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Influence of pH solution on photoluminescence of porous silicon
Sensors and Actuators A 137 (2007) 345–349
Influence of pH solution on photoluminescence of porous silicon A. Benilov a,b,∗ , I. Gavrilchenko b , I. Benilova c , V. Skryshevsky b , M. Cabrera a a
Laboratoire d’Electronique, Opto´electronique et Microsyst`emes, Ecole Centrale de Lyon, BP 163-69131, Ecully Cedex, France b Radiophysics Department, Kyiv National Taras Shevchenko University, 64 Volodymyrska, 01033 Kyiv, Ukraine c Institute of Molecular Biology and Genetics NAS of Ukraine, 150 Zabolotnogo, 03143 Kyiv, Ukraine Received 21 September 2006; received in revised form 28 February 2007; accepted 28 February 2007 Available online 6 March 2007
Abstract The radiative lifetime of as-prepared and modified layers of porous silicon (por-Si) were studied in liquid solutions with different pH. It was observed that por-Si photoluminescence (PL) intensity and decay lifetime strongly depend on pH value. This phenomenon is explained by competition of the following processes: UV-induced hydrogen effusion, hydrogen adsorption from the buffer solution, and por-Si oxidation. Regarding the phenomenon a pH change sensor can be proposed. Por-Si layer degradation can be decreased somehow by protective PEDOT layer. © 2007 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Photoluminescence; pH sensor
1. Introduction Porous silicon (por-Si) has attracted much attention as a new optoelectronic material since first of all observation of its efficient photoluminescence (PL) at room temperature. Low cost of a product, possibility of improved performance and optical properties caused that the investigation of por-Si luminescence and its optical properties have been made in many laboratories [1,2]. Por-Si is also claimed to be a promised material for implementation in various sensing devices. Chemical sensors based on por-Si displays some advantages comparing with other transducers. Thus, it is observed the reversible quenching of PL intensity in presence of different gases; the quenching grows with partial pressure and dipole momentum of adsorbed molecules [3]. PL intensity of por-Si drops by three orders of magnitude when porSi is immersed in organic solvent of dielectric constant varying from 2 to 20 [4]. However, varying por-Si samples preparation process, and conditions of PL measurement the contradictory effects (increasing of PL intensity in gas atmosphere, independence of PL intensity from dielectric constant, etc.) are observed too [3]. Currently por-Si stabilization is an important problem to be solved in order to create an efficient por-Si based sensor systems. ∗
Corresponding author. E-mail address: [email protected] (A. Benilov).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.02.032
Etching of por-Si in aggressive environments (acids and bases) [5] and its oxidation on open air and in aqueous solutions [6] are the most probable processes leading to por-Si electrical (loss of conductivity) and optical (PL quenching) degradation [7–9]. Many works are dedicated to stabilization of por-Si layer applying different treatment methods [10]. These include chemical treatment [11–13], mechanical protection by polymers [14], or a preliminary treatment [15–17] in order to stabilize near-surface layer of por-Si. In this work, the influence of pH buffer solutions on por-Si PL was studied in details. A PEDOT polymer deposition was used to reduce por-Si degradation. As a result of the investigation of pH-dependent por-Si PL intensity and decay lifetime, a pH change sensor can be proposed. 2. Samples and experiment Por-Si samples were prepared using routine anodization process by applying a constant anodic current to p-doped silicon slabs (∼10 cm) with (1 0 0) crystallographic orientation. The etching had been carried out in an etching solution consisting hydrofluoric acid (48%):ethanol = 1:1 (vol.). Porosity of the samples obtained was estimated to be near 60%, por-Si layer thickness was approximately 1 m. Some samples were covered by PEDOT polymer by a spin coating to prevent por-Si luminescence degradation in aqueous solutions.
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A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349
Fig. 2. Setup for PL intensity and decay time measurement. Fig. 1. Transient PL spectra of por-Si measured in analog mode (without deconvolution).
