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The physical principles behind HS-LEIS are similar to those of a game of billiards. Like a cue ball, noble gas ions are fired at the surface of a sample. An ion interacts with a sample’s surface atom the same way a cue ball strikes another pool ball – it bounces straight back or is deflected at an angle. In the process, a fraction of its energy is transferred to the surface atom.
The amount of energy lost is directly related to the atomic weight of the surface atom. The spectrometer measures the energy of the rebounding noble gas ions to determine the identity of the atom from which it was scattered.
A toroidal energy analyzer in Lehigh’s HS-LEIS spectrometer includes a positionsensitive detector and time-of-flight mass filter that provide a 3,000-fold improvement in sensitivity over its predecessors. It also allows for elemental 2-D surface mapping that complements the elemental 2-D near-surfaceregion mapping capabilities of the ESCA 300.
A new Fourier Transform-Infrared (FT-IR) 8700 spectrometer enhances Lehigh’s surface analysis capabilities. Lehigh is one of the first research facilities to acquire this instrument, which collects signals in as little as 10 nanoseconds and can study liquid-solid and gas-solid interfaces.
The FT-IR 8700 provides molecular-level information critical to the photocatalytic splitting of water into oxygen and hydrogen, a clean fuel. The splitting occurs in just the nanoseconds that it takes for light-excited electrons to hop from the valence to the conduction band of a solid semiconductor mixed oxide material, and back.
“Many photocatalytic reactions and chemical processes happen in time scales on the order of nanoseconds,” says Charles A. Roberts, a Ph.D. student in chemical engineering. “FT-IR lets us monitor the rapid electron and chemical transformations that occur during these processes.”
Two views are better than one
To rapidly obtain a 3-D picture of a material’s surface and the location, width, height and depth of its bumps and indentations, scientists rely on atomic force microscopy (AFM).
An atomic force microscope consists of a probe, or needle, that scans a surface like an old record player stylus, measuring the height and recording the position of its topographical features. The needle can also detect surface modulations, magnetic and chemical forces, and atomic and electronic structure.
The resulting representation, says Richard Vinci, resembles a hiker’s topographical map. “AFM is a wonderful visualization tool to imagine what a surface looks like,” says Vinci, professor of materials science and engineering. “You have to imagine because you never really see the surface; what you see is a computer reconstruction of what the surface looks like based on the interaction between the surface and the moving probe.”
Unlike an electron microscope, which operates in a vacuum, an AFM can characterize materials in liquid or air and is thus well-suited to study bio- and nano-materials.
Vinci and Slava V. Rotkin, associate professor of physics in the College of Arts and Sciences, recently acquired an NTEGRA-Spectra, which couples an AFM manufacturered by the Russian company NT-MDT with an optical microscope made by Olympus.
By positioning an AFM atop an inverted optical microscope, the NTEGRA allows researchers to examine materials in multiple ways simultaneously. One option is to examine a specimen from below with the optical microscope while probing it from above with AFM. Another is to stimulate a specimen with a laser through the Olympus optics while the AFM measures its properties from above.