- Email: [email protected]
Development of humidity sensors based on nanostructured carbon films
Sensors and Actuators B 111–112 (2005) 140–144
Development of humidity sensors based on nanostructured carbon films S. Miglio a,∗ , M. Bruzzi a , M. Scaringella a , D. Menichelli a , E. Leandri a , A. Baldi a , G. Bongiorno b , P. Piseri b , P. Milani b b
a INFM-Dipartimento di Energetica, Via S. Marta 3, 50139 Firenze, Italy INFM-Dipartimento di Fisica and Centro Interdisciplinare Materiali e Interfacce Nanostrutturati, Universit`a di Milano, Via Celoria 16, 20133 Milan, Italy
Available online 11 August 2005
Abstract Humidity sensors based on cluster assembled nanostructured carbon films in capacitive configuration have been fabricated and characterized. An electronic read-out system has been designed and implemented in order to measure the complex impedance of the device under operation. The sensor and read-out electronics provide a fully integrated and cost-effective system. Devices under test show a fast dynamic response and a good sensitivity compared to capacitive commercial systems. A linear response is observed for relative humidity in the range up to 70%. Hysteresis is practically absent in this range, while for higher values it is at most in the order of 2%. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructured carbon; Supersonic cluster beam deposition; Capacitive humidity sensors
1. Introduction The measurement and control of humidity is an important and ticklish matter as water molecules are easily adsorbed on almost any surface, where they could be present as a mono- or multimolecular layer of molecules. The determination of humidity is thus imperative for improving quality of life and enhancing industrial processes. Commercial sensors are mostly based on metal oxides, such as Al2 O3 [1] and TiO2 [2]. Since now the advanced base-materials proposed for humidity sensing devices are porous silicon [3–6] and polymers [1,7,8]. In these materials the adsorption of water vapour drastically change the electrical properties of the device, which is usually in the form of a resistor or a capacitance. The investigation is focussed on the improvement of requirements as: short response time, high sensitivity, negligible hysteresis, resistance against contaminants, good long-term reproducibility and possibly a wide operating range for both humidity and temperature. All these requirements not always meet concurrently (or match each other). Low cost, easiness of manufacture and small-sized sensors ∗
Corresponding author. Tel.: +39 055 4796350; fax: +39 055 4796342. E-mail address: [email protected] (S. Miglio).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.044
are also required; so it has become mandatory to search for new materials fulfilling all these characteristics. Supersonic cluster beam deposition (SCBD) can be used to grow nanostructured thin films where the original cluster structure is substantially maintained after deposition [9]. The random stacking of the precursor clusters leads to a high porosity texture, with a mesoscale granularity with pores in the range ˚ and high specific surface areas [10]. The porous 20–500 A structure of this material suggests humidity sensing as a promising application [11]. Preliminary measurements performed by us on nanostructured carbon (ns-C) films equipped with electric contacts have shown that they are sensitive to relative humidity [12,13]. In this work we discuss recent results on the development of ns-C humidity sensors in capacitive configuration, with particular concern to: device geometry, design and manufacture of a suitable electronic read out system, tests in controlled atmosphere.
2. Experimental procedure The films have been produced by deposition of a supersonic beam of neutral carbon clusters at Dipartimento di Fisica, University of Milan, by using a pulsed microplasma
S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
141
Fig. 1. (a) Sketch of the layers composing the capacitive sensor and (b) sketch of the sensor structure.
cluster source (PMCS) [14,15]. In a PMCS, an aerodynamically confined plasma discharge ablates a graphite rod; the vaporized carbon atoms are quenched in a condensation cavity by a pulse of helium and coalesce to form clusters. The helium-cluster mixture is then expanded through the source nozzle into a high-vacuum chamber (P ∼ 10−5 Pa) to form a supersonic beam [15]. The sensor concept has been described in [13]. A sketch of the device is given in Fig. 1a and b, while a picture of the manufactured sensor is shown in Fig. 2. Capacitance measurements have been performed by means of an electronic read-out system designed and manufactured at Dipartimento di Energetica, University of Firenze. This has been implemented to provide a fully integrated, cost-effective system. The complex impedance Zs of the sample is measured using the circuit described in Fig. 3. An integrated waveform generator produces the sine volt-
Fig. 2. Picture of the capacitive sensor based on ns-C.
age wave which excites the sample. The excitation frequency can be varied in the range 10–1000 kHz. The current flowing through the sample is converted into the voltage Vi by a transimpedance amplifier. The voltage drop across the sample is attenuated to obtain a signal VV with the correct signal amplitude at the input of the gain phase detector (GPD). The attenuating network is designed to introduce the same phase shift as the amplifier. The GPD performs the comparison between Vi and VV , producing two analog outputs: VG and VP . VG ∝ VV /Vi is proportional to the modulus of Zs . VP is proportional to the phase difference between VV and Vi and in turn to the phase of Zs . The outputs of the GPD are finally read by a data acquisition board and transmitted to a personal computer for recording and processing. Relative humidity has been monitored with a commercial hygrometer, consisting of a capacitive-type sensor produced by AHLBORN, Germany equipped with Almemo FH A 6467 read-out electronics. Measurements have been performed in air with controlled humidity and temperature at atmospheric pressure. The apparatus (Fig. 4) consists of a humidifier and a small measurement chamber (volume: 400 cm3 ). The relative
Fig. 3. Sketch of the read-out system manufactured by us and used to perform complex impedance measurements.
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S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
Fig. 4. Sketch of the humidification system.
humidity value is obtained by bubbling dry air in the humidifier, which contains distilled water that can be heated from 20 to 80 ◦ C by means of a resistor. The relative humidity percentage inside the measurement chamber is controlled by two inlet needle valves. During measurements the relative humidity and the temperature of the sample are monitored. It is also possible to heat the sample in vacuum in order to achieve the complete desorption of residual water vapour molecules and thus to refresh the capacitive device. The evacuation of the chamber is performed by a diaphragm pump.
3. Results and discussion Capacitance measurements have been performed for different relative humidity values, at atmospheric pressure and 300 K. In Fig. 5 the capacitance versus relative humidity plot is shown. The starting point of the adsorption curve is 2% RH, from where the RH was gradually raised up to 90% and then it was gradually reduced back to 2% to obtain the desorption curve. A linear behaviour up to 70% RH is observed. A superlinear trend is observed for higher relative humidity values. The sensitivity of the device: S = (C − C0 )/RH%, where C0 is the capacitance in dry air (C0 = 87.25 pF), is
Fig. 5. Capacitance vs. relative humidity curves in the range 2–90% RH.
0.23 pF/%RH, comparable to the values reported for commercial sensors and for sensors based on polymers [8]. Higher relative changes in capacitance are reported in literature in the case of a nonlinear response [5], in our sensor a capacitance increase of 40 pF is achieved over the whole investigated range. In the range 2–80% no hysteresis is observed, while for higher RH% a slight displacement from the adsorption values is recorded performing measurements in desorption regime. Although more measurements would be necessary to quantify the effect, a first estimation gives a maximum variation of 2%. From data sheets, a 1% hysteresis characterizes the commercial device used as a reference. Higher hysteresis have been reported in literature [5] when devices characterized by higher sensitivity and non-linear response are measured in the same RH range. Fast and reversible changes in the capacitance have been observed as the relative humidity is varied, as it is evidenced in Fig. 6, where capacitance versus time is shown. The dynamical response of the ns-C system and the reference one are compared, in adsorption and desorption regimes, respectively in Figs. 7 and 8. The plots evidence the fast response of the ns-C system to humidity changes. Evaluating the time necessary to reach the 50% of the difference between two stable values of RH, our sensor response is
Fig. 6. Adsorption curves for ns-C sensor and commercial humidity sensor (AHLBORN-Almemo) as a function of time.
