pubs.acs.org/JPCL

LetterCarbon Monoxide Stripe Motion Driven by Correlated Lateral
Hopping in a 1.4 × 1.4 Monolayer Phase on Cu(111)Nana K. M. Nazriq, Peter Krüger, and Toyo Kazu Yamada*Cite This: J. Phys. Chem. Lett. 2020, 11, 1753−1761

Downloaded via CHIBA UNIV on February 18, 2020 at 23:15:36 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

ACCESS

Metrics & More

Read Online

Article Recommendations

sı Supporting Information*

ABSTRACT: We report an ultra-high-vacuum low-temperature (4.6
K) scanning tunneling microscopy study of the molecular structureand dynamics of a carbon monoxide (CO) monolayer adsorbed at 20
K on Cu(111). We observe the well-known 1.4 × 1.4 phase of CO/
Cu(111) for the ﬁrst time in real-space imaging. At 4.6 K, thehexagonal symmetry of the monolayer is locally broken by theformation of stripes made of single and double CO rows of diﬀerent
apparent heights. Using density functional theory calculations, weassign the high rows to CO molecules adsorbed mostly at oﬀ-center
top sites and the low rows to bridge sites. Groups of three or four very
high molecules appear randomly and are assigned to nearestneighbor, titled top site molecules. We observe simultaneous hoppingof a few CO molecules between adjacent top and bridge sites, which produces the apparent motion of the stripe pattern.
observed by LEED, including 1.5 × 1.5 R18 (Θ = 0.44)25 and1.4 × 1.4 (Θ = 0.51).18 In these cases, CO molecules adsorb
not only at top sites but also at bridge sites,19,28,29 but
adsorption at hollow sites is unlikely.26 Reﬂection adsorption
infrared spectroscopy (RAIRS) results obtained for the CO/
Cu(111) surface at approximately 15 K indicate that all CO
chemisorbs at top sites up to a coverage of Θ = 0.33, whilebridge sites also become occupied for higher coverages.27 In
the adsorption temperature regime of 7−77 K, the ratio ofbridge to top sites strongly increases with coverage such that at
Θ = 0.5 there are approximately as many bridge as top sites.27
Surprisingly, real-space evidence for the dense phases of CO/
Cu(111) is scarce. The only scanning tunneling microscopy(STM) results for coverages Θ > 0.4 were reported by Bartels
et al.,31 who observed a dense monolayer at 15 K with a p(4 ×4) periodicity. They proposed a model of four top and ﬁve
bridge site CO molecules in the 4 × 4 cell, corresponding to Θ
= 0.56 and an approximate 1.33 × 1.33 periodicity, which isclearly diﬀerent from those of the structures identiﬁed by
LEED. Considering the tremendous progress in STM imaging
over the past several decades and the role of CO/Cu(111) as aprototype system of surface science, it is most surprising thatthe molecular structure of well-established low-temperaturephases, especially 1.4 × 1.4, has not been revealed to date.

T

he adsorption of carbon monoxide (CO) has beenstudied extensively in the past several decades because a
precise understanding of the structure formation andadsorption site on metal surfaces is an important key forunveiling catalysis and molecular surface interaction.1−3 Morerecently, greatly diverse research on CO was conducted. Single
CO molecules adsorbed on noble metal substrates at cryogenic
temperatures under ultrahigh vacuum (UHV) conditions areused as a standard sample to demonstrate single-molecule
manipulation4,5 and also as a building block to produce
artiﬁcial graphene nanoribbon molecules6 or kagome lattices
with exotic electronic properties of a higher-order topologicalinsulator.7−10 CO adsorbed on diﬀerent substrates can form
monolayers with a variety of lattice structures.11−16 For theextensively studied CO/Cu(111) system,17−30 several apparently simple monolayer structures (such as 1.4 × 1.4) arereadily observed via low-energy electron diﬀraction (LEED)
but have never been conﬁrmed using a real-space imaging
technique. Here, we solve this puzzle that has lasted for
decades.
CO adsorbs preferentially at the top site of Cu(111). Kole etal. measured the potential energy surface of CO at low
coverage on Cu(111) at 170−210 K using helium spin−echo
spectroscopy.26 The top adsorption site was found to be themost stable, followed by the bridge site (38 meV), while thehollow site is a local maximum of the potential energy surface
and thus unstable. For CO/Cu(111), various monolayer
phases have been found.17−30 At an adsorption temperatureof 85 K, CO forms a (√3 × √3)R30 structure, with asaturation coverage of Θ = 0.33, where one of three top sites is
occupied.24,25,31 Below 80 K, several denser phases have been
© XXXX American Chemical Society

Received: December 10, 2019
Accepted: February 9, 2020
Published: February 9, 2020

1753

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

LetterFigure 1. UHV STM study on the 1.4 × 1.4 CO monolayer ﬁlm on Cu(111) at 4.6 K with a CO-terminated W tip. (A) STM topographic image (7nm × 8.6 nm; Vs = 40 mV; I = 0.73 nA). The inset represents an STM image corresponding to underlying fcc Cu(111) atoms. a⃗ and b⃗ denote a 5 ×
5 CO molecule unit cell of the 1.4 × 1.4 phase corresponding to a 7 × 7 cell of the Cu(111) substrate. (B) Enlarged STM images (2 nm × 1.9 nm)
taken from panel A. The stripes along a⃗ are lower CO rows (L) alternating with either single (S) or double (D) higher rows. (C) Height proﬁlealong the white arrow in panel A.

CO⃗⃗
Figure 2. (A) Fourier transform (FFT) image obtained from Figure 1A. Six symmetric spots are observed. Reciprocal lattice vectors kCO
a and kb
CO
CO
CO
⃗
⃗
⃗
⃗
correspond to 2π/(a⃗/5) and 2π/(b/5), respectively, and kc = kb − ka is shown. (B) FFT image of the clean Cu(111) substrate. (C) Three line
CO
CO
Cu
⃗
⃗
⃗
⃗
proﬁles along kCO
a (black), kb (blue), and kc (red) in panel A. The line proﬁle along ka obtained from panel B is also shown as a dotted blackline. Regions I−III denote the main peaks, submain peaks, and Γ point regions, respectively. (D) Inverse FFT images obtained from regions I−III
in the FFT image obtained on the CO surface.

