Session F04: Advances in Moiré Assembly

Ultraclean Layer Transfer and Atomic Reconstruction in Twisted TMD Lattices

Layer-by-layer assembly of van der Waals (vdW) heterostructures underpins new discoveries in solid state physics, material science and chemistry. Despite successes, all current assembly techniques use polymeric supports which limit their cleanliness, ultimate electronic performance, and potential for optoelectronic applications. In the first part of the talk, I will introduce a polymer-free platform for heterostructure assembly using re-usable flexible silicon nitride membranes. This approach enables production of heterostructures with interfaces free from interlayer contamination and correspondingly excellent (opto)electronic behaviour. In addition, eliminating polymeric supports allows new possibilities for vdW heterostructure fabrication: assembly at temperatures up to 600°C, and in different environments including ultra-high vacuum (UHV) and liquid submersion. We demonstrate UHV heterostructure assembly for the first time and show the reliable creation of moiré superlattices with over x10 improvement in homogeneity. In the second part of the talk, I will review our recent progress on twisted bilayer TMDs bilayers in the reconstruction regime. I will discuss their atomic structure observed in STEM, which is strikingly different for aligned (R) and anti-aligned (H) TMD monolayer orientations. This difference gives rise to correspondingly different optoelectronic properties, with ferroelectric domains arising in the former case and piezoelectric charge texturing in the latter. I will report our recent investigation of sliding ferroelectricity, and its switching behaviour observed in double gated structures using the electron channelling contrast SEM.   

In-Place Exfoliation of 2D Materials

Conventionally, adhesive tape is used to exfoliate layers from 2D materials like graphene or hexagonal boron nitride (hBN). Selected ideal flakes—ultra flat, with the desired thickness, and typically measuring tens of micrometers—are then transferred via a polymer film for stacking and device fabrication. The ratio of the desired ultrathin large flakes to thicker flakes is extremely low, so this conventional approach yields few usable flakes. Moreover, the polymer-assisted transfer often damages ultrathin flakes and leaves an undesirable residue, potentially compromising the reliability of measurements.

Here, we introduce a method for exfoliating a relatively thicker flake in-place. Instead of melting the polymer on the final substrate, a final exfoliation is performed at the precise location of interest. The result is an ultrathin flake positioned without polymer that requires removal. By applying this technique, we have successfully placed a suspended monolayer of hBN over a specifically designed hole (10 μm x 10 μm) in an SiO₂/Si substrate.   

Mapping twist and strain evolution during thermal relaxation of twisted hexagonal boron nitride interfaces

Precise control over both twist angle and strain is necessary to design novel properties originating from moiré superlattices in van der Waals heterostructures. However, the heterostructure assembly process typically imparts twist angle inhomogeneity and strain, and the origin of these unintentional distortions is often not explicitly known. Post-stacking thermal annealing has been shown to reduce the twist angle due to gradual relaxation and rotation of the moiré interface and might also reduce spatial inhomogeneity. Here, we apply scanning probe microscopy to elucidate the thermal relaxation of twist angle and strain within ferroelectric hexagonal boron nitride structures. We develop and compare real-space and FFT-based twist/strain extraction algorithms for hBN interfaces with nominal twist angles ranging from 0.08º to 0.28º. Applying successive annealing steps under ambient and UHV conditions, at temperatures up to 400 ºC, reveal spatially-dependent evolution of twist and strain, and suggest potential sources of strain generation and pinning within fabricated stacks.   

Embedding Air-Sensitive Magnetic Heterostructures Inside an Optical Micro-Cavity for Magnetically Tunable Polaritons

The assembly of two-dimensional heterostructures using van der Waals (vdW) materials has created opportunities to harness layer-specific properties in engineered devices. A prime example is layer-dependent magnetism, as observed in CrI₃, paving the way for devices with tailored layer-dependent magnetic properties.  For example, combining CrI₃ with transition metal dichalcogenide (TMD) monolayers results in interfacial charge transfer that bestows magnetic characteristics upon the TMD trion emission. Past demonstrations of exciton-polaritons in vdW materials embedded in micro-cavities have indicated regimes where polariton optical response can be magnetically modulated. Although magnetic responses of polaritons in layered bulk 2D magnetic materials have been reported, the unique features of magnetic heterostructures remain relatively unexplored. A significant challenge is the air sensitivity of many magnetic vdW materials, which has supported an emphasis on bulk attributes over layer-dependent or monolayer effects in polaritons. Here, we report on the fabrication of magnetic layered heterostructures designed for cavity polaritons. A strategy is presented to address assembly challenges with embedding air-sensitive CrI₃/WSe₂ heterostructures within an optical micro-cavity.   

Homogeneous moiré exciton emission in MoSe2/WSe2 heterolayers

Vertical stacking of two atomic layers with a small angle rotation and/or slight lattice mismatch give birth to moiré superlattice, which is a fertile ground for research of novel phases of excitons and electrons. However, the fabrication of high-quality moiré superlattice, particularly the interface involving transition metal dichalcogenides (TMD), has been a long-standing challenge. The complex inhomogeneous spectra put huge difficulty in analyzing the properties of moiré exciton. In this presentation, we will report homogeneous moiré exciton emission in large-area atomically clean TMD heterobilayers assembled under ultra-high vacuum, shed light on the origins of emission lines in moiré potentials. For a presentative MoSe₂/WSe₂ sample, distortions of moiré wavelength are found to be only a few percentages across the sample area about 10 μm². Furthermore, we observe homogeneous single narrow peak emission in two different samples, which show similar PL spectra at higher excitation power. The biexciton emission with attractive dipolar interaction emerges due to the delocalization of excitons in moiré superlattices. On the high energy side of exciton, we observe an exciton-phonon line. Our findings provide insights into moiré physics, accelerate further exploration of the properties and applications of TMD heterobilayers.   

Tuning Superlattice Potential in Triple-Gated Bilayer Graphene

Motivated by Moiré superlattice in twisted bilayer graphene, an alternative approach of gate-defined superlattice has been used to create periodic superlattice potentials on 2d materials, with greater flexibility and tunability. Previously we have demonstrated signatures of correlated insulator phases in dual-gated Bernal-stacked bilayer graphene modulated by such gate-defined superlattice potential, manifested by a set of resistance peaks centered at carrier densities of integer multiples of single electron per unit cell of the superlattice potential.  Such dual-gate device only allows access to part of the superlattice-displacement field-doping parameter space. Here we discuss our recent work on fabricating and characterizing a triple-gate device, which allows better tuning of the potential profiles, potentially allowing nontrivial band topology and its interplay with the interaction effect.   

Introducing Superconductivity in 2D Topological Chalcogenides

A class of two-dimensional (2D) transition metal dichalcogenides (TMDs) and their twist stacks has recently emerged as a rich platform for exploring topological quantum phases. The integration of the diverse 2D TMD phases with superconductors is pivotal in engineering new electronic states and crafting superconducting (SC) quantum devices. Nevertheless, this has been challenging using conventional methods especially for air sensitive 2D materials. In this presentation, we introduce an innovative technique which converts a class of atomically thin TMDs into robust superconductors, offering a unique route for the creation and investigation of superconductivity and superconducting junctions in 2D topological phases.   

Engineering heterostrain to strongly distort moire lattices: direct imaging and transport properties, Part II

Moire superlattices in vdW heterostructures have been a popular and fruitful area of study due to the interesting physics they can exhibit. These studies usually rely on twisting layers during the stacking process to set the moire wavelength, which is a key dimension modifying electronic behavior.

Heterostrain of vdW devices offers an additional method to tune the moire wavelength, extending the range of accessible wavelengths or even altering the symmetries of the superlattice.

To explore the effects of heterostrain on moire devices, we’ve developed methods to mechanically reconfigure vdW heterostructures using microfabricated metal features which can strongly grip, slide, and stretch layers relative to each other, allowing us to substantially heterostrain devices.

In this 2 part talk, we will discuss our progress towards fabricating aligned graphene-hBN heterostructures, applying ~1% strain to the graphene layer, directly imaging the resultant extreme distortion of strained moire superlattices, and performing electronic transport characterization.   

Engineering heterostrain to strongly distort moire lattices: direct imaging and transport properties, Part I

Moire superlattices in vdW heterostructures have been a popular and fruitful area of study due to the interesting physics they can exhibit. These studies usually rely on twisting layers during the stacking process to set the moire wavelength, which is a key dimension modifying electronic behavior.

Heterostrain of vdW devices offers an additional method to tune the moire wavelength, extending the range of accessible wavelengths or even altering the symmetries of the superlattice.

To explore the effects of heterostrain on moire devices, we’ve developed methods to mechanically reconfigure vdW heterostructures using microfabricated metal features which can strongly grip, slide, and stretch layers relative to each other, allowing us to substantially heterostrain devices.

In this 2 part talk, we will discuss our progress towards fabricating aligned graphene-hBN heterostructures, applying ~1% strain to the graphene layer, directly imaging the resultant extreme distortion of strained moire superlattices, and performing electronic transport characterization.   

2D Quantum Materials Pipelines for automated production of 2D heterostructures at the MonArk NSF Quantum Foundry

Layered two-dimensional (2D) materials enable seemingly endless opportunities to engineer 2D heterostructures by assembling stacks of atomically thin 2D sheets with distinct structural, optical, electronic, and magnetic properties. With interlayer van der Waals bonding, the interfaces between the layers can be pristine, and the selection of the atomic sheets to be integrated as well as their assembly do not need to account for chemical and structural compatibility between the individual layers. Untethered from such limitations and equipped with a vast library of 2D materials, the design and fabrication of novel 2D heterostructures to explore new emergent phenomena or precisely tailor functionalities for technological applications can be intensely pursued with unprecedented freedom. However, the current procedures for fabricating 2D heterostructures are manual, heavily relying on humans to perform the critical processes of mechanically exfoliating bulk crystals, identifying single layers, and stacking individual layers on top of one another. The reliance on manual processes makes 2D heterostructure fabrication tedious, plagues it with low yields, and limits the rate at which results can be reproduced and confirmed. In this talk, I will provide an overview of the 2D Quantum Materials Pipelines (2D-QMaPs) of the MonArk NSF Quantum Foundry that aim to overcome these challenges by automating the assembly of 2D heterostructures with robotic devices for mechanical exfoliation, optical identification of single layers, and stacking of layers into an assembled heterostructures. By leveraging industrial automation technologies and implementing machine learning and artificial intelligence, the 2D-QMaPs significantly accelerate the rate at which these key steps are performed. Select use cases demonstrate how substantially improved reliability and repeatability facilitate faster and higher-quality production of 2D heterostructures as well as more rapid exploration of new 2D material systems. I will discuss how the 2D-QMaPs are integrated with nanofabrication capabilities to realize an end-to-end assembly line for 2D quantum devices that is ultimately intended to provide samples and devices to the overall community of 2D materials researchers.