In-situ Observation of Surface Graphitization of Gallium Droplet and Concentration of Carbon in Liquid Gallium

Abstract

Although carbon has been recognized to be insoluble in gallium, we found that the outermost surface of gallium has unexpectedly high carbon solubility, particularly the limited region of about a few nanometers in depth. Our in-situ transmission electron microscope observations revealed that a graphene layer was precipitated at the surface of a gallium droplet simultaneously with gallium evaporation, and some of the droplets created an internal graphitic layer. On the basis of these experimental data, we evaluated a substantial carbon solubility that seemed to exceed about 50 at. %, but was realized in a very thin surface region of about 4 nm in depth. We believe that this high carbon solubility at the gallium surface is the key mechanism for the catalytic ability of gallium that was observed at the interface between liquid gallium and solid amorphous carbon.

Kinetics of Ga droplet decay on thin carbon films

“…In contrast with previous in situ TEM observations, we do not observe graphitic shell formation around the droplets. 19,21,22,24 We do, however, observe small remnants that do not change in size after the bulk of the droplets dissolve. While the images lack sufficient resolution to identify these features, presumably they are graphitic shells as has been reported in the literature.…”

“…While carbon is expected to be insoluble in liquid gallium, previous studies have suggested otherwise and due to the large difference in the surface energies, liquid gallium does not wet amorphous carbon and forms droplets with wetting angles exceeding 120 . 19,[21][22][23] In this letter, we present in situ transmission electron microscopy (TEM) studies of the decay behavior of Ga droplets on amorphous carbon thin films during annealing. From the time-lapsed TEM images, we find that smaller droplets shrink and the volume decay rates depend on the local environment, characteristic of surface-diffusion-limited Ostwald ripening.…”

“…Energy dispersive spectroscopy (EDS) measurements acquired from the XTEM sample, both before and after annealing, indicate that a significant amount of Ga was present within the carbon film prior to heating and in the droplets observed post-annealing after air-exposure. EDS data also indicated a significant fraction of oxygen but this is likely due to air exposure and not expected in the droplets since gallium oxide does not melt at 773 K. The presence of carbon, if any, 19,[21][22][23] within the droplets could not be determined accurately from EDS due to the large background signal of carbon from the surrounding region. This procedure of forming Ga droplets is highly reproducible and we have obtained similar results, i.e., Ga droplets on carbon films on FIB-prepared XTEM samples of C/Ti/Si, Zr/Al 2 O 3 (0001), and bare Al 2 O 3 (0001) substrates.…”

Near room temperature chemical vapor deposition of graphene with diluted methane and molten gallium catalyst

“…We have demonstrated previously that the outermost surface of molten gallium can absorb up to 50 wt% of carbon in a ~5 nm thin skin layer 23 , despite a low carbon solubility in gallium. At ~1000 °C, carbon atoms diffusing through bulk liquid gallium is also possible 42 .…”

Direct Growth of Graphene over Insulators by Gaseous‐Promotor‐Assisted CVD: Progress and Prospects

“…Along with the typical metal catalysts for growing graphene (copper and nickel), it is proved that Ga catalyst provides favorable controllability in the layer numbers of graphene because of the lower solubility of carbon . In addition, the saturated vapor pressure of low‐melting‐point Ga is nearly one order of magnitude higher than that of Cu under the same condition, which would provide more catalyst atoms in the graphene growth on insulating substrates .…”