Transformation of cluster structure at initial stage
Introduction
In the nano era, many efforts have been made to investigate the self-assembly of nanometer-scale clusters [1], [2], [3]. In order to control the growth structure, it is necessary to understand growth properties. A basic understanding of metal films on metal substrates is of interest not only in fundamental research, but also in technologically bimetallic applicable areas [4]. Transition metals of the platinum family adsorbed on the W (1 1 0) plane show interesting properties. For example, the adsorbed atoms prefer to form linear chains at the initial growth stage. The growth of these material systems has been investigated with various surface analysis techniques and has attracted continuing study for many years [5], [6], [7], [8], [9], [10], [11]. However, few studies have investigated the structural transformation between different dimensional clusters at initial growth.
Studying cluster surface structures using field ion microscopy (FIM) is an ideal way. First, it is possible to observe individual atoms and monitor their diffusion directly. Second, the symmetry of the problem can be reduced to two dimensions and the entropy of the system is negligibly small for the small ensemble of atoms [12], [13], [14]. In addition, the size limitation of the FIM samples causes nucleation in the vicinity of surface steps and the growth effect of the terrace boundary to be more obvious.
In the present paper, we report some FIM observations of structural transformation of Pd and Ir clusters on the W (1 1 0) surface. The transformation of one-dimensional (1D) linear chain to two-dimensional (2D) compact island with three-dimensional (3D) growth affected by surface steps, and a commensurate bcc (1 1 0) to incommensurate fcc (1 1 1) structural phase transition in the Pd overlayer are shown. The important factors triggering structural transformation are discussed.
Section snippets
Experiment
All investigations were made with a home-built atomic resolution ultra-high vacuum-field ion microscopy (UHV-FIM), which has already been described in detail elsewhere [15], [16], [17]. A tungsten tip was prepared from a poly-crystal wire of 0.1 mm in diameter by electrochemical etching in aqueous KOH (concentration 2 M) and was cleaned in a UHV environment by a combination of thermal degassing, neon cathode sputtering, and field evaporation. Pd or Ir atoms were deposited from well-outgassed
Cluster structures at initial stage
At initial growth stage, a small cluster with more than three atoms can form a 1D linear chain, and various types of 2D or 3D structures. For palladium, previous observations reported that clusters containing as many as eight atoms formed chains [18], chains of nine Pd atoms were metastable [8], and structural rearrangements from 1D linear to 2D compact shapes were reversible for Pd8 [19]. Our results are consistent with that of previous research. Fig. 1 shows the FIM images of Pdx on W (1 1 0)
Conclusion
The atomic resolved field ion images of the chain and 2D island structures show conclusively that Pd and Ir overlayers on W (1 1 0) have pseudomorphic bcc (1 1 0) structures. Both Pd and Ir chains are always oriented in a close-packed <1 1 1> direction on W (1 1 0) surfaces. The structure transforms from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) at higher adsorbate layers. When the growth structure meets the terrace boundary, the chain cluster may turn the growth direction and the 2D
Acknowledgement
The research is supported by National Science Council of ROC under grant no. NSC 95-2112-M-003-022-MY3.
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