Chapter 2: Design of Hollow–to–Solid Moth–Eye Structures Using Anodic
2.4 Summary
We have investigated tantala nanodot and nanocones arrays through the fundamental research. Artificial Moth-eye structures by using hollow-to-solid nanocones were successfully fabricated for high-performance AR coatings.
Quasi-closely packed arrays of cone-like nanostructures are fabricated by the anodization of Al/Ta bilayers coated on substrates. We proposed the mechanism of
formation of the hollow NCA due to the stress of gaseous O2. The porosity of the hollow nanostructures is calculatedly controlled by varying the deposited thickness of Ta film. The transmissions of the glass substrates coated with hollow NCA of 65.5%
porosity reached greater than 97.8% over the whole visible range. Besides, the reflectivity was suppressed greatly in a broadband omnidirection and reached as low as 10% at an incident angle of 70°. In other cases of porosity, NCA coatings were also found as an efficient ARS on sapphire and AlN substrates. Good mechanical stability makes NCA suitable for protective coatings for optical transparency. The availability of the hollow nanostructures has allowed the design of artificial optical properties as new building blocks (e. g. diffractive layer, diffusers, or reflectors) in future photonic devices.
Moreover, we also have fabricated hollow TiO2 nanocones under PAA featuring TiO2 NTs within directly on FTO glass and used them as new working electrodes in DSSCs. By depositing a thin layer (15-nm-thick) of Ti on a FTO substrate prior to anodization, we obtained self-organized hollow TiO2 nanocones, with improved contact between the FTO substrate and the overlaying Al, thereby solving the problems of delamination of an undesirable barrier. This more-stable PAA/FTO structure was highly suitable for use in subsequent sol–gel processing of Ti(OiPr)4. The novel structure combines two types of TiO2 materials—0-D nanocones and 1-D NTs—to benefits from a large contact area, direct electron transport path, and slow recombination of electrons. The unique morphology provided a photocurrent of 5.15 mA/cm2, an open circuit voltage of 0.64 V, and an IPCE peak of 26% from an 800-nm-thick NT array. The relatively short NT array results in a considerably lower photoabsorption than, for example, the current DSSC ―gold standard‖ featuring a tens-of-micrometers-thick layer of TiO2 NTs. We suspect that increasing the length of the NT array on the electrode might allow us to further improve the efficiencies of
such DSSCs. The facile synthesis of this novel architecture may allow the design of new nanostructures for use as new building blocks in future electro-optical devices.
Figure 2.1 The illustrated geometry of the RCWA configuration.
Figure 2.2 Top-view SEM image of PAA film anodizing in 0.3 M oxalic acid at 40 V after a pore-widening treatment.
200 nm
Figure 2.3 A typically chronoamperical curve was recorded during the anodization.
Figure 2.4 Top-view SEM images of the specimens terminated at point (a) A, (b) B, (c) C and (d) D in Figure 2.3. The scale bar is 200 nm.
Figure 2.5 (a) Cross-sectional SEM image of the nanodots embedded in porous alumina film. (b) Slide-view SEM image of nanodot arrays after removing alumina film.
The scale bars are 100 nm.
Figure 2.6 Schematic diagrams showing the principal steps of modification of the underlying metal/alumina interface.
Figure 2.7 XPS depth-profile analysis of Ta 4f spectrum for tantalum oxide nanodots on Si substrates at normal incidence. The bombardment of Ar+ sputtering was used to examine the inner composition; each sputter cycle lasted 5 min.
Figure 2.8 (a) Slide-view SEM image of tantalum oxide NCA in unfinished anodization. (b) Slide-view SEM image of tantalum oxide NCA in complete anodization. (c) Top-view SEM image of tantalum oxide NCA. (d) TEM image of the NCA under the porous alumina film.
Figure 2.9 A schematic diagram to describe the formation mechanism of the tantalum oxide NCA at the Ta/Al interface.
Figure 2.10 XPS depth-profile analysis of Ta 4f spectrum for tantalum oxide NCA on Si substrates at normal incidence. The Ar+ sputtering of 5 and 30 min was used to examine the inner composition.
Figure 2.11 The spectroscopic measurements of broadband reflectance for the blank silicon wafer, the anodic tantalum oxide film and tantalum oxide NCA, respectively.
Figure 2.12 Experimental reflectivities of NCAs on AlN substrates, formed from Ta films having dimensions ranging from 7 to 30 nm.
Figure 2.13 A typically chronoamperical curve was recorded during the anodization of Al on glass substrates with tTa = 0 (general PAA), 10 (hollow NCA), and 30 nm (solid NCA).
Figure 2.14 SEM and TEM images of cone-like nanostructure arrays. (a) Side-view SEM image (scale bar: 500 nm) of hollow NCA after alumina had been removed selectively. Inset: Magnified image. (b) Top-view SEM image (scale bar: 1 μm) of hollow NCA that had been ground using a diamond emery paper. TEM images of hollow (c), (d) and solid (e) nanostructures under porous alumina (scale bar: 100 nm).
Insert (e): SAED pattern revealing the amorphous composition. (f) Schematic representation of a close-packed hexagonal NPC for theoretical calculation: period, 200 nm; base diameter, 200 nm; height, 200 nm.
Figure 2.15 Schematic representation of the mechanism of the formation of the hollow nanocones. (a) Tantalum oxide nucleus formed as the anodization approached the Ta–Al interface. (b) Tantalum oxide hillocks grew, leaving voids caused by O2 pressure.
