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.

In this work we study the optoelectronic properties of individual TiO2 fibres produced through coupled sol-gel and electrospinning, by depositing them onto pre-patterned Ti/Au electrodes on SiO2/Si substrates. Transport measurements in the dark give a conductivity above 2·10 -5 S, which increases up to 8·10 -5 S in vacuum. Photocurrent measurements under UV-irradiation show high sensitivity (responsivity of 90 A·W -1 for 375 nm wavelength) and a response time to illumination of ~ 5 s, which is superior to state-of-the-art TiO2-based UV photodetectors. Both responsivity and response speed are higher in air than in vacuum, due to oxygen adsorbed on the TiO2 surface which traps photoexcited free electrons in the conduction band, thus reducing the recombination processes. The photodetectors are sensitive to light polarization, with an anisotropy ratio of 12%. These results highlight the interesting combination of large surface area and low 1D transport resistance in electrospun TiO2 fibres. The simplicity of the sol-gel/electrospinning synthesis method, combined with a fast response and high responsivity makes them attractive candidates for UV-photodetection in ambient conditions. We anticipate their high (photo) conductance is also relevant for photocatalysis and dye-sensitized solar cells.

The research field of two dimensional (2D) materials strongly relies on optical 21 microscopy characterization tools to identify atomically thin materials and to determine 22 their number of layers. Moreover, optical microscopy-based techniques also opened the 23 door to study the optical properties of these nanomaterials. We present a comprehensive 24 study of the differential reflectance spectra of 2D semiconducting transition metal 25 dichalcogenides (TMDCs), MoS2, MoSe2, WS2 and WSe2, with thickness ranging from 26 one layer up to six layers. We analyze the thickness-dependent energy of the different 27 excitonic features, indicating the change in the band structure of the different TMDC 28 materials with the number of layers. Our work provides a route to employ differential 29 reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2 and 30 WSe2. 31 32The isolation of atomically thin semiconducting TMDCs by mechanical exfoliation of 33 bulk layered crystals has aroused the interest of the nanoscience and nanotechnology transmittance with different number of layers in a wide spectral range, which is still 10 lacking. 11Here we systematically study the differential reflectance of single-and few-layer MoS2, 12MoSe2, WS2 and WSe2 from the near-infrared (1.4 eV) to the near-ultraviolet (3.0 eV). 13The differential reflectance spectra show prominent features due to excitons. The 14 thickness dependence of these excitonic features is analyzed. The optical spectra of the fabricated flakes are characterized by using a homebuilt micro- transmittance measurements acquired on the same sample. 9The differential reflectance spectrum is calculated as (R-R0)/R and it is related to the 10 absorption coefficient of the material α(λ) as [34,35] 11where R is the intensity reflected by the flake, R0 the intensity reflected by the substrate, 13n is the refractive index of the flake under study and n0 is the refractive index of the 14 substrate. Figure 2 shows the differential reflectance spectra measured on the single-and Brillouin zone. [18][19][20] This feature is the most studied one, as it is also the dominant one TMDCs, the origin of this higher energy transition at the K point is related to the splitting 5 of the valence band due to the spin-orbit interaction. For multilayer systems, the splitting 6 of the valence band is driven by a combination of spin-orbit-and interlayer interaction. 7Apart from the narrow A and B exciton peaks, the differential reflectance spectra of 8 MoS2, MoSe2 and WS2 also show other broader spectroscopic features in an energy range 9 from 2.5 eV to 2.9 eV (referred to as C exciton peak), which is due to singularities in the 10 joint density of states between the first valence and conduction bands in a circle around 18The differential reflectance spectra have been fit to a sum of Gaussian/Lorentzian peaks Interestingly, we also find that the C exciton shows a prominent shift with the thickness, Peer-reviewed version available at Nanomaterials 2018, 8, 725; doi:10.33...