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Characterization of CdS and AgPt nanofillers used in organic capacitors
Synthetic Metals 223 (2017) 26–33
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Characterization of CdS and AgPt nanofillers used in organic capacitors Mohammad Y. Al-Haika,* , Yousef Haikb , Muhammad R. Hajja a b
Department of Biomedical Engineering and Mechanics, Virginia Tech, Blacksburg, VA, 24060, USA Department of Mechanical Engineering, Qatar University, Doha, Qatar
A R T I C L E I N F O
Article history: Received 2 May 2016 Accepted 26 November 2016 Available online xxx Keywords: Organic capacitor Storage device Charge transfer Conductive nanoparticles Conducting polymer
A B S T R A C T
This study compares two charge storage nanoparticles (CdS and AgPt) in organic storage devices. The charge elements are embedded in an active conductive polymeric layer composed of poly vinyl alcohol (PVA) and poly acrylic acid (PAA) doped with glycerol. The size distribution of the synthesized particles was characterized using SEM and Zeta analyzer. Capacitance–voltage measurements of the fabricated storage devices revealed hysteresis loop with large window gates indicating their charge storage capacity. Centimeter-scale micro wind turbine is used to generate charge to be stored in the storage devices and to determine their storage characteristics and time constants. The results showed that the semiconductive charge storage CdS discharges 23% faster than the conductive bimetallic AgPt. © 2016 Published by Elsevier B.V.
1. Introduction The ability to perform nanoparticle syntheses, characterization and assessment has provided the opportunity to utilize them in various electronic applications that include metal-insulatorsemiconductor (MIS) devices, thin film transistors (TFTs), solar panels, self-powered sensors and electric storage devices by applying them as charge storage elements [1–7]. Nanoparticles, as the ones mentioned in this paper, are commonly used in many application ranging from medical to nuclear and solar technology [8] due to their good thermal stability, chemical resistance and high opacity [9]. Both semiconducting and bimetallic nanoparticles are currently being studied for their great electrical properties. Although single metal nanoparticles such as gold nanoparticles, most commonly used, have been reported to have a high effect on electrical characterization in terms of charge storage capabilities [2,3,7,10] when sandwiched in MIS devices. There has been recent interest in the development of high density, high speed, low cost and low power organic memory devices that can be used in electronic applications in many fields [11–13]. Due to the various electrical properties that nanoparticles exhibit, they have recently received increased attention to the size of the window gate of a capacitance-voltage (C-V) hysteresis when
* Corresponding author at: Department of Biomedical Engineering and Mechanics, Virginia Tech, 495 Old Turner Street, Norris Hall, Room 122, Virginia, USA. E-mail address: [email protected] (M.Y. Al-Haik). http://dx.doi.org/10.1016/j.synthmet.2016.11.037 0379-6779/© 2016 Published by Elsevier B.V.
utilized as storage elements. Metallic nanoparticle filler are being investigated due to their strong influence they possess on the electronic charge properties [14]. Researchers utilizing gold nanoparticles as charge elements reported their excellent charging capabilities and the potential to embed gold nanoparticles in various memory applications [15]. Leong et al. [10] and Mabrook et al. [7] fabricated metal-insulator-semiconductor (MIS) memory devices with gold nanoparticles as charge storage elements. Mabrook et al. [7] used three different organic dielectric materials, poly(methyl methacrylate), pentacene and cadmium arachidate, in fabricating MIS structures. Their C-V measurements revealed relatively large window gates which were dependent on the voltage range applied. They also noted the presence of hysteresis loops regardless of the silicon substrate used whereas Leong et al. carried out the fabrication process only on an n-type silicon wafer. Both of the above investigations reported charging and discharging characteristics resembling the MIS structure properties. Patil et al. [16] investigated the electron charge transport properties of gold nanoparticles with synthesized polymeric films. They studied the effect of charging with different coating configuration. The study shows different C-V behaviors when drop casting the polymer was followed by a layer of gold nanoparticles or when gold nanoparticles are coated with the polymer. Their results revealed that having the polymer first provides a slightly larger hysteresis gate and a longer voltage span. The direction of the hysteresis was reported to have an impact depending on which is coated first. The direction of the flat band voltage is proportional to orientation of
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the stacking sequence. They also reported that a negative flat band depicts the holes are trapping the charge and would result in a counter-clockwise direction. Conductive nanofillers in polymeric matrix are being favored for microelectronic application due to their high dielectric constant, high component density, design flexibility [17] and reduced cost. Embedded capacitor applications require composites with high dielectric constant and low dielectric loss. The fabrication of organic storage devices with utilization of inorganic nanoparticles has increased the interest of many researchers in the development and discovery of newly synthesized nanoparticles for various applications in many industrial and biomedical fields [13]. Many synthesized nanoparticles such as gold [7,10], silver [18], silver copper (AgCu) [1], carbon nanotubes (CNTs) [13] and several other elements have been investigated for their charging characteristics in fabrication of electric storage devices. It has been observed that the storage capability of nanoparticles is correlated to the size, composition and the shape [19]. In this effort, we characterize the charge storage capabilities of two different compositions of synthesized nanoparticles and investigate their storage performance when integrated with a microscale energy harvesting device. The effect of the particles conductivity on their storage capabilities is investigated. To our knowledge the two synthesized nanoparticles have not been reported as storage element nor has their capacitive function been investigated. Both particles are thermally and chemically stable and both have high work function, which gives them the potential to be used as charge storage elements. 2. Materials and methods 2.1. Semiconducting polymer process and characterization We have previously reported the preparation of a semiconducting polymer made of a blend of two non-conductive polymers, namely, poly(vinyl) alcohol (PVA) and poly-acrylamide-co-acrylic acid (PAA) with average molecular weights of 61,000 g/mol and 5,000,000 g/mol, respectively, and doped with glycerol [2]. The preparation procedure is discussed briefly. The semiconducting polymer was obtained by separately dissolving five grams of each polymer (PVA and PAA), in beakers filled with 100 mL of deionized water. Each beaker was placed over a hot plate set at 90 C for one hour and was subjected to extreme mixture using a magnetic stirrer. After an hour, both solutions were blended together and the blend was stirred for an additional 20 min. Afterwards, the blend was left to cool until a clear homogenous lubricant type substance was observed, leaving no air bubbles in the mixture. Then, 20 mL of polymer-blended solution was mixed with 2 wt% of glycerol, to obtain the organic semiconducting polymer. The addition of doping glycerol increases a small quantity of impurity atoms classifying the organic polymers as extrinsic semiconductors. We reported [1,2,20] the effect of adding glycerol and how it modulates the resistivity of the polymeric blend. We noted that as the glycerol percentage is increased, the resistivity decreased from 3.28 105 to 4.75 101 Vm. The impedance varies with the inverse of frequency in the range 103–106 Hz, and for frequencies less than 102 Hz, the ac impedance is almost independent of the frequency. For the phase angle varies from 90 to 0 in most cases, which is typical for films that are equivalent to an RC network in parallel [21,22]. Also, the activation energy (Ea) decreased by 67% when glycerol was added. This decrease implies that as the glycerol is added, the resistivity decreases, decreasing the activation energy and as a result, the conductivity increases [2]. This increase in electrical conductivity provides excess holes in p-type semiconductors.
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2.2. Preparation of synthesized nanoparticles The two nanoparticles used in this study were synthesized by chemical co-precipitation technique. For the synthesis of cadmium sulfide (CdS) nanoparticles, 100 mL of 1 mM of cadmium chloride (CdCl2) was added to 100 mL of sodium citrate dihydrate and magnetically stirred for 10 min. To this solution, 100 mL of sodium sulfide nonahydrate was added and stirred for 15 min. The yellow precipitate obtained was washed three times with deionized water and dried at 60 C. For the synthesis of silver-platinum (AgPt) nanoparticles, 25 mL of 1.32 mM solution of potassium tetrachloroplatinate (II) and 25 mL of 1.32 mM silver nitrate were added to poly(vinyl) pyrrolidone (PVP) solution (0.7335 g in 50 mL) at room temperature. The solution was magnetically stirred for 30 min. To this solution, 20 mL of 16.5 mM sodium borohydride (NaBH4) was added within five seconds. The addition of NaBH4 results in the formation of dark brown or black colloidal suspension of Ag-Pt nanoparticles. The solution was magnetically stirred for one hour. The particles were washed three times using a centrifuge and vacuum dried at 40 C for 48 h. 2.3. Organic storage device setup The storage devices consisted of sequential layers of the organic dielectric and novel nanoparticles. Fig. 1 shows a schematic of the layer-by-layer setup of the organic capacitor. The fabrication of the organic capacitor was carried out as followed: Two glass substrates of dimensions 1 cm 1 cm were cleaned by the standard cleaning method inside an ultrasonic cleaner using acetone, ethanol and deionized water, respectively; one after the other for three times, and dried with nitrogen gas after each stage. The first layer, bottom electrode of the device, was formed by thermally depositing aluminum (Al) powder as a thin layer of film onto the glass substrates with a thickness of 100 nm using a thermal evaporation system. An insulating polymer was developed as a separator between the metallic electrodes and particles to avoid a short circuit from occurring during testing. The insulating polymer namely, poly(methyl methacrylate) (PMMA), was prepared by diluting 24 mg of PMMA powder (molecular weight 97,000 g/mol) in 3 mL of chloroform and continuously stirred overnight to ensure a perfect dissolution. Two drops of the PMMA solution were spun
Fig. 1. Layer-by-layer schematic of the device.
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Fig. 2. Images of (a) SEM micrograph for CdS nanoparticles showing tiny clusters over the substrate. (b) Size distribution by volume using Zeta analyzer varying particle size. (c) EDS spectra showing CdS particle’s elemental percentage.
coated on the substrates (on the surface of the bottom Al electrode) initially at 500 rpm for 10 s, then at 5000 rpm for 50 s. The samples were then annealed at 120 C for 20 min. The nanoparticles were then spread over the PMMA layer by spin coating for 90 s at a speed of 5000 rpm then dried at 50 C for 10 min. Two drops of the polymer blend, PVA-PAA-glycerol, was then spun coated at 5000 rpm for two minutes and dried at 80 C for five minutes. Before depositing the top aluminum electrode, Teflon tape was wrapped around the edges of the glass substrate to ensure that no contact occurs between the top and bottom Al electrodes. Once the Teflon is wrapped, the top aluminum electrode was thermally evaporated to a thickness of 100 nm. 3. Results and discussion 3.1. Nanoparticle morphological characterization Scanning electron microscopy (SEM) using an LEO (Zeiss) 1550 field-emission SEM, was performed to investigate the surface morphology and distribution of the nanoparticles. A Malvern Instruments Zeta analyzer was also used to determine the size of the particles. The composition of the nanoparticles was measured using an energy dispersive spectroscopy (EDS) that was coupled with the SEM machine. Compositional contrast that results from different atomic number elements and their distribution were generated from the back-scattering electron images displayed by the SEM. The EDS analyzer provided the identification of what particular elements are contained in the tested sample and their relative proportions. To determine the elemental contents within the nanoparticles, a drop of the solution containing the nanoparticles was spread over a nickel plated substrate and left overnight to dry. The figures of the performed EDS below depict the average elemental percentages for the nanoparticles for the corresponding X-ray spectra that was generated over a localized area. The Y-axis of the spectra shows the number of X-rays, counts, received and processed by the detector and the X-axis shows the energy levels of the X-rays received [23]. For both nanoparticle characterization, SEM, EDS and Zeta analyzer were performed and displayed.
Fig. 2(a) shows the SEM images for cadmium sulfide, CdS, nanoparticles. The image exhibits very small clusters scattered over the substrate. The size distribution by volume is verified by using the Zeta analyzer, which shows an average diameter size approximately 27 nm as depicted in Fig. 2(b). Fig. 2(c) shows the composition of the nanoparticles of the Cd and S elements weight percentage are 81.14% and 18.86%, respectively, with a 2381 ray counts received. Therefore, Cd81.14S18.86 will be used to describe the correct composition of the nanoparticle. Fig. 3(a) shows the SEM images for silver platinum, AgPt, nanoparticles. The images show larger clusters scattered over the substrate. These large clusters contain many smaller particles. The size distribution was verified with the Zeta analyzer, which showed an average diameter size approximately 2.3 nm as depicted in Fig. 3(b). Fig. 3(c) shows the EDS spectra for the AgPt nanoparticles that the composition of the nanoparticles of the Ag and Pt have weight percentages of 56.92% and 43.08%, respectively, with a 3322 ray counts received. Therefore, Ag56.92Pt43.08 will be used to describe the correct composition of the nanoparticle. It was observed that during the SEM testing, the two composition of nanoparticles were noticed to glow, indicating that the nanoparticles were charging. Due to the limitations of EDS, it is only used to determine the actual chemical compounds. The percentage mentioned above is related to the overall detector precision of each nanoparticle synthesized. Because of the glomerations observed in the SEM testing, both nanofillers were utilized in the fabrication of storage devices and tested for their charge storage capabilities. Due to their thermal and chemical stability and their high work function, the nanoparticles tested in this study have the potential to be used as charge storage elements in storage devices. 3.2. C-V measurements Capacitance versus voltage (C-V) sweeps of the storage devices were performed using a Keithley 4200 semiconductor characterization system (SCS) computer controlled parameter analyzer to investigate the storage potential of the nanoparticles. The measurements were performed with a dual voltage sweep in the range of 20 V to +20 V and back to 20 V at a scan rate of 0.5 V/
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Fig. 3. Images of (a) SEM micrograph for AgPt nanoparticles showing tiny clusters over the substrate. (b) Size distribution by volume using Zeta analyzer varying particle size. (c) EDS spectra showing AgPt particle’s elemental percentage.
Fig. 4. C-V Characteristic of: (a) CdS embedded which shows a hysteresis loop of 38 V flat band voltage gate, (b) AgPt embedded which shows hysteresis loop of 30 V flat band voltage gate and (c) reference device with no nanoparticles; no hysteresis loop was observed.
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s. A higher sweep rate will produce a larger hysteresis window because any stored charge would not have had time to leak away, apart from allowing a quicker test. To ensure the devices work properly under ambient conditions, all C-V measurements were performed at room temperature, 298 K. The C-V measurements, plotted in Fig. 4(a) and (b), show the conventional accumulation (capacitor is charged) and inversion (capacitor is discharged) characteristics that are similar to a typical metal-insulatorsemiconductor (MIS) device based on a p-type semiconductor [7]. Fig. 4(c) shows the C-V plot for a reference device that did not contain nanoparticles. It is interesting to note that the C-V hysteresis loops were reproducible and in most cases a maximum voltage of 38 V was obtained for CdS and 30 V for the AgPt when a 20 V sweep was performed. During the charging process, as the voltage is applied to the gate electrode, the nanoparticles are charged by electron transport through the insulating layer of the device [24]. These devices have charge storage elements, also known as trapping layers. As the voltage is applied, electrons begin to flow, so the charge carriers store the charge in the nanoparticle storage layers [25]. Under the band theory of solids, metallic and semiconducting metals have different behaviors. The Bimetallic (AgPt) nanoparticles known for their metallic conductivity are reported [26] to have a higher capacitance value as observed from Fig. 4(b). The band gap in metals is small and thus the electrons can easily take a leap to the conduction band and conduct electricity. Whereas for the semiconducting (CdS) nanoparticle, the band gap is bigger however small enough for a few electrons to jump between the valence band and the conduction band. During the C-V tests, the electrons are heated which provides the electrons with enough energy to move from the lower valence band to the higher conduction band. The wide window gates observed in the hysteresis plots are a clear indication of the charge storage capability of the nanoparticles. This large hysteresis window is mainly linked to the high charge transport mobility of the carriers. Because the reference device with no particles did not show a hysteresis with a window gap, it can be concluded that the majority of the charge is being stored in the nanoparticles and not in the organic conductive polymeric layer [27–29]. It is also noticed that these storage elements revealed an order of magnitude larger capacitance value of the hysteresis loops than that of pervious work where gold nanoparticles were utilized [2]. As the C-V measurements system is connected, induced electric fields [24,30,31] allow the electrons to pass from the top aluminum electrode through the semiconducting polymeric layer [32,33] and into the nanoparticles, eventually storing the charge generated. The C-V plots show counter-clockwise direction of the accumulation and inversion regions of the hysteresis loop. The flat band voltage for the reference device (Fig. 4(c)), has an accumulation capacitance of 4.5pF. Whereas the other devices with embedded nanoparticles show much higher capacitances values. The direction of the hysteresis loop is linked to the flow of electrons passing through the device. As the current flows from the top electrode, the charge passes through the organic conducting polymers which is then stored in the nanoparticles. The accumulation region at the positive voltage of the scan represents the positive charges within the organic polymeric semiconducting layer. As the voltage is applied, electrons are forced to flow through the dielectric material of the capacitor, PMMA, and as a result, charging the nanoparticles. The negative charges attract the holes in the polymeric semiconductor, which leads to the accumulation of holes in the polymer interface, comparable to a p-type semiconductor characteristics [7,34–36], hence, making them extremely suitable for electronic applications.
The large surface to volume ratio of nanoparticles qualifies them for use in charge storage devices. Due to the resistance toward electron transfer in nanoparticles, the conductivity of the nanoparticles is usually lower than the conductivity of the bulk material. The particle size influences the capacitive properties of the nanostructured materials. There are four different known mechanisms that classify hysteresis developed by C-V measurements. We characterize the studied storage devices as carrier injection mechanism because of the observed counter-clockwise loop in both storage devices and the presence of a flat band voltage (VFB). This mechanism usually has a non-uniform hysteresis as observed in Fig. 4. The presence of the flat band voltage also explains the negative shift observed in the C-V measurements. The direction of the hysteresis indicates that the electrons are injected into the nanoparticles if the flat band voltage is negative and are extracted from the nanoparticles if the voltage is positive [37]. From the C-V measurements test, it is evident that the electrons are injected into the nanoparticles from the bottom aluminum electrode, thus charging the nanoparticles. It is also evident that the absence of the hysteresis from the reference device indicates that the charging of the nanoparticles are trapping the electrons. The polymeric semiconducting organic polymer also plays a vital role in the performance of the storage device where the electrons are released. According to the band theory of solids, the electrons or holes of the semiconductive polymer, the polarity of the induced voltage depending on the flow direction causes this counter-clockwise hysteresis loop [16]. The majority of carriers in p-type semiconductors are holes which have the ability to move in the valence band. This explains the movement of the conventional current direction as observed in the C-V plots. The flow of electricity is associated to the transport of mobile charges, referred to as carriers, through the material. In metals, these mobile charges, are usually viewed as electrons. However in semiconductors, they can either be electrons or holes. The charge mobility of electrons varies between various materials, for instance, insulating materials have zero charge mobility whereas metals have the highest charge mobility due to their conductivity content. The organic semiconducting polymer significantly influences the transportation of the charge carriers. As the temperature increases the charge mobility reduces and as such, the conductivity is reduced. This is correlated by the impurity levels and valence electrons being ionized. This implies that, at low temperatures, the extrinsic semiconducting polymers have higher conductivity. The temperature relation is given by: E r ¼ r0 exp a KBT where ro is the pre-exponential factor (constant), KB is the Boltzmann’s constant, T is the absolute temperature and Ea is the activation energy. The linear dependence of Ln(r) on (1000/T) can be used to calculate the activation energy for the doped polymer films. Plotting the natural log of Ln(r) versus (1000/T) produces a straight line. The slope of the line gives the activation energy Ea [2]. The activation energy for this polymer composition was reported to be 0.22 eV. This implies a 67% increase in the activation energy from pure polymer blend without the dopant of glycerol. Increasing the temperature of the doped films increases the carrier concentration inside the film because of the electron transfer from the valance band to the conduction band. This indicates that increasing the temperature of the polymer increases the activation energy thus allowing the electrons to jump freely from band to band. When heating the semiconductive polymer films above room temperature, a second semi-circle of the resistivity Nyquist plot appears, this is due to the ions having
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storage capacity, it is not practical to conclude which nanoparticle would store more charge based on the size of the two particles studied because they have different compositions. It would be practical however if the same particle was studied with different sizes. The layer-by-layer setup of the device fabrication process was sequentially laid out this way due to the fact that the amount of charge stored increases when there is a direct contact between the organic conductive polymeric semiconductor and the nanoparticle charge storage elements [1,12]. 4. Energy harvesting and storage
Fig. 5. Five centimeter propeller mounted on to a CSMWT.
