Friction Change Induced by Single Mev Ion Impact Measured by Scanning Probe Microscope
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Mat. Res. Soc. Symp. Proc. Vol. 396 01996 Materials Research Society
Target Chamber
Scanner Accelerator
Analyzing Magnet
Analyzing Magnet
Figure 1 Schematic diagram of ion irradiating system generation by nuclear collision and electronic stopping power, then compared the calculated results to the observed probabilities of ion track creation. EXPERIMENTS The specimens
-
5x5 mm2 HOPG substrates -
were cleaved immediately before being
placed in a vacuum target chamber at 1 0 4 Pa. The ion irradiation was conducted on a tandem type accelerator (National Electrostatic Corp. 3SDH-4). Figure 1 shows the schematic diagram. A monochromatic beam was ensured by the use of analyzing magnets and an electrostatic beam deflector for removing neutral particles. The ion species used were Si++, Cu*+, Ag•, and Au**, the ion energy was 3.1 MeV, and the dose, measured by integrating the current from the specimen to ground, was 2.3xl0 11cm-2. The specimen was surrounded by negative bias (-300 V) electrodes to suppress secondary electron emission, making the current nearly equal to the ion beam current. We observed specimen surfaces using a scanning probe microscope (SEIKO instrument Inc. SP13700) in air. The deflection and the torsion of the cantilever were simultaneously measured by an optical probe using a four-segmented photodiode. When the scanning direction was perpendicular to the cantilever, topographies were detected by the cantilever deflection (AFM mode), and the lateral force between the tip and surface was detected by the torsion of the cantilever (FFM mode). A tip of the commercial cantilever used was made of silicon nitride. RESULTS FFMobservation
The observed image of the Au ion-irradiated HOPG surface in atomic resolution using FFM shows high friction force between the tip and surface as a dark colour (Figure 2). The region disordered by ion impact, dark as a whole, was observable, with no disordered areas seen in unimplanted HOPG surfaces. This colour indicates
ordred region
Figure 2 Frictional force image (10xl0 nm2) of the ion track on the HOPG surface, ion energy: 3.1 MeV, ion sepecies: Au. 668
that the frictional force between the surface and tip increased in the disordered region, namely, MeV ion irradiation created a disordered, and high-friction region 4 to 8 nm in diameter. This region caused by ion impact is here after called an ion track. We observed HOPG specimens individually implanted 3.1 MeV Au, Ag, Cu, and Si ions using FFM in order to examine the effect on the ion species (Figure 3, 500x500 nm2 image area). Ion tracks appear as dots on a surface observed at low magnification, and are seen in every image. However, the surface density of ion tracks for each ion-irradiated surface differed, being about 1000 jim-2 at the Au-irradiated surface, 880 jm-2 at the Ag-irradiated surface, 372 Jim-2 at the Cuirradiated surface and 44 gim-2 at the Si-irradiated surface. Then, the each probability of ion track creation was deducted from the ratio of the observed number density to the incident ion dose.
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