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Herein we present nanoporous transparent and conducting films prepared by sputtering controlled amounts of ITO onto and into the interconnected pores of SiO2 and SnO2 nanoparticle films. The sheet resistance of these nanoporous transparent conducting films made with either SiO2 or SnO2 nanoparticles is less than 103 Ω/□. Furthermore, we demonstrate entirely nanoporous one-dimensional photonic crystals by alternately stacking nanoporous transparent and conducting films made from SiO2 and SnO2 nanoparticles.
© 2013 Optical Society of America
1. Introduction
Various methods of fabricating porous transparent and conducting films have been reported in the literature. For example, Fattakhova et al. have used block copolymers in the Evaporation- Induced Self-Assembly process [1] to prepare crystalline indium-tin oxide (ITO) layers with well ordered, accessible mesoporosity and good electrical conductivity [2]. Porous transparent conducting oxide (TCO) films have also been prepared using vacuum-based glancing angle deposition [3]. In this process a thin film of plural columns, referred to as “seed posts”, are initially formed. The growth direction of the columns can be controlled by altering the oblique angle from which the vapor flux arrives. Nanoporous transparent conducting films have also been made by infiltrating a nanoporous structure with a conducting material using electrodeposition [4]. In this three step process a transparent conducting layer is first deposited onto a substrate. Secondly, a film with interconnected nanopores is deposited onto the transparent conducting layer. In the final step the pores are infiltrated with a transparent and conducting material via electrodeposition. Another method of depositing nanoporous transparent conducting films is by spin- or dip-coating dispersions of conductive nanoparticles (NPs) [5,6]. In other work Lin et al. have prepared microporous films by using metallorganic chemical vapour deposition to deposit ITO into the void space of a porous glass (PG) matrix [7]. However, the transmittance of these PG/ITO composite films is low because visible light is scattered by its micropores. In contrast to microporous films, nanoporous films can be made with high transparency because their small pores (< 50nm) do not significantly scatter visible light.
Herein we present a new method of fabricating nanoporous, transparent and conducting (n-TC) films with interconnected pores. Moreover, these films can be used as “building blocks” to fabricate entirely nanoporous selectively transparent and conducting photonic crystals (n-STCPCs). This new method of fabricating n-TC films involves sputtering controlled amounts of a transparent conducting material onto and into a transparent nanoporous material. Advantages of the fabrication technique presented in this work are that the transparent nanoporous material does not necessarily have to be conductive itself and that an initial deposition of a TCO film or a set of seed posts is not required.
2. Fabrication methods
The n-TC films presented herein are prepared by sputtering ITO onto and into the pores of a silica NP film. For example, an SEM image of a bare silica NP film is shown in Fig. 1(a) while n-TC films prepared from silica NPs (determined to be ~10nm from SEM imaging) are shown in Figs. 1(b), 1(c) and 1(d). The ~100nm thick NP film shown in Fig. 1(a) was prepared from a solution of SiO2 NPs, purchased from Aldrich (Ludox SM-30, 30 wt%) that was diluted in deionized water (3:1 deionized water/Ludox), and then spin-coated onto a ~2.5cm x 2.5cm Corning glass substrate. Subsequently, 10nm of ITO is deposited onto the SiO2 NP film by RF magnetron sputtering to fabricate the n-TC film shown in Fig. 1(b). In comparing Figs. 1(a) and 1(b) it is evident that after sputtering 10nm of ITO onto the silica NP films the top layer of SiO2 NPs retain a nearly spherical shape and porous regions are still visible in the regions where the spheres are less tightly packed together. Further details about the methods used to perform the spin-coating and sputtering depositions are available in the literature [8].
Fig. 1 Top-view SEM images of (a) a bare silica NP film and (b) a n-TC film prepared by sputtering 10nm of ITO onto a SiO2 nanoparticle film. Cross-sectional SEM images of a n-TC films prepared by alternately spin-coating films from a SiO2 nanoparticle dispersion with a deionized water:Ludox concentration of (c) [3:1] and (d) [5:1] and sputter-depositing ITO films, repeated five times over. TOF-SIMs depth profiles for the samples in (c) and (d) are shown overlaying their cross-sectional SEM image (rotated by 90°) in (e) and (f), respectively; the vertical scale represents the signal counts.
