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Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices. ; This project has received funding from the European Research Council (ERC) under European Union's Horizon 2020 research and innovation programme (grant agreement no 755655, ERC-StG 2017 project 2D-TOPSENSE). R. F. acknowledges the support from the Spanish Ministry of Economy, Industry and Competitiveness through a Juan de la Cierva-formación fellowship 2017 FJCI-2017-32919. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

Two-dimensional indium selenide (InSe) has attracted extensive attention recently due to its record-high charge carrier mobility and photoresponsivity in the fields of electronics and optoelectronics. Nevertheless, the mechanical properties of this material in the ultra-thin regime have not been investigated yet. Here, we present our efforts to determine the Young's modulus of thin InSe (∼1-2 layers to ∼34 layers) flakes experimentally by using a buckling-based methodology. We find that the Young's modulus has a value of 23.1 ± 5.2 GPa, one of the lowest values reported to date for crystalline two-dimensional materials. This superior flexibility can be very attractive for different applications, such as strain engineering and flexible electronics. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 755655, ERC-StG 2017 project 2D-TOPSENSE). EU Graphene Flagship funding (Grant Graphene Core 2, 785219) is acknowledged. RF acknowledges support from the Spanish Ministry of Economy, Industry and Competitiveness through a Juan de la Cierva-formación fellowship (2017 FJCI-2017-32919). QHZ acknowledges the grant from the China Scholarship Council (CSC) under no. 201700290035. TW acknowledges support from the National Natural Science Foundation of China: 51672216. ; We acknowledge support of the publication fee by the CSIC Open Access Support Initiative through its Unit of Information Resources for Research (URICI ; Peer reviewed

Due to the excellent electrical transport properties and optoelectronic performance, thin indium selenide (InSe) has recently attracted attention in the field of 2D semiconducting materials. However, the mechanism behind the photocurrent generation in thin InSe photodetectors remains elusive. Here, we present a set of experiments aimed at explaining the strong scattering in the photoresponsivity values reported in the literature for thin InSe photodetectors. By performing optoelectronic measurements on thin InSe-based photodetectors operated under different environmental conditions we find that the photoresponsivity, the response time and the photocurrent power dependency are strongly correlated in this material. This observation indicates that the photogating effect plays an imporant role in thin InSe flakes, and it is the dominant mechanism in the ultra-high photoresponsivity of pristine InSe devices. In addition, when exposing the pristine InSe photodetectors to the ambient environment we observe a fast and irreversible change in the photoresponse, with a decrease in the photoresponsivity accompanied by an increase of the operating speed. We attribute this photodetector performance change (upon atmospheric exposure) to the decrease in the density of the traps present in InSe, due to the passivation of selenium vacancies by atmospheric oxygen species. This passivation is accompanied by a downward shift of the InSe Fermi level and by a decrease of the Fermi level pinning, which leads to an increase of the Schottky barrier between Au and InSe. Our study reveals the important role of traps induced by defects in tailoring the properties of devices based on 2D materials and offers a controllable route to design and functionalize thin InSe photodetectors to realize devices with either ultrahigh photoresposivity or fast operation speed. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no 755655, ERC-StG 2017 project 2D-TOPSENSE). EU Graphene Flagship funding (Grant Graphene Core 2, 785219) is acknowledged. RF acknowledges support from the Netherlands Organization for Scientific Research (NWO) through the research program Rubicon with project number 680-50-1515 and from the Spanish Ministry of Economy, Industry and Competitiveness through a Juan de la Cierva-formación fellowship (2017 FJCI-2017-32919). QHZ acknowledges the grant from China Scholarship Council (CSC) under No. 201700290035. TW acknowledges support from the National Natural Science Foundation of China: 51672216 ; We acknowledge support of the publication fee by the CSIC Open Access Support Initiative through its Unit of Information Resources for Research (URICI)

The isolation of air-sensitive two-dimensional (2D) materials and the race to achieve a better control of the interfaces in van der Waals heterostructures has pushed the scientific community towards the development of experimental setups that allow to exfoliate and transfer 2D materials under inert atmospheric conditions. These systems are typically based on over pressurized N2 of Ar gloveboxes that require the use of very thick gloves to operate within the chamber or the implementation of several motorized micro-manipulators. Here, we set up a deterministic transfer system for 2D materials within a gloveless anaerobic chamber. Unlike other setups based on over-pressurized gloveboxes, in our system the operator can manipulate the 2D materials within the chamber with bare hands. This experimental setup allows us to exfoliate 2D materials and to deterministically place them at a desired location with accuracy in a controlled O2-free and very low humidity (<2% RH) atmosphere. We illustrate the potential of this system to work with air-sensitive 2D materials by comparing the stability of black phosphorus and perovskite flakes inside and outside the anaerobic chamber ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755655, ERC-StG 2017 project 2D-TOPSENSE). EU Graphene Flagship funding (Grant Graphene Core 2, 785219) is acknowledged. RF acknowledges the support from the Spanish Ministry of Economy, Industry and Competitiveness through a Juan de la Cierva-formación fellowship 2017 FJCI2017-32919. QHZ acknowledges the grant from China Scholarship Council (CSC) under No. 201700290035. MS acknowledges the financial support of a fellowship from 'la Caixa' Foundation (ID 100010434). The fellowship code is LCF/BQ/IN17/11620040. MS has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 713673. FP acknowledges financial support from the Spanish Ministry of Economy and Competitiveness through the 'María de Maeztu' Program for Units of Excellence in R and D (MDM-2014-0377)

Van der Waals materials with narrow energy gaps and efficient response over a broadband optical spectral range are key to widen the energy window of nanoscale optoelectronic devices. Here, we characterize FePS3 as an appealing narrow-gap p-type semiconductor with an efficient broadband photo-response, a high refractive index, and a remarkable resilience against air and light exposure. To enable fast prototyping, we provide a straightforward guideline to determine the thickness of few-layered FePS3 nanosheets extracted from the optical transmission characteristics of several flakes. The analysis of the electrical photo-response of FePS3 devices as a function of the excitation energy confirms a narrow gap suitable for near IR detection (1.23 eV) and, more interestingly, reveals a broad spectral responsivity up to the ultraviolet region. The experimental estimate for the gap energy is corroborated by ab-initio calculations. An analysis of photocurrent as a function of gate voltage and incident power reveals a photo-response dominated by photogating effects. Finally, aging studies of FePS3 nanosheets under ambient conditions show a limited reactivity of the outermost layers of flakes in long exposures to air. ; The present work has received funding from the European Union's Horizon 2020 research and innovation program through the European Research Council grants ERC-2017-StG-755655 2D-TOPSENSE, ERC-2018-AdG-788222 MOL-2D, under the Graphene Flagship through projects GrapheneCore2 (grant nr. 785219), and GrapheneCore3 (grant nr. 881603) and through the COST Action CA15128 MOLSPIN. The authors also acknowledge funding from the Spanish Government through the Unit of Excellence "María de Maeztu" MDM-2015-0538, through projects MAT2017-88377-C2-2-R (AEI/FEDER), MAT2017-88377, MAT2017-89528 (AEI/FEDER), and R.F.'s Juan de la Cierva-formación fellowship FJCI-2017-32919. This work has been supported by the Generalitat Valenciana, through grants CIDEGENT/2018/004, CIDEGENT/2018/005, CDEIGENT/2019/022, the Prometeo Program of Excellence, and M.R.'s APOSTD/2020/249 fellowship.

We fabricated large-area atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) by sulfurization at relatively low temperatures. The obtained MoS2 sheets are polycrystalline (~10–20 nm single-crystal domain size) with areas of up to 300 × 300 µm2, 2–4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements. ; ACG acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755655, ERC-StG 2017 project 2D-TOPSENSE) and from the EU Graphene Flagship funding (Grant Graphene Core 2, 785219). JAA thanks the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) for funding the project MAT2017-84496-R. RF acknowledges support from MINECO through a "Juan de la Cierva: formación" fellowship (2017 FJCI-2017-32919). GSS acknowledges financial support from MINECO (Juan de la Cierva 2015 program, FJCI-2015-25427). ; Peer reviewed

This article belongs to the Special Issue Low Dimensional Materials for Environmental and Biomedical Applications. ; The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2. ; This project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research, innovation programme (grant agreement No. 755655, ERC-StG 2017 project 2D-TOPSENSE), the EU Graphene Flagship funding (Grant Graphene Core 2, 785219), the Netherlands Organisation for Scientific Research (NWO) through the research program Rubicon with project number 680-50-1515, the MINECO (program FIS2015-67367-C2-1-P), and the China Scholarship Council (File NO. 201506120102).