Large Area, Multilayer Graphene Films as a Flexible Electronic Material.

Chemically reduced graphene oxide possesses unique properties and leads to a secure processing method for many applications. The electrical and optical properties of graphene oxide are strongly dependent on the chemical and atomic structure. In the present work, the reduction of synthesized multilayer graphene oxide sheets by both chemical and thermal methods to use them as a substrate in the field of molecular electronic device fabrication is reported. 1-Dodecanethiol molecules are used to covalently bond on the surface atoms of reduced graphene oxide to constitute molecular electronic devices. The metal-organic molecules-reduced graphene oxide-metal junctions show a significant reduction in current levels and weak diode behavior. The observations confirm the tunneling as the conduction mechanism. The sheets are low cost, highly flexible, and can be used as a substrate to build the molecular electronic junctions.

Introduction

Rapid development in molecular electronics yields an entirely new class of electronic devices with unique functionalities than conventional devices.[1] Substrates like silicon, indium tin oxide (ITO), and gold have been used for the development of electronic devices, wherein specific organic molecules are covalently bound to form a monolayer on the surface of substrates.[2] It is known that multilayer graphene can be used as an electrode/substrate material in various applications.[3] Graphene is a 2-D material with a single layer of carbon atoms arranged in a honeycomb structure and is extensively used to describe properties of many carbon-based materials, including graphite, fullerenes, and nanotubes.[4] Graphene is widely used in displays,[5] chemical/biological sensors,[6] energy conversion/storage,[7] thin-film transistors,[8] nanoelectrochemical resonators,[9] and nonvolatile memories due to its carrier mobility and a range of unusual phenomena arising from the linear energy dispersion.[10] Charge carriers in an ideal graphene sheet behave as massless Dirac fermions and hence lead to high mobility. Graphene showed remarkable mechanical, thermal, chemical, and optical properties.[10]

The synthesis of graphene oxide and its conversion to multilayer graphene is by the colloidal route, and it is an attractive approach for several reasons. First, natural graphite is an inexpensive and ubiquitous resource to produce low-cost materials. Second, the average yield in a reaction is relatively high (more than 95%). Third, the solution phase exfoliation generates more functionality for the organic moieties on the edges/surfaces. It provides a better way to create hybrid materials for various applications. Fourth, excellent colloidal dispersion of graphene oxide in different aqueous and non-aqueous solvents permits fabrication of the paper-like films that possess high mechanical flexibility and tunable optoelectronic properties in an effortless way. Such films find numerous applications as transparent conductors,[5] chemical/biological sensors,[6] electrode materials for energy conversion and storage,[7,11−15] thin-film transistors,[8] nanoelectromechanical resonators, nonvolatile memories, and so forth.[9] The surface atoms of carbon materials are well known for their affinity toward other organic/biological molecules by covalent bonding. The present work aims to synthesize flexible and multilayer reduced graphene oxide sheets and their surface decoration with 1-dodecanethiol to constitute molecular electronic devices to understand transport phenomena by current–voltage characterizations. The self-assembly of these molecules in a large area enables chemical characterization to understand the structure–function or charge transport mechanisms. The significant aspect is the notion of self-assembly or a thermodynamically driven process that can lead to perfect ordering.

Results and Discussions

The synthesized graphene oxide sheet showed a sheet resistance of 2 MΩ/cm2 because of the functional groups attached to the atoms of carbon in graphene oxide and the trapped water molecules between the graphene oxide layers. These functional groups and water molecules are removed by thermal reduction. Chemical reduction is another method to reduce the graphene oxide, which involves the exposure of the graphene oxide sheet to reducing chemicals like hydrazine,[16] hydrides,[17] hydroquinone,[18] and phenylenediamine.[19]

In the present work, hydrazine hydrate is used as a reducing agent for the chemical reduction of thermally reduced graphene oxide sheets. Hydrazine is useful for the removal of in-plane functional groups such as epoxy and hydroxyls but leaves the edge moieties such as carboxyl and carbonyl intact. The properties of graphene oxide (GO) and reduced graphene oxide (RGO) films are possible to tune by varying the coverage of sheets, film thickness, chemical composition, average flake size, and film morphology. The observed sheet resistance is 1.12 kΩ/cm2 after the reduction by both thermal and chemical methods.

