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Organic-inorganic hybrid nanomaterials for advanced light dependent resistors
Materials Chemistry and Physics 202 (2017) 169e176
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Organic-inorganic hybrid nanomaterials for advanced light dependent resistors Ishpal Rawal a, *, Neeraj Dwivedi b, Ravi Kant Tripathi c, O.S. Panwar d, Hitendra K. Malik e a
Department of Physics, Kirori Mal College, University of Delhi, Delhi 110007, India Department of Electrical and Computer Engineering, National University of Singapore, 117583 Singapore c Polymorphic Carbon Thin Films Group, Physics of Energy Harvesting Division, CSIR-National, Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India d Department of Physics, Semiconductor-Large Area and Flexible Electronics Laboratory, BML Munjal University, Gurgaon 122413, India e Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Development of Organic/Inorganic hybrid Nanomaterials for Light Dependent Resistors. Designing of Light Dependent Resistors (LDRs) for white/visible light. To study the role of ZnO NPs incorporation on the LDR device performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2017 Received in revised form 24 July 2017 Accepted 15 September 2017 Available online 17 September 2017
We report the photoconduction behaviour of hybrid organic (polypyrrole) - inorganic (ZnO) nanostructures, synthesized by the surfactant mediated solution based chemical method at room temperature, for the energy saving light dependent resistor (LDR) applications. The formation of organicinorganic hybrid nanostructures is confirmed by the combination of high resolution transmission electron microscopy and scanning electron microscopy, which revealed creation of cubical shaped wurtzite ZnO nanoparticles with different preferential orientations and transformation of clip-like nanofibres form of pure polypyrrole (PPy) into worm-like nanoribbons having embedded ZnO nanoparticles in it. Given p-type nature of PPy and n-type nature of ZnO, the embedment of ZnO nanoparticles in PPy is expected to create several localized discrete p-n junctions, which is why Raman spectroscopic results reveal the change in molecular structure of PPy due to its nanoscale interaction with ZnO. The photoresponse of PPy is found to continuously enhance from ~4.5 to 10.7% with increase in ZnO concentration from 0 to 10% at the constant illumination intensity of 100 mW/cm2. The improved photoresponse can be attributed to the formation of localized p-n junctions (p-type PPy/n-type ZnO) in the structure. The
Keywords: Polypyrrole ZnO Photoconductivity PPy/ZnO nanohybrids Light dependent resistor
* Corresponding author. E-mail address: [email protected] (I. Rawal). http://dx.doi.org/10.1016/j.matchemphys.2017.09.026 0254-0584/© 2017 Elsevier B.V. All rights reserved.
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photoresponse of PPy and its nanocomposites also increases with increase in illumination intensity from 30 to 100 mW/cm2, which is attributed to the increased number of photo-generated electron-hole pairs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nanoscale hybridization of organic conjugated polymer and inorganic metal oxide semiconductor has received considerable recognition in the field of nanodevice fabrication. Owing to their synergistic effect, these hybrid nanomaterials can open a platform for the development of high performance electronic devices such as transistors, light emitting diodes, gas sensor and solar cells. The efficiency of these devices can significantly depend on the shape and size of the nanomaterials due to increase in surface to volume ratio at nanoscale [1e8]. Today, the generation of renewable and clean energy, like solar energy, is the point of concern to sort out the problems of energy crises and environmental pollution, because the conventional energy sources like coal, petroleum are going to exhaust and their burning generates environment pollutant at high level, resulting into serious health concerns. Apart from the generation of clean and renewable energy, energy saving is also equally important. Unawareness of the energy saving strategies and high cost of automatic switches, which is mainly due to the use of sophisticated and expensive techniques for the fabrication of light dependent resistors (LDRs), seem to be the main reasons for electrical energy wastage. Furthermore, the precursor materials used for LDRs are highly toxic, leaving long term hazardous environmental effects [9]. Thus, the search of new and efficient low cost environment friendly materials for light dependent resistors always remains at the centre of attention. With the advent of micro/nanomaterials formation techniques, a plethora of nanostructured metal oxides (ZnO, TiO2, CdO) and metal sulphides (ZnS, CdS, etc.) are exclusively investigated for photoconductors, nanophotodetectors, photoconducting light switches, ultra violet light sensor, field emission and other photonic applications [10e15]. Out of these inorganic materials, ZnO nanostructures have received considerable attention due to their high thermal stability, large optical band gap, high mechanical strength and high resistance against oxidation in callous environments. The optical band gap of ZnO lies in ultra-violet region that makes it suitable for UV- photodetectors [14,15]. On the other hand, polypyrrole (PPy) is recognized as one of the most stable polymeric systems with wide range of applications in gas sensors, super capacitors, batteries, EMI shielding, etc., because of its tuneable optical band gap [16e19]. Although few reports [19e21]are available on PPy and its nanocomposites for photodetectors, most of them were restricted to UV-visible sensors. There is a dearth of data on white light detectors and light dependent resistors for visible light. Moreover, there is lack of data on the parameters like responsivity, detectivity, photocurrent enhancement factor and trap depth, etc., associated with the performance of a photodetector, which are also the centres of attention of the present study. Owing to combined effect and the nanoscale interaction, the organic-inorganic (PPyZnO) nanocomposites can show promising light dependent response in the white light region of the spectrum with significant enhancement in nanodevice parameters responsivity, detectivity and photocurrent enhancement factor. These facts have motivated us to carry out the present work. Thus, to investigate the effect of nanoscale hybridization on the performance of white light detectors, nanocomposites of PPy and ZnO were prepared by solution based chemistry method in different morphological forms, towards their non-toxic and cost effective light dependent resistor applications. The nanoscale interaction of ZnO with PPy significantly
alters the surface morphology of PPy and resulting in augment photoresponse characteristics. 2. Experimental details 2.1. Synthesis of ZnO nanoparticles and PPy/ZnO nanocomposites Cubical shaped ZnO nanoparticles were prepared by the surfactant assisted hydrothermal process, the detailed synthetic approach has been described elsewhere [22]. In a typical synthesis process, salt solutions of Zn(NO3)2$6H2O, capping agent (cetyltrimethylammonium bromide, CTAB), reducing agent (NaOH) were prepared separately in double distilled water. Then these solutions were mixed together under constant stirring and refluxing at a temperature of 90 C for 2 h in the presence of 1 M ammonia, added to increase the pH value (~9) of the solution. After the completion of the reaction, the solution was allowed to naturally cool down at room temperature, and ZnO nanoparticles were separated out, washed and dried overnight at 60 C. 2.2. Synthesis of PPy/ZnO nanocomposites The detailed synthesis process for pure polypyrrole (Z0) and its nanocomposites with 5 wt % (Z1) and 10 (Z2) wt. % of ZnO nanoparticles has been reported elsewhere [23]. In a typical synthesis process, the salt solutions of the capping agent (CTAB) and oxidizing agent (K2S2O8) were separately prepared in double distilled water containing 1 M HCl solution and a desired quantity of ZnO nanoparticles (5 wt% or 10 wt %). Then these solutions were well homogenized under constant magnetic stirring. After ensuring the homogenous mixing of above solutions, a desired quantity of double distilled pyrrole monomer was added to it and the reaction was carried out for 12 h. Finally reaction was terminated by adding sufficient amount of methanol and the polypyrrole powder was then separated out, washed and dried at 60 C for overnight. 2.3. Sample characterization The morphological analysis of ZnO nanoparticles, PPy and PPy/ ZnO nanocomposites was performed using high resolution transmission electron microscopy (HRTEM,FEI, Tecnai G2 F30- STWIN) and scanning electron microscope (SEM Leo Electron Microscopeemodel LEO 440). Structural analysis of PPy/ZnO nanocomposite was carried out using Raman spectroscopy at Renishaw InVia Reflex micro Raman spectrometer, where air cooled argon laser of wavelength ~514.5 nm is used as an excitation source at a power of 10 mW. The room temperature dc conductivity measurements were carried out using Keithley 2410 SMU on all the samples in sandwich structure (Au/PPy/Au), where gold electrode of 3 mm were deposited on both sides of the pellets of the samples via thermal evaporation technique. The photoconductivity measurements were performed using Keithley 2410 SMU on the cubical shaped pellets of same sizes (~1 10 5 mm3) of the samples in parallel electrode geometry at room temperature and at different illumination intensities. Before performing the experiment, parallel gold electrodes (electrode gap is ~ 2 mm) were deposited by thermal evaporation techniques under high vacuum (~1.2 105 torr) condition. To avoid the heating effect in these samples, a spacing of 25 cm was always
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maintained between the light source and the sample. Such effects were further reduced by performing the experiments in pulses of 30s for on/off the light. Moreover, the temperature of the sample was continuously observed with the help of Eurotherm’ 3216 temperature controller and no significant heating effects were observed in the samples. Schematic of the photoconductivity measurements is shown in Fig. 1.
3. Results and discussion 3.1. Structural and morphological studies Fig. 2(a)-2(c) show the SEM micrograph of pure PPy sample (Z0), and PPy with 5 wt % ZnO (sample Z1) and 10 wt % ZnO nanoparticles (sample Z2). The incorporation of cubical shaped ZnO nanoparticles (inset of Fig. 2(a)) of size about 45 nm in PPy matrix changed the surface morphology of PPy from clip-like nanofibres (Z0) to worm-like nanoribbons (Z1 and Z2). A detailed mechanism of formation of such polypyrrole nanostructures under surfactant mediated synthesis approach is reported by us [23] and other workers [24,25]. The PPy/ZnO nanoribbons were formed via nucleation of PPy over the ZnO crystals and side by side overlapping of PPy nanofibers embedded with ZnO nanoparticles, which can be clearly seen in the high resolution SEM image of sample Z2 (Fig. 2(d)). Fig. 3 (a) displays the Raman spectra of PPy, PPy/ZnO nanocomposites and pure ZnO nanoparticles (inset of Fig. 3 (a)). The Raman spectra of PPy/ZnO nanocomposites comprise all the fundamental bands of pure PPy, however, no significant features of ZnO are observed in the spectra. However, the application of ZnO nanoparticles significantly shifts the peak positions of PPy. The band associated with C¼C stretching vibrations is found to down shift from ~1590 cm1 in pure PPy to ~1580 cm1 in PPy/ZnO. In the case of highly oxidized PPy samples, C¼C stretching vibration band generally appears toward higher wavenumber (~1592 cm1). However, in lower oxidized state of PPy, this band generally appears toward lower wavenumber (~1560 cm1). In our case, the downward shift of C¼C vibration peak is attributed to the nanoscale interaction of ZnO with PPy which results in the reduction of PPy chains or the transformation of highly oxidized to lower oxidized state of PPy. Moreover, with increase in ZnO concentration, the band associated with the polaronic states of C-H out of plane deformations shifts from~1050 to ~1043 cm1 and the polaronic band of ring deformation shifts from ~970 to 980 cm1 [23,26e29]. These shifting in polaronic bands further suggest the decrease in the oxidation level of PPy via the interaction of ZnO and PPy. However, the peak positions of various Raman bands also depend on the
Fig. 1. Schematic representation of photoconductivity measurements with varying illumination intensities.
Fig. 2. (a) Scanning electron micrographs of pure PPy(a), PPy/ZnO nanocomposite samples with ZnO concentration of (b) 5 wt% and (c) 10 wt% and (d) high resolution SEM micrograph of PPy/ZnO nanocomposite with 10 wt% ZnO and inset show the HRTEM image of wurtzite ZnO nanoparticles.
conjugation length and may also be responsible factor for such changes. The inset of Fig. 3(a) shows Raman spectrum of pure ZnO that possesses all the fundamental optical bands of ZnO at ~586, 524, 437, 385, 330, and 98 cm1, which correspond to E1 (LO), E1 (TO), A1 (LO), A1 (TO), E2 (high)eE2 (low), and E2 (low) modes, respectively [22,29,30]. The occurrence of highly augmented band at ~437 cm1, associated with the IR active non-polar optical phonon E2 (high) mode of wurtzite ZnO crystals, suggests the growth of good quality ZnO crystallites [31]. The band observed at ~98 cm1, associated with E2 (low) mode, can be attributed to the defects which are induced during the synthesis or the vibrational band of heavy Zn sub-lattice. Besides these bands, the bands observed around 974 and 1050 cm1 are associated with the carbon content (carboxylate, etc.) present in ZnO sample [22,30,32]. Next, the optical property (optical band gap) of the prepared samples was examined. Fig. 3(b) shows the Tauc plot (obtained by UV-Vis spectroscopy) of pure PPy (Z0) and PPy/ZnO nanocomposite with 10 wt% of ZnO (Z2). The optical band gaps of samples were estimated using the Tauc's relation [31,33e37], a ¼ A(hn - Eg)n, where, hn is the photon energy and Eg is the band gap in eV, A is the constant of proportionality and a is the absorbance coefficient. The power index n ¼ 1/2 is used for direct band gap. The optical band gaps are found to be 2.10 and 2.41 eV for the pure PPy (Z0) and PPy/ ZnO nanocomposite (Z2) samples, respectively. The band gap of PPy is found to increase with the increase in ZnO concentration which is attributed to the strong interaction of higher band gap (ZnO) material with relatively lower band gap (PPy) material. A similar change in optical band gap of PPy with 15 wt% doping of ZnO nanowires has been observed by Jain et al. [38] due to Synergistic effect of ZnO. The band gap of pure PPy and its nanocomposites with ZnO lie in the visible region of the radiation spectrum, which can be useful for white light detectors application. The room temperature (at 298 K) electrical conductivity of the pure PPy nanofibers measured by two probe method in sandwich structure is found to be 5.32 102 S/cm. The electrical conductivity of PPy is found to decrease with 5 wt% and 10 wt% doping of ZnO nanoparticles (NPs) and achieved to 7.61 104 and 1.15 104 S/cm values, respectively. The decrease in conductivity value in doped samples can be attributed to the strong interaction between the highly conducing PPy and lower conducting/insulating ZnO NPs. The change in electrical conductivity, optical band gap and the position Raman bands with ZnO doping are well
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Fig. 3. (a) Raman spectra of pure PPy and PPy/ZnO nanocomposite samples Z1 and Z2 and inset shows Raman spectrum of cubical shaped ZnO nanoparticles, (b) UV-Vis. spectra of pure PPy(Z0) and PPy/ZnO2 nanocomposite (Z2).
corroborated with each other and confirm the strong interaction between organic and inorganic nanomaterials. 3.2. Photoconduction behaviour of PPy/ZnO nanomaterials The photoconducting properties of the prepared PPy and its nanocomposites with ZnO were investigated in the white light region of the radiation spectrum for their possible applications in energy saving LDRs. The electrical resistance of the samples was measured in dark as well as with the illumination intensity of ~100 mW/cm2 at room temperature. It is observed that the electrical resistance of all the samples decreases on the illumination of white light. The recorded change in the electrical resistance of the samples is converted into the photo response by the formula,
Response ¼
Rd R [ x100 Rd
(1)
where, Rd and R[ are the resistances before and after illumination of the light. Fig. 4(a-c) show the photo response of samples Z0, Z1 and Z2. The photoresponse of PPy continuously increases from ~4.5 to 6.8% and then to 10.7% with the increase in ZnO concentration from 0 to 5% and then to 10%, respectively. In a surfactant directed synthesis of PPy/ZnO nanocomposites, ZnO nanoparticles are trapped in the cages of surfactant micelles over which the PPy chains are grown. The coupling of these organic-inorganic semiconductors might results in the formation of large number of localized p-n junctions. The formation of such local p-n junction between organic-inorganic hybrid materials is well established and attributed to the improve properties of the prepared nanocomposites [31,38,39]. Thus, the enhanced photoresponse of PPy/ZnO nanocomposites may be attributed to the formation of localized p-n junctions at p-type PPy and n-type ZnO interfaces. The photoconductive response of a material depends on two parameters: (1) generation of sufficient number of electron-hole pairs by the interaction of light photon with the materials and (2) transport of photo-generated charge carriers to the electrodes without significant loss/recombination at defect states in between the electrodes. The numbers of photo-generated carriers are responsible for photocurrent n(r) and follow exponential decay function while moving towards the electrodes. If r is the distance between the electrode and illumination centre, and L is the diffusion length, then n(r) ¼ n0e-r/L
(2)
where, n0 is the number of electron-hole pair generated on the
illumination of the sample. Thus, n0 is the key parameter in improving the photocurrent generation. When white light photons fall on the junctions between the PPy and ZnO, the electron-hole pairs are generated. The photogenerated electron-hole pairs are separated out by the junction field before their recombination that leads to the generation of photocurrent. The rises and falls of the response curves can help in determining the dynamics of photoconduction in the prepared samples via abrupt switching on and off light at room temperature. The rise and fall in the photoresponse of the material were determined by the characteristics of traps and recombination centres present inside the sample. A slower rise in the photoresponse of pure PPy sample can be attributed to the presence of large number of recombination centres, trap levels and defect states in the band gap region [12,19,40,41]. As soon as the light is switched off; the decay in the photoresponse is started due to the suppression of light induced charge carriers. To describe the transient behaviour of the photocurrent in disordered systems in which transfer of energy takes place between disordered structures, Kohlrausch stretched exponential function is generally used. The Kohlrausch function is more convenient and flexible fitting function due to its stretching parameters that make it distinct from the conventional classical exponential decay. The decay in photocurrent of the prepared samples after turning off the light can be given by the Kohlrausch function expressed as g
Ip ¼ Id þ Aeðt=tÞ
(3)
Here, Ip and Id are the photo and dark currents, A is a constant, t is decay time constant and g (0
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Fig. 4. The photoconductive responses of the (a) pure PPy, Z0 and PPy/ZnO nanocomposites with (b) 5 wt% of ZnO nanoparticles, Z1 and (c) 10 wt% of ZnO nanoparticles Z2 at the illumination intensity of 100 mW/cm2, (d) fitted decay curves for the samples Z0, Z1 and Z2.
Table 1 Fitted parameters of the photoresponse curves. Sample Name Z0 Z1 Z2
A 0.0088 0.0016 0.0067
Τ 1.3513 1.6212 1.7446
G 0.1376 0.1141 0.1634
be attributed to the dominance of bulk phenomena over the surface processes [20]. It is observed that the decay time constant of the PPy/ZnO nanocomposites increases with the ZnO concentration (Table - 1), suggesting the slower recovery of the nanocomposite samples. The mechanism of slower recovery in nanocomposite samples can be understood as follows. There are large numbers of discrete p-n junctions formed in PPy/ZnO nanocomposites. On switching off the light, the p-n junctions take some time to establish stable equilibrium of dark through the junction potential. The presence of the p-n junctions provides physical severance of electrons and holes that leads to the significant enhancement in the life time of the photo carriers. This causes the slower recovery and significant persistent photoconductivity in the case of nanocomposite samples (Z1 and Z2) [13,37]. Fig. 5(a-c) show the variation in photoresponses of these samples as a function of illumination intensity from 30 to 100 mW/cm2. The photoresponse of the prepared samples is found to enhance with the increasing ZnO concentration as well as the illumination intensity. It is observed from Fig. 5(d) that the photoresponse of all the samples increases almost linearly with the illumination intensity. The increase in illumination intensity results in an increase in the number of incident photon, which causes generation of enhanced electron-hole pairs and hence, an improved photocurrent. The recombination of the photo-generated electron-hole pairs
Responsivity (mA/W)
Detectivity (cmHz1/2/W) 10
1.58 10 2.05 1010 2.78 1010
0.1805 0.225 0.2634
Trap depth (eV) 0.61 0.62 0.66
is prevented by the built in field of p-n junctions. This explains why PPy/ZnO nanocomposites show enhanced photoresponse. The behaviour of photodetectors can be further analyzed in terms of the responsivity (R) which can be defined as the ratio of photocurrent density (JP) to power density (Pd) or illumination intensity [12,13,40,41].
R¼
Jp Pd
(4)
Fig. 6(a) shows the responsivity curves as a function of time for all the samples where the illumination intensity is kept constant at ~100 mW/cm2. The responsivity of the prepared samples is found to increase from ~0.1805 to 0.2634 mA/W with the increase in ZnO concentration from 0 to 10 wt % (Table 1). The detectivity (D) of light detector is another important parameter, which defines the characteristic behaviour of the photodetectors and is given by Refs. [37,40].
R D ¼ pffiffiffiffiffiffiffiffiffiffiffiffi 2q Jd
(5)
Here q is the electronic charge and Jd is the dark current density. Fig. 7(b) shows the detectivity curves for the samples Z0, Z1 and Z2 which is evaluated using the expression (3) from the
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Fig. 5. The photoconductive responses of the (a) pure PPy, Z0 and PPy/ZnO nanocomposites with (b) 5 wt% of ZnO nanoparticles, Z1 and (c) 10 wt% of ZnO nanoparticles Z2 at different illumination intensities varying from 30 to 100 mW/cm2, (d) variation of photoconductive response for samples Z0, Z1 and Z2 with illumination intensities.
