Observation of different charge transport processes and origin of

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C: Energy Conversion and Storage; Energy and Charge Transport

Observation of different charge transport processes and origin of magnetism in rGO and rGO-ZnSe composite Abdulla Bin Rahaman, Atri Sarkar, Koushik Chakraborty, Jonaki Mukherjee, Tanusri Pal, Surajit Ghosh, and Debamalya Banerjee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02710 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 3, 2019

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The Journal of Physical Chemistry

Observation of different charge transport processes and origin of magnetism in rGO and rGO-ZnSe composite Abdulla Bin Rahaman‡a , Atri Sarkar‡a , Koushik Chakrabortyb , Jonaki Mukherjeea , Tanusri Palc , Surajit Ghoshb , and Debamalya Banerjee∗a a

Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India.

b

Department of Physics, Vidyasagar University, Midnapore, 721102, WB, India. c

Department of Physics, Midnapore College, Midnapore, 721101, WB, India. May 31, 2019

∗

corresponding author: [email protected] These authors contributed equally to this work.

0‡

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Abstract

Effect of nanoparticle incorporation inside reduced graphene oxide (rGO) layers on the electrical transport properties has been investigated by temperature dependent resistivity measurements on rGO and rGO - zinc selenide (rGO-ZnSe) thin films in a wide temperature range of 84K-473K. The fraction of ZnSe in rGO-ZnSe composite has been varied from 26% to 98%. Resistance curve derivative analysis (RCDA) reveals a conduction mechanism consistent with Mott two-dimensional (2D) variable range hopping (VRH) in rGO films whereas different rGO-ZnSe samples exhibit three-dimensional (3D) VRH at lower temperatures (84K-280K). At higher temperatures (290K-473K), Arrhenius-like transport was observed for all the samples. A model where ZnSe nanoparticles are incorporated inside wrinkled rGO layers to facilitate inter-layer connections, thus 3D charge transport, has been proposed. Room temperature carrier mobility values, calculated using trap free space charge limited current (SCLC) conduction model under dark, goes through a maximum at 54 wt.% ZnSe content. Vibrating sample magnetometer (VSM) and electron paramagnetic resonance (EPR) measurements reveal paramagnetic nature of the composites, in contrast to the diamagnetic behaviour of pure rGO and ZnSe at room temperature. Evidences from X-ray photoelectron spectroscopy and Fourier transform infrared spectra suggest that the unreduced carbonyl groups present at the edge of rGO sheets could be held responsible for observed paramagnetism in rGO-ZnSe composites.

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2

Introduction

Out of different graphitic materials graphene is getting attention and becomes a subject of interest in scientific community due to its excellent electrical [1], thermal [2] and mechanical [3] properties. In spite of all these exciting properties, it is not attractive in optoelectronic device applications as graphene is a zero band gap semiconductor [4]. Graphene oxide (GO) is a two dimensional single layer of carbon atoms in hexagonal structure with various oxygen functional groups (OFGs) like carbonyl, epoxides, hydroxyl etc. on it. These functional groups disrupt the sp2 network on the graphene surface and reduce the electrical conductivity of GO [5]. To regain the electrical conductivity in GO, the common practice is to remove the OFGs by thermal treatment [6], hydrogen or ammonia plasma treatment [7], epitaxial growth [8], chemical reduction [9] of exfoliated graphene oxide etc. Among all these methods chemical reduction of exfoliated GO to rGO is the popular method due to easy processing, low cost and efficient production [10]. The OFGs that were present in GO can act as nucleation centres for the nanoparticles in a typical rGO-nanomaterial composite. This way, stable and dispersed nanoparticles can be formed on rGO sheets, which in turn can increase the interplanar spacing and maintain properties of individual rGO sheets [11]. Semiconductor nanocrystals go through exciton recombination directly, lowering device efficiency for optoelectronic applications. This limitation can be overcome by combining the semiconductor with an electron acceptor like rGO sheets [12, 13]. Recent studies show that synergistic effect of rGO and nanoparticles can give rise to enhanced performance or can lead to new properties which are not present in the individual counterparts [14–17]. Zinc based semiconductor nanostructures have attracted much more attention among various semiconductors due to application in the field of optoelectronics [18, 19]. Zinc selenide (ZnSe, band gap 2.67 eV), has been considered as an excellent candidate for photodetectors, blue laser diodes, light emitting diodes, sensors and photocatalysts [15, 20–22]. However, the low electrical conductivity and carrier mobility in ZnSe is the main disadvantage in the field of optoelectronic application. Recent studies have shown that forming composite with carbon nanotubes or graphene is a pathway to sort out this problem [12, 15, 23]. Charge carrier mobility (µ), like other transport parameters, is of great importance for semiconductor device applications. It has been reported that the incorporation of rGO improves the µ values in rGO-CdZnS and rGO-TiO2 composites as 3



