10th End Semester Presentation
Structural, electronic and transport properties of electron-doped
LaCoO3 thin films.
by
Ashok Kumar
Ph.D. Scholar, 10th Sem.
Department of Physics
NIT, Hamirpur (H.P)
under supervision of
Prof. Ravi Kumar
(HOD) Centre for Material Science & Engg.
NIT, Hamirpur, (H.P)
Dr. Rajesh Kumar
Department of Physics
NIT, Hamirpur, (H.P)
Contents
 Abstract
 Introduction
 Structural Study
 Electronic Structure Study
 Transport Study
 Conclusion
 References
Abstract
We have deposited the thin films of LaCoO3 and LaCo.7Ni.3O3 (doped with Ni 30\%) by pulsed laser
deposition method on LaAlO3 (100) single crystal substrate. The XRD characterization of thin films
confirms that both the films are single crystalline in nature and orientated in (001) direction. In a
transport study, we observe that temperature dependence of resistivity shows the semiconducting
behaviour in the whole temperature range for LaCoO3 and LaCo.7Ni.3O3 films. We found a drastic
change of four hundred folds in resistivity (ρ) with Ni substitution. At room temperature 300 K, the
resistivity (ρ) of thin film decrease from 7.024 Ω cm to 0.0154 Ω cm. This decrease in resistivity was
mainly attributed to an increase in the number of charge carriers (electrons) in the 3d orbital of
transition metal ion and which has been confirmed by the x-ray absorption studies. The conduction
mechanism is not same for both the films for whole temperature range.
.
1. Introduction
The electronic structure of LaCoO3 thin film is relatively insensitive to the lattice strain, but magnetic
properties displayed strong dependence. This insensitiveness against lattice strain might be due to the
unusual electronic structure of LaCoO3 [1].
According to DMFT study of LaCoO3 thin film on strain dependence, the strain mostly influence the O 2p
energies but not much the Co 3d energies. This stated that valence and consequently ionic size of Co3+
can hardly change due to external stresses[2].
From the literature study, we observe that strain (tensile or compressive) may not change the transport
behaviour of LaCoO3 thin film. Also, oxygen non-stoichiometry may not affect the insulating state of the
LaCoO3 thin films[3].
In La1-xSrxCoO3 bulk and thin films, Sr+2 (for x =.18 composition) induce the charge disproportion in Co, and
that change the insulating state into metallic one[4]. However, oxygen non-stoichiometry in
𝐿𝑎1−𝑥 𝑆𝑟𝑥𝐶𝑜𝑂3−𝛿 thin films, metal to insulator transition has been observed at room temperature[5].
The hole doped (Ca, Sr) LaCoO3 thin film were studied by the quite a lot of researchers, but there was
found rare report on transport studies of electron-doped LaCoO3 thin film[4,5].
So in this work, we have investigated the effect of compressive lattice strain and Ni substitution on the
structural and transport properties of LaCoO3 thin films.
2. Structural properties
 Thin films of LaCoO3 (LCO) and LaCo.7Ni.3O3 (LCNO) were grown in the (100) direction on single crystalline LaAlO3 (LAO) (100)
substrate.
 These films are crystalline, oriented (001) and of single phase as can be seen from their X-ray diffraction patterns given in figures 1
(a) and (b).
 Both films show clearly (100) and (200) reflections of the pseudo-cubic structure.
 Also, no indication of impurities and disorientation was detected.
 The out of plane lattice parameter of LCO film on LAO was calculated as 3.826Å , which agrees with the expected value for the
fully strained state.
 The lattice mismatch calculated for these films came out to be 0.7%, due to which the epitaxial LCO film experiences lattice strain.
 Percentage lattice strain (out-of-plane) is calculated as = (as - af)/af, where as, af are lattice constant of substrate and film
respectively. It was found to be 0.94%.
 The lattice constant increases with Ni substitution and found to be 3.833Å. This may be due to the difference in the ionic radii of
Ni and Co, which gives rise to the expanded lattice.
(a)
(b)
Figure 1 XRD patterns of (a) LaCoO3 thin film and (b) LaCo.7Ni.3O3 thin film on LaAlO3 (100) single crystal substrate.
3.1 Electronic Properties
 To probe the valence state of cobalt, we have taken the XAS
of bulk LaCoO3, LaCoO3 and LaCo.7Ni.3O3 thin film samples.
 Co L edge is highly sensitive to the valence state and it
probes 3d the state.
 In bulk LaCoO3 XAS, energy near ~ 782ev a small peak was
detected which confirms the Co low spin (LS) state and
insulator at room temperature.
 From XAS of Co L2,3 edge of bulk LaCoO3, LaCoO3 and
LaCo.7Ni.3O3 thin film samples, we found that films XAS are
shifted to higher values of ~.1ev as compared to bulk.
 This shift in energy towards higher value correlates with an
increase in the valence state towards Co+4. This type feature
was also reported for bulk La1-xSrxO3 with x~.1. In LaCoO3
case, it may be attributed to La or oxygen vacancies induced
during PLD process. But this hole concentration is not
sufficient to create change in transport properties.
 In XAS of thin films LaCoO3 and LaCo.7Ni.3O3, two peaks
appeared (pre and post-edge) at energy 779.1ev and 782ev
respectively.
 The pre-edge corresponds to intermediate spin (IS) or high
spin (HS) and post-edge correspond to low spin (LS).
 The intensity of pre-edge increased with Ni doping and it
attributes to an increase in the number of electrons in 3d
orbital .
Co L 32
LS
(a)
LS
Co L 32
(b)
LS
Figure 2 (a) Normalized spectra for Co L32 edge of LaCoO3,
LaCo.7Ni.3O3 thin film on LaAlO3 (100) single crystal substrate
and (b) for Co L32 edge of Bulk LaCoO3 at 300 K temperature
Continue…
Continue…
3.2 Electronic Properties
𝑒𝑔
❖ The intensity of peak A and B is assigned to the unoccupied states of Co/Ni 3d and
𝑡2𝑔
6
Co +3 (d6)→ 𝑡2𝑔
, 𝑒𝑔0
(a) Low Spin (LS)
O 2p states generated by Co/Ni 3d and O 2p Hybridization. Intensity linearly
proportional to the number of empty states i.e. t2g and eg .
❖ In bulk t2g state may be fully occupied and eg state fully unoccupied, as result
intensity around A is small and at B it is high.
❖ Thin film of LCO on LAO 100, intensity at A i.e. t2g increased due to increase in
unoccupied states and at B decrease i.e. eg due decrease in unoccupied states.
❖ Ni 30% substitution with Co in LCNO, the number of unoccupied states decreases
(t2g) slightly as compared to LCO with increased number of 3d electrons (eg).
5
Co +3 (d6)→ 𝑡2𝑔
, 𝑒𝑔1
(b) Intermediate Spin (IS)
Co 4sp
La 5d
Co 3d
6
Ni +3 (d7)→ 𝑡2𝑔
, 𝑒𝑔1
(c) Low Spin (LS)
Figure 3. Spin states of Co, (a),
(b), & Ni (c).
Figure 4. Normalized O K edge XASNES spectra of LaCoO3 , LaCo.7Ni.3O3 thin film on
LaAlO3 (100) single crystal substrate and Bulk LaCoO3 at 300 K temperature.
7
4. Transport Properties





To observe the effect of strain and electron doping on the conduction mechanism, temperature dependent dc resistivity were made in the
temperature range 100 K to 300 K.
Temperature dependence of resistivity shows the semiconducting behaviour, in the whole temperature range for film LaCoO3, and film
LaC0.7Ni0.3O3.
The resistivity (ρ) of LaCoO3 thin film varies from very large at 100 K temperature to 7.024 Ω cm at room temperature 300 K.
With Ni substitution resistivity (ρ) of LaC0.7Ni0.3O3 thin film varies from 0.30 Ω cm at 100 K temperature to 0.0154 Ω cm at room temperature
300 K.
We observed the drastic change of four hundred folds in resistivity (ρ) at 100 K and room temperature 300 K with Ni substitution.
Continue…
Figure 5. Electrical resistivity versus temperature plots of LaCoO3 , LaCo.7Ni.3O3 thin film on LaAlO3 (100) single crystal
substrate.
8
4. Transport Properties
Continue…
 Arrhenius law stated as
ρ = ρ0 exp(Ea/kBT) where ρ0 being pre exponential factor, Ea is the activation energy and kB is the Boltzmann
constant. This law describe the thermally activated behaviour due to presence of energy gap Ea.
 In case of disordered solids, at least locally, there is deviation from the Arrhenius law. In such a case Efros Shklovski’s (ES)
Variable Range Hopping (VRH) occurs
ρ = ρo exp[(To/T )1/2 ] where the parameter To is characteristics temperature coefficient that describe the
variable range hopping of conduction mechanism.

