Matrix assisted pulsed laser evaporation of Mn12(Propionate) thin films
Highlights
► Deposition of Mn12(Propionate) films using MAPLE. ► Identification of growth conditions for continuous and smooth films. ► Unusual steps in magnetic hysteresis loop of deposited films.
Introduction
Single molecule magnets (SMMs) have recently attracted a lot of interest because of their unusual magnetic properties [1], [2] and potential applications in quantum computing, molecular spintronic devices and ultrahigh density magnetic storage. SMMs possess a large ground state spin value (S), a large and negative Ising (or easy axis) type of magnetoanisotropy as measured by the axial zero field splitting parameter (D), and negligible intermolecular magnetic interactions. Such species display the classical properties of a macroscopic magnet below their blocking temperature, i.e., hysteresis in magnetization versus dc field scans [2]. After the discovery of this unusual magnetic behavior in the archetypal SMM Mn12(Acetate): [Mn12O12(O2CCH3)16(H2O)4] [3], [4], many derivative compounds have been reported, including Mn12(Benzoate): [Mn12O12(O2CPh)16(H2O)4] [5] and Mn12(Propionate): [Mn12O12(O2CC2H5)16(H2O)n] [6], [7] complexes.
Several of the potential applications for which SMMs could be exploited require these materials to be prepared in thin film form. Mn12-based SMMs are chemically fragile with low decomposition temperature, for example, Mn12(Acetate) decomposes at ∼65 °C [8]. This makes it difficult to process them using traditional physical vapor deposition techniques like e-beam evaporation, thermal evaporation, sputtering and pulsed laser deposition (PLD). Unlike PLD, evaporation rather than ablation of the target takes place during the matrix assisted pulsed laser evaporation (MAPLE) process. MAPLE has been successfully used for the deposition of a variety of fragile molecules such as biomaterials [9], [10], [11] and polymers [12]. This technique has recently also been used for film deposition of other SMMs such as Mn12(Benzoate) [13] and Mn12(Acetate) [8] films.
In this paper we report on the deposition conditions for growth of Mn12(Propionate) films on Si and glass substrates without fragmentation of the transferred molecules. Surface morphology, chemical composition and magnetic properties of MAPLE deposited Mn12(Propionate) films were investigated using Atomic Force Microscopy (AFM), Fourier Transform Infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Superconducting Quantum Interference Device (SQUID) magnetometry.
Section snippets
Experimental
Single crystals of Mn12(Propionate) were prepared as described elsewhere [6], [7]. The crystals were carefully pulverized using an agate mortar and pestle to produce fine powders for the preparation of MAPLE targets. Both chloroform and toluene were tested as potential solvents; toluene proved to be the best choice in terms of creating a solution and not a suspension. Solutions of 0.25 mg Mn12(Propionate) in 5 ml toluene were prepared. This concentration is below the solubility limit. Part of the
Results and discussion
The surface morphology of the SMM31 film, is presented in the AFM images of Fig. 1. The root-mean-square (RMS) roughness for an area of 10 μm × 10 μm is 9.8 and 8.15 nm for the films fabricated on glass (see Fig. 1a) and Si (see Fig. 1b) substrates, respectively. Hence, the RMS roughness of the as-deposited Mn12(Propionate) films does not depend significantly on substrate type.
Fig. 2 shows the FTIR spectra for the Mn12(Propionate) powder and the SMM33 film. The top spectrum represents the
Conclusions
The successful deposition of SMM Mn12(Propionate) films on Si and glass substrates via MAPLE was reported. The optimization of the deposition conditions was necessary to avoid fragmentation of the molecule during the deposition process. The best films were produced using toluene as solvent, fluence of 200 mJ cm−2 and Tsub of 30 °C. The FTIR and XPS spectroscopy revealed that a number of Mn12(Propionate) molecules remain intact during MAPLE deposition process, suggesting that this technique could
Acknowledgement
This work was supported by the Cyprus Research Promotion Foundation, Contact No.: Bilateral/CY-RO/0609/11.
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