current status of micro and nano structured optical fiber sensors

Accepted Manuscript Title: Transport and gas sensing properties of In 2 O 3 nanocrystalline thick films: a Hall effect based approach Authors: A. Oprea, A. Gurlo, N. B ˆ arsan, U. Weimar PII: S0925-4005(09)00204-4 DOI: doi:10.1016/j.snb.2009.03.002 Reference: SNB 11382 To appear in: Sensors and Actuators B Received date: 14-8-2008 Revised date: 15-12-2008 Accepted date: 5-3-2009 Please cite this article as: A. Oprea, A. Gurlo, N. B ˆ arsan, U. Weimar, Transport and gas sensing properties of In 2 O 3 nanocrystalline thick films: a Hall effect based approach, Sensors and Actuators B: Chemical (2008), doi:10.1016/j.snb.2009.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. 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 proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Page 1 of 27 Accepted Manuscript 1 Transport and gas sensing properties of In 2 O 3 nanocrystalline thick films: a Hall effect based approach A.Oprea* 1 , A. Gurlo 2 , N. Bârsan 1 , U. Weimar 1 1 Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 8, 72076 Tuebingen, Germany 2 Fachbereich Material- und Geowissenschaften, Technische Universitaet Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany Abstract Undoped nanosized In 2 O 3 with n-type conduction was produced in both polymorphic forms (cubic and rhombohedral) and deposited by screen printing as thick films. These films show high sensitivity to low O 3 concentration levels. They have been investigated by four point conductance and Hall Effect measurements under sensor operating conditions (elevated temperature and ozone exposure). The effective values of the charge carrier concentration and mobility have been calculated from the experimental records using the recipe for the single crystals. The response to O 3 is discussed in the frame of the standard models for gas sensors. The observed deviations from the model are explained in connection with the film crystalline structure and microscopic parameters spread. Keywords: In 2 O 3 , mobility; gas sensitivity 1. Introduction In 2 O 3 is investigated since decades. At the beginning the scientific interest was, more or less, the reason and the stimulus of investigations. However, remarkable optical transmission and the metallic like conduction (when suitably impurified or having * Corresponding author: alexandru.oprea@ipc.uni-tuebingen.de Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-12), July 13-16, 2008, Columbus, OH, USA revised Manuscript Page 2 of 27 Accepted Manuscript 2 stoichiometric deficiencies) of indium oxide led soon to practical application. Either alone or, most frequently, in combination with other oxides of transition metals, it became the most utilised transparent conducting oxide (TCO) in optoelectronics and related fields. For such purposes the material is produced as thin and compact layers with low structural defect concentration. The literature abounds in studies performed on films of this type, deposited by different means on a large variety of substrates. Some comprehensive overviews are also available [1]. After observing the high sensitivity of In 2 O 3 towards ozone [2-4] (see also survey in [5]) or other oxidising gases [6-13] and in direct connection with the rising concern about the ozone negative effects on ambient quality and human health the interest for gas sensors based on this compound begun to increase. Since then several pertinent investigations [5, 14] on the O 3 -sensing properties of In 2 O 3 have been performed resulting in laboratory versions of chemoresistive gas sensors with detection limits in few parts per billion (ppb) range. The material utilised in gas sensing should have a very large specific area and therefore it is typically prepared as porous thin/thick layer. The most employed manufacturing paths are making use of powder technologies, at least in their last step. Morphologies with grain / crystallite dimensions spread over many order of magnitude (few nanometer to micrometer) [6, 10, 15, 16] or nanostructures [17] have been reported. A good understanding of electrical conduction in films with such structures is indispensable for the optimal design of the sensing devices. In the same time, there is a scientific interest on this topic as well, due to the strong connection between the combined electrical transport mechanisms, taking place in and across the material grains and the surface interactions with the gaseous ambient, mainly at elevated temperatures (250 - 450°C) where the films have to operate as gas sensing elements. In spite of this principle scientific interest we did not find any articles dealing with the electrical transport in granular In 2 O 3 layers for gas sensing and, to the best