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Seeing Atoms Dave Gohlke COSI Materials Day August 14, 2010 Gupta Research Lab Hi! My name is Dave and I am a student researcher at Ohio State University, working in the research laboratory of Dr. Jay Gupta. Today, I will talk about the weird world of quantum mechanics, and how we can use our knowledge of it to see atoms and also move them around.
Scanning Tunneling Microscope • Microscope that can “see” atoms! • Operated in vacuum (no contaminants) • Cold! (So atoms stay put) In our lab, we use scientific instruments called scanning tunneling microsopes, or STM. These are microscopes that allow us look at individual atoms on a surface. Our microscopes are inside of large vacuum chambers, about the size of a table. These vacuum chambers have all the air taken out of them, so that no other atoms get in the way. Also, when atoms get hot, they jump around. We don't want anything moving, so we do all of our experiments at about -450 degrees Fahrenheit.
Electric Current • Electric current is the flow of charged particles from one location to another. • Electrons are small charged particles that are part of atoms. For our experiments, we're concerned about how electricity flows. A movement of small charged particles called electrons gives us an electric current. You might have seen the Bohr model of the atom before, where the electrons are spinning around a nucleus that has protons and neutrons. This model is sort of accurate, but it turns out that the electrons are jumping around so quickly that they actually just look like a blurry cloud.
Smaller andsmallerandsmaller andsmallerandsmallerandsmaller… Zooming out 10 times Zooming out 100 times Zooming out 1,000,000 times Zooming out 1,000 times Zooming out 100,000,000 times Zooming out 10,000,000 times Zooming out 100,000 times Zooming out 10,000 times Zooming out 1,000,000,000 times Zooming in 10 times Zooming in 100,000,000 times Zooming in 100 times Zooming in 1,000,000,000 times Zooming in 1,000 times Zooming in 1,000,000 times Zooming in 100,000 times Zooming in 10,000 times Zooming in 10,000,000 times From Powers Of Ten, by Ray and Charles Eames These atoms are reallysmall. There are over a trillion trillion atoms in your pinky finger, and atoms are less than a billionth of a meter across. To get an idea of how small this really is, let's zoom in and out one billion times, starting from a man napping on a blanket. Zooming out one-hundred thousand times shows us all of the Chicago area, but zooming in 100,000x, we can see a single cell. If we keep going, we can see the whole planet and strands of DNA. Zooming in one billion times, we begin to see the atoms that make up DNA, while if we zoom out that much, we'd see the entire orbit of the moon around the earth.
Quantum Mechanics! • Study of nanometer-size objects • Things act a little different than we are used to The thing about really small things, like atoms, is that we need to use quantum mechanics to describe them. I know that might be a scary phrase, but basically all it says is that small particles act differently than large objects. If we had a ball rolling back and forth, it would never jump over a hump, no matter how many times it tried, unless something gave it a push. In quantum physics though, there's a very small chance that ball might actually jump across to the other dip. This jump is called tunneling, because it's like the ball has found a hidden tunnel to take. Think about how weird this is though. If you had a cup full of water, then the water would all tunnel outside of the glass on to the table. Luckily for us, large objects are very unlikely to tunnel.
Mario as electron e- Even though I'm used to things that are much larger than atoms, it helps me understand quantum physics better when I think about something I know as a tunneling electron. Growing up playing video games, I'm pretty familiar with Super Mario, so I'll pretend that he is an electron. Scientists often use the letter e with a minus sign as an abbreviation for electron since electrons have negative charge.
‘Classical’ Mario Let's look at Super Mario Bros 1, from way back in 1985. This 'Classical' Mario can run around and avoid small obstacles in his way, but he is unable to jump away from the ground for more than a second or so, and he can’t get over large walls, no matter how much he tries.
‘Quantum’ Mario In Super Mario Galaxy, from 2007, on the other hand, Mario can occasionally jump high enough to get from one asteroid surface to another. In this game, then, it's like 'Quantum' Mario is an electron that is able to tunnel, just like the quantum ball that was discussed earlier.
‘Quantum’ Mario If the ground that Mario stands on was a metal surface, and a neighboring asteroid was the end of a metal wire, then we can do some pretty cool science using our STM.
Metal Tip Sample Scanning Tunneling Microscope scanning tunneling microscope In its simplest form, a scanning tunneling microscope is just a metal surface known as a sample, and a metal needle called a tip. If we zoom way in to the very end of our tip, then we have a few atoms which are very close to the surface.
z-servo ~1nm electron tunneling Metal Tip V I We use fancy electronics to hold the tip about one nanometer from the surface. Since we're looking at the atoms on the tip, there are bunches of electrons surrounding each atom. If we apply a voltage to our system, more electrons end up crowding the tip, until they start to tunnel to the surface. Once electrons start flowing, then we have an electric current, which scientists abbreviate with the letter I.
imaging • keep I constant This 'tunnel current' depends very strongly on the distance between the tip and sample. If we move the tip along the surface, while keeping the current constant, the tip will trace out the surface, as seen in this animation created by IBM. If something happens to be on the surface, the tip will retract far away. If we do this over many rows of atoms, we can map out the surface. All of this data is sent to a computer, and we can zoom in on interesting features.
imaging The computer can then make a 3D model of our surface. Doing this allows us to "see" atoms, even though a single atom is one thousand times smaller than the smallest beam of visible light.
atom manipulation Another cool thing that we can do is actually move atoms. If we bring the tip close enough to the surface, the tip can partially bond with an atom. Then, we can drag the atom along the surface to a new atomic site. Finally, we can pull the tip away, and the atom will stay where we have left it.
Actual Movement • Movement of cobalt atoms by tip 3.0 nm To see this in action, we look at the creation of what could be considered the world's tiniest wire - 4 cobalt atoms brought into contact with each other. The cobalt atoms are the white dots in this picture, but we can also view this as a 3D image.
Atomic Graffiti • Creation of Script Ohio, by Taeyoung Choi We can also make images that are a little more fun, as in this well known band formation, created by Taeyoung Choi.
spectroscopy • measure current • measure resistance Ohm’s Law: V = I * R +Vac V I+Iac Earlier, I mentioned that we are concerned with the flow of electricity. A formula called Ohm's Law tells us that the voltage is equal to the resistance times the current, or V=IR.
Homes for electrons • Electrons will flow from the tip if there is space for them on the surface • We change the voltage and see when electrons flow the best and worst We are often interested in knowing when the resistance is high or low. The peaks in this graph show that electrons with certain energies are much more likely to tunnel than other electrons, or that there are "homes" in the sample for electrons with the right energy. The scientific term for this is the 'density of states' of the sample, which usually lets us know new and interesting science.
Group Members And to finish, here are some pictures of some of the people involved in this research when we're not in the lab. I hope you enjoyed the show!