Metal nanoparticles exhibit improved properties at the nanoscale and are extensively studied for applications in catalysis, plasmonics and nanoelectronics. Their characteristics are largely based on the surface-to-volume ratio that, on the nanoscale, is significantly higher than for bulk materials. This considerably improves the impact of the surface on the behaviour and properties of nanoparticles.
While using these materials in novel and already existing applications, it is critical to understand these properties. The melting point of nanoparticles in relation to bulk is considerably decreased by the high surface-to-volume ratio and particle curvature, which affect sintering behaviour and melting.
Essential properties on a nanoparticle by nanoparticle basis can be resolved by directly observing the sintering and melting behaviour of metal nanoparticles in the TEM using in situ heating experiments. This cannot be done with bulk techniques. In this article, the Aduro in situ heating and electrical biasing system was used for studying the melting and sintering behavior of silver nanoparticles in the TEM.
The ceramic heating membrane of semiconductor devices known as E-chips™ behaves as the sample support and active area. The monolithic design and small size of the E-chips, considerably minimises thermal drift, allowing imaging at high resolution at temperatures up to 1200°C.
Effective compensation by users for thermal drift and study of the same particles over a large temperature excursion (i.e. at room temperature to 1200°C) is possible without changing the field of view.
Individual calibration of each E-chip ensures that the temperature of the heated material is accurately known. This, in turn, facilitates quantitative interpretation and elucidation of results.
In this experiment, silver nanoparticles were studied in order to determine sintering properties as well as vaporization and melting temperatures in terms of size. The selection of silver was based on its resistance to oxidation. Direct dispersion of nanoparticles was performed on an E-chip’s heating membrane, which includes a thin carbon layer for additional support.
The vaporization and melting properties of silver nanoparticles with sizes from 4 to 50nm were studied in the first experiment. In the second experiment, sintering properties of nanoparticles with sizes from 14 to 40nm were studied. Dihedral angle and neck radius were the sintering parameters measured. From these measured parameters, surface diffusion coefficients were determined.
The silver nanoparticles featured an organic layer for prevention of agglomeration at room temperature and were bought from Nanotechnologies, Inc. The experiments were conducted in a JEOL 2010F in Dr. Paulo Ferreira’s laboratory at the University of Texas at Austin. The microscope was run in the bright field mode at 200kV.
Results and Discussion
In the first experiment, the vaporization and melting points were determined as a function of size. Figure 1 shows a series of images at increasing temperature from room temperature to 600°C. It was observed that small particles vaporised and melted before larger particles. Along particular crystallographic planes in the nanoparticle, shrinkage was seen due to surface energy anisotropy or interactions with the carbon support film.
Figure 1. Sequence of images at increasing temperature from room temperature to 600 °C.
In the next set of experiments, the sintering behavior of the silver nanoparticles was observed. Figure 2 shows two 15nm diameter nanoparticles in the process of sintering. During this experiment, the temperature was maintained at 200°C. The neck radius was determined as 4.2nm in the centre figure and in the dihedral angle was determined to be 100°.
Figure 2. Two 15nm diameter nanoparticles in the process of sintering
The surface diffusion coefficient can be determined from these parameters and is based on the change in neck radius over time, the temperature and the particle radius. Here, the range was 1.04 to 1.55 x 10-21 m2/s. The organic capping layer is believed to reduce the surface diffusion coefficient.
Metal nanoparticles are an essential material used for an extensive range of applications. It is critical to ascertain how they behave at high temperatures particularly in the case of nanoparticles being used as catalysts.
The behaviour of small particles from room temperature to 1200°C is determined using the Aduro heating and electrical biasing system on a particle-by-particle basis, at resolutions down to the atomic scale.
Protochips, Inc. is a rapidly growing early-stage company focused on providing the world's leading materials and life sciences research breakthrough analytical tools for targeted research and development of nano-scale materials.
Using its proprietary technology, Protochips is addressing the market need by transforming the most widely used tools in nanotechnology – electron and optical microscopes - from cameras into complete nano-scale laboratories.
Protochips' core competency lies in the application of semiconductor techniques to development of MEMS devices capable of providing heat, electrical, liquid and gas environments to samples in situ.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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