of our knowledge, Page 3 of 27 Accepted Manuscript 3 there are no references addressing the concentration and the mobility of the charge carriers in such layers. The present paper deal with the electrical conduction of O 3 sensing films deposited by screen printing from undoped nanosized In 2 O 3 powders. Both In 2 O 3 polymorphs, i.e. bixbyite- type c-In 2 O 3 (cubic, C-type structure of rare-earth oxides, space group 3Ia , No. 204) and corundum-type rh-In 2 O 3 (rhombohedral, space group cR3 , No. 167) were studied. To the best of our knowledge the O 3 sensing properties of the undoped corundum-type rh-In 2 O 3 and the electrical parameters under operation conditions of O 3 sensing layers from both polymorphs have not been evaluated until now. Only two articles report the gas sensing investigations of corundum-type ITO [18] and rh-In 2 O 3 [19]. Therefore the investigations, based on Hall Effect [20-22] and four point conductance measurements, have been performed in synthetic atmospheres with controlled composition and temperature. They aim to provide the missing information concerning the charge carrier concentration and mobility in In 2 O 3 thick porous layers for gas sensing and, by using it, to sketch some features of the interplay between the electrical transport and sensing properties in such films. 2. Experimental As just stated above, the experimental basis of the investigations consists of Hall Effect and four point conductance measurements on nano-granular screen-printed In 2 O 3 thick films. The samples, heated at temperatures appropriate for gas sensing, have been exposed during the electrical tests to synthetic atmospheres containing nitrogen, oxygen, ozone and water vapour (humidity) prepared with a dynamic gas mixing station [23]. In this way it is possible to either reproduce the condition in which a sensor made from the same material is usually operated (that is, to make operando investigations) [24], or to create a completely unusual atmosphere, relevant for gas sensing mechanisms and their relation with the electrical transport [25]. Page 4 of 27 Accepted Manuscript 4 2.1. Synthesis, structural characterisation of the materials and sample preparation. Synthesis was performed by the sol-gel method based on the ammonia-induced hydrolysis of indium nitrate in methanol with acetylacetone as a complexing agent (acetylacetone route hereafter) and without acetylacetone (hydrolysis route hereafter) [26]. The processed powders (calcined in air at 500°C for 1 h) were used for the structural characterization and for the screen-printing. For the present approach it is important to point out that both In 2 O 3 polymorphs, have been obtained through the above referred sol-gel routes. The acetylacetone route resulted in the bixbyite-type c-In 2 O 3 , while the hydrolysis route leads to the corundum-type rh-In 2 O 3 . The rh-In 2 O 3 reveals nanorhombohedra terminated by planes with a size ranging between 50 – 100 nm; the c-In 2 O 3 possess much smaller, highly agglomerated, crystallites with a size below 20 nm. The structural and morphological characterisation of c-In 2 O 3 is reported in Ref. [14], those of rh-In 2 O 3 , in Ref. [27, 28]; the detailed characterisation of the sensing properties of the cordundum-type rh-In 2 O 3 will be presented in detail elsewhere [29]. In the last technological step thick (~ 20 µm) sensing films have been screen printed [14, 30] on substrates suitable for Hall Effect and electrical measurements. They are provided with platinum electrodes on the layer side and with a platinum heating meander on the opposite one. The description of the sample geometry and electrode functionality can be found in [31, 32]. 2.2. Measuring system The main part of the measuring system has already been described elsewhere [31, 32]. It consists in a gas mixing station which supplies with gaseous test mixture a flat measuring chamber placed between the polar pieces of a Brucker electromagnet. A computer driven power electronics ensure magnetic field with the strength up to 1 T and required time dependencies. For the present study some supplementary facilities have been added to the above referred experimental setup. They are related to the ozone generation and humidity Page 5 of 27 Accepted Manuscript 5 level control. O 3 , the main target gas in the investigations was produced with an Anseros ozone test system SIM 6000 with an integrated ozone generator and MP UV ozone analyzer; its concentration in the carrier gas was determined before and after sample exposure by using the Anseros MP UV ozone analyzer and Environics Series 300 computerized UV ozone analyzers, respectively. Because the wide pulse modulation (WPM) regime of the O 3 generator was factory-set in the low frequency range, it was