This process, wherein a SiO2 NP layer is deposited and subsequently controlled amounts of ITO are sputtered onto the film, can be successively repeated in order to build up a nanoporous film comprised of multiple layers to achieve a desired thickness. For example, the cross-sectional SEM images in Figs. 1(c) and 1(d) show n-TC films prepared by successively spin-coating SiO2 NP films from dispersions with a deionized water:Ludox concentration of [3:1] and [5:1], respectively, and subsequently sputtering 10nm of ITO onto these films, repeated five times over. Each NP layer in the film prepared from the more concentrated [3:1] dispersion is ~100nm and the overall thickness of the film is ~500nm. Likewise, each layer in the film prepared from the less concentrated [5:1] dispersion is ~60nm and its overall thickness is ~300nm. Moreover, thin ITO overlayers reside on top of the five NP layers within each film. The porosity of these n-TC films will be discussed afterwards with reference to Fig. 4.
2. TOF-SIMS experiments
We performed Time-of-flight Secondary-Ion Mass-Spectroscopy (TOF-SIMS) experiments on the n-TC films shown in Figs. 1(c) and 1(d) to investigate the depth profile of the ITO throughout these structures. Specifically, the compositional depth profile, measured using TOF-SIMs as a function of time, for each sample shown in Figs. 1(c) and 1(d) is shown overlaying its cross-sectional SEM image (rotated by 90°) in Figs. 1(e) and 1(f). The depth profiles of Si and In indicate the relative concentrations of silica and ITO within the n-TC films, respectively. The peaks of the In profile align perfectly with the thin ITO over-layers in the film. Furthermore, the height of these peaks decreases while their base increases for successive layers in the film because some ITO from the over-layer diffuses into the pores of the NP film during the annealing process [8]. Since the annealing treatment is performed after each silica layer is deposited, the ITO over-layers towards the bottom of the n-TC film (closer to the substrate) experience longer over-all annealing times. The Si depth profile has the same periodicity as the In profile, but shifted to deeper values as expected.
3. Optical characterization of nanoporous transparent and conducting films
The transmittance spectra of n-TC SiO2 films prepared from dispersions with deionized water:Ludox concentrations of [3:1] and [5:1] are plotted in Figs. 2(a) and 2(b), respectively. The transmittance spectrum through a bare glass substrate is also plotted in each of these figures for comparison. It is interesting to note that the transmittance spectra shown in Fig. 2(a) exhibit a dip at ~400nm. This dip is the side-lobe of a Bragg-reflectance peak; Bragg-reflectance occurs from the thin ITO over-layers residing on top of each SiO2 nanoparticle film. Bragg-reflectance is not evident for the sample prepared from the solution with the [5:1] concentration because the stop-gap for this one-dimensional photonic crystal (1DPC) structure is positioned at wavelengths much lower than the absorption edge of the glass substrate. For photons of wavelength greater than ~500nm the transmittance through the n-TC films is close to that of the glass substrate.
Fig. 2 The transmittance spectra of n-TC films with 1- or 5- layers prepared from dispersions with deionized water:Ludox concentrations of (a) [3:1] and (b) [5:1] and sputtered ITO. The transmittance of a bare glass substrate is plotted as the dashed line.
4. Electrical characterization of nanoporous transparent and conducting films
We also measured the sheet resistance of n-TC films with 1-, 3-, or 5- layers using 4pt-probe conductivity measurements (Four Dimensions, Inc. Model 101C) and the results are plotted in Fig. 3.The sheet resistance of films with 1-layer prepared from SiO2 dispersions with deionized water:Ludox concentrations of [3:1] and [5:1] and sputtered ITO are 3.1x104 Ω/□ and 4.2x104 Ω/□, respectively. As expected, the sheet resistance of these films decreases as the number of NP film layers in their overall structure increases. For example the sheet resistances of n-TC films prepared from deionized water:Ludox concentrations of [3:1] and [5:1] with 5-layers are 1.4x104 Ω/□ and 8.5x102 Ω/□, respectively.
Fig. 3 The sheet resistances of nanoporous transparent conducting films with 1-, 3- or 5- layers prepared from dispersions with deionized water:Ludox concentrations of [3:1] and [5:1] and from SnO2 NPs with and without the addition of sputtered ITO.
