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Atomic Layer Deposition of TiO2 to Bond Free-Standing Nanoporous Alumina Templates to Gold-Coated Substrates as Planar Optical Waveguide Sensors Lee Kheng Tan,† Han Gao,*,† Yun Zong,*,† and Wolfgang Knoll‡ Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, and Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed: August 8, 2008; ReVised Manuscript ReceiVed: September 12, 2008
We demonstrate a method to intimately bond a free-standing nanoporous anodic aluminum oxide (AAO) template onto a piece of gold-coated high reflective index glass (LaSFN9) substrate by an atomic layer deposition (ALD) technique. The capability of ALD as “nanoglue” is well demonstrated by the optical waveguide behavior of the bonded sample. We monitored the optical behavior before and after ALD of TiO2 with no exciting waveguide modes in the sample before ALD. Quantitative analysis of the ALD TiO2 thickness layer using the Fresnel calculation is in good agreement with the expected growth rate of the ALD setup. The subangstrom sensitivity of the waveguide sensor for bio- or chemical sensing is demonstrated with bovine serum albumin (BSA) adsorption/desorption processes at various pH values. Furthermore, a layer-by-layer polyelectrolyte multilayer deposition is performed to distinguish processes that occurred inside and outside the nanopores. This novel function of ALD as “nanoglue” finds a wide range of applications in areas such as bonding or adhering nanostructures to various substrates. Introduction Nanoporous anodic aluminum oxide (AAO) films prepared by anodization of Al foils have been intensively studied and used as templates for fabricating nanostructured materials and their arrays such as metals, semiconductors, carbons, and polymers.1-6 This self-organization nanofabrication method has several favorable characteristics over expensive lithographic techniques, in particular for the fabrication of sub-100 nm nanostructured arrays.7,8 First, the template is prepared at ease and low cost over large areas since the nanosized pores are naturally occurred during Al anodization. Second, the dimension of AAO can be easily tuned by varying anodization conditions. Lastly, the template is easily removed by wet etching method. Recent advancements in this area such as preparing perfectly ordered templates,9 fabrication of multiplies AAO templates,10 and simple and fast fabrication of ultrathick compositiontuneable highly ordered AAO by hard anodization11,12 will spur more studies on their novel applications. For practical applications, it is highly desired to integrate the AAO templates with a wide range of substrates. Several methods have been proposed for addressing this challenge. For example, by directly attaching them onto the substrate, ultrathin freestanding AAO templates were frequently used as masks for evaporation of nanoparticle arrays on substrates.13 Sander and Tan demonstrated direct growth of AAO on substrates by in situ anodization of an Al thin layer evaporated on the substrate.7 This method allows for both vapor- and solution-based fabrication for a great number of nanostructured materials because of the intimate contact between the template and the substrate.7,9,14 Jung et al. developed a method to bond a 400 nm thick freestanding AAO template to a Si wafer for growing carbon nanotubes.15 In this report, we employed atomic layer deposition * To whom correspondence should be addressed. Tel.: (65) 6872 7526. Fax: (65) 6772 7744. E-mail:
[email protected] (H.G.); y-zong@ imre.a-star.edu.sg (Y.Z.). † Institute of Materials Research and Engineering. ‡ Max Planck Institute for Polymer Research.
(ALD) of TiO2 as “nanoglue” to bond a free-standing AAO template to a piece of gold-coated high-refractive-index glass (LaSFN9) substrate. We then demonstrate that the bonded sample can serve as a planar optical waveguide sensor for monitoring interfacial association/dissociation reactions, which shows subangstrom-level sensitivity and a capability of distinguishing processes that occurred inside and outside nanopores. Our strategy to bond the free-standing AAO onto the substrate relies on the unique features of the ALD technique. ALD is a simple and layer-by-layer coating technique developed in the 1970s that allows for atomic layer growth of thin films and conformal coating even on a surface covered with nanoscaled features.16,17 This technique involves alternate, repeated exposures of gaseous precursors to a sample that undergoes a gasphase self-limiting reaction on the surface, leading to a layerby-layer conformal coating with precisely controlled thickness. These novel features make the ALD technique suitable for delivering conformal coatings on or replications of nanostructured objectives, such as nanoporous templates,18-22 semiconductor nanowires,23,24 as well as photonic crystals with tunable band gaps.25-27 Being a nondestructive low-temperature deposition process, ALD also allows for replicating and functionalizing biomaterials,28 such as butterfly wings29 and tobacco mosaic virus.30 The recent emerging applications of the ALD technique in nanofabrication and nanotechnology were reviewed by Knez et al.28 In this work, we take advantage of the conformal coating and atomic layer growth capabilities of the ALD technique to “nanoglue” the gaps and voids between the free-standing AAO template and the substrate for the intimate contact interfaces in favor of optical waveguide sensing applications. In turn, the optical behavior of the template before and after ALD well demonstrates the excellent ALD capability as “nanoglue”. This is also a novel application of ALD in nanofabrication and nanotechnology, which has never been demonstrated, to the best of our knowledge. This novel function of “nanoglue” might find a wide range of applications in the areas such as bonding nanostructures to wafers,15 adhering and packing carbon nano-
10.1021/jp8070794 CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
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tubes and/or semiconductor nanowires into a hybrid CMOS circuit,31 and bonding the nanosphere assembly into photonic crystals, etc.32 Experimental Methods Adhering a Free-Standing AAO Template onto a Substrate by TiO2 ALD. Free-standing AAO templates were prepared by a two-step anodization of high-purity aluminum foils (99.999%, Goodfellow Cambridge) as published previously.6 Briefly, the electrochemically polished aluminum foil was anodized in 0.3 M oxalic acid at room temperature for 6-10 h at 40 V. A solution of 3.5 vol % H3PO4 and 45 g/L CrO3 at 60 °C was used to remove the AAO film prepared at the first step, and a second anodization in 0.3 M oxalic acid at 40 V and 2 °C was performed. The template thickness could be tuned by controlling the duration of anodization at a growth rate of ∼40 nm/min. The pores of the as-prepared template were then widened in 5 wt % H3PO4 for ∼50 min and followed by applying a protective poly(methyl methacrylate) (PMMA) layer, which dried at 120 °C for ∼30 min.9 The AAO film was separated from the aluminum foil with 0.1 M CuSO4 and 10 wt % HCl. The barrier layer at the bottom of the template was removed in 5 wt % H3PO4 solution for another 30-40 min. Finally, the template was carefully attached to a high-refractiveindex LaSFN9 glass slide (n ) 1.85 at λ ) 633 nm, Schott Glass GmbH) which was thermally evaporated (R-DEC Co., Ltd., Japan) with 2 nm of chromium, 48 nm of gold (rms ) 0.818 nm) and pretreated with mild O2 plasma (80 sccm O2, 80 mTorr, 150 W) for ease of attaching the AAO film. The PMMA protective layer was removed by UV-ozone in a dry stripper (Samco UV-1) at 200 °C for ∼30 min. Finally, drops of acetone were added to the attached template to further reduce any voids between the AAO template and the substrate. Atomic Layer Deposition of TiO2. Free-standing AAO templates attached on the substrates were deposited with TiO2 films in a home-built viscous flow ALD setup at 150 °C. The substrates were exposed to alternating vapors of TiCl4 (Merck, g99%) and deionized H2O at a base pressure of 1 Torr in a 200 sccm N2 flow. Both precursors have a 0.5 s exposure time and 60 s N2 purge between the two exposures. The film thickness was easily controlled by the number of ALD cycles. The growth rate was measured using a variable angle spectroscopic ellipsometer (J. A. Woollam Company) and further confirmed by high-resolution transmission electron microscopy (HRTEM) (Philips CM300 FEGTEM). Structural and Optical Waveguide Spectroscopy Characterization. Field emission scanning electron microscopy (SEM) (JEOL-6700F) was used to characterize morphology of the AAO films on the substrate before and after TiO2 ALD. The sample was refractive-index-matched to a LaSFN9 triangular prism to form an optically continuous medium for optical waveguide spectroscopy (OWS) measurement in the Kretschmann coupling scheme.33 Both p- and s-light from a He-Ne laser with λ ) 633 nm were used to excite TM and TE modes in the waveguide layer, respectively. The reflectivity (R) was measured as a function of the incident angles (θ). For bovine serum albumin (BSA) adsorption/desorption studies solutions of 5 mM phosphate-buffered saline (PBS) of different pH values were prepared by adding calculated amounts of 85% H3PO4 (98%, APS Ajax Finechem) to 0.20 M PBS solution, prior to dilution with Milli-Q water (18.2 MΩ · cm). BSA (96%, Sigma-Aldrich Co.) was dissolved in the respectively prepared PBS solutions at a concentration of 0.33 wt %. In the layerby-layer polyelectrolyte multilayer buildup experiment poly-
Figure 1. Schematic depiction of using the ALD technique to “nanoglue” a freestanding AAO template onto a gold-coated LaSFN9 substrate as a planar optical waveguide sensor (A), and the Kretschmann prism coupling setup for OWS measurement (B). Free-standing AAO template is attached onto the substrate with the assistance of a protective layer. The voids/gaps between the template and the substrate are then filled by TiO2 ALD which the AAO template thus adhered on the substrate as an optical waveguide sensor.
(diallyl dimethyl ammonium chloride) (PDADMAC; MW 200 000-350 000, 20 wt % aqueous solution, Sigma-Aldrich Co.) and sodium polystyrene sulfonate (PSS; MW 70 000, 30 wt % aqueous solution, Sigma-Aldrich Co.) solutions were used. The two aqueous solution samples were 0.01 M with an ionic strength of 0.18 M, prepared by diluting one portion of their respective 0.10 M aqueous solution with nine portions (v/v) of 0.20 M PBS buffer. Fresnel calculations were carried out using the WINSPALL software (version 2.0) developed by the Max Planck Institute for Polymer Research in Mainz, Germany. Results and Discussion Our strategy for preparing AAO waveguide sensors is schematically illustrated in Figure 1A, which involves attaching a free-standing AAO template to a gold-coated LaSFN9 substrate with assistance of a protective PMMA layer9 and applying an ALD process of TiO2 to the sample after removal of the protective layer. After ALD, a conformal TiO2 layer with the desired thickness is coated around the entire AAO template and, meanwhile, penetrates and fills the voids/gaps between the template and the substrate owing to vapor-phase reaction and non-line-of-sight conformal growth of the ALD technique. As a consequence, this free-standing AAO template is firmly “nanoglued” onto the substrate, rendering a ready planar optical waveguide sensor for detection of interfacial association/ dissociation reactions. The Kretschmann prism coupling technique is used for the OWS measurement, as shown in Figure 1B. Briefly, the gold-coated LaSFN9 substrate with TiO2adhered AAO is refractive-index-matched to a LaSFN9 triangular prism to form an optically continuous medium, and the excited optical waveguide modes are then recorded by detecting the corresponding reflectivity (R) as sweeping a linearly polarized laser beam over a range of incident angles.33-35 Optical waveguide sensors made of AAO templates for biological and chemical sensing were first proposed by Lau et al. in 2003.33 They have demonstrated subangstrom-level sensitivity of the AAO waveguide sensors for detecting chemical and biological processes occurring inside the pores. The application is,
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Figure 2. SEM images of the AAO template adhering to the substrate after 100 cycles of TiO2 ALD: (A) uniform and hexagonally packed pores with an average diameter of ∼55 nm; (B) oblique-view SEM image revealing the intimately attached template on the substrate after ALD. The tubular residuals of TiO2 indicated by arrows and the dot arrays on the substrate after removal of the template strongly suggest conformal coating of ALD around the entire template and on the substrate which eventually bonds the template onto the substrate.
