Solidification of Gold Nanoparticles in Carbon Nanotubes

The structure and the solidification of gold nanoparticles in a carbon nanotube are investigated using molecular dynamics simulations. The simulations indicate that the predicted solidification temperature of the enclosed particle is lower than its bulk counterpart, but higher than that observed for clusters placed in vacuum. A comparison with a phenomenological model indicates that, in the considered range of tube radii ($R_{\mathrm{CNT}}$) of $0.5<R_{\mathrm{CNT}}<1.6\mathrm{nm}$, the solidification temperature depends mainly on the length of the particle with a minor dependence on $R_{\mathrm{CNT}}$.

Figure 1

(a) Time averaged atomic configuration of the Au nanoparticle and section of the carbon nanotube for the cases ($N_{\mathrm{Au}}=775$, $n=30$) at 800 K (solid, left side) and 1400 K (liquid, right side). $L$ denotes the length of the cylindrical core, $R$ the radius, $\vartheta$ the contact angle, and $h$ the height of the cap. (b) Instantaneous view along the axis of the CNT for the system of ($N_{\mathrm{Au}}=775$, $n=30$) at four different temperatures (400, 800, 1200, and 1400 K). The first two correspond to the solid phase, while the last two correspond to a liquid layered phase. The representation of the atomic radius is decreased with respect to (a) to distinguish the different layers.

Figure 2

Radial density profiles for the case at different temperatures. Circles, $T=1100\mathrm{K}$; squares, $T=1200\mathrm{K}$; triangles, $T=1400\mathrm{K}$. (a) The case ($N_{\mathrm{Au}}=775$, $n=30$), with $T_S=1150\mathrm{K}$, and (b) the case ($N_{\mathrm{Au}}=4983$, $n=40$), with $T_S=1215\mathrm{K}$.

Figure 3

Two-dimensional orientation order parameter ${\mathrm{Q}}_6$ at different temperatures: Case ($N_{\mathrm{Au}}=1316$, $n=30$). Circles: first layer (innermost); squares: second layer; up triangles: third layer; down triangles: fourth layer (outermost).

Figure 4

Solidification curve as a function of particle length for all the studied cases. Markers, MD simulations: triangle, CNT $(20,0)$; circles, CNT $(30,0)$; squares, CNT $(40,0)$. Continuous lines, corresponding predictions of the second term (II) in the rhs of (3): dash-dotted line, CNT $(20,0)$; continuous line, CNT $(30,0)$; dashed line, CNT $(40,0)$. Inset: Solidification curve vs number of gold atoms in the particle. Closed squares: particles in vacuum; triangles: particles enclosed in a CNT $(20,0)$; circles: particles enclosed in a CNT $(30,0)$; squares: particles enclosed in a CNT $(40,0)$.