Here we report the ﬁrst real-space observation of the 1.4 ×
1.4 phase of CO/Cu(111) using STM experiments performed
at 4.6 K under UHV. The CO monolayer was grown on an
atomically ﬂat and clean Cu(111) surface held at approximately 20 K. The STM analysis provides detailed insight into
the local arrangement of the CO molecules, which complements the lattice information obtained by LEED.19,28
Signiﬁcantly, we observe that the CO monolayer consists ofdense CO rows, made of higher single and double (S and D,
respectively) rows alternating with lower (L) rows. This leads
to a one-dimensional, quasi-periodic patterning of the STM
contrast, i.e., the formation of stripes with an average width of
2.5 CO rows as clearly established by Fourier analysis. From
the STM contrast and STM spectroscopy dI/dV measurements, the L rows are attributed to CO molecules adsorbed on
Cu(111) bridge sites and the S and D rows to molecules
1754

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

Letterprincipal directions ka⃗ (black line), kb⃗ (blue line), and kc⃗ (redline) as depicted in Figure 2C. Main peaks are located around
25 nm−1 for all three proﬁles, while stronger subdomain peaks
at 10 nm−1 are observed only in the ka⃗ direction. For bareCu(111), we plotted the data as dotted lines in Figure 2C. To
determine the lattice constant of the CO monolayer precisely
and remove any systematic error due to the drift andimperfection of experimental piezo calibration, we take theCO⃗⃗ (i = a−c) individually for ka⃗ (black), kb⃗
ratios Ri = kCu
i /ki(blue), and kc⃗ (red). We obtain an Ra of 1.42 (=35.0/24.6), an
Rb of 1.39 (=34.6/24.9), and an Rc of 1.41 (=36.0/25.5) withan error of ±0.02 (see Figure 2C). These results are clear
evidence that the observed CO monolayer has a 1.4 × 1.4
structure. To the best of our knowledge, the STM image in
Figure 1A is the ﬁrst real-space evidence of the 1.4 × 1.4 phase
discovered by Pritchard et al. using LEED.18
It should be noted that the ratio R = kC⃗ u/kC⃗ O is slightly
diﬀerent in the three directions; in particular, Ra/Rb = 1.02.
This means that the CO row distance in the ka⃗ direction (i.e.,the distance between CO rows running along b⃗ ) is ∼2% larger
than in the kb⃗ direction. While this diﬀerence is hardly above
the error bar, it is consistent with the fact that the stripe
formation breaks the 3-fold symmetry and indicates that the
CO molecules are a little more densely packed along thestripes than perpendicular to them.
Stripe Formation. The stripe pattern formation was further
investigated by analyzing the FFT line proﬁles (Figure 2C).
⃗
The line proﬁle along kCO
a displays an intense subpeak at 9.97
−1
± 0.13 nm . The wavenumber ratio with respect to the main
−1⃗
peak (kCOa = 24.60 ± 0.10 nm ) is 0.405 ± 0.006. This peak at⃗a corresponds to a periodic intensity modulation with a
0.4kCO
wavelength of 1/0.4 = 2.5 times the main wavelength λCO [COrow distance (see Figure 1A)]. Because the 0.4kC⃗ O subpeak isabsent or much weaker in the other two directions, we expect
one-dimensional patterning, i.e., the appearance of stripes⃗
perpendicular to kCO
a , in agreement with the real-space picture.
In Figure 1A, the distance between two L rows is either 2λCOor 3λCO, corresponding to the LSL or LDL sequence,
respectively. The occurrence of S and D is irregular; i.e., S
and D rows are not always alternating (as in ···LSLDLSLD···).
However, there are about as many D as S rows, and the average
distance between L rows is 2.5λCO, which explains the peak at
0.4 (=1/2.5) in the FFT line proﬁle (see Figure 2C).
Therefore, the dominant subpeaks at 0.4 correspond to(blue/green) stripe formation along the ka⃗ (or 2a⃗ + b⃗ )direction in the real-space STM image. Note that if the CO
overlayer was in a perfectly periodic, commensurate 1.4 × 1.4
superstructure on Cu, we would expect some modulation witha period of 5λCO [=7 λCu (see the inset in Figure 1A)] that⃗ in the FFT. The
would show up as a sharp peak at 0.2kCOaabsence of a such a peak shows that the CO row sequence isnonperiodic, in agreement with the real-space image.
Hence, through FFT analysis, we could understand that the
STM image in Figure 1A consists of three main components,
namely, region I (k ∼ 25 nm−1) (1.4 × 1.4 structure), region II(k ∼ 10 nm−1) (stripe rows), and region III (k ∼ 0 nm−1)(nonsymmetric patterns including the BS features). The
inverse FFT of regions I−III, shown in Figure 2D, provides
direct visual support for these assignments.
As mentioned above, the S and D rows are separated by L
rows and alternate in a nonperiodic but not completely
random way. The S/D sequence resembles the quasi-periodic“silver mean” sequence on Ag ﬁlms discussed by Smith etadsorbed on mostly oﬀ-center top sites. The higher D rows are
not uniform but contain clusters of three or four CO molecules[named bright spots (BS)] that appear to be signiﬁcantly
higher (+15 pm). By comparison with CO/Cu(100),32,33 theseclusters can be attributed to tilted CO molecules on nearestneighbor top sites. The stripe patterning is nonperiodic, and D
rows frequently evolve into S rows (and vice versa) along therow direction.
The time evolution of the CO monolayer is studied bytaking STM images at 1.5 min intervals. The connection point
between some single and double rows is found to move
between images. This leads to an apparent, one-dimensional
wave motion of the stripe pattern and is explained as acorrelated hopping of several CO molecules from bridge to top
sites in the direction perpendicular to the rows.
STM on the 1.4 × 1.4 CO Phase. We adsorbed CO on theCu(111) substrate at approximately 20 K and carefully
monitored the overlayer structure as a function of the amountof CO. At a CO dosage of 0.1 L, only randomly distributed
single molecules are found (see Figure S1A and ref 18). At adose of 0.5 L, the well-known (√3 × √3) R30° phase appears
and covers ∼50% of the surface, while the rest of the surface
shows disordered CO structures. A drastic change occurs at 1.0
L, when the whole surface is covered by a dense, ordered COmonolayer. Figure 1A shows an STM image of this monolayer,
obtained with a CO-functionalized W tip under UHV at 4.6 K.
Each bright spot corresponds to a single CO molecule. The
inset (bottom left corner) in Figure 1A shows the atomic
image of the underlying Cu(111) substrate. The lattice
constant ratio between CO and Cu is very nearly 7/5 = 1.4
along all three axes of the hexagonal lattice, such that the COlayer can clearly be identiﬁed as the 1.4 × 1.4 phase, which had
previously been observed only by LEED.18 The commensurate
1.4 × 1.4 phase corresponds to a supercell with unit vectors a⃗
and b⃗ shown in Figure 1A, along the [011] and [101]directions of fcc Cu, respectively. The supercell consists of 7 ×
7 Cu surface cells and contains 5 × 5 = 25 CO molecules. In
addition to the 1.4 × 1.4 phase in molecular resolution, theSTM image shows strong contrast modulations, in particular astripe patterning along b⃗ . This modulation is best observed
when looking at lower CO rows (blue color scale); we namethem L (for lower). Two neighboring L rows are separated byeither one or two higher CO rows. These regions are called S
and D for single and double higher rows, respectively (seeFigure 1B).
A height proﬁle along the white arrow in Figure 1A is shown in
Figure 1C. Clearly, the S and D rows are 15 pm higher thanthe L rows. Moreover, within the D rows, we frequently
observe signiﬁcant brighter spots (named “BS”) made of threeor rarely four CO molecules. The height proﬁle in Figure 1C
reveals that the BS spots are 15 pm higher than the D rows.
Approximately 10 BS features are shown in Figure 1A.
The periodicity of the CO monolayer was carefully analyzed
by taking the fast Fourier transform (FFT) of the STM
topographic images. Figure 2A shows the FFT image of Figure
1A. The six bright spots forming a hexagon correspond to the
1.4 × 1.4 CO lattice. On the basis of the direct hexagonallattice vectors a⃗ and b⃗ in Figure 1A, we deﬁne the reciprocal
CO
CO
⃗
⃗
⃗
lattice vectors kCO
a , kb , and kc in Figure 2A. The FFT imageof the bare Cu(111) surface (Figure 2B) also shows six
Cu
Cu
⃗
⃗
⃗
hexagonally symmetric spots (named kCu
a , kb , and kc ) in thesame directions. To see the details of the periodicity in the COmonolayer stripe patterns, line proﬁles were taken along the1755

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

LetterFigure 3. Time evolution of STM images at 4.6 K. (A) Successive STM topographic images obtained at the same area on the densely packed COmonolayer ﬁlm on Cu(111). Each image was obtained with a 1.5 min interval. Four bright spots (BS) are marked by red triangles. (B) Timedependent molecular hopping events. Six STM topographic images were obtained at 0.0, 1.5, 4.5, 6.0, 15, and 18 min. Corresponding FFT and
IFFT images obtained using regions I−III in the FFT images are shown. (C) Time dependence of the coverage percentages for the lower (L, blackdots) and higher (S, D, and BS, white dots) rows. The time dependence of the hopping probability (red circles) reveals approximately 9% of the
CO molecules on the surface hop, where N and ΔN denote the total numbers of CO molecules in the STM image and numbers of hopping CO
molecules in the STM image, respectively.

al.45−47 We ﬁnd that the S/D sequence is consistent with thesilver mean over a ﬁnite length of only ≤7 nm (see Figure S2
for details). While it is possible that the CO overlayer lattice
has a quasi-crystal-like behavior of the silver mean type, we did
not ﬁnd enough experimental evidence to support thisconclusion.
Correlated Molecular Hopping. We have studied the time
evolution of the CO monolayer by taking successive STM
images over a period of hours at a ﬁxed temperature of 4.6 K
under UHV. Interestingly, we found an apparent motion of the
stripe pattern on a time scale of minutes. Such dynamic motion
has not been reported before on any CO monolayer surface
phase.
Figure 3A shows subsequent STM images of the same
surface area taken at a time interval of 1.5 min, where four BS
features (1−4) marked by red triangles are observed. From 0.0
to 1.5 min, feature 3 moved upward along the row direction.
Then, from 1.5 to 4.5 min, feature 1 moved upward, andsimultaneously, feature 2 changed the three spots upside down;no change was observed between 4.5 and 6.0 min. This type of
CO molecular motion occurred continuously and randomly
from one scan to the other on a time scale of minutes. The
hopping events are clearly visible in the successive STM images[t = 0.0−43.5 min (see Figures S3 and S4)] and videos(Videos 1−5).
To further investigate the lateral hopping event on the COmonolayer surface, we carefully checked the STM images inthe same area and corresponding FFT and the inverse FFT(IFFT) images produced with the FFT spots located in regions
I−III. Figure 3B shows results at 0.0, 1.5, 4.5, 6.0, 15, and 18
min. We focus on the area marked by the circles in Figure 3B,
where one BS feature hops from left to right between 0.0 and
1.5 min. Then, this BS repeatedly appeared and disappearedfor 1.5−18 min. Between these events, the FFT, IFFT ofregion I (1.4 × 1.4 phase), and IFFT of region II (stripe
waves) remain essentially unchanged, which means that the BS
hopping dynamics does not aﬀect the 1.4 × 1.4 phase or the
stripe row periodicity. However, the IFFT of region III clearlyshows the hopping events. The relative number of CO
molecules in lower (L) and higher (S, D, and BS) rows isplotted in Figure 3C as a function of time. Although the
hopping events occur frequently and at each time step, some
5% of the CO molecules switch between lower and higher
sites, and the hopping does not change the statistical
distribution of the molecules, which is very stable over 40
min: 52.5% in lower rows and 47.5% in higher rows (the latterbeing composed of 28.5% S, 12% D, and 7% BS).
These observations suggest that CO hopping is a stochastic
process as expected for thermal or quantum ﬂuctuations. We
counted how many BS hopped every Δt = 90 s and normalized
the hopping BS number (ΔN) by the total numbers of BS in
the area (N). The hopping probability so obtained (ΔN/N) is
shown as red dots in Figure 3C, where the average indicates1756

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

LetterFigure 4. Models of the 1.4 × 1.4 CO phase on Cu(111). Black and white circles denote CO molecules and Cu atoms, respectively. (A) The left
panel shows a coincidence lattice, with COs on perfect 1.4 × 1.4 lattice sites. The right panel shows 13 CO molecules located on the bridge sites
and 12 CO molecules located on the oﬀ-center top sites. Yellow and blue regions denote higher and lower rows, respectively, reproducing theexperimentally observed L, S, D, and BS. The BS are marked by red triangles. (B) Only one and two COs shift from oﬀ-center top to bridge sites,
producing lower rows. Original CO positions are marked by dotted circles. (C) Models before and after the correlated hopping from top to bridgeand bridge to top, maintaining the 13:12 ratio. (D) Models reproducing the experimentally obtained STM images before and after the correlated
hopping.

8.9%. This yields a hopping rate = (ΔN/N)/Δt = 0.089/90 s−1∼ 1 mHz.
The hopping of CO on Cu(111) at temperatures of a few K
is a complex phenomenon that may be caused by thermal
diﬀusion, quantum mechanical tunneling, or tip eﬀects. If the
hopping were due to thermal diﬀusion alone, we could
estimate the potential barrier from the measured hopping rate
by Arrhenius’ law. With a typical value for the attempt
frequency of 1012−1013 s−1, we obtain a barrier of 14 ± 1 meV.
However, this is much lower than the value of ∼100 meVreported for single-CO diﬀusion at low coverage.26 A possible
reason for the discrepancy is that at the measurement
temperature of 4.6 K, CO hopping is partially due to quantum
tunneling. For small CO clusters on Cu(111), it was found thatquantum tunneling dominates below 6 K.4 Finally, the hopping
could be tip-induced. We did not see any evidence of strong tip
eﬀects such as a clear diﬀerence between STM images obtainedin forward or backward scan directions (see Figure S7), whiletiny scratches are observed, which may be tip-induced. The
forward and backward scans show identical BS features.
Nonetheless, the possibility that CO hopping is triggered
either by the electric ﬁeld between tip CO and surface CO34 orby vibrational heating through inelastic electron scattering48
cannot be fully ruled out.
Then the question arises arises why the 1.4 × 1.4 phase iseasily seen in LEED, even at much higher temperatures (20−70 K). Contrary to STM, which measures the absolute atomic

positions on a time scale much slower than that of molecularmotion (picosecond), LEED is a fast reciprocal-space probe
and thus sensitive only to the instantaneous position of ascattering atom relative to its neighbors (i.e., it measures thepair correlation function). Consequently, the LEED patternhardly changes when CO molecules hop from bridge to top
sites, as long as the statistical distribution of the distancevectors between neighboring molecules is preserved, which isthe case for collective CO hopping events observed in Figure 3.
Adsorption Models. We now discuss the molecular structureof the 1.4 × 1.4 phase of CO/Cu(111) in view of the models
suggested previously by Pritchard.18 Figure 4A shows theschematic model of the 1.4 × 1.4 phase. The super cell unit
contains 5 × 5 = 25 CO molecules and 7 × 7 = 49 Cu
substrate atoms (circles). Using unit vectors of the substrate
1 − 3⃗ = 1 , 3 , theand bCufcc(111) Cu lattice, aCu
⃗ = ,

(

2

2

)

(

2

2

)

supercell unit vectors can be described as a⃗ = 7a⃗Cu = 5a⃗CO andb⃗ = 7b⃗Cu = 5b⃗CO (see Figure S6 for notation clariﬁcation). CO
molecules adsorbed on oﬀ-center top (T) and bridge (B) sites
are shown in Figure 4 as red and black dots, respectively.
We start with the coincidence structure, i.e., a regular 1.4 ×
1.4 lattice of CO simply overlaid on the substrate lattice (seeFigure 4A, left panel). This model contains top sites, oﬀ-center
top sites, bridge sites, and a few hollow sites. A coincidence
lattice is expected for a completely ﬂat potential energy surface,
but this is not realistic. According to Pritchard18 and Raval et1757

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

LetterFigure 5. (A) Top view (center) of the calculated CO/Cu(111) structure. Oxygen (red), carbon (green), and surface layer copper atoms (brown)
are shown. The 7 × 7 unit cell (dashed line) contains 25 CO molecules. CO rows attributed to higher (S and D) lines are shaded yellow, and lower(L) rows are shaded blue. (B) Histogram of the lateral O−O distance in the calculated CO/Cu(111) structure. It is seen that the O−O distances
are narrowly distributed around 1.4dCu, corresponding to a perfect 1.4 × 1.4 superstructure.

al.,28 relaxation of the CO positions is crucial such that only
top and 2-fold bridge sites are occupied with as many top sites
as possible, because the top site is clearly preferred on Cu. The
model suggested by Pritchard has a 13:12 ratio of top:bridge
sites and is shown in Figure 4A (right panel). Note theappearance of groups of three or four molecules on adjacentoﬀ-center top sites (red dots). As seen in the right panel of
Figure 4A, there are rows with only bridge sites (L, shadedblue) and rows with a majority of top sites (S or D, shaded
yellow).
It was pointed out that the coincidence structure (Figure 4A,
left panel) and the relaxed structure (Figure 4A, right panel)would be indistinguishable in LEED,28 and to date, there has
been no direct experimental evidence for either model.
However, our STM data contain evidence favoring the relaxed
model. In Figure 1A, there are bright (“BS”) features of three(sometimes four) CO molecules, which we attribute to CO on
adjacent top sites. The apparent height of the BS molecules is∼15 pm larger than the other spots in single or double higher
rows (see Figure 1C). For CO/Cu(100) in the c(7√2 × √2)
phase, it was shown that CO molecules on nearest-neighbor
top sites appear much brighter than CO molecules on next
nearest-neighbor top sites.30 The diﬀerence in apparent height
was found to be 30 pm, in good agreement with the diﬀerenceof 15 pm observed here. CO molecules on the nearestneighbor Cu(001) top sites were shown to be strongly tilted,33
which reduces the Pauli repulsion. The enhanced apparent
height in STM images of the tilted molecules was attributed to
an increase in O pz and C pz orbital character of the statesaround the Fermi level, which is the dominant tunnelingchannel in the case of a CO tip. The distance between the next
nearest-neighbor sites on Cu(100) is √2dCu, which is only 1%larger than the distance between CO molecules in the 1.4 ×
1.4 phase in CO/Cu(111). Thus, the eﬀect of compression
and adjacent site occupation should be quite comparable
between the two systems. Our density functional theory(DFT) calculations (see Figure 5) show that also for CO/
Cu(111), CO molecules adsorbed on the nearest-neighbor top
sites are strongly tilted (by about 10°). Due to the triangularsymmetry of Cu(111), three such molecules naturally form aregular triangle, which explains the appearance of bright BSfeatures.
Note that our experimental results are compatible with the
hexagonal 1.4 × 1.4 model proposed by Pritchard, which also
features ﬁve diﬀerent rows per cell and clusters of three or four
top atoms. However, we clearly observe a small, local breakingof the 3-fold symmetry by stripe formation. This does notcontradict the observation of regular 1.4 × 1.4 LEED patterns,
because the hexagonal lattice distortion is very small and in a
LEED experiment the average over equivalent domains is seen.
An important new feature observed in this phase is thecorrelated switching between the top (T) and bridge (B) sites.
Due to the repulsive intermolecular interaction of the CO
overlayer, the rows where the CO molecules are closely
arranged together would be more likely to switch adsorption
site, which is the rows with top sites adjacent to each other.
In the left panel of Figure 4B, one of the top site CO marked
by the arrow shifts to the near bridge site, and a row of bridge
sites is formed (blue shade along the b⃗ direction),corresponding to the experimentally observed lower row.
This shift (from top to bridge) is portrayed by the dotted red
circle. The right panel in Figure 4B represents another hopping
model of two COs marked by arrows from top to bridge site.
These models give us important insights into the stripe
motion; namely, only one or two COs hopping between top
and bridge sites control stripe formation.
Figure 4C depicts the situation before and after thecorrelated hopping. Two COs shift from top to bridge sites,and subsequently, two neighboring COs shift from bridge to
top sites (see CO molecules marked by arrows). As aconsequence, the bright BS feature located at the center ofthe model has vanished. Also, the original SLDLS rows switch
to SLSLD rows. Collectively, the switching of adsorption sites
results in stripe motion, which is the shifting of a D row to an S
row.
Finally, Figure 4D shows models corresponding to the real
hopping occurrence in the STM images. BS features 1−4 canbe observed before the hopping, while feature 4 shifts the
position after the hopping. The corresponding models suggest
three COs shift from top to bridge sites and three COs shift
from bridge to top sites.
1758

