applied sciences

Article Indium Tin Oxide Nanoparticle: TiO2: Air Layers for One-Dimensional Multilayer Photonic Structures

Ilka Kriegel 1 and Francesco Scotognella 2,3,*

1 Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT), 16163 Genova, Italy; [email protected] 2 Dipartimento di Fisica, Politecnico di Milano, 20133 Milano, Italy 3 Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, 20133 Milan, Italy * Correspondence: [email protected]



Received: 17 May 2019; Accepted: 20 June 2019; Published: 24 June 2019


Abstract: In this work we study the optical properties of one-dimensional photonic crystals in which layers of silica nanoparticles are alternated with layers of indium tin oxide nanoparticle (ITO)/titania nanoparticle mixture, using the transfer matrix method. The dielectric function of the mixed ITO/TiO2 nanoparticle layer is carefully accounted for with a generalized Rayleigh equation for the ternary mixture ITO:TiO2:air. We studied the light transmission of the multilayer photonic crystal as a function of the ITO/TiO2 ratio. We observe that, by increasing the ITO content and its carrier density in the three-phase mixture, the intensity of the plasmon resonance in the near infrared (NIR) increases and the intensity of the photonic band gap (visible) decreases. Thus, our study is of major importance for the realization of electrochromic smart windows, in which separate and independent NIR and visible light control is required.

Keywords: photonic crystal; indium tin oxide/titanium dioxide (ITO/TiO2) heterojunction; effective medium approximation

1. Introduction Porous nanoparticle based multilayer photonic crystals are produced by very simple and versatile fabrication methods, as for example spin coating [1,2] and sputtering [3]. Their photonic band gap [4–6] can be modified by exploiting the properties of the constituent nanoparticles as well as the multilayer porosity, of interest for sensing, lasing, and switching [7,8]. The refractive index contrast between the two layers accounts for the photonic stop band and can be adjusted to deliver a targeted optical response. This becomes particularly interesting when implementing refractive index tunable materials, such as transparent conducting oxides (TCOs) [9–11]. A very interesting property of such materials is 26 27 3 the near infrared (NIR) plasmonic response due to the free carrier density in the range of 10 –10 m− . This latter can be manipulated by the applying an electric [12] or an electrochemical potential [13]. In this way, carriers are injected in a capacitive manner adding to the predefined carrier density, resulting in enhanced NIR absorption [9]. In a porous multilayer photonic crystal, this modification of the dielectric response automatically affects the refractive index contrast and therefore provides a means to actively manipulate the photonic stop band [14,15]. Gaining the independent control over the photonic bandgap and the plasmonic absorption (in the NIR) would be of major interest for the application in electrochromic smart windows that can expel separately either heat or sunlight by applying a potential. Recent works addressed this target by implementing NbOx glasses as matrix [16]. This involves an additional chemical step in the coloration, and thus, the devices become more prone to degradation. Implementing a physical effect such as the photonic bandgap in the coloration therefore is a highly attractive approach. In this work, we study the light transmission properties of a multilayer photonic

Appl. Sci. 2019, 9, 2564; doi:10.3390/app9122564 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2564 2 of 8 crystal made by alternating silica nanoparticle layers and of electrochemically tunable indium tin oxide (ITO) nanoparticle/titania (TiO2) nanoparticle layers. The implementation of the three material mixture (ITO:TiO2:air) delivers an additional ingredient to fine tune the optical response of the entire structure due to the option to further modify the refractive index of this layer. We predict that the tuning of the ITO nanoparticle dielectric function in the three-phase mixture, and thus the overall refractive index of this layer, will modify both the plasmon resonance of ITO in the NIR and the photonic band gap of the overall structure. Remarkably, for the ITO:TiO2:air 60:10:30 ratio, the increase of the number of carriers of ITO induces a simultaneous increase of the plasmon resonance intensity and decrease of the photonic band gap intensity, thus taking a step towards the independent control over the optical signatures of our multilayer structure.