necessary to smear off the O 3 concentration variations with a 1 l buffer. The time constants of the buffer itself, of about 10 min at 100 sccm carrier gas flow, are strongly reflected by the sample response, but did not affect the present investigations, intended for near equilibrium conditions. For reasons not clarified yet, in the O 3 delivered by the generator a non negligible level of humidity was present, mainly at high O 3 generation rates. The exact values could not be determined because of the incompatibility between the existing psychometers and the O 3 containing atmosphere. In many experiments this parasitic humidity and the humidity background accidentally present in the gas circuitry, with important consequences in the sample response towards strong oxidising gases, was reduced below 30 ppm (parts per million) with a cryogenic N 2 vapour trap (to avoid the oxygen condensation). However, controlled amounts of humidity have been provided by a dedicated channel of the gas manifold, when required. The O 3 loss by adsorption and reaction in the pipe lines and measuring chambers, which would randomly modify the test mixture composition, was prevented by using adequate materials as perfluoroalkoxy (PFA) and polytetrafluoroethylene (PTFE, Teflon ® ). 2.3. Measurement procedure and acquired data In order to gather data relevant for the influence of the surrounding atmosphere on the conduction mechanisms in the investigated material three main types of operando measurements have been carried out: I-V characteristics, four point conductance and dc Hall voltage. In all of them four point sample geometry and electrical set-ups have been utilised. Page 6 of 27 Accepted Manuscript 6 During the investigations the samples have been exposed to N 2 / O 2 mixtures with a mixing ratio varying from 100 ppm O 2 / N 2 to 100% O 2 and, additionally, to O 3 dosed in concentrations from 10 ppb to 2 ppm in some O 2 / N 2 selected combinations. Due to set-up functional limitations the mixtures containing 50% r.h. (relative humidity) contained up to 1ppm O 3 only. The experimental data have been usually recorded very near to the thermodynamic equilibrium of the gaseous and solid interacting phases. Waiting times of 12 – 24 h between the gas exposure steps have been typically used. In the routine tests, intended to check the material suitability for low level O 3 sensing, significantly shorter intervals of 2 – 4 h appeared to be sufficient. The parasitic thermoelectric and thermomagnetic effects occurring together with the Hall Effect, especially at elevated operation temperature, have been eliminated by making use of reversing magnetic fields and polarisation voltages. The time dependency of the magnetic field, a trapezoidal one, avoids, on one hand, large transient Eddy currents in the electromagnet coils, and, on the other hand, allows the direct visualisation of the sample response linearity. Therefore the electrical and magneto-electrical measurements extended over both stationary and transitory regions. 3. Results and discussions The outputs of the performed measurements are three types of raw data: the I-V characteristics, four point conductance and trapezoidally shaped Hall voltages, all of them depending on the ambient composition and working temperature. Once the linearity of the responses confirmed by the transitory regions of the records, only the steady state values of the electrical parameters and the specific sensitivity curves have been considered in the further analysis of the experimental results. Thus, in the first step of data evaluation, the effective charge carrier concentration and mobility have been obtained by using the recipe for single crystals [21, 22, 33-35]. They give an intuitive picture of the material behaviour and provide the basis for subsequent analysis and discussions. Page 7 of 27 Accepted Manuscript 7 The discussions on the results are starting from the general accepted models for gas sensing and conduction in MOX semiconductors (as presented and commented in [32, 36, 37]). 3.1. I-V characteristics For each material and measuring condition an I-V characteristic was determined. All of them are very linear over more than 4 orders of magnitude, proving, by that, the pure ohmic character of the samples, at least under all particular condition occurring during the investigations. Fig. 1 shows typical I-V plots in both standard linear scales (left panel) and logarithmic scales (right panel) for rh-In 2 O 3 samples heated at 200°C. One has to briefly remark the wide range of slopes encountered in the graph with linear scales reflecting the strong dependence of the material resistance on the O 3 concentration. In the graph with logarithmic scales the slope is always the same, and expresses the