Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials. This process is often used in the semiconductor industry to produce thin films. The development of CVD for synthetic single-crystalline graphene at the wafer-scale without grain boundaries between graphene domains, which, otherwise leads to the degradation of the graphene’s electrical, thermal and mechanical properties, is vital. At present, two approaches have been explored for this purpose. The first strategy involves reducing the nucleation density to grow a “giant” single domain of monolayer graphene; while as, the second strategy involves the unidirectional alignment of multiple graphene domains which coalesce to form a monocrystalline graphene layer over substrates which are either in a single-crystal phase or liquid phase. Both strategies possess shortfalls that hinder their applications in various ways. Researchers globally have shown that during coalescence, the orientation of adjacent grains can easily adjust to match each other when over a liquid Cu surface. The driving force is attributed to the energy minimization at the stitched region.
Observing the patterns of nature presents a facile approach to assessing various complex scientific problems. One notable example is that of Christiaan Huygens in 1665 when he first noticed how two pendulums, regardless of their initial state, would synchronize. Moreover, it is known that the universe is full of complex self-organizing systems, from neural networks to correlated materials. Building on this, researchers from the Soochow University in China led by Professor Mark H. Rümmeli, together with Dr. Thomas Gemming at the Leibniz Institute for Solid State and Materials Research Dresden in Germany, Professor Barbara Trzebicka at the Polish Academy of Sciences, Dr. David Perello at The University of Manchester and Professor Slava Rotkin at The Pennsylvania State University examined flakes forming over a contiguous polycrystalline graphene layer [Stranski–Krastanov (SK) growth)] by chemical vapor deposition (CVD). Their aim was to observe how graphene flakes, nucleated over a polycrystalline graphene film, synchronize during growth so as to ultimately yield a common crystal orientation at the macroscale. Their work is currently published in the research journal, Advanced Materials.
In their approach, Stranski–Krastanov-like graphene was synthesized using an atmospheric pressure CVD system over polished copper with methane as the feedstock. After growth, a poly (methyl methacrylate) (PMMA) solution was spin-coated on the graphene/Cu at 1000 rpm for 60 s to protect the graphene film during transfer process. A confocal Raman CRM 200 was implemented for Raman spectroscopy and mapping. Further, for SEM characterization, a field emission scanning microscope was used.
Following this approach, the authors were able to show the remarkable and ubiquitous self-alignment of secondary graphene domains after nucleation during SK growth over millimeter areas over an initial base layer of polycrystalline graphene, namely: large area single-crystal graphene forms over a polycrystalline substrate. Moreover, extensive statistical analysis showed that the nucleating secondary graphene domains always initially formed as hexagons, presumably due to epitaxial considerations.
In summary, the multinational collaborative study showed that CVD-grown graphene crystals forming over a graphene substrate during Stranski– Krastanov growth always nucleate as hexagons and in the lowest energy stacking configuration, namely, Bernal stacking. Overall, the research team demonstrated that graphene synthesis could be advanced to control the nucleated crystal shape, registry, and relative alignment between graphene crystals for large area, that is, a single-crystal bilayer, and (AB-stacked) few-layer graphene could be grown at the wafer scale. In a statement to Advances in Engineering, Professor Mark H. Rümmeli explained their findings provided astonishing insight into graphene formation and demonstrated that global cross-talk between growing graphene crystals is possible and could pave the way for advanced synthetic graphene (and other 2D materials) procedures in terms of crystal shape, registry and relative alignments between graphene crystals at the macroscale and large-area single-crystal 2D material fabrication.
Huy Quang Ta, Alicja Bachmatiuk, Rafael Gregorio Mendes, David J. Perello, Liang Zhao, Barbara Trzebicka, Thomas Gemming, Slava V. Rotkin, Mark H. Rümmeli. Large-Area Single-Crystal Graphene via Self-Organization at the Macroscale. Advanced Materials 2020; volume 32 – issue 45, 2002755.