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Silver Nano-platelet Precursors for Ultra-Thin Conductors
Brendan Farrell, Sarah Rowland & Dan V. Goia
Center for Advanced Materials Processing
Eric Groat
NanoDynamics Inc.
Introduction
Metallic spheres or flakes are used extensively in the electronics industry to create thin, conductive layers. This paper describes a novel approach that produces silver nano - thickness flakes (nanoplatelets) cost-effectively in large scale through chemical precipitation. These nanoplatelets are excellent candidates for generating uniform conductive metallic layers either by conventional screen printing or by inkjet printing, flexography, or spin coating. Their unique properties offer the possibility of sintered as well as non-sintered metallic conductive structures, opening avenues to new electronic architectures.
Particle preparation and characterization
The silver nanoplatelets described in this paper were produced by precipitation using an environmentally friendly compound to reduce silver ions in aqueous solution in the presence of an organic dispersant. By controlling the nucleation and growth of the metallic phase using principles described in our previous published work1 this novel method can generate either isometric or anisotropic silver particles. Advantages include simple equipment, benign experimental conditions, and a high volumetric concentration. A patent has been applied for and will be licensed to NanoDynamics Inc.
Particle Properties
Figure 1 shows the anisotropic dimensions of the silver platelets, with thickness averaging about 60nm and the largest dimension averaging about 900nm.
 Figure 1a. FE-SEM images of Ag nanoplatelets at magnification of 50K
Figure 1b. FE-SEM images of Ag nanoplatelets at magnification of 100K
Only five to ten per cent are isometric, too few to prevent densely packed platelet structures oriented preferentially along the underlying substrate. Figure 2 shows a size distribution of the silver nanoplatelets with two distinct peaks, at ~ 80 nm and ~900 nm, corresponding to the two major dimensions of the particles.
Figure 2. Particle size distribution of Ag nanoplatelets
The particle size and electron micrographs suggest that the particles of silver are fully dispersed. Figure 3a shows nanoplatelets deposited on a glass slide as thin, uniform, and well packed deposits.
 Figure 3a. FESEM images of cross section of the deposited dry film of Ag nanoplatelets
Figure 3b. FESEM images of top view of the deposited dry film of Ag nanoplatelets
Fig. 3b shows that the silver nanoplatelets tend to orient along the direction of the support, giving a measurable electrical conductivity. Films with isometric particles of similar size, degree of dispersion, crystalline structure, and surface properties do not show this conductivity. Curing the film at 200ºC decreases the initial electrical resistivity of about 100 mΩ/in2by at least an order of magnitude. This may result from both a rearrangement of the silver nanoplatelets and a partial removal of the dispersant from the particle surfaces.
The high purity of all chemical compounds used results in low levels (less than 10 ppm) of inorganic impurities in the final silver nanoplatelets. Such levels of residual organics are typical of similar precursors presently used for conductive metallic layers in multilayer ceramic capacitors (MLCC), low-temperature co-fired ceramics (LTCC), plasma display panels (PDP), solar cells, and other electronic applications. Sintering the silver particles leads to near - total elimination of the pores in the packed structure. Sintering temperatures are comparable to those of isometric silver particles of similar size.
Conclusions
Highly dispersed silver platelets with thickness in the nano domain (~60 nm) and a largest dimension around 0.9 micrometers can be prepared by a simple and cost effective precipitation process in aqueous homogeneous solutions. The particle characteristics suggest that these materials could be used to construct thin, continuous conductive metallic layers. Because of their anisotropy, these materials could form thin yet relatively conductive 'green' deposits that can find applications in non-sintered thick film applications. In addition, these materials could also offer unique sintering properties which could be helpful in generating smaller and more complex electronic architectures.
References
1. Goia, D.V.; Preparation and Formation Mechanisms of Uniform Metallic Particles in Homogeneous Solutions. Journal of Materials Chemistry. 2003, 14, 451-458
FOR MORE INFORMATION
Author Contacts
Brendan Farrell, Sarah Rowland & Dan V. Goia
Center for Advanced Materials Processing, Potsdam, NY 13699-5814, USA
Tel: 1-315-268-4411; Fax: 1-315-268-6567; email: goiadanv@clarkson.edu
Eric Groat
NanoDynamics Inc., 901 Fuhrmann Boulevard, Buffalo NY 14203, USA
Tel: 1-716-853-4900; email: egroat@nanodynamics.com
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