Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.

Transmittance and photocurrent (PC) spectroscopy has been used to study absorption in bulk 2H-MoS2 at energies close to its direct bandgap at the K-point of the Brillouin zone. Spectral lineshape analysis using the hydrogenic exciton model, together with temperature dependence of absorption and PC spectrum, and also bias dependence of PC, suggests that the feature previously identified as the n = 2 excited state transition of the A exciton, and used to estimate its binding energy Eb, has a different origin. The feature is reproduced in simulations only after including the recently identified H-point exciton transition. A consistent picture, which explains Eb in terms of other experimentally and theoretically determined parameters, emerges when excitons in bulk MoS2 are considered as quasi 2-dimensional with Eb∼84 meV for the A exciton at the K-point. This value when scaled appropriately matches fairly with a measured Eb of the A exciton in monolayer MoS2.

Apart from the defect related emission peak which lies ∼100 meV below the A exciton/trion peak and is labeled D1 here, this study shows that there is another distinct feature D2 lying ∼200 meV below A in the photoluminescence spectrum of the exfoliated monolayer MoS2 on SiO2/Si substrates. The D2 feature is explicitly resolved at low temperature only in few samples. Both D1 and D2 do not show circular polarization anisotropy for 633 nm excitation. Both decay with the increase in temperature in a seemingly activated manner with similar activation energy of ∼50 meV, but D1 decays earlier and therefore D2 dominates at high temperature in all samples. Annealing in vacuum increases both D1 and D2 emission intensities while annealing under sulfur vapour decreases them. Comparison with reported theoretical studies on defects in monolayer MoS2 suggests that these two emissions possibly involve excitons bound to single and double sulphur vacancies, the latter binding excitons more strongly.

This study describes the optoelectronic characteristics of CuFeS2/Si nanocrystal/bulk heterojunctions. These heterojunctions show a strong photocurrent response under ambient conditions upon excitation from a wide optical spectrum, from 460 to 2200 nm. The devices comprise of a heterojunction formed between heavily doped n‐type silicon (1–100 Ω cm) and copper iron sulfide (CuFeS2) nanocrystal films. Over the spectral range 460–2200 nm the device shows a fast response (20 µs at NIR wavelengths), along with responsivity and detectivity of 4.68 mA W−1 and 5.29 × 109 Jones at 1900 nm wavelength. The photocurrent is further observed to be a nonlinear function of power. These properties of the devices are discussed in terms of a defect filling mechanism. Besides their regular photoresponse described above, the devices also exhibit a slower photothermal response, allowing these to also sense hot objects (450 K; excess 6 mW incident onto the device) within the focal plane, thereby extending the useful sensing range of the devices deeper into the infrared.

Reflectance spectrum measured using an optical microscope with a large numerical aperture objective lens is shown to get modified. The change is most prominent when there are optical interference related features in the spectrum. This modification is shown to arise primarily due to the wide range of angles of incidence involved in the measurement and a simple formulation is provided to correct for this in simulations. The importance of such analysis is brought out through a reflectance contrast spectroscopy based study for identifying mono-layer and bi-layer graphene and MoS2.