Preparation of monolayers of [MnIII6CrIII]3+ single-molecule magnets on HOPG, mica and silicon surfaces and characterization by means of non-contact AFM
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We report on the characterization of various salts of [Mn III 6 Cr III ] 3+ complexes prepared on substrates such as highly oriented pyrolytic graphite (HOPG), mica, SiO2, and Si3N4. [Mn III 6 Cr III ] 3+ is a single-molecule magnet, i.e., a superparamagnetic molecule, with a blocking temperature around 2 K. The three positive charges of [Mn III 6 Cr III ] 3+ were electrically neutralized by use of various anions such as tetraphenylborate (BPh4-), lactate (C3H5O3-), or perchlorate (ClO4-). The molecule was prepared on the substrates out of solution using the droplet technique. The main subject of investigation was how the anions and substrates influence the emerging surface topology during and after the preparation. Regarding HOPG and SiO2, flat island-like and hemispheric-shaped structures were created. We observed a strong correlation between the electronic properties of the substrate and the analyzed structures, especially in the case of mica where we observed a gradient in the analyzed structures across the surface.
KeywordsHighly Orient Pyrolytic Graphite Step Edge BPh4 Tetraphenylborate Highly Orient Pyrolytic Graphite Surface
The strongest interaction is the antiferromagnetic coupling of the central CrIII ion with the six terminal MnIII ions which results in a spin ground state of the molecule of St = 21/2. This high-spin ground state in combination with a strong easy-axis magnetic anisotropy and a C3 symmetry results in an energy barrier for spin-reversal, which leads to a slow relaxation of the magnetization at low temperatures (single-molecule magnetism behavior, i.e., molecular superparamagnetism [10, 11]). [Mn III 6 Cr III ] 3+ has a blocking temperature around 2 K [6, 7]. Recent experimental spin resolved photoemission results of [Mn III 6 Cr III ] 3+ single-molecule magnet (SMM) , X-ray magnetic circular dichroism (XMCD) at a Fe-SMM-adsorbed molecule  and cross-comparison between spin-resolved photoemission and XMCD in Mn-based molecular adsorbates have been published elsewhere . The three positive charges of [Mn III 6 Cr III ] 3+ can be neutralized by various anionic counterions. Herein, the three salts [Mn III 6 Cr III ](BPh4)3, [Mn III 6 Cr III ](C3H5O3)3, and [Mn III 6 Cr III ](ClO4)3 were investigated using as three anions either tetraphenylborate (BPh4-), lactate (C3H5O3-), or perchlorate (ClO4-), respectively. Being able to choose between three different anions for the same core compound allowed us to study the influence of the anions with respect to the whole molecule-substrate-system.
Investigation in this regime is best done via non-contact atomic force microscope (AFM) [14, 15]. Due to [Mn III 6 Cr III ] 3+ simply physisorbing onto the surface, the use of non-contact (nc)-AFM allows us to observe the molecule with a decreased risk of manipulating the molecule during this process. Of special interest are the thin layers of [Mn III 6 Cr III ] 3+ and whether these layers are crystalline or amorphous [16, 17, 18, 19].
Preparation was carried out in air at room temperature (21 ± 1°C) and air moisture between 40% and 60% via the droplet technique using an amount of 10 μl and a concentration of 10-5 mol/l of the solution. As the solvent, we used dichloromethane for [Mn III 6 Cr III ](BPh4)3 and methanol for [Mn III 6 Cr III ](C3H5O3)3 and [Mn III 6 Cr III ](ClO4)3. Either the selected concentration and amount of solution, or the number of molecules, was sufficient for the creation of approximately one monolayer. During preparation the sample was held at an angle of 57° which led to a more homogeneous wetting. Substrates (10 × 10 mm2) were affixed onto Omicron carriers (Omicron NanoTechnology GmbH, Taunusstein, Germany).
The surface topography of the samples was analyzed by means of non-contact atomic force microscopy in ultra-high vacuum (UHV) (Omicron UHV-AFM/STM). The pressure of the vacuum chamber was approximately 10-7 Pa and the measurements were taken at room temperature.
We used silicon non-contact cantilevers (NSC15, MikroMasch, San Jose, CA, USA) with a resonance frequency of approximately 325 kHz. The microscope was operated at a frequency shift between 20 and 80 Hz below the vacuum resonance frequency.
