APPLIED PHYSICS LETTERSVOLUME 76, NUMBER 5

31 JANUARY 2000

Scanning tunneling microscopy on the formation of lipoamide-cyclodextrin
monolayer on Au„111…
Satoshi Yasuda and Hidemi Shigekawaa)
Institute of Applied Physics, and CREST (Japan Science and Technology Corporation),
University of Tsukuba, Tsukuba 305-8573, Japan

Iwao Suzuki
Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578,
Japan

Tohru Nakamura and Mutsuyoshi Matsumoto
Department of Molecular Engineering, National Institute of Materials and Chemical Research, Tsukuba,305-0046, Japan

Makoto Komiyamab)
Department of Chemistry and Biotechnology, Graduate School Engineering, University of Tokyo,
Tokyo 113-8656, Japan

共Received 24 May 1999; accepted for publication 30 November 1999兲

␤-cyclodextrin molecules modified with lipoamide residue 共LP-␤-CyD兲 were self-assembled on an
Au共111兲 surface in ethanol solution, and the growth process was studied by scanning tunnelingmicroscopy. At the initial stage, adsorption sites were not only random, but also partially linear
ordering, which suggests the existence of some influence by the herringbone structure of theAu共111兲 surface. According to the macroscopic analysis, the subsequent growth process wasexplained by the Elovich model, which is based on the repulsive interaction between adsorbed
molecules. However, when the immersion time increased, island structures began forming. This
result suggests the interaction between LP-␤-CyD molecules as attractive, which in fact is moreprobable in consideration of the possibility of the hydrophobic and the hydrogen bonding
interactions between CyD molecules. Finally, formation of a single LP-␤-CyD layer was clearlyconfirmed. © 2000 American Institute of Physics. 关S0003-6951共00兲01405-4兴

In order to design and develop multifunctional moleculardevices, the formation of thin/monolayer films using the selfassembling technique has been the main focus of many
researchers.1–20 Among them, the formation of cyclodextrin
共CyD兲 derivative monolayers is attracting considerable attention because of their high potential for applications such as
in novel molecular recognition sensors. CyDs are donutshaped cyclic oligosaccharides consisting of several glucopyranose units 关Fig. 1共a兲兴, and are able to form host–guest
complexes by including various molecules 共guest molecules兲
in their cavities.6 In addition, the inclusion process can becontrolled by chemical modification of CyDs with additionalresidues. For example, when an azobenzene molecule, whichshows a cis-trans transformation by photostimulation, is attached to the top of the CyD cavity,7 the inclusion process
becomes controllable by photoswitching. Therefore, if theformation of the CyD monolayer is achieved, a multifunctional molecular device can be realized.
Recently, Rojas et al. showed the high potential of CyD
molecules as a molecular receptor by cyclic voltammetry forthiolated CyDs self-assembled on Au.8 Since thiolated CyDs
are adsorbed onto the Au surface as a result of the chemicalbonding between S atoms in the molecules and the Au surface, they are considered to be located on the Au surface

with their cavity axis aligned perpendicular to the surface.
The conformation was confirmed by the analysis of the fourier transform infrared-reflection absorption spectroscopy.9When a Au electrode was covered by only pentanethiol molecules, no redox signal of the electroactive marker, ferrocenea兲

Electronic mail: hidemi@ims.tsukuba.ac.jp,
Web: http://dora.ims.tsukuba.ac.jpb兲
Electronic mail: mkomi@chembio.t.u-tokyo.ac.jp

FIG. 1. Schematic structures of 共a兲 ␤-CyD, 共b兲 LP, 共c兲 ␤-CyD modified withLP, and 共d兲 molecular recognition by LP-␤-CyD monolayer.

0003-6951/2000/76(5)/643/3/$17.00
643© 2000 American Institute of Physics
Downloaded 03 Mar 2003 to 130.158.147.14. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

644

Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000

molecules, was observed because the pentanethiol molecules
prevented the electroactive marker from reaching the Au surface. However, when the Au electrode was covered by both
pentanethiol and thiolated ␤-CyD molecules, the reversible
redox current could be measured. Furthermore, using CyDs
modified by lipoamide residue, Suzuki et al. demonstrated
that the signal of the redox current by the ferrocene carboxylic acid changed with the concentration and a species of theguest molecules in the solution.10–12 Namely, inclusion of
the guest molecules prevented the electroactive marker fromreaching the Au surface through the cavity 关Fig. 1共d兲兴. Theseresults led to the suggestion that CyD monolayers wereformed, and the existence of guest molecules could be identified due to the reduction in the current. However, from theanalysis of a similar system by plasmon and surface polariton spectroscopy,13 a quasidoublelayer structural model wasproposed to explain the observed change in the dielectric
constant.14 Furthermore, the formation process of the molecular layer was explained by the Elovich mechanism which
includes a repulsive interaction between molecules. Since,
CyDs contain many hydroxyl groups 关Fig. 1共a兲兴, attractive
interaction between CyDs due to the hydrogen bonds is considered to be more probable.3–5 Hydrophobic interaction is
supposed to work as attractive, too. As has been described,
despite the important characteristics of the CyD/Au system,some contradictions exist among the obtained results. Therefore, in order to utilize the high potentiality of this system, it
is extremely important to clarify this point. As is wellknown, since macroscopic measurements give information
averaged over the surface, the results sometimes differ fromthe microscopic structure. For example, regarding the growth
process of the self-assembled alkanethiols on Au共111兲, macroscopic analyses such as contact angle measurement andellipsometry predicted it to be according to the Langmuirmodel, i.e., initial random and homogeneous adsorption.15 In
addition, the molecular arrangement determined by He atom
diffraction and other method was a )⫻) structure.16 However, the microscopic analysis using scanning tunneling microscopy 共STM兲 and atomic force microscopy revealed that anucleation and domain growth mode existed in the process,and coexistence of the closed packed c(4⫻2) and n ⫻) 共n:integer兲 structures was confirmed at the initial stage.17,18 Furthermore, formation of numerous pit-like defects was observed during the growth process.
In consideration of these facts, in order to utilize thehigh potentiality of the CyDs that has been demonstrated byRojas et al., structural analysis of CyD/Au共111兲 from a microscopic viewpoint is urgently required. Therefore, we performed the first microscopic analysis of this system using
STM. As considered, the obtained results were completelydifferent from those predicted by the macroscopic analyses.
Figure 1 shows the schematic structures of 共a兲 ␤-CyD共consisting of seven glucopyranose units兲, 共b兲 lipoamide residue 共LP兲, and 共c兲 thiolated ␤-CyD derivative used in thisstudy. As shown in Fig. 1共c兲, CyDs were modified with alipoamide residue chemically added onto the primary hydroxyl side 共LP-␤-CyD兲.11 The molecules were grown on theAu substrates by immersing the substrates into the 0.01 mM
ethanol solution with LP-␤-CyD molecules at room temperature. The Au共111兲 substrates were prepared by the vapor

Yasuda et al.

FIG. 2. STM images of LP-␤-CyD on Au共111兲 (150 nm⫻150 nm) formedby different immersion times, 共a兲 3 s (Vs⫽1.0 V, It⫽0.2 nA), 共b兲 6 s(Vs⫽2.0 V, It⫽0.3 nA), 共c兲 60 s (Vs⫽1.0 V, It⫽0.2 nA), and 共d兲 10 min(Vs⫽2.0 V, It⫽0.2 nA), respectively.