S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
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is more hydrophobic, i.e. for contact angles >90◦ . Previous investigations on similar ns-C films showed that a hydrophobic surface is achievable by increasing the average slope of the surface, which should be correlated with the steepness (and roughness) of the film surface [17]. Film roughness is therefore another important topic to control sensors efficiency. To reduce the superlinear trend and hysteresis it could be also useful to heat the sample in order to avoid capillary condensation inside smaller pores: a constant temperature operation is nevertheless far more reliable for humidity measurements.
4. Conclusions
Fig. 7. Comparison of the dynamic response measured at room temperature for the ns-C based device and the commercial humidity sensor (AHLBORNAlmemo) (rising-edge).
advancing the commercial one by 3s in adsorption and 5s in desorption. One possible explanation is that the reference system is characterized by a slower read-out electronics, which, compared to our system, requires additional time to process data (e.g. pressure and temperature compensation, eventual linearization of the sensor response). Taking into account of this delay, the rising and falling times of the ns-C sensing system are similar to those of the commercial hygrometric system. A further engineering of the ns-C sensor is now under way to extend linearity in the whole RH% range and further reduce hysteresis. As superlinearity and hysteresis could be ascribed to water condensation inside pores [5,16], pore size is a key parameter to improve the overall sensing performance. Capillary condensation occurs for radii smaller then the Kelvin radius rK [16], which becomes smaller when the surface
We have studied the properties of ns-C based humidity sensors at room temperature and atmospheric pressure. Read-out electronics have been designed and implemented on purpose to provide a fully integrated, cost-effective system. We measured the capacitance variations induced by changes in relative humidity up to 90%. A linear behaviour up to 70% RH has been observed while the device showed a superlinear trend for higher relative humidity values. The sensitivity is 0.23 pF/%RH, comparable to values reported for commercial sensors and for sensors based on polymers. No hysteresis has been observed in the range 2–80% while for higher RH values a slight hysteresis has been recorded, with a maximum variation of 2%. The dynamical response of the ns-C system has been compared to the reference one, in adsorption and desorption regimes. Fast and reversible changes in the capacitance have been measured as the relative humidity is varied. The ns-C system is responding faster, advancing the commercial one by 3–5 s, while the rising and falling times are similar for the two systems. Further engineering of the ns-C sensor is now under way to improve sensitivity and linearity in the whole RH% range. Electrode geometry as well as material morphology are considered as key parameters to optimize the performance of the device.
Acknowledgement This work has been partially supported by MIUR under project FIRB “Carbon micro and nanostructures”.
References
Fig. 8. Comparison of the dynamic response measured at room temperature for the ns-C based device and the commercial humidity sensor (AHLBORNAlmemo) (falling-edge).
[1] Z.M. Rittersma, Recent achievements in miniaturised humidity sensors-a review of transduction techniques, Sens. Actuators A: Phys. 96 (2002) 196–210. [2] O.K. Varghese, C.A. Grimes, Metal oxide nanoarchitectures for environmental sensing, J. Nanosci. Nanotech. 3 (2003) 277–293. [3] Z.M. Rittersma, A. Splinter, A. B¨odecker, W. Benecke, A novel surface-micromachined capacitive porous silicon humidity sensor, Sens. Actuators B: Chem. 68 (2000) 210–217. [4] E.J. Connolly, G.M. O’Halloran, H.T.M. Pham, P.M. Sarro, P.J. French, Comparison of porous silicon, porous polysilicon and porous
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S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144 silicon carbide as materials for humidity sensing applications, Sens. Actuators A: Phys. 99 (2002) 25–30. M. Bj¨orkqvist, J. Salonen, J. Paski, E. Laine, Characterization of thermally carbonized porous silicon humidity sensor, Sens. Actuators A: Phys. 112 (2004) 244–247. J. Das, S.M. Hossain, S. Chakraborty, H. Saha, Role of parasitics in humidity sensing by porous silicon, Sens. Actuators A: Phys. 94 (2001) 44–52. F. Kraus, S. Cruz, J. M¨uller, Plasmapolymerized silicon organic thin films from HMDSN for capacitive humidity sensors, Sens. Actuators B: Chem. 88 (2003) 300–311. P.M. Harrey, B.J. Ramsey, P.S.A. Evans, D.J. Harrison, Capacitivetype humidity sensors fabricated using the offset lithographic printing process, Sens. Actuators B: Chem. 87 (2002) 226–232. P. Milani, S. Iannotta, Cluster Beam Synthesis of Nanostructured Materials, Springer Verlag, Berlin, 1999. C. Lenardi, E. Barborini, V. Briosi, L. Lucarelli, P. Piseri, P. Milani, NEXAFS characterization of nanostructured carbon thinfilms exposed to hydrogen, Diam. Related Mater. 10 (2001) 1195–1200. N. Yamazoe, N. Miura, Some Basic Aspects of Semiconductor Gas Sensors in Chemical Sensor Technology, Elsevier, Amsterdam, 1992. M. Bruzzi, P. Piseri, S. Miglio, G. Bongiorno, E. Barborini, C. Ducati, J. Robertson, P. Milani, Electrical conduction in nanostructured carbon and carbon-metal films grown by supersonic cluster beam deposition, Eur. Phys. J. B 36 (2003) 3–13. M. Bruzzi, S. Miglio, M. Scaringella, G. Bongiorno, P. Piseri, A. Podest`a, P. Milani, First study of humidity sensors based on nanostructured carbon films produced by supersonic cluster beam deposition, Sens. Actuators B: Chem. 100 (2004) 173–176. E. Barborini, P. Piseri, P. Milani, A pulsed microplasma source of high intensity supersonic carbon cluster beams, J. Phys. D: Appl. Phys. 32 (1999) L105–L109. P. Piseri, A. Podest`a, E. Barborini, P. Milani, Production and characterization of highly intense and collimated cluster beams by inertial focusing in supersonic expansions, Rev. Sci. Instrum. 72 (2001) 2261–2267. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London, 1982. L. Ostrovskaya, A. Podest`a, P. Milani, V. Ralchenko, Europhys. Lett. 63 (2003) 401–408.
Biographies Stefania Miglio was born in Castrovillari, Italy, in 1971 and received her MSc degree in physics from the University of Florence in 2000, defending a thesis on electrical and optical properties of neutron irradiated chemical vapour deposited (CVD) diamond detectors. She is presently working on electrical characterization of nanostructured carbon films obtained by supersonic cluster beam deposition and on their sensing applications. She is now completing her PhD in Materials Engineering at the University of Florence.
Mara Bruzzi is Associate Professor in Physics at the University of Florence. She is mainly involved in the development of advanced materials for sensing applications in nuclear, high energy, environmental and medical physics. She is Spokesperson of the CERN international collaboration RD50: “Development of Radiation Hard Semiconductor Detectors for Very High Luminosity Colliders”. Monica Scaringella was born in Canosa di Puglia, (Italy) in 1976 and received her MSc degree in electronic engineering from the University of Florence in 2002, defending a thesis on characterization of semiconductors defects by thermal spectroscopy. She is presently working on radiation damage of silicon detectors and on applications of nanostructured carbon films obtained by Supersonic Cluster Beam Deposition. She is now a PhD Student in Materials Engineering at the University of Florence. David Menichelli was born in Italy in 1971 and received his MSc degree in electronic engineering from the University of Florence in 1998, defending a thesis on radiation damage in silicon particle detectors. In 2001 he received his PhD after a research about optoacoustics. He is presently working at University of Florence about the development of particle detection systems for medical applications and dosimetry. Moreover, he is interested in radiation hardness of silicon, diamond and silicon carbide materials. Ettore Leandri was born in Arezzo (Italy) in 1976. He has got a bachelor degree in Informational Engineering at the University of Florence in 2002 and presently he is working to read-out electronics for nanostructured carbon sensors to obtain the MSc degree in Electronic Engineering. Andrea Baldi was born in Florence in 1961. He achieved his High school technical degree at the Technical Institute “Leonardo da Vinci” in Florence. Since 1984 he works at the Dipartimento di Energetica, University of Florence, as technician. His activity is particularly concerned in giving an important contribution to the designs and fabrication of measurement apparatus and in technical troubleshooting. Gero Bongiorno was born in Milan in 1977. In 2001 he graduated from University of Milan in Physics with a thesis on the characterization of electric transport properties in nanostructured materials. He is now completing his PhD in Physics at the University of Milan working on the synthesis and characterization of nanocomposite materials. Paolo Piseri is researcher at the Universit`a degli Studi di Milano since 2002. He graduated in Physics in 1995 and is PhD in Physics since 2000 with the Thesis: supersonic cluster beam deposition for the synthesis of nanophase materials. He is active at the molecular beams and nanocrystalline materials laboratory (LGM) of the Universit`a degli Studi di Milano since its foundation in 1993 and brought an important contribution to the development of the original synthesis and characterization techniques of LGM. Paolo Milani is Full Professor at the Department of Physics of the University of Milan, Italy. He currently serves as Director of the Interdisciplinary Centre of Nanostructured Materials and Interfaces of the University of Milan.
Development of humidity sensors based on nanostructured carbon films S. Miglio a,∗ , M. Bruzzi a , M. Scaringella a , D. Menichelli a , E. Leandri a , A. Baldi a , G. Bongiorno b , P. Piseri b , P. Milani b b
a INFM-Dipartimento di Energetica, Via S. Marta 3, 50139 Firenze, Italy INFM-Dipartimento di Fisica and Centro Interdisciplinare Materiali e Interfacce Nanostrutturati, Universit`a di Milano, Via Celoria 16, 20133 Milan, Italy
Available online 11 August 2005
Abstract Humidity sensors based on cluster assembled nanostructured carbon films in capacitive configuration have been fabricated and characterized. An electronic read-out system has been designed and implemented in order to measure the complex impedance of the device under operation. The sensor and read-out electronics provide a fully integrated and cost-effective system. Devices under test show a fast dynamic response and a good sensitivity compared to capacitive commercial systems. A linear response is observed for relative humidity in the range up to 70%. Hysteresis is practically absent in this range, while for higher values it is at most in the order of 2%. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructured carbon; Supersonic cluster beam deposition; Capacitive humidity sensors
1. Introduction The measurement and control of humidity is an important and ticklish matter as water molecules are easily adsorbed on almost any surface, where they could be present as a mono- or multimolecular layer of molecules. The determination of humidity is thus imperative for improving quality of life and enhancing industrial processes. Commercial sensors are mostly based on metal oxides, such as Al2 O3 [1] and TiO2 [2]. Since now the advanced base-materials proposed for humidity sensing devices are porous silicon [3–6] and polymers [1,7,8]. In these materials the adsorption of water vapour drastically change the electrical properties of the device, which is usually in the form of a resistor or a capacitance. The investigation is focussed on the improvement of requirements as: short response time, high sensitivity, negligible hysteresis, resistance against contaminants, good long-term reproducibility and possibly a wide operating range for both humidity and temperature. All these requirements not always meet concurrently (or match each other). Low cost, easiness of manufacture and small-sized sensors ∗
Corresponding author. Tel.: +39 055 4796350; fax: +39 055 4796342. E-mail address: [email protected] (S. Miglio).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.044
are also required; so it has become mandatory to search for new materials fulfilling all these characteristics. Supersonic cluster beam deposition (SCBD) can be used to grow nanostructured thin films where the original cluster structure is substantially maintained after deposition [9]. The random stacking of the precursor clusters leads to a high porosity texture, with a mesoscale granularity with pores in the range ˚ and high specific surface areas [10]. The porous 20–500 A structure of this material suggests humidity sensing as a promising application [11]. Preliminary measurements performed by us on nanostructured carbon (ns-C) films equipped with electric contacts have shown that they are sensitive to relative humidity [12,13]. In this work we discuss recent results on the development of ns-C humidity sensors in capacitive configuration, with particular concern to: device geometry, design and manufacture of a suitable electronic read out system, tests in controlled atmosphere.
