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The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2. [ABSTRACT FROM AUTHOR]

We fabricated large-area atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) by sulfurization at relatively low temperatures. The obtained MoS2 sheets are polycrystalline (~10–20 nm single-crystal domain size) with areas of up to 300 × 300 µm2, 2–4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements. [ABSTRACT FROM AUTHOR]

Gallium selenide (GaSe) is a novel 2D material, which belongs to the layered III–VIA semiconductors family and attracted interest recently as it displays single‐photon emitters at room temperature and strong optical nonlinearity. Nonetheless, few‐layer GaSe is not stable under ambient conditions and it tends to degrade over time. Here atomic force microscopy, Raman spectroscopy, and optoelectronic measurements are combined in photodetectors based on thin GaSe to study its long‐term stability. It is found that the GaSe flakes exposed to air tend to decompose forming first amorphous selenium and Ga2Se3 and subsequently Ga2O3. While the first stage is accompanied by an increase in photocurrent, in the second stage, a decrease in photocurrent is observed, which leads to the final failure of GaSe photodetectors. Additionally, it is found that the encapsulation of the GaSe photodetectors with hexagonal boron nitride (h‐BN) can protect the GaSe from degradation and can help to achieve long‐term stability of the devices. The degradation of thin GaSe in air is a complex process that involves photooxidation and generates various byproducts such as elemental selenium and Ga2O3. This degradation has profound influences on the optoelectronic properties of devices based on this layered material. [ABSTRACT FROM AUTHOR]

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2. [ABSTRACT FROM AUTHOR]