Tunable Switching Characteristics of Low Operating Voltage Organic

Feb 22, 2017 - The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, ...
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Tunable Switching Characteristics of Low Operating Voltage Organic Bistable Memory Devices Based on Gold Nanoparticles and Copper Phthalocyanine Thin Films Narayanan Padma,*,† Chirayath A. Betty,‡ Soumen Samanta,† and Akash Nigam§ †

Technical Physics Division and ‡Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Electrical Engineering, Centre of Excellence in Nanoelectronics, IIT Bombay, Mumbai 400076, India

§

S Supporting Information *

ABSTRACT: The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, using copper phthalocyanine and gold nanoparticle thin films, was investigated using Au, Al, and Hg electrodes. While the ON/OFF ratio higher than 105 was achieved for all the devices, the nature of the memory behavior was found to be dependent on the top electrodes. Thermally evaporated Au and Al electrodes resulted in write-once−read-many times (WORM) behavior, whereas Hg drop soft contact led to write−read−erase−read (rewritable) characteristics, with the device retaining the ON state in the former and returning to the OFF state in the latter. The switching voltage was found to be influenced by the top electrode with the devices switching to the ON state at around 2 V for Hg and close to 1 V for Au and Al electrodes. Additionally, though the ON state conduction mechanism was dominated by Fowler−Nordheim (FN) tunneling through AuNP trap states in all the devices, the dynamics of switching was found to be dependent on the top electrode, showing abrupt switching to the ON state for Au and Al electrodes. In contrast, a gradual increase in current at the onset of FN tunneling before switching was observed for devices with Hg electrodes. Such a significant influence of the top electrodes was mainly attributed to the difference in injection barriers between the top electrode/active layer and gold nanoparticle/active layer junctions. Devices exhibit rewritable behavior when the former is higher than the latter, while they change to WORM behavior when the two are equal. The study shows that the same device structure can be tuned to exhibit WORM or rewritable memory behavior by employing the top electrode with the work function in suitable combination with that of the nanoparticles forming trap states in the bulk of the film.

1. INTRODUCTION Progress in information technology in the last few decades has forced the scientific community to search for methods and materials to achieve high density data storage devices which could also be cost-effective. This has accelerated the research on organic memory devices which could meet the requirements such as large storage capacity, flexibility, easy processability, and low cost. Memory devices can be generally classified as volatile and nonvolatile, where the information is lost once the power is switched off in the former and is retained in the later. There are many studies in the literature employing organic molecules and polymers in two terminal or three terminal transistor structures, demonstrating volatile or nonvolatile performances1−8 with the main focus on improving the ON/OFF ratio and long-term stability.7 Memory characteristics can be tuned by the gate voltage and by the choice of the ferroelectric or metal nanoparticles incorporated dielectrics and semiconductors in the three terminal transistor structures. But the two terminal resistor structure in which the memory cell is confined to the area of contact between two crossed lines is the simplest and highly scalable structure compared to the transistor based memories.8 A number of approaches such as fully organic, organic−inorganic hybrid, organic/metal oxide nanoparticles © XXXX American Chemical Society

and organic/metal nanoparticle composites, ferroelectric switching, and redox based switching have been employed for constructing such two terminal devices.1,3,5,9−14 Several mechanisms including conformational change, charge transfer, charge trapping, and filament conduction mechanisms have been proposed for the bistability behavior of these devices.15 Electric field induced charge transfer between metal nanoparticles and the organic semiconductor resulting in oxidation or reduction of the latter has been proposed in one of the devices.11 Wang et al. have proposed the origin of the interface dipole between p-type CuPc and n-type F16CuPc as the plausible mechanism behind the memory observed in their study.16 Song et al. reported charge trapping and detrapping at the interface between PMMA and P3HT to be causing bistable behavior in the heterostructure device.17 Bhansali et al. have demonstrated memory behavior in all polymer devices using PEDOT:PSS film, attributing it to molecular alignment and the change in morphology under electric field.18 Formation and destruction of the interface dipole is also reported to cause Received: September 17, 2016 Revised: January 28, 2017 Published: February 22, 2017 A

