Thionine Self-Assembled Structures on Graphene: Formation

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Interface-Rich Materials and Assemblies

Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping Thiago Sousa, Thales Fernando Damasceno Fernandes, Matheus J.S. Matos, Eduardo Nery Duarte Araujo, Mario S. C. Mazzoni, Bernardo R. A. Neves, and Flávio Plentz Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00506 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping

Thiago A. S. L. de Sousa*,†, §, Thales F. D. Fernandes†,§, Matheus J. S. Matos‡, Eduardo N. D. Araujoǁ, Mario S. C. Mazzoni†, Bernardo R. A. Neves†, and Flávio Plentz†

†Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Ave. Presidente Antônio Carlos 6627, Belo Horizonte, CEP 31270-901, Brasil ‡Departamento de Física, ICEB, Universidade Federal de Ouro Preto, R. Diogo de Vasconcelos 122, Ouro Preto, CEP 35400-000, Brasil ǁDepartamento de Física, CCE, Universidade Federal de Viçosa, Avenida Peter Henry Rolfs, s/n, Viçosa, CEP 36570-900, Brasil

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ABSTRACT

The association of organic molecules with 2D materials, creating hybrid systems with mutual influences, constitutes an important testbed for both basic science self-assembly studies and perspective applications. Following this concept, in this work, we show a rich phenomenology that is involved in the interaction of thionine with graphene, leading to a hybrid material formed by well-organized self-assembled structures atop graphene. This composite system is investigated by atomic force microscopy, electric transport measurements, Raman spectroscopy, and first principles calculations, which show: 1) an interesting time evolution of thionine selfassembled structures atop graphene; 2) the final molecular assembly is highly oriented (in accordance with the underlying graphene surface symmetry); and 3) thionine introduces a strong n-type doping effect in graphene. The nature of the thionine-substrate interaction is further analyzed in experiments using mica as a polar substrate. The present results may help pave the way to achieving tailored 2D material hybrid devices via properly chosen molecular selfassembly processes.

KEYWORDS: graphene, thionine, self-assembly, functionalization, n-doping


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INTRODUCTION

The breakthrough that graphene investigation represents to materials science relies mostly on its two-dimensional (2D) nature and on the massless Dirac Fermions, which dominate its low energy electronic properties.1,2 The interplay between these two aspects is behind a fascinating possibility, which is the use of graphene as a platform to assist the formation of self-assembled compounds, leading to novel 2D hybrid materials.3-9 Such studies present a twofold motivation: the elucidation of the effect of graphene’s idiosyncrasies on the conformation of the adsorbed species and the search for a mechanism to control graphene doping levels. For instance, phosphonic acids have been shown to self-assemble atop graphene, forming crystals oriented along the underlying armchair direction and producing a hole-doping effect.10 Similarly, p-type doping has been detected for several other adsorbates, such as fluoroalkyl trichlorosilane,11 tetracyanoquinodimethane,12 tetrasulfonic acid,13 and melamine.14 However, for purposes related to the application in nanodevice construction, it is also desirable to find controlled mechanisms to provide n-type doping. The present work addresses this challenge by employing a combination of experimental and theoretical techniques to propose, synthesize, and characterize a hybrid material formed by organized self-assembled molecules following specific crystallographic directions in graphene, with the ability to strongly induce an n-type doping effect. To accomplish this goal, we have chosen the thionine molecule, C12H10N3S, whose structure is shown in Fig. 1. Thionine is a small and planar molecule, with two NH2 groups symmetrically distributed on each side. Importantly, it acts as a cationic component in ionic compounds through an electron donor site centered at the sulphur atom (represented in yellow in Fig. 1), which interacts with anions such as acetate (CH3CO2-) or chloride (Cl-). A similar situation may occur


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in thionine-graphene association, with a charge transfer mechanism allowing a n-doped graphene to play the role of the anionic species. Additionally, the π-conjugated units of thionine introduce a competing interaction mechanism with the underlying graphene sheet, eventually yielding interesting self-assembly routes.

Figure 1. Molecular structure of thionine. Sulphur, nitrogen, carbon, and hydrogen atoms are represented by yellow, blue, grey and white spheres, respectively.15 The grey scale bar represents 2 Å. Incidentally, the non-covalent functionalization of carbon materials with thionine molecules have been reported in the literature. For instance, thionine molecules have been shown to enrich the surface chemistry of carbon nanotubes by mediating their interaction with other chemical species.16 A controlled modification of the nanotube surface by the attachment of negatively charged metallic nanoparticles to the nanotube sidewalls may be achieved with this mechanism,17 which may also lead to applications related to the construction of biosensors.18-21 Similar studies have indicated a strong adhesion of thionine molecules on graphene, suggesting π-π interactions and charge transfer as the main contributions to the binding energy.22 Several applications in the field of electrochemical biosensing have been proposed for the resulting composite. Indeed, efficient electrochemical biosensors for detection of a variety of molecules of