Por-Si photoluminescence spectra were investigated using time-resolved spectrometry carried out at room temperature. The samples’ PL was excited with a pulse nitrogen laser (λ = 337 nm, FWHM = 8 ns, power in pulse 20 kW) or with blue LED (λ = 406 nm), powered in pulse mode. The proper band of PL emission was selected by monochromator (MS2004, SOLAR T II), signal was registered by photomultiplier (HAMAMATSU C6270) used in analog mode or photon counting photomultiplier (HAMAMATSU H7360-03). Samples prepared manifested a visible luminescence with a maximal intensity at 650–750 nm band (Fig. 1) which corresponds to, so-called, S-band (slow) of PL [3]. To study the pH influence on PL we prepared several buffer solutions with different pH values. We used a multi-component “polymix” buffer with following composition: 2.5 mM Tris, 2.5 mM citric acid, 2.5 mM sodium tetraborate, 2.5 mM potassium phosphate, pH was adjusted with 1 M HCl or 1 M NaOH solutions in order to obtain the expected pH value. Its buffer capacity is stable over a wide range of pH (5–9). This buffer is used for studying of pH influence on the ISFET responses (especially enzyme-modified ISFETs) [18]. In range of pH (2.2–3.6) we applied acidic 0.1 M buffer composed of glycine and HCl. pH was adjusted by 1 M HCl. PL spectra shapes for homogeneous por-Si layers were not greatly affected by different pH buffers, so only integral intensity and lifetime at maximum emission wavelength were studied. PL measurement setup for application as pH sensor can be much simplified. Instead of a monochromator band-pass filter with transmission from 670 nm to 730 nm was used. Por-Si sample was placed in flow-through cell with a window providing PL excitation and registration (Fig. 2).
while immersion into basic solutions leads to fast PL quenching. The exponential law of S-band decay is observed only at late time of decay. For early time it reveals more complex multiexponential behavior. Thus, the proper parameter of decay lifetime can be introduced for longtime tail of PL decay curve. Cyclic change of buffer in the flow cell leads to adequate change of por-Si PL intensity and decay lifetime (Fig. 4). However, PL quenching was observed while the samples being exposed to different pH buffers. This quenching can be explained by partial por-Si oxidation in aqueous solutions (as far as all buffers consist of more than 90% of water). It is known, that por-Si being immersed in water begins to oxidize that leads to improvement of por-Si surface wettability [19]. The presence of silicon oxide bonds in this case can easily been monitored with FTIR spectroscopy. Por-Si oxidation leads to PL quenching [7]. Furthermore, a permanent UV illumination of por-Si sample immersed in water leads to even stronger PL quenching (Fig. 5). This phenomenon is well described by UV-induced hydrogen effusion from por-Si and defect generation in near-surface region of por-Si [19,20].
3. Results and discussion Por-Si PL intensity and decay lifetime appeared to be very sensitive to the pH value of the solution passed through the flow cell (Fig. 3). PL appears to be more intense in acid environment
Fig. 3. The longtime tail of PL decays of as-prepared por-Si in several pH buffers.
A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349
347
Fig. 4. PL intensity and PL decay time for as-prepared por-Si cyclically exposed to buffer solutions of pH 8 (low PL intensity and decay time), and pH 5 (high PL intensity and decay time). PL quenching time ∼300 min (dashed curve).
Fig. 5. PL intensity of as-prepared por-Si sample, the same sample that was treated for 10 min in pH 6 buffer in the dark, and the same sample treated for 10 min in pH 6 buffer while permanent UV (400 nm LED) illumination.
A photooxidation process also takes place while the sample is exposed to UV illumination [8]. UV irradiation leads to an increase in the intensity of Si O stretching bands and the H Si O3 deformation and stretching bands in IR absorption. The decrease in intensity of these four bands after etching in HF
indicates the formation of an oxide layer under UV illumination. Its appearance is also accompanied by a reduction in the intensity of all Si H bands (664, 906, 2087, 2106, and 2140 cm−1 ). Fig. 6 shows the increase of oxide bands and the decrease of Si Hn bonds upon UV illumination of the por-Si layer. PL intensity and decay time growth while the sample being exposed to acid buffers (pH < 7.4) can be explained by siliconhydrogen bands restoration in the acid buffer solution. Thus, two processes take place simultaneously: hydrogen effusion caused by UV illumination of por-Si sample, and hydrogen adsorption from the buffer solution. Obviously, a detailed equilibrium of these two processes leads to a definite value of PL intensity and decay time (which degrade due to the oxidation process, taking place for every buffer solution). Moreover, a constant degradation of por-Si layer takes place due to oxidation process and por-Si etching in basic solutions. Covering the samples by PEDOT polymer in order to protect the porous surface, decreases somehow the degradation of PL intensity (Fig. 7), but the sample degrades nevertheless while being treated in solution for several hours. Semipermeable polymers, such as PEDOT seems to be applicable for stabilizing of por-Si [14] preventing somehow por-Si layer oxidation and etching and allowing at the same time free access of ions (H+ in our case) to the surface. However, thin polymer layers cannot protect
Fig. 6. IR normal-incidence transmission spectra of (1) Si-substrate and (2–8) por-Si samples upon UV illumination for various times: (2) 0, (3) 10, (4) 20, (5) 30, (6) 40, (7) 60, and (8) 100 min.
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A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349
Fig. 7. PL intensity and PL decay time for PEDOT covered por-Si cyclically exposed to buffer solutions of pH 8 (low PL intensity and decay time), and pH 5 (high PL intensity and decay time). PL quenching time ∼450 min (dashed curve). Table 1 Comparing sensing capabilities of as-prepared and PEDOT covered samples Sample
PL intensity I/I (pH = 1)
PL lifetime τ/τ (pH = 1)
PL quenching time (min)
As-prepared PEDOT covered
0.1 0.17
0.22 0.22
300 450
the surface against aggressive environment (acids or bases) for a long time, and thick polymer layers are not permeable enough to ensure por-Si based sensor sufficient sensitivity. In contrast to known por-Si based pH-sensible systems, based on field effect [21–24], using por-Si PL measurements allows to perform a contact-free pH changes monitoring. An average sensitivity (signal change I or τ to absolute signal value I or τ ratio) of the pH sensor proposed is showed in the Table 1 (as compared with [21–24], field effect transducers have I/I ∼ 0.6–1.2).
sponding changes in PL intensity and decay time. For all that high PL intensity and long decay time correspond to lower pH (acids) while low PL intensity and short decays correspond to higher pH (bases) (Fig. 8). This phenomenon is well explained by competition of hydrogen effusion-adsorption and oxidation processes while UV illumination. The increase of PL quenching takes place for any pH solution used due to the por-Si oxidation in water. Por-Si layer degradation (etching) is observed while the samples being placed in alkaline solution (pH > 7.4). Using PEDOT as protective layer allows slowing down the porSi degradation process but can’t protect the surface for durable treatment. Acknowledgements This work has been funded in part by INTAS program (project 05-1000005-7729), Science & Technology Center in Ukraine under contract STCU N3819, A.B. thanks CROUS (fellowship from the French Government) for funding this research.
4. Conclusions Por-Si luminescence seems to be very sensitive to the pH level of buffer solutions. Changes of pH value lead to corre-
Fig. 8. PL intensity of por-Si sample exposed to different buffer solutions.
References [1] Z. Lukasiak, P. Dalasinski, W. Bala, Kinetics of photoluminescence of porous silicon studied by photo-luminescence excitation spectroscopy and time-resolved spectroscopy, Optoelectron. Rev. 11 (2) (2003) 113–117. [2] A. Bratkowski, et al., Novel gas sensor based on porous silicon measured by photovoltage, photoluminescence, and admittance spectroscopy, Optoelectron. Rev. 13 (1) (2005) 35–38. [3] L.T. Canham (Ed.), Properties of Porous Silicon, INSPEC, The Institution of Electrical Engineers, London, UK, 1997. [4] S. Fellah, et al., Porous silicon in solvents: constant-lifetime PL quenching and confirmation of dielectric effects, Phys. Status Solidi (a) 182 (2000) 367–372. [5] D. Hamm, T. Sakka, Y.H. Ogata, Etching of porous silicon in basic solution, Phys. Status Solidi (a) 197 (1) (2003) 175–179. [6] Y.H. Ogata, et al., Oxidation of porous silicon in dry and wet environments under mild temperature conditions, J. Porous Mater. 7 (2000) 63–66. [7] M.A. Tischler, et al., Luminescence degradation in porous silicon, Appl. Phys. Lett. 60 (5) (1992) 639–641. [8] A.I. Belogorokhov, L.I. Belogorokhova, Optical properties of porous silicon layers processed with HF:HCl:C2 H5 OH electrolyte, Semiconductors 33 (2) (1999) 169–174. [9] N. Hadj Zoubir, et al., Interpretation of the luminescence quenching in chemically etched porous silicon by the desorption of SiH3 species, Appl. Phys. Lett. 65 (1) (1994) 82–84.
A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349 [10] M. Bj¨orkqvist, et al., Comparison of stabilizing treatments on porous silicon for sensor applications, Phys. Status Solidi (a) 197 (2) (2003) 374–377. [11] J.L. Gole, et al., Chloride salt enhancement and stabilization of the photoluminescence from a porous silicon surface, Phys. Rev. B 61 (8) (2000) 5615–5631. [12] W.D. Kirkey, et al., Quasi-reversible photoluminescence quenching of stable dispersions of silicon nanoparticles, J. Mater. Chem. 15 (2005) 2028–2034. [13] J.M. Lauerhaas, et al., Reversible luminescence quenching of porous Si by solvents, J. Am. Chem. Soc. 114 (1992) 1911–1912. [14] J.W.P. Bakker, et al., Improvement of porous silicon based gas sensors by polymer modification, Phys. Status Solidi (a) 197 (2) (2003) 378–381. [15] E. Massera, et al., Improvement of stability and recovery time in poroussilicon-based NO2 sensor, Sens. Actuators B 102 (2004) 195–197. [16] E. Garrone, et al., Porous silicon in NO2 : a chemisorption mechanism for enhansed electrical conductivity, Phys. Status Solidi (a) 197 (1) (2003) 103–106. [17] Y. Zhao, D. Li, D. Yang, Effect of post-treatment processes on the photoluminescence of porous silicon, Phys. B 364 (2005) 180–185. [18] A.P. Soldatkin, et al., Glucose-sensitive field-effect transistor with additional Nafion membrane: reduction of influence of buffer capacity on the sensor response and extension of its dynamic range, Anal. Chem. Acta 283 (2) (1993) 695–701. [19] V.P. Tolstoy, I.V. Chernyshova, V.A. Skryshevsky, Handbook of Infrared Spectroscopy of Ultrathin Films, John Wiley & Sons, Inc., 2003. [20] R.T. Collins, M.A. Tischler, J.H. Stathis, Photoinduced hydrogen loss from porous silicon, Appl. Phys. Lett. 61 (14) (1992) 1649–1651. [21] M. Thust, et al., Porous silicon as a substrate material for potentiometric biosensors, Meas. Sci. Technol. 7 (1996) 26–29. [22] M.J. Sch¨oning, et al., A (bio-)chemical field-effect sensor with macroporous Si as substrate material and a SiO2 /LPCVD-Si3 N4 double layer as pH transducer, Sensors 2 (2002) 11–22. [23] M.J. Sch¨oning, et al., Miniaturization of potentiometric sensors using porous silicon microtechnology, Electrochim. Acta 42 (20–22) (1997) 3185–3193.
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[24] H. L¨uth, et al., Biochemical sensors with structured and porous silicon capacitors, Mater. Sci. Eng. B 69–70 (2000) 104–108.
