"Write your name in atoms!" he said. (Part 2)

Posted in: MPhys Project

So we've used up all the storage space in our last post, but here is the rest of the story...

Week 8: Oh what a lovely crumble!

So this week we needed to sort out the jolting in our images resulting from the chairs of the caffeine fueled drinkers in the cafe upstairs. To do this high density rubber pads were placed under the feet of the STM to dampen the vibrations. But nothing is ever that simple, and so we needed to re-align the STM again so ensure the microscope could hang freely on its springs. As the centre of mass is not in the middle of this set-up, this led to a fun game of turning each leg a quarter of a turn in various combinations until we could see the bubble of the spirit level move.

As we are getting closer to starting our experiment and now we have more of a handle on all the terminology, it was time for us to look into understanding how the tunneling actually works and causes the Toluene on our surface to float away. So there were lots and lots of diagrams going  on!20131125_113043Then it was time to start dosing the surface with Toluene. To do this, another LabVIEW program is used to control the opening of the test tube's value and measure the amount of the chemical dosed. Only the stepper motor doesn't seem to want to talk to LabVIEW at the moment so Duncan had to open the value manually. We dosed 5 langmuirs onto the surface which saturated the Silicon so it can no longer bond with more. The maximum coverage is actually 50% of the surface, because the Toluene needs to form 2 bonds, so a maximum of 6 Toluene molecules per unit cell are seen. Scanning the surface now looks like it is really dirty, but this is because the Toluene appears as dark blobs. This is why it was so important to get a clean sample in the first place, so now we know the dark blobs are actually Toluene and not some other dirt like we had been seeing previously.

We took lots of images with this set-up and tried to check our thermal drift still works... you guessed it... we made another movie!! Yippeeee!

With the Toluene splattered all over our sample, we had a go at some non-local desorbing. Peter fished out his old program to do this, and walked us through how it runs. We had a few goes at different injection voltages, trying to find the threshold. Here's one example of how things come and go over time.


The final excitement for the week was getting to try out Peters local LabVIEW program, where we can click an Toluene molecule to desorb and the tip is automatically moved to this selected position to carry out an injection! Snazzy!

Week 9: Tuesday's Trauma

Monday brought along more playing around with the injection program, but more practicing than taking actual data. It's cool to be able to see how much more comfortable we are with 'playing' with things in the lab now. When we first got here, watching Peter and Duncan fiddle with buttons and voltages and programs looked totally out of our depth! It's funny how happy we are now with things such as perfecting the tip by digging up silicon from the surface with a high negative bias, or shaking it off with a high positive bias. In fact we're now left alone to move the sample here and there, and still haven't dropped it!

On Tuesday evening just before we left we were retracting the tip and about to pluck the sample plate out of its enclosure (casually), when we looked a little closer and noticed things weren't quite as happy as they normally are. The tip seemed to be at an odd angle to the sample, and we knew enough to know that misalignment could be a big deal! As happy as we are with things, we still know when it's best for us to just walk away. We left the skewed microscope be, and hastily rang up to Peter. He  confirmed our hesitancy, and told us to leave it to him. We spent Wednesday in emotional turmoil...after all these weeks of set up had we gone and broken it?! (We'd barely touched it so we weren't sure how we could have!). We hid in the lab reading, until Thursday morning, when Peter and Duncan came and reassured us that everything was fine. Something had just slipped off its track, but was easily fixable without having to open everything up (YIPPEE!!!).

With things back on track (!) we got back to the job in hand: physics. The point and click injection program was now ready to go, so our questions about local desorption could finally be investigated. On Friday we toyed with this a little, ready to take proper measurements next week.

Also this week: we dabbled in LabVIEW a little more, proof-read an Athena SWAN (encouraging woman in physics) application (causing much debate among our peers), and yet more paper reading.

Week 10: The Final Countdown

With only 10 days to go it's time for some serious data collection. Now the multiple-point-'n'-shoot program works we can collect 5 current injections within one scan making everything so much quicker! In the time it took us to take 30 injection last week was now have around 150! We accidentally learned how OCD we can be about things, when without meaning to noticed just how beautifully ordered the data was recorded in the log book! Although it's just the not too important recording of what each file saved contains, everyone is very aware of the importance of keeping to the right coloured pens when. So much so that if someone dares to make a mistake, it is soon corrected as best as can. It's just a shame we're not as fussed about keeping it in straight lines...


So you can see we are recording the experiment number, and then the parameters set for each. We are alternating between the values the bias is rammed to: a choice of 1.5, 1.8, and 2 V, all below the threshold for non-local desorptions. Current values are picked accordingly. At first for all voltages, a current of 200 pA was used, but we were seeing so few desorptions for 1.5 V and 1.8 V it was decided to increase the number of electrons so events were more likely to happen. All the injections last 15s. At first we played around with the number of injections to do for each data set, so kept a note of this, but even since deciding to stick to five every time we have insisted on keeping our pretty pattern with writing it down. We also just keep a note of which ones of those 5 we think actually did desorb. The program saves the tip height as a function of time, so desorptions can be seen. Because the toluene molecule does not have empty states below the scanning voltage, they are 'seen' by the tip as reason to move closer to the surface in order to maintain the tunneling current. So when a molecule flies away, the tip is now exposed to the juicy p-orbital of the Silicon atom underneath, which much more readily accepts the electrons. This causes the tunneling current to go up, so the tip pulls away to balance the feedback loop again. So watching the tip height as a function of time, we can tell exactly when the desorption occurs. Clever, ay?

point and click ps better

Posted in: MPhys Project


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