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Nonconventional phthalocyanines for field effect gas detection
Sensors and Actuators B 179 (2013) 54–60
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nonconventional phthalocyanines for field effect gas detection W. Simmendinger ∗ , A. Oprea, N. Bârsan, U. Weimar Institute of Physical and Theoretical Chemistry, auf der Morgenstelle 15, 72076 Tuebingen, Germany
a r t i c l e
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Article history: Received 12 July 2012 Received in revised form 1 October 2012 Accepted 11 October 2012 Available online 22 October 2012 Keywords: Metal phthalocyanine NO2 sensing Work function Kelvin Probe
a b s t r a c t Metal (TiO and Zn) phthalocyanines that are not currently investigated for gas sensing applications have been deposited by thermal vacuum evaporation as thin layers and evaluated in operando Kelvin Probe setup as NO2 sensing materials. From the raw data their calibration curves have been inferred and some sensing parameters determined. Comparative conductometric evaluations have also been performed. On the basis of the experimental information the main peculiarities of the gas responses have been identified and discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction After their initial use as pigment or dye it was found that the metal phthalocyanines (MPcs) change their electrical properties under the sorption and desorption of oxidizing gases [1–4]. Therefore, the gas sensing community tried to utilize the MPcs as sensitive layer for e.g. NO2 -detection, where it is definitely need of monitoring due to its poisonous and carcinogenic properties. Nevertheless, up to now, there is no commercially available gas sensor based on MPcs on the market, although they offer several advantages, like cost effective production, low power consumption, thermal and chemical stability, etc. These features are seldom encountered at the already established NO2 gas sensors, such as the metal oxide based ones, which unfortunately have reduced selectivity and, moreover, operate at rather high temperatures or at the more specific electrochemical cells that are expensive and have a limited lifetime. The gas sensing characteristics of the MPcs depend on the nature of the central metallic atom [5,6], on the substituents at the organic ring and also on the crystalline structure of the material, which can be influenced by chemical/thermal pre-treatments [7–9]. Fine tuning of Pc’s composition and morphology can result in films with increased gas sensitivity, optimized for different gas sensing application such as ambient surveillance, fire detection and diseases recognition [10,11]. Meanwhile NO2 concentrations down to 10 ppb are experimentally measurable with MPc based field effect sensors [11].
∗ Corresponding author. E-mail address: [email protected] (W. Simmendinger). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.064
The interaction between the MPcs and the analytes takes place in different ways with different strengths. It ranges from unspecific and weak dispersion interactions, (common to organic compounds) which causes small but fast changes in the layers properties (effect is mostly utilized in gravimetric measurements [12–15]), to strong interactions with oxidizing gases, leading to the formation of free charge carriers (usually holes) via charge transfer complexes [16–18]. The last process subsequently alters the electrical properties of the semiconducting MPcs (usually having a p-type conduction) resulting in a good sensitivity and fair selectivity. For a successful implementation in practical applications some known drawbacks of the gas sensing with Pcs have to be, however, overcame. The main one is the long recovery time due to the strong interactions between MPcs and the oxidizing gases (addressed above). Also the long term stability of some MPc layers that depreciate after few weeks or even days has to be improved. In such cases, convenient application types and suited readout scenarios/algorithms might be required. The MPcs gas sensors proposed in the literature are mainly chemoresistors [5,6] or field effect devices [10,11]. The field effect readout has several advantages: • Uses signals produced by the sensing layer surface [19–21], which, in principle, should increase the specificity of the sensor gas response, eliminating the contribution of the dispersion interaction (significant for the volume effects based transducers like the gravimetric ones). Simultaneously, a certain decrease in the response and recovery times is expected because the diffusion times into and out of the sensing material are avoided. • Very low power consumption, as in the case of any electrostatic readout.
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• Easily incorporation in application specific integrated circuits (ASICs) with increased market potential. The earliest and most investigated MPc is the CuPc [6]. Oxidizing gases (NO2 , O3 ) and halogens (Cl) have been detected with conductometric (down to 10 ppb [22–24]) and work function (down to ∼1 ppb [11,25]) sensors. With the time the number of available MPcs increased, stimulating the interest for related gas sensing devices (PbPc, NiPc, CoPc [5,6,26–30]). However, there are several MPcs, whose suitability as gas sensing materials have been, until now, not very intensively investigated. For instance, only few conductivity [31,32] and no Kelvin Probe (KP) measurements gas exposure have been performed on titanylphthalocyanine (TiOPc), even though field effect measurements for organic electronics have been reported [33,34]. Actually, in [33] a comparison between the output transfer characteristics of an organic TiOPc field effect transistor in NO2 atmosphere (20 ppm in N2 ) and vacuum has been made. Therefore no/scarce information about the KP/work function response of the TiOPc to oxidizing gases like NO2 is available. Also ZnPc lacks the data about KP gas effects in spite of the fact that it is since longer time in the attention of gas sensing research [6,35–39]. In this context seemed us useful to make some combined operando investigations on TiOPc and ZnPc films. The operando method [20,40] supposes the evaluation of the device or material samples under conditions similar to those being employed during the foreseen application. Thus KP, conductometric, gravimetric and infrared absorption measurements under controlled atmosphere with focus on the field effect applications/implementations have been performed. The gravimetric and infrared data are not reported here, the measurements being still in progress. 2. Experimental 2.1. Sample preparation Because of their high thermal and chemical stability, but poor solubility the TiOPc and ZnPc layers have been deposited by vacuum thermal evaporation on appropriate substrates. The studied MPcs have been purchased from Sigma–Aldrich and purified before the deposition through a bake-out procedure up to 400 ◦ C. For the KP samples 7 mm × 9 mm silicon substrates covered with a 800 nm Au/20 nm Ti electrodes have been produced (Ti layer improved the adhesion of the Au electrode to the Si). The dc- conductance samples required interdigitated transducers on sapphire substrates and having 50 m line spacing and width. In a deposition batch have been included all types of substrates by using a dedicated holder and a common shadow mask. The process has been performed under 3 × 10−7 mbar, with a deposition rate of 0.02 nm/s at a temperature of 400–450 ◦ C, resulting in ∼100 nm MPc layers. Since the substrates have not been intentionally heated, the amorphous phase of the TiOPc and the ␣-modification of the ZnPc have been obtained. Under a 24 h exposure to saturated ethanol vapour it was possible to transform the amorphous TiOPc-layer into its ␣-modification [41,42]. Finally the samples for the Kelvin Probe and dc-measurements have been mounted on heatable sockets in order to enable measurements at temperatures higher than the room one.
Fig. 1. Experimental setup, consisting of a gas mixing system, operando measuring chambers and investigation apparatus (McAllister Kelvin Probe and Hewlett Packard picoampermeter).
dynamic (continuous gas flow) gas exposure, the samples have been mounted into dedicated Teflon® (or Teflon® coated) measuring chambers. The main investigation tool was the non-locking KP 6500 from McAllister Technical Services. The results are reported as delivered by the apparatus, that is, as contact potential differences [CPD] [19,20]. To convert them to work function changes a multiplication with the electron charge (−e) has to be carried out. The conductometric measurements have been performed with the HP 4140B picoampere metre (pA-metre) and Keithley 617 programmable Electrometer. The chamber for the dc-measurements has been connected downstream to the KP measuring chamber, but no noticeable influence (under steady state conditions) of the exposure order was observed. This happens because the sensing mechanisms leading to the detection of oxidizing gases with phthalocyanines do not suppose the chemical conversion of the analyte to other compounds and the analyte concentration remains constant along the whole measurement chain. This would be not the case for other sensors, as, for example, the metal oxide chemo-resistors, where the reducing gases catalytically burn on the sensor surface, so that the downstream concentration of the analyte is smaller than the upstream one. In all performed measurements (KP and conductometric) has been employed the gas exposure protocol presented in Fig. 2. 6.0 purity dry N2 was the carrier gas in the first half of the exposure sequence (1; 3; 5 ppm NO2 ). In the second half of the exposure sequence (with the same NO2 concentration steps) 50% relative humidity (r.h.) background has been added to N2 (through a high
2.2. Sample characterization The characterization of the samples has been made, as accounted for above, in an operando evaluation set-up, including a computer controlled gas mixing system and remote operated measurement instruments (Fig. 1). In order to enable their
Fig. 2. Gas exposure protocol utilized for the KP and conductrometric evaluation of TiOPc und ZnPc.
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rate vaporizer included in a dedicated channel of the gas mixing system). 3. Results and discussion From the raw data acquired during standard NO2 exposures (performed mainly at room temperature) have been derived the calibration curves of the MPc samples in the presence and absence of the background humidity. On their basis the main feature of the field effect sensing with the selected MPcs have been identified and shortly addressed. Few additional examples concerning the dc conductometric response of samples from the same batches have been added for comparison. It is important to underline here that KP data have been recorded in successive KP runs, because only one KP sample could be evaluated with the KP hardware in one exposure sequence. As accounted for in the McAllister KP6500 manual and explained elsewhere [20] only the work function changes are physically and chemically relevant. Therefore, the position of the work function baseline (KP sample response to the carrier gas) has no significance. 3.1. TiOPc The as prepared TiOPc layers are amorphous. As pointed out in the experimental section, some of them have been also converted, under ethanol saturated atmosphere, to the ␣- modification, which is crystalline (triclinic) [42]. The crystalline structure of the converted layers has been confirmed by UV–VIS spectroscopy [41,42]. Both morphologies have been investigated. The room temperature KP responses to NO2 from twin samples (same deposition batch, one amorphous, the other crystalline) are depicted in Fig. 3 (only one pair presented). First of all one observes large CPD changes upon 1 ppm NO2 exposure in dry air for both amorphous and crystalline samples (>120 mV) as well as a pronounced nonlinearity. The time constant of the response ( R63 ∼8 min) is acceptable for the room temperature characterization of the samples. The steady state is, however, hard to achieve. For the amorphous specimen, which is the fastest one, the response time for 90% from the steady state value ( R90 ) is quite large (∼1 h). The recovery times are also pretty large. This feature is common among non-heated phthalocyanines and is often reported in the literature [43]. With humidity background, which would be the “normal” operation condition for an atmospheric
Fig. 3. The room temperature KP response of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
sensor, the TiOPc behaviour significantly changes: the amorphous samples roughly keep the response direction and magnitude but the crystalline ones do not; in their case the CPD changes became positive, and a little bit smaller in module. The associated time constants decreased to some extent (∼4 min for the amorphous phase) and the steady state has been almost achieved during the exposure times being used (2 h). All the addressed features are also visible on the calibration curves (Fig. 4). They have been traced for two samples of each kind (two twins) in two consecutive exposure sequences (two times the sequence without humidity background and two times with 50% r.h. background). In the limit of the statistical spread, the main attributes of samples are reproduced among the class to which they belong (amorphous or ␣-crystalline) and to the exposure conditions being employed (with and without background humidity). Two new additional elements are obvious in Fig. 4: the significant decrease of the responses in the second exposure under dry ambient and the rough preservation of the responses under humid ambient (50% r.h). The first one is only apparent. Due to the slow analyte desorption rate, the baseline incompletely recovers (in the given time of 2 h) formally resulting in a response decrease. The second element is real and favourable in gas sensing application.
Fig. 4. The room temperature KP calibration curves of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
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Fig. 6. KP responses of the ZnPc layers to NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 .
Fig. 5. The room temperature conductometric calibration curves of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm. (First run only).
layers to NO2 are much smaller under humid conditions in comparison with those obtained at 50 ◦ C (discussion will follow). The calibration curves (Fig. 7) at room temperature and 50 ◦ C for successive exposures are quit reproducible; the 100 ◦ C ones show almost no response (as the KP raw data in Fig. 6 already indicated). As in the case of TiOPc significant differences appear in the conductometric responses (Figs. 8 and 9) in respect with the field effect ones. For example the best sensitivity is achieved at 100 ◦ C, while the sensitivities at the room temperature and 50 ◦ C are very small. This trend has been already underlined by other authors [6]. 3.3. Discussions
In the comparative conductivity measurements on TiOPc layers a different behaviour came into sight (see Fig. 5). Here the responses for amorphous samples in the presence of background humidity are much lower than those obtained in dry atmosphere (a 100 times decay was observed at 5 ppm NO2 ). Moreover, the humidity does not switch the direction and, to a large extent, does not modify the value of the conductometric responses from the crystalline ␣-TiOPc-layers (dissimilar to the KP samples) (explanation and comments later on). The conductometric response and recovery times are similar to that of the KP ones; the presence of the humidity accelerates the sensing processes and improves reproducibility. Judging the performance presented above, the amorphous phase would be more suited for gas sensing applications, but it is very unstable, lasting only a couple of test procedures. The ␣modification is stable, has enough sensitivity and is therefore, in spite of the longer time constants, much better suited for field effect gas sensing.
The striking non-equivalence between the results obtained in KP experiments and the conductometric ones, even if up-to-date not thoroughly elucidated and fairly modelled, seem to stem from several principle readout dissimilarities: 1. The KP generally measures the changes in the work function arising from two contributions [20]: a. The changes in the electron affinity of the sample material in the case of any conductor. b. The changes in the band banding, in the case of semiconductors. If the semiconductor surface hosts a dipolar layer, a contribution from the affinity has to be also considered. 2. The conductometric response has also two main contributions: a. The direct changes in the charge carrier concentration. b. The changes in the conduction mechanism affecting the effective mobility of the charge carriers.
3.2. ZnPc The as prepared ZnPc resulted in the ␣-modification; no additional treatments have been performed before characterization of the samples. The ZnPc layers show no clear KP response to NO2 (see Fig. 6) under dry ambient conditions; even the temperature increase has no obvious influence. 50% r.h. background causes a drop of the CPD, like in the case of TiOPc, and, more important, unambiguous KP responses (CPD changes) upon NO2 exposure came into play for room and slightly increased temperatures (50 ◦ C). The steady state was almost reached during the exposure due to time constants significantly smaller (especially at 50 ◦ C) in respect with those encountered at TiOPc. At 100 ◦ C the response of the ZnPc
The electron affinity actually expresses the energy required to bring an electron from the edge of the conduction band to the completely free state (vacuum level). If a dipolar layer covers the surface of a conducting material, less/more energy will be then needed to accomplish this task because the electron has now to pass throughout a favourable/unfavourable additional electrical field (the evaluation is given, for example, in [20]). Fig. 10 schematically presents the described process. In the case of semiconducting sensing films the analyte can be ionosorbed, exchanging mobile charge with the absorbent conduction/valence band or its delocalised electron system. Consequently the amount of free charge carriers and/or their effective mobility is modulated resulting in conductance responses
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Fig. 7. The KP calibration curves of ␣-ZnPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
to the gas exposure. The surface trapped charge is compensated by volume spatial charge (under the surface) leading to band bending (please refer to Fig. 4 from [20] for a graphical approach). The affinity changes are not visible in the conductivity of the samples because the bands are shifted upwards or downwards as a whole. The work function, however, includes these changes and is very sensitive to them. Therefore the background humidity, contributing mainly with a dipolar effect to the work function (through electron affinity) is very efficient in modulating the KP responses. The humidity dependence of the affinity does not affect the conduction, because it is an electrostatic effect. Nevertheless, the humidity can produce changes of the conduction either directly [15], acting as an electron donator [44], or in more complex ways, involving competitive absorption processes. On the basis of the above considerations one can understand the change of the KP response direction for the TiOPc as follows: The exposure of the TiOPc samples to the background humidity drastically modifies the surface coverage with water dipoles and results in a negative change in the CPD value (∼−130 mV). In the first moment the presence of the NO2 analyte produces the same type of response as in dry atmosphere through its direct action (the
Fig. 8. Conductometric responses of the ZnPc layers to NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 .
first spike downwards in Fig. 3). With the time (but very fast) the NO2 adsorption becomes a competitive process to the H2 O adsorption, reducing the H2 O adsorption rate, and shifting the surface balance to a state with less dipolar (H2 O) coverage. In consequence the CPD increases. The temperature also acts partially unlike in the case of the work function and conductometric responses: 1. Increased temperature favours the diffusion and, through it, the gas ab/ad-sorption and desorption.
