Oxide based materials are often used for relevant technological applications in fields as catalysis, corrosion protection, micro- and nano-electronics, sensoristics, spintronics, drug delivery, etc. [1]. This justifies the great effort for a complete characterisation of such materials [1,2]. Ultrathin oxide films are of utmost importance, since they may show peculiar electronic and chemical properties different from those of the corresponding bulk materials and which are well known to depend on film structure and defectivity [1,3,4]. The ability to engineer nearly perfect ultrathin oxide layers, up to the limit of monolayer thickness, is therefore a key issue for nano-technological applications [1]. MgO, in particular, has become the focus of intensive research for its role as an interface material [5], with applications e.g. as high k-dielectric in electronic devices and in magnetic tunnel junctions [6]. Moreover, the simple cubic structure and the small distortion of the MgO layers grown on Mo(100) or Ag(100) make it an ideal model system for the study of electronic and catalytic properties of oxide films and of oxide-supported metal nanoclusters [7].
In-spite of intensive research, however, some very important issues are still unclear. Indeed, no uniform, high quality, ultrathin MgO layers suitable to be exploited as substrates for the deposition of further materials (nanoclusters, admolecules, etc...) were so far achieved and contrasting information is present in literature about the optimal deposition conditions, the geometry, the height and even the orientation of MgO islands grown on Ag(100) [8-12].
In our work we characterize submonolayer MgO films grown by reactive deposition on Ag(100) using different preparation protocols and considering, for the first time, post-deposition treatments. The morphology of the films is analyzed by Scanning Tunneling Microscopy (STM), while their chemical composition is determined by XPS. The analysis of vibrational modes also provides information on the film structure. In addition to the already established influence of the growth temperature (Tg) on growth mode, we find an unexpected dependence of the film morphology on the cooling process after deposition, a parameter which had been disregarded so far. The poor reproducibility of the existing data is thus due to the strong dependence of sample cooling on the design of the different experimental setups. With suitable after-growth treatments, i.e. by controlling so far neglected thermodynamics constrains, we succeeded in tuning the shape of the oxide films from irregular, nm-sized, monolayer thick islands, to slightly larger, perfectly squared bilayer islands, to extended monolayers limited apparently only by substrate steps see Fig. 1). Our method can be valid not only for the production of oxide thin films but also for many other layered systems. Since the film structure influences both chemical and electronic properties of the layers, a full control of all experimental parameters opens important perspectives for applications in catalysis and for the use of ultrathin oxide films as support for the further deposition of organic and inorganic nano-objects.
Financial help from Compagnia S. Paolo and from ICTP (through a post-doctoral grant) is acknowledged.
Jagriti Pal1,2, Marco Smerieri1, Edvige Celasco1,2, Letizia Savio1*, Luca Vattuone1,2, Mario Rocca1,2
1 IMEM-CNR, U.O.S. Genova, Via Dodecaneso 33, 16146 Genova, IT
2 Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, IT
*corresponce to : letizia.savio@cnr.it, savio@fisica.unige.it
For more information see:
Phys. Rev. Lett. 112, 126102 (2014), DOI: 10.1103/PhysRevLett.112.126102
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Fig.1. Columns (a-c): STM Images of MgO films of 0.7 ML nominal thickness grown under different conditions. (a) Tg=450 K, Fast Cooling (FC); (b) Tg=773 K, FC; (c) Tg=773 K, Slow Cooling (SC). For all panels, image size 21x21 nm2; I=0.2 nA. Panel (d): Atomically resolved image of clean Ag(100), used for calibration. High symmetry directions are marked by arrows. Image size: (2.4x2.4) nm2 , V=0.1, I=0.2 nA. Panel (e): height profiles of the different MgO structures cut along the lines marked in the bottom panels (a-c) (<001> direction, topographic conditions).
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