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Organic Electronics

journal homepage: www.elsevier.com/locate/orgel

Electrical switching behaviour of a metalloporphyrin in Langmuir-Blodgettfilm

Bapi Deya, Sekhar Chakrabortya, Santanu Chakrabortyb, Debajyoti Bhattacharjeea, Inamuddinc,d,Anish Khanc,d, Syed Arshad Hussaina,∗

a Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar, 799022, Tripura, IndiabDepartment of Physics, NIT Agartala, Jiraniya, 799046, West Tripura, Tripura, Indiac Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabiad Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, 21589, Saudi Arabia

A R T I C L E I N F O

Keywords:Organic electronicsBipolar switchingThreshold switchingMnTPPSLB films

A B S T R A C T

Here we report the resistive switching behaviour of an water soluble anionic metalloporphyrin 5,10,15,20-Tetrakis(4-sulfonatophenyl)-21H,23H-porphine manganese(III) chloride (MnTPPS) assembled onto spin coatedas well as in Langmuir-Blodgett (LB) film. To prepare LB film, water soluble MnTPPS molecules were in-corporated into monolayers of two cationic matrix molecules Octadecyltrimethylammonium bromide (OTAB)and N-Cetyl-N,N,N, trimethyl ammonium bromide (CTAB) through electrostatic interaction. Successful in-corporation of MnTPPS molecules into the matrix (OTAB/CTAB) monolayers has been confirmed by measuring π– A isotherm, π – t curve and BAM investigations at air-water interface. From I – V characteristic it was foundthat by adjusting the measurement protocols (compliance current, sweeping direction) all the devices fabricatedby using spin coated as well as LB films exhibit outstanding bipolar switching and threshold switching behaviourat room temperature. Presence of electron acceptor groups (SO3H) and π – electron clouds on the MnTPPSmolecules mainly play the crucial role for such observed switching behaviour onto ultrathin films. This type ofbipolar memory switching and threshold switching in a single device is technologically very important to use asan active component for the non-volatile information storage and future optoelectronics devices.

1. Introduction

Electronic and optoelectronic devices are the leading tools for themodern society [1–7]. There is a growing interest for the developmentof nanoscale devices with new functionality and/or greatly enhancedperformance. In recent years organic electronics deals with carbon-based conductive organic materials and are extensively studied due totheir interesting optical, electrical, photoelectrical conducting, semi-conducting, memory, storage and magnetic properties [8–12]. Thereare numerous potential applications of organic materials in the devel-opment of electronic devices such as sensors, solar cells, field-effecttransistors (FET), switching devices, optical data storage, organic lightemitting diode (OLED) etc [13–19].

Of late molecular electronics has emerged as an important tech-nology which deals with the manipulation of organic materials at thenanoscale level to realize devices that will store and/or process in-formation [20–22]. Here single molecule or an assembly of moleculewill be used for the fabrication of electronic components. It has been

observed that several organic molecules show rectification, switching,semiconducting and even metallic properties under certain conditions[23–26]. Therefore molecules that are probably suitable for the appli-cations in molecular-electronic devices have recently been the subjectof current interest.

On the other hand Langmuir – Blodgett (LB) technique is one of thefew methods for preparing nanoscale organized molecular assemblieswhich are the pre-requisite for the realization of molecular electronicdevices [27,28]. LB technique provides the opportunity to exercisemolecular level control over the structure of organic thin layer of mo-lecules [29–31].

Porphyrin derivatives have interesting characteristics such as rigidplanar structure, high stability, intense absorption and small HOMO-LUMO energy gap. These characteristics make porphyrins a class ofsynthetic building blocks for functional nano materials suitable formolecular electronics [32]. Due to their interesting properties and ex-tended π – conjugated structure, porphyrins have potential applicationsin nonlinear photonic devices [33–35] and for investigations of

https://doi.org/10.1016/j.orgel.2017.12.038Received 11 October 2017; Received in revised form 21 December 2017; Accepted 24 December 2017

∗ Corresponding author.E-mail addresses: sa_h153@hotmail.com, sahussain@tripurauniv.in (S.A. Hussain).

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artificial light harvesting systems [36]. Among the most commonlystudied porphyrins the metalloporphyrins are the important class ofcompounds. The metalloporphyrin ring is found in variety of biologicalsystem and can be used to mimic many biological process [37]. Theyhave wide applications in molecular electronic devices [38], as a cat-alysts in chemical reactions [39], photosensitizers [40], artificial pho-tosynthesis [41] etc. Beletskaya and co-workers [42] suggested thatthese materials form organized multilayers which are used as modelsfor biological processes and catalysts for many reactions due to axial co-ordination via central metal ion. Porphyrins form aggregates (J-or H-aggregates) under various conditions such as at specific pH, con-centration, ionic strength etc [43–47].

In this manuscript we report the results of our investigations on theelectrical switching behaviour of an anionic metallo porphyrin5,10,15,20-Tetrakis(4-sulfonatophenyl)-21H,23H-porphine manganese(III) chloride (MnTPPS) assembled onto spin coated as well as in LBfilm. Presence of electron acceptor groups (SO3H) and π – electronclouds on the MnTPPS molecules trigger us to study the electricalproperties of this molecule onto thin films. Our investigations revealedthat this porphyrin derivative show two types of resistive switching –non-volatile bipolar switching and threshold switching. This kind ofobserved bipolar memory switching and threshold switching in ultrathin films are the promising candidate for potential applications to thenext generation non-volatile memory devices and/or logic circuits [48].

2. Experimental section

2.1. Materials

5,10,15,20-Tetrakis(4-sulfonatophenyl)-21H,23H-porphine manga-nese(III) chloride (MnTPPS), Octadecyltrimethylammonium bromide(OTAB) and N-Cetyl-N,N,N, trimethyl ammonium bromide (CTAB) hasbeen purchased from Sigma Aldrich chemical Co., USA and were usedas received. Chemical sctructures of MnTPPS, OTAB and CTAB areshown in Fig. 1. Spectroscopic grade chloroform (SRL, India) has beenused as solvent. Ultra pure Milli-Q water (18.2 MΩ–cm) is used assubphase.