enough energy to diffuse towards the electrodes under the action of low frequency electric field and accumulate there. The accumulated charges on the electrodes resist the motion of new ions which increases the interfacial charge transfer resistivity. As the voltage sweep is reversed, the positive voltage leaves the holes from the polymeric semiconducting interface leading to the depletion and eventually to the inversion of the hysteresis curve. It can be observed from the hysteresis loops that the capacitance values for the CdS and AgPt embedded devices have a capacitance difference of 14 nF and 10 nF, respectively. Although the capacitance for the CdS is larger, the possibility for leakage is higher due to the fact that it is a semiconducting nanoparticle. Therefore, AgPt is anticipated to retain more charge as it is a conducting nanoparticle. This difference in capacitance can be related to the particle size of each element. As the particle size increases, the capacitance increases and thus the storage capability increases. Although, the size of the particles plays a significant role in the
Combining the two fabricated organic storage devices with energy harvesting technology provides limitless energy for autonomous devices. There are numerous methods for energy harvesting that includes piezoelectric actuators, wind turbines and thermoelectric generators. For this effort, we chose a miniature wind turbine that generates relatively high levels of power. We use a five centimeter, four-bladed fan type propeller that was attached to an AC generated centimeter microscale wind turbine (CSMWT) to investigate the performance of an integrated harvesting and storage system and consider the two storage devices described above. Fig. 5 shows the CSMWT with the propeller attached. The micro wind turbine was mounted on to a stand with an airflow speed of approximately 3.5 m/s generated by a larger fan. As the airflow is generated, the CSMWT begins to rotate, converting the mechanical energy of the shaft into electrical charge. The wind turbine was connected to a circuit board allowing the generated charge to flow through a diode and stored in the capacitors. The diode was introduced to allow only the positive voltage through. The optimum resistor of 220 V was reported by Al-Haik et al. [38] and was used in this experiment. Fig. 6 reveals the charging behavior of the two various nanoparticles storing the generated charge. It is observed that the two fabricated organic capacitors show variations of the allowable stored charge. This implies that the composition of the nanoparticles play a vital role in storing the
Fig. 6. Variation of charge storage within each capacitor.
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generated charge. Because AgPt is a conductive nanoparticle, we earlier anticipated that it would store in more charge. The storage characteristics as revealed in Fig. 6 show that both the CdS and AgPt devices store the same maximum however the CdS leaks the charge faster than the AgPt. The results also show that the AgPt embedded capacitor retains the charge slightly more than that of the CdS embedded capacitor. This is because CdS are semiconductive particles whereas AgPt are metallic nanoparticles. This implies that semiconductive nanoparticles have a faster discharge rate. At a certain point of the downward voltage, the capacitors are disconnected from the harvester and begin to discharge due to the diode only allowing the positive voltage to flow. The capacitors were reported to have different time constants and are depicted by the discharge rates as observed in Fig. 6. The time constant reported for each capacitor is 0.0127 s and 0.0103 s for AgPt and CdS respectively. The larger the time constant the longer the capacitor retains the charge. This difference is tolerable to the capacitance value of each capacitor. However, as soon as the harvester reaches the point where the charge is allowed to flow through the diode, the capacitors begin to charge again. The small variations noticed between the capacitor storage and the harvester is attributed to the losses caused by the diode. 5. Conclusion We evaluated two different nanoparticles as charge storage elements that can be embedded within layers of organic polymeric semiconductors and PMMA. Both CdS and AgPt nanoparticles show a potential for use as charge storage elements in storage devices since they are chemically stable and have a high work function. SEM images showed glowing elements depicting the charging of the nanoparticles. The average size of the nanoparticles and their composition were evaluated and reported. The two synthesized nanoparticles were able to show charge storage capabilities. Large hysteresis window gaps were observed when the nanoparticles were deposited indicating storage within the nanoparticles. A CSMWT energy harvester was connected to the capacitors to determine the storage behavior. The time constants of the two organic capacitors revealed that AgPt embedded nanoparticles retains the charge slightly longer than the CdS nanoparticles due to the electrical characteristics, chemical composition and size of the nanoparticles. Due to their low cost and ease of processing, such devices have the potential to be used on a larger scale and in sense, could be implemented as organic capacitors and complemented with an energy harvesting system to power up small sensors or actuators autonomously. Acknowledgment This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] A.I. Ayesh, S. Qadri, V.J. Baboo, M.Y. Al-Haik, Y. Haik, Nano-floating gate organic memory devices utilizing Ag–Cu nanoparticles embedded in PVA-PAAglycerol polymer, Synth. Met. 183 (2013) 24–28. [2] M.Y. Haik, A.I. Ayesh, T. Abdulrehman, Y. Haik, Novel organic memory devices using Au–Pt–Ag nanoparticles as charge storage elements, Mater. Lett. 124 (2014) 67–72. [3] R.R. Nejm, A.I. Ayesh, D.A. Zeze, A. Sleiman, M.F. Mabrook, A. Al-Ghaferi, M. Hussein, Electrical characteristics of hybrid-organic memory devices based on Au nanoparticles, J. Electron. Mater. 44 (2015) 2835–2841. [4] M.R. Perez, I. Mejia, A.L. Salas-Villasenor, H. Stiegler, I. Trachtenberg, B.E. Gnade, M.A. Quevedo-Lopez, Hybrid CMOS thin-film devices based on solution-processed CdS n-TFTs and TIPS-pentacene p-TFTs, Org. Electron. 13 (2012) 3045–3049.
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Characterization of CdS and AgPt nanofillers used in organic capacitors Mohammad Y. Al-Haika,* , Yousef Haikb , Muhammad R. Hajja a b
Department of Biomedical Engineering and Mechanics, Virginia Tech, Blacksburg, VA, 24060, USA Department of Mechanical Engineering, Qatar University, Doha, Qatar
A R T I C L E I N F O
Article history: Received 2 May 2016 Accepted 26 November 2016 Available online xxx Keywords: Organic capacitor Storage device Charge transfer Conductive nanoparticles Conducting polymer
A B S T R A C T
This study compares two charge storage nanoparticles (CdS and AgPt) in organic storage devices. The charge elements are embedded in an active conductive polymeric layer composed of poly vinyl alcohol (PVA) and poly acrylic acid (PAA) doped with glycerol. The size distribution of the synthesized particles was characterized using SEM and Zeta analyzer. Capacitance–voltage measurements of the fabricated storage devices revealed hysteresis loop with large window gates indicating their charge storage capacity. Centimeter-scale micro wind turbine is used to generate charge to be stored in the storage devices and to determine their storage characteristics and time constants. The results showed that the semiconductive charge storage CdS discharges 23% faster than the conductive bimetallic AgPt. © 2016 Published by Elsevier B.V.
1. Introduction The ability to perform nanoparticle syntheses, characterization and assessment has provided the opportunity to utilize them in various electronic applications that include metal-insulatorsemiconductor (MIS) devices, thin film transistors (TFTs), solar panels, self-powered sensors and electric storage devices by applying them as charge storage elements [1–7]. Nanoparticles, as the ones mentioned in this paper, are commonly used in many application ranging from medical to nuclear and solar technology [8] due to their good thermal stability, chemical resistance and high opacity [9]. Both semiconducting and bimetallic nanoparticles are currently being studied for their great electrical properties. Although single metal nanoparticles such as gold nanoparticles, most commonly used, have been reported to have a high effect on electrical characterization in terms of charge storage capabilities [2,3,7,10] when sandwiched in MIS devices. There has been recent interest in the development of high density, high speed, low cost and low power organic memory devices that can be used in electronic applications in many fields [11–13]. Due to the various electrical properties that nanoparticles exhibit, they have recently received increased attention to the size of the window gate of a capacitance-voltage (C-V) hysteresis when
* Corresponding author at: Department of Biomedical Engineering and Mechanics, Virginia Tech, 495 Old Turner Street, Norris Hall, Room 122, Virginia, USA. E-mail address: [email protected] (M.Y. Al-Haik). http://dx.doi.org/10.1016/j.synthmet.2016.11.037 0379-6779/© 2016 Published by Elsevier B.V.
utilized as storage elements. Metallic nanoparticle filler are being investigated due to their strong influence they possess on the electronic charge properties [14]. Researchers utilizing gold nanoparticles as charge elements reported their excellent charging capabilities and the potential to embed gold nanoparticles in various memory applications [15]. Leong et al. [10] and Mabrook et al. [7] fabricated metal-insulator-semiconductor (MIS) memory devices with gold nanoparticles as charge storage elements. Mabrook et al. [7] used three different organic dielectric materials, poly(methyl methacrylate), pentacene and cadmium arachidate, in fabricating MIS structures. Their C-V measurements revealed relatively large window gates which were dependent on the voltage range applied. They also noted the presence of hysteresis loops regardless of the silicon substrate used whereas Leong et al. carried out the fabrication process only on an n-type silicon wafer. Both of the above investigations reported charging and discharging characteristics resembling the MIS structure properties. Patil et al. [16] investigated the electron charge transport properties of gold nanoparticles with synthesized polymeric films. They studied the effect of charging with different coating configuration. The study shows different C-V behaviors when drop casting the polymer was followed by a layer of gold nanoparticles or when gold nanoparticles are coated with the polymer. Their results revealed that having the polymer first provides a slightly larger hysteresis gate and a longer voltage span. The direction of the hysteresis was reported to have an impact depending on which is coated first. The direction of the flat band voltage is proportional to orientation of
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the stacking sequence. They also reported that a negative flat band depicts the holes are trapping the charge and would result in a counter-clockwise direction. Conductive nanofillers in polymeric matrix are being favored for microelectronic application due to their high dielectric constant, high component density, design flexibility [17] and reduced cost. Embedded capacitor applications require composites with high dielectric constant and low dielectric loss. The fabrication of organic storage devices with utilization of inorganic nanoparticles has increased the interest of many researchers in the development and discovery of newly synthesized nanoparticles for various applications in many industrial and biomedical fields [13]. Many synthesized nanoparticles such as gold [7,10], silver [18], silver copper (AgCu) [1], carbon nanotubes (CNTs) [13] and several other elements have been investigated for their charging characteristics in fabrication of electric storage devices. It has been observed that the storage capability of nanoparticles is correlated to the size, composition and the shape [19]. In this effort, we characterize the charge storage capabilities of two different compositions of synthesized nanoparticles and investigate their storage performance when integrated with a microscale energy harvesting device. The effect of the particles conductivity on their storage capabilities is investigated. To our knowledge the two synthesized nanoparticles have not been reported as storage element nor has their capacitive function been investigated. Both particles are thermally and chemically stable and both have high work function, which gives them the potential to be used as charge storage elements. 2. Materials and methods 2.1. Semiconducting polymer process and characterization We have previously reported the preparation of a semiconducting polymer made of a blend of two non-conductive polymers, namely, poly(vinyl) alcohol (PVA) and poly-acrylamide-co-acrylic acid (PAA) with average molecular weights of 61,000 g/mol and 5,000,000 g/mol, respectively, and doped with glycerol [2]. The preparation procedure is discussed briefly. The semiconducting polymer was obtained by separately dissolving five grams of each polymer (PVA and PAA), in beakers filled with 100 mL of deionized water. Each beaker was placed over a hot plate set at 90 C for one hour and was subjected to extreme mixture using a magnetic stirrer. After an hour, both solutions were blended together and the blend was stirred for an additional 20 min. Afterwards, the blend was left to cool until a clear homogenous lubricant type substance was observed, leaving no air bubbles in the mixture. Then, 20 mL of polymer-blended solution was mixed with 2 wt% of glycerol, to obtain the organic semiconducting polymer. The addition of doping glycerol increases a small quantity of impurity atoms classifying the organic polymers as extrinsic semiconductors. We reported [1,2,20] the effect of adding glycerol and how it modulates the resistivity of the polymeric blend. We noted that as the glycerol percentage is increased, the resistivity decreased from 3.28 105 to 4.75 101 Vm. The impedance varies with the inverse of frequency in the range 103–106 Hz, and for frequencies less than 102 Hz, the ac impedance is almost independent of the frequency. For the phase angle varies from 90 to 0 in most cases, which is typical for films that are equivalent to an RC network in parallel [21,22]. Also, the activation energy (Ea) decreased by 67% when glycerol was added. This decrease implies that as the glycerol is added, the resistivity decreases, decreasing the activation energy and as a result, the conductivity increases [2]. This increase in electrical conductivity provides excess holes in p-type semiconductors.
