polypyrrole hybrid structure with enhanced electrical and optical characteristics

Journal of Electroanalytical Chemistry 729 (2014) 68–74

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Electrochemical formation of a novel porous silicon/polypyrrole hybrid structure with enhanced electrical and optical characteristics Farid A. Harraz ⇑ Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt

a r t i c l e

i n f o

Article history: Received 4 May 2014 Received in revised form 3 July 2014 Accepted 10 July 2014 Available online 18 July 2014 Keywords: Porous silicon Polypyrrole Hybrid structure Electrochemical Photoluminescence

a b s t r a c t The electrochemical formation of a hybrid structure composed of a redox-active polypyrrole (PPy) and porous silicon (PSi) layer is demonstrated. PSi layers with pore sizes (200 nm) are prepared by the electrochemical anodization of n-type Si wafer in HF-based solution, whereas the PPy as a p-type conducting polymer was prepared by electro-oxidative polymerization of parent monomers in acetonitrile solution using PSi as a working electrode. Uniform and vertically-oriented PPy nanorod arrays were successfully fabricated inside the porous matrix. After pore filling, the electrical conductivity and photoluminescence of as-fabricated hybrid structure were remarkably enhanced. The PSi/PPy heterojunction was fully characterized by a variety of techniques including field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. Thermal properties were also evaluated using thermo-gravimetric analysis (TGA) measurements. Formation mechanism and surface properties of such hybrid structure along with its electrical and optical characteristics are addressed and thoroughly discussed. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical etching of single crystalline silicon in hydrofluoric acid (HF)-based solutions leads to the formation of various nano/macro-pore arrays, known as porous silicon (PSi) [1–5]. The formation of such various pore arrays are essentially dependent on the characteristics of starting silicon substrates, the anodizing solution chemistry and the applied electrochemical parameters [6–9]. The electrochemical etching could be performed either under constant current or potential, however the galvanostatic anodization is preferable as it gives a better control of the layer porosity [10,11]. The extremely large specific surface area, high tendency for oxidation and capability for surface functionalization open the door for a wide range of technological interests in PSi. These include, for examples, chemical and gas sensors [12–17], biosensors [18], optoelectronic and biomedical applications [19,20], a sacrificial layer in micromachining [21,22], templates for micro- and nanofabrication [23–25] and photovoltaic device applications [26–29]. However, the PSi itself is not an ideal ⇑ Address: Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia. Tel.: +966 75428888; fax: +966 75428887. E-mail address: [email protected] http://dx.doi.org/10.1016/j.jelechem.2014.07.015 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.

material for devices due to poor electrical and mechanical properties [30]. Incorporation of other conducting materials into the porous matrix may improve the deficient properties of PSi and forms a hybrid structure that offers a unique combination of electrical and optical properties [31,32]. Conducting polymers are potential candidates that could offer the promise of achieving a new generation of materials, exhibiting electrical and optical properties of semiconductors and retaining the attractive processing advantages of polymers. Polypyrrole (PPy) is one of the most extensively studied polymers due to its possible uses in sensors and actuators [33–35], material for corrosion protections [36] and photovoltaic devices [37,38]. The high charge carrier mobility is a substantial condition for several applications of conducting polymers. It is a common observation that the electron mobility in most conducting polymers is quite low compared to the hole mobility in either p-type conducting polymers or the electron mobility in inorganic materials basedsystems. Hence, there is an increasing attention towards the formation of hybrid structures of conducting polymers and inorganic materials, that expecting to represent a synergic approach to overcome the limitations of conducting polymers based-devices, keeping at the same time their beneficial processability. The combination of conducting polymers and inorganic materials is recently studied for various electronic applications [39,40].

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

Particularly, in photovoltaic devices [41–43] such a combination is indispensable to enhance the electron transport properties and would eventually lead to a yet newer class of hybrid photovoltaic devices with competitive efficiencies. The aim of the present work is to provide a solid base of knowledge for the electrochemical synthesis of PSi/PPy hybrid structures that would have future potential application as a photovoltaic device. Here, we demonstrate the successful fabrication of vertically oriented PPy nanorods inside 200 nm pore arrays of PSi matrix, with emphasis on its electrical and optical performance. The hybrid structure was fabricated via a double-step electrochemical approach; electrochemical etching for PSi formation followed by electrochemical polymerization to deposit the conducting polymer. The surface properties, the electrochemical characteristics, along with thermal and photoluminescence performance are evaluated and thoroughly described. 2. Experimental 2.1. Formation of PSi Porous silicon (PSi) layers were fabricated, according to our previous procedures [24,44], by a galvanostatic anodization of heavily doped n-type Si (1 0 0) wafer with a resistivity of 0.01–0.018 X cm in 6 wt.% aqueous HF + 8 mM KMnO4 as an oxidizing agent + 3000 ppm NCW-1001 surfactant (Wako Pure Chemical Ind.). The wafer was initially washed by deionized water and sonicated in acetone for 15 min, followed by dipping in 5 wt.% aq. HF to remove the native oxides. The electrochemical etching was conducted in a two-electrode set up with 0.78 cm2 Si working electrode and a Pt rod served as a counter electrode. The anodization process was performed with applied current density of 27 mA/ cm2 for 180 s, producing pore arrays with sizes in the range of 150–200 nm and layer thickness of 4.5 lm. 2.2. Electrochemical polymerization of pyrrole into PSi layer The electrochemical polymerization was performed on the wet porous layer after rinsing in ethanol. Pyrrole (C4H5N) monomers with a concentration of 0.1 M were polymerized at a constant current of 150 lA in acetonitrile (CH3CN) solution in the presence of 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) as a supporting electrolyte. A three-electrode cell was connected to a computerized potentiostat–galvanostat. A Pt wire was used as a quasi reference electrode calibrated with Ag/AgCl (potential of Pt wire = 0.21 ± 0.02 V vs. Ag/AgCl). In order to verify that the entire volume of porous matrix was uniformly infiltrated with polymer, the corresponding potential transient of the galvanostatic process was recorded.

69

calorimetry (DSC) were applied to assess the thermal stability of the polymer using SETARAM instrumentation TG-DSC16, at a heating rate of 10 K/min from 100 to 800 °C. Photoluminescence (PL) was measured at room temperature by means of Spectrofluorophotometer, SHIMATDZU RF-5301PC using a 150 W xenon lamp as an excitation source.

3. Results and discussion 3.1. Electropolymerization of pyrrole The polymerization process was conducted under a galvanostatic condition using 150 lA. A typical potential transient (E–t characteristic) is shown in Fig. 1. Different characteristic stages of galvanostatic polymerization could be recognized. At the early stage of polymerization, polymer nucleation takes place at the pore tips, simultaneously with a partial oxidation of PSi. This stage is accompanied by a rapid potential rise. An intermediate period of a nearly steady state of potential is related to the polymer growth inside the pores. A stage transition from polymerization inside the pores to polymerization at outer porous surface is accompanied with an appreciable potential rise of 0.14 V; this step is indicated by ‘‘transition stage’’. A potential rise means an increase of overpotential, which is likely observed when actual electrode area is decreased. The transition stage is not sharp and it takes 5 min polymerization time to reach onto the top surface. The charge consumed during this stage was 14% from the total polymerization charge. Such a behavior can be either related to the different pore lengths or to the inhomogeneous PPy growth inside the porous matrix. Generally, the amount of polymer deposited in the interior of the pores can be controlled by the value of polymer-formation charge passing before transition stage. Similar deposition transients have been observed previously for the electrodeposition of poly(bithiophene) [45], poly(3-dodecylthiophene) [46] and PPy [5,32,47] into p-type PSi. It is likely that after the transition stage the pores are completely filled with polymer. A thin film of dark brown PPy was visually detected after the transition stage. A constant potential is consequently observed during the polymerization at the outer surface, the process here seems to be superficial. Compared to our previous works using smaller mesopore sizes, one could conclude that as the pore size increases in the current work to 200 nm, the amount of charge needed to deposit the polymer and completely fill the pores increases, and consequently the polymerization time is extremely prolonged. Additionally, it is worthy

2.3. Characterization of PSi/PPy hybrid structure Morphological investigation for as-formed hybrid structure was done using field emission-scanning electron microscopy, FE-SEM (JSM-7600F-JEOL) on the cleaved samples, coupled with energy dispersive X-ray spectroscopy (EDS) for chemical analysis. Surface analysis was performed using X-ray photoelectron spectroscopy (XPS); a thermo scientific K-ALPHA, XPS, England. The recorded binding energies have been calibrated by taking the C1s peak at 285.0 eV as a reference. The electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and current–voltage (I–V) curves were measured using the electrochemical workstation, ZahnerZennium, Germany. The measurements parameters and the data acquisition and storage were controlled with Thales software. Thermo-gravimetric analysis (TGA) and differential scanning

Fig. 1. Potential vs. time (E–t) characteristics recorded during the electrodeposition of PPy into PSi layer in a solution composed of 0.1 M pyrrole monomers + 0.1 M Bu4NClO4/CH3CN, at an applied current of 150 lA.

70

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

to note that applying high anodic current during the galvanostatic polymerization led to plugging at pore openings, poor pore filling with PPy and is usually accompanied by oxidative degradation of polymer. Based on this result, the PPy could be electrochemically deposited into PSi matrix under a galvanostatic condition with distinguishable deposition steps; (i) polymer nucleation with a partial silicon oxidation, (ii) polymer deposition inside the pores and (iii) polymer deposition on the top surface.

3.2. Morphological investigation and surface analysis The electrochemically deposited PPy into (200 nm pore size, 4.5 lm layer thickness) was then inspected using FE-SEM. The cross-sectional view FE-SEM micrographs taken at two magnifications are shown in Fig. 2. It is revealed that the porous matrix is filled with PPy polymer. The shape of PPy is a true replica of the starting PSi layer. It is likely that the polymerization of parent monomers started preferentially at the pore tip and propagated to the outer surface as explained in potential transient of Fig. 1. The length of the PPy nanorods electrodeposited in PSi is appreciably uniform and can be easily controlled by simply altering the polymerization time. The PPy rods reached at the top surface began to form over-filled PPy film on the outer surface; the bulk PPy grown at the outer surface could be observed in the FE-SEM image (a). Energy dispersive X-ray spectroscopy (EDS) is measured for the cleaved sample at a two selected area of analysis; inside the porous matrix near to the pore bottom, region I of Fig. 2(a), and at the pore opening and outer surface, region II as labeled in Fig. 2(a). Fig. 2 shows the recorded EDS spectra at the two specified regions. The spectral peaks for Si, C, N, O and Cl are detected within the porous matrix as well as at outer surface. The detection of N indicates the presence of PPy polymer inside the pores. About 6 at.% of N content is measured inside the porous layer, whereas at top surface the

analysis exhibits a higher N content of about 18 at.% as well as a higher carbon content. This is ascribed to the deposition of a thin polymer film on outer surface. Surface oxide formation was also detected. To acquire in-depth information on the surface analysis of the hybrid structure, the X-ray photoelectron spectroscopy (XPS) technique was employed to analyze the specific surface composition of the PSi after filling with PPy. The recorded spectra for both low resolution (survey) and high resolution are shown in Fig. 3. The charge correction was made by setting the binding energy of C1s to 285 eV as an energy reference. The main Si2p, C1s, and O1s peaks are centered at 99, 285, and 532 eV, respectively. As can be observed the surface is oxidized. The Si2p is identified as a doublet at 99 and 103 eV, which are related consequently to the two different oxidation states; Si(0), and Si(4); i.e. corresponding to elemental Si and SiO2. The detection of oxide is consistent with the above EDS analysis. Of particular importance is the detection of N1s peak at around 400 eV, originating from the polymer ring. It is also of interest to detect a Cl2p at around 209 eV arising from the ClO 4 dopant ion. This suggests that the doping of the polymer occurs simultaneously with the film formation. The doping level of polymer with the ClO 4 counter ion can be determined by the Cl/N ratio, giving a value of around 35%. This means that the extra number of electrons consumed in the oxidation process is therefore equal to 0.35. Thus a total number of 2.35 electrons are transferred during the whole polymerization process. This value is in a good agreement with the previously reported value of 2.25 for PPy [48].

3.3. Electrochemical behavior of PSi/PPy The electrochemical behavior of the PSi/PPy hybrid structure in contact with a monomer-free solution of 0.1 M Bu4NClO4/CH3CN was evaluated by means of cyclic voltammetry technique. Cyclic

Fig. 2. Cross-sectional view FE-SEM images of PSi (pore size 200 nm) filled with PPy by the electrochemical polymerization technique; (a) low magnification and (b) same sample at a higher magnification. EDS analysis taken at two different regions within the porous structure (Region I) and at the top surface (Region II) are also shown.

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

71

Fig. 4. Cyclic voltammograms of PPy-electrodeposited PSi electrode, measured in a blank solution of 0.1 M Bu4NClO4/CH3CN. Scan rate 50 mV/s with examined potential range from 4 to 7 V vs. Pt wire.

of surrounding porous matrix, some kinetic limitations in the redox reaction of PPy may arise leading to different voltammetric behavior of the present system. The finding of Fig. 4 reveals also that the stability of the PSi/PPy hybrid structure is gradually declined as the number of electrochemical cycles is repeatedly increased. The redox peaks were found to drift from their original positions with cycling, together with a decrease of recorded current. The drop in the current amplitudes is likely ascribed to structural organization in the polymer [52], which would lead to film deterioration after a few cycles. Additionally, such electrochemical behavior could be related to the formation of an insulating layer of silicon oxides on the PSi electrode, which acts as a blocking layer for charge transfer [53]. 3.4. Electrical properties of PSi/PPy hybrid structure

Fig. 3. XPS analysis of PPy electrodeposited within PSi; (a) Low resolution (survey) spectrum and (b) high resolution spectra.

voltammograms have been measured for five consecutive cycles and the results are shown in Fig. 4. It is found that the hybrid structure could be electrochemically switched between the oxidized (conducting) form and the reduced (insulating) form. The reaction of the PPy usually involves the oxidation of the extended p-system of polymer as it is driven between the oxidized and the reduced state [49], and hence the redox reaction produced a color change in the film from a brownish-black when oxidized to faint yellow in the reduced state. This behavior is an inherent property of the electroactive conducting polymers including PPy [32]. Under this condition, contraction or expansion of the bulk polymer is associated with these redox reactions as ClO 4 counter ions are incorporated or expelled from the polymeric chains. One could also observe that the voltammetric behavior of the hybrid structure is irreversible and not identical with the normal voltammograms obtained for PPy films [49]. It has been reported that the voltammetric characteristics of PPy are depending on film thickness and type of substrate [50], along with the electrolyte type [49]. Additionally, the electrochemical performance of conducting polymer is significantly influenced by the kinetics of doping and de-doping processes of ions [51]. Based on the above and due to the presence

The electrical properties of PSi filled with PPy are measured. Aluminum (Al) contact acting as a cathode was thermally evaporated onto the rear, unpolished side of the bulk silicon. A top thin film of gold (Au) electrode was evaporated on the top polymer layer and serving as an anode. The heterojunction diodes for as prepared PSi and polymer modified PSi have, respectively the following structures: Au/PSi/Si/Al and Au/PPy–PSi/Si/Al. Schematic diagram of the fabricated heterojunctions is depicted in Fig. 5(a). Fig. 5(b) and (c) show the I–V characteristics for the heterostructures measured in the dark and plotted in linear and Log scale. The curve of PSi diode with no polymer deposition (unmodified) is multiplied by 200. A significant increase of the forward current for the structure with PPy deposition is clearly noticed in comparison to the PSi matrix. To understand the behavior of conductivity changes, I–V characteristics of such diodes can be expressed by the thermionic emission theory according to the following equation [54]:

I ¼ I0 ½expðqV  IRs =nkTÞ where I0 denotes the saturation current, q is the elementary charge, V is the dropped bias over the heterojunction, Rs is the series resistance, k is the Boltzmann constant, T is the absolute temperature and n is the ideality factor which equals unity for an ideal heterojunction. The significant improvement in conductivity after conducting polymer deposition can now be attributed to a reduction in series resistance and the ideality factor. As a bulk effect, the enhancement of the electrical conductivity is likely related to the

72

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

Fig. 5. (a) Schematic diagram of Al/PSi/PPy/Au heterojunction. I–V characteristics of the heterojunction plotted on a linear (b) and log scale (c). In both cases, the red squares denote PSi diode with deposited PPy, whereas the blue triangles correspond to PSi without PPy.

The conductivity of as-synthesized hybrid structure was also evaluated by measuring the electrochemical impedance spectroscopy (EIS). Impedance characteristics were measured in the dark at 0.0 V for as-prepared PSi and PSi filled with PPy. The measurement was conducted over frequency range 100 mHz to 100 kHz with a signal amplitude 5 mV. Fig. 6 shows a typical Bode plot for both samples with an inset descripting the a magnified part. After PPy deposition, the electronic resistance decreases compared to PSi sample with no polymer deposition. Enhancement of the electrical properties of the hybrid structure is an indication that the polymer was deposited preferentially inside the pores. The findings of both I–V and EIS measurements (Figs. 5 and 6) are consistent and indicate also that the complete oxidation of PSi layer is not expected during the electrochemical polymerization event as explained previously in Fig. 1.

3.5. Thermal properties and photoluminescence performance

Fig. 6. Bode plot of EIS measured in the dark at 0.0 V vs Pt wire, in 0.1 M Bu4NClO4/ CH3CN for as-prepared PSi and PSi/PPy hybrid structure. Inset shows a magnified part at high frequency. The potential amplitude was 5 mV.

presence of conducting polymer inside the porous matrix, that would form efficient, conducting sites in the PSi structure which led to facilitate the transport of the charge carriers.

Thermal stability of the hybrid structure is of both fundamental and technological importance due to its possible application at elevated temperature. Generally, exposure of conducting polymers to elevated temperature is known to induce changes in the molecular structure. The changes may be related to the interaction between the charged polymer backbone and the counter ion (ClO 4 in this study). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to assess the thermal stability of the polymer and the obtained results are shown in Fig. 7. The TG (blue1 curve) shows a small weight loss of around 1 For interpretation of color in Figs. 4-6, the reader is referred to the web version of this article.

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

Fig. 7. TGA and DSC data measured for PSi/PPy sample at elevated temperature up to 800 °C.

2% below 220 °C. A gradual weight loss of another 18% occurs between 220 and 400 °C. The slight weight loss of 2% may be related to the removal of trapped solvent (CH3CN) from the porous structure, which led to a complete drying of the structure. The second weight loss of about 18% is inferred to be due to the removal of dopant ion (counter ion: ClO 4 ) from the polymer. The dopant elimination is likely accompanied by polymer decomposition. The DSC thermogram (red curve) shows an endothermic transition at about 152 °C (point 1), probably due to the removal of the CH3CN solvent. Between point 2 and 3 (temperature range: 350–480 °C), a broad exothermic transition appears which could be due to an organizational process that is likely accompanied by dopant removal, in agreement with a recent report [55]. During this stage, PPy polymer starts to gradually decompose. The data presented here for both TGA and DSC are found to support each other. The photoluminescence (PL) spectra measurement provide useful information about the charge carrier’s recombination mechanism. The corresponding PL spectra measured for as-formed PSi

73

and PSi filled with conducting PPy are shown in Fig. 8. Both samples were exited at 350 nm excitation wavelength. As can be seen, the unmodified PSi layer exhibits a PL peak in the red region of the visible spectrum at 700 nm (1.77 eV). After the electropolymerization of PPy, the PL intensity was remarkably enhanced (1.5 times larger), together with a red-shift of the spectral peak to 722 nm (1.72 eV). It is worthy to note that the PPy itself does not show a PL peak at around 720 nm [56]. A question arises as to understand the reason of PL enhancement after polymer deposition. The PL is a process in which a substance absorbs photons and then re-radiates photons. Absorption of photons led to excitation to a higher energy state, and then a return to a lower energy state which is usually accompanied by photons emission. Ultimately, the available energy states in the substance under investigation and the allowed transitions between states are determined by quantum mechanics rules. Accordingly, the enhancement of PL intensity observed after PPy deposition may be attributed to the reduction of dangling bonds due to the passivation of surface states in PSi. The electrodeposited PPy likely passivates the nonradiative channels of PSi matrix. As a result, the closely spaced surface states of PSi may act as continuous energy levels, transferring the energy to the conduction band of PPy giving off the emission. Again, the polymerization process was accompanied by the formation of a partial oxide layer (SiO2), which could generate radiative recombination centers. The increase in PL intensity after PPy deposition may be related also to the recombination of photoexcited carriers within the nanocrystallites [57,58]. Furthermore, due to the presence of pores, the polymeric molecules are separated to each other which in turn would decrease the polymer collision and increase the conjugation of molecules [59], which finally would enhance the emission of PPy. As a common finding observed during the measurement of PL of PSi layers, the PL spectral shift of PSi is often detected upon ageing the sample in air due to layer oxidation. The shift direction depends essentially on porosity degrees: lower porosity layers would exhibit a red shift, while higher porosity samples would induce a blue shift [60]. The partial oxidation of PSi layer observed during the polymer deposition and detected by EDS and XPS analysis is proposed to induce the spectral red shift in Fig. 8 and is an indication on a porosity modification of the hybrid structure. Further optimization of the experimental conditions is now underway, including the length and diameter of the polymeric nanorods, along with the creation of efficient electrical contacts and improvement of the interface between PSi and the conducting polymer for possible photovoltaic and sensing applications. 4. Conclusions In summary, we have successfully infiltrated PPy conducting polymer into PSi matrix with pore sizes 200 nm using the electrochemical technique under a galvanostatic condition. The polymerization process exhibits different deposition stages that starting from the pore bottom and propagating towards the pore opening. A novel hybrid structure of PSi/PPy with vertically oriented polymeric nanorods has been achieved. XPS and EDS analysis indicated some silicon oxide formation during the electropolymerization. The as-synthesized hybrid structure exhibits a remarkable increase in both electrical conductivity and photoluminescence spectral emission. We believe that the present hybrid PSi/PPy structure is expected to be beneficial in various current and future technologies including photovoltaic and sensing applications, which needs further investigation. Conflict of interest

Fig. 8. Room temperature photoluminescence (PL) spectra of as-formed PSi (without polymer) and PPy-electrodeposited PSi.

The author declares no conflict of interest.

74

F.A. Harraz / Journal of Electroanalytical Chemistry 729 (2014) 68–74

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

A.G. Cullis, L.T. Canham, P.D. Calcott, J. Appl. Phys. 82 (1997) 909. V. Lehmann, J. Electrochem. Soc. 140 (1993) 2836. E.K. Propst, P.A. Kohl, J. Electrochem. Soc. 141 (1994) 1006. F.A. Harraz, T. Sakka, Y.H. Ogata, Electrochim. Acta 50 (27) (2005) 5340. K. Fukami, F.A. Harraz, T. Yamauchi, T. Sakka, Y.H. Ogata, Electrochem. Commun. 10 (2008) 56. Y.H. Ogata, A. Koyama, F.A. Harraz, M.S. Salem, T. Sakka, Electrochemistry 75 (3) (2007) 270. F.A. Harraz, M.S. Salem, T. Sakka, Y.H. Ogata, Electrochim. Acta 53 (2008) 3734. S. Ottow, V. Lehmann, H. Föll, J. Electrochem. Soc. 143 (1996) 385. K. Kobayashi, F.A. Harraz, S. Izuo, T. Sakka, Y.H. Ogata, Phys. Stat. Sol. (a) 204 (5) (2007) 1321. Y.H. Ogata, J. Sasano, J. Jorne, T. Tsuboi, F.A. Harraz, T. Sakka, Phys. Stat. Sol. (a) 182 (2000) 71. F.A. Harraz, J. Sasano, T. Sakka, Y.H. Ogata, J. Electrochem. Soc. 150 (5) (2003) C277. F.A. Harraz, A.A. Ismail, H. Bouzid, S.A. Al-Sayari, A. Al-Hajry, M.S. Al-Assiri, Appl. Surf. Sci. 307 (2014) 704. T. Tahercio, M Dilhan, E Massone, A Foucaran, A.M. Gue, T. Bretagnon, B. Fraisse, L. Montes, Sensor Actuat A-Phys. 46 (1995) 43. M.S. Salem, M.J. Sailor, F.A. Harraz, T. Sakka, Y.H. Ogata, J. Appl. Phys. 100 (2006) 083520. M.S. Salem, M.J. Sailor, F.A. Harraz, T. Sakka, Y.H. Ogata, Phys. Stat. Sol. (c) 4 (6) (2007) 2073. M. Li, M. Hu, P. Zeng, S. Ma, W. Yan, X. Qin, Electrochem. Acta 108 (2013) 167. F.A. Harraz, A.A. Ismail, H. Bouzid, S.A. Al-Sayari, A. Al-Hajry, M.S. Al-Assiri, Int. J. Electrochem. Sci. 9 (2014) 2149. F.A. Harraz, Sensor Actuat. B-Chem. 202 (2014) 897. C. Mazzdeni, L. Pavesi, Appl. Phys. Lett. 67 (1995) 2983. L.T. Canham, C.L. Reeves, A. Loni, M.R. Houlton, J.P. Newey, A.J. Simons, T.I. Cox, Thin Solid Films 297 (1–2) (1997) 304. W. Lang, P. Steiner, A. Richter, K. Marusczyk, G. Weimann, H. Sandmaier, Sensor Actuat A-Phys. 43 (1994) 239. P.C. Searson, Appl. Phys. Lett. 59 (1991) 832. F.A. Harraz, K. Kamada, J. Sasano, S. Izuo, T. Sakka, Y.H. Ogata, Phys. Stat. Sol. (a) 202 (8) (2005) 1683. F.A. Harraz, S.M. El-Sheikh, T. Sakka, Y.H. Ogata, Electrochim. Acta 53 (2008) 6444. F.A. Harraz, Phys. Stat. Sol. (c) 8 (6) (2011) 1883. A. Ramizy, W.J. Aziz, Z. Hassan, K. Omar, K. Ibrahim, Optik 122 (2011) 2075. B.R. Mohamed, H. Anouar, B. Brahim, Sol. Energy 86 (2012) 1411. S. Sa´nchez de la Morena, G. Recio-Sa´nchez, V. Torres-Costa, R.J. Martı´n-Palma, Scripta Mater. 74 (2014) 33. M. Atyaoui, W. Dimassi, A. Atyaoui, J. Elyagoubi, R. Ouertani, H. Ezzaouia, J. Lumin. 141 (2013) 1.

