Effect of lanthanum doping on modulating the thermochromic properties of VO2 thin films

Article abstract of DOI:10.1039/c6ra09514c

Lanthanum (La) doped vanadium dioxide (VO₂) thin films were fabricated through a facile sol-gel technique. The La doping was found to be effective for reducing the phase transition temperature (τ_c) of VO₂ below the 4 at% doping level, with a reducing rate of -1.1 °C per at%. In addition, below the 3 at% La doping level, the band gap (E_g) decreased steadily with the increasing addition of La dopants. 5 at% La doping can enhance both the integrated visible transmission (T_lum) and the solar modulating ability (ΔT_sol) simultaneously. The combination of T_lum = 50.1% and ΔT_sol = 10.3% represents the best values for VO₂ continuous thin films.

Full text of DOI:10.1039/c6ra09514c

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Effect of Lanthanum Doping on Modulating Thermochromic
Properties of VO2 Thin Films
Ning Wang,a Nigel Tan Chew Shun,a Martial Duchamp,b Rafal E Dunin-Borkowski,b Zhong Li,c and Yi
Long a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Lanthanum (La) doped vanadium dioxide (VO2) thin films were fabricated through a facile sol-gel technique. The La doping
was found to be effective in reducing the phase transition temperature (τc) of VO2 below the 4 at.% doping level, with a
reducing rate of -1.1 °C/at.%. In addition, below the 3 at.% La doping level, the band gap (Eg) decreased steadily with the
increasing addition of La dopants. The 5 at.% La doping can enhance both integrated visible transmission (Tlum) and solar
modulating ability (ΔTsol) simultaneously. The combination of Tlum=50.1% and ΔTsol=10.3% represents the best value for VO2
continuous thin films.
www.rsc.org/
22
contents of VO2 and/or surface plasmon resonance (SPR)10, 23 in
Introduction
some special surface/interface conditions.
Rare earth (RE) elements are characterized with large ion radii and
abundant valence electrons, which always contribute to the
enhancement of physical properties as dopants.24-26 Europium
cation (Eu3+) has been found to be effective in reducing τc of VO2
with a rate of -6.5 °C/at.%, and a moderate combination of Tlum
=54%, ΔTsol=6.7% was achieved.13 Terbium cation (Tb3+) was
reported to be helpful in increasing the Tlum from ~45% to ~79%,
but only a relative low τc reducing rate (-1.5 °C/at.%) could be
obtained.27 As predicted by Chao Sun et al. using the density
functional theory (DFT) calculation,28 the doping of lanthanum
cation (La3+) could cause the V-V distance change and great V-V
dimer distortion, which may result in a decrease of τc. In this paper,
La3+ cations were firstly doped into VO2 lattice at different doping
levels experimentally from 1 to 5 at.%. It was found that the τc of
MIT could be reduced in a rate of -1.1 °C/at.% and a combination of
Tlum =50%, ΔTsol=10.3% could be achieved at the 4 at.% doping level.
Vanadium dioxide (VO2) is an interesting phase transition material
at a critical temperature (τc) around 68 °C, accompanied by a
reversible metal to insulator transition (MIT).1, 2 With regard to the
pure VO2, the insulating phase (τc<68 °C) with a monoclinic P 21/c
crystal system shows strong transmission at the infrared (IR)/near
infrared (NIR) range, while the metallic counterpart (τc >68 °C) with
a
tetragonal
transmission across IR/NIR.3, The thermal modulating abilities in
IR/NIR and electrical conductivity contrast across τc make VO2
P 42/mnm crystal system exhibits depressed
4
a
promising candidate in thermochromic smart window and various
smart sensor applications.5-8 Especially for the smart window, the
IR/NIR light can be transmitted (τ < τc) or absorbed (τ > τc)
automatically according to the out-door temperature, maintaining
the comfortable indoor environment, which meets the nowadays
energy-saving requirements.9
However, the high τc, low visible transmission (Tlum) and the low
solar modulating abilities (ΔTsol) of pure VO2 are three main
drawbacks to be overcome.9, 10 Moreover, there is always a trade-
off between enhancing both Tlum and ΔTsol simultaneously.11 With
respect to τc, the extra strain energy12 and doped by metal cations
with large ion radii/large valence state13-15 have been proved to be
effective in reducing τc. As for Tlum, the microroughness,16 widening
band gap (Eg),17 biomimetic nanostructure,11 nanogrid7 and porous
Experimental section
All of the chemicals V2O5 (99.6%, Alfa Aesar), La2O3 (99.9%, Alfa
Aesar and H2O2 (30 wt.%, Sigma-Aldrich) were used as received
without any further purification.
Precursor preparation. 182 mg V2O5 and weighed La2O3 powder
were added into 5 mL hot H2O2 (30 wt.%, 90 °C) solution under
vigorous stirring. After a violent evaporation and cooling down to
room temperature, another 15 mL H2O2 (30 wt.%) was added into
the suspension, and a clear brown color precursor solution was
attained after stirring for 4 minutes. Then the precursor was moved
to do the dip coating at once. It should be noted that the reaction
should be done in a 300 mL beaker due to the strong heat release
during the reaction between V2O5 and hot H2O2.
morphology18-21 have been reported to be able to increase Tlum
.
Lastly, the ΔTsol is always believed to be correlated to the solid
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VO2 thin film fabrication.
A fused silica substrate with the temperatures ranging from 20 to 100 °C. In order to attain the
dimension 15×15×0.5 mm3 was dipped into the precursor solution phase transition temperature τc, the temperature-dependent
at a rate 50 mm/min. After immersing for 20 seconds, the substrate heating and cooling %T(τ) data were fitted using a sigmoidal
was lifted up vertically at a withdrawing rate 200 mm/min. After function of the form:31
drying in air, a brownish thin film with the thickness ~60 nm/side
%T(τ)=A2+(A1-A2)/[1+exp(τ-τc)/B] (2)
was got on the substrate. Then the thin film was moved into a tube
where τ is the temperature in °C and A1, A2, τc and B are fitting
furnace and annealed at 550 °C for 2h in an Ar flow (200 mL/min).
parameters. Two phase transition temperatures τc,h and τc,c were
Characterization. The phase of the products was characterized with
obtained in the heating and cooling cycles, respectively and the
a Shimadzu XRD-6000 X-ray diffractometer (Cu-Kα, λ = 0.15406 nm)
average phase transition temperature was defined as τc = (τc,h
+
using a voltage of 40 kV and a current of 30 mA at an X-ray grazing
angle of 1.0°. Their morphology and La doping level were
determined using a field emission scanning electron microscope
(FESEM, JSM-7600F, JEOL, Japan) with an INCA EDX detector at an
τc,c)/2, Δ τc = τc,h – τc,c. The solar modulating ability is termed as
ΔTsol=Tsol (20 °C)- Tsol (90 °C).
accelerating voltage of
5 kV and 20 kV, respectively. For
Results and discussion
conventional transmission electron microscopy (TEM) studies, i.e.,
using selected area electron diffraction (SAED), bright field (BF)
imaging, an JEOL 2010 (JEOL Company, Japan) microscope was used
at an accelerating voltage of 200 kV. Transmittance spectra in the
Synthesis of La doped VO2 thin films
Figure 1a shows the XRD patterns of the pristine and La doped VO2
thin films. The bump around 20° (2θ) in the XRD patterns should be
ascribed to the diffraction of the amorphous fused silica substrates.
All of the thin films exhibit the characteristic (011) XRD peak of VO2
(monoclinic, P 21/c, JCPDS #82-661) around 28° (2θ), and no other
impurity phase can be found, indicating the successful synthesis of
VO2 phase. Compared with the pristine VO2, the (011) peak position
in the 5 at.% La doped sample is left-shifted by 0.2° from 28.0° to
27.8°, which indicates the lattice expansion arising from the
replacement of V by La with relatively larger ionic radius (1.032 Å
v.s. 0.58 Å) in accord with the DFT calculation.28 The La doping
range 250-2500 nm were measured using
spectrophotometer (Cary 5000, Agilent Ltd, USA) equipped with a
Linkam PE120 system Peltier heating cooling stage. The
a UV-Vis-NIR
&
integrated visible transmittance (Tlum, 380-780 nm) and solar/IR
transmittance (Tsol, 280-2500 nm; TIR, 780-2500 nm) were calculated
based on the recorded %T spectra using the expression
Tlum/sol=ʃφlum/sol(λ)T(λ)d λ/ ʃφlum/sol(λ) d λ (1)
where T(λ) is the recorded film transmittance, φlum is the standard
luminous efficiency function for the photopic vision of human
eyes29 and φsol is the solar irradiance spectrum for air mass 1.5
(corresponding to the sun standing 37˚ above the horizon).30 The
hysteresis loop of %T at a wavelength of 2000 nm was measured at
levels were further confirmed by EDX analysis with
a 10%
derivation. As depicted in Figure 1b, the EDX results agree well with
the expected doping levels.
(a)
(b)
(011)
6
5
5 at.%
4 at.%
3 at.%
2 at.%
4
3
2
1 at.%
1
pristine
0
ꢀ1
0
1
2
3
4
5
10 15 20 25 30 35 40 45 50 55 60 65 70
Expected doping level (at.%)
2θ (deg)
Fig. 1 (a) XRD patterns of the pristine and La doped (1-5 at.%) VO2 thin films. (b) La doping levels determined by EDX characterization with a
10% derivation.
Figure 2 shows the morphology evolution of the pristine and La films have been detected using AFM under tapping mode, which
doped VO2 thin films tested using FESEM. All of the samples show have been tabulated in Table 1. The thickness of the La-doped
nanograin packed morphology, and the average grain size for the 0- samples shows a derivation compared to the pristine VO2 thin film
5 at.% La doped samples is 61, 72, 84, 79, 62 and 73 nm, by ~±14 nm, while the roughness exhibits a slight increase by 1.1-
respectively, which shows that the La doping gives rise to a slight 5.6 nm. The TEM characterization of the 3 at.% La doped sample
increase of the grain size. The roughness and thickness of the thin has been shown in Figure 3. As shown in Figure 3a-b, the bright-
2 | J. Name., 2016, 00, 1-3
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field TEM images reveal that the average grain size of the sample is high resolution STEM image, where the La atoms with larger atomic
~80 nm, consistent with the SEM result (Figure 2). The SAED as number are brighter than the V atoms, arising from their different
shown in Figure 3c exhibits diffraction rings that can be indexed by contributions to the elastic scattering signals collected by the
(011), (200), (021) and (022) faces of VO2, which indicates the detector. In addition, the STEM-EDX elemental mapping (Figure 3f)
polycrystalline nature of the La doped VO2 thin film. The doping of related to the view of HAADF (high angle annular dark field) shows
La was further investigated through STEM. As shown in Figure 3d, that the La atoms are incorporated in the VO2 lattice as the
the atom replacement of V by La could be clearly observed in the distribution of La signal matches the lattice plane arrangement.
Fig. 2 FESEM images for the pristine (a) and La doped (1-5 at.%, b-f) VO2 thin films. The scale bar in the images is 500 nm.
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Fig. 3 TEM (a) and the enlarged TEM image (b) of the 3 at.% La doped VO2 grains. (c) SAED pattern of the polycrystalline 3 at.% La doped
VO2 grains. (d) STEM image of the 3 at.% La doped VO2 grain. (e) HAADF image of the doped sample. (f) The La element mapping related to
(e).
Thermochromic properties
recorded at the wavelength 2000 nm in a temperature range 20-
100 °C. As shown in Figure 4c, after doping with La, the MIT tends
to move toward lower temperature until the doping level of 4 at.%,
which is similar to the case of Eu doping.13 Plots of doping level
dependent τc were shown in Figure 4d, and a τc reducing rate of -1.1
°C/at.% could be attained below the doping level of 4 at.%. This is
the first experiment to prove the efficiency of La doping in reducing
τc as indicated in simulation work using DFT calculation.28 With
respect to the τc reduction mechanism, besides the lattice distortion
from the substitution of large La atoms calculated by DFT,28 the
Figure 4a shows the transmittance (%T) spectra (250<λ<2500 nm)
of the samples at the temperature 20 and 90 °C, respectively. Upon
heating from 20 to 90 °C, a large %T contrast in the IR range could
be observed for all of the samples, which should be ascribed to the
MIT.20, 32, 33 The integrated Tlum and ΔTsol were tabulated in Table 1
and Figure 4b shows the doping level dependent average Tlum and
ΔTsol. It can be observed that the best combination of Tlum=50.1%,
ΔTsol=10.3% was achieved at the La doping level 4 at.%, which
represents the best value for RE-doped VO2 thin films,13, 27, 34 and is
comparable to other doping cases without RE.35-40 It is of interest
that both of average Tlum and ΔTsol at the La doping level 5 at.% are
higher than that of pristine VO2 (69.2 %, 9.0 % v.s. 64.8%, 7.3 %),
which should be due to a surface plasmon resonance peak around
1250 nm arising from the La doping. The %T hysteresis loop was
increase of hole carrier density (h+) from V5+ propagating caused by
5+
La doping (V4+
V
xLa3+xO2) should also contribute to the
1-2x
adjustment of τc.41 The competition between these two
mechanisms should be the reason why τc starts to increase at
higher La doping level.
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(a)
100
80
60
40
20
0
(b)
20
15
10
5
20°C
70
65
60
55
50
45
40
35
30
pristine
2at.%
4at.%
1at.%
3at.%
5at.%
90°C
(c) 0
100
90
(d)
69
500 1000 1500 2000 2500
λ (nm)
0
1
2
3
4
5
Doping level (at.%)
68
67
66
65
64
63
62
80
-
1
70
.
1
pristine
1at.%
2at.%
3at.%
4at.%
5at.%
°
C
/
a
60
t
.
%
50
40
30
20
0
1
2
3
4
5
20
40
60
τ (°C)
80
100
Doping level (at.%)
Fig. 4 (a) UV-Vis-NIR spectra of the pristine and La doped (1-5 at.%) VO2 thin films under the temperature 20 and 90 °C. (b) Average Tlum and
the ΔTsol of the VO2 thin films with different La doping levels. (c) %T hysteresis loop of the pristine and La doped (1-5 at.%) VO2 thin films.
(d) Plots of τc v.s. La doping level.
As listed in Table 2, compared to other two reported rare earth Figure 5a shows the %A and %R spectra of the pristine and La
dopants Eu3+ and Tb3+, the La3+ cation has the largest ionic radius. doped VO2 thin films. It can be found that the %A and %R could be
The average Tlum of the RE-doped VO2 thin films decrease with the changed by the La doping, where the 5 at.% and 4 at.% La doped
ascending ionic radius of dopants and this is consistent with the fact samples exhibit the lowest and highest %A / %R, resulting in their
that larger ions result in greater electronic polarization and slower highest and lowest %T, respectively. In order to further investigate
velocity of light, leading to a larger refractive index. From Tb3+ to the La doping effects on the optical properties, the direct bandgap
Eu3+ dopant, the ionic radius increases from 0.92 to 0.95 Å and the (Eg) of the VO2 thin films was calculated by fitting the linear part of
τc reducing rate is increased from 1.5 to 6.5 °C/at% which should be (αћω)2 v.s. E curves (Figure 5b) based on the equation (αħω)2=
due to the larger lattice distortion. However, the largest size La3+ A(ħω-Eg),44, 45 where α is the absorption coefficient (αd=-ln(T/1-R)),
doping shows the lowest τc reducing rate and the reason remains A is a constant and ħω is the photon energy. As depicted in the
unknown, but some dot defects in the form of La insertion in the inset of Figure 5b, the Eg of the VO2 thin films is gradually declined
seriously distorted lattice may be partially attributed to this upon doping with La and the lowest value 2.42 eV is got at the
phenomenon. In addition, compared to the previous reported W- doping level of 3 at.%, then it returns back with further doping. The
/Mg-doped VO2 continuous thin films (Table 2), the La-doped thin decrease of Eg below the doping level 3 at.% is similar to the case
film shows the best combination of Tlum (50.1%) and ΔTsol (10.3%).
under tungsten (W) doping arising from the larger ionic radius,46
and the following Eg widening should be due to the competition
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between the effect of large ionic radius and h+ carrier density. Since to the change of band structure resulted from the La doping in the
the change of band gap is of electronic nature,44 the Eg narrowing of lattice.
