Help? Gaps between quads

Hi all! I've run into an interesting but annoying problem recently.

I'm trying to make a Minecraft clone in Unity just for fun, but I discovered that if I place two quads next to each other, a thin gap appears between them.

It first appeared when I generated the meshes for the terrain chunks. I thought it was because of the way that I set my mesh up. So being lazy, I decided to avoid fixing the problem and went on to make the menu.

But when I'm making a panorama for the main menu, I found that even when the quads are in different game objects, the gap still appears. There shouldn't be any gaps between them at all. I placed six quads, each facing towards the centre and is one unit away from the centre. I set their scale to 2 on every axis.

And then I found that light can actually pass through the gaps. That gave me a new way of troubleshooting. So I tried to disable alpha clipping on the texture, but the gaps didn't go away.

I don't know how to solve this problem, and I don't even know if it is Unity's problem or is it my problem.

Can anyone please help me?