compared to pure CdZnS and TiO2 , respectively [24, 25]. So, the addition of rGO may be a key factor for device performances as it greatly improves the transport properties of the system. Temperature dependence of electrical transport properties of rGO and rGO based nanocomposite system has been reported by several groups. Most of the studies have found either Mott 2-dimensional (2D) variable range hopping (VRH) or Efros-Shklovskii (ES) VRH as the primary transport mechanism at low temperatures. In particular, R. Negishi et.al. observed a crossover of transport mechanism from Mott 2D-VRH to Arrhenius behaviour on rGO films [26]. They have observed different crossover temperatures in 60K-170K range depending on sp2 and disordered fraction present in their samples. Similar crossover at a temperature of 180K has been obsereved by B. Muchharla et.al. on drop casted rGO thin films [27]. Joung et.al. reported ES-VRH in chemically reduced graphene oxide sheets in the temperature range of 4.2K-295K [5]. On the other hand, X. Li et.al. observed Mott 2D-VRH in nitrogen-doped GO (NGO) in 10K-300K [28]. Kim et.al. reported Mott 3D-VRH with quantum tunneling model in thick reduced graphene oxide film [29]. Put together, most studies have found that the dominant transport mechanism in rGO at low temperature is Mott 2D-VRH. In this discussion we have not included other ordered carbon materials like nanotubes in which the transport mechanism may be ES, Mott 2D or 3D-VRH [30]. Similar measurements on rGO based nanocomposites have been conducted to reveal the dominant tranposrt mechanism of those systems. M. Knite et.al. reported a crossover from Mott-VRH to nearest neighbour hopping (Arrhenius) on polyisoprene/nanostructured carbon composite at ∼230K [31]. In a limited temperature range (300K-373K), Nagesh Kumar et.al. observed Mott 3D-VRH transport process in rGO-ZnO nanocomposite [32]. Song et.al. used parallel register model in order to represent the temerature dependence of resistivity across metal-insulator transition in a hexagonal boron nitride-graphene hybrid composite and reported ES-VRH as the dominant transport process only in the insulating region of temperature dependence [33]. M. Mitra et.al. observed ES-VRH transport process in rGO-PANI composite in 300K-420K range [17]. Lastly, T. N. Narayanan et.al. studied electronic transport properties of rGO-Fe3 O4 nanocomposite in 200K-300K and suggested Mott 2D-VRH transport mechanism in the entire temperature range [34]. The above discussion suggests that there are good amount of interest among scientific

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community to determine the underlying transport processes in both rGO and rGO based nanocomposites. Some of these studies have also reported a crossover in the transport process in form of VRH to Arrhenius transport. It will be fair to assume that any such transition happening near or above room temperature might has been missed because of the scarcity of data points at higher temperatures. In this work, we have synthesized rGO and rGO-ZnSe composites with different ZnSe fractions through solvothermal method. The structural and morphological properties have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA), Fourier-transform infrared spectrum (FTIR) and electron paramagnetic resonance (EPR) analysis. Temperature dependent electrical properties have been studied over a wide temperature range (84K-473K). Our study have confirmed that the transport processes are strongly dependent on the crosslinking of rGO layers through ZnSe microspheres in the composites. Charge carrier mobility values have been estimated in trap free SCLC regime under dark. Moreover, the composites have been found to be paramagnetic despite the diamagnetic nature of the individual components. Unreduced OFGs in rGO-ZnSe is determined to be the main reason behind this paramagnetism.

3

Experimental Details

GO was synthesized from graphite powder by modified Hummer's method [35]. In a typical method to synthesize rGO-ZnSe composite, 40 mg of GO was dispersed in a mixture of ethyleneglycol (C2 H6 O2 ; EG) and ethylenediamine (C2 H8 N2 ; ED) (EG: ED = 24:1 volume ratio) followed by ultrasonication for 1 h. Same amount of zinc acetate dihydrate [Zn(CH3 COO)2 .2H2 O] and sodium selenite [Na2 SeO3 ] and 0.4 gm polyvinylpyrrolidone (PVP) were added to the suspension of GO and stirred for 30 min. The mixture was then transferred in a teflon lined stainless-steel autoclave and kept in an oven at 180o C for 12 h. The resulting product was centrifuged and washed with distilled water and ethanol for several times. Washed samples were dried under vacuum at room temperature for 6h and the obtained product was defined as rGO-ZnSe. We have prepared different rGO-ZnSe composites with varying weight fraction of ZnSe and labeled as sample A, B, C, D, E, F

