Article
Cite This: J. Phys. Chem. C 2019, 123, 18939−18950

pubs.acs.org/JPCC

Well-Ordered Monolayer Growth of Crown-Ether Ring Molecules on
Cu(111) in Ultra-High Vacuum: An STM, UPS, and DFT Study
Ryohei Nemoto,† Peter Krüger,†,‡ Ayu Novita Putri Hartini,† Takuya Hosokai,§ Masaki Horie,∥
Satoshi Kera,†,⊥ and Toyo Kazu Yamada*,†,‡†

Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan§
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1
Umezono, Tsukuba, Ibaraki 305-8568, Japan∥
Department of Chemical Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu, 30013, Taiwan⊥
Institute for Molecular Science, Myodaiji, Okazaki 444-8585 Japan

Downloaded via CHIBA UNIV on August 9, 2019 at 01:55:51 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information*

ABSTRACT: Crown-ether (CR) ring molecules are known as
host molecules for capturing guest species inside the ring. So far CR
molecular ﬁlms have only been grown by drop-casting a CR
solution on an inert substrate in air which oﬀers little control overthe molecular structure. Here we report the successful growth of awell-ordered CR molecular array on an atomically ﬂat and clean
noble metal Cu(111) substrate at 300 K in ultra-high vacuum(UHV), using 4,4′,5,5′-tetrabromodibenzo[18] crown-6 ether (BrCR). The adsorption, self-assembly, and electronic structures of BrCR were studied by means of UHV low-temperature scanningtunneling microscopy and spectroscopy, low electron energy
diﬀraction, and angle-resolved ultraviolet photoemission spectroscopy. We found that (1) the Br-CR ring, which is bent both in the crystal and gas phase, ﬂattens upon adsorption on Cu(111).
Density functional theory reveals that the two benzene groups of the molecule lie ﬂat on the surface such as to maximize thesubstrate-molecule interaction. (2) The moderate molecule−substrate interaction allows thermal diﬀusion of the Br-CR
molecules, resulting in the formation of self-assembled monolayer islands with 7 × 4 superstructure. (3) While the deposition of
0.05 ML Br-CR forms multidomain islands with disordered defects, a drastic improvement occurred at 0.25 ML, where only
atomically ﬂat single-domain islands were grown. This Br-CR ﬂat ring cavity array could become a template for designing novel
two-dimensional arrays of desired guest atoms, ions, or functionalized molecules.

1. INTRODUCTION
The growth of organic ﬁlms by molecular evaporation anddeposition on atomically clean substrates in ultrahigh vacuum(UHV) has been studied intensively in the last decades.1−16
UHV growth has several advantages with respect to chemical
growth methods such as drop-casting a molecular solution on
the substrate in air.17−20 First, contamination of the ﬁlm withsolvent residue, a common problem with solution-based
processes, is excluded in UHV. Second, the formation ofwell-ordered molecular arrays can be controlled more easily inmolecular beam epitaxy under UHV conditions, because of thecleanliness of the substrate and control of deposition rate andsubstrate temperature. As a consequence, molecular ﬁlms
grown in UHV contain much fewer defects due to impurities,
improving the molecular crystallinity, electronic properties andthus the device quality.21
Since atomically ﬂat and clean (impurity <1%) substratesurfaces are obtained in UHV, new novel molecularnanostructures can be designed using the atomic precise© 2019 American Chemical Society

control of the substrate surface crystalline structures. So far,
UHV growth of two-dimensional molecular arrays has mainly
focused on ﬂat plane π-conjugated organic molecules, such asphthalocyanine (Pc), porphyrin (TPP), pentacene, perylenetracarboxylic dianhydride (PTCDA), and tetracyanoquinodimethane (TCNQ).1−16 These rigid molecules are notdeformed or deconstructed during the sublimation and heating
to 400−600 K, which allows for controllable sublimation frommolecular powders in an aluminum or quartz crucible in UHV.
On atomically ﬂat noble metal substrates, such ﬂat, πconjugated molecules can self-assemble into well-ordered
arrays.
Crown-ether (CR) cyclic ring molecules are known22−33 fortheir ability to capture guest metal atoms, ions, or molecules
inside the ring. Therefore, CR molecular ﬁlms are very
Received: April 10, 2019
Revised: June 25, 2019
Published: July 15, 2019
18939

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

Figure 1. STM/LEED results obtained on bare Cu(111) surface and 0.05 ML Br-CR on Cu(111). (A) STM image obtained on a clean Cu(111)atomic terraces (100 × 70 nm2, Vs = −1.0 V, It = 300 pA). Line proﬁle along the arrow shows the step height of ∼210 pm in good agreement with
fcc-Cu(111) layer distance of 208 pm. (B) Atomically resolved STM image and (C) LEED spots (beam energy: 182.1 eV) obtained on the Cu
terrace. Cu atoms follow the fcc(111) hexagonal symmetry. (D−F) STM images obtained on the Cu(111) surface covered by a 0.05 ML Br-CR:(D) 60 × 60 nm2, Vs = +1.0 V, It = 20 pA, (E) 30 × 30 nm2, Vs = −0.7 V, It = 100 pA, (F) 20 × 20 nm2, Vs = −0.7 V, It = 100 pA. The inset in (D)denotes LEED spots obtained on the surface. Beam energy of 28.9 eV was used. White circles denote new 6-fold symmetry spots appeared after
0.05 ML Br-CR deposition. In (F), three domains were observed in one island. The stripe pattern in each domain follows the ⟨211⟩ direction. (G)Line proﬁles along the blue and red arrows in (F).

all previous reports on UHV measurements of CR molecularﬁlms, the ﬁlms were grown by drop-casting the CR solution on
an inert Au or highly oriented pyrolytic graphite (HOPG)
surface.26,31
Here we demonstrate the successful UHV growth of a ﬂatring array using a 4,4′,5,5′- tetrabromodibenzo[18] crown-6
ether (Br-CR) on an atomically ﬂat and clean Cu(111)substrate at 300 K. To the best of our knowledge, this is thepromising template materials for creating atomically controlled
networks of captured guest species, in view of low-dimensional
device applications.33−42
However, CR ring molecules are more delicate for UHV
growth than the aforementioned ﬂat π-conjugated molecules,
because the CR ring is soft and ﬂexible, i.e., it can easily be
deformed and possibly deconstructed during the sublimationand heating process involved in UHV growth. We note that in
18940

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C
ﬁrst demonstration of CR ﬁlm growth under UHV conditions.
Precise molecule crystalline structures were obtained using
atomically resolved UHV scanning tunneling microscopy(STM) and high-sensitive low-energy electron diﬀraction(LEED). Further, the electronic state of the array was analyzed
using scanning tunneling microscopy (STS) and angle-resolved
ultraviolet photoemission spectroscopy (UPS). Experimentallyobtained crystalline and electronic structures of the ring array
were successfully reproduced by density functional theory(DFT) calculations, and the energetically most stablemolecular structure was identiﬁed.
In this work, we show that (1) the Br-CR ring, which is bent
both in the crystal and gas phase, adopts a ﬂat conformation
upon adsorption on Cu(111). DFT reveals that the twobenzene groups of the molecule are parallel to the surface suchas to maximize the substrate-molecule interaction. (2) The
moderate molecule−substrate interaction allows thermal
diﬀusion of the Br-CR molecules, resulting in the formationof self-assembled monolayer islands with a 7 × 4 periodicity on
Cu(111). (3) While the deposition of 0.05 ML Br-CR forms
multidomain islands with disordered defects, a drastic
improvement occurs at 0.25 ML, where atomically ﬂat singledomain islands are obtained.

The crystal orientation of the Cu(111) substrate was obtainedby measuring the STM atomic image (see Figure 1B) and
LEED spots (see Figure 1C). Scanning tunneling spectroscopy(STS) showed the surface state peak at −0.35 eV and angleresolved UPS also showed the s-band surface state peak around−0.4 eV and d-band peak at −2.3 eV (see SI Figures S2 andS4).
2.4. STM and STS Measurements. A home-built STM
combined with the Nanonis SPM controller BP4 was used toobtain topographic images of sample surfaces in a constantcurrent mode. Since the tunneling current (I) detected by the
tip is exponentially proportional to the tip−sample separation(z) via exp[−8m(Φ ± eVs/2) z /ℏ], where m: electron mass,ℏ: Planck constant, Φ: apparent barrier height between tip andsample, and Vs: set point sample bias voltage, we can measure
the surface corrugations in the order of 1 pm.
STS can measure sample surface local density of states(LDOS). Tunneling current (I) as a function of sample bias
voltages (typically, from −2 V to +2 V) was measured under
feedback oﬀ condition (one single curve was measured within
50−200 ms) by ﬁxing the tip−sample separation (z = zc),
which was determined by the set point voltage (V = Vs) andthe current (I = Is). The obtained I(V) curves were numericallydiﬀerentiated, and diﬀerential conductance (dI/dV) curves
were obtained: dI/dV = ρ·exp[−8m(Φ ± eVs/2) z /ℏ], whereρ denotes sample surface LDOS. The obtained STM/STS data
were analyzed using WSxM 5.0 Develop 9.0 software45 andGwyddion 2.53.
2.5. UHV, ARUPS, and LEED Measurements. UPS andLEED measurements were conducted in the home-built UHV
apparatus (see SI Figures S1, S4, and S7). The angle-resolved
UPS (ARUPS) and angle-integrated UPS were performed at
295 K using an ultralow-background, high-sensitivity UPS
apparatus with a hemispherical electron energy analyzer (MBSA-1) and two monochromators (MBS-M-1) for two wavelength regions as setup in the SI Figure S1A. The ﬁrst one is for
HeIα (hν = 21.218 eV) and HeIIα (hν = 40.814 eV) radiation
sources by a high-density plasma lamp (MBS-L1) with Al ﬁlter.
The second one covers the low-photon energy range, and XeIα(hν = 8.437 eV) is used as a radiation source by a capillarydischarge lamp (Omicron HIS13) with a LiF single-crystal
ﬁlter.46
The Cu(111) was cleaned at the preparation chamber (<5.0
× 10−8 Pa), and Br-CR molecules were deposited in thedeposition chamber (<6.0 × 10−7 Pa). LEED with amultichannel plate (MCP) equipped in the preparationchamber was used to detect weak electron signal from themonolayer molecular ﬁlm (see Figure 2). To avoid damages tothe deposited molecular ﬁlm, we used 10−40 eV beam energy.
High-resolution spot-proﬁle analysis LEED (SPA-LEED) was
performed for the annealed ﬁlm (see SI Figure S7). UPS was
measured in the chamber No.1 (<1.0 × 10−8 Pa) in SI Figure
S1A. ARUPS spectra were measured with an acceptance angleof ±18° (see SI Figure S4). Angle-integrated UPS spectra wererecorded by a transmission (spatial) mode. The total
instrumental energy resolution of the measurements was set
to 30 meV. A bias of −5 V was applied to the sample in orderto detect the secondary cutoﬀ. The binding energy scale isreferred to the Fermi level (EF) measured on a metal substrate.
2.6. Br-CR Sublimation Control in UHV. We used two
diﬀerent UHV setups: one for ARUPS and LEED, and anotherone for low-temperature STM (see SI Figure S1). Therefore, it
was necessary to control precisely the amount of deposited

2. METHODS
2.1. Low-Temperature UHV STM Setup. STM/STS
measurements were performed by using a home-built
apparatus (see Supporting Information, SI Figure S1), which
consists of STM (No.1 in Figure S1B), preparation (No.2),
and deposition chambers (No.3). All chambers have a basepressure below 2.0 × 10−8 Pa. Tips and samples weretransferred between chambers using transfer rods without
breaking the UHV. Each chamber was separated by gate valves.
A UHV cryostat (CryoVAC) located on the STM chamber
was used to cool down the whole STM setup. We ﬁlled liquid
nitrogen for both inner and outer tanks in the cryostat. STM/
STS measurements were performed at 78 K in UHV. STM
probe tips and samples were introduced by opening only thedeposition chamber (No.3).
2.2. W Tip Probe. Tungsten tips were used as a probe. A
tungsten wire (diameter 0.3 mm, purity 99.95%) was
chemically etched with aqueous KOH solution (∼2 N) in
air, and subsequently rinsed by hot water and acetone. The
apex was monitored by scanning electron microscopy (10−2 Pa,
Tiny-SEM, Technex Co., Ltd.). Only sharp cone-shape tips
were introduced into the preparation chamber (No.2, see SI
Figure S1B). Proper ﬂashing treatments were performed in the
preparation chamber to remove the oxide ﬁlm coating on the
W tip apex.43 A clean and sharp W tip was set into the STM
head in the STM chamber (No.1).
2.3. Cu(111) Substrate. We used two Cu(111) single
crystals, one for UPS/LEED and another one for STM. BothCu(111) single crystals (diameter 6 mm, MatecK, 99.999%)
were carefully sputtered and annealed to obtain clean andatomically ﬂat surfaces (see Figure 1A−C and Supporting
Information, SI Figure S2).44 Cleaning parameters were Ar+
sputtering (+1.0 keV, 420 nA, Ulvac PHI ﬂoating type iongun) and annealing (∼820 K) for the STM setup, and Ar+
sputtering (+1.0 keV, 600 nA) and annealing (∼770 K) for the
UPS setup. During the sputtering, the pressure around thesample was maintained below 3.0 × 10−8 Pa by diﬀerential
pumping. Br-CR molecules were deposited on the clean andatomically ﬂat Cu(111) surface in the introduction chamber.
18941

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

2.7. Synthesis of 4,4′,5,5′-Tetrabromodibenzo[18]crown-6 Ether (Br-CR). Br-CR was prepared according to
modiﬁcation of the literature method.47 In a 500 mL threenecked ﬂask with a reﬂux condenser connected to a gas
absorption trap, a mixture of bromine (3.92 g, 24.5 mmol) andacetic acid (25 mL) was added to a chloroform solution (300mL) of dibenzo[18]crown-6 (2.00 g, 5.55 mmol) at room
temperature. The mixture was reﬂuxed for 12 h. After cooling,
the orange powder was ﬁltered and washed with diethyl ether(50 mL × 5 times) to give a white powder. This was puriﬁed
by recrystallization from methanol solution to aﬀord Br-CR as
white crystals (1.4 g, 50%). Anal. Calcd for C20H20Br4O6: C,
35.54; H, 2.98; Br, 47.28%. Found: C, 35.47; H, 2.96; Br,
47.10%. 1H NMR (400 MHz, CDCl3, r. t.): δ 3.96 (t, 8H,
CH2, J(HH) = 4.4 Hz), 4.11 (t, 8H, CH2, J(HH) = 4.4 Hz),7.05 (s, 4H, aromatic). FABMS: m/z = 676 [M]+. CCDC
1901496 contain the supplementary crystallographic data forthis paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: + 44 1223 336033.
2.8. DFT Calculations. Density functional theory (DFT)
calculations with the PBE exchange-correlation potential were
performed using the VASP code (see SI Figures S5 and S6).48
The plane-wave energy cutoﬀ was 400 eV and the k-point
spacing was about 0.2 Å −1 (e.g., 3 × 5 × 1 k-mesh for a 17.8 ×
8.8 × 25.0 Å unit cell). Dispersion corrections were included
using the Grimme DFT-D2 scheme.49 The C6 coeﬃcient of
Cu was reduced by 50% (to 5.4 J nm6 mol−1) to correct for thesystematic overestimation of molecule−metal adsorptionenergy in DFT-D2.50 With the chosen C6 value, the Cu bulk
lattice constant (3.605 Å) and adsorption energy of benzene/
Cu(111) (0.62 eV) are in excellent agreement with experiment(3.603 Å and 0.71 eV, respectively).51 The Cu(111) substrate
was modeled with a slab of four atomic layers separated by 17
Å of vacuum.52 The atomic structure was optimized until all
forces were below 0.01 eV/Å. The experimentally observed
ij a÷◊÷′yz
jj ÷◊÷ zz = 4 2 a ⃗ surface cell was used for the Br-CR/
jb′z
0 7 b⃗k {Cu(111) system, where a⃗ and b⃗ denote the unit vectors on theCu(111) substrate. STM pictures were simulated using the
Tersoﬀ-Hamann approximation with the experimental bias.
For the density of states (DOS) plot, the partial Cu-d DOS
was down-shifted by 0.5 eV to correct for the underestimationof the Cu-3d band binding energy in DFT.53

Figure 2. LEED results obtained on 0.25 ML Br-CR on Cu(111). (A,B) LEED spots obtained by illuminating beam energy of (A) 37.9 eVand (B) 48.9 eV. White circles denote new 6-fold symmetry spots
appeared after 0.25 ML Br-CR deposition. In the circle, three spots
are observed. (C) A unit cell obtained from the LEED spots in (A, B):a=⃗ 0.88 nm, b⃗ = 1.79 nm, θ = 90°. (D) Simulated LEED spots using
the unit cell in (C). Blue, green, and red dots correspond to eachdomain in Figure 1F. These spots well reproduced three spots in thecircles in (A, B).

molecules on the Cu(111) substrate at 300 K in both setups.
We used the same quartz SiO2 crucible and the same homebuilt molecular evaporator (thermal radiation heating) toobtain the same crucible temperature and the same pressure (1× 10−6 Pa, see SI Figure S1C,D). For the quartz crystal
microbalance (QCM), the same parameters (z-ratio = 1,
density = 1) were taken to operate QCM and set QCM at thesame distance from the crucible (110 mm). The quarts
crucible was radiatively heated by ﬂowing current through atungsten wire ﬁlament (diameter 0.3 mm). The crucible
temperature was monitored by using the almel-chromel
thermocouple contacting to the bottom of the crucible. The
Cu(111) substrate was put ∼110 mm above the crucible.
Before the deposition, we always checked the Br-CR
evaporation rate by setting a QCM at the same distanceposition. SI Figure S1E shows one example of the obtained
QCM data. We carefully increased the crucible temperature intime, and exactly above the crucible temperature of 84.0 °C(∼357 K), QCM detected the molecule deposition. From theslope in Figure S1E, the evaporation speed of 0.03 nm/min
and 0.08 nm/min were reproducibly obtained at the crucible
temperature of 87.6 °C (∼361 K) and 95.9 °C (∼369 K),
respectively. We used the former evaporation rate. Since the
QCM parameters of z-ratio and molecule density, theestimated deposition amounts as well as the adsorption
probability on the substrate were unknown, the molecule
thickness estimated by QCM was only used as an index.
Precise deposited molecular monolayer (ML) was checked by
STM.

(

)( )

3. RESULTS AND DISCUSSION
In this study, we examined the adsorption and self-assembly of
Br-CR ring molecules on an atomically ﬂat Cu(111) substrate.
To obtain a well-order molecular array, two conditions shouldbe met. (1) The molecules have a deﬁned adsorption
geometry. (2) The surface diﬀusion barriers are low enoughto enable rearrangement and self-assembly of the molecules. Asa ring molecule, 4,4′,5,5′-tetrabromodibenzo[18] crown-6
ether (Br-CR) was used (see Methods 2.6 and 2.7). The BrCR ring is known to be bent both in the crystal and gas phase(see SI Figure S1F). The Br-CR molecules were reproducibly
sublimated with a stable deposition rate of 0.03 nm/min at a
crucible temperature of 87.6 °C in UHV (see SI Figure S1E).
A choice of the substrate is crucial factor for the UHV onsurface growth since the energy balance between the18942

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

Figure 3. STM results obtained on 0.25 ML Br-CR on Cu(111). (A) STM topographic image (300 × 300 nm2, Vs = −0.7 V, It = 10 pA). 25% of
the Cu(111) surface was covered by the molecular islands. Here each island includes only one domain. (B, C) Enlarged images of the boxed areasin (A). From the stripe patterns domain I in (B) and domain II in (C) are identiﬁed. (D) STM image of the domain III island (30 × 30 nm2, Vs =−0.7 V, It = 50 pA). The inset in (D) denotes the Fourier transformed image obtained from the STM image in (D). Clearly, two symmetric spots
originated by the one-dimensional stripe pattern are observed. (E) Enlarged STM image obtained on the island in (D) (6 × 6 nm2, Vs = −0.7 V, It= 100 pA). The stripe patterns are marked by the black arrows. A unit cell obtained from the STM image in (E): a ⃗ = 0.90 nm, b ⃗ = 1.74 nm, θ =88.9° is shown. The line proﬁle along the green arrow is shown at the right-hand side panel. (F) STM image (300 × 70 nm2). Chains are observed.
A line proﬁle along the blue arrow is shown in the lower panel.

molecule−substrate interactions and the intermolecularinteractions controls the molecular array nanostructure. Here,

we used a Cu(111) surface. Indeed, the well-known inert
Au(111) or HOPG(0001) surface could be suitable as a18943