The resistivity data was fitted using the Arrhenius law as well Efros Shklovski’s (ES) Variable Range Hopping (VRH) model.

The conduction mechanism is not same for both the films for whole temperature range.
lco
lcno
8
8
lcno
Linear Fit of Sheet1 ln(p)
(a)
(b)
6
6
4
ln(p)(cm)
ln(p) (cm)
4
2
0
2
0
-2
-2
-4
-4
0.003
0.004
0.005
0.006
-1
0.007
-1
T (K )
0.008
0.009
0.010
0.05
0.06
0.07
0.08
T
-1/2
0.09
0.10
-1/2
(K
)
Fig. 5. (a) Plot of 𝑙𝑛𝜌 versus reciprocal of temperature (1/T) and (b) 𝑙𝑛𝜌 versus reciprocal of square of temperature 1/ 𝑇 for
LaCoO3 , LaCo.7Ni.3O3 thin film on LaAlO3 (100) single crystal substrate
5. Conclusion

LaCoO3, andLaC0.7Ni0.3O3 thin films grown on LAO (100) single crystalline substrate by PLD method are crystalline and oriented in
the (001) direction. The films strained compressively .

From XAS studies of bulk LaCoO3, LaCoO3 andLaC0.7Ni0.3O3 thin films , we observe that
 in bulk LaCoO3 Co+3 in low spin state at room temperature.
 In LaCoO3 and LaC0.7Ni0.3O3 thin films Co+3 in mixed spin state i.e. low spin (LS), intermediate spin (IS) or high spin (HS) .
 number of charge carrier in 3d orbital increases with Ni substitution .

From transport studies, we found that
 temperature dependence of resistivity shows the semiconducting behaviour, in the whole temperature range for film
LaCoO3, and film LaC0.7Ni0.3O3
 at room temperature 300 K, the resistivity (ρ) of thin film decrease from 7.024 Ω cm to 0.0154 Ω cm with Ni substitution
 drastic change of four hundred folds in resistivity (ρ) at 100 K and room temperature 300 K with Ni substitution.
 conduction mechanism is not same for both the films for whole temperature range.
5. References
1.
W. S. Choi et al. https://doi.org/10.1021/nl302562f | Nano Lett. 2012, 12, 4966−4970.
2.
D. W. Jeong et al. https//:doi:10.1038/srep06124.
3.
N. Biskup https://doi:10.1103/PhysRevLett.112.087202.
4.
H. Taguchi et al. Journal of Solid State Chemistry, 33, 169-172 (1980)
5.
B Liu et al. Journal of Applied Physics, 120, 154103 (2016).
6.
B Liu et al. Materials and Design 89 (2016) 715–720.
7.
N. Choudhary et al. Journal of Superconductivity and Novel Magnetism.
https://doi.org/10.1007/s10948-018-4665-0.
8. https://doi:10.4028/www.scientific.net/DDF.341.155.
9. https://doi.org/10.1063/1.5028906.
Continue…
Continue…
5. References
10. R. N. Parmar et al. DOI: 10.1063/1.2359291 · Source: IEEE Xplore .
11. Kumar et al. Journal of Applied Physics. 114, 073704 (2013).
12. Kumar et al. Journal of Alloys and Compounds 574 (2013) 316–319.