Alfa Chemistry is a pioneer in the research of two-dimensional (2D) materials and can provide customers like large R&D labs and universities throughout the world unique 2D material heterojunction creation services. We demonstrate our research capabilities in a variety of substrate and heterojunction arrangements, including control of surfaces and interfaces. Experimental strategies regarding our retention and customization of 2D heterostructure interfaces include minimization of vertical interface contamination, control of vertical interfaces through intercalation, and control of 2D lateral interfaces.
Minimizing Contamination at Vertical Interfaces
Alfa Chemistry has two general methodologies for generating vertical two-dimensional heterostructures: mechanical transfer and direct growth, both of which produce atomically clean interfaces locally under ideal conditions. Nevertheless, methods that minimize contamination are highly desired, especially in large-area applications requiring high spatial homogeneity, such as integrated circuit technology.
To avoid contamination, our experimental technique for customizing 2D heterostructured interfaces is to avoid direct contact between 2D materials and polymers. A 2D flake, for example, can be taken up by another 2D material at a clean interface due to strong van der Waals interactions. This pickup operation can be repeated until the entire heterogeneous structure is transferred collectively onto the substrate. We recently employed this technology to manufacture wafer-level vertical heterostructures made up of MOCVD-grown TMDC monolayers using a modified vacuum stacking process.
Fig 1. a) Schematic of a vacuum-based transfer method. b) A fabricated heterostructure of three layers of MoS2 with large-scale uniformity (left). Cross-sectional TEM image of a MoSe2/ MoS2/WS2 heterostructure (right). (Kang K, et al. 2017)
Controlling Vertical Interfaces Through Intercalation
Between layers of 2D materials, Van der Waals gaps have the ability to accommodate inserted atomic and molecular species. Intercalation chemistry has been widely employed by Alfa Chemistry to change interfacial interactions in 2D systems and to stimulate the creation of novel materials. For example, our high-temperature annealing of epitaxial graphene environments in promotes hydrogen embedding between the buffer layer and the SiC substrate, leading to passivation of Si dangling bonds. This process transforms the buffer layer into quasi-independent graphene with recovered electronic properties.
Control of 2D Transverse Interfaces
Direct growth and post-growth alterations are used to create 2D lateral heterostructures, as opposed to vertical heterostructures. The risk of passivating and contaminating the first material's growth front, which could break the resulting interface or impede subsequent growth, is a difficulty in constructing lateral heterostructures. Furthermore, alterations in the second material's growing circumstances may cause damage to the already formed material.
Alfa Chemistry has successfully developed experimentally TMDC-based and graphene-hBN-based 2D lateral heterostructures using our many years of research knowledge and related technological support.
Fig 2. TEM image and schematic of a WSe2 - MoS2 lateral heterostructure with an atomically sharp interface. (Li M. Y, et al. 2015)
TMDC Lateral Interfaces
With the advancement of monolayer TMDC CVD growing techniques, we use a comparable one-step CVD method to create 2D TMDC-based transverse heterostructures. At a lower temperature of 650°C, MoO3, W, and S solids are utilized as precursors to form WS2 - MoS2 lateral heterostructures with atomically defined lateral interfaces.
Graphene-hBN Lateral Interfaces
Graphene-hBN lateral heterostructures have considerable electrical discontinuities at the interface as compared to semiconductor TMDC heterostructures, which makes them appealing for lateral tunneling device applications.
CVD growth of graphene and hBN on metal substrates has been widely accepted. A scalable process called "patterned regrowth" enables transverse hBN growth using aminoborane between pre-patterned graphene. The resulting spatially well-defined lateral heterostructures can then be transferred from the copper growth substrate to the desired dielectric substrate for large-area electronic circuit fabrication.
References
- Kang K, et al. (2017). "Layer-by-Layer Assembly of Two-Dimensional Materials into Wafer-Scale Heterostructures." Nature. 550: 229.
- Li M. Y, et al. (2015). "Epitaxial Growth of A Monolayer WSe2-MoS2 Lateral p-n Junction with An Atomically Sharp Interface." Science. 349: 524.
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