The notable transformation of the electronic properties of transition-metal dichalcogenides (TMDs) when reduced to a single X-M-X plane (X: chalcogen; M: metal) [1] makes them suitable for flexible, innovative optoelectronic devices, [2][3][4] and transistors. [5] Like graphene, few-layer TMDs can also withstand surprisingly large mechanical deformations, [6][7][8][9] which, coupled to the material's electronic structure, would enable the observation of nondissipative topological transport, provided a periodic modulation of strain is attained. [10][11][12][13] TMD monolayers (MLs) and nanostructures are also important for their catalytic role in the cost-effective production of hydrogen. [14][15][16] These examples share the need to achieve spatial control of the material's properties, over sample regions with size ranging from the nano [14,16] to the micrometer [16] scale lengths.In this study, we present a route toward the patterning of TMDs based on the effects of low-energy proton irradiation [17] on the structural and electronic properties of bulk WS 2 , WSe 2 , WTe 2 , MoS 2 , MoSe 2 , and MoTe 2 . Suitable irradiation conditions trigger the production and accumulation of H 2 just beneath the first X-M-X basal plane, leading to the localized exfoliation of the topmost monolayer and to the formation of spherically shaped domes. Structural and optical characterizations confirm that these domes are typically one ML-thick and contain H 2 at pressures in the 10-100 atm range, depending on their size. Such high pressures induce strong and complex strain fields acting on the curved X-M-X planes, that are evaluated by means of a mechanical model. The domes' morphological characteristics can be tuned by lithographically controlling the area of the sample basal plane participating in the hydrogen production process. This results in the unprecedented fabrication of robust domes with controlled position/density and sizes tunable from the nanometer to the micrometer scale, that, by virtue of their inherently strained nature and geometry, might prompt a variety of applications.The samples, consisting of thick (tens to hundreds of MLs) TMD flakes, were obtained by mechanical exfoliation, deposited on Si substrates, and afterwards proton-irradiated using a Kaufman source (see the Experimental Methods). Differently from the other works in the literature concerning protonirradiation of TMDs-where beams with energies ≥10 5 eV are used, [18] aiming at the controlled formation of defects in the irradiated samples-here we irradiate the flakes with low energy At the few-atom-thick limit, transition-metal dichalcogenides (TMDs) exhibit strongly interconnected structural and optoelectronic properties. The possibility to tailor the latter by controlling the former is expected to have a great impact on applied and fundamental research. As shown here, proton irradiation deeply affects the surface morphology of bulk TMD crystals. Protons penetrate the top layer, resulting in the production and progressive accumulation of molecular hydr...

The band-anticrossing (BAC) model has been widely applied to analyse the electronic structure of dilute nitride III-V-N alloys such as GaN x As 1−x . The BAC model describes the strong band gap bowing observed at low N composition in GaN x As 1−x in terms of an interaction between the GaAs host matrix conduction band edge and a higher lying band of localized N resonant states. In practice, replacing As by N introduces a range of N-related defect levels, associated with isolated N atoms, N-N pairs and larger clusters of N atoms. We show that the effect of such defect levels on the alloy conduction band structure is strongly dependent on the relative energy of the defect levels and the host conduction band edge. We first consider GaN x As 1−x , where we show that the unexpectedly large electron effective mass and gyromagnetic ratio, and their non-monotonic variation with x, are due to hybridization between the conduction band edge and specific nitrogen states close to the band edge. The N-related defect levels lie below the conduction band edge in GaN x P 1−x . We must therefore explicitly treat the interaction between the higher lying GaP host conduction band minimum and defect states associated with a random distribution of N atoms in order to obtain a good description of the lowest conduction states in disordered GaPN alloys. Turning to other alloys, N-related defect levels should generally lie well above the conduction band minimum in InNSb, with the band dispersion of InNSb then well described by a two-level BAC model. Both InP and InAs are intermediate between InSb and GaAs. By contrast, we calculate that N-related defect levels lie close to the conduction band minimum in GaNSb, and will therefore strongly perturb the lowest conduction states in this alloy. Overall, we conclude that the BAC model provides a good qualitative explanation of the electronic properties of dilute nitride alloys, but that it is in many cases necessary to include the details of the distribution of N-related defect levels to obtain a quantitative understanding of the conduction band structure in dilute nitride alloys.

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Recently, it was shown that lasing from epitaxial Ge quantum dots (QDs) on Si substrates can be obtained if they are partially amorphized by Ge ion bombardment (GIB). Here, we present a model for the microscopic origin of the radiative transitions leading to enhanced photoluminescence (PL) from such GIB-QDs. We provide an energy level scheme for GIB-QDs in a crystalline Si matrix that is based on atomistic modeling with Monte Carlo (MC) analysis and density functional theory (DFT). The level scheme is consistent with a broad variety of PL experiments performed on as-grown and annealed GIB-QDs. Our results show that an extended point defect consisting of a split-[110] self-interstitial surrounded by a distorted crystal lattice of about 45 atoms leads to electronic states at the Γ-point of the Brillouin zone well below the conduction band minimum of crystalline Ge. Such defects in Ge QDs allow direct transitions of electrons localized at the split-interstitial with holes confined in the Ge QD. We identify the relevant growth and annealing parameters that will let GIB-QDs be employed as an efficient laser active medium.