Emission pattern of photoelectrons from the (111) surface of diamond, measured in two seconds at the ALS.
Synchrotron radiation is emitted by electrons orbiting in a storage ring. It provides ultra-bright ultraviolet light and soft X-rays which are used for a wide range of analytical techniques. As surface scientists we are primarily interested in finding out how atoms are bonded at a surface and how their electrons determine the bonding. Analyzing photoelectrons ejected from a surface by soft X-rays provides chemical analysis with optimum surface sensitivity because the photoelectrons are emitted at their minimum escape depth (only a few atomic layers deep). The energy spectrum of photoelectrons reflects the energy levels of surface atoms, which identify an atom and its neighbors via chemical shifts.
Overview of photoelectron spectroscopy: F.J. Himpsel and I. Lindau, Ch. 9, Photoemission and Photoelectron Spectra, Encyclopedia of Applied Spectroscopy, Edited by David L. Andrews, Wiley-VCH, (Weinheim 2009). ISBN: 978-3-527-40773-6.
Undulators are periodic magnet structures that represent the state of the art in producing synchrotron radiation. They exhibit a spectral brilliance up to 10,000 times higher than conventional bending magnets. Undulators at the Advanced Light Source (ALS) cover the spectral range from 100-1000 eV photon energy, which contains the sharpest core level of each element. Undulators at the Synchrotron Radiation Center (SRC) are optimized for the 10-100 eV energy range, which represents the best energies for exciting the valence electrons.
A special photoelectron analyzer displays an image of the photoelectrons emitted in various directions. With the high brightness at an undulator at the ALS such images take only a few seconds to acquire. That makes it possible to quickly explore electrons emitted in all directions and thereby obtain a complete picture of electrons in solids and at surfaces.
Emission pattern of photoelectrons from the (111) surface of diamond,
measured in two seconds at the ALS.
The most common method for describing electrons in solids is local density theory. The 1998 Nobel Prize in Chemistry was awarded for the development of this method. It works so well that it has been difficult to test its limits and to find ways to take its accuracy to the next level. As a fundamental characteristic of the electrons one can choose their band width, which reflects the strength of their interactions. This number can be obtained from a series of images of photoelectrons emitted by synchrotron radiation at different energies, such as those shown below. These data from diamond achieve 1% accuracy, ten times better than previous experiments. It is found that the band width is 7% too small in local density theory, but well-described by quasiparticle calculation, the latest theory of electrons in solids. This type of measurement has been performed for protopes of an insulator (lithium fluoride), a semiconductor (diamond), and a semi-metal (graphite).
Examples: Himpsel et al., Phys. Rev. Lett. 68, 3611 (1992); Jimenez et al., Phys. Rev. B 56, 7215 (1997); Heske et al., Phys. Rev. B 59, 4680 (1999).
Images of photoelectrons from diamond at various energies below the
top of the valence band (in eV).
Many properties of a solid are determined by electrons close to the Fermi level, which represents the highest occupied energy level. Examples are the electrical conductivity, magnetoresistance, superconductivity, and magnetism. Only a narrow energy slice around the Fermi level is relevant for these properties. The width of this slice is about 3.5 kT, corresponding to the states that are thermally accessible (kT=25 meV at room temperature). In recent years the resolution of photoemission experiments has improved such that electrons within a few meV around Fermi level can be distinguished. Angle-resolved photoemission provides the information about the momentum of these electrons, which is not accessible by measuring conductivity and magnetism directly.
Examples:
Altmann et al., Phys. Rev. Lett. 87, 137201 (2001).
For an overview on magnetoelectronics:
Himpsel et al., J. Magn. Magn. Mat. 200, 456 (1999).
Energy bands of nickel close to the Fermi level, measured with
high-resolution at the SRC.
The magnetic splitting between spin up and spin down electrons is clearly resolved near the Fermi level (energy zero).
The slope of the dark lines corresponds to the electron velocity.
Core level absorption spectroscopy with fluorescence detection is one of the techniques that became possible with the leap in spectral brilliance using undulators. The ALS is suited particularly well for the core levels of organic and biomaterials, and it covers the sharpest core levels of all elements. Synchrotron light selects a transition from the core level of a well-defined atom into a specific valence orbital. The atom subsequently emits soft X-rays as the electron falls back into the core hole.
This technique makes it possible to look at nanostructures assembled at a surface, such as nanocrystals, self-assembled monolayers of organic molecules, and all the way to DNA and proteins immobilized at a surface for DNA chips and bio-sensors. Small amounts of impurities can be detected as well, such as buried layers of boron dopants in silicon devices. The next level of sophistication is a micro-focused fluorescence probe with efficient detectors. Such an instrument makes it possible to look at small samples of new materials and to scan graded overlayers with varying composition and thickness. This enables high throughput combinatorial analysis methods of thin film structures. The nearby Molecular Foundry provides many capabilities for synthesizing nanostructures. That closes the synthesis-analysis feedback loop for rapid development of new "designer" materials. For example, a recent project has the goal to improve the efficiency of inexpensive dye-sensitized solar cells by tailoring the energy levels of the dye molecules.
Examples:
Buried layers of dopants:
Carlisle et al., Appl. Phys. Lett. 67, 34 (1995)
Nanocrystals assembled at surfaces:
Pérez-Dieste et al., Appl. Phys. Lett. 83, 5053 (2003)
Immobilized DNA:
Petrovykh et al., J. Am. Chem. Soc. 128, 2 (2006)
Immobilized protein:
Liu et al., Langmuir 22, 7719 (2006)
Dyes for biomimetic solar cells:
Cook et al., J. Chem. Phys. 131, 194701 (2009).
The sharpest core levels of the elements and their binding energies.
Images of a new chamber for photon in / photon out spectroscopy of nanostructures at the
ALS.
Concept on the left, the real thing in action on the right.