Precision synthesis
Monolayer and Thin Film Growth
Materials synthesis is the foundation of all advanced science and engineering.
Much of the progress in 2D materials has been enabled by micromechanical exfoliation — the original Scotch Tape method. This is a simple and inexpensive way to generate high-quality crystals. Unfortunately, the resulting samples are small and unpredictable.
High quality thin film growth is key to future technologies.
Our lab is equipped with state-of-the-art synthesis facilities — some of it home-built, all of it customized — to enable detailed investigation of growth processes and produce material for advanced device fabrication. Many projects in our lab focus on developing improved methods for to grow high-quality crystals, or to control their properties via doping and polytype modification. Our work focuses on two primary synthesis techniques:
Molecular Beam Epitaxy (MBE)
MBE involves physical vapor deposition onto atomically-clean surfaces under ultra-high vacuum (UHV, pressures typically < 10-10 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 modern, commercial MBE chamber for growing 2D chalcogenide and pnictide materials. We are developing surface chemistry methods to enhance the quality and structural selectivity during growth of layered quantum materials in UHV.
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. We use several variations of this versatile concept:
Metal-organic chemical vapor deposition (MOCVD) is a subset of CVD which uses volatile organometallic precursors to grow materials, with the potential for high uniformity and scalability, low temperature synthesis, and high crystal quality. Our lab operates one MOCVD (4 inch wafers) for WS2 and WSe2.
Ongoing projects include methods for polytype control of TMDCs, precise doping, selective area growth, and growth of ~100 nm thick films for optical applications.
Our group recently developed a Hybrid MOCVD process which uses inorganic salts alongside conventional MOCVD precursor delivery. This simplifies the process of growing new materials and testing new growth chemistries. We recently built a custom 2" reactor dedicated to exploring new growth chemistries for sulfide materials.
See this technique in action at Z. Zhang, L. Hoang, et al., ACS Nano (2024).
Ultra-high vacuum (UHV) CVD operates at low pressures to minimize contamination and precursor flux. We built and operate an advanced UHV-CVD system for hBN growth on 2" wafers, using a custom heater which can reach up to 2000ºC!
We additionally can run ad-hoc UHV-CVD processes in the UHV prep chambers of our STM/MBE system.
We also have access to several conventional tube-furnace CVD systems for graphene and hBN growth.