Experimental

Fine-grain titanium from Goodfellow (99.6%) was used. Titanium rods (5 mm radius) were ground and then electropolished according to [16]. Thin TiC>2 films were achieved by anodic oxidation in 0.5 m sulfuric acid according to [12]. TiN layers were preparad by ion implantation (3 keV, implanted dose 2 x 1017 N2+/cm2 into the fine-grain titanium surfaces. Sputter-Ti surfaces were 3 pm Ti sputtered on a silicon wafer (without grains and grain boundaries). Oxide films on these surfaces were prepared by the same technique as described above. STM and contact-mode AFM investigations were performed on a Topometrix TMX 2000 Discoverer in ambient air or in a nitrogen atmosphere. Spectroscopy measurements were recorded on a modified NanoScope I STM (described in more detail in [20]). All spectra were recorded while the STM was in a black box without light, since light has a significant influence on the tunnel current on semiconducting surfaces [21]. AFM tips were standard tips with a radius of 50nm (Si3N4 with a spring constant of 0.032 N/m). STM tips were prepared electrochemically (Au in 3 M KCN solution and W in 2 M NaOH with 100 Hz sine and 2-5 Vpp).

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