0.5 Å). A dip can be observed exactly at the surface
CO position. The CO monomer (red line) and CO pair (blueline) have a potential dip of ΔE ∼0.4 eV and ΔE ∼0.7 eV,
respectively. The DFT calculation model is shown in supplementary information, ﬁgure S3. The larger ΔE of the pair
indicates that the CO in the pair has a larger dipole, which
could produce a large repulsive force against the CO-tip.
This interaction can be detected using the I–z curve (seeﬁgure 3(d)). The entire behaviour of the I–z curve is similar tothat of the CO monomer case in ﬁgure 2. During theapproach, ﬁrst (z=0 to −1.3 Å), the tunnelling current
̃
exponentially increased. But, at z−1.3
Å, a sudden drop in
the current (∼30 nA down) was observed. Here, the target CO
molecule below the CO-tip hopped laterally. Notably, thecurrent drop was observed at a shorter tip displacementcompared with the CO monomer case (the current drop at z∼−1.60 Å in ﬁgure 2(d)). This is strong experimental evidence that the CO-tip feels a stronger repulsive interaction for
the CO pair case.
Similar to the interaction between the CO-tip and the CO
monomer, shown in ﬁgure 3(d), before and after lateral̃hopping, the decay rate changed to κ3.13
Å−1 while−1̃approaching and κ2.53
Å while retracting. That is, thebarrier height between the CO-tip and the CO in the pair was∼9.34 eV (see ﬁgure 3(e)), while the barrier height between
the CO-tip and the CO molecule after hopping was ∼6.07 eV(∼1 eV higher than the barrier height between the CO-tip andthe bare Cu(111) surface). The larger barrier height decreases
the electron tunnelling probability. Therefore, CO manipulation by the CO-tip could be due to the repulsive force interaction rather than electron injection or excitation driven
hopping.

event occurred by approaching the CO-tip 160 pm to thetarget CO molecule, the hopping event occurred at S∼4.4 Å.
Although the short-range atomic force becomes dominant
when tip-sample separation <300 pm [19], at such a longrange regime, two interactions mainly exist: attractive vdW
interaction and repulsive electrostatic dipole interaction.
Thus, it is plausible that the long-range dipole–dipole repulsive interaction is the major trigger of the repulsive CO-tip
manipulation.
In the CO-tip and the CO molecule on metal substratecase, both CO molecules have the same polarity. Althoughseveral possible dipole directions have been discussed inprevious studies [15, 20, 34–36], these inconclusive arguments are beyond the scope of this study.
From the slope of the I–z curve during tip retraction(z=−2 Å to 0 Å) in ﬁgure 2(d), a barrier height of f∼5.81
eV was obtained (see ﬁgure 2(e)), which is ∼0.8 eV higher
than the barrier height of f∼4.97 eV between the CO-tip
and the bare Cu(111). As discussed above, the CO lateral
hopping event occurred at the tip-sample separation ofS∼4.4 Å. When the CO hopped ∼5 Å, the separation
between two CO molecules increased to S ∼6.7 Å. The
experimentally obtained larger barrier height even after the
hopping means that the CO-tip still experiences the repulsive
force via the long-range dipole–dipole interaction. This could
be also one evidence that the dipole–dipole repulsive interaction is the major trigger of the CO-tip manipulation.

