SUPPLEMENTARY INFORMATION
Probing optical band gap at nanoscale in NiFe2O4 and CoFe2O4
epitaxial films by high resolution electron energy loss spectroscopy
K. Dileep,1,2 B. Loukya,1,2 and R. Datta1,2
1
International Centre for Materials Science, 2Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre
for Advanced Scientific Research, Bangalore 560064, India
N. Pachauri3 and A. Gupta3
3
Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487, USA
Experimental spectra acquisition:
The spectra were collected by exciting the gun monochromator in a FEI TITAN 80-300 keV
aberration corrected transmission electron microscope (TEM). Fig 1(a) shows the electron beam
stretched by electric field of the monochromator. The center part of the beam, which is
monochromated with an energy resolution of 0.18 eV, is used to acquire spectra at different
regions of the samples as shown in fig. 1 (c).
Cerencov radiation is known to create artifacts in the acquired spectra, which affect the indirect
band gap measurement significantly. But, the Cherencov intensity is known to have a very low
angular dispersion (<20 µrad). So, the intensities from it can be digitally removed from the ω-q
maps. The procedure for removing the Cerencov artifact is given in fig 1(b).
FIG. 1. (a) The beam after the monochromator excitation, which gives a resolution better than 0.18 eV. (b) The
spectrum image mode used only during indirect band gap measurement. The intensity profile across the aperture is
shown to the left side of the beam. The spectra are collected from either side of the center of the GIF aperture for
indirect band gap measurement to avoid the contribution of Cerenkov radiation. (c) Spectrum are collected from
different areas of the sample.
Electronic structure simulation:
The 5 different types of structures for NFO that were simulated are given in fig. 2.
FIG. 2. Schematics of simulated structures of NFO. (a) Inverse spinel structure, (b) 50% spinelinverse spinel mixed structure, (c) normal spinel stucture, (d) 50% A site cation vacancies and (e)
100% A site cation vacancies.
The schematics of the 5 different CFO structures are given below.
FIG. 3. Schematics of simulated structures of CFO. (a) Inverse spinel structure, (b) 50% spinel-inverse spinel mixed
structure, (c) normal spinel stucture, (d) 50% A site cation vacancies and (e) 100% A site cation vacancies.
(a)
(b)
FIG. 4. DOS for normal spinel (a) NFO and (b) CFO.
In normal spinel NFO, the tetrahedral Ni is in high spin (2.25 µB/atom) +2+δ charge state.
Octahedral Fe is in low spin (0.39 µB/atom) +2+δ’ charge state, making the tetrahedral atoms
carry the majority spin. In majority channel, the high band gap (3.34 eV) is determined by the
crystal field split of octahedral Fe d states. In minority channel, the unfilled tetrahedral Ni d
states form the bottom of the conduction band and octahedral Fe t2g states form the top of the
valence band (band gap = 1.42 eV).
In normal spinel CFO, the tetrahedral Co (2.54 µB/ atom) is crystal field split and determines the
very high band gap (5.5 eV) in majority channel. In the minority channel,Fe (4.23µB/ atom) is in
a high spin state and the nearly empty Fe d minority states form the bottom of the conduction
band rendering a smaller (1.7 eV) band gap. The net moment is 4 µB/ f.u. and the majority spins
are carried by the octahedral high spin Fe atoms.
The band structure plots for normal spinel NFO and NFO with 100% A site cation vacancy are
given in figure 5. The fundamental band gap transitions are marked.
EF
FIG. 5. (a) The band structure plots for (a) normal spinel NFO and (b) 100% A site cation vacancies in NFO.
The band structure plots for normal spinel CFO and CFO with 100% A site cation vacancy are
given in figure 6. The fundamental band gap transitions are marked.
FIG. 6. (a) The band structure plots for (a) normal spinel CFO and (b) 100% A site cation vacancies in CFO.