Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (R). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS and MoTe by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS and MoTe, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that R is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

Coherent motion of the electrons in the Bloch states is one of the fundamental concepts of the charge conduction in solid state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by e.g. large interlayer separation. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite, and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene.PACS numbers: 71.20.Ps, 71.18.+y, 72.20.My In many layered systems, the interlayer coupling is one of the key parameters for altering their electronic properties [1][2][3][4][5][6]. When a thick insulating block is inserted between the metallic layers, the interlayer coupling can be significantly reduced, leading to breakdown of the interlayer coherence, as nicely demonstrated in two dimensional electron gas (2DEG) in semiconductor superlattice [6]. Such an "interlayer version" of the Mott-Ioffe-Regel limit is realized when the layer separation exceeds the mean free path across the layers, which is evidenced by qualitatively different temperature dependences of the intralayer (metallic) and the interlayer (semiconducting) resistivities. The intriguing interlayer conduction has been observed in various systems including high-T c cuprates [1], organic crystals [2], dichalcogenides [3], graphite [4,5] and semiconductor superlattices [6], but its underlying mechanism related to the interlayer incoherence has been under debate in last decades [7].Graphene, an ideal 2DEG system, also exhibits rich electronic properties depending on how it is stacked on top of another graphene layer [8,9]. While bilayer graphene in Bernal stacking has massive charge carriers with a zero band gap, twisted bilayer graphene with a random orientation of the layers has a massless electronic dispersion similar to that of monolayer graphene [10]. Twisted bilayer graphene is of particular interest because several intriguing properties such as renormalization of Fermi velocity [10,11], van Hove singularities [12], and electronic localization [11,13] were recently discovered. Experimental studies including angle-resolved photoemission spectroscopy [14], scanning tunneling spectroscopy [12], Raman spectroscopy [15,16], and in-plane transport [17,18], suggest that the layers are decoupled in twisted bilayer graphene, and the misoriented layers are often considered as being electrically isolated. However, it is still not clear what sense the layers are decoupled on an atomic length scale of the layer separation, and how strong the interlayer coupling is in twisted bilayer graphene [9,1...

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By using four-terminal configurations, we investigated the dependence of longitudinal and diagonal resistances of a graphene p-n interface on the quantum-Hall edge-state equilibration position. The resistance of a p-n device in our four-terminal scheme is asymmetric with respect to the zero point where the filling factor (ν) of the entire graphene vanishes. This resistance asymmetry is caused by the chiral-direction-dependent change of the equilibration position and leads to a deeper insight into the equilibration process of the quantum-Hall edge states in a bipolar graphene system.