Chemical and biological sensors based on defect-engineered graphene mesh field-effect transistors
© The Author(s) 2016
Received: 12 March 2016
Accepted: 26 May 2016
Published: 11 July 2016
Graphene has been intensively studied for applications to high-performance sensors, but the sensing characteristics of graphene devices have varied from case to case, and the sensing mechanism has not been satisfactorily determined thus far. In this review, we describe recent progress in engineering of the defects in graphene grown by a silica-assisted chemical vapor deposition technique and elucidate the effect of the defects upon the electrical response of graphene sensors. This review provides guidelines for engineering and/or passivating defects to improve sensor performance and reliability.
KeywordsGraphene Graphene mesh Edge-defect Defect-passivation Nanosensor Biosensor
Graphene, a two-dimensional (2D) zero-gap semiconductor, has drawn great interest as a promising platform for novel electronic, optoelectronic, and energy harvesting systems [1–9]. In particular, application to sensors has been explored because graphene’s one-atomic 2D nature allows its electrical characteristics to be sensitively influenced by the surrounding chemical and biological environment [10–20]. Moreover, graphene has excellent electrical conductivity and mobility [5, 21–23] as well as a low level of 1/f noise [10, 12], which might even enable the real-time electrical detection of single-molecular binding events.
However, the performance of graphene sensors has varied greatly among reported works [12, 24–28]. This variation has been ascribed to the quality of graphene, which is determined by the synthesis and fabrication processes used; yet the relevant mechanisms, and especially the role of defects, have remained poorly understood thus far. For example, Ang et al. reported large Dirac point shifts of graphene field-effect transistors under changes in acidity (i.e., pH response) of 99 mV/pH, which is even higher than the Nernst limit of 59 mV/pH  others observed much smaller pH response when the defects in graphene were passivated with hydrophobic fluorobenzene molecules . On the other hand, Tan et al. found a significant enhancement of the pH response in graphene nanoribbon sensors . The sensitivity improvement was attributed to binding of OH− ions to edge defect sites, but the binding characteristic was not thoroughly determined. Nevertheless, existing methods for introducing defects in graphene entail difficulties in controlling the quantity of the defects and/or avoiding contamination from external substances . Due to such unavoidable side effects, the specific influence of defects upon sensing characteristics and sensing mechanism remain largely unclear.
In an effort to address this issue, a new fabrication strategy was developed to directly synthesize graphene mesh structures . This approach allows the engineering of graphene defects and enables further investigation of their effect upon graphene-based sensor characteristics. Sensors based on graphene mesh have shown unprecedented detection characteristics compared to those of normal graphene sensors. For example, in the case of gas sensors based on Pd nanoparticle-decorated graphene mesh (Pd-GM), defects lowered the energy barrier during carrier injection at the Pd/graphene junction, thereby enhancing sensitivity and allowing faster response and recovery . On the other hand, under a physiological environment where the graphene surface was directly exposed to electrolyte solutions, ion species were directly bound to the defect sites by means of strong chemisorption . This reaction was proven to be irreversible and thus would limit its application in multiple-cycle sensor operations.
2 Graphene mesh: synthesis and properties
Because the graphene meshes are directly synthesized, their edges are less vulnerable to contamination than those produced by lithographic methods. To further examine the characteristics of the edge defects, doping of graphene meshes was explored by high-temperature thermal annealing under NH3 atmosphere. In the drain current versus gate voltage (Id–Vg) curves of graphene mesh FETs, the Dirac point was shifted greatly in the negative direction after N doping. During the annealing process, physisorbed molecules such as O species [52, 53] were desorbed and N elements were covalently functionalized at the edges of graphene meshes, moving the Dirac point to the left-hand side (Fig. 2d). Such successful N doping results can be associated with the clean and abrupt nature of the graphene mesh edges, which allows them to be chemically reactive. The stable doping through strong C–N bonds in the N-doped graphene mesh was also confirmed by a distinct N peak in Auger electron spectroscopy (AES) spectra that remained even after additional vacuum annealing (Fig. 2e).