Fig. 9. Conductometric calibration curves of the ZnPc layers against NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 . RB designates the base line value.
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Fig. 10. Conventional sketch presenting the effect of a dipolar layer adsorption on a semiconductor surface. The additional dipole charges change the microscopic potential in the vicinity of the surface leading to a new position of the vacuum level in respect with the Fermi level (considered as energy reference). The change of the electron affinity leads to an equal change in the work function .
2. At higher temperatures the analyte physical/chemical binding to the sensing material can be easier overcame by the thermal activation and, consequently, the amount of bonded analyte to the host sensing layer should decrease with the temperature increase. 3. The particular 100 ◦ C value corresponds to the boiling point of the free condensed water. Above this temperature it is less probable to have absorbed water in the sensing layer. In consequence the influence of the ambient humidity decreases and the sensing layer behaviour resembles the dry conditions one. All these influences from the operation temperature come into play with different weights and in connection with the nature of the work function changes. Furthermore, the nonspecific interaction of the analyte with the sensing material (like the dispersion one [45]), which can be disregarded for the crystalline phases, might be important for the amorphous phases. So, at 100 ◦ C the amount of the water adsorbed on the ZnPc layer is much less than that adsorbed at room temperature or 50 ◦ C. Therefore, also the CPD downwards jump caused by the humidity is significantly smaller at reduced temperatures (room temperature or 50 ◦ C) than at 100 ◦ C. The similitude in the CPD behaviour in dry N2 and humid one at 100 ◦ C originates also from there. If one reconsiders now the KP responses of the ZnPc samples (always crystalline) towards NO2 , with humidity background and at low temperature (25 or 50 ◦ C), which show the same trend with the ␣-TiOPc samples, one can ascertain that same competitive adsorption of H2 O and NO2 should be involved. The conductometric response of the amorphous TiOPc in the presence of the water vapours is very poor in respect with the one recorded in dry carrier gas (N2 ). For the moment a pertinent explanation is missing but the feature is probable due to a volume (even if the layer is so thin) sorption process with the modification of the TiOPc layer morphology which eventually results in a drastic degradation of the material itself. The crystalline phases are more stable and less sensitive to the background humidity. Nevertheless the ZnPc at relatively low temperatures (<70 ◦ C) is not too well suited for NO2 detection. The linear shape of the calibration curves in logarithmic scales (for KP samples) and double logarithmic ones (for conductometric samples) seems to support the models based on the charge/partial charge transfer processes accompanying the adsorption of the analyte [46]. This assumption is however very speculative at this stage of the investigation and additional data (mainly operando IR spectra) are required in order to identify the species residing on the sensing layer surface during the gas exposure and to conclude on the main gas sensing mechanisms.
4. Conclusion The investigated MPcs displayed good NO2 sensitivities, mainly in field effect sensing setups. Their crystalline modifications showed fair chemical stability during the evaluation and reversible interaction with the analyte. However, the relative long response and recovery time is hindering a repetitive operation at room temperature. Application that would require only single detection events, preceded by long standby periods (like the fire detection) or those that would allow short time thermal refreshment might open the way for the implementation of TiOPc and ZnPc in gas sensing devices. References [1] C.C. Leznoff, A.B.P. Lever, Phthalocyanines: Properties and Applications, VCH Publishers Ltd, New York, Weinheim, Cambridge, 1989. [2] G. Guillaud, J. Simon, J.P. Germain, Metallophthalocyanines gas sensors, resistors and field effect devices, Coordination Chemistry Reviews 178–180 (1998) 1433–1484. [3] J. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Progress in Surface Science 31 (1989) 1–60. [4] R.D. Gould, Structure and electrical conduction properties of phthalocyanine thin films, Coordination Chemistry Reviews 156 (1996) 237–274. [5] B. Bott, T.A. Jones, A highly sensitive NO2 sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators 5 (1984) 43–53. [6] T.A. Jones, B. Bott, Gas-induced electrical conductivity changes in metal phthalocyanines, Sensors and Actuators 9 (27) (1996) 27–37. [7] Y. Sadaoka, M. Matsuguchi, Y. Sakai, Y. Mori, W. Göpel, Effect of crystal form on the conductance in oxidative gases of metal-free and some metal phthalocyanines, Sensors and Actuators B 4 (1991) 495–498. [8] Y. Sadaoka, T.A. Jones, G.S. Revell, W. Göpel, Effects of morphology on NO2 detection in air at room temperature with phthalocyanine thin films, Journal Of Materials Science 25 (1990) 5257–5268. [9] C. Fietzek, H.G. Mack, Influence of different transition metals, in phthalocyanines on their, interaction energies with volatile organic compounds: an experimental and computational study, Journal of Molecular Modeling 13 (2007) 11–17. [10] R. Pohle, E. Simon, R. Schneider, M. Fleischer, R. Sollacher, H. Gao, K. Müller, P. Jauch, M. Loepfe, H.-P. Frerichs, C. Wilbertz, Fire detection with low power fet gas sensors, Sensors and Actuators B 129 (2007) 669–672. [11] M. Fleischer, E. Simon, E. Rumpel, H. Ulmer, M. Harbeck, M. Wandel, C. Fietzek, U. Weimar, H. Meixner, Detection of volatile compounds correlated to human diseases through breath analysis with chemical sensors, Sensors and Actuators B 83 (2002) 245–249. [12] C. Fietzek, Präferenzielle Einlagerung flüchtiger organischer Verbindungen in polykristalline Phthalocyanin-Schichten: Mechanismus und Anwendung als chemischer Sensor, PhD Thesis, Tübingen, 2001. [13] A. Ozmen, F. Fekce, M.A. Ebeoglu, C. Tasaltin, Z.Z. Öztürk, Finding the composition of gas mixtures by a phthalocyanine-coated QCM sensor array and an artificial neural network, Sensors and Actuators B 115 (2006) 450–454. [14] D.D. Erbahar, I. Gürol, V. Ahsen, Z. Öztürk, E. Musluoglu, M. Harbeck, Explosives detection in sea water with phthalocyanine quartz crystal microbalance sensors, Sensors Letters 9 (2011) 745–748. [15] F.I. Bohrer, C.N. Colesniuc, J. Park, M. Ruidaz, I.K. Schuller, A.C. Kummel, W.C. Trogler, Comparative gas sensing in cobalt, nickel, copper, zinc and metal-free phthalocyanine chemiresistors, Journal of the American Chemical Society 131 (2009) 478–485. [16] J. Claußen, Elektronische Struktur von phthalocyaninen vor und nach der Wechselwirkung mit NO2 , Diploma Thesis, Tübingen, 2002.
