Chapter 6. Acquire Spectroscopy Data
Scanning Tunneling Microscopy measurements can give us high resolution topographic and current
images by maintaining a fixed bias voltage while using the tunneling current between the tip and sample
as the feedback signal. However, the tunneling current is a function of the bias voltage, local density of
states, and barrier heights.
The R9 Controller allows the user to perform a variety of spectroscopic procedures. These procedures
involve ramping a variable parameter, such as the bias voltage or Z position, while measuring another
dependent signal, such as the current, or dI/dV. The STM spectroscopic measurements allow the user to
study the local density of states, energy gap, electron path during electron transfer, and probe metal
oxidation states.
During an STM spectroscopy measurement:
1. The tip will stop at certain locations on the sample.
2. The Z feedback loop is disabled.
3. The bias voltage or Z position is ramped.
4. While ramping, the Current, or Rate of Change of the Current is measured.
5. The bias voltage or Z position is returned to its pre-ramped value.
6. The Z feedback loop is enabled.
Recording the differential value of current (dI/dV) can be obtained by either:
• Numerically differentiating the current with respect to either the ramping bias (I/V).
• Detected with a lock-in amplifier by modulating the bias.
When measuring I/V or dI/dV, the distance between the tip and sample can be kept constant at the
imaging Setpoint, or adjusted to a target distance by changing the Setpoint and allowing the feedback
loop to adjust the tip-sample separation.
There are many other techniques to obtain STM spectroscopy measurements than this manual can
cover. The R9 Software provides the user the flexibility to program and reprogram unique measurements
with all the possibilities. Please contact RHK Support if you have any questions about spectroscopy
techniques.
6.1 Configuring the I/V Ramp Spectroscopy Settings
A standard Scanning Tunneling Spectroscopy (STS) measurement involves locating the tunneling probe
at a feature on the sample, opening the feedback loop, and ramping the bias voltage while measuring the
tunneling current. All RHK preconfigured STM .ihl files include I/V and I/Z spectroscopy procedures
already enabled and configured. All the user needs to do is adjust the settings appropriate for the
experiment.
These settings are contained in a special hardware item called Ramp Spectroscopy Settings. This
hardware item connects between the signal being used as the output for the ramping parameter (typically
CH1 Drive or the Z output of the Scan Processor).
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