Biographies Arthur Benilov is PhD student in the field of electronics and microelectronics. He received his master degree in 2003 from Taras Shevchenko Kiev University. His current scientific interests deal with porous silicon based sensors, multisensor systems, automated control systems, and digital signal processing. Iryna Gavrilchenko graduated from Taras Shevchenko Kiev University in 2001 and received master degree. Now she works on the position of engineer at Radiophysics Department. The current interests are chemical sensors based on porous silicon. Iryna Benilova is PhD student in the field of biotechnology. She received her MS degree in biochemistry from Taras Shevchenko Kiev University in 2004. Her current scientific activities: study on molecular basis of the interaction of enzymes with substrates and inhibitors using pH-sensitive and impedimetric biosensors; investigation of the recognition specificity of olfactory receptors as a recognition part of the olfactory biological sensors. Valeri Skryshevsky He graduated from Kiev Taras Shevchenko University with honors in 1978, received PhD degree in 1984, degree of Doctor of Science in 2001, professor of Radiophysics Department of National Kiev Taras Shevchenko University from 2002. The current interests concern with the semiconductor sensors (chemical, bio and radiation), nanophysics, porous silicon, optoelectronics. Michel Cabrera is researcher at the Centre National de la Recherche Scientifiqe at the Institut des Nanotechnologies de Lyon (INL). He got an engineering degree at the Ecole Sup´erieure d’Electricit´e, and got his PhD degree in 1986. His current interest is in the development of non-conventional rapid machining techniques to prepare microfluidics and biosensor devices including micro contact printing and micro and nano electrical machining.
Influence of pH solution on photoluminescence of porous silicon A. Benilov a,b,∗ , I. Gavrilchenko b , I. Benilova c , V. Skryshevsky b , M. Cabrera a a
Laboratoire d’Electronique, Opto´electronique et Microsyst`emes, Ecole Centrale de Lyon, BP 163-69131, Ecully Cedex, France b Radiophysics Department, Kyiv National Taras Shevchenko University, 64 Volodymyrska, 01033 Kyiv, Ukraine c Institute of Molecular Biology and Genetics NAS of Ukraine, 150 Zabolotnogo, 03143 Kyiv, Ukraine Received 21 September 2006; received in revised form 28 February 2007; accepted 28 February 2007 Available online 6 March 2007
Abstract The radiative lifetime of as-prepared and modified layers of porous silicon (por-Si) were studied in liquid solutions with different pH. It was observed that por-Si photoluminescence (PL) intensity and decay lifetime strongly depend on pH value. This phenomenon is explained by competition of the following processes: UV-induced hydrogen effusion, hydrogen adsorption from the buffer solution, and por-Si oxidation. Regarding the phenomenon a pH change sensor can be proposed. Por-Si layer degradation can be decreased somehow by protective PEDOT layer. © 2007 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Photoluminescence; pH sensor
1. Introduction Porous silicon (por-Si) has attracted much attention as a new optoelectronic material since first of all observation of its efficient photoluminescence (PL) at room temperature. Low cost of a product, possibility of improved performance and optical properties caused that the investigation of por-Si luminescence and its optical properties have been made in many laboratories [1,2]. Por-Si is also claimed to be a promised material for implementation in various sensing devices. Chemical sensors based on por-Si displays some advantages comparing with other transducers. Thus, it is observed the reversible quenching of PL intensity in presence of different gases; the quenching grows with partial pressure and dipole momentum of adsorbed molecules [3]. PL intensity of por-Si drops by three orders of magnitude when porSi is immersed in organic solvent of dielectric constant varying from 2 to 20 [4]. However, varying por-Si samples preparation process, and conditions of PL measurement the contradictory effects (increasing of PL intensity in gas atmosphere, independence of PL intensity from dielectric constant, etc.) are observed too [3]. Currently por-Si stabilization is an important problem to be solved in order to create an efficient por-Si based sensor systems. ∗
Corresponding author. E-mail address: [email protected] (A. Benilov).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.02.032
Etching of por-Si in aggressive environments (acids and bases) [5] and its oxidation on open air and in aqueous solutions [6] are the most probable processes leading to por-Si electrical (loss of conductivity) and optical (PL quenching) degradation [7–9]. Many works are dedicated to stabilization of por-Si layer applying different treatment methods [10]. These include chemical treatment [11–13], mechanical protection by polymers [14], or a preliminary treatment [15–17] in order to stabilize near-surface layer of por-Si. In this work, the influence of pH buffer solutions on por-Si PL was studied in details. A PEDOT polymer deposition was used to reduce por-Si degradation. As a result of the investigation of pH-dependent por-Si PL intensity and decay lifetime, a pH change sensor can be proposed. 2. Samples and experiment Por-Si samples were prepared using routine anodization process by applying a constant anodic current to p-doped silicon slabs (∼10 cm) with (1 0 0) crystallographic orientation. The etching had been carried out in an etching solution consisting hydrofluoric acid (48%):ethanol = 1:1 (vol.). Porosity of the samples obtained was estimated to be near 60%, por-Si layer thickness was approximately 1 m. Some samples were covered by PEDOT polymer by a spin coating to prevent por-Si luminescence degradation in aqueous solutions.
