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Organic Electronics 9 (2008) 39–44

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Switching between different conformers of a molecule:Multilevel memory elements

Bikas C. Das, Amlan J. Pal *

Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata 700 032, India

Received 21 May 2007; received in revised form 22 June 2007; accepted 24 July 2007Available online 15 August 2007

Abstract

We report voltage-driven electrical bistability in an organic semiconductor, namely Ponceau SS. Conductance switchingto different levels or ‘‘multilevel switching’’ in devices based on thin-films is due to different density of high-conductingmolecules. In a monolayer of Ponceau SS, we have observed one low-conducting and two high-conducting states. Thisis due to three configurable planes of the molecule exhibiting at least two stable high-conducting conformers. Apart fromestablishing conductance switching to be a molecular phenomenon, the multilevel conductance in a monolayer shows thata single molecule can exhibit multilevel memory application.� 2007 Elsevier B.V. All rights reserved.

PACS: 73.61.Ph; 82.37.Gk; 85.35.�p; 85.65.+h

Keywords: Conductance switching; Memory phenomenon; Multilevel memory; Molecular multilevel memory

1. Introduction

With the success of organic semiconductors inthin-film transistors [1], light-emitting devices [2],sensors [3] and photovoltaic solar cells [4], the mate-rials are being considered for electronic memory ele-ments [5–14]. Apart from acting as switchingelements in integrated-circuits, the conjugatedorganics are expected to offer high-density memoryapplications to meet the need of the future. Duringthe last few years, several classes of organic materi-als exhibited such applications, which occurred due

1566-1199/$ - see front matter � 2007 Elsevier B.V. All rights reserved

doi:10.1016/j.orgel.2007.07.008

* Corresponding author. Tel.: +91 33 24734971; fax: +91 3324732805.

E-mail address: sspajp@iacs.res.in (A.J. Pal).

to electrical bistability. In general, a suitable voltagechanges the conformer of a molecule [5,15]. Whenboth the conformers are stable with a large differ-ence in their conductivities, the molecules exhibitelectrical bistability. It is manifested as two currentvalues at a voltage, with the preceding voltage pulsedetermining the conducting state. By controlling thedensity of high-conducting molecules in a device,multilevel conductivity has also been achieved inorganic memory devices [13,14].

In devices based on thin-films of organic materi-als with electrode metals having low electronegativ-ity, the bistability sometimes arises due to reversiblegrowth of metal filaments through redox reactions[16]. When individual molecules (or a 2D molecularlayer) with noble metal electrodes exhibit electrical

.

40 B.C. Das, A.J. Pal / Organic Electronics 9 (2008) 39–44

bistabilty, the phenomenon is explained in terms ofconformational change [5] and/or electroreductionof the molecule [17]. The bistability in such casesis hence a molecular phenomenon. With a suitablematerial, such systems may yield multilevel conduc-tivity for multibit memory storage. The chosen mol-ecules will then have to possess several stableconformers. In this article, we introduce such a mol-ecule, which, due to its multiplaner structure, yieldsmultilevel conductivity with associated memoryphenomenon in the molecular scale.

2. Experimental

The molecule for the present work is Ponceau SS(Acid red 150), which was purchased from AldrichChemical Co. Molecular structure of the moleculeis shown in the inset of Fig. 1. Both monolayerand spin-cast thin-films of the material were depos-ited and characterized. Devices based on spun-castfilms of Ponceau SS were fabricated on indium tinoxide (ITO) coated glass substrates (sheet resis-tance = 12 X/h). Ponceau SS in methanol (2 mg/ml) was spun at a speed of 1000 rpm resulting in afilm thickness of about 70 nm. The films were driedin vacuum at 60 �C for 12 h. Aluminum (Al) wasthermally evaporated at a pressure of 1 · 10�6 Torrto act as the top electrode. Area of a typical devicewas 6 mm2.

The devices were placed in a shielded vacuumchamber before current–voltage (I–V) characteris-tics were recorded at room temperature with aKeithley 486 picoammeter and Yokogawa 7651 dcsource. I–V characteristics were recorded at two

Fig. 1. I–V characteristics of a device based on spun-cast film ofPonceau SS in three loops. Inset shows the molecular structure ofa Ponceau SS molecule.

sweep directions (from +VMax to �VMax and from�VMax to +VMax) and also in voltage loops (from0 to +VMax to �VMax to +VMax). Here VMax repre-sents amplitude of voltage up to which bias wasswept. For read-only memory (ROM) and ran-dom-access memory (RAM) applications, voltagepulse of suitable amplitude and width were gener-ated by fast switching transistors triggered by a PC.

