Recently, self-assembled nanostructures have attracted tremendous interest. Among them, the ones with regular spatial array, uniform and well-defined geometric and structural characters, are especially desirable. We can get precise information on the detailed material properties of such nano-systems even using macroscopic detection methods which have been developed for bulk systems. However, it is still a challenge to prepare such high-quality nanostructures.
In the present paper, we describe the fabrication of self-assembled manganese nitride nano-islands on the Cu(001) surface and propose a new mechanism of the nano-self-assembling. [1]
Each island shows a square shape and has a well-defined size of 3 nm x 3 nm. They are regularly arranged with a periodicity of 3.5 nm and form a two-dimensional square superstructure. The depth of the trench between the islands is the same as the step height of the surface or the height of the island on the superstructure. The nano-islands adopt a NaCl-like structure which is oriented in the same way as the fcc Cu(001) substrate. They are reproducibly prepared in three steps in ultrahigh vacuum by Mn deposition, atomic nitrogen exposure and annealing. The prepared nano-islands are thermally stable at least upon being heated at 800 K.
The stoichiometry of the manganese nitride islands has been studied by in-situ X-ray photoemission spectroscopy. It is determined that the formula of the manganese nitride is MnN. This stoichiometry is the same as that of the bulk MnN crystal, which has a face-centered tetragonal structure (distorted NaCl structure) with a lattice constant lager than that of Cu(001) [2]. Thus, the nanostructure formation is attributed to strain-relief at the interface. However, different from the conventional stress-domain dominated self-assembly, the shape, size, and periodicity of the MnN islands do not change even when they coexist with the clean Cu(001) surface. This indicates that the self-assembling is driven by a short-range mechanism.
Reference:
[1] X. Liu, B. Lu, T. Iimori, K. Nakatsuji and F. Komori, Phys. Rev. Lett. 98, 066103 (2007).
[2] K. Suzuki, T. Keneko, H. Yoshida, Y. Obi, H. Fujimori, and H. Morita, J. Alloys Comp. 306, 66 (2000).
|