Atomically thin 2D materials incorporated into van der Waals heterostructures are a promising platform to engineer quantum materials with atomically resolved thickness and abrupt interfaces across macroscopic length scales, resulting in excellent material properties.
Because 2D materials exhibit a wide range of electronic characteristics with properties that often rival conventional electronic materials — e.g., metals, semiconductors, insulators, and superconductors — it is possible to combine them in virtually infinite variety to achieve diverse heterostructures.
Many of these heterostructures would be otherwise infeasible through direct growth.
We have developed a unique set of in-house capabilities to study 2D materials and van der Waals heterostructures.
Monolayer and Thin Film Growth
Materials growth is the foundation for all subsequent science and engineering. Much of the progress in 2D materials has been enabled by micromechanical exfoliation, a facile but stochastic method of sample preparation. The Mannix lab is equipped with extensive sample growth facilities to enable deterministic study of the key parameters for high-quality, reproducible growth. We intend to employ two primary techniques for the synthesis of atomically thin films:
Molecular Beam Epitaxy (MBE)
MBE involves physical vapor deposition onto atomically-clean surfaces under ultra-high vacuum (UHV, pressures < 10-9 mbar) from solid or gas sources with precisely calibrated fluxes. MBE is amenable to in-situ monitoring during growth (e.g., RHEED, QMS, QCM, BFM) and samples can be characterized without leaving vacuum (e.g., STM). The Mannix lab is equipped with a state-of-the-art MBE chamber for growing 2D chalcogenide and pnictide materials.
Chemical Vapor Deposition (CVD)
CVD involves the deposition of vapor-phase precursors (supplied by solid, liquid, or gas sources) on a growth substrate at elevated temperatures. Metal-organic chemical vapor deposition (MOCVD) is a subset of CVD which provides access to a diverse array of precursors to grow a variety of materials. The Mannix lab is starting out with one MOCVD (4 inch wafers) and one UHV-CVD system (2 inch wafers) to grow a wide range of 2D materials (e.g., mono- and dichalcogenide semiconductors and doped hBN). We can also perform UHV-CVD directly in our STM and MBE preparation chambers.
Our MOCVD and CVD capabilities include synthesis of monolayer and thin-film WS2, WSe2, and hBN.. We have demonstrated inclusion of diverse dopants in these CVD processes.
Atomic-scale and Hyperspectral Characterization
We use and develop characterization techniques to understand the structure and properties of our materials, and to optimize the quality of our synthesis and device fabrication, with in-house technique development of scanning probe microscopy and hyperspectral optical microscopy techniques.
Defects dominate the properties of electronic materials, but it has been challenging to elucidate the role that specific defects play in many 2D materials. This is especially important for functional defects, such as the quantum emitters observed in hBN and transition metal dichalcogenides (e.g., WS2). The Mannix group has a cryogenic, UHV scanning tunneling microscope (STM) to characterize these defects, and correlate their presence with the operation of devices. The STM is equipped with a quartz tuning fork AFM to provide complementary topographical characterization.
Complementary information is obtained from (scanning) transmission electron microscopy (S/TEM) in the SNSF user facilities.
We also develop hyperspectral microscopy techniques to measure spatially-dependent material properties and their evolution over time.
Assembly of van der Waals Heterostructures (vdWHS)
Atomically-resolved solids constructed sequentially from layered 2D materials have generated a flood of discoveries in condensed matter physics. Any future applications of these materials require large-scale fabrication, which remains a critical challenge. The Mannix group is building automated stacking equipment to enable the scalable production of vdWHS.
Nature Nanotechnology, 2022.
The intersection between these techniques will enable us to deterministically study and engineer the properties of 2D materials and their heterostructures, thus providing a platform for engineering the properties of matter at the atomic scale. The resulting capabilities will be of great relevance to emerging platforms for quantum information science based on defects or topological matter.