2.2. Isotherm measurement and film formation

A commercially available Langmuir-Blodgett (LB) film depositioninstrument (Apex 2006C, Apex Instruments Co., India) was used forsurface pressure vs area per molecule (π-A) isotherms measurement,surface pressure vs. time (π-t) characteristic study, and LB films

preparation. The concentration of the stock solutions for OTAB, CTABwere 10−3 M and for MnTPPS aqueous solution concentration was10−4 M. In order to measure the isotherm and film formation dilutechloroform solution of 80 μl OTAB or CTAB were spread by a microsyringe on water subphase of pure Milli-Q water (18.2 MΩ–cm) andsubphase containing aqueous solution of MnTPPS at room temperature.After complete evaporation of volatile solvent, the barrier was com-pressed at a rate of 5 mm/min to record the surface pressure−area permolecule isotherms. The surface pressure (π) versus average areaavailable for one molecule (A) was measured by a Wilhelmy plate ar-rangement [49]. Each isotherm was repeated a number of times anddata for surface pressure−area per molecule isotherms were obtainedby a computer interfaced with the LB instrument Before each isothermmeasurement, the trough and barrier were cleaned with ethanol andthen rinsed by Milli-Q water. Surface pressure vs. time (π-t) curve wasrecorded to monitor the progress of reaction. 80 μl chloroform solutionof OTAB or CTAB were spread on the subphase containing aqueoussolution of MnTPPS in different volume. The increase in surface pres-sure with time was recorded to have the π-t curve. After the completionof reaction kinetics the film was lifted vertically at that saturatedpressure with the fixed position of the barrier. Smooth fluorescencegrade quartz plates (for spectroscopy) and fresh cleaned ITO-coatedglass (for electrical characteristic) were used as solid substrate. Y-typedeposition at a particular surface pressure was followed to transferLangmuir films at a deposition speed of 5 mm/min. The transfer ratiowas estimated by calculating the ratio of decrease in subphase area toactual area on the substrate coated by the layer and was found to be0.98 ± 0.02.

Spin coated film of MnTPPS has been prepared by using spin coaterunit, Model: EZ spin-SD, Apex Instruments Co., India. For spin coatingfilm the MnTPPS solution concentration was 10−4 M. Before film pre-paration the solution was stirred for almost 10 h. After that 500 μlaqueous solution of MnTPPS was spread onto ITO coated glass substratedrop wise. After each 1–2 drop the substrate was spun at a rotatingspeed of 1800 rpm for 30s. The resulted spin coating film was dried formore than 10 h in vacuum at room temperature followed by I-V char-acteristics measurement.

2.3. Brewster angle microscopy (BAM) imaging

The morphology of the film at the air-water interface was observedusing a Brewster Angle Microscope (Accurion nanofilm_EP4-BAM,Serial No. 1601EP4030) equipped with a 30 mW laser emitting p-po-larized light at 532 nm wavelength which was reflected off the air/

Fig. 1. Chemical structure of MnTPPS, OTAB & CTAB.

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water interface at the Brewster angle (53.1°). This reflected beam passthrough a focal lens, into an analyzer at a known angle of incidentpolarization, and finally to a CCD camera, which measures gray levelinstead of relative intensity. The lateral resolution of the microscopewas 2 μm, the shutter speed was 1/50 s and the images were digitalized.

2.4. UV-vis spectroscopy

UV–vis absorption of pure solutions, as well as those of mixed LBfilms, were recorded using absorption spectrophotometer (PerkinElmer,Lambda 25). The absorption spectra were recorded at 90° incidenceusing a clean quartz slide as reference.

2.5. I-V characteristic measurement

The current–voltage (I–V) characteristics were measured by usingKeithley 2401 source meter. 60 layer LB films of mixed MnTPPS+OTAB and MnTPPS+CTAB were deposited onto ITO-coated glasssubstrates at 15 mN/m surface pressure. Pure MnTPPS thin films ontoITO coated glass slide was prepared using spin coating technique. HereITO-coated glass substrate acts as bottom electrode and a gold tip, softlytouching the surface of thin films was used as top electrode giving anactive area of 1 mm2 for measurement. The solutions were stirred foralmost 10 h before the preparation of the films and the prepared thinfilms were dried in a vacuum desiccator for more than 10 h beforecharacterization. During the experiment, the samples were not allowedto be irradiated by any light and always kept at room temperature. TheI-V curves were recorded by applying different sweep voltage at avoltages scan rate of 2.5 mV/s.

3. Results and discussion

3.1. Monolayer characteristics at the air-water interface

The incorporation and packing of MnTPPS in ionic monolayers wasinvestigated at the air–water interface by using LB technique. The tetraanionic MnTPPS is water-soluble and does not form a stable monolayerby itself. This has been confirmed by observing almost no surfacepressure change upon compression of barrier in the Langmuir troughcontaining MnTPPS solution subphase.

Ideally, LB technique is suitable for preparation of mono- or mul-tilayer thin film using water insoluble material [29,49]. However, thereare several reports where water soluble ionic molecules were in-corporated onto LB films through electrostatic interaction with a pre-formed ionic insoluble monolayer [50]. In the present case the moleculeMnTPPS under investigation is anionic in nature with four SO3

−

groups. Therefore it is expected that MnTPPS can be incorporated ontoultrathin films through electrostatic interaction with a water insolublemonolayer of cationic matrix molecule. Accordingly, two cationic sur-factants OTAB and CTAB were chosen as matrix molecule. Here, thematrix molecules (both OTAB and CTAB) were spread onto the LBtrough containing MnTPPS solution (1.5 × 10−6 M) subphase. Theincorporation of MnTPPS onto the floating matrix layer was in-vestigated by recording the surface pressure – area per molecule (π – A)isotherm as well as the surface pressure – time (π – t) characteristics.

