Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.
Controlled
substitutional doping of two-dimensional transition-metal
dichalcogenides (TMDs) is of fundamental importance for their applications
in electronics and optoelectronics. However, achieving p-type conductivity in MoS2 and WS2 is challenging
because of their natural tendency to form n-type
vacancy defects. Here, we report versatile growth of p-type monolayer WS2 by liquid-phase mixing of a host tungsten
source and niobium dopant. We show that crystallites of WS2 with different concentrations of substitutionally doped Nb up to
1014 cm–2 can be grown by reacting solution-deposited
precursor film with sulfur vapor at 850 °C, reflecting the good
miscibility of the precursors in the liquid phase. Atomic-resolution
characterization with aberration-corrected scanning transmission electron
microscopy reveals that the Nb concentration along the outer edge
region of the flakes increases consistently with the molar concentration
of Nb in the precursor solution. We further demonstrate that ambipolar
field-effect transistors can be fabricated based on Nb-doped monolayer
WS2.
Defects
are commonly found in two-dimensional (2D) transition-metal
dichalcogenide (TMD) materials. Such defects usually dictate the optical
and electrical properties of TMDs. It is thus important to develop
techniques to characterize the defects directly with good spatial
resolution, specificity, and throughput. Herein, we demonstrate that
Kelvin probe force microscopy (KPFM) is a versatile technique for
this task. It is able to unveil defect heterogeneity of 2D materials
with a spatial resolution of 10 nm and energy sensitivity better than
10 meV. KPFM mappings of monolayer WS2 exhibit interesting
work function variances that are associated with defects distribution.
This finding is verified by aberration-corrected scanning transmission
electron microscopy and density functional theory calculations. In
particular, a strong correlation among the work function, electrical
and optical responses to the defects is revealed. Our findings demonstrate
the potential of KPFM as an effective tool for exploring the intrinsic
defects in TMDs.
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