Fig. 6. Behaviour of (a) responsivity and (b) detectivity of the samples Z0, Z1 and Z2 at the fixed illumination intensity of ~100 mW/cm2, and behaviour of (c) responsivity and (d) detectivity of the samples Z0, Z1 and Z2 at different illumination intensities varying from 30 to 100 mW/cm2.
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Fig. 7. Behaviour of (a) responsivity and (b) detectivity of the samples Z0, Z1 and Z2 at different illumination intensities.
photoresponse recorded at an illumination intensity of ~100 mW/ cm2. The detectivity values for pure PPy (Z0) and its nanocomposite samples Z1 and Z2 are found to be ~1.58 1010, 2.05 1010 and 2.78 1010 cmHz1/2/W, respectively. The detectivity of the PPy sample increases with the increase in ZnO concentration, which can be attributed to the synergistic effect of ZnO and formation of p-n junctions between PPy and ZnO nanoparticles. The observed values of the responsivity and detectivity are in agreement with the reported values for different inorganic nanomaterials [37,40,41]. The responsivity and detectivity of the prepared samples were measured at different illumination intensities from 30 to 100 mW/ cm2 (Fig. 6(c) and (d)). It is observed that both the responsivity and detectivity are increased with the increase in illumination intensity for all the samples. In addition, both the responsivity and detectivity have almost linear variation with the illumination intensity for all the samples, as shown in Fig. 7(a) and (b). The linearity of responsivity and detectivity is the resultant of linear variation of the photocurrent with illumination intensity. The photocurrent enhancement factor (Ep) has also been calculated using the relation [40].
Ep ¼
IZ[d IP[d
(6)
where, IZ[d is the ratio of the photocurrent to dark current for nanocomposite samples (PPy/ZnO), whereas IP[d is the ratio of photocurrent to dark current for the pure PPy sample. The value of photocurrent enhancement factor is found to increase from 1.367 (Z1) to 2.906 (Z2) with the increase in ZnO concentration from 5 to 10 wt % [37,40e42]. 3.3. Determination of trap depth The photoconductive behaviour of the prepared samples can be strongly dependent on the concentration of trap centre and the trap depth or activation energy. In semiconducting materials, there can be two types of trap centers. One of that is available above the Fermi level or just below the conduction band, which remains empty in the equilibrium state and another one is below the Fermi level or just above the valence band, which are completely filled with electrons in equilibrium state. These intermediate localized electron energy states between the valence band and conduction band generally arise due to some lattice irregularities or structural disorder or via the doping of impurities in the host materials. At normal temperature, it may be possible that both types of the traps are filled with free electrons. On the illumination of the sample with radiant energy greater than the trap depth or activation energy, the trapped electrons are given back to the conduction band,
which causes to increase in the charge carrier concentration in the conduction band and hence, an increase in the electric current in the sample. If the energy state of trapped electron is laid E eV below the conduction band, then the electron must absorb at least energy equal to E eV to escape from the trap. By the analogy of a ball in a hole under the influence of gravitation field, E is called the trap depth that can be defined as the amount of energy required to eliminate an electron from the trap. This can be evaluated from the decay curves using the expression [12,13].
0
1 B ln II0 C B C C lnf ln E ¼ kT B B t C @ A
(7)
where, T is the temperature, k is the Boltzmann constant and f is the frequency factor, which can be defined as the product of the number of attempts per second that the quanta from lattice vibration (phonon) made to eject the electrons from the traps and the transition probability of escaped electrons in the conduction band [43]. If we consider the trap as a potential box, then it can be expressed as the product of the frequency with which the electron strikes with the walls of box and the reflection coefficient. It has a value of ~109 Hz at room temperature. The values of the trap depths are found to be 0.61, 0.62, 0.66 eV for the samples Z0, Z1 and Z2, respectively, and the observed values of trap depth are found to be comparable to the reported values for TiO2 and its nanocomposites with Mn [13]. The transient current (I) at any instant of decay time can be given by as [12,13].
I ¼ I0 ept
(8)
where, I0 is the saturated value of the photocurrent or the current at which the light is switched off and p is the probability of ejection of an electron from the trap of depth E at temperature T and has a form of
p ¼ feE=kT
(9)
Thus, by using the frequency factor (f) and trap depth (E) in the expression (7), the probability of ejection of electron for all the samples has been calculated and found to be 0.055, 0.050 and 0.009 for the samples Z0, Z1 and Z2, respectively. It has been observed that the probability of the electron ejection decreases, whereas the trap depth increases with the ZnO doping concentration, indicating the presence of defect level in the deep of band gap region of PPy. However, it is interesting to note that despite greater trap depth observed in PPy/ZnO composites than pure PPy the better
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photoresponse is observed in PPy/ZnO composites. This suggests that favourable interaction of ZnO with PPy overcome the greater energy barrier of electrons in PPy/ZnO composites, thus enables the formation of organic-inorganic hybrid materials-based advanced light dependent resistors. 4. Conclusions Cubical shaped ZnO nanoparticles of sizes ~40e50 nm were synthesized by the surfactant assisted hydrothermal process, whereas polypyrrole and its nanocomposites with ZnO nanoparticles were prepared by the chemical oxidation method. The introduction of ZnO nanoparticles in PPy is found to significantly tailor the morphological and structural properties. The dark conductivity of the prepared nanocomposites is found to decrease with the increase in ZnO concentration due to nanoscale interfacial interaction between the PPy and ZnO. The strong coupling between the PPy and ZnO alters the electronic band structure of PPy and the formation of localized p-n junctions between them, which influences the photosensitivity of PPy/ZnO nanocomposites, measured at different illumination intensities from 30 to 100 mW/ cm2, owing to synergistic effects. The photoresponse, responsivity and detectivity of the synthesized samples are found to increase with increase in illumination intensities, and at fixed illumination intensity of 100 mW/cm2. Moreover, all these parameters are further improved with increase in ZnO concentration from 0 to 10 wt%. Thus, the ZnO concentration dependent visible light photoresponse is observed in these samples, which may lead to make them the potential candidates for the futuristic non-toxic LDR applications. Acknowledgements Ishpal Rawal is grateful to the Principal, Kirori Mal College, University of Delhi, Delhi-110007 (India) for his support during work. References [1] R. Koenenkamp, R.C. Word, M. Godinez, Nano Lett. 5 (2005) 2005. [2] M.S. Hammer, C. Deibel, J. Pflaum, V. Dyakonov, Org. Electron 11 (2010) 1569. [3] S.D. Oosterhout, M.M. Wienk, S.S.N. Bavel, R. Thiedmann, L.J.A. Koster, J. Gilot, J. Loos, V. Schmidt, R.A.J. Janssen, Nat. Mater 8 (2009) 818. [4] T. Marimuthu, M.R. Mahmoudian, S. Mohamad, Y. Alias, Sens. Actuat B202
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Organic-inorganic hybrid nanomaterials for advanced light dependent resistors Ishpal Rawal a, *, Neeraj Dwivedi b, Ravi Kant Tripathi c, O.S. Panwar d, Hitendra K. Malik e a
Department of Physics, Kirori Mal College, University of Delhi, Delhi 110007, India Department of Electrical and Computer Engineering, National University of Singapore, 117583 Singapore c Polymorphic Carbon Thin Films Group, Physics of Energy Harvesting Division, CSIR-National, Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India d Department of Physics, Semiconductor-Large Area and Flexible Electronics Laboratory, BML Munjal University, Gurgaon 122413, India e Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Development of Organic/Inorganic hybrid Nanomaterials for Light Dependent Resistors. Designing of Light Dependent Resistors (LDRs) for white/visible light. To study the role of ZnO NPs incorporation on the LDR device performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2017 Received in revised form 24 July 2017 Accepted 15 September 2017 Available online 17 September 2017
We report the photoconduction behaviour of hybrid organic (polypyrrole) - inorganic (ZnO) nanostructures, synthesized by the surfactant mediated solution based chemical method at room temperature, for the energy saving light dependent resistor (LDR) applications. The formation of organicinorganic hybrid nanostructures is confirmed by the combination of high resolution transmission electron microscopy and scanning electron microscopy, which revealed creation of cubical shaped wurtzite ZnO nanoparticles with different preferential orientations and transformation of clip-like nanofibres form of pure polypyrrole (PPy) into worm-like nanoribbons having embedded ZnO nanoparticles in it. Given p-type nature of PPy and n-type nature of ZnO, the embedment of ZnO nanoparticles in PPy is expected to create several localized discrete p-n junctions, which is why Raman spectroscopic results reveal the change in molecular structure of PPy due to its nanoscale interaction with ZnO. The photoresponse of PPy is found to continuously enhance from ~4.5 to 10.7% with increase in ZnO concentration from 0 to 10% at the constant illumination intensity of 100 mW/cm2. The improved photoresponse can be attributed to the formation of localized p-n junctions (p-type PPy/n-type ZnO) in the structure. The
Keywords: Polypyrrole ZnO Photoconductivity PPy/ZnO nanohybrids Light dependent resistor
* Corresponding author. E-mail address: [email protected] (I. Rawal). http://dx.doi.org/10.1016/j.matchemphys.2017.09.026 0254-0584/© 2017 Elsevier B.V. All rights reserved.
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photoresponse of PPy and its nanocomposites also increases with increase in illumination intensity from 30 to 100 mW/cm2, which is attributed to the increased number of photo-generated electron-hole pairs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nanoscale hybridization of organic conjugated polymer and inorganic metal oxide semiconductor has received considerable recognition in the field of nanodevice fabrication. Owing to their synergistic effect, these hybrid nanomaterials can open a platform for the development of high performance electronic devices such as transistors, light emitting diodes, gas sensor and solar cells. The efficiency of these devices can significantly depend on the shape and size of the nanomaterials due to increase in surface to volume ratio at nanoscale [1e8]. Today, the generation of renewable and clean energy, like solar energy, is the point of concern to sort out the problems of energy crises and environmental pollution, because the conventional energy sources like coal, petroleum are going to exhaust and their burning generates environment pollutant at high level, resulting into serious health concerns. Apart from the generation of clean and renewable energy, energy saving is also equally important. Unawareness of the energy saving strategies and high cost of automatic switches, which is mainly due to the use of sophisticated and expensive techniques for the fabrication of light dependent resistors (LDRs), seem to be the main reasons for electrical energy wastage. Furthermore, the precursor materials used for LDRs are highly toxic, leaving long term hazardous environmental effects [9]. Thus, the search of new and efficient low cost environment friendly materials for light dependent resistors always remains at the centre of attention. With the advent of micro/nanomaterials formation techniques, a plethora of nanostructured metal oxides (ZnO, TiO2, CdO) and metal sulphides (ZnS, CdS, etc.) are exclusively investigated for photoconductors, nanophotodetectors, photoconducting light switches, ultra violet light sensor, field emission and other photonic applications [10e15]. Out of these inorganic materials, ZnO nanostructures have received considerable attention due to their high thermal stability, large optical band gap, high mechanical strength and high resistance against oxidation in callous environments. The optical band gap of ZnO lies in ultra-violet region that makes it suitable for UV- photodetectors [14,15]. On the other hand, polypyrrole (PPy) is recognized as one of the most stable polymeric systems with wide range of applications in gas sensors, super capacitors, batteries, EMI shielding, etc., because of its tuneable optical band gap [16e19]. Although few reports [19e21]are available on PPy and its nanocomposites for photodetectors, most of them were restricted to UV-visible sensors. There is a dearth of data on white light detectors and light dependent resistors for visible light. Moreover, there is lack of data on the parameters like responsivity, detectivity, photocurrent enhancement factor and trap depth, etc., associated with the performance of a photodetector, which are also the centres of attention of the present study. Owing to combined effect and the nanoscale interaction, the organic-inorganic (PPyZnO) nanocomposites can show promising light dependent response in the white light region of the spectrum with significant enhancement in nanodevice parameters responsivity, detectivity and photocurrent enhancement factor. These facts have motivated us to carry out the present work. Thus, to investigate the effect of nanoscale hybridization on the performance of white light detectors, nanocomposites of PPy and ZnO were prepared by solution based chemistry method in different morphological forms, towards their non-toxic and cost effective light dependent resistor applications. The nanoscale interaction of ZnO with PPy significantly
alters the surface morphology of PPy and resulting in augment photoresponse characteristics. 2. Experimental details 2.1. Synthesis of ZnO nanoparticles and PPy/ZnO nanocomposites Cubical shaped ZnO nanoparticles were prepared by the surfactant assisted hydrothermal process, the detailed synthetic approach has been described elsewhere [22]. In a typical synthesis process, salt solutions of Zn(NO3)2$6H2O, capping agent (cetyltrimethylammonium bromide, CTAB), reducing agent (NaOH) were prepared separately in double distilled water. Then these solutions were mixed together under constant stirring and refluxing at a temperature of 90 C for 2 h in the presence of 1 M ammonia, added to increase the pH value (~9) of the solution. After the completion of the reaction, the solution was allowed to naturally cool down at room temperature, and ZnO nanoparticles were separated out, washed and dried overnight at 60 C. 2.2. Synthesis of PPy/ZnO nanocomposites The detailed synthesis process for pure polypyrrole (Z0) and its nanocomposites with 5 wt % (Z1) and 10 (Z2) wt. % of ZnO nanoparticles has been reported elsewhere [23]. In a typical synthesis process, the salt solutions of the capping agent (CTAB) and oxidizing agent (K2S2O8) were separately prepared in double distilled water containing 1 M HCl solution and a desired quantity of ZnO nanoparticles (5 wt% or 10 wt %). Then these solutions were well homogenized under constant magnetic stirring. After ensuring the homogenous mixing of above solutions, a desired quantity of double distilled pyrrole monomer was added to it and the reaction was carried out for 12 h. Finally reaction was terminated by adding sufficient amount of methanol and the polypyrrole powder was then separated out, washed and dried at 60 C for overnight. 2.3. Sample characterization The morphological analysis of ZnO nanoparticles, PPy and PPy/ ZnO nanocomposites was performed using high resolution transmission electron microscopy (HRTEM,FEI, Tecnai G2 F30- STWIN) and scanning electron microscope (SEM Leo Electron Microscopeemodel LEO 440). Structural analysis of PPy/ZnO nanocomposite was carried out using Raman spectroscopy at Renishaw InVia Reflex micro Raman spectrometer, where air cooled argon laser of wavelength ~514.5 nm is used as an excitation source at a power of 10 mW. The room temperature dc conductivity measurements were carried out using Keithley 2410 SMU on all the samples in sandwich structure (Au/PPy/Au), where gold electrode of 3 mm were deposited on both sides of the pellets of the samples via thermal evaporation technique. The photoconductivity measurements were performed using Keithley 2410 SMU on the cubical shaped pellets of same sizes (~1 10 5 mm3) of the samples in parallel electrode geometry at room temperature and at different illumination intensities. Before performing the experiment, parallel gold electrodes (electrode gap is ~ 2 mm) were deposited by thermal evaporation techniques under high vacuum (~1.2 105 torr) condition. To avoid the heating effect in these samples, a spacing of 25 cm was always
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maintained between the light source and the sample. Such effects were further reduced by performing the experiments in pulses of 30s for on/off the light. Moreover, the temperature of the sample was continuously observed with the help of Eurotherm’ 3216 temperature controller and no significant heating effects were observed in the samples. Schematic of the photoconductivity measurements is shown in Fig. 1.