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2.2. Preparation of synthesized nanoparticles The two nanoparticles used in this study were synthesized by chemical co-precipitation technique. For the synthesis of cadmium sulfide (CdS) nanoparticles, 100 mL of 1 mM of cadmium chloride (CdCl2) was added to 100 mL of sodium citrate dihydrate and magnetically stirred for 10 min. To this solution, 100 mL of sodium sulfide nonahydrate was added and stirred for 15 min. The yellow precipitate obtained was washed three times with deionized water and dried at 60 C. For the synthesis of silver-platinum (AgPt) nanoparticles, 25 mL of 1.32 mM solution of potassium tetrachloroplatinate (II) and 25 mL of 1.32 mM silver nitrate were added to poly(vinyl) pyrrolidone (PVP) solution (0.7335 g in 50 mL) at room temperature. The solution was magnetically stirred for 30 min. To this solution, 20 mL of 16.5 mM sodium borohydride (NaBH4) was added within five seconds. The addition of NaBH4 results in the formation of dark brown or black colloidal suspension of Ag-Pt nanoparticles. The solution was magnetically stirred for one hour. The particles were washed three times using a centrifuge and vacuum dried at 40 C for 48 h. 2.3. Organic storage device setup The storage devices consisted of sequential layers of the organic dielectric and novel nanoparticles. Fig. 1 shows a schematic of the layer-by-layer setup of the organic capacitor. The fabrication of the organic capacitor was carried out as followed: Two glass substrates of dimensions 1 cm 1 cm were cleaned by the standard cleaning method inside an ultrasonic cleaner using acetone, ethanol and deionized water, respectively; one after the other for three times, and dried with nitrogen gas after each stage. The first layer, bottom electrode of the device, was formed by thermally depositing aluminum (Al) powder as a thin layer of film onto the glass substrates with a thickness of 100 nm using a thermal evaporation system. An insulating polymer was developed as a separator between the metallic electrodes and particles to avoid a short circuit from occurring during testing. The insulating polymer namely, poly(methyl methacrylate) (PMMA), was prepared by diluting 24 mg of PMMA powder (molecular weight 97,000 g/mol) in 3 mL of chloroform and continuously stirred overnight to ensure a perfect dissolution. Two drops of the PMMA solution were spun
Fig. 1. Layer-by-layer schematic of the device.
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Fig. 2. Images of (a) SEM micrograph for CdS nanoparticles showing tiny clusters over the substrate. (b) Size distribution by volume using Zeta analyzer varying particle size. (c) EDS spectra showing CdS particle’s elemental percentage.
coated on the substrates (on the surface of the bottom Al electrode) initially at 500 rpm for 10 s, then at 5000 rpm for 50 s. The samples were then annealed at 120 C for 20 min. The nanoparticles were then spread over the PMMA layer by spin coating for 90 s at a speed of 5000 rpm then dried at 50 C for 10 min. Two drops of the polymer blend, PVA-PAA-glycerol, was then spun coated at 5000 rpm for two minutes and dried at 80 C for five minutes. Before depositing the top aluminum electrode, Teflon tape was wrapped around the edges of the glass substrate to ensure that no contact occurs between the top and bottom Al electrodes. Once the Teflon is wrapped, the top aluminum electrode was thermally evaporated to a thickness of 100 nm. 3. Results and discussion 3.1. Nanoparticle morphological characterization Scanning electron microscopy (SEM) using an LEO (Zeiss) 1550 field-emission SEM, was performed to investigate the surface morphology and distribution of the nanoparticles. A Malvern Instruments Zeta analyzer was also used to determine the size of the particles. The composition of the nanoparticles was measured using an energy dispersive spectroscopy (EDS) that was coupled with the SEM machine. Compositional contrast that results from different atomic number elements and their distribution were generated from the back-scattering electron images displayed by the SEM. The EDS analyzer provided the identification of what particular elements are contained in the tested sample and their relative proportions. To determine the elemental contents within the nanoparticles, a drop of the solution containing the nanoparticles was spread over a nickel plated substrate and left overnight to dry. The figures of the performed EDS below depict the average elemental percentages for the nanoparticles for the corresponding X-ray spectra that was generated over a localized area. The Y-axis of the spectra shows the number of X-rays, counts, received and processed by the detector and the X-axis shows the energy levels of the X-rays received [23]. For both nanoparticle characterization, SEM, EDS and Zeta analyzer were performed and displayed.
Fig. 2(a) shows the SEM images for cadmium sulfide, CdS, nanoparticles. The image exhibits very small clusters scattered over the substrate. The size distribution by volume is verified by using the Zeta analyzer, which shows an average diameter size approximately 27 nm as depicted in Fig. 2(b). Fig. 2(c) shows the composition of the nanoparticles of the Cd and S elements weight percentage are 81.14% and 18.86%, respectively, with a 2381 ray counts received. Therefore, Cd81.14S18.86 will be used to describe the correct composition of the nanoparticle. Fig. 3(a) shows the SEM images for silver platinum, AgPt, nanoparticles. The images show larger clusters scattered over the substrate. These large clusters contain many smaller particles. The size distribution was verified with the Zeta analyzer, which showed an average diameter size approximately 2.3 nm as depicted in Fig. 3(b). Fig. 3(c) shows the EDS spectra for the AgPt nanoparticles that the composition of the nanoparticles of the Ag and Pt have weight percentages of 56.92% and 43.08%, respectively, with a 3322 ray counts received. Therefore, Ag56.92Pt43.08 will be used to describe the correct composition of the nanoparticle. It was observed that during the SEM testing, the two composition of nanoparticles were noticed to glow, indicating that the nanoparticles were charging. Due to the limitations of EDS, it is only used to determine the actual chemical compounds. The percentage mentioned above is related to the overall detector precision of each nanoparticle synthesized. Because of the glomerations observed in the SEM testing, both nanofillers were utilized in the fabrication of storage devices and tested for their charge storage capabilities. Due to their thermal and chemical stability and their high work function, the nanoparticles tested in this study have the potential to be used as charge storage elements in storage devices. 3.2. C-V measurements Capacitance versus voltage (C-V) sweeps of the storage devices were performed using a Keithley 4200 semiconductor characterization system (SCS) computer controlled parameter analyzer to investigate the storage potential of the nanoparticles. The measurements were performed with a dual voltage sweep in the range of 20 V to +20 V and back to 20 V at a scan rate of 0.5 V/
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Fig. 3. Images of (a) SEM micrograph for AgPt nanoparticles showing tiny clusters over the substrate. (b) Size distribution by volume using Zeta analyzer varying particle size. (c) EDS spectra showing AgPt particle’s elemental percentage.