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The Journal of Physical Chemistry C bistability in some other studies.1,19 In most of the devices, the filamentary conduction mechanism, either by metallic bridge formation due to diffusion of electrodes or carbon-rich filament formation due to degradation of organic films, is reported to be dominating.1,15 Pan et al. have provided a detailed review of different switching mechanisms, focusing mainly on the filamentary conduction.20 Among the many approaches mentioned before, incorporation of nanoparticles plays an important role in improving the electrical bistability characteristics of devices, especially in enhancing the ON/OFF current ratio, low power consumption, easy processability, etc.11,21,22 where the presence of nanopartcicles is expected to influence the charge transfer by trapping/hopping mechanisms.8,23 Apart from the ON/OFF ratio, operating power is another essential parameter to focus upon to ensure practical use of these devices. Though some studies demonstrate switching at voltages close to 2 V,7,10,15,24 reports on those operating at voltages lower than that are scarce.1,4 In the present study, we have employed a simple device structure using gold nanoparticles (AuNPs) and copper phthalocyanine (CuPc) thin films, and have demonstrated bistability at voltages close to as low as 1 V. Switching between the conducting states can be either reversible or irreversible, where the former can be used as the rewritable or write−read−erase−read device while the latter behaves as write-once−read-many-times (WORM) devices that are applicable as the noneditable permanent data storage device.,16,25 Most of the reports in the literature demonstrate a single memory behavior of the device studied, i.e., volatile or nonvolatile, write-once−read-many-times (WORM) or rewritable/write−read−erase−read behavior. Tuning of the memory property using the same device structure with minor modifications has been scarcely reported.26−29 Chen et al. have reported switching between nonvolatile and volatile behavior of the device using aromatic polyimides by modifying the electron withdrawing moieties.28 In another study, Wu et al. have reported tunable memory properties in their study using poly(triphenylamines) by introducing suitable substituent acceptors and modifying charge transfer properties.29 Zhou et al. have tuned the memory properties from WORM to DRAM etc. by using suitable donor moieties in combination with the same triazole acceptor.27 Though WORM behavior is sufficient for certain applications, rewritability is also essential for most data storage applications, increasing the versatility of nonvolatile devices.30 In the literature, there are studies reporting devices exhibiting either rewritability1,4,9,15 or WORM behavior.10,16,19,24 Chandrakishore et al. have shown a change in WORM behavior to rewritable behavior by varying the carbon nanotube concentration.31 In our study we have shown the tunable memory behavior of the same device by changing merely the top metal electrode without altering any materials or the structure of the device. We have shown that by thermally evaporating gold or aluminium electrode, the device exhibits WORM behavior with the device continuing to remain in the ON state after switching, while with soft deposition of a drop of mercury as the top electrode, the same device exhibits rewritability. A few studies have attempted to establish correlation between the memory characteristics and the work function of the top electrode.25,32−36 Bozano et al. have reported that the memory behavior in their study is insensitive to the electrode work function.35 Other studies have concluded that the memory behavior is affected by the work function of the

electrode. But even those studies report their influence only on the threshold voltage for switching, not on the nature of the memory characteristics itself.25,32−34,36 To our knowledge there are no studies reporting change in the memory characteristics from WORM to rewritable behavior, merely due to the change in work function of the top electrode. Our study shows that, by using the suitable combination of the work function of the top electrode and the nanoparticles, the memory characteristics can be tuned from WORM to rewritability.

