Scanning Tunneling Microscopy and Atomic Force Microscopy of Au Implanted in Highly Oriented Pyrolytic Graphite
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233 Mat. Res. Soc. Symp. Proc. Vol. 396 01996 Materials Research Society
of two different gold implantation doses(6.8x I0' 6/cm2 and 3.2x10' 6/cm2 ). 10000E E. §00
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DEPTH (ANGSTROM) Figure 1: Rutherford backscattering depth profiles of the implanted gold in graphite of two different gold implantation doses.
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Figure 2 Constant current STM image of 6.8x1016/cm 2 implanted HOPG.
According to Figure 1, the gold concentration on the surface of the 6.8x 10 `6/cm 2 gold implanted graphite should be much higher than the 3.2x 0'0 6/cm 2 implanted graphite. Figure 2 is the constant current STM image of the 6.8x10' 6/cm 2 implanted sample. Round shaped features are over most of the sample surfaces with lateral diameters in the range of 7-16 nm. Similar features were also observed by the tapping mode AFM and z-heights of these features are around 7-10 nm. In a previous study, graphite samples implanted at doses of l x 0 "2cm 2 gold at energies of 530 KeV or 4.5 MeV were studied by STM.[21 Hillocks with diameters - 1.8 nm are the major features formed on the surface. Due to the much larger sizes of the round features (7-16 nm), they are probably not hillocks but gold colloids. The TappingMode AFM suggests the existence of gold colloids with diameters around 3-4 nm on the surface of 3.2x10 6" /cm 2 ample. Based on the above observation, the sizes of gold colloids formed depends on the ion dose which is consistent with our previous observation of gold colloids formed in gold ion implanted mica.[3] To further characterize the round shaped features observed on the graphite surfaces, we applied a bias voltage in the range of +3.5-4 V which modifies the surface. Recently, a number of presentations have dealt with the modification of gold surfaces by scanning tunneling microscopy.[6-81 Typically mounds of 100-200 A in diameters and heights of 20-30 A were formed on gold surfaces by applying pulses of 600 ns and +3.6 V.[6] Both field evaporation and mechanical contact were proposed as mechanisms for creating the mounds or craters. On the other hand, a different mechanism is proposed by Ohto et. al..[8] The authors applied a bias of ±2-3 V, a current of 20 nA, and the duration is 1 minute instead of nano seconds to modify gold films of different thicknesses deposited on HOPG. During this one minute application of ±2-3 V, the feedback maintained a tunneling resistance of 0.1 GO. Under these conditions it is unlikely that there was mechanical contact between the tip and the sample. Depending on the thickness of the film and the tip voltage, either craters or hillocks were formed on the gold surfaces. The authors explain the phenomenon by migration of gold. The migration mechanism is attributed to electro- and thermo-migration. We performed similar experiments on the gold implanted samples. Figure 3 a shows a STM image of a sample dosed with
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3.2x10l6/cm' Au ions and scanned under a bias current of 1 nA and bia
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