The sheet resistance of the films prepared from the more dilute SiO2 NP dispersion decreases to a greater extent with increasing number of NP films in the overall structure because there is a stronger electrical connection between the conductive ITO over-layers at the topside of each NP film. That is, during the sputter deposition some of the ITO infiltrates the porous region of the NP film and creates a continuous electrically conductive pathway throughout the structure. For thinner SiO2 layers (prepared from more dilute NP dispersions) the ITO over-layers are closer together and there is a stronger electrical connection between them. This is in agreement with the TOF-SIMS measurements in Figs. 1(e) and 1(f) which show that the volume fraction of ITO in the porous region of the NP films is larger for the sample prepared from the more dilute deionized water:Ludox concentration of [5:1]. Thus, it is possible to reduce the sheet resistance of the n-TC films with 3-, or 5- layers reported herein by reducing the thickness of each layer within the structure. This could be accomplished without compromising film quality by increasing the ramp rate and final rotation speed of the spin-coating process [9]. However, decreasing the thickness of the each layer with the n-TC films with 3-, or 5- layers will also affect the optical properties of the film. That is, decreasing the thickness of the individual layers will increase the effective refractive index of the film since a greater portion of the film will comprise sputtered ITO.
We also investigated the sheet resistance of n-TC films prepared from conductive SnO2 NPs. The size of the SnO2 NPs and the film thickness were determined to be ~8nm and ~70nm, respectively, from cross-sectional SEM imaging. The details about the spin-coating procedure used to fabricate the SnO2 NP films are available in the literature [10]. The sheet resistance of n-TC films prepared identically to those shown in Fig. 1, but with SnO2 rather than SiO2 NP films, is plotted as the open black triangles in Fig. 3 as a function of the number of layers in the structure. The sheet resistance of the n-TC films made from SnO2 films is comparable to that of n-TC films made from SiO2 NPs. Furthermore, we also measured the sheet resistance of SnO2 NP films that were not subjected to ITO sputter deposition after each NP film was spin-coated and the results are plotted as the solid black triangles in Fig. 3. The sheet resistance of these samples is greater than 1x107 Ω/□ and does not decrease significantly as the number of layers in the structure increases. We also prepared reference SiO2 NP films without sputtered ITO, however the sheet resistance of these films was too large to measure. These results show that sputtered ITO provides the primary conductive pathway through the n-TC films. For example, as shown in Fig. 3, the sheet resistance of the SnO2 NP films with 5-layers is reduced by four orders of magnitude, from ~1x107 Ω/□ to ~1x103 Ω/□, when the 10nm ITO layers are sputtered onto the structure after each spin-coating step.
5. Transparent and conducting nanoporous photonic crystals
In recently reported work we have fabricated selectively transparent and conducting photonic crystals (STCPCs) from alternating layers of spin-coated NP films and sputtered ITO layers [8, 11] The n-TC films shown in Fig. 1 can be used as “building blocks” in a bottoms-up approach to fabricate entirely nanoporous STCPCs (n-STCPCs). Specifically, the SiO2 and SnO2 n-TC films presented in the previous section were alternately deposited in order to build up a substantially porous 1DPC, otherwise known as a Bragg-reflector [12]. Figures 4(a) and 4(b) show cross-sectional SEM images of a n-STCPC comprised of five alternating bi-layers of SnO2 and SiO2 n-TC films prepared with a deionized water:Ludox concentration of [5:1] and [3:1], respectively. The reflection spectra of the n-STCPCs imaged in Figs. 4(a) and 4(b) are plotted in Fig. 4(c) and exhibit broad and intense reflection peaks. For example, the reflection spectrum of the n-STCPC shown in Fig. 4(b) has peak value of 82% and a full-width at half-maximum of 170nm. Moreover, the transmission spectrum of this n-STCPC, plotted as the blue line in Fig. 4(c), demonstrates high transparency in the spectral regions outside the stop-gap. Furthermore, the sheet resistance of the n-STCPCs prepared from dispersions with deionized water:Ludox concentrations of [5:1] and [3:1] are 1.4x103 Ω/□ and 2.6x103 Ω/□, respectively.
