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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 .

After the recent finding that CrI 3 , displays ferromagnetic order down to its monolayer, extensive studies have followed to pursue new two-dimensional (2D) magnetic materials. In this article, we report on the growth of single crystal CrCl 3 in the layered monoclinic phase. The system after mechanical exfoliation exhibits stability in ambient air (the degradation occurs on a time scale at least four orders of magnitude longer than is observed for CrI 3 ). By means of mechanical cleavage and atomic force microscopy (AFM) combined with optical identification, we demonstrate the systematic isolation of single and few layer flakes onto 270 nm and 285 nm SiO 2 /Si (100) substrates with lateral size larger than graphene flakes isolated with the same method. The layer number identification has been carried with statistically significant data, quantifying the optical contrast as a function of the number of layers for up to six layers. Layer dependent optical contrast data have been fitted within the Fresnel equation formalism determining the real and imaginary part of the wavelength dependent refractive index of the material. A layer dependent (532 nm) micro-Raman study has been carried out down to two layers with no detectable spectral shifts as a function of the layer number and with respect to the bulk.

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.

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.

By means of theory and experiments, the application capability of nickel ditelluride (NiTe 2 ) transition-metal dichalcogenide in catalysis and nanoelectronics is assessed. The Te surface termination forms a TeO 2 skin in an oxygen environment. In ambient atmosphere, passivation is achieved in less than 30 min with the TeO 2 skin having a thickness of about 7 Å. NiTe 2 shows outstanding tolerance to CO exposure and stability in water environment, with subsequent good performance in both hydrogen and oxygen evolution reactions. NiTe 2 -based devices consistently demonstrate superb ambient stability over a timescale as long as one month. Specifically, NiTe 2 has been implemented in a device that exhibits both superior performance and environmental stability at frequencies above 40 GHz, with possible applications as a receiver beyond the cutoff frequency of a nanotransistor.

Palladium ditelluride (PdTe 2 ) is a novel transition-metal dichalcogenide exhibiting type-II Dirac fermions and topological superconductivity. To assess its potential in technology, its chemical and thermal stability is investigated by means of surfacescience techniques, complemented by density functional theory, with successive implementation in electronics, specifically in a millimeter-wave receiver. While water adsorption is energetically unfavorable at room temperature, due to a differential Gibbs free energy of ≈+12 kJ mol −1 , the presence of Te vacancies makes PdTe 2 surfaces unstable toward surface oxidation with the emergence of a TeO 2 skin, whose thickness remains sub-nanometric even after one year in air. Correspondingly, the measured photocurrent of PdTe 2 -based optoelectronic devices shows negligible changes (below 4%) in a timescale of one month, thus excluding the need of encapsulation in the nanofabrication process. Remarkably, the responsivity of a PdTe 2 -based millimeter-wave receiver is 13 and 21 times higher than similar devices based on black phosphorus and graphene in the same operational conditions, respectively. It is also discovered that pristine PdTe 2 is thermally stable in a temperature range extending even above 500 K, thus paving the way toward PdTe 2 -based high-temperature electronics. Finally, it is shown that the TeO 2 skin, formed upon air exposure, can be removed by thermal reduction via heating in vacuum.Chemical and thermal stability represent crucial bottlenecks in the prospect of technological exploitation of materials "beyond graphene." [7] Definitely, chemical instability is usually associated with the chemical reactivity of the surface [8] and to presence of intrinsic [9] or extrinsic [8]

The advent of tin diselenide (SnSe 2 ) enables novel pathways for optoelectronics, due to its reduced cost, ultralow thermal conductivity and high potential for thermoelectricity. To date, SnSe 2 -based optoelectronic devices have been focused on the visible and infrared range of the electromagnetic spectrum, with efficiency sharply decreasing at longer wavelength. Here, we present SnSe 2 photodetectors with exfoliated SnSe 2 nanosheets extended in the range of THz frequency, exhibiting high responsivity (170 V W −1 ), fast speed (2.2 µs), as well as room-temperature operation, based on efficient production of hotelectrons under deep-subwavelength electromagnetic focus, which outperform thermal-based photodetectors. Our SnSe 2 -based detectors show high-contrast imaging from terahertz (THz) up to visible. The outstanding ambient stability of our broadband photodetectors in a timescale of months is due to the chemical inertness of stoichiometric SnSe 2 crystals, validated by surface-science experiments. Our results demonstrate the suitability of SnSe 2 for multispectral sensing and real-time imaging.