SnS2/MoS2 van der Waals Heterostructure Photodetector with Ultrahigh Responsivity Realized by a Photogating Effect.

Photoresponsivity is a fundamental parameter used to quantify the ability of photoelectric conversion of a photodetector device. High-responsivity photodetectors are essential for numerous optoelectronic applications. Due to the strong light-matter interactions and the high carrier mobility, two-dimensional (2D) materials are promising candidates for the next-generation photodetectors. However, poor light absorption, lack of photoconductive gain, and the interfacial recombination lead to the relatively low responsivity of 2D photodetectors. The photogating effect, which extends the lifetime of photoexcited carriers, provides a simple approach to enhance responsivity in photodetector devices. Here, the O2 plasma treatment introduced surface traps on the SnS2 surface, leading to a gate-tunable photogating effect in SnS2/MoS2 heterojunctions. The heterojunction device exhibits an ultrahigh responsibility of up to 28 A/W. Moreover, the photodetector possesses a wide spectral photoresponse spanning from 300 to 1100 nm and a high specific detectivity (D*) of 4 × 1011 Jones under a 532 nm laser at VDS = 1 V. These results demonstrate that O2 plasma treatment is an efficient and simple avenue to achieve photogating effects, which can be employed to enhance the performance of van der Waals heterostructure photodetector devices and make them suitable for future integration into advanced electronic and optoelectronic systems.

Visualized and Nondestructive Quality Identification of Two-Dimensional MoS2 Based on Principal Component Analysis.

To date, the common quality characterizations for MoS2 are inefficient or cause irreversible damage to the samples, which have limited scalability and low throughput. Here, we propose a visualized and nondestructive approach to evaluate the quality of MoS2 based on the PCA machine learning method. Through PCA processing of PL mapping, the CVD grown MoS2 with different edge defect densities can be well distinguished. Furthermore, six twin GBs along the sulfur zigzag direction of the six pointed MoS2 stars are also successfully identified. To verify the correctness of the identification results, we measured the lifetime mapping and thermal expansion coefficient of the synthesized MoS2 samples. It is found that the high quality MoS2 samples have a shorter carrier lifetime (∼0.291 ns) and lower thermal expansion coefficient (∼2.03 × 10-5K-1). Therefore, our work offers a new approach to evaluate the quality of MoS2 to drive their practical application.

Strong Anisotropic Optical Properties by Rolling up MoS2 Nanoflake.

Although anisotropic two-dimensional materials have attracted great scientific interest, the anisotropy of those materials is limited to particular crystallographic directions. Herein, with dimension confining, MoS2 nanoscrolls are successfully fabricated by a rolling-up process after dropping an ethanol-water solution on a chemical vapor deposition-grown MoS2 monolayer. The anisotropic vibrational and optical properties are systematically studied by angle-resolved polarized spectroscopy, including Raman, photoluminescence, and reflection measurements. Upon comparing the photoluminescence results between MoS2 nanoscrolls and nanosheets, an obvious PL quenching phenomenon is observed, indicating the efficient separation of photon-induced carriers. Moreover, the time-resolved PL test identifying the lifetime of the carriers is decreased to 303 ps in the nanoscrolls, indicating a higher carrier-transfer efficiency. In summary, our work demonstrates the strong anisotropic optical properties of MoS2 nanorolls, showing the nanoscrolls are a promising candidate for the fabrication of multifunctional devices.