2. EXPERIMENTAL SECTION The synthesis of gold nanoparticles has already been reported earlier.37,38 In brief, gold nanoparticles (AuNPs) were prepared by using the citrate reduction method. In this method, 20 mL of 1 mM HAuCl4 (Aldrich) solution was refluxed with continuous stirring. When the solution began to boil, 2 mL of 38.8 mM trisodium citrate (Aldrich) was added rapidly. The solution was allowed to boil for another 15 min, during which the color of the solution changed from pale yellow to purple. The solution was cooled to room temperature with constant stirring. About 30 μL of the as-prepared gold nanoparticle solution was drop-cast on a freshly cleaned n-type silicon substrate (0.01−0.05 Ω cm and (100) orientation) of size around 5 mm × 5 mm, with about 5 nm thick native oxide. This was then allowed to dry naturally, thereby forming a rough film of AuNPs. No other treatment of the substrates was carried out. Copper phthalocyanine (CuPc) purchased from SigmaAldrich was deposited by thermal evaporation under the base pressure of about 2 × 10−5 mbar on the silicon substrate with and without gold nanoparticles. The thickness and the evaporation rate were maintained to be 30 nm and 0.5−1 Å/ s as monitored using a quartz crystal monitor. About 500 × 500 μm size gold (Au) or aluminum (Al) electrodes are thermally evaporated on CuPc film using a shadow mask. A mercury top electrode of diameter around 0.5−0.7 mm was deposited by a simple drop-cast method. The morphology of the CuPc films and that of gold nanoparticles were verified using scanning electron microscopy (SEM; Vega TC). A cross-sectional image of the device structure was verified using an SEM from SEC Engineering, Korea (Model SNE 3000M). Current−voltage characteristics were measured using a Keithley 6487 picoammeter/voltage source, in air and at room temperature under normal room light conditions. The impedance characteristics were measured using a potentiostat/galvanostat 273 system. 3. RESULTS AND DISCUSSION The size of the gold nanoparticles estimated using SEM imaging, already discussed in the previous studies, was found to be about 50 nm (Figure S1 in the Supporting Information),37,38 which was also confirmed by UV−vis absorption measurements (Figure S2). SEM imaging for the drop-cast film in the present study showed the size of the nanoparticles to be ranging from (Figure 1a) 50 nm to more than 100 nm, which could be due to the agglomeration of the same. The morphology of CuPc films verified by SEM shows a granular structure on the Si substrate without gold nanoparticles (Figure 1b), while it is converted to nanofibrous structure when deposited on Si with gold nanoparticles (Figure 1c). In our previous study, atomic force microscopic (AFM) imaging of a 30 nm thick CuPc film on SiO2 (200 nm thickness, without gold nanoparticles) had shown a peak−peak surface roughness of about 4−5 nm (Figure S3).39 The cross-sectional B

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The Journal of Physical Chemistry C 1. Thermionic emission (TE) model:9 ⎡ ⎛ q3V ⎞1/2 ⎤ qϕb ⎢ ⎟ ⎥ I ∝ A*T exp − + q⎜ ⎢ kT πε 4 ⎝ ⎠ ⎥⎦ ⎣ 2

(1)

16

2. Direct tunneling model: When the charge carrier injection barrier (tunneling barrier) between the electrode/ CuPc is trapezoidal, I−V characteristics follow the direct tunneling model given by

Figure 1. (a) SEM imaging of gold nanoparticles. (b and c) SEM imaging of CuPc film on native oxide Si without and with gold nanoparticles.

⎛ 2d 2mϕ b I ∝ V exp⎜⎜ − h ⎝

SEM image of the device with gold nanoparticles, in the present study, is shown in Figure S4. The surface roughness of the CuPc film deposited on gold nanoparticle film, as seen from Figure S4, was found to increase significantly compared to that without gold nanoparticle film. This could be because of the random aggregation and hence increased roughness of the AuNP film. The schematics of the device structures without and with gold nanoparticles, based on the surface roughness seen from AFM and SEM images shown in Figures S3 and S4, and with gold/aluminum/Hg top electrodes are shown in Figure 2, parts a and b, respectively. The I−V characteristics of the device without gold nanoparticle and with Au top electrode, shown in Figure 3a exhibits rectification. This is because of the work function difference between Au (5.1 eV) and Si (4.1 eV).40,41 When negative voltage is applied to the silicon substrate, the Fermi level of Au electrode moves close to the highest occupied molecular orbital (HOMO) of CuPc allowing injection of charge carriers into CuPc. During positive voltage to silicon, the barrier between the Fermi level of Au and the HOMO of CuPc increases, preventing injection. This would cause heavy current to flow in the former case and negligible current in the latter case, resulting in rectification. Figure 3b shows the I−V characteristics of the device with gold nanoparticles and with a gold top electrode, where initially for the voltage scan from 0 to −0.95 V (region 1) the device exhibits a very low current and hence a low conducting state. At −0.95 V, the current abruptly increases switching the device to a high conducting state, equivalent to the “writing” process. As the voltage is increased above −0.95 V, the current further increases, reaching the saturation level of the picoammeter (region 2). For the voltage scan in the reverse direction from −1.3 to +1.3 V, and for subsequent multiple scans (regions 3, 4, and 5), the device continues to be in the high conducting state, allowing high current to flow. This clearly shows that the device exhibits WORM behavior with the current ON/OFF ratio higher than 105. The conduction mechanism causing the switching behavior was investigated by analyzing the I−V characteristics under low and high bias conditions with different models:

⎞ ⎟ ⎟ ⎠

(2)

or ln

2d 2mϕb I 1 ∝ ln − 2 V h V

(3) 16

3. Fowler−Nordheim tunneling model: When the injection barrier changes from trapezoidal to triangular, I−V characteristics follow the Fowler−Nordheim tunneling model given by ⎛ 4d 2mϕ 3 ⎞ b ⎟ I ∝ V exp⎜ − ⎜ 3ℏqV ⎟ ⎝ ⎠

(4)

⎛ 3 ⎞ 1 ⎜ 4d 2mϕb ⎟ I ∝ − ⎟ 3ℏq V⎜ V2 ⎝ ⎠

(5)

2

or ln

4. Space charge limited current conduction (SCLC) model:9,16 I ∝ Vα

(6)

where m, k, A*, T, ε, ϕb, ℏ, q, and d are the effective mass of the charge carrier, Boltzmann’s constant, Richardson’s constant, absolute temperature, dielectric permittivity, barrier height, Planck’s constant divided by 2π, electronic charge, and thickness of the active layer, respectively. Considering the I−V characteristics of the device with the Au electrode, fitting of the experimental data at low voltage regions before switching (0 to −0.95 V) did not satisfy the TE model (figure not shown), implying that the conduction mechanism before switching is not by thermionic emission. The plot of ln(I/V2) vs ln(1/V) for the same voltage range (0 to −0.95 V, region 1) offers an excellent linear fit (Figure 4a) indicating the conduction to be by direct tunneling. The plot of ln(I/V2) vs (1/V) above −0.95 V (Figure 4b), in the ON state (region 2), shows a linear fit in the region before the current value reaches

Figure 2. Schematics of devices (a) without AuNP and (b) with AuNP. C

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Figure 3. I−V characteristics of devices (a) without AuNP and (b) with AuNP.

Figure 4. (a) ln(I/V2) vs ln(I/V) from 0 to −0.95 V before switching (region 1). (b) ln(I/V2) vs (1/V) (FN tunneling) from −0.96 to −1.02 V (ON state, region 2); inset, FN plot from 0 to −0.95 V. (c) FN plot of reverse scan from −1.3 to 0 V (region 3). (d) Direct tunneling plot of reverse scan from −1.3 to 0 V (region 3). Dashed lines are the theoretically fitted lines.

Figure 5. (a) Energy level diagram corresponding to memory device structure, (b) direct tunneling of charge carriers at low voltages, and (c) Fowler−Nordheim tunneling at high voltages.

+1.3 V (region 4), where the device maintains the high conducting state, Fowler−Nordheim tunneling at high bias and direct tunneling at low bias (below −0.14 V) are still satisfied (Figure 4c,d). It should be mentioned that the ON state data

the saturation of the picoammeter, indicating the conduction mechanism in the ON state to be due to Fowler−Nordheim tunneling. The inset in Figure 4b shows the FN plot for 0 to −0.95 V. During the scan from −1.3 to 0 V (region 3) and to D

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Figure 6. (a) I−V characteristics of memory devices on Si without native oxide. (b) FN tunneling model after switching (ON state, from −0.58 to −0.68 V); inset, FN tunneling before switching (OFF state, from 0 to −0.56 V). Dashed lines are the fitted lines.

Figure 7. Impedance measurements of the device for the frequency range 2 MHz−280 Hz in the (a) OFF state at −0.8 V (inset, equivalent circuit employed to fit experimental data in the OFF state) and (b) ON state at −3 V. Solid lines with symbols represent the measured data, and the dashed line represents the fitted data.

did not fit with the ohmic conduction model, thereby ruling out filament formation. Based on the fitting of the experimental data with the above models, the conduction mechanism can be explained as follows: Even though the gold nanoparticle layer is first deposited before thermally evaporating CuPc, the rough and porous surface of the AuNP layer (Figure 1a) could be allowing the CuPc molecules to be occupying the gap between the AuNPs, resulting in extension of gold nanoparticles well into the CuPc film. The energy level diagram corresponding to the memory device structure is shown in Figure 5a. Even though ohmic contact is expected between Au and CuPc as the Fermi level of Au is about 5.1 eV, very close to the HOMO level of CuPc (5.05 eV),40 an interface dipole (Δ) is reported to be formed between Au and phthalocyanines causing a barrier for hole transport.42 Lindner et al. and Peisert et al. have reported a Δ value of −1.2 eV at the Au/CuPc interface (electron transfer from CuPc to Au), causing an increase in injection barrier for holes from Au to CuPc.42,43 Also, the presence of AuNPs could be forming trap states between the HOMO and lowest unoccupied molecular orbital (LUMO) of CuPc leading to multiple interfaces and hence injection barriers between AuNPs and CuPc, leading to the trapping of the injected charge carriers at low voltages initially. With increase in voltage and hence injection current, the Fermi level of the top electrode could be aligning further with the trap states leading to filling of an increasing number of traps and hence allowing only low current to flow through the device. Since at such low voltages the injection barrier between AuNP

and CuPc could be forming a trapezoidal barrier, the charge carriers could be directly tunneling between one AuNP trap to another, thereby satisfying the direct tunneling model (Figure 5b). Around −0.95 V, the injection of charge carriers could be sufficient to fill all the traps. Additionally the injection barrier could also be getting modified from trapezoidal to triangular barrier (Figure 5c). The charge carriers could now be easily tunneled through the triangular barrier flowing unobtrusively as heavy current since traps are already filled. This could be resulting in switching of the device to a high conductance state, thereby satisfying the Fowler−Nordheim tunneling model in the ON state (region 2). It should be mentioned here that, at high voltages, the native SiO2 layer generally breaks down offering a direct path to charge carriers.41 Fit of the data in the reverse scan from −1.3 to 0 V (region 3) by FN modeling shows a clear point of inflection indicating the transition from direct tunneling to FN tunneling at about −0.14 V (Figure 4c). It should be noted that, during the forward scan, the transition takes place at about −0.95 V while in the reverse scan FN conduction begins at a much lesser voltage of −0.23 V. This could be because the trapped carriers in the AuNPs might be retained during the reverse scan, thereby screening the applied voltage. This could be causing the injection barrier to be triangular even at low voltages and hence shifting the transition voltage to lower values than that in the forward scan before switching. At very low voltages, the injection barrier could again be becoming trapezoidal allowing charge carriers to be transported by direct tunneling (Figure 4d). The presence of the trapped carriers could still be reducing E

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Figure 8. (a) I−V characteristics of the device with Al electrodes. (b) ln(I/V2) vs (ln(1/V) (direct tunneling before switching (region 1). (c) ln(I/ V2) vs 1/V (FN tunneling) before switching (region 1); inset, expanded view of FN tunneling model for voltages between −0.7 and −1.2 V. (d) FN model for reverse scan from −2.0 to 0 V (region 3). (e) Direct tunneling model for reverse scan from −2.0 to 0 V (region 3).