To obtain a monolayer of Ponceau SS via elec-trostatic assembly, layer-by-layer (LbL) films weredeposited with poly(allylamine hydrochloride)(PAH) as a polycation. A deprotonated n-typeSi(1 11) substrate (resistivity = 3–10 mX cm) wasfirst dipped in the polycationic bath (pH 6.5) for15 min followed by through rinsing in deionizedwater baths. The Si substrate was then dipped inthe Ponceau SS bath (5 mM) for 15 min followedby the same rinsing protocol in a separate set ofwater baths. This resulted in a monolayer of Pon-ceau SS due to electrostatic binding through itsSO�3 moieties. The dipping sequence was repeatedto obtain multilayer films (on quartz) to record elec-tronic absorption spectra.

Before recording scanning tunneling microscope(STM) images, the films were annealed in vacuumat 100 �C. Pt/Ir tip of the STM was lowered till acurrent of 0.5 nA was achieved at 0.5 V. The tipposition was then fixed to measure a set of I–V char-acteristics. Bias was swept in both directions. In cer-tain cases, a suitable voltage pulse was appliedbefore recording the I–V characteristics. Measure-ments were carried at room temperature and inambient condition.

3. Results and discussion

3.1. Conductance switching

Fig. 1 shows a typical I–V plot for a spun-castfilm of Ponceau SS sandwiched between ITO andAl electrodes. Ponceau SS exhibits electrical bista-bility. The I–V characteristics depend on the voltagesweep direction. The magnitude of device current ata voltage is higher during the sweep from a positivevoltage as compared to that from a negative one. Inother words, a suitable positive bias induces ahigher conducting state. The higher state is retainedeven when the bias is removed from the devicesterming the bistability as a memory-switching phe-nomenon. The On/Off ratio, the ratio between cur-rent values during the two voltage-sweeps, reachesup to 3000. When the bias was applied in loops,

Fig. 2. I–V characteristics of a device based on spun-cast film ofPonceau SS from different +VMaxs. For VMax = 2.7 V, currentvalue reached the limit of measuring instrument during thevoltage sweep. Inset shows current at 0.8 V as a function of VMax.

B.C. Das, A.J. Pal / Organic Electronics 9 (2008) 39–44 41

the I–V characteristics retraced for both the con-ducting states showing (high) reproducible natureof the electrical bistability in Ponceau SS.

The degree of bistability, or the conductivity ofthe high-state depends on the amplitude of VMax.Plots for +VMax to 0 V for different VMax are shownin Fig. 2. The inset of the plot summarizes theresults showing the dependence of device current(at a voltage) as a function of VMax. The plot showsthe current (at 0.8 V) increases with VMax – the rateof rise initially being low – giving rise to multilevelconductivity. Such dependence in a thin-film devicemay be due to switching of more number of mole-cules by the application of higher voltage amplitude.It may also arise if the molecule has more than onehigh-conducting (stable) conformers.

Fig. 3. (a) Multilevel Read-Only Memory of a device based on spun-cas2 s; duty cycle = 17 %); for (11), current values were divided by six for cthe same device. Current under ‘‘write-read-erase-read’’ voltage sequen

3.2. Multilevel memory

Multilevel memory has indeed been observed inthis system. To demonstrate multilevel read-onlymemory (MROM), we applied different pump volt-ages and probed the states by applying a small volt-age. Fig. 3a shows the current under probe voltageas a function of time after a suitable pump voltagepulse was applied. Here, pump voltages were ±1.8,±2.2 and ±2.6 V (width = 10 s). While the positivevoltages induced high-conducting states, the nega-tive ones reinstated the low-state. The figure showsthat current under probe voltage for the high-statedepends on the magnitude of preceding pump pulse.In probing the low-conducting state, the currentremained unaltered for the three cases. It furthershows that we could successfully reinstate the low-conducting state every time, showing reproducibilityof conductance switching. Here, the low-state maybe referred to as (00), whereas the high-conductingstates pumped by +1.8, +2.2 and +2.6 V may betermed as (01), (10) and (11), respectively, resem-bling two-bit memory elements in a single device.

