Further insight into the electronic properties of Mn12-ac submonolayers deposited on Au(111) and Ag(111) was obtained by using X-ray photoelectron spectroscopy (XPS). In the following, we present a quantitative analysis of the C 1s, O 1s, Mn 2p and Mn 3s core level spectra.
C 1s and O 1s Core Level Spectra
Due to the similarity of the results achieved on both noble metal surfaces, the C 1s and O 1s core level spectra are discussed only for Mn12-ac/Au(111). Figure 9.19a shows the spectral shape of the C 1s contribution at a surface coverage of 0.4 ML.
The measurement exhibits a low binding energy peak at 285.2 eV and a high binding energy peak at 288.7 eV. According to the binding configuration, we attribute the two peaks to the C atoms of the methyl (CH3) and carboxylate (COO) groups of the acetate ligands, respectively. The energy separation between both peaks is consistent with the range of 3.5–4 eV reported in literature [225].
C1s
O1s
Mn2p, Au4p (a)
(b)
(c)
Clean Au 0.2ML 0.4ML 0.7ML
Mn2p1/2 Au4p1/2 Mn2p3/2
Figure 9.19 | X-ray photoelectron spectra (Mg Kα) of Mn12-ac/Au(111).
(a)C 1s and(b)O 1s core level spectra at 0.4 ML coverage. The data were corrected by subtracting a linear and a polynomial background, respectively. Solid red and dashed black lines are fits to the experimental data as described in the text.(c)The Mn 2p spectrum is superimposed by the Au 4p1/2 contribution. An increase of the Mn 2p1/2 peak is clearly visible as the coverage increases from 0 and 0.7 ML.
The peak areas were estimated by fitting the spectrum with a sum of pseudo-Voigt functions. The resulting area ratio of the methyl to carboxyl peak is 1.2, which is to be compared to a 1:1 ratio of the corresponding carbon atoms in Mn12-ac. The slightly higher value of the area ratio can be explained by considering two different effects: First, the methyl groups of the upper-lying acetate ligands, which contribute
strongest to the signal, are closer to the top of the sample surface than their carboxyl groups, meaning that the methyl contribution is less attenuated. Secondly, organic contaminations present on the sample surface mainly contribute to the methyl peak.
For the latter reason, significantly larger methyl to carboxyl ratios were reported in other experiments [200,203]. In contrast to these former studies, the almost identical peak areas observed here demonstrate that only small amounts of impurities are present on the sample surface.
The O 1s core level spectrum, obtained on the same sample, is shown Figure 9.19b. The measurement exhibits a main peak at high binding energies, which is attributed to the oxygen atoms of the carboxylate groups and water molecules.
An additional shoulder visible at smaller binding energies can be ascribed to the contribution of the Mn12O12 core [200]. Fitting the profile with two pseudo-Voigt functions yields peak positions of 531.8 eV and 530.1 eV, respectively. No attempts were made to separate the carboxylate and water contributions in the fit, as the spectral shape does not provide enough detail to reliably estimate the additional degrees of freedom that would result from a three-peaks fitting model.
From the best fit of the experimental spectrum, an area ratio of 3.0 between the high and low binding energy contribution is calculated. This value is in perfect agreement with the abundance of the different oxygen species when it is assumed that water and acetic acid molecules of crystallization are not present on the sample surface, i.e. only the cluster Mn12O12(CH3COO)16(H2O)4 is deposited.
The Mn Oxidation State
XPS allows for an identification of the Mn oxidation state based on three different parameters [230]. First, the binding energy of the Mn 2p3/2 peak is increasing as the oxidation state increases. Secondly, the position of the Mn 2p shake-up satellite with respect to the main peak is significantly different for Mn(II) compared to other oxidation states. Finally, the exchange splitting of the Mn 3s level is a decreasing function of the oxidation state. Reference values of all three parameters were reported for Mn12-ac bulk material [231], as well as for different manganese oxides [230, 232]
and are summarized in Table 9.1.
