Monolayer formation of 6-deoxy-6-thiol-␤-cyclodextrin on a Au„111… surface
studied by scanning tunneling microscopy
Satoshi Yasuda, Don Norimi Futaba, and Osamu Takeuchi
Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japan

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

Kiyoshi Yase
UltraThin Films Group, Photonics Research Institute, National Institute of Advanced Industrial Scienceand Technology (AIST), Tsukuba 305-8561, JapanJun Sumaoka and Makoto Komiyama
Research Center for Advanced Science and Technology, University of Tokyo, Komaba,Tokyo 153-8904, JapanHidemi Shigekawaa)
Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japan

共Received 9 October 2000; accepted 26 March 2001兲
Mono共6-deoxy-6-thiol兲 ␤-cyclodextrins 共␤-CyD–SH兲 were grown on a Au共111兲 surface in ethanol
solution, and monolayer formation process of which was studied by scanning tunneling microscopy
for the first time. At the initial stage, ␤-CyD–SH molecules were adsorbed on the Au共111兲 surface
not only randomly, but also partially in a linear ordering. In the following stage, islands were formed
and resulted in a uniform monolayer. The observed mechanism is completely different from themodel predicted on the basis of the macroscopic result, the Langmuir model. An attractive
interaction exists between ␤-CyD–SHs, contrary to the prediction by the model. The observed
growth processes are quite similar to what was previously observed for lipoamide– ␤-cyclodextrin
共LP– ␤-CyD兲. However, in the case of ␤-CyD–SH, the islands did not grow in size as compared tothe case of LP– ␤-CyD, and the number of the islands increased to form a uniform monolayer. Thischaracteristic indicates that the interaction strength ratio of CyD–Au to CyD–CyD is larger for
␤-CyD–SH than that for LP– ␤-CyD. © 2001 American Vacuum Society.
关DOI: 10.1116/1.1372906兴

I. INTRODUCTION
Cyclodextrins 共CyDs兲 have a donut-shaped structure consisting of several glucopyranose units as shown in Fig. 1共a兲,and are able to include various guest molecules in theircavities.1 Due to this characteristic, they play important rolesin the field of catalysts, artificial enzymes, and biosensor
devices, such as drug deliverers and material removers withhigh selectivity. Since structures of CyDs are very simple,
they are also important as the models for the analysis of thefunctions of more complicated molecules.
On the other hand, self-assembled monolayers 共SAMs兲 oforganic molecules have been studied extensively in developing the interfacial systems that provide various functional
properties on solid surfaces.2 From a practical point of viewfor the industrial applications, formation of SAMs on the Au
surface by using the molecular chains such as thiols has been
most attractive. This technique easily provides stable andhighly packed monolayers of functional molecules by simple
treatments such as immersion or vapor deposition processes.
With this advantage, high potential is expected to realize
new molecular devices by fabricating the micromorphologya兲Electronic
mail:dora.ims.tsukuba.ac.jp/1266

hidemi@ims.tsukuba.ac.jp;

www:J. Vac. Sci. Technol. A 19„4…, JulÕAug 2001

http//of materials; microcontact printing,3 lithography with ultraviolet light, and so on. Therefore, in order to use the highpotentiality of CyD materials, it is important to establish thistechnique for the case of CyDs.
Recently, Rojas et al. confirmed, by using the electrochemical method, that SAMs of the thiolated CyD derivatives work as molecular receptors to achieve molecular recognition sensors.4 Thiolated CyD derivatives are adsorbed
onto the Au surface by the chemical bonding between sulfur
atoms in the molecular chains of thiols and the Au surface,resulting in the CyDs having their cavity axis aligned perpendicular to the Au surface. The conformation of CyD/Au
gives functions as a recognition sensor, since CyDs can selectively include molecules into their cavities.
In our recent work, we performed the first microscopic
analysis on the structure and formation process of
lipoamide– ␤-cyclodextrin 共LP– ␤-CyD兲 on the Au共111兲 surface by using scanning tunneling microscopy 共STM兲, andconfirmed that LP– ␤-CyD molecules uniformly formed amonolayer on the Au substrate.5 However, in the case of
thiolated CyDs modified with long molecular chains, such as
LP– ␤-CyD, the flexibility of the long molecular chain induces unstable CyD motions, resulting in the lower efficiency of the function. Therefore, in order to utilize the high0734-2101Õ2001Õ19„4…Õ1266Õ4Õ$18.00©2001 American Vacuum Society

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FIG. 1. Schematics of: 共a兲 ␤-cyclodextrin 共CyD兲, 共b兲 ␤-CyD–SH, and 共c兲
LP– ␤-CyD.

potentiality of the material further, it is necessary to clarify
the possibility of stabilizing the Au/CyD system using thiolated CyD derivatives with shorter molecular chains.
Among such derivatives, since mono共6-deoxy-6-thiol兲
␤-CyD 共␤-CyD–SH兲 has only one thiol group on the primaryhydroxyl side, the CyD/Au system using ␤-CyD–SH is expected to be one of the most favorable candidates to realize astable structure to have a high efficient molecular recognition
system.
In consideration of the facts described above, we performed STM analysis on the monolayer formation process of
␤-CyD–SH molecules on the Au共111兲 surface. The results
obtained were compared with those obtained for the case of
LP– ␤-CyD, which has a longer molecular chain.

