Preliminary experiments on Mn12monolayers were carried out using a wet-chemical preparation routine, in which Mn12 is grafted on Au(111) by means of functional thiol groups in the carboxylate ligands. Since the Mn12core is oxidatively unstable in the presence of free thiols, ligand exchange in solution prior to the deposition cannot be performed in a direct manner1. The sample preparation is therefore realized in a two step process, following a preparation protocol that was originally introduced by Vosset al.in 2006 [222, 223]. First, the Au(111) surface is covered by a monolayer of thiol-carboxylate linkers, which are forming a strong bond to the gold surface via the sulfur atom of the thiol group. Subsequently, the Mn12molecules are attached to the
1Synthesis of Mn12 derivatives that include thiol groups is possible by using acetyl-protected thiols for the ligand exchange and deprotecting the thiol group afterwards [182].
Au(111) Au(111)
Au(111)
(a) (b) (c)
Figure 9.4 | Schematic illustration of the preparation process used for a wet-chemical grafting of Mn12. (a) Functionalization of Au(111) with a layer of 4-MOBCA linkers.(b)Immersion of the functionalized surface in a solution of Mn12 -pfb. (c) Binding of Mn12-pfb to Au(111) via ligand exchange with the 4-MOBCA linkers. Images by M. Fonin, S. Voss and M. Burgert.
prefunctionalized surface in a ligand exchange reaction, according to the following equation:
(Ausur−S−R3−COOH)n+ Mn12O12(R1COO)16(R2OH)x−−→
(Ausur−S−R3−COO)n−Mn12O12(R1COO)16−n(R2OH)x+ n·R1COOH.
Sample Preparation
Clean Au(111) surfaces were prepared as described in chapter 7.1. For the experi-ments presented here, the Au surface was prefunctionalized by immersing the gold single crystal in a 2 mM solution of 40-mercapto-octafluorobiphenyl-4-carboxylic acid (4-MOBCA) in ethanol for 5–10 min. After removing the crystal from solution, the surface was rinsed with ethanol and dried in nitrogen gas. Deposition of Mn12 was then performed by dipping the functionalized surface in a 3×10−5M solution of Mn12O12(C6H4FCOO)16(EtOH)4 (Mn12-pfb) in dichloromethane (DCM) for 6 min.
Finally, the surface was rinsed with DCM, dried in nitrogen atmosphere and intro-duced into the UHV chamber. The preparation process is schematically illustrated in Figure 9.4.
Results and Discussion
In order to validate the deposition process, STM measurements were performed both before and after immersing the prefunctionalized Au crystal in the Mn12-pfb solution. After deposition of Mn12, STM images reveal a homogeneous distribution of elliptically shaped objects that are separated by 2–3 nm (Figure 9.5c). In contrast,
25 nm 25 nm 25 nm
(a) Au(111) (b) 4-MOBCA / Au(111) (c) Mn -pfb / 4-MOBCA12 / Au(111)
Figure 9.5 |STM images obtained on(a)the clean Au(111) substrate before depo-sition,(b) the 4-MOBCA functionalization layer and(c) the Mn12-pfb monolayer.
Scanning parameters: (a)V = 0.5 V, I = 1 nA, T = 9 K, (b) V = 2 V, I = 20 pA, room temperature, (c)V = 3 V,I = 10 pA, T = 12 K.
no objects of comparable size and shape were found on the prefunctionalized surface prior to Mn12 deposition (Figure 9.5b), or after immersion of the Au single crystal in pure solvents. The clusters are therefore attributed to the individual Mn12-pfb molecules. No signs of fragmentation, such as significantly smaller objects or larger accumulations of material, are evident from the STM measurements.
Insight into the chemical composition of the Mn12-pfb monolayer is provided by X-ray photoelectron spectroscopy. Figure 9.6a shows an XPS overview spectrum for binding energies in the range of 0–1000 eV. The measurement demonstrates that all elements expected to be observed in the system are present on the sample surface.
Specifically, these are Au (from the substrate), C and O (from 4-MOBCA, Mn12-pfb and organic impurities), F (from 4-MOBCA and Mn12-pfb) and Mn (from Mn12 -pfb). The S atom of the 4-MOBCA linker is not resolved in the overview spectrum, due to the low abundance and the small photoelectron cross section [192]. Peaks labeled as s.p. arise from Mo and Ta contributions that are present in the sample plate.
The observation of strong Au peaks in the XPS overview indicates that the substrate is covered only by a relatively small amount of material. An inhomogeneous coverage of the sample, i.e. the occurrence of native Au(111) areas, can be excluded based on the STM measurements. Therefore, the XPS data support the assumption that only a single layer of Mn12 is formed during the preparation process.
High resolution measurements of the C 1s, F 1s and Mn 2p core level spectra are shown in Figure 9.6b–d. The C 1s spectrum comprises a complex structure, which is caused by a superposition of peaks from C atoms with different chemi-cal environments. The individual contributions are ascribed to the phenyl rings and aliphatic carbon atoms (284.7 eV [119]), C−S species (286.0 eV [191]), C−OH species (286.4 eV [224]), carboxylate groups (288.6 eV [191]) and fluorinated carbon atoms (288.7 eV [119]). Adventitious carbon contaminations from the atmosphere are
ex-Overview C1s
F1s Mn2p, Au4p
VB Au4f
s.p.
C1s Au4d O1s
Au4p3/2 Au4p1/2
Mn2p F1s OKVV1
FKLL
Mn2p1/2
Au4p1/2 Mn2p3/2
(a) (b)
(c) (d)
Figure 9.6 | XPS spectra (Mg Kα) obtained on Mn12-pfb/4-MOBCA/Au(111).
(a)Overview spectrum. (b–d)High resolution spectra of the C 1s, F 1s and Mn 2p core level contributions.
pected to contribute to the 284.7 eV peak. In contrast, the F 1s spectrum comprises a single peak structure, confirming the identical chemical environment of all F atoms in the 4-MOBCA linkers and Mn12-pfb ligands. The spectral shape of the Mn 2p level is not discussed here, since a quantitative analysis is hampered by an overlap with the Au 4p1/2 peak.
In conclusion, both the STM and XPS measurements are in good agreement with previously published results [188, 223], demonstrating that the preparation routine of Voss et al. was successfully reproduced. It was originally planned to extend the available STM, XPS and XAS investigations by low temperature and magnetic field dependent STS measurements on individual Mn12-pfb molecules. However, this goal turned out to be extremely challenging for several reasons. Since the deposition is performed ex situ, impurities on the sample surface result in frequent changes of the tip state during the measurement. Treating the tip after such contamination is difficult, since common procedures, such as application of voltage pulses or moderate ramping of the tip into the substrate, are hampered by the absence of clean metal spots. Furthermore, a characterization of the tip state via reference measurements on the metallic substrate is prevented the full monolayer coverage as well.
Because of the mentioned disadvantages, it was decided to change the preparation routine from wet-chemical grafting to electrospray deposition (ESD). The results obtained by means of ESD are described in the following section.
9.4 Study of Mn12-ac Submonolayers and Individual Molecules