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Energy Express

  • Editor: Christian Seassal
  • Vol. 21, Iss. S5 — Sep. 9, 2013
  • pp: A750–A764
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A bioinspired solution for spectrally selective thermochromic VO2 coated intelligent glazing

Alaric Taylor, Ivan Parkin, Nuruzzaman Noor, Clemens Tummeltshammer, Mark S Brown, and Ioannis Papakonstantinou  »View Author Affiliations


Optics Express, Vol. 21, Issue S5, pp. A750-A764 (2013)
http://dx.doi.org/10.1364/OE.21.00A750


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Abstract

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO2) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications. Our method deviates from conventional approaches and utilizes subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarization insensitive and wide-angle antireflection properties. The moth-eye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean. We develop a set of design rules for the moth-eye surface nanostructures and, following this, numerically optimize their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations. We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8. Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmittance of up to 70.3%. The performance metrics of the windows presented in this paper are the highest calculated for VO2 based smart-windows.

© 2013 OSA

1. Introduction

A large proportion of global energy is spent on either heating or cooling domestic and industrial buildings. Studies have estimated this energy usage to be anywhere between 30 and 40% of our primary energy spend [1

1. United Nations Environment Programme, Buildings and climate change - status, challenges and opportunities (UNEP, 2007).

]. Pressure to improve building efficiency has increased as the environmental impacts of unwarranted energy consumption have become unacceptable and the economic incentives to improve efficiency become increasingly attractive. Incorporation into architectural designs of windows that are spectrally selective and switch their transmittance characteristics depending upon the ambient temperature has generated significant interest. The role of these windows is to modulate the solar thermal energy entering a building by switching between a hot or cold state. These are referred to as either intelligent- or smart-windows. In its cold state a smart-window is expected to transmit a large portion of the visible and thermal infrared spectrum whilst in its hot state transmittance of infrared energy should be decreased.

A new class of smart-window using moth-eye nanostructures

The super-hydrophobicity induced by the aspect ratio of the moth-eye surface structures and the intrinsic hydrophobicity of VO2[8

8. T. D. Manning, I. P. Parkin, R. J. H. Clark, D. Sheel, M. E. Pemble, and D. Vernadou, “Intelligent window coatings: atmospheric pressure chemical vapour deposition of vanadium oxides,” J. Mater. Chem. 12, 2936–2939 (2002) [CrossRef] .

, 9

9. C. Piccirillo, R. Binions, and I. P. Parkin, “Synthesis and functional properties of vanadium oxides: V2O3, VO2, and V2O5 deposited on glass by aerosol-assisted CVD,” Chem. Vap. Deposition 13, 145–151 (2007) [CrossRef] .

] may lend an additional self-cleaning mechanism to the smart-window, without any additional treatment. This negates the requirement for chemical or physical cleaning and therefore reduces building maintenance costs.

Densely packed, tapered cones form a highly efficient moth-eye optical impedance matching antireflection layer that is broadband, polarization insensitive and wide-angle. [3

3. K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.” ACS Nano 6, 3789–99 (2012) [CrossRef] [PubMed] .

] Antireflection structures of this type can be found on a variety of naturally occurring surfaces including the eyeballs of moths [10

10. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the ’moth eye’ principle,” Nature 244, 281–282 (1973) [CrossRef] .

] and wings of cicada and butterflies [11

11. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

]. Under the quasistatic condition, that the pitch of the structures is smaller than the wavelength of incident light, the tapered cones are not resolved and therefore appear to the incident radiation as an effective medium with a gradually varying refractive index [11

11. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

, 12

12. W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” JOSA A 8, 549–553 (1991) [CrossRef] .

]. Reflectance can be suppressed by more than 99% over a broad range of wavelengths provided that the height of the cones is more than 0.4λ[13

13. S. J. Wilson and M. C. Hutley, “The optical properties of ’moth eye’ antireflection surfaces,” Optica Acta 29, 993–1009 (1982) [CrossRef] .

]. We show that by coating a thin layer of VO2 over a moth-eye nano-structured window surface the undesirable planar Fresnel reflections arising from the high refractive index of VO2 are reduced and the window’s transmittance characteristics significantly improved.

We begin in section 2 by defining a consistent set of metrics used to benchmark each smart-window system and parameter set. Following this, in section 3, we discuss the optical properties of VO2 and the challenges these pose to smart-window implementation. We dedicate section 4 to contemporary solutions that have been advanced to combat the challenges of VO2 based smart-windows. For consistency and benchmarking purposes we make independent calculations of the smart-window metrics for a selection of these contemporary designs. Directly after this, in section 5, we examine the key geometrical design parameters for our moth-eye class smart-window and identify a domain for further investigation. Our FDTD simulations are presented and discussed in sections 5.7 and 6, we conclude in section 7.

2. Smart-window metrics

The metrics used to describe the transmittance characteristics for smart-windows vary throughout the literature. In some cases transmittance and its cold to hot state switching-depth are quoted for only a single wavelength [14

14. C. S. Blackman, C. Piccirillo, R. Binions, and I. P. Parkin, “Atmospheric pressure chemical vapour deposition of thermochromic tungsten doped vanadium dioxide thin films for use in architectural glazing,” Thin Solid Films 517, 4565–4570 (2009) [CrossRef] .

19

19. M. Saeli, R. Binions, C. Piccirillo, and I. P. Parkin, “Templated growth of smart coatings: hybrid chemical vapour deposition of vanadyl acetylacetonate with tetraoctyl ammonium bromide,” Applied Surface Science 255, 7291–7295 (2009) [CrossRef] .

] whereas in others [5

5. M. Tazawa, K. Yoshimura, P. Jin, and G. Xu, “Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings,” Appl. Phys. A Mater. Sci. Process. 77, 455–459 (2003) [CrossRef] .

, 7

7. G. Xu, P. Jin, M. Tazawa, and K. Yoshimura, “Optimization of antireflection coating for VO2-based energy efficient window,” Sol. Energ. Mat. Sol. Cells 83, 29–37 (2004) [CrossRef] .

, 20

20. Z. Zhang, Y. Gao, H. Luo, L. Kang, Z. Chen, J. Du, M. Kanehira, Y. Zhang, and Z. L. Wang, “Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications,” Energy & Environmental Science 4, 4290 (2011).

22

22. C. G. Granqvist, “Transparent conductors as solar energy materials: a panoramic review,” Sol. Energ. Mat. Sol. Cells 91, 1529–1598 (2007) [CrossRef] .

] the transmittance spectra are weighted by a secondary function and then integrated to produce a single broadband metric. We favor the latter weighted mean approach as it gives a more complete description of the energy transmittance characteristics for a smart-window. Our motivation to explicitly define a set of metrics arises from the need for direct comparison between alternative designs and our parameter sets.

In order to quantify the amount of visible light useful for human vision under normal conditions we define in Eq. 1 the photopically averaged transmittance Tlum. Similarly, in order to quantify the amount of solar thermal energy entering a building we define the solar averaged transmittance Tsol. Building upon these we quantify the modulation of transmitted visible and solar energy in Eq. 2 as ΔTlum and ΔTsol.
Tlumσ=λ=380nm780nmy¯(λ)Tσ(λ)dλλ=380nm780nmy¯(λ)dλTsolσ=λ=300nm2500nmAM1.5(λ)Tσ(λ)dλλ=300nm2500nmAM1.5(λ)dλ
(1)
ΔTlum=TlumcoldTlumhotΔTsol=TsolcoldTsolhot
(2)

In Eq. 1 σ represents the temperature state of the window (i.e. hot or cold state). The CIE photopic luminous efficiency of the human eye [23

23. T. Smith and J. Guild, “The C.I.E. colorimetric standards and their use,” Trans. of the Opt. Soc. 22, 73 (1931) [CrossRef] .

], ȳ(λ), and the AM1.5 solar irradiance spectrum [24

24. American Society for Testing and Materials, “ASTM G173-03 reference spectra,” (2013), http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html.

] are used as weighting functions for the wavelength dependent transmittance coefficients, see Fig. 1. The wavelength range used for our photopically averaged calculations was 380 nm ≤ λ ≤ 780 nm corresponding to the limits of human vision. The AM1.5 weighting spectrum was chosen for our solar averaged transmittance 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 was 300 nm ≤ λ ≤ 2500 nm which accounts for 99.2% of terrestrial solar energy.

Fig. 1 Spectra used to weight the transmittance functions in Eqs. 1. The photopic luminous efficiency of the human eye [23], ȳ(λ), and the AM1.5 solar irradiance spectrum [24].

The denominators in Eq. 1 normalize Tlum and Tsol which subsequently take values between zero and unity corresponding to no transmittance and full transmittance respectively. Large positive ΔTsol (Eq. 2) facilitates strong room temperature stabilization and is desirable for smart-window applications. The window’s hot state dimming is quantified as ΔTlum. Although dimming is not a requirement, it may be desirable in some applications. We are primarily concerned with transmittance ; however, equivalent metrics for reflectance and absorbance are reached through the substitution of Tσ with either Rσ or Aσ in Eq. 1 respectively.

2.1. Color calculations

We use the Commission Internationale de l’Eclairage’s (CIE) 1931 standard observer color matching functions [23

23. T. Smith and J. Guild, “The C.I.E. colorimetric standards and their use,” Trans. of the Opt. Soc. 22, 73 (1931) [CrossRef] .

] to obtain the window’s tint based upon its transmittance spectrum (TRGB). This provides a qualitative description of transmittance and is helpful for understanding a smart-window’s aesthetic properties. Within our colorimetric calculations the CIE Standard Illuminant D65 defines the white point as a representative daylight illuminant against which the transmittance spectra are normalized [26

26. N. Ohta and A. R. Robertson, CIE standard colorimetric system in colorimetry: fundamentals and applications (John Wiley & Sons, Ltd, Chichester, UK, 2006).

]. Displayable iRGB color hex-strings are extracted from the XYZ tristimulus values using standard functions. Our calculations of the color code assume display chromacities and physical gamma components as defined in the sRGB standard.

3. Thermochromic VO2

VO2 has invited strong interest as a potential thermochromic material for use in smart-window applications as its optical properties change as a function of temperature which facilitates a self-stabilizing smart-window with no requirement for control circuitry. The phase change from semiconductor (monoclinic structure at low-temperatures) to metallic (tetragonal rutile structure at high-temperatures) is abrupt, fully reversible and passive; altering the infra-red transmittance from transparent to blocking, leaving visible transmittance largely unchanged [27

27. H. W. Verleur, J. A. S. Barker, and C. N. Berglund, “Optical Properties of VO2 between 0.25 and 5 eV,” Phys. Rev. 172, 172 (1968) [CrossRef] .

].

The persistent challenge for VO2 based smart-windows has been to simultaneously maximize the transmittance of visible light (Tlum, Eq. 1) and the window’s solar energy modulation (ΔTsol, Eq. 2). VO2 has a high refractive index in the visible region (2.25 < n < 2.75, Fig. 2). For planar VO2 films the refractive index discontinuity between air-VO2 and VO2-glass leads to high Fresnel reflection and consequently a low Tlum. The window’s solar energy modulation, ΔTsol, is influenced by two competing processes. In its hot state VO2 has a significantly increased infra-red extinction coefficient (k, see Fig. 2) resulting in both strong reflectance and absorptance, thus providing potential for good solar modulation. However, the high cold state refractive index reduces Tsolcold as a result of high Fresnel reflections and thus ΔTsol is limited. Discontinuities at the interfaces of a planar VO2 thin-film form a resonant cavity. A careful selection of the film thickness can lead to resonances that improve Tlum and ΔTsol for specific angles of incidence. However, absorptance penalties for increased thickness remain a problem.

Fig. 2 The optical model for VO2 as used in our FDTD simulations.

In order to illustrate these trade-offs we simulated glass with planar VO2 coatings using an FDTD method. The model used for these planar simulations was a modification upon the method presented in section 5.7. As we see in Fig. 3 the highest achieved solar modulation (ΔTsol ∼ 15%) comes with a penalty of low visible transmittance (Tlum ∼ 20%). Whereas, for more acceptable visible transmittance (Tlum > 60%) the solar energy modulation is low (ΔTsol < 10%). Our simulations were in agreement with measurements of 90 nm [6

6. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

] and 100 nm [28

28. H. Kakiuchida, P. Jin, and M. Tazawa, “Control of thermochromic spectrum in vanadium dioxide by amorphous silicon suboxide layer,” Sol. Energ. Mat. Sol. Cells 92, 1279–1284 (2008) [CrossRef] .

] planar VO2 films which showed an increase in film thickness was associated with a drop in visible transmittance ( Tlumcold) from 38.2% to 32.0% and an increase in solar energy modulation (ΔTsol) from 6.6% to 7.4%. The color bars along the top of Fig. 3 show the difference between the hot and cold state visible transmittance (dimming) to be low. It is clear that the transmittance color, TRGB, becomes unacceptably dark for VO2 coatings with thicknesses in the order of 100 nm and above.

Fig. 3 FDTD simulations of planar VO2 window systems with different thin-film thicknesses showing the associated changes in Tlumcold and ΔTsol. The transmittance color in both the hot and cold phases are shown in the upper bars.

4. Contemporary VO2 smart-window systems

There are several distinct approaches that have previously been adopted towards improving the performance characteristics of VO2 based smart-windows. The two most common are intermediary dielectric antireflection layers and band-gap widening via doping with metal ions.

4.1. Dielectric antireflection layers

Multilayered antireflection coatings have been investigated extensively with regards to VO2 smart-windows. In principle they operate by creating resonant cavities in which constructive and destructive interference bands are tuned to enhance Tlum and ΔTsol. These type of antireflection layers are typically formed from titanium dioxide [5

5. M. Tazawa, K. Yoshimura, P. Jin, and G. Xu, “Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings,” Appl. Phys. A Mater. Sci. Process. 77, 455–459 (2003) [CrossRef] .

, 29

29. Z. Chen, Y. Gao, L. Kang, J. Du, Z. Zhang, H. Luo, H. Miao, and G. Tan, “VO2-based double-layered films for smart windows: optical design, all-solution preparation and improved properties,” Sol. Energ. Mat. Sol. Cells 95, 2677–2684 (2011) [CrossRef] .

]. However, zirconium dioxide [7

7. G. Xu, P. Jin, M. Tazawa, and K. Yoshimura, “Optimization of antireflection coating for VO2-based energy efficient window,” Sol. Energ. Mat. Sol. Cells 83, 29–37 (2004) [CrossRef] .

], zinc oxide [30

30. K. Kato, P. K. Song, H. Odaka, and Y. Shigesato, “Study on thermochromic VO2 films grown on ZnO-coated glass substrates for “smart windows” Jpn. J. Appl. Phys. 42, 6523–6531 (2003) [CrossRef] .

] and silicon oxides [29

29. Z. Chen, Y. Gao, L. Kang, J. Du, Z. Zhang, H. Luo, H. Miao, and G. Tan, “VO2-based double-layered films for smart windows: optical design, all-solution preparation and improved properties,” Sol. Energ. Mat. Sol. Cells 95, 2677–2684 (2011) [CrossRef] .

] have also been used. Early examples of antireflection coatings for VO2 smart-window coatings utilized just two layers [31

31. P. Jin, G. Xu, M. Tazawa, and K. Yoshimura, “A VO2-based multifunctional window with highly improved luminous transmittance,” Jpn. J. Appl. Phys. 41, L278–L280 (2002) [CrossRef] .

], whereas the most recent and highest performing multilayer antireflection systems incorporate five layers [6

6. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

]. The dielectric antireflection layer approach is ultimately limited by the cavity process it relies upon as the resonant conditions are narrow-band. Furthermore, these systems are inherently sensitive to the angle of incidence and polarization of light. The incorporation of additional dielectric antireflection layers can improve the window performance, however, this is at the expense of fabrication cost and complexity [6

6. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

].

Within our review of antireflective multilayer VO2 smart-windows (see Table 1) we found no examples of systems with Tlumcold>52% or ΔTsol > 12%. These champion values were not exhibited in the same system. The most impressive performance characteristics were found in a five-layer TiO2/VO2/TiO2/VO2/TiO2 system [6

6. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

] which, based upon our calculations, had a high solar modulation ΔTsol = 11.79% and visible transmittance of Tlumcold=45.32%. Visible transmittance for almost all of the multilayer systems was too low for practical fenestration and the solar modulation not high enough to induce energy saving that would attract widespread investment in this type of smart-window.

Table 1. A comparison of smart-window transmittance metrics for a variety of VO2 based multilayer smart-window systems. Eqs. 1 and 2 are used to calculate the reported metrics using broadband transmittance data extracted from the cited publications where appropriate. All dimensions are in nanometers, Tsol, Tlum and ΔTsol are all presented as a percentage of energy transmittance.

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4.2. Doping

It has been shown that doping with either magnesium or fluorine [32

32. W. Burkhardt, T. Christmann, B. Meyer, W. Niessner, D. Schalch, and A. Scharmann, “W- and F-doped VO2 films studied by photoelectron spectrometry,” Thin Solid Films 345, 229–235 (1999) [CrossRef] .

] can lead to an increase in the visible transmittance of a VO2 thin film. Fluorine doping is not favorable for smart-window applications as it is known to widen the phase-transition hysteresis and can increase the metallic-phase transmittance and thus decrease the solar energy modulation [32

32. W. Burkhardt, T. Christmann, B. Meyer, W. Niessner, D. Schalch, and A. Scharmann, “W- and F-doped VO2 films studied by photoelectron spectrometry,” Thin Solid Films 345, 229–235 (1999) [CrossRef] .

]. On the other hand, magnesium doping is of interest as it increases Tlum by widening the fundamental band gap in the VO2 film [33

33. S.-Y. Li, G. Niklasson, and C. Granqvist, “Thermochromic fenestration with VO2-based materials: three challenges and how they can be met,” Thin Solid Films 520, 3823–3828 (2012) [CrossRef] .

] and can improve the transmittance color. Magnesium doping of a 50 nm thick VO2 film [34

34. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature,” Appl. Phys. Lett. 95, 171909 (2009) [CrossRef] .

] was shown to result in a thin film with Tlumcold=50.9% and ΔTsol = 3.7%. Double-doping using both tungsten and titanium has also been investigated [35

35. I. Takahash, M. Hibino, and T. Kudo, “Thermochromic properties of double-doped VO2 thin films prepared by a wet coating method using polyvanadate-based sols containing W and Mo or W and Ti,” Jpn. J. Appl. Phys. 40, 1391–1395 (2001) [CrossRef] .

]. Based upon spectra extracted from [35

35. I. Takahash, M. Hibino, and T. Kudo, “Thermochromic properties of double-doped VO2 thin films prepared by a wet coating method using polyvanadate-based sols containing W and Mo or W and Ti,” Jpn. J. Appl. Phys. 40, 1391–1395 (2001) [CrossRef] .

] our calculations showed a 100 nm double-doped thin film to have reasonable solar energy modulation ΔTsol = 7.2% but poor visible transmittance Tlumcold=27.7%.

5. Moth-eye smart-window optical design parameters

Fig. 4 Side (a) and top (b) elevations of a nanotextured surface with hexagonally arranged circular paraboloid cones. Pitch P, base-width W and VO2 coating thickness C. As in (b), when W=P432C both the peak-to-trough height difference and the areal density at the air-VO2 interface are maximized.

5.1. Pitch (P)

In order for the array of surface structures not to be resolved by incident light the separation of cones, P, must satisfy the following inequality derived from the grating equation where we require a suppression of diffracted orders within the material [13

13. S. J. Wilson and M. C. Hutley, “The optical properties of ’moth eye’ antireflection surfaces,” Optica Acta 29, 993–1009 (1982) [CrossRef] .

, 36

36. C. Aydin, A. Zaslavsky, G. J. Sonek, and J. Goldstein, “Reduction of reflection losses in ZnGeP2 using motheye antireflection surface relief structures,” Appl. Phys. Lett. 80, 2242 (2002) [CrossRef] .

],
Pλminn1+n0sin(θi)
(3)
where n0 is the refractive index of air, θi is the angle light entering the structure makes with the surface normal and λmin is the shortest wavelength for which this inequality is satisfied.

In Table 2 we present calculations that relate the nanostructure pitch to the angle of incidence and portion of visible light for which Eq. 3 is satisfied. In these calculations we assign n1 as the maximum refractive index within the specified wavelength interval for VO2 in both its metallic and semiconductor states. Based upon these calculations we identified a suitable range of pitches for VO2 coated moth-eye nanostructures to be between 115.1 and 183 nm. Nanostructure pitches of 150 nm satisfy the quasistatic inequality for a large portion of the visible spectrum up to incident angles of 20° In order to satisfy the inequality over a larger range of incident angles a smaller pitch is required. The cones are expected to be tall; therefore, a carefully considered choice of pitch is required in order to account for challenges in fabricating high aspect ratio nanostructures. For our work we chose P = 130 nm since it satisfies Eq. 3 for 99.9% of visible light up to angles of 25.9° and 99% of visible light up to 42.0°. This would indicate angle-invariant window characteristics for solid angles of at least 1.60sr.

Table 2. Calculations of the maximum pitch for moth-eye surface nanostructures that satisfy Eq. 3 over different wavelength intervals and angles of incidence. The moth-eye surface is assumed to be in air (n0 = 1). Values for n1 are calculated as the maximum refractive index for metallic and semiconductor VO2 within the wavelength range. All angles are in degrees, wavelengths and pitches are quoted in nanometers

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5.2. Surface distribution

There are three common systems under which moth-eye antireflection nanostructures are commonly arranged on a substrate’s surface: random [37

37. J. Hao, N. Lu, H. Xu, W. Wang, L. Gao, and L. Chi, “Langmuir-Blodgett monolayer masked chemical etching: an approach to broadband antireflective surfaces,” Chem. Mater. 21, 1802–1805 (2009) [CrossRef] .

], square [3

3. K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.” ACS Nano 6, 3789–99 (2012) [CrossRef] [PubMed] .

, 12

12. W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” JOSA A 8, 549–553 (1991) [CrossRef] .

, 13

13. S. J. Wilson and M. C. Hutley, “The optical properties of ’moth eye’ antireflection surfaces,” Optica Acta 29, 993–1009 (1982) [CrossRef] .

, 38

38. L. Yang, Q. Feng, B. Ng, X. Luo, and M. Hong, “Hybrid moth-eye structures for enhanced broadband antireflection characteristics,” Appl. Phys. Express 3, 102602 (2010) [CrossRef] .

, 39

39. O. Deparis, N. Khuzayim, A. Parker, and J. Vigneron, “Assessment of the antireflection property of moth wings by three-dimensional transfer-matrix optical simulations,” Phys. Rev. E 79, 1–7 (2009) [CrossRef] .

] and hexagonal arrays [2

2. H. Deniz, T. Khudiyev, F. Buyukserin, and M. Bayindir, “Room temperature large-area nanoimprinting for broadband biomimetic antireflection surfaces,” Appl. Phys. Lett. 99, 183107 (2011) [CrossRef] .

, 4

4. W.-L. Min, B. Jiang, and P. Jiang, “Bioinspired self-cleaning antireflection coatings,” Adv. Mater. 20, 3914–3918 (2008) [CrossRef] .

, 11

11. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

, 39

39. O. Deparis, N. Khuzayim, A. Parker, and J. Vigneron, “Assessment of the antireflection property of moth wings by three-dimensional transfer-matrix optical simulations,” Phys. Rev. E 79, 1–7 (2009) [CrossRef] .

, 40

40. C.-H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92, 061112 (2008) [CrossRef] .

]. Each surface distribution lends itself towards a different method of fabrication. Naturally occurring antireflection nanostructures are often found to have a local hexagonal arrangement. We investigated a hexagonal surface distribution as in conjunction with circular base nanostructures it permits very high areal densities.

5.3. Base-width (W)

The relationship between the base-width of the nanostructures, their distribution and pitch fully defines the areal density of the moth-eye surface. Smooth refractive index matching is achieved when the areal density is maximized, i.e. when planar discontinuities between the base of the tapered cones and the bulk substrate are minimized. If the base-widths are allowed to become larger than the pitch of the cones (i.e. if the structures bleed into each other) then the discontinuity in the refractive index profile can be eliminated. For a hexagonal array of circular base paraboloids the highest areal density that maintains the peak-to-trough height difference is achieved when W=P4/3. However, if one was to coat an array of paraboloids with this base-width the height difference between the peaks and troughs at the VO2-air interface would be less than that of the uncoated surface. In order to maintain both the highest areal density and peak-to-trough height difference for the VO2-air interface the SiO2 surface paraboloid base-widths (W) must hold the following relationship to their pitch (P) and VO2 coating thickness (C), see Fig. 4,
W=P432C
(4)

5.4. Feature profile

In order to perform smooth optical impedance matching the nanostructures must taper so that light traveling into the surface encounters gradually more high index material, this criteria is essential. Several studies have examined the relationship between the profile of uncoated surface protrusions and reflectance suppression [11

11. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

, 12

12. W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” JOSA A 8, 549–553 (1991) [CrossRef] .

], however, the optimum rate of taper for coated moth-eye nanostructures is not explicitly defined. The surface protrusions investigated in this research were circular-base paraboloids. This was a bioinspired choice mimicking the tapered cone and rounded cap commonly found in natural antireflection structures that give rise to an approximately linear refractive index gradient for uncoated nanostructures [11

11. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

].

5.5. Height (H)

The degree to which reflectance is suppressed by the tapered cone is strongly influenced by the height of the cone. Destructive interference occurs as small portions of the electromagnetic wave are reflected with an associated phase shift as it travels through the conical structure. It has been shown that reflectance can be reduced to below 0.25% when Hλ0.4[13

13. S. J. Wilson and M. C. Hutley, “The optical properties of ’moth eye’ antireflection surfaces,” Optica Acta 29, 993–1009 (1982) [CrossRef] .

]. Using this information we identified 250 and 1200 nm as a suitable range of heights for the nanostructures. Within this range the shortest cones are antireflective for wavelengths up to 625 nm (93.6% of visible light, but not infra-red radiation) whilst the longest cones are antireflective for wavelengths up to 3000 nm (99.3% of the AM1.5 solar spectrum).

5.6. VO2 coating thickness (C)

The moth-eye structures have a greatly increased surface area. The smart-window switching behavior is induced by the metal to semiconductor phase change of the VO2 covering the nanostructures A high volume of VO2 facilitates strong solar energy modulation, however, this is traded off against degraded window transmittance. It is important that the coating thickness is not so great that the graduated refractive index profile becomes too steep as this would counteract the operation of the moth-eye antireflection nanostructures. We investigated thin, conformal VO2 coatings with thicknesses between 5 and 50 nm.

5.7. FDTD simulations

Calculations were made using an FDTD method implemented through proprietary software (Lumerical). The hexagonal moth-eye smart-window surface was decomposed into rectangular simulation cells of area P by 3P with translational symmetry in the x and y dimensions (shaded region in Fig. 4(b)). Bloch boundary conditions were applied in both the x and y directions. The bandwidth of the source was chosen to include wavelengths between 300 and 2500 nm accounting for 99.2% of AM1.5 integrated solar irradiance. Two flux planes were placed one behind the source (in air) and the other below the moth-eyes (within the substrate). Data from these flux planes were used to calculate broadband reflectance, absorptance and transmittance coefficients for the surface; we extracted the smart-window metrics using Eqs. 1 and 2. Following convergence testing, the spatial resolution of each simulation was chosen to satisfy both wavelength and spatial frequency sampling conditions and was typically close to 1.5 nm.

Our fully dispersive multi-coefficient (Drude-Lorentz-Debye) material model for VO2 was constructed using refractive index data from a thin-film produced via reactive DC magnetron sputtering [6

6. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

], see Fig. 2.

6. FDTD analysis

As discussed in section 5 we conducted parameter sweeps for a hexagonal surface distribution of paraboloid of moth-eye structures with a constant pitch of 130 nm. Our primary parameters for optimization were the VO2 coating thickness which was varied between 5 and 50 nm and the surface paraboloid height which varied between 250 and 1200 nm (aspect ratios between 1.9 and 9.2). The base-widths of the surface paraboloids were varied according to Eq. 4.

We calculated the visible transmittance and solar energy modulation for each of the parameters sets - see Fig. 5. The volume of VO2 covering the surface structures is dependent upon both the coating thickness and height of the paraboloids. Our simulations showed that increases in either of these two parameters leads to a deterioration in the visible transmittance, Tlum, as absorptance becomes significant. Visible transmittance of Tlumcold=80% was achieved for 250 nm tall nanocones with an 8 nm VO2 coating. Tlumcold decreased to 50% if either the nanostructure height was increased to 1000 nm or the coating thickness was increased to 25 nm.

Fig. 5 FDTD parameter search for hexagonally arranged VO2 coated paraboloid nanostructures in which W=43P2C. Contours lines of Tlum are overlaid upon a heat-map of ΔTsol and vice-versa. Six parameter sets are chosen [A–F] (detailed in Table 3)

High volumes of VO2 increase the gradient of the effective refractive index and lead to undesirable levels of absorptance in the cold state, preventing strong solar thermal modulation. However, without a reasonable volume of VO2 transmittance switching between the hot and cold states is insignificant. An optimal band exists where the trade-off between the volume of VO2 the surface structures’ antireflective properties enable strong solar thermal transmittance modulation. For example, we calculated that ΔTsol > 23% could be achieved for 750 nm tall structures coated with 14 nm of VO2; this is unprecedented for a VO2 based smart-window.

Structures taller than 1000 nm offered little improvement, as by this point the antireflective impedance matching criteria has been satisfied for almost all of the solar spectrum. Structures with heights of 500 nm and below also performed well since they are antireflective for visible light and the most energy dense regions of the solar spectrum.

We used overlaid contour plots of Tlum and ΔTsol to select six parameter sets which we believe to be of interest. These are presented in Table 3 which includes the specific dimensions for each case. Case A (H = 1150 nm, C = 9 nm) is notable for its exceptionally high solar energy modulation, ΔTsol = 23.1%. However, its visible transmittance was low, Tlumcold=39.4%, and it had the highest aspect ratio high of 8.8. Cases B (H = 850 nm, C = 9 nm) and C (H = 700 nm, C = 8 nm) offered improved visible transmittance Tlumcold=49.5% and Tlumcold=59.9%, whilst maintaining strong solar energy modulation ΔTsol = 21.9% and ΔTsol = 19.4% respectively. Case D (H = 500 nm, C = 7.5 nm) offered very high visible transmittance Tlumcold=70.3% with relatively impressive solar modulation Tsolcold=15.5% and an aspect ratio of only 3.8. Cases E (H = 340 nm, C = 19 nm) and F (H = 250 nm, C = 40 nm) had the lowest aspect ratios (2.6 and 1.9) yet both offered good solar modulation ΔTsol = 19.8% at the expense of average and low visible transmittance Tlumcold=50.4% and Tlumcold=37.0% respectively. All of the selected cases offered higher solar modulation than previously reported VO2 based smart-window systems. Cases C and D are notable as they struck what we believe to be a good balance between visible transmittance, solar energy modulation and achievable aspect ratios.

Table 3. Smart-window transmittance metrics for selected moth-eye VO2 smart-windows systems. Equations 1 and 2 are used to calculate the reported metrics. All dimensions are in nanometers, Tsol, Tlum and ΔTsol are all presented as a percentage of energy transmittance.

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The reflectance, absorptance and transmittance spectra for case C are presented in Fig. 6. Our calculations show that less than 5% of light is reflected for all wavelengths; it is either absorbed or transmitted depending upon the phase of the vanadium. Transmittance in the visible region varies between 15% and 70%, at 555 nm (peak of human visible efficiency) transmittance is 60% in its cold state and 56% in its hot state. Infrared transmittance in its cold state varies between 70 and 90% whilst in its hot state this decreases to between 28 and 50%. The spectrally averaged modulation is very high, ΔTsol = 19.4%, and is significantly stronger for infrared wavelengths than visible wavelengths. We therefore expect that this window would have a significant impact upon the energy consumption of a building.

Fig. 6 [C] Normal incidence transmittance, absorptance and reflectance spectra for a moth-eye smart-window, case C: H = 700 nm, C = 8 nm, P = 130 nm and W = 134 nm.

The 62% transmittance modulation observed at 2500 nm for case C is almost entirely accounted for by an increase in absorptance in its hot state. We predict that this will lead to heating of the window pane. Provided that the smart-window uses a multiple-glazing system, and the moth-eye layer is not deployed on an internal surface, conductive heating of the room by the window in its hot state is expected to be low. However, isotropic blackbody emission into the building is unavoidable and could counteract the smart-window’s solar thermal modulation. Increased absorptance may present an advantage for the moth-eye class of smart-window as the window’s temperature will be strongly influenced by light intensity, not just conductive temperature. This may afford the window additional photochromic properties.

When film thicknesses approach the length scale of the conduction electron mean free path we expect that the imaginary part of the hot state (metallic) dielectric constant will increase, leading to higher absorptance [41

41. E. a. Coronado and G. C. Schatz, “Surface plasmon broadening for arbitrary shape nanoparticles: a geometrical probability approach,” J. Chem. Phys. 119, 3926 (2003) [CrossRef] .

]. This thickness dependent absorptance effect is stronger for long wavelengths. Our VO2 optical model did not take effect into account. However, we expect that including this effect would lead to an increased solar energy modulation as the cold (semiconductor) state VO2 optical model should remain unaffected. This increased solar modulation could be a significant advantage exclusive to the moth-eye system facilitated by its exceptionally high surface area.

It is also worth noting that the phase-transition temperature for VO2 is reduced by geometric confinement [42

42. B. Viswanath, Changhyun Ko, Z. Yang, and S. Ramanathan, “Geometric confinement effects on the metal-insulator transition temperature and stress relaxation in VO2 thin films grown on silicon,” Appl. Phys. 109, 063512 (2011).

] and strain [43

43. J. Narayan and V. M. Bhosle, “Phase transition and critical issues in structure-property correlations of vanadium oxide,” Appl. Phys. 100, 103524 (2006).

, 44

44. C. Piccirillo, R. Binions, and I. P. Parkin, “Synthesis and characterisation of W-doped VO2 by aerosol assisted chemical vapour deposition,” Thin Solid Films 516, 1992–1997 (2008) [CrossRef] .

] such that very thin films require lower metal-ion doping concentrations in order to bring the phase transition to an optimum level at room temperature. High doping concentration can increase the phase-transition hysteresis [15

15. R. Binions, C. Piccirillo, and I. P. Parkin, “Tungsten doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride,” Surface and Coatings Tech. 201, 9369–9372 (2007) [CrossRef] .

]. Therefore, the moth-eye system’s potentially lower requirement for doping means that the system will be able to switch more abruptly between its hot and cold states.

Wide-angle simulations

Fig. 7 Wide-angle FDTD simulations of the transmittance through (a) a moth-eye smart-window surface (case C) and (b) a planar 50 nm thick VO2 smart-window with a 40 nm TiO2 antireflection coating applied to the surface.

In contrast, the planar system in Fig. 7 shows strong angular and polarization dependence. For p(TM)-polarized light ΔTsol increases from 6% at normal incidence to 22% at 70°, whereas for s(TE)-polarized light ΔTsol decreases to 3% at 70° incidence. Similar angular and polarization relationships hold for the visible transmittance, Tlum. The planar system shows angular variation in both Tlum and Tsol as the resonant cavity lengths between the interfaces change. Transmittance of s(TE)-polarized light is decreased as oscillating charge at the interfaces respond to the incoming electric field and reflect light from the window’s surface.

The moth-eye system exhibits a stronger dimming effect (difference between hot and cold state visible transmittance ) than the planar system. Our calculations indicate that the moth-eye window will be 8% dimmer in its hot state, when solar intensity is at its peak.

The moth-eye system has a fractionally lower transmittance (< 5%) at normal incidence when compared against shallow angles of around 10°. We attribute this to a lower gradient in the quasistatic refractive index profile at shallow angles of incidence as the change in optical density occurs over a longer path length. As the angle of incidence increases from 10° to 50° increased path length leads to losses in the absorbing coated moth-eye layer. Transmittance is, to a good degree, constant as a balance is struck between increased path length and lower refractive index gradient. For angles of incidence greater than 50°, Eq. 3 is satisfied by smaller portions of the solar and photopic spectra. At high angles of incidence the shortest wavelengths are able to resolve the high index material and are diffracted thus reducing specular transmittance. Slightly higher levels of absorptance are seen for s(TE)-polarized light at angles between 30° and 50°. This is due to a stronger in-plane interaction between the electric field and the absorbing coating on the side-walls of the nanocones. It is interesting to note that p(TM)-absorptance losses for Tlum between 30° and 50° are low. We attribute this to a combination of the previously described processes: low refractive index gradient, satisfaction of Eq. 3 for the entire photopic efficiency spectrum and low in-plane interaction of the electric field with the absorbing VO2 layer.

7. Conclusions and outlook

We have outlined our concept for a new class of VO2 bioinspired smart-window. FDTD simulations were used to identify design rules and select six different cases with optimized dimensions for the surface structures. Our calculations indicated that this class of smart-window offers much improved performance when compared against contemporary dielectric multilayer designs: 96% better solar modulation when compared against the best multilayer system. Doping is compatible with our system and could further improve the window transmittance characteristics.

The moth-eye surface may also induce desirable super-hydrophobic self-cleaning properties. However, in order to verify our optical calculations and confirm this surface wettability it would be necessary to fabricate a prototype. We envisage two key steps in this process. First, a functionalization of the surface in which the moth-eye nanostructures are patterned upon the window substrate, this procedure has been demonstrated previously using both interference [3

3. K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.” ACS Nano 6, 3789–99 (2012) [CrossRef] [PubMed] .

] and colloidal lithography [40

40. C.-H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92, 061112 (2008) [CrossRef] .

]. Second, a single deposition of VO2 potentially using a slow growth rate chemical vapor deposition process [45

45. M. Saeli, C. Piccirillo, I. P. Parkin, I. Ridley, and R. Binions, “Nano-composite thermochromic thin films and their application in energy-efficient glazing,” Sol. Energ. Mat. Sol. Cells 94, 141–151 (2010) [CrossRef] .

