Atomic Force Microscopy (AFM)

[Music] [Applause] [Music] hi my name is Jennifer McLeod and today I'm going to show you how we do atomic force microscopy which is a type of microscopy that gives us really detailed information about the surface of samples at the nano scale this is our atomic force microscope let's get started with an experiment the first thing we're going to need to do is to install our sample into the sample holder we're going to handle everything with tweezers because the sample was prepared in vacuum and is rather clean for AFM our sample holder just has a bit of sticky tape on the base this keeps the samples stable while we're working on it next we bring it to the instrument where we have to remove the microscope head in order to install the sample it slides in we put the head back into position secure the cabling and we're ready to get started the next thing we're going to need to do is to turn on all the bits of equipment that are necessary our first step is to vibrationally isolate the microscope atomic force microscopy is really sensitive to vibrations so we use this active damping stage to make sure that any vibrations are damped out the next thing we do is to turn on the microscope controller next we're going to use an optical microscope to locate our AFM tip now we're ready to turn on the software as the tip scans it moves up and down to follow the contours of the surface of nano structures and the cantilever is deflected accordingly the magnitude of this very minut deflection is then recorded by the position of the reflected laser beam in the position detector our software lets us see the laser signal in that four quadrant detector what we can do now is try to optimize the reflection of the laser off the cantilever by moving the cantilever a bit this is giving us a little better intensity but we've moved the reflection off the center of our four quadrant detector that's fine because we can move the detector a bit too now we've got a nice strong laser signal centered in our detector that's exactly the way we want to start the experiment we'll do one quick check to make sure everything is operational and then we're ready to bring the probe close to the surface looks good so we can get started so I've set us up now so that we have a better view of the laser bouncing off the cantilever we're going to start approaching the surface and as we do I'll explain what's happening we initiate the approach by hitting the button that says landing what's happening now is that our probe is staying fixed and the surface is very slowly being brought close to it as the surface comes closer and closer will approach the range where Vander Waals forces will begin to act on our very sharp probe tip when this happens the placement of the tip will change slightly we can measure that with the laser we've now detected the surface through this very slight displacement of our cantilever and now we're ready to start mapping the topography of the surface we do that in a different dialogue what we want to do is to monitor what our probe sees as we raster it first forward and then backwards across the surface so we'll set up to acquire data in both directions we'll start with an image that's 20 microns by 20 microns now we're acquiring data in the optical monitor you can see this happens by keeping our probe tip still and moving our sample back and forth in a 20 micron trajectory it doesn't look like much on the monitor to begin with that's because we have a slope on the sample we'll do a little bit of slope correction and now we see the topography of the surface showing up on our screen here in the monitor you can see that this sample has little islands on it it's a silicon sample onto which we've deposited a bit of silver and that silver has taken on what's called a Stransky crafton oov growth mode these islands range in height from a few nanometers to a few hundred nanometers and so what we'd like to do is try to position ourselves over one of the islands so that we can collect some data about its height and its width that's where we'll go next one of the really nice things about AFM is that we can use our optical image to decide which parts of the sample we'd like to investigate this sample has little islands all over it and using the optical image we can choose individual islands on which to make our measurements in order to do that we need to reposition the sample underneath the tip it's a bit dangerous to do that when our tip is very close to our sample remember vanderwaal's distances are on the order of tens of nanometers we really don't want to move our sample when the tip is that close so what we'll do is retract our sample from the tip pick an island and then move the sample so that it's positioned under the tip we can click the fast move out button in the software to move our sample away from the tip when we do that it D focuses in our optical image so the best thing to do is to refocus our optical image so that we can see the islands a bit better now that we can see them we can use these micrometers to move the sample under the probe tip the probe tip is probably a few hundred microns from the sample now so it's fairly safe to do this once we've found an island we like we can really so the data are going to show up really rapidly on the screen here at least for this technique in atomic force microscopy we're keeping our probe close to the surface and moving the surface underneath our probe those Vander Waals forces are going to vary as the topography on the surface brings the surface closer to the tip and further away we're monitoring those forces in real time and we're keeping track of the topography of the surface based on how those forces act on the tip remember we're reading the tip position with a laser here we have a very good idea of how the tip is responding to topography here you can see the beginning of our island image coming up on the screen atomic force microscopy can be conducted really quickly like this so that we get a very rapid idea of what's going on on the surface here we can see that our surface topography is varying on the order of 400 nanometers we are reading really incredible height variations at the nano scale really quickly as the image builds up we see what's happened on our surface we have a silver island here with a rhombohedral this has got a bunch of little decorations on it which are probably due to the way the silver grew as it was being deposited here our background is likely mostly formed from silicon with a very thin layer of silver on it this is the way Stransky question off growth works once the image finishes acquiring we'll be able to do a little analysis on it to get a little better idea of what the image dimensions are we tend to do our analysis in a different software this is a free software that lets us make measurements of the size of features in our images for example we can draw a line profile that goes from the substrate onto the island and this lets us make a measurement of how high that island is in this case it's about a hundred and sixty nanometers one of the nice things about microscopy data is that you can have a lot of fun processing the data there's not a lot of science in making the data look like this but there is a lot of information in the image when we process it mathematically we've learned today about atomic force microscopy which remember uses van der Waals interactions to map the shapes at a surface this is part of a whole family of techniques that can be used to map all sorts of different surface variables we can map potential we can map other electronic properties we can map friction this is a really powerful suite of techniques that give us information into what's going on at the nano scale I hope you found what we learned today interesting [Music]

Channel: LD SEF

Views: 31,801

Rating: 4.9230771 out of 5

Keywords: PVB321, nanotechnology, nanotech, atomic force microscopy, QUT, AFM

Id: jRAqhFdwt20

Length: 10min 22sec (622 seconds)

Published: Wed Apr 10 2019