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Nanoscale modification of electronic states induced by the impact of slow highly charged ion (HCI) onto highly oriented pyrotytic graphite (HOPG) surfaces is reviewed. The high potential energy of slow Ar induces multiple emission of electrons from the surface, providing a strong modification of the electronic states of the local area of HOPG surfaces. Size of created protrusions created by the Ar-impact was about 1 nm in diameter with 400 eV of the kinetic energy, and the subsequent surface treatment by electron injection from a scanning tunneling microscope (STM) induced a localized transition from sp to sp hybridization at the center of the protrusion, which considered to result in the formation of nano-diamond-like structures. Figs. 1(a)-(d) summarize the variation of STM images of HOPG after the single ion impact with some optional treatment. Fig. 1(a) shows symmetric triangular features due to electronic perturbation in the case of Ar impact, indicating multiple point which were typically seen for the single charged ion irradiation onto HOPG surfaces. On the other hand, in the case of Ar irradiation, a larger protrusion-like feature was observed due to the contribution of the Coulomb potential of ions as shown in Fig. 1(b). Similar protrusion-like features created on HOPG surface by HCI impact have been observed in the different kinetic energy region. When electrons were injected from an STM tip into the protrusion created by the impact of Ar, the center part was observed as a dark region as shown in Fig. 1(c). Here the electron injection was carried out by applying bias voltage in an STM. This transformation indicates that electronic states of the center had been modified by electron injection. The I-V characteristics measured by scanning tunneling spectroscopy (Fig. 1(e)) represent that the dark center becomes nonconductive while the area surrounding the dark center possesses metallic nature, suggesting the formation of a large band gap via interaction with Ar. Fig. 1(d) represents an STM image of the Ar-impact area followed by electron injection and the additional thermal treatment in hydrogen ambient. This treatment was carried out at 600 °C for 30 min in 76 Torr of H2 pressure. This structure is found to show I-V characteristics similar to the CVD-grown poly-diamond in the field emission measurement by means of STM. The possible structure of the Ar-impact site is considered to be a planar-type sp structure with a vacancy, which is expected easily to transfer from the planar structure of sp graphite. Though the details of the formed structure are still being studied, HCI has potential to create novel surfaces with attractive nature completely different from the case of singly charged ion.

Effects of Slow Highly Charged Ion Impact Upon Highly Oriented Pyrolytic Graphite-Nanoscale Modification of Electronic States of Graphite Surface-