Deciphering the Effect of Polymer-Assisted Doping on the

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Deciphering the Effect of Polymer Assisted Doping on Optoelectronic Properties of Block Copolymer Anchored Graphene Oxide Nabasmita Maity, Atanu Kuila, and Arun K. Nandi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03923 • Publication Date (Web): 22 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Deciphering the Effect of Polymer Assisted Doping on Optoelectronic Properties of Block Copolymer Anchored Graphene Oxide Nabasmita Maity, Atanu Kuila and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata700 032, INDIA

ABSTRACT: Doping facilitate the tuning of band gap providing an opportunity to tailor the optoelectronic properties of graphene in a simple way and polymer assisted doping is a new route to combine the optoelectronic properties of graphene with the properties of polymer. In this endeavour, a linear diblock copolymer, polycaprolactone–block–poly(dimethyl aminoethyl methacrylate)

(PCL13-b-PDMAEMA117) is grafted from GO surface (GPCLD) via

consecutive ring opening and atom transfer radical polymerization. GPCLD is characterized from

1

H NMR, Fourier transformed infrared spectroscopy, atomic force microscopy,

thermogravimetric analysis, X-ray photoelectron and Raman spectroscopy. The phase transition behaviour of GPCLD solution with varying temperature and pH is monitored from fluorescence spectroscopy and dynamic light scattering. The temperature dependent 1H NMR spectra at pH 9.2 indicate the influence of temperature on the interaction between GPCLD and solvent (water) molecules causing the phase separation. Fluorescence spectra at pH 4 and pH 9.2 give the evidence of localized p and n – type doping of graphene assisted by the pendant PDMAEMA chains. In the impedance spectra of GPCLD films the Nyquist plots vary with pH; at pH 4, it exhibits a semicircle at higher frequencies and a spike at lower frequencies; at pH 7.0 the spike is replaced by an arc and at pH 9.2 the semicircle at higher

*

For correspondence: Arun K. Nandi, Email: [email protected] 1 ACS Paragon Plus Environment

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frequencies vanishes and only a spike is noticed suggesting different type of doping of graphene at different pH. The dc-conductivity also varies with pH and temperature due to the different types of doping. The current (I)-voltage (V) property of GPCLD at different pH is very unique, at pH 9.2 an interesting feature of negative differential resistance (NDR), at pH 7 rectification property and at pH 4 again the NDR property is observed. The temperature dependent I-V property at pH 7 and 9.2 clearly indicates a signature of doping, de-doping and re-doping due to change of interaction of GO with the grafted polymer arising from coiling and de-coiling of polymer chains. INTRODUCTION: Over the past few years, graphene oxide (GO)1 has spurred intensive scientific and industrial interest owing to its extraordinary chemical, mechanical, electronic and optical properties.2-9 It consists of a monolayer network of sp2-hybridized carbon atoms decorated with covalently attached hydroxyl, carboxyl and epoxy functionalities either at the edge or on the basal plane making it dispersible in water as well as in a wide range of polar solvents.10,11 Unlike graphene, GO possesses both the conducting π-state of sp2-hybridized carbons and an energy gap between the σ-state of sp3-hybridized carbons.12 Graphene, being a zero band gap material is non fluorescent and is difficult to disperse which limits its applications in many fields. However, easy solution processability, existence of finite band gap combined with the fluorescence property makes GO a promising candidate for its wider applications e.g., optoelectronic devices, solar cells, biotechnology, drug delivery, imaging, chemical and biological sensing etc.13-19 Moreover, the availability of oxygen-containing functional groups and large aspect ratio of GO facilitates further functionalization that provides a way of band gap modulation, thus allowing tuning of its optoelectronic properties.20,21 Recently, chemical functionalization of GO by polymers18,22,23 and block copolymers,24,25 has aroused great interest because it results in a smart polymeric material integrated with both the properties of GO and polymers/block copolymers. Compared to the

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homopolymers, block copolymers exhibit attractive phase behaviour and exciting selfaggregation properties yielding switchable morphologies under different stimulants.26 So, it would be interesting to observe any alteration in the optical as well as electronic properties of GO which may be tuneable with block copolymer aggregation. Doping also has a significant effect on the band gap tuning as it leads to a shift of the Fermi level, therefore providing an opportunity to tailor the optical and electronic properties of graphene in a simple way.27 Tuning the optoelectronic properties of graphene by band gap modulation using chemical functionalization and/or doping would provide a large impact on its properties and applications. There are several fantastic reports in the literature regarding doping of graphene where graphene has been doped chemically, electrochemically or by electrostatic backgating.28-31 However, a detailed study elucidating the polymer assisted doping phenomena and its effect on the optical, especially on the electronic properties of graphene is still rare. In an attempt, we have previously reported a method for the synthesis of an amphiphilic reduced graphene oxide (rGO) via grafting of poly(dimethylaminoethyl methacrylate) (PDMAEMA) and its fluorescence and electrical properties are investigated. The PDMAEMA chains, by virtue of their -NMe2 groups, are found to be involved in p- and n-type doping of graphene, thus giving a new way of polymer assisted doping by simply changing the pH of the medium.32 Motivated by our previous work, we now report the optoelectronic properties of GO grafted linear diblock copolymer having poly(εcaprolactone) (PCL) as hydrophobic segment and PDMAEMA as dual (thermo/pH) responsive hydrophilic segment under varying temperatures and pHs, in detail. Owing to the hydrophobic-hydrophilic balance of the block copolymer chains, the system undergoes phase transition, affecting the extent of doping and hence the properties differently. In this context, we have used electrochemical impedance spectroscopy to recognize the polymer assisted 3 ACS Paragon Plus Environment

