Single crystals of metals have superior electronic properties compared to their polycrystalline phases, owing to electron scattering at grain boundaries in the latter. Metal single crystals are used as substrates to grow grain boundary-free graphene through chemical vapor deposition (CVD). However, growing large-area single crystals as either bulk crystals or thin films has been a consistent challenge because of heterogeneous nucleation during the growth process. Researchers at the Institute of Basic Science (IBS), Ulsan National Institute of Science & Technology (UNIST), and Sungkyunkwan University have now introduced a method to make single-crystal metal foils from polycrystalline metal foils through a contact-free annealing (CFA) process. Their research was published in a recent issue of Science.

Traditional methods to grow single crystals are expensive and restricted to small areas. Large-area single-crystal metal foils are needed for growing large-area graphene, diamond, hexagonal boron nitride, and cubic boron nitride. Rodney S. Ruoff, from IBS and UNIST, says, “Cu(111), Ni(111), and Co(0001) surfaces are relatively closely lattice matched to these materials.” While preparing substrates for the growth of these materials, they discovered a novel CFA technique which can be used to grow large-area single crystals of metals such as copper, nickel, cobalt, platinum, and palladium. All of these metals exhibit a face-centered cubic (fcc) structure; however, Ruoff says that “the types of single-crystal metal foils will be expanded to include those metals that have body-centered cubic (bcc), hexagonal closed pack (hcp), and other crystal structures”.

As-received Cu foils typically exhibit elongated grains in the rolling direction. During the CFA process, a metal foil is suspended and exposed to high temperatures (near the melting point of the metal) and under a hydrogen atmosphere.  The elongated grains, present in the as-received foils, recrystallize to form large polygonal grains within an hour. However, only some of the grains attain large, centimeter-sized areas (cm). Grains located at the edge of the foil grow abnormally large with atypical speeds of 70 mm/s by consuming the nearby smaller grains. This could be an effect of energy stored at the edge of the foil due to previous cutting or handling.

X-ray diffraction experiments confirmed the presence of single crystals grown through CFA process, with similar signatures for in-plane and out-of-plane scattering. Through the electron backscatter diffraction technique, the researchers confirmed the uniform presence of (111) planes in the sample.  In the suspended metal foil, any deformation from interfacial contact that results in thermal stress is minimized. The experiments demonstrate that hydrogen atoms in the environment strongly stabilize the vacancies in the bulk of all of the metals studied, except for Pt which remains unaffected by the hydrogen atmosphere. The researchers successfully prepared Cu(111), Ni(111), Co(0001), Pt(111), and Pd(111) single-crystal foils with large grain sizes of up to 32 cm² using CFA.

Anthony Rollett of Carnegie Mellon University wrote in his commentary on this work in Science, “The grain boundaries must overcome many physical barriers in order to migrate over distances comparable to the desired grain size. Indeed, it is accepted that thin films rarely will coarsen beyond the thickness of the film itself. Thus, the study by Jin et al. is very surprising because it demonstrates that grain boundaries can be moved in foils of metals such as copper and platinum over distances on the order of centimeters.”

Read the abstract in Science.