In this house, we OBEY the laws of thermodynamics!
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:
Some Recent Publications:
- 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?
The Icons below lead to some specific examples
- 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
- 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
- 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
- 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.
- 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.