Angle resolved XPS

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

These conclusions were supported by results obtained from angle-resolved XPS. The band near 932.4 eV in the Cu(2pV2) photoelectron spectrum of the mirror coated with y-APS increased in intensity relative to that near 934.9 eV when the take-off angle was increased from 15° to 75°. Similarly, the band near 336.8 eV in the Auger spectrum also increased in intensity relative to that near 340.0 eV. Such behavior would be expected if the bands near 932.4 eV in the photoelectron spectrum and near 336.8 eV in the Auger spectrum were related to an oxide that was covered by a thin film of silane. 

Results. It was found that XPS spectra of mixed oxides could be interpreted using only the U + and U + oxidation states. A 1.3 + 0.2 eV shift existed between these two states which was maintained for mixed valence oxides. The U(4f ) photoelectron spectra for UO2 is shown in Fig. 4a while the spectra for a surface oxidized at - -300 mV is shown in Fig. 4b. The peak fits obtained, maintaining the 1.3 eV separation between UO2 (+4) and U03(-)-6), show good fits to the data and were used to determine the ratio of U +/U + in the surface oxide. Angle resolved XPS measurements indicated that the surface oxide was sufficiently thick after two minutes of oxidation to prevent any U +(U02) contribution to the spectrum from the UO2 substrate. 

Fig. 4 Distributions of SMA hydrophobic stearic [Ci8] endgroups vs. hydrophilic PEG spacer arms at various depth of MSPEO-modified PEU surface layer. Ratios of Ci8/Cis vs. cosine value of XPS takeoff angle [TOA] for SMAs with various-sized PEG spacers (2300 and 12 000 Da) respectively on air and water interfaces, as determined by angle-resolved XPS [76], Reproduced from [174]...

Though a typical XPS detector collects all emitted photons, regardless of their ejection angles, it should be noted that angle-resolved XPS (ARPES) and UPS (ARUPS) may also be carried out. By detecting photoelectrons emitted from a surface at different emission angles, one obtains the energy of the electrons as a function... 

The layer thickness of PVFA-co-PVAm-95 adsorbed on a silicon wafer was also estimated from angle-resolved XPS and ellipsometric measurements. From XPS measurements the layer thickness was determined to be 1.26 nm, and values obtained from ellipsometric measurements ranged between 1.0 and 1.5 nm [73]. These additional results correspond well with values determined from AFM scratch experiments (Table 1). 

In aqueous solutions the carboxylate groups of the grafted mercapto components can act as anchors for PVFA-co-PVAm molecules. The adsorption from differently concentrated PVFA-co-PVAm solutions (0.3 pmol L-1 to 3000 pmol L-1) gave polyelectrolyte layer thicknesses between 0.9 and 2.7 nm (determined by ellipsometry and angle-resolved XPS). The values found are in good agreement with the layer thicknesses on bare silicon wafer surfaces [97]. The layer thicknesses of the pre-adsorbed mercapto compounds range from 0.5 to 0.9 nm. That means that at very low polyelectroyte concentrations in the stock solution no increase in layer thickness was ob-... 

Tilting the sample so that the analyser collects electrons that are emitted at a more grazing angle increases the surface sensitivity of the technique. By acquiring spectra at several different angles, which have different surface sensitivities, quantitative information about the depth distribution of the elements in the top few nanometers of the sample can be extracted, this is know as angle resolved XPS (ARXPS). 

Table 6. Angle-resolved XPS atomic composition of corona-treated PDMS...

---