Scanning Probe Microscopy DRAFT

Overview of Machine

AFM base, environmental chamber, afm scanner, stm scanner, nose - contact mode nose (2 pins, 9 degree tilt), ac mode nose, cs-afm nose (4-pins), piezo nose, fludic sample nose( has 9degree tilt ?). Scanner stand, spring clip tool, nose remover, sample plate, liquid sample plate addon, sample plate holder

For liquid sample we mainly use contact mode, but if you want to use the AC mode in liquid sample (which adds noise because of entire liquid in vibration) you need to take out the AC block from the AC nose and put it into the liquid nose. It has 9degree tilt because in liquid beam bents and go out of the detector range, making it 9degree brings it back to the detector range.

Nose

Nine-degree nose assemblies are used for general purpose imaging. Nine and eight-degree nose assemblies may be used for imaging in liquid.

There is a notch filte which only reflect red light to the cantilever.

Blue cable is low voltage bias to scanner, laser and tip (for CS-AFM), sensor

Red cable is 250V high voltage for piezo inside the scanner.

There is an led in the scanner to provide some more light. There are two window cap for the scanner, the blue cap has no lens and the black cap has a lense. Black cap can be used when you interface optical microscope into it.

STM scanner pre-amplifier sensitivity - 1nA/V (blue)
Gain switch x10 or x1

To calculate the final sensitivity, use this equation:

Final Sensitivity = (Preamp Sensitivity) / (Gain)

For HOPG imaging - Set final sensitivity to 1nA/V (In my case it will be gain switch to 1x)

There is a lens inside the scanner which makes inverted image of the cantilever

Warning

No lateral force on scanner as it can damange the piezo material inside the scanner.

Keep the “spring clip tool” connected to the magnet provided in the scanner stand

When done unmount scanner, nose and keep it back in the nose box.

Use gloves while handling any parts of the afm

Don;t pressurize or create vacuum inside the enviromental chamber.

3 AFM Imaging Steps

3.1 Parts required

Decide which mode you want to use and select appropriate nose. scanner, scanner stand, spring clip tool, nose remover

3.2 Install Nose in scanner

Select the desired nose. There is one nose with 12 degree angle which is used for general imaging. The nose with 9 degree angle is used for liquid samples and liquid further deviate the laser beam and we need to bring it back to the detector. General purpose nose has two pins for piezo assembly that is used for AC mode. In principle the AC mode piezo can be transfered from general purpose nose to liquid sample nose for liquid sample AC imaging but it will be noisy as there will be many resonant peak since the tip is in the sample. For most of the imaing select the 12 degree two pin nose.

1- Put the scanner on scanner holder as shown in figure

2- Slide the nose in proper orientation. The needle should go inside the hole in the scanner.

3- Gently press it

3.3 Install the cantilever

1- Raise the box and hold cantilever with twizer such that cantelver make acute angle with the twizer (its importat) and close the cantilever box

2- Using spring to raise the spring, retract it until the spring tool touches the scanner

3- Gently place the cantiliver at the grew provided and release the screw. Make sure that the cantilever is at proper distance inside

Slide the nose into the scanner opening using guided slots. Push gently inside till the small rubber o-ring. Be careful not to apply any force laterally as this may damage the piezo material. If its difficult to insert the nose, apply some grece in the o-ring.

To take out the nose, slide the nose inside the slot of the nose reomver tool and gently pull the handle upwards

3.4 Install the Scanner

1- Center the stage, reading on both the moving stage screws should read 5/5

2 - Slide the scanner inside, be carefull about the led switch, rotate the scanner slightly to mare some room for the led switch

3- Tighten the side screw.

4- Connect the red and blue cable

3.5 Align the laser

Note : If you see the full circle Sum = 10.00V chances are that detector cable is not connected. Make sure that the cable is connected and proceed with alignment.

1 - Insert the detector assembly and connecte the cable.

2- Maximise the laser alignment window. See if you can see the laser spot at the bottom of the afm. If yes clockwise rotate the right knob on scanner until the spot disaperar and clock wise to make it visible again. This is the base of the cantilever.

If no move it close wise until you see the spot. Then do as described above.

3- Use the left knob to bring the spot to the top of the cantilver - cockwise rotation move it righ and counter-clockwise move it left. When image dissapera it means its on top of the catilever bring the image back. This is the position when beam is at bottom and at the side of the cantiliver.

4- Use right knob to bring the spot at the tip of the cantilever

3.6 Insert the detector

1 - Gently slide the detector in completely and make sure that laser power supply cable (green cable) is connected

2 - If Laser alighment window is not already open go to controls -> Laser alignment and the laser alignment window will appear. Have a look at the Deflection, friction and sum values. and the laser spot. If the spot is hollow circle then beam is off the detector and signal is very low. If the circle is solid then signal is good. Use the two knobs in the detecot to bring the red spot at the center of the cross (if ac mode) or at intersection of yellow and green light (if using contact mode)

First bring the spot at horizontal center using side screw. Hold the detector assemble with your left hand thumb and use the finger to rotate the knob. Rotating the side knob away from you will move the spot from left to right

Clockwise rotation of fron knob will bring the laser up

Note - There are four switchs for the gain in detector assembly. It reads “NO gain” don't get confused it with “ON”

3.7 Mount the Sample

1- Put your sample on the sample disk. Put the disk on top of the sample mouting

2 - Open the lid - hold bottom with your right hand and and use left hand to open

3- Retract the three motors all the way down using the open switch in EHB. Visualise the plane passing through the two ball rings this plane should be at safe distance from the cantilever tip.

