Instruction manual
Scanning Tunneling Microscopy (STM)
Low-Current STM
156 MultiMode SPM Instruction Manual Rev. B
9.5 Low-Current STM
The following sections provide detailed instructions for performing low-current (sub-picoamp)
STM using two Veeco devices: the Low Current STM Converter for MultiMode AFM (Model
#MMSTMLC), or with the Basic Extender Module (Model #MMSTMLCE).
9.5.1 Description
Operation of conventional STMs with current (I
tun
) in the nanoamp range is characterized by strong
tip-sample force interactions. The forces applied to the sample during STM imaging in air actually
exceed the forces in atomic force microscopy (AFM). The effects of high STM forces were found
when imaging materials which included graphite [1a], inorganic layered compounds[1b], organic
conducting crystals [1c] and organic adsorbates on conducting substrates [1d].
For metallic surfaces, the tip-sample gap resistance, , can be used as a qualitative
measure of tip-sample separation. Where
In general, decreasing will increase the tip-sample separation and decrease the tip-sample
force.
By operating at in the GΩ range, for example, and the tip-
sample distance increases enough to allow less-destructive imaging of a variety of surfaces.
Picoampere-level STM provides the opportunity to image semi-insulating samples deposited on
conducting substrates. Indeed, it was shown that using STM at ultra high R
gap
allows imaging of
the topmost layers of alkanethiol adsorbates on gold surfaces and to reveal earlier unknown surface
features.
Check sample conductivity There are two problems associated with sample conductivity:
1). The bulk conductivity of the sample may make it difficult to image. If the resistance of the
sample is greater than 1KΩ/cm, higher bias voltages should be tried. If the resistance is greater
than 1MΩ, bias voltages of 100mV or more should be used. Samples with resistances 1MΩ or
greater will be difficult to image even with high bias voltages. These types of semiconducting
materials are not suitable for STM measurement.
2). Oxide and contamination layers on the sample surface can make imaging very difficult.
Higher bias voltages are required for these types of samples.
Head/Tip Problem Troubleshooting Tip
R
gap
V
bias
I
tun
⁄=
V
bias
bias voltage=
I
tun
setpoint current=
I
tun
R
gap
V
bias
1.0 V=
I
tun
1 pA=