Scanning Tunneling Microscopy (STM)
Low-Current STM
160
MultiMode SPM Instruction Manual
Rev. B
9.5.5 Operation
The operation of the low-current STM head is typically checked in the atomic-scale imaging of
graphite and the large-scale imaging of a gold-coated grating. The atomic-lattice of graphite is well
resolved in the images obtained with I
tun
in the 2-20pA and V
bias
in the 20-100mV range.
Mechanically-sharpened Pt/Ir tips are generally used for such measurements, and a tripod is good
enough for the vibrational isolation of the SPM base.
Peculiarities
The transition to low current brings some limitations, which are absent in STM measurements with
standard heads. The bandwidth of the ampli
fi
er and, consequently, of feedback frequency response
is substantially narrower than that used for standard STM measurements in the nA range (12kHz).
This must be taken into account during imaging. The scan rate is typically reduced by at least half
as compared to scan speeds normally used in the standard STM system.
However, the low current STM head can be used for imaging of atomically
fl
at surfaces such as
graphite and inorganic layered crystals at scan rates up to 50Hz and still achieve atomic-scale
resolution in the Current data type.
Illustrative Examples
Several images obtained using low-current STM are presented below.
Preamp Setting
Current Sensitivity Setting
10
10
0.1nA/V
10
11
0.01nA/V
