Morphology and Photoelectrical Properties of Solution Processable

Jun 9, 2010 - E-mail: [email protected] (L.V.); [email protected] (B.R.K.)., †. Università di Perugia. , ‡. American University of Beirut. , ...
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Morphology and Photoelectrical Properties of Solution Processable Butylamine-Modified Graphene- and Pyrene-Based Organic Semiconductor Silvia Bittolo Bon,† Luca Valentini,*,† Rasha M. Moustafa,‡ Fadi M. Jradi,‡ Bilal R. Kaafarani,*,‡ Raquel Verdejo,§ Miguel Angel Lopez-Manchado,§ and Jose` M. Kenny†,§ Dipartimento di Ingegneria CiVile e Ambientale, UniVersita` di Perugia, INSTM, UdR Perugia, 05100 Terni, Italy, Department of Chemistry, American UniVersity of Beirut, Beirut 1107-2020, Lebanon, and Instituto de Ciencia y Tecnologia de Polimeros, CSIC, Juan de la CierVa, 3, 28006 Madrid, Spain ReceiVed: February 19, 2010; ReVised Manuscript ReceiVed: May 25, 2010

The morphological and electrical properties of 2,11-di-tert-butyl-6,7,15,16-tetrakis(hexylthio)quinoxalino[2′,3′: 9,10]phenanthro[4,5-abc]phenazine (TQPP) organic molecule combined with butylamine (BAM)-modified graphene sheets (GSs) are described. The grafting of the amine onto the graphene sheet plane promotes its compatibility with this BAM soluble molecule, leading to the preparation of homogeneous films containing exfoliated graphene sheets, as demonstrated by infrared spectroscopy, field emission electron microscopy, and Raman spectroscopy. The film deposited from the BAM-modified GSs/TQPP blend showed a photoelectrical response higher than those prepared with neat molecule and GSs/TQPP blend, respectively. Introduction Graphene, which consists of atom-thick sheets of carbon, organized in a honeycomb structure, has attracted tremendous attention from the research community because of its long-range π-conjugation, yielding extraordinary thermal, mechanical, and electrical properties.1 Graphene is usually prepared as individual flakes by micromechanical cleavage of graphite. Such method is ideal for preparing sample with controllable layering for nanoscale electrical,2 spectroscopic,3,4 or nanomechanical characterization.5 However, the yield of this technique is low for any large scale applications that require graphene in thin-film form1,6-9 as a transparent conductor or as a filler in composites.10,11 For such applications, the exfoliation in liquid phase of graphene is needed. In this regard, the technical difficulty is to fabricate graphene-based devices by spin- or blade-coating a solution in organic solvent. Van der Waals attractive forces aggregate the graphene sheets (GSs), and thus there is a pronounced interest in developing methods for preparation of soluble solutionprocessable graphene derivatives.12 Reduction of exfoliated graphite oxide (GO) was used to produce GSs stabilized by amphiphilic polymers to form a stable dispersion.13 GSs were chemically modified by alkylamine14 and well-dispersed GSs were obtained by utilizing the hydrophilic carboxyl groups on graphene surface.15 The chemical interaction between alkylamines and graphene/GO has been also extensively debated in the literature.16-21 The treatment of GO with alkylamine can lead to the derivatization of both the edge carboxyl and surface epoxy functional groups, as suggested by Bourlinos et al.,18 who proposed covalent bonding of amine on epoxy groups, or by Mastuo et al.,17 who suggested hydrogen bonding between amines and hydroxyl groups on the basal plane. Che et al.21 discussed a one-step method consisting of chemical reduction by ethylenediamine as well as dispersions in N,Ndimethylformamide. * Corresponding authors. E-mail: [email protected] (L.V.); bilal.kaafarani@ aub.edu.lb (B.R.K.). † Universita` di Perugia. ‡ American University of Beirut. § Instituto de Ciencia y Tecnologia de Polimeros.

A novel approach to achieve stable colloidal suspensions of quasi-2D carbon sheets through plasma-enhanced chemical vapor deposition was recently developed.22 It has been demonstrated that the direct fluorination of GSs and their subsequent derivatization provide a versatile tool for the preparation and manipulation of graphenes with variable sidewall functionalities. The functionalization of graphene has been considered to be important for improving their solubility and applications in devices.23 Noncovalent functionalization of GSs through π-π interactions using aromatic organic molecules has been addressed in literature; for example, stable aqueous dispersions of GSs using a water-soluble pyrene derivative were prepared.24,25 The noncovalent modification of the graphene films with pyrene butanoic acid succidymidyl ester was recently used for largearea production of continuous, transparent, and highly conducting few-layered graphene films for application in photovoltaic devices.26 The interest in bisphenazine discoid molecules has recently increased. The synthesis, photophysical properties, mesophase behavior, and device properties of 6,7,15,16-tetrakis(alkoxy/ alkythio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine was reported.27-32 Lee et al. reported the substituent effect on the morphology and self-assembly of these molecules.33,34 Molecular ribbons,35 nanofibers of asymmetrically substituted bisphenazine through organogelation,36 and diradicals37 of these discoid molecules were also reported. Recently, it was reported how graphite powder can be exfoliated by sonicating in an aqueous solution of pyrene molecules that had been previously modified with water-soluble groups.38 Along this line, our current interest is designing new soluble pyrene-modified graphene materials that can be directly used to fabricate graphene-based molecular photoresponsive devices. For these reasons, we prepared chemically derived GSs that were combined with 2,11-di-tert-butyl-6,7,15,16-tetrakis(hexylthio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine, TQPP[t-Bu]2-[SC6H13]4, organic molecule (Scheme 1).

10.1021/jp101518n  2010 American Chemical Society Published on Web 06/09/2010

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SCHEME 1

TQPP-[t-Bu]2-[SC6H13]4 was found to be soluble in butylamine (BAM) and thus combined with plasma-fluorinated GSs, leading to a stable dispersion of layered GSs when they were exposed to the BAM. From such a blend, we deposited a film with improved photoelectrical response with respect to the films cast from the neat molecule and the unmodified graphene/ molecule blend. Experimental Section The general synthesis of TQPP-[t-Bu]2-[SR]4 is described elsewhere.27 After column chromatography using CHCl3 as eluent, the compound was recrystallized from toluene. Spectroscopic data of TQPP-[t-Bu]2-[SC6H13]4 compound hereinafter named TQPP are reported elsewhere28 (Scheme 1). GSs were produced starting from natural graphite powder (universal grade, 200 mesh, 99.9995%), as previously reported.39 (See the Supporting Information.) Fluorinated GSs were obtained by the plasma-assisted decomposition of CF4 employing a 13.56 MHz radiofrequency plasma source.22 A commercially available grade of BAM (CH3(CH2)3NH2) supplied by Sigma-Aldrich Chemicals, was used in this research. To get BAM-modified GSs, fluorinated GSs were dispersed by immersing an ultrasonication probe for 1 h into the liquid amine held at 5 °C on a thermostatic bath to avoid the evaporation of the amine. Neat GSs and BAMmodified GSs were characterized by scanning electron microscopy. (See the Supporting Information.) Solutions consisting of TQPP dispersed in anhydrous N-methyl2-pyrrolidone (NMP, purchased from Aldrich, 1 mg/1 mL) and BAM (1 mg/1 mL) and a combination of the graphene and TQPP moiety (1(GSs):1(TQPP) in NMP and 1(BAM modified GSs):1(TQPP) wt %) were sonicated (750W, 60% amplitude) for 1 h. The vials containing such dispersions are shown in Figure 1. The prepared solutions were drop-cast onto indium tin-oxide (ITO)-coated poly(ethylene terephthalate) (PET) substrates (Kintec) with the electrical sheet resistivity of 14 Ω/sq. The samples were left to evaporate at 50 °C for 1 h. The infrared (IR) spectra of the deposited film were recorded in the 500-4000 cm-1 range. The morphologies of the prepared samples were investigated by field-emission scanning electron (FE-SEM) microscopy and atomic force microscopy (AFM). AFM images were obtained in tapping mode equipped with an integrated silicon tip/cantilever having a resonance frequency ∼300 kHz. UV-vis measurements of the deposited films were carried out with a Perkin-Elmer spectrometer Lambda 35; for all samples, an ITO-coated PET slide was used as the reference. Raman spectra of samples were measured using a Renishaw Invia microscope using an excitation wavelength of 514.5 nm.

Figure 1. Vials containing dispersions (from left to right; 1 mg/mL) of TQPP in NMP, GSs/TQPP in NMP, TQPP in BAM, and TQPP dispersed in BAM-modified GSs.

For the sheet resistance measurements of spin-cast samples, before the film deposition, four stripes of ITO spaced of ∼0.1 mm were removed mechanically from the PET substrate, and four ITO electrodes not short-circuited between them and connected to a computer-controlled Keithley 4200 source measure unit were obtained. Photoelectrical measurements were obtained for the films over several ON/OFF light illumination cycles. The conductivity of the films was monitored as a function of exposure time through a solar simulator AM1.5D with an input power of 100 mW cm-2. Results and Discussion Regarding the preparation process, the solubility of TQPP in NMP and BAM is a critical factor. Four dispersions, namely, TQPP in NMP, GSs/TQPP in NMP, TQPP in BAM, and TQPP dispersed in BAM-modified GSs were observed (Figure 1). The vial with TQPP dispersed into NMP contains visible precipitates, whereas TQPP in BAM remained well-dispersed in the amino phase. This finding indicated the better solubility of TQPP in BAM. In accordance with these results, a dark-brown dispersion of the TQPP dispersed in BAM-modified GSs with no visible precipitates has been obtained, whereas the GSs/TQPP dispersion in NMP led to flocculation. The first investigation was devoted to understand whether BAM can be used as solvent for the TQPP film casting; such information is very important in view of recent experiments that demonstrated how the addition of this amine led to an exfoliation of plasma fluorinated GSs. For comparison, a TQPP film was deposited from NMP solution because of the fact that the NMP is considered one of the most used solvents for graphene.40 FE-SEM was utilized to analyze the surface morphology of the TQPP films drop-cast from NMP and BAM solutions, respectively. The FE-SEM analysis of the TQPP film drop-cast from NMP (Figure 2a) shows that the morphology consists of isolated fibrils leaving large area of uncovered substrate. On the contrary, the film deposited starting from the BAM solution shows (Figure 2b) a continuous and homogeneous cauliflowerlike structure. The IR spectrum of the TQPP film drop-cast from BAM solution shows the features of the NH2 bending and N-H stretching vibrations of the primary amine at 1605 cm-1 and between 3290 and 3370 cm-1, respectively (Figure 3a).41 When TQPP was mixed with the fluorinated GSs, an attenuation of these signals was observed (Figure 3b). This finding is consistent

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Figure 3. (a) FTIR spectrum of the TQPP film drop-cast from BAM solution. (b) FTIR spectrum of the BAM modified GSs/TQPP film.

Figure 2. FE-SEM images of the TQPP films drop-cast from (a) NMP and (b) BAM solutions.

with a reaction between fluorinated graphenes and primary amine via elimination of fluorine and the observation of a N-H bond reported elsewhere.22,42,43 (See the Supporting Information.) The addition of neat graphene to the TQPP moiety in NMP solution does not alter the peak features of TQPP. (See the Supporting Information.) From these findings, it is reasonable to suppose that the BAMmodified GSs could be the best candidates for their compatibility with such BAM soluble organic molecule. To investigate the role played by the solvent when the GSs are combined with TQPP, FE-SEM analysis of thin films consisting of GSs/TQPP and BAM-modified GSs/TQPP deposited from NMP and BAM, respectively, are reported in Figure 4. The film drop-cast from the NMP solution of graphene and TQPP moiety presents a phase separation consisting of fibrils (i.e., the organic compound as reported in Figure 2a) and flakes (i.e., graphene) (Figure 4a). This morphological aspect is typical of graphene film deposited from NMP solution (Supporting Information), whereas film

prepared from BAM-modified GSs/TQPP blend is homogeneous and morphologically similar to that of TQPP (Figures 4b and 2b). To have more possibilities of imaging phases with different electrical conductivity,44 the FE-SEM analysis of the film reported in Figure 4b was performed with the inlens configuration; in this configuration, it is possible to observe the presence of dispersed GSs (Figure 4c). The role played by the TQPP molecule for the modification of graphene with BAM is very similar to the recently proposed mechanism by Zhang et al.38 The authors proposed that the large planar aromatic structures of water-soluble pyrene molecules anchor themselves onto the hydrophobic surface of GSs via π-π interactions and yield stable solutions of graphene/pyrene hybrids.38 Analogously, in our case, the solubility of our pyrenebased compound in BAM makes such moiety able to interact with exfoliated amino-modified GSs. Previous results performed by AFM demonstrated that such amino modified GSs have an average thickness of 0.7 to 0.9 nm, which is characteristic of exfoliated GS.22,45,46 On the contrary, drop-cast graphene was found to agglomerate to form isolated clusters 50-100 nm thick. (See the Supporting Information.) Neat and amino-modified GSs were further characterized by Raman spectroscopy, as reported in Figure 5. A typical Raman spectrum of our graphene samples displays four main peaks: the G band at ∼1590 cm-1, the disorder-related D peak at ∼1350 cm-1, the 2D peak at ∼2700 cm-1, and the D+G peak at 2950 cm-1, respectively.47-50 For the BAM-modified graphene film (Figure 5b), the I(2D)/I(D+G) ratio, reported in the inset, is in agreement with the thickness evaluation measured through AFM measurements reported elsewhere,22 suggesting an exfoliation induced by the amino functionalization of the GSs. The peak ratios of our Raman spectra are similar to those obtained by Su et al.,51 and the lineshapes of the two components at 2700 and 2950 cm-1 are typical of defected GS.50

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Figure 5. Raman spectra of (a) graphene- and (b) BAM-modified graphene. The inset of Figure 4b reports the ratio between the 2D and D+G peak areas.

Figure 4. FE-SEM images of (a) GSs/TQPP, (b) BAM-modified GSs/ TQPP films, and (c) BAM-modified GSs/TQPP film obtained with inlens secondary detector. The arrows indicate the graphene.

The TQPP films deposited from NMP and BAM solutions showed two absorption regions, one in the range of 400-460

nm and another at 350 nm (Figure 6a,b), respectively. The addition of BAM-modified GSs to TQPP indicates a broadening of the region between 400 and 550 nm (Figure 6c). The incorporation of graphene onto TQPP had the most direct effect on electrical properties. The sheet resistance of TQPP film deposited from BAM solution was ∼12 × 106 Ω/sq, and those of GSs/TQPP and BAM -modified GSs/TQPP films were ∼103 and 10 × 106 Ω/sq, respectively. The low sheet resistance value measured for the GSs/TQPP film is due to the molecule phase separation with respect the graphene (Figure 4a); the low electrical resistance reflects the electrical transport through the highly conducting GSs. The addition of BAM-modified graphene to TQPP resulted in an overall decrease in the electrical conductivity. The decrease in the electrical conductivity is consistent with both the introduction of defect centers onto the graphene plane after the plasma treatment, which decreases the graphene charge mobility, and with the presence of the alkyl chains maintaining a larger intersheet spacing preventing the graphene ordering.20,52 XRD analysis is currently under investigation to support this hypothesis. Photoconductivity experiments on graphene films prepared by water-soluble graphene oxide were reported by Chen et al.52 The relationships between the photoconductivity and different preparation methods were also investigated.52 The current of TQPP and those of GSs/TQPP and BAMmodified GSs/TQPP films were monitored as a function of time as the light was switched ON (i.e., under illumination) and OFF (i.e., in the dark). The analysis of the diagrams, shown in Figure 7, indicates that the change of the conductance under illumination of the neat TQPP (Figure 7a) drop-cast from BAM solution was lower than that of its blend with amino-modified GSs (Figure 7b). Moreover, this analysis shows that the ON and OFF normalized current variations of the GSs/TQPP film obtained from NMP solution (Figure 7c) are markedly lower than those observed from the BAM -modified GSs/TQPP sample. The

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Figure 6. UV-vis spectra of the TQPP films drop-cast from (a) NMP and (b) BAM solutions. (c) UV-vis spectrum of the BAM modified GSs/TQPP film.

transport phenomena from the GSs/TQPP film obtained from NMP solution can be understood in the following model. The poorer dispersion of the GSs induces the formation of a network of graphene/graphene junctions. The change of conductance under illumination for neat GSs deposited from NMP solution has been also investigated. (See the Supporting Information.) With respect to previously reported results,52 a poorer photocurrent variation has been recorded. A possible explanation could be the different preparation method; that is, Lv et al.52 started by graphene oxide solubilized in water, spin-cast onto the substrate, and reduced with hydrazine. We retain that our preparation method that starts from GSs in NMP leads to the formation of crossed graphene junctions in the drop cast film that act as a gate for photogenerated carriers to move in the mat. In the blend obtained from BAM solution, these junctions are replaced by exfoliated GSs, which promote a better charge transfer between the molecule and the GSs. Conclusions In summary, chemically derived GSs prepared by plasma fluorination and subsequent reaction with a primary amine can be combined with an amino-soluble pyrene-based semiconduct-

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Figure 7. Evolution of the current under light-on and light-off steps (normalized with respect to the dark current) of the (a) TQPP film drop cast from BAM solution, (b) BAM-modified GSs/TQPP film, and (c) GSs/TQPP film drop-cast from NMP solution.

ing organic molecule. It was demonstrated how such aminomodified GSs have a better dispersion in the synthesized pyrenebased molecule and how this phenomenon promotes the generation of charge carriers transportation in this hybrid semiconducting molecule under illumination. Acknowledgment. The work at the American University of Beirut was supported by the Petroleum Research Fund (PRF) of the American Chemical Society (grant no. 47343-B10) and the Munib and Angela Masri Institute of Energy and Natural Resources. We are grateful for this support. We are also grateful for the financial support of the Spanish Ministry of Science and Innovation (MiCINN) through the project MAT2007-61116. Supporting Information Available: The experimental methods, FTIR spectra of TQPP film drop cast, FE-SEM and AFM images, XRD patterns, the proposed reaction mechanism for fluorine modified GSs, and the evolution of the current under light-on and light-off steps are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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