Graphene Nanoribbon Thin Films Using Layer-by-Layer Assembly

Oct 15, 2010 - Described here is a room temperature procedure to fabricate graphene nanoribbon (GNR) thin films. The GNRs, synthesized by unzipping ca...
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Graphene Nanoribbon Thin Films Using Layer-by-Layer Assembly Yu Zhu and James M. Tour* Departments of Chemistry and Mechanical Engineering and Materials Science and the Smalley Institute for Nanoscale Science and Technology, Rice University, MS 222, 6100 Main Street, Houston, Texas 77005, United States ABSTRACT Described here is a room temperature procedure to fabricate graphene nanoribbon (GNR) thin films. The GNRs, synthesized by unzipping carbon nanotubes, were reduced and functionalized. The functionalized GNRs are negatively or positively charged, which are suitable to assemble thin films by electrostatic layer-by-layer absorption. The homogenous full GNR films were fabricated on various substrates with controllable thicknesses. By assembling the GNRs films on silicon oxide/silicon surfaces, bottomgated GNR thin-film transistors were fabricated in a solution processed technique. KEYWORDS Graphene nanoribbons, thin film, layer-by-layer, transistor

solubility of the f-GNRs is still only ∼0.5 mg/mL.21 This solubility is high enough for characterization and for some applications such as forming graphene/polymer composites. But it is still difficult to form homogeneous f-GNRs films by spin-coating due to the higher concentration required by this technique where typically over 10 mg/mL is required. Other film-formation techniques such as spray coating, drop casting, blade coating, and filtration, though possible, are sometimes undesired due to the film morphology obtained or the cumbersome transfer techniques necessary to make films. In this work, we used the layer-by-layer assembly method22 to prepare full f-GNR films. This method is advantageous because it can be used to assemble ultrathin films of a variety of organic and inorganic compounds in a simple and inexpensive manner, with thickness control in the nanometer range. Most importantly, it works well with the solubility of the present f-GNRs. In this work, the f-GNRs were functionalized with anionic and cationic moieties. The full GNR thin films were then fabricated by the layer-by-layer assembly technique at room temperature. The electronic properties of the thin films were recorded. GNRs were prepared by using the reported carbon nanotube unzipping technique where multiwalled carbon nanotubes were treated with potassium permanganate in acid.8,9 This procedure, as a high-throughput method, exhibits advantages over other methods10,13,23,24 in situations where large quantities are required. Solution-phase hydrazine reduction of surfactant-wrapped graphene oxide nanoribbons was followed by diazonium chemistry based functionalization to prepare charged f-GNRs. The functionalization route is shown in Scheme 1.20,21 X-ray photoelectron spectroscopy (XPS) was used to verify the success of functionalization reactions. Previous work9,16,17,20,28 indicated that oxygen-containing groups on GNRs are significantly removed by hydrazine treatment. Similar results were observed by high-resolution XPS C 1s

G

raphene nanoribbons (GNRs), which gradually transform from semiconducting to semimetals as their width increases,1-7 represent a particularly versatile variety of graphene. Recently, the longitudinal unzipping of nanotubes to form GNRs was discovered, which provided a new way to produce GNRs on a large scale.8,9 Due to their high surface areas, high aspect ratios, and interesting electronic properties, GNRs are promising candidates for applications in composite materials, field-effect transistors (FETs), transparent electrodes, hydrogen storage media, and other semiconductor devices. FETs made from single GNRs have been constructed using lithographic methods.10-13 However, the production of a GNR film, which might be a promising thin film transistor (TFT) material, has yet to be reported. The limited yield of lithographically derived GNRs, and the highly oxidized state of unzipped GNRs, are the main obstacles to fabricating GNR TFTs. Due to processing difficulties of pristine graphene, solution-processed graphene functional thin films were fabricated by using either graphene oxide14-17 or graphene/polymer composites.18,19 In those cases, high-temperature annealing (∼1100 °C)14-16 or anhydrous hydrazine reduction17 were used to convert the surface-bound graphene oxide to the more conductive chemically converted graphene. These methods, however, are unlikely to be compatible with fabrication techniques used for most electronic applications. Hence, more processable GNRs are studied here for fabricating full GNR thin films under mild condition and their applications as FET active layers. Covalent functionalization of graphene and GNRs was demonstrated in our previous work.20,21 The functionalized GNRs (f-GNRs) are soluble and free of most oxygen-containing groups. Unfortunately, even after functionalization, the * Corresponding author. E-mail: [email protected]. Received for review: 05/12/2010 Published on Web: 10/15/2010 © 2010 American Chemical Society

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SCHEME 1.

(a) Preparation of Diazonium Salts21,25,26 and (b) Preparation of Functionalized Graphene Nanoribbonsa 27

a The oxidized starting material nanoribbons bear oxygen-containing functionalities such as carbonyls, carboxyls, and hydroxyls (not shown) and were surfactant-treated with sodium dodecyl sulfate (SDS). Reduction with hydrazine followed by covalent attachment of the aryl moieties was not limited to the edges of the nanoribbons as depicted for functionalized products f-GNR1 and f-GNR2; attachment at the basal plane is also expected though a higher majority tend toward the edges.

spectra of the f-GNRs, which show a significant decrease of signals at 286-288 eV, indicating the loss of C-O and CdO functionalities (Figure 1a). Upon treatment with the diazonium salts, a substantial percentage of sulfur (for benzenesulfonic acid group-containing f-GNR1) and nitrogen (for anilinic group containing f-GNR2) are detected, implying that the surface has been successfully functionalized (Figure 1b-d). High-resolution XPS of f-GNRs give the atomic percentages shown in Table 1. Considering that the sulfonic group is an oxygen-containing group (with S:O ) 1:3), the residual surface oxygen (not belonging to the addends) after functionalization was ∼13% in both f-GRNs. The surface oxygen is thought to be due to the edge carboxyl units, which undergo little reductive loss upon hydrazine treatments.9 The functional groups render the two f-GNRs soluble in polar solvents. f-GNR1 is readily soluble in water with a solubility up to 0.5 mg/mL (in pure water it is anionic due to the acidity of the -SO3H moiety), while f-GNR2, although not soluble in water directly, can be protonated to form a © 2010 American Chemical Society

clear solution in a mixture of acid and DMF, rendering both f-GNRs suitable for electrostatic layer-by-layer deposition.22 IR and Raman spectra were used to confirm successful functionalization of the GNRs. Attenuated total reflectance infrared (ATR-IR) spectra of functionalized GNRs are shown in Figure 2. The signal at 3250 cm-1 in the spectrum of f-GNR2 belongs to the NH2 addend as the double peak is characteristic of a primary amine. The peaks at 1600-1650 cm-1 are signals from the bending of the N-H. In the ATRIR spectrum of f-GNR1, the broad peak at 3100-3400 cm-1 indicates the presence of a OH due to the sulfonic acid moiety. The sulfonic group stretch appears at ∼1200 cm-1, which is difficult to identify in the spectrum due to its location in the fingerprint region of the IR spectrum. The spectrum of the reduced GNRs (not shown here),9 however, is devoid of any informative signal between 1500 and 4000 cm-1. The Raman spectra of bulk f-GNRs and reduced GNRs using 633 nm laser excitation (Figure S1 in Supporting Information) show a profile similar to that of the previously reported functionalized graphene and GNRs.21 The diamon4357

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FIGURE 1. (a) High-resolution XPS C 1s spectra of oxidized GNRs (red), reduced GNRs (black), f-GNR1 (blue), and f-GNR2 (green) showing significant loss of CsO and CdO groups after reduction. (b, c) High-resolution XPS S 2p and O 1s spectra of f-GNR1 showing evidence of a sulfonic group. (d) High-resolution XPS N 1s spectrum of f-GNR1 showing evidence of the aniline moiety. The base pressure of the system was at 5 × 10-9 Torr. A monochromatic Al X-ray source at 100 W was used with a pass energy of 26 eV and with a 45° takeoff angle. The beam diameter was 100.0 µm. Binding energy values were referenced externally to a gold 4f peak at 84.00 eV and internally to a carbon 1s binding energy of 280.50 eV (NIST XPS Database). TABLE 1. Atomic Concentration of Different GNRs and f-GNRs Filma atomic concentration by XPS samples

C (%)

N (%)

O (%)

S (%)

GO ribbons reduced GNR f-GNR1 f-GNR2 f-GNR film

50.2 79.1 77.4 78.9 69.3

1.3 3.5 7.9 3.9

35.8 17.3 20.2 13.2 24.8

2.4 2.0

a The data for GO ribbons and reduced GNR are from the literature.9 The dash signifies that the value was 1 µm in 4358

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FIGURE 3. AFM and SEM images of f-GNR1. (a) AFM image of a homogenator-cut f-GNR1 and (b) its amplitude image for the same ribbon. (c, d) Height profiles along the long and short directions, respectively, indicated in the AFM image. (e, f) Monolayer ribbons with sharp edges and identical contrast. (g-j) Few-layered ribbons exhibit distorted edges and varied contrast in different regions. The scale bars in the SEM images are 100 nm. The original length of the ribbons can be ∼5 µm, but they are cut during the homogenization step.8,9

length with the height between 1.5 and 3 nm. The theoretical height for a graphene sheet functionalized on both sides is ∼2.2 nm, assuming that the height of the bare graphene sheets is 1 nm16,30,31 with the substituted aromatic groups contributing 0.6 nm per side in height. On the basis of this, the ribbon in Figure 3a is single- or double-layered GNR. The SEM imaging (Figure 3e-j) emphasizes the high aspect ratio ribbon structure of the f-GNRs. The typical nanoribbons have lengths of 1-2 µm and widths from 80 to 320 nm. Panels e and f of Figure 3 show monolayer nanoribbons. Longer fewlayered ribbons such as those found in panels g-j of Figures 3 are representative of many that were imaged, with lengths >2 µm and widths 100-300 nm. For film formation, pretreated quartz substrates and silicon wafer supports32 were used (for detailed pretreatment procedure, see the Supporting Information). The dipping procedures are shown in Scheme 2a. The pretreated supports were first dipped into a solution of 0.1 mg/mL anionic f-GNR1 in water (step I). The GNRs were adsorbed on the positively charged amino-functionalized surface (see Supporting Information), and the surface charge was inverted. After immersion for 3 min, the substrate was removed from the solution and soaked in a water bath twice each for 1 min (steps II and III). After the coated substrate was blown dry by nitrogen and baked in a warm chamber for 1 min (step IV), it was dipped into a solution of f-GNR2 in a 9:1 v/v mixture of N,N-dimethylformamide (DMF) and 6 M HCl (step V). The dipping process produced a complex formation of f-GNR1 and f-GNR2, and the first bilayer of f-GNRs was adsorbed. After 3 min, the substrate was removed from the flask and soaked in DMF (step VI), followed by water (step VII), for 1 min each, and then the substrate was dried (step VIII). Dipping the substrate into the f-GNR1 solution, step I, © 2010 American Chemical Society

began the deposition process again. After film formation, the film was finally dried by heating in a vacuum oven at 65 °C overnight. The sequence of steps depicted in Scheme 2a led to adsorption of the f-GNRs films. The complex formation is illustrated in Scheme 2b. The process described above was performed by an Asymtek automatic dispensing system. The successful formation of the film is indicated by the UV-vis absorption spectrum of the adsorbed film on quartz. The spectra in Figure 4a indicate that the absorption bands at 292 nm grow continuously in intensity when the number of dipping cycles is increased. Ellipsometry data were collected when the film was grown on a silicon wafer (Figure 4b). After 80 dipping cycles, the thickness of the f-GNR film was approximately 80 nm, that is, each cycle deposited a film ∼1 nm thick, on average. High-resolution XPS analysis (Figure S2 in the Supporting Information) of the film exhibits the atomic percentages shown in Table 1. Compared to the XPS data for f-GNR1 and f-GNR2, the oxygen concentration was slightly increased, which may be due to trapped water. Atomic force microscopy (AFM) indicates a smooth and homogeneous surface structure, with few aggregates distributed over the substrate (Figure 5, panels a and b). The thickness of the film was measured by AFM (Figure 5e), and the results are consistent with the ellipsometry data of 80 nm for 80 dipping cycles. The thin f-GNRs film on the silicon wafer substrate was patterned into stripes by using photolithography and an oxygen plasma etching. Standard e-beam lithography was used to define the source and drain electrodes. The fabrication process is shown in Figure 6a. A scanning electron 4359

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SCHEME 2. (a) The Layer-by-Layer Assembly Procedure of f-GNRsa and (b) An Illustration of the Complex Formation between f-GNR1 and f-GNR2b

a The pretreated substrates (quartz or Si wafer) are taken through the different deposition steps in order from I to VIII. After each cycle from I to VIII, bilayers of f-GNRs were adsorbed on the surface. b The pretreated substrates are positively charged. The red and blue layers represent the negative charged f-GNR1 and positive charged f-GNR2, respectively.

FIGURE 4. (a) Absorbance spectrum of a f-GNRs thin film on quartz. There is an automated grating change at ∼800 nm. (b) f-GNRs film thicknesses measured by ellipsometry. Each cycle described in Scheme 2a produced one bilayer.

microscopy (SEM) image of the device is shown in the inset of Figure 6b. The thin-film FET properties of the f-GNR film devices were further characterized at room temperature under high vacuum (10-5 Torr) to minimize adsorption-induced effects from air exposure. Unlike many ambipolar graphene or graphene oxide FETs,15 the f-GNR thin film FETs show only p-type behavior. The gradual decrease of the magnitude of the gate voltage in the negative region (from -40 to 20 V) leads to the decrease of the conductivity (Figure 6b). Even after being kept in a vacuum chamber for 48 h, there is still no obvious neutrality point for drain-source current in the observed range (-40 V < Vg < 40 V), which indicates the intrinsic p-type state of the f-GNRs film. The total effect of all functional groups contributing to the GNRs thin film, © 2010 American Chemical Society

therefore, could be regarded as p-doping. The conductivity of the f-GNRs film is much better than that of the graphene oxide based devices,12,15 but still a few orders of magnitude lower than those from the graphene devices based on the Scotch-tape derived graphene, chemical vapor deposition of graphene, or epitaxially grown graphene.33-36 For the sample shown in Figure 6b, the transconductance, ∆Isd/∆Vg, is 4.5 × 10-8 A/V in the linear Isd-Vg regime for hole mobility. Under a source-drain bias voltage of 40 V, it is estimated that the hole mobility µh is 0.2 cm2/(V s). The typical values of the f-GNRs film devices fabricated in this work are between 0.1 and 0.5 cm2/(V s). The mobility is comparable with typical graphene oxide devices after high temperature annealing. As shown in Figure 6c, the Ids-Vds behavior of the devices shows significant nonlinearity, which is a typical 4360

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FIGURE 5. AFM images of a f-GNR film. (a) A 5 µm2 film presented in a three-dimensional profile. (b) A 2 µm2 film presented in a threedimensional profile. (c) An AFM image of the edge of the film formed after 80 dipping cycles. The edge has been formed by photolithography and oxygen plasma etching. (d) An AFM amplitude image of the edge of the film. (e) A height profile of the edge of the film. (f) A threedimensional presentation of the film edge.

FIGURE 6. (a) Schematic process of f-GNRs thin-film device fabrication. (b) Ids-Vg curves recorded at Vds ) 40 V for the device. The inset shows the SEM image of the device, where the two Pt electrodes are aligned vertically and the f-GNR thin film is perpendicular to the electrodes. (c) Ids-Vds curve at Vg ) 0 V shows significant nonlinear behavior.

many layered graphene devices,15 is small (Figure S3 in the Supporting Information). Nevertheless, the use of functionalized graphene nanoribbons improved the process ability of graphene-based material. The thin-film GNR devices could be fabricated under ambient conditions, a positive attribute for many flexible-substrate-based applications. In summary, we have synthesized f-GNRs with negative or positively charged groups. By using electrostatic layer-bylayer deposition, the homogenous full GNR films were fabricated on quartz and silicon wafer substrates with controllable thicknesses. Bottom gated thin film FETs were

characteristic of a semiconductor. Although the GNRs used in this work have large widths (over 100 nm), which excludes the possibility of generating band gaps through quantum confinement, the introduction of functional groups on the edge and basal surface of the GNRs can change their electronic properties. Previous research37 suggested graphene oxide has a band gap due to the existence of oxygencontaining groups, which distort the lattice, break the symmetry of the system, and induce a band gap. Likewise, the I-V curves from the f-GNRs films have a semiconductor behavior. However, the on/off ratio of the FETs, as with © 2010 American Chemical Society

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fabricated and exhibited mobilities of 0.1-0.5 cm2/(V s), which is comparable with typical graphene oxide devices prepared after high-temperature annealing. The entire fabrication procedure was operated under ambient conditions, which renders this an interesting technique to prepare graphene-based electronics on various flexible substrates.

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Acknowledgment. The authors thank P. Cox, J. Lomeda, A. Maier, and B. Chen for helpful discussions. The work was funded by the Office of Naval Research Graphene MURI program (00006766), the Air Force Research Laboratory through University Technology Corporation, 09-S568-06401-C1, and the Air Force Office of Scientific Research, FA9550-09-1-0581. Supporting Information Available. Experimental procedures, Raman and XPS spectra, I-V curves, and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2)

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