Tuning intrinsic photoluminescence from light-emitting multispectral nanoporous anodic alumina photonic crystals

Solid-state light-driven luminescence—henceforth “photoluminescence” or PL—is a class of light–matter interaction in which incident photons absorbed by a material’s atoms excite electrons to higher energy levels. Once excited, electrons undergo certain relaxation processes through which photons are re-radiated or emitted in the form of a light beam.¹ The properties of the emitted photoluminescent beam rely on the electronic band structure of the photo-excited material.² Characteristically, PL emissions from solid-state materials with defects in their electronic band structure are spectrally broad, featuring a Gaussian-like emission centered within the vicinity of the exciton resonance—typically at a longer wavelength than that of the excitation input.³ Broad PL emissions can be narrowed by distinct strategies to fulfill requirements for specific applications, such as lasing,⁴ LEDs,^5,6 bioimaging,⁷ and diagnostics and sensing.^8,9 Of all these, engineering the structure of light-emitting materials at the nanoscale in the form of photonic crystals (PCs) enables highly directional emission of light with tunable wavelength by confining, enhancing, and attenuating emitted light at specific, narrow spectral regions.^10,11 PCs are optical structures with allowed and forbidden photonic stopbands (PSBs) that modify the group velocity of incident or emitted photons when these flow through the PCs’ structure. Light–matter interactions in PCs can be tuned with precision by engineering their architecture so that regions with high and low refractive indices are periodically and spatially distributed following specific patterns.¹² A variety of PC forms based on a broad range of materials can be produced to control electromagnetic waves with versatility over the broad spectrum from UV to IR. Currently, PCs have multiple applications, including chemical sensing and biosensing,^13–18 photonic encoding,¹⁹ lasing,^20–25 and photocatalysis.^26,27 Nanoporous PCs are particularly suitable platform materials to tailor photo-induced light emissions both extrinsically and intrinsically. In the former mode, a host non-photoluminescent nanoporous PC is infiltrated with a photoluminescent material to enhance, filter, or guide its light emission,²⁷ while in the latter configuration, a photoluminescent nanoporous PC material filters its own PL emission.²⁸ Nanoporous anodic alumina (NAA) has long been devised as an ideal nanoporous PC platform material due to its cylindrical nanopores with well-defined and highly controllable geometric features.^29–32 NAA is produced by electrochemical oxidation—anodization—of aluminum, which is a cost-effective and fully scalable top–down industrial process compatible with conventional micro- and nanofabrication. Although architecture, dimensions, and distribution of nanopores in NAA can be precisely engineered in a variety of NAA-based PCs (NAA–PCs) during anodization,^33–35 its intrinsic optical properties—refractive index and photoluminescence—are critically determined by the anodizing acid electrolyte, anodization regime, and post-anodization treatments, such as annealing.³⁶ The origin of PL in NAA is thought to be attributable to two types of photoluminescent centers: (i) F⁺ centers^37–39 related to ionized oxygen vacancies and (ii) F⁻ centers associated with carboxylate impurities incorporated from the acid electrolyte during anodization.^40,41 The position and intensity of NAA’s PL emission depend intrinsically on the quantity of F⁺ and F⁻ centers and their respective distribution across the structure of NAA. NAA films produced in oxalic acid feature more intense PL emission than that of its counterparts fabricated in sulfuric or phosphoric acids.^42–46 NAA’s PL intensity also decreases with annealing temperature due to the elimination of oxygen vacancies and impurities in its structure through crystallographic organization and burning under oxygen atmosphere.^47–52 Pioneering studies on 2D NAA–PCs featuring straight nanopores demonstrated their application as host materials to tune PL emissions from fluorescent dyes infiltrated in their nanoporous matrix.^57,58 In this system, the red edge of the characteristic photonic stopband (PSB) of NAA–PCs reduces the group velocity of incident photons at that spectral region by the so-called “slow photon” effect. As a result, PL emission is enhanced due to increasing frequency of interactions between exciting photons and light-emitting molecules at that spectral region—increase in the radiative rate.²⁵ An alternative approach would be to tailor the intrinsic photoluminescence of NAA by engineering its structure in the form NAA–PCs.^53,54 Under this configuration, alignment of the NAA–PC’s PSB with the Gaussian-like PL emission of NAA would narrow its linewidth by internally forbidding light propagation at specific spectral regions. Surprisingly, to the best of our knowledge, this system has not been explored yet.