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

Density Functional Theory Calculations. We have performed
DFT calculations on the 1.4 × 1.4 monolayer phase of CO/
Cu(111). Figure 5A shows the optimized structure of the 1.4 ×
1.4 phase. Among the 25 CO molecules in the unit cell, 19 areadsorbed on top (T), three on bridge (B), and three on hollow(H) sites. However, only one of the top site COs is straight(Ts) on Cu, while 18 COs are tilted (Tt), most of them in the
direction of a Cu−Cu bond, such that the x−y position of the
O atom is between the top and bridge site. We now attribute
the CO rows of the DFT calculation from top to bottom to theexperimentally observed rows L, S, and D. Row 1 (top row in
Figure 5, marked S) has the adsorption site sequence Ts Tt Tt
Tt Tt. It contains only top sites and is attributed to the higher
single row (S). Rows 3 (Tt Tt Tt Tt H) and 4 (Tt B Tt Tt Tt)
are two adjacent rows with 80% tilted top sites and can be
attributed to the higher double row (D). The remaining rows,
2 (Tt B Tt B Tt) and 5 (Tt Tt H Tt H), separate the single anddouble rows. They have the largest number (40%) of nontop
sites. Moreover, the three top sites in each row are verystrongly tilted such that the O atoms are closer to bridge sites
than top sites. Considering the fact that the bridge site energy
is overestimated in the present DFT+U scheme, we think thatthese three strongly tilted top site molecules might in reality beadsorbed on bridge sites. Therefore, we attribute rows 2 and 5
to the lower rows (L).
Figure 5B shows the statistical distribution of the lateral O−
O distances in the optimized structure. Importantly, all lateralO−O distances are very close to the 1.4 × Cu−Cu distance.
Thus, the O positions alone form an almost perfect 1.4 × 1.4
structure. The tilting of the CO molecules clearly indicates that
the O atoms of neighboring CO molecules repel each other. As
adsorption at a top site has the lowest energy, the system tries
to maximize the number of top sites, while keeping the O
atoms as far apart as possible. This leads to a regular lattice ofO positions and tilting of the CO top sites. It is important tonote that the computed structure should appear as a very
regular 1.4 × 1.4 superstructure in both STM and LEED,
because these experiments essentially probe only the topmost
atomic layer, i.e., the O positions.
In conclusion, the DFT simulations conﬁrm the 1.4 × 1.4
superstructure and are consistent with the observation of single
and double top site rows alternating with rows of mainly bridge
sites.
Finally, we have analyzed the dI/dV curves of CO molecules
in a lower and higher row (see Figure S5). We ﬁnd that all
lower row COs have a very similar dI/dV signal while
approximately half of the higher row COs have a distinctly
diﬀerent signal. This corroborates the models in Figure 4
where the lower rows are adsorbed at a unique site (bridge)
while the higher rows are a mixture of oﬀ-center top and bridge
sites.
In summary, we have achieved the ﬁrst real-spaceobservation of the 1.4 × 1.4 phase of CO/Cu(111) using
STM at 4.6 K. Locally, the hexagonal symmetry is broken by
the formation of stripes between CO rows of diﬀerent apparent
heights. We distinguish three types of adsorption states,
namely, lower (L) single CO rows, higher single (S) or double(D) rows, and bright spots (BS) made of three or four CO
molecules, which appear randomly within the D rows. On thebasis of DFT calculations, we assign the L rows to bridgeadsorption sites, the S and D rows to a mixture of top andbridge sites, with a majority of oﬀ-center top sites, and the BSfeatures to three nearest-neighbor, titled top site molecules. By

Letterstudying the time evolution of the STM images, we haveobserved an apparent motion of the stripe pattern that is
explained by the correlated hopping of one to three COmolecules between adjacent top and bridge sites.