3. Results and discussion 3.1. Structural and morphological studies Fig. 2(a)-2(c) show the SEM micrograph of pure PPy sample (Z0), and PPy with 5 wt % ZnO (sample Z1) and 10 wt % ZnO nanoparticles (sample Z2). The incorporation of cubical shaped ZnO nanoparticles (inset of Fig. 2(a)) of size about 45 nm in PPy matrix changed the surface morphology of PPy from clip-like nanofibres (Z0) to worm-like nanoribbons (Z1 and Z2). A detailed mechanism of formation of such polypyrrole nanostructures under surfactant mediated synthesis approach is reported by us [23] and other workers [24,25]. The PPy/ZnO nanoribbons were formed via nucleation of PPy over the ZnO crystals and side by side overlapping of PPy nanofibers embedded with ZnO nanoparticles, which can be clearly seen in the high resolution SEM image of sample Z2 (Fig. 2(d)). Fig. 3 (a) displays the Raman spectra of PPy, PPy/ZnO nanocomposites and pure ZnO nanoparticles (inset of Fig. 3 (a)). The Raman spectra of PPy/ZnO nanocomposites comprise all the fundamental bands of pure PPy, however, no significant features of ZnO are observed in the spectra. However, the application of ZnO nanoparticles significantly shifts the peak positions of PPy. The band associated with C¼C stretching vibrations is found to down shift from ~1590 cm1 in pure PPy to ~1580 cm1 in PPy/ZnO. In the case of highly oxidized PPy samples, C¼C stretching vibration band generally appears toward higher wavenumber (~1592 cm1). However, in lower oxidized state of PPy, this band generally appears toward lower wavenumber (~1560 cm1). In our case, the downward shift of C¼C vibration peak is attributed to the nanoscale interaction of ZnO with PPy which results in the reduction of PPy chains or the transformation of highly oxidized to lower oxidized state of PPy. Moreover, with increase in ZnO concentration, the band associated with the polaronic states of C-H out of plane deformations shifts from~1050 to ~1043 cm1 and the polaronic band of ring deformation shifts from ~970 to 980 cm1 [23,26e29]. These shifting in polaronic bands further suggest the decrease in the oxidation level of PPy via the interaction of ZnO and PPy. However, the peak positions of various Raman bands also depend on the
Fig. 1. Schematic representation of photoconductivity measurements with varying illumination intensities.
Fig. 2. (a) Scanning electron micrographs of pure PPy(a), PPy/ZnO nanocomposite samples with ZnO concentration of (b) 5 wt% and (c) 10 wt% and (d) high resolution SEM micrograph of PPy/ZnO nanocomposite with 10 wt% ZnO and inset show the HRTEM image of wurtzite ZnO nanoparticles.
conjugation length and may also be responsible factor for such changes. The inset of Fig. 3(a) shows Raman spectrum of pure ZnO that possesses all the fundamental optical bands of ZnO at ~586, 524, 437, 385, 330, and 98 cm1, which correspond to E1 (LO), E1 (TO), A1 (LO), A1 (TO), E2 (high)eE2 (low), and E2 (low) modes, respectively [22,29,30]. The occurrence of highly augmented band at ~437 cm1, associated with the IR active non-polar optical phonon E2 (high) mode of wurtzite ZnO crystals, suggests the growth of good quality ZnO crystallites [31]. The band observed at ~98 cm1, associated with E2 (low) mode, can be attributed to the defects which are induced during the synthesis or the vibrational band of heavy Zn sub-lattice. Besides these bands, the bands observed around 974 and 1050 cm1 are associated with the carbon content (carboxylate, etc.) present in ZnO sample [22,30,32]. Next, the optical property (optical band gap) of the prepared samples was examined. Fig. 3(b) shows the Tauc plot (obtained by UV-Vis spectroscopy) of pure PPy (Z0) and PPy/ZnO nanocomposite with 10 wt% of ZnO (Z2). The optical band gaps of samples were estimated using the Tauc's relation [31,33e37], a ¼ A(hn - Eg)n, where, hn is the photon energy and Eg is the band gap in eV, A is the constant of proportionality and a is the absorbance coefficient. The power index n ¼ 1/2 is used for direct band gap. The optical band gaps are found to be 2.10 and 2.41 eV for the pure PPy (Z0) and PPy/ ZnO nanocomposite (Z2) samples, respectively. The band gap of PPy is found to increase with the increase in ZnO concentration which is attributed to the strong interaction of higher band gap (ZnO) material with relatively lower band gap (PPy) material. A similar change in optical band gap of PPy with 15 wt% doping of ZnO nanowires has been observed by Jain et al. [38] due to Synergistic effect of ZnO. The band gap of pure PPy and its nanocomposites with ZnO lie in the visible region of the radiation spectrum, which can be useful for white light detectors application. The room temperature (at 298 K) electrical conductivity of the pure PPy nanofibers measured by two probe method in sandwich structure is found to be 5.32 102 S/cm. The electrical conductivity of PPy is found to decrease with 5 wt% and 10 wt% doping of ZnO nanoparticles (NPs) and achieved to 7.61 104 and 1.15 104 S/cm values, respectively. The decrease in conductivity value in doped samples can be attributed to the strong interaction between the highly conducing PPy and lower conducting/insulating ZnO NPs. The change in electrical conductivity, optical band gap and the position Raman bands with ZnO doping are well
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Fig. 3. (a) Raman spectra of pure PPy and PPy/ZnO nanocomposite samples Z1 and Z2 and inset shows Raman spectrum of cubical shaped ZnO nanoparticles, (b) UV-Vis. spectra of pure PPy(Z0) and PPy/ZnO2 nanocomposite (Z2).
