We report that an enhancement in electrical bistability in devices based on organic molecules can be achieved by the introduction of semiconducting nanoparticles. Here, devices based on alternate layers of a dye in the xanthene class and CdSe nanoparticles have been compared with devices based on the individual components. Results from dye/CdSe devices have yielded an appreciable enhancement in electrical bistability compared with those based on the dye or the nanoparticles. The enhancement is due to augmented carrier transport through the nanoparticles to the dye that consequently undergoes a change in its conformation, having a higher conductivity. We have evidenced read-only and random-access memory applications in the dye/nanoparticle hybrid system.
In recent years, organic semiconductors have been chosen for different electronic and opto-electronic applications ranging from field-effect transistors to light-emitting diodes. The range widened further with the work showing organic memory for data-storage applications (Ma et al. 2000; Bandhopadhyay & Pal 2003; Yang et al. 2004). Organic memory is manifested owing to electrical bistability of molecules and devices (Donhauser et al. 2001; Solak et al. 2002; Ssenyange et al. 2006). In an electrically bistable device, current–voltage (I–V) characteristics are dependent on the preceding voltage pulses or voltage-sweep directions. This leads to two I–V characteristics with an associated memory phenomenon. Conformation change and/or electroreduction of organic molecules plays a key role in changing the conductivity of the devices (Donhauser et al. 2001; Solak et al. 2002; Bandhopadhyay & Pal 2003; Ssenyange et al. 2006). In devices with soft metals as electrodes, filament formation may occur to yield conductance switching.
Recent work on organic memory focused on several aspects, including: (i) understanding of the mechanism (Bozano et al. 2004; Bandyopadhyay & Pal 2005; Chen & Ma 2006; Rath et al. 2006; Paul 2007); (ii) improvement of device parameters by choosing suitable systems (Bandhopadhyay & Pal 2003; Jiang et al. 2008; Portney et al. 2008); and (iii) addressability for data-storage applications (Jakobsson et al. 2005; Mukherjee & Pal 2007). While electrical and dielectric properties at different temperatures have been studied for understanding the mechanism, donor/acceptor systems or molecules with suitable functional groups have been chosen to improve device parameters (Chu et al. 2005; Jakobsson et al. 2005; Mukherjee & Pal 2007), such as on/off ratio, retention time, threshold voltage and so on. Here on/off ratio means the ratio between conductivities of a high- and a low-conducting state measured at a particular voltage.
Improvement in electrical conduction in organic semiconductors has so far been hindered mostly due to their poor mobility. Since the use of single crystals is never a practical solution from the application point of view, it is imperative to supplement the conductivity with suitable systems. Semiconducting nanoparticles may come in handy in this regard. In this article, we introduce CdSe nanoparticles in organic memory devices to enhance electrical bistability for improved organic memory applications. As the organic molecule, we have chosen a conventional molecule in the xanthene class. Molecules in this class are known to exhibit bistability and memory phenomena in thin films and as a molecule itself (Bandhopadhyay & Pal 2003; Rath et al. 2006).
Phloxine B (PB), mercaptoacetic acid (MAA) and poly(allylamine hydrochloride) (PAH, Mw=70 000) were purchased from Aldrich Chemical Co. All the chemicals were used without further purification. To synthesize MAA-capped CdSe nanoparticles, cadmium acetate (Cd(COOH)2⋅2H2O) was used as the metal precursor. Sodium selenosulphate (Na2SeSO3), obtained by refluxing 2 g of selenium powder and 4.83 g of sodium sulphite (Na2SO3) in 100 ml of water for 10 h, acted as the selenium source. MAA was the stabilizing agent for the growth of CdSe nanoparticles. The pH of the solutions was balanced with dilute NH4OH solution. Milli-Q water of resistivity 18.2 M2 cm was used for all the experiments. Indium tin oxide (ITO)-coated glass substrates with a sheet resistance of 12 2 cm−2 were purchased from Optical Filters Ltd, UK.
(b) Synthesis and characteristics of CdSe nanoparticles
To synthesize anionic-capped CdSe nanoparticles, we first prepared an aqueous solution of cadmium acetate (108.44 mg in 80 ml). We then added MAA (32 mg) to this solution and mixed thoroughly with magnetic stirring. The pH of the solution was adjusted to 12.5 by adding dilute NH4OH solution. Freshly prepared sodium selenosulphate (1 ml) was then added to the solution under continuous stirring. After 30 min, the mixed solution turned clear and greenish yellow in colour; the solution was stirred continuously for 12 h. To remove the product (MAA-capped CdSe nanoparticles) from unreacted chemicals, the resultant solution was cooled to 4°C and centrifuged at 14 000 r.p.m. The product was isolated, redispersed and centrifuged in three cycles. Finally, the nanoparticles were dried in a vacuum oven at 60°C for 12 h. The acid-stabilized nanoparticles were finally redispersed in deionized water under sonication to obtain an optically homogeneous solution for use as an anionic bath during layer-by-layer (LbL) film deposition. The nanoparticles were characterized by electronic absorption spectroscopy (Shimadzu UV-2550 spectrophotometer), high-resolution transmission electron microscopy (HR-TEM, JEOL-JEM 2010) with energy dispersive X-ray (EDX) analyses (INCA, Oxford) and powder X-ray diffraction (XRD, Rich-Seifert XRD 3000P).
(c) Device fabrication
To fabricate the devices, LbL films were deposited on ITO-coated glass substrates. The LbL method relies on surface charge reversal during deposition of every polyionic layer (Decher & Hong 1991; Kotov et al. 1995; Yoo et al. 1997; Das & Pal 2002; Lee et al. 2006; Ariga et al. 2007). While PAH was used as the polycation, PB or MAA-capped CdSe nanoparticles acted as the anionic source. At first, ITO-coated glass substrates (strips of ITO) were deprotonated with H2O, H2O2 and NH4OH solution (v/v ratio 5:2:2). The ITO substrates were first dipped into the cationic PAH solution for 15 min, followed by rinsing in neutral water three times. They were then dipped into an anionic solution bath for 15 min, followed by the same rinsing protocol. By repeating the dipping sequence, we could obtain the desired number of bilayers of LbL films. In the present study, we deposited the following three films: (PAH/PB)20, (PAH/CdSe)20 and (PAH/PB/PAH/CdSe)10. In the latter, PB molecules and CdSe nanoparticles were deposited in alternate layers with the inert PAH layer in between them. The films were annealed at 120°C in vacuum overnight. To complete device fabrication, aluminium was thermally evaporated at a pressure below 10−5 torr as top electrode. The area of the devices was 2 mm×3 mm.
(d) Device characterization
Devices were kept in a shielded vacuum chamber to record current–voltage (I–V) characteristics. A Yokogawa 7651 DC voltage source and Keithley 486 picoammeter were used for this purpose. Bias was applied with respect to the aluminium electrode. Sweep speed was 50 mV s−1 to reduce displacement current. For pulsed mode measurements, the DC voltage source was coupled to fast switching transistors to generate voltage pulses of different widths and amplitudes. The rise and fall times of the voltage pulse were less than 100 ns. The instruments were controlled by a PC via a general-purpose interface bus.
3. Results and discussion
Figure 1a shows the electronic absorption spectrum of CdSe nanoparticles in dispersed solution. The spectrum shows a band at 413 nm. The diameter of the nanoparticles, as calculated from the peak wavelength, effective mass of an electron and band gap of the bulk CdSe (=1.84 eV), is found to be 4.5 nm. The diameter obtained from HR-TEM images over a larger area has been projected to be approximately 4.8 nm. The values obtained from the two methods matched quite well. The HR-TEM images, a typical one of which is presented in the inset of figure 1a, show lattice spacing of CdSe crystals (0.35 nm). The values match very well with the lattice spacing for (0 0 2) plane of CdSe crystals, as mentioned in JCPDS PDF no. 08-0459. Formation of CdSe nanocrystals has further been verified by the XRD spectrum. While the EDX analyses evidence composition of the nanoparticles, the XRD spectrum, as presented in figure 1b, shows the crystal planes of CdSe. All the results evidenced the formation of CdSe nanocrystals.