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doping of graphene using thin film of sample (casted at different pH) sandwiched between two ITO electrodes. Whilst deciphering the electrical properties, the current (I)-voltage (V) characteristics of the system are investigated in detail and possible explanation for the observed properties of negative differential resistance (NDR) and rectification is delineated. Comparisons of the doping are also made by varying chain lengths of hydrophilic and hydrophobic blocks grafted from GO and also with the GO-g-PDMAEMA (GD). Herein, our primary objective is to inspect how chemical functionalization with a thermo and pH responsive block copolymer can revolutionize the doping and optoelectronic properties of graphene. EXPERIMENTAL Synthesis: Three polycaprolactone-block-poly(dimethyl aminoethyl methacrylate) with different chain lengths of the blocks grafted from GO [GO-g-(PCLx-b-PDMAEMAy)] were synthesised and characterized by 1H NMR spectroscopy (Figure S1A, S1B). The degree of polymerization (DP) of each blocks of the GPCLDs were calculated from 1H NMR spectra.. We have synthesized GO-g-(PCL13-b-PDMAEMA117), GO-g-(PCL13-b-PDMAEMA158) and GO-g-(PCL3-b-PDMAEMA60) which are termed as [GPCLD], [GPCLD1] and [GPCLD2], respectively. The details of the synthesis33 are presented in supplementary information (SI). Characterization Microscopy: The atomic force microscopy (AFM) was conducted in the non-contact mode at a resonance frequency of the tip end ~250 KHz and the morphology and thickness of the GO and GPCLD were studied using atomic force microscope (Veeco, model AP 0100). Spectroscopy: The 1H NMR spectra of the samples were recorded using a Bruker 500 MHz NMR spectrometer in CDCl3. Temperature variable 1H NMR study was conducted using a Bruker 300 MHz NMR spectrometer in D2O at pH 9.2. FTIR studies of the samples were 4 ACS Paragon Plus Environment

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performed using a Shimadzu FT-IR instrument (model 8400S). The photoluminescence (PL) spectra were obtained using a quartz cell of 1 cm path length for solution and depositing sample on a quartz plate for solid sample with a Fluoromax-3 fluorimeter (Horiva Jovin Yvon) at an excitation wavelength of 260 nm using a slit width of 5 nm. Raman spectra of the samples were recorded using a Lab Spec Raman spectroscope (JY T6400) with 514 nm argon laser with a scanning duration of 40 s. Dynamic Light Scattering: The DLS experiments of the aqueous solutions of GPCLD (pH 4, 7 and 9.2) at different temperatures were carried out in a Malvern instrument at a scattering angle of 173o. X-ray diffraction (XRD) analysis: Wide angle x-ray scattering (WAXS) spectra of the samples were recorded using a Bruker AXS diffractometer (model D8 advance) operated at a generator voltage of 40 kV and a current of 40 mA. Samples were scanned in the range of 2θ = 5-35o at a scan rate of 0.5 s per step with a step width of 0.02o. Thermal study: The thermal stabilities of GO, GPCL-OH and GPCLD were measured using a Perkin-Elmer TGA instrument (Pyris Diamond TG/DTA, model SDT Q600) under nitrogen atmosphere at a heating rate of 10 oC min-1. Impedance spectroscopy: Electrochemical impedance spectroscopic measurements of GPCLD at pH 4, 7 and 9.2 were recorded at 25 oC using a Solarton SI 1260 impedance analyzer (Solarton, U.K.) at a frequency range of 32 MHz - 1 mHz with an oscillation voltage of 100 mV. dc Conductivity measurement and current-voltage study: The dc-conductivities of the samples were measured by two-probe method. Each sample was sandwiched between two indium-tin oxide (ITO) conducting strips of 2 mm width placed perpendicularly. The area of the sample was 0.04 cm2 and the thickness of the sample was measured by a digital slide 5 ACS Paragon Plus Environment

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calipers. The conductivities of the samples were measured by an electrometer (Keithley, model 2401) using the equation:

σ=

1 l × R a

where ‘ l ’ is the thickness, ‘ a ’ is the area and ‘R’ is the resistance. The current-voltage (I-V) studies were performed by applying the voltage from -5 to +5 V and measuring the current at each applied voltage. In order to study the temperature effect, all the studies regarding dcconductivities and I-V measurements were performed by keeping the sample sandwiched between two ITO strips in a closed chamber (Optistat DN, Oxford instruments) under argon atmosphere controlled through a temperature controller (Oxford instruments, model ITC 503S). RESULTS AND DISCUSSION We have synthesised GO by modified Hummers method1 and performed pH metric titration of GO with 0.01 M NaOH solution. From the titration curve (Figure S2) the amount of – COOH and, -OH groups are determined as 1.11 mmol/gm, 2.38 mmol/gm, respectively.34 The grafting of a linear diblock copolymer, PCL13-b-PDMAEMA117 from GO surface involves ring opening polymerization (ROP) of CL and atom transfer radical polymerization (ATRP) of DMAEMA in a consecutive manner (Scheme 1). In the 1st step, we have synthesized GPCL-OH via ROP of CL initiated from –OH groups of graphene oxide33,35,36 in presence of Sn(Oct)2 catalyst. Figure S1A(a) shows the typical 1H-NMR spectrum of GPCLOH and the degree of polymerization (DP) of grafted PCL chains is calculated to be 13. In the 2nd step, GPCL-OH is coupled with an ATRP initiator, 2-bromoisobutyryl bromide (BIB) producing GPCLI, which is then used for the ATRP of DMAEMA in presence of CuCl/HMTETA catalyst-ligand system (3rd step). Figure S1A(b) and S1A(c) show the 1H-


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NMR spectra of GPCLI and GPCLD respectively and the DP has been calculated from Figure S1A(c) for DMAEMA as 117.33 Scheme 1. Synthesis of graphene oxide grafted block copolymer (GPCLD).