In this work, we demonstrate for the first time how the intrinsic photoluminescence of NAA can be tuned by engineering its structure in the form of multispectral PCs (MS–NAA–PCs)—NAA–PCs featuring multiple PSBs at specific spectral positions (see Fig. 1). Spectral alignment between the three characteristic PSBs of MS–NAA–PCs and the photoluminescence emission of NAA [see Fig. 1(a)] makes it possible to judiciously narrow its linewidth with precision by inhibiting light propagation at specific spectral positions [see Fig. 1(b)]. Our results create exciting new opportunities to modulate light emission from NAA-based structures, which could have broad implications across optoelectronic disciplines, such as lasing, sensing, energy harvesting, and photocatalysis.

High purity aluminum circular chips (Al) (0.5 mm thickness and 99.99% purity) were acquired from Goodfellow Cambridge Ltd. (UK). Acetone [(CH₃)₂CO], ethanol (EtOH; C₂H₅OH), perchloric acid (HClO₄), oxalic acid (H₂C₂O₄), hydrochloric acid (HCl), sulfuric acid (H₂SO₄), chromic acid (H₂CrO₄), phosphoric acid (H₃PO₄), and copper chloride (CuCl₂) were provided by Sigma-Aldrich (Spain). Double deionized water (DI) (18.2 M$Ω$ cm) was used to prepare all aqueous solutions, unless otherwise specified.

A set of NAA films featuring straight cylindrical nanopores from the top to the bottom were used as a reference material to quantify bulk PL emission. Al disks were anodized by the two-step process, full details of which are reported elsewhere.^52–56 Briefly, before anodization, Al chips were electropolished in a mixture of EtOH and HClO₄ 4:1 (v:v) at 20 V and 5 °C for 3 min. Then, the first anodization step was carried out in 0.3M H₂C₂O₄ aqueous electrolyte at 40 V and 5 °C for 20 h. Next, the resulting NAA layer was selectively removed by wet chemical etching in a mixture of 0.2M H₂CrO₄ and 0.4M H₃PO₄ for 3 h at 70 °C. After this, the second anodization step was performed under the same anodization conditions for a total charge density (integrated current density throughout time per unit of area) of 113.2 C cm⁻² under voltage control conditions at 40 V to generate a NAA film of ∼50 µm featuring straight cylindrical nanopores.

Fabrication of MS–NAA–PCs was performed following a modified two-step anodization process. Electropolished Al chips were first anodized in a 0.3M sulfuric acid electrolyte at 5 °C under constant voltage at 25 V for 20 h. The NAA film was then chemically dissolved in a mixture of 0.2M H₂CrO₄ and 0.4M H₃PO₄ for 3 h at 70 °C. Then, a 10-min second step in the same acid electrolyte at 25 V for 10 min was applied to generate a shuttle layer for homogeneous nanopore growth. Then, the anodization process was switched to sinusoidal pulse anodization under current density control conditions. During this process, a sinusoidal current density input profile was applied to modulate the nanopore diameter in depth and generate gradient-index filter PC structures.⁵⁹ MS–NAA–PCs consisted of a NAA layer featuring three gradient-index filters generated by judiciously modulating the anodizing current density waveform input into sequential sinusoids with three different pulse periods (i.e., T₁, T₂, and T₃), as described by the following equation:

where J(t) is the anodization current density at time t (in seconds), J₁ is the current density amplitude (in mA cm⁻²), T_i (i = 1, 2, 3) is the anodization period i (in seconds), and J₀ is the offset current density (in mA cm⁻²). The offset current density (J₀), current density amplitude (J₁), and number of periods (N) were kept constant throughout the anodization process for all MS–NAA–PCs fabricated in this study, while the anodization periods were varied as T₁ = 125 s, T₂ = 150 s, and T₃ = 175 s for N = 100 pulses per period (N₁, N₂, and N₃) (i.e., total of 300 pulses). After this, a 50 µm thick NAA layer produced in 0.3M oxalic electrolyte (i.e., light-emitting layer) was generated under potentiostatic conditions at 40 V (i.e., third anodization step). Table I summarizes the fabrication conditions used to fabricate MS–NAA–PCs. After anodization, the aluminum substrate remaining at the backside of MS–NAA–PCs was removed by selective chemical etching in a saturated solution of HCl and CuCl₂ for optical characterization.