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and G (see table ST(I) in supporting information). To synthesize pure rGO, same process has been followed except the addition of precursors containing Zn and Se. Similar procedure was carried out without adding GO to synthesize ZnSe. Thermogravimetric analysis (TGA) was performed from room temperature to 800o C at 10o C/min heating rate with TA-SDT Q-600 thermal analyzer under air flow. The crystalline structures of the samples were determined by X-ray diffraction (XRD, RigakuMiniflex II) with Cu-Kα radiation (λ=1.5418 ˚ A) operated at 30 kV and 10 mA. The surface morphology of the samples were performed using field emission scanning electron microscope (FE-SEM, Zeiss, operated at 5 keV) and transmission electron microscope (TEM, JEOL-JEM 2100F, operated at 200 keV). For a qualitative understanding of the reduction process, we have performed Raman spectroscopy of GO, rGO and rGO-ZnSe composite using a micro-Raman spectrometer (TRIZX550, JY, France) equipped with a 514 nm exciton laser. X-ray photoelectron spectroscopy (XPS) was recorded by ULVACPHI 5000 Versa Probe II spectrometer using a monochromatic Al-Kα radiation (photon energy 1486.6 eV) operated at 25 watt and 15 kV. The UV-vis absorbance spectra were recorded on a Shimadzu UV-1700 spectrophotometer. Fourier-transform infrared (FTIR) spectra were carried out with a Perkin Elmer Spectrum 100 spectrometer using KBr pellet. CW EPR measurements were performed using X-band Bruker ELEXSYS 550 with flexline cavity. For EPR measurements on different rGO-ZnSe fractions, almost equal dry masses were loaded in quartz tubes. Sufficient no of scans were accumulated to get an acceptable signal to noise. The EPR data were divided by respective number of scans. For electrical conductivity measurements in a side-by-side top electrode configuration, the film was prepared on glass substrates and Ag was deposited as electrodes using thermal evaporation through a shadow mask under 5×10−6 mbar vacuum pressure. The active sample area was 6 mm2 . For SCLC measurements in top-bottom configuration, the sample was spin coated at 1000 rpm for 30 sec on ITO coated glass substrates and Al electrode was deposited on top. The film thickness were measured by 3D-optical surface profiler (3D-OSP, Bruker, Contour GT). An Agilent 4294A impedance analyzer has been used to measure the capacitance-frequency response of the samples in 40Hz 70MHz frequency range. We have used Keithley 2450 source measure unit to measure current-voltage (I-V) characteristics of the samples in temperature range 84K-473K. For temperature dependent measurements, the samples were mounted in a liquid nitrogen

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cooled dipstick cryostat filled with small amount of He exchange gas. Temperature was controlled using a homemade temperature controller built around an industrial PID unit (Honeywell 1010). The controller offers temperature stability of ±0.5K throughout the whole range. Magnetization of the samples were measured using vibrating sample magnetometer (VSM, Lake Shore cryotronics, USA, 7400 series). The composition and different measurements performed on the samples are tabulated in table ST(I) (see SI).

4

Results and Discussions

4.1

Materials Characterization

To quantify rGO and ZnSe contents in rGO-ZnSe composites, TGA was carried out. The TGA curves (figure S(I) in supporting information) of rGO-ZnSe composites indicate a weight loss from 200o C to 600o C, owing to the segregation of OFGs and decomposition of carbon species present in the samples [36–38]. By comparing the weight loss of pure ZnSe and rGO-ZnSe, the ZnSe contents have been calculated to be 26, 38, 54, 72, 81, 94 and 98% by weight in sample A, B, C, D, E, F and G respectively.

(c)

(d)

(e)

(f)

(g)

(h)

(a)

(b)

Figure 1: TEM image of (a) rGO sheets and (b) rGO-ZnSe (sample D), inset shows lattice fringe pattern of ZnSe nanoparticle. (c)-(h) SEM image of sample A, B, C, D, E and F respectively. Figure 1(a) shows TEM image of rGO which indicates transparent wrinkled sheet. It 7



is observed from figure 1(b) that the ZnSe nanoparticles appear as dark microspheres, wrapped on wrinkled rGO sheets. Inset of figure 1(b) shows the lattice fringe pattern of ZnSe with a spacing of 0.33 nm, consistent with the (111) crystal plane of cubic zinc blende structure of ZnSe as observed in XRD data (figure S(II) in SI). Size of individual nanoparticle inside the microspheres is found approximately 7 nm using Debye–Scherrer formula. SEM image of rGO-ZnSe composites (figure 1(c)-1(h)) indicates that the ZnSe microspheres are incorporated within individual layer of wrinkled rGO sheet. It is also observed from the SEM images that the size of the ZnSe microspheres in the composite progressively increase with the increase in ZnSe fraction, but individual ZnSe nanoparticle size remains approximately same (7-10 nm) as can be seen from FWHM of XRD peak. (a)

rGO D

Expt D band G band Fit

G

GO

Counts (a.u.)

Intensity (a.u.)

rGO-ZnSe (sample D)

1200 1500 1800 -1 Raman Shift (cm )

280

C1s

Experiment C-C C-O C=O O-C=O Fit

284 288 Binding energy (eV)

292


(b)

C1s

280

Counts (a.u.)

(c) Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

C1s

280

292


292

Figure 2: (a) Raman spectra of GO, rGO and rGO-ZnSe (sample D). The intensity ratios of ID /IG have been calculated by deconvolution of the spectra. The deconvoluted spectrum of rGO is shown in the figure. High resolution XPS spectra with C1s deconvoluted peak for sample (b) C, (c) D and (d) E respectively. The reduction of GO to rGO was confirmed by Raman spectroscopy (figure 2(a)). The D band centred at 1351 cm−1 comes primarily from sp3 fraction, and other disorderness present in the lattice as well. The G band at 1599 cm−1 is vibration mode due to sp2 bonded ordered graphitic carbons. The intensity ratio (ID /IG ), which is the ratio of deconvoluted peak area of the D and G bands, helps to determine the graphitization degree of the samples. Low value of this ratio means that degree of graphitization is 8