Fig. 2 shows the surface pressure –area (π-A) isotherms of OTAB andCTAB monolayer spread on to the aqueous subphase containing1.5 × 10−6 M of MnTPPS for different waiting time after spreading.The isotherms of pure OTAB and CTAB measured on water subphase(Fig. 2) shows a very steep nature with mean molecular area 0.17 nm2

and 0.15 nm2. The shapes as well as nature of these isotherms areconsistent with reported literature [51,52]. After spreading the matrixmolecules onto the MnTPPS subphase, certain time was allowed for theinteraction to occur followed by compression of the barrier to recordthe (π – A) isotherm. Interestingly, for all the cases in presence ofMnTPPS subphase the isotherms were shifted to larger area for both the

matrix molecules. The enlargement of molecular areas of isotherm onMnTPPS subphase indicates that there exists a strong interaction be-tween the amphiphiles in monolayer and MnTPPS in the subphase andthe consequent incorporation of MnTPPS molecule on the floating ionicfilms. Anionic MnTPPS molecules, being water soluble, remain in theaqueous subphase of the LB trough. When a Langmuir film of cationicmatrix (OTAB or CTAB) was prepared at air – water interface, MnTPPSmolecules from the aqueous subphase were adsorbed electrostaticallyon the floating matrix layer. With the passage of time matrix molecules(OTAB/CTAB) in the preformed Langmuir film was replaced by theMnTPPS – matrix complex i.e. either MnTPPS – OTAB or MnTPPS –CTAB complex at air – water interface. The size of these complex islarger compared to either OTAB or CTAB. Accordingly, the area permolecule in case of complex film shifted to the larger area.

It has been observed that with increase in waiting time, the iso-therms were shifted to the larger area and for both the cases the iso-therm attain maximum area per molecule with waiting time 2 h.Isotherm recorded with waiting time greater than 2 h did not show anyfurther increase in area per molecule. The mean molecular area notedfor all the complex isotherms are listed in Table 1. It has been observedthat the mean molecular areas for both MnTPPS – OTAB and MnTPPS –CTAB complex isotherm reached its maximum value recorded withwaiting time 2 h or more. Considering the MnTPPS molecule as squarewhose side length is ca. 1.3 nm according to CPK model, the area ofMnTPPS molecule under flat surface conformation should be 1.69 nm2

[53]. Again in the complex monolayer (MnTPPS – OTAB or MnTPPS –CTAB) long alkyl chain of OTAB or CTAB molecule was oriented outsidethe air – water interface and MnTPPS molecule occupied an area at theinterface. Accordingly the mean molecular area in the complex iso-therm should be 1.69 nm2. However, in the present case the observedmean molecular area was less than this value for all the cases evenwhen the complex isotherm attains maximum area per molecule. Thisindicates that the macrocyclic ring of the MnTPPS molecule do notremain flat at air-water interface rather become inclined to the plane ofthe complex monolayer [54].

Also it was observed that the mean molecular area changed withchange in waiting time until stable configuration was attained. Thisindicates that change in orientation of the macrocyclic ring of theMnTPPS molecules occurred with time. Analysis of compressibility aswell as BAM studies of the complex monolayer also suggested phasechange in the complex monolayer upon change in waiting time.Corresponding results have been presented in the later section of thismanuscript. However, with waiting time 2 h, orientation of macrocyclicring become stabilized, showing no further change in mean moleculararea with further increase in waiting time.

3.2. Studies of compressibility at the air-water interface

Compressibility of Langmuir film as a function of surface pressurecan be calculated from the π – A isotherm using the following standardthermodynamic relation in two dimension [55] –

C = – (1/A)(dA/dπ)

Where “A” is the area per molecule at the surface pressure of π.Idea about monolayer phases can be obtained from the compressi-

bility versus surface pressure (C – π) curve [30]. The states of Langmuirfilm or the phase transition in the Langmuir film are reflected as thepeaks in the C – π curve. Generally presence of peaks in the curve re-presents the inherent phase transitions involved in such systems [56].

From Fig. 3 it has been observed that compressibility curve for pureOTAB and pure CTAB show almost similar profile with two identicalpeaks in the range 0–5 mN/m and 15–22 mN/m regions. Here the twopeaks correspond to the phase changes from gaseous to liquid and li-quid to solid phase respectively. Similar nature of C – π curve for bothpure OTAB and pure CTAB indicate that phase changes in pure matrix

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monolayers are almost similar for both the cases. However, asymmetryof the peaks in the different C – π curves corresponding to complexmonolayer for both the matrices clearly indicates that phase transitionsof different complex monolayers are strongly dependent on the extentof interaction between the MnTPPS molecule and the matrix molecule(OTAB or CTAB). Also change in the curve for the complex monolayerwith different waiting time clearly suggest that change of monolayerphases occurred in the complex film with time due to increase ofMnTPPS molecules onto the complex film as well as their reorientationwith time until final stable phase is reached after 2 h.

3.3. Surface pressure vs. time (π-t) characteristic studies at the air-waterinterface

In the present work we have incorporated the water solubleMnTPPS molecules onto ultrathin films using LB technique [50]. Herethe water soluble MnTPPS molecules were adsorbed onto preformedfloating ionic layer of OTAB or CTAB through electrostatic interaction.In order to monitor the reaction kinetics the change in surface pressure(π) with the passage of time (t) during the adsorption process was re-corded.

Fig. 4 shows the corresponding surface pressure (π) vs. time (t)curves. Here in order to monitor the reaction kinetics, 80 μl chloroformsolution of OTAB or CTAB (10−3 M) has been spread onto the LB troughcontaining MnTPPS solution (1.5 × 10−6 M) subphase. In all the cases,the starting point was the nominal surface pressure of 0.5 mN/m. Fromthe figure it has been observed that as time passed, surface pressuregradually increased with the progress of adsorption. Despite of thebarrier was kept fixed at a particular position (where the control

Fig. 2. The surface pressure-area isotherms (a) OTAB and (b) CTAB on water subphase and recorded onto MnTPPS solution subphase with waiting time 10 min, 2 h and 3 h afterspreading.

Table 1Mean molecular area for different waiting time of OATB+MnTPPS and CTAB+MnTPPSisotherm.

Isotherms Mean molecular area for different waiting time (nm2)

10 Min 2 h 3 h

OTAB+MnTPPS 0.95 1.09 1.09CTAB+MnTPPS 1.04 1.13 1.13

Fig. 3. C-π graphs of pure OTAB, CTAB along with the OTAB-MnTPPS and CTAB-MnTPPScomplex monolayer for different waiting time (10 min & 2 h).

Fig. 4. Surface pressure – time (π – t) graph of complex OTAB+MnTPPS (black) andCTAB+MnTPPS (red) monolayer. For both the cases, barrier was kept fixed at initialstarting pressure 0.5 mN/m. (For interpretation of the references to colour in this figurelegend, the reader is referred to the Web version of this article.)

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pressure was arrived) the increase in surface pressure with time is anindication of the formation of MnTPPS+OTAB or MnTPPS+CTABhybrid monolayer at the air-water interface. Initially the monolayer wasoccupied by only the OTAB or CTAB molecules and finally the mono-layer was occupied by the complex molecules. Since the area per mo-lecule of this complex is greater than the area per molecule of pureOTAB or CTAB molecules, hence the area per molecule of the complexmonolayer tends to increase. But as the barrier was kept fixed at aparticular position, the increase in the area per molecule was conse-quently manifested as the increase in surface pressure. From Fig. 4 itwas also observed that as time progresses the rate of increase in surfacepressure came down slowly. After 3 h the surface pressure reached itsmaximum value and became almost steady and parallel to the time axis,indicating the completion of reaction. At this stage the floating complexfilm was deposited onto solid substrate for further studies. It has alsobeen observed that the increase in surface pressure as well as maximumsurface pressure reached after the completion of adsorption process ishigher in case of OTAB-MnTPPS film. This may be due to the fact thatchain length of OTAB is longer than CTAB. Accordingly, size of thecomplex in case of OTAB is also bigger, affecting the adsorption processresulting attainment of higher surface pressure.

3.4. Brewster angle microscopy (BAM) imaging at air-water interface

In-situ Brewster Angle Microscopic (BAM) investigation is one of themost reliable tools to have visual idea about the microstructure of thecomplex monolayer at air-water interface. Domains of different sizesand shapes in the BAM images of Langmuir monolayer indicates thephase transition in the floating layer. In the present case, BAM imagesof MnTPPS+OTAB/CTAB complex Langmuir monolayer were taken atdifferent interaction time during π-t measurement as well as at differentsurface pressure during π-A isotherm measurement at air-water inter-face.

BAM image of empty water surface showed completely dark imageindicating no reflection of light (figure shown in supporting informa-tion, Fig. S1A). Again BAM image recorded for the MnTPPS aqueoussubphase also showed almost black image (Fig. S1B). This is because,being water soluble, the MnTPPS molecules remain within the subphaseof LB trough and hardly any MnTPPS molecule lie on the air-waterinterface. On the other hand BAM image recorded for the matrixmonolayer (OTAB or CTAB) spread onto pure aqueous subphaseshowed almost similar continuous small circular domains having di-mensions ranging from 5 μm–15 μm throughout the surface (Fig. S1Cand D).

The images recorded just after spreading (waiting time 0 min)of thematrix molecules (OTAB or CTAB) onto MnTPPS subphase is also al-most similar to that of pure matrix monolayer (Figs. 5A and 6A).However, with passage of time (Figs. 5B–D & 6B – D) marked changes inthe BAM images occurred for both the matrix molecules. Here, uponpassage of interaction time the number of domains, domain structuresas well as background contrast in the BAM images changed gradually.BAM images recorded after waiting time 2 h showed maximum con-densed domains with maximum surface coverage. Images recorded withwaiting time higher than 2 h did not show any further marked change.

In the present case the water soluble anionic MnTPPS moleculeadsorbed onto floating cationic matrix monolayer (OTAB/CTAB)through electrostatic interaction. Number of MnTPPS molecules ad-sorbed onto floating matrix layer increases with waiting time. Ourisotherm study as well as compressibility analysis also suggested thatwith increase in waiting time the number of MnTPPS molecules ad-sorbed onto floating complex monolayer increased and reorientation ofthe MnTPPS molecules occurred. However, after passage of 2 h thecomplex monolayer gets stabilized and no further reorientation ofmolecules occurred. Consequently, BAM images recorded at differentwaiting time upto 2 h after spreading of matrix molecules showedgradual change and for the images recorded with waiting time more

than 2 h remain almost unchanged.BAM images recorded at different stages of compression during π –

A isotherm measurement (Figs. S2 and S3) for the complex monolayersalso showed that initially at lower surface pressure the BAM imageswere almost similar to the pure matrix monolayer (OTAB/CTAB).However, at higher surface pressure (> 10 mN/m) the BAM imagesshowed marked change and became almost similar to that recordedduring π – t measurement with waiting time 2 h or more. This indicatesthe formation of condensed monolayer of MnTPPS-OTAB or MnTPPS-CTAB complex at air – water interface. As a whole BAM study givescompelling visual evidence of adsorption process as well as complexfilm formation at air-water interface.

3.5. UV–vis absorption spectroscopy

In order to check the possible aggregation nature of complex OTAB+MnTPPS and CTAB+MnTPPS in LB film we have recorded theUV–Vis absorption spectra of complex MnTPPS LB films along withpure MnTPPS and MnTPPS+OTAB/CTAB solution for comparison.

Fig. 7 shows the corresponding UV–Vis absorption spectra. It is re-levant to mention in this context that the absorption bands of por-phyrins are ascribed to the in-plane π–π* transitions and can be ex-plained by four orbital model of Gouterman [57]. Electronic transitionsbetween these orbitals (two HOMO and two LUMO) results into twoexcited states which are split up resulting in two distinct transitionbands one with higher energy called the Soret band or B band (S0→ S2)and the other with lower energy called the Q band (S0→ S1). There aresome additional weak bands (N, L, M bands) in the UV range [58,59].MnTPPS aqueous solution spectrum (curve 1, Fig. 7a and b) showed theintense Soret band at around 466 nm, the Q bands ranging from 525 to670 nm and the additional N, L, M bands in the range 350–450 nmhaving peak positions 423, 400, 378 nm respectively. The soret bandposition is sensitive to substituent groups and highly sensitive to mi-croenvironments [59]. The absorption band of the MnTPPS+OTABmixed solution (curve 2, Fig. 7a) shows the intense Soret band ataround 470 nm which is nearly 4 nm red shifted with respect to pureMnTPPS solution. Also the bands in the lower wavelength (N, L, M) arebroadened with respect to pure solution absorption spectrum. The ob-served 4 nm red shift of the soret band in the MnTPPS+OTAB mixedsolution is owing to the closer association of the MnTPPS moleculeswhile tagged onto the backbone of OTAB molecule and consequentlyformation of aggregates. It may be mentioned in this context that tetraanionic, MnTPPS molecule may not form aggregates in aqueous solu-tion due to predominance of repulsive force among the MnTPPS mo-lecules [59–61]. However, when MnTPPS molecules are adsorbed onthe backbone of OTAB in the floating amphiphile layer, they may comeclose to each other due to electrostatic interaction and form traces of J-aggregates. Our isotherm studies also suggested that MnTPPS moleculesobliquely stacked into the complex layer to form favourable conditionof J-aggregate. It is well known that 5,10,15,20- Tetraphenylporphinetetra sulfonic acid is a water soluble porphyrin which formed J-ag-gregates in acidic water or in the presence of various organic and in-organic cations [62–64]. Although Takagi et al. [65] suggested thatsmall red shift of 3–4 nm may also be due to the change in solventpolarity of the solution.

Here we have prepared the complex LB films following two proce-dure – (i) In the first case OTAB solution was spread onto the LB trough(expanded condition) containing MnTPPS aqueous solution subphase.After waiting 2 h the floating layer was compressed upto the surfacepressure of 15 mN/m to deposite the LB monolayer film. (ii) In thesecond case OTAB solution was spread onto the LB trough containingMnTPPS aqueous solution subphase. The barrier was compressed tosurface pressure of 0.5 mN/m and the reaction between MnTPPS andOTAB was monitored by observing the π-t curve. After the completionof the reaction (pressure become parallel to the time axis) for waitingtime five and half an hour the film was deposited onto solid substrate.

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After that the absorption spectra for the LB films were recorded.OTAB+MnTPPS complex LB film absorption spectrum showed in-

teresting feature. The absorption spectra of the LB film prepared byprocedure (i) (curve 3, Fig. 7a) showed prominent and intense band

with peak position at 420 nm, which was almost a weak hump in case ofsolution absorption spectrum. Also the soret band decreased largelycompared to its solution counter part with a shift of about 4 nm. The Qband intensity also decreased. On the other hand the absorption

Fig. 5. BAM images of the complex OTAB + MnTPPSmonolayer recorded (A) immediately after spreading, (B)after 30 min, (C) after 1.5 h, (D) after 2 h during π-t mea-surement at air-water interface.

Fig. 6. BAM images of the complex CTAB + MnTPPSmonolayer recorded (A) immediately after spreading, (B)after 30 min, (C) after 1.5 h, (D) after 2 h during π-t mea-surement at air-water interface.

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spectrum of the LB film prepared following procedure (ii) (curve 4,Fig. 7a) showed almost similar spectral profile except the soret bandwas totally disappeared. It is relevant to mention in this context thatisotherm studies revealed that when MnTPPS molecule comes onto air-water interface via electrostatic interaction with OTAB molecules, themacrocyclic ring of the MnTPPS molecule do not remain flat at air-water interface rather become inclined to the plane of the complexmonolayer. Accordingly, when this floating complex monolayer istransferred onto solid support the staking of MnTPPS molecules areaffected. This intern effects the in plane π - π* transitions in case of LBfilm absorption spectra. Also the energy levels are deformed to a certainextent due to this change in organization of MnTPPS molecules incomplex LB films resulting the observed change in the absorptionspectra of LB films. However, the observed difference in the soret bandfor two LB film spectra may be due to the change in extent of de-formation in energy level for the films prepared following two differentprocedure. As a whole the nature of LB film absorption spectra clearlyindicate the formation of aggregates of MnTPPS molecule onto LB films.The absorption spectra of CTAB+MnTPPS LB films showed almost si-milar spectral profile to that of OTAB+MnTPPS LB films (Fig. 7b).

3.6. Resistive switching behaviour

Materials suitable for molecular scale switching devices, which canbe used in both logic and memory circuits have recently been subject ofmuch attention. A resistive switching device can be switched betweentwo states, a high resistance (OFF) and a low resistance (ON) state bysuitable applied voltage [66–70]. Based on volatility, there are twotypes of resistive switching (RS) - nonvolatile memory switching andthreshold switching. In nonvolatile memory switching after the removalof external voltage both the low resistance state (LRS) and high re-sistance state (HRS) can be retained, whereas in threshold switchingonly the HRS is stable at low applied voltages [71]. This type ofmemory and threshold switching in a single device is technologicallyvery important where the former is the key to realize the nonvolatileinformation storage and the later one plays a vital role to solve thesneak path problem in cross-point or multistack structures as a selectorin series with a memory cell [72–75].

In the present case we expected the switching behaviour of MnTPPSdue to the presence of electron-acceptor groups (SO3H) and π-electron

clouds over the surface of the MnTPPS molecule. Accordingly we havedesigned three devices – (i) Au/MnTPPS/ITO (ii) Au/MnTPPS+OTAB/ITO (iii) Au/MnTPPS+CTAB/ITO and investigated their bipolar re-sistive memory switching as well as the threshold switching behaviourby adjusting the measurement protocols [76]. The configurations of thedevices are shown in supporting information (Fig. S4). It is possible tohave different switching mode by adjusting the device configurationand measurement protocols (compliance current, Sweeping direction).These types of bistable organic switching devices are the promisingcandidates for next generation information storage and future optoe-lectronic devices [77,78].

3.6.1. I-V characteristic of MnTPPS onto spin coated filmIn order to study the switching behaviour of pure MnTPPS a device

having configuration Au/MnTPPS/ITO was prepared. For this purposepure MnTPPS thin film was prepared onto ITO coated glass substrateusing spin coating. Here ITO coated glass substrate served as the bottomelectrode for the device. A gold tip softly touching the surface of theMnTPPS thin films acts as top electrode for the device.

The switching behaviour of this device has been studied by re-cording the I – V characteristics in two sweep directions by applyingscanning voltage from +Vmax to -Vmax followed by a separate reversescan from –Vmax to +Vmax. The typical I-V characteristics of Au/MnTPPS/ITO device for scan range +4.5 V to −4.5 V is shown inFig. 8A.

Interestingly it has been observed that while scaning from +Vmax to–Vmax initially the device exhibits its low conducting state (OFF state),but when the voltage approaches to the so-called threshold voltage(about 4.1 V) the current increases rapidly, showing its high conductingON state and again the device shows a transition from ON to OFF stateafter reaching the threshold voltage during scaning from –Vmax

to +Vmax (Fig. 8A). We have changed the scaning range by increasingVmax with increment of 0.1 V in each time and exactly similar bipolarswitching behaviour was obtained upto Vmax = 5 V. However, whenVmax exceeds certain value (beyond 5 V) driving the device to its ONstate (scaning from +Vmax to –Vmax), does not return back to its OFFstate during scaning from –Vmax to +Vmax. (Fig. 8B). Observed one wayswitching for higher Vmax = 5.1 V may be explained as due to the shortcircuit of the organic layer between the electrodes in the ON state,when higher Vmax was applied [79]. Here in both the cases the current

Fig. 7. UV–Vis spectra of pure MnTPPS solution (a & b, curve 1), MnTPPS solution with OTAB and CTAB (a & b, curve 2), the LB films of OTAB and CTAB with MnTPPS transferred at15 mN/m pressure after 10 min of interaction time (a & b, curve 3) and the LB films of OTAB and CTAB with MnTPPS transferred to solid substrate after 2 h of interaction time (a & b,curve 4).

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in the OFF state are due to the low leakage current. The calculatedvalues of OFF state and ON state resistances are listed in Table 2.

Apart from the bipolar resistive switching behaviour we have alsoinvestigated the threshold switching of the same device by adjusting thecompliance current as well as sweeping direction. The thresholdswitching for both forward and reverse bias was observed when thecompliance current was fixed at 10 mA. Fig. 8C represents the I-Vcharacteristic for threshold switching of Au/MnTPPS/ITO device withscanning range 0–5 V. It has been observed that compliance current of10 mA was the upper limiting value for having such threshold switching

for this device. Also this type of threshold switching was observed bylowering the compliance current upto 5 mA. However, increasing thescan range (greater than 5.1 V) does not effect this threshold switching.This is because here increase in sweep voltage does not increase thecurrent beyond compliance level. Accordingly the device does not getshort circuited.

In the present manuscript for all devices the active junction area was1 mm2 for I-V measurement. However, we have also measured the I-Vcharacteristic for different devices by varying the active junction areabut no significant variation is observed (result not shown). There are

Fig. 8. I-V characteristic for bipolar resistive switching (A &B) and threshold switching (C) of the Au/MnTPPS/ITO device based on spin-cast films of MnTPPS for two sweep direction.Initial sweep voltages were (A) ± 4.5 V, (B) ± 5.1 V and (C) ± 5 V at 10 mA compliance current. Arrows show the sweep direction of applied voltage.

Table 2Different parameters of I – V characteristics for Au/MnTPPS/ITO, Au/MnTPPS+ OTAB/ITO and Au/MnTPPS+ CTAB/ITO devices.

Device configuration Bipolar switching Threshold switching Resistances at

Thresholdvoltage (V)

Maximum allowedScanning Voltage

(V)

ShortedScanning

Voltage (V)

Thresholdvoltage (V)

Maximumcompliance current

(mA)

Minimumcompliance current

(mA)

OFF state (Ω) ON state (Ω)

Au/MnTPPS/ITO 4.1 5 5.1 4.1 10 5 10.9× 104 38×101

Au/MnTPPS+ OTAB/ITO (Procedure – i)

2.7 3.3 3.4 2.7 6 4 6×104 43×101

Au/MnTPPS+ CTAB/ITO (Procedure – i)

2.05 2.5 2.6 2.05 4 2 4.8× 104 49×101

Au/MnTPPS+ OTAB/ITO (Procedure – ii)

2.5 3 3.1 2.5 5 4 6.2× 104 43.4× 101

Au/MnTPPS+ CTAB/ITO (Procedure – ii)

1.5 2 2.1 1.5 3 2 4.6× 104 48.8× 101

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similar reports where it has been shown that variation in the activejunction area did not affect the switching behaviour significantly [79].