3 Pd–graphene mesh hybrid gas sensors
Pd shows high reactivity and resistance change upon exposure to H2, even at room temperature [54–57], making it a promising material for H2 sensing. However, rigid Pd films undergo structural degradation during reaction with H2 [54, 57–60]. To overcome this problem of Pd film sensors, the use of Pd nanoparticles-semiconductor hybrid structures has been proposed such as Pd-GaAs , Pd-Si , and Pd-CNT [57, 63–65]. In these sensors, the charge carriers generated during Pd hydridation are transported to the semiconductor channels and modulate the resistance. More recently, as an alternative to the existing channel materials, graphene was proposed to take advantage of its 2D nature. In the resulting Pd-graphene (Pd-Gr) sensors, however, charge carrier injection from Pd to the chemically inert graphene surface was limited by the relatively high contact barrier [66, 67]. Accordingly, the presence of defects in the graphene modulates the contact barrier and thus plays an important role in the sensing characteristics.
It is also noteworthy that the response of Pd-GM sensors to H2 was reversible; their resistances returned to their initial values when the reactor was purged with air (Fig. 3a, b). This indicates that the Pd hydridation reaction is reversible and also that the edge defects are not directly associated with the reaction with H2.
4 Graphene mesh pH sensors
To investigate the effect of defects in graphene-based FET pH sensors, electrolyte-gated graphene-FET (Gr-FET) and graphene mesh–FET (GM-FET) devices have been tested . Both sensors exhibit negative Dirac point shifts upon decreases in pH. Whereas Gr-FETs have shown sensitivities of ~16.2 mV/pH, those of GM-FETs were significantly higher, at ~89.7 mV/pH (Fig. 4c, d). Typically, the pH sensitivities of GM-FETs were on average ~3 times higher than that of normal Gr-FETs, and also they frequently exceeded the thermodynamically allowed maximum limit (i.e., the Nernst limit) of 59 mV/pH. This result illustrates that the Dirac point shift is not driven solely by the electrostatic gating effect arising from physisorption of H+ ions on the Gr surface; rather, this result suggests an additional defect doping effect whereby unsaturated C atoms at the graphene mesh edges (i.e., defect sites) provide binding sites for the H+ ions, thereby further increasing the Dirac point shifts in the GM-FETs.
Gr-FETs have shown results consistent with those of GM-FETs: there was an irreversible portion of the response that gradually decreased with cycling, and only a reversible component remained after five cycles (Fig. 5d–f). The magnitude of the irreversible component of the Dirac point shift in Gr-FETs (~30 mV) was considerably smaller than that in GM-FETs (~90 mV), whereas the remaining reversible component was similar for both FETs (~7.0 mV/pH). This reflects the fact that both intrinsic and extrinsic defects provided interaction sites for H+ ions, whereas the increased number of defects at the mesh edges enhanced the pH response of the GM-FETs during the initial cycles. Furthermore, the irreversible response is believed to be caused by the direct adsorption of ions onto defects, which presumably involved strong chemisorption. It has been reported that H+ ions were attached so strongly to the edge defects and have only been detached after high temperature annealing . Such strong interactions were rarely reversed, and thus led to passivation of the defects upon repeated exposure to acidic solutions. This passivation eliminated the irreversible component of the Dirac point shift, thereby enabling stable and reversible pH sensing.
In this review, we investigated the defects of graphene meshes and their influence upon various sensor applications. Direct growth of graphene mesh by silica-assisted CVD is an excellent way to produce graphene with contamination-free defect sites. When these defects are introduced to Pd-GM gas sensors, the lowered energy barrier at the junctions between Pd nanoparticles and edge defects enhance indirect charge carrier injection into the graphene channel. As a result, the sensitivity and response time of Pd-GM gas sensors have been greatly improved over those of other graphene-based sensors. In contrast, graphene mesh pH sensors respond directly to H+ ions at edge defects, which results in increased sensitivities that sometimes exceeded the Nernst limit. Unfortunately, such direct interactions involve irreversible covalent bonding, and therefore are not preferred in multiple-cycle sensor operations. For graphene mesh pH sensors, this issue has been solved through a simple process of passivating the edge sites. These results suggest how graphene mesh edge defects can improve sensors’ sensitivity and response time and also enable stable multiple-cycle operation through indirect carrier injection. As an example, biological sensors can be constructed by including graphene mesh for enhanced sensing characteristics, while improving stability by attaching appropriate receptors to the graphene edge sites to inhibit direct carrier injection.
All authors have contributed to the writing of the manuscript. All authors read and approved the final manuscript.
This work was supported by the Basic Science Research Program (2015R1A2A2A11001426) and the International Research and Development Program (2013K1A3A1A32035393) of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea.
The authors declare that they have no competing interests.
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