however, limited by their fabrication approach in which the AAO template is directly fabricated by anodizing a thin Al layer evaporated onto the gold-coated LaSFN9 substrate. The direct anodization approach faces a number of challenges during the waveguide substrate fabrication: (1) peeling-off of the AAO films from the substrate during the anodization; (2) oxidation of the gold coupling layer, resulting in demolition of the coupling medium; (3) difficulties in preparing highly ordered, uniform AAO templates that are indispensable to the high detection sensitivity.9 Direct anodization of evaporated Al layer on the substrate tends to contaminate the substrate surface and alter and roughen the surface morphology, in particular, on a conductive substrate such as Au on LaSFN9.18 In comparison with the direct anodization method, the first advantage of this strategy is the conservation of the intact gold coupling layer during the process because of the elimination of the direct anodization step on the substrate. Second, uniform pore structures that will facilitate the coupling are easily obtained as the preparation of highly ordered and even ideally ordered freestanding AAO templates are well-established.36,37 Third, the TiO2 layers deposited are chemically stable and biocompatible. Figure 2 shows SEM images of the highly ordered freestanding AAO template “glued” by TiO2 ALD on the goldcoated LaSFN9 glass substrate. After 100 cycles of TiO2 ALD, the pore size is reduced from ∼75 to ∼55 nm (Figure 2A) which is slightly smaller than the actual pore size. This is because the sample had to undergo gold coating for SEM observation which further reduced the pore sizes. The pores shown in the image are hexagonally packed and highly uniform. An oblique view of the AAO template shows no noticeable voids or gaps along the interfaces between the template and the substrate (Figure 2B) after cleavage of the sample for SEM, indicating an intimately attached template on the gold surface after the ALD process. Moreover, the broken nanotubes inserted in the AAO pores (indicated by arrows in Figure 2B) and the white dot arrays on the substrate after removal of the template evidence that the deposition of TiO2 films occurs both inside the pores and on the substrate. All these findings confirm that after attaching the AAO template onto the substrate, the voids or gaps between the interfaces are filled by ALD which firmly glued the template onto the gold-coated LaSFN9 substrate. The intimate of adhesion of AAO by ALD can be further confirmed by the optical behavior of the samples before and after the ALD process, as shown in Figure 3. With the pristine sample (without undergoing TiO2 ALD) a typical surface plasmon resonance (SPR) spectrum is observed under ppolarization (transverse magnetic or TM) with the resonance angle located at θ ) 25.8°, which is typical for a blank gold-
Figure 3. Optical waveguide spectra (scatters) with an AAO template before (A) and after (B) TiO2 ALD and their theoretical simulations (solid lines). Only a typical SPR pattern is seen before ALD, whereas both TM and TE waveguide modes are present after ALD. This different optical behavior is direct evidenced for the “nanoglue” capability of the ALD technique.
coated LaSFN9 substrate in air according to Fresnel calculations (Figure 3A). The failure of exciting waveguide modes in the sample before ALD is also supported by the absence of these modes in OWS under s-polarization (transverse electric or TE). These results strongly suggest that without an ALD process the AAO template is loosely attached to the substrate and unable to guide the light. Upon 100 cycles of TiO2 ALD, however, typical waveguide modes are apparent in the angular scans with both s- and p-polarized laser beams (dotted curves in Figure 3B). By comparing the optical behavior before and after ALD, the capability of ALD as “nanoglue” to firmly attach the AAO on the substrate is confirmed. To the best of our knowledge, this is the first demonstration on the use of the ALD technique as “nanoglue” to intimately attach nanostructured components to substrates. A quantitative analysis of the optical waveguide experimental data shown in Figure 3B is performed using the Fresnel
Atomic Layer Deposition of TiO2 Nanoglue
Figure 4. Optical waveguide spectra (symbols) of the bonded AAO films after the adsorption of zero, two, four, and six polyelectrolyte bilayers, respectively, and their theoretical simulations (solid lines): (A) TE mode; (B) TM mode. The resonance angles are shifting to higher values as the polyelectrolyte was deposited, but the angle shifts in (A) gradually reduced. The adsorption occurring inside and outside the pores can be unambiguously distinguished.