■

METHODS
Home-Built Low-Temperature UHV STM Setup. The experiment was performed with a home-built low-temperature STM
instrument at 4.6 K under UHV. The setup consists ofintroduction, preparation, and analytical chambers with a basepressure of <10−8 Pa (see details in refs 34−37). A UHV
cryostat (CryoVac), including an outer liquid nitrogen tank (8L) and an inner helium tank (4 L), is docked on the analyticalchamber. The STM instrument was placed at the center of theanalytical chamber thermally contacting the cryostat. Heat
radiation from the analytical chamber to the STM instrument
was cut with two cylindrical cooling shields, which maintained
the STM temperature of 4.6 K for 40 h using 4 L of liquid
helium.
STM/STS Measurements. STM and scanning tunnelingspectroscopy (STS) measurements were performed with a
combined Nanonis SPM controller and software. The
topographic images were obtained with a constant current
mode. STS was performed by measuring tunneling current as afunction sample bias voltage [I(V)] at each pixel position in an
STM topographic image (feedback oﬀ grid mode). Diﬀerential
conductance (dI/dV) curves, which are proportional to thesample local density of states (LDOS), were obtained using
WSxM 5.0 Develop 9.0 software38 via numerical diﬀerentiationof I(V) by the sample bias voltage (V).
Fabrication of the CO-Functionalized W Tip. STM tips were
fabricated from polycrystalline W wires with a radius of 0.3
mm (99.95% pure) via electrochemical etching using aqueous
KOH and subsequently transferred into the UHV preparationchamber. Then, the tip apex was ﬂashed at 2000 K to exclude
oxide layers.39 The cleaned W tip was again transferred to the
UHV analytical chamber without breaking UHV and placed in
the STM instrument using a wobble stick. The W tip was
approached on the Cu(111), and then CO gas was dosed. We
prepared a CO-functionalized W tip by picking up an adsorbed
CO single molecule from the Cu(111) surface on the W tip
apex33,34 or direct adsorption of a CO gas molecule on the W
tip apex.
Cu(111) Preparation. The Cu(111) single crystal was
cleaned by repeated cycles of Ar+ sputtering (1.0 kV, 0.80μA) for 15−30 min with subsequent annealing (820 K) (seedetails in refs 34 and 37). The cleanliness of the substrate waschecked using LEED. The cleaned Cu(111) was cooled ﬁrst at
the 100 K cooling stage for 15 min and second at the STM
sample stage while the temperature of 4.6 K was maintained
for at least 3 h. This avoided unnecessary thermal drift during
approach and scanning. In most cases, immediately after thesurface had been cleaned, atomically ﬂat terraces (>100 nm)
were produced with an impurity concentration of <1%. dI/dV
curves were measured to check the Cu(111) surface state peakat −0.35 eV below the Fermi energy.
CO Molecule Adsorption at Cryogenic Temperatures. The
cleaned Cu(111) in the preparation chamber of the UHVSTM setup was subsequently transferred to the STM analyticalchamber through the gate valve without breaking UHV. Then,
we opened the two cooling shields cutting the radiation heatfrom the 300 K chambers and set the Cu(111) sample into the
STM sample stage maintained at 5 K. We closed the shield1759

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL

doors and waited 2−3 h to cool the sample to 5 K. We again
opened the shield doors shortly (which heats the sample stage
temperature from 5 to ∼20 K) and gently dosed the CO gas
molecules into the STM stage. CO gas was exposed to the
STM chamber in an amount of 1.0 L (Langmuir, 1.33 × 10−6
mbar s) from the CO gas cylinder (0.9 MPa) through avariable leak valve. Then, the CO/Cu(111) sample was again
allowed to cool, and after the temperature has stabilized at 4.6
K, STM measurement was carried out.
Density Functional Theory Calculations. Density functional
theory (DFT) calculations were performed using the VASP
code in the DFT+U scheme with the PBE exchangecorrelation functional. Following ref 40, we took U = 6 eV
for the 2p orbitals of C and O. A plane wave energy cutoﬀ of
400 eV and a 2 × 2 × 1 k-point mesh were used. The Cu(111)substrate was modeled with a four-atom layer slab in the 7 × 7
surface cell. The unit cell contains 196 Cu atoms and 25 CO
molecules. Slabs were separated by 15 Å vacuum. The lower
two Cu layers were frozen. The coordinates of all other atoms
were optimized until the forces were <0.1 eV/atom.
The CO/Cu system is a notorious case in which pure DFT
functionals fail to predict the correct (top) adsorption site.41
Correlation eﬀects beyond DFT need to be taken into account,by using conﬁguration interaction,42 the random phaseapproximation,43 or the computationally much less demanding
DFT+U approach.40,41 Following Eran et al.,40 we have chosenwith the PBE+U functional a U value of 6 eV for the 2p
orbtials of both C and O atoms. This functional predicts the
top site to be most stable with an adsorption energy of −0.5
eV, in good agreement with experiment. It is generally believedthat the hollow site is less stable than the bridge site,27,44 but
the experimental evidence is rather limited and indirect,
because hollow and bridge sites can be diﬃcult to distinguish,e.g., in RAIRS.27 Within the present PBE+U approach, we haveE(top) < E(hollow) < E(bridge); i.e., the order of bridge andhollow sites appears to be reversed as compared to experiment.

■■

LetterVideo 2 (MP4)Video 3 (MP4)Video 4 (MP4)Video 5 (MP4)

AUTHOR INFORMATION

Corresponding Author

Toyo Kazu Yamada − Department of Materials Science andMolecular Chirality Research Centre, Chiba University, Chiba
263-8522, Japan; orcid.org/0000-0001-5185-6472;
Email: toyoyamada@faculty.chiba-u.jp

Authors

Nana K. M. Nazriq − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan
Peter Krüger − Department of Materials Science and Molecular
Chirality Research Centre, Chiba University, Chiba 263-8522,
Japan; orcid.org/0000-0002-1247-9886

Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpclett.9b03645
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grants
19H05789, 17K19023 15K13357, 15H03531, 15H06092,
25110011, and 25600012 and the Murata Science Foundation.
The authors thank Prof. Wolf Widdra for a fruitful discussionof the quasi-periodic silver mean and Mr. Tetsushi Yamada forassistance at an early stage of the DFT calculations.

