Figure 4

A schematic model of the STM experiment on HOPG(0001) with the [001]-oriented W tip (a). STM images (6×6 Ų) measured with the W[001] tip shown in Fig. 3 at a sample bias voltage of −50 mV and tunneling currents of 0.15 nA (b), 0.2 nA (c) and 0.4 nA (d). The [100] and [010] crystallographic directions of the tungsten tip in the experiment coincide with the x and y axes of the STM scanner.