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

has an interval of ∼1.7 nm with ∼50 pm corrugation. Seven toeight stripe lines are included in one single domain.
Although molecules inside each domain follow in periodic
order, molecules around the domain boundaries are disturbed,
forming disordered areas. This imperfection in the island could
be the origin of the broadening of the LEED spots depicted in
Figure 1D. The bumps (∼100 pm higher than the island
surface) marked by the circle in Figure 1F were located at thedisordered domain boundary. The bump could be buckled BrCR molecule or a cluster due to the stress between domains.
Summarizing the results of low (0.05 ML) deposition at 300
K, we found that Br-CR on Cu(111) forms monolayer islands
via thermal diﬀusion. The monolayer shows three domains
oriented along the <211> equivalent directions. The island
surface is not uniform, but features some disorder androughness, and the domain size is small (2−5 nm). Theseﬁndings show the diﬃculty of growing a well-ordered ringarray using soft crown ether molecules. However, a drasticchange occurred when the amounts of deposited molecules
was increased to 0.25 ML as shown in the following.
3.2. Br-CR Films on Cu(111) at 0.25 ML Coverage.
Upon increasing the coverage of 0.25 ML, a drastic
improvement of the Br-CR molecule alignment in the island
was observed. Much sharper LEED spots are seen in Figure 2A(EB = 37.9 eV) and B (EB = 48.9 eV), in which 6-foldsymmetry spots marked by circles are identical to the blunterspots in Figure 1D, but here inside one circle, additional two
satellite spots are visible adjacent to the strong spot. A unit cellof the Br-CR molecules alignment was obtained by using the
LEED spots in Figure 2A and B: |a ⃗ ′| = 0.88 nm, |b ⃗ ′| = 1.79
nm, θ = 90.0° (see Figure 2C), i.e., we found the Br-CR
ordered array has a uniform periodic pattern of÷◊÷
jij a÷◊÷′zyz = 4 2 a ⃗ , where a⃗ and b⃗ denote the unit vectors
jj zz
0 7 b⃗kb′{
on the Cu(111) substrate. By rotating this rectangular unit cellby 60° and 120° around the (00) spot, we recovered theexperimentally obtained LEED spots as shown in Figure 2D,
where ﬁlled circles mean observable spots in Figure 2A and B.
Namely, the LEED spots tell us the existence of three domains
on 0.25 ML Br-CR surface.
Figure 3A shows an STM topographic image (300 × 300nm2) obtained on the Cu(111) surface deposited by 0.25 ML
Br-CR. Now the islands are much larger (>50 nm) compared
to Figure 1D. One remarkable point of these islands is that, at
0.25 ML, only one type of the stripe pattern is observed inside
one island (see Figure 3B−D), i.e., the islands are single
domains. Also, no bump exists in the island, i.e., the island has
a ﬂat uniform well-ordered structure.
Figure 3B−D shows STM topographic images of three
diﬀerent islands consisting of domain I, II, and III, where stripe
patterns parallel to [12̅1], [1̅1̅2], and [2̅11] directions,
respectively. The Fourier transformed image obtained fromthe STM image in Figure 3D clearly shows two symmetricspots originated from the one-dimensional stripe pattern (seethe inset in Figure 3D, marked by circles).
The regularity in the Br-CR molecule at 0.25 ML was
conﬁrmed by visualizing molecular structures inside the island.
Figure 3E shows a high-resolution STM image. Stripe patternson the island follow the [2̅11] direction, but now orderedbright spots are also observed. From the periodicity in the
STM image, we obtained the unit cell of |a ⃗ ′| = 0.90 nm, |b ⃗ ′|= 1.74 nm, θ = 88.9° (see Figure 3E). These values are in

substrate since these inert surfaces are energetically stable atroom temperature in air, liquid, and UHV. One disadvantage isa weak molecule−substrate interaction, i.e., the adsorbed
molecule property is similar to gas phase.54 However, the BrCR molecule we use here has the bent ring structure in the
crystal and gas phase. Our idea is to use the stronger
molecule−substrate interaction, which could ﬂatten the ringwhen the Br-CR is adsorbed on the substrate surface. The
noble metal Cu(111) surface, which has 3d and 4s bands in thevicinity of the Fermi energy, could provide stronger
interactions than the molecule-Au(111) interaction. Also, theCu(111) surface has no surface reconstruction, and thecleaning and annealing processes to obtain atomically ﬂan
clean (impurity <1%) are established.44
The good quality of the Cu(111) substrate can be seen on
the STM image in Figure 1. The terrace width is >50 nm andthe step height is ∼210 pm (Figure 1A), in agreement with thebulk (111) layer distance of 208 pm. On the enlarged image(Figure 1B) with atomic resolution, the ⟨211⟩ crystal axes are
identiﬁed, and the impurity concentration was found below
1%. The surface was also checked by LEED and 6-fold
fcc(111) spots were clearly observed (see Figure 1C).
3.1. Br-CR Films on Cu(111) at 0.05 ML Coverage.
First, we deposited only 0.05 ML of Br-CR at 300 K tounderstand how the molecules adsorb on Cu(111) andwhether they form islands via thermal diﬀusion. The electronicinteractions (charge transfer, dipole, or van der Waals)
between adsorbed molecules and the substrate govern theadsorption energy and diﬀusion barrier height. The subtle
balance between these energy terms determines diﬀusion rates
and directions and thereby the shapes of self-assembled
molecular islands.
Figure 1D shows an STM image obtained on the Cu(111)
surface after depositing 0.05 ML Br-CR (60 × 60 nm2). Several
atomic terraces from bottom to top, and molecular islands areobserved. Typical island sizes are 10 × 10−30 × 30 nm2 with aconstant height of ∼240 pm. The growth of the monolayerheight islands implies that the adsorbed Br-CR molecules candiﬀuse on the Cu substrate at 300 K, which suggests low
diﬀusion barriers and weak molecule substrate bonding. Also,
one might note that one edge of the islands always, even at
diﬀerent sizes, follows the Cu(111) monatomic step, whichindicates that the island growth occurs from the descending or
ascending step. When single Br-CR molecules are adsorbed on
the terrace, they immediately diﬀuse. Since the islands werealways located at the step edge, the step could have a larger
potential barrier enough to generate the nucleation. This is thewell-known Ehrlich-Schwoebel barrier for descending a stepedge.55 The LEED image obtained on the same surface (seethe inset in Figure 1D) shows that sharp 6-fold LEED spots in
Figure 1C which originated by the fcc-Cu(111) symmetry
disappeared after deposition of 0.05 ML of Br-CR, while newbright spots marked by the circles can be observed.
We focus on one island to see the details of the molecular
structure inside the island (see Figure 1E). We found some
stripe patterns and several bumps in the island, i.e., at 0.05 ML,
the island surface is not continuously ﬂat and ordered.
Upon closer inspection, we found three domains (see Figure1F). Domain I, II, and III have stripe patterns parallel to [12̅1],[1̅1̅2], and [2̅11] directions. Line proﬁles along the blue andred arrows in Figure 1F are shown in Figure 1G. The Br-CR
islands have a constant height of ∼240 pm. The stripe pattern

(

18944

)( )

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

Figure 4. UPS and STS measurements for Br-CR on Cu(111). (A, B) Angle-resolved UPS results obtained on Cu(111) surface (A) before and (B)after the 0.25 ML Br-CR deposition. He Iα 21.2 eV light was used and the angle is measured from the surface normal. (C) Angle-integrated UPS
results obtained at Br-CR coverages of 0.00, 0.01, 0.05, and 0.25 ML on Cu(111) with Xe 8.43 eV light. WF denotes work function. A new HOMO
peak is appeared at −1.48 eV. (D) STS dI/dV curve obtained on the surface of 0.25 ML Br-CR on Cu(111). A HOMO peak is observed at −1.54
eV. The inset shows a dI/dV curve obtained on a bare Cu(111). The surface state peak at −0.4 eV is observed.