calculation, resulting in the dielectric constants (ε) and the thickness (d) of the waveguide layer.16 One can see that the dips in the spectra are broader and shallower in comparison to the calculated curves. This is mainly due to the nonuniform distribution of the nanopores and the surface roughness of the film.33,34 They are, however, more regular than those from samples prepared by directly anodizing a deposited aluminum layer on the substrate.33 With a pore volume (fpore) of ∼30.4% and an average pore diameter of ∼75 nm measured by SEM, the thickness of the TiO2 layer (dtitania) is obtained from the Fresnel calculation to be ∼8.2 nm, which is in good agreement with the layer thickness of 100 cycles of ALD at the growth rate of 0.88 Å/cycle in our setup. The calculated total thickness of 326 nm (8.2 nm TiO2 + 317.8 nm AAO) also matches well with the thickness of 318 nm for the AAO layer measured by the SEM. Since the pores in AAO are open at the top which can be filled at will by different optical media, slight changes in the pore materials can be sensed as shifts of the waveguide coupling modes in OWS due to the corresponding changes in εeff and, thus, constitute the principle of an AAO waveguide sensor.33,34 As the pores were filled with PBS solution of pH 7.4, i.e., changing the dielectric constant inside the pores from εair ) 1 to εPBS ) 1.774, the resonance angles shifted from θ ) 42.51° and θ ) 57.99 (in Figure 3B) to θ ) 58.67° and θ ) 68.99° (in Figure 4, parts A and B, leftmost modes) in TM and TE mode, respectively. With a pore medium of 5 mM PBS solution at various pH values, BSA adsorption/desorption was investigated, and the subangstrom sensitivity was seen as that in the previous report.33 In addition, we performed a layer-by-layer polyelectrolyte multilayer deposition onto the AAO template in order to demonstrate the capability of monitoring processes inside the pores. We anticipated that the different adsorption behaviors
J. Phys. Chem. C, Vol. 112, No. 45, 2008 17579 of the polycation/polyanion self-assembly inside and outside the pores could be distinguished. The electrostatically driven self-assembly of polyelectrolytes is an ideal system for the study as the layer thickness is tunable so that the pores can be easily filled after only several cycles of adsorption.38,39 Figure 4 shows the resonance angle shifts of TE (Figure 4A) and TM (Figure 4B) waveguide modes after different numbers of polycation/ polyanion bilayers’ adsorption. The shift ∆θ was 1.3°, 1.1°, and 0.9°, respectively, for the TM mode (Figure 4B), and ∆θ ) 1.5°, 0.5°, and 0.4°, respectively, for the TE mode (Figure 4A) after two, four, and six cycles of bilayer deposition. It is noteworthy that the magnitudes of angle shift for TM modes barely vary, whereas those for TE modes display an abrupt change after four cycles of bilayer deposition. The Fresnel calculation reveals a thickness of ∼4 nm for a polyelectrolyte bilayer on the top of AAO surfaces that is similar to the previous report in which similar experimental conditions are used to deposit polyelectrolyte bilayers onto a planar substrate.40 The deposition occurring inside the pores, however, was completely different: at the first two cycles of adsorption, the effective thickness of the polyelectrolyte coating the inner walls of the pores are the same as that occurring outside, indicating that at this stage the layer-by-layer polyelectrolyte deposition is a uniform process across the entire template; nevertheless, at the second and third two cycles of adsorption, the thicknesses of the film coated inside the pores increased only by 4 and 1 nm, respectively. This different adsorption behavior occurring inside and outside the pores may be attributed to the reduced pore size after adsorption which cumbers the diffusion of the polyelectrolytes into the pores at the following steps and leads to a reduced deposition inside the pores. Thus, we demonstrate the utility of AAO waveguides to directly monitor the adsorption process inside the pores in situ. Conclusions In summary, we have demonstrated the use of TiO2 ALD as “nanoglue” to bond a free-standing AAO template onto a piece of gold-coated high-refractive-index LaSFN9 substrate which can be generalized for adhering nanocomponents such as nanowires, nanotubes, and nanoparticles onto respective substrates for device applications. The adhesion capability of the ALD technique is evidenced by the optical behavior of the substrate before and after ALD. Free-standing AAO template, intimately attached onto LaSFN9 substrate with multicycles of TiO2 ALD, can serve as a planar optical waveguide sensor whose performance is documented by monitoring the layer-bylayer polyelectrolyte multilayer buildup. It is shown that the process occurring inside and outside the pores can be unambiguously distinguished. Such waveguide sensors with subangstrom sensitivity can be fabricated with ease, and their fabrication is substrate-independent. In addition, biocompatibility of TiO2 coatings facilitates sensing on biomolecules. Acknowledgment. We thank the Institute of Materials Research and Engineering under the Agency for Science, Technology and Research (A*STAR), Singapore, for the financial support. References and Notes (1) Wang, Y.; Wu, K. J. Am. Chem. Soc. 2005, 127, 9686. (2) Tian, M. L.; Wang, J. U.; Kurtz, J.; Mallouk, T. E.; Chan, M. H. W. Nano Lett. 2003, 3, 919. (3) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Gosele, U. Science 2002, 296, 1997.
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