Advances in Aerosol Processing Furthering the Utilization of Aerosols in Nanomanufacturing

Reviewed by Dr. M. Ostraat, RTI International

• S. L. Girshick, "Aerosol Processing for Nanomanufacturing," J. Nanopart. Res. 10: 935-945 (invited paper for special issue on nanomanufacturing, 2008). DOI: 10.1007/s11051-007-9331-6
• Liu, P., P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, 1995, "Generating particle beams of controlled dimensions and divergence: I. Theory of particle motion in aerodynamic lenses and nozzle expansions," Aerosol Sci. Technol. 22: 293-313. DOI: 10.1080/02786829408959748
• Liu, P., P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, 1995, "Generating particle beams of controlled dimensions and divergence: II. Experimental evaluation of particle motion in aerodynamic lenses and nozzle expansions," Aerosol Sci. Technol. 22:314-324. DOI: 10.1080/02786829408959749

The ability to create materials from the bottom-up is a powerful tool in nanotechnology.  By synthesizing materials with a bottom-up approach, the nanotechnologist can specify the composition, arrangement, and morphology of the ultimate nanostructure by tuning synthesis and processing conditions, thus fabricating the desired material that exhibits the desired property.  Aerosol synthesis, the creation of liquid or solid particles suspended in a gas, is one tool capable of generating nanomaterials with a bottom-up approach. 

In a recent research review paper, Professor Steven Girshick from the University of Minnesota details several advances in aerosol processing that can further the utilization of aerosols in nanomanufacturing (J. Nanopart. Res. 10: 935-945).  He offers the following advantages of aerosol synthesis over liquid synthesis of nanomaterials, including higher purity, continuous versus batch synthesis, higher throughput, and solvent-less processes.  Girshick provides an overview of three different technologies developed in his laboratories to advance the gas-phase processing of nanoparticles, namely a) hypersonic impaction of nanoparticles onto surfaces to produce nanoparticle coatings, b) focusing techniques to align nanoparticles for deposition in nanofabrication applications, and c) photoinduced chemical vapor deposition (CVD) of coating materials onto nanoparticles to modify nanoparticle surfaces.

In hypersonic impaction of nanoparticles, Girshick describes a hypersonic plasma particle deposition (HPPD) process in which nanostructured film growth occurs by a combination of nanoparticle impaction and CVD, resulting in a film in which nanoscale grains and interfacial material between those grains is controlled.  Although challenges remain, including thickness and particle concentration uniformities, areas that may utilize the advantages of HPPD include applications in which dense, nonporous films that resist crack propagation are required.

Girshick then reviews progress made in the ability to “write” two-dimensional lines and patterns and three-dimensional structures of nanoparticles.  By using aerodynamic lenses developed by McMurry and coworkers (Aerosol Sci. Technol. 22: 293-313; Aerosol Sci. Technol. 22:314-324), Girshick adapts focused beam technology into micromechanical systems (MEMS) fabrication.  By combining nanoparticles in microfabrication, Girshick recognizes the potential to exploit nanoparticle functionality, such as photoluminescence, into MEMS devices.  An illustration of the integration of aerosol synthesis, aerodynamic lens focusing, and MEMS fabrication is depicted in Figure 1.

In the final overview, Girshick presents results on the coating of individual nanoparticles through photoinduced CVD.  Either by coating nanoparticles to passivate their surface or to change their surface chemistry, photoinduced CVD is advantageous in that in addition to high temperature and high pressure environments, it can also be conducted at room temperature and at atmospheric pressure.  Using known aerosol instrumentation, the coating thickness applied to the particles can be directly measured.

From the perspective of nanomanufacturing, aerosol processing remains a capability with great potential that simultaneously offers considerable flexibility in system configuration and integration.  Indeed, Girshick concludes with a vision that “many types of nanomanufacturing will require multiple nanoparticle processing steps.  From this viewpoint, sequential operations that utilize a continuous aerosol flow stream would have obvious advantages.”