linearity of the response; the intercept on a vertical axis is decreasing with the resistance increase. The linear behaviour results from the low voltage drop on each grain, less than 3 mV, which, at the working temperature of 150°C - 270°C, is well below the thermal voltage 5035  e Tk V B T mV (where: T , e , B k denote respectively the absolute temperature, elementary charge and Boltzmann constant). The evaluation has been done in the most unfavourable conditions, that is, maximal applied voltage of 100 V, and maximal crystallite diameter of 100 nm, by using the actual electrode spacing of the samples of 3 mm. At the above specified polarisation the double barriers associated to each grain to grain contact never reach the Schottky diode operation region (due to polarisation) and the ohmic response is the “normal” one. Therefore, for each measuring conditions the sample resistance / conductance is the only one significant parameter resulting from an I-V characteristic (actually due to its high linearity). The experimental values of the conductance determined during the investigation will be given later on in the §3.4. Page 8 of 27 Accepted Manuscript 8 3.2. Four point resistance Self-standing four point resistance measurements have been performed only to prove the sensitivity of the sensing layers and to provide a “sensor fingerprint” familiar in the field of gas sensing. Fig. 2 is well stressing the general trend of the studied materials, namely a high response to reduced concentrations of O 3 . The saturation trend of the curves is enhanced to some extent by the presence of the residual humidity, not removed for these routine tests. We have to point out that we have recently reported unusual O 3 -sensing properties for c- In 2 O 3 , i.e. we observed that the screen-printed c-In 2 O 3 sensors showed saturation in the two- point conductance measurements even at low O 3 concentrations [14]. This effect was explained by possible influence of adsorbed oxygen, similar effects were also observed for - Fe 2 O 3 [38, 39]. An exhaustive discussion concerning the different sensor parameter (gas response, sensitivity, selectivity, reproducibility, time constants) will be provided in a dedicated paper (including also film preparation and morphology), currently under preparation [29]. The results from four point resistance / conductance measurements acquired together with the Hall voltages during the Hall Effect measurements are included in common graphs (Fig. 3 and Fig. 4) 3.3. Hall voltage The steady state values of the Hall voltage have themselves no direct meaning if taken alone, but they can provide a rough estimate of the effective concentration of the majority charge carrier, when using the standard single crystal recipe [21, 22, 33-35] in the first data evaluation step. In the same frame one can also calculate the effective Hall mobility of the majority charge carriers from the conductance records in a subsequent step. The mobility extraction procedure is susceptible of some errors as long as one utilises the ratio of two quantities extending over many order of magnitude (with a roughly exponential dependency on the O 3 concentration) and therefore the rapid variations of the obtained values have to be Page 9 of 27 Accepted Manuscript 9 considered with care. Independent of the mobility value, the Hall measurements confirmed the negative sign of the free charge carriers participating to the electrical transport in our In 2 O 3 samples; this feature was already known from the decrease of the material resistivity under reducing gases exposure [40]. The effective parameters give a more “friendly” description of the transport properties presented by the investigated materials at macroscopic scale, close to the classical Drude model [41-44]. To, however, relate them to the processes taking place at microscopic level and in relation with the sample structure / morphology is a difficult - if still solvable - task [31, 32, 35]. 3.4. The influence of the ambient atmosphere on the effective electrical parameters In the following some significant experimental dependencies of the effective electrical parameters (single crystal recipe) are presented (Fig. 3 and Fig. 4) and shortly commented. The trends in the behaviour of the samples are better visible on Fig. 5, where the normalised (relative) values of the considered parameter are included. Before addressing the original results of the investigation it is important to shortly comment on the experimental dependency of the conductance (and implicitly electron concentration) on temperature. As Fig. 4 shows the material under consideration follows some general trends of n type MOX. In a first stage, with the increasing of the temperature over the room value, the shallow donor bulk levels ionise more and more towards complete ionisation, if possible. Deeper donor levels will follow at higher temperatures. In parallel with these electronic processes other physical and chemical processes are activated by the increase of the temperature. So, the ambient oxygen adsorbs at the semiconductor surface, at the beginning (below 150°C), as molecular ions and then (above 150°C), when O 2 molecule dissociate, as atomic ions by trapping conduction electrons. In this way occupied surface levels appear that are not available in the absence of the adsorbat. The release of the electrons and the desorption processes are necessarily occurring together (for a thorough description in relation [...]