60
W. Simmendinger et al. / Sensors and Actuators B 179 (2013) 54–60
[17] M. Passard, A. Pauly, J.-P. Blanc, S. Dogo, J.-P. Germain, C. Maleysson, Doping mechanisms of phthalocyanines by oxidizing gases: application to gas sensors, Thin Solid Films 237 (1994) 272–276. [18] T.V. Basova, E.K. Kolt’sov, I.K. Igumenov, Spectral investigation of interaction of copper phthalocyanine with nitrogen dioxide, Sensors and Actuators B 105 (2005) 259–265. [19] A. D’Amico, C. Di Natale, R. Paolesse, A. Mantini, C. Goletti, F. Davide, G. Filosofi, Chemical sensing materials characterization by Kelvin Probe technique, Sensors and Actuators B 70 (2000) 254–262. [20] A. Oprea, N. Barsan, U. Weimar, Work function changes in gas sensitive materials: fundamentals and application, Sensors and Actuators B 142 (2009) 470–493. [21] M. Bouvet, Phthalocyanine based field-effect transistors as gas sensors, Analytical and Bioanalytical Chemistry 384 (2006) 366–373. [22] J. Brunet, V. Parra Garcia, A. Pauly, C. Varenne, B. Lauron, An optimized gas sensor microsystem for accurate and real-time measurement of nitrogen dioxide at ppb level, Sensors and Actuators B 134 (2008) 632–639. [23] J. Brunet, A. Pauly, L. Mazet, J.P. Germain, M. Bouvet, B. Malezieux, Improvement in real time detection and selectivity of phthalocyanine gas sensors dedicated to oxidizing pollutants evaluation, Thin Solid Films 490 (2005) 28–35. [24] T. Miyata, S. Kawagushi, M. Ishii, T. Minami, High sensitivity chlorine gas sensor using Cu-phthalocyanine thin films, Thin Solid Films 425 (2003) 255–259. [25] A. Oprea, U. Weimar, E. Simon, M. Fleischer, H.-P. Frerichs, Ch. Wilbertz, M. Lehmann, Copper phthalocyanine suspended gate field effect transistors for NO2 detection, Sensors and Actuators B 118 (2006) 249–254. [26] W. Jakubik, M. Urbancyk, A. Opilski, Sensor properties of lead phthalocyanine in a surface acoustic wave system, Ultrasonics 39 (2001) 227–232. [27] P.M. Burr, P.D. Jeffery, J.D. Benjamin, M.J. Uren, A gas-sensitive field effect transistor utilizing a thin film of lead phthalocyanine as the gate material, Thin Solid Films 151 (1987) L111–L113. [28] B. Holcroft, G.G. Roberts, Surface acoustic wave sensors incorporating Langmuir–Blodgett films, Thin Solid Films 160 (1988) 445. [29] T. Miyata, T. Minami, Chlorine gas sensors with high sensitivity using Mgphthalocyanine thin films, Applied Surface Science 244 (2005) 563–567. [30] I. Muzikante, V. Parra, R. Dobulans, E. Fonavs, J. Latvels, M. Bouvet, A novel gas sensor based on phthalocyanine heterojunction devices, Sensors 7 (2007) 2984–2996. [31] C.J. Liu, S.Y. Wang, J.C. Hsieh, Y.H. Ju, Gas sensing properties of vacuumdeposited titanyl phthalocyanine film, Sensors and Actuators B 65 (2000) 371–374. [32] C.J. Liu, S.Y. Wang, Y.H. Ju, J.C. Hsieh, Titanyl phthalocyanine gas sensor for NO2 detection, Sensors and Actuators B 52 (1998) 264–269. [33] H. Touda, H. Tada, K. Matsushige, Evaluation of carrier mobility of phthalocyanine films in NH3 and NO2 gas atmosphere by field effect measurement, Molecular Crystals and Liquid Crystals 327 (1999) 287–290. [34] L. Li, Q. Tang, H. Li, X. Yang, W. Hu, Y. Song, Z. Shuai, W. Xu, Y. Liu, D. Zhu, An ultra closely (-stacked organic semiconductor for high performance field-effect transistors, Advanced Materials 19 (2007) 2613–2617. [35] J. Spadavecchia, G. Ciccarella, R. Rella, Optical characterization and analysis of the gas/surface adsorption phenomena on phthalocyanines thin films for gas sensing application, Sensors and Actuators B 106 (2005) 212–220. [36] R.A. Collins, K.A. Mohammed, Electrical, structural and sensing properties of zinc phthalocyanine thin films, Thin Solid Films 145 (1986) 133–145.
[37] X. Ma, H. Chen, M. Shi, G. Wu, M. Wang, J. Huang, High gas-sensitivity and selectivity of fluorinated zinc phthalocyanine film to some non-oxidizing gases at room temperature, Thin Solid Films 489 (2005) 257–261. [38] J.P. Germain, A. Pauly, C. Maleysson, J.P. Blanc, B. Schöllhorn, Influence of peripheral electron-withdrawing substituents on the conductivity of zinc phthalocyanine in the presence of gases. Part 1. Reducing gases, Thin Solid Films 326 (1998) 245–250. [39] J.P. Germain, A. Pauly, C. Maleysson, J.P. Blanc, B. Schöllhorn, Influence of peripheral electron-withdrawing substituents on the conductivity of zinc phthalocyanine in the presence of gases. Part 2. Oxidizing gases, Thin Solid Films 333 (1998) 235–239. [40] N. Barsan, D. Koziej, U. Weimar, Metal oxide based gas sensors research: how to? Sensors and Actuators B 121 (2007) 18–35. [41] T. Tsuzuki, Y. Kuwabara, N. Noma, Y. Shirota, M.R. Willis, Effect of morphology on photovoltaic properties of titanyl phthalocyanine, Japanese Journal of Applied Physics 35 (1996) 447–450. [42] J. Miziguchi, G. Rihs, H.R. Karfunkel, Solid-state spectra of titanylphthalocyanine as viewed from molecular distortion, Journal of Physical Chemistry 99 (1995) 16217–16227. [43] C.J. Liu, J.C. Hsieh, Y.H. Ju, Response characteristics of lead phthalocyanine gas sensor: effect of operating temperature and postdeposition annealing, Journal of Vacuum Science and Technology A 14 (1996) 753. [44] A. Belghachi, R.A. Collins, The effects of humidity on phthalocyanine NO2 and NH3 sensors, Journal of Physics D: Applied Physics 23 (1990) 223–227. [45] J.W. Grate, M.H. Abraham, Solubility interactions and the design of chemically selective coatings for chemical sensors and arrays, Sensors and Actuators B 3 (1991) 85–111. [46] J. Janata, M. Josowics, Chemical modulation of work function as a transduction mechanism for chemical sensors, Accounts of Chemical Research 31 (1998) 241–248.