deposition of about 75-nm-thick gold films onto freshly
cleaved mica sheets. During Au deposition, temperature ofthe mica substrates was kept at 450 °C. The deposition rate
was 0.3–1.0 Å/s and vacuum pressure was 1.0⫻10⫺7 Torr.
Following the deposition, they were annealed at 550 °C for 2h. Brief flame annealing was performed before each experiment. The Au(111)-22⫻) herringbone structure wasclearly observed on these surfaces by STM.
In order to analyze the molecular structure and thegrowth mechanism, the LP-␤-CyD/Au samples with different molecular densities were prepared by changing the immersion time from 3 s to 12 h. They were repeatedly rinsed
with ethanol solution and dried with nitrogen gas flow. According to previous studies,14 physisorbed molecules can beclearly removed by the rinsing process, and only chemisorbed molecules remain on the surface. All STM observations were performed in constant current mode in air at room
temperature using Pt/Ir tips.
Figure 2 shows STM images of the LP-␤-CyD molecules adsorbed on Au共111兲 for different immersion times;
共a兲 3, 共b兲 6, 共c兲 60 s, and 共d兲 10 min. Cross sections along
A–B, C–D, E–F, and G–H in STM images are also shown.
The amount of adsorbed LP-␤-CyD molecules increased
with the immersion time, as predicted. However, the detailed
adsorption process, as observed by STM, was completelydifferent from that predicted from the macroscopic analyses.
First, at the initial stage, a few LP-␤-CyD molecules
adsorbed on the Au共111兲 surface. These adsorption sites

Downloaded 03 Mar 2003 to 130.158.147.14. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

Yasuda et al.

Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000

were not only random, but looked in part linearly ordered, as
shown in Fig. 2共a兲. According to the recent STM study of theself-assembled 4-mercaptopyridine molecules similarly
formed on the Au共111兲 surface, a similar molecular nucleation along the ordered lines was observed.19 Similarly ordered nucleation was also observed for a Ni atom on theAu共111兲 formed by vapor deposition.20 These results indicatethat the herringbone reconstructed structure of the Au共111兲
surface strongly influences the molecular adsorption at theinitial stage. In consideration of these facts, the adsorption
site of the LP-␤-CyD molecules may be related to the herringbone structure as in the other cases. Since the herringbone structure is relaxed rapidly by the molecular adsorption,
further study is necessary to clarify its effect on the growthmode in the very early stage.
Next, we analyze the growth process in further detail.
According to the macroscopic measurements,14 the growth
process of CyD derivatives is considered to be governed by
the Elovich growth model;d⌰/dt⬃ 共 1⫺⌰ 兲 exp 共 ⫺ ␣ ⌰/RT 兲
where, ⌰, ␣, and R represent surface coverage, activationenergy, and gas constant, respectively. The rate equation isbased on the repulsive interaction between molecules as isrepresented by the ⌰-dependent exponential factor. Increasein the number of unoriented molecules on the surface is an
example for the suppression. However, as is shown in Fig.
2共b兲, islands of LP-␤-CyD molecules are formed on
Au共111兲. Since island formation is caused by the attractive
interaction between molecules, the observed structure can
not be explained by the Elovich or the simple Langmuir ( ␣→0) model. In fact, in consideration of the fact that CyDs
contain many hydroxyl groups 关Fig. 1共a兲兴, attractive interaction between LP-␤-CyDs due to the hydrogen bonds is moreprobable. This difference may be due to the uncertainty ofthe macroscopic analysis. In fact, there exist large error bars
in the data on which the Elovich model was based.14 Therefore, ruling out the island growth mode from the data only
appears to be difficult.
As shown in the cross sections, the height of the molecules in Figs. 2共a兲–2共c兲 is ⬃0.6 nm, and is in good agreement with the size of a single CyD. Since STM measurements were performed in air, CyD molecules were directlyon the substrate. When the molecular density is saturated
关Fig. 2共d兲兴, molecules are almost uniformly adsorbed, andtriangularly shaped terraces are observed on Au共111兲, whichreflects the surface structure of the Au共111兲 substrate. And
the height difference between two neighboring terraces is0.25 nm, which is close to the value of an Au共111兲 stepheight. Therefore, contrary to the quasidoublelayer structure
predicted on the basis of the macroscopic analysis, a monolayer structure is formed on the surface.
As has been discussed, the structure and growth process645

of the self-assembled LP-␤-CyD on Au共111兲 were studied
using STM. The obtained results were different from thosepredicted by the macroscopic analysis. At the initial stage,
LP-␤-CyD molecules, as being partially influenced by theherringbone structure, adsorbed on the Au surface. Since island formation was observed in the intermediate stage, interaction between the LP-␤-CyD molecules must be attractive,
which differs from the previous predictions of the Elovich
model. Finally, distinct monolayer structural formation of
LP-␤-CyD molecules was observed on the surface. Since theother works were performed using Au substrates without
confirming their surface structures, atomic scale structure of
the Au surface may play an important role in the observed
formation process. In any case, since LP-␤-CyD molecules
form a monolayer on Au, application of the LP-␤-CyD/Au
system for the fabrication of multifunctional molecular devices has become more probable.
This work was supported by Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sportsand Culture of Japan. One of the authors 共S.Y.兲 was financially supported by the Society for the Promotion of Science
共JSPS兲 Fellowships for Japanese Junior Scientists.

1D. M. Cyr, B. Venkataraman, G. W. Flynn, A. Black, and G. M. Whitesides, J. Phys. Chem. 100, 13747 共1996兲.
F. Chara and J. Cousty, Phys. Rev. Lett. 80, 1682 共1998兲.
3
S. Yasuda, K. Miyake, Y. Goto, M. Ishida, K. Hata, M. Fujita, M.
Yoshida, J. Sumaoka, M. Komiyama, and H. Shigekawa, Jpn. J. Appl.
Phys., Part 1 37, 3844 共1998兲.
4
S. Yasuda, K. Miyake, J. Sumaoka, M. Komiyama, and H. Shigekawa,Jpn. J. Appl. Phys., Part 1 38, 3888 共1999兲.
5
H. Shigekawa, T. Morizumi, M. Komiyama, M. Yoshimura, A. Kawazu,and Y. Saito, J. Vac. Sci. Technol. B 9, 1189 共1991兲.
6
J. Szejtli, in Comprehensive Supramolecular Chemistry, 1st ed., edited byJ.-M. Lehn 共Pergamon, London, 1996兲 Vol. 3, Chap. 2, p. 5.
7
A. Ueno, H. Yoshimura, R. Saka, and T. Osa, J. Am. Chem. Soc. 101,2779 共1979兲.
8
M. T. Rojas, R. Königer, J. F. Stoddart, and A. E. Kaifer, J. Am. Chem.
Soc. 117, 336 共1995兲.
9G. Nelles, M. Weisser, R. Back, P. Wohlfart, G. Wenz, and S. M.-Neher,
J. Am. Chem. Soc. 118, 5039 共1996兲.
10
P. He, J. Ye, Y. Fang, I. Suzuki, and T. Osa, Anal. Chim. Acta 337, 217
共1997兲.
11
P. He, J. Ye, Y. Fang, I. Suzuki, and T. Osa, Electroanalysis 9, 68 共1997兲.
12
I. Suzuki, K. Murakami, J. Anzai, T. Osa, P. He, and Y. Fang, Mater. Sci.
Eng., C 6, 19 共1998兲.
13
J. Quian, R. Hentschke, and W. Knoll, Langmuir 13, 7092 共1997兲.
14
M. Weisser, G. Nelles, P. Wohlfart, G. Wenz, and S. M-Neher, J. Phys.
Chem. 100, 17893 共1996兲.
15
R. G. Nuzzo and D. L. Allara, J. Am. Chem. Soc. 105, 4481 共1983兲.
16
N. Camillone III, C. E. D. Chidsey, G.-Y. Liu, T. M. Putvinski, and G.
Scoles, J. Phys. Chem. 94, 8493 共1991兲.
17G. E. Poirier and E. D. Pylant, Science 272, 1145 共1996兲.
18
K. Tamada, M. Hara, H. Sasabe, and W. Knoll, Langmuir 13, 1558
共1997兲.
19
M. Hara, H. Sasabe, and W. Knoll, Thin Solid Films 273, 66 共1996兲.
20
D. D. Chambliss, R. J. Wilson, and S. Chiang, Phys. Rev. Lett. 66, 1721
共1991兲.
2

Downloaded 03 Mar 2003 to 130.158.147.14. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp