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Oxidative nano-writing by a conducting AFM tip to Si substrates has so far been ascribed to an anodic electrochemical process. However, there is evidence that thermal effects may play a role. More recently it has been demonstrated that conducting diamond-like carbon (DLC) films can be nano-machined by a biased conducting tip; the process is evidently due to local thermally activated oxidation and formation of CO2. Thus the prevailing description in terms of an athermal anodic oxidative mechanism will need to be revisited and possibly revised. The physico-chemical state of the tip is known to affect the rate and efficiency of the process in some unspecified manner. It is plausible to ascribe variations in the I-V characteristics and thermal transport to tip effects that are operational at the point of contact. However, so far the focus has been on the surface, and little attention has been given to a detailed and relevant characterization of the tip and its efficacy as a manipulative probe. Tips were prepared with a range of known initial conditions (i.e. doped Si plus H-termination, Au-coating, or native oxide). I-V characteristics were correlated against oxidative efficiency versus clean H-terminated Si, Si plus native oxide, Si plus 2.5 nm thermal oxide and DLC. The outcomes at positive sample bias can variously be described by ohmic transport, by Fowler-Nordheim, and direct tunnelling. Likewise, tip alteration resulting from oxidative surface manipulation has been monitored by subsequent characterization. The evidence favours a thermal mechanism of oxidation arising from a repetitive, but interrupted, deposition of thermal energy at the tip-to-surface junction, where inelastic tunnelling completes the circuit. Thus there is a narrow temperature window of optimum conditions. The tip will deteriorate irreversibly if the peak temperature is too high, while oxidation of the surface will not take place if the peak temperature is too low.
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