Hydrogenated Silicon Nanoclusters with a Permanent Electric Dipole Moment for the Controlled Assembly of Silicon-Based Nanostructures

Abstract

While silicon nanoclusters have extensively been used for their outstanding properties for many decades, never before their dipole moment has been exploited for any application. Here, we have succeeded in producing hydrogenated silicon nanoclusters with a strong permanent electric dipole moment. This dipole moment allows us to use electric fields in order to orient and guide individual clusters. As a first example, we 1 demonstrate the catalyst-free one-by-one self-assembly of one of the thinnest silicon nanowires yet observed. As a second example, we show that the simple presence of those nanoclusters on LaB 6 cathodes leads to a 30-fold enhancement of the thermionic electron current density over pristine LaB 6 . Last but not least, the nanoclusters provide a protective layer against chemical and mechanical attack and largely prevent the evaporation of substrate materials potentially increasing the operational lifetime of cathodes substantially.

Scaling of the permanent electric dipole moment in isolated silicon clusters with near-spherical shape

“…31,32 Interestingly, hydrogenated Si-NCs also show dipole moments, which are important for the controlled assembly of silicon-based nanostructures. 37 With the work presented here, it is now additionally shown that isolated Si N nanoclusters in the gas phase are also polar despite their compact, near-spherical shape.…”

Electric Field-Induced Nano-Assembly Formation: First Evidence of Silicon Superclusters with a Giant Permanent Dipole Moment

“…This means that all SiNCs considered here are neutral in the afterglow. We underline that we have already given experimental proof for this conclusion regarding our specific nanoclusters [8]: After their creation, our SiNCs had to travel a certain distance before laser detection. Even if they had been charged with only one single elementary charge, they should have traveled this distance within 9 µs because of the applied electric DC field.…”

“…Since the experimental details have been published elsewhere [8], we will only concentrate on some major features here. Inside a 0.02 m 3 vacuum chamber, an RF (13.56 MHz) CCP discharge was generated (Cesar 133 RF power supply) at low pressure (10 to 15 Pascals) between a grounded and a powered electrode.…”

“…The simple presence of these SiNCs on a LaB 6 cathode actually increased the thermionic electron density emission by a factor of up to 30. To this end, it was necessary, however, to deposit the polar SiNCs in a well-oriented manner, which was simply accomplished with the application of an electric field during their surface deposition on the cathode [8]. Interestingly, Schäfer et al have very recently confirmed the existence of silicon clusters with a permanent electric dipole moment by means of electric molecular beam deflection experiments at cryogenic temperatures for clusters with 30 to 90 atoms [9], following the theoretical work of Jackson and Jellinek [10].…”

“…To make sure that our nanoclusters do not have a crystalline structure, we thus adopted a scheme that is exactly contrary to the one of Kortshagen et al Specifically, we created the same plasma environment as the one that already permitted us to produce one-to-twonanometer amorphous-like SiNCs with a relatively strong permanent dipole moment [8]. Furthermore, we increased the on-time of our pulsed plasma from about 2 s to 8 s, which was sufficient time for the tiny SiNCs to agglomerate before the end of the plasma pulse.…”