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biological interest have been described.23-28 Nonetheless, a more systematic study on how the thionine assembly takes place atop graphene is still lacking. In the present work, we focus on the hybrid thionine-graphene compound and we show that it constitutes a complex system whose structure depends on the functionalization time, forming either monolayer islands or well-organized filaments. Mechanical exfoliation, atomic force microscopy (AFM) experiments, electric transport measurements, Raman spectroscopy and density functional theory (DFT) calculations are among the techniques employed to investigate the composite thionine-graphene system, allowing us to regard the proposed strategy as a tool to design novel nanoelectronic devices through selective doping control. EXPERIMENTAL SECTION

Mechanically exfoliated graphene samples were obtained by micromechanical cleavage of natural graphite crystals (purchased from Nacional de Grafite) and were deposited on the surface of a 300 nm-thick SiO2 layer on top of a silicon substrate. CVD graphene devices were used for the electrical characterization of the effect of thionine on the electric transport properties of graphene. The CVD graphene devices were produced by direct laser writing photolithography, as previously developed by our group.29 We made use of commercial CVD monolayer graphene samples grown on copper foils (purchased from Graphene Platform) and we transferred them to a 300 nm-thick SiO2 layer, over a highly p-doped Si substrate, using the conventional PMMA transfer technique.30 The electric contact was fabricated by thermal evaporation of 5 nm of Cr, followed by 100 nm of Au.


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The use of exfoliated graphene in the AFM measurements is necessary since transferred CVD graphene does not have the necessary clean and smooth surface, which is a requirement for the observation of the thionine self-assembled structures. On the other hand, the use of CVD graphene for the fabrication of the device grants a much easier and scalable method for the device production, which allows a large number of devices to be fabricated and tested using different functionalization conditions. Additionally, Raman spectroscopy measurements were carried out in order to elucidate the nature of thionine-graphene interaction. A pronounced graphene D band is expected if covalent bonds are formed. These measurements were performed in samples of exfoliated graphene since they do not feature a D band prior to functionalization. Graphene and mica (the latter was purchased from SPI Supplies) were functionalized by dipping in a thionine chloride (purchased from Santa Cruz Biotechnology) solution, for which deionized (DI) water was used as a solvent, with a concentration of 10 . The functionalization was carried out at (25.0 ± 0.5)℃, maintained by a controlled temperature water bath, and after this process the samples were rinsed in DI water, followed by drying with dry nitrogen flow. Exfoliated graphene samples used in AFM measurements were functionalized for 2, 5, 10, 20, 30, 60, 70, 80, 90, 100, 120 and 180 minutes. For Raman spectroscopy investigations five exfoliated graphene samples were used and they were functionalized for 10, 30, 90, 120 and 180 minutes. Mica samples were functionalized for 2, 5, 10, 15, 20, 30, 60, 120 and 180 minutes, and, finally, the CVD graphene devices were functionalized for 2, 10, 30, 60, 80, 90, 100, 120 and 180 minutes. The AFM measurements were made with a Nanoscope V Multimode SPM (Veeco Instruments) on Peak Force Tapping mode using conventional silicon nitride cantilevers. The Raman


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spectroscopy investigations were carried out with a WITec alpha300 RA, using a 532 nm laser line with a constant power of 1.4 mW. Ab initio calculations were performed within the pseudopotential DFT31 formalism based on the SIESTA32 implementation. We employed the vdW-DF/DRSLL parametrization for the exchange-correlation functional, which takes into account van der Waals interactions in a selfconsistent way.33-35 Troullier-Martins norm-conserving pseudopotentials36 in the KleinmanBylander factorized form37 were chosen, and the Kohn-Sham eigenstates were expanded in a finite-range double-ζ basis set augmented by polarization orbitals. All geometries were optimized until the maximum force component in any atom was less than 10 meV/Å. RESULTS AND DISCUSSION

The self-assembly of thionine molecules atop of graphene follows a non-trivial behavior, strongly dependent on the functionalization time and resulting in well-organized bilayer-like filaments. The full evolution of thionine self-assembly on graphene is illustrated in the topographic AFM images presented in Fig. 2. From a–l, twelve images corresponding to distinct graphene samples, each one associated with a specific functionalization time (ranging from 2 to 180 minutes), are shown. In each image, the white scale bar represents 500 nm. The heights of the SiO2 substrate and of the graphene flakes (G), as well as the heights corresponding to the length (1) and twice the length (2) of a thionine molecule, are indicated in the scale bar. The evolution pattern that emerges from the images can be summarized in the following four steps: (i) growth of initial nucleation sites with 2 minutes of functionalization (Fig. 2 a);38,39 (ii) formation of randomly distributed self-assembled monolayer (SAM) islands up to 70 minutes (Figs. 2 b–g); (iii) transition to a distinct phenomenology with 80 minutes of functionalization, 7 ACS Paragon Plus Environment

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in which randomly oriented self-assembled linear structures become apparent (Fig. 2 h); and, finally, (iv) consolidation of this new conformation after 90 minutes and longer (Figs. 2 i–l). Interestingly, in this last step, as time evolves, these filaments undergo an alignment rearrangement with specific directions over the graphene sheet. We shall return to this point for a quantitative description later on in our analysis.