, 46

46. T. D. Manning, I. P. Parkin, C. Blackman, and U. Qureshi, “APCVD of thermochromic vanadium dioxide thin films-solid solutions V2-xMxO2 (M = Mo, Nb) or composites VO2 : SnO2,” J. Mater. Chem. 15, 4560 (2005) [CrossRef] .

] or atomic layer deposition [47

47. G. Rampelberg, M. Schaekers, K. Martens, Q. Xie, D. Deduytsche, B. De Schutter, N. Blasco, J. Kittl, and C. Detavernier, “Semiconductor-metal transition in thin VO2 films grown by ozone based atomic layer deposition,” Appl. Phys. Lett. 98, 162902 (2011) [CrossRef] .

] as these techniques are not line-of-sight processes and have been shown to produce high quality films that conformally coat a textured surface. Thin-film VO2 deposition on moth-eye textured surfaces has not been reported previously; fabrication of this type of surface requires demonstration and will be the subject of future studies.

Acknowledgments

Alaric Taylor would like to thank the Photonic Systems Development Centre for Doctoral Training for its award of a research scholarship. We would also like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for awarding a grant towards the fabrication of prototype VO2 moth-eye smart-windows (EP/K015354/1).

References and links

1.

United Nations Environment Programme, Buildings and climate change - status, challenges and opportunities (UNEP, 2007).

2.

H. Deniz, T. Khudiyev, F. Buyukserin, and M. Bayindir, “Room temperature large-area nanoimprinting for broadband biomimetic antireflection surfaces,” Appl. Phys. Lett. 99, 183107 (2011) [CrossRef] .

3.

K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.” ACS Nano 6, 3789–99 (2012) [CrossRef] [PubMed] .

4.

W.-L. Min, B. Jiang, and P. Jiang, “Bioinspired self-cleaning antireflection coatings,” Adv. Mater. 20, 3914–3918 (2008) [CrossRef] .

5.

M. Tazawa, K. Yoshimura, P. Jin, and G. Xu, “Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings,” Appl. Phys. A Mater. Sci. Process. 77, 455–459 (2003) [CrossRef] .

6.

N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi (a) 206, 2155–2160 (2009) [CrossRef] .

7.

G. Xu, P. Jin, M. Tazawa, and K. Yoshimura, “Optimization of antireflection coating for VO2-based energy efficient window,” Sol. Energ. Mat. Sol. Cells 83, 29–37 (2004) [CrossRef] .

8.

T. D. Manning, I. P. Parkin, R. J. H. Clark, D. Sheel, M. E. Pemble, and D. Vernadou, “Intelligent window coatings: atmospheric pressure chemical vapour deposition of vanadium oxides,” J. Mater. Chem. 12, 2936–2939 (2002) [CrossRef] .

9.

C. Piccirillo, R. Binions, and I. P. Parkin, “Synthesis and functional properties of vanadium oxides: V2O3, VO2, and V2O5 deposited on glass by aerosol-assisted CVD,” Chem. Vap. Deposition 13, 145–151 (2007) [CrossRef] .

10.

P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the ’moth eye’ principle,” Nature 244, 281–282 (1973) [CrossRef] .

11.

D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies.” Proc. R. Soc. B. 273, 661–7 (2006) [CrossRef] [PubMed] .

12.

W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” JOSA A 8, 549–553 (1991) [CrossRef] .

13.

S. J. Wilson and M. C. Hutley, “The optical properties of ’moth eye’ antireflection surfaces,” Optica Acta 29, 993–1009 (1982) [CrossRef] .

14.

C. S. Blackman, C. Piccirillo, R. Binions, and I. P. Parkin, “Atmospheric pressure chemical vapour deposition of thermochromic tungsten doped vanadium dioxide thin films for use in architectural glazing,” Thin Solid Films 517, 4565–4570 (2009) [CrossRef] .

15.

R. Binions, C. Piccirillo, and I. P. Parkin, “Tungsten doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride,” Surface and Coatings Tech. 201, 9369–9372 (2007) [CrossRef] .

16.

I. P. Parkin, R. Binions, C. Piccirillo, C. S. Blackman, and T. D. Manning, “Thermochromic coatings for intelligent architectural glazing,” Nano Res. 2, 1–20 (2008) [CrossRef] .

17.

R. Binions, G. Hyett, C. Piccirillo, and I. P. Parkin, “Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties,” J. Mater. Chem. 17, 4652 (2007) [CrossRef] .

18.

M. E. Warwick, C. W. Dunnill, J. Goodall, J. A. Darr, and R. Binions, “Hybrid chemical vapour and nanoceramic aerosol assisted deposition for multifunctional nanocomposite thin films,” Thin Solid Films 519, 5942–5948 (2011) [CrossRef] .

19.

M. Saeli, R. Binions, C. Piccirillo, and I. P. Parkin, “Templated growth of smart coatings: hybrid chemical vapour deposition of vanadyl acetylacetonate with tetraoctyl ammonium bromide,” Applied Surface Science 255, 7291–7295 (2009) [CrossRef] .

20.

Z. Zhang, Y. Gao, H. Luo, L. Kang, Z. Chen, J. Du, M. Kanehira, Y. Zhang, and Z. L. Wang, “Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications,” Energy & Environmental Science 4, 4290 (2011).

21.

N. Mlyuka, G. Niklasson, and C. Granqvist, “Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance,” Sol. Energ. Mat. Sol. Cells 93, 1685–1687 (2009) [CrossRef] .

22.

C. G. Granqvist, “Transparent conductors as solar energy materials: a panoramic review,” Sol. Energ. Mat. Sol. Cells 91, 1529–1598 (2007) [CrossRef] .

23.

T. Smith and J. Guild, “The C.I.E. colorimetric standards and their use,” Trans. of the Opt. Soc. 22, 73 (1931) [CrossRef] .

24.

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25.

M. Saeli, C. Piccirillo, I. P. Parkin, R. Binions, and I. Ridley, “Energy modelling studies of thermochromic glazing,” Energy and Buildings 42, 1666–1673 (2010) [CrossRef] .

26.

N. Ohta and A. R. Robertson, CIE standard colorimetric system in colorimetry: fundamentals and applications (John Wiley & Sons, Ltd, Chichester, UK, 2006).

27.

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28.

H. Kakiuchida, P. Jin, and M. Tazawa, “Control of thermochromic spectrum in vanadium dioxide by amorphous silicon suboxide layer,” Sol. Energ. Mat. Sol. Cells 92, 1279–1284 (2008) [CrossRef] .

29.

Z. Chen, Y. Gao, L. Kang, J. Du, Z. Zhang, H. Luo, H. Miao, and G. Tan, “VO2-based double-layered films for smart windows: optical design, all-solution preparation and improved properties,” Sol. Energ. Mat. Sol. Cells 95, 2677–2684 (2011) [CrossRef] .

30.

K. Kato, P. K. Song, H. Odaka, and Y. Shigesato, “Study on thermochromic VO2 films grown on ZnO-coated glass substrates for “smart windows” Jpn. J. Appl. Phys. 42, 6523–6531 (2003) [CrossRef] .

31.

P. Jin, G. Xu, M. Tazawa, and K. Yoshimura, “A VO2-based multifunctional window with highly improved luminous transmittance,” Jpn. J. Appl. Phys. 41, L278–L280 (2002) [CrossRef] .

32.

W. Burkhardt, T. Christmann, B. Meyer, W. Niessner, D. Schalch, and A. Scharmann, “W- and F-doped VO2 films studied by photoelectron spectrometry,” Thin Solid Films 345, 229–235 (1999) [CrossRef] .

33.

S.-Y. Li, G. Niklasson, and C. Granqvist, “Thermochromic fenestration with VO2-based materials: three challenges and how they can be met,” Thin Solid Films 520, 3823–3828 (2012) [CrossRef] .

34.

N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature,” Appl. Phys. Lett. 95, 171909 (2009) [CrossRef] .

35.

I. Takahash, M. Hibino, and T. Kudo, “Thermochromic properties of double-doped VO2 thin films prepared by a wet coating method using polyvanadate-based sols containing W and Mo or W and Ti,” Jpn. J. Appl. Phys. 40, 1391–1395 (2001) [CrossRef] .

36.

C. Aydin, A. Zaslavsky, G. J. Sonek, and J. Goldstein, “Reduction of reflection losses in ZnGeP2 using motheye antireflection surface relief structures,” Appl. Phys. Lett. 80, 2242 (2002) [CrossRef] .

37.

J. Hao, N. Lu, H. Xu, W. Wang, L. Gao, and L. Chi, “Langmuir-Blodgett monolayer masked chemical etching: an approach to broadband antireflective surfaces,” Chem. Mater. 21, 1802–1805 (2009) [CrossRef] .

38.

L. Yang, Q. Feng, B. Ng, X. Luo, and M. Hong, “Hybrid moth-eye structures for enhanced broadband antireflection characteristics,” Appl. Phys. Express 3, 102602 (2010) [CrossRef] .

39.

O. Deparis, N. Khuzayim, A. Parker, and J. Vigneron, “Assessment of the antireflection property of moth wings by three-dimensional transfer-matrix optical simulations,” Phys. Rev. E 79, 1–7 (2009) [CrossRef] .

40.

C.-H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92, 061112 (2008) [CrossRef] .

41.

E. a. Coronado and G. C. Schatz, “Surface plasmon broadening for arbitrary shape nanoparticles: a geometrical probability approach,” J. Chem. Phys. 119, 3926 (2003) [CrossRef] .

42.

B. Viswanath, Changhyun Ko, Z. Yang, and S. Ramanathan, “Geometric confinement effects on the metal-insulator transition temperature and stress relaxation in VO2 thin films grown on silicon,” Appl. Phys. 109, 063512 (2011).

43.

J. Narayan and V. M. Bhosle, “Phase transition and critical issues in structure-property correlations of vanadium oxide,” Appl. Phys. 100, 103524 (2006).

44.

C. Piccirillo, R. Binions, and I. P. Parkin, “Synthesis and characterisation of W-doped VO2 by aerosol assisted chemical vapour deposition,” Thin Solid Films 516, 1992–1997 (2008) [CrossRef] .

45.

M. Saeli, C. Piccirillo, I. P. Parkin, I. Ridley, and R. Binions, “Nano-composite thermochromic thin films and their application in energy-efficient glazing,” Sol. Energ. Mat. Sol. Cells 94, 141–151 (2010) [CrossRef] .

46.

T. D. Manning, I. P. Parkin, C. Blackman, and U. Qureshi, “APCVD of thermochromic vanadium dioxide thin films-solid solutions V2-xMxO2 (M = Mo, Nb) or composites VO2 : SnO2,” J. Mater. Chem. 15, 4560 (2005) [CrossRef] .

47.

G. Rampelberg, M. Schaekers, K. Martens, Q. Xie, D. Deduytsche, B. De Schutter, N. Blasco, J. Kittl, and C. Detavernier, “Semiconductor-metal transition in thin VO2 films grown by ozone based atomic layer deposition,” Appl. Phys. Lett. 98, 162902 (2011) [CrossRef] .

OCIS Codes
(160.6840) Materials : Thermo-optical materials
(350.6050) Other areas of optics : Solar energy
(310.3915) Thin films : Metallic, opaque, and absorbing coatings
(310.5448) Thin films : Polarization, other optical properties
(310.6628) Thin films : Subwavelength structures, nanostructures
(310.6805) Thin films : Theory and design

ToC Category:
Thermo-optics

History
Original Manuscript: May 31, 2013
Revised Manuscript: June 26, 2013
Manuscript Accepted: June 27, 2013
Published: July 10, 2013

Citation
Alaric Taylor, Ivan Parkin, Nuruzzaman Noor, Clemens Tummeltshammer, Mark S Brown, and Ioannis Papakonstantinou, "A bioinspired solution for spectrally selective thermochromic VO2 coated intelligent glazing," Opt. Express 21, A750-A764 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S5-A750


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References

  1. United Nations Environment Programme, Buildings and climate change - status, challenges and opportunities (UNEP, 2007).
  2. H. Deniz, T. Khudiyev, F. Buyukserin, and M. Bayindir, “Room temperature large-area nanoimprinting for broadband biomimetic antireflection surfaces,” Appl. Phys. Lett.99, 183107 (2011). [CrossRef]
  3. K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.” ACS Nano6, 3789–99 (2012). [CrossRef] [PubMed]
  4. W.-L. Min, B. Jiang, and P. Jiang, “Bioinspired self-cleaning antireflection coatings,” Adv. Mater.20, 3914–3918 (2008). [CrossRef]
  5. M. Tazawa, K. Yoshimura, P. Jin, and G. Xu, “Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings,” Appl. Phys. A Mater. Sci. Process.77, 455–459 (2003). [CrossRef]
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