We have deposited LbL films of PB and CdSe nanoparticles with PAH as the polycation. A dipping sequence of PAH/CdSe/PAH/PB resulted in LbL films of the dye with nanoparticles. For comparison, LbL films of the individual components were also deposited with the same polycation. Electronic absorption spectra of the (PAH/CdSe/PAH/PB)10, (PAH/CdSe)20 and (PAH/PB)20 LbL films are shown in figure 2. Here, the suffixes represent the number of dipping cycles. The figure shows that the materials were adsorbed and deposited as LbL films. Spectra of CdSe and PB films show signatures of the respective material. The spectrum of the (PAH/CdSe/PAH/PB)10 film is mostly an algebraic sum of the spectra of the components. This shows that adsorptions of CdSe nanoparticles and PB molecules were uniform during LbL film deposition.
We have recorded I–V characteristics of the devices based on LbL films of PB and CdSe nanoparticles deposited in the alternate layers. For comparison, devices based on the components, that is, LbL films of PB and CdSe nanoparticles separately, were also characterized. I–V characteristics of an ITO/(PAH/CdSe/PAH/PB)10/Al device under multiple voltage loops are presented in figure 3. The characteristics show electrical bistability. I–V characteristics show that the conductivity of the film is higher when the voltage is swept from a negative value, compared with the case when it is scanned from a positive one. That is, switching from a low- to a high-conducting state of the device occurs at a negative bias. The bistability is, moreover, repeatable over multiple voltage loops.
In xanthene class molecules, of which PB is a typical example, bistability is, in general, due to conformational change with consequent change in the band gap. A change in the conformation occurs owing to the biplanar structure of the molecule (inset of figure 3). Considering the electrode combination used, electron injection is favourable from the ITO electrode. With the polarity of voltage used in the experiment, electron injection from the ITO electrode occurs in the negative bias. With injection of electrons, some of the molecules change their conformation to one having a lower band gap. This allows facile carrier injection with more and more molecules switching to their high-conducting state. When a percolative network is formed between the two electrodes, the device yields a higher current that is termed as high-conducting state. With slow withdrawal of bias, the percolative network of high-conducting molecules is retained to some extent. Finally, at an opposite voltage, the initial state of the molecule is re-established, resulting in a break in the networks and consequently reinstating the low-conducting state of the device. Such a cycling between the low and the high states was achieved by applying bias in multiple loops.
It is imperative to compare the I–V characteristics of the ITO/(PAH/CdSe/PAH/PB)10/Al device with those of its components, namely ITO/(PAH/CdSe)20/Al and ITO/(PAH/PB)20/Al devices. The latter two devices also show bistability. A comparison of the characteristics of the three devices is presented in figure 4a, with the magnitude of the current being plotted as a function of voltage. When compared with devices based on CdSe or PB, the CdSe/PB devices yield low off-state and high on-state current. CdSe nanoparticles in between PB layers must have augmented conductance switching of PB molecules. Carrier confinement in nanoparticles (Jung et al. 2006; Mohanta et al. 2006; Das et al. 2007; Li et al. 2007; Verbakel et al. 2007) may also result in higher on/off ratio in devices based on CdSe/PB. Degree of bistability can be quantified by calculating the on/off ratio as a function of voltage (figure 4b). The ratio in the CdSe/PB device is much higher than that in the devices based on the components. The ratio in ITO/(PAH/CdSe/PAH/PB)10/Al reaches more than 104. This is a couple of orders higher than that in ITO/(PAH/CdSe)20/Al and ITO/(PAH/PB)20/Al devices. Such a high ratio is itself of importance, so that misreading rate will be low during the read processes. Moreover, higher on/off ratios in ITO/(PAH/CdSe/PAH/PB)10/Al films when compared with ITO/(PAH/CdSe)20/Al and ITO/(PAH/PB)20/Al show that introduction of nanoparticles in organic bistable devices can be considered to be a novel route to supplement electrical switching.