II. EXPERIMENT
Figure 1 shows the schematic structures of: 共a兲 ␤-CyD
共consisting of seven glucopyranose units兲 and 共b兲 mono共6deoxy-6-thiol兲 ␤-cyclodextrin 共␤-CyD–SH兲 used in thisstudy. In order to compare the characteristic of ␤-CyD–SH
with the case of CyD modified with longer molecular chains,
CyD modified with a lipoamide residue 关Fig. 1共c兲兴 was alsoprepared, and STM observation was performed again.
These molecules were grown on Au substrates at room
temperature by immersing the substrates into the 0.01 mM
ethanol solution containing the molecules. Au共111兲 substrates were prepared by the vapor deposition of about 75nm-thick gold films onto freshly cleaved mica sheets. During
Au deposition, the temperature of the mica substrates waskept at 450 °C. The deposition rate was 0.3–1.0 Å/s andvacuum pressure was 1.0⫻10⫺7 Torr. Following the deposition, they were annealed at 550 °C for 2 h. Each substrate
was briefly flamed using a H2 torch before each experiment.
The Au共111兲-22⫻冑3 herringbone structure was clearly observed by STM. In order to analyze the molecular structure
and the growth mechanism, the ␤-CyD–SH/Au samples withdifferent molecular densities were prepared by changing theimmersion time from 3 s to 6 h. The samples were repeatedly
rinsed with ethanol solution and dried with nitrogen gas
flow. According to the previous studies, physisorbed molecules can be clearly removed by the rinsing process, andonly chemisorbed molecules remain on the surface.6 All
STM observations were performed in constant current modein air at room temperature using a Pt/Ir tip.
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FIG. 2. STM images of ␤-CyD–SH on Au共111兲 formed by different immersion times 共100 nm⫻100 nm, V s ⫽0.5 V, I t ⫽0.3 nA兲: 共a兲 3 s, 共b兲 6 s, 共c兲 60s, 共d兲 10 min and 共e兲 1 h, respectively, 共f兲 magnification of 共c兲. 共g兲 and 共h兲cross sections along A – B in 共a兲 and C – D in 共b兲, respectively.

III. RESULTS
Figure 2 shows STM images of ␤-CyD–SH molecules
adsorbed on the Au surface for different immersion times of共a兲 3 s, 共b兲 6 s, 共c兲 60 s, 共d兲 10 min, and 共e兲 1 h, respectively.
For comparison, results obtained for LP– ␤-CyD are shownin Figs. 3共a兲 1 s, 3共b兲 6 s, 3共c兲 60 s, 3共d兲 10 min, 3共e兲 1 h, and3共f兲 3h.
At the initial stage, ␤-CyD–SH molecules adsorbed on
the Au surface not only randomly, but also in partially linear
ordering, as shown Fig. 2共a兲. Similar structure was observedat the initial adsorption stage of LP– ␤-CyD on the Au sur-FIG. 3. STM images of LP– ␤-CyD on Au共111兲 formed by different immersion times 共100 nm⫻100 nm, V s ⫽0.5 V, I t ⫽0.3 nA兲: 共a兲 3 s, 共b兲 6 s, 共c兲 60s, 共d兲 10 min, 共e兲 1 h, and 共f兲 6 h, respectively. 共g兲 and 共h兲 show the crosssections along A – B in 共a兲 and C – D in 共b兲, respectively.

