Many aspects of environmental problems, from soil contamination and transport through the groundwater to catalytic remediation schemes, involve the adsorption of molecules on a surface. The scanning tunneling microscope (STM) is capable of imaging solid surfaces and molecular adsorbates with atomic-scale resolution. As a real space probe, the STM can directly determine the role of surface structure, especially at defects such as steps, vacancies, etc., in the adsorption and heterogeneous chemistry of molecules on surfaces.
Unfortunately, many surfaces of relevance to environmental chemistry are unsuitable for detailed studies due to the insulating nature of many oxides, the lack of large, single-phase crystals, and/or difficulties associated with surface preparation. These problems can be obviated by using ultrathin films grown epitaxially on well-characterized, single crystal, conducting substrates.
Figure 1. A 1000Å x 1000Å image of MgO on Mo(001) grown at 970K. Single atomic steps are evident in the Mo substrate. The edges of the square MgO islands are oriented along the MgO(100) direction. Tunneling conditions: 3.3V, 0.2nA. (M.C. Gallagher, M.S. Fyfield, J.P. Cowin, and S.A. Joyce, Surf. Sci. 339, L909, (1995).)
The growth of microscopically thin films on conducting substrates overcomes the electrical charging problems associated with bulk insulators. To this end we have used in-situ STM to investigate the growth and electronic properties of MgO thin films deposited on Mo(001). We have chosen the growth of magnesium oxide on molybdenum to study for a number of reasons: MgO, with a bandgap of 8 eV, is a prototypical ionic solid insulator, the epitaxial growth on Mo(100) has been demonstrated1, and several groups at PNL, both experimental and theoretical, have extensively studied the structure, growth, and chemistry of MgO. The films were grown by evaporating Mg metal in a background pressure of oxygen. We have successfully imaged films as thick as 25 Å, clearly demonstrating the feasibility of this method. Films were grown at substrate temperatures between 300 and 1050 K. Low temperature growth produced smooth, uniform films with small MgO islands of between 20 and 100 Å.
Annealing these films at 1100K results in domain coalescence and a higher degree of crystalline orientation of the domain edges. Annealing, therefore, significantly reduces the number of edge defects in the films. Images of films grown at high temperature reveal nonwetting behavior with large three-dimensional islands indicating a Volmer-Weber growth mode. Figure 1 displays a region of MgO film grown on Mo(100) at 970K. The rectangular shaped islands are crystallites of MgO(100) in registry with the underlying and partially exposed Mo substrate. The total vertical relief is ~ 9Å, indicating that the films are relatively smooth over macroscopic distances. In contrast to films deposited in a background pressure of oxygen, growth of Mg metal on Mo(001) was pseudomorphic at low coverage. At higher coverage a transition to three-dimensional Mg hexagonal islands was observed, which is consistent with a Stranski-Krastanov growth mode.
Figure 2. A 250Å x 250Å image of TiOx on W(110). The TiOx coverage is 1 monolayer and the annealing temperature was 1350K. There are three steps running from the lower left to upper right. The direction along the step is [001]. Tunneling conditions: 1.6V, 0.15nA. (G.S. Herman, M.C. Gallagher, S.A. Joyce, and C.H.F. Peden, J. Vac. Sci. Technol. B14, 1126 (1996).)
Macroscopic single crystals of the rocksalt oxide,TiOx~1, are not available. Long range, periodic structures of ultrathin film TiOx, however, can be grown on the surface of W(110) substrates.2 Films are produced by depositing Ti metal on a room temperature substrate, dosing with molecular oxygen and then annealing. STM and low energy electron diffraction studies reveal a rich structural "phase diagram" for this system which is sensitive to the amount of Ti deposited and to the final annealing temperature. Most of the observed diffraction patterns are quite complex and can only be reasonably interpreted as multiple scattering from both the TiOx overlayer and the W(110) substrate. An image of a one monolayer film of TiOx annealed to 1350K is shown in figure 2. The complex, mesoscopic features in the image can be understood in terms of the atomic-scale coincidences which occur between the TiOx and the underlying substrate.
This work was performed at the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
(1) M. Wu, J. S. Corneille,
J. W. He, C. A. Estrada, D. W. Goodman, J. Vac. Sci. Technol. A 10, 1467
(1992).
(2) G. S. Herman,
C. H. F. Peden, J. Vac. Sci. Techno. A 12, 2087 (1994).
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