Surface Energetics of Iron Pyrite
Gabriel Jurado, Tula R . Paudel, and Evgeny Y. Tsymbal
Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience,
University of Nebraska - Lincoln, USA
Iron pyrite structure
Introduction
We studied the surface energetics of iron pyrite to determine
the most stable surface configuration. Interest in this research
stems from current application of iron pyrite as a photovoltaic.
Problems arise due to iron pyrites low Open Circuit Voltage
(OCV). The open-circuit voltage VOC, is the maximum voltage
available from a solar cell, at zero current. We would like to
understand if surface stoichiometry is responsible for the low
OCV. Density Functional Theory calculation as implemented in
VASP (Vienna Ab-initio Simulation Package) was performed
using PAW potentials and the GGA+Hubbard U correction was
used. The plane wave energy cutoff as 310eV, and for each cycle
the total energy was converged within 1E-06 eV. A Monkhorst
pack k-mesh of 9x9x9 was used for bulk relaxations,
and a 4x4x1 k-mesh for the surface. DOS and band structure
were ran first by a self consistent calculation, and then a nonself consistent run using CHGCAR with a k-mesh of 12x12x1.
Bulk (single crystal) of iron pyrite is known to be an n-type
semiconductor, and the Fermi level lies closer to the conduction
band. While the surface (thin film) is commonly a p-type semi
conductor and the Fermi level lies closer to the valance band.
Formation energy of intrinsic vacancies in iron
pyrite bulk
Density of states of bulk iron pyrite
Band structure for bulk iron pyrite
[1]
0.83 eV
[5]
 Surfaces (100) was created by adding a 10angstrom vacuum. Termination on this plane
allows for surfaces modeled by FeS(2-x), where
x is 0, 1, and 2.
Density of states of iron pyrite charge defects
• The valence band (VB) is split into two
regions. The region between 0 and 1.4 is made of of primarily non
bonding fully occupied Fe 3d states.
• The region of the VB between -1.4
and -7.8 eV consists of predominately
S 3p states.
Band structure of iron pyrite surface defects
• The band gap was found to be 0.83 eV close to
widely accepted value of 0.95 eV [3]. This
underestimation is attributed to the GGA
method with often under binds atoms.
Formation energy in (100) iron
pyrite surface
Fe vancany (neutral )
S vacancy (neutral)
0.26-1.67 eV
2.13-1.42 eV
Schottky defect
4.51 eV
• It can be seen that the formation
energies of the neutral vacancies are [2]
much lower than that of the bulk.
• Further research on surface
energetics should focus on defect
sates on the (100) surface.
[2]
 Predominance map depicting the regions in the
µS vs εF space where the different types of
vacancies have the least formation energy, and
thus, the highest dominance [2]. The bulk
formation energy is high, VS=2.27 eV whereas in
the surface it drops to 0.27 eV.
• It is shown that the most stable defects of iron
pyrite bulk space are Vs when Fermi level is close
to conduction bands and Vfe, when Fermi level is
close to valance bands.
[2]
• The Fermi level can be changed due to the
presence of charged defects, some of
whose density of states (DOS) does not
differ significantly from that of neutral
defects except for the location of the Fermi
level.
• The predominance of neutral charge state
defects in bulk pyrite provides a motivation
for investigating neutral defects on the
(100) surface.
[4]
• Due to the subtraction of sulfur from the bulk
surface, we can see the band gap change from
0.83-0.95 eV for the bulk, to around 0.4 ± 0.1 eV
at the surface [4].
• A sulfur vacancy creates two distinct defect
levels in the band gap. These defect states result
from the breakage of two distinct types of
bonds: the Fe–S bond, and the S-S dimer bond.
If enough of either defect is created, they can
cause a change in the position of the Fermi level
from near the center of the band gap towards
one of the band edges [2].
Acknowledgments and citations
California State University San Bernardino
University of Nebraska-Lincoln
MRSEC
CREST
UNL-REU
NSF
[1] Brian Kolb and Alexie M. Kolpak, Ultrafast band-gap oscillations in iron pyrite. PHYSICAL
REVIEW B 88, 235208 (2013)
[2]A Krishnamoorthy et al Electronic states of intrinsic surface and
bulk vacancies in FeS2, J. Phys.: Condens. Matter 25 (2013) 045004
[3]A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonsovante, K. Buker, M. Bronold, C. Hopfner,
H. Tributsch, Sol. Energy Mater. Sol. Cells, 29 , 289 (1983)
[4]F.W. Herbert, et al., Quantification of electronic band gap and surface states on
FeS2(100), Surf. Sci. (2013)
[5]G. U. von Oertzen, W. M. Skinner, and H. W. Nesbitt, Phys. Rev. B 72, 235427 (2005).