Photoconductivity Switching in MoTe2/Graphene Heterostructure by Trap-Assisted Photogating.

Negative photoconductivity (NPC), a reduction in photoconductivity under light illumination, could provide low power consumption and high-speed frequency response. The NPC has been generally observed in low-dimensional materials, which can be easily affected by the trapping of photocarriers. However, a gradual transition between NPC and positive photoconductivity (PPC) by controlling the light intensity has not been reported. In this study, a gradual and reversible switching between NPC and PPC is achieved in a van der Waals heterostructure of graphene and MoTe2. The initially observed NPC state becomes a PPC state with the increase in light intensity. The switching between NPC and PPC is considered to originate from the hole trapping in MoTe2. The hole trapping can induce a shift in the Fermi level of MoTe2 and thus change the junction characteristics between the graphene and MoTe2, which determine the photoresponse type (NPC or PPC). Notably, the switching from one state to the other can also be reversed, depending on the gate bias. The stable and reversible effect upon light illumination and application of a gate voltage could be used in optoelectronic devices and optical communications.

Ultrasensitive Phototransistor Based on WSe2-MoS2 van der Waals Heterojunction.

Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe2 and MoS2 was developed. The MoS2 was utilized as the channel for a phototransistor, whereas the WSe2-MoS2 PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 106. The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 1011 Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

High-Performance Field-Effect Transistor and Logic Gates Based on GaS-MoS2 van der Waals Heterostructure.

This work demonstrates a high-performance and hysteresis-free field-effect transistor based on two-dimensional (2D) semiconductors featuring a van der Waals heterostructure, MoS2 channel, and GaS gate insulator. The transistor exhibits a subthreshold swing of 63 mV/dec, an on/off ratio over 106 within a gate voltage of 0.4 V, and peak mobility of 83 cm2/(V s) at room temperature. The low-frequency noise characteristics were investigated and described by the Hooge mobility fluctuation model. The results suggest that the van der Waals heterostructure of 2D semiconductors can produce a high-performing interface without dangling bonds and defects caused by lattice mismatch. Furthermore, a logic inverter and a NAND gate are demonstrated, with an inverter voltage gain of 14.5, which is higher than previously reported by MoS2-based transistors with oxide dielectrics. Therefore, this transistor based on van der Waals heterostructure exhibits considerable potential in digital logic applications with low-power integrated circuits.

Si-MoS2 Vertical Heterojunction for a Photodetector with High Responsivity and Low Noise Equivalent Power.

In this study, we propose the fabrication of a photodetector based on the heterostructure of p-type Si and n-type MoS2. Mechanically exfoliated MoS2 flakes are transferred onto a Si layer; the resulting Si-MoS2 p-n photodiode shows excellent performance with a responsivity ( R) and detectivity ( D*) of 76.1 A/W and 1012 Jones, respectively. In addition, the effect of the thickness of the depletion layer of the Si-MoS2 heterojunction on performance is investigated using the depletion layer model; based on the obtained results, we optimize the photoresponse of the device by varying the MoS2 thickness. Furthermore, low-frequency noise measurement is performed for the fabricated devices. The optimized device shows a low noise equivalent power (NEP) of 7.82 × 10-15 W Hz-1/2. Therefore, our proposed device could be utilized for various optoelectronic devices for low-light detection.

Vertical-Tunnel Field-Effect Transistor Based on a Silicon-MoS2 Three-Dimensional-Two-Dimensional Heterostructure.

We present a tunneling field-effect transistor based on a vertical heterostructure of highly p-doped silicon and n-type MoS2. The resulting p-n heterojunction shows a staggered band alignment in which the quantum mechanical band-to-band tunneling probability is enhanced. The device functions in both tunneling transistor and conventional transistor modes, depending on whether the p-n junction is forward or reverse biased, and exhibits a minimum subthreshold swing of 15 mV/dec, an average of 77 mV/dec for four decades of the drain current, a high on/off current ratio of approximately 107 at a drain voltage of 1 V, and fully suppressed ambipolar behavior. Furthermore, low-temperature electrical measurements demonstrated that both trap-assisted and band-to-band tunneling contribute to the drain current. The presence of traps was attributed to defects within the interfacial oxide between silicon and MoS2.

Atomic-scale etching of hexagonal boron nitride for device integration based on two-dimensional materials.

Hexagonal boron nitride (h-BN) is considered an ideal template for electronics based on two-dimensional (2D) materials, owing to its unique properties as a dielectric film. Most studies involving h-BN and its application to electronics have focused on its synthesis using techniques such as chemical vapor deposition, the electrical analysis of its surface state, and the evaluation of its performance. Meanwhile, processing techniques including etching methods have not been widely studied despite their necessity for device fabrication processes. In this study, we propose the atomic-scale etching of h-BN for integration into devices based on 2D materials, using Ar plasma at room temperature. A controllable etching rate, less than 1 nm min-1, was achieved and the low reactivity of the Ar plasma enabled the atomic-scale etching of h-BN down to a monolayer in this top-down approach. Based on the h-BN etching technique for achieving electrical contact with the underlying molybdenum disulfide (MoS2) layer of an h-BN/MoS2 heterostructure, a top-gate MoS2 field-effect transistor (FET) with h-BN gate dielectric was fabricated and characterized by high electrical performance based on the on/off current ratio and carrier mobility.

Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction.

We report a simple approach to fabricate a pyridinic-N-doped graphene film (N-pGF) without high-temperature heat treatment from perforated graphene oxide (pGO). pGO is produced by a short etching treatment with hydrogen peroxide. GO perforation predominated in a short etching time (∼1 h), inducing larger holes and defects compared to pristine GO. The pGO is advantageous to the formation of a pyridinic N-doped graphene because of strong NH3 adsorption on vacancies with oxygen functional groups during the nitrogen-doping process, and the pyridinic-N-doped graphene exhibits good electrocatalytic activity for oxygen reduction reaction (ORR). Using rotating-disk electrode measurements, we confirm that N-pGF undergoes a four-electron-transfer process during the ORR in alkaline and acidic media by possessing sufficient diffusion pathways and readily available ORR active sites for efficient mass transport. A comparison between Pt/N-pGF and commercial Pt/C shows that Pt/N-pGF has superior performance, based on its more positive onset potential and higher limiting diffusion current at -0.5 V.