Fullerenes DOI: 10.1002/smll.200701242 Reversibility-Controlled Single Molecular Level Chemical Reaction in a C 60 Monolayer via Ionization Induced by Scanning Transmission Microscopy Masato Nakaya, Yuji Kuwahara, Masakazu Aono, and Tomonobu Nakayama* Controlling chemical reactions at the single-molecule level, especially controlling reversibility of such a reaction, is of great interest for realizing stable switching devices, nonvolatile memory cells, and so on at the single-molecular scale in future nanoelectronics and molecular electronics. [1–5] For this purpose, it is also important to realize such control using molecules that are tolerant of repetition of a reversible reaction. From these viewpoints, the association of two C 60 molecules into one C 60 dimer and the dissociation of the dimer into two molecules, respectively termed the polymerization and depolymerization of C 60 molecules, [6–8] are extremely attractive not only because of the stability of the C 60 molec- ule [8] but also because of the reversibility of the intermolecular reaction between the C 60 molecules. [9,10] In the present work, we show that the polymerization and depolymerization in a C 60 monolayer film on a highly oriented pyrolitic graphite (HOPG) surface can be selectively induced at the single molecular level resolution and precision under the tip of a scanning tunneling microscope at room temperature. The applications of appropriate negative and positive sample bias voltages result in the polymerization and depolymerization, respectively. We conclude that such scanning tunneling microscopy (STM)-induced polymerization and depolymer- ization occur via STM-induced ionization of C 60 molecules; in other words, reversibility-controlled single molecular level chemical reactions between C 60 molecules are realized. All the experiments described in this Communication were carried out in an ultrahigh vacuum environment. We prepared well-ordered and uniform C 60 monolayer films on a HOPG surface and approached the tip of the STM instrument, as schematically shown in Figure 1a. In pristine monolayer films, all the C 60 molecules exhibit smooth hemispherical features, as shown in STM observations in Figure 1b, because of their free rotation at room temperature. After fixing the position of the STM tip above the point indicated by the cross in Figure 1b, a voltage pulse with V s ¼2.6 V and and current I t ¼ 300 pA for 1 s was applied to the sample. As a result, two C 60 dimers were formed, as indicated by the white arrows in Figure 1c. We found that these dimers could be depolymerized by applying a 1-s voltage pulse with V s ¼þ3.5 V and I t ¼ 100 pA. The depolymerization process will be described again later. After depolymerization, we again applied a 1-s voltage pulse with V s ¼2.5 V and I t ¼ 300 pA and observed the STM image as shown in Figure 1d. A single C 60 dimer was created in the C 60 monolayer film as indicated by the white arrow in the figure. The appearance of C 60 molecules apparently changes after the polymerization: Two molecules forming a dimer show their internal structure corresponding to the spatial distribution of the probed molecular orbitals, [11] indicating that the free rotation of the molecules is inhibited. And the intermolecular distance is reduced to about 90% of its initial distance, which is in good agreement with the reduction ratio that have been measured by X-ray diffraction in the case of the C 60 dimer crystal [12] and by STM in the case of the photo- induced polymerization in a C 60 monolayer on the Si(111) H3–Ag surface. [13] Figure 2 is a series of STM images that show the depolymerization of dimers in a C 60 monolayer on the HOPG surface. Here, we first scanned the STM tip over the designated area at V s ¼2.7 V and I t ¼ 100 pA to induce the polymerization, which results in the creation of nine C 60 dimers as indicated by the white arrows in Figure 2a. Next, we scanned the tip over the same area at V s ¼þ2.3 V and I t ¼ 100 pA, resulting in the depolymerization of three dimers, as denoted by black arrows in Figure 2b. Then, we scanned the tip over the same area at V s ¼þ2.5 V and I t ¼ 100 pA, resulting in the depolymerization of all the dimers as shown in Figure 2c. It is noted that, during these depolymerization experiments at positive V s , no polymerization was observed. An important finding is that it is possible to intentionally induce one of the two chemical reactions, polymerization and depolymerization, as follows: negative V s induces polymeriza- tion whereas positive V s induces depolymerization in a C 60 monolayer film on the HOPG surface. This means that the STM-induced chemical reaction between fullerene C 60 molecules is switchable from polymerization to depolymer- ization and vice versa just by switching the polarity of the bias voltage over the tunneling gap in the STM: namely, reversi- bility of the chemical reaction is controlled. Small V s values in the range of –1.5 to 1.5 V can safely prevent the C 60 molecules from polymerization and depolymerization. The positions where the chemical reactions are induced are almost under- neath the tip. The positional deviations between the tip position and the location of reacted molecules were typically smaller than 2 nm in our experiments. The plausible reasons for such deviation are twofold: the tip shape and the probabilistic nature of the reactions. C 60 monolayer films were prepared on a Si(111)H3–Ag surface, [14–16] but STM-induced polymerization and depoly- merization were not realized in these films. To understand the communications [  ] Prof. T. Nakayama, M. Nakaya, Prof. M. Aono National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) Fax: (þ81)-29-860-4793 E-mail: nakayama.tomonobu@nims.go.jp M. Nakaya, Prof. Y. Kuwahara Department of Material and Life Science Graduate School of Engineering, Osaka University 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan) Prof. T. Nakayama, Prof. Y. Kuwahara, Prof. M. Aono Nanoscale Quantum Conductor Array Project, ICORP Japan Science and Technology Agency 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan) 538 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 5, 538–541