The interplay of the massless Dirac fermions in graphene and the Cooper pair states in a superconductor has the potential to give rise to exotic physical phenomena and useful device applications. But to date, the junctions formed between graphene and superconductors on conventional substrates have been highly disordered. Charge scattering and potential fluctuations caused by such disorder are believed to have prevented the emergence or observation of new physics. Here we propose to address this problem by forming suspended graphene-superconductor junctions. We demonstrate the fabrication of high-quality suspended monolayer graphene-NbN Josephson junctions with device mobility in excess of 150,000 cm 2 per Vs, minimum carrier density below 10 10 cm À 2 , and the flow of a supercurrent at critical temperatures greater than 2 K. The characteristics of our Josephson junctions are consistent with ballistic transport, with a linear dependence on the Fermi energy that reflects of linear dispersion of massless Dirac fermions.

The implementation of aberration-corrected electron beam lithography (AC-EBL) in a 200 keV scanning transmission electron microscope (STEM) is a novel technique that could be used for the fabrication of quantum devices based on 2D atomic crystals with single nanometer critical dimensions, allowing to observe more robust quantum effects. In this work we study electron beam sculpturing of nanostructures on suspended graphene field effect transistors using AC-EBL, focusing on the in situ characterization of the impact of electron beam exposure on device electronic transport quality. When AC-EBL is performed on a graphene channel (local exposure) or on the outside vicinity of a graphene channel (non-local exposure), the charge transport characteristics of graphene can be significantly affected due to charge doping and scattering. While the detrimental effect of non-local exposure can be largely removed by vigorous annealing, local-exposure induced damage is irreversible and cannot be fixed by annealing. We discuss the possible causes of the observed exposure effects. Our results provide guidance to the future development of high-energy electron beam lithography for nanomaterial device fabrication.

Two-dimensional atomic crystals (2DACs) can be mechanically assembled with precision for the fabrication of heterostructures, allowing for the combination of material building blocks with great flexibility. In addition, while conventional nanolithography can be detrimental to most of the 2DACs which are not sufficiently inert, mechanical assembly potentially minimizes the nanofabrication processing and preserves the intrinsic physical properties of the 2DACs. In this work we study the interfacial charge transport between various 2DACs and electrical contacts, by fabricating and characterizing 2DAC-superconductor junctions through mechanical transfer. Compared to devices fabricated with conventional nanolithography, mechanically assembled devices show comparable or better interface transparency. Surface roughness at the electrical contacts is identified to be a major limitation to the interface quality.The study of two-dimensional (2D) atomic crystals [1] and their heterostructures spans over the past decade from graphene [2, 3] to a variety of semiconductors [4], insulators [5], superconductors [6], topological materials [7,8], etc. Through the widely practiced approach of mechanical co-lamination/transfer [9, 10], 2D atomic crystals can be directly stacked together with well-adjusted spatial and angular alignment relative to one another [10], even for material combinations which are not possible to synthesize through conventional methods. Such flexible combination of different 2D atomic crystals (2DACs) allows for the discovery of novel devices and artificial materials, which greatly broadens the horizon of low dimensional material research.An important concern when working with 2DACs and their heterostructures is to minimize the impact of the fabrication process on the intrinsic properties of these materials. Except for a few 2DACs (e.g. graphene, hBN), a majority of the recently explored 2D materials are air-sensitive [10][11][12]. Hence, the conventional nanofabrication process often causes degradation to these materials. On the other hand, following the basic approach of co-lamination, device fabrication can also be achieved through mechanical assembly, i.e. direct transfer of 2D materials/heterostructure onto predefined leads. It is therefore important to understand, compared to conventional lithography, how electrically transparent a contact/2DAC interface can be achieved through mechanical assembly, and what the limiting factors to the interface transparency are. In this work, we systematically studied the approach of creating transparent electrical contacts through mechanical assembly, hence minimizing the detrimental effects from conventional nanolithography on some of the air-sensitive 2DACs.Mechanical assembly of 2DAC devices is based on van der Waals interactions between the 2D crystals and the electric contacts, which may be sufficiently strong so that the interlayer distance is small and charge transfer OPEN ACCESS RECEIVED