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Graphene and novel graphitic ZnO and ZnS nanofilms: the energy landscape, non-stoichiometry and water dissociation

Research output: Contribution to journalArticle

  • Sergio Conejeros
  • Neil Allan
  • Judy Hart
  • Frederick Claeyssens
Original languageEnglish
Pages (from-to)1924-1935
Number of pages12
JournalNanoscale Advances
Volume1
Issue number5
Early online date1 Apr 2019
DOIs
DateAccepted/In press - 22 Mar 2019
DateE-pub ahead of print - 1 Apr 2019
DatePublished (current) - 1 May 2019

Abstract

The energy landscapes of ultra-thin nanofilms of ZnO and ZnS are examined in detail using periodic hybrid density functional calculations. We predict new staggered graphitic forms, which, are stable only for the thinnest films and are of particular interest as the electronic structure shows a spontaneous symmetry breaking across the film and consequently a marked decrease in band gap with thickness. The relative energies of the various forms, their structural and electronic properties and their variation with film thickness are discussed. Possible kinetic pathways for transitions from the graphitic forms are examined by explicit evaluation of transition state energies.
For polar surfaces, such as (0001) würtzite and (111) zinc blende, many different mechanisms operate to remove or reduce the surface dipole depending on the number of layers in the nanofilm. The polar ZnS nanofilms, but not the polar ZnO analogues or any non-polar film, are predicted to spontaneously become non-stoichiometric by loss of zinc atoms from the surface.
The behaviour of adsorbed water on the ultra-thin films is also examined. There is no dissociation on any ZnS film. For ZnO, dissociation into OH- and H+ takes place not only on (101¯0) würtzite, but also on (110) zinc blende. This result that does not appear to have been reported previously and deserves future experimental study.
While we concentrate on ZnO and ZnS, similar energy landscapes are expected for any oxide or sulphide which adopts the würtzite or zinc blende structure in the bulk
The energy landscapes of ultra-thin nanofilms of ZnO and ZnS are examined in detail using periodic hybrid density functional calculations. We predict new staggered graphitic forms, which, are stable only for the thinnest films and are of particular interest as the electronic structure shows a spontaneous symmetry breaking across the film and consequently a marked decrease in band gap with thickness. The relative energies of the various forms, their structural and electronic properties and their variation with film thickness are discussed. Possible kinetic pathways for transitions from the graphitic forms are examined by explicit evaluation of transition state energies.
For polar surfaces, such as (0001) würtzite and (111) zinc blende, many different mechanisms operate to remove or reduce the surface dipole depending on the number of layers in the nanofilm. The polar ZnS nanofilms, but not the polar ZnO analogues or any non-polar film, are predicted to spontaneously become non-stoichiometric by loss of zinc atoms from the surface.
The behaviour of adsorbed water on the ultra-thin films is also examined. There is no dissociation on any ZnS film. For ZnO, dissociation into OH- and H+ takes place not only on (101¯0) würtzite, but also on (110) zinc blende. This result that does not appear to have been reported previously and deserves future experimental study.
While we concentrate on ZnO and ZnS, similar energy landscapes are expected for any oxide or sulphide which adopts the würtzite or zinc blende structure in the bulk

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