Photoelectron diffraction, X-ray

The sample must be a single crystal semiconductor or metal and its surface should be smooth and flat so that the electron emission angle can be accurately determined. High surface sensitivity makes the technique vulnerable to contamination, necessitating the use of UHV procedures. However this surface sensitivity combined with the element specific crystallographic information that XPD provides makes the technique ideal for studying the structure of adsorbed molecular layers on single crystal substrates. 

Element specific crystallographic information about the near surface region. 

Typically content of a few percent at the surface can be detected, depending strongly on the photoemission cross-section of the particular elements present Sensitivity decreases rapidly with depth below the surface and is often negligible below a depth of approximately 5—10 nm, depending on electron energy. All elements can be detected directly with the exception of hydrogen and helium. 

The composition of the surface as probed by XPD may not be completely representative of the bulk material due to surface segregation or relaxation of the lattice. However the surface specific crystallographic information provided by XPD means that it is a particularly useful technique for studying such surface reconstructions. 

XPD is used to probe the element specific, or chemical state specific, crystallographic structure and near neighbour atomic arrangement of atoms in the near surface region of a crystalhne material or of adsorbate molecules on its surface. 

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This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

Figure 1 Simplistic schematic illustration of the scattering mechanism upon which X-ray photoelectron diffraction (XPD) is based. An intensity increase is expected in the forward scattering direction, where the scattered and primary waves constructively interfere.

X-Ray Photoelearon Spectroscopy X-Ray Photoemission Spectroscopy Electron Spectroscopy for Chemical Analysis X-Ray Photoelectron Diffraction Photoelectron Diffraction Kinetic Energy... 

Cecconi, T. Atrei, A. Bardi, U. Forni, F. Innocenti, M. Loglio F. Foresti M. Rovida G. 2001. X-ray photoelectron diffraction (XPD) study of the atomic structure of the ultrathin CdS phase deposited on Ag(lll) by electrochemical atomic layer epitaxy (ECALE). J. Electron Spectrosc. Relat. Phenom. 114-116 563-568. 

The second example concerns heptahehcene. Figure 4.17 shows the structure of left-handed M-heptahelicene, where M stands for minus. On Cu(lll) surfaces the M-heptahelicene molecules are found to adsorb in a geometry with their terminal phenanthrene group (the first three carbon rings) oriented parallel to the (111) faces and to successively spiral away from the surface from the fourth ring on, as determined by X-ray photoelectron diffraction experiments (Fasel et al, 2001). 

A (1x1) LEED pattern is generally observed upon sputtering and annealing in UHV. To my knowledge no quantitative LEED study has been reported, probably because of the difficulty of creating defects when the sample is bombarded with electrons (see section 3.1.4). A medium-energy electron diffraction study (MEED) study of Ti02(l 10) employed an ESDI AD optics with a channelplate this setup is more sensitive than a conventional LEED apparatus, and allows for very small electron currents to be used. The results of this study are basically consistent with the (1x1) structure depicted in Fig. 3b. X-ray photoelectron diffraction (XPD) spectra also fit the expected (1x1) termination [33], as do STM and AFM results (which are discussed in section 3.1.3 below). 

As reported by numerous workers using a range of techniques including MEISS, thermal desorption of probe molecules (CO) and polar X-ray photoelectron diffraction (XPD) [47-50], there is significant heterogeneity in the Cu 100 -c(2x2)-Pd alloy with a significant portion of the deposited Pd resides in second layer substitutional sites and domains of pure Cu in the outermost layer. 

Galeotti M, Atrei A, Bardi U, Rovida G, Torrini M (1994) Surface alloying at the Sn-Pt(l 11) interface a study by x-ray photoelectron diffraction. Surf Sd 313 349... 

Li Y, Voss MR, Swami N, Tsai YL, Koel BE (1997) Probing the structures of bimetallic Sn/Rh(lll) surfaces alkali ion scattering and X-ray photoelectron diffraction studies. Phys Rev B 56 15982... 

Although infrared absorption intensities are very sensitive to molecular orientation, deriving quantitative information about molecular orientation is not easy [33,34], On the other hand, photoelectron diffraction is relatively easily interpreted to yield adsorbate orientations on surfaces. X-ray photoelectron diffraction and density functional theory calculations have been used in tandem to study the orientations of D- and L-cysteine adsorbed on the Au(17, 11, 9) surface (Fig. 4.9) [35]. Cysteine is an amino acid with the functional group R = CH SH. On the gold surface the S-H bond dissociates to give a thiolate bond to the surface. X-ray photoelectron diffraction of the N Is level indicates that the N-C bond in o-cysteine is oriented roughly parallel to the step edge on the Au(17, 11, 9) surface while the N-C bond in... 

For films of titania grown on metal substrates, it is readily apparent that the overlayer is an oxide rather than an alloy as the superstructures only form in the presence of oxygen and oxygen is seen in XPS. X-ray photoelectron diffraction (XPD) has also been employed to probe the layer structure and most phases (including all of the hexagonal phases) the overlayers are proposed to form with a Pt-Ti-0 stacking sequence in the monolayer [29],... 

Nowicki M, Emundts A, Werner J, Pimg G, Bonzel HP (2000) X-ray photoelectron diffraction study of a long-range-ordered acetate layer on Ni(llO). Surf Rev Lett 7 25... 

X-ray photoelectron diffraction is the coherent superposition of a directly photo-emitted electron wave with the elastically scattered waves from near-neighboring atoms. This gives element-specific structural information about the near surface atoms in a single crystal [8-10]. The short inelastic mean free path of the electron waves at the kinetic energies of interest (15 to 1000 eV) leads to surface sensitivity and determination of the atomic geometry of the emitting atom. The known energies of narrow XPS core-level peaks lead to element specificity. The resolution of surface peaks and chemical shifts may even sometimes lead to a chemical state-specific structure determination. 

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