Transition metal dichalcogenide two-dimensional materials have attracted significant attention due to their unique optical, mechanical, and electronic properties. For example, molybdenum disulfide (MoS) exhibits a tunable band gap that strongly depends on the numbers of layers, which makes it an attractive material for optoelectronic applications. In addition, recent reports have shown that laser thinning can be used to engineer an MoS monolayer with specific shapes and dimensions. Here, we study laser-thinned MoS in both ambient and vacuum conditions via confocal μ-Raman spectroscopy, imaging X-ray photoelectron spectroscopy (i-XPS), and atomic force microscopy (AFM). For low laser powers in ambient environments, there is insufficient energy to oxidize MoS, which leads to etching and redeposition of amorphous MoS on the nanosheet as confirmed by AFM. At high powers in ambient, the laser energy and oxygen environment enable both MoS nanoparticle formation and nanosheet oxidation as revealed in AFM and i-XPS. At comparable laser power densities in vacuum, MoS oxidation is suppressed and the particle density is reduced as compared to ambient. The extent of nanoparticle formation and nanosheet oxidation in each of these regimes is found to be dependent on the number of layers and laser treatment time. Our results can shed some light on the underlying mechanism of which atomically thin MoS nanosheets exhibit under high incident laser power for future optoelectronic applications.

Layered hafnium diselenide (HfSe2) is an emerging Van der Waals semiconductor in which a hafnium layer is sandwiched between two selenium layers. Owning to its indirect band gap with magnitudes close to silicon's band gap and high predicted carrier mobility, hafnium diselenide material is a strong candidate for device applications. Here, the effect of laser treatment on 2H‐ HfSe2 devices is shown in ambient conditions using µ‐Raman spectroscopy. It is shown that an emerging Raman peak evolves with increasing laser exposure time. It is also shown that top‐down fabricated 2H‐HfSe2 devices exhibit an anomalous p‐type behavior post laser treatment, with Ion/Ioff ratio as high as 103. This anomalous conductivity change can be observed after thermal and electrical annealing. For bottom‐up devices, it is observed that p‐type conductivity with remarkable Ion/Ioff ratio reaching 104. This conductivity switch can also be shown on 1T‐HfSe2 devices post laser irradiation and high Vds bias treatments. Based on the circuit model, this conductivity switch is attributed to contact doping caused by an increase in the Schottky barrier height at each contact, which shifts the Fermi energy closer to the valance band. These results demonstrate a unique conductivity switching mechanism for HfSe2‐FET devices.

No abstract

No abstract