Guanine as a gate dielectric/passivation layer for graphene-based sensors

DNA shows promise as an organic component for biosensors, particularly when it is combined with graphene. Specifically, when deposited onto graphene, DNA has the otherwise irreproducible charge carrier mobilities of the graphene in addition to those of non-uniform DNA films. Our previous studies of these films for biosensor applications identified the DNA nucleobase guanine as particularly resilient. For example, we found that the degradation temperature of guanine (460°C, which we identified by thermogravimetric analysis) is higher than that of DNA, and its other nucleobases thymine, cytosine, and adenine. When we deposited guanine onto monolayers of graphene, we were able to produce films that were not only uniform, but also had reproducible, consistent charge carrier mobilities of the graphene.

In this work, we have sought to maintain the bulk charge carrier mobility of graphene after deposition of the gate dielectric or passivation layer used in the fabrication of transistor devices.¹ We introduced a thin film of guanine, deposited by physical vapor deposition, onto layers of chemical vapor deposition (CVD)-grown graphene, and transferred this from thermal release tape onto willow glass, a flexible substrate. We fabricated two test platforms, one with guanine as a stand-alone gate dielectric (the control)—platform A—and the other with guanine as a passivation layer between graphene and poly(methyl methacrylate) (PMMA): platform B. Using platform A, we compared guanine with PMMA as a reference gate dielectric, and found that the PMMA reduced graphene charge carrier mobilities at room temperature (see Figure 1).

Figure 1. Electrical characterization of substrates with graphene only and with poly(methyl methacrylate) (PMMA) on graphene, compared with graphene only and guanine-on-graphene. (Image courtesy of Williams et al., 2015.¹)

We showed that the bulk charge carrier mobility of graphene was maintained and most stable with guanine as a gate dielectric and a passivation layer (between the graphene and PMMA). We also demonstrated PMMA fabricated as a material with 2D graphene: see the gate dielectric in Figure 2(b). The bulk charge carrier mobilities were maintained with guanine as a standalone gate dielectric—see Figure 2(a)—and with a passivation layer: see Figure 2(b).

Figure 2. (a) Schematic of graphene test platform A, with guanine as a dielectric layer. (b) Test platform B, with guanine as a passivation layer and PMMA as a dielectric layer. (Image reprinted with permission from Williams et al., 2015.¹)

Next, we studied the stability of the graphene on willow glass at room temperature in air for up to six days. This investigation would enable test platforms for transistors in bio-based applications. We determined that the guanine as a passivation layer (platform B) maintained graphene's mobilities (see Figure 3). We took three samples from the same batch of CVD-grown graphene and carried out Hall effect measurements at room temperature in air. We used this characterization technique for the mobility of graphene for all the samples studied.

Figure 3. Test platform B: Bulk charge carrier mobility of graphene over six days at room temperature in air. Samples 1–3 were all from the same batch of chemical vapor deposition-grown graphene: 60nm PMMA/10nm guanine/4-multi-layered graphene (4-MLG)/willow glass (WG). (Image courtesy of Williams et al., 2015.¹)

We compared the current-voltage (I-V) measurements for test platforms A and B (see Figure 4). This determined any type of leakage current typical for direct-current (DC) voltage (which is commonly used for gating organic field effect transistors). Now, in platform A, guanine was the stand-alone gate dielectric, whereas in platform B, PMMA was the gate dielectric layer on top of the guanine-passivation layer.

Figure 4. Current-voltage (I-V) curve of test platform A compared with test platform B. (Image courtesy of Williams et al., 2015.¹)

We have successfully shown that PMMA can be used with graphene—a conductive 2D material—and that guanine as a passivation layer is much less conductive, with no shorting through the PMMA at lower voltages. Unlike in platform A, guanine as the stand-alone gate dielectric proved to be a less effective insulator.

Our future research will be to analyze the interfacial layers of the gate dielectric or passivation layer and the graphene, in order to better optimize the performance of such a transistor, with a view to its use as a bio-based sensor. In addition, we will study another DNA nucleobase purine, adenine, as a potential stand-alone gate dielectric and passivation layer. These transistors could have applications in human sweat monitoring. We have also investigated other transport properties, such as charge carrier concentration, conductivity type, and electrical resistivity. This is an important first step to realizing high-performance graphene-based transistors for potential use in bio- and environmental sensors, computer processing, and electronics.