2. Methods

2.1. ITO:TiO2:Air Layer Refractive Index According to the Drude model to describe the plasmonic response [17], the frequency dependent complex dielectric function of ITO can be written as:

εITO,ω = ε1,ω + iε2,ω (1)

where 2 ωP ε1,ω = ε (2) ∞ − (ω2 Γ2) − and 2 ωPΓ ε2,ω = (3) ω(ω2 Γ2) − where ω is the plasma frequency and Γ is the free carrier damping. The plasma frequency is p P 2 ωP = Ne /m∗ε0 , with N number of charges, e the electron charge, m∗ the effective mass, and ε0 the vacuum dielectric constant. For ITO, we use N = 2.49 1026 charges/m3 (and we start from this value × when we increase the number of carriers in this study), m = 0.4 m , with m = 9.1 10 31 kg. ∗ × 0 0 × − For the free carrier damping, we use Γ = 0.1132 eV. The parameters for ITO nanoparticles are taken from [9,18]; the value of the high frequency dielectric constant, ε = 4, is taken from [19]. ∞ The wavelength dependent refractive index of TiO2 can be written [20]:

 1 1 1/2 nTiO (λ) = 4.99 + + (4) 2 96.6λ1.1 4.60λ1.95

2 where λ is the wavelength [in micrometers, and εTiO (λ) = n (λ)]. 2 TiO2 To determine the dielectric function of the layer made of ITO nanoparticles, TiO2 nanoparticles, and air—ITO:TiO2:air (we call it εe f f 1,ω)—we use a generalized Rayleigh mixing formula for a three-phase mixture [21]:   ε ε   fITO(εITO,ω εAir) + fTiO t12,ω εTiO ,ω εAir e f f 1,ω − Air − 2 2 − = fITO + fTiO2   (5) εe f f 1,ω + 2εAir ( ) fITO εITO,ω + 2εAir + fTiO2 t12,ω εTiO2,ω + 2εAir

with 3εITO,ω t12,ω = (6) εTiO2,ω + 2εITO,ω where fITO is the volume fraction (filling factor) occupied by ITO nanoparticles and fTiO2 is the volume fraction occupied by TiO2 nanoparticles. The frequency dependent complex refractive index of the ITO:TiO2:air layer is determined considering that ne f f 1,ω = √εe f f 1,ω. Appl. Sci. 2019, 9, 2564 3 of 8

2.2. SiO2:Air Layer Refractive Index

The wavelength dependent refractive index of SiO2 can be described by the following Sellmeier Equation [22]: 1.9558λ2 1.345λ2 10.41λ2 n2 (λ) 1 = + + (7) SiO2 − λ2 0.154942 λ2 0.06342 λ2 27.122 − − − where λ is the wavelength in micrometers. To determine the effective dielectric function of the SiO2:air layer (we call it εe f f 2,ω) we use the Bruggeman effective medium approximation [23,24]:

ε ε   ε ε Air − e f f 2,ω SiO2,ω − e f f 2,ω fSiO2   + 1 fSiO2   = 0 (8) ε + Lm ε ε − ε + ε ε /3 e f f 2,ω Air − e f f 2,ω e f f 2,ω SiO2,ω − e f f 2,ω where fSiO2 is the filling factor of the SiO2 nanoparticles (i.e., the fraction of layer volume occupied by the nanoparticles) and in this study we choose fSiO2 = 0.7. Lm is the depolarization factor, equal to 1/3 if we assume a film of SiO2 spherical nanoparticles. The photon energy dependent complex refractive index of the SiO2:air layer was determined considering that ne f f 2,ω = √εe f f 2,ω . The ITO:TiO2:air layers and the SiO2:air layers can be considered as random networks of nanoparticles. Thus, a filling factor up to 0.7 is a reliable value [25].

2.3. Transmission of the Multilayer Photonic Crystal

We used the two refractive indexes ne f f 1,ω and ne f f 2,ω for the ITO:TiO2:air layers and for the SiO2:air layers, respectively, to study the light transmission through the photonic structure by employ the transfer matrix method [8,26,27]. For a transverse electric (TE) wave the transfer matrix for the kth layer is given by:       cos 2π n d i sin 2π n d  λ k k nk λ k k  Mk =    −    (9)  in sin 2π n d cos 2π n d  − k λ k k λ k k with nk the refractive index and dk the thickness of the layer. In this study the thickness of the ITO:TiO :Air layers is 185.63 nm, while the thickness of the SiO :Air layers is 238.46 nm. 2 " #2 m11 m12 The product M = M1 M2 ... Mk ... Ms = gives the matrix of the multilayer (of · · · · · m21 m22 s layers). The transmission coefficient is:

2n t = s (10) (m11 + m12n0)ns + (m21 + m22n0) with ns the refractive index of the substrate (in this study ns = 1.46) and n0 the refractive index of air. Thus, the light transmission of the multilayer photonic crystal is: n T = 0 t 2 (11) ns | |

In this work the number of bilayers selected was 10 (i.e., the ITO:TiO2:air/SiO2:air bilayer repeated 10 times), which means 20 layers and a total thickness of about 4,241 nm.

3. Results and Discussion

Figure1a represents a sketch of a ITO:TiO 2:air/SiO2:air multilayer structure and a magnification of an ITO:TiO2:air layer. The blue dots indicate the ITO nanoparticles, while the grey dots indicate the TiO2 nanoparticles. The white interstitial volume depicts the air voids between the nanoparticles. To determine the effective refractive index of the layer we used a generalized Rayleigh Equation for a three-phase mixture (Equation (5)). Appl. Sci. 2019, 9, x 2564 4 of 8

Appl. Sci. 2019, 9, x 4 of 8

(a) (b)

Figure 1. (a) Sketch of a ann indium tin oxide (ITO):TiO(ITO):TiO2:air:air layer, layer, with ITO nanoparticles (blue (blue dots), dots), (a) 2 (b) TiO2 nanoparticlesnanoparticles (grey (grey dots) dots),, and and air air (white (white interstitial interstitial volume) volume);; (b) (realb) real part part (solid (solid black black curve) curve) and andimaginary imaginaryFigure part1. (a part) (solidSketch (solid redof a redncurve indium curve)) of tin ofthe oxide the dielectric dielectric (ITO):TiO function function2:air layer, of of witha aITO:TiO ITO:TiO ITO nanoparticles2:air2:air (20:50:30 (20:50:30 (blue ratio) ratio) dots), layer layer, , calculatedTiO2 nanoparticles with the Rayleigh (grey dots) mixing , and formula,air (white interstitial and real part volume) (dashed(dashed; (b) real black part curve)curve (solid) black and imaginary curve) and part (dashedimaginary redred curve)curve) part ofof(solid aa ITO:airITO:air red curve (20:80(20:80) of ratio)ratio) the dielectric layer,layer, calculatedcalculated function of withwith a ITO:TiO thethe BruggemanBruggeman2:air (20:50:30 approximation.approximation ratio) layer, . calculated with the Rayleigh mixing formula, and real part (dashed black curve) and imaginary part In Figure Figu(dashedre1 b, 1 redb the, thecurve) dashed dashed of a curveITO:air curve represents(20:80 represent ratio) layer thes real , calculatedthe (black) real with (black) and the imaginary Bruggemanand imaginary (red) approximation part (red) of the. part dielectric of the function of an ITO:air two-phase mixture, which is a dispersion of ITO nanoparticles, with a filling dielectric Infunction Figure of 1b ,a nthe ITO:air dashed two curve-phase represent mixture,s the which real (black)is a dispersion and imaginary of ITO (red) nanoparticles, part of the with factora fillingdielectric of 0.2,factor infunction of air. 0.2, Thisof inan two-phaseITO:air air. This two two- mixturephase-phase mixture, refractive mixture which index is refractive a dispersion is determined index of ITO is nanoparticles, determined with the Bruggeman with the eBruggemanffectivea filling medium effective factor theory of medium 0.2, [ 23in, 24 airtheory].. Instead,This [23,24] two the-phase. Inste solid mixturead, curves the refractivesolid in Figure curves index1b in represent Figure is determined 1 theb represent real with (black) thethe andreal imaginary(black)Bruggeman and (red) imaginary effective part of (red) themedium e ffpartective theory of dielectricthe [23,24] effective. Inste function dielectricad, the of solid an function ITO:TiO curves inof2:air Figurean mixture.ITO:TiO 1b represent2 It:air is clearmixture. the real that It the is resonanceclear (black)that the related and resonance imaginary to the ITOrelated (red) plasmonic partto the of ITOthe peak effective plasmonic shows dielectric a splitting, peak showsfunction and a one splittingof a resonancen ITO:TiO, and2 shifts:airone mixture. resonanc towards It eis higher shifts energiestowardsclear inhigher that the the three-material energies resonance in related the mixture three to the-materialevidencing ITO plasmonic mixture the peak possibility evidencing shows a tosplitting the further possibility, and tailor one the toresonanc further opticale shifts tailor response the ofoptical ITOtowards nanostructures.response higher of ITOenergies nanostructures. in the three-material mixture evidencing the possibility to further tailor the Inoptical Figure response2 2 we we showshow of ITO thethe nanostructures. didifferencefference between a photonic crystal with layers of silica nanoparticles In Figure 2 we show the difference between a photonic crystal with layers of silica nanoparticles alternatedalternated withwith layerslayers of of titania titania nanoparticles nanoparticles (black (black curve), curve), and and a photonic a photonic crystal crystal with with layers layers of silica of alternated with layers of titania nanoparticles (black curve), and a photonic crystal with layers of nanoparticlessilica nanoparticles alternated alternated with awith layer a oflayer ITO of nanoparticles ITO nanoparticles (red curve). (red curve). silica nanoparticles alternated with a layer of ITO nanoparticles (red curve).

Figure 2. Light transmission spectrum of a SiO2 nanoparticle/TiO2 nanoparticle photonic crystal (black Figure 2. Light transmission spectrum of a SiO2 nanoparticle/TiO2 nanoparticle photonic crystal (black curve) both with a filling factor of 0.7, and light transmission spectrum of a SiO2 nanoparticle/ITO curve) both with a filling factor of 0.7, and light transmission spectrum of a SiO2 nanoparticle/ITO Figurenanoparticle 2. Light transmission photonic crystal spectrum (red curve) of a SiOboth2 withnanoparticle/TiO a filling factor2 of nanoparticle 0.7. photonic crystal (black nanoparticle photonic crystal (red curve) both with a filling factor of 0.7. curve) both with a filling factor of 0.7, and light transmission spectrum of a SiO2 nanoparticle/ITO nanoparticle photonic crystal (red curve) both with a filling factor of 0.7. Appl. Sci. 2019, 9, x 5 of 8 Appl. Sci. 2019, 9, 2564 5 of 8 Appl. Sci. 2019, 9, x 5 of 8 The silica/ITO photonic crystal shows a strong transmission valley at lower energy due to the plasmonThe resonance silica/ITO of photoni ITO. Moreover,c crystal shows the silica/ITO a strong transmission photonic crystal valley shows at lower a less energy intense due photonic to the The silica/ITO photonic crystal shows a strong transmission valley at lower energy due to the bandplasmon gap with resonance respect of to ITO. the oneMoreover, of the silica/titaniathe silica/ITO photonic photonic crystal crystal because shows a of less the intense smaller photonic refractive plasmon resonance of ITO. Moreover, the silica/ITO photonic crystal shows a less intense photonic indexband contrast. gap with W respecte can now to the relate one of to the the silica/titania two spectra photonic in Figure crystal 2 as the because two extremes of the smaller of a threerefractive-phase band gap with respect to the one of the silica/titania photonic crystal because of the smaller refractive mixtureindex contrast.of ITO nanoparticles, We can now relate TiO2 to nanoparticles the two spectra, and in Figureair in which 2 as the we two keep extremes constant of a thethree volume-phase of index contrast. We can now relate to the two spectra in Figure2 as the two extremes of a three-phase airmixture and we of change ITO nanoparticles, the ITO:TiO 2TiO ratio2 nanoparticles. We show this, and ratio air variationin which wein Figurekeep constant 3, where the the volume increasing of mixture of ITO nanoparticles, TiO2 nanoparticles, and air in which we keep constant the volume of contentair and of we ITO change relates the ITO:TiO to an increase2 ratio. We in show the transmissionthis ratio variation valley in Figure intensity 3, where due tothe theincreasing plasmon air and we change the ITO:TiO ratio. We show this ratio variation in Figure3, where the increasing resonance.content ofInterestingly, ITO relates the to anplasmon2 increase resonance in the transmissionis blue shifted valley with intensityrespect to due the toone the of the plasmon ITO:air content of ITO relates to an increase in the transmission valley intensity due to the plasmon resonance. layer.resonance. Interestingly, the plasmon resonance is blue shifted with respect to the one of the ITO:air Interestingly,layer. the plasmon resonance is blue shifted with respect to the one of the ITO:air layer.