Fig. 4. C-V Characteristic of: (a) CdS embedded which shows a hysteresis loop of 38 V flat band voltage gate, (b) AgPt embedded which shows hysteresis loop of 30 V flat band voltage gate and (c) reference device with no nanoparticles; no hysteresis loop was observed.
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s. A higher sweep rate will produce a larger hysteresis window because any stored charge would not have had time to leak away, apart from allowing a quicker test. To ensure the devices work properly under ambient conditions, all C-V measurements were performed at room temperature, 298 K. The C-V measurements, plotted in Fig. 4(a) and (b), show the conventional accumulation (capacitor is charged) and inversion (capacitor is discharged) characteristics that are similar to a typical metal-insulatorsemiconductor (MIS) device based on a p-type semiconductor [7]. Fig. 4(c) shows the C-V plot for a reference device that did not contain nanoparticles. It is interesting to note that the C-V hysteresis loops were reproducible and in most cases a maximum voltage of 38 V was obtained for CdS and 30 V for the AgPt when a 20 V sweep was performed. During the charging process, as the voltage is applied to the gate electrode, the nanoparticles are charged by electron transport through the insulating layer of the device [24]. These devices have charge storage elements, also known as trapping layers. As the voltage is applied, electrons begin to flow, so the charge carriers store the charge in the nanoparticle storage layers [25]. Under the band theory of solids, metallic and semiconducting metals have different behaviors. The Bimetallic (AgPt) nanoparticles known for their metallic conductivity are reported [26] to have a higher capacitance value as observed from Fig. 4(b). The band gap in metals is small and thus the electrons can easily take a leap to the conduction band and conduct electricity. Whereas for the semiconducting (CdS) nanoparticle, the band gap is bigger however small enough for a few electrons to jump between the valence band and the conduction band. During the C-V tests, the electrons are heated which provides the electrons with enough energy to move from the lower valence band to the higher conduction band. The wide window gates observed in the hysteresis plots are a clear indication of the charge storage capability of the nanoparticles. This large hysteresis window is mainly linked to the high charge transport mobility of the carriers. Because the reference device with no particles did not show a hysteresis with a window gap, it can be concluded that the majority of the charge is being stored in the nanoparticles and not in the organic conductive polymeric layer [27–29]. It is also noticed that these storage elements revealed an order of magnitude larger capacitance value of the hysteresis loops than that of pervious work where gold nanoparticles were utilized [2]. As the C-V measurements system is connected, induced electric fields [24,30,31] allow the electrons to pass from the top aluminum electrode through the semiconducting polymeric layer [32,33] and into the nanoparticles, eventually storing the charge generated. The C-V plots show counter-clockwise direction of the accumulation and inversion regions of the hysteresis loop. The flat band voltage for the reference device (Fig. 4(c)), has an accumulation capacitance of 4.5pF. Whereas the other devices with embedded nanoparticles show much higher capacitances values. The direction of the hysteresis loop is linked to the flow of electrons passing through the device. As the current flows from the top electrode, the charge passes through the organic conducting polymers which is then stored in the nanoparticles. The accumulation region at the positive voltage of the scan represents the positive charges within the organic polymeric semiconducting layer. As the voltage is applied, electrons are forced to flow through the dielectric material of the capacitor, PMMA, and as a result, charging the nanoparticles. The negative charges attract the holes in the polymeric semiconductor, which leads to the accumulation of holes in the polymer interface, comparable to a p-type semiconductor characteristics [7,34–36], hence, making them extremely suitable for electronic applications.
The large surface to volume ratio of nanoparticles qualifies them for use in charge storage devices. Due to the resistance toward electron transfer in nanoparticles, the conductivity of the nanoparticles is usually lower than the conductivity of the bulk material. The particle size influences the capacitive properties of the nanostructured materials. There are four different known mechanisms that classify hysteresis developed by C-V measurements. We characterize the studied storage devices as carrier injection mechanism because of the observed counter-clockwise loop in both storage devices and the presence of a flat band voltage (VFB). This mechanism usually has a non-uniform hysteresis as observed in Fig. 4. The presence of the flat band voltage also explains the negative shift observed in the C-V measurements. The direction of the hysteresis indicates that the electrons are injected into the nanoparticles if the flat band voltage is negative and are extracted from the nanoparticles if the voltage is positive [37]. From the C-V measurements test, it is evident that the electrons are injected into the nanoparticles from the bottom aluminum electrode, thus charging the nanoparticles. It is also evident that the absence of the hysteresis from the reference device indicates that the charging of the nanoparticles are trapping the electrons. The polymeric semiconducting organic polymer also plays a vital role in the performance of the storage device where the electrons are released. According to the band theory of solids, the electrons or holes of the semiconductive polymer, the polarity of the induced voltage depending on the flow direction causes this counter-clockwise hysteresis loop [16]. The majority of carriers in p-type semiconductors are holes which have the ability to move in the valence band. This explains the movement of the conventional current direction as observed in the C-V plots. The flow of electricity is associated to the transport of mobile charges, referred to as carriers, through the material. In metals, these mobile charges, are usually viewed as electrons. However in semiconductors, they can either be electrons or holes. The charge mobility of electrons varies between various materials, for instance, insulating materials have zero charge mobility whereas metals have the highest charge mobility due to their conductivity content. The organic semiconducting polymer significantly influences the transportation of the charge carriers. As the temperature increases the charge mobility reduces and as such, the conductivity is reduced. This is correlated by the impurity levels and valence electrons being ionized. This implies that, at low temperatures, the extrinsic semiconducting polymers have higher conductivity. The temperature relation is given by: E r ¼ r0 exp a KBT where ro is the pre-exponential factor (constant), KB is the Boltzmann’s constant, T is the absolute temperature and Ea is the activation energy. The linear dependence of Ln(r) on (1000/T) can be used to calculate the activation energy for the doped polymer films. Plotting the natural log of Ln(r) versus (1000/T) produces a straight line. The slope of the line gives the activation energy Ea [2]. The activation energy for this polymer composition was reported to be 0.22 eV. This implies a 67% increase in the activation energy from pure polymer blend without the dopant of glycerol. Increasing the temperature of the doped films increases the carrier concentration inside the film because of the electron transfer from the valance band to the conduction band. This indicates that increasing the temperature of the polymer increases the activation energy thus allowing the electrons to jump freely from band to band. When heating the semiconductive polymer films above room temperature, a second semi-circle of the resistivity Nyquist plot appears, this is due to the ions having
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storage capacity, it is not practical to conclude which nanoparticle would store more charge based on the size of the two particles studied because they have different compositions. It would be practical however if the same particle was studied with different sizes. The layer-by-layer setup of the device fabrication process was sequentially laid out this way due to the fact that the amount of charge stored increases when there is a direct contact between the organic conductive polymeric semiconductor and the nanoparticle charge storage elements [1,12]. 4. Energy harvesting and storage
Fig. 5. Five centimeter propeller mounted on to a CSMWT.
