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A Supramolecular Nanofibre Based Passive Memory Device for Remembering Past Humidity Umesha Mogera, Murali Gedda, Subi J. George, and Giridhar U. Kulkarni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10732 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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A Supramolecular Nanofibre Based Passive Memory Device for Remembering Past Humidity Umesha Mogera,†,χ Murali Gedda,†,χ Subi J George‡ and Giridhar U Kulkarni*,ψ ,ξ †

Chemistry and Physics of Materials Unit and Thematic Unit on Nanochemistry, Jawaharlal

Nehru Centre for Advanced Scientific Research Bangalore 560064 (India) ‡

Supramolecular Chemistry Laboratory, New Chemistry Unit Jawaharlal Nehru Centre for

Advanced Scientific Research, Bangalore 560064 (India)

ψ

Centre for Nano and Soft Matter Sciences, Bangalore, 560013 (India)

ξ

On lien from Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560064

(India)

KEYWORDS memory device, passive memory, humidity sensor, supramolecule, nanofibre.

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ABSTRACT Memorizing the magnitude of a physical parameter such as relative humidity in a consignment, may be useful while maintaining recommended condition over a period of time. In relation to cost and energy considerations, it is important that the memorising device works in the unpowered passive state. In this article, we report the fabrication of a humidity-responsive device that can memorize the humidity condition it had experienced while being unpowered. The device makes use of supramolecular nanofibers obtained from the self-assembly of donor– acceptor (D-A) molecules, coronene tetracarboxylate salt (CS) and dodecyl methyl viologen (DMV) respectively, from aqueous medium. The fibres while being highly sensitive to humidity tend to develop electrically induced disorder under constant voltage, leading to increased resistance with time. The conducting state can be regained via self-assembly by exposing the device to humidity in the absence of applied voltage, the extent of recovery depending on the magnitude of the humidity applied under no bias. This nature of the fibres has been exploited in reading the humidity memory state which interestingly, is independent of the lapsed time since the humidity exposure as well as the duration of exposure. Importantly, the device is capable of differentiating the profiles of varying humidity conditions from its memory. The device finds use in applications requiring stringent condition monitoring.

1. INTRODUCTION Conventionally, memory pertaining to a signal is stored either digitally1-3 in the form of 0 and 1 or in the analogue form, for instance, as an electric signal on a magnetic tape.4,5 Both invariably require an active circuit working alongside which, considering energy supply and cost, may not always be affordable and practical. Depending on context, especially relating to internet

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of things (IoT),6,7 a signal to be stored may relate to local changes in physical quantities such as temperature, pressure, sound, light, humidity and so on. There have not been many efforts in the literature to design devices which memorise past states passively (unpowered) with the active element not powered. Nonavailability of the relevant sensory materials may be one reason for the lack of progress in this direction. It is only recently that there is an upsurge in this field of passive devices recording physical variables of the past as memory. As examples, there are shock8,9 and pressure sensors1013

which when attached to a package, can sense and memorise any shock or tension exerted on

the package. New types of labels and indicators have entered the market to enable monitoring of health of vegetables and fruits.14-16 Of late, Ham et. al. using gold nanoparticles embedded in a polymer matrix reported a colorimetric sensor which remembers the stress it has undergone.17 Stimuli-responsive materials, especially shape memory polymers, have been investigated in which the shape of polymer element changes when a physical quantity such as temperature,18 light,19 pH,20 heat,21 or electric field21 is varied. These sensors are being increasingly deployed in industries particularly in smart packaging used in transport and distribution.22 Smart packaging which involves time-temperature indicators, ripeness indicators, chemical sensors and radio frequency identification (RFID) as main components, has found immense application while ascertaining the authenticity, traceability, tampering, theft as well as safety of goods.22-24 Thus, the field of passive sensors is fast progressing. In this study, we report a humidity based passive memory device, which to the best of our knowledge, is first of its kind. The device has been realised using a supramolecular 1D nanofibre system made up of coronene tetracarboxylate (CS) and dodecyl methyl viologen derivative (DMV)) based donor-acceptor (D-A) pairs. These nanofibers spread across µm spaced electrodes

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act as the resistive element in the sensor. Earlier reports from this laboratory have established ultrafast humidity sensing25 using this nanofibre system as active element. In another instance, it has served as a field effect transistor (FET) element.26 In this report, we describe a memory action wherein the device is made to remember the past humidity states it was exposed to. 2. RESULTS AND DISCUSSION

Figure 1. Schematic of the device fabrication. (a) Schematic representation of sensor fabrication and its active elements. AFM image of the nanofibers is shown overlapping the electrodes. (b) Schematic of the self-assembled D-A molecules to form nanofibers. (c) An optical microscopy image of the nanofibre film spread across the Au gap (centre) electrodes. Scale bar: 100 µm. (d) A high magnification FESEM image of the nanofibre film, depicting its 1D fibrous geometry. Scale bar: 200 nm.