Figure 3. Light transmission spectra of ITO:TiO2:air/SiO2:air multilayer photonic crystals with FigureFigure 3. 3.Light Light transmission transmission spectra spectra of ITO:TiO of ITO:TiO2:air/SiO2:air/2:airSiO multilayer2:air multilayer photonic photonic crystals with crystals diff erent with different ITO:TiO2 ratios, keeping constant the sum of 푓 + 푓 = 0.7. 2 f + f 퐼푇푂= 푇푖푂2 ITO:TiOdifferent2 ITO:TiOratios, keeping ratios, constant keeping theconstant sum of theITO sum ofTiO 푓퐼푇푂2 +0.7.푓푇푖푂2 = 0.7. In Figure 4 we show the transmission value in correspondence of the plasmon resonance and InIn FigureFigure4 4we we show show the the transmission transmission value value in correspondencein correspondence of theof the plasmon plasmon resonance resonance and theand the photonic band gap of the structure as a function of the ITO content, highlighting a cross over photonicthe photonic band band gap ofgap the of structure the structure as a function as a function of the ITOof the content, ITO content, highlighting highlighting a cross over a cross between over f between= 0.2 and푓퐼푇푂 f= 0.2= and0.3 푓 (corresponding퐼푇푂 = 0.3 (corresponding to the red to and the red blue and curves blue incurves Figure in3 Figure). 3). betweenITO 푓퐼푇푂 = 0ITO.2 and 푓퐼푇푂 = 0.3 (corresponding to the red and blue curves in Figure 3).

Figure 4. Transmission minima of the plasmon resonance and of the photonic band gap as a function

of the ITO nanoparticles, keeping constant the sum of 푓퐼푇푂 + 푓푇푖푂2 = 0.7 . FigureFigure 4.4. Transmission minimaminima ofof thethe plasmonplasmon resonanceresonance andand ofof thethe photonicphotonic bandband gapgap asas aa functionfunction

of the ITO nanoparticles, keeping constant the sum of 푓퐼푇푂 + 푓푇푖푂2 = 0.7 . of the ITO nanoparticles, keeping constant the sum of fITO + fTiO2 = 0.7. Appl. Sci. 2019, 9, 2564 6 of 8 Appl. Sci. 2019, 9, x 6 of 8

By changingchanging the carrier densitydensity in ITOITO wewe cancan furtherfurther modulatemodulate thethe lightlight transmissiontransmission ofof thethe photonicphotonic crystal,crystal, sincesince the change in the number of carriers leads to a change change in in the the plasma plasma frequency frequency,, and thus the dielectric function of ITO. This results in a simultaneous change of the plasmon resonance Appl.and Sci. thus 2019 the, 9, x dielectric function of ITO. This results in a simultaneous change of the plas6mon of 8 andresonance of the photonicand of the band photonic gap. band gap. ByFigure changing5 depicts the changes carrier density to modulation in ITO we in the can number further ofmodulate carriers the in ITO light for transmission photonic crystals of the photonicwhere the crystal, ratio insince the the ITO:TiO change2:air in the layer number is 10:60:30 of carriers (Figure leads5a), to 30:40:30 a change (Figure in the 5plasmab), and frequency 60:10:30 , (Figure5c). We change the number of carriers from 2.49 to 5.49 1026 charges/m3. This change in the and thus the dielectric function of ITO. This results in a simultaneous× change of the plasmon resonancenumber of and carriers of the can photonic be for example band gap. induced via an electrochemical [18] or optical [28] way.

