Density Functional Modelling

A critical component for understanding the properties of materials, and enabling the development of new materials is the ability to characterize them in detail as well as understand why they form. While this may appear to be a combination of two disparate concepts, in many respects they are not and should be considered as synergistic. Approached from the characterization side, better tools allow one to answer more fundamental scientific questions about why a particular structure is formed. Approached from the other side, the underlying scientific questions can drive what types of characterization is needed. Frequently the underlying science can be best revealed by theoretical calculations, particularly density functional calculations which despite some limitations can probe many important questions.

On particular area of interest is surface structures. It has now become almost conventional when proposing a model for a surface reconstruction to perform a density functional theory (DFT) calculation. What one wants to know is whether the proposed positions are plausible, i.e. the difference between them and refined DFT positions is not too large, as well as whether energetically the structure is plausible. For this one needs to have answered three fundamental questions:

• What is the most appropriate DFT functional to use? There are many in the literature.
  
  
• What are the errors in the energies? These numbers are rarely analyzed or published and from an experimental viewpoint a measurement without errors is marginal. Obviously only with knowledge of the errors in the energies can one determine if a structure is plausible.
  
  
• What are the basic simple structures against which one wants to compare a reconstruction? Since often reconstructions are variants/superstructures based upon simple 1x1 units, this information is also needed a-priori to aid in solving reconstructions, particularly ones with large unit cells.
  
  
• Do the theoretical calculations agreement between experimental and theoretical results for surface structures, particularly oxides, and in many cases they do not. Are the surface structures obtained experimentally kinetically metastable which, if annealed for long enough, would transform? Are there fundamental problems with the theoretical calculations?

Recent Publications:

1. Water adsorption on SrTiO₃(001): II. Water, water, everywhere
   A. E. Becerra-Toledo, J. A. Enterkin, D. M Kienzle and L. D. Marks
   Surface Science (2012), doi:10.1016/j.susc.2012.01.010
   
   
2. Surface and Defect Structure of Oxide Nanowires on SrTiO₃
   M. S. J. Marshall, A. E. Becerra-Toledo, L. D. Marks and M. R. Castell
   Physical Review Letters 107 (2011) 086102
   
   
3. Vacant-Site Octahedral Tilings on SrTiO₃ (001), the (sqrt(13)xsqrt(13))R33.7 Surface, and Related Structures
   D. M. Kienzle, A. E. Becerra-Toledo and L. D. Marks
   Physical Review Letters 106 (2011) 176102
   
   
4. A homologous series of structures on the surface of SrTiO₃(110)
   J.A. Enterkin, A.K. Subramanian, B.C. Russell, M.R. Castell, K.R. Poeppelmeier and L.D. Marks
   Nature Materials 9 (2010) 245.
   
   
5. Why the case for clean surfaces does not hold water: Structure and morphology of hydroxylated nickel oxide (111)
   J. Ciston, A. Subramanian, D. Kienzle, L. D. Marks
   Surface Science 604 (2010) 155.

The icons below lead to some specific examples: