Vanadium dioxide nanogrid films for high
transparency smart architectural window
applications
Chang Liu,1,4 Igal Balin,2,4 Shlomo Magdassi,3 Ibrahim Abdulhalim2,5 and Yi Long1,*
1
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue,639798,
Singapore
2
Department of Electro-optical Engineering, Ben-Gurion University of the Negev, Beer Sheva,84105, Israel
3
Institute of Chemistry and center for nanoscience and nanotechnology, Faculty of Science, The Hebrew University,
Jerusalem, 91904, Israel
4
These authors contributed equally
5
abdulhlm@bgu.ac.il
*
longyi@ntu.edu.sg
Abstract: This study presents a novel approach towards achieving high
luminous transmittance (Tlum) for vanadium dioxide (VO2) thermochromic
nanogrid films whilst maintaining the solar modulation ability (ΔTsol). The
perforated VO2-based films employ orderly-patterned nano-holes, which
are able to favorably transmit visible light dramatically but retain large
near-infrared modulation, thereby enhancing ΔTsol. Numerical optimizations
using parameter search algorithms have implemented through a series of
Finite Difference Time Domain (FDTD) simulations by varying film
thickness, cell periodicity, grid dimensions and variations of grid
arrangement. The best performing results of Tlum (76.5%) and ΔTsol (14.0%)
are comparable, if not superior, to the results calculated from
nanothermochromism, nanoporosity and biomimic nanostructuring. It opens
up a new approach for thermochromic smart window applications.
© 2015 Optical Society of America
OCIS codes: (160.4236) Nanomaterials; (160.6840) Thermo-optical materials; (310.6628)
Subwavelength structures, nanostructures; (310.6805) Theory and design; (310.6860) Thin
films, optical properties; (350.6050) Solar energy.
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1. Introduction
The energy usage of buildings in the US is between 20% and 40% of US primary energy
demand and approximately half of this portion is actually spent on heating or cooling
buildings [1]. In order to cut down the energy consumption of air conditioning, incorporation
of intelligent architectural windows is considered as a green solution to improve building
efficiency. Thermochromic smart windows can modulate solar thermal energy through
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adaptive response to weather conditions and vanadium dioxide (VO2) is one of leading
materials that could be used in the applications of thermochromic smart windows [2–4]. VO2
has an excellent temperature-responsive behavior of switching near-infrared (NIR)
transmittance upon a critical transition temperature (τc) of 341 K (68 °C) near room
temperature, whereas maintaining luminous transmittance (Tlum) from 380 nm to 780 nm [5,
6]. The practical architectural windows requires Tlum larger than 60% [7, 8] while
concurrently maintaining moderately high solar modulation (ΔTsol). A dilemma to
simultaneously improve both Tlum and ΔTsol of inorganic VO2 films has been tackled by a
range of approaches via simulation as shown in Table 1 and the efficacy of such simulations
have been proved experimentally [9–13].
Table 1. A comparison of simulated thermochromic performances presented by
contemporary VO2 smart window systems
Porosity
Simulation
ΔTsol
Tlum(low)
Tlum(high)
Remark
6%
81%
-
15 nm thickness with 35%
porosity [12]
Nanocomposite
(embedded in
dielectric matrix)
Simulation
10%
82%
77.5%
Dilute spheroidal nanoparticles
with filling factor of 0.01 in a
dielectric matrix (dielectric
constant εm = 2.25) [14]
Biomimic motheye nanostructures
Simulation
15.5%
70%
67%
500 nm tall 135 nm base-width
nanostructures coated with 7.5
nm of VO2 [15]
Ref.: continuous
films
Simulation
13.5%
33.5%
26%
100 nm thin solid films [16]
Introducing pores into VO2 films [12] reduces the optical constants, thereby overcoming
the physical limits of continuous films [14]. By imbedding VO2 nanoparticles in dielectric
matrix [10, 11, 14, 17] large luminous absorption of VO2 at both high and low temperature in
the continuous film can be alleviated to enhance the Tlum. Biomimetic surface [13, 15] with
high aspect ratio paraboloid cones provides antireflection functionality leading to largely
improved Tlum while larger volume of VO2 contained in such structure offered enhanced ΔTsol.
However, the process of synthesizing good crystalline VO2 nanoparticles or achieving
extremely high aspect ratio nanostructures usually involves complex procedures, low
reproducibility and poor durability [11, 17, 18]. The more recent pure organic [5] and hybrid
thermochromic coatings [19] enjoyed unprecedented combination of high Tlum and ΔTsol,
however the loss of transparency at high temperature is not tolerated in many applications.
To maximize Tlum without the expense of ΔTsol, this study pioneers a novel smart window
design using nano-patterned VO2-based perforated films. Since ΔTsol of VO2 films is largely
contributed by the NIR modulation (ΔTNIR) instead of luminous modulation (ΔTlum) [2, 20],
nanogrid structures physically allow more short-wavelength visible light to pass through the
holes, while retaining the advantageous long-wavelength NIR modulating abilities of VO2.
Meanwhile, the abundant ridge area is capable of holding a high volume of VO2, which could
possibly intend to facilitate strong ΔTsol. The dimension and shape of nanogrid design, as well
as cell periodicity and film thickness were investigated by using Finite Difference Time
Domain (FDTD) simulations. The calculated data demonstrate that nano-patterned films offer
significant improvement of Tlum without deteriorating ΔTsol, which open up a new approach to
enhance thermochromic properties for smart windows in the real-life applications.
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2. Optical design for VO2-based nanogrid films
2.1 Structural models
In order for the perforated nanogrid not to diffract visible light or be resolved by incident
light, the cell periodicity (see Fig. 1), which is defined as the distance between two adjacent
holes, should be smaller than the wavelength of the visible light in the medium [21, 22]. The
coating thickness should not be too large to degrade window transmittance due to light
absorption [15]. Three structural models used in current optical simulation work are
schematically illustrated in Fig. 1, namely square cells with square holes (Fig. 1(a)) [21, 23],
square cells with circular holes (Fig. 1(b)) [24–26] and hexagonal cells with circular holes
(the most densely packed) (Fig. 1(c)) [15, 27, 28].
Fig. 1. Three different 3-dimensional models of VO2-based perforated thin films, (a) square
holes in square lattices, (b) circular holes in square lattices and (c) circular holes in hexagonal
lattices are densely packed with defined periodicity and hole length/radius as labeled.
2.2 FDTD simulation
We performed 3D finite difference time domain (FDTD) simulations using commercial
software (FDTD solution, Lumerical Inc. Vancouver, Canada). A uniform mesh size of 3 nm
(x, y, and z directions) was chosen. The optical constants of materials were taken in the
spectrum range from 300 nm to 2500 nm [29]. The dielectric dispersion profiles of the
materials were fitted by the multi-coefficient model that relies on an extensive set of basis
functions. To calculate the frequency dependent transmittance, PML (perfectly matched
layer) boundary conditions were set for the z direction, and Bloch boundary conditions were
applied to x and y directions of the simulation region. The VO2 nanogrid films were
supported by a glass substrate, the entire system was suspended in air/vacuum, and the
incident beam was modeled as a plane wave traveling along the z-axis. Field monitors were
placed at fixed z position below the surface of the VO2 nanogrid film to detect the intensity of
the transmitted beam.
2.3 Optical calculations of thermochromic properties
In order to quantify the amount of visible light useful for human vision under normal
conditions, a photopically averaged transmittance has been defined as the integrated luminous
transmittance Tlum. Similarly, the terms named as “integrated NIR transmittance TNIR” and
“integrated solar transmittance Tsol” are to define the NIR light and solar averaged
transmittance respectively, as to quantify the amount of NIR or solar thermal energy entering
a building. Upon these determinations, the modulations of transmitted luminous, NIR and
solar energy are characterized as ΔTlum, ΔTNIR and ∆Tsol and they can be integrated by using
the mathematical relationships stated below as Eq. (1) and Eq. (2).
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780 nm
Tlum

= λ
φ T (λ ) d λ
= 380 nm lum
780 nm
λ
φ dλ
2500 nm
TNIR
= 380 nm lum

= λ
AM 1.5 (λ )T (λ )d λ
= 780 nm
2500 nm
λ
= 780 nm
AM 1.5 (λ )d λ
2500 nm
Tsol

= λ
AM 1.5 (λ )T (λ )d λ
= 300 nm
2500 nm
λ
= 300 nm
AM 1.5 (λ )d λ
(1)
ΔTlum = Tlum (low) − Tlum ( high ) ΔTNIR = TNIR (low) − TNIR ( high ) ΔTsol = Tsol (low) − Tsol( high )
(2)
Where T(λ) represents the measured film transmittance. The CIE (represents
“Commission Internationale de I’eclairage”) photopic luminous efðciency of the human eye
[30], Φlum, and the air mass (AM) 1.5 solar irradiance spectrum distribution [31] are used as
weighting functions for the wavelength dependent transmittance coefðcients. The dimming
effect of a smart window between cold and hot state is quantified as ∆Tlum. The wavelength
range used for Tlum is 380 nm - 780 nm corresponding to the limits of human vision. The
AM1.5 weighting spectrum is chosen for TNIR and Tsol calculations as it represents an overall
yearly average for mid-latitudes including diffuse light from the ground and sky on a south
facing surface tilted 37° from horizontal. The wavelength range used for these calculations is
300 nm - 2500 nm which accounts for 99.2% of terrestrial solar energy.
3. Computed optical data and discussion
3.1 Square holes in square cells
Fig. 2. Thermochroimc properteis for square holes perforated in VO2 square cells at different
values of periodicity and square hole length. (a) Contour lines (red dashed) of ΔTsol are
overlaid upon a coutour map of Tlum, (b) thermochomic properties of selected samples in table
form.
The contour (see Fig. 2(a)) suggests that with fixed periodicity, larger holes lead to increased
Tlum and depressed ΔTsol, which is consistent with other observations [10, 20]. The table in
Fig. 2(b) shows three selected samples and it suggested that Tlum could be enhanced without
much sacrifice on ΔTsol compared with continuous thin films. For unknown reason, the
sample with the period size of 180 nm and hole size of 80 nm as highlighted gives both
enhanced Tlum(avg) (29.0 v.s. 26.0%) and ΔTsol (14.7 v.s 11.1%) compared with continuous
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films. This proves the efficiencies of nanogrid VO2 nanostructures, which has the great
potential to enhance both Tlum and ΔTsol.
3.2 Structure design for nanogrid
Different nanostructures may induce different optical performance due to light interaction
with different shapes of holes and cells. To exploit the shape and size effects, the nanogrid
films are fixed at 110 nm thickness and 180 nm period. The computational results for each
system are schematically displayed in Figs. 3(a)-3(d) respectively.
Fig. 3. Relationship between nanogrid parameters and thermochromic properties in terms of (a)
square holes (length varied from 30 nm to 150 nm) in square cells; (b) circular holes (radius
varied from 20 nm to 80 nm) in square cells; (c) circular holes (radius varied from 20 nm to 80
nm) in hexagonal cells; (d) four different structural systems with similar fill factor; (e) solar
transmittance and (f) light modulation ability as a function of fill factor ( =
A ( hole )
A( cell )
) of
perforated films.
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According to Fig. 3(a), by inserting square holes into square cells with increasing hole
length, the Tlum at both low and high temperature (black and red bar) are increased
dramatically when the fill factor (FF) reaches to ~70% at the sacrifice of the ΔTsol (blue bar).
Tlum(low) elevates dramatically from hole length of 30 nm to hole length of 150 nm, whereas
the value is surpassed by Tlum(high) once hole length is greater than 90 nm, leading to a
undesirable spectra overlap in the visible range (i.e., negative ∆Tlum). Since the shortwavelength visible light carries half of energy in the entire solar spectra [19], the negative
∆Tlum induces an observable degradation in ∆Tsol, which is undesirable for a good
thermochromic performance.
The large increments of Tlum with increasing hole size also occur in the circular hole
design systems either in square cells or in hexagonal cells as illustrated in Figs. 3(b) and 3(c).
By comparing with square holes, the circular one reveals better thermochromic behaviors,
because Tlum is doubled without jeopardizing ΔTsol appreciably. The highest averaged Tlum has
been enhanced to ~63%, together with securely maintained ΔTsol at 9.22%, when 80 nm
radius circular holes arrange in 180 nm period hexagonal cells.
By summarizing the relationship between FF and the transmission, it suggested that the
Tlum, TNIR and Tsol at both low and high temperature are dominantly controlled by FF instead
of the hole size or shape. This gives a general guideline for enhancing Tlum. In contrast,
increasing the FF has much more complicated effects on the transmission difference between
high and low temperatures as shown in Fig. 3(f), which is to the advantage of the possibility
to enhance both Tlum and ΔTsol simultaneously via increasing the FF. It is of interest to observe
that the ΔTNIR, ΔTsol and ΔTsol relate with the FF in a parallel way.
3.3 Circular holes in hexagonal cells
3.3.1 Thickness effect
As Tlum could be largely enhanced by increasing the FF, circular holes in hexagonal cells are
chosen for further study due to the closest packed characteristics of such structure. In order to
achieve ΔTsol much higher than that of continuous VO2 thin film, thickening the perforated
nano-patterned film could be a feasible solution to raise VO2 content and compensate the
slightly sacrificed ΔTsol when the FF is increased (Fig. 3(c)). With fixed periodicity and hole
size, ΔTsol and Tlum were studied by increasing film thickness. The highest ΔTsol achievable is
16.7% if the film thickness is as thick as 400 nm (see Fig. 4(a)). Tlum keeps decreasing as the
thickness increases. It is of great interest that as compared to continuous thin films (see Fig.
4(b)), the turning point (defined by the point where thickening loses its effect on increasing
ΔTsol) of the perforated sample can be much thicker (400 nm) than that of the continuous film
(160 nm).
Fig. 4. Computed (a) thickness effect on ΔTsol (black) and Tlum(avg) (blue) for nanogrid films
with period = 180 nm and hole radius = 80 nm, (b) thickness effect on continuous thin films
(dashed black) and perforated films (solid red).
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3.3.2 Periodicity and fill factor
The influences of periodicity and FF on the optical behaviors are presented in Fig. 5(a) with
fixed 120 nm hole radius and 300 nm film thickness. An obvious blue shift of the
transmission peaks could be observed with reducing periodicity and increasing FF;
meanwhile both transmission at high and low temperature increases dramatically. This is
indicated in the Fig. 5(b) and transmission at both visible and NIR ranges could be enhanced
significantly. Compared with continuous film, Tlum increases from 4.3 to 12.6% when the FF
increases to 0.666. As the hole is touching the edge of cell with corresponding FF of 0.907,
Tlum can be abruptly enhanced to nearly three times as compared with the sample where the
FF is 0.773. Similar trends can be observed in both NIR transmission and solar transmission
which suggests that with touching holes, the whole solar transmission could be boosted
enormously. The periodicity effects on modulation (Fig. 5(c)) are more complicated.
ΔTlum/ΔTNIR can be increased from 0.3%/18.8% (continuous film) to 5.0%/24.3% (the
touching holes sample with corresponding largest FF). This, to large extent, favors the largely
enhanced ΔTsol (8.6 v.s 14.0%) compared with continuous films.
Fig. 5. (a) Computed spectra of transmittance for perforated VO2 films with different
periodicity and FF as compared to continuous thin films. (All film thickness = 300 nm) (b)
solar transmittance and (c) light modulation ability as a function of periodicity.
3.4 Summary of parameter optimizations
The thermochromic properties of best performing samples are listed in Table 2. As hexagonal
cells/circular holes provide the largest possible FF, good combinations of high transparency
(> 70%) with moderately high ΔTsol (>10%) could be obtained with touching holes design.
This finding surpasses the best reported simulation results such as nanoporous thin film and
nanothermochromic coatings (Table 1). Although the ΔTsol is slightly lower (14.0%
v.s.15.5%) than the moth-eye nanostructuring, the averaged Tlum is higher (76.5% v.s 68.5%).
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It should be noted that such biomimetic moth-eye structure with 500 nm height and 135 nm
base-width nanostructures is of great technical challenge to achieve in practice.
Table 2. Typical thermochromic properties for the samples with optimized nanopatterning parameters
Hexagonal cells +
Circular holes
Tlum (20 °C)
Tlum (100 °C)
ΔTlum
TNIR (20 °C)
TNIR (100 °C)
ΔTNIR
Tsol (20 °C)
Tsol (100 °C)
ΔTsol
Thickness d = 300 nm
Periodicity = 160 nm
Periodicity = 240 nm
radius = 80 nm
radius = 120 nm
FF: 0.907
FF: 0.907
78.8
75.7
3.1
89.9
64.7
25.2
80.3
67.2
13.0
79.2
74.2
5.0
90.8
66.4
24.3
80.5
66.6
14.0
Finally it should be mentioned that diffraction effects from the periodic structures were
considered and found to be negligible. For window applications it is essential that the light
transmittance will be direct, i.e. the window should not be diffractive. In order to evaluate
possible diffraction effects in VO2 perforated film, the transmitted diffraction efficiencies at
different orders were calculated using Rigorous Coupled Wave Analysis (RCWA) [22]. In the
simulation, the circular holes were replaced with rectangular holes of the same surface area to
reduce computation time. The calculations were performed for the optimum structure with the
largest period. The diffraction depends on the ratio of the period to the wavelength and hence
the highest effect is supposed to occur in the largest period and the shortest wavelengths.
Therefore, the sample tested was with the period size of 240 nm, square hole size 212 nm, and
thickness 300 nm. The transmission contribution is found mainly due to the zeroth order
whereas the higher diffraction order efficiencies are up to 5% in the 500-600 nm range while
it is less than 0.5% throughout the wavelength range 550-2500 nm.
4. Conclusion
Numerical optimizations using parameter search algorithms were implemented through a
series of FDTD simulations with taking account of film thickness, geometrical design of
nano-holes and cells as well as periodicity and FF. This study presents a novel approach
towards achieving high Tlum for thermochromic smart window applications via VO2-based
nanogrid perforated films whilst maintaining moderate ΔTsol as required for smart
architectural window applications. Increasing the FF could tremendously enhance Tlum and the
largest possible Tlum (~80%) could be acquired via touching holes configuration in hexagonal
cells with circular hole design. Thickness effects on enhancing ΔTsol is limited to less than
200 nm in continuous films, whereas it is extended up to 400 nm by adopting periodic
perforating structure. This opens up a new concept for thermochromic VO2-based films to
satisfy the real-life requirements for smart window applications.
Acknowledgments
This Research was conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water
Management Programme under the Campus for Research Excellence and Technological
Enterprise (CREATE), that is supported by the National Research Foundation, Prime
Minister’s Office, Singapore.
#228622 - $15.00 USD
(C) 2015 OSA
Received 26 Nov 2014; revised 19 Jan 2015; accepted 19 Jan 2015; published 28 Jan 2015
9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.00A124 | OPTICS EXPRESS A132