(c) Cone-like hollow nanostructures formed with pore-wall obstructing.
Figure 2.16 XPS depth-profile analysis of tantalum oxide NCAs on glass substrates at normal incidence. (a) Ta 4f spectrum of solid NCA (tTa = 30 nm); (b) Ta 4f spectrum of hollow NCA (tTa = 10 nm).
Figure 2.17 Refractive index profiles—for samples of various tTa—through air, the nanostructure and the substrate (wavelength: 488 nm). The x-axis represents the distance from the interface of the nanostructure and the substrate.
Figure 2.18 Schematic diagrams for effective refractive-index calculation.
Figure 2.19 RCWA simulation of transmisstance for single-sided coated Pyrex glass with various porosity and sizes NCAs. The nanocone structures was assumed to be a hexagonal close-packed period of (a) 100 nm, (b) 200 nm and (c) 300 nm with a height of (a)100 nm, (b) 200 nm and (c) 300 nm. A NCA owning a period and height of 200 nm and a porosity of 65% obtains the best transmisstance.
Figure 2.20 The optical performance of antireflective properties for NCA-coating glasses. (a) Measured transmissions of bare glass (green line) and NCA-coated (single side) glass substrates of initial Ta thicknesses of 10 nm (red line), 20 nm (blue line), 30 nm (black line) and 10 nm for double sides (orange line). Inset: Photographs of the structured (right) and unstructured (left) glass samples (30 mm × 30 mm) demonstrate the antireflective effect at a large tilted angle. (b) Measured wavelength and angle resolved absolute reflectance. The glass substrates were coated without (upper curve) and with double-sided NCA coating (lower curve).
Figure 2.21 The improvement of reflectivity before and after the treatment for NCA coated on (a) sapphire and (b) AlN substrates. Experimental (solid) and RCWA-simulated (dotted) specular reflectivities at normal incidence were compared for a blank substrate (red lines), NCA coating (blue line) and NCA coating with treatment (green line).
Figure 2.22 Schematic representation of our assembled DSSC featuring TiO2 NTs and nanocones within PAA.
Figure 2.23 Typical anodization results of Al/FTO and Al/Ti/FTO samples. (a) Chronoamperical curve recorded during the anodization process. (b, c) Photographs of systems prepared in the (b) absence and (c) presence of Ti as the adhesion layer between the Al layer and the FTO substrate.
Figure 2.24 (a) Top-view SEM image (scale bar: 500 nm) of hollow TiO2 nanocone arrays after alumina had been removed selectively. In the left part of the image, focused ion beam (FIB) milling had been used to reveal the hollow inner sections of the nanocones. (b) Cross-sectional TEM image of hollow TiO2 nanocones (scale bar: 100 nm). (c–e) Schematic representation of the mechanism of formation of the hollow nanocones. (c) TiO2 nucleus formed as the anodization approached the Ti–Al interface.
(d) TiO2 hillocks grew, leaving voids caused by O2 pressure. (e) Cone-like hollow nanostructures formed with obstructing pore-walls.
Figure 2.25 Grazing incident X-ray diffraction (GIXRD) analysis for TiO2 nanocones on FTO.
Figure 2.26 (a–c) Morphology of the TiO2 NTs grown onto the pore walls of PAA. (a, b) Top-view SEM images (scale bar: 500 nm) of PAA (a) before and (b) after deposition of the TiO2 NTs. (c) Cross-sectional SEM image (scale bar: 200 nm) of the sample in (b).
(d) GIXRD analysis of a TiO2 NT, revealing the desirable crystalline anatase phase.
Figure 2.27 Schematic and microscopic images of samples NT–A–NC and NT–A. (a) Schematic representation of sample NT–A. (b) The AFM image of TiO2 film onto FTO substrate. The arithmetic average roughness (Ra) and root mean square roughness (RRMS) were 2.45 and 5.13 nm, respectively. The thickness of TiO2 film is 100 nm from cross-sectional SEM observation. (c) Schematic representation of sample NT. (d) Top-view SEM image (scale bar: 500 nm) of anodic TiO2 NTs of sample NT. The average inner diameter of the nanotube was 110 nm and outer diameter 160 nm. The depth of anodic TiO2 NTs was estimated at about 700 nm from cross-sectional SEM observation. As we can see, cracks and interconnections appear in TiO2 NTs.
Figure 2.28 Photovoltage performances of cells constructed with various TiO2
morphologies as electrodes: hollow TiO2 nanocones under porous alumina and TiO2
NTs (NT-A-NC); a TiO2 film under porous alumina and TiO2 NTs (NT-A); and anodic TiO2 NTs without alumina (NT). (a) Current–voltage characteristics of the three DSSC devices. (b) The incident photon–to–current conversion efficiency (IPCE) spectra of the three DSSC devices.
Sample Jsc (mA/cm2) Voc (V) FF (%) η (%)
NT-A-NC 5.15 0.64 59 1.71
NT-A 4.78 0.59 46 1.45
NT 3.62 0.60 47 1.22
Table 2.1 DSSC performance metrics.
Figure 2.29 Absorption spectra of N719 sensitizer in absolute ethanol solution. The molar extinction coefficients of MLCT absorption band for N719 dye is 1.36 × 104 M−1 cm−1, which is as well as that of ILCT absorption band (1.36 × 104 M−1 cm−1).