I want to solve a non linear equation for an unkown parameter phi0.

This equation represents the electronuetrality condition where LHS is the no. of electrons withdrown from the volume of a spherical grain and RHS is the elerons trapped at the surface of the spherical grain.

Where V is the volume of the sphere (I assume this has to be mentioned in spherical cordinates; volume integral or triple integration over the limits of grain size of diameter 20 nm).

Nd = 1e24;
e  = 1e-12;
r  = 50*1e-9;
R  = 20*1e-9;
Et_Ef = 0.8*1.6*1e-19;
K  = 1.3807e-23;
T  = 600;
Nt = 1e15;
q  = 1.6*e-19;
Ld = 14 *e-9

I want to reproduce these kind of relationship between phi0 and Nt parameters. This can be done by solving the above mentioned equation univocally to get phi0.