Reduced graphene oxide sheets are characterized by scanning electron microscopy (SEM) (JS-6360, JEOL, Japan) and X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) equipped with Cu Kα radiation at a scan rate of 2°/min in the 10–60° range. The FT-IR spectra are recorded on a PerkinElmer Frontier MIR/FIR spectrometer, USA in the form of GO, RGO, 1-dodecanethiol (1-DT), and 1-dodecanethiol-treated reduced graphene oxide (1-DT + RGO) in a KBr pellet. Figure a shows the SEM image of the graphene oxide sheet before the reduction. Figure b shows the highly porous surface and reveals the crumpled/rippled morphology of reduced graphene oxide nanosheets. The reduced graphene oxide nanosheets are layer structured, irregular, folded, and entangled with each other and shown in the inset of Figure b. Such surface morphology of reduced graphene oxide sheets is reported in the literature.[20,21]

Figure 1

Scanning electron microscopy images of the (a) pristine graphene oxide sheet, (b) reduced graphene oxide (inset: at higher magnification), and (c) X-ray diffraction pattern of graphene oxide (GO) and reduced graphene oxide (RGO).

The transformation of GO to RGO by thermal and chemical reduction is confirmed using XRD profiles, as shown in Figure c. For GO (red line), peaks appear at 9.54°, which are assigned to (002) reflection. The small peaks observed at 20.1 and 25.5° indicate that GO is not fully interconnected with oxygen atoms. In the case of RGO (black line), a broad diffraction peak (002) is shifted to a higher angle of 24.11° due to the short-range order in stacked stacks. The broadening of the peak from 24 to 26.58° is due to the short-range order in stacked layers. It confirms that the reduction methods used in this work help remove the oxygen functional groups for the preparation of multilayer graphene from graphene oxide. Similar patterns of X-ray diffraction have been reported in the literature.[22]

Figure a shows the FT-IR spectra of (i) GO (black line), (ii) RGO (red line), (iii) 1-DT (blue line), and (iv) 1-DT + RGO (magenta line). In Figure a (i), the GO curve shows that the absorbance peaks between 800 and 1330 cm–1 were attributed to a C–O in C–OH or C–O–C functional groups, the absorption band at 1632.4 cm–1 is attributed to the asymmetric vibrations of the carboxylate groups, and broad bands at 3000–3700 cm–1 were attributed to the presence of the free and associated hydroxyl groups due to the absorbed and inhibited water molecules that broadens the band associated with C–OH at ∼3000–3700 cm–1. The Figure a (ii) RGO curve shows the peak at ∼620 cm–1 attributed to a C–O functional group; decomposition of carbonyls is minor as evidenced by a small loss of infrared absorbance at ∼1632.2 and ∼3000–3700 cm–1; peak ∼2800 cm–1 is for −CH2 stretching, and ∼2900 cm–1 corresponds to stretching and bending of −CH3; and the C–OH and COOH bands are narrowed because of the evaporation of water molecules. The Figure a (iii) 1-DT curve shows the peak at ∼1466 cm–1 attributed to a carbonyl group, ∼2600 cm–1 corresponds to the S–H functional group, peak ∼2862 cm–1 is for −CH2 stretching, and ∼2934 cm–1 corresponds to stretching and bending of −CH3. The Figure a (iv) 1-DT + RGO curve shows all the peaks of RGO except the peak at ∼1200 cm–1, which attributes to thioketals C=S formed between the carbonyl group in RGO and −SH of dodecanethiol. This peak is not appeared in 1-DT confirms the covalent interaction between 1-DT and RGO.

Figure 2

(a) FT-IR spectra of (i) GO, (ii) RGO, (iii) dodecane thiol (1-DT), and (iv) 1-dodecanethiol-treated RGO (1-DT + RGO). (b) Enlarged FT-IR spectra in the range of the wavenumber for the thiol peak.

Further, for the individual comparisons, the reported S–H bond in 1-DT appears at ∼2563 cm–1,[23] and in the present case, it is observed at ∼2522 cm–1, as shown in Figure b (i). As seen in Figure b (i), this peak does not appear in the 1-dodecanethiol-treated RGO spectra as shown in Figure b (ii). This is a clear indication that the thiol has successfully conjugated on to the RGO surface via the S–H bond.

The contact angle, the angle subtended by a drop of liquid concerning the surface under investigation, is an essential parameter that can be used to characterize the chemically modified surfaces of a given material. It is a simple and low-cost technique to check surface modification. The measurements are carried out for the pristine graphene oxide sheet, reduced graphene oxide sheet, and 1-dodecanethiol-decorated reduced graphene oxide sheet using a lab-made setup. A web camera is attached to an eyepiece of a traveling microscope, the sample placed parallel to the objective, and a syringe is used to place a drop of water (2 μL) on the surface of the sample. The pristine graphene oxide sheet shows a hydrophilic nature as shown in Figure a with the contact angle ϕ = 44.83° due to the presence of functional groups, trapped water molecules, and the interplanar forces between the layers of carbon atoms in the graphene oxide sheets. In Figure b, the thermally and chemically reduced graphene oxide sheet is shown with the contact angle ϕ = 115.14° as a result of the removal of functional groups and trapped water molecules during the reduction process. The 1-dodecanethiol-decorated reduced graphene oxide sheet exhibited a high contact angle of ϕ = 141.44°, indicating a hydrophobic nature as shown in Figure c.

Figure 3

Contact angle measurements of the (a) pristine graphene oxide sheet, (b) reduced graphene oxide sheet, and (c) 1-dodecanethiol-decorated reduced graphene oxide sheet. (d) Plot of contact angle versus the type of graphene oxide sheet.

In the process of chemical modification or decoration of the reduced graphene oxide sheet using 1-dodecanethiol, the first is the affinity of the sulfur-end moiety for the carbon atoms of reduced graphene oxide, and the molecules would bind in vertical alignment by forming a S–C chemical bond that gives an ordered monolayer of molecules. The sulfur atom and carbons in the methylene groups act as the main driving force of 1-dodecanethiol for the ordered monolayer. The surface that is created by the other end of these molecules will give the C–H terminating structure, which is hydrophobic, and contact angle measurements confirm it. Further, Figure d shows the plot of the contact angle versus the samples of GO, RGO, and 1-DT + RGO, and linear behavior represents the incremental contact angle with the samples. This simple technique helps to determine the hydrophilic or hydrophobic nature of the surfaces. Also, it confirms the deposition of 1-dodecanethiol molecules on the surface of the reduced graphene oxide sheet.

Figure a shows the linear current–voltage graph of the reduced graphene oxide sheet decoded as Cu/RGO/Hg and the 1-dodecanethiol-decorated reduced graphene oxide sheet decoded as Cu/RGO/1-DT/Hg because of the device structure as shown in Figure S1. The logarithmic current–voltage measurements of Cu/RGO/Hg and Cu/RGO/1-DT/Hg measured in a potential window of ±1 V at 300 K is shown in Figure b. Similarly, Figure c shows the linear current–voltage plot of Cu/RGO/1-DT/Hg. As evidenced from the figures, the reduced graphene oxide sheet shows high conductivity for an applied voltage of 1 V, and the observed current density is 0.872 mA/cm2. The curve (Figure a) is symmetric in forward and reverse bias regions, indicating the ohmic charge transport mechanism. This transport mechanism is investigated with the help of a metal energy band diagram. The characteristics of reduced graphene oxide include a low resistivity—the energy band diagram for this is shown in two forms. Figure S2a shows the partially filled band, and many valence electrons are available for the electrical conduction so that the material can exhibit high electrical conductivity. Figure S2b shows another possible band diagram for the reduced graphene oxide sheet in which the conduction and valence bands overlap; hence, there are more free electrons and massive number energy levels available for the charge transfer, which results in the high electrical conductivity in the material.

Figure 4

(a) Linear current–voltage graph of the reduced graphene oxide sheet and 1-dodecanethiol-decorated reduced graphene oxide sheet. (b) Semilogarithmic current–voltage characteristics of the reduced graphene oxide sheet and 1-dodecanethiol-decorated reduced graphene oxide sheet. (c) Linear current–voltage graph of 1-dodecanethiol-decorated reduced graphene oxide.

The 1-dodecanethiol decoration on the surface of the reduced graphene oxide sheet reduces the current levels significantly in comparison to the current levels of the reduced graphene oxide sheet. The current–voltage plot of Cu/RGO/1-DT/Hg is weakly rectifying or sigmoidal (as shown in Figure c). Hg contact on 1-DT is made through nonrectifying metal–semiconductor junctions or ohmic contact. An ohmic contact is a low-resistance junction providing current conduction in both forward and reverse regions.

The band diagram of Cu/RGO/1-DT/Hg is shown in Figure . The work function of Cu is 4.7 eV, RGO is 4.6 eV, and Hg is 4.5 eV; the Eg (HOMO–LUMO gap) of 1-DT between π–π* stacks is 8 eV with a band edge ΦB of 1.39 eV selected from the literature.[24] The height of the potential barrier is influenced by the band bending between the interface of Cu/RGO/1-DT/Hg. Based on these facts, it is assumed that in Figure c, positive bias corresponds to electrons injected from the Hg contact (top contact) into the 1-DT molecules. RGO, with the unique 2D π-conjugated dish-like shape with the desired interface with 1-DT, could facilitate the active charge transport. The holes collected at the Hg from the HOMO level of 1-DT, and the electrons easily transferred from the LUMO level of 1-DT to Cu through conducting RGO. The grafting of 1-DT molecules on RGO decreased carrier mobility because of a large number of scattering centers created by 1-DT molecules, which hinders the flow of charges in the Cu/RGO/1-DT/Hg device, causing overall increment in the resistance and resulting in the 10–9A/cm2 current density at 1 V bias as shown in Figure c.[25]

Figure 5

Band diagram of the Cu/RGO/1-DT/Hg device

Under forward bias, the electrons tunnel through the barrier from Hg to RGO, and the barrier is slightly larger in the case of reverse bias for the hole tunneling. As a result, the junction exhibits a weak rectification property, as shown in Figure c. There is no reliable experimental data on the Fermi level alignment in these metal–1-DT–metal systems; the ΦB value is obtained by Wang et al.[24] for the 1-DT molecules, and it is used in the present work to draw the band diagram. The study revealed that no significant temperature dependence is observed for the potential range of 0–1.0 V between 300 and 80 K, and by the Arrhenius plot (ln(I) vs. 1/T) of 1-DT on Au contact showed small temperature dependence at different biases within the potential range of 0–1.0 V, depicting the absence of thermal activation.[24] Therefore, in the present study, it is concluded that the conduction mechanism through 1-DT molecules is tunneling.[24] Based on the applied potential as compared with the barrier height ΦB, the tunneling through a 1-DT is understood either by direct (V < ΦB/e) or Fowler–Nordheim (V > ΦB/e) through-bond tunneling. Within the bias range of 0–1.0 V, no significant voltage dependence is observed by Wang et al.,[24] which revealed no obvious Fowler–Nordheim transport behavior as it needs higher bias. Hence, the observations confirm tunneling as the conduction mechanism, and the obtained results are compared with experimental I–V data with theoretical calculations from the tunneling model. When the Fermi level (EF), as shown in Figure , is aligned closer to one energy level (HOMO or LUMO), the effect of the other distant energy level on the tunneling transport is negligible, hence the Simmons model is widely used as an excellent approximation.[26−28]

The tunneling barrier of the 1-dodecanethiol molecules with long methylene chains (CH2) is high enough to compare with the Simmons model.[23] Current–voltage characteristics measured for the Cu/RGO/DT/Hg device at the low bias range ± 1 V are considered to fit the Simmons model. Seon et al. calculated the data for the Simmons model using the experimental parameters using scanning tunneling microscope (STM) junctions, the current measured across a molecular junction of an STM Pt/Ir tip–1-dodecanethiol–gold substrate.[29−31]

The comparison between the experimental current–voltage data (black line) and theoretically calculated data (blue line) is shown in Figure . Both the curves are similar. According to the Simmons model, in the low bias regime (0–0.3 V), the tunneling current depends on the barrier width; similarly in the higher bias (0.5-1 V), the tunneling current depends on the barrier-lowering effect, and the same is observed by the calculation of Wang et al.[24] Hence, it is proved that the obtained experimental data of the I–V curve shows low current at low bias voltage and high current at high bias voltage.

Figure 6

Current–voltage graph of experimental data and Simmons metal–insulator–metal fit data.

Another essential aspect of the current–voltage curve is the rectification ratio. It is the ratio of the forward current (IF) to the reverse current (IR) at a specific applied voltage. The estimated rectification ratio (IF/IR) at 1 V is 1.12. The significant difference in current density values is observed between Cu/RGO/Hg and Cu/RGO/1-DT/Hg devices. The current has been reduced by almost six orders of magnitude (Figure b). Such a reduction is rare in the case of molecular junctions as per the literature. As an example, silicon–molecules–metal junctions by a transfer printing method showed three to five order differences in the current density.[32] A similar type of work, the metal–molecule–metal junctions characterized by conducting probe atomic force microscopy, showed the seven order difference in the current density.[33]

Using the current–voltage measurements further probed to investigate the nature of the conduction mechanism in the Cu/RGO/1-DT/Hg structure. In current–voltage characteristics, the forward and reverse biases obtained with Hg as top contact and Cu as bottom contact. The symmetrical nature of the curve is attributed to the work function of the electrode, implying the same barriers at the Cu/RGO/1-DT/Hg interface, as shown in Figure . At low voltage, the slope of the ln(J) versus ln(mV) in Figure a,b plots is approximately equal to unity representing the ohmic conduction. In contrast, at higher voltages, the slope is found to be 2.1, and 2 depicts the space charge limiting current (SCLC). I–V characteristics of metal–organic molecules are described by two basic processes: (a) injection of charge carriers from electrodes into organic molecules and vice versa, and (b) transport of the charge carriers in the bulk film. The Hg contact with 1-DT forms a high barrier at low voltages, and the injected charge density becomes small so that the overall behavior is observed as ohmic conduction.[34] As the voltage increases, the number of injected carriers increases so that space charge accumulates, limiting the current, and hence at a higher voltage, the SCLC conduction mechanism is observed.[34]

Figure 7

Current–voltage characteristics of (a) forward bias Hg/DT/RGO/Cu (ln(J) vs. ln(mV)) and (b) reverse bias of Hg/DT/RGO/Cu (ln (J) vs. ln (mV)).

It is well known that a monolayer of organic molecules on metal/semiconductor surfaces would act as an insulating layer through which the injected charge carriers have to pass through to complete the circuit. The role of molecules is to modify the electron states/surface potential of a substrate by the aligned static dipole moments. The latter would make a large electric field on the surface that creates field-induced effects. In silicon–molecules–metal junctions[32] and other junctions,[33] it is reported that this effect systematically controls the molecular junction properties. On bare indium tin oxide-coated glass electrodes, the work function is significantly tuned by binding different organic molecules that possess significant dipole moments.[35] Another role of organic molecules is that the bound monolayer acts as an insulating layer through which charge carriers tunnel under the applied bias voltage. The effect of dipole moment and the length of organic molecules, as an insulating layer, have been formulated in the current–voltage equation of semiconductor–molecules–metal junctions.[36] The present work clearly shows that paper-like multilayer reduced graphene oxide films can be an excellent choice to bind organic molecules to create molecular junctions. Further work in this direction is essential to understand the role of molecular parameters in controlling the molecular junctions that created on low cost and flexible multilayer graphene.

Conclusions

In summary, the present work includes the role of an attached monolayer of organic molecules on the surface of a reduced multilayer graphene oxide substrate. This work depicts that the reduced graphene oxide can be a suitable substrate material to constitute molecular junctions using a wide range of organic molecules and bioentities. The contact angle measurement was used as a simple and low-cost method to differentiate the RGO and 1-DT + RGO. The organic molecules were firmly attached on the surface-reduced graphene oxide sheet by the simple electrografting process. The 1-DT decoration on the surface of RGO sheet reduces the current levels significantly in comparison to the current levels of RGO sheet. The study of I–V curves shows the domination of ohmic and SCLC transport at both low and high voltages, respectively. The band diagram confirms tunneling as the conduction mechanism. It can be extended to other kinds of organic molecules/biomolecules to study the role of molecules on the charge transport property injunctions. Hence, the reduced multilayer graphene oxide can become a promising substrate in the field of future molecular electronics.

Experimental Methods

All the chemicals and reagents are used as received without any further purification. Graphite powder, sulfuric acid (H2SO4), potassium permanganate (KMnO4), ethanol (C2H6O), and hydrazine hydrate (N2H4) are obtained from Himedia Pvt. Ltd., India, whereas 1-dodecanethiol (CH3 (CH2)11SH) is procured from Sigma-Aldrich Germany. Milli-Q triple distilled water is used for all the reaction and washing purposes. Graphene oxide is prepared according to the Hummers method.[37,38] In detail, 3 g of graphite powder is added to a conical flask, then 64.8 mL of H2SO4 was added to graphite powder and stirred in an ice bath for 30 min. Next, 9 g of KMnO4 is added slowly to graphite powder and H2SO4 slurry under continuous stirring in an ice bath; due to the exothermic reaction, which occurs during the reaction, the temperature of the ice bath is maintained below 5 °C to avoid damage. The suspension is stirred and allowed to react for 2 h in an ice bath, and distilled water is later added slowly and continuously to increase the volume of the suspension to 500 mL. The product is centrifuged and washed with distilled water. Finally, the brown-colored slurry is obtained and used to prepare the flexible sheets by the doctor blade method.

Synthesized graphene oxide slurry is poured on a clean glass plate and allowed to dry to obtain a thin paper-like sheet. Then, the film is peeled off and cut to the required size for further investigations. Figure shows the schematic representation of the preparation of reduced graphene oxide sheets. The thermal reduction of the above graphene oxide sheets is carried out at 350 °C by placing between two thick glass plates. Efficient chemical reduction by hydrazine hydrate is also carried out for the thermally reduced graphene oxide (RGO) sheets since both sides of the sheets can interact with the reducing agent.

Figure 8

Schematic representation of the preparation of multilayer graphene oxide sheets.

The 17 mM/dm3 concentration of 1-dodecanethiol molecules in ethanol is used to decorate on the surface of reduced graphene oxide by the electrografting method at a constant current of 100 μA for 2 h. In this process, the graphite rod acted as an anode, and the reduced graphene oxide sheet served as a cathode. The molecule-grafted sheets are washed with ethanol and dried. In this, the sulfur-end moiety of the chosen molecule is bound covalently to the surface atoms of reduced graphene oxide by forming a C–S chemical bond.

The current–voltage measurements are carried out by a Keithley 2450 EC workstation equipped with a computer-controlled LabView program. The structures of junctions are as follows: (a) metal/reduced graphene oxide/metal (Hg/RGO/Cu) and (b) metal/dodecanethiol/reduced graphene oxide/metal (Hg/DT/RGO/Cu) in the case of the decorated surface. These structures are shown schematically in the Supporting Information, Figure S1.