Controlling nanoparticle shapes electrochemically

Most nanoparticle synthesis methods result in nanoparticles bounded by low-index, low-energy faces such as the {111} or {100} atomic planes. This makes intuitive sense, as any high-energy face should grow itself out of existence, leaving particles bound by more stable faces. Unfortunately, particles with mostly low-energy surfaces contain a low percentage of atomic edge and corner sites. The synthetic method of Tian et al. produces particles capped by {730} faces, a surface structure that contains a relatively high density of atomic step edges (see the right panel of the figure). The authors calculated that 43% of the total number of surface atoms reside along steps, which can be compared to 6%, 13%, and 35% for 5-nm-diameter platinum cubes, spheres, and tetrahedral particles, respectively.

From this perspective article of David L Feldheim; the synthetic method of Tian et al in question is an electrochemical method (electrodeposition). Some of the particle shapes shown in the paper of Tian et al reminded me of the particle shapes seen in the simulations of Saswata Bhattacharyya et al (See pp.18-19 of this pdf file for example) — I will try and get a preprint of their paper uploaded on the net, and link here. Here is the draft of the paper by Saswata Bhattacharyya et al on roughening transitions.

Growth above and below eutectic temperatures

The perspective article by Volker Schmidt and Ulrich Goesle summarises the results rather neatly:

As expected, above the eutectic temperature, nanowire growth involves a liquid droplet on top of the germanium nanowires (…). However, when the authors reduced the temperature to below the eutectic temperature while keeping the supply of germanium constant, they observed two distinctly different phenomena (…). Some gold nanodroplets remained liquid even though the temperature was, in one case, more than 100°C below the of 361°C. The authors observed this VLS-type growth mostly for nanowires with relatively large diameters.

In contrast, for nanowires with relatively small diameters, the gold droplet became solid as the temperature fell below . The nanowires continued to grow, but did so much more slowly than in the case of VLS growth (…). Further cooling experiments showed that the transformation of the gold caps from liquid to solid at temperature below could be delayed for tens of minutes. Kodambaka et al. show that this delay depends on various parameters, such as the vapor pressure, the temperature, and the diameter of the nanowires.

The bibliographic details of the paper referred to above are as follows:

Title: Germanium nanowire growth below the eutectic temperature

Authors: S Kodambaka, J Tersoff, M C Reuter, and F M Ross

Source: Science May 4 2007. Vol. 316, No. 5825, pp. 729-732.

Abstract: Nanowires are conventionally assumed to grow via the vapor-liquid-solid process, in which material from the vapor is incorporated into the growing nanowire via a liquid catalyst, commonly a low–melting point eutectic alloy. However, nanowires have been observed to grow below the eutectic temperature, and the state of the catalyst remains controversial. Using in situ microscopy, we showed that, for the classic Ge/Au system, nanowire growth can occur below the eutectic temperature with either liquid or solid catalysts at the same temperature. We found, unexpectedly, that the catalyst state depends on the growth pressure and thermal history. We suggest that these phenomena may be due to kinetic enrichment of the eutectic alloy composition and expect these results to be relevant for other nanowire systems.