3.6.2. I-V characteristic of MnTPPS on LB filmsKeeping in mind the importance of LB technique towards molecular

electronics [26,29] we have designed two devices, Au/MnTPPS+OTAB/ITO and Au/MnTPPS+CTAB/ITO devices. Here MnTPPS+OTAB as well as MnTPPS+CTAB thin films have been prepared usingLB technique. These MnTPPS+OTAB and MnTPPS+CTAB LB filmshave been prepared following two procedures - (i) LB film deposited at15 mN/m during compression. (ii) LB film deposited after completion ofthe reaction during π – t curve measurement. Details of these twoprocedures have been mentioned in the UV–Vis absorption spectro-scopy section of this manuscript. I – V characteristics for the deviceshave been investigated to study the bipolar resistive switching as wellas the threshold switching behaviour.

Figs. 9A and 10A show the typical representative I – V character-istics for two devices respectively where active layer (LB films) wereprepared following procedure–(i). I – V characteristics for the deviceswhere active layer (LB films) were prepared following procedure–(ii)are shown in supporting information (Figs. S5 and S6). The values ofdifferent parameters of I – V characteristics are listed in Table 2. In-terestingly for both the devices switching occurred at lower thresholdvoltages compared to the device consisting of pure MnTPPS spin coated

thin film.Also for the device composed of mixed LB films the short circuit of

the devices occurred at lower voltages compared to the pure MnTPPSdevice (Figs. 9B and 10B, Table 2). However, among the devices con-sisting of mixed LB films, in case of MnTPPS+CTAB thin film device theswitching as well as short circuit occurred at lower values than that ofMnTPPS+OTAB thin film device.

We have also observed the threshold switching for both forward andreverse bias for the devices consisting of MnTPPS+CTAB and MnTPPS+OTAB LB films. Here the switching was observed at lower compliancecurrent range for all the devices compared to that of pure MnTPPS spincoated device. However, differences in threshold voltage as well as inmeasurement protocol were also observed for the devices where activelayer (LB films) were prepared following two different procedure(Table 2).

We have also prepared the active layer (LB films) of the devices bychanging the concentration of OTAB/CTAB and MnTPPS solution.However, no significant change in the switching behaviour was ob-served. It is relevant to mention in this context that in the present casewater soluble MnTPPS molecules were incorporated onto LB film whenthey interacted electrostatically with the floating preformed film ofeither CTAB or OTAB. Thus complex monolayer of either OTAB-MnTPPS or CTAB-MnTPPS is formed. In this process one MnTPPS mo-lecule interact with four OTAB or CTAB molecules. Progress of reaction

Fig. 9. I-V characteristic for bipolar resistive switching (A &B) and threshold switching (C) for two sweep direction of the LB film based Au/MnTPPS+OTAB/ITO device prepared byusing procedure (i). Initial sweep voltages were (A) ± 3 V, (B) ± 3.4 V and (C) ± 5 V at 6 mA compliance current. Arrows show the sweep direction of applied voltage.

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was monitored by observing the surface pressure vs time characteristicscurve (Fig. 4). When plateau region (Fig. 4) was achieved then reactionwas completed. Film was deposited onto ITO coated glass substratewhen reaction was completed. After completion of reaction the area permolecule of OTAB-MnTPPS complex as well as CTAB-MnTPPS complexis fixed. During LB film preparation the deposition was done at a fixedarea per molecule/fixed surface pressure. Therefore the concentrationof OTAB or CTAB and MnTPPS did not affect the film structure as wellas their properties. Accordingly no change in the switching behaviourwas observed.

3.6.3. Discussion on switching behaviourA number of mechanisms and strategies responsible for the two

switching states in a variety of organic molecules have been proposed,such as conformational changing, rotation of functional group, chargetransfer, and reduction–oxidation process, filamentary conduction,space charge and traps, ionic conduction etc [70–73].

In the present case, observed bipolar switching behaviour may beexplained by reduction-oxidation process. The electron acceptor group(SO3H) in MnTPPS molecule attracts π electron clouds towards themand largely affects conjugation in the molecules. As a result the mole-cule shows very low OFF state leakage current. When the device isreverse biased to a certain value during scaning from +Vmax to –Vmax

the molecule gets electron from external source, which reduces theeffects of electron withdrawing group. Accordingly, the electron density

in the benzene rings rearranges so as to restore the electron conjugationin the molecule. As a result the π electron cloud extends over the entiremolecule and opens up a conduction pathway to result in high con-ducting ON state. Again the device returns to its OFF state duringscaning from -Vmax to +Vmax when the voltage reached to a suitableforward bias voltage, which oxidizes the molecule by withdrawingextra electron previously added.

In case of threshold switching the difference of work functions be-tween the metals used as electrodes and resulting built-in internal fielddetermine the bias direction as well as the amplitude at which theelectron injection to LUMO level of the organic layer is favourable.Therefore depending on the bias the conjugation in the backbone of themolecule is presumably extended and/or HOMO–LUMO gap reduced toresult in the high-conducting state. The carrier conduction involvesboth injection and transport. In other words, electro-reduction of themolecules in the device involves: (i) electron injection overcoming thebarrier potential and (ii) hopping transport, which requires suitablefield. With ITO and Au electrodes, the slope of LUMO level of MnTPPSin after contact condition disfavours electron transport from the ITOside. Hence, in these devices, even if electron barrier height is low inthe reverse bias direction, a combined effect of barrier height andelectron transport favours electron injection from Au electrode in theforward bias as compared to that from ITO in the reverse bias. Hence,electro-reduction of MnTPPS molecules occurred and the deviceswitches to its high state at a forward bias. In these devices, electrons

Fig. 10. I-V characteristic for bipolar resistive switching (A &B) and threshold switching (C) for two sweep direction of the LB film based Au/MnTPPS+CTAB/ITO device prepared byusing procedure (i). Initial sweep voltages were (A) ± 2.2 V, (B) ± 2.6 V and (C) ± 5 V at 5 mA compliance current. Arrows show the sweep direction of applied voltage.

