Terahertz multispectral imaging for the analysis of gold nanoparticles’ size and the number of unit cells in comparison with other techniques

Advances in the controlled assembly of nanoscale building blocks, such as gold and silver nanoscale spheres, and quantum dots (QDs) have resulted in functional devices, such as nano-optoelectronic components, bio-photonics, nanosensors, and novel contrast probes for molecular imaging.1,2 In the assembled structures, the photophysical properties of nanomaterials are a function of the number and the size of the nanomaterials and the distances among them. For instance, the fluorescence lifetime of a quantum dot cluster depends on the number of the QDs, due to the energy transfer between them.3 The surface enhanced Raman spectroscopy (SERS) signal in noble metal nanoparticle arrays depends not only on the properties of the building blocks, but also on the geometric characteristics of the whole array, such as array size. The absorption band of gold nanoparticles is a function of the size and the different fractal structures of the gold nanoparticles and depends on electron-photon relaxation rate.4 Therefore, accurate determination of the size parameter of the nanomaterials is important in order to control the photophysical properties of these nanomaterials. Techniques currently deployed in the determination of particle size include Dynamic Light Scattering,5 Transmission Electron microscope,3 Scanning Electron Microscope7 and Atomic Force Microscope.8 The measured diameter of nanoparticles usually varies depending on the type of instrumentation used for respective measurements. For TEM and SEM, the samples must be small and must remain in high vacuum. Also, for TEM, the samples must be thin enough for electron transparency; as such the sample preparation involves tedious and time consuming steps. AFM imaging is also conducted only on small samples and is strictly a surface imaging technique. In this research, we utilized the terahertz technique as a nondestructive tool capable of non-contact probing and measuring both the size and size distribution of the nanoparticles. Especially, the technique gives one beneficial advantage compared to the AFM, SEM and TEM. It allows us not only to measure the photophysical properties of materials but also to determine the size dimension of some materials9,10 as well as spectroscopic analysis leading to unique identification capabilities.