CO-tip manipulation of CO-pair
The experimental and theoretical results in ﬁgure 2 tell us that
the CO-tip is able to push the target CO molecule on a metal
substrate with atomic-scale accuracy. Such a repulsive interaction could be due to dipole–dipole interactions between the
two CO molecules. This type of delicate manipulation was
further tested. As a target, we used the same CO molecule, but
now another CO molecule was located at a distance of ∼0.5
nm (=2d) away. If CO molecules on a Cu(111) surface have
dipoles and the local surface potential is modiﬁed, we may
see different behaviour when manipulating CO moleculeswith the CO-tip. Figure 3(a) shows four single CO molecules.
Two molecules sit next each other (separated by a distance of∼0.5 nm (=2d)). We refer to these two molecules as a ‘pair’.
The CO tip was brought to approach one CO in the pair; thisis marked by a black triangle in ﬁgure 3(a). After theapproach, we scanned the same area and obtained ﬁgure 3(b).
The target molecule moved to a nearby atop-site position witha hopping distance of 2d along the [110] direction. Notably,
only the target molecule moved, while the other three molecules were not moved even though one of the three molecules
was located only ∼0.5 nm away from the target molecule.
This is also strong evidence that the CO-tip can be used forhighly accurate manipulation. Moreover, owing to therepulsive interaction between the CO molecules in the pair,the target CO molecule did not hop towards the neighbouringmolecule. The stronger repulsive force for the pair was conﬁrmed using the calculated electrostatic potentials in

Statistical analysis
In this section, we comment on the statistics of the hopping
events. As observed in ﬁgures 2 and 3, the hopping distance,
direction, and adsorption sites were not uniform. The energetically stable hopping position was investigated by repeating I–z measurements to obtain a statistical distribution. The
hopping-distance histogram from when the CO-tip approached a CO monomer is shown in ﬁgure 4(a). Clearly, thenearest neighbour distance, x = 1d = a / 2 = 0.260 nm, isthe energetically stable position: 55% of hops ended at 1d,
21% at 2d, 13% at 3d, 5% at 4d, and 5% at 5d. This suggeststhat the CO-tip is capable of gentle manipulation at the atomic
level. Vertical hopping was never observed. The experimentally obtained two-dimensional scattering map in ﬁgure 4(b)shows no clear directional dependence. The pushed CO
monomers are located randomly. In ﬁgure 4(b), the centre
position was the original position before scattering. Adsorption sites after hopping were also plotted (see supplementary
information, ﬁgure S4(b)). Markedly, under this repulsive
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Figure 3. CO-tip manipulation of a CO-pair. (a), (b) STM topography images of four single CO molecules adsorbed on Cu(111) obtained by

the CO-tip (Vs=+500 mV, I=500 pA). The black lines denote the Cu(111) fcc lattices as a guide for the eyes. In (a) the tip approached at
the black triangular position. After the hopping event in (b), only the CO molecule marked by the black triangular moved to the red triangular
position. In (a) and (b), the CO molecule marked by the green triangle was not moved. (c) Calculated electrostatic potential for the CO
monomer (red) and the CO pair (blue) adsorbed on Cu(111). (d) I–z spectroscopy curve. Blue-red triangles were measured between (a) and(b). The lateral hopping event occurred at the current drop position. The decay rate (κ) was obtained by the least squares ﬁtting of I=I0 exp[−κz] to the experimentally obtained I–z data points: κ=3.133±0.026 Å−1 for approaching and κ=2.526±0.010 Å−1 for retracting.
(e) Measured barrier height between the CO-tip and the CO pair on Cu(111). Work function of Cu(111) (4.55 eV) and W(110) (4.80 eV) are
shown [41]. EF and VL denote the Fermi energy and vacuum level, respectively.

The majority of the distances were found to be between 2d
and 4d (23%–31%) instead of the nearest 1d (15%). The COmolecule in the pair prefers to hop more than two atomic
distances from the initial position. Therefore, the CO-tip
approaching the CO pair experienced a larger repulsive
pushing force due to the neighbouring CO molecule. The
scattering map is shown in ﬁgure 4(d). No clear directional
dependence is observed.
Further, this stronger repulsive force for the pair casecauses some randomness in the adsorption site after hopping.
CO monomers prefer to hop to the atop site (see supplementary information, ﬁgure S4(b)), but for the CO-pair,bridge and atop sites are comparable; therefore, no clear site
dependence is observed.
However, when the CO-tip approached the pair, manipulation was successful more than 90% of the time (see supplementary information, ﬁgure S4(a)). This experimental

