Optoelectronic Thinning of Transition Metal Dichalcogenides for Device Fabrication

Since the discovery of field effect of graphene, atomically thin two-dimensional (2D) materials have attracted strong interests. Unlike the zero-band-gap graphene, atomic monolayers of transition metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2 and WSe2 feature direct band gap. With their excellent electrical and optical properties, robust mechanical deformation, large surface-to-volume ratio, strong spin-orbit coupling and spin-valley locking, atomically thin TMDCs are promising for applications in photodetection, valleytronics, sensing, biomedical imaging, and drug delivery.

Many methods have been developed to obtain atomic monolayers of TMDCs, including mechanical exfoliation, liquid-phase exfoliation, electrochemical deposition, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE). However, the mechanical exfoliation method is not scalable and usually obtains monolayers of small sizes. The liquid-phase exfoliation can cause unwanted phase change of TMDCs. MBE and CVD can achieve sizable monolayers of TMDCs. However, the electrical properties of TMDCs grown by the bottom-up approaches are not as good as those by the top-down methods. MBE and CVD require complex and expensive instruments. Recently, a different top-down approach towards thinning bulk flakes of TMDCs to atomic monolayers has been demonstrated. Laser thinning is particularly promising because of its capability of low-cost, site-selective and on-demand patterning of atomic layers of TMDCs. Oxidation and sublimation caused by the laser opto-thermal effect were firstly used to remove materials at the shined site. The basal layers were remained as a result of poor interlayer thermal transfer of TMDCs and the thermal sink effect of the substrate. But the high temperatures (usually over 400 ℃) will result in undesired property alterations of the fabricated monolayers. Electrochemical laser thinning were demonstrated by promoting electrochemical reactions of TMDCs with the optically pumping electrons to the conduct band from the direct band gap. The laser power consumption and the working temperature are lower than the thermal oxidation and sublimation mechanisms, but very careful cautions are needed to take to obtain monolayer as the final layer is easy to be removed too.

Herein, we report a self-limiting optoelectronic thinning method where optically activated electrochemical reaction will stop automatically once TMDCs flakes are thinned into monolayers. The self-limiting property is enabled by only pumping electrons from the indirect band gap of multi- and few-layer TMDCs. Monolayers of MoS₂ with qualities as good as mechanically exfoliated ones have been obtained without our elaborate control of experimental conditions. Furthermore, the monolayers can be patterned by controlling the laser beam to drive the site-selective chemical reaction. With the capability of robust thinning, arbitrary patterning, and self-limiting, the optoelectronic method will enable mass production and patterning of high-quality monolayers of TMDCs for electronic and photonic nanodevices.