Researchpaper ... Clay Sciencexxx(2017)xxx-xxx Contents lists available at ScienceDirect Applied Clay Science ... bDepartmentofPhysics,NITAgartala,Jiraniya,799046,Tripura,India

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Contents lists available at ScienceDirect

Applied Clay Sciencejournal homepage: www.elsevier.com

Effect of nano clay Laponite on stability of SHG active J-aggregate of a thiacyanine dyeonto LB filmsPintu Debnath a, Santanu Chakraborty b, Subrata Debc, J. Nathd, B. Deya, D. Bhattacharjee a, Honami Soda e,Makoto Tominaga e, Yasutaka Suzuki e, Jun Kawamata e, Syed Arshad Hussain a, ⁎

a Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar, 799022, Tripura, Indiab Department of Physics, NIT Agartala, Jiraniya, 799046, Tripura, Indiac Department of Physics, Women's College, Agartala, 799001, Tripura, Indiad Department of Chemistry, Tripura University, Suryamaninagar, 799022, Tripura, Indiae Department of Chemistry, Faculty of Science, Yamaguchi University, Yoshida, Yamaguchi 753-8512, Japan

A R T I C L E I N F O

Keywords:J-aggregateLangmuir-Blodgett filmsSecond Harmonic Generation

A B S T R A C T

We have investigated the stability of Second Harmonic Generation (SHG) active J-aggregate of a thiacyanine dyeN, N′-dioctadecylthiacyanine perchlorate (TC18) in Langmuir-Blodgett (LB) films in presence and absence of asynthetic clay mineral Laponite. Surface pressure vs area per molecule isotherm was taken to observe the mono-layer stability and UV–vis absorption, deconvolution of the absorption spectra, Atomic Force Microscopy (AFM),X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) etc. have been employed to investigatethe stability. It has been observed that pure TC18 J-aggregates degrades with irradiation of light, passage of timeas well as post heat treatment of the TC18 J-aggregated LB films. However, in case of TC18 – Laponite hybridfilms the decrease in J-aggregate is minimal. Interestingly, it was observed that the relative humidity plays amajor role in the reconstruction of the J-aggregate in the treated film and thereby stabilizes the J-aggregate.Both pure TC18 and TC18 – Laponite hybrid LB monolayer films are found to be SHG active. In case of TC18 –Laponite hybrid film SHG signal shows better stability.

1. Introduction

Organic dye has a special property of self association or self assem-bling, which lead them to form different types of technology friendly ag-gregates in solution as well as in ultrathin films (Burdett, 1993; Chibisovet al., 2004; Miljanic et al., 2002). Depending on the orientation of thedye molecules in the aggregates, there exists different kinds of aggre-gating species viz. H-aggregates, J-aggregates, excimer etc. The extentof aggregation may strongly be influenced by various parameters likedye concentration, structure, ionic strengths, humidity, temperature andpresence of organic solvents etc. (Burdett, 1993). Varying these para-meters, orientational or conformational change of molecules can be ob-tained, which leads to the formation of various types of aggregates. Dyeaggregates are often found to play an important role in fundamental sci

ence as well as technological applications such as optical memory, or-ganic solar cells, sensors and nonlinear optical device applications etc.(Borsenberger and Weiss, 1993; Herrera et al., 2014; Mizuno et al.,2016; Walker et al., 2011). Surface driven migratory behaviour of aggre-gates leads to the coordinated movement within the aggregate (Ogawaet al., 2016).

Depending on the molecular orientation or arrangement in the ag-gregate, different types of aggregated species were found to form inultrathin films. Highly ordered molecular head to tail arrangement oftransition dipole moments results in the appearance of narrow ab-sorption band, shifted towards longer wavelength region and with al-most zero stokes shift; these are J-aggregates. Non-covalently coupledmolecules or dye molecules such as cyanine, porphyrines, squaranineetc. form well - ordered nano-scale J-aggregates (El-Hachemi et al.,2013; Li et al., 2015; Völker et al., 2014). Exciton delocalization lengthof J-aggregate governed its

⁎ Corresponding author.Email address: [email protected] (S.A. Hussain)

http://dx.doi.org/10.1016/j.clay.2017.07.013Received 3 November 2016; Received in revised form 3 July 2017; Accepted 6 July 2017Available online xxx0169-1317/ © 2017.

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spectral features, which is usually up to tens monomers, rather than itsphysical size (Völker et al., 2014). Due to excitonic nature of electronicexcitations, J-aggregates reveal a number of unique spectral propertiessuch as narrow sharp bands, large extinction coefficients, very largescale second and third-order optical nonlinearities up to 10− 5 esu andso on (Kobayashi, 1996). Such optical properties make the J-aggregatesa very promising candidate for many technological applications, for e.g.spectral sensitization in photovoltaic cells, optical waveguide, nonlin-ear optical devices, luminescent, optical switch, probes in biology andmedicine and so on (Borsenberger et al. 1993; El-Hachemi et al., 2013;Herrera et al., 2014; Imae, 2007; Kobayashi, 1996; Kuroda et al., 2002;Li et al., 2015; Mizuno et al., 2010; Völker et al., 2015; Walker et al.,2011).

Cyanines and their derivatives are probably the best known selfassociating dyes and have attracted considerable interest as buildingblocks for the construction of new functional nanoscale aggregates forelectronics and optoelectronics (Calzolari et al., 2009). In addition tothis, Langmuir-Blodgett (LB) technique is one of the best techniquesfor preparation of such aggregate in ultrathin films (Chakraborty et al.,2013; Hussain and Bhattacharjee, 2009; Kawasaki et al., 2014; Mobius,1995). The most important advantage of this technique is that onecan easily control the organization, orientation and hence the prop-erties of molecules as a function of molar composition, temperature,humidity, surface pressure, layer thickness etc. (Bhattacharjee et al.,2010; Chakraborty et al., 2013; Debnath et al., 2015; Hussain andBhattacharjee, 2009; Hussain et al., 2011; Kobayashi, 1996; Kawasakiet al., 2014; Mobius, 1995).

Aggregating behaviors of different cyanine and thiacyanine mole-cules assembled onto LB films have already been studied in our labo-ratory (Bhattacharjee et al., 2010; Chakraborty et al., 2015; Debnathet al., 2015; Hussain and Schoonheydt, 2010; Hussain et al., 2011). Ithas been observed that thiacyanine J-aggregate in LB films can be con-trolled by incorporating clay mineral nanolayers (Bhattacharjee et al.,2010), irradiation of UV-light decays J-aggregate to H-aggregate andmonomers (Hussain et al., 2011), SHG active J-aggregate of a thiacya-nine dye in LB film has also been studied (Chakraborty et al., 2015). Ithas been observed that the stability of J-aggregate increases in presenceof clay mineral nanolayers (Chakraborty et al., 2015). Reversible transi-tions between two different kinds of aggregates i.e. J-aggregate and ex-cimer were observed in LB films (Debnath et al., 2015). Different typesof J-aggregate/H-aggregates adsorbed on single silver nano aggregateswere extensively studied (Ogawa et al., 2010). Clay minerals are naturalnano particles with large surface area, layered structure and cation ex-change capacity (Schoonheydt, 2002, 2016). Accordingly, clay materi-als are considered as ideal host materials for charged as well as neutralorganic materials (Ras et al., 2007). Marked changes of spectral prop-erties along with change in functionality were observed when dye mol-ecules were adsorbed on to clay layers (Boháč et al., 2016; Bujdak andIyi, 2008; Sas et al., 2015; Schoonheydt, 2014).

Organic dye J-aggregates are generally found to degrade with thepassage of time (Debnath et al., 2016; Tani et al., 2008). However, it isvery important to curb this degradation or to find a suitable techniquesuch that the faded J-aggregate may be retraced back. It is extremelyimportant as the J-aggregated films may be used in practical purposesfor long time.

In one of our earlier reports, we have demonstrated the depen-dence of subphase temperature and concentration of a thiacyanineJ-aggregate in LB films (Chakraborty et al., 2015). Here we are in-terested to study the stability of TC18 J-aggregate in LB films. Con-ventional LB films are mechanically unstable as the molecules in thistype of films are held together by the van der Waals force.

So, in the quest of greater stability, we have prepared TC18 J-aggre-gates in presence of Laponite in LB films. In the organo-clay hybrid film,greater stability is expected due to the presence of electro-static forcebetween TC18 and Laponite. In this present work, we have presented re-sults of our investigations on the effect of irradiation of light, passage oftime (aging), temperature and humidity on TC18 J-aggregate in pure/hybrid LB films.

It was found that TC18 J-aggregate in LB films show SHG activity.Interestingly, it was observed that the use of clay minerals enhances thestability of J-aggregation in the hybrid films. Considering the wide ap-plication of nanoscale J-aggregates in ultrathin films, this result is verymuch important from application points of view.

2. Experimental

N, N′-dioctadecylthiacyanine perchlorate (TC18) purchased fromHayashibara Biochemical Laboratories Inc. was used as received with-out further purification. This dye was dissolved in HPLC graded chloro-form (99% Aldrich, stabilized by 0.5%–1% ethanol). The clay mineralused in the present work was Laponite, obtained from Laponite Inorgan-ics, UK, and used as received.

A commercially available Langmuir-Blodgett (LB) film deposition in-strument (Apex 2000C, Apex Instruments Co., India) was used for thepreparation of monolayer LB film. Ultra pure Milli-Q water of resistiv-ity 18.2 MΩ-cm was used as subphase. The concentration of the stocksolutions for TC18 was 0.5 mg/ml. In order to prepare LB film, 80 μl ofTC18 solution was spread onto the subphase with the help of a micro sy-ringe. After complete evaporation of volatile solvent (chloroform), bar-rier was compressed at a rate of 12.33 mm2/s to prepare the monolayerfilm. Smooth fluorescence grade quartz plates (for spectroscopy) andSi-wafer (for AFM studies) were used as solid substrate. Y-type deposi-tion at a particular surface pressure was followed to transfer Langmuirfilms at a deposition speed of 5 mm/min. All the films were deposited ata surface pressure of 15 mN/m. Details about the LB technique has beendescribed in our previous works (Hussain and Bhattacharjee, 2009).Hybrid Laponite films were prepared by spreading the TC18 solutiononto Laponite dispersion (1 ppm) subphase. Half an hour was waitedfor evaporation of the solvent as well as to complete the adsorption ofLaponite particle onto the floating TC18 layer followed by barrier com-pression as well as film transfer onto solid substrate. For AFM measure-ment, a single layer was deposited. The transfer ratio was estimated bycalculating the ratio of decrease in subphase area to actual area on thesubstrate coated by the layer and was found to be 0.98 ± 0.02.

UV–vis absorption of LB films was recorded using absorption spec-trophotometer (Perkin Elmer, Lambda 25). The absorption spectra wererecorded at 90° incidence and using a clean quartz slide as reference.

A homemade glass chamber was used for the post heat treatmentof the J-aggregated thin films, in which films were placed at the mid-dle of the chamber from a hanging support. UV–vis absorption spectrawere recorded immediately after the heat treatment. The temperaturedependence of J-aggregates in LB films was investigated by recordingthe UV–visible absorption spectra immediately after the preparation ofthe sample to avoid the aging effects of the films. We had used anotherhomemade glass chamber to investigate the effect of humidity on aggre-gates. Films were suspended from a support and UV–vis absorption wasrecorded immediately after the treatment.

The atomic force microscopic (AFM) image of monolayer film wastaken with a commercial AFM system, Innova AFM system

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(Bruker AXS Pte Ltd.) by using silicon cantilevers with a sharp, highapex ratio tip (Veeco Instruments). The AFM images presented herewas obtained in intermittent-contact (“tapping”) mode. X-ray diffraction(Bruker D8 advance) data was obtained using monochromatic copper Kαradiation (wavelength 1.54 ) and 2θ step of 0.02°. The ATR-FTIR spec-tra of TC18 LB films deposited onto ZnSe single crystal were recordedusing an FTIR spectrophotometer (PerkinElmer, Model No. Spectrum100, USA).

The incident angle dependence of the optical Second Harmonic Gen-eration (SHG) intensities of the films were measured using a pulsedbeam from a repetitively Q-switched Nd:YAG laser (Lee, Model818TQ,1 kHz) at a wavelength of 1064 nm with a typical pulse durationof 140 ns and a pulsed laser with peak power 5 kW. The samples weremounted on a motor-controlled rotating stage and the angle of incidenceof the laser beam relative to the film varied from 0° to 60°. The sam-ples were illuminated with the p-polarized (perpendicular to the rota-tion axis) laser beam, and the p-polarized SH light was detected (p–p po-larization measurement) by a photomultiplier tube (Hamamatsu, ModelR212). The spot diameter of focused laser is about 100 μm (Chakrabortyet al., 2015; Chandra et al., 2005; Kawamata and Hasegawa, 2006). Thesignal from the photomultiplier tube was processed using a boxcar aver-age (Stanford Research, Model SR250).

3. Results and discussion

3.1. Monolayer characteristics

To have an idea about the thermodynamic behaviour we have stud-ied the monolayer characteristics of TC18 at the air water interfaceas well as onto Laponite dispersion interface. Pure TC18 andTC18-Laponite form stable monolayer at the air water interface. Fig.1 shows the corresponding surface pressure – area per moleculeisotherms. The molecular structure of TC18 is shown in

Fig. 1. Surface pressure vs area per molecule isotherm of (a) pure TC18 in water subphaseand (b) TC18 at Laponite subphase. Inset shows the molecular structure of TC18.

the inset of Fig. 1. Each isotherm is a compilation of three compres-sions, each from three independent sets of experiments. The lift off areaof pure TC18 is about 1.12 nm2, in agreement with reported results(Chakraborty et al., 2015). The liftoff area is basically the mean area permolecule when surface pressure starts rising just above the zero surfacepressure, determined by the method described by Ras et al. (2004). PureTC18 isotherm shows liquid condensed and solid phase before collapse.However no sharp collapse was observed.

On the other hand TC18 isotherm measured onto Laponite disper-sion (1 ppm) subphase show marked changes with respect to that inpure water subphase. Here the lift off area is larger (1.26 nm2). At thebeginning liquid – expanded phase was observed and no clear collapsewas seen at the end of solid phase. Observed changes in the pressure– area isotherm in presence of Laponite dispersion subphase comparedto that in pure water subphase clearly indicates the incorporation ofLaponite layers onto the floating TC18 film and formation of TC18 –Laponite hybrid monolayer at the air – water interface (Chakraborty etal., 2015; Hussain and Schoonheydt, 2010).

3.2. Formation of TC18 J-aggregate

In solution, TC18 predominantly remains as monomer with promi-nent monomer absorption band at around 430 nm along with a veryweak high energy shoulder at 410 nm (Chakraborty et al., 2015;Hussain et al., 2011) (Fig. 2a, inset). This weak shoulder is attrib-uted due to minute amount of H-aggregate in solution (Chakrabortyet al., 2015; Hussain et al., 2011). Fig. 2a shows the UV–vis absorp-tion spectrum of pure monolayer TC18 LB film lifted at a surface pres-sure of 15 mN/m. The subphase temperature was 25 °C and the spread-ing volume was 80 μl. Concentration of the TC18 solution was 0.5 mg/ml. The absorption spectrum of conventional TC18 LB film possessesa prominent peak at 461 nm and two weak humps at 433 nm and410 nm. The 461 nm band and 433 nm band are assigned to J-aggre-gate and monomer respectively (Chakraborty et al., 2015; Yamaguchiet al., 2005). The 410 nm absorption band of TC18 LB films has beenassigned as due to the formation of minute amount of H-aggregation(Hussain and Schoonheydt, 2010 and Yamaguchi et al., 2005). Decon-volution of the pure TC18 LB film absorption spectrum (Fig. 2(b)) re-veals the presence of three Gaussian curves corresponding to the J-ag-gregate, monomer band and H-dimer band confirming the adequate de-termination of the existence of different species present in the pureTC18 LB film. From the absorption and its deconvolution spectra, it isclear that TC18 J-aggregate predominates in LB films compared to TC18monomer and H-aggregate. Our research group already demonstratedthat TC18 forms strong J-aggregate in LB films under various conditions(Hussain et al., 2011). It is well known that the head-to-tail like arrange-ment of molecules in J-aggregate makes it an essential candidate fornonlinear optical study due to its non centrosymmetric structure. Thishead-to-tail arrangement of molecules in J-aggregate is responsible forits non-centrosymmetric structure in monolayer LB film. Multilayer ofsuch films results in centrosymmetric structure, whereas non-centrosym-metry is the prerequisite to possess SHG activity. Therefore, J-aggregatein monolayer LB films is quite interesting with respect to its nonlinearapplications.

3.3. Effect of laser irradiation

It has already been reported by several authors that J-aggregatesin pure/hybrid LB films could decay to monomers and H-aggregatesupon laser irradiation and irradiation of monochromatic

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Fig. 2. (a) UV–vis absorption spectra of pure TC18 J-aggregate in LB film lifted at a surface pressure of 15 mN/m, inset shows the solution spectra of pure TC18 and (b) Deconvolution ofUV–vis absorption spectra of pure TC18 in LB film.

light in LB film (Chandra et al., 2005; Deb et al., 2005; Kawaguchi andIwata, 1988; Kishida et al., 2008;). The SHG measurement process it-self involves laser irradiation and was found to perturb the assembly ofmolecules in the ultrathin film (Kawamata and Hasegawa, 2006). So itis important to check the stability of J-aggregates of TC18 in pure andhybrid LB films in order to find their potential use as effective nonlinearoptical active materials.

Accordingly, we have investigated the effect of laser irradiation onTC18 J-aggregate in both pure and hybrid LB films. For this the LB filmswere exposed to laser beam of peak power 5 kW for different time inter-vals, followed by absorption spectrum measurement. Fig. 3(a) and (b)show the corresponding absorption spectra for TC18 J-aggregate in pureand hybrid LB films respectively. From Fig. 3(a) it is seen that the in-tensity of J-aggregate band decreases with increase in irradiation time.TC18 J-aggregate absorbance becomes less than 50% of its initial ab-sorbance for an irradiation time of 45 min. In our SHG measurements,the films have to be exposed to laser beam for a period of 25–30 min ap-proximately. Accordingly the effect of laser irradiation up to 45 min onLB films were checked. The deconvolution of the corresponding absorp-tion spectra (Fig. S1) also exhibits the decrease of J-aggregated bandintensity with increase in irradiation exposure time. However, the de-crease in J-aggregated band absorbance for hybrid TC18 film due tolaser irradiation is less than 10% even after 45 min of irradiation (Fig.3(b)). Deconvolution of the corresponding absorption spectra also repli-cates the same (Fig. S2). A comparison of relative decrease in the ab-sorbance of J-aggregated band for both pure and hybrid monolayer LBfilm was shown in Fig. 3(c). From the figure it is obvious that stabilityof TC18 J-aggregate is higher in hybrid LB film in comparison to purefilm. Therefore, it can be concluded that incorporation of Laponite en-hances the stability of J-aggregate in LB film.

The coherent domain size or spectroscopic aggregation number cor-responding to J-aggregate is very important to have an idea about theextent of J-aggregate formation (Koti et al., 2003; Tani et al., 2008).In addition, the nonlinear behaviour of J-aggregation is highly influ-enced by the spectroscopic aggregation number (Koti et al., 2003).The value of aggregation number is higher when the extent of J-ag-gregation is larger and vice versa. The coherent size

of the J-aggregate can be calculated from absorption spectra ofmonomer and J-aggregate as follows (Tani et al., 2008; Koti et al.,2003),

where

• N = Spectroscopic aggregation number.• ∆ ν1/2(M) = full width half maxima of monomer.• ∆ ν1/2(J) = full width half maxima of J-aggregate.

In the present study, the spectroscopic aggregation numbers are cal-culated and listed in Table 1. From the table, it has been observedthat spectroscopic aggregation number/coherent size decreases with in-crease in irradiation time for both pure and hybrid LB films. However,in case of hybrid film the decrease is much lower. The values are in wellagreement with the observed changes of J-aggregate band with irradia-tion time.

It is relevant to mention in this context that Laponite possesses neg-atively charged surface with a cation exchange capacity (CEC) and lay-ered structure (Singla et al., 2012). In the process of formation of -Laponite hybrid LB films, the cationic TC18 molecules were adsorbedonto Laponite surface through electrostatic/ion – exchange reaction. Asa result, the TC18 molecules were strongly fixed onto the Laponite tem-plate resulting in an enhancement in its stability. There are several re-ports where it has been shown that molecules in the hybrid films aremore stable compared to their pure counterpart (Frindy et al., 2016).

3.4. Aging effect

Since the structure of J-aggregate is non-centrosymmetric, it is pos-sible to have SHG active films by forming J-aggregates in LB mono-layer (Chakraborty et al., 2015), even if the single crystal of the mol-ecule is SHG inactive (Chakraborty et al., 2015). Such films may besuitable for various nonlinear optical devices (Kobayashi, 1996). How-ever, any kind of organizational/orientational changes in the filmswith the passage of time is undesirable for real time de

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Fig. 3. UV–vis absorption spectra of 1 layer LB films measured after irradiating for different time interval with laser light – (a) pure TC18 J-aggregate, (b) TC18-Laponite J-aggregate and(c) Relative decrease in J-aggregate Intensity due to irradiation in pure TC18 and TC18-Laponite LB film. Duration of irradiation time is mentioned in the corresponding figure.

