From Quantum Confinement in Carbon Nanotubes to Helium Droplet-Mediated Deposition of Metallic Nanoparticles

Abstract

High-surface areas and precisely tuned pores of carbon nanotubes make them relevant materials for applications such as in gas adsorption, selective separation of light isotopes, and nanoreactors for quasi one-dimensional confinement of metal nanoparticles. Understanding the role of quantum nuclear effects and intramolecular interactions in the motion of molecules in carbon nanotubes is deeply fundamental. Very recent experimental measurements at low temperatures (2-5 K) of Ohba [1] revealed that much more molecules of nitrogen than helium atoms absorb in small diameter (below 0.7 nm) carbon nanopores, despite of the larger kinetic diameter of the former. Using the helium density-functional formulation for a large 4He droplet containing a carbon nanotubes inside, we first show that the experiment can be understood by considering very large zeropoint effects in the helium motion, which includes the formation of cavities with zero helium densities [2]. Second, we present an ad-hoc developed nuclear wave-function treatment to provide a detailed insight into the effects of quantum confinement for both N2 and 4He clusters in carbon nanotubes as a function of the tube diameter [3]. Third, we will present a pairwise potential model [3] to describe the gas adsorption to carbon materials which relies on DFT-based symmetry-adapted perturbation theory [4]. Finally, we propose a mixed approach combining nuclear density functional and wave-function treatments [3].

The ultra-cold 4He droplet-assisted soft deposition of nanoscale metallic particles will be the second topic of the talk. This topic attracts nowadays strong attention due to both the exciting fundamental physics behind, including quantum vorticity in superfluid 4He droplets [6], and to the applications of deposited metallic and bimetallic core-shell nanoparticles and nanowires [7, 8, 9], such as in catalysis of TiO2-supported gold nanoparticles [9]. Our recent theoretical study demonstrated that the collision of atomic gold immersed in a 4He300 droplet onto the TiO2(110) surface can be characterized as a softlanding process if the quantum - time-dependent helium density functional-based - description of the helium droplet is applied [10]. We will present molecular dynamics calculations on the aggregation and deposition of a few-nm sized silver particles (up to 10000 Ag atoms) embedded in large 4He droplets (up to 100000 4He atoms) and considering two carbon-based surfaces (graphene and amorphous carbon) at room temperature. Our focus will be in comparing simulations with and without inclusion of the 4He droplet dynamics, and in analysing their consistency with recent experimental measurements of Vilesov¿s group [11]. It will be shown how the experimental measurements can be explained through large-scale atomistic simulations when ab-initio-based van-der- Waals-dominated interaction forces are used.