CRYSTAL GROWTH & DESIGN
Controlled Growth and Sensing Properties of In2O3 Nanowires Alberto Vomiero,* Sebastiano Bianchi, Elisabetta Comini, Guido Faglia, Matteo Ferroni, and Giorgio Sberveglieri INFM-CNR SENSOR Laboratory and Dipartimento di Fisica e Chimica per l’Ingegneria e i Materiali Via Valotti 9, 25133 Brescia, Italy
2007 VOL. 7, NO. 12 2500–2504
ReceiVed March 2, 2007; ReVised Manuscript ReceiVed August 30, 2007
ABSTRACT: Single-crystalline nanowires of indium oxide were produced under controlled conditions via condensation from the vapor phase. Seeding of the substrate through dispersion of metallic indium nanoparticles was proven effective in promoting a nanowire nucleation driven by the vapor-solid process. Nanowires with as small an average lateral dimension as 110 nm were produced, allowing the exploitation of size reduction effects on the electrical properties and the response to gases. Preparation and microstructural and electrical characterization of nanowires are presented, and the peculiarities of these innovative structures for the development of nanodevices are highlighted. Introduction The fabrication of one-dimensional (1D) nanostructures has stimulated intense research due to their novel physical properties and feasibility as building blocks for nanotechnology.1–3 In particular, increasing attention has been devoted to the preparation of metal-oxide (MOX) nanowires, nanorods, and nanobelts for their peculiar electronic properties.4,5 MOX nanowires are robust candidates for a next generation of sensors: single-wire-based devices can be designed, thus reducing dimension to the nanoscale and achieving top performance in terms of sensitivity and stability.6,7 Indeed, crystalline nanowires feature exceptionally high surface-to-volume ratio, which is the basis of the sensing mechanism for semiconducting MOXs sensors.8 In addition, the transverse dimension of nanowires may be even smaller than the Debye length associated with the surface space-charge region, and the detection of surface interactions with reactive species may reach very high efficiency.9 Thermal evaporation and condensation10 resulted in a very effective method for synthesis of MOX 1D nanostructures such asZnO11 andSnO2.12 Intheframeworkofevaporation-condensation growth, two basic mechanisms have been invoked, namely, vapor-solid (VS)13–15 and vapor–liquid–solid (VLS) condensation.16,17 Several methodologies have also been exploited for the synthesis of In2O3 nanowires.5,16–21 In2O3 is wide band gap transparent semiconductor (direct band gap energy about 3.6 eV), which is employed in microelectronic applications such as window heaters, solar cells, and flat-panel displays;22–26 its properties as a gas sensing material have been also investigated. Potential applications of In2O3 1D nanostructures as gas sensing elements have been recently investigated.27–29 Synthesis of In2O3 is particularly demanding because the decomposition of precursor powders occurs in the 1400–1500 °C temperature range.19 Such high temperature requires extreme chemical stability of the deposition environment; highly pure polycrystalline alumina and sapphire are the most common materials used as substrate. Addition of heterogeneous catalysts either in the precursor powder or on the substrates is a common way to reduce the decomposition temperature of the precursors or to control nanowire nucleation and growth, respectively. The effect of reducing compounds mixed with In2O3 or metallic In precursor * Corresponding author. E-mail address:
[email protected]. Phone: +39 030 371 5404. Fax: +39 030 2091271.