2. Experimental procedure The films have been produced by deposition of a supersonic beam of neutral carbon clusters at Dipartimento di Fisica, University of Milan, by using a pulsed microplasma
S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
141
Fig. 1. (a) Sketch of the layers composing the capacitive sensor and (b) sketch of the sensor structure.
cluster source (PMCS) [14,15]. In a PMCS, an aerodynamically confined plasma discharge ablates a graphite rod; the vaporized carbon atoms are quenched in a condensation cavity by a pulse of helium and coalesce to form clusters. The helium-cluster mixture is then expanded through the source nozzle into a high-vacuum chamber (P ∼ 10−5 Pa) to form a supersonic beam [15]. The sensor concept has been described in [13]. A sketch of the device is given in Fig. 1a and b, while a picture of the manufactured sensor is shown in Fig. 2. Capacitance measurements have been performed by means of an electronic read-out system designed and manufactured at Dipartimento di Energetica, University of Firenze. This has been implemented to provide a fully integrated, cost-effective system. The complex impedance Zs of the sample is measured using the circuit described in Fig. 3. An integrated waveform generator produces the sine volt-
Fig. 2. Picture of the capacitive sensor based on ns-C.
age wave which excites the sample. The excitation frequency can be varied in the range 10–1000 kHz. The current flowing through the sample is converted into the voltage Vi by a transimpedance amplifier. The voltage drop across the sample is attenuated to obtain a signal VV with the correct signal amplitude at the input of the gain phase detector (GPD). The attenuating network is designed to introduce the same phase shift as the amplifier. The GPD performs the comparison between Vi and VV , producing two analog outputs: VG and VP . VG ∝ VV /Vi is proportional to the modulus of Zs . VP is proportional to the phase difference between VV and Vi and in turn to the phase of Zs . The outputs of the GPD are finally read by a data acquisition board and transmitted to a personal computer for recording and processing. Relative humidity has been monitored with a commercial hygrometer, consisting of a capacitive-type sensor produced by AHLBORN, Germany equipped with Almemo FH A 6467 read-out electronics. Measurements have been performed in air with controlled humidity and temperature at atmospheric pressure. The apparatus (Fig. 4) consists of a humidifier and a small measurement chamber (volume: 400 cm3 ). The relative
Fig. 3. Sketch of the read-out system manufactured by us and used to perform complex impedance measurements.
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S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
Fig. 4. Sketch of the humidification system.
humidity value is obtained by bubbling dry air in the humidifier, which contains distilled water that can be heated from 20 to 80 ◦ C by means of a resistor. The relative humidity percentage inside the measurement chamber is controlled by two inlet needle valves. During measurements the relative humidity and the temperature of the sample are monitored. It is also possible to heat the sample in vacuum in order to achieve the complete desorption of residual water vapour molecules and thus to refresh the capacitive device. The evacuation of the chamber is performed by a diaphragm pump.
3. Results and discussion Capacitance measurements have been performed for different relative humidity values, at atmospheric pressure and 300 K. In Fig. 5 the capacitance versus relative humidity plot is shown. The starting point of the adsorption curve is 2% RH, from where the RH was gradually raised up to 90% and then it was gradually reduced back to 2% to obtain the desorption curve. A linear behaviour up to 70% RH is observed. A superlinear trend is observed for higher relative humidity values. The sensitivity of the device: S = (C − C0 )/RH%, where C0 is the capacitance in dry air (C0 = 87.25 pF), is
Fig. 5. Capacitance vs. relative humidity curves in the range 2–90% RH.
