The I(z) Spectroscopy is related to LBH spectroscopy and can be used for providing an information about the z-dependence of the microscopic work function of the surface. Next important use of the I(z) Spectroscopy is concerned with for testing of the STM tip quality.
The tunneling current IT in STM exponentially decays with the tip-sample separation z as
IT ~ exp(-2kz),
where the decay constant is given by
2k = 2(2mU/h2)1/2.
U is the average work function Uav = (Us + Ut)/2, where Ut and Us are the tip and sample work functions, respectively.
In the I(z) Spectroscopy, we measure the tunnel current versus tip-sample separation at each pixel of an STM image.
For Uav = 1 eV, 2k = 1.025 A-1eV-1. Sharp I(z) dependence helps in determining of tip quality. As is empirically established if tunnel current UT drop to one-half with Z < 3 A the tip is considered to be very good, if with Z < 10 A, then using this tip it is possible to have an atomic resolution on HOPG.
If this takes place with Z > 20 A this tip should not be used and must be replaced.
- G. Binnig and H. Rohrer: Surf. Sci. 126 (1983) 236. Rep. Prog. Phys. 55, 1165-1240 (1992).