J. Phys. Chem. C 2009, 113, 6381–6389
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Titania Nanofilm with Electrical Switching Effects upon Hydrogen/Air Exposure at Room Temperature Manippady K. Kumar,† Lee K. Tan,† Nitya N. Gosvami,‡ and Han Gao*,† Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore, and Department of Mechanical Engineering, National UniVersity of Singapore, Singapore 117543, Singapore ReceiVed: October 17, 2008; ReVised Manuscript ReceiVed: December 17, 2008
We report that a very thin film of titania, with thickness controlled to the order of its Debye length, exhibits reversible on/off electrical switching effects upon hydrogen/air exposure at room temperature. Such extreme changes in electrical conduction are usually observed only in the “nanosensors” made of their nanoscaled counterparts. The nanofilm of titania with compact, “monograin-equivalent” film structure is fabricated by atomic layer deposition (ALD) with atomic layer control over its thickness. For the first time, the switching effects of the titania nanofilm capped with catalytic Pd nanoparticles are in situ observed with conducting atomic force microscopy (C-AFM) technique. We further demonstrate that the switching effects can be achieved only when the thickness of the nanofilm is reduced to less than 15 nm. In addition, arrays of TiO2 nanowires are fabricated to demonstrate the compatibility of ALD with the planar technology. Introduction Semiconducting metal oxides (SMO) have attracted intense research efforts over the past several decades toward their application in solid state gas sensors due to their low cost and power consumption, simplicity of fabrication and use, versatility in detecting a wide range of toxic/flammable gases, and stability in harsh environments.1-3 SMO gas sensors rely on reversible changes in electrical conductivity upon adsorption/desorption of gas molecules at their surfaces.2,4 These surface mechanism induced properties upon gas adsorption in SMO such as SnO2, TiO2, WO3, ZnO, Fe2O3, and In2O3 in different forms have been studied and the benefits from the addition of noble metals such as Pd, Pt, Au, Ag, etc. in improving the selectivity and stability have been understood.5-10 In general, the oxygen adsorption on an n-type SMO can trap mobile carriers (electrons) from the bulk, resulting in an electron-depleted surface layer (i.e., space-charge layer). If the size of the sensing element is comparable with the depth of the electron-depleted layer (depending on the Debye length of materials), the entire sensing element can be electron-depleted, creating a totally nonconducting (off) state. In contrast, hydrogen adsorption results in the depletion of the space-charge layer and hence a conducting (on) state.11-14 A pioneering experimental demonstration of such size effects was reported by Yamazoe et al.15 They observed a steep increase in gas sensitivity upon decreasing the crystallite size of porous sintered SnO2 elements from 32 to 5 nm. The size effects were further confirmed by Rothschild et al. from theoretical simulations of SnO2 grains with diameters ranging from 80 to 5 nm.16 Such size effects suggest the technical need to prepare stable and small crystallites with large accessible surface areas.13 Recent advancements in nanoscience and nanotechnology provide novel strategies to address this technical need.12,13,17 Quasi-1D nanostructures are one such ideal sensing platform * To whom correspondence should be addressed. Phone: (65) 6872 7526. Fax: (65) 6772 7744. E-mail:
[email protected]. † Institute of Materials Research and Engineering. ‡ National University of Singapore.
because of their large surface-to-volume ratio, nanoscale dimensions on the order of the Debye length, and configurable 1D architecture.18-23 For instance, Grimes et al. have demonstrated TiO2 nanotube arrays anodized from Ti foils to show 8.7 orders of magnitude sensitivity toward 1000 ppm H2 at room temperature.20,24 SMO with three dimensionally (3D) interconnected nanoparticles have also been developed to achieve nanosized crystallites with enhanced accessibility.25 For example, nanocrystalline mesoporous PdO-SnO2, prepared by a surfactanttemplated process, showed rapid response with ∼105 conductivity change to 5000 ppm H2 at room temperature.26 3D networks of TiO2 nanosponges by wet oxidization exhibit response to even 1 ppm H2 at room temperature.27 In addition, nanotubes, 18,24 nanowires,12,19,23,28,29 nanorods,21 nanofibers,22 and nanobelts28 with intriguing properties, compositions, and morphologies have been extensively studied. By using the nanostructured “mesowires” of Pd, the room temperature operational hydrogen sensor has been demonstrated.30,31 The sensing response is due to the closure of the nanoscopic gaps in wires created by the dilation of the palladium grains undergoing hydrogen absorption.30 All these demonstrate the unprecedented increase in sensitivity and decrease in operational temperature upon shrinking the crystallite size to the nanoscale. In nanomaterial, the sensitivity is predominantly controlled by chemical processes at the surfaces, which might enable us to detect even molecular-level gas adsorption.12,13 A majority of thick and thin film titania-based hydrogen gas sensors work at elevated temperatures.32-36 Recently, the fabrication of sensors based on nanomaterials of titania have now led to their operation at room temperature.20,24,37 It is, however, anticipated to have similar sensing performance by a planar technology compatible fabrication method, and yet with the performance of the nanoscaled counterparts. We were interested to investigate whether SMO thin films with their film thickness on the order of their Debye length can demonstrate such extremely good gas sensing properties as observed in the “nanosensors”. We were motivated by the prevailing thin-film technology widely used in industries due to its simplicity of preparing compact and long-term stable films, ability to provide the modulation of the materials composition,
10.1021/jp809183y CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
6382 J. Phys. Chem. C, Vol. 113, No. 16, 2009 compatibility to the planar technology, and ability in preparing miniature devices with low cost and low power consumption.38 We denote our thin films as “nanofilms”, as they differ from the thick and thin films due to the absence of any underneath SMO grains in them, i.e., monograin-equivalent films.39-41 In these nanofilms, the thickness of the film is just on the order of the Debye length and when exposed to the oxidizing gas, the space charge layer can encompass the entire film. Unlike a conventional thick- or thin-film sensor, the off state of a nanofilm sensor would not have any contribution from the grains below the surface layer.8 Specifically, we report on a TiO2 nanofilm (∼5 nm) catalyzed with Pd nanoparticles, showing reversible on-off electrical switching effects at room temperature upon exposure to hydrogen and air ambience. The film was prepared by atomic layer deposition (ALD). We note that obtaining a compact, uniform film of thickness less than 15 nm is a significant challenge using the standard thin-film technologies.8 ALD enables atomic layer control over the thickness of the deposited films, which involves sequential exposure of the substrate to gaseous species which undergo self-limiting reactions on the surface. ALD is a simple process that results in highly conformal films and enables excellent control over film thickness.42-44 By taking advantage of precise thickness control of the ALD technique, we can further demonstrate that the switching effects can be achieved only when the thickness of the nanofilms is reduced to less than 15 nm. Moreover, due to the compatibility of ALD with planar technology, these nanofilms can be patterned at ease for nanodevice applications.45,46 For the first time, the switching effects and the process of switching are observed in situ by conducting atomic force microscopy (CAFM). These nanofilms provide SMOs having the operational advantages of their nanoscale counterparts, and with the handling ease of thin-film technology. Experimental Section ALD of TiO2 Nanofilm. ALD of the TiO2 film was performed in a viscous flow ALD setup equipped with a mechanical rotary pump and computer-controlled solenoid valves.47 The base pressure of the chamber was maintained at 1 Torr in a 200 sccm N2 flow. Si(100) substrates (1 cm × 1 cm) with thermally oxidized SiO2 (50 nm in thickness) were used for the ALD. Atomic layer controlled films of TiO2 were obtained from alternate exposures of the substrate to vapors of two precursors, TiCl4 (Merck, g99%) and deionized H2O, at 150 °C. Both precursors have a 0.5 s exposure and 60 s N2 purge between the two exposures. The film thickness was easily controlled by counting the number of ALD cycles. The growth rate was measured by using a variable angle spectroscopic ellipsometer (J. A. Woollam Company) and further confirmed by high-resolution transmission electron microscopy (HRTEM, model Philips CM300 FEGTEM). The as-prepared TiO2 film was annealed in air at 500 °C for 2 h. Fabrication of Pd Nanoparticles on Titania Nanofilm. A thin Pd film was sputtered at room temperature (Denton Discovery 18 Magnetron Sputtering system) onto the asannealed TiO2 for 4 s from a high-purity Pd target ((99.999%), at a sputtering power of 100 W, an working pressure of 5 mTorr, and an Air flow of 10 sccm. Pd nanoparticles were obtained by an activation procedure performed in a tubular quartz chamber. The activation procedure involved a heat treatment of the sample to 300 °C in hydrogen (1000 ppm H2 in N2 balance) at atmospheric pressure for 2 min and thereafter 3 cycles of air and hydrogen exposures. The gas flow rate into the chamber
Kumar et al. was 500 sccm during the entire process. The activation process was closely monitored by the electrical conduction measurements of the Pd overlayer. The conductivity decreased sharply due to the segregation of Pd overlayer leading to the formation of Pd nanoparticles. Measurements of On/Off Switching Effects. C-AFM was used for direct observation of on-off switching effects and in situ study of the local electrical features of the samples. C-AFM was carried out at atmospheric pressure under air and 100 ppm H2 (balanced in N2) with a commercial AFM (Molecular Image) in contact mode. The local conductivity and topography were probed by using a commercial silicon cantilever (from Nano World) coated with doped n-type diamond having high wear resistance. The current, from the tip to the sample was measured with a Keithley picoammeter (model 6485). A constant voltage of 1.5V was applied between the tip and the sample. On-off switching effects were also confirmed by electrical conduction measurements, using a two-probe configuration. The sensing system consists of three parts namely gas mixing manifold, sensor test chamber, and data acquisition unit. All the electrical characterizations were performed in a quartz chamber at atmospheric pressure. The current flow through the sample upon application of a constant voltage of 1.5 V was monitored by using a Keithley sourcemeter (model 2400) that was computer interfaced with LABVIEW. Surface and Cross-Sectional Morphology Characterization. The surface morphology of the ultrathin TiO2 film and the film capped with Pd nanoparticles were characterized by AFM (Multimode-Digital instruments) in contact mode. The cross-sectional view of the Pd-capped TiO2 film was studied by HRTEM at 300 kV. The HRTEM sample preparation involved bonding of 2 pieces (3 mm × 1 mm) of samples with epoxy glue. This was then followed by curing and polishing with graphite lapping film to 10-50 µm thick. The sample was finally dimpled and ion milled for TEM observation. E-Beam Lithography To Prepare Nanofilm Ribbons. PMMA resist was first spin-coated on 500 nm SiO2/Si and was patterned subsequently by using ELS-7000 e-beam lithography at a bias of 100 kV and a charge density of 960 mC/cm2. The patterned Si was then treated with oxygen plasma for 30 s to remove any undeveloped resist in the patterned areas and to increase the hydrophilicity of the surface for ALD. After ALD of TiO2 at room temperature, the sample was soaked and sonicated in acetone to lift-off the resist. Results and Discussion Structure and Morphology of TiO2 Nanofilm. TiO2 nanofilms were deposited with ALD on 50 nm thick SiO2 obtained by thermal oxidization of Si (100) substrates. The distinct advantage of ALD lies in its ability to provide precise thickness control, i.e., the thickness control can be realized simply by counting the number of ALD cycles. Figure 1a shows a linearly proportional relation between the film thickness and the number of ALD cycles with a growth rate of ∼0.88 Å/cycle.48 The asprepared films were annealed in air at 500 °C to achieve anatase TiO2 phase and characterized by X-ray diffraction (XRD) pattern to confirm crystalline structure.49 The atomic force microscopy (AFM) image (with a scan area of 1 µm × 1 µm) of the annealed TiO2 film (Figure 1b) shows smooth, uniform, and pinhole-free surface morphology with a root-mean-square (rms) surface roughness