(a) (b) (c)

Figure 5. Light transmission spectra of the photonic crystal as a function of the doping level 26 ⁄ 3 26 ⁄ 3 26 ⁄ 3 26 ⁄ 3 (2.49 × 10 charges m , 3.49 × 10 charges m , 4.49 × 10 charges m , 5.49 × 10 charges m ) for the ITO:TiO2 ratio (a) 10:60, (b) 30:40, and (c) 60:10. (a) (b) (c)

FigureFigure 5. 5.5 LightdepictsLight transmission transmission changes spectrato spectramodulation of theof photonic the in photonic the crystal number crystal as a functionof as carriers a function of the indoping ITO of for the level photonic doping (2.49 level10 crystals26 where(2 .49the× ratio10263 chargesin the ⁄ITO:TiOm263 , 3.492×:air10 326layercharges is 10:60:30⁄26m3 , 4.49 (Figure× 103 26 charges5a), 3026⁄:40:30m3 , 5 .49(Figure×31026 5b),charges and×⁄ m60:10:303 ) charges/m , 3.49 10 charges/m , 4.49 10 charges/m , 5.49 10 charges/m ) for the ITO:TiO2 × × × 26 3 (Figureforratio the5c) (a ITO:TiO.) 10:60,We change (b2 ratio) 30:40, the(a) and 10:60, number (c) ( 60:10.b) 30:40, of carriers and (c ) from60:10. 2 .49 to 5.49 × 10 charges⁄m . This change in the number of carriers can be for example induced via an electrochemical [18] or optical [28] way. FigureIfIf wewe useuse 5 depicts thethe photonicphotonic change crystalcrystals to modulation wherewhere thethe in ITO:TiOITO:TiO the number22:air layerlayer of carriers has aa ratioratio in ITO 60:10:3060:10:30 for photonic (Figure(Figure 5 crystals5c)c) asas aa wherefilterfilter forfor the thethe ratio SunSun in irradiation the ITO:TiO (direct2:air and layer circumsolar is 10:60:30 [[29] 29(Figure]),), wewe see5a),see thatthat30:40:30 inin thethe (Figure threethree bandsbands 5b), and inin thethe 60:10:30 rangerange (Figurebetween 5c) 0.6. We andand change 1.11.1 eVeV thethethe light lightnumber transmissiontransmission of carriers isis fromswitchedswitched 2.49 for forto 5 twotwo.49 doping×doping1026 charges levelslevels (Figure⁄(Figurem3. This6 ).6) . change in the number of carriers can be for example induced via an electrochemical [18] or optical [28] way. If we use the photonic crystal where the ITO:TiO2:air layer has a ratio 60:10:30 (Figure 5c) as a filter for the Sun irradiation (direct and circumsolar [29]), we see that in the three bands in the range between 0.6 and 1.1 eV the light transmission is switched for two doping levels (Figure 6).

Figure 6. 6. Transmission of the Sun (direct and circumsolar circumsolar irradiation [[29]29])) withoutwithout thethe photonic photonic crystal filter (blue curve), with an ITO:TiO2 60:10 sample characterized by a doping level 26 of crystal filter (blue curve), with an ITO:TiO2 60:10 sample characterized by a doping level of 2.49 10 26 3 26 3 × charges2.49 × 10/m3charges(red curve)⁄m and(red 5.49curve)10 and26 charges 5.49 × 10/m3 charges(yellow⁄ curve).m (yellow curve). × Figure 6. Transmission of the Sun (direct and circumsolar irradiation [29]) without the photonic crystal filter (blue curve), with an ITO:TiO2 60:10 sample characterized by a doping level of 2.49 × 1026 charges⁄m3 (red curve) and 5.49 × 1026 charges⁄m3 (yellow curve). Appl. Sci. 2019, 9, 2564 7 of 8

We need to consider that a layer in which ITO nanoparticles and TiO2 nanoparticles are in close contact can be seen as a bulk heterojunction, with a large contact surface between the two materials, in which plasmon-induced hot electron generation can occur [30,31]. In a future work, for a more precise description of this interesting entangled multicomponent nanostructure, we should take into account this type of interaction.