powders was studied.18 The effect of substrates catalyzed with Au nanoparticles was also investigated;17 the noble metal particles assist the growth of nanowires through a VLS mechanism but also result in a heterogeneous structure because most of the nanowires exhibited the gold nanoparticle at the apex.16 For the purpose of functional characterization, presence of catalytically active metal particles is expected to strongly affect the interaction with adsorbed gases.30,31 The present paper reports on the controlled growth of In2O3 nanowires seeded by metallic indium nanoparticles deposited over Al2O3 substrates, with the aim to produce thin, densely distributed, free from heterogeneous catalysts, and structurally homogeneous nanowires. The sensing properties toward acetone and NO2, an oxidizing gas of great importance for air-quality monitoring in urban areas, have been tested over a wide concentration and temperature range. Experimental Section The growth of the nanowires in controlled conditions has been achieved through a two-step procedure. First, metallic indium particles were sputtered on heated 3 × 3 mm2 alumina substrate via rheotaxial growth method:32 substrate heating at 300 °C, which is twice the melting point of In (∼157 °C), resulted in formation of nanosized metallic seeds. Seeding of the alumina substrate was carried out by direct-current sputtering from a 4 in. metallic In source operated at 50 W power, in 0.5 Pa inert atmosphere of Ar. Afterward, condensation of vaporized precursors from In2O3 powder on seeded substrates took place in a tubular furnace and led to nanowire growth. The transient heating of the furnace before starting wire growth plays a fundamental role in oxidizing the metallic indium seeds and allows formation of effective nucleation centers. In2O3 powder (purity 99.9999%) supplied by SigmaAldrich was used as precursor, and evaporation took place by heating the source at 1500 °C. Mass transport was obtained by flowing 100 sccm of argon at 2 × 104 Pa. Residual oxygen in the gas transport system allowed growth of oxide nanowires. Furnace temperature was raised from RT to 1500 °C in 2 h, and the deposition was maintained for 1.5 h. During heating and cooling transients, the direction of the Ar flow was reversed in order to avoid uncontrolled condensation. Nucleation and growth of nanowires occurred between 750 and 800 °C. Morphological investigation was carried out by LEO 1525 SEM equipped with field emission gun and in-lens secondary electrons detector. The enhanced performance of the system allows for the observation at 2.7 keV of uncoated specimens of nanowires over the insulating substrates.
10.1021/cg070209p CCC: $37.00 2007 American Chemical Society Published on Web 11/17/2007
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Figure 1. Dispersion of In seeds over alumina: (a) the bare alumina substrate; (b-d) comparison of the dispersion for different durations of the In deposition; (e-g) effect of thermal treatment at 750 °C. TEM investigation was carried out with a FEI Tecnai F20 microscope equipped with field emission source, supertwin objective lens with 0.19 nm point resolution, and 0.14 nm information limit for high resolution imaging. For the characterization of the gas-sensing device, platinum electrical contacts and heater were fabricated above the nanowires and below the substrate, respectively. The operating temperature of the gas sensors was maintained through a feedback circuit. The sensors were biased by 1 V direct voltage, and the electrical current was measured. The flow-through technique was used: the reference atmosphere of synthetic air was maintained at the constant condition of 0.3 L/min flow, 20 °C temperature, and 40% relative humidity. Nitrogen dioxide and acetone were mixed to the synthetic air flow in controlled concentration.
Results and Discussion A. Substrate Seeding. Figure 1 shows the effect of the duration of sputtering deposition on the dispersion of metallic In particles. After 5 s sputtering, the finest steps and facets of the alumina grains were covered; 10 s deposition resulted in formation of dispersed indium droplets. After 20 s deposition, a continuous film with coarse morphology is achieved. The sample seeded via 5 s sputtering appears slightly modified after heating at 750 °C in Ar flow (see Figure 1e), as tiny bright features suggest that partial oxidation of the In particles may have occurred due to residual oxygen in the environment. No other significant modifications have been recorded by observation of the samples. B. Growth of Nanowires. Figure 2 shows the effect of substrate seeding on the growth of wires. The wires grow over the bare alumina and the substrate seeded by dispersed In nanoparticles. In the first case, micrometer-sized wires are present (Figure 2a), while indium seeding promoted formation of a larger quantity of connected nanowires, up to tens of micrometers in length (Figure 2b). Growth of nanowires did not occur over the 20 s sputtered substrate, a polycrystalline In-based layer being formed instead (not shown). The early stage of nanowire nucleation on a seeded substrate was studied through a short condensation run (5 min, see Figure 2c). Nanowires nucleate from indium-based clusters, and the lateral dimension of the nanowire was smaller than the size of the cluster. The distribution of lateral dimension for the wires is reported in Figure 3. The average values of the distribution for wire growth over the unseeded and the seeded substrate are 560 nm (rms 200 nm) and 110 nm (rms 10 nm), respectively. The seeding process and consequent nanowire nucleation from
Figure 2. The growth of the In2O3 wires: (a) thick wires over bare alumina; (b) thin In2O3 nanowires over 5 s seeded substrate; (c) nucleation of nanowires from indium-based clusters after short (5 min) condensation run.
