Self-Assembly of Nanometer-Scale Magnetic Dots with Narrow Size Distributions on an Insulating Substrate

Abstract

The self-assembly of iron dots on the insulating surface of NaCl (001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer. Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.PACS numbers: 81.07. Ta,61.46.+w,68.37.Ps,68.35.Md Clustering on surfaces by nucleation and growth during atom deposition has been an important subject in basic and applied science for decades [1,2]. Recent efforts have been focused on searching for methods to obtain nanometer-scale clusters with narrow size distributions. Such clusters or quantum dots are potentially valuable for optical, electronic, and magnetic device applications, but mass production of such structures by lithography or etching-based fabrication has proved to be exceptionally challenging [3,4]. Alternatively, it has been realized that the strain energy associated with the lattice mismatch between the dot and the substrate materials can be exploited to induce self-assembled formation of quantum dots with narrow size distributions. This has generated much excitement, particularly in the area of semiconductor quantum dots [3,4,5,6,7,8]. In such cases, the growth of the dots often proceeds in the StranskiKrastanow (SK) mode, which is characterized by the presence of a wetting layer prior to three-dimensional (3D) clustering. To date, the precise mechanism for size selection in semiconductor quantum dot systems remains a subject of active debate [4,5,6,9]: some attribute them to strain-induced thermodynamic equilibrium states, while others associate them with metastable configurations due to kinetic limitations.Although improved size uniformity can be achieved in quantum dot formation via the SK growth mode, the presence of a wetting layer is often undesirable, particularly for electronic and magnetic device applications of metallic/magnetic quantum dots. For this reason, it is preferred to fabricate quantum dots in the VolmerWeber (VW) growth mode, which is characterized by immediate 3D clustering on the substrate surface. Indeed, considerable recent efforts have been devoted to metallic/magnetic quantum dot formation on various substrates [10,11,12], but no significant size uniformity has been achieved in such studies.In this Letter we investigate the self-assembly of iron dots on NaCl(001), an insulating substrate, by thermal deposition and variable-temperature atomic force microscopy in ultrahigh vacuum. We show that, by properly choosing the growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting l...

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