X-Ray Photoelectron Spectroscopy
Two different XPS instruments were used. One is a commercial set-up (SPECS) offering a non- monochromatic Mg or Al Kα photon source (hν = 1253.6eV and hν = 1486.6eV, respectively). A Phoibos 150 (SPECS) analyzer was implemented in the system. Spectra were recorded with a pass Energy of EP ass= 20 eV. The SPECS set-up using the Mg Kα photon source was mainly
utilized for data presented in Chap. 5. XPS data is normalized on the maximum intensity of the Au 4f7/2core level.
Additionally, in Chap. 6 another experimental set-up was utilized which was operated by the interface analysis service department, Max Planck Institute for Solid State Research, Stuttgart. The x-ray source is a monochromatic Al Kα line (Kratos Ultra, Axis). Survey scans and detailed spectra were recorded using EP ass = 80 eV and EP ass = 20 eV, respectively. Note: the same
sample preparation was followed as described for LEED measurements (section 2.5.3). The utilized XPS source and set-up is identified at the spectra in Chap. 6.
XPS data was analyzed using XPSpeak4.1 software.
X-Ray Absorption Spectroscopy
All XAS experiments were performed at the X-Treme beamline of the Swiss Light Source, Paul Scherrer Institute, Switzerland[133]. Linear horizontal (σh) or linear vertical (σv) polarized x-rays were generated by a synchrotron source. Spectra were recorded at 300 K in grazing incidence (θ = 60◦) using TEY mode. A negligible magnetic field of 50 mT was applied helping
the electrons to leave the surface resulting in a reduced noise on the measured drain cur- rent. Contrary to the notation explained in section 1.3.2,σhrefers to out-of-plane, whileσv represents in-plane excitation due to a different definition at this specific beamline.
Post EC samples investigated by XAS were prepared in Stuttgart and transferred using a vacuum suitcase to the Swiss Light Source. The transfer is described in section 2.1.1.
760
770
780
790
800
810
820
Photon Energy (eV)
Intensity (a.u.)
FeTPyP+Co (raw)
Fit
Co Background/Au(111)
Figure 2.11: Exemplary background subtraction of XAS raw data. Color code: raw data of FeTPyP+Co at Co L2,3-absorption edge forσh(black); manual fit (purple); measured background of Co L2,3-edge
on Au(111) forσh(grey).
The data was analyzed with Igor and OriginPro. The background differed for each sample, thus subtracting a baseline was rather complicated. The baseline was fitted manually in OriginPro for each polarization individually. A representative example is shown in Fig. 2.11. Subsequently, the background correctedσhandσvwere summed (in the following referred as XAS). The XLD is calculated by subtractingσhfromσv. Data is normalized to the integral of the corresponding XAS. However, the N K-edge was normalized to the integral of XAS of the corresponding Fe L2,3-edge. Furthermore, the Cu L2,3-edge is normalized on the integral of the CuTPyP for a better comparison of the changed intensities due to a change in occupation from d9to d10.
3
Metal Exchange in Porphyrins at the
Vacuum/Solid Interface
Controlled synthesis and modification of organic nanostructures is key for designing new materials for, e.g., sensors[135], (electro-)catalysts[39,42]or optoelectronics[136]. Inspired by nature, porphyrins are promising molecules to generate highly reactive, stable and flexible technical devices. A prominent example of naturally occuring porphyrins is the iron porphyrin incorporated in the heme-group serving as co-factor for enzymes involved in transport and storage of respirator gases, as well as constituting the active site of cytochrome P450.[137,138] Another example is the Mg porphyrin active site of chlorophyll, which is involved in the photosynthesis process.[139]
Porphyrins are large organic molecules consisting of a rigid tetrapyrrole macrocycle with a large conjugatedπ-system leading to high chemical and mechanical stability as well as a planar geometry. The macrocycle acts as a tetradentate ligand explaining the strong metal- organic coordination bond which is responsible for the extraordinary stability of the complex. Moreover, the tetrapyrrole ring provides a coordination cage for many metal center of the periodic table enabling a great variety of complexes. The planar geometry provides an easy access of reactants to the active metal cation supporting the reactivity of the MP. Further- more, the planarity makes it attractive to the field of surface science due to the utilization as organic structural element for a bottom-up approach of novel nanostructured materials and its investigation on the molecular scale by scanning probe techniques. In addition to the large number of metal centers applicable to the macrocycle, the organic backbone can be modified by suitable functional groups connected in meso-position at the methine bridges or at the pyrrolicβ site. Thus, the variations of this molecular class are seemingly unlimited offering a large toolbox for tailoring the molecular building block towards the needs of the specific applications and promoting research on porphyrins and related molecules in thin films, as monolayers, and at the single-molecule level.[45,46,140–142]
In order to exploit the full capacity of the MP’s variety, suitable synthetic pathways to in- corporate the active metal center are of great importance. Typical techniques utilized are
metalation, demetalation, and transmetalation, which have been studied extensively in liquid environment.[143–148]Metal incorporation (metalation) in a free-base porphyrin proceeds by substitution of the two pyrrolic hydrogens by the metal cation, generally present as metal salt in the reaction flask.[143,144]Demetalation is performed by adding diluted acetic acid or water following a hydrolysis reaction pathway. An alternative synthetic route is the so-called transmetalation process describing the exchange of two different metal cations.
In some cases transmetalation offers a more efficient synthesis pathway for a specific MP. For instance the formation of Zn-tetraphenylporphyrin (TPP) demonstrated a faster reaction rate starting from HgTPP than from the free-base porphyrin.[147]This observation also excludes the free-base prophyrin as intermediate for the transmetalation reaction mechanism in this context.[144]The possibility of a transmetalation reaction strongly depends on the properties of the metal cation such as electronegativity, charge, bond strength, solvation energy and size of the cation. Alongside qualitative studies, reaction rates and mechanisms of metalation and transmetalation have been investigated for a better understanding of the process.[145–150] However, the concepts in liquid environment are not easy transferable to surface science in UHV but serves as inspiration.
The flat geometry making the active center accessible for a variety of reactants also results in a strong influence of the surface on the central metal atom changing the catalytic properties of the MP, additionaly.[151]Hence, the interest in finding in situ synthesis and modification of MPs on surfaces is rising. There are two experimental pathways to generate self-assembled MP networks in UHV: 1) sublimation of the desired MP directly onto the substrate, 2) on-surface metalation of free-base porphyrin by up-taking metal atoms at the vacuum/solid interface. In the latter case, the metalation can occur through the incorporation of readily available metal atoms from the metallic support,[152]or alternatively through the allocation of a different metal by thermal evaporation.[123,153,154]The metalation can therefore be achieved with various metal species and is routinely used to synthesize MPs on surfaces on demand.[45,140]
While metalation of free-base porphyrin became a standard procedure in vacuum, an on- surface transmetalation process is rarely reported. Conflicting publications demonstrate the complexity of on-surface transmetalation by showing both the formation of a bimetal- lic MOCN[127,128]and a cationic exchange on the surface[155,156]. Two studies describe the replacement of Ni or Co by Cu, which is supplied through the adatom gas on the Cu(111) surface in ultra-high vacuum.[155,156]A third study describes the exchange at the solid/liquid interface, in which Zn is replaced by Cu.[157]The process underlying the metal exchange is far from being understood which is exemplified by two publications: on the one hand Franke et al.[157]report that Zn is substituted by Cu in a TPP network at the solid-liquid interface. On the other hand, Shi et al.[128]report the formation of a bimetallic coordination network for Zn-TPP and co-deposited Cu instead of an exchange. These two publications highlight that the underlying process of metal exchange is highly complex and critically depending on the exact nature of the environment, i.e. vacuum vs. solution, and the metal species involved as well as the supporting surface. A recent study on a Cu-pyrphyrin network on a