As an important supplementary material to graphene in the optoelectronics field, molybdenum disulfide (MoS₂) has attracted attention from researchers due to its good light absorption capacity and adjustable bandgap. In this paper, MoS₂ layers are respectively grown on SiO₂/Si and sapphire substrates by atmospheric pressure chemical vapor deposition (APCVD). Atomic force microscopy, optical microscopy, and Raman and photoluminescence spectroscopy are used to probe the optical properties of MoS₂ on SiO₂/Si and sapphire substrates systematically. The peak shift between the characteristic A_1g and E¹_2g peaks increases, and the I peak of the PL spectrum on the SiO₂/Si substrate redshifts slightly when the layer numbers were increased, which can help in obtaining the layer number and peak position of MoS₂. Moreover, the difference from monolayer MoS₂ on the SiO₂/Si substrate is that the B peak of the PL spectrum has a blueshift of 56 meV and the characteristic E¹_2g peak of the Raman spectrum has no blueshift. The 1- and 2-layer MoS₂ on a sapphire substrate had a higher PL peak intensity than that of the SiO₂/Si substrate. When the laser wavelength is transformed from 532 to 633 nm, the position of I exciton peak has a blueshift of 16 meV, and the PL intensity of monolayer MoS₂ on the SiO₂/Si substrate increases. The optical properties of MoS₂ can be obtained, which is helpful for the fabrication of optoelectronic devices.

By adopting the first-principles plane wave pseudopotential method based on density functional theory, the electronic structure properties of single-layer MoS₂ (molybdenum disulfide) crystals under biaxial strain are studied. The calculation results in this paper show that when a small strain is applied to a single-layer MoS₂, its band structure changes from a direct band gap to an indirect band gap. As the strain increases, the energy band still maintains the characteristics of the indirect band gap, and the band gap shows a linear downward trend. Through further analysis of the density of states, sub-orbital density of states, thermodynamic parameters and Raman spectroscopy, it revealed the variation of single-layer MoS₂ with strain. This provides a theoretical basis for realizing the strain regulation of MoS₂.

To improve the on-state current and reduce the miller capacitance of the conventional junction-less tunneling field effect transistor (JLTFET), the junction-less TFET with Ge/Si_0.3Ge_0.7/Si heterojunction and heterogeneous gate dielectric (H-JLTFET) is investigated by the Technology Computer Aided Design (TCAD) simulation in this paper. The source region uses the narrow bandgap semiconductor material germanium to obtain the higher on-state current; the gate dielectric adjacent to the drain region adopts the low-k dielectric material SiO₂, which is considered to reduce the gate-to-drain capacitance effectively. Moreover, the gap region uses the Si_0.3Ge_0.7 material to decrease the tunneling distance. In addition, the effects of the device sizes, doping concentration and work function on the performance of the H-JLTFET are analyzed systematically. The optimal on-state current and switching ratio of the H-JLTFET can reach 6 µA/µm and 2.6 × 10¹², which are one order of magnitude and four orders of magnitude larger than the conventional JLTFET, respectively. Meanwhile, the gate-to-drain capacitance, off-state current and power consumption of the H-JLTFET can be effectively suppressed, so it will have a great potential in future ultra-low power integrated circuit applications.

A variety of hetero-junctions can be constructed to form the basic structural units in the different optoelectronic devices, such as the photo-detectors, solar cells, sensors and light-emitting diodes. In our research, the large-area high-quality MoS₂/WS₂ vertical hetero-junction are prepared by the two-step atmospheric pressure chemical vapor deposition (APCVD) methods and the dry transfer method, and the corresponding optimal reaction conditions of MoS₂/WS₂ vertical hetero-junction are obtained. The morphology, composition and optical properties of MoS₂/WS₂ vertical hetero-junction are systematically characterized by the optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and the field emission scanning electron microscopy. Compared to the mechanical transfer method, the MoS₂/WS₂ vertical hetero-junction sample obtained by the APCVD and dry transfer methods have lower impurity content, cleaner interfaces and tighter interlayer coupling. Besides, the strong interlayer coupling and effective interlayer charge transfer of MoS₂/WS₂ vertical hetero-junction are also further studied. The photoluminescence intensity of MoS₂/WS₂ vertical hetero-junction is significantly reduced compared to the single MoS₂ or WS₂ material. In general, this research can help to achieve the large-scale preparation of various Van der Waals hetero-junctions, which can lay the foundation for the new application of optoelectronic devices.

Transition metal dichalcogenides (TMDCs) have received wide attention as a new generation of semiconductor materials. However, there are still many problems to be solved, such as low carrier mobility, contact characteristics between metal and two-dimensional materials, and complicated fabrication processes. In order to overcome these problems, a large amount of research has been carried out so that the performance of the device has been greatly improved. However, most of these studies are based on complicated fabrication processes which are not conducive to the improvement of integration. In view of this problem, a horizontal-gate monolayer MoS₂ transistor based on image force barrier reduction is proposed, in which the gate is in the same plane as the source and drain and comparable to back-gated transistors on-off ratios up to 1 × 10⁴ have been obtained. Subsequently, by combining the Y-Function method (YFM) and the proposed diode equivalent model, it is verified that Schottky barrier height reduction is the main reason giving rise to the observed source-drain current variations. The proposed structure of the device not only provides a new idea for the high integration of two-dimensional devices, but also provides some help for the study of contact characteristics between two-dimensional materials and metals.

Self-powered photodetectors that do not require external power support are expected to play a key role in future photodetectors due to their low power characteristics, but achieving high responsivity remains a challenge. 2D van der Waals heterojunctions are a promising technology for high-performance self-powered photodetectors due to their excellent optical and electrical properties. Here, we fabricate a self-powered photodetector based on In₂Se₃/WSe₂/ReS₂ van der Waals heterojunction self-powered photodetector. Due to the presence of ReS₂ layer, photocurrent is enhanced as a result of the increase in light absorption efficiency and the effective region for generating photogenerated carriers. The built-in electric field is enhanced by a negative ‘back-gate voltage’ along the p–n junction vertical direction generated by the electrons in the photo-generated electrons accumulation layer. Accordingly, the optical responsivity and the photoresponse speed of this heterojunction self-powered photodetector are greatly boosted. The proposed self-powered photodetector based on the In₂Se₃/WSe₂/ReS₂ heterojunction exhibits a high responsivity of 438 mA W⁻¹, which is 17 times higher compared to the In₂Se₃/WSe₂ photodetector, a self-powered current (1.1 nA) that is an order of magnitude higher than that of the In₂Se₃/WSe₂ photodetector, and a fast response time that is 250% faster. Thus the self-powered photodetector with a stronger built-in electric field and a wider depletion zone can provide a new technological support for the fabrication of high responsivity, low power consumption and high speed self-powered photodetectors based on van der Waals heterojunctions.