[30] F.A. Harraz, T. Sakka, Y.H. Ogata, Electrochim. Acta 46 (2001) 2805. [31] S.P. Duttagupta, X.L. Chen, S.A. Jenekhe, P.M. Fauchet, Solid State Commun. 101 (1997) 33. [32] F.A. Harraz, J. Electrochem. Soc. 153 (2006) C349. [33] S.T. Navale, A.T. Mane, M.A. Chougule, R.D. Sakhare, S.R. Nalage, V.B. Patil, Synthetic Met. 189 (2014) 94. [34] J.-S. Do, S.-H. Wang, Sensor Actuat B-Chem. 185 (2013) 39. [35] R. Temmer, I. Must, F. Kaasik, A. Aabloo, T. Tamm, Sensor Actuat B-Chem. 166– 167 (2012) 411. [36] D.O. Flamini, M. Saugo, S.B. Saidman, Corros. Sci. 81 (2014) 36. [37] B. He, Q. Tang, J. Luo, Q. Li, X. Chen, H. Cai, J. Power Sources 256 (2014) 170. [38] M. Sookhakian, Y.M. Amin, S. Baradaran, M.T. Tajabadi, A. Moradi, Thin Solid Films 552 (2014) 204. [39] P.G. -Romero, Adv. Mater. 13 (2001) 163. [40] C. Sanchez, G.S. -Illia, F. Ribot, T. Lalot, C.R. Mayer, V. Cabuil, Chem. Mater. 13 (2001) 3061. [41] W.U. Huynh, j.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [42] E. Arici, H. Hoppe, F. Schäffler, D. Meissner, M.A. Malik, N.S. Sariciftci, Thin Solid Films 451–452 (2004) 612. [43] C.Y. Kwong, A.B. Djurišic´, P.C. Chui, K.W. Cheng, W.K. Chan, Chem. Phys. Lett. 384 (2004) 372. [44] F.A. Harraz, A.M. Salem, B.A. Mohamed, A. Kandil, I.A. Ibrahim, Appl. Surf. Sci. 264 (2013) 391. [45] J.W. Schultze, K.G. Jung, Electrochim. Acta 40 (1995) 1369. [46] N. Errien, G. Froyer, G. Louarn, P. Retho, Synth. Met. 150 (2005) 255. [47] F.A. Harraz, A.M. Salem, Scripta Mater. 68 (2013) 683. [48] A.F. Diaz, J.I. Castillo, J. Chem. Soc., Chem. Commun. 9 (1980) 397. [49] A.F. Diaz, J.I. Castillo, J.A. Logan, W.-Y. Lee, J. Electroanal. Chem. 129 (1981) 115. [50] J.H. Chen, Z.P. Huang, D.Z. Wang, S.X. Yang, W.Z. Li, J.G. Wen, Z.F. Ren, Synth. Met. 125 (2001) 289. [51] P. Novak, K. Muller, K.V.S. Santhanam, O. Hass, Chem. Rev. 97 (1997) 207. [52] T.F. Otero, M.J. Ariza, J Phys. Chem. B 107 (2003) 13954. [53] C.M. Intelmann, V. Syritski, D. Tsankov, K. Hinrichs, J. Rappich, Electrochim. Acta 53 (2008) 4046. [54] S.M Sze, Modern Semiconductor Device Physics, John Wiley & Sons. Inc, 1998. p. 87. [55] M. Ak, G.K. Çılgı, F.D. Kuru, H. Cetisßli, J. Therm. Anal. Calorim. 111 (2013) 1627. [56] P. Galárˇ, B. Dzurnˇák, P. Maly´, J. Cˇermák, A. Kromka, M. Omastová, B. Rezek, Int. J. Electrochem. Sci. 8 (2013) 57. [57] R. Boukherroub, D.D.M. Wayner, D.J. Lockwood, Appl. Phys. Lett. 81 (2002) 601. [58] R. Boukherroub, D.D.M. Wayner, G.I. Sproule, D.J. Lockwood, L.T. Canham, Nano Lett. 1 (2001) 713. [59] D. Li, D. Yang, C. Zhou, D. Que, Mater. Sci. Eng., B 121 (2005) 229. [60] X.B. Bao, X. He, T. Gao, F. Yan, H.L. Chen, Solid State Commun. 109 (1999) 169.