nanosized grains determine reduction of average lateral size and narrowing of its distribution with respect to unseeded substrate. C. Structural Characterization. The morphology and the crystalline arrangement of the nanowires were investigated. Figure 4 reveals the sharp parallel lateral sides of the nanowires grown on seeded substrates, their uniform section, and pyramidal termination. The conventional TEM image in Figure 5a shows dark contrast features due to the bending of the nanowire, which is only 30 nm in width. No evidence of extended line defects across the wire was recorded. Figure 5b,c also shows the nanowire at high resolution, where the uniform fringe contrast indicates that the projected thickness is constant across the wire, according to its regular section. Accurate analysis of crystallinity is important for the discussion of the growth mechanism and to assess the wire thermodynamic stability. For the purpose of precise determination of unit cell, lattice parameters, and space group, convergent beam
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Figure 3. Distribution of lateral size for thin nanowires (a) and thick wires (b).
Figure 4. SEM images of the crystal habit for the In2O3 nanowires grown on seeded substrate. The section appears to be squared, and the apex of the wires is tapered. No metallic droplet is present over the tapered tips, according to a VS growth mechanism.
electron diffraction and analysis of zero-order and higher-order Laue-zone diffraction was carried out. Figure 5d shows the whole-electron diffraction pattern obtained by focusing a convergent beam of electrons over as small a nanowire area as 10 nm. The central part of the pattern contains the conventional spot pattern that is usually obtained by selected-area diffraction techniques. The outer part of the pattern is formed by additional diffraction disks, corresponding to electron trajectories generated by diffraction at large angles (high-order Laue zones). The solid lines and dots superimposed with the recorded image highlight the complete agreement between the experimental pattern and the one expected for the Ia3j body-centered cubic In2O3 structure (spatial group 206). The
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nanowires grow along the [100] vector of the cubic In2O3 crystalline cell. Electron diffraction has been carried out over several selected areas of the sample, confirming the cubic phase of indium oxide for the nanowires. High-resolution TEM analysis confirmed that most of the nanowires exhibited single-crystalline arrangement with atomically sharp termination of the lateral sides. The lateral sides are also parallel to the wire growth direction, an indication of the stable nanowire crystalline habit. It is generally accepted that high-index crystalline planes as lateral termination and microfaceting may indicate a nanowire’s growth driven by either catalysts or extended crystal defects.33 The facets of the pyramidal termination of the nanowires are crystal planes with higher index with respect to the lateral sides and indicate that the vapor-solid (VS) mechanism was predominant.14 Complementary TEM-energy-dispersive X-ray microanalysis showed that no impurities are present in the nanowires within the sensitivity of the technique. An insight into the mechanism of nanowire growth can be rationalized from the experimental evidence as follows: the combined effect of substrate seeding and of the rate of condensation of indium over the substrate plays a fundamental role in determining the density of nucleation sites for the nanowires. The metallic indium seeds oxidize into crystalline nanograins during the preliminary heating stage and act as selfcatalyst for the growth of nanowires during the condensation of In cations from the Ar flow. The nanowires were found to nucleate from indium-based clusters, and the lateral dimension of the nanowire was smaller than the size of the cluster. Differently, the early stage of In condensation over the unseeded substrate causes the formation of grains, which increase their size due to the high mass flow before nucleation of wires. Thus, the lateral dimensions of the wires are larger as experimentally observed. In the case of complete substrate coverage, the polycrystalline layer prevented wires from growing. The wires of indium oxide are expected to possess similar stoichiometry and density of point defects because the thermodynamic conditions of growth were identical. In such a condition, a comparative investigation of the gas-sensing behavior would highlight the effect of lateral dimension on the gas sensitivity. D. Gas Sensing Properties. In general, the sensing capability of a MOX-based sensor relies on surface reactions: in the presence of a reducing gas that causes a transfer of electronic charge at the surface, the reference value of conductance, G0, in an n-type semiconductor changes within the characteristic response time to a final value, Gf, which depends primarily both on gas concentration and on operating temperature. The relative variation in conductance (resistance) is usually defined as the response for reducing (oxidizing) gases.8,34 The wires of cubic indium oxide feature a high degree of crystalline ordering and the same crystalline habit. The different size between nanowires and thick wires is expected to play a key role in determining the amplitude of the response to reactive gases. Nanowires may also behave differently because their electrical conductivity could depart from the diffusive regime of ionic crystalline semiconductors. Therefore, variation in size for the nanowires could significantly affect both electrical conductivity and gas sensitivity. The electrical response of the In2O3 nanowires toward acetone and nitrogen dioxide was investigated. The effect of different acetone concentrations on the electrical conductance of nanowires or thick wires (as shown in Figure 2a,b, respectively) is
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Figure 5. (a) TEM images of a wire 30 nm in width, (b, c) high-resolution TEM imaging, and (d) convergent beam electron diffraction.
