Van der Waals Effects on semiconductor clusters

Abstract

© 2015 Wiley Periodicals, Inc. Van der Waals (vdW) interactions play an important role on semiconductors in nanoscale. Here, we utilized first-principles calculations based on density functional theory to demonstrate the growth mode transition from prolate to multiunit configurations for Ge<inf>n</inf> (n=10-50) clusters. In agreement with the injected ion drift tube techniques that "clusters with n < 70 can be thought of as loosely bound assemblies of small strongly bound fragments (such as Ge<inf>7</inf> and Ge<inf>10</inf>)," we found these stable fragments are connected by Ge<inf>6</inf>, Ge<inf>9</inf>, or Ge<inf>10</inf> unit (from bulk diamond), via strong covalent bonds. Our calculated cations usually fragment to Ge<inf>7</inf> and Ge<inf>10</inf> clusters, in accordance with the experiment results that the spectra Ge<inf>7</inf> and Ge<inf>10</inf> correspond to the mass abundance spectra. By controlling a germanium cluster with vdW interactions parameters in the program or not, we found that the vdW effects strengthen the covalent bond from different units more strikingly than that in a single unit. With more bonds between units than the threadlike structures, the multiunit structures have larger vdW energies, explaining why the isolated nanowires are harder to produce.