How can I make the viewed image (in sunlight) on my Nikon Camera LCD brighter?
One solution for this is to use a hood.
It's basically a black box that you place on your display. You look into it through a little eye piece that often also magnifies.
Take a look at what's available at B&H for example.
As you can see, making the image brighter is not a good solution. It will eat up more battery while still being unable to compete with the sun.
Instead, make the surroundings darker.
If you understand that principle, you should be able to even create a DIY solution with a bit of cardboard that's probably more compact and a lot cheaper than the commercially available hoods.