4. Conclusions In this work we show the light transmission properties of a one-dimensional multilayer photonic crystal made by alternating silica nanoparticle layers and ITO nanoparticle:TiO2 nanoparticle layers. We have calculated the effective refractive index of the silica layer considering it as a two-phase mixture and the one of the ITO:titania layer, considering it as a three-phase mixture. We use the calculated effective refractive indexes in the transfer matrix method to determine the light transmission of the multilayer structure. The ITO content and carrier density in the ITO:TiO2:air mixture affects both the intensity of the plasmon resonance and the photonic band gap, highlighting their use as tunable filters where light transmission can be switched, depending on the carrier density. Future works can be focused on the interaction, in terms of plasmon-induced hot electron extraction, in indium tin oxide—TiO2 nanoheterojunctions.

Author Contributions: Conceptualization, I.K. and F.S.; Methodology, I.K. and F.S.; Writing—original draft, I.K. and F.S.; Writing—review & editing, F.S. Funding: This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. [816313]). Conflicts of Interest: The authors declare no conflict of interest.

References

1. von Freymann, G.; Kitaev, V.; Lotsch, B.V.; Ozin, G.A. Bottom-up assembly of photonic crystals. Chem. Soc. Rev. 2013, 42, 2528–2554. [CrossRef][PubMed] 2. Lova, P.; Manfredi, G.; Comoretto, D. Advances in Functional Solution Processed Planar 1D Photonic Crystals. Adv. Opt. Mater. 2018, 6, 1800730. [CrossRef] 3. Chiasera, A.; Tosello, C.; Moser, E.; Montagna, M.; Belli, R.; Gonçalves, R.R.; Righini, G.C.; Pelli, S.; Chiappini, A.; Zampedri, L.; et al. Er3+/Yb3+-activated silica–titania planar waveguides for EDPWAs fabricated by rf-sputtering. J. Non-Cryst. Solids 2003, 322, 289–294. [CrossRef] 4. John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett. 1987, 58, 2486–2489. [CrossRef][PubMed] 5. Yablonovitch, E. Inhibited Spontaneous Emission in Solid-State Physics and Electronics. Phys. Rev. Lett. 1987, 58, 2059–2062. [CrossRef][PubMed] 6. Joannopoulos, J.D. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, USA, 2008. 7. Nucara, L.; Greco, F.; Mattoli, V. Electrically responsive photonic crystals: A review. J. Mater. Chem. C 2015, 3, 8449–8467. [CrossRef] 8. Bellingeri, M.; Chiasera, A.; Kriegel, I.; Scotognella, F. Optical properties of periodic, quasi-periodic, and disordered one-dimensional photonic structures. Opt. Mater. 2017, 72, 403–421. [CrossRef] 9. Kriegel, I.; Scotognella, F.; Manna, L. Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives. Phys. Rep. 2017, 674, 1–52. [CrossRef] 10. Naik, G.V.; Shalaev, V.M.; Boltasseva, A. Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 2013, 25, 3264–3294. [CrossRef] 11. Pedroni, M.; Canetti, M.; Chiarello, G.L.; Cremona, A.; Inzoli, F.; Luzzati, S.; Pietralunga, S.M.; Tagliaferri, A.; Zani, M.; Vassallo, E.; et al. Tungsten oxide thin film photo-anodes by reactive RF diode sputtering. Thin Solid Film. 2016, 616, 375–380. [CrossRef] 12. Feigenbaum, E.; Diest, K.; Atwater, H.A. Unity-Order Index Change in Transparent Conducting Oxides at Visible Frequencies. Nano Lett. 2010, 10, 2111–2116. [CrossRef][PubMed] Appl. Sci. 2019, 9, 2564 8 of 8

