For this purpose, X-ray photoelectron spectroscopy (XPS) is one of the most attractive techniques, and as such has been widely used to characterize doped materials. A typical sample must therefore be composed of innumerable such nanostructures in order to reach a measurable quantity, and thus any variability in their properties or in the distribution of dopants poses additional challenges for characterization.įurthermore, although local methods such as scanning tunneling microscopy (STM) and transmission electron microscopy based electron energy loss spectroscopy (TEM/EELS) can these days be used to directly visualize and study individual atoms (e.g., ), it is vital to obtain information about the distribution of the dopants in an entire sample. Compared to bulk solids, individual nano-objects are composed of far fewer atoms, and thus usual dopant concentrations correspond to a rather limited number of heteroatoms in the lattice. One of the key issues for controlled heteroatom doping is the detection and identification of the dopant atoms, and the further analysis of their concentration and atomic bonding environment in the studied materials. Heteroatom doping, the intentional replacement of some carbon atoms of the lattice with other elements, has been long studied for this and other purposes. However, for many real applications, additional control over the intrinsic properties of a material is needed.
Furthermore, the charge carriers in graphene behave as massless Dirac fermions, leading to unparalleled mobility and a number of exotic quantum phenomena. Finally, atomically thin single-layer graphene is extremely elastic yet impermeable, and the stiffest and strongest material ever measured. Moreover, the single-walled types are either semiconducting or metallic ballistic conductors even at room temperature, and capable of sustaining current densities 1000 times higher than copper. Carbon nanotubes have the highest length-to-diameter ratio of any material, with an extremely high specific strength. Fullerenes are very stable nanocontainers, exhibiting interesting selective surface reactivity. These materials each have superb intrinsic properties. ĭue to the unique nature of sp 2 hybridization, strong σ bonds are formed between carbon atoms in fullerenes, nanotubes and graphene ( Figure 1), along with delocalized π orbitals. Three recent stages have received major attention, starting with the discovery of fullerenes in the late 1980s, followed by the proliferation of carbon nanotube research from the early 1990s, and coming finally to the latest stage when graphene rose into prominence in the mid-2000s. Although two naturally occurring forms of carbon, graphite and diamond, have been known for millennia, several new carbon nanomaterials have been created and identified in the last decades. Graphitic carbon nanomaterials consist of carbon bonded via sp 2-hybridized covalent bonds into structures with dimensionalities in the nanometer scale. Keywords: carbon nanotubes core level photoemission graphene substitutional doping X-ray photoelectron spectroscopy (XPS) Starting from the characteristics of pristine materials, this review provides a practical guide for interpreting X-ray photoelectron spectra of doped graphitic carbon nanomaterials, and a reference for their binding energies that are vital for compositional analysis via XPS. If this is not the case, incorrect conclusions can easily be drawn, especially in the assignment of measured binding energies into specific atomic configurations. In general, care should be taken in the preparation of a suitable sample, consideration of the intrinsic photoemission response of the material in question, and the appropriate spectral analysis. Although the majority of works has concentrated on nitrogen, important work is still ongoing to identify its precise atomic bonding configurations.
Hundreds of studies have used XPS for analyzing the concentration and bonding of dopants in various materials. Towards this aim, the most studied dopants are neighbors to carbon in the periodic table, nitrogen and boron, with phosphorus starting to emerge as an interesting new alternative. Although these materials have superb intrinsic properties, these often need to be modified in a controlled way for specific applications. X-ray photoelectron spectroscopy (XPS) is one of the best tools for studying the chemical modification of surfaces, and in particular the distribution and bonding of heteroatom dopants in carbon nanomaterials such as graphene and carbon nanotubes.