I am currently an EE/Physics undergrad doing scanning tunneling microscopy research at my local university. For future experiments, it would be helpful to employ a higher gain preamp than we currently have, and given that other groups have built them, I was looking into doing the same.
We're hoping to measure current signals from about 0.05-10pA from DC to a few kHz (though more bandwidth with a low noise floor is certainly welcome), requiring a transimpedance gain on the order of 0.1-1 teraohm. This certainly seems possible as it was done at the University of Florence (circuit below), as well as in a number of other papers. A few other topologies are presented here.
I was able to perform a noise analysis of the circuit in LTspice, but I'm a little uncertain of how to translate the spectrum of uV/rtHz vs frequency into a average value for noise floor as it seems to blow up at low frequency with 1/f noise.
Additionally, the LTC6268 seems to have a very low input bias and reasonable noise characteristics, so I was considering using it in place of the above for the first stage.