Biographies Wolfram Simmendinger received his diploma in chemistry from the Eberhard Karls University of Tübingen in 2007. Since 2007 he is working on his PhD in the field of gas sensing with organic materials at the same university, in the gas sensor group. Alexandru Oprea received the diploma in physics from the University of Bucharest in 1976 and the PhD in solid state physics from the Central Institute of Physics, Bucharest, Romania in 1996. Since 2001 he is senior researcher in the Gas Sensor Group of the University of Tübingen, Germany. The research fields: thin films solar cells, high field electroluminescent devices, polymer and metal oxide gas sensors. Nicolae Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his PhD in Solid State Physics from the Institute of Atomic Physics, Bucharest, Romania. Since 1995 he is a senior researcher at the Institute of Physical Chemistry of the University of Tübingen where, currently, is leading together with Udo Weimar the Gas Sensor research group. Udo Weimar received his diploma in physics 1989, his PhD in chemistry 1993 and his Habilitation 2002 from the University of Tübingen. He is currently a full Professor at the Faculty of Science of the University of Tübingen. His research interest focuses on chemical sensors as well as on multicomponent analysis and pattern recognition.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nonconventional phthalocyanines for field effect gas detection W. Simmendinger ∗ , A. Oprea, N. Bârsan, U. Weimar Institute of Physical and Theoretical Chemistry, auf der Morgenstelle 15, 72076 Tuebingen, Germany
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Article history: Received 12 July 2012 Received in revised form 1 October 2012 Accepted 11 October 2012 Available online 22 October 2012 Keywords: Metal phthalocyanine NO2 sensing Work function Kelvin Probe
a b s t r a c t Metal (TiO and Zn) phthalocyanines that are not currently investigated for gas sensing applications have been deposited by thermal vacuum evaporation as thin layers and evaluated in operando Kelvin Probe setup as NO2 sensing materials. From the raw data their calibration curves have been inferred and some sensing parameters determined. Comparative conductometric evaluations have also been performed. On the basis of the experimental information the main peculiarities of the gas responses have been identified and discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction After their initial use as pigment or dye it was found that the metal phthalocyanines (MPcs) change their electrical properties under the sorption and desorption of oxidizing gases [1–4]. Therefore, the gas sensing community tried to utilize the MPcs as sensitive layer for e.g. NO2 -detection, where it is definitely need of monitoring due to its poisonous and carcinogenic properties. Nevertheless, up to now, there is no commercially available gas sensor based on MPcs on the market, although they offer several advantages, like cost effective production, low power consumption, thermal and chemical stability, etc. These features are seldom encountered at the already established NO2 gas sensors, such as the metal oxide based ones, which unfortunately have reduced selectivity and, moreover, operate at rather high temperatures or at the more specific electrochemical cells that are expensive and have a limited lifetime. The gas sensing characteristics of the MPcs depend on the nature of the central metallic atom [5,6], on the substituents at the organic ring and also on the crystalline structure of the material, which can be influenced by chemical/thermal pre-treatments [7–9]. Fine tuning of Pc’s composition and morphology can result in films with increased gas sensitivity, optimized for different gas sensing application such as ambient surveillance, fire detection and diseases recognition [10,11]. Meanwhile NO2 concentrations down to 10 ppb are experimentally measurable with MPc based field effect sensors [11].
∗ Corresponding author. E-mail address: [email protected] (W. Simmendinger). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.064
The interaction between the MPcs and the analytes takes place in different ways with different strengths. It ranges from unspecific and weak dispersion interactions, (common to organic compounds) which causes small but fast changes in the layers properties (effect is mostly utilized in gravimetric measurements [12–15]), to strong interactions with oxidizing gases, leading to the formation of free charge carriers (usually holes) via charge transfer complexes [16–18]. The last process subsequently alters the electrical properties of the semiconducting MPcs (usually having a p-type conduction) resulting in a good sensitivity and fair selectivity. For a successful implementation in practical applications some known drawbacks of the gas sensing with Pcs have to be, however, overcame. The main one is the long recovery time due to the strong interactions between MPcs and the oxidizing gases (addressed above). Also the long term stability of some MPc layers that depreciate after few weeks or even days has to be improved. In such cases, convenient application types and suited readout scenarios/algorithms might be required. The MPcs gas sensors proposed in the literature are mainly chemoresistors [5,6] or field effect devices [10,11]. The field effect readout has several advantages: • Uses signals produced by the sensing layer surface [19–21], which, in principle, should increase the specificity of the sensor gas response, eliminating the contribution of the dispersion interaction (significant for the volume effects based transducers like the gravimetric ones). Simultaneously, a certain decrease in the response and recovery times is expected because the diffusion times into and out of the sensing material are avoided. • Very low power consumption, as in the case of any electrostatic readout.
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• Easily incorporation in application specific integrated circuits (ASICs) with increased market potential. The earliest and most investigated MPc is the CuPc [6]. Oxidizing gases (NO2 , O3 ) and halogens (Cl) have been detected with conductometric (down to 10 ppb [22–24]) and work function (down to ∼1 ppb [11,25]) sensors. With the time the number of available MPcs increased, stimulating the interest for related gas sensing devices (PbPc, NiPc, CoPc [5,6,26–30]). However, there are several MPcs, whose suitability as gas sensing materials have been, until now, not very intensively investigated. For instance, only few conductivity [31,32] and no Kelvin Probe (KP) measurements gas exposure have been performed on titanylphthalocyanine (TiOPc), even though field effect measurements for organic electronics have been reported [33,34]. Actually, in [33] a comparison between the output transfer characteristics of an organic TiOPc field effect transistor in NO2 atmosphere (20 ppm in N2 ) and vacuum has been made. Therefore no/scarce information about the KP/work function response of the TiOPc to oxidizing gases like NO2 is available. Also ZnPc lacks the data about KP gas effects in spite of the fact that it is since longer time in the attention of gas sensing research [6,35–39]. In this context seemed us useful to make some combined operando investigations on TiOPc and ZnPc films. The operando method [20,40] supposes the evaluation of the device or material samples under conditions similar to those being employed during the foreseen application. Thus KP, conductometric, gravimetric and infrared absorption measurements under controlled atmosphere with focus on the field effect applications/implementations have been performed. The gravimetric and infrared data are not reported here, the measurements being still in progress. 2. Experimental 2.1. Sample preparation Because of their high thermal and chemical stability, but poor solubility the TiOPc and ZnPc layers have been deposited by vacuum thermal evaporation on appropriate substrates. The studied MPcs have been purchased from Sigma–Aldrich and purified before the deposition through a bake-out procedure up to 400 ◦ C. For the KP samples 7 mm × 9 mm silicon substrates covered with a 800 nm Au/20 nm Ti electrodes have been produced (Ti layer improved the adhesion of the Au electrode to the Si). The dc- conductance samples required interdigitated transducers on sapphire substrates and having 50 m line spacing and width. In a deposition batch have been included all types of substrates by using a dedicated holder and a common shadow mask. The process has been performed under 3 × 10−7 mbar, with a deposition rate of 0.02 nm/s at a temperature of 400–450 ◦ C, resulting in ∼100 nm MPc layers. Since the substrates have not been intentionally heated, the amorphous phase of the TiOPc and the ␣-modification of the ZnPc have been obtained. Under a 24 h exposure to saturated ethanol vapour it was possible to transform the amorphous TiOPc-layer into its ␣-modification [41,42]. Finally the samples for the Kelvin Probe and dc-measurements have been mounted on heatable sockets in order to enable measurements at temperatures higher than the room one.
Fig. 1. Experimental setup, consisting of a gas mixing system, operando measuring chambers and investigation apparatus (McAllister Kelvin Probe and Hewlett Packard picoampermeter).
dynamic (continuous gas flow) gas exposure, the samples have been mounted into dedicated Teflon® (or Teflon® coated) measuring chambers. The main investigation tool was the non-locking KP 6500 from McAllister Technical Services. The results are reported as delivered by the apparatus, that is, as contact potential differences [CPD] [19,20]. To convert them to work function changes a multiplication with the electron charge (−e) has to be carried out. The conductometric measurements have been performed with the HP 4140B picoampere metre (pA-metre) and Keithley 617 programmable Electrometer. The chamber for the dc-measurements has been connected downstream to the KP measuring chamber, but no noticeable influence (under steady state conditions) of the exposure order was observed. This happens because the sensing mechanisms leading to the detection of oxidizing gases with phthalocyanines do not suppose the chemical conversion of the analyte to other compounds and the analyte concentration remains constant along the whole measurement chain. This would be not the case for other sensors, as, for example, the metal oxide chemo-resistors, where the reducing gases catalytically burn on the sensor surface, so that the downstream concentration of the analyte is smaller than the upstream one. In all performed measurements (KP and conductometric) has been employed the gas exposure protocol presented in Fig. 2. 6.0 purity dry N2 was the carrier gas in the first half of the exposure sequence (1; 3; 5 ppm NO2 ). In the second half of the exposure sequence (with the same NO2 concentration steps) 50% relative humidity (r.h.) background has been added to N2 (through a high
2.2. Sample characterization The characterization of the samples has been made, as accounted for above, in an operando evaluation set-up, including a computer controlled gas mixing system and remote operated measurement instruments (Fig. 1). In order to enable their
Fig. 2. Gas exposure protocol utilized for the KP and conductrometric evaluation of TiOPc und ZnPc.