manipulation, we observed that the atop site is the most
preferred adsorption site, followed by bridge, hollow-hcp, andhollow-fcc sites. In other words, the closest atop site isenergetically stable after hopping.
We note that the CO-tip successfully moved the target
CO molecule with 72% chance (see supplementary information, ﬁgure S4(a)). The 28% failure rate can probably be
attributed to the following factors. One is that the position
actually approached is slightly shifted from the exact centre
position of the CO molecule during tracking and targeting topoke the single CO molecule on the STM image. Anotherfactor could be the tilting of the CO molecule on the W
tip apex.
Statistical experiments clearly showed a difference
between the CO monomer and the CO pair cases. The CO-tip
was made to approach the target CO molecule in the pair. The
obtained hopping-distance histogram is shown in ﬁgure 4(c).
7Nanotechnology 29 (2018) 495701

N K M Nazriq et al

Figure 4. Statistical measurements obtained by repeating the lateral hopping events for (a), (b) CO-monomer and (c), (d) CO-pair cases. 1dcorresponds to a/ 2 , 0.26 nm, where a denotes the Cu lattice constant. (a), (c) Hop distribution as a function of the distance measured fromthe original position. (b), (d) Two-dimensional X–Y plots of the hopped CO. X and Y axes are parallel to the á 110ñ and á 211ñ directions,
respectively. The red dot shows the original position. The white dot shows the neighbouring CO in the pair.

the I–z curve. From the exponential slope in the I–z curve, alarge barrier height was measured between the CO-tip and thetarget CO molecule, which decreases the electron tunnellingprobability between the tip and sample; therefore, electrondriven manipulation cannot be a major trigger for CO–CO
repulsive manipulation. The CO on the tip apex was neverobserved to hop.
Statistical distributions of the lateral hopping distance ofthe target CO monomer after being approached by the CO-tip
showed that the energetically stable position is the nearest
neighbour atop site (∼0.26 nm =1d). These statistical measurements also found that the CO-tip feels a larger repulsive
interaction when the target CO molecule is part of a pair (twoCOs located next each other). In the pair case, the target CO
hopped more than twice the distance without any site
dependence.
This CO-tip repulsive manipulation technique can be
useful for fabricating artiﬁcial two-dimensional CO molecule
networks and CO molecular logic gates, or for achieving abetter understanding of catalytic oxidation processes.

evidence suggests stronger repulsive interactions for the COpair case, which is likely due to dipole–dipole interactions.

Summary
We demonstrated repulsive lateral manipulation of CO
molecules on a Cu(111) surface with atomic-scale accuracy
by using the interactions between a CO/W-tip and the target
CO molecule. Dipole–dipole interactions could be the key tothis phenomenon. The CO-tip is able to precisely manipulate
one target CO molecule without moving another CO locatedeven at a distance of ∼0.5 nm. Vertical hopping to the tip wasnever observed. All the STM experiments were performed in
UHV at cryogenic temperature.
The lateral hopping event was directly monitored bysimultaneously measuring the I–z curve. When the repulsive
interaction overcomes the adsorption energy of CO on
Cu(111), the target CO molecule was observed to hop laterally. This event was observed as a sudden current drop in

8

Nanotechnology 29 (2018) 495701

N K M Nazriq et al

Experimental methods

Theoretical calculations

Tip preparations

DFT calculations were performed using the Vienna ab initio
simulation package [38, 39] with the projected augmented
wave method [40]. The cutoff energy for the plane wave
expansion was set to 400 eV. The exchange and correlation
were described at the level of the generalised gradientapproximation. For the calculation including the STM tip, we
modelled the Cu(111) surface with the STM tip using a 10layer thickness slab model with a (2×2) supercell where an
additional Cu atom representing the STM tip was placed at
the bottom surface. The distance between the tip and surface
was controlled by changing the lattice constant of the z-axis.
The atomic positions were optimised until the forces on therespective atoms were less than 0.01 eV A−1. The Brillouinzone was sampled using a 7×7×1 Monkhorst–Packk-mesh. The atoms in the bottommost Cu layer and the
additional Cu atom were ﬁxed during the optimisation. For
the calculation of the electrostatic potential, the Cu(111)
surface was modelled with a ﬁve-layer thickness slab modelwith a (7×7) supercell. Owing to the large dimensions of
the supercell, the Brillouin zone was sampled with a single
k-point only at the Γ point. In all cases, we used the hollow
site as the stable adsorption site of CO.