4- Attach the sample cable. The ball bearings are magnetic. First attache the plate to the bottom ball bearing then the one at the front. Now looking at the sample and the tip distance gradualing slide the posts inside the sample plate.

and close the lid

5- Bring the sample plate close to the tip using the close button in EHB. Make sure that the sample and tip don't collide. If the sample is reflective do it until you observe the reflection of the tip in the sample.

3.8 Approach

3.8.1 Contact Mode

The AFM tip is brought into gentle contact with the sample and then scanned in raster fashion across the sample surface. Because of this reason the initial deflection in AC mode is set to a non zero value (force vs distance figure, as the tip come closer to the smaple first there is “non-contact” attraction force and then “intermittent contact” repulsive force).The system will either maintain a constant force on the tip or will maintain the tip at constant height (higher resolution). Constant height mode is used for very flat surface.

Controller maintants a constant deflection of the tip based on the specified Setpoint voltage. The deflection signal is the differece between setpoint voltage and the actual cantilever defelection measured in voltage by the photodetector.

1. Select contact Mode in PicoView software Mode -> Contact AFM

2. Click Controls -> Imaging to open the Servo window. Set the setpoint value slightly more positive than the current deflection reading. This is the deflection that the feedback loop will achieve and maintain.

3. Click on the approach button. The scanner will retract (approach range) see if the sample is there come back and then stage will be. The aproach range can be set by Controls->Setup-> Microscope. Currently its 400nm.

4. Make sure that the IGain and PGain is set to 10% in servo window (more on it later)

5. Once you click the Aproach button a windows will appear “ Apprach In Progress”. This window will show you how much your tip is moved and will dissapear when the approach is done. You can see the yellow bar in Servo window moving up and down. This is the motion of the tip.

6. Once the approach is done click the Scan arrow in “Scan and Motor” Window .

7. In the “Realtime Images” window select the “Topography, deflection and Friction

3.8.2 AC Mode

1. Find out the resonant frequency of the tip - Go to controls -> AC mode tune, AC tune window will apear. select Auto, look at the datasheet of the catilever and set the start and stop value accordingly, Set the peak applitude to be 4V (default value) and click on Auto Tune. The software will change the drive values to get the specified amplitude. typtically drive is less the 1%. Using slider slide the red dotted line little bit left of the peak . Close the window

2- In the scan and motor window set the stop at - 95%, what is will do is that the feedback loop will start at 95% of the peak voltage (in this case 4V) and click on the approch button. YOu can look into the servo window location of the tip. Wait until the Approach window disapear. When done the oscilloscope windows shows the differece between set point and actual point

3- Now change the setvalue voltage. Smaller the voltage more is the interaction with the sample and more is the resolution of the image. Set the setpoint to 3V.

4- In the scan and motor window go to scan tab. Select the scan size (max 10um), offset etc. click the up-arrow button to start the scan.

In the race/retrace window see if race and retrace curves are following each other. Increase the I-gane so that both curve follow each other. If you observe high frequency noise in the curves decrase I-gain little bit.

STM Mode

 First check the conducting side of the substrate and make sure during mounting it, the conducting side faces upside. Mount the sample on sample plate and attach the electrode from bias point to substrate.Connect 3-cable pin connector to sample holder. Set the bias value in software and check if you are getting sample value at sample (use serial port outer casing as ground)

Closing Up Session

- Retract AFM Tip and Open stage (increase distance between tip and stage)

- Turn off Laser, remove scanner cable and detector cable.

- Close software and turn-off PC.

- Turn-off AFM controller and AC Mode III.

Consumables 

1) Mica Disk 10mm diameter (Ted pella Product# 50)
2) Atomic flat gold substrate
3) AFM/STM Metal Specimen Discs 15mm, Part# 16218
4) Metal Speciman Disk 15mm Part# 16218-G 
5) General lab supply to keep small sample - Prod# 1395-10

AFM Probes

http://www.nanoworld.com/afm-probes-catalog

For contact mode (used in mica atomic resolution)
5) AFM probe contact mode,  Part# PNP-TR-20

http://www.nanoworld.com/pyrex-nitride-triangular-silicon-nitride-cantilever-afm-tip-pnp-tr

 

Part I AFM Calibration

I ordered gold calibration kit for AFM calibration[AFMCalibrationKit]. It has spheres of size of many size

References http://www.tedpella.com/calibrat_html/16200.htm

Maintanace

1) Updating to PicoView 1.18.2 version.