Image fields up to 720 × 720 nm2 were recorded with a scan speed of approximately 350 nm/s and 300 lines per image. Standard image processing was performed using a polynomial background correction by means of Gwyddion (version 2.19) and SPIP (version 5.0.6), in order to flatten the image plane.
The X-ray photoelectron spectroscopy measurements were recorded using a PHI 5600ci multitechnique spectrometer (Physical Electronics, Chanhassen, MN, USA) with a monochromatic Al Kα (hν = 1,486.6 eV) radiation of 0.3 eV FWHM bandwidth. The sample was kept at room temperature. The resolution of the analyzer depended on the pass energy. During these measurements, the pass energy was 187.85 eV, leading to a resolution 0.44 eV. All spectra were obtained using a 400 μm diameter analysis area. During the measurements, the pressure in the main chamber was kept within the range of 10-7 Pa.
The samples were oriented at a surface-normal angle of 45° to the X-ray source and -45° to the analyzer for all core-level X-ray photoelectron spectroscopy (XPS) measurements.
[Mn III 6 Cr III ](BPh4)3 prepared on highly oriented pyrolytic graphite (HOPG) leads to flat island-like structures with height of about 2 nm. These structures appear in sizes from 10 nm diameter up to several hundred nanometers and even ones covering nearly the whole scanned area. Two main structures can be distinguished:
Free islands which do not have any lateral contact. These show most often the tendency to appear in a circular manner.
Islands attached to a step edge. Again these tend to form a circle-like structure but are hindered by the edge. The islands do not continue their extension on the other side of the edge but seem to be cut off. No tendency can be seen as to whether these cut islands appear more often on the upper or lower side of the step edges.
Agglomeration along the step edges with no preference relating to upper or lower step edges.
In Figure 4 (left hand side), 9.8% of the area was covered by 316 [Mn III 6 Cr III ](BPh4)3 particles. The average size of the particles was 11.9 nm at 161 nm2.
In Figure 4 (center), we moved along the gradient where the number of particles dropped down to 68, covering 8.4% of the surface. The mean particle size increased by a factor of 2 to 23.4 nm while the area covered rose to 640 nm2, and the particle height reached 1.1 nm.
In Figure 4 (right hand side), [Mn III 6 Cr III ](BPh4)3 can be seen to form larger structures. The number of particles did not change. The covered area rose up to 17.1% while the average particle size reached 30.3 nm at 1270 nm2. Again, the height of the particles reached 1.1 nm leading to the conclusion that the gradient influences the covered area only and not the thickness of the layers.
Silicon (SiO2, Si3N4)
We observed no difference in the investigated silicon-based materials such as SiO2, Si3N4. Furthermore, we used different oxide layers of SiO2 with thicknesses of 200 and 500 nm without any significant change.
Influence of the anions
Switching the anions to lactate on HOPG leads to a change in the emerging structures compared to the ones created with [Mn III 6 Cr III ](BPh4)3. No islands are visible but the whole surface appears to be coated. It was not possible to measure the height of this film due to there being no trenches or other marks which would have allowed such an analysis. Due to non-existent islands, it is likely there is neither order in the film nor any kind of monolayer.
XPS Data from [Mn III 6 Cr III ](BPh4)3 on HOPG
Measured value ± error
Normalized to Mn
Influence of the substrate
The adsorption of any [Mn III 6 Cr III ] 3+ salt is strongly influenced by the substrate on which it is prepared. Since [Mn III 6 Cr III ] 3+ is a cation, it is crucial to neutralize its electric charge. In solution, the neutralization occurs through the anions which may move freely.
In the presence of a surface, we suggest the [Mn III 6 Cr III ] 3+ trication could adsorb on the surface without the need of interaction with anions and bind to available adsorption sites on the substrate. An explanation for this speculation is the formation of mirror charges on the surface which assume the function of the anions.
Molecule-substrate interaction being stronger than molecule-molecule interaction.
Molecule-substrate interaction being equal to or weaker than molecule-molecule interaction.
On the one hand, HOPG shows metallic properties which may allow [Mn III 6 Cr III ] 3+ to build up mirror charges solely existing in the top graphene sheet causing a strong electrostatic interaction . This would lead to the observed behavior of [Mn III 6 Cr III ] 3+ trying to gain as much contact with the surface as possible. Nevertheless, this does not explain double-layers of [Mn III 6 Cr III ] 3+ . As the trications would experience a strong electrostatic repulsion without interstitial anions, the close proximity of the anions in these double-layers appears to be very likely.
The interaction between the bottom [Mn III 6 Cr III ] 3+ layer and the substrate may rely on the emerged mirror charges created by the positive charge of the SMM. This system is already stable at ambient conditions at room temperature. On HOPG we observe different heights for the first and second layer. This may be due to different van-der-Waals or mirror-charge interaction between two SMM layers in respect to the interaction between the substrate and the first SMM layer.
The first layer of the SMM is stabilized through the mirror charge. Thus a layer of anions can place itself on top of the [Mn III 6 Cr III ] 3+ layer. By creating a negative charge at the surface, a second layer of [Mn III 6 Cr III ] 3+ SMMs is attracted. If this is the case, it is unclear why this only takes place for a second layer of [Mn III 6 Cr III ] 3+ . The anions can stabilize the SMM by themselves, thus the mirror charge created in the HOPG may simply be needed just at the start of the process. In this case, a second layer of anions is needed on top (Figure 10a).
Anions mixed with SMMs
It is more likely that a stronger interaction between the SMM and the anions leads to the anions being embedded inside a [Mn III 6 Cr III ] 3+ layer. Also, this leads to lower levels of energy and higher levels of entropy inside the layer. However, we cannot distinguish whether the anions are needed in the bottom layer because of the mirror-charge effect. Nevertheless, we expect the anions to be in the top layer (Figure 10b).
Anions mixed with SMMs without anions in the first layer
Our results have shown a significant change in heights between the first and the following layers. This difference can be explained by a neutralization of charge of [Mn III 6 Cr III ] 3+ caused by the mirror-charge effect in the first layer but by anions in the other ones (Figure 10c).
Mica on the other hand is an insulator, but being cleaved, the K+ ions in the crystal are separated due to a weak binding to the close aluminosilicate  thus leading to surface potentials up to -130 V . This potential becomes neutralized in air within a few minutes  but there are still enough negatively charged sites to allow [Mn III 6 Cr III ] 3+ to adsorb at the surface. Further layers neutralize their charge the same way as with HOPG. Anions are in between the SMMs in one layer.
Using lactate or perchlorate as the anion, we have not yet been able to observe such a gradient. We expect the mobility of the anion to have an influence on the way [Mn III 6 Cr III ] 3+ orders itself on the surface.
Influence of the anions
The anions are crucial for the stability of the whole complex. As we have shown, changes in the anions may cause a drastic variation in the way [Mn III 6 Cr III ] 3+ is absorbed on top of the surface.
The biggest difference can be seen between tetraphenylborate/perchlorate and lactate. The former ones show a strong influence by the substrate. Depending on which substrate is used various kinds of structures can be observed: flat islands, multistackings, big clusters, and even the homogeneous coverage of large areas. The latter shows just one structure. This is the coverage of the whole sample with an inhomogeneous but continuous film.
FFT performed on any of the systems did not reveal a crystalline structure resulting in [Mn III 6 Cr III ] 3+ or its anions which is why we expect no epitactical growth.
XPS data gained on [Mn III 6 Cr III ](BPh4)3 confirmed the existence of a layer of the SMM on the HOPG surface.
The ratios between the elements, including four solvent molecules are close to the expected values for [Mn III 6 Cr III ](BPh4)3. The errors of the ratios given in Table 1 are mainly due to the uncertainty of background substraction.
We have demonstrated a strong influence of the electric properties of the used substrates on the ordering of [Mn III 6 Cr III ] 3+ on the surface. Substrates allowing [Mn III 6 Cr III ] 3+ to neutralize its charge cause more flat structures than the others on which [Mn III 6 Cr III ] 3+ tends to form high clusters. Furthermore, we have investigated different anions used with [Mn III 6 Cr III ] 3+ and observed a drastic change in occurrences on surfaces when lactate instead of tetraphenylborate or perchlorate is used.
This work is supported by the Deutsche Forschungsgemeinschaft within Research Unit 945.
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