STM tips were fabricated from polycrystalline W wires with aradius of 0.3 mm (purity: 99.95%) using an electrochemical
etching process, and subsequently transferred into UHV andcleaned using a tip-ﬂashing process and checked using a ﬁeldemission technique [21]. The CO tip was fabricated bypicking up a single CO molecule from the Cu(111) substrate
onto a bare W tip in UHV at 5 K. Note that, in our study, CO
molecules on the Cu(111) surface always shifted laterally, but
the CO single molecule adsorbed on the W tip apex never
moved, which was experimentally demonstrated as the COmolecules on Cu(111) were always observed as protrusions.
This suggests that the CO π-orbital bonds more strongly with
W 5d states than Cu 4 s states.

I–z curve

The variation of the tunnelling current (I) as a function of the
tip-apex displacement (z) was measured at a constant bias
voltage. The zero position of the displacement (z=0) was set
as the initial position of the tip during the approach. The
measured tip current was divided by the applied bias voltage,
Vs, to calculate the conductance. The measured conductance
was normalised by Go=2e2/h, where e is the electron massand h is Planck’s constant. The I–z method requires the
feedback to be turned off, and the tip was gradually moved
toward the target molecule precisely [37].

Acknowledgments
This work was supported by JSPS KAKENHI Grant Nos.
23681018, 25110011 and the Asahi Glass Foundation. We
thank to Dr Eiichi Inami for valuable discussions. Dr Naoka
Ohta provided support for experiments of H2Pc/Cu(111) with
the CO-tip.

Sample preparations

The Cu(111) surface was cleaned using repeated cycles of
Ar+ sputtering (+1.0 kV, +0.80 μA) for 15–30 min withsubsequent annealing (∼820 K). The cleanliness of the substrate was veriﬁed using low-energy electron diffraction.
After cleaning, the Cu(111) substrate was left to cool for
30 min, and thereafter placed on a pre-cooling stage (100 K)
for 15 min before insertion into the STM stage. The Cu(111)substrate was thereafter left to cool to 5 K on the STM stage
for at least 3 h. Before depositing CO molecules, the surface
was examined using STM and spectroscopy measurement(supplementary information, ﬁgure S5). In most cases,
immediately after cleaning the surface, terraces of >100 nm
were produced with impurity concentrations of <1% (seeﬁgures S5(a), (b), (d)). From differential conductance (dI/dV )spectra measurements, the surface state peak of Cu(111)
could be observed at −0.35 eV below the Fermi energy, EF(see ﬁgure S5(c)). After conﬁrming the state of the Cu(111)
surface, CO single molecules were adsorbed onto the surface.
The STM chamber was exposed to 0.1 L (1 L–1.33×10−6
mbar s) of CO gas (purity 99.9%) from a variable leak valve(see supplementary information, ﬁgure S6). During theexposure (100 s at 10−7 Pa), the shield covering the STM was
opened, which caused the substrate temperature to increaseslightly (∼10 K, base temperature 5 K).

