Imaging and Manipulation of Gold Nanorods with an Atomic Force Microscope

Shuchen Hsieh, Sheffer Meltzer, C.R. Chris Wang, Aristides A.G. Requicha, Mark E. Thompson and Bruce E. Koel

Laboratory for Molecular Robotics, University of Southern California, Los Angeles, California 90089-0482

The fabrication of nanodevices depends on the ability to synthesize, deposit and position nanoscale building blocks on suitable substrates. Much of the recent effort in this field has focused on using carbon nanotubes, nanowires and spherical nanoparticles. These materials are promising and their mechanical and physical properties can be tuned by alterations in the synthesis methods.
Scanning Probe Microscopy (SPM) techniques has been shown to provide powerful tools for nanomanipulation. We have shown the ability to controllably manipulate gold particles, link particles and use gold particles as templates for subsequent deposition processes.
The shape and size of metal nanorods make them excellent models to study the electrical and optical properties of nanowires, as well as friction forces and nanometer-scale mechanics. Several groups have demonstrated the ability to synthesize colloidal nanorods with well-defined sizes and shapes using different materials. Most of these studies have used TEM and spectroscopic techniques to analyze the size and shape distributions of the synthesized rods. To the best of our knowledge, AFM has not been used yet as a complementary characterization technique in part because of difficulties in sample preparation and imaging. In spite of the wide range of research that has dealt with synthesis issues, very little research has been devoted to the self-assembly of rod-shaped nanocrystals or the robotic manipulation and assembly of nanorods (other than carbon nanotubes). One of the few examples available is from El-Sayed’s lab where the dimensions of self-assembled, gold-rod structures were shown to be controlled by experimental conditions.
We report here on studies of deposition and manipulation of electrochemically prepared, micelle-capped Au nanorods deposited on silicon dioxide (SiO2) surfaces. Both sample preparation and manipulation of nanorods differ from their counterparts for (spherical) Au particles. A thiol-terminated silane (3-Mercaptopropylmethyldimethoxysilane, MPMDMS) was used as an active interface for gold nanorod assembly. Scanning force microscope (SFM) and XPS analysis were used to confirm the presence of gold on the substrate. We have used our Probe Control Software to image and manipulate individual Au-nanorods. It was found that mechanical movement of the rods depends on the location of the pushing point along the rod and whether the pushing manipulation is taking place along or across the rod.
By tracking the tip amplitude and deflection signals during the manipulation operation we were able to estimate the mechanical threshold needed for manipulation. These signals can also be used to compare interactions between different particles and adhesive layers.

SFM images (500 nm  500 nm scan size) displaying the result of manipulation of four gold nanorods.