Elsevier

Applied Surface Science

Volume 256, Issue 4, 30 November 2009, Pages 1128-1131
Applied Surface Science

Transformation of cluster structure at initial stage

https://doi.org/10.1016/j.apsusc.2009.05.064Get rights and content

Abstract

The initial stages of cluster nucleation for Pd or Ir adatoms on a W (1 1 0) surface and in the vicinity of surface steps are directly imaged by a field ion microscope (FIM). Three types of structures are observed. One is a one-dimensional linear chain, which is parallel to the nearest neighbor-stacking directions of the substrate. Another is a two-dimensional compact island, which is a pseudomorphic structure like the substrate. The other is a three-dimensional cluster, which shows a structural transition from bcc (1 1 0) to fcc (1 1 1). Factors affecting the structural transformation include coverage of atoms or atom chains, temperature of heat treatments and boundary of the substrate terrace.

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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