Graphene is a two-dimensional (2D) atomic crystal lattice with outstanding material properties such as mechanical stiffness, high electrical and thermal conductivity, and optical detection, etc.1−7
In recent years, graphene nanoribbons (GNRs) with a high on/off ratio have received significant attention as semiconductor materials in the field-effect transistor (FET). Many production techniques such as unzipping of carbon nanotubes, lithographic, and chemical methods have been reported to prepare GNRs.8−11
However, the fabrication of the aligned GNR arrays in a large scale by the previous methods is very hard. Herein, we report a simple yet efficient method to produce largescale aligned GNRs with sub-30 nm width. In this process, V2O5 nanowires (NWs) were aligned via spraying a solution of V2O5 NWs on a graphene layer and used as a shadow mask during the reactive ion etching (RIE) process. We measured the atomic force microscopy (AFM) topographic images of the aligned and the randomly oriented GNRs on a SiO2 substrate and compared their orientation. Highly-oriented large-scale patterns of the GNRs on the SiO2 substrate were obtained. Furthermore, we successfully fabricated the FET with the aligned GNRs and measured its electrical performance. Because of the ease of synthesis and high yield of the aligned GNRs, this technique maybe useful for further applications such as electrical and mechanical devices.
The graphene layers used for patterning the aligned GNRs were mechanically exfoliated on a ~100 nm SiO2/ Si substrate, as reported previously.12−16
We have used the pristine graphene in the assembly process of V2O5 NWs. Any kind of surface functionalization of the graphene layer is not required to assemble the V2O5 NWs on the graphene surface. V2O5 solutions were prepared using ammonium meta-vanadate (Aldrich) and acidic ion-exchange resin (DOWEX 50WX8-100, Aldrich) in deionized water. A solution of V2O5 NWs was sprayed via a syringe, resulting in a flow along one of the direction of the V2O5 NWs on the graphene surface (Fig. 1(a)). The V2O5 NWs with the alignment of the flow direction were adsorbed on the graphene surface. The adsorbed V2O5 NWs were used as a shadow mask to protect the underlying graphene region from the RIE process (60 W, 2 min) as shown in Fig. 1(b). When the sample was placed in a solution of HCl (~5–10%) for ~1 min, the V2O5 NWs were removed, and the aligned GNRs remained on the SiO2 surface (Fig. 1(c)). Then, the Au and Pd (10 nm/30 nm) films were deposited by the photolithography followed by thermal evaporation under high vacuum condition (~2×10−6 Torr). Finally, the FET with aligned GNRs was achieved (Fig. 1(d)).
Figure 1.Schematic diagram showing the fabrication of a GNRFET.
RESULTS AND DISCUSSION
Figs. 2(a) and (b) show the AFM topographic images of the aligned GNRs generated by the spray method. The white arrow in Figs. 2(a) indicates the flow direction of the spray of the V2O5 NWs solution using a syringe. The AFM images show that almost all the GNRs were aligned along the flow direction of the spray on the SiO2 substrate over the surface region as large as 1 cm.
Figure 2.AFM topographic images of the aligned GNRs on the SiO2 substrate. The white arrow in (a) indicates the flow direction of the spray of the V2O5 NW solution via a syringe.
For a comparison, randomly oriented GNRs were prepared by dipping the graphene/SiO2 substrate in a solution of the V2O5 NWs. Fig. 3 shows the AFM topographic image of the GNRs with a random orientation after the RIE process and HCl treatment.
Figure 3.AFM topographic image of the randomly oriented GNRs on the SiO2 substrate.
The orientation of the individual GNRs in the aligned GNR pattern was measured with respect to the flow direction of the spray. Fig. 4 shows the histogram (red bar) of the orientation of individual GNRs in the aligned GNR pattern. The orientation of the aligned GNRs is defined as an absolute value of the angle between the direction of the long axis of the GNR and the flow direction of the spray. The statistical analysis showed that over 80% GNRs were aligned along the flow direction of the spray with the deviation less than 10° indicating that the spray procedure allowed to assemble the aligned GNRs with a high degree of alignment of the individual GNRs in a large scale on the substrate. The other histogram (blue bar) in Fig. 4 shows the angle distribution of the randomly oriented GNRs, indicating a broad distribution of the angle for the GNRs.
Figure 4.Histogram of the orientation of the aligned (red) and randomly oriented (blue) GNRs.
To thoroughly probe the removal of the V2O5 NW on the GNRs, we compared the height profiles before and after the HCl treatment of the sample. Figs. 5(a) and (b) shows the AFM topographic images and the height profiles of the GNR before (green) and after (gray) the HCl treatment. The height profiles indicate that the HCl treatment removed the V2O5 NWs on the graphene. Fig. 5(c) shows the histogram of the height before and after placing the sample in the HCl solution. The solid curves are the Gaussian fits for each histogram. The inset depicts the height for the GNR and the V2O5 NW on the GNR. The peaks of the height in the histogram are ~4 nm for the GNR with the V2O5 NW (before the HCl treatment, green) and ~2 nm for the GNR (after the HCl treatment, gray) from the Gaussian fittings indicating that the V2O5 NWs were removed from the GNRs via the HCl treatment.17
Figure 5.AFM images (a) and its height profile (b) of the GNN before and after the HCl treatment. The height profiles indicate that the HCl treatment removed the V2O5 NWs on graphene layer. (c) Histogram of the height before (green) and after (gray) the HCl treatment.
Fig. 6(a) shows the transmission electron microscope (TEM) image of the V2O5 NWs, with a diameter and length in the ranges ~20.50 nm and ~1.10 μm, respectively. Fig. 6(b) shows the histogram of the width of the GNRs. The red solid curve is the Gaussian fit to the width distribution. The sub-30 nm width of the GNRs was obtained from the center of the Gaussian fit, attributed to the diameter of the V2O5 NWs.
Figure 6.(a) Transmission electron microscope (TEM) image of the V2O5 NWs. (b) Histogram of the width of the GNRs.
We successfully fabricated the FET using the aligned GNR patterns. Fig. 7(a) shows the AFM topographic image of the channel with the aligned GNRs between Pd/Au electrodes. The source and drain electrodes were fabricated using the conventional photolithography followed by the thermal evaporation of Pd/Au (10 nm/30 nm) and a lift-off process. The white regions represent the metal electrodes. Fig. 7(b) shows the Ids–Vg curves of the aligned GNR based FET. The applied bias was 1V for the measurement. The FET with the GNRs showed the p-type characteristics due to the oxygen edge groups, generated by the RIE process.18
Figure 7.(a) AFM topographic image of the FET channel with the aligned GNRs and the metal electrodes. (b) Gating effect of a FET based on the aligned GNRs.
In conclusion, we developed a method to assemble the aligned GNRs over a large scale surface region. In this process, the V2O5 NWs were aligned on the graphene layer via spraying the V2O5 NWs solution, and they were used as a shadow mask in the RIE process. This method enabled us to prepare the aligned GNRs on the desired surface with a large scale. The orientation of the alignment for the aligned and the randomly oriented GNRs were compared by the AFM images. We found that the V2O5 NWs were properly removed after the HCl treatment by measuring the AFM height profile for the GNR and GNR with the V2O5 NW. The width of the GNRs was ~30 nm, attributed to the width of the V2O5 NWs. The FET devices based on GNRs were fabricated, exhibiting the p-type characteristics of the Ids−Vg measurement. This method could be a powerful tool for the mass-production of the aligned GNR-based devices and in various practical applications such as biological sensor or opto-electronics.