■

REFERENCES(1) Lan, J.; Hutter, J.; Iannuzzi, M. First-Principles Simulations of an
Aqueous CO/Pt(111) Interface. J. Phys. Chem. C 2018, 122, 24068−
24076.
(2) Zhang, K.; Li, L.; Shaikhutdinov, S.; Freund, H. J. Carbon
Monoxide Oxidation on Metal-Supported Monolayer Oxide Films:
Establishing Which Interface is Active. Angew. Chem., Int. Ed. 2018,57, 1261−1265.
(3) Liu, S.; Ishimoto, T.; Koyama, M. How Oxides Affect theStretching Modes of Carbon Monoxide Adsorbed on Ni Catalyst.
Appl. Surf. Sci. 2019, 478, 1074−1080.
(4) Heinrich, A. J.; Lutz, C. P.; Gupta, J. A.; Eigler, D. M. Moleculecascades. Science 2002, 298, 1381−7.
(5) Kuz'min, M. V.; Mittsev, M. A. Influence of CO Admolecules on
the Electronic State of Ytterbium Nanofilms Grown on Silicon
Substrates. Tech. Phys. 2019, 64, 1024−1028.
(6) Gomes, K. K.; Mar, W.; Ko, W.; Guinea, F.; Manoharan, H. C.
Designer Dirac Fermions and Topological Phases in Molecular
Graphene. Nature 2012, 483, 306−310.
(7) Kempkes, S. N.; Slot, M. R.; van den Broeke, J. J.; Capiod, P.;
Benalcazar, W. A.; Vanmaekelbergh, D.; Bercioux, D.; Swart, I.;
Morais Smith, C. Robust Zero-Energy Modes in an Electronic
Higher-Order Topological Insulator. Nat. Mater. 2019, 18, 1292−
1297.
(8) Drost, R.; Ojanen, T.; Harju, A.; Liljeroth, P. Topological States
in Engineered Atomic Lattices. Nat. Phys. 2017, 13, 668−671.
(9) Collins, L. C.; Witte, T. G.; Silverman, R.; Green, D. B.; Gomes,
K. K. Imaging Quasiperiodic Electronic States in a Synthetic Penrose
Tiling. Nat. Commun. 2017, 8, 15961.
(10) Slot, M. R.; Kempkes, S. N.; Knol, E. J.; Van Weerdenburg, W.
M. J.; Van Den Broeke, J. J.; Wegner, D.; Vanmaekelbergh, D.;
Khajetoorians, A. A.; Morais Smith, C.; Swart, I. P. Band Engineeringin Artificial Electronic Lattices. Phys. Rev. X 2019, 9, 011009.

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpclett.9b03645.
UHV STM topogprahic images obtained on theCu(111) surface after dosing 0.1 and 0.5 L of CO(Langmuir) (Figure S1), discussion of the Silver mean
sequence for the CO stripe row alignment (Figure S2),successive STM topographic images (t = 0.0−43.5 min,time interval of 1.5 min per image) obtained on the 1.4
× 1.4 phase of CO/Cu(111) using STM at 4.6 K(Figure S3), successive inverse FFT images (t = 0.0−
43.5 min, time interval of 1.5 min per image) using thesubmain peaks in the FFT images, showing clearly the
stripe pattern formation and dynamical wave motion,
obtained on the 1.4 × 1.4 phase of CO/Cu(111) using
STM at 4.6 K (Figure S4), STM spectroscopy dI/dV
results obtained on the stripe rows in the 1.4 × 1.4 phaseof CO/Cu(111) using STM at 4.6 K (Figure S5),
schematic diagram depicting the Cu lattice unit vectors
and the corresponding CO unit lattice (Figure S6), andSTM images on the CO 1.4 × 1.4 phase when the STM
tip scanned from left to right (forward) and right to left(backward) (Figure S7) (PDF)
Video 1 (MP4)1760

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761

The Journal of Physical Chemistry Letterspubs.acs.org/JPCL(11) Leon, C. C.; Liu, Q.; Ceyer, S. T. CO Adsorption on Gold
Nickel Au-Ni(111) Surface Alloys. J. Phys. Chem. C 2019, 123, 9041−
9058.
(12) Manzi, S.; Brites Helú, M. A.; Tysoe, W. T.; Calaza, F. C.
Combining IR Spectroscopy and Monte Carlo Simulations to Identify
CO Adsorption Sites on Bimetallic Alloys. J. Phys. Chem. C 2019, 123,
8406−8420.
(13) Gunasooriya, G. T. K. K.; Saeys, M. CO Adsorption on
Pt(111): From Isolated Molecules to Ordered High-Coverage
Structures. ACS Catal. 2018, 8, 10225−10233.
(14) López, Y. C. V.; Carrión, S. M.; López, M. B. Adsorption andOxidation of Carbon Monoxide in a Sheet of β-Graphyne: Theoretical
Study. Rev. Mater. 2018, 23, No. e12111.
(15) You, H.; Fain, S. C. Structure of Carbon Monoxide Monolayers
Physisorbed on Graphite. Surf. Sci. 1985, 151, 361−373.
(16) Wu, H.; Ren, P.; Zhao, P.; Gong, Z.; Wen, X.; Cui, Y.; Fu, Q.;
Bao, X. Dynamic Nanoscale Imaging of Enriched CO Adlayer onPt(111) Confined under h-BN Monolayer in Ambient Pressure
Atmospheres. Nano Res. 2019, 12, 85−90.
(17) Hollins, P.; Pritchard, J. Interactions of CO Molecules
Adsorbed on Cu(111). Surf. Sci. 1979, 89, 486−495.
(18) Pritchard, J. On the Structure of CO Adlayers on Cu(100) andCu(111). Surf. Sci. 1979, 79, 231−244.
(19) Szabó, A.; Kiskinova, M.; Yates, J. T. Carbon Monoxide−
Oxygen Interaction on the Pt(111) Surface: an Electron Stimulated
Desorption Ion Angular Distribution (ESDIAD) Study. J. Chem. Phys.
1989, 90, 4604−4612.
(20) Eve, J.; McCash, E. Low-Temperature Adsorption of CO on
Cu(111) Studied by RAIRS. Chem. Phys. Lett. 2002, 360, 202−208.
(21) Uvdal, P.; Karlsson, P.-A.; Nyberg, C.; Andersson, S.;
Richardson, N. V. On the Structure of Dense CO Overlayers. Surf.
Sci. 1988, 202, 167−182.
(22) Ishi, S.; Ohno, Y.; Viswanathan, B. An Overview on theElectronic and Vibrational Properties of Adsorbed CO. Surf. Sci.
1985, 161, 349−372.
(23) Eve, J.; McCash, E. Low-Temperature Adsorption of CO on
Cu(111) Studied by Reflection Absorption Infrared Spectroscopy.
Chem. Phys. Lett. 1999, 313, 575−581.
(24) Ishi, S.; Ohno, Y.; Viswanathan, B. An Overview on theElectronic and Vibrational Properties of Adsorbed CO. Surf. Sci.
1985, 161, 349−372.
(25) Hollins, P.; Pritchard, J. Interactions of CO Molecules
Adsorbed on Cu(111). Surf. Sci. 1979, 89, 486−495.
(26) Kole, P. R.; Hedgeland, H.; Jardine, A. P.; Allison, W.; Ellis, J.;
Alexandrowicz, G. Probing the Non-Pairwise Interactions between
CO Molecules Moving on a Cu(111) Surface. J. Phys.: Condens.
Matter 2012, 24, 104016.
(27) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M. An Infrared
Spectroscopic Study of CO on Cu(111): The Linear, Bridging andPhysisorbed Species. Surf. Sci. 1985, 155, 553−566.
(28) Raval, R.; Parker, S. F.; Pemble, M. E.; Hollins, P.; Pritchard, J.;
Chesters, M. A. FT-Rairs, Eels and Leed Studies of the Adsorption of
Carbon Monoxide on Cu(111). Surf. Sci. 1988, 203, 353−377.
(29) Tüshaus, M.; Berndt, W.; Conrad, H.; Bradshaw, A. M.;
Persson, B. Understanding the Structure of High Coverage CO
Adlayers. Appl. Phys. A: Solids Surf. 1990, 51, 91−98.
(30) Persson, B. N. J.; Tüshaus, M.; Bradshaw, A. M. On the Natureof Dense CO Adlayers. J. Chem. Phys. 1990, 92, 5034−5046.
(31) Bartels, L.; Meyer, G.; Rieder, K.-H. The Evolution of CO
Adsorption on Cu(111) as Studied with Bare and CO-Functionalized
Scanning Tunneling Tips. Surf. Sci. 1999, 432, L621−L626.
(32) Biberian, J. P.; Van Hove, M. A. A New Model for CO
Ordering at High Coverages on Low Index Metal Surfaces: aCorrelation between LEED, HREELS and IRS: I. CO Adsorbed on
fcc (100) Surfaces. Surf. Sci. 1982, 118, 443−464.
(33) Thamankar, R.; Meyerheim, H. L.; Ernst, A.; Ostanin, S.;
Maznichenko, I. V.; Soyka, E.; Mertig, I.; Kirschner, J. Tilting,
Bending, and Nonterminal Sites in CO/Cu (001). Phys. Rev. Lett.
2011, 106, 106101.