The device consists of two gold electrodes, separated by 10 µm channel length on a glass substrate. A dispersion of supramolecular nanofiber (1 mM, 10 µL) was placed on it and allowed to vacuum evaporate overnight (see schematic in Figure 1a). The details of self-assembly of the

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D-A molecules to form nanofiber are reported elsewhere.27 Briefly, it involves charge-transfer driven alternate co-assembly of CS and DMV based D-A molecules. In water, these two components follow a surfactant-like assembly to form cylindrical micelles of diameter ~ 6 nm. This forms bilayers of charge transfer amphiphiles arranged radially with the D and A pairs stacked face-to-face along the length of the nanofiber (see Figure 1b). The nanofibre dispersion once drop coated on the Au electrodes nicely spread across all over including the gap region (see Figure 1c), providing a conduction path between the two electrodes. An important feature of the nanofibre system is its 1D geometry (see Figure 1d) though which the conduction takes place.28,29

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Figure 2. Decay behaviour of nanofiber with humidity. Temporal change in conductivity of nanofiber with RH cycles (blue curve) and constant RH (red dotted curve). The current states S1 and S2 are indicated.

The nanofibre conductivity is humidity dependent (Figure S1, Supporting Information) which is central to its humidity sensing action.25 For a given humidity (RH ~ 70%), the current through the fibre exhibits a jump initially, followed by a gradual decay to reach a nearly steady value on continued exposure to humidity. The initial jump is termed as state 1 (briefly S1) and the nearly steady current state, as S2. In our previous work, this stable current response region of the device (which we now term as S2 state) was used for humidity sensing over a wide range, 590% RH, with millisecond response and recovery times. The focus of the present study is on the nature of S1 to S2 transition. Figure 2 shows the data from RH cycles (5% to 70% and back to 5%). During the first RH cycle, as the RH value increased, the current also increased rapidly to S1 and decreased to zero as RH was withdrawn, due to the humidity sensing action (see blue curve). The response and recovery times being ~ few ms,25 the curve closely follows the RH pulse. For subsequent RH cycles, the magnitude of current jump upon exposing to RH decreased gradually. Interestingly, the decrease in the jump value was such that it closely mimicked the decay behaviour obtained when exposed to a constant RH (red dotted line). Thus, the nanofiber behaves as though it remembers the jump value of the preceding RH cycle. Importantly, this ‘memory effect’ does not seem to depend on the time width of the RH cycle, the sequence of the time periods as well as the time gap, if any, between the cycles (see Figure 2). Based on this striking behaviour, a humidity memory device was conceived. Before taking up discussion on memory action, two RH based terms are defined here, namely the curing RH pulse (CP) and the testing RH pulse (TP). CP is defined as the pulse whose magnitude decides the memory state of the device, while TP is that used for probing the

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effect of CP, in other words, a reading pulse. Thus, reading with TP is done with a fixed voltage (1 V) while during CP, the voltage is made zero. In this study, TP is held at 70% RH, unless otherwise mentioned. The pulse widths are typically few seconds.

Figure 3. Memory action of the device. The change in the conductive behaviour of the nanofiber during time spans 1 - 5 as marked. The sequence of voltage (orange curve) and RH values are shown; on the latter, TP (blue) and CP (green) are clearly marked. The dotted red line is the expected behaviour in presence of constant humidity.

The memory characteristic action of the device is shown in Figure 3. In time span 1corresponding to the readings with two TPs separated by ~ 8 seconds, the S1 and S2 states similar to those in Figure 2 are seen. The applied voltage (1 V) was then turned OFF for 10 seconds (time span 2) and turned ON again for 30 seconds (time span 3). Subsequent

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introduction of a TP in this span 3, gave rise to a S2 type signal (purple curve), reinforcing the memory action from the passive state of the device. However, when a CP (70%) was introduced with the voltage OFF (time span 4), the subsequent reading with a TP (time span 5) showed an S1 type signal (magenta curve) implying erasing past memory and registering new memory in the passive state of the device. In other words, the device could remember the event from time span 4 and forget the S2 type signal from time span 3, the rewriting being done by intervening CP. The CP term used is thus justifiable. An insight into the memory action of the nanofiber is explained below. Humidity brings a tighter packing of the D-A pairs25,30 leading to a conductive state depending on the magnitude of humidity. More the humidity, the better is the conducting nature of the nanofiber as detailed in our previous work.25 This is reflected in the initial current flow (jump) corresponding to an applied voltage during time span 1. Once having been subjected to a given voltage, an electric field induced stress sets within the nanofiber, perhaps disturbing the molecular orientation thus increasing the resistance of the nanofiber. It appears that this disorder once induced, spreads across the nanofiber leading to a gradual decrease in the current (or decay current) with time. This spreading of the disorder continues even in the presence of constant humidity (see red dotted curves in Figures 2 and 3) and even after the voltage is withdrawn as seen from the behaviour in time spans 2 and 3 in Figure 3. This dynamic behavior is not entirely surprising for a supramolecular system hosting non-covalent interactions with low activation barriers and indeed stands as a signature of the self-assembly process.31-34 It is clear that the disorder can be lifted from the nanofiber only when the voltage is held OFF. Importantly, the extent to which the recovery takes place would depend on the magnitude of the humidity the nanofiber gets exposed to, here done through a CP. Accordingly, when a CP of 70% was given with voltage being OFF

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(time span 4), the device recovered back to the S1 state (time span 5), as though it remembered the CP. Such a property would be unique to supramolecular systems which are known to exhibit self-repairing behavior.26,35-37

Figure 4. CP magnitude dependent response of the device. Device response (black curve) measured with TP of 70% (blue curve) with different CPs (green curve). The voltage is ON only around the TPs. All plots follow same time scale. The vertical lines are drawn to bring clarity.

The influence of CP in terms of the number, magnitude, sequence as well as exposure time has been studied in detail as described below. As explained previously, the extent to which the voltage stress can be recovered depends on the magnitude of the CP introduced (note that while introducing CP, the voltage is always OFF) as shown in Figure 4. In order to examine this aspect, the device was fed with CPs of random values (green curve). One may readily see that

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the resultant TP-induced current variation (black curve) simply follows the magnitude of the preceding CP. This dissimilar response obtained corresponding to different CP values (TP value remained the same at 70%) can give an alternative way to assess, whether the measured RH is a true value or it was influenced a-priori by another RH condition (here it is CP).

Figure 5. Calibration and extraction of CP. (a) Measured current response for different TP = CP values. (b) Extracted CP value (from a) versus the applied CP.

In order to obtain a calibration of the measured current responses from different CPs, the current responses were measured by feeding a CP at the same value as TP for different values of the latter (see Figure S2 in supporting information). The average jump current values were calculated over many cycles for each TP (= CP) values and a calibration curve was obtained as shown in Figure 5a. Using this curve, the apparent RH values (or measured RH) were extracted

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against each CP as given in Figure 5b. Interestingly, the measured RH values vary linearly with the CP values for a given TP. Now using this curve, an unknown RH value, to which the device might have been exposed in the past, can be backtracked and measured (see Figure 5b). Further, the ability of the device to memorize multiple RHs in different sequences has been investigated. Three CPs (30%, 50% and 70%) were exposed in succession and the responses were measured using a single TP of 70% RH (see Figure 6). The CP sequences (‘%’ removed for brevity) used are (30, 50, 70), (50, 30, 70), (50, 70, 30), (30, 70, 50), (70, 50, 30) and (70, 30, 50) and these responses have been compared with the one measured with a single CP of 70% (see Figure 6a). From Figure 6b, it is observed that the responses for (30, 50, 70) and (50, 30, 70) sequences are indeed comparable to that of single CP of 70% from Figure 6a. This implies that the first two lower values CPs (30% and 50%) had no observable effect on the response of the device. Figure 6c shows responses for the sequences of (50, 70, 30) and (30, 70, 50). Noticeably, the response decreased in both the cases when compared to the one measured with a single CP of 70% (Figure 6a). However, the response to the sequence of (50, 70, 30) was found to be lower than that of (30, 70, 50) as shown in Figure 6c. In the remaining sets (70, 50, 30) and (70, 30, 50), the response decreased considerably compared to the previous cases (Figure 6d).

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Figure 6. Influence of CP sequence on the device response. (a-d) Three CPs namely 30%, 50% and 70% RH were introduced for 10 s each but in different sequences. The TP was kept constant at 70% RH. The black line is the current response of the device whereas green and orange lines represent CP and voltage variations respectively. The axis values are given below (a). (e) Histogram of measured RH for different sequences of CPs, best matched with average values of CP+TP.