Table 1Variation of Spectroscopic aggregation number of TC18 and TC18- Laponite J-aggregatedue to laser irradiation.

ConditionAggregationnumber (N) Condition

Aggregationnumber (N)

Pure TC18 2.97 TC18-Laponiteinitial

5.16

Initial 2.61 5 min 5.135 min 2.47 15 min 5.1315 min 2.36 25 min 5.1025 min 1.82 35 min 5.0435 min 1.82 45 min 4.6245 min 1.89

vice applications. Generally, it has been observed that some molecu-lar movements occurred in LB films with passage of time (Miura et al.,2013), which leads to change in molecular organization as well as prop-erties of the film. So the information regarding the stability of J-aggre-gate with the passage of time in pure/hybrid LB films is very important.Accordingly, we have studied the properties of TC18 J-aggregate in bothpure and hybrid LB films with the passage of time.

In order to study the aging behaviour, we have prepared TC18 J-ag-gregate in pure and hybrid LB monolayer and measured the absorp-tion spectra with the passage of time. Fig. 4(a) and (b) show the cor-responding absorption spectra for pure and hybrid LB monolayer filmsrespectively. From Fig. 4(a), it is seen that the intensity of 461 nmband, which corresponds to J-aggregates, decreases with the passageof time. Almost 80% decrease in TC18 J-aggregate absorbance oc-curred within four days. However, after four days the TC18 J-aggre-gate film becomes stable and no further decrease was observed. Cal-culated values of spectroscopic aggregation number or coherent size(Table – S1) as well as the deconvolution of the corresponding UV–visabsorption spectra also replicate the same (Fig. S3). On the other hand,J–aggregate formed in hybrid LB film (Fig. 4(b)) was found to bemuch more stable than the pure TC18 J-aggregate. Here also up tofour days the intensity of J-aggregated band decreases and then it be-comes stable. Interestingly here the extent of decrease is of the or-der of 10–15% with respect to its initial absorbance. Calculated val-ues of aggregation numbers as well as deconvolution of the correspond-ing absorption spectra also confirm this (Fig. S4). A comparison be-tween the relative decreases of intensity of J-aggregate in both thepure and hybrid films (Fig. 4(c)) shows that the J-aggregate of pureTC18 film is very unstable with the passage of time whereas the TC18-

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Fig. 4. UV–vis absorption spectra of 1 layer LB films measured with the passage of time – (a) pure TC18 J-aggregate, (b) TC18-Laponite J-aggregate and (c) Relative decrease in J-aggre-gate intensity of pure TC18 and TC18- Laponite LB film with the passage of time.

Laponite film J-aggregate is much better with respect to its stability.Thus use of Laponite restricts the molecular movements within the J-ag-gregate in hybrid LB films up to a certain extent.

3.5. Effect of temperature on J-aggregate

For practical device application molecules assembled into ultrathinfilms should be stable up to a certain temperature otherwise the prop-erties may be affected. Therefore, it would be interesting to have ideaabout the effect of increasing temperature on the TC18 J-aggregate inLB monolayer. Accordingly both pure and hybrid films of TC18 J-aggre-gate were placed in a glass chamber for 1 min at different temperaturesfollowed by absorption spectroscopic measurement. The temperature ofthe chamber were varied from 25 °C to 115 °C. Corresponding spectrawere given in Fig. 5(a) and (b) respectively for pure and hybrid LB films.

It has been observed that for both pure TC18 and TC18-Laponitehybrid LB films the intensity of J-aggregate band decreases upon in-creasing temperature. However, for pure film the extent of decay ismuch more than that in hybrid LB film. In pure LB films the J-aggre-gate band becomes almost a weak hump at 115 °C, whereas, in hy-brid film the J-aggregate band was distinct and prominent. The rel-ative decrease in J-aggregated band intensity in case of pure TC18film was 60% and that in hybrid film was

20% (approx). Interestingly in case of hybrid films J-aggregate band in-tensity become almost stable beyond 90 °C and remained almost un-changed up to 115 °C. Calculated values of coherent size/SpectroscopicAggregation Number (Table S1) as well as the deconvolution spectraalso support the same (Figs.S5 and S6).

The initial decrease in J-aggregated band intensity in both pure andhybrid LB films are almost similar up to 80 °C (approx) (Fig. 5c). How-ever beyond this temperature for hybrid films no significant decreasewas observed although for pure TC18 films significant decrease in J-ag-gregated band intensity was observed. Initially there may exist watermolecules in the LB film (Hanley et al., 1996). Upon heating the watermolecules evaporate resulting in re-orientation (molecular movement)of TC18 molecules in both pure and hybrid LB films. The size or shapeof J-aggregate domain may change resulting in a change in J-aggre-gated band intensity in absorption spectra. However at higher temper-ature in case of pure TC18 J-aggregate film dissociation of J-aggregateoccurs and thereby increases the non-radiative transition rate, which re-sults in decrease in J-aggregated band intensity. It has been reportedthat upon heating merocyanine J-aggregate in LB film dissociated withsimultaneous increase in monomer band (Fukui et al., 1983). Pseudo-cyanine J-aggregate decayed to monomer in LB films at higher temper-ature (Shelkovnikov et al., 1995).

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Fig. 5. UV–vis absorption spectra of 1 layer LB film kept at different temperatures ranging from 25 °C to 115 °C for a fixed time period of 1 min – (a) pure TC18 LB films, (b) TC18-Laponite hybrid films and (c) Relative decrease in J-aggregate intensity of pure TC18 and TC18- Laponite LB film with increase in temperature.

In case of hybrid films, the cationic molecules are attached on to thenegatively charged Laponite surface through ion exchange/electrosta-tic interaction. Owing to this they are strongly bounded in the hybridfilms compared to that in absence of Laponite. This Laponite inducedstrong bonding of TC18 molecules in TC18 J-aggregate in case of hybridfilm opposes the dissociation of the J-aggregated species. As a result, theJ-aggregated band absorbance remained almost unchanged.

3.6. Reconstruction of J-aggregate

In the previous section of the manuscript it has been observed thatJ-aggregate in LB films decays with passage of time as well as with in-crease in temperature. Although in TC18-Laponite hybrid films the ex-tent of decay was less compared to that in pure TC18 LB films. However,stable J-aggregate in LB films is expected for the application purposesespecially, if the application is based on J-aggregate, e.g. non-linear op-tical application (Herrera et al., 2014). It is believed that the evapora-tion of water molecules in the film or dissociation of J-aggregated do-mains may be responsible for such degradation.

Therefore, in an attempt to regain the TC18 J-aggregate it wouldbe interesting to check the effect of water molecule on the films (withdecreased J-aggregate). Accordingly, thermally treated

as well as aged films (pure TC18 and TC18-Laponite films) were exposedto different humidity level in a chamber with a controlled manner. Allthe films were kept under humidity for five minutes. After that the filmswere taken out and kept at ambient environment (to dry out the ex-tra water in the film surface) followed by their absorption spectroscopicmeasurements. Corresponding spectra is shown in Fig. 6(a) and (b) re-spectively for pure and hybrid films.

Interestingly it has been observed that J-aggregate band intensity in-creases with increase in humidity level and reaches maximum at therelative humidity level 99% for both pure and hybrid films. It was pos-sible to regain 80% of the intensity of freshly prepared TC18 J-aggre-gate (Fig. 6(c)). Calculation of spectroscopic aggregation number/coher-ent size (Table – S1) as well as deconvolution spectra also supports this(Figs.S7 and S8). Increase in J-aggregated band intensity after humiditytreatment suggests that J-aggregate domains get reconstructed in the LBfilms.

In order to check whether this reconstruction of J-aggregate in LBfilms is reversible or not, the films (in which J-aggregate regained bythe humidity treatment) were again heated. Accordingly, the J-aggre-gate decays as observed from the absorption spectroscopic measure-ment (Fig. S9). Now this films (both pure and hybrid) were again ex-posed to different humidity levels. Interestingly it has been observedthat TC18 J-aggregate regains in the films. We have checked few cy-cles of such experiments and observed

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Fig. 6. UV–vis absorption spectra of 1 layer LB film kept at different humidity for a fixed time period of 5 min – (a) pure TC18 LB films, (b) TC18- Laponite hybrid LB films (c) Relativeincrease in J-aggregate intensity due to increase in humidity of pure TC18 and TC18- Laponite LB film and (d) Variation of J-aggregate intensity of pure TC18 and TC18-laponite LB filmdue to heat and humidity treatment.

that in each case 80% of the TC18 J-aggregate intensity regained afterhumidity treatment (Fig. 6(d)).

In order to check whether there is any deformation/change in theTC18 molecules after heat treatment, the treated film was dissolved inchloroform and absorption spectra were recorded. Spectrum was foundto be exactly the similar to that of pure TC18 solution absorption spec-trum (Fig. S10) which suggests that there was no modification in TC18molecular structure due to heat treatment or aging.

In order to check whether there is any structural changes occurredin the TC18-Laponite hybrid films during humidity as well as heattreatment, the hybrid LB films were studied using XRD. The XRD pat-terns used here are of out-of- plane patterns. Fig. 7 shows the XRDpatterns of multilayered LB film of (a) TC18 untreated (b) TC18 afterheat treatment (c) TC18 after humidity treatment (d) TC18 – Laponite(e) TC18 – Laponite after heat treatment and (f) TC18 – Laponite af-ter humidity treatment. From figure it is clear that the peak positionsof all the XRD spectra ((a)–(f)) are same except some small changesin the intensity. This indicates that no significant structural changeoccurred in the pure TC18 LB film and in the TC18 – Laponite hy-brid LB film due to heat as well as humidity treatment. However,slight change in intensity indicates reorientation of molecules in theLB film resulting in change in the de

gree of orderness. This is explained as follows; before heating therewas hydrophilic area near the substrate with water molecules and hy-drophobic area near the outer surface in the film, which could orientthe amphiphilic molecules in one direction and when the water mole-cules were removed by heat treatment the difference in hydrophilicity– hydrophobocity decreased and as a result the TC18 molecules will bedisordered. Again a careful observation of all the XRD patterns revealsthat the loss of intensity in the hybrid film is less compared to that ofpure TC18 LB film. This confirms the greater stability in the hybrid filmcompared to that of pure TC18 LB film.

It may be mentioned in this context that the layer distance for themultilayer film having layered structures can be calculated based on theposition of the first diffraction peak (Umemura et al., 2002). In the pre-sent case the peak position for first diffraction peak (2θ = 2.02) is al-most same for all the spectra. The layer distance calculated based on thisis d = 4.4 nm. This value is consistent with the thickness of the singlelayer film as observed from the AFM images given in the later section ofthis manuscript.

It is interesting to note that in LB film formation process firstlythe TC18 molecules were spread at air-water interface and monolayerfilm of TC18 molecules were formed. Later this floating film was trans-ferred onto solid support for preparation of LB mono

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Fig. 7. XRD spectra of LB film of (a) pure TC18, (b) TC18 after heat treatment, (c) TC18 after humidity treatment, (d) TC18 – Laponite, (e) TC18 – Laponite after heat treatment and (f)TC18 – Laponite after humidity treatment.

layer films. At the air-water interface several TC18 molecules may comecloser through hydrogen bonding. Later when this is transferred ontoquartz substrate domains of TC18 J-aggregate were formed. There areseveral reports where it has been demonstrated that due to hydro-gen bonding different kinds of nano structure were formed in LB films(Correia et al., 2012; Jiang et al., 2006; Rosoff, 2001). When this TC18J-aggregate film is heated the water molecules get evaporated result-ing in the dissociation in the J-aggregated domains. Accordingly a de-crease in the intensity of the J-aggregated band is observed. Againwhen the films were exposed to different humidity levels, water mole-cules penetrate on to the films. As a result TC18 J-aggregate domainswere reformed in the LB films. In order to confirm the presence ofwater molecule in the LB film we have studied the FTIR spectra ofTC18 LB film after heat treatment as well as after exposing to humid-ity (Table 2). Corresponding spectra are shown in Fig. 8. Interestingly,peaks in the region 3000–4000 cm− 1 and 1650 cm− 1 were observedafter exposing to humidity which were absent in the heat treated film.

Table 2Important band assignments for FTIR spectra.

Bandposition(cm− 1) Assignment

TC18 LB filmafter heattreatment

TC18 LB filmafter humiditytreatment

3455 O H Absent Present2920 CH2 stretching

(asymmetric)Present Present

2846 CH2 stretching(symmetric)

1635 C C Absent Present

Presence of 3455 cm− 1 band confirms the presence of water moleculein the LB film. Therefore it can be concluded that the presence of watermolecule in the film play a crucial rule in the formation of TC18 J-ag-gregate.

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Fig. 8. FTIR of TC18 LB film after heat treatment (black line) and after humidity treat-ment (red line). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

3.7. Second Harmonic Generation (SHG) response of TC18 andTC18-Laponite J-aggregate

J-aggregate in LB monolayer films possesses non-centrosymmetricstructure (Kajikawa et al., 1991). Accordingly J-aggregate in LB mono-layer shows nonlinear optical activity (NLO) (Feller et al., 1997; Furukiet al., 1999). This technique is suitable to prepare NLO active ultra-thin films even with intrinsically NLO inactive materials. In our previ-ous investigation, it was demonstrated that the molecule TC18 formsprominent J-aggregate structure in LB monolayer, which was SHG ac-tive (Chakraborty et al., 2015). Hence in the present manuscript the sta-bility of TC18 J-aggregate in LB monolayer under various conditionswas explored. In the previous section it was assumed that J-aggregatedecays due to aging as well as light irradiation and at high tempera-ture. However, J-aggregate in TC18 – Laponite hybrid film was morestable. Therefore, it would be interesting to investigate the SHG activityof TC18 J-aggregate under various conditions.

In order to check the effect of aggregation on SHG activity wehave measured the SHG signal of the recently prepared film as wellas aged film. It has been observed that the SHG signal intensity grad-ually decreased due to aging up to few days. All the films older thanfour days possess almost same intensity. Representative figure show-ing the dependence of the SHG intensity on the incident angle ofthe laser beam for recent and several days old pure TC18 J-aggre-gate monolayer LB film has been shown in Fig. 9(a). Observed de-crease in SHG intensity is in agreement with our previous spectro-scopic studies of the TC18 J-aggregate LB films with the passage

of time. Initially due to change in monolayer orientation or monolayermovement the structure of J-aggregate domains changes with time.However after four days the J-aggregate structure in LB film becomesstable and as expected no further decrease in SHG intensity was ob-served.

In case of TC18-Laponite hybrid films (Fig. 9(b)) the extent of de-crease in SHG intensity is much less. Here, the decrease was of the orderof only 10% compared to 75% in case of pure TC18 LB films. We havealso checked the effect of laser irradiation and heating on SHG inten-sity of both pure and hybrid LB films (figure not shown). For both pureand hybrid monolayer LB films the SHG intensity decreased due to laserirradiation as well as for the films kept at high temperature. In case ofpure LB films the decrease in SHG intensity was of the order of 70–75%.However, in case of hybrid films, the extent of decrease was of the or-der of 10–15%. Absorption spectroscopic studies (given in previous sec-tion) also support the same. This observation suggests that clay mineralcan be considered as ideal host material to stabilize the J-aggregate inLB films to use the same as an efficient SHG active material in ultrathinfilms.

3.8. Atomic Force Microscopy (AFM)

To get visual confirmation about the changes in surface morphol-ogy of the LB monolayer films due to aging, heat treatment as wellas humidity treatment, the films were investigated using Atomic ForceMicroscopy (AFM). Fig. 10(a) shows the AFM image of monolayer LBfilm of pure TC18 deposited onto silicon substrate at 20 °C. Disc likeobjects were observed on the AFM image of TC18 LB film. The di-ameter and height of the disc ranges in between 125 and 160 nm and2–4 nm respectively. Almost all the surface of the film was covered withsuch objects. In the previous section, it has been observed that strongTC18 J-aggregate is formed in the film prepared under similar condi-tion. Therefore, this nano disc like structure may be due to the forma-tion of TC18 J-aggregate domains in LB monolayer. Circular shaped do-mains of other cyanine derivatives in LB monolayer has already beenreported by several other researchers (Miura et al., 2013; Ozcelik et al.,2004). In one of our previous works, it has been seen that indocarbo-cyanine derivative in LB monolayer forms such circular J-aggregate do-mains in LB monolayer when mixed with fatty acid under specific con-ditions (Debnath et al., 2015).