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from Au electrode hop to the organic layer of the device due to fa-vourable LUMO slope. During the transport, the electrons electro-re-duce MnTPPS molecules to its anionic state, which has much lowerHOMO–LUMO gap. The lower LUMO level of the reduced species, inturn, further offers lower barrier height for electron injection from theAu electrode. Enhanced electron injection and conduction via hoppingthrough the percolating redox centers continue to result higher con-ductivity in the forward bias. When bias was swept to negative direc-tion, the molecules in the organic layer of the device still remain re-duced until opposite bias is large enough to remove the extra electronand oxidize them to their neutral state. Considering the thinness of thefilm, electrons can be tunnelled directly to the LUMO level of any of themolecules. Irrespective of the slope of LUMO level in after contactcondition, electron tunnelling becomes possible in both bias directions.When amplitude of bias exceeds a certain value (threshold voltage)during scanning, electrons are injected reducing MnTPPS molecules andhence the LUMO level decreases in energy and band-gap(HOMO–LUMO gap) lowers. As a result the enhancement in chargecarrier density caused by this injection mechanism occurs, the currentdensity increases rapidly to switch the device to the ON state. Thusthreshold switching in both bias directions is therefore observed insingle molecular layer of MnTPPS.

The threshold voltage decreases for the devices consists of MnTPPS+OTAB or MnTPPS+CTAB LB films compared to pure MnTPPS spincoated films. Here anionic MnTPPS molecules were incorporated ontocationic matrix (OTAB or CTAB) layer through electrostatic interaction.As a result during LB film formation a strong electrostatic interactionbetween the anonic MnTPPS and cataonic matrix molecules (OTAB orCTAB) occurred and facilitates a structural modification i.e. con-formational changes in MnTPPS-OTAB or MnTPPS-CTAB system, whichreduces the effects of the electron accepting groups in the MnTPPSmoiety. That is why after applying the bias in device consisting of LBfilms, the high conducting switching state (ON state) occurs earliercompared to that device consisting of pure MnTPPS spin coated film. Inorder to confirm the ionic interaction between MnTPPS and OTAB orCTAB we have also measured the pH value for both pure and mixedsolution. For pure MnTPPS solution the pH value was 5.92 whereas forthe mixed MnTPPS+OTAB and MnTPPS+CTAB solution it was 5 and5.2 respectively. This lower pH value for the mixed solutions comparedto that of pure MnTPPS solution clearly indicates the strong ionic in-teraction between the anonic MnTPPS and cataionic matrix molecules.

Again LB technique is one of the best technique to prepare orga-nized molecular assembly of almost uniform thickness ideally havingone molecule thick [29]. On the other hand spin coated films are quitethick and have less control over the organization of molecules onto thethin films. Therefore lower thickness as well as difference in organi-zation due to film formation technique may also play crucial role in theobserved change in threshold voltage as well as compliance current inthe device consisting of LB films.

It was observed that the device get short circuited when the scaningvoltage exceeds beyond certain maximum limiting values. During ONstate when current passes through the devices, the organic layer in thedevice heated up. With increase in current due to increase in biasvoltage extent of heating also increases. As a result depending on biasvoltage, when current passing through the device crosses certainmaximum value the organic layer in the device damages resulting inshort circuit of the device. In case of LB film based devices short circuitoccurred at lower voltages compared to the devices consisting of spincoated films. This is because due to lower thickness of LB film, theorganic layer damages earlier i.e. at lower bias voltages. The heat tol-erance due to passage of current is lower in case of LB films since itsthickness is very low compared to spin coated films. Observed differ-ence in the measurement protocol as well as threshold voltage for LBfilm based devices where LB films were prepared following two dif-ferent procedure is due to the difference in packing as well as con-formation of the MnTPPS molecule onto complex LB films during LB

film formation.

4. Conclusion

In summary, we have designed different resistive switching devicesby using active layer of an anionic metallo porphyrin MnTPPS, MnTPPS+OTAB and MnTPPS+CTAB thin films. The active layer was preparedusing spin coating and LB technique. Water soluble anionic MnTPPSmolecules were incorporated onto LB films through electrostatic inter-action with preformed cationic surfactant (OTAB or CTAB) Langmuirmonolayer. π – A isotherm, π – t curve and BAM measurement clearlyrevealed the formation of complex monolayer of MnTPPS+OTAB &MnTPPS+CTAB as well as the strong electrostatic interaction betweenthe binary molecules in complex monolayer at air-water interface.Successful transformation of the floating monolayer onto solid substratehas been confirmed by absorption spectroscopic investigation.

All the devices showed bipolar switching as well as thresholdswitching under certain conditions. Here presence of electron-acceptorgroups (SO3H) and π-electron clouds over the surface of the MnTPPSmolecule play the vital role for the observed switching behaviour. Theobserved switching behaviour has been explained in terms of re-duction–oxidation process and conformational changes. Based on activelayer formation techniques as well as composition different thresholdvoltages of the devices have been observed at different measurementprotocol. This type of resistive bipolar and threshold switching in thinfilms are very important due to their promising application as the activecomponents for the memory device and/or logic circuits to realizemolecular electronics.

Author contributions

B.D., and S.A.H. designed the work. B.D. and S.C.a performed allexperiments and data analysis. S.A.H. D.B and B.D. wrote the manu-script with input from Inamuddin, A.K., S.C.b

Acknowledgment

The authors are grateful to DST, Govt. of India, for financial supportto carry out this research work through DST project Ref: EMR/2014/000234 and FIST DST project Ref. SR/FST/PSI-191/2014. The authorsare also grateful to UGC, Govt. of India for financial support to carry outthis research work through financial assistance under UGC – SAP pro-gram 2016.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.orgel.2017.12.038.

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