An n-type MoS monolayer grown by chemical vapor deposition method was partially hybridized with an organic semiconducting p-type tetracene thin film. The photoluminescence (PL) intensity in the hybrid region of the MoS/tetracene is clearly lower than that of pristine tetracene because of the charge-transfer effect, which was confirmed by the decrease in exciton lifetimes. Decrease in the temperature led to blue-shift in the PL peak position of MoS layers and, consequently, the PL intensities of both tetracene and MoS considerably increased owing to the decrease in phonon interaction. The PL spectra of bound excitons in the hybrid region were clearly observed at low temperatures, indicating the formation of trap states. The lateral-type n-p heterojunction field-effect transistors (FETs) using the MoS/tetracene hybrid as an active layer showed gate-tunable rectification I- V and anti-ambipolar field-effect characteristics with hysteresis effect. The charge transport characteristics across the n-p heterojunction of the hybrid region of the FET can be explained in terms of the Shockley-Read-Hall trap-intermediated tunneling and Langevin recombination mechanisms. To improve the performance of MoS/tetracene-based FET, a dielectric hexagonal boron nitride (h-BN) thin layer was inserted between the SiO surface and the active MoS layer. We observed the decrease in the hysteresis effect and threshold voltage of the h-BN/MoS/tetracene-based FETs due to the decrease in the number of traps at the interface. The performance of h-BN/MoS/tetracene FET device was also enhanced after the annealing process.

The electrical and optical characteristics of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be improved by surface modification. In this study, distinctive field-effect transistors (FETs) were realized by forming cross-type 2D WSe2/MoS2 p–n heterojunctions through surface treatment using poly­(methyl methacrylate-co-methacrylic acid) (PMMA-co-PMAA). The FETs were applied to new ternary inverters as multivalued logic circuits (MVLCs). Laser confocal microscope photoluminescence spectroscopy indicated the generation of trions in the WSe2 and MoS2 layers, and the intensity decreased after PMMA-co-PMAA treatment. For the cross-type WSe2/MoS2 p–n heterojunction FETs subjected to PMMA-co-PMAA treatment, the channel current and the region of anti-ambipolar transistor characteristics increased considerably, and ternary inverter characteristics with three stable logic states, “1”, “1/2”, and “0”, were realized. Interestingly, the intermediate logic state 1/2, which results from the negative differential transconductance characteristics, was realized by the turn-on of all component FETs, as the current of the FETs increased after PMMA-co-PMAA treatment. The electron-rich carboxyl acid moieties in PMMA-co-PMAA can undergo coordination with the metal Mo or W atoms present in the Se or S vacancies, respectively, resulting in the modulation of charge density. These features yielded distinctive FETs and ternary inverters for MVLCs using cross-type WSe2/MoS2 heterojunctions.

We demonstrate the fine control of the nanoscale photoluminescence (PL) and optical waveguiding characteristics of light-emitting organic rubrene nanorods (NRs) through focused electron-beam (E-beam) irradiation. Nanoscale laser confocal microscope PL intensity and spectra of the focused E-beam-treated compartments of the rubrene NR drastically varied with Ebeam dose, and yellow-white light emission from those compartments with a specific E-beam dose was observed. In optical waveguiding experiments, the propagation characteristics of optical signals along the single NR were dependent on the crystalline structure of the rubrene.For the E-beam treated NR, the waveguiding PL signals along the crystalline a-axis had relatively higher output, indicating the major axis for the optical energy transfer. Additionally, the waveguiding characteristics such as decay constant along the crystalline b-axis and c-axis of the NR considerably varied at the boundary of the E-beam treatment.

Van der Waals (vdW) heterojunctions, which consist of p-type and n-type semiconductors, have provided new features for transition metal dichalcogenides (TMDs). In this work, a negative differential transconductance (NDT) transistor based on a MoSe2-WSe2 heterojunction (MoSe2-WSe2 H-TR) is proposed. The MoSe2-WSe2 H-TR provides desirable device characteristics for ternary circuit operation with a switching behavior of off-state / p-type turn-on / NDT region / p-type turn-on. As a result, a 100% output voltage (VOUT) swing inverter can be achieved in a ternary inverter, which consists of the proposed MoSe2-WSe2 vdW-H-TR and a MoS2 floating-gate transistor. Furthermore, a tunable intermediate-logic ternary circuit operation is demonstrated by controlling the threshold voltage (VTH) in a pull-down n-type MoS2 floating-gate transistor. We also investigated that a light-induced operation on the MoSe2-WSe2 vdW-H-TR offers control of the VOUT amplitude at the intermediate-logic state. Based on the proposed MoSe2-WSe2 vdW-H-TR, this work suggests a strategy to obtain a tunable ternary circuit, thus providing a new concept of heterojunction electronics using layered TMDs.

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