Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 4901–4904
Part 1, No. 7B, July 2003
#2003 The Japan Society of Applied Physics

Structures of Self-Assembled Monolayer of Squaraine Molecules
Adsorbed on Highly Oriented Pyrolytic Graphite and Au(111) Substrates
Satoshi YASUDA and Hidemi SHIGEKAWA
Institute of Applied Physics, 21st Century COE, University of Tsukuba Tsukuba 305-8573, Japan(Received March 13, 2003; accepted for publication March 19, 2003)

Molecular arrangements of squaraine (SQ) molecules formed on highly oriented pyrolytic graphite (HOPG) and Au(111)substrates in phenyloctane solution were studied by scanning tunneling microscopy (STM). On HOPG, SQ molecules withouthydroxyl groups formed row arrangements, in contrast to the herringbone structures previously observed for the SQ moleculeswith hydroxyl groups, indicating that the chemically added hydroxyl groups play an important role in the formation of theherringbone arrangement. On the other hand, the observed molecular structures on the Au(111) substrate were completely
diﬀerent from those on the HOPG substrate, which was attributed to the speciﬁc interactions between the two anilino moieties
in a SQ molecule and Au(111) lattices. The obtained results indicate the possibility of structural control of SQ molecules,
namely, the photo-absorption property, by changing the kinds of functional groups and substrates.
[DOI: 10.1143/JJAP.42.4901]
KEYWORDS: STM, squaraine, photoactive, intermolecular charge transfer, lattice matching1.

Introduction

Development of the organic optical devices is one of themain goals of current researchers. Among the many organic
materials, which have been studied to realize this purpose,squaraine (SQ) molecules (Fig. 1) are interesting because of
their highly potential optical characteristics.1) Since SQ
molecules are photoconductive and show a narrow-band
photoabsorption property with high eﬃciency, ie., molar
extinction coeﬃcient of 3  105 cm1 M1 , they are of great
importance in industrial applications such as optical recording media.
The two amine groups on the both sides and the central
four-membered ring (C4 O4 ) of a SQ molecule work aselectron donors and an electron acceptor, respectively. The
donor-acceptor-donor (D-A-D) structure in a SQ molecule
induces an intramolecular charge transfer (CT). According
to the theoretical studies, intramolecular CT occurs in thephotoexcited state as well as in the ground state.2) Moreover,since excited electrons are transferred from oxygen atoms to
the central C4 O2 unit, SQ molecules in the excited stateshow a higher localized electronic state compared to theground state. This localization of the electronic transition
plays an important role in the optical characteristics of thismaterial.

Fig. 1. Schematic molecular structures of bis(4-dibutylamino-2-phyenyl)
squaraine (SQ) molecule with (a) and without (b) hydroxyl groups.



http://dora.ims.tsukuba.ac.jp

Recently, fabrication of SQ molecules by the LangmuirBlodgett (LB) technique has been studied extensively.3,4)
During photoillumination, an apparent photocurrent wasobserved, and the high potential of the SQ molecule as a thinorganic solar cell was clearly shown. These results accelerated the study of novel SQ interface systems to achieve thedevelopment of molecular devices such as photoconductive
wires and photodiodes, which has become one of the most
attractive trends in the molecular electronics researches.
However, a remarkable point is that the observed photoabsorption property crucially depends on the moleculararrangement of the SQ molecules. Therefore, detailed
understanding and control of the molecular arrangement of
the interface is urgently needed to realize the development
and optimal control of those novel SQ interfacial systems.
According to the scanning tunneling microscopy (STM)
study of the molecular arrangement of SQ derivatives withshort alkyl tails adsorbed on highly oriented pyrolytic
graphite (HOPG),5) the molecules formed a herringbone
arrangement, and no ordered structure was formed inrelation to the symmetry of the HOPG lattice. Therefore, it
was concluded that the SQ-HOPG interaction was weakcompared to the SQ-SQ interaction induced by the intermolecular donor-acceptor CT in a molecule. However, since
the SQ molecule used in the experiment was chemically
modiﬁed with hydroxyl groups added onto the benzene rings
as shown in Fig. 1(b), not only Coulomb force but alsohydrogen bonding between hydroxyl groups must haveplayed an important role in the formation of the herringbone
structure. Therefore, ﬁrst of all, study on the SQ molecules
without hydroxyl groups is necessary.
In addition, it is also important to understand the SQsubstrate interactions as well as the SQ-SQ interaction tocontrol the SQ molecular arrangement for the future
development of optical devices. For example, SQ molecules
on a Au substrate may form an arrangement diﬀerent fromthat on a HOPG substrate, through the interactions between
the amine groups in the SQ molecule and the lattice of theAu substrate.
In order to establish the potential of the SQ molecularinterface system, we studied SQ-substrate and SQ-SQ
interactions by changing the substrate and SQ derivative.

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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7B

First, an SQ molecule without hydroxyl groups was used inorder to clarify the eﬀect of the hydrogen bonding
interaction between SQ molecules. Secondly, a Au(111)substrate was used to examine the inﬂuence of the substrate
upon the molecular arrangement of the self-assembled
monolayer SQ molecules.
2.