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A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349
Fig. 2. Setup for PL intensity and decay time measurement. Fig. 1. Transient PL spectra of por-Si measured in analog mode (without deconvolution).
Por-Si photoluminescence spectra were investigated using time-resolved spectrometry carried out at room temperature. The samples’ PL was excited with a pulse nitrogen laser (λ = 337 nm, FWHM = 8 ns, power in pulse 20 kW) or with blue LED (λ = 406 nm), powered in pulse mode. The proper band of PL emission was selected by monochromator (MS2004, SOLAR T II), signal was registered by photomultiplier (HAMAMATSU C6270) used in analog mode or photon counting photomultiplier (HAMAMATSU H7360-03). Samples prepared manifested a visible luminescence with a maximal intensity at 650–750 nm band (Fig. 1) which corresponds to, so-called, S-band (slow) of PL [3]. To study the pH influence on PL we prepared several buffer solutions with different pH values. We used a multi-component “polymix” buffer with following composition: 2.5 mM Tris, 2.5 mM citric acid, 2.5 mM sodium tetraborate, 2.5 mM potassium phosphate, pH was adjusted with 1 M HCl or 1 M NaOH solutions in order to obtain the expected pH value. Its buffer capacity is stable over a wide range of pH (5–9). This buffer is used for studying of pH influence on the ISFET responses (especially enzyme-modified ISFETs) [18]. In range of pH (2.2–3.6) we applied acidic 0.1 M buffer composed of glycine and HCl. pH was adjusted by 1 M HCl. PL spectra shapes for homogeneous por-Si layers were not greatly affected by different pH buffers, so only integral intensity and lifetime at maximum emission wavelength were studied. PL measurement setup for application as pH sensor can be much simplified. Instead of a monochromator band-pass filter with transmission from 670 nm to 730 nm was used. Por-Si sample was placed in flow-through cell with a window providing PL excitation and registration (Fig. 2).
while immersion into basic solutions leads to fast PL quenching. The exponential law of S-band decay is observed only at late time of decay. For early time it reveals more complex multiexponential behavior. Thus, the proper parameter of decay lifetime can be introduced for longtime tail of PL decay curve. Cyclic change of buffer in the flow cell leads to adequate change of por-Si PL intensity and decay lifetime (Fig. 4). However, PL quenching was observed while the samples being exposed to different pH buffers. This quenching can be explained by partial por-Si oxidation in aqueous solutions (as far as all buffers consist of more than 90% of water). It is known, that por-Si being immersed in water begins to oxidize that leads to improvement of por-Si surface wettability [19]. The presence of silicon oxide bonds in this case can easily been monitored with FTIR spectroscopy. Por-Si oxidation leads to PL quenching [7]. Furthermore, a permanent UV illumination of por-Si sample immersed in water leads to even stronger PL quenching (Fig. 5). This phenomenon is well described by UV-induced hydrogen effusion from por-Si and defect generation in near-surface region of por-Si [19,20].
3. Results and discussion Por-Si PL intensity and decay lifetime appeared to be very sensitive to the pH value of the solution passed through the flow cell (Fig. 3). PL appears to be more intense in acid environment
Fig. 3. The longtime tail of PL decays of as-prepared por-Si in several pH buffers.
A. Benilov et al. / Sensors and Actuators A 137 (2007) 345–349
347
Fig. 4. PL intensity and PL decay time for as-prepared por-Si cyclically exposed to buffer solutions of pH 8 (low PL intensity and decay time), and pH 5 (high PL intensity and decay time). PL quenching time ∼300 min (dashed curve).
Fig. 5. PL intensity of as-prepared por-Si sample, the same sample that was treated for 10 min in pH 6 buffer in the dark, and the same sample treated for 10 min in pH 6 buffer while permanent UV (400 nm LED) illumination.
A photooxidation process also takes place while the sample is exposed to UV illumination [8]. UV irradiation leads to an increase in the intensity of Si O stretching bands and the H Si O3 deformation and stretching bands in IR absorption. The decrease in intensity of these four bands after etching in HF
indicates the formation of an oxide layer under UV illumination. Its appearance is also accompanied by a reduction in the intensity of all Si H bands (664, 906, 2087, 2106, and 2140 cm−1 ). Fig. 6 shows the increase of oxide bands and the decrease of Si Hn bonds upon UV illumination of the por-Si layer. PL intensity and decay time growth while the sample being exposed to acid buffers (pH < 7.4) can be explained by siliconhydrogen bands restoration in the acid buffer solution. Thus, two processes take place simultaneously: hydrogen effusion caused by UV illumination of por-Si sample, and hydrogen adsorption from the buffer solution. Obviously, a detailed equilibrium of these two processes leads to a definite value of PL intensity and decay time (which degrade due to the oxidation process, taking place for every buffer solution). Moreover, a constant degradation of por-Si layer takes place due to oxidation process and por-Si etching in basic solutions. Covering the samples by PEDOT polymer in order to protect the porous surface, decreases somehow the degradation of PL intensity (Fig. 7), but the sample degrades nevertheless while being treated in solution for several hours. Semipermeable polymers, such as PEDOT seems to be applicable for stabilizing of por-Si [14] preventing somehow por-Si layer oxidation and etching and allowing at the same time free access of ions (H+ in our case) to the surface. However, thin polymer layers cannot protect
Fig. 6. IR normal-incidence transmission spectra of (1) Si-substrate and (2–8) por-Si samples upon UV illumination for various times: (2) 0, (3) 10, (4) 20, (5) 30, (6) 40, (7) 60, and (8) 100 min.
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Fig. 7. PL intensity and PL decay time for PEDOT covered por-Si cyclically exposed to buffer solutions of pH 8 (low PL intensity and decay time), and pH 5 (high PL intensity and decay time). PL quenching time ∼450 min (dashed curve). Table 1 Comparing sensing capabilities of as-prepared and PEDOT covered samples Sample
PL intensity I/I (pH = 1)
PL lifetime τ/τ (pH = 1)
PL quenching time (min)
As-prepared PEDOT covered
0.1 0.17
0.22 0.22
300 450
the surface against aggressive environment (acids or bases) for a long time, and thick polymer layers are not permeable enough to ensure por-Si based sensor sufficient sensitivity. In contrast to known por-Si based pH-sensible systems, based on field effect [21–24], using por-Si PL measurements allows to perform a contact-free pH changes monitoring. An average sensitivity (signal change I or τ to absolute signal value I or τ ratio) of the pH sensor proposed is showed in the Table 1 (as compared with [21–24], field effect transducers have I/I ∼ 0.6–1.2).
sponding changes in PL intensity and decay time. For all that high PL intensity and long decay time correspond to lower pH (acids) while low PL intensity and short decays correspond to higher pH (bases) (Fig. 8). This phenomenon is well explained by competition of hydrogen effusion-adsorption and oxidation processes while UV illumination. The increase of PL quenching takes place for any pH solution used due to the por-Si oxidation in water. Por-Si layer degradation (etching) is observed while the samples being placed in alkaline solution (pH > 7.4). Using PEDOT as protective layer allows slowing down the porSi degradation process but can’t protect the surface for durable treatment. Acknowledgements This work has been funded in part by INTAS program (project 05-1000005-7729), Science & Technology Center in Ukraine under contract STCU N3819, A.B. thanks CROUS (fellowship from the French Government) for funding this research.
