Formation of tungsten silicide on an STM tip during atom manipulation

We have investigated compositional changes of a tip before and after atom manipulation by a scanning tunnelling microscope (STM) combined with an atom probe (AP). On clean Si(0 0 1) : (2×1), the surface was modified by the STM with a bias of +5 V and 2 nA at the sample. The AP mass spectrum clearly showed that Si as well as W atoms were detected. The detected ratio of W to Si is 1 : 2 at first and then 5 : 3, which indicates that two types of stable tungsten silicides, WSi2 and W5Si3, respectively, are formed on the STM tip. Direct evidence of silicide formation during atom manipulation was clearly shown by STM combined with AP(AP-STM).