No abstract

Gallium selenide (GaSe) is a van der Waals semiconductor widely used for optoelectronic devices, whose performances are dictated by bulk properties, including band-gap energy. However, recent experimental observations that the exfoliation of GaSe into atomically thin layers enhances performances in electrochemistry and photocatalysis have opened new avenues for its applications in the fields of energy and catalysis. Here, it is demonstrated by surface-science experiments and density functional theory (DFT) that the oxidation of GaSe into Ga 2 O 3 , driven by Se vacancies and edge sites created in the exfoliation process, plays a pivotal role in catalytic processes. Specifically, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are energetically unfavorable in pristine GaSe, due to energy barriers of 1.9 and 5.7-7.4 eV, respectively. On the contrary, energy barriers are reduced concurrently with surface oxidation. Especially, the Heyrovsky step (H ads + H + + e − → H 2) of HER becomes energetically favorable only in sub-stoichiometric Ga 2 O 2.97 (−0.3 eV/H +). It is also discovered that the same mechanisms occur for the case of the parental compound indium selenide (InSe), thus ensuring the validity of the model for the broad class of III-VI layered semiconductors.

The successful growth of high-quality SnSe 2 crystals by chemical vapour transport (CVT) and epitaxial films by molecular beam epitaxy (MBE) was reported by several groups 1-2 . However, the productivity by CVT and MBE is far below than that by the Bridgman technique. Compared with the high cost of equipment of MBE, both CVT and Bridgman methods are relatively faster and more economic. However, the CVT growth would introduce a trace of transport agent, with an obvious influence on the carrier density for semiconductor, and would also produce chalcogen vacancies at high temperature. For the CVT-grown SnSe 2 crystal, the suppression of Se vacancies can be achieved by adding excess Se and lowering the temperature gradient. The Se vacancies can be controlled by turning the source beam flux ratio in the MBE growth, while the lattice-matched substrate must be carefully chosen. Therefore, we can conclude the Bridgman growth is the most suitable method for up scaling for the case of SnSe 2 .

Liquid-phase exfoliation is the most suitable platform for large-scale production of two-dimensional materials. One of the main open challenges is related to the quest of green and bioderived solvents to replace state-of-the-art dispersion media, which suffer several toxicity issues. Here, we demonstrate the suitability of methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv Polarclean) for sonication-assisted liquid-phase exfoliation of layered materials for the case-study examples of WS 2 , MoS 2 , and graphene. We performed a direct comparison, in the same processing conditions, with liquid-phase exfoliation using N -methyl-2-pyrrolidone (NMP) solvent. The amount of few-layer flakes (with thickness <5 nm) obtained with Polarclean is increased by ∼350% with respect to the case of liquid-phase exfoliation using NMP, maintaining comparable values of the average lateral size, which even reaches ∼10 μm for the case of graphene produced by exfoliation in Polarclean, and of the yield (∼40%). Correspondingly, the density of defects is reduced by 1 order of magnitude by Polarclean-assisted exfoliation, as evidenced by the I (D)/ I (G) ratio in Raman spectra of graphene as low as 0.07 ± 0.01. Considering the various advantages of Polarclean over state-of-the-art solvents, including the absence of toxicity and its biodegradability, the validation of superior performances of Polarclean in liquid-phase exfoliation paves the way for sustainable large-scale production of nanosheets of layered materials and for extending their use in application fields to date inhibited by toxicity of solvents (e.g., agri-food industry and desalination), with a subsequent superb impact on the commercial potential of their technological applications.

DNA-stabilized silver nanoclusters (DNA-AgNCs) are promising fluorophores whose photophysical properties and synthesis procedures have received increased attention in the literature. However, depending on the preparation conditions and the DNA sequence, the DNA-AgNC samples can host a range of different emitters, which can influence the reproducibility of the optical response and the evolution over time of the populations of these emitters. We have developed a simple method to characterize the spectral heterogeneity and time evolution of these emissive species at any given point in time after preparation, by plotting the average decay time as a function of emission wavelength. These so-called average decay time spectra were acquired for different excitation wavelengths of AgNCs stabilized by an oligonucleotide containing 24 cytosines (C24-AgNCs). The average decay time spectra allowed the comparison of sample preparation and the judgment of reproducibility. Therefore, we propose the use of the average decay time spectra as a robust and easy tool to characterize and compare different as-synthesized DNA-AgNC samples. The average decay time spectra can in general also be used to characterize the spectral heterogeneity of other fluorophores, such as luminescent colloidal nanoparticles, and to assess the reproducibility of a synthetic procedure containing an unknown distribution of emissive species.