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Thin films, surfaces and nanomaterials

Let γs be the surface energy of the substrate, γf that of the film and γi the film–substrate interfacial energy. Under which condition is island (Volmer–Weber) growth likely?

  1. AWhen γs > γf + γi
  2. BWhen γs < γf + γi
  3. CWhen the deposition rate is slow, regardless of the relative surface energies
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Answer: B. When γs < γf + γi

Whether a film wets and spreads over the substrate is determined by the balance of surface energies. Covering the substrate with film replaces the substrate surface (γs) with the film surface (γf) and the interface (γi), so if γs < γf + γi, the energy is lower without covering, and the film balls up into three-dimensional islands. This is Volmer–Weber growth. Conversely, if γs > γf + γi, covering with film lowers the energy, and the film grows layer by layer. This is why metals with high surface energy deposited on oxide or glass substrates with low surface energy tend to form islands. Deposition rate and temperature affect the density and size of the islands, but the basic growth mode is set by the relative energies.

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