13. Agrawal, A.; Kriegel, I.; Runnerstrom, E.L.; Scotognella, F.; Llordes, A.; Milliron, D.J. Rationalizing the Impact of Surface Depletion on Electrochemical Modulation of Plasmon Resonance Absorption in Metal Oxide Nanocrystals. ACS Photonics 2018, 5, 2044–2050. [CrossRef] 14. Kriegel, I.; Scotognella, F. Tunable light filtering by a Bragg mirror/heavily doped semiconducting nanocrystal composite. Beilstein J. Nanotechnol. 2015, 6, 193–200. [CrossRef][PubMed] 15. Guduru, S.S.K.; Kriegel, I.; Ramponi, R.; Scotognella, F. Plasmonic Heavily-Doped Semiconductor Nanocrystal Dielectrics: Making Static Photonic Crystals Dynamic. J. Phys. Chem. C 2015, 119, 2775–2782. [CrossRef] 16. Llordés, A.; Garcia, G.; Gazquez, J.; Milliron, D.J. Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites. Nature 2013, 500, 323–326. [CrossRef] 17. Müller, J.; Sönnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T.A.; Feldmann, J. Electrically controlled light scattering with single metal nanoparticles. Appl. Phys. Lett. 2002, 81, 171–173. [CrossRef] 18. Lounis, S.D.; Runnerstrom, E.L.; Bergerud, A.; Nordlund, D.; Milliron, D.J. Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals. J. Am. Chem. Soc. 2014, 136, 7110–7116. [CrossRef] 19. Mendelsberg, R.J.; Garcia, G.; Li, H.; Manna, L.; Milliron, D.J. Understanding the Plasmon Resonance in Ensembles of Degenerately Doped Semiconductor Nanocrystals. J. Phys. Chem. C 2012, 116, 12226–12231. [CrossRef] 20. Scotognella, F.; Chiasera, A.; Criante, L.; Aluicio-Sarduy, E.; Varas, S.; Pelli, S.; Łukowiak, A.; Righini, G.C.; Ramponi, R.; Ferrari, M.; et al. Metal oxide one dimensional photonic crystals made by RF sputtering and spin coating. Ceram. Int. 2015, 41, 8655–8659. [CrossRef] 21. Sihvola, A.; Lindell, I.V.Polarizability and Effective Permittivity of Layered and Continuously Inhomogeneous Dielectric Spheres. J. Electromagn. Waves Appl. 1989, 3, 37–60. [CrossRef] 22. Malitson, I.H. Interspecimen Comparison of the Refractive Index of Fused Silica. J. Opt. Soc. Am. 1965, 55, 1205–1208. [CrossRef]

23. Niklasson, G.A.; Granqvist, C.G. Optical properties and solar selectivity of coevaporated Co-Al2O3 composite films. J. Appl. Phys. 1984, 55, 3382–3410. [CrossRef] 24. Smith, G.B.; Niklasson, G.A.; Svensson, J.S.E.M.; Granqvist, C.G. Noble-metal-based transparent infrared reflectors: Experiments and theoretical analyses for very thin gold films. J. Appl. Phys. 1986, 59, 571–581. [CrossRef] 25. Scotognella, F.; Puzzo, D.P.; Monguzzi, A.; Wiersma, D.S.; Maschke, D.; Tubino, R.; Ozin, G.A. Nanoparticle One-Dimensional Photonic-Crystal Dye Laser. Small 2009, 5, 2048–2052. [CrossRef][PubMed] 26. Born, M.; Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light; Cambridge University Press: Cambridge, UK, 2000. 27. Xiao, X.; Wenjun, W.; Shuhong, L.; Wanquan, Z.; Dong, Z.; Qianqian, D.; Xuexi, G.; Bingyuan, Z. Investigation

of defect modes with Al2O3 and TiO2 in one-dimensional photonic crystals. Optik 2016, 127, 135–138. [CrossRef] 28. Paternò, G.M.; Iseppon, C.; D’Altri, A.; Fasanotti, C.; Merati, G.; Randi, M.; Desii, A.; Pogna, E.A.A.; Viola, D.; Cerullo, G.; et al. Solution Processable and Optically Switchable 1D Photonic Structures. ArXiv171103192 Cond-Mat Physicsphysics. Available online: http://arxiv.org/abs/1711.03192 (accessed on 5 December 2017). 29. Solar Spectral Irradiance: Air Mass 1.5, (n.d.). Available online: http://rredc.nrel.gov/solar/spectra/am1.5/ (accessed on 14 February 2018). 30. Clavero, C. Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices. Nat. Photonics 2014, 8, 95–103. [CrossRef] 31. Leenheer, A.J.; Narang, P.; Lewis, N.S.; Atwater, H.A. Solar energy conversion via hot electron internal photoemission in metallic nanostructures: Efficiency estimates. J. Appl. Phys. 2014, 115, 134301. [CrossRef]

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