Trigger function key using applescript

I'm trying to trigger a function key using applescript. The increase volume 'key'.

I wrote the following applescript but it isn't effective:

tell application "System Events"
    key down 63 -- fn
    key code 111 -- f12
    key up 63
end tell

just executing 'key code 111' gives me the same result as the code above,

 key down 63 

doesn't seem to effect the behaviour of the script.

Is there anything like

 using function down / using fn down

just like there is for the command key

 using command down

or any way to achieve this?

Thanks,

Rik