Reflection spectra from MS–NAA–PCs were measured from 250 to 900 nm with a resolution of 2 nm at varying angle of incidence from 8° to 65° in a PerkinElmer UV–visible–NIR Lambda 950 spectrophotometer. The reflection intensity (R_PSB), position of central wavelength (λ_PSB), and full width at half maximum (FWHM_PSB) of the photonic stopbands (PSBs) of MS–NAA–PCs were estimated from reflection spectra through Gaussian fittings performed in OriginPro8.5^®. PL spectra from MS–NAA–PCs were acquired in a fluorescence spectrophotometer (Photon Technology International Inc., Division of Horiba, USA) equipped with a Xe lamp as an excitation light source at room temperature and at an excitation wavelength (λ_ex) of 355 nm. PL measurements were performed from the top side of reference NAA films and MS–NAA–PCs using a commercial bandpass filter with a cutoff wavelength of 350 nm.

Morphological and structural features of NAA films and MS–NAA–PCs were characterized by a field-emission gun scanning electron microscope (FEG-SEM FEI Quanta 450) operating at an accelerating voltage of 20–25 keV. Characteristic geometric features of MS–NAA–PCs were quantified by analyzing FEG-SEM images in ImageJ software.⁶⁰ 

Figure 2(a) illustrates the fabrication process used to produce the light-filtering layer of MS–NAA–PCs under sinusoidal current density conditions, including a full-view of a representative anodization profile. Figure 2(b) shows magnified views of this sinusoidal pulse anodization process in which the current density period (T_i)—time between consecutive sinusoidal pulses—is increased from 125 to 175 s with a step size ΔTi = 25 s after each 100 pulses. This input current density profile results in three stacked layers of NAA featuring a modulated nanopore diameter in depth, each of which represents a gradient-index filter with its characteristic PSB. As such, the optical response of the composite NAA–PC features three PSBs, one for each NAA gradient-index filter composing the overall PC structure. It is apparent from these graphs that under the fabrication conditions used in our study, the sinusoidal current density input with J₀ = 1.98 mA cm⁻², J₁ = 1.41 mA cm⁻², T₁ = 125 s, T₂ = 150 s, and T₃ = 175 s for a total of 300 pulses is precisely translated into a sinusoidal voltage output, which mimics the frequency of the input profile. Analysis of this anodization profile indicates that the amplitude of the sinusoidal voltage output slightly increases with increasing T_i from ∼20–25 V at T₁ = 125 s to ∼20–29 V at T₃.