We have studied the transport mechanism in the low- and high-conducting states. We find that the characteristics of the low-conducting state can be fitted to an injection-dominated mechanism. As per the thermionic emission mechanism, the current density is given by where A* is Richardson’s constant, T is the absolute temperature, q is the magnitude of electronic charge, ϕB is the barrier height, E is the electric field, ϵ is the dielectric permittivity and k is Boltzmann’s constant. A plot of versus V1/2, as shown in figure 5a, shows a linear fit, showing the validity of the thermionic emission model in the low-conducting state of the device. It may be added that there was no change in the mechanism upon addition of the nanoparticles.
Current in the high-conducting state, on the other hand, did not fit to an injection-dominated mechanism. We could fit it to a bulk-dominated mechanism, namely space charge-limited conduction (SCLC). Space charge-limited current, which dominates when the concentration of the injected carriers exceeds the carriers produced by thermal excitation, requires ohmic contact with the electrode. Ohmic contact is a possibility in the high-conducting state owing to a decrease in the band gap of PB upon electroreduction. According to the SCLC mechanism in the absence of traps, current density is given by where μ is the carrier mobility and d is the thickness of the device. A plot of versus is shown in figure 5b. The plot fits to a straight line with a slope of 2.3, showing the validity of the SCLC mechanism for the high-conducting state. Here, while the addition of the nanoparticles did not change the transport mechanism, they enhanced the magnitude of the current owing to facile charge confinement.
Electrical bistability under multiple voltage loops is a prerequisite for organic memory. Since we observed such a bistability (figure 3), we examined the devices for possible read-only memory (ROM) and random-access memory (RAM) applications. For ROM applications, we first induced a high or a low state and then probed the state of the device by measuring current under small voltage pulses. Figure 6a shows a plot of the device current at −0.9 V as a function of time after a high or a low state was induced. The magnitude of the current remained clearly different for the two conducting states, substantiating ROM applications for several hours. Such endurance makes the material suitable for organic memory devices. Since electrical bistability was observed under multiple voltage loops, the states could also be flip-flopped between the two conducting states. For RAM applications, we induced (write) the two conducting states in sequence and read them in between doing so. In figure 6b, we show plots of voltage sequence and corresponding current in a CdSe/PB-based device. The figure shows that while probing the high and the low states in repeated cycles, the current values remained clearly separable evidencing RAM applications.
In conclusion, we have fabricated devices based on alternate layers of an organic dye (PB) and semiconducting nanoparticles (CdSe) deposited via LbL electrostatic assembly. Owing to the introduction of nanoparticles, there is an enhancement in electrical bistability of PB. The on/off ratio in CdSe/PB systems is about two orders of magnitude greater than that in devices based on dye or nanoparticles. We have shown that the bistability is augmented because of facile carrier transport through the nanoparticles. The bistability in organic molecules is, in general, due to their conformational change. During the switching from a low- to a high-conducting state, the mechanism of conduction changes from an injection dominated to a bulk one. The devices based on CdSe/PB systems evidenced RAM and ROM applications.
B.C.D. acknowledges CSIR-NET Fellowship no. 09/080(0504)/2006-EMR-I, roll no. 503982. The authors acknowledge financial support through projects SR/S2/RFCMP-02/2005 and 2007/37/2/BRNS.
One contribution of 8 to a Theme Issue ‘Making nano-bits remember: a recent development in organic electronic memory devices’.
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