By plasma-enhanced chemical vapor deposition, a molybdenum disulfide (MoS2 ) thin film is synthesized directly on a wafer-scale plastic substrate at below 300 °C. The carrier mobility of the films is 3.74 cm(2) V(-1) s(-1) . Also, humidity is successfully detected with MoS2 -based sensors fabricated on the flexible substrate, which reveals its potential for flexible sensing devices.

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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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.

Direct contacts of a metal with atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been found to suppress device performance by producing a high contact resistance. NbS is a 2D TMDC and a conductor. It is expected to form ohmic contacts with 2D semiconductors because of its high work function and the van der Waals interface it forms with the semiconductor, with such an interface resulting in weak Fermi level pinning. Despite the usefulness of NbS as an electrode, previous synthesis methods could not control the thickness, uniformity, and shape of the NbS film and hence could not find practical applications in electronics. Here, we report a patternable method for carrying out the synthesis of NbS films in which the number of NbS layers formed over a large area was successfully controlled, which is necessary for the production of customized electrodes. The synthesized NbS films were shown to be highly transparent and uniform in thickness and conductivity over the large area. Furthermore, the synthesized NbS showed half the contact resistance than did the molybdenum metal in MoS field effect transistors (FETs) on a large transparent quartz substrate. The MoS device with NbS showed an electron mobility as high as 12.7 cm V s, which was three times higher than that found for the corresponding molybdenum-contacted MoS device. This result showed the high potential of the NbS thin film as a transparent electrode for 2D transition metal dichalcogenide (TMDC) semiconductors with low contact resistance.