ij a÷◊÷′yz
jj ÷◊÷ zz = 4 2 a ⃗ , where a⃗ and b⃗ denote the unit vectors
jb′z
0 7 b⃗k {on the Cu(111) surface.
In order to better understand the electronic coupling
between the adsorbed molecules and the substrate as well as
between the molecules in the monolayer, we have performed insitu photoemission measurements. In particular, we checked
whether the periodic arrangement of the molecules leads toband dispersion of the molecular levels and their position withrespect to substrate bands.
Figure 4A shows angle resolved UPS maps of the bareCu(111) surface measured with He Iα light (21.2 eV). The
binding energy at 0 eV corresponds to the Fermi energy.
Cu 3d-bands around −2.5 eV and the parabolic band
originated from the 4s surface state are clearly observed. We
also checked with He IIα light (40.8 eV) since the molecularstates may appear clearly using more than He Iα laser. We
measured UPS with not only the angles between ±15° but also
from 5° to 35° to see sp band (see Figure S4B).
The UPS map changes signiﬁcantly upon deposition of 0.25
ML Br-CR (Figure 4B). We measured ARUPS using both He
Iα and He IIα light (see Figures 4B and SI S4C for He Iα andSI S4D for He IIα). In particular, the surface state hasdisappeared and a slight increase of the intensity around −1.5

agreement with those obtained by LEED in Figure 2 (|a ⃗ ′| =0.88 nm, |b ⃗ ′| = 1.79 nm, θ = 90.0°), where the same structure
was obtained for the annealed monolayer ﬁlm conﬁrmed by
SPA-LEED (Figure S7). In Figure 3E, one unit cell includes
four bright protrusions, indicating that a single Br-CR molecule
has four buckled positions. The ﬂatness of the Br-CR array waschecked by the line proﬁle along the green arrow in Figure 3E,
which shows that the stripe dips have a depth of ∼50 pm andthe protrusions have a height of ∼40 pm. Namely, one Br-CR
molecule has a corrugation within 40 pm. The line proﬁleacross the 80 nm-size island in Figure 3F clearly shows theﬂatness of the Br-CR array.
We note that apart from the monolayer islands, onedimensional features are frequently observed on the terraces(see Figure 3F and SI Figure S3C). These features haveconstant height (∼100 pm) and a width of ∼2 nm, comparablewith the size of a Br-CR molecule and might be made of single
molecular chains.
From the experimentally obtained STM and LEED results in
Figures 2 and 3, we found that the growth of 0.25 ML Br-CR
on Cu(111) at 300 K in UHV provides atomically ﬂat wellordered ring molecular array with a unit of

(

18945

)( )

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

Figure 5. DFT calculations of Br-CR on Cu(111). (A) Three Br-CR two-dimensional structures on Cu(111) were tested. The structure in (A) was
found to be the most energetically stable conﬁguration: top (left) and side (right) views. For Br-CR, black, red, brown, and pink spheres denote C,
O, Br, and H atoms, respectively. Hexagonal lines denote the Cu substrate lattices. (B, C) Simulated and experimentally obtained STM images at Vs= −0.7 eV (It = 100 pA). Two buckled carbon atoms and Br atoms are observed as bright spots. (D) Calculated DOS of Br-CR on Cu(111). The
Br-CR HOMO peak is observed at −1.45 eV.

eV is observed. While there is some intensity modulation, band
dispersion is not seen, indicating that intermolecular hybridization is small. The slight change of the Cu d-band shape
could be due to surface scattering and diﬀerent cross sections
between the pristine Cu and the Br-CR coated Cu.
Angle-integrated UPS spectra for a Br-CR coverage of 0.00
ML, 0.01 ML, 0.05 ML, and 0.25 ML, obtained with hν = 8.4
eV Xe Iα light, are shown in Figure 4C. Upon increasing themolecular coverage from 0.01 to 0.25 ML, the work function
substantially decreases by 0.39 eV. The Cu(111) surface state
below −0.4 eV weakens and completely vanishes at 0.25 ML
coverage. At the same time the wide bump centered at −1.7 eVevolves into a sharper feature with maximum at −1.48 eV.

Although the latter changes are small, we tentatively assign thepeak at −1.48 eV to a molecular level of Br-CR. The HOMO
peak is more clearly observed in scanning tunneling spectroscopy (STS). Figure 4D shows a dI/dV curve, which is
proportional to the sample LDOS, obtained on the Br-CR
island in Figure 3. All islands show identical dI/dV curves. A
LDOS peak at −1.54 eV is clearly observed, while the Cu(111)
surface state peak is absent (compare with the bare Cu(111)
data in the inset of Figure 4D). In summary, the combined useof STS and UPS shows that upon Br-CR deposition, the work
function decreases, and the Cu surface state vanishes, reﬂecting
the formation of a continuous molecular ﬁlm. At 0.25 ML
18946

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C
coverage, a new LDOS peak appears at −1.5 eV which we
assign to a Br-CR molecular level.
3.3. DFT Calculations of Adsorbed Br-CR/Cu(111). The
experimental results in Figures 1−3 show that 0.25 ML Br-CR
on Cu(111) forms monolayer islands in a well-deﬁned 40 27
superstructure. On the basis of these structural parameters, wehave performed DFT calculations to identify the energeticallystable conformation and arrangement of Br-CR on Cu(111).
Crown ethers often have several competing conformations
due to the high ﬂexibility of the crown ring.26 In the gas phase,the free molecule is bent (see SI Figure S1F) with a calculated
angle between the two benzene groups of θ = 112°. A rather
small energy of 0.7 eV is needed to make the molecule ﬂat (θ =180°, see SI Figure S5). In the bulk crystal phase of Br-CR, themolecules have a step-like shape (SI Figure S1F). For an
isolated molecule, this “stepped” conformation is 0.23 eVhigher in energy than the stable “bent” conformation.
The structural optimization of the Br-CR monolayer on
Cu(111) was performed in two steps. First, calculations weredone for free-standing molecule layer in the same twodimensional unit cell as the adsorbed layer 40 27 by startingfrom either the bent or the stepped conformation with several
diﬀerent molecular orientations. After optimization, the threemost stable structures were put on the Cu(111) and optimized
again. They are shown in Figure 5A. The adsorption energy ofthe most stable structure is −2.57 eV (see the left panel in
Figure 5A). Other structures are 0.42 and 0.69 eV higher inenergy (see Figure 5A). In the most stable structure, bothbenzene groups lie almost ﬂat on the surface which maximizes
the attractive van der Waals interaction between molecules andsubstrate. The crown ring is asymmetric and very diﬀerent
from both the bent and stepped conformation of the freemolecule (see SI Figure S1F). This distortion enables themolecules to arrange themselves in a dense rectangular lattice.
The two other less stable structures, have a kink, similar to thebulk stepped conformation of the Br-CR crystal (see SI FigureS5). As a result, one of the benzene groups is tilted away fromthe Cu surface and so the molecule−substrate interaction
becomes weaker. Also, the Br atoms of neighboring molecules
are farther apart than in the most energetically stable structure,thus reducing the attractive intermolecular interaction.
We have also calculated monolayers with comparable
molecular density but diﬀerent surface cells (5 × 5, 5 × 6, 6
× 6). The energies of these structures were found to be at least
0.22 eV higher than the stable structure shown in Figure 5A,
conﬁrming that the experimentally observed 40 27 arrangement aﬀords the most stable monolayer of Br-CR on Cu(111).
In the stable structure (Figure 5A), the molecule is lying very
ﬂat on the substrate and the four Br atoms are at 3.1 ± 0.1 Åabove the Cu surface plane. The molecule−substrateinteraction is maximized in this geometry and overcomes theenergy cost (∼0.7 eV) required to “ﬂatten” the free molecule
structure (see SI Figure S5). The self-assembled monolayer
structure is dictated by the delicate balance between
intermolecular van der Waals and molecule−substrateinteractions and the conformational ﬂexibility of the crownring.
Figure 5B shows the DFT STM simulation image obtained
for the energetically stable ﬂat ring structure in Figure 5A andthe experimental bias. By comparing the DFT and STM((

