Alternate mechanism for the spontaneous formation of freestanding Ga nanoribbons on Cr 2 GaC surfaces
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Michel W. Barsoumb) Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 (Received 1 March 2006; accepted 20 April 2006)
Herein we propose an alternate simpler mechanism for a new phenomenon we reported recently in this journal [Z.M. Sun, S. Gupta, H. Ye, and M.W. Barsoum, J. Mater. Res. 20, 2618 (2005)]. The presence of freestanding Ga nanoribbons on Cr2GaC surfaces were found to be the leftover skins of Ga whiskers or spheres that were reabsorbed into the Cr2GaC grain boundaries, most likely as a result of their melting.
In a recent paper in this journal,1 we reported on the room-temperature spontaneous growth of Ga nanoribbons—sometimes longer than several millimeters and at most 10 nm thick—from the grain boundaries of polycrystalline samples of Cr2GaC. These nanoribbons were found to be different from those reported so far in the literature.2–8 The growth mechanism of these nanoribbons was assumed to be due to surface wetting of Ga, followed by an oxygen-induced dewetting. Upon further observation, however, it now appears the formation of these nanoribbons is, in fact, much simpler; they are the leftover skins of the whiskers that form spontaneously on the surfaces of Cr2GaC, when the latter are exposed to air. In other words, the nanoribbons are analogous to the skins of molting snakes. Figures 1 and 2 clearly show this intriguing formation process in real time. In Fig. 1(a), a Ga whisker (which is a single crystal as will be discussed elsewhere9) was found at the root of a nanoribbon. It can be clearly seen in a magnified view [Fig. 1(c)] of the whisker that the nanoribbon is on top of the whisker. However, 6 min later when we returned to the same location after observing some neighboring locations, the Ga whisker at the root of the nanoribbon had disappeared [Figs. 1(b) and 1(d)], leaving behind a longer nanoribbon. Essentially the same phenomenon was observed at another location, where this time two relatively large Ga
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2006.0220 J. Mater. Res., Vol. 21, No. 7, Jul 2006
spheres [Figs. 2(a) and 2(c)], were seen to disappear into the grain boundary of the matrix, leaving behind a bundle of nanoribbons [Figs. 2(b) and 2(d)]. In this case, the sample was kept in the scanning electron microscope (SEM) overnight without venting the chamber, and the time between the two sets of micrographs was ∼20 h. Such Ga spheres were frequently observed on the sample surface, and their spherical geometries were confirmed by tilting the sample stage in the SEM chamber. The fact that both whiskers and spheres disappear essentially into the grain boundaries of the Cr2GaC matrix, accounts for both the morphologies of the straight, long nanoribbons (Fig. 1) and the ones that are more two dimensional or scarf-like (Fig. 2). The spontaneous room-temperature growth of low melting point or soft metal whiskers, such as Sn, Cd, Bi, and Zn,10–13 is a well-established
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