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Although tight oil reservoirs have abundant resources, their recovery efficiency is generally low. In recent years, CO2 injection huff-n-puff has become an effective method for improving oil recovery on the basis of depleted production of volume-fracturing horizontal wells in tight oil reservoirs. In order to study the effects of CO2 huff-n-puff (CO2-HnP) on production, a compositional numerical simulation study of CO2 huff-n-puff (CO2-HnP) was conducted in tight oil reservoirs with complex fractures. Embedded discrete fracture model technology was used in the simulations to characterize complex fractures. The process of CO2 huff-n-puff (CO2-HnP) was simulated, which consists of CO2 injection, CO2 soaking, and CO2 production. Taking into account the threshold pressure gradient and stress sensitivity in the model, we conducted a series of numerical simulations with different production condition parameters, such as bottom-hole pressure, CO2 injection rate, injection time, soaking time, and the number of cycles of CO2 huff-n-puff (CO2-HnP). Then, the effects of these sensitivity parameters on the cumulative oil production (COP) were studied. The results indicate that the threshold pressure gradient and rock stress sensitivity factors greatly affect the pressure field of tight reservoirs and the cumulative oil production (COP) of multistage-fracturing horizontal wells. The production parameters all have an impact on the COP. The injection rate and circulation number both have optimal values, and the injection time and soak time tend to have less significant effects on the growth of cumulative oil production over time. According to the numerical simulation, the optimal solution is 5 × 104 m3/day injection rate per cycle, 25 days of injection time, 35 days of soaking time, three cycles, and production for 5 years, which can obtain the optimal cumulative oil production. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) vertical heterojunctions possess peculiar electronic and optoelectronic properties according to their complementary composition, ultrathin thickness, and unique geometry. However, it is still a challenge to synthesize 2D vertical heterojunctions based on non-layered materials due to strong covalent bonds and abundant surface dangling bonds. Here we developed a partial cation exchange method for the synthesis of 2D vertical Cu2S/CdS heterojunctions. Large-size and single-crystalline CdS nanosheets prepared by a van der Waals epitaxy method served as host lattices for cation exchange reactions. The evolution of the nanosheets in terms of their morphology, composition and crystal structure was elaborately characterized. The as-prepared 2D vertical heterojunctions have two-layer ultra-thin structures and form large-area junctions at their interfaces. Under light irradiation, the photogenerated electron–hole pairs can be quickly separated under the electric field in the junction region, minimizing the possibility of charge recombination. As a result, the device based on 2D vertical Cu2S/CdS heterojunctions exhibits a higher photoelectric conversion capability compared with the device based on single-component 2D CdS or Cu2S Schottky junctions. Our work opens up opportunities to explore 2D vertical heterojunctions based on non-layered materials for high-performance optoelectronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) MoS2 has important applications in the fields of photodetection, photovoltaics, and light-emitting diodes (LEDs) due to its superior electronic and optoelectronic properties. However, the planar thin-layered MoS2 usually suffers from low light-absorbing ability because of the short transmission path of incident light, which limits the photoelectric conversion quantum efficiency of the thin-layered MoS2-based photodetectors. Herein, we demonstrate a high-performance ZnO–MoS2 core–shell nanopillar (NP) array-based photodetector by depositing thin-layered MoS2 on ZnO NP arrays via magnetron sputtering. With the aid of the ZnO NP arrays, which show a strong light trapping effect, the light-absorbing ability of thin-layered MoS2 is greatly enhanced. As a result, the ZnO–MoS2 core–shell NP array-based photodetector exhibits a broadband response spanning from the ultraviolet (UV) to the visible and to the near-infrared (NIR) light region. Compared with the ZnO–MoS2 planar heterojunction, the photocurrent of the NP array-based photodetector under illumination is improved by up to 60 times. Our work opens up new opportunities for the development of high-performance, broadband photodetectors based on 2D semiconductor materials. [ABSTRACT FROM AUTHOR]

Nano-field-effect transistor (nano-FET)-based nonvolatile memory (NVM) devices fabricated from one-dimensional (1D) nanostructures have attracted much attention due to their superior memory performance. However, the construction of NVM devices is relatively complex due to the multi-step process needed to fabricate the trapping/tunneling layers. Here, we report a one-step fabrication of CdS:Mo–CdMoO4 core–shell nanoribbons (NRs) for high-performance nano-FET-based NVM devices. The CdMoO4 shell could serve as both the charge storage media and tunneling layer, thus greatly facilitating the device construction. The resultant NVM devices exhibited a large memory window of 60 V as well as a long retention time of 3600 s. Significantly, the devices showed an excellent resistance switching behavior with a current ON/OFF ratio as high as 106, which is larger than those of most 1D nanostructure-based nano-FET memories. A mechanism associated with defect states caused by oxygen vacancies in the CdMoO4 shell was proposed to interpret the memory characteristics. Given the excellent memory performance, along with the simple one-step fabrication process, CdS:Mo–CdMoO4 NR-based NVM devices will have important applications in new-generation high-performance NVM devices. [ABSTRACT FROM AUTHOR]