images (see Figure 5B and C), the four bright protrusions
observed in the STM image were identiﬁed. From Figure 5A,
the Br-CR ring is ﬂat to the Cu(111), but only two carbon
atoms located at the side of the ring (see also the side view inFigure 5A) are slightly buckled from the molecule plane.
Exactly at these two positions, the STM image shows two
protrusions. The other two bright protrusions were found at
the Br atom positions.
We further compared the DFT results with the experimentally obtained LDOS peak. Figure 5D shows thecalculated DOS which is in good agreement with UPS andSTS results in Figure 4. The DFT-DOS shows a peak at −1.45
eV which is mainly of Carbon-pz orbital character and whoseenergy position is very close to the HOMO peak at −1.48 eV
in UPS and −1.54 eV in STS. These good agreements between
the theoretical and experimental results, not only for themolecular arrangements but also the LDOS peak positions,prove the high reliability of our study.
Owing to the electronic coupling between Br-CR andCu(111), the bent structure in free-standing gas phase (see SI
Figure S1F) was forced to make a ﬂat ring, successfully
producing a ﬂat ring cavity array.
As seen from the theoretical analysis, Br-CR lies ﬂat on
Cu(111) such as the interaction between the two benzene
groups and Cu is maximized. From this result, we expect thatmost crown ether molecules with two benzene will tend to lie
ﬂat on Cu(111), but because of the subtle balance between
bending energy and adsorption energy, a bent conformation
similar to the free molecule may be favored for certain
molecules.
We now comment on the possibility of Br dehalogenation in
our experiments. It is known that Br dehalogenation can occur
at 300−400 K, but ruthenium or palladium catalytic reactions
are required.56−59 The noble metal Cu(111) surface used hereis much less reactive than Ru or Pd. Further, in our study, allexperiments were performed in UHV, and no catalytic reaction
can happen during the sublimation process. As the C−Br
bonding is rather strong and stable even at 500 K and theboiling temperature of the 1,2-dibrobenzene is 497 K, Br-CR
should not be deconstructed in our experiments. We kept the
crucible temperature below 400 K. Moreover, the result of
SPALEED shown in Figure S7 demonstrates that the eﬀects of
the annealing on the diﬀraction pattern are not observed
indicating any changes in the chemical structure of themolecule are hardly expected at 500 K.
3.4. Work Function Variation. We found a decrease of
the work function with an increase of Br-CR deposition on
Cu(111): 4.86 eV for 0 ML, 4.86 eV for 0.01 ML, 4.69 eV for
0.05 ML, and 4.47 eV for 0.25 ML (see Figure 4C). Such a
decrease of the work function has been often reported for πconjugated molecules adsorbed on coinage substrate surfaces.
This is ascribed to a subtle interplay among intramolecular
dipole moments, push-back eﬀect (Pauli repulsion of metal
surface electrons) and charge-transfer between the adsorbates
and underlying substrates atoms. The strong push-back eﬀect
reduces a work function of coinage substrate surfaces in arange of 0.5 to 1.0 eV, while the eﬀect is compensated by the
charge-transfer interaction.60 For the Br-CR/Cu system case,
the Cu work function is continuously decreased by thedepositions and the shift amount of −0.39 eV at 0.25 ML issigniﬁcant and is in line with many π-conjugated molecules/
metal systems reported before. This suggests that the main
origin of the work function change is the push-back eﬀect and

)

)

(

)

18947

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C

Foundation. We thank Dr. Eiichi Inami for valuable
discussions, and Dr. Takahiro Ueba and Mr. Hiroyuki Machidafor supporting UPS measurements.

its compensator, i.e., charge-transfer is less eﬀective. Indeed,
the DFT calculation (see SI Figure S6B) predicts a dipole layerfrom the substrate to the molecular layer in the ﬂat molecularconformation, where the intramolecular dipole moment is
negligible. The DFT calculation shows that the charge transfer
occurs between Br-CR and Cu(111), providing a dipole
pointing normal to the surface.

■(1) Tsukahara, N.; Noto, K. I.; Ohara, M.; Shiraki, S.; Takagi, N.;
Takata, Y.; Miyawaki, J.; Taguchi, M.; Chainani, A.; Shin, S.; et al.
Adsorption-Induced Switching of Magnetic Anisotropy in a Single
Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface.
Phys. Rev. Lett. 2009, 102, 167203.
(2) Imada, H.; Miwa, K.; Imai-Imada, M.; Kawahara, S.; Kimura, K.;
Kim, Y. Real-Space Investigation of Energy Transfer in Heterogeneous Molecular Dimers. Nature 2016, 538, 364−368.
(3) Mänz, A.; Hauke, A. A.; Witte, G. Copper Phthalocyanine asContact Layers for Pentacene Films Grown on Coinage Metals. J.
Phys. Chem. C 2018, 122, 2165−2172.
(4) Robles, R.; Lorente, N.; Isshiki, H.; Liu, J.; Katoh, K.; Breedlove,
B. K.; Yamashita, M.; Komeda, T. Spin Doping of Individual
Molecules by Using Single-Atom Manipulation. Nano Lett. 2012, 12,
3609−3612.
(5) Yamagishi, Y.; Nakashima, S.; Oiso, K.; Yamada, T. K. Recovery
of Nanomolecular Electronic States from Tunneling Spectroscopy:
LDOS of Low-Dimensional Phthalocyanine Molecular Structures on
Cu(111). Nanotechnology 2013, 24, 395704.
(6) Gottfried, J. M. Surface Chemistry of Porphyrins andPhthalocyanines. Surf. Sci. Rep. 2015, 70, 259−379.
(7) 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.
(8) Yamada, T. K.; Yamagishi, Y.; Nakashima, S.; Kitaoka, Y.;
Nakamura, K. Role of π-d Hybridization in a 300-K Organic-Magnetic
Interface: Metal-Free Phthalocyanine Single Molecules on a Bcc
Fe(001) Whisker. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94,
195437.
(9) Otero, R.; Vázquez de Parga, A. L.; Gallego, J. M. Electronic,
Structural, and Chemical Effects of Charge-Transfer at Organic/
Inorganic Interfaces. Surf. Sci. Rep. 2017, 72, 105−145.
(10) Pham, V. D.; Repain, V.; Chacon, C.; Bellec, A.; Girard, Y.;
Rousset, S.; Abad, E.; Dappe, Y. J.; Smogunov, A.; Lagoute, J. Tuning
the Electronic and Dynamical Properties of a Molecule by Atom
Trapping Chemistry. ACS Nano 2017, 11 (11), 10742−10749.
(11) Li, Y.; Ngo, A. T.; DiLullo, A.; Latt, K. Z.; Kersell, H.; Fisher,
B.; Zapol, P.; Ulloa, S. E.; Hla, S.-W. Anomalous Kondo ResonanceMediated by Semiconducting Graphene Nanoribbons in a Molecular
Heterostructure. Nat. Commun. 2017, 8, 946.
(12) 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.
(13) Lu, J.; Ruan, Z.; Guan, Y.; Bao, D. L.; Lin, X.; Hao, Z.; Zhang,
H.; Song, L.; Yan, C.; Cai, J.; Du, S.; Gao, H. J. Controllable Densityof Atomic Bromine in a Two-Dimensional Hydrogen Bond Network.
J. Phys. Chem. C 2018, 122, 25681−25684.
(14) Garnica, M.; Stradi, D.; Barja, S.; Calleja, F.; Díaz, C.; Alcamí,M.; Martín, N.; Vázquez de Parga, A. L.; Martín, F.; Miranda, R.
Long-Range Magnetic Order in a Purely Organic 2D Layer Adsorbed
on Epitaxial Graphene. Nat. Phys. 2013, 9, 368−374.
(15) Esat, T.; Friedrich, N.; Tautz, F. S.; Temirov, R. A Standing
Molecule as a Single-Electron Field Emitter. Nature 2018, 558, 573.
(16) Stöhr, M.; Gabriel, M.; Möller, R. Investigation of the Growthof PTCDA on Cu(110): an STM Study. Surf. Sci. 2002, 507, 330−
334.
(17) Tanaka, H.; Arima, R.; Fukumori, M.; Tanaka, D.; Negishi, R.;
Kobayashi, Y.; Kasai, S.; Yamada, T. K.; Ogawa, T. Method forControlling Electrical Properties of Single-Layer Graphene Nanoribbons via Adsorbed Planar Molecular Nanoparticles. Sci. Rep. 2015, 5,12341.

