Charged clusters in thin film growth

Abstract

A cauliflower structure is a granular film composed of spherical particles similar in size, each with numerous nanoscale nodules on its surface. The structure is produced during certain chemical vapour deposition (CVD) processes for diamond and silicon thin film growth. A classical account in terms of atomic unit deposition fails to explain the growth of such a cauliflower structure, as it requires a gas phase of much higher supersaturation than for onset of diffusion controlled growth. Another interesting and somewhat puzzling phenomenon encountered during a diamond CVD process is that while diamond is depositing on a graphite substrate, carbon atoms in the graphite itself are etched away into the vapour phase; that is, experience evaporation. Again, an elementary kinetic barrier mechanism fails to explain such CVD deposition of a less stable diamond phase combined with simultaneous evaporation of a stable graphite phase. In order to account for such puzzling CVD phenomena and others, a theory of charged clusters has been developed over the past decade as a new paradigm for thin film growth. The theory and its applications are reviewed in this work.

Generation of Charged Nanoparticles during Synthesis of ZnO Nanowires by Carbothermal Reduction

“…One is that ions are formed first by surface ionization in the reactor and nanoparticles are formed in the gas phase by ion-induced nucleation, which is the case of formation of charged nanoparticles in hot filament CVD (Hwang et al 1996;Hwang et al 2000;Hwang & Kim 2004;Kim et al 2006) or plasma CVD (Girshick et al 1996;Haaland et al 1996;Bleecker et al 2004). The temperature of the reactor is so low that the formation of ions would be negligible.…”

“…The twodimensional growth of nanosheets is also expected to come from the electrostatic energy originated from the charge carried by ZnO nanoparticles. Besides, the electrostatic energy previously suggested to explain the growth mechanism of the self-coiling process of ZnO nanorings and nanobelts (Wang 2004;Kong et al 2004) and silicon nanowires Hwang & Kim 2004) should come from the charge carried by nanoparticles such as those shown in Figures 3 and 5. And the charged nanoparticles seem to be also involved in the one-dimensional growth of oxides such as Nb 2 O 5 and V 2 O 5 (Mozetič et al 2005).…”

“…The temperature of the reactor is so low that the formation of ions would be negligible. The other possible mechanism is that nucleation takes place first and then the nuclei undergo surface ionization on any surface such as the quartz tube of the reactor (Hwang & Kim 2004). Magnusson et al (1999) reported similar unusual charging of gold nanoparticles during reshaping at high temperature.…”

“…In order to explain the one-dimensional growth of nanowires and nanotubes prepared by CVD in the absence of catalytic metal particles, Hwang et al Hwang & Kim 2004) suggested the electrostatic energy arising from the charged clusters or nanoparticles generated in the CVD processes as a main driving force. Considering these results, the electrostatic energy that was suggested to play a critical role in the growth of ZnO nanostructures such as nanobelts and nanorings (Kong et al 2004) might be originated from the charge carried by the nanoparticles.…”

“…Recently, Kim et al (2007) showed that the charged nanoparticles were generated during the syntheses of silicon films, ZnO nanowires and carbon nanotubes (CNTs) by thermal CVD by measuring the size distribution using a differential mobility analyzer (DMA). These measurements were done based on the previous study of Hwang et al (Hwang et al 1996;Hwang et al 2000;Hwang & Kim 2004;Huh et al 2005;Song et al 2007), who suggested that these charged nanoparticles should be a main flux for the growth of films, nanowires, and nanotubes. The generation of charged nanoparticles in the gas phase was experimentally confirmed in various CVD processes (Cheong et al 1999;Jeon et al 1999;Jeon et al 2000;Jeon et al 2001;Ahn et al 2002;Kim et al 2006).…”