Synthesis of polyacrylamide hydrogels as p-n-junction with ionic charge carriers.
Today, although the developments of organic-based electronic equipments are an attractive research area, inorganic semiconductors are still the most frequently used materials in electronic devices. The fact that the conjugated polymers become electrically conducting [1-4] initiated many fundamental investigations, and created the possibility of a new kind of materials combining the typical properties of plastics with the electrical conductivity of metals. The effect of electrically conductive polymers and their advantages continue to enhance applications in wide range of arising technologies from polymer-based electronics to nanotechnologies.
Electronic devices, which were produced by organic semiconductors, can be scalable, less expensive, and less complex. Therefore, polymeric materials have potential for integration in inexpensive microelectronic devices. Moreover, these organic electronic devices may directly reside on, or be implanted inside animal and human tissue and could perform various sensing.
The great advantage of conjugated polymer semiconductors is their ability to quickly change the carrier density electrochemically in the presence of mobile counter ions. These features of organic semiconductors provide new mechanism for time-dependent and field-dependent properties ,
There are plenty of studies which show that conjugated semi-conducting polymer composites--that contain immobile anions as counter ions for the oxidized form of the polymer and mobile cations--can be used for electronic junctions and devices [5-19]. In these studies, while some of the devices can work temporary [15, 20] the others work more stable such as frozen junction [18, 21]. For constructing these junctions, electrochemical disproportionation and trapping methods  or radical induced polymerization of ion-pair monomers as counter ions have been used .
The current rectifying properties of these junctions arise from redox reactions occurring either across a membrane  or at the interface with the electrodes [23, 24], Within some polymeric diodes known as electrolyte diode [25-27], a uniform hydrogel or film transporting the ion freely, separates two depots containing acidic and basic media.
The disadvantage of bipolar membranes or these electrolyte diodes is that they include water-filled rooms. Therefore the gradients of electrolyte are affected on their operation, for example at over time these devices will stop rectifying. But, still these devices can be used for special applications since they contain the potential of biocompatibility.
Recently we synthesized p- and n-type polyacrylamide (PAAm) gels using some doping molecules having counter ions . These ions move through the gel and carry the current under applied voltage when the gel is in swollen state [28, 29].
In the present work, three new designs for p-n junctions were formed with p-and n-type polyacrylamide hydrogels (PAAm) doped with positive ([Na.sup.+]) and negative ([Br.sup.-]) ions as charge carriers. The designs and their rectification efficiencies of all these junctions were discussed in detail in below sections.
Acrylamide (AAm), bisacrylamide (BIS), ammonium persulfate (APS) and 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (Pyranine) were supplied by MERCK and were used as-received. Tetraallil ammonium bromide (TAAB) was synthesized by Filiz Senkal and the details of reaction given in the literature [30, 31].
Electrical measurements were carried out at 21[degrees]C. The samples were placed between two platinum electrodes that were fixed gently in a cylindrical glass tube. The open ends of the tube were sealed with a Teflon stopper. The length of the tube was kept short enough to make the open space between the electrodes small enough to prevent the drying of the gels during the measurements. The voltage was applied between these electrodes and the current thorough the gel was measured using a Keithley 6487 Multimeter.
RESULTS AND DISCUSSION
p-n Junction With Physical Contact
Synthesis. Gels were synthesized via free-radical crosslinking copolymerization by using AAm as monomer and BIS as cross-linker and APS as initiator. Pyranine was used as the positively charged molecule, having [Na.sup.+] ions as charge carrier, and added to pregel solution to form the p-type PAAm gel, as discussed in detail in reference , Similarly, TAAB molecules, having [Br.sup.-] as negative counter ion, was added to the pregel solution to form the n-type PAAm gel. Compositions used for ionic p-type and ionic n-type PAAm gels are listed in Tables 1 and 2, respectively.
The pregel solutions were deoxygenated by bubbling nitrogen for 15 min. Synthesis was performed at 60[degrees]C. These gelation processes are explained in detail in the literature [28-30, 32-37].
After the gelation processes, gels were washed and dried following the standard procedure [38, 39]. For washing the gels they were put into pure water. The water refreshed every day during 15 days. By this way, unreacted chemicals washed out from the gel, and only pyranine and TAAB molecules bound to the polymer strands chemically, were stayed in the gel. Finally, the gels were dried in a furnace kept at 40[degrees]C.
Thus only the charged molecules bound chemically to the polymer strands remained in the gel. After the gels dried they were cut into thin slices of ~1 mm thickness and swollen in pure water to some certain swelling ratios (the mass ratio, m/[m.sub.0], of the swollen to the dried gel) for electrical measurements.
Design and Experimental Results of Physically Contacted p-n Junction. For preparing p-n junction p- and n-type sliced gels were superimposed and a small force was applied on the metal electrode softly to stick the gels together. Scheme 1 shows the schematic representation and working mechanism of this p-n junction. As seen from Scheme 1, under forward bias [Na.sup.+] and [Br.sup.-] ions can move through the opposite sides and the ions can pass through the interface of p- and n type gels to reach the opposite electrodes. When an ion reaches the opposite electrode it attracts or repels the electrons from the metal electrodes. Thus a current will flows through the circuit. But if reverse bias is applied to the gel, the ions start to move thorough nearest electrode, therefore they cannot pass thorough the gel. Thus the current cannot occur.
Figure 1 shows the current density per unit mass, J/[m.sub.gel], versus applied voltage, U, for junctions formed with both two neat gels (undoped gels) and two oppositely doped gels having different monomer molarities. When the junction was constructed by using neat gels, no rectification is observed on the current density (Fig. la). However, considerable rectification is observed when the junctions are formed with p-and n-type gels. Moreover the rectification ratio increased with increasing doping concentration (Fig. lb and c). Swelling ratio of gels was kept fixed around 1.3 in these measurements.
J/[m.sub.gel]-U characters of the p-n junctions were also tested for varying swelling ratios of the gels. Figure 2 represents how the diode character of the junction constructed with doped gels varies with swelling ratio of the gel. Here one side of the junction was chosen as Sample 1 in Table 1 and the other side as Sample 1 in Table 2 so as to keep the free ion concentration equal on each side. It can be seen from Fig. 2 that the rectification ratio first increases and then decreases with increasing water uptake. The rectification becomes completely unnoticeable when the mass ratio reaches to 1.63. This shows that above a certain water uptake, [H.sup.+] and [OH.sup.-] ions of the water suppresses the effect of doped ions, [Na.sup.+] and [Br.sup.-]. Thus relatively high residual currents in the backward direction were observed, as seen in Fig. 2, for the mass ratios except for 1.19 and 1.32. This is an important parameter for applications of these junctions.
Up to some water uptake, analogously the mass ratio, the mobility of the counter ions contributing the current will increase and thus the intensity of the current will increase. When a critical value for the water uptake is reached, the water ions reach a considerable number to contribute the current. This contribution can decrease the rectification the direction dependence of the current will disappear as the water ions are increased.
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It is clearly seen that the J/[m.sub.gel]-U characteristics of these p-n junctions are adjustable by changing the doping concentration and the mass ratio of the gels.
p-n Junction With Chemical Contact
Synthesis and Design. In this part, p-n junction was formed by synthesizing the n-type gel on the top of shortly before synthesized p-type gel. The same materials and the same method for the synthesis were used as in previous part. First pregel solution of p-type gel was poured into a glass pot and then this solution was turned to the gel form as a layer (Scheme 2) at 60[degrees]C. Thereafter, pregel solution of n-type gel was poured onto the shortly before-synthesized p-type gel layer. Then the second layer, the n-type pregel, was turned to the gel. Thus, gels with different doping agents stuck together. During the synthesis of second layer some polymer strands must have been entangled each other at the interface. When p-n junction is formed via chemical connections, it becomes more resistive due to the entanglements of the strands at the interface of p- and n- type parts. That is, it keeps its form very long time against the drying. After measurements, when the gels are set for drying, or during the measurements the system may be destroyed due to the difference in response of the gels against drying, i.e., one side can be dried faster than the other one. p- and n- parts of the physically contacted junction thus may be separated easily upon drying because no entanglement occurs at the interface of this junction. For similar reasons, the rectification ratio for chemically connected junction can be expected to be much better than that of physically connected one.
After synthesis samples for measurements were cut by using a cylindrical cutter. For electrical measurements, bottom and top sides of cylindrical junction were smoothed after sample was dried. Schematically representation of the synthesis is seen in Scheme 2.
Compositions of PAAm gels used for construction of chemically contacted junction are given in Table 3.
Experimental Results of Chemically Contacted p-n-Junction.
J/[m.sub.gel] versus U was measured for varying swelling ratios. Similar to the physically contacted p-n junction, chemically contacted p-n junctions showed also different response for different swelling ratio. Again, the current rectification increases with increasing water content up to a threshold swelling degree, then, it decreases with further increase in water content. Bui, the threshold swelling degree is different for physically and chemically connected junctions. For chemically contacted junction the swelling degree for best current rectification increases with increasing doping concentration (see Fig. 3).
Synthesizing p and n Types Together in the Same Gel Under Electric Field
Synthesis and Design. In this section, current rectifying gel was synthesized in one synthesis in a composite form. That is, both the p and n sides of the gels were synthesized in a single gel together during the gelation process. To do this while preparing the pregel solution, pregel solutions for p- and n-types were mixed together. Then this solution was placed in an electric field for separation of the dopants at opposite sides. Schematic representation of experimental setup for synthesis of this composite gel is shown in Scheme 3.
When the pregel solution placed in electrical field, positive and negative ions will start to move in opposite directions. In this case, upper and lower sides of this slap gel would be rich with different ions so that the p-n junction is to be formed spontaneously in the same gel. These phenomena are shown schematically in Scheme 4 and compositions of prepared current rectifying gels are summarized in Table 4.
In this case we have only one gel contained both positive and negative sites in itself. A considerable variation in the internal morphology and thus an unexpected increase in the number of defects at the interface would not be expected for this junction, i.e., p- and n-types are indiscernible at the interface from the point of the polymer density. Therefore, when the ions are moving through the gel, scattering from the defects would be less enough compared with other junctions; the physically contacted one and chemically connected one.
Results of Current Rectifying Gel. Figure 4 shows J/[m.sub.gel]-U characters of all samples given in Table 4. For comparison, one of the samples (sample 1) in this table is synthesized under zero electric field. No rectification is observed for this sample for any swelling degree as seen in Fig. 4a. However, considerable current rectifications are observable for all the other samples. This is an expected result because the sample synthesized under zero electric field cannot form p-and n-type regions.
The samples in the first set of Table 4, where the doping concentration is kept fixed, were used to study the effect of external field during the synthesis. Almost no current is observed under reverse bias for Sample 2, as can be seen from Fig. 4b. The rectification ratio--the rate of current density under forward bias to reverse bias--of these samples were calculated as 9.4 for sample 2, 4.5 for sample 3, 5.2 for sample 4 and 4.9 for sample 5. From these results it is seen that sample 2 (synthesized under 5 V) gives best rectification.
The fact that the rectification is best for sample 2 can be explained as below: higher external voltage may result in a widening of the depletion region in which neither p nor n type exists. This region is similar to an uncharged PAAm gel (traditional PAAm gel). Increasing width of this region will result in deviation to be a rectifier because in this case this region will include more water. As the water content is increased the current can be created not only in one but also in two directions which cause some decrease in the rectification. Therefore there must be an optimum value for the external voltage. In our experiments it seems it is 5 V.
As seen from Fig. 4b the leakage current (measured current in reverse biased) increases as the external field is increased. This can be explained as follow. When the voltage is increased the water molecules trapped in some region can be dissociated into free ions, [H.sup.+] and [OH.sup.-]. The presence of water ions through the gel would result in some distortion of the rectification, as discussed above. Of course increasing voltage will increase the current but it will cause at the same time the increase of leakage current which results in decreasing rectification. So it is expected that the rectification should take a maximum value for a specific concentration of polymer and doping agent, swelling degree, and external field.
In the second set of Table 4 the doping concentration is varied and the external voltage is kept fixed at 5V. The results are summarized in Fig. 4c. It is seen that when the doping concentration is increased the rectification increases. The doping concentration of Sample 6 is highly bigger than the others. Therefore the rectification becomes maximum for sample 6.
It is seen from Fig. 4b that the current densities for some samples (for example for Sample 2 and Sample 3) pass through a maximum, i.e., the current densities start to decrease after some higher voltages. Reason of this behavior could be due to the overestimating the applied voltage. Above a threshold value of the applied voltage it is expected that a short time later most of the charges will be accumulated on the surfaces of the gel, which will result in a decrease in the current.
The best result is obtained for Sample 6 where there is almost zero as seen from Fig. 4 where current at reverse bias and the rectification is tremendously bigger than that of the ones formed upon chemical or physical constructions.
Here we demonstrated that adjustable current rectifying diode junctions, of which the charge carriers are ions, can be fabricated by interfacing PAAm gels doped with oppositely charged counter ions. The synthesis of these gels and the construction of the junctions are simpler, cheaper, and more scalable than that of traditional organic and inorganic p-n junctions. Moreover, these junctions can be used in flexible and biocompatible electronic circuits. The current-voltage characteristic of the polymeric p-n junction can be changed by changing the doping concentration and the swelling ratio. Chemically contacted gel diode is more substantial than the physically contacted junction. But the best results for steady current and current rectification are obtainable for the composite gel synthesized under electric field where the charge transport can be expected to be more effective compared to the other forms. The potential usage of our gel diodes especially in flexible and biocompatible electronic circuits is clear, and it could be further developed for such applications.
The authors thank Prof. Dr. Filiz Senkal for synthesizing TAAB molecule for them.
