Manipulation and light-induced agglomeration of carbon nanotubes through optical trapping of attached silver nanoparticles

Chao Shi, Yi Zhang, Claire Gu, Leo Seballos and Jin Z Zhang

Nanotechnology 19 (2008).  p. 215304-1-4.

 Purpose of the study

 Evaluate a method of using lasers to control carbon nanotube position and agglomeration through the use of silver nanoparticles.

 Methods

 Silver nanoparticles with an average diameter of 25nm were fabricated by reducing silver nitrate with sodium citrate.  Size was determined using transmission electron microscopy (TEM).  Silver was chosen as the base metal because it has a surface plasmon resonance (SPR) in the visible region thereby simplifying the technique.  Multi-walled carbon nanotubes (MWCNTs) were synthesized by thermal enchanced chemical vapor deposition with Al₂O₃ -coated (200nm) silicon wafers as the substrate.  MWCNTs were subsequently scraped off the silicon substrate for mixture with SNPs.  High resolution TEM and Raman scattering were used to measure dimensions (30micron length; 20nm diameter) and multi-walled structure.  To verify adhesion of SNPs to MWCNTs, a TEM image was taken before and after adding SNPs to MWCNTs in an isopropyl alcohol solvent.  Optical trapping was achieved by applying a TEM₀₀ mode laser (532nm wavelength) to a sample (water-based solution of SNP-attached MWCNTs) placed on a glass slide.  Trapping was confirmed by taking TEM images before and after moving the slide.  An un-trapped MWCNT bundle is used as a reference point on the slide demonstrating relative movement of the trapped bundle to the un-trapped bundle. 

 Key Findings

1. SNPs appear on or at junctions of MWCNT implying binding; most likely due to electrostatic and/or van der Waals forces.
2. Optical trapping was not observed with MWCNTs when SNPs were not present.
3. Two-dimensional optical trapping was observed when SNPs were bound.
4. The minimum volume ratio of SNPs/CNTs to achieve optical trapping is 1:4.
5. Hydrodynamic drag force determines the maximum speed at which a CNT bundle can remain optically trapped:     F_drag = hdu    (h = viscosity; d = longest dimension of the moving object; u = object’s velocity relative to the fluid)
6. MWCNT cannot exceed 24.8micron s-1 and remain optically trapped all else remaining equal (water solution, size/shape of the MWCNT, fixed laser power).
7. MWCNT agglomeration was observed under the laser beam in a donut-shaped disk with the outer diameter reaching 100micron and center hole diameter reaching 20micron.  Size of the microstructure as well as formation rate increased with increased laser power.

Definitions

Surface plasmon resonance:  Light-induced excitation of surface electron gas surrounding lattice metallic structures.

Optical trapping:  method of stabilizing and manipulating position of nanoparticles through use of laser-induced gradient forces.

Transverse ElectroMagnetic Mode (TEM₀₀ mode laser): type of laser in which there are no electric or magnetic fields in the direction of propagation.