We report a method that uses van der Waals interactions to transfer continuous, high-quality graphene films from Ge(110) to a different substrate held by hexagonal boron nitride carriers in a clean, dry environment. The transferred films are uniform and continuous with low defect density and few charge puddles. The transfer is effective because of the weak interfacial adhesion energy between graphene and Ge. Based on the minimum strain energy required for the isolation of film, the upper limit of the interfacial adhesion energy is estimated to be 23 meV per carbon atom, which makes graphene/Ge(110) the first as-grown graphene film that has a substrate adhesion energy lower than that of typical van der Waals interactions between layered materials. Our results suggest that graphene on Ge can serve as an ideal material platform to be integrated with other material systems by a clean assembly process.

Crystalline films offer various physical properties based on the modulation of their thicknesses and atomic structures. The layer-by-layer assembly of atomically thin crystals provides a powerful means to arbitrarily design films at the atomic level, which are unattainable with existing growth technologies. However, atomically clean assembly of the materials with high scalability and reproducibility remains challenging. We report programmed crystal assembly of graphene and monolayer hexagonal boron nitride, assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield. The atomic configurations of the films are tailored with layer-resolved compositions and in-plane crystalline orientations. We demonstrate batch-fabricated tunnel device arrays with modulation of the resistance over orders of magnitude by thickness control of the hexagonal boron nitride barrier with single-atom precision and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. Our results constitute an important development in the artificial design of large-scale films.

To utilize the intrinsic properties of 2D materials, it is important to control both interlayer interfaces and intralayer dislocations. Significant efforts have been made mostly to suppress the formation of crystalline disorders by developing advanced material growths [10][11][12] and integration techniques, [13] resulting in single-crystalline materials with atomically clean interfaces. However, structural boundaries can provide exciting control knobs to program the material properties beyond what is available in the thermodynamically most stable forms if the boundaries are fabricated controllably. Prototypical examples are the electrical doping of materials by introducing impurities [14] and optimizing device performances for targeted functionalities by forming heterojunctions. [15] Furthermore, 2D materials of van der Waals (vdW) structures arbitrarily enable the control of their atomic configurations due to the weak interlayer interactions. Therefore, various types of structural boundaries with different crystalline symmetries and band structures have been reported even in a single material platform by atomic displacements, [16,17] crystalline misorientation, [18] and distortions of chemical bonding [19,20] without the introduction of foreign materials. They have provided testbeds to discover novel electrical properties, [21] which are inaccessible in perfect crystals. However, direct applications of the material properties are elusive due to the lack of techniques to precisely control the boundaries over technologically relevant scales.In this review, we discuss about the emerging properties of structural boundaries in vdW structures and the developments of techniques to control the boundaries at the atomic scale. We focus on boundary structures caused by atomic displacements within a single-crystalline material, rather than heterogeneous boundaries with chemical complexity, which have been summarized in detail elsewhere. [22] We discuss the remaining critical issues to reproduce functional boundaries by a designer approach for the discovery of properties and applications in electronics and provide our outlook on the directions in the field. Boundary Types and Related Properties in vdW SolidsA grain boundary is an interfacial plane that exists between two perfect crystallites. The boundaries are not randomly formed, and the resultant structures are restricted by the crystalline lattices, in which they are embedded. The structural defects emerge at the boundaries of misaligned crystalline domains, whose rotation axis most likely points to the out-of-plane Engineering the boundary structures in 2D materials provides an unprecedented opportunity to program the physical properties of the materials with extensive tunability and realize innovative devices with advanced functionalities. However, structural engineering technology is still in its infancy, and creating artificial boundary structures with high reproducibility remains difficult. In this review, various emergent properties of 2D materials with different ...

Metal (M) contact with a semiconductor (S) introduces metal-induced gap states (MIGS), which makes it difficult to study the intrinsic electrical properties of S. A bilayer of metal with graphene (Gr), i.e., a M/Gr bilayer, may form a contact with S to minimize MIGS. However, it has been challenging to realize the pristine M/Gr/S junctions without interfacial contaminants, which result in additional interfacial states. Here, we successfully demonstrate the atomically clean M/Gr/n-type silicon (Si) junctions via all-dry transfer of M/Gr bilayers onto Si. The fabricated M/Gr/Si junctions significantly increase the current density J at reverse bias, compared to those of M/Si junctions without a Gr interlayer (e.g., by 105 times for M = Au in Si(111)). The increase of the reverse J by a Gr interlayer is more prominent in Si(111) than in Si(100), whereas in M/Si junctions, J is independent of the type of Si surface. The different transport data between M/Gr/Si(111) and M/Gr/Si(100) are consistent with Fermi-level pinning by different surface states of Si(111) and Si(100). Our findings suggest the effective way to suppress MIGS by an introduction of the clean Gr interlayer, which paves the way to study intrinsic electrical properties of various materials.