La doped VO2 thin films below 3 at.% doping should be also related
(a)
(b)
22
pristine
3at.%
1at.%
4at.%
2at.%
5at.%
2.60
7
80
2.55
2.50
2.45
2.40
pristine
2at.%
4at.%
1at.%
3at.%
5at.%
0
1
2
3
4
5
La doping level (at.%)
0
0.6
1.2
1.8
2.4
3.0
500
1000
1500
2000
λ (nm)
E (eV)
Fig. 5 (a) %A and %R spectra of the pristine and La doped (1-5 at.%) VO2 thin films at room temperature. (b) Curve of (αћω)2 v.s. E and the
band gap Eg (inset) under different La doping levels.
Table 1. Thermochromic properties of the La doped VO2 thin films.
Doping level/at.% Thickness/nm Raa/nm Tlum(20°C/90°C)/% ΔTsolb/% τc(heating/cooling)/°C
0
1
2
3
4
5
74
83
60
72
90
80
3.0
4.4
8.6
4.4
4.1
5.5
64.8/64.8
61.1/61.3
67.5/67.7
55.8/56.6
49.9/50.4
69.5/68.9
7.3
9.1
79.9/57.1
76.1/52.4
79.6/54.2
76.2/54.9
75.9/49.2
80.2/55.2
6.2
8.4
10.3
9.0
a Ra is the average roughness; b ΔTsol=Tsol(20°C)- Tsol(90°C).
Table 2. Doping effects on the thermochromic performance of VO2 continuous films.
RE doping
cation Ionic Radius/Å Doping /at.% Ave. Tlum/% ΔTsol/% d τc/d at.%
Ref.
13
Eu3+
Tb3+
La3+
0.95
0.92
1.03
4
4
4
54
65.9
6.7
4.6
-6.5
-1.5
-1.1
27
50.1
10.3
This work
Other dopings
W6+
Ti4+
0.6
0.605
0.72
2
-
45.1
40
6.9
4.6
4.8
20
-
42
43
10
Mg2+
5
82.1
1.4
6 | J. Name., 2016, 00, 1-3
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Notes: The thickness, roughness and grain size of the RE-doped VO2 thin films are at the same level.
6. Prasad, A. K.; Amirthapandian, S.; Dhara, S.; Dash, S.; Murali, N.;
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Conclusions
In summary, a series of La doped VO2 thin films were prepared
through a facile sol-gel method, and the thin films exhibited
8. Hu, B.; Ding, Y.; Chen, W.; Kulkarni, D.; Shen, Y.; Tsukruk, V. V.;
Wang, Z. L. Adv. Mater. 2010, 22, 5134-5139.
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Especially, the 4 at.% La doped sample shows the best
combination of Tlum=50.1% and ΔTsol=10.3% compared with
other reported VO2 continuous thin films. Compared with
pristine VO2, 5 at.% La doped sample can increase Tlum and
ΔTsol simultaneously. La doping was firstly proved to be able to
reduce the τc of MIT in experiment which is consistent with
the simulation work, and it followed a reducing rate of -1.1
°C/at.% below the doping level of 4 at.%. A narrowing down of
band gap was observed below the doping level of 3 at.%,
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caused by the doping in VO2 lattice with large La atoms. This
investigation of La doping effects on thermochromic
properties of VO2 thin films should be useful for other element
doping researches with large ionic radius and low valence
state.
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Acknowledgements
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This research was supported by the Singapore National
Research Foundation under the CREATE programme:
Nanomaterials for Energy and Water Management, Singapore
Ministry of Education (MOE) Academic Research Fund Tier 1
RG101/13, JTC-sponsored FYP program AY 2015-16 and an
MSE-ERC Julich collaboration-MAP project grant. XRD, FESEM
and TEM characterization was performed at the Facility for
Analysis, Characterization, Testing and Simulation (FACTS) in
Nanyang Technological University, Singapore and in the Ernst
Ruska-Centre for Microscopy and Spectroscopy with Electrons
(ER-C) in Forschungszentrum Jülich, Germany. The authors
acknowledge financial support from the European Union under
the Seventh Framework Programme under a contract for an
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Graphical Abstract
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0
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Doping level (at.%)
Doping level (at.%)
La doping is found to be effective in enhancing the luminous transmission and solar modulating
abilities simultaneously for VO2 thin films, and exhibits a low τc reducing rate of -1.1 °C/at.%.