... ideal systems for the standard modelling procedures of the gas sensors Moreover the analysis of the interplay of the microscopic parameters allowed to additionally explain the trends of the samples when operated in regions situated outside of the standard model validity 14 Page 14 of 27 References [1] I Hamberg,C.G Granqvist, Evaporated Sn-doped In2O3 Films: Basic optical properties and applications to... structures on gas sensing properties of nanocrystalline ITO thick films Sensors and Actuators BChemical, 89 (2003) 180-186 [19] D.W Chu, Y.P Zeng, D.L Jiang, J.Q Xu, Tuning the phase and morphology of In2O3 nanocrystals via simple solution routes Nanotechnology, 18 (2007) - 16 Page 16 of 27 [20] E.H Hall, On a new action of the magnet on electrical current American Journal of Mathematics, 2 (1879) 287-292... Polycrystalline and Powdered ip t [33] N Barsan,U Weimar, Conduction model of metal oxide gas sensors Journal of us [36] cr Semiconductors Reports on Progress in Physics, 43 (1980) 1263-1307 Electroceramics, 7 (2001) 143-167 N Barsan,U Weimar, Understanding the fundamental principles of metal oxide based an [37] gas sensors; the example of CO sensing with SnO2 sensors in the presence of [38] M humidity Journal of. .. 1993 and his Habilitation 2002 from the University of Tübingen He is currently the head of Gas Sensors Group at the University of Tübingen His research interest focuses on chemical sensors as well as on multicomponent analysis and pattern recognition 20 Page 20 of 27 Captions Fig 1 Example of I-V characteristics for rh-In2O3 under different O3 exposure levels: 0% r.h background (upper panel) and 50%... gained from the analysis of the three microscopic parameters, l grain ,  and LD , shows that is almost impossible to model cr the sensing layer response under increased analyte concentrations, but gives a fair explanation us of the observed trends an Conclusions The preparation and operando investigation of the two polymorphic forms of the M nanosized - In2O3 brought novel and interesting results:... complications in the understanding and modelling of the 12 Page 12 of 27 sensing layer response towards the target gases As already shown in literature [31, 32, 35, 36], the interplay of the grain size (lgrain), mean free path (l), and Debye length ( LD ) controls the amount of the free charge still available in each grain, the depletion level of the grains and the height of the associated Schottky... diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his Ph.D in Solid State Physics from the Institute of ed Atomic Physics, Bucharest, Romania Since 1995 he is a researcher at the Institute of Physical Chemistry of the University of Tübingen and actually is in charge with the developments in pt the field of metal oxides based gas sensors Ac ce UdoWeimar received his... well-shaped blocks of indium oxide obtained by the sol-gel method and their gas-sensing properties Chemistry of Materials, 15 (2003) 4377-4383 M Ivanovskaya, A Gurlo, P Bogdanov, Mechanism of O3 and NO2 detection and cr [12] [13] A Gurlo, M Ivanovskaya, A Pfau, U Weimar, W Gopel, Sol-gel prepared In2O3 an thin films Thin Solid Films, 307 (1997) 288-293 [14] us selectivity of In2O3 sensors Sensors and Actuators... Riedel, Nanocubes or Ac ce [28] pt European Journal, 14 (2008) 3306-3310 nanorhombohedra? Unusual crystal shape of corundum-type indium oxide Jounal of Physical Chemistry C, 112 (2008) 9209-9213 [29] [30] A Gurlo, A Oprea, N Barsan, U Weimar, (in preparation) M Graf, A Gurlo, N Barsan, U Weimar, A Hierlemann, Microfabricated gas sensor systems with sensitive nanocrystalline metal-oxide films Journal of Nanoparticle... Barsan, M Ivanovskaya, U Weimar, W Gopel, In2O3 and MoO3- M [5] In2O3 thin film semiconductor sensors: interaction with NO2 and O3 Sensors and [6] ed Actuators B - Chemical, 47 (1998) 92-99 W.Y Chung, G Sakai, K Shimanoe, N Miura, D.D Lee, N Yamazoe, Preparation of pt indium oxide thin film by spin-coating method and its gas-sensing properties Sensors [7] Ac ce and Actuators B - Chemical, 46 (1998) 139-145