RESUMEN
Interfacial water molecules affect carrier transportation within graphene and related applications. Without proper tools, however, most of the previous works focus on simulation modeling rather than experimental validation. To overcome this obstacle, a series of graphene field-effect transistors (GFETs) with suspended (substrate-free, SF) and supported (oxide-supported, OS) configurations are developed to investigate the graphene-water interface under different hydrophilic conditions. With deionized water environments, in our experiments, the electrical transportation behaviors of the graphene mainly originate from the evolution of the interfacial water-molecule arrangement. Also, these current-voltage behaviors can be used to elucidate the first-water layer at the graphene-water interface. For SF-GFET, our experimental results show positive hysteresis in electrical transportation. These imply highly ordered interfacial water molecules with a separated-ionic distributed structure. For OS-GFET, on the contrary, the negative hysteresis shows the formation of the hydrogen-bond interaction between the interfacial water layer and the SiO2 substrate under the graphene. This interaction further promotes current conduction through the graphene/water interface. In addition, the net current-voltage relationship also indicates the energy required to change the orientation of the first-layer water molecules during electro-potential change. Therefore, our work gives an insight into graphene-water interfacial evolution with field-effect modulation. Furthermore, this experimental architecture also paves the way for investigating 2D solid-liquid interfacial features.
RESUMEN
We investigate growth of twisted bilayer graphene through two-stage chemical vapor deposition (CVD). Exploiting the synergetic nucleation and growth dynamics involving carbon sources from the residual carbon impurities in Cu bulk and gaseous CHx, sub-millimeter-sized single crystalline graphene grains with multiple merged adlayer grains formed underneath are grown on Cu substrate. The distribution of the twist angles is investigated through a computer algorithm utilizing spectral features from micro-Raman mapping. Besides the more thermodynamically stable AB-stacking (AB-BLG) or large angle (>15°) decoupled bilayer graphene (DC-BLG) configurations, there are some bilayer regions that contain specific twist angles (3-8°, 8-13°, and 11-15°) (termed as TBLG). The statistics show no TBLG formation for BLG with single nucleation center. The formation probability of TBLG is strongly dependent on the relative orientation of merging adlayer grains. Significant defects are found at the grain boundaries formed in AB-DC merging event without creating TBLG domain. The areal fraction of TBLG increases as H2/CH4 ratio increases. The growth mechanism of TBLG is discussed in light of the interactions between the second layer grains with consideration of strain generation during merging of adlayers.