4. CONCLUSIONS
We succeeded in growing a ring array using a 4,4′,5,5′tetrabromodibenzo[18] crown-6 ether (Br-CR) molecule on
an atomically ﬂat Cu(111) substrate at 300 K, which wereobtained by combined experimental studies of STM, LEED,and UPS with DFT theoretical calculation supports.
We found (1) the Br-CR ring, which is bent in bulk crystal
and gas phases, becomes ﬂat upon adsorption on Cu(111)owing to molecule−substrate electronic interactions. (2) A
moderate energy balance between intermolecular and molecule−substrate interactions allows Br-CR molecule thermal
diﬀusion on Cu(111) at room temperature, forming a wellordered self-assembly ring array with a periodic unit ofij a÷◊÷′yza ⃗ , where a⃗ and b⃗ denote the unit vectors on
jjj ÷◊÷ zzz = 40 27
⃗
b
b
′
k {
the Cu(111) substrate. (3) While the deposition of 0.05 ML
Br-CR forms multidomain islands with disordered defects, adrastic improvement occurred at 0.25 ML, where only
atomically ﬂat single-domain islands were grown. This Br-CR
ﬂat ring cavity array is a promising new template for capturing
guest species for making artiﬁcial array network toward the
UHV on-surface based electronic devices.

(

■

)( )

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.9b03335.
Figure S1: UHV-STM and UHV-UPS experimental
setups and the molecule evaporator with QCM rate
check; Figure S2: UPS, STM atomic image, and STS
surface state results obtained on the Cu(111) substrate;
Figure S3: STM images of extra adsorbates on Cu(111);
Figure S4: Angle-resolved UPS results; Figure S5: DFT
calculations of Br−CR monolayer on Cu(111); Figure
S6: UPS and DFT results of work function variation due
to Br-CR adsorption on Cu(111); and Figure S7:
SPALEED results of Br-CR ﬁlm on Cu(111) (PDF)■

REFERENCES

AUTHOR INFORMATION

Corresponding Author*E-mail: toyoyamada@faculty.chiba-u.jp (T.K.Y.).
ORCID

Peter Krüger: 0000-0002-1247-9886
Masaki Horie: 0000-0002-7734-5694
Satoshi Kera: 0000-0003-0353-5863
Toyo Kazu Yamada: 0000-0001-5185-6472
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Number
23681018, 25110011, and 26248062, and the Asahi Glass
18948

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C(18) Yamada, T. K.; Fukuda, H.; Fujiwara, T.; Liu, P.; Nakamura, K.;
Kasai, S.; Vazquez de Parga, A. L.; Tanaka, H. Energy Gap Opening
by Crossing Drop Cast Single-Layer Graphene Nanoribbons.
Nanotechnology 2018, 29, 315705.
(19) Azzam, W.; Zharnikov, M.; Rohwerde, M.; Bashir, A.
Functional Group Selective STM Imaging in Self-Assembled
Monolayers: Benzeneselenol on Au(111). Appl. Surf. Sci. 2018, 427,
581−586.
(20) Thomas, J. C.; Goronzy, D. P.; Serino, A. C.; Auluck, H. S.;
Irving, O. R.; Jimenez-Izal, E.; Deirmenjian, J. M.; Machácě k, J.;
Sautet, P.; Alexandrova, A. N.; Baše, T.; Weiss, P. S. Acid−BaseControl of Valency within Carboranedithiol Self-Assembled Monolayers: Molecules Do the Can-Can. ACS Nano 2018, 12, 2211−2221.
(21) Machida, S.-i.; Nakayama, Y.; Duhm, S.; Xin, Q.; Funakoshi, A.;
Ogawa, N.; Kera, S.; Ueno, N.; Ishii, H. Highest-Occupied-MolecularOrbital Band Dispersion of Rubrene Single Crystal as Observed byAngle-Resolved Ultraviolet Photoelectron Spectroscopy. Phys. Rev.
Lett. 2010, 104, 156401.
(22) Pedersen, C. J. Cyclic Polyethers and Their Complexes with
Metal Salts. J. Am. Chem. Soc. 1967, 89, 7017−7036.
(23) Cram, D. J. The Design of Molecular Hosts, Guests, and Their
Complexes. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009−1020.
(24) Pedersen, C. J. The Discovery of Crown Ethers. Science 1988,
241, 536−540.
(25) Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. 2DSupramolecular Arrangements f dibenzo-18-crown-6-ether and Its
Inclusion Complex with Potassium Ion by Potential Controlled
Adsorption. Org. Biomol. Chem. 2003, 1, 251−253.
(26) Hosokai, T.; Horie, M.; Aoki, T.; Nagamatsu, S.; Kera, S.;
Okudaira, K. K.; Ueno, N. Change in Molecular Conformation ofDibenzo-Crown Ether Induced by Weak Molecule-Substrate Interaction. J. Phys. Chem. C 2008, 112, 4643−4648.
(27) Schmaltz, B.; Rouhanipour, A.; Rader, H. J.; Pisula, W.; Mullen,
K. Filling the Cavity of Conjugated Carbazole Macrocycles with
Graphene Molecules: Monolayers Formed by Physisorption Serve as a
Surface for Pulsed Laser Deposition. Angew. Chem., Int. Ed. 2009, 48,
720−724.
(28) Kusaka, R.; Inokuchi, Y.; Ebata, T. Structure of Hydrated
Clusters of Dibenzo-18-Crown-6-Ether in a Supersonic Jet-Encapsulation of Water Molecules in the Crown Cavity. Phys. Chem. Chem.
Phys. 2008, 10, 6238−6244.
(29) Bruns, C. J.; Stoddart, J. F. Rotaxane-Based Molecular Muscles.
Acc. Chem. Res. 2014, 47, 2186−2199.
(30) Guo, J.; Lee, J.; Contescu, C. I.; Gallego, N. C.; Pantelides, S.
T.; Pennycook, S. J.; Moyer, B. A.; Chisholm, M. F. Crown Ethers in
Graphene. Nat. Commun. 2014, 5, 5389.
(31) Lu, H.; Kwak, I.; Park, J. H.; O’Neill, K.; Furuyama, T.;
Kobayashi, N.; Seabaugh, A.; Kummel, A.; Fullerton-Shirey, S. K.
Solution-Cast Monolayers of Cobalt Crown Ether Phthalocyanine on
Highly Ordered Pyrolytic Graphite. J. Phys. Chem. C 2015, 119,21992.
(32) Cheng, S.-C.; Chen, K.-J.; Suzaki, Y.; Tsuchido, Y.; Kuo, T.-S.;
Osakada, K.; Horie, M. Reversible Laser-Induced Bending ofPseudorotaxane Crystals. J. Am. Chem. Soc. 2018, 140, 90−93.
(33) Xu, K.; Lu, H.; Kinder, E. W.; Seabaugh, A.; Fullerton-Shirey, S.
K. Monolayer Solid-State Electrolyte for Electric Double Layer Gatingof Graphene Field-Effect Transistor. ACS Nano 2017, 11, 5453−
5464.
(34) Zhang, W.; Jin, W.; Fukushima, T.; Saeki, A.; Seki, S.; Aida, T.
Supramolecular Linear Hetero-junction Composed of Graphite-likeSemiconducting Nanotubular Segments. Science 2011, 334, 340−343.
(35) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C.
Highly Efficient Organic Light-Emitting Diodes from Delayed
Fluorescence. Nature 2012, 492, 234−238.
(36) Kim, R. H.; Kim, H. J.; Bae, I.; Hwang, S. K.; Velusamy, D. B.;
Cho, S. M.; Takaishi, K.; Muto, T.; Hashizume, D.; Uchiyama, M.;
Andre, P.; Mathevet, F.; Heinrich, B.; Aoyama, T.; Kim, D.; Lee, H.;
Ribierre, J. C.; Park, C. Non-Volatile Organic Memory with SubMillimetre Bending Radius. Nat. Commun. 2014, 5, 3583.

