The nucleation, radial growth, and bonding of TiO2 deposited via atomic layer deposition on single-walled carbon nanotubes

Abstract

The ability to determine the radial growth rate of atomic layer deposited (ALD) films on quasi-inert surfaces can enable precise control of decorative particle size or film closure for specific applications. The radial-growth-per-cycle (rGPC) is proposed in this work to quantify how thin films nucleate and grow from defect sites on carbon surfaces until film closure. The ALD of TiO2 on non-functionalized singled-walled carbon nanotubes (SWCNTs) was monitored using in-situ Raman measurements, which collects signal of hundreds of SWCNTs during deposition. The gradual increase of the intensity ratio between the D- and G-band ID/IG from the sp2-to-sp3 hybridization reveals the progression in which TiO2 nuclei grow from surface defects towards the pristine surface until nuclei coalescence. The radial overgrowth of TiO2 along the inert surface is mainly bound by Van der Waals forces. Chemical bonding to the surface only occurred on average every ~12 nm, equivalent to only ~1% of the carbon atoms chemically bonded to the film. The in-situ Raman measurements determined a nuclei rGPC of ~0.19 ± 0.02 nm at temperatures lower than 120 °C, the point of film closure at the SWCNT circumference and axis, and a finite induced compressive stress of ~1 GPa to the nanotubes.