ORCID iDs
T K Yamada

https://orcid.org/0000-0001-5185-6472

References[1] Komeda T, Kim Y, Kawai M, Persson B N J and Ueba H 2002
Lateral hopping of molecules induced by excitation ofinternal vibration mode Science 295 2055–8[2] Heinrich A J, Lutz C P, Gupta J A and Eigler D M 2002
Molecule cascades Science 298 1381–7[3] Gomes K K, Mar W, Ko W, Guinea F and Manoharan H C
2012 Designer Dirac fermions and topological phases inmolecular graphene Nature 483 306–10[4] Bartels L, Meyer G, Rieder K-H, Velic D, Knoesel E,
Hotzel A, Wolf M and Ertl G 1998 Dynamics of electroninduced manipulation of individual CO molecules on
Cu(111) Phys. Rev. Lett. 80 2004–7[5] Welker J and Giessibl F J 2012 Revealing the angularsymmetry of chemical bonds by atomic force microscopy
Science 336 444–9[6] Bartels L, Meyer G and Rieder K-H 1997 Controlled verticalmanipulation of single CO molecules with the scanningtunneling microscope: a route to chemical contrast Appl.
Phys. Lett. 71 2
9

Nanotechnology 29 (2018) 495701

N K M Nazriq et al[24] Lauhon L J and Ho W 1999 Single-molecule vibrational
spectroscopy and microscopy: CO on Cu(001) and Cu(110)Phys. Rev. B 60 R8525[25] Fielicke A, Gruene P, Meijer G and Rayner D M 2009 The
adsorption of CO on transition metal clusters: a case study ofcluster surface chemistry Surf. Sci. 603 1427–33[26] Zhang R, Hu Z, Li B and Yang J 2014 Efﬁcient method for fast
simulation of scanning tunneling microscopy with a tip
effect J. Phys. Chem. A 118 8953–9[27] Drakova D, Nedjalkova M and Doyen G 2006 Theory of tipdependent imaging of adsorbates in the STM: CO on
Cu(111) Int. J. Quantum Chem. 106 1419–31[28] Aizawa H and Tsuneyuki S 1997 First-principles study of CO
bondong to Pt(111): validity of the Blyholder model Surf.
Sci. 399 L364–70[29] Vitali L, Ohmann R, Kern K, Garcia-Lekue A, Frederiksen T,
Sanchez-Portal D and Arnau A 2010 Surveying molecularvibrations during the formation of metal-molecule
nanocontacts Nano Lett. 10 657[30] Yamada T K, Fujii J and Mizoguchi T 2001 STM, STS, andlocal work function study of Cs/p-GaAs (110) Surf. Sci. 479
33–42[31] Uvdal P, Karlsson P-A, Nyberg C, Andersson S andRichardson N V 1988 On the structure of dense CO
overlayers Surf. Sci. 202 167–82[32] Froitzheim H, Ibach H and Lehwald S 1977 Surface vibrationsof CO on W(100) Surf. Sci. 63 56–66[33] Held G, Schuler J, Sklarek W and Steinrück H-P 1998
Determination of adsorption sites of pure and coadsorbed
CO on Ni(111) by high resolution x-ray photoelectron
spectroscopy Surf. Sci. 398 154–71[34] Hapala P, Kichin G, Wagner C, Tautz S, Temirov R and
Jelinek P 2014 Mechanism of high-resolution STM/AFM
imaging with functionalized tips Phys. Rev. B 90
085421[35] Hapala P, Temirov R, Tautz S and Jelinek P 2014 Origin of
high-resolution IETS-STM images of organic moleculeswith functionalized tips Phys. Rev. Lett. 113 226101[36] Hoffman T, Pielmeier F and Giessibl F J 2014 Chemical andcrystallographic characterization of the tip apex in scanningprobe microscopy Phys. Rev. Lett. 112 066101[37] Kitaguchi Y, Habuka S, Okuyama H, Hatta S, Aruga T,
Frederiksen T, Paulsson M and Ueba H 2015 Controlledswitching of single-molecule junctions by mechanicalmotion of a phenyl ring Beil. J. Nanotechnol. 6 2088–95[38] Kresse G and Furthmüller J 1996 Efﬁcient iterative schemes
for ab initio total-energy calculations using plane-wave basis
set Phys. Rev. B 54 11169–86[39] Kresse G and Furthmüller J 1996 Efﬁciency of ab-initio totalenergy calculations for metals and semiconductors using aplane-wave basis set Comput. Mater. Sci. 6 15–50[40] Blöchl P E 1994 Projector augmented-wave method Phys. Rev.
B 50 17953–79[41] Miedema A R, Chatel P F and Boer F R 1980 Cohesion inalloys—fundamentals of semi-empirical model Physica
1000B 1–28

