Sensors and actuators

The experimental results suggest a competing evaporation and condensation process, which converts the nanocrystalline SnO2-films into single crystalline SnO2-nanowires directly on the ch

Authors: Anton K¨ock, Alexandra Tischner, Thomas Maier,

Michael Kast, Christian Edtmaier, Christian Gspan, Gerald

Please cite this article as: A K¨ock, A Tischner, T Maier, M Kast, C Edtmaier,

C Gspan, G Kothleitner, Atmospheric pressure fabrication of SnO2-nanowires forhighly sensitive CO and CH4 detection, Sensors and Actuators B: Chemical (2008),

doi:10.1016/j.snb.2009.02.055

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As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain

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Anton Köck* 1 , Alexandra Tischner 1 , Thomas Maier 1 , Michael Kast 1 , Christian Edtmaier 2 , Christian

Gspan 3 , and Gerald Kothleitner 3

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nanowires for gas sensing applications based on a combined spray pyrolysis and annealing process The SnO2-nanowires are grown on SiO2-coated Si-substrates and exhibit diameters of 30 - 400 nm and lengths up to several 100 µm The whole SnO2-nanowire fabrication procedure is performed at atmospheric pressure and requires no vacuum The experimental results suggest a competing evaporation and condensation process, which converts the nanocrystalline SnO2-films into single crystalline SnO2-nanowires directly on the chip For the realization of gas sensors the SnO2-nanowires are transferred to another SiO2-coated Si-substrate Evaporation of Ti/Au contact pads on both ends of single SnO2-nanowires enables their direct use as sensing elements The devices are very sensitive, are able to detect humidity, and concentrations of CO and CH4 as low as a few ppm at operating temperatures of 200 – 250°C We believe that our fabrication procedure might be the technology of choice for the controlled fabrication of SnO2-nanowires as highly sensitive gas sensing elements on a wafer scale

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highly sensitive gas detecting devices Metal oxide based gas sensors, which rely on changes of electrical conductance due to the interaction with the surrounding gas, have been developed over the years to established devices Especially the employment of thin films has improved the sensor performance [1–4] Among all metal oxides SnO2has become the most prominent sensing material and many SnO2-based sensor devices have been realized so far [5–7] The implementation of MEMS technology has further advanced the gas sensing device performance Thermally insulated micro hotplates, for example, have been integrated as platforms on CMOS-chips for the realization of sensor arrays comprising different polycrystalline materials and allow for adjustment of different temperatures

to provide a certain level of selectivity [8–10]

A most powerful strategy to improve sensor performance is the implementation of nanostructured materials, such as nanocrystalline films or nanowires, which have a high surface to volume ratio and thus a strong interaction between the surrounding gas and the material Several gas detecting devices utilizing metal oxide nanocrystals [11, 12], nanorods [13–15], or nanosheets [16], for example, have been realized Nanocrystalline sensing films, however, may have the problem of long-term sensor poisoning Therefore with respect to device stability single crystalline nanowires are favorable Numerous fascinating devices based on nanowires as sensing probes for both chemical and as well as biological analysis have been demonstrated so far [17, 18] Excellent reviews of nanowire sensor devices are given in references [19, 20] The range of nanowire materials encompasses metals [21, 22], semiconductors [17, 23–25], metal oxides [26, 27], carbon nanotubes [28, 29], and polymers [30, 31]

For gas sensing applications, which often require measurements in harsh environments, metal oxide

nanowires are of particular importance because they have a high chemical resistance and thermal stability Gas sensing has been successfully achieved with a variety of metal oxide nanowires, such as

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hydrogen versus CO monoxide discrimination as a step towards the electronic nose [37]