Figure 6. (a) Kinetic variation of current of In2O3 nanowires (red) and thick wires (black) towards 25, 50, and 100 ppm of acetone (dotted line) at 40% relative humidity, 20 °C ambient temperature, and 400 °C operating temperature; (b) values of steady-state electrical conductance of nanowires (red) and thick wires (black) as a function of temperature in synthetic air (solid symbols) or after the introduction of 100 ppm of acetone (open symbols); and (c) response of nanowires (red) and thick wires (black) as a function of acetone concentration at 40% relative humidity, 20 °C ambient temperature, and 400 °C operating temperature.
shown in Figure 6. The electrical conductance is higher for the thick wires. Such effect can be attributed to the surface space charge region extending on the lateral sides of the wires and exposed to ambient atmosphere. The enhanced surface-tovolume ratio in thin wires determines a reduction of the
conductance, as electron flow is highly reduced in the surfacedepleted region. Both samples show similar behavior of the electric conductance in air, which achieves its minimum at 400 °C (Figure 6b). This value corresponds to the maximum response for the sensor, maybe due to maximized oxygen chemisorption over the nanowires. The exposure to acetone increases the electric current, as expected for an n-type semiconductor, because of reduction of the space-charge region. The response as a function of different acetone concentrations at 40% relative humidity, 20 °C ambient temperature, and 400 °C operating temperature is shown in Figure 6c. Figure 7a shows that the response to acetone increases with the operating temperature up to a maximum located at about 400 °C, where the nanowires feature a response about 7 times higher than the one of thick wires. This result has to be attributed to the higher surface-to-volume ratio of nanowires, which causes more extended surface interaction with gas molecules, and testifies that the control of lateral dimension of the wires is the key point for achievement of high sensitivities. Similar behavior has been documented for polycrystalline thin films,35,36 in which gas response dramatically falls as grain size increases, since small grain size allows a higher fraction of the sensing layer being involved in the charge transfer process. As for NO2 detection, the interaction of NO2 is described as a direct adsorption of the oxidizing molecule at the surface of indium oxide.8 In the case of nitrogen dioxide, Figure 7b reports the response to 500 ppb. The response decreases upon increasing the working temperature; the same behavior was reported for thin-film-based gas sensors.37 In thick wires, the response vanishes at temperatures higher than 300 °C, while nanowires are active in NO2 detection up to 400 °C. As well as in the case of acetone, nanowires exhibit higher response with respect to thicker ones. Conclusions In2O3 wires were synthesized through the thermal evaporation and condensation method starting from pure In2O3 powders, and the promoting role of metallic In nanoparticles deposited on substrates before evaporation was investigated. We showed that a suitable choice of substrate seeding is a key issue for tailoring density distribution and lateral dimensions of the wires, which are the main parameters controlling the gas sensing properties: properly treated substrates resulted in growth of thin and densely
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Figure 7. Response of In2O3 nanowires and thick wires as a function of the operating temperature toward (a) 25 ppm of acetone and (b) 500 ppb of NO2.
concentrated In2O3 nanowires. The wires exhibit uniform section, atomically sharp lateral facets, and pyramidal termination, typical of a VS growth mechanism assisted by oxidized nanocrystalline seeds. The nanowires resulted single crystals oriented to the [100] direction of the Ia3j body-centered cubic In2O3 structure. The gas sensing properties of the wires have been tested toward acetone and NO2. The control of the lateral dimensions of the wires proved crucial for obtaining high sensitivity, owning to the increase of the surface-to-volume ratio. Acknowledgment. This work was supported by European Union (Nanostructured solid-state gas sensors with superior performance-NANOS4 STREP Project No. 001528), and MIUR (Nanostructured semiconductors for chemical sensing PRIN Project 2004 and Quasi mono dimensional nanosensors for label free ultra sensitive biological detection PRIN Project 2005).
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