Figure 3 shows a set of top and cross-sectional FEG-SEM images of a representative MS–NAA–PC. The structure of these heterogeneous NAA–PCs features a top layer composed of three NAA gradient-index filters with varying period length (L_TP)—distance between adjacent nanopore modulations in depth—and a bottom NAA layer as a light-emitting film. Figure 3(a) shows a top view FEG-SEM image of this NAA–PC structure, which features hexagonally arranged, homogeneously distributed nanopores across its surface. The average nanopore diameter estimated from FEG-SEM image analysis was 17 ± 4 nm, while the interpore distance—distance between the center of adjacent nanopores—was measured to be 62 ± 5 nm. Figure 3(b) shows a general cross-sectional view FEG-SEM image of these NAA–PCs from which it is possible to discern two main layers: (i) a top light-filtering layer formed by three NAA gradient index filters labeled NAA–PC–A (T₁ = 125 s), NAA–PC–B (T₂ = 150 s), and NAA–PC–C (T₃ = 175 s), which feature the modulated nanopore diameter in depth; and (ii) a bottom light-emitting layer featuring straight cylindrical nanopores at the bottom labeled NAA–Ox. The thicknesses of the top and bottom layers measured by FEG-SEM image analysis were 58 ± 1 and 50 ± 1 µm, respectively. Figure 3(b) shows a set of magnified cross-sectional views showing details of nanopores at specific positions of the top and bottom layers of the MS–NAA–PC structure. FEG-SEM images of the NAA gradient-index filters forming the top light-filtering layer reveal nanopore modulations, the period length (L_TP) of which increases with the input anodization period (T_i). Analysis of this geometric parameter for NAA–PC–A, NAA–PC–B, and NAA–PC–C reveals that L_TP increases linearly with T_i at a rate of 2.1 ± 0.1 nm s⁻¹, having values of 210 ± 4, 265 ± 6, and 313 ± 8 nm for T₁, T₂, and T₃, respectively. Figure 3(c) also shows a magnified cross-sectional view of the light-emitting layer at the bottom of the MS–NAA–PC structure, which was fabricated in 0.3M oxalic acid electrolyte at 40 V for a total of ∼16 h. This layer features straight cylindrical nanopores with a constant diameter from the top to the bottom. A set of FEG-SEM images of this film from a control NAA–Ox sample used to characterize the bulk PL emission of this layer are shown in Fig. S1 of the supplementary material. The 50 μm-thick layer features straight cylindrical nanopores with an average diameter of 40 ± 3 nm and an interpore distance of 101 ± 6 nm.

MS–NAA–PCs can be optically described as heterogeneous PC structures with a spectroscopic signature that results from the multiple contributions of the NAA-based gradient-index filters composing their structure (i.e., NAA–PC–A, NAA–PC–B, and NAA–PC–C). As such, the reflection spectrum of MS–NAA–PCs features three well-resolved, intense PSBs, the features of which are critically determined by the structural features of each NAA gradient-index filter engineered by the input current density profile. The light-filtering layer on the top side of MS–NAA–PCs filters the photon-stimulated PL emission from the light-emitting layer at the bottom side of the MS–NAA–PC structure. Therefore, it is critical to characterize and understand the angle-dependence of both light reflection and light emission at the light-filtering and light-emitting layers of the MS–NAA–PC structure, respectively.

The effect of the incidence or excitation angle (θ) on the optical features of the PSBs of MS–NAA–PCs was systematically evaluated by reflection spectroscopy [see Fig. 4(a)]. Figure 4(b) shows the reflection spectra of MS–NAA–PCs at different incidence angles (i.e., 8°, 10°, 15° 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, and 65°). Table S1 of the supplementary material summarizes the optical features of the three characteristic PSBs of NAA–PC–A, NAA–PC–B, and NAA–PC–C labeled PSB₁, PSB₂, and PSB₃, respectively. These include the central wavelength position (λ_PSB), reflection intensity (R_PSB), and full width at half maximum (FWHM_PSB), all estimated from Gaussian fittings. The position of the three PSBs in the reflection spectrum of MS–NAA–PCs blue shifts, while their intensity decreases with increasing angle of incidence. This result is in good agreement with the Bragg–Snell law in which the wavelength of the diffracted light depends on the angle of incidence, the periodicity, and the effective refractive index^61–63 of the nanoporous PC as expressed in the following equation:

where λ_PSB is the central wavelength of the PSB, m is the order of the PSB, L_TP is the structure periodicity, θ is the angle of incidence, and n_eff is the effective refractive index. Analysis of this dependence, summarized in Fig. 4(c), shows that λ_PSB is blue shifted at rates of −1.22 ± 0.04, −1.49 ± 0.05, and −1.78 ± 0.06 nm deg⁻¹ for PSB₁, PSB₂, and PSB₃, respectively. Fig. S2 of the supplementary material shows the dependence of R_PSB and FWHM_PSB as a function of the incidence angle, which indicates that the reflection intensity of the PSB decreases with increasing angle of incidence. Analysis of the fitting lines shown in Fig. S2a of the supplementary material denotes that R_PSB decreases at rates of −0.37 ± 0.02, −0.39 ± 0.007, and −0.38 ± 0.016 a.u. deg⁻¹ for PSB₁, PSB₂, and PSB₃, respectively. It is apparent from Fig. S2b of the supplementary material that the dependence of FWHM_PSB with the angle of incidence follows a Gaussian-like trend, where this optical feature increases until 40° and dramatically decreases from >45° to 65°. Trends for FWHM_PSB’s PSB₁, PSB₂, and PSB₃ were determined to be FWHM₁ = 7.53 + ((64.63)/(34.64 · p/2)·Exp(−2·(θ − 36.59/(34.65)), FWHM₂ = −4.74 + ((2363.2)/(123.63 · p/2)·Exp(−2·(θ − 33.84/(123.63)), and FWHM₃ = 9.19 + ((125.62)/(33.0 · p/2)·Exp(−2·(θ − 35.92/(33.0)) from Gaussian fittings, respectively.

Photoluminescence of NAA is attributable to oxygen vacancies distributed across the onion-like composition of anodic alumina and oxalic impurities incorporated into its structure from the acid electrolyte during the anodization process. Of all types, NAA produced in oxalic acid electrolyte has the highest PL emission.^49–51 Conversely, the characteristic PL emission from NAA produced in sulfuric acid electrolyte is weak and almost negligible to that of NAA structures produced in oxalic acid electrolyte. For this reason, we devised the structure of MS–NAA–PCs featuring a top light-filtering layer having an almost negligible PL emission and a bottom light-emitting layer with strong PL emission. Figure 5(a) shows a schematic illustration of the experimental setup used to characterize PL emissions from MS–NAA–PCs. Figure 5(b) shows the characteristic PL emission at different angles of incidence from 15° to 85° with an interval of 10° from a representative 50 μm-thick NAA film fabricated by two-step anodization in oxalic acid electrolyte. The emission spectrum of the NAA film is characterized by a broad, intense, Gaussian-like emission band across the 380–600 nm range for all the angles of incidence analyzed. It is apparent from Fig. 5(c) that the position of the PL band (λ_PL) is shifted toward longer wavelengths (i.e., red shift) with increasing angle of incidence, following a sigmoidal trend where λ_PL = 475.04 + (445.8 − 475.04)/1 + Exp (θ − 51.67/8.82). This graph indicates a slight red shift of λ_PL with θ from 445 nm at 15° to 448 nm at 35°. However, λ_PL undergoes a significant shift between 35° and 75°, in which λ_PL red shifts its position from 448 to 479 nm, respectively, and it stabilizes its position for θ > 75°. Figure 5(d) shows the dependence of FWHM_PL with the angle of incidence. It is apparent that FWHM_PL follows a Gaussian-like trend with θ, where this optical feature initially increases with angle of incidence until 45°. At this angle, FWHM_PL achieves its maximum value of 115 ± 1 nm. For θ > 45°, FWHM_PL starts to decrease until a minimum of 98 ± 2 nm at an angle of 85°. Table S2 of the supplementary material summarizes the dependence of λ_PL and FWHM_PL with the angle of incidence for a representative light-emitting NAA film produced in oxalic acid electrolyte.