corroborated with each other and confirm the strong interaction between organic and inorganic nanomaterials. 3.2. Photoconduction behaviour of PPy/ZnO nanomaterials The photoconducting properties of the prepared PPy and its nanocomposites with ZnO were investigated in the white light region of the radiation spectrum for their possible applications in energy saving LDRs. The electrical resistance of the samples was measured in dark as well as with the illumination intensity of ~100 mW/cm2 at room temperature. It is observed that the electrical resistance of all the samples decreases on the illumination of white light. The recorded change in the electrical resistance of the samples is converted into the photo response by the formula,
Response ¼
Rd R [ x100 Rd
(1)
where, Rd and R[ are the resistances before and after illumination of the light. Fig. 4(a-c) show the photo response of samples Z0, Z1 and Z2. The photoresponse of PPy continuously increases from ~4.5 to 6.8% and then to 10.7% with the increase in ZnO concentration from 0 to 5% and then to 10%, respectively. In a surfactant directed synthesis of PPy/ZnO nanocomposites, ZnO nanoparticles are trapped in the cages of surfactant micelles over which the PPy chains are grown. The coupling of these organic-inorganic semiconductors might results in the formation of large number of localized p-n junctions. The formation of such local p-n junction between organic-inorganic hybrid materials is well established and attributed to the improve properties of the prepared nanocomposites [31,38,39]. Thus, the enhanced photoresponse of PPy/ZnO nanocomposites may be attributed to the formation of localized p-n junctions at p-type PPy and n-type ZnO interfaces. The photoconductive response of a material depends on two parameters: (1) generation of sufficient number of electron-hole pairs by the interaction of light photon with the materials and (2) transport of photo-generated charge carriers to the electrodes without significant loss/recombination at defect states in between the electrodes. The numbers of photo-generated carriers are responsible for photocurrent n(r) and follow exponential decay function while moving towards the electrodes. If r is the distance between the electrode and illumination centre, and L is the diffusion length, then n(r) ¼ n0e-r/L
(2)
where, n0 is the number of electron-hole pair generated on the
illumination of the sample. Thus, n0 is the key parameter in improving the photocurrent generation. When white light photons fall on the junctions between the PPy and ZnO, the electron-hole pairs are generated. The photogenerated electron-hole pairs are separated out by the junction field before their recombination that leads to the generation of photocurrent. The rises and falls of the response curves can help in determining the dynamics of photoconduction in the prepared samples via abrupt switching on and off light at room temperature. The rise and fall in the photoresponse of the material were determined by the characteristics of traps and recombination centres present inside the sample. A slower rise in the photoresponse of pure PPy sample can be attributed to the presence of large number of recombination centres, trap levels and defect states in the band gap region [12,19,40,41]. As soon as the light is switched off; the decay in the photoresponse is started due to the suppression of light induced charge carriers. To describe the transient behaviour of the photocurrent in disordered systems in which transfer of energy takes place between disordered structures, Kohlrausch stretched exponential function is generally used. The Kohlrausch function is more convenient and flexible fitting function due to its stretching parameters that make it distinct from the conventional classical exponential decay. The decay in photocurrent of the prepared samples after turning off the light can be given by the Kohlrausch function expressed as g
Ip ¼ Id þ Aeðt=tÞ
(3)
Here, Ip and Id are the photo and dark currents, A is a constant, t is decay time constant and g (0
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Fig. 4. The photoconductive responses of the (a) pure PPy, Z0 and PPy/ZnO nanocomposites with (b) 5 wt% of ZnO nanoparticles, Z1 and (c) 10 wt% of ZnO nanoparticles Z2 at the illumination intensity of 100 mW/cm2, (d) fitted decay curves for the samples Z0, Z1 and Z2.
Table 1 Fitted parameters of the photoresponse curves. Sample Name Z0 Z1 Z2
A 0.0088 0.0016 0.0067
Τ 1.3513 1.6212 1.7446
G 0.1376 0.1141 0.1634
be attributed to the dominance of bulk phenomena over the surface processes [20]. It is observed that the decay time constant of the PPy/ZnO nanocomposites increases with the ZnO concentration (Table - 1), suggesting the slower recovery of the nanocomposite samples. The mechanism of slower recovery in nanocomposite samples can be understood as follows. There are large numbers of discrete p-n junctions formed in PPy/ZnO nanocomposites. On switching off the light, the p-n junctions take some time to establish stable equilibrium of dark through the junction potential. The presence of the p-n junctions provides physical severance of electrons and holes that leads to the significant enhancement in the life time of the photo carriers. This causes the slower recovery and significant persistent photoconductivity in the case of nanocomposite samples (Z1 and Z2) [13,37]. Fig. 5(a-c) show the variation in photoresponses of these samples as a function of illumination intensity from 30 to 100 mW/cm2. The photoresponse of the prepared samples is found to enhance with the increasing ZnO concentration as well as the illumination intensity. It is observed from Fig. 5(d) that the photoresponse of all the samples increases almost linearly with the illumination intensity. The increase in illumination intensity results in an increase in the number of incident photon, which causes generation of enhanced electron-hole pairs and hence, an improved photocurrent. The recombination of the photo-generated electron-hole pairs
Responsivity (mA/W)
Detectivity (cmHz1/2/W) 10
1.58 10 2.05 1010 2.78 1010
0.1805 0.225 0.2634
Trap depth (eV) 0.61 0.62 0.66
is prevented by the built in field of p-n junctions. This explains why PPy/ZnO nanocomposites show enhanced photoresponse. The behaviour of photodetectors can be further analyzed in terms of the responsivity (R) which can be defined as the ratio of photocurrent density (JP) to power density (Pd) or illumination intensity [12,13,40,41].
R¼
Jp Pd
(4)
Fig. 6(a) shows the responsivity curves as a function of time for all the samples where the illumination intensity is kept constant at ~100 mW/cm2. The responsivity of the prepared samples is found to increase from ~0.1805 to 0.2634 mA/W with the increase in ZnO concentration from 0 to 10 wt % (Table 1). The detectivity (D) of light detector is another important parameter, which defines the characteristic behaviour of the photodetectors and is given by Refs. [37,40].
R D ¼ pffiffiffiffiffiffiffiffiffiffiffiffi 2q Jd
(5)
Here q is the electronic charge and Jd is the dark current density. Fig. 7(b) shows the detectivity curves for the samples Z0, Z1 and Z2 which is evaluated using the expression (3) from the
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Fig. 5. The photoconductive responses of the (a) pure PPy, Z0 and PPy/ZnO nanocomposites with (b) 5 wt% of ZnO nanoparticles, Z1 and (c) 10 wt% of ZnO nanoparticles Z2 at different illumination intensities varying from 30 to 100 mW/cm2, (d) variation of photoconductive response for samples Z0, Z1 and Z2 with illumination intensities.
