[R] how to make point character thicker in xyplot?

[array chip]

Is there anyway to make plotting point character being thicker in xyplot? I mean 
not larger which can achieved by "cex=2", but thicker. I tried lwd=2, but it 
didn't work. I know "lwd" works in regular plot() not only for lines, but also 
for points. For example

plot(1:10, lwd=2)

Thanks

John