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Hi I’m Alex Veneman of the Armstrong group here at the University of Arizona. Today we’re
going to talk about how to perform conducting tip atomic force microscopy experiments. The
first thing we’re going to do is prepare our sample.
Our sample in this case is indium-tin oxide which is a conducting transparent metal oxide.
We’re going to start off by painting silver paste onto the backside to make contact with
our instrument. Just a little strip on the top.
Make sure it goes over the sides and on to the bottom.
Leave that there to dry and cover. Next we’re going to prepare and mount our
cantilever on to our mount for the instrument. Since these particular chips come with three
individual cantilevers and we’re doing electrical Measurements, we must knock off two of the
excess tips. So we’ll go in and take a chip and bring
in our tweezers and use them to knock off two of the excess tips, leaving just one on
the chip. First off we will check to make sure there
is no dust or any particulates sitting on the surface of our mount. And we will gently
pick up a cantilever chip and place it on the mount.
Make sure the chip is centered on the mount. And we’re done.
While the silver paint on my sample is drying, I will install the conducting tip module and
prepare the instrument to take measurements. I start off by grounding myself because I
am going to be dealing with sensitive electronics and we don’t want to damage them.
This is the conducting tip module, which I plug into the appropriate port on the instrument.
Then I calibrate. We perform the calibration by plugging in
a precision wire-wound resistor, adjust the amplifier offset in the module using a screwdriver,
make a small change and observe the change on the screen until my amplifier is zeroed
out. We start by setting the microscope mode into
the TUNA mode which is the tunneling AFM mode. And then to calibrate our amplifier, we will
use the oscilloscope mode which allows us to directly view the signal from our amplifiers.
We will turn on channel 2 as the TUNA current and select the appropriate current sensitivity
range. For our experiments we usually use the 1 nanoamp per volt range.
This is the current signal coming from the conducting tip module and since we have no
bias applied across the resistor, this current should be zero. So we will adjust the current
offset in the amplifier using a screw driver and the potentiometer in there.
Now the current will read zero and the zero-offset is correct.
We will now set the slope of the amplifier by applying a test bias across the resistor.
We are applying 4 volts across our one gigaohm resistor which should give us a current of
4 nanoamps. We are slightly under that and we will adjust the slope of the amplifier’s
response using our screwdriver again. So now it reads 4 nanoamps, and so we turn
off our bias off and it goes back to zero current so now we know that our amplifier
is correctly zeroed and it has the appropriate response.
Now that we have zeroed out the conducting tip module, I can remove my calibration resistor
and mount the module to the side of the scan head.
Now that the conducting tip module is mounted on the scan head we can mount the cantilever
mount on the scan head so we tighten this thumb screw to release the scanner head,
gently flip it upside down, being careful not to put a thumbprint on the laser window.
The cantilever mount is seated on the four pins on the very top of the piezo scan head.
You must be very careful not to bend these so very gently push the cantilever mount straight
down. Make the electrical attachment to the amplifier
using a pair of tweezers and being very gentle. The scan head can then be placed back in its
dovetail and the thumb screw released to lock it into place.
Now that the tip is mounted in the system, we can align the laser beam with the backside
of the tip and we will do that by using these knobs to move the laser across the cantilever,
observing the reflection of the laser in this window.
We can tell when our laser beam is on the back of the cantilever because moving it side
to side our reflection will disappear very quickly
but moving it front to back, we can move along the cantilever fairly well.
We will now adjust the reflection of the laser beam off the back of the cantilever so that
it lines up with the detector. We do that by adjusting these knobs on the
left and observing the change in the signal from the detector on our screen.