Langmuir 2007, 23, 9831-9835
9831
Conductance Switching in an Organic Material: From Bulk to Monolayer Arup K. Rath and Amlan J. Pal* Indian Association for the CultiVation of Science, Department of Solid State Physics and Centre for AdVanced Materials, JadaVpur, Kolkata 700032, India ReceiVed April 18, 2007. In Final Form: July 9, 2007 Fluorescein sodium, which does not exhibit electrical bistability in thin films, can be switched to a high conducting state by the introduction of carbon nanotubes as channels for carrier transport. Thin films based on fluorescein sodium/carbon nanotubes display memory switching phenomenon among a low conducting state and several high conducting states. Read-only and random-access memory applications between the states resulted in multilevel memory in these systems. Results in thin films and in a monolayer (deposited via layer-by-layer assembly) show that instead of different molecular conformers, multilevel conducting states arise from the different density of high conducting fluorescein molecules.
Introduction To fulfill the technological demand of ultrahigh-density memory elements, focus has now shifted to organic molecules. Certain organic molecules exhibit electrical bistability in their current-voltage (I-V) characteristics.1-13 In an electrical bistable device, at least two conducting states are observed at a particular voltage. A suitable voltage pulse induces a high conducting state. Electrical bistability in devices based on organic materials has been explained in terms of conformational change,8,9 electroreduction,4,10 the formation of metal filaments through redox reactions,11-13 and so forth. Some of the molecules, where the high state is stable after withdrawing the bias, can be used for memory applications.3-7 In recent years, read-only and randomaccess memory applications (ROM and RAM, respectively) have been demonstrated in systems based on organic molecules.3-7 Approaches have been made to make memory elements addressable.14 Although there are in-depth reports on the measurement of conductivity of nanostructures or single molecules15-17 using nanogaps fabricated by electron beam * To whom correspondence should be addressed. Tel: +91-33-24734971. E-mail:
[email protected]. (1) Gao, H. J.; Sohlberg, K.; Xue, Z. Q.; Chen, H. Y.; Hou, S. M.; Ma, L. P.; Fang, X. W.; Pang, S. J.; Pennycook, S. J. Phys. ReV. Lett. 2000, 84, 1780. (2) Terai, M.; Fujita, K.; Tsutsui, T. Jpn. J. Appl. Phys. 2006, 45, 3754. (3) Ma, L. P.; Liu, J.; Yang, Y. Appl. Phys. Lett. 2002, 80, 2997. (4) Bandhopadhyay, A.; Pal, A. J. J. Phys. Chem. B 2003, 107, 2531. (5) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (6) Lewis, P. A.; Inman, C. E.; Yao, Y. X.; Tour, J. M.; Hutchison, J. E.; Weiss, P. S. J. Am. Chem. Soc. 2004, 126, 12214. (7) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid State Lett. 2002, 5, E43. (8) Ssenyange, S.; Yan, H. J.; McCreery, R. L. Langmuir 2006, 22, 10689. (9) Colle, M.; Buchel, M.; de, Leeuw, D. M. Org. Electron. 2006, 7, 305. (10) Yang, Y.; Ma, L. P.; Wu, J. H. MRS Bull. 2004, 29, 833. (11) Chen, J. S.; Ma, D. G. Appl. Phys. Lett. 2005, 87, 023505. (12) Lauters, M.; McCarthy, B.; Sarid, D.; Jabbour, G. E. Appl. Phys. Lett. 2006, 89, 013507. (13) Graves-Abe, T.; Sturm, J. C. Appl. Phys. Lett. 2005, 87, 133502. (14) Tanaka, H.; Yajima, T.; Matsumoto, T.; Otsuka, Y.; Ogawa, T. AdV. Mater. 2006, 18, 1411. (15) Xue, Y. Q.; Datta, S.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. Phys. ReV. B 1999, 59, R7852. (16) Donhauser, Z. J.; Mantooth, B. A.; Pearl, T. P.; Kelly, K. F.; Nanayakkara, S. U.; Weiss, P. S. Jpn. J. Appl. Phys. 2002, 41, 4871. (17) Zandbergen, H. W.; van Duuren, R. J. H. A.; Alkemade, P. F. A.; Lientschnig, G.; Vasquez, O.; Dekker, C.; Tichelaar, F. D. Nano Lett. 2005, 5, 549.
lithography, break junctions, electrochemical growth, electromigration, and so forth, it is always difficult to address a nanoscale object properly. Hence, instead of addressing a single molecule, finite-sized composite systems (with nanomaterials) as active elements may provide a route to the addressability of memory elements.18,19 In this article, we show that the nanomaterials in the composites play a dual role: apart from offering the addressability of memory elements, carriers may be transported through the nanotubes to switch organic molecules in the bulk of the device. In this article, we show how a nanomaterial may trigger or accelerate conductance switching in organic devices, which in the absence of the same would not have exhibited electrical bistability at all. Experimental Section Fluorescein sodium, multiwalled carbon nanotubes (CNTs), poly(allylamine hydrochloride) (Mw ) 70 000 g/mol), and polyvinyl alcohol (99+ % hydrolyzed, Mw ) 85 000-146 000 g/mol) were purchased from Aldrich Chemical Co. and were used without further purification. The outer diameter, wall thickness, length, and purity of CNTs were 20-30 nm, 1-2 nm, 0.5-2.0 µm, and >95%, respectively. The CNTs were functionalized following the route proposed by Qin et al.20 They were converted to their acid form by sonicating in a concentrated nitric acid/sulfuric acid mixture (1:3) at 50 °C for 6 h. Excess acid was removed by centrifuging in deionized water repeatedly until the pH of water was around 7. Finally, the centrifuged product was dried in vacuum at 60 °C. During the chemical treatments involved in the functionalization process, the impurities present in the pristine CNTs were also removed. Devices were fabricated on stripped indium tin oxide (ITO)coated glass substrates, which were cleaned and processed following standard protocol. To obtain spun-cast films, fluorescein sodium was first dissolved in methanol (2 mg/mL). Measured amounts of acidified CNTs was added to fluorescein sodium solution and sonicated for 4 h to obtain a homogeneous mixture of fluorescein sodium/CNT. In the mixed solution, concentration of CNTs in fluorescein sodium varied between 0 and 15 wt %. To obtain films of CNTs without fluorescein sodium, a methanol solution containing polyvinyl alcohol (2 mg/mL) and CNTs (20 wt %) was sonicated thoroughly. Fluorescein sodium, fluorescein sodium/CNT (5, 10, (18) Mukherjee, B.; Batabyal, S. K.; Pal, A. J. AdV. Mater. 2007, 19, 717. (19) Mukherjee, B.; Mohanta, K.; Pal, A. J. Chem. Mater. 2006, 18, 3302. (20) Qin, Y.; Shi, J.; Wu, W.; Li, X.; Guo, Z. X.; Zhu, D. J. J. Phys. Chem. B 2003, 107, 12899.
10.1021/la701132f CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
9832 Langmuir, Vol. 23, No. 19, 2007 and 15 wt %), and polyvinyl alcohol/CNT (20 wt %) solutions were spun at 1000 rpm. The thin films, which had a thickness of 50-70 nm, were annealed at 60 °C in vacuum (10-3 Torr) for 2 h. Aluminum (Al) was vacuum evaporated on top of the annealed films from a tungsten filament basket at a pressure below 10-5 Torr. A mechanical shutter protected the films from excess heat before and after Al deposition. The active area of each of the devices, determined by the overlap of ITO and Al strips, was 6 mm2. To obtain a monolayer of fluorescein via electrostatic assembly, layer-by-layer (LbL) films were deposited with poly(allylamine hydrochloride) (PAH) as a polycation. A deprotonated n-type Si(111) substrate (resistivity 3-10 mΩ‚cm) was first dipped in the polycationic bath (concentration 10 mM, pH 6.5) for 15 min, followed by thorough rinsing in deionized water baths. The Si substrate was then dipped in the fluorescein sodium bath (5 mM) for 15 min, followed by the same rinsing protocol in a separate set of water baths. This resulted in a bilayer of PAH/fluorescein where a monolayer of fluorescein was electrostatically bound to the PAH monolayer through the -COO- and -O- moieties of fluorescein sodium (in aqueous solution). The dipping sequence was repeated to obtain a different number of bilayer films on quartz to record electronic absorption spectra (with a Shimadzu UV-2550 UV-visible spectrophotometer). To study the electrical characteristics, the sandwiched structures (ITO/film/Al) were kept in a shielded vacuum chamber. For each type of device, a number of them were characterized to check the results’ reproducibility. Measurements were carried out at room temperature. I-V characteristics of the devices were recorded with a Keithley 6517 electrometer. Bias was applied to the ITO electrode with respect to the Al one. The voltage step was 0.05 V, and the current was measured after 2 s, resulting in a scan speed of 25 mV/s. A dc source (Yokogawa 7651), coupled with fast-switching transistors, generated a voltage pulse of different widths and amplitudes, which were used to “write” or “read” or “erase” a state of the devices. The instruments were controlled by a PC via a generalpurpose interface bus (GPIB). Measurements were carried out with LabView software. A bilayer of PAH/fluorescein (on doped Si) was characterized with the Pt/Ir tip of an STM (Nanosurf easyScan2) controller in noncontact (tunneling) mode under ambient condition. Bias was applied to the tip with respect to the base Si electrode.
Results and Discussion Electrical Bistability in Fluorescein Sodium/CNT. It is intriguing to observe that a device based on spun-cast films of fluorescein sodium does not exhibit electrical bistability whereas those based on fluorescein sodium/CNT do. Figure 1 shows I-V characteristics of five devices based on different spun-cast films in a voltage loop. Although the devices based on fluorescein sodium and CNTs in polyvinyl alcohol do not exhibit bistability, the three devices with fluorescein sodium/CNT (5, 10, and 15 wt %) show electrical bistability with associated memory phenomenon. All three devices (based on fluorescein sodium/ CNT) switch at a negative voltage. The on/off ratio, which is the ratio between the current in the two states at a certain voltage, is expectedly voltage-dependent. Because the high-conductivity state is retained even without any bias, the electrical bistability in the present system can be termed a memory-switching phenomenon. The on/off ratio is the highest in an intermediate case. At a higher CNT concentration, the ratio decreases, which can be due to an increase in the off-state current. We aimed to investigate the difference in the nature of I-V characteristics in the fluorescein sodium and fluorescein sodium/ CNT systems. Though most of the biplanar molecules in the xanthene class exhibit electrical bistability, fluorescein sodium does not.4 This is due to the inherent low conductivity of the molecule, which, in contrast to other xanthene class molecules, does not contain any electron-accepting functional groups such as iodine, chlorine, bromine, and so forth. The CNTs in fluorescein
Rath and Pal
Figure 1. I-V characteristics of devices based on fluorescein sodium, fluorescein sodium/CNT, and CNT in a polyvinyl alcohol (PVA) matrix in a voltage loop. Arrows show the direction of the voltage scan.
sodium/CNT systems may hence provide channels for carrier transport to fluorescein sodium in the bulk of the device. As in other xanthene molecules, the carriers can electroreduce fluorescein sodium molecules, possibly with a change in their conformation. The reduced molecules, which are stable and have higher conductivity, yield a high conducting on state in the fluorescein sodium/CNT film-based devices. With an increase in the CNT concentration, the density of carrier-transporting channels increases, resulting in a rise in both high- and low-state currents. The intermediate CNT concentration provides an optimum number of channels maximizing the on/off ratio. The CNTs hence provide a route or channels to switch an organic material with the further possibility of tuning the on/off ratio. The switching between the two states is reversible in nature and occurs in cycles. The reproducibility of the measurements was high, as evidenced by little variation in the I-V characteristics from a cell to others (Supporting Information). Figure 2a shows the I-V characteristics of the device based on fluorescein sodium/ CNT (10%) in three consecutive loops. The Figure shows that when bias is applied in loops the electrochemical reaction processes also cycle reversibly, resulting in retraceable I-V characteristics. The Figure also shows that the on/off ratio remains invariant from one loop to the other. The on/off ratio, however, depends on the voltage up to which the device is biased. Figure 2b shows the reverse bias section of the I-V curves of a fluorescein sodium/CNT (10%) device switched by different voltages. Here, voltage was scanned in a loop up to different maximum voltage (VMax). The results show that whereas the off-state current (after induced by +VMax) remains mostly the same the on-state current responds strongly to -VMax. That is, the amplitude of current at any bias increases with (the amplitude of) the switching voltage. The inset of Figure 2b sums up the results as a plot of current at a particular voltage versus the magnitude of VMax. The plot shows a monotonic increase in the on-state current with the amplitude of -VMax that induces the high conducting states. For the low conducting off state, as induced by +VMax values, the current at a certain voltage does not depend on VMax. Such I-V characteristics may lead to multilevel conductance and multilevel memory elements.21,22 (21) Mukherjee, B.; Pal, A. J. Appl. Phys. Lett. 2004, 85, 2116. (22) Lauters, M.; McCarthy, B.; Sarid, D.; Jabbour, G. E. Appl. Phys. Lett. 2005, 87, 231105.
Conductance Switching in an Organic Material
Langmuir, Vol. 23, No. 19, 2007 9833
Figure 2. I-V characteristics of devices based on fluorescein sodium/CNT (10 wt %) (a) in three consecutive voltage loops and (b) up to different voltages, VMax (shown as legends). Only the negative voltage region is shown in part b. The molecular structure of fluorescein sodium is shown in part a. The inset of part b sums up the I-V characteristics showing a current at -1.5 V for the low state (sweep toward -VMax) and different high states (sweep toward +VMax). Arrows show the direction of the voltage scan.
Figure 3. (a) Multilevel ROM and (b) multilevel RAM application of a fluorescein sodium/CNT (10 wt %)-based device. In part a, the amplitude of the current under -0.8 V is plotted. In part b, the voltage pulse sequence is shown in the upper panel. The states, as identified by the current under the probe voltage, have been shown as broken lines and legends.
Multilevel ROM and RAM. In fluorescein sodium/CNT devices, because the current at a certain voltage depends on the preceding voltage pulse’s amplitude, one can make use of this property for ROM and RAM applications. Moreover, the monotonic increase in current (at a certain voltage) as a function of -VMax, as shown in the inset of Figure 2b, may lead to multilevel memory applications.21,22 For ROM applications, we chose four voltage pulses, namely, -2.0, -2.3, -2.6, and -2.9 V (width 30 s) to induce four different high conducting states. In addition, by applying a +2.1 V pulse of the same width, we reinstated the off state. The time response of all of the states has been probed under a -0.8 V pulse (width 2 s, duty cycle 2%). Figure 3a shows that whereas the current under a probe voltage is low for the off state the current is increasingly higher for the on states. The states are clearly distinguishable for several hours and depict multilevel ROM applications Because the states can be flip-flopped among them, we aimed for multilevel RAM applications. Here, after inducing and probing each of the high states, the initial low conducting state was reinstated and probed again. In Figure 3b, we show current under a -0.8 V probe voltage pulse (width 2 s, duty cycle 33%) after inducing the four high conducting states in sequence. Here, the low conducting state has been reinstated after probing each of the high states. The sequence of voltage applied to the device is presented in the upper panel of the Figure. The results show that the current under a probe voltage depends on the state that is last induced. The states can be flip-flopped sequentially for
many cycles, providing evidence of multilevel RAM. The high states can also be flip-flopped randomly (figure not shown). If we label four of the five states as 00, 01, 10, and 11, then the results presented in Figure 3b are an example of a two-bit RAM application. Transport Mechanisms. The different high conducting states may be due to either more than one high conducting conformer of fluorescein sodium or a different density of high conducting moieties. To identify the origin of more than two states, we modeled the I-V characteristics in their low and high states. Figure 4 shows a reverse bias section of the I-V plots on a double logarithmic scale displaying both off and on states. Here, plots of different VMax values, which induce different high conducting states, have been summed up. The I-V curves for the low conducting state for different +VMax values fit to pure space-charge limited conduction are given by23 9
J)
/8qr0µV2 d3
where J is the current density, q is the electronic change, r is the relative dielectric constant of the material, 0 is the permittivity of free space, d is the thickness of the film, and µ is the carrier mobility. The plots for different VMax values yield a slope of 1.9, (23) Lampert, M. A.; Mark, P. Current Injection in Solids; Academic Press: New York, 1970.
9834 Langmuir, Vol. 23, No. 19, 2007
Figure 4. I-V plots of a fluorescein sodium/CNT (10 wt %) device on a double logarithmic scale for different VMaxvalues. The straight lines show linear fits to the off and on states with slopes of 1.9 and 1.5, respectively. The symbols have the same meaning as in Figure 2b.
which is close to that expected from the model. The I-V plots for the high states deviated from the space-charge limited conduction (SCLC) model. The plots for the high states, as induced by different VMax values, however, yield a single slope of 1.5, suggesting that an identical conduction mechanism is applicable to all of them. This points toward a system where different high conducting states may not have resulted from different conformers of fluorescein sodium. Different molecular conformers, which also tune the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), would have resulted in variations in the band gap, barrier height for electrons and holes with electrodes, and hence conduction mechanisms. A slope of 1.5 for the I-V plot indicates that ohmic conduction may be partially applicable in all of the high states of the devices. Bistability in a PAH/Fluorescein Bilayer. To focus further on the origin of more than one high conducting state, we chose a monolayer of fluorescein on the PAH monolayer deposited via LbL electrostatic assembly so that progressive switching along the depth of the device does not occur. Here, doped Si(111) wafers, on which the monolayers were grown, act as the bottom electrodes. To confirm the deposition of fluorescein during LbL deposition, we recorded the electronic absorption spectra of the films on a quartz substrate. Because the value of absorbance for a monolayer should be low, we confirmed film formation by monitoring its progress during multilayer LbL film deposition. Electronic absorption spectra of different numbers of bilayers of PAH/fluorescein LbL films are shown in Figure 5. All of the spectra show a peak at 506 nm, with the intensity of the band increasing with the number of layers deposited. The band at 506 nm represents the electronic absorption of fluorescein. The inset of Figure 5 shows the absorbance of the film at 506 nm as a function of the number of layers. A linear plot through the origin with a slope of unity confirms that the fluorescein molecules were adsorbed uniformly during the deposition of every bilayer via LbL assembly. Topographic images of a bare wafer and a monolayer of fluorescein (on a PAH monolayer) are presented in insets a and b of Figure 5, respectively. A clear difference between the images is certainly due to the deposition of PAH/ fluorescein during electrostatic adsorption via binding through the -COO- and -O- groups. We have characterized 2D arrays of a bilayer of PAH/ fluorescein by an STM tip (Figure 6). As control experiments,
Rath and Pal
Figure 5. Electronic absorption spectra of LbL films of different numbers of bilayers of PAH/fluorescein. The rightmost inset shows the absorbance at 506 nm as a function of the number of bilayers. The other insets show topographic STM images of (a) a bare Si substrate and (b) a bilayer of PAH/fluorescein on Si. The STM measurements were recorded after the tip was approached in current feedback mode (0.5 nA at a 0.5 V bias). The displayed scan area is 50 nm × 50 nm in both cases.
Figure 6. I-V characteristics with an STM tip of a monolayer of fluorescein. Voltage sweeps were carried out after the application of a voltage pulse of different amplitudes.
we have measured the I-V characteristics of a native SiO2 film and a PAH monolayer on Si substrates. Although the results from the control experiments do not show any transition in electrical conductance (Supporting Information), a monolayer of fluorescein exhibits electrical bistability. The bistability in a monolayer of fluorescein contrasts with its absence in a spuncast film (Figure 1). The absence of bistability in the later case must then be due to the difficulty of carrier transport that has been supplemented by CNTs. To record the I-V characteristics of the low and other possible high conducting states, we first applied a suitable voltage pulse and then scanned the I-V characteristics in a small voltage range (Figure 6). In this experiment, the width of the pulse was kept the same (0.2 ms). When a positive voltage pulse was applied, it induced a low conducting state as evidenced by low current in the I-V characteristics. To achieve a high state, the threshold voltage was -2.8 V. The I-V plots show that a negative voltage
Conductance Switching in an Organic Material
pulse of different amplitudes induces a single high conducting state. That is, the I-V characteristics do not depend on the amplitude of bias that induces the high state. This is in contrast to the I-V characteristics of devices based on spun-cast films of fluorescein. Such I-V characteristics in a monolayer of fluorescein could be possible if the fluorescein molecules have only one high conducting state. Because the STM measurements involve a number of molecules in parallel, the two I-V curves suggest that at best two conducting states of fluorescein are possible. This implies that the multilevel conductance in fluorescein sodium/CNT thin films should be due to a different density of high conducting moieties.
Conclusions We have observed that whereas a monolayer of fluorescein exhibited electrical bistability its thin-film form did not do so. We have presented a route to switch the thin film of organic molecules. CNTs introduced into the bulk of the thin film of the organic molecule provided channels for carriers that electroreduced and switched the conductivity of fluorescein sodium. The bistability is associated with a memory phenomenon. In the present
Langmuir, Vol. 23, No. 19, 2007 9835
system (fluorescein sodium/CNT), we have observed multilevel ROM and RAM applications. The appearance of more than one high conducting state is not due to different conformers of the molecules. Identical transport mechanisms for all of the high conducting states and the observation of a single high state in a monolayer point to the following: instead of different conformers, different densities of high conducting molecules result in different high states. Acknowledgment. A.K.R. acknowledges a CSIR Junior Research Fellowship (no. 09/080(0505)/2006-EMR-I, roll no. 503974). The Department of Science & Technology, Government of India, financially supported this work through a Ramanna Fellowship (SR/S2/RFCMP-02/2005). Supporting Information Available: Current-voltage characteristics of three sandwiched structures based on fluorescein sodium/ CNT (10%) thin films, a native SiO2 film, and a PAH monolayer on Si substrates by an STM tip. This material is available free of charge via the Internet http://pubs.acs.org. LA701132F
