MATLAB: How to solve the max iterations exceeded

I'm trying to solve root finding problem by using Newton method when I run the code I got this message. How can I avoid this message? Is there a problem with my code?
max_iter=10; err=1; tol=10^-6; n_iter=0;
while err>tol
    n_iter=n_iter+1;
    delta=-F(r)/dF(r);
    r=r+delta;
    err=abs(delta/r);
    if n_iter>max_iter
        warning('max iterations exceeded')
        break
    end
end 
end 
%%%%%%%%%%%%%%%%%%%%%
function out=F(r)
K=9*10^9; e= 1.6*10^-19; p=0.33*10^-10; Alpha=1.74637*10^-16;
out=-K*(e^2./r)+ Alpha*exp(-r./p); 
end
%%%%%%%%%%%%%%
function out=dF(r)
K=9*10^9; e= 1.6*10^-19; p=0.33*10^-10; Alpha=1.74637*10^-16;
out=K*(e^2./r^2)- (Alpha/p)*exp(-r./p);
end

Best Answer

Had to hunt for awhile to find a place where the function switches signs, but try this for a starting point:
r = 0.1e-9
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