(37) Ishii, H.; Sugiyama, K.; Ito, R.; Seki, K. Energy Level Alignment
and Interfacial Electronic Structures at Organic/Metal and Organic/
Organic Interfaces. Adv. Mater. 1999, 11, 605−625.
(38) Kawano, K.; Adachi, C. H. Evaluating Carrier Accumulation in
Degraded Bulk Heterojunction Organic Solar Cells by a Thermally
Stimulated Current Technique. Adv. Funct. Mater. 2009, 19, 3934−
3940.
(39) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Interface
Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20,1371−1388.
(40) Schmaus, S.; Bagrets, A.; Nahas, Y.; Yamada, T. K.; Bork, A.;
Bowen, M.; Beaurepaire, E.; Evers, F.; Wulfhekel, W. Giant
Magnetoresistance Through a Single Molecule. Nat. Nanotechnol.
2011, 6, 185−189.
(41) Bagrets, A.; Schmaus, S.; Jaafar, A.; Kramczynski, D.; Yamada,T. K.; Alouani, M.; Wulfhekel, W.; Evers, F. Single Molecule
Magnetoresistance with Combined Antiferromagnetic and Ferromagnetic Electrodes. Nano Lett. 2012, 12, 5131−5136.
(42) Kawabe, T.; Shimose, K.; Goto, M.; Suzuki, Y.; Miwa, S.
Magnetic Tunnel Junction with Fe(001)/Co Phthalocyanine/
MgO(001) Single-Crystal Multilayer. Appl. Phys. Express 2018, 11,013201.
(43) 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.
(44) Nazriq, N. K. M.; Minamitani, R.; Yamada, T. K. CO-Tip
Manipulation Using Repulsive Interactions. Nanotechnology 2018, 29,495701.
(45) 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.
(46) Yang, J. P.; Bussolotti, F.; Kera, S.; Ueno, N. Origin and Role ofGap States in Organic Semiconductor Studied by UPS: as the Natureof Organic Molecular Crystals. J. Phys. D: Appl. Phys. 2017, 50,
423002.
(47) Dubois, G.; Reye, C.; Corriu, R. J. P.; Chuit, C. OrganicInorganic Hybrid Materials. Preparation and Properties of Dibenzo18-Crown-6 Ether-Bridged Polysilsesquioxanes. J. Mater. Chem. 2000,
10, 1091−1098.
(48) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for abinitio Total Energy Calculations using a Plane-Wave Basis Set. Phys.
Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169.
(49) Grimme, S. Semi-Empirical GGA-Type Density Functional
Constructed with a Long-Range Dispersion Correction. J. Comput.
Chem. 2006, 27, 1787.
(50) Tonigold, K.; Groß, A. Adsorption of Small Aromatic
Molecules on the (111) Surfaces of Noble Metals: A Density
Functional Theory Study with Semiempirical Corrections for
Dispersion Effects. J. Chem. Phys. 2010, 132, 224701.
(51) Liu, W.; Ruiz, V. G.; Zhang, G. X.; Santra, B.; Ren, X.; Scheffler,M.; Tkatchenko, A. Structure and Energetics of Benzene Adsorbed on
Transition-Metal Surfaces: Density-Functional Theory with Van der
Waals Interactions Including Collective Substrate Response. New J.
Phys. 2013, 15, 053046.
(52) Krüger, P.; Petukhov, M.; Domenichini, B.; Berko, A.;
Bourgeois, S. Monolayer Formation of Molybdenum Carbonyl on
Cu(111) Revealed by Scanning Tunneling Microscopy and Density
Functional Theory. J. Phys. Chem. C 2012, 116, 10617−10622.
(53) Krüger, P.; da Pieve, F.; Osterwalder, J. Real-Spaces Multiple
Scattering Method for Angle-Resolved Photoemission and ValenceBand Photoelectron Diffraction at Its Application to Cu(111). Phys.
Rev. B: Condens. Matter Mater. Phys. 2011, 83, 115437.
(54) Nakashima, S.; Yamagishi, Y.; Oiso, K.; Yamada, T. K. How
Contacting Electrode Affect Single π-Conjugated Molecular Electronic States: Local Density of States of Phthalocyanine Nanomolecules on MgO(001), Cu(111), Ag(001), Fe(001), and Mn(001).
Jpn. J. Appl. Phys. 2013, 52, 110115.
18949

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950

Article

The Journal of Physical Chemistry C(55) Schwoebel, R. L.; Shipsey, E. J. Step Motion on Crystal
Surfaces. J. Appl. Phys. 1966, 37, 3682−3686.
(56) Zhong, H.; Liu, C.; Zhou, H.; Wang, Y.; Wang, R.
Prefunctionalized Porous Organic Polymers: Effective Supports ofSurface Palladium Nanoparticles for the Enhancement of Catalytic
Performances in Dehalogenation. Chem. - Eur. J. 2016, 22, 12533−
12541.
(57) You, T.; Wang, Z.; Chen, J.; Xia, Y. Transfer HydroDehalogenation of Organic Halides Catalyzed by Ruthenium(II)Complex. J. Org. Chem. 2017, 82, 1340−1346.
(58) Sabater, S.; Mata, J. A.; Peris, E. Hydrodefluorination of
Carbon-Fluorine Bonds by The Synergistic Action of a RutheniumPalladium Catalyst. Nat. Commun. 2013, 4, 2553.
(59) Guo, X.; Yu, C.; Yin, Z.; Sun, S.; Seto, C. T. Hydrodehalogenation of Polyhalogenated Aromatics Catalyzed by NiPd
Nanoparticles Supported on Nitrogen-Doped Graphene. Chem. Sus.
Chem. 2018, 11, 1617−1620.
(60) Duhm, S.; Gerlach, A.; Salzmann, I.; Broker, B.; Johnson, R. L.;
Schreiber, F.; Koch, N. PTCDA on Au(111), Ag(111) and Cu(111):
Correlation of Interface Charge Transfer to Bonding Distance. Org.
Electron. 2008, 9, 111.

18950

DOI: 10.1021/acs.jpcc.9b03335
J. Phys. Chem. C 2019, 123, 18939−18950