enough energy to diffuse towards the electrodes under the action of low frequency electric field and accumulate there. The accumulated charges on the electrodes resist the motion of new ions which increases the interfacial charge transfer resistivity. As the voltage sweep is reversed, the positive voltage leaves the holes from the polymeric semiconducting interface leading to the depletion and eventually to the inversion of the hysteresis curve. It can be observed from the hysteresis loops that the capacitance values for the CdS and AgPt embedded devices have a capacitance difference of 14 nF and 10 nF, respectively. Although the capacitance for the CdS is larger, the possibility for leakage is higher due to the fact that it is a semiconducting nanoparticle. Therefore, AgPt is anticipated to retain more charge as it is a conducting nanoparticle. This difference in capacitance can be related to the particle size of each element. As the particle size increases, the capacitance increases and thus the storage capability increases. Although, the size of the particles plays a significant role in the
Combining the two fabricated organic storage devices with energy harvesting technology provides limitless energy for autonomous devices. There are numerous methods for energy harvesting that includes piezoelectric actuators, wind turbines and thermoelectric generators. For this effort, we chose a miniature wind turbine that generates relatively high levels of power. We use a five centimeter, four-bladed fan type propeller that was attached to an AC generated centimeter microscale wind turbine (CSMWT) to investigate the performance of an integrated harvesting and storage system and consider the two storage devices described above. Fig. 5 shows the CSMWT with the propeller attached. The micro wind turbine was mounted on to a stand with an airflow speed of approximately 3.5 m/s generated by a larger fan. As the airflow is generated, the CSMWT begins to rotate, converting the mechanical energy of the shaft into electrical charge. The wind turbine was connected to a circuit board allowing the generated charge to flow through a diode and stored in the capacitors. The diode was introduced to allow only the positive voltage through. The optimum resistor of 220 V was reported by Al-Haik et al. [38] and was used in this experiment. Fig. 6 reveals the charging behavior of the two various nanoparticles storing the generated charge. It is observed that the two fabricated organic capacitors show variations of the allowable stored charge. This implies that the composition of the nanoparticles play a vital role in storing the
Fig. 6. Variation of charge storage within each capacitor.
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generated charge. Because AgPt is a conductive nanoparticle, we earlier anticipated that it would store in more charge. The storage characteristics as revealed in Fig. 6 show that both the CdS and AgPt devices store the same maximum however the CdS leaks the charge faster than the AgPt. The results also show that the AgPt embedded capacitor retains the charge slightly more than that of the CdS embedded capacitor. This is because CdS are semiconductive particles whereas AgPt are metallic nanoparticles. This implies that semiconductive nanoparticles have a faster discharge rate. At a certain point of the downward voltage, the capacitors are disconnected from the harvester and begin to discharge due to the diode only allowing the positive voltage to flow. The capacitors were reported to have different time constants and are depicted by the discharge rates as observed in Fig. 6. The time constant reported for each capacitor is 0.0127 s and 0.0103 s for AgPt and CdS respectively. The larger the time constant the longer the capacitor retains the charge. This difference is tolerable to the capacitance value of each capacitor. However, as soon as the harvester reaches the point where the charge is allowed to flow through the diode, the capacitors begin to charge again. The small variations noticed between the capacitor storage and the harvester is attributed to the losses caused by the diode. 5. Conclusion We evaluated two different nanoparticles as charge storage elements that can be embedded within layers of organic polymeric semiconductors and PMMA. Both CdS and AgPt nanoparticles show a potential for use as charge storage elements in storage devices since they are chemically stable and have a high work function. SEM images showed glowing elements depicting the charging of the nanoparticles. The average size of the nanoparticles and their composition were evaluated and reported. The two synthesized nanoparticles were able to show charge storage capabilities. Large hysteresis window gaps were observed when the nanoparticles were deposited indicating storage within the nanoparticles. A CSMWT energy harvester was connected to the capacitors to determine the storage behavior. The time constants of the two organic capacitors revealed that AgPt embedded nanoparticles retains the charge slightly longer than the CdS nanoparticles due to the electrical characteristics, chemical composition and size of the nanoparticles. Due to their low cost and ease of processing, such devices have the potential to be used on a larger scale and in sense, could be implemented as organic capacitors and complemented with an energy harvesting system to power up small sensors or actuators autonomously. Acknowledgment This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] A.I. Ayesh, S. Qadri, V.J. Baboo, M.Y. Al-Haik, Y. Haik, Nano-floating gate organic memory devices utilizing Ag–Cu nanoparticles embedded in PVA-PAAglycerol polymer, Synth. Met. 183 (2013) 24–28. [2] M.Y. Haik, A.I. Ayesh, T. Abdulrehman, Y. Haik, Novel organic memory devices using Au–Pt–Ag nanoparticles as charge storage elements, Mater. Lett. 124 (2014) 67–72. [3] R.R. Nejm, A.I. Ayesh, D.A. Zeze, A. Sleiman, M.F. Mabrook, A. Al-Ghaferi, M. Hussein, Electrical characteristics of hybrid-organic memory devices based on Au nanoparticles, J. Electron. Mater. 44 (2015) 2835–2841. [4] M.R. Perez, I. Mejia, A.L. Salas-Villasenor, H. Stiegler, I. Trachtenberg, B.E. Gnade, M.A. Quevedo-Lopez, Hybrid CMOS thin-film devices based on solution-processed CdS n-TFTs and TIPS-pentacene p-TFTs, Org. Electron. 13 (2012) 3045–3049.
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