Can someone explain van der Waals planes

Mutauelectron

Hello all,

I'm participating in an REU this summer in Condensed Matter and one project is building a device out of tungsten disulfide. Anyway, in my reading I've come across the term "van der Waals plane" and the paper discusses planes perpendicular and parallel to the substrate (with respect to WS2 growth on something like glass or quartz).

Can someone explain this to me? Is it talking about the orientation of the binding forces within the WS2 molecules?

Thanks,
Aaron