Nanoscale Selective Passivation of Electrodes Contacting a 2D Semiconductor

Abstract

2D semiconducting materials have become the central component of various nanoelectronic devices and sensors. For sensors operating in liquid, it is crucial to efficiently block the electron transfer that occurs between the electrodes contacting the 2D material and the interfering redox species. This reduces current leakages and preserves a good signal-to-noise ratio. Here, a simple electrochemical method is presented for passivating the electrodes contacting a monolayer of MoS 2 , a representative of transition metal dichalcogenide semiconductors. The method is based on blocking the electrode surface by a thin and compact layer of electronically nonconductive poly(phenylene oxide), PPO, formed by electrochemical polymerization of phenol. Since the phenol polymerization occurs in the potential window where MoS 2 is electrochemically inactive, the PPO deposition is area-selective, limited to the electrode surface. The deposited PPO film is characterized by electrochemical, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy techniques. The applicability of this method is demonstrated by coating the electrodes of a MoS 2based field-effect transistor coupled with a nanopore. The highly selective deposition, the simple approach, and the compatibility with MoS 2 makes this method a good strategy for efficient insulation of micro-and nanoelectrodes contacting 2D semiconductor-based devices.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

“…Regarding the band gap, the zero-band-gap structure makes graphene highly sensitive to external conditions such as electric field and foreign doping impurities. , However, this band structure, in turn, results in a moderate switch ON/OFF ratio of GFETs . The 2D materials (for example, s-TMDs and phosphorene) have dramatic changes in their band gap as they become thinner from bulk to single layer, leading to a broad energy range. , For instance, monolayer TMDs including WSe 2 (1.4 eV), MoS 2 (1.7–1.9 eV), WS 2 (1.8 eV), MoSe 2 (1.4 eV), PtSe 2 (1.2 eV), and MoTe 2 (1.1 eV) have a direct band gap, whereas their bulk phases have a smaller indirect band gap. ,− These direct band gaps enable TMDs to complement graphene in low-power applications such as phototransistors and energy harvesting. ,, Similarly, the band gap of BP increases monotonically to ∼2 eV in the monolayer form by reducing the layer counts . This thickness-dependent band gap adds the promise of BP in photodetection applications …”

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

“…As the number of layers decreases, the quantum confinement effect becomes more pronounced while the interlayer interaction becomes less effective. , These two effects not only affect the band gap value but also influence the direct and indirect nature of the band gap. − A reduction in the number of layers, particularly in TMDs such as MoS 2 , leads to a gradual increase in the energy of indirect excitonic transition, whereas the direct band gap at the K point of the Brillouin zone barely changes. Interestingly, as the thickness of MoS 2 decreases to a monolayer, the indirect band gap becomes higher than the direct transition energy and the material converts into a direct bandgap 2D semiconductor ( E g ∼1.9 eV). , Similarly, the monolayer form of several other TMDs such MoSe 2 (∼1.5 eV), WS 2 (∼2.1 eV), WSe 2 (∼1.6 eV), PtSe 2 (∼1.8 eV), and MoTe 2 (∼1.1 eV) exhibits a direct band gap nature, whereas their bulk phases exhibit an indirect bandgap nature with smaller values (Figure b). − The existence of direct band gaps makes TMDs the most promising candidates for various optoelectronic applications such as photosensors and energy harvesters. Among all other TMDs, the superficial synthesis and extraordinary physical, physicochemical, and electronic properties of MoS 2 make it the most auspicious material for flexible electronic applications. , BP is another promising layered semiconductor that exhibits a thickness-dependent direct bandgap in the range of 2.0–0.3 eV (from monolayer - bulk) and an experimental mobility of ∼1000 cm 2 V –1 s –1 . ,, The strain and layer number-dependent tuning in its band gap makes it also a very interesting candidate for the optoelectronic applications in the visible to mid-infra-red (MIR) wavelength range.…”

Long-Range Electron Transfer through Ultrathin Polyelectrolyte Complex Films: A Hopping Model

“…Probing electron transport in the 1–100 nm range requires high-quality (pinhole free) conformal, rugged, insulating films of uniform thickness. Methods to produce pinhole-free films in the 1–10 nm thickness range include the use of self-assembled monolayers, , vacuum evaporation, chemical vapor deposition, and electrochemical polymerization. , Polyelectrolyte multilayers (PEMUs) are conformal ultrathin films made via an iterative deposition of polycations and polyanions on substrates using the layer-by-layer (LBL) technique . The spontaneous complexation of oppositely charged polymers is entropically driven by the release of their counterions .…”