Experimental and Results

Figure 1(a) shows the molecular structures of the bis(4dibutylamino-2-phyenyl) squaraine (SQ) molecule used inthis study. The SQ molecules used in the previous study
were chemically modiﬁed with a hydroxyl group added onto
each benzene ring as shown in Fig. 1(b). On the other hand,
there is no hydroxyl group in the case of our SQ molecules(Fig. 1(a)). HOPG and atomically ﬂat Au(111) surfaces wereused as substrates. Au(111) substrates were prepared byvapor deposition of approximately 75-nm-thick Au ﬁlms
onto freshly cleaved mica sheets. During the Au deposition,
the temperature of the mica substrates was kept at 400 C.
The deposition rate was 0.8–1.0 A/s
and the vacuum
pressure was maintained below 8:0  104 Pa. Following
the deposition, the mica substrate were annealed at 400 C
for 1 h. Brief ﬂame-annealing was performed before eachexperiment. After conﬁrmation of the substrate surface
structure in a ﬂuid cell, 1 mM phenyloctane solution
containing SQ molecules was introduced into the cell.
Mechanically cut Pt/Ir wire was used as a STM tip. All
images could be acquired in the solution as in air, indicating
the possibility of the future investigation of structural control
by such as electrochemical techniques and solvent eﬀect.
Figure 2 shows typical STM images of the SQ molecular

S. YASUDA and H. S HIGEKAWA
Table I.

Unit cell

Packing
structure


a (A)


b (A)

 (deg)row

19  210  246  2row

17  29273  2Herringbone

22  220  282  3row

32  28290  2

Substrate

HOPG
Au(111)

Fig. 3. STM images of typically observed SQ monolayer adsorbed on
Au(111) substrate (a) and its magniﬁcation (Vs ¼ þ1:0 V, It ¼ 0:1 nA).

structures on HOPG substrate and their schematic models. In
contrast to the previous results,5) SQ molecules adsorbed on
HOPG substrate formed three diﬀerent types as shown in
Figs. 2(a) to 2(c), namely, two row-type structures ((a) and(b)) and one herringbone-type structure ((c)). The observed
unit cells, the data of which are listed in Table I, areschematically shown in the ﬁgure.
The row-type molecular arrangements were dominant,and the herringbone-type ones could hardly be observed.
These molecular structures were stably observed under thecondition of tunnel resistance above 50 G, however, when
the STM tip was moved closer to the sample, the molecules
became unstable, and the HOPG substrate became visible.
From the analysis of the observation, the observed molecular
structures were conﬁrmed not as multilayer but as amonolayer.
Figure 3(a) shows a STM image of SQ molecules
adsorbed on a Au(111) substrate. The SQ molecules seemed
uniformly adsorbed, with their axes parallel to the Au(111)substrate. However, contrary to the ordered homogeneous
molecular arrangements on the HOPG substrate, many
domains existed, and the molecular arrangements wereapparently diﬀerent from those observed on the HOPG
substrate, suggesting that SQ-substrate interaction plays animportant role in the case of the Au substrate. Observed
lattice constants are also listed in Table I.
3.

Fig. 2. STM images of typically observed SQ molecular arrangements on
HOPG substrate (Vs ¼ þ1:0 V, It ¼ 0:2 nA) and their schematic models.

Unit cell data experimentally obtained in this study.

Discussion

3.1 SQ molecules on HOPG substrate
As was mentioned, the previous STM result using SQ
molecules with hydroxyl groups has shown that there is no
lattice matching between the SQ molecules and HOPG
substrate, and that SQ-HOPG interaction is not a dominantfactor in determining the molecular arrangements.5)
According to the previous STM study, SQ molecules withhydroxyl groups form a herringbone structure. The driving
force is attributed to the Coulomb force induced by theintermolecular CT between the donor amine groups andJpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7B

acceptor C4 O2 ring in a molecule. In contrast to that result,
SQ molecules without hydroxyl groups formed rows asshown in Fig. 2, and the herringbone structure could hardly
be observed.
To conﬁrm the origin of the interaction resulting in theobserved structures, lattice matching between SQ moleculesand HOPG lattices was analyzed in detail, in considerationof the  stacking between the two aromatic rings in a SQ
molecule and the graphite rings. As a result, the lattice
matching could not be found between SQ molecules and thesubstrate. Thus, SQ-SQ interaction must be dominantcompared to the SQ-HOPG interaction in the case of SQ
molecules without hydroxyl groups as well as in the case of
the previous results obtained for the SQ molecules withhydroxyl groups. Therefore, it is strongly suggested that theexistence of hydroxyl groups plays an important role in theformation of the herringbone structure. However, in consideration of the row structures, hydrogen bonding between
adjacent molecules is possible even in the row arrangements.
Then, the observed diﬀerence may arise from the change inthe electronic structure induced by the modiﬁcation ofhydroxyl groups instead of the eﬀect of simple hydrogenbonding. In the case of inclusion complexes, eﬀect of the
included molecular dipole on the modiﬁcation of thehydrogen bonding interaction has been reported.6,7)
Further experimental and theoretical studies are necessary
to understand this eﬀect. However, in any case, we haverevealed that the existence of hydroxyl groups, namely, themodiﬁcation of the molecule, crucially inﬂuences the SQ
molecular arrangement.
3.2 SQ molecules on Au(111) surface
As shown in Fig. 3, the domain structures of the SQ
molecules on the Au(111) surface were observed, and themolecular arrangements were completely diﬀerent fromthose on HOPG. The molecular rows in the domains were
almost in good agreement with the three fold symmetry of
the underlying Au(111) substrate. Figures 4(a) and 4(b)show a magniﬁed STM image of the typically observed row
structure and its schematic model. Molecules in the alternatively arranged rows are tilted from the row axes as 79  2

