Kobe EBIS を用いた多価イオンと物質の相互作用におけるポテンシャル効果に関する研究

概要

A highly charged ion (HCI) has large potential energy increasing with its charge state. In order to remove electrons from an atom, it is necessary to give an energy equal to its binding energy, so a HCI produced after removal of many electrons has a large potential energy which is the sum of binding energy for each ionization step. Since the binding energy increases as the shell becomes deep one, the potential energy increases exponentially as the valence increases, and its value is approximately proportional to the
2.8th power of the valence. The interaction between a HCI and the surface results in emission of photons in the range of visible to X-ray, hundreds of secondary electrons, sputtering of secondary ions and the modification of the surface structure in the nanometer scale. The effects of HCI on the surface are different from those of singly charged ion (SCI), since SCI deposits only its kinetic energy. The effect characteristic to the potential energy deposition is called as ‘potential effect’. Many experiments of irradiating materials with HCIs have been performed. However, the fluence (number of irradiated ions per unit area, expressed as ~ ions/cm2) of HCI employed in most of the experiments is relatively small (~1011 ions/cm2). In the present study, the interaction of HCI (Ar8+ ~ Ar16+) with surfaces and the potential effect on various materials were evaluated at an experimental condition of relatively high fluence up to 1014 ions/cm2.

In the present study, HCIs were produced with an electron beam ion source called Kobe EBIS. Kobe EBIS, which has been developed at the Kobe University, is an electron beam ion source (EBIS)-type ion source of HCIs with higher ion beam intensity than existent similar type ion sources. Kobe EBIS uses a focused electron beam to remove electrons around an atom to generate HCIs. The Kobe EBIS consists of four parts; an electron gun, drift tubes, an electron collector and a magnet. In the Kobe EBIS the drift tubes are at room temperature during operation. The magnet is a commercially available superconducting magnet which is cooled by a closed cycle refrigerator. The magnet has Helmholz coils in order to expand the flat region of magnetic field strength (3 T along the electron beam axis).

Recent machine study on the Kobe EBIS revealed that periodic intermission of electron beam improves charge state distribution of extracted ions. This finding is useful for studying potential effect in the interaction of HCI with surfaces because the potential effect is emphasized for the irradiation with HCI in higher charge state. The period of intermission is in the order of 100 ms, and the width of beam‐off time is 1 ms or less. This operation makes it possible to produce Ar15+ to Ar17+ effectively, whereas the charge is limited less than 14+ under the ordinary operational mode with DC electron beam. A spike of HCIs with a peak current in the order of nA is also produced at each moment of electron beam off. The measurement on the time evolution of the intensity of Ar16+ around the timing of mode changes revealed that the intensity of extracted Ar16+ changes slowly after mode change with a time constant of few seconds, whereas Ar16+ is created rapidly within a few tens ms after switching on. This means that it needs several seconds to reach equilibrium charge state distribution of extracted ions after the operational mode is changed.

The purposes of the present study are to investigate the effects of HCI irradiation by conducting various studies in the region of experimental condition with high ion dose by utilizing the advantages of Kobe EBIS.

Carbon based materials such as carbon nanotube (CNT) are promising materials for nanodevices and circuits. Investigations on the modification of structural and electric/magnetic properties of carbon based materials by the irradiation with HCIs are expected to provide valuable information for applying HCIs to such nanotechnology as well as understanding the interaction of HCIs with the materials. Several experiments which exhibit the potential effect in various irradiation phenomena, such as modifications on structural, electric, and magnetic properties, and photon emission, was performed in this study with carbon based materials.

Structural modification of multi-walled carbon nanotubes (MWCNTs) irradiated with HCIs has been analyzed using a transmission electron microscope (TEM) and Raman spectroscopy. It has been thought to be difficult to observe the identical place on the nanometer scale because samples were transferred between irradiation at Kobe EBIS and observation with TEM. However, we have established a procedure for observing the same location in the sample before and after irradiation for accurate comparisons. We investigated the difference of TEM images at the identical position between samples before and after irradiation with HCIs. TEM images of MWCNTs irradiated with Ar14+ exhibit uneven surfaces at the fluence of 6 × 1013 ions/cm2. Outer diameters are decreased, outlines of surfaces become uneven and hollow regions disappeared by the irradiation. These results are different from the case of irradiation with SCIs. It is considered that potential sputtering occurred on the surfaces of MWCNTs. From Raman spectroscopy, there is no outstanding dependence on charge state. Since the D/G ratio increases as the fluence of HCIs increases, it is considered that many defects are introduced due to the kinetic energy of HCIs. It is considered that the number of defects is increased by irradiation. The Raman spectrum is considered to show the result of the average information about the defect of MWCNTs due to the beam size of the laser used. Therefore, the contribution of kinetic energy is large in Raman spectroscopy. On the other hand, since a fine structure in atomic scale can be observed by TEM observation, it is considered that the potential effect of HCI is observable.

