Forget about the Independent Spirit Awards. IBM just made the ultimate small production: A Boy and His Atom. The 60-s movie contains 242 unique frames, each produced by painstakingly positioning individual carbon monoxide (CO) molecules nearly 10,000 times. The team at IBM Research Almaden labs in San Jose, California, crafted the film with the same scanning tunneling microscope (STM) they used in 2012 to demonstrate a 12-atom-per-bit memory. We spoke with research scientist and team member Chris Lutz to find out how they made the magic happen.
The work has its roots in the 1981 development of the STM by IBM researchers Gerd Binnig and Heinrich Rohrer, work for which they later received a Nobel Prize. In an STM, an atomically narrow metal tip is placed in proximity to the sample and a bias applied. When the probe gets within a few angstroms of the sample surface, the electrons tunnel across the gap, establishing a tunneling current. The level of the current varies depending on the tip-to-sample separation; that data is processed to create three-dimensional images with sub-nanometer spatial resolution.
When the tip-to-sample distance shrinks further, the probe tip exerts sufficient force to overcome the bond of the atom with the substrate and the atom can be moved to a new position. The process requires a fiendish level of positioning control and thermal and vibration stabilization, as well as a great deal of skill. In 1989, IBM Fellow Don Eigler used the STM to position 35 xenon atoms to spell out the company’s name. The movie business could hardly be far behind.
The image above shows the sample being placed in its holding fixture prior to installation in the sample chamber. For this project, STM must be used in vacuum and cooled to -268° C before work begins, a process that can take days.
Before they switched to moviemaking, the team was working to develop technology for the memory of the future. In 2012, they used the STM to position individual iron atoms on a copper substrate coated with copper nitride. By adjusting the voltage applied to the STM probe, they were able to switch the magnetic orientation of all twelve individual atoms. All 12 atoms switch direction together. The checkerboard pattern of blue and white is the measured direction of the magnetism, which alternates from atom to atom to form an antiferromagnet. “They’re really like classical magnets in that they sit there holding their direction of magnetization for a long time,” says Lutz. “We probe them using spin polarized currents from the STM tip in order to determine which way they’re pointed.” The result was a 12-atom nonvolatile memory bit.
In the STM, a copper-tipped iridium wire with a tip ending in just one atom (upside down pyramid) positioned in three dimensions by piezoelectric actuators can drag an atom or molecule (blue ball) from one position to another on a copper substrate (bottom).
Building the sample
The team started with a copper 111 substrate, chosen for its tightly packed surface. They deposited the CO molecules by introducing a flow of CO into the room-temperature part of the vacuum chamber at a pressure of 0.1 mPa for a few minutes. The cold copper surface adsorbed the molecules. The team uses a similar technique for other materials. In the case of the memory experiment, they heated an iron wire to evaporate atoms off the wire and onto the copper.
Before they could start making the film, they had to map the sample surface to find a sufficiently large defect-free area. It took multiple tries to develop a sample ideal for the film.
The screen shows the initial random arrangement of molecules as they appear when an operator views a new area of the copper surface. Each dark spot is one CO molecule, and larger dark spots are two or three molecules close together. They are placed where they were adsorbed by the copper substrate. Individual molecules need to be dragged into position to make the desired frame of the movie, and any remaining CO needs to be dragged out of the frame, molecule by molecule. The large spots in the upper right-hand corner are nanoscale contamination that needs to be avoided when selecting a place for a new frame.
In the grand tradition of Hollywood films, which almost always shoot out of order, the process started with the final frame—the company logo, which had the largest number of molecules. From there, the team progressed directly from one frame to the next, to minimize the number of moves. Although moving an atom takes seconds, positioning the tip takes far longer. Producing a single frame of the film varied from a few minutes to several hours. The entire movie was a result of several weeks of devoted effort by a team of four.
The nano ruler
To create a realistic sense of motion, the molecules needed to be displaced by a specific distance from frame to frame. The team built a molecular “ruler” that helped them gauge distance. Although the projections appear to be single spheres, they’re actually each a single CO molecule viewed end on. The carbon bonds to the copper and the oxygen projects toward the viewer. “It has just the right balance for us of holding still for imaging, being easy to move into new locations, and being very forgiving so that if we bump them into each other we can pull them apart again,” says Lutz.
The sound of motion
The red dot (center-right) indicates the position of the STM tip. The blue circles indicate the desired positions of individual molecules. Current flow changes with the sample-to-tip spacing, which in turn increases and decreases as the CO molecule is dragged over the copper from one binding site to the next. Run through an amplifier, that current variation generates a characteristic scritchy sound. Because the same STM probe tip is used for imaging mode and positioning mode, the team cannot observe the molecules at the same time as they move them. While they move the molecules, the last image scanned remains on the screen to help them navigate; to track their progress, though, they must rely on the sound.
A frame from the movie shows the CO molecules surrounded by ripples, which are not measurement artifacts but reflect a real physical property. The crystal face of copper naturally has some of the electrons confined to the surface so they form a two-dimensional electron gas. Electrons in the copper, especially those confined to the surface, bounce off of the CO molecules and interfere at a quantum mechanical level to form a standing wave (if you ever wanted proof of the dual wave/particle nature of matter, you see it here). Put another way, the ripples represent regions of alternately high and low probability of finding electrons. The ripples are visible to the STM because it measures the flow of tunneling current, which is in turn affected by the concentrations of electrons in the copper surface.
What’s next for the team? They’re continuing research into classical magnetic behavior at the atomic level but also studying quantum-level effects that allow electrons to simultaneously exist in spin up and spin down states. The team has spent most of the past year building an instrument that will allow them to observe and control this behavior, Lutz says.
To boldly go where no STM has gone before…
Of course, the team hasn’t by any means given up the film business. In addition to building that entirely new instrument, they’ve been busy crafting a series of Star Trek-themed images and animations for Paramount’s Star Trek Into Darkness
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