analyses of the dominant Raman signal in the range between 900 and 1000 cm–1 (Fig. 3.2, inset).
A sharp single line at 962 cm–1, attributed to the symmetrical phosphate stretch, is characteristic of crystalline HA; the presence of a relatively small, almost indistinct shoulder toward lower wave numbers is attributable to a not well crystallized apatite structure (Fig. 3.2e).
Additional bands at 947 and 970 cm–1indicate the existence of b-TCP and a-TCP phases (Fig. 3.2c).
A doublet at 941 and 947 cm–1confirms the presence of tetracalcium phos-phate (TTCP) phases in the coating.
A broad band centered at 950 cm–1is characteristic of an amorphous cap structure (Fig. 3.2a).
Although Raman spectroscopy is a powerful, nondestructive characteriza-tion procedure, the technique also has some disadvantages. The interpretacharacteriza-tion of Raman spectra of CaP coatings is a complicated task because the intensity of the Raman signals is weak compared to those of bulk materials. Additionally, precautions should be taken in the spectrum interpretation as fluorescence and resonance effects can be present. Normal Raman bands and fluorescence can be discriminated by observing the anti-Stokes and Stokes spectra [9].
the latter being the energy needed by the photoelectron to escape the sample in the direction of the detector.
3.4.1 Qualitatitive and Quantitative XPS
Because f and hn are known in a setup and the kinetic energy is measured using the XPS analyzer, Ebindcan be calculated directly. A strongpoint of XPS is that all emitted electrons have binding energies that are specific for each element, so qualitative and quantitative determinations of the emitting elements are usually straightforward. In principle, the area under an XPS peak relates to the abun-dance of a species in the sample. However, the cross section for the photoelectric effect varies among the elements, and a correction factor must be applied to the measured peak to adjust the area and provide a quantitative result. This factor, also called the sensitivity factor, is usually derived from measuring thick stoi-chiometric reference samples. Caution should be practiced in simply copying these factors from system to system as they depend on the specific character-istics of the electron analyzer being used.
Another topic that complicates quantitative analysis to some extent but is often neglected is the varying escape depth, ‘‘attenuation length,’’ of the elec-trons. Although ionization through absorption of the x-rays occurs to a depth of a few micrometers, only those electrons that are created within a few nanometers below the solid surface can potentially leave the sample to be detected. The attenuation length of electrons emitted from CaP coatings is known and depends on their kinetic energy [15], which is derived from the electron binding energies (Eq. 3.1) of the most significant species (Ca, P, O), in the sample. Imagine a thin film (1–2 nm), for example, in an interface study of a CaP coating on a substrate. Most electrons that are emitted in the right direction escape the material. However, when the coating thickness is increased, the electrons with lower kinetic energies have a reduced chance of escaping. As a result, if no correction is made for varying attenuation lengths, an apparent gradient in coating composition can be measured that is not real. Also due to the same effect, a thin layer of (organic) contamination on top of the CaP coating (e.g., after cell culture experiments) influences the coating composition determination. The abundance of the species emitting electrons with low kinetic energy might be underestimated. Finally, caution should be practiced during quantification when the sample roughness has a considerable effect on the attenuation length of the electrons.
3.4.2 Chemical Shifts
The chemical environment of an atom influences the valence shell orbital, which in turn influences the binding energy of the core electrons. As a result, a shift in the kinetic energy of the photoelectrons can be measured that provides a tool
for determining the chemical structure of the analyzed sample. During XPS analysis, insulating samples charge and the XPS peaks shift accordingly.
A possible misinterpretation can be avoided by shifting all spectra such that a known reference material in or on top of the sample is at the expected position.
Usually the position of the carbon contamination (which is virtually always present) on top of a sample is used; alternatively, one can use an additional element, such as gold, that has been deposited on the surface for this purpose.
Unfortunately, there is some discrepancy in the literature regarding the refer-ence value that should be used for the carbon referrefer-ence; 284.6 eV and 285.0 eV are most widely used. In case of using binding energies found by other authors, it should be determined to which binding energies these values were measured.
3.4.3 Application Example
Figure 3.3A depicts an example of carbon C(1 s) spectra of polystyrene (PS) with and without 2 nm CaP that were measured in an interface study of CaP on
Fig. 3.3 A X-ray photoelec-tron spectroscopy (XPS) spectra of polystyrene (PS) with and without 2 nm of CaP (CaP). B C–O/C–C ratio as a function of CaP coating thickness for PS and polyydimethylsiloxane (PDMS). Insets Likely configurations of the initial bonding of CaP to PS and PDMS
polymers for better understanding coating adhesion. The CaP coatings of varying thickness were deposited by radiofrequency (RF) magnetron sputter deposition. The charging during XPS analysis was corrected for by shifting the main C–C peak to 284.6 eV.
The untreated PS shows two peaks, the main one representing the C–C/C–H bonds (further abbreviated as C–C) and a weak peak around 292 eV associated with the aromatic group of PS (not further considered here). When a 2 nm CaP coating is deposited, there is a strong decrease of the C–C peak, a result of the PS getting out of probing depth. At 288 eV, a clear side peak appears that is originated by different carbon-oxygen configurations (further abbreviated as C–O). This experiment was performed for varying CaP thickness using both PS and polydimethylsiloxane (PDMS) (silicone rubber) as substrate. The derived C–O/C–C ratios are plotted in Fig. 3.3B, revealing that for PS the C–O/C–C ratio increases with coating thickness from about zero to 0.6 for a 9 nm thick coating, decreasing again with increasing thickness. Obviously, many C–O bonds are formed during deposition of the CaP coating on PS, and the decrease of the C–O/C–C ratio is caused by the fact that the interface is getting out of probing reach. Further supported by the observation that P enrichment was found near the interface [16], these results proved that the CaP coating estab-lished a bond to the carbon chains of the PS via the phosphate groups (Fig. 3.3B, inset). Interestingly, when the same experiment is performed on PDMS, the C–O/C–C ratio remains low (Fig. 3.3B), and the relative P abun-dance near the interface was again found to be high [17]. This result indicates that no direct bond between the C of the PDMS and the phosphate groups is established now and that the carbon side groups of the PDMS are removed (Fig. 3.3B, inset), implying that a direct bond between Si of the PDMS and the phosphate groups has been formed. This example clearly shows how powerful XPS can be in studying thin layers of CaP (e.g., in an interface study).
3.4.4 Depth Information
The previous example suggests that XPS is suitable only for studying extremely thin layers or interfaces of CaP. This is certainly not the case. First, when the top few nanometers of a CaP coating is representative of the whole coating, XPS can of course be used to study coating composition. A rather sophisticated method to influence the probing depth during the analysis is to vary the angle between the sample surface and the analyzer in a technique called angle-resolved XPS. Of course, the analyzed region still never exceeds 10 nm as the attenuation lengths of the electrons are limited. Deeper regions of a sample can only be analyzed using controlled erosion of the surface by ion sputtering.
Which kind of ions is used for this purpose depends on the type of specimen and the desired depth profile characteristic. Soft erosion might be most suitable for determining sharp interfaces, and strong sputtering is more suitable for
analyzing regions deeper in the material. However, during such a depth profile analysis, one should be aware of the risk of preferential sputtering of the weaker bonded species, as this process might lead to erroneous quantification. More-over, the impact of the ions on the sample can charge its surface and modify the energy of the emitted photoelectrons as well as enhance the roughness of the surface. Furthermore, the measured composition of the sample can be misin-terpreted owing to the ‘‘knock-on’’ effect (the process of the sputtering ion
‘‘knocking’’ an element deeper into the sample). In addition to depth profiling, the sputtering effect is widely used to gently clean the surface of a sample to remove ambient contamination prior to XPS analysis.