Soft-x-ray spectroscopy probes nanomaterial-based devices

Abstract

Synchrotron radiation-based spectroscopic techniques provide information about the characteristic electronic structure of nanostructured materials.Synchrotron radiation is a photon light source generated by high-energy electrons that are centripetally accelerated in the magnetic fields of a storage ring, as shown in Figure 1. This radiation is extremely intense over a broad range of wavelengths extending from the infrared through visible light and ultraviolet into the soft and hard x-rays of the electromagnetic spectrum. 1 The wavelength of soft x-rays (1∼10nm) is most suitable for the analysis of nanomaterials. Such analysis provides information about the materials' electronic structures that can be used to optimize nanomaterial-based devices.Among the soft-x-ray spectroscopic techniques, x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) probe the energy distribution of electronic states in atoms, molecules, and solid state materials. The basic concepts, shown in Figure 2, involve the interaction of x-rays and matter as explained by molecular orbital theory. In XAS, the absorption of photons excites electrons from deep core levels, such as 1s, of a selected atom in a molecule to unoccupied states, leaving behind a core hole. In XES, the core hole is filled by a valence electron causing the emission of an x-ray photon. XES gives information about chemical bonding in the molecule. By combining XAS and XES, one can obtain information about unoccupied states (conduction band) and occupied states (valence band). The difference in energy between the conduction band and the valence band is called the band gap.As semiconductor devices become smaller and smaller, the dimensions of new electronics components are reaching nanometer scale. Understanding the fundamental properties of these new devices, such as their electronic structures and band gaps, provides opportunities to design advanced materials and to fabricate devices for future applications. In certain semiconductors, when electrons fall from the conduction band to the valence

Soft x-ray spectroscopy for probing electronic and chemical states of battery materials

“…A schematic representation of soft x-ray absorption spectroscopy and emission spectroscopy processes. [21] sXAS and XES signals correspond to the unoccupied and occupied states, respectively.…”