Recently, led by the team of Professor Gao Libo of the School of Physics, Nanjing University, and collaborating with the team of four young scholars in the school, the topic of "proton-assisted growth of ultra-flat graphene thin films" was published in "Nature" magazine using proton-assisted growth for high quality Research results of graphene preparation. This work not only explored a method for the controlled growth of ultra-flat graphene films, but more importantly, the team also discovered the internal mechanism of this growth method, that is, proton assist, which is expected to be extended to Flexible electronics, high-frequency transistors and other important research areas.
It is reported that the chemical vapor deposition (CVD) growth graph involved in this achievement is currently the most important method for preparing large-area, high-quality single crystal grains or thin films. However, due to the strong coupling between graphene and the matrix material, graphene will form wrinkles during the growth process. This phenomenon severely limits the preparation of large-scale uniform thin films and hinders the further development and application of two-dimensional materials.
"The wrinkles in CVD graphene are an important bottleneck that affects its physical properties." Gao Libo told reporters that the wrinkles in CVD graphene are derived from the difference in the thermal expansion rates of graphene and the growth substrate. Graphene grows on growth substrates such as copper or platinum The growth temperature is mostly above 600 degrees. After the growth is completed, the temperature drops to room temperature, which causes wrinkles of graphene. The presence of wrinkles affects the excellent properties of graphene. However, there is no complete comparative data on how far it can affect its performance. "How to completely eliminate wrinkles and prepare ultra-flat graphene film has gradually become the focus and difficulty of its quality leapfrog improvement." Gao Libo said.
The research team tried a variety of methods to eliminate wrinkles, but the results were not satisfactory, leaving only the only way to weaken the coupling between graphene and the growing substrate. On the basis of summarizing a large number of experiments, the Gao Libo team found that a high proportion of hot hydrogen (H2) will weaken the coupling between graphene and the growing substrate to a certain extent. At the same time, the researchers found through theoretical simulations that the hydrogen between the graphene and the copper matrix can weaken the coupling of the two under high-concentration and high-temperature conditions. Among the components of hot hydrogen, protons and electrons can freely shuttle through the honeycomb lattice of graphene. Therefore, the researchers speculate that after protons penetrate graphene, there is a certain probability that they will combine with electrons again to form hydrogen.
"The team verified the proposed model by comparing the effects of hydrogen, deuterium (D2), and helium (He) plasma." Gao Libo introduced that increasing the proton density has become a key way to weaken the coupling between the two. In view of this, the research team used hydrogen plasma to treat the folded graphene film, supplemented by high temperature, and gradually weakened and eliminated the graphene wrinkles. If hydrogen plasma is introduced while growing graphene, the graphene grown is completely free of wrinkles.
In order to fully characterize the graphene film without wrinkling, through a variety of physical properties, including scanning tunneling microscope (STM) to observe the moiré fringes and scanning tunneling spectroscopy (STS), angle-resolved photoelectron spectroscopy (ARPES) to visually observe graphene and copper The change in the coupling effect of the substrate and the difference in thermal expansion rate characterized by variable temperature Raman spectroscopy all indicate that this ultra-flat graphene film is in a state of decoupling and undoping from the growing substrate. Due to the ultra-flat properties of graphene films, when removing other substances on the surface of graphene, especially the transfer medium PMMA remaining during the transfer of graphene, it shows the advantage of being extremely easy to clean.
In addition, in order to highlight the advantages of ultra-flat graphene films, that is, large size and high quality, the researchers also conducted graphene quantum Hall effect measurements at different line widths, line widths were 2μm, 20μm, 100μm, 500μm. Previously, because of the uniformity of large-size graphene samples, the maximum line width of the graphene quantum Hall effect was 50 μm, and the ultra-flat graphene film grown from the threshold conditions for the appearance of the quantum Hall effect, and the 1 μm line The intrinsic graphene measured at wide time is almost equivalent. More importantly, for different line width measurements, their platform appearance threshold is almost unchanged. "This shows that only by eliminating wrinkles, can the homogenization and high quality of large-sized graphene be achieved to the greatest extent." Gao Libo said that the proton-assisted CVD method can not only maintain the inherent properties of graphene as much as possible, but also prepare for the future. Other types of nanomaterials are universal. (Author: reporter newspaper correspondent Su Yan Wu Yuhao)
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