Fig. 6. Behaviour of (a) responsivity and (b) detectivity of the samples Z0, Z1 and Z2 at the fixed illumination intensity of ~100 mW/cm2, and behaviour of (c) responsivity and (d) detectivity of the samples Z0, Z1 and Z2 at different illumination intensities varying from 30 to 100 mW/cm2.
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Fig. 7. Behaviour of (a) responsivity and (b) detectivity of the samples Z0, Z1 and Z2 at different illumination intensities.
photoresponse recorded at an illumination intensity of ~100 mW/ cm2. The detectivity values for pure PPy (Z0) and its nanocomposite samples Z1 and Z2 are found to be ~1.58 1010, 2.05 1010 and 2.78 1010 cmHz1/2/W, respectively. The detectivity of the PPy sample increases with the increase in ZnO concentration, which can be attributed to the synergistic effect of ZnO and formation of p-n junctions between PPy and ZnO nanoparticles. The observed values of the responsivity and detectivity are in agreement with the reported values for different inorganic nanomaterials [37,40,41]. The responsivity and detectivity of the prepared samples were measured at different illumination intensities from 30 to 100 mW/ cm2 (Fig. 6(c) and (d)). It is observed that both the responsivity and detectivity are increased with the increase in illumination intensity for all the samples. In addition, both the responsivity and detectivity have almost linear variation with the illumination intensity for all the samples, as shown in Fig. 7(a) and (b). The linearity of responsivity and detectivity is the resultant of linear variation of the photocurrent with illumination intensity. The photocurrent enhancement factor (Ep) has also been calculated using the relation [40].
Ep ¼
IZ[d IP[d
(6)
where, IZ[d is the ratio of the photocurrent to dark current for nanocomposite samples (PPy/ZnO), whereas IP[d is the ratio of photocurrent to dark current for the pure PPy sample. The value of photocurrent enhancement factor is found to increase from 1.367 (Z1) to 2.906 (Z2) with the increase in ZnO concentration from 5 to 10 wt % [37,40e42]. 3.3. Determination of trap depth The photoconductive behaviour of the prepared samples can be strongly dependent on the concentration of trap centre and the trap depth or activation energy. In semiconducting materials, there can be two types of trap centers. One of that is available above the Fermi level or just below the conduction band, which remains empty in the equilibrium state and another one is below the Fermi level or just above the valence band, which are completely filled with electrons in equilibrium state. These intermediate localized electron energy states between the valence band and conduction band generally arise due to some lattice irregularities or structural disorder or via the doping of impurities in the host materials. At normal temperature, it may be possible that both types of the traps are filled with free electrons. On the illumination of the sample with radiant energy greater than the trap depth or activation energy, the trapped electrons are given back to the conduction band,
which causes to increase in the charge carrier concentration in the conduction band and hence, an increase in the electric current in the sample. If the energy state of trapped electron is laid E eV below the conduction band, then the electron must absorb at least energy equal to E eV to escape from the trap. By the analogy of a ball in a hole under the influence of gravitation field, E is called the trap depth that can be defined as the amount of energy required to eliminate an electron from the trap. This can be evaluated from the decay curves using the expression [12,13].
0
1 B ln II0 C B C C lnf ln E ¼ kT B B t C @ A
(7)
where, T is the temperature, k is the Boltzmann constant and f is the frequency factor, which can be defined as the product of the number of attempts per second that the quanta from lattice vibration (phonon) made to eject the electrons from the traps and the transition probability of escaped electrons in the conduction band [43]. If we consider the trap as a potential box, then it can be expressed as the product of the frequency with which the electron strikes with the walls of box and the reflection coefficient. It has a value of ~109 Hz at room temperature. The values of the trap depths are found to be 0.61, 0.62, 0.66 eV for the samples Z0, Z1 and Z2, respectively, and the observed values of trap depth are found to be comparable to the reported values for TiO2 and its nanocomposites with Mn [13]. The transient current (I) at any instant of decay time can be given by as [12,13].
I ¼ I0 ept
(8)
where, I0 is the saturated value of the photocurrent or the current at which the light is switched off and p is the probability of ejection of an electron from the trap of depth E at temperature T and has a form of
p ¼ feE=kT
(9)
Thus, by using the frequency factor (f) and trap depth (E) in the expression (7), the probability of ejection of electron for all the samples has been calculated and found to be 0.055, 0.050 and 0.009 for the samples Z0, Z1 and Z2, respectively. It has been observed that the probability of the electron ejection decreases, whereas the trap depth increases with the ZnO doping concentration, indicating the presence of defect level in the deep of band gap region of PPy. However, it is interesting to note that despite greater trap depth observed in PPy/ZnO composites than pure PPy the better
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photoresponse is observed in PPy/ZnO composites. This suggests that favourable interaction of ZnO with PPy overcome the greater energy barrier of electrons in PPy/ZnO composites, thus enables the formation of organic-inorganic hybrid materials-based advanced light dependent resistors. 4. Conclusions Cubical shaped ZnO nanoparticles of sizes ~40e50 nm were synthesized by the surfactant assisted hydrothermal process, whereas polypyrrole and its nanocomposites with ZnO nanoparticles were prepared by the chemical oxidation method. The introduction of ZnO nanoparticles in PPy is found to significantly tailor the morphological and structural properties. The dark conductivity of the prepared nanocomposites is found to decrease with the increase in ZnO concentration due to nanoscale interfacial interaction between the PPy and ZnO. The strong coupling between the PPy and ZnO alters the electronic band structure of PPy and the formation of localized p-n junctions between them, which influences the photosensitivity of PPy/ZnO nanocomposites, measured at different illumination intensities from 30 to 100 mW/ cm2, owing to synergistic effects. The photoresponse, responsivity and detectivity of the synthesized samples are found to increase with increase in illumination intensities, and at fixed illumination intensity of 100 mW/cm2. Moreover, all these parameters are further improved with increase in ZnO concentration from 0 to 10 wt%. Thus, the ZnO concentration dependent visible light photoresponse is observed in these samples, which may lead to make them the potential candidates for the futuristic non-toxic LDR applications. Acknowledgements Ishpal Rawal is grateful to the Principal, Kirori Mal College, University of Delhi, Delhi-110007 (India) for his support during work. References [1] R. Koenenkamp, R.C. Word, M. Godinez, Nano Lett. 5 (2005) 2005. [2] M.S. Hammer, C. Deibel, J. Pflaum, V. Dyakonov, Org. Electron 11 (2010) 1569. [3] S.D. Oosterhout, M.M. Wienk, S.S.N. Bavel, R. Thiedmann, L.J.A. Koster, J. Gilot, J. Loos, V. Schmidt, R.A.J. Janssen, Nat. Mater 8 (2009) 818. [4] T. Marimuthu, M.R. Mahmoudian, S. Mohamad, Y. Alias, Sens. Actuat B202
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