the barrier height, as compared to that before the traps were filled in the forward scan, resulting in higher current and hence a high conducting state. Additionally, field induced migration of gold nanoparticles from the top electrode into the CuPc film could be during the forward scan, thereby increasing the trap concentration. Availability of additional trap states during the reverse scan, which were absent in the forward scan, could be resulting in the aligning of the Fermi level of the top electrode with the trap states even at low voltages. This could further be enabling easier tunneling of the charge carriers allowing higher current to flow even at very low voltages and hence maintaining the device at high conductance state. In order to identify the role played by the native oxide in FN tunneling and memory behavior, devices with Au top electrodes were also fabricated after etching the native oxide. For this purpose Si substrates were immersed in 2% HF for about 1 min before drop-casting AuNP film. The I−V characteristics showed memory behavior similar to that with native oxide (Figure 6a). It can be seen that the switching to high conducting state occurs at voltages as low as −0.56 V, which could be attributed to an increase in the effective field across the film brought about by the absence of native oxide. Similar to the devices with

native oxide, the ON state conduction mechanism is found to be dominated by FN tunneling (Figure 6b and inset), thereby confirming that the tunneling is mainly through the AuNP trap states. Involvement of AuNP/CuPc interface in the conduction mechanisms was further confirmed by carrying out impedance measurements at voltages corresponding to that before and after switching. In the OFF state, two semicircles, with the one in the high frequency being suppressed, are observed as shown in Figure 7a though the second semicircle is not explicitly shown in the figure. Figure 7a also shows the fitting of the data for the frequency range 2 MHz−280 Hz with the equivalent circuit shown in the inset. The equivalent circuit mainly consists of R1 parallel to Q1 (constant phase element) representing the Au/CuPc contact and another (R2 parallel to Q2) representing the distributed AuNP/CuPc contacts. The low frequency region normally represents the Au/CuPc interface, while the high frequency region correlates with the bulk resistance which includes here AuNP/CuPc interfaces. The equivalent circuit is reduced to almost negligible impedance after switching as seen in Figure 7b with a significant decrease (of the order of 105) in total impedance. Such a decrease in total impedance can be associated with a F

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Figure 9. (a) I−V characteristics of devices with Hg electrode; (b) FN tunneling model for voltage from 0 to −1.78 V before switching (region 1); (c) direct tunneling model for voltages from 0 to −1.78 V before switching (region 1); (d) FN tunneling model for voltages from −2.5 to 0 V in reverse scan (region 3); (e) SCLC model for voltages between −0.5 and 0 V (region 3).

tunneling below −1.1 V (region 1), switching to FN tunneling in the ON state above −1.18 V (Figure 8b,c). In the inset of Figure 8c, the region between −0.7 and −1.18 V is expanded for a better view of the onset of FN tunneling. In the reverse scan the plot of ln(I/V2) vs (1/V) (region 3) becomes linear above −0.26 V, indicating domination of FN tunneling above this voltage and that of direct tunneling below −0.14 V with the point of inflection at about −0.18 V (Figure 8d,e). These voltages, which are close to that observed for Au electrode, in spite of the lower work function of Al, are in agreement with the assumption on the formation of an interface dipole favoring hole injection. Similar to the Au electrode case, field induced metal diffusion could be occurring also in the case of Al electrode aiding in the WORM behavior of the devices. When the top Au electrode is replaced by Hg drop, the device returns to low conducting state during the reverse scan (from −2.5 to 0 V) after switching to high conducting state during the forward scan (from 0 to −2.5 V) (Figure 9a). The switching between high conductance and low conductance states is observed for many scans (not shown here). For the scan in the opposite direction (from 0 to +2.5 V) no switching is observed where the current increases/decreases with

reduction in both Au/CuPc and AuNP/CuPc interface barriers correlating with the FN tunneling mentioned earlier. The impedance spectrum reduces to predominantly capacitive behavior with negative reactance in the frequency range 117 Hz−755 kHz indicating filling up or storing of charges in the trap states.44 The devices with Al electrodes also exhibit WORM behavior as shown in Figure 8a. Even though Al has a work function of 4.2 eV exhibiting a high injection barrier with the HOMO of CuPc, the devices still allow heavy current to flow. Clear hole injection from Al to hole transporting materials such as pentacene and polyvinyl carbazole (PVK) has been reported in earlier studies where the orientation of the interface dipole is said to be in a suitable direction to push up the energy band of pentacene and PVK with respect to the vacuum level of the metal, leading to significant reduction in the hole injection barrier.19,45 A similar interface dipole between Al and CuPc could have been formed also in the present study, reducing the barrier significantly and aiding in injection of holes. Similar to the conduction mechanisms observed for the devices with Au electrodes, charge transport in the OFF state is by direct G