0.23 pF/%RH, comparable to the values reported for commercial sensors and for sensors based on polymers [8]. Higher relative changes in capacitance are reported in literature in the case of a nonlinear response [5], in our sensor a capacitance increase of 40 pF is achieved over the whole investigated range. In the range 2–80% no hysteresis is observed, while for higher RH% a slight displacement from the adsorption values is recorded performing measurements in desorption regime. Although more measurements would be necessary to quantify the effect, a first estimation gives a maximum variation of 2%. From data sheets, a 1% hysteresis characterizes the commercial device used as a reference. Higher hysteresis have been reported in literature [5] when devices characterized by higher sensitivity and non-linear response are measured in the same RH range. Fast and reversible changes in the capacitance have been observed as the relative humidity is varied, as it is evidenced in Fig. 6, where capacitance versus time is shown. The dynamical response of the ns-C system and the reference one are compared, in adsorption and desorption regimes, respectively in Figs. 7 and 8. The plots evidence the fast response of the ns-C system to humidity changes. Evaluating the time necessary to reach the 50% of the difference between two stable values of RH, our sensor response is
Fig. 6. Adsorption curves for ns-C sensor and commercial humidity sensor (AHLBORN-Almemo) as a function of time.
S. Miglio et al. / Sensors and Actuators B 111–112 (2005) 140–144
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is more hydrophobic, i.e. for contact angles >90◦ . Previous investigations on similar ns-C films showed that a hydrophobic surface is achievable by increasing the average slope of the surface, which should be correlated with the steepness (and roughness) of the film surface [17]. Film roughness is therefore another important topic to control sensors efficiency. To reduce the superlinear trend and hysteresis it could be also useful to heat the sample in order to avoid capillary condensation inside smaller pores: a constant temperature operation is nevertheless far more reliable for humidity measurements.
4. Conclusions
Fig. 7. Comparison of the dynamic response measured at room temperature for the ns-C based device and the commercial humidity sensor (AHLBORNAlmemo) (rising-edge).
advancing the commercial one by 3s in adsorption and 5s in desorption. One possible explanation is that the reference system is characterized by a slower read-out electronics, which, compared to our system, requires additional time to process data (e.g. pressure and temperature compensation, eventual linearization of the sensor response). Taking into account of this delay, the rising and falling times of the ns-C sensing system are similar to those of the commercial hygrometric system. A further engineering of the ns-C sensor is now under way to extend linearity in the whole RH% range and further reduce hysteresis. As superlinearity and hysteresis could be ascribed to water condensation inside pores [5,16], pore size is a key parameter to improve the overall sensing performance. Capillary condensation occurs for radii smaller then the Kelvin radius rK [16], which becomes smaller when the surface
We have studied the properties of ns-C based humidity sensors at room temperature and atmospheric pressure. Read-out electronics have been designed and implemented on purpose to provide a fully integrated, cost-effective system. We measured the capacitance variations induced by changes in relative humidity up to 90%. A linear behaviour up to 70% RH has been observed while the device showed a superlinear trend for higher relative humidity values. The sensitivity is 0.23 pF/%RH, comparable to values reported for commercial sensors and for sensors based on polymers. No hysteresis has been observed in the range 2–80% while for higher RH values a slight hysteresis has been recorded, with a maximum variation of 2%. The dynamical response of the ns-C system has been compared to the reference one, in adsorption and desorption regimes. Fast and reversible changes in the capacitance have been measured as the relative humidity is varied. The ns-C system is responding faster, advancing the commercial one by 3–5 s, while the rising and falling times are similar for the two systems. Further engineering of the ns-C sensor is now under way to improve sensitivity and linearity in the whole RH% range. Electrode geometry as well as material morphology are considered as key parameters to optimize the performance of the device.
Acknowledgement This work has been partially supported by MIUR under project FIRB “Carbon micro and nanostructures”.
References
Fig. 8. Comparison of the dynamic response measured at room temperature for the ns-C based device and the commercial humidity sensor (AHLBORNAlmemo) (falling-edge).
