A jellium model analysis on quantum growth of metal nanowires and nanomesas

Abstract

A simple jellium model is used to investigate the stability of a metal nanowire as a function of its size. The theoretical results from the model indicate the quantum selectivity of preferable radii of nanowires, in apparent agreement with the experimental observations. It is consequently suggested that a series of stable "magic numbers" and "instability gaps" observed in the synthesis experiments of Au nanowires is mainly attributed to the quantum-mechanical behavior. These stable radii can be achieved by rearranging atoms during the formation of nanowires. The model is also used to analyze the growth of Au nanomesas on a graphite surface, and the puzzling growth behavior of Au nanomesas can be reasonably explained.Keywords jellium model, free electrons, quantum size effects, metal nanowires, metal nanomesas PACS numbers 81.10. Aj, 36.40.Qv ½ ÁÒØÖÓ Ù Ø ÓÒA metal nanostructure often exhibits a size-dependent feature arising from quantum confinement when its structural dimension is comparable to the electron Fermi wavelength, a phenomenon referred to as the quantum size effect (QSE). The QSE can have two distinct sizedependent manifestations: a property anomaly at certain selected size, i.e., the so-called "magic" size, or an oscillatory size-dependent property. For example, metal nanoclusters are anomalously stable when they contain a magic number of atoms [1−5]; a chain of transition metal atoms shows an oscillatory magnetic moment with increasing chain length [6]; and, metal nanofilms display a periodic oscillation of stability with increasing film thickness [7,8]. The size-dependency of physical properties of metal nanostructures has attracted much interest because it is of importance not only in fundamental physics but also in many targeted potential applications, e.g., electronic device technology, nanocatalysts, sensors, biological interfaces, and functional materials. However, the measurement and interpretation of properties relevant to metal nanostructures manifests a considerable theoretical and experimental challenge [2−5] because, as mentioned above, the big difference in properties often exists between a metal nanostructure and its bulk counterpart.Nanowires, belonging to one type of nanostructure, can be produced by employing various experimental techniques that have burgeoned in the past decade. Such experiments on alkali metals [9−11], noble metals [12−14], Al [15][16][17] and Bi [18] have revealed an oscillatory behavior in nanowire electrical conductivity as a function of its diameter. Suspended Au nanowires formed in an ultra-high vacuum have been reported as exhibiting a series of stable "magic numbers" and "instability gaps" [19]. Although some theoretical analyses on metal nanowires have been made to connect this magic sequence to the electron shell structure, the physical origin is still not very clear [16, 20−25].Nanomesas, belonging to another type of nanostructure, can grow on a semiconductor or insulator surface in vacuum evaporation-deposition experiments, and often adopt a flat-to...

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