Optically Directed Assembly of Continuous Mesoscale Filaments

We demonstrate irreversible continuous filament formation when a weak laser focus is positioned near the edge of an evaporating colloidal droplet containing carbon and gold nanoparticles. Optical trapping, hydrothermal, and chemical interactions lead to controlled colloidal synthesis of stable, irreversible mesoscale filaments of arbitrary shape and size. Mechanisms for this optically directed assembly are discussed with fluid dynamics, molecular dynamics, and lattice kinetic Monte Carlo calculations.

Figure 1

ODA of mesoscale structures. (a) Optical image of a gold-carbon filament obtained with a stationary optical trap. The thick black feature is a $7\mu \mathrm{m}$ diameter carbon fiber. (b) Hook-shaped gold-carbon structure form by slow translation of the trap. (c) Lobe-shaped filament form by translating the trap with varying speeds. Slower speeds cause larger filament diameters and vice versa. (d) TEM image of a filament grown on a TEM grid. The sharp tip is caused when the filament stops growing due to local colloid depletion. (e) Close-up of (d) showing a dense “neck.” This illustrates how filament radial dimensions are influenced by the radial component of the trap potential. (f) Schematic ODA model: NPs drawn into the optical trap from below by convection irreversibly bind at the focus.

Figure 2

Subthreshold filament growth in the steady-state ODA mode (stationary trap). Two vertically displaced traces of transmitted laser light illustrate the characteristic slow attenuation region I, corresponding to diffusional trap loading, followed by an enhanced attenuation region  II due to the faster hydrothermal loading.

Figure 3

CFD results for laser heating of a carbon NP. The droplet radius is $1300\mu \mathrm{m}$. (a) 2D steady-state fluid vector plots from heating a carbon NP (filled white circle) located on the glass surface, with a focused 1.5 mW laser ($\mathrm{m}/\mathrm{s}$). Vortices form by the toroidal circulation. The velocity vectors rotate anticlockwise in the left blowup and clockwise in the right blowup, consistent with funneling just above the NP between these regions. (b) Resulting temperature profiles (in K). A hydrothermal jet can be seen extending vertically above the particle.

Figure 4

Gold-carbon NP interactions. (a) TEM image of the extreme tip of the gold-carbon filament in Fig. 1d. (b) TEM image of encapsulated gold NPs within the tip. (c) Initial gold-carbon NP configuration for a MD simulation. (d) MD result after 10 ns showing wetting of a gold NP by carbon atoms in the 450 K range. These results indicate the possibility of encapsulation of gold NPs by carbon. (e) Initial configuration of gold-gold interaction for the MD simulation. (f) Snapshot showing result after 10 ns at 500 K.

Figure 5

LKMC and experimental deposition for laser powers 0.5 (top), 1 (middle), and 2 mW (bottom). Two different experimental data sets are displayed for mixed colloids (all at fixed gold NP concentration $7.2\times {10}^{10}{\mathrm{cm}}^{-3}$).