Imaging the Invisible

Advanced scanning probe microscopy tools are opening new worlds of nanofabrication.
 

Some mark the dawn of the nanoscience era in 1982, when two IBM researchers invented the scanning tunneling microscope. The STM for the first time provided real-space imaging of single atoms, suddenly making possible the study and manipulation of the electronic and atomic structure of materials at the atomic level. Two decades later, imaging detailed features of surfaces using the STM helps physicists improve semiconductor and microelectronic devices, enables chemists to see how well a catalyst stimulates chemical reactions on surfaces, and allows biologists to examine DNA molecules.

Using the STM, ORNL researchers study the structure of a conductive surface by employing an extremely sharp probe that scans the surface at a distance on the order of an atomic radius. Electrons tunnel between the surface atoms and the single atom forming the probe tip, producing an electrical current. Slowly scanning across the surface, the probe rises and falls to keep the current constant, thus maintaining the distance. The probe's vertical movements are recorded, creating a profile of the surface that the computer translates into a two-dimensional map.

John Wendelken of ORNL's Condensed Matter Sciences Division has led the development of novel techniques for growing and manipulating nanostructures using the STM as an imaging and fabrication tool. Wendelken and his team have used the STM to crack iron-containing molecules to make magnetic iron wires only 5 nanometers wide, establishing a pathway for STM-assisted chemical vapor deposition. Thus, fabricated nanostructures, as well as buffer-layer-grown iron nanoparticles, are shown by the STM to be in alignment with the surface crystal structure of a copper substrate.

Wendelken's team also demonstrated in separate experiments that cyclopentadiene (C₅H₅) molecules deposited on a silver substrate form "rings" that represent the substrate's electronic response to the C₅H₅ molecules. At a certain STM tip voltage-current setting, the C₅H₅ molecule jumps to the next lattice site before the ring image is completed. The resulting zigzag tracks show the paths of the molecules being "swept" across the surface in the so-called "molecular broom" effect. "Understanding this tip-controlled molecular dynamics holds the key to future molecular fabrication technologies," Wendelken says.

Wendelken, Ward Plummer, and colleagues at ORNL are working with STM designers to build advanced ultrahigh-vacuum scanning microscopy probes for the Department of Energy's Center for Nanophase Materials Sciences at ORNL.

"We are buying and helping develop a cryogenic four-probe STM," says Wendelken. "This instrument will enable researchers to grow, prepare, and manipulate samples. The instrument is so unique that we expect to attract researchers from the world's outstanding universities."

This STM will enable studies of quantum transport in nanoscale systems and fabrication and characterization of nanoscale devices. Combining cutting-edge imaging capabilities, the STM will guide deposition of metallic and semiconducting films using molecular beam epitaxy.

Another instrument to be housed at the CNMS will be the low-temperature, high-field "ultimate" STM. ORNL, the University of Tennessee, and the University of Houston are fabricating this CNMS partner instrument. The ultimate STM will be used for single-atom and single-molecule spectroscopy, the generation of atomic-resolved spectroscopy maps, and studies of quantum responses of nano-objects.

ORNL's Sergei Kalinin helped pioneer several advanced scanning probe microscopies that will be available to users at CNMS. These techniques include electromechanical imaging of biological systems and scanning impedance microscopy, designed to address frequency-dependent electromechanical properties and electron transport in systems as diverse as carbon nanotubes and oxide nanowires, electronic devices, and biological systems. The nanocenter will also have ambient and ultrahigh-vacuum microscopes capable of piezoresponse force microscopy and atomic force acoustic microscopy.

Kalinin and colleagues used atomic force acoustic microscopy to study a butterfly wing. When the wing "sample" was vibrated mechanically, acoustical waves—tiny blasts of sound—were transmitted to the probe tip and detected. The contrast between the hard and soft regions of the wing provided insights on the butterfly wing's elasticity and durability.

As if on the wings of a butterfly, ORNL's researchers will probe into worlds their predecessors could only imagine.

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