The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Atomically thin binary two-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure, and thickness. By adding another element in these materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffraction. Transmission electron microscopy showed that preferentially cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy and exhibited significant dependence on thickness of crystals exfoliated down to a single layer. Optical absorption determined by reflectance contrast was dominated by d-d-type transitions localized at Cr ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness dependence and hinted at a high photochemical activity for single-layer CrPS. The current structural and optical investigation will provide a firm basis for future study and application of this kind of atomically thin magnetic semiconductors.

devices based on heterojunctions can exhibit diverse functionality upon stacked 2D materials with different electron affinities and different bandgaps. [4,[6][7][8] Accordingly, heterojunctions can be classified into three distinct types based on their band-structure alignment: straddlinggap (type-I), staggered-gap (type-II), and broken-gap (type-III). In particular, broken-gap heterojunctions are interesting as there is no overlap between the energy bands of the two stacked materials, resulting in some exotic phenomena. [9][10][11][12] As apparent in recent reports, [6,13] change in current-transport across broken-gap heterojunctions can only be achieved by varying the combination of 2D materials employed. Hence, the majority of reported electronic devices exhibit inadequate control of multifunctional operations across the particular broken-gap heterojunction being used. Thus, diverse current-transport across a broken-gap heterojunction appears to be a primarily material-dependent phenomenon. In this article, we demonstrate that a black phosphorus (BP)-based broken-gap heterojunction can exhibit tunable current-transport characteristics. To elucidate our findings, we have chosen BP/rhenium disulfide (ReS 2 ) van der Waals (vdW) heterostructures for the following reasons: i) In addition to having majority charge carriers of opposite polarity (i.e., p-type BP and n-type ReS 2 ), BP/ ReS 2 forms a type-III broken-gap alignment at the heterojunction (Figure 1) and ii) BP flake work function exhibits substantial dependence on the respective flake thickness (Figure 1). Moreover, BP electronic structure features a lone pair of electrons at each phosphorus atom, which can interact strongly with out-of-plane atoms. [14][15][16] Consequently, BP has the potential to exhibit higher out-of-plane conductivities compared to other 2D materials and could be useful to be employed in heterostructures where vertical transport is usually dominant. [17,18] An advantage over epitaxial thinfilm-based heterojunctions is that the vdW gap enables band alignment at the heterojunction without requiring a tunneling barrier. [12,19,20] Our results demonstrate that current-transport across a BP/ ReS 2 broken-gap heterojunction can be tuned in a controlled way to function as a current-rectifying p-n junction diode-Esaki diode-backward-rectifying diode-nonrectifying device by gradually increasing the BP layer thickness from 5 to 100 nm or more. This is explained by the emergence of various The finite energy band-offset that appears between band structures of employed materials in a broken-gap heterojunction exhibits several interesting phenomena. Here, by employing a black phosphorus (BP)/ rhenium disulfide (ReS 2 ) heterojunction, the tunability of the BP work function (Φ BP ) with variation in flake thickness is exploited in order to demonstrate that a BP-based broken-gap heterojunction can manifest diverse current-transport characteristics such as gate tunable rectifying p-n junction diodes, Esaki diodes, backward-rectifying diodes, and nonr...

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Heterostructures that combine graphene and transition metal dichalcogenides, such as MoS , MoTe , and WS , have attracted attention due to their high performances in optoelectronic devices compared to homogeneous systems. Here, a photodevice based on a hybrid van der Waals heterostructure of rhenium disulfide (ReS ) and graphene is fabricated using the stacking method. The device presents a remarkable ultrahigh photoresponsivity of 7 × 10 A W and a detectivity of 1.9 × 10 Jones, along with a fast response time of less than 30 ms. Tremendous photocurrents are generated in the heterostructure due to the direct bandgap, high quantum efficiency, and strong light absorption by the multilayer ReS and the high carrier mobility of graphene. The ReS /graphene heterostructured device displays a high photocurrent under the applied gate voltages due to the photogating effect induced by the junction between graphene and ReS . The ReS /graphene heterostructure may find promising applications in future optoelectronic devices, providing a high sensitivity, flexibility, and transparency.