Understanding the Link Between Photonic and Electronic Performance of 2D Semiconducting Layers

Susan Fullerton and her colleagues wrote a scientific report on deconvoluting the photonic and electronic response of two-dimensional (2D) materials for the case of molybdenum disulfide (MoS2).

What are the main criteria which provide evidence that the material is “high quality”? Are the photonic properties or electronic performance? Susan Fullerton and her colleagues have studied the MoS2 materials and their devices to answer this question and to find the correlation between electronic and optical properties in 2D materials. In their study, they used Raman, photoluminescence (PL), time-resolved photoluminescence (TRPL), high-resolution scanning transmission electron microscopy (HR-STEM), X-ray photoelectron spectroscopy (XPS), field effect transistors (FET) fabrication electrolyte gate application methods to characterize MoS2.

Their PL measurements demonstrate that the degree of domain alignment has the strong influence on the optical properties of the grain boundaries. They found that when the crystalline domains are not aligned, PL intensity exhibit dramatic enhancement.  Furthermore, their TRPL results revealed the exciton carrier lifetime enhancement along the grain-boundary region when domains are misaligned.  They found that photoluminescence and exciton carrier lifetime enhancement in MoS2 is the result of the presence of defective regions near the boundaries of misaligned domains and non-radiative recombination through their STEM and PL characterizations. The reason behind the defect induced enhancement in PL correlated to the local charge transfer between absorbed oxygen and Mo vacancies. They stated that such defect-mediated compensation doping yields g a larger population of neutral excitons with higher PL quantum yield and reduces the formation rate of trions. To explore the reason behind the decrease in the formation of trions they applied micro Raman studies. However, they didn’t detect any strain effect which may reduce the formation of trions.

They showed that the phenomenon of defect-induced PL enhancement can be directly engineered across the entire MoS2 layer by utilizing an O-terminated substrate surface (r-sapphire). The typical c-plane oriented sapphire (c-sapphire) exhibits an aluminium (Al) surface termination whereas r-plane sapphire is typically oxygen terminated. In the c-sapphire termination can be tuned between Al and a mixed Al-O termination which effects the electron doping to the MoS2 film due to the presence of Al-O bonds. The reduction in the free-electron transfer is likely responsible for the enhanced PL and carrier lifetimes noted at domain boundaries, and therefore, engineering this property may be desirable. They noted that this can be done by further increasing the fraction of O termination by considering different orientations of sapphire, such as r-plane sapphire. Their study on r-plane sapphire led to dramatic enhancements in the photophysical properties of monolayer MoS2 and its excited state lifetime. They demonstrated that Mo-O bonding responsible for enhanced photonic performance.

In addition, they studied the electronic properties of MoS2 FETs on c-plane and r-plane sapphire and evaluate the transport properties using a solid-polymer electrolyte gate to assess how photonic and electronic properties are linked. They found out that monolayer MoS2 on r-sapphire exhibits reduced transport properties compared with c-sapphire. They attributed this to the additional interaction at the MoS2/r-sapphire interface such as Mo-O bonding. Furthermore, they demonstrated higher sheet resistance and contact resistance on MoS2/r-sapphire than MoS2/c-sapphire.

In conclusion, they demonstrated that the Mo-O bonding responsible for enhanced photonic performance actually results in reduced electronic performance. Hence, they concluded that optical properties may not be correlated with electronic properties in 2D materials.

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