On the other hand the AFM image (Fig. 10(b)) of monolayer TC18LB film kept at 110 °C for 1 min does not possess such circular do-mains of TC18 J-aggregate. AFM image clearly indicating that the TC18J-aggregate domains were destroyed due to heat treatment. Here thefilm possesses uniform surface with lower height profile. Our previ-ous spectroscopic studies (Fig. 5) also indicated that TC18 J-aggre-gate decayed due to heat treatment. The

Fig. 9. Dependence of the SHG signal intensity on the incident angle of the (a) recently prepared pure TC18 J-aggregate film and 7 days old pure TC18 J-aggregate film and (b) recentlyprepared TC18- Laponite J-aggregate film and 7 days old TC18- Laponite J-aggregate film.

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Fig. 10. AFM image of TC18 J-aggregate measured after exposing to (a) 25 °C, (b) 110 °C temperature and (c) after exposing the heat treated film to 99% humidity, (d) pure TC18-laponite,(e) TC18-Laponite after exposing the film to 110 °C, (f) TC18-Laponite film after exposing the heat treated film to humidity.

observed difference in the AFM images of TC18 monolayer LB filmsbefore and after heat treatment gives a compelling visual evidence ofthe change in the TC18 aggregation in LB films. The AFM images ofrecent TC18 LB monolayer film and the same for 7 days old TC18 LBmonolayer LB films (Fig not shown) also gives visual evidences for thechanges in TC18 J-aggregate domains due to aging.

In the previous sections of the manuscript it was also found that theTC18 J-aggregate dissociated or destroyed due to heat treatment can berecovered by humidity treatment of the same film. This suggests thatcircular shaped TC18 J-aggregate domains can be reconstructed in thefilm after humidity treatment. Although, the TC18 J-aggregate domainswere destroyed due to heating but the AFM image (Fig. 10(c)) of TC18LB monolayer kept at 110 °C for 1 min followed by humidity treatment(99%, 5 min) clearly shows the presence of circular shaped TC18 J-ag-gregate domains. This observation supports that TC18 J-aggregate wasreconstructed when the films were exposed to humidity.

The AFM image of TC18 – Laponite LB film (Fig. 10d) is quite dif-ferent from that of pure TC18 LB film. In LB technique the Laponiteparticles were adsorbed onto floating TC18 layer through electrostaticinteraction/cation exchange reaction and thus hybrid TC18 – Laponitelayer was formed. In this layer ideally the Laponite particles were cov-ered with the TC18 molecules. However, since the dimension of the dyeis beyond the resolution of the AFM system, it's not possible to identifythe dye and Laponite separately. However, dye aggregates are clearlyvisible in the AFM images. As a whole the AFM image possess two fea-tures – (i) Laponite particle covered with TC18 (randomly oriented) and(ii) Laponite particle with dye aggregates. These two features are clearlydistinguishable. The height of TC18 aggregate onto Laponite is around5 nm.

On the other hand the AFM image measured after heat treatment(Fig. 10e) followed by humidity exposure and AFM image

(Fig. 10f) showed no significant change in TC18 aggregate in hybrid LBfilm. All the films possess almost similar morphology. This suggests thatJ-aggregate in TC18 – Laponite film is much more stable.

4. Conclusion

The stability of TC18 J-aggregate was investigated under variousconditions in presence and absence of Laponite. We have observed thatthe intensity of pure TC18 J-aggregate decreases with irradiation oflight, passage of time and with increase in temperature. But under iden-tical situation, TC18-Laponite J-aggregated films exhibit quite betterstability. After exposing these films to humidity (RH 99%), we observedthat the decayed J-aggregate structure gets reconstructed in both thepure and hybrid J-aggregated films. Almost 80% of the decayed J-ag-gregated structure reconstructed back in both the cases. It has been ob-served that the decayed J-aggregate can be retraced back up to 80% dur-ing several cycles of heat and humidity treatment. Monolayers of TC18J-aggregate in both pure TC18 and TC18-Laponite LB films were foundto be SHG active. In case of TC18 J-aggregate LB film 75% decrease inSHG intensity occurred after 7 days. However, in case of TC18-Laponitehybrid films the SHG intensity remained almost same even after 7 days.Thus, use of Laponite enhances the stability of TC18 J-aggregate andhence the SHG behaviour.

Uncited references

Fatnassi and Es-Souni, 2015Ghasemi et al., 2004Jiang et al., 2016Kitahama et al., 2010Mchedlov-Petrossyan and Kholin, 2004Micheau et al., 2004Su et al., 2004

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Acknowledgement

The authors are grateful to DST, for financial support to carry outthis research work through FIST DST project ref. SR/FST/PSI-191/2014.The author SAH is grateful to DST, for financial support to carryout this research work through DST, Govt of India project ref. No. EMR/2014/000234.The author Mr. Pintu Debnath acknowledges the financialsupport through INSPIRE fellowship of DST, Govt. of India.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.clay.2017.07.013.

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