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increased which leads to better electrical conductivity [39]. The ID /IG ratio of rGO and rGO-ZnSe composite (sample D) is notably lower than that of GO (ID /IG = 1.38, 1.25, 1.29 for GO, rGO and rGO-ZnSe, respectively). This signifies recovery of graphitic structure and the removal of OFGs during solvothermal process [40]. The full width at half maximum (FWHM) of the D band peak is ∼ 110 cm−1 for both rGO and rGO-ZnSe. It also shows that the D band peak remains unaltered and the G band peak of both rGO and rGO-ZnSe downshifts from 1599 cm−1 to 1587 cm−1 due to restoration of sp2 network of carbon atoms [40,41]. The micro-Raman spectrum of rGO-ZnSe composite (D) (figure S(III) in SI) indicates that the crystallinity of ZnSe remains unchanged (transverse optic (TO) and longitudinal optic (LO) phonon modes) after incorporating ZnSe in rGO matrix which is also consistent with the XRD data. To identify the chemical composition and different types of functional groups present in the composite, XPS analysis was performed. The full surface survey spectrum (figure S(IV) in SI) was recorded in a wide range of binding energy (0-1100 eV) and the result confirms the presence of Zn, Se, C and O elements. The deconvoluted C1s peaks of rGO-ZnSe composites (figure 2(b)-2(d)) consist of functional groups such as sp2 (CC), epoxyl/hydroxyl (C-O), carbonyl (C=O) and carboxylates (O-C=O). The individual deconvoluted C1s peak indicates that the composite consists of a main peak centred at ∼ 284.7 eV (C-C) which signifies the presence of rGO [15]. The UV-vis absorption spectra of ZnSe and rGO-ZnSe (D) are depicted in figure S(V) in SI. Pure ZnSe shows a characteristic absorption peak at ∼ 290 nm. The attachment of rGO sheets with ZnSe nanoparticle enhances the absorption coefficient of rGO-ZnSe composite than pure ZnSe. Inset of figure S(V) shows the tauc plot with a band gap of ∼ 2.7 eV for ZnSe and ∼ 1.9 eV for rGO-ZnSe. FTIR spectrum of GO (figure 3(a)) shows the presence of OFGs at 990 cm−1 (C-O stretching), 1170 cm−1 (epoxide C-O-C or phenolic C-O-H), 1350 cm−1 (alcoholic C-OH bending), 1560 cm−1 (C=C stretching), 1660 cm−1 (carbonyl C=O stretching) [42, 43]. These OFGs are vanishingly small in rGO (traces of C-O-C and C-O are present) which signifies the reduction of GO to rGO in hydrothermal treatment. The shift in C=C bond is due to the reconstruction of sp2 structure caused by the removal of OFGs [44]. The FTIR spectra for rGO-ZnSe indicate that the OFGs are present in reduced amount in the composites.

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A F rGO5

B G

C-O C-O-H

C GO5

D

E

Mn2+ hyperfine lines

C-OH C=C C=O

GO

Intensity (a.u.)

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a)

rGO ZnSe rGO-ZnSe (D) at 300K 10

(b)

rGO-ZnSe (D) at 4.7K

1000

1250 1500 -1 1750 300 Wavenumber (cm )

320 340 360 380 Magnetic field (mT)

Figure 3: (a) FTIR spectra of GO, rGO and rGO-ZnSe. The stretching band at ∼ 1660 cm−1 (marked) is seen to intensify from sample A to D and then weaken for higher ZnSe contents (b) X-band CW EPR spectra of GO, rGO and rGO-ZnSe composite (D). EPR investigations were done to confirm the reduction of GO to rGO and to probe the underlying paramagnetic nature of the samples. Figure 3(b) shows characteristic M n2+ hyperfine splitting centred at g ∼1.996, arising from central electronic transition Ms = −1/2 ↔ Ms = +1/2. The splitting is due to trace amount of M n+2 impurity which comes from preparation stage of GO, where KM nO4 is used as oxidizing reagent [45]. Apart from this, one can observe a strong and sharp EPR signal which comes from carbonyl and carboxylic OFGs in GO (see below) [46]. After chemical reduction, these OFGs were mostly removed, resulting an almost quenched EPR signal of rGO as shown in figure 3(b) [47]. M. Sepioni et.al. reported diamagnetic behaviour of graphene-like systems at room temperature [48] and it is consistent with our EPR signal of rGO. While rGO shows no trance of paramagnetic centers, rGO-ZnSe composite (sample D) exhibits a pronounced EPR signal. The EPR signal primarily contains a single resonance line at g ∼ 1.996 residing on a broad background. The composite also remains paramagnetic at 4.7K. The only noticeable difference of the low temperature EPR signal with the room temperature one is in the intensity, which is a direct outcome of greater population difference achieved at lower temperatures.

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4.2

Temperature Dependent Charge Transport Process

To understand the charge transport mechanism in rGO and rGO-ZnSe composites, temperature (T ) dependent current-voltage (I-V) responses were recorded in temperature range 84K-473K (shown in figure S(VI) in SI). Figure S(VII) indicates that the resistance of rGO-ZnSe sample depends approximately linearly on the distance between two electrodes which signifies that the charge transport behaviour is primarily controlled by the sample and not by the contact resistance. Figure 4(c) shows that resistance (R) decreases with T , which indicates semiconducting behaviour of the samples [49]. The resistance shows nonlinear (stretched exponential) dependence on temperature and it can be fitted with an equation of the form

R = R0 exp(

T0 p ) T

(1)

where R0 is a prefactor, T0 is characteristic Mott temperature related to the energy needed for charge carrier hopping, p = 1/(d + 1), (d is the dimensionality of the system). These temperature dependences are signatures of VRH mechanism [5, 50, 51]. VRH denotes a mechanism of charge transport by means of charge carrier hopping between two localized states around Fermi level (EF ) of the system [52, 53]. Mott considered the density of states (DOS) near EF to be constant [5, 51] and derived a relation similar to equation 1 with p values of either 1/3 or 1/4. The corresponding d values are 2 and 3, respectively, and consequently these p values were associated with 2D and 3D-VRH. But Efros and Shklovskii pointed out that DOS may not be constant around EF . Instead, DOS may decrease due to electron-electron Coulomb interaction that creates a Coulomb gap at EF and results in a non-constant value of DOS [5, 50]. Their model gives p = 1/2 and does not connect directly with the dimensionality of the system. The common way to determine the transport mechanism of charge carriers, i.e. the value of p, is to plot ln( RR0 ) vs T −p . This plot should be straight line for the suitable p value. But such plots can be misleading as often same data can be shown to behave in an approximate linear way for different values of p (see figure S(VIII) in SI). Resistance curve derivative analysis (RCDA) [54] is a very useful method to avoid such confusion. In this method, the value of p is obtained from the slope of lnw (w = | d(lnR) |) vs lnT plot (see d(lnT ) details in SI). Before describing our results we summarize the reports on RCDA analysis of graphene based system. B. Muchharla et.al. studied electrical transport properties of drop 11



casted rGO thin films that showed Mott 2D-VRH (p = 1/3) transport process in 50K180K and Arrhenius like temperature dependence in 180K-400K temperature range [27]. On similar system, Joung et.al. reported ES VRH (p = 1/2) transport mechanism in 4.2K-295K temperature range [5]. Both studies were conducted on chemically reduced rGO with different reduction route (reference (27) used ascorbic acid as a reducing agent and reference (5) used hydrazine hydrate). In general, RCDA is applicable on any temperature dependent resistance data. For single component systems where the DOS around EF is known, the parameter T0 obtained from RCDA may be used to estimate a correlation length (ξ) of hopping. Such analysis is in general not possible for a composite/hybrid system where DOS is not known. But even for doped systems if accurate measurement of DOS is available ξ may be estimated. Indeed, the first set of RCDA analysis was performed on a Ga doped n-Ge system where DOS data was available [54]. But one can still apply RCDA in order to reveal underlying transport processes on systems where DOS is not defined. To our knowledge, there is no RCDA analysis on nanocomposites of rGO. In our study, nonlinear behaviours of resistance with temperature of rGO and rGOZnSe composites have been observed in two different temperature regimes, shown in figure 4(c). In region I (84K–280K), rGO shows 2D-VRH behaviour (p ∼ 1/3) whereas rGO-ZnSe composites with different ZnSe fraction exhibit 3D-VRH (p ∼ 1/4). Even for the lowest fraction of ZnSe (in our case 26% by weight), 3D-VRH transport process has been observed (see table ST(IV) in SI). The correlation length (ξ) for rGO obtained from T0 value is 3.73 nm (see SI for details) which is in agreement with the previous reports [27]. For rGO-ZnSe system we are unable to calculate ξ as the DOS is not known in this hybrid system. The VRH behaviour suggests that there exists localized electronic states around Fermi level. In region II (290K–473K), p ∼ 1 was observed for both rGO and rGO-ZnSe. It indicates band gap dominated transport by means of nearest neighbour hopping (Arrhenius like trasnport) [55]. The results were confirmed by several reproducible runs (see table ST(II) and ST(III) in SI). The average values of 1/p for rGO obtained from different observations are (3.08±0.03) and (1±0.09) for region I and region II, respectively. The 1/p values for other rGO-ZnSe composites are tabulated in table ST(IV). It is observed that the 1/p values in region I for rGO-ZnSe composites are ∼ 4 even in case of lowest % ZnSe content (sample A). From Arrhenius equation of resistance

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Glass substrate

(b)

5

D

12

region II

C

0.3 1/p = 3.07 4.5 5.0 lnT

5.5

region I Experimental data Linear fit

4

Temperature range = 85K-280K

0.16

A

14

6

ln(R/R0)

13

rGO

1.0

11

5.0 lnT

(f)

1.2

1/p = 0.98 5.8

(e)

Experimental data Linear fit

2.4

2.7 -13.0 1000/T (K )

200 300 400 Temperature (K)

500

Temperature range = 84K-280K 10 0.24 0.27 0.30 0.33 -1/4 -1/4 T (K )

3.3

Linear fit

1/p = 0.96 5.8

6.0 lnT

7

(g)

6.2

region II Experimental data Linear fit

6

100

region II Experimental data Linear fit

2.1

8

region I

6.2


2.1

1.8

5.5

6.0 lnT

4

9

1/p=3.98 4.5

Linear fit

1.6

5

0.22

Linear fit

1.2

12

region I

0.18 0.20 -1/3 -1/3 T (K )

lnw

6

(d)

9 B

3

Linear fit

ln(R/R0)

Ag

0.6

lnw

sample

6

ln(R/R0)

Ag

15 E

3D-VRH 2D-VRH

lnw

A

(a)

(c)

F

lnw

V

ln(R/R0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ln(R/R0)

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2.1

2.4

2.7 -1 3.0 1000/T (K )

3.3

Figure 4: (a) Schematic diagram of the samples to measure current-voltage (I-V) characteristics. (b) Photograph of rGO and rGO-ZnSe (sample D) dispersed in water. (c) Temperature dependent resistance of rGO, rGO-ZnSe composites indicating two different charge transport regions. Experimental data are fitted by 2D-VRH and 3D-VRH for rGO and different rGO-ZnSe samples respectively (solid lines). (d) Semilog plot of resistance (R) vs T−1/3 , inset shows the exponent value (1/p) = 3.07 indicating 2D-VRH for rGO. (e), (g) Arrhenius plot of resistance, inset shows 1/p = 0.98 for rGO and 1/p = 0.96 for rGO-ZnSe (D) indicating Arrhenius like behaviour. (f) Semilog plot of resistance (R) vs T−1/4 , inset shows (1/p) = 3.98 indicating 3D-VRH for rGO-ZnSe composite. (see SI) we got activation energy (EA ) = (152.05 ± 0.54) meV for rGO and EA = (235.83 ± 0.62) meV for rGO-ZnSe composite (D). Similar EA values were obtained for other composites as well. In their detailed investigation on rGO thin film, R. Negishi et.al. have described a similar crossover in transport mechanism (from 2D-VRH to Arrhenius) in terms of the ratio of sp2 and disordered fraction [26]. They proposed that with increasing disorder (mainly sp3 like domains) the band gap opens up. For electrical transport at low temperature, this gap forces the electrons to hop through localized states around EF (VRH mechanism). As the temperature increases, band transport becomes more probable due to the availability of thermal energy and a crossover to band gap dominated transport (Arrhenius mechanism) is likely. By comparing ID /IG ratio (∼ 1) and FWHM (∼ 110cm−1 for both rGO and rGO-ZnSe) with the reported values in literature [26], it can be concluded that large disordered fractions are present in our samples. These fractions restrain the onset 13



Page 14 of 28

of band gap dominated transport in our system and we see Arrhenius like behaviour only when available thermal energy is high. The presence of 2D- or 3D-VRH as the dominant transport mechanism, on the other hand, depends strongly on the nanoscale morphology of the system (see below for details). Kim et.al. observed a finite conductivity value at the zero temperature extrapolation in thick reduced graphene oxide films [29]. They have considered the quantum mechanical tunneling in addition to the 3D-VRH term to explain the finite conductivity as T→0. Our temperature range does not include that very low temperature regime, so the quantum tunneling term, if any, could not be included in the analysis.

4.3

Space charge limited current (SCLC) conduction

Charge carrier mobility of rGO and different rGO-ZnSe compositions have been measured from current density-voltage (J − V ) characteristics at room temperature with a top-bottom electrode configuration (inset of figure 5(b)). Figure 5(a) shows the J − V characteristics of rGO (inset) and rGO-ZnSe compositions (sample A and C) in dark in double logarithmic scale. The individual J − V curve follows J ∼ V m power law in two regions. Region-I is characterized by Ohmic conduction J ∼ V (m ∼ 1). In region-II, at higher bias voltage (≥∼ 1V ), injected carriers dominate over intrinsic carriers and we have m ∼ 2 [56, 57]. This region is known as SCLC region. A value of m ∼ 2 signifies that our samples are free from exponentially distributed trap states within the band gap (i.e. trap free SCLC conduction) [56]. The values of m for rGO and different rGO-ZnSe composites are tabulated in table ST(V) in SI. The J − V relation in a low mobility sample with trap free SCLC conduction is described with the Mott-Gurney equation [58]: J=

90 r µV 2 8d3

(2)

where 0 is the permittivity of free space, r represents dielectric constant of active material (r = 3.2 for rGO and 4.5 for rGO-ZnSe, see figure S(X) in SI), µ is the charge carrier mobility and d stands for active layer thickness (see table ST(VI) in SI). Mobility may be extracted from the slope of J vs V 2 plot as has been done for several disorder semiconductors and rGO composites [24, 25, 59]. The calculated electron mobility for rGO is 4.3×10−2 cm2 V −1 s−1 . The µ values for 14


10

10

2

10

1

experiment m~1.99 linear fit

m~1.04

rGO

(a) m~1.96

0.1 1 Voltage (V)

m~1.98

1

II

m~1.03

region I

10

0

10

-2

sample A sample C linear fit

m~1.05

0.1

1

V

A

-1 -1

3

Al

10

-3

A

Voltage (V)

(b)

rGO: ZnSe ITO substrate

2

2

10

Mobility (cm V s )

10

3 -2

10

J (Am )

-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current density (Am )

Page 15 of 28

B C D E rGO: ZnSe composition

Figure 5: (a) J − V characteristics of rGO (inset) and rGO-ZnSe in dark at room temperature. Solid lines (red) are fits to J ∼ V m . (b) Electron mobilities of different rGO-ZnSe samples with error bar. µ values have been calculated from the slope of region-II (m ∼ 2). Inset shows schematic diagram of samples that has been used to measure carrier mobility. rGO-ZnSe samples are typically one order lower than pure rGO mobility as shown in figure 5(b). Our µ values (∼ 10−3 cm2 V−1 s−1 ) are in agreement with other reported rGO based inorganic composites [24, 25]. The error bars were estimated from values obtained in separate measurements on identical samples. Interestingly, the mobility values for composites reach a maximum for ZnSe content of ∼54% (sample C).

4.4

Magnetic Properties of rGO, ZnSe and rGO-ZnSe composites

The paramagnetic nature of rGO-ZnSe (sample D) has been observed by EPR for the entire temperature range despite having diamagnetic behaviour of rGO and ZnSe (figure 3(b)). A previous detailed investigation has revealed the diamagnetic nature of pure rGO films [48]. The room temperature EPR spectra (figure 6(a)) of different rGOZnSe samples indicate paramagnetic nature. The EPR signal for all the samples (A-G) contain a single resonance at g ∼ 1.996. Although efforts were made to maintain similar experimental condition for different samples (A - G), the absolute signal intensities may not be compared. Nonetheless, there is a clear trend in signal intensity. At first, the signal becomes stronger with increasing ZnSe fraction upto ∼72% of ZnSe content (sample D), then with further increase in ZnSe content, signal intensity gradually decreases, and becomes vanishingly small in pure ZnSe (figure 3(b) and SI figure S(XII)). For samples E, F and G, some additional lines were seen on both sides of this main resonance which 15



G

ZnSe

1

F E

M (emu/g)

0.05 0.00

-0.05 -4

D C

(a)

M (emu/g)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

-2

0 H (T)

2

4

(b) 0 A B C D E ZnSe

B A

-1

330

340

350

-1

Magnetic field (mT)

0 H (T)

1

Figure 6: (a) X-band EPR spectra of different rGO-ZnSe composites. The signal of sample D has been divided by 5 to fit in the scale. (b) Magnetization as a function of magnetic field (M-H) of rGO-ZnSe and ZnSe samples at room temperature. Inset shows diamagnetic behaviour of pure ZnSe. was assigned to trace amount of M n2+ (also observed in GO, figure 3(b)) that may be present in some of the samples. The hyperfine lines are absent in the background scan and in pure ZnSe sample (figure S(XII) in SI). VSM magnetization at room temperature (figure 6(b)) also confirms paramagnetism of rGO-ZnSe composites and diamagnetism of pure ZnSe. Once again, it is observed that the paramagnetic moment of rGO-ZnSe increases with the increase in ZnSe fraction upto ∼72% in the composite. But with further increase in ZnSe contents, the magnetic moment decreases (sample E). In case of sample F and G, the paramagnetic moment becomes so less due to higher contents of ZnSe nanoparticles that it appears diamagnetic in VSM measurement. But much more sensitive EPR measurement indicates their paramagnetic nature with relatively weak signal intensity (figure 6(a)). Moreover, figure S(XI) (in SI) shows zero-field-cooled (ZFC) magnetization curve of rGO-ZnSe (sample D). For entire temperature range between 5K to 330K the composite exhibits paramagnetism which is in accordance with the low temperature EPR data (figure 3(b)). OFGs present in graphene oxide (GO) plays a vital role in controlling the growth of ZnSe microspheres in the synthesis of rGO-ZnSe composites. In the in-situ hydrothermal synthesis, positively charged Zn2+ – PVP complex interacts electrostatically with the negatively charged GO sheet, and it forms the nucleation site for ZnSe microspheres

16


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in the presence of N a2 SeO3 [60]. This also restricts the ZnSe microspheres from self agglomeration which assists the formation of smaller size as compared to ZnSe synthesis in the absence of GO [61, 62]. On the other hand ZnSe microspheres formed at the sites of OFGs also hinder the restacking and collapse of the graphene sheets in this synthesis process. In rGO, like our result most of the previous reports have found 2D-VRH charge transport. In GO there are fewer reports on transport mechanism. L´ opez et.al. have found 2D-VRH transport in chemical vapor deposited GO monolayer [63]. Interestingly, Jilani et.al. have reported 3D-VRH transport in a drop casted film of GO [52]. This finding indicates to a difference in charge conduction process between GO and rGO. Structurally, the difference between these two forms of graphite is the presence of OFGs and interplanar spacing (of (002) plane) as seen from the XRD data. Despite the fact that stacking of graphite sheets is more in rGO than GO, a 3D-VRH mechanism in GO as opposed to 2D-VRH in rGO points to possible participation of OFGs in charge transport. GO is rich with different OFGs (epoxy, hydroxyl on the basal plane and carboxyl and carbonyl groups on the edge of individual graphene sheets) [64]. rGO, on the other hand, has only small concentration of epoxy and alkoxy group on the basal plane and negligible presence of carboxyl and carbonyl at the edges(figure 3(a)). So, although the interplanar spacing in rGO is less compared to GO, the charge transport in rGO is primarily confined along the basal plane and we see 2D-VRH (figure 7(a)). In rGO-ZnSe nanocomposite, the interplanar contact among rGO flakes is established by incorporated ZnSe microspheres between graphene sheets(figure 7(b)), which are semiconducting in nature. As a result, we not only see 3D-VRH in rGO-ZnSe nanocomposite, but the resistance increases by almost 3 orders of magnitude as compared to pure rGO films (figure 4(c)). Such change is impossible to explain unless we consider active participation of ZnSe microspheres in charge transport process. Indeed, such incorporation of ZnSe microspheres is evident from the TEM and SEM images (figure 1(b)-(h)). Moreover, we see that although all the rGO-ZnSe samples follow same charge conduction mechanism (3D-VRH), the mobility of sample C is highest. This provides a way of choosing ZnSe concentration in case we look for fabrication of rGO-ZnSe based devices. OFGs play another important role in GO in terms of its magnetic property. Paramagnetism in GO is mainly due to OFGs, specially the ones that are at the edges of

17


Electrode

Page 18 of 28

Electrode

Electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electrode


(b)

(a) Current Voltage

Current Voltage

e-

ZnSe

Figure 7: Proposed charge carrier transport mechanism of (a) rGO and (b) rGO-ZnSe. graphene sheet, namely carbonyl and carboxyl groups [46]. As rGO is mostly free from those functional groups, as seen from the FTIR data (figure 3(a)), we see no evidence of paramagnetism in rGO. The paramagnetism in rGO-ZnSe nanocomposite could be because of a) presence of carbonyl and/or carboxyl groups or b) any surface defect that may be present in the sample. Given that individually synthesized ZnSe is also diamagnetic (figure 3(b) and 6(b)) and that FTIR and XPS shows presence of carbonyl groups in nanocomposites, we think the origin of paramagnetism in rGO-ZnSe nanocomposite is the OFGs present in the edge of graphene sheet. In an effort to quantify the role of carbonyl group in magnetization for samples C, D and E we carried out comparison of (i) the ratios of saturation magnetization from VSM data (figure 6(b)) (ii) The ratios of cabonyl (C=O) groups weighted by total area under C1s peak of XPS data (figure 2(b)-(d)). The ratios are 0.48 : 1.0 : 0.11 for VSM and 0.52 : 1.0 : 0.39 for XPS. Although the absolute values are not in agreement, the trend is very similar, i .e. the value decreases on either side of ∼72 wt% of ZnSe content. A closer look into FTIR (figure 3(a)) and EPR (figure 6(a)) data also agrees to this finding, that both the carbonyl group stretching band (at ∼ 1660 cm−1 ) and the paramagnetic resonance line (at ∼ 3443G) intensifies till a ZnSe content of ∼72 wt%, and weakens with higher ZnSe content. Unfortunately, these results cannot be quantified as achieving a normalization for signal level between different sets of EPR or FTIR measurement is very difficult. In brief, the above results indicate to the fact that the paramagnetism in rGO-ZnSe nanocomposites are primarily due to carbonyl (C=O) groups, which may be present because of incomplete reduction of the GO. However, we could not establish any link (or correlation) between temperature dependence of charge transport processes and 18


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observed paramagnetism in rGO-ZnSe composites.

5

Conclusions

In summary, rGO and rGO-ZnSe composites were synthesized by the modified Hummer's method and chemical reduction technique. Temperature dependent electrical conductivity of rGO and rGO-ZnSe samples have been measured. It is observed that Mott 2D-VRH is the charge transport mechanism for rGO whereas for rGO-ZnSe, it follows Mott 3DVRH in 84K-280K temperature range as the ZnSe microspheres act as linker between rGO layers. The charge transfer mechanism follows band gap dominated activated transport process in 290K-473K for all the samples. Calculated mobility values, combined with our conductivity data, provide the optimal composition of rGO and ZnSe for optoelectronic applications. Our EPR results and VSM data shows room temperature paramagnetism for all the composite samples as opposed to the diamagnetic behaviour of pure rGO and ZnSe. The EPR signal intensities (for sample A to E) are in line with the trend we get from VSM magnetization. By analyzing results from XPS and FTIR, the carbonyl functional group is held responsible for paramagnetism in rGO-ZnSe nanocomposites.

Supporting Information TGA, XRD data, spectroscopic analysis, temperature dependent I-V data and analysis, capacitance-frequency response, sample thickness, ZFC magnetization curve of rGO-ZnSe (sample D) are shown in supporting information.

Acknowledgement DB acknowledges support from IIT Kharagpur through ISIRD funding. ABR and AS acknowledge MHRD, India for research fellowship. Authors are thankful to Materials Characterization laboratory at the Materials Science Centre, IIT Kharagpur for TGA measurements, DST-FIST facility at department of Physics, IIT Kharagpur for XPS measurements, central research facility, IIT Kharagpur for SEM, TEM, XRD, Raman, VSM, EPR, 3D-OSP and FTIR measurements.

19



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Table of Contents (TOC) Image rGO

Electrode

2D-VRH

Electrode

Electrode

rGO-ZnSe

Electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3D-VRH

Current Voltage

Current Voltage

e-

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ZnSe


Observation of different charge transport processes and origin of

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