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...

The broken-gap (type III) van der Waals heterojunction is of particular interest, as there is no overlap between energy bands of its two stacked materials. Despite several studies on straddling-gap (type I) and staggered-gap (type II) vdW heterojunctions, comprehensive understanding of current transport and optoelectronic effects in a type-III heterojunction remains elusive. Here, we report gate-tunable current rectifying characteristics in a black phosphorus (BP)/rhenium disulfide (ReS2) type-III p–n heterojunction diode. Current transport in this heterojunction was modeled using the Simmons approximation through direct tunneling and Fowler–Nordheim tunneling in lower- and higher-bias regimes, respectively. We showed that a p–n diode based on a type-III heterojunction is mainly governed by tunneling-mediated transport, but that transport in a type-I p–n heterojunction is dominated by majority carrier diffusion in the higher-bias regime. Upon illumination with a 532 nm wavelength laser, the BP/ReS2 type-III p–n heterojunction showed a photo responsivity of 8 mA/W at a laser power as high as 100 μW and photovoltaic energy conversion with an external peak quantum efficiency of 0.3%. Finally, we demonstrated a binary inverter consisting of BP p-channel and ReS2 n-channel thin film transistors for logic applications.

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Recently, multivalued logic (MVL) circuits have attracted tremendous interest due to their ability to process more data by increasing the number of logic states rather than the integration density. Here, we fabricate logic circuits based on molybdenum telluride (MoTe2)/black phosphorus (BP) van der Waals heterojunctions with different structural phases of MoTe2. Owing to the different electrical properties of the 2H and mixed 2H +1T′ phases of MoTe2, tunable logic devices have been realized. A logic circuit based on a BP field-effect transistor (FET) and a BP/MoTe2 (2H + 1T′) heterojunction FET displays the characteristics of binary logic. However, a drain voltage-controlled transition from binary to ternary logic has been observed in BP FET- and BP/ MoTe2 (2H) heterojunction FET-based logic circuits. Also, a change from binary to ternary characteristics has been observed in BP/MoTe2 (2H)-based inverters at low temperature below 240 K. We believe that this work will stimulate the assessment of the structural phase transition in metal dichalcogenides toward advanced logic circuits and offer a pathway to substantialize the circuit standards for future MVL systems.

The recent discovery of magnetic van der Waals (vdW) materials provides a platform to answer fundamental questions on the two-dimensional (2D) limit of magnetic phenomena and applications. An important question in magnetism is the ultimate limit of the antiferromagnetic layer thickness in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures to observe the exchange bias (EB) effect, of which origin has been subject to a long-standing debate. Here, we report that the EB effect is maintained down to the atomic bilayer of AFM in the FM (Fe3GeTe2)/AFM (CrPS4) vdW heterostructure, but it vanishes at the single-layer limit. Given that CrPS4 is of A-type AFM and, thus, the bilayer is the smallest unit to form an AFM, this result clearly demonstrates the 2D limit of EB; only one unit of AFM ordering is sufficient for a finite EB effect. Moreover, the semiconducting property of AFM CrPS4 allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices.