Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS₂ Heterostructures

Abstract

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene-WS heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene-WS heterojunctions as-grown on sapphire and transferred to SiO with and without thermal annealing. Both p-n and n-n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene-WS than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene-WS junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.

Dramatic Impact of Dimensionality on the Electrostatics of P-N Junctions and Its Sensing and Switching Applications

“…The equations governing the potential profiles and depletion width of 3D, 2D and 1D systems are presented. Experiments on PN junctions demonstrate an enormous impact of dimensionality on the electrostatics [30], [31], [37]. The rapid progress in doping thin flakes of 2D materials and 1D nanowires over the last two decades [38]- [40] and its role in low dimensional MOSFETs [41]- [43] and tunnel transistors [11]- [17] call for a thorough analytical understanding of the low dimensional PN junctions, well validated against numerical simulations.…”

“…et al have measured depletion widths (W D ) of a 2D PN junction as a function of reverse bias using optical techniques and have shown the difference in behavior of W D of 2D junctions as against their 3D counterparts [30]. More recently, Zheng et al have used Kelvin Probe Force Microscopy (KPFM) technique to show that the potential profile itself changes significantly in low dimensions[31].Several theoretical models have been developed to explain some of these anomalous behavior in electrostatics of low dimensional systems. For 1D systems, Leonard and Tersoff at…”

Flexible and High‐Performance All‐2D Photodetector for Wearable Devices

“…As a result, the heterointerface is atomically regular, and the band edge is sharp . Thus far, extensive theoretical and experimental efforts have been devoted to 2D contacts, and all‐2D devices including transistors, light‐emitting diodes, humidity sensors, complementary inverters, field emission tunnel diodes, mechanical resonators, and tactile sensors . Recently, there emerged several attempts on all‐2D photodetectors .…”

Reliable Synthesis of Large‐Area Monolayer WS2 Single Crystals, Films, and Heterostructures with Extraordinary Photoluminescence Induced by Water Intercalation

“…Intensive studies of the growth of monolayer MoS 2 , WS 2 , MoSe 2 , and WSe 2 in the TMD family have been conducted in recent years, with yields of crystals of various size and quality. Moreover, in‐plane heterostructures of monolayer TMDs with graphene have been achieved recently on MoS 2 and WS 2 . Despite these exciting advances, challenges remain to the achievement of controllable synthesis.…”