Figure 2. AFM images of the graphene samples functionalized with thionine . Images a–l represent, respectively, the samples functionalized for 2, 5, 10, 20, 30, 60, 70, 80, 90, 100, 120 and 180 minutes. In the color scale are shown the height of the SiO2 substrate, the height of the graphene flakes (G), and the heights of thionine monolayers (1) and bilayers (2). In all the


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images, the white scale bars represent 500 nm. Selected lines along which height profiles are determined are shown in panels g, h and l. Height profiles provide valuable information on thionine conformation onto graphene. We made use of NanoScope Analysis software to trace them along the selected lines shown in Figs. 2 g–h and Fig. 2 l. The red solid lines intersect island regions, while the green solid lines intersect filaments regions. Fig. 3 a-c show the measured height profiles: top, middle and bottom panels correspond, respectively, to the thionine island intersected by the red line in Fig. 2 g, both island and filament regions of Fig. 2 h, and three distinct thionine filaments along the green line of Fig. 2 l. The height of the graphene surface was chosen to be at 0 nm. The profiles suggest that the thickness of thionine filaments (~3.0 nm) is fairly twice the thickness of thionine islands (~1.5 nm).

Figure 3. Height profiles of thionine islands (red profiles) and filaments (green profiles). The image a shows the height profile of the red solid line drawn in Fig. 2 g, which intersects a thionine island region. The image b shows the height profiles of the red and green solid lines drawn in Fig. 2 h, which intersect, respectively, island and filament regions. Finally, the image c


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presents the height profile of the green solid line drawn in Fig. 2 l, which, in turn, intersects three different thionine filaments. In order to validate the above description of the evolution pattern and to have a more accurate determination of the characteristic heights, we have used Gwyddion software for image processing to determine the height distributions for the images in Fig. 2. Figs. 4 a–c show, respectively, a representative height distribution histogram for each detected regime: islands, transition, and filaments. The first peak on the left in all panels represents the height of the graphene flake, chosen to be at 0 nm. Therefore, the position of the other peaks is associated with the thickness of thionine self-assembled structures over the graphene surface in islands or in filaments conformation. Fig. 4 a shows that the thickness distribution of thionine structures in SAM island conformation is centered at 1.1 nm (which is similar to the molecular length, see scale bar in Fig. 1). In the transient regime (Fig. 4 b), the height distribution has two peaks, one for the SAM islands, close to 1.6 nm, and another for the SAM filaments, which is close to 3.0 nm. Finally, Fig. 4 c shows that the thickness of the filament conformation is 2.4 nm, which is twice the effective length of thionine molecules. These results strongly suggest that the islands are formed by thionine monolayers laying perpendicular to the graphene surface, whereas the filaments are formed by thionine “double-layers” in the same upright molecular orientation.


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Figure 4. Representative height distributions for thionine molecules over graphene surface. The curve in a represents the typical height distribution for thionine monolayers in island conformation; in b, the transition from islands to filaments; and in c, the height distribution for thionine self-assembled structures in the filament regime. In all three height distributions, the first peak on the left represents the height of the graphene flake and it was chosen to be at 0 nm. The general picture is consistent with the self-assembly model for geometrically frustrated elements described by Lenz and Witten 40,41. In their scheme, the competition between geometric frustration, which may be unfavourable interactions between molecules, and the molecules’ attractive interactions, guides the aggregation process. The outcomes may be non-periodic 2D tree-like structures or bulk formation for low and high frustration regimes, respectively. An intermediate regime is also predicted, characterized by the formation of quasi-1D domains of fiber-like structures which maintain regularity up to large lengths. We believe that thionine self-assembly on graphene follows a similar trend, in which the thionine’s positive charge acts as a geometrical constraint which plays a prominent role in the transition from monolayer islands (tree-like aggregates) to bilayer filaments (fiber-like


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aggregates). Initially, the formation of thionine islands is favoured by the little frustration regime, since there is little electrostatic repulsion between the molecules. But as the selfassembly process goes on, more thionine molecules are aggregated to the preexisting islands and upon lateral stacking of these molecules,38,39 the whole structure is rearranged in order to minimize the energy cost associated with this addition.40 As a result, at a critical point in which the energies associated with island aggregation and island deformation are comparable, the lateral stacking may favour thinner structures, leading to the formation of the bilayer filaments. The concentration of the solution is expected to affect the morphologies of self-assembled structures since it is directly related to the adsorption rate.42,43 To verify its role in the present problem, two exfoliated graphene samples were functionalized with thionine for 10 and 30 seconds, respectively. The concentration of the solution (1mM) was chosen to be one hundred times larger than before. The functionalization process was carried out at a constant temperature of 20ºC, so that any change in the morphology of the final self-assembled structure in comparison with the structures shown in Fig. 2 may be ascribed to the concentration of the solution. Fig. S1 (Fig. S2) shows AFM topography images of the graphene flake functionalized for 10 seconds (30 seconds) and an example of height profile relative to the graphene surface of a thionine agglomerate. A high percentage of surface coverage (84%) was achieved, even for a relatively short functionalization time (30 seconds). The molecules were found perpendicular to the graphene surface (see Table S1), but only self-assembled monolayers were formed. We have not detected any transition from thionine islands to filaments, as observed in the case of a more diluted solution. A more uniform molecular film was formed, though. Additionally, to further characterize the thionine self-assembly evolution presented in Fig. 2, we employed NanoScope Analysis software to measure the angles between the thionine filaments 12 ACS Paragon Plus Environment

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shown in Figs. 2 h–l. The normalized histogram plots shown in Figs. 5 a–e clearly indicate that the transition detected in the topography images, and quantified by the height distribution, is accompanied by the alignment of thionine filaments in specific directions. Fig. 5 a shows a broad angle distribution between thionine filaments at the beginning of the transition (corresponding to 80 minutes of functionalization). The next step is the appearance of protuberant shoulders at certain angles (Fig. 5 b), culminating in a highly organized configuration in which the angle distribution becomes characterized by sharp peaks (Fig. 5 c-e). The location of the peaks indicates that the molecular filaments are oriented in two directions, which make an angle of ±60º with each other. It is worth noting that this behavior shows the large length regularity of the predicted self-assembled fibers by Lenz and Witten and also follows some restricted directions.40 For more details on how the angles between the thionine lines were measured and on how the histograms were obtained as well, see Figures S3–S7 and Tables S2–S6 in Supporting Information.


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Figure 5. Normalized distributions of the angles between self-assembled thionine filaments present in the graphene samples functionalized for 80 minutes a, 90 minutes b, 100 minutes c, 120 minutes d and for 180 minutes e. These results show that, if thionine molecules have enough time to settle over the graphene surface, they tend to form well-organized self-assembled filaments whose diameter (or thickness) is compatible with two thionine molecules lying perpendicular to the graphene surface. Moreover, these filaments are aligned in specific directions which follow the symmetry of the graphene surface crystallographic orientation. But how dependent is this configuration on the choice of substrate? Fontes and Neves,44 for instance, have reported that the orientation of phosphonic acid self-assembled bilayers depends strongly on whether the substrate is nonpolar (graphite) or polar (mica). Therefore, we conducted additional experiments using mica as a polar


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substrate to compare the influence of the substrate polarity in the conformation of thionine molecules. Topographic images of distinct mica samples, each one functionalized with a specific functionalization time, are shown in Figs. 6 a–h. The white scale bar in each image represents 500 nm. The average height of thionine structures in each panel, assumed to be the center value of each normal distribution function fitted in the height distributions shown in Fig. S8, is shown in Fig. 6 i. Two distinct ways in which thionine molecules settle over mica are apparent from Fig. 6: they either form film-like structures (seen in some regions of Figs. 6 b, f and h), or, more frequently, self-assembled lines, as observed in most of the images. However, unlike the graphene case, all self-assembled thionine lines in a given mica sample are aligned in a single direction, instead of forming 60º angles with each other. For these mica samples, we observed experimentally that there is a good correlation between a particular line orientation and the direction of the N2 flow used to remove solution excess and dry the samples out. An interesting feature can be seen in Fig. 6 i, which shows that the average thickness of thionine structures over mica does not vary significantly over functionalization time. Indeed, the thickness is fairly constant, with an average value of (0.40 ± 0.02) nm. This result indicates that, regardless the functionalization time, thionine molecules self-assemble onto mica laying parallel to its surface.


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Figure 6. AFM images of the mica samples functionalized with thionine . Images a–h represent, respectively, the samples functionalized for 2, 5, 15, 20, 30, 60, 120 and 180 minutes. The color scale shows the height of the mica substrate and the height of thionine self-assembled structures (1). In all the images, the white scale bars represent 500 nm. i. Average height of thionine structures over mica for each functionalization time (log scale). All height distributions are shown in Supporting Information, as well as the definition of the error bars.

The dependence of molecular orientation on substrate polarity has been previously observed.44 In that case, phosphonic acids with long alkyl chains were found to self-assemble parallel to a nonpolar substrate and perpendicular to a polar substrate. Since thionine is a positively charged molecule, we found the opposite trend with the negatively charged mica surface: strong Coulomb 16 ACS Paragon Plus Environment

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interactions between charged sites keep the molecule close to the polar substrate, rendering a parallel configuration. Alternatively, the absence of this mechanism in graphene allows the molecule to lay perpendicular to the substrate, following crystallographic directions, in a behavior that reflects not only the molecule-substrate interaction, but also the molecule-molecule interaction in the self-assembled structure. An important ingredient in the description of the composite system is related to the question of how the self-assembled structures affect the electronic properties of graphene. To address this issue, we performed electrical transport measurements (graphene resistivity versus gate voltage, ) for multiple devices before and after functionalization with thionine. To each device corresponds a different functionalization time. A typical CVD graphene device built for this purpose is shown in the Supporting Information (Fig. S9), and the electrical transport measurements results are presented in Fig. S10. From Fig. S10 one can see that graphene’s charge neutrality point before functionalization varies significantly between devices. However, Casiraghi et al. showed via a systematic Raman spectroscopy investigation for several monolayer graphene samples, that nominally identical graphene samples produced in the same way need not necessarily have identical initial doping features.45 We found that for all functionalization times (2, 10, 30, 60, 80, 90, 100, 120 and 180 minutes), thionine molecules have promoted an n-type doping effect in graphene. From these data, we have calculated the amount of electron transfer (∆) from thionine to graphene for each functionalization time as shown in Fig. 7 a. Since = − , where is the neutrality point and = 7.2 × 10 ,2 the electron transfer is given by ∆ = ! , being ! the left-shift of the graphene’s neutrality point after the functionalization process (see Fig. S10). The absence of a direct measurement of for our samples results in approximate absolute values 17 ACS Paragon Plus Environment

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of charge transfer. Nevertheless, our data properly reflect the charge transfer and its relation with the functionalization time. The error bars in Fig. 7 a are attributed to an uncertainty of ±0.07 cm2

in the electron transfer (see Supporting Information). An interesting feature in the charge

transfer process is its saturation as a function of functionalization time. This behavior is strongly correlated with the surface coverage, as shown in Fig. 7 b. In other words, the saturation in the coverage leads to the saturation in the charge transfer. (see Supporting Information, Fig, S11, for the image processing methodology used to evaluate the covered area fraction). From 2 minutes of functionalization time up to 180 minutes, the area coverage of graphene increases from ~14% up to ~55%. Therefore, even though the surface coverage saturates at a somewhat low value (a bit more than half of the available graphene surface) in this specific case, it is possible to envisage even higher doping levels. It may be accomplished by an increase in the thionine surface coverage, as we have shown to be plausible for a more concentrated thionine solution (see Fig. S2).


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Figure 7. a Electron transfer from thionine to CVD graphene devices functionalized with thionine as a function of the functionalization time. b Surface coverage with thionine of the exfoliated graphene samples functionalized as a function of the functionalization time. Additionally, Raman spectroscopy measurements were conducted for mechanically exfoliated graphene with different degrees of thionine adsorption in order to provide information into the nature of the interaction of thionine with graphene. Fig. 8 a shows Raman spectra of different graphene samples, each functionalized with thionine for 0 (non-functionalized), 10, 30, 90, 120 and 180 minutes, represented by the black, red, blue, green, purple and orange solid lines, respectively. All the spectra are normalized by the G band of graphene. A first piece of information which may be extracted from the Raman data concerns the type (electron or hole) and amount of doping in graphene, since the positions of graphene G and 2D


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bands are sensitive to shifts in the Fermi level.46 However the changes are known to be negligible in a low doping regime (< 5 × 10 ),46 as it is supposed to be the present case according to the electrical transport measurements previously described (an estimate of 1.3 × 10 for the longest functionalization time – 180 minutes - and highest surface coverage, as shown in Fig. 7 a). This is in full agreement with the results presented in Fig. 8 a, which shows negligible changes in positions of the G and 2D bands upon functionalization (see also Fig. S12 and Tables S10-S14 in Supporting Information). A second important aspect of Raman spectroscopy in graphene is related to the existence of a D band (~1350 ) induced by disorder,47 which can be used to infer whether the interaction with thionine is covalent or non-covalent.48 Fig. 8 a reveals a series of peaks in the Raman spectra, whose intensities increase with functionalization time. Fig. 8 b highlights the Raman spectrum of the sample functionalized for 180 minutes in the range 1100 − 1800, where these new bands become apparent. The positions of the Raman peaks are in very good agreement with the results reported by Hutchinson et al,49 who performed Raman spectroscopy of thionine on gold and assigned the observed bands to specific bonds present in the thionine molecule, as it can be verified in Table S15 of the Supporting Information. In particular, such comparison reveals that the blue (1152 and 1289 ), red (1387 and 1428 ) and orange (1504 ) peaks in Fig. 8 b correspond to thionine C–H, C–N , and NH2 groups, respectively, while the green (1583 ) and pink (1627 ) peaks represent the graphene G band and the thionine C=C bond, respectively. The important point is that, apart from graphene G band, all other peaks seem to have a correspondence with Hutchinson et al. assignments,49 which implies the absence of a defective D band. Physically, it means that the functionalization does not introduce defects in the graphene lattice, confirming the non-covalent interaction hypothesis.48 20 ACS Paragon Plus Environment

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Figure 8. a Raman spectra of six different graphene samples functionalized with thionine for 0 (non-functionalized), 10, 30, 90, 120 and 180 minutes, respectively associated with the black, red, blue, green, purple and orange solid lines. All spectra are normalized by the G band of graphene. b Raman spectrum of the graphene sample functionalized for 180 min (orange solid line in a) in the range 1100 − 1800 , where is possible to observe Raman peaks related to thionine. The blue peaks correspond to C−H bond in thionine, the red peaks correspond to C−N bond, the orange peak is associated with the NH2 functional group, the green peak is related to the G band of graphene, while the pink peak is attributed to the C=C bond in thionine. Inset is shown the thionine molecular structure for comparison.47 In order to corroborate and better understand these experimental results, DFT calculations were performed to determine if a perpendicular orientation for the thionine molecules relative to the graphene sheet is consistent with the experimental height profile and also to show how the 21 ACS Paragon Plus Environment

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functionalization affects the electronic properties of graphene. Fig. 8 a shows the result of a complete geometry relaxation for a pair of thionine molecules on top of graphene. The presence of water vapor in ambient conditions makes it very likely to have water molecules surrounding the system. In particular, hydrogen bonds may connect the two -NH2 ends of the thionine molecules, as shown in Fig. 8 a. We found that the O(H2O)-H(NH2) (d1) and H(H2O)-N(NH2) (d2) distances are both 1.9 Å, consistent with this idea. The total height of the system in this configuration is 2.8 nm, which is in good agreement with the experimental result. Moreover, the calculation suggests a very important feature: the lower thionine molecule interacts with the doped graphene sheet via hydrogen bonds. In fact, the HNH2-Cgraphene distance (d3) is 2.4 Å, closer than it should be expected if only van der Waals interactions were present and consistent with Hmediated interactions. This may result in a higher binding energy per area when more molecules are packed together relative to a parallel van der Waals configuration, which gives a possible explanation for the experimental results. This is indeed what we have found by comparing the binding energy per unit area of thionine molecules interacting with graphene in both configurations, parallel and perpendicular to the carbon sheet. For a single molecule, the binding energies per unit area are degenerate (within less than 1 meV/Å2); for two molecules - one on top of the other in the parallel configuration and one distant 3.35 Å from the other in the perpendicular configuration -, the latter arrangement is more stable than the former by more than 12.5 meV/Å2. Concerning electronic properties of graphene, the experimentally verified fact that the thionine pair highly dopes the graphene sheet (n-type doping) is clearly shown in Fig. 7 9 b, which presents the band structure of the hybrid system featuring the Dirac point downshifted by 0.25 eV. Therefore, the Fermi level is placed inside the conduction band of graphene, which results in 22 ACS Paragon Plus Environment

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the observed n-type doping. This is actually true irrespective of the orientation of the thionine molecules relative to graphene. In fact, planar arrangements lead to similar results, as it can be seen in Figs. S13 and S14 in the Supporting Information. It reinforces the idea that functionalization with thionine may be an important tool for the implementation of nanoelectronic devices through selective doping control.

Figure 9. a Complete geometry relaxation for a pair of thionine molecules on top of graphene and in the presence of water vapor. In this image, d1 represents the O(H2O)-H(NH2) distance, d2 is the H(H2O)-N(NH2) distance, while d3 represents the H(NH2)-Cgraphene distance. b Band structure of the hybrid system shown in a.

CONCLUSIONS The investigation of the interaction of thionine molecules with graphene led to a rich phenomenology with a wide range of potential applications. In addition, we showed that a mechanism mediated by charge transfer and hydrogen bonds is responsible for a self-assembly of thionine molecules arranged in filaments which follow crystallographic directions in the underlying graphene sheet, laying perpendicular to it. This conclusion was reached after a systematic analysis of topographic images, electric transport measurements and first principles 23 ACS Paragon Plus Environment

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calculations. The composite system may be regarded as a novel hybrid material, in which the graphene is subjected to a strong n-doping effect and induces specific assembly pathways to thionine. As a perspective, the present study suggests that the presence of a self-assembled thionine structure on top of a bilayer graphene may break the symmetry between layers, opening up an energy gap at the Fermi level. Overall, the results suggest that thionine-graphene compound may play an important role in the construction of nanodevice circuits. Additionally, adequate choices of thionine solution’s concentrations may promote larger surface coverages, which may be important in applications related to biosensing.


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ASSOCIATED CONTENT Supporting Information Exfoliated graphene samples functionalized with thionine for 10 and 30 seconds, with a 1 mM concentration solution. Histograms of angles between thionine lines in graphene samples functionalized for 80, 90, 100, 120 and 180 minutes. Height distributions of thionine molecules over mica samples functionalized for 2, 5, 15, 20, 30, 60, 120 and 180 minutes. Optical image of our typical CVD graphene device, a schematic illustration of the device structure and ohmic behavior of the devices. Electrical transport measurements (ρ x VG) performed on CVD graphene devices before and after functionalization with thionine for 2, 10, 30, 60, 80, 90, 100, 120 and 180 minutes. Percentage surface coverage of the exfoliated graphene samples functionalized with thionine for 2, 10, 30, 60, 80, 90, 100, 120 and 180 minutes. Raman spectroscopy on mechanically exfoliated graphene samples functionalized with thionine for 0, 10, 30, 90, 120 and 180 minutes. DFT calculations for five different planar arrangements of thionine on the graphene surface.

AUTHOR INFORMATION Corresponding Author *E-mail address for corresponding authors: [email protected] Author Contributions FP proposed the project. BRAN designed the AFM experiments. TASLS and TFDF carried out the AFM experiments. TASLS and ENDA carried out the electric measurements, including sample fabrication. TASLS and FP carried out the Raman spectroscopy measurements. MJSM


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and MSCM performed the ab initio calculations. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Brazilian funding agencies CNPq, CAPES and FAPEMIG, by the National Institute of Science and Technology of Carbon Nanomaterials, INCT-NanoCarbono, by the Banco Nacional de Desenvolvimento Ecônomico e Social, BNDES, and by SEVA Engenharia S/A.


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REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666-669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Tw-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197-200. (3) Wang, Q. H.; Hersam, M. C. Room-Temparature Molecular-Resolution Characterization of Self-Assembled Organic Monolayers on Epitaxial Graphene. Nat. Chem. 2009, 1, 206-211. (4) Gobbi, M.; Bonacchi, S.; Lian, J. X.; Liu, Y.; Wang, X-Y.; Stoeckel, M-A.; Squillaci, M. A.; D’Avino, G.; Narita, A.; Müllen, K.; Feng, X.; Olivier, Y.; Beljonne, D.; Samorì, P.; Orgiu, E. Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene. Nat. Commun. 2017, 8, 1-8. (5) Zhang, T.; Cheng, Z.; Wang, Y.; Li, Z.; Wang, C.; Li, Y.; Fang, Y. Self-Assembled 1Octadecanethiol Monolayers on Graphene for Mercury Detection. Nano Lett. 2010, 10, 47384741. (6) Deshpande A.; Sham, C-H.; Alaboson, J. M. P.; Mullin, J. M.; Schatz, G. C.; Hersam, M. C. Self-Assembly and Photopolymerization of Sub-2 nm One-Dimensional Organic Nanostructures on Graphene. J. Am. Chem. Soc. 2012, 134, 16759-16764.


Langmuir 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 28 of 34

(7) Alaboson, J. M. P.; Sham, C-H.; Kewalramani, S.; Emery, J. D.; Johns, J. E.; Deshpande, A.; Chien, T. Y.; Bedzyk, M. J.; Elam, J. W.; Pellin, M. J.; Hersam, M. C. Templating Sub-10 nm Atomic Layer Deposited Oxide Nanostructures on Graphene via One-Dimensional Organic Self-Assembled Monolayers. Nano Lettt. 2013, 13, 5763-5770. (8) Li, B.; Tahara, K.; Adisoejoso, J.; Vanderlinden, W.; Mali, K. S.; de Gendt, S.; Tobe, Y.; de Feyter, S. Self-Assembled Air-Stable Supramolecular Porous Networks on Graphene. ACS Nano 2013, 7, 10764-10772. (9) MacLeod, J. M.; Rosei, F. Molecular Self-Assembly on Graphene. Small 2014, 10, 10381049. (10) Prado, M. C.; Nascimento, R.; Moura, L. G.; Matos, M. J. S.; Mazzoni, M, S, C.; Cancado, L, G.; Chacham, H.; Neves, B. R. A. Two-Dimensional Molecular Crystals of Phosphonic Acids on Graphene. ACS Nano 2011, 5, 394-398. (11) Lee, B.; Chen, Y.; Duerr, F.; Mastrogiovanni, D.; Garfunkel, E.; Andrei, E. Y.; Podzorov, V. Modification of Electronic Properties of Graphene with Self-Assembled Monolayers. Nano Lett. 2010, 10, 2427-2432. (12) Hsu, C-L.; Lin, C-T. Huang, J-H.; Chu, C-W.; Wei, K-H.; Li, L-J. Layer-by-Layer Graphene/TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells. ACS Nano 2012, 6, 5031-5039. (13) Dong, X.; Fu, D.; Fang, W. Shi, Y.; Chen, P.; Li, L-J. Doping Single-Layer Graphene with Aromatic Molecules. Small 2009, 5, 1422-1426.


Page 29 of 34 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

Langmuir

(14) Cervenka, J.; Budi, A.; Dontschuk, N.; Stacey, A.; Tadich, A.; Rietwyk, K. J.; Schenk, A. Edmonds, M. T.; Yin, Y.; Medhekar, N.; Kalbac, M.; Pakes, C. I. Graphene field effect transistors as a probe of electronic structure and charge transfer at organic molecule-graphene interfaces. Nanoscale 2015, 7, 1471-1478. (15) Knizia, G.; Klein, J. E. M. N. Electron Flow in Reaction Mechanisms-Revealed from First Principles. Angew. Chem., Int. Ed. 2015, 54, 5518-5522. (16) Li, Q.; Zhang, J.; Yan, H.; He, M.; Liu, Z. Thionine-mediated chemistry of carbon nanotubes. Carbon 2004, 42, 287-291. (17) Wang, Z.; Li, M.; Zhang, Y.; Yuan, J.; Shen, Y.; Niu, L.; Ivaska, A. Thionine-interlinked multi-walled carbon nanotube/gold nanoparticle composites. Carbon 2007, 45, 2111-2115. (18) Jeykumari, D. R. S.; Ramaprabhu, S.; Narayanan, S. S. A thionine functionalized multiwalled carbon nanotube modified electrode for the determination of hydrogen peroxide. Carbon 2007, 45, 1340-1353. (19) Shahrokhian, S.; Asadian, E. Simultaneous voltametric determination of ascorbic acid, acetaminophen and isoniazid using thionine immobilized multi-walled carbon nanotube modified carbon paste electrode. Electrochim. Acta 2010, 55, 666-672. (20) Salimi, A.; Noorbakhsh, A.; Ghadermarzi, M. Amperometric detection of nitrite, iodate and periodate at glassy carbon electrode modified with catalase and multi-wall carbon nanotubes. Sens. Actuators, B 2007, 123, 530-537.


Langmuir 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 30 of 34

(21) Shahrokhian, S.; Zare-Mehrjardi, H. R. Application of thionine-nafion supported on multiwalled carbon nanotube for preparation of a modified electrode in simultaneous voltametric detection of dopamine and ascorbic acid. Electrochim. Acta 2007, 52, 6310-6317. (22) Chen, C.; Zhai, W.; Lu, D.; Zhang, H.; Zheng, W. A facile method to prepare stable noncovalent functionalized graphene solution with thionine. Mater. Res. Bull. 2011, 46, 583-587. (23) Yang, M.; Javadi, A.; Li, H.; Gong, S. Ultrasensitive immunosensor for the detection of cancer biomarker based on graphene sheet. Biosens. Bioelectron. 2010, 26, 560-565. (24) Han, J.; Ma, J.; Ma, Z. One-step synthesis of graphene oxide-thionine-Au nanocomposites and its applications for electrochemical immunosensing. Biosens. Bioelectron. 2013, 14, 243247. (25) Zhu, L.; Luo, L.; Wang, Z. DNA electrochemical biosensor based on thionine-graphene nanocomposite. Biosens. Bioelectron. 2012, 35, 507-511. (26) Yu, S.; Wei, Q.; Du, B.; Wu, D.; Li, H.; Yan, L.; Ma, H.; Zhang, Y. Label-free immunosensor for the detection of kanamycin using Ag@Fe3O4 nanoparticles and thionine mixed graphene sheet. Biosens. Bioelectron. 2013, 48, 224-229. (27) Guo, L.; Zhang, Q.; Huang, Y.; Han, Q.; Wang, Y.; Fu, Y. The application of thioninegraphene nanocomposite in chiral sensing for Tryptophan enantiomers. Bioelectrochemistry 2013, 94, 87-93.


Page 31 of 34 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

Langmuir

(28) Sun, Z.; Fu, H.; Deng, L.; Wang, J. Redox-active thionine-graphene oxide hybrid nanosheet: One-pot, rapid synthesis, and application as a sensing platform for uric acid. Anal. Chim. Acta 2013, 761, 84-91. (29) Leon, J. A.; Alves, E. S.; Elias, D. C.; Brant, J. C.; Barbosa, T. C.; Malard, L. M.; Pimenta, M. A.; Ribeiro, G. M.; Plentz, F. Rapid Fabrication of Bilayer Graphene Devices Using Direct Laser Writing Photolithography. J. Vac. Sci. Techol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2011, 29, 021204. (30) Suk, J. W.; Kitt, A.; Magnuson, C. W.; Hao, Y.; Ahmed, S.; An, J.; Swan, A. K.; Goldberg, B. B.; Ruoff, R. S. Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates. ACS Nano 2011, 5, 6916-6924. (31) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133. (32) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA Method for ab initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745-2779. (33) Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B, I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. (24) Thonhauser, T.; Cooper, V. R.; Li, S.; Puzder, A.; Hyldgaard, P.; Langreth D. C. Van der Waals Density Functional: Self-Consistent Potential and the Nature of the van der Waals Bond. Phys. Rev. B 2007, 76, 125112.


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Page 32 of 34

(35) Román-Pérez, G.; Soler, J. M. Efficient Implementation of a van der Waals Density Functional: Application to Double-Wall Carbon Nanotubes. Phys. Rev. Lett. 2009, 103, 096102. (36) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev B 1993, 43, 1993-2006. (37) Kleinman, L.; Bylander, D. M. Efficacious Form for Model Pseudopotentials. Phys. Rev. Lett. 1982, 48, 1425. (38) Doudevski, I.; Haynes, W. A.; Schwartz, D. K. Submonolayer Island Nucleation and Growth Kinetics during Self-Assembled Monolayer Formation. Phys. Rev. Lett. 1998, 81, 4927. (39) Doudevski, I.; Schwartz, D. K. Dynamic scaling of the submonolayer island size distribution during self-assembled monolayer growth. Phys. Rev. B 1999, 60, 14-17. (40) Lenz, M.; Witten, T. A. Geometrical frustration yields fibre formation in self-assembly. Nat. Phys. 2017, 13, 1100-1104. (41) Grason, G. M. Self-assembly: Misfits unite. Nat. Phys. 2017, 13, 1149-1150. (42) Bartelt, M. C.; Evans, J. W. Scaling analysis of diffusion-mediated island growth in surface adsorption processes. Phys. Rev. B 1992, 46, 12675. (43) Amar, J. G.; Family, F.; Lam, P-M. Dynamic scaling of the island-size distribution and percolation in a model of submonolayer molecular-beam epitaxy. Phys. Rev. B 1994, 50, 8781. (44) Fontes, G. N.; Neves, B. R. A. Effects of Substrate Polarity and Chain Length on Conformational and Thermal Properties of Phosphonic Acid Self-Assembled Bilayers. Langmuir 2005, 21, 11113-11118. 32 ACS Paragon Plus Environment

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Langmuir

(45) Casiraghi, C.; Pisana, S.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Raman fingerprint of charged impurities in graphene. Appl. Phys. Lett. 2007, 91, 233108. (46) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U, V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat. Nanotechnol. 2008, 3, 210-215. (47) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (48) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; Zhang, H.; Shepperd, K.; Hicks, J.; Sprinkle, M.; Berger, C.; Lau, C. N.; deHeer, W. A.; Conrad, E. H.; Haddon, R. C. Spectroscopy of Covalently Functionalized Graphene. Nano Lett. 2010, 10, 4061-4066. (49) Hutchinson, K.; Hester, R. E.; Albery, W. J.; Hillman, A. R. Raman Spectroscopic Studies of a Thionine-modified Electrode. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2053-2071.


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Thionine Self-Assembled Structures on Graphene: Formation

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