Generally, tethering of polymer chains on GO surface would affect its thickness and therefore, AFM topographic image and its corresponding height profiles are taken as an important tool to characterize the grafted polymer chains on GO. As depicted in the AFM topography and the corresponding height profile of GO (Figure S3), the measured thickness is very uniform (~0.8 nm), which indicate single-layer structure. The Scheme 1 has been made as simple as possible to make it distinct for easy understanding. The real situation may be different and the PCL chains are anchored from different numbers of –OH groups of 7 ACS Paragon Plus Environment

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different GO sheets. AFM topography image of GPCL-OH is presented in Figure S4 and the height profile shows an average thickness of 1.83 nm, which is 1.05 nm higher than that of GO. So, appreciable amounts of PCL chains became attached on to GO surface. However, from Figure 1 the thickness of GPCLD is found to be ~3.6 nm and the increased thickness signify the tethering of PDMAEMA chains from the GPCL-OH.17,32

Figure 1. AFM topographic image and corresponding height profile of GPCLD To further confirm the surface functionalization on GO, we have performed XPS analysis for GPCL-OH and GPCLD. In Figure S5a, the peaks at 287.3 eV (C1s) and 535.3 eV (O1s) are observed for GPCL-OH. But in case of GPCLD (Figure S5b), peak for N1s (401.4 eV) is observed along with the peaks for C1s (287.6 eV) and O1s (535.2 eV) level. The FTIR spectra of GO, GPCL-OH and GPCLD are presented in Figure 2 where GO has its characteristic peaks at 1720, 3400, 1040 and 1625 cm-1 for >C=O stretching, -OH stretching, C-O stretching of epoxide group and C=C stretching vibrations of the unoxidized graphitic domains respectively. In case of GPCL-OH the bands appearing at 1727, 3448, 1048 and 1622 cm-1 represent vibrations correspond to the >C=O stretching, O-H stretching, C-O stretching and C=C stretching respectively. Besides, the peaks in the FTIR of GPCL-OH at 1295, 1243 and 1186 cm-1 for C-O stretching of ester and at 2946 and 2865 cm-1 for aliphatic C–H stretching indicate the characteristic peaks of PCL grafted from GO. Interestingly, the intensity of the band for C=C stretching vibrations which is assigned as 8 ACS Paragon Plus Environment

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originating from GO decreases in GPCL-OH due to formation of PCL. The spectrum of GPCLD exhibits the characteristic peaks of PDMAEMA at 1152 cm-1 for C–O stretching vibrations and at 2770, 2823 and 2940 cm-1 owing to methyl and methylene vibrations adjacent to the nitrogen atom. As the concentration of GO decreases from GPCL-OH to GPCLD, the intensity of the band for C=C stretching vibrations starts decreasing and finally arises as a small hump.

1625 GO Transmittance (%)

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2946 GPCL-OH

GPCLD 2940 4000

3500

3000

2865

1622 1727

1152

2770 2500

2000

1500

1000

500

-1

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Figure 2. FTIR spectra of GO, GPCL-OH and GPCLD. In the FTIR spectra the stretching vibration of –OH group occurs at 3410, 3439 and 3427 cm1

, for GO, GPCL-OH and GPCLD, respectively. The shifts towards higher energy in both

GPCL-OH and GPCLD arise due to H-bonding (bridge type) between carboxylic –OH of GO and the >C=O groups of the polymer chains. But, in case of GPCLD the shift is somewhat lower due to lesser availability of >C=O group for H-bonding than GPCL-OH as a large amount of PDMAEMA component is present there. In order to study the structural changes of GO during chemical functionalization, Raman spectroscopy is used as another important analytical technique. The Raman spectrum of GPCL-OH (Figure S6) exhibits two bands at 1345 (D band) and 1605 cm-1 (G band), arising from the disorder-induced and Raman allowed phonon mode of vibrations of GO,


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respectively. The blue shift (25 cm-1) of G band from GO (1580 cm-1) to GPCL-OH may be a sign of decrease in the sp2 domains size upon polymer grafting. Moreover, compared to the ID/IG ratio of GO (0.97), the value increases to 1.21 in GPCL-OH. Actually, the ID/IG ratio is a measure of clusters size of sp2 carbons within a network of sp2- and sp3- hybridized carbon atoms.

According

to

Tuinstra

and

Koenig

relation:

where, La is the average size of the sp2 carbon clusters,37 the higher ID/IG ratio of GPCL-OH gives an indication of formation of comparatively smaller sp2 domain size than GO suggesting that GO gets partially reduced during attachment with PCL. In case of GPCLD due to very less amount of GO, these D and G bands are difficult to recognise for noisy nature of the spectra coming from the large PDMAEMA content. The effect of tethering of polymer chains on GO can also be followed with the help of X-ray diffraction (XRD) study. As indicated from the wide-angle XRD patterns (Figure S7), GO shows its characteristic peak appearing at 2θ = 11.9o, corresponding to a d-spacing of 7.43 Å arising from the stacked nature of the GO sheets. GPCL-OH exhibits diffraction peaks only for crystals of PCL (folded chain) with no characteristic peak for GO signifying the disruption of stacked arrangement of GO i.e. the formation of fully exfoliated structures into individual sheets.8 But in GPCLD, characteristic peaks both for GO and PCL disappear and a broad diffraction peak (2θ = 17.7o ) appears indicating its amorphous nature. In order to investigate the effect of polymer grafting on thermal stability, we have analysed the TGA thermograms of GO, GPCL-OH and GPCLD (Figure S8a). The TGA thermogram shows that GO starts weight losing right from the beginning of heating due to the loss of adsorbed water in its π-stacked structure. The major weight loss (~ 42 wt %) occurs at ~ 215 oC because of the decomposition of labile oxygen containing functional


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groups. The thermal stability enhances remarkably upon grafting of the PCL chains from the graphene surface. The thermogram of GPCL-OH shows only a single and large break in the region between 250 oC to 350 oC due to the decomposition of grafted PCL chains. The TGA curve of GPCLD is very much similar as that of PDMAEMA,38 which is expected in light of appreciably high weight fraction of PDMAEMA in GPCLD. The first break here occurs near 300 oC, which may be attributed to the loss of pendent chains and the PCL chain. In the second stage at about 395 oC the weight loss is attributed to the backbone degradation of PDMAEMA chains. After 450 oC, the thermogram does not show any further weight loss with temperature, indicating a major reduction of graphene sheets during reaction proceedings. To support the above contention the derivatograms of the TGA of GPCL-OH and GPCLD are shown in Figure S8b and it is very much apparent that the first derivatogram of GPCLD coincides with that of the GPCL-OH indicating that the pendent groups of PDMAEMA and the PCL chain degrades concurrently. To get an idea about the effect of polymer grafting on the fluorescence behaviour of GO, we have performed the photoluminescence (PL) emission spectral studies of GO, GPCLOH and GPCLD at pH 9.2 at 25 oC normalised with respect to GO content (Figure S9). From the Figure it is quite obvious that the fluorescence intensity, originating from GO, enhances drastically upon step-wise grafting from GPCL-OH to GPCLD. Having oxygenated functional groups, GO exhibits comparatively weak emission spectra due to large proportion of non-radiative decay of excitons.39 However, the amount of hydroxyl groups of GO is reduced largely during ROP causing reduction of non-radiative excitonic decay path and grafting prevents the restacking of GO sheets which collectively contributes to the enhancement of fluorescence intensity. In case of GPCLD, grafting of PDMAEMA makes the system readily soluble in water. PDMAEMA chains being engaged in H-bonding with


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water molecules at 25 oC, restrict the motion of solvent molecules decreasing the quenching of excitons of GO with water, hence resulting a hike of fluorescence intensity. To gain further insight of graphene-polymer interactions we have recorded the PL emission spectra of GPCLD at 25 oC in different pH environment (pH 4, 7 and 9.2). It is clear from Figure 3 that GPCLD exhibits a distinct emission peak at 467 nm for all three pH buffer mediums along with another broad peak at 386 nm for pH 4 and at 407 nm for pH 7 and 9.2. Although the findings are quite unusual, a probable reason may be given in terms of doping on the basis of protonation/deprotonation of pendent –NMe2 groups of PDMAEMA chains.32 However, modification in synthetic strategy makes our present study somewhat different from the previous one. The band at 467 nm arises from the radiative recombination of electron-hole pairs of confined sp2 clusters of GO. The size of sp2 clusters decides the local band gap which determines the wave length of emitted fluorescence.12 Under acidic conditions (pH 4), the –NMe2 groups of PDMAEMA side chains remain in fully protonated state which can dope the graphitic ring, generating secondary holes in sp2 domains. This ptype doping may influence the radiative recombination of electron-hole to proceed through an 2.5

2.0

pH 9.2 pH 7 pH 4

467

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386

PL Intensity X 10

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407

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Figure 3. PL spectra of GPCLD at pH 4, 7 and 9.2 at 25 oC. 12 ACS Paragon Plus Environment

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additional path resulting in the appearance of the band at 386 nm. The red shift of the band position from 386 nm at pH 4 to 407 nm at pH 7 and 9.2 can be explained in terms of n-type doping occurring mainly from non bonding electrons of –NMe2 groups in its deprotonated state. 40 Again, the removal of protons from –NMe2 groups reduce the possibility of nonradiative decay of excitons, causing an increase in the PL intensity with increase of pH at main emission peak (467 nm).32 Being curious we have also studied the PL spectra of GPCLD at different pH (pH 4, 7 and 9.2) with varying temperature. In Figure S10 the PL intensities at 467 nm are plotted against temperature and unlike our previous report32 the spectrum at pH 9.2 exhibits a hike (about 9 fold) of fluorescence intensity during heating and reverse during cooling, but remain almost invariant for others (pH 4, 7). It is quite obvious from the Figure that the plot at pH 9.2 has an inflection point at 32 oC indicating the LCST type phase separation of PDMAEMA in the system. Above 30 oC, the –NMe2 groups of PDMAEMA start collapsing as the Hbonds between –NMe2 groups and water molecules rupture, thereby decreasing the nonradiative excitonic decay of GO with the solvent molecules. This causes a sharp hike in the PL-intensity with temperature and also cause phase separation due to self-aggregation of the polymer chains. Here, attachment of PDMAEMA with ~10 mol% hydrophobic PCL causes lowering of its LCST (32 oC) instead of 60 oC. The main reason behind this large decrease of LCST of PDMAEMA is hydrophobic PCL block facilitating its phase separation to occur earlier than in case of pure PDMAEMA.33 In order to understand the effect of temperature, the fluorescence properties of GPCLD are also studied at the solid state at temperatures lower (25 oC) and higher (35 and 45 o

C) than the LCST of GPCLD solution at pH 9.2 (Figure S11). The solid state fluorescence

spectra exhibit two distinct peaks at 455 and 410 nm for 25 oC and for 35 and 45oC the 455nm peak shows a red shift to 459 nm, although the peak position of 410 nm remains 13 ACS Paragon Plus Environment

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unaltered. The fluorescence in GPCLD arises from the passivation of GO sheets due to anchored block copolymer chain. With increase of temperature above LCST the H-bonding present between –COOH of GO and >C=O group of the block copolymer (cf. FTIR spectra) breaks and thus GO becomes more planar decreasing the band gap showing the red shift of 455 nm peak. The intensity of 410 nm peak shows a significant decrease (almost flat) when the temperature is raised to 35 oC. This peak originated from the n-type doping of GO sheet becomes disfavored due to onset of macro phase separation at ~35 oC. With further increase of temperature by 10 oC de-coiling of polymer chain starts and doping from the -NMe2 groups become again operative, showing increase of intensity of the 410 nm peak. It is to be noted that the intensity of 455 nm peak decreases with increase of temperature and one possible reason may lie in the aggregation of grafted chain causing the graphene sheets to be nearer to each other causing nonradiative decay of excitons decreasing the fluorescence intensity. In order to check the phase separation behaviour we have further performed the temperature dependent DLS analysis of GPCLD solution at pH 4, 7 and 9.2 (Figure S12). Upon heating, the z-average size increases for GPCLD at pH 9.2 indicating its LCST at 32 oC which is in accordance to its temperature variable PL study. On cooling, the reverse process follows the same path signifying the reversible nature of the phase separation process. However, no distinct change in the z-average size is observed for GPCLD solutions at pH 4 and 7. Hence, the results from both the PL and DLS study indicate the thermo as well as pHresponsiveness of GPCLD. To understand the effect of temperature on molecular interactions we have recorded temperature variable 1H NMR spectra of GPCLD solution in D2O at pH 9.2. In Figure 4, the signal (at δ= 2.17-2.30 ppm) assigned for methyl protons (a) of pendant –NMe2 groups of PDMAEMA block appears comparatively sharp and visible, but becomes broader and lesser intense and finally disappears upon stepwise heating above 25 oC. From the observed result it 14 ACS Paragon Plus Environment

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is obvious that below 25 oC, –NMe2 groups remain in solvated state in D2O through strong Hbonding interactions. As temperature increases and approaches near to LCST, these H-bonds become weak resulting in the desolvation of the side chains of PDMAEMA which reflects in the observed lowering of peak intensities. Additionally, the signal (a) exhibits a downfield shift of 0.13 ppm with rise in temperature, but definite reason for this fact is unknown. A probable reason might be given from the H-bonding interactions between –NMe2 groups and oxygenated functional groups of GO to some extent.

0

35 C

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a 0

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a

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a 0

20 C 3.0

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ppm

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Figure 4. Temperature dependent 1H NMR spectra of GPCLD in D2O at pH 9.2. In a quest to investigate the electrical properties of the GPCLD samples at different pH values, we have carried out ac impedance spectroscopy. Figure 5 and 6 depict the Nyquist plots of the GPCLD samples at pH 4 (Figure 5a), 9.2 (Figure 5b) and pH 7 (Figure 6a), respectively. Surprisingly, the same GPCLD samples exhibit different nature in Nyquist plots at different pH values. At pH 4, the Nyquist plot exhibits a semicircle at higher frequencies and a spike at lower frequency. The semicircle corresponds to the bulk electronic resistance probably arising from the graphene π-electrons.41 The spike represents semi15 ACS Paragon Plus Environment

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infinite Warburg impedance that occurs due to diffusion of charge carriers at the electrode. At pH 4, the graphene sheet is p-type doped thus generating secondary holes in the sp2 domains. These holes can diffuse on application of AC voltage causing the Warburg impedance. A slightly different scenario is observed in the Nyquist plot of GPCLD sample at pH 9.2 (Figure 5b) where the semicircle is absent. 8

10

pH 4 pH 9.2

8

5

Z'' Ω6 X 10

5

6

Z'' ΩΩX 10

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4

6 5

Z'ΖΩ ΖX 10

0

8

1

Rct

W

2

3

4

5

6

4

Z' Ω X 10

c CPE Figure 5. Nyquist plots of GPCLD at (a) pH 4 and (b) 9.2 at 25 oC and (c) their corresponding equivalent circuit diagram. Here the non-protonated –NMe2 groups can cause n-type doping of the graphene rings. The graphene rings then behave like an n-type semiconductor and the free electrons diffuse upon application of AC voltage. The Nyquist plots of both the system could be fitted on the basis of modified Randle equivalent circuit (Figure 5c) consisting of a Faradic impedance in parallel with a constant phase element (CPE). The Faradic impedance is a combination of Rct and ZW, where Rct is related to kinetically controlled processes and ZW corresponds to the diffusion or mass controlled processes. Invisibility of the semicircle at pH 9.2 indicates that the mass transfer i.e. diffusion process is slower at this pH and consequently ZW is higher compared to that at pH 4.42 At pH 7 (Figure 6a), a somewhat different feature is observed. Here, the spike is replaced by an arc at lower frequencies. This type of nature in Nyquist plot


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corresponds to the bounded Randle equivalent circuit (Figure 6b) where the Warburg element is replaced by the bounded Warburg element (BW) which describes linear diffusion in a homogeneous layer with finite thickness and at lower frequencies its impedance (ZBW) tends to a finite value.42 At this pH both p- and n-type doping of graphene sheets occur through the protonated and non-protonated –NMe2 groups. This causes the concomitant presence of holes and electrons in the system. Some of these holes and electrons may recombine which mitigates the amount of electroactive species. This limits the mass transport /diffusion process within the system thus causing the finite diffusion resulting a second arc in the Nyquist plot. This hypothesis can be strengthened by the dc-conductivity values of GPCLD which indicate the highest value (1.5 X 10-7 S/cm) at pH 4 and moderate value (5.5 X 10-8 S/cm) at pH 9.2 because of charge diffusion and the lowest value of 1.5 X 10-8 S/cm at pH 7 due to charge recombination. 24

20

a

pH 7

4

16 Z''(Ω ) X 10

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8

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0 0

1

2

3

4

5

5

Z'(Ω) X 10

BW

Rct

b CPE Figure 6. (a) Nyquist plot of GPCLD at pH 7 at 25 oC and (b) corresponding equivalent circuit diagram. It is clear from the result that doping of GO at different level control the conductivity exclusively. In order to explain the overall impedance behaviour of GPCLD at different pH in a simple way, we have used a model compiling the observed impedance spectra (Scheme 2)


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where, the Nyquist plot exhibits a semicircle at higher frequencies and a spike at lower frequencies at pH 4 but at pH 7 the spike is replaced by another arc and at pH 9.2 the semicircle at higher frequencies vanishes and only a spike is observed.

Scheme 2. Model of the combined impedance behaviour for GPCLD at different pH values. In order to understand the conducivity property more clearly, we have studied the thermal variation of conductivity and logσ vs 1000/T is plotted (Figure S13) in which a deviation from Arrhenius type conductivity is observed at ~ 29 oC for pH 7 and 9.2. At pH 4 in GPCLD system GO is doped well with –NHMe2+ of PDMAEMA behaving as an extrinsic semiconductor, so with rise in temperature conductivity increases according to Arrhenius theory facilitating the flow of charges. At pH 7 with increase of temperature initially the Arrhenius principle is obeyed, though the absolute value is lower than that of pH 4. This is because there is some annihilation of charges due to both p and n type doping at this pH. A peculiar phenomenon of sudden decrease of conductivity occurs when the temperature is increased to 29 oC which is near the LCST of the GPCLD solution arising from the breaking of H-bonds of GPCLD with water. Similar breaking of H-bond between carboxylic –OH group of GO and the >C=O groups of the polymer chain is possible in the solid state (cf. FTIR spectra) causing onset of macro heterogeneity. This causes a difficulty of charge flow due to obstacle in the charge hopping process. With further rise of temperature the segmental 18 ACS Paragon Plus Environment

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motion of the chains increases, causing de-coiling and gradually permitting the flow of more charge carriers with rise in temperature. The behaviour at pH 9.2 is quite similar to that at pH 7; however the conductivity value is somewhat higher because of absence of any charge annihilation as here only n-type doping is present. In this context, further inspections of the temperature variable current (I)-voltage (V) property of GPCLD at different pH may be certainly beneficial. The I-V trace at pH 9.2 (Figure 7a) exhibits a resistive switching behaviour with an interesting feature of negative differential resistance (NDR) in positive as well as in negative bias through a voltage sweep between -5.0 V to +5.0 V at 27 oC. At the positive bias, with increasing applied voltage the IV trace follows an exponential control function until the voltage reaches to 3.2 V. Then the current starts to decrease, showing the NDR. The curve exhibits another exponential control function with decreasing voltage. As the voltage increases at the negative bias, an increase in current is observed before another NDR (-1.6 V). From the spectroscopic evidence, as n-type doping of GO is there at pH 9.2, the observed behaviour can be well explained considering the trapped charges stabilised by the GO at a specific band gap. These stabilised charges obstruct the flow of current until its release at higher voltage showing the NDR. Here, we have determined the energy gap of 4.44 eV using ultraviolet-visible absorption spectrum (band at 280 nm) on the basis of the following form of the Tauc’s Formulation:43

where ω is the angular frequency of the incident radiation, ε is the absorption intensity and Eg is the optical band gap. Now, plotting ε1/2/λ against energy (Figure S14) and extrapolating the linear region to the x-axis, we have measured the optical band gap. From the observed I-V characteristics, it can be proposed that besides charge trapping a charge detrapping process is also present signifying the occurrence of two-sided NDR.44 But the


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trapping and detrapping process may not occur in the same rate, demonstrating the unsymmetrical nature of the I-V trace. When we have increased the temperature of ITO/GPCLD/ITO device to 37 oC, surprisingly, the I-V trace (Figure 7b) transforms into a loop having a negative hysteresis but no NDR is noticed in this case. On further increase of temperature (42 oC), a twist happen and NDR is visible again in both the forward and backward bias (Figure S15). An explanation for this peculiar behaviour may be proposed considering the interactions between polymer chains and GO. At low temperatures (27o and 32 oC), doping of GO by the non-bonding electrons of –NMe2 groups of PDMAEMA is reflected in their corresponding I-V traces (Figure 7a, Figure S16). As the temperature starts increasing, LCST type phase transition (onset of macro heterogeneity) is occurring from breaking of H-bond of polymer with GO, thus affecting the electronic properties of GPCLD at pH 9.2. The coiling and de-coiling of these polymer chains with rise of temperature would alter the doping, hence the electronic properties resulting in the observed I-V trace at 37 oC. On further heating increased thermal energy facilitates the de coiling of polymer and the GOpolymer interactions again take part to some extent (as stated above) causing reappearance of NDR at 42 oC. At 27 oC

1.5

At 37 OC

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Current (amp) X 10-8

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-3 -6

-4

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2

4

-6

6

-4

-2

Voltage (V)

0

2

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Voltage (V)

Figure 7. Current–voltage (I–V) characteristic curves of GPCLD at (a) 27 oC and (b) 37 oC at pH 9.2. 20 ACS Paragon Plus Environment

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However, on changing the pH from 9.2 to 7, besides small NDR characteristics in the positive bias, the asymmetric I-V trace (Figure 8a) also exhibits a rectification property in the negative bias with a rectification ratio (RR) of 6.1 at 27 oC. This behaviour can be explained on the basis of intrinsic p/n junction assemblies which may arise from partially protonated –NMe2 groups of polymer chains that participate in p-type doping of GO, thus converting GO to a p-type semiconductor. At this pH a portion of –NMe2 groups which remain non-protonated get engaged into n-type doping of GO through their non-bonding electrons, thus converting GO to n-type semiconductor and are responsible for the formation of p/n junction giving rectification characteristics. With the elevated temperature from 27 o to 37 oC (Figure 8b) through 32 oC (Figure S17), RR decreases gradually and a signature of NDR is visible again. Since, Figure S13 indicates the similar behaviour of GPCLD under temperature dependent dc-conductivities measurement in both the pH 9.2 and 7, the nature may be explained correlating the I-V traces observed in both the cases. As temperature increases, interaction between polymer chains increases and thus charge neutralisation between polymer chains takes place before doping of GO causing gradual disappearance of rectification. Surprisingly, a rectification is observed again in the I-V trace at 42 oC (Figure S18) supporting our hypothesis of polymer assisted redoping of graphene 0.6

At 27 oC

0.5

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At 37 C

0.4

0.0

Current (amp) X 10-8

Current (amp) X 10 -8

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-0.5 -1.0

a -1.5

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-4

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-2

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Voltage (V)

Voltage (V)

Figure 8. Current–voltage (I–V) characteristic curves of GPCLD at (a) 27 oC and (b) 37 oC at pH 7. 21 ACS Paragon Plus Environment

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due to de-coiling of polymer chains. The curve at pH 4 (Figure S19) also exhibits interesting NDR phenomena in negative as well as in positive bias (though small). As the GO is doped by a large number of holes at this pH, the NDR property arises from the charge trapping by GO in GPCLD. But, in this case no prominent temperature effect of the I-V behaviour is noticed as observed from the previous studies. In this system the interaction between GO and doped polymer is so prominent that temperature effect is overcome particularly at the temperature range studied here. DISCUSSION: GO is less fluorescent for having oxygenated functional groups, causing large proportion of non-radiative decay of excitons,39 but passivation of the functional groups of GO causes a detectable amount of fluorescence in GO. At pH7 and pH 9.2 the –COOH groups of GO remains partially and fully ionized, respectively

39,45

and the transfer of

electrons of ionized carboxyl group to the holes causes a non- radiative decay of GO excitons. At pH 4 the –COOH group remains unionized diminishing non radiative decay of excitons and hence increasing the fluorescence of intensity by ~ 8 times than that at pH 7 and 9.2 (Figure S20). On attachment of PCL, GO is reduced largely during ROP (cf. Raman Spectra) causing reduction of non-radiative excitonic decay path. Also grafting prevents the restacking of GO sheets. Both of these factors contribute to the enhancement of fluorescence intensity than that of GO at pH 9.2 (Figure S9). The PL-spectrum of GO grafted PDMAEMA (GD) exhibits an emission peak at 451 nm for pH 9.2 (Figure S9). The PLintensity of GD is higher from GO and GPCL-OH. The grafting of PDMAEMA from the GO surface results n-type doping of GO32 causing an enhancement in fluorescence intensity of the GD. Due to formation GPCLD the fluorescence property has changed significantly at pH 9.2. Here we observed a new peak at 407 nm due to ‘n’ type doping and a sharp peak at 467 nm due to the radiative recombination of electron-hole pairs of confined sp2 clusters. In case of GPCL-OH there is no question of doping, however, in GD there also exist ‘n’ type doping 22 ACS Paragon Plus Environment

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which is a cause for intensity increase from that of GPCL-OH at pH9.2. The two distinct peaks in GPCLD arise due to block copolymer formation because the PCL block acts as a spacer and the –N(CH3)2 groups of PDMAEMA, therefore, selectively dope a particular region of GO. But in case of GD this doping would occur throughout the GO sheet giving broad peak at 451 nm. GO at pH 7 has a dc conductivity of 0.11 x 10

-4

S/cm at 27 oC and is a semiconducting

material as evident from the I-V curve (Figure S21). GPCL-OH, GD and GPCLD exhibit dc conductivity values of 0.30 x 10 -5, 0.80 x 10-6 and 0.15 X 10-7 S/cm, respectively, measured under similar condition. The results indicate due to grafting of non-conducting polymer chains the dc conductivity decreases and the decrease is higher for the block copolymer system. The I-V characteristic curves of GPCL-OH and GD at pH 9.2 and at 27 oC are presented in Figure S22(a, b) and both exhibit typical semiconducting behavior with a hysteresis. The I-V curves of GPCL-OH and GD are almost similar, both exhibiting better semiconducting behavior than GO (Figure S21). The I-V trace of GPCLD at pH 9.2 (Figure 7a) exhibits a NDR behavior both in positive and negative bias at 27 oC. At pH 9.2, in GPCLD n-type doping of GO is present, so the observed NDR behavior can be well explained considering the trapped charges stabilized by the GO obstructing the current flow until its release at higher voltage showing the NDR. In case of GD ‘n’-type doping also occurs as evident from fluorescence spectra (Figure S9). In the GPCLD due to the spacer PCL block the doping by PDMAEMA block is selective to a particular region of GO (i.e. localized doping) whereas in GD the doping is occurring throughout the GO sheet in a continuous fashion (i.e delocalized doping). The localized doping in GPCLD does not allow the doped charges to flow under a potential gradient upto 3.2 causing NDR. In GD the delocalized doping allow the flow of charges in whole region of GO under the potential difference showing a semiconducting nature. 23 ACS Paragon Plus Environment

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In order to understand the effect of graft length of PCL and PDMAEMA on the optoelectronic properties of GO in the GPCLD system we have synthesized two other GPCLD polymers GPCL13D158 and GPCL3D60 which are termed as GPCLD1 and GPCLD2, respectively. The GPCLD (i.e. GPCL13D117) has LCST at 32 oC, but GPCLD1 exhibit LCST at 38 oC (Figure S23). This is due to the increase of hydrophilic PDMAEMA chain length that increases the LCST. In case of GPCLD2 both the chain lengths are shortened, however, the hydrophobic-hydrophilic ratio has decreased more compared to GPCLD showing higher LCST (44 oC, Figure S24). The fluorescence spectrum at pH 9.2 of GPCLD1 is different from that of GPCLD (Figure 3). Here the emission peak at 410 nm arising due to localized n-type doping has decreased significantly and mainly delocalized n-type doping occurs showing a broad peak at 451 nm (Figure S23), the later peak is very much similar to that of GD (Figure S9) where the PDMAEMA chain dopes GO in a continuous fashion. The fluorescence spectrum of GPCLD2 (Figure S24) shows emission peak at 416 nm due localized n-type doping, but here the relative GO content is much higher than that of GPCLD and GPCLD1, attributing mainly the emission peak of sp2 clusters of GO at 467 nm. The IV properties of GPCLD1 exhibit typical semiconducting behavior at 27 oC at pH 9.2 (Figure S25a), mainly arising from the delocalized doping as evident from fluorescence spectrum. On the other hand like GPCLD, GPCLD2 exhibit a NDR behavior (Figure S25b) arising from the localized n-type doping by trapping the charges which decreases the current flow till a certain higher voltage when the normal semiconducting behavior is noticed. CONCLUSION: So, the behaviour of GPCLD with pH and temperature in solution as well as in solid state is interesting both from self-aggregation and doping changing the optoelectronic properties. The temperature dependent 1H NMR spectra at pH 9.2 indicate the influence of temperature on the interaction between GPCLD and water molecules causing the LCST-type phase separation. Fluorescence spectra of GPCLD solution at pH4 and pH9.2 24 ACS Paragon Plus Environment

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indicate p and n – type doping of graphene assisted by the pendant PDMAEMA chain. The different type of doping in the solid state is evident from the impedance spectra of GPCLD films. At pH 4, the Nyquist plot exhibits a semicircle at higher frequencies and a spike at lower frequencies; at pH 7.0 the spike is replaced by an arc and at pH 9.2 the semicircle at higher frequencies vanishes and only a spike is noticed suggesting different type of doping of graphene at different pH. The variation of dc- conductivity with pH and temperature is due to the different types of doping caused from coiling and de-coiling of the polymer chains. In the I-V property of GPCLD at pH 9.2 an interesting feature of NDR, at pH 7 rectification property and at pH 4 again the NDR property occur. The temperature dependent I-V property validates a signature of doping, de-doping and re-doping of graphene by the grafted polymer arising from coiling and de-coiling of polymer chains. The influence of block architecture on the doping of GO is established from a comparison with the optoelectronic behaviour of GO-g-PDMAEMA and it suggests that PCL block helps in localized doping of GO causing NDR behaviour in GPCLD at pH 9.2. The greater the block length of hydrophilic chain of GPCLD system lesser is the localized doping. SUPPORTING INFORMATIONS Additional figures (Figure S1-S25) are presented in supporting information. The Supporting Information is available free of charge on http://pubs.acs.org/. ACKNOWLEDGEMENTS We gratefully acknowledge SERB New Delhi (grant no. SB/SI/OC-11/2013) for financial support. N.M. and A.K. acknowledge CSIR for providing fellowships.

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38. Meyer, F.; Minoia, A.; Raquez, J. M.; Spasova, M.; Lazzaroni, R.; Dubois, P. Poly(amino-methacrylate) as Versatile Agent for Carbon Nanotube Dispersion: an Experimental, Theoretical and Application Study. J. Mater. Chem. 2010, 20, 68736880. 39. Kundu, A.; Layek, R. K.; Nandi, A. K. Enhanced Fluorescent Intensity of Graphene Oxide-Methyl Cellulose Hybrid in Acidic Medium: Sensing of Nitro-Aromatics. J. Mater. Chem. 2012, 22, 8139-8144. 40. Zhao, W.; Song, C.; Pehrsson, P. E. Water-Soluble and Optically pH-Sensitive SingleWalled Carbon Nanotubes from Surface Modification. J. Am. Chem. Soc. 2002, 124, 12418-12419. 41. Maity, N.; Kuila, A.; Das, S.; Mandal, D.; Shit, A.; Nandi, A. K. Optoelectronic and Photovoltaic Properties of Graphene Quantum Dot-Polyaniline Nanostructures. J. Mater. Chem. A 2015, 3, 20736-20748. 42. Yuan, X.-Z.; Song, C.; Wang, H.; Zhang, J. Electrochemical Impedance Spectroscopy in PEM Fuel Cells: Fundamentals and Applications; Springer: London, U.K., 2010. 43. Mathkar, A.; Tozier, D.; Cox, P.; Ong, P.; Galande, C.; Balakrishnan, K.; Reddy, A. L. M.; Ajayan, P. M. Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide. J. Phys. Chem. Lett. 2012, 3, 986-991. 44. Du, Y.; Pan, H.; Wang, S.; Wu, T.; Feng, Y. P.; Pan, J.; Wee, A. T. S. Symmetrical Negative Differential Resistance Behavior of a Resistive Switching Device. ACS Nano 2012, 6, 2517-2523. 45. Kundu, A.; Layek, R. K.; Kuila, A.; Nandi, A. K. Highly Fluorescent Graphene OxidePoly(vinyl alcohol) Hybrid: An Effective Material for Specific Au3+ Ion Sensors. ACS Appl. Mater. Interfaces 2012, 4, 5576-5582.


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