As SnO2 has been the most prominent sensing material in conventional gas sensors, the major focus has been put on the fabrication and implementation of SnO2nanobelts and nanowires as sensing probes Nanosensors based on SnO2-belts with high sensitivity for CO, NO2 and Ethanol have been reported [26] Detection of CO and O2has been achieved with template grown SnO2nanowires, while enhanced gas sensing has been demonstrated with Pd functionalized SnO2 nanowires [38] Tin oxide nanobelts have been integrated with microsystems for nerve agent detection [39] Modified SnO2nanoribbons and nanowires have been developed for the detection of H2S [40]; CO and humidity have been detected with single SnO2nanowires [41]

In this paper we report on atmospheric pressure synthesis of single crystalline SnO2-nanowires directly

on the Si-chip by spray pyrolysis and subsequent annealing Our two-step SnO2-nanowire fabrication procedure is very simple, requires no vacuum, and allows for straightforward upscaling the possible substrate to 6”-wafer size We believe that our fabrication procedure might be the technology of choice for the controlled fabrication of SnO2-nanowires as highly sensitive gas sensing elements on a wafer scale

2 Experimental

The SnO2-nanowires are fabricated in a two-step procedure: In a first step nanocrystalline SnO2-films are fabricated by a spray pyrolysis process These SnO2films have already been employed by us as gas sensing elements Recently we have reported on SnO2thin-films gas sensors (thickness 50 nm - 100 nm)

that show high sensitivity to humidity and are able to detect carbon monoxide down to a concentration

of less than 5 ppm [42] The experimental setup, shown in Fig 1, consists of a 30 x 30 cm2 hotplate and

a siphon-fed spray setup with an air atomizing spray nozzle (QuickMist QMJML, Spraying Systems Co.), which is positioned on the side of the hot plate allowing the atomized spray to flow parallel to the

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Three different types of samples were prepared: The 2 cm x 2 cm large substrates were either directly used for SnO2 deposition or sputter coated with a 40 nm thick Cu-layer, or 5 nm thick Au-layer, respectively, before SnO2 deposition in order to study possible influence of catalytic metal films The samples were directly placed on the hotplate heated up to a temperature of 500°C Spraying a 0.28 molar solution of tin chloride pentahydrate in ethyl acetate results in the formation of nanocrystalline SnO2-

layers on the substrates with a deposition rate of ~100 nm/min according to the (simplified) chemical reaction

SnCl4+ 2H2O = SnO2+ 4HCl (1)

Spray duration was 2 min so that the resulting SnO2film thickness on all samples was around 200 nm Subsequent to the deposition process, the samples were removed from the hotplate and immediately cooled to room temperature The whole deposition procedure was performed in ambient air

The second SnO2-nanowire fabrication step is an annealing process: a tube furnace was heated up to temperatures of 800°C, 900°C and 1000°C The SnO2-coated samples were placed into the heated furnace; due to the small thermal mass ramp-up time was negligible The annealing process was performed in Ar-gas at atmospheric pressure, a slight Ar-gas flow was used during the process Annealing time was 60 min for all samples, only the temperature was varied as critical parameter in order to find the best conditions for synthesizing ultra-long SnO2nanowires for sensor applications All samples were annealed individually to exclude any mutual material growth or deposition of vaporized products Immediately after the annealing process the samples were removed from the furnace and cooled in ambient air to room temperature

3 Results and Discussion

3.1 Tin oxide nanowire characterization

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samples A scanning electron microscopy image (SEM) shows SnO2nanowires with diameters of 30

-400 nm and lengths up to several 100 µm directly on the samples (Fig 2) The nanowire density was lower and nanowires tend to be shorter towards the middle of the samples Growth of nanowires with lengths up to several 10 µm was also observed on samples without metallic intermediate layer Areal density of nanowires, however, was orders of magnitude less than compared to samples with metallic intermediate layer Annealing at a temperature of 1000°C showed also wool-like nanowire growth on samples with intermediate Cu- and Au layers, but SEM analysis reveal that the nanowires significantly start branching and interconnecting to each other, which makes further processing of single nanowires as gas sensing elements difficult A few nanowires are also found randomly distributed on samples without intermediate metal layers, but nanowires are only a few µm long

The SnO2-nanowires have been investigated with a transmission electron microscope (TEM) Bright field images and electron diffraction patterns were acquired using a Tecnai F20 with a field emission gun (FEG) operating at 200 kV The microscope has a post column energy filter (Gatan Imaging Filter, GIF) and the images were recorded in zero-loss filtered mode, using a 10 eV wide slit (i.e elastically scattered electrons only) The selected area electron diffraction pattern of a typical SnO2-nanowire fabricated at annealing temperatures of 900°C is shown in Fig 3 and represents a cubic crystal with orientation [001] The SnO2-nanowires are found to grow either straight or alternating along two distinct directions, as is obvious from the TEM bright field image shown in Fig 4 Fig 5 represents the high resolution (HR) TEM detail of the SnO2-nanowire indicated in the inset of Fig 4 The SnO2-nanowire is single crystalline and the surface is very smooth The Fourier transformation (FFT) of the HRTEM image shows two preferred growth directions [100] and [110] As a result of these two growth directions some nanowires show 45° degree facets (Fig 5) The TEM-micrograph of a nanowire with a diameter of

solid mechanism, have been elaborated and are summarized in some excellent reviews [48-50].

Among all technologies our fabrication scheme seems to be mostly related to the vapour-solid-process

as described in Ref 48 Normally, the source material is thermally vaporized, a carrier gas such as Ar is used for the transport, and the resultant vapour phase condenses on a substrate, which is placed downstream of the source material In our case, however, the SnO2-nanowires are fabricated on a single chip, which might indicate a competing SnO2 evaporation and subsequent condensation process immediately above the sample surface: The nanocrystalline SnO2 film, which has been deposited by spray pyrolysis on the Si-chip, is acting as the source material X-ray photoelectron spectroscopy analysis clearly proves that the deposited films are composed of SnO2and no other oxidation state, such

as SnO, is contained in the films [42] When the samples are heated at atmospheric pressure, the SnO2

starts to vaporize into molecular species As no carrier gas is used in our case for possible vapour phase transportation, the SnO2-molecules can immediately start to precipitate on specific crystal facets of the nanocrystalline SnO2 grains, which are statistically distributed in all crystal orientations on the Si-

substrate The newly arrived molecules continue to deposit on the formed nucleus and the side surfaces that have lower energy start to form Due to the high mobility of the molecules, the next molecules diffuse on the surface, move to the lower energy sites at the growth front and form the nanowires

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been performed at higher pressure of 1.33 mbar in an environmental scanning electron microscope (ESEM) with an integrated furnace in order to study the nanowire growth process in-situ By stepwise increasing the temperature, a structural change of the nanocrystalline SnO2-film was observable for temperatures above 850°C However, the SnO2-material simply vaporized, no nanowire growth due to condensation was observed From these experimental results we conclude that the annealing process requires conditions around atmospheric pressure, which suppresses the evaporation of the SnO2-material from the surface and favours the subsequent condensation in order to form the nanowires

While these results are an indication for the proposed competing evaporation and condensation process directly on the surface of the sample, the influence of the intermediate metal layers Cu and Au,

is not understood It is obvious from the experiments that these metal layers strongly trigger the nanowire growth, which indicates a metal-catalytic vapour-liquid-solid (VLS) growth mechanism [49]

The metal layers are underneath the SnO2-nanowire at the interface to the SiO2-layer, where basically no vapour can influence the growth Due to the nanocrystalline structure the metal might diffuse along grain boundaries to the surface and start to trigger a VLS-growth In the TEM studies, however, we have not found any evidence for Cu or Au clusters at the tips of the SnO2-nanowires so far, which would be a clear evidence for a VLS growth mechanism First experiments with Cu and Au-layers on top of the SnO2-films have also been performed and SnO2-nanowire growth has also been achieved The nanowires, however, are much shorter than in case of the intermediate metal films, and are strongly branched and interconnected Additional nanowire growth experiments will be carried out in order to fully understand the growth mechanism and to clarify whether it is a VLS dominated mechanism or not

3.2 Tin oxide nanowire sensors