Letter(34) Nazriq, N. K. M.; Minamitani, E.; Yamada, T. K. CO-Tip
Manipulation Using Repulsive Interactions. Nanotechnology 2018, 29,495701.
(35) Inami, E.; Yamaguchi, M.; Yamaguchi, T.; Shimasaki, M.;
Yamada, T. K. Controlled Deposition Number of Organic Molecules
Using Quartz Crystal Microbalance Evaluated by Scanning TunnelingMicroscopy Single-Molecule-Counting. Anal. Chem. 2018, 90, 8954−
8959.
(36) Inami, E.; Shimasaki, M.; Yorimitsu, H.; Yamada, T. K. Room
Temperature Stable Film Formation of π-Conjugated Organic
Molecules on 3d Magnetic Substrate. Sci. Rep. 2018, 8, 353.
(37) Nemoto, R.; Kruger, P.; Putri Hartini, A. N.; Hosokai, T.;
Horie, M.; Kera, S.; Yamada, T. K. Well-Ordered Monolayer Growthof Crown-Ether Ring Molecules on Cu(111) in Ultra-High Vacuum:
An STM, UPS, and DFT Study. J. Phys. Chem. C 2019, 123, 18939−
18950.
(38) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero,J.; Gómez-Herrero, J.; Baro, A. M. WSXM?: A Software for Scanning
Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum.
2007, 78, 013705.
(39) Yamada, T. K.; Abe, T.; Nazriq, N. M. K.; Irisawa, T. ElectronBombarded <110>-Oriented Tungsten Tips for Stable TunnelingElectron Emission. Rev. Sci. Instrum. 2016, 87, 033703.
(40) Eren, B.; Zherebetskyy, D.; Patera, L. L.; Wu, C. H.; Bluhm, H.;
Africh, C.; Wang, L. W.; Somorjai, G. A.; Salmeron, M. Activation ofCu(111) Surface by Decomposition into Nanoclusters Driven by COAdsorption. Science 2016, 351, 475−478.
(41) Gajdoš, M.; Hafner, J. CO Adsorption on Cu(111) andCu(001) Surfaces: Improving Site Preference in DFT Salculations.
Surf. Sci. 2005, 590, 117−126.
(42) Sharifzadeh, S.; Huang, P.; Carter, E. Embedded Configuration
Interaction Description of CO on Cu(111):? Resolution of the Site
Preference Conundrum. J. Phys. Chem. C 2008, 112, 4649−4657.
(43) Ren, X.; Rinke, P.; Scheffler, M. Exploring the Random Phase
Approximation: Application to CO Adsorbed on Cu(111). Phys. Rev.
B: Condens. Matter Mater. Phys. 2009, 80, 045402.
(44) Kole, P. R.; Hedgeland, H.; Jardine, A. P.; Allison, W.; Ellis, J.;
Alexandrowicz, G. Probing the Non-Pairwise Interactions between
CO Molecules Moving on a Cu(111) Surface. J. Phys.: Condens.
Matter 2012, 24, 104016.
(45) Smith, A. R.; Chao, K.-J.; Niu, Q.; Shih, C.-K. Formation of
Atomically Flat Silver Films on GaAs with a Silver Mean Quasi
Periodicity. Science 1996, 273, 226−8.
(46) Ali, M. K.; Gumbs, G. Quasiperiodic Dynamics for a
Generalized Third-Order Fibonacci Series. Phys. Rev. B: Condens.
Matter Mater. Phys. 1988, 38, 7091−7093.
(47) Gumbs, G.; Ali, M. K. Dynamical Maps, Cantor Spectra, andLocalization for Fibonacci and Related Quasiperiodic Lattices. Phys.
Rev. Lett. 1988, 60, 1081−1084.
(48) Stroscio, J. A.; Celotta, R. J. Controlling the Dynamics of aSingle Atom in Lateral Atom Manipulation. Science 2004, 306, 242−
247.

1761

https://dx.doi.org/10.1021/acs.jpclett.9b03645
J. Phys. Chem. Lett. 2020, 11, 1753−1761