Fig. 4 Cross-sectional SEM images of n-STCPCs made from alternating layers of SnO2 and SiO2 NP films prepared from dispersions with deionized water:Ludox concentrations of (a) [5:1] and (b) [3:1]. The top and bottom layers in the 1DPC are comprised of SnO2 and SiO2 nanoparticles, respectively. (c) The reflection spectra of the n-STCPCs shown in (a) and (b). A Bragg-reflector made of alternating layers of SnO2 and SiO2 NP films (d) and a n-STCPC made similarly to the Bragg-reflector shown in (d) but with 10nm of ITO sputtered on top of each NP film within the structure (e). The transmittance spectra of the NP Bragg-stack shown in (d) when air, water, and toluene are the surrounding medium is plotted as the solid, dashed and dotted light-blue lines, respectively (f). Likewise, the transmittance spectra of the n-STCPC shown in (e) when air, water, and toluene are the surrounding medium is plotted as the solid, dashed and dotted black lines, respectively. It can be noted that the SEM image of the 1DPC with sputtered ITO (e) is clearer than that of the 1DPC without sputtered ITO (d) because the Bragg-reflector without sputtered ITO charges to a greater extent during SEM imaging.
The porosity of SiO2 and SnO2 NP films, similar to those presented herein but without the addition of sputtered ITO, were determined to be ~28% from ellipsometric porosimetry analysis (EPA) in previous work [8,10]. However, we were unable to determine the porosity of the n-TC films presented herein using EPA measurements; it was not possible to perform the optical analysis on account of the complex depth profile of the ITO throughout the structure, as shown in the TOF-SIMS plots in Fig. 1.
In order to gain insight into the nature of their porosity we prepared two n-STCPC films similar to that shown in Fig. 4a, however, the sputtered ITO films was not included for one of them. Cross-sectional SEM images of these 1DPCs comprised of alternating layers of SiO2 and SnO2 NP films, without and with sputtered ITO, are shown in Figs. 4(d) and 4(e), respectively. We measured the transmission of these 1DPCs while they were immersed in air (n = 1), water (n ~1.33), and toluene (n~1.5) and the results are plotted in Fig. 4(f). As expected, the transmission dips red-shift and become less pronounced as the index of refraction of the back-ground media increases. Moreover, the transmission spectra of the 1DPC with the ITO are similar to that of the 1DPC without sputtered ITO, but red-shifted because the periodicity of the n-STCPC is larger on account of the thickness of the sputtered ITO over-layers. From comparing Figs. 4(d) and 4(e), on average the ITO layers are ~5nm thick. Thus, given that the porosity of the 1DPC without sputtered ITO is 28% and that roughly half of the 10nm thick ITO layers infiltrate the pores of the NP films they are sputtered onto one can estimate that the porosity of the n-STCPC shown in Fig. 4(a), which is 677nm thick in total, is ~20%. However, it is noted that this is a rough estimate and that future work is required to investigate the pore sizes and distribution throughout the n-STCPCs presented in this work.
6. Conclusion
In conclusion, we have presented a novel method of fabricating nanoporous transparent and conducting films by sputtering small and controlled amounts of ITO onto and into a transparent nanoporous film with interconnected pores. Their built-in continuous network of nanoscale voids provides a distinct advantage over conventional solid thin-film transparent conducting oxides; their continuously interconnected pore volume can be infiltrated with numerous different reactive species to fabricate devices. Furthermore, two different types of nanoporous transparent and conducting films comprised of nanoparticles with different indices of refraction can be alternately stacked in order to form a one-dimensional nanoporous transparent and conducting photonic crystal. Given their unique combination of mesoporosity, conductivity, optical properties and large specific surface area these 1DPCs could potentially be utilized to enhance numerous devices such as dye-sensitized solar cells [13], photoelectrochemical sensors [14], as structures that host chemical reactions amplified by slow-photons [15], to design nanostructured catalytic electrodes with enhanced charge transport for effectively reducing CO2 to hydrocarbon fuels or converting water to hydrogen [16].
Acknowledgments
This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Ontario Research Fund – Research Excellence program. GAO is Government of Canada Research Chair in Materials Chemistry and Nanochemistry. We are grateful to Dr. Sodhi at Surface Interface Ontraio (SIO) at the University of Toronto for some useful discussions.
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