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unlike the abrupt switching observed at the onset of FN tunneling for the devices with the Au and Al electrodes. This could be because, at higher voltages, even though the injection barrier at the Hg/CuPc interface could still be higher, the barrier between AuNP trap states and the HOMO level of CuPc could be decreasing significantly, modifying to a triangular barrier. This could be allowing easier transport of charge carriers between the AuNP trap states by FN tunneling, thereby increasing the current significantly before switching (Figure 10b). As the voltage crosses a certain level, the injection barrier at the Hg/CuPc interface itself could be reducing such as to allow FN tunneling at this interface to dominate the conduction mechanism, switching the device to the high conducting state (Figure 10c). During the reverse scan from −2.5 to 0 V, the fit with the FN tunneling model shows linear behavior above −0.5 V (Figures 9d and 10d), again lower than that in the forward scan as observed for Au and Al electrodes. This could again be due to the trapped holes screening the applied voltage. At voltages below −0.5 V, the conduction mechanism is mainly by space charge limited current (SCLC) (Figure 9e). At low voltages, the Fermi level of the Hg electrode could be moving out of alignment with Au trap states such that the number of trap states through which tunneling can take place could be reducing. Simultaneously, release of the previously trapped charge carriers could be allowing the current to flow, thereby emptying trap states. Below very low voltages (−0.08 V), the unfilled trap levels and the lack of trap states in alignment with the Hg Fermi level could be returning the device to a condition almost similar to that at the beginning of scan, as shown in Figure 10a. This could be forcing the device to switch back to a low conducting state. Also, diffusion of metal ions forming additional trap states could be almost absent as placing a drop of the Hg electrode would not damage the film. Additionally, the higher surface tension of the Hg drop would also be preventing the field induced metal ion diffusion into the film.24 In order to investigate the endurance of the devices, a write− read sequence test was carried out for the devices with Au electrodes as shown in Figure 11a,b, where the device current was read at −0.5 V after applying a writing voltage of −2 V and an erase voltage of +0.5 V. Each voltage was applied for about 10 s. Since the device exhibits WORM behavior, the erase voltage does not reduce the current drastically; hence even after

increase/decrease in voltage. The mechanism behind the switching behavior is again attributed to filling of the traps until the transition to the high conductance state, similar to that observed with Au and Al electrodes. The fitting of the data with direct and FN tunneling models are shown in Figure 9b−e. The corresponding energy level diagrams depicting the flow of charge carriers at respective voltage ranges are shown in Figure 10.

Figure 10. Energy level diagrams and flow of charge carriers for voltages (a) below −0.5 V in OFF state before switching (region 1), (b) between −0.9 and −1.76 V before switching (onset of FN tunneling; region 1), (c) above −1.76 V in ON state (region 2), and (d) from −2.5 to −0.5 V in reverse scan (region 3).

The fit of the OFF state data (before switching) shows linear behavior with the FN model above −0.94 V (Figures 9b and 10b) and direct tunneling below about −0.5 V (Figures 9c and 10a), with the point of inflection at about −0.56 V. It can be seen that the device requires higher voltage to switch to the high conductance state which could be because of the lower work function of Hg causing a higher injection barrier as compared to that with Au and Al electrodes. It can also be seen that the current increases gradually after the onset of FN tunneling before switching to the high conductance state,

Figure 11. Endurance tests of memory devices with (b) Au and (d) Hg electrodes; (a and c) corresponding voltage cycles. H

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Figure 12. Memory retention characteristics of devices with (a) Au, (b) Al, and (c) Hg electrodes.

