STM Imaging Techniques

Quick Description: There are tons of places on the internet that you can go to in order to understand the principles under which STMs operate. However, achieving a good image can be a bit trickier and less ideal than these explanations. Over the past three years, I've worked with 5 different STMs in 2 different labs and have googled a lot in fits of desperation. I've never found really good documentation on this sort of stuff, so I thought I'd include it here, even if it is a bit tangential to other posts in the blog. 

The Point: Getting good STM images.

Prerequisites: A working STM and patience. (If I ever have time, I'd also like to write a post on debugging STM problems.)

Notes: Some techniques are system specific, but just in case it is useful to you, I'll include all the tips and tricks I've picked up over time here. Remember that each STM and sample are unique and require different parameters; at the end of the day, you probably will need to play with all the parameters and see what works for you.

• Low voltage scanning:  A lower voltage brings the tip closer the sample, allowing the probe to more closely follow small features. This provides a higher resolution sample. BUT, you must be careful to have a clean and flat surface. In this mode, the tip is grazing the surface of the sample and can easily be damaged or pick up dirt.

• Shaking the voltage/tip: Quickly pulsing the tip from high to low voltage and back literally shakes the tip and also changes its state dramatically. This can allow you to shake off dirt and/or unwanted instabilities at the tip's apex. You can manually change the voltage very fast, but some STMs have a software feature that will do this for you.

• High voltage scanning (for moving molecules or for cleaning the tip): You may want to scan at higher voltages in two situations. One, if you want to clean your tip, you can sort of blast the tip (~10V) and sample with a high voltage over an area a few times before running away from this likely dirty location and reducing back down to a normal scanning voltage. Two, if you have a lot of molecules free flowing on your surface, your tunneling current and feedback loop can be disturbed but this "sea of molcules". Back your tip away with a high voltage and try using a large gain to distinguish the features of your surface.

• Crashing the tip: This is a bit of a sketchy technique, but if used very carefully and in moderation, you can use this technique to brush off your tip, thereby sharpening and/or cleaning it. To do so, lower the tip until the tunneling current disappears and immediately retract away from the sample.

• Scanning at the edges of your piezos: One trick that we find useful in our lab is to scan towards the limit of the x and y raster piezos. This often reduces noise (perhaps by stiffening the piezos a bit more).

• Gain values: "Optimize the I-Gain and P-Gain values. These values control the feedback loop that adjusts the tip to maintain a constant tunneling current. Increasing these values will give a sharper image up to a point; increasing them too much leads to instability and a noisy image." - Cornell CNS's Intro to STM PDF

• Scan speed: "When you are in a scan range and z range close to where you should see atoms, a good thing to do is speed up the time/line value to avoid these little temperature changes.  A suggested value is 0.06 s/line.  If you can't get this value, make sure you experiment by using one lower than the time/line value you previously were using for the larger scan range values." - Cornell CNS's Intro to STM PDF

• Check for Noise: This may seem obvious, but still... check that you don't have any dangling wires (or any wires with tension on them), things in the surrounding area causing vibration, or the like. If your system uses any sort of spring or air flotation noise suppression, check that those are in place properly. And if other people, experiments, whatever are causing vibrations, try to figure out a way around having that interfere with your experiments. Try looking at the FFT of your current to see if there is a certain frequency contributing to your noisy images. This may clue you in to the source of the noise. Of course this can be filtered out later, but it's better not to have at all in the first place. 
• 60 Hz is likely electrical noise, perhaps coming from a grounding issue
• If you have a inner/outer cryostat cooling system, then it's possible that the inner cryostat (essentially a long thin tube) is misaligned and therefor crashing into the side of the outer cryo. This can be fixed by either rebalancing the table or adjusting the bellow top from which the inner cryo hangs.

Have any more tips and tricks? Please drop a comment! :)