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Photoelectronic scoping of adatoms, atomic vacancies, and the outermost layer of a surface

An effective yet simple means disclosed herewith has allowed us to gain the atomistic, local, and quantitative information of bonds and electrons at sites surrounding undercoordinated atoms, complementing the scanning tunneling microscopy/spectroscopy and photoelectron spectroscopy (XPS). Examining Rh and Pt surfaces with and without adatoms and graphite surface with and without atomic vacancies, we confirmed that: i) bonds between undercoordinated atoms become shorter and stronger; ii) subjective polarization happens to the valence electrons of defects or adatoms by the densely entrapped bonding electrons, which in turn screens and splits the crystal field and hence the core band of the specimen.

Bonds and electrons annexed the undercoordinated atoms dictate the unusual performance of materials at surface, defect, and at the nanoscale in many applications such as photoelectron emission, 1 local charge density, 2 mechanical strength and work function, 3 surface magnetism, 4 catalytic reactivity, [5][6] crystal growth, 7 adsorption, 8 oxidation, 9 decomposition, 10 doping, 11 interface formation, [12][13] wettability, [14][15] thermal stability, 16 etc. Although the chemistry and physics of materials associated with under-coordinated atoms have been extensively investigated, the laws governing the performance of such local bonds and electrons remain as yet to be established. [17][18] Collection and purification of such local, atomistic zone selective, and quantitative information having been increasingly demanded but it remains as yet the "dead corner" of the community.
As a powerful means, scanning tunneling micro/spectroscopy (STM/S) maps local electrons in the open side of a too-thin subatomic layer of a surface with energies of a few eVs cross Fermi energy (E F ). Understanding the intriguing STM/S attributes of high protrusions and the additional resonant spectral peak nearby E F at graphite atomic vacancy 19 and graphene edge, 20 for instances, as well as the driving force for the unusual protrusions and the E F resonance remains challenge. One urgently needs to identify what the "root" of the STM/S attributes is and what the quantitative information could be about the length and strength of the annexed bonds and the energies of the associated electrons. On the other hand, a photoelectron spectroscopy (PES such as ultraviolet or x-ray as the sources called UPS and XPS, respectively) probes statistic and volumetric information of electrons with binding energy in the valence band and below within a too-thick layer of 10 nanometers or thicker. 18,[21][22][23] In order to cope with this challenge, we have developed a special yet simple technique of zone-resolved photoelectron spectroscopy (ZPS), which has enabled us to realize the dreams.
We need to clarify first the reference point and the direction of the core level shift upon bulk formation and atomic undercoordination, as well as correlation between the bond length, bond energy and the core level shift as measured using XPS. According to the energy band theory of tight binding (TB) approach, 24 the energy shift of a specific th core band from that of an isolated atom is proportional to the crystal potential energy at equilibrium or the cohesive energy per bond, The BOLS-TB decomposition of the C 1s core level shifts of graphene edge, graphene, graphite, and diamond, 26 the 4f 7/2 level of Pt, 27 Figure 1 illustrate the correlation between the E  (0), the E  (12), and the z-dependent shift in comparison with those derivative from carbon allotropes. 25 These findings provide the foundations for the photoelectronic skinning of the adatoms, defects and the outermost atomic layers of a surface as demonstrated thus.
Instead of the tedious processing of XPS decomposition, we can purify the spectra due to atomic undercoordination using the ZPS method upon the spectral background correction and the peak area normalization. The difference between the Pt and the Rh spectra coincides exceedingly well with the BOLS expectation that only the otherwise conductive half-filled s-electron Rh(4d 8 5s 1 ) can be polarized and anchored as monopoles to the adatoms. The possible reason for the loss of the initially trapped surface charge at z = 3 is that the electrons of adatoms are fully polarized, moving the otherwise entrapped states from z = 3 to z = 4 ~ 6 associated with the P states.
The raw XPS spectra, shown in Figure 3, were collected from (a) clean graphite (0001) surface at 25 and 75 emission angles (between the electronic beam and the surface normal) and from (b) the surface with and without Ar + -spraying induced vacancy defects at 55. One can hardly tell anything from the raw data but the ZPS brought a great difference. As an XPS collects more information from surface at large emission angles than those at small angles, the difference between the two spectra can discriminate the surface from the bulk, as shown in Figure 3(c).
Likewise, the difference between the spectra collected before and after defect formation purifies the defect states as compared in Figure 3