Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of β-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g L . Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. It is demonstrated that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few micrometers, and thicknesses ranging from 1 to 20 nm, with a maximum population centered at ≈5 nm, corresponding to 4 Se-In-In-Se quaternary layers. It is also shown that no formation of further InSe-based compounds (such as In Se ) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for the hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. The dependence of the InSe flakes' morphologies, i.e., surface area and thickness, on the HER performances is highlighted, achieving the best efficiencies with small flakes offering predominant edge effects. The theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes.

At terahertz (THz) frequencies, scattering-type scanning near-field optical microscopy (s-SNOM) based on continuous wave sources mostly relies on cryogenic and bulky detectors, which represents a major constraint for its practical application. Here, we devise a THz s-SNOM system that provides both amplitude and phase contrast and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a quantum cascade laser that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique. We demonstrate its performance by probing a phonon-polariton-resonant CsBr crystal and doped black phosphorus flakes.

Layered semiconductors of the IIIA-VIA group have attracted considerable attention in (opto)electronic applications thanks to their atomically thin structures and their thickness-dependent optical and electronic properties, which promise ultrafast response and high sensitivity. In particular, 2D indium selenide (InSe) has emerged as a promising candidate for the realization of thin-film field effect transistors and phototransistors due to its high intrinsic mobility (>10 2 cm 2 V −1 s −1 ) and the direct optical transitions in an energy range suitable for visible and near-infrared light detection. A key requirement for the exploitation of large-scale (opto)electronic applications relies on the development of low-cost and industrially relevant 2D material production processes, such as liquid phase exfoliation, combined with the availability of high-throughput device fabrication methods. Here, a β polymorph of indium selenide (β-InSe) is exfoliated in isopropanol and spray-coated InSe-based photodetectors are demonstrated, exhibiting high responsivity to visible light (maximum value of 274 A W −1 under blue excitation 455 nm) and fast response time (15 ms). The devices show a gate-dependent conduction with an n-channel transistor behavior.Overall, this study establishes that liquid phase exfoliated β-InSe is a valid candidate for printed high-performance photodetectors, which is critical for the development of industrial-scale 2D material-based optoelectronic devices.

Despite their huge application capabilities, millimeter-and terahertzwave photodetectors still face challenges in the detection scheme. Topological insulators (TIs) are predicted to be promising candidates for long-wavelength photodetection, due to the presence of Dirac fermions in their topologically protected surface states. However, photodetection based on TIs is usually hindered by the large dark current, originating from the mixing of bulk states with topological surface states (TSSs) in most realistic samples of TIs. Here millimeter and terahertz detectors based on a subwavelength metal-TI-metal (MTM) heterostructure are demonstrated. The achieved photoresponse stems from the asymmetric scattering of TSS, driven by the localized surface plasmon-induced terahertz field, which ultimately produces direct photocarriers beyond the interband limit. The device enables high responsivity in both the self-powered and bias modes even at room temperature. The achieved responsivity is over 75 A/W, with response time shorter than 60 ms in the self-powered mode. Remarkably, the responsivity increases by several orders of magnitude in the biased configuration, with the noise-equivalent power (NEP) of 3.6 × 10 −13 W Hz −1/2 and a detectivity of 2.17 × 10 11 cm Hz −1/2 W −1 at room temperature. The detection performances open a way toward realistic exploitation of TIs for large-area, real-time imaging within long-wavelength optoelectronics.

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Despite the considerable effort, fast and highly sensitive photodetection is not widely available at the low-photon-energy range (~meV) of the electromagnetic spectrum, owing to the challenging light funneling into small active areas with efficient conversion into an electrical signal. Here, we provide an alternative strategy by efficiently integrating and manipulating at the nanoscale the optoelectronic properties of topological Dirac semimetal PtSe2 and its van der Waals heterostructures. Explicitly, we realize strong plasmonic antenna coupling to semimetal states near the skin-depth regime (λ/104), featuring colossal photoresponse by in-plane symmetry breaking. The observed spontaneous and polarization-sensitive photocurrent are correlated to strong coupling with the nonequilibrium states in PtSe2 Dirac semimetal, yielding efficient light absorption in the photon range below 1.24 meV with responsivity exceeding ∼0.2 A/W and noise-equivalent power (NEP) less than ~38 pW/Hz0.5, as well as superb ambient stability. Present results pave the way to efficient engineering of a topological semimetal for high-speed and low-energy photon harvesting in areas such as biomedical imaging, remote sensing or security applications.

The recent discoveries related to the efficient light-to-heat conversion in nanomaterials have enabled the implementation of sunlight-driven Membrane Distillation for a desalination at the water-energy nexus.

photoejected hot carriers excited through photon absorption in metal structures and extracted via internal photoemission [1,2] with the intriguing prospect of direct below-bandgap photodetection at room temperature (RT). However, the collection efficiency of photoejected hot carriers at metal-semiconductor (MS) interface [3] or metal-insulator-metal (MIM) junctions [4] is severely hindered by (i) the fast internal relaxation process, (ii) momentum mismatch, and (iii) the lack of effective lighttrapping mechanisms. To date, plasmonic modes in metals have been widely utilized to enhance photoemission of hot electrons since they can concentrate photon energy in a deep subwavelength region, where extensive amounts of hot electrons can be generated. [5][6][7] Therefore, the working wavelength can be tuned by simply adjusting the frequency of metal plasmonic resonance rather than the bandgap of materials. Hot carriers can be generated in a femtosecond timescale via the Laudau damping of plasmonic modes [8] and they are lifted from electronic states below the Fermi level with appropriate energy hυ. However, they lose their energy very fast, as a consequence, only a small portion of them is able to escape from the MS or MIM interface. Despite the presence of Photodetectors exploiting photoejected hot electrons have the potential to achieve ultrahigh sensitivity and broadband detection capabilities, which are controlled by the structure of the device rather than the bandgap of the employed materials. However, the achievement of photodetectors of long-wavelength photons with both high responsivity and bandwidth is still challenging. Here, a novel class of high-gain photodetectors based on the manipulation of intrinsic hot carriers by exploiting the electromagnetic engineering of a graphene-based active channel is presented. Light field is focused in a split-finger gated structure to create a potential gradient in the channel, which is able to trap and detrap the charges laterally transferred from low resistive Au-graphene interface, finally leading to a high photoconductive gain. Correspondingly, the device activity can be easily switched from photovoltaic to photoconductive, depending on the photoinduced hot-carrier distribution, just by controlling the electric field. The device shows tunable sensitivity, higher energy efficiency, and photoconductive gain. In particular, the responsivity (0.6-6.0 kV W −1 ) and the noise-equivalent power (less than 0.1 nW Hz −0.5 at room temperature) are significantly improved even at low-energy terahertz band with respect to state-of-the-art devices based on extrinsically coupled hot carriers operating in the near infrared.