Instruction manual

Scanning Tunneling Microscopy (STM)
Spectroscopy with the STM
Rev. B MultiMode SPM Instruction Manual 153
2. Prior to engaging the microscope it is necessary to set the Bias voltage and Current
Setpoint. A good typical Current Setpoint is 1-2nA. A good recommended Bias voltage is
20-50mv for highly conductive samples like graphite or gold. Try using Bias voltages of
100-500mv for less conductive materials. Usually if Scan Size is greater than 1µm, altering
the values of Current and Bias voltage do little. Varying the setpoint current on the bias
voltage can help improve imaging quality on smaller scan sizes where atomic resolution is
desired.
3. Set the integral and proportional gain setting in the feedback controls to values typically used
for Contact AFM.
4. Set the Scan Rate to a value typically used for Contact AFM.
9.3 Spectroscopy with the STM
The NanoScope STM performs spectroscopic operations under the two scanning tunneling
spectroscopy STS modes of operation. The variation of the tunneling current due to variations of
the bias voltage or tip-to-sample separation can be tested and recorded at a single point with the
View > STS Plot modes. These are discussed in this section.
9.3.1 STS plot modes
In the STS Plot modes, the tip is positioned at a point on the surface, and a spectroscopic plot is
acquired and displayed in a scope format. Between plots, the feedback is run to establish the
tunneling current to the setpoint value. The different types of STS Plots that can be acquired are:
• STS i(v)—The tunneling current as a function of the bias voltage is displayed. The tip
height is held constant while the current (I) versus voltage (V) plot is being acquired. In
addition to I versus V, it is also possible to plot the following:
• STS i(s)—The tunneling current as a function of the tip height is displayed. The bias
voltage is held constant while the current (I) versus tip height (S) plot is being acquired.
In addition to I versus S, it is also possible to plot versus S.
di
dv
------
versus V
di()ln
dv
---------------
versus V
di()ln
dv()ln
----------------
versus V
I()ln