The current responses were converted into measured RH values using the calibration plot from Figure 5b. The measured RH values have been plotted as a bar diagram against different CP sequences (see Figure 6e and Figure S3, Supporting Information). Additionally, the average values of CPs (one, two, three) plus TP, termed as CP-TP average for brevity, were calculated and correlated with the measured RH values. Interestingly, for every measured RH value corresponding to a CP sequence, a matching CP-TP average could be found. From these observations, it appears that the position of the maximum CP (70% here) in the sequence of CPs influences the response of the device. For example, for a sequence of (70, 50, 30), the measured RH value matched with the average value considering all three CP+TP values whereas for (50, 30, 70), the measured RH value matched with the average value considering only the last CP + TP value. Thus, the measured RH matches closely with the average of CP+TP, when CPs

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following the highest CP are taken into account (see Table S1, Supporting Information, for more details). As detailed above, the memory effect from the nanofibre device can be realised in two ways. Under an applied voltage, the humidity response involving S1 and S2 states and the response generated after exposing fibre to a given humidity. The former relies on the electrical stress and resulting disorder whereas the latter corresponds to recovery of the nanofibre aided by exposure to humidity (with electrical stress). The later situation is interesting in that it corresponds to registration of humidity effect with the device in the passive or unpowered state. Thus, it has the promise to be used as a device accompanying a cargo whose internal humidity condition is critical. The present device with its calibration established with a recommended TP, can reveal possible humidity variations in the cargo which could be taking place during transit. Importantly, the time of exposure and time intervals (Figure S4, Supporting Information) are irrelevant which enhances the scope of the device. There are also other contexts where such device finds applications. For example maintaining and ensuring constant humidity is critical in fields such as scientific laboratories, electronic fabrication, pharma industry etc. The present device being a simple resistive sensor will be compatible with alarming systems and wifi, and thus can be very well integrated with IoT concepts. 3. CONCLUSIONS In this study, using supramolecular nanofibre as active material, we have developed a memory device that can remember the humidity it had been subjected to in the past. The supramolecular nanofibres were self-assembled from coronene tetracarboxylate salt (D) and dodecyl methyl viologen (D) molecules whose assembly and associated conductivity are very

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sensitive to external stimuli namely humidity and electric field, the combination leading to an efficient humidity sensor. However, under continued electrical stress, the fibres show a steady decay in conductivity due to increasing disorder in the molecular assembly. This electrically induced disorder can be completely removed by introducing humidity in the absence of voltage (passive state), the extent of removal depending on the magnitude of humidity. The observation of recovery of conductivity in the passive state led us to design a humidity device which in a practical context, say as an unpowered sensor fitted to a consignment, can reveal possible undesired humidity conditions the consignment might have gone through. If the humidity were to have varied, the sensor would remember the average value from the highest exposed humidity. The device may find use in various applications that demand constant humidity condition for a longer time. Further, the present work has also demonstrated an approach to use properties of the supramolecular system to construct unique functional devices that are conventionally unfeasible. 4. EXPERIMENTAL SECTION 1. Synthesis of CS-DMV nanofibres. The detailed synthesis procedure of the nanofibres are reported in Reference 27. In brief, Coronene Tetracarboxylate (CS) was synthesized by a Diels– Alder reaction of perylene with N-ethyl maleimide followed by hydrolysis with KOH in methanol. Dodecyl Methyl Viologen (DMV) was synthesized from 4,4’-bipyridine by controlled reaction with dodecyl bromide to give mono pyridinium ion and subsequently treating it with methyl iodide to amphiphllic dicationic bipyridine. The charge-transfer fibres are then assembled from the introduction of a methanol solution of DMV to the aqueous solution containing free CS molecules by injection method.

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2. Device fabrication. The glass substrates were cleaned in piranha solution followed by washing with distilled water several times. Metallic contacts were made by physical vapor deposition of Au by shadow masking using a resistive thermal evaporator (HindHivac, India) at a base pressure of 10-6 Torr. After removing the shadow mask, the resulting device consists of two gold electrodes, separated by 10 µm channel length. A dispersion of already prepared supramolecular nanofiber (1 mM, 10 µL) was drop coated on it and allowed to vacuum evaporate overnight to remove maximum water content coated dispersion. For rapid switching of RH, a humidity cell was constructed with containing a gas inlet and outlet. Switching between two RH levels was done using a Tee with a flow rate of ~ 500 sccm. To rapidly turn on and turn off the voltage, a switch was used in the circuit. A commercial humidity meter Testo 410-2 was used to measure the obtained RH. A Keithley Semiconductor Characterization System 4200 was used to measure response characteristics.

ASSOCIATED CONTENT Supporting Information. It contains the lot of current Vs. RH of the nanofibre, responses of the device for the same CP and TP, a histogram of correlation of measured RH with different CPs and response of the device exposed to CP of different time intervals. The following files are available free of charge. Supporting Information (PDF file)

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. χThese authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Prof. C.N.R. Rao for his constant encouragement and support. This research work is supported by Department of Science and Technology (DST), New Delhi, India. U.M. thank CSIR, India and M.G. thank DST-Nanomission, India for the research fellowship. REFERENCES 1.

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