S. YASUDA and H. S HIGEKAWA

4903

Fig. 4. (a) High-resolution STM image of SQ molecules adsorbed on
Au(111) substrate. (b) Schematic model for the structure in (a).

and 113  2 , respectively, as shown in the schematic
model. To clarify the commensuration of the structure withthe substrate, we analyzed the lattice matching between the
SQ molecules and Au(111) substrate.
Since nitrogen atoms are expected to interact with theAu(111) substrate,8–10) the speciﬁc interaction between theamine group in a molecule and the lattice structure of the Au
substrate was considered. The top, bridge and hollow sites
shown in Fig. 5 were considered as the three candidates forthe adsorption sites. As shown in the upper part of Fig. 5, thetwo amine groups in a SQ molecule have a good lattice
matching with the Au(111) substrate lattice for all the threeadsorption models. Also, as indicated by the arrows in thelower part, there exist many similar matching relationships
between the SQ molecules and each lattice structure.
However, among the three models, only the hollow sitemodel reproduces the lattice constant and tilt angles of themolecules observed in the STM images.
The molecular packing structure of the hollow site modelis shown in Fig. 6. As shown in Fig. 6, the tilt angles ofFig. 5. Positional relationships between SQ molecule and Au(111) substrate for three diﬀerent adsorption sites of (a) top, (b) hollow
and (c) bridge sites. Figures in the upper side show the lattice matching between the two amine groups in the SQ molecule and Au
lattices for each model. The arrows in the lower side show the possible lattice matchings.

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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7B

S. YASUDA and H. S HIGEKAWA

4.

Fig. 6. Proposed molecular packing structures adsorbed on the hollow
site.

molecules related to the molecular rows, 78 and 111 , and b ¼ 7:7 A,
 and  ¼ 90
the lattice constants of a ¼ 32:2 A,
are in good agreement with the experimentally obtained
values.
Domains with SQ molecules arranged in rows seemdiﬀerent from the three fold symmetry axes as shown by thewhite dotted area in Fig. 3(b). However, since there aremany possible molecular directions to induce the lattice
matching as shown in Fig. 5, these structures can also be
reproduced well by the hollow site model.
As is well known, molecular arrangement depends on thebalance between molecule-molecule and molecule-substrateinteractions. In this case, no etch pit structures wereobserved, suggesting a weak coupling of electronic structures between SQ molecules and Au substrate. However,existence of the lattice matching indicates the clear eﬀect ofthe molecule-substrate interaction. As the modulation of theinteractions by modifying the surface electronic structure byadsorption of other atoms11,12) or applying electrochemical
potential has been reported,8,9) Au surface is more controllable for the practical arrangement of the adsorbed molecules. Since the optical properties of SQ molecules cruciallydepend on the molecular arrangement, future developmentof this material as an organic optical device is expected.
Further experiment is now in progress.

Conclusion

Molecular arrangements of SQ molecules formed on
HOPG and Au(111) substrates in phenyloctane solution
were studied by STM. On HOPG substrate, SQ molecules
without hydroxyl groups formed row arrangements, incontrast to the herringbone structures previously observed
for the SQ molecules with hydroxyl groups, indicating thatthe chemically added hydroxyl groups play an important rolein the formation of the herringbone structure. On the other
hand, the observed molecular structures on the Au(111)substrate were completely diﬀerent from those on the HOPG
substrate, which was attributed to the speciﬁc interactions
between the two amine groups in a SQ molecule andAu(111) lattices. With consideration of the lattice matching
between the two amine groups with the three sites, top,
bridge and hollow sites, the hollow site model showed thebest agreement with the experimental results.
The obtained results indicate the possibility of structuralcontrol of SQ molecules, namely, the photoabsorption
property, by changing the kinds of functional groups andsubstrates.
Acknowledgement
This work was supported by a Grant-in-Aid for ScientiﬁcResearch from the Ministry of Education, Science andCulture of Japan. One of the authors (S.Y.) was ﬁnancially
supported by the Society for the Promotion of Science(JSPS) Fellowships for Japanese Junior Scientists.
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