Photoemission spectroscopy surface sensitivity

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Four UHV spectroscopies used for the compositional and chemical analysis of surfaces are discussed. These are X-ray Photoemission, Auger Spectroscopy, Secondary Ion Mass Spectroscopy, and Ion Scattering (both low and high energy). Descriptions of the basic processes and information contents are given, followed by a comparative discussion of the surface sensitivities, advantages and disadvantages of each spectroscopy. 

Photoemission spectroscopy involves measurement of the energy distribution of electrons emitted from a solid under irradiation with mono-energetic photons. In-house experiments are usually performed with He gas discharge lamps which generate vacuum UV photons at 21.2 eV (He la radiation) or 40.8 eV (He Ila radiation ) or with Mg Ka (hv=1284.6 eV) or A1 Ka (hv=1486.6eV) soft X-ray sources. UV photoemission is restricted to the study of valence and conduction band states, but XPS allows in addition the study of core levels. Alternatively photoemission experiments may be performed at national synchrotron radiation facilities. With suitable choice of monochromators it is possible to cover the complete photon energy range from about 5 eV upward to in excess of 1000 eV. The surface sensitivity of photoemission derives from the relatively short inelastic mean free path of electrons in solids, which reaches a minimum of about 5A for electron energies of the order 50-100 eV. 

In terms of predictive capabilities, this distinction creates problems as most of the studies of hot-carrier relaxation have been confined to bulk processes by the virtue of the experimental limitations associated with all optical methods. The majority of the carriers are generated and probed in the bulk of the crystal. Fortunately, in the last few years significant progress has been made in the development of femtosecond photoemission spectroscopy (Bokor et al, 1986 Goldman and Prybyla, 1994 Haight, 1996 Schmuttenmaer et al, 1996) which is sensitive to the near-surface carrier relaxation processes. 

Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons. 

Apparently in contradiction the photoemission spectra (see Fig. 5.3) exhibit a dominance of majority electrons near the Fermi level. However, keeping in mind the calculation of Wu et al. [50] one can now easily realize the distinct surface sensitivity of metastable de-excitation spectroscopy (MDS) which gives predominantly information from the topmost surface layer whereas in photoemission experiments the information depth is a few layers. 

Unlike the absorption process, fluorescence of QDs is highly sensitive to the surface chemistry. Surface states within the semiconductor optical gap may introduce several effective radiative or nonradiative decay pathways that reduce PL quantum yields. Surface traps can be passivated by a variety of techniques including judicious selection of capping ligands, or the growth of inorganic larger band gap shell layers or photochemical passivation. The complex surface chemistry of nanocrystals has also been studied by NMR spectroscopy and X-ray photoemission spectroscopy (XPS). ... 

This chapter describes fundamental aspects of the electronic structure of organic semiconductors (small molecules and polymers), and their interfaces, and the method to bridge the electronic structure and electrical property more directly using ultraviolet photoemission spectroscopy (UPS). Penning ionization electron spectroscopy (PIES), which is the most surface-sensitive method, is also introduced for study of electronic states at outermost surfaces of solids, which are responsible for charge exchange through the interfaces between different materials when they get contact to form a hybrid system. 

The structures and registries of chemisorbed benzene on Rh(lll) have been thoroughly scrutinized by surface-sensitive techniques such as LEED, EELS, and angle-resolved UV photoemission spectroscopy (ARUPS) in UHV [43]. These previous studies revealed various structures for benzene, including well-known structures such as c 2 i x 4)rect and (3 x 3), depending on whether CO was present unintentionally or intentionally in the UHV chambers. Although it has been repeatedly demonstrated that the adlayer structures of benzene on Rh and Pt were greatly affected by the presence of CO in the adlayer, the structure of the pure benzene adlayer has not yet been fully understood. Neuber et al. reported that a completely new structure with a /19 x yi9)i 23.4° symmetry appeared for the pure benzene adsorption on Rh(lll) under cleaner UHV conditions in the absence of CO, and the previously known structures of 0(2 3 x 4)rect and (3 x 3) were found to appear upon admission of CO [48]. 

Having seen the power (and limitation) of nexafs spectroscopy in the preceding sections, one can readily envision the enhanced utility of nexafs spectroscopy as a characterization tool that would result from the addition of high spatial resolution capabilities. Since the spectroscopic sensitivity to specific moieties and functional groups can in many or even most cases be exceeded by ir, nmr, and Raman spectroscopies, nexafs microscopy will have to exceed the spatial resolution of these other spectroscopy techniques in order to be truly useful. To date, nexafs microscopy has surpassed a spatial resolution of 50 nm both in transmission to measure bulk properties (75-77) and in a reflection geometry to study surfaces (78,79). This level of spatial resolution is at least an order of magnitude better than what can be accomplished with complementary compositional analysis techniques. Future developments in nexafs microscopy might achieve a spatial resolution of a few nanometers (80,81). In addition, nexafs microscopy has exceptional surface sensitivity of about 10 nm, a sensitivity that could be improved to about 1 nm with photoemission electron microscopes (peem s) that incorporate a bandpass filter (80-82). 

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