Suspended 2D Material Biosensor for Reliable Sensing
Authors: Masurkar Nirul , Thangavel Naresh Kumar , Yurgelevic Sally , Auner Gregory W. , Arava Leela Mohana Reddy 
Published: February 2020

TMDs such as MoS2 is playing an important role in the field of FETs, photodetectors, thin film transistors and efficient biosensors because of their direct band-gap, high mobility, and biocompatibility. Despite these strengths, the performance and reliability of such atomic layer are easily influenced by supporting substrate. Interaction between the supporting substrate and MoS2 implies that interface control is vital for performance of devices consisting of monolayer MoS2. In particular, the Silicon dioxide (SiO2) supporting substrate has an uneven morphology and is chemically active because of trapped environmental gases, unknown functional groups, chemical adsorbates, and charges. Thus, adding another layer of MoS2 on the top of SiO2 cannot contribute charge transport clearly, which leads to the unreliable function of every single device. To solve the interface problem, suspended 2D layer devices have been reported by wet etching silicon di oxide underneath the monolayer. Freestanding MoS2 has shown 10 times greater back gate electronic mobility than the supporting on the SiO2 substrate, which is ~0.9 cm2/Vs [1]. However, the existing SiO2 requires hazardous chemical etchants such as hydrofluoric acid (HF), which is difficult to handle and affects the 2D film structure and purity [2]. Secondly, freestanding MoS2 sags between the two electrodes because of the high spacing (~ 2 µm), which makes it impossible to coat another layer such as hafnium oxide (HfO2) and antibodies on top of monolayer. Therefore, this structure impedes making top gate FET biosensors, which allows for only back gating. However, back gate mobility is far lesser than the top gate mobility which hinders making a highly sensitive FET-based biosensor because the sensitivity of a sensor depends on its mobility [3]. Moreover, when linkers/antibodies are directly attached to the bare channel, the transduction mechanism is the combination of the electrostatic gating, direct charge transfer and mobility modulation. Therefore, it is desirable to eliminate the density of defects by covering the bare channel material with insulating material and then functionalizing linkers and antibodies on the top of the insulator [3, 4].
In this work, CVD grown MoS2 channel material is transferred on self-assembled photolithographically patterned nano-gaps to achieve suspension and is covered with HfO2 to eliminate the direct functionalization of channel material. These nano-gap arrays provide mechanical strength to the monolayer and do not allow the supporting substrate to touch after coating another thin insulating layer as well as linkers/antibodies. HfO2 can be easily functionalized by linkers (Benzene 1, 4 dithiols) and antibodies (E-coli antibodies) to bring variation to the suspended 2D material by targeting a charged biomolecule (E-coli). In addition, termination of the supporting substrate leads to increments of electron mobility which are directly proportional to the sensitivity of the FET biosensor. The proposed FET biosensor has the capability to detect one molecule because of its single atomic layer as a channel material, its scalability due to the involvement of optical photolithography, and its fast response because of higher mobility.

References

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