Fig. 3b shows that multilevel random-accessmemory (MRAM) applications can also beobserved in these devices. We ‘‘write’’ the threehigh-conducting states and ‘‘erase’’ them to thelow-state every time. After establishing one of thefour states, it is ‘‘read’’ by the application of a smallvoltage. In effect, the device undergoes a ‘‘write-read-erase-read’’ voltage pulse sequence with‘‘write’’ pulse amplitude of +1.8, +2.2 and +2.6 V(width = 10 s). While the ‘‘erase’’ pulse has a valueof –2.6 V (width 10 s), +0.8 V was applied as ‘‘read’’voltage pulse. The results show that the current

t film of Ponceau SS. Current was read under +0.8 V pulse (widthomparison. (b) Multilevel Random-Access Memory application ofce is presented.

42 B.C. Das, A.J. Pal / Organic Electronics 9 (2008) 39–44

under the probe voltage for the low- and three high-conducting states differed distinctively. The fourstates, namely (00), (01), (10) and (11), can hencebe achieved and probed in a device for RAM appli-cations between two-bits. The results further showreversible nature of conductance switching to thethree high-conducting states.

3.3. Formation of a monolayer

Though the multilevel memory in devices basedon Ponceau SS may arise due to varied density ofhigh-conducting molecules, we investigated the pos-sibility of more than two stable conformers in Pon-ceau SS. In doing so, we chose a monolayer ofPonceau SS on doped Si(1 11) wafers deposited viaelectrostatic assembly, so that progressive switchingalong the depth of the device does not occur. Topo-graphic image of the bare wafer and monolayer ofPonceau SS are presented in insets (a) and (b) ofFig. 4, respectively. A clear difference between theimages is certainly due to deposition of PonceauSS during electrostatic adsorption via bindingthrough the two SO�3 groups.

To further confirm deposition of Ponceau SSduring LbL deposition, we recorded electronicabsorption spectra of the films on quartz substrate.Since the value of absorbance for a monolayer waslow, we confirmed film formation by monitoring itsprogress during multilayer deposition. Electronic

Fig. 4. Electronic absorption spectra of LbL films of Ponceau SSfor different number of layers. Insets show topographic STMimage of (a) a bare Si substrate and (b) a monolayer of PonceauSS on Si. The STM measurements were recorded in a constantcurrent mode (0.5 nA) at a 0.5 V bias. The displayed scan area is85 nm · 85 nm in both the cases. Inset (c) shows absorbance at515 nm as a function of number of layers.

absorption spectra of different number of layers ofLbL films are shown in Fig. 4. All the spectra showa peak at 515 nm – the intensity of the band increas-ing with number of layers deposited. The band at515 nm is very close to that in Ponceau SS solution(516 nm). Such a low shift further shows that Pon-ceau SS did not form aggregates in LbL films. Theinset (c) of Fig. 4 shows the absorbance of the filmat 515 nm as a function of number of Ponceau SSlayers. A linear plot through the origin with a slopeof unity confirms that the Ponceau SS moleculeswere adsorbed uniformly during deposition of everylayer via LbL assembly.

3.4. Multilevel memory in a monolayer

We characterized the 2D array of Ponceau SS bySTM tip. Here since Ponceau SS molecules areattached to the substrate by electrostatic bindingvia the two SO�3 groups, the two planes containingbenzene rings connected through N@N remainsfreely configurable (to reach a local low-energy con-figuration). To record I–V characteristics of the low-and other possible different high-conducting states,we first applied a suitable voltage pulse and thenscanned I–V characteristics in a small voltage rangein loops. VMax for the I–V characteristics rangedfrom ±1.0 to ±1.5 V. In this experiment, while thewidth of the pulse was kept the same (10 ms), theamplitude of the pulse varied up to 8.5 V (Fig. 5).The figure shows that the current in the forwardbias depends strongly on the preceding ‘‘write’’ volt-age pulse. There was however little difference in thereverse bias current. At any forward voltage, highercurrent was observed when amplitude of the ‘‘write’’pulse was higher. The increase in current was notmonotonic (in contrast to the case of the devicebased on spun-cast film) with the amplitude of thepulse. The I–V plots were clubbed or ‘‘bunched’’.The results are summarized in inset (a) of Fig. 5as a plot of current (at 0.8 V) as a function of ampli-tude of the preceding voltage pulse. The currentshows three steps. By the application of a reversebias pulse, the low-conducting state was induced.Depending on the amplitude of voltage pulse, twodistinctively different probe current was observed.This shows that multiple conducting states in amonolayer can be achieved due to different high-conducting configurations of a single molecule. Inother words, one low- (00) and at least two high-conducting states (two of 01, 10, 11) could beobserved in a single Ponceau SS molecule. Absence