Irradiation effects on the resistivity of single MWCNT supported on micrometer scale bridge pattern was also measured. The as-prepared MWCNTs were dispersed onto a Si substrate. The source/drain contacts were patterned by depositing 30 nm of Pd followed by 10 nm of Au. The resistance between the contacts was measured using a DC two-terminal method. The resistance was ~10 kΩ at room temperature in most of the MWCNT channels before the irradiation. We evaluated the electric characteristics of MWCNTs irradiated with the HCIs of Ar8+, Ar11+, and Ar14+. Each MWCNT located on the high dope Si substrate had a source/drain contact, forming a back-gate FET configuration. The source to drain resistance of the MWCNT channel tended to increase with increasing fluence and charge state of the HCIs. The current-voltage curves were nonlinear at low temperatures for the irradiated samples, and Coulomb oscillations were observed in the gate voltage dependence of the drain current. At a cryogenic temperature (1.6 K), Coulomb diamonds and periodic Coulomb oscillations were observed, and it is suggested that a single quantum dot (QD) was formed. Fabrication of QD with HCI irradiation is an epochal achievement. This result is full-scale nanotechnology research using atomic physics techniques such as HCI irradiation, which has not been realized before the present research.

We have measured magnetic structure of highly oriented pyrolytic graphite (HOPG) samples irradiated with HCIs using electron spin resonance (ESR) at low temperature. HOPG samples were irradiated with Ar8+ and Ar14+. There are two resonance lines in the spectra at low temperatures. A Dysonian line (D1 line) was observed in lower resonance field. A Lorentzian line (L1 line) appeared at low temperatures in higher resonance field. The L1 line was not observed in unirradiated

HOPG sample. This line is considered to be caused by dangling bonds produced by irradiation with HCIs. The L1 line intensity of Ar14+ is larger than that of Ar8+ at the same fluence of 1 × 1014 ions/cm2. This is because of the effect of the potential energy of HCIs. The L1 line intensity increases almost proportional to the fluence. Therefore, it is suggested that the defects of HOPG responsible for the magnetization is roughly proportional to the fluence of HCIs and increases with the charge state. Potential effects on the interaction of HCI with the surface are observed. The detection of ESR peak due to HCI irradiation is limited to our research group which uses Kobe EBIS.

For photon emission measurements, we observed spatial and spectral distribution of visible light emission from the surface during irradiation with HCIs. Wavelength distribution for the range of 420–670 nm and spatial distribution of horizontal direction were measured from the two-dimensional CCD image. From the emission spectra, it is suggested that these peaks are not derived from irradiated ions or samples since three emitted light lines appear in common. The analysis of ions produced by irradiation with HCIs was performed using a QMS to identify the origin of the light emission. From these results, it seems that emitted light lines observed by spectroscopic measurements come from H and Na, i.e. Balmer series (Hα and Hβ) and the resonance line of Na (D line). The origin of luminescence was identified as hydrogen atom as the main light emission species from the spectroscopic measurement and SIMS. To investigate the nature of Balmer light produced by the injection of HCIs, the emission intensities at the 656 ± 5 nm region (Hα) were evaluated for various experimental conditions. The emission intensity of Balmer light increases with the charge state of HCIs and decreases as the irradiation continues. The Balmer light emission is strongest near the surface irradiated with HCIs. In addition, temporal change in the Balmer light emission of the ABS resin irradiated with HCIs showed that H contained in the chemical composition of the ABS resin did not contribute significantly to the Balmer light than H derived from water or hydrogen. Our observation of Balmer light emitted from excited hydrogen atoms produced by the interaction of HCI with the surface is also a unique achievement in the world.

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