Probing average size of metal nanoparticles

Many nanotechnology and biotechnology applications rely on how tiny metal particles arrange themselves on flat surfaces. These patterned structures on the nanometre scale are of great importance in making novel electronic, magnetic and photonic devices. Therefore being able to accurately characterise them is vital. However, this can be very challenging and sometimes requires several different techniques to find the detailed shape and chemical composition of the nanoparticles. A group of researchers from Diamond employed a technique called X-ray standing wave (XSW) on Diamond’s Test Beamline B16 to probe the average vertical size of metal nanoparticles on a substrate surface with accuracies of better than 1nm. This work has been published in the journal Physical Review B.

In Detail

Dispersion of metal nanoparticles on flat surfaces has attracted considerable interest in several nanotechnology, biotechnology and quantum dot applications. For example, molecular printing of nanoparticles is highlighted as a method for creating an organized precursor structure on a substrate surface for locating nanowires and carbon nanotubes. To fully exploit the surface it is vital that it can be adequately characterised. Currently the most commonly used techniques for visualising these patterned nanostructures are scanning electron microscopy (SEM) and atomic force microscopy (AFM). However these techniques are time consuming and do not provide any information on the physical and chemical state of the nanoparticles. As a result complementary methods are frequently required.

X-ray Standing Waves (XSW) have already been used in surface condensed matter physics to characterise heavy ions deposited on organic monolayers, in polymer films and in thick Langmuir-Blodgett films However, it has not been used to examine the structure of metal nanoparticles on surfaces. In this study bright X-rays emitted from a bending magnet on beamline B16 at Diamond, monochromatized from a Si (111) double crystal monochromator, were allowed to excite metal nanoparticle samples at grazing incidence angles.

The new approach provides element specific analysis of the nanoparticles and therefore is applicable for analyzing all kinds of metal and metalloid mixture of nanoparticles distribution on a flat surface, for their surface morphology and chemical compositions. The method does not depend on the crystalline or amorphous nature of the nanoparticles. The method also has the advantage of being able to examine nanoparticle distribution in a liquid medium, or buried inside a coating layer. A single measurement determines the average size of the nanoparticles over large surface areas, obviating the need to perform several measurements over small regions of the specimen, as is commonly required in more conventional probes like atomic force microscopy and optical micro-interferometery.

M. K. Tiwari, K. J. S. Sawhney, Tien-Lin Lee, S. G. Alcock and G. S. Lodha, Probing the average size of self-assembled metal nanoparticles using x-ray standing waves,  Phys Rev B, 80 (July 2009)

DOI: 10.1103/PhysRevB.80.035434