Fig. 5. I–V characteristics with a STM tip of a monolayer of Ponceau SS. Voltage-sweeps were carried out after application of voltagepulse of different amplitudes. For the off-state, pulse amplitude was up to 2.5 V. For the first and second on-states, the amplitude rangedfrom 3.0 to 5.5 V and 6.0 to 8.5 V, respectively. Insets show (a) tunneling current at +0.8 V as a function of amplitude of preceding voltagepulse (fixed width of 10 ms) and (b) tunneling current at +0.8 V as a function of width of preceding voltage pulse (amplitude being 5.0 and7.0 V representing first and second on-states, respectively).

B.C. Das, A.J. Pal / Organic Electronics 9 (2008) 39–44 43

of a monotonic increase in inset (a) of Fig. 5 showsthat even though the STM measurements involve anumber of molecules in parallel, different high-con-ducting states did not arise due to switching of morenumber of molecules in the 2D plane. We may addthat by application of a suitable negative voltagepulse, the pristine low-conducting state can alwaysbe reinstated showing reproducibility of switchingin a monolayer or a molecule of Ponceau SS. Con-trol experiments on only bare Si wafer yielded muchhigher current as compared to that with Ponceau SSmonolayer. The observation of electrical bistabilityin a monolayer with STM tip further excludes thepossibility of metal filament formation upon diffu-sion of metal cations.

To verify if the width of voltage pulse has anyrole in inducing different high-conducting states,we have carried out the following experiment. Bykeeping the amplitude of voltage pulse the same,we varied the width of the pulse that precedes theI–V sweep. Inset (b) of Fig. 5 shows the current at+0.8 V (from I–V sweep) as a function of pulsewidth that ranged from 1 to 1000 ms. Measurementswere carried out for the two high-states by applying

5.0 and 7.0 V amplitude pulses, respectively. The fig-ure shows that the current did not depend on thewidth of the preceding pulse. In each of the twohigh-conducting states, all the molecules must haveswitched to a particular configuration by the appli-cation of a pulse width of more than 5 ms. The high-state induced by a 7.0 V pulse cannot be achieved byapplying a 5.0 V pulse of very long width. Similarly,a very short pulse of 7.0 V cannot induce an equiv-alent state induced by a 5.0 V pulse. From the insetsof Fig. 5, we conclude that the amplitude of voltagepulse is the key factor in obtaining different con-former or conducting state in a molecule.

4. Conclusions

In summary, we have shown that Ponceau SSmolecules exhibit multilevel memory-switchingproperty. When a monolayer of the molecule ischaracterized by STM, the molecules exhibit onelow- and two high-conducting states. The threestates arise due to different conformers of the mole-cules. Amplitude of voltage pulse determines theconformer or corresponding high-state of the

44 B.C. Das, A.J. Pal / Organic Electronics 9 (2008) 39–44

molecule. The width of the pulse has little or noeffect in the 1–1000 ms range. In thin-film baseddevices, amplitude of voltage pulse determines thedensity of high-conducting molecules and hencethe level of high-conducting states. Such devicesexhibit multilevel ROM and RAM phenomena.Our results show that while in a spun-cast filmbased device multilevel memory is a bulk property,for a monolayer of Ponceau SS, it appears due todifferent conformers of a single molecule itself.The results open up a route to achieve multilevelmemory elements in the molecular scale.

Acknowledgments

B.C.D. acknowledges CSIR Junior Research Fel-lowship No. 09/080(0504)/2006-EMR-I, Roll No.503982. The Department of Science and Technol-ogy, Government of India, financially supportedthe work through Ramanna Fellowship SR/S2/RFCMP-02/2005.

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