[1] Z.M. Rittersma, Recent achievements in miniaturised humidity sensors-a review of transduction techniques, Sens. Actuators A: Phys. 96 (2002) 196–210. [2] O.K. Varghese, C.A. Grimes, Metal oxide nanoarchitectures for environmental sensing, J. Nanosci. Nanotech. 3 (2003) 277–293. [3] Z.M. Rittersma, A. Splinter, A. B¨odecker, W. Benecke, A novel surface-micromachined capacitive porous silicon humidity sensor, Sens. Actuators B: Chem. 68 (2000) 210–217. [4] E.J. Connolly, G.M. O’Halloran, H.T.M. Pham, P.M. Sarro, P.J. French, Comparison of porous silicon, porous polysilicon and porous
144
[5]
[6]
[7]
[8]
[9] [10]
[11] [12]
[13]
[14]
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Biographies Stefania Miglio was born in Castrovillari, Italy, in 1971 and received her MSc degree in physics from the University of Florence in 2000, defending a thesis on electrical and optical properties of neutron irradiated chemical vapour deposited (CVD) diamond detectors. She is presently working on electrical characterization of nanostructured carbon films obtained by supersonic cluster beam deposition and on their sensing applications. She is now completing her PhD in Materials Engineering at the University of Florence.
Mara Bruzzi is Associate Professor in Physics at the University of Florence. She is mainly involved in the development of advanced materials for sensing applications in nuclear, high energy, environmental and medical physics. She is Spokesperson of the CERN international collaboration RD50: “Development of Radiation Hard Semiconductor Detectors for Very High Luminosity Colliders”. Monica Scaringella was born in Canosa di Puglia, (Italy) in 1976 and received her MSc degree in electronic engineering from the University of Florence in 2002, defending a thesis on characterization of semiconductors defects by thermal spectroscopy. She is presently working on radiation damage of silicon detectors and on applications of nanostructured carbon films obtained by Supersonic Cluster Beam Deposition. She is now a PhD Student in Materials Engineering at the University of Florence. David Menichelli was born in Italy in 1971 and received his MSc degree in electronic engineering from the University of Florence in 1998, defending a thesis on radiation damage in silicon particle detectors. In 2001 he received his PhD after a research about optoacoustics. He is presently working at University of Florence about the development of particle detection systems for medical applications and dosimetry. Moreover, he is interested in radiation hardness of silicon, diamond and silicon carbide materials. Ettore Leandri was born in Arezzo (Italy) in 1976. He has got a bachelor degree in Informational Engineering at the University of Florence in 2002 and presently he is working to read-out electronics for nanostructured carbon sensors to obtain the MSc degree in Electronic Engineering. Andrea Baldi was born in Florence in 1961. He achieved his High school technical degree at the Technical Institute “Leonardo da Vinci” in Florence. Since 1984 he works at the Dipartimento di Energetica, University of Florence, as technician. His activity is particularly concerned in giving an important contribution to the designs and fabrication of measurement apparatus and in technical troubleshooting. Gero Bongiorno was born in Milan in 1977. In 2001 he graduated from University of Milan in Physics with a thesis on the characterization of electric transport properties in nanostructured materials. He is now completing his PhD in Physics at the University of Milan working on the synthesis and characterization of nanocomposite materials. Paolo Piseri is researcher at the Universit`a degli Studi di Milano since 2002. He graduated in Physics in 1995 and is PhD in Physics since 2000 with the Thesis: supersonic cluster beam deposition for the synthesis of nanophase materials. He is active at the molecular beams and nanocrystalline materials laboratory (LGM) of the Universit`a degli Studi di Milano since its foundation in 1993 and brought an important contribution to the development of the original synthesis and characterization techniques of LGM. Paolo Milani is Full Professor at the Department of Physics of the University of Milan, Italy. He currently serves as Director of the Interdisciplinary Centre of Nanostructured Materials and Interfaces of the University of Milan.