Java charAt() or substring? Which is faster?

I want to go through each character in a String and pass each character of the String as a String to another function.

String s = "abcdefg";
for(int i = 0; i < s.length(); i++){
    newFunction(s.substring(i, i+1));}

or

String s = "abcdefg";
for(int i = 0; i < s.length(); i++){
    newFunction(Character.toString(s.charAt(i)));}

The final result needs to be a String.
So any idea which will be faster or more efficient?