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Esra Alveroglu, Yasar Yilmaz
Department of Physics Engineering, Faculty of Science and Letters, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
Correspondence to: Esra Alveroglu; e-mail; firstname.lastname@example.org
Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Compositions of p-type gels used for physical contact. AAm Pyranine Gel (mol (mg (mol (mg number [L.sup.-1]) [mL.sup.-1]) [L.sup.-1]) [mL.sup.-1]) 1 4 284 [10.sup.-2] 5.3 2 4 284 [10.sup.-4] 5.3 x [10.sup.-2] 3 4 284 [10.sup.-6] 5.3 x [10.sup.-4] 4 4 284 -- -- 5 2 142 [10.sup.-2] 5.3 6 2 142 [10.sup.-4] 5.3 x [10.sup.-2] 7 2 142 [10.sup.-6] 5.3 x [10.sup.-4] 8 2 142 -- -- BIS APS Gel (mol (mg (mol (mg number [L.sup.-1]) [mL.sup.-1]) [L.sup.-1]) [mL.sup.-1]) 1 0.060 9.3 0.029 6.6 2 0.060 9.3 0.029 6.6 3 0.060 9.3 0.029 6.6 4 0.060 9.3 0.029 6.6 5 0.030 4.6 0.014 3.3 6 0.030 4.6 0.014 3.3 7 0.030 4.6 0.014 3.3 8 0.030 4.6 0.014 3.3 TABLE 2. Compositions of p-type gels used for physical contact. PAAm TAAB Gel (mol (mg (mol (mg number [L.sup.-1]) [mL.sup.-1] [L.sup.-1]) [mL.sup.-1] 1 4 284 3 X [10.sup.-2] 8 2 4 284 3 X [10.sup.-4] 8 X [10.sup.-2] 3 4 284 3 x [10.sup.-6] 8 X [10.sup.-4] 4 4 284 -- -- 5 2 142 3 x [10.sup.-2] 8 6 2 142 3 X [10.sup.-4] 8 X [10.sup.-2] 7 2 142 3 X [10.sup.-6] 8 X [10.sup.-4] 8 2 142 -- -- BIS APS Gel (mol (mg (mol (mg number [L.sup.-1]) [mL.sup.-1] [L.sup.-1]) [mL.sup.-1] 1 0.060 9.3 0.029 6.6 2 0.060 9.3 0.029 6.6 3 0.060 9.3 0.029 6.6 4 0.060 9.3 0.029 6.6 5 0.030 4.6 0.014 3.3 6 0.030 4.6 0.014 3.3 7 0.030 4.6 0.014 3.3 8 0.030 4.6 0.014 3.3 TABLE 3. Compositions of p-type and n-type gels used in chemically contacted p-n-junction. p-type Pyranine Junction AAm (mol BIS APS (mol number L.sup.-1) (mg) (mg) [L.sup.-1]) 1 4 0.056 0.040 [10.sup.-3] 2 4 0.056 0.040 [10.sup.-4] 3 4 0.056 0.040 [10.sup.-5] 4 4 0.056 0.040 [10.sup.-6] n-type TAAB Junction AAm (mol BIS APS (mol number L.sup.-1) (mg) (mg) [L.sup.-1]) 1 4 0.056 0.040 3 X [10.sup.-3] 2 4 0.056 0.040 3 X [10.sup.-4] 3 4 0.056 0.040 3 X [10.sup.-5] 4 4 0.056 0.040 3 X [10.sup.-6] TABLE 4. Compositions of current rectifying gels synthesized under electric field. Applied Sample voltage AAm (mol Pyranine (mol number (V) [L.sup.-1]) [L.sup.-1]) 1 0 4 [10.sup.-2] 2 5 4 [10.sup.-2] 3 10 4 [10.sup.-2] 4 15 4 [10.sup.-2] 5 20 4 [10.sup.-2] 6 5 2 [10.sup.-3] 7 5 2 [10.sup.-4] 8 5 2 [10.sup.-5] Sample TAAB (mol BIS (mol APS (mol number [L.sup.-1]) [L.sup.-1]) [L.sup.-1]) 1 3 x [10.sup.-2] 0.06 0.029 2 3 x [10.sup.-2] 0.06 0.029 3 3 x [10.sup.-2] 0.06 0.029 4 3 x [10.sup.-2] 0.06 0.029 5 3 x [10.sup.-2] 0.06 0.029 6 3 x [10.sup.-1] 0.03 0.014 7 3 x [10.sup.-4] 0.03 0.014 8 3 x [10.sup.-5] 0.03 0.014
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|Author:||Alveroglu, Esra; Yilmaz, Yasar|
|Publication:||Polymer Engineering and Science|
|Date:||Dec 1, 2014|
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