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face. These results indicate that the herringbone reconstructed structure of the Au共111兲 surface must have an influence on the molecular adsorption at the initial stage.
As shown in the cross sections in Figs. 2 and 3, the apparent height of ␤-CyD–SH and LP– ␤-CyD molecules wasabout 0.4 and 0.6 nm, respectively. The difference in theheight was confirmed by measuring the height of the single
Au atomic steps, 0.25 nm. The height difference may berelated to the difference of the molecules used for the modification. For example, it may be due to the fact that theLP– ␤-CyD molecule has a stable lipoamide residue. However, further analysis including theoretical calculation isneeded to understand the details.
In the following stage, many island structures were observed as shown in Fig. 2共b兲. As the immersion times wereincreased, the Au surface was fully covered with ␤-CyD–SH
关Fig. 2共c兲兴, and ␤-CyD–SH molecules were adsorbed
densely as expected.
The observed initial growth processes of ␤-CyD–SH arealmost the same as that of the LP– ␤-CyD case shown in Fig.
3. However, details were different. Island size of LP– ␤-CyD
grew with the immersion time until the formation of a monolayer. On the other hand, in the case of ␤-CyD–SH, theislands did not grow in size, and the number of islands increased to form a monolayer 关Fig. 2共c兲兴. Growth speed andtime to complete the monolayer formation were shorter forthe case of ␤-CyD–SH. With further increase of the immersion time after monolayer formation, many etch pit-like defects appeared for both cases 关Figs. 2共d兲–2共e兲 and 3共f兲兴.
IV. DISCUSSIONAs shown Figs. 2共c兲 and 2共f兲, ␤-CyD–SH molecules adsorbed on the Au surface and formed a monolayer structurewith high density as expected. According to the macroscopic
analysis, the growth process of the ␤-CyD–SH molecule wasexplained by the Langmuir mechanism, which is the growthmodel without any interaction between molecules.6 However, in the observed growth process of ␤-CyD–SH, manyisland structures were observed 关Fig. 2共b兲兴 as well as the caseof LP– ␤-CyD molecules 关Fig. 3共b兲兴. Since island formation
is induced by the attractive interaction between molecules,
the observed process cannot be explained by the Langmuirmodel. In consideration of the fact that CyDs contain manyhydroxyl groups 关Fig. 1共a兲兴, attractive interaction between
CyD molecules is expected to occur due to the hydrogenbonding as observed experimentally.
The wrong prediction on the basis of the previous works
is thought to be due to the uncertainty of the macroscopic
analysis. Since macroscopic analyses only give information
averaged over the surface, accurate analysis of the microscopic structure is difficult. Our observation of the
␤-CyD–SH growth mode strongly indicates the importanceof the microscopic analysis as previously pointed out
through the STM experiment on LP– ␤-CyD.
In the case of LP– ␤-CyDs, as the immersion time increased, the island size of LP– ␤-CyD grew until the completion of the monolayer formation. On the other hand, in theJ. Vac. Sci. Technol. A, Vol. 19, No. 4, JulÕAug 2001

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FIG. 4. Histogram of observed island size of ␤-CyD–SH and LP– ␤-CyD,respectively. The sampling of dates performed the results of STM images
for 6 s immersion times.

case of ␤-CyD–SH, the island did not grow in size, but thenumber of island increased to form a monolayer. Figure 4
shows the histogram of the observed island sizes of
␤-CyD–SH 共gray bar兲 and LP– ␤-CyD 共white bar兲 molecules, respectively. The data were extracted from the STM
images obtained for 6 s immersion time 关Figs. 2共b兲 and 3共b兲兴,respectively. The island size and dispersion are smaller forthe case of ␤-CyD–SHs. In the case of ␤-CyD–SH, the observed island size was saturated through the growth 共average
diameter was about 6.6⫻6.6 nm2兲, and monolayer wasformed by the increase in the number of islands.
Generally, the diffusion length of molecules on the Au
surface is a crucial factor to govern the island size, and isdetermined by the relation of the intermolecular interaction
between CyD molecules and the chemical reaction process
between Au and S atoms in molecular chains. In consideration of the observed smaller island size for ␤-CyD–SH, it isconcluded that the strength ratio of interactions, CyD–Au to
CyD–CyD, is larger for ␤-CyD–SH than that for LP– ␤CyD.
Etch pit-like defects in the alkanethiol monolayer are considered to be formed by the release of Au atoms due to S–Auinteraction.7 Since the time to form the monolayer and theappearance of the etch pit-like defect were faster for the caseof ␤-CyD–SH, ␤-CyD–SH molecules are more strongly adsorbed on the Au surface compared to LP– ␤-CyD.
V. CONCLUSIONThe structure and growth process of ␤-CyD–SH molecules adsorbed from ethanol onto the Au共111兲 surface werestudied microscopically using STM for the first time. Theresults obtained were completely different from those predicted on the basis of the macroscopic analyses. The observed interaction between ␤-CyD–SH molecules was attractive, which differs from the previous predictions of theLangmuir model of no interaction between the molecules.
From our STM analysis, structure and growth of the CyD
adlayer depend on the kind of CyD derivatives, suggesting
the existence of controllability of the functional molecular

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structures on the Au surface by changing the immersion time
and the kind of molecular chains.
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, and Culture of Japan.
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