[7] Wintterlin J, Volkening S, Janssens T V W, Zambelli T and
Ertl G 1997 Atomic and macroscopic reaction rates of asurface-catalyzed reaction Science 278 1931
[8] Hahn J R and Ho W 2001 Oxidation of a single carbon
monoxide molecule manipulated and induced with ascanning tunneling microscope Phys. Rev. Lett. 87 166102[9] Bonn M, Funk S, Hess C, Denzler D N, Stampß C,
Schefßer M, Wolf M and Ertl G 1999 Phonon-versus
electron-mediated desorption and oxidation of CO on
Ru(0001) Science 285 1042
[10] Slot M R, Gardenier T S, Jacobse P H, van Miert G C P,
Kempkes S N, Zevenhuizen S J M, Smith C M,
Vanmaekelbergh D and Swart I 2017 Experimental
realization and characterization of an electronic Lieb latticeNat. Phys. 13 672[11] Drost R, Ojanen T, Harju A and Liljeroth P 2017 Topologicalstates in engineered atomic lattices Nat. Phys. 13 668[12] Collins L C, Witte T G, Silverman R, Green D B and
Gomes K K 2017 Imaging quasiperiodic electronic states ina synthetic Penrose tiling Nat. Commun. 8 15961[13] Emmrich M, Schneiderbauer M, Huber F, Weymouth A J,
Okabayashi N and Giessibl F J 2015 Force ﬁeld analysis
suggests a lowering of diffusion barriers in atomic
manipulation due to presence of STM tip Phys. Rev. Lett.
114 146101[14] Gross L, Mohn F, Moll N, Liljeroth P and Meyer G 2009 The
chemical structure of a molecule resolved by atomic force
microscopy Science 325 1110–4[15] Ellner M, Pavlicek N, Pou P, Schuler B, Moll N, Meyer G,
Gross L and Perez R 2016 The electric ﬁeld of CO tips andits relevance for atomic force microscopy Nano Lett. 16
1974–80[16] Moll N, Gross L, Mohn F, Curioni A and Meyer G 2010 The
mechanisms underlying the enhanced resolution of atomic
force microscopy with functionalized tips New J. Phys. 12125020[17] Sweetman A, Rashid M A, Jarvis S P, Dunn J L, Rahe P and
Moriarty P 2016 Visualizing the orientational dependence ofan intermolecular potential Nat. Commun. 7 10621[18] Weymouth A J, Hofmann T and Giessibl F J 2014 Quantifying
molecular stiffness and interaction with lateral force
microscopy Science 343 1120–2[19] Sun Z, Boneschanscher M P, Swart I, Vanmaekelbergh D and
Liljeroth P 2011 Quantitative atomic force microscopy withcarbon monoxide terminated tips Phys. Rev. Lett. 106 046104[20] Scwarz A, Kohler A, Grenz J and Weisendanger R 2014
Detecting the dipole moment of a single carbon monoxide
molecule Appl. Phys. Lett. 105 011606[21] Yamada T K, Abe T, Nazriq N K M and Irisawa T 2016
Electron-bombarded á 110ñ -oriented tungsten tips for stable
tunneling electron emission Rev. Sci. Instrum. 87 033703[22] Bartels L, Meyer G and Rieder K-H 1999 The evolution of CO
adsorption on Cu(111) as studied with bare and COfunctionalized scanning tunneling tips Surf. Sci. 432 621–6[23] Blyholder G 1964 Molecular orbital view of chemisorbed
carbon monoxide J. Phys. Chem. 68 2772

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