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rate vaporizer included in a dedicated channel of the gas mixing system). 3. Results and discussion From the raw data acquired during standard NO2 exposures (performed mainly at room temperature) have been derived the calibration curves of the MPc samples in the presence and absence of the background humidity. On their basis the main feature of the field effect sensing with the selected MPcs have been identified and shortly addressed. Few additional examples concerning the dc conductometric response of samples from the same batches have been added for comparison. It is important to underline here that KP data have been recorded in successive KP runs, because only one KP sample could be evaluated with the KP hardware in one exposure sequence. As accounted for in the McAllister KP6500 manual and explained elsewhere [20] only the work function changes are physically and chemically relevant. Therefore, the position of the work function baseline (KP sample response to the carrier gas) has no significance. 3.1. TiOPc The as prepared TiOPc layers are amorphous. As pointed out in the experimental section, some of them have been also converted, under ethanol saturated atmosphere, to the ␣- modification, which is crystalline (triclinic) [42]. The crystalline structure of the converted layers has been confirmed by UV–VIS spectroscopy [41,42]. Both morphologies have been investigated. The room temperature KP responses to NO2 from twin samples (same deposition batch, one amorphous, the other crystalline) are depicted in Fig. 3 (only one pair presented). First of all one observes large CPD changes upon 1 ppm NO2 exposure in dry air for both amorphous and crystalline samples (>120 mV) as well as a pronounced nonlinearity. The time constant of the response ( R63 ∼8 min) is acceptable for the room temperature characterization of the samples. The steady state is, however, hard to achieve. For the amorphous specimen, which is the fastest one, the response time for 90% from the steady state value ( R90 ) is quite large (∼1 h). The recovery times are also pretty large. This feature is common among non-heated phthalocyanines and is often reported in the literature [43]. With humidity background, which would be the “normal” operation condition for an atmospheric
Fig. 3. The room temperature KP response of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
sensor, the TiOPc behaviour significantly changes: the amorphous samples roughly keep the response direction and magnitude but the crystalline ones do not; in their case the CPD changes became positive, and a little bit smaller in module. The associated time constants decreased to some extent (∼4 min for the amorphous phase) and the steady state has been almost achieved during the exposure times being used (2 h). All the addressed features are also visible on the calibration curves (Fig. 4). They have been traced for two samples of each kind (two twins) in two consecutive exposure sequences (two times the sequence without humidity background and two times with 50% r.h. background). In the limit of the statistical spread, the main attributes of samples are reproduced among the class to which they belong (amorphous or ␣-crystalline) and to the exposure conditions being employed (with and without background humidity). Two new additional elements are obvious in Fig. 4: the significant decrease of the responses in the second exposure under dry ambient and the rough preservation of the responses under humid ambient (50% r.h). The first one is only apparent. Due to the slow analyte desorption rate, the baseline incompletely recovers (in the given time of 2 h) formally resulting in a response decrease. The second element is real and favourable in gas sensing application.
Fig. 4. The room temperature KP calibration curves of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
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Fig. 6. KP responses of the ZnPc layers to NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 .
Fig. 5. The room temperature conductometric calibration curves of amorphous and ␣-TiOPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm. (First run only).
layers to NO2 are much smaller under humid conditions in comparison with those obtained at 50 ◦ C (discussion will follow). The calibration curves (Fig. 7) at room temperature and 50 ◦ C for successive exposures are quit reproducible; the 100 ◦ C ones show almost no response (as the KP raw data in Fig. 6 already indicated). As in the case of TiOPc significant differences appear in the conductometric responses (Figs. 8 and 9) in respect with the field effect ones. For example the best sensitivity is achieved at 100 ◦ C, while the sensitivities at the room temperature and 50 ◦ C are very small. This trend has been already underlined by other authors [6]. 3.3. Discussions
In the comparative conductivity measurements on TiOPc layers a different behaviour came into sight (see Fig. 5). Here the responses for amorphous samples in the presence of background humidity are much lower than those obtained in dry atmosphere (a 100 times decay was observed at 5 ppm NO2 ). Moreover, the humidity does not switch the direction and, to a large extent, does not modify the value of the conductometric responses from the crystalline ␣-TiOPc-layers (dissimilar to the KP samples) (explanation and comments later on). The conductometric response and recovery times are similar to that of the KP ones; the presence of the humidity accelerates the sensing processes and improves reproducibility. Judging the performance presented above, the amorphous phase would be more suited for gas sensing applications, but it is very unstable, lasting only a couple of test procedures. The ␣modification is stable, has enough sensitivity and is therefore, in spite of the longer time constants, much better suited for field effect gas sensing.
The striking non-equivalence between the results obtained in KP experiments and the conductometric ones, even if up-to-date not thoroughly elucidated and fairly modelled, seem to stem from several principle readout dissimilarities: 1. The KP generally measures the changes in the work function arising from two contributions [20]: a. The changes in the electron affinity of the sample material in the case of any conductor. b. The changes in the band banding, in the case of semiconductors. If the semiconductor surface hosts a dipolar layer, a contribution from the affinity has to be also considered. 2. The conductometric response has also two main contributions: a. The direct changes in the charge carrier concentration. b. The changes in the conduction mechanism affecting the effective mobility of the charge carriers.
3.2. ZnPc The as prepared ZnPc resulted in the ␣-modification; no additional treatments have been performed before characterization of the samples. The ZnPc layers show no clear KP response to NO2 (see Fig. 6) under dry ambient conditions; even the temperature increase has no obvious influence. 50% r.h. background causes a drop of the CPD, like in the case of TiOPc, and, more important, unambiguous KP responses (CPD changes) upon NO2 exposure came into play for room and slightly increased temperatures (50 ◦ C). The steady state was almost reached during the exposure due to time constants significantly smaller (especially at 50 ◦ C) in respect with those encountered at TiOPc. At 100 ◦ C the response of the ZnPc
The electron affinity actually expresses the energy required to bring an electron from the edge of the conduction band to the completely free state (vacuum level). If a dipolar layer covers the surface of a conducting material, less/more energy will be then needed to accomplish this task because the electron has now to pass throughout a favourable/unfavourable additional electrical field (the evaluation is given, for example, in [20]). Fig. 10 schematically presents the described process. In the case of semiconducting sensing films the analyte can be ionosorbed, exchanging mobile charge with the absorbent conduction/valence band or its delocalised electron system. Consequently the amount of free charge carriers and/or their effective mobility is modulated resulting in conductance responses
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Fig. 7. The KP calibration curves of ␣-ZnPc to NO2 in dry and humid 6.0 pure nitrogen, 200 sccm.
to the gas exposure. The surface trapped charge is compensated by volume spatial charge (under the surface) leading to band bending (please refer to Fig. 4 from [20] for a graphical approach). The affinity changes are not visible in the conductivity of the samples because the bands are shifted upwards or downwards as a whole. The work function, however, includes these changes and is very sensitive to them. Therefore the background humidity, contributing mainly with a dipolar effect to the work function (through electron affinity) is very efficient in modulating the KP responses. The humidity dependence of the affinity does not affect the conduction, because it is an electrostatic effect. Nevertheless, the humidity can produce changes of the conduction either directly [15], acting as an electron donator [44], or in more complex ways, involving competitive absorption processes. On the basis of the above considerations one can understand the change of the KP response direction for the TiOPc as follows: The exposure of the TiOPc samples to the background humidity drastically modifies the surface coverage with water dipoles and results in a negative change in the CPD value (∼−130 mV). In the first moment the presence of the NO2 analyte produces the same type of response as in dry atmosphere through its direct action (the
Fig. 8. Conductometric responses of the ZnPc layers to NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 .
first spike downwards in Fig. 3). With the time (but very fast) the NO2 adsorption becomes a competitive process to the H2 O adsorption, reducing the H2 O adsorption rate, and shifting the surface balance to a state with less dipolar (H2 O) coverage. In consequence the CPD increases. The temperature also acts partially unlike in the case of the work function and conductometric responses: 1. Increased temperature favours the diffusion and, through it, the gas ab/ad-sorption and desorption.
