The Step Doubling Effect

The Step Doubling Effect of Oxygen on Terraced Metal Surfaces

Stepped single crystal surfaces are very informative model systems for probing the properties of imperfect or "real" surfaces. By cutting a single crystal in certain crystallographic directions, one can create a surface with controlled numbers and types of defects. One example is the Ni(977) surface which consists of eight atom wide terraces separated by single atomic height steps. When small amounts of oxygen are adsorbed on a Ni(977) surface between the temperatures of 370-470 K, the surface "doubles", i.e. the steps become two atoms high and the terraces fifteen atoms wide. Upon addition of more oxygen, removal of all oxygen, or an increase in surface temperature above 470 K, the surface reverts to the single-stepped structure. Figure 1 schematically illustrates this process [1].

The Sibener group has examined the kinetics of this reconstruction using helium atom diffraction with the apparatus shown in Figure 2.

These measurements monitor the angular dependence of the elastically scattered helium atom flux. The position of the diffraction peak depends on the step height, so that a double-stepped surface has a different diffraction spectrum than a single stepped surface. Figure 3 shows the diffraction peak due to the formation of the double-stepped surface as a function of time after oxygen exposure. This reconstruction has also been modeled by the Einstein group at the University of Maryland [2].

The kinetics of step-doubling and -singling give us information on the thermodynamic properties of the surface. The double stepped surface forms because it is stabilized by oxygen adsorbed at the step edge. Removing the oxygen, through desorption or dissolution into the bulk, causes the surface to revert to the single-stepped structure. This, along with entropic considerations, accounts for the high temperature limit to the doubling process. At temperatures below 370 K, the limited mobility of Ni atoms on the surface prevents the movement necessary for two single steps to form a double step. Currently an STM lab is being constructed to directly image the doubling process. This will allow us to further examine the mechanism responsible for step doubling in real time.

Figure 3

This study complements our previous work on step-localized and surface phonons on Ni(977) [3], [4]. Information on force constants extracted from phonon measurements is related to the relative stabilities of the single- and double-stepped surfaces. These experiments deepen our understanding of structural changes of the Ni(977) surface, in this case during the earliest stages of oxidation.

Reference:

  1. "Reconstruction kinetics of a stepped metallic surface: step doubling and singling of Ni(977) induced by low oxygen coverages"
    L. Niu, D. D. Koleske, D. J. Gaspar, S. F. King, and S. J. Sibener, Surf. Sci. 356 144 (1996)
  2. "Dynamics of Step Doubling: Simulations for a Simple Model and Comparison with Experiment"
    S. V. Khare, T. L. Einstein, and N. C. Bartelt, Surf. Sci. 339, 353-362(1995).
  3. "Phonons Localized at Step Edges: A Route to Understanding Forces at Extended Surface Defects"
    L. Niu, D. J. Gaspar, and S. J. Sibener, Science 268, 847 (1995).
  4. "Vibrational Dynamics of a Stepped Metallic Surface: Step-Edge Phonons and Terrace Softening on Ni(977)"
    L. Niu, D.D. Koleske, D.J. Gaspar, and S.J. Sibener, J. Chem. Phys. 102, 9077 (1995).