+0.5 V, high current is measured at −0.5 V. In the case of the device with Hg electrode, a write voltage of −3 V and an erase voltage of +0.5 V were applied and the current was measured at −1.2 V each time thereby exhibiting a write−read−erase−read sequence (Figure 11c,d). To further confirm the memory retention characteristics, the ON and OFF currents of the devices were measured for the devices with Au and Al electrodes at reading voltages of −0.5 and −0.7 V respectively for a period of 1 h after applying a write voltage of −2 V in both cases (Figure 12a,b). For the devices with Hg electrode, a write voltage of −3 V and an erase voltage of +0.5 V were applied before reading at −1.2 V (Figure 12c) in the ON and OFF states, respectively. Successful endurance and retention tests of both devices indicate their reproducible memory behavior and their long-term retention capabilities. As mentioned in the Introduction, there are many studies demonstrating bistable memory behavior in devices incorporating metal nanoparticles in the bulk of the organic active layer.20−22 But those studies demonstrate only a single type of memory for a particular device structure, either WORM or rewritable. Tunable memory characteristics have been reported by varying the constituent acceptor groups, tuning the moiety group, etc.27−29 But the above results show that the same memory device structure incorporating metal nanoparticles in the bulk of the film can operate in different memory modes by simply employing top contact electrodes with different work functions. The studies studying the effect of metal electrodes have found their work functions to affect only the threshold voltage of the switching characteristics.32−34,36 When the injection barrier at the top electrode/active layer junction matches that of the metal nanoparticle/active layer interface, the device can operate in WORM mode (Figure 5). When the work function of the top electrode is lower than that of nanoparticles, i.e., the injection barrier at top electrode/active layer interface is higher than that of nanoparticle/active layer

interface, the device can return to the low conducting/erase state (Figure 9a) resulting in rewritable memory mode. Even though the devices exhibited even lower operating voltage after removal of the native SiO2, reproducibility had deteriorated due to additional surface roughness introduced during etching. Therefore, the devices were demonstrated on the substrates with native SiO2. For each electrode type, performances of about 25 devices were verified out of which about 75% of the devices exhibited similar performances with the switching voltage varying within the margin of ±0.15 V. The present study implies that, employing a top electrode with a work function in suitable combination with that of the metal nanoparticle, the memory mode of such a device can be tuned from noneditable to rewritable mode.

4. CONCLUSIONS The top contact electrodes of an organic bistable memory device based on gold nanoparticles and copper phthalocyanine thin films were found to have a significant influence on the switching characteristics. The devices exhibited WORM behavior with Au and Al top contact electrodes while they modified to read−write−erase type memory with Hg electrode. All the devices exhibited an ON/OFF ratio of about 105−106, excellent endurance and memory retention characteristics. Charge carrier transport was determined to be primarily by Fowler−Nordheim tunneling in high voltages and by direct tunneling at low voltages for all devices. The presence of switching behavior for the devices with inert top electrodes such as Au and for those without native SiO2 clearly indicated the insignificant role played by any interfacial oxides if present as well as the native SiO2 in the memory mechanisms. The tunneling conduction was attributed to be taking place primarily through the trap states formed by gold nanoparticles. The switching dynamics was also found to be varying with the top electrode, showing abrupt switching for devices with Au and Al electrodes and a gradual increase in current before I

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switching for Hg electrodes. Such differences in the memory behavior, i.e. WORM and rewritable modes as well as the switching dynamics with top electrodes, was attributed to the difference in the injection barrier at the top electrode/active layer and trap state forming nanoparticle/active layer junctions. The study shows that, by using a top contact electrode with the work function in suitable combination with that of the nanoparticles forming trap states in the bulk of the film, the memory behavior can be tuned from noneditable to rewritable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09404. SEM image and UV−vis absorption spectrum of freshly prepared gold nanoparticle (AuNP), AFM image of CuPc film on SiO2/Si without AuNP, cross-sectional SEM image of CuPc/AuNP/native SiO2/Si (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Narayanan Padma: 0000-0002-6886-1849 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to express their sincere thanks to Dr. S. P. Koiry, Technical Physics Division, BARC, for synthesizing gold nanoparticles required for this study. The authors would also like to thank the Glass & Advanced Materials Division, BARC, for providing cross-sectional SEM image of the device structure.

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