A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.

Efficient evolution of hydrogen through electrocatalysis at low overpotentials holds tremendous promise for clean energy. Hydrogen evolution can be easily achieved by electrolysis at large potentials that can be lowered with expensive platinum-based catalysts. Replacement of Pt with inexpensive, earth-abundant electrocatalysts would be significantly beneficial for clean and efficient hydrogen evolution. To this end, promising results have been reported using 2H (trigonal prismatic) XS₂ (where X  =  Mo or W) nanoparticles with a high concentration of metallic edges. The key challenges for XS₂ are increasing the number and catalytic activity of active sites. Here we report monolayered nanosheets of chemically exfoliated WS₂ as efficient catalysts for hydrogen evolution with very low overpotentials. Analyses indicate that the enhanced electrocatalytic activity of WS₂ is associated with the high concentration of the strained metallic 1T (octahedral) phase in the as-exfoliated nanosheets. Our results suggest that chemically exfoliated WS₂ nanosheets are interesting catalysts for hydrogen evolution.

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Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.

Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of atomic layers of InSe. Correlation between resonance Raman spectroscopy and photoconductivity measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few atomic layers. Analysis of photoconductivity spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temperature-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized pz-like orbitals, and these orbitals couple strongly with the in-plane E' and E″ phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications.

Transport properties of progressively reduced graphene oxide (GO) are described.Evolution of the electronic properties reveals that as-synthesized GO undergoes insulator-semiconductor-semi-metal transitions with reduction. The apparent transport gap ranges from 10 ~ 50 meV and approaches zero with extensive reduction. Measurements at varying degrees of reduction reveal that transport in reduced GO occurs via variable-range hopping and further reduction leads to increased number of available hopping sites.

Tuning to G(O) flat: Photoluminescence in graphene oxide (GO) suspensions can be tuned from red to blue emission (see scheme) by gradually changing the amounts of sp2‐ and sp3‐bonded carbon atoms through reduction of the surface oxide groups. Electron–hole recombination from two different types of excited states is proposed to explain the luminescence in GO at varying degrees of reduction.

We performed low temperature photoluminescence (PL) studies on individual oxygen-doped single-walled carbon nanotubes (SWCNTs) and correlated our observations to electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the E11 bright exciton peak. Our simulation suggests an association of these peaks with deep trap states tied to different specific chemical adducts. In addition, oxygen doping is also observed to split the E11 exciton into two or more states with an energy splitting <40 meV. We attribute these states to dark states that are brightened through defect-induced symmetry breaking. While the wave functions of these brightened states are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with our experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from interaction between pinned excitons and one-dimensional phonons. Exciton pinning also increases the sensitivity of the deep traps to the local dielectric environment, leading to a large inhomogeneous broadening. Observations of multiple spectral features on single nanotubes indicate the possibility of different chemical adducts coexisting on a given nanotube.