MS–NAA–PCs were fabricated to feature three well-resolved PSBs at specific spectral positions within the PL emission of NAA–Ox films (i.e., ∼400–600 nm). To this end, the structure of MS–NAA–PCs was judiciously engineered with three stacked gradient-index filters embedded within the same heterogeneous PC structure. These heterogeneous NAA-based PCs were used as model platforms to study for the first time the light-filtering properties of NAA–PCs, harnessing the intrinsic light-emitting properties of NAA. MS–NAA–PCs were mounted on a rotating stage, which allows the PL signal to be measured at different angles of excitation and emission. Excitation light at 355 nm was shone onto the center of MS–NAA–PCs and the emission was collected from 380 to 600 nm at varying angle of incidence. Figure 6 shows the PL emission of a reference NAA–Ox film (black line), the reflection (red line) spectrum, and PL emission (blue line) of MS–NAA–PCs at varying angle of incidence (i.e., 65°, 70°, 75°, and 80° for PL and complementary reflection angles of 25°, 20°, 15°, and 10° for reflection). It is apparent that the generation of three gradient-index filters on top of the light-emitting NAA–Ox layer filters efficiently the PL emission from the NAA–Ox layer by narrowing FWHM_PL and shifting λ_PL following an angle-dependent pattern. FWHM_PL of the PL emission of the NAA–Ox film at 80°, 75°, 70°, and 65° was measured to be 99 ± 2, 100 ± 2, 103 ± 1, and 105 ± 1 nm, respectively [see Figs. 6(a)–6(d)]. Upon generation of MS–NAA–PCs on top of the light-emitting NAA–Ox layer, FWHM_PL is narrowed to 93 ± 2, 88 ± 2, 97 ± 3, and 88 ± 6 nm at 80°, 75°, 70°, and 65°, respectively. This result indicates that light emission is filtered by the top gradient-index filters. It is also found that λ_PL red shifts its position with decreasing angle of incidence, having values of 443 ± 1, 447 ± 1, 451 ± 1, and 450 ± 1 nm at 80°, 75°, 70°, and 65°, respectively [see Figs. 6(a)–6(d)]. For instance, the PL emission of MS–NAA–PCs at 80° [see Fig. 6(a)] shows two local minima or shoulders at 402 and 473 nm, which would correspond to light-emitting inhibition associated with PSB₁ (389 nm) and PSB₂ (461 nm), respectively. At that angle of incidence, PSB₃ (551 nm) is far from the main Gaussian-like PL emission. Therefore, it is inferred that this gradient-index filter does not contribute to the filtered PL emission from the MS–NAA–PC structure within this spectral region. Upon reduction in the angle of incidence, the characteristic PBSs of the MS–NAA–PC structure undergo a blue shift, which, in turn, changes the characteristics of the light beam emitted from the underlying NAA–Ox film. Figure 6(b) shows that the local minima of the PL emission from MS–NAA–PCs at 75° feature two local minima at 412 and 486 nm, which would correspond to light-emitting inhibition associated with PSB₁ (387 nm) and PSB₂ (466 nm), respectively. At that angle of incidence, PSB₃ is located at 548 nm, which is still far from the spectral position of the PL emission and does not contribute significantly to filtering. Figure 6(c) shows that the local minima of the PL emission of MS–NAA–PCs at 70° feature two local minima at 421 and 497 nm, which would correspond to light-emitting inhibition associated with PSB₁ (382 nm) and PSB₂ (466 nm), respectively. At that angle of incidence, PSB₃ is located at 541 nm, which is still far from the spectral position of the PL emission and does not contribute significantly to filtering. Figure 6(d) shows that the local minima of the PL emission of MS–NAA–PCs at 65° feature two local minima at 430 and 507 nm, which would correspond to light-emitting inhibition associated with PSB₁ (377 nm) and PSB₂ (455 nm), respectively. At that angle of incidence, PSB₃ is located at 541 nm, which is still far from the spectral position of the PL emission and does not contribute significantly to PL emission filtering. The optical properties of MS-NAA-PCs were modeled by combining an effective medium approximation (EMA) model and the transfer matrix method (TMM). The effective refractive index of each layer within MS–NAA–PCs’ structure was estimated by the Looyenga–Landau–Lifshitz (3L) EMA model. Figure S3 of the supplementary material shows a comparison between experimental and simulation data. These data demonstrate that simulations can predict the spectral position (λ) and the reflection (R) of the PSB of MS–NAA–PCs with excellent accuracy with a deviation of 0.68% (λ) for the spectral position and 1.7% for the reflection (%R).

