3.3 WO2/W(110)

The high-temperature oxidation of W(110) results in the formation of a thin, strained-commensurate WO2(010) structure at the surface [94]. A typical STM image and a LEED pattern taken from the WO2/W(110) surface are shown in Figures 3.6a and 3.6b, respectively.


Figure 3.6: (a) Low-temperature STM image of the WO2/W(110) surface: Vsample-0.06 V, It = 0.10 nA, size 6.5 nm × 6.5 nm, 78 K. The green parallelogram shows the oxide’s oblique unit cell with dimensions a = 2.5nm and b = 1.3nm. (b) LEED pattern from the WO2/W(110) surface, acquired at a primary beam energy of 70 eV

WO2(010) has an O–W–O trilayer structure and forms well-ordered oxide nanorows separated by 2.5 nm on the surface (Figure 3.6a). These rows appear as bright regions with dark depressions in between. The atomic structure of the WO2 overlayer is shown in Figure 3.7. The trilayer structure can be clearly seen in the side view schematic, looking along the [001] direction of the substrate.


Figure 3.7: Top and side view of the WO2/W(110) surface. Red atoms indicate oxygen, with the large atoms forming the topmost layer. Blue atoms represent the tungsten sandwiched in the WO2 trilayer, and grey atoms make up the W(110) surface. The side view is looking along the [001] direction of the W(110) surface, parallel to the W–W short bonds in the oxide. The green parallelogram highlights the unit cell of one nanorow.

The LEED pattern (Figure 3.6b) shows characteristic satellite spots around each primary W(110) spot, representing two equivalent overlayer domains on the surface. The WO2 nanorows follow either the [337] or the [337] directions of the W(110) substrate depending on the domain [94]. The WO2 overlayer has an oblique unit cell with vectors a = 2.5nm and b = 1.3nm, as obtained by STM and confirmed by LEED, shown on Figure 3.6a as a green parallelogram.

The presence of oxide nanorows can influence the growth of a C60 molecular overlayer, and so the WO2/W(110) surface has been chosen as an interesting nanostructured template for their self-assembly.