Fig. 9. Conductometric calibration curves of the ZnPc layers against NO2 at different temperatures without and with background humidity. Carrier gas: 200 sccm 6.0 N2 . RB designates the base line value.
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Fig. 10. Conventional sketch presenting the effect of a dipolar layer adsorption on a semiconductor surface. The additional dipole charges change the microscopic potential in the vicinity of the surface leading to a new position of the vacuum level in respect with the Fermi level (considered as energy reference). The change of the electron affinity leads to an equal change in the work function .
2. At higher temperatures the analyte physical/chemical binding to the sensing material can be easier overcame by the thermal activation and, consequently, the amount of bonded analyte to the host sensing layer should decrease with the temperature increase. 3. The particular 100 ◦ C value corresponds to the boiling point of the free condensed water. Above this temperature it is less probable to have absorbed water in the sensing layer. In consequence the influence of the ambient humidity decreases and the sensing layer behaviour resembles the dry conditions one. All these influences from the operation temperature come into play with different weights and in connection with the nature of the work function changes. Furthermore, the nonspecific interaction of the analyte with the sensing material (like the dispersion one [45]), which can be disregarded for the crystalline phases, might be important for the amorphous phases. So, at 100 ◦ C the amount of the water adsorbed on the ZnPc layer is much less than that adsorbed at room temperature or 50 ◦ C. Therefore, also the CPD downwards jump caused by the humidity is significantly smaller at reduced temperatures (room temperature or 50 ◦ C) than at 100 ◦ C. The similitude in the CPD behaviour in dry N2 and humid one at 100 ◦ C originates also from there. If one reconsiders now the KP responses of the ZnPc samples (always crystalline) towards NO2 , with humidity background and at low temperature (25 or 50 ◦ C), which show the same trend with the ␣-TiOPc samples, one can ascertain that same competitive adsorption of H2 O and NO2 should be involved. The conductometric response of the amorphous TiOPc in the presence of the water vapours is very poor in respect with the one recorded in dry carrier gas (N2 ). For the moment a pertinent explanation is missing but the feature is probable due to a volume (even if the layer is so thin) sorption process with the modification of the TiOPc layer morphology which eventually results in a drastic degradation of the material itself. The crystalline phases are more stable and less sensitive to the background humidity. Nevertheless the ZnPc at relatively low temperatures (<70 ◦ C) is not too well suited for NO2 detection. The linear shape of the calibration curves in logarithmic scales (for KP samples) and double logarithmic ones (for conductometric samples) seems to support the models based on the charge/partial charge transfer processes accompanying the adsorption of the analyte [46]. This assumption is however very speculative at this stage of the investigation and additional data (mainly operando IR spectra) are required in order to identify the species residing on the sensing layer surface during the gas exposure and to conclude on the main gas sensing mechanisms.
4. Conclusion The investigated MPcs displayed good NO2 sensitivities, mainly in field effect sensing setups. Their crystalline modifications showed fair chemical stability during the evaluation and reversible interaction with the analyte. However, the relative long response and recovery time is hindering a repetitive operation at room temperature. Application that would require only single detection events, preceded by long standby periods (like the fire detection) or those that would allow short time thermal refreshment might open the way for the implementation of TiOPc and ZnPc in gas sensing devices. References [1] C.C. Leznoff, A.B.P. Lever, Phthalocyanines: Properties and Applications, VCH Publishers Ltd, New York, Weinheim, Cambridge, 1989. [2] G. Guillaud, J. Simon, J.P. Germain, Metallophthalocyanines gas sensors, resistors and field effect devices, Coordination Chemistry Reviews 178–180 (1998) 1433–1484. [3] J. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Progress in Surface Science 31 (1989) 1–60. [4] R.D. Gould, Structure and electrical conduction properties of phthalocyanine thin films, Coordination Chemistry Reviews 156 (1996) 237–274. [5] B. Bott, T.A. Jones, A highly sensitive NO2 sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators 5 (1984) 43–53. [6] T.A. Jones, B. Bott, Gas-induced electrical conductivity changes in metal phthalocyanines, Sensors and Actuators 9 (27) (1996) 27–37. [7] Y. Sadaoka, M. Matsuguchi, Y. Sakai, Y. Mori, W. Göpel, Effect of crystal form on the conductance in oxidative gases of metal-free and some metal phthalocyanines, Sensors and Actuators B 4 (1991) 495–498. [8] Y. Sadaoka, T.A. Jones, G.S. Revell, W. Göpel, Effects of morphology on NO2 detection in air at room temperature with phthalocyanine thin films, Journal Of Materials Science 25 (1990) 5257–5268. [9] C. Fietzek, H.G. Mack, Influence of different transition metals, in phthalocyanines on their, interaction energies with volatile organic compounds: an experimental and computational study, Journal of Molecular Modeling 13 (2007) 11–17. [10] R. Pohle, E. Simon, R. Schneider, M. Fleischer, R. Sollacher, H. Gao, K. Müller, P. Jauch, M. Loepfe, H.-P. Frerichs, C. Wilbertz, Fire detection with low power fet gas sensors, Sensors and Actuators B 129 (2007) 669–672. [11] M. Fleischer, E. Simon, E. Rumpel, H. Ulmer, M. Harbeck, M. Wandel, C. Fietzek, U. Weimar, H. Meixner, Detection of volatile compounds correlated to human diseases through breath analysis with chemical sensors, Sensors and Actuators B 83 (2002) 245–249. [12] C. Fietzek, Präferenzielle Einlagerung flüchtiger organischer Verbindungen in polykristalline Phthalocyanin-Schichten: Mechanismus und Anwendung als chemischer Sensor, PhD Thesis, Tübingen, 2001. [13] A. Ozmen, F. Fekce, M.A. Ebeoglu, C. Tasaltin, Z.Z. Öztürk, Finding the composition of gas mixtures by a phthalocyanine-coated QCM sensor array and an artificial neural network, Sensors and Actuators B 115 (2006) 450–454. [14] D.D. Erbahar, I. Gürol, V. Ahsen, Z. Öztürk, E. Musluoglu, M. Harbeck, Explosives detection in sea water with phthalocyanine quartz crystal microbalance sensors, Sensors Letters 9 (2011) 745–748. [15] F.I. Bohrer, C.N. Colesniuc, J. Park, M. Ruidaz, I.K. Schuller, A.C. Kummel, W.C. Trogler, Comparative gas sensing in cobalt, nickel, copper, zinc and metal-free phthalocyanine chemiresistors, Journal of the American Chemical Society 131 (2009) 478–485. [16] J. Claußen, Elektronische Struktur von phthalocyaninen vor und nach der Wechselwirkung mit NO2 , Diploma Thesis, Tübingen, 2002.
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W. Simmendinger et al. / Sensors and Actuators B 179 (2013) 54–60
[17] M. Passard, A. Pauly, J.-P. Blanc, S. Dogo, J.-P. Germain, C. Maleysson, Doping mechanisms of phthalocyanines by oxidizing gases: application to gas sensors, Thin Solid Films 237 (1994) 272–276. [18] T.V. Basova, E.K. Kolt’sov, I.K. Igumenov, Spectral investigation of interaction of copper phthalocyanine with nitrogen dioxide, Sensors and Actuators B 105 (2005) 259–265. [19] A. D’Amico, C. Di Natale, R. Paolesse, A. Mantini, C. Goletti, F. Davide, G. Filosofi, Chemical sensing materials characterization by Kelvin Probe technique, Sensors and Actuators B 70 (2000) 254–262. [20] A. Oprea, N. Barsan, U. Weimar, Work function changes in gas sensitive materials: fundamentals and application, Sensors and Actuators B 142 (2009) 470–493. [21] M. Bouvet, Phthalocyanine based field-effect transistors as gas sensors, Analytical and Bioanalytical Chemistry 384 (2006) 366–373. [22] J. Brunet, V. Parra Garcia, A. Pauly, C. Varenne, B. Lauron, An optimized gas sensor microsystem for accurate and real-time measurement of nitrogen dioxide at ppb level, Sensors and Actuators B 134 (2008) 632–639. [23] J. Brunet, A. Pauly, L. Mazet, J.P. Germain, M. Bouvet, B. Malezieux, Improvement in real time detection and selectivity of phthalocyanine gas sensors dedicated to oxidizing pollutants evaluation, Thin Solid Films 490 (2005) 28–35. [24] T. Miyata, S. Kawagushi, M. Ishii, T. Minami, High sensitivity chlorine gas sensor using Cu-phthalocyanine thin films, Thin Solid Films 425 (2003) 255–259. [25] A. Oprea, U. Weimar, E. Simon, M. Fleischer, H.-P. Frerichs, Ch. Wilbertz, M. Lehmann, Copper phthalocyanine suspended gate field effect transistors for NO2 detection, Sensors and Actuators B 118 (2006) 249–254. [26] W. Jakubik, M. Urbancyk, A. Opilski, Sensor properties of lead phthalocyanine in a surface acoustic wave system, Ultrasonics 39 (2001) 227–232. [27] P.M. Burr, P.D. Jeffery, J.D. Benjamin, M.J. Uren, A gas-sensitive field effect transistor utilizing a thin film of lead phthalocyanine as the gate material, Thin Solid Films 151 (1987) L111–L113. [28] B. Holcroft, G.G. Roberts, Surface acoustic wave sensors incorporating Langmuir–Blodgett films, Thin Solid Films 160 (1988) 445. [29] T. Miyata, T. Minami, Chlorine gas sensors with high sensitivity using Mgphthalocyanine thin films, Applied Surface Science 244 (2005) 563–567. [30] I. Muzikante, V. Parra, R. Dobulans, E. Fonavs, J. Latvels, M. Bouvet, A novel gas sensor based on phthalocyanine heterojunction devices, Sensors 7 (2007) 2984–2996. [31] C.J. Liu, S.Y. Wang, J.C. Hsieh, Y.H. Ju, Gas sensing properties of vacuumdeposited titanyl phthalocyanine film, Sensors and Actuators B 65 (2000) 371–374. [32] C.J. Liu, S.Y. Wang, Y.H. Ju, J.C. Hsieh, Titanyl phthalocyanine gas sensor for NO2 detection, Sensors and Actuators B 52 (1998) 264–269. [33] H. Touda, H. Tada, K. Matsushige, Evaluation of carrier mobility of phthalocyanine films in NH3 and NO2 gas atmosphere by field effect measurement, Molecular Crystals and Liquid Crystals 327 (1999) 287–290. [34] L. Li, Q. Tang, H. Li, X. Yang, W. Hu, Y. Song, Z. Shuai, W. Xu, Y. Liu, D. Zhu, An ultra closely (-stacked organic semiconductor for high performance field-effect transistors, Advanced Materials 19 (2007) 2613–2617. [35] J. Spadavecchia, G. Ciccarella, R. Rella, Optical characterization and analysis of the gas/surface adsorption phenomena on phthalocyanines thin films for gas sensing application, Sensors and Actuators B 106 (2005) 212–220. [36] R.A. Collins, K.A. Mohammed, Electrical, structural and sensing properties of zinc phthalocyanine thin films, Thin Solid Films 145 (1986) 133–145.
[37] X. Ma, H. Chen, M. Shi, G. Wu, M. Wang, J. Huang, High gas-sensitivity and selectivity of fluorinated zinc phthalocyanine film to some non-oxidizing gases at room temperature, Thin Solid Films 489 (2005) 257–261. [38] J.P. Germain, A. Pauly, C. Maleysson, J.P. Blanc, B. Schöllhorn, Influence of peripheral electron-withdrawing substituents on the conductivity of zinc phthalocyanine in the presence of gases. Part 1. Reducing gases, Thin Solid Films 326 (1998) 245–250. [39] J.P. Germain, A. Pauly, C. Maleysson, J.P. Blanc, B. Schöllhorn, Influence of peripheral electron-withdrawing substituents on the conductivity of zinc phthalocyanine in the presence of gases. Part 2. Oxidizing gases, Thin Solid Films 333 (1998) 235–239. [40] N. Barsan, D. Koziej, U. Weimar, Metal oxide based gas sensors research: how to? Sensors and Actuators B 121 (2007) 18–35. [41] T. Tsuzuki, Y. Kuwabara, N. Noma, Y. Shirota, M.R. Willis, Effect of morphology on photovoltaic properties of titanyl phthalocyanine, Japanese Journal of Applied Physics 35 (1996) 447–450. [42] J. Miziguchi, G. Rihs, H.R. Karfunkel, Solid-state spectra of titanylphthalocyanine as viewed from molecular distortion, Journal of Physical Chemistry 99 (1995) 16217–16227. [43] C.J. Liu, J.C. Hsieh, Y.H. Ju, Response characteristics of lead phthalocyanine gas sensor: effect of operating temperature and postdeposition annealing, Journal of Vacuum Science and Technology A 14 (1996) 753. [44] A. Belghachi, R.A. Collins, The effects of humidity on phthalocyanine NO2 and NH3 sensors, Journal of Physics D: Applied Physics 23 (1990) 223–227. [45] J.W. Grate, M.H. Abraham, Solubility interactions and the design of chemically selective coatings for chemical sensors and arrays, Sensors and Actuators B 3 (1991) 85–111. [46] J. Janata, M. Josowics, Chemical modulation of work function as a transduction mechanism for chemical sensors, Accounts of Chemical Research 31 (1998) 241–248.
Biographies Wolfram Simmendinger received his diploma in chemistry from the Eberhard Karls University of Tübingen in 2007. Since 2007 he is working on his PhD in the field of gas sensing with organic materials at the same university, in the gas sensor group. Alexandru Oprea received the diploma in physics from the University of Bucharest in 1976 and the PhD in solid state physics from the Central Institute of Physics, Bucharest, Romania in 1996. Since 2001 he is senior researcher in the Gas Sensor Group of the University of Tübingen, Germany. The research fields: thin films solar cells, high field electroluminescent devices, polymer and metal oxide gas sensors. Nicolae Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his PhD in Solid State Physics from the Institute of Atomic Physics, Bucharest, Romania. Since 1995 he is a senior researcher at the Institute of Physical Chemistry of the University of Tübingen where, currently, is leading together with Udo Weimar the Gas Sensor research group. Udo Weimar received his diploma in physics 1989, his PhD in chemistry 1993 and his Habilitation 2002 from the University of Tübingen. He is currently a full Professor at the Faculty of Science of the University of Tübingen. His research interest focuses on chemical sensors as well as on multicomponent analysis and pattern recognition.