Unfortunately, in case of Mn12-ac/Au(111) both the Mn 2p and Mn 3s core level contributions are superimposed by highly intensive substrate peaks, making a quan-titative analysis of their structure impossible (see Figure 9.19c). The Mn oxidation state is therefore only discussed for Mn12-ac/Ag(111). All data presented here were obtained on a sample with ∼0.8 ML coverage.
The Mn 2p and Mn 3s core level spectra of the Mn12-ac/Ag(111) submonolayer are depicted in Figure 9.20. The maximum of the Mn 2p3/2 peak is measured at a binding energy of 641.7 eV. Mn 2p shake-up satellites are clearly visible at around 5–
6 eV with respect to the Mn 2p1/2 and Mn 2p3/2 main peaks. In order to estimate the
Valency Mn 2p3/2 (eV) ∆E sat. (eV) ∆E Mn 3s (eV)
MnO 2 640.4–641.7 5.4–5.7 5.8–6.2
Mn3O4 2.6 641.4–641.5 10.5–11.3 5.3–5.6
Mn2O3 3 641.8–641.9 10.0–10.5 5.2–5.5
MnO2 4 642.2–642.6 11.2–12.9 4.5–4.7
Mn12-ac 3.3 642.4±0.2a 10.6±0.2a 5.4±0.15
Table 9.1 | Literature values of the Mn 2p3/2 binding energy, the Mn 2p shake-up satellite position and the Mn 3s exchange splitting, reported for manganese oxides with different Mn oxidation states and for Mn12-ac bulk material. References: [230–
232].aValue estimated from the plot of the Mn 2p core level spectrum.
Mn 3s exchange splitting, great care was taken to perform an appropriate background correction. In detail, the background was described as a sum of a Shirley profile and four Doniach-Sunjic peaks, accounting for the nonlinear contributions of the adjacent Ag 4s, Ag 4p1/2 and Ag 4p3/2 levels, as well as for a Ag 4s satellite caused by the non monochromatic X-ray source. The data shown in Figure 9.20b were corrected for the background contribution. The best fit of the Mn 3s double-peak yields an exchange splitting of (5.7±0.2) eV.
In comparison with the reference values reported for both Mn12-ac bulk ma-terial and different manganese oxides, the observed position of the Mn 2p shake-up satellites clearly demonstrates a partial reduction of the manganese ions in the Mn12-ac/Ag(111) submonolayer from +III/+IV to +II. This finding is corroborated by the binding energy of the Mn 2p3/2 peak, which is significantly smaller than the value expected for an Mn(III)/Mn(IV) mixed valence compound. The same trend is observed for the Mn 3s exchange splitting. Compared to manganese oxide reference materials, the exchange splitting of the Mn12-ac submonolayer indicates a reduction the Mn12O12 core. However, we note that the deviation of the exchange splitting from the value reported for Mn12-ac bulk material lies within the accuracy of our measurement.
In conclusion, the XPS Mn 2p and Mn 3s core level spectra indicate a reduction of manganese in Mn12-ac/Ag(111). Two different effects need to be taken into account to explain this finding. First, a reduction of the manganese ions might result from a charge transfer between substrate and molecule. Secondly, a change of the oxidation state can generally be caused by radiation damage, i.e. a degradation or charging of the molecules caused by the ray radiation. From STM images reacord after X-ray photoelectron spectroscopy, we found no significant changes in the topographic appearance of the samples. Therefore, we consider a charge transfer process to be the most likely explanation.
(a) (b)
Mn2p Mn3s
Mn2p1/2 Mn2p3/2
Figure 9.20 | X-ray photoelectron spectra (Mg Kα) of approximately 0.8 ML Mn12-ac on Ag(111). (a)Mn 2p core level spectrum. Arrows indicate the positions of shake-up satellites.(b)Exchange splitting of the Mn 3s level. Data were background corrected as described in the text. Solid red and dashed black lines are fits to the data based on Voigt functions.