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The mechanism of graphene delamination from a Pt catalyst growth surface with electrochemical methods is studied. After a water intercalation step, an electrochemical graphene delamination process is done with a variety of different electrolytes. It is shown that (hydrogen or oxygen) bubble formation is not the main driving force to decouple graphene from its catalyst growth substrate. Ion intercalation is identified as the primary component for a fast graphene delamination process from its catalytic growth substrate. When the Pt/graphene sample is negatively charged, cations will intercalate, assuming they do not reduce within the electrochemical window of the solvent. This cation intercalation does result in graphene delamination. In the same way, anions intercalate in positively charged Pt/graphene samples when they do not react within the electrochemical window of the solvent. Furthermore, it is shown that applying a potential is sufficient (current is not needed) to induce ion intercalation and, as a result, graphene delamination. These findings open the door to avoid Na+ or K+ contamination introduced during currently described electrochemical graphene delamination. Alternative electrolytes (i.e. ammonium hydroxide and tetraethylammonium hydroxide) are proposed, due to the absence of alkali contaminants and rapid cation intercalation to delaminate graphene.

Despite the fact that two-dimensional MoS2 films continue to be of interest for novel device concepts and beyond silicon technologies, there is still a lack of understanding on the carrier injection at metal/MoS2 interface and effective mitigation of the contact resistance. In this work, we develop a semi-classical model to identify the main mechanisms and trajectories for carrier injection at MoS2 contacts. The proposed model successfully captures the experimentally observed contact behavior and the overall electrical behavior of MoS2 field effect transistors. Using this model, we evaluate the injection trajectories for different MoS2 thicknesses and bias conditions. We find for multilayer (>2) MoS2, the contribution of injection at the contact edge and injection under the contact increase with lateral and perpendicular fields, respectively. Furthermore, we identify that the carriers are predominantly injected at the edge of the contact metal for monolayer and bilayer MoS2. Following these insights, we have found that the transmission line model could significantly overestimate the transfer length and hence the contact resistivity for monolayer and bilayer MoS2. Finally, we evaluate different contact strategies to improve the contact resistance considering the limiting injection trajectory.

Despite rapid progress in 2D molybdenum disulfide (MoS2) research in recent years, MoS2 field-effect transistors (FETs) still suffer from a high metal-to-MoS2 contact resistance and low intrinsic mobility, which are major hindrances to their future application. We report an efficient technique to dope thin-film MoS2 FETs using a poly(vinyl-alcohol) (PVA) polymeric coating. This results in a reduction of the contact resistance by up to 30% as well as a reduction in the channel resistance to 20 kΩ sq−1. Using a dehydration process, we were able to effectively control the surface interactions between MoS2 and the more electropositive hydroxyl groups (–OH) of PVA, which provided a controllable and yet reversible increase in the charge carrier density to a value of 8.0 × 1012 cm−2. The non-covalent, thus non-destructive, PVA doping of MoS2 increases the carrier concentration without degrading the mobility, which shows a monotonic increase while enhancing the doping effect. The PVA doping technique is then exploited to create heavily doped access regions to the intrinsic MoS2 channel, which yields 200% increase of the ON-state source–drain current. This establishes PVA doping as an effective approach to enhance the transport properties of MoS2 FETs for a variety of applications.

Atomically-thin, inherently 2D semiconductors offer thickness scaling of nanoelectronic devices and excellent response to light for low-power versatile applications. Using small exfoliated flakes, advanced devices and integrated circuits have already been realized, showing great potential to impact nanoelectronics. Here, high-quality single-crystal MoSe2 is grown by molecular beam epitaxy on AlN(0001)/Si(111), showing the potential for scaling up growth to low-cost, large-area substrates for mass production. The MoSe2 layers are epitaxially aligned with the aluminum nitride (AlN) lattice, showing a uniform, smooth surface and interfaces with no reaction or intermixing, and with sufficiently high band offsets. High-quality single-layer MoSe2 is obtained, with a direct gap evidenced by angle-resolved photoemission spectroscopy and further confirmed by Raman and intense room temperature photoluminescence. The successful growth of high-quality MoSe2/Bi2Se3 multilayers on AlN shows promise for novel devices exploiting the non-trivial topological properties of Bi2Se3.

In this paper, we report the formation of self-assembled monolayers (SAMs) of oleylamine (OA) on highly oriented pyrolytic graphite (HOPG) and graphene surfaces and demonstrate the potential of using such organic SAMs to tailor the electronic properties of graphene. Molecular resolution Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) images reveal the detailed molecular ordering. The electrical measurements show that OA strongly interacts with graphene leading to n-doping effects in graphene devices. The doping levels are tunable by varying the OA deposition conditions. Importantly, neither hole nor electron mobilities are decreased by the OA modification. As a benefit from this noncovalent modification strategy, the pristine characteristics of the device are recoverable upon OA removal. From this study, one can envision the possibility to correlate the graphene-based device performance with the molecular structure and supramolecular ordering of the organic dopant.