4. Conclusions Por-Si luminescence seems to be very sensitive to the pH level of buffer solutions. Changes of pH value lead to corre-
Fig. 8. PL intensity of por-Si sample exposed to different buffer solutions.
References [1] Z. Lukasiak, P. Dalasinski, W. Bala, Kinetics of photoluminescence of porous silicon studied by photo-luminescence excitation spectroscopy and time-resolved spectroscopy, Optoelectron. Rev. 11 (2) (2003) 113–117. [2] A. Bratkowski, et al., Novel gas sensor based on porous silicon measured by photovoltage, photoluminescence, and admittance spectroscopy, Optoelectron. Rev. 13 (1) (2005) 35–38. [3] L.T. Canham (Ed.), Properties of Porous Silicon, INSPEC, The Institution of Electrical Engineers, London, UK, 1997. [4] S. Fellah, et al., Porous silicon in solvents: constant-lifetime PL quenching and confirmation of dielectric effects, Phys. Status Solidi (a) 182 (2000) 367–372. [5] D. Hamm, T. Sakka, Y.H. Ogata, Etching of porous silicon in basic solution, Phys. Status Solidi (a) 197 (1) (2003) 175–179. [6] Y.H. Ogata, et al., Oxidation of porous silicon in dry and wet environments under mild temperature conditions, J. Porous Mater. 7 (2000) 63–66. [7] M.A. Tischler, et al., Luminescence degradation in porous silicon, Appl. Phys. Lett. 60 (5) (1992) 639–641. [8] A.I. Belogorokhov, L.I. Belogorokhova, Optical properties of porous silicon layers processed with HF:HCl:C2 H5 OH electrolyte, Semiconductors 33 (2) (1999) 169–174. [9] N. Hadj Zoubir, et al., Interpretation of the luminescence quenching in chemically etched porous silicon by the desorption of SiH3 species, Appl. Phys. Lett. 65 (1) (1994) 82–84.
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Biographies Arthur Benilov is PhD student in the field of electronics and microelectronics. He received his master degree in 2003 from Taras Shevchenko Kiev University. His current scientific interests deal with porous silicon based sensors, multisensor systems, automated control systems, and digital signal processing. Iryna Gavrilchenko graduated from Taras Shevchenko Kiev University in 2001 and received master degree. Now she works on the position of engineer at Radiophysics Department. The current interests are chemical sensors based on porous silicon. Iryna Benilova is PhD student in the field of biotechnology. She received her MS degree in biochemistry from Taras Shevchenko Kiev University in 2004. Her current scientific activities: study on molecular basis of the interaction of enzymes with substrates and inhibitors using pH-sensitive and impedimetric biosensors; investigation of the recognition specificity of olfactory receptors as a recognition part of the olfactory biological sensors. Valeri Skryshevsky He graduated from Kiev Taras Shevchenko University with honors in 1978, received PhD degree in 1984, degree of Doctor of Science in 2001, professor of Radiophysics Department of National Kiev Taras Shevchenko University from 2002. The current interests concern with the semiconductor sensors (chemical, bio and radiation), nanophysics, porous silicon, optoelectronics. Michel Cabrera is researcher at the Centre National de la Recherche Scientifiqe at the Institut des Nanotechnologies de Lyon (INL). He got an engineering degree at the Ecole Sup´erieure d’Electricit´e, and got his PhD degree in 1986. His current interest is in the development of non-conventional rapid machining techniques to prepare microfluidics and biosensor devices including micro contact printing and micro and nano electrical machining.