To gain further insight into the light-filtering properties of MS–NAA–PCs and the dependence of PL emission, we analyzed the optical features of PL emission at distinct angle of incidence. At first glance, Fig. 7(a) reveals an interesting effect where light emitted from the underlying photoluminescent layer is narrowed and shifted with the angle of incidence. Figures 7(b) and 7(c) summarize the positions of the characteristic PSB₁, PSB₂, and PSB₃ and the features of the PL emission (FWHM_PL and λ_PL) from MS–NAA–PCs. It is apparent that the position of PL emission (λ_PL) undergoes a slight red shift when the angle of incidence is increased from 65° to 70° (i.e., from 450 to 451 nm, respectively) [see Fig. 7(b)]. After this point, the light-filtering layer of MS–NAA–PCs blue shifts the position of the Gaussian-like PL emission from the light-emitting NAA–Ox layer at the bottom of the heterogeneous NAA–PC structure. PL emission from 70° to 85° is found to blue shift linearly from 451 to 437 nm at a rate of −0.93 nm deg⁻¹. Dependence of FWHM_PL with the angle of incidence is found to follow a qualitatively similar trend. Initially, PL emission is slightly widened from 65° to 70°, where this optical feature increases from 92 to 97 nm, respectively [see Fig. 7(c)]. However, the light-filtering layer of MS–NAA–PCs can efficiently narrow PL emission from 70° to 85°, where FWHM_PL is found to be sharply and linearly narrowed from 97 to 54 nm at a rate of −3.11 nm deg⁻¹. These results clearly demonstrate that the structural design of NAA–PCs can be judiciously harnessed to tune and control the features of the intrinsic photoluminescence emission of this highly tailorable platform material. NAA is a platform material that offers unique properties, such as chemical, mechanical, and thermal stability, under different conditions. As such, light-emitting NAA-based PC structures are an alternative platform for other porous materials, such as porous silicon, which is limited by its low chemical stability without additional passivation steps and its fragile mechanical strength. Consequently, our findings provide new opportunities to develop NAA-based optical systems for a broad range of photonic technologies. It is also worth noting that the intrinsic properties of NAA can be further modified to modulate its light-emitting properties. Recent studies have demonstrated that doping with rare earths enables strong enhancement of the intrinsic electroluminescence of this material, paving the way for new light-emitting devices and systems.^64,65

To the best of our knowledge, this study is the first demonstration of structural tuning of intrinsic photoluminescence emissions from nanoporous anodic alumina photonic crystals. We have harnessed a smart structural design in which a non-emitting, light-filtering layer in the form of multi-spectral NAA–PC is combined with an intrinsically light-emitting layer of NAA. MS–NAA–PCs feature three intense, well-resolved photonic stopbands, the positions of which can be spaced across the visible spectrum from ∼380 to 560 nm. This approach makes it possible to engineer the three photonic stopbands in the top light-filtering layer to effectively narrow and tune photoluminescence emission from the underlying light-emitting layer. Alignment of the PSBs makes it possible to narrow the width of its photoluminescence emission up to ∼50 nm and blue shift its position for ∼15 nm. Inhibition of light emission is accomplished by harnessing forbidden light propagation through the characteristic photonic stopbands of the top light-filtering layer. MS–NAA–PCs enable control over intrinsic photoluminescence of NAA without the use of external PL emitters, such as dyes and fluorophores. Our findings provide exciting new opportunities to engineer the intrinsic light-emitting properties of NAA-based photonic crystals structures, which have implications across a variety of photonic technologies, such as sensing and biosensing, lasing and light sources, photodetection, photocatalysis, green energy generation, and solar light harvesting.

See the supplementary material for the structural characterization of NAA films, effect of the incidence angle on the optical features of MS-NAA-PCs, and summary of the optical features of NAA and MS-NAA PCs.

This work was supported by the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN/FEDER) (Grant No. RTI2018-094040-B-I00) and by the Agency for Management of University and Research Grants (AGAUR) (Reference No. 2017-SGR-1527). The authors acknowledge the support provided by the Australian Research Council under Grant Nos. CE140100003 and DP200102614, the School of Chemical Engineering, The University of Adelaide, the Institute for Photonics and Advanced Sensing, and the ARC Centre of Excellence for Nanoscale BioPhotonics.

The authors have no conflicts to disclose.

Dr. L.K.A. and Dr. C.S.L. carried out the experimental part. Professor L.F.M. conceived the idea and designed the experimental part of this work in collaboration with Dr. A.S. and Dr. J.F.-B. The obtained results were discussed and analyzed by all the authors. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The data that support the findings of this study are available from the corresponding author upon reasonable request.