We present a detailed study on the electronic properties of monolayers of Mn 12 derivatives chemically grafted on clean as well as on functionalized Au͑111͒ surfaces. Scanning tunneling microscopy and x-ray photoelectron spectroscopy were employed to ensure the successful monolayer deposition. Unoccupied and occupied valence band states in the electronic structure of Mn 12 -clusters were probed by means of x-ray absorption spectroscopy ͑XAS͒ and resonant photoelectron spectroscopy ͑RPES͒ at the Mn 2p-3d absorption edge, respectively. XAS measurements reveal a significant difference between the Mn oxidation states of Mn 12 cores bound to the Au surface compared with the single crystal environment.

The authors report on the electronic properties of individual molecules of two Mn 12 derivatives chemically grafted on the functionalized Au͑111͒ surface studied by means of ultrahigh vacuum scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from both Mn 12 molecules, showing a well defined wide band gap. In agreement with the tunneling spectroscopy results, the bias voltage variation upon scanning leads to apparent height changes of the Mn 12 clusters. The authors discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn 12 molecules.

Abstract:A new approach to the deposition of Mn12 single-molecule magnet monolayers on the functionalized Au(111) surface optimized for the investigation by means of scanning tunneling spectroscopy was developed. To demonstrate this method, the new Mn 12 complex [Mn12O12(O2CC6H4F)16(EtOH)4]‚ 4.4CHCl3 was synthesized and characterized. In MALDI-TOF mass spectra the isotopic distribution of the molecular ion peak of the latter complex was revealed. The complex was grafted to Au(111) surfaces via two different short conducting linker molecules. The Mn 12 molecules deposited on the functionalized surface were characterized by means of scanning tunneling microscopy showing homogeneous monolayers of highest quality. Scanning tunneling spectroscopy measurements over a wider energy range compared with previous results could be performed because of the optimized Au(111) surface functionalization. Furthermore, the results substantiate the general suitability of short acidic linker molecules for the preparation of Mn 12 monolayers via ligand exchange and represent a crucial step toward addressing the magnetic properties of individual Mn12 single-molecule magnets.

Individual Mn 12 single-molecule magnets have been investigated by means of scanning tunneling spectroscopy at room temperature. Current-voltage characteristics of a Mn 12 derivative are studied in detail and compared with simulations. A few-parameter scalar model for ballistic current flow through a single energy level is sufficient to describe the main features observed in scanning tunneling spectra of individual Mn 12 molecules and offers a deeper insight into the electronic transport properties of this class of single-molecule magnets. In addition, distance-voltage spectroscopy performed on individual Mn 12 molecules reveals a possibility to identify the orientation of the molecular easy axis. The results indicate a preferential orientation of the easy axis of the molecules nearly perpendicular to the surface.

Different approaches to the deposition of Mn 12 single molecule magnets on the Au(111) surface and their characterization by a broad variety of techniques are investigated with respect to their suitability for a profound corroboration of the integrity of the Mn 12 core. In this context, the most recent improvements in the experimental approaches are presented and the latest results on the electronic properties of Mn 12 are linked to each other. The results confirm